U.S. patent application number 17/828574 was filed with the patent office on 2022-09-22 for application of immobilized enzymes for nanopore library construction.
This patent application is currently assigned to New England Biolabs, Inc.. The applicant listed for this patent is New England Biolabs, Inc.. Invention is credited to Yi Fang, Luo Sun, Ming-Qun Xu, Aihua Zhang.
Application Number | 20220298504 17/828574 |
Document ID | / |
Family ID | 1000006377823 |
Filed Date | 2022-09-22 |
United States Patent
Application |
20220298504 |
Kind Code |
A1 |
Xu; Ming-Qun ; et
al. |
September 22, 2022 |
Application of Immobilized Enzymes for Nanopore Library
Construction
Abstract
The present disclosure relates, according to some embodiments,
to methods for preparing a library for sequencing. For example, a
method may comprise (a) in a coupled reaction, (i) contacting a
population of nucleic acid fragments with a tailing enzyme to
produce tailed fragments, and (ii) ligating to the tailed fragments
a sequencing adapter with a ligase to produce adapter-tagged
fragments; and/or separating adapter-tagged fragments from the
tailing enzyme and the ligase to produce separated adapter-tagged
fragments and, optionally, separated tailing enzyme and/or
separated ligase. In some embodiments, a tailing enzyme and/or a
ligase used in library preparation may be immobilized enzymes.
Inventors: |
Xu; Ming-Qun; (Hamilton,
MA) ; Fang; Yi; (Ipswich, MA) ; Zhang;
Aihua; (Hamilton, MA) ; Sun; Luo; (Hamilton,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
New England Biolabs, Inc. |
Ipswich |
MA |
US |
|
|
Assignee: |
New England Biolabs, Inc.
Ipswich
MA
|
Family ID: |
1000006377823 |
Appl. No.: |
17/828574 |
Filed: |
May 31, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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17018862 |
Sep 11, 2020 |
11377654 |
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17828574 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C12Q 1/6869 20130101;
C12N 15/1065 20130101; C12Q 1/6806 20130101 |
International
Class: |
C12N 15/10 20060101
C12N015/10; C12Q 1/6806 20060101 C12Q001/6806; C12Q 1/6869 20060101
C12Q001/6869 |
Claims
1. A method of preparing a library for sequencing, comprising: (a)
in a coupled reaction, (i) contacting a population of nucleic acid
fragments with a tailing enzyme to produce tailed fragments, and
(ii) ligating to the tailed fragments a sequencing adapter with a
ligase to produce adapter-tagged fragments; and (b) separating
adapter-tagged fragments from the tailing enzyme and the ligase to
produce separated adapter-tagged fragments and, optionally,
separated tailing enzyme and/or separated ligase.
2. A method according to claim 1, wherein the tailing enzyme and
the ligase are immobilized enzymes.
3. A method according to claim 1, wherein the tailing enzyme is
immobilized on a magnetic bead.
4. A method according to claim 3, wherein the separating the
adapter tagged fragments further comprises subjecting the coupled
reaction to a magnetic field.
5. A method according to claim 1, wherein the ligase is immobilized
on a magnetic bead.
6. A method according to claim 5, wherein the separating the
adapter tagged fragments further comprises subjecting the coupled
reaction to a magnetic field.
7. A method according to claim 1, wherein the tailing enzyme and
the ligase are immobilized on separate supports.
8. A method according to claim 1, wherein the coupled reaction
steps occur in a single tube, well, capillary, flow cell or
surface.
9. A method according to claim 1, wherein the tailing enzyme and
the ligase are soluble enzymes.
10. A method according to claim 1, wherein the population of
nucleic acid fragments comprise ribonucleic acid fragments.
11. A method according to claim 1, wherein the population of
nucleic acid fragments comprise deoxyribonucleic acid
fragments.
12. A method according to claim 1, wherein the population of
nucleic acid fragments has less than 100 ng of nucleic acids.
13. A method according to claim 1, wherein the population of
nucleic acid fragments has less than 10 ng of nucleic acids.
14. A method according to claim 1, further comprising: (c) in a
second coupled reaction, (i) contacting a second population of
nucleic acid fragments with the separated tailing enzyme to produce
additional tailed fragments, and (ii) ligating to the additional
tailed fragments a second sequencing adapter with the separated
ligase to produce additional adapter-tagged fragments, and (d)
separating the additional adapter-tagged fragments from the
separated tailing enzyme and the separated ligase to produce
separated additional adapter-tagged fragments, separated tailing
enzyme, and separated ligase.
15. A method according to claim 14, further comprising: (e)
translocating the separated adapter-tagged fragments through one or
more transmembrane pores; (f) detecting electrical changes as the
one or more separated adapter-tagged fragments are translocated
through the one or more transmembrane pores in an insulating
membrane to produce an electrical signal; and (g) analyzing the
electrical signal to generate a sequence read.
16. A method according to claim 15, wherein the one or more
transmembrane pores retain about 90% of their initial activity
after two hours.
17. A method according to claim 15, wherein the one or more
transmembrane pores retain about 50% of their initial activity
after 8 hours.
18. A method according to claim 15, wherein the one or more
transmembrane pores produce at least 900 sequence reads per
transmembrane pore.
19. A method according to claim 1, wherein the sequencing adapter
is a single stranded adapter comprising: a leader sequence; and a
first sequence and a second sequence, wherein the first and second
sequences are complementary to each other and define a hairpin,
wherein the leader sequence is configured to thread into the one or
more transmembrane pores.
Description
SEQUENCE LISTING STATEMENT
[0001] This disclosure includes a Sequence Listing submitted
electronically in ascii format under the file name
"NEB-424_ST25.txt". This Sequence Listing is incorporated herein in
its entirety by this reference.
BACKGROUND
[0002] The nanopore sequencing platform provided by Oxford Nanopore
Technologies (ONT) is a third-generation sequencing approach to
sequence long DNA/RNA molecules through the change of electrical
signals as the DNA/RNA passes through the nanopore on a membrane
(Jaworski, E. and A. Routh, Parallel ClickSeq.RTM. and Nanopore
sequencing elucidates the rapid evolution of defective-interfering
RNAs in Flock House virus. PLoS pathogens, 2017. 13(5): p.
e1006365; Weirather, J. L., et al., Comprehensive comparison of
Pacific Biosciences and Oxford Nanopore Technologies and their
applications to transcriptome analysis. F1000Research, 2017. 6;
Wongsurawat, T., et al., Rapid sequencing of multiple RNA viruses
in their native form. Frontiers in microbiology, 2019. 10: p. 260;
Zhao, L., et al., Analysis of transcriptome and epitranscriptome in
plants using PacBio Iso-Seq.RTM. and nanopore-based direct RNA
sequencing. Frontiers in Genetics, 2019. 10: p. 253). Nanopore
direct RNA sequencing permits generation of full length,
strand-specific RNA sequence reads. However, library prep practices
with multiple bead purification steps demand relatively high input
of RNA or DNA, at least in part, because significant sample loss
can occur during these steps. This bead purification procedure may
also produce bias in binding and elution of nucleic acid substrates
of various lengths so that the output doesn't precisely represent
the input library. In particular, polynucleotides (e.g., long or
ultralong RNA and DNA templates) may be subjected to breakage and
precipitation during bead (e.g., AMPure.RTM. bead)
purification.
SUMMARY
[0003] The present disclosure provides methods for preparing a
library for sequencing. For example, a method may comprise (a) in a
coupled reaction, (i) contacting a population of nucleic acid
fragments with a tailing enzyme to produce tailed fragments, and
(ii) ligating to the tailed fragments a sequencing adapter with a
ligase to produce adapter-tagged fragments; and/or separating
adapter-tagged fragments from the tailing enzyme and the ligase to
produce separated adapter-tagged fragments and, optionally,
separated tailing enzyme and/or separated ligase. In some
embodiments, a tailing enzyme and/or a ligase used in library
preparation may be immobilized enzymes. For example, a tailing
enzyme may be immobilized on a magnetic bead and/or a ligase may be
immobilized on a magnetic bead. Optionally, a tailing enzyme and a
ligase may be immobilized on the separate supports or
co-immobilized on a single support. A tailing enzyme and/or a
ligase, according to some embodiments, may be soluble enzymes. In
some embodiments, separating adapter tagged fragments may further
comprise subjecting the coupled reaction to a magnetic field (e.g.,
bringing the sample to a magnet, bringing a magnet to the sample,
activating an electromagnetic field). A population of nucleic acid
fragments may comprise ribonucleic acid fragments and/or may
comprise deoxyribonucleic acid fragments. In some embodiments,
methods may be capable of producing sequencing libraries with
little input RNA. For example, methods may use a population of
nucleic acid fragments having less than 100 ng of nucleic acids or
a population of nucleic acid fragments having less than 10 ng of
nucleic acids.
[0004] The present disclosure further provides methods for
preparing sequencing libraries comprise any combination of steps
(a) and (b) and further comprise: (c) in a second coupled reaction,
(i) contacting a second population of nucleic acid fragments with
the separated tailing enzyme to produce additional tailed
fragments, and (ii) ligating to the additional tailed fragments a
second sequencing adapter with the separated ligase to produce
additional adapter-tagged fragments, and/or (d) separating the
additional adapter-tagged fragments from the separated tailing
enzyme and the separated ligase to produce separated additional
adapter-tagged fragments, separated tailing enzyme, and separated
ligase. In some embodiments, a method comprise any combination of
steps (a), (b), (c), and (d) and may further comprise (e)
translocating the separated adapter-tagged fragments through one or
more transmembrane pores; (f) detecting electrical changes as the
one or more separated adapter-tagged fragments are translocated
through the one or more transmembrane pores in an insulating
membrane to produce an electrical signal; and/or (g) analyzing the
electrical signal to generate a sequence read. In some embodiments,
one or more transmembrane pores may retain about 90% of their
initial activity after two hours and/or may retain about 50% of
their initial activity after 8 hours. One or more transmembrane
pores, according to some embodiments of the disclosure, may produce
at least 900 sequence reads per transmembrane pore. In some
embodiments, a sequencing adapter may be a single-stranded adapter
and may comprise a leader sequence; and a first sequence and a
second sequence, wherein the first and second sequences are
complementary to each other and define a hairpin, wherein the
leader sequence is configured to thread into the one or more
transmembrane pores.
BRIEF DESCRIPTION OF THE FIGURES
[0005] The file of this patent contains at least one drawing
executed in color. Copies of this patent with color drawing(s) will
be provided by the Patent and Trademark Office upon request and
payment of the necessary fee.
[0006] FIG. 1A-E schematically illustrates five methods for
preparing nucleic acid libraries for sequencing (e.g., Nanopore
MinION.RTM. sequencing). FIG. 1A shows library construction in two
sequential enzymatic steps using soluble enzymes without
AMPure.RTM. bead purification (Sol without BP). FIG. 1B shows
library construction in two sequential enzymatic steps using
soluble enzymes with AMPure.RTM. bead purification (Sol-seq). FIG.
1C shows library construction in a coupled enzymatic reaction using
soluble enzymes with AMPure.RTM. bead purification (Sol-cpl). FIG.
1D shows library construction in two sequential enzymatic steps
using immobilized enzymes without bead purification (Im-seq). FIG.
1E shows library construction in a coupled enzymatic reaction using
immobilized enzymes without bead purification (Im-cpl).
[0007] FIG. 2 shows that poly(A) extension is dependent on poly(A)
polymerase concentration. An RNA 45-mer oligo strand was treated
with different concentrations of untagged poly(A) polymerase from
NEB and poly(A) tailing activity of poly(A) polymerase was
evaluated by capillary electrophoresis (CE).
[0008] FIG. 3 shows that poly(A) extension is dependent on the
concentration of poly(A) polymerase-SNAP fusion protein. An RNA
45-mer oligo strand was treated with different concentrations of a
poly(A) polymerase-SNAP fusion protein and poly(A) tailing activity
of was evaluated by capillary electrophoresis (CE).
[0009] FIG. 4 shows poly(A) polymerase immobilization on
PEG.sub.750 coated O.sup.6-benzylguainine (B G) beads.
[0010] FIG. 5 shows that poly(A) extension is dependent on the
concentration of an immobilized poly(A) polymerase. An RNA 45-mer
oligo strand was treated with different concentrations of an
immobilized poly(A) polymerase and poly(A) tailing activity was
evaluated by capillary electrophoresis (CE).
[0011] FIG. 6 shows that poly(A) tailing activity from soluble or
immobilized SNAP-tagged poly(A) polymerase (PAP). The following
four reaction samples (from top to bottom) were analyzed by CE
technique: Sample 1, 45-mer RNA oligo substrate without enzymatic
treatment; Sample 2, substrate treated with soluble PAP; Sample 3,
substrate reacted with PAP immobilized to BG-magnetic beads without
PEG.sub.750-coated; sample 4: substrate reacted with PAP
immobilized to PEG.sub.750-coated BG magnetic beads. Sample 2 and
Sample 4 displayed poly(A) tailing activity.
[0012] FIG. 7 shows relative activity of T4 DNA ligase immobilized
on magnetic beads stored at -20.degree. C. or 25.degree. C. over a
period of 7 days. The activity at day 1 is normalized to 100%.
[0013] FIG. 8 shows that T4 DNA ligase immobilized on agarose beads
is more thermostable than two soluble T4 DNA ligases. Heat
treatment was conducted with two soluble T4 DNA Ligases, untagged
T4 DNA Ligase (NEB M0202) and SNAP-tagged T4 DNA Ligase (HS-T4 DNA
Ligase), and immobilized T4 DNA Ligase (HS-T4 DNA Ligase-Agarose).
Ligase activity was monitored using FMA-labeled synthetic
double-stranded DNA (FAM-dsDNA). The substrate/enzyme mixtures were
treated at various temperatures (40.degree. C.-100.degree. C.),
followed by incubation at 4.degree. C. overnight. Fluorescent gel
scanning was used to visualize substrate and ligation products
(including the major product, termed Product), as detected in
positive controls when the reactions were pre-treated at 4.degree.
C. but were absent in the negative controls (NO Enzyme).
[0014] FIG. 9 shows that SNAP-tagged T4 DNA Ligase immobilized onto
agarose beads displayed ligase activity after heat treatment at
45.degree. C. for 30 min but the soluble form showed little or no
ligase activity under the same conditions. Aliquots of HS-T4 DNA
Ligase conjugated to BG-Agarose beads were incubated for 30 min at
4.degree. C. (A), 37.degree. C. (B) or 45.degree. C. (C), followed
by ligation reactions at room temperature (23.degree. C.) for 2
hours. The samples (in the same order) were loaded onto three PAGE
gels for electrophoretic separation, followed by fluorescent gel
scanning. 1, No Enzyme; 2, Ligase (untagged T4 DNA Ligase, NEB
M0202); 3, HS-Ligase (SNAP-tagged T4 DNA Ligase); 4,
Ligase-Agarose; 5, Ligase-Chitin; 6, Ligase-Mag; 7, Ligase-SiM.
Arrows indicate the expected positions of ligation product for
soluble T4 DNA Ligase (arrows in lane 2) or Ligase-Agarose (arrows
in lane 4).
[0015] FIG. 10 shows that SNAP-tagged T4 DNA Ligase immobilized
onto agarose beads displayed more ligase activity after heat
treatment at 55.degree. C. or 65.degree. C. than the soluble form,
which had little or no ligase activity under the same conditions.
DNA ligation was monitored using a fluorophore (FAM)-labeled DNA
substrate. The reactions were incubated for 10 min at 4.degree. C.,
55.degree. C. or 65.degree. C., followed by ligation reactions at
23.degree. C. for 2 hours (FIG. 10A) or at 4.degree. C. overnight
(FIG. 10B). The samples were electrophoresed on PAGE gels, followed
by fluorescent gel scanning Ligase, untagged T4 DNA Ligase, NEB
M0202; HS-Ligase, SNAP-tagged T4 DNA Ligase; Ligase-Agarose, HS-T4
DNA Ligase immobilized to BG-Agarose Beads; Ligase-Mag, HS-T4 DNA
Ligase conjugated to BG-Magnetic Beads. Arrows indicate the major
ligation product from the reactions with Ligase-Agarose.
[0016] FIG. 11 shows that immobilized enzymes can be reused for
consecutive reactions. Data from reactions #1, #10, and #20 of
twenty consecutive ligation reactions catalyzed by a single
preparation of immobilized T4 DNA ligase. Performance over these
twenty reactions matches the results observed in a single reaction
with soluble T4 DNA ligase.
[0017] FIG. 12 shows number of direct RNA sequencing reads from
libraries prepared by two methods. In the first, labeled Sol-seq,
the library was prepared by two sequential steps of poly(A) tailing
and adaptor ligation. In the second, labeled Sol-cpl, the library
was prepared by carrying out poly(A) tailing and adaptor ligation
simultaneously.
[0018] FIG. 13 shows the poly(A) tailing and adaptor ligation
activity from immobilized poly(A) polymerase and immobilized T4 DNA
Ligase. Four samples were examined by CE (from top to bottom):
Sample 1, untreated FAM-labeled RNA substrate showing a distinct
peak; sample 2, FAM-labeled RNA substrate treated by immobilized
Poly(A) polymerase; Sample 3, Sample 2 treated with immobilized T4
DNA ligase and RTA-poly(dT).sub.15; Sample 4, Sample 2 treated with
immobilized T4 DNA ligase and RTA-poly(dT).sub.10. A bell-shaped
peak in Sample 2 represents addition of 3' poly(A) tails of various
length to the RNA substrate (Sample 1). Ligation of an RTA adaptor
to the poly(A) tailed products generated higher molecular mass
products resulting in a shift of the bell-shaped peak to the
right.
[0019] FIG. 14 compares Nanopore RNA sequence reads obtained with
libraries prepared by different methods. Each library was prepared
using soluble enzymes without bead purification (Sol w/o BP),
soluble enzymes with sequential poly(A) tailing and ligation with
bead purification (Sol-seq), or immobilized enzymes with sequential
poly(A) tailing and ligation protocol without bead purification
(Im-seq). 164 ng of RNA library was used for each sequencing
run.
[0020] FIG. 15 shows total sequence reads from Nanopore direct RNA
sequencing of RNA libraries constructed with 500 ng input RNA using
immobilized enzymes without AMPure.RTM. bead purification following
either sequential reaction protocol (Im-seq) or coupled reaction
protocol (Im-cpl). After enzymatic treatment enzyme-conjugated
beads were pelleted on a magnetic rack and the supernatants were
transferred to a fresh tube. 105 ng of total RNA from each library
was mixed with the solution provided by ONT before loading onto
MinION.RTM. R9.1.4 flow cells for direct RNA sequencing.
[0021] FIG. 16A-B shows that co-immobilized enzymes displayed both
poly(A) polymerase activity and T4 DNA ligase activity. FIG. 16A
shows that immobilized poly(A) polymerase, co-immobilized with T4
DNA ligase, is active in a poly(A) tailing assay in which a poly(A)
tail is added to a 35-mer RNA (lower panel), but not a
corresponding control (upper panel). FIG. 16B shows that
immobilized T4 DNA ligase, co-immobilized with poly(A) polymerase
on BG-modified beads (BGPL), displayed activity in an adapter
ligation assay in which adapters RTA and RMX were ligated to each
other (lower panel), but not a control with RTA alone (upper
panel).
[0022] FIG. 17 shows an example of fully automated Nanopore
sequencing workflow which includes library construction catalyzed
by immobilized enzymes.
[0023] FIG. 18A-B shows the number of Nanopore RNA sequencing reads
obtained with low-input RNA libraries prepared with immobilized
enzymes. FIG. 18A shows the number of sequencing reads obtained
from 100 ng of Listeria monocytogenes RNA libraries prepared using
immobilized enzymes following either a sequential reaction method
("Im-seq 100"; Example 4D) or a coupled reaction method ("Im-cpl
100"; Example 4E) without AMPure.RTM. bead purification. FIG. 18B
shows the number of sequencing reads obtained from 100 ng of
mammalian RNA libraries prepared using immobilized ligase ("ImL")
or immobilized polymerase and immobilized ligase ("ImP & ImL")
without AMPure.RTM. bead purification. Each output library was
loaded onto Nanopore R9.4.1. flow cells for direct RNA
sequencing.
[0024] FIG. 19 shows a comparison of the number of Nanopore RNA
sequencing reads obtained with RNA libraries prepared according to
one of five methods: Sol-seq libraries were prepared with soluble
enzymes and bead purification using a sequential reaction protocol;
Sol-cpl libraries were prepared with soluble enzymes and bead
purification using a coupled reaction protocol; Sol w/o BP
libraries prepared with soluble enzymes sequentially without bead
purification; Im-seq libraries were prepared with immobilized
enzymes using the sequential reaction protocol without bead
purification; Im-cpl libraries were prepared with immobilized
enzymes using the coupled reaction protocol without bead
purification. The sequencing reads shown were obtained after 2 hour
run time.
[0025] FIG. 20 shows a comparison of functional nanopores over time
during direct RNA sequencing runs in ONT flow cells. Duplicate
libraries were prepared using each of the five protocols
illustrated in FIG. 1A-E and the resulting sequencing reads were
displayed in FIG. 19.
[0026] FIG. 21 shows the sequence reads per nanopore from libraries
prepared by the coupled reaction method (square dots between 900
and 1100 of normalized reads) in comparison with those from the
libraries prepared by the sequential reaction method with
immobilized enzymes (circular dots between 200 and 500 of
normalized reads). The orange curve aligns the reads/pore data
points from three sequencing samples with 83 ng, 109 ng or 136.5 ng
loaded on a flow cell. The blue curve aligns the reads/pore data
points from four sequencing samples with various amount of RNA (38
ng, 39 ng, 105 ng and 164.4 ng) per flow cell.
[0027] FIG. 22 shows proposed DNA library preparation workflow for
ONT sequencing platform. The workflow is comprised of three
reactions, poly(dA) tailing catalyzed by terminal deoxynucleotidyl
transferase (TdT), ligation of a 3' Poly(dT)-containing adaptor
with motor protein (red dot), and gap-filling and nick sealing with
DNA polymerase (Pol) and DNA Ligase. Relevant soluble or
immobilized enzymes can be utilized to catalyze each enzymatic
treatment. Enzymes may be removed, inactivated or present in the
final sequencing library.
[0028] FIG. 23 shows a two-step reaction of poly(dA) tailing of a
synthetic double-stranded DNA substrate (possessing a FAM probe)
catalyzed by TdT and subsequent ligation with a synthetic adaptor,
RTA-poly(dT) possessing a ROX probe. A ligated product can be
detected by CE analysis due to the presence of both FAM and ROX
probes (as shown in FIG. 18C).
[0029] FIG. 24A-C shows sequential poly(dA) and ligation reactions
with immobilized enzymes. FIG. 24A shows poly(dA) tailing of
5'FAM-labeled DNA substrate by TdT in two different
substrate-to-dATP ratios (1:100 and 1:200). Incorporation of dAMP
at the 3' termini of 5'FAM-labeled DNA strand and the length or
range of poly(dA) can be detected by CE analysis. FIG. 24B shows
detection of a FAM-labeled DNA substrate ligated to ROX-labeled
RTA-Poly(dT) using FAM-detecting channel by CE analysis. Top:
FAM-labeled DNA substrate; Middle: FAM-labeled DNA substrate
treated with TdT shows multiple species corresponding to various
poly(dA) length; Bottom: the Poly(dA)-tailed DNA mixture further
treated with T4 DNA Ligase in the presence of RTA-Poly(dT) exhibits
a shift to a pool of higher molecular mass species with various
length of poly(dA) tails. FIG. 24C shows detection of a FAM-labeled
DNA substrate ligated to ROX-labeled RTA-Poly(dT) by CE analysis.
Top: FAM-labeled DNA substrate without enzyme treatment; Middle:
FAM-labeled DNA substrate treated with TdT shows multiple species
corresponding to various length of poly(dA) tails; Bottom:
detection of the ligation products using both FAM- and
ROX-detecting channels (depicted in blue and red, respectively).
The Poly(dA)-tailed DNA mixture (as presented in the middle graph)
treated with T4 DNA Ligase in the presence of RTA-Poly(dT) exhibits
a shift to a pool of higher molecular mass species with various
length of poly(dA) tails, with overlapping signals from FAM and ROX
probes.
[0030] FIG. 25 shows CE analysis of sequential Poly(dA) tailing and
adaptor ligation reaction products catalyzed by soluble and
immobilized T4 DNA ligase. FAM-labeled DNA substrate ligated to
ROX-labeled RTA-Poly(dT) by TdT in a substrate-to-dATP ratio of
1:100. Subsequently, the reaction medium containing the
poly(dA)-tailed DNA products (pool), was incubated with either
soluble or immobilized T4 DNA ligase and RTA-poly(dT) adaptor
possessing 3' poly(dT) and ROX probe. DNA substrate: FAM-labeled
DNA substrate without enzyme treatment; TdT: FAM-labeled DNA
substrate treated with TdT showing multiple species corresponding
to various length of poly(dA) tails; TdT+Ligase: Poly(dA) tailed
DNA treated by T4 DNA Ligase was examined with both FAM- and
ROX-detecting channels (depicted in blue and red, respectively).
TdT+IM-Ligase: TdT-treated DNA was treated with immobilized T4 DNA
Ligase and examined with both FAM- and ROX-detecting channels
(depicted in blue and red, respectively). RTA-Poly(dT): adaptor
without enzymatic treatment. Co-localization of the fluorescence
signals of FAM (blue) and ROX (red) indicates ligation of the 5'
FAM-labeled DNA pool to the 3' ROX-labeled strand of the
adaptor.
[0031] FIG. 26 shows schematic diagrams of two methods of DNA
library construction. FIG. 26A shows library construction using
soluble enzymes with an AMPure.RTM. bead purification step. FIG.
26B shows library construction using immobilized DNA modifying
enzymes without AMPure.RTM. bead purification.
[0032] FIG. 27 shows end repair, dA-tailing and adaptor ligation of
synthetic DNA modified using immobilized enzymes with products of
each step subjected to CE analysis. This method is designed for
construction of Nanopore DNA library without use of AMPure.RTM.
bead purification and PEG-based buffer.
DETAILED DESCRIPTION
[0033] The present disclosure generally relates to methods and
compositions for preparing polynucleotide libraries. Polynucleotide
libraries, in some embodiments, may be prepared for sequencing
using the disclosed methods and compositions. In some embodiments,
compositions comprising polynucleotides (e.g., fragments) may be
subjected to coupled reactions in which soluble enzymes,
immobilized enzymes, or both soluble and immobilized enzymes repair
or condition the ends of the polynucleotides, tail one or both
ends, and/or ligate the polynucleotides to a sequencing adapter.
One or more of the enzymes used may be immobilized on a bead (e.g.,
a magnetic bead) or other solid support. For example, in a coupled
reaction comprising a tailing reaction and a ligation reaction, a
tailing enzyme and a ligase may be immobilized on separate supports
or co-immobilized on a common support Immobilized enzymes may
reduce or obviate the need for damaging bead purification steps.
Bead purification may be used to remove soluble enzymes and other
compounds in the reaction media, but may also damage the
polynucleotides being purified and may introduce contaminating
chemicals present on the beads or in required wash solutions (e.g.,
ethanol and PEG among others) that may interfere with subsequent
uses of the purified polynucleotides (e.g., sequencing). Library
preparation methods using immobilized enzymes may require lower
amounts of input polynucleotides to achieve the same number of
sequencing reads and may better preserve the activity of
transmembrane pores used in sequencing. Library preparation and
sequencing workflows using immobilized enzymes may be automated and
may include reuse of immobilized enzymes, preserving reagents and
lowering costs.
[0034] Aspects of the present disclosure can be further understood
in light of the embodiments, section headings, figures,
descriptions and examples, none of which should be construed as
limiting the entire scope of the present disclosure in any way.
Accordingly, the claims set forth below should be construed in view
of the full breadth and spirit of the disclosure.
[0035] Each of the individual embodiments described and illustrated
herein has discrete components and features which can be readily
separated from or combined with the features of any of the other
several embodiments without departing from the scope or spirit of
the present teachings. Any recited method can be carried out in the
order of events recited or in any other order which is logically
possible.
[0036] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this disclosure belongs. Still,
certain terms are defined herein with respect to embodiments of the
disclosure and for the sake of clarity and ease of reference.
[0037] Sources of commonly understood terms and symbols may
include: standard treatises and texts such as Kornberg and Baker,
DNA Replication, Second Edition (W.H. Freeman, New York, 1992);
Lehninger, Biochemistry, Second Edition (Worth Publishers, New
York, 1975); Strachan and Read, Human Molecular Genetics, Second
Edition (Wiley-Liss, New York, 1999); Eckstein, editor,
Oligonucleotides and Analogs: A Practical Approach (Oxford
University Press, New York, 1991); Gait, editor, Oligonucleotide
Synthesis: A Practical Approach (IRL Press, Oxford, 1984);
Singleton, et al., Dictionary of Microbiology and Molecular
biology, 2d ed., John Wiley and Sons, New York (1994), and Hale
& Markham, the Harper Collins Dictionary of Biology, Harper
Perennial, N.Y. (1991) and the like.
[0038] As used herein and in the appended claims, the singular
forms "a" and "an" include plural referents unless the context
clearly dictates otherwise. For example, the term "a protein"
refers to one or more proteins, i.e., a single protein and multiple
proteins. It is further noted that the claims can be drafted to
exclude any optional element. As such, this statement is intended
to serve as antecedent basis for use of such exclusive terminology
as "solely," "only" and the like in connection with the recitation
of claim elements or use of a "negative" limitation.
[0039] Numeric ranges are inclusive of the numbers defining the
range. All numbers should be understood to encompass the midpoint
of the integer above and below the integer i.e., the number 2
encompasses 1.5-2.5. The number 2.5 encompasses 2.45-2.55 etc. When
sample numerical values are provided, each alone may represent an
intermediate value in a range of values and together may represent
the extremes of a range unless specified.
[0040] In the context of the present disclosure, "adapter" refers
to a sequence that is joined to or can be joined to another
molecule (e.g., ligated or copied onto via primer extension). An
adapter can be DNA or RNA, or a mixture of the two. An adapter may
be 15 to 100 bases, e.g., 50 to 70 bases, although adapters outside
of this range are envisioned. In a library of polynucleotide
molecules that contain an adapter (e.g., a 3' or 5' adapter, the
adapter sequence used is not present in the DNA sequences under
examination (i.e., the sequence in between the adapters). For
example, if the library of polynucleotide molecules contains
sequences derived from mammalian genomic DNA, cDNA or RNA, then the
sequences of the adapters are not present in the mammalian genome
under study. In many cases, the 5' and 3' adapters are of a
different sequence and are not complementary. In many cases, an
adapter will not contain a contiguous sequence of at least 8, 10 or
12 nucleotides that is found in the DNA under examination. Adapters
may be designed to serve a specific purpose. For example, adapters
may be designed for use in sequencing applications. Sequencing
adapters may comprise, for example, an oligo-(dT) overhang, a
barcode sequence, an overhang (other than oligo-(dT)) to anneal to
another adapter, a site for anchoring a motor protein, and a
sequence to bind to tethering oligos with affinity to polymer
membrane for guiding a DNA or RNA fragment (on which it resides) to
the vicinity of a nanopore, and combinations thereof.
[0041] In the context of the present disclosure,
"adapter-containing" refers to either a nucleic acid that has been
ligated to an adapter, or to a nucleic acid to which an adapter has
been added by primer extension. In some embodiments, the adapters
of a library of nucleic acid molecules may be made by ligating
oligonucleotides to the 5' and 3' ends of the molecules (or
specific sequences of the same) in an initial nucleic acid sample,
e.g., DNA or genomic DNA, cDNA.
[0042] In the context of the present disclosure, "bead
purification" refers to use of magnetic beads to preferentially
adsorb polynucleotide molecules (e.g., RNA, DNA) away from soluble
enzymes (and optionally, other components) through a series of
binding, washing, and elution steps.
[0043] In the context of the present disclosure, "coupled reaction"
refers to a reaction in which two or more reaction steps occur in a
single reaction mixture and in a single reaction vessel (e.g., a
tube, a well, a capillary, a flow cell, a surface). Sequential
reaction steps in a coupled reaction may begin and/or continue
without changes to reaction conditions (e.g., without addition or
removal of reagents, changes in temperature, pH, volume, or
washing) beyond those that arise or follow from the reactions
themselves. For example, a coupled reaction may include a reaction
in which a polymerase (e.g., an immobilized polymerase) is combined
in a single reaction vessel with a ligase (e.g., an immobilized
ligase) and both tailing and ligation reactions proceed in the same
mixture (e.g., without an intervening bead purification). For
clarity, coupled reactions include reactions in which
microenvironments may exist (e.g., on the surface of individual
microbeads in the reaction mixture).
[0044] In the context of the present disclosure, "fragment" refers
to a polynucleotide. A fragment may originate from in vitro or in
vivo synthetic processes. A population of fragments may include
full-length polynucleotides (as originally synthesized) and/or
smaller portions of such full-length sequences resulting from
mechanical, chemical, and/or enzymatic breakage.
[0045] In the context of the present disclosure, "immobilized"
refers to covalent attachment of an enzyme to a solid support with
or without a linker. Examples of solid supports include beads
(e.g., magnetic, agarose, polystyrene, polyacrylamide, chitin).
Beads may include one or more surface modifications (e.g.,
O.sup.6-benzyleguanine, polyethylene glycol) that facilitate
covalent attachment and/or activity of an enzyme of interest.
Non-covalent attachment (e.g., avidin:biotin, chitin:CBP) may also
be useful in some embodiments, for example, where the level of
dissociation of the binding partner is deemed tolerable.
[0046] In the context of the present disclosure, "library" or
"polynucleotide library" refers to a mixture of different
molecules. A library may comprise DNA and/or RNA (e.g., genomic
DNA, organelle DNA, cDNA, mRNA, microRNA, long non-coding RNAs or
other RNAs of interest) or fragments thereof from any desired
source (e.g., human, non-human mammal, plant, microbe, virus, or
synthetic). A library may have any desired number of different
polynucleotides. For example, a library may have more than
10.sup.4, 10.sup.5, 10.sup.6 or 10.sup.7 different nucleic acid
molecules. A library may have fewer different molecules, for
example, where the molecules collectively have more than 10.sup.4,
10.sup.5, 10.sup.6 or 10.sup.7 or more nucleotides. In some
embodiments, a library of polynucleotide molecules may be an
enriched library, in which case the library may have a complexity
of less than 10%, less than 5%, less than 1%, less than 0.5%, or
less than 0.1%, less than 0.01%, less than 0.001% or less than
0.0001% relative to the unenriched sample (e.g., a sample made from
total RNA or total genomic DNA from a eukaryotic cell sample.
Molecules can be enriched by methods such as described in
US2014/0287468 or US 2015/0119261. A library, in some embodiments,
may include member polynucleotides that are tagged with an
adapter.
[0047] In the context of the present disclosure, "ligase" refers to
enzymes that join polynucleotide ends together. Ligases include
ATP-dependent double-strand polynucleotide ligases, NAD+-dependent
double-strand DNA or RNA ligases and single-strand polynucleotide
ligases. Ligases may include any of the ligases described in EC
6.5.1.1 (ATP-dependent ligases), EC 6.5.1.2 (NAD+-dependent
ligases), EC 6.5.1.3 (RNA ligases) (see ExPASy Bioinformatics
Resource Portal having a URL of enzyme.expasy.org which is a
repository of information concerning nomenclature of enzymes based
on the recommendations of the Nomenclature Committee of the
International Union of Biochemistry and Molecular Biology (IUBMB)
describing each type of characterized enzyme for which an EC
(Enzyme Commission) number has been provided. Specific examples of
ligases include bacterial ligases such as E. coli DNA ligase and
Taq DNA ligase, Ampligase.RTM. thermostable DNA ligase
(Epicentre.RTM. Technologies Corp., part of Illumina.RTM., Madison,
Wis.) and phage ligases such as T3 DNA ligase, T4 DNA ligase, T7
DNA ligase, 9.degree. N DNA ligase, and mutants thereof. In some
embodiments, a ligase may be included in a fusion protein with a
SNAP-tag protein.
[0048] In the context of the present disclosure, "magnetically
gathering" refers to application of a magnetic field to a subject
surface or container. A magnetic field may be applied by forming a
magnetic field at or near a surface or container, or by bringing a
surface or container into the effective range of an existing
magnetic field, for example, by moving the surface or container
near the existing field and/or by reshaping a field. Magnetically
gathering immobilized enzymes into a group may include forming a
pellet of immobilized enzyme. Such pellet may be sufficiently well
formed and stable to tolerate manipulation or removal of a fluid,
composition, or reaction mixture adjoining and/or in contact with
the pellet.
[0049] In the context of the present disclosure, "non-naturally
occurring" refers to a polynucleotide, polypeptide, carbohydrate,
lipid, or composition that does not exist in nature. Such a
polynucleotide, polypeptide, carbohydrate, lipid, or composition
may differ from naturally occurring polynucleotides polypeptides,
carbohydrates, lipids, or compositions in one or more respects. For
example, a polymer (e.g., a polynucleotide, polypeptide, or
carbohydrate) may differ in the kind and arrangement of the
component building blocks (e.g., nucleotide sequence, amino acid
sequence, or sugar molecules). A polymer may differ from a
naturally occurring polymer with respect to the molecule(s) to
which it is linked. For example, a "non-naturally occurring"
protein may differ from naturally occurring proteins in its
secondary, tertiary, or quaternary structure, by having a chemical
bond (e.g., a covalent bond including a peptide bond, a phosphate
bond, a disulfide bond, an ester bond, and ether bond, and others)
to a polypeptide (e.g., a fusion protein), a lipid, a carbohydrate,
or any other molecule. Similarly, a "non-naturally occurring"
polynucleotide or nucleic acid may contain one or more other
modifications (e.g., an added label or other moiety) to the 5'-end,
the 3' end, and/or between the 5'- and 3'-ends (e.g., methylation)
of the nucleic acid. A "non-naturally occurring" composition may
differ from naturally occurring compositions in one or more of the
following respects: (a) having components that are not combined in
nature, (b) having components in concentrations not found in
nature, (c) omitting one or components otherwise found in naturally
occurring compositions, (d) having a form not found in nature,
e.g., dried, freeze dried, crystalline, aqueous, and (e) having one
or more additional components beyond those found in nature (e.g.,
buffering agents, a detergent, a dye, a solvent or a
preservative).
[0050] In the context of the present disclosure, "tailing enzyme"
refers to template-independent enzymes (e.g., polymerases,
transferases) that add one or more nucleotides or ribonucleotides
to the 3' end of a polynucleotide. Tailing enzymes may add one or
more As, one or more Gs, one or more Ts, one or more Cs, or one or
more Us. Tailing enzymes may be selected for specific applications
based on their preference for adding a particular nucleotide or
ribonucleotide, for example, to compliment the end of an adapter to
which the tailed polynucleotide. Examples of tailing enzymes
include poly(A) polymerases, poly(G) polymerases, poly(U)
polymerases, and terminal deoxynucleotidyl transferase (TdT). In
some embodiments, a tailing enzyme may be included in a fusion
protein with a SNAP-tag protein.
[0051] In the context of the present disclosure, "transmembrane
pore" refers to protein pores and solid state pores. A
transmembrane pore may be a nanopore. Transmembrane protein pores
may be or comprise hemolysin, leucocidin, lysenin, a Mycobacterium
smegmatis porin (e.g., MspA, MspB, MspC, MspD), CsgG, an outer
membrane porin (e.g., OmpF, OmpG), outer membrane phospholipase A,
Neisseria autotransporter lipoprotein (NalP), WZA, or variants
thereof.
[0052] In the context of the present disclosure, "unique molecule
identifier" (UMI) refers to a random unique sequence of at least 6
nucleotides (6N). Longer random unique sequences may be used, for
example, 2-15 nucleotides, 6-12 nucleotides, or 8-12 nucleotides.
UMIs may have sufficient sequence diversity to distinguish the
molecule of which they are a part (e.g., an adapter or a tagged
fragment) from other molecules in a mixture.
[0053] All publications (including all co-published supplemental
and supporting information), patents, and patent applications
mentioned in this specification are herein incorporated by
reference to the same extent as if each individual publication,
patent, or patent application was specifically and individually
indicated to be incorporated by reference.
[0054] FIG. 1A-E illustrates some embodiments of methods and
compositions disclosed herein. For example, a method may include
sequentially contacting an RNA composition (e.g., comprising one or
more species of RNA molecules) and a soluble tailing enzyme
(optionally in the presence of a buffer) to produce a polyA tailed
RNA composition, contacting the polyA tailed RNA composition with
an adapter and a soluble ligase to produce ligated products,
washing the ligated products, eluting the ligated products, and
sequencing the ligated products (FIG. 1A).
[0055] A coupled method may include contacting an RNA composition
(e.g., comprising one or more species of RNA molecules), a soluble
tailing enzyme, an adapter, and a soluble ligase (optionally in the
presence of a buffer) to produce ligated products, washing the
ligated products, eluting the ligated products, and sequencing the
ligated products (FIG. 1B).
[0056] A method, in some embodiments, may include sequentially
contacting an RNA composition (e.g., comprising one or more species
of RNA molecules) and a soluble tailing enzyme (optionally in the
presence of a buffer) to produce a polyA tailed RNA composition,
contacting the polyA tailed RNA composition with an adapter and a
soluble ligase to produce ligated products, and directly (e.g.,
without washing or elution) sequencing the ligated products (FIG.
1C).
[0057] In some embodiments, a method may include sequentially
contacting an RNA composition (e.g., comprising one or more species
of RNA molecules) and an immobilized tailing enzyme (optionally in
the presence of a buffer) to produce a polyA tailed RNA
composition, removing the immobilized tailing enzyme (e.g., in the
case of enzymes bound to magnetic beads, magnetically gathering the
magnetic beads into a group and taking away the polyA tailed
composition, for example, by pipetting away from the beads),
contacting the polyA tailed RNA composition with an adapter and an
immobilized ligase to produce ligated products, removing the
immobilized ligase (e.g., in the case of enzymes bound to magnetic
beads, magnetically gathering the magnetic beads into a group and
taking away the ligated products, for example, by pipetting away
from the beads), and directly (e.g., without further washing or
elution) sequencing the ligated products (FIG. 1D).
[0058] A coupled method, in some embodiments, may include
contacting an RNA composition (e.g., comprising one or more species
of RNA molecules), an immobilized tailing enzyme, an adapter, and
an immobilized ligase (optionally in the presence of a buffer) to
produce ligated products, removing the immobilized tailing enzyme
and the immobilized ligase (e.g., in the case of enzymes bound to
magnetic beads, magnetically gathering the magnetic beads into a
group and taking away the ligated products, for example, by
pipetting away from the beads), and directly (e.g., without further
washing or elution) sequencing the ligated products (FIG. 1E).
[0059] With respect to its corresponding soluble enzyme, an
immobilized enzyme is physically constrained to a support which
defines a microenvironment for the immobilized enzyme molecules and
its substrates. Surface environments (e.g. charges, functional
groups, morphology, hydrophilicity) of the support materials can
effect the enzymatic rate and immobilized enzyme stability.
Therefore, improving and optimizing this microenvironment may
enhance or maximize enzymatic activities upon immobilization. One
or more strategies to alter surface microenvironment may be used to
improve activity of immobilized enzymes. A single optimization
solution may not applicable to all the enzymes. In some
embodiments, various blocking groups or bead coatings (ethanolamine
and polyethylene glycol-PEG of different lengths) can be utilized
to modify hydrophilicity of support surface. For example,
polyethylene glycol (PEG) moieties can be used to modify the
surface of BG-functionalized magnetic beads. This PEG coating
strategy has been shown to be effective in enhancing activity of
several enzymes validated, including T4 DNA polymerase, Taq DNA
polymerase and T4 DNA ligase) (Li et al. 2018). According to some
embodiments, the distance between the immobilized enzyme and the
beads surface may play a key role in retaining or reducing
enzymatic activity. By using the proper conjugation chemistry,
polyethylene glycol (PEG) linkage groups with variable length can
be applied as a spacer in between a SNAP-reactive BG and the bead
surface. Various benzylguanine (BG) moieties (with PEGylated or
non-PEGylated linkers) may confer different spatial arrangement of
conjugated enzyme molecules. In some embodiments, solid phase
catalysis strategically considers the substrate properties and
accessibility which can be affected by surface properties and
enzyme orientation. In addition, CLIP-reactive benzylcytosine (BC)
moieties can be utilized to substitute for BG moieties on solid
support because BC moieties are considered to be more hydrophilic
than BG moieties. With this strategy, a target enzyme is fused to
CLIP-tag instead of SNAP-tag. According to some embodiments, a
bio-orthogonal conjugation strategy can simultaneously
co-immobilize two enzymes in a desired molar ratio onto beads
functionalized with SNAP-reactive BG and CLIP-reactive BC moieties.
Selection of support materials and proper modifications may enhance
enzymatic activity and thermostability. The surface properties can
modulate refolding upon relaxation and denaturation of enzyme
globular structures thereby maintaining or regaining activity after
storage and heat treatment.
[0060] In RNA sequencing reactions or other applications using
nanopores, the nanopores may be clogged, inactivated, and/or
otherwise compromised by proteins that may be present in the
compositions contacted with the nanopores. Accordingly, methods,
applications, protocols and workflows including nanopores may
comprise removing proteins (e.g., soluble proteins) by bead
purification to alleviate such fouling. In some embodiments, the
need for bead purification may be reduced or obviated by optimizing
enzymatic reactions, for example, by reducing the amounts of
enzymes used (e.g., effectively decreasing the ratio of enzyme to
product in a reaction). Reducing the amount of enzyme(s) may reduce
nanopore fouling thereby extending the functional time of nanopores
in flow cells. While a reduction in the quantity of enzymes used
may apply to all proteins or all enzymes in a reaction, since each
protein and enzyme may interact with a given nanopore differently,
reductions may be made on a more selective basis, targeting those
that are more prone to fouling. As explained in more detail in the
Example section below, FIG. 14 & FIG. 19 show that it is
possible to generate sequence reads from libraries without bead
purification, confirming that optimization of reactions with
soluble enzymes enhance library preparation and/or performance.
[0061] In some embodiments, a wide range of enzymes may be
immobilized without loss or without substantial loss of activity
including, for example, Taq DNA polymerase, T4 DNA polymerase (T4
DNA pol), T4 polynucleotide kinase (T4 PNK), T4 DNA ligase, polyA
polymerase, Klenow (Exo-), T4 BGT, 9.degree. N DNA ligase, Taq DNA
ligase, Bst DNA pol 2.0, phi29 DNA pol, Vvn (nuclease), Gka Reverse
transcriptase, Tbr Reverse transcriptase, RNase A (catalytic
mutants), PolyA polymerase, Beta-galactosidase, PNGaseF, Endo H,
Endo S, Sialidase, Human carbonyl reductase, Human Aldose
reductase, Drosophila aldehyde-ketone reductase (AKR).
[0062] For example, the current ONT protocol uses 3 ul of T4 DNA
Ligase (NEB M202M 2000 units/ul) or 6000 units for 500 ng input
library. Use of immobilized enzymes and/or coupled reactions may
reduce the amount of soluble T4 DNA Ligase by 90% (i.e. use of 600
units of ligase) because the immobilized enzyme protocols validated
used only 180 units. For low input RNA library (<100 ng input),
enzyme consumption can be further lowered.
[0063] Enzyme immobilization may provide opportunities to enhance
performance of enzymatic processes, for example, by allowing faster
and more efficient production of products, at least in part, by
reducing or eliminating purification steps needed for corresponding
soluble enzyme processes, by reducing reactant and/or product
losses from washing steps, and/or by allowing enzymes to be reused
in subsequent reaction cycles Immobilization may imbue bound
enzymes with additional thermostability and/or thermoactivity. For
example, immobilized enzymes may tolerate higher temperatures (even
if they are not catalytically active at such higher temperatures),
which could be useful for applications in which enzymes are reused.
In some embodiments, enzyme immobilization may allow soluble enzyme
processes to be automated (or automated more efficiently)
Immobilization may also allow processes to be more effective and/or
efficient by reducing enzyme carry over to subsequent steps.
[0064] In some embodiments, methods including immobilized enzymes
may omit or exclude heat treatments to inactivate enzymes, bead
purification steps, and/or sequencing pore clogging. Heat stress
can lead to the accumulation of 8-oxoguanine, deaminated cytosine,
and apurinic DNA sites (AP-sites) in a cell (Bruskov V. I.,
Malakhova L. V., Masalimov Z. K., Chernikov A. V.//Nucleic Acids
Res. 2002. V. 30. P. 1354-1363. 19. Lindahl T., Nyberg
B.//Biochemistry. 1974. V. 13. P. 3405-3410. 20. Warters R. L.,
Brizgys L. M.//J. Cell Physiol. 1987. V. 133. P. 144-150.
Elimination of bead purification may result in more uniformly sized
fragments in a library to be sequenced. Bead purification may
result in alteration of a library such as size distribution; For
example, large or small species may be lost more than the species
in the middle size range due to either less binding (leading to
more loss) or tighter binding resulting in lower elution
efficiencies. This step may also introduce impurities (present in
loading and wash solutions) that may affect performance or
parameters of nanopores such as signals or functioning time.
[0065] According to some embodiments, methods including immobilized
enzymes may be adapted to and performed in microfluidic,
lab-on-a-chip formats with enzymes immobilized on surfaces. For
example, systems for single-cell RNA sequencing that produce RNA of
a single cell may be adapted to contact such RNA with a tailing
enzyme and a ligase (coupled or sequentially) on a surface or in a
microfluidics device.
[0066] The present disclosure provides embodiments in which
purification of nucleic acids is facilitated by combining enzymatic
steps into a single reaction and/or immobilizing enzymes on
magnetic beads or other supports. The present disclosure further
provides embodiments in which enzyme activity and/or
thermostability is enhanced by immobilization on magnetic beads or
other supports.
[0067] In some embodiments, a method of preparing a library (e.g.,
a DNA library, an RNA library) for sequencing (e.g., ONT
sequencing) may include in a coupled reaction, (a) contacting a
population of nucleic acid fragments with a tailing enzyme to
produce tailed fragments, and/or (b) ligating to the tailed
fragments a sequencing adapter with a ligase to produce
adapter-tagged fragments. A method may further include separating
adapter-tagged fragments from the tailing enzyme and the ligase to
produce separated adapter-tagged fragments and optionally separated
tailing enzyme and/or separated ligase. A tailing enzyme, in some
embodiments, may be or comprise immobilized tailing enzyme. A
ligase, in some embodiments, may be or comprise immobilized ligase.
For example, a tailing enzyme may be immobilized on a bead (e.g., a
magnetic bead) and/or a ligase may be immobilized on a bead (e.g.,
a magnetic bead). Each immobilized enzyme may be attached to a
separate support or may be combined on a common support.
Optionally, a tailing enzyme and a ligase each may be immobilized
on their own separate support or both may be co-immobilized on a
single support. In some embodiments, one or more enzymes (e.g., a
tailing enzyme and/or a ligase) may be soluble enzymes. For
example, a method may include contacting one or more soluble
enzymes with one or more substrates in a liquid (e.g., aqueous)
media. In some embodiments, a method may include contacting two
enzymes (e.g., a tailing enzyme and a ligase) with at least one
substrate for at least one of the two enzymes (e.g., DNA or RNA) in
a coupled reaction. In some embodiments of a coupled reaction, at
least one product of one of the enzymes is a substrate of the other
enzyme. It may be desirable to select reaction conditions to favor
production of the product(s) that are substrates of the other
enzyme and minimize or avoid production of anything that reduces
the efficiency of any of the coupled reaction enzymes, but
conditions may be adjusted to tolerate the presence of some
unwanted products.
[0068] In some embodiments, separating adapter tagged fragments
(e.g., where one or more enzymes used are immobilized on magnetic
beads) may further comprise subjecting the coupled reaction to a
magnetic field. Subjecting a coupled reaction to a magnetic field
may include accomplished in any manner desired. For example, a
coupled reaction may be moved into an existing magnetic field, an
existing magnet may be moved into effective range of a coupled
reaction, or a magnetic field may be applied, for example, by
switching on an electromagnet within an effective distance of a
coupled reaction. In some embodiments, subjecting a coupled
reaction to a magnetic field gathers magnetic beads in the coupled
reaction forming a liquid fraction comprising, for example,
reaction products, buffers, and solvent, but few, if any, magnetic
beads) and a bead fraction comprising, for example, magnetic beads,
enzymes, and solvent, but few, if any, reaction products. Gathered
magnetic beads may form a pellet or other aggregate that
facilitates separation (e.g., removal) of other reaction components
(e.g., components remaining in solution).
[0069] A population of nucleic acid fragments may comprise
ribonucleic acid fragments and/or may comprise deoxyribonucleic
acid fragments. Fragments may be of any desired size. For example,
a population of nucleic acid fragments may comprise fragments
ranging in length from 100 to 1000 nts, 200 to 2000 nts, 500 to
5000 nts, 1,000 to 10,000 nts, 2,000 to 20,000 nts, 5,000 to 50,000
nts, 10,000 to 100,000 nts, or combinations thereof. A population
of nucleic acid fragments may comprise fragments from any desirable
source including, for example, fragments synthesized or assembled
in vitro and/or fragments of polynucleotides from microbes (e.g.,
yeast, bacteria, viruses, phage), fungi, plants, amphibians,
reptiles, fish, mammals, birds, or any other organism.
[0070] In some embodiments, methods may be capable of producing
sequencing libraries with little input RNA. For example, methods
may use a population of nucleic acid fragments having less than 100
ng of nucleic acids or a population of nucleic acid fragments
having less than 10 ng of nucleic acids. In some embodiments,
methods including coupled reactions and/or immobilized enzymes may
produce more sequencing reads per mass of input DNA or RNA when
compared with corresponding methods that do not include coupled
reactions and/or immobilized enzymes. For example, methods
including a coupled reaction and/or an immobilized enzyme may
produce 2.times., 3.times., 4.times., 5.times., 6.times., 7.times.,
8.times., 9.times., 10.times., 11.times., 12.times. or more
sequencing reads compared to methods including only sequential
reactions with soluble enzymes and bead purification.
[0071] The present disclosure further provides methods for
preparing sequencing libraries comprise any combination of tailing
and/or ligating steps and further comprising reusing the tailing
enzyme and/or the ligase. For example, a method may include, in a
second reaction (e.g., a second coupled reaction), contacting a
second population of nucleic acid fragments with the separated
tailing enzyme (produced from the first reaction) to produce
additional tailed fragments, and ligating (optionally, with a
ligase also recycled from the first reaction) to the additional
tailed fragments a second sequencing adapter with the separated
ligase to produce additional adapter-tagged fragments. The
additional adapter-tagged fragments may be separated from the
separated tailing enzyme and the separated ligase to produce
separated additional adapter-tagged fragments, separated tailing
enzyme, and/or separated ligase. In some embodiments, a method
contacting separated adapter-tagged fragments with one or more
transmembrane pores (e.g., ONT nanopores) for sequencing. For
example, a method may comprise translocating separated
adapter-tagged fragments through one or more transmembrane pores,
(f) detecting electrical changes as the one or more separated
adapter-tagged fragments are translocated through the one or more
transmembrane pores in an insulating membrane to produce an
electrical signal; and/or analyzing the electrical signal to
generate a sequence read. In some embodiments, one or more
transmembrane pores (e.g., in contact with a population of
adapter-tagged fragments) may retain about 90% of their initial
activity (e.g., translocation activity) after two hours and/or may
retain about 50% of their initial activity after 8 hours. One or
more transmembrane pores, according to some embodiments of the
disclosure, may produce at least 900 sequence reads per
transmembrane pore. For example, the number of sequencing reads of
a population of nanopores (e.g., in contact with a population of
adapter-tagged fragments) may be, on average, at least 900. In some
embodiments, a sequencing adapter may be a single-stranded adapter
and may comprise a leader sequence; and a first sequence and a
second sequence, wherein the first and second sequences are
complementary to each other and define a hairpin, wherein the
leader sequence is configured to thread into the one or more
transmembrane pores.
Kits
[0072] The present disclosure further relates to kits including
immobilized enzymes. For example, a kit may include an immobilized
tailing enzyme, an immobilized ligase, a polynucleotide (e.g., a
population of polynucleotides) dNTPs, rNTPs, primers, buffering
agents, and/or combinations thereof. Immobilized enzymes may be
included in a storage buffer (e.g., comprising glycerol and a
buffering agent). A kit may include a reaction buffer which may be
in concentrated form, and the buffer may contain additives (e.g.
glycerol), salt (e.g. KCl), reducing agent, EDTA or detergents,
among others. A kit comprising dNTPs may include one, two, three of
all four of dATP, dTTP, dGTP and dCTP. A kit comprising rNTPs may
include one, two, three of all four of rATP, rUTP, rGTP and rCTP. A
kit may further comprise one or more modified nucleotides. A kit
may optionally comprise one or more primers (random primers, bump
primers, exonuclease-resistant primers, chemically-modified
primers, custom sequence primers, or combinations thereof). One or
more components of a kit may be included in one container for a
single step reaction, or one or more components may be contained in
one container, but separated from other components for sequential
use or parallel use. The contents of a kit may be formulated for
use in a desired method or process.
[0073] A kit is provided that contains: (i) an immobilized tailing
enzyme; and (ii) a buffer or (i) an immobilized tailing enzyme;
(ii) an immobilized ligase, and (iii) a buffer. An immobilized
enzyme may have a lyophilized form or may be included in a buffer
(e.g., an aqueous buffer, a storage buffer or a reaction buffer in
concentrated form). A kit may contain the immobilized enzyme in a
mastermix suitable for receiving and amplifying a template nucleic
acid. An immobilized enzyme may be a purified enzyme so as to
contain substantially no DNA or RNA and/or no nucleases. A reaction
buffer for and/or storage buffers containing an immobilized enzyme
may include non-ionic, ionic e.g. anionic or zwitterionic
surfactants and crowding agents. A kit may include an immobilized
enzyme and a reaction buffer in a single tube or in different
tubes.
[0074] A subject kit may further include instructions for using the
components of the kit to practice a desired method. The
instructions may be recorded on a suitable recording medium. For
example, instructions may be printed on a substrate, such as paper
or plastic, etc. As such, the instructions may be present in the
kits as a package insert, in the labeling of the container of the
kit or components thereof (i.e., associated with the packaging or
sub-packaging) etc. Instructions may be present as an electronic
storage data file residing on a suitable computer readable storage
medium (e.g. a CD-ROM, a flash drive). Instructions may be provided
remotely using, for example, cloud or internet resources with a
link or other access instructions provided in or with a kit.
EXAMPLES
[0075] Some embodiments may be illustrated by one or more of the
examples provided herein.
Example 1: Immobilization of Poly(A) Polymerase and Kinetics
Study
Example 1A. Soluble Poly(A) Polymerase Alone
[0076] Poly(A) polymerase catalyzes poly(A) tailing at the 3' end
of RNA and the resulting tails can be hybridized with and ligated
to adapters (e.g., Nanopore Adaptors) for direct RNA sequencing.
The kinetics of poly(A) polymerase (NEB M0276) at different
concentrations was studied as described in this Example 1. Reaction
components (6 .mu.L nuclease-free water, 1 .mu.L, 10.times. poly(A)
polymerase reaction buffer (NEB), 1 .mu.L 10 mM ATP, 0.5 .mu.L
RNase inhibitor, 1 .mu.L 1 .mu.M RNA 45-mer oligo and 0.5 .mu.L
poly(A) polymerase (at 12 nM, 24 nM, 60 nM or 120 nM)) were mixed
and incubated at 37.degree. C. for 20 min to allow poly(A) tailing.
Each reaction was quenched by addition of 10 .mu.L 50 mM EDTA with
0.7% Tween-20, diluted to a final volume of 200 .mu.L, and sent for
capillary electrophoresis (CE) analysis.
[0077] Results shown in FIG. 2 demonstrate that RNA oligo strands
were extended by addition of poly(A) tails at the 3' end of the RNA
with the presence of poly(A) polymerase. More extensive strand
extension was observed with increasing the concentration of poly(A)
polymerase from 0.6 nM to 6 nM final concentration.
Example 1B. Poly(A) Polymerase-SNAP-Tag.RTM. Fusion
[0078] Cells expressing a poly(A) polymerase-SNAP-Tag.RTM. fusion
were harvested by centrifugation and lysed by sonication on ice.
The resulting lysate was centrifuged and the clarified crude
extract produced was purified on a nickel column. After loading,
the column was washed and the fusion protein was eluted and
dialyzed overnight. The enzyme concentration was determined using
Bradford assay.
[0079] The activity of the expressed fusion protein was evaluated
according to Example 1A. Results shown in FIG. 3 demonstrate RNA
45-mer oligo strand extension by the purified poly(A)
polymerase-SNAP fusion protein. Comparing FIG. 2 and FIG. 3
demonstrates that RNA 45-mer oligo strand extension by the poly(A)
polymerase-SNAP fusion protein aligned well with the soluble NEB
poly(A) polymerase.
Example 1C. Poly(A) Polymerase Immobilization on
O.sup.6-Benzylguainine (BG) Magnetic Beads
[0080] O.sup.6-benzylguainine (BG) functionalized magnetic beads
coated with PEG.sub.750 (100 .mu.L of a 25% (v/v) slurry) were
washed five times with 250 .mu.L buffer (1.times.PBS, #9808, Cell
Signaling, 1 mM DTT, 300 mM NaCl) for 5 times. Poly(A)
polymerase-SNAP fusion protein (25 .mu.g) in 125 .mu.L buffer
(1.times.PBS with 300 mM NaCl), was mixed with the pre-washed BG
beads, and incubated at 4.degree. C. overnight to immobilize the
fusion protein (FIG. 4). The enzyme bead mixture was washed with
the same buffer 8 times to remove unbound protein. Diluent C buffer
without BSA (NEB) was used to resuspend the beads with immobilized
fusion protein for storage at -20.degree. C.
[0081] The activity of the immobilized poly(A) polymerase was
evaluated according to Example 1A. Results shown in FIG. 5
demonstrate RNA 45-mer oligo strand extension by the immobilized
poly(A) polymerase-SNAP fusion protein.
Example 2: Immobilized Poly(A) Polymerase Displays Stability
Including Thermostability
[0082] This example shows how to improve microenvironment for
immobilized enzymes by increasing hydrophilicity of bead surface by
PEG coating. Poly(A) polymerase was immobilized to two types of
O.sup.6-benzylguainine (BG) functionalized magnetic beads coated
with or without PEG.sub.750 generally as described in Li, S et al,
"Enhancing Multistep DNA Processing by Solid-Phase Enzyme Catalysis
on Polyethylene Glycol Coated Beads" Bioconjugate Chem. 2018, 29,
7, 2316-2324 An aliquot of 100 .mu.L of 25% (v/v) bead slurry was
washed five times with 250 .mu.L buffer (1.times.PBS, #9808, Cell
Signaling, 1 mM DTT, 300 mM NaCl) for 5 times. Poly(A)
polymerase-SNAP fusion protein (25 .mu.g) was dissolved in 125
.mu.L buffer (1.times.PBS with 300 mM NaCl), combined with the
washed BG beads, and incubated at 4.degree. C. overnight to
immobilize the fusion protein on the beads. The immobilized poly(A)
polymerase-SNAP fusion protein beads were washed with the same
buffer 8 times to remove any unbound protein. Diluent C buffer
(NEB) with no BSA was used to resuspend the beads with immobilized
fusion protein for storage at -80.degree. C.
[0083] Poly(A) tailing reactions were performed using the soluble
and immobilized poly(A) polymerase according to the protocols
described in Example 1. The data shown in FIG. 6 demonstrate that
SNAP-tagged poly(A) polymerase immobilized to PEG.sub.750-coated
magnetic beads displayed poly(A) tailing activity on a 45-mer RNA
oligo whereas the same fusion protein immobilized to magnetic beads
without PEG.sub.750 coating displayed little, if any, detectable
poly(A) tailing.
Example 3: Immobilized T4 DNA Ligase Displays Stability Including
Thermostability
Example 3A. T4 DNA Ligase Immobilization on Magnetic Beads and
Stability Assays
[0084] This example provides immobilization of SNAP-tagged T4 DNA
Ligase to BG-magnetic Beads and validation of storage stability at
-20.degree. C. and 25.degree. C.
[0085] HS-T4 DNA Ligase protein was immobilized to
BG-Magnetic-Beads by mixing 100 .mu.g protein per 400 .mu.l of 25%
(V/V) bead slurry at 4.degree. C. overnight in 1.times.PBS buffer
containing 1 mM DTT, followed by extensive wash (8.times.). The
resulting immobilized enzyme was termed BG-HS-T4 DNA Ligase and
stored at -20.degree. C. or 25.degree. C. for 7 days. Activity
testing was performed according to the Determination of the Unit
Activity of T4 DNA Ligase by Capillary Electrophoresis (CE)
activity assay (One unit is defined as the amount of enzyme
required to give 40% to 70% (55%.+-.15%) ligation of 0.12 .mu.M of
synthesized double-stranded DNA oligos with Hind III ends in 20
minutes at 16.degree. C.
[0086] Results are shown in FIG. 7. No detectable decrease in
enzyme activity was observed at -20.degree. C. and an approximately
30% reduction in ligase activity at 25.degree. C. during the
storage period.
Example 3B. T4 DNA Ligase Immobilization on Agarose Beads
[0087] This example demonstrates that immobilization can improve
thermostability of SNAP-tagged T4 DNA ligase conjugated to
BG-Agarose beads (HS-T4 DNA Ligase Agarose) compared to free
SNAP-tagged T4 DNA ligase (HS-T4 DNA Ligase) or untagged T4 DNA
ligase (NEB M0202). HS-T4 DNA Ligase protein was immobilized to
SNAP-Capture Pull-Down Resin (a highly crosslinked agarose, NEB
S9144), termed BG-Agarose, by mixing 100 .mu.g protein per 100
.mu.l of 50% bead slurry at 4.degree. C. overnight in 1.times.PBS
buffer containing 1 mM DTT, followed by extensive wash. The
resulting immobilized enzyme was termed HS-T4 DNA Ligase Agarose.
Each immobilized enzyme master mixture was made by mixing 32 .mu.L
of HS-T4 DNA Ligase Agarose, 20 .mu.L of 10.times.T4 DNA Ligase
Reaction Buffer and 74.64 .mu.L of H2O; Two types of soluble enzyme
master mixtures were made by mixing 8 .mu.L of T4 DNA Ligase (NEB
M0202) or HS-T4 DNA Ligase, 20 .mu.L of 10.times.T4 DNA Ligase
Reaction Buffer and 98.64 .mu.L of H2O.
Example 3C. Comparison of Ligase Activity of Soluble and
Immobilized Ligases
[0088] A FAM-labeled DNA duplex was formed by annealing synthetic
oligomer, Gene32FAM-fw3'A, /56-FAMN/CA TGG TGA TTA CGA TTC TTG CCC
AGT ATG TCA ATA CAT CAG TAA AAA TA (SEQ ID NO:1) and Gene32-rv5'p,
/5Phos/AT TTT TAC TGA TGT ATT GAC ATA CTG GGC AAG AAT CGT AAT CAC
CATG (SEQ ID NO:2). A DNA substrate mixture was prepared by mixing
60 .mu.L of 10 .mu.M 5TAM-labeled DNA duplex with 3'A and 160.08
.mu.L of 15 .mu.M TA-Adaptor possessing a 3'T, 5'75Phos/GAT CGG AAG
AGC ACA CGT CTG AAC TCC AGT C/ideoxyu/A CAC TCT TTC CCT ACA CGA CGC
TCT TCC GAT CT-3' (SEQ ID NO:3).
[0089] For heat treatment, an aliquot of 15.83 .mu.L from an enzyme
master mixture was incubated at 4, 40, 60, 80, 90, 95 or
100.degree. C. for 10 min, followed by addition of 9.17 .mu.L of
the DNA substrate mixture. All the ligation reactions were carried
out at 4.degree. C. overnight in a shaker. The samples were
analyzed by electrophoresis on a 12% Tris-Glycine PAGE
(Novex/Invitrogen) in 1.times.TAE Buffer for 2.5 hours at 25 mA
(current). Results are shown in FIG. 8. The DNA species possessing
FAM probe signal was detected by scanning the PAGE gel with an 488
nm excitation wavelength on Typhoon Imager (GE Healthcare). DNA
ligation resulted in formation of new species of larger molecular
mass, absent in the control reactions without ligase (No Enzyme).
Both untagged T4 DNA ligase and soluble HS-T4 DNA Ligase showed no
detectable ligase activity after treatment in the temperature range
of 60-100.degree. C., indicating that soluble form was subjected to
irreversible denaturation. In contrast, HS-T4 DNA Ligase
immobilized to BG-Agarose beads retained enzymatic activity after
treatment in the same range of elevated temperature tested.
Example 3D. Effect of Heat Treatment on Various Soluble and
Immobilized Products of T4 DNA Ligases
[0090] Four types of beads, Agarose (SNAP-Capture Pull-Down Resin,
NEB S9144), Chitin, Magnetic beads (Mag), SiMag beads (SiM) were
modified to possess benzylguanine ligand, a substrate of SNAP-tag.
SNAP-tagged T4 DNA Ligase (HS-T4 DNA Ligase) protein was
immobilized to each type of benzylguanine-functionalized beads. A
typical immobilization reaction was performed by mixing 100 .mu.g
protein with an Agarose bead slurry at 4.degree. C. overnight,
followed by extensive wash. The resulting immobilized enzyme was
termed Ligase-Agarose, Ligase-Chitin, Ligase-Mag and Ligase-SiM,
respectively. Ligation reactions were set up by mixing the
following components in a final volume of 20 .mu.L containing
1.times.T4 DNA Ligase Reaction Buffer, 0.5 .mu.M FAM-labeled DNA
duplex, 3.75 .mu.M adaptor, and 1 .mu.L of immobilized HS-T4 DNA
Ligase or HS-T4 DNA Ligase (HS-Ligase) or T4 DNA Ligase (Ligase,
NEB M0202S). The reaction mixtures were incubated for 30 min at
4.degree. C. (A), 37.degree. C. (B) or 45.degree. C. (C).
Subsequently, all the reaction mixtures were incubated for 2 hours
at 23.degree. C. for DNA ligation to proceed. The samples were
analyzed by electrophoresis on a 12% Tris-Glycine PAGE
(Novex/Invitrogen) in 1.times.TAE Buffer for 2.5 hours at 25 mA
(current). The DNA species possessing FAM signal was visualized by
scanning the PAGE gel with an 488 nm excitation wavelength on
Typhoon Imager (GE Healthcare). Results are shown in FIG. 9. DNA
ligation resulted in formation of a product of higher molecular
mass, which is absent in the control reactions without ligase (No
Enzyme).
[0091] All ligase-containing reactions except for Ligase-SiM showed
ligase activity after treatment at 4.degree. C. (FIG. 9A) or
37.degree. C. (FIG. 9B) for 30 min. FIG. 9C shows that after heat
treatment at 45.degree. C. HS-T4 DNA Ligase immobilized to
BG-Agarose beads retained higher enzymatic activity compared to the
other immobilized ligase products. Both untagged T4 DNA ligase
(Ligase) and soluble HS-T4 DNA Ligase (HS-Ligase), however, showed
no or residual ligase activity after treatment at 45.degree. C. for
30 min, indicating that these soluble form ligases was subjected to
irreversible denaturation.
Example 3E. Effect of Heat Treatment on Various Soluble and
Immobilized Products of T4 DNA Ligases
[0092] SNAP-tagged T4 DNA Ligase (HS-T4 DNA Ligase) protein was
immobilized to BG-Agarose Beads (SNAP-Capture Pull-Down Resin, NEB
59144) and BG-Magnetic Beads (Mag, 1 .mu.m) functionalized with
benzylguanine ligand. The resulting immobilized enzyme was termed
Ligase-Agarose and Ligase-Mag, respectively. Ligation reactions
were set up by mixing the following components in a final volume of
20 .mu.L containing 1.times.T4 DNA Ligase Reaction Buffer, 0.5
.mu.M FAM-labeled DNA duplex, 3.75 .mu.M adaptor, and 1 .mu.L of
immobilized HS-T4 DNA Ligase or HS-T4 DNA Ligase (HS-Ligase) or T4
DNA Ligase (Ligase, NEB M0202S). The reaction mixtures were
incubated for 10 min at 4.degree. C., 55.degree. C. (B) or
65.degree. C. Subsequently, the reaction mixtures were incubated
either at 23.degree. C. for 2 hours (FIG. 10A) or at 4.degree. C.
overnight (FIG. 10B). The samples were electrophoresed on a 12%
Tris-Glycine PAGE (Novex/Invitrogen) in 1.times.TAE Buffer for 2
hours at 25 mA/gel. The DNA species possessing FAM probe were
visualized by scanning the PAGE gel with an 488 nm excitation
wavelength on Typhoon Imager (GE Healthcare). Results are shown in
FIG. 10. All positive control reactions (4.degree. C.) displayed
ligase activity. Untagged T4 DNA ligase (Ligase) and soluble HS-T4
DNA Ligase (HS-Ligase) as well as HS-T4 DNA Ligase immobilized onto
Magnetic Beads (Ligase-Mag) showed no or residual ligase activity
after treatment at 55.degree. C. or 65.degree. C. In contrast,
HS-T4 DNA Ligase immobilized to BG-Agarose Beads exhibited similar
enzymatic activity for each series of ligation reactions when
pre-treated at 4.degree. C., 55.degree. C. or 65.degree. C.,
indicating that immobilization to BG-Agarose Beads improved heat
resistance of T4 DNA Ligase.
Example 3F. Reusing T4 DNA Ligase to Incorporate Unique Molecular
Identifiers (UMIs)
[0093] For next-generation sequencing, barcoding is an effective
and commonly used approach in multiplexed deep sequencing
experiments. During the demultiplexing step, identification of UMIs
(barcodes) enables calling and quantification of the individual
libraries which are pooled for a single sequencing run.
Furthermore, UMIs are increasingly used to track nucleic acids from
individual cells and to quantitatively assess their clonal
contributions over time. This example provides a workflow for
efficiently producing libraries with UMIs that reuses immobilized
enzymes.
[0094] A typical library preparation protocol may consist of (a)
repairing the ends of the members of a population of nucleic acids,
(b) A/dA-tailing repaired members of the population, (c) ligating
adapters to A/dA tailed members of the population, and (d) bead
purification of adapter-tagged members of the population. Using
immobilized enzymes in accordance with this example obviates the
need for bead purification and allows enzymes to be reused in
subsequent cycles of library preparation.
[0095] In each cycle, a nucleic acid library may be ligated to an
adapter with a bar code using immobilized enzymes in accordance
with Example 4E to produce an adapter tagged library. Immobilized
enzyme beads (IM-Poly(A) polymerase and IM-ligase) are extensively
washed, for example, at least 5 times to remove residual barcoded
adaptor, as demonstrated in the experiment below and retained for
reuse in the next cycle. A wash step can be incorporated to wash
away residual bar-coded adaptor in each cycle before an adaptor
possessing a different barcode is ligated to RNA species from a
fresh RNA sample. The number of cycles may be varied, and all
resulting adapter-tagged libraries may be pooled for multiplex
sequencing.
[0096] In this example, a preparation of 300 units of T4 DNA ligase
immobilized onto magnetic beads was utilized to perform repeated
ligation of two adaptors used for library construction for Nanopore
direct RNA sequencing. One of the adaptor sequences was labeled
with a 5' FAM probe to detect and quantify the ligation product
using capillary electrophoresis. In each reaction cycle, (a) an RNA
library and the adapters were added to a vial containing the
immobilized ligase and incubated at 25.degree. C. for 10 min; (b)
the enzyme-bearing beads were pelleted on magnetic rack; (c) the
product-containing supernatant was removed from the vial and
transferred for CE analysis; and (d) the pelleted beads were washed
5 times in conjunction with micro-centrifugation in preparation for
the next adaptor ligation cycle.
[0097] Results are shown in FIG. 11 and demonstrate efficient
ligation in 20 consecutive ligation reactions, which is indicative
of reliability and reproducibility of immobilized T4 DNA ligase. In
the control, soluble T4 DNA ligase was used to carry out a single
ligation reaction for the same adaptor substrates.
Example 4: Library Preparation Using Soluble and Immobilized
Enzymes and Nanopore Direct RNA Sequencing
[0098] Nanopore direct RNA sequencing was performed using libraries
prepared according to one of the five methods described in this
example and illustrated in FIG. 1A-E. Total RNA from Listeria
monocytogenes was extracted using NEB Monarch Total RNA Miniprep
Kit (NEB #T2010) and DNase I pack (NEB #T2019L) according the
protocols of the manufacturer. The concentration of purified total
RNA was measured using Invitrogen Qubit.TM. RNA High-sensitivity
Assay Kit (cat. Number: Q32852).
[0099] Details of RNA library preparation for each approach are
discussed below. In all cases in this Example 4, sequencing
preparation began with 500 ng of each RNA as recommended by Oxford
Nanopore Technologies. For the libraries prepared with soluble
enzyme with bead purification, Nanopore's bead purification
protocol was adopted. After bead purification 20 .mu.L of the
resulting RNA library was mixed with 17.5 .mu.L of nuclease-free
water and 37.5 .mu.L of RNA running buffer (provided by ONT) to a
final volume of 75 .mu.L before loading into a flow cell for direct
RNA sequencing. For the libraries prepared with soluble enzyme
without bead purification, and immobilized enzymes, a portion of
each 40 .mu.L RNA library was supplemented with nuclease-free water
to 37.5 .mu.L, and mixed with 37.5 .mu.L RNA running buffer to a
final volume of 75 .mu.L.
[0100] Direct RNA sequencing was performed on a MinION.RTM. MkIb
with R9.4 flow cells. MinKNOW.RTM. instrument software (ONT)
recorded the nanopore current as each strand of an adaptor-ligated
RNA translocated through a nanopore. Albacore 1.2.1 (ONT) was used
to perform base-calling. A report that displayed the major data
sets was generated for each sequencing. Major parameters, such as
direct RNA reads and average read length, were compared.
Example 4A. Soluble Poly(A) Polymerase without Bead Purification
(FIG. 1A)
[0101] For poly(A) tailing, mix 8 .mu.L quick ligation buffer, 1.2
.mu.L 5 M NaCl solution, 0.5 .mu.L poly(A) polymerase (NEB M0276),
and 500 ng total Listeria monocytogenes RNA, supplemented with
nuclease-free water to 30 .mu.L in a 0.2 mL thin-walled PCR tube.
Incubate the reaction at 37.degree. C. for 20 min. Next, for
adaptor ligation, add 1.0 .mu.L RT Adaptor (RTA), 6.0 .mu.L RNA
Adaptor (RMX) and 3.0 .mu.L T4 DNA ligase (NEB M0202M) to the
poly(A) tailed RNA sample to make a final volume of 40 .mu.L.
Incubate the reaction at 25.degree. C. for 10 min RNA concentration
was measured using the Qubit method after the enzymatic reaction
(FIG. 1A). An aliquot of 40 .mu.L RNA sample was used for further
library prep as described above.
Example 4B. Sequential Reactions with Soluble Enzymes and Bead
Purification (FIG. 1B)
[0102] Library preparation according to Example 4A was repeated
with the addition of a bead purification step after the enzymatic
reactions. Specifically, 40 .mu.L of resuspended NEBNext Sample
Purification beads (E7104S) were combined with the adapter ligation
reaction (40 .mu.L) and mixed by pipetting and incubated on a Hula
mixer (rotator mixer) at room temperature for 5 min. Samples were
spun and pelleted on a magnet. Supernatant was pipetted off while
pellets were retained on a magnet. Beads were combined with 150
.mu.L of Wash Buffer (WSB) (150 .mu.L) and resuspended by flicking
the tubes. Tubes were returned to the magnetic rack to allow beads
to pellet and supernatant was removed by pipette. The wash step was
repeated and the supernatant was removed. Each pellet was
resuspended in 21 .mu.L Elution Buffer by gently flicking the tube
after removal from the magnetic rack. Each tube was incubated at
room temperature for 10 min to allow the elution of RNA. Beads were
then pelleted on a magnet until the eluate was clear and colorless.
21 .mu.L of each eluate was removed and retained in a clean
Eppendorf DNA LoBind.RTM. tube. 1 .mu.L of RNA was used for
concentration measurement using Qubit Assay Kit. Final yield and
recovery rate were determined. All 20 .mu.L RNA samples were used
for further library prep as described above.
Example 4C. Coupled Reactions with Soluble Poly(A) Polymerase and
Bead Purification (FIG. 1C)
[0103] For the coupled reactions approach using soluble enzymes,
mix 8 .mu.L quick ligation buffer, 1.2 .mu.L 5 M NaCl solution, 0.5
.mu.L poly(A) polymerase (NEB M0276), 500 ng total RNA, 1.0 .mu.L
RT Adaptor (RTA), 6.0 .mu.L RNA Adaptor (RMX) and 3.0 .mu.L T4 DNA
ligase (NEB M0202M) supplemented with nuclease-free water to 40
.mu.L in a 0.2 mL thin-walled PCR tube. Incubate the reaction at
37.degree. C. for 20 min followed by 25.degree. C. for 10 min to
allow the simultaneous poly(A) tailing and adaptor ligation. Sample
purification, RNA concentration determination, and further RNA
library prep (using all 20 .mu.L) were carried out as described in
Example 4B.
[0104] Comparison of Nanopore direct RNA sequencing reads. Each
library was prepared using soluble enzyme with bead purification by
sequential (Sol-seq) and coupled (Sol-cpl) reaction protocols for
poly(A) tailing and adaptor ligation. Results are shown in FIG.
12.
Example 4D. Sequential Reactions with Immobilized Enzymes (FIG.
1D)
[0105] A model study was conducted by CE analysis of sequential
treatment of FAM-labeled RNA oligo (35mer) with immobilized poly(A)
polymerase and immobilized T4 DNA ligase. Poly(A) tailing was
performed at 37.degree. C. for 20 min after mixing 6 .mu.L
nuclease-free water, 1 .mu.L 10.times. poly(A) polymerase reaction
buffer (NEB), 1 .mu.L 10 mM ATP, 0.5 .mu.L RNase inhibitor, 1 .mu.L
1 .mu.M RNA 35-mer oligo (100 nM final concentration) and 0.5 .mu.L
immobilized poly(A) polymerase (EXAMPLE 1C). Subsequently, after
removal of immobilized PAP ligation was carried out at 25.degree.
C. for 10 min with the addition of immobilized T4 DNA Ligase
(provided by NEB, 60 units/.mu.L) and RTA-poly(dT).sub.15 or
RTA-poly(dT).sub.10 (300 nM). Positive results were observed by CE
analysis of the samples taken from the poly(A) tailing reaction and
adaptor ligation reactions (FIG. 13).
[0106] An RNA library was also prepared using immobilized enzymes
using the same workflow as described in Example 4B above except
that the soluble enzymes (i.e. poly(A) polymerase and T4 DNA
ligase) were replaced with their immobilized counterparts. Briefly,
poly(A) tailing and ligation were carried out sequentially by
incubating RNA with 2.5 .mu.L immobilized Poly(A) polymerase at
37.degree. C. for 20 min., removing the beads, incubating the
supernatant with 3.0 .mu.L immobilized T4 DNA ligase at 25.degree.
C. for 10 min., and removing the beads with immobilized T4 DNA
ligase Immobilized enzymes, poly(A) polymerase in the first step
and T4 DNA ligase in the second step, were separated from the
reaction medium on a magnetic rack and the supernatant containing
the products and other soluble components were transferred to a
fresh tube for the subsequent reaction. No bead purification was
performed after the ligation of RTA and RMX adapters. The RNA
concentration in the supernatant was determined using Qubit method.
A portion of the 40 .mu.L RNA library was supplemented with
nuclease-free water to 37.5 .mu.L, and mixed with 37.5 .mu.L RNA
running buffer to a final volume of 75 .mu.L before loading into a
flow cell for direct RNA sequencing.
[0107] FIG. 14 shows that using immobilized enzymes yielded total
reads and sequence length comparable to both soluble enzymes and
bead purification, indicating that immobilized enzymes can be used
to substitute soluble enzymes in catalyzing poly(A) tailing and
adaptor ligation reactions. In addition, immobilized enzymes
generated many more sequence reads than the soluble enzyme protocol
incorporating no bead purification. Thus, removal of the enzyme
components from the RNA library appears to be sufficient for
generation of high sequence reads in nanopore sequencing presumably
by avoiding clogging of nanopores by enzyme molecules. The soluble
enzyme protocol without bead purification yielded fewer reads,
suggesting that proteins or other components in the reaction
mixture may cause nanopore fouling. Soluble enzyme protocol with
bead purification also displayed fewer reads probably due to
impurities as the result of bead purification.
Example 4E. Coupled Reactions with Immobilized Enzymes (FIG.
1E)
[0108] The sequential poly(A) tailing and ligation steps (shown in
Example 4D) were combined into a single, coupled reaction as shown
in FIG. 1E. Poly(A) tailing and ligation were carried out by using
2.5 .mu.L immobilized Poly(A) polymerase and 3.0 .mu.L immobilized
T4 DNA ligase together at 37.degree. C. for 20 min followed by
25.degree. C. incubation for 10 min. The immobilized enzyme beads
were separated from the reaction medium on magnetic rack and the
supernatant containing the products and other soluble components
were transferred to a fresh tube. A library was also prepared using
the sequential reaction protocol with immobilized enzymes described
in EXAMPLE 4D.
[0109] The RNA concentration in the resulting libraries was
determined using Qubit method. The same amount of RNA from each
library was used to prepare the sequencing mixtures supplemented
with nuclease-free water to a volume of 37.5 .mu.L and another 37.5
.mu.L of RNA running buffer (RRB) were used to prepare 75 .mu.L
sample for RNA sequencing according to Example 2D. Direct RNA
sequencing for both sequential (Example 2C) and coupled reaction
(this example) were performed on a MinION.RTM. MkIb with R9.4 flow
cells as introduced before.
[0110] Results shown in FIG. 15 contrast the number of sequencing
read from the coupled reaction protocol to that of the sequential
reaction protocol using immobilized enzymes. The library prepared
using a sequential reaction strategy (Example 4D) with an RNA
sequencing input of 105 ng of RNA yielded 459 K reads, which is
comparable to the results presented in Example 4C with final yield
of 488.5 K reads from 164.4 ng of RNA input. However, the library
prepared in this Example using a coupled reaction protocol with
immobilized enzymes with similar amount of RNA input produced
almost a 3-fold increase in RNA sequencing reads compared to a
sequential reaction protocol using immobilized enzymes.
Example 4F. Coupled Reactions with Co-Immobilized Enzymes
[0111] O.sup.6-benzylguainine (BG) functionalized magnetic beads
coated with PEG.sub.750 (100 .mu.L of a 25% (v/v) slurry) were used
for enzyme co-immobilization. Poly(A) polymerase-SNAP fusion
protein and T4 DNA ligase-SNAP fusion protein (12.5 .mu.g of each)
were dissolved in 125 .mu.L buffer (1.times.PBS with 300 mM NaCl),
combined with the washed BG beads, and incubated at 4.degree. C.
overnight to immobilize the fusion protein on the beads, according
to the procedure described in EXAMPLE 1C. The co-immobilized beads
were washed with the same buffer 8 times to remove any unbound
enzyme molecules. Diluent A buffer without BSA (NEB) with 100 mM
NaCl was used to resuspend the beads with immobilized fusion
protein for storage at -80.degree. C.
[0112] Poly(A) tailing and adaptor ligation activities were
measured for the bead sample co-immobilized with poly(A) polymerase
and T4 DNA ligase. 5 .mu.L of the co-immobilized enzyme bead
mixture were used to replace 2.5 .mu.L immobilized poly(A)
polymerase and 3 .mu.L immobilized T4 DNA ligase in each activity
assay. The co-immobilized enzymes displayed both poly(A) polymerase
activity (FIG. 16A) and T4 DNA ligase activity (FIG. 16B).
Example 5: Automated Library Construction and Sequencing
[0113] While next-generation sequencing (NGS) has greatly advancing
biological research and clinical diagnostics, the process would
benefit from automation from library construction to sequencing
libraries and data analysis. The preceding examples demonstrate
that application of immobilized enzymes in multi-reaction library
construction workflows avoids bead based nucleic acid purification
which may cause sample loss and bias in fragment distribution.
Combining the single-reaction preparation of Example 4 with NGS
(e.g., using robotics and/or microfluidics) advances automation in
sequencing. Specifically, otherwise cumbersome steps of adding
adapters to nucleic acid libraries to be sequenced may be in a
single reaction vessel and fed directly into sequencing platforms.
The concomitant reduction in handling will reduce error rates and
variations in high-throughput research and clinical application of
Nanopore and other NGS technologies (FIG. 17).
[0114] As shown, magnetic beads bearing enzymes are positioned in
proper enclosed chamber and tunnel to process an input or
intermediate library. There is no extra purification step required
for separation of the enzymes and the resulting products, between
or after enzymatic reaction steps. The input library can be
produced by a method, for example, RNA extraction, that
incorporates a properly designed automated workflow. An output
library can be properly formulated for direct sequencing on a
nanopore sequencing device, i.e. flow cell, such as currently
available R9.4.1. or R10. Ultimately, this workflow is linked to
locally based or cloud-based computer software to provide a fully
automated sequencing solution.
Example 6: Direct RNA Sequencing with Low Input RNA is Possible
when Immobilized Enzymes are Used in Library Prep
[0115] All sequencing reads using the libraries described in
Example 4 were performed with 500 ng of RNA as suggested by Oxford
Nanopore Technologies. This example demonstrates construction and
successful sequencing of duplicate libraries from a lower initial
input (100 ng) of Listeria monocytogenes RNA using either a
sequential or coupled reaction protocol with immobilized poly(A)
polymerase and T4 DNA Ligase. For sequential reaction protocol, 3
.mu.L quick ligation buffer, 0.45 .mu.L 5 M NaCl solution, 1.5
.mu.L immobilized poly(A) polymerase, and 100 ng total RNA,
supplemented with nuclease-free water to 10 .mu.L were mixed in a
0.2 mL thin-walled PCR tube and incubated at 37.degree. C. for 20
min for poly(A) tailing. After immobilized poly(A) polymerase beads
were removed by placing the tube on the magnetic rack, 0.5 .mu.L RT
Adaptor (RTA), 3.0 .mu.L RNA Adaptor (RMX) and 1.5 .mu.L
immobilized T4 DNA ligase were added to the poly(A) tailed RNA
sample to yield a final volume of 15 .mu.L. This mixture was
incubated at 25.degree. C. for 10 min for adaptor ligation. The
immobilized ligase was removed using the magnet. For the coupled
reaction protocol, the same amounts of enzymes and buffer were
utilized as disclosed above (except that all the components were
combined in a single tube). The mixtures were incubated at
37.degree. C. for 20 min and 25.degree. C. for 10 min,
consecutively, and both immobilized enzymes were removed in a
single step on the magnetic rack. The RNA yields of the prepared
libraries were determined by high sensitivity RNA Qubit assay.
[0116] With the initial RNA input of 100 ng, the sequential and
coupled reaction methods resulted in recovery of an average of 38
ng and 83 ng, respectively, from duplicate libraries. Thus, the
recovery rate for RNA library prep using the coupled reaction
protocol is 83% of the initial input, much higher than that of the
sequential reaction protocol (FIG. 18A). The results also suggest
that a coupled reaction process generates more reads compared to
the sequential reaction. In addition, a sequential reaction
generates 568 K reads on average, higher than that obtained from
the protocol using soluble enzymes with bead purification. The
results demonstrate that low input RNA library construction can be
achieved by using both sequential and coupled reaction protocols
with immobilized enzymes (FIG. 18A).
[0117] Similar results were obtained with library construction from
a low initial input mammalian human brain RNA (100 ng). FIG. 18B
shows direct RNA sequencing of low input of poly(A) tailed human
RNA (100 ng PolyA+RNA) prepared by ligation to RTA and RMX adaptors
with immobilized T4 DNA Ligase (ImL) Furthermore, total mammalian
RNA library (100 ng input), prepared with immobilized poly(A)
polymerase and immobilized T4 DNA ligase (ImP and ImL) using the
coupled reaction protocol described above in this example, was
successfully used for direct RNA sequencing.
Example 7: Metrics of the ONT Direct RNA-Seq Datasets from Various
Library Preparation Protocols
[0118] Duplicate libraries using each of the five RNA library
preparation protocols were made with 500 ng input RNA extracted
from Listeria monocytogenes (ATCC 1115) culture. The final volume
for the enzymatic reactions was 40 .mu.l prior to bead
purification; for soluble enzyme protocols, Sol-seq and Sol-cpl,
each RNA library was further purified with RNA binding beads and
eluted in 20 .mu.l volume. For immobilized enzyme protocols enzyme
removal was performed without purification using RNA binding beads
prior to use of a fraction of the library sample for Nanopore
direct RNA sequencing. Results are shown in TABLE 1.
TABLE-US-00001 TABLE 1 500 ng input libraries Sol-seq Sol-cpl
Im-seq Replicate R1 R2 Avg Std R1 R2 Avg Std R1 R2 FAM9 FAM9 FAM9
FAN2 FAN2 FAN2 6082 4718 4789 3186 7746 7699 Recovery 29% 39% 34%
34% 34% 34% 31% 47% rate Loading 176 197 186 196 222 209 88 116
(ng) Loading 20 20 20 20 20 20 (.mu.L) Reads 134 149 141 8 642 757
699 58 903 798 generated (K) Bases 136 168 152 16 681 816 748 68
836 737 generated (Mb) Estimated 157 200 178 21 829 962 895 66 1030
896 generated (Mb) Bases/ 0.86 0.84 0.85 0.82 0.85 0.84 0.81 0.82
Estimated Run 18 22 20 45 53 49 40 40 Length (hr) 500 ng input
libraries Im-seq Im-cpl Sol w/o BP Replicate Avg Std R1 R2 Avg Std
R1 R2 Avg Std FAM9 FAN0 FAL6 FAL7 4697 3228 8474 0280 Recovery 39%
100% 117% 108% rate Loading 102 176 197 186 (ng) Loading 11 15.6
(.mu.L) Reads 850 52 1580 1070 1325 255 430 226 328 102 generated
(K) Bases 786 50 1610 1120 1365 245 427 146 286 140 generated (Mb)
Estimated 963 67 2000 1390 1695 305 526 192 359 167 generated (Mb)
Bases/ 0.82 0.81 0.81 0.81 0.81 0.76 0.80 Estimated Run 40 72 72 72
43 21 32 Length (hr)
Example 8: Comparison of the Major Metrics of the ONT Direct
RNA-Seq Datasets from Various Library Preparation Methods
[0119] The average of the major metrics from duplicate libraries
prepared using each of the five RNA library preparation protocols
were analyzed. Each library was made with 500 ng input RNA
extracted from Listeria monocytogenes (ATCC 1115) culture. The
final volume for the enzymatic reactions was 40 .mu.l prior to bead
purification; for soluble enzyme protocols, Sol-eq and Sol-cpl,
each RNA library was further purified with RNA binding beads and
eluted in 20 .mu.l volume. For immobilized enzyme protocols enzyme
removal was performed without purification using RNA binding beads
prior to use of a fraction of the library sample for Nanopore
direct RNA sequencing.
[0120] Averages of two replicate runs are shown in TABLE 2.
TABLE-US-00002 TABLE 2 500 ng input libraries Sol-seq Sol-cpl
Im-seq Im-cpl Recovery rate 0.34 0.34 0.39 1.08 Loading (ng) 186.4
208.8 102.0 186.4 Loading (.mu.L) 20 20 20 13.3 Reads generated (K)
141.42 699.33 850.42 1325 Bases generated (Mb) 151.58 748.15 786.34
1365 Estimated generated 178.2 895.4 963.11 1695 (Mb)
Bases/Estimated 0.85 0.84 0.82 0.81 Run Length (hr) 20.2 48.8 40.3
72.0 Mean read length 1145.3 1156.4 1065.9 1107.9 (nt) Median read
length 1157 1028 950 951 (nt) Read length N50 1493 1520 1464 1511
Mean read quality 10.2 10.1 10.2 10.2 Median read quality 10.4 10.2
10.4 10.3 Mapping 99.6% 99.2% 99.3% 99.1% Mapped reads 140.84
693.52 844.55 1313.08 Expected Reads 141.4 699.3 1615.8 3902.4
(K)/Library Expected bases 151.6 748.2 1494.0 4020.2 (Mb)/Library
Ratio of Expected 1 4.9 11.4 27.6 Reads (K)/Library
Example 9: Comparison of Nanopore Direct RNA Sequencing Reads
[0121] Duplicate libraries were prepared using the five protocols
illustrated in FIG. 1A-E. Each library was made with 500 ng input
RNA extracted from Listeria monocytogenes (ATCC 1115) culture. For
Sol-seq and Sol-cpl, each RNA library was further purified with RNA
binding beads prior to Nanopore sequencing. Both immobilized enzyme
protocols did not utilize a purification step with RNA-binding
beads after enzymatic treatment, and enzyme removal was performed
with magnetic rack prior to sequencing. Results are shown in FIG.
19.
Example 10: Reduced Inactivation of Nanopores Using Libraries
Prepared with Immobilized Enzymes
[0122] Data presented in Examples 7-9 demonstrates that bead
purification following library preparation with soluble enzymes is
associated with a considerable loss of library RNA. Example 13
shows further that omitting a bead purification step (to avoid this
loss of RNA) may not be a favorable solution in that the resulting
libraries produce substantially fewer sequence reads from the same
amount of input RNA compared to the immobilized enzyme protocols. A
possible reason for fewer sequence reads from the libraries
produced by the soluble enzyme protocol without bead purification
is that residual polymerase and/or ligase may occlude nanopores. On
the other hand, bead purification may affect library quality due to
loss of RNA (or certain RNA types) and may also produce impurity
derived from the wash solutions. In this example, the activity of
nanopores was monitored over the course of a sequencing run from
MinKNOW.RTM. report. Results shown in FIG. 20 demonstrate that more
nanopores remain active (upper trace) when the libraries were
prepared with immobilized enzymes compared to those prepared with
soluble enzymes. For example, after two hours of sequencing, about
90% of the pores processing immobilized enzyme libraries remained
active while only about 65% of the pores processing soluble enzyme
libraries remained active. At 8 hours, about half of the pores
processing immobilized enzyme libraries remained active while only
about 10% of the pores processing soluble enzyme libraries remained
active. These results demonstrate that the use of immobilized
enzymes in library construction can increase nanopore sequencing
output, possibly by reducing nanopore fouling.
[0123] The number of reads per pore was also evaluated for the
coupled reaction method. Normalized reads shown in FIG. 21 were
generated from dividing the reads from sequencing a Listeria RNA
library by the number of pores.
[0124] Three Listeria RNA libraries were prepared using the coupled
reaction protocol using immobilized enzymes (orange dots). Two low
input RNA libraries were prepared in 15 uL following the coupled
reaction protocol as described in Example 5, resulting in direct
RNA sequencing of 83 ng and 136.5 ng RNA per flow cell,
respectively. The third library was prepared as described in
Example 4E with 500 ng input RNA in 40 uL and only part of the
resulting library (109 ng) was loaded for sequencing.
[0125] The number of reads per pore was also examined for a set of
four libraries prepared using the sequential reaction protocol
using immobilized enzymes (blue dots). Two low input RNA libraries
were prepared in 15 uL as described in Example 5, resulting in
sequencing 38 ng and 39 ng RNA per flow cell, respectively. Two 500
ng input RNA libraries were made in 40 uL as described in Example
4D and a portion of each resulting library, 105 ng and 164.4 ng,
respectively, was used for loading on a flow cell and direct RNA
sequencing.
[0126] Results indicate that the coupled reaction protocol can
generate a significantly higher reads per nanopore compared to the
sequential reaction protocol using the same set of immobilized
enzymes and conditions (i.e. buffer, total reaction time and
volume).
Example 11: DNA Library Construction Workflow for Nanopore
Sequencing of Ultra-Long Templates without Bead Purification
[0127] This example describes a new strategy for preparation of DNA
libraries for nanopore DNA sequencing. The current ONT protocol, as
depicted in FIG. 26 and Example 13, utilizes a set of four
DNA-modifying enzymes to perform end-polishing, dA-tailing and
adaptor ligation, in conjunction with bead purification to produce
a library for long-read sequencing. Addition of a single 3'A in
dA-tailing demands use of PEG in the subsequent adaptor ligation
because T/A pairing is inefficient in the absence of PEG or other
enhancers. However, use of PEG in conjunction with use of
AMPure.RTM. beads may not be ideal since PEG can cause DNA
compaction onto beads. In addition, application of bead
purification can result in shearing of long DNA templates thereby
adversely affecting the ability of ultra-long sequence reads by
nanopore sequencing.
[0128] A new method is proposed here and illustrated in FIG. 22. As
shown, the proposed method does not use bead purification and/or
may include or exclude use PEG during DNA library preparation. This
workflow is comprised of three major enzymatic steps. First,
Terminal deoxynucleotidyl transferase (TdT) is employed to catalyze
poly(dA) tailing at 3' end of DNA fragments possibly pre-treated
with end-polishing enzyme(s). The oligo(dA) overhang can then
efficiently ligate with an adaptor with a 3' Poly(dT) overhang and
motor protein, in the presence or absence of PEG in the reaction
medium. Next, gap-filling and nick sealing can be accomplished with
DNA polymerase and DNA Ligase. Enzymes may be removed, inactivated
or present in the final sequencing library. Breakage of DNA
molecules may be reduced and/or recovery of long DNA templates may
be improved by avoiding use of bead purification and/or PEG. As
practitioners having the benefit of this disclosure will
appreciate, TdT can add other types of oligos, such as poly(dT) or
poly(dG) to be suitable for other adaptor ligation strategies in
the absence of PEG or DNA-compacting factor. Thus, different
enzymes and tailing approaches can be designed to prepare
DNA-adaptor molecules anchored with motor protein and other
features required for nanopore sequencing.
Example 12: DNA Library Preparation
[0129] This example demonstrates poly(dA) tailing of a synthetic
DNA substrate and subsequent ligation of the products possessing
various lengths of 3' poly(dA) sequences to an adaptor having a 3'
poly(dT) overhang as illustrated in FIG. 23.
Example 6A. Poly(dA)-Tailing Mediated by Terminal Deoxynucleotidyl
Transferase (TdT)
[0130] This example demonstrates poly(dA) tailing of a synthetic
DNA substrate by Terminal deoxynucleotidyl Transferase (TdT). A
double-stranded DNA substrate was formed by annealing two
oligonucleotides, with one possessing a 5' fluorophore probe, FAM
and 3' protruding overhang for addition of Poly(dA) tails.
5'FAM-labeled double-stranded DNA was treated with TdT in the
presence of various concentrations of dATP to create different
substrate to dATP ratios (e.g. 1:100 and 1:200). CE analysis was
performed to assess the incorporation of dAMP at the 3' termini of
5'FAM-labeled DNA strand and estimation of the lengths (or range)
of poly(dA) tails.
[0131] 3' poly(dA) tailing was carried out in a 30 .mu.l reaction
volume in the presence of 0.1 .mu.M of the DNA substrate, 0.5 .mu.l
(20 units) of TdT (NEB, M0315S, 40,000 units/ml,), 1.times.TdT
Reaction Buffer (NEB), 0.25 mM CoCl.sub.2 and 10 or 20 .mu.M of
dATP. The reactions were performed at 37.degree. C. for 30 min,
followed by treatment at 70.degree. C. for 10 min in a T-100
Thermocycler (Bio-Rad Laboratories, Hercules, Calif.). The
reactions were terminated by diluting in 1:1 ratio in 50 mM EDTA
and 0.1% Tween-20, and analyzed by CE technique and Peak Scanner
software. Results shown in FIG. 24A demonstrate that poly(dA)
tailing can be performed in 1.times.TdT buffer supplemented with
CoCl.sub.2. Efficient conversion of the DNA substrate to poly(dA)
tailed products of various lengths was observed after treatment
with TdT and the length of the poly(dA) can be modulated by the
control of substrate-to-dATP ratio.
Example 12B. Sequential Poly(dA) Tailing and Adaptor Ligation
[0132] The FAM-labeled double-stranded DNA substrate was assayed
for sequential poly(dA) tailing with soluble TdT as described in
EXAMPLE 12A and adaptor ligation with soluble T4 DNA Ligase.
[0133] A modified RTA adaptor, RTA-Poly(dT) was made by annealing
two oligonucleotides, derived from the sequences of RTA (provided
by ONT), with one oligonucleotide containing 5' phospho group and
3' ROX probe, and the second one being modified to possess 3'
poly(dT).3' poly(dA) tailing was carried out in a 30 .mu.l reaction
volume in the presence of 0.1 .mu.M of the DNA substrate, 1 unit of
Terminal Deoxynucleotide Transferase (NEB, M0315S, 40,000
units/ml,), 1.times.TdT Reaction Buffer (NEB), 0.25 mM CoCl.sub.2
and 10, 20 or 50 .mu.M of dATP. The reactions were performed at
37.degree. C. for 30 min, followed by treatment at 70.degree. C.
for 10 min in a T-100 Thermocycler (Bio-Rad Laboratories, Hercules,
Calif.). Next, adaptor ligation was performed at 25.degree. C. for
30 min after addition of 100 nM RTA-poly(dT), 1 mM ATP and 1 .mu.l
of T4 DNA Ligase (NEB, M0202S, 400,000 units/.mu.l) to the
TdT-treated samples. The reactions were terminated by diluting in
50 mM EDTA and 0.1% Tween-20, and analyzed by CE technique and Peak
Scanner software. Efficient conversion of the DNA substrate to
poly(dA) tailed products of various lengths was observed after
treatment with TdT. Joining of poly(dA) tailed products and
modified RTA was also detected because of the shift of the FAM
labeled products and the co-localization of FAM and ROX signals
after ligation reaction, in comparison with the TdT-treated
sample.
[0134] FIG. 24B shows a peak formed by a range of FAM-labeled
products (in blue), representing various 3' poly(dA) lengths.
Subsequently, the reaction medium containing the poly(dA)-tailed
DNA products, was incubated with T4 DNA ligase and RTA-poly(dT)
adaptor possessing 3' poly(dT) and 5' ROX. FIG. 24C shows
co-localization of the fluorescence signals of FAM (blue) and ROX
(red) indicates ligation of the 5'FAM-labeled DNA species to the 3'
ROX-labeled strand of the adaptor. Successful ligation also
resulted in a shift of the FAM-labeled species (major peak in TdT
sample) to higher molecular products (major Peak in TdT/T4 DNA
Ligase sample).
[0135] Results shown FIG. 24B and FIG. 24C demonstrate that both
poly(dA) and ligation reactions can be performed in 1.times.TdT
buffer supplemented with CoCl.sub.2.
Example 12C. Ligation of Poly(dA) Tailed DNA with Adaptor with
Soluble and Immobilized Ligase
[0136] As shown in FIG. 25, a FAM-labeled double-stranded DNA
substrate was first tailed using soluble TdT as described in
EXAMPLE 6A and then ligated to an adapter with either soluble or
immobilized T4 DNA Ligase. 3' poly(dA) tailing was carried out in a
30 .mu.l reaction volume in the presence of 0.1 .mu.M of the DNA
substrate, 1 unit of Terminal Deoxynucleotide Transferase (NEB,
M0315S, 40,000 units/ml,), 1.times.TdT Reaction Buffer (NEB), 0.25
mM CoCl.sub.2 and 10, 20 or 50 .mu.M of dATP. The reactions were
performed at 37.degree. C. for 30 min, followed by treatment at
70.degree. C. for 10 min in a T-100 Thermocycler (Bio-Rad
Laboratories, Hercules, Calif.). Next, adaptor ligation was
performed at 25.degree. C. for 30 min after addition of 100 nM
RTA-poly(dT), 1 mM ATP and 1 .mu.l of T4 DNA Ligase (NEB, M0202S,
400,000 units/.mu.l) or immobilized T4 DNA Ligase (NEB, Production
Lot 1, 60 units/.mu.l) to the TdT-treated samples. The reactions
were terminated by diluting in 50 mM EDTA and 0.1% Tween-20, and
analyzed by CE technique and Peak Scanner software. Efficient
conversion of the DNA substrate to poly(dA) tailed products of
various lengths was observed after treatment with TdT. Poly(dA)
tailed products and modified RTA were detected with either soluble
or immobilized T4 DNA ligase because of the shift of the FAM
labeled products and the co-localization of FAM and ROX signals
after ligation reaction, in comparison with the TdT-treated sample.
These results show that both poly(dA) and ligation reactions can be
performed in 1.times.TdT buffer supplemented with CoCl.sub.2.
Example 13: DNA Library Construction Using Immobilized DNA
Modifying Enzymes
[0137] Many existing methods rely on steps (e.g., AMPure.RTM. bead
purification) that shear long DNA molecules and are detrimental to
long-read sequencing. In addition to use for RNA library
preparation for sequencing, immobilized enzymes can be used to
construct DNA libraries. FIG. 26 shows a schematic of DNA library
construction using a set of four immobilized DNA modifying enzymes
(IM-T4 DNA polymerase IM-T4 PNK, IM-Taq DNA Pol, IM-T4 DNA Ligase).
Soluble forms of these enzymes are currently used for Nanopore DNA
library construction. The following example sets forth the use of
relevant immobilized enzymes to generate a DNA library by using an
oligo DNA model system with a CE technique to conduct step-by-step
analyses.
Example 14: DNA Library Construction Using Immobilized DNA
Modifying Enzymes
[0138] A DNA library construction protocol for nanopore sequencing
may include fragmentation, end repair (blunting and 5'
phosphorylation), 3' A-tailing and adaptor ligation. Once the
sample DNA has been sheared, the fragment ends are repaired by
blunting and 5' phosphorylation with a mixture of enzymes, such as
T4 polynucleotide kinase (PNK) and T4 DNA polymerase (T4 DNA pol).
This end repair step is followed by 3' A-tailing at 37.degree. C.
using a mesophilic polymerase such as Klenow Fragment 3'-5'
exonuclease minus.sup.11, or at elevated temperatures using a
thermophilic polymerase such as Taq DNA polymerase (Taq DNA pol)
(Head, S. R. et al. Library construction for next-generation
sequencing: overviews and challenges. BioTechniques 56, 61-64, 66,
68, passim (2014); Star, B. et al. Palindromic Sequence Artifacts
Generated during Next Generation Sequencing Library Preparation
from Historic and Ancient DNA. PLOS ONE 9, e89676 (2014)). 3'
A-tailed DNA fragments are ligated to an adaptor using a T/A
ligation method and purified using AMPure.RTM. beads prior to
nanopore sequencing. Bead-based purification step(s) may result in
shearing large DNA which is detrimental to long read sequencing. In
addition, T/A ligation efficiency is highly dependent on the
presence of crowding agent, such as PEG, however, use of a crowding
agent, namely PEG, appears to cause large DNA molecules to compact
(Warren M. Mardoum, Stephanie M. Gorczyca, Kathryn E. Regan,
Tsai-Chin Wu, and Rae M. Robertson-Anderson. Crowding Induces
Entropically-Driven Changes to DNA Dynamics That Depend on Crowder
Structure and Ionic Conditions. Front Phys. 2018; 6: 53; Heikki
Ojal, Gabija Ziedait, Anders E. Wallin, Dennis H. Bamford, Edward
H.ae butted.ggstrom. Optical tweezers reveal force plateau and
internal friction in PEG-induced DNA condensation. European
Biophysics Journal, March 2014, Volume 43, Issue 2-3, pp 71-79).
Consequently, the PEG-induced DNA compaction may reduce DNA elution
from AMPure.RTM. beads, resulting in low library yield of large
DNA.
[0139] An enzyme immobilization strategy was previously utilized to
perform DNA library construction for the sequencing on the Illumina
platform (Zhang, Aihua, et al. Solid-phase enzyme catalysis of DNA
end repair and 3' A-tailing reduces GC-bias in next-generation
sequencing of human genomic DNA. Scientific reports 8.1 (2018):
1-11.). The relevant DNA-modifying enzymes were produced as
SNAP-tagged fusion proteins and immobilized by covalent conjugation
onto magnetic beads functionalized with benzyl guanine ligand (the
substrate of SNAP-tag). These immobilized enzymes were successfully
applied to Illumina DNA library construction in place their soluble
counterparts. One of the major of the major advantages is that the
enzymes can be removed without heat treatment or AMPure.RTM. bead
purification.
[0140] This example demonstrates that the same set of enzymes can
be used for the current workflow of Nanopore DNA library
construction. In this example, each enzymatic reaction step was
monitored by fluorescence capillary gel electrophoresis (CE) using
a synthetic double stranded DNA end-labeled with a fluorescent
probe, FAM. DNA was end-repaired for 30 min at 20.degree. C. using
immobilized T4 DNA pol and T4 polynucleotide kinase in a 20
reaction in the presence of 1.times.NEBNext End Repair Buffer II.
These end-repair enzymes were pelleted on a magnetic rack and the
supernatant was transferred to a new tube for 3' A-tailing with
immobilized Taq DNA pol at 37.degree. C. for 30 min. The resulting
product was ligated to an adaptor with an end possessing 5' phospho
group and 3'T. The reactions were terminated by diluting in 50 mM
EDTA and 0.1% Tween-20. The reactions were performed in a T-100
Thermocycler (Bio-Rad Laboratories, Hercules, Calif.) and analyzed
by CE technique and Peak Scanner software. A partial ligation of
the DNA substrate to the adaptor is shown in FIG. 27, indicating
that T/A ligation can be performed in 1.times.NEBNext End Repair
Buffer II without supplement with PEG.
Sequence CWU 1
1
3149DNAArtificial SequenceSynthetic
constructmisc_featureGene32FAM-fwmisc_feature(2)..(2)6-FAMN
1catggtgatt acgattcttg cccagtatgt caatacatca gtaaaaata
49248DNAArtificial SequenceSynthetic
constructmisc_featureGene32-rvmisc_feature(1)..(1)p 5Phos
2atttttactg atgtattgac atactgggca agaatcgtaa tcaccatg
48364DNAArtificial SequenceSynthetic constructmisc_feature(1)..(1)p
5Phosmisc_feature(32)..(32)ideoxyu 3gatcggaaga gcacacgtct
gaactccagt cacactcttt ccctacacga cgctcttccg 60atct 64
* * * * *