LIGASE-ASSISTED NUCLEIC ACID
CIRCULARIZATION AND AMPLIFICATION
FIELD OF INVENTION
[0001] The invention generally relates to nucleic acid assays that involve the
generation of a single-stranded DNA circle from a single-stranded or double-stranded
linear DNA via template-independent single-stranded DNA ligation. It further relates
to the amplification and/or detection of the single-stranded DNA circle via rolling
circle amplification. Generation of single-stranded DNA circles and subsequent DNA
amplification are performed in a single reaction vessel without any intervening
isolation and/or purification steps. Kits for performing the methods are also provided.
BACKGROUND
[0002] DNA amplification is a process of replicating a target double-stranded
DNA (dsDNA) to generate multiple copies of it. Since individual strands of a dsDNA
are antiparallel and complementary, each strand may serve as a template strand for the
production of its complementary strand. The template strand is preserved as a whole
or as a truncated portion and the complementary strand is assembled from
deoxynucleoside triphosphates (dNTPs) by a DNA polymerase. The complementary
strand synthesis proceeds in 5' 3' direction starting from the 3' terminal end of a
primer sequence that is hybridized to the template strand.
[0003] Whole-genome amplification (WGA) involves non-specific
amplification of a target DNA. WGA is often achieved by multiple displacement
amplification (MDA) techniques employing random oligonucleotide primers for
priming the DNA synthesis at multiple locations of the target DNA along with a high
fidelity DNA polymerase having a strand displacing activity (e.g., Phi29 polymerase).
Even though currently available commercial WGA systems such as GenomiPhi (GE
Healthcare, USA) and RepliG (Qiagen) kits provide optimal results with high
molecular weight target DNA, performance of these systems is poor when the target
DNA is short and/or highly fragmented. When the target DNA is fragmented and the
sequence length is less than about 1000 nucleotides, amplification of the target DNA
using conventional methods results in decreased amplification speed, significant
sequence dropout especially near the ends of the target DNA, and highly sequencebiased
amplification. As the length of the template DNA is decreased, the likelihood
of that strand being primed multiple times decreases in the MDA reaction. This
decreases the amplification potential of these shorter fragments. Efficient methods for
non-specifically amplifying short, fragmented DNA are therefore highly desirable.
[0004] Ligation-mediated polymerase chain reaction (PCR) has been used to
amplify fragmented dsDNA. However, only a small fraction of the fragmented DNA
gets amplified in these reactions leading to inadequate genome coverage. To
efficiently amplify fragmented, target dsDNA, they may first be repaired and then be
concatamerized by blunt-end ligation to generate sequences that are longer than 1000
base pairs (bp). However, a relatively higher concentration of the target DNA is often
required to promote concatamerization and subsequent amplification. Circularization
of double-stranded target DNA has also been employed in various nucleic acid based
assays including MDA, WGA, hyper-branched rolling circle amplification (RCA) and
massively parallel DNA sequencing. To effectively circularize and amplify
fragmented dsDNA, the double-stranded ends of the fragmented DNA are first
repaired, followed by blunt-end ligation to form double-stranded DNA circles.
However, it is difficult to circularize double-stranded DNA fragments that are less
than 500 bp in length.
[0005] The double- stranded DNA may be denatured to produce singlestranded
DNA (ssDNA), which may further be circularized in a template-dependent
intra-molecular ligation reaction using a ligase. However, prior sequence information
of the target DNA is required to perform a template-dependent circularization.
Template-independent intra-molecular ligation of ssDNA has also been documented.
For example, TS2126 RNA ligase (commercially available under the trademarks
ThermoPhage™ RNA ligase II or ThermoPhage™ ssDNA ligase (Prokaria, Matis,
Iceland) or CircLigase™ ssDNA ligase (Epicenter Biotechnologies, Wisconsin, USA)
has been used for making digital DNA balls, and/or locus-specific cleavage and
amplification of DNA, such as genomic DNA. Linear, single-stranded
complementary DNA (cDNA) molecules prepared from 5'-end fragments of mRNA
have also been amplified via rolling circle replication after circularization using
TS2126 RNA ligase. By appropriately incorporating a sense RNA polymerase
promoter sequence in to the cDNA, the circularized cDNA template has shown to act
as a transcription substrate and thus effect the amplification of the mRNA molecules
in a biological sample. Further, the TS2126 RNA ligase has been used for amplifying
the cDNA ends for random amplification of cDNA ends (RACE). From limited
amounts of fragmented DNA, DNA template for rolling circle amplification has also
been generated by employing TS2126 RNA ligase. The method involved denaturing
the linear, fragmented dsDNA to obtain linear ssDNA fragments, ligating the linear
ssDNA with CircLigase™ ssDNA ligase to obtain single- stranded DNA circle, and
then amplifying the single-stranded DNA circle using random primers and Phi29
DNA polymerase via RCA. However, even after optimizing the reaction conditions,
the amount of generated single-stranded circular DNA was highly variable and
sequence dependent. For example, oligonucleotides comprising a 5'G and a 3'T
nucleotide ligated significantly better than its complementary oligonucleotide
comprising a 5Ά and a 3'C under identical ligation conditions. Further, intra
molecular ligation efficiency varied among linear ssDNA sequences having identical
or very similar sizes but with small differences in nucleotide sequence. The
efficiency also varied among linear ssDNA sequences of different sizes (e.g.,
sequence length ranging from 100 bases to kilobases in size). Moreover, all attempts
of ligation-amplification reactions involved intermediate isolation, purification and/or
cleaning steps, thus making the ligation-amplification workflow cumbersome. For
example, analysis of forensic samples of fragmented DNA by circularization followed
by rolling circle amplification was carried out in multiple steps comprising 5' DNA
phosphorylation, adapter ligation, DNA circularization, and whole-genome
amplification. Each step reactions were subjected to a reaction clean-up before
performing the next step. No amplification advantage was observed when ligation
and amplification was performed in single reaction vessel. However, the multi-step
process often resulted in the loss of template DNA and led to failed analysis.
Efficient methods for non-specifically amplifying short DNA sequences in a single
reaction vessel without any sequence bias and any intervening cleaning steps are
therefore highly desirable.
BRIEF DESCRIPTION
[0006] In some embodiments, a method for generating a single-stranded DNA
circle from a linear DNA is provided. The method comprises the steps of providing a
linear DNA, end-repairing the linear DNA by incubating it with a polynucleotide
kinase in the presence of a phosphate donor to generate a ligatable DNA sequence
having a phosphate group at a 5' terminal end and a hydroxyl group at a 3' terminal
end, and performing an intra-molecular ligation of the repaired, ligatable DNA
sequence with a ligase in order to generate the single-stranded DNA circle. All steps
of the method are performed in single reaction vessel without any intervening
isolation or purification steps. The phosphate donor may be a guanosine triphosphate
(OTP), a cytidine triphosphate (CTP), a uridine triphosphate (UTP), a
deoxythymidine triphosphate (dTTP) or a combination thereof. The linear DNA may
either be double-stranded or single-stranded DNA. DNA may be a fragmented DNA
such as circulating DNA. The ligatable DNA, if in double-stranded form, needs to be
denatured prior to intra-molecular ligation reaction. A pre-adenylated ligase that is
capable of template-independent, intra-molecular ligation of single-stranded DNA
sequences may be employed for the ligation reaction.
[0007] In some embodiments, a method for generating a single-stranded DNA
circle from a linear DNA is provided, wherein the method employs a DNA preadenylation
step prior to an intra-molecular ligation step. The linear DNA may
optionally be incubated with a polynucleotide kinase in the presence of adenosine
triphosphate (ATP) to generate a ligatable DNA sequence that comprises a phosphate
group at a 5' terminal end and a hydroxyl group at a 3' terminal end. Generation of
ligatable DNA sequence from the linear DNA may be preferred if the linear DNA is
in a highly fragmented form. The linear DNA or the ligatable DNA sequence is then
incubated with an adenylating enzyme in presence of ATP to generate a 5' adenylated
DNA sequence. The 5' adenylated DNA sequence is then incubated with a nonadenylated
ligase, which is capable of template-independent intra-molecular ligation
of the 5' adenylated DNA sequence to generate the single-stranded DNA circle. All
steps of the method are performed in a single reaction vessel without any intervening
isolation or purification steps. ATP may have to be removed from the reaction
mixture (e.g., by treating the reaction mixture with a phosphatase) before the intra
molecular ligation reaction if the non-adenylated ligase is an ATP-dependent ligase.
If the 5' adenylated DNA is in double- stranded form, it needs to be denatured prior to
the intra-molecular ligation reaction.
DRAWINGS
[0008] These and other features, aspects and advantages of the invention will
become better understood when the following detailed description is read with
reference to the accompanying figures.
[0009] FIG. 1 illustrates a schematic representation of an embodiment of a
ligase-assisted whole-genome amplification of a fragmented dsDNA.
[0010] FIG. 2 illustrates size profiles of circulating DNA isolated from blood
plasma of healthy individuals.
[0011] FIG. 3 illustrates ligase-assisted whole-genome amplification of
circulating DNA extracted from blood using different ligases.
[0012] FIG. 4 illustrates the effectiveness of ligase-assisted whole-genome
amplification for sensitive and balanced DNA amplification of four different CODIS
loci.
[0013] FIG. 5 illustrates the effectiveness of ligase-assisted whole-genome
amplification for sensitive and balanced DNA amplification of twelve different
CODIS loci.
[0014] FIG. 6 illustrates the efficiencies of ligase-assisted whole-genome
amplification in different reaction and buffer conditions.
[0015] FIG. 7 illustrates the inhibition of amplification of high molecular
weight genomic DNA in ligase-assisted whole-genome amplification.
[0016] FIG. 8 illustrates a schematic representation of ligase-assisted wholegenome
amplification that includes the processing (e.g., end-repair) of a fragmented
DNA using a polynucleotide kinase followed by ligase-assisted amplification of the
processed fragmented DNA.
[0017] FIG. 9 illustrates a schematic representation of a single-tube reaction
of ligase-assisted amplification of fragmented DNA employing PNK and CircLigase
II™ in the presence of GTP.
[0018] FIG. 10 illustrates a single-tube ligase-assisted amplification reaction
using male-female plasma/blood, wherein DYS14 male-specific marker is detected
using a library created from the input DNA.
[0019] FIG. 11 illustrates a schematic representation of phosphorylation and
pre-adenlyation of fragmented DNA followed by ligation using a substantially nonadenylated
ligase.
[0020] FIG. 12 illustrates the enhanced efficiency of circularization of a preadenylated
DNA sequence using a substantially non-adenylated ligase.
[0021] FIG. 13 illustrates the enhanced efficiency of ligase-assisted wholegenome
amplification when the target DNA sequence was pre-adenylated and when
the ligation was performed using a non-adenylated ligase.
DETAILED DESCRIPTION
[0022] The following detailed description is exemplary and not intended to
limit the invention or uses of the invention. Throughout the specification,
exemplification of specific terms should be considered as non-limiting examples. The
singular forms "a", "an" and "the" include plural referents unless the context clearly
dictates otherwise. Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative representation
that could permissibly vary without resulting in a change in the basic function to
which it is related. Accordingly, a value modified by a term such as "about" is not to
be limited to the precise value specified. Unless otherwise indicated, all numbers
expressing quantities of ingredients, properties such as molecular weight, reaction
conditions, so forth used in the specification and claims are to be understood as being
modified in all instances by the term "about." Accordingly, unless indicated to the
contrary, the numerical parameters set forth in the following specification and
attached claims are approximations that may vary depending upon the desired
properties sought to be obtained by the present invention. At the very least, and not as
an attempt to limit the application of the doctrine of equivalents to the scope of the
claims, each numerical parameter should at least be construed in light of the number
of reported significant digits and by applying ordinary rounding techniques. Where
necessary, ranges have been supplied, and those ranges are inclusive of all sub-ranges
there between. To more clearly and concisely describe and point out the subject
matter of the claimed invention, the following definitions are provided for specific
terms, which are used in the following description and the appended claims.
[0023] As used herein, the term "nucleoside" refers to a glycosylamine
compound wherein a nucleic acid base (nucleobase) is linked to a sugar moiety. A
"nucleotide" refers to a nucleoside phosphate. A nucleotide may be represented using
alphabetical letters (letter designation) corresponding to its nucleoside as described in
Table 1. For example, A denotes adenosine (a nucleoside containing the nucleobase,
adenine), C denotes cytidine, G denotes guanosine, U denotes uridine, and T denotes
thymidine (5-methyl uridine). W denotes either A or T/U, and S denotes either G or
C. N represents a random nucleoside and dNTP refers to deoxyribonucleoside
triphosphate. N may be any of A, C, G, or T/U.
[0024] Table 1: Letter designations of various nucleotides.
u U
R G or A
Y T/U or C
M A or C
K G or T/U
S G or C
w A or T/U
H A or C or T/U
B G or T/U or C
V G or C or A
D G or A or T/U
N G or A or T/U or C
[0025] As used herein, the term "nucleotide analogue" refers to compounds
that are structurally analogous to naturally occurring nucleotides. The nucleotide
analogue may have an altered phosphate backbone, sugar moiety, nucleobase, or
combinations thereof. Nucleotide analogues may be a natural nucleotide, a synthetic
nucleotide, a modified nucleotide, or a surrogate replacement moiety (e.g., inosine).
Generally, nucleotide analogues with altered nucleobases confer, among other things,
different base pairing and base stacking proprieties. As used herein, the term "LNA
(Locked Nucleic Acid) nucleotide" refers to a nucleotide analogue, wherein the sugar
moiety of the nucleotide contains a bicyclic furanose unit locked in a ribonucleic acid
(RNA)-mimicking sugar conformation. The structural change from a
deoxyribonucleotide (or a ribonucleotide) to the LNA nucleotide is limited from a
chemical perspective, namely the introduction of an additional linkage between
carbon atoms at 2' position and 4' position (e.g., 2'-C, 4'-C-oxymethylene linkage;
see, for example, Singh, S. K., et. al., Chem. Comm., 4, 455-456, 1998, or Koshkin,
A. A., et. al., Tetrahedron, 54, 3607-3630, 1998.)). The 2' and 4' position of the
furanose unit in the LNA nucleotide may be linked by an O-methylene (e.g., oxy-
LNA: 2'-0, 4'-C-methylene-P-D-ribofuranosyl nucleotide), a S-methylene (thio-
LNA), or a NH-methylene moiety (amino-LNA), and the like. Such linkages restrict
the conformational freedom of the furanose ring. LNA oligonucleotides display
enhanced hybridization affinity toward complementary single-stranded RNA, and
complementary single- or double-stranded DNA. The LNA oligonucleotides may
induce A-type (RNA-like) duplex conformations. Nucleotide analogues having
altered phosphate-sugar backbone (e.g., PNA, LNA) often modify, among other
things, the chain properties such as secondary structure formation. A star (*) sign
preceding a letter designation denotes that the nucleotide designated by the letter is a
phosphorothioate modified nucleotide. For example, *N represents a
phosphorothioate modified random nucleotide. A plus (+) sign preceding a letter
designation denotes that the nucleotide designated by the letter is a LNA nucleotide.
For example, +A represents an adenosine LNA nucleotide, and +N represents a
locked random nucleotide (i.e., a random LNA nucleotide).
[0026] As used herein, the term "oligonucleotide" refers to oligomers of
nucleotides. The term "nucleic acid" as used herein refers to polymers of nucleotides.
The term "sequence" as used herein refers to a nucleotide sequence of an
oligonucleotide or a nucleic acid. Throughout the specification, whenever an
oligonucleotide or nucleic acid is represented by a sequence of letters, the nucleotides
are in 5 ®3 order from left to right. For example, an oligonucleotide represented by
a letter sequence (W) (N)y(S) , wherein x=2, y=3 and z=l, represents an
oligonucleotide sequence WWNNNS, wherein W is the 5' terminal nucleotide and S
is the 3' terminal nucleotide. The oligonucleotides or nucleic acids may be a DNA, an
RNA, or their analogues (e.g., phosphorothioate analogue). The oligonucleotides or
nucleic acids may also include modified bases and/or backbones (e.g., modified
phosphate linkage or modified sugar moiety). Non-limiting examples of synthetic
backbones that confer stability and/or other advantages to the nucleic acids may
include phosphorothioate linkages, peptide nucleic acid, locked nucleic acid, xylose
nucleic acid, or analogues thereof.
[0027] As used herein, the term "primer" refers to a short linear
oligonucleotide that hybridizes to a target nucleic acid sequence (e.g., a DNA
template to be amplified) to prime a nucleic acid synthesis reaction. The primer may
be an RNA oligonucleotide, a DNA oligonucleotide, or a chimeric sequence. The
primer may contain natural, synthetic, or modified nucleotides. Both the upper and
lower limits of the length of the primer are empirically determined. The lower limit
on primer length is the minimum length that is required to form a stable duplex upon
hybridization with the target nucleic acid under nucleic acid amplification reaction
conditions. Very short primers (usually less than 3 nucleotides long) do not form
thermodynamically stable duplexes with target nucleic acid under such hybridization
conditions. The upper limit is often determined by the possibility of having a duplex
formation in a region other than the pre-determined nucleic acid sequence in the target
nucleic acid. Generally, suitable primer lengths are in the range of about 3
nucleotides long to about 40 nucleotides long.
[0028] As used herein, the term "random primer" refers to a mixture of primer
sequences, generated by randomizing a nucleotide at any given location in an
oligonucleotide sequence in such a way that the given location may consist of any of
the possible nucleotides or their analogues (complete randomization). Thus the
random primer is a random mixture of oligonucleotide sequences, consisting of every
possible combination of nucleotides within the sequence. For example, a hexamer
random primer may be represented by a sequence NNNNNN or (N)6. A hexamer
random DNA primer consists of every possible hexamer combinations of 4 DNA
nucleotides, A, C, G and T, resulting in a random mixture comprising 46 (4,096)
unique hexamer DNA oligonucleotide sequences. Random primers may be
effectively used to prime a nucleic acid synthesis reaction when the target nucleic
acid' s sequence is unknown or for whole-genome amplification reaction.
[0029] As described herein, the term "partially constrained primer" refers to a
mixture of primer sequences, generated by completely randomizing some of the
nucleotides of an oligonucleotide sequence (i.e., the nucleotide may be any of A, T/U,
C, G, or their analogues) while restricting the complete randomization of some other
nucleotides (i.e., the randomization of nucleotides at certain locations are to a lesser
extent than the possible combinations A, T/U, C, G, or their analogues). For example,
a partially constrained DNA hexamer primer represented by WNNNNN, represents a
mixture of primer sequences wherein the 5' terminal nucleotide of all the sequences in
the mixture is either A or T. Here, the 5' terminal nucleotide is constrained to two
possible combinations (A or T) in contrast to the maximum four possible
combinations (A, T, G or C) of a completely random DNA primer (NNNNNN).
Suitable primer lengths of a partially constrained primer may be in the range of about
3 nucleotides long to about 15 nucleotides long.
[0030] As described herein, the term "partially constrained primer having a
terminal mismatch primer-dimer structure" refers to a partially constrained primer
sequence, wherein when two individual primer sequences in the partially constrained
primer hybridize each other inter-molecularly, with an internal homology of three or
more nucleotides, to form a primer-dimer structure having no recessed ends, or a
primer-dimer structure having a single-nucleotide base 3' recessed ends, or a primerdimer
structure having a two-nucleotide base 3' recessed ends, there exists a
nucleotide mismatch (i.e., nucleotides do not base-pair) at both the 3' terminal
nucleotides in the primer-dimer structure. For example, a partially constrained
pentamer primer represented by WNNNS provides a terminal mismatch at both the 3'
terminal nucleotides when it is inter-molecularly hybridized to form a primer-dimer
structure having no recessed ends. In the primer-dimer structure, there exists an
internal homology of three nucleotides (i.e., the three random nucleotides in WNNNS
may base-pair with each other when the primer-dimer structure having no recessed
ends is formed by inter-molecular hybridization). However, this primer example does
not provide a terminal mismatch when it is inter-molecularly hybridized to form a
primer-dimer structure with single-nucleotide base 3' recessed ends. Similarly, a
partially constrained hexamer primer represented by WWNNNS provides a terminal
mismatch at both the 3' terminal nucleotides when it is inter-molecularly hybridized
to form a primer-dimer structure having no recessed ends. Moreover, this primer
example provides a terminal mismatch at both the 3' terminal nucleotides even when
it is inter-molecularly hybridized to form a primer-dimer structure having a singlenucleotide
base 3' recessed ends. A partially constrained heptamer primer represented
by WWWNNNS provides a terminal mismatch at both the 3' terminal nucleotides
when it is inter-molecularly hybridized to form a primer-dimer structure having no
recessed ends. Further, this primer example provides a terminal mismatch at both the
3' terminal nucleotides when it is inter-molecularly hybridized to form a primer-dimer
structure having a single-nucleotide base 3' recessed ends, or to form a primer-dimer
structure having a two-nucleotide base 3' recessed ends.
[0031] As used herein, the term "rolling circle amplification (RCA)" refers to
a nucleic acid amplification reaction that amplifies a circular nucleic acid template
(e.g., single stranded DNA circles) via a rolling circle mechanism. Rolling circle
amplification reaction is initiated by the hybridization of a primer to a circular, often
single-stranded, nucleic acid template. The nucleic acid polymerase then extends the
primer that is hybridized to the circular nucleic acid template by continuously
progressing around the circular nucleic acid template to replicate the sequence of the
nucleic acid template over and over again (rolling circle mechanism). The rolling
circle amplification typically produces concatamers comprising tandem repeat units of
the circular nucleic acid template sequence. The rolling circle amplification may be a
linear RCA (LRCA), exhibiting linear amplification kinetics (e.g., RCA using a single
specific primer), or may be an exponential RCA (ERCA) exhibiting exponential
amplification kinetics. Rolling circle amplification may also be performed using
multiple primers (multiply primed rolling circle amplification or MPRCA) leading to
hyper-branched concatamers. For example, in a double-primed RCA, one primer may
be complementary, as in the linear RCA, to the circular nucleic acid template,
whereas the other may be complementary to the tandem repeat unit nucleic acid
sequences of the RCA product. Consequently, the double-primed RCA may proceed
as a chain reaction with exponential (geometric) amplification kinetics featuring a
ramifying cascade of multiple-hybridization, primer-extension, and stranddisplacement
events involving both the primers. This often generates a discrete set of
concatemeric, double-stranded nucleic acid amplification products. The rolling circle
amplification may be performed in-vitro under isothermal conditions using a suitable
nucleic acid polymerase such as Phi29 DNA polymerase.
[0032] As used herein, multiple displacement amplification (MDA) refers to a
nucleic acid amplification method, wherein the amplification involves the steps of
annealing a primer to a denatured nucleic acid followed by a strand displacement
nucleic acid synthesis. As nucleic acid is synthesized by strand displacement, a
gradually increasing number of priming events occur, forming a network of hyperbranched
nucleic acid structures. MDA is highly useful for whole-genome
amplification for generating high-molecular weight DNA with limited sequence bias
from a small amount of genomic DNA sample. Any strand displacing nucleic acid
polymerase that has a strand displacement activity apart from its nucleic acid
synthesis activity such as a Phi29 DNA polymerase or a large fragment of the Bst
DNA polymerase may be used in MDA. MDA is often performed under isothermal
reaction conditions, using random primers for achieving amplification with limited
sequence bias.
[0033] As used herein, the term "pre-adenylated ligase" refers to a ligase that
is in its adenylated form. The adenylated form of a ligase is capable of intra
molecular ligation of a linear, ssDNA molecule having a 5' phosphoryl group and a 3'
hydroxyl group in the absence of ATP or dATP. A ligation using a pre-adenylated
ligase refers to a ligation reaction wherein a high proportion of the ligase molecules
that are used in the reaction are in their adenylated form. Generally more than 60% of
the ligase molecules may be in their adenylated form. In some embodiments, when a
ligation reaction is performed using a pre-adenylated ligase, more than 70% of the
ligase molecules employed for the reaction may be in their adenylated form. In some
other embodiments, when a ligation reaction is performed using a pre-adenylated
ligase, more than 80%, 90%, or 95% of the ligase molecules employed for the
reaction may be in their adenylated form.
[0034] As used herein the term "adenylating enzyme" refers to an enzyme that
is capable of adenylating a nucleic sequence to generate a 5' adenylated nucleic acid.
The 5'adenylated nucleic acid as used herein refers to a nucleic acid sequence that has
a hydroxyl group at its 3' end and has adenylated terminal nucleotide at its 5' end.
For example, a 5' adenylated DNA (AppDNA), refers to a DNA sequence that is
adenylated at its 5' end and has a hydroxyl group at its 3' end.
[0035] As used herein the term "non-adenylated ligase" refers to a ligase that
is in their non-adenylated form. The non-adenylated form of the ligase is capable of
intra-molecular ligation of a linear, 5'-adenylated ssDNA molecule having a 3'
hydroxyl group in the absence of ATP or dATP. A ligation using a non-adenylated
ligase refers to a ligation reaction wherein a high proportion of the ligase molecules
that are used in the reaction are in their non-adenylated form. Generally more than
60% of the ligase molecules may be in their non-adenylated form. In some
embodiments, when a ligation reaction is performed using a non-adenylated ligase,
more than 70% of the ligase molecules employed for the reaction may be in their unadenylated
form. In some other embodiments, when a ligation reaction is performed
using a non-adenylated ligase, more than 80%, 90% or 95% of the ligase molecules
employed for the reaction may be in their un-adenylated form.
[0036] In some embodiments, a method for generating a single- stranded DNA
circle from a linear DNA is provided. The linear DNA may be a fragmented, linear
DNA. The fragmented DNA may be a circulating DNA, an ancient DNA or a DNA
degraded by environmental exposure, or a formalin-fixed DNA. The length of a
fragmented, linear DNA may range from 15 nucleotides to 21000 nucleotides. The
fragmented, linear DNA may comprise sequences that have non-ligatable terminal
ends. For example, the linear DNA may have either a 5' hydroxyl group or a 3'
phosphoryl group or both. In some embodiments, the method comprises the steps of
providing a linear DNA, end-repairing the linear DNA by incubating it with a
polynucleotide kinase (PNK) in the presence of a phosphate donor to generate a
ligatable DNA sequence having a phosphate group at a 5' terminal end and a hydroxyl
group at a 3' terminal end, and performing an intra-molecular ligation of the ligatable
DNA sequence with a ligase to generate the single-stranded DNA circle. End repair
may include phosphorylation of a 5' terminal nucleotide, de-phosphorylation of a 3'
terminal nucleotide or both to generate a ligatable DNA sequence. The end-repaired,
ligatable DNA, if in double- stranded form, needs to be denatured prior to the intra
molecular ligation reaction. In some embodiments, DNA is denatured prior to PNK
reaction. Phosphorylation or dephosphorylation of single-stranded DNA is generally
more efficient than that of a double- stranded blunt or 5'-recessed ends. The
phosphate donor and its concentration in the reaction mixture are selected such that it
does not inhibit the subsequent intra-molecular ligation reaction. For example, any
suitable phosphate donor other than adenosine triphosphate (ATP) or deoxyadenosine
triphosphate (dATP) may be used for the end-repair reaction using PNK. Suitable
phosphate donors include, but are not limited to, guanosine triphosphate (GTP),
cytidine triphosphate (CTP), uridine triphosphate (UTP) or dexoythymine
triphosphate (dTTP). In some embodiments, a pre-adenylated ligase is used for the
ligation reaction. Any pre-adenylated ligase that is capable of template-independent,
single-stranded DNA sequences may be employed. In some embodiments, a
substantially adenylated form of TS2126 RNA ligase is used for the templateindependent,
intra-molecular ligation reaction. The kinase reaction and the ligation
reaction are performed in the absence of ATP and/or dATP. All the steps of the
method are performed in single reaction vessel without any intervening isolation or
purification steps. The individual steps of the methods may be performed
simultaneously or in sequential manner without any intermediate purification or
isolation steps. For example, PNK along with GTP may be added to a reaction vessel
(e.g., eppendorf tube) containing a nucleic acid solution comprising the linear target
DNA to facilitate the end-repair of the linear target DNA. Any PNK that has a 5'
phosphorylation and a 3' phosphatase activity (e.g., T4 PNK) may be used for the
end-repair reaction. A combination of PNKs each of which has 5' phosphorylation or
a 3' phosphatase may also be used for the end-repair reaction. Once the kinase
reaction is completed, a pre-adenylated ligase may be added to the same reaction
vessel to facilitate the intra-molecular ligation reaction.
[0037] The linear DNA may be a double-stranded or single-stranded DNA of
either natural or synthetic origin. The DNA may be obtained from a biological
sample (e.g., a sample obtained from a biological subject) or discovered from
unknown objects (e.g., DNA obtained during a forensic investigation) in vivo or in
vitro. For example, it may be obtained from, but not limited to, bodily fluid (e.g.,
blood, blood plasma, serum, urine, milk, cerebrospinal fluid, pleural fluid, lymph,
tear, sputum, saliva, stool, lung aspirate, throat or genital swabs), organs, tissues, cell
cultures, cell fractions, sections (e.g., sectional portions of an organ or tissue) or cells
isolated from the biological subject or from a particular region (e.g., a region
containing diseased cells, or circulating tumor cells) of the biological subject. The
biological sample that contains or suspected to contain the target linear DNA (i.e.,
linear DNA of interest) may be of eukaryotic origin, prokaryotic origin, viral origin or
bacteriophage origin. For example, the target linear DNA may be obtained from an
insect, a protozoa, a bird, a fish, a reptile, a mammal (e.g., rat, mouse, cow, dog,
guinea pig, or rabbit), or a primate (e.g., chimpanzee or human). The linear DNA
may be a genomic DNA or a cDNA (complementary DNA). The cDNA may be
generated from an RNA template (e.g., mRNA, ribosomal RNA) using a reverse
transcriptase enzyme. The linear DNA may be a fragmented DNA and may have
non-ligatable terminal nucleotides. For example, linear DNA may comprise a 5'
hydroxyl group and/or a 3'phosphate group such that a DNA ligase cannot perform an
intra-molecular ligation reaction. The linear DNA may be dispersed in solution or
may be immobilized on a solid support, such as in blots, assays, arrays, glass slides,
microtiter plates or ELISA plates. For example, the linear DNA may be immobilized
on a substrate through a primer and then may be circularized and amplified.
[0038] When the linear DNA is in a double-stranded form, it needs be
denatured to a single-stranded form prior to the intra-molecular ligation reaction.
This may be achieved by using any of the art-recognized methods for the conversion
of dsDNA to ssDNA sequences. For example, the dsDNA may be thermally
denatured, chemically denatured, or both thermally and chemically denatured. The
dsDNA may be chemically denatured using a denaturant (e.g., glycerol, ethylene
glycol, formamide, urea or a combination thereof) that reduces the melting
temperature of dsDNA. The denaturant may reduce the melting temperature by 5°C
to 6°C for every 10% (vol./vol.) of the denaturant added to the reaction mixture. The
denaturant or combination of denaturants (e.g., 10% glycerol and 6-7% ethylene
glycol) may comprise 1%, 5%, 10%, 15%, 20%, or 25% of reaction mixture
(vol./vol.). Salts that reduce hybridization stringency may be included in the reaction
buffers at low concentrations to chemically denature the dsDNA at low temperatures.
The dsDNA may be thermally denatured by heating the dsDNA, for example, at 95°C.
[0039] After the denaturing step, the generated ssDNA may be treated with a
DNA or RNA ligase that is capable of intra-molecular ligation of ssDNA substrates in
the absence of a template to form the single-stranded DNA circles. Suitable ligases
that may be used for the ligation reaction include, but are not limited to, TS2126 RNA
ligase, T4 DNA ligase, T3 DNA ligase or E. coli DNA ligase. The conversion of
linear, single-stranded DNA molecules to single- stranded DNA circles is
conventionally performed via a template-dependent intra-molecular ligation reaction
using a ligation enzyme such as T4 RNA ligase. However, template-dependent intra
molecular ligation of single- stranded DNA or single-stranded RNA has met only with
limited success, particularly when the circularization of ssDNA molecules is to be
performed in a population of ssDNA molecules of unknown sequence and/or size.
Even though bacteriophage T4 RNA ligase I exhibits a template-independent intra
molecular ligation activity, this activity is far too low and inefficient for practical use
in generating circular ssDNA molecules from linear ssDNA molecules.
[0040] In some embodiments, conversion of the ssDNA to single- stranded
DNA circle is performed with a thermostable RNA ligase that has good templateindependent,
intra-molecular ligation activity for linear ssDNA and/or ssRNA
substrates that have 5' phosphoryl and 3' hydroxyl groups. The ligase may be in a
substantially pre-adenylated form. For example, TS2126 RNA ligase derived from
the Thermus bacteriophage TS2126 that infects the thermophilic bacterium, Thermus
scotoductus may be employed for template-independent circularization of the
fragmented linear ssDNA to circular ssDNA. TS2126 RNA ligase is more
thermostable (stable up to about 75 °C.) than many of the mesophilic RNA ligases
such as the T4 RNA ligase. The range of temperature for TS2126 RNA ligase activity
can be greater than about 40 °C, for example, from about 50 °C to about 75 °C. Due
to this, TS2126 RNA ligase may be used at higher temperatures, which further reduce
undesirable secondary structures of ssDNA. The circularization of linear ssDNA may
also be achieved by a ligase other than TS2126 RNA ligase or by employing any
other enzyme having DNA joining activity such as topoisomerase. In some
embodiments, the circularization of fragmented, single stranded DNA molecule is
achieved by an RNA ligase 1 derived from thermophilic archeabacteria,
Methanobacterium thermoautotrophicum (Mth RNA ligase) that has high templateindependent
ligase activity in circularizing linear, fragmented ssDNA molecules.
[0041] In some embodiments, a method for improving the efficiency of
circularization of ssDNA by TS2126 RNA ligase is provided. Use of HEPES buffer
having a pH of 8.0 for the ligation reaction increased the ligation efficiency.
Template-independent ssDNA ligation was inefficient when the reaction was
performed in TRIS buffer (e.g., For CircLigase™ II, the suggested lOx reaction buffer
by EpiCenter comprises 0.33 M TRIS-Acetate (pH 7.5), 0.66 M potassium acetate,
and 5mM DTT). Further, manganese, an essential co-factor for the ligation reaction,
is rapidly oxidized under alkaline conditions and forms a precipitate in the presence of
TRIS. Air oxidation of Mn + to Mn + may be facilitated by the anions that can
strongly complex the Mn + ions. For example, when equal volumes of 0.2 mol/liter
Tris with pH appropriately adjusted with HC1 and 2 mmol/liter MnCl 2 were mixed,
the color change was immediate at pH 9.3 (the pH of TRIS base alone); had an initial
time lag of about 3 minutes at pH 8.5; and was not detectable in 1 hour at pH values
below 8.3. Although the reaction did not occur at lower pH, the changes observed at
higher pH were not reversed by adding acid. Due to rapid oxidation of manganese in
TRIS buffer, a higher concentration of manganese is essential for the ligation reaction
(e.g., addition of MnCl2 to a final concentration of 2.5 mM) when the intra-molecular
ligation is performed in TRIS buffer. Further, it becomes difficult to accurately
predict the working concentration of manganese in the reaction as the manganese
concentration continues to decrease over time. Higher concentrations of manganese
may lead to higher error-rate of the polymerase during amplification when the ligation
and amplification is performed in a single reaction vessel. By substituting TRIS
buffer with HEPES buffer in the ligation reaction, effective intra-molecular ligation
may be achieved with manganese ion concentration less than 0.5 mM. Apart from
HEPES, any of other the Good's buffers (see, for example, Good, Norman et al.
Biochemistry, 5 (2): 467-477, 1966; and Good, Norman et al., Methods Enzymol.,
24: 53-68, 1972.) may be employed for the intra-molecular ligation reaction.
[0042] The ssDNA circles in the ligation reaction mixture may be amplified
under isothermal conditions via rolling circle amplification (RCA) methods. The
amplification reagents including DNA polymerase, primers and dNTPs may be added
to the same reaction vessel to produce an amplification reaction mixture and to initiate
an RCA reaction. The amplification reaction mixture may further include reagents
such as single-stranded DNA binding proteins and/or suitable amplification reaction
buffers. The amplification of ssDNA circles is performed in the same reaction vessel
in which ligation is performed. Isolation or purification of the ssDNA circles and/or
removal of the ligase is not necessary prior to the amplification reaction. The
amplified DNA may be detected by any of the currently known methods for DNA
detection.
[0043] RCA may be performed by using any of the DNA polymerases that are
known in the art such as a Phi29 DNA polymerase. It may be performed using a
random primer mixture or by using a specific primer. In some embodiments, random
primers are used for the RCA reaction. Primer sequences comprising one or more
nucleotide analogues (e.g., LNA nucleotides, 2-Amino-A, or 2-Thio T modification)
may also be used. In some embodiments, nuclease-resistant primers (e.g., primer
sequences comprising phosphorothioate groups at appropriate positions) are employed
for the amplification reactions (e.g., NNNN*N*N). In some embodiments, RCA may
be performed by contacting the ssDNA circles with a primer solution comprising a
random primer mixture to form a nucleic acid template-primer complex; contacting
the nucleic acid template-primer complex with a DNA polymerase and
deoxyribonucleoside triphosphates; and amplifying the nucleic acid template. In
some embodiments, the primer solution comprises a partially constrained primer such
as WWNNS. The partially constrained primer may have a terminally mismatched
primer-dimer structure. In some embodiments, a partially constrained primer that
consists of a nucleotide sequence (W) (N)y(S) , wherein x, y and z are integer values
independent of each other, and wherein value of x is 2 or 3, value of y is 2, 3 or 4, and
value of z is 1 or 2 are used for the RCA reaction. The partially constrained primer
may comprise one or more nucleotide analogues. In some embodiments, a nucleaseresistant,
partially constrained primer comprising a modified nucleotide, and having
terminal mismatch primer-dimer structure is employed for RCA reaction. Suitable
primer sequences include, but are not limited to, +W+WNNS, W+W+NNS,
+W+WNNNS, W+W+NNNS, W+W+NN*S, +W+WNN*S, W+W+NNN*S,
+W+WNNN*S, W+W+N*N*S, +W+WN*N*S, W+W+NN*N*S, or
+W+WNN*N*S. In some embodiments, RCA reaction is performed by contacting
the ssDNA circle with a primer solution that consists essentially of a partially
constrained primer mixture comprising a terminal mismatch primer-dimer structure
and amplifying the ssDNA circle. In some other embodiments, RCA reaction is
performed by contacting the ssDNA circle with a primer solution that consists
essentially of a partially constrained primer mixture comprising a nucleotide analogue
and amplifying the ssDNA circle. RCA of ssDNA circles produces large quantities of
DNA with reduced sequence dropout and reduced amplification bias. The entire
process of ssDNA ligation and amplification may be performed in a single tube
without any intermediate purification or isolation steps.
[0044] In some embodiments, methods for amplification of limiting quantities
of linear fragmented DNA via multiple displacement amplification (MDA) are
provided. Conventional methods of MDA, when attempted on a linear fragmented
DNA, result in decreased amplification speed and highly sequence-biased
amplification. Moreover, significant sequence dropout is often observed particularly
near the ends of the fragmented DNA. To overcome these limitations, the fragmented
dsDNA is first converted to ssDNA. The ssDNA is then converted to single-stranded,
circular DNA (i.e., DNA circle) via a template-independent intra-molecular ligation
reaction, thereby eliminating the problematic DNA ends. Even ssDNA sequences that
are shorter than 500 bp may be circularized using template-independent intra
molecular ligation of ssDNA. Further, no prior knowledge of the target sequence is
needed to create DNA circles when the ligation of the ssDNA is performed in a
template-independent manner. Prior to circularization, fragmented DNA may be
treated with a PNK to repair the non-ligatable terminal ends. After circularization of
the fragmented ssDNA, MDA is performed on the circularized DNA. The
amplification reaction may be performed under isothermal conditions via employing
rolling circle amplification (RCA) methods. RCA may be performed using
commercially available RCA amplification kits such as TempliPhi™ RCA kit (GE
Healthcare). The TempliPhi™ rolling-circle amplification employs locked nucleic
acid-containing random primers, which provide higher sensitivity and amplification
balance. In some embodiments, nuclease-resistant primers are used for RCA reaction.
The methods disclosed herein improve amplification sensitivity, reduce sequence
dropout and allow more balanced amplification. Since template-independent
circularization of single-stranded fragmented DNA may be achieved on shorter
sequences even at lower concentrations, a more balanced DNA amplification with
faster kinetics and improved sequence coverage may be achieved when ligase-assisted
whole-genome amplification is employed for amplification of highly fragmented
DNA (e.g. circulating DNA in blood plasma). For example, the persistence length of
ssDNA may be as low as 15 nucleotides for template-independent circularization of
ssDNA. When CircLigase™ is employed for ligation reaction, under standard
conditions, virtually no linear concatamers or circular concatmers are produced.
Further, both the circularization and amplification reactions may be performed in a
single reaction vessel without any intermediated purification or isolation steps thereby
reducing the chances of contamination and simplifying the amplification workflow.
Ligase-assisted whole-genome amplification methods may be employed for, but not
limited to, analyzing circulating plasma DNA, fragmented DNA isolated from
formalin fixed paraffin-embedded (FFPE) samples, forensics DNA samples that have
been exposed to environmental conditions or ancient DNA samples. The amplified
library may further be used for targeted detection of amplified sequences via qPCR or
sequencing.
[0045] Various ligation-assisted whole-genome amplification methods
described herein that comprise prior ligation of ssDNA fragments to DNA circles
followed by rolling circle amplification, provide preferential amplification of a
fragmented DNA over a high molecular weight genomic DNA. For example, plasma
preparations comprising circulating DNA may often be contaminated with genomic
DNA that are released from blood cells during the purification process. Conventional
methods of whole-genome amplification via MDA amplify both the circulating DNA
and the genomic DNA. In contrast, when fragmented, circulating DNA molecules are
first circularized with TS2126 RNA followed by amplification of the circularized
DNA molecules via RCA employing a Phi29 DNA polymerase the circulating DNA
was preferentially amplified over the high molecular weight genomic DNA. Such
preferential amplification of fragmented DNA over the genomic DNA is particularly
suitable for diagnostic applications since diagnostically relevant DNA may be
preferentially amplified for downstream analysis (see, Example 4). Further, ligaseassisted
whole-genome amplification allows more robust amplification of fragmented
DNA when compared to conventional MDA-based whole-genome amplification.
[0046] FIG. 1 depicts a schematic representation of an embodiment of ligaseassisted
whole-genome amplification of a fragmented dsDNA. The persistence length
of double-stranded DNA is much higher (-150 bp) and its innate stiffness makes
circularization of fragments less than 500 bp highly inefficient. Further, with small
double-stranded fragmented DNA molecules of about 250 bp range, circularization is
inefficient unless the ends are in proper alignment (-10.5 bp/turn). In contrast, the
persistence length of the circularization of single-stranded fragmented DNA is very
small, approximately 15 nucleotides, when compared to the double-stranded
fragmented DNA. As depicted in FIG. 1, in ligase-assisted whole-genome
amplification, fragmented dsDNA is first converted into single-stranded DNA circles.
This may be achieved by incubating the fragmented double-stranded DNA at 95 °C
for a sufficient period to denature the dsDNA into single strands. The fragmented
ssDNA is then treated with a DNA or RNA ligase that is capable of templateindependent,
intra-molecular ligation of single-stranded DNA substrates to generate
the single-stranded DNA circles. Non-limiting examples of ligases that may be used
for intra-molecular ligation includes, CircLigase™, T3 DNA ligase, T4 RNA ligase,
Mth RNA ligase (MthRnll), or E. coli ligase. Amplification reagents, including DNA
polymerase, random primers, and dNTPs are then added to initiate a RCA reaction on
the single-stranded DNA circles. This ligase-assisted whole-genome amplification
employing RCA produces large quantities of DNA with reduced sequence dropout
and amplification bias in contrast to the conventional whole-genome amplification
methods. Therefore, it may be used to amplify and detect even highly fragmented
DNA. The entire process of generation of the single-stranded DNA circles and its
subsequent amplification by RCA is done in a single tube without any intervening
purification steps.
[0047] In some embodiments, a single-tube workflow is provided for ligaseassisted
whole-genome amplification of fragmented DNA that includes processing of
a fragmented DNA to repair the non-ligatable DNA ends. For example, if a
fragmented single- stranded DNA does not contain a 5' phosphoryl group and a 3'
hydroxyl group, it may not get ligated in an intra-molecular ligation reaction.
Presence of such non-ligatable DNA sequences may cause an amplification bias in the
ligase-assisted whole-genome amplification. For example, as schematically
represented FIG. 8., DNA fragments that are generated by DNAse II digestion during
cell death may contain a 5' hydroxyl group, a 3' phosphoryl group. The singlestranded
DNA fragments originating from such double-stranded DNA fragments that
contain a 5' hydroxyl group, a 3' phosphoryl group will not get circularized in an
intra-molecular ligation reaction. Thus DNAse II type breaks are likely to be underrepresented
in whole-genome amplification. In some embodiments, the fragmented
DNA is treated with a kinase (e.g., a T4 Polynucleotide Kinase, TPK) to
phosphorylate the 5' hydroxyl groups and/or dephosphorylate the 3' phosphoryl group
of the fragmented DNA. Inclusion of kinase in the reaction allows efficient
circularization of fragments in a pool that do not contain a 5' phosphate.
Phosphorylating the 5' ends of the fragmented DNA with a kinase followed by
amplification of the fragmented DNA creates a more representative library.
[0048] In some embodiments, phosphorylation repair of the fragmented
dsDNA may be performed by using a T4 PNK kinase. The phosphorylation repair
may either be performed on the fragmented dsDNA or on the denatured fragmented
ssDNA. If the phosphorylation repair is performed on the dsDNA, repaired dsDNA
may then be denatured to linear ssDNA, which may be subsequently circularized
using a CircLigase II™ (abbreviated as CLII). CircLigase II comprises a
substantially adenylated form of TS2126 RNA ligase. Template-independent intra
molecular ligation of ssDNA by CircLigase II™ is inhibited by higher concentrations
of ATP or dATP. However, the phosphorylation repair by kinase often requires the
presence of ATP. Further, it may not be easy to remove ATP from the reaction
mixture without damaging the DNA. For example, a phosphatase treatment of the
reaction mixture to remove ATP will also result dephosphorylation of DNA (unless
the DNA is protected, for example, by pre-adenylation), thus making the DNA strands
un-ligatable. As a result, performing a phosphorylation repair of the fragmented DNA
and generation of ssDNA circles in a single tube without any intervening purification
or isolation steps is often difficult. The methods provided herein employ GTP, CTP,
UTP or dTTP instead of ATP during the kinase reaction. Since CircLigase II™ is
more tolerant to GTP or an alternate phosphate donor (e.g., CTP or UTP), the kinase
repair step and the ligation step may be conducted in a single reaction vessel without
any intervening purification and/or isolation steps. The kinase reaction mixture may
further comprise additional reagents such as manganese salts and betaine (zwitterionic
trimethylglycine). Once ligated, the ssDNA circles may be amplified. By conducting
the ligation and amplification reaction at a relatively low concentration of GTP, the
single-tube workflow described herein avoids the intermittent clean-up steps between
enzymatic treatments and minimizes the DNA template loss (see FIG. 9 for a
schematic representation a single-tube workflow involving kinase repair, ligation and
amplification).
[0049] In some embodiments, an alternative method for generating a singlestranded
DNA circle from a linear DNA is provided, wherein the method employs a
DNA pre-adenylation step prior to intra-molecular ligation step. First, the linear DNA
may be incubated with a polynucleotide kinase in the presence of ATP to generate a
ligatable DNA sequence that comprises a phosphate group at 5' terminal end and a
hydroxyl group at 3' terminal end. The ligatable DNA sequence is then incubated
with an adenylating enzyme in presence of adenosine triphosphate to generate a 5'
adenylated DNA sequence. The 5' adenylated DNA sequence has a free 3' hydroxyl
group. The concentration of ATP is in the ligation reaction is selected such that no
adenylation happens at the 3' end of the ligatable DNA sequence. The 5' adenylated
DNA sequence is then incubated with a non-adenylated ligase, which is capable of
template-independent intra-molecular ligation of the 5' adenylated DNA sequence, to
generate the single-stranded DNA circle. If an ATP-dependent non-adenylated ligase
is employed for the intra-molecular ligation reaction, the ATP may have to be
removed from the reaction mixture by treating the reaction mixture with a
phosphatase prior to the intra-molecular ligation reaction. The 5' phosphate at the
terminal nucleotide of the DNA, which would normally be removed by a phosphatase,
is protected from the phosphatase treatment because of the pre-adenylation. If the
DNA is in double-stranded form, it needs to be denatured prior to intra-molecular
ligation reaction. All the steps of the method are performed in single reaction vessel
without any intervening isolation or purification steps.
[0050] In some embodiments, an RNA ligase such as RNA ligase I derived
from thermophilic archeabacteria, Methanobacterium thermoautotrophicum (Mth
RNA ligase 1) is used in the presence of ATP to generate the adenylated form of the
linear DNA. A mutant or suitably engineered ATP-independent ligase that is
defective in self-adenylation, de-adenylation and/or adenylate transfer may be used
for the intra-molecular ligation reaction of the adenylated linear DNA to generate the
single-stranded DNA circle. For example, a motif V lysine mutant (K246A) of Mth
RNA ligase may be employed. This mutant has full ligation activity with preadenylated
substrates. Mth RNA ligase mutant that has an alanine substitution for the
catalytic lysine in motif I (K97A) may also be employed. The activity of the K97A
mutant is similar with either pre-adenylated RNA or single-stranded DNA (ssDNA)
as donor substrates but has a two-fold preference for RNA as an acceptor substrate
compared to ssDNA with an identical sequence. If ATP-dependent ligases such as
TS2126 RNA ligase are employed for intra-molecular ligation reaction of the 5'
adenylated DNA sequences, the ATP in the reaction may have to be removed prior to
the ligation reaction.
[0051] In some embodiments, ligase-assisted whole-genome amplification
employing the alternative workflow is provided. A schematic representation of this
workflow is provided in FIG. 11. The method comprises the repair of fragmented
DNA with a kinase and pre-adenylating the fragmented DNA at the 5' end with an
RNA ligase or DNA ligase in presence of ATP prior to ligation and amplification.
Fragmented DNA comprising sequences that have non-ligatable ends (e.g., sequences
comprising 5' hydroxyl and/or 3' phosphoryl groups) are phosphorylated at 5' ends
and de-phosphorylated at 3' ends by treating with a kinase to generate a ligatable
DNA sequence. The ligatable DNA sequence may then adenylated using an RNA
ligase such as Mth RNA ligase (MthRnl 1), in the presence of ATP to generate an
adenylated form of the fragmented DNA. The ATP is subsequently removed from the
reaction mixture by treating the reaction mixture with a phosphatase (e.g., shrimp
alkaline phosphatase (SAP)). Any method that is available in the art for 5'
adenylation of a DNA may be employed (e.g., RNA ligase, DNA ligase or synthetic
methods). The pre-adenylated single-stranded linear DNA is then treated with an
RNA ligase that has a low degree of adenylation such as CircLigase I™ to generate
DNA circles via intra-molecular ligation. The DNA circles are then amplified using
RCA. In embodiments where CircLigase I™ to generated DNA circles via intra
molecular ligation, the intra-molecular DNA ligation and subsequent amplification
reaction are performed in the absence of ATP. Elimination of ATP from the reaction
mixture after kinase treatment and pre-adenylation reaction is essential since
circularization of pre-adenylated ssDNA by CircLigase I is inhibited by ATP. In
some embodiments, ATP is converted to adenosine and phosphate by treatment with a
phosphatase. Even though adenosine is not inhibitory to the circularization reaction,
the resultant phosphate may inhibit the intra-molecular ligation reaction. The
generated phosphate may be further removed by treating the reaction mixture with
phosphate-sequestering enzymes or with reagents that precipitate or remove
phosphate (e.g., phosphate binding resin such as LayneRT resin) from the solution.
Phosphate removal may also be achieved by treating the reaction mixture with an
enzyme such as maltose phosphorylase which catalyzes conversion of maltose to
glucose and glucose- 1-phosphate, thereby removing the phosphate from the solution.
Inclusion of kinase in the reaction allows circularization and amplification of DNA
fragments in a pool that does not contain a 5' phosphate and/or 3' hydroxyl groups,
thereby creating a more representative library via ligase-assisted amplification. Preadenylation
of target DNA facilitates the use of ligases having low degree of
adenylation (e.g., CircLigase I™, which is about 30% adenylated) for intra-molecular
ligation reaction. This may be of interest since ligases having high degree of
adenylation (e.g., CircLigase II™) ligate un-adenylated DNA only a single time.
Thus, a stoichiometric amount of ligase is often required to drive an intra-molecular
ligation reaction to completion. In contrast, ligases that have a low degree of
adenylation (such as CircLigase I™) have high turn-over, and can reversibly and
catalytically or repeatedly act on multiple pre-adenylated DNA molecules. This
increases ligation kinetics, reduces the quantity of ligase required, and potentially
allows for increased circularization of more difficult or complex DNA templates.
[0052] In some embodiments, methods for ligase-assisted, whole-genome
amplification is used for amplification and subsequent detection of circulating nucleic
acids (e.g., circulating DNA from the non-cellular fraction of a biological sample) in a
biological sample such as whole blood or urine. Circulating nucleic acids may
originate from apoptotic or necrotic cells, or may be actively released from cells.
Since cellular nucleases break down the high molecular weight genomic DNA into
small, nucleosome-sized fragments, circulating nucleic acids are naturally highly
fragmented. Highly fragmented circulating nucleic acid is often not amenable for
conventional nucleic acid amplification methods. Further, circulating nucleic acids
are present in very low quantities in the bloodstream. Standard rolling circle
amplification (RCA) of double-stranded circulating linear nucleic acids is inefficient
and highly biased. Separating the circulating nucleic acids to single-strands and
circularizing with a ligase prior to rolling circle amplification improves efficiency and
leads to less bias. To enable good RCA kinetics and high sensitivity with such dilute
DNA template, RCA methods employing primers comprising LNAs are employed.
This improved RCA has been optimized for trace DNA and single-cell amplification.
[0053] In some embodiments, a method of amplifying circulating DNA from
the whole blood is provided. Circulating DNA is amplified from the non-cellular
fraction of the whole blood (e.g., plasma or serum). This method comprises the steps
of collecting the non-cellular fraction of the whole blood, collecting the circulating
DNA (mostly presented in its native double-stranded form) from the non-cellular
fraction, denaturing the double-stranded DNA to generate linear single-stranded
DNA, circularizing the circulating single-stranded DNA molecule to generated singlestranded
DNA circles, and amplifying the single-stranded DNA circles via rolling
circle amplification. Due to persistence length, it is not generally possible to
circularize dsDNA that has a sequence length smaller than 150 bp, and it is very
difficult to circularize dsDNA until the DNA is longer than 200 bp. In contrast, linear
ssDNA molecules having a sequence length of 15 nucleotides (nt) or more are very
efficiently circularized by a suitable ligase as long as the 5' end is phosphorylated and
the 3' end is hydroxylated. The circularization of the single-stranded DNA to
generate single-stranded DNA circle is achieved by employing a ligase that is capable
of template-independent intra-molecular ligation of single-stranded DNA. In some
embodiments, the circularization of the single-stranded DNA molecules is performed
by treating the single-stranded linear DNA with an RNA ligase such as CircLigase
II™.
[0054] In some embodiments, sensitivity of circulating DNA detection is
further increased by phosphorylating the circulating nucleic acids with polynucleotide
kinase (PNK) prior to the ssDNA ligation step and RCA. Upon incorporating the
PNK step in the work flow, ligase-assisted whole-genome amplification methods
presented herein could detect male circulating DNA in female whole blood when
spiked at 1% levels (triplicate repeats). Template-independent intra-molecular
ligation cannot be achieved unless the ssDNA template has a 5' phosphate group and
a 3' hydroxyl group. A variety of conditions produce 5' hydroxyls in DNA (including
DNase II enzymatic cleavage, and phosphatase activity in blood). The PNK treatment
eliminates this problem and improves the diversity of rolling-circle amplified CNA
library.
[0055] In some embodiments, kits for generation of a single-stranded DNA
circle from a linear DNA are provided. In one embodiment, the kit comprises a
polynucleotide kinase, a phosphate donor and a pre-adenylated ligase that is capable
of template-independent, intra-molecular ligation of ssDNA sequence, packaged
together. The polynucleotide kinase may be a T4 PNK. The phosphate donor may be
chosen from OTP, UTP, CTP or dTTP. In one embodiment, the kit may include a
TS2126 ligase. More than 60% of the TS2126 ligase may be pre-adenylated. The kit
may further comprise buffers (e.g., HEPES), DNA amplification regents (e.g., DNA
polymerase, primers, dNTPs) and other reagents (e.g., MnCl2, betaine) that are
employed for the generation of single-stranded DNA circle by the provided methods.
In some embodiments, the kit may include a Phi29 DNA polymerase and
random/partially constrained primers. In another embodiment, the kit comprises an
adenylating enzyme, a phosphatase and a non-adenylated ligase packaged together.
The kit may further comprise a polynucleotide kinase and/or a phosphate donor. The
adenylating enzyme may be an RNA ligase I derived from Methanobacterium
thermoautotropicum (Mth RNA ligase). The non-adenylated ligase may be a
composition of TS2126 ligase, wherein more than 60% of the ligase is in the nonadenylated
form. The kits may further include instruction for generation of singlestranded
DNA circle from a linear DNA.
[0056] Practice of the invention will be still more fully understood from the
following examples, which are presented herein for illustration only and should not be
construed as limiting the scope of the present invention as defined by the appended
claims. Some abbreviations used in the examples section are expanded as follows:
"mg": milligrams; "ng": nanograms; "pg": picograms; "fg": femtograms; "mL":
milliliters; "mg/mL": milligrams per milliliter; "mM": millimolar; "mmol":
millimoles; "pM": picomolar; "pmol": picomoles; "mI7': microliters; "min.": minutes
and "h.": hours.
[0057] EXAMPLES:
[0058] Example 1: Whole-genome amplification of circulating nucleic acid
from blood plasma:
[0059] Circulating DNA was isolated from citrate-phosphate-dextrose (CPD) -
stabilized blood plasma of apparently healthy individuals using the Wako DNA
extractor SP kit (Wako Pure Chemical Industries). Approximately 1.3 ng was
analyzed by electrophoresis through a 2% agarose gel using TBE buffer, stained with
SYBR Gold and visualized using a Typhoon imager. As depicted in FIG. 2, the
majority of the circulating DNA was approximately 180 bp in length, with an
additional smaller amount of sequences that were approximately 370 bp long, and a
substantially smaller amount of higher molecular weight sequences.
[0060] 350 pg circulating DNA from plasma was heated at 95 °C to denature
the template. The denatured, single-stranded DNA template was then treated with an
RNA or DNA ligase to generated single-stranded DNA circles. ATP-dependent T4
DNA ligase, cell-encoded NAD-dependent E. coli DNA ligase or a thermostable
RNA ligase (CircLigase II) was used for the ligation reaction. 100 pg of DNA ligated
single-stranded DNA circles were then subjected to whole-genome amplification
using GenomiPhi kit (GE Healthcare) employing a Phi29 DNA polymerase. The
amplification was performed using the primer mixture +N+N(at N)(at N)(at N)*N
where the "at N" represents a random mixture containing 2-amino dA, 2-thio-dT,
normal G and normal C. Real-time amplification was performed by adding a small
amount of SYBR green I to the amplification mixture and monitoring the fluorescence
signal increase over time in a Tecan plate reader (Tecan SNiPer, Amersham-
Pharmacia Biotech). For comparison, an equivalent concentration of un-treated
genomic DNA, untreated plasma DNA, and a sample without DNA template (No
template amplification) were included.
[0061] As depicted in FIG. 3, the amplification kinetics of the untreated,
fragmented plasma DNA were much lower when compared to an equivalent amount
of high molecular weight genomic DNA, indicating a defect in amplification.
However, when the fragmented plasma DNA was pre-treated and converted to singlestranded
DNA circles using the CircLigase II™, rapid amplification kinetics were
achieved (FIG. 3A). The ligases, including the ATP-dependent T4 DNA ligase (FIG.
3B) and the cell-encoded NAD-dependent E. coli DNA ligase (FIG. 3C) were also
effective, but with less efficiency, in restoring amplification kinetics of the
fragmented plasma DNA. In these examples, the relative increase in amplification
kinetics indicates the effectiveness of each of the ligases in promoting the intra
molecular ligation of the single-stranded DNA template.
[0062] Example 2 : Analysis of amplified circulating nucleic acids from blood
plasma by ligase- assisted whole-genome amplification.
[0063] The amplified DNA generated in Example 1 was further analyzed by
quantitative PCR using primers targeting four different CODIS loci (vWA, TPOX,
D8S1129, and D13S317) to sample the effectiveness of the ligase-assisted wholegenome
amplification method for promoting sensitive and balanced DNA
amplification. These DNA levels were compared with the values from unamplified
DNA to determine the relative representation levels after amplification. As illustrated
in FIG. 4, in both examples, the amplification of untreated plasma DNA led to
sequence dropout or produced DNA that was highly under-represented at the tested
loci. In contrast, including either CircLigase II™ or T4 DNA ligase in the method
prevented the sequence dropout of the four loci and produced DNA that was more
similar in representation to the amplified high molecular weight genomic DNA. In
the example using CircLigase II™ as the single-stranded DNA ligase, out of 12
different CODIS loci tested by quantitative PCR (qPCR) using primers targeting 12
different CODIS loci, 11 were recovered after amplification, whereas only 4 were
present in the amplified untreated plasma DNA (FIG. 5). In FIG. 5, the Ct values
reported are an average of two replicates. PCR reactions where the Ct value was
undetermined are marked by an "X".
[0064] Example 3 : Optimization of reaction conditions for ligase-assisted
whole-genome amplification.
[0065] The ligase-assisted DNA amplification reaction was further optimized
by optimizing the efficiency of ligation reaction of single stranded DNA molecule by
TS2126 RNA ligase. The presence of metal ion was essential for the ligation reaction
since eliminating manganese from the standard manufacturer recommended buffer
reduced amplification rates to background levels. Untreated genomic DNA and
untreated plasma DNA were compared with CircLigase II™ -treated plasma DNA
samples using modified buffer conditions (FIG. 6). All buffer conditions contained
33 mM KOAc, 0.5 mM DTT, and 1M betaine. Where indicated, buffers contained 33
mM Tris-acetate (pH 7.5) or 33 mM HEPES-KOH (pH 8.0) and additionally
contained 2.5 mM MgCl2 or 2.5 mM MnCl2. Real-time amplification was performed
by adding a small amount of SYBR green I to the amplification mixture and
monitoring fluorescence increase over time in a Tecan plate reader. The amplification
threshold is the time at which fluorescence rises above background levels (2000
RFU).
[0066] Comparison of amplification kinetics of ligase-assisted whole-genome
amplification reactions (100 pg samples) is depicted in FIG. 6. Both magnesium and
manganese promoted similar effects in the presence of the standard TRIS buffer, but it
was observed that the combination of manganese and magnesium in the presence of
HEPES buffer, pH 8.0 was most effective in promoting high amplification rates.
HEPES buffer increased circularization efficiency of the plasma DNA in this reaction
condition may be due reduced oxidation of the manganese cation in the HEPES
buffer.
[0067] Example 4 : Inhibition of amplification of high molecular weight
genomic DNA in ligase-assisted whole-genome amplification.
[0068] The amplification kinetics of whole-genome amplification reactions of
untreated genomic DNA was compared with CircLigase I™ and CircLigase II -
treated genomic DNA samples (100 pg samples). The results are illustrated in FIG. 7.
As depicted in FIG. 7, CircLigase treatment of genomic DNA produced an inhibitory
effect on the amplification rate of high molecular weight genomic DNA (unlike the
positive effects on plasma DNA). The inhibition was apparent for both CircLigase I
and CircLigase II™.
[0069] To investigate if Phi29-based amplification was inhibited by the ligase,
untreated genomic DNA was amplified in the presence of active ligase. Real-time
amplification was performed by adding a small amount of SYBR green I to the
amplification mixture and monitoring fluorescence increase over time in a Tecan plate
reader. Amplification threshold is the time at which fluorescence rises above
background levels (2000 RFU). It was observed that the genomic DNA amplification
inhibition was not an effect of active ligase being present during the amplification.
[0070] A preference for the amplification of circulating over high molecular
weight genomic DNA might be an advantage for certain applications, as genomic
DNA from blood cells often contaminates preparations of circulating nucleic acids,
and is of less diagnostic value.
[0071] Example 5 : Single-tube amplification of fragmented DNA employing
ligase-assisted whole-genome amplification - Effect of phosphorylation of circulating
DNA fragments with kinase prior to intra-molecular ligation.
[0072] Phosphorylation of circulating DNA fragments with kinase allowed
more sensitive detection of circulating DNA in blood plasma. A male-female
plasma/blood mixing experiment was performed to establish that the library created
from the input DNA treated with kinase was more representative, allowing for more
sensitive detection of the DYS14 male-specific marker (FIG. 10, 3/3 replicates,
whereas only 1/3 was detected if phosphorylation was not done). 100 m of
blood/plasma mixtures were prepared as follows: 100A: 100% male plasma; 5A-C:
male plasma spiked into female whole blood at 5% v/v; 1A-C: male plasma spiked
into female whole blood at 1% v/v; and OA: 100% female blood. The plasma was
separated from the blood cells by lateral flow through an MF1 membrane (Whatman)
followed by collection onto a cellulose pad that was dried and stored overnight. The
circulating DNA was then isolated from the cellulose pad by a modification of the
Wako extractor SP kit (Wako Pure Chemical Industries), a standard sodium
iodide/detergent based method. Approximately 1.8 ng of DNA was then treated with
or without T4 polynucleotide kinase in the presence of OTP, manganese, and betaine
and then treated with CircLigase II™ to circularize the single-stranded DNA
fragments. DNA was then subjected to GenomiPhi whole-genome amplification (GE
Healthcare) and products were analyzed by quantitative PCR to assess the detection of
two markers: Dysl4, which is a multi-copy gene located on the Y-chromosome and
should be detectible from the male fraction only, and D16S539 which is an STR locus
located on chromosome 16 and should be detectible from both male and female
fractions. The reaction was performed in a single reaction vessel, without any
intermediate purification or isolation steps in the workflow. This was achieved by
performing the phosphorylation reaction at a relatively low concentration of GTP.
[0073] FIG. 10 illustrates that inclusion of a kinase in the reaction allows the
circularization and amplification of DNA fragments in a pool that do not contain a 5'
phosphate, thereby creating a more representative library. This would include DNA
fragments containing a 5' hydroxyl, which are specifically generated by DNase II
digestion during cell death. Using a male-female plasma/blood mixing experiment, it
is demonstrated that the library created from the input DNA treated with kinase was
more representative, allowing for more sensitive detection of the DYS14 malespecific
marker (3/3 replicates, whereas only 1/3 was detected if phosphorylation was
not done).
[0074] Example 6 : Effect of pre-adenylation of fragmented DNA prior to
circularization reaction.
[0075] The efficiency of circularization of a small DNA fragment that is either
phosphorylated or pre-adenylated in 40 minutes is assessed with different amounts of
CircLigase enzyme. 2.5 pmol of a 64-mer oligonucleotide containing either a
phosphate group or an adenylation at the 5' position was treated with increasing
amounts of CircLigase I™ or CircLigase II™ for 40 minutes at 60°C. The percent
circularization was determined by scanning the intensity of the bands at the linear and
circular positions. As depicted in FIG. 12, pre-adenlyation of fragmented DNA
improved the ligation and amplification kinetics. In FIG. 12, P-64mer represents a 5'-
phosphorylated 64-nt oligonucleotide; and ad-64 represents pre-adenylated 64-nt
oligonucleotide. Pre-adenylated DNA was circularized more rapidly than the
standard phosphorylated DNA. Further, the ligation enzyme, which has low degree of
adenylation catalyzed the ligation of a molar excess of substrate indicating that the
ligase has multiple opportunities to ligate the pre-adenylated DNA molecule, which
increases ligation kinetics and potentially allows for increased circularization of more
difficult templates.
[0076] Example 7 : Circularization of 5'-phosphate and 5'-hydroxyl-containing
oligonucleotides using the pre-adenylation workflow.
[0077] Reactions containing 5 pmol of a 64-mer oligonucleotide with either a
phosphate group or a hydroxyl group at the 5' position were treated with 1.25 U of T4
polynucleotide kinase at 37 °C where indicated. Following incubation with 25 pmol
Mth RNA ligase at 65°C, reactions were treated with 0.25 units of shrimp alkaline
phosphatase. Since Mth RNA ligase is very sensitive to ATP concentration, at
standard 100 mM ATP concentration, Mth RNA ligase almost exclusively adenylate
DNA ends. No intra-molecular ligation happens by Mth RNA ligase at this ATP
concentration. Enzymes were heat-inactivated after each incubation. Finally,
reactions were treated with 50 units of CircLigase I where indicated and incubated for
60 minutes at 60°C. The percent circularization was determined by scanning the
intensity of the bands at the linear and circular positions (FIG. 13). P-64mer
represents a 5'-phosphorylated 64-nt oligonucleotide and ad-64mer represents a preadenylated
64-nt oligonucleotide.
[0078] Figure 11 shows a "single-tube" pre-adenylation workflow in which
linear oligonucleotides containing a 5'-phosphate or a 5'-hydroxyl group are converted
to circular forms. In this "single-tube" process substrates are successively treated
with polynucleotide kinase, Mth RNA ligase, shrimp alkaline phosphatase, and
CircLigase I™ without any intermediate purification steps.
[0079] The claimed invention may be embodied in other specific forms
without departing from the spirit or essential characteristics thereof. The foregoing
embodiments are selected embodiments or examples from a manifold of all possible
embodiments or examples. The foregoing embodiments are therefore to be
considered in all respects as illustrative rather than limiting on the invention described
herein. While only certain features of the claimed invention have been illustrated and
described herein, it is to be understood that one skilled in the art, given the benefit of
this disclosure, will be able to identify, select, optimize or modify suitable
conditions/parameters for using the methods in accordance with the principles of the
present invention, suitable for these and other types of applications. The precise use,
choice of reagents, choice of variables such as concentration, volume, incubation
time, incubation temperature, and the like may depend in large part on the particular
application for which it is intended. It is, therefore, to be understood that the
appended claims are intended to cover all modifications and changes that fall within
the true spirit of the invention. Further, all changes that come within the meaning and
range of equivalency of the claims are intended to be embraced therein.

CLAIMS:
1. A method for generating a single-stranded DNA circle from a linear DNA, the
method comprising:
(a) providing the linear DNA;
(b) incubating the linear DNA with a polynucleotide kinase in presence of a
phosphate donor to generate a ligatable DNA sequence having a phosphate group at a
5' terminal end and a hydroxyl group at a 3' terminal end; and
(c) incubating the ligatable DNA sequence with a ligase that is capable of
template-independent, intra-molecular ligation of a single-stranded DNA sequence to
generate the single-stranded DNA circle,
wherein all the steps of the method are performed in a single reaction vessel
without any intervening isolation or purification steps.
2. The method of claim 1, wherein step (b) is performed using a phosphate donor
other than adenosine triphosphate or deoxyadenosine triphosphate.
3. The method of claim 2, wherein the phosphate donor is selected form a group
consisting of guanosine triphosphate, cytidine triphosphate, uridine triphosphate,
deoxythymidine triphosphate and combinations thereof.
4. The method of claim 3, where in the phosphate donor is guanosine
triphosphate.
5. The method of claim 4, wherein the linear DNA is incubated with the
polynucleotide kinase in presence of less than 200 mM of guanosine triphosphate.
6. The method of claim 5, wherein the linear DNA is incubated with the
polynucleotide kinase in presence of up to 30 mM of guanosine triphosphate and up to
2.5 mM of manganese ion.
7. The method of claim 4, further comprising denaturing the ligatable DNA
sequence prior to step (c) if the ligatable DNA sequence is in double-stranded form.
8. The method of claim 7, wherein the ligase is a pre-adenylated ligase.
9. The method of claim 8, wherein the pre-adenylated ligase is a pre-adenylated
TS2126 RNA ligase.
10. The method of claim 9, wherein steps (a) to (c) are performed in a sequential
manner in the single reaction vessel.
11. The method of claim 10, wherein all the steps of the method are performed in
absence of adenosine triphosphate or deoxyadenosine triphosphate.
12. The method of claim 11, wherein all the steps of the method are performed in
HEPES buffer.
13. The method of claim 1, wherein the linear DNA is a circulating DNA, a DNA
isolated from formalin fixed paraffin-embedded samples, a forensic DNA sample that
have been exposed to environmental conditions, or an ancient DNA sample.
14. The method of claim 1, further comprising amplifying the single-stranded
DNA circle under isothermal conditions.
15. The method of claim 14, wherein the single-stranded DNA circle is amplified
via rolling circle amplification.
16. The method of claim 15, wherein the rolling circle amplification is performed
using a nuclease-resistant primer.
17. The method of claim 16, wherein the nuclease-resitant primer comprises at
least one modified nucleotide.
18. The method of claim 15, wherein the single-stranded DNA circle is amplified
using a random primer mixture.
19. The method of claim 15, wherein the single-stranded DNA circle is amplified
using a primer solution that consists essentially of a partially constrained primer
mixture comprising a terminal mismatch primer-dimer structure.
20. A method of generating a single-stranded DNA circle from a linear DNA, the
method comprising:
(a) providing the linear DNA;
(b) optionally incubating the linear DNA with a polynucleotide kinase in
presence of adenosine triphosphate to generate a ligatable DNA sequence having a
phosphate group at a 5' terminal end and a hydroxyl group at a 3' terminal end;
(c) incubating the linear DNA sequence or the ligatable DNA sequence with
an adenylating enzyme in presence of adenosine triphosphate to generate a 5'
adenylated DNA sequence; and
(d) incubating the 5' adenylated DNA sequence with a non-adenylated
ligase, which is capable of template-independent intra-molecular ligation of a 5'
adenylated, single-stranded DNA sequence to generate the single-stranded DNA
circle,
wherein all the steps of the method are performed in single reaction vessel
without any intervening isolation or purification steps.
21. The method of claim 20, further comprising denaturing the 5' adenylated DNA
sequence prior to step (d) if the 5' adenylated DNA sequence is in double-stranded
form.
22. The method of claim 21, wherein the adenylating enzyme is an RNA ligase 1
derived from thermophilic archeabacteria, Methanobacterium thermoautotrophicum.
23. The method of claim 22, wherein the non-adenylated ligase is a mutant or
engineered ligase, which is adenosine-triphosphate-independent and is defective in
self-adenlyation, de-adenylation and adenylate transfer.
24. The method of claim 23, wherein the mutant ligase is a mutant of RNA ligase
1 derived from thermophilic archeabacteria, Methanobacterium
thermoautotrophicum.
25. The method claim 22, further comprising incubating the reaction mixture of
step (c) comprising the 5' adenylated DNA sequence with a phosphatase to eliminate
the adenosine triphosphate from the reaction mixture.
26. The method of claim 25, wherein the non-adenylated ligase is a TS2126 RNA
ligase derived from thermus bacteriophage, TS2126.
27. The method of claim 26, wherein steps (a) to (d) are performed in a sequential
manner in the single reaction vessel.
28. The method of claim 20, wherein the linear DNA is a circulating DNA, a
DNA isolated from formalin fixed paraffin-embedded samples, a forensic DNA
sample that have been exposed to environmental conditions, or an ancient DNA
sample.
29. The method of claim 20, further comprising amplifying the single-stranded
DNA circle under isothermal conditions.
30. The method of claim 29, wherein the single-stranded DNA circle is amplified
via rolling circle amplification.
31. The method of claim 30, wherein the single-stranded DNA circle is amplified
using a nuclease-resistant primer.
32. The method of claim 31, wherein the nuclease-resitant primer comprises at
least one modified nucleotide.
33. The method of claim 30, wherein the single-stranded DNA circle is amplified
using a random primer mixture.
34. The method of claim 30, wherein the single-stranded DNA circle is amplified
using a primer solution that consists essentially of a partially constrained primer
mixture comprising a terminal mismatch primer-dimer structure.