High-throughput droplet digital PCR system for absolute quantitation of DNA copy number.
ABSTRACT Digital PCR enables the absolute quantitation of nucleic acids in a sample. The lack of scalable and practical technologies for digital PCR implementation has hampered the widespread adoption of this inherently powerful technique. Here we describe a high-throughput droplet digital PCR (ddPCR) system that enables processing of ~2 million PCR reactions using conventional TaqMan assays with a 96-well plate workflow. Three applications demonstrate that the massive partitioning afforded by our ddPCR system provides orders of magnitude more precision and sensitivity than real-time PCR. First, we show the accurate measurement of germline copy number variation. Second, for rare alleles, we show sensitive detection of mutant DNA in a 100,000-fold excess of wildtype background. Third, we demonstrate absolute quantitation of circulating fetal and maternal DNA from cell-free plasma. We anticipate this ddPCR system will allow researchers to explore complex genetic landscapes, discover and validate new disease associations, and define a new era of molecular diagnostics.
- SourceAvailable from: columbia.edu[show abstract] [hide abstract]
ABSTRACT: BEAMing allows the one-to-one conversion of a population of DNA fragments into a population of beads. We used rolling circle amplification to increase the number of copies bound to such beads by more than 100-fold. This allowed enumeration of mutant and wild-type sequences even when they were present at ratios less than 1:10,000 and was sensitive enough to directly quantify the error rate of DNA polymerases used for PCR.Nature Methods 03/2006; 3(2):95-7. · 23.57 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The use of fetal DNA in maternal plasma for noninvasive prenatal diagnosis of trisomy 21 (T21) is an actively researched area. We propose a novel method of T21 detection that combines fetal-specific epigenetic and genetic markers. We used combined bisulfite restriction analysis to search for fetal DNA markers on chromosome 21 that were differentially methylated in the placenta and maternal blood cells and confirmed any target locus with bisulfite sequencing. We then used methylation-sensitive restriction endonuclease digestion followed by microfluidics digital PCR analysis to investigate the identified marker. Chromosome-dosage analysis was performed by comparing the dosage of this epigenetic marker with that of the ZFY (zinc finger protein, Y-linked) gene on chromosome Y. The putative promoter of the HLCS (holocarboxylase synthetase) gene was hypermethylated in the placenta and hypomethylated in maternal blood cells. A chromosome-dosage comparison of the hypermethylated HLCS and ZFY loci could distinguish samples of T21 and euploid placental DNA. Twenty-four maternal plasma samples from euploid pregnancies and 5 maternal plasma samples from T21 pregnancies were analyzed. All but 1 of the euploid samples were correctly classified. The epigenetic-genetic chromosome-dosage approach is a new method for noninvasive prenatal detection of T21. The epigenetic part of the analysis can be applied to all pregnancies. Because the genetic part of the analysis uses paternally inherited, fetal-specific genetic markers that are abundant in the genome, broad population coverage should be readily achievable. This approach has the potential to become a generally usable technique for noninvasive prenatal diagnosis.Clinical Chemistry 10/2009; 56(1):90-8. · 7.15 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Gene inventory and metagenomic techniques have allowed rapid exploration of bacterial diversity and the potential physiologies present within microbial communities. However, it remains nontrivial to discover the identities of environmental bacteria carrying two or more genes of interest. We have used microfluidic digital polymerase chain reaction (PCR) to amplify and analyze multiple, different genes obtained from single bacterial cells harvested from nature. A gene encoding a key enzyme involved in the mutualistic symbiosis occurring between termites and their gut microbiota was used as an experimental hook to discover the previously unknown ribosomal RNA-based species identity of several symbionts. The ability to systematically identify bacteria carrying a particular gene and to link any two or more genes of interest to single species residing in complex ecosystems opens up new opportunities for research on the environment.Science 01/2007; 314(5804):1464-7. · 31.20 Impact Factor
Published:October 28, 2011
r2011 American Chemical Society
dx.doi.org/10.1021/ac202028g|Anal. Chem. 2011, 83, 8604–8610
High-Throughput Droplet Digital PCR System for Absolute
Quantitation of DNA Copy Number
Benjamin J. Hindson,*,†Kevin D. Ness,†Donald A. Masquelier,†Phillip Belgrader,†Nicholas J. Heredia,†
Anthony J. Makarewicz,†Isaac J. Bright,†Michael Y. Lucero,†Amy L. Hiddessen,†Tina C. Legler,†
Tyler K. Kitano,†Michael R. Hodel,†Jonathan F. Petersen,†Paul W. Wyatt,†Erin R. Steenblock,†
Pallavi H. Shah,†Luc J. Bousse,†Camille B. Troup,†Jeffrey C. Mellen,†Dean K. Wittmann,†
Nicholas G. Erndt,†Thomas H. Cauley,†Ryan T. Koehler,†Austin P. So,†Simant Dube,†Klint A. Rose,†
Luz Montesclaros,†Shenglong Wang,†David P. Stumbo,†Shawn P. Hodges,†Steven Romine,†
Fred P. Milanovich,†Helen E. White,‡John F. Regan,†George A. Karlin-Neumann,†
Christopher M. Hindson,†Serge Saxonov,†and Bill W. Colston†
†Bio-Rad Laboratories, Inc., 7068 Koll Center Parkway, Pleasanton, California 94566, United States
S Supporting Information
growing number of molecular diagnostic tests. The first genera-
tion of PCR users performed end-point analysis by gel electro-
phoresis to obtain qualitative results. The advent of real-time
PCR spawned a second generation that enabled quantitation by
monitoring the progression of amplification after each cycle
using fluorescence probes. In real-time PCR, quantitative infor-
mation is obtained from the cycle threshold (CT), a point on the
analogue fluorescence curve where the signal increases above
background. External calibrators or normalization to endogen-
ous controls are required to estimate the concentration of an
unknown. Imperfect amplification efficiencies affect CTvalues
which in-turn limits the accuracy of this technique for absolute
Early pioneers1recognized that the combination of limiting
dilution, end-point PCR, and Poisson statistics could yield an
absolutemeasure ofnucleic acidconcentration,anapproachthat
later became known as digital PCR.2In digital PCR, target DNA
molecules are distributed across multiple replicate reactions at a
level where there are some reactions that have no template and
etection and quantitation of specific nucleic acid sequences
using PCR is fundamental to a large body of research and a
others that have one or more template copies present. After
amplification to the terminal plateau phase of PCR, reactions
containing one or more templates yield positive end-points,
whereas those without template remain negative. The number
of target DNA molecules present can be calculated from the
fraction of positive end-point reactions using Poisson statistics,
according to eq 1,
λ ¼ ? lnð1 ?pÞ
where λ is the average number of target DNA molecules per
replicate reaction and p is the fraction of positive end-point
reactions. From λ, together with the volume of each replicate
PCR and the total number of replicates analyzed, an estimate of
the absolute target DNA concentration is calculated. In digital
PCR, the number of replicates, or partitions, largely defines
the dynamic range of target DNA quantitation, where an
order of magnitude increase in the number of replicates yields
August 3, 2011
October 5, 2011
ABSTRACT: Digital PCR enables the absolute quantitation of nucleic
acids in a sample. The lack of scalable and practical technologies for
digital PCR implementation has hampered the widespread adoption of
droplet digital PCR (ddPCR) system that enables processing of ∼2
million PCR reactions using conventional TaqMan assays with a
96-well plate workflow. Three applications demonstrate that the massive
partitioning afforded by our ddPCR system provides orders of magni-
tude more precision and sensitivity than real-time PCR. First, we show the accurate measurement of germline copy number
variation. Second, for rare alleles, we show sensitive detection of mutant DNA in a 100000-fold excess of wildtype background.
era of molecular diagnostics.
dx.doi.org/10.1021/ac202028g |Anal. Chem. 2011, 83, 8604–8610
approximately an order of magnitude increase in dynamic range.
Increasing the number of partitions also improves precision and
therefore enables resolution of small concentration differences
the relationshipbetweenthe number of pixels and the resolution
of a digital image. As digital PCR relies on a binary end-point
threshold to assign each replicate reaction as either positive or
negative (one or zero, respectively), it can tolerate wide varia-
tions in amplification efficiencies without affecting DNA copy
range, digital PCR by limiting dilution in microwell plates is still
used today.3A practical and low-cost embodiment will unlock
the potential of digital PCR and establish a third generation of
PCR users and applications.
Currently there are two approaches used by commercially
available digital PCR systems. The first approach uses micro-
wells5or microfluidic chambers6?8to split the sample into
hundreds of nanoliter partitions. Microfluidic chips simplify
reaction setup but are challenging to scale to achieve high-
throughput. The second approach, called BEAMing,9,10is based
on emulsion PCR, where templates are clonally amplified in the
presence of beads. Post-PCR, the emulsion is broken to recover
the beads, which are subsequently labeled with a fluorescent
hybridization probe and read by conventional flow-cytometry.
BEAMing requires specialized heterogeneous assay schemes
to a few applications including rare allele detection and DNA
methylation.11?13Overall, high costs, limited throughput, and
complicated workflows have hampered the adoption of
We have developed an approach that uses water-in-oil
put digital PCR in a low-cost and practical format. Our approach
takes advantage of simple microfluidic circuits and surfactant
support PCR amplification of single template molecules using
homogeneous assay chemistries and workflows similar to those
widely used for real-time PCR applications (i.e., TaqMan). An
PCR at a rate of 32 wells per hour.
’RESULTS AND DISCUSSION
The droplet digital PCR (ddPCR) workflow requires the
following steps (Figure 1): Eight assembled PCR reactions, each
comprising template, ddPCR Mastermix and TaqMan reagents,
are loaded into individual wells of a single-use injection molded
cartridge. Next, droplet generation oil containing stabilizing
surfactants is loaded and the cartridge placed into the droplet
generator. By application of vacuum to the outlet wells, sample
and oil are drawn through a flow-focusing junction where mono-
disperse droplets are generated at a rate of ∼1000 per second.
The surfactant-stabilized droplets flow to a collection well where
they quickly concentrate due to density differences between the
oil and aqueous phases, forming a packed bed above the excess
oil. The densely packed droplets are pipet transferred to a
After thermal cycling, the plate is transferred to a droplet reader.
Here, droplets from each well are aspiratedand streamed toward
the detector where, en route, the injection of a spacer fluid
separates and aligns them for single-file simultaneous two-color
detection. TaqMan assays provide specific duplexed detection of
target and reference genes. All droplets are gated based on
detector peak width to exclude rare outliers (e.g., doublets,
triplets). Each droplet has an intrinsic fluorescence signal result-
ing from the imperfect quenching of the fluorogenic probes
Figure 1. Droplet digital PCR workflow: (a) Samples and droplet
generation oil are loaded into an eight-channel droplet generator
cartridge. (b) A vacuum is applied to the droplet well, which draws
droplets are formed. In <2 min, eight samples are converted into eight
sets of 20000 droplets. (c) The surfactant-stabilized droplets are pipet
transferred to a 96-well PCR plate. (d) Droplet PCR amplification to
(e) The plate is loaded onto a reader which sips droplets from each well
and streams them single-file past a two-color detector at the rate of
∼1000 per second. (f) Droplets are assigned as positive or negative
based on their fluorescence amplitude. The number of positive and
negative droplets in each channel is used to calculate the concentration
based 95% confidence intervals.
dx.doi.org/10.1021/ac202028g |Anal. Chem. 2011, 83, 8604–8610
template, specific cleavage of TaqMan probes generates a strong
fluorescence signal. On the basis of fluorescence amplitude, a
simple threshold assigns each droplet as positive or negative. As
the droplet volume is known, the fraction of positive droplets is
then used to calculate the absolute concentration of the target
sequence. For 20000 droplets, the dynamic range for absolute
human genomic DNA, this equates to an input DNA mass
ranging from 3.3 fg to 330 ng per 20 μL reaction. As templates
are randomly distributed across the droplet partitions, a Poisson
correction extends the dynamic range into the realm where on
average there are multiple copies per droplet. Statistical models
are applied to calculate confidence limits of the concentration
estimates and their ratios.4,17
To demonstrate the immediate utility of this ddPCR system,
we present data on three application areas of increasing interest
to researchers: determination of copy number variation (CNV),
detection of rare alleles and the absolute quantitation of circulat-
ing DNA in cell-free plasma. Each application was selected to
highlight a distinct advantage that massive droplet partitioning
affords to digital PCR. For CNV, the large number of replicates
provides sufficient precision to accurately measure copy number
states. For the detection of rare alleles, partitioning the target
mutant DNA away from highly homologous wildtype DNA
increases sensitivity. Finally, droplet partitioning enables accurate
quantitation of nucleic acids from clinical samples over a wide
CNVs are deletions and amplifications of genome segments
ranging from hundreds to millions of base pairs in length that
have been implicated in a broad spectrum of human disease.18
Microarrays and the next-generation sequencing technologies
have enabled and accelerated the discovery of new CNVs,19
thereby further increasing the need for a high-throughput, low-
cost approach to making precise CNV measurements with
increased dynamic range for validation and follow-up studies.
Although microarray technologies are valuable tools for CNV
discovery,20they have limited dynamic range and are expensive
to scale to large numbers of samples for population studies.
Multiplex ligation-dependent probe amplification (MLPA)21is
an assaythat allows resolution of deletions or duplications for up
to 40 targets but requires selection from a predefined test menu
or extensive upfront assay optimization for new target panels.
CNV investigators using methods based on real-time PCR have
reported technical difficulty obtaining accurate copy number
measurements.22Real-time PCR measurements are inherently
imprecise, and copy number estimates can drift between cases
samples by ddPCR. Because increases in gene copy number are
often the result of tandem gene duplications, we used restriction
enzymes to predictably and efficiently separate linked copies of
the target gene such that each sequence is encapsulated into its
own droplet and counted separately. Restriction enzymes were
selected to cut either side of the amplicon sequences avoiding
known mutation sites23and methylation sensitivities. Physically
shearing DNA using ultrasound or microfluidic devices is less
attractive as it reduces the amount of target that can be amplified
without specialized equipment. Preamplification, an alternative
strategy for separation of linked copies24has the potential to
introduce bias between the target and reference genes.
Seven HapMap samples were screened for CNVs for three
target genes. Each ddPCR reaction contained duplex TaqMan
the copy number states from 1 up to 6 were completely resolved
from the results of a single well for each sample (Figure 2a).
Lower CNV states for CYP2D6 and Chromosome X were also
easily resolved, as shown. For 13 HapMap samples, our system
estimated the copy number of CCL3L1, a gene associated with
HIV-1/AIDS susceptibility18(Figure 2b). For DNA sample
NA18507, next-generation sequencing estimated the CCL3L1
copy number to be 5.725whereas our ddPCR system estimated
billions of reads of a next-generation sequencing run are dis-
only 30?. Thus, once target genes have been identified, greater
precision can readily be achieved with ddPCR since the number
of reads can be scaled almost arbitrarily. The current ddPCR
system can achieve read depths of up to 20000? for two genes
from a single well. These data show that our ddPCR system is
well suited for CNV population studies as it enables large
numbers of samples to be tested against smaller sets of genes.
Figure 2. Determination of copy number variation states by droplet digital PCR. (a) Measured copy number variation states in HapMap samples for
MRGPRX1, Chromosome X, CYP2D6, and (b) CCL3L1. (c) Correlation of measured copy number alterations of GRB7 and ERBB2 in DNA extracted
from normal and tumorous breast tissues. Each marker represents a CNV measurement from a single ddPCR well of ∼20000 droplets. Error bars
indicate the Poisson 95% confidence intervals for each copy number determination.
dx.doi.org/10.1021/ac202028g |Anal. Chem. 2011, 83, 8604–8610
Sample heterogeneity can attenuate the measurement of copy
number amplifications, which requires more precise measure-
ments to discriminate smaller differences from normal. Somatic
high-throughput technology for precise copy number quantita-
diagnosing amplifications and deletions as this technique affords
and guide therapy. For example, Her2 positive breast tumors
respond to Trastuzumab (Herceptin). For a set of normal and
tumor breast tissue samples, the measured copy numbers of
ERBB2 and GRB7 correlated with the exception of two samples
were expected as the GRB7 gene is part of the HER2 amplicon
and is coamplified in almost all breast tumors with 17q11-21
amplification.27,28This ddPCR method provides the ability to
scale the number of partitions by combining replicate wells to
could foreseeably form the basis of more efficient diagnostic tests.
Figure 3. Detection of the BRAF V600E rare mutant allele in the
presence of homologous wildtype DNA by droplet digital PCR. Serial
dilutions of the mutant cell line DNA were prepared in a constant
background of wildtype human genomic DNA. Droplet partitioning
reduces competitive amplification effects allowing detection down to
0.001% mutant fraction, 1000 times lower than real-time PCR. The
mutant cell line contains 35% BRAF V600E, as measured by ddPCR.
Figure 4. Absolute quantitation of circulating fetal and maternal DNA
from cell-free plasma for male and female fetuses. (a) Quantitation of
fetal DNA concentration using SRY (red bar) and hypermethylated
RASSF1 (blue bar). The RASSF1 gene of circulating fetal DNA is
hypermethylated whereas maternal DNA is hypomethylated. Methyla-
tion sensitive restriction enzymes selectively digested away the hypo-
methylated fraction, leaving the hypermethylated fetal DNA that was
quantified. (b) Quantitation of total DNA concentration (black bar)
represented as the weighted average from six independent assay
measurements including undigested RASSF1 and β-actin as well as
RNaseP and TERT. (c) Fetal loads as determined from the ratio of SRY
to total (male fetuses only) and RASSF1 to total (male and female
fetuses). For male fetuses, the Pearson’s correlation coefficient between
SRY and RASSF1 fetal loads was 97.3%. Fetal DNA is not completely
hypermethylated; therefore, the RASSF1 fetal loads measured for some
the case of fetal load estimates.
dx.doi.org/10.1021/ac202028g |Anal. Chem. 2011, 83, 8604–8610
The second application demonstrates improved detection of
rare mutant alleles by drastically reducing competitive PCR pro-
cesses that occur in the presence of a highly homologous wild-
assays, ddPCR partitions the competing background away from
the mutant, effectively increasing the average mutant-to-wild-
type ratio by 20000 times. On average, the effective enrichment
of the mutant molecules per PCR reaction is proportional to the
number of sample partitions used. For a duplex TaqMan assay
targeting the BRAF V600E mutation,29we show droplet parti-
tioning detects 0.001% mutant fraction, 1000 times lower than
mentary Figure 1 in the Supporting Information). With depen-
dence on the amount recovered from clinical samples, more
limits down to even lower levels. This approach enables re-
turn lead to the improved detection of minimal residual disease
and less invasive diagnostics.
We next evaluated the ability of this ddPCR system to quan-
titate DNA in clinical samples. Circulating DNA in cell-free
plasma30has received increasing levels of attention as a sample
type for developing noninvasive prenatal31and oncology32
diagnostics. The cell-free DNA in plasma is highly fragmented33
and present at low levels, which present challenges for quantita-
tion. We enumerated fetal and total DNA in maternal cell-free
plasma. For 19 maternal plasma samples taken between 10 and
20 weeks gestational age, the level of fetal (Figure 4a) and total
DNA (Figure 4b) were measured for both male and female
fetuses. A selective methylation-sensitive digest enabled the low-
levels of hypermethylated RASSF1 fetal DNA34to be accurately
SRY, RASSF1, and total DNA concentrations, the fetal load for
each sample was calculated (Figure 4c). For male fetuses, a
correlation of 93.7% between the hypermethylated RASSF1 fetal
DNA and SRY fetal loads provided confidence in the estimates
for female fetuses. On the basis of RASSF1 alone, fetal loads
ranged from 2.1 to 11.9% and were in general agreement with
those data collected by next-generation sequencing35that is
currently limited to estimating fetal loads from male fetuses.
This application demonstrates the capability of absolute quanti-
tation of highly fragmented cell-free DNA in clinical samples.
Overall, these data show that ddPCR offers a practical
solution to realize precise estimates of DNA copy number
with high-throughput. We anticipate this system will unlock
the inherent power of digital PCR to more researchers for
Droplet Digital PCR Workflow. The ddPCR workflow was
described in Figure 1. The TaqMan PCR reaction mixture was
andprobes (finalconcentrationsof 900 and250 nM, respectively)
and template (variable volume) in a final volume of 20 μL. Each
assembled ddPCR reaction mixture was then loaded into the
sample well of an eight-channel disposable droplet generator
cartridge (Bio-Rad). A volume of 60 μL of droplet generation oil
(Bio-Rad) was loaded into the oil well for each channel. The
cartridge was placed into the droplet generator (Bio-Rad). The
cartridge was removed from the droplet generator, where the
droplets that collected in the droplet well were then manually
transferred with a multichannel pipet to a 96-well PCR plate. The
plate was heat-sealed with a foil seal and then placed on a
conventional thermal cycler and amplified to the end-point
(40?55 cycles). After PCR, the 96-well PCR plate was loaded
on the droplet reader (Bio-Rad), which automatically reads
the droplets from each well of the plate (32 wells/h). Analysis
of the ddPCR data was performed with QuantaSoft analysis
software (Bio-Rad) that accompanied the droplet reader.
Determination of Copy Number Variation in HapMap
Samples. For MRGPRX1, ChromosomeX, and CYP2D6, 4.4 μg
of each purified human genomic DNA sample (Coriell) was
digested with 10 units of RsaI (NEB) in 50 μL for 1 h at 37 ?C.
The digest was diluted 8-fold to 400 μL with TE buffer (pH 8.0)
then 33 ng (3 μL) was assayed per 20 μL ddPCR reaction. For
CCL3L1, 815 ng of each purified human genomic DNA sample
(Coriell) was digested with 7.5 units of MseI (NEB) in 10 μL for
1 h at 37 ?C. The digest was diluted 3.5-fold to 35 μL with TE
buffer and then 69 ng (3 μL) was assayed per 20 μL ddPCR
reaction. MRGPRX1 assay sequences36were (forward primer)
50-TTAAGCTTCATCAGTATCCCCCA-30, (reverse primer)
50-CAAAGTAGGAAAACATCATCACAGGA-30, and (probe)
6FAM-ACCATCTCTAAAATCCT-MGBNFQ. Chromosome X
assay sequences37were (forward primer) 50-GATGAGGAAGG-
CAATGATCC-30, (reverse primer) 50-TTGGCTTTTACCA-
AATAGGG-30, and (probe) 50-FAM-TGTTTCTCTCTGCC-
TGCACTGG-BHQ1-30(Integrated DNA Technologies). The
CYPD2D6(Hs00010001_cn) waspurchased asa20?premixof
primers and FAM-MGBNFQ probe (Applied Biosystems). Mod-
ifiedCCL3L1 assaysequences19were (forward primer) 50-GGG-
TCCAGAAATACGTCAGT-30, (reverse primer) 50-CATGTT-
CCCAAGGCTCAG-30, and (probe) 6FAM-TTCGAGGCC-
CAGCGACCTCA-MGBNFQ. All CNV assays were duplexed
with an RPP30 reference assay (forward primer) 50-GATTTG-
GACCTGCGAGCG-30, (reverse primer) 50-GCGGCTGTCT-
CCACAAGT-30, and (probe) VIC-CTGACCTGAAGGCTCT-
MGBNFQ. Thermal cycling conditions were 95 ?C ? 10 min
(1 cycle), 94 ?C ? 30 s and 60 ?C ? 60 s (40 cycles), 98 ?C ?
10 min (1 cycle), and 12 ?C hold.
Determination of GRB7 and ERBB2 Copy Number Altera-
tions. Purified DNA (20 ng) from each normal and tumorous
breast tissue sample (D8235086-1, Biochain) was digested with
0.2 units of NlaIII in 10 μL for 1 h at 37 ?C. The restricted DNA
was added directly to ddPCR Mastermix at 8.8 ng (4.4 μL) per
20 μL of ddPCR reaction. ERBB2 (Hs02803918_cn) and GRB7
(Hs02139994_cn) assays were purchased as 20? premixes of
primers and FAM-MGBNFQ probe (Applied Biosystems) and
duplexed with the RPP30 reference assay described above.
Thermal cycling conditions were 95 ?C ? 10 min (1 cycle),
94 ?C ? 30 s and 60 ?C ? 60 s (40 cycles), 98 ?C ? 10 min
(1 cycle), and 12 ?C hold.
Rare Allele Detection. A dilution series of BRAF V600E
mutant DNA (HTB-38D) from a HT-29 cell line (ATCC) was
prepared in a high, constant background (5000 copies/μL) of
wildtype DNA (NA19205, Coriell). For ddPCR, when the
concentration of intact human genomic DNA is >66 ng/20 μL
reaction, the accompanying increase in viscosity can cause the
average droplet volume to change, which in turn could affect the
accuracy of DNA quantitation. Therefore, for samples of this
nature, restriction enzyme digestion is recommended to frag-
ment the DNA and reduce solution viscosity. In our experience,
once fragmented, the human genomic DNA concentration can
dx.doi.org/10.1021/ac202028g |Anal. Chem. 2011, 83, 8604–8610
volume. Therefore, prior to ddPCR, each sample of the dilution
series was digested with 40 U of HaeIII (NEB) in 100 μL con-
taining 1? NEB buffer 4 and BSA. The BRAF V600E/wildtype
duplex TaqMan assay used common primers (forward) 50-CTA-
50-ATCCAGACAACTGTTCAAACTGATG-30, and specific
probes (BRAF V600E) 6FAM-TAGCTACAGAGAAATC-MG-
NFQ. Eight ddPCR wells were used for each sample of the
dilution series. Thermal cycling conditions were 95?C?10min
(1 cycle), 94 ?C ? 30 s and 62.7 ?C ? 60 s (55 cycles), and
12 ?C hold.
Quantitation ofCell-FreeFetalandTotal DNAin Maternal
Plasma. Whole blood (3 ? 10 mL) was collected (ProMedDx)
from healthy pregnant donors, between 10 and 20 weeks of
gestational age, by venipuncture into cell-free DNA BCT tubes
(Streck) according to the manufacturer’s instructions. Fetus
gender was determined by ultrasound within 6 weeks of sample
collection. The tubes were stored for up to 48 h at room
temperature then shipped overnight at 4 ?C to Bio-Rad where
they were processed upon receipt. The whole blood was cen-
trifuged for 10 min at 1600g, the supernatant removed and
transferred to a new tube, centrifuged for 10 min at 16000g, the
supernatant removed, and transferred to a new tube, then the
cell-free plasma was stored at ?80 ?C. Cell-free plasma (5 mL)
was thawed and cell-free DNA isolated using the QIAmp
Circulating Nucleic Acid Kit (Qiagen) according to the manu-
factuer's protocol and eluted in AVE buffer (150 μL). A portion
of the eluate (99 μL) was subjected to a single-tube digest
containing HhaI (30 U), HpaII (60 U), and BstUI (30 U) in 1?
eluate (33 μL) was used in a no-digest control mixture where
restriction enzymes were substituted for water. The mixtures
were incubated for 37 ?C for 2 h, 60 ?C for 2 h, then 65 ?C for
20 min. The restriction enzyme digested mixture was split and
subjected to three ddPCR duplexed assays of SRY/TERT,
RASSF1/RNaseP, and RASSF1/β-actin. The restriction enzyme
mixture cuts unmethylated RASSF1 and β-actin TaqMan tem-
plates but not SRY, RNaseP, or TERT. The no-digest control
RASSF1/RNaseP and RASSF1/β-actin. β-Actin is hypomethy-
lated in both fetal and maternal DNA and is completely digested
by the enzyme cocktail.
RASSF134and SRY37assays were reported previously. RNaseP
and TERT copy number reference assays were purchased com-
mercially (Applied Biosystems). The β-actin assay was modified
from Chan et al. (forward primer) 50- GCAAAGGCGAGGC-
TTATGG-30, and (probe) VIC-ACCGCCGAGACCGCGTC-
MGBNFQ. For RASSF1/RNaseP and RASSF1/β-actin duplexes,
1? GC-Rich Solution (Roche) was used as a component of the
were 95 ?C ? 10 min (1 cycle), 95 ?C ? 30 s and 60 ?C ? 60 s
(45 cycles), and 4 ?C hold.
For each sample, six independent assay measurements of total
DNA concentration (G.E/mL) were made from one TERT, one
β-actin, two RASSF1, and two RNaseP assays. Each assay mea-
surement comprised data from seven replicate ddPCR wells. We
combined the droplet counts (positive and negative) from all
seven replicate wells to yield a single “metawell”. The concentra-
tion and confidence intervals for each of the 6 measurement
metawells were computed.4The appropriate dilution factors
were applied to yield total cell-free DNA concentration (G.E./mL)
and the confidence interval is scaled accordingly. The weighted
mean of the six total measurements was calculated, where
weights are inverses of confidence interval variances of these
measurements.For digested RASSF1, there aretwoindependent
assay measurements, which are also combined in the same
manner. For SRY, there is one measurement that was used
directly, with scaling by a factor of 2 to account for haploidy.
95% confidence intervals.
noted in text. This material is available free of charge via the
Internet at http://pubs.acs.org.
Additional information as
The project described was supported by Grant Number
R01EB010106 from the National Institute of Biomedical Ima-
ging and Bioengineering. The content is solely the responsibility
of the authors and does not necessarily represent the official
views of the National Institute of Biomedical Imaging and
Bioengineering or the National Institutes of Health.
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