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Genomics

MicroscopyóMay 2

Big DataóJune 13

Digital Lab ManagementóJuly 25

Upcoming Features

Researchers studying rare variant biomarkers often find

themselves on the lookout for faint genetic signals against an

overwhelming background, sometimes as little as a single positive

in 100,000 negatives or more. Such a situation cries out for

polymerase chain reaction (PCR), but standard PCR wonít do.

These days, thereís digital PCR (dPCR). By discretizing those

100,000 molecules in a large number of individual reactions,

dPCR makes the rare positive surprisingly easy to find.

See the full story on page 212.

The Digital PCR Revolution

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changesódistinguishing six and seven copies, for instanceóand limited

sensitivity.

The resulting uncertainty can complicate, for instance, data sharingóa

critical limitation when setting up multi-institutional trials, says Muneesh

Tewari, associate professor at the University of Michigan and formerly

associate member of the Fred Hutchinson Cancer Research Center

(FHCRC), who uses dPCR for microRNA biomarker discovery. ìReal-time

PCR just doesnít have the day-to-day precision, or sometimes even the pre-

cision within the same day that we would like,î he says.

dPCR circumvents these issues using a ìdivide-and-conquerî strategy. A

mixture of molecules is discretized or compartmentalized into a large num-

ber of reaction chambersóthe more the betterósuch that each chamber

has on average either one target nucleic acid or none. Following PCR am-

plification of each compartment, Poisson statistics can be used to convert

the count of positive signals into an absolute number.

Kinzler likens the process to Sanger sequencing. Sequencing a mixture

of 100 template molecules, one of which is mutant, in a single tube would

yield an electropherogram in which the dominant signal at the mutated

position would be the wild-type base. ìYou probably wouldnít even be able

to tell there was a little hump underneath [the peak] for the alternative base

read, because it represents too small a percentage of the molecules.î But

diluting the molecules and distributing them in multiple reactions would

produce 99 wild-type signals and one clear mutant óa digital, and abso-

lutely quantitative, result.

Digital PCR essentially lifts weak signals out of the noise, and research-

ers today are using that strategy for everything from detecting copy number

variations and circulating tumor DNA to HIV viral load monitoring and fe-

tal aneuploidy testing. Clinical applications, in particular, seem most likely

to benefit from the technology, assuming it can be made simple enough for

clinical laboratory workflows.

MicroscopyóMay 2

Big DataóJune 13

Digital Lab ManagementóJuly 25

Upcoming Features

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Kenneth Kinzler, co-director of the

Ludwig Center at Johns Hop-

kins University, who (with col-

league Bert Vogelstein) first coined the

term ìdigital PCR,î says he and Vogelstein

developed the approach to better identify

rare cancer mutations. ìThe thought that

occurred to us was that the most sensitive

[detection] you can get is by looking at

single molecules,î Kinzler explains. ìWhen

you start with single molecules, [reactions]

are either 100% mutant or 100% wild type,

which makes the distinction of a tumor

from wild type much easier.î

Of course, seeing even very rare events

using just standard PCR should be possible.

After all, PCR excels at plucking needles

from molecular haystacks. In theory, given

the right primers and cycling conditions,

the reaction can copy a single piece of

template DNA into millions upon millions

of daughter strands, enough to clone, se-

quence, or detect on a gel. But because the

process isnít quantitative, researchers can-

not infer the DNA content of the starting

sample from the number of molecules pres-

ent at the end of the reaction.

qPCR addresses that problem by quan-

tifying the reaction as it runs. By charting

fluorescence intensity over time, research-

ers can compare, for instance, the relative

expression of a given gene from sample

to sample. But absolute quantification by

qPCR isnít straightforward. It generally

requires standard curves to convert abun-

dance into absolute concentrations, and

those concentrations can sometimes vary

day to day and across labs. qPCR also strug-

gles with detecting subtle copy-number

The Digital PCR RevolutionìLike finding a needle in a haystackî is an overused expression, but when it comes to some

biological scavenger hunts, it fits. Researchers studying rare variant biomarkers often find

themselves on the lookout for faint genetic signals against an overwhelming background,

sometimes as little as a single positive in 100,000 negatives or more. Such a situation cries

out for polymerase chain reaction (PCR), a technique uniquely capable of capturing the

proverbial needle. But standard PCR wonít doóit is a qualitative techniqueóand neither

will quantitative real-time PCR (qPCR), which often lacks the necessary accuracy and

sensitivity. These days, thereís a third and increasingly popular option: digital PCR (dPCR).

By discretizing those 100,000 molecules in a large number of individual reactions, dPCR

makes the rare positive surprisingly easy to find. By Jeffrey M. Perkel

Digital PCR

essentially lifts

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out of the noise,

and researchers

today are using

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everything from

detecting copy

number variations

and circulating

tumor DNA to

HIV viral load

monitoring.

Genomics

Life Science Technologies Produced by the Science��������������������� �����

statistics describe a random distribution. As

long the compartments arenít saturated, us-

ers can back-calculate how many molecules

they started with, even if some wells actually

receive more than one molecule (an event

that, in dPCR, reads as a single count). ìYou

tell me how many compartments or how

many droplets you have, [and] what fraction

of those are negative. That will tell me es-

sentially what concentration I started with

that would give me that ratio of positive

compartments to negative compartments,î

he says.

Reginald Beer, medical diagnostics initiative leader at the Lawrence

Livermore National Laboratory, who as a graduate student published

the first report of dPCR in monodisperse (i.e., identically sized) droplets,

says the advantage of the droplet-based approach lies in its scalability: Itís

relatively simple to increase the number of reaction vessels, which in turn

increases data quality. ìPoisson accuracy greatly improves as you scale or

increase the number of your reactor vessels,î he explains.

Published reports establish the efficacy of both Bio-Radís and Rain-

Danceís dPCR platforms as well as the diversity of applications amenable to

dPCR. RainDanceís platform, for instance, has been used to detect mutant

gene transcripts in cerebrospinal fluid from glioma patients and KRAS mu-

tations in sera from colorectal cancer patients.

Jason Bielas, associate member of the FHCRC, used Bio-Radís system

to quantify tumor-infiltrating lymphocytes in ovarian cancer, while Keith

Jerome, professor and head of the Virology Division in the Department

of Laboratory Medicine at the University of Washington and associ-

ate member of the FHCRC, used it to develop a way to distinguish active

HHV-6 viremia from latent, genome-integrated virus. (The former must be

treated if it occurs during the course of bone marrow transplantation, but

not the latter; the two are distinguished by the ratio of viral sequence copies

to genome equivalents in blood, which in the case of genome integration

should be 1.0.)

Bielas says his team was able to use their own method, termed QuanTILfy,

to reproducibly resolve as little as one T lymphocyte in 10,000 cancer

cellsóenough to detect an association between T-cell tumor infiltration and

patient survival. That method is considerably more quantitative, he explains,

than the current standard of immunohistochemistry. ìIn

THE ORIGINAL DIGITAL PCR

Published in 1999, Kinzler and Vogelsteinís original ìdigital PCRî approach

was a tedious, manual affair. Looking to detect and quantify K-RAS

mutations in stool samples from colorectal cancer patients, the pair diluted

and aliquoted genomic DNA into each well of a 384-well microtiter plate,

such that the entire plate represented a single sample. They then amplified

the gene region that encompassed the mutational hotspots they were

looking for.

Once the reactions came to completionódPCR generally is an end-

point PCR assay, though it doesnít have to beóthey hybridized two fluores-

cent probes, a control that should always hybridize and a second that only

binds to wild-type sequences, and read the results. Just over 100 wells had

genomic DNA, of which four appeared to be mutants.

The results, Kinzler says, indicated dPCR was ìa very robust, reliable,

and accurateî method for rare mutation detection. But it also was labor-

intensive and difficult to scale. ìIt was often impossible to convince a grad

student to do it,î he says.

In 2003, Kinzler and Vogelstein described an improved protocol, which

they called ìBEAMingî for its reliance on ìbeads, emulsion, amplification,

and magnetics.î In place of manual sample discretizing in a microtiter plate,

BEAMing turns the individual droplets of a water-in-oil emulsion into re-

action vessels. Using an emulsion PCR process thatís now used widely in

next generation DNA sequence library preparation, BEAMing distributes

a mixture of template, primers, PCR reagents, and magnetic beads into

droplets, again under conditions such that most droplets will contain either

zero or one template. Following PCR, during which the amplified prod-

uct is coupled to the bead via a biotin-streptavidin linkage, the emulsion is

broken and the beads read by hybridization with detection oligonucleotides

and flow cytometry.

Instead of manual separation on a plate, BEAMing ìcan do the equivalent

of 100,000 wells,î says Kinzler, who still uses the technology and (with

Vogelstein and other colleagues) cofounded Inostics to commercialize it.

(Inostics subsequently was acquired by Sysmex, and today Kinzler holds no

equity in the company.)

Droplet-based strategies have also been commercialized by Bio-Rad

Laboratories and RainDance Technologies. Instead of magnetic

beads, both companies discretize (or, as RainDance President and Chief Ex-

ecutive Officer S. Roopom Banerjee puts it, ìdropletizeî) the PCR mixture

into either 20,000 nanoliter-sized (Bio-Radís QX200) or 10 million pico-

liter-sized (RainDance RainDrop) droplets. Following PCR amplification,

the reaction results are read by flowing the droplets past a fluorescence

excitation source and detector, like a flow cytometer without the cells.

ìThink of it as like a digital camera,î says Banerjee, who says RainDanceís

RainDrop system was designed with single-button, ìApple-like simplicity.

You have a complicated picture with lots of colors, lots of diversity. What

weíre doing is basically pixelizing that picture down to miniscule-sized pix-

els and then we literally read out every single pixel in order to say exactly

whatís in that biological picture.î In RainDanceís case, it takes four hours to

read 80 million such ìpixels,î the output of eight parallel dPCR reactions.

POWERFUL POISSON

Because dPCR is an endpoint assay, the resulting concentration could in the-

ory be thrown off if the target molecules didnít discretize perfectlyóthat

is, some compartments ended up with more than one target. Thatís where

Poisson statistics come in. According to George Karlin-Neumann, director

of scientific affairs at Bio-Rad Laboratoriesí Digital Biology Center, Poisson

The technique

clearly offers

considerable

advantages for a

growing number

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Produced by the Science�������������� ������������� Genomics

Life Science Technologies

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213

Jeffrey M. Perkel is a freelance science writer based in Pocatello, Idaho.

www.sciencemag.org/products

essence, youíre counting T-cell genomes.î

Hanlee Ji, senior associate director of the Stanford Genome

Technology Center, also uses Bio-Rad, both to validate genetic

aberrations from genome-sequencing studies and more recently to quality

control next-gen libraries prior to sequencing. For his group, the advantage

of dPCR is largely simplicity: He can get an undergrad in the lab up to speed

on dPCR in weeks, whereas qPCR can take considerably longer.

ìOne of my tests of technology is: If I take an undergrad who has rea-

sonably good bench skills and sit them in front of an instrument, can they

generate robust results from the positive and negative controls that theyíre

asked to do within a matter of weeks? If so, that says a lot regarding how

well the system works.î

DIGITAL PCR-BY-SEQUENCING

In 2011, Kinzler and Vogelstein described yet another approach to dPCR

for rare allele detection, this time based on sequencing. Called Safe-

Sequencing System (Safe-SeqS), the strategy entails tagging individual

template molecules with unique identifiers (or barcodes), which are then

amplified and read out by next generation DNA sequencing.

According to Nickolas Papadopoulos, director of translational genetics

at the Ludwig Center at Johns Hopkins, professor of oncology at Johns

Hopkins Medical Institutes, and coauthor of the study: Safe-SeqS enables

researchers to extend the capacity of dPCR to many more locióup to 30

or moreóthan BEAMing can reasonably handle.

ìMassively parallel sequencing represents a particularly powerful form

of dPCR in that hundreds of millions of template molecules can be analyzed

one by one,î explain Papadopoulos and his coauthors in their 2011 study. ìIt

has the advantage over conventional dPCR methods in that multiple bases

can be queried sequentially and easily in an automated fashion.î But NGS

also has an unacceptably high inherent error rate caused by mistakes in am-

plification, sequencing, and detection. By tagging each starting molecule

with a unique identifier, Safe-SeqS enables researchers to differentiate true

mutations from procedural artifacts.

In the original study, the authors assessed mutations in the CTN-

NB1 gene in 100,000 normal human cells. Raw Illumina sequencing

reads produced an error rate 2.1x10-4. Taking into account the Safe-SeqS

barcodes lowered that rate 24-fold, to 9x10-6. When applied to mitochon-

drial DNA, Safe-SeqS dropped the observed mutation rate 15-fold.

This past year, the team extended Safe-SeqS to search for multiplexed

signatures of ovarian, endometrial, and cervical cancer in the scraped cervi-

cal material that comes from routine Pap testing, a first step in developing

an early detection system for gynecologic tumors. In this case, the team

used the assay to assess mutations in 46 gene regions from 12 cancer-asso-

ciated genes.

Papadopoulos, who discussed the method at a recent digital biology con-

ference in San Diego, says Safe-SeqS may be particularly useful for early

tumor detection from plasma. Overall, his team could detect mutations in

more than 80% of tumor plasma samples. But some tumor types worked

better than others. Particularly difficult, he says, were malignancies in

brain. ìCurrently, we can detect mutations in the plasma from only 10% of

the patients with these tumor types,î he says.

ARRAY-BASED STRATEGIES

For Andrzej Pietrzykowski, assistant professor of animal sciences at

Rutgers University, who studies the molecular and genetic bases of

alcoholism, dPCRís precision is particularly beneficial. Many of the changes

he sees are less than twofold and thus difficult to discern by qPCR. That

difference is readily detected digitally, he says.

But Pietrzykowski also cites another advantage. He has identified a par-

ticular microRNA that seems to be associated with alcoholism, and can

quantify that molecule using qPCR. But qPCR assays for microRNA pre-

cursors are harder to pull off, as identifying reliable controls for standard

curve calibration can be problematic. ìYou need to Ö have that housekeep-

ing gene double or triple-checked that it doesnít change due to conditions.î

Thatís a non-issue with dPCR, though, as the technique doesnít require a

standard curve.

Pietrzykowski was one of five Innovation Grant recipients of Life

Technologiesí Digital PCR Applications Grant program, which in late

2013 provided him with the companyís QuantStudio 3-D chip reader, chip

loader, and thermo cycler. The QuantStudio 3D Digital PCR system is like a

souped-up version of Kinzlerís original dPCR implementation. Researchers

use a tool akin to a windshield ìsqueegeeî to discretize samples onto 20,000

wells etched into the surface of a silicon wafer. Fluidigm, too, uses this

physical array strategy; its qdPCR 37K ìintegrated fluidic circuitî leverages

the companyís microfluidics expertise to distribute 48 samples into 770

chambers each (though a single sample can be spread over multiple sets of

chambers to increase accuracy, up to 37,000).

The technique clearly offers considerable advantages for a growing num-

ber of specific (and often clinical) applications. But donít discard your real-

time thermocycler just yet. For most researchers dPCR represents a com-

plement to qPCR, not a replacement. Indeed, for many applications, says

Beer, qPCR is ìby far sufficientîó plus, the technology is far more mature

and the assays established. ìqPCR certainly still has a significant advantage

from a throughput perspective,î says Iain Russell, senior product manager

for dPCR at Life Technologies (recently purchased by Thermo Fisher Scien-

tific), ìand to be quite honest, for a large number of applications out there

it meets all the needs of the customer.î

Says Pietrzykowski, ìIt really depends on what kind of question youíre

asking.î

Featured Participants

Bio-Rad Laboratorieswww.bio-rad.com

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Ludwig Center at Johns Hopkins ������������� � �����������������������������center

214

DOI: 10.1126/science.opms.p1400084

RainDance Technologieswww.raindancetech.com

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DNA LIBRARY KIT

The Accel-NGS 2S DNA Library Kit for Illumina Next Generation Se-quencing (NGS) systems provides linear yields from inputs ranging from 10 pg to 1 µg. Polymerase chain reaction-free libraries can be produced from as little as 100 ng. Unlike other commercially available kits, exceptional quality and evenness of coverage are obtained even at very low levels of DNA input. The product is also is ideally suited for clinical samples such as FFPE tissues and plasma. Accel-NGS 2S enables NGS laboratories to stock and use a single kit for their varied DNA library needs, ranging from high molecular weight DNA to limited quantity, damaged FFPE samples. In ad-dition to whole genome sequencing, Accel-NGS 2S is also compatible with the leading hybridization capture products and is suitable for ChIP-Seq. Built with usability in mind, the 2S protocol is readily automatable for those with high throughput applications. The wide dynamic range of input makes pre-library quantification optional, further streamlining workflow.Swift Biosciences

For info: 734-330-2568 www.swiftbiosci.com

NGS LIBRARY PREP KITS

Current methods used for epigenetic analysis of DNA methylation are deficient in differentiating 5-methylcytosine from 5-hydroxymethylcyto-sine. The new Pico Methyl-Seq Library Prep and RRHP 5-hmC Library Prep kits are designed for NGS-based, whole-genome analysis of 5-meth-lycytosine and genome-wide analysis of 5-hydroxymethylcytosine in DNA, respectively. These unique products feature streamlined workflows and in-clude all the technologies required for consistent and robust library con-struction. Pico Methyl-Seq is a post-bisulfite library preparation method that can accommodate DNA inputs as low as 10 pg. Alternatively, RRHP allows for strand-specific, single-base profiling of 5-hmC from DNA in-puts as low as 100 ng by utilizing a bisulfite-free, enzymatic-based method. When combined, both technologies will elucidate genomic-scale methyla-tion and hydroxymethylation profiles for a particular DNA sample from any species. Zymo Research Corporation

For info: 888-882-9682 www.zymoresearch.com

POST-BISULFITE DNA LIBRARY PREPARATION

The new EpiNext Post-Bisulfite DNA Library Preparation Kit is designed for constructing DNA libraries directly from bisulfite-treated DNA for whole genome bisulfite sequencing. Bisulfite conversion is an essential pro-cess of modifying DNA so that methylated cytosine bases can be detected via sequencing. Currently used whole genome bisulfite sequencing meth-ods need rather large amounts of DNA (great than 1 µg) as input mate-rial, as well as requiring the need to first shear DNA and ligate adapters to DNA fragments prior to bisulfite conversion. The innovative technology developed by Epigentek allows bisulfite-treated DNA to be directly used for ligation, thereby eliminating the possibility of breaking adapter-ligated fragments, which can often occur in currently used next generation bisul-fite sequencing methods. The commercial kit, based on this technology, has high sensitivity and efficiency, enabling input DNA to be as low as 1 ng, and could be used for precious or limited biological samples.EpigentekFor info: 877-374-4368 www.epigentek.com

GENE-EDITING KITSThe new GENASSIST range of gene-editing kits and reagents enable easier, robust implementation of CRISPR and rAAV gene editing experiments. The current GENASSIST offering comprises both off-the-shelf reagents for using CRISPR editing technology and a unique kit combination of these reagents to allow customers to generate their own CRISPR-ready cell lines that constitutively express Cas9-nickase. Using such cell lines provides a quick start for customers, enabling them to make further modifications to the cell line more efficiently than if they were starting fresh each time. Ho-rizon is also launching a new service for the design, manufacture and most importantly validation of CRISPR RNA guides, in order to maximize the likelihood that gene editing will occur as expected. The new GENASSIST kits, combined with access to rAAV, ZFN, and CRISPR technologies, give researchers an invaluable suite of tools to determine the function of endog-enous gene alterations and their effect on disease and therapeutic responses. Horizon Discovery

For info: +44-(0)-1223-655580 www.horizondiscovery.com

DESKTOP WHOLE-GENOME SEQUENCER

The NextSeq 500 System delivers the power of high throughput sequencing with the load-and-go simplicity of a desktop sequencer, effectively transforming a broad range of high throughput appli-cations into affordable, everyday research tools. Its push-button operation delivers a one-day turn-around for a number of popular sequencing applications, including one whole human genome and up to 16 exomes, up to 20 noninvasive prenatal testing samples, up to 20 transcriptomes, up to 48 gene expression samples, and up to 96 targeted panels. With its streamlined informatics, sequencing data can be run through a range of open -ource or commercial pipelines or instantly transferred, analyzed, and stored securely in BaseSpace or the new BaseSpace OnSite for researchers needing an on-premises solution. Users also have the flexibility to switch to lower throughput sequencing as needed, and while other platforms require several pieces of specialized equipment, the NextSeq 500 System integrates cluster generation and sequencing into a single instrument. Illumina For info: 800-809-4566 www.illumina.com/nextseq

Science��������������������� ���� New Products: Genomics

Life Science Technologies

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September 29 - October 1, 2014HudsonAlpha Biotechnology Campus

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Bringing together preeminent leaders and thinkers

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Mary Ellen ConleyFederal Express Chair of Excellence and Professor, Departmentof Pediatrics, University of Tennessee, College of Medicine, Memphis

Mark DavisInvestigator, Howard Hughes Medical Institute; Professor, Departmentof Microbiology and Immunology; Director, Institute for Immunity,Transplantation, and Infections, Stanford University School of Medicine

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