Assays have been developed that rely on analyzing nucleic acid molecules from bodily fluids for the presence of mutations, thus leading to early diagnosis of certain diseases such as cancer. In a typical bodily fluid sample however, any abnormal nucleic acids containing mutations of interest are often present in small amounts (e.g., less than 1%) relative to a total amount of nucleic acid in the bodily fluid sample. This can result in a failure to detect the small amount of abnormal nucleic acid due to stochastic sampling bias.
The advent of PCR and real-time PCR methodologies has greatly improved the analysis of nucleic acids from both throughput and quantitative perspectives. While traditional PCR techniques typically rely on end-point, and sometimes semi-quantitative, analysis of amplified DNA targets via agarose gel electrophoresis, real-time PCR (or qPCR) methods are geared toward accurately quantifying exponential amplification as the reaction progresses. qPCR reactions are monitored either using a variety of highly sequence specific fluorescent probe technologies, or by using non-specific DNA intercalating fluorogenic dyes.
As the need for higher throughput in analyzing multiple targets in parallel continues to escalate in the fields of genomics and genetics, and as the need for more efficient use of sample grows in medically related fields such as diagnostics, the ability to perform and quantify multiple amplifications simultaneously within the same reaction volume (multiplexing) is paramount for both PCR and qPCR. While end-point PCR can support a high level of amplicon multiplexing, such ample capacity for multiplexing probe-based qPCR reactions remains elusive for a number of reasons. For example, most commercial real-time thermal cyclers only support up to four differently colored fluorophores for detection as a consequence of the limited spectral resolution of common fluorophores, translating into a multiplexing capacity of 4×. Additionally, while optimization of single target primer/probe reactions is now standard practice, combining primers and probes for multiple reactions changes the thermodynamic efficiencies and/or chemical kinetics, necessitating potentially extensive troubleshooting and optimization. Very high multiplexing of greater than 100× has been demonstrated in a “one of many” detection format for pathogen identification using “sloppy” molecular beacons and melting points as fingerprints, however the approach is restricted to applications with a slim likelihood of the presence of multiple simultaneous targets. A half-multiplexing method achieved 19× in a two step reaction with general multiplexed preamplification in the first step, followed by separate single-plex quantitative PCR in the second step. However a general purpose single-pot solution to qPCR multiplexing does not yet exist.
Digital PCR (dPCR) is an alternative quantitation method in which dilute samples are divided into many separate reactions. See for example, Brown et al. (U.S. Pat. Nos. 6,143,496 and 6,391,559) and Vogelstein et al. (U.S. Pat. Nos. 6,440,706, 6,753,147, and 7,824,889), the content of each of which is incorporated by reference herein in its entirety. The distribution from background of target DNA molecules among the reactions follows Poisson statistics, and at so called “terminal dilution” the vast majority of reactions contain either one or zero target DNA molecules for practical intents and purposes. In another case, at so called “limiting dilution” some reactions contain zero DNA molecules, some reactions contain one molecule, and frequently some other reactions contain multiple molecules, following the Poisson distribution. It is understood that terminal dilution and limiting dilution are useful concepts for describing DNA loading in reaction vessels, but they have no formal mathematical definition, nor are they necessarily mutually exclusive. Ideally, at terminal dilution, the number of PCR positive reactions (PCR(+)) equals the number of template molecules originally present. At limiting dilution, Poisson statistics are used to uncover the underlying amount of DNA. The principle advantage of digital compared to qPCR is that it avoids any need to interpret the time dependence of fluorescence intensity—an analog signal—along with the main underlying uncertainty of non-exponential amplification during early cycles.
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