|Digital polymerase chain reaction (digital PCR) can be used for early detection of circulating tumor DNA in many types of cancer even in the absence of any circulating tumor cells.|
Digital PCR is a refinement of conventional PCR methods that can be used to directly quantify and clonally amplify nucleic acids including DNA, cDNA, or RNA. The key difference between digital PCR and traditional PCR lies in the method of measuring nucleic acids amounts, with the former being a more precise method than PCR. PCR carries out one reaction per single sample. Digital PCR also carries out a single reaction within a sample, however the sample is separated into a large number of partitions, and the reaction is carried out in each partition individually. This separation allows a more reliable collection and sensitive measurement of nucleic acid amounts. The method has been demonstrated as useful for studying variations in gene sequences—such as copy number variants and point mutations.
Investigators at Johns Hopkins University (Baltimore, MD, USA) used digital PCR to evaluate the ability of circulating tumor DNA (ctDNA) to reveal tumors in 640 patients with various cancer types. They found that ctDNA was detectable in more than 75% of patients with advanced pancreatic, ovarian, colorectal, bladder, gastroesophageal, breast, melanoma, hepatocellular, and head and neck cancers, but in less than 50% of primary brain, renal, prostate, or thyroid cancers.
In patients with localized tumors, ctDNA was detected in 73, 57, 48, and 50% of patients with colorectal cancer, gastroesophageal cancer, pancreatic cancer, and breast adenocarcinoma, respectively. ctDNA was often present in patients without detectable circulating tumor cells, suggesting that these two biomarkers were distinct entities. In a separate panel of 206 patients with metastatic colorectal cancers, the investigators found that the sensitivity of ctDNA for detection of clinically relevant KRAS gene mutations was 87.2% and its specificity was 99.2%.
Finally, the investigators determined whether ctDNA could provide clues into the mechanisms underlying resistance to epidermal growth factor receptor (EGFR) blockade in 24 patients who initially responded to therapy but subsequently relapsed. Twenty-three (96%) of these patients developed one or more mutations in genes involved in the mitogen-activated protein kinase pathway.
The results reported in this study suggest that ctDNA is a broadly applicable, sensitive, and specific biomarker that can be used for a variety of clinical and research purposes in patients with multiple different types of cancer.
“We are already very good at treating and curing cancer when it is localized,” said first author Dr. Chetan Bettegowda, assistant professor of oncology and neurological surgery at Johns Hopkins University. “But we wanted to develop a noninvasive technology to enhance detection of cancer at an early stage, and we feel this is an exciting starting point for further work using this method.”
The study was published in the February 19, 2014, online edition of the journal Science Translational Medicine.