Enthusiasm has emerged for the potential of liquid biopsies to provide easily accessible genetic biomarkers for early diagnosis and mutational cancer characterization. We here systematically investigated the suitability of circulating cell-free DNA (cfDNA) analysis for mutation detection in colorectal cancer (CRC) patients with respect to clinicopathological disease stage. Droplet Digital PCR (ddPCR) was performed to detect common point mutations in the KRAS and BRAF oncogenes in cfDNA from 65 patients and compared to mutations in tumor tissue. Stage of disease was classified according to UICC (Union for International Cancer Control) criteria. In tumor tissue, KRAS or BRAF mutations were present in 35 of 65 cases (44% UICC stage I, 50% stage II, 47% stage III, and 62% stage IV). Although cfDNA was detected in 100% of patients, ddPCR displayed the tumor tissue mutation in only 1 of 6 (17%) stage II patients, whereas 10 of 18 (56%) reported variants were verified in cfDNA samples of the stage IV cohort. No BRAF or KRAS mutation was detected in cfDNA from patients with wild-type tumor tissue. In one case of mutant stage II colon cancer (KRAS-G12C), the G12D variant was detected in cfDNA instead. Further workup revealed that circulating tumor-derived DNA and liver metastases originated from a synchronous KRAS-mutated cancer of the pancreas. Our results demonstrate that ddPCR-based analysis is highly specific and useful for mutation monitoring, but the sensitivity limits its usefulness for early cancer detection.
