Circulating tumor DNA (ctDNA) has been known to be released from tumor cells and evaluated as potential biomarkers for therapeutic responses. In our previous study, we proved applicability for KRAS mutation as a prognostic marker in pancreatic ductal adenocarcinoma (PDA) through the quantitative analysis of ctDNA using KRAS multiplex kit which covers 7 sites. In this study, we analyzed distribution of each single mutation in tumor and ctDNA by performed droplet digital PCR (ddPCR).
Total of 126 PDA patients were enrolled and analyzed for tumor DNA. We found 94 patients have samples for matched ctDNA analysis. Plasma was separated by established method and ctDNA were extracted using QIAamp Circulating Nucleic Acid Kit. QX200 Droplet Digital PCR System was applied to measure frequency of KRAS mutations using KRAS screening multiplex kit and probe for G12D, G12V and G12C in ctDNA and tumor DNA. Then, we compared the frequency between multiplex and single point mutation of KRAS.
The positivity of mutation of tumor and ctDNA in multiplex ddPCR were 96.0% (n = 121/126) and 60.3% (n = 76/126), respectively. Positive rates of G12D, G12V and G12C were 59.6%, 24.5% and 22.3% in tumor and 36.2%, 16.0% and 17.0% in ctDNA, respectively. There was a correlation between multiplex and the sum of single point mutation (r = 0.805). In the single point mutation results, 44 of the samples detected in both tumor and ctDNA mutations were detected in the same site (75.9%), 12 of them were partially identical (19.0%), and 3 were inconsistent (5.2%). Comparing the frequency with other studies, G12D and G12V showed no difference in frequency, but G12C showed a higher positive rate than the previous studies even though we did not analysis for G12R. Multiple KRAS mutations were found in 17% and 6.4% in tumor and ctDNA.
These results show that the frequency of KRAS mutation correlated with tumor tissue and ctDNA. Also multiple mutations were detected high proportion compared to the previous studies. Further studies are needed to determine G12R proportion. We further evaluate whether the single point mutation detected by single probe is more valuable than sum of mutations from KRAS multiplex kit as a biomarker for predicting the prognosis of PDA.
Sun-Young Kong.
National Cancer Center and the Ministry of Health & Welfare, Republic of Korea.
All authors have declared no conflicts of interest.