Tumor somatic mutations also existing as germline polymorphisms may help to identify functional SNPs from genome-wide association studies

We hypothesized that a joint analysis of cancer risk-associated single-nucleotide polymorphism (SNP) and somatic mutations in tumor samples can predict functional and potentially causal SNPs from GWASs. We used mutations reported in the Catalog of Somatic Mutations in Cancer (COSMIC). Confirmed somatic mutations were subdivided into two groups: (1) mutations reported as SNPs, which we call mutational/SNPs and (2) somatic mutations that are not reported as SNPs, which we call mutational/noSNPs. It is generally accepted that the number of times a somatic mutation is reported in COSMIC correlates with its selective advantage to tumors, with more frequently reported mutations being more functional and providing a stronger selective advantage to the tumor cell. We found that mutations reported ≥10 times in COSMIC—frequent mutational/SNPs (fmSNPs) are likely to be functional. We identified 12 cancer risk-associated SNPs reported in the Catalog of published GWASs at least 10 times as confirmed somatic mutations and therefore deemed to be functional. Additionally, we have identified 42 SNPs that are tightly linked (R2 ≥ 0.8) to SNPs reported in the Catalog of published GWASs as cancer risk associated and that are also reported as fmSNPs. As a result, 54 candidate functional/potentially causal cancer risk associated SNPs were identified. We found that fmSNPs are more likely to be located in evolutionarily conserved regions compared with cancer risk associated SNPs that are not fmSNPs. We also found that fmSNPs also underwent positive selection, which can explain why they exist as population polymorphisms.

In silico saturation mutagenesis of cancer genes

SummaryExtensive bioinformatics analysis of datasets of tumor somatic mutations data have revealed the presence of some 500-600 cancer driver genes. The identification of all potential driver mutations affecting cancer genes is essential to implement precision cancer medicine and to understand the interplay of mutation probability and selection in tumor development. Here, we present an in silico saturation mutagenesis approach to identify all driver mutations in 568 cancer genes across 66 tumor types. For most cancer genes the mutation probability across tissues --underpinned by active mutational processes-- influences which driver variants have been observed, although this differs significantly between tumor suppressor and oncogenes. The role of selection is apparent in some of the latter, the observed and unobserved driver mutations of which are equally likely to occur. The number of potential driver mutations in a cancer gene roughly determines how many mutations are available for detection across newly sequenced tumors.

Clonal hematopoiesis mutations in plasma cfDNA RAS/BRAF genotyping of metastatic colorectal cancer.

e15083 Background: Clonal hematopoiesis (CH) leads to blood-derived somatic mutations in KRAS, NRAS and BRAF. Our aim is to identify the prevalence of CH-derived mutations in these three genes in metastatic colorectal cancer (mCRC) patients and reveal the practical clinical implication of these mutations on plasma genotyping. Methods: We analyzed KRAS, NRAS and BRAF genotypes in plasma and matched tumor tissues of 236 mCRC patients through next-generation sequencing (NGS) and polymerase chain reaction. Suspected CH mutations were defined as those only detected in plasma with variant allelic frequencies (AFs) of <5% and were confirmed by paired peripheral blood cells (PBCs) using droplet digital PCR (ddPCR). The hemopoietic lineage harboring a CH-derived mutation was analyzed through flow cytometry. Results: We identified suspected CH mutations in twenty patients (8.4%, 20/236). Three of these patients (1.27%, 3/236) had a CH-derived KRAS mutation, which was confirmed present in paired PBCs. Two of them had a KRAS G12X and the third had a KRAS Q61H. We did not detect CH-derived NRAS or BRAF mutations in our cohort. All three patients harboring a CH-derived mutation previously received chemotherapy treatment. In a selected CH-derived KRAS G12X case, the mutation was enriched in lymphocytes and persisted in plasma cell-free DNA (cfDNA) over the course of 4 months of therapy. Conclusions: In summary, we confirmed the existence of CH-derived KRAS mutations in a small proportion of mCRC patients. This should be noted to prevent misclassification as tumor somatic mutations when performing cfDNA sequencing in the absence of genotyping matched PBCs.