RAS/RAF mutations and their associations with epigenetic alterations for distinct pathways in Vietnamese colorectal cancer

Abstract

KRAS, NRAS, and BRAF are potential tumor-driven genes that are involved in the RAS/RAF/MAPK signaling pathway. RAS/RAF mutations importantly contribute to colorectal tumorigenesis since they remain the activated status of downstream pathways without regulation of the upstream EGFR signal. However, it has not been unclear how epigenetic alterations involved in colorectal tumorigenesis mediated by KRAS, NRAS, or BRAF mutations. Therefore, in this study, we investigated the frequency and distribution of KRAS/NRAS/BRAF mutations in Vietnamese colorectal cancer (CRC) and explored the relationship between genetic and epigenetic abnormalities in 156 tumors of CRC. Somatic mutations of KRAS (exon 2, codon 12/13; exon 3, codon 61), NRAS (exon 2, codon 12/13; exon 3, codon 61), and BRAF (exon 15, codon 600) was determined by Cobas® KRAS Mutation Test, Therascreen NRAS Pyro Kit and Cobas® 4800 BRAF V600 Mutation Test, respectively. Methylation status of BRCA1, MLH1, MGMT, p16, RASSF1A, and APC was detected by methylation-specific PCR. Distribution of each abnormality in clinicopathological features was also analyzed. Results showed the mutation rates of KRAS, NRAS, and BRAF were 41.0 %, 9.6 %, 8.3 % respectively, while the methylation rates of BRCA1, MLH1, MGMT, p16, RASSF1A, and APC were 16.7 %, 16.7 %, 32.7 %, 30.1 %, 30.1 %, and 37.2 % respectively. The distribution of KRAS mutation was mutually exclusive against that of NRAS (p < 0.001) and BRAF (p < 0.001) mutations in CRC. RAS/RAF mutations were more common in adenocarcinoma subtype (p = 0.020), whereas RASSF1A methylation was more frequent in mucinous adenocarcinoma subtype (p = 0.007). In addition, the frequency of having KRAS mutations was significantly higher in MGMT (p = 0.035) or RASSF1A (p = 0.043) methylated cases than in those without methylation. BRAF mutations were positively associated with MLH1 hypermethylation (p = 0.028) but were inversely associated with APC hypermethylation (p = 0.032). Overall, our results show specific interactions of genetic and epigenetic alterations and suggest the presence of independent oncogenic pathways in tumorigenesis of CRC.