Intermediate Methylation Epigenotype and Its Correlation to KRAS Mutation in Conventional Colorectal Adenoma

A subset of colorectal cancer shows significant accumulation of aberrant promoter methylation. Previously, we developed two groups of methylation markers that classified colorectal cancer into three epigenotypes: i) high-, ii) intermediate-, and iii) low-methylation epigenotypes. High-methylation epigenotype, with methylation of both group 1 and group 2 markers, correlates to BRAF-mutation(+). Intermediate-methylation epigenotype, with methylation of group 2 markers, but not group 1, correlates to KRAS-mutation(+). To gain insight into epigenotype development in colorectal carcinogenesis, especially intermediate-methylation epigenotype and its correlation to KRAS-mutation(+) in adenoma, we analyzed methylation levels of group 1 and group 2 markers quantitatively by matrix assisted laser desorption ionization-time of flight mass spectrometry, in 51 adenomas, 13 aberrant crypt foci, and 26 normal mucosa samples, and we compared them to 149 previously analyzed colorectal cancer samples. Three serrated adenomas were all BRAF-mutation(+), showing great methylation of group 1 and group 2 markers, thus high-methylation epigenotype. Forty-eight conventional adenomas were not methylated in group 1 markers and were classified into two clusters with higher and lower methylation of group 2 markers, thus into intermediate- and low-methylation epigenotypes, respectively. Adenoma with intermediate-methylation epigenotype correlated to KRAS-mutation(+). Methylation levels of group 2 markers in adenoma were higher than aberrant crypt foci and normal samples, but similar to cancer. These data suggested that epigenotype development occur at an earlier stage than carcinoma formation, and already be completed at the adenoma stage. Intermediate methylation epigenotype and its correlation to KRAS-mutation(+) were developed in conventional adenoma.