While sporadic colorectal cancer (CRC) is classified into several molecular subtypes, stratification of familial colorectal tumors is yet to be well investigated. We previously established two groups of methylation markers through genome-wide DNA methylation analysis, which classified sporadic CRC and adenoma into three distinct subgroups: high-, intermediate-, and low-methylation epigenotypes. Here, we investigated familial adenomatous polyposis (FAP), through quantitative methylation analysis of 127 samples (16 cancers, 96 adenomas, and 15 benign mucosa from 14 patients with FAP) using six Group-1 and 14 Group-2 methylation markers, APC, BRAF, and KRAS mutation analysis, and CTNNB1 and TP53 immunohistochemical analysis. All the 14 patients presented with APC germline mutation. Three were from the same family and presented the same APC mutation. FAP tumors lacked BRAF-mutation(+) high-methylation epigenotype and were classified into two methylation epigenotypes. While 24 of 112 tumor samples showed intermediate-methylation epigenotype significantly correlating with KRAS-mutation(+) (P=3×10-4), 88 tumor samples showed low-methylation epigenotype correlating with the absence of KRAS- and BRAF-mutations. Similar to sporadic CRC, CTNNB1 was frequently activated at the adenoma stage, and TP53 mutation occurred during cancer development from adenoma. Whereas some patients showed a single epigenotype in all tumors throughout the colon, tumors with two distinct epigenotypes developed within a family with the same APC mutation or even within one patient. Methylation accumulation significantly correlated with proximal location and older age. These results indicate that there are at least two distinct molecular subtypes of FAP tumors, resembling sporadic CRC and independent from the APC germline mutation status.
Keywords: CIMP; DNA methylation; KRAS; colorectal cancer; familial adenomatous polyposis (FAP).