The Raf/MEK/ERK (MAPK) signal transduction cascade is an important mediator of a number of cellular fates including growth, proliferation and survival. The BRAF gene is activated by oncogenic Ras, leading to cooperative effects in cells responding to growth factor signals. Our study was performed to elucidate a possible function of BRAF in ulcerative colitis (UC)-related colorectal carcinogenesis. Mutations of BRAF and KRAS were determined in 33 UC-related colorectal cancers by direct DNA sequencing analyses after microdissection. Mismatch-repair deficiency was assessed by immunohistochemistry for major mismatch-repair proteins hMLH1, hMSH2 and hMSH6 and microsatellite analyses of the BAT25 and BAT26 loci. Hypermethylation of the hMLH1 promoter was also tested. The results obtained were correlated with histopathologic variables. Activating BRAF missense mutations were identified in 3/33 UC-related cancers (9%), 2 of which exhibited a loss of hMLH1-protein expression and hypermethylation of the hMLH1 promoter. Corresponding nondysplastic UC-mucosa of these patients did not show BRAF mutations. KRAS mutations were found in 6/33 (18%) UC cancers. All 6 UC cancers with KRAS mutations had an intact BRAF gene as the 3 cancers with BRAF mutations had an intact KRAS gene. There was no significant correlation between BRAF or KRAS status and clinicopathologic variables. Our data indicate that BRAF mutations are not an initiating event in UC-related carcinogenesis and are associated with mismatch-repair deficiency through hMLH1-promoter hypermethylation. Disruption of the Raf/MEK/ERK (MAPK) kinase pathway-either through RAS or BRAF mutation-was detected in 27% of all UC-related cancers and thus plays an important role in UC-related carcinogenesis.
(c) 2005 Wiley-Liss, Inc.