MLH1-deficient Colorectal Carcinoma With Wild-type BRAF and MLH1 Promoter Hypermethylation Harbor KRAS Mutations and Arise From Conventional Adenomas.

Between 10% and 15% of colorectal carcinomas demonstrate sporadic DNA mismatch-repair protein deficiency as a result of MLH1 promoter methylation and are thought to arise from sessile serrated adenomas, termed the serrated neoplasia pathway. Although the presence of the BRAF V600E mutation is indicative of a sporadic cancer, up to 30% to 50% of colorectal carcinomas with MLH1 promoter hypermethylation will lack a BRAF mutation. We report the clinicopathologic and molecular features of MLH1-deficient colorectal carcinoma with wild-type BRAF and MLH1 promoter hypermethylation (referred to as MLH1-hypermethylated BRAF wild-type colorectal carcinoma, n=36) in comparison with MLH1-deficient BRAF-mutated colorectal carcinoma (n=113) and Lynch syndrome-associated colorectal carcinoma (n=36). KRAS mutations were identified in 31% of MLH1-hypermethylated BRAF wild-type colorectal carcinomas compared with 0% of MLH1-deficient BRAF-mutated colorectal carcinomas and 37% of Lynch syndrome-associated colorectal carcinomas. When a precursor polyp was identified, MLH1-hypermethylated BRAF wild-type colorectal carcinomas arose from precursor polyps resembling conventional tubular/tubulovillous adenomas in contrast to MLH1-deficient BRAF-mutated colorectal carcinomas, which arose from precursor sessile serrated adenomas (P<0.001). Both MLH1-hypermethylated BRAF wild-type colorectal carcinoma and MLH1-deficient BRAF-mutated colorectal carcinoma had a predilection for the right colon compared with Lynch syndrome-associated colorectal carcinoma (86% vs. 92% vs. 49%, P<0.001). There was no significant difference in mucinous differentiation, tumor-infiltrating lymphocytes, Crohn-like reaction, and medullary differentiation between the 3 tumor groups. Using Kaplan-Meier survival functions, there was no significant difference in disease-specific survival between the 3 patient groups (P>0.05). In conclusion, our results indicate that MLH1-hypermethylated BRAF wild-type colorectal carcinomas can harbor KRAS mutations and arise from precursor polyps resembling conventional tubular/tubulovillous adenomas.