Serrated adenoma of the stomach: a clinicopathologic, immunohistochemical, and molecular study of nine cases.

Gastric serrated adenoma is a recently recognized entity that has been rarely described and poorly characterized. To examine whether gastric serrated adenoma shares the same immunophenotypic and molecular features of its colorectal traditional serrated adenoma, the clinicopathologic features, expression of mucin proteins (MUC2, MUC5AC, CD10, MUC6) and mismatch repair protein (MLH1), and mutations of BRAF and KRAS genes were studied. The nine serrated adenomas were obtained from five men and four women, with a mean age of 67 years. Seven (78%) serrated adenomas were located in the body of the stomach. The endoscopic findings were not sufficiently characteristic to diagnose serrated adenoma or serrated adenocarcinoma; however, most were elevated lesions. The initial biopsy material was available in all cases and the serrated features were evident in 6 cases diagnosed as adenoma. Among the nine cases, seven (78%) were associated with invasive adenocarcinoma within the serrated adenoma. MUC5AC was expressed in 6 serrated adenomas (67%). Expression of MUC5AC was observed in all tumors located in the lower third of the stomach. Focal MUC6 expression was observed in the basal part of two serrated adenomas. MLH1 expression was lost in two cases (22%). KRAS mutations were observed in three cases (33%) while BRAF mutations were not detected in any of the cases. Gastric serrated adenoma does not completely share the same immunophenotypic and molecular features of its colorectal counterpart. Gastric serrated adenomas are frequently associated with adenocarcinoma. When serrated adenoma is encountered in a gastric biopsy specimen, the possibility of associated adenocarcinoma should be considered in the adjacent stomach.