Histologic and molecular analyses of colonic perineurial-like proliferations in serrated polyps: Perineurial-like stromal proliferations are seen in sessile serrated adenomas

Colonic perineuriomas are recently described benign mucosal polyps that are composed of a bland spindle cell proliferation surrounding crypts that often demonstrate hyperplastic/serrated epithelial changes. However, the origin of this unique stromal proliferation is still unclear, and the association with serrated polyps, including sessile serrated adenomas, has not been fully described. We evaluated the pathologic and molecular features of colonic polyps associated with perineurial-like proliferations in 2 retrospective cohorts: (1) a series of 198 consecutive sessile serrated adenomas and (2) 20 colonic polyps diagnosed as a perineurioma irrespective of the presence of serrated colonic crypts. Thirteen of 198 (6.5%) sessile serrated adenomas demonstrated a perineurial-like stromal proliferation, with most (12 of 13, 92%) involving the right (9 cases) and transverse colon (3 cases). In all 13 cases, the perineurial-like proliferation surrounded serrated colonic crypts and typically involved only a small area of the sessile serrated adenoma (average 9% of polyp size; range, 2% to 19%). All 11 polyps evaluated for epithelial membrane antigen (EMA) expression demonstrated stromal EMA staining limited to the perineurial-like proliferation. Twelve of 13 (92%) sessile serrated adenomas with perineurial-like proliferations demonstrated a pV600E BRAF mutation. Of the 20 colonic polyps diagnosed as a perineurioma, 18 (90%) demonstrated serrated crypts intimately associated with the perineurial-like proliferation. In 13 of 18 polyps with associated serrated crypts, all serrated crypts were invested with the perineurial proliferation. In 5 cases, serrated crypts were seen away from the perineurial proliferation. Of these 18 polyps, the majority (16 of 18, 89%) were microvesicular hyperplastic polyps involving the left colon. However, 2 (11%) polyps in the right colon demonstrated histologic features diagnostic of sessile serrated adenoma. All 18 polyps with serrated crypts demonstrated a pV600E BRAF mutation. In contrast, the 2 polyps not associated with serrated crypts were negative for a BRAF mutation. Our results show for the first time that perineurial-like stromal proliferations frequently occur in sessile serrated adenomas. The presence of focal perineurial-like stromal proliferations in sessile serrated adenomas and the common finding of serrated crypts in colonic perineuriomas are likely indicative of an epithelial-stromal interaction, possibly related to some factor elaborated by the serrated epithelium.