BRAF Mutation in Colorectal Rhabdoid and Poorly Differentiated Medullary Carcinomas

Colorectal rhabdoid carcinomas (CRbCs) are very rare and aggressive cancers. The BRAF mutation and CpG island methylator phenotype have been reported to be common features of CRbCs. This study reviews the literature about CRbCs and analyzes the clinicopathological and molecular profiles of seven CRbCs characterized by large discohesive cells with abundant eosinophilic cytoplasm, showing hyaline inclusions and large rounded to bean-shaped nuclei. For comparison, we included four poorly differentiated medullary carcinomas (PDMCs) with focal aspects mimicking rhabdoid features. Overall survival was poor in both subsets, with 78% of patients dying of disease within 2–11 months. The main features of CRbCs were: Loss of/reduced SMARCB1/INI expression, intense vimentin immunostaining, and dense neutrophilic infiltration. The PDMCs were positive for pancytokeratin but negative for vimentin and showed moderate peritumoral/intratumoral CD8+ lymphocytes. All PDMCs showed SMARCB1(INI-1) expression. The coexistence of BRAF and TP53 mutations was observed in 80% of CRbCs and PDMCs. PDMCs always showed microsatellite instability and CpG island methylator phenotype (CIMP), while CRbCs were CIMP negative and exhibited microsatellite instability (MSI) in two out of seven cases. CRbCs are characterized by BRAF and TP53 mutations. Loss/reduced expression of nuclear SMARCB1/INI, intense vimentin immunostaining, dense neutrophilic infiltration, and low frequency of CIMP are useful markers to recognize these rare aggressive tumors.