Aberrant pattern of the cytokeratin 7/cytokeratin 20 immunophenotype in colorectal adenocarcinomas with BRAF mutations.

Cytokeratin 7 (CK7) and 20 (CK20) are used for the differential diagnosis of metastases from colorectal carcinomas (CRC), which are usually CK7-/CK20+, and other tumors. In our study, we performed immunohistochemical staining with CK7 and CK20 in 52 randomly selected cases of CRC and analyzed microsatellite instability status and BRAF mutations to identify those factors that may determine the changing pattern of CK7/CK20 immunophenotype in these tumors. CK7 was negative in all microsatellite stable tumors (MSS), but all carcinomas presenting microsatellite instability (MSI) and BRAF mutations were diffusely positive for this marker. CK20 was diffusely expressed in 79.06% of MSS tumors. Regarding MSI, in case with no BRAF mutations, a progressive decrease in CK20 expression was noted, and in BRAF-mutated adenocarcinomas, no expression of CK20 was observed. It seems that in case of MSI located on the proximal colon, which also presents BRAF mutations, CK20/CK7 may present a changing immunophenotype pattern, which may complicate the differential diagnosis of metastatic tumors. This is the first reported study of the relationship between CK20/CK7 immunophenotype, BRAF mutations and microsatellite status in CRC.