Mismatch repair proteins immunohistochemical null phenotype in colon medullary carcinoma

Introduction In mismatch repair (MMR) immunohistochemistry, four MMR proteins’ staining pattern reveals which particular gene may be defective. However, in the null phenotype, four MMR proteins are lost; consequently, it will be challenging to assume the target gene by immunohistochemistry and to determine whether deficient MMR was sporadic or germline. Case report A 70-year-old man underwent right hemicolectomy with the diagnosis of ascending colon cancer. The postoperative histopathology revealed the diagnosis of medullary carcinoma and the loss of all four MMR expressions in immunohistochemistry. The mutation analysis using a peripheral blood sample showed no germline mutations in the four genes. Discussion This clinical case presents an unusual colon carcinoma that showed a MMR protein immunohistochemistry null phenotype. The cause of expression loss of MMR proteins can be explained by the loss of MLH1 and MSH2 functions associated with somatic loss of function mutations, functional loss in all four MMR proteins associated with somatic loss of function mutations, or Lynch-like syndrome. Correct interpretation and accumulation of relevant cases are necessary to unveil unusual cases in the era of universal screening.