Microsatellite instability in colorectal carcinoma: The comparison of immunohistochemistry and molecular biology suggests role for hMLH6 immunostaining

Microsatellite instability (MSI) due to defective mismatch repair (MMR) genes has been reported in the majority of colorectal tumors from patients with hereditary nonpolyposis colorectal cancer syndrome and in 10% to 15% of sporadic colorectal cancers. The identification of cancers associated with MSI requires classical molecular testing as the gold standard. The aim of this study was to evaluate the role of immunohistochemistry with antibodies directed against 4 MMR proteins as a screening tool for carcinomas with MSI. In this study, 204 formalin-fixed, paraffin-embedded colorectal carcinomas were examined for MMR protein expression (hMLH1, hMSH2, hMSH6, and hPMS2) and analyzed for MSI (MSI-H indicates at least 2 of 6 markers affected). These results were correlated with histopathologic parameters. Immunohistochemical analysis revealed that loss of expression of at least 1 protein was present in 17% of cases. One hundred percent of carcinomas that showed high instability (MSI-H) showed loss of expression of hMLH1, hMSH2, or hMSH6. Loss of expression of 2 proteins was present in 59.4% of MSI-H cases, with only 2 combinations, namely, hMLH1/hPMS2 and hMSH2/hMSH6. Isolated loss of hMSH6 expression was present in 2 MSI-H cases. These findings confirm that examination of MMR protein expression by immunohistochemistry is a simple method to diagnose colorectal cancer with MSI. Our data suggest that the study of hMSH6 may be useful, in addition to hMLH1 and hMSH2. Moreover, immunohistochemistry could represent a screening method with which to direct research on the mutations of MMR genes observed in hereditary nonpolyposis colorectal cancer syndrome.