Immunohistochemical pattern of MLH1/MSH2 expression is related to clinical and pathological features in colorectal adenocarcinomas with microsatellite instability.

Detection of colorectal carcinomas with high-frequency microsatellite instability (MSI-H) is clinically important for several reasons. Recent studies suggested that immunohistochemical analysis of MLH1 and MSH2 expression is a rapid and accurate method for identifying large bowel tumors of the MSI-H phenotype. In this study, we evaluated by immunohistochemistry MLH1 and MSH2 protein expression in 132 MSI-H, 23 MSI-L (low-frequency MSI), and 150 microsatellite stable (MSS) colorectal adenocarcinomas. Loss of MLH1 or MSH2 expression was detected in 120 (90.9%) MSI-H carcinomas, whereas all MSI-L and MSS tumors showed normal expression of both proteins. Lack of MLH1 nuclear staining was observed much more often than absence of MSH2 nuclear staining (106 and 14 cases, respectively). Among MSI-H carcinomas, MLH1/MSH2 pattern of expression was significantly related to several clinical and pathological variables. In particular, MSI-H MLH1/MSH2-positive carcinomas were more often located in th...