Evaluation of Microsatellite Instability in Colorectal Adenomas and Carcinomas by Immunohistochemistry and a Comparison of Histopathological Features

Objective: Approximately 15% of sporadic colorectal carcinomas (CRCs) develop along the microsatellite instability (MSI) pathway. In this study, we compared the MLH1, MSH2, Ki-67, and p53 immunostaining properties with histopathological features of colorectal adenomas and CRCs. Methods: A total of 102 cases were selected, including 50 adenomatous polyps, 25 adenocarcinomas, 10 adenocarcinomas with mucinous component, 14 mucinous adenocarcinomas, and three signet-ring cell carcinomas. The tissues were stained for MLH1, MSH2, p53, and Ki-67 primary antibodies. Results: Negative staining was observed for MLH1 in 25% and MSH2 in 3.8% of all CRC cases. Compared with adenocarcinoma not otherwise specified (NOS), mucinous adenocarcinomas showed weaker staining for MLH1, which was statistically significant. There was also a statistically significant difference between adenocarcinoma NOS and signet-ring cell carcinomas in terms of negative staining for MLH1. A total of 69.2% of the MLH1-negative cases were high-grade. There was a statistically significant relationship between the histological grade and MLH1 negativity. A positive correlation was found between the grade of dysplasia and p53 staining intensity and Ki-67 proliferation index. No negative staining was observed for MLH1 and MSH2 in any of the adenomatous polyps. Conclusion: For the histopathological examination of CRCs, in the presence of mucinous and poorly differentiated morphology, tumor-infiltrating lymphocytes and Crohn-like inflammatory response, immunohistochemical staining for MLH1, and MSH2 antibodies may be useful in the detection of tumors showing MSI.