Purpose: Epigenetic silencing of the DNA mismatch repair genes has been poorly described in colorectal carcinomas showing the classic adenoma-carcinoma pathway of carcinogenesis. The aim of this study was to investigate the methylation status of MutL homolog 1 (hMLH1), MutS homolog 2 (hMSH2), and O-6-methylguanine-DNA methyltransferase (MGMT) in a series of colorectal carcinomas that contain both adenomas and carcinomas.
Methods: Promoter methylation of hMLH1, hMSH2, and MGMT was evaluated in normal mucosa, adenoma, and carcinoma samples from 112 colorectal cancer patients. Methylation was assessed by bisulfite modification and methylation-specific PCR. Expression of the gene products was also examined by immunohistochemistry.
Results: Of the 112 adenomas, methylation was detected for hMLH1 (2, 1.8%), hMSH2 (9, 8.0%), and MGMT (38, 33.9%). In the carcinoma samples, methylation was seen in hMLH1 (2, 1.8%), hMSH2 (15, 13.4%), and MGMT (53, 47.3%). In normal mucosa, hMSH2 (6, 5.4%) and MGMT (12, 10.7%) were methylated, whereas hMLH1 was not. Immunohistochemical analysis revealed abnormal hMLH1 (14, 12.5%), hMSH2 (11, 9.8%), and MGMT (53, 47.3%) expression with a significant correlation between aberrant MGMT methylation and a loss of MGMT expression.
Conclusions: These data suggest that CpG island methylation in hMSH2 and MGMT, but not hMLH1, is closely related to carcinogenesis in colorectal carcinomas presenting with a conventional adenoma-carcinoma sequence. Therefore, the detection of hMSH2 and MGMT methylation may have clinical significance in the evaluation of colon cancer patients and in tumor-specific management of the disease.