Methylation pattern of different regions of the MLH1 promoter and silencing of gene expression in hereditary and sporadic colorectal cancer

Nonrandom, widespread promoter methylation of tumor suppressor genes is a c ommon mechanism of gene inactivation during tumorigenesis. We examined the methylation status of two distinct regions of the MLH1 promoter (proximal a nd distal to the transcription start sire) and the MLH1 gene expression by methylation-specific PCR and immunohistochemistry. A total of 72 colorectal tumors, both with (n = 51, 22 affected by hereditary nonpolyposis colorect al cancer, HNPCC, defined according to the international clinical criteria and 29 sporadic cases) and without microsatellite instability (MS1) (n = 21 ) were studied. Methylation was present in at least one of the two promoter regions in 86% of the sporadic MS1 cases, in 33% of the cases lacking MS1, and in 23% of the HNPCC tumors. In the HNPCC cases with a known MLH1 mutat ion (n = 10) none of the two promoter regions was methylated. Hypermethylat ion in both MLH1 promoter regions was seen in 45% of the MS1 sporadic cases vs. 5% of the MS1-negative cases and 0% of the HNPCC cases. The overall co ncordance between the two promoter regions regarding methylation status was good (P = 0.009), but no significant correlation between methylation and s uppression of the MLH1 immunohistochemical expression was found. Our data c onfirm that mutation and hypermethylation are mutually exclusive mechanisms in inducing mismatch repair deficiency and support the hypothesis of methy lation as a process evenly distributed along the different regions of the p romoter. (C) 2001 Wiley-Liss.