Analysis of MLH1 promoter methylation in colorectal carcinomas with microsatellite instability

Background Microsatellite analysis and immunohistochemistry for DNA mismatch repair proteins (MMRPs) demonstrated great utility in the identification of Lynch syndrome. However, the majority of MMR-deficient colorectal carcinomas are sporadic and produced by hypermethylation of the MLH1 promoter. The aim of our study was to evaluate the role of MLH1 promoter methylation analysis in the distinction between MLH1- negative sporadic and hereditary carcinomas. Methods The study included 370 colorectal adenocarcinomas. Microsatellite analysis was performed using the five markers of Bethesda plus BAT40 and a fluorescence based PCR method. MMRPs expression (MLH1, MSH2, MSH6, PMS2) was evaluated by immunohistochemistry. MLH1 promoter methylation (C-region, proximal relative to the transcription start of the MLH1 gene) was determined by methylation-specific PCR. Results MLH1 promoter methylation was detected in 198 of 272 (72.8%) MSI-H carcinomas, whereas all the 98 MSS/MSI-L tumors analyzed were unmethylated. Among MSI-H tumors, MLH1 methylation was found in 196/222 (88.3%) MLH1-negative carcinomas and in 2 of 50 (4%) MLH1-positive carcinomas (p<0.001). MLH1-negative tumors of patients aging <56 years were less frequently methylated (8/14, 57.1%) than tumors of patients aging 56–70 years (46/55, 83.6%) and of patients older than 70 years (142/153, 92.8%) (p<0.001). Conclusions Our data confirm that MLH1 promoter methylation is the major mechanism leading to MSI-H in colorectal cancer. Analysis of MLH1 methylation in conjunction with clinical data and MMRPs expression pattern may be relevant in the selection of patients with suspected Lynch syndrome for genetic testing.