MGMT expression in colorectal cancer: relation to clinicopathologic and molecular features

Background: The O6-methylguanine-DNA-methyltransferase (MGMT) is a DNA repair protein that removes mutagenic and cytotoxic adducts from O6-guanine in DNA. MGMT inactivation by promoter methylation is thought to play a major role in the development of a subgroup of colorectal tumors. Aim of this study was to evaluate whether colorectal adenocarcinomas (CRCs) with MGMT inactivation display distinctive clinical, pathologic and molecular features. Methods: MGMT protein expression was evaluated by immunohistochemistry in a series of 196 surgically resected CRCs using the anti-MGMT monoclonal antibody MT3.1. Tumor DNA mismatch repair (MMR) status was determined by immunohistochemical evaluation of MLH1, MSH2 and MSH6 expression and microsatellite genotyping. In addition, MGMT and MLH1 promoter methylation were assessed in 57 tumors using methylation-specific PCR. Results: Loss of MGMT expression was detected in 44 (22.4%) of the 196 tumors examined. In most cases (40/44) a complete loss of MGMT immunoreactivity was observed. MGMT expression was closely related to MGMT methylation status. MGMT promoter methylation was observed in 11 of the 13 MGMT-negative (84.6%) and in 6 of the 44 MGMT-positive (13.6%) carcinomas examined (P< 0.001). MGMT-negative carcinomas occurred more frequently in the proximal colon (P= 0.02) and among patients older than 70 years (P= 0.01). No significant relationship was found with other clinical and pathologic features. MGMT expression was found to be related to the MMR status of the tumors. Infact, loss of MGMT reactivity was observed in 13 of 40 (32.5%) MMR-defective tumors and in 31 of 156 (19.9%) MMR- proficient carcinomas. Specifically, loss of MGMT expression occurred in 33 of 168 (19.6%) MLH1-positive carcinomas and in 11/28 (39.3%) MLH1-negative carcinomas (P= 0.02). To more properly define the relationship between MMR status and MGMT expression, we examined further 63 MSI-H colorectal adenocarcinomas and evaluated MLH1 promoter methylation in a subset of cases. Lack of MGMT expression was detected in 34/82 (41.5%) MSI-H MLH1-negative and in 1/21(4.7%) MSI-H MSH2 or MSH6-negative carcinomas (P= 0.01). A significant relationship between MGMT and MLH1 promoter methylation was observed in the 57 cases examined by methylation-specific PCR (P< 0.01). Conclusion: Our results indicate that MGMT- negative CRCs are characterized by certain clinical and molecular features. The role of MGMT expression in cancer recurrence and in disease-free and overall survival after curative surgery and chemotherapy needs to be carefully evaluated.