Inactivation of the DNA repair gene O⁶-methylguanine-DNA methyltransferase by promoter hypermethylation and its relationship to aflatoxin B₁-DNA adducts and p53 mutation in hepatocellular carcinoma

O⁶-methylguanine-DNA methyltransferase (MGMT) is a repair protein that specifically removes promutagenic alkyl groups from the O⁶ position of guanine in DNA. MGMT is transcriptionally silenced by promoter hypermethylation in several human cancers. Methylation-specific PCR (MSP) was used to analyze the MGMT promoter methylation status of 83 hepatocellular carcinomas (HCC) and 2 HCC cell lines (HepG2 and Hep3B). Hypermethylation was detected in 32 of 83 (39%) HCC tissues, but it was not found in either HCC cell line. We also analyzed MGMT expression by immunohistochemical analysis of HCC tissue samples. The presence of aberrant hypermethylation was associated with loss of MGMT protein. The relationship between methylation status and risk factors and tumor markers including environmental exposure to aflatoxin B₁ (AFB₁), measured as DNA adducts, and status of tumor suppressor gene p53 was also investigated. A statistically significant association was found between MGMT promoter hypermethylation and high level of AFB₁-DNA adducts in tumor tissues (OR = 5.05, 95% CI = 1.29-19.73). A significant association was also found between methylation and p53 mutation status (OR = 2.97, 95% CI = 1.09-8.11). These results suggest that epigenetic inactivation of MGMT plays an important role in the development of HCC and exposure to environmental carcinogens may be related to altered methylation of genes involved in cancer development. The role of chemical carcinogens in hypermethylation needs further investigation.