Repair of DNA lesion O6-methylguanine in hepatocellular carcinogenesis

Evidence from both experimental carcinogenesis and studies in human cirrhotic liver suggest that defective repair of the promutagenic DNA base lesion, O6-methylguanine, is a factor in the multistep process of hepatocellular carcinogenesis. Ubiquitous environmental alkylating agents such as N-nitroso compounds can produce O6-methylguanine in cellular DNA. Unrepaired, O6-methylguanine can lead to the formation of G --> A transition mutations, a known mechanism of human oncogene activation and tumour suppressor gene inactivation. Combined treatment of rodents with an agent producing O6-methylguanine in DNA, and an agent promoting cell proliferation, leads to development of hepatic nodules and hepatocellular carcinoma (HCC), cell division, hence DNA replication, being required for the propagation of tumorigenic mutation(s) in hepatocyte DNA. The paramount importance of O6-methylguanine in hepatocellular carcinogenesis is indicated by the observation that transgenic mice engineered to have increased hepatic levels of repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) are significantly less prone to hepatocellular carcinogenesis following alkylating agent treatment. Cirrhosis is a universal risk factor for development of human HCC, and a condition that is characterized by increased hepatocyte proliferation as a result of tissue regeneration. Levels of the human repairing enzyme for O6-methylguanine were found to be significantly lower in cirrhotic liver than in normal tissue. In accord with findings from animal models, this suggested a mechanism in which persistence of O6-methylguanine due to defective DNA repair by MGMT, together with increased hepatocyte proliferation, might lead to specific gene mutation(s) and hepatocellular carcinogenesis. Screening for the presence and persistence of O6-methylguanine in human DNA presently involves formidable technical difficulty. Indications are that such limitations might be overcome by the use of an ultrasensitive method such as immuno-polymerase chain reaction (PCR). This approach should allow parallel measurement of DNA adduct and repair enzyme in routine liver biopsy samples. It might also enable investigation of O6-methylguanine in human genes specifically associated with hepatocellular carcinogenesis. Given the wide variation in human MGMT levels observed between individuals, tissues, and cells, this technology should be adapted to permit the ultrasensitive localisation and measurement of adducts and repairing enzyme in liver biopsy tissue sections. Ability to ultrasensitively measure O6-methylguanine, and its repair enzyme, should prove valuable in the risk assessment of cirrhotic patients for developing hepatocellular carcinoma.