DNA Repair and Damage Response Following Exposure of Cells to Alkylating Carcinogens

Abstract

Alkylating carcinogens are widely distributed in the environment and are present in food, beverages and tobacco. They are also endogenously formed in stomach and gut. These agents induce a dozen different DNA lesions, and some of them have been identified to be carcinogenic, clastogenic, recombinogenic and cytotoxic. A critical DNA adduct is O6-methylguanine (O6MeG). This damage causes mutations and is responsible for most of the carcinogenic effects of simple alkylating agents. At the same time, O6MeG is a highly powerful cytotoxic lesion, giving rise to the induction of apoptosis, necrosis and autophagy. The damage is repaired by the suicide enzyme alkyltransferase (MGMT), which is a very important first-line defense mechanism and biomarker of alkylating drug resistance, both in normal tissue and tumors (therefore it also plays a key role in tumor therapy). MGMT knockout mice respond to alkylating agent treatment with a high yield of colon cancer. The same is true for MPG ko mice defective in base excision repair, indicating that not only O6MeG, but also non-repaired N-alkylation lesions give rise to mutations and cancer. Elimination of pre-transformed cells by apoptosis counteracts this process. We have shown that O6MeG is a very powerful trigger of apoptosis, which is executed via the death receptor and the mitochondrial damage pathway. The apoptotic response is downstream, triggered by DNA double-strand breaks (DSB) that are formed during the mismatch repair dependent processing of O6MeG. These O6MeG-induced DSBs are repaired by homologous recombination (HR), which is a second-line defense against O6MeG triggered cell death. Other players involved in DSB recognition and HR are NBS-1, ATM, Rad51, XRCC2 and XRCC3. In some cell types, the efficiency of O6MeG to trigger the p53 dependent death receptor pathway is higher than the p53 independent endogenous mitochondrial pathway, which rests on p53 driven death receptor upregulation. However, p53 is also able to upregulate DNA repair genes thus protecting against mutations and cell death. The implications for human defense against environmental carcinogens will be discussed. Work was supported by DFG KA724.