DNA Repair of Alkylation Damage and Its Relevance to Mutation Fixation in Mammalian Cells
Simple monofunctional alkylating agents such as Nmethyl-N’-nitro-N-nitrosoguanidine (MNNG) and N-ethyl-Nnitrosourea (ENU) act directly on oxygen and ring nitrogen atoms of DNA bases, and on oxygen atoms of the phosphate groups to generate a variety of adducts. Two major pathways for the repair of alkylation damage have been identified. The removal of O6-methylguanine (O6-meG) adducts proceeds via the transfer of the alkyl group to a cysteine residue of an acceptor protein, O6-alkylguanine-DNA methyltransferase (MT), which thereby becomes irreversibly inactivated 1,2. Other alkyl adducts are excised through the action of DNA glycosylases which hydrolyze the N-glycosylic bond linking the damaged base to the sugar phosphate backbone. The pathways for the repair of alkylation damage in mammalian cells have not been characterized to the same extent as in bacteria, although the basic strategies employed are the same in prokaryotes and eukaryotes. For example, an alkyltransferase which transfers in a stoichiometric fashion the methyl group from the oxygen 6 of guanine to a cysteine thiol in the protein has been identified3,4 along with a glycosylase activity capable of excising alkyl groups from nitrogen 3 of adenine and guanine and from nitrogen 7 of guanine5. Among the lesions produced by alkylating agents considerable evidence suggests that O6 -alkylguanine (O6 -alkG) is a major premutagenic lesion responsible for carcinogenicity and mutagenicity. Accumulation and persistence of O6 -meG and O6 -ethylguanine (O6 -etG) in target organs for carcinogenesis by alkylating agents has been demonstrated6,7. Comparative mutagenicity studies using a series of human fibroblast cell lines that differ in their levels of MT have shown a low mutagenicity in lines with high capacity to remove O6 -alkG8. In contrast some reports suggest that other lesions may be implicated as well in the mutagenic process9. Good candidates are the Oalkylpyrimidines (O4 -alkylthymine and O2 -alkylthymine)10.
KeywordsChinese Hamster Ovary Cell Methyl Transferase Cysteine Thiol Methylating Agent Sugar Phosphate Backbone
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