title: Enhanced in vivo repair of O(4)-methylthymine by a mutant human DNA alkyltransferase.
abstract: The repair of O(6)-methylguanine (m(6)G) by human O(6)-alkylguanine-DNA alkyltransferase (hAGT) is approximately 5000-fold greater than that for O(4)-methylthymine (m(4)T). To evaluate each adduct's contribution to mutagenesis, we previously created a mutant hAGT with increased specificity for m(4)T in vitro. The mutant and wild-type (WT) hAGT have now been expressed in bacterial strains that allow for the specific detection of A:T-->G:C and G:C-->A:T mutations induced by m(4)T and m(6)G, respectively. After exposure to the mutagenic methylating agent, N-methyl-N'-nitro-N-nitrosoguanidine, A:T-->G:C substitutions were reduced >4-fold in cells expressing the mutant hAGT compared with 1. 1-fold for WT hAGT. G:C-->A:T substitutions were decreased >2.5-fold in cells expressing the mutant hAGT, whereas WT hAGT totally prevented G:C-->A:T mutations. These results demonstrate that the altered substrate specificity of hAGT observed in vitro also occurs in vivo, and that it is responsible for the observed differences in mutations.
authors.affiliation: The Joseph Gottstein Memorial Cancer Research Laboratory, Departments of Pathology and Biochemistry, University of Washington School of Medicine, Seattle, WA 98195-7705 USA.
authors.forename: L P
authors.initials: LP
authors.lastname: Encell
authors.forename: L A
authors.initials: LA
authors.lastname: Loeb
mesh_terms: D004249:DNA Damage; D004260:DNA Repair / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D007763:Lac Operon; D008769:Methylnitronitrosoguanidine / Q000633:toxicity; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism*; D010641:Phenotype; D017354:Point Mutation; D013379:Substrate Specificity; D013941:Thymine / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; C064976:O(6)-benzylguanine; D008769:Methylnitronitrosoguanidine; C021223:O-4-methylthymine; D006147:Guanine; C008449:O-(6)-methylguanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D013941:Thymine