Genetic evidence for nucleotide excision repair of O6-alkylguanine in mammalian cells.

Human cells that lack O6-alkylguanine DNA alkyltransferase (AT) activity can remove O6-butylguanine (O6-nBuG) produced in cellular DNA by exposure to N-n-butyl-N-nitrosourea as determined by radioimmunoassay of enzyme digests of DNA. Fibroblasts from xeroderma pigmentosum (XP) complementation groups A and G that show less than 5% unscheduled DNA synthesis following exposure to UVC failed to remove O6-nBuG. Hence it appears that O6-alkylguanine is repaired in cells that lack AT by a process that is defective in XP cells, presumably nucleotide excision repair. Neither V79 nor V79/79 Chinese hamster cell lines have AT activity and both are able to remove O6-nBuG from DNA. However, only V79/79 is able to remove O6MeG, suggesting some substrate specificity of the excision repair process. Comparison of relative levels of O6-alkylation by N-methyl-, N-ethyl-, N-propyl- and N-n-butyl-nitrosourea indicate that approximately equal levels of O6-alkylation are produced by equitoxic doses of these agents.