Transfectant CHO cells expressing O6-alkylguanine-DNA-alkyltransferase display increased resistance to DNA damage other than O6-guanine alkylation

BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea, Carmustine] is anitrosourea that crosslinks DNA and is useful in cancer chemotherapy.Tumor cells resistant to BCNU produce high levels of O⁶-alkylguanine-DNA-alkltransferase(AT), a protein that removes the O⁶-guanine adduct formed byBCNU prior to crosslinking. By the transfection of a human cosmidlibrary into the Chinese hamster ovary cell line AA8, severaltransgenic cell lines which express the AT gene have been constructed.These ‘BR’ cells were isolated on the basis of theirresistance to G-418 and BCNU. Like human mer⁺ strains, BR cells(relative to the parental AA8 cells) are {small tilde}500 timesmore resistant to the cytotoxic effects of 80 µM BCNU.Treatment with exogenous O⁶-methylguanine (O⁶MG), which depletescellular AT, abolishes their BCNU resistance. Also consistentwith the mer⁺ phenotype, BR cells are resistant to the mutagenicand killing activity of N-methyl-N'-nitro-N-nitrosoguanidine(MNNG). Treatment with exogenous O⁶MG, while reversing the resistanceto MNNG mutation, does not reverse the resistance to MNNG killing.Unexpectedly, BR cells also exhibit resistance to killing bydimethylsulfate (DMS). The BR cells are not, however, detectablyresistant to UV light. These results suggest that AT activityin mammalian cells is dosely linked to the activity of otherDNA repair pathways.