Alkylation damage, DNA repair and mutagenesis in human cells.

17 human cell lines that differ significantly in level of O6-alkylguanine-DNA alkyltransferase (AGT) activity were identified by comparing their sensitivity to the cytotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and determining the level of AGT activity in cell extracts from the various lines by measuring the decrease in radiolabeled O6-methylguanine from DNA, using high-performance liquid chromatography. 9 lines exhibited high levels of AGT activity, 2 showed an intermediate level (25-50% of the mean of those with the higher levels), and 6 exhibited very low or virtually undetectable levels of AGT. Included were several lines that are very deficient in capacity for nucleotide excision repair. When representatives from the 3 categories of cell lines defined by the level of AGT activity were compared for sensitivity to the cytotoxic and mutagenic effect of MNNG, they showed an inverse correlation between the degree of cell killing and frequency of mutants induced and the level of AGT activity. The cells' capacity for nucleotide excision repair did not affect these results. Exposure of cells with a high level of AGT activity to O6-methylguanine in the medium reduced the AGT activity 60-80%. These pre-treated cells exhibited a significantly higher frequency of MNNG-induced mutants than did cells that were not pre-treated, suggesting that the O6-methylguanine lesion in DNA is responsible for a significant proportion of the mutations induced. Cell strains containing substrates for assaying intrachromosomal homologous recombination were constructed using parental cell lines from each of the 3 categories of AGT activity. These strains showed an inverse correlation between the level of AGT activity and the frequency of MNNG-induced recombination. When various cell lines representing the 3 categories of AGT activity were compared for sensitivity to ethylnitrosourea, the results were consistent with AGT and nucleotide excision repair playing a role in preventing cell killing and mutation induction by this agent.