Analysis of 4-nitroquinoline-1-oxide induced mutations at the hprt locus in mammalian cells: possible involvement of preferential DNA repair.

Mutation spectra induced by 4-nitroquinoline 1-oxide (4NQO) at the hprt locus for both normal (AA8) and 4NQO-sensitive (UV5) Chinese hamster ovary cells were determined to investigate the effect of DNA repair on the nature of induced mutations. The UV5 cell line is three times more sensitive to 4NQO than the AA8 parental cell line. In UV5 cells, the dGuo-N2-AQO adduct, which is considered to be the most toxic and mutagenic adduct in Escherichia coli, is poorly repaired. The molecular nature of 30 hprt mutants isolated from AA8 and 20 isolated from UV5 cells was determined by sequence analysis of in vitro amplified hprt cDNA. Both similarities and differences emerged. In both cell lines we found that (i) 4NQO is basically a base substitution mutagen acting almost exclusively at G residues and (ii) G transversions are prevalent over G transitions in both cell lines, independently from the ability to repair dGuo-N2-AQO. A high proportion (13/25) of splice mutations was observed in AA8 cells, statistically different (P < 0.04, Fisher's exact test) from the incidence of splice mutants in UV5 cells (4/20). In AA8 mutants, all but two of the point mutations were due to lesions localized on the non-transcribed strand, suggesting preferential repair of the transcribed strand. Compared with AA8, the proportion of mutants due to lesions present on the transcribed strand was higher in UV5 cells, as expected if a preferential repair mechanism was impaired in the sensitive cell line. Our data are consistent with the molecular defect in DNA repair recently characterized in UV5.