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 hpr t locus for both normal (AA8) and 4NQO-sensitive (UV5) Chinese hamster ovary cells were determined to investigate the effect of DNA repair o n the nature of induced mutations. The UV5 cell line is three times mo re sensitive to 4NQO than the AA8 parental cell line. In UV5 cells, th e dGuo-N2 - AQO adduct, which is considered to be the most toxic and m utagenic 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 c DNA. Both similarities and differences emerged. In both cell lines we found that (i) 4NQO is basically a base substitution mutagen acting al most 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 lesion s localized on the non-transcribed strand, suggesting preferential rep air of the transcribed strand. Compared with AA8, the proportion of mu tants due to lesions present on the transcribed strand was higher in U V5 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.