Mutagenic selectivity at the HPRT locus in V-79 cells: comparison of mutations caused by bay-region benzo[a]pyrene 7,8-diol-9,10-epoxide enantiomers with high and low carcinogenic activity

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Abstract
Earlier studies from our laboratories characterized the mutation profile of the optically active (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-BPDE—the ultimate carcinogenic metabolite of benzo[a]pyrene] in the coding region of the hypoxanthine (guanine) phosphoribosyltransferase (HPRT) gene of Chinese hamster V-79 cells. In the present study, we evaluated the mutation profile of (−)-7S,8R-dihydroxy-9R, 10S-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(−)-BPDE—a weakly carcinogenic or inactive enantiomer] and compared its mutation profile with that of (+)-BPDE. In both diol epoxide enantiomers, the benzylic 7-hydroxy group and epoxide oxygen are trans. The mutation frequency for V-79 cells treated with DMSO vehicle or with a low, non-cytotoxic dose (0.5 μM) or a high cytotoxic dose (2.0 μM) of (−)-BPDE was 1, 25 or 185 8-azaguanine-resistant colonies/105 survivors, respectively. Independent 8-azaguanine-resistant clones were isolated, and complementary DNAs were prepared by reverse transcription. The coding region of the HPRT gene was amplified by the polymerase chain reaction and sequenced. Altogether, 92 (−)-BPDE-induced mutant clones were examined. At both doses, base substitutions were the most prevalent mutations observed (present in −;70% of the mutant clones), followed by exon deletions (present in −22% of the mutant clones) and frame shift mutations (present in −6% of the mutant clones) in the cDNAs analyzed. At the high cytotoxic dose, 5 out of 36 base substitutions occurred at AT base pairs (14%) and 31 at GC base pairs (86%). At the low, non-cytotoxic dose, 7 out of 34 base substitutions were at AT base pairs (21%) and 27 were at GC base pairs (79%). Although there was a trend towards an increase in the proportion of mutations at AT base pairs when the dose of (−)-BPDE was decreased, this trend was not statistically significant. The data also indicated no dose-dependent differences in the kinds of base substitutions or exon deletions in cDNAs induced by (−)-BPDE. Ninety-one per cent of the (−)-BPDE-induced mutations that occurred at guanine were on the non-transcribed strand of DNA and 9% were on the transcribed strand. In contrast to these results, 50% of the (−)-BPDE-induced mutations that occurred at adenine were on the transcribed strand and 50− on the non-transcribed strand. A comparison of the mutation profiles of (−)- and (+)-BPDE revealed that (i) (+)-BPDE is more mutagenic than (−)-BPDE, (ii) (+)-BPDE caused a significantly higher proportion of mutations at AT base pairs as the dose decreased, but this was not observed for (−)-BPDE, (iii) (+)-BPDE showed high selectivity for GC → TA transversions, but considerably less selectivity was observed for (−)-BPDE, and (iv) (−)-BPDE had different hot spots for base substitutions than did (+)-BPDE.

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