Dose-dependent differences in the profile of mutations induced by an ultimate carcinogen from benzo[a]pyrene

Mutations in the coding region of the hypoxanthine (guanine) phosphoribosyltransferase (HPRT) gene of Chinese hamster V-79 cells were examined after exposure of the cells to a high cytotoxic dose (0.48 μM; 35% survival) and a low noncytotoxic dose (0.04 μM; 100% survival) of the ultimate carcinogen (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-BPDE]. Independent 8-azagnanine-resistant colonies were isolated and cDNAs were prepared by reverse transcription. The coding region of the cDNA of the HPRT gene was amplified by the polymerase chain reaction and sequenced. An examination of the DNA base sequence changes induced by different doses of (+)-BPDE demonstrated that the high dose of (+)-BPDE caused base substitution mutations almost exclusively at G·C base pairs whereas the low dose of (+)-BPDE caused mutations at both G·C and A·T base pairs. Thus, use of a low dose of (+)-BPDE allowed the detection of mutations (at A·T base pairs) that were not readily observed with a high dose of (+)-BPDE. The data also suggest that the low dose of (+)-BPDE may have caused a different profile of base substitutions at G·C base pairs and exon deletions than the high dose. The results indicate dose-dependent differences in the profile of mutations for an ultimate carcinogen.