LacI mutation spectra following benzo[a]pyrene treatment of Big Blue (R) mice

The mutation spectrum of the lad gene from the liver of C57B16 Big Blue(R) transgenic mice treated with benzo[a]pyrene (B[a]P) has been compared with the spectrum of spontaneous mutations observed in the liver of untreated Bi g Blue(R) mice, Mice were treated with B[a]P for 3 days followed by a parti al hepatectomy one day after the last injection. Liver tissue was removed f or analysis at hepatectomy and, again, 3 days later at the time of sacrific e. Earlier, we reported that the lacI mutant frequency in these B[a]P-treat ed mice was elevated in the liver both at the time of hepatectomy and at sa crifice; however, a statistically significant increase in the mutant freque ncy was observed only at sacrifice. In this study, the DNA sequence spectra of lad mutations observed in the liver of B[a]P-treated Big Blue(R) mice a t hepatectomy and at time of sacrifice were compared with each other and wi th the spectrum of spontaneous liver mutations. No differences were observe d between the two B[a]P-treatment spectra, However, mutation frequencies of both GC->TA and GC->CG at the time of hepatectomy and at sacrifice were si gnificantly elevated compared with the spontaneous frequency of these same transversions, Also, the frequency of AT->TA transversions was significantl y higher than the spontaneous frequency at the time of hepatectomy but not at sacrifice. The frequency of all other classes of mutations scored was no t significantly different from the frequency of these same events in the sp ontaneous spectra, These data support the view that B[a]P treatment results in the induction of GC->TA and GC->CG transversions within 1 day of the la st injection and they provide insights regarding the relative roles of benz o[a]pyrene-7,8-diol-9,10-epoxide and radical cations of B[a]P in B[a]P-indu ced mutagenesis in vivo. Finally, these data provide evidence for B[a]P-ind uced mutagenesis under conditions where no statistical increase in mutant f requency could be shown.