Mutations induced by benzo[a]pyrene and dibenzo[a,l]pyrene in lacI transgenic B6C3F1 mouse lung result from stable DNA adducts.

Dibenzo[a,l]pyrene (DB[a,l]P) and benzo[a]pyrene (B[a]P) are carcinogenic polycyclic aromatic hydrocarbons (PAHs) that are each capable of forming a variety of covalent adducts with DNA. Some of the DNA adducts formed by these PAHs have been demonstrated to spontaneously depurinate, producing apurinic (AP) sites. The significance of the formation of AP sites as a key event in the production of mutations and tumours by PAHs has been a subject of ongoing investigations. Because cells have efficient and accurate mechanisms for repairing background levels of AP sites, the contribution of PAH-induced AP site mutagenesis is expected to be maximal in conditions where those induced AP sites are produced in significant excess of the endogenous AP sites. In this study, we investigated the effect of two dosing regimens on the mutagenicity of DB[a,l]P and B[a]P in vivo using the Big Blue(R) transgenic mouse system. We compared administration of a single highly tumorigenic dose of each PAH with a fractionated delivery of the same total dose administered over 5 days, with the expectation that PAH-induced AP sites would be produced at a greater margin above background levels in animals receiving the high single dose than in the animals receiving the fractionated doses. Treatment with DB[a,l]P yielded a 2.5-fold (single dose) to 3-fold (fractionated dose) increase in mutant frequencies relative to controls. Both single-dose and fractionated dose treatment regimens with B[a]P produced about a 15-fold increase in mutant frequencies compared to controls. The mutations induced by B[a]P and DB[a,l]P correlated with the stable covalent DNA adducts produced by each. These mutation results are consistent with the previously identified stable covalent DNA adducts being the promutagenic lesions produced by these two PAHs and do not support a major role for depurinating adducts, contributing to PAH-induced mutagenesis in mouse lung in vivo.