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Mutations Induced by Benzo[a]pyrene and Dibenzo[a,l]pyrene in lacI Transgenic B6C3F1 Mouse Lung Result from Stable DNA Adducts
Citation:
LEAVITT, S. A., M. H. GEORGE, T. MOORE, AND J. A. ROSS. Mutations Induced by Benzo[a]pyrene and Dibenzo[a,l]pyrene in lacI Transgenic B6C3F1 Mouse Lung Result from Stable DNA Adducts. MUTAGENESIS. Oxford University Press, Cary, NC, 23(6):4445-450, (2008).
Impact/Purpose:
PAHs are ubiquitous environmental carcinogens, and many research studies have demonstrated that the formation of covalent PAH-DNA adducts is a critical early key event in their carcinogenic mode of action. Recent studies have indicated that other types of DNA damage may also be induced by PAHs, and that these may also contribute to carcinogenesis. This paper evaluates mutation induction by two carcinogenic environmental PAHs in vivo to assess the role of these alternate pathways in a target tissue.
Description:
Dibenzo[a,l]pyrene (DB[a,l]P) and benzo[a]pyrene (B[a]P) are carcinogenic polycyclic aromatic hydrocarbons (PAH) 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 tumors 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® 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 five 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. Six animals per treatment group were dosed ip either with 6 mg/kg DB[a,l]P once, or 1.2 mg/kg DB[a,l]P on five consecutive days, or with 200 mg/kg B[a]P once, or 40 mg/kg B[a]P on five consecutive days. Animals were sacrificed after 31 days and lungs were removed. DNA extracted from the lungs was assayed for the presence of mutations in the lacI gene. Solvent treated control mice exhibited a mutant frequency of 3.18 x 10-5. In contrast, 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 treatment regimens with B[a]P produced about a 15-fold increase in mutant frequencies compared to controls. The most prevalent mutation class recovered from control mice was GC to AT transitions (73%). The mutation spectrum recovered from mice treated with 6 mg/kg DB[a,l]P contained GC to AT transitions (30%), GC to TA transversions (28%) and AT to TA transversions (12%). For mice treated with 1.2 mg/kg DB[a,l]P for five days, GC to AT transitions (24%), GC to TA transversions (12%) and AT to TA (19%) transversions dominated the mutation spectrum. The mutation spectrum obtained from mice treated with 200 mg/kg B[a]P featured predominantly GC to TA transversions (50%) and GC to AT transitions (21%). For mice treated with 40 mg/kg B[a]P for five days, the prevalent types of mutations were: GC to AT transitions (20%), GC to TA transversions (44%) and GC to CG transversions (13%). 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.
What is the study?
This study compares the mutational yield and spectra in mouse lung for two polycyclic aromatic hydrocarbons (PAHs) administered as single or fractionated doses.
Why was it done?
PAHs are ubiquitous environmental carcinogens, and many research studies have demonstrated that the formation of covalent PAH-DNA adducts is a critical early key event in their carcinogenic mode of action. Recent studies have indicated that other types of DNA damage may also be induced by PAHs, and that these may also contribute to carcinogenesis. This paper evaluates mutation induction by two carcinogenic environmental PAHs in vivo to assess the role of these alternate pathways in a target tissue.
What is the impact to the field and the agency?
Risk assessment of PAHs requires accurate dose-response information for key molecular events in the low-dose region. Identification of which putative key events are actually operant in target tissues is necessary to guide the collection and interpretation of low dose dose-response data. Further, this study addresses a current area of controversy in the field of PAH carcinogenesis, and provides key information to differentiate between possible key events in PAH modes of action.
