Synthesis and Studies of Site-Specific Carcinogen-DNA Lesions

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Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment and they pose a cancer threat to humans as they are products of activities in a modern society. Alternant bay and fjord region PAHs are metabolized to 4 isomeric diol epoxides that are potent DNA alkylating agents. Reaction of these diol epoxides with cellular DNA results in the formation of 16 nucleoside adducts; 8 isomers from deoxyadenosine (dA) and correspondingly 8 from deoxyguanosine (dG). The isomeric diol epoxides have markedly different tumorigenicities that implies differences in intracellular recognition, replication and repair the individual diol epoxide-nucleoside adducts. Thus, an understanding of the structural differences in the individual diol epoxide-DNA lesions in relation to the biological responses is expected to provide a better basis for understanding chemical carcinogenesis at the molecular level. The proposed studies are on two topologically different structural paradigms; the bay region represented by benzo[a]pyrene (BaP) and the fjord region represented by benzo[c]phenanthrene (BcPh) the DNA adducts of which are expected to elicit markedly different structural properties. One aim of the project is to complete delineation of novel, unequivocal synthesis of all diol epoxide-nucleoside adducts of these two PAHs. The methods that so evolve will potentially have general applicability for studies with any bay or fjord region PAH diol epoxide. The adducts will be incorporated into suitable DNA sequences (primarily human N-ras for the dA adducts and c-Ki-ras for the dG) for comparative studies within the individual sequence contexts. These include: 7m studies with normal and mismatched complementary strands, and temperature-dependent CD studies of the duplexes. The proposed synthesis encompasses ample flexibility to allow for the modification of any sequence or the incorporation of MeC adjacent to the dG adducts. Thus sequence context effects can also be probed. In-house collaboration will be the NMR structural evaluations of the modified duplexes. External collaborations are also planned for UvrABC repair experiments and single molecule studies on protein-DNA complexes. This concerted synthesis and structural evaluation is anticipated to contribute to a greater global understanding of the causative effects in PAH carcinogenesis.