Carcinogen-DNA Adducts: Topology, Conformation and Repair (Renewal)

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A category of non-planar, twisted polycyclic aromatic hydrocarbons (PAHs), termed fjord region compounds, are extremely potent tumorigens; they include dibenzo[a,l]pyrene which has recently been cited as the most tumorigenic PAH yet identified. These pollutants are released into the environment as combustion products of a variety of fuels, and they contaminate food crops. They are biologically active at the low concentrations present in foods and urban air and are hazardous to the population at large. The origin of the extraordinary carcinogenic potencies of fjord PAHs remains unknown. However, it has recently been shown that several of the bulky DNA adducts that they produce after metabolic activation to diol epoxides, are resistant to nucleotide excision repair (NER), the principal cellular defense against such DNA lesions. Resistance to DNA repair of these adducts is deemed a critical cause for the extraordinary tumorigenicity of the parent chemicals, as they cause the mutations which initiate cancer. However, each environmental fjord PAH gives rise to a complex mixture of stereoisomeric guanine and adenine DNA adducts. Furthermore, the NER susceptibility of each such adduct may vary with DNA base sequence. In this multitude of lesions, the key repair-resistant ones that lead to cancer remain unidentified. Our broad, long-term objective is, working in tandem with our experimental collaborator Prof. N. Geacintov, to identify the NER-resistant adducts and their characteristics using innovative and state-of-the-art modeling methods: we hypothesize that NER-resistance is governed by the structural, dynamic and thermodynamic properties of the PAH-modified DNA. The fjord PAHs selected for detailed study are dibenzo[a,l]pyrene, benzo[g]chrysene, and benzo[c]phenanthrene; we investigate their adducts produced via the well established diol epoxide metabolic activation pathway. These PAHs represent aromatic systems of 6, 5, and 4 rings, respectively, a range optimal for the induction of tumors. We aim to investigate the many diol epoxide adducts of the three parent PAHs in selected sequences that we hypothesize will alter their NER-susceptibilities. We further aim to determine the characteristic properties and NER susceptibilities of lesions when organized within the histone protein environment of the nucleosome, the basic unit of chromatin structure in the cellular environment. This is an essential first step towards elucidating the functioning of the complex NER machinery in the context of chromatin. We will work hand-in-hand with our experimental collaborator Prof. N. Geacintov: NER data with human cell extracts and including lesion- containing nucleosomes will provide anchors for directly linking our findings with the experimental observations, and our analyses will point to important predictions that will be tested in his laboratory. Our studies will provide the next-generation of biomarkers for PAH exposure, facilitate design of better NER- resistant chemotherapeutics through our gained understanding of NER mechanisms, and advance our capability for genotoxic screening of adducts derived from PAHs present in our environment.