Epigenetic effects on susceptibility to heavy metal & PAH induced DNA damage

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Heavy metals such as arsenic, nickel, and chromium as well as polycyclic aromatic hydrocarbons (PAHs) as benzo(a)pyrene are common environmental contaminants. Human exposure to such toxicants greatly increases cancer risk. The carcinogenicity of PAHs has been attributed to their covalent binding to DNA, which can result in mutations that ultimately lead to carcinogenesis. The mechanisms of metal-induced carcinogenesis are less clear. It has been found that nickel and arsenic exposure can induce DNA hypermethylation. Such non-mutational epigenetic changes could also result in suppression of tumor suppressor genes, such as the p53 gene, triggering tumorigenesis. Recently, we have found that the guanines at C5 cytosine methylated CpG sites are the major DNA targets of many PAHs, including benzo(a)pyrene diol epoxide (BPDE). In fact, we have found that the BPDE binding spectrum in the tumor suppressor p53 gene coincides with the mutation spectrum of this gene in lung cancer. It has been found that a mutated p53 gene is sufficient to trigger carcinogenesis in animal models, and that more importantly, 50% of human cancers have a mutation in the p53 gene. These findings have lead us to hypothesize that targeted DNA damage, rather than selection, is the major determinant of the p53 mutation spectrum in cancer, and that the CpG selection, is the major determinant of the p53 mutation spectrum in human cancer, and that the CpG methylation status of an individual p53 gene may determine the susceptibility of this gene to DNA damage and mutation. These findings have led us to hypothesize that targeted DNA damage, rather than selection, is the major determinant of the p53 mutation spectrum in human cancer, and that the CpG methylation status of an individual p53 gene may determine the susceptibility of this gene to DNA damage and mutation. In light of these findings, we propose that the carcinogenicity of nickel, arsenic and chromium may be partly due to their ability to induce DNA hypermethylation, which in turn enhances the susceptibility of methylated tumor suppressor genes and protooncogenes to bulky carcinogen-induced DNA damage and mutations. The objective of this research is to test the hypothesis using two state-of-the-art technologies; UvrABC-ligation-mediated-PCR to map DNA adducts at the single nucleotide level, and the p53 GeneChip to detect mutations and cytosine methylation. We will determine: 1) the p53 gene methylation status in lymphocytes of different individuals and assess its relationship with the susceptibility to BDE damage, 2) whether nickel, arsenic, and chromium induce changes in the methylation status of the p53 gene and whether nickel, arsenic, and chromium induce changes in the methylation status of the p53 gene and, consequently, in the susceptibility of this gene to BPDE induced-DNA damage, and 3) the effect of nickel, arsenic and chromium treatment on the repair of BPDE-DNA adducts in the p53, beta-actin and HPRT gene.