Superfund Research Program
Investigations of DNA Damage by Heavy Metals and PAHs
Human exposure to environmental contaminants such as polycyclic aromatic hydrocarbons (PAHs) and the heavy metals arsenic, nickel and chromium 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 not yet understood. Interestingly, epidemiologic studies have shown that individuals exposed to both metals and PAHs have increased incidences of cancer, especially lung cancer.
Dr. Moon-shong Tang at the New York University School of Medicine is conducting studies focused to determine if metal and PAH exposures exert any synergistic effects on mutagenesis and to identify the molecular basis of these synergistic effects. He hypothesizes that exposure to arsenic, nickel or chromium enhances DNA methylation. DNA methylation tags cytosine, one of the four chemical bases that make up the genetic code, with a methyl group (CH4). This epigenetic change (one that influences the behavior of a cell without directly affecting its DNA) acts like a switch in the control of gene activity. DNA methylation is generally associated with silencing of gene expression, whereas active genes are usually unmethylated. Dr. Tang proposes that metal-induced changes in DNA methylation result in increased susceptibility of genomic DNA to PAH-induced damage.
To test this hypothesis, Dr. Tang's research group has developed methods to determine the epigenetic changes at the gene and sequence level, and to map and quantify metal and PAH-induced DNA damage at single-nucleotide resolution. These approaches apply state-of-the-art technologies including the UvrABC nuclease incision method in combination with ligation-mediated polymerase chain reaction (LMPCR) and DNA sequencing.
Dr. Tang's group concentrates their studies on two genes:
- p53 tumor suppressor gene: p53 safeguards cellular genomic integrity by coordinating cellular responses to DNA damage - inducing either cell cycle arrest or programmed cell death (apoptosis) to avoid accumulation of mutated progeny cells. More than 50% of human cancers have mutations in the p53 gene. p53 mutations found in human cancers not only occur at different sequences in the p53 gene, but also occur as various types of mutations that seem to be specific to the etiological agent.
- K-ras gene: Mutations in the ras family of genes (H-, K- and N-) reduce intrinsic GTPase activity, triggering a signal pathway regulating growth and cell development. Mutations of codon 12 of K-ras are by far the most common mutations in human cancers - occurring in 90% of pancreatic cancers, 50% of colon cancers, and more than 30% of smoking-related lung cancers (though only 5% of lung cancers from nonsmokers contain this mutation).
In studies to examine the impact of chromate or nickel exposure on PAH-induced DNA damage in the p53 and K-ras genes, Dr. Tang's research group showed that metal pre-exposure increases the susceptibility of mammalian cells to PAH-induced mutations. They learned that chromate pre-exposure enhances binding of DNA and benzo(a)pyrene diol epoxide (BPDE, the metabolically activated derivative of benzo(a)pyrene, a PAH found in smoke generated by cigarettes and fossil fuel and solid waste combustion). The enhancement was found to be sequence-specific; it did not increase total BPDE-DNA at the genomic DNA level, but greatly enhanced binding at exons 7 and 8 of the p53 gene, especially at previously identified "lung cancer hotspots". The researchers also determined that the enhancement is caused by the effect of chromate on chromosomal structure rather than by chromate-DNA interactions. They found that pre-exposure to nickel chloride also enhances BPDE-induced mutation frequency, but to a much smaller degree than potassium chromate.
In associated studies, Dr. Tang's group has demonstrated that:
- BPDE selectively binds to codon 12 of the K-ras gene (but not H- or N-ras), and that the DNA damage is poorly repaired compared to damage at other codons. They believe that metal pre-exposure enhances cytosine methylation, increasing genomic DNA susceptibility to PAH-induced damage.
- N-OH-4-ABP (a metabolically activated derivative of 4-aminobiphenyl, a component of cigarette smoke and an environmental and industrial contaminant, which is considered the major etiological agent for human bladder cancer) interacts with DNA to form adducts with sequence specificity. They identified preferential p53 binding sites for N-OH-4-ABP, and found that this carcinogen preferentially binds at bladder cancer p53 mutational hotspots at codons 280 and 285. In addition, they also found that cytosine methylation greatly enhances N-OH-4-ABP binding at codons 175 and 248 of the p53 gene, two other mutational hotspots commonly observed in human bladder cancer. Their results suggest that mutations at these codons in bladder cancer are likely caused by the binding of 4-aminobiphenyl.
These studies, identifying preferential binding sites and clarifying the role of cytosine methylation, increase our understanding of the mutational spectra of the p53 and K-ras genes in human cancers and suggest that these genes are not only a relevant, but also sensitive, biomarkers for detecting both endogenous- and exogenous-induced DNA damage. Such information is of great importance not only for understanding carcinogenesis, but also for cancer risk assessment and for cancer prevention.
For More Information Contact:
To learn more about this research, please refer to the following sources:
- Feng Z, Hu W, Rom WN, Costa M, Tang M. 2003. Chromium(VI) exposure enhances polycyclic aromatic hydrocarbon-DNA binding at the p53 gene in human lung cells. Carcinogenesis 24(4):771-778. PMID:12727806
- Feng Z, Hu W, Chen JX, Pao A, Li H, Rom WN, Hung M, Tang M. 2002. Preferential DNA damage and poor repair determine ras gene mutational hotspot in human cancer. J Natl Cancer Inst 94(20):1527-1536. PMID:12381705
- Feng Z, Hu W, Rom WN, Beland FA, Tang M. 2002. 4-aminobiphenyl is a major etiological agent of human bladder cancer: evidence from its DNA binding spectrum in human p53 gene. Carcinogenesis 23(10):1721-1727. PMID:12376482
- Feng Z, Hu W, Rom WN, Beland FA, Tang M. 2002. N-hydroxy-4-aminobiphenyl-DNA binding in human p53 gene: sequence preference and the effect of C5 cytosine methylation. Biochemistry 41(20):6414-6421. PMID:12009904
- Tang M, Pfeifer GP, Denissenko MF, Feng Z, Hu W, Pao A, Zheng Y, Zheng JB, Li H, Chen JX. 2002. Mapping polycyclic aromatic hydrocarbon and aromatic amine-induced DNA damage in cancer-related genes at the sequence level. Int J Hyg Environ Health 205(1-2):103-113. PMID:12018002
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