Hepatocellular carcinoma (HCC) has been correlated with specific p53 mutations and exposure to aflatoxin B1 (AFB1). Individual response to liver carcinogens shows great variability. High risk factors include chronic infection with hepatitis B (HBV) and C viruses (HCV), while additional risk factors include exposure to tobacco smoke. Cytochrome P450 genes, such as CYP1A1, CYP1A2, CYP3A4, and CYP3A5 encode proteins that activate potent liver carcinogens into genotoxic epoxides. Heterocyclic aromatic amines (HAs) require additional activation by N,O-acetyltransferase (NAT2). Specific cytochrome P450 polymorphisms and NAT2 polymorphisms contribute to risk of specific cancers, such as breast, pancreatic and colon. However, determining the risk of P450 and NAT polymorphisms in HCC is complicated by many modifying factors that can lead to detoxification of metabolites. To determine whether P450 polymorphisms per se are sufficient to increase the genotoxicity of carcinogens, we have expressed the P450 genes in Saccharomyces cerevisiae (yeast), which lacks the detoxification enzymes. We previously observed that expression of specific P450 genes in yeast is sufficient to stimulate carcinogen-associated mutation and recombination, the transcriptional induction of DNA repair genes after AFB1 exposure, and AFB1-associated activation of the checkpoint gene Rad53 (CHK2). We propose to further screen CYP1A1, CYP1A2, CYP3A4 and CYP3A5 polymorphisms that are associated with increased cancer risk. In the first specific aim, we will determine whether a subset of CYP1A1 and CYP1A2 polymorphisms affect frequencies of genetic recombination and mutation and the DNA damage response to AFB1 and liver carcinogens. In the second specific aim, we will determine whether CYP1A2 and NAT2 polymorphisms affect the metabolic activation of HAs in yeast. In the third specific aim, we will develop an expression system for CYP3A4 and CYP3A5 in yeast that will enable us to detect metabolic activation of carcinogens mediated by CYP3A3 and CYP3A5 polymorphisms. These studies will thus provide a new strategy for determining the potential risk of cytochrome P450 polymorphisms in liver cancer. Ultimately, the information will aid public health practitioners and clinicians to identify which individuals are at highest risk for liver cancer, and guide prevention and early detection efforts.
