This exploratory research grant in the area of digestive diseases and nutrition has an overriding aim to identify a gene/nutrient interaction that will predispose animals to disease, cancer. The goal of this research grant is to elucidate the molecular mechanism by which folate deficiency increases tumorigenicity, i.e., we will determine whether genes in the DNA repair cascade, in particular base excision repair (BER), are involved in susceptibility to diseases of the colon and liver as a result of folate deficiency. Folate deficiency causes massive incorporation of uracil into DNA and also results in increased levels of DNA single strand breaks and chromosomal aberrations. These strand breaks could reflect incomplete DNA repair of uracil as a result of saturation of the BER pathway and stalling at the rate-limiting step of BER. By using an animal with a diminished BER capacity, we can directly test whether the genotoxic effects of folate deficiency and the synergy between folate deficiency and carcinogen exposure are due to impairment in BER capacity. In folate deficiency, we suggest that the ability of the BER pathway to process the massive amount of DNA damage is exceeded, and that any additional stress presented to the system is poorly handled. Our central hypothesis is that oxidizing and alkylating agents are more damaging in the face of folate deficiency as a result of saturation of the base excision repair pathway, and that a synergism between folate deficiency and inefficient BER will predispose to diseases of the colon and/or liver. The model of BER deficiency used in these studies is the DNA polymerase Beta heterozygous knockout mouse. The specific aims of the research described in this proposal are as follows: (1) To determine the sensitivity of a BER deficient animal to folate deficiency: the incidence of DNA damage and mutation frequency in response to folate deficiency in a BER compromised environment will be measured. (2) To determine whether folate deficiency within the context of BER deficiency will result in greater accumulation of biomarkers of disease in the dimethylhydrazine model of colon carcinogenesis. (3) To determine whether folate deficiency within the context of BER deficiency will result in greater accumulation of biomarkers of disease in the 2-nitropropane model of hepatocarcinogenesis. Achieving these objectives will provide molecular insights into the synergy between nutritional deficiency and genetic polymorphisms resulting in mutation induction and will further elucidate the relationship between diet and environmental carcinogens.