Increased susceptibility to breast cancer is associated with polymorphism in genes encoding proteins involved in DNA repair. Interindividual differences in DNA repair capacity may be critical determinants of breast carcinogenesis or disease progression under conditions of nutritional stress. We are studying a potential gene-environment interaction and hypothesize that the effects of folic acid on genome stability are modifed by two polymorphic genes that are implicated in folic acid metabolism. The reduced folate, 5,10-methylenetetrahydrofolate (MTHF), is a critical metabolite involved in one-carbon transfers and is partitioned between two pathways that play an important role in genome stability. In one, thymidylate synthase (TS) utilizes MTHF in the synthesis of thymidylate (TMP); depletion of TMP is associated with misincorporation of uracil in DNA and nucleotide pool imbalance. In the second pathway, methylenetetrahydrofolate reductase (MTHFR) utilizes MTHF for the synthesis of methionine, which supplies the methyl group for DNA methylation. We developed a highly sensitive assay for analysis of MTHF levels in erythrocytes from patients (n=40) with breast cancer. The levels of MTHF varied by 10-fold. We also analyzed common polymorphisms in the genes encoding TS and MTHFR using DNA isolated from either whole blood or lymphocytes. We analyzed alleles that are associated with difference in either mRNA expression (TYMS; 2R/3RC-G promoter polymorphisms) or enzyme activity (MTHFR; C677T). We observed higher levels of MTHF in patients with a T/T allele in MTHFR, compared to patients with C/C or C/T alleles. No association between TYMS 2R/3R polymorphism and MTHF was observed. The data indicate that MTHF levels are higher in patients homozygous for T alleles. The strength of this association is being tested in a larger sample population.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA