Genetic predisposition plays a significant role in tumorigenesis and cancer prevention. A common polymorphism 677C->T in methylenetetrahydrofolate reductase (MTHFR) is associated with altered risk of certain types of cancer, but the mechanism remains to be determined. MTHFR catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the methyl donor for the synthesis of methionine from homocysteine. The homozygous C677T mutation in the MTHFR gene results in reduction in enzyme activity and changes in cellular composition of folates. In the present study we investigate if the folate form redistribution results in compromised homocysteine remethylation and elevated de novo purine synthesis, both of which may be potential regulatory pathways involved in tumorigenesis. Metabolic fluxes of homocysteine remethylation and de novo purine synthesis were studied in Epstein-Barr virus transformed lymphoblasts expressing common polymorphic allele 677 C and 677T of the MTHFR gene. Using different stable isotopic tracers, specific enrichments in intracellular amino acid pool and nucleotides were measured by Gas Chromatography/Mass Spectrometry under various nutritional conditions. Our preliminary data indicated that when cellular folate was adequate, the MTHFR genotype did not affect the folate dependent homocysteine remethylation pathways (relative enrichments of methionine m+1 specie labeled from 13C-serine tracer in CC vs. TT=0.45 ± 0.28 vs. 0.40 ± 0.11, p=0.83). Only when cells became folate deficient, the folate dependent remethylation pathway in TT genotype was significantly reduced compared to the CC genotypes (CC vs. TT = 0.048 ± 0.011 vs. 0.008 ± 0.006, p=0.043), presumably due to limited methylated folates. Using 13C-serine as the tracer, de novo purine synthesis in TT genotype was found to be elevated when folate status is adequate (relative enrichments in the purine +1 specie in CC vs. TT = 0.48 ± 0.11 vs. 0.71 ± 0.06, p<0.0001), presumably due to increased availability of formylated folates. When folate is restricted, no difference in purine synthesis was found between the 2 genotypes, suggesting the advantage in purine synthesis only exist when folate suuply is adequate. Our data suggested that the impacts of MTHFR 677T polymorphism are closely related to nutritional conditions, and such alterations may modulate metabolic pathways involved in disease onset/progression. The advantage of de novo purine synthesis found in the MTHFR TT genotype may account for the protective effect of MTHFR in certain type of cancers. These transformed cells are potential models for studying the consequences of human genetic variation in MTHFR and their relationship to pathways involved in cancer pathogenesis or progression. Supported by NSC-93-2320-B-005-006 and NSC-93-2313-B005-083 (E-P Chiang)

[Proc Amer Assoc Cancer Res, Volume 46, 2005]