The causal metabolic pathways underlying associations between folate and risk for colorectal cancer (CRC) have yet to be established. Folate-mediated one-carbon metabolism is required for the de novo synthesis of purines, thymidylate and methionine. Methionine is converted to S-adenosylmethionine (AdoMet), the major one-carbon donor for cellular methylation reactions. Impairments in folate metabolism can modify DNA synthesis, genomic stability and gene expression, characteristics associated with tumorigenesis. The Mthfd1 gene product, C1-tetrahydrofolate synthase, is a trifunctional enzyme that generates one-carbon substituted tetrahydrofolate cofactors for one-carbon metabolism. In this study, we use Mthfd1gt/+ mice, which demonstrate a 50% reduction in C1-tetrahydrofolate synthase, to determine its influence on tumor development in two mouse models of intestinal cancer, crosses between Mthfd1gt/+ and Apcmin/+ mice and azoxymethane (AOM)-induced colon cancer in Mthfd1gt/+ mice. Mthfd1 hemizygosity did not affect colon tumor incidence, number or load in Apcmin/+ mice. However, Mthfd1 deficiency increased tumor incidence 2.5-fold, tumor number 3.5-fold and tumor load 2-fold in AOM-treated mice. DNA uracil content in the colon was lower in Mthfd1gt/+ mice, indicating that thymidylate biosynthesis capacity does not play a significant role in AOM-induced colon tumorigenesis. Mthfd1 deficiency-modified cellular methylation potential, as indicated by the AdoMet: S-adenosylhomocysteine ratio and gene expression profiles, suggesting that changes in the transcriptome and/or decreased de novo purine biosynthesis and associated mutability cause cellular transformation in the AOM CRC model. This study emphasizes the impact and complexity of gene–nutrient interactions with respect to the relationships among folate metabolism and colon cancer initiation and progression
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