Mthfd1 Is an Essential Gene in Mice and Alters Biomarkers of Impaired One-carbon Metabolism

Cytoplasmic folate-mediated one carbon (1C) metabolism functions to carry and activate single carbons for the de novo synthesis of purines, thymidylate, and for the remethylation of homocysteine to methionine. C1 tetrahydrofolate (THF) synthase, encoded by Mthfd1, is an entry point of 1Cs into folate metabolism through its formyl-THF synthetase (FTHFS) activity that catalyzes the ATP-dependent conversion of formate and THF to 10-formyl-THF. Disruption of FTHFS activity by the insertion of a gene trap vector into the Mthfd1 gene results in embryonic lethality in mice. Mthfd1(gt/+) mice demonstrated lower hepatic adenosylmethionine levels, which is consistent with formate serving as a source of 1Cs for cellular methylation reactions. Surprisingly, Mthfd1(gt/+) mice exhibited decreased levels of uracil in nuclear DNA, indicating enhanced de novo thymidylate synthesis, and suggesting that serine hydroxymethyltransferase and FTHFS compete for a limiting pool of unsubstituted THF. This study demonstrates the essentiality of the Mthfd1 gene and indicates that formate-derived 1Cs are utilized for de novo purine synthesis and the remethylation of homocysteine in liver. Further, the depletion of cytoplasmic FTHFS activity enhances thymidylate synthesis, affirming the competition between thymidylate synthesis and homocysteine remethylation for THF cofactors.