MTHFR

MTHFR is the metabolic enzyme that irreversibly converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the folate co-substrate for homocysteine remethylation to methionine, thereby partitioning one-carbon units between DNA synthesis (thymidylate/purine) and methionine regeneration (PMID:11282420). The catalytically active ~70 kDa enzyme is encoded by an 11-exon gene whose product was confirmed by cDNA expression (PMID:9680386). Deep mutational scanning across folinate environments resolved its regulatory architecture, identifying variants that escape S-adenosylmethionine feedback inhibition and a disordered loop that retains the FAD cofactor at the active site (PMID:34214447). Enzyme output is additionally controlled through the cell cycle: CDK1/Cyclin B1 phosphorylates MTHFR at Thr34, peaking in mitosis and lowering catalytic activity (PMID:24769206). Beyond its canonical metabolic role, MTHFR supports centromeric heterochromatin maintenance, as its knockdown reduces H3K9me3 and de-represses centromeric transcription (PMID:24769206), and it influences vascular function through a SIRT1-linked pathway that operates independently of homocysteine levels (PMID:35821533). The common C677T (Ala222Val) variant produces a thermolabile enzyme with reduced activity, elevating plasma homocysteine in a folate-dependent manner (PMID:11282420, PMID:25449138), and functional-impact scores correlate with age of onset in severe MTHFR deficiency (PMID:34214447).

1. 1998 High

   Establishing the gene structure and confirming that the cloned cDNA produces a catalytically active enzyme provided the molecular foundation for all subsequent mechanistic work.

   Evidence Genomic clone isolation and cDNA expression yielding an active ~70 kDa enzyme, with cross-species conservation between human and mouse

   PMID:9680386

   Open questions at the time

   • No structural model of the active site at this stage
   • Regulatory mechanisms not addressed
2. 2001 High

   Defining MTHFR as the committed, irreversible step directing folate toward homocysteine remethylation versus nucleotide synthesis placed it at a metabolic branch point and linked the C677T variant to elevated homocysteine.

   Evidence Enzyme activity and thermolability assays integrated with population metabolic studies correlating genotype, folate species, and homocysteine

   PMID:11282420

   Open questions at the time

   • Structural basis of thermolability not resolved
   • Cofactor and feedback regulation not mechanistically defined
3. 2008 Medium

   Loss-of-function studies tested whether MTHFR activity is required for cell survival, showing methionine-dependent tumor cells rely on it and that VPA can transcriptionally upregulate the enzyme.

   Evidence Antisense inhibition with cytotoxicity and xenograft assays (PMID 18473861); MTHFR promoter-reporter, mRNA/protein, and in vivo VPA studies in Mthfr+/- mice (PMID 18615588)

   PMID:18473861 PMID:18615588

   Open questions at the time

   • Mechanism of VPA-driven promoter activation not mapped to specific elements
   • Generality of tumor methionine dependence beyond tested lines unestablished
4. 2014 Medium

   Discovery of CDK1/Cyclin B1-mediated Thr34 phosphorylation and a heterochromatin phenotype revealed cell-cycle regulation of MTHFR activity and a moonlighting role beyond folate metabolism.

   Evidence Phospho-specific antibody, cell-cycle synchronization, in vitro kinase assays, and siRNA knockdown with H3K9me3 immunoblotting and centromeric RT-PCR

   PMID:24769206

   Open questions at the time

   • Single-lab reciprocal Co-IP without independent confirmation
   • How reduced MTHFR activity mechanistically links to H3K9me3 maintenance is unresolved
5. 2020 Medium

   Direct metabolite measurement in genotype-stratified cells clarified that low-activity MTHFR limits conversion of folic acid to 5-methyltetrahydrofolate, with implications for supplementation strategy.

   Evidence LC-MS/MS quantification of intracellular 5-Me-THF in genotyped lymphoblastoid lines under folic acid versus 5-Me-THF supplementation, with folate-deprivation cell-cycle assays

   PMID:32887268

   Open questions at the time

   • Single cell-type system
   • In vivo relevance of supplementation findings not established
6. 2021 High

   A genome-scale variant atlas resolved the enzyme's regulatory features—SAM feedback inhibition and FAD retention by a disordered loop—and made variant effects environment- and folate-dependent, linking functional scores to clinical onset.

   Evidence Saturation genome editing / deep mutational scanning of 98,336 variant assays across four folinate environments with correlation to disease onset

   PMID:34214447

   Open questions at the time

   • Atomic-resolution structure of the disordered FAD-retaining loop not provided
   • Mechanism by which escape variants bypass SAM inhibition not structurally defined
7. 2022 Medium

   A homocysteine-independent vascular axis was established, showing MTHFR deficiency causes endothelial dysfunction and thrombosis via SIRT1 downregulation that is pharmacologically reversible.

   Evidence Mthfr+/- mouse thrombosis and endothelial function models plus human C677T flow-mediated dilatation and aggregometry, with SIRT1 activator rescue and inhibitor controls

   PMID:35821533

   Open questions at the time

   • Molecular link between MTHFR activity and SIRT1 expression not defined
   • Single-lab findings

• How epigenetic regulation of MTHFR (promoter hypermethylation in preeclampsia and sperm) causally affects expression and phenotype remains unresolved.
• Promoter methylation associations are correlative without functional proof of causality
• No mechanistic link between methylation state and downstream pathology demonstrated