MTR

MTR encodes methionine synthase, a methylcobalamin-dependent enzyme that catalyzes the remethylation of homocysteine to methionine, linking one-carbon folate metabolism to the methionine cycle. The 1265-amino-acid protein contains a cobalamin-binding domain and a C-terminal S-adenosylmethionine (AdoMet)-binding domain that mediates reductive reactivation of the enzyme; patient mutations such as P1173L in the AdoMet domain disrupt this activation mechanism, while mutations in the cobalamin-binding domain impair cofactor retention (PMID:8968737, PMID:8968736, PMID:1627355). MTR physically interacts with the cobalamin chaperone MMACHC to acquire its methylcobalamin cofactor, and this interaction is diminished in cblC-deficient cells (PMID:23825108). Loss-of-function mutations in MTR cause cblG complementation group disease, characterized by hyperhomocysteinemia, megaloblastic anemia, and neurological impairment (PMID:3384945, PMID:9683607).

1. 1988 High

   Defining complementation groups resolved that functional methionine synthase deficiency comprises two genetically distinct disorders (cblE and cblG), with cblG mapping to the synthase itself and cblE to an upstream activating step.

   Evidence Genetic complementation analysis and enzyme activity assays in cultured patient fibroblasts

   PMID:3384945

   Open questions at the time

   • The gene had not yet been cloned, so the molecular basis of cblG remained unknown
   • No structural information on the human enzyme was available
2. 1992 Medium

   Demonstrating that a subset of cblG lines fail to accumulate cobalamin on methionine synthase revealed heterogeneity within cblG and implicated the cofactor-binding capacity of the enzyme as a distinct disease mechanism.

   Evidence Radiolabeled cobalamin binding and subcellular fractionation in patient fibroblasts

   PMID:1627355

   Open questions at the time

   • Single-lab study; not independently confirmed at the time
   • Molecular identity of mutations causing impaired cofactor binding was unknown
3. 1996 High

   Cloning of the full-length human MTR cDNA and mapping to 1q43 established the molecular identity of the enzyme and enabled genotype–phenotype correlation in cblG patients, while structure-based analysis of patient mutations (P1173L, ΔI881) located the reductive activation function to the C-terminal AdoMet-binding domain.

   Evidence RT-PCR cloning, FISH mapping, SSCP mutation screening, structural inference from E. coli methionine synthase crystal structure

   PMID:8968736 PMID:8968737

   Open questions at the time

   • No crystal structure of the human enzyme existed; domain assignments relied on bacterial homolog
   • Mechanism of reductive activation was inferred from structural modeling rather than direct reconstitution
4. 1998 High

   Identification of functionally null MTR alleles (frameshifts, splice-site disruptions) causing mRNA instability and complete loss of protein confirmed that absence of methionine synthase is compatible with survival but causes severe cblG disease.

   Evidence Northern blotting, RT-PCR, and sequence analysis of patient fibroblasts

   PMID:9683607

   Open questions at the time

   • Residual in vivo methionine synthesis in null patients was not quantified
   • No rescue experiment demonstrated restoration of enzyme activity upon re-expression
5. 2002 High

   Complete elucidation of the 33-exon MTR gene structure and identification of P1173L as a recurrent CpG-site mutation on independent haplotypes expanded the mutational spectrum and explained the high frequency of this allele in cblG.

   Evidence Genomic exon amplification, sequencing, and haplotype analysis in 24 cblG patients

   PMID:12068375

   Open questions at the time

   • Functional impact of many novel missense variants was not biochemically validated
   • Genotype–phenotype severity correlations remained incomplete
6. 2013 High

   Demonstrating a physical interaction between MTR and the cobalamin chaperone MMACHC established a direct cofactor-transfer mechanism and linked the cblC and cblG pathways at the protein level.

   Evidence Reciprocal co-immunoprecipitation, proximity ligation assay, and siRNA knockdown in human cells

   PMID:23825108

   Open questions at the time

   • In vitro reconstitution of cofactor transfer from MMACHC to MTR has not been shown
   • Stoichiometry and kinetics of the MTR–MMACHC complex are undefined
   • Whether other chaperones participate in cofactor loading onto MTR is unknown

• A high-resolution structure of human MTR, the precise mechanism of cofactor transfer from MMACHC, and the structural basis for how specific patient mutations differentially affect cobalamin binding versus reductive activation remain to be determined.
• No crystal or cryo-EM structure of human methionine synthase
• In vitro reconstitution of cofactor transfer from MMACHC to MTR is lacking
• Quantitative genotype–phenotype model for cblG mutations is absent