MTRR

MTRR encodes methionine synthase reductase, a dual-flavin (FMN/FAD) and NADPH-dependent electron transferase of the ferredoxin-NADP⁺ reductase family that maintains the methylcobalamin cofactor of methionine synthase (MTR) in its active, reduced state, thereby sustaining the folate/cobalamin-dependent remethylation of homocysteine to methionine (PMID:10484769, PMID:2688421). Loss-of-function mutations in MTRR, distributed across FMN-, FAD-, and NADPH-binding domains as well as deep intronic positions that activate pseudoexons through SF2/ASF-dependent exonic splicing enhancers, cause the cblE complementation group of homocystinuria characterized by megaloblastic anemia, hyperhomocysteinemia, and impaired methylcobalamin synthesis (PMID:3384945, PMID:10484769, PMID:20120036). Splice-shifting oligonucleotides targeting the aberrant exonic splicing enhancer restore MTRR transcript integrity and recover approximately 50% of enzymatic activity in patient fibroblasts, establishing the pseudoexon mechanism as therapeutically reversible (PMID:25878036). In mice, hypomorphic Mtrr disruption depletes hepatic glycogen stores and impairs fatty acid β-oxidation, linking MTRR-dependent one-carbon metabolism to broader fuel homeostasis (PMID:32257815).

1. 1988 High

   The existence of two distinct genetic loci required for cobalamin-dependent methionine biosynthesis was established by defining the cblE and cblG complementation groups, revealing that cblE cells retain methionine synthase catalytic activity but fail to generate methylcobalamin.

   Evidence Somatic cell complementation analysis and enzymatic/isotope incorporation assays in patient fibroblasts

   PMID:3384945

   Open questions at the time

   • The cblE gene product was not identified
   • Whether cblE defect is in reductase activity versus cobalamin trafficking was unresolved
2. 1989 High

   The cblE defect was pinpointed to reductive activation/maintenance of methionine synthase rather than the catalytic enzyme itself, because methionine synthase activity in cblE cells was normal under excess reducing agent but collapsed under limiting reductant.

   Evidence Methionine synthase activity assayed under variable dithiothreitol concentrations in patient fibroblasts with radiolabeled cobalamin distribution

   PMID:2688421

   Open questions at the time

   • The reductase protein remained unidentified
   • No molecular basis for the reductive activation deficiency
3. 1999 High

   Cloning of MTRR identified the cblE gene as a novel FNR-family electron transferase containing FMN, FAD, and NADPH binding domains, and 11 patient mutations distributed across all three cofactor-binding regions established that each domain is functionally essential.

   Evidence RT-PCR cloning, SSCP/heteroduplex analysis, and sequencing of patient cDNA with domain mapping

   PMID:10484769

   Open questions at the time

   • No crystal structure to interpret mutations at atomic level
   • Kinetic properties of wild-type versus mutant MTRR not characterized in vitro
4. 2002 High

   Identification of a pathogenic 140-bp pseudoexon insertion caused by an intronic T>C transition in MTRR revealed pre-mRNA mis-splicing as a major disease mechanism in cblE homocystinuria.

   Evidence Enzymatic complementation studies and MTRR sequencing in Czech cblE patients

   PMID:12555939

   Open questions at the time

   • The splicing regulatory elements responsible were not characterized
   • Frequency of this mutation in the cblE population was unclear
5. 2003 Medium

   A homozygous S454L mutation in the FAD-binding domain was shown to cause a mild cblE phenotype (megaloblastic anemia without neurological disease), establishing that partial loss of MTRR function produces an attenuated clinical spectrum.

   Evidence Enzymatic analysis, complementation, and methylcobalamin formation assay in patient fibroblasts

   PMID:12971424

   Open questions at the time

   • Residual enzymatic activity of S454L not quantified in purified protein
   • Genotype–phenotype correlation across the full mutation spectrum not systematically assessed
6. 2008 Medium

   The MTRR His595Tyr (rs10380) variant was shown to reduce MTRR protein levels, elevate homocysteine secretion, and decrease LINE-1 methylation in stable transfectants, linking common MTRR variation to genome-wide DNA methylation status.

   Evidence Stable transfection of risk vs. wild-type MTRR haplotypes with homocysteine, protein expression, and methylation analysis

   PMID:18515090

   Open questions at the time

   • Mechanism of reduced protein expression (stability vs. translation) not determined
   • In vivo validation in human tissues lacking
7. 2010 High

   The molecular mechanism of the most frequent cblE mutation (c.903+469T>C) was resolved: the mutation creates an SF2/ASF (SRSF1)-dependent exonic splicing enhancer within a cryptic pseudoexon, and blocking SF2/ASF binding eliminates pseudoexon inclusion.

   Evidence Minigene splicing assays, siRNA knockdown of SF2/ASF, in vitro RNA-binding assays

   PMID:20120036

   Open questions at the time

   • Therapeutic reversibility not demonstrated
   • Contribution of other SR proteins not fully excluded
8. 2015 High

   Splice-shifting oligonucleotides targeting the mutant ESE restored correct MTRR splicing and recovered ~50% of enzymatic activity in patient fibroblasts, and the pseudoexon was found to be maintained near recognition threshold by a balance between an hnRNP A1-binding silencer and two ESEs.

   Evidence SSO transfection in patient cells, minigene assays, endogenous transcript and enzymatic activity measurement

   PMID:25878036

   Open questions at the time

   • In vivo delivery and efficacy of SSOs not tested
   • Long-term stability of correction unknown
9. 2015 Medium

   MTRR knockdown in ovarian cancer cells inhibited proliferation, induced apoptosis via caspase activation, suppressed autophagy, and sensitized cells to cisplatin both in vitro and in xenografts, implicating MTRR-dependent one-carbon metabolism in chemoresistance through mTOR signaling.

   Evidence RNAi knockdown in cancer cell lines with proliferation, apoptosis, autophagy assays and in vivo xenograft model

   PMID:26550452

   Open questions at the time

   • Whether the effect is through methionine synthase reactivation or alternative MTRR targets is unclear
   • Pathway connection to mTOR is correlative
10. 2020 Medium

    Hypomorphic Mtrr mutation in mice depleted hepatic glycogen stores, downregulated glycogen synthesis genes (Ugp2, Gsk3a), and impaired fatty acid β-oxidation, extending MTRR's metabolic role beyond homocysteine remethylation to fuel storage homeostasis.

    Evidence Histology, gene expression, lipidomics, and high-resolution respirometry in Mtrr-mutant mouse livers

    PMID:32257815

    Open questions at the time

    • Whether hepatic phenotype is a direct consequence of impaired methionine synthase activation or secondary to altered methyl donor availability is unresolved
    • Rescue by methionine or betaine supplementation not tested

• No high-resolution structural model of MTRR exists, the electron transfer mechanism from NADPH through FAD and FMN to the cobalamin cofactor of MTR has not been reconstituted with purified components at kinetic resolution, and the basis for genotype-phenotype severity differences across MTRR mutations remains poorly defined.
• No crystal or cryo-EM structure
• Electron transfer pathway kinetics not reconstituted in vitro with purified MTRR and MTR
• Systematic genotype–phenotype correlation across all known mutations lacking