The N-terminus of MTRR plays a role in MTR reactivation cycle beyond electron transfer

Case report: Rare variants in the MTRR gene, 66GG and 524TT cause hyperhomocysteinemia and folic acid deficiency linked to schizophrenia

We present an adult patient with schizophrenia who was later found to have hyperhomocysteinemia, a condition that increases the risk of several diseases, due to a deficiency in folic acid. Although folic acid supplementation quickly normalized the hyperhomocysteinemia and folic acid levels, it did not significantly improve the overall mental and cognitive health. Genotype analysis was performed and the patient was found to have two pathogenic variants in the MTRR gene, 66GG and 524TT, which encodes for methionine synthase reductase (MSR), an enzyme crucial for homocysteine metabolism. The results can shed light on the reasons behind the patient's hyperhomocysteinemia and folic acid deficiency. Hyperhomocysteinemia confers an increased risk of several diseases. Indeed, the patient has neurodevelopment and cardiovascular health problems for decades. Given the rarity of the condition and the nonspecific nature of the symptoms, the detection of hyperhomocysteinemia or MSR deficiency can often be delayed or overlooked. Considering the potential irreversible and detrimental consequences of prolonged hyperhomocysteinemia and folic acid deficiency that our patient is likely experiencing, we suggest that clinicians be vigilant for associated signs when they encounter adolescents exhibiting psychotic symptoms, especially those with additional physical symptoms and a history of resistance to treatment.

Effect of Methylfolate, Pyridoxal-5'-Phosphate, and Methylcobalamin (SolowaysTM) Supplementation on Homocysteine and Low-Density Lipoprotein Cholesterol Levels in Patients with Methylenetetrahydrofolate Reductase, Methionine Synthase, and Methionine Synthase Reductase Polymorphisms: A Randomized Controlled Trial

Exploring the link between genetic polymorphisms in folate metabolism genes (MTHFR, MTR, and MTRR) and cardiovascular disease (CVD), this study evaluates the effect of B vitamin supplements (methylfolate, pyridoxal-5'-phosphate, and methylcobalamin) on homocysteine and lipid levels, potentially guiding personalized CVD risk management. In a randomized, double-blind, placebo-controlled trial, 54 patients aged 40-75 with elevated homocysteine and moderate LDL-C levels were divided based on MTHFR, MTR, and MTRR genetic polymorphisms. Over six months, they received either a combination of methylfolate, P5P, and methylcobalamin, or a placebo. At the 6 months follow-up, the treatment group demonstrated a significant reduction in homocysteine levels by 30.0% (95% CI: -39.7% to -20.3%) and LDL-C by 7.5% (95% CI: -10.3% to -4.7%), compared to the placebo (p < 0.01 for all). In the subgroup analysis, Homozygous Minor Allele Carriers showed a more significant reduction in homocysteine levels (48.3%, 95% CI: -62.3% to -34.3%, p < 0.01) compared to mixed allele carriers (18.6%, 95% CI: -25.6% to -11.6%, p < 0.01), with a notable intergroup difference (29.7%, 95% CI: -50.7% to -8.7%, p < 0.01). LDL-C levels decreased by 11.8% in homozygous carriers (95% CI: -15.8% to -7.8%, p < 0.01) and 4.8% in mixed allele carriers (95% CI: -6.8% to -2.8%, p < 0.01), with a significant between-group difference (7.0%, 95% CI: -13.0% to -1.0%, p < 0.01). Methylfolate, P5P, and methylcobalamin supplementation tailored to genetic profiles effectively reduced homocysteine and LDL-C levels in patients with specific MTHFR, MTR, and MTRR polymorphisms, particularly with homozygous minor allele polymorphisms.

The complex machinery of human cobalamin metabolism

Vitamin B₁₂ (cobalamin, Cbl) is required as a cofactor by two human enzymes, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) and methylmalonyl-CoA mutase (MMUT). Within the body, a vast array of transporters, enzymes and chaperones are required for the generation and delivery of these cofactor forms. How they perform these functions is dictated by the structure and interactions of the proteins involved, the molecular bases of which are only now being elucidated. In this review, we highlight recent insights into human Cbl metabolism and address open questions in the field by employing a protein structure and interactome based perspective. We discuss how three very similar proteins-haptocorrin, intrinsic factor and transcobalamin-exploit slight structural differences and unique ligand receptor interactions to effect selective Cbl absorption and internalisation. We describe recent advances in the understanding of how endocytosed Cbl is transported across the lysosomal membrane and the implications of the recently solved ABCD4 structure. We detail how MMACHC and MMADHC cooperate to modify and target cytosolic Cbl to the client enzymes MTR and MMUT using ingenious modifications to an ancient nitroreductase fold, and how MTR and MMUT link with their accessory enzymes to sustainably harness the supernucleophilic potential of Cbl. Finally, we provide an outlook on how future studies may combine structural and interactome based approaches and incorporate knowledge of post-translational modifications to bring further insights.

Causes and consequences of impaired methionine synthase activity in acquired and inherited disorders of vitamin B12 metabolism

Methyl-Cobalamin (Cbl) derives from dietary vitamin B₁₂ and acts as a cofactor of methionine synthase (MS) in mammals. MS encoded by MTR catalyzes the remethylation of homocysteine to generate methionine and tetrahydrofolate, which fuel methionine and cytoplasmic folate cycles, respectively. Methionine is the precursor of S-adenosyl methionine (SAM), the universal methyl donor of transmethylation reactions. Impaired MS activity results from inadequate dietary intake or malabsorption of B₁₂ and inborn errors of Cbl metabolism (IECM). The mechanisms at the origin of the high variability of clinical presentation of impaired MS activity are classically considered as the consequence of the disruption of the folate cycle and related synthesis of purines and pyrimidines and the decreased synthesis of endogenous methionine and SAM. For one decade, data on cellular and animal models of B₁₂ deficiency and IECM have highlighted other key pathomechanisms, including altered interactome of MS with methionine synthase reductase, MMACHC, and MMADHC, endoplasmic reticulum stress, altered cell signaling, and genomic/epigenomic dysregulations. Decreased MS activity increases catalytic protein phosphatase 2A (PP2A) and produces imbalanced phosphorylation/methylation of nucleocytoplasmic RNA binding proteins, including ELAVL1/HuR protein, with subsequent nuclear sequestration of mRNAs and dramatic alteration of gene expression, including SIRT1. Decreased SAM and SIRT1 activity induce ER stress through impaired SIRT1-deacetylation of HSF1 and hypomethylation/hyperacetylation of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), which deactivate nuclear receptors and lead to impaired energy metabolism and neuroplasticity. The reversibility of these pathomechanisms by SIRT1 agonists opens promising perspectives in the treatment of IECM outcomes resistant to conventional supplementation therapies.