5-10-methylenetetrahydrofolic-acid and Metabolism--Inborn-Errors

The prevalence of hyperhomocysteinemia in renal disease patients, its treatment by folate administration, and its aggravation by the 677 C-->T mutation of methylene-tetrahydrofolate (methylene-THF) reductase has established the folate cycle as an important factor in the pathogenesis and management of renal disease. Proper function of the folate cycle depends on normal function of involved enzymes adequate of the vitamin and its correct disposition within the body. Vital processes in folate disposition include conversion of dietary folylpolyglutamates to monoglutamates, intestinal absorption, receptor and carrier-mediated transport across cell membranes, and cellular export. Folate coenzymes are responsible for the one-carbon unit transfer in intermediary metabolism and are required for several reactions in key metabolic processes, for example of purine, pyrimidine and methionine synthesis, and glycine and serine metabolism. Methionine synthase and its recently discovered reducing protein as well as methylene tetrahydrofolate reductase are key folate enzymes in homocysteine metabolism. Deficiencies of these enzymes are important causes of severe disease in the rare remethylation defects causing homocystinuria. Knowledge of their catalytic and molecular properties is important in understanding possible causes of moderate hyperhomocysteinemia, as for example, the well-known 677 C-->T transition of methylene tetrahydrofolate reductase.

Topics: 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; 5,10-Methylenetetrahydrofolate Reductase (FADH2); Aminohydrolases; Biological Transport, Active; Folic Acid; Formate-Tetrahydrofolate Ligase; Histidine; Homeostasis; Homocysteine; Humans; Intestinal Mucosa; Kidney Diseases; Metabolism, Inborn Errors; Methylenetetrahydrofolate Dehydrogenase (NADP); Methylenetetrahydrofolate Reductase (NADPH2); Multienzyme Complexes; Oxidoreductases; Polyglutamic Acid; Purine Nucleotides; Serine; Tetrahydrofolates

5-Methyltetrahydrofolate, the major form of folate in plasma, is a carbon donor for the remethylation of homocysteine to methionine. This form of folate is generated from 5,10-methylenetetrahydrofolate through the action of 5,10-methylenetetrahydrofolate reductase (MTHFR), a cytosolic flavoprotein. Patients with an autosomal recessive severe deficiency of MTHFR have homocystinuria and a wide range of neurological and vascular disturbances. We have recently described the isolation of a cDNA for MTHFR and the identification of two mutations in patients with severe MTHFR deficiency. We report here the characterization of seven novel mutations in this gene: six missense mutations and a 5' splice-site defect that activates a cryptic splice site in the coding sequence. We also present a preliminary analysis of the relationship between genotype and phenotype for all nine mutations identified thus far in this gene. A nonsense mutation and two missense mutations (proline to leucine and threonine to methionine) in the homozygous state are associated with extremely low activity (0%-3%) and onset of symptoms within the 1st year of age. Other missense mutations (arginine to cysteine and arginine to glutamine) are associated with higher enzyme activity and later onset of symptoms.

Topics: Adolescent; Adult; Age of Onset; Amino Acid Sequence; Base Sequence; Child, Preschool; Female; Genotype; Homozygote; Humans; Infant; Male; Metabolism, Inborn Errors; Methylenetetrahydrofolate Reductase (NADPH2); Molecular Sequence Data; Mutation; Oxidoreductases Acting on CH-NH Group Donors; Phenotype; RNA Splicing; Sequence Homology, Amino Acid; Tetrahydrofolate Dehydrogenase; Tetrahydrofolates