Characterization of mutations in severe methylenetetrahydrofolate reductase deficiency reveals an FAD-responsive mutation.
Methylenetetrahydrofolate reductase (MTHFR) synthesizes 5-methyltetrahydrofolate, a major methyl donor for homocysteine remethylation to methionine. Severe MTHFR deficiency results in marked hyperhomocysteinemia and homocystinuria. Patients display developmental delay and a variety of neurological and vascular symptoms. Cloning of the human cDNA and gene has enabled the identification of 29 rare mutations in homocystinuric patients and two common variants [677C>T (A222V) and 1298A>C (E429A)] with mild enzymatic deficiency. Homozygosity for 677C>T or combined heterozygosity for both polymorphisms is associated with mild hyperhomocysteinemia. In this communication, we describe four novel mutations in patients with homocystinuria: two missense mutations (471C>G, I153M; 1025T>C, M338T), a nonsense mutation (1274G>A, W421X), and a 2-bp deletion (1553delAG). We expressed the 1025T>C mutation as well as two previously reported amino acid substitutions [983A>G (N324S) and 1027T>G (W339G)] and observed decreased enzyme activity at 10%, 36%, and 21% of control levels, respectively, with little or no effect on affinity for 5-methyltetrahydrofolate. One of these mutations, 983A>G (N324S), showed flavin adenine dinucleotide (FAD) responsiveness in vitro. Expression of these mutations in cis with the 677C>T polymorphism, as observed in the patients, resulted in an additional 50% decrease in enzyme activity. This report brings the total to 33 severe mutations identified in patients with severe MTHFR deficiency.
Sibani
Sahar
S
Department of Biology, McGill University, Montreal, Canada.
Leclerc
Daniel
D
Weisberg
Ilan S
IS
O'Ferrall
Erin
E
Watkins
David
D
Artigas
Carmen
C
Rosenblatt
David S
DS
Rozen
Rima
R
D017668:Age of Onset; D004247:DNA / Q000737:chemistry / Q000235:genetics; D004252:DNA Mutational Analysis; D004795:Enzyme Stability / Q000187:drug effects / Q000235:genetics; D005192:Family Health; D005260:Female; D005182:Flavin-Adenine Dinucleotide / Q000494:pharmacology; D006712:Homocystinuria / Q000201:enzymology / Q000235:genetics*; D006801:Humans; D008297:Male; D042965:Methylenetetrahydrofolate Reductase (NADPH2); D009154:Mutation; D000587:Oxidoreductases Acting on CH-NH Group Donors / Q000172:deficiency* / Q000235:genetics* / Q000378:metabolism; D010375:Pedigree
D005182:Flavin-Adenine Dinucleotide; D004247:DNA; D000587:Oxidoreductases Acting on CH-NH Group Donors; D042965:Methylenetetrahydrofolate Reductase (NADPH2)