Mice deficient in methylenetetrahydrofolate reductase exhibit hyperhomocysteinemia and decreased methylation capacity, with neuropathology and aortic lipid deposition

Abstract

Hyperhomocysteinemia, a risk factor for cardiovascular disease, is caused by nutritional and/or genetic disruptions in homocysteine metabolism. The most common genetic cause of hyperhomocysteinemia is the 677C-->T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene. This variant, with mild enzymatic deficiency, is associated with an increased risk for neural tube defects and pregnancy complications and with a decreased risk for colon cancer and leukemia. Although many studies have reported that this variant is also a risk factor for vascular disease, this area of investigation is still controversial. Severe MTHFR deficiency results in homocystinuria, an inborn error of metabolism with neurological and vascular complications. To investigate the in vivo pathogenetic mechanisms of MTHFR deficiency, we generated mice with a knockout of MTHFR: Plasma total homocysteine levels in heterozygous and homozygous knockout mice are 1.6- and 10-fold higher than those in wild-type littermates, respectively. Both heterozygous and homozygous knockouts have either significantly decreased S-adenosylmethionine levels or significantly increased S-adenosylhomocysteine levels, or both, with global DNA hypomethylation. The heterozygous knockout mice appear normal, whereas the homozygotes are smaller and show developmental retardation with cerebellar pathology. Abnormal lipid deposition in the proximal portion of the aorta was observed in older heterozygotes and homozygotes, alluding to an atherogenic effect of hyperhomocysteinemia in these mice.

Investigation of inter-individual variability of the one-carbon folate pathway: a bioinformatic and genetic review

“…Studies in models of folate deficiency both in vitro 84 and in vivo 85 have shown decreased DNA methylation capacity. Associations between global DNA hypomethylation in peripheral leukocytes and homozygosity for the MTHFR C677T polymorphism have also been described.…”

“…Knockout mouse pathogenesis CBS [137][138][139][140] Hyperhomocysteinemia Hyperkeratosis MAT1A 89 Spontaneous hepatocellular carcinoma and impaired liver regeneration MTHFR 85,141 Hyperhomocysteinemia (À/+ and À/À) Developmental retardation with cerebellar pathology (À/À) Aortic lipid deposition (À/+ and À/À) Abnormal spermatogenesis (À/À) FOLR1 (Folbp1 Embryonic lethality (À/À) mouse homolog) 90 reversed by folinic acid supplementation of (À/+) dams…”

A new perspective on neural tube defects: Folic acid and microRNA misexpression

“…Folic acid donates methyl groups by a complex pathway where it is first converted to tetrahydrofolate (THF) by dihydrofolate reductase (DHFR) and, through a series of steps, from THF to 5-methyl-THF by N 5 N 10 -methylenetetrahydrofolate reductase (MTHFR; Chen et al, 2001). 5-methyl-THF serves to replenish the methylated form of Vitamin B12, a cofactor for methionine synthase.…”

“…Methylation is essential for synthesis of membrane phospholipids, myelin basic protein, and neurotransmitters as well as for establishing proper gene expression in embryogenesis (Biniszkiewicz et al, 2002;Chen et al, 2001). During embryogenesis, all three major DNA methyltransferases (Dnmt1, Dnmt3a, and Dnmt3b) are expressed, the most important being Dnmt1 and Dnmt3b in establishing and maintaining proper genomic methylation (Fan et al, 2001;Jackson et al, 2004).…”

“…After donating its methyl group, SAM becomes SAH, which is converted to homocysteine via SAH hydrolase by a reversible reaction (Bohnsack and Hirschi, 2004;Chen et al, 2001). If homocysteine is not converted to methionine then it may be converted back to SAH, which inhibits DNA methyltransferases because they have an increased affinity for SAH over SAM (Jackson et al, 2004).…”