Targeted Disruption of the Methionine Synthase Gene in Mice

Abstract

Alterations in homocysteine, methionine, folate, and/or B₁₂ homeostasis have been associated with neural tube defects, cardiovascular disease, and cancer. Methionine synthase, one of only two mammalian enzymes known to require vitamin B₁₂ as a cofactor, lies at the intersection of these metabolic pathways. This enzyme catalyzes the transfer of a methyl group from 5-methyl-tetrahydrofolate to homocysteine, generating tetrahydrofolate and methionine. Human patients with methionine synthase deficiency exhibit homocysteinemia, homocysteinuria, and hypomethioninemia. They suffer from megaloblastic anemia with or without some degree of neural dysfunction and mental retardation. To better study the pathophysiology of methionine synthase deficiency, we utilized gene-targeting technology to inactivate the methionine synthase gene in mice. On average, heterozygous knockout mice from an outbred background have slightly elevated plasma homocysteine and methionine compared to wild-type mice but seem to be otherwise indistinguishable. Homozygous knockout embryos survive through implantation but die soon thereafter. Nutritional supplementation during pregnancy was unable to rescue embryos that were completely deficient in methionine synthase. Whether any human patients with methionine synthase deficiency have a complete absence of enzyme activity is unclear. These results demonstrate the importance of this enzyme for early development in mice and suggest either that methionine synthase-deficient patients have residual methionine synthase activity or that humans have a compensatory mechanism that is absent in mice.

ACKNOWLEDGMENTS

We thank Celina Chang (University of California—Berkeley) for her assistance in some of the studies and Amy Chen, Theresa Hernandez, and Anthony Wynshaw-Boris for advice and technical assistance (National Human Genome Research Institute). We are indebted to T. Swanson-Linville for his patience.

This work was supported by NIH grant HL58984 to R.B. and DK42033 and HL58991 to B.S. R.B. is an Established Investigator of the American Heart Association.