Defects in homocysteine metabolism: diversity among hyperhomocyst(e)inemias

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There are now four genetic mouse models that induce hyperhomocyst(e)inemia by decreasing the activity of an enzyme involved in homocysteine metabolism: cystathionine β-synthase, methylenetetrahydrofolate reductase, methionine synthase and methionine synthase reductase. While each enzyme deficiency leads to murine hyperhomocyst(e)inemia, the accompanying metabolic profiles are significantly and often unexpectedly, different. Deficiencies in cystathionine β-synthase lead to elevated plasma methionine, while deficiencies of the remaining three enzymes lead to hypomethioninemia. The liver [S-adenosylmethionine]/[S-adenosylhomocysteine] ratio is decreased in mice lacking methylenetetrahydrofolate reductase or cystathionine β-synthase, but unexpectedly increased in mice with deficiencies in methionine synthase or methionine synthase reductase. Folate pool imbalances are observed in complete methylenetetrahydrofolate reductase deficiency, where methyltetrahydrofolate is a minor component, and in methionine synthase reductase deficiency, where methyltetrahydrofolate is increased relative to wild-type mice. These differences illustrate the potential diversity among human patients with hyperhomocyst(e)-inemia, and strengthen the argument that the pathologies associated with the dissimilar forms of the condition will require different treatments.

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