Hyperhomocysteinemia refers to above-normal concentrations of plasma/serum homocysteine. Plasma/serum homocysteine is the sum of the thiol-containing amino acid homocysteine and the homocysteinyl moiety of the disulfides homocystine and cysteine-homocysteine, whether free or bound to proteins (Malinow and Stampfer, 1994).
Hyperhomocysteinemia in isolation may be associated with an increased risk of atherosclerosis and recurrent arterial and venous thrombosis usually in the third or fourth decade of life (review by Welch and Loscalzo, 1998).
Homocysteinemia is also a feature of several inherited metabolic disorders, including homocystinuria (236200), due to mutation in the CBS gene (613381), and N(5,10)-methylenetetrahydrofolate reductase deficiency (236250), caused by mutation in the MTHFR gene (607093). Homocysteinemia/homocystinuria and megaloblastic anemia can result from defects in vitamin B12 (cobalamin; cbl) metabolism, which have been classified according to complementation groups of cells in vitro; see cblE (236270) and cblG (250940). See also the various forms of combined methylmalonic aciduria (MMA) and homocystinuria due to disorders of cobalamin: cblC (277400), cblD (277410), and cblF (277380). [from
OMIM]