[From gene to disease; from homocysteine to hyperhomocysteinemia].

Homocysteine is a sulfhydryl containing amino acid which is produced as an intermediate product in the metabolism of the essential amino acid methionine. Apart from environmental factors such as the intake of folate and other B vitamins, the level of homocysteine in the blood is determined by the genetically based activities of several enzymes involved in the methionine or folate cycle. The well-known congenital defect homocystinuria is due to homozygosity for mutated cystathionine beta synthase. It is characterized by severe hyperhomocysteinemia, which leads to arterial and venous disease at a very young age. Mild to moderate hyperhomocysteinemia, due to either heterozygosity for severe mutations in the genes of enzymes involved or based upon homozygosity for more mild mutations, has also been recognized as a risk factor for vascular disease in the last decade. However, ongoing clinical intervention studies still need to demonstrate a casual role of mildly increased homocysteine levels in vascular disease.