The homocystinurias is a general term for a group of disorders that cause elevated levels of the amino acid homocystine in the urine. These disorders primarily occur because of deficiency of any of several enzymes required for the proper degradation or reformation of an essential amino acid called methionine. Elevated levels of homocysteine and abnormal levels of methionine can cause a variety of health problems.
The three primary homocystinurias are genetic metabolic disorders:
1. Classic homocystinuria (homocystinuria due to cystathionine (β -synthase deficiency)
2. Methylene tetrahydrofolate reductase (MTHFR) deficiency
3. Homocystinuria due to cobalamin (vitamin B12 ) metabolic defects