Homocystinuria is an amino acid disorder. It is a hereditary disease that is most often caused by a missing enzyme (cystathionine b-synthase) that is required to digest methionine and homocysteine. These are amino acids found in protein.
The gene defect for homocystinuria is inherited when both parents have the same abnormal gene and unknowingly pass it on to their baby. A parent who has the abnormal gene, but not the disease, is called a carrier. The health of a carrier is rarely affected. However, for each pregnancy of two such carriers, there is a 1 in 4 chance that the child will be born with the disease. Homocystinuria occurs in about 1 in every 100,000 births in Australia.
Homocystinuria can cause mental retardation, seizures, psychiatric disturbances, dislocation of the lens of the eye, abnormal thinning and weakness of the bones, and an increased risk for blood clots in the veins and arteries, which may lead to life-threatening complications.
Treatment aims to decrease the level of homocysteine in the blood and to prevent or delay the onset of symptoms. About half the patients will respond to medication with vitamin B6 (pyridoxine) and folate. The other patients will also need treatment with oral betaine. Babies are usually started on a low-methionine diet with methionine-free amino acid supplement. All patients will also need extra vitamin B12.
Detection can depend on the amount of protein ingested by the infant. Not all infants may have methionine levels high enough to be detected in the first week of life, especially infants with the vitamin B6-responsive form. Homocystinuria due to the remethylation defects (Cbl C, D, E, F, and G) may also be missed by screening as methionine is not increased in these.