It is an inborn error of Methionine metabolism in which there is deficiency of enzyme cystathionine b-synthase

Homocysteine is converted to methionine. In homocystinuria there is impaired conversion of homocysteine to methionine, The sulfur atom of the essential amino acid methionine is transferred ultimately to cysteine by the transsulfuration pathway. In one of these steps, homocysteine condenses with serine to form cystathionine. This reaction is catalyzed by the pyridoxal phosphate-dependent enzyme cystathionine b-synthase.

 Homocysteine and methionine accumulate in cells and body fluids; cysteine synthesis is impaired, resulting in reduced concentrations of this amino acid and its disulfide form cystine.

Homocystinuria is relatively common in Ireland (1 in 60,000 births) but rare elsewhere (less than 1 in 200,000 births).

This genetic defect is also seen in consanguineous marriages.

Features:
Mental retardation
Venous thromboses- a major cause of morbidity
Osteoporosis
Hair often fine & brittle. Malar flush. Ectopia lentis, myopia.
Convulsions in about 10%. Psychiatric disturbances.
Skeletal features resembling Marfan syndrome - in 50 % of cases.

Diagnosis:
Detection of Homocysteine in urine.

Treatment:
Pyridoxine 20-300 mg per day.

Restricted intake of methionine plus cystine supplements.
Administration of choline or betaine to enhance remethylation of homocystine