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Unlabelled: Homocystinuria due to cystathionine beta-synthase (CBS) deficiency is an autosomal recessive disease of sulphur amino acid metabolism. Major clinical manifestations include disorders of the eye, the skeleton, the central nervous system and the vascular system. A wide clinical spectrum of the disease has been reported. We discuss the role of genetic factors (e.g. different mutations of the CBS gene and a variable genetic background) and the importance of environmental factors (e.g. diet, vitamins, perinatal factors and drugs) in explaining the phenotypic variability observed in homocystinuria., Conclusion: Homocystinuria represents a good model to explain the clinical differences frequently observed among patients affected by monogenic diseases.

In a 3-year-old mentally retarded girl with homocystinuria due to 5,10-methylenetetrahydrofolate reductase deficiency among different therapeutic approaches only treatment with betaine (15-20 g/day) resulted in a satisfactory biochemical response. Betaine improved homocysteine remethylation and thus lowered plasma homocystine to trace amounts and normalized the previously very low plasma methionine concentration. This biochemical response was associated with a clinical improvement although she remained mentally retarded.