In homocystinuria, homocysteine is not catabolized to cystathionine due to a deficiency of the enzyme cystathionine-P-synthase (CBS). As a result, homocys-teine accumulates in the plasma and is excreted in the urine. The mutant gene for CBS maps to chromosome 21. The disease is autosomal recessive and develops gradually, affecting multiple organs. Characteristic features are arterial and venous thromboses, myocardial infarct, skeletal abnormalities, lens dislocation and myopia, and livido reticularis. Mental retardation, recurrent strokes, seizures, psychiatric symptoms, and focal deficits characterize the nervous system involvement. The cerebral pathology consists of vascular endo-thelial proliferation and arterial and venous infarcts.

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