Genetic Defects

Severe elevations in plasma homocysteine (concentrations as high as several hundred mmoll-1) are observed in individuals with homozygous genetic defects affecting cystathionine ^-synthase, MTHFR, or any of several enzymes responsible for the conversion of vitamin B12 to its methionine synthase-associated cofactor form. These autosomal recessive genetic disorders, collectively termed homocystin-uria because homocysteine accumulates in the urine as well as the blood, are associated with severe premature vascular disease, including thrombosis and atherosclerosis, mental retardation, dislocation of the eye lens (ectopia lentis), and skeletal malformations. Premature death (often in childhood) usually results from a major thrombotic or embolic event. Notably, one of the genetic defects that afflicts a significant proportion of homocystinuria patients reduces the affinity of cystathionine ^-synthase for its vitamin B6 cofactor, PLP. For these patients, the metabolic defect can be overcome to some extent with high-dose vitamin B6 supplements, which significantly lower plasma homo-cysteine levels, reduce morbidity, and increase life expectancy. Interestingly, for other genetic defects involving cystathionine ^-synthase that cause homo-cystinuria independent of the affinity of the enzyme for PLP, high-dose vitamin B6 supplements nonetheless have a therapeutic effect despite having little or no influence on plasma homocysteine levels.

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