Familial Thrombophilia Associated With Homozygosity for the Cystathionine b-Synthase 833T3C Mutation

Severe hyperhomocysteinemia due to cystathionine b-synthase (CBS) deficiency is a strong risk factor for premature cardiovascular disease. Among untreated patients, '50% have suffered a thromboembolic event by 30 years of age. We report on 3 sisters with severe hyperhomocysteinemia due to homozygosity for the CBS 833T3C mutation. These patients, who displayed no other known thrombophilic predisposition, had suffered single or multiple venous thrombosis before CBS deficiency was diagnosed relatively late in life. In this family, homozygosity for the 833T3C mutation was associated with a mild phenotype with respect to other sequelae of CBS deficiency. Consequently, our results indicate that most cases with this genotype may remain undiagnosed. Investigated family members heterozygous for the 833T3C mutation displayed normal total homocysteine in plasma (tHcy) levels, even when they were homozygous for the methylenetetrahydrofolate reductase 677C3T polymorphism. The prevalence of homozygosity for the 833T3C mutation has previously been estimated at no less than 1:20 500 in our population. Because a reduction of the severely elevated levels of tHcy in CBS deficiency reduces cardiovascular risk and because homozygosity for the 833T3C mutation is more prevalent than previously thought, our results emphasize the importance of measuring tHcy routinely in thrombophilia screening. (Arterioscler Thromb Vasc Biol. 2000;20:1392-1395.)