FUNCTIONAL-MODELING OF VITAMIN RESPONSIVENESS IN YEAST - A COMMON PYRIDOXINE-RESPONSIVE CYSTATHIONINE BETA-SYNTHASE MUTATION IN HOMOCYSTINURIA

Cystathionine beta-synthase (CBS) deficiency is an autosomal recessive disorder which results in extremely elevated levels of total plasma h omocysteine (tHcy) and high risk of thromboembolic events. About half of all patients diagnosed with CBS deficiency respond to pyridoxine tr eatment with a significant lowering of tHcy levels. We examined 12 CBS -deficient patients from 10 Norwegian families for mutations in the CB S gene and identified mutations in 18 of the 20 CBS alleles. Five of t he seven patients classified as pyridoxine-responsive contain the newl y identified point mutation, G(797)A (R266K). This point mutation is t ightly linked with a previously identified 'benign' 68 bp duplication of the intron 7-exon 8 boundary within the CBS gene. We tested the eff ect of all of the mutations identified on human CBS function utilizing a yeast system. Five of the six mutations had a distinguishable pheno type in yeast, indicating that they were in fact pathogenic. Interesti ngly, the G(797)A allele had no phenotype when the yeast were grown in high concentrations of pyridoxine, but a severe phenotype when grown in low concentrations, thus mirroring the behavior in humans. These st udies show that the G(797)A mutation is an important cause of pyridoxi ne-responsive CBS deficiency and demonstrate the utility of yeast func tional assays in the analysis of human mutations.