A 31 bp VNTR in the cystathionine beta-synthase (CBS) gene is associated with reduced CBS activity and elevated post-load homocysteine levels

Molecular defects in genes encoding enzymes involved in homocysteine metabo lism may account for mild hyperhomocysteinaemia, an independent and graded risk factor for cardiovascular disease (CVD). Although heterozygosity for c ystathionine beta -synthase (CBS) deficiency has been excluded as a major g enetic cause of mild hype rhomocysteinaemia in vascular disease, mutations in (non-)coding DNA sequences may lead to a mildly decreased CBS expression and, consequently, to elevated plasma homocysteine levels. We assessed the association between a 31 bp VNTR, that spans the exon 13-intron 13 boundar y of the CBS gene, and fasting, post-methionine load and increase upon meth ionine load plasma homocysteine levels in 190 patients with arterial occlus ive disease, and in 381 controls. The 31 bp VNTR consists of 16,17,18,19 or 21 repeat units and shows a significant increase in plasma homocysteine co ncentrations with an increasing number of repeat elements, in particular af ter methionine loading. In 26 vascular disease patients the relationship be tween this 31 bp VNTR and CBS enzyme activity in cultured fibroblasts was s tudied. The CBS enzyme activity decreased with increasing number of repeat units of the 31 bp VNTR. RT-PCR experiments showed evidence of alternative splicing at the exon 13-intron 13 splice junction site. The 31 bp VNTR in t he CBS gene is associated with post-methionine load hyperhomocysteinaemia t hat may predispose individuals to an increased risk of cardiovascular disea ses.