Tsai M Y, Bignell M, Schwichtenberg K, Hanson N Q
Department of Laboratory Medicine and Pathology, University of Minnesota Medical School, Minneapolis, USA.
Am J Hum Genet. 1996 Dec;59(6):1262-7.
We found that a mutation previously described by Sebastio et al., involving a 68-bp insertion in the coding region of exon 8 of the cystathionine-beta-synthase (CBS) gene in a single patient with homocystinuria, is highly prevalent. In our control population, 11.7% (9/77) of the individuals were heterozygous carriers of this mutation. In contrast to the previous report, which assumed that the 68-bp insertion introduced a premature-termination codon and resulted in a nonfunctional CBS enzyme, we found that the presence of this mutation is not associated with hyperhomocysteinemia. Assay of CBS activity in transformed lymphocytes from individuals who were heterozygous or homozygous for this mutation showed normal activity. Furthermore, reverse-transcripion-PCR showed that individuals carrying this mutation have normal size mRNA. Our results suggest that the insertion creates an alternate splicing site, which eliminates not only the inserted intronic sequences but also the T833C mutation associated with this insertion. The net result is the generation of both quantitatively and qualitatively normal mRNA and CBS enzyme. Although the mutation does not seem to affect the activity of the CBS enzyme, the prevalence is somewhat increased in patients with premature coronary-artery disease, although the difference is not statistically significant.
我们发现，塞巴斯蒂奥等人先前描述的一种突变在高胱氨酸尿症患者中高度普遍，该突变涉及胱硫醚-β-合酶（CBS）基因第8外显子编码区的68个碱基对插入。在我们的对照人群中，11.7%（9/77）的个体是这种突变的杂合携带者。与之前的报告不同，之前的报告认为68个碱基对的插入引入了一个提前终止密码子并导致CBS酶无功能，我们发现这种突变的存在与高同型半胱氨酸血症无关。对这种突变的杂合或纯合个体的转化淋巴细胞中的CBS活性进行检测，结果显示活性正常。此外，逆转录聚合酶链反应表明，携带这种突变的个体有正常大小的信使核糖核酸。我们的结果表明，该插入产生了一个替代剪接位点，这不仅消除了插入的内含子序列，还消除了与该插入相关的T833C突变。最终结果是产生了数量和质量上均正常的信使核糖核酸和CBS酶。尽管该突变似乎不影响CBS酶的活性，但在早发性冠状动脉疾病患者中其发生率有所增加，不过差异无统计学意义。
