Schepers U, Glombitza G, Lemm T, Hoffmann A, Chabas A, Ozand P, Sandhoff K
Institut für Organische Chemie und Biochemie der Universität Bonn.
Am J Hum Genet. 1996 Nov;59(5):1048-56.
Lysosomal degradation of ganglioside GM2 by beta-hexosaminidase A (hex A) requires the presence of the GM2 activator protein (GM2AP) as an essential cofactor. A deficiency of the GM2 activator causes the AB variant of GM2 gangliosidosis, a recessively inherited disorder characterized by excessive neuronal accumulation of GM2 and related glycolipids. Two novel mutations in the GM2 activator gene (GM2A) have been identified by the reverse-transcriptase-PCR method--a three-base deletion, AAG262-264, resulting in a deletion of Lys88, and a single-base deletion, A410, that causes a frameshift. The latter results in substitution of 33 amino acids and the loss of another 24 amino acid residues. Both patients are homoallelic for their respective mutations inherited from their parents, who are heteroallelic at the GM2A locus. Although the cultured fibroblasts of both patients produce normal levels of activator mRNA, they lack a lysosomal form of GM2AP. Pulse/chase labeling of cultured fibroblasts of the patients, in presence and absence of brefeldin A, indicates a premature degradation of both--mutant and truncated--GM2APs in the endoplasmic reticulum or Golgi. These results were supported by in vitro translation experiments and expression of the mutated proteins. When the mutated GM2APs were expressed in Escherichia coli, both mature GM2AP forms turned proved to exhibit only residual activities in an in vitro assay.
β-己糖胺酶A（己糖胺酶A）对神经节苷脂GM2的溶酶体降解需要GM2激活蛋白（GM2AP）作为必需的辅助因子。GM2激活蛋白缺乏会导致GM2神经节苷脂沉积症的AB变异型，这是一种隐性遗传疾病，其特征是GM2和相关糖脂在神经元中过度积累。通过逆转录-聚合酶链反应方法在GM2激活基因（GM2A）中鉴定出两个新突变——一个三碱基缺失，AAG262 - 264，导致Lys88缺失；另一个单碱基缺失，A410，导致移码。后者导致33个氨基酸的替代以及另外24个氨基酸残基的丢失。两名患者对于从父母遗传而来的各自突变都是纯合等位基因，其父母在GM2A位点是杂合等位基因。尽管两名患者培养的成纤维细胞产生正常水平的激活蛋白mRNA，但它们缺乏溶酶体形式的GM2AP。在有和没有布雷菲德菌素A的情况下对患者培养的成纤维细胞进行脉冲/追踪标记表明，在内质网或高尔基体中，突变型和截短型GM2AP都会过早降解。体外翻译实验和突变蛋白的表达支持了这些结果。当突变的GM2AP在大肠杆菌中表达时，在体外测定中两种成熟形式的GM2AP都仅表现出残余活性。
