Wreden Christopher C, Wlizla Marcin, Reimer Richard J
Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305, USA.
J Biol Chem. 2005 Jan 14;280(2):1408-16. doi: 10.1074/jbc.M411295200. Epub 2004 Oct 29.
Salla disease and infantile sialic acid storage disorder are autosomal recessive neurodegenerative diseases characterized by loss of a lysosomal sialic acid transport activity and the resultant accumulation of free sialic acid in lysosomes. Genetic analysis of these diseases has identified several unique mutations in a single gene encoding a protein designated sialin (Verheijen, F. W., Verbeek, E., Aula, N., Beerens, C. E., Havelaar, A. C., Joosse, M., Peltonen, L., Aula, P., Galjaard, H., van der Spek, P. J., and Mancini, G. M. (1999) Nat. Genet. 23, 462-465; Aula, N., Salomaki, P., Timonen, R., Verheijen, F., Mancini, G., Mansson, J. E., Aula, P., and Peltonen, L. (2000) Am. J. Hum. Genet. 67, 832-840). From the biochemical phenotype of the diseases and the predicted polytopic structure of the protein, it has been suggested that sialin functions as a lysosomal sialic acid transporter. Here we directly demonstrate that this activity is mediated by sialin and that the recombinant protein has functional characteristics similar to the native lysosomal sialic acid transport system. Furthermore, we describe the effect of disease-causing mutations on the protein. We find that the majority of the mutations are associated with a complete loss of activity, while the mutations associated with the milder forms of the disease lead to reduced, but residual, function. Thus, there is a direct correlation between sialin function and the disease state. In addition, we find with one mutation that the protein is retained in the endoplasmic reticulum, indicating that altered trafficking of sialin is also associated with disease. This analysis of the molecular mechanism of sialic acid storage disorders is a further step in identifying therapeutic approaches to these diseases.
萨勒病和婴儿型唾液酸贮积症是常染色体隐性神经退行性疾病，其特征为溶酶体唾液酸转运活性丧失，导致游离唾液酸在溶酶体中蓄积。对这些疾病的基因分析已在一个编码名为唾液酸转运蛋白的基因中鉴定出多个独特突变（Verheijen, F. W., Verbeek, E., Aula, N., Beerens, C. E., Havelaar, A. C., Joosse, M., Peltonen, L., Aula, P., Galjaard, H., van der Spek, P. J., and Mancini, G. M. (1999)《自然遗传学》23卷，462 - 465页；Aula, N., Salomaki, P., Timonen, R., Verheijen, F., Mancini, G., Mansson, J. E., Aula, P., and Peltonen, L. (2000)《美国人类遗传学杂志》67卷，832 - 840页）。从这些疾病的生化表型以及该蛋白预测的多结构域结构来看，有人提出唾液酸转运蛋白作为溶酶体唾液酸转运体发挥作用。在此我们直接证明这种活性由唾液酸转运蛋白介导，且重组蛋白具有与天然溶酶体唾液酸转运系统相似的功能特性。此外，我们描述了致病突变对该蛋白的影响。我们发现大多数突变与活性完全丧失相关，而与疾病较轻形式相关的突变导致功能降低但仍有残余。因此，唾液酸转运蛋白功能与疾病状态之间存在直接关联。另外，我们发现一个突变使该蛋白滞留在内质网中，这表明唾液酸转运蛋白转运改变也与疾病相关。对唾液酸贮积症分子机制的这种分析是确定这些疾病治疗方法的又一进展。
