Ceru Slavko, Rabzelj Sabina, Kopitar-Jerala Natasa, Turk Vito, Zerovnik Eva
Department of Biochemistry and Molecular Biology, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia.
Med Hypotheses. 2005;64(5):955-9. doi: 10.1016/j.mehy.2004.11.038.
Loss of function mutations in the gene (CSTB) encoding human cystatin B, a widely expressed cysteine protease inhibitor, are responsible for a severe neurological disorder known as an Unverricht-Lundborg disease (EPM1). EPM1 had been linked to chromosome 21q22.3 in Finnish families and it is an autosomal recessive inherited disorder with a homozygous minisatellite expansion in the cystatin B gene (stefin B gene). This disease is difficult to treat because it is refractory to most antiepileptic drugs and using multiple medications had been unsuccessful so far. To come a step closer to understanding of the nature of this disease, especially about the events on the molecular level, in vitro properties of missense EPM1 mutant G4R were determined. It was observed that the mutant has a prolonged lag phase of fibrillation at the same protein stability, which could indicate it were more toxic to the cells. Similar experiments with the N-terminal fragment of 67 aminoacid residues are ongoing, showing higher propensity to aggregate. Therefore, a hypothesis is launched that at least in a subset of Unverricht-Lundborg disease patients, cystatin B protein may aggregate in the cell. Protein aggregation can be secondary to external insults or aging, however, inherited forms of neurodegenerative diseases, such as familial Parkinson's, Huntington's or familial Alzheimer's disease, are directly linked to the mutant proteins aggregation. Protein aggregates in the form of amyloid plaques, neurofibrilary tangles, intra-cytoplasmic or intra-nuclear inclusions lead to increased production of the reactive oxygen species and dysfunction of the ubiquitine/proteasome system. Finally, mitochondrial dysfunction and cell death are observed. Certainly, it remains to be checked by experiments whether overexpression in cell culture of the missense mutants G4R and N-terminal fragment to residue 68 lead to cellular inclusions and the accompanying changes characteristic for the conformational disorders.
编码人胱抑素B（一种广泛表达的半胱氨酸蛋白酶抑制剂）的基因（CSTB）功能丧失突变，是导致一种严重神经疾病——昂韦里希特-伦德伯格病（EPM1）的原因。在芬兰家族中，EPM1与21号染色体q22.3区域相关联，它是一种常染色体隐性遗传疾病，在胱抑素B基因（斯替芬B基因）中存在纯合微卫星扩增。这种疾病难以治疗，因为它对大多数抗癫痫药物都有耐药性，而且到目前为止，使用多种药物治疗均未成功。为了更深入地了解这种疾病的本质，特别是分子水平上的事件，我们测定了错义EPM1突变体G4R的体外特性。结果发现，该突变体在相同蛋白质稳定性下具有延长的纤维蛋白原形成延迟期，这可能表明它对细胞毒性更大。目前正在对67个氨基酸残基的N端片段进行类似实验，结果显示其聚集倾向更高。因此，我们提出一个假设，即至少在一部分昂韦里希特-伦德伯格病患者中，胱抑素B蛋白可能在细胞内聚集。蛋白质聚集可能继发于外部损伤或衰老，然而，神经退行性疾病的遗传形式，如家族性帕金森病、亨廷顿病或家族性阿尔茨海默病，都与突变蛋白聚集直接相关。淀粉样斑块、神经原纤维缠结、胞浆内或核内包涵体形式的蛋白质聚集体会导致活性氧生成增加以及泛素/蛋白酶体系统功能障碍。最终会观察到线粒体功能障碍和细胞死亡。当然，错义突变体G4R和68位残基的N端片段在细胞培养中的过表达是否会导致细胞内包涵体以及伴随构象障碍的特征性变化，仍有待通过实验进行验证。
