離子通道功能異常導致小腦性運動失調症之分子機制

Abstract

脊髓小腦性運動失調症是一群以影響小腦功能為主的遺傳性神經退化性疾病，其臨床表徵與致病基因具有高度多樣性，診斷此類疾病的要領在於了解病人之疾病史與家族病史，並由身體檢查評估病人是否具有小腦功能障礙，搭配其他特定的臨床病徵可作為診斷之線索，基因檢測是確定診斷的方式之一。多麩醯胺酸疾病是脊髓小腦性運動失調症中最常見的一群，這類疾病於臨床、病理、與疾病分子機制有許多相似之處。近年的研究發現已有將近五十種基因變異與脊髓小腦性運動失調症相關，使此疾病的病生理分子機制得以被更深入的了解。 神經細胞中表現著許多種類的離子通道蛋白，各式各樣的離子通道以及其相關的分子精確的分佈在神經細胞中特定的位置並執行其專責的功能。由於基因定序技術快速的發展，有越來越多的病人被發現是由於離子通道或相關分子功能變異所致。這些研究揭示由於離子通道蛋白功能變異所致的脊髓小腦性運動失調症在臨床表現與分子基因層面上都具有高度的歧異性，凸顯診斷這類疾病的困難性。然而，針對探討這類疾病的病生理機轉的研究仍然很有限。這些離子通道蛋白功能變異如何引起本身功能的改變，進而影響與其他分子交互作用的改變，並產生對神經細胞生理功能乃至於神經突觸功能恆定的影響，甚或產生增強或衰減整個神經網絡活性的變化，最終導致神經功能退化的過程，以及對病人臨床表徵的影響…等，這些問題都是神經科學與神經醫學研究中非常有趣的議題。 這份研究論文旨在探討本土脊髓小腦性運動失調症病人中是否存在由於離子通道蛋白變異所致的個案，進而分析其臨床表徵之特性並尋找是否有明確的基因型與表現型的關聯性。透過細胞電生理實驗、生物化學實驗、以及免疫螢光實驗，我們可以探討這些變異對離子通道功能的影響，進一步探索其與相關分子的交互作用以及對於神經生理恆定的影響。此外，我們期能建立與此疾病之病生理機轉相關聯的疾病模式動物，藉此研究更深入複雜的神經突觸或神經網絡之議題。 透過次世代基因定序，我們發現了離子通道蛋白變異所致之脊髓小腦性運動失調症的本土個案，這些病患在臨床表現上具有高度的變異性，除了小腦性運動失調症以外，這些病人有很高的比例具有認知功能障礙的特徵，然而其基因型與表現型並不具有明顯的關聯性。我們也深入的探索由於這些變異對離子通道蛋白功能產生的影響，並成功的建立與此疾病有高度關聯性的疾病模式果蠅。未來可望能透過更清楚了解這些疾病的病生理機轉與小腦及相關細胞生理調控機制，探索相關的分子作用並藉由調控目標分子的功能作為發展新治療的方針。

Spinocerebellar ataxia (SCA) is a group of hereditary neurodegenerative diseases with mainly affecting the function of the cerebellum and its network. The clinical features and the causes of SCA are highly diverse. In combination of understanding the clinical presentations and the family histories, a comprehensive physical examination to assess the cerebellar function and the other specific clinical features can provide insights to determine the diagnosis of SCA. The genetic testing is the gold standard to establish the definite diagnosis of SCA. The group of polyglutamine disorders comprises the most frequent causes of SCA and shares the clinical, pathological and molecular similarities. To date, there are approximate fifty disease-causing genes or chromosomal loci have been mapped to be associated with SCA. The identification of disease-causing mutations has also contributed to a deeper understanding for the molecular mechanism and pathophysiology of this group of diseases. There are numerous subtypes of ion channel proteins expressing in the nervous systems. These ion channels and their related molecules are precisely distributed in the subcellular locations to be responsible for their special functions. Due to the recent advance in genetic sequencing technologies, there are more and more SCA subtypes linked to ion channel dysfunctions. Those studies have revealed that the clinical and molecular features of SCA subtypes associated with ion channel dysfunction are heterogeneous, which highlights the difficulty in diagnosing such diseases. However, studies focusing on exploring the pathogenesis of cerebellar ataxias associated with ion channel dysfunction are still sparse. Whether the sequence variants resulting in ion channel dysfunction or not is still unclear. There are still open questions that how do ion channel dysfunctions lead to alternation of molecular network, disruption of synaptic homeostasis, perturbation of neuronal activities, induction of neurodegenerative processes, and consequent clinical presentations. Numerous intriguing issues remained to be further elucidated in the neuroscience and clinical neurology fields. In this study, we aim to investigate whether there are patients with SCA associated with ion channel gene mutations in Taiwanese population. We also intend to evaluate the molecular mechanism of these disorders by using multidisciplinary experiments. Utilizing the next-generation sequencing technologies, we found that there are cases of SCA caused by mutations in ion channel genes. The clinical presentations of this group of patients are variable. Cerebellar ataxia is exclusively manifested by the patients and a notable proportion of the patients are characterized by cognitive dysfunction as. However, there is not definite genotype-phenotype correlation for the patients. Besides functional characterizations for the ion channel mutations in vitro, and we have also developed a disease-relevant model using Drosophila melanogaster as the model organism. By means of in vivo exploring the molecular mechanism underlying these disorders we will able to conduct more in-depth investigation for the complex issues such as neural synapses homeostasis or neural network in regarding to the ion channel functions in the future.