藉由族群遺傳分析與基因轉殖小鼠模式探討TBP基因上CAG三核苷酸重複序列對神經退化性疾病之影響

Abstract

TATA binding protein (TBP) 為細胞中一種主要的轉錄因子，其在主導基因轉錄的起始過程中扮演著重要的角色。人類TBP基因位於染色體6q27，其5’端包含一段CAG三核苷酸重複序列，轉譯出的蛋白質N端上會形成一段多麩醯胺(polyglutamine, polyQ) 的片段。在神經退化性疾病研究中發現TBP功能異常與疾病的發生有關，包括杭丁頓氏舞蹈症 (Huntington’s disease, HD)、阿茲罕默氏症 (Alzheimer’s disease, AD) 以及第十七型脊髓小腦運動失調症(SCA17)。SCA17為一種體染色體顯性遺傳之神經退化性疾病，屬於眾多類型的脊髓小腦運動失調症 (SCA) 其中一型，臨床上患者有小腦萎縮、吞嚥困難、智力退化、以及錐體外徑路 (extrapyramidal tract) 等症狀。目前已知SCA17致病原因與TBP基因之CAG重複序列擴增有關，正常族群中重複次數為31到42個，患病者則擴增為43到66個。為探討TBP基因上CAG重複序列在台灣人族群中之分布情況，我們針對正常人族群及不同類型的神經退化性病人進行TBP基因型分析，結果發現在各族群中最常見之CAG重複個數為36個。此外分別檢測出4位失智症 (Dementia) 患者與2位帕金森氏症 (Parkinson’s disease, PD) 患者帶有擴增之等位基因 (allele)。為建立SCA17之動物模式，我們分別將帶有36與109個CAG重複之人類TBP (hTBP) 基因，以pcp2/L7組織專一性啟動子大量表現於小鼠小腦的Purkinje細胞，目前成功建立7株基因轉殖小鼠。由分生分析中証實，轉殖的hTBP在基因轉殖小鼠體內均可正常表現。基因轉殖小鼠行為觀察實驗發現，hTBP109Q line-16與line-54小鼠出現clasping症狀，line-54與line-69更分別於6到9個月大時出現運動失調之症狀。經由rota-rod滾輪測試小鼠之運動能力發現，在line-54與line-69之基因轉殖小鼠，其平衡感與協調性表現明顯劣於非基因轉殖小鼠，而hTBP36Q之基因轉殖小鼠則未出現此一差異。由組織切片觀察中發現hTBP109Q基因轉殖小鼠之小腦Purkinje細胞確實有受損之情形。本篇論文之實驗結果一方面建立國人族群遺傳資料庫，另一方面建立SCA17疾病動物模式，提供後續致病機轉探討與臨床治療、篩檢藥物研究之利器。

TATA binding protein (TBP) is a general transcription factor that plays an important role in initiation of transcription. TBP gene is located in chromosome 6q27 and contains a CAG/CAA trinucleotide repeats region in 5’ end which encodes a polyglutamine tract. It was reported that TBP is involved in numerous neurodegenerative diseases including Huntington’s disease (HD), Alzheimer’s disease (AD) and spinocerebellar ataxia type 17 (SCA17). SCA17 is an autosomal dominant cerebellar ataxia (ADCA). It has been known that the length of polyglutamine tract encoded by the CAA/CAG repeats is related to the disease progression. The range of CAG repeats of TBP gene is 31- 42 in normal population and 43 - 63 in SCA17 patients. To investigate the TBP trinucleotide expansion effect on neurodegeneration, we conducted genotyping analysis in both normal and neurodegenerative disease populations in Taiwan. We found that the most common TBP allele contains 36 repeats. We identified six individuals with expanded CAG repeats from two families originally diagnosed as PD and Dementia, respectively. To establish SCA17 disease animal model, we generate transgenic mice expressing the human TBP gene with either normal or expanded CAA/CAG tracts under the control of Purkinje cell-specific promoter, Pcp2/L7 promoter. Seven transgenic mouse lines have been identified. The existence of transgene in the mouse genome was confirmed by Southern blot analysis. RNA and protein expressions were detected by RT-PCR ad Western blot analyses, respectively. By behavior observation, we found that hTBP109Q line-16 and line-54 transgenic mice have a hind-limb clasping phenotype, which was also reported by Huntington’s disease transgenic mice. Among the 7 transgenic lines, line-69 and line-54 showed significant reduction in the Rota-rod performance compared to their wild-type littermates. We also observe ataxia phenotype of these two lines in their elder stage. Immunohistochemical analysis has shown that the Purkinje cell in line-69 transgenic mouse cerebellum were lost severly. In conclusion, we have successfully generated the hTBP transgenic mice as SCA17 animal model. This model should help us to gain insight about the role of TBP in neurodegeneration and eventually could lead to rational therapeutic protocol designing.