多種人類的神經肌肉退化性遺傳疾病是由(CAG/CTG)n三聯核酸重複序列擴增突變所引起。CAG擴增突變通常發生在密碼區，而CTG擴增突變則在不轉譯區。由於基因轉殖動物的疾病模式對致病基因的生理功能可提供直接且有力的證據，在之前我們即開始研究將不同長度的CAG重複序列(CAG0、CAG200)接在綠螢光蛋白enhanced green fluorescence protein(EGFP)基因之3’端不轉譯區，並以gamma-sarcoglycan(GSG)基因之驅動子來驅動EGFP在小鼠肌肉專一性表達。本論文主要是分析較短長度的CAG重複序列(CAG23) 在小鼠中造成的影響，並與之前的結果比較，探討CAG重複序列長度導致生理缺陷的關係。實驗結果發現(CAG)23 轉殖基因鼠在肌肉組織型態、病理切片、活動力、骨骼肌電生理及基因表達各方面所呈現的表型都是類似(CAG)0轉殖基因鼠的情況，也就是說(CAG)23並不會造成病變。本論文並進一步探討之前(CAG)200小鼠繁殖力降低的原因，發現(CAG)200小鼠的精子數目不變而活動力卻大幅減低。以電顯觀察，證實精子鞭毛的微管及粒線體結構異常。故經由不同長度的(CAG)n重複序列轉殖基因鼠的研究，我們確定了擴增的CAG重複序列雖位於不轉譯區仍可造成小鼠生理缺陷，暗示可能有未知的人類神經肌肉或其他方面的疾病是由擴增的CAG RNA所引起，進一步研究將找出可能受到影響的下游分子。 The most frequent trinucleotide repeats found in human disorders are CAG and its complementary sequence, CTG. CAG repeats are almost always found in coding regions whereas CTG expansions are located in untranslated regions (UTRs). Transgenic animal models of human diseases usually provide direct and strong evidence in understanding the physiological function of disease genes. To investigate the possible pathogenic effect of untranslated (CAG)n trinucleotide repeat in mammals, we made transgenic mice expressing a muscle-specific transcript of enhanced green fluorescence protein (EGFP) gene with (CAG)200 inserted in its 3’-UTR previously. In this study, we made mice expressing a shorter repeat (CAG23), analyzed their phenotypes and compared them to the previous results to explore the relationship between the repeat length and the pathogenicity. We found that (CAG)23 transgenic mice displayed similar phenotypes as of (CAG)0 mice, in aspects of muscle morphology, histochemical staining, cage activity, muscle electrophysiology and gene expression. Previouly (CAG)200 mice showed reduced breeding efficiency. Further analysis revealed that these mice had normal sperm counts but impaired sperm motility. Electromicrography of the sperm tails from (CAG)200 mice revealed structural defects in axonemes and mitochondria. By studying transgenic mice with different lengths of CAG repeat, we demonstrate, for the first time, that untranslated CAG repeat expansion can result in pathogenic effects in mice. It also provides a model for dissecting the mechanism by which untranslated CAG expansions may lead to human disorders.
