Pathogenic Mechanisms in Fragile X Tremor Ataxia Syndrome

描述（由申请人提供）：脆性X震颤共济失调综合征（FXTAS）是一种常见的遗传性原因，影响超过1:800的男性步态障碍和震颤。FXTAS是由脆性X智力低下基因FMR1的5'非翻译区扩增的“CGG”核苷酸重复引起的。在患者和动物模型中，这种扩增的CGG重复序列与FMR1 mRNA表达升高、神经退行性变和泛素阳性核内神经元包涵体（含有扩增的CGG重复序列mRNA和几种蛋白质）相关。迄今为止的工作主要集中在CGG重复序列通过RNA功能获得机制引发神经变性的潜在作用上。然而，疾病病理的关键方面不容易通过纯粹的mRNA介导的隔离过程来解释。我们的中心假设是CGG重复扩增通过多个重叠的分子机制促进FXTAS的神经元变性。这些机制包括顺式中FMR1基因的转录失调和RNA介导的功能毒性获得，导致异常的RNA剪接事件和蛋白质质量控制途径的改变。我们检验这一假设的提议分为三个目的。目的1将研究FMR1 mRNA转录增加的机制，重点关注FMR1位点的组蛋白乙酰化状态。目的2将研究泛素蛋白酶体系统在FXTAS发病机制中的作用。目标一和目标二都将利用果蝇FXTAS模型以及患者来源的淋巴细胞。目的3将使用从FXTAS患者成纤维细胞中分化成神经元的诱导多能干细胞。我们将确定这些人类FXTAS神经元的替代转录组谱，以解决FXTAS中RNA结合蛋白的隔离导致mRNA剪接异常的假设。综上所述，这些研究将大大扩展我们对这种神经退行性疾病的理解，并有助于确定合适的候选治疗靶点。

DESCRIPTION (provided by applicant): Fragile X Tremor Ataxia Syndrome (FXTAS) is a common erited cause of gait disorder and tremor affecting upwards of 1:800 males. FXTAS is caused by an expanded "CGG" nucleotide repeat in the 5' untranslated region of the Fragile X Mental Retardation gene, FMR1. In patients and animal models, this expanded CGG repeat is associated with elevated FMR1 mRNA expression, neurodegeneration, and ubiquitin- positive intranuclear neuronal inclusions that contain the expanded CGG repeat mRNA and several proteins. Work to date has focused on the potential role of the CGG repeat in eliciting neurodegeneration via an RNA gain-of-function mechanism. However, critical aspects of disease pathology are not easily explained by a pure mRNA mediated sequestration process. Our central hypothesis is that the CGG repeat expansion contributes to neuronal degeneration in FXTAS via multiple, overlapping molecular mechanisms. These mechanisms include transcriptional dysregulation of the FMR1 gene in cis and RNA mediated gain of function toxicity leading to anomalous RNA splicing events and alterations in protein quality control pathways. Our proposal to test this hypothesis is divided into three aims. Aim 1 will examine the mechanisms by which FMR1 mRNA transcription is increased, focusing on the histone acetylation status at the FMR1 locus. Aim 2 will examine the role of the ubiquitin proteasome system in FXTAS pathogenesis. Both aims one and two will utilize a drosophila model of FXTAS as well as patient derived lymphoblasts. Aim 3 will employ inducible pluripotent stem cells from FXTAS patient fibroblasts that are differentiated into neurons. We will determine the alternative transcriptome profile of these human FXTAS neurons to address the hypothesis that sequestration of RNA binding proteins in FXTAS leads to aberrant mRNA splicing. Taken together, these studies should significantly extend our understanding of this neurodegenerative disorder and assist in identification appropriate candidate therapeutic targets. PUBLIC HEALTH RELEVANCE: Fragile X Tremor Ataxia syndrome (FXTAS) is a common inherited rodegenerative disorder that causes difficulties with walking, shaking of the limbs and dementia in older men. Our work proposes to test the idea that the genetic cause of FXTAS leads to neuronal death by two overlapping mechanisms: it increases the transcription of the mutant gene into RNA and it makes that RNA toxic to neurons, such that problems develop with the way these cells process and degrade other RNAs and proteins. To test our hypothesis we will utilize a fruit fly model of the disorder and we will also derive inducible pluripotent stem cells from patients with the disease so that we can study human derived neurons with the causative mutation. Taken together, we feel these studies will tell us a lot about what causes FXTAS and what we need to do to develop treatments for it.