Novel genetic determinants of the neuromuscular SMA phenotype

描述（由申请方提供）：脊髓性肌萎缩症（SMA）是一种常见的、经常致死的常染色体隐性遗传疾病，由运动神经元1（SMN 1）基因的纯合突变引起，导致SMN蛋白缺乏。残留蛋白由SMN 2（SMN 1基因的部分功能同源物）表达。目前还没有治愈SMA的方法。目前可用的治疗方法充其量是姑息性的。尽管已经了解了很多关于人类疾病的病理学和自然史，并且尽管概念验证研究证明了通过将SMN恢复到疾病的小鼠模型来挽救SMA表型，但将低水平的蛋白质与神经变性联系起来的生化途径仍然不清楚。SMN在协调snRNP生物发生中的单一既定功能未能充分阐明SMA中观察到的运动神经元表型，促使寻找将SMN缺乏和破坏的snRNP生物发生与神经肌肉疾病联系起来的蛋白质和/或基因的其他功能。增加SMN 2拷贝数可导致患者和突变小鼠中SMN蛋白水平升高，并导致表型变轻。然而，在极少数情况下，SMN 2拷贝与疾病严重程度之间的相关性不再成立，这意味着存在SMA表型的其他遗传修饰因子。鉴定这些修饰物是揭示SMN蛋白新的疾病相关功能或揭示snRNP生物发生中断导致SMA表型的效应基因的一种方法。在NIH的资金申请中，我们概述了两个相关目的的实验，以利用SMA小鼠模型中疾病表型的修饰来绘制和鉴定修饰基因座。在目的1中，将创建SMA小鼠的同类品系以精确定义不同遗传背景如何影响突变表型。此外，将通过分子、细胞和表型方法生成并表征来自同源SMA载体之间的确定菌株间杂交的突变体。在目标2中，具有最不同疾病表型的突变体将用于连锁研究以定位并最终鉴定修饰基因座。为了证实所鉴定的基因座的疾病修饰作用，我们将它们重新引入表现出“典型”疾病表型的SMA小鼠中。我们的研究将产生两个重要结果。首先，他们将发现新的疾病相关的生化途径，从而为脊髓性肌萎缩症的潜在生物学提供信息。其次，他们将确定可以作为未来SMA治疗新分子靶点的基因。我们的实验结果将为SMA患者设计安全有效的治疗方法迈出重要一步。 公共卫生相关性：SMA是一种由SMN蛋白减少引起的使人衰弱、经常致死、无法治愈的人类神经肌肉疾病。我们希望确定从SMN减少到功能障碍和疾病的途径。为此，我们制作了小鼠模型，使我们能够识别这些途径和新的相关基因。确定这些通路和基因不仅可以更好地了解SMA，还可以确定安全有效治疗该疾病的潜在靶点。

DESCRIPTION (provided by applicant): Spinal muscular atrophy (SMA) is a common, frequently fatal, autosomal recessive disorder caused by homozygous mutations in the Survival of Motor Neuron 1 (SMN1) gene that lead to a deficiency of the SMN protein. Residual protein is expressed from SMN2, a partially functional homologue of the SMN1 gene. There is presently no cure for SMA. Currently available treatments are palliative at best. Although much has been learned about the pathology and natural history of the human disease and notwithstanding proof-of-concept studies demonstrating rescue of an SMA phenotype by restoring SMN to mouse models of the disease, the biochemical pathway(s) linking low levels of the protein to neurodegeneration remain(s) obscure. The single established function of SMN in orchestrating snRNP biogenesis has failed to shed adequate light on the motor neuron phenotype observed in SMA, prompting the search for additional functions of the protein and/or genes linking SMN paucity and disrupted snRNP biogenesis to neuromuscular disease. Increasing SMN2 copy number leads to higher levels of the SMN protein in patients and mutant mice and results in milder phenotypes. However, in rare instances the correlation between SMN2 copies and disease severity no longer holds, implying the existence of additional genetic modifiers of the SMA phenotype. Identifying such modifiers is one way to uncover new, disease-relevant functions of the SMN protein or reveal effector genes through which a disruption in snRNP biogenesis causes the SMA phenotype. In this application for funding to the NIH, we have outlined experiments in two related aims to exploit a modification of the disease phenotype in mouse models of SMA to map and identify modifying loci. In aim 1 congenic strains of SMA mice will be created to precisely define how different genetic backgrounds affect the mutant phenotype. Additionally, mutants from defined inter-strain crosses between the congenic SMA carriers will be generated and characterized by molecular, cellular and phenotypic means. In aim 2, mutants with the most distinct disease phenotypes will be used in linkage studies to map and eventually identify modifier loci. To confirm the disease modifying effects of the identified loci we will re-introduce them into SMA mice exhibiting a "typical" disease phenotype. Our studies will have two important outcomes. First, they will uncover novel, disease-relevant biochemical pathways and thus inform the underlying biology of spinal muscular atrophy. Second, they will identify genes that could serve as new molecular targets for future SMA therapies. The results of our experiments will constitute an important step toward the design of safe and effective treatments for SMA patients. PUBLIC HEALTH RELEVANCE: SMA is a debilitating, frequently fatal, incurable human neuromuscular disorder caused by reduced SMN protein. We wish to define pathways that lead from reduced SMN to dysfunction and disease. To do so we have made mouse models that allows us to identify such pathways and novel associated genes. Defining the pathways and genes will not only lead to a better understanding of SMA but also serve to identify potential targets for safe and effective treatments for the disease.