Novel genetic determinants of the neuromuscular SMA phenotype

描述(申请人提供)：脊髓性肌萎缩症(SMA)是一种常见的，通常是致命的常染色体隐性遗传病，由运动神经元存活1(SMN1)基因的纯合子突变导致SMN蛋白缺陷引起。残留蛋白是从SMN1基因的部分功能同源物SMN2中表达的。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治疗。尽管已经对人类疾病的病理和自然历史有了很多了解，尽管概念验证研究表明通过将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退变联系起来的生化途径(S)仍然不清楚(S)。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，促使人们寻找将SMN缺乏和中断的SnRNP生物发生与神经肌肉疾病联系起来的蛋白质和/或基因的额外功能。增加SMN2拷贝数会导致患者和突变小鼠中SMN蛋白的水平更高，并导致更温和的表型。然而，在极少数情况下，SMN2拷贝和疾病严重程度之间的相关性不再成立，这意味着存在SMA表型的额外遗传修饰物。识别这些修饰物是发现SMN蛋白新的疾病相关功能或揭示效应基因的一种方式，通过这些基因，SnRNP生物发生的中断会导致SMA表型。在这份给NIH的资金申请中，我们概述了两个相关的实验，目的是利用SMA小鼠模型中疾病表型的修饰来定位和识别修饰基因。在目标1中，将创建SMA小鼠的同源品系，以精确地定义不同的遗传背景如何影响突变表型。此外，来自同源SMA携带者之间定义的菌株间杂交的突变体将被产生，并通过分子、细胞和表型手段进行表征。在目标2中，具有最明显疾病表型的突变体将用于连锁研究，以定位并最终识别修饰基因座。为了确认已识别的基因座的疾病改善效果，我们将把它们重新引入表现出“典型”疾病表型的SMA小鼠。我们的研究将有两个重要的结果。首先，他们将发现新的、与疾病相关的生化途径，从而揭示脊髓性肌肉萎缩的潜在生物学。其次，他们将识别可以作为未来SMA疗法新分子靶点的基因。我们的实验结果将成为设计安全有效的SMA患者治疗方法的重要一步。

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.