Novel genetic determinants of the neuromuscular SMA phenotype

描述（由申请人提供）：脊髓性肌萎缩症（SMA）是一种常见的、经常致命的常染色体隐性遗传病，由运动神经元1 （SMN1）存活基因的纯合突变导致SMN蛋白缺乏引起。残余蛋白由SMN2表达，SMN1基因的部分功能同源物。目前还没有治愈SMA的方法。目前可用的治疗方法最多只能起到缓和作用。尽管对人类疾病的病理和自然历史已经了解了很多，尽管概念验证研究表明通过在疾病的小鼠模型中恢复SMN来拯救SMA表型，但将低水平的蛋白质与神经退行性变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，这促使人们寻找将SMN缺乏和snRNP生物发生中断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SMN2拷贝数导致患者和突变小鼠中SMN蛋白水平升高，并导致较温和的表型。然而，在极少数情况下，SMN2拷贝数与疾病严重程度之间的相关性不再有效，这意味着存在额外的SMA表型遗传修饰因子。鉴定这些修饰因子是发现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。在向美国国立卫生研究院申请资金的过程中，我们概述了两个相关目标的实验，以利用SMA小鼠模型中疾病表型的修饰来绘制和识别修饰位点。在目标1中，将创建SMA小鼠的同源菌株，以精确定义不同的遗传背景如何影响突变表型。此外，同源SMA携带者之间定义的株间杂交将产生突变体，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，以绘制并最终确定修饰位点。为了确认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。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，从而为脊髓性肌萎缩症的潜在生物学提供信息。其次，他们将识别出可以作为未来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的一步。

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.