Survival Motor Neuron Genes in Spinal Muscular Atrophy

描述（由申请人提供）：脊髓性肌萎缩症（SMA）是人类第二常见的常染色体隐性遗传疾病，也是婴儿死亡的最常见遗传原因。它是由运动神经元1存活基因（SMN1）而不是其复制基因SMN2的缺失引起的。这两个几乎相同的基因在外显子7上有一个核苷酸的不同，这改变了外显子7剪接增强子的活性。因此，大多数来自SMN2的转录本缺乏外显子7 (delta7 SMN)，而来自SMN1的转录本大部分是全长（FLSMN）。delta7 SMN蛋白产物不稳定，降解迅速。因此，SMN2只产生低水平的SMN蛋白。低水平的SMN蛋白导致运动神经元变性，这是SMA的一个特征。小鼠缺乏SMN2，纯合子敲除小鼠Stun是胚胎致死的。我们通过在缺乏小鼠眩晕的小鼠中表达人类SMN2基因，建立了小鼠SMA模型。在无Smn背景下携带1或2个SMN2基因拷贝的小鼠表现出严重（I型）SMA的所有症状，而8个SMN2拷贝则完全挽救了疾病表型。这表明来自SMN2的足够的SMN蛋白可以预防运动神经元的丢失和疾病表型。因此，包括我们自己在内的许多研究小组已经启动了高通量药物筛选，旨在识别能够刺激SMN2中SMN表达的分子。我们最近报道了一种这样的化合物，清楚地证明了药物筛选的可行性。然而，导致SMA最终治疗的重要问题仍然存在。我们在这项资助中的目标是：1)表征轻度SMA的小鼠模型，以确定运动神经元丢失的时间；2)确定在疾病过程的哪个阶段需要高水平的SMN来挽救SMA表型。3)在小鼠中确定在SMA相关形式（SMA伴有呼吸窘迫）中发现的表型修饰因子是否也会改变SMA的表型；4)确定在细胞培养中改变SMN2表达的药物化合物是否也在全动物（体内）中起作用。该提案的结果有望成为SMA有效治疗的过渡。

DESCRIPTION (provided by applicant): Spinal muscular atrophy (SMA) is the second most common autosomal recessive inherited disorder in humans, and the most common genetic cause of infant death. It is caused by loss of the survival motor neuron 1 gene(SMN1) but not its copy gene, SMN2. These two virtually identical genes differ by a single nucleotide in exon 7 which alters the activity of an exon 7 splice enhancer. Consequently, a majority of the transcripts from SMN2 lacks exon 7 (delta7 SMN), whereas most of the transcript from SMN1 is full length (FLSMN). The protein product of delta7 SMN is unstable and is rapidly degraded. Thus, SMN2 produces only low levels of SMN protein. Low levels of SMN protein result in motor neuron degeneration, a characteristic feature of SMA. Mice lack SMN2 and a homozygous knockout of murine Stun is embryonic lethal. We have created mouse models of SMA by expressing the human SMN2 gene in mice lacking murine Stun. Mice carrying 1 or 2 copies of the SMN2 transgene on a null Smn background exhibit all of the symptoms of severe (type I) SMA, whereas 8 copies of the SMN2 completely rescue the disease phenotype. This indicates that sufficient SMN protein from SMN2 can prevent motor neuron loss and the disease phenotype. Numerous groups including our own have therefore initiated high through-put drug screens designed to identify molecules capable of stimulating SMN expression from SMN2. We have recently reported one such compound clearly demonstrating the feasibility of the drug screens. However, important questions leading to the eventual treatment of SMA remain. Our aims in this grant are to 1) characterize a mouse model of mild SMA, in order to determine the timing of motor neuron loss, 2) determine at what stage of the disease process high levels of SMN are required to rescue the SMA phenotype, 3) determine in mice whether a phenotypic modifier identified in a related form of SMA (SMA with respiratory distress) also modifies the SMA phenotype and 4) determine whether drug compounds that alter SMN2 expression in cell culture also function in whole animals (in vivo). The results of this proposal are hoped to act as a transition to an effective treatment of SMA.