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。它是由运动神经元生存基因1（SMN1）的丢失引起的，而不是其拷贝基因SMN2。这两个几乎相同的基因的区别在于外显子7中的单个核苷酸，其改变外显子7剪接增强子的活性。因此，来自SMN2的大部分转录物缺乏外显子7（delta7 SMN），而来自SMN1的大部分转录物是全长（FLSMN）。 delta7 SMN的蛋白产物不稳定，并迅速降解。因此，SMN2仅产生低水平的SMN蛋白。低水平的SMN蛋白导致运动神经元变性，这是SMA的特征性特征。小鼠缺乏SMN2，并且小鼠Stun的纯合敲除是胚胎致死的。我们通过在缺乏鼠Stun的小鼠中表达人SMN2基因来创建SMA小鼠模型。在无Smn背景下携带1或2个SMN2转基因拷贝的小鼠表现出重度（I型）SMA的所有症状，而8个SMN2拷贝完全挽救了疾病表型。这表明来自SMN2的足够的SMN蛋白可以防止运动神经元损失和疾病表型。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，旨在鉴定能够刺激SMN 2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的是1）表征轻度SMA的小鼠模型，以确定运动神经元损失的时间，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的相关形式中鉴定的表型修饰物是否与SMA的相关形式中鉴定的表型修饰物相同。（SMA伴呼吸窘迫）也改变SMA表型和4）确定改变细胞培养物中SMN2表达的药物化合物是否也在整个动物（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.