using SMN gene replacement to address biological questions

Spinal Muscular Atrophy (SMA), an autosomal recessive neuromuscular disorder, is the leading genetic cause of infant mortality. SMA is caused by the functional, homozygous loss of the Survival Motor Neuron-1 (SMN1) gene which encodes for the ubiquitously expressed Survival Motor Neuron (SMN) protein. Here we utilize SMA mouse models to demonstrate that self-complementary Adeno-associated virus encoding SMN1 (scAAV-SMN) is a promising therapeutic for the treatment of SMA. We show that pre-symptomatic, ubiquitous restoration of SMN via scAAV is able to dramatically improve the SMA phenotype extending lifespan from ̃14 to 200+ days. We show that direct delivery of scAAV-SMN into the central nervous system is able to more robustly rescue the phenotype of a mouse model of SMA compared to systemic, intravascular delivery. After determining the most efficacious route of injection, we seek to determine the therapeutic window and investigate the effectiveness of viral delivery after the onset of disease symptoms. We report that early, pre-symptomatic scAAV-SMN delivery produces a better phenotypic rescue than treatment after the onset of symptoms, emphasizing the need for early therapeutic intervention. We assess the need for SMN in a specific cell population, upper cortical motor neurons, reporting that restoration of SMN to solely to these cells is not sufficient to modify the SMA phenotype. Lastly, we use the scAAV vector to delivery mutant forms of SMN and other genes which may be able to modify the SMA phenotype. In this experiment, we identify several modifying genes which are able to improve the phenotype of an intermediate mouse model. Collectively, these studies help to provide insight into the use of scAAV-SMN as a therapeutic and help to address clinically relevant questions about the temporal and spatial requirements of SMN protein during disease progression.Includes bibliographical references (pages 128-144)

Self-oligomerization Regulates Stability of Survival Motor Neuron Protein Isoforms by Sequestering an SCF\u3csup\u3eSlmb\u3c/sup\u3e Degron

Spinal muscular atrophy (SMA) is caused by homozygous mutations in human SMN1. Expression of a duplicate gene (SMN2) primarily results in skipping of exon 7 and production of an unstable protein isoform, SMNΔ7. Although SMN2 exon skipping is the principal contributor to SMA severity, mechanisms governing stability of survival motor neuron (SMN) isoforms are poorly understood. We used a Drosophila model system and label-free proteomics to identify the SCFSlmb ubiquitin E3 ligase complex as a novel SMN binding partner. SCFSlmb interacts with a phosphor degron embedded within the human and fruitfly SMN YG-box oligomerization domains. Substitution of a conserved serine (S270A) interferes with SCFSlmb binding and stabilizes SMNΔ7. SMA-causing missense mutations that block multimerization of full-length SMN are also stabilized in the degron mutant background. Overexpression of SMNΔ7S270A, but not wild-type (WT) SMNΔ7, provides a protective effect in SMA model mice and human motor neuron cell culture systems. Our findings support a model wherein the degron is exposed when SMN is monomeric and sequestered when SMN forms higher-order multimers

Self-oligomerization regulates stability of survival motor neuron protein isoforms by sequestering an SCF^Slmb degron

Spinal muscular atrophy (SMA) is caused by homozygous mutations in human SMN1. Expression of a duplicate gene (SMN2) primarily results in skipping of exon 7 and production of an unstable protein isoform, SMNΔ7. Although SMN2 exon skipping is the principal contributor to SMA severity, mechanisms governing stability of survival motor neuron (SMN) isoforms are poorly understood. We used a Drosophila model system and label-free proteomics to identify the SCFSlmb ubiquitin E3 ligase complex as a novel SMN binding partner. SCFSlmb interacts with a phosphor degron embedded within the human and fruitfly SMN YG-box oligomerization domains. Substitution of a conserved serine (S270A) interferes with SCFSlmb binding and stabilizes SMNΔ7. SMA-causing missense mutations that block multimerization of full-length SMN are also stabilized in the degron mutant background. Overexpression of SMNΔ7S270A, but not wild-type (WT) SMNΔ7, provides a protective effect in SMA model mice and human motor neuron cell culture systems. Our findings support a model wherein the degron is exposed when SMN is monomeric and sequestered when SMN forms higher-order multimers

Restoration of SMN to Emx-1 expressing cortical neurons is not sufficient to provide benefit to a severe mouse model of Spinal Muscular Atrophy.

Spinal Muscular Atrophy (SMA), an autosomal recessive neuromuscular disorder, is a leading genetic cause of infant mortality. SMA is caused by the homozygous loss of Survival Motor Neuron-1 (SMN1). However, low, but essential, levels of SMN protein are produced by a nearly identical copy gene called SMN2. Detailed analysis of neuromuscular junctions in SMA mice has revealed a selective vulnerability in a subset of muscle targets, suggesting that while SMN is reduced uniformly, the functional deficits manifest sporadically. Additionally, in severe SMA models, it is becoming increasing apparent that SMA is not restricted solely to motor neurons. Rather, additional tissues including the heart, vasculature, and the pancreas contribute to the complete SMA-associated pathology. Recently, transgenic models have been utilized to examine the tissue-specific requirements of SMN, including selective depletion and restoration of SMN in motor neurons. To determine whether the cortical neuronal populations expressing the Emx-1 promoter are involved in SMA pathology, we generated a novel SMA mouse model in which SMN expression was specifically induced in Emx-1 expressing cortical neurons utilizing an Emx-1-Cre transgene. While SMN expression was robust in the central nervous system as expected, SMA mice did not live longer. Weight and time-to-right motor function were not significantly improved. Transgenic Res 2013 Oct; 22(5):1029-36