Determinants that regulate splicing of SMN

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Spinal muscular atrophy (SMA) is the leading genetic cause of infantile death, yet there currently is no cure. SMA is a neuromuscular disorder resulting from the loss of survival motor neuron 1 (SMN1). A nearly identical copy gene exists, SMN2, however, it cannot provide protection from disease development in the absence of SMN1. Remarkably, both SMN1 and 2 encode identical proteins, however, due to a single silent mutation, the vast majority of SMN2 transcripts are alternatively spliced and the final coding exon (exon 7: 54 nts) is removed. Therefore, the molecular basis for this devastating disease is an alternative splicing event that results in the production of a truncated and unstable SMN protein (called SMN-delta7). The restoration of full-length SMN expression by modulating SMN2 splicing patterns represents an exciting prospect for therapeutic intervention. The molecular genetics of SMA make this disease especially amenable to therapeutic strategies that promote full-length expression from SMN2. 1) nearly 99% of all SMA cases are caused by a single gene; 2) SMN2 encodes an identical protein; 3) the SMA population is remarkably homogenous with regards to SMN2. Individuals have not been identified that are homozygous null for SMN1 and SMN2 - presumably because this condition would be lethal (consistent with the knock- out mouse model). Therefore, essentially all SMA patients carry at least one SMN2 gene; and 4) transcripts generated from SMN2 are stable. The primary goal of this proposal is to develop recombinant adeno-associated virus (rAAV) vectors that express short RNAs that promote stimulate full-length SMN expression by promoting the inclusion of SMN2 exon 7. A step-wise evaluation process will be used to identify the most effective rAAV-derived RNAs in cell- based models. Finally, the most effective rAAV vectors will be evaluated in a mild SMA mouse model to determine whether SMN2 splicing can be altered in vivo and whether this increase ameliorates the well- characterized SMA phenotype. While the experiments described in this proposal have immediate implications for the development of a SMA therapy, the results could be used as a model for a broad range of genetic disorders in which correcting a splicing defect would restore functionality to a disease-causing gene.