Temporally inducible SMN expression and splicing modulation of the SMN2 gene in SMA mouse models

Proximal spinal muscular atrophy (SMA) is a progressive neurodegenerative disease that is caused by low levels of Survival Motor Neuron (SMN) protein. SMN protein is encoded by two genes in humans, SMN1 and SMN2. SMA patients have lost SMN1 expression by mutation or deletion, and SMN2 only partially compensates for SMN1 due to inefficient splicing of exon 7. The inefficiency of SMN2 exon 7 splicing is due to a C>T mutation in exon 7 that reduces exon definition by the splicing machinery. Increase survival in severe SMA mouse models by splicing correction of SMN2 and SMN replacement therapies has proven successful, however, the timing of SMN reintroduction will be crucial in therapeutic efficacy and may vary between severities of SMA.;To address the temporal requirement of SMN replacement we generated an inducible SMN mouse model under the control of Cre-recombinase. Induction of SMN expression at varying time-points during development was verified using tamoxifen inducible Cre expression. We further evaluated the ability of our inducible SMN mouse to modulate survival of a severe SMA mouse model. Our lab previously developed a mild mouse model of SMA by introducing the SMN2 C>T mutation into the murine Smn gene, the homologue of SMN1. This mouse model exhibits motor function and behavioral deficits that recapitulate mild SMA. We further characterized these mice for neuropathological defects that may serve as markers for therapeutic benefit when SMN is reintroduced to determine if the same therapeutic window is required for mild SMA mouse models.;Disease progression in severe SMA mice is associated with global alterations in splicing and induction of stress related genes. One such gene that is altered by splicing and increased expression is the hypoxia inducible hif3alpha gene. We observed induced skipping of SMN2 exon 7 under hypoxic stress and reduction of SMN protein. SMA mice treated with hyperoxia showed an improvement in motor function and inclusion of SMN2 exon 7. Respiratory assistance is a primary treatment in SMA and is capable of extending survival and quality of life of SMA patients. The role of hypoxic stress induced skipping of SMN2 exon 7 may shed light on additional therapeutic benefits of respiratory assistance in SMA patients, beyond compensation of progressively weakening respiratory muscles.