O.7 Spinal muscular atrophy: How it works and therapeutic targets

Spinal muscular atrophy is caused by loss of SMN1 and retention of SMN2. This results in low SMN levels. The SMN protein functions in assembling of Sm proteins onto snRNAs this stabilizes the snRNA and results in transport of the snRNP into the nucleus where they are critical in splicing of genes. While there is currently a debate on whether snRNP assembly or some unknown function in axons is responsible for SMA. We have found direct correlation of snRNP assembly and SMA severity in SMA mice. Furthermore we find no axonal or axon patterning defects in mice and SMN missense mutations that do correct axonal defects in SMN reduced Zebrafish do correct SMA mice and snRNP assembly in these mice. Thus currently we favor disruption of splicing of a select number of target genes in motor neurons as the molecular mechanism. In this regards we have performed laser microdisection of motor neurons from SMA mice and identified a limited number of changes (20) which are specific to SMN deficiency and not due to the state of the motor neuron. These changes are specific to motor neurons and not detectable in total spinal cord extracts we are currently investigating whether restoration gives rescue of the SMA phenotype thus indicating these changes as alternative therapeutic targets. We have also found that mild C and N terminal mutation complement each other and completely rescue Smn-/- mice with no SMA phenotype indicating that there are critical proteins bound by the N and C terminus for the function of SMN in SMA. The SMN function disrupted in SMA is unlikely to be the axonal complex. We are looking at specific back mutation to determine conclusively whether Sm assembly is the critical function affected in SMA. Lastly we have optimized antisense oligonucleotide treatment of SMA by treating at P0-P3 followed by re-administration of the ASO at 30days in a formulation that allows wide spread distribution of the morpholino this results in extended survival beyond 150 days.