A Role for SMN Exon 7 Splicing in the Selective Vulnerability of Motor Neurons in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an inherited motor neuron disease caused by homozygous loss of theSurvival Motor Neuron 1 (SMN1 ) gene. In the absence ofSMN1 , inefficient inclusion of exon 7 in transcripts from the nearly identicalSMN2 gene results in ubiquitous SMN decrease but selective motor neuron degeneration. Here we investigated whether cell type-specific differences in the efficiency of exon 7 splicing contribute to the vulnerability of SMA motor neurons. We show that normal motor neurons express markedly lower levels of full-length SMN mRNA fromSMN2 than do other cells in the spinal cord. This is due to inefficient exon 7 splicing that is intrinsic to motor neurons under normal conditions. We also find that SMN depletion in mammalian cells decreases exon 7 inclusion through a negative feedback loop affecting the splicing of its own mRNA. This mechanism is activein vivo and further decreases the efficiency of exon 7 inclusion specifically in motor neurons of severe-SMA mice. Consistent with expression of lower levels of full-length SMN, we find that SMN-dependent downstream molecular defects are exacerbated in SMA motor neurons. These findings suggest a mechanism to explain the selective vulnerability of motor neurons to loss ofSMN1 .