Investigating the pre-mRNA splicing of the Survival Motor Neuron genes to model the Spinal Muscular Atrophy disease phenotype

Proximal spinal muscular atrophy (SMA) is a neurodegenerative disease caused by low levels of the Survival Motor Neuron (SMN) protein. In humans SMN1 and SMN2 encode the SMN protein. In SMA patients the SMN1 gene is lost and the remaining SMN2 gene only partially compensates. Mediated by a C>T nucleotide transition in SMN2i, the inefficient recognition of exon 7 by the splicing machinery results in low levels of SMN. Exon 7 splicing is regulated by a number of splicing regulatory sequences.;Here we show that mouse Smn and human SMN minigenes are regulated similarly by conserved elements within exon 7 and its downstream intron. Importantly, the C>T mutation is sufficient to induce exon 7 skipping in the mouse minigene as in the human SMN2 gene. Furthermore, by examining evolutionarily conserved sequences in the SMN genes from a number of different species we found two conserved elements that affect exon 7 splicing in our human minigene system. Additional analysis of one of these regions showed decreased inclusion of exon 7 in SMN transcripts when deletions or mutations were introduced. These results describe a novel intronic splicing enhancer sequence located in the final intron of the SMN genes.;A better understanding of the way SMN pre-mRNA is spliced can lead to the development of new therapies aimed at correcting SMN2 splicing. Because the SMN2 gene is capable of expressing SMN protein, correction of SMN2 splicing is an attractive therapeutic option. While current mouse models of SMA characterized by Smn knockout alleles in combination with SMN2 transgenes adequately model the disease phenotype, their complex genetics and short lifespan have hindered the development and testing of therapies aimed at SMN2 splicing correction. When the mouse Smn gene was humanized to carry the SMN2 C>T mutation, keeping it under the control of the endogenous promoter and in the natural genomic context, the resulting mice exhibit exon 7 skipping and mild adult onset SMA characterized by muscle weakness, decreased activity, and an increased incidence of atrophic muscle fibers. This new Smn C>T mouse model represents an adult onset form of SMA in which therapies aimed at correction of SMN2 splicing can be easily tested.