Proximal spinal muscular atrophy (SMA) is an autosomal recessive neuro-degenerative disease that is the primary genetic cause of infant mortality in the United States. Absence of the survival of motor neuron 1 (SMN1) gene product leads to the disease. The SMN1 protein is necessary for motor neuron survival. The human genome contains a nearly identical gene. The SMN2 gene, however, is expressed at greatly reduced levels due to reduced processing of the SMN2 RNA product. Specifically, the SMN2 varies from the SMN1 gene at a single nucleotide positioned in exon 7. This altered nucleotide leads to decreased recognition of exon 7 by the splicing machinery and results in skipping of the exon and the generation of a non-functional protein product. Correction of the SMN2 splicing phenotype is therefore a powerful therapeutic option. However, no animal models exist to test this therapeutic option. We hypothesize that the introduction of the human SMN2 exon 7 point mutation in the mouse SMN gene will allow for exon 7 skipping in the mouse and model the SMA phenotype. We further hypothesize that this mouse model will more accurately recapitulate the human SMA condition relative to splicing of the SMN2 gene. In order to test the mouse SMN gene's susceptibility to skipping of exon 7 in the presence of the human SMN2 exon 7 point mutation, we constructed a series of mouse SMN mini-genes comprised of SMN exon 6 to exon 8. As seen with the human SMN minigene, the mouse minigene transcript containing the native ESE sequences yields mRNA containing all three exons (exon 6, 7, and 8) while the minigene containing the exon 7 point mutation predominantly contains the exon 7 skipped mRNA in both human and mouse cell lines. These experiments support the idea that a point mutation in exon 7 of the mouse gene will affect SMN splicing in the mouse tissues, as in humans. We plan on completing aditional experments to demonstrate that a mouse model of Proximal Spinal Muscular Atrophy is appropriate to adequately test therapies. We will construct mouse and human cell lines stably expressing our SMN mini-genes and compare their splicing profile while testing a promising therapy. We hypothesize that both the mouse and human cells will respond in the same way. Additionally, we have designed a targeting construct to generate the exon 7 point mutation in the mouse ES cells by homologous recombination. These ES cells will subsequently be utilized to generate a mouse harboring the SMN C>T mutation in exon 7, potentially modeling the SMA disease phenotype. In the future this mouse model can be used to address potential therapies aimed at correcting SMN2 splicing. ? ? ? ?
| Bebee, Thomas W; Gladman, Jordan T; Chandler, Dawn S (2011) Generation of a tamoxifen inducible SMN mouse for temporal SMN replacement. Genesis 49:927-34 |
| Bebee, Thomas W; Gladman, Jordan T; Chandler, Dawn S (2010) Splicing of the Survival Motor Neuron genes and implications for treatment of SMA Front Biosci (Landmark Ed) 15:1191-1204 |
| Gladman, Jordan T; Bebee, Thomas W; Edwards, Chris et al. (2010) A humanized Smn gene containing the SMN2 nucleotide alteration in exon 7 mimics SMN2 splicing and the SMA disease phenotype. Hum Mol Genet 19:4239-52 |
| Gladman, Jordan T; Chandler, Dawn S (2009) Intron 7 conserved sequence elements regulate the splicing of the SMN genes. Hum Genet 126:833-41 |
