Résumé :
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Spinal Muscular Atrophy (SMA) is caused by deletion/inactivation of the SMN1 (survival of motoneurons) gene resulting in a degeneration of ?-motoneurons. A second, nearly identical gene (SMN2) allows patients to survive into early childhood, but its ability to complement the defect is incomplete, as a single base difference causes an almost total exclusion of exon 7 from SMN2 mRNA. It has been hypothesised that boosting SMN2 exon 7 inclusion could represent an effective SMA therapy. However, although exon 7 inclusion can be stimulated in cell culture by oligonucleotides or intracellularly expressed RNAs, it remains to be proven that this can alleviate SMA symptoms in vivo. We have described a modified U7 snRNA that can supply an exogenous exonic splicing enhancer sequence to SMN2 pre-mRNA. One such construct caused an almost complete exon 7 inclusion from an SMN2 reporter gene, both in transient transfection and stably after lentiviral transduction. Moreover, a permanent splicing correction of the endogenous SMN2 pre-mRNA in SMA type I patient fibroblasts was achieved, resulting in an increase and correct localisation of SMN protein. We have now generated transgenic mice carrying the corrective U7 snRNA gene and have cross-bred them into the most severe SMA mouse model. In these animals, the mouse smn gene is deleted and two copies of the human SMN2 gene allow survival up to 6 days after birth. Preliminary results show a clear complementation of this very severe SMA phenotype. Depending on the number of U7 gene copies, this complementation is either moderate with death occurring between 17 and >75 days or strong with 3 out of 3 mice surviving beyond day 75. Besides being an important milestone towards the development of a somatic gene therapy for SMA, this result effectively proves the therapeutic value of an SMN2 exon 7 inclusion strategy.
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