Résumé :
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Myotonic dystrophy type 1 (DM1) is a dominant neuromuscular disorder affecting 1/8000 individuals worldwide. This disease is caused by an unstable expansion of CTG trinucleotide repeats located in the 3' untranslated region of the dystrophy myotonic protein kinase (DMPK) gene (Fu et al, 1992). In muscle cells, the mutated DMPK transcript is retained in nuclear foci (Davis et al, 1997) where it sequesters the splicing factor MBNL1 (Mankodi et al, 2001). In addition, the activation of the protein kinase C in DM1 muscle cells leads to an increased stability of the splicing factor CUGBP1 (Kuyumcu-Martinez et al, 2007).Thanks to site-specific transgenesis, we generated a Drosophila model of DM1 expressing non-coding RNAs with 240, 480, 600 or 960 CUG interrupted repeats. When non coding RNAs carrying CUG repeats were specifically expressed in somatic muscles, foci can be detected by in situ hybridization in all nuclei of muscle fibres of 3rd instar larvae and colocalize with the MBNL1 drosophila orthologue, Mbl. In order to better understand the role played by MBNL1 and CUGBP1 in this disease, 2 lines were tested in parallel with DM1 lines: (i) bru-3 gain-of-function line that mimics CUGBP1 stabilization in muscle cells of DM1 patients (ii) line with attenuated mbl expression that mimics the decreased availability of MBNL1 in muscle cells of DM1 patients.Antibody staining confirmed both cytoplasmic and nuclear expression of Mbl and Bru-3 in larval and adult somatic and cardiac muscles. Heart physiology analyses were performed on semi-intact preparation of 1 week-old and 5 week-old flies. Increasing bru-3 in cardioblasts does not affect cardiac parameters in flies. However, decreasing mbl in cardioblasts leads to significantly increased heart period (both diastolic and systolic intervals increased), increased arrythmia and asystoles associated with dilated cardiomyopathy (increased diastolic and systolic diameters, decreased fractional shortening). Surprisingly, the DM1 line with 960 repeats displays fibrillation and increased diastolic diameter (but not systolic diameter).To conclude, DM1 lines and mbl attenuated line display different cardiac phenotypes, which can be both found in DM1 patients. Taken together, our results suggest there is another factor in DM1 line, different from Mbl and Bru-3, whose availability or expression is altered.
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