MOLECULAR BASIS OF SKELETAL MUSCLE ATROPHY IN MYOTONIC DYSTROPHY
Creators: name: RENNA, LAURA VALENTINA
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: Myotonic dystrophy (DM) is an autosomal dominant multisystemic disorder characterized by a variety of multisystemic features including myotonia, muscular dystrophy, cardiac dysfunctions, cataracts and insulin-resistance. DM1 is caused by an expanded (CTG)n in the 3? UTR of the DMPK gene, while DM2 is caused by the expansion of a (CCTG)n repeat in the intron 1 of the CNBP gene. In both forms, the mutant transcripts accumulate in nuclear foci altering the function of some alternative splicing regulators which are necessary for the physiological processing of mRNAs. However, the downstream pathways by which these RNA binding proteins cause skeletal muscle alteration are not well understood. For these reasons the aim of my PhD project was to analyze the molecular mechanisms behind DM skeletal muscle atrophy. In the first part of my PhD we have performed different studies to better define the molecular pathogenesis of DM. In particular, we have analysed the histopathological and biomolecular features of skeletal muscle biopsies from a cohort of DM1 and DM2 patients presenting different phenotypes. The results indicated that the splicing and muscle pathological alterations observed are related to the clinical DM1 and DM2 phenotype and that CUGBP1 seems to play a role only in DM1, confirming that the molecular pathomechanism of DM is more complex than the one actually suggested. These data were confirmed by the analysis of two different biopsies obtained from 5 DM2 patients that showed that morphological alterations evolve more rapidly over time than the molecular changes suggesting that the molecular mechanisms that drive to skeletal muscle atrophy are still unclear and that these features cannot be explained only by spliceopathy. For all these reasons we decided to analyse DM satellite cells activity in vitro. Satellite cells are the muscle fibre precursor cells and our data indicated that both DM1 and DM2 skeletal muscle cells have lower proliferative capability than control myoblasts. Moreover, the premature proliferative growth arrest observed in DM cells appears to be caused by an overexpression of p16 in DM1 muscle cells, while DM2 muscle cells stop dividing with telomeres shorter than controls, suggesting that in these cells the signaling involved in premature senescence depend on a telomere-driven pathway. Finally, we decided to analyze the insulin pathway which is involved in the regulation of skeletal muscle atrophy. Our data have shown that DM1 and DM2 cells exhibit a lower glucose uptake and a lower proteins activation after 10 nM insulin stimulation when compared to controls suggesting that also this pathway could play a role in the molecular mechanisms that drive skeletal muscle atrophy in DM patients. In conclusion, we have shown that the molecular mechanisms behind skeletal muscle atrophy in DM1 and DM2 patients are more complicated than that previously suggested and further analysis are necessary to understand why skeletal muscle atrophy affect mainly type 1 fibres in DM1 patients, while on the contrary it affects selectively type 2 fibres in DM2 patients.