In vitro ageing of myotubes derived from myoblasts of patients affected by myotonic dystrophy type 2: ultrastructural evidence

Myotonic dystrophy type 2 (DM2) is a dominantly inherited autosomal disease with multi-systemic clinical features and is caused by expansion of a CCTG tetranucleotide repeat in the first intron of the zinc finger protein 9 (ZNF9) gene in 3q21. The expanded- CCUG-containing transcripts are retained in cell nuclear domains (called foci) which specifically sequester some splicing factors, thus causing a general alteration of the pre-mRNA post-transcriptional pathway that is likely responsible for the multifactorial phenotype of DM2 patients. However, at the skeletal muscle level, there is still no mechanistic explanation for the muscle weakness and atrophy of DM2 patients. It has been noted that in DM2 patients skeletal muscle regeneration is decreased, suggesting an impaired responsiveness of satellite cells to regeneration stimuli as much as it occurs in ageing muscles. In order to investigate the differentiation potential of senesceing DM2 myoblasts and the development of the derived myotubes, we developed a model system of cell ageing in vitro: by this approach, the structural features of myotubes derived from DM2 myoblasts grown in culture for increasing times have been investigated by fluorescence and transmission electron microscopy. Apparent alterations of several cytoplasmic features have been observed in the myotubes derived from myoblasts at higher passages. This strongly argues in favour of the involvement of satellite cell senescence in the reduced regenerative potential of dystrophic muscles.