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Description

Idiopathic inflammatory myopathies (IIM) constitute a heterogeneous group of diseases defined by non-infectious nor toxic acquired myopathies. Amongst them, dermatomyositis (DM) is the most frequent IIM, occurring during child and adulthood, and is potentially life-threatening. Its diagnosis is based on the association of muscle weakness, inflammatory infiltrates on histological muscle sections and auto-antibodies in the serum. Etiopathogenesis, that has mainly focused on adaptative immunity, remains largely unknown. In DM, the necrosis/regeneration balance is detrimental to muscle integrity, with a chronic course and poor clinical outcome for some patients. Nevertheless, the capacity of muscle regeneration has been barely studied. In normal injured muscle, regeneration is achieved by myogenic precursor cells (MPCs) that undergo myogenesis process to build new myofibers. The aim of our project was to evaluate DM-derived MPC capacity to undertake myogenesis. Methods. After establishing specific selection criteria, I selected DM (severe and mild ), and healthy controls (HC , sex and age paired) from the Hospices Civils de Lyon database and obtained corresponding samples from the hospital biobank. Supplemental investigation was performed on other IIM derived-MPCs: immune-mediated necrotizing myopathy (IMNM) (n=3), inclusion body myositis (IBM) (n=3), and anti-synthetase syndrome (ASS) (n=3). Myogenesis was explored in vitro using specific MPC culture for the evaluation of proliferation, differentiation and fusion. Proliferation was evaluated as the incorporation of a thymidine analog into DNA during proliferation. Terminal myogenic differentiation was evaluated by myogenin expression, an essential transcription factor of myogenic differentiation. Cell fusion was quantified as the number of nuclei incorporated into myotubes. Quantitative analysis was performed using Image J software, and statistical analysis was performed by mean comparison (t test or ANOVA). Similar methods were used to evaluate the impact of interferon blockers (JAK / STAT inhibitor) on myogenesis. Senescence was analyzed by flow cytometry with fluorogenic substrate of SA-βGalactosidase, a senescence-specific marker. Results. Analysis of MPC proliferation indicates that DM-derived MPCs exhibited a significantly lower proliferation than HC-derived MPCs (p=0.0020). Terminal myogenic differentiation was also strongly impaired in severe DM-derived MPCs while less altered in mild-DM-derived cells (p=0.001). MPC fusion was significantly altered in DM-derived MPCs with a dramatic decreased observed in severe DM and to a lesser degree in mild DM (p<0.0001 for each). These results were recapitulated for other IIM, with altered myogenesis identified for IMNM (n=3), IBM (n=3), but not for ASS (n=3). Through cellular and molecular investigations, we found increased senescence in DM derived-MPCs, and potential involvement of interferon type I in myogenesis impairment. Indeed, sDM and mDM derived-MPCs myogenesis was improved even restored when culture was performed with JAK/STAT inhibitor, a downstream interferon pathway blocker or IFN receptor blocking antibody. Conclusion. These results show that skeletal muscle regeneration is impaired in DM due to altered capacities of MPCs to proliferate, to differentiate and to fuse. This may explain in part the muscle alterations and muscle weakness that are observed in patients. These results support the hypothesis of intrinsic alterations of MPC properties in DM, potentially linked with interferon pathway, and participating to the pathogenesis of the disease.