Background Dermatomysoitis (DM) has been related to high type I interferon (IFN-I) signaling in skeletal muscle which is thought to play a pivotal role in muscle inflammation and impairment. However, the mechanisms by which muscle dysfunctions occurred remain unknown.
Objectives We assessed the involvement of mitochondria in the muscle of recent onset untreated DM patients using transcriptomic, morphological and in situ functional studies. We also unraveled the link between inflammation, reactive oxygen species (ROS) and mitochondrial dysfunctions using animal model and cell system.
Methods Ten early (<6 months) and untreated DM (according to ENMC) patients were prospectively included and compared to controls. C2C12 cells were exposed to IFN-β in the presence and the absence N-acetyl-cystein (NAC), a potent ROS scavenger. BALB/c mice were immunized with PBS (controls) or myosin (EAM) emulsified in CFA. The treated group received 300 mg/kg/day of NAC until sacrifice. We recorded aerobic capacity in patients and muscle strength in animals. Transcriptomic analysis were confirmed using qPCR. Mitochondria integrity and function was further assessed by oxygraphic oxygen consumption recording, histoenzymological staining of oxidative enzymes and electronic microscopy (EM). Reactive oxygen species (ROS) production was quantified using electron paramagnetic resonance spectroscopy or H202 detection with spectrofluorometry.
Results The main up-regulated cluster of transcripts in DM patients (n=3) was composed of genes encoding proteins involved in inflammatory responses (especially IFN-I induced genes) while the first cluster of the down-regulated transcripts was composed of genes encoding proteins involved in mitochondrial integrity and functions. Numerous mitochondrial abnormalities were found on EM and abnormal histochemical staining of oxidative enzymes was noted in all DM muscles (n=10). In situ oxygen consumption of DM muscle samples (n=10) was about 40% decreased whatever the mitochondrial substrate used while H2O2 production was increased (p<0.01). Oxygen consumption in the muscle correlated with maximal aerobic capacities on cycle ergometer (Spearman r=0.90).
After 24 hours exposition, IFN-β induced a 30% decrease in mitochondrial respiration of C2C12 cells that was prevented by ROS scavenging with NAC (p<0.05).
Muscle of EAM animals exhibited 2-fold increase in ROS production and about 1.5-fold decrease in mitochondrial respiration that was partially prevented by NAC. NAC also prevented EAM muscle weakness (p<0.05) and the increase of muscle transcripts of several genes implied in inflammation including IFIT3 (p<0.05).
Conclusions Mitochondrial dysfunctions, contributing to poor aerobic capacity, occur early in DM muscle and might be mediated by high ROS production triggered by IFN-β. In turn, ROS might participate in IFN-I inducible genes expression and inflammation, which can become self-sustained.
Disclosure of Interest None declared