Muscle weakness in myositis: microRNAs mediate dystrophin reduction in MHC class I transgenic mouse model and human muscle biopsies

OBJECTIVE: Muscle inflammation is a feature in myositis and Duchenne Muscular Dystrophy (DMD). Autoimmune mechanisms are thought to contribute to myositis muscle weakness. However, the lack of correlation between inflammatory infiltrates and muscle weakness indicates a role for non-immune pathological mechanisms. We previously identified two microRNA sets elevated in DMD muscle. One is an "inflammatory" set that is dampened with glucocorticoids; the other is a "dystrophin-targeting" set that inhibits dystrophin translation. Here we test the hypothesis that these miRNAs are similarly dysregulated in myositis muscle and contribute to muscle weakness and disease severity. METHODS: We utilized the MHC class I transgenic myositis mouse model and validated findings in 6 myositis patient muscle biopsies. Mice were classified as mild or severe based on transgene expression, weight, histological severity, and muscle strength/weakness. RESULTS: Severe myositis mice show mononuclear cell infiltration along with elevated expression of Type 1 IFN and NF-B-regulated genes including Tlr7 (3.8-fold, p<0.05). Severe mice show elevated inflammatory miRNAs (miR-146a/miR-142-3p/miR-142-5p/miR-455-3p/miR-455-5p, ~3-40-fold increase, p<0.05) and dystrophin-targeting miRNAs (miR-146a/miR-146b/miR-31/miR-223, ~3-38-fold increase, p<0.05). ChIP-seq bioinformatics identify at least one NF-B consensus element within the promoter/enhancer regions of these miRNAs. Western blot and immunofluorescence demonstrate reduced dystrophin levels in severe myositis muscle. We also observe elevated NF-B-regulated genes, TLR7, and miRNAs along with reduced dystrophin in histologically severe human myositis muscle biopsies. These data demonstrate an acquired dystrophin deficiency may occur through NF-B-regulated miRNAs in myositis and suggest a "unifying theme" for perpetuation of muscle injury, inflammation and weakness in both myositis and DMD.