Mitochondrial damage is associated with an early immune response in inclusion body myositis

Polymyositis with mitochondrial pathology (PM-Mito) was first identified in 1997 as a subtype of idiopathic inflammatory myopathy. Recent findings demonstrated significant molecular similarities between PM-Mito and inclusion body myositis (IBM), suggesting a trajectory from early to late IBM and prompting the inclusion of PM-Mito as an IBM precursor (early IBM) within the IBM spectrum. Both PM-Mito and IBM show mitochondrial abnormalities, suggesting that mitochondrial disturbance is a crucial element of IBM pathogenesis. The primary objective of this cross-sectional study was to characterize the mitochondrial phenotype in PM-Mito at histological, ultrastructural and molecular levels and to study the interplay between mitochondrial dysfunction and inflammation. Skeletal muscle biopsies of 27 patients with PM-Mito and 27 with typical IBM were included for morphological and ultrastructural analysis. Mitochondrial DNA (mtDNA) copy number and deletions were assessed by quantitative PCR and long-range PCR, respectively. In addition, full-length single-molecule sequencing of the mtDNA enabled precise mapping of deletions. Protein and RNA levels were studied using unbiased proteomic profiling, immunoblotting and bulk RNA sequencing. Cell-free mtDNA was measured in the serum of IBM patients. We found widespread mitochondrial abnormalities in both PM-Mito and IBM, illustrated by elevated numbers of cytochrome c oxidase (COX)-negative and succinate dehydrogenase (SDH)-positive fibres and prominent ultrastructural abnormalities, with disorganized and concentric cristae within enlarged and dysmorphic mitochondria. mtDNA copy numbers were significantly reduced, and multiple large-scale mtDNA deletions were already evident in PM-Mito compared with healthy age-matched controls, similar to the IBM group. The canonical cyclic GMP-AMP synthase/stimulator of interferon genes inflammatory pathway was activated in PM-Mito and IBM, and we detected elevated levels of circulating cell-free mtDNA indicative of mtDNA leakage. In PM-Mito and IBM, these findings were accompanied by dysregulation of proteins and transcripts linked to the mitochondrial membranes. In summary, we identified that mitochondrial dysfunction with multiple mtDNA deletions and depletion, disturbed mitochondrial ultrastructure and defects of the inner mitochondrial membrane are features of PM-Mito and IBM, underlining the concept of an IBM-spectrum disease. Notably, mitochondrial abnormalities precede tissue remodelling and infiltration by specific T-cell subpopulations (e.g. KLRG1+) characteristic of late IBM. The activation of inflammatory, DNA-sensing pathways might be related to mtDNA release, which would indicate a significant role of mitochondria-associated inflammation in the pathogenesis of IBM-spectrum disease. This study highlights the crucial role of early mitochondrial abnormalities in the pathomechanism of IBM, which might lead to new approaches to therapy.