Spatial transcriptomic analysis of muscle biopsy from patients with treatment-naive juvenile dermatomyositis reveals mitochondrial abnormalities despite disease-related interferon-driven signature

Objectives: This study aimed to investigate the spatial transcriptomic landscape of muscle tissue from treatment-naïve juvenile dermatomyositis (JDM) patients in comparison to healthy paediatric muscle tissue. // Methods: Muscle biopsies from three JDM patients and three age-matched controls were analysed using the Nanostring GeoMx® Digital Spatial Profiler. Regions of interest were selected based on muscle fibres without immune cells, immune cell infiltration and CD68+ macrophage enrichment. Differential gene expression, pathway analysis and pathways clustering analysis were conducted. Key findings were validated in 19 cases of JDM using immunohistochemistry and chemical stains, and a bulk RNAseq dataset of four cases of JDM. // Results: JDM muscle tissues exhibited significant interferon pathway activation and mitochondrial dysfunction compared to controls. A 15-gene interferon signature was significantly elevated in JDM muscle and macrophage-enriched regions, correlating with clinical weakness. In contrast, mitochondrial dysregulation, characterized by downregulated respiratory chain pathways, was present regardless of interferon activity or muscle strength. The interferondriven and mitochondrial signatures were replicated in an independent RNAseq dataset from JDM muscle; lack of association between interferon signature and mitochondrial dysregulation was validated in 19 cases by conventional staining methods. Clustering analysis revealed distinct transcriptomic profiles between JDM and control tissues, as well as between JDM patients with varying clinical phenotypes. // Conclusions: This study highlights mitochondrial dysfunction as a consistent pathological feature in JDM muscle, which may be independent of interferon-driven inflammation. These findings highlight the potential for mitochondrial-targeted therapies in JDM management and emphasise the need for further studies to explore their therapeutic value.