Mitochondrial Respiratory Chain Function is crucial for Muscle Toxicity in Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is driven by DUX4-induced toxicity, yet the pathomechanisms remain unclear. Here, we identify persistent transcriptional suppression of the mitochondrial respiratory chain and metabolic rewiring in FSHD muscle biopsies and myotubes. Using DUX4-inducible human myogenic cells, we show that DUX4 target gene activation is accompanied by mitochondrial function impairment and Caspase 9-mediated apoptosis. Reverse electron transfer (RET) at complex I is the dominant oxidative stress generating mechanism in FSHD muscle cells. RET-driven mitochondrial reactive oxygen species (mitoROS) are the trigger for oxidative stress and apoptosis, which is a unique feature of FSHD mitochondria. Pharmacological inhibition of RET suppresses mitoROS, reduces Caspase 9 activation, and rescues abnormal myogenesis in FSHD cell lines. Importantly, DUX4 is non-toxic in oxidative phosphorylation-deficient human myogenic cells. Our findings identify the mitochondrial respiratory chain as a key mediator of DUX4 toxicity and highlight RET inhibition as a potential therapeutic approach for FSHD.