Mutations leading to the loss of protein dysferlin cause limb girdle muscular dystrophy type R2 (formerly LGMD2B). The loss of dysferlin persistently impairs cellular calcium handling and lysosome fusion, and leads to poor repair of skeletal muscle cells. However, despite the existence of these cellular pathologies, the onset of clinical symptoms in LGMDR2 does not arise until late adolescence. Hence the mechanism of pathogenesis in LGMDR2 remains enigmatic, which has hampered therapeutic development.
To gain insight, we compared the repair ability of myofibers from young (asymptomatic) and old (symptomatic) dysferlin-deficient mice, finding that myofiber repair ability is increasingly compromised with disease progression. We further investigated the cellular mechanisms for delayed onset of membrane repair deficit and of clinical symptoms in LGMDR2 muscles, by performing longitudinal (same muscle over different ages) and cross-sectional (different muscles at same age) proteomic analysis in healthy and LGMDR2 mouse models. Both, longitudinal and cross-sectional muscle samples showed upregulation of proteins involved in vesicle trafficking and membrane repair. However, simultaneously they suffered a downregulation of mitochondrial electron transport chain (ETC) subunits, without a loss of cellular mitochondrial content. Mitochondrial ROS signaling is activated by injury and is required for sarcolemmal repair, but the reduced mitochondrial ETC capacity in LGMDR2 muscles reduces injury-triggered mitochondrial ROS production, creating a second hit that exacerbates the repair deficit in dysferlin-deficient myofibers. A similar deficit can be caused by cytosolic accumulation of mitochondria-derived ROS. This offers a comprehensive model for LGMDR2 disease onset, where progressive mitochondrial deficit leads to the second hit on top of the primary dysferlin deficiency, causing an insurmountable loss of myofiber repair capacity. These insights into the onset of LGMDR2 identify rebalancing of mitochondrial ROS production as a new therapeutic target to delay or prevent disease onset in LGMD2B.