Frataxin deficiency lowers lean mass and triggers the integrated stress response in skeletal muscle.

Friedreich's ataxia (FRDA) is an inherited disorder caused by reduced levels of frataxin (FXN), which is required for iron-sulfur cluster biogenesis. Neurological and cardiac comorbidities are prominent and have been a major focus of study. Skeletal muscle has received less attention despite indications that FXN loss affects it. Here, we show that lean mass is lower, whereas body mass index is unaltered, in separate cohorts of adults and children with FRDA. In adults, lower lean mass correlated with disease severity. To further investigate FXN loss in skeletal muscle, we used a transgenic mouse model of whole-body inducible and progressive FXN depletion. There was little impact of FXN loss when FXN was approximately 20% of control levels. When residual FXN was approximately 5% of control levels, muscle mass was lower along with absolute grip strength. When we examined mechanisms that can affect muscle mass, only global protein translation was lower, accompanied by integrated stress response (ISR) activation. Also in mice, aerobic exercise training, initiated prior to the muscle mass difference, improved running capacity, yet, muscle mass and the ISR remained as in untrained mice. Thus, FXN loss can lead to lower lean mass, with ISR activation, both of which are insensitive to exercise training.