Overexpression of γ-sarcoglycan induces severe muscular dystrophy. Implications for the regulation of sarcoglycan assembly

The sarcoglycan complex is found normally at the plasma membrane of muscle. Disruption of the sarcoglycan complex, through primary gene mutations in dystrophin or sarcoglycan subunits, produces membrane instability and muscular dystrophy. Restoration of the sarcoglycan complex at the plasma membrane requires reintroduction of the mutant sarcoglycan subunit in a manner that will permit normal assembly of the entire sarcoglycan complex. To study sarcoglycan gene replacement, we introduced transgenes expressing murine γ-sarcoglycan into muscle of normal mice. Mice expressing high levels of γ-sarcoglycan, under the control of the muscle-specific creatine kinase promoter, developed a severe muscular dystrophy with greatly reduced muscle mass and early lethality. Marked γ-sarcoglycan overexpression produced cytoplasmic aggregates that interfered with normal membrane targeting of γ-sarcoglycan. Overexpression of γ-sarcoglycan lead to the up-regulation of α- and β-sarcoglycan. These data suggest that increased γ-sarcoglycan and/or mislocalization of γ-sarcoglycan to the cytoplasm is sufficient to induce muscle damage and provides a new model of muscular dystrophy that highlights the importance of this protein in the assembly, function, and downstream signaling of the sarcoglycan complex. Most importantly, gene dosage and promoter strength should be given serious consideration in replacement gene therapy to ensure safety in human clinical trials.