Codon-optimized human Smad7 gene therapy enhances skeletal muscle mass and function in a murine model of Duchenne muscular dystrophy

Commercial development of gene therapeutics often requires transitioning to human payload genes as initial proof-of-concept studies in animal models often use taxa-specific orthologs. Such transitions also provide opportunities to address potential secondary structure and immune-related subsequences as with human Smad7 cDNA, which was optimized by removing several repeats, potential hairpins and negative cis elements. Thermodynamic modeling at or above minimal free energy states revealed substantial improvements in secondary structure with fewer hairpins and improved diversity scores. Serotype 6 adeno-associated viral vectors with optimized human Smad7 (AVGN7.2) expression constructs were equally or more effective than those with wild-type mouse Smad7 in stimulating skeletal muscle hypertrophy and enhancing isometric torque of hind-limb dorsiflexor muscles in vivo. In murine models of Duchenne Muscular dystrophy, where deficits in muscle mass and disproportionate declines in force are pathognomonic, AVGN7.2 proportionally increased muscle mass and isometric torque while normalizing contractile kinetics. Such improvements occurred without deleterious impacts on serum creatine kinase, fibrosis or myofiber central nucleation. These data suggest that AVGN7.2 is capable of enhancing dystrophic muscle function without exacerbating muscle degeneration. Although these functional effects were partial, they resembled those of several dystrophin-targeting drugs and suggest that combinatorial approaches may safely yield further benefit.