Exacerbated Skeletal Muscle Phenotype in Mice with ‘Homotypic’ Expression of the Tubular Aggregate Myopathy ORAI1 G100S Mutation

<b>Background:</b> Tubular aggregate myopathy (TAM) is an autosomal dominant myopathy that results from gain-of-function mutations in the <i>STIM1</i> and <i>ORAI1</i> genes, which encode the two key proteins that coordinate store-operated Ca²⁺ entry in skeletal muscle and other cell types. Knock-in mice heterozygous for a glycine-to-serine point mutation in the ORAI1 pore (<i>ORAI1^G100S/+</i> or GS mice) phenocopy several key aspects of TAM in humans with the analogous mutation including muscle weakness, exercise intolerance, elevated CK levels, hypocalcemia, and the presence of tubular aggregates. <b>Methods:</b> Since homozygous inheritance of the ORAI1-G100S mutation is embryonic lethal, we assessed the impact of homotypic ORAI1-G100S expression in skeletal muscle by crossing GS mice with constitutive, muscle-specific ORAI1 knock-in mice (cORAI1-KO). <b>Results:</b> Compound cORAI1-KO/GS mice exhibit only one active <i>ORAI1</i> (GS) allele, and thus only express ORAI1-G100S monomers in skeletal muscle ('homotypic' GS mice). Homotypic GS mice exhibit an earlier onset and more severe muscle phenotype than age-matched heterotypic GS mice with both WT and GS alleles. Specifically, homotypic GS mice exhibit TAs at an earlier age, as well as significantly reduced <i>in vivo</i> muscle performance (grip strength, treadmill endurance, and rotarod endurance), maximal specific force production, and respiratory function, compared to those observed for both WT and heterotypic GS mice. <b>Conclusions:</b> These findings indicate that homotypic expression of the ORAI1-G100S mutation in skeletal muscle results in an earlier-onset and more severe muscle phenotype.