Background: Becker muscular dystrophy (BMD) is a genetic neuromuscular disease of growing importance caused by in-frame, partial loss-of-function mutations in the dystrophin (DMD) gene. BMD presents with reduced severity compared to Duchenne muscular dystrophy (DMD), the allelic disorder of complete dystrophin deficiency. Significant therapeutic advancements have been made in DMD, including 4 FDA-approved drugs. BMD, however, is understudied and underserved—there are no drugs and few clinical trials. Discordance in therapeutic efforts is due in part to the absence of a mouse model of BMD. Such a model would enable greater understanding of disease pathophysiology, and de-risk potential therapeutics before first-in-human trials. Importantly, a BMD mouse model is becoming increasingly critical as emerging DMD dystrophin restoration therapies strive to convert a DMD genotype into a BMD phenotype.
Methods: We use CRISPR-Cas9 technology to generate bmx (Becker muscular dystrophy, X-linked) mice, which express an in-frame ~40,000bp deletion of exons 45-47 in the murine Dmd gene, reproducing the most common BMD patient mutation.
Results: Overall, bmx mice present with significant muscle weakness and heart dysfunction in comparison to wild-type (WT) mice, despite a substantial improvement in pathology over dystrophin-null mdx52 mice. bmx mice show impaired motor function in grip strength, wire hang time, and in vivo isometric force. Echocardiography reveals a decline in heart function through reduced fractional shortening. Histologically, bmx muscles display increased myofiber size variability and centrally located nuclei indicating degeneration/regeneration bmx muscles also display dystrophic pathology, however levels of the following parameters are moderate in comparison to mdx52: inflammatory/necrotic foci, collagen deposition, and sarcolemmal damage measured by intracellular IgM. bmx mice show reduced dystrophin protein (~20-50% of WT) in skeletal and cardiac muscles, while Dmd transcript levels are unchanged. At the molecular level, bmx muscles show increased expression of inflammatory genes and miRNAs, and fibrosis genes.
Conclusions: The bmx mouse recapitulates BMD disease phenotypes with histological, molecular and functional deficits. This novel model will enable further characterization of BMD disease progression, identification of biomarkers and therapeutic targets, and new preclinical drug studies aimed at developing therapies for BMD patient