The X‐linked Becker muscular dystrophy (bmx) mouse models Becker muscular dystrophy via deletion of murine dystrophin exons 45–47
Authors: Heier Christopher R. , McCormack Nikki M. , Tully Christopher B. , Novak James S. , Newell‐Stamper Breanne L. , Russell Alan J. , Fiorillo Alyson A. 
Published: January 2023

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 with Duchenne muscular dystrophy (DMD), the allelic disorder of complete dystrophin deficiency. Significant therapeutic advancements have been made in DMD, including four 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 lack of a BMD mouse model which would enable greater understanding of disease 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 aim to convert a DMD genotype into a BMD phenotype.

Methods

We use CRISPR/Cas9 technology to generatebmx(Beckermuscular dystrophy,X‐linked) mice, which express an in‐frame ~40 000 bpdeletion of exons 45–47 in the murineDmdgene, reproducing the most common BMD patient mutation. Here, we characterize muscle pathogenesis using molecular and histological techniques and then test skeletal muscle and cardiac function using muscle function assays and echocardiography.

Results

Overall,bmxmice present with significant muscle weakness and heart dysfunction versus wild‐type (WT) mice, despite a substantial improvement in pathology over dystrophin‐nullmdx52mice.bmxmice show impaired motor function in grip strength (−39%,P < 0.0001), wire hang (P = 0.0025), andin vivoas well asex vivoforce assays. In agedbmx, echocardiography reveals decreased heart function through reduced fractional shortening (−25%,P = 0.0036). Additionally, muscle‐specific serum CK is increased >60‐fold (P < 0.0001), indicating increased muscle damage. Histologically,bmxmuscles display increased myofibre size variability (minimal Feret's diameter:P = 0.0017) and centrally located nuclei indicating degeneration/regeneration (P < 0.0001).bmxmuscles also display dystrophic pathology; however, levels of the following parameters are moderate in comparison withmdx52: inflammatory/necrotic foci (P < 0.0001), collagen deposition (+1.4‐fold,P = 0.0217), and sarcolemmal damage measured by intracellular IgM (P = 0.0878). Like BMD patients,bmxmuscles show reduced dystrophin protein levels (~20–50% of WT), whereasDmdtranscript levels are unchanged. At the molecular level,bmxmuscles express increased levels of inflammatory genes, inflammatory miRNAs and fibrosis genes.

Conclusions

Thebmxmouse recapitulates BMD disease phenotypes with histological, molecular and functional deficits. Importantly, it can inform both BMD pathology and DMD dystrophin restoration therapies. This novel model will enable further characterization of BMD disease progression, identification of biomarkers, identification of therapeutic targets and new preclinical drug studies aimed at developing therapies for BMD patients.