A new dystrophin deficient rat model mirroring exon skipping in patients with DMD exon 45 deletions
Mutations in the dystrophin (DMD) gene can cause muscle-wasting disorders ranging from the milder Becker muscular dystrophy (BMD) to the more severe Duchenne muscular dystrophy (DMD). Exon 45 deletion is the most frequently reported single-exon deletion in DMD patients worldwide. In this study, we generated a novel rat model with an exon 45 deletion using the CRISPR/Cas9 technology. The Dmd del45 rat recapitulate key clinical and molecular features of DMD, including progressive skeletal muscle degeneration, impaired muscle and cardiac function, cognitive deficits, elevated circulating muscle damage biomarkers and an overall reduced lifespan. Transcriptomics analyses confirmed the deletion of exon 45 and revealed gene expression patterns consistent with dystrophin deficiency. In the skeletal muscle, RNA-seq profiles demonstrated a transition from early stress responses and regenerative activity at 6 months to chronic inflammation, fibrosis, and metabolic dysfunction by 12 months. Similarly, the cardiac transcriptomic shifted from an early inflammatory and stress-responsive state to one characterized by fibrotic remodelling and metabolic impairment. Despite these pathological features, the Dmd del45 rats exhibited a milder phenotype than other DMD rat models. This attenuation may be attributed to spontaneous exon 44 skipping, which partially restores the reading frame and results in an age-dependent increase in revertant dystrophin-positive fibres. Further analysis indicated downregulation of spliceosome-related genes, suggesting a potential mechanism driving exon skipping in this model. In summary, the Dmd del45 rat represents a valuable model for investigating both the molecular determinants of phenotypic variability and the endogenous mechanisms of exon skipping. These findings offer important insights for the development of personalized exon-skipping therapies, particularly for DMD patients with exon 45 deletions.