Timing matters: Exon skipping therapy is most effective when initiated early in a mouse model of Duchenne muscular dystrophy.

Exon skipping is a leading therapeutic approach for Duchenne muscular dystrophy (DMD), whereby modulation of pre-mRNA splicing is used to restore the dystrophin translation reading frame. Four exon skipping drugs have received FDA accelerated approval, despite limited clinical efficacy. To investigate how treatment timing influences exon skipping outcomes, dystrophin-deficient mdx mice were injected with peptide-conjugated phosphorodiamidate morpholino oligonucleotide (PPMO) exon skipping conjugates beginning at adult (12-week-old) or aged (75-week-old) stages, followed by biochemical and transcriptomic analyses in tibialis anterior muscles. Mean Dmd exon 23 skipping was 79% in adults and 44% in aged PPMO-treated mdx mice, whereas dystrophin protein restoration was 35% and 8%, respectively. Histopathological improvements were evident only in the adult treated mice. PPMO-treatment in adult mdx mice induced a broad transcriptomic shift toward a wild-type signature, whereas treatment in aged mice resulted in negligible gene expression changes, indicating that late intervention fails to reverse disease-associated pathologies despite low-level dystrophin restoration. Increased expression of the dystrophin-repressing microRNA miR-31-5p, which was more strongly upregulated in aged mdx muscle, provides a potential mechanistic explanation. In conclusion, PPMO-mediated exon skipping is substantially more effective when initiated in adult vs. aged dystrophic muscle, supporting early therapeutic intervention in DMD-affected individuals.