A fusion peptide directs enhanced systemic dystrophin exon skipping and functional restoration in dystrophin-deficient mdx mice

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene that abolish the synthesis of dystrophin protein. Antisense oligonucleotides (AOs) targeted to trigger excision of an exon bearing a mutant premature stop codon in the DMD transcript have been shown to skip the mutated exon and partially restore functional dystrophin protein in dystrophin-deficient mdx mice. To fully exploit the therapeutic potential of this method requires highly efficient systemic AO delivery to multiple muscle groups, to modify the disease process and restore muscle function. While systemic delivery of naked AOs in DMD animal models requires high doses and is of relatively poor efficiency, we and others have recently shown that short arginine-rich peptide-AO conjugates can dramatically improve in vivo DMD splice correction. Here we report for the first time that a chimeric fusion peptide (B-MSP-PMO) consisting of a muscle-targeting heptapeptide (MSP) fused to an arginine-rich cell-penetrating peptide (B-peptide) and conjugated to a morpholino oligomer (PMO) AO directs highly efficient systemic dystrophin splice correction in mdx mice. With very low systemic doses, we demonstrate that B-MSP-PMO restores high-level, uniform dystrophin protein expression in multiple peripheral muscle groups, yielding functional correction and improvement of the mdx dystrophic phenotype. Our data demonstrate proof-of-concept for this chimeric peptide approach in DMD splice correction therapy and is likely to have broad application.