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Most mutations that truncate the reading frame of the DMD gene result in loss of dystrophin expression and lead to the most common childhood muscle disease, the severe and progressive Duchenne muscular dystrophy. However, frame-truncating mutations within the first five exons of the DMD gene typically do not result in Duchenne muscular dystrophy, but instead result in milder dystrophinopathy syndromes as originally observed in patients with very mild clinical features despite nonsense mutations in exon 1. We have previously shown that amelioration of disease severity result from the expression of a highly functional N-truncated dystrophin beginning in exon 6 of the DMD. Here we demonstrate that this protein represents a novel dystrophin isoform resulting from usage of an internal ribosome entry site (IRES) within exon 5 that is glucocorticoid-inducible. In vitro studies with bicistronic reporter assays demonstrate translation at levels approximately 60% of the well-known viral IRES (eMCV), suggesting a relatively strong activity. Activity in humans was confirmed in patient muscle tissues using ribosome profiling and mass-spectrometric peptide sequencing. The resultant N-truncated dystrophin protein produced from this IRES, lacks the first calponin homology domain of the canonical actin binding domain 1. Nevertheless, it is highly functional, raising the possibility of the therapeutic use of this isoform. We use a novel out-of-frame exon-skipping approach to generate a truncated reading frame upstream of the IRES in both patient-derived cell lines and in a new DMD mouse model, leading to synthesis of a functional N-truncated isoform. In the mouse, this expression protects muscle from contraction-induced injury and corrects muscle force to the same level as control mice. Together these results support a novel therapeutic approach for patients with mutations within the 5' exons of DMD.

Objective The degenerative muscle diseases Duchenne (DMD) and Becker muscular dystrophy result from mutations in the DMD gene, which encodes the dystrophin protein. Recent improvements in mutational analysis techniques have resulted in the increasing identification of deep intronic point mutations, which alter splicing such that intronic sequences are included in the messenger RNA as “pseudoexons.” We sought to test the hypothesis that the clinical phenotype correlates with splicing efficiency of these mutations, and to test the feasibility of antisense oligonucleotide (AON)–mediated pseudoexon skipping. Methods We identified three pseudoexon insertion mutations in dystrophinopathy patients, two of whom had tissue available for further analysis. For these two out-of-frame pseudoexon mutations (one associated with Becker muscular dystrophy and one with DMD), mutation-induced splicing was tested by quantitative reverse transcription polymerase chain reaction; pseudoexon skipping was tested using AONs composed of 2′-O-methyl–modified bases on a phosphorothioate backbone to treat cultured primary myoblasts. Results Variable amounts of pseudoexon inclusion correlates with the severity of the dystrophinopathy phenotype in these two patients. AON treatment directed at the pseudoexon results in the expression of full-length dystrophin in a DMD myoblast line. Interpretation Both DMD and Becker muscular dystrophy can result from out-of-frame pseudoexons, with the difference in phenotype being due to variable efficiency of the newly generated splicing signal. AON-mediated pseudoexon skipping therapy is a viable approach to these patients and would be predicted to result in increased expression of wild-type dystrophin protein.