Duchenne muscular dystrophy is a fatal neuromuscular disorder affecting around one in 3,500–5,000 male births that is characterized by progressive muscular deterioration. It is inherited in an X-linked recessive fashion and is caused by loss-of-function mutations in the DMD gene coding for dystrophin, a cytoskeletal protein that stabilizes the plasma membrane of muscle fibers. In September 2016, the US Food and Drug Administration granted accelerated approval for eteplirsen (or Exondys 51), a drug that acts to promote dystrophin production by restoring the translational reading frame of DMD through specific skipping of exon 51 in defective gene variants. Eteplirsen is applicable for approximately 14% of patients with DMD mutations. This article extensively reviews and discusses the available information on eteplirsen to date, focusing on pharmacological, efficacy, safety, and tolerability data from preclinical and clinical trials. Issues faced by eteplirsen, particularly those relating to its efficacy, will be identified. Finally, the place of eteplirsen and exon skipping as a general therapeutic strategy in Duchenne muscular dystrophy treatment will be discussed.

Duchenne muscular dystrophy (DMD) is a lethal genetic disorder caused by an absence of the dystrophin protein in bodywide muscles, including the heart. Cardiomyopathy is a leading cause of death in DMD. Exon skipping via synthetic phosphorodiamidate morpholino oligomers (PMOs) represents one of the most promising therapeutic options, yet PMOs have shown very little efficacy in cardiac muscle. To increase therapeutic potency in cardiac muscle, we tested a next-generation morpholino: arginine-rich, cell-penetrating peptide-conjugated PMOs (PPMOs) in the canine X-linked muscular dystrophy in Japan (CXMD_J) dog model of DMD. A PPMO cocktail designed to skip dystrophin exons 6 and 8 was injected intramuscularly, intracoronarily, or intravenously into CXMD_J dogs. Intravenous injections with PPMOs restored dystrophin expression in the myocardium and cardiac Purkinje fibers, as well as skeletal muscles. Vacuole degeneration of cardiac Purkinje fibers, as seen in DMD patients, was ameliorated in PPMO-treated dogs. Although symptoms and functions in skeletal muscle were not ameliorated by i.v. treatment, electrocardiogram abnormalities (increased Q-amplitude and Q/R ratio) were improved in CXMD_J dogs after intracoronary or i.v. administration. No obvious evidence of toxicity was found in blood tests throughout the monitoring period of one or four systemic treatments with the PPMO cocktail (12 mg/kg/injection). The present study reports the rescue of dystrophin expression and recovery of the conduction system in the heart of dystrophic dogs by PPMO-mediated multiexon skipping. We demonstrate that rescued dystrophin expression in the Purkinje fibers leads to the improvement/prevention of cardiac conduction abnormalities in the dystrophic heart. Significance Duchenne muscular dystrophy (DMD) is a lethal genetic disorder caused by a lack of dystrophin protein. Cardiomyopathy is a leading cause of death in DMD. Although exon skipping with antisense morpholino oligonucleotides is very promising, morpholino-mediated exon skipping has induced barely detectable dystrophin levels in the hearts of DMD animal models. Here, we show that systemic multiexon skipping using a cocktail of peptide-conjugated morpholinos (PPMOs) rescued dystrophin expression in the myocardium and cardiac Purkinje fibers in a dystrophic dog model. The treatment reduced vacuole degeneration in Purkinje fibers and improved electrocardiogram abnormalities without apparent toxicity. These results indicate that PPMO-mediated multiexon skipping is a therapeutic approach for treating cardiac conduction abnormalities in DMD.

Duchenne muscular dystrophy (DMD), a fatal X-linked recessive disorder, is caused mostly by frame-disrupting, out-of-frame deletions in the dystrophin (DMD) gene. Antisense oligonucleotide-mediated exon skipping is a promising therapy for DMD. Exon skipping aims to convert out-of-frame mRNA to in-frame mRNA and induce the production of internally-deleted dystrophin as seen in the less severe Becker muscular dystrophy. Currently, multiple exon skipping has gained special interest as a new therapeutic modality for this approach. Previous retrospective database studies represented a potential therapeutic application of multiple exon skipping. Since then, public DMD databases have become more useful with an increase in patient registration and advances in molecular diagnosis. Here, we provide an update on DMD genotype-phenotype associations using a global DMD database and further provide the rationale for multiple exon skipping development, particularly for exons 45–55 skipping and an emerging therapeutic concept, exons 3–9 skipping. Importantly, this review highlights the potential of multiple exon skipping for enabling the production of functionally-corrected dystrophin and for treating symptomatic patients not only with out-of-frame deletions but also those with in-frame deletions. We will also discuss prospects and challenges in multiple exon skipping therapy, referring to recent progress in antisense chemistry and design, as well as disease models.

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive neuromuscular disease prevalent in 1 in 3500 to 5000 males worldwide. As a result of mutations that interrupt the reading frame of the dystrophin gene (DMD), DMD is characterized by a loss of dystrophin protein that leads to decreased muscle membrane integrity, which increases susceptibility to degeneration. CRISPR/Cas9 technology has garnered interest as an avenue for DMD therapy due to its potential for permanent exon skipping, which can restore the disrupted DMD reading frame in DMD and lead to dystrophin restoration. An RNA-guided DNA endonuclease system, CRISPR/Cas9 allows for the targeted editing of specific sequences in the genome. The efficacy and safety of CRISPR/Cas9 as a therapy for DMD has been evaluated by numerous studies in vitro and in vivo, with varying rates of success. Despite the potential of CRISPR/Cas9-mediated gene editing for the long-term treatment of DMD, its translation into the clinic is currently challenged by issues such as off-targeting, immune response activation, and sub-optimal in vivo delivery. Its nature as being mostly a personalized form of therapy also limits applicability to DMD patients, who exhibit a wide spectrum of mutations. This review summarizes the various CRISPR/Cas9 strategies that have been tested in vitro and in vivo for the treatment of DMD. Perspectives on the approach will be provided, and the challenges faced by CRISPR/Cas9 in its road to the clinic will be briefly discussed.

Facioscapulohumeral muscular dystrophy (FSHD) is a disabling inherited muscular disorder characterized by asymmetric, progressive muscle weakness and degeneration. Patients display widely variable disease onset and severity, and sometimes present with extra-muscular symptoms. There is a consensus that FSHD is caused by the aberrant production of the double homeobox protein 4 (DUX4) transcription factor in skeletal muscle. DUX4 is normally expressed during early embryonic development, and is then effectively silenced in all tissues except the testis and thymus. Its reactivation in skeletal muscle disrupts numerous signalling pathways that mostly converge on cell death. Here, we review studies on DUX4-affected pathways in skeletal muscle and provide insights into how understanding these could help explain the unique pathogenesis of FSHD.

Significance: Duchenne muscular dystrophy (DMD) is a fatal disorder of progressive body-wide muscle weakness, considered the most common muscular dystrophy worldwide. Most patients have out-of-frame deletions in the DMD gene, leading to dystrophin absence in muscle. There is no cure for DMD, but exon skipping is emerging as a potential therapy that uses antisense oligonucleotides to convert out-of-frame to in-frame mutations, enabling the production of truncated, partially functional dystrophin. Currently approved exon skipping therapies, however, have limited applicability and efficacy. Here, we developed a more economical approach to skip DMD exons 45 to 55 (a strategy that could treat nearly half of all DMD patients) and identified DG9 peptide conjugation as a powerful way to improve exon skipping efficiencies in vivo.