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Gene replacement using adeno-associated virus (AAV) vectors is a promising therapeutic approach for many diseases 1 , 2 . However, this therapeutic modality is challenged by the packaging capacity of AAVs (approximately 4.7 kilobases) 3 , limiting its application for disorders involving large coding sequences, such as Duchenne muscular dystrophy, with a 14 kilobase messenger RNA. Here we developed a new method for expressing large dystrophins by utilizing the protein trans -splicing mechanism mediated by split inteins. We identified several split intein pairs that efficiently join two or three fragments to generate a large midi-dystrophin or the full-length protein. We show that delivery of two or three AAVs into dystrophic mice results in robust expression of large dystrophins and significant physiological improvements compared with micro-dystrophins. Moreover, using the potent myotropic AAVMYO 4 , we demonstrate that low total doses (2 × 10 13  viral genomes per kg) are sufficient to express large dystrophins in striated muscles body-wide with significant physiological corrections in dystrophic mice. Our data show a clear functional superiority of large dystrophins over micro-dystrophins that are being tested in clinical trials. This method could benefit many patients with Duchenne or Becker muscular dystrophy, regardless of genotype, and could be adapted to numerous other disorders caused by mutations in large genes that exceed the AAV capacity. A method is developed for expressing large dystrophins to enhance muscle function in mouse models of muscular dystrophy, with potential clinical benefits for numerous disorders caused by mutations in large genes that exceed the adeno-associated virus capacity.

The dystrophin glycoprotein complex (DGC) is a multimeric protein assembly associated with either the X-linked cytoskeletal protein dystrophin or its autosomal homologue utrophin. In striated muscle cells, the DGC links the extracellular matrix to the actin cytoskeleton and mediates three major functions: structural stability of the plasma membrane, ion homeostasis, and transmembrane signaling. Mutations affecting the DGC underlie major forms of congenital muscle dystrophies. The DGC is prominent also in the central and peripheral nervous system and in tissues with a secretory function or which form barriers between functional compartments, such as the blood-brain barrier, choroid plexus, or kidney. A considerable molecular heterogeneity arises from cell-specific expression of its constituent proteins, notably short C-terminal isoforms of dystrophin. Experimentally, the generation of mice carrying targeted gene deletions affecting the DGC has clarified the interdependence of DGC proteins for assembly of the complex and revealed its importance for brain development and regulation of the 'milieu intérieur. Here, we focus on recent studies of the DGC in brain, blood-brain barrier and choroid plexus, retina, and kidney and discuss the role of dystrophin isoforms and utrophin for assembly of the complex in these tissues.

Mutations that disrupt the open reading frame and prevent full translation of DMD, the gene that encodes dystrophin, underlie the fatal X-linked disease Duchenne muscular dystrophy. Oligonucleotides targeted to splicing elements (splice switching oligonucleotides) in DMD pre-mRNA can lead to exon skipping, restoration of the open reading frame, and the production of functional dystrophin in vitro and in vivo, which could benefit patients with this disorder. We did a single-blind, placebo-controlled, dose-escalation study in patients with DMD recruited nationally, to assess the safety and biochemical efficacy of an intramuscular morpholino splice-switching oligonucleotide (AVI-4658) that skips exon 51 in dystrophin mRNA. Seven patients with Duchenne muscular dystrophy with deletions in the open reading frame of DMD that are responsive to exon 51 skipping were selected on the basis of the preservation of their extensor digitorum brevis (EDB) muscle seen on MRI and the response of cultured fibroblasts from a skin biopsy to AVI-4658. AVI-4658 was injected into the EDB muscle; the contralateral muscle received saline. Muscles were biopsied between 3 and 4 weeks after injection. The primary endpoint was the safety of AVI-4658 and the secondary endpoint was its biochemical efficacy. This trial is registered, number NCT00159250. Two patients received 0·09 mg AVI-4658 in 900 μL (0·9%) saline and five patients received 0·9 mg AVI-4658 in 900 μL saline. No adverse events related to AVI-4658 administration were reported. Intramuscular injection of the higher-dose of AVI-4658 resulted in increased dystrophin expression in all treated EDB muscles, although the results of the immunostaining of EDB-treated muscle for dystrophin were not uniform. In the areas of the immunostained sections that were adjacent to the needle track through which AVI-4658 was given, 44–79% of myofibres had increased expression of dystrophin. In randomly chosen sections of treated EDB muscles, the mean intensity of dystrophin staining ranged from 22% to 32% of the mean intensity of dystrophin in healthy control muscles (mean 26·4%), and the mean intensity was 17% (range 11–21%) greater than the intensity in the contralateral saline-treated muscle (one-sample paired t test p=0·002). In the dystrophin-positive fibres, the intensity of dystrophin staining was up to 42% of that in healthy muscle. We showed expression of dystrophin at the expected molecular weight in the AVI-4658-treated muscle by immunoblot. Intramuscular AVI-4658 was safe and induced the expression of dystrophin locally within treated muscles. This proof-of-concept study has led to an ongoing systemic clinical trial of AVI-4658 in patients with DMD. UK Department of Health.

Duchenne muscular dystrophy (DMD) is a severe X-linked recessive disorder marked by progressive muscle wasting leading to premature mortality 1 , 2 . Discovery of the DMD gene encoding dystrophin both revealed the cause of DMD and helped identify a family of at least ten dystrophin-associated proteins at the muscle cell membrane, collectively forming the dystrophin–glycoprotein complex (DGC) 3 , 4 , 5 , 6 , 7 , 8 – 9 . The DGC links the extracellular matrix to the cytoskeleton, but, despite its importance, its molecular architecture has remained elusive. Here we determined the native cryo-electron microscopy structure of rabbit DGC and conducted biochemical analyses to reveal its intricate molecular configuration. An unexpected β-helix comprising β-, γ- and δ-sarcoglycan forms an extracellular platform that interacts with α-dystroglycan, β-dystroglycan and α-sarcoglycan, allowing α-dystroglycan to contact the extracellular matrix. In the membrane, sarcospan anchors β-dystroglycan to the β-, γ- and δ-sarcoglycan trimer, while in the cytoplasm, β-dystroglycan’s juxtamembrane fragment binds dystrophin’s ZZ domain. Through these interactions, the DGC links laminin 2 to intracellular actin. Additionally, dystrophin’s WW domain, along with its EF-hand 1 domain, interacts with α-dystrobrevin. A disease-causing mutation mapping to the WW domain weakens this interaction, as confirmed by deletion of the WW domain in biochemical assays. Our findings rationalize more than 110 mutations affecting single residues associated with various muscular dystrophy subtypes and contribute to ongoing therapeutic developments, including protein restoration, upregulation of compensatory genes and gene replacement. The detailed structure of the native dystrophin–glycoprotein complex provides mechanistic insights into mutations underlying muscular dystrophy.

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle weakness and a shortened life span. The disease is caused by mutations that reduce or prevent expression of dystrophin, an essential structural protein in skeletal and heart muscle. The gene editing technology CRISPR-Cas9 can correct disease-causing mutations and has yielded promising results in mouse models of DMD. In a small, short-term study, Amoasii et al. tested this strategy in a dog model of DMD that exhibits many features of the human disease. Intramuscular or systemic delivery of the gene editing components resulted in a substantial increase in dystrophin protein levels in skeletal and heart muscle. Restoration of dystrophin expression was accompanied by improved muscle histology. Science , this issue p. 86 Successful CRISPR correction of a dystrophin mutation in dogs increases dystrophin protein expression in skeletal and heart muscle. Mutations in the gene encoding dystrophin, a protein that maintains muscle integrity and function, cause Duchenne muscular dystrophy (DMD). The deltaE50-MD dog model of DMD harbors a mutation corresponding to a mutational “hotspot” in the human DMD gene. We used adeno-associated viruses to deliver CRISPR gene editing components to four dogs and examined dystrophin protein expression 6 weeks after intramuscular delivery ( n = 2) or 8 weeks after systemic delivery ( n = 2). After systemic delivery in skeletal muscle, dystrophin was restored to levels ranging from 3 to 90% of normal, depending on muscle type. In cardiac muscle, dystrophin levels in the dog receiving the highest dose reached 92% of normal. The treated dogs also showed improved muscle histology. These large-animal data support the concept that, with further development, gene editing approaches may prove clinically useful for the treatment of DMD.

Efficient and widespread gene transfer is required for successful treatment of Duchenne muscular dystrophy (DMD). Here, we performed the first clinical trial using a chimeric adeno-associated virus (AAV) capsid variant (designated AAV2.5) derived from a rational design strategy. AAV2.5 was generated from the AAV2 capsid with five mutations from AAV1. The novel chimeric vector combines the improved muscle transduction capacity of AAV1 with reduced antigenic crossreactivity against both parental serotypes, while keeping the AAV2 receptor binding. In a randomized double-blind placebo-controlled phase I clinical study in DMD boys, AAV2.5 vector was injected into the bicep muscle in one arm, with saline control in the contralateral arm. A subset of patients received AAV empty capsid instead of saline in an effort to distinguish an immune response to vector versus minidystrophin transgene. Recombinant AAV genomes were detected in all patients with up to 2.56 vector copies per diploid genome. There was no cellular immune response to AAV2.5 capsid. This trial established that rationally designed AAV2.5 vector was safe and well tolerated, lays the foundation of customizing AAV vectors that best suit the clinical objective (e.g., limb infusion gene delivery) and should usher in the next generation of viral delivery systems for human gene transfer.

After injury in the heart, postnatal mouse hearts deficient in the Hippo pathway show efficient repair, and in the hearts of Mdx mice (a model of Duchenne muscular dystrophy) Hippo deficiency protects against overload-induced heart failure. Heart cells hindered by DCG The dystrophin glycoprotein complex (DCG) is a multicomponent complex found in the cell membrane of cardiac and skeletal cells, which mechanically links the actin cytoskeleton to the extracellular matrix. DCG deficiency in humans results in muscular dystrophy. James Martin and colleagues reveal a role for the DCG in regulating cardiomyocyte proliferation via the Hippo pathway. They show that the DCG sequesters phosphorylated Yap — a Hippo pathway effector — to prevent its transcriptional activity and inhibit cardiomyocyte proliferation. Hippo-pathway-deficient adult mouse hearts are able to regenerate after damage. The authors show that hearts that are deficient in both the Hippo pathway and dystrophin regenerate with excessive myocardial overgrowth and enhanced cardiomyocyte cytokinesis. Subjecting dystrophin-deficient mice to transverse aortic constriction leads to dilated cardiomyopathy. They find that, in this model, the Hippo pathway seems to be maladaptive and knocking down the Hippo pathway function boosts cardiomyocyte proliferation and rescues heart function. The regenerative capacity of the adult mammalian heart is limited, because of the reduced ability of cardiomyocytes to progress through mitosis 1 . Endogenous cardiomyocytes have regenerative capacity at birth but this capacity is lost postnatally, with subsequent organ growth occurring through cardiomyocyte hypertrophy 2 , 3 . The Hippo pathway, a conserved kinase cascade, inhibits cardiomyocyte proliferation in the developing heart to control heart size and prevents regeneration in the adult heart 4 , 5 . The dystrophin–glycoprotein complex (DGC), a multicomponent transmembrane complex linking the actin cytoskeleton to extracellular matrix, is essential for cardiomyocyte homeostasis. DGC deficiency in humans results in muscular dystrophy, including the lethal Duchenne muscular dystrophy. Here we show that the DGC component dystroglycan 1 (Dag1) directly binds to the Hippo pathway effector Yap to inhibit cardiomyocyte proliferation in mice. The Yap–Dag1 interaction was enhanced by Hippo-induced Yap phosphorylation, revealing a connection between Hippo pathway function and the DGC. After injury, Hippo-deficient postnatal mouse hearts maintained organ size control by repairing the defect with correct dimensions, whereas postnatal hearts deficient in both Hippo and the DGC showed cardiomyocyte overproliferation at the injury site. In the hearts of mature Mdx mice (which have a point mutation in Dmd )—a model of Duchenne muscular dystrophy—Hippo deficiency protected against overload-induced heart failure.

Genome editing therapy for Duchenne muscular dystrophy (DMD) holds great promise, however, one major obstacle is delivery of the CRISPR-Cas9/sgRNA system to skeletal muscle tissues. In general, AAV vectors are used for in vivo delivery, but AAV injections cannot be repeated because of neutralization antibodies. Here we report a chemically defined lipid nanoparticle (LNP) system which is able to deliver Cas9 mRNA and sgRNA into skeletal muscle by repeated intramuscular injections. Although the expressions of Cas9 protein and sgRNA were transient, our LNP system could induce stable genomic exon skipping and restore dystrophin protein in a DMD mouse model that harbors a humanized exon sequence. Furthermore, administration of our LNP via limb perfusion method enables to target multiple muscle groups. The repeated administration and low immunogenicity of our LNP system are promising features for a delivery vehicle of CRISPR-Cas9 to treat skeletal muscle disorders. In vivo delivery of CRISPR-Cas9 holds promise for treating muscular dystrophy, however, AAV delivery is known to be immunogenic. Here, the authors show that LNP delivery of CRISPR-Cas9 enables repeated injections into skeletal muscle and leads to restored dystrophin expression in multiple muscle groups.

Duchenne muscular dystrophy (DMD) is a severe, progressive, and rare neuromuscular, X-linked recessive disease. Dystrophin deficiency is the underlying cause of disease; therefore, mutation-specific therapies aimed at restoring dystrophin protein production are being explored. We aimed to assess the efficacy and safety of ataluren in ambulatory boys with nonsense mutation DMD. We did this multicentre, randomised, double-blind, placebo-controlled, phase 3 trial at 54 sites in 18 countries located in North America, Europe, the Asia-Pacific region, and Latin America. Boys aged 7–16 years with nonsense mutation DMD and a baseline 6-minute walk distance (6MWD) of 150 m or more and 80% or less of the predicted normal value for age and height were randomly assigned (1:1), via permuted block randomisation (block size of four) using an interactive voice-response or web-response system, to receive ataluren orally three times daily (40 mg/kg per day) or matching placebo. Randomisation was stratified by age (<9 years vs ≥9 years), duration of previous corticosteroid use (6 months to <12 months vs ≥12 months), and baseline 6MWD (<350 m vs ≥350 m). Patients, parents and caregivers, investigational site personnel, PTC Therapeutics employees, and all other study personnel were masked to group allocation until after database lock. The primary endpoint was change in 6MWD from baseline to week 48. We additionally did a prespecified subgroup analysis of the primary endpoint, based on baseline 6MWD, which is reflective of anticipated rates of disease progression over 1 year. The primary analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT01826487. Between March 26, 2013, and Aug 26, 2014, we randomly assigned 230 patients to receive ataluren (n=115) or placebo (n=115); 228 patients comprised the intention-to-treat population. The least-squares mean change in 6MWD from baseline to week 48 was −47·7 m (SE 9·3) for ataluren-treated patients and −60·7 m (9·3) for placebo-treated patients (difference 13·0 m [SE 10·4], 95% CI −7·4 to 33·4; p=0·213). The least-squares mean change for ataluren versus placebo in the prespecified subgroups was −7·7 m (SE 24·1, 95% CI −54·9 to 39·5; p=0·749) in the group with a 6MWD of less than 300 m, 42·9 m (15·9, 11·8–74·0; p=0·007) in the group with a 6MWD of 300 m or more to less than 400 m, and −9·5 m (17·2, −43·2 to 24·2; p=0·580) in the group with a 6MWD of 400 m or more. Ataluren was generally well tolerated and most treatment-emergent adverse events were mild to moderate in severity. Eight (3%) patients (n=4 per group) reported serious adverse events; all except one event in the placebo group (abnormal hepatic function deemed possibly related to treatment) were deemed unrelated to treatment. Change in 6MWD did not differ significantly between patients in the ataluren group and those in the placebo group, neither in the intention-to-treat population nor in the prespecified subgroups with a baseline 6MWD of less than 300 m or 400 m or more. However, we recorded a significant effect of ataluren in the prespecified subgroup of patients with a baseline 6MWD of 300 m or more to less than 400 m. Baseline 6MWD values within this range were associated with a more predictable rate of decline over 1 year; this finding has implications for the design of future DMD trials with the 6-minute walk test as the endpoint. PTC Therapeutics.