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Concept: Duchenne muscular dystrophy

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Identification of a systemically acting and universal small molecule therapy for Duchenne muscular dystrophy would be an enormous advance for this condition. Based on evidence gained from studies on mouse genetic models we have identified tyrosine phosphorylation and degradation of β-dystroglycan as a key event in the aetiology of Duchenne muscular dystrophy. Thus preventing tyrosine phosphorylation and degradation of β-dystroglycan presents itself as a potential therapeutic strategy. Using the dystrophic sapje zebrafish we have investigated the use of tyrosine kinase and other inhibitors to treat the dystrophic symptoms in this model of Duchenne muscular dystrophy. Dasatinib, a potent and specific Src tyrosine kinase inhibitor was found to decrease the levels of β-dystroglycan phosphorylation on tyrosine and increase the relative levels of non-phosphorylated β-dystroglycan in sapje zebrafish. Furthermore, dasatinib treatment resulted in the improved physical appearance of the sapje zebrafish musculature and increased swimming ability as measured by both duration and distance of swimming dasatinib treated fish compared to control animals. These data suggest great promise for pharmacological agents that prevent the phosphorylation of β-dystroglycan on tyrosine and subsequent steps in the degradation pathway as therapeutic targets for the treatment of Duchenne muscular dystrophy.

Concepts: Signal transduction, Protein kinase, Phosphorylation, Enzyme inhibitor, Electromyography, Muscular dystrophy, Duchenne muscular dystrophy, Morpholino

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Genome editing with CRISPR/Cas9 is a promising new approach for correcting or mitigating disease-causing mutations. Duchenne muscular dystrophy (DMD) is associated with lethal degeneration of cardiac and skeletal muscle caused by more than 3000 different mutations in the X-linked dystrophin gene (DMD). Most of these mutations are clustered in “hotspots.” There is a fortuitous correspondence between the eukaryotic splice acceptor and splice donor sequences and the protospacer adjacent motif sequences that govern prokaryotic CRISPR/Cas9 target gene recognition and cleavage. Taking advantage of this correspondence, we screened for optimal guide RNAs capable of introducing insertion/deletion (indel) mutations by nonhomologous end joining that abolish conserved RNA splice sites in 12 exons that potentially allow skipping of the most common mutant or out-of-frame DMD exons within or nearby mutational hotspots. We refer to the correction of DMD mutations by exon skipping as myoediting. In proof-of-concept studies, we performed myoediting in representative induced pluripotent stem cells from multiple patients with large deletions, point mutations, or duplications within the DMD gene and efficiently restored dystrophin protein expression in derivative cardiomyocytes. In three-dimensional engineered heart muscle (EHM), myoediting of DMD mutations restored dystrophin expression and the corresponding mechanical force of contraction. Correcting only a subset of cardiomyocytes (30 to 50%) was sufficient to rescue the mutant EHM phenotype to near-normal control levels. We conclude that abolishing conserved RNA splicing acceptor/donor sites and directing the splicing machinery to skip mutant or out-of-frame exons through myoediting allow correction of the cardiac abnormalities associated with DMD by eliminating the underlying genetic basis of the disease.

Concepts: DNA, Gene, RNA, Messenger RNA, Intron, Spliceosome, RNA splicing, Duchenne muscular dystrophy

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Gene replacement therapies utilizing adeno-associated viral (AAV) vectors hold great promise for treating Duchenne muscular dystrophy (DMD). A related approach uses AAV vectors to edit specific regions of the DMD gene using CRISPR/Cas9. Here we develop multiple approaches for editing the mutation in dystrophic mdx(4cv) mice using single and dual AAV vector delivery of a muscle-specific Cas9 cassette together with single-guide RNA cassettes and, in one approach, a dystrophin homology region to fully correct the mutation. Muscle-restricted Cas9 expression enables direct editing of the mutation, multi-exon deletion or complete gene correction via homologous recombination in myogenic cells. Treated muscles express dystrophin in up to 70% of the myogenic area and increased force generation following intramuscular delivery. Furthermore, systemic administration of the vectors results in widespread expression of dystrophin in both skeletal and cardiac muscles. Our results demonstrate that AAV-mediated muscle-specific gene editing has significant potential for therapy of neuromuscular disorders.

Concepts: DNA, Gene, Genetics, Evolution, Heart, Muscular dystrophy, Duchenne muscular dystrophy, Neuromuscular disease

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In Duchenne muscular dystrophy (DMD), progressive loss of respiratory function leads to restrictive pulmonary disease and places patients at significant risk for severe respiratory complications. Of particular concern are ineffective cough, secretion retention and recurrent respiratory tract infections. In a Phase 3 randomized controlled study (DMD Long-term Idebenone Study, DELOS) in DMD patients 10-18 years of age and not taking concomitant glucocorticoid steroids, idebenone (900 mg/day) reduced significantly the loss of respiratory function over a 1-year study period. In a post-hoc analysis of DELOS we found that more patients in the placebo group compared to the idebenone group experienced bronchopulmonary adverse events (BAEs): placebo: 17 of 33 patients, 28 events; idebenone: 6 of 31 patients, 7 events. The hazard ratios (HR) calculated “by patient” (HR 0.33, p = 0.0187) and for “all BAEs” (HR 0.28, p = 0.0026) indicated a clear idebenone treatment effect. The overall duration of BAEs was 222 days (placebo) vs. 82 days (idebenone). In addition, there was also a difference in the use of systemic antibiotics utilized for the treatment of BAEs. In the placebo group, 13 patients (39.4%) reported 17 episodes of antibiotic use compared to 7 patients (22.6%) reporting 8 episodes of antibiotic use in the idebenone group. Furthermore, patients in the placebo group used systemic antibiotics for longer (105 days) compared to patients in the idebenone group (65 days). This post-hoc analysis of DELOS indicates that the protective effect of idebenone on respiratory function is associated with a reduced risk of bronchopulmonary complications and a reduced need for systemic antibiotics.

Concepts: Medicine, Clinical trial, Bacteria, Respiratory system, Clinical research, Antibiotic, Muscular dystrophy, Duchenne muscular dystrophy

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Duchenne muscular dystrophy (DMD) is a debilitating X-linked disorder that is fatal. DMD patients lack the expression of the structural protein dystrophin caused by mutations within the DMD gene. The absence of functional dystrophin protein results in excessive damage from normal muscle use due to the compromised structural integrity of the dystrophin associated glycoprotein complex. As a result, DMD patients exhibit ongoing cycles of muscle destruction and regeneration that promote inflammation, fibrosis, mitochondrial dysfunction, satellite cell (SC) exhaustion and loss of skeletal and cardiac muscle function. The nuclear receptor REV-ERB suppresses myoblast differentiation and recently we have demonstrated that the REV-ERB antagonist, SR8278, stimulates muscle regeneration after acute injury. Therefore, we decided to explore whether the REV-ERB antagonist SR8278 could slow the progression of muscular dystrophy. In mdx mice SR8278 increased lean mass and muscle function, and decreased muscle fibrosis and muscle protein degradation. Interestingly, we also found that SR8278 increased the SC pool through stimulation of Notch and Wnt signaling. These results suggest that REV-ERB is a potent target for the treatment of DMD.

Concepts: Metabolism, Heart, Muscle, Cardiac muscle, Electromyography, Muscular system, Muscular dystrophy, Duchenne muscular dystrophy

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OBJECTIVES:The purpose of this study was to assess health-related quality of life (QoL) in children with Duchenne muscular dystrophy (DMD), including development and field-testing of a DMD-specific module integrated with the core Pediatric Quality of Life Inventory (PedsQL).METHODS:The PedsQL 4.0 Generic Core and DMD Module Scales were completed by 203 families, including 200 parents and 117 boys with DMD. Scores on the PedsQL Core Scales were compared with those of matched healthy children. Relationships between PedsQL scores and patient characteristics were examined.RESULTS:By both parent report and child self-report, mean PedsQL scores for boys with DMD were significantly lower than those for healthy children for physical and psychosocial QoL (P < .0001), with significantly impaired psychosocial QoL scores self-reported by 57%. Psychosocial QoL, by self-report only, tended to be higher in the older boys (13-18 years) than in younger boys (8-12 years; P = .05) and was not significantly associated with use of mobility aids. Although parents reported higher Daily Activities scores in boys receiving steroids (P = .01), boys receiving steroids reported no difference in Daily Activities but significantly less worry (P = .004). Parent-child concordance was generally in the fair to poor range. Internal consistency reliability coefficients for PedsQL DMD module scales ranged from 0.66 to 0.86.CONCLUSIONS:Overall, boys with DMD reported significantly lower QoL than their healthy peers. Despite decreased physical functioning, older boys seem to perceive better psychosocial QoL than perceived by their parents and by younger boys, unrelated to their need for mobility aids.

Concepts: Quality, Electromyography, Genetic disorders, Muscular dystrophy, Duchenne muscular dystrophy, Muscular Dystrophy Association, Internal consistency, DJ Qualls

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To review current approaches for obtaining patient data in Duchenne muscular dystrophy (DMD) and consider how monitoring and comparing outcome measures across DMD clinics could facilitate standardized and improved patient care.

Concepts: Hospital, Electromyography, Genetic disorders, Muscular dystrophy, Duchenne muscular dystrophy, Muscular Dystrophy Association

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Duchenne muscular dystrophy (DMD) is a devastating disease affecting about 1 out of 5000 male births and caused by mutations in the dystrophin gene. Genome editing has the potential to restore expression of a modified dystrophin gene from the native locus to modulate disease progression. In this study, adeno-associated virus was used to deliver the CRISPR/Cas9 system to the mdx mouse model of DMD to remove the mutated exon 23 from the dystrophin gene. This includes local and systemic delivery to adult mice and systemic delivery to neonatal mice. Exon 23 deletion by CRISPR/Cas9 resulted in expression of the modified dystrophin gene, partial recovery of functional dystrophin protein in skeletal myofibers and cardiac muscle, improvement of muscle biochemistry, and significant enhancement of muscle force. This work establishes CRISPR/Cas9-based genome editing as a potential therapy to treat DMD.

Concepts: DNA, Gene, Genetics, Evolution, Heart, Muscle, Muscular dystrophy, Duchenne muscular dystrophy

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In Duchenne muscular dystrophy, progressive loss of muscle tissue is accompanied by fibrosis, chronic inflammation and reduced muscle regenerative capacity. Although much is known about the development of fibrosis and chronic inflammation in muscular dystrophy, less is known about how they are mechanistically linked to loss of muscle regenerative capacity. We have developed a proteomics method to discover dystrophy-associated changes in the muscle progenitor cell niche, which identified serine proteases, and especially neutrophil elastase, as candidates. We show that elastase activity is increased in dystrophic (mdx(4cv)) muscle and impairs myoblast survival in culture. While the effect of elastase on C2C12 cell survival correlates with the kinetics of elastase-mediated degradation of the substrate to which the cells adhere, the effect of elastase on satellite cell-derived primary myoblast growth and differentiation is substrate-independent and even more dramatic than the effect on C2C12 cells, suggesting a detrimental role for elastase on myogenesis in vivo. Additionally, elastase impairs differentiation of both primary and C2C12 myoblasts into myotubes. Our findings evidence the importance of neutrophil-mediated inflammation in muscular dystrophy and indicate elastase-mediated regulation of myoblast behaviour as a potential mechanism underlying loss of regenerative capacity in dystrophic muscle.

Concepts: Enzyme, Glucose, Muscle, Serine protease, Electromyography, Muscular dystrophy, Duchenne muscular dystrophy, Elastase