Delivery of AAV2/9-microdystrophin genes incorporating helix 1 of the coiled-coil motif in the C-terminal domain of dystrophin improves muscle pathology and restores the level of α1-syntrophin and α-dystrobrevin in skeletal muscles of mdx mice

In Silico Structural Prediction for the Generation of Novel Performant Midi-Dystrophins Based on Intein-Mediated Dual AAV Approach

Duchenne Muscular Dystrophy (DMD) is a pediatric disorder characterized by progressive muscle degeneration and premature death, and has no current cure. The current, most promising therapeutic avenue is based on gene replacement mediated by adeno-associated viruses (AAVs) using a shortened, but still functional, version of dystrophin, known as micro-dystrophin (µDys), to fit AAV capacity. The limited improvements observed in clinical trials suggest a sub-optimal performance of µDys in the human context that could be due to the lack of key domains in the protein. Therefore, expressing larger dystrophin proteins may be necessary for a more complete correction of the disease phenotype. In this study, we developed three novel midi-dystrophin constructs using a dual-AAV approach, leveraging split-intein-based protein trans-splicing. The midi-dystrophins include additional domains compared to µDys, such as the central cytoskeleton-binding domain, nNOS and Par1b interacting domains, and a complete C-terminal region. Given the limited capacity of each AAV vector, we strategically partially reduced hinge regions while ensuring that the structural stability of the protein remains intact. We predicted the interactions between the two halves of the split midi-Dys proteins thanks to the deep learning algorithm AphaFold3. We observed strong associations between the N- and C-termini in midi-Dys 1 and 2, while a weaker interaction in midi-Dys 3 was revealed. Our subsequent experiments confirmed the efficient protein trans-splicing both in vitro and in vivo in DBA2/mdx mice of the midi-Dys 1 and 2 and not in midi-Dys 3 as expected from the structural prediction. Additionally, we demonstrated that midi-Dys 1 and 2 exhibit significant therapeutic efficacy in DBA2/mdx mice, highlighting their potential as therapeutic agents for DMD. Overall, these findings highlight the potential of deep learning-based structural modeling for the generation of intein-based dystrophin versions and pose the basis for further investigation of these new midi-dystrophins versions for clinical studies.

Assessment of Therapeutic Potential of a Dual AAV Approach for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a yet incurable rare genetic disease that affects the skeletal and cardiac muscles, leading to progressive muscle wasting and premature death. DMD is caused by the lack of dystrophin, a muscle protein essential for the biochemical support and integrity of muscle fibers. Gene replacement strategies for Duchenne muscular dystrophy (DMD) employing the adeno-associated virus (AAV) face the challenge imposed by the limited packaging capacity of AAV, only allowing the accommodation of a short version of dystrophin (µDys) that is still far removed from correcting human disease. The need to develop strategies leading to the expression of a best performing dystrophin variant led to only few studies reporting on the use of dual vectors, but none reported on a method to assess in vivo transgene reconstitution efficiency, the degree of which directly affects the use of safe AAV dosing. We report here on the generation of a dual AAV vector approach for the expression of a larger dystrophin version (quasidystrophin) based on homologous recombination, and the development of a methodology employing a strategic droplet digital PCR design, to determine the recombination efficiency as well as the occurrence of unwanted concatemerization events or aberrant expression from the single vectors. We demonstrated that, upon systemic delivery in the dystrophic D2.B10-Dmd^mdx/J (DBA2mdx) mice, our dual AAV approach led to high transgene reconstitution efficiency and negligible Inverted Terminal Repeats (ITR)-dependent concatemerization, with consequent remarkable protein restoration in muscles and improvement of muscle pathology. This evidence supports the suitability of our system for gene therapy application and the potential of this methodology to assess and improve the feasibility for therapeutic translation of multiple vector approaches.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Microdystrophin Gene Addition Significantly Improves Muscle Functionality and Diaphragm Muscle Histopathology in a Fibrotic Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare neuromuscular disease affecting 1:5000 newborn males. No cure is currently available, but gene addition therapy, based on the adeno-associated viral (AAV) vector-mediated delivery of microdystrophin transgenes, is currently being tested in clinical trials. The muscles of DMD boys present significant fibrotic and adipogenic tissue deposition at the time the treatment starts. The presence of fibrosis not only worsens the disease pathology, but also diminishes the efficacy of gene therapy treatments. To gain an understanding of the efficacy of AAV-based microdystrophin gene addition in a relevant, fibrotic animal model of DMD, we conducted a systemic study in juvenile D2.mdx mice using the single intravenous administration of an AAV8 system expressing a sequence-optimized murine microdystrophin, named MD1 (AAV8-MD1). We mainly focused our study on the diaphragm, a respiratory muscle that is crucial for DMD pathology and that has never been analyzed after treatment with AAV-microdystrophin in this mouse model. We provide strong evidence here that the delivery of AAV8-MD1 provides significant improvement in body-wide muscle function. This is associated with the protection of the hindlimb muscle from contraction-induced damage and the prevention of fibrosis deposition in the diaphragm muscle. Our work corroborates the observation that the administration of gene therapy in DMD is beneficial in preventing muscle fibrosis.

Gene Therapy for Regenerative Medicine

The development of biological methods over the past decade has stimulated great interest in the possibility to regenerate human tissues. Advances in stem cell research, gene therapy, and tissue engineering have accelerated the technology in tissue and organ regeneration. However, despite significant progress in this area, there are still several technical issues that must be addressed, especially in the clinical use of gene therapy. The aims of gene therapy include utilising cells to produce a suitable protein, silencing over-producing proteins, and genetically modifying and repairing cell functions that may affect disease conditions. While most current gene therapy clinical trials are based on cell- and viral-mediated approaches, non-viral gene transfection agents are emerging as potentially safe and effective in the treatment of a wide variety of genetic and acquired diseases. Gene therapy based on viral vectors may induce pathogenicity and immunogenicity. Therefore, significant efforts are being invested in non-viral vectors to enhance their efficiency to a level comparable to the viral vector. Non-viral technologies consist of plasmid-based expression systems containing a gene encoding, a therapeutic protein, and synthetic gene delivery systems. One possible approach to enhance non-viral vector ability or to be an alternative to viral vectors would be to use tissue engineering technology for regenerative medicine therapy. This review provides a critical view of gene therapy with a major focus on the development of regenerative medicine technologies to control the in vivo location and function of administered genes.

Clinical potential of microdystrophin as a surrogate endpoint

Accelerated approval based on a likely surrogate endpoint can be life-changing for patients suffering from a rare progressive disease with unmet medical need, as it substantially hastens access to potentially lifesaving therapies. In one such example, antisense morpholinos were approved to treat Duchenne muscular dystrophy (DMD) based on measurement of shortened dystrophin in skeletal muscle biopsies as a surrogate biomarker. New, promising therapeutics for DMD include AAV gene therapy to restore another form of dystrophin termed mini- or microdystrophin. AAV-microdystrophins are currently in clinical trials but have yet to be accepted by regulatory agencies as reasonably likely surrogate endpoints. To evaluate microdystrophin expression as a reasonably likely surrogate endpoint for DMD, this review highlights dystrophin biology in the context of functional and clinical benefit to support the argument that microdystrophin proteins have a high probability of providing clinical benefit based on their rational design. Unlike exon-skipping based strategies, the approach of rational design allows for functional capabilities (i.e. quality) of the protein to be maximized with every patient receiving the same optimized microdystrophin. Therefore, the presence of rationally designed microdystrophin in a muscle biopsy is likely to predict clinical benefit and is consequently a strong candidate for a surrogate endpoint analysis to support accelerated approval.

Evaluation of the dystrophin carboxy-terminal domain for micro-dystrophin gene therapy in cardiac and skeletal muscles in the DMDmdx rat model

Duchenne muscular dystrophy (DMD) is a muscle wasting disorder caused by mutations in the gene encoding dystrophin. Gene therapy using micro-dystrophin (MD) transgenes and recombinant adeno-associated virus (rAAV) vectors hold great promise. To overcome the limited packaging capacity of rAAV vectors, most MD do not include dystrophin carboxy-terminal (CT) domain. Yet, the CT domain is known to recruit α1- and β1-syntrophins and α-dystrobrevin, a part of the dystrophin-associated protein complex (DAPC), which is a signaling and structural mediator of muscle cells. In this study, we explored the impact of inclusion of the dystrophin CT domain on ΔR4-23/ΔCT MD (MD1), in DMD^mdx rats, which allows for relevant evaluations at muscular and cardiac levels. We showed by LC-MS/MS that MD1 expression is sufficient to restore the interactions at a physiological level of most DAPC partners in skeletal and cardiac muscles, and that inclusion of the CT domain increases the recruitment of some DAPC partners at supra-physiological levels. In parallel, we demonstrated that inclusion of the CT domain does not improve MD1 therapeutic efficacy on DMD muscle and cardiac pathologies. Our work highlights new evidences of the therapeutic potential of MD1 and strengthens the relevance of this candidate for gene therapy of DMD.

TRPC3, but not TRPC1, as a good therapeutic target for standalone or complementary treatment of DMD

Background

Duchenne muscular dystrophy (DMD) is an X-linked inherited disease caused by mutations in the gene encoding dystrophin that leads to a severe and ultimately life limiting muscle-wasting condition. Recombinant adeno-associated vector (rAAV)-based gene therapy is promising, but the size of the full-length dystrophin cDNA exceeds the packaging capacity of a rAAV. Alternative or complementary strategies that could treat DMD patients are thus needed. Intracellular calcium overload due to a sarcolemma permeability to calcium (SPCa) increase is an early and critical step of the DMD pathogenesis. We assessed herein whether TRPC1 and TRPC3 calcium channels may be involved in skeletal muscle SPCa alterations and could represent therapeutic targets to treat DMD.

Methods

All experiments were conducted in the DMD^mdx rat, an animal model that closely reproduces the human DMD disease. We measured the cytosolic calcium concentration ([Ca²⁺]_c) and SPCa in EDL (Extensor Digitorum Longus) muscle fibers from age-matched WT and DMD^mdx rats of 1.5 to 7 months old. TRPC1 and TRPC3 expressions were measured in the EDL muscles at both the mRNA and protein levels, by RT-qPCR, western blot and immunocytofluorescence analysis.

Results

As expected from the malignant hyperthermia like episodes observed in several DMD^mdx rats, calcium homeostasis alterations were confirmed by measurements of early increases in [Ca²⁺]_c and SPCa in muscle fibers. TRPC3 and TRPC1 protein levels were increased in DMD^mdx rats. This was observed as soon as 1.5 months of age for TRPC3 but only at 7 months of age for TRPC1. A slight but reliable shift of the TRPC3 apparent molecular weight was observed in DMD^mdx rat muscles. Intracellular localization of both channels was not altered. We thus focused our attention on TRPC3. Application of Pyr10, a specific inhibitor of TRPC3, abolished the differences between SPCa values measured in WT and DMD^mdx. Finally, we showed that a rAAV-microdystrophin based treatment induced a high microdystrophin expression but only partial prevention of calcium homeostasis alterations, skeletal muscle force and TRPC3 protein increase.

Conclusions

All together our results show that correcting TRPC3 channel expression and/or activity appear to be a promising approach as a single or as a rAAV-based complementary therapy to treat DMD.

Dose-Dependent Microdystrophin Expression Enhancement in Cardiac Muscle by a Cardiac-Specific Regulatory Element

Duchenne muscular dystrophy (DMD) is an X-linked recessive disease that affects 1:5,000 live male births and is characterized by muscle wasting. By the age of 13 years, affected individuals are often wheelchair bound and suffer from respiratory and cardiac failure, which results in premature death. Although the administration of corticosteroids and ventilation can relieve the symptoms and extend the patients' lifespan, currently no cure exists for DMD. Among the different approaches under preclinical and clinical testing, gene therapy, using adeno-associated viral (AAV) vectors, is one of the most promising. In this study, we delivered intravenously AAV9 vectors expressing the microdystrophin MD1 (ΔR4-R23/ΔCT) under control of the synthetic muscle-specific promoter Spc5-12 and assessed the effect of adding a cardiac-specific cis-regulatory module (designated as CS-CRM4) on its expression profile in skeletal and cardiac muscles. Results show that Spc5-12 promoter, in combination with an AAV serotype that has high tropism for the heart, drives high MD1 expression levels in cardiac muscle in mdx mice. The additional regulatory element CS-CRM4 can further improve MD1 expression in cardiac muscles, but its effect is dose dependent and enhancement becomes evident only at lower vector doses.

The PKA-p38MAPK-NFAT5-Organic Osmolytes Pathway in Duchenne Muscular Dystrophy: From Essential Player in Osmotic Homeostasis, Inflammation and Skeletal Muscle Regeneration to Therapeutic Target

In Duchenne muscular dystrophy (DMD), the absence of dystrophin from the dystrophin-associated protein complex (DAPC) causes muscle membrane instability, which leads to myofiber necrosis, hampered regeneration, and chronic inflammation. The resulting disabled DAPC-associated cellular pathways have been described both at the molecular and the therapeutical level, with the Toll-like receptor nuclear factor kappa-light-chain-enhancer of activated B cells pathway (NF-ƘB), Janus kinase/signal transducer and activator of transcription proteins, and the transforming growth factor-β pathways receiving the most attention. In this review, we specifically focus on the protein kinase A/ mitogen-activated protein kinase/nuclear factor of activated T-cells 5/organic osmolytes (PKA-p38MAPK-NFAT5-organic osmolytes) pathway. This pathway plays an important role in osmotic homeostasis essential to normal cell physiology via its regulation of the influx/efflux of organic osmolytes. Besides, NFAT5 plays an essential role in cell survival under hyperosmolar conditions, in skeletal muscle regeneration, and in tissue inflammation, closely interacting with the master regulator of inflammation NF-ƘB. We describe the involvement of the PKA-p38MAPK-NFAT5-organic osmolytes pathway in DMD pathophysiology and provide a clear overview of which therapeutic molecules could be of potential benefit to DMD patients. We conclude that modulation of the PKA-p38MAPK-NFAT5-organic osmolytes pathway could be developed as supportive treatment for DMD in conjunction with genetic therapy.

Next-generation muscle-directed gene therapy by in silico vector design

There is an urgent need to develop the next-generation vectors for gene therapy of muscle disorders, given the relatively modest advances in clinical trials. These vectors should express substantially higher levels of the therapeutic transgene, enabling the use of lower and safer vector doses. In the current study, we identify potent muscle-specific transcriptional cis-regulatory modules (CRMs), containing clusters of transcription factor binding sites, using a genome-wide data-mining strategy. These novel muscle-specific CRMs result in a substantial increase in muscle-specific gene transcription (up to 400-fold) when delivered using adeno-associated viral vectors in mice. Significantly higher and sustained human micro-dystrophin and follistatin expression levels are attained than when conventional promoters are used. This results in robust phenotypic correction in dystrophic mice, without triggering apoptosis or evoking an immune response. This multidisciplinary approach has potentially broad implications for augmenting the efficacy and safety of muscle-directed gene therapy.

Adeno-associated viral vectors (AAV) are being developed for gene therapy of skeletal muscle, but it is a challenge to achieve robust gene expression. Here, the authors identify muscle-specific cisregulatory elements that lead to a substantial increase in micro-dystrophin and follistatin expression, resulting in a safe and sustainable rescue of the dystrophic phenotype in mouse models.

Improvement of Dystrophic Muscle Fragility by Short-Term Voluntary Exercise through Activation of Calcineurin Pathway in mdx Mice

Dystrophin deficiency in mdx mice, a model for Duchenne muscular dystrophy, leads to muscle weakness revealed by a reduced specific maximal force as well as fragility (ie, higher susceptibility to contraction-induced injury, as shown by a greater force decrease after lengthening contractions). Both symptoms could be improved with dystrophin restoration-based therapies and long-term (months) voluntary exercise. Herein, we evaluated the effect of short-term (1-week) voluntary wheel running. We found that running improved fragility of tibialis anterior muscle (TA), but not plantaris muscle, independently of utrophin up-regulation, without affecting weakness. Moreover, TA muscle excitability was also preserved by running, as shown by compound muscle action potential measurements after lengthening contractions. Of interest, the calcineurin inhibitor cyclosporin A prevented the effect of running on both muscle fragility and excitability. Cyclosporin also prevented the running-induced changes in expression of genes involved in excitability (Scn4a and Cacna1s) and slower contractile phenotype (Myh2 and Tnni1) in TA muscle. In conclusion, short-term voluntary exercise improves TA muscle fragility in mdx mice, without worsening weakness. Its effect was related to preserved excitability, calcineurin pathway activation, and changes in the program of genes involved in excitability and slower contractile phenotype. Thus, remediation of muscle fragility of Duchenne muscular dystrophy patients through appropriate exercise training deserves to be explored in more detail.

Systemic AAV Micro-dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin gene mutation. Conceptually, replacing the mutated gene with a normal one would cure the disease. However, this task has encountered significant challenges due to the enormous size of the gene and the distribution of muscle throughout the body. The former creates a hurdle for viral vector packaging and the latter begs for whole-body therapy. To address these obstacles, investigators have invented the highly abbreviated micro-dystrophin gene and developed body-wide systemic gene transfer with adeno-associated virus (AAV). Numerous microgene configurations and various AAV serotypes have been explored in animal models in many laboratories. Preclinical data suggests that intravascular AAV micro-dystrophin delivery can significantly ameliorate muscle pathology, enhance muscle force, and attenuate dystrophic cardiomyopathy in animals. Against this backdrop, several clinical trials have been initiated to test the safety and tolerability of this promising therapy in DMD patients. While these trials are not powered to reach a conclusion on clinical efficacy, findings will inform the field on the prospects of body-wide DMD therapy with a synthetic micro-dystrophin AAV vector. This review discusses the history, current status, and future directions of systemic AAV micro-dystrophin therapy.

Single Intramuscular Injection of AAV-shRNA Reduces DNM2 and Prevents Myotubular Myopathy in Mice

Myotubular myopathy, or X-linked centronuclear myopathy, is a severe muscle disorder representing a significant burden for patients and their families. It is clinically characterized by neonatal and severe muscle weakness and atrophy. Mutations in the myotubularin (MTM1) gene cause myotubular myopathy, and no specific curative treatment is available. We previously found that dynamin 2 (DNM2) is upregulated in both Mtm1 knockout and patient muscle samples, whereas its reduction through antisense oligonucleotides rescues the clinical and histopathological features of this myopathy in mice. Here, we propose a novel approach targeting Dnm2 mRNA. We screened and validated in vitro and in vivo several short hairpin RNA (shRNA) sequences that efficiently target Dnm2 mRNA. A single intramuscular injection of AAV-shDnm2 resulted in long-term reduction of DNM2 protein level and restored muscle force, mass, histology, and myofiber ultrastructure and prevented molecular defects linked to the disease. Our results demonstrate a robust DNM2 knockdown and provide an alternative strategy based on reduction of DNM2 to treat myotubular myopathy.

Dual AAV Gene Therapy for Duchenne Muscular Dystrophy with a 7-kb Mini-Dystrophin Gene in the Canine Model

Dual adeno-associated virus (AAV) technology was developed in 2000 to double the packaging capacity of the AAV vector. The proof of principle has been demonstrated in various mouse models. Yet, pivotal evidence is lacking in large animal models of human diseases. Here we report expression of a 7-kb canine ΔH2-R15 mini-dystrophin gene using a pair of dual AAV vectors in the canine model of Duchenne muscular dystrophy (DMD). The ΔH2-R15 minigene is by far the most potent synthetic dystrophin gene engineered for DMD gene therapy. We packaged minigene dual vectors in Y731F tyrosine-modified AAV-9 and delivered to the extensor carpi ulnaris muscle of a 12-month-old affected dog at the dose of 2 × 10¹³ viral genome particles/vector/muscle. Widespread mini-dystrophin expression was observed 2 months after gene transfer. The missing dystrophin-associated glycoprotein complex was restored. Treatment also reduced muscle degeneration and fibrosis and improved myofiber size distribution. Importantly, dual AAV therapy greatly protected the muscle from eccentric contraction-induced force loss. Our data provide the first clear evidence that dual AAV therapy can be translated to a diseased large mammal. Further development of dual AAV technology may lead to effective therapies for DMD and many other diseases in human patients.

Long-term microdystrophin gene therapy is effective in a canine model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an incurable X-linked muscle-wasting disease caused by mutations in the dystrophin gene. Gene therapy using highly functional microdystrophin genes and recombinant adeno-associated virus (rAAV) vectors is an attractive strategy to treat DMD. Here we show that locoregional and systemic delivery of a rAAV2/8 vector expressing a canine microdystrophin (cMD1) is effective in restoring dystrophin expression and stabilizing clinical symptoms in studies performed on a total of 12 treated golden retriever muscular dystrophy (GRMD) dogs. Locoregional delivery induces high levels of microdystrophin expression in limb musculature and significant amelioration of histological and functional parameters. Systemic intravenous administration without immunosuppression results in significant and sustained levels of microdystrophin in skeletal muscles and reduces dystrophic symptoms for over 2 years. No toxicity or adverse immune consequences of vector administration are observed. These studies indicate safety and efficacy of systemic rAAV-cMD1 delivery in a large animal model of DMD, and pave the way towards clinical trials of rAAV–microdystrophin gene therapy in DMD patients.

Duchenne muscular dystrophy is a progressive degenerative disease of muscles caused by mutations in the dystrophin gene. Here the authors use AAV vectors to deliver microdystrophin to dogs with muscular dystrophy, and show restoration of dystrophin expression and reduction of symptoms up to 26 months of age.

Nanotherapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked childhood muscle wasting disease caused by mutations in the dystrophin gene. Nanobiotechnology-based therapies (such as synthetic nanoparticles and naturally existing viral and nonviral nanoparticles) hold great promise to replace and repair the mutated dystrophin gene and significantly change the disease course. While a majority of DMD nanotherapies are still in early preclinical development, several [such as adeno-associated virus (AAV)-mediated systemic micro-dystrophin gene therapy] are advancing for phase I clinical trials. Recent regulatory approval of Ataluren (a nonsense mutation read-through chemical) in Europe and Exondys51 (an exon-skipping antisense oligonucleotide drug) in the United States shall offer critical insight in how to move DMD nanotherapy to human patients. Progress in novel, optimized nano-delivery systems may further improve emerging molecular therapeutic modalities for DMD. Despite these progresses, DMD nanotherapy faces a number of unique challenges. Specifically, the dystrophin gene is one of the largest genes in the genome while nanoparticles have an inherent size limitation per definition. Furthermore, muscle is the largest tissue in the body and accounts for 40% of the body mass. How to achieve efficient bodywide muscle targeting in human patients with nanomedication remains a significant translational hurdle. New creative approaches in the design of the miniature micro-dystrophin gene, engineering of muscle-specific synthetic AAV capsids, and novel nanoparticle-mediated exon-skipping are likely to result in major breakthroughs in DMD therapy. WIREs Nanomed Nanobiotechnol 2018, 10:e1472. doi: 10.1002/wnan.1472 This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Advances in gene therapy for muscular dystrophies

Duchenne muscular dystrophy (DMD) is a recessive lethal inherited muscular dystrophy caused by mutations in the gene encoding dystrophin, a protein required for muscle fibre integrity. So far, many approaches have been tested from the traditional gene addition to newer advanced approaches based on manipulation of the cellular machinery either at the gene transcription, mRNA processing or translation levels. Unfortunately, despite all these efforts, no efficient treatments for DMD are currently available. In this review, we highlight the most advanced therapeutic strategies under investigation as potential DMD treatments.

Dystrophin restoration therapy improves both the reduced excitability and the force drop induced by lengthening contractions in dystrophic mdx skeletal muscle

Background

The greater susceptibility to contraction-induced skeletal muscle injury (fragility) is an important dystrophic feature and tool for testing preclinic dystrophin-based therapies for Duchenne muscular dystrophy. However, how these therapies reduce the muscle fragility is not clear.

Methods

To address this question, we first determined the event(s) of the excitation-contraction cycle which is/are altered following lengthening (eccentric) contractions in the mdx muscle.

Results

We found that the immediate force drop following lengthening contractions, a widely used measure of muscle fragility, was associated with reduced muscle excitability. Moreover, the force drop can be mimicked by an experimental reduction in muscle excitation of uninjured muscle. Furthermore, the force drop was not related to major neuromuscular transmission failure, excitation-contraction uncoupling, and myofibrillar impairment. Secondly, and importantly, the re-expression of functional truncated dystrophin in the muscle of mdx mice using an exon skipping strategy partially prevented the reductions in both force drop and muscle excitability following lengthening contractions.

Conclusion

We demonstrated for the first time that (i) the increased susceptibility to contraction-induced muscle injury in mdx mice is mainly attributable to reduced muscle excitability; (ii) dystrophin-based therapy improves fragility of the dystrophic skeletal muscle by preventing reduction in muscle excitability.

Full-length Dysferlin Transfer by the Hyperactive Sleeping Beauty Transposase Restores Dysferlin-deficient Muscle

Dysferlin-deficient muscular dystrophy is a progressive disease characterized by muscle weakness and wasting for which there is no treatment. It is caused by mutations in DYSF, a large, multiexonic gene that forms a coding sequence of 6.2 kb. Sleeping Beauty (SB) transposon is a nonviral gene transfer vector, already used in clinical trials. The hyperactive SB system consists of a transposon DNA sequence and a transposase protein, SB100X, that can integrate DNA over 10 kb into the target genome. We constructed an SB transposon-based vector to deliver full-length human DYSF cDNA into dysferlin-deficient H2K A/J myoblasts. We demonstrate proper dysferlin expression as well as highly efficient engraftment (>1,100 donor-derived fibers) of the engineered myoblasts in the skeletal muscle of dysferlin- and immunodeficient B6.Cg-Dysf(prmd) Prkdc(scid)/J (Scid/BLA/J) mice. Nonviral gene delivery of full-length human dysferlin into muscle cells, along with a successful and efficient transplantation into skeletal muscle are important advances towards successful gene therapy of dysferlin-deficient muscular dystrophy.

piggyBac transposons expressing full-length human dystrophin enable genetic correction of dystrophic mesoangioblasts

Duchenne muscular dystrophy (DMD) is a genetic neuromuscular disorder caused by the absence of dystrophin. We developed a novel gene therapy approach based on the use of the piggyBac (PB) transposon system to deliver the coding DNA sequence (CDS) of either full-length human dystrophin (DYS: 11.1 kb) or truncated microdystrophins (MD1: 3.6 kb; MD2: 4 kb). PB transposons encoding microdystrophins were transfected in C2C12 myoblasts, yielding 65±2% MD1 and 66±2% MD2 expression in differentiated multinucleated myotubes. A hyperactive PB (hyPB) transposase was then deployed to enable transposition of the large-size PB transposon (17 kb) encoding the full-length DYS and green fluorescence protein (GFP). Stable GFP expression attaining 78±3% could be achieved in the C2C12 myoblasts that had undergone transposition. Western blot analysis demonstrated expression of the full-length human DYS protein in myotubes. Subsequently, dystrophic mesoangioblasts from a Golden Retriever muscular dystrophy dog were transfected with the large-size PB transposon resulting in 50±5% GFP-expressing cells after stable transposition. This was consistent with correction of the differentiated dystrophic mesoangioblasts following expression of full-length human DYS. These results pave the way toward a novel non-viral gene therapy approach for DMD using PB transposons underscoring their potential to deliver large therapeutic genes.

Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder. It is caused by loss-of-function mutations in the dystrophin gene. Currently, there is no cure. A highly promising therapeutic strategy is to replace or repair the defective dystrophin gene by gene therapy. Numerous animal models of DMD have been developed over the last 30 years, ranging from invertebrate to large mammalian models. mdx mice are the most commonly employed models in DMD research and have been used to lay the groundwork for DMD gene therapy. After ~30 years of development, the field has reached the stage at which the results in mdx mice can be validated and scaled-up in symptomatic large animals. The canine DMD (cDMD) model will be excellent for these studies. In this article, we review the animal models for DMD, the pros and cons of each model system, and the history and progress of preclinical DMD gene therapy research in the animal models. We also discuss the current and emerging challenges in this field and ways to address these challenges using animal models, in particular cDMD dogs.

Effect of voluntary physical activity initiated at age 7 months on skeletal hindlimb and cardiac muscle function in mdx mice of both genders

INTRODUCTION

The effects of voluntary activity initiated in adult mdx (C57BL/10ScSc-DMD(mdx) /J) mice on skeletal and cardiac muscle function have not been studied extensively.

METHODS

We studied the effects of 3 months of voluntary wheel running initiated at age 7 months on hindlimb muscle weakness, increased susceptibility to muscle contraction-induced injury, and left ventricular function in mdx mice.

RESULTS

We found that voluntary wheel running did not worsen the deficit in force-generating capacity and the force drop after lengthening contractions in either mdx mouse gender. It increased the absolute maximal force of skeletal muscle in female mdx mice. Moreover, it did not affect left ventricular function, structural heart dimensions, cardiac gene expression of inflammation, fibrosis, or remodeling markers.

CONCLUSION

These results indicate that voluntary activity initiated at age 7 months had no detrimental effects on skeletal or cardiac muscles in either mdx mouse gender.

Abnormal splicing switch of DMD's penultimate exon compromises muscle fibre maintenance in myotonic dystrophy

Myotonic Dystrophy type 1 (DM1) is a dominant neuromuscular disease caused by nuclear-retained RNAs containing expanded CUG repeats. These toxic RNAs alter the activities of RNA splicing factors resulting in alternative splicing misregulation and muscular dysfunction. Here we show that the abnormal splicing of DMD exon 78 found in dystrophic muscles of DM1 patients is due to the functional loss of MBNL1 and leads to the re-expression of an embryonic dystrophin in place of the adult isoform. Forced expression of embryonic dystrophin in zebrafish using an exon-skipping approach severely impairs the mobility and muscle architecture. Moreover, reproducing Dmd exon 78 missplicing switch in mice induces muscle fibre remodelling and ultrastructural abnormalities including ringed fibres, sarcoplasmic masses or Z-band disorganization, which are characteristic features of dystrophic DM1 skeletal muscles. Thus, we propose that splicing misregulation of DMD exon 78 compromises muscle fibre maintenance and contributes to the progressive dystrophic process in DM1.

Dystrobrevin increases dystrophin's binding to the dystrophin-glycoprotein complex and provides protection during cardiac stress

Duchenne muscular dystrophy is a fatal progressive disease of both cardiac and skeletal muscle resulting from the mutations in the DMD gene and loss of the protein dystrophin. Alpha-dystrobrevin (α-DB) tightly associates with dystrophin but the significance of this interaction within cardiac myocytes is poorly understood. In the current study, the functional role of α-DB in cardiomyocytes and its implications for dystrophin function are examined. Cardiac stress testing demonstrated significant heart disease in α-DB null (adbn(-/-)) mice, which displayed mortality and lesion sizes that were equivalent to those seen in dystrophin-deficient mdx mice. Despite normal expression and subcellular localization of dystrophin in the adbn(-/-) heart, there is a significant decrease in the strength of dystrophin's interaction with the membrane-bound dystrophin-associated glycoprotein complex (DGC). A similar weakening of the dystrophin-membrane interface was observed in mice lacking the sarcoglycan complex. Cardiomyocytes from adbn(-/-) mice were smaller and responded less to adrenergic receptor induced hypertrophy. The basal decrease in size could not be attributed to aberrant Akt activation. In addition, the organization of the microtubule network was significantly altered in adbn(-/-) cardiac myocytes, while the total expression of tubulin was unchanged in adbn(-/-) hearts. These studies demonstrate that α-DB is a multifunctional protein that increases dystrophin's binding to the dystrophin-glycoprotein complex, and is critical for the full functionality of dystrophin.

Phosphorylation within the cysteine-rich region of dystrophin enhances its association with β-dystroglycan and identifies a potential novel therapeutic target for skeletal muscle wasting

Mutations in dystrophin lead to Duchenne muscular dystrophy, which is among the most common human genetic disorders. Dystrophin nucleates assembly of the dystrophin-glycoprotein complex (DGC), and a defective DGC disrupts an essential link between the intracellular cytoskeleton and the basal lamina, leading to progressive muscle wasting. In vitro studies have suggested that dystrophin phosphorylation may affect interactions with actin or syntrophin, yet whether this occurs in vivo or affects protein function remains unknown. Utilizing nanoflow liquid chromatography mass spectrometry, we identified 18 phosphorylated residues within endogenous dystrophin. Mutagenesis revealed that phosphorylation at S3059 enhances the dystrophin-dystroglycan interaction and 3D modeling utilizing the Rosetta software program provided a structural model for how phosphorylation enhances this interaction. These findings demonstrate that phosphorylation is a key mechanism regulating the interaction between dystrophin and the DGC and reveal that posttranslational modification of a single amino acid directly modulates the function of dystrophin.

Conserved regions of the DMD 3' UTR regulate translation and mRNA abundance in cultured myotubes

Duchenne muscular dystrophy (DMD), a severe muscle-wasting disease, is caused by mutations in the DMD gene, which encodes for the protein dystrophin. Its regulation is of therapeutic interest as even small changes in expression of functional dystrophin can significantly impact the severity of DMD. While tissue-specific distribution and transcriptional regulation of several DMD mRNA isoforms has been well characterized, the post-transcriptional regulation of dystrophin synthesis is not well understood. Here, we utilize qRTPCR and a quantitative dual-luciferase reporter assay to examine the effects of isoform specific DMD 5' UTRs and the highly conserved DMD 3' UTR on mRNA abundance and translational control of gene expression in C2C12 cells. The 5' UTRs were shown to initiate translation with low efficiency in both myoblasts and myotubes. Whereas, two large highly conserved elements in the 3' UTR, which overlap the previously described Lemaire A and D regions, increase mRNA levels and enhance translation upon differentiation of myoblasts into myotubes. The results presented here implicate an important role for DMD UTRs in dystrophin expression and delineate the cis-acting elements required for the myotube-specific regulation of steady-state mRNA levels and translational enhancer activity found in the DMD 3' UTR.

Long-term episomal transgene expression from mitotically stable integration-deficient lentiviral vectors

Nonintegrating gene delivery vectors have an improved safety profile compared with integrating vectors, but transgene retention is problematic as nonreplicating episomes are progressively and rapidly diluted out through cell division. We have developed an integration-deficient lentiviral vector (IDLV) system generating mitotically stable episomes capable of long-term transgene expression. We found that a transient cell cycle arrest at the time of transduction with IDLVs resulted in 13-45% of Chinese hamster ovary (CHO) cells expressing the transgene for over 100 cell generations in the absence of selection. The use of a scaffold/matrix attachment region did not result in improved episomal retention in this system, and episomes did not form after transduction with adeno-associated viral or minicircle vectors under the same conditions. Investigations into the episomal status of the vector genome using (1) linear amplification-mediated polymerase chain reaction followed by deep sequencing of vector-genome junctions, (2) Southern blotting, and (3) fluorescent in situ hybridization strongly suggest that the vector is not integrated in the vast majority of cells. In conclusion, we have developed an IDLV procedure generating mitotically stable episomes capable of long-term transgene expression. The application of this approach to stem cell populations could significantly improve the safety profile of a range of stem and progenitor cell gene therapies.

Full-length dystrophin reconstitution with adeno-associated viral vectors

Duchenne muscular dystrophy (DMD) is the most common lethal muscle disorder in children. It is caused by mutations of the dystrophin gene. Adeno-associated virus (AAV)-mediated gene replacement therapy has been actively pursued to treat DMD. However, this promising therapeutic modality has been challenged by the small packaging capacity of the AAV vector. The size of the full-length dystrophin cDNA is >11 kb, while an AAV virus can carry only a 5 kb genome. Innovative high-capacity AAV vectors may offer an opportunity to express the full-length dystrophin coding sequence. Here we describe several sets of tri-AAV vectors for full-length human dystrophin delivery. In each set, the full-length human dystrophin cDNA was split into three fragments and independently packaged into separate recombinant AAV vectors. Each vector was engineered with unique recombination signals for directional recombination. Tri-AAV vectors were coinjected into the tibialis anterior muscle of dystrophin-deficient mdx4cv mice. Thirty-five days after injection, dystrophin expression was examined by immunofluorescence staining. Despite low reconstitution efficiency, full-length human dystrophin was successfully expressed from the tri-AAV vectors. Our results suggest that AAV can be engineered to express an extra-large (up to 15 kb) gene that is approximately three times the size of the wild-type AAV genome. Further optimization of the trivector strategy may expand the utility of AAV for human gene therapy.

Skipping multiple exons of dystrophin transcripts using cocktail antisense oligonucleotides

Duchenne muscular dystrophy (DMD) is one of the most common and lethal genetic disorders, with 20,000 children per year born with DMD globally. DMD is caused by mutations in the dystrophin (DMD) gene. Antisense-mediated exon skipping therapy is a promising therapeutic approach that uses short DNA-like molecules called antisense oligonucleotides (AOs) to skip over/splice out the mutated part of the gene to produce a shortened but functional dystrophin protein. One major challenge has been its limited applicability. Multiple exon skipping has recently emerged as a potential solution. Indeed, many DMD patients need exon skipping of multiple exons in order to restore the reading frame, depending on how many base pairs the mutated exon(s) and adjacent exons have. Theoretically, multiple exon skipping could be used to treat approximately 90%, 80%, and 98% of DMD patients with deletion, duplication, and nonsense mutations, respectively. In addition, multiple exon skipping could be used to select deletions that optimize the functionality of the truncated dystrophin protein. The proof of concept of systemic multiple exon skipping using a cocktail of AOs has been demonstrated in dystrophic dog and mouse models. Remaining challenges include the insufficient efficacy of systemic treatment, especially for therapies that target the heart, and limited long-term safety data. Here we review recent preclinical developments in AO-mediated multiple exon skipping and discuss the remaining challenges.

Triple trans-splicing adeno-associated virus vectors capable of transferring the coding sequence for full-length dystrophin protein into dystrophic mice

Recombinant adeno-associated virus (rAAV) vectors have been shown to permit very efficient widespread transgene expression in skeletal muscle after systemic delivery, making these increasingly attractive as vectors for Duchenne muscular dystrophy (DMD) gene therapy. DMD is a severe muscle-wasting disorder caused by DMD gene mutations leading to complete loss of dystrophin protein. One of the major issues associated with delivery of the DMD gene, as a therapeutic approach for DMD, is its large open reading frame (ORF; 11.1 kb). A series of truncated microdystrophin cDNAs (delivered via a single AAV) and minidystrophin cDNAs (delivered via dual-AAV trans-spliced/overlapping reconstitution) have thus been extensively tested in DMD animal models. However, critical rod and hinge domains of dystrophin required for interaction with components of the dystrophin-associated protein complex, such as neuronal nitric oxide synthase, syntrophin, and dystrobrevin, are missing; these dystrophin domains may still need to be incorporated to increase dystrophin functionality and stabilize membrane rigidity. Full-length DMD gene delivery using AAV vectors remains elusive because of the limited single-AAV packaging capacity (4.7 kb). Here we developed a novel method for the delivery of the full-length DMD coding sequence to skeletal muscles in dystrophic mdx mice using a triple-AAV trans-splicing vector system. We report for the first time that three independent AAV vectors carrying "in tandem" sequential exonic parts of the human DMD coding sequence enable the expression of the full-length protein as a result of trans-splicing events cojoining three vectors via their inverted terminal repeat sequences. This method of triple-AAV-mediated trans-splicing could be applicable to the delivery of any large therapeutic gene (≥11 kb ORF) into postmitotic tissues (muscles or neurons) for the treatment of various inherited metabolic and genetic diseases.

New developments in the use of gene therapy to treat Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a lethal X-linked inherited disorder characterised by progressive muscle weakness, wasting and degeneration. Although the gene affected in DMD was identified over 25 years ago, there is still no effective treatment.

AREAS COVERED

Here we review some of the genetic-based strategies aimed at amelioration of the DMD phenotype. A number of Phase II/III clinical trials of antisense oligonucleotide-induced exon skipping for restoration of the open reading frame (ORF) of the DMD gene have recently been completed. The potential strategies for overcoming the hurdles that appear to prevent exon skipping becoming an effective treatment for DMD currently are discussed.

EXPERT OPINION

The applicability of exon skipping as a therapy to DMD is restricted and the development of alternative strategies that are more encompassing is needed. The rapid pre-clinical advances that are being made in the field of adeno-associated virus (AAV)-based delivery of micro-dystrophin would address this. The obstacles to be faced with gene replacement strategies would include the need for high viral titres, efficient muscle targeting and avoidance of immune response to vector and transgene. The new emerging field of gene editing could potentially provide permanent correction of the DMD gene and the feasibility of such an approach to DMD is discussed.

Multiple recombinant adeno-associated viral vector serotypes display persistent in vivo gene expression in vector-transduced rat stifle joints

Our aim was to investigate serotype-specific cell and tissue-transduction tropisms, transgene expression levels and longevity, and immunogenicity of candidate rAAV serotypes in rat osteochondral cells, tissues, and stifle joints. In vitro, we used six rAAV serotypes and two promoters to transduce synoviocytes and chondrocytes. Serotypes rAAV2/5 and 2/2 yielded the highest transduction efficiency 4 days after transduction. No differences were detected between cytomegalovirus and chicken β-actin promoters. In vivo, intra-articular injection was used to introduce four rAAV serotypes into 4-month-old rats in the left stifle joint. Eleven months later, serotype 2/5 vector, diluted with saline or surfactant, was injected into the right stifle joint of the same rats. Rats were analyzed up to 12 months after initial injection. Bioluminescence was detected at 7 days and all serotypes tested displayed bioluminescence above controls after 1 year in the left stifle. Gene expression was detected in the right stifle joints of all rats with the exception of rats previously injected with serotype 2/5. We observed no difference irrespective of whether the luciferin was injected subcutaneously or intraperitoneally. However, surfactant-diluted vectors led to increased gene expression compared with saline-diluted vectors. Cell- and tissue-specific transduction was observed in rat stifles injected with an nLacZ-containing rAAV. Transduction was greatest in stromal tissues and mesenchymal cell types. Exposure to a specific serotype did not inhibit subsequent transduction with a different serotype at a second vector injection. Including surfactant as a vector diluent increased gene expression within the stifle joint and should be considered for in vivo gene therapy applications.

The Rag2⁻Il2rb⁻Dmd⁻ mouse: a novel dystrophic and immunodeficient model to assess innovating therapeutic strategies for muscular dystrophies

The development of innovative therapeutic strategies for muscular dystrophies, particularly cell-based approaches, is still a developing field. Although positive results have been obtained in animal models, they have rarely been confirmed in patients and resulted in very limited clinical improvements, suggesting some specificity in humans. These findings emphasized the need for an appropriate animal model (i.e., immunodeficient and dystrophic) to investigate in vivo the behavior of transplanted human myogenic stem cells. We report a new model, the Rag2(-)Il2rb(-)Dmd(-) mouse, which lacks T, B, and NK cells, and also carries a mutant Dmd allele that prevents the production of any dystrophin isoform. The dystrophic features of this new model are comparable with those of the classically used mdx mouse, but with the total absence of any revertant dystrophin positive fiber. We show that Rag2(-)Il2rb(-)Dmd(-) mice allow long-term xenografts of human myogenic cells. Altogether, our findings indicate that the Rag2(-)Il2rb(-)Dmd(-) mouse represents an ideal model to gain further insights into the behavior of human myogenic stem cells in a dystrophic context, and can be used to assess innovative therapeutic strategies for muscular dystrophies.

Long-term miR-669a therapy alleviates chronic dilated cardiomyopathy in dystrophic mice

BACKGROUND

Dilated cardiomyopathy (DCM) is a leading cause of chronic morbidity and mortality in muscular dystrophy (MD) patients. Current pharmacological treatments are not yet able to counteract chronic myocardial wastage, thus novel therapies are being intensely explored. MicroRNAs have been implicated as fine regulators of cardiomyopathic progression. Previously, miR-669a downregulation has been linked to the severe DCM progression displayed by Sgcb-null dystrophic mice. However, the impact of long-term overexpression of miR-669a on muscle structure and functionality of the dystrophic heart is yet unknown.

METHODS AND RESULTS

Here, we demonstrate that intraventricular delivery of adeno-associated viral (AAV) vectors induces long-term (18 months) miR-669a overexpression and improves survival of Sgcb-null mice. Treated hearts display significant decrease in hypertrophic remodeling, fibrosis, and cardiomyocyte apoptosis. Moreover, miR-669a treatment increases sarcomere organization, reduces ventricular atrial natriuretic peptide (ANP) levels, and ameliorates gene/miRNA profile of DCM markers. Furthermore, long-term miR-669a overexpression significantly reduces adverse remodeling and enhances systolic fractional shortening of the left ventricle in treated dystrophic mice, without significant detrimental consequences on skeletal muscle wastage.

CONCLUSIONS

Our findings provide the first evidence of long-term beneficial impact of AAV-mediated miRNA therapy in a transgenic model of severe, chronic MD-associated DCM.

Clinical trials using antisense oligonucleotides in duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by mutations in the DMD gene, affecting 1 in 3500 newborn males. Complete loss of muscle dystrophin protein causes progressive muscle weakness and heart and respiratory failure, leading to premature death. Antisense oligonucleotides (AONs) that bind to complementary sequences of the dystrophin pre-mRNA to induce skipping of the targeted exon by modulating pre-mRNA splicing are promising therapeutic agents for DMD. Such AONs can restore the open reading frame of the DMD gene and produce internally deleted, yet partially functional dystrophin protein isoforms in skeletal muscle. Within the last few years, clinical trials using AONs have made considerable progress demonstrating the restoration of functional dystrophin protein and acceptable safety profiles following both local and systemic delivery in DMD patients. However, improvement of AON delivery and efficacy, along with the development of multiple AONs to treat as many DMD patients as possible needs to be addressed for this approach to fulfill its potential. Here, we review the recent progress made in clinical trials using AONs to treat DMD and discuss the current challenges to the development of AON-based therapy for DMD.

Protective effect of female gender-related factors on muscle force-generating capacity and fragility in the dystrophic mdx mouse

INTRODUCTION

The dystrophic features in hindlimb skeletal muscles of female mdx mice are unclear.

METHODS

We analyzed force-generating capacity and force decline after lengthening contraction-induced damage (fragility).

RESULTS

Young (6-month-old) female mdx mice displayed reduced force-generating capacity (-18%) and higher fragility (23% force decline) compared with female age-matched wild-type mice. These 2 dystrophic features were less accentuated in young female than in young male mdx mice (-32% and 42% force drop). With advancing age, force-generating capacity decreased and fragility increased in old (20 month) female mdx mice (-21% and 57% force decline), but they were unchanged in old male mdx mice. Moreover, estradiol treatment had no effect in old female mdx mice.

CONCLUSIONS

Female gender-related factors mitigate dystrophic features in young but not old mdx mice. Further studies are warranted to identify the beneficial gender-related factor in dystrophic muscle.

Voluntary physical activity protects from susceptibility to skeletal muscle contraction-induced injury but worsens heart function in mdx mice

It is well known that inactivity/activity influences skeletal muscle physiological characteristics. However, the effects of inactivity/activity on muscle weakness and increased susceptibility to muscle contraction-induced injury have not been extensively studied in mdx mice, a murine model of Duchenne muscular dystrophy with dystrophin deficiency. In the present study, we demonstrate that inactivity (ie, leg immobilization) worsened the muscle weakness and the susceptibility to contraction-induced injury in mdx mice. Inactivity also mimicked these two dystrophic features in wild-type mice. In contrast, we demonstrate that these parameters can be improved by activity (ie, voluntary wheel running) in mdx mice. Biochemical analyses indicate that the changes induced by inactivity/activity were not related to fiber-type transition but were associated with altered expression of different genes involved in fiber growth (GDF8), structure (Actg1), and calcium homeostasis (Stim1 and Jph1). However, activity reduced left ventricular function (ie, ejection and shortening fractions) in mdx, but not C57, mice. Altogether, our study suggests that muscle weakness and susceptibility to contraction-induced injury in dystrophic muscle could be attributable, at least in part, to inactivity. It also suggests that activity exerts a beneficial effect on dystrophic skeletal muscle but not on the heart.

Gene replacement therapies for duchenne muscular dystrophy using adeno-associated viral vectors

The muscular dystrophies collectively represent a major health challenge, as few significant treatment options currently exist for any of these disorders. Recent years have witnessed a proliferation of novel approaches to therapy, spanning increased testing of existing and new pharmaceuticals, DNA delivery (both anti-sense oligonucleotides and plasmid DNA), gene therapies and stem cell technologies. While none of these has reached the point of being used in clinical practice, all show promise for being able to impact different types of muscular dystrophies. Our group has focused on developing direct gene replacement strategies to treat recessively inherited forms of muscular dystrophy, particularly Duchenne and Becker muscular dystrophy (DMD/BMD). Both forms of dystrophy are caused by mutations in the dystrophin gene and all cases can in theory be treated by gene replacement using synthetic forms of the dystrophin gene. The major challenges for success of this approach are the development of a suitable gene delivery shuttle, generating a suitable gene expression cassette able to be carried by such a shuttle, and achieving safe and effective delivery without elicitation of a destructive immune response. This review summarizes the current state of the art in terms of using adeno-associated viral vectors to deliver synthetic dystrophin genes for the purpose of developing gene therapy for DMD.

The beneficial effect of myostatin deficiency on maximal muscle force and power is attenuated with age

The prolonged effect of myostatin deficiency on muscle performance in knockout mice has as yet been only poorly investigated. We have demonstrated that absolute maximal force is increased in 6-month old female and male knockout mice and 2-year old female knockout mice as compared to age- and sex-matched wildtype mice. Similarly, absolute maximal power is increased by myostatin deficiency in 6-month old female and male knockout mice but not in 2-year old female knockout mice. The increases we observed were greater in 6-month old female than in male knockout mice and can primarily result from muscle hypertrophy. In contrast, fatigue resistance was decreased in 6-month old knockout mice of both sexes as compared to age- and sex-matched wildtype mice. Moreover, in contrast to 2-year old female wildtype mice, aging in 2-year old knockout mice reduced absolute maximal force and power of both sexes as compared to their younger counterparts, although muscle weight did not change. These age-related decreases were lower in 2-year old female than in 2-year old male knockout mice. Together these results suggest that the beneficial effect of myostatin deficiency on absolute maximal force and power is greater in young (versus old) mice and female (versus male) mice. Most of these effects of myostatin deficiency are related neither to changes in the concentration of myofibrillar proteins nor to the slow to fast fiber type transition.

Genetic therapeutic approaches for Duchenne muscular dystrophy

Despite an expansive wealth of research following the discovery of the DMD gene 25 years ago, there is still no curative treatment for Duchenne muscular dystrophy. However, there are currently many promising lines of research, including cell-based therapies and pharmacological reagents to upregulate dystrophin via readthrough of nonsense mutations or by upregulation of the dystrophin homolog utrophin. Here we review genetic-based therapeutic strategies aimed at the amelioration of the DMD phenotype. These include the reintroduction of a copy of the DMD gene into an affected tissue by means of a viral vector; correction of the mutated DMD transcript by antisense oligonucleotide-induced exon skipping to restore the open reading frame; and direct modification of the DMD gene at a chromosomal level through genome editing. All these approaches are discussed in terms of the more recent advances, and the hurdles to be overcome if a comprehensive and effective treatment for DMD is to be found. These hurdles include the need to target all musculature of the body. Therefore any potential treatment would need to be administered systemically. In addition, any treatment needs to have a long-term effect, with the possibility of readministration, while avoiding any potentially detrimental immune response to the vector or transgene.

Recent advances in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), an allelic X-linked progressive muscle-wasting disease, is one of the most common single-gene disorders in the developed world. Despite knowledge of the underlying genetic causation and resultant pathophysiology from lack of dystrophin protein at the muscle sarcolemma, clinical intervention is currently restricted to symptom management. In recent years, however, unprecedented advances in strategies devised to correct the primary defect through gene- and cell-based therapeutics hold particular promise for treating dystrophic muscle. Conventional gene replacement and endogenous modification strategies have greatly benefited from continued improvements in encapsidation capacity, transduction efficiency, and systemic delivery. In particular, RNA-based modifying approaches such as exon skipping enable expression of a shorter but functional dystrophin protein and rapid progress toward clinical application. Emerging combined gene- and cell-therapy strategies also illustrate particular promise in enabling ex vivo genetic correction and autologous transplantation to circumvent a number of immune challenges. These approaches are complemented by a vast array of pharmacological approaches, in particular the successful identification of molecules that enable functional replacement or ameliorate secondary DMD pathology. Animal models have been instrumental in providing proof of principle for many of these strategies, leading to several recent trials that have investigated their efficacy in DMD patients. Although none has reached the point of clinical use, rapid improvements in experimental technology and design draw this goal ever closer. Here, we review therapeutic approaches to DMD, with particular emphasis on recent progress in strategic development, preclinical evaluation and establishment of clinical efficacy. Further, we discuss the numerous challenges faced and synergistic approaches being devised to combat dystrophic pathology effectively.

Impaired adaptive response to mechanical overloading in dystrophic skeletal muscle

Dystrophin contributes to force transmission and has a protein-scaffolding role for a variety of signaling complexes in skeletal muscle. In the present study, we tested the hypothesis that the muscle adaptive response following mechanical overloading (ML) would be decreased in MDX dystrophic muscle lacking dystrophin. We found that the gains in muscle maximal force production and fatigue resistance in response to ML were both reduced in MDX mice as compared to healthy mice. MDX muscle also exhibited decreased cellular and molecular muscle remodeling (hypertrophy and promotion of slower/oxidative fiber type) in response to ML, and altered intracellular signalings involved in muscle growth and maintenance (mTOR, myostatin, follistatin, AMPKα1, REDD1, atrogin-1, Bnip3). Moreover, dystrophin rescue via exon skipping restored the adaptive response to ML. Therefore our results demonstrate that the adaptive response in response to ML is impaired in dystrophic MDX muscle, most likely because of the dystrophin crucial role.

Long-term preservation of cardiac structure and function after adeno-associated virus serotype 9-mediated microdystrophin gene transfer in mdx mice

Dystrophin plays an important role in muscle contraction, linking the intracellular cytoskeleton to the extracellular matrix. Mutations of the dystrophin gene leading to a complete loss of the protein cause Duchenne muscular dystrophy (DMD), frequently associated with severe cardiomyopathy. Early clinical trials in DMD using gene transfer to skeletal muscle are underway, but gene transfer to dystrophic cardiac muscle has not yet been tested in humans. The aim of this study was to develop an optimized protocol for cardiac gene therapy in the mouse model of dystrophin deficiency (mdx), using a cardiac promoter for expression of a microdystrophin (μDys) transgene packaged into an adeno-associated virus serotype 9 vector (AAV9). In this study adult mdx mice were intravenously injected with 1×10(12) genomic particles of AAV9 vectors carrying a cDNA encoding μDys under the control of either a ubiquitously active cytomegalovirus (CMV) promoter or a cardiac-specific CMV-enhanced myosin light chain (MLC0.26) promoter. After 10 months, both AAV9 vectors led to sustained μDys expression in cardiac muscle, but the MLC promoter conferred about 4-fold higher protein levels. AAV9-CMV-MLC0.26-μDys resulted in significant protection of cardiac morphology and function as assessed by histopathology, echocardiography, and left ventricular catheterization. In conclusion, we established an AAV9-mediated gene transfer approach for efficient and specific long-term μDys expression in the hearts of mdx mice, resulting in a sustained therapeutic effect. Thus, this approach might be a basis for further translation into a treatment strategy for DMD-associated cardiomyopathy.