Phenotypic improvement of dystrophic muscles by rAAV/microdystrophin vectors is augmented by Igf1 codelivery

Therapeutic approaches for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a monogenic muscle-wasting disorder and a priority candidate for molecular and cellular therapeutics. Although rare, it is the most common inherited myopathy affecting children and so has been the focus of intense research activity. It is caused by mutations that disrupt production of the dystrophin protein, and a plethora of drug development approaches are under way that aim to restore dystrophin function, including exon skipping, stop codon readthrough, gene replacement, cell therapy and gene editing. These efforts have led to the clinical approval of four exon skipping antisense oligonucleotides, one stop codon readthrough drug and one gene therapy product, with other approvals likely soon. Here, we discuss the latest therapeutic strategies that are under development and being deployed to treat DMD. Lessons from these drug development programmes are likely to have a major impact on the DMD field, but also on molecular and cellular medicine more generally. Thus, DMD is a pioneer disease at the forefront of future drug discovery efforts, with these experimental treatments paving the way for therapies using similar mechanisms of action being developed for other genetic diseases.

Current Strategies of Muscular Dystrophy Therapeutics: An Overview

Muscular dystrophies are a group of genetic disorders characterized by varying degrees of progressive muscle weakness and degeneration. They are clinically and genetically heterogeneous but share the common histological features of dystrophic muscle. There is currently no cure for muscular dystrophies, which is of particular concern for the more disabling and/or lethal forms of the disease. Through the years, several therapies have encouragingly been developed for muscular dystrophies and include genetic, cellular, and pharmacological approaches. In this chapter, we undertake a comprehensive exploration of muscular dystrophy therapeutics under current development. Our review includes antisense therapy, CRISPR, gene replacement, cell therapy, nonsense suppression, and disease-modifying small molecule compounds.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Striated muscle activator of Rho signalling (STARS) overexpression in the mdx mouse enhances muscle functional capacity and regulates the actin cytoskeleton and oxidative phosphorylation pathways

NEW FINDINGS

What is the central question of this study? Striated muscle activator of rho signalling (STARS) is an actin-binding protein that regulates transcriptional pathways controlling muscle function, growth and myogenesis, processes that are impaired in dystrophic muscle: what is the regulation of the STARS pathway in Duchenne muscular dystrophy (DMD)? What is the main finding and its importance? Members of the STARS signalling pathway are reduced in the quadriceps of patients with DMD and in mouse models of muscular dystrophy. Overexpression of STARS in the dystrophic deficient mdx mouse model increased maximal isometric specific force and upregulated members of the actin cytoskeleton and oxidative phosphorylation pathways. Regulating STARS may be a therapeutic approach to enhance muscle health.

ABSTRACT

Duchenne muscular dystrophy (DMD) is characterised by impaired cytoskeleton organisation, cytosolic calcium handling, oxidative stress and mitochondrial dysfunction. This results in progressive muscle damage, wasting and weakness and premature death. The striated muscle activator of rho signalling (STARS) is an actin-binding protein that activates the myocardin-related transcription factor-A (MRTFA)/serum response factor (SRF) transcriptional pathway, a pathway regulating cytoskeletal structure and muscle function, growth and repair. We investigated the regulation of the STARS pathway in the quadriceps muscle from patients with DMD and in the tibialis anterior (TA) muscle from the dystrophin-deficient mdx and dko (utrophin and dystrophin null) mice. Protein levels of STARS, SRF and RHOA were reduced in patients with DMD. STARS, SRF and MRTFA mRNA levels were also decreased in DMD muscle, while Stars mRNA levels were decreased in the mdx mice and Srf and Mrtfa mRNAs decreased in the dko mice. Overexpressing human STARS (hSTARS) in the TA muscles of mdx mice increased maximal isometric specific force by 13% (P < 0.05). This was not associated with changes in muscle mass, fibre cross-sectional area, fibre type, centralised nuclei or collagen deposition. Proteomics screening followed by pathway enrichment analysis identified that hSTARS overexpression resulted in 31 upregulated and 22 downregulated proteins belonging to the actin cytoskeleton and oxidative phosphorylation pathways. These pathways are impaired in dystrophic muscle and regulate processes that are vital for muscle function. Increasing the STARS protein in dystrophic muscle improves muscle force production, potentially via synergistic regulation of cytoskeletal structure and energy production.

Micro-dystrophin Gene Therapy Partially Enhances Exercise Capacity in Older Adult mdx Mice

Micro-dystrophin (μDys) gene therapeutics can improve striated muscle structure and function in different animal models of Duchenne muscular dystrophy. Most studies, however, used young mdx mice that lack a pronounced dystrophic phenotype, short treatment periods, and limited muscle function tests. We, therefore, determined the relative efficacy of two previously described μDys gene therapeutics (rAAV6:μDysH3 and rAAV6:μDys5) in 6-month-old mdx mice using a 6-month treatment regimen and forced exercise. Forelimb and hindlimb grip strength, metabolic rate (VO2 max), running efficiency (energy expenditure), and serum creatine kinase levels similarly improved in mdx mice treated with either vector. Both vectors produced nearly identical dose-responses in all assays. They also partially prevented the degenerative effects of repeated high-intensity exercise on muscle histology, although none of the metrics examined was restored to normal wild-type levels. Moreover, neither vector had any consistent effect on respiration while exercising. These data together suggest that, although μDys gene therapy can improve isolated and systemic muscle function, it may be only partially effective when dystrophinopathies are advanced or when muscle structure is significantly challenged, as with high-intensity exercise. This further suggests that restoring muscle function to near-normal levels will likely require ancillary or combinatorial treatments capable of enhancing muscle strength.

Gene doping and genomic science in sports: where are we?

The misuse of sport-related gene transfer methods in elite athletes is a real and growing concern. The success of gene therapy in the treatment of hereditary diseases has been most evident since targets in gene therapy products can be used in healthy individuals to improve sports performance. Performing these practices threatens the sporting character of competitions and may pose potential health hazards. Since the World Anti-Doping Agency pronouncement on the prohibition of such practices in 2003, several researchers have been trying to address the challenge of developing an effective method for the detection of genetic doping. This review presents an overview of the published methods developed for this purpose, the advantages and limitations of technologies and the putative target genes. At last, we present the perspective related to the application of the detection methods in the doping control field.

Functional muscle hypertrophy by increased insulin-like growth factor 1 does not require dysferlin

INTRODUCTION

Dysferlin loss-of-function mutations cause muscular dystrophy, accompanied by impaired membrane repair and muscle weakness. Growth promoting strategies including insulin-like growth factor 1 (IGF-1) could provide benefit but may cause strength loss or be ineffective. The objective of this study was to determine whether locally increased IGF-1 promotes functional muscle hypertrophy in dysferlin-null (Dysf-/- ) mice.

METHODS

Muscle-specific transgenic expression and postnatal viral delivery of Igf1 were used in Dysf-/- and control mice. Increased IGF-1 levels were confirmed by enzyme-linked immunosorbent assay. Testing for skeletal muscle mass and function was performed in male and female mice.

RESULTS

Muscle hypertrophy occurred in response to increased IGF-1 in mice with and without dysferlin. Male mice showed a more robust response compared with females. Increased IGF-1 did not cause loss of force per cross-sectional area in Dysf-/- muscles.

DISCUSSION

We conclude that increased local IGF-1 promotes functional hypertrophy when dysferlin is absent and reestablishes IGF-1 as a potential therapeutic for dysferlinopathies.

The potential of utrophin and dystrophin combination therapies for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal neuromuscular disorder caused by loss of dystrophin. Several therapeutic modalities are currently in clinical trials but none will achieve maximum functional rescue and full disease correction. Therefore, we explored the potential of combining the benefits of dystrophin with increases of utrophin, an autosomal paralogue of dystrophin. Utrophin and dystrophin can be co-expressed and co-localized at the same muscle membrane. Wild-type (wt) levels of dystrophin are not significantly affected by a moderate increase of utrophin whereas higher levels of utrophin reduce wt dystrophin, suggesting a finite number of actin binding sites at the sarcolemma. Thus, utrophin upregulation strategies may be applied to the more mildly affected Becker patients with lower dystrophin levels. Whereas increased dystrophin in wt animals does not offer functional improvement, overexpression of utrophin in wt mice results in a significant supra-functional benefit over wt. These findings highlight an additive benefit of the combined therapy and potential new unique roles of utrophin. Finally, we show a 30% restoration of wt dystrophin levels, using exon-skipping, together with increased utrophin levels restores dystrophic muscle function to wt levels offering greater therapeutic benefit than either single approach alone. Thus, this combination therapy results in additive functional benefit and paves the way for potential future combinations of dystrophin- and utrophin-based strategies.

Combined Therapies for Duchenne Muscular Dystrophy to Optimize Treatment Efficacy

Duchene Muscular Dystrophy (DMD) is the most frequent muscular dystrophy and one of the most severe due to the absence of the dystrophin protein. Typical pathological features include muscle weakness, muscle wasting, degeneration, and inflammation. At advanced stages DMD muscles present exacerbated extracellular matrix and fat accumulation. Recent progress in therapeutic approaches has allowed new strategies to be investigated, including pharmacological, gene-based and cell-based therapies. Gene and cell-based therapies are still limited by poor targeting and low efficiency in fibrotic dystrophic muscle, therefore it is increasingly evident that future treatments will have to include “combined therapies” to reach maximal efficiency. The scope of this mini-review is to provide an overview of the current literature on such combined therapies for DMD. By “combined therapies” we mean those that include both a therapy to correct the genetic defect and an additional one to address one of the secondary pathological features of the disease. In this mini-review, we will not provide a comprehensive view of the literature on therapies for DMD, since many such reviews already exist, but we will focus on the characteristics, efficiency, and potential of such combined therapeutic strategies that have been described so far for DMD.

Stem Cells and Tissue Niche: Two Faces of the Same Coin of Muscle Regeneration

Capacity of adult muscle to regenerate in response to injury stimuli represents an important homeostatic process. Regeneration is a highly coordinated program that partially recapitulates the embryonic developmental program. However, muscle regeneration is severely compromised in several pathological conditions. It is likely that the restricted tissue repair program under pathological conditions is due to either progressive loss of stem cell populations or to missing signals that limit the damaged tissues to efficiently activate a regenerative program. It is therefore plausible that loss of control over these cell fates might lead to a pathological cell transdifferentiation, limiting the ability of a pathological muscle to sustain an efficient regenerative process. The critical role of microenvironment on stem cells activity and muscle regeneration is discussed.

MicroRNAs modulated by local mIGF-1 expression in mdx dystrophic mice

Duchenne muscular dystrophy (DMD) is a X-linked genetic disease in which the absence of dystrophin leads to progressive lethal skeletal muscle degeneration. It has been demonstrated that among genes which are important for proper muscle development and function, micro-RNAs (miRNAs) play a crucial role. Moreover, altered levels of miRNAs were found in several muscular disorders, including DMD. A specific group of miRNAs, whose expression depends on dystrophin levels and whose deregulation explains several DMD pathogenetic traits, has been identified. Here, we addressed whether the anabolic activity of mIGF-1 on dystrophic muscle is associated with modulation of microRNAs expression. We demonstrated that some microRNAs are strictly linked to the dystrophin expression and are not modulated by mIGF-1 expression. In contrast, local expression of mIGF-1 promotes the modulation of other microRNAs, such as miR-206 and miR-24, along with the modulation of muscle specific genes, which are associated with maturation of regenerating fibers and with the stabilization of the differentiated muscle phenotype. These data suggest that mIGF-1, modifying the expression of some of the active players of muscle homeostasis, is able, even in absence of dystrophin expression, to activate circuitries that confer robustness to dystrophic muscle.

Prospect for pharmacological therapies to treat skeletal muscle dysfunction

Skeletal muscle weakness is a leading cause of mobility disability in the elderly (sarcopenia), as a complication of acute or chronic illness (cachexia), and due to inherited or acquired muscle diseases (muscular dystrophies, myositides, etc.). As of now, there are no approved drugs that can reliably increase muscle strength and function. However, with our understanding of the regulation of myocyte signaling and homeostasis evolving rapidly, experimental treatments are now entering the clinic. We review the current status of clinical research in pharmacological therapies for muscle disorders.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

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.

Reduced IGF signaling prevents muscle cell death in a Caenorhabditis elegans model of muscular dystrophy

Duchenne muscular dystrophy, a fatal degenerative muscle disease, is caused by mutations in the dystrophin gene. Loss of dystrophin in the muscle cell membrane causes muscle fiber necrosis. Previously, loss-of-function mutations in dys-1, the Caenorhabditis elegans dystrophin ortholog, were shown to cause a contractile defect and mild fiber degeneration in striated body wall muscle. Here, we show that loss of dystrophin function in C. elegans results in a shorter lifespan and stochastic, age-dependent muscle-cell death. Reduction of dystrophin function also accelerated age-dependent protein aggregation in muscle cells, suggesting a defect in proteostasis. Both muscle cell death and protein aggregation showed wide variability among the muscle cells. These observations suggest that muscle cell death in dys-1 mutants is greatly influenced by cellular environments. Thus, the manipulation of the cellular environment may provide an opportunity to thwart the cell death initiated by the loss of dystrophin. We found that reduced insulin-like growth factor (IGF) signaling, which rejuvenates the cellular environment to protect cells from a variety of age-dependent pathologies, prevented muscle cell death in the dys-1 mutants in a daf-16-dependent manner. Our study suggests that manipulation of the IGF signaling pathways in muscle cells could be a potent intervention for muscular dystrophy.

Micro-dystrophin and follistatin co-delivery restores muscle function in aged DMD model

Pharmacologic strategies have provided modest improvement in the devastating muscle-wasting disease, Duchenne muscular dystrophy (DMD). Pre-clinical gene therapy studies have shown promise in the mdx mouse model; however, studies conducted after disease onset fall short of fully correcting muscle strength or protecting against contraction-induced injury. Here we examine the treatment effect on muscle physiology in aged dystrophic mice with significant disease pathology by combining two promising therapies: micro-dystrophin gene replacement and muscle enhancement with follistatin, a potent myostatin inhibitor. Individual treatments with micro-dystrophin and follistatin demonstrated marked improvement in mdx mice but were insufficient to fully restore muscle strength and response to injury to wild-type levels. Strikingly, when combined, micro-dystrophin/follistatin treatment restored force generation and conferred resistance to contraction-induced injury in aged mdx mice. Pre-clinical studies with miniature dystrophins have failed to demonstrate full correction of the physiological defects seen in mdx mice. Importantly, the addition of a muscle enhancement strategy with delivery of follistatin in combination with micro-dystrophin gene therapy completely restored resistance to eccentric contraction-induced injury and improved force. Eccentric contraction-induced injury is a pre-clinical parameter relevant to the exercise induced injury that occurs in DMD patients, and herein, we demonstrate compelling evidence for the therapeutic potential of micro-dystrophin/follistatin combinatorial therapy.

Gene and cell-mediated therapies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating muscle disorder that affects 1 in 3,500 boys. Despite years of research and considerable progress in understanding the molecular mechanism of the disease and advancement of therapeutic approaches, there is no cure for DMD. The current treatment options are limited to physiotherapy and corticosteroids, and although they provide a substantial improvement in affected children, they only slow the course of the disorder. On a more optimistic note, more recent approaches either significantly alleviate or eliminate muscular dystrophy in murine and canine models of DMD and importantly, many of them are being tested in early phase human clinical trials. This review summarizes advancements that have been made in viral and nonviral gene therapy as well as stem cell therapy for DMD with a focus on the replacement and repair of the affected dystrophin gene.

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.

Molecular cardiology in translation: gene, cell and chemical-based experimental therapeutics for the failing heart

Acquired and inherited diseases of the heart represent a major health care issue in this country and throughout the World. Clinical medicine has made important advancements in the past quarter century to enable several effective treatment regimes for cardiac patients. Nevertheless, it is apparent that even with the best care, current treatment strategies and therapeutics are inadequate for treating heart disease, leaving it arguably the most pressing health issue today. In this context it is important to seek new approaches to redress the functional deficits in failing myocardium. This review focuses on several recent gene, cell and chemical-based experimental therapeutics currently being developed in the laboratory for potential translation to patient care. For example, new advances in bio-sensing inducible gene expression systems offer the potential for designer cardio-protective proteins to be expressed only during hypoxia/ischemia in the heart. Stem cells continue to offer the promise of cardiac repair, and some recent advances are discussed here. In addition, discovery and applications of synthetic polymers are presented as a chemical-based strategy for acute and chronic treatment of diseased and failing cardiac tissue. Collectively, these approaches serve as the front lines in basic biomedical research, with an eye toward translation of these findings to clinically meaningful applications in cardiac disease.

Soluble TNF-α receptor secretion from healthy or dystrophic mice after AAV6-mediated muscle gene transfer

Muscle is an attractive target because it is easily accessible; it also offers a permissive environment for adeno-associated virus (AAV)-mediated gene transfer and has an abundant blood vascular supply providing an efficient transport system for the secretion of proteins. However, gene therapy of dystrophic muscle may be more difficult than that of healthy tissue because of degenerative-regenerative processes, and also because of the inflammatory context. In this study we followed the expression levels of secreted inhibitors of the proinflammatory tumor necrosis factor (TNF) cytokine after intramuscular (i.m.) injection of AAV6 into dystrophic mdx and healthy C57BL/10 mice. We used two chimeric proteins, namely, the human or murine TNF-soluble receptor I fused with the murine heavy immunoglobulin chain. We conducted an AAV6 dose-response study and determined the kinetics of transgene expression. In addition, we followed the antibody response against the transgenes and studied their expression pattern in the muscle. Our results show that transduction efficiency is reduced in dystrophic muscles as compared with healthy ones. Furthermore, we found that the immune response against the secreted protein is stronger in mdx mice. Together, our results underscore that the pathological state of the muscle has to be taken into consideration when designing gene therapy approaches.

Follistatin gene delivery enhances muscle growth and strength in nonhuman primates

Antagonists of myostatin, a blood-borne negative regulator of muscle growth produced in muscle cells, have shown considerable promise for enhancing muscle mass and strength in rodent studies and could serve as potential therapeutic agents for human muscle diseases. One of the most potent of these agents, follistatin, is both safe and effective in mice, but similar tests have not been performed in nonhuman primates. To assess this important criterion for clinical translation, we tested an alternatively spliced form of human follistatin that affects skeletal muscle but that has only minimal effects on nonmuscle cells. When injected into the quadriceps of cynomolgus macaque monkeys, a follistatin isoform expressed from an adeno-associated virus serotype 1 vector, AAV1-FS344, induced pronounced and durable increases in muscle size and strength. Long-term expression of the transgene did not produce any abnormal changes in the morphology or function of key organs, indicating the safety of gene delivery by intramuscular injection of an AAV1 vector. Our results, together with the findings in mice, suggest that therapy with AAV1-FS344 may improve muscle mass and function in patients with certain degenerative muscle disorders.

Combination of myostatin pathway interference and dystrophin rescue enhances tetanic and specific force in dystrophic mdx mice

Duchenne muscular dystrophy is characterized by muscular atrophy, fibrosis, and fat accumulation. Several groups have demonstrated that in the mdx mouse, the exon-skipping strategy can restore a quasi-dystrophin in almost 100% of the muscle fibers. On the other hand, inhibition of the myostatin pathway in adult mice has been described to enhance muscle growth and improve muscle force. Our aim was to combine these two strategies to evaluate a possible additive effect. We have chosen to inhibit the myostatin pathway using the technique of RNA interference directed against the myostatin receptor AcvRIIb mRNA (sh-AcvRIIb). The restoration of a quasi-dystrophin was mediated by the vectorized U7 exon-skipping technique (U7-DYS). Adeno-associated vectors carrying either the sh-AcvrIIb construct alone, the U7-DYS construct alone, or a combination of both constructs were injected in the tibialis anterior (TA) muscle of dystrophic mdx mice. We show that even if each separate approach has some effects on muscle physiology, the combination of the dystrophin rescue and the downregulation of the myostatin receptor is required to massively improve both the tetanic force and the specific force. This study provides a novel pharmacogenetic strategy for treatment of certain neuromuscular diseases associated with muscle wasting.

Limb-girdle muscular dystrophy type 2D gene therapy restores alpha-sarcoglycan and associated proteins

OBJECTIVE

alpha-Sarcoglycan deficiency results in a severe form of muscular dystrophy (limb-girdle muscular dystrophy type 2D [LGMD2D]) without treatment. Gene replacement represents a strategy for correcting the underlying defect. Questions related to this approach were addressed in this clinical trial, particularly the need for immunotherapy and persistence of gene expression.

METHODS

A double-blind, randomized controlled trial using rAAV1.tMCK.hSGCA injected into the extensor digitorum brevis muscle was conducted. Control sides received saline. A 3-day course of methylprednisolone accompanied gene transfer without further immune suppression.

RESULTS

No adverse events were encountered. SGCA gene expression increased 4-5-fold over control sides when examined at 6 weeks (2 subjects) and 3 months (1 subject). The full sarcoglycan complex was restored in all subjects, and muscle fiber size was increased in the 3-month subject. Adeno-associated virus serotype 1 (AAV1)-neutralizing antibodies were seen as early as 2 weeks. Neither CD4+ nor CD8+ cells were increased over contralateral sides. Scattered foci of inflammation could be found, but showed features of programmed cell death. Enzyme-linked immunospot (ELISpot) showed no interferon-gamma response to alpha-SG or AAV1 capsid peptide pools, with the exception of a minimal capsid response in 1 subject. Restimulation to detect low-frequency capsid-specific T cells by ELISpot assays was negative. Results of the first 3 subjects successfully achieved study aims, precluding the need for additional enrollment.

INTERPRETATION

The finding of this gene replacement study in LGMD2D has important implications for muscular dystrophy. Sustained gene expression was seen, but studies over longer time periods without immunotherapy will be required for design of vascular delivery gene therapy trials.

Producing recombinant adeno-associated virus in foster cells: overcoming production limitations using a baculovirus-insect cell expression strategy

Establishing pharmacological parameters, such as efficacy, routes of administration, and toxicity, for recombinant adeno-associated virus (rAAV) vectors is a prerequisite for gaining acceptance for clinical applications. In fact, even a therapeutic window, that is, the dose range between therapeutic efficacy and toxicity, has yet to be determined for rAAV in vivo. Multiphase clinical trials investigating the safety and efficacy of recombinant AAV-based therapeutics will require unprecedented vector production capacity to meet the needs of preclinical toxicology studies, and the progressive clinical protocol phases of safety/dose escalation (phase I), efficacy (phase II), and high-enrollment, multicenter evaluations (phase III). Methods of rAAV production capable of supporting such trials must be scalable, robust, and efficient. We have taken advantage of the ease of scalability of nonadherent cell culture techniques coupled with the inherent efficiency of viral infection to develop an rAAV production method based on recombinant baculovirus-mediated expression of AAV components in insect-derived suspension cells.

Overexpression of Galgt2 reduces dystrophic pathology in the skeletal muscles of alpha sarcoglycan-deficient mice

Recent studies have shown that a number of genes that are not mutated in various forms of muscular dystrophy may serve as surrogates to protect skeletal myofibers from injury. One such gene is Galgt2, which is also called cytotoxic T cell GalNAc transferase in mice. In this study, we show that Galgt2 overexpression reduces the development of dystrophic pathology in the skeletal muscles of mice lacking alpha sarcoglycan (Sgca), a mouse model for limb girdle muscular dystrophy 2D. Galgt2 transgenic Sgca(-/-) mice showed reduced levels of myofiber damage, as evidenced by i) normal levels of serum creatine kinase activity, ii) a lack of Evans blue dye uptake into myofibers, iii) normal levels of mouse locomotor activity, and iv) near normal percentages of myofibers with centrally located nuclei. In addition, the overexpression of Galgt2 in the early postnatal period using an adeno-associated virus gene therapy vector protected Sgca(-/-) myofibers from damage, as observed using histopathology measurements. Galgt2 transgenic Sgca(-/-) mice also had increased levels of glycosylation of alpha dystroglycan with the CT carbohydrate, but showed no up-regulation of beta, gamma, delta, or epsilon sarcoglycan. These data, coupled with results from our previous studies, show that Galgt2 has therapeutic effects in three distinct forms of muscular dystrophy and may, therefore, have a broad spectrum of therapeutic potential for the treatment of various myopathies.

Counteracting muscle wasting in aging and neuromuscular diseases: the critical role of IGF-1

Most muscle pathologies are characterized by the progressive loss of muscle tissue due to chronic degeneration combined with the inability of regeneration machinery to replace the damaged muscle. These pathological changes, known as muscle wasting, can be attributed to the activation of several proteolytic systems, such as calpain, ubiquitin-proteasome and caspases, and to the alteration in muscle growth factors. Among them, insulin-like growth factor-1 (IGF-1) has been implicated in the control of skeletal muscle growth, differentiation, survival, and regeneration and has been considered a promising therapeutic agent in staving off the advance of muscle weakness. Here we review the molecular basis of muscle wasting associated with diseases, such as sarcopenia, muscular dystrophy and Amyotrophic Lateral Sclerosis, and discuss the potential therapeutic role of local IGF-1 isoforms in muscle aging and diseases.

Systemic human minidystrophin gene transfer improves functions and life span of dystrophin and dystrophin/utrophin-deficient mice

Duchenne muscular dystrophy (DMD) is the most common and lethal genetic muscle disease, caused by mutations in the dystrophin gene. No efficacious treatment is currently available. Here we report AAV vector systemic delivery and therapeutic benefits of the functional human minidystrophin gene in a severe and more reliable DMD mouse model, the dystrophin/utrophin double deficiency mouse (dys-/-:utrn-/-, dKO). These mice show many pathologic and phenotypic signs typical of DMD in humans including kyphosis and shorter life span, all of which are not seen in the mdx mice due to their utrophin upregulation that partially compensates the loss of dystrophin functions and leads to mild phenotypes. The therapeutic value of this new approach was demonstrated in both mdx and dKO murine models, in which we observed highly efficient minidystrophin gene expression, ameliorated muscle pathologies, improvement in growth and motility, inhibition of spine and limb deformation, and prolongation of life span.

Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease

In most cases, pharmacologic strategies to treat genetic muscle disorders and certain acquired disorders, such as sporadic inclusion body myositis, have produced modest clinical benefits. In these conditions, inhibition of the myostatin pathway represents an alternative strategy to improve functional outcomes. Preclinical data that support this approach clearly demonstrate the potential for blocking the myostatin pathway. Follistatin has emerged as a powerful antagonist of myostatin that can increase muscle mass and strength. Follistatin was first isolated from the ovary and is known to suppress follicle-stimulating hormone. This raises concerns for potential adverse effects on the hypothalamic-pituitary-gonadal axis and possible reproductive capabilities. In this review we demonstrate a strategy to bypass off-target effects using an alternatively spliced cDNA of follistatin (FS344) delivered by adeno-associated virus (AAV) to muscle. The transgene product is a peptide of 315 amino acids that is secreted from the muscle and circulates in the serum, thus avoiding cell-surface binding sites. Using this approach our translational studies show increased muscle size and strength in species ranging from mice to monkeys. Adverse effects are avoided, and no organ system pathology or change in reproductive capabilities has been seen. These findings provide the impetus to move toward gene therapy clinical trials with delivery of AAV-FS344 to increase size and function of muscle in patients with neuromuscular disease.

Construction and analysis of compact muscle-specific promoters for AAV vectors

Adeno-associated viral (AAV) vectors have been broadly used for gene transfer in vivo for various applications. However, AAV precludes the use of most of the original large-sized tissue-specific promoters for expression of transgenes. Efforts are made to develop highly compact, active and yet tissue-specific promoters for use in AAV vectors. In this study, we further abbreviated the muscle creatine kinase (MCK) promoter by ligating a double or triple tandem of MCK enhancer (206-bp) to its 87-bp basal promoter, generating the dMCK (509-bp) and tMCK (720-bp) promoters. The dMCK promoter is shorter but stronger than some previously developed MCK-based promoters such as the enh358MCK (584-bp) and CK6 (589-bp) in vitro in C2C12 myotubes and in vivo in skeletal muscles. The tMCK promoter is the strongest that we tested here, more active than the promiscuous cytomegalovirus (CMV) promoter. Furthermore, both the dMCK and tMCK promoters are essentially inactive in nonmuscle cell lines as well as in the mouse liver (>200-fold weaker than the CMV promoter). The dMCK promoter was further tested in a few lines of transgenic mice. Expression of LacZ or minidystrophin gene was detected in skeletal muscles throughout the body, but was weak in the diaphragm, and undetectable in the heart and other tissues. Similar to other miniature MCK promoters, the dMCK promoter also shows preference for fast-twitch myofibers. As a result, we further examined a short, synthetic muscle promoter C5-12 (312-bp). It is active in both skeletal and cardiac muscles but lacks apparent preference on myofiber types. Combination of a MCK enhancer to promoter C5-12 has increased its strength in muscle by two- to threefold. The above-mentioned compact muscle-specific promoters are well suited for AAV vectors in muscle-directed gene therapy studies.

Toward exascale production of recombinant adeno-associated virus for gene transfer applications

To gain acceptance as a medical treatment, adeno-associated virus (AAV) vectors require a scalable and economical production method. Recent developments indicate that recombinant AAV (rAAV) production in insect cells is compatible with current good manufacturing practice production on an industrial scale. This platform can fully support development of rAAV therapeutics from tissue culture to small animal models, to large animal models, to toxicology studies, to Phase I clinical trials and beyond. Efforts to characterize, optimize and develop insect cell-based rAAV production have culminated in successful bioreactor-scale production of rAAV, with total yields potentially capable of approaching the exa-(10(18)) scale. These advances in large-scale AAV production will allow us to address specific catastrophic, intractable human diseases such as Duchenne muscular dystrophy, for which large amounts of recombinant vector are essential for successful outcome.

Efficient gene transfer in skeletal muscle with AAV-derived bicistronic vector using the FGF-1 IRES

IRESs (internal ribosome entry sites) are RNA elements behaving as translational enhancers in conditions of global translation blockade. IRESs are also useful in biotechnological applications as they allow expression of several genes from a single mRNA. Up to now, most IRES-containing vectors use the IRES from encephalomyocarditis virus (EMCV), highly active in transiently transfected cells but long and not flexible in its positioning relative to the gene of interest. In contrast, several IRESs identified in cellular mRNAs are short and flexible and may therefore be advantageous in gene transfer vectors such as those derived from the adeno-associated virus (AAV), where the size of the transgene expression cassette is limited. Here, we have tested bicistronic AAV-derived vectors expressing two luciferase genes separated by the EMCV- or fibroblast growth factor 1 (FGF-1) IRES. We demonstrate that the AAV vector with the FGF-1 IRES, when administrated into the mouse muscle, leads to efficient expression of both transgenes with a stable stoechiometry, for at least 120 days. Interestingly, the bicistronic mRNA containing the FGF-1 IRES leads to transgene expression 10 times superior to that observed with EMCV, in vivo. AAV vectors featuring the FGF-1 IRES may thus be advantageous for gene therapy approaches in skeletal muscle involving coexpression of genes of interest.

Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors

Increasing the size and strength of muscles represents a promising therapeutic strategy for musculoskeletal disorders, and interest has focused on myostatin, a negative regulator of muscle growth. Various myostatin inhibitor approaches have been identified and tested in models of muscle disease with varying efficacies, depending on the age at which myostatin inhibition occurs. Here, we describe a one-time gene administration of myostatin-inhibitor-proteins to enhance muscle mass and strength in normal and dystrophic mouse models for >2 years, even when delivered in aged animals. These results demonstrate a promising therapeutic strategy that warrants consideration for clinical trials in human muscle diseases.

Long term expression of bicistronic vector driven by the FGF-1 IRES in mouse muscle

Background

Electrotransfer of plasmid DNA into skeletal muscle is a promising strategy for the delivery of therapeutic molecules targeting various muscular diseases, cancer and lower-limb ischemia. Internal Ribosome Entry Sites (IRESs) allow co-expression of proteins of interest from a single transcriptional unit. IRESs are RNA elements that have been found in viral RNAs as well as a variety of cellular mRNAs with long 5' untranslated regions. While the encephalomyocarditis virus (EMCV) IRES is often used in expression vectors, we have shown that the FGF-1 IRES is equally active to drive short term transgene expression in mouse muscle. To compare the ability of the FGF-1 IRES to drive long term expression against the EMCV and FGF-2 IRESs, we performed analyses of expression kinetics using bicistronic vectors that express the bioluminescent renilla and firefly luciferase reporter genes. Long term expression of bicistronic vectors was also compared to that of monocistronic vectors. Bioluminescence was quantified ex vivo using a luminometer and in vivo using a CCD camera that monitors luminescence within live animals.

Results

Our data demonstrate that the efficiency of the FGF-1 IRES is comparable to that of the EMCV IRES for long term expression of bicistronic transgenes in mouse muscle, whereas the FGF-2 IRES has a very poor activity. Interestingly, we show that despite the global decrease of vector expression over time, the ratio of firefly to renilla luciferase remains stable with bicistronic vectors containing the FGF-1 or FGF-2 IRES and is slightly affected with the EMCV IRES, whereas it is clearly unstable for mixed monocistronic vectors. In addition, long term expression more drastically decreases with monocistronic vectors, and is different for single or mixed vector injection.

Conclusion

These data validate the use of bicistronic vectors rather than mixed monocistronic vectors for long term expression, and support the use of the FGF-1 IRES. The use of a cellular IRES over one of viral origin is of particular interest in the goal of eliminating viral sequences from transgenic vectors. In addition, the FGF-1 IRES, compared to the EMCV IRES, has a more stable activity, is shorter in length and more flexible in terms of downstream cloning of second cistrons. Finally, the FGF-1 IRES is very attractive to develop multicistronic expression cassettes for gene transfer in mouse muscle.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Gene therapy progress and prospects: Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder affecting 1/3500 male births. There is currently no effective treatment, but gene therapy approaches are offering viable avenues for treatment development. The last 10 years have seen the development of a number of strategies and tools for muscle gene therapy. However, the major hurdle has been the inability to deliver vectors at high enough efficiency via a systemic route. The last 2-3 years (reviewed here) have seen unrivalled progress in efficient systemic delivery of viral and non-viral gene transfer agents and antisense oligonucleotides. This progress, coupled with the successful completion of the first gene therapy clinical trial for DMD, has led to three more clinical trials planned for the immediate future.

Systemic administration of IGF-I enhances oxidative status and reduces contraction-induced injury in skeletal muscles of mdx dystrophic mice

The absence of dystrophin and resultant disruption of the dystrophin glycoprotein complex renders skeletal muscles of dystrophic patients and dystrophic mdx mice susceptible to contraction-induced injury. Strategies to reduce contraction-induced injury are of critical importance, because this mode of damage contributes to the etiology of myofiber breakdown in the dystrophic pathology. Transgenic overexpression of insulin-like growth factor-I (IGF-I) causes myofiber hypertrophy, increases force production, and can improve the dystrophic pathology in mdx mice. In contrast, the predominant effect of continuous exogenous administration of IGF-I to mdx mice at a low dose (1.0-1.5 mg.kg(-1).day(-1)) is a shift in muscle phenotype from fast glycolytic toward a more oxidative, fatigue-resistant, slow muscle without alterations in myofiber cross-sectional area, muscle mass, or maximum force-producing capacity. We found that exogenous administration of IGF-I to mdx mice increased myofiber succinate dehydrogenase activity, shifted the overall myosin heavy chain isoform composition toward a slower phenotype, and, most importantly, reduced contraction-induced damage in tibialis anterior muscles. The deficit in force-producing capacity after two damaging lengthening contractions was reduced significantly in tibialis anterior muscles of IGF-I-treated (53 +/- 4%) compared with untreated mdx mice (70 +/- 5%, P < 0.05). The results provide further evidence that IGF-I administration can enhance the functional properties of dystrophic skeletal muscle and, compared with results in transgenic mice or virus-mediated overexpression, highlight the disparities in different models of endocrine factor delivery.

Gene therapy with anabolic growth factors to prevent muscle atrophy

PURPOSE OF REVIEW

Many situations cause muscle atrophy. When severe, muscle atrophy is associated with an increase in morbidity and mortality. This loss of muscle mass is thought to be due to an imbalance between catabolic and anabolic pathways, resulting in an increase of muscle protein proteolysis and in a decrease in protein synthesis. Changes in muscle levels of muscle growth factors are thought to play a major role in this imbalance. Despite recent better understanding of the metabolic and molecular derangements leading to muscle wasting, therapy of muscle atrophy still has a poor success rate.

RECENT FINDINGS

The recent demonstration that changes in local growth factors, such as insulin-like growth factor-I and myostatin, occur during muscle atrophy has stimulated research interest to prevent muscle mass loss by delivering these growth factors or their inhibitors into the muscle. During the last few years, several advances in the field of muscle gene transfer, using electroporation or recombinant adeno-associated viral vectors, have opened novel therapeutic ways to deliver growth factors able to counteract the loss of muscle mass.

SUMMARY

Preventing decrease of insulin-like growth factor-I muscle, or inhibiting myostatin action by local genes over-expression, may provide a clinically relevant avenue for the preservation, attenuation or reversal of disease-related muscle loss.

Treatment of human disease by adeno-associated viral gene transfer

During the past decade, in vivo administration of viral gene transfer vectors for treatment of numerous human diseases has been brought from bench to bedside in the form of clinical trials, mostly aimed at establishing the safety of the protocol. In preclinical studies in animal models of human disease, adeno-associated viral (AAV) vectors have emerged as a favored gene transfer system for this approach. These vectors are derived from a replication-deficient, non-pathogenic parvovirus with a single-stranded DNA genome. Efficient gene transfer to numerous target cells and tissues has been described. AAV is particularly efficient in transduction of non-dividing cells, and the vector genome persists predominantly in episomal forms. Substantial correction, and in some instances complete cure, of genetic disease has been obtained in animal models of hemophilia, lysosomal storage disorders, retinal diseases, disorders of the central nervous system, and other diseases. Therapeutic expression often lasted for months to years. Treatments of genetic disorders, cancer, and other acquired diseases are summarized in this review. Vector development, results in animals, early clinical experience, as well as potential hurdles and challenges are discussed.

A highly functional mini-dystrophin/GFP fusion gene for cell and gene therapy studies of Duchenne muscular dystrophy

A promising approach for treating Duchenne muscular dystrophy (DMD) is by autologous cell transplantation of myogenic stem cells transduced with a therapeutic expression cassette. Development of this method has been hampered by a low frequency of cellular engraftment, the difficulty of tracing transplanted cells, the rapid loss of autologous cells carrying marker genes that are unable to halt muscle necrosis and the difficulty of stable transfer of a large dystrophin gene into myogenic stem cells. We engineered a 5.7 kb miniDys-GFP fusion gene by replacing the dystrophin C-terminal domain (DeltaCT) with an eGFP coding sequence and removing much of the dystrophin central rod domain (DeltaH2-R19). In a transgenic mdx(4Cv) mouse expressing the miniDys-GFP fusion protein under the control of a skeletal muscle-specific promoter, the green fusion protein localized on the sarcolemma, where it assembled the dystrophin-glycoprotein complex and completely prevented the development of dystrophy in transgenic mdx(4Cv) muscles. When myogenic and other stem cells from these mice were transplanted into mdx(4Cv) recipients, donor cells can be readily identified in skeletal muscle by direct green fluorescence or by using antibodies against GFP or dystrophin. In mdx(4Cv) mice reconstituted with bone marrow cells from the transgenic mice, we monitored engraftment in various muscle groups and found the number of miniDys-GFP(+) fibers increased with time. We suggest that these transgenic mdx(4Cv) mice are highly useful for developing autologous cell therapies for DMD.

C-terminal-truncated microdystrophin recruits dystrobrevin and syntrophin to the dystrophin-associated glycoprotein complex and reduces muscular dystrophy in symptomatic utrophin/dystrophin double-knockout mice

C-terminal-truncated (DeltaC) microdystrophin is being developed for Duchenne muscular dystrophy gene therapy. Encouraging results have been achieved in the mdx mouse model. Unfortunately, mdx mice do not display the same phenotype as human patients. Evaluating DeltaC microdystrophin in a symptomatic model will be of significant relevance to human trials. Utrophin/dystrophin double-knockout (u-dko) mice were developed to model severe dystrophic changes in human patients. In this study we evaluated the therapeutic effect of the DeltaR4-R23/DeltaC microdystrophin gene (DeltaR4/DeltaC) after serotype-6 adeno-associated virus-mediated gene transfer in neonatal u-dko muscle. At 2 months after gene transfer, the percentage of centrally nucleated myofiber was reduced from 89.2 to 3.4% and muscle weight was normalized. Furthermore, we have demonstrated for the first time that DeltaC microdystrophin can eliminate interstitial fibrosis and macrophage infiltration and restore dystrobrevin and syntrophin to the dystrophin-associated glycoprotein complex. Interestingly neuronal nitric oxide synthase was not restored. The most impressive results were achieved in muscle force measurement. Neonatal gene therapy increased twitch- and tetanic-specific force. It also brought the response to eccentric contraction-induced injury to the normal range. In summary, our results suggest that the DeltaR4/DeltaC microgene holds great promise in preventing muscular dystrophy.

Gene Therapy Strategies for Duchenne Muscular Dystrophy Utilizing Recombinant Adeno-associated Virus Vectors

Gene transfer vectors based on adeno-associated virus (AAV) are now widely used in the field of gene therapy. These vectors have been studied for their potential use in treating many diseases, among them the muscular dystrophies, the most common of which is Duchenne muscular dystrophy (DMD). Several recent advances in the areas of AAV serotype analysis, transgene engineering, and vector delivery to muscle, together with novel means of rescuing mutant mRNA transcripts, have yielded impressive results in animal models of DMD. This minireview focuses on these recent advances and their implications for potential treatments for DMD and other neuromuscular disorders.