Induction of T-Cell Infiltration and Programmed Death Ligand 2 Expression by Adeno-Associated Virus in Rhesus Macaque Skeletal Muscle and Modulation by Prednisone

Use of adeno-associated virus (AAV) to transduce genes into skeletal muscles can be associated with T-cell responses to viral capsid and/or to transgenic protein. Intramuscular mononuclear cell infiltrates primarily consisting of CD8+ T cells and also containing FOXP3+ regulatory T cells were present in rhesus macaque skeletal muscle treated with rAAVrh74.MCK.GALGT2 by vascular delivery. Administration of oral prednisone prior to AAV gene delivery and throughout the study reduced such infiltrates by 60% at 24 weeks post AAV delivery compared with AAV-treated animals not receiving prednisone, regardless of the presence of pre-existing AAV serum antibodies at the time of treatment. The majority of CD8+ T cells in AAV-treated muscles expressed activated caspase 3 and programmed cell death protein 1 (PD1), suggesting ongoing programmed cell death. AAV-transduced skeletal muscles also had elevated expression of programmed death ligand 2 (PDL2) on skeletal myofibers, and this increase in expression extended to muscles where transgene was not overexpressed. These data demonstrate that prednisone can reduce the extent of intramuscular T-cell infiltrates in AAV-treated muscles, which may aid in achieving long-term transgene expression, as may the induction of PDL2 expression on skeletal myofibers to promote PD1-mediated programmed T-cell death.

Systemic AAV-Mediated β-Sarcoglycan Delivery Targeting Cardiac and Skeletal Muscle Ameliorates Histological and Functional Deficits in LGMD2E Mice

Limb-girdle muscular dystrophy type 2E (LGMD2E), resulting from mutations in β-sarcoglycan (SGCB), is a progressive dystrophy with deteriorating muscle function, respiratory failure, and cardiomyopathy in 50% or more of LGMD2E patients. SGCB knockout mice share many of the phenotypic deficiencies of LGMD2E patients. To investigate systemic SGCB gene transfer to treat skeletal and cardiac muscle deficits, we designed a self-complementary AAVrh74 vector containing a codon-optimized human SGCB transgene driven by a muscle-specific promoter. We delivered scAAV.MHCK7.hSGCB through the tail vein of SGCB^-/- mice to provide a rationale for a clinical trial that would lead to clinically meaningful results. This led to 98.1% transgene expression across all muscles that was accompanied by improvements in histopathology. Serum creatine kinase (CK) levels were reduced following treatment by 85.5%. Diaphragm force production increased by 94.4%, kyphoscoliosis of the spine was significantly reduced by 48.1%, overall ambulation increased by 57%, and vertical rearing increased dramatically by 132% following treatment. Importantly, no adverse effects were seen in muscle of wild-type mice injected systemically with scAAV.hSGCB. In this well-defined model of LGMD2E, we have demonstrated the efficacy and safety of systemic scAAV.hSGCB delivery, and these findings have established a path for clinically beneficial AAV-mediated gene therapy for LGMD2E.

Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes

Sporadic inclusion body myositis, a variant of inflammatory myopathy, has features distinct from polymyositis/dermatomyositis. The disease affects men more than women, most commonly after age 50. Clinical features include weakness of the quadriceps, finger flexors, ankle dorsiflexors, and dysphagia. The distribution of weakness is similar to Becker muscular dystrophy, where we previously reported improvement following intramuscular injection of an isoform of follistatin (FS344) by AAV1. For this clinical trial, rAAV1.CMV.huFS344, 6 × 10¹¹ vg/kg, was delivered to the quadriceps muscles of both legs of six sporadic inclusion body myositis subjects. The primary outcome for this trial was distance traveled for the 6-min walk test. The protocol included an exercise regimen for each participant. Performance, annualized to a median 1-year change, improved +56.0 m/year for treated subjects compared to a decline of -25.8 m/year (p = 0.01) in untreated subjects (n = 8), matched for age, gender, and baseline measures. Four of the six treated subjects showed increases ranging from 58-153 m, whereas two were minimally improved (5-23 m). Treatment effects included decreased fibrosis and improved regeneration. These findings show promise for follistatin gene therapy for mild to moderately affected, ambulatory sporadic inclusion body myositis patients. More advanced disease with discernible muscle loss poses challenges.

Duchenne muscular dystrophy: CRISPR/Cas9 treatment

A novel approach to gene correction by genome editing shows great promise as a treatment for Duchenne muscular dystrophy (DMD). CRISPR/Cas9 delivered by adeno-associated virus to a mouse model for DMD demonstrated improvement in function and histology.

Defective membrane fusion and repair in Anoctamin5-deficient muscular dystrophy

Limb-girdle muscular dystrophies are a genetically diverse group of diseases characterized by chronic muscle wasting and weakness. Recessive mutations in ANO5 (TMEM16E) have been directly linked to several clinical phenotypes including limb-girdle muscular dystrophy type 2L and Miyoshi myopathy type 3, although the pathogenic mechanism has remained elusive. ANO5 is a member of the Anoctamin/TMEM16 superfamily that encodes both ion channels and regulators of membrane phospholipid scrambling. The phenotypic overlap of ANO5 myopathies with dysferlin-associated muscular dystrophies has inspired the hypothesis that ANO5, like dysferlin, may be involved in the repair of muscle membranes following injury. Here we show that Ano5-deficient mice have reduced capacity to repair the sarcolemma following laser-induced damage, exhibit delayed regeneration after cardiotoxin injury and suffer from defective myoblast fusion necessary for the proper repair and regeneration of multinucleated myotubes. Together, these data suggest that ANO5 plays an important role in sarcolemmal membrane dynamics. Genbank Mouse Genome Informatics accession no. 3576659.

Follistatin Gene Therapy Improves Ambulation in Becker Muscular Dystrophy

Follistatin is a ubiquitous secretory propeptide that functions as a potent inhibitor of the myostatin pathway, resulting in an increase in skeletal muscle mass. Its ability to interact with the pituitary activin-inhibin axis and suppress the secretion of follicle-stimulating hormone (FSH) called for caution in its clinical applicability. This limitation was circumvented by the use of one of the alternatively spliced follistatin variants, FS344, undergoing post-translational modification to FS315. This follistatin isoform is serum-based, and has a 10-fold lower affinity to activin compared to FS288. Preclinical studies of intramuscular delivery of the follistatin gene demonstrated safety and efficacy in enhancing muscle mass. We herein review the evidence supporting the utility of follistatin as a genetic enhancer to improve cellular performance. In addition, we shed light on the results of the first clinical gene transfer trial using the FS344 isoform of follistatin in subjects with Becker muscular dystrophy as well as the future directions for clinical gene therapy trials using follistatin.

β-Sarcoglycan gene transfer decreases fibrosis and restores force in LGMD2E mice

Limb-girdle muscular dystrophy type 2E (LGMD2E) results from mutations in the β-sarcoglycan (SGCB) gene causing loss of functional protein and concomitant loss of dystrophin-associated proteins. The disease phenotype is characterized by muscle weakness and wasting, and dystrophic features including muscle fiber necrosis, inflammation and fibrosis. The Sgcb-null mouse recapitulates the clinical phenotype with significant endomysial fibrosis providing a relevant model to test whether gene replacement will be efficacious. We directly addressed this question using a codon optimized human β-sarcoglycan gene (hSGCB) driven by a muscle-specific tMCK promoter (scAAVrh74.tMCK.hSGCB). Following isolated limb delivery (5 × 10(11) vector genome (vg)), 91.2% of muscle fibers in the lower limb expressed β-sarcoglycan, restoring assembly of the sarcoglycan complex and protecting the membrane from Evans blue dye leakage. Histological outcomes were significantly improved including decreased central nucleation, normalization of muscle fiber size, decreased macrophages and inflammatory mononuclear cells, and an average of a 43% reduction in collagen deposition in treated muscle compared with untreated muscle at end point. These measures correlated with improvement of tetanic force and resistance to eccentric contraction. In 6-month-old mice, as indicated by collagen staining, scAAVrh74.tMCK.hSGCB treatment reduced fibrosis by 42%. This study demonstrates the potential for gene replacement to reverse debilitating fibrosis, typical of muscular dystrophy, thereby providing compelling evidence for movement to clinical gene replacement for LGMD2E.

Human α7 Integrin Gene (ITGA7) Delivered by Adeno-Associated Virus Extends Survival of Severely Affected Dystrophin/Utrophin-Deficient Mice

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene. It is the most common, severe childhood form of muscular dystrophy. We investigated an alternative to dystrophin replacement by overexpressing ITGA7 using adeno-associated virus (AAV) delivery. ITGA7 is a laminin receptor in skeletal muscle that, like the dystrophin-glycoprotein complex, links the extracellular matrix to the internal actin cytoskeleton. ITGA7 is expressed in DMD patients and overexpression does not elicit an immune response to the transgene. We delivered rAAVrh.74.MCK.ITGA7 systemically at 5-7 days of age to the mdx/utrn(-/-) mouse deficient for dystrophin and utrophin, a severe mouse model of DMD. At 8 weeks postinjection, widespread expression of ITGA7 was observed at the sarcolemma of multiple muscle groups following gene transfer. The increased expression of ITGA7 significantly extended longevity and reduced common features of the mdx/utrn(-/-) mouse, including kyphosis. Overexpression of α7 expression protected against loss of force following contraction-induced damage and increased specific force in the diaphragm and EDL muscles 8 weeks after gene transfer. Taken together, these results further support the use of α7 integrin as a potential therapy for DMD.

AAV.Dysferlin Overlap Vectors Restore Function in Dysferlinopathy Animal Models

OBJECTIVE

Dysferlinopathies are a family of untreatable muscle disorders caused by mutations in the dysferlin gene. Lack of dysferlin protein results in progressive dystrophy with chronic muscle fiber loss, inflammation, fat replacement, and fibrosis; leading to deteriorating muscle weakness. The objective of this work is to demonstrate efficient and safe restoration of dysferlin expression following gene therapy treatment.

METHODS

Traditional gene therapy is restricted by the packaging capacity limit of adeno-associated virus (AAV), however, use of a dual vector strategy allows for delivery of over-sized genes, including dysferlin. The two vector system (AAV.DYSF.DV) packages the dysferlin cDNA utilizing AAV serotype rh.74 through the use of two discrete vectors defined by a 1 kb region of homology. Delivery of AAV.DYSF.DV via intramuscular and vascular delivery routes in dysferlin deficient mice and nonhuman primates was compared for efficiency and safety.

RESULTS

Treated muscles were tested for dysferlin expression, overall muscle histology, and ability to repair following injury. High levels of dysferlin overexpression was shown for all muscle groups treated as well as restoration of functional outcome measures (membrane repair ability and diaphragm specific force) to wild-type levels. In primates, strong dysferlin expression was demonstrated with no safety concerns.

INTERPRETATION

Treated muscles showed high levels of dysferlin expression with functional restoration with no evidence of toxicity or immune response providing proof of principle for translation to dysferlinopathy patients.

Plasmapheresis eliminates the negative impact of AAV antibodies on microdystrophin gene expression following vascular delivery

Duchenne muscular dystrophy is a monogenic disease potentially treatable by gene replacement. Use of recombinant adeno-associated virus (AAV) will ultimately require a vascular approach to broadly transduce muscle cells. We tested the impact of preexisting AAV antibodies on microdystrophin expression following vascular delivery to nonhuman primates. Rhesus macaques were treated by isolated limb perfusion using a fluoroscopically guided catheter. In addition to serostatus stratification, the animals were placed into one of the three immune suppression groups: no immune suppression, prednisone, and triple immune suppression (prednisone, tacrolimus, and mycophenolate mofetil). The animals were analyzed for transgene expression at 3 or 6 months. Microdystrophin expression was visualized in AAV, rhesus serotype 74 sero-negative animals (mean: 48.0 ± 20.8%) that was attenuated in sero-positive animals (19.6 ± 18.7%). Immunosuppression did not affect transgene expression. Importantly, removal of AAV binding antibodies by plasmapheresis in AAV sero-positive animals resulted in high-level transduction (60.8 ± 18.0%), which is comparable with that of AAV sero-negative animals (53.7 ± 7.6%), whereas non-pheresed sero-positive animals demonstrated significantly lower transduction levels (10.1 ± 6.0%). These data support the hypothesis that removal of AAV binding antibodies by plasmapheresis permits successful and sustained gene transfer in the presence of preexisting immunity (natural infection) to AAV.

Eteplirsen for the treatment of Duchenne muscular dystrophy

OBJECTIVE

In prior open-label studies, eteplirsen, a phosphorodiamidate morpholino oligomer, enabled dystrophin production in Duchenne muscular dystrophy (DMD) with genetic mutations amenable to skipping exon 51. The present study used a double-blind placebo-controlled protocol to test eteplirsen's ability to induce dystrophin production and improve distance walked on the 6-minute walk test (6MWT).

METHODS

DMD boys aged 7 to 13 years, with confirmed deletions correctable by skipping exon 51 and ability to walk 200 to 400 m on 6 MWT, were randomized to weekly intravenous infusions of 30 or 50 mg/kg/wk eteplirsen or placebo for 24 weeks (n = 4/group). Placebo patients switched to 30 or 50 mg/kg eteplirsen (n=2/group) at week 25; treatment was open label thereafter. All patients had muscle biopsies at baseline and week 48. Efficacy included dystrophin-positive fibers and distance walked on the 6MWT.

RESULTS

At week 24, the 30 mg/kg eteplirsen patients were biopsied, and percentage of dystrophin-positive fibers was increased to 23% of normal; no increases were detected in placebo-treated patients (p≤0.002). Even greater increases occurred at week 48 (52% and 43% in the 30 and 50 mg/kg cohorts, respectively), suggesting that dystrophin increases with longer treatment. Restoration of functional dystrophin was confirmed by detection of sarcoglycans and neuronal nitric oxide synthase at the sarcolemma. Ambulation-evaluable eteplirsen-treated patients experienced a 67.3 m benefit compared to placebo/delayed patients (p≤0.001).

INTERPRETATION

Eteplirsen restored dystrophin in the 30 and 50 mg/kg/wk cohorts, and in subsequently treated, placebo-controlled subjects. Duration, more than dose, accounted for dystrophin production, also resulting in ambulation stability. No severe adverse events were encountered.

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.

Update on the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy is the most severe childhood form of muscular dystrophy caused by mutations in the gene responsible for dystrophin production. There is no cure, and treatment is limited to glucocorticoids that prolong ambulation and drugs to treat the cardiomyopathy. Multiple treatment strategies are under investigation and have shown promise for Duchenne muscular dystrophy. Use of molecular-based therapies that replace or correct the missing or nonfunctional dystrophin protein has gained momentum. These strategies include gene replacement with adeno-associated virus, exon skipping with antisense oligonucleotides, and mutation suppression with compounds that "read through" stop codon mutations. Other strategies include cell therapy and surrogate gene products to compensate for the loss of dystrophin. All of these approaches are discussed in this review, with particular emphasis on the most recent advances made in each therapeutic discipline. The advantages of each approach and challenges in translation are outlined in detail. Individually or in combination, all of these therapeutic strategies hold great promise for treatment of this devastating childhood disease.

Impaired regeneration in LGMD2A supported by increased PAX7-positive satellite cell content and muscle-specific microrna dysregulation

INTRODUCTION

Recent in vitro studies suggest that CAPN3 deficiency leads initially to accelerated myofiber formation followed by depletion of satellite cells (SC). In normal muscle, up-regulation of miR-1 and miR-206 facilitates transition from proliferating SCs to differentiating myogenic progenitors.

METHODS

We examined the histopathological stages, Pax7 SC content, and muscle-specific microRNA expression in biopsy specimens from well-characterized LGMD 2A patients to gain insight into disease pathogenesis.

RESULTS

Three distinct stages of pathological changes were identified that represented the continuum of the dystrophic process from prominent inflammation with necrosis and regeneration to prominent fibrosis, which correlated with age and disease duration. Pax7-positive SCs were highest in the fibrotic group and correlated with down-regulation of miR-1, miR-133a, and miR-206.

CONCLUSIONS

These observations, and other published reports, are consistent with microRNA dysregulation leading to inability of Pax7-positive SCs to transit from proliferation to differentiation. This results in impaired regeneration and fibrosis.

Emerging drugs for Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is the most common, severe childhood form of muscular dystrophy. Treatment is limited to glucocorticoids that have the benefit of prolonging ambulation by approximately 2 years and preventing scoliosis. Finding a more satisfactory treatment should focus on maintaining long-term efficacy with a minimal side effect profile.

AREAS COVERED

Authors discuss different therapeutic strategies that have been used in pre-clinical and clinical settings.

EXPERT OPINION

Multiple treatment approaches have emerged. Most attractive are molecular-based therapies that can express the missing dystrophin protein (exon skipping or mutation suppression) or a surrogate gene product (utrophin). Other approaches include increasing the strength of muscles (myostatin inhibitors), reducing muscle fibrosis and decreasing oxidative stress. Additional targets include inhibiting NF-κB to reduce inflammation or promoting skeletal muscle blood flow and muscle contractility using phosphodiesterase inhibitors or nitric oxide (NO) donors. The potential for each of these treatment strategies to enter clinical trials is a central theme of discussion. The review emphasizes that the goal of treatment should be to find a product at least as good as glucocorticoids with a lower side effect profile or with a significant glucocorticoid sparing effect.

mdx(⁵cv) mice manifest more severe muscle dysfunction and diaphragm force deficits than do mdx Mice

Duchenne muscular dystrophy (DMD) is characterized by progressive skeletal muscle dysfunction leading to premature death by the third decade of life. The mdx mouse, the most widely used animal model of DMD, has been extremely useful to study disease mechanisms and to screen new therapeutics. However, unlike patients with DMD, mdx mice have a very mild motor function deficit, posing significant limitations for its use as a platform to assess the impact of treatments on motor function. It has been suggested that an mdx variant, the mdx(5cv) mouse, might be more severely affected. Here, we compared the motor activity, histopathology, and individual muscle force measurements of mdx and mdx(⁵cv) mice. Our study revealed that mdx(⁵cv) mice showed more severe exercise-induced fatigue, Rotarod performance deficits, and gait anomalies than mdx mice and that these deficits began at a younger age. Muscle force studies showed more severe strength deficits in the diaphragm of mdx(⁵cv) mice compared to mdx mice, but similar force generation in the extensor digitorum longus. Muscle histology was similar between the two strains. Differences in genetic background (genetic modifiers) probably account for these functional differences between mdx strains. Overall, our findings indicate that the mdx and mdx(⁵cv) mouse models of DMD are not interchangeable and identify the mdx(⁵cv) mouse as a valuable platform for preclinical studies that require assessment of muscle function in live animals.

Aminoglycoside-induced mutation suppression (stop codon readthrough) as a therapeutic strategy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common, lethal, X-linked genetic disease, affecting 1 in 3500 newborn males. It is caused by mutations in the DMD gene. Owing to the large size of the gene, the mutation rate in both germline and somatic cells is very high. Nearly 13-15% of DMD cases are caused by nonsense mutations leading to premature termination codons in the reading frame that results in truncated dystrophin protein. Currently there is no cure for DMD. The only available treatment is the use of glucocorticoids that have modest beneficial effects accompanied by significant side effects. Different therapeutic strategies have been developed ranging from gene therapy to exon skipping and nonsense mutation suppression to produce the full-length protein. These strategies have shown promise in the mdx mouse model of muscular dystrophy where they have been reported to ameliorate the dystrophic phenotype and correct the physiological defects in the membrane. Each of these molecular approaches are being investigated in clinical trials. Here we review nonsense mutation suppression by aminoglycosides as a therapeutic strategy to treat DMD with special emphasis on gentamicin-induced readthrough of disease-causing premature termination codons.

AAV-mediated gene therapy to the isolated limb in rhesus macaques

The development of a nonhuman primate (NHP) model for vascular delivery of therapeutic transgenes with adeno-associated viral (AAV) vectors is crucial for successfully treating muscular dystrophies. Current animal models for Duchenne muscular dystrophy (DMD) gene therapy have species limitations related to assessing function, immune response, and distribution of the micro- and minidystrophin transgenes in a clinically relevant manner. In addition, there are many forms of muscular dystrophy for which there are no available disease models. NHPs provide the ideal model to optimize vector delivery across a vascular barrier and provide accurate dose estimates for local or broadly targeted gene therapy studies. The vascular anatomy NHPs more clearly parallels humans providing an appropriate substrate for translational experiments. Here we outline the development of a rhesus macaque isolated focal limb perfusion (IFLP) protocol targeting the vascular bed of the gastrocnemius. This protocol serves as a model with broad implications for other muscle diseases along with the capability of targeting multiple muscle groups. To overcome the partial homogeneity between portions of the human microdystrophin transgene and those of the NHP dystrophin gene, we utilized a FLAG tag for tracking distribution of microdystrophin. We also provide methods for assessing transduction efficiency of microdystrophin.FLAG following the IFLP vascular delivery protocol.

Sustained alpha-sarcoglycan gene expression after gene transfer in limb-girdle muscular dystrophy, type 2D

OBJECTIVE

The aim of this study was to attain long-lasting alpha-sarcoglycan gene expression in limb-girdle muscular dystrophy, type 2D (LGMD2D) subjects mediated by adeno-associated virus (AAV) gene transfer under control of a muscle specific promoter (tMCK).

METHODS

rAAV1.tMCK.hSGCA (3.25 × 10¹¹ vector genomes) was delivered to the extensor digitorum brevis muscle of 3 subjects with documented SGCA mutations via a double-blind, randomized, placebo controlled trial. Control sides received saline. The blind was not broken until the study was completed at 6 months and all results were reported to the oversight committee.

RESULTS

Persistent alpha-sarcoglycan gene expression was achieved for 6 months in 2 of 3 LGMD2D subjects. Markers for muscle fiber transduction other than alpha-sarcoglycan included expression of major histocompatibility complex I, increase in muscle fiber size, and restoration of the full sarcoglycan complex. Mononuclear inflammatory cells recruited to the site of gene transfer appeared to undergo programmed cell death, demonstrated by terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick-end labeling and caspase-3 staining. A patient failing gene transfer demonstrated an early rise in neutralizing antibody titers and T-cell immunity to AAV, validated by enzyme-linked immunospot on the second day after gene injection. This was in clear distinction to other participants with satisfactory gene expression.

INTERPRETATION

The findings of this gene replacement study in LGMD2D subjects have important implications not previously demonstrated in muscular dystrophy. Long-term, sustainable gene expression of alpha-sarcoglycan was observed following gene transfer mediated by AAV. The merit of a muscle-specific tMCK promoter, not previously used in a clinical trial, was evident, and the potential for reversal of disease was displayed.

Molecular therapeutic strategies targeting Duchenne muscular dystrophy

Since the discovery of the gene for Duchenne muscular dystrophy more than 20 years ago, scientists have worked to apply molecular principles for restoration of the dystrophin protein and correction of the underlying physiologic defect that predisposes muscle fibers to injury. Recent studies provide realistic hope that molecular therapies may help patients who have this disorder. At present, only corticosteroids can improve walking ability and increase quality of life for boys with this disease. The results are modest and encumbered by side effects. The authors review 3 molecular therapeutic approaches that have been introduced into the clinic: (1) gene replacement therapy, (2) mutation suppression, and (3) exon skipping.

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.