Sustained AAV-mediated Dystrophin Expression in a Canine Model of Duchenne Muscular Dystrophy with a Brief Course of Immunosuppression

Immunity and AAV-Mediated Gene Therapy for Muscular Dystrophies in Large Animal Models and Human Trials

Adeno-associated viral (AAV) vector-mediated gene replacement for the treatment of muscular dystrophy represents a promising therapeutic strategy in modern medicine. One major obstacle in using AAV vectors for in vivo gene delivery is the development of host immune responses to the viral capsid protein and transgene products as evidenced in animal models and human trials for a range of genetic diseases. Here, we review immunity against AAV vector and transgene in the context of gene delivery specific to muscles for treating muscular dystrophies and non-muscle diseases in large animal models and human trials, factors that influence the intensity of the immune responses, and immune modulatory strategies to prevent unwanted immune responses and induce tolerance to the vector and therapeutic gene for a successful gene therapy.

Long-term systemic myostatin inhibition via liver-targeted gene transfer in golden retriever muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, X-linked recessive disease affecting 1 in 3,500 newborn boys for which there is no effective treatment or cure. One novel strategy that has therapeutic potential for DMD is inhibition of myostatin, a negative regulator of skeletal muscle mass that may also promote fibrosis. Therefore, our goal in this study was to evaluate systemic myostatin inhibition in the golden retriever model of DMD (GRMD). GRMD canines underwent liver-directed gene transfer of a self-complementary adeno-associated virus type 8 vector designed to express a secreted dominant-negative myostatin peptide (n = 4) and were compared with age-matched, untreated GRMD controls (n = 3). Dogs were followed with serial magnetic resonance imaging (MRI) for 13 months to assess cross-sectional area and volume of skeletal muscle, then euthanized so that tissue could be harvested for morphological and histological analysis. We found that systemic myostatin inhibition resulted in increased muscle mass in GRMD dogs as assessed by MRI and confirmed at tissue harvest. We also found that hypertrophy of type IIA fibers was largely responsible for the increased muscle mass and that reductions in serum creatine kinase and muscle fibrosis were associated with long-term myostatin inhibition in GRMD. This is the first report describing the effects of long-term, systemic myostatin inhibition in a large-animal model of DMD, and we believe that the simple and effective nature of our liver-directed gene-transfer strategy makes it an ideal candidate for evaluation as a novel therapeutic approach for DMD patients.

AAV6-mediated systemic shRNA delivery reverses disease in a mouse model of facioscapulohumeral muscular dystrophy

Treatment of dominantly inherited muscle disorders remains a difficult task considering the need to eliminate the pathogenic gene product in a body-wide fashion. We show here that it is possible to reverse dominant muscle disease in a mouse model of facioscapulohumeral muscular dystrophy (FSHD). FSHD is a common form of muscular dystrophy associated with a complex cascade of epigenetic events following reduction in copy number of D4Z4 macrosatellite repeats located on chromosome 4q35. Several 4q35 genes have been examined for their role in disease, including FRG1. Overexpression of FRG1 causes features related to FSHD in transgenic mice and the FRG1 mouse is currently the only available mouse model of FSHD. Here we show that systemic delivery of RNA interference expression cassettes in the FRG1 mouse, after the onset of disease, led to a dose-dependent long-term FRG1 knockdown without signs of toxicity. Histological features including centrally nucleated fibers, fiber size reduction, fibrosis, adipocyte accumulation, and inflammation were all significantly improved. FRG1 mRNA knockdown resulted in a dramatic restoration of muscle function. Through RNA interference (RNAi) expression cassette redesign, our method is amenable to targeting any pathogenic gene offering a viable option for long-term, body-wide treatment of dominant muscle disease in humans.

Targeting RNA to treat neuromuscular disease

The development of effective therapies for neuromuscular disorders such as Duchenne muscular dystrophy (DMD) is hampered by considerable challenges: skeletal muscle is the most abundant tissue in the body, and many neuromuscular disorders are multisystemic conditions. However, despite these barriers there has recently been substantial progress in the search for novel treatments. In particular, the use of antisense oligonucleotides, which are designed to target RNA and modulate pre-mRNA splicing to restore functional protein isoforms or directly inhibit the toxic effects of pathogenic RNAs, offers great promise and these approaches are now being tested in the clinic. Here, we review recent advances in the development of such antisense oligonucleotides and other promising novel approaches, including the induction of readthrough nonsense mutations.

Local gene delivery and methods to control immune responses in muscles of normal and dystrophic dogs

Adeno-associated viral vector (AAV)-mediated gene transfer represents a promising gene replacement strategy for treating Duchenne muscular dystrophy (DMD). However, recent studies demonstrated cellular immunity specific to AAV capsid proteins in animal models, which resulted in liver toxicity and elimination of transgene expression in a human trial of hemophilia B. We have recently developed immunosuppressive strategies to prevent such immunity for successful long-term transgene expression in dog muscle. Here, we describe in detail the immunosuppressive regimens employed in both normal and DMD dogs and provide methods for evaluating the efficiency of the regimens following intramuscular injection of AAV in dogs.

Age-matched comparison reveals early electrocardiography and echocardiography changes in dystrophin-deficient dogs

The absence of dystrophin in the heart leads to Duchenne cardiomyopathy. Dystrophin-deficient dogs represent a critical model to translate novel therapies developed in mice to humans. Unfortunately, little is known about cardiophysiology changes in these dogs. We performed prospective electrocardiographic and echocardiographic examinations at 3, 6 and 12 months of age in four normal and three affected dogs obtained from the same litter. Affected dogs showed growth retardation and serum creatine kinase elevation. Necropsy confirmed cardiac dystrophin deficiency and histopathology. Q/R ratio elevation and diastolic left ventricular (LV) internal diameter reduction were the most consistent findings in affected dogs at all ages. At 6 and 12 months, dystrophic dogs also showed significant reduction of PR intervals, LV end diastolic/systolic volumes and systolic LV internal diameters. Epicardial and endocardial slope times were significantly reduced in affected dogs at 12 months. These results establish the baseline for evaluating experimental therapies in the future.

State-of-the-art gene-based therapies: the road ahead

Improvements in the gene transfer vectors used in therapeutic trials have led to substantial clinical successes in patients with serious genetic conditions, such as immunodeficiency syndromes, blindness and some cancer types. Several barriers need to be overcome before this type of therapy becomes a widely accepted treatment for a broad group of medical diseases. However, recent progress in the field is finally realizing some of the promises made more than 20 years ago, providing optimism for additional successes in the near future.

Current status of pharmaceutical and genetic therapeutic approaches to treat DMD

Duchenne muscular dystrophy (DMD) is a genetic disease affecting about one in every 3,500 boys. This X-linked pathology is due to the absence of dystrophin in muscle fibers. This lack of dystrophin leads to the progressive muscle degeneration that is often responsible for the death of the DMD patients during the third decade of their life. There are currently no curative treatments for this disease but different therapeutic approaches are being studied. Gene therapy consists of introducing a transgene coding for full-length or a truncated version of dystrophin complementary DNA (cDNA) in muscles, whereas pharmaceutical therapy includes the use of chemical/biochemical substances to restore dystrophin expression or alleviate the DMD phenotype. Over the past years, many potential drugs were explored. This led to several clinical trials for gentamicin and ataluren (PTC124) allowing stop codon read-through. An alternative approach is to induce the expression of an internally deleted, partially functional dystrophin protein through exon skipping. The vectors and the methods used in gene therapy have been continually improving in order to obtain greater encapsidation capacity and better transduction efficiency. The most promising experimental approaches using pharmaceutical and gene therapies are reviewed in this article.

Therapeutic approaches to muscular dystrophy

Muscular dystrophies are a heterogeneous group of genetic disorders characterized by muscle weakness and wasting. Duchenne muscular dystrophy (DMD) is the most common and severe form of muscular dystrophy, and although the molecular mechanisms of the disease have been extensively investigated since the discovery of the gene in 1986, there is currently no effective treatment. However, new gene-based therapies have recently emerged with particular noted advances in using conventional gene replacement strategies, RNA-based technology and pharmacological approaches. While the proof of principle has been demonstrated in animal models, several clinical trials have recently been undertaken to investigate the feasibility of these strategies in patients. In particular, antisense-mediated exon skipping has shown encouraging results and holds promise for the treatment of dystrophic muscle. Here, we summarize the recent progress in therapeutic approaches to muscular dystrophies, with an emphasis on gene therapy and exon skipping for DMD.

Electrotransfer of the full-length dog dystrophin into mouse and dystrophic dog muscles

Duchenne muscular dystrophy (DMD) is an X-linked genetic disease characterized by the absence of dystrophin (427 kDa). An approach to eventually restore this protein in patients with DMD is to introduce into their muscles a plasmid encoding dystrophin cDNA. Because the phenotype of the dystrophic dog is closer to the human phenotype than is the mdx mouse phenotype, we have studied the electrotransfer of a plasmid carrying the full-length dog dystrophin (FLDYS(dog)) in dystrophic dog muscle. To achieve this nonviral delivery, the FLDYS(dog) cDNA was cloned in two plasmids containing either a cytomegalovirus or a muscle creatine kinase promoter. In both cases, our results showed that the electrotransfer of these large plasmids (∼17 kb) into mouse muscle allowed FLDYS(dog) expression in the treated muscle. The electrotransfer of pCMV.FLDYS(dog) in a dystrophic dog muscle also led to the expression of dystrophin. In conclusion, introduction of the full-length dog dystrophin cDNA by electrotransfer into dystrophic dog muscle is a potential approach to restore dystrophin in patients with DMD. However, the electrotransfer procedure should be improved before applying it to humans.

Duchenne muscular dystrophy gene therapy: Lost in translation?

A milestone of molecular medicine is the identification of dystrophin gene mutation as the cause of Duchenne muscular dystrophy (DMD). Over the last 2 decades, major advances in dystrophin biology and gene delivery technology have created an opportunity to treat DMD with gene therapy. Remarkable success has been achieved in treating dystrophic mice. Several gene therapy strategies, including plasmid transfer, exon skipping, and adeno-associated virus-mediated microdystrophin therapy, have entered clinical trials. However, therapeutic benefit has not been realized in DMD patients. Bridging the gap between mice and humans is no doubt the most pressing issue facing DMD gene therapy now. In contrast to mice, dystrophin-deficient dogs are genetically and phenotypically similar to human patients. Preliminary gene therapy studies in the canine model may offer critical insights that cannot be obtained from murine studies. It is clear that the canine DMD model may represent an important link between mice and humans. Unfortunately, our current knowledge of dystrophic dogs is limited, and the full picture of disease progression remains to be clearly defined. We also lack rigorous outcome measures (such as in situ force measurement) to monitor therapeutic efficacy in dystrophic dogs. Undoubtedly, maintaining a dystrophic dog colony is technically demanding, and the cost of dog studies cannot be underestimated. A carefully coordinated effort from the entire DMD community is needed to make the best use of the precious dog resource. Successful DMD gene therapy may depend on valid translational studies in dystrophin-deficient dogs.

PEG-modulated column chromatography for purification of recombinant adeno-associated virus serotype 9

Column chromatography has been described for purification of recombinant adeno-associated viral vectors (rAAV) serotypes 1, 2, 5, 6 and 8. Some of these purification processes have been used in manufacturing pre-clinical grade and clinical grade rAAV vectors. Recently, recombinant AAV9 has been reported to be highly efficient in transducing cardiac muscle in animal models. Systemic or cardiac gene delivery and other applications may require large quantities of rAAV9 vectors, thus a scalable method supporting large scale purification of rAAV9 is needed for clinical development. However, column chromatography-based purification has not been reported to date for rAAV9. This study reports a polyethylene glycol (PEG) modulated chromatography process for purification of AAV9 vectors. Inclusion of PEG in chromatography buffers modulated rAAV9 elution profiles in a manner that resulted in significantly improved resin binding capacity, vector purity and yield. PEG-modulated methods were developed and optimized for hydroxyapatite and ion exchange chromatography, and shown to result in vectors of high purity and functional activity.

Post-Natal knockdown of fukutin-related protein expression in muscle by long-termRNA interference induces dystrophic pathology [corrected]

Limb-girdle muscular dystrophy 2I (LGMD2I) is caused by mutations in the fukutin-related protein (FKRP) gene. Unlike its severe allelic forms, LGMD2I usually involves slower onset and milder course without defects in the central nervous system. The lack of viable animal models that closely recapitulate LGMD2I clinical phenotypes led us to use RNA interference technology to knock down FKRP expression via postnatal gene delivery so as to circumvent embryonic lethality. Specifically, an adeno-associated viral vector was used to deliver short hairpin (shRNA) genes to healthy ICR mice. Adeno-associated viral vectors expressing a single shRNA or two different shRNAs were injected one time into the hind limb muscles. We showed that FKRP expression at 10 months postinjection was reduced by about 50% with a single shRNA and by 75% with the dual shRNA cassette. Dual-cassette injection also reduced a-dystroglycan glycosylation and its affinity to laminin by up to 70% and induced α-dystrophic pathology, including fibrosis and central nucleation, in more than 50% of the myofibers at 10 months after injection. These results suggest that the reduction of approximately or more than 75% of the normal level of FKRP expression induces chronic dystrophic phenotypes in skeletal muscles. Furthermore, the restoration of about 25% of the normal FKRP level could be sufficient for LGMD2I therapy to correct the genetic deficiency effectively and prevent dystrophic pathology.

Capsid-expressing DNA in AAV vectors and its elimination by use of an oversize capsid gene for vector production

Adeno-associated virus vectors have been shown to mediate persistent transduction in animal models of gene therapy. However, clinical trials with AAV vectors have shown that an immune response to AAV capsid protein can result in clearance of transduced cells. One source of capsid antigen is from the delivered vector virions, but expression of cap DNA impurities in AAV vector preparations might provide an alternative and persistent source of capsid antigen. Here we show that DNA without any AAV sequences can be packaged in AAV virions, and that both cap and rep DNA are packaged into AAV vectors produced by standard methods. Using a sensitive complementation assay, we also observed significant expression of capsid in cultured cells transduced with AAV vectors. In an attempt to solve this problem, we inserted a large intron into the cap gene to generate a capsid expression cassette (captron) that is too large for packaging into AAV virions. Both complementation assays and quantitative reverse-transcription PCR analysis showed that cultured cells infected with AAV vectors made with the captron plasmid expressed no detectible capsid. Elimination of transfer of capsid-expressing DNA may reduce immune responses to AAV vector-transduced cells and promote long-term expression of therapeutic proteins.

The status of exon skipping as a therapeutic approach to duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is associated with mutations in the dystrophin gene that disrupt the open reading frame whereas the milder Becker's form is associated with mutations which leave an in-frame mRNA transcript that can be translated into a protein that includes the N- and C- terminal functional domains. It has been shown that by excluding specific exons at, or adjacent to, frame-shifting mutations, open reading frame can be restored to an out-of-frame mRNA, leading to the production of a partially functional Becker-like dystrophin protein. Such targeted exclusion can be achieved by administration of oligonucleotides that are complementary to sequences that are crucial to normal splicing of the exon into the transcript. This principle has been validated in mouse and canine models of DMD with a number of variants of oligonucleotide analogue chemistries and by transduction with adeno-associated virus (AAV)-small nuclear RNA (snRNA) reagents encoding the antisense sequence. Two different oligonucleotide agents are now being investigated in human trials for splicing out of exon 51 with some early indications of success at the biochemical level.

AAV's anatomy: roadmap for optimizing vectors for translational success

Adeno-Associated Virus based vectors (rAAV) are advantageous for human gene therapy due to low inflammatory responses, lack of toxicity, natural persistence, and ability to transencapsidate the genome allowing large variations in vector biology and tropism. Over sixty clinical trials have been conducted using rAAV serotype 2 for gene delivery with a number demonstrating success in immunoprivileged sites, including the retina and the CNS. Furthermore, an increasing number of trials have been initiated utilizing other serotypes of AAV to exploit vector tropism, trafficking, and expression efficiency. While these trials have demonstrated success in safety with emerging success in clinical outcomes, one benefit has been identification of issues associated with vector administration in humans (e.g. the role of pre-existing antibody responses, loss of transgene expression in non-immunoprivileged sites, and low transgene expression levels). For these reasons, several strategies are being used to optimize rAAV vectors, ranging from addition of exogenous agents for immune evasion to optimization of the transgene cassette for enhanced therapeutic output. By far, the vast majority of approaches have focused on genetic manipulation of the viral capsid. These methods include rational mutagenesis, engineering of targeting peptides, generation of chimeric particles, library and directed evolution approaches, as well as immune evasion modifications. Overall, these modifications have created a new repertoire of AAV vectors with improved targeting, transgene expression, and immune evasion. Continued work in these areas should synergize strategies to improve capsids and transgene cassettes that will eventually lead to optimized vectors ideally suited for translational success.

Preclinical studies for gene therapy of Duchenne muscular dystrophy

The muscular dystrophies are a diverse group of genetic disorders without an effective treatment. Because they are caused by mutations in various genes, the most direct way to treat them involves correcting the underlying gene defect (ie, gene therapy). Such a gene therapy approach involves delivering a therapeutic gene cassette to essentially all the muscles of the body in a safe and efficacious manner. The authors describe gene delivery methods using vectors derived from adeno-associated virus that are showing great promise in preclinical studies for treatment of Duchenne muscular dystrophy. It is hoped that variations on these methods might be applicable for most, if not all, of the different types of muscular dystrophy.

Inhibition of the IKK/NF-κB pathway by AAV gene transfer improves muscle regeneration in older mdx mice

The IκB kinase (IKKα, β and the regulatory subunit IKKγ) complex regulates nuclear factor of κB (NF-κB) transcriptional activity, which is upregulated in many chronic inflammatory diseases. NF-κB signaling promotes inflammation and limits muscle regeneration in Duchenne muscular dystrophy (DMD), resulting in fibrotic and fatty tissue replacement of muscle that exacerbates the wasting process in dystrophic muscles. Here, we examined whether dominant-negative forms of IKKα (IKKα-dn) and IKKβ (IKKβ-dn) delivered by adeno-associated viral (AAV) vectors to the gastrocnemius (GAS) and tibialis anterior (TA) muscles of 1, 2 and 11-month-old mdx mice, a murine DMD model, block NF-κB activation and increase muscle regeneration. At 1 month post-treatment, the levels of nuclear NF-κB in locally treated muscle were decreased by gene transfer with either AAV-CMV-IKKα-dn or AAV-CMV-IKKβ-dn, but not by IKK wild-type controls (IKKα and β) or phosphate-buffered saline (PBS). Although treatment with AAV-IKKα-dn or AAV-IKKβ-dn vectors had no significant effect on muscle regeneration in young mdx mice treated at 1 and 2 months of age and collected 1 month later, treatment of old (11 months) mdx with AAV-CMV-IKKα-dn or AAV-CMV-IKKβ-dn significantly increased levels of muscle regeneration. In addition, there was a significant decrease in myofiber necrosis in the AAV-IKKα-dn- and AAV-IKKβ-dn-treated mdx muscle in both young and old mice. These results demonstrate that inhibition of IKKα or IKKβ in dystrophic muscle reduces the adverse effects of NF-κB signaling, resulting in a therapeutic effect. Moreover, these results clearly demonstrate the therapeutic benefits of inhibiting NF-κB activation by AAV gene transfer in dystrophic muscle to promote regeneration, particularly in older mdx mice, and block necrosis.

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.

Exon skipping and duchenne muscular dystrophy therapy: selection of the most active U1 snRNA antisense able to induce dystrophin exon 51 skipping

One promising approach for the gene therapy of Duchenne muscular dystrophy (DMD) is exon skipping. When thinking of possible intervention on human, it is very crucial to identify the most appropriate antisense sequences able to provide the highest possible skipping efficiency. In this article, we compared the exon 51 skipping activity of 10 different antisense molecules, raised against splice junctions and/or exonic splicing enhancers (ESEs), expressed as part of the U1 small nuclear RNA (snRNA). The effectiveness of each construct was tested in human DMD myoblasts carrying the deletion of exons 48-50, which can be treated with skipping of exon 51. Our results show that the highest skipping activity and dystrophin rescue is achieved upon expression of a U1 snRNA-derived antisense molecule targeting exon 51 splice sites in combination with an internal exon sequence. The efficacy of this molecule was further proven on an exon 45-50 deletion background, utilizing patient's fibroblasts transdifferentiated into myoblasts. In this system, we showed that the selected antisense was able to produce 50% skipping of exon 51.

Peripheral transvenular delivery of adeno-associated viral vectors to skeletal muscle as a novel therapy for hemophilia B

Muscle represents an important tissue target for adeno-associated viral (AAV) vector-mediated gene transfer of the factor IX (FIX) gene in hemophilia B (HB) subjects with advanced liver disease. Previous studies of direct intramuscular administration of an AAV-FIX vector in humans showed limited efficacy. Here we adapted an intravascular delivery system of AAV vectors encoding the FIX transgene to skeletal muscle of HB dogs. The procedure, performed under transient immunosuppression (IS), resulted in widespread transduction of muscle and sustained, dose-dependent therapeutic levels of canine FIX transgene up to 10-fold higher than those obtained by intramuscular delivery. Correction of bleeding time correlated clinically with a dramatic reduction of spontaneous bleeding episodes. None of the dogs (n = 14) receiving the AAV vector under transient IS developed inhibitory antibodies to canine FIX; transient inhibitor was detected after vector delivery without IS. The use of AAV serotypes with high tropism for muscle and low susceptibility to anti-AAV2 antibodies allowed for efficient vector administration in naive dogs and in the presence of low- but not high-titer anti-AAV2 antibodies. Collectively, these results demonstrate the feasibility of this approach for treatment of HB and highlight the importance of IS to prevent immune responses to the FIX transgene product.

Widespread muscle expression of an AAV9 human mini-dystrophin vector after intravenous injection in neonatal dystrophin-deficient dogs

Duchenne (DMD) and golden retriever (GRMD) muscular dystrophy are caused by genetic mutations in the dystrophin gene and afflict striated muscles. We investigated systemic gene delivery in 4-day-old GRMD dogs given a single intravenous injection of an AAV9 vector (1.5 x 10(14) vector genomes/kg) carrying a human codon-optimized human mini-dystrophin gene under control of the cytomegalovirus (CMV) promoter. One of the three treated dogs was euthanized 9 days later due to pre-existing conditions. Scattered mini-dystrophin-positive myofibers were seen by immunofluorescent (IF) staining in numerous muscles. At the end of the 16-week study, the other two dogs showed generalized muscle expression of mini-dystrophin in ~15% to nearly 100% of myofibers. Western blot and vector DNA quantitative PCR results agreed with the IF data. Delayed growth and pelvic limb muscle atrophy and contractures were seen several weeks after vector delivery. T-2 weighted magnetic resonance imaging (MRI) at 8 weeks showed increased signal intensity compatible with inflammation in several pelvic limb muscles. This marked early inflammatory response raised concerns regarding methodology. Use of the ubiquitous CMV promoter, extra-high vector dose, and marked expression of a human protein in canine muscles may have contributed to the pathologic changes seen in the pelvic limbs.

Gene therapy for muscle disease

The molecular mechanisms of Duchenne muscular dystrophy (DMD) have been extensively investigated since the discovery of the dystrophin gene in 1986. Nonetheless, there is currently no effective treatment for DMD. Recent reports, however, indicate that adenoassociated viral (AAV) vector-mediated transfer of a functional dystrophin cDNA into the affected muscle is a promising strategy. In addition, antisense-mediated exon skipping technology has been emerging as another promising approach to restore dystrophin expression in DMD muscle. Ongoing clinical trials show restoration of dystrophin in DMD patients without serious side effects. Here, we summarize the recent progress in gene therapy, with an emphasis on exon skipping for DMD.

Dystrophin: more than just the sum of its parts

Dystrophin is one of a number of large cytoskeleton associated proteins that connect between various cytoskeletal elements and often are tethered to the membrane through other transmembrane protein complexes. These cytolinker proteins often provide structure and support to the cells where they are expressed, and mutations in genes encoding these proteins frequently gives rise to disease. Dystrophin is no exception in any of these respects, providing connections between a transmembrane complex known as the dystrophin-glycoprotein complex and the underlying cytoskeleton. The most established connection and possibly the most important is that to F-actin, but more recently evidence has been forthcoming of connections to membrane phospholipids, intermediate filaments and microtubules. Moreover it is becoming increasingly clear that the multiple spectrin-like repeats in the centre of the molecule, that had hitherto been thought to be largely redundant, harbour binding activities that have a significant impact on dystrophin functionality. This functionality is particularly apparent when assessed by the ability to rescue the dystrophic phenotype in mdx mice. This review will focus on the relatively neglected but functionally vital coiled-coil region of dystrophin, highlighting the structural relationships and interactions of the coiled-coil region and providing new insights into the functional role of this region.

AAV-directed muscular dystrophy gene therapy

IMPORTANCE OF THE FIELD

Muscle-directed gene therapy for genetic muscle diseases can be performed by the recombinant adeno-associated viral (rAAV) vector delivery system to achieve long-term therapeutic gene transfer in all affected muscles.

AREAS COVERED IN THIS REVIEW

Recent progress in rAAV-vector-mediated muscle-directed gene transfer and associated techniques for the treatment of muscular dystrophies (MD). The review covers literature from the past 2 - 3 years.

WHAT THE READER WILL GAIN

rAAV-directed muscular dystrophy gene therapy can be achieved by mini-dystrophin replacement and exon-skipping strategies. The additional strategies of enhancing muscle regeneration and reducing inflammation in the muscle micro-environment should be useful to optimize therapeutic efficacy. This review compares the merits and shortcomings of different administration methods, promoters and experimental animals that will guide the choice of the appropriate strategy for clinical trials.

TAKE HOME MESSAGE

Restoration of muscle histopathology and function has been performed using rAAV systemic gene delivery. In addition, the combination of gene replacement and adjuvant therapies in the future may be beneficial with regard to improving muscle regeneration and decreasing myofiber necrosis. The challenges faced by large animal model studies and in human trials arise from gene transfer efficiency and immune response, which may be overcome by optimizing the rAAV vectors utilized and the administration methods.

Meganucleases can restore the reading frame of a mutated dystrophin

Mutations in Duchenne muscular dystrophy (DMD) are either inducing a nonsense codon or a frameshift. Meganucleases (MGNs) can be engineered to induce double-strand breaks (DSBs) at specific DNA sequences. These breaks are repaired by homologous recombination or by non-homologous end joining (NHEJ), which results in insertions or deletions (indels) of a few base pairs. To verify whether MGNs could be used to restore the normal reading frame of a dystrophin gene with a frameshift mutation, we inserted in a plasmid coding for the dog micro-dystrophin sequences containing a MGN target. The number of base pairs in these inserted sequences changed the reading frame. One of these modified target micro-dystrophin plasmids and an appropriate MGN were then transfected in 293FT cells. The MGN induced micro-deletion or micro-insertion in the micro-dystrophin that restored dystrophin expression. MGNs also restored micro-dystrophin expression in myoblasts in vitro and in muscle fibers in vivo. The mutation of the targeted micro-dystrophin was confirmed by PCR amplification followed by digestion with the Surveyor enzyme and by cloning and sequencing of the amplicons. These experiments are thus a proof of principle that MGNs that are adequately engineered to target appropriate sequences in the human dystrophin gene should be able to restore the normal reading frame of that gene in DMD patients with an out-of-frame deletion. New MGNs engineered to target a sequence including or near nonsense mutation could also be used to delete it.

Expression of human alpha1-antitrypsin in mice and dogs following AAV6 vector-mediated gene transfer to the lungs

We evaluated the potential of lung-directed gene therapy for alpha1-antitrypsin (AAT) deficiency using an adeno-associated virus type 6 (AAV6) vector containing a human AAT (hAAT) complementary DNA (cDNA) delivered to the lungs of mice and dogs. The results in normal and immune-deficient mice showed that hAAT concentrations were much higher in lung fluid than in plasma, and therapeutic levels were obtained even in normal mice. However, in normal mice an immune response against the vector and/or transgene limited long-term gene expression. An AAV6 vector expressing a marker protein verified that AAV6 vectors efficiently transduced lung cells in dogs. Delivery of AAV6-hAAT resulted in low levels of hAAT in dog serum but therapeutic levels in the lung that persisted for at least 58 days to 4 months in three immunosuppressed dogs. Expression in the serum was not detectable after 45 days in one nonimmune suppressed dog. A lymphoproliferative response to AAV capsid but not to hAAT was detected even after immunosuppression. These results in mice and dogs show the feasibility of expression of therapeutic levels of AAT in the lungs after AAV vector delivery, and advocate for approaches to prevent cellular immune responses to AAV capsid proteins for persistence of gene expression in humans.

Cardiomyopathy in Duchenne muscular dystrophy: pathogenesis and therapeutics

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by the absence of dystrophin, a sarcolemmal protein which links the cytoskeleton to the extracellular matrix by interacting with a large number of proteins. Heart failure is a classic complication of this disease. The authors review the pathogenesis and therapeutics of cardiac involvement in DMD.

Chronic administration of membrane sealant prevents severe cardiac injury and ventricular dilatation in dystrophic dogs

Duchenne muscular dystrophy (DMD) is a fatal disease of striated muscle deterioration caused by lack of the cytoskeletal protein dystrophin. Dystrophin deficiency causes muscle membrane instability, skeletal muscle wasting, cardiomyopathy, and heart failure. Advances in palliative respiratory care have increased the incidence of heart disease in DMD patients, for which there is no cure or effective therapy. Here we have shown that chronic infusion of membrane-sealing poloxamer to severely affected dystrophic dogs reduced myocardial fibrosis, blocked increased serum cardiac troponin I (cTnI) and brain type natriuretic peptide (BNP), and fully prevented left-ventricular remodeling. Mechanistically, we observed a markedly greater primary defect of reduced cell compliance in dystrophic canine myocytes than in the mildly affected mdx mouse myocytes, and this was associated with a lack of utrophin upregulation in the dystrophic canine cardiac myocytes. Interestingly, after chronic poloxamer treatment, the poor compliance of isolated canine myocytes remained evident, but this could be restored to normal upon direct application of poloxamer. Collectively, these findings indicate that dystrophin and utrophin are critical to membrane stability-dependent cardiac myocyte mechanical compliance and that poloxamer confers a highly effective membrane-stabilizing chemical surrogate in dystrophin/utrophin deficiency. We propose that membrane sealant therapy is a potential treatment modality for DMD heart disease and possibly other disorders with membrane defect etiologies.

Expression of dog microdystrophin in mouse and dog muscles by gene therapy

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. Several previous studies demonstrated the feasibility of delivering microdystrophin complementary DNA (cDNA) into mouse and normal nonhuman primate muscles by ex vivo gene therapy. However, these animal models do not reproduce completely the human DMD phenotype, while the dystrophic dog model does. To progress toward the use of the best animal model of DMD, a dog microdystrophin was transduced into human and dystrophic dog muscle precursor cells (MPCs) with a lentivirus before their transplantation into mouse muscles. One month following MPC transplantation, myofibers expressing the dog microdystrophin were observed. We also used another approach to introduce this transgene into myofibers, i.e., the electrotransfer of a plasmid coding for the dog microdystrophin. The plasmid was injected into mouse and dog muscles, and brief electric pulses were applied in the region of injection. Two weeks later, the transgene was detected in both animals. Therefore, ex vivo gene therapy and electrotransfer are two possible methods to introduce a truncated version of dystrophin into myofibers of animal models and eventually into myofibers of DMD patients.

Targeted systemic gene therapy and molecular imaging of cancer contribution of the vascular-targeted AAVP vector

Gene therapy and molecular-genetic imaging have faced a major problem: the lack of an efficient systemic gene delivery vector. Unquestionably, eukaryotic viruses have been the vectors of choice for gene delivery to mammalian cells; however, they have had limited success in systemic gene therapy. This is mainly due to undesired uptake by the liver and reticuloendothelial system, broad tropism for mammalian cells causing toxicity, and their immunogenicity. On the other hand, prokaryotic viruses such as bacteriophage (phage) have no tropism for mammalian cells, but can be engineered to deliver genes to these cells. However, phage-based vectors have inherently been considered poor vectors for mammalian cells. We have reported a new generation of vascular-targeted systemic hybrid prokaryotic-eukaryotic vectors as chimeras between an adeno-associated virus (AAV) and targeted bacteriophage (termed AAV/phage; AAVP). In this hybrid vector, the targeted bacteriophage serves as a shuttle to deliver the AAV transgene cassette inserted in an intergenomic region of the phage DNA genome. As a proof of concept, we assessed the in vivo efficacy of vector in animal models of cancer by displaying on the phage capsid the cyclic Arg-Gly-Asp (RGD-4C) ligand that binds to alphav integrin receptors specifically expressed on the angiogenic blood vessels of tumors. The ligand-directed vector was able to specifically deliver imaging and therapeutic transgenes to tumors in mice, rats, and dogs while sparing the normal organs. This chapter reviews some gene transfer strategies and the potential of the vascular-targeted AAVP vector for enhancing the effectiveness of existing systemic gene delivery and genetic-imaging technologies.

Immune responses to AAV in canine muscle monitored by cellular assays and noninvasive imaging

We previously demonstrated that direct intramuscular injection of rAAV2 or rAAV6 in wild-type dogs resulted in robust T-cell responses to viral capsid proteins, and others have shown that cellular immunity to adeno-associated virus (AAV) capsid proteins coincided with liver toxicity and elimination of transgene expression in a human trial of hemophilia B. Here, we show that the heparin-binding ability of a given AAV serotype does not determine the induction of T-cell responses following intramuscular injection in dogs, and identify multiple epitopes in the AAV capsid protein that are recognized by T cells elicited by AAV injection. We also demonstrate that noninvasive magnetic resonance imaging (MRI) can accurately detect local inflammatory responses following intramuscular rAAV injection in dogs. These studies suggest that pseudotyping rAAV vectors to remove heparin-binding activity will not be sufficient to abrogate immunogenicity, and validate the utility of enzyme-linked immunosorbent spot (ELISpot) assay and MRI for monitoring immune and inflammatory responses following intramuscular injection of rAAV vectors in preclinical studies in dogs. These assays should be incorporated into future human clinical trials of AAV gene therapy to monitor immune responses.

Progress and prospects: immune responses to viral vectors

Viral vectors are potent gene delivery platforms used for the treatment of genetic and acquired diseases. However, just as viruses have evolved to infect cells efficiently, the immune system has evolved to fight off what it perceives as invading pathogens. Therefore, innate immunity and antigen-specific adaptive immune responses against vector-derived antigens reduce the efficacy and stability of in vivo gene transfer. In addition, a number of vectors are derived from parent viruses that humans encounter through natural infection, resulting in preexisting antibodies and possibly in memory responses against vector antigens. Similarly, antibody and T-cell responses may be directed against therapeutic gene products that often differ from the endogenous nonfunctional or absent protein that is being replaced. As details and mechanisms of such immune reactions are uncovered, novel strategies are being developed, and vectors are being specifically engineered to avoid, suppress or manipulate the response, ideally resulting in sustained expression and immune tolerance to the transgene product. This review provides a summary of our current knowledge of the interactions between the immune system adeno-associated virus, adenoviral and lentiviral vectors, and their transgene products.

Cardiac consequences to skeletal muscle-centric therapeutics for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disease of muscle deterioration. Duchenne muscular dystrophy affects all striated muscles in the body, including the heart. Recent advances in palliative care, largely directed at improving respiratory function, have extended life but paradoxically further unmasked emergent heart disease in DMD patients. New experimental strategies have shown promise in restoring dystrophin in the skeletal muscles of dystrophin- deficient animals. These strategies often have little or no capacity for restitution of dystrophin in the hearts of these animals. This article draws on both clinical data and recent experimental data to posit that effective skeletal muscle restricted therapies for DMD will paradoxically heighten cardiomyopathy and heart failure in these patients.

Efficient transduction of non-human primate motor neurons after intramuscular delivery of recombinant AAV serotype 6

Retrograde transport of viral vectors in the rodent spinal cord provides a powerful means to administer a therapeutic transgene from the innervated musculature. With the aim of scaling up this approach to non-human primates, we have injected recombinant adeno-associated vectors (rAAV) serotype 6 expressing enhanced green fluorescent protein (eGFP) into the gastrocnemius muscle of African green monkeys to determine whether this results in efficient transgene delivery to lumbar motor neurons. Cells expressing eGFP were observed across more than 1 cm of the spinal cord 4 weeks after intramuscular injection, reaching more than half of motor neurons in some cross-sections. Furthermore, quantitative PCR on the spinal cord tissue confirmed that eGFP expression within motor neurons was due to bona fide retrograde transport of the vector genome from the muscle. Although infiltrations of macrophages and lymphocytes were observed in the rAAV2/6-injected muscle, there was no detectable immune response within the transduced region of the spinal cord. These findings imply that retrograde delivery of rAAV serotype 6 in a primate species constitutes a non-invasive and robust approach to transduce motor neurons, a crucial target cell population in neurodegenerative disorders, such as amyotrophic lateral sclerosis and spinal muscular atrophy.

Subretinal delivery of adeno-associated virus serotype 2 results in minimal immune responses that allow repeat vector administration in immunocompetent mice

Background

Adeno-associated virus serotype 2 (AAV2) vectors show considerable promise for ocular gene transfer. However, one potential barrier to efficacious long-term therapy is the development of immune responses against the vector or transgene product.

Methods

We evaluated cellular and humoural responses in mice following both single and repeated subretinal administration of AAV2, and examined their effects on RPE65 and green fluorescent protein transgene expression.

Results

Following subretinal administration of vector, splenocytes and T-cells from draining lymph nodes showed minimal activation following stimulation by co-culture with AAV2. Neutralizing antibodies (NAbs) were not detected in the ocular fluids of any mice receiving AAV2 or in the serum of mice receiving a lower dose. NAbs were present in the serum of a proportion of mice receiving a higher dose of the vector. Furthermore, no differences in immunoglobulin titre in serum or ocular fluids against RPE65 protein or AAV2 capsid between treated and control mice were detected. Histological examination showed no evidence of retinal toxicity or leukocyte infiltration compared to uninjected eyes. Repeat administration of low-dose AAV.hRPE65.hRPE65 to both eyes of RPE65^−/− mice resulted in transgene expression and functional rescue, but re-administration of high-dose AAV2 resulted in boosted NAb titres and variable transgene expression in the second injected eye.

Conclusions

These data, which were obtained in mice, suggest that, following subretinal injection, immune responses to AAV2 are dose-dependent. Low-dose AAV2 is well tolerated in the eye, with minimal immune responses, and transgene expression after repeat administration of vector is achievable. Higher doses lead to the expression of NAbs that reduce the efficacy of repeated vector administration. Copyright © 2009 John Wiley & Sons, Ltd.

Evaluation of vascular delivery methodologies to enhance rAAV6-mediated gene transfer to canine striated musculature

A growing body of research supports the development of recombinant adeno-associated viral (rAAV) vectors for delivery of gene expression cassettes to striated musculature as a method of treating severe neuromuscular conditions. However, it is unclear whether delivery protocols that achieve extensive gene transfer in mice can be adapted to produce similarly extensive gene transfer in larger mammals and ultimately patients. Consequently, we sought to investigate methodological modifications that would facilitate rAAV-mediated gene transfer to the striated musculature of canines. A simple procedure incorporating acute (i) occlusion of limb blood flow, (ii) exsanguination via compression bandage, and (iii) vector "dwell" time of <20 minutes, markedly enhanced the transduction of limb muscles, compared with a simple bolus limb infusion of vector. A complementary method whereby vector was infused into the jugular vein led to efficient transduction of cardiomyocytes and to a lesser degree the diaphragm. Together these methods can be used to achieve transgene expression in heart, diaphragm, and limb muscles of juvenile dogs using rAAV6 vectors. These results establish that rAAV-mediated gene delivery is a viable approach to achieving systemic transduction of striated musculature in mammals approaching the dimensions of newborn humans.

Strategies to modulate immune responses: a new frontier for gene therapy

The success of gene therapy strategies to cure disease relies on the control of unwanted immune responses to transgene products, genetically modified cells and/or to the vector. Effective treatment of an established immune response is much harder to achieve than prevention of a response before it has had a chance to develop. However, preventive strategies are not always effective in avoiding immune responses, thus the use of drugs to induce immunosuppression (IS) is required. The growing discovery of novel drugs provides a conceptual shift from using generalized, moderately intensive immunosuppressive regimens towards a refined approach to attain the optimal balance of naive cells, effector cells, memory cells, and regulatory cells, harnessing the natural tolerance mechanisms of the body. We review several strategies based on transient IS coupled with gene therapy for sustained immune tolerance induction to the therapeutic transgene.

Emerging strategies for cell and gene therapy of the muscular dystrophies

The muscular dystrophies are a heterogeneous group of over 40 disorders that are characterised by muscle weakness and wasting. The most common are Duchenne muscular dystrophy and Becker muscular dystrophy, which result from mutations within the gene encoding dystrophin; myotonic dystrophy type 1, which results from an expanded trinucleotide repeat in the myotonic dystrophy protein kinase gene; and facioscapulohumeral dystrophy, which is associated with contractions in the subtelomeric region of human chromosome 1. Currently the only treatments involve clinical management of symptoms, although several promising experimental strategies are emerging. These include gene therapy using adeno-associated viral, lentiviral and adenoviral vectors and nonviral vectors, such as plasmid DNA. Exon-skipping and cell-based therapies have also shown promise in the effective treatment and regeneration of dystrophic muscle. The availability of numerous animal models for Duchenne muscular dystrophy has enabled extensive testing of a wide range of therapeutic approaches for this type of disorder. Consequently, we focus here on the therapeutic developments for Duchenne muscular dystrophy as a model of the types of approaches being considered for various types of dystrophy. We discuss the advantages and limitations of each therapeutic strategy, as well as prospects and recent successes in the context of future clinical applications.

Gene therapy in large animal models of muscular dystrophy

The muscular dystrophies are a group of genetically and phenotypically heterogeneously inherited diseases characterized by progressive muscle wasting, which can lead to premature death in severe forms such as Duchenne muscular dystrophy (DMD). In many cases they are caused by the absence of proteins that are critical components of the dystrophin-glycoprotein complex, which links the cytoskeleton and the basal lamina. There is no effective treatment for these disorders at present, but several novel strategies for replacing or repairing the defective gene are in development, with early encouraging results from animal models. We review these strategies, which include the use of stem cells of different tissue origins, gene replacement therapies mediated by various viral vectors, and transcript repair treatments using exon skipping strategies. We comment on their advantages and on limitations that must be overcome before successful application to human patients. Our focus is on studies in a clinically relevant large canine model of DMD. Recent advances in the field suggest that effective therapies for muscular dystrophies are on the horizon. Because of the complex nature of these diseases, it may be necessary to combine multiple approaches to achieve a successful treatment.

Therapy for neuromuscular disorders

Research into therapeutic approaches for both recessive and dominant neuromuscular disorders has made great progress over the past few years. In the field of gene therapy, antisense-mediated exon skipping is being applied to bypass deleterious mutations in the dystrophin gene and restore dystrophin expression in animal models of muscular dystrophy. Approaches for the dominant genetic muscle diseases have turned toward elimination of the mutant gene product with anti-sense oligonucleotide therapy and RNA interference techniques. Refinements of adeno-associated viral vectors and strategies for their delivery are also leading towards future clinical trials. The discovery of new, multipotent cell lineages, some of which possess the ability to successfully engraft muscle following vascular delivery, presents exciting prospects for the field of stem cell therapy. These discoveries represent steady progress towards the development of effective therapies for a wide range of neuromuscular disorders.

Host and vector-dependent effects on the risk of germline transmission of AAV vectors

The assessment of the risk of germline transmission of vector-coded sequences is critical for clinical translation of gene transfer strategies. We used rabbit models to analyze the risk of germline transmission of adeno-associated viral (AAV) vectors. Intravenous injection of AAV-2 or AAV-8 resulted in liver-mediated, long-term expression of therapeutic levels of human factor IX (hFIX) in a dose-dependent manner. In high-dose cohorts, AAV-8 resulted in twofold higher levels of circulating hFIX and of vector DNA in liver compared to AAV-2. Vector sequences were found in the semen of all rabbits. The kinetics of vector clearance from semen was dose- and time-dependent but serotype-independent. No late recurrence of AAV-8 sequences was found in the semen over several consecutive cycles of spermatogenesis. In a novel rabbit model, AAV-2 or AAV-8 sequences were detected in the semen of vasectomized animals that lack germ cells. Therefore, structures of the genitourinary (GU) tract, as well as the testis, contribute significantly to vector shedding in the semen. Collectively, data from these two models suggest that the risk of inadvertent germline transmission in males by AAV-8 vectors is low, similar to that of AAV-2, and that AAV dissemination to the semen is in part modulated by host-dependent factors.

The value of mammalian models for duchenne muscular dystrophy in developing therapeutic strategies

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy. There is no effective treatment and patients typically die in approximately the third decade. DMD is an X-linked recessive disease caused by mutations in the dystrophin gene. There are three mammalian models of DMD that have been used to understand better the pathogenesis of disease and develop therapeutic strategies. The mdx mouse is the most widely used model of DMD that displays some features of muscle degeneration, but the pathogenesis of disease is comparatively mild. The severity of disease in mice lacking both dystrophin and utrophin is similar to DMD, but one has to account for the discrete functions of utrophin. Canine X-linked muscular dystrophy (cxmd) is the best representation of DMD, but the phenotype of the most widely used golden retriever (GRMD) model is variable, making functional endpoints difficult to ascertain. Although each mammalian model has its limitations, together they have been essential for the development of several treatment strategies for DMD that target dystrophin replacement, disease progression, and muscle regeneration.

Biological gene delivery vehicles: beyond viral vectors

Gene therapy covers a broad spectrum of applications, from gene replacement and knockdown for genetic or acquired diseases such as cancer, to vaccination, each with different requirements for gene delivery. Viral vectors and synthetic liposomes have emerged as the vehicles of choice for many applications today, but both have limitations and risks, including complexity of production, limited packaging capacity, and unfavorable immunological features, which restrict gene therapy applications and hold back the potential for preventive gene therapy. While continuing to improve these vectors, it is important to investigate other options, particularly nonviral biological agents which include bacteria, bacteriophage, virus-like particles (VLPs), erythrocyte ghosts, and exosomes. Exploiting the natural properties of these biological entities for specific gene delivery applications will expand the repertoire of gene therapy vectors available for clinical use. Here, we review the prospects for nonviral biological delivery vehicles as gene therapy agents with focus on their unique evolved biological properties and respective limitations and potential applications. The potential of these nonviral biological entities to act as clinical gene therapy delivery vehicles has already been shown in clinical trials using bacteria-mediated gene transfer and with sufficient development, these entities will complement the established delivery techniques for gene therapy applications.