Immune responses to dystropin: implications for gene therapy of Duchenne muscular dystrophy

T Cell Responses to Dystrophin in a Natural History Study of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin, but many patients have rare revertant fibers that express dystrophin. The skeletal muscle pathology of DMD patients includes immune cell infiltration and inflammatory cascades. There are several strategies to restore dystrophin in skeletal muscles of patients, including exon skipping and gene therapy. There is some evidence that dystrophin restoration leads to a reduction in immune cells, but dystrophin epitopes expressed in revertant fibers or following genome editing, cell therapy, or microdystrophin delivery after adeno-associated viral gene therapy may elicit T cell production in patients. This may affect the efficacy of the therapeutic intervention, and potentially lead to serious adverse events. To confirm and extend previous studies, we performed annual enzyme- linked immunospot interferon-gamma assays on peripheral blood mononuclear cells from 77 pediatric boys with DMD recruited into a natural history study, 69 of whom (89.6%) were treated with corticosteroids. T cell responses to dystrophin were quantified using a total of 368 peptides spanning the entire dystrophin protein, organized into nine peptide pools. Peptide mapping pools were used to further localize the immune response in one positive patient. Six (7.8%) patients had a T cell-mediated immune response to dystrophin at at least one time point. All patients who had a positive result had been treated with corticosteroids, either prednisolone or prednisone. Our results show that ∼8% of DMD individuals in our cohort have a pre-existing T cell-mediated immune response to dystrophin, despite steroid treatment. Although these responses are relatively low level, this information should be considered a useful immunological baseline before undertaking clinical trials and future DMD studies. We further highlight the importance for a robust, reproducible standard operating procedure for collecting, storing, and shipping samples from multiple centers to minimize the number of inconclusive data.

Assessment of systemic AAV-microdystrophin gene therapy in the GRMD model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disease caused by the absence of dystrophin, a membrane-stabilizing protein encoded by the DMD gene. Although mouse models of DMD provide insight into the potential of a corrective therapy, data from genetically homologous large animals, such as the dystrophin-deficient golden retriever muscular dystrophy (GRMD) model, may more readily translate to humans. To evaluate the clinical translatability of an adeno-associated virus serotype 9 vector (AAV9)-microdystrophin (μDys5) construct, we performed a blinded, placebo-controlled study in which 12 GRMD dogs were divided among four dose groups [control, 1 × 1013 vector genomes per kilogram (vg/kg), 1 × 1014 vg/kg, and 2 × 1014 vg/kg; n = 3 each], treated intravenously at 3 months of age with a canine codon-optimized microdystrophin construct, rAAV9-CK8e-c-μDys5, and followed for 90 days after dosing. All dogs received prednisone (1 milligram/kilogram) for a total of 5 weeks from day -7 through day 28. We observed dose-dependent increases in tissue vector genome copy numbers; μDys5 protein in multiple appendicular muscles, the diaphragm, and heart; limb and respiratory muscle functional improvement; and reduction of histopathologic lesions. As expected, given that a truncated dystrophin protein was generated, phenotypic test results and histopathologic lesions did not fully normalize. All administrations were well tolerated, and adverse events were not seen. These data suggest that systemically administered AAV-microdystrophin may be dosed safely and could provide therapeutic benefit for patients with DMD.

The Effect of Immunomodulatory Treatments on Anti-Dystrophin Immune Response After AAV Gene Therapy in Dystrophin Deficient mdx Mice

BACKGROUND

AAV-based gene therapy is an attractive approach to treat Duchenne muscular dystrophy (DMD) patients. Although the long-term consequences of a gene therapy approach for DMD are unknown, there is evidence in both DMD patients and animal models that dystrophin replacement by gene therapy leads to an anti-dystrophin immune response that is likely to limit the long-term use of these therapeutic strategies.

OBJECTIVE

Our objective is to test whether the anti-dystrophin immune response is affected by immunomodulatory drugs in mdx mice after rAAV gene therapy.

METHODS

mdx mice were treated with rAAV microdystrophin alone or in combination with immunomodulatory drugs. Dystrophin expression in skeletal muscle was assessed by mass spectrometry. Immune responses were assessed by immunophenotyping, western blot for anti-dystrophin antibodies and flow cytometry assays for antigen-specific T-cell cytokine expression. The impact on muscle was measured by grip strength assessment, in vivo torque, optical imaging for inflammation and H&E staining of sections to assess muscle damage.

RESULTS

We found that AAV-9-microdystrophin gene therapy induced expression of microdystrophin, anti-dystrophin antibodies, and T-cell cytokine responses. Immunomodulatory treatments, rituximab and VBP-6 completely abrogated the anti-dystrophin antibody response. Prednisolone, CTLA4-Ig, and Eplerenone showed variable efficacy in blocking the anti-dystrophin immune response. In contrast, none of the drugs completely abrogated the antigen specific IFN-γ response. AAV-microdystrophin treatment significantly reduced inflammation in both forelimbs and hindlimbs, and the addition of prednisolone and VBP6 further reduced muscle inflammation. Treatment with immunomodulatory drugs, except eplerenone, enhanced the beneficial effects of AAV-microdystrophin therapy in terms of force generation.

CONCLUSIONS

Our data suggest that AAV-microdystrophin treatment results in anti-dystrophin antibody and T-cell responses, and immunomodulatory treatments have variable efficacy on these responses.

Role of Immunoglobulins in Muscular Dystrophies and Inflammatory Myopathies

Muscular dystrophies and inflammatory myopathies are heterogeneous muscular disorders characterized by progressive muscle weakness and mass loss. Despite the high variability of etiology, inflammation and involvement of both innate and adaptive immune response are shared features. The best understood immune mechanisms involved in these pathologies include complement cascade activation, auto-antibodies directed against muscular proteins or de-novo expressed antigens in myofibers, MHC-I overexpression in myofibers, and lymphocytes-mediated cytotoxicity. Intravenous immunoglobulins (IVIGs) administration could represent a suitable immunomodulator with this respect. Here we focus on mechanisms of action of immunoglobulins in muscular dystrophies and inflammatory myopathies highlighting results of IVIGs from pre-clinical and case reports evidences.

Genetic Disease and Therapy

Genetic diseases cause numerous complex and intractable pathologies. DNA sequences encoding each human's complexity and many disease risks are contained in the mitochondrial genome, nuclear genome, and microbial metagenome. Diagnosis of these diseases has unified around applications of next-generation DNA sequencing. However, translating specific genetic diagnoses into targeted genetic therapies remains a central goal. To date, genetic therapies have fallen into three broad categories: bulk replacement of affected genetic compartments with a new exogenous genome, nontargeted addition of exogenous genetic material to compensate for genetic errors, and most recently, direct correction of causative genetic alterations using gene editing. Generalized methods of diagnosis, therapy, and reagent delivery into each genetic compartment will accelerate the next generations of curative genetic therapies. We discuss the structure and variability of the mitochondrial, nuclear, and microbial metagenomic compartments, as well as the historical development and current practice of genetic diagnostics and gene therapies targeting each compartment.

Role of Regulatory T Cell and Effector T Cell Exhaustion in Liver-Mediated Transgene Tolerance in Muscle

The pro-tolerogenic environment of the liver makes this tissue an ideal target for gene replacement strategies. In other peripheral tissues such as the skeletal muscle, anti-transgene immune response can result in partial or complete clearance of the transduced fibers. Here, we characterized liver-induced transgene tolerance after simultaneous transduction of liver and muscle. A clinically relevant transgene, α-sarcoglycan, mutated in limb-girdle muscular dystrophy type 2D, was fused with the SIINFEKL epitope (hSGCA-SIIN) and expressed with adeno-associated virus vectors (AAV-hSGCA-SIIN). Intramuscular delivery of AAV-hSGCA-SIIN resulted in a strong inflammatory response, which could be prevented and reversed by concomitant liver expression of the same antigen. Regulatory T cells and upregulation of checkpoint inhibitor receptors were required to establish and maintain liver-mediated peripheral tolerance. This study identifies the fundamental role of the synergy between Tregs and upregulation of checkpoint inhibitor receptors in the liver-mediated control of anti-transgene immunity triggered by muscle-directed gene transfer.

Morpholino-induced exon skipping stimulates cell-mediated and humoral responses to dystrophin in mdx mice

Exon skipping is a promising genetic therapeutic strategy for restoring dystrophin expression in the treatment of Duchenne muscular dystrophy (DMD). The potential for newly synthesized dystrophin to trigger an immune response in DMD patients, however, is not well established. We have evaluated the effect of chronic phosphorodiamidate morpholino oligomer (PMO) treatment on skeletal muscle pathology and asked whether sustained dystrophin expression elicits a dystrophin-specific autoimmune response. Here, two independent cohorts of dystrophic mdx mice were treated chronically with either 800 mg/kg/month PMO for 6 months (n = 8) or 100 mg/kg/week PMO for 12 weeks (n = 11). We found that significant muscle inflammation persisted after exon skipping in skeletal muscle. Evaluation of humoral responses showed serum-circulating antibodies directed against de novo dystrophin in a subset of mice, as assessed both by Western blotting and immunofluorescent staining; however, no dystrophin-specific antibodies were observed in the control saline-treated mdx cohorts (n = 8) or in aged (12-month-old) mdx mice with expanded 'revertant' dystrophin-expressing fibers. Reactive antibodies recognized both full-length and truncated exon-skipped dystrophin isoforms in mouse skeletal muscle. We found more antigen-specific T-cell cytokine responses (e.g. IFN-g, IL-2) in dystrophin antibody-positive mice than in dystrophin antibody-negative mice. We also found expression of major histocompatibility complex class I on some of the dystrophin-expressing fibers along with CD8+ and perforin-positive T cells in the vicinity, suggesting an activation of cell-mediated damage had occurred in the muscle. Evaluation of complement membrane attack complex (MAC) deposition on the muscle fibers further revealed lower MAC deposition on muscle fibers of dystrophin antibody-negative mice than on those of dystrophin antibody-positive mice. Our results indicate that de novo dystrophin expression after exon skipping can trigger both cell-mediated and humoral immune responses in mdx mice. Our data highlights the need to further investigate the autoimmune response and its long-term consequences after exon-skipping therapy. Copyright © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Myoediting: Toward Prevention of Muscular Dystrophy by Therapeutic Genome Editing

Muscular dystrophies represent a large group of genetic disorders that significantly impair quality of life and often progress to premature death. There is no effective treatment for these debilitating diseases. Most therapies, developed to date, focus on alleviating the symptoms or targeting the secondary effects, while the underlying gene mutation is still present in the human genome. The discovery and application of programmable nucleases for site-specific DNA double-stranded breaks provides a powerful tool for precise genome engineering. In particular, the CRISPR/Cas system has revolutionized the genome editing field and is providing a new path for disease treatment by targeting the disease-causing genetic mutations. In this review, we provide a historical overview of genome-editing technologies, summarize the most recent advances, and discuss potential strategies and challenges for permanently correcting genetic mutations that cause muscular dystrophies.

Dystrophin Cardiomyopathies: Clinical Management, Molecular Pathogenesis and Evolution towards Precision Medicine

Duchenne’s muscular dystrophy is an X-linked neuromuscular disease that manifests as muscle atrophy and cardiomyopathy in young boys. However, a considerable percentage of carrier females are often diagnosed with cardiomyopathy at an advanced stage. Existing therapy is not disease-specific and has limited effect, thus many patients and symptomatic carrier females prematurely die due to heart failure. Early detection is one of the major challenges that muscular dystrophy patients, carrier females, family members and, research and medical teams face in the complex course of dystrophic cardiomyopathy management. Despite the widespread adoption of advanced imaging modalities such as cardiac magnetic resonance, there is much scope for refining the diagnosis and treatment of dystrophic cardiomyopathy. This comprehensive review will focus on the pertinent clinical aspects of cardiac disease in muscular dystrophy while also providing a detailed consideration of the known and developing concepts in the pathophysiology of muscular dystrophy and forthcoming therapeutic options.

Overexpression of Mutant FKRP Restores Functional Glycosylation and Improves Dystrophic Phenotype in FKRP Mutant Mice

Autosomal recessive homozygous or compound heterozygous mutations in FKRP result in forms of muscular dystrophy-dystroglycanopathy varying in age of onset, clinical presentation, and disease progression, ranging from the severe Walker-Warburg, type A,5 (MDDGA5), muscle-eye-brain (MDDGB5) with or without cognitive deficit, to limb-girdle type 2I (MDDGC5). Phenotypic variation indicates degrees of functionality of individual FKRP mutation, which has been supported by the presence of residual expression of functionally glycosylated α-dystroglycan (DG) in muscles of both animal models and patients. However, direct evidence showing enhancement in glycosylation of α-DG by mutant FKRP is lacking. Using AAV9-mediated overexpression of mutant human FKRP bearing the P448L mutation (mhFKRP-P448L) associated with severe congenital muscular dystrophy (CMD), we demonstrate the restoration of functional glycosylation of α-DG and reduction in markers of disease progression. Expression of mhFKRP-P448L also corrects dystrophic phenotypes in the models of L276I mutation with mild disease phenotype and causes no obvious histological or biomarker alteration in C57BL/6 normal mice. Our results confirm the existing function of mutant FKRP. The results also suggest that mutant FKRP could be an alternative approach for potential gene therapy should normal FKRP gene products be immunogenic.

Immunoglobulin therapy ameliorates the phenotype and increases lifespan in the severely affected dystrophin-utrophin double knockout mice

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder, caused by mutations in the dystrophin gene, affecting 1:3500-5000 boys worldwide. The lack of dystrophin induces degeneration of muscle cells and elicits an immune response characterized by an intensive secretion of pro-inflammatory cytokines. Immunoglobulins modulate the inflammatory response through several mechanisms and have been widely used as an adjuvant therapy for autoimmune diseases. Here we evaluated the effect of immunoglobulin G (IG) injected intraperitoneally in a severely affected double knockout (dko) mouse model for Duchenne muscular dystrophy. The IG dko treated mice were compared regarding activity rates, survival and histopathology with a control untreated group. Additionally, dendritic cells and naïve lymphocytes from these two groups and WT mice were obtained to study in vitro the role of the immune system associated to DMD pathophysiology. We show that IG therapy significantly enhances activity rate and lifespan of dko mice. It diminishes muscle tissue inflammation by decreasing the expression of costimulatory molecules MHC, CD86 and CD40 and reducing Th1-related cytokines IFN-γ, IL-1β and TNF-α release. IG therapy dampens the effector immune responses supporting the hypothesis according to which the immune response accelerates DMD progression. As IG therapy is already approved by FDA for treating autoimmune disorders, with less side-effects than currently used glucocorticoids, our results may open a new therapeutic option aiming to improve life quality and lifespan of DMD patients.

Dystrophic Cardiomyopathy-Potential Role of Calcium in Pathogenesis, Treatment and Novel Therapies

Duchenne muscular dystrophy (DMD) is caused by defects in the DMD gene and results in progressive wasting of skeletal and cardiac muscle due to an absence of functional dystrophin. Cardiomyopathy is prominent in DMD patients, and contributes significantly to mortality. This is particularly true following respiratory interventions that reduce death rate and increase ambulation and consequently cardiac load. Cardiomyopathy shows an increasing prevalence with age and disease progression, and over 95% of patients exhibit dilated cardiomyopathy by the time they reach adulthood. Development of the myopathy is complex, and elevations in intracellular calcium, functional muscle ischemia, and mitochondrial dysfunction characterise the pathophysiology. Current therapies are limited to treating symptoms of the disease and there is therefore an urgent need to treat the underlying genetic defect. Several novel therapies are outlined here, and the unprecedented success of phosphorodiamidate morpholino oligomers (PMOs) in preclinical and clinical studies is overviewed.

Influence of immune responses in gene/stem cell therapies for muscular dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of diseases, caused by mutations in different components of sarcolemma, extracellular matrix, or enzymes. Inflammation and innate or adaptive immune response activation are prominent features of MDs. Various therapies under development are directed toward rescuing the dystrophic muscle damage using gene transfer or cell therapy. Here we discussed current knowledge about involvement of immune system responses to experimental therapies in MDs.

Genetic therapeutic approaches for Duchenne muscular dystrophy

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

Effect of rapamycin on immunity induced by vector-mediated dystrophin expression in mdx skeletal muscle

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene. Therapeutic gene replacement of a dystrophin cDNA into dystrophic muscle can provide functional dystrophin protein to the tissue. However, vector-mediated gene transfer is limited by anti-vector and anti-transgene host immunity that causes rejection of the therapeutic protein. We hypothesized that rapamycin (RAPA) would diminish immunity due to vector-delivered recombinant dystrophin in the adult mdx mouse model for DMD. To test this hypothesis, we injected limb muscle of mdx mice with RAPA-containing, poly-lactic-co-glycolic acid (PLGA) microparticles prior to dystrophin gene transfer and analyzed treated tissue after 6 weeks. RAPA decreased host immunity against vector-mediated dystrophin protein, as demonstrated by decreased cellular infiltrates and decreased anti-dystrophin antibody production. The interpretation of the effect of RAPA on recombinant dystrophin expression was complex because of an effect of PLGA microparticles.

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.

Novel compounds for the treatment of Duchenne muscular dystrophy: emerging therapeutic agents

The identification of dystrophin and the causative role of mutations in this gene in Duchenne and Becker muscular dystrophies (D/BMD) was expected to lead to timely development of effective therapies. Despite over 20 years of research, corticosteroids remain the only available pharmacological treatment for DMD, although significant benefits and extended life have resulted from advances in the clinical care and management of DMD individuals. Effective treatment of DMD will require dystrophin restitution in skeletal, cardiac, and smooth muscles and nonmuscle tissues; however, modulation of muscle loss and regeneration has the potential to play an important role in altering the natural history of DMD, particularly in combination with other treatments. Emerging biological, molecular, and small molecule therapeutics are showing promise in ameliorating this devastating disease, and it is anticipated that regulatory environments will need to display some flexibility in order to accommodate the new treatment paradigms.

Dystrophin immunity in Duchenne's muscular dystrophy

We report on delivery of a functional dystrophin transgene to skeletal muscle in six patients with Duchenne's muscular dystrophy. Dystrophin-specific T cells were detected after treatment, providing evidence of transgene expression even when the functional protein was not visualized in skeletal muscle. Circulating dystrophin-specific T cells were unexpectedly detected in two patients before vector treatment. Revertant dystrophin fibers, which expressed functional, truncated dystrophin from the deleted endogenous gene after spontaneous in-frame splicing, contained epitopes targeted by the autoreactive T cells. The potential for T-cell immunity to self and nonself dystrophin epitopes should be considered in designing and monitoring experimental therapies for this disease. (Funded by the Muscular Dystrophy Association and others; ClinicalTrials.gov number, NCT00428935.).

Effect of tissue-specific promoters and microRNA recognition elements on stability of transgene expression after hydrodynamic naked plasmid DNA delivery

Intravenous hydrodynamic injections into the liver and skeletal muscle have increased the efficacy of naked DNA delivery to a level that makes therapeutically relevant gene transfer attainable. Although there are no concerns about the immunogenicity of the delivered DNA itself, transgene products that are foreign to the host can trigger an immune response and hamper the therapeutic effect. Our goal was to determine whether and to what extent some known preventive measures are applicable to these delivery methods in order to achieve longterm expression of foreign proteins in immunocompetent mice. We designed plasmid DNA vectors that expressed a marker gene under the control of either a ubiquitous or a tissue-specific promoter. We also included microRNA (miR) target sites in the transcripts in order to silence expression in antigen-presenting cells (APCs). The constructs were delivered either into muscle or liver, using outbred ICR and inbred C57BL=6 mice. The data suggest that firefly luciferase, a potent immunogen, triggered a uniform immune response only in outbred ICR mice, and only when expressed from a ubiquitous promoter. This response could not be prevented by including APC-specific miR target sites in the transcript. In contrast, the probability of immune rejection in ICR mice could be significantly diminished by using tissue-specific promoters, and under these circumstances, the silencing of transgene expression in APCs did confer some benefits. After a single hydrodynamic injection, inbred mice did not reject luciferase under any of the tested conditions for at least 8 weeks. To test whether they became tolerized, they were challenged with a second boost of a cytomegalovirus promoter-driven luciferase construct. This triggered a strong immune response, suggesting that luciferase-reactive cells from the animals' T and B cell repertoire had not been eliminated. This secondary reaction could not be prevented by silencing expression in APCs. In conclusion, for the clinical application of hydrodynamic naked DNA delivery the use of tissue-specific promoters in combination with silencing expression in APCs will increase the probability of long-term expression, but the most desirable outcome, the establishment of transgene tolerance, appears unlikely to be achieved by any of these measures.

Identification of noncollagenous sites encoding specific interactions and quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen: implications for Alport gene therapy

Defective assembly of alpha 3 alpha 4 alpha 5(IV) collagen in the glomerular basement membrane causes Alport syndrome, a hereditary glomerulonephritis progressing to end-stage kidney failure. Assembly of collagen IV chains into heterotrimeric molecules and networks is driven by their noncollagenous (NC1) domains, but the sites encoding the specificity of these interactions are not known. To identify the sites directing quaternary assembly of alpha 3 alpha 4 alpha 5(IV) collagen, correctly folded NC1 chimeras were produced, and their interactions with other NC1 monomers were evaluated. All alpha1/alpha 5 chimeras containing alpha 5 NC1 residues 188-227 replicated the ability of alpha 5 NC1 to bind to alpha3NC1 and co-assemble into NC1 hexamers. Conversely, substitution of alpha 5 NC1 residues 188-227 by alpha1NC1 abolished these quaternary interactions. The amino-terminal 58 residues of alpha3NC1 encoded binding to alpha 5 NC1, but this interaction was not sufficient for hexamer co-assembly. Because alpha 5 NC1 residues 188-227 are necessary and sufficient for assembly into alpha 3 alpha 4 alpha 5 NC1 hexamers, whereas the immunodominant alloantigenic sites of alpha 5 NC1 do not encode specific quaternary interactions, the findings provide a basis for the rational design of less immunogenic alpha 5(IV) collagen constructs for the gene therapy of X-linked Alport patients.

Correction of multiple striated muscles in murine Pompe disease through adeno-associated virus-mediated gene therapy

Glycogen storage disease type II (Pompe disease; MIM 232300) stems from the deficiency of acid alpha-glucosidase (GAA; acid maltase; EC 3.2.1.20), which primarily involves cardiac and skeletal muscles. An adeno-associated virus 2/8 (AAV2/8) vector containing the muscle creatine kinase (MCK) (CK1) reduced glycogen content by approximately 50% in the heart and quadriceps in GAA-knockout (GAA-KO) mice; furthermore, an AAV2/8 vector containing the hybrid alpha-myosin heavy chain enhancer-/MCK enhancer-promoter (MHCK7) cassette reduced glycogen content by >95% in heart and >75% in the diaphragm and quadriceps. Transduction with an AAV2/8 vector was higher in the quadriceps than in the gastrocnemius. An AAV2/9 vector containing the MHCK7 cassette corrected GAA deficiency in the distal hindlimb, and glycogen accumulations were substantially cleared by human GAA (hGAA) expression therein; however, the analogous AAV2/7 vector achieved much lower efficacy. Administration of the MHCK7-containing vectors significantly increased striated muscle function as assessed by increased Rotarod times at 18 weeks after injection, whereas the CK1-containing vector did not increase Rotarod performance. Importantly, type IIb myofibers in the extensor digitalis longus (EDL) were transduced, thereby correcting a myofiber type that is unresponsive to enzyme replacement therapy. In summary, AAV8 and AAV9-pseudotyped vectors containing the MHCK7 regulatory cassette achieved enhanced efficacy in Pompe disease mice.

Gene delivery to dystrophic muscle

Electroporation is a powerful method for gene delivery to dystrophic muscle in the mdx mouse model of Duchenne muscular dystrophy. Successful transfer of reporter and therapeutic plasmids and antisense oligonucleotides has been demonstrated. However, the efficiency falls with increasing plasmid size. Although it is unlikely that the electrotransfer approach will be useful clinically, it is an important experimental tool, particularly in testing potential immune responses to gene transfer in the absence of vector proteins.

How not to be seen: immune-evasion strategies in gene therapy

The development of efficient and safe vectors for gene delivery paved the way for evolution of gene therapy as a new modality for treatment of various inherited disorders and for cancer. The current vectors, viral and non-viral, have their limitations. Innate and adaptive immune responses to vector particles and components may restrict the efficiency of gene transfer and the persistence of expression of the transgene. Results from preclinical studies in animals and more recently data from clinical studies have demonstrated the potential impact of the cellular and the humoral immune response on the therapeutic efficacy. Not only the vector components, but also the transgene products may induce an immune response that negatively affects the therapeutic efficacy. The induction of a cytotoxic T-cell response to transgene-encoded peptides, as well as the production of antibodies directed against secreted proteins have been reported in preclinical and clinical studies, and these may thwart those applications that require long-term expression. Here we will review some of the options to blunt the acquired immune responses to transgene-encoded polypeptides.

Non-viral gene therapy for Duchenne muscular dystrophy: Progress and challenges

Duchenne muscular dystrophy (DMD) is one of the most common lethal, hereditary diseases of childhood. Since the identification of the genetic basis of this disorder, there has been the hope that a cure would be developed in the form of gene therapy. This has yet to be realized, but many different gene therapy approaches have seen dramatic advances in recent years. Although viral-mediated gene therapy has been at the forefront of the field, several non-viral gene therapy approaches have been applied to animal and cellular models of DMD. These include plasmid-mediated gene delivery, antisense-mediated exon skipping, and oligonucleotide-mediated gene editing. In the past several years, non-viral gene therapy has moved from the laboratory to the clinic. Advances in vector design, formulation, and delivery are likely to lead to even more rapid advances in the coming decade. Given the relative simplicity, safety, and cost-effectiveness of these methodologies, non-viral gene therapy continues to have great promise for future gene therapy approaches to the treatment of DMD.

Redirecting Splicing to Address Dystrophin Mutations: Molecular By-pass Surgery

Mutations in the dystrophin gene that prevent synthesis of a functional protein lead to Duchenne muscular dystrophy (DMD), the most common serious childhood muscular dystrophy. The major isoform is produced in skeletal muscle and the size of the dystrophin gene and complexity of expression have posed great challenges to the development of a therapy for DMD. Considerable progress has been made in the areas of gene and cell replacement, yet it appears that any potential therapy for DMD is still some years away. Other approaches are being considered, and one that has generated substantial interest over the last few years is induced exon skipping. Antisense oligonucleotides have been used to block abnormal splice sites and force pre-mRNA processing back to the normal patterns. This approach is re-interpreted to address the more common dystrophin mutations, where normal splice sites are targeted to induce abnormal splicing, resulting in specific exon exclusion. Selected exon removal during processing of the dystrophin pre-mRNA can by-pass nonsense mutations or restore a disrupted reading frame arising from genomic deletions or duplications. Attributes of the dystrophin gene that have hampered gene replacement therapy may be regarded as positive features for induced exon skipping, which may be regarded as a form of by-pass surgery at the molecular level. In humans, antisense oligonucleotides have been more generally applied to down-regulate specific gene expression, for the treatment of acquired conditions such as malignancies and viral infections. From interesting in vitro experiments several years ago, the dystrophin exon-skipping field has progressed to the stage of planning for clinical trials.

Targeting the respiratory muscles of fetal sheep for prenatal gene therapy for Duchenne muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a lethal degenerative muscular disease. Fetal gene therapy may correct the primary genetic defect. Our aim was to achieve expression of a reporter gene in the respiratory muscles of early gestation fetal sheep.

STUDY DESIGN

An adenovirus vector containing the beta-galactosidase reporter gene (AdRSVbetagal) was injected into the thoracic musculature (n = 3) and pleural cavity (n = 6) of fetal sheep (61-67 days' gestation) under ultrasound guidance. Tissues were harvested after 48 hours and site and intensity of beta-galactosidase expression were assessed.

RESULTS

Limited transgene expression observed after a single injection was improved by multiple injections, but remained localized. Ultrasound-guided creation of a hydrothorax led to an increase in the intensity of beta-galactosidase expression (ELISA). X-gal staining and immunohistochemistry showed that vector spread was confined to the innermost intercostal musculature.

CONCLUSION

Ultrasound-guided injection can deliver gene therapy vectors to the fetal pleural cavity and achieve transduction of the respiratory muscles.

Long-term correction of hyperbilirubinemia in the Gunn rat by repeated intravenous delivery of naked plasmid DNA into muscle

We evaluated nonviral gene delivery into skeletal muscle via femoral artery and great saphenous vein for correction of hyperbilirubinemia in the Gunn rat, the animal model of Crigler-Najjar syndrome type I. A single injection of pDNA expressing hUGT1A1 under the CMV promoter resulted in excretion of bilirubin glucuronides in bile and a significant decrease in serum bilirubin for at least 2 or 4 weeks, respectively. Loss of metabolic effect was associated with a decrease in recombinant protein in muscle, while pDNA and transcript were detectable 4 weeks after gene delivery. Monthly intravenous gene delivery maintained metabolic correction for at least 5 months. Fibrosis around vessels in the arterial group limited the number of successful repeat gene transfer sessions to 3. Animals expressing hUGT1A1 developed anti-hUGT1A1 antibodies and lymphocytic infiltrate in muscle. Immunosuppression abrogated antibody response, ameliorated lymphocytic inflammation, and enhanced metabolic correction but did not prevent a decrease in the amount of recombinant protein. In conclusion, repeated intravenous delivery of pDNA into muscle enables long-term correction of hyperbilirubinemia in the Gunn rat. The procedure is safe and simple, with great clinical potential. Further studies are needed to explain the mechanisms of loss and improve the stability of recombinant hUGT1A1 in muscle.

Correction of glycogen storage disease type II by an adeno-associated virus vector containing a muscle-specific promoter

Glycogen storage disease type II (Pompe disease) causes death in infancy from cardiorespiratory failure due to acid alpha-glucosidase (GAA; acid maltase) deficiency. An AAV2 vector pseudotyped as AAV6 (AAV2/6 vector) transiently expressed high-level human GAA in GAA-knockout (GAA-KO) mice without reducing glycogen storage; however, in immunodeficient GAA-KO/SCID mice the AAV2/6 vector expressed high-level GAA and reduced the glycogen content of the injected muscle for 24 weeks. A CD4+/CD8+ lymphocytic infiltrate was observed in response to the AAV2/6 vector in immunocompetent GAA-KO mice. When a muscle-specific creatine kinase promoter was substituted for the CB promoter (AAV-MCKhGAApA), that AAV2/6 vector expressed high-level GAA and reduced glycogen content in immunocompetent GAA-KO mice. Muscle-restricted expression of hGAA provoked only a humoral (not cellular) immune response. Intravenous administration of a high number of particles of AAV-MCKhGAApA as AAV2/7 reduced the glycogen content of the heart and skeletal muscle and corrected individual myofibers in immunocompetent GAA-KO mice 24 weeks postinjection. In summary, persistent correction of muscle glycogen content was achieved with an AAV vector containing a muscle-specific promoter in GAA-KO mice, and this approach should be considered for muscle-targeted gene therapy in Pompe disease.

Transplantation of a multipotent cell population from human adipose tissue induces dystrophin expression in the immunocompetent mdx mouse

Here, we report the isolation of a human multipotent adipose-derived stem (hMADS) cell population from adipose tissue of young donors. hMADS cells display normal karyotype; have active telomerase; proliferate >200 population doublings; and differentiate into adipocytes, osteoblasts, and myoblasts. Flow cytometry analysis indicates that hMADS cells are CD44+, CD49b+, CD105+, CD90+, CD13+, Stro-1(-), CD34-, CD15-, CD117-, Flk-1(-), gly-A(-), CD133-, HLA-DR(-), and HLA-I(low). Transplantation of hMADS cells into the mdx mouse, an animal model of Duchenne muscular dystrophy, results in substantial expression of human dystrophin in the injected tibialis anterior and the adjacent gastrocnemius muscle. Long-term engraftment of hMADS cells takes place in nonimmunocompromised animals. Based on the small amounts of an easily available tissue source, their strong capacity for expansion ex vivo, their multipotent differentiation, and their immune-privileged behavior, our results suggest that hMADS cells will be an important tool for muscle cell-mediated therapy.

Function induced modifications of gene expression: an alternative approach to gene therapy of Duchenne muscular dystrophy

In Duchenne muscular dystrophy a large gene that codes for dystrophin is altered. The possibility that the defective gene/protein could be at least in part substituted by other molecules that the diseased muscle is able to produce and that have a function similar to that of dystrophin is being discussed. Muscle fibres have a tremendous adaptive potential, and the expression of several protein isoforms can be induced by either stretch or long-term change of activity. The exploitation of this ability of muscle cells to express new genes, which would code for proteins that will not be alien to the individual, for treatment of Duchenne muscular dystrophy is being considered. The argument for this approach is strengthened by results that in patients with Duchenne muscular dystrophy the progress of the disease can be slowed with changes of muscle activity.

Intraarterial delivery of naked plasmid DNA expressing full-length mouse dystrophin in the mdx mouse model of duchenne muscular dystrophy

Our previous studies have demonstrated that the intraarterial delivery of naked plasmid DNA leads to high levels of foreign gene expression throughout the muscles of the targeted limb. Although the procedure was first developed in rats and then extended to nonhuman primates, the present study has successfully implemented the procedure in normal mice and the mdx mouse model for Duchenne muscular dystrophy. After intraarterial delivery of plasmid DNA expressing the normal, full-length mouse dystrophin from either the cytomegalovirus promoter or a muscle-specific human desmin gene control region, mdx mouse muscle stably expressed dystrophin in 1-5% of the myofibers of the injected hind limb for at least 6 months. This expression generated an antibody response but no apparent cellular response.

Initial failure in myoblast transplantation therapy has led the way toward the isolation of muscle stem cells: potential for tissue regeneration

Myoblast transfer therapy can restore dystrophin expressing myofibers in mdx mice and patients with Duchenne muscular dystrophy (DMD). However, the effectiveness of this technique is hindered by numerous limitations, including minimal distribution of cells after injection, immune rejection, and poor cell survival. Initial studies revealed that only a small population of cells was responsible for muscle regeneration. Compared with myoblast transplantation, the injection of a population of myogenic cells purified with the pre-plate technique results in a superior regeneration of dystrophin-expressing myofibers. These postnatal muscle-derived stem cells (MDSC) undergo self-renewal, display long-term proliferation, and differentiate into multiple lineages. This review examines the initial obstacles encountered in myoblast transplantation, the regenerative properties of MDSC, and the potential use of these stem cells not only for DMD therapy but also for multiple applications, including bone repair and blood reconstitution.

Recombinant adeno-associated viral (rAAV) vectors as therapeutic tools for Duchenne muscular dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a lethal genetic muscle disorder caused by recessive mutations in the dystrophin gene. The size of the gene (2.4 Mb) and mRNA (14 kb) in addition to immunogenicity problems and inefficient transduction of mature myofibres by currently available vector systems are formidable obstacles to the development of efficient gene therapy approaches. Adeno-associated viral (AAV) vectors overcome many of the problems associated with other vector systems (nonpathogenicity and minimal immunogenicity, extensive cell and tissue tropism) but accommodate limited transgene capacity (<5 kb). As a result of these observations, a number of laboratories worldwide have engineered a series of microdystrophin cDNAs based on genotype-phenotype relationship in Duchenne (DMD) and Becker (BMD) dystrophic patients, and transgenic studies in mdx mice. Recent progress in characterization of AAV serotypes from various species has demonstrated that alternative AAV serotypes are far more efficient in transducing muscle than the traditionally used AAV2. This article summarizes the current progress in the field of recombinant adeno-associated viral (rAAV) delivery for DMD, including optimization of recombinant AAV-microdystrophin vector systems/cassettes targeting the skeletal and cardiac musculature.

Phase I Study of Dystrophin Plasmid-Based Gene Therapy in Duchenne/Becker Muscular Dystrophy

Nine patients with Duchenne or Becker muscular dystrophy were injected via the radialis muscle with a full-length human dystrophin plasmid, either once with 200 or 600 microg of DNA or twice, 2 weeks apart, with 600 microg of DNA. In the biopsies taken 3 weeks after the initial injection, the vector was detected at the injection site in all patients. Immunohistochemistry and nested reverse transcription-polymerase chain reaction indicated dystrophin expression in six of nine patients. The level of expression was low (up to 6% weak, but complete sarcolemmal dystrophin staining, and up to 26% partial sarcolemmal labeling). No side effects were observed, nor any cellular or humoral anti-dystrophin responses. These results suggest that exogenous dystrophin expression can be obtained in Duchenne/Becker patients after intramuscular transfer of plasmid, without adverse effects, hence paving the way for future developments in gene therapy of hereditary muscular diseases.

Long-term expression of full-length human dystrophin in transgenic mdx mice expressing internally deleted human dystrophins

One of the possible therapies for Duchenne muscular dystrophy (DMD) is the introduction of a functional copy of the dystrophin gene into the patient. For this approach to be effective, therapeutic levels and long-term expression of the protein need to be achieved. However, immune responses to the newly expressed dystrophin have been predicted, particularly in DMD patients who express no dystrophin or only very truncated versions. In a previous study, we demonstrated a strong humoral and cytotoxic immune response to human dystrophin in the mdx mouse. However, the mdx mouse was tolerant to murine dystrophin, possibly due to the endogenous expression of dystrophin in revertant fibres or the other nonmuscle dystrophin isoforms. In the present study, we delivered human and murine dystrophin plasmids by electrotransfer after hyaluronidase pretreatment to increase gene transfer efficiencies. Tolerance to murine dystrophin was still seen with this improved gene delivery. Tolerance to exogenous recombinant full-length human dystrophin was seen in mdx transgenic lines expressing internally deleted versions of human dystrophin. These results suggest that the presence of revertant fibres may prevent the development of serious immune responses in patients undergoing dystrophin gene therapy.

Long-term transgene expression in cardiac and skeletal muscle following fetal administration of adenoviral or adeno-associated viral vectors in mice

BACKGROUND

In utero gene transfer may provide advantages for the correction of congenital genetic disorders. In the present study we compare the ability of adenovirus (AdCMVLacZ), and two serotypes of adeno-associated virus (AAVCMVLacZ serotypes 2 and 2/5), to target cardiac and skeletal muscle after prenatal systemic or intramuscular injection in mice and assess the immune response to the vectors.

METHODS

Day 14 gestation fetal mice underwent direct intraperitoneal or intramuscular injection of AdCMVLacZ, and AAVCMVLacZ serotypes 2 and 2/5 vectors. Tissues were processed for beta-galactosidase expression in frozen or high-resolution thin plastic sections at early and late time points. Neutralizing antibodies to Ad and AAV were analyzed in separate fetal experimental and neonatal or adult control groups after administration and re-administration of the vectors.

RESULTS

A single injection of each vector in utero resulted in sustained expression of beta-galactosidase transgene in skeletal and cardiac muscle. Transgene expression was detected for the length of the study, i.e. 86, 58, and 31 weeks after birth for AdCMVLacZ, and AAVCMVLacZ serotypes 2 and 2/5, respectively. High-level expression in the myocardium was observed independent of the vector or route of administration. Neutralizing antibody responses to AAV and Ad antigens were reduced and long-term expression in muscle was not ablated on postnatal re-administration of vector.

CONCLUSIONS

Sustained, high-level cardiac and skeletal muscle transgene expression can be obtained after prenatal gene transfer with each of these vectors. The potential for immune response to viral antigens is altered, but not entirely ablated after in utero exposure.

Enhanced in vivo delivery of antisense oligonucleotides to restore dystrophin expression in adult mdx mouse muscle

The use of antisense oligonucleotides (AOs) to induce exon skipping leading to generation of an in-frame dystrophin protein product could be of benefit in around 70% of Duchenne muscular dystrophy patients. We describe the use of hyaluronidase enhanced electrotransfer to deliver uncomplexed 2'-O-methyl modified phosphorothioate AO to adult dystrophic mouse muscle, resulting in dystrophin expression in 20-30% of fibres in tibialis anterior muscle after a single injection. Although expression was transient, many of the corrected fibres initially showed levels of dystrophin expression well above the 20% of endogenous previously shown to be necessary for phenotypic correction of the dystrophic phenotype.

The Hopes and Fears of In Utero Gene Therapy for Genetic Disease—A Review

Somatic gene delivery in utero is a novel approach to gene therapy for genetic disease. It is based on the concept that application of gene therapy vectors to the fetus in utero may prevent the development of early disease related tissue damage, may allow targeting of otherwise inaccessible organs, tissues and still expanding stem cell populations and may also provide postnatal tolerance against the therapeutic transgenic protein. This review outlines the hypothesis and scientific background of in utero gene therapy and addresses some of the frequently expressed concerns raised by this still experimental, potentially preventive gene therapy approach. We describe and discuss the choice of vectors, of animal models and routes of administration to the fetus. We address potential risk factors of prenatal gene therapy such as vector toxicity, inadvertent germ line modification, developmental aberration and oncogenesis as well as specific risks of this procedure for the fetus and mother and discuss their ethical implications.

Identification of an HLA-A*0201-restricted epitopic peptide from human dystrophin: application in duchenne muscular dystrophy gene therapy

Dystrophin-based gene therapy treatments aimed at correcting the Duchenne muscular dystrophy phenotype require stable expression of normal dystrophin (DYST) protein in myocytes without immune responses, which would compromise long-term expression. To predict cytotoxic T-cell-mediated responses elicited by transgenes, we used here H-2-negative HLA-A*0201 transgenic mice and identified human DYST epitopes, which elicit HLA-A*0201-restricted cytotoxic T cell activities. Among a series of eight peptides predicted from the human DYST sequence, not shared with the endogenous mouse DYST sequence, four of them were able to bind to HLA-A*0201 molecules and to induce cytotoxic T lymphocyte (CTL) responses. After human DYST DNA transfer in muscle of HLA-A*0201 mice, only the human DYST1281 epitope, located in the spectrin-like repeat 9 domain, induced strong CD8(+) CTL responses. Using the corresponding human DYST1281 peptide/HLA-A*0201 tetramer, we detected human DYST1281-specific CD8(+) T cells in peripheral lymphoid organs and blood of HLA-A*0201 mice injected with human DYST DNA. Our results demonstrate that muscle injection with human DYST DNA systematically triggers CTL responses against this HLA-A*0201-restricted human DYST1281 peptide, which is present in long human DYST isoforms. Identification of such immunodominant human DYST epitopes and use of peptide/HLA tetramers will allow the immunomonitoring of CTL responses in HLA-phenotyped Duchenne muscular dystrophy patients undergoing gene therapy. Finally, the knowledge of HLA-A*0201-restricted human DYST peptides will be of importance to test, in mouse models, new immunomodulatory interventions allowing long-term engraftment of human dystrophin.

Expression of dystrophin driven by the 1.35-kb MCK promoter ameliorates muscular dystrophy in fast, but not in slow muscles of transgenic mdx mice

Successful gene therapy of Duchenne muscular dystrophy may require the lifelong expression of a therapeutic gene in all affected muscles. The most promising gene delivery vehicles, viral vectors, suffer from several limitations, including immunogenicity, loss of therapeutic gene expression, and a limited packaging capacity. Therefore, various efforts were previously undertaken to use small therapeutic genes and to place them under the control of a strong and muscle-specific promoter. Here we report the effects of a minidystrophin (6.3 kb) under the control of a short muscle-specific promoter (MCK 1.35 kb) over most of the lifetime (4-20 months) of a transgenic mouse model. Dystrophin expression remained stable and muscle-specific at all ages. The dystrophic phenotype was greatly ameliorated and, most importantly, muscle function in limb muscles was significantly improved not only in young adult but also in aged mice compared to nontransgenic littermates. Dystrophin expression was strong in fast-twitch skeletal muscles such as tibialis anterior and extensor digitorum longus, but weak or absent in heart, diaphragm, and slow-twitch muscles. Additionally, expression was strong in glycolytic but weak in oxidative fibers of fast-twitch muscles. This study may have important implications for the design of future gene therapy trials for muscular dystrophy.

Dystrophic phenotype of canine X-linked muscular dystrophy is mitigated by adenovirus-mediated utrophin gene transfer

Utrophin is highly homologous and structurally similar to dystrophin, and in gene delivery experiments in mdx mice was able to functionally replace dystrophin. We performed mini-utrophin gene transfer in Golden Retriever dogs with canine muscular dystrophy (CXMD). Unlike the mouse model, the clinicopathological phenotype of CXMD is similar to that of Duchenne muscular dystrophy (DMD). We injected an adenoviral vector expressing a synthetic utrophin into tibialis anterior muscles of newborn dogs affected with CXMD and examined transgene expression by RNA and protein analysis at 10, 30 and 60 days postinjection in cyclosporin-treated and -untreated animals. Immunosuppression by cyclosporin was required to mitigate the immune response to viral and transgene antigens. RT-PCR analysis showed the presence of the exogenous transcript in the muscle of cyclosporin-treated and -untreated animals. The transgenic utrophin was efficiently expressed at the extrajunctional membrane in immunosuppressed dogs and this expression was stable for at least 60 days. We found reduced fibrosis and increased expression of dystrophin-associated proteins (DAPs) in association with muscle areas expressing the utrophin minigene, indicating that mini-utrophin can functionally compensate for lack of dystrophin in injected muscles. For this reason, utrophin transfer to dystrophin-deficient muscle appears as a promising therapeutic approach to DMD.