Correction of the Dystrophic Phenotype byIn VivoTargeting of Muscle Progenitor Cells

Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by viral fusogenic proteins that drive membrane rearrangements needed for fusion between viral and target membranes. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens but do not structurally or functionally resemble classical viral fusogens. We asked whether the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver μDystrophin to skeletal muscle of a mouse model of Duchenne muscular dystrophy and alleviate pathology. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by fusogenic proteins that form a complex between membranes to drive rearrangements needed for fusion. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens, but do not structurally or functionally resemble classical viral fusogens. We asked if the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver micro-Dystrophin (μDys) to skeletal muscle of a mouse model of Duchenne muscular dystrophy. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

Targeting Muscle-Resident Single Cells Through in vivo Electro-Enhanced Plasmid Transfer in Healthy and Compromised Skeletal Muscle

Skeletal muscle is composed of syncytial muscle fibers, and by various mononucleated cellular types, such as muscle stem cells, immune cells, interstitial and stromal progenitors. These cell populations play a crucial role during muscle regeneration, and alterations of their phenotypic properties have been associated with defective repair and fibrosis in aging and dystrophic muscle. Studies involving in vivo gene modulation are valuable to investigate the mechanisms underlining cell function and dysfunction in complex pathophysiological settings. Electro-enhanced transfer of plasmids using square-wave generating devices represents a cost-effective approach that is widely used to transport DNA to muscle fibers efficiently. Still, it is not clear if this method can also be applied to mononuclear cells present in muscle. We demonstrate here that it is possible to efficiently deliver DNA into different muscle–resident cell populations in vivo. We evaluated the efficiency of this approach not only in healthy muscle but also in muscles of aging and dystrophic animal models. As an exemplificative application of this method, we used a strategy relying on a reporter gene-based plasmid containing regulatory sequences from the collagen 1 locus, and we determined collagen expression in various cell types reportedly involved in the production of fibrotic tissue in the dystrophic settings. The results enclosed in this manuscript reveal the suitability in applying electro-enhanced transfer of plasmid DNA to mononucleated muscle-resident cells to get insights into the molecular events governing diseased muscle physiology.

Nanotherapy for Duchenne muscular dystrophy

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

Duchenne muscular dystrophy: current cell therapies

Duchenne muscular dystrophy is a genetically determined X-linked disease and the most common, progressive pediatric muscle disorder. For decades, research has been conducted to find an effective therapy. This review presents current therapeutic methods for Duchenne muscular dystrophy, based on scientific articles in English published mainly in the period 2000 to 2014. We used the PubMed database to identify and review the most important studies. An analysis of contemporary studies of stem cell therapy and the use of granulocyte colony-stimulating factor (G-CSF) in muscular dystrophy was performed.

Ongoing therapeutic trials and outcome measures for Duchenne muscular dystrophy

Muscular dystrophy is a heterogeneous group of genetic disorders characterised by progressive muscle tissue degeneration. No effective treatment has been discovered for these diseases. Preclinical and clinical studies aimed at the development of new therapeutic approaches have been carried out, primarily in subjects affected with dystrophinopathies (Duchenne and Becker muscular dystrophy). In this review, we outline the current therapeutic approaches and past and ongoing clinical trials, highlighting both the advantages and limits of each one. The experimental designs of these trials were based on different rationales, including immunomodulation, readthrough strategies, exon skipping, gene therapy, and cell therapy. We also provide an overview of available outcome measures, focusing on their reliability in estimating meaningful clinical improvement in order to aid in the design of future trials. This perspective is extremely relevant to the field considering the recent development of novel therapeutic approaches that will result in an increasing number of clinical studies over the next few years.

Strength and muscle specificity of a compact promoter derived from the slow troponin I gene in the context of episomal (gutless adenovirus) and integrating (lentiviral) vectors

BACKGROUND

Gutless adenovirus (helper-dependent adenoviral vector; HDAd) and lentiviral vectors (LV) are attractive vectors for the gene therapy of muscle diseases. Because the organization of their DNA (episomal versus integrated) differs, we investigated whether the strength and specificity of ΔUSEx3, a novel muscle-specific promoter previously tested with plasmid, were maintained in the context of these vectors.

METHODS

Two HDAds expressing β-galactosidase regulated by ΔUSEx3 or CAG [cytomegalovirus (CMV) enhancer/β-actin promoter], and three LV expressing green fluorescent protein regulated by ΔUSEx3, CMV or a modified skeletal α-actin promoter (SPcΔ5-12), were constructed. Gene expression was compared in cell culture and after intravenous (HDAd only) and intramuscular injection of mice.

RESULTS

Irrespective of the vector used, ΔUSEx3 remained poorly active in nonmuscle cells and tissues. In myotubes, ΔUSEx3 was as strong as CMV and SPcΔ5-12, although it was ten-fold weaker than CAG, a proven powerful promoter in muscle. In cell culture, ΔUSEx3 activity in the context of LV was more stable than CMV, indicating it is less prone to silencing. In the context of HDAd, the behavior of ΔUSEx3 in skeletal muscle mirrored that of cell culture (10% of the CAG activity and half the number of transduced fibers). Surprisingly, in muscles treated with LV, ΔUSEx3 activity was five-fold lower than SPcΔ5-12.

CONCLUSIONS

The data obtained in the present study confirm that ΔUSEx3 is a strong and robust muscle-specific promoter in the context of HDAd (cell culture and in vivo) and LV (cell culture). However, it was less efficient in vivo in the context of LV.

Repair or replace? Exploiting novel gene and cell therapy strategies for muscular dystrophies

Muscular dystrophies are genetic disorders characterized by skeletal muscle wasting and weakness. Although there is no effective therapy, a number of experimental strategies have been developed over recent years and some of them are undergoing clinical investigation. In this review, we highlight recent developments and key challenges for strategies based upon gene replacement and gene/expression repair, including exon-skipping, vector-mediated gene therapy and cell therapy. Therapeutic strategies for different forms of muscular dystrophy are discussed, with an emphasis on Duchenne muscular dystrophy, given the severity and the relatively advanced status of clinical studies for this disease.

Cell-matrix interactions in muscle disease

The extracellular matrix (ECM) provides a solid scaffold and signals to cells through ECM receptors. The cell-matrix interactions are crucial for normal biological processes and when disrupted they may lead to pathological processes. In particular, the biological importance of ECM-cell membrane-cytoskeleton interactions in skeletal muscle is accentuated by the number of inherited muscle diseases caused by mutations in proteins conferring these interactions. In this review we introduce laminins, collagens, dystroglycan, integrins, dystrophin and sarcoglycans. Mutations in corresponding genes cause various forms of muscular dystrophy. The muscle disorders are presented as well as advances toward the development of treatment.

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.

Muscle fiber type-predominant promoter activity in lentiviral-mediated transgenic mouse

Variations in gene promoter/enhancer activity in different muscle fiber types after gene transduction was noticed previously, but poorly analyzed. The murine stem cell virus (MSCV) promoter drives strong, stable gene expression in hematopoietic stem cells and several other cells, including cerebellar Purkinje cells, but it has not been studied in muscle. We injected a lentiviral vector carrying an MSCV-EGFP cassette (LvMSCV-EGFP) into tibialis anterior muscles and observed strong EGFP expression in muscle fibers, primary cultured myoblasts, and myotubes isolated from injected muscles. We also generated lentiviral-mediated transgenic mice carrying the MSCV-EGFP cassette and detected transgene expression in striated muscles. LvMSCV-EGFP transgenic mice showed fiber type-dependent variations in expression: highest in types I and IIA, intermediate in type IID/X, and lowest in type IIB fibers. The soleus and diaphragm muscles, consisting mainly of types I and IIA, are most severely affected in the mdx mouse model of muscular dystrophy. Further analysis of this promoter may have the potential to achieve certain gene expression in severely affected muscles of mdx mice. The Lv-mediated transgenic mouse may prove a useful tool for assessing the enhancer/promoter activities of a variety of different regulatory cassettes.

Interventions for muscular dystrophy: molecular medicines entering the clinic

Muscular dystrophies are individually rare genetic disorders that cause much chronic disability, affecting young children and adults. In the past 20 years, more than 30 genetic types of muscular dystrophy have been defined. During this time, precise diagnosis, genetic counselling, and medical management have improved. These advances in medical practice have occurred while definitive therapies based on an improved knowledge of disease pathogenesis are awaited. A wide range of therapeutic options have been tested in animal models, and some are being tested in clinical trials. Various therapeutic targets are being investigated, from personalised medicines targeting specific mutations and drugs targeting cellular pathways to gene-based and cell-based therapies.

Desmin-regulated lentiviral vectors for skeletal muscle gene transfer

Lentiviral vectors (LVs) are highly attractive as a gene therapy agent as they are able to stably integrate their genomes in both dividing and nondividing cells and, in principle, provide long-term therapeutic benefit. However, their performance in skeletal muscle in adult animals has, to date, been disappointing. In order to gain clearer insight into their utility in this tissue type, we have conducted an extensive quantitative comparison of constitutive and muscle-specific promoter activities in skeletal muscle and nonmuscle systems following LV delivery in cell lines and neonatal mice. Our data show that LV delivery to hind leg skeletal muscle of neonatal mouse results in long-term transgene expression in adulthood. We find that the human desmin (DES) promoter/enhancer is the first muscle-specific control region to match the activity of the highly active constitutive human cytomegalovirus (hCMV) promoter/enhancer in skeletal muscle within a LV context both in vitro and in vivo. Furthermore, the DES promoter/enhancer provides six- to eightfold greater expression per viral copy than the muscle-specific human muscle creatine kinase (CKM) promoter/enhancer. DES also confers a more reproducible and tissue-specific transgene expression profile compared to CKM and is therefore a highly attractive regulatory element for use in muscle gene therapy vectors.

Dystrophin delivery to muscles of mdx mice using lentiviral vectors leads to myogenic progenitor targeting and stable gene expression

To explore whether stable transduction of myogenic stem cells using lentiviral vectors could be of benefit for treating dystrophic muscles, we generated vectors expressing a functional microdystrophin/enhanced green fluorescence protein fusion (microDys/eGFP) gene. Lentiviral vector injection into neonatal mdx(4cv) muscles resulted in widespread and stable expression of dystrophin for at least 2 years. This expression resulted in a significant amelioration of muscle pathophysiology as assessed by a variety of histological and functional assays. To assess whether this long-term expression was accompanied by stable transduction of satellite cells, we harvested muscle mononuclear cells 1 year after vector injection. Up to 20% of the cultured myoblast colonies expressed the microDys/eGFP transgene following myotube formation. Furthermore, transplantation of the muscle mononuclear cells into secondary mdx(4cv) recipients showed their ability to regenerate dystrophin-expressing myofibers in vivo. The ability to isolate myogenic cells able to form dystrophin-positive myotubes or myofibers in vitro and in vivo >1 year postinjection indicates that the vectors stably transduced muscle satellite cells, or a progenitor of such cells, in neonatal mdx(4cv) muscles. These studies suggest that integrating lentiviral vectors have potential utility for gene therapy of muscular dystrophy.

Mesenchymal stem cells: emerging therapy for Duchenne muscular dystrophy

Multipotent cells that can give rise to bone, cartilage, fat, connective tissue, and skeletal and cardiac muscle are termed mesenchymal stem cells. These cells were first identified in the bone marrow, distinct from blood-forming stem cells. Based on the embryologic derivation, availability, and various pro-regenerative characteristics, research exploring their use in cell therapy shows great promise for patients with degenerative muscle diseases and a number of other conditions. In this review, the authors explore the potential for mesenchymal stem cell therapy in the emerging field of regenerative medicine with a focus on treatment for Duchenne muscular dystrophy.

In vivo and ex vivo gene transfer in thymocytes and thymocyte precursors

The thymus provides a specialized environment allowing the differentiation of T lymphocytes from bone marrow-derived progenitor cells. We and others have demonstrated that gene transfer into distinct thymocyte populations can be obtained, both in vivo and ex vivo, using lentiviral vectors. Here, we describe techniques for intrathymic lentiviral transduction in mice, using a surgical approach wherein the thoracic cavity is exposed as well as a significantly less invasive strategy wherein virions are directly injected through the skin. Moreover, thymocyte differentiation from murine and human progenitors is now feasible in vitro, under conditions wherein the Notch and IL-7 signaling pathways are activated. We describe methods allowing transduction of murine and human progenitors and their subsequent differentiation into more mature thymocytes. Conditions for lentiviral gene transfer into more differentiated human thymocyte subsets are also presented. Optimization of technologies for HIV-based gene transfer into murine and human thymocyte progenitors will advance strategies aimed at modulating T-cell differentiation and function in-vivo; approaches potentially targeting patients with genetic and acquired immunodeficiencies as well as immune-sensitive tumors. Furthermore, this technology will foster the progression of basic research aimed at elucidating molecular aspects of T-cell differentiation in mice and humans.

Beta and gamma-sarcoglycans are decreased in the detrusor smooth muscle cells of the partially obstructed rabbit bladder

PURPOSE

We evaluated and quantified the levels of sarcoglycans present in the detrusor muscle layer of rabbits with partial bladder outlet obstruction.

MATERIALS AND METHODS

Rabbits underwent surgery, as previously described, to partially obstruct the urethra. One, 3, 7 and 14 days after obstruction the detrusor muscle layer was dissected free of the remaining bladder tissue and extracted with detergent to isolate the transmembrane components of the dystroglycan-glycoprotein complex. Several components of the dystroglycan-glycoprotein complex were characterized and quantified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting.

RESULTS

Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis several bands were noted on gels with a molecular weight (43 and 35 kDa, respectively) corresponding to beta and gamma-sarcoglycan. As obstruction progressed longitudinally, the levels of beta and gamma-sarcoglycan showed progressive decrease at the protein level with beta-sarcoglycan levels recovering at later time points. Bladders with a functional physiology that showed more advanced symptoms of dysfunction had a greater decrease in beta and gamma-sarcoglycan protein.

CONCLUSIONS

The levels of beta and gamma-sarcoglycan progressively change with obstruction with greater changes occurring in the levels of gamma-sarcoglycan. It is likely that alterations in the dystroglycan-glycoprotein complex are responsible for some of the changes in muscle physiology that occur as a consequence of obstruction.

Genetic modification of cells for transplantation

Progress in gene therapy has produced promising results that translate experimental research into clinical treatment. Gene modification has been extensively employed in cell transplantation. The main barrier is an effective gene delivery system. Several viral vectors were utilized in end-stage differentiated cells. Recently, successful applications were described with adenovirus-associated vectors. As an alternative, embryonic stem cell- and stem cell-like systems were established for generation of tissue-specified gene-modified cells. Owing to the feasibility for genetic manipulations and the self-renewing potency of these cells they can be used in a way enabling large-scale in vitro production. This approach offers the establishment of in vitro cell culture systems that will deliver sufficient amounts of highly purified, immunoautologous cells suitable for application in regenerative medicine. In this review, the current technology of gene delivery systems to cells is recapitulated and the latest developments for cell transplantation are discussed.

Autologous transplantation of SM/C-2.6(+) satellite cells transduced with micro-dystrophin CS1 cDNA by lentiviral vector into mdx mice

Duchenne muscular dystrophy (DMD) is a lethal muscle disorder caused by mutations in the dystrophin gene. Transplantation of autologous myogenic cells genetically corrected ex vivo is a possible treatment for this disorder. In order to test the regenerative efficiency of freshly isolated satellite cells, we purified quiescent satellite cells from limb muscles of 8-12-week-old green fluorescent protein-transgenic (GFP-Tg) mice using SM/C-2.6 (a recently developed monoclonal antibody) and flow cytometry. Freshly isolated satellite cells were shown to participate in muscle regeneration more efficiently than satellite cell-derived myoblasts passaged in vitro do, when transplanted into tibialis anterior (TA) muscles of 8-12-week-old cardiotoxin-injected C57BL/6 mice and 5-week-old dystrophin-deficient mdx mice, and analyzed at 4 weeks after injection. Importantly, expansion of freshly isolated satellite cells in vitro without passaging had no detrimental effects on their regenerative capacity. Therefore we directly isolated satellite cells from 5-week-old mdx mice using SM/C-2.6 antibody and cultured them with lentiviral vectors expressing micro-dystrophin CS1. The transduced cells were injected into TA muscles of 5-week-old mdx mice. At 4 weeks after transplantation, the grafted cells efficiently contributed to regeneration of mdx dystrophic muscles and expressed micro-dystrophin at the sarcolemma. These results suggest that there is potential for lentiviral vector-mediated ex vivo gene therapy for DMD.

Upregulation of the creatine synthetic pathway in skeletal muscles of mature mdx mice

Duchenne muscular dystrophy (DMD) is a fatal neuromuscular human disease caused by dystrophin deficiency. The mdx mouse lacks dystrophin protein, yet does not exhibit the debilitating DMD phenotype. Investigating compensatory mechanisms in the mdx mouse may shed new insights into modifying DMD pathogenesis. This study targets two metabolic genes, guanidinoacetate methyltransferase (GAMT) and arginine:glycine amidinotransferase (AGAT) which are required for creatine synthesis. We show that GAMT and AGAT mRNA are up-regulated 5.4- and 1.9-fold respectively in adult mdx muscle compared to C57. In addition, GAMT protein expression is up-regulated at least 2.5-fold in five different muscles of mdx vs. control. Furthermore, we find GAMT immunoreactivity in up to 80% of mature mdx muscle fibers in addition to small regenerating fibers and rare revertants; while GAMT immunoreactivity is equal to background levels in all muscle fibers of mature C57 mice. The up-regulation of the creatine synthetic pathway may help maintain muscle creatine levels and limit cellular energy failure in leaky mdx skeletal muscles. These results may help better understand the mild phenotype of the mdx mouse and may offer new treatment horizons for DMD.

Multiexon skipping leading to an artificial DMD protein lacking amino acids from exons 45 through 55 could rescue up to 63% of patients with Duchenne muscular dystrophy

Approximately two-thirds of Duchenne muscular dystrophy (DMD) patients show intragenic deletions ranging from one to several exons of the DMD gene and leading to a premature stop codon. Other deletions that maintain the translational reading frame of the gene result in the milder Becker muscular dystrophy (BMD) form of the disease. Thus the opportunity to transform a DMD phenotype into a BMD phenotype appeared as a new treatment strategy with the development of antisense oligonucleotides technology, which is able to induce an exon skipping at the pre-mRNA level in order to restore an open reading frame. Because the DMD gene contains 79 exons, thousands of potential transcripts could be produced by exon skipping and should be investigated. The conventional approach considers skipping of a single exon. Here we report the comparison of single- and multiple-exon skipping strategies based on bioinformatic analysis. By using the Universal Mutation Database (UMD)-DMD, we predict that an optimal multiexon skipping leading to the del45-55 artificial dystrophin (c.6439_8217del) could transform the DMD phenotype into the asymptomatic or mild BMD phenotype. This multiple-exon skipping could theoretically rescue up to 63% of DMD patients with a deletion, while the optimal monoskipping of exon 51 would rescue only 16% of patients.

X-chromosome linked inhibitor of apoptosis protein inhibits muscle proteolysis in insulin-deficient mice

Loss of muscle protein is a serious complication of catabolic diseases and contributes substantially to patients' morbidity and mortality. This muscle loss is mediated largely by the activation of the ubiquitin-proteasome system; however, caspase-3 catalyzes an initial step in this process by cleaving actomyosin into small protein fragments that are rapidly degraded by the proteasome-dependent proteolytic pathway. We hypothesized that X-chromosome linked inhibitor of apoptosis protein (XIAP), an endogenous caspase-3 inhibitor, would block this first step in the cleavage of actomyosin that would make XIAP a candidate for treating muscle wasting. To determine if XIAP could attenuate muscle protein degradation, we used a recombinant lentivirus (Len-XIAP) encoding the full-length human XIAP cDNA to express XIAP in vivo. In muscle of streptozotocin-treated insulin-deficient mice, total muscle protein degradation, caspase-3 activity, and myofibril destruction were increased while XIAP was decreased. Overexpression of XIAP in these mice attenuated the excessive muscle protein degradation. Increased proteasome activity, caspase-3 activity and myofibril protein breakdown were all reduced. The ability of XIAP to prevent the loss of muscle protein suggests that XIAP could be a therapeutic reagent for muscle atrophy in catabolic diseases.

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.

Autologous Transplantation of Muscle Precursor Cells Modified with a Lentivirus for Muscular Dystrophy: Human Cells and Primate Models

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. We tested the ability of lentiviral vectors to deliver a transgene into myogenic cells before their transplantation. Enhanced green fluorescent protein (eGFP) transgene was efficiently transferred into cells and eGFP-positive fibers were generated following transplantation. An eGFP-micro-dystrophin transgene under the control of a cytomegalovirus promoter was then transferred with the same viral vector but caused some toxicity to the mono-nucleated cells. We then used instead a muscle creatine kinase promoter. Dystrophin expression was observed in the muscle fibers after the transplantation of such genetically modified cells into mdx and severe combined immunodeficient mice. Micro-dystrophin expression was also observed in monkey muscles a month after allogenic or autologous transplantation of genetically modified myoblasts. Therapeutic exon skipping was induced by infecting myoblasts of a DMD patient, deleted for dystrophin exons 49 and 50, with a lentivirus expressing a U7 small nuclear RNA containing antisense sequences against exon 51. The modification led to correct exon skipping and to the expression of a quasi-dystrophin in vitro and in vivo. These results demonstrate the feasibility of lentiviral-based ex vivo gene therapy for DMD.

Gene therapy for Duchenne muscular dystrophy

Gene therapy has great potential to treat Duchenne muscular dystrophy. Among many proposed strategies to deliver a therapeutic gene to muscle, recombinant adeno-associated virus-mediated gene transfer is the most promising. The recent isolation of new adeno-associated virus serotypes from human and nonhuman primates provides the opportunity to develop vectors that can achieve the long-term expression of a therapeutic gene in muscles of the entire body without detrimental effects. To translate the results from small animal models to clinical trials in humans, further work using larger animal models, such as dystrophic dogs or nonhuman primates, is required. This review also discusses recent progress in other gene transfer-related therapeutic approaches, including targeted exon skipping and gene correction.

Dystrophin expression in host muscle following transplantation of muscle precursor cells modified with the phiC31 integrase

Duchenne muscular dystrophy (DMD) is the most severe muscular dystrophy. It is caused by the absence of dystrophin in muscle fibers. The autologous transplantation of genetically corrected muscle precursor cells (MPCs) is a possible cure for DMD. A non-viral method of genetic modification was tested in this study. The co-transfection (nucleofection) of a phiC31 integrase and a transgene expressing plasmid in MPCs led to an increased stable expression in vitro. The stable expression of a small transgene (eGFP) in muscle fibers was initially demonstrated following the transplantation of the genetically modified cells. The stable expression of a truncated version of dystrophin as well as the full-length dystrophin fused with eGFP was then demonstrated in MPCs obtained from an mdx mice. The transplantation of these cells led not only to the expression of these fusion proteins in muscle fibers but also to the reconstitution of the dystrophin complex. Human MPCs were also genetically modified with a plasmid coding for the full-length human dystrophin gene fused with eGFP and transplanted in severe combined immuno deficient mice leading to the expression of eGFP dystrophin in muscle fibers. This work indicates that cell transplantation after correction of MPCs with phiC31 integrase is a possible approach to treat DMD.

Therapeutic restoration of dystrophin expression in Duchenne muscular dystrophy

It is 20 years since the discovery of the genetic defect causing Duchenne muscular dystrophy (DMD). This X-linked progressive and fatal myopathy is due to the absence of a functional version of a critical sub-sarcolemmal protein called dystrophin that appears to act both as a structural and as a signalling molecule in the muscle fibre. A number of molecular approaches have been developed to restore the expression of dystrophin in DMD patients. Pre-clinical experiments have demonstrated the potential of delivery of recombinant versions of the DMD gene using viral or non-viral vectors and importantly several of these systems are compatible with vascular delivery, an essential feature as all muscles are affected in this condition. Other studies have shown that antisense oligonucleotides can modify the splicing of the primary transcript to provide an internally truncated but still functional protein. Alternatively, in approximately 10-20% of cases it is possible to chemically persuade the translational machinery to read-through a pre-mature stop codon. The pre-clinical results of the last 4 years have encouraged the development of clinical trials for all of the above.

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

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

Lentiviral vectors

We review the use of lentiviral vectors in current human gene therapy applications that involve genetic modification of nondividing tissues with integrated transgenes. Safety issues, including insertional mutagenesis and replication-competent retroviruses, are discussed. Innate cellular defenses against retroviruses and their implications for human gene therapy with different lentiviral vectors are also addressed.

Stable transduction of myogenic cells with lentiviral vectors expressing a minidystrophin

Gene therapy for Duchenne muscular dystrophy (DMD) will require sustained expression of therapeutic dystrophins in striated muscles. Lentiviral vectors have a relatively large transgene carrying capacity and can integrate into nondividing cells. We therefore explored the use of lentiviral vectors for transferring genes into mouse skeletal muscle cells. These vectors successfully transferred a minidystrophin expression cassette into mdx muscles, and minidystrophin expression persisted and prevented subsequent muscle fiber degeneration for at least 6 months. However, only low to moderate levels of skeletal muscle transduction could be obtained by intramuscular injection of the highest currently available lentiviral doses. Using cultured cells, the lentiviral vectors effectively transduced proliferating and terminally differentiated muscle cells, indicating that cell cycling is not essential for transduction of myogenic cells. We further showed that lentiviral vectors efficiently transduced both primary myoblasts and multipotent adult progenitor cells (MAPCs) in vitro, and the cells persistently expressed transgenes without any obvious toxicity. When mdx primary myoblasts were genetically modified with minidystrophin vectors and transplanted into mdx skeletal muscles, significant numbers of dystrophin-expressing myofibers formed. Finally, we showed that a short, highly active CK6 regulatory cassette directed muscle-specific activity in the context of the lentiviral vectors. The ability of lentiviral vectors to transduce myogenic progenitors using a minidystrophin cassette regulated by a muscle-specific promoter suggests that this system could be useful for ex vivo gene therapy of muscular dystrophy.

In vivo correction of ZAP-70 immunodeficiency by intrathymic gene transfer

SCID patients have been successfully treated by administration of ex vivo gene-corrected stem cells. However, despite its proven efficacy, such treatment carries specific risks and difficulties. We hypothesized that some of these drawbacks may be overcome by in situ gene correction of T lymphoid progenitors in the thymus. Indeed, in vivo intrathymic transfer of a gene that provides a selective advantage for transduced prothymocytes should result in the generation of functional T lymphocyte progeny, allowing long-term immune reconstitution. We assessed the feasibility of this approach in a murine model of ZAP-70-deficient SCID. A T cell-specific ZAP-70-expressing lentiviral vector was injected into thymi of adult ZAP-70-/- mice without prior conditioning. This resulted in the long-term differentiation of mature TCR-alphabeta+ thymocytes, indicating that the vector had integrated into progenitor cells. Moreover, peripheral ZAP-70-expressing T cells demonstrated a partially diversified receptor repertoire and were responsive to alloantigens in vitro and in vivo. Improved treatment efficacy was achieved in infant ZAP-70-/- mice, in which the thymus is proportionately larger and a higher percentage of prothymocytes are in cycle. Thus, intrathymic injection of a lentiviral vector could represent a simplified and potentially safer alternative to ex vivo gene-modified hematopoietic stem cell transplantation for gene therapy of T cell immunodeficiencies.

Newborn liver gene transfer by an HIV-2-based lentiviral vector

Newborn gene therapy, because it can prevent the damage caused by the onset of a disease, deserves specific attention. To evaluate gene transfer in tissues of newborn mice, we used a human immunodeficiency virus (HIV)-2 based lentiviral vector pseudotyped with vesicular stomatitis virus G glycoprotein expressing the green fluorescent protein reporter gene under the control of the cytomegalovirus promoter. We found that very low doses of HIV-2 could infect and be expressed in newborn mice. Under these conditions, the virus was preferentially expressed in the liver and hepatocytes were the predominant target. The treatment was not toxic, the infected liver cells proliferated and the transduced gene was stably expressed. Adult mice could be infected by HIV-2, but the vector was detected in the liver only utilizing the sensitive method of polymerase chain reaction coupled with Southern blot. Direct comparison between newborn and adult recipients demonstrated a much greater efficiency of liver transduction in the newborn mouse. These results indicate that the combination of early intervention and low multiplicity of infection may be a strategy for preferentially and efficiently targeting newborn liver for gene therapy applications.

Gene transfer into skeletal muscle using novel AAV serotypes

BACKGROUND

Skeletal muscle is an interesting target for gene delivery because of its mass and because the vectors can be delivered in a noninvasive way. Adeno-associated virus (AAV) vectors are capable of transducing skeletal muscle fibers and achieving stable and safe transgene expression. To date, most animal experiments using AAV have been based on AAV serotype 2, but some recent studies have demonstrated that AAV1 is more efficient than AAV2/2 in transducing muscle fibers. Recently, novel AAVs (AAV7 and AAV8) were isolated from rhesus macaques.

METHODS

We injected three different muscles (gastrocnemius, soleus, biceps femoris) of immunocompetent C57BL/6 mice with different pseudotyped AAV serotypes (AAV2/1, AAV2/2, AAV2/5, AAV2/7 and AAV2/8) and quantitatively compared the different gene transfer efficiencies.

RESULTS

The efficiencies of transduction in skeletal muscle with AAV2/7 and AAV2/8 were similar to AAV2/1, and higher than that seen with AAV2/2 and AAV2/5. All serotypes were able to transduce both slow and fast muscle fibers similarly at the vector titer used (1x10(11) genome copies per mouse). Despite a limited inflammatory response (slightly higher when using AAV2/2, AAV2/7 and AAV2/8 vectors than AAV2/1 and AAV2/5), transgene expression was observed throughout the length of the experiment.

DISCUSSION

These results show that AAV2/7 and AAV2/8 are able to transduce muscle fibers of immunocompetent mice very efficiently, offering new perspectives in gene transfer of skeletal muscle.

Gene transfer for neurologic disease

Advances in molecular biology have contributed to a growing interest in gene therapy as a form of management for neurologic diseases. However, implementation requires knowledge of the regulatory policies governing this field of research, especially in view of the greater stringency imposed by the serious adverse events affecting some patients participating in gene therapy protocols. Educational resources for neurologists, or any clinicians, who hope to serve as potential principal investigators for a gene therapy protocol are not available through any single source, requiring considerable effort to discover appropriate guidance. Summarized here are the regulatory agencies and their requirements, the phases of clinical development with emphasis on a Phase I study, and specific steps leading to an Investigational New Drug application for a biologic product to be used in a gene therapy clinical trial. The links provided to all appropriate Web sites will facilitate the process for the clinician investigator.

Myotubes differentiate optimally on substrates with tissue-like stiffness: pathological implications for soft or stiff microenvironments

Contractile myocytes provide a test of the hypothesis that cells sense their mechanical as well as molecular microenvironment, altering expression, organization, and/or morphology accordingly. Here, myoblasts were cultured on collagen strips attached to glass or polymer gels of varied elasticity. Subsequent fusion into myotubes occurs independent of substrate flexibility. However, myosin/actin striations emerge later only on gels with stiffness typical of normal muscle (passive Young's modulus, E approximately 12 kPa). On glass and much softer or stiffer gels, including gels emulating stiff dystrophic muscle, cells do not striate. In addition, myotubes grown on top of a compliant bottom layer of glass-attached myotubes (but not softer fibroblasts) will striate, whereas the bottom cells will only assemble stress fibers and vinculin-rich adhesions. Unlike sarcomere formation, adhesion strength increases monotonically versus substrate stiffness with strongest adhesion on glass. These findings have major implications for in vivo introduction of stem cells into diseased or damaged striated muscle of altered mechanical composition.

Highly efficient EIAV-mediated in utero gene transfer and expression in the major muscle groups affected by Duchenne muscular dystrophy

Gene therapy for Duchenne muscular dystrophy has so far not been successful because of the difficulty in achieving efficient and permanent gene transfer to the large number of affected muscles and the development of immune reactions against vector and transgenic protein. In addition, the prenatal onset of disease complicates postnatal gene therapy. We have therefore proposed a fetal approach to overcome these barriers. We have applied beta-galactosidase expressing equine infectious anaemia virus (EIAV) lentiviruses pseudotyped with VSV-G by single or combined injection via different routes to the MF1 mouse fetus on day 15 of gestation and describe substantial gene delivery to the musculature. Highly efficient gene transfer to skeletal muscles, including the diaphragm and intercostal muscles, as well as to cardiac myocytes was observed and gene expression persisted for at least 15 months after administration of this integrating vector. These findings support the concept of in utero gene delivery for therapeutic and long-term prevention/correction of muscular dystrophies and pave the way for a future application in the clinic.

Sustained Improvement of Muscle Function One Year After Full-Length Dystrophin Gene Transfer intomdxMice by a Gutted Helper-Dependent Adenoviral Vector

Dystrophin gene transfer using helper-dependent adenoviral vectors (HDAd) deleted of all viral genes is a promising option to treat muscles in Duchenne muscular dystrophy (DMD). Previously, we reported high-level dystrophin expression and functional correction of dystrophin-deficient (mdx) mouse muscle 60 days after gene transfer with an HDAd encoding two full-length murine dystrophin cDNAs (referred to as HDCBDysM). In the present study, we tested the long-term efficacy of HDCBDysM by examining muscle contractility parameters and the stability of dystrophin expression 1 year after injection into neonatal mdx muscles. At this point, HDCBDysM-treated muscles averaged 52% dystrophin-positive fibers. Treated muscles also displayed significantly greater isometric force production as well as greater resistance to the force deficits and damage caused by eccentric contractions. The level of protection against eccentric contraction-induced force deficits correlated with the percentage of dystrophin-positive fibers. Furthermore, HDCBDysM treatment restored the dystrophin-glycoprotein complex (DGC) to the sarcolemma and improved other aspects of mdx muscle histopathology examined (central nucleation, muscle hypertrophy, and mononuclear [phagocytic] cell infiltration). These improvements occurred despite the induction of a humoral response against murine dystrophin. Our results indicate that major therapeutic benefits of HDCBDysM are maintained for a long period of the animals' lifespan and suggest that HDCBDys holds promise for treating DMD by gene therapy.