Rescue of skeletal muscles of gamma-sarcoglycan-deficient mice with adeno-associated virus-mediated gene transfer

NFI-C2 negatively regulates α-sarcoglycan promoter activity in C2C12 myoblasts

alpha-Sarcoglycan striated muscle-specific protein is a member of the sarcoglycan-sarcospan complex. Positive and negative transcriptional regulation of sarcoglycan genes are important in sarcoglycan's intracellular localization and sarcolemmal stability. In the present work we assessed the function of NFI transcription factors in the regulation of alpha-sarcoglycan promoter through the C2C12 cell line differentiation. NFI factors act alternatively as activators and negative modulators of alpha-sarcoglycan promoter activity. In myoblasts NFI-A1.1 and NFI-B2 are activators, whereas NFI-C2 and NFI-X2 are negative regulators. In myotubes, all NFI members are activators, being NFI-C2 the less potent. We identified the alpha-sarcoglycan promoter NFI-C2 response element by testing progressive deletion constructs and point mutations in C2C12 cells over-expressing NFI-C2. Gel-shift and chromatin immunoprecipitation experiments demonstrated that NFI factors are indeed interacting in vitro and in vivo with the binding sequence. These results suggest a NFI role in C2C12 cell differentiation.

Efficient and long-term intracardiac gene transfer in delta-sarcoglycan-deficiency hamster by adeno-associated virus-2 vectors

Intracardiac gene transfer and gene therapy have been investigated with different vector systems. Here we used adeno-associated virus (AAV) vectors to deliver either a reporter gene or a therapeutic gene into the heart of golden Syrian hamsters. The method of gene delivery was direct infusion of the AAV2 vectors into the coronary artery ex vivo in a heterotopically transplanted heart. When an AAV2 vector carrying the Lac-Z gene driven by CMV promoter was delivered into the heart of healthy hamsters, effective gene transfer was achieved in up to 90% of the cardiomyocytes. Lac-Z gene expression persisted for more than 1 year without immune rejection or promoter shutoff. Furthermore, when an AAV2 vector carrying human delta-sarcoglycan gene was similarly delivered into the heart of Bio14.6 Syrian hamster, a congestive heart failure and limb girdle muscular dystrophy animal model, widespread therapeutic gene transfer was achieved in a majority of the cardiomyocytes. Efficient expression of the human delta-sarcoglycan gene in the dystrophic hamster hearts restored the entire sarcoglycan complex that was missing due to the primary deficiency of delta-sarcoglycan. Transgene expression persisted for 4 months (the duration of the study) without immune rejection or promoter shutoff. These results indicate that AAV is a promising vector system for cardiac gene therapy.

Tetracycline-inducible system for regulation of skeletal muscle-specific gene expression in transgenic mice

Tightly regulated control of over-expression is often necessary to study one aspect or time point of gene function and, in transgenesis, may help to avoid lethal effects and complications caused by ubiquitous over-expression. We have utilized the benefits of an optimized tet-on system and a modified muscle creatine kinase (MCK) promoter to generate a skeletal muscle-specific, doxycycline (Dox) controlled over-expression system in transgenic mice. A DNA construct was generated in which the codon optimized reverse tetracycline transactivator (rtTA) was placed under control of a skeletal muscle-specific version of the mouse MCK promoter. Transgenic mice containing this construct expressed rtTA almost exclusively in skeletal muscles. These mice were crossed to a second transgenic line containing a bi-directional promoter centered on a tet responder element driving both a luciferase reporter gene and a tagged gene of interest; in this case the calpain inhibitor calpastatin. Compound hemizygous mice showed high level, Dox dependent muscle-specific luciferase activity often exceeding 10,000-fold over non-muscle tissues of the same mouse. Western and immunocytochemical analysis demonstrated similar Dox dependent muscle-specific induction of the tagged calpastatin protein. These findings demonstrate the effectiveness and flexibility of the tet-on system to provide a tightly regulated over-expression system in adult skeletal muscle. The MCKrtTA transgenic lines can be combined with other transgenic responder lines for skeletal muscle-specific over-expression of any target gene of interest.

Adeno-associated virus vector-mediated gene transfer into dystrophin-deficient skeletal muscles evokes enhanced immune response against the transgene product

Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscular disorder caused by a defect in the DMD gene. AAV vector-mediated micro-dystrophin cDNA transfer is an attractive approach to treatment of DMD. To establish effective gene transfer into skeletal muscle, we examined the transduction efficiency of an AAV vector in skeletal muscles of dystrophin-deficient mdx mice. When an AAV vector encoding the LacZ gene driven by a CMV promoter (AAV-CMVLacZ) was introduced, beta-galactosidase expression markedly decreased in mdx muscle 4 weeks after injection due to immune responses against the transgene product. We also injected AAV-CMVLacZ into skeletal muscles of mini-dystrophin-transgenic mdx mice (CVBA3'), which show ameliorated phenotypes without overt signs of muscle degeneration. AAV vector administration, however, evoked substantial immune responses in CVBA3' muscle. Importantly, AAV vector using muscle-specific MCK promoter also elicited responses in mdx muscle, but at a considerably later period. These results suggested that neo-antigens introduced by AAV vectors could evoke immune reactions in mdx muscle, since increased permeability allowed a leakage of neo-antigens from the dystrophin-deficient sarcolemma of muscle fibers. However, resident antigen-presenting cells, such as myoblasts, myotubes and regenerating immature myofibers, might also play a role in the immune response.

Metabolic profiles of dystrophin and utrophin expression in mouse models of Duchenne muscular dystrophy

Metabolic profiles from (1)H nuclear magnetic resonance spectroscopy have been used to describe both one and two protein systems in four mouse models related to Duchenne muscular dystrophy using the pattern recognition technique partial least squares. Robust statistical models were built for extracts and intact cardiac tissue, distinguishing mice according to expression of dystrophin. Using metabolic profiles of diaphragm, models were built describing dystrophin and utrophin, a dystrophin related protein, expression. Increased utrophin expression counteracted some of the deficits associated with dystrophic tissue. This suggests the method may be ideal for following treatment regimes such as gene therapy.

Adeno-associated virus-mediated vascular endothelial growth factor gene therapy in skeletal muscle before transplantation promotes revascularization of regenerating muscle

Successful clinical transplantation of whole skeletal muscles can be limited by impaired muscle revascularization and regeneration. The aim of this study was to enhance the revascularization (and hence speed of regeneration) of transplanted whole muscles by transducing muscles with the vascular endothelial growth factor (VEGF) gene before transplantation, using a recombinant adeno-associated virus (rAAV). The rAAV encoding VEGF and green fluorescent protein (GFP) (rAAV.VEGF.GFP) was injected into the tibialis anterior muscles of adult BALB/c mice. One month after injection whole muscle autotransplantation was performed. Muscles were sampled 7 days after autografting. GFP expression was examined as an indicator of persistent transgene expression after grafting, and immunohistochemistry was used to identify VEGF, blood vessels, and newly formed myotubes. After grafting, GFP expression persisted only in a few surviving myofibers in the periphery of rAAV.VEGF.GFP-pretreated muscles, although abundant VEGF expression was seen in myogenic cells in all grafted muscles. Quantitative analysis demonstrated that, although only small numbers of rAAV.VEGF.GFP-transduced myofibers were present, whole muscle grafts preinjected with rAAV.VEGF.GFP were significantly more vascular than saline-injected and uninjected control muscle grafts. Furthermore, rAAV.VEGF.GFP-injected whole muscle transplants were further advanced in terms of regeneration (myotube formation) compared with the uninjected control muscle transplants. This study clearly shows that rAAV-mediated VEGF expression persists only in myofibers that survive the necrosis induced by muscle transplantation; however, this amount of VEGF results in significantly increased revascularization and regeneration of whole muscle transplants.

Delivery of alpha- and beta-sarcoglycan by recombinant adeno-associated virus: efficient rescue of muscle, but differential toxicity

The sarcoglycanopathies are a group of four autosomal recessive limb girdle muscular dystrophies (LGMD 2D, 2E, 2C, and 2F), caused by mutations of the alpha-, beta-, gamma-, or delta-sarcoglycan genes, respectively. The delta-sarcoglycan-deficient hamster has been the most utilized model for gene delivery to muscle by recombinant adeno-associated virus (AAV) vectors; however, human patients with delta-sarcoglycan deficiency are exceedingly rare, with only two patients described in the United States. Here, we report construction and use of AAV vectors expressing either alpha- or beta-sarcoglycan, the genes responsible for the most common forms of the human sarcoglycanopathies. Both vectors showed successful short-term genetic, biochemical, and histological rescue of both alpha- and beta-sarcoglycan-deficient mouse muscle. However, comparison of persistence of expression in 51 injected mice showed substantial differences between AAV alpha-sarcoglycan (alpha-SG) and beta-sarcoglycan (beta-SG) vectors. AAV-beta-SG showed long-term expression with no decrease in expression for more than 21 months after injection, whereas AAV-alpha-SG showed a dramatic loss of positive fibers between 28 and 41 days post-injection (p = 0.006). Loss of immunopositive myofibers was correlated with significant inflammatory cell infiltrate, primarily macrophages. To determine whether the loss of alpha-sarcoglycan-positive fibers was due to an immune response or cytotoxic effect of alpha-sarcoglycan overexpression, severe combined immunodeficient (SCID) mouse muscle was assayed for cytotoxicity after injection with AAV-alpha-SG, AAV-beta-SG, or phosphate-buffered saline. The results were consistent with overexpression of alpha-sarcoglycan causing significant cytotoxicity. The cytotoxicity of alpha-sarcoglycan, and not beta- or delta-sarcoglycan overexpression, was consistent with biochemical studies of the hierarchical order of assembly of the sarcoglycan complex. Our data suggest that even closely related proteins might require different levels of expression to avoid toxicity and achieve long-term tissue rescue.

Adeno-associated virus vector-mediated minidystrophin gene therapy improves dystrophic muscle contractile function in mdx mice

Duchenne muscular dystrophy (DMD) is the most common disabling and lethal genetic muscle disorder, afflicting 1 of every 3500 males. Patients with DMD experience progressive muscle degeneration and weakness and succumb to respiratory or cardiac failure by their early twenties. No treatment is currently available for DMD. Mutations in the dystrophin gene result in lack of a functional dystrophin protein in striated muscle, which induces instability in the muscle cell membrane leading to persistent muscle injury after contraction. We have previously created novel minidystrophin genes and demonstrated that adeno-associated virus (AAV)-mediated intramuscular delivery of the minigenes effectively ameliorated mdx dystrophic histopathology and led to normal cell membrane integrity for more than 1 year. In this paper, we investigated whether AAV-minidystrophin could also improve mdx muscle contractile function. Two-month-old adult male mdx mice, with established muscular dystrophy, were given a single-dose injection of an AAV-minidystrophin vector in the tibialis anterior (TA) muscle of one leg, with the untreated contralateral leg used as a control. The treated TA muscle showed both (1) a significant increase in isometric force generation and (2) a significant increase in resistance to lengthening activation-induced muscle force decrements. We conclude that AAV-minidystrophin gene treatment is effective in improving mdx muscle contractile function.

Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that connects the cytoskeleton of a muscle fiber to its surrounding extracellular matrix. Mutations in the DGC disrupt the complex and lead to muscular dystrophy. There are a few naturally occurring animal models of DGC-associated muscular dystrophy (e.g. the dystrophin-deficient mdx mouse, dystrophic golden retriever dog, HFMD cat and the delta-sarcoglycan-deficient BIO 14.6 cardiomyopathic hamster) that share common genetic protein abnormalities similar to those of the human disease. However, the naturally occurring animal models only partially resemble human disease. In addition, no naturally occurring mouse models associated with loss of other DGC components are available. This has encouraged the generation of genetically engineered mouse models for DGC-linked muscular dystrophy. Not only have analyses of these mice led to a significant improvement in our understanding of the pathogenetic mechanisms for the development of muscular dystrophy, but they will also be immensely valuable tools for the development of novel therapeutic approaches for these incapacitating diseases.

Cardiomyocyte-specific gene expression following recombinant adeno-associated viral vector transduction

Recombinant adeno-associated viral (rAAV) vectors hold promise for delivering genes for heart diseases, but cardiac-specific expression by the use of rAAV has not been demonstrated. To achieve this goal rAAV vectors were generated expressing marker or potentially therapeutic genes under the control of the cardiac muscle-specific alpha myosin heavy chain (MHC) gene promoter. The rAAV-MHC vectors expressed in primary cardiomyocytes with similar kinetics to rAAV-CMV; however, expression by the rAAV-MHC vectors was restricted to cardiomyocytes. rAAV vectors have low cytotoxicity, and it is demonstrated here that rAAV fails to induce apoptosis in cardiomyocytes compared with a recombinant adenoviral vector. rAAV-MHC or rAAV-CMV vectors were administered to mice to determine the specificity of expression in vivo. The rAAV-MHC vectors expressed specifically in cardiomyocytes, whereas the control rAAV-CMV vector expressed in heart, skeletal muscle, and brain. rAAV-MHC transduction resulted in long term (16 weeks) expression of human growth hormone following intracardiac, yet not intramuscular, injection. Finally, we defined the minimal MHC enhancer/promoter sequences required for specific and robust in vivo expression in the context of a rAAV vector. For the first time we describe a panel of rAAV vectors capable of long term cardiac specific expression of intracellular and secreted proteins.

Correction of the enzymatic and functional deficits in a model of Pompe disease using adeno-associated virus vectors

Pompe disease is a lysosomal storage disease caused by the absence of acid alpha-1,4 glucosidase (GAA). The pathophysiology of Pompe disease includes generalized myopathy of both cardiac and skeletal muscle. We sought to use recombinant adeno-associated virus (rAAV) vectors to deliver functional GAA genes in vitro and in vivo. Myotubes and fibroblasts from Pompe patients were transduced in vitro with rAAV2-GAA. At 14 days postinfection, GAA activities were at least fourfold higher than in their respective untransduced controls, with a 10-fold increase observed in GAA-deficient myotubes. BALB/c and Gaa(-/-) mice were also treated with rAAV vectors. Persistent expression of vector-derived human GAA was observed in BALB/c mice up to 6 months after treatment. In Gaa(-/-) mice, intramuscular and intramyocardial delivery of rAAV2-Gaa (carrying the mouse Gaa cDNA) resulted in near-normal enzyme activities. Skeletal muscle contractility was partially restored in the soleus muscles of treated Gaa(-/-) mice, indicating the potential for vector-mediated restoration of both enzymatic activity and muscle function. Furthermore, intramuscular treatment with a recombinant AAV serotype 1 vector (rAAV1-Gaa) led to nearly eight times normal enzymatic activity in Gaa(-/-) mice, with concomitant glycogen clearance as assessed in vitro and by proton magnetic resonance spectroscopy.

Molecular pathophysiology and targeted therapeutics for muscular dystrophy

Experimental therapeutics of the muscular dystrophies has made impressive advances on several fronts. Adeno-associated virus has emerged as the clear 'vector of choice' for muscle gene delivery, with successful functional rescue of dystrophic muscle in rodent models. Correction of the dystrophin gene mutation in a dog model has been reported, and several reports of progress on myogenic stem cell characterization are resurrecting cell transplantation as a possible therapeutic approach. The downstream consequences of dystrophin deficiency are being defined quickly using microarray experiments, and drugs targeting specific biochemical pathways are being tested rapidly in animal models. Such targeted drug discoveries, which are discussed in this article, have begun to be implemented in human clinical trials.

Enhancement of muscle gene delivery with pseudotyped adeno-associated virus type 5 correlates with myoblast differentiation

Adeno-associated virus (AAV)-based muscle gene therapy has achieved tremendous success in numerous animal models of human diseases. Recent clinical trials with this vector have also demonstrated great promise. However, to achieve therapeutic benefit in patients, large inocula of virus will likely be necessary to establish the required level of transgene expression. For these reasons, efforts aimed at increasing the efficacy of AAV-mediated gene delivery to muscle have the potential for improving the safety and therapeutic benefit in clinical trials. In the present study, we compared the efficiency of gene delivery to mouse muscle cells for recombinant AAV type 2 (rAAV-2) and rAAV-2cap5 (AAV-2 genomes pseudo-packaged into AAV-5 capsids). Despite similar levels of transduction by these two vectors in undifferentiated myoblasts, pseudotyped rAAV-2cap5 demonstrated dramatically enhanced transduction in differentiated myocytes in vitro (>500-fold) and in skeletal muscle in vivo (>200-fold) compared to rAAV-2. Serotype-specific differences in transduction efficiency did not directly correlate with viral binding to muscle cells but rather appeared to involve endocytic or intracellular barriers to infection. Furthermore, application of this pseudotyped virus in a mouse model of Duchenne's muscular dystrophy also demonstrated significantly improved transduction efficiency. These findings should have a significant impact on improving rAAV-mediated gene therapy in muscle.

Overexpression of gamma-sarcoglycan induces severe muscular dystrophy. Implications for the regulation of Sarcoglycan assembly

The sarcoglycan complex is found normally at the plasma membrane of muscle. Disruption of the sarcoglycan complex, through primary gene mutations in dystrophin or sarcoglycan subunits, produces membrane instability and muscular dystrophy. Restoration of the sarcoglycan complex at the plasma membrane requires reintroduction of the mutant sarcoglycan subunit in a manner that will permit normal assembly of the entire sarcoglycan complex. To study sarcoglycan gene replacement, we introduced transgenes expressing murine gamma-sarcoglycan into muscle of normal mice. Mice expressing high levels of gamma-sarcoglycan, under the control of the muscle-specific creatine kinase promoter, developed a severe muscular dystrophy with greatly reduced muscle mass and early lethality. Marked gamma-sarcoglycan overexpression produced cytoplasmic aggregates that interfered with normal membrane targeting of gamma-sarcoglycan. Overexpression of gamma-sarcoglycan lead to the up-regulation of alpha- and beta-sarcoglycan. These data suggest that increased gamma-sarcoglycan and/or mislocalization of gamma-sarcoglycan to the cytoplasm is sufficient to induce muscle damage and provides a new model of muscular dystrophy that highlights the importance of this protein in the assembly, function, and downstream signaling of the sarcoglycan complex. Most importantly, gene dosage and promoter strength should be given serious consideration in replacement gene therapy to ensure safety in human clinical trials.

Gene therapy: a strategy for the treatment of inherited muscle diseases?

The emergence of new vectors of viral origin (recombinant adeno-associated viruses, second and third generation adenoviruses) and a new potential source of cells for transplantation (muscle-derived stem cells) are broadening the panel of therapeutic options for myopathies. Although the perfect gene-transfer method(s) have not yet been found, recent findings will certainly constitute a strong knowledge base for future clinical trials.

Analysis of the spatial, temporal and tissue-specific transcription of gamma-sarcoglycan gene using a transgenic mouse

To evaluate the promoter function of the 5'-flanking sequence of mouse gamma-sarcoglycan (gamma-SG) gene in vivo, we generated transgenic mice harboring this sequence fused with enhanced green fluorescent protein reporter gene. The reporter expression was restricted in striated muscles and particularly strong in all myofibers in skeletal muscles. Using these mice, we examine the spatial and temporal transcriptional patterns of the gamma-SG gene during mouse skeletal muscle development. The expression of basic helix loop helix transcriptional factors preceded that of the reporter. Differences between the expression of reporter and endogenous gamma-SG genes in non-muscle tissues suggested the existence of additional promoter elements in the endogenous gene, and the analysis of endogenous mRNAs demonstrated the existence of a novel upstream exon and promoter active in non-muscle tissues.

Harnessing the potential of dystrophin-related proteins for ameliorating Duchenne's muscular dystrophy

Duchenne's muscular dystrophy (DMD) is a fatal disease caused by mutations in the DMD gene that lead to quantitative and qualitative disturbances in dystrophin expression. Dystrophin is a member of the spectrin superfamily of proteins. Dystrophin itself is closely related to three proteins that constitute a family of dystrophin-related proteins (DRPs): the chromosome 6-encoded DRP or utrophin, the chromosome-X encoded, DRP2 and the chromosome-18 encoded, dystrobrevin. These proteins share sequence similarity and functional motifs with dystrophin. Current attempts at somatic gene therapy of DMD face numerous technical problems. An alternative strategy for DMD therapy, that circumvents many of these problems, has arisen from the demonstration that the DRP utrophin can functionally substitute for the missing dystrophin and its overexpression can rescue dystrophin-deficient muscle. Currently, a promising avenue of research consists of identifying molecules that would increase the expression of utrophin and the delivery of these molecules to dystrophin-deficient tissues as a means of DMD therapy. In this review, we will focus on DRPs from the perspective of strategies and issues related to upregulating utrophin expression for DMD therapy. Additionally, we will address the techniques used for anatomical, biochemical and physiological evaluation of the potential benefits of this and other forms of DMD therapy in dystrophin-deficient animal models.

Muscle-specific promoters may be necessary for adeno-associated virus-mediated gene transfer in the treatment of muscular dystrophies

Recombinant adeno-associated virus (rAAV) vectors allow efficient gene transfer and expression in the muscle; therefore, rAAVs represent a potential gene therapy vector for muscular dystrophies. For further investigations, we used a mouse muscular dystrophy model (gsg(-/-) mice) gamma-sarcoglycan, a subunit of the dystrophin-glycoprotein complex, is missing. gsg(-/-) mice develop progressive dystrophy representative of a severe human phenotype disease. We previously showed high levels and stable expression of gamma-sarcoglycan in myofibers after direct muscle injection into gsg(-/-) mice of a recombinant AAV vector (AAV.dMCK.gSG) carrying the gamma-sarcoglycan cDNA driven by a muscle-specific promoter (truncated version of muscle creatine kinase). Here, we show that when gamma-sarcoglycan expression is driven by the ubiquitous cytomegalovirus (CMV) promoter (AAV.CMV.gSG), lower levels of transgene expression are observed and are associated with a humoral response to gamma-sarcoglycan. When using an rAAV vector, expressing the highly immunogenic product gamma-galactosidase under the CMV promoter (AAV.CMV.LacZ), we measured a strong cellular and humoral immune response to the transgene after intramuscular injection into gsg(-/-) mice. This study suggests that restriction of transgene expression to the muscle is an important criterion for the treatment of muscular dystrophies and will aid in the design of protocols for gene therapy.

Adeno-associated virus vector carrying human minidystrophin genes effectively ameliorates muscular dystrophy in mdx mouse model

Duchenne muscular dystrophy (DMD) is the most common and lethal genetic muscle disorder, caused by recessive mutations in the dystrophin gene. One of every 3,500 males suffers from DMD, yet no treatment is currently available. Genetic therapeutic approaches, using primarily myoblast transplantation and adenovirus-mediated gene transfer, have met with limited success. Adeno-associated virus (AAV) vectors, although proven superior for muscle gene transfer, are too small (5 kb) to package the 14-kb dystrophin cDNA. Here we have created a series of minidystrophin genes (<4.2 kb) under the control of a muscle-specific promoter that readily package into AAV vectors. When injected into the muscle of mdx mice (a DMD model), two of the minigenes resulted in efficient and stable expression in a majority of the myofibers, restoring the missing dystrophin and dystrophin-associated protein complexes onto the plasma membrane. More importantly, this AAV treatment ameliorated dystrophic pathology in mdx muscle and led to normal myofiber morphology, histology, and cell membrane integrity. Thus, we have defined minimal functional dystrophin units and demonstrated the effectiveness of using AAV to deliver the minigenes in vivo, offering a promising avenue for DMD gene therapy.