The potential for gene therapy in Duchenne muscular dystrophy and other genetic muscle diseases

Skipping multiple exons of dystrophin transcripts using cocktail antisense oligonucleotides

Duchenne muscular dystrophy (DMD) is one of the most common and lethal genetic disorders, with 20,000 children per year born with DMD globally. DMD is caused by mutations in the dystrophin (DMD) gene. Antisense-mediated exon skipping therapy is a promising therapeutic approach that uses short DNA-like molecules called antisense oligonucleotides (AOs) to skip over/splice out the mutated part of the gene to produce a shortened but functional dystrophin protein. One major challenge has been its limited applicability. Multiple exon skipping has recently emerged as a potential solution. Indeed, many DMD patients need exon skipping of multiple exons in order to restore the reading frame, depending on how many base pairs the mutated exon(s) and adjacent exons have. Theoretically, multiple exon skipping could be used to treat approximately 90%, 80%, and 98% of DMD patients with deletion, duplication, and nonsense mutations, respectively. In addition, multiple exon skipping could be used to select deletions that optimize the functionality of the truncated dystrophin protein. The proof of concept of systemic multiple exon skipping using a cocktail of AOs has been demonstrated in dystrophic dog and mouse models. Remaining challenges include the insufficient efficacy of systemic treatment, especially for therapies that target the heart, and limited long-term safety data. Here we review recent preclinical developments in AO-mediated multiple exon skipping and discuss the remaining challenges.

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.

β–Amino Ester Polymers Facilitate in Vivo DNA Transfection and Adjuvant Plasmid DNA Immunization

Increased in vivo expression of intramuscularly delivered plasmid DNA will be essential for clinical success in gene therapy and plasmid DNA vaccination. We screened polymers from a library of beta-amino esters for their ability to augment transgene expression as measured by beta-galactosidase activity and cellular immune responses. Among the candidates identified in this screen, poly[(1,6-di(acryloxyethoxy)hexane)-co-(4-aminobutanol)] enhanced plasmid DNA transgene expression by sevenfold (P=0.0001) and its immunogenicity by 70% (P=0.03). We found that polymers with moderately hydrophobic backbones and terminal alcohol groups facilitated transfection most effectively in vivo. We also observed a log-linear correlation (R2=0.93) between peak cellular immune responses and transgene activity in all evaluated polymer-plasmid DNA formulations, clarifying the relationship between immunogenicity and the quantity of expressed antigen.

Distribution and effects of a single intramuscular injection of India ink in mice

Our goal has been to study the distribution and effects following a single intramuscular injection of a substance using India ink as a tracer. We injected 30 microl India ink in the gastrocnemius muscle group of C57Bl10 mice. Hematoxylin-Eosin, Trichrome stains and polyclonal anti-laminin, anti-collagen-IV and anti-dystrophin were used. The liquid spreads in all directions mainly following the perimysial and epimysial septae. The collagen bundles act as physical barriers preventing passage of the ink particles. In the area of the injection site, necrosis of the fibres is associated with disruption of the basement membrane. In the zones adjacent and distal to the injection site, the liquid progresses by pushing the muscle fibres apart with preservation of the basement membrane. Research based on intramuscular injection of substances should take the following into consideration: a) anatomy of the muscle group injected, b) routes of distribution of the substance, c) types of lesions produced with relation to the site of injection of the liquid, and d) size of the particles of the injected substance.

Restoration of dystrophin expression in cultured hybrid myotubes

Absence of dystrophin, as found in Duchenne boys, mdx mice and HFMD cats, leads to destabilization of the sarcolemmal-associated protein complex. Gene and cell therapy strategies aim to restore the dystrophin-associated protein complex. In order to better understand the cellular events involved in such therapy in feline and human muscular dystrophy, we asked whether dystrophin-deficient myoblasts would fuse with myoblasts expressing normal dystrophin, and whether the complex would be restored after such a fusion. Cat and human myoblasts were isolated from skeletal muscle of normal subjects and of patients with dystrophin deficiency and proliferated well. After co-culture with normal myoblasts, they fused to form hybrid myotubes. These hybrid myotubes expressed dystrophin, utrophin and dystrophin- associated proteins. Expression of these proteins were restored also in the vicinity of nuclei from dystrophin-deficient donors. These results demonstrate that dystrophin can be expressed and handled normally by hybrid myotubes. They show that myoblasts with a normal dystrophin gene can restore dystrophin expression in dystrophin-deficient myoblasts.

Administration of chinese herbal medicines facilitates the locomotor activity in dystrophin-deficient mice

The purpose of this study was to access the effects of chinese herbal medicines on Duchenne muscular dystrophy (DMD). We use dystrophin-deficient mice (mdx), an animal model of DMD, to evaluate the effect of chinese herbal medicines on locomotor activity. The consumption of water for each mouse was controlled during the three-month experimental session. Each mouse was allowed to drink 3 ml water with or without herbal medicines daily for three months. The estimated intake of chinese herbal medicine in adult mdx mouse with 30 g weight is 100 mg/kg per day, close to a dose used in human. The locomotor activity of the mdx mice was measured every month. Monitoring the locomotor activity of mdx mice after three-month administration of chinese herbal medicines, the results showed that liu-wei-di-huang-wan (LDW) and san-lin-pai-tsu-san (SPS) can facilitate locomotor activity with the parameters of horizontal activity, total distance, number of movements, movement time, vertical activity, number of vertical movements, vertical movement time, stereotypy, number of stereotyped movements, and stereotyped movement time. These results suggest that either LDW or SPS can act as a potent herbal medicine for the pharmacological treatment of DMD patients.

Biodistribution studies of 99mTc-labeled myoblasts in a murine model of muscular dystrophy

The purpose of this study was twofold: first, to evaluate the myoblast labeling of various 99mTc complexes and to select the complex that best accomplishes this labeling, and second to evaluate the biodistribution of myoblasts labeled with this complex using mice with MDX muscular dystrophy (the murine homologue of Duchenne's muscular dystrophy). The following ligands were used to prepare the corresponding 99mTc complexes: hexakis-methoxy-isobutyl-isonitrile (MIBI), bis(2-ethoxyethyl)diphosphinoethane (Tf), (RR,SS)-4,8-diaza-3,6,6,9-tetramethyl-undecane-2,10-dione-bisoxime (HM-PAO), bis(N-ethyl)dithiocarbamate (NEt), and bis(N-ethoxy, N-ethyl)dithiocarbamate (NOEt). One million murine myoblasts were incubated for 30-60 minutes with 5 mCi of each of the 99mTc complexes prepared from the above ligands. Viability was assessed by microscopic counting after trypan blue staining, and the radioactivity absorbed in the cells was measured after centrifugation. The compound with the highest uptake in cellular pellets was [99mTc]N-NOEt. The biodistribution of myoblasts labeled with this complex was evaluated after intraaortic injection in dystrophic mice. Such an approach has the potential of effecting widespread gene transfer through the bloodstream to muscles lacking dystrophin.

Biology of E1-deleted adenovirus vectors in nonhuman primate muscle

Adenovirus vectors have been studied as vehicles for gene transfer to skeletal muscle, an attractive target for gene therapies for inherited and acquired diseases. In this setting, immune responses to viral proteins and/or transgene products cause inflammation and lead to loss of transgene expression. A few studies in murine models have suggested that the destructive cell-mediated immune response to virally encoded proteins of E1-deleted adenovirus may not contribute to the elimination of transgene-expressing cells. However, the impact of immune responses following intramuscular administration of adenovirus vectors on transgene stability has not been elucidated in larger animal models such as nonhuman primates. Here we demonstrate that intramuscular administration of E1-deleted adenovirus vector expressing rhesus monkey erythropoietin or growth hormone to rhesus monkeys results in generation of a Th1-dependent cytotoxic T-cell response to adenovirus proteins. Transgene expression dropped significantly over time but was still detectable in some animals after 6 months. Systemic levels of adenovirus-specific neutralizing antibodies were generated, which blocked vector readministration. These studies indicate that the cellular and humoral immune response generated to adenovirus proteins, in the context of transgenes encoding self-proteins, hinders long-term transgene expression and readministration with first-generation vectors.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is not treatable; there is no cure. More than a decade ago, randomized trials demonstrated that oral steroid therapy was of benefit to DMD patients by prolonging ambulation. Although few significant side effects were reported, study patients were followed for 18 months or less. However, when treating DMD with steroids, the clinician must consider beginning treatment in mid-childhood and continuing until adolescence or longer, a total of at least 10 years. There is no evidence that steroids are associated with prolonged life or with improved pulmonary or cardiac function in DMD. It is clear that the risk of side effects increases with duration of use of oral steroids. Therefore, oral steroids are not recommended for treatment of DMD on a routine basis. If, in certain cases, one does institute therapy, the patient should be monitored carefully for side effects, maintain dietary restriction, and exercise regularly.

Thymic myoid cells as a source of cells for myoblast transfer

Transplantation of disaggregated myoblasts from normal donor to the muscles of a diseased host, or reimplantation of genetically modified host myoblasts, has been suggested as a possible route to therapy for inherited myopathies such as Duchenne muscular dystrophy, or to supply missing proteins that are required systemically in diseases such as hemophilia. With two exceptions, studies of myoblast transfer in the mouse have involved transplantation of donor myoblasts isolated from adult or neonatal skeletal muscle satellite cells. In this study we present evidence that thymic myoid cells are capable of participating in the regeneration of postnatal skeletal muscle, resulting in the expression of donor-derived proteins such as dystrophin and retrovirally encoded proteins such as beta-galactosidase within host muscles. This leads us to conclude that thymic myoid cells may provide an alternative to myoblasts derived from skeletal muscle as a source of myogenic cells for myoblast transfer.

Targeted delivery of plasmid DNA to myogenic cells via transferrin-conjugated peptide nucleic acid

We describe a novel approach to conjugate a targeting ligand to plasmid DNA without affecting either its supercoiled conformation or its ability to be efficiently transcribed. A 14-mer peptide nucleic acid (PNA) containing lysine and cysteine on each end was designed to target to a unique sequence located at the antibiotic resistance gene of the plasmid. The binding of PNA to the plasmid was found to be dose-dependent and sequence-specific and not to change the conformation of the plasmid. Transferrin (Tf) was conjugated with PNA via a reversible disulfide bond using N-succinimidyl-3-(2-pyridyldithio)propionate. Tf-PNA retained the ability to the plasmid in a sequence-specific manner. The efficiency of this bioconjugate for delivering plasmid was examined in cultured myoblasts and myotubes. Naked DNA and Tf-PNA/DNA showed no transfection activity in either myoblasts or myotubes. Polyethyleneimine (PEI) is required for significant increase of the transfection efficiency. At N:P ratio of 5, Tf-PNA enhanced gene transfection about fourfold over that of the DNA/PEI complex in both myoblasts and myotubes. This enhancement could be inhibited by excess free Tf, indicating that the enhancement of transfection was through Tf-mediated endocytosis. These findings suggest that this targeting system may have the potential for gene transfer to myogenic cells in vivo.

Gene suture--a novel method for intramuscular gene transfer and its application in hypertension therapy

In this report, reporter gene beta-galactosidase (LacZ) was chosen to compare two different intramuscular gene transfer methods, direct injection and gene suture. Evidence showed that gene suture can produce a higher foreign gene express efficiency in skeletal muscle compared with the direct injection method. The highly efficient eukaryotic expressing vectors of human atrial natriuretic factor (ANF) were constructed (pcD2/pAdVAntage/hANF and pcDNA3/hANF), and in vivo ANF gene delivery was performed by intramuscular gene suture. The effects of ANF gene transfer on blood pressure and renal sodium and water excretion were studied in three models of hypertensive animals. Results showed that a marked decrease of mean arterial pressure (MAP) and a significant increase of urine volume and urinary sodium excretion was produced in rats receiving the hANF construct due to the local expression of ANF and its secretion into plasma. Taken together, these results indicate that gene suture may represent a novel gene delivery modality in gene therapy.

Efficient infection of mature skeletal muscle with herpes simplex virus vectors by using dextran sulfate as a co-receptor

The use of herpes simplex virus (HSV) vectors for gene delivery to skeletal muscle is hampered by a maturation-dependent loss of muscle fiber infectivity. Previous studies of HSV type 1 (HSV-1) infection in the rodent show that the loss of infectivity may be due, at least in part, to the development of the basal lamina throughout the course of maturation, which may block the initial events in HSV infection. To initiate infection, HSV normally attaches to cell surface heparan sulfate, which stabilizes the virus such that it can interact with secondary protein receptors required for entry into host cells. In this study, we demonstrate that heparan sulfate biosynthesis is downregulated during skeletal muscle maturation. When myofibers were treated with a variety of enzymes, including collagenase type IV or chondroitin ABC lyase, HSV infection was restored, which suggests that virus secondary receptors were present but not readily accessible to the virus in the intact myofiber. Surprisingly, we also found that HSV-1 infectivity could be restored in vitro and in vivo by exposing myofibers to low concentrations of the glycosaminoglycan analog dextran sulfate, which appears to act as a surrogate receptor to stabilize the virus at the myofiber surface such that HSV can engage additional receptors. This demonstration that the basal lamina is not an absolute block to HSV-1 infection is remarkable because it allows for the nondestructive targeting of HSV-1 to mature myofibers and greatly expands the usefulness of HSV as a gene therapy vector for the treatment of inherited and acquired diseases.

Aminoglycoside antibiotics restore dystrophin function to skeletal muscles of mdx mice

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene, leading to the absence of the dystrophin protein in striated muscle. A significant number of these mutations are premature stop codons. On the basis of the observation that aminoglycoside treatment can suppress stop codons in cultured cells, we tested the effect of gentamicin on cultured muscle cells from the mdx mouse - an animal model for DMD that possesses a premature stop codon in the dystrophin gene. Exposure of mdx myotubes to gentamicin led to the expression and localization of dystrophin to the cell membrane. We then evaluated the effects of differing dosages of gentamicin on expression and functional protection of the muscles of mdx mice. We identified a treatment regimen that resulted in the presence of dystrophin in the cell membrane in all striated muscles examined and that provided functional protection against muscular injury. To our knowledge, our results are the first to demonstrate that aminoglycosides can suppress stop codons not only in vitro but also in vivo. Furthermore, these results raise the possibility of a novel treatment regimen for muscular dystrophy and other diseases caused by premature stop codon mutations. This treatment could prove effective in up to 15% of patients with DMD.

Adenovirus-mediated transfer of human acid maltase gene reduces glycogen accumulation in skeletal muscle of Japanese quail with acid maltase deficiency

Acid maltase deficiency (AMD) causes a lysosomal glycogenosis inherited as an autosomal recessive trait. The infantile type of AMD (Pompe disease) leads to early death due to severe dysfunction of cardiac and respiratory muscles and no effective therapy is available. Replication-defective adenovirus vectors offer a promising tool for in vivo gene delivery and gene therapy. We constructed a recombinant adenovirus containing the human acid maltase (AM) cDNA downstream of the CAG promoter, composed of modified chicken beta-actin promoter and CMV IE enhancer (AxCANAM). Japanese quail with AMD was used for this study as an animal model for human AMD. When cultured fibroblasts from AMD quail were infected with AxCANAM, AM activity in the cells increased in proportion to the multiplicity of infection (MOI). When AxCANAM (4.5 x 10(8) PFU) was injected into unilateral superficial pectoral muscle of AMD quail, PAS staining showed that glycogenosomes disappeared and stainability of acid phosphatase was reduced in the injected area as compared with the contralateral muscle of the same birds. Biochemically, AM activity increased and glycogen content decreased in the injected muscle. Western blot analysis showed that AMD quail muscle injected with AxCANAM expressed human AM protein processed to active forms. These results suggest that the human AM cDNA transferred by an adenovirus vector was sufficiently expressed, leading to a marked reduction of the glycogen accumulation in the skeletal muscle of AMD quail.

Implications of maturation for viral gene delivery to skeletal muscle

Different viral vectors have been analyzed as gene delivery vehicles to skeletal muscle for potentially therapeutic purposes. In this review, we evaluate the application of retroviral, adenoviral, and herpes simplex viral vectors to deliver genes to skeletal muscle and focus on the dramatic loss of viral transduction detected throughout muscle maturation. Recent results suggested that there are several factors involved in the reduced viral transducibility of mature skeletal muscle: muscle cells become post-mitotic in an early stage, the extracellular matrix develops into a physical barrier, and a loss of myoblast mediation occurs since myoblasts progressively become quiescent. Approaches to improve viral gene delivery to mature skeletal muscle may include the use of particular enzymes to increase the permeability of the extracellular matrix, the pre-treatment of the muscle with a myonecrotic agent to induce myoblast mediation, or the application of the myoblast-mediated ex vivo gene transfer.

Plasmid delivery to muscle: Recent advances in polymer delivery systems

Preclinical studies involving intramuscular injection of plasmid into animals have revealed at least four significant variables that effect levels of gene expression (i.e., >fivefold effect over controls), including the formulation, injection technique, species and pretreatment of the muscle with myotoxic agents to induce muscle damage. The uptake of plasmid formulated in saline has been shown to be a saturable process, most likely via a receptor-mediated event involving the T tubules and caveolae. Pharmacokinetic studies have demonstrated that the bioavailability of injected plasmid to muscle cells is very low, due to rapid and extensive plasmid degradation by extracellular nucleases. We have developed protective, interactive, non-condensing (PINC) delivery systems designed to complex plasmids and to (i) protect plasmids from rapid nuclease degradation, (ii) disperse and retain intact plasmid in the muscle and (iii) facilitate the uptake of plasmid by muscle cells. PINC systems result in up to at least a one log increase in both the extent and levels of gene expression over plasmid formulated in saline. We have combined the PINC delivery systems with two different muscle-specific expression plasmids. After direct intramuscular injection of these gene medicines, we have shown both local myotrophic and neurotrophic effects of expressed human insulin-like growth factor (hIGF-I) and the secretion of biologically active human growth hormone (hGH) into the systemic circulation.

Nuclear magnetic resonance spectroscopy study of muscle growth, mdx dystrophy and glucocorticoid treatments: correlation with repair

Proton nuclear magnetic resonance spectroscopy (1H NMR) can be used to study skeletal muscle metabolism. The mdx mouse is a unique animal for studies of muscle regeneration, and models the disease of Duchenne muscular dystrophy (DMD). The goals of this study were to determine the potential of 1H NMR spectroscopy as an alternative to conventional histology in monitoring: (1) normal growth in control muscle and the progression of dystrophy in mdx muscle, and (2) beneficial treatments (glucocorticoids) on mdx dystrophy. Ex vivo 1H NMR spectra of limb and diaphragm muscles were obtained from different ages of control and mdx mice, and from mice which were treated with prednisone or deflazacort. Peaks with contributions from creatine, taurine and lipids were examined. Lower levels of taurine and creatine characterized predystrophy and active dystrophy intervals in mdx muscle compared to control. Levels of taurine increased with stabilization of the disease by repair. A measure of accumulated muscle repair, fiber centronucleation and many spectral peaks were highly and significantly correlated. Greater amounts of lipids were found in the diaphragm compared to limb spectra. Treatment of dystrophy, which improved muscle phenotype, resulted in greater levels of taurine and creatine, especially in the limb muscle. Therefore, 1H NMR differentially discriminates: (1) control and mdx muscle; (2) the progression of mdx dystrophy and developmental stages in normal growth; (3) mild and severe dystrophic phenotypes (diaphragm vs limb); and (4) changes associated with improved muscle phenotype and regeneration (due to treatment or injury). The results focus on monitoring muscle repair, not degeneration. We conclude that 1H NMR is a reliable tool in the objective investigation of muscle repair status during muscular dystrophy.

Transplantation of myoblasts from a transgenic mouse overexpressing dystrophin prduced only a relatively small increase of dystrophin-positive membrane

Myoblast cultures from normal and Tg-MDA (transgenic mouse overexpressing dystrophin 50-fold) mice were transplanted into dystrophin-deficient mdx mouse muscles. Four weeks after transplantation, dystrophin-positive fibers were observed four times more frequently in cross sections of muscles injected with Tg-MDA. Myoblasts from Tg-MDA mice also expressing the beta-gal transgene (Tg-MDA/beta-gal) and myoblasts from beta-gal transgenic mice containing one normal dystrophin gene (normal/beta-gal) were also transplanted into mdx mouse muscles. Four weeks after transplantation, the fiber length positive for dystrophin (nuclear domain) was shorter (439 +/- 326 microm) than the beta-gal nuclear domain (1466 +/- 713 microm) of the same fiber when normal/beta-gal myoblasts were transplanted, but increased (1302 +/- 487 microm) when Tg-MDA/beta-gal myoblasts were used. These experiments show that despite the presence in Tg-MDA myoblasts of constructions which lead in vivo in transgenic mice to an overexpression of dystrophin 50-fold, the membrane area over which dystrophin was expressed was increased only threefold. This observation is also expected for vector-mediated gene therapy.

Study of adenovirus-mediated dystrophin minigene transfer to skeletal muscle by combined microscopic display of adenoviral DNA and dystrophin

In situ DNA hybridization of an E4 adenoviral sequence amplified by in situ polymerase chain reaction (PCR) was used to mark adenovirus-containing myonuclei in muscles of immunocompetent and immunosuppressed mdx mice following intramuscular injection of adenoviral recombinants. The adenoviral recombinants contained a 6.3-kb dystrophin cDNA (minigene) driven by a cytomegalovirus (CMV) promoter/enhancer and thus, immunostaining for dystrophin of the same sections permitted correlation of adenoviral recombinant-containing myonuclei with dystrophin positivity of the same muscle fiber segments. As early as 2 hr post-injection of adenoviral recombinant, an appreciable number of adenoviral recombinant-positive (AVR+) myonuclei, and some partial dystrophin positive (pdys+) fibers were observed. Some fully dystrophin-positive (dys+) muscle fibers were present as early as 6 hr. The maximum number of fibers containing AVR+ myonuclei (observed by 72 hr) was maintained until 60 days in immunosuppressed, but not in immunocompetent, animals. In immunocompetent animals, the maximum number of dys+ fibers was observed at 10 days. The vast majority of these fibers contained AVR+ myonuclei; however, by 60 days, dys+ fibers disappeared with some AVR+ myonuclei persisting. Our studies suggest that widespread delayed inactivation of the dystrophin expression cassette is probably unlikely. Thus, optimization of immunosuppression could assure successful long-term dystrophin gene transfer for gene therapy.

Recombinant adeno-associated viral vectors mediate long-term transgene expression in muscle

Gene transfer to muscle holds overt promise for the treatment of inherited myopathies, lysosomal storage disorders, and serum protein deficiencies. In addition, muscle could provide a reservoir for delivery of therapeutic molecules like blood clotting factors, erythropoietin, or insulin. To date, successful gene transfer to muscle has been limited by the inefficiency of the vector delivery systems and the transient nature of gene expression. In this paper, we show that a vector based on recombinant adeno-associated virus (rAAV) can efficiently transduce adult mouse skeletal muscle. Transduced myofibers escape immune elimination and transgene expression is robust beyond 5 months. Importantly, input vector DNA appears to undergo conversion from single-stranded genomes to high-molecular-weight concatameric forms. These data suggest that rAAV might have a significant advantage over many other viral and nonviral gene delivery methods, and holds significant promise as a vector for gene transfer to mature muscle.

Electrical properties of diaphragm and EDL muscles during the life of dystrophic mice

The membrane electrical properties of diaphragm and extensor digitorum longus (EDL) muscle fibers of dystrophic mdx and control mice from 4 wk to 14-19 mo of age were recorded with the intracellular microelectrode technique. Up to 8 wk of age, the diaphragm and EDL muscles did not differ between the two strains. From 8 up to 20 wk, the mdx diaphragm fibers showed a higher membrane resistance (Rm), which was due to significantly lower values of resting chloride conductance (GCl) and an overexcitability with respect to age-matched controls. Oppositely, the mdx EDL muscle fibers had significantly lower Rm and higher GCl values than age-related controls at 8, 10, and 13 wk, along with a decreased membrane excitability. These differences were no longer detectable at 20 wk. The diaphragm and EDL muscles from 14- to 19-mo-old controls showed a decrease of GCl and an increase of potassium conductance with respect to adult animals. In aged mdx animals, these changes were very dramatic in diaphragm fibers, whereas no differences, with respect to adults, were found in the EDL muscle. Thus GCl is an index of the dystrophic condition of mdx muscles. In the degenerating diaphragm, the impairment of GCl can account for some of the pathological features of the muscle. In the EDL muscle, the changes of GCl can follow the high regenerative potential of the hindlimb muscles of the mdx phenotype.

E-box sites and a proximal regulatory region of the muscle creatine kinase gene differentially regulate expression in diverse skeletal muscles and cardiac muscle of transgenic mice

Previous analysis of the muscle creatine kinase (MCK) gene indicated that control elements required for transcription in adult mouse muscle differed from those required in cell culture, suggesting that distinct modes of muscle gene regulation occur in vivo. To examine this further, we measured the activity of MCK transgenes containing E-box and promoter deletions in a variety of striated muscles. Simultaneous mutation of three E boxes in the 1,256-bp MCK 5' region, which abolished transcription in muscle cultures, had strikingly different effects in mice. The mutations abolished transgene expression in cardiac and tongue muscle and caused a reduction in expression in the soleus muscle (a muscle with many slow fibers) but did not affect expression in predominantly fast muscles: quadriceps, abdominals, and extensor digitorum longus. Other regulatory sequences with muscle-type-specific activities were found within the 358-bp 5'-flanking region. This proximal region conferred relatively strong expression in limb and abdominal skeletal muscles but was inactive in cardiac and tongue muscles. However, when the 206-bp 5' enhancer was ligated to the 358-bp region, high levels of tissue-specific expression were restored in all muscle types. These results indicate that E boxes and a proximal regulatory region are differentially required for maximal MCK transgene expression in different striated muscles. The overall results also imply that within skeletal muscles, the steady-state expression of the MCK gene and possibly other muscle genes depends on transcriptional mechanisms that differ between fast and slow fibers as well as between the anatomical and physiological attributes of each specific muscle.

Local production of the p40 subunit of interleukin 12 suppresses T-helper 1-mediated immune responses and prevents allogeneic myoblast rejection

The p40 subunit of interleukin 12 (IL-12p40) has been known to act as an IL-12 antagonist in vitro. We here describe the immunosuppressive effect of IL-12p40 in vivo. A murine myoblast cell line, C2C12, was transduced with retro-virus vectors carrying the lacZ gene as a marker and the IL-12p40 gene. IL-12p40 secreted from the transfectant inhibited the IL-12-induced interferon gamma (IFN-gamma) production by splenocytes in vitro. Survival of C2C12 transplanted into allogeneic recipients was substantially prolonged when transduced with IL-12p40. Cytokine (IL-2 and IFN-gamma) production and cytotoxic T lymphocyte induction against allogeneic C2C12 were impaired in the recipients transplanted with the IL-12p40 transfectant. Delayed-type hypersensitivity response against C2C12 was also diminished in the IL-12p40 recipients. Furthermore, serum antibodies against beta-galactosidase of the T-helper 1-dependent isotypes (IgG2 and IgG3) were decreased in the IL-12p40 recipients. These results indicate that locally produced IL-12p40 exerts a potent immunosuppressive effect on T-helper 1-mediated immune responses that lead to allograft rejection. Therefore, IL-12p40 gene transduction would be useful for preventing the rejection of allografts and genetically modified own cells that are transduced with potentially antigenic molecules in gene therapy.

Dystrophinopathies

The discovery of the subsarcolemmal muscle fiber protein dystrophin has, to a certain extent, replaced former nosological terms of Duchenne (DMD) and Becker (BMD) muscular dystrophies by the term dystrophinopathies. The immunohistochemical and Western blot analysis of dystrophin has not only enlarged the clinical spectrum of dystrophinopathies, but has also made carrier detection of DMD more reliable, particularly in manifesting carriers without family history. Moreover, prenatal muscle biopsy, under selected circumstances, can show presence or absence of dystrophin, ie, in the latter case an affected male fetus. Molecular genetics have provided a wealth of genetic details in the dystrophinopathies, but therapy has not yet succeeded to a similar extent, on the contrary, myoblast transplantation has not resulted in any clinical improvement.

Dystrophin expression in muscles of mdx mice after adenovirus-mediated in vivo gene transfer

We have generated high-titer adenoviral recombinants (AVR) expressing a 6.3-kb partial dystrophin cDNA insert under the control of either the Rous sarcoma virus (RSV) or cytomegalovirus (CMV) promoter. These AVR preparations were free of both E1-containing AVR and AVR with a nonfunctional dystrophin expression cassette. With these optimal AVR preparations, we have obtained a high degree of short-term (10 days) expression of a truncated (approximately 200 kD) dystrophin in dystrophin-deficient mdx muscles injected in the neonatal period; a lesser degree of expression of dystrophin was found in muscles injected in the young adult age and in old animals. Microscopic indices of muscle damage revealed that the truncated dystrophin provided a significant protection of the transduced muscle fibers. However, by 60 days post-injection, a substantial reduction of the number of dystrophin-positive fibers was noted, even in the neonatally injected muscles, and near-total elimination of dystrophin-positive fibers occurred in muscles injected in the adult age. These effects appeared to be brought about by the activity of CD8+ cytotoxic lymphocytes directed against the transduced cells, leading to their eventual elimination. In severe combined immunodeficiency (SCID) mice, lacking both humoral and cellular immune competence, muscles transduced (either in the neonatal or adult age) by AVR containing a CMV-LacZ expression cassette maintained the early (10 day) transduction level up to 30 days post-injection. Systemic administration of AVR (i.e., into the left ventricle of the heart) led in 5 days to a high number of dystrophin-positive fibers in heart, diaphragm, and intercostal muscles but not in limb muscles.

The therapy of respiratory chain encephalomyopathy: a critical review of the past and current perspective

The mitochondrial respiratory chain encephalomyopathies represent an important group of multisystem disorders. No curative treatment is currently available. A number of measures have been reported to have a theoretical potential to improve respiratory function. These treatment strategies have variable scientific support, many reports being anecdotal. We critically review the various therapeutic measures employed and suggest future treatment directions.

Effects of beta 1-integrin antisense phosphorothioate-modified oligonucleotide on myoblast behaviour in vitro

Myoblasts gene-engineered in vitro and then injected in vivo are safe, efficient options for gene therapy. While isolation of satellite cells is routinely achieved, their proliferation potential in vitro remains a limiting factor for cell transplantation under clinical conditions. We have studied the role of reversible inhibition of gene expression by antisense oligonucleotides on the proliferation of the myogenic cells. Addition of antisense oligonucleotides to myoblast cultures has been used to inhibit specifically the expression of the beta 1-integrin subunit gene. Here we show that the effects of multiple pulses of a phosphorothioate oligodeoxinucleotide antisense on the attachment to substrata and on the proliferation of myoblasts are dose-dependent. The addition of antisense to rat myoblasts caused rounding up of the cells and most of the cells became detached after several days in culture. A single pulse did not show any consistent effect, while in the presence of continuously administered antisense, the relative numbers of myoblasts in the treated muscle culture increased. We have no evidence of inhibition of myoblast fusion under these conditions. On the other hand, [3H]-TdR incorporation, total DNA and total number of cells decreased in antisense-treated cultures thus demonstrating an inhibitory effect of the phosphorothioate oligonucleotides on DNA synthesis. These side-effects could be overcome by substituting the phosphorothioate by unmodified oligonucleotides, so decreasing the half-life of the antisense, but also its toxicity. The overall results suggest a potential role of integrin antisense strategy in modulating the potential of myoblasts to proliferate.

Muscular degeneration in Duchenne's dystrophy may be caused by a mitochondrial defect

Duchenne's dystrophy (DMD), a recessive chromosome X-related disease, is the most common and severe form of myopathy. The different theories (vascular, neurogenic, membraneous, calcic and auto-immune) formulated to account for this disease have not been swept away by the discovery of the DMD gene and the deficient protein, dystrophin, since the exact cellular role played by the latter is still unknown. Our work on skeletal muscle has demonstrated a mitochondrial deficiency of the calcium-specific protein, calmitine, in degenerating muscle of myopathic persons and animals. Considering its great affinity for calcium, this protein specific to skeletal muscle could be essential to mitochondrial calcium regulation and thus to the functioning of the entire muscle cell. Its deficiency in Duchenne's and Becker type muscular dystrophy could be due to a mitochondrial genome alteration solely accountable for muscular degeneration. This hypothesis challenges the supposedly essential but still undefined role that researchers have attributed to dystrophin.

Antioxidant lazaroids enhance differentiation of C2 skeletal muscle cells

We have examined the incidence of new antioxidant compounds, so-called lazaroids, on the morphological and biochemical aspects of differentiation of C2 mouse skeletal muscle cells. We show that three lazaroids (U-74006F, U-74389F and U-83836E) enhance fusion of myogenic cells when added at the day of fusion. A parallel increase in nicotinic acetylcholine receptor expression and skeletal muscle alpha-actin content was observed. This promoting effect of lazaroids on myogenesis could be related to inhibition of free radical-mediated muscle damage. Therefore, one could expect that lazaroids might be useful in the treatment of degenerating muscle diseases such as Duchenne muscular dystrophy.

Emergence of early region 1-containing replication-competent adenovirus in stocks of replication-defective adenovirus recombinants (delta E1 + delta E3) during multiple passages in 293 cells

Early region 1 (E1)-deleted human adenovirus (AV) recombinants have been shown to be powerful tools of gene transfer in vivo and in vitro and are considered for application in human gene therapy. We could detect increasing titers of E1-containing adenovirus in two independent E1 + E3-deleted recombinant AV stocks during multiple passages in 293 cells, most likely due to a recombinant event with the host cell genome. We show the deleterious effects of this E1-containing, mostly replication-competent AV subpopulation in vivo and compare different screening methods of AV stocks for its detection. These considerations are important for the safety of human gene therapy trials.

Gene transfer into skeletal muscles by isogenic myoblasts

The best way to overcome immunorejection in heterologous myoblast transfer (HMT) is by the use of immunodeficient and/or highly immunosuppressed mice as hosts. The same may be attained by autologous myoblast transfer (AMT). In this paper, we describe myoblast transfer in mdx and normal mice where the donor myogenic cells originated from highly inbred litter mates that are considered to be isogenic and thus the procedure is analogous to AMT. The myoblasts were marked in vitro with Rous Sarcoma Virus (RSV)-luciferase (Lux) or RSV-beta-galactosidase (LacZ) reporter genes through transduction mediated by an autonomously replication-defective recombinant human adenovirus. This permitted us to follow their fate after transplantation. mdx and normal mice were irradiated with 20 Gray gamma rays; necrosis and regeneration were induced by intramuscular notexin prior to myoblast injection. In both mdx and normal mice, the expression of luciferase rapidly declined after the injection implying that a large portion of the injected myoblasts were lost by 48 hr, due to undetermined cause(s). The surviving, injected myoblasts well-mosaicized large groups of host fibers but only in the immediate vicinity of the injection. Substantial expression of the reporter gene continued up to 1 month post-transplantation in normal mice, but there was a gradual decline and eventual disappearance of the reporter gene expression in mdx mice. This latter phenomenon was due to the ongoing intense necrosis of muscle fibers in mdx. There was no evidence of immunorejection. These experiments indicate that even in the absence of immunorejection, myoblast transfer suffers from important negative features: major loss of myoblasts within 48 hr after the injection and lack of significant spread of the injected cells from the injection site in the host muscle. These factors, plus the limited proliferative and fusion capacity of Duchenne muscular dystrophy (DMD) myoblasts, make them less than an ideal vector for the dystrophin cDNA for dystrophin gene replacement therapy in DMD.

The immunobiology of muscle

Skeletal muscle can be both the site and target of immune reactions. Here, Reinhard Hohlfeld and Andrew Engel consider the role of muscle as an immunological microenvironment and discuss the immunological properties of human muscle cells. Furthermore, they provide a brief overview of autoimmune diseases of muscle and of other conditions in which intramuscular immune reactions play a role. Finally, they discuss the immunological problems of novel gene therapies that rely on muscle cells as vehicles for gene transfer.