Mutation Types and Aging Differently Affect Revertant Fiber Expansion in Dystrophic Mdx and Mdx52 Mice

Duchenne muscular dystrophy (DMD), one of the most common and lethal genetic disorders, and the mdx mouse myopathies are caused by a lack of dystrophin protein. These dystrophic muscles contain sporadic clusters of dystrophin-expressing revertant fibers (RFs), as detected by immunohistochemistry. RFs are known to arise from muscle precursor cells with spontaneous exon skipping (alternative splicing) and clonally expand in size with increasing age through the process of muscle degeneration/regeneration. The expansion of revertant clusters is thought to represent the cumulative history of muscle regeneration and proliferation of such precursor cells. However, the precise mechanisms by which RFs arise and expand are poorly understood. Here, to test the effects of mutation types and aging on RF expansion and muscle regeneration, we examined the number of RFs in mdx mice (containing a nonsense mutation in exon 23) and mdx52 mice (containing deletion mutation of exon 52) with the same C57BL/6 background at 2, 6, 12, and 18months of age. Mdx mice displayed a significantly higher number of RFs compared to mdx52 mice in all age groups, suggesting that revertant fiber expansion largely depends on the type of mutation and/or location in the gene. A significant increase in the expression and clustering levels of RFs was found beginning at 6months of age in mdx mice compared with mdx52 mice. In contrast to the significant expansion of RFs with increasing age, the number of centrally nucleated fibers and embryonic myosin heavy chain-positive fibers (indicative of cumulative and current muscle regeneration, respectively) decreased with age in both mouse strains. These results suggest that mutation types and aging differently affect revertant fiber expansion in mdx and mdx52 mice.     

Towards a therapeutic inhibition of dystrophin exon 23 splicing in mdx mouse muscle induced by antisense oligoribonucleotides (splicomers): target sequence optimisation using oligonucleotide arrays

BACKGROUND: The activity of synthetic antisense oligonucleotides (splicomers) designed to block pre-mRNA splicing at specific exons has been demonstrated in a number of model systems, including constitutively spliced exons in mouse dystrophin RNA. Splicomer reagents directed to Duchenne muscular dystrophy (DMD) RNAs might thus circumvent nonsense or frame-shifting mutations, leading to therapeutic expression of partially functional dystrophin, as occurs in the milder, allelic (Becker) form of the disease (BMD). METHODS: Functional and hybridisation array screens have been used to select optimised splicomers directed to exon 23 of dystrophin mRNA which carries a nonsense mutation in the mdx mouse. Splicomers were transfected into cultured primary muscle cells, and dystrophin mRNA assessed for exon exclusion. Splicomers were also administered to the muscles of mdx mice. RESULTS: Oligonucleotide array analyses with dystrophin pre-mRNA probes revealed strong and highly specific hybridisation patterns spanning the exon 23/intron 23 boundary, indicating an open secondary structure conformation in this region of the RNA. Functional screening of splicomer arrays by direct analysis of exon 23 RNA splicing in mdx muscle cultures identified a subset of biologically active reagents which target sequence elements associated with the 5' splice site region of dystrophin intron 23; splicomer-mediated exclusion of exon 23 was specific and dose-responsive up to a level exceeding 50% of dystrophin mRNA, and Western blotting demonstrated de novo expression of dystrophin protein at 2-5% of wild-type levels. Direct intramuscular administration of optimised splicomer reagents in vivo resulted in the reappearance of sarcolemmal dystrophin immunoreactivity in > 30% of muscle fibres in the mdx mouse CONCLUSIONS: These results suggest that correctly designed splicomers may have direct therapeutic value in vivo, not only for DMD, but also for a range of other genetic disorders.     

Systemic delivery of morpholino oligonucleotide restores dystrophin expression bodywide and improves dystrophic pathology

For the majority of Duchenne muscular dystrophy (DMD) mutations, antisense oligonucleotide (AON)-mediated exon skipping has the potential to restore a functional protein. Here we show that weekly intravenous injections of morpholino phosphorodiamidate (morpholino) AONs induce expression of functional levels of dystrophin in body-wide skeletal muscles of the dystrophic mdx mouse, with resulting improvement in muscle function. Although the level of dystrophin expression achieved varies considerably between muscles, antisense therapy may provide a realistic hope for the treatment of a majority of individuals with DMD.     

Frameshift deletions of exons 3-7 and revertant fibers in Duchenne muscular dystrophy: mechanisms of dystrophin production.

Duchenne muscular dystrophy (DMD) patients with mutations that disrupt the translational reading frame produce little or no dystrophin. Two exceptions are the deletion of exons 3-7 and the occurrence of rare dystrophin-positive fibers (revertant fibers) in muscle of DMD patients. Antibodies directed against the amino-terminus and the 5' end of exon 8 did not detect dystrophin in muscle from patients who have a deletion of exons 3-7. However, in all cases, dystrophin was detected with an antibody directed against the 3' end of exon 8. The most likely method of dystrophin production in these cases is initiation at a new start codon in exon 8. We also studied two patients who have revertant fibers: one had an inherited duplication of exons 5-7, which, on immunostaining, showed two types of revertant fibers; and the second patient had a 2-bp nonsense mutation in exon 51, which creates a cryptic splice site. An in-frame mRNA that uses this splice site in exon 51 was detected. Immunostaining demonstrated the presence of the 3' end of exon 51, which is in agreement with the use of this mRNA in revertant fibers. The most likely method of dystrophin production in these fibers is a second mutation that restores the reading frame.     

Restoration of human dystrophin following transplantation of exon-skipping-engineered DMD patient stem cells into dystrophic mice

Duchenne muscular dystrophy (DMD) is a hereditary disease caused by mutations that disrupt the dystrophin mRNA reading frame. In some cases, forced exclusion (skipping) of a single exon can restore the reading frame, giving rise to a shorter, but still functional, protein. In this study, we constructed lentiviral vectors expressing antisense oligonucleotides in order to induce an efficient exon skipping and to correct the initial frameshift caused by the DMD deletion of CD133+ stem cells. The intramuscular and intra-arterial delivery of genetically corrected CD133 expressing myogenic progenitors isolated from the blood and muscle of DMD patients results in a significant recovery of muscle morphology, function, and dystrophin expression in scid/mdx mice. These data demonstrate that autologous engrafting of blood or muscle-derived CD133+ cells, previously genetically modified to reexpress a functional dystrophin, represents a promising approach for DMD.     

Modular flexibility of dystrophin: implications for gene therapy of Duchenne muscular dystrophy

Attempts to develop gene therapy for Duchenne muscular dystrophy (DMD) have been complicated by the enormous size of the dystrophin gene. We have performed a detailed functional analysis of dystrophin structural domains and show that multiple regions of the protein can be deleted in various combinations to generate highly functional mini- and micro-dystrophins. Studies in transgenic mdx mice, a model for DMD, reveal that a wide variety of functional characteristics of dystrophy are prevented by some of these truncated dystrophins. Muscles expressing the smallest dystrophins are fully protected against damage caused by muscle activity and are not morphologically different from normal muscle. Moreover, injection of adeno-associated viruses carrying micro-dystrophins into dystrophic muscles of immunocompetent mdx mice results in a striking reversal of histopathological features of this disease. These results demonstrate that the dystrophic pathology can be both prevented and reversed by gene therapy using micro-dystrophins.     

Deletion of Dystrophin In-Frame Exon 5 Leads to a Severe Phenotype: Guidance for Exon Skipping Strategies

Duchenne and Becker muscular dystrophy severity depends upon the nature and location of the DMD gene lesion and generally correlates with the dystrophin open reading frame. However, there are striking exceptions where an in-frame genomic deletion leads to severe pathology or protein-truncating mutations (nonsense or frame-shifting indels) manifest as mild disease. Exceptions to the dystrophin reading frame rule are usually resolved after molecular diagnosis on muscle RNA. We report a moderate/severe Becker muscular dystrophy patient with an in-frame genomic deletion of DMD exon 5. This mutation has been reported by others as resulting in Duchenne or Intermediate muscular dystrophy, and the loss of this in-frame exon in one patient led to multiple splicing events, including omission of exon 6, that disrupts the open reading frame and is consistent with a severe phenotype. The patient described has a deletion of dystrophin exon 5 that does not compromise recognition of exon 6, and although the deletion does not disrupt the reading frame, his clinical presentation is more severe than would be expected for classical Becker muscular dystrophy. We suggest that the dystrophin isoform lacking the actin-binding sequence encoded by exon 5 is compromised, reflected by the phenotype resulting from induction of this dystrophin isoform in mouse muscle in vivo. Hence, exon skipping to address DMD-causing mutations within DMD exon 5 may not yield an isoform that confers marked clinical benefit. Additional studies will be required to determine whether multi-exon skipping strategies could yield more functional dystrophin isoforms, since some BMD patients with larger in-frame deletions in this region have been reported with mild phenotypes.     

Suppression of nonsense mutations in the Dystrophin gene by a suppressor tRNA gene

Nonsense mutations in the dystrophin gene are the cause of Duchenne muscular dystrophy (DMD) in 10-15% of patients. In such an event, one approach to gene therapy for DMD is the use of suppressor tRNAs to overcome the premature termination of translation of the mutant mRNA. We have carried out cotransfection of the HeLa cell culture with constructs containing a suptRNA gene (pcDNA3suptRNA) and a marker LacZ gene (pNTLacZhis) using their polymer VSST-525 complexes. It was found that the number of cells producing beta-galactosidase depends inversely on the dose of the suptRNA gene. A single in vivo injection of the construct providing for expression of the suptRNAochre gene into mdx mouse muscle resulted in the production of dystrophin in 2.5% of fibers. This suggests that suppressor tRNAs are applicable in gene therapy for hereditary diseases caused by nonsense mutations.     

Antisense-induced multiexon skipping for Duchenne muscular dystrophy makes more sense

Dystrophin deficiency, which leads to severe and progressive muscle degeneration in patients with Duchenne muscular dystrophy (DMD), is caused by frameshifting mutations in the dystrophin gene. A relatively new therapeutic strategy is based on antisense oligonucleotides (AONs) that induce the specific skipping of a single exon, such that the reading frame is restored. This allows the synthesis of a largely functional dystrophin, associated with a milder Becker muscular dystrophy phenotype. We have previously successfully targeted 20 different DMD exons that would, theoretically, be beneficial for >75% of all patients. To further enlarge this proportion, we here studied the feasibility of double and multiexon skipping. Using a combination of AONs, double skipping of exon 43 and 44 was induced, and dystrophin synthesis was restored in myotubes from one patient affected by a nonsense mutation in exon 43. For another patient, with an exon 46-50 deletion, the therapeutic double skipping of exon 45 and 51 was achieved. Remarkably, in control myotubes, the latter combination of AONs caused the skipping of the entire stretch of exons from 45 through 51. This in-frame multiexon skipping would be therapeutic for a series of patients carrying different DMD-causing mutations. In fact, we here demonstrate its feasibility in myotubes from a patient with an exon 48-50 deletion. The application of multiexon skipping may provide a more uniform methodology for a larger group of patients with DMD.     

Extensive and prolonged restoration of dystrophin expression with vivo-morpholino-mediated multiple exon skipping in dystrophic dogs

Duchenne muscular dystrophy (DMD) is a severe and the most prevalent form of muscular dystrophy, characterized by rapid progression of muscle degeneration. Antisense-mediated exon skipping is currently one of the most promising therapeutic options for DMD. However, unmodified antisense oligos such as morpholinos require frequent (weekly or bi-weekly) injections. Recently, new generation morpholinos such as vivo-morpholinos are reported to lead to extensive and prolonged dystrophin expression in the dystrophic mdx mouse, an animal model of DMD. The vivo-morpholino contains a cell-penetrating moiety, octa-guanidine dendrimer. Here, we sought to test the efficacy of multiple exon skipping of exons 6-8 with vivo-morpholinos in the canine X-linked muscular dystrophy, which harbors a splice site mutation at the boundary of intron 6 and exon 7. We designed and optimized novel antisense cocktail sequences and combinations for exon 8 skipping and demonstrated effective exon skipping in dystrophic dogs in vivo. Intramuscular injections with newly designed cocktail oligos led to high levels of dystrophin expression, with some samples similar to wild-type levels. This is the first report of successful rescue of dystrophin expression with morpholino conjugates in dystrophic dogs. Our results show the potential of phosphorodiamidate morpholino oligomer conjugates as therapeutic agents for DMD.     

Therapeutic gene transfer to dystrophic diaphragm by an adenoviral vector deleted of all viral genes

Duchenne muscular dystrophy is caused by defects in the dystrophin gene, and the mdx mouse is the most frequently employed genetic model of this disease. It is well known that different muscle groups do not respond in the same way to dystrophin deficiency. In particular, the mdx mouse diaphragm exhibits severe morphological and functional changes not found in other mdx muscles. Use of early generation adenoviral vectors to deliver genes to the diaphragm in immunocompetent mdx mice has been associated with substantial functional toxicity and a rapid loss of transgene expression. Here we determined the response to dystrophin gene replacement in the mdx diaphragm using a "gutted" adenoviral vector that contains the coding sequence of two full-length dystrophin genes and is deleted of most viral DNA sequences. At 1 wk postdelivery of the vector, 23.6 +/- 4% of total fibers in the injected diaphragm bundle expressed dystrophin at the sarcolemma, which remained stable over the study duration of 30 days without the need for continuous immunosuppression. Treated diaphragms showed a significantly improved resistance to the abnormal force deficits induced by high-stress muscle contractions, the latter being a functional hallmark of dystrophin-deficient muscle. This functional amelioration was achieved despite the presence of mildly increased inflammation (CD4+ and CD8+ lymphocytes) within the vector-treated diaphragms. To our knowledge, this is the first demonstration that a viral vector can achieve reversal of functional abnormalities in the dystrophic diaphragm via therapeutic dystrophin gene transfer without the need for sustained immunosuppressive therapy.     

Muscular dystrophy in mice caused by two different alterations of dystrophin gene

The study addressed the question of the functional significance of various isoforms of dystrophin. Mice bearing two different alterations of the dystrophin gene, mdx and mdx-beta geo, were examined. Dystrophic mice with mdx mutation do not express full-length dystrophins, while they express short dystrophins; on the other hand, the dystrophic mice with mdx-beta geo mutation express neither full-length nor short dystrophins. We found pathological changes typical for muscular dystrophy in the diaphragm of both mutant strains. No pathological changes were found in brains of either mdx or mdx-beta geo mutants. We concluded tentatively that short isoforms of dystrophin do not contribute to the muscular dystrophy and that they do not play any role in the development and maintenance of histological structure of the brain.     

Negamycin restores dystrophin expression in skeletal and cardiac muscles of mdx mice

The ability of aminoglycoside antibiotics to promote read-through of nonsense mutations has attracted interest in these drugs as potential therapeutic agents in genetic diseases. However, the toxicity of aminoglycoside antibiotics may result in severe side effects during long-term treatment. In this paper, we report that negamycin, a dipeptide antibiotic, also restores dystrophin expression in skeletal and cardiac muscles of the mdx mouse, an animal model of Duchenne muscular dystrophy (DMD) with a nonsense mutation in the dystrophin gene, and in cultured mdx myotubes. Dystrophin expression was confirmed by immunohistochemistry and immunoblotting. We also compared the toxicity of negamycin and gentamicin, and found negamycin to be less toxic. Furthermore, we demonstrate that negamycin binds to a partial sequence of the eukaryotic rRNA-decoding A-site. We conclude that negamycin is a promising new therapeutic candidate for DMD and other genetic diseases caused by nonsense mutations.     

Long-term administration of antisense oligonucleotides into the paraspinal muscles of mdx mice reduces kyphosis

The mdx mouse model of muscular dystrophy has a premature stop codon preventing production of dystrophin. This results in a progressive phenotype causing centronucleation of skeletal muscle fibers, muscle weakness, and fibrosis and kyphosis. Antisense oligonucleotides alter RNA splicing to exclude the nonsense mutation, while still maintaining the open reading frame to produce a shorter, but partially functional dystrophin protein that should ameliorate the extent of pathology. The present study investigated the benefits of chronic treatment of mdx mice by once-monthly deep intramuscular injections of antisense oligonucleotides into paraspinal muscles. After 8 mo of treatment, mdx mice had reduced development of kyphosis relative to untreated mdx mice, a benefit that was retained until completion of the study at 18 mo of age (16 mo of treatment). This was accompanied by reduced centronucleation in the latissimus dorsi and intercostals muscles and reduced fibrosis in the diaphragm and latissimus dorsi. These benefits were accompanied by a significant increase in dystrophin production. In conclusion, chronic antisense oligonucleotide treatment provides clear and ongoing benefits to paralumbar skeletal muscle, with associated marked reduction in kyphosis.