In-frame Dystrophin Following Exon 51-Skipping Improves Muscle Pathology and Function in the Exon 52–Deficient mdx Mouse
Yoshitsugu AokiAkinori NakamuraToshifumi YokotaTakashi SaitoHitoshi OkazawaTetsuya NagataShin’ichi Takeda
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A promising therapeutic approach for Duchenne muscular dystrophy (DMD) is exon skipping using antisense oligonucleotides (AOs). In-frame deletions of the hinge 3 region of the dystrophin protein, which is encoded by exons 50 and 51, are predicted to cause a variety of phenotypes. Here, we performed functional analyses of muscle in the exon 52-deleted mdx (mdx52) mouse, to predict the function of in-frame dystrophin following exon 51-skipping, which leads to a protein lacking most of hinge 3. A series of AOs based on phosphorodiamidate morpholino oligomers was screened by intramuscular injection into mdx52 mice. The highest splicing efficiency was generated by a two-oligonucleotide cocktail targeting both the 5' and 3' splice sites of exon 51. After a dose-escalation study, we systemically delivered this cocktail into mdx52 mice seven times at weekly intervals. This induced 20-30% of wild-type (WT) dystrophin expression levels in all muscles, and was accompanied by amelioration of the dystrophic pathology and improvement of skeletal muscle function. Because the structure of the restored in-frame dystrophin resembles human dystrophin following exon 51-skipping, our results are encouraging for the ongoing clinical trials for DMD. Moreover, the therapeutic dose required can provide a suggestion of the theoretical equivalent dose for humans.Keywords:
Exon skipping
Morpholino
mdx mouse
Exon skipping
mdx mouse
Utrophin
Adeno-associated virus
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Duchenne and Becker muscular dystrophies are allelic disorders arising from mutations in the dystrophin gene. Duchenne muscular dystrophy is characterised by an absence of functional protein, while Becker muscular dystrophy, commonly caused by in-frame deletions, shows synthesis of partially functional protein. Antisense oligonucleotides can induce specific exon removal during processing of the dystrophin primary transcript, whilst maintaining or restoring the reading frame, and thereby overcome proteintruncating mutations. The mdx mouse has a nonsense mutation in exon 23 of the dystrophin gene that precludes functional dystrophin production, and this model has been used in the development of treatment strategies for dystrophinopathies. A phosphorodiamidate morpholino oligomer has previously been shown to exclude exon 23 from the dystrophin gene transcript and induce dystrophin expression in the mdx mouse, in vivo and in vitro. A cell-penetrating peptide-conjugated oligomer, targeted to the mouse dystrophin exon 23 donor splice site, was administered to mdx mice by intraperitoneal injection. We demonstrate dystrophin expression and near-normal muscle architecture in all muscles examined, except for cardiac muscle. The cell penetrating peptide greatly enhanced uptake of the phosphorodiamidate morpholino oligomer, resulting in widespread dystrophin expression. Treatment of neonatal mdx mice induced dystrophin expression and averted the onset of the dystrophic process that normally begins shortly before 3 weeks of age.
Morpholino
Exon skipping
mdx mouse
Utrophin
Nonsense mutation
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Duchenne muscular dystrophy (DMD) is a severe neuromuscular disorder caused by mutations in the dystrophin gene that result in the absence of functional protein. Antisense-mediated exon-skipping is one of the most promising approaches for the treatment of DMD because of its capacity to correct the reading frame and restore dystrophin expression, which has been demonstrated in vitro and in vivo. In particular, peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs) have recently been shown to induce widespread high levels of dystrophin expression in the mdx mouse model. Here, we report the efficiency of the PPMO-mediated exon-skipping approach in the utrophin/dystrophin double-knockout mouse (dKO) mouse, which is a much more severe and progressive mouse model of DMD. Repeated intraperitoneal (i.p.) injections of a PPMO targeted to exon 23 of dystrophin pre-mRNA in dKO mice induce a near-normal level of dystrophin expression in all muscles examined, except for the cardiac muscle, resulting in a considerable improvement of their muscle function and dystrophic pathology. These findings suggest great potential for PPMOs in systemic treatment of the DMD phenotype. Duchenne muscular dystrophy (DMD) is a severe neuromuscular disorder caused by mutations in the dystrophin gene that result in the absence of functional protein. Antisense-mediated exon-skipping is one of the most promising approaches for the treatment of DMD because of its capacity to correct the reading frame and restore dystrophin expression, which has been demonstrated in vitro and in vivo. In particular, peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs) have recently been shown to induce widespread high levels of dystrophin expression in the mdx mouse model. Here, we report the efficiency of the PPMO-mediated exon-skipping approach in the utrophin/dystrophin double-knockout mouse (dKO) mouse, which is a much more severe and progressive mouse model of DMD. Repeated intraperitoneal (i.p.) injections of a PPMO targeted to exon 23 of dystrophin pre-mRNA in dKO mice induce a near-normal level of dystrophin expression in all muscles examined, except for the cardiac muscle, resulting in a considerable improvement of their muscle function and dystrophic pathology. These findings suggest great potential for PPMOs in systemic treatment of the DMD phenotype.
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Exon skipping
Utrophin
Oligomer
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mdx mouse
Exon skipping
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Induced splice modulation of pre-mRNAs shows promise to correct aberrant disease transcripts and restore functional protein and thus has therapeutic potential. Duchenne muscular dystrophy (DMD) results from mutations that disrupt the DMD gene open reading frame causing an absence of dystrophin protein. Antisense oligonucleotide (AO)-mediated exon skipping has been shown to restore functional dystrophin in mdx mice and DMD patients treated intramuscularly in two recent phase 1 clinical trials. Critical to the therapeutic success of AO-based treatment will be the ability to deliver AOs systemically to all affected tissues including the heart. Here, we report identification of a series of transduction peptides (Pip5) as AO conjugates for enhanced systemic and particularly cardiac delivery. One of the lead peptide-AO conjugates, Pip5e-AO, showed highly efficient exon skipping and dystrophin production in mdx mice with complete correction of the aberrant DMD transcript in heart, leading to >50% of the normal level of dystrophin in heart. Mechanistic studies indicated that the enhanced activity of Pip5e-phosphorodiamidate morpholino (PMO) is partly explained by more efficient nuclear delivery. Pip5 series derivatives therefore have significant potential for advancing the development of exon skipping therapies for DMD and may have application for enhanced cardiac delivery of other biotherapeutics. Induced splice modulation of pre-mRNAs shows promise to correct aberrant disease transcripts and restore functional protein and thus has therapeutic potential. Duchenne muscular dystrophy (DMD) results from mutations that disrupt the DMD gene open reading frame causing an absence of dystrophin protein. Antisense oligonucleotide (AO)-mediated exon skipping has been shown to restore functional dystrophin in mdx mice and DMD patients treated intramuscularly in two recent phase 1 clinical trials. Critical to the therapeutic success of AO-based treatment will be the ability to deliver AOs systemically to all affected tissues including the heart. Here, we report identification of a series of transduction peptides (Pip5) as AO conjugates for enhanced systemic and particularly cardiac delivery. One of the lead peptide-AO conjugates, Pip5e-AO, showed highly efficient exon skipping and dystrophin production in mdx mice with complete correction of the aberrant DMD transcript in heart, leading to >50% of the normal level of dystrophin in heart. Mechanistic studies indicated that the enhanced activity of Pip5e-phosphorodiamidate morpholino (PMO) is partly explained by more efficient nuclear delivery. Pip5 series derivatives therefore have significant potential for advancing the development of exon skipping therapies for DMD and may have application for enhanced cardiac delivery of other biotherapeutics.
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mdx mouse
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Duchenne muscular dystrophy (DMD) is a severe, progressive, X-linked myopathy involving cycles of muscle cell degeneration, regeneration, and inflammation. DMD results from mutations in the dystrophin gene that result in a consequent failure to translate dystrophin protein. Exon skipping is a promising therapeutic strategy employing antisense oligonucleotides (AO) to exclude exons that disrupt the open reading frame, so as to produce a truncated, partially functional dystrophin protein. Several different AOs, including the phosphorodiamidate morpholino (PMO) and the 2'O-methyl phosphorothioate (2'OMe), have been shown to induce dystrophin expression and are currently being investigated in clinical trials. However, drug uptake and efficacy is inconsistent and highly-variable between and within individual muscles. Our objective was to identify factors within dystrophic muscle responsible for the observed variability in myofiber penetration, exon skipping, and dystrophin expression following systemic AO administration. Specifically, we investigated the role of myofiber regeneration on exon skipping. By treating dystrophin-null mdx mice with a single high-dose of the PMO together with staggered pulses of bromodeoxyuridine (BrdU) we can precisely identify timeframes of myocyte proliferation and relate this to the efficiency of PMO delivery. Initially, we utilized a tagged-PMO to track its efficiency of entry into muscle fibers and determine how long after administration it persists. Intriguingly, we observed that the efficiency of entry of PMO into myofibers is strongly associated with the late stages of differentiation and fusion of satellite cells into regenerating dystrophic mdx myofibers. We also observed a specific co-localization of BrdU-positive myonuclei with dystrophin-positive myofibers when the pulse of BrdU immediately preceded PMO delivery, implying that satellite cell proliferation and fusion into regenerating myofibers play a crucial role in the effectiveness of PMO-mediated exon skipping. Our investigation of the mechanisms involved in PMO uptake and efficacy will provide valuable insights for optimizing protocols for this promising approach to therapy for DMD.
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Exon skipping
mdx mouse
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Morpholino
Exon skipping
mdx mouse
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Antisense oligonucleotide (AO)-mediated exon-skipping therapeutics shows great promise for Duchenne muscular dystrophy (DMD) patients. However, recent failure with drisapersen, an AO candidate drug in phase 3 trial, highlights the importance of exploring other effective AO chemistries for DMD. Previously, we demonstrated the appreciable biological activity of peptide nucleic acid (PNA) AOs in restoring dystrophin expression in dystrophin-deficient mdx mice intramuscularly. Here, we further explore the systemic potential and feasibility of PNA AOs in mediating exon skipping in mdx mice as a comprehensive systemic evaluation remains lacking. Systemic delivery of PNA AOs resulted in therapeutic level of dystrophin expression in body-wide peripheral muscles and improved dystrophic pathology in mdx mice without any detectable toxicity. Up to 40% of dystrophin restoration was achieved in gastrocnemius, to a less extent with other skeletal muscles, with no dystrophin in heart. Notably, comparable systemic activity was obtained between PNA AOs and phosphorodiamidate morpholino oligomer, a DMD AO chemistry in phase 3 clinical trial, under an identical dosing regimen. Overall, our data demonstrate that PNA is viable for DMD exon-skipping therapeutics with 20 mer showing the best combination of activity, solubility, and safety and further modifications to increase PNA aqueous solubility can enable longer, more effective therapeutics without the associated toxicity.
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mdx mouse
Peptide nucleic acid
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Abstract Objective Duchenne muscular dystrophy (DMD) is caused by the inability to produce dystrophin protein at the myofiber membrane. A method to rescue dystrophin production by antisense oligonucleotides, termed exon‐skipping, has been reported for the mdx mouse and in four DMD patients by local intramuscular injection. We sought to test efficacy and toxicity of intravenous oligonucleotide (morpholino)‐induced exon skipping in the DMD dog model. Methods We tested a series of antisense drugs singly and as cocktails, both in primary cell culture, and two in vivo delivery methods (intramuscular injection and systemic intravenous injection). The efficiency and efficacy of multiexon skipping (exons 6–9) were tested at the messenger RNA, protein, histological, and clinical levels. Results Weekly or biweekly systemic intravenous injections with a three‐morpholino cocktail over the course of 5 to 22 weeks induced therapeutic levels of dystrophin expression throughout the body, with an average of about 26% normal levels. This was accompanied by reduced inflammatory signals examined by magnetic resonance imaging and histology, improved or stabilized timed running tests, and clinical symptoms. Blood tests indicated no evidence of toxicity. Interpretation This is the first report of widespread rescue of dystrophin expression to therapeutic levels in the dog model of DMD. This study also provides a proof of concept for systemic multiexon‐skipping therapy. Use of cocktails of morpholino, as shown here, allows broader application of this approach to a greater proportion of DMD patients (90%) and also offers the prospect of selecting deletions that optimize the functionality of the dystrophin protein. Ann Neurol 2009
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Exon skipping
mdx mouse
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