Long-term efficacy of systemic multiexon skipping targeting dystrophin exons 45-55 with a cocktail of vivo-morpholinos in mdx52 mice

A Novel CRISPR-Cas9 Strategy to Target DYSTROPHIN Mutations Downstream of Exon 44 in Patient-Specific DMD iPSCs

Mutations in the DMD gene cause fatal Duchenne Muscular Dystrophy (DMD). An attractive therapeutic approach is autologous cell transplantation utilizing myogenic progenitors derived from induced pluripotent stem cells (iPSCs). Given that a significant number of DMD mutations occur between exons 45 and 55, we developed a gene knock-in approach to correct any mutations downstream of exon 44. We applied this approach to two DMD patient-specific iPSC lines carrying mutations in exons 45 and 51 and confirmed mini-DYSTROPHIN (mini-DYS) protein expression in corrected myotubes by western blot and immunofluorescence staining. Transplantation of gene-edited DMD iPSC-derived myogenic progenitors into NSG/mdx4Cv mice produced donor-derived myofibers, as shown by the dual expression of human DYSTROPHIN and LAMIN A/C. These findings further provide proof-of-concept for the use of programmable nucleases for the development of autologous iPSC-based therapy for muscular dystrophies.

Networking to Optimize Dmd exon 53 Skipping in the Brain of mdx52 Mouse Model

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene that disrupt the open reading frame and thus prevent production of functional dystrophin proteins. Recent advances in DMD treatment, notably exon skipping and AAV gene therapy, have achieved some success aimed at alleviating the symptoms related to progressive muscle damage. However, they do not address the brain comorbidities associated with DMD, which remains a critical aspect of the disease. The mdx52 mouse model recapitulates one of the most frequent genetic pathogenic variants associated with brain involvement in DMD. Deletion of exon 52 impedes expression of two brain dystrophins, Dp427 and Dp140, expressed from distinct promoters. Interestingly, this mutation is eligible for exon skipping strategies aimed at excluding exon 51 or 53 from dystrophin mRNA. We previously showed that exon 51 skipping can restore partial expression of internally deleted yet functional Dp427 in the brain following intracerebroventricular (ICV) injection of antisense oligonucleotides (ASO). This was associated with a partial improvement of anxiety traits, unconditioned fear response, and Pavlovian fear learning and memory in the mdx52 mouse model. In the present study, we investigated in the same mouse model the skipping of exon 53 in order to restore expression of both Dp427 and Dp140. However, in contrast to exon 51, we found that exon 53 skipping was particularly difficult in mdx52 mice and a combination of multiple ASOs had to be used simultaneously to reach substantial levels of exon 53 skipping, regardless of their chemistry (tcDNA, PMO, or 2'MOE). Following ICV injection of a combination of ASO sequences, we measured up to 25% of exon 53 skipping in the hippocampus of treated mdx52 mice, but this did not elicit significant protein restoration. These findings indicate that skipping mouse dystrophin exon 53 is challenging. As such, it has not yet been possible to answer the pertinent question whether rescuing both Dp427 and Dp140 in the brain is imperative to more optimal treatment of neurological aspects of dystrophinopathy.

Self-transfecting GMO-PMO chimera targeting Nanog enable gene silencing in vitro and suppresses tumor growth in 4T1 allografts in mouse

Phosphorodiamidate morpholino oligonucleotide (PMO)-based antisense reagents cannot enter cells without the help of a delivery technique, which limits their clinical applications. To overcome this problem, self-transfecting guanidinium-linked morpholino (GMO)-PMO or PMO-GMO chimeras have been explored as antisense agents. GMO facilitates cellular internalization and participates in Watson-Crick base pairing. Targeting NANOG in MCF7 cells resulted in decline of the whole epithelial to mesenchymal transition (EMT) and stemness pathway, evident through its phenotypic manifestations, all of which were promulgated in combination with Taxol due to downregulation of MDR1 and ABCG2. GMO-PMO-mediated knockdown of no tail gene resulted in desired phenotypes in zebrafish even upon delivery after 16-cell stages. In BALB/c mice, 4T1 allografts were found to regress via intra-tumoral administration of NANOG GMO-PMO antisense oligonucleotides (ASOs), which was associated with occurrence of necrotic regions. GMO-PMO-mediated tumor regression restored histopathological damage in liver, kidney, and spleen caused by 4T1 mammary carcinoma. Serum parameters of systemic toxicity indicated that GMO-PMO chimeras are safe. To the best of our knowledge, self-transfecting antisense reagent is the first report since the discovery of guanidinium-linked DNA (DNG), which could be useful as a combination cancer therapy and, in principle, can render inhibition of any target gene without using any delivery vehicle.

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

Dystrophinopathy Phenotypes and Modifying Factors in DMD Exon 45-55 Deletion

OBJECTIVE

Duchenne muscular dystrophy (DMD) exon 45-55 deletion (del45-55) has been postulated as a model that could treat up to 60% of DMD patients, but the associated clinical variability and complications require clarification. We aimed to understand the phenotypes and potential modifying factors of this dystrophinopathy subset.

METHODS

This cross-sectional, multicenter cohort study applied clinical and functional evaluation. Next generation sequencing was employed to identify intronic breakpoints and their impact on the Dp140 promotor, intronic long noncoding RNA, and regulatory splicing sequences. DMD modifiers (SPP1, LTBP4, ACTN3) and concomitant mutations were also assessed. Haplotypes were built using DMD single nucleotide polymorphisms. Dystrophin expression was evaluated via immunostaining, Western blotting, reverse transcription polymerase chain reaction (PCR), and droplet digital PCR in 9 muscle biopsies.

RESULTS

The series comprised 57 subjects (23 index) expressing Becker phenotype (28%), isolated cardiopathy (19%), and asymptomatic features (53%). Cognitive impairment occurred in 90% of children. Patients were classified according to 10 distinct index-case breakpoints; 4 of them were recurrent due to founder events. A specific breakpoint (D5) was associated with severity, but no significant effect was appreciated due to the changes in intronic sequences. All biopsies showed dystrophin expression of >67% and traces of alternative del45-57 transcript that were not deemed pathogenically relevant. Only the LTBP4 haplotype appeared associated the presence of cardiopathy among the explored extragenic factors.

INTERPRETATION

We confirmed that del45-55 segregates a high proportion of benign phenotypes, severe cases, and isolated cardiac and cognitive presentations. Although some influence of the intronic breakpoint position and the LTBP4 modifier may exist, the pathomechanisms responsible for the phenotypic variability remain largely unresolved. ANN NEUROL 2022;92:793-806.

Development of Therapeutic RNA Manipulation for Muscular Dystrophy

Approval of therapeutic RNA molecules, including RNA vaccines, has paved the way for next-generation treatment strategies for various diseases. Oligonucleotide-based therapeutics hold particular promise for treating incurable muscular dystrophies, including Duchenne muscular dystrophy (DMD). DMD is a severe monogenic disease triggered by deletions, duplications, or point mutations in the DMD gene, which encodes a membrane-linked cytoskeletal protein to protect muscle fibers from contraction-induced injury. Patients with DMD inevitably succumb to muscle degeneration and atrophy early in life, leading to premature death from cardiac and respiratory failure. Thus far, the disease has thwarted all curative strategies. Transcriptomic manipulation, employing exon skipping using antisense oligonucleotides (ASO), has made significant progress in the search for DMD therapeutics. Several exon-skipping drugs employing RNA manipulation technology have been approved by regulatory agencies and have shown promise in clinical trials. This review summarizes recent scientific and clinical progress of ASO and other novel RNA manipulations, including RNA-based editing using MS2 coat protein-conjugated adenosine deaminase acting on the RNA (MCP-ADAR) system illustrating the efficacy and limitations of therapies to restore dystrophin. Perhaps lessons from this review will encourage the application of RNA-editing therapy to other neuromuscular disorders.

Antisense and Gene Therapy Options for Duchenne Muscular Dystrophy Arising from Mutations in the N-Terminal Hotspot

Duchenne muscular dystrophy (DMD) is a fatal genetic disease affecting children that is caused by a mutation in the gene encoding for dystrophin. In the absence of functional dystrophin, patients experience progressive muscle deterioration, leaving them wheelchair-bound by age 12 and with few patients surviving beyond their third decade of life as the disease advances and causes cardiac and respiratory difficulties. In recent years, an increasing number of antisense and gene therapies have been studied for the treatment of muscular dystrophy; however, few of these therapies focus on treating mutations arising in the N-terminal encoding region of the dystrophin gene. This review summarizes the current state of development of N-terminal antisense and gene therapies for DMD, mainly focusing on exon-skipping therapy for duplications and deletions, as well as microdystrophin therapy.

A Dystrophin Exon-52 Deleted Miniature Pig Model of Duchenne Muscular Dystrophy and Evaluation of Exon Skipping

Duchenne muscular dystrophy (DMD) is a lethal X-linked recessive disorder caused by mutations in the DMD gene and the subsequent lack of dystrophin protein. Recently, phosphorodiamidate morpholino oligomer (PMO)-antisense oligonucleotides (ASOs) targeting exon 51 or 53 to reestablish the DMD reading frame have received regulatory approval as commercially available drugs. However, their applicability and efficacy remain limited to particular patients. Large animal models and exon skipping evaluation are essential to facilitate ASO development together with a deeper understanding of dystrophinopathies. Using recombinant adeno-associated virus-mediated gene targeting and somatic cell nuclear transfer, we generated a Yucatan miniature pig model of DMD with an exon 52 deletion mutation equivalent to one of the most common mutations seen in patients. Exon 52-deleted mRNA expression and dystrophin deficiency were confirmed in the skeletal and cardiac muscles of DMD pigs. Accordingly, dystrophin-associated proteins failed to be recruited to the sarcolemma. The DMD pigs manifested early disease onset with severe bodywide skeletal muscle degeneration and with poor growth accompanied by a physical abnormality, but with no obvious cardiac phenotype. We also demonstrated that in primary DMD pig skeletal muscle cells, the genetically engineered exon-52 deleted pig DMD gene enables the evaluation of exon 51 or 53 skipping with PMO and its advanced technology, peptide-conjugated PMO. The results show that the DMD pigs developed here can be an appropriate large animal model for evaluating in vivo exon skipping efficacy.

Current Pharmacological Strategies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, X-linked neuromuscular disorder caused by the absence of dystrophin protein, which is essential for muscle fiber integrity. Loss of dystrophin protein leads to recurrent myofiber damage, chronic inflammation, progressive fibrosis, and dysfunction of muscle stem cells. There is still no cure for DMD so far and the standard of care is principally limited to symptom relief through glucocorticoids treatments. Current therapeutic strategies could be divided into two lines. Dystrophin-targeted therapeutic strategies that aim at restoring the expression and/or function of dystrophin, including gene-based, cell-based and protein replacement therapies. The other line of therapeutic strategies aims to improve muscle function and quality by targeting the downstream pathological changes, including inflammation, fibrosis, and muscle atrophy. This review introduces the important developments in these two lines of strategies, especially those that have entered the clinical phase and/or have great potential for clinical translation. The rationale and efficacy of each agent in pre-clinical or clinical studies are presented. Furthermore, a meta-analysis of gene profiling in DMD patients has been performed to understand the molecular mechanisms of DMD.

Animal models for researching approaches to therapy of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a relatively widespread genetic disease which develops as a result of a mutation in the gene DMD encoding dystrophin. In this review, animal models of DMD are described. These models are used in preclinical studies to elucidate the pathogenesis of the disease or to develop effective treatments; each animal model has its own advantages and disadvantages. For instance, Caenorhabditis elegans, Drosophila melanogaster, and zebrafish (sapje) are suitable for large-scale chemical screening of large numbers of small molecules, but their disease phenotype differs from that of mammals. The use of larger animals is important for understanding of the potential efficacy of various treatments for DMD. While mdx mice have their advantages, they exhibit a milder disease phenotype compared to humans or dogs, making it difficult to evaluate the efficacy of new treatment for DMD. The disease in dogs and pigs is more severe and progresses faster than in mice, but it is more difficult to breed and obtain sufficient numbers of specimens in order to achieve statistically significant results. Moreover, working with large animals is also more labor-intensive. Therefore, when choosing the optimal animal model for research, it is worth considering all the goals and objectives.

Systemic antisense therapeutics inhibiting DUX4 expression ameliorates FSHD-like pathology in an FSHD mouse model

Aberrant expression of the double homeobox 4 (DUX4) gene in skeletal muscle causes muscle deterioration and weakness in Facioscapulohumeral muscular dystrophy (FSHD). Since the presence of a permissive pLAM1 polyadenylation signal is essential for stabilization of DUX4 mRNA and translation of DUX4 protein, disrupting the function of this structure can prevent expression of DUX4. We and others have shown promising results using antisense approaches to reduce DUX4 expression in vitro and in vivo following local intramuscular administration. Here we demonstrate that further development of the antisense chemistries enhances in vitro antisense efficacy. The optimal chemistry was conjugated to a cell-penetrating moiety and was systemically administered into the tamoxifen-inducible Cre-driver FLExDUX4 double-transgenic mouse model of FSHD. After four weekly treatments, mRNA quantities of DUX4 and target genes were reduced by 50% that led to 12% amelioration in muscle atrophy, 52% improvement in in situ muscle strength, 17% reduction in muscle fibrosis and prevention of shift in the myofiber type profile. Systemic DUX4 inhibition also significantly improved the locomotor activity and reduced the fatigue level by 22%. Our data demonstrate that the optimized antisense approach has potential of being further developed as a therapeutic strategy for FSHD.

Antisense Oligonucleotide Treatment in a Humanized Mouse Model of Duchenne Muscular Dystrophy and Highly Sensitive Detection of Dystrophin Using Western Blotting

Duchenne muscular dystrophy (DMD) is a devastating X-linked muscle disorder affecting many children. The disease is caused by the lack of dystrophin production and characterized by muscle wasting. The most common causes of death are respiratory failure and heart failure. Antisense oligonucleotide-mediated exon skipping using a phosphorodiamidate morpholino oligomer (PMO) is a promising therapeutic approach for the treatment of DMD. In preclinical studies, dystrophic mouse models are commonly used for the development of therapeutic oligos. We employ a humanized model carrying the full-length human DMD transgene along with the complete knockout of the mouse Dmd gene. In this model, the effects of human-targeting AOs can be tested without cross-reaction between mouse sequences and human sequences (note that mdx, a conventional dystrophic mouse model, carries a nonsense point mutation in exon 23 and express the full-length mouse Dmd mRNA, which is a significant complicating factor). To determine if dystrophin expression is restored, the Western blotting analysis is commonly performed; however, due to the extremely large protein size of dystrophin (427 kDa), detection and accurate quantification of full-length dystrophin can be a challenge. Here, we present methodologies to systemically inject PMOs into humanized DMD model mice and determine levels of dystrophin restoration via Western blotting. Using a tris-acetate gradient SDS gel and semi-dry transfer with three buffers, including the Concentrated Anode Buffer, Anode Buffer, and Cathode Buffer, less than 1% normal levels of dystrophin expression are easily detectable. This method is fast, easy, and sensitive enough for the detection of dystrophin from both cultured muscle cells and muscle biopsy samples.

Advances in Treatments in Muscular Dystrophies and Motor Neuron Disorders

Increased understanding of disease pathophysiology and advances in gene therapies and drug technologies are revolutionizing treatment of muscular dystrophies and motor neuron disorders (MNDs). New drugs have been approved for Duchenne muscular dystrophy, spinal muscular atrophy, and amyotrophic lateral sclerosis. For other diseases, new targets have been identified, and new therapies are in clinical trials. The impact of such therapies will be fully understood only in the next decades. Cost burden and accessibility are major challenges in the wide application of new drugs. This article reviews advances in gene therapies, newly approved drugs, and therapeutic promises in muscular dystrophies and MNDs.

Inotersen for the Treatment of Hereditary Transthyretin Amyloidosis

Hereditary transthyretin amyloidosis (hATTR) is a rare autosomal dominant condition in which mutations in the transthyretin gene cause amyloid fibrils to develop and deposit into tissues, affecting primarily the nerves and heart causing polyneuropathy and cardiomyopathy respectively. Standard treatment has been liver transplants to try and eliminate the mutated transthyretin products as the liver is the main source of transthyretin production. A new drug named inotersen (brand name Tagsedi), also known as IONIS-TTR_RX, has been approved by the United States Food and Drug Agency, Health Canada, and European Commission in 2018, and introduced to the market for patients in stage 1 and stage 2 hATTR polyneuropathy. Inotersen is a second-generation antisense oligonucleotide with 2'-O-methoxyethyl modification designed to bind to the 3' untranslated region of the transthyretin mRNA in the nucleus of the liver cells. By doing so, it prevents the production of the mutant and wild-type forms of transthyretin, impeding the progression of the disease. In this article, the mechanism of action and safety profile of inotersen will be discussed along with some future directions following its approval.

Knocking Down Long Noncoding RNAs Using Antisense Oligonucleotide Gapmers

Long noncoding RNAs (lncRNAs) are a class of RNA with 200 nucleotides or longer that are not translated into protein. lncRNAs are highly abundant; a study estimates that at least four times more lncRNAs are typically present than coding RNAs in humans. However, function of more than 95% of human lncRNAs are still unknown. Synthetic antisense oligonucleotides called gapmers are powerful tools for lncRNA loss-of-function studies. Gapmers contain a central DNA part, which activates RNase H-mediated RNA degradation, flanked by modified oligonucleotides, such as 2'-O-methyl RNA (2'OMe), 2'-O-methoxyethyl RNA (2'MOE), constrained ethyl nucleosides (cEt), and locked nucleic acids (LNAs). In contrast to siRNA or RNAi-based methods, antisense oligonucleotide gapmer-based knockdown is often more effective against nuclear-localized lncRNA targets, since RNase H is mainly localized in nuclei. As such, gapmers are also potentially a powerful tool for therapeutics targeting lncRNAs in various diseases, including cancer, cardiovascular diseases, lung fibrosis, and neurological/neuromuscular diseases. This chapter will discuss the development and applications of gapmers for lncRNA loss-of-function studies and tips to design effective antisense oligonucleotides.

Gene modification strategies using AO‐mediated exon skipping and CRISPR/Cas9 as potential therapies for Duchenne muscular dystrophy patients

Duchenne muscular dystrophy (DMD) is an X-linked genetic disease affecting 1 in 5000 young males worldwide annually. Patients experience muscle weakness and loss of ambulation at an early age, with ∼75% reduced life expectancy. Recently developed genetic editing strategies aim to convert severe DMD phenotypes to a milder disease course. Among these, the antisense oligonucleotide (AO)-mediated exon skipping and the adeno-associated viral-delivered clustered regularly interspaced short palindromic repeat (CRISPR) associated protein 9 (adeno-associated viral (AAV)-delivered CRISPR/Cas9) gene editing have shown promising results in restoring dystrophin protein expression and functionality in skeletal and heart muscle in both animals and human cells in vivo and in vitro. However, therapeutic benefits currently remain unclear. The aim of this review is to compare the potential therapeutic benefits, efficacy, safety, and clinical progress of AO-mediated exon skipping and CRISPR/Cas9 gene-editing strategies. Both techniques have demonstrated therapeutic benefit and long-term efficacy in clinical trials. AAV-delivery of CRISPR/Cas9 may potentially correct disease-causing mutations following a single treatment compared to the required continuous AO/PMO-delivery of exon skipping drugs. The latter has the potential to increase the dystrophin expression in skeletal/heart muscle with sustained effects. However, therapeutic challenges including the need for optimised delivery must be overcome in to advance current clinical data.

Mutation-independent Proteomic Signatures of Pathological Progression in Murine Models of Duchenne Muscular Dystrophy

The absence of the dystrophin protein in Duchenne muscular dystrophy (DMD) results in myofiber fragility and a plethora of downstream secondary pathologies. Although a variety of experimental therapies are in development, achieving effective treatments for DMD remains exceptionally challenging, not least because the pathological consequences of dystrophin loss are incompletely understood. Here we have performed proteome profiling in tibialis anterior muscles from two murine DMD models (mdx and mdx52) at three ages (8, 16, and 80 weeks of age), all n = 3. High-resolution isoelectric focusing liquid chromatography-tandem MS (HiRIEF-LC-MS/MS) was used to quantify the expression of 4974 proteins across all 27 samples. The two dystrophic models were found to be highly similar, whereas multiple proteins were differentially expressed relative to WT (C57BL/6) controls at each age. Furthermore, 1795 proteins were differentially expressed when samples were pooled across ages and dystrophic strains. These included numerous proteins associated with the extracellular matrix and muscle function that have not been reported previously. Pathway analysis revealed multiple perturbed pathways and predicted upstream regulators, which together are indicative of cross-talk between inflammatory, metabolic, and muscle growth pathways (e.g. TNF, INFγ, NF-κB, SIRT1, AMPK, PGC-1α, PPARs, ILK, and AKT/PI3K). Upregulation of CAV3, MVP and PAK1 protein expression was validated in dystrophic muscle by Western blot. Furthermore, MVP was upregulated during, but not required for, the differentiation of C2C12 myoblasts suggesting that this protein may affect muscle regeneration. This study provides novel insights into mutation-independent proteomic signatures characteristic of the dystrophic phenotype and its progression with aging.

A novel mouse model of Duchenne muscular dystrophy carrying a multi-exonic Dmd deletion exhibits progressive muscular dystrophy and early-onset cardiomyopathy

Duchenne muscular dystrophy (DMD) is a life-threatening neuromuscular disease caused by the lack of dystrophin, resulting in progressive muscle wasting and locomotor dysfunctions. By adulthood, almost all patients also develop cardiomyopathy, which is the primary cause of death in DMD. Although there has been extensive effort in creating animal models to study treatment strategies for DMD, most fail to recapitulate the complete skeletal and cardiac disease manifestations that are presented in affected patients. Here, we generated a mouse model mirroring a patient deletion mutation of exons 52-54 (Dmd Δ52-54). The Dmd Δ52-54 mutation led to the absence of dystrophin, resulting in progressive muscle deterioration with weakened muscle strength. Moreover, Dmd Δ52-54 mice present with early-onset hypertrophic cardiomyopathy, which is absent in current pre-clinical dystrophin-deficient mouse models. Therefore, Dmd Δ52-54 presents itself as an excellent pre-clinical model to evaluate the impact on skeletal and cardiac muscles for both mutation-dependent and -independent approaches.

Enhanced dimethylarginine degradation improves coronary flow reserve and exercise tolerance in Duchenne muscular dystrophy carrier mice

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by null mutations in dystrophin and characterized by muscle degeneration. Cardiomyopathy is common and often prevalent at similar frequency in female DMD carriers irrespective of whether they manifest skeletal muscle disease. Impaired muscle nitric oxide (NO) production in DMD disrupts muscle blood flow regulation and exaggerates postexercise fatigue. We show that circulating levels of endogenous methylated arginines including asymmetric dimethylarginine (ADMA), which act as NO synthase inhibitors, are elevated by acute necrotic muscle damage and in chronically necrotic dystrophin-deficient mice. We therefore hypothesized that excessive ADMA impairs muscle NO production and diminishes exercise tolerance in DMD. We used transgenic expression of dimethylarginine dimethylaminohydrolase 1 (DDAH), which degrades methylated arginines, to investigate their contribution to exercise-induced fatigue in DMD. Although infusion of exogenous ADMA was sufficient to impair exercise performance in wild-type mice, transgenic DDAH expression did not rescue exercise-induced fatigue in dystrophin-deficient male mdx mice. Surprisingly, DDAH transgene expression did attenuate exercise-induced fatigue in dystrophin-heterozygous female mdx carrier mice. Improved exercise tolerance was associated with reduced heart weight and improved cardiac β-adrenergic responsiveness in DDAH-transgenic mdx carriers. We conclude that DDAH overexpression increases exercise tolerance in female DMD carriers, possibly by limiting cardiac pathology and preserving the heart's responses to changes in physiological demand. Methylated arginine metabolism may be a new target to improve exercise tolerance and cardiac function in DMD carriers or act as an adjuvant to promote NO signaling alongside therapies that partially restore dystrophin expression in patients with DMD.NEW & NOTEWORTHY Duchenne muscular dystrophy (DMD) carriers are at risk for cardiomyopathy. The nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) is released from damaged muscle in DMD and impairs exercise performance. Transgenic expression of dimethylarginine dimethylaminohydrolase to degrade ADMA prevents cardiac hypertrophy, improves cardiac function, and improves exercise tolerance in DMD carrier mice. These findings highlight the relevance of ADMA to muscular dystrophy and have important implications for therapies targeting nitric oxide in patients with DMD and DMD carriers.

Ex vivo cell-based CRISPR/Cas9 genome editing for therapeutic applications

The recently developed CRISPR/Cas9 technology has revolutionized the genome engineering field. Since 2016, increasing number of studies regarding CRISPR therapeutics have entered clinical trials, most of which are focusing on the ex vivo genome editing. In this review, we highlight the ex vivo cell-based CRISPR/Cas9 genome editing for therapeutic applications. In these studies, CRISPR/Cas9 tools were used to edit cells in vitro and the successfully edited cells were considered as therapeutics, which can be introduced into patients to treat diseases. Considering a large number of previous reviews have been focused on the CRISPR/Cas9 delivery methods and materials, this review provides a different perspective, by mainly introducing the targeted conditions and design strategies for ex vivo CRISPR/Cas9 therapeutics. Brief descriptions of the history, functionality, and applications of CRISPR/Cas9 systems will be introduced first, followed by the design strategies and most significant results from previous research that used ex vivo CRISPR/Cas9 genome editing for the treatment of conditions or diseases. The last part of this review includes general information about the status of CRISPR/Cas9 therapeutics in clinical trials. We also discuss some of the challenges as well as the opportunities in this research area.

Duchenne muscular dystrophy animal models for high-throughput drug discovery and precision medicine

Introduction: Duchenne muscular dystrophy (DMD) is an X-linked handicapping disease due to the loss of an essential muscle protein dystrophin. Dystrophin-null animals have been extensively used to study disease mechanisms and to develop experimental therapeutics. Despite decades of research, however, treatment options for DMD remain very limited.Areas covered: High-throughput high-content screening and precision medicine offer exciting new opportunities. Here, the authors review animal models that are suitable for these studies.Expert opinion: Nonmammalian models (worm, fruit fly, and zebrafish) are particularly attractive for cost-effective large-scale drug screening. Several promising lead compounds have been discovered using these models. Precision medicine for DMD aims at developing mutation-specific therapies such as exon-skipping and genome editing. To meet these needs, models with patient-like mutations have been established in different species. Models that harbor hotspot mutations are very attractive because the drugs developed in these models can bring mutation-specific therapies to a large population of patients. Humanized hDMD mice carry the entire human dystrophin gene in the mouse genome. Reagents developed in the hDMD mouse-based models are directly translatable to human patients.

Exons 45-55 Skipping Using Mutation-Tailored Cocktails of Antisense Morpholinos in the DMD Gene

Mutations in the dystrophin (DMD) gene and consequent loss of dystrophin cause Duchenne muscular dystrophy (DMD). A promising therapy for DMD, single-exon skipping using antisense phosphorodiamidate morpholino oligomers (PMOs), currently confronts major issues in that an antisense drug induces the production of functionally undefined dystrophin and may not be similarly efficacious among patients with different mutations. Accordingly, the applicability of this approach is limited to out-of-frame mutations. Here, using an exon-skipping efficiency predictive tool, we designed three different PMO cocktail sets for exons 45-55 skipping aiming to produce a dystrophin variant with preserved functionality as seen in milder or asymptomatic individuals with an in-frame exons 45-55 deletion. Of them, the most effective set was composed of select PMOs that each efficiently skips an assigned exon in cell-based screening. These combinational PMOs fitted to different deletions of immortalized DMD patient muscle cells significantly induced exons 45-55 skipping with removing 3, 8, or 10 exons and dystrophin restoration as represented by western blotting. In vivo skipping of the maximum 11 human DMD exons was confirmed in humanized mice. The finding indicates that our PMO set can be used to create mutation-tailored cocktails for exons 45-55 skipping and treat over 65% of DMD patients carrying out-of-frame or in-frame deletions.

Peptide-conjugate antisense based splice-correction for Duchenne muscular dystrophy and other neuromuscular diseases

Duchenne muscular dystrophy (DMD) is an X-linked disorder characterized by progressive muscle degeneration, caused by the absence of dystrophin. Exon skipping by antisense oligonucleotides (ASOs) has recently gained recognition as therapeutic approach in DMD. Conjugation of a peptide to the phosphorodiamidate morpholino backbone (PMO) of ASOs generated the peptide-conjugated PMOs (PPMOs) that exhibit a dramatically improved pharmacokinetic profile. When tested in animal models, PPMOs demonstrate effective exon skipping in target muscles and prolonged duration of dystrophin restoration after a treatment regime. Herein we summarize the main pathophysiological features of DMD and the emergence of PPMOs as promising exon skipping agents aiming to rescue defective gene expression in DMD and other neuromuscular diseases. The listed PPMO laboratory findings correspond to latest trends in the field and highlight the obstacles that must be overcome prior to translating the animal-based research into clinical trials tailored to the needs of patients suffering from neuromuscular diseases.

In Vivo Evaluation of Multiple Exon Skipping with Peptide-PMOs in Cardiac and Skeletal Muscles in Dystrophic Dogs

Exon skipping is an emerging approach to treating Duchenne muscular dystrophy (DMD), one of the most common lethal genetic disorders. Exon skipping uses synthetic antisense oligonucleotides (AONs) to splice out frame-disrupting exon(s) of DMD mRNA to restore the reading frame of the gene products and produce truncated yet functional proteins. The FDA conditionally approved the first exon-skipping AON, called eteplirsen (brand name ExonDys51), targeting exon 51 of the DMD gene, in late 2016. Using a cocktail of AONs, multiple exons can be skipped, which can theoretically treat 80-90% of patients with DMD. Although the success of multiple exon skipping in a DMD dog model has made a significant impact on the development of therapeutics for DMD, unmodified AONs such as phosphorodiamidate morpholino oligomers (PMOs) have little efficacy in cardiac muscles. Here, we describe our technique of intravenous injection of a cocktail of peptide-conjugated PMOs (PPMOs) to skip multiple exons, exons 6 and 8, in both skeletal and cardiac muscles in dystrophic dogs and the evaluation of the efficacy and toxicity.

Tips to Design Effective Splice-Switching Antisense Oligonucleotides for Exon Skipping and Exon Inclusion

Antisense-mediated exon skipping and exon inclusion have proven to be powerful tools for treating neuromuscular diseases. The approval of Exondys 51 (eteplirsen) and Spinraza (nusinersen) for the treatment of patients with Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA) was the most noteworthy accomplishment in 2016. Exon skipping uses short DNA-like molecules called antisense oligonucleotides (AONs) to correct the disrupted reading frame, allowing the production of functional quasi-dystrophin proteins, and ameliorate the progression of the disease. Exon inclusion for SMA employs an AON targeting an intronic splice silencer site to include an exon which is otherwise spliced out. Recently, these strategies have also been explored in many other genetic disorders, including dysferlin-deficient muscular dystrophy (e.g., Miyoshi myopathy; MM, limb-girdle muscular dystrophy type 2B; LGMD2B, and distal myopathy with anterior tibial onset; DMAT), laminin α2 chain (merosin)-deficient congenital muscular dystrophy (MDC1A), sarcoglycanopathy (e.g., limb-girdle muscular dystrophy type 2C; LGMD2C), and Fukuyama congenital muscular dystrophy (FCMD). A major challenge in exon skipping and exon inclusion is the difficulty in designing effective AONs. The mechanism of mRNA splicing is highly complex, and the efficacy of AONs is often unpredictable. We will discuss the design of effective AONs for exon skipping and exon inclusion in this chapter.

In Vivo Evaluation of Single-Exon and Multiexon Skipping in mdx52 Mice

Exon-skipping therapy is an emerging approach that uses synthetic DNA-like molecules called antisense oligonucleotides (ASOs) to splice out frame-disrupting parts of mRNA, restore the reading frame, and produce truncated yet functional proteins. Phosphorodiamidate morpholino oligomer (PMO) is one of the safest among therapeutic ASOs for patients and has recently been approved under the accelerated approval program by the US Food and Drug Administration (FDA) as the first ASO-based drug for Duchenne muscular dystrophy (DMD). Multi-exon skipping utilizing ASOs can theoretically treat 80-90% of patients with DMD. Here, we describe the systemic delivery of a cocktail of ASOs to skip exon 51 and exons 45-55 in the mdx52 mouse, an exon 52 deletion model of DMD produced by gene targeting, and the evaluation of their efficacies in vivo.

Genome Editing in Induced Pluripotent Stem Cells using CRISPR/Cas9

The development of the reprogramming technology led to generation of induced Pluripotent Stem Cells (iPSC) from a variety of somatic cells. Ever since, fast growing knowledge of different efficient protocols enabled the differentiation of these iPSCs into different cells types utilized for disease modeling. Indeed, iPSC-derived cells have been increasingly used for investigating molecular and cellular pathophysiological mechanisms underlying inherited diseases. However, a major barrier in the field of iPSC-based disease modeling relies on discriminating between the effects of the causative mutation and the genetic background of these cells. In the past decade, researchers have made great improvement in genome editing techniques, with one of the latest being CRISPR/Cas9. Using a single non-sequence specific protein combined with a small guiding RNA molecule, this state-of-the-art approach enables modifications of genes with high efficiency and accuracy. By so doing, this technique enables the generation of isogenic controls or isogenic mutated cell lines in order to focus on the pathologies caused by a specific mutation. In this article, we review the latest studies combining iPSC and CRISPR/Cas9 technologies for the investigation of the molecular and cellular mechanisms underlying inherited diseases including immunological, metabolic, hematological, neurodegenerative and cardiac diseases.

Multiple Exon Skipping in the Duchenne Muscular Dystrophy Hot Spots: Prospects and Challenges

Duchenne muscular dystrophy (DMD), a fatal X-linked recessive disorder, is caused mostly by frame-disrupting, out-of-frame deletions in the dystrophin (DMD) gene. Antisense oligonucleotide-mediated exon skipping is a promising therapy for DMD. Exon skipping aims to convert out-of-frame mRNA to in-frame mRNA and induce the production of internally-deleted dystrophin as seen in the less severe Becker muscular dystrophy. Currently, multiple exon skipping has gained special interest as a new therapeutic modality for this approach. Previous retrospective database studies represented a potential therapeutic application of multiple exon skipping. Since then, public DMD databases have become more useful with an increase in patient registration and advances in molecular diagnosis. Here, we provide an update on DMD genotype-phenotype associations using a global DMD database and further provide the rationale for multiple exon skipping development, particularly for exons 45–55 skipping and an emerging therapeutic concept, exons 3–9 skipping. Importantly, this review highlights the potential of multiple exon skipping for enabling the production of functionally-corrected dystrophin and for treating symptomatic patients not only with out-of-frame deletions but also those with in-frame deletions. We will also discuss prospects and challenges in multiple exon skipping therapy, referring to recent progress in antisense chemistry and design, as well as disease models.

Designing Effective Antisense Oligonucleotides for Exon Skipping

During the past 10 years, antisense oligonucleotide-mediated exon skipping and splice modulation have proven to be powerful tools for correction of mRNA splicing in genetic diseases. In 2016, the US Food and Drug Administration (FDA)-approved Exondys 51 (eteplirsen) and Spinraza (nusinersen), the first exon skipping and exon inclusion drugs, to treat patients with Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA), respectively. The exon skipping of DMD mRNA aims to restore the disrupted reading frame using antisense oligonucleotides (AONs), allowing the production of truncated but partly functional dystrophin proteins, and slow down the progression of the disease. This approach has also been explored in several other genetic disorders, including laminin α2 chain-deficient congenital muscular dystrophy, dysferlin-deficient muscular dystrophy (e.g., Miyoshi myopathy and limb-girdle muscular dystrophy type 2B), sarcoglycanopathy (limb-girdle muscular dystrophy type 2C), and Fukuyama congenital muscular dystrophy. Antisense-mediated exon skipping is also a powerful tool to examine the function of genes and exons. A significant challenge in exon skipping is how to design effective AONs. The mechanism of mRNA splicing is highly complex with many factors involved. The selection of target sites, the length of AONs, the AON chemistry, and the melting temperature versus the RNA strand play important roles. A cocktail of AONs can be employed to skip multiples exons. In this chapter, we discuss the design of effective AONs for exon skipping.

Antisense PMO cocktails effectively skip dystrophin exons 45-55 in myotubes transdifferentiated from DMD patient fibroblasts

Antisense-mediated exon skipping has made significant progress as a therapeutic platform in recent years, especially in the case of Duchenne muscular dystrophy (DMD). Despite FDA approval of eteplirsen-the first-ever antisense drug clinically marketed for DMD-exon skipping therapy still faces the significant hurdles of limited applicability and unknown truncated protein function. In-frame exon skipping of dystrophin exons 45-55 represents a significant approach to treating DMD, as a large proportion of patients harbor mutations within this "hotspot" region. Additionally, patients harboring dystrophin exons 45-55 deletion mutations are reported to have exceptionally mild to asymptomatic phenotypes. Here, we demonstrate that a cocktail of phosphorodiamidate morpholino oligomers can effectively skip dystrophin exons 45-55 in vitro in myotubes transdifferentiated from DMD patient fibroblast cells. This is the first report of substantive exons 45-55 skipping in DMD patient cells. These findings help validate the use of transdifferentiated patient fibroblast cells as a suitable cell model for dystrophin exon skipping assays and further emphasize the feasibility of dystrophin exons 45-55 skipping in patients.

Autologous Cell Therapy Approach for Duchenne Muscular Dystrophy using PiggyBac Transposons and Mesoangioblasts

Duchenne muscular dystrophy (DMD) is a lethal muscle-wasting disease currently without cure. We investigated the use of the PiggyBac transposon for full-length dystrophin expression in murine mesoangioblast (MABs) progenitor cells. DMD murine MABs were transfected with transposable expression vectors for full-length dystrophin and transplanted intramuscularly or intra-arterially into mdx/SCID mice. Intra-arterial delivery indicated that the MABs could migrate to regenerating muscles to mediate dystrophin expression. Intramuscular transplantation yielded dystrophin expression in 11%-44% of myofibers in murine muscles, which remained stable for the assessed period of 5 months. The satellite cells isolated from transplanted muscles comprised a fraction of MAB-derived cells, indicating that the transfected MABs may colonize the satellite stem cell niche. Transposon integration site mapping by whole-genome sequencing indicated that 70% of the integrations were intergenic, while none was observed in an exon. Muscle resistance assessment by atomic force microscopy indicated that 80% of fibers showed elasticity properties restored to those of wild-type muscles. As measured in vivo, transplanted muscles became more resistant to fatigue. This study thus provides a proof-of-principle that PiggyBac transposon vectors may mediate full-length dystrophin expression as well as functional amelioration of the dystrophic muscles within a potential autologous cell-based therapeutic approach of DMD.

Current Translational Research and Murine Models For Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked genetic disorder characterized by progressive muscle degeneration. Mutations in the DMD gene result in the absence of dystrophin, a protein required for muscle strength and stability. Currently, there is no cure for DMD. Since murine models are relatively easy to genetically manipulate, cost effective, and easily reproducible due to their short generation time, they have helped to elucidate the pathobiology of dystrophin deficiency and to assess therapies for treating DMD. Recently, several murine models have been developed by our group and others to be more representative of the human DMD mutation types and phenotypes. For instance, mdx mice on a DBA/2 genetic background, developed by Fukada et al., have lower regenerative capacity and exhibit very severe phenotype. Cmah-deficient mdx mice display an accelerated disease onset and severe cardiac phenotype due to differences in glycosylation between humans and mice. Other novel murine models include mdx52, which harbors a deletion mutation in exon 52, a hot spot region in humans, and dystrophin/utrophin double-deficient (dko), which displays a severe dystrophic phenotype due the absence of utrophin, a dystrophin homolog. This paper reviews the pathological manifestations and recent therapeutic developments in murine models of DMD such as standard mdx (C57BL/10), mdx on C57BL/6 background (C57BL/6-mdx), mdx52, dystrophin/utrophin double-deficient (dko), mdxβgeo, Dmd-null, humanized DMD (hDMD), mdx on DBA/2 background (DBA/2-mdx), Cmah-mdx, and mdx/mTRKO murine models.

Systemic Delivery of Morpholinos to Skip Multiple Exons in a Dog Model of Duchenne Muscular Dystrophy

Exon-skipping therapy is an emerging approach that uses synthetic DNA-like molecules called antisense oligonucleotides (AONs) to splice out frame-disrupting parts of mRNA, restore the reading frame, and produce truncated yet functional proteins. Multiple exon skipping utilizing a cocktail of AONs can theoretically treat 80-90% of patients with Duchenne muscular dystrophy (DMD). The success of multiple exon skipping by the systemic delivery of a cocktail of AONs called phosphorodiamidate morpholino oligomers (PMOs) in a DMD dog model has made a significant impact on the development of therapeutics for DMD, leading to clinical trials of PMO-based drugs. Here, we describe the systemic delivery of a cocktail of PMOs to skip multiple exons in dystrophic dogs and the evaluation of the efficacies and toxicity in vivo.

A dystrophic Duchenne mouse model for testing human antisense oligonucleotides

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disease generally caused by reading frame disrupting mutations in the DMD gene resulting in loss of functional dystrophin protein. The reading frame can be restored by antisense oligonucleotide (AON)-mediated exon skipping, allowing production of internally deleted, but partially functional dystrophin proteins as found in the less severe Becker muscular dystrophy. Due to genetic variation between species, mouse models with mutations in the murine genes are of limited use to test and further optimize human specific AONs in vivo. To address this we have generated the del52hDMD/mdx mouse. This model carries both murine and human DMD genes. However, mouse dystrophin expression is abolished due to a stop mutation in exon 23, while the expression of human dystrophin is abolished due to a deletion of exon 52. The del52hDMD/mdx model, like mdx, shows signs of muscle dystrophy on a histological level and phenotypically mild functional impairment. Local administration of human specific vivo morpholinos induces exon skipping and dystrophin restoration in these mice. Depending on the number of mismatches, occasional skipping of the murine Dmd gene, albeit at low levels, could be observed. Unlike previous models, the del52hDMD/mdx model enables the in vivo analysis of human specific AONs targeting exon 51 or exon 53 on RNA and protein level and muscle quality and function. Therefore, it will be a valuable tool for optimizing human specific AONs and genome editing approaches for DMD.

Skipping Multiple Exons to Treat DMD-Promises and Challenges

Duchenne muscular dystrophy (DMD) is a lethal disorder caused by mutations in the DMD gene. Antisense-mediated exon-skipping is a promising therapeutic strategy that makes use of synthetic nucleic acids to skip frame-disrupting exon(s) and allows for short but functional protein expression by restoring the reading frame. In 2016, the U.S. Food and Drug Administration (FDA) approved eteplirsen, which skips DMD exon 51 and is applicable to approximately 13% of DMD patients. Multiple exon skipping, which is theoretically applicable to 80-90% of DMD patients in total, have been demonstrated in animal models, including dystrophic mice and dogs, using cocktail antisense oligonucleotides (AOs). Although promising, current drug approval systems pose challenges for the use of a cocktail AO. For example, both exons 6 and 8 need to be skipped to restore the reading frame in dystrophic dogs. Therefore, the cocktail of AOs targeting these exons has a combined therapeutic effect and each AO does not have a therapeutic effect by itself. The current drug approval system is not designed to evaluate such circumstances, which are completely different from cocktail drug approaches in other fields. Significant changes are needed in the drug approval process to promote the cocktail AO approach.

Restoration of Dystrophin Protein Expression by Exon Skipping Utilizing CRISPR-Cas9 in Myoblasts Derived from DMD Patient iPS Cells

Duchenne muscular dystrophy (DMD) is a congenital X-linked disease caused by mutations in the gene encoding the dystrophin protein, which is required for myofiber integrity. Exon skipping therapy is an emerging strategy for restoring the open reading frame of the dystrophin gene to produce functional protein in DMD patients by skipping single or multiple exons. Although antisense oligonucleotides are able to target pre-mRNA for exon skipping, their half-lives are short and any therapeutic benefit is transient. In contrast, genome editing by DNA nucleases, such as the CRISPR-Cas9 system, could offer permanent correction by targeting genomic DNA. Our laboratory previously reported that disrupting the splicing acceptor site in exon 45 by plasmid delivery of the CRISPR-Cas9 system in iPS cells, derived from a DMD patient lacking exon 44, successfully restored dystrophin protein expression in differentiated myoblasts. Herein, we describe an optimized methodology to prepare myoblasts differentiated from iPS cells by mRNA transfection of the CRISPR-Cas9 system to skip exon 45 in myoblasts, and evaluate the restored dystrophin by RT-PCR and Western blotting.

An Overview of Recent Advances and Clinical Applications of Exon Skipping and Splice Modulation for Muscular Dystrophy and Various Genetic Diseases

Exon skipping is a therapeutic approach that is feasible for various genetic diseases and has been studied and developed for over two decades. This approach uses antisense oligonucleotides (AON) to modify the splicing of pre-mRNA to correct the mutation responsible for a disease, or to suppress a particular gene expression, as in allergic diseases. Antisense-mediated exon skipping is most extensively studied in Duchenne muscular dystrophy (DMD) and has developed from in vitro proof-of-concept studies to clinical trials targeting various single exons such as exon 45 (casimersen), exon 53 (NS-065/NCNP-01, golodirsen), and exon 51 (eteplirsen). Eteplirsen (brand name Exondys 51), is the first approved antisense therapy for DMD in the USA, and provides a treatment option for ~14% of all DMD patients, who are amenable to exon 51 skipping. Eteplirsen is granted accelerated approval and marketing authorization by the US Food and Drug Administration (FDA), on the condition that additional postapproval trials show clinical benefit. Permanent exon skipping achieved at the DNA level using clustered regularly interspaced short palindromic repeats (CRISPR) technology holds promise in current preclinical trials for DMD. In hopes of achieving clinical success parallel to DMD, exon skipping and splice modulation are also being studied in other muscular dystrophies, such as Fukuyama congenital muscular dystrophy (FCMD), dysferlinopathy including limb-girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy (MM), and distal anterior compartment myopathy (DMAT), myotonic dystrophy, and merosin-deficient congenital muscular dystrophy type 1A (MDC1A). This chapter also summarizes the development of antisense-mediated exon skipping therapy in diseases such as Usher syndrome, dystrophic epidermolysis bullosa, fibrodysplasia ossificans progressiva (FOP), and allergic diseases.

Making sense of antisense oligonucleotides: A narrative review

Synthetic nucleic acid sequences that bind to ribonucleic acid (RNA) through Watson-Crick base pairing are known as antisense oligonucleotides (ASOs) because they are complementary to "sense strand" nucleic acids. ASOs bind to selected sequences of RNA and regulate the expression of genes by several mechanisms depending on their chemical properties and targets. They can be used to restore deficient protein expression, reduce the expression of a toxic protein, modify functional effects of proteins, or reduce toxicity of mutant proteins. Two ASOs were approved by the U.S. Food and Drug Administration in 2016: eteplirsen for Duchenne muscular dystrophy and nusinersen for spinal muscular atrophy. Clinical trials in amyotrophic lateral sclerosis and familial amyloid polyneuropathy are ongoing. We review the chemistry, pharmacology, and mechanisms of action of ASOs, preclinical data, and clinical trials in neuromuscular diseases and discuss some ethical, regulatory, and policy considerations in the clinical development and use of ASOs. Muscle Nerve 57: 356-370, 2018.

Dystrophin Exon 29 Nonsense Mutations Cause a Variably Mild Phenotype

BACKGROUND

Nonsense mutations in the dystrophin gene usually result in a severe Duchenne muscular dystrophy phenotype.

FINDINGS

We describe a 7-year-old boy with a rare pathogenic mutation in exon 29 c.3940C>T p.(Arg1314Ter) resulting in exon skipping, in turn rescuing the phenotype from a severe Duchenne type to a milder Becker muscular dystrophy type. No adults have been described with this mutation to date.

CONCLUSIONS

Exon skipping of exon 29 results in a higher level of functional dystrophin. Some cases of muscular dystrophy may still require muscle biopsy to determine optimal management and pharmaceutical treatment options.

The AAV-mediated and RNA-guided CRISPR/Cas9 system for gene therapy of DMD and BMD

Mutations in the dystrophin gene (Dmd) result in Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD), which afflict many newborn boys. In 2016, Brain and Development published several interesting articles on DMD treatment with antisense oligonucleotide, kinase inhibitor, and prednisolone. Even more strikingly, three articles in the issue 6271 of Science in 2016 provide new insights into gene therapy of DMD and BMD via the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). In brief, adeno-associated virus (AAV) vectors transport guided RNAs (gRNAs) and Cas9 into mdx mouse model, gRNAs recognize the mutated Dmd exon 23 (having a stop codon), and Cas9 cut the mutated exon 23 off the Dmd gene. These manipulations restored expression of truncated but partially functional dystrophin, improved skeletal and cardiac muscle function, and increased survival of mdx mice significantly. This review concisely summarized the related advancements and discussed their primary implications in the future gene therapy of DMD, including AAV-vector selection, gRNA designing, Cas9 optimization, dystrophin-restoration efficiency, administration routes, and systemic and long-term therapeutic efficacy. Future orientations, including off-target effects, safety concerns, immune responses, precision medicine, and Dmd-editing in the brain (potentially blocked by the blood-brain barrier) were also elucidated briefly. Collectively, the AAV-mediated and RNA-guided CRISPR/Cas9 system has major superiorities compared with traditional gene therapy, and might contribute to the treatment of DMD and BMD substantially in the near future.

Quantitative Antisense Screening and Optimization for Exon 51 Skipping in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), the most common lethal genetic disorder, is caused by mutations in the dystrophin (DMD) gene. Exon skipping is a therapeutic approach that uses antisense oligonucleotides (AOs) to modulate splicing and restore the reading frame, leading to truncated, yet functional protein expression. In 2016, the US Food and Drug Administration (FDA) conditionally approved the first phosphorodiamidate morpholino oligomer (morpholino)-based AO drug, eteplirsen, developed for DMD exon 51 skipping. Eteplirsen remains controversial with insufficient evidence of its therapeutic effect in patients. We recently developed an in silico tool to design antisense morpholino sequences for exon skipping. Here, we designed morpholino AOs targeting DMD exon 51 using the in silico tool and quantitatively evaluated the effects in immortalized DMD muscle cells in vitro. To our surprise, most of the newly designed morpholinos induced exon 51 skipping more efficiently compared with the eteplirsen sequence. The efficacy of exon 51 skipping and rescue of dystrophin protein expression were increased by up to more than 12-fold and 7-fold, respectively, compared with the eteplirsen sequence. Significant in vivo efficacy of the most effective morpholino, determined in vitro, was confirmed in mice carrying the human DMD gene. These findings underscore the importance of AO sequence optimization for exon skipping.

Nanotherapy for Duchenne muscular dystrophy

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

Effects of systemic multiexon skipping with peptide-conjugated morpholinos in the heart of a dog model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal genetic disorder caused by an absence of the dystrophin protein in bodywide muscles, including the heart. Cardiomyopathy is a leading cause of death in DMD. Exon skipping via synthetic phosphorodiamidate morpholino oligomers (PMOs) represents one of the most promising therapeutic options, yet PMOs have shown very little efficacy in cardiac muscle. To increase therapeutic potency in cardiac muscle, we tested a next-generation morpholino: arginine-rich, cell-penetrating peptide-conjugated PMOs (PPMOs) in the canine X-linked muscular dystrophy in Japan (CXMD_J) dog model of DMD. A PPMO cocktail designed to skip dystrophin exons 6 and 8 was injected intramuscularly, intracoronarily, or intravenously into CXMD_J dogs. Intravenous injections with PPMOs restored dystrophin expression in the myocardium and cardiac Purkinje fibers, as well as skeletal muscles. Vacuole degeneration of cardiac Purkinje fibers, as seen in DMD patients, was ameliorated in PPMO-treated dogs. Although symptoms and functions in skeletal muscle were not ameliorated by i.v. treatment, electrocardiogram abnormalities (increased Q-amplitude and Q/R ratio) were improved in CXMD_J dogs after intracoronary or i.v. administration. No obvious evidence of toxicity was found in blood tests throughout the monitoring period of one or four systemic treatments with the PPMO cocktail (12 mg/kg/injection). The present study reports the rescue of dystrophin expression and recovery of the conduction system in the heart of dystrophic dogs by PPMO-mediated multiexon skipping. We demonstrate that rescued dystrophin expression in the Purkinje fibers leads to the improvement/prevention of cardiac conduction abnormalities in the dystrophic heart.

Exon skipping: a first in class strategy for Duchenne muscular dystrophy

INTRODUCTION

Exon skipping is a therapeutic approach for Duchenne muscular dystrophy (DMD) that has been in development for close to two decades. This approach uses antisense oligonucleotides (AONs) to modulate pre-mRNA splicing of dystrophin transcripts to restore the disrupted DMD reading frame. The approach has moved from in vitro proof of concept studies to the clinical trial phase and marketing authorization applications with regulators. The first AON (eteplirsen) has recently received accelerated approval by the Food and Drug Administration in the US. Areas covered: In this review the authors explain the antisense-mediated exon skipping approach, outline how it needs be tailored for different DMD mutation types and describe the challenges and opportunities for each mutation type. The authors summarize the clinical development of antisense-mediated exon 51 skipping, and discuss methods to improve efficiency. Finally, the authors provide their opinion on current developments and identify topics for future prioritization. Expert opinion: Exon skipping development has been a learning experience for all those involved. Aside from an approved therapy, its development has yielded side benefits including the development of tools for clinical trials and has increased collaboration between academics, patients, industry and regulators.

Systemic Antisense Therapeutics for Dystrophin and Myostatin Exon Splice Modulation Improve Muscle Pathology of Adult mdx Mice

Antisense-mediated exon skipping is a promising approach for the treatment of Duchenne muscular dystrophy (DMD), a rare life-threatening genetic disease due to dystrophin deficiency. Such an approach can restore the disrupted reading frame of dystrophin pre-mRNA, generating a truncated form of the protein. Alternatively, antisense therapy can be used to induce destructive exon skipping of myostatin pre-mRNA, knocking down myostatin expression to enhance muscle strength and reduce fibrosis. We have reported previously that intramuscular or intraperitoneal antisense administration inducing dual exon skipping of dystrophin and myostatin pre-mRNAs was beneficial in mdx mice, a mouse model of DMD, although therapeutic effects were muscle type restricted, possibly due to the delivery routes used. Here, following systemic intravascular antisense treatment, muscle strength and body activity of treated adult mdx mice increased to the levels of healthy controls. Importantly, hallmarks of muscular dystrophy were greatly improved in mice receiving the combined exon-skipping therapy, as compared to those receiving dystrophin antisense therapy alone. Our results support the translation of antisense therapy for dystrophin restoration and myostatin inhibition into the clinical setting for DMD.

Impaired regenerative capacity and lower revertant fibre expansion in dystrophin-deficient mdx muscles on DBA/2 background

Duchenne muscular dystrophy, one of the most common lethal genetic disorders, is caused by mutations in the DMD gene and a lack of dystrophin protein. In most DMD patients and animal models, sporadic dystrophin-positive muscle fibres, called revertant fibres (RFs), are observed in otherwise dystrophin-negative backgrounds. RFs are thought to arise from skeletal muscle precursor cells and clonally expand with age due to the frequent regeneration of necrotic fibres. Here we examined the effects of genetic background on muscle regeneration and RF expansion by comparing dystrophin-deficient mdx mice on the C57BL/6 background (mdx-B6) with those on the DBA/2 background (mdx-DBA), which have a more severe phenotype. Interestingly, mdx-DBA muscles had significantly lower RF expansion than mdx-B6 in all age groups, including 2, 6, 12, and 18 months. The percentage of centrally nucleated fibres was also significantly lower in mdx-DBA mice compared to mdx-B6, indicating that less muscle regeneration occurs in mdx-DBA. Our study aligns with the model that RF expansion reflects the activity of precursor cells in skeletal muscles, and it serves as an index of muscle regeneration capacity.

The short-lived African turquoise killifish: an emerging experimental model for ageing

Human ageing is a fundamental biological process that leads to functional decay, increased risk for various diseases and, ultimately, death. Some of the basic biological mechanisms underlying human ageing are shared with other organisms; thus, animal models have been invaluable in providing key mechanistic and molecular insights into the common bases of biological ageing. In this Review, we briefly summarise the major applications of the most commonly used model organisms adopted in ageing research and highlight their relevance in understanding human ageing. We compare the strengths and limitations of different model organisms and discuss in detail an emerging ageing model, the short-lived African turquoise killifish. We review the recent progress made in using the turquoise killifish to study the biology of ageing and discuss potential future applications of this promising animal model.