Identification of Novel Antisense-Mediated Exon Skipping Targets in DYSF for Therapeutic Treatment of Dysferlinopathy

Heparan Sulfate Chain-Conjugated Laminin-E8 Fragments Advance Paraxial Mesodermal Differentiation Followed by High Myogenic Induction from hiPSCs

Human-induced pluripotent stem cells (hiPSCs) have great therapeutic potential. The cell source differentiated from hiPSCs requires xeno-free and robust methods for lineage-specific differentiation. Here, a system is described for differentiating hiPSCs on new generation laminin fragments (NGLFs), a recombinant form of a laminin E8 fragment conjugated to the heparan sulfate chains (HS) attachment domain of perlecan. Using NGLFs, hiPSCs are highly promoted to direct differentiation into a paraxial mesoderm state with high-efficiency muscle lineage generation. HS conjugation to the C-terminus of Laminin E8 fragments brings fibroblast growth factors (FGFs) bound to the HS close to the cell surface of hiPSCs, thereby facilitating stronger FGF signaling pathways stimulation and initiating HOX gene expression, which triggers the paraxial mesoderm differentiation of hiPSCs. This highly efficient differentiation system can provide a roadmap for paraxial mesoderm development and an infinite source of myocytes and muscle stem cells for disease modeling and regenerative medicine.

A novel homozygous variant (c.5876T > C: p. Leu1959Pro) in DYSF segregates with limb-girdle muscular dystrophy: a case report

BACKGROUND

Limb girdle muscular dystrophies (LGMDs) constitute a heterogeneous group of neuromuscular disorders with a very variable clinical presentation and overlapping traits. The clinical symptoms of LGMD typically appear in adolescence or early adulthood. Genetic variation in the dysferlin gene (DYSF) has been associated with LGMD.

METHODS

We characterized a recessive LGMD in a young adult from consanguineous Irani families using whole-exome sequencing (WES) technology. Sanger sequencing was performed to verify the identified variant. Computational modeling and protein-protein docking were used to investigate the impact of the variant on the structure and function of the DYSF protein.

RESULTS

By WES, we identified a novel homozygous missense variant in DYSF (NM_003494.4: c.5876T > C: p. Leu1959Pro) previously been associated with LGMD phenotypes.

CONCLUSIONS

The identification and validation of new pathogenic DYSF variant in the present study further highlight the importance of this gene in LGMD.

Unexpected extra exon skipping in the DYSF gene during restoring the reading frame by CRISPR/Cas9

The DYSF gene encoding dysferlin protein is one of the largest and has many transcripts. Pathogenic variants in the gene can lead to various types of myopathies, which makes it a good object for studying the events occurring in it during genome editing by the CRISPR/Cas method. In this study, we evaluated the possibility of permanent skipping of exons 3-4, and 26-27 which deletion does not violate the reading frame and allows to eliminate truncated variants within exons. Editing was performed with simultaneous transfection of two sgRNA- and sa/spCas9-containing plasmids on HEK293T cell cultures and healthy donor myoblasts. Skipping of exons 3-4 was performed by destroying the splicing acceptor sites, and exons 26-27 by cuts in the flanking exons with the corresponding deletion in the DNA. Some unexpected results were obtained, when exons 26-27 were skipped, exon 30 was also absent in the transcript, although it is not alternatively spliced and is normally present in all transcripts. This event indicates that DNA changes near splicing sites can affect adjacent exons and the whole gene. However, this fact requires further study.

Portrait of Dysferlinopathy: Diagnosis and Development of Therapy

Dysferlinopathy is a disease caused by a dysferlin deficiency due to mutations in the DYSF gene. Dysferlin is a membrane protein in the sarcolemma and is involved in different functions, such as membrane repair and vesicle fusion, T-tubule development and maintenance, Ca2+ signalling, and the regulation of various molecules. Miyoshi Myopathy type 1 (MMD1) and Limb-Girdle Muscular Dystrophy 2B/R2 (LGMD2B/LGMDR2) are two possible clinical presentations, yet the same mutations can cause both presentations in the same family. They are therefore grouped under the name dysferlinopathy. Onset is typically during the teenage years or young adulthood and is characterized by a loss of Achilles tendon reflexes and difficulty in standing on tiptoes or climbing stairs, followed by a slow progressive loss of strength in limb muscles. The MRI pattern of patient muscles and their biopsies show various fibre sizes, necrotic and regenerative fibres, and fat and connective tissue accumulation. Recent tools were developed for diagnosis and research, especially to evaluate the evolution of the patient condition and to prevent misdiagnosis caused by similarities with polymyositis and Charcot-Marie-Tooth disease. The specific characteristic of dysferlinopathy is dysferlin deficiency. Recently, mouse models with patient mutations were developed to study genetic approaches to treat dysferlinopathy. The research fields for dysferlinopathy therapy include symptomatic treatments, as well as antisense-mediated exon skipping, myoblast transplantation, and gene editing.

Limb-Girdle Muscular Dystrophies Classification and Therapies

Limb-girdle muscular dystrophies (LGMDs) are caused by mutations in multiple genes. This review article presents 39 genes associated with LGMDs. Some forms are inherited in a dominant fashion, while for others this occurs recessively. The classification of LGMDs has evolved through time. Lately, to be considered an LGMD, the mutation has to cause a predominant proximal muscle weakness and must be found in two or more unrelated families. This article also presents therapies for LGMDs, examining both available treatments and those in development. For now, only symptomatic treatments are available for patients. The goal is now to solve the problem at the root of LGMDs instead of treating each symptom individually. In the last decade, multiple other potential treatments were developed and studied, such as stem-cell transplantation, exon skipping, gene delivery, RNAi, and gene editing.

Limb-Girdle Muscular Dystrophy Type 2B (LGMD2B) caused by Pathogenic Splice and Missense Variants of DYSF Gene among Iranians with Muscular Dystrophy

Background

The phenotypic range of limb-girdle muscular dystrophies (LGMDs) varies significantly because of genetic heterogeneity ranging from very mild to severe forms. Molecular analysis of the DYSF gene is challenging due to the wide range of mutations and associated complications in interpretations of novel DYSF variants with uncertain significance. Thus, in the current study, we performed the NGS analysis and its results are confirmed with Sanger sequencing to find the plausible disease-causing variants in patients with muscular dystrophy and their relatives via segregation analysis.

Materials and Methods

Nine patients with LGMD type 2B (LGMD2B) characteristics were screened for putative mutations by the whole-exome sequencing (WES) test. Either the patients themselves or their parents and first relatives were investigated in the segregation analysis through Sanger sequencing. The majority of variants were classified as pathogenic through American College of Medical Genetics and Genomics (ACMG) guidelines, segregation results, and in silico predictions.

Results

Results revealed eight variants in DYSF gene, including three splicing (c.1149+4A>G, c.2864+1G>A, and c.5785-7G>A), two nonsense (p.Gln112Ter and p.Trp2084Ter), two missense (p.Thr1546Pro and p.Tyr1032Cys), and one frameshift (p.Asp1067Ilefs), among nine Iranian families. One of the eight identified variants was novel, including p.Asp1067Ilefs, which was predicted to be likely pathogenic based on the ACMG guidelines. Notably, prediction tools suggested the damaging effects of studied variants on dysferlin structure.

Conclusion

Conclusively, the current report introduced eight variants including a novel frameshift in DYSF gene with noticeable pathogenic effects. This study significantly can broaden the diagnostic spectrum of LGMD2B in combination with previous reports about DYSF mutations and may pave the way for a rapidly high-ranked identification of the accurate type of dysferlinopathy.

Ensemble-Learning and Feature Selection Techniques for Enhanced Antisense Oligonucleotide Efficacy Prediction in Exon Skipping

Antisense oligonucleotide (ASO)-mediated exon skipping has become a valuable tool for investigating gene function and developing gene therapy. Machine-learning-based computational methods, such as eSkip-Finder, have been developed to predict the efficacy of ASOs via exon skipping. However, these methods are computationally demanding, and the accuracy of predictions remains suboptimal. In this study, we propose a new approach to reduce the computational burden and improve the prediction performance by using feature selection within machine-learning algorithms and ensemble-learning techniques. We evaluated our approach using a dataset of experimentally validated exon-skipping events, dividing it into training and testing sets. Our results demonstrate that using a three-way-voting approach with random forest, gradient boosting, and XGBoost can significantly reduce the computation time to under ten seconds while improving prediction performance, as measured by R² for both 2'-O-methyl nucleotides (2OMe) and phosphorodiamidate morpholino oligomers (PMOs). Additionally, the feature importance ranking derived from our approach is in good agreement with previously published results. Our findings suggest that our approach has the potential to enhance the accuracy and efficiency of predicting ASO efficacy via exon skipping. It could also facilitate the development of novel therapeutic strategies. This study could contribute to the ongoing efforts to improve ASO design and optimize gene therapy approaches.

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

Minimal expression of dysferlin prevents development of dysferlinopathy in dysferlin exon 40a knockout mice

Dysferlin is a Ca²⁺-activated lipid binding protein implicated in muscle membrane repair. Recessive variants in DYSF result in dysferlinopathy, a progressive muscular dystrophy. We showed previously that calpain cleavage within a motif encoded by alternatively spliced exon 40a releases a 72 kDa C-terminal minidysferlin recruited to injured sarcolemma. Herein we use CRISPR/Cas9 gene editing to knock out murine Dysf exon 40a, to specifically assess its role in membrane repair and development of dysferlinopathy. We created three Dysf exon 40a knockout (40aKO) mouse lines that each express different levels of dysferlin protein ranging from ~ 90%, ~ 50% and ~ 10-20% levels of wild-type. Histopathological analysis of skeletal muscles from all 12-month-old 40aKO lines showed virtual absence of dystrophic features and normal membrane repair capacity for all three 40aKO lines, as compared with dysferlin-null BLAJ mice. Further, lipidomic and proteomic analyses on 18wk old quadriceps show all three 40aKO lines are spared the profound lipidomic/proteomic imbalance that characterises dysferlin-deficient BLAJ muscles. Collective results indicate that membrane repair does not depend upon calpain cleavage within exon 40a and that ~ 10-20% of WT dysferlin protein expression is sufficient to maintain the muscle lipidome, proteome and membrane repair capacity to crucially prevent development of dysferlinopathy.

Morpholino-Mediated Exons 28-29 Skipping of Dysferlin and Characterization of Multiexon-skipped Dysferlin using RT-PCR, Immunoblotting, and Membrane Wounding Assay

Dysferlinopathies are a group of disabling muscular dystrophies  that includes limb girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy, and distal myopathy with anterior tibial onset (DMAT) as the main phenotypes. They are associated with molecular defects in DYSF, which encodes dysferlin, a key player in sarcolemmal homeostasis. Previous investigations have suggested that exon skipping may be a promising therapy for many patients with dysferlinopathies. It was reported that exons 28-29 of DYSF are dispensable for dysferlin functions. Here, we present a method for multiexon skipping of DYSF exons 28-29 using a cocktail of two phosphorodiamidate morpholino oligomers (PMOs) on cells derived from a dystrophinopathy patient. Also, we describe assays to characterize the multiexon skipped dysferlin at several levels by using one-step RT-PCR, immunoblotting, and a membrane wounding assay.

Current Strategies of Muscular Dystrophy Therapeutics: An Overview

Muscular dystrophies are a group of genetic disorders characterized by varying degrees of progressive muscle weakness and degeneration. They are clinically and genetically heterogeneous but share the common histological features of dystrophic muscle. There is currently no cure for muscular dystrophies, which is of particular concern for the more disabling and/or lethal forms of the disease. Through the years, several therapies have encouragingly been developed for muscular dystrophies and include genetic, cellular, and pharmacological approaches. In this chapter, we undertake a comprehensive exploration of muscular dystrophy therapeutics under current development. Our review includes antisense therapy, CRISPR, gene replacement, cell therapy, nonsense suppression, and disease-modifying small molecule compounds.

eSkip-Finder: a machine learning-based web application and database to identify the optimal sequences of antisense oligonucleotides for exon skipping

Exon skipping using antisense oligonucleotides (ASOs) has recently proven to be a powerful tool for mRNA splicing modulation. Several exon-skipping ASOs have been approved to treat genetic diseases worldwide. However, a significant challenge is the difficulty in selecting an optimal sequence for exon skipping. The efficacy of ASOs is often unpredictable, because of the numerous factors involved in exon skipping. To address this gap, we have developed a computational method using machine-learning algorithms that factors in many parameters as well as experimental data to design highly effective ASOs for exon skipping. eSkip-Finder (https://eskip-finder.org) is the first web-based resource for helping researchers identify effective exon skipping ASOs. eSkip-Finder features two sections: (i) a predictor of the exon skipping efficacy of novel ASOs and (ii) a database of exon skipping ASOs. The predictor facilitates rapid analysis of a given set of exon/intron sequences and ASO lengths to identify effective ASOs for exon skipping based on a machine learning model trained by experimental data. We confirmed that predictions correlated well with in vitro skipping efficacy of sequences that were not included in the training data. The database enables users to search for ASOs using queries such as gene name, species, and exon number.

Frequent DYSF rare variants/mutations in 152 Han Chinese samples with ovarian endometriosis

PURPOSE

Endometriosis is a common chronic gynecological disease greatly affecting women health. Prior studies have implicated that dysferlin (DYSF) aberration might be involved in the pathogenesis of ovarian endometriosis. In the present study, we explore the potential presence of DYSF mutations in a total of 152 Han Chinese samples with ovarian endometriosis.

METHODS

We analyze the potential presence of DYSF mutations by direct DNA sequencing.

RESULTS

A total of seven rare variants/mutations in the DYSF gene in 10 out of 152 samples (6.6%) were identified, including 5 rare variants and 2 novel mutations. For the 5 rare variants, p.R334W and p.G941S existed in 2 samples, p.R865W, p.R1173H and p.G1531S existed in single sample, respectively; for the two novel mutations, p.W352* and p.I1642F, they were identified in three patients. These rare variants/mutations were absent or existed at extremely low frequency either in our 1006 local control women without endometriosis, or in the China Metabolic Analytics Project (ChinaMAP) and Genome Aggregation Database (gnomAD) databases. Evolutionary conservation analysis results suggested that all of these rare variants/mutations were evolutionarily conserved among 11 vertebrate species from Human to Fox. Furthermore, in silico analysis results suggested these rare variants/mutations were disease-causing. Nevertheless, we find no significant association between DYSF rare variants/mutations and the clinical features in our patients. To our knowledge, this is the first report revealing frequent DYSF mutations in ovarian endometriosis.

CONCLUSION

We identified a high frequency of DYSF rare variants/mutations in ovarian endometriosis for the first time. This study suggests a new correlation between DYSF rare variants/mutations and ovarian endometriosis, implicating DYSF rare variants/mutations might be positively involved in the pathogenesis of ovarian endometriosis.

Functional recovery of a novel knockin mouse model of dysferlinopathy by readthrough of nonsense mutation

Biallelic mutations in the dysferlin gene cause limb-girdle muscular dystrophy 2B or Miyoshi distal myopathy. We found that nonsense mutations are the most common mutation type among Korean patients with dysferlinopathy; more than half of the patients have at least one nonsense allele, which may be amenable to readthrough therapy. We generated a knockin mouse, dqx, harboring DYSF p.Q832∗ mutation. Homozygous dqx mice lacked dysferlin in skeletal muscle, while 2 weeks of oral ataluren restored dysferlin expression and ameliorated skeletal muscle pathology. Their physical performance improved, and protection against eccentric contractions was noted. The improvement was most evident in mice treated with oral ataluren of 0.9 mg/mL. These improvements were sustained for 8 weeks in ataluren-treated dqx mice, while the parameters of A/J mice treated with ataluren over the same period did not improve. These results support that readthrough therapy by oral ataluren may also be applicable to dysferlinopathy patients with nonsense mutation.

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.

Proteasome inhibitors reduce thrombospondin-1 release in human dysferlin-deficient myotubes

Background

Dysferlinopathies are a group of muscle disorders causing muscle weakness and absence or low levels of dysferlin, a type-II transmembrane protein and the causative gene of these dystrophies. Dysferlin is implicated in vesicle fusion, trafficking, and membrane repair. Muscle biopsy of patients with dysferlinopathy is characterized by the presence of inflammatory infiltrates. Studies in the muscle of both human and mouse models of dysferlinopathy suggest dysferlin deficient muscle plays a role in this inflammation by releasing thrombospondin-1. It has also been reported that vitamin D3 treatment enhances dysferlin expression. The ubiquitin-proteasome system recognizes and removes proteins that fail to fold or assemble properly and previous studies suggest that its inhibition could have a therapeutic effect in muscle dystrophies. Here we assessed whether inhibition of the ubiquitin proteasome system prevented degradation of dysferlin in immortalized myoblasts from a patients with two missense mutations in exon 44.

Methods

To assess proteasome inhibition we treated dysferlin deficient myotubes with EB1089, a vitamin D3 analog, oprozomib and ixazomib. Western blot was performed to analyze the effect of these treatments on the recovery of dysferlin and myogenin expression. TSP-1 was quantified using the enzyme-linked immunosorbent assay to analyze the effect of these drugs on its release. A membrane repair assay was designed to assess the ability of treated myotubes to recover after membrane injury and fusion index was also measured with the different treatments. Data were analyzed using a one-way ANOVA test followed by Tukey post hoc test and analysis of variance. A p ≤ 0.05 was considered statistically significant.

Results

Treatment with proteasome inhibitors and EB1089 resulted in a trend towards an increase in dysferlin and myogenin expression. Furthermore, EB1089 and proteasome inhibitors reduced the release of TSP-1 in myotubes. However, no effect was observed on the repair of muscle membrane after injury.

Conclusions

Our findings indicate that the ubiquitin-proteasome system might not be the main mechanism of mutant dysferlin degradation. However, its inhibition could help to improve muscle inflammation by reducing TSP-1 release.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-020-03756-7.

The ties that bind: functional clusters in limb-girdle muscular dystrophy

The limb-girdle muscular dystrophies (LGMDs) are a genetically pleiomorphic class of inherited muscle diseases that are known to share phenotypic features. Selected LGMD genetic subtypes have been studied extensively in affected humans and various animal models. In some cases, these investigations have led to human clinical trials of potential disease-modifying therapies, including gene replacement strategies for individual subtypes using adeno-associated virus (AAV) vectors. The cellular localizations of most proteins associated with LGMD have been determined. However, the functions of these proteins are less uniformly characterized, thus limiting our knowledge of potential common disease mechanisms across subtype boundaries. Correspondingly, broad therapeutic strategies that could each target multiple LGMD subtypes remain less developed. We believe that three major "functional clusters" of subcellular activities relevant to LGMD merit further investigation. The best known of these is the glycosylation modifications associated with the dystroglycan complex. The other two, mechanical signaling and mitochondrial dysfunction, have been studied less systematically but are just as promising with respect to the identification of significant mechanistic subgroups of LGMD. A deeper understanding of these disease pathways could yield a new generation of precision therapies that would each be expected to treat a broader range of LGMD patients than a single subtype, thus expanding the scope of the molecular medicines that may be developed for this complex array of muscular dystrophies.

The genetic profile of dysferlinopathy in a cohort of 209 cases: Genotype-phenotype relationship and a hotspot on the inner DysF domain

Dysferlinopathy is a group of autosomal recessive muscular dystrophies caused by variants in the dysferlin gene (DYSF), with variable proximal and distal muscle involvement. We performed DYSF gene analyses of 200 cases suspected of having dysferlinopathy (Cohort 1), and identified diagnostic variants in 129/200 cases, including 19 novel variants. To achieve a comprehensive genetic profile of dysferlinopathy, we analyzed the variant data from 209 affected cases from unrelated 209 families, including 80 previously diagnosed and 129 newly diagnosed cases (Cohort 2). Among the 90 types of variants identified in 209 cases, the NM_003494.3: c.2997G>T; p.Trp999Cys, was the most frequent (96/420; 22.9%), followed by c.1566C>G; p.Tyr522* (45/420; 10.7%) on an allele base. p.Trp999Cys was found in 70/209 cases (33.5%), including 20/104 cases (19.2%) with the Miyoshi muscular phenotype and 43/82 cases (52.4%) with the limb-girdle phenotype. In the analysis of missense variants, p.Trp992Arg, p.Trp999Arg, p.Trp999Cys, p.Ser1000Phe, p.Arg1040Trp, and p.Arg1046His were located in the inner DysF domain, representing in 113/160 missense variants (70.6%). This large cohort highlighted the frequent missense variants located in the inner DysF domain as a hotspot for missense variants among our cohort of 209 cases (>95%, Japanese) and hinted at their potential as targets for future therapeutic strategies.

Antisense-Mediated Skipping of Dysferlin Exons in Control and Dysferlinopathy Patient-Derived Cells

Dysferlinopathies encompass a spectrum of progressive muscular dystrophies caused by the lack of dysferlin due to missense mutations in the dysferlin gene or mutations causing premature truncation of protein translation. Dysferlin is a modular protein, and dysferlins lacking one or more repetitive domains have been shown to retain functionality. As such, antisense-mediated exon skipping has been proposed as a therapy for dysferlinopathy. By skipping the mutated exon, the reading frame would be maintained, while the mutation would be bypassed, thus allowing production of an internally deleted, but partially functional, dysferlin. We previously showed that dysferlin exon skipping is feasible in control cell lines. We here evaluated exon skipping and dysferlin protein restoration in patient-derived cells requiring the skipping of exon 9, 29, 30, or 34. Exon 30 skipping was possible at high efficiency, but did not result in increased dysferlin. We discovered that the alleged exon 30 mutation was in fact a polymorphism and identified a splicing mutation in intron 28 as the disease-causing mutation. While exon skipping was feasible for each of the other cell lines, no increases in dysferlin protein could be detected by western blotting.

Recent advancements in exon-skipping therapies using antisense oligonucleotides and genome editing for the treatment of various muscular dystrophies

Muscular dystrophy is a group of genetic disorders characterised by degeneration of muscles. Different forms of muscular dystrophy can show varying phenotypes with a wide range of age, severity and location of muscle deterioration. Many palliative care options are available for muscular dystrophy patients, but no curative treatment is available. Exon-skipping therapy aims to induce skipping of exons with disease-causing mutations and/or nearby exons to restore the reading frame, which results in an internally truncated, partially functional protein. In antisense-mediated exon-skipping synthetic antisense oligonucleotide binds to pre-mRNA to induce exon skipping. Recent advances in exon skipping have yielded promising results; the US Food and Drug Administration (FDA) approved eteplirsen (Exondys51) as the first exon-skipping drug for the treatment of Duchenne muscular dystrophy, and in vivo exon skipping has been demonstrated in animal models of dysferlinopathy, limb-girdle muscular dystrophy type 2C and congenital muscular dystrophy type 1A. Novel methods that induce exon skipping utilizing Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are also being developed where splice site mutations are created within the genome to induce exon skipping. Challenges remain as exon-skipping agents can have deleterious non-specific effects and different in-frame deletions show phenotypic variance. This article reviews the state of the art of exon skipping for treating muscular dystrophy and discusses challenges and future prospects.

AAV-based gene therapies for the muscular dystrophies

Muscular dystrophy (MD) is a group of progressive genetic diseases affecting the musculature that are characterized by inflammatory infiltrates, necrosis and connective tissue and fat replacement of the affected muscles. Unfortunately, treatments do not exist for the vast majority of MD patients. Adeno-associated viral vector (AAV)-based gene therapy is thus emerging as a potential treatment for many types of MD. Treatments strategies based on AAV are being adapted for replacement of mutant disease-causing genes, knockdown of dominant disease-causing genes using antisense oligonucleotides or inhibitory RNAs, delivery of gene editing tools such as clustered regularly interspaced short palindromic repeats/Cas9 and effecting alterations in pre-mRNA splicing and by manipulating expression levels of modifier genes. Translational and clinical trial work focused on these types of AAV treatments for Duchenne MD, various limb girdle MDs, myotonic dystrophy 1, facioscapulohumeral MD, dysferlinopathies and congenital MDs are discussed here, with a focus on recent studies, pre-clinical large animal work and many promising ongoing and upcoming AAV clinical trials.

CRISPR for Neuromuscular Disorders: Gene Editing and Beyond

This is a review describing advances in CRISPR/Cas-mediated therapies for neuromuscular disorders (NMDs). We explore both CRISPR-mediated editing and dead Cas approaches as potential therapeutic strategies for multiple NMDs. Last, therapeutic considerations, including delivery and off-target effects, are also discussed.

Engineered skeletal muscles for disease modeling and drug discovery

Skeletal muscle is the largest organ of human body with several important roles in everyday movement and metabolic homeostasis. The limited ability of small animal models of muscle disease to accurately predict drug efficacy and toxicity in humans has prompted the development in vitro models of human skeletal muscle that fatefully recapitulate cell and tissue level functions and drug responses. We first review methods for development of three-dimensional engineered muscle tissues and organ-on-a-chip microphysiological systems and discuss their potential utility in drug discovery research and development of new regenerative therapies. Furthermore, we describe strategies to increase the functional maturation of engineered muscle, and motivate the importance of incorporating multiple tissue types on the same chip to model organ cross-talk and generate more predictive drug development platforms. Finally, we review the ability of available in vitro systems to model diseases such as type II diabetes, Duchenne muscular dystrophy, Pompe disease, and dysferlinopathy.

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.