What Can We Learn From Clinical Trials of Exon Skipping for DMD?

Targeting a Novel Site in Exon 51 with Antisense Oligonucleotides Induces Enhanced Exon Skipping in a Mouse Model of Duchenne Muscular Dystrophy

Exon skipping with antisense oligonucleotides (ASOs) can correct disease-causing mutations of Duchenne muscular dystrophy (DMD) through RNA-targeted splice correction. This correction restores the reading frame and supports expression of near full-length dystrophin. First-generation exon 51-skipping ASOs targeted the same binding site, with limited clinical efficacy. We characterized a novel binding site within exon 51 that induced highly efficient exon skipping. A precursor ASO (AON-C12) and clinical ASO (BMN 351) were designed using 2'-O-methyl-modified phosphorothioate (2'OMePS) RNA and locked nucleic acids. hDMDdel52/mdx mice were given AON-C12 or BMN 351 for 13 weeks and evaluated for molecular and phenotypic correction of dystrophin deficiency. BMN 351 treatment induced durable, dose-dependent levels of exon skipping and dystrophin production in all muscles evaluated. In the heart, 8 weeks after the last BMN 351 dose at 18 mg/kg, exon-skipped transcripts remained at 44.3% of total, and dystrophin levels were 21.8% of wild type. BMN 351 reached higher tissue concentrations and percent exon skipping in the heart than a clinically relevant peptide-conjugated phosphorodiamidate morpholino oligomer comparator. BMN 351 also improved gait scores and clinical and anatomical muscle pathology parameters compared with vehicle-treated hDMDdel52/mdx mice. The pharmacologic activity and safety of BMN 351 warrant further nonclinical and clinical development.

Evaluation of Chemically Modified Nucleic Acid Analogues for Splice Switching Application

In recent years, several splice switching antisense oligonucleotide (ASO)-based therapeutics have gained significant interest, and several candidates received approval for clinical use for treating rare diseases, in particular, Duchenne muscular dystrophy and spinal muscular atrophy. These ASOs are fully modified; in other words, they are composed of chemically modified nucleic acid analogues instead of natural RNA oligomers. This has significantly improved drug-like properties of these ASOs in terms of efficacy, stability, pharmacokinetics, and safety. Although chemical modifications of oligonucleotides have been discussed previously for numerous applications including nucleic acid aptamers, small interfering RNA, DNAzyme, and ASO, to the best of our knowledge, none of them have solely focused on the analogues that have been utilized for splice switching applications. To this end, we present here a comprehensive review of different modified nucleic acid analogues that have been explored for developing splice switching ASOs. In addition to the antisense chemistry, we also endeavor to provide a brief historical overview of the approved spice switching ASO drugs, including a list of drugs that have entered human clinical trials. We hope this work will inspire further investigations into expanding the potential of novel nucleic acid analogues for constructing splice switching ASOs.

Mimicking sarcolemmal damagein vitro: a contractile 3D model of skeletal muscle for drug testing in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most prevalent neuromuscular disease diagnosed in childhood. It is a progressive and wasting disease, characterized by a degeneration of skeletal and cardiac muscles caused by the lack of dystrophin protein. The absence of this crucial structural protein leads to sarcolemmal fragility, resulting in muscle fiber damage during contraction. Despite ongoing efforts, there is no cure available for DMD patients. One of the primary challenges is the limited efficacy of current preclinical tools, which fail in modeling the biological complexity of the disease. Human-based three-dimensional (3D) cell culture methods appear as a novel approach to accelerate preclinical research by enhancing the reproduction of pathophysiological processes in skeletal muscle. In this work, we developed a patient-derived functional 3D skeletal muscle model of DMD that reproduces the sarcolemmal damage found in the native DMD muscle. These bioengineered skeletal muscle tissues exhibit contractile functionality, as they responded to electrical pulse stimulation. Sustained contractile regimes induced the loss of myotube integrity, mirroring the pathological myotube breakdown inherent in DMD due to sarcolemmal instability. Moreover, damaged DMD tissues showed disease functional phenotypes, such as tetanic fatigue. We also evaluated the therapeutic effect of utrophin upregulator drug candidates on the functionality of the skeletal muscle tissues, thus providing deeper insight into the real impact of these treatments. Overall, our findings underscore the potential of bioengineered 3D skeletal muscle technology to advance DMD research and facilitate the development of novel therapies for DMD and related neuromuscular disorders.

Longitudinal Alterations in Gait Features in Growing Children With Duchenne Muscular Dystrophy

Prolonging ambulation is an important treatment goal in children with Duchenne muscular dystrophy (DMD). Three-dimensional gait analysis (3DGA) could provide sensitive parameters to study the efficacy of clinical trials aiming to preserve ambulation. However, quantitative descriptions of the natural history of gait features in DMD are first required. The overall goal was to provide a full delineation of the progressive gait pathology in children with DMD, covering the entire period of ambulation, by performing a so-called mixed cross-sectional longitudinal study. Firstly, to make our results comparable with previous literature, we aimed to cross-sectionally compare 31 predefined gait features between children with DMD and a typically developing (TD) database (1). Secondly, we aimed to explore the longitudinal changes in the 31 predefined gait features in growing boys with DMD using follow-up 3DGA sessions (2). 3DGA-sessions (n = 124) at self-selected speed were collected in 27 boys with DMD (baseline age: 4.6-15 years). They were repeatedly measured over a varying follow-up period (range: 6 months-5 years). The TD group consisted of 27 children (age: 5.4-15.6 years). Per measurement session, the spatiotemporal parameters, and the kinematic and kinetic waveforms were averaged over the selected gait cycles. From the averaged waveforms, discrete gait features (e.g., maxima and minima) were extracted. Mann-Whitney U tests were performed to cross-sectionally analyze the differences between DMD at baseline and TD (1). Linear mixed effect models were performed to assess the changes in gait features in the same group of children with DMD from both a longitudinal (i.e., increasing time) as well as a cross-sectional perspective (i.e., increasing baseline age) (2). At baseline, the boys with DMD differed from the TD children in 17 gait features. Additionally, 21 gait features evolved longitudinally when following-up the same boys with DMD and 25 gait features presented a significant cross-sectional baseline age-effect. The current study quantitatively described the longitudinal alterations in gait features in boys with DMD, thereby providing detailed insight into how DMD gait deteriorates. Additionally, our results highlight that gait features extracted from 3DGA are promising outcome measures for future clinical trials to quantify the efficacy of novel therapeutic strategies.

Effects of Chronic, Maximal Phosphorodiamidate Morpholino Oligomer (PMO) Dosing on Muscle Function and Dystrophin Restoration in a Mouse Model of Duchenne Muscular Dystrophy

BACKGROUND

Phosphorodiamidate morpholino oligomer (PMO)-mediated exon skipping is currently used in clinical development to treat Duchenne muscular dystrophy (DMD), with four exon-skipping drugs achieving regulatory approval. Exon skipping elicits a truncated, but semi-functional dystrophin protein, similar to the truncated dystrophin expressed in patients with Becker Muscular dystrophy (BMD) where the disease phenotype is less severe than DMD. Despite promising results in both dystrophic animal models and DMD boys, restoration of dystrophin by exon skipping is highly variable, leading to contradictory functional outcomes in clinical trials.

OBJECTIVE

To develop optimal PMO dosing protocols that result in increased dystrophin and improved outcome measures in preclinical models of DMD.

METHODS

Tested effectiveness of multiple chronic, high dose PMO regimens using biochemical, histological, molecular, and imaging techniques in mdx mice.

RESULTS

A chronic, monthly regimen of high dose PMO increased dystrophin rescue in mdx mice and improved specific force in the extensor digitorum longus (EDL) muscle. However, monthly high dose PMO administration still results in variable dystrophin expression localized throughout various muscles.

CONCLUSIONS

High dose monthly PMO administration restores dystrophin expression and increases muscle force; however, the variability of dystrophin expression at both the inter-and intramuscular level remains. Additional strategies to optimize PMO uptake including increased dosing frequencies or combination treatments with other yet-to-be-defined therapies may be necessary to achieve uniform dystrophin restoration and increases in muscle function.

Evaluating the potential of novel genetic approaches for the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle-wasting disorder that is caused by a lack of functional dystrophin, a cytoplasmic protein necessary for the structural integrity of muscle. As variants in the dystrophin gene lead to a disruption of the reading frame, pharmacological treatments have only limited efficacy; there is currently no effective therapy and consequently, a significant unmet clinical need for DMD. Recently, novel genetic approaches have shown real promise in treating DMD, with advancements in the efficacy and tropism of exon skipping and surrogate gene therapy. CRISPR-Cas9 has the potential to be a 'one-hit' curative treatment in the coming decade. The current limitations of gene editing, such as off-target effects and immunogenicity, are in fact partly constraints of the delivery method itself, and thus research focus has shifted to improving the viral vector. In order to halt the loss of ambulation, early diagnosis and treatment will be pivotal. In an era where genetic sequencing is increasingly utilised in the clinic, genetic therapies will play a progressively central role in DMD therapy. This review delineates the relative merits of cutting-edge genetic approaches, as well as the challenges that still need to be overcome before they become clinically viable.

Development of outcome measures according to dystrophic phenotypes in canine X-linked muscular dystrophy in Japan

Duchenne muscular dystrophy (DMD) is an X-linked lethal muscle disorder characterized by primary muscle degeneration. Therapeutic strategies for DMD have been extensively explored, and some are in the stage of human clinical trials. Along with the development of new therapies, sensitive outcome measures are needed to monitor the effects of new treatments. Therefore, we investigated outcome measures such as biomarkers and motor function evaluation in a dystrophic model of beagle dogs, canine X-linked muscular dystrophy in Japan (CXMD_J). Osteopontin (OPN), a myogenic inflammatory cytokine, was explored as a potential biomarker in dystrophic dogs over the disease course. The serum OPN levels of CXMD_J dystrophic dogs were elevated, even in the early disease phase, and this could be related to the presence of regenerating muscle fibers; as such, OPN would be a promising biomarker for muscle regeneration. Next, accelerometry, which is an efficient method to quantify performance in validated tasks, was used to evaluate motor function longitudinally in dystrophic dogs. We measured three-axis acceleration and angular velocity with wireless hybrid sensors during gait evaluations. Multiple parameters of acceleration and angular velocity showed notedly lower values in dystrophic dogs compared with wild-type dogs, even at the onset of muscle weakness. These parameters accordingly decreased with exacerbation of clinical manifestations along with the disease course. Multiple parameters also indicated gait abnormalities in dystrophic dogs, such as a waddling gait. These outcome measures could be applicable in clinical trials of patients with DMD or other muscle disorders.

Molecular correction of Duchenne muscular dystrophy by splice modulation and gene editing

Duchenne muscular dystrophy (DMD) is a currently incurable X-linked neuromuscular disorder, characterized by progressive muscle wasting and premature death, typically as a consequence of cardiac failure. DMD-causing mutations in the dystrophin gene are highly diverse, meaning that the development of a universally-applicable therapy to treat all patients is very challenging. The leading therapeutic strategy for DMD is antisense oligonucleotide-mediated splice modulation, whereby one or more specific exons are excluded from the mature dystrophin mRNA in order to correct the translation reading frame. Indeed, three exon skipping oligonucleotides have received FDA approval for use in DMD patients. Second-generation exon skipping drugs (i.e. peptide-antisense oligonucleotide conjugates) exhibit enhanced potency, and also induce dystrophin restoration in the heart. Similarly, multiple additional antisense oligonucleotide drugs targeting various exons are in clinical development in order to treat a greater proportion of DMD patient mutations. Relatively recent advances in the field of genome engineering (specifically, the development of the CRISPR/Cas system) have provided multiple promising therapeutic approaches for the RNA-directed genetic correction of DMD, including exon excision, exon reframing via the introduction of insertion/deletion mutations, disruption of splice signals to promote exon skipping, and the templated correction of point mutations by seamless homology directed repair or base editing technology. Potential limitations to the clinical translation of the splice modulation and gene editing approaches are discussed, including drug delivery, the importance of uniform dystrophin expression in corrected myofibres, safety issues (e.g. renal toxicity, viral vector immunogenicity, and off-target gene editing), and the high cost of therapy.

Developing DMD therapeutics: a review of the effectiveness of small molecules, stop-codon readthrough, dystrophin gene replacement, and exon-skipping therapies

INTRODUCTION

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the dystrophin (DMD) gene. Most patients die from respiratory failure or cardiomyopathy. There are significant unmet needs for treatments for DMD as the standard of care is principally limited to symptom relief through treatments including steroids.

AREAS COVERED

This review summarizes safety and efficacy in promising areas of DMD therapeutics - small molecules, stop codon readthrough, gene replacement, and exon skipping - under clinical examination from 2015-2020 as demonstrated in the NIH Clinical Trials and PubMed search engines.

EXPERT OPINION

Currently, steroids persist as the most accessible medicine for DMD. Stop-codon readthrough, gene replacement, and exon-skipping therapies all aim to restore dystrophin expression. Of these strategies, gene replacement therapy has recently gained momentum while exon-skipping retains great traction. The  FDA approval of three exon-skipping antisense oligonucleotides illustrate this regulatory momentum, though the effectiveness and sequence design of eteplirsen remain controversial. Cell-penetrating peptides promise to more efficaciously treat DMD-related cardiomyopathy.The recent success of antisense therapies, however, poses major regulatory challenges. To fully realize the benefits of exon-skipping, including cocktail oligonucleotide-mediated multiple exon-skipping and oligonucleotide drugs for very rare mutations, regulatory challenges need to be addressed in coordination with scientific advances.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Synthesis of N-substituted morpholine nucleoside derivatives

Herein, we report the synthesis of substituted morpholino nucleoside derivatives starting from ribonucleosides. The present protocol shows high functional group tolerance, uses mild reaction conditions, and gives moderate to good yields. This transformation is based on two sequential pathways: (i) the oxidation of the ribonucleosides to the corresponding dialdehyde using sodium periodate and (ii) the reductive amination of the in situ generated dialdehydes with the hydrochloride salts of various the alkylamines.

Anchor peptide captures, targets, and loads exosomes of diverse origins for diagnostics and therapy

Exosomes are circulating nanovesicular carriers of macromolecules, increasingly used for diagnostics and therapeutics. The ability to load and target patient-derived exosomes without altering exosomal surfaces is key to unlocking their therapeutic potential. We demonstrate that a peptide (CP05) identified by phage display enables targeting, cargo loading, and capture of exosomes from diverse origins, including patient-derived exosomes, through binding to CD63-an exosomal surface protein. Systemic administration of exosomes loaded with CP05-modified, dystrophin splice-correcting phosphorodiamidate morpholino oligomer (EXO_PMO) increased dystrophin protein 18-fold in quadriceps of dystrophin-deficient mdx mice compared to CP05-PMO. Loading CP05-muscle-targeting peptide on EXO_PMO further increased dystrophin expression in muscle with functional improvement without any detectable toxicity. Our study demonstrates that an exosomal anchor peptide enables direct, effective functionalization and capture of exosomes, thus providing a tool for exosome engineering, probing gene function in vivo, and targeted therapeutic drug delivery.

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.

Direct Reprogramming of Human DMD Fibroblasts into Myotubes for In Vitro Evaluation of Antisense-Mediated Exon Skipping and Exons 45-55 Skipping Accompanied by Rescue of Dystrophin Expression

Antisense oligonucleotide-mediated exon skipping is a promising therapeutic approach for the treatment of various genetic diseases and a therapy which has gained significant traction in recent years following FDA approval of new antisense-based drugs. Exon skipping for Duchenne muscular dystrophy (DMD) works by modulating dystrophin pre-mRNA splicing, preventing incorporation of frame-disrupting exons into the final mRNA product while maintaining the open reading frame, to produce a shortened-yet-functional protein as seen in milder Becker muscular dystrophy (BMD) patients. Exons 45-55 skipping in dystrophin is potentially applicable to approximately 47% of DMD patients because many mutations occur within this "mutation hotspot." In addition, patients naturally harboring a dystrophin exons 45-55 in-frame deletion mutation have an asymptomatic or exceptionally mild phenotype compared to shorter in-frame deletion mutations in this region. As such, exons 45-55 skipping could transform the DMD phenotype into an asymptomatic or very mild BMD phenotype and rescue nearly a half of DMD patients. In addition, this strategy is potentially applicable to some BMD patients as well, who have in-frame deletion mutations in this region. As the degree of exon skipping correlates with therapeutic outcomes, reliable measurements of exon skipping efficiencies are essential to the development of novel antisense-mediated exon skipping therapeutics. In the case of DMD, researchers have often relied upon human muscle fibers obtained from muscle biopsies for testing; however, this method is highly invasive and patient myofibers can display limited proliferative ability. To overcome these challenges, researchers can generate myofibers from patient fibroblast cells by transducing the cells with a viral vector containing MyoD, a myogenic regulatory factor. Here, we describe a methodology for assessing dystrophin exons 45-55 multiple skipping efficiency using antisense oligonucleotides in human muscle cells derived from DMD patient fibroblast cells.

Use of Tricyclo-DNA Antisense Oligonucleotides for Exon Skipping

Antisense oligonucleotides (AONs) have been actively developed for more than 30 years as a form of molecular medicine and represent promising therapeutic tools for many disorders. Significant progress has been made toward their clinical development in particular for splice switching AONs for the treatment of neuromuscular disorders such as Duchenne muscular dystrophy (DMD). Many different chemistries of AONs can be used for splice switching modulation, and some of them have now reached regulatory approval. However, despite advances in AON chemistry and design, systemic use of AONs is limited due to poor tissue uptake and sufficient therapeutic efficacy is difficult to achieve. Therefore, there is still a critical need to develop efficient AONs able to target all relevant tissues and international efforts are currently on going to advance new compounds or alternative chemistries with higher therapeutic potential. Here we describe the methods to evaluate the potency of tricyclo-DNA (tcDNA)-AONs, a novel class of AONs which displays unique pharmacological properties and unprecedented uptake in many tissues after systemic administration (Goyenvalle et al., Nat Med 21:270-275, 2015; Goyenvalle et al., J Neuromuscul Dis 3:157-167, 2016; Relizani et al., Mol Ther Nucleic Acids 8:144-157, 2017; Robin et al., Mol Ther Nucleic Acids 7:81-89, 2017). We will focus on the preclinical evaluation of these tcDNA for DMD, specifically targeting the exon 51 of the human dystrophin gene. We will first detail methods to analyze their efficacy both in vitro in human myoblasts and in vivo in the hDMD and mdx52 mouse models and then describe means to evaluate their potential renal toxicity.

Do porcine Sertoli cells represent an opportunity for Duchenne muscular dystrophy?

Sertoli cells (SeC) are responsible for the immunoprivileged status of the testis thanks to which allogeneic or xenogeneic engraftments can survive without pharmacological immune suppression if co‐injected with SeC. This peculiar ability of SeC is dependent on secretion of a plethora of factors including maturation factors, hormones, growth factors, cytokines and immunomodulatory factors. The anti‐inflammatory and trophic properties of SeC have been largely exploited in several experimental models of diseases, diabetes being the most studied. Duchenne muscular dystrophy (DMD) is a lethal X‐linked recessive pathology in which lack of functional dystrophin leads to progressive muscle degeneration culminating in loss of locomotion and premature death. Despite a huge effort to find a cure, DMD patients are currently treated with anti‐inflammatory steroids. Recently, encapsulated porcine SeC (MC‐SeC) have been injected ip in the absence of immunosuppression in an animal model of DMD resulting in reduction of muscle inflammation and amelioration of muscle morphology and functionality, thus opening an additional avenue in the treatment of DMD. The novel protocol is endowed with the advantage of being potentially applicable to all the cohort of DMD patients regardless of the mutation. This mini‐review addresses several issues linked to the possible use of MC‐SeC injected ip in dystrophic people.

Accelerometric outcomes of motor function related to clinical evaluations and muscle involvement in dystrophic dogs

Duchenne muscular dystrophy (DMD) is an X-linked muscle disorder characterized by primary muscle degeneration. Patients with DMD reveal progressive muscle weakness leading to ambulatory dysfunction. Novel outcome measures are needed for more sensitive evaluation of therapeutic effects in clinical trials. Multiple parameters of acceleration and angular velocity are used as efficient indicators to quantify the motion of subjects, and these parameters have been recently applied for evaluation of motor function in DMD. In the present study, we evaluated gait in a dystrophic dog model, CXMDJ, by measuring three-axial acceleration and angular velocity over the course of months. Hybrid sensors were placed on the dorsal thoracic and lumbar regions of dogs to detect a wide range of acceleration (±8 G) and angular velocity (±1000 degrees per second). Multiple parameters showed lower values in dystrophic dogs compared to wild-type (WT) dogs, and declined over the course of months. Acceleration magnitude (AM) at the thoracic region in dystrophic dogs was prominently lower compared with WT dogs, even at the age of 2 months, the onset of muscle weakness, whereas AM at the lumbar region drastically declined throughout the disease course. The angular velocity index in the vertical direction in the lumbar region increased in dystrophic dogs, suggesting waddling at the girdle. These parameters also accordingly decreased with exacerbation of clinical manifestations and a decrease in spontaneous locomotor activity. The AM of dystrophic dogs was analyzed with magnetic resonance imaging to look for a correlation with crus muscle involvement. Results showed that acceleration and angular velocity are multifaceted kinematic indices that can be applied to assess outcomes in clinical trials for hereditary neuromuscular disorders including DMD.

Shorter Phosphorodiamidate Morpholino Splice-Switching Oligonucleotides May Increase Exon-Skipping Efficacy in DMD

Duchenne muscular dystrophy is a fatal muscle disease, caused by mutations in DMD, leading to loss of dystrophin expression. Phosphorodiamidate morpholino splice-switching oligonucleotides (PMO-SSOs) have been used to elicit the restoration of a partially functional truncated dystrophin by excluding disruptive exons from the DMD messenger. The 30-mer PMO eteplirsen (EXONDYS51) developed for exon 51 skipping is the first dystrophin-restoring, conditionally FDA-approved drug in history. Clinical trials had shown a dose-dependent variable and patchy dystrophin restoration. The main obstacle for efficient dystrophin restoration is the inadequate uptake of PMOs into skeletal muscle fibers at low doses. The excessive cost of longer PMOs has limited the utilization of higher dosing. We designed shorter 25-mer PMOs directed to the same eteplirsen-targeted region of exon 51 and compared their efficacies in vitro and in vivo in the mdx52 murine model. Our results showed that skipped-dystrophin induction was comparable between the 30-mer PMO sequence of eteplirsen and one of the shorter PMOs, while the other 25-mer PMOs showed lower exon-skipping efficacies. Shorter PMOs would make higher doses economically feasible, and high dosing would result in better drug uptake into muscle, induce higher levels of dystrophin restoration in DMD muscle, and, ultimately, increase the clinical efficacy.

Muscle membrane integrity in Duchenne muscular dystrophy: recent advances in copolymer-based muscle membrane stabilizers

The scientific premise, design, and structure-function analysis of chemical-based muscle membrane stabilizing block copolymers are reviewed here for applications in striated muscle membrane injury. Synthetic block copolymers have a rich history and wide array of applications from industry to biology. Potential for discovery is enabled by a large chemical space for block copolymers, including modifications in block copolymer mass, composition, and molecular architecture. Collectively, this presents an impressive chemical landscape to leverage distinct structure-function outcomes. Of particular relevance to biology and medicine, stabilization of damaged phospholipid membranes using amphiphilic block copolymers, classified as poloxamers or pluronics, has been the subject of increasing scientific inquiry. This review focuses on implementing block copolymers to protect fragile muscle membranes against mechanical stress. The review highlights interventions in Duchenne muscular dystrophy, a fatal disease of progressive muscle deterioration owing to marked instability of the striated muscle membrane. Biophysical and chemical engineering advances are presented that delineate and expand upon current understanding of copolymer-lipid membrane interactions and the mechanism of stabilization. The studies presented here serve to underscore the utility of copolymer discovery leading toward the therapeutic application of block copolymers in Duchenne muscular dystrophy and potentially other biomedical applications in which membrane integrity is compromised.

Comprehensive RNA-Sequencing Analysis in Serum and Muscle Reveals Novel Small RNA Signatures with Biomarker Potential for DMD

Extracellular small RNAs (sRNAs), including microRNAs (miRNAs), are promising biomarkers for diseases such as Duchenne muscular dystrophy (DMD), although their biological relevance is largely unknown. To investigate the relationship between intracellular and extracellular sRNA levels on a global scale, we performed sRNA sequencing in four muscle types and serum from wild-type, dystrophic mdx, and mdx mice in which dystrophin protein expression was restored by exon skipping. Differentially abundant sRNAs were identified in serum (mapping to miRNA, small nuclear RNA [snRNA], and PIWI-interacting RNA [piRNA] loci). One novel candidate biomarker, miR-483, was increased in both mdx serum and muscle, and also elevated in DMD patient sera. Dystrophin restoration induced global shifts in miRNA (including miR-483) and snRNA-fragment abundance toward wild-type levels. Specific serum piRNA-like sRNAs also responded to exon skipping therapy. Absolute miRNA expression in muscle was positively correlated with abundance in the circulation, although multiple highly expressed miRNAs in muscle were not elevated in mdx serum, suggesting that both passive and selective release mechanisms contribute to serum miRNA levels. In conclusion, this study has revealed new insights into the sRNA biology of dystrophin deficiency and identified novel DMD biomarkers.

At the Crossroads of Clinical and Preclinical Research for Muscular Dystrophy-Are We Closer to Effective Treatment for Patients?

Among diseases affecting skeletal muscle, muscular dystrophy is one of the most devastating and complex disorders. The term ‘muscular dystrophy’ refers to a heterogeneous group of genetic diseases associated with a primary muscle defect that leads to progressive muscle wasting and consequent loss of muscle function. Muscular dystrophies are accompanied by numerous clinical complications and abnormalities in other tissues that cause extreme discomfort in everyday life. The fact that muscular dystrophy often takes its toll on babies and small children, and that many patients die at a young age, adds to the cruel character of the disease. Clinicians all over the world are facing the same problem: they have no therapy to offer except for symptom-relieving interventions. Patients, their families, but also clinicians, are in urgent need of an effective cure. Despite advances in genetics, increased understanding of molecular mechanisms underlying muscle disease, despite a sweeping range of successful preclinical strategies and relative progress of their implementation in the clinic, therapy for patients is currently out of reach. Only a greater comprehension of disease mechanisms, new preclinical studies, development of novel technologies, and tight collaboration between scientists and physicians can help improve clinical treatment. Fortunately, inventiveness in research is rapidly extending the limits and setting new standards for treatment design. This review provides a synopsis of muscular dystrophy and considers the steps of preclinical and clinical research that are taking the muscular dystrophy community towards the fundamental goal of combating the traumatic disease.

Skeletal muscle magnetic resonance biomarkers correlate with function and sentinel events in Duchenne muscular dystrophy

OBJECTIVE

To provide evidence for quantitative magnetic resonance (qMR) biomarkers in Duchenne muscular dystrophy by investigating the relationship between qMR measures of lower extremity muscle pathology and functional endpoints in a large ambulatory cohort using a multicenter study design.

METHODS

MR spectroscopy and quantitative imaging were implemented to measure intramuscular fat fraction and the transverse magnetization relaxation time constant (T2) in lower extremity muscles of 136 participants with Duchenne muscular dystrophy. Measures were collected at 554 visits over 48 months at one of three imaging sites. Fat fraction was measured in the soleus and vastus lateralis using MR spectroscopy, while T2 was assessed using MRI in eight lower extremity muscles. Ambulatory function was measured using the 10m walk/run, climb four stairs, supine to stand, and six minute walk tests.

RESULTS

Significant correlations were found between all qMR and functional measures. Vastus lateralis qMR measures correlated most strongly to functional endpoints (|ρ| = 0.68-0.78), although measures in other rapidly progressing muscles including the biceps femoris (|ρ| = 0.63-0.73) and peroneals (|ρ| = 0.59-0.72) also showed strong correlations. Quantitative MR biomarkers were excellent indicators of loss of functional ability and correlated with qualitative measures of function. A VL FF of 0.40 was an approximate lower threshold of muscle pathology associated with loss of ambulation.

DISCUSSION

Lower extremity qMR biomarkers have a robust relationship to clinically meaningful measures of ambulatory function in Duchenne muscular dystrophy. These results provide strong supporting evidence for qMR biomarkers and set the stage for their potential use as surrogate outcomes in clinical trials.

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.

Assisted delivery of antisense therapeutics in animal models of heritable neurodegenerative and neuromuscular disorders: a systematic review and meta-analysis

Antisense oligonucleotide (AON)-based therapies hold promise for a range of neurodegenerative and neuromuscular diseases and have shown benefit in animal models and patients. Success in the clinic is nevertheless still limited, due to unfavourable biodistribution and poor cellular uptake of AONs. Extensive research is currently being conducted into the formulation of AONs to improve delivery, but thus far there is no consensus on which of those strategies will be the most effective. This systematic review was designed to answer in an unbiased manner which delivery strategies most strongly enhance the efficacy of AONs in animal models of heritable neurodegenerative and neuromuscular diseases. In total, 95 primary studies met the predefined inclusion criteria. Study characteristics and data on biodistribution and toxicity were extracted and reporting quality and risk of bias were assessed. Twenty studies were eligible for meta-analysis. We found that even though the use of delivery systems provides an advantage over naked AONs, it is not yet possible to select the most promising strategies. Importantly, standardisation of experimental procedures is warranted in order to reach conclusions about the most efficient delivery strategies. Our best practice guidelines for future experiments serve as a step in that direction.

Duchenne muscular dystrophy: an updated review of common available therapies

BACKGROUND AND PURPOSE

Duchenne muscular dystrophy (DMD) is a lethal progressive pediatric muscle disorder and genetically inherited as an X-linked disease that caused by mutations in the dystrophin gene. DMD leads to progressive muscle weakness, degeneration, and wasting; finally, follows with the premature demise in affected individuals due to respiratory and/or cardiac failure typically by age of 30. For decades, scientists tried massively to find an effective therapy method, but there is no absolute cure currently for patients with DMD, nevertheless, recent advanced progressions on the treatment of DMD will be hopeful in the future. Several promising gene therapies are currently under investigation. These include gene replacement, exon skipping, suppression of stop codons. More recently, a promising gene editing tool referred to as CRISPR/Cas9 offers exciting perspectives for restoring dystrophin expression in patients with DMD. This review intents to briefly describe these methods and comment on their advances. Since DMD is a genetic disorder, it should be treated by replacing the deficient DMD copy with a functional one. However, there are different types of mutations in this gene, so such therapeutic approaches are highly mutation specific and thus are personalized. Therefore, DMD has arisen as a model of genetic disorder for understanding and overcoming of the challenges of developing personalized genetic medicines, consequently, the lessons learned from these approaches will be applicable to many other disorders.

CONCLUSIONS

This review provides an update on the recent gene therapies for DMD that aim to compensate for dystrophin deficiency and the related clinical trials.

Reversible immortalisation enables genetic correction of human muscle progenitors and engineering of next-generation human artificial chromosomes for Duchenne muscular dystrophy

Transferring large or multiple genes into primary human stem/progenitor cells is challenged by restrictions in vector capacity, and this hurdle limits the success of gene therapy. A paradigm is Duchenne muscular dystrophy (DMD), an incurable disorder caused by mutations in the largest human gene: dystrophin. The combination of large-capacity vectors, such as human artificial chromosomes (HACs), with stem/progenitor cells may overcome this limitation. We previously reported amelioration of the dystrophic phenotype in mice transplanted with murine muscle progenitors containing a HAC with the entire dystrophin locus (DYS-HAC). However, translation of this strategy to human muscle progenitors requires extension of their proliferative potential to withstand clonal cell expansion after HAC transfer. Here, we show that reversible cell immortalisation mediated by lentivirally delivered excisable hTERT and Bmi1 transgenes extended cell proliferation, enabling transfer of a novel DYS-HAC into DMD satellite cell-derived myoblasts and perivascular cell-derived mesoangioblasts. Genetically corrected cells maintained a stable karyotype, did not undergo tumorigenic transformation and retained their migration ability. Cells remained myogenic in vitro (spontaneously or upon MyoD induction) and engrafted murine skeletal muscle upon transplantation. Finally, we combined the aforementioned functions into a next-generation HAC capable of delivering reversible immortalisation, complete genetic correction, additional dystrophin expression, inducible differentiation and controllable cell death. This work establishes a novel platform for complex gene transfer into clinically relevant human muscle progenitors for DMD gene therapy.

Tween 85-Modified Low Molecular Weight PEI Enhances Exon-Skipping of Antisense Morpholino Oligomer In Vitro and in mdx Mice

We investigated a series of Tween 85 modified low molecular weight polyethylenimine (LPEI, 0.8k/1.2k/2.0k)-copolymers (Zs) through simple formulation and covalent conjugation with phosphorodiamidate morpholino oligomer (PMO) for their potential to enhance delivery in vitro and in dystrophic mdx mice. Z polymers significantly enhanced PMO-induced exon-skipping in a GFP reporter-based cell culture system. Application of optimized formulations of Zs with PMO targeted to dystrophin exon 23 demonstrated a significant increase in exon-skipping efficiency in mdx mice. Consistent with our observations in vitro, optimization of molecular size and hydropholic-lipopholic balance (HLB) of polymers are important factors to achieve enhanced PMO delivery in vivo. The best formulation of Zs enhanced PMO delivery with 20- and 6-fold over PMO alone in vitro and in vivo, respectively. Further, chemical conjugation of the polymer and PMO exhibits greater benefit than polymer/PMO simple formulation in PMO delivery efficiency. Observed cytotoxicity of the Zs was lower than Endo-porter and PEI 25k in vitro, and no tissue toxicity was clearly detected with the Zs at the dosage tested. These results indicate the potential of the Zs as effective and safe PMO delivery carriers for treating diseases such as muscular dystrophy.

Myoblasts and macrophages are required for therapeutic morpholino antisense oligonucleotide delivery to dystrophic muscle

Exon skipping is a promising therapeutic strategy for Duchenne muscular dystrophy (DMD), employing morpholino antisense oligonucleotides (PMO-AO) to exclude disruptive exons from the mutant DMD transcript and elicit production of truncated dystrophin protein. Clinical trials for PMO show variable and sporadic dystrophin rescue. Here, we show that robust PMO uptake and efficient production of dystrophin following PMO administration coincide with areas of myofiber regeneration and inflammation. PMO localization is sustained in inflammatory foci where it enters macrophages, actively differentiating myoblasts and newly forming myotubes. We conclude that efficient PMO delivery into muscle requires two concomitant events: first, accumulation and retention of PMO within inflammatory foci associated with dystrophic lesions, and second, fusion of PMO-loaded myoblasts into repairing myofibers. Identification of these factors accounts for the variability in clinical trials and suggests strategies to improve this therapeutic approach to DMD.Exon skipping is a strategy for the treatment of Duchenne muscular dystrophy, but has variable efficacy. Here, the authors show that dystrophin restoration occurs preferentially in areas of myofiber regeneration, where antisense oligonucleotides are stored in macrophages and delivered to myoblasts and newly formed myotubes.

Biomarker Potential of Extracellular miRNAs in Duchenne Muscular Dystrophy

miRNAs are small, noncoding RNAs that not only regulate gene expression within cells, but might also constitute promising extracellular biomarkers for a variety of pathologies, including the progressive muscle-wasting disorder Duchenne Muscular Dystrophy (DMD). A set of muscle-enriched miRNAs, the myomiRs (miR-1, miR-133, and miR-206) are highly elevated in the serum of patients with DMD and in dystrophin-deficient animal models. Furthermore, circulating myomiRs might be used as pharmacodynamic biomarkers, given that their levels can be restored towards wild-type levels following exon skipping therapy in dystrophic mice. The relationship between muscle pathology and extracellular myomiR release is complex, and incompletely understood. Here, we discuss current progress leading towards the clinical utility of extracellular miRNAs as putative DMD biomarkers, and their possible contribution to muscle physiology.

Biological applications of xeno nucleic acids

Xeno nucleic acids (XNAs) are a group of chemically modified nucleic acid analogues that have been applied to various biological technologies such as antisense oligonucleotides, siRNAs and aptamers.

Efficacy and Safety Profile of Tricyclo-DNA Antisense Oligonucleotides in Duchenne Muscular Dystrophy Mouse Model

Antisense oligonucleotides (AONs) hold promise for therapeutic splice-switching correction in many genetic diseases. However, despite advances in AON chemistry and design, systemic use of AONs is limited due to poor tissue uptake and sufficient therapeutic efficacy is still difficult to achieve. A novel class of AONs made of tricyclo-DNA (tcDNA) is considered very promising for the treatment of Duchenne muscular dystrophy (DMD), a neuromuscular disease typically caused by frameshifting deletions or nonsense mutations in the gene-encoding dystrophin and characterized by progressive muscle weakness, cardiomyopathy, and respiratory failure in addition to cognitive impairment. Herein, we report the efficacy and toxicology profile of a 13-mer tcDNA in mdx mice. We show that systemic delivery of 13-mer tcDNA allows restoration of dystrophin in skeletal muscles and to a lower extent in the brain, leading to muscle function improvement and correction of behavioral features linked to the emotional/cognitive deficiency. More importantly, tcDNA treatment was generally limited to minimal glomerular changes and few cell necroses in proximal tubules, with only slight variation in serum and urinary kidney toxicity biomarker levels. These results demonstrate an encouraging safety profile for tcDNA, albeit typical of phosphorothiate AONs, and confirm its therapeutic potential for the systemic treatment of DMD patients.

Evaluation of Amphiphilic Peptide Modified Antisense Morpholino Oligonucleotides In Vitro and in Dystrophic mdx Mice

A series of amphiphilic peptides modified PMO (Pt-PMO) were prepared, and their antisense effect and toxicity were evaluated both in vitro and in mdx mice. The results showed that the exon-skipping performance of Pt-PMO are relative to the structure of the conjugated peptide: the Pt3/Pt4 composed of six/seven arginines and one myristoylation modified PMO showed more efficacy and with less toxicity as compared to others, confirming that appropriate hydrophilic-lipophilic balance (HLB) and cationic sequence numbers play a crucial role in improving cell uptake and corresponding exon-skipping efficiency. This was observed particularly in enhanced delivery efficiency of PMO comparable to B-PMO in vitro, while 6-fold improved exon-skipping was achieved against naked PMO in vivo. The multi-PMO modified Pt8-PMO also showed improved exon-skipping both in vitro and in vivo, though there is lower efficiency in systemic delivery as compared to Pt4-PMO. These data suggest that with optimization of peptide in component, charge density has clear potential for exploration towards achieving higher efficiency of antisense oligonucleotide systemic delivery, and thus is more applicable for clinical application.

Gene Editing With CRISPR/Cas9 RNA-Directed Nuclease

Genetic engineering of model organisms and cultured cells has for decades provided important insights into the mechanisms underlying cardiovascular development and disease. In the past few years the development of several nuclease systems has broadened the range of model/cell systems that can be engineered. Of these, the CRISPR (clustered regularly interspersed short palindromic repeats)/Cas9 (CRISPR-associated protein 9) system has become the favorite for its ease of application. Here we will review this RNA-guided nuclease system for gene editing with respect to its usefulness for cardiovascular studies and with an eye toward potential therapy. Studies on its off-target activity, along with approaches to minimize this activity will be given. The advantages of gene editing versus gene targeting in embryonic stem cells, including the breadth of species and cell types to which it is applicable, will be discussed. We will also cover its use in iPSC for research and possible therapeutic purposes; and we will review its use in muscular dystrophy studies where considerable progress has been made toward dystrophin correction in mice. The CRISPR/Ca9s system is also being used for high-throughput screening of genes, gene regulatory regions, and long noncoding RNAs. In addition, the CRISPR system is being used for nongene-editing purposes such as activation and inhibition of gene expression, as well as for fluorescence tagging of chromosomal regions and individual mRNAs to track their cellular location. Finally, an approach to circumvent the inability of post-mitotic cells to support homologous recombination-based gene editing will be presented. In conclusion, applications of the CRISPR/Cas system are expanding at a breath-taking pace and are revolutionizing approaches to gain a better understanding of human diseases.

Non-coding RNAs as drug targets

Most of the human genome encodes RNAs that do not code for proteins. These non-coding RNAs (ncRNAs) may affect normal gene expression and disease progression, making them a new class of targets for drug discovery. Because their mechanisms of action are often novel, developing drugs to target ncRNAs will involve equally novel challenges. However, many potential problems may already have been solved during the development of technologies to target mRNA. Here, we discuss the growing field of ncRNA - including microRNA, intronic RNA, repetitive RNA and long non-coding RNA - and assess the potential and challenges in their therapeutic exploitation.

Correction of the Exon 2 Duplication in DMD Myoblasts by a Single CRISPR/Cas9 System

Exonic duplications account for 10%-15% of all mutations in Duchenne muscular dystrophy (DMD), a severe hereditary neuromuscular disorder. We report a CRISPR (clustered regularly interspaced short palindromic repeat)/Cas9-based strategy to correct the most frequent (exon 2) duplication in the DMD gene by targeted deletion, and tested the efficacy of such an approach in patient-derived myogenic cells. We demonstrate restoration of wild-type dystrophin expression at transcriptional and protein level in myotubes derived from genome-edited myoblasts in the absence of selection. Removal of the duplicated exon was achieved by the use of only one guide RNA (gRNA) directed against an intronic duplicated region, thereby increasing editing efficiency and reducing the risk of off-target effects. This study opens a novel therapeutic perspective for patients carrying disease-causing duplications.

Patient reported outcomes in GNE myopathy: incorporating a valid assessment of physical function in a rare disease

BACKGROUND

The aim of this analysis was to evaluate the psychometric properties of three patient reported outcome (PRO) measures characterizing physical function in GNE myopathy: the Human Activity Profile, the Inclusion Body Myositis Functional Rating Scale, and the Activities-specific Balance Confidence scale.

METHODS

This analysis used data from 35 GNE myopathy subjects participating in a natural history study. For construct validity, correlational and known-group analyses were between the PROs and physical assessments. Reliability of the PROs between baseline and 6 months was evaluated using the intra-class correlation coefficient model; internal consistency was tested with Cronbach's alpha.

RESULTS

The hypothesized moderate positive correlations for construct validity were supported; the strongest correlation was between the human activity profile adjusted activity score and the adult myopathy assessment endurance subscale score (r = 0.81; p < 0.0001). The PROs were able to discriminate between known high and low functioning groups for the adult myopathy assessment tool. Internal consistency of the PROs was high (α > 0.8) and there was strong reliability (ICC >0.62).

CONCLUSION

The PROs are valid and reliable measures of physical function in GNE myopathy and should be incorporated in investigations to better understand the impact of progressive muscle weakness on physical function in this rare disease population. Implications for Rehabilitation GNE myopathy is a rare muscle disease that results in slow progressive muscle atrophy and weakness, ultimately leading to wheelchair use and dependence on a caregiver. There is limited knowledge on the impact of this disease on the health-related quality of life, specifically physical function, of this rare disease population. Three patient reported outcomes have been shown to be valid and reliable in GNE myopathy subjects and should be incorporated in future investigations to better understand how progressive muscle weakness impacts physical functions in this rare disease population. The patient reported outcome scores of GNE myopathy patients indicate a high risk for falls and impaired physical functioning, so it is important clinicians assess and provide interventions for these subjects to maintain their functional capacity.

Evidence for ACTN3 as a genetic modifier of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized by muscle degeneration and progressive weakness. There is considerable inter-patient variability in disease onset and progression, which can confound the results of clinical trials. Here we show that a common null polymorphism (R577X) in ACTN3 results in significantly reduced muscle strength and a longer 10 m walk test time in young, ambulant patients with DMD; both of which are primary outcome measures in clinical trials. We have developed a double knockout mouse model, which also shows reduced muscle strength, but is protected from stretch-induced eccentric damage with age. This suggests that α-actinin-3 deficiency reduces muscle performance at baseline, but ameliorates the progression of dystrophic pathology. Mechanistically, we show that α-actinin-3 deficiency triggers an increase in oxidative muscle metabolism through activation of calcineurin, which likely confers the protective effect. Our studies suggest that ACTN3 R577X genotype is a modifier of clinical phenotype in DMD patients.

RNA-based therapies for genodermatoses

Genetic disorders affecting the skin, genodermatoses, constitute a large and heterogeneous group of diseases, for which treatment is generally limited to management of symptoms. RNA-based therapies are emerging as a powerful tool to treat genodermatoses. In this review, we discuss in detail RNA splicing modulation by antisense oligonucleotides and RNA trans-splicing, transcript replacement and genome editing by in vitro-transcribed mRNAs, and gene knockdown by small interfering RNA and antisense oligonucleotides. We present the current state of these therapeutic approaches and critically discuss their opportunities, limitations and the challenges that remain to be solved. The aim of this review was to set the stage for the development of new and better therapies to improve the lives of patients and families affected by a genodermatosis.

Genetic diagnosis as a tool for personalized treatment of Duchenne muscular dystrophy

Accurate definition of genetic mutations causing Duchenne muscular dystrophy (DMD) has always been relevant in order to provide genetic counseling to patients and families, and helps to establish the prognosis in the case where the distinction between Duchenne, Becker, or intermediate muscular dystrophy is not obvious. As molecular treatments aimed at dystrophin restoration in DMD are increasingly available as commercialized drugs or within clinical trials, genetic diagnosis has become an indispensable tool in order to determine eligibility for these treatments. DMD patients in which multiplex ligation-dependent probe amplification (MLPA) or similar techniques show a deletion suitable to exon skipping of exons 44, 45, 51, or 53, may be currently treated with AONs targeting these exons, in the context of clinical trials, or, as is the case for exon 51 skipping in the United States, with the first commercialized drug (eteplirsen). Patients who test negative at MLPA, but in whom DMD gene sequencing shows a nonsense mutation, may be amenable for treatment with stop codon readthrough compounds such as ataluren. Novel molecular approaches such as CRISPR-Cas9 targeting of specific DMD mutations are still in the preclinical stages, but appear promising. In conclusion, an accurate genetic diagnosis represents the entrance into a new scenario of personalized medicine in DMD.

Translational development of splice-modifying antisense oligomers

INTRODUCTION

Antisense nucleic acid analogues can interact with pre-mRNA motifs and influence exon or splice site selection and thereby alter gene expression. Design of antisense molecules to target specific motifs can result in either exon exclusion or exon inclusion during splicing. Novel drugs exploiting the antisense concept are targeting rare, life-limiting diseases; however, the potential exists to treat a wide range of conditions by antisense-mediated splice intervention. Areas covered: In this review, the authors discuss the clinical translation of novel molecular therapeutics to address the fatal neuromuscular disorders Duchenne muscular dystrophy and spinal muscular atrophy. The review also highlights difficulties posed by issues pertaining to restricted participant numbers, variable phenotype and disease progression, and the identification and validation of study endpoints. Expert opinion: Translation of novel therapeutics for Duchenne muscular dystrophy and spinal muscular atrophy has been greatly advanced by multidisciplinary research, academic-industry partnerships and in particular, the engagement and support of the patient community. Sponsors, supporters and regulators are cooperating to deliver new drugs and identify and define meaningful outcome measures. Non-conventional and adaptive trial design could be particularly suited to clinical evaluation of novel therapeutics and strategies to treat serious, rare diseases that may be problematic to study using more conventional clinical trial structures.

Clinical potential of oligonucleotide-based therapeutics in the respiratory system

The discovery of an ever-expanding plethora of coding and non-coding RNAs with nodal and causal roles in the regulation of lung physiology and disease is reinvigorating interest in the clinical utility of the oligonucleotide therapeutic class. This is strongly supported through recent advances in nucleic acids chemistry, synthetic oligonucleotide delivery and viral gene therapy that have succeeded in bringing to market at least three nucleic acid-based drugs. As a consequence, multiple new candidates such as RNA interference modulators, antisense, and splice switching compounds are now progressing through clinical evaluation. Here, manipulation of RNA for the treatment of lung disease is explored, with emphasis on robust pharmacological evidence aligned to the five pillars of drug development: exposure to the appropriate tissue, binding to the desired molecular target, evidence of the expected mode of action, activity in the relevant patient population and commercially viable value proposition.

Advances in gene therapy for muscular dystrophies

Duchenne muscular dystrophy (DMD) is a recessive lethal inherited muscular dystrophy caused by mutations in the gene encoding dystrophin, a protein required for muscle fibre integrity. So far, many approaches have been tested from the traditional gene addition to newer advanced approaches based on manipulation of the cellular machinery either at the gene transcription, mRNA processing or translation levels. Unfortunately, despite all these efforts, no efficient treatments for DMD are currently available. In this review, we highlight the most advanced therapeutic strategies under investigation as potential DMD treatments.

Poly(ester amine) Composed of Polyethylenimine and Pluronic Enhance Delivery of Antisense Oligonucleotides In Vitro and in Dystrophic mdx Mice

A series of poly(esteramine)s (PEAs) constructed from low molecular weight polyethyleneimine (LPEI) and Pluronic were evaluated for the delivery of antisense oligonuclotides (AOs), 2'-O-methyl phosphorothioate RNA (2'-OMePS) and phosphorodiamidate morpholino oligomer (PMO) in cell culture and dystrophic mdx mice. Improved exon-skipping efficiency of both 2'-OMePS and PMO was observed in the C2C12E50 cell line with all PEA polymers compared with PEI 25k or LF-2k. The degree of efficiency was found in the order of PEA 01, PEA 04 > PEA 05 > others. The in vivo study in mdx mice demonstrated enhanced exon-skipping of 2'-OMePS with the order of PEA 06 > PEA 04, PEA 07 > PEA 03 > PEA 01 > others, and much higher than PEI 25k formulated 2'-OMePS. Exon-skipping efficiency of PMO in formulation with the PEAs were significantly enhanced in the order of PEA 02 > PEA 10 > PEA 01, PEA 03 > PEA 05, PEA 07, PEA 08 > others, with PEA 02 reaching fourfold of Endo-porter formulated PMO. PEAs improve PMO delivery more effectively than 2'-OMePS delivery in vivo, and the systemic delivery evaluation further highlight the efficiency of PEA for PMO delivery in all skeletal muscle. The results suggest that the flexibility of PEA polymers could be explored for delivery of different AO chemistries, especially for antisense therapy.

Targeting mRNA for the treatment of facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is an inherited autosomal dominant disorder characterized clinically by progressive muscle degeneration. Currently, no curative treatment for this disorder exists. FSHD patients are managed through physiotherapy to improve function and quality of life. Over the last two decades, FSHD has been better understood as a disease genetically characterized by a pathogenic contraction of a subset of macrosatellite repeats on chromosome 4. Specifically, several studies support an FSHD pathogenesis model involving the aberrant expression of the double homeobox protein 4 (DUX4) gene. Hence, potential therapies revolving around inhibition of DUX4 have been explored. One of the potential treatment options is the use of effective antisense oligonucleotides (AOs) to knockdown expression of the myopathic DUX4 gene and its downstream molecules including paired-like homeodomain transcription factor 1 (PITX1). Success in the suppression of PITX1 expression has already been demonstrated systemically in vivo in recent studies. In this article, we will review the pathogenesis of FSHD and the latest research involving the use of antisense knockdown therapy.

Mechanistic aspects of the formation of α-dystroglycan and therapeutic research for the treatment of α-dystroglycanopathy: A review

α-Dystroglycanopathy, an autosomal recessive disease, is associated with the development of a variety of diseases, including muscular dystrophy. In humans, α-dystroglycanopathy includes various types of congenital muscular dystrophy such as Fukuyama type congenital muscular dystrophy (FCMD), muscle eye brain disease (MEB), and the Walker Warburg syndrome (WWS), and types of limb girdle muscular dystrophy 2I (LGMD2I). α-Dystroglycanopathy share a common etiology, since it is invariably caused by gene mutations that are associated with the O-mannose glycosylation pathway of α-dystroglycan (α-DG). α-DG is a central member of the dystrophin glycoprotein complex (DGC) family in peripheral membranes, and the proper glycosylation of α-DG is essential for it to bind to extracellular matrix proteins, such as laminin, to cell components. The disruption of this ligand-binding is thought to result in damage to cell membrane integration, leading to the development of muscular dystrophy. Clinical manifestations of α-dystroglycanopathy frequently include mild to severe alterations in the central nervous system and optical manifestations in addition to muscular dystrophy. Eighteen causative genes for α-dystroglycanopathy have been identified to date, and it is likely that more will be reported in the near future. These findings have stimulated extensive and energetic investigations in this research field, and novel glycosylation pathways have been implicated in the process. At the same time, the use of gene therapy, antisense therapy, and enzymatic supplementation have been evaluated as therapeutic possibilities for some types of α-dystroglycanopathy. Here we review the molecular and clinical findings associated with α-dystroglycanopathy and the development of therapeutic approaches, by comparing the approaches with the development of Duchenne muscular dystrophy.

Antisense-Based Progerin Downregulation in HGPS-Like Patients' Cells

Progeroid laminopathies, including Hutchinson-Gilford Progeria Syndrome (HGPS, OMIM #176670), are premature and accelerated aging diseases caused by defects in nuclear A-type Lamins. Most HGPS patients carry a de novo point mutation within exon 11 of the LMNA gene encoding A-type Lamins. This mutation activates a cryptic splice site leading to the deletion of 50 amino acids at its carboxy-terminal domain, resulting in a truncated and permanently farnesylated Prelamin A called Prelamin A Δ50 or Progerin. Some patients carry other LMNA mutations affecting exon 11 splicing and are named “HGPS-like” patients. They also produce Progerin and/or other truncated Prelamin A isoforms (Δ35 and Δ90) at the transcriptional and/or protein level. The results we present show that morpholino antisense oligonucleotides (AON) prevent pathogenic LMNA splicing, markedly reducing the accumulation of Progerin and/or other truncated Prelamin A isoforms (Prelamin A Δ35, Prelamin A Δ90) in HGPS-like patients’ cells. Finally, a patient affected with Mandibuloacral Dysplasia type B (MAD-B, carrying a homozygous mutation in ZMPSTE24, encoding an enzyme involved in Prelamin A maturation, leading to accumulation of wild type farnesylated Prelamin A), was also included in this study. These results provide preclinical proof of principle for the use of a personalized antisense approach in HGPS-like and MAD-B patients, who may therefore be eligible for inclusion in a therapeutic trial based on this approach, together with classical HGPS patients.

Hip kinetics during gait are clinically meaningful outcomes in young boys with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked genetic neuromuscular disorder characterized by progressive proximal to distal muscle weakness. The success of randomized clinical trials for novel therapeutics depends on outcome measurements that are sensitive to change. As the development of motor skills may lead to functional improvements in young boys with DMD, their inclusion may potentially confound clinical trials. Three-dimensional gait analysis is an under-utilized approach that can quantify joint moments and powers, which reflect functional muscle strength. In this study, gait kinetics, kinematics, spatial-temporal parameters, and timed functional tests were quantified over a one-year period for 21 boys between 4 and 8 years old who were enrolled in a multisite natural history study. At baseline, hip moments and powers were inadequate. Between the two visits, 12 boys began a corticosteroid regimen (mean duration 10.8±2.4 months) while 9 boys remained steroid-naïve. Significant between-group differences favoring steroid use were found for primary kinetic outcomes (peak hip extensor moments (p=.007), duration of hip extensor moments (p=.007), peak hip power generation (p=.028)), and spatial-temporal parameters (walking speed (p=.016) and cadence (p=.021)). Significant between-group differences were not found for kinematics or timed functional tests with the exception of the 10m walk test (p=.03), which improves in typically developing children within this age range. These results indicate that hip joint kinetics can be used to identify weakness in young boys with DMD and are sensitive to corticosteroid intervention. Inclusion of gait analysis may enhance detection of a treatment effect in clinical trials particularly for young boys with more preserved muscle function.