Efficacy of Multi-exon Skipping Treatment in Duchenne Muscular Dystrophy Dog Model Neonates

Antisense oligonucleotide-mediated exon 27 skipping restores dysferlin function in dysferlinopathy patient-derived muscle cells

Dysferlinopathies are debilitating autosomal recessive muscular dystrophies caused by mutations in the DYSF gene, encoding dysferlin, a protein crucial for sarcolemmal homeostasis and membrane resealing. Currently, no therapies exist for dysferlinopathies. Dysferlin features a modular structure with multiple calcium-dependent C2 lipid-binding domains. Clinical reports of mild, late-onset phenotypes suggest partial retention of functionality despite missing C2 domains, supporting exon-skipping therapies using antisense oligonucleotides (ASOs). In this study, we identified a patient-derived muscle cell line with a splice site mutation in DYSF intron 26, causing exon 26 exclusion, an out-of-frame transcript, and no detectable dysferlin protein. We hypothesized that skipping DYSF exon 27 could restore the reading frame and membrane repair function. Using an in-house in silico tool, we designed ASOs targeting exon 27. Treatment resulted in 65%-92% exon 27 skipping in myoblasts and myotubes, leading to a 39%-51% rescue of normal dysferlin expression, demonstrating robust efficacy of our designed ASOs. Two-photon laser-based assays indicated functional membrane repair. Additionally, we observed improved myotube fusion, cell vitality, and reduced apoptosis levels post-treatment. These findings provide proof of concept that DYSF exon 27 skipping restores functional dysferlin in patient-derived cells, paving the way for future in vivo and clinical studies.

Heteroduplex oligonucleotide technology boosts oligonucleotide splice switching activity of morpholino oligomers in a Duchenne muscular dystrophy mouse model

The approval of splice-switching oligonucleotides with phosphorodiamidate morpholino oligomers (PMOs) for treating Duchenne muscular dystrophy (DMD) has advanced the field of oligonucleotide therapy. Despite this progress, PMOs encounter challenges such as poor tissue uptake, particularly in the heart, diaphragm, and central nervous system (CNS), thereby affecting patient's prognosis and quality of life. To address these limitations, we have developed a PMOs-based heteroduplex oligonucleotide (HDO) technology. This innovation involves a lipid-ligand-conjugated complementary strand hybridized with PMOs, significantly enhancing delivery to key tissues in mdx mice, normalizing motor functions, muscle pathology, and serum creatine kinase by restoring internal deleted dystrophin expression. Additionally, PMOs-based HDOs normalized cardiac and CNS abnormalities without adverse effects. Our technology increases serum albumin binding to PMOs and improves blood retention and cellular uptake. Here we show that PMOs-based HDOs address the limitations in oligonucleotide therapy for DMD and offer a promising approach for diseases amenable to exon-skipping therapy.

Duchenne and Becker muscular dystrophy: Cellular mechanisms, image analysis, and computational models: A review

The muscle is the principal tissue that is capable to transform potential energy into kinetic energy. This process is due to the transformation of chemical energy into mechanical energy to enhance the movements and all the daily activities. However, muscular tissues can be affected by some pathologies associated with genetic alterations that affect the expression of proteins. As the muscle is a highly organized structure in which most of the signaling pathways and proteins are related to one another, pathologies may overlap. Duchenne muscular dystrophy (DMD) is one of the most severe muscle pathologies triggering degeneration and muscle necrosis. Several mathematical models have been developed to predict muscle response to different scenarios and pathologies. The aim of this review is to describe DMD and Becker muscular dystrophy in terms of cellular behavior and molecular disorders and to present an overview of the computational models implemented to understand muscle behavior with the aim of improving regenerative therapy.

Recent Trends in Antisense Therapies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a debilitating and fatal genetic disease affecting 1/5000 boys globally, characterized by progressive muscle breakdown and eventual death, with an average lifespan in the mid-late twenties. While no cure yet exists for DMD, gene and antisense therapies have been heavily explored in recent years to better treat this disease. Four antisense therapies have received conditional FDA approval, and many more exist in varying stages of clinical trials. These upcoming therapies often utilize novel drug chemistries to address limitations of existing therapies, and their development could herald the next generation of antisense therapy. This review article aims to summarize the current state of development for antisense-based therapies for the treatment of Duchenne muscular dystrophy, exploring candidates designed for both exon skipping and gene knockdown.

Restoring Dystrophin Expression by Skipping Exons 6 and 8 in Neonatal Dystrophic Dogs

Duchenne muscular dystrophy (DMD) is caused by the mutations in the DMD gene resulting in no dystrophin production. Skipping DMD exons using phosphorodiamidate morpholino oligomers (PMOs) is an emerging treatment strategy that can restore the reading frame of the mutated gene and produce truncated but functional dystrophin protein. To date, four PMOs, including eteplirsen, casimersen, viltolarsen, and golodirsen, have been conditionally approved by the FDA for the treatment of DMD. Since degeneration of muscle fibers and irreversible fibrosis occur from childhood, the earlier treatment is preferred. The canine X-linked muscular dystrophy in Japan (CXMDj), a dog model of DMD, produces no dystrophin and exhibits a severe phenotype similar to human patients from early childhood. As such, CXMDj, which harbors a splice site mutation in intron 6, is a useful model for examining the long-term effects of early PMO treatment. In this chapter, we describe the systemic delivery of a cocktail of four PMOs that can successfully induce multiple exon skipping (exons 6-9) in neonatal dystrophic dogs. We also describe the procedures to evaluate the efficacy and toxicity, including clinical grading of dystrophic dogs, ELISA-based quantification of PMOs, histology, RT-PCR, and western blotting.

Viltolarsen: From Preclinical Studies to FDA Approval

Viltolarsen is a phosphorodiamidate morpholino antisense oligonucleotide (PMO) designed to skip exon 53 of the DMD gene for the treatment of Duchenne muscular dystrophy (DMD), one of the most common lethal genetic disorders characterized by progressive degeneration of skeletal muscles and cardiomyopathy. It was developed by Nippon Shinyaku in collaboration with the National Center of Neurology and Psychiatry (NCNP) in Japan based on the preclinical studies conducted in the DMD dog model at the NCNP. After showing hopeful results in pre-clinical trials and several clinical trials across North America and Japan, it received US Food and Drug Administration (FDA) approval for DMD in 2020. Viltolarsen restores the reading frame of the DMD gene by skipping  exon 53 and produces a truncated but functional form of dystrophin. It can treat approximately 8-10% of the DMD patient population. This paper aims to summarize the development of viltolarsen from preclinical trials to clinical trials to, finally, FDA approval, and discusses the challenges that come with fighting DMD using antisense therapy.

Development of DG9 peptide-conjugated single- and multi-exon skipping therapies for the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disorder of progressive body-wide muscle weakness, considered the most common muscular dystrophy worldwide. Most patients have out-of-frame deletions in the DMD gene, leading to dystrophin absence in muscle. There is no cure for DMD, but exon skipping is emerging as a potential therapy that uses antisense oligonucleotides to convert out-of-frame to in-frame mutations, enabling the production of truncated, partially functional dystrophin. Currently approved exon skipping therapies, however, have limited applicability and efficacy. Here, we developed a more economical approach to skip DMD exons 45 to 55 (a strategy that could treat nearly half of all DMD patients) and identified DG9 peptide conjugation as a powerful way to improve exon skipping efficiencies in vivo.

Duchenne muscular dystrophy (DMD) is primarily caused by out-of-frame deletions in the dystrophin gene. Exon skipping using phosphorodiamidate morpholino oligomers (PMOs) converts out-of-frame to in-frame mutations, producing partially functional dystrophin. Four single-exon skipping PMOs are approved for DMD but treat only 8 to 14% of patients each, and some exhibit poor efficacy. Alternatively, exons 45 to 55 skipping could treat 40 to 47% of all patients and is associated with improved clinical outcomes. Here, we report the development of peptide-conjugated PMOs for exons 45 to 55 skipping. Experiments with immortalized patient myotubes revealed that exons 45 to 55 could be skipped by targeting as few as five exons. We also found that conjugating DG9, a cell-penetrating peptide, to PMOs improved single-exon 51 skipping, dystrophin restoration, and muscle function in hDMDdel52;mdx mice. Local administration of a minimized exons 45 to 55–skipping DG9-PMO mixture restored dystrophin production. This study provides proof of concept toward the development of a more economical and effective exons 45 to 55–skipping DMD therapy.

Pharmacology and toxicology of eteplirsen and SRP-5051 for DMD exon 51 skipping: an update

Duchenne muscular dystrophy (DMD) afflicts 1 in 5000 newborn males, leading to progressive muscle weakening and the loss of ambulation between the ages of 8 and 12. Typically, DMD patients pass away from heart failure or respiratory failure. Currently, there is no cure, though exon-skipping therapy including eteplirsen (brand name Exondys 51), a synthetic antisense oligonucleotide designed to skip exon 51 of the dystrophin gene, is considered especially promising. Applicable to approximately 14% of DMD patients, a phosphorodiamidate morpholino oligomer (PMO) antisense oligonucleotide eteplirsen received accelerated approval by the US Food and Drug Administration (FDA) in 2016. Throughout clinical trials, eteplirsen has been well tolerated by patients with no serious drug-related adverse events. The most common events observed are balance disorder, vomiting, and skin rash. Despite its safety and promise of functional benefits, eteplirsen remains controversial due to its low production of dystrophin. In addition, unmodified PMOs have limited efficacy in the heart. To address these concerns of efficacy, eteplirsen has been conjugated to a proprietary cell-penetrating peptide; the conjugate is called SRP-5051. Compared to eteplirsen, SRP-5051 aims to better prompt exon-skipping and dystrophin production but may have greater toxicity concerns. This paper reviews and discusses the available information on the efficacy, safety, and tolerability data of eteplirsen and SRP-5051 from preclinical and clinical trials. Issues faced by eteplirsen and SRP-5051, including efficacy and safety, are identified. Lastly, the current state of eteplirsen and exon-skipping therapy in general as a strategy for the treatment of DMD are discussed.

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.

Restoring Protein Expression in Neuromuscular Conditions: A Review Assessing the Current State of Exon Skipping/Inclusion and Gene Therapies for Duchenne Muscular Dystrophy and Spinal Muscular Atrophy

The debilitating neuromuscular disorders Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA), which harm 1 in 5000 newborn males and 1 in 11,000 newborns, respectively, are marked by progressive muscle wasting among other complications. While DMD causes generalized muscle weakness due to the absence of the dystrophin protein, SMA patients generally face motor neuron degeneration because of the lack of the survival motor neuron (SMN) protein. Many of the most promising therapies for both conditions restore the absent proteins dystrophin and SMN. Antisense oligonucleotide-mediated exon skipping and inclusion therapies are advancing clinically with the approved DMD therapies casimersen, eteplirsen, golodirsen, and viltolarsen, and the SMA therapy nusinersen. Existing antisense therapies focus on skeletal muscle for DMD and motor neurons for SMA, respectively. Through innovative techniques, such as peptide conjugation and multi-exon skipping, these therapies could be optimized for efficacy and applicability. By contrast, gene replacement therapy is administered only once to patients during treatment. Currently, only onasemnogene abeparvovec for SMA has been approved. Safety shortcomings remain a major challenge for gene therapy. Nevertheless, gene therapy for DMD has strong potential to restore dystrophin expression in patients. In light of promising functional improvements, antisense and gene therapies stand poised to elevate the lives of patients with DMD and SMA.

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

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

Detection of Locked Nucleic Acid Gapmers from Mouse Muscle Samples Using ELISA

Antisense oligonucleotide (ASO)-mediated therapy is promising for the treatment of a variety of genetic disorders, such as Duchenne muscular dystrophy. As more ASOs advance in therapeutic development and enter clinical trials, it becomes necessary to have a means of quantifying their amounts in biological samples post-treatment. This information will be valuable for evaluating the safety and pharmacokinetic profiles of ASOs, and in deciding how the efficacy of these drugs can be improved. Gapmers are a class of ASOs characterized by having a central DNA portion that is surrounded by chemically modified nucleotides on both ends. While relatively simple and accessible methods to quantify other ASOs such as phosphorodiamidate morpholino oligomers (PMOs) using enzyme-linked immunosorbent assay (ELISA)-based techniques are available and have been used for in vivo studies, no such method is available for gapmers to our knowledge. Here, we describe a sensitive ELISA protocol that can be used to quantify the levels of locked nucleic acid (LNA) gapmers in mouse muscle tissue.

Genetic Approaches for the Treatment of Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder characterized by progressive, asymmetric muscle weakness at the face, shoulders, and upper limbs, which spreads to the lower body with age. It is the third most common inherited muscular disorder worldwide. Around 20% of patients are wheelchair-bound, and some present with extramuscular manifestations. FSHD is caused by aberrant expression of the double homeobox protein 4 (DUX4) gene in muscle. DUX4 codes for a transcription factor which, in skeletal muscle, dysregulates numerous signaling activities that culminate in cytotoxicity. Potential treatments for FSHD therefore aim to reduce the expression of DUX4 or the activity of its toxic protein product. In this article, we review how genetic approaches such as those based on oligonucleotide and genome editing technologies have been developed to achieve these goals. We also outline the challenges these therapies are facing on the road to translation, and discuss possible solutions and future directions.

A Genotype-Phenotype Correlation Study of Exon Skip-Equivalent In-Frame Deletions and Exon Skip-Amenable Out-of-Frame Deletions across the DMD Gene to Simulate the Effects of Exon-Skipping Therapies: A Meta-Analysis

Dystrophinopathies are caused by mutations in the DMD gene. Out-of-frame deletions represent most mutational events in severe Duchenne muscular dystrophy (DMD), while in-frame deletions typically lead to milder Becker muscular dystrophy (BMD). Antisense oligonucleotide-mediated exon skipping converts an out-of-frame transcript to an in-frame one, inducing a truncated but partially functional dystrophin protein. The reading frame rule, however, has many exceptions. We thus sought to simulate clinical outcomes of exon-skipping therapies for DMD exons from clinical data of exon skip-equivalent in-frame deletions, in which the expressed quasi-dystrophins are comparable to those resulting from exon-skipping therapies. We identified a total of 1298 unique patients with exon skip-equivalent mutations in patient registries and the existing literature. We classified them into skip-equivalent deletions of each exon and statistically compared the ratio of DMD/BMD and asymptomatic individuals across the DMD gene. Our analysis identified that five exons are associated with significantly milder phenotypes than all other exons when corresponding exon skip-equivalent in-frame deletion mutations occur. Most exon skip-equivalent in-frame deletions were associated with a significantly milder phenotype compared to corresponding exon skip-amenable out-of-frame mutations. This study indicates the importance of genotype-phenotype correlation studies in the rational design of exon-skipping therapies.

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.

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

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

Advances in Genetic Characterization and Genotype-Phenotype Correlation of Duchenne and Becker Muscular Dystrophy in the Personalized Medicine Era

Currently, Duchenne muscular dystrophy (DMD) and the related condition Becker muscular dystrophy (BMD) can be usually diagnosed using physical examination and genetic testing. While BMD features partially functional dystrophin protein due to in-frame mutations, DMD largely features no dystrophin production because of out-of-frame mutations. However, BMD can feature a range of phenotypes from mild to borderline DMD, indicating a complex genotype-phenotype relationship. Despite two mutational hot spots in dystrophin, mutations can arise across the gene. The use of multiplex ligation amplification (MLPA) can easily assess the copy number of all exons, while next-generation sequencing (NGS) can uncover novel or confirm hard-to-detect mutations. Exon-skipping therapy, which targets specific regions of the dystrophin gene based on a patient's mutation, is an especially prominent example of personalized medicine for DMD. To maximize the benefit of exon-skipping therapies, accurate genetic diagnosis and characterization including genotype-phenotype correlation studies are becoming increasingly important. In this article, we present the recent progress in the collection of mutational data and optimization of exon-skipping therapy for DMD/BMD.

A gene-edited mouse model of limb-girdle muscular dystrophy 2C for testing exon skipping

Limb-girdle muscular dystrophy type 2C is caused by autosomal recessive mutations in the γ-sarcoglycan (SGCG) gene. The most common SGCG mutation is a single nucleotide deletion from a stretch of five thymine residues in SGCG exon 6 (521ΔT). This founder mutation disrupts the transcript reading frame, abolishing protein expression. An antisense oligonucleotide exon-skipping method to reframe the human 521ΔT transcript requires skipping four exons to generate a functional, internally truncated protein. In vivo evaluation of this multi-exon skipping, antisense-mediated therapy requires a genetically appropriate mouse model. The human and mouse γ-sarcoglycan genes are highly homologous in sequence and gene structure, including the exon 6 region harboring the founder mutation. Herein, we describe a new mouse model of this form of limb-girdle muscular dystrophy generated using CRISPR/Cas9-mediated gene editing to introduce a single thymine deletion in murine exon 6, recreating the 521ΔT point mutation in Sgcg These mice express the 521ΔT transcript, lack γ-sarcoglycan protein and exhibit a severe dystrophic phenotype. Phenotypic characterization demonstrated reduced muscle mass, increased sarcolemmal leak and fragility, and decreased muscle function, consistent with the human pathological findings. Furthermore, we showed that intramuscular administration of a murine-specific multiple exon-directed antisense oligonucleotide cocktail effectively corrected the 521ΔT reading frame. These data demonstrate a molecularly and pathologically suitable model for in vivo testing of a multi-exon skipping strategy to advance preclinical development of this genetic correction approach.

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

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

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

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

Patent landscape of molecular and cellular targeted therapies for recessive dystrophic epidermolysis bullosa

INTRODUCTION

Recessive Dystrophic Epidermolysis Bullosa (RDEB) is a monogenetic inherited genodermatosis associated with deleterious mutations in the gene encoding type VII collagen (COL7A1). COL7A1 is essential for promoting attachment of the epidermis to the dermis, and its dysfunction may lead to generalized mucosal and cutaneous blistering associated to severe deformities. Currently, management of RDEB patients is limited to supportive care, being aimed at treating and preventing common complications associated with this condition. There is a great demand to develop targeted therapies for this devastating disease and RDEB research advances are currently being translated into clinical trials.

AREAS COVERED

Based on the literature and patent search, the authors have grouped the RDEB targeted therapies into five categories: a) cell-based therapies; b) gene therapy; c) protein replacement therapy; d) molecular therapy based on exon skipping; and e) drug-mediated premature termination codon read-through. The patent searching strategy involved inquiring Google and USPTO patent databases to reveal companies and institutions that are active in the area of RDEB targeted therapies.

EXPERT OPINION

The patent landscape related to targeted therapies for RDEB is quite heterogeneous, with each targeted therapeutic approach being associated with its own challenges in achieving robust patent protection and identifying opportunities for future development.

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

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