A phase I trial of adeno-associated virus serotype 1-γ-sarcoglycan gene therapy for limb girdle muscular dystrophy type 2C

Advanced therapeutic approaches in sarcoglycanopathies

Sarcoglycanopathies are rare autosomal recessive diseases belonging to the family of limb-girdle muscular dystrophies. They are caused by mutations in the genes coding for α-, β-, γ-, and δ-sarcoglycan. The mutations impair the assembly of a key structural complex, which normally protects the sarcolemma of striated muscle from contraction-derived stress. Although heterogeneous, sarcoglycanopathies are characterized by progressive muscle degeneration, increased serum creatine kinase levels, loss of ambulation often during adolescence, and variable cardio-respiratory impairment. Genetic defects can impair sarcoglycan synthesis or produce a protein that is defective in folding. There is currently no effective treatment available; however, both gene replacement strategy and small molecule-based approaches show great promise and have entered or are starting to enter clinical trials.

Gene therapy for primary myopathies: literature review and prospects

Gene therapy has emerged as a promising frontier in the pursuit of effective treatments for primary myopathies. This scientific review explores the application of viral vectors and more specifically of recombinant adeno-associated virus (rAAV) vectors as a potent gene delivery tool in the context of primary myopathies, highlighting its transformative potential. Focusing on primary myopathies, including Duchenne muscular dystrophy (DMD), limb-girdle muscular dystrophies (LGMDs), X-linked myotubular myopathy (XLMTM), and Pompe disease, we review the ongoing pre-clinical and clinical trials that underscore the therapeutic promise of rAAV-based gene therapies. Recent developments in gene therapy have unveiled innovative gene transfer approaches, particularly with rAAV vectors. These vectors offer a well-tolerated and efficient means of delivering corrective genetic material to diseased muscles, thereby addressing the root causes of primary myopathies. Encouraging data from pre-clinical studies and early clinical trials have demonstrated the potential to ameliorate muscle function, reduce pathological manifestations, and enhance the quality of life for patients afflicted with these devastating diseases. However, the transition from bench to bedside is not without challenges. This review emphasizes the critical need for a comprehensive risk management strategy to better handle potential side effects and immune responses associated with gene therapy. As the field of gene therapy for primary myopathies is advancing, it is imperative to refine and optimize safety measures, ensuring that the transformative potential of these therapies is realized while the risks are minimized. © 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.

The ice age - A review on formulation of Adeno-associated virus therapeutics

Gene therapies offer promising therapeutic alternatives for many disorders that currently lack efficient treatment options. Due to their chemical nature and physico-chemical properties, delivery of polynucleic acids into target cells and subcellular compartments remains a significant challenge. Adeno-associated viruses (AAV) have gained a lot of interest for the efficient delivery of therapeutic single-stranded DNA (ssDNA) genomes over the past decades. More than a hundred products have been tested in clinical settings and three products have received market authorization by the US FDA in recent years. A lot of effort is being made to generate potent recombinant AAV (rAAV) vectors that show favorable safety and immunogenicity profiles for either local or systemic administration. Manufacturing processes are gradually being optimized to deliver a consistently high product quality and to serve potential market needs beyond rare indications. In contrast to protein therapeutics, most rAAV products are still supplied as frozen liquids within rather simple formulation buffers to enable sufficient product shelf life, significantly hampering global distribution and access. In this review, we aim to outline the hurdles of rAAV drug product development and discuss critical formulation and composition aspects of rAAV products under clinical evaluation. Further, we highlight recent development efforts in order to achieve stable liquid or lyophilized products. This review therefore provides a comprehensive overview on current state-of-the-art rAAV formulations and can further serve as a map for rational formulation development activities in the future.

Immune Responses to Muscle-Directed Adeno-Associated Viral Gene Transfer in Clinical Studies

Muscle-directed gene therapy with adeno-associated viral (AAV) vectors is undergoing clinical development for treating neuromuscular disorders and for systemic delivery of therapeutic proteins. Although these approaches show considerable therapeutic benefits, they are also prone to induce potent immune responses against vector or transgene products owing to the immunogenic nature of the intramuscular delivery route, or the high doses required for systemic delivery to muscle. Major immunological concerns include antibody formation against viral capsid, complement activation, and cytotoxic T cell responses against capsid or transgene products. They can negate therapy and even lead to life-threatening immunotoxicities. Herein we review clinical observations and provide an outlook for how the field addresses these problems through a combination of vector engineering and immune modulation.

Systemic γ-sarcoglycan AAV gene transfer results in dose-dependent correction of muscle deficits in the LGMD 2C/R5 mouse model

Limb-girdle muscular dystrophy (LGMD) type 2C/R5 results from mutations in the γ-sarcoglycan (SGCG) gene and is characterized by muscle weakness and progressive wasting. Loss of functional γ-sarcoglycan protein in the dystrophin-associated protein complex destabilizes the sarcolemma, leading to eventual myofiber death. The SGCG knockout mouse (SGCG ^-/-) has clinical-pathological features that replicate the human disease, making it an ideal model for translational studies. We designed a self-complementary rAAVrh74 vector containing a codon-optimized human SGCG transgene driven by the muscle-specific MHCK7 promoter (SRP-9005) to investigate adeno-associated virus (AAV)-mediated SGCG gene transfer in SGCG ^-/- mice as proof of principle for LGMD 2C/R5. Gene transfer therapy resulted in widespread transgene expression in skeletal muscle and heart, improvements in muscle histopathology characterized by decreased central nuclei and fibrosis, and normalized fiber size. Histopathologic improvements were accompanied by functional improvements, including increased ambulation and force production and resistance to injury of the tibialis anterior and diaphragm muscles. This study demonstrates successful systemic delivery of the hSGCG transgene in SGCG ^-/- mice, with functional protein expression, reconstitution of the sarcoglycan complex, and corresponding physiological and functional improvements, which will help establish a minimal effective dose for translation of SRP-9005 gene transfer therapy in patients with LGMD 2C/R5.

Current Strategies of Muscular Dystrophy Therapeutics: An Overview

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

Immunofluorescence signal intensity measurements as a semi-quantitative tool to assess sarcoglycan complex expression in muscle biopsy

Sarcoglycanopathies are highly heterogeneous in terms of disease progression, muscular weakness, loss of ambulation and cardiac/respiratory involvement. Their clinical severity usually correlates with the residual protein amount, which makes protein quantification extremely relevant. Sarcoglycanopathy diagnosis is genetic, but skeletal muscle analysis - by both immunohistochemistry and Western blot (WB) - is still mandatory to establish the correct diagnostic process. Unfortunately, however, WB analysis cannot be performed if the bioptic specimen is scarce. This study provides a sensitive tool for semi-quantification of residual amount of sarcoglycans in patients affected by sarcoglycanopathies, based on immunofluorescence staining on skeletal muscle sections, image acquisition and software elaboration. We applied this method to eleven sarcoglycanopathies, seven Becker muscular dystrophies, as pathological control group, and four age-matched controls. Fluorescence data showed a significantly reduced expression of the mutated sarcoglycan in all patients when compared to their respective age-matched healthy controls, and a variable reduction of the other sarcoglycans. The reduction is due to the effect of gene mutation and not to the increasing age of controls. Fluorescence normalized data analyzed in relation to the age of onset of the disease, showed a negative correlation of a-sarcoglycan fluorescence signal vs fibrosis in patients with an early age of onset and a negative correlation between d-sarcoglycan signal and fibrosis in both intermediate and late age of onset groups. The availability of a method that allows objective quantification of the sarcolemmal proteins, faster and less consuming than WB analysis and able to detect low residual sarcoglycan expression with great sensitivity, proves useful also in view of possible inferences on disease prognosis. The proposed method could be employed also to monitor the efficacy of therapeutic interventions and during clinical trials.

Advanced Gene-Targeting Therapies for Motor Neuron Diseases and Muscular Dystrophies

Gene therapy is a revolutionary, cutting-edge approach to permanently ameliorate or amend many neuromuscular diseases by targeting their genetic origins. Motor neuron diseases and muscular dystrophies, whose genetic causes are well known, are the frontiers of this research revolution. Several genetic treatments, with diverse mechanisms of action and delivery methods, have been approved during the past decade and have demonstrated remarkable results. However, despite the high number of genetic treatments studied preclinically, those that have been advanced to clinical trials are significantly fewer. The most clinically advanced treatments include adeno-associated virus gene replacement therapy, antisense oligonucleotides, and RNA interference. This review provides a comprehensive overview of the advanced gene therapies for motor neuron diseases (i.e., amyotrophic lateral sclerosis and spinal muscular atrophy) and muscular dystrophies (i.e., Duchenne muscular dystrophy, limb-girdle muscular dystrophy, and myotonic dystrophy) tested in clinical trials. Emphasis has been placed on those methods that are a few steps away from their authoritative approval.

Spectrum of Genetic Diseases in Tunisia: Current Situation and Main Milestones Achieved

Genetic diseases in Tunisia are a real public health problem given their chronicity and the lack of knowledge concerning their prevalence and etiology, and the high rates of consanguinity. Hence, we performed systematic reviews of the literature in order to provide a more recent spectrum of these disorders and to expose the challenges that still exist to tackle these kinds of diseases. A manual textual data mining was conducted using MeSH and PubMed databases. Collected data were classified according to the CIM-10 classification and the transmission mode. The spectrum of these diseases is estimated to be 589 entities. This suggests remarkable progress through the development of biomedical health research activities and building capacities. Sixty percent of the reported disorders are autosomal recessive, which could be explained by the high prevalence of endogamous mating. Congenital malformations (29.54%) are the major disease group, followed by metabolic diseases (22%). Sixty percent of the genetic diseases have a known molecular etiology. We also reported additional cases of comorbidity that seem to be a common phenomenon in our population. We also noticed that epidemiological data are scarce. Newborn and carrier screening was only limited to pilot projects for a few genetic diseases. Collected data are being integrated into a database under construction that will be a valuable decision-making tool. This study provides the current situation of genetic diseases in Tunisia and highlights their particularities. Early detection of the disease is important to initiate critical intervention and to reduce morbidity and mortality.

Monobac System-A Single Baculovirus for the Production of rAAV

Large-scale manufacturing of rAAV is a bottleneck for the development of genetic disease treatments. The baculovirus/Sf9 cell system underpins the first rAAV treatment approved by EMA and remains one of the most advanced platforms for rAAV manufacturing. Despite early successes, rAAV is still a complex biomaterial to produce. Efficient production of the recombinant viral vector requires that AAV replicase and capsid genes be co-located with the recombinant AAV genome. Here, we present the Monobac system, a singular, modified baculovirus genome that contains all of these functions. To assess the relative yields between the dual baculovirus and Monobac systems, we prepared each system with a transgene encoding γSGC and evaluated vectors’ potency in vivo. Our results show that rAAV production using the Monobac system not only yields higher titers of rAAV vector but also a lower amount of DNA contamination from baculovirus.

Sarcoglycanopathies: an update

Sarcoglycanopathies are the most severe forms of autosomal recessive limb-girdle muscular dystrophies (LGMDs), constituting about 10-25% of LGMDs. The clinical phenotype is variable, but onset is usually in the first decade of life. Patients present muscle hypertrophy, elevated CK, variable muscle weaknesses, and progressive loss of ambulation. Four subtypes are known: LGMDR3, LGMDR4, LGMDR5 and LGMDR6, caused, respectively, by mutations in the SGCA, SGCB,SGCG and SGCD genes. Their four coded proteins, α-SG, ß-SG, λ-SG and δ-SG are part of the dystrophin-glycoprotein complex (DGC) present in muscle sarcolemma, which acts as a linker between the cytoskeleton of the muscle fiber and the extracellular matrix, providing mechanical support to the sarcolemma during myofiber contraction. Many different mutations have already been identified in all the sarcoglycan genes, with a predominance of some mutations in different populations. The diagnosis is currently based on the molecular screening for these mutations. Therapeutic approaches include the strategy of gene replacement mediated by a vector derived from adeno-associated virus (AAV). Pre-clinical studies have shown detectable levels of SG proteins in the muscle, and some improvement in the phenotype, in animal models. Therapeutic trials in humans are ongoing.

A Journey with LGMD: From Protein Abnormalities to Patient Impact

The limb-girdle muscular dystrophies (LGMD) are a collection of genetic diseases united in their phenotypical expression of pelvic and shoulder area weakness and wasting. More than 30 subtypes have been identified, five dominant and 26 recessive. The increase in the characterization of new genotypes in the family of LGMDs further adds to the heterogeneity of the disease. Meanwhile, better understanding of the phenotype led to the reconsideration of the disease definition, which resulted in eight old subtypes to be no longer recognized officially as LGMD and five new diseases to be added to the LGMD family. The unique variabilities of LGMD stem from genetic mutations, which then lead to protein and ultimately muscle dysfunction. Herein, we review the LGMD pathway, starting with the genetic mutations that encode proteins involved in muscle maintenance and repair, and including the genotype–phenotype relationship of the disease, the epidemiology, disease progression, burden of illness, and emerging treatments.

[Sarcoglycanopathies: state of the art and therapeutic perspectives]

Sarcoglycanopathies are the third most common cause of autosomal recessive limb girdle muscular dystrophies (LGMD). They are the result of a deficiency in one of the sarcoglycans a, b, g, or d. The usual clinical presentation is that of a symmetrical involvement of the muscles of the pelvic and scapular girdles as well as of the trunk, associated with more or less severe cardio-respiratory impairment and a marked increase of serum CK levels. The first symptoms appear during the first decade, the loss of ambulation occurring often during the second decade. Lesions observed on the muscle biopsy are dystrophic. This is associated with a decrease or an absence of immunostaining of the sarcoglycan corresponding to the mutated gene and, to a lesser degree, of the other three sarcoglycans. Many mutations have been reported in the four incriminated genes and some of them are prevalent in certain populations. To date, there is no curative treatment, which does not prevent the development of many clinical trials, especially in gene therapy.

Current Genetic Survey and Potential Gene-Targeting Therapeutics for Neuromuscular Diseases

Neuromuscular diseases (NMDs) belong to a class of functional impairments that cause dysfunctions of the motor neuron-muscle functional axis components. Inherited monogenic neuromuscular disorders encompass both muscular dystrophies and motor neuron diseases. Understanding of their causative genetic defects and pathological genetic mechanisms has led to the unprecedented clinical translation of genetic therapies. Challenged by a broad range of gene defect types, researchers have developed different approaches to tackle mutations by hijacking the cellular gene expression machinery to minimize the mutational damage and produce the functional target proteins. Such manipulations may be directed to any point of the gene expression axis, such as classical gene augmentation, modulating premature termination codon ribosomal bypass, splicing modification of pre-mRNA, etc. With the soar of the CRISPR-based gene editing systems, researchers now gravitate toward genome surgery in tackling NMDs by directly correcting the mutational defects at the genome level and expanding the scope of targetable NMDs. In this article, we will review the current development of gene therapy and focus on NMDs that are available in published reports, including Duchenne Muscular Dystrophy (DMD), Becker muscular dystrophy (BMD), X-linked myotubular myopathy (XLMTM), Spinal Muscular Atrophy (SMA), and Limb-girdle muscular dystrophy Type 2C (LGMD2C).

The ties that bind: functional clusters in limb-girdle muscular dystrophy. Skelet Muscle. 2020;10:22. [PMID: 32727611 PMCID: PMC7389686 DOI: 10.1186/s13395-020-00240-7]

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

Limb-girdle Muscular Dystrophy and Therapy: Insights into Cell and Gene-based Approaches

The Limb-Girdle Muscular Dystrophies (LGMD) are genetically heterogeneous disorders, responsible for muscle wasting and severe form of dystrophies. Despite the critical developments in the insight and information of pathomechanisms of limb-girdle muscular dystrophy, any definitive treatments do not exist, and current strategies are only based on the improvement of the signs of disorder and to enhance the life quality without resolving an underlying cause. There is a crucial relationship between pharmacological therapy and different consequences; therefore, other treatment strategies will be required. New approaches, such as gene replacement, gene transfer, exon skipping, siRNA knockdown, and anti-myostatin therapy, which can target specific cellular or molecular mechanism of LGMD, could be a promising avenue for the treatment. Recently, genome engineering strategies with a focus on molecular tools such as CRISPR-Cas9 are used to different types of neuromuscular disorders and show the highest potential for clinical translation of these therapies. Thus, recent advancements and challenges in the field will be reviewed in this paper.

New Therapeutics Options for Pediatric Neuromuscular Disorders

Neuromuscular disorders (NMDs) of Childhood onset are a genetically heterogeneous group of diseases affecting the anterior horn cell, the peripheral nerve, the neuromuscular junction, or the muscle. For many decades, treatment of NMDs has been exclusively symptomatic. But this has changed fundamentally in recent years due to the development of new drugs attempting either to ameliorate secondary pathophysiologic consequences or to modify the underlying genetic defect itself. While the effects on the course of disease are still modest in some NMDs (e.g., Duchenne muscular dystrophy), new therapies have substantially prolonged life expectancy and improved motor function in others (e.g., spinal muscular atrophy and infantile onset Pompe disease). This review summarizes recently approved medicaments and provides an outlook for new therapies that are on the horizon in this field.

A Muscle Hybrid Promoter as a Novel Tool for Gene Therapy

Gene therapy is a promising strategy to cure rare diseases. The lack of regulatory sequences ensuring specific and robust expression in skeletal and cardiac muscle is a substantial limitation of gene therapy efficiency targeting the muscle tissue. Here we describe a novel muscle hybrid (MH) promoter that is highly active in both skeletal and cardiac muscle cells. It has an easily exchangeable modular structure, including an intronic module that highly enhances the expression of the gene driven by it. In cultured myoblasts, myotubes, and cardiomyocytes, the MH promoter gives relatively stable expression as well as higher activity and protein levels than the standard CMV and desmin gene promoters or the previously developed synthetic or CKM-based promoters. Combined with AAV2/9, the MH promoter also provides a high in vivo expression level in skeletal muscle and the heart after both intramuscular and systemic delivery. It is much more efficient than the desmin-encoding gene promoter, and it maintains the same specificity. This novel promoter has potential for gene therapy in muscle cells. It can provide stable transgene expression, ensuring high levels of therapeutic protein, and limited side effects because of its specificity. This constitutes an improvement in the efficiency of genetic disease therapy.

An AAV-SGCG Dose-Response Study in a γ-Sarcoglycanopathy Mouse Model in the Context of Mechanical Stress

Sarcoglycanopathies are rare autosomic limb girdle muscular dystrophies caused by mutations in one of the genes coding for sarcoglycans. Sarcoglycans form a complex, which is an important part of the dystrophin-associated glycoprotein complex and which protects the sarcolemma against muscle contraction-induced damage. Absence of one of the sarcoglycans on the plasma membrane reduces the stability of the whole complex and perturbs muscle fiber membrane integrity. There is currently no curative treatment for any of the sarcoglycanopathies. A first clinical trial to evaluate the safety of a recombinant AAV2/1 vector expressing γ-sarcoglycan using an intramuscular route of administration showed limited expression of the transgene and good tolerance of the approach. In this report, we undertook a dose-effect study in mice to evaluate the efficiency of an AAV2/8-expressing γ-sarcoglycan controlled by a muscle-specific promoter with a systemic mode of administration. We observed a dose-related efficiency with a nearly complete restoration of gamma sarcoglycan (SGCG) expression, histological appearance, biomarker level, and whole-body strength at the highest dose tested. In addition, our data suggest that a high expression threshold level must be achieved for effective protection of the transduced muscle, while a suboptimal transgene expression level might be less protective in the context of mechanical stress.

Hutchinson-Gilford Progeria Syndrome-Current Status and Prospects for Gene Therapy Treatment

Hutchinson-Gilford progeria syndrome (HGPS) is one of the most severe disorders among laminopathies—a heterogeneous group of genetic diseases with a molecular background based on mutations in the LMNA gene and genes coding for interacting proteins. HGPS is characterized by the presence of aging-associated symptoms, including lack of subcutaneous fat, alopecia, swollen veins, growth retardation, age spots, joint contractures, osteoporosis, cardiovascular pathology, and death due to heart attacks and strokes in childhood. LMNA codes for two major, alternatively spliced transcripts, give rise to lamin A and lamin C proteins. Mutations in the LMNA gene alone, depending on the nature and location, may result in the expression of abnormal protein or loss of protein expression and cause at least 11 disease phenotypes, differing in severity and affected tissue. LMNA gene-related HGPS is caused by a single mutation in the LMNA gene in exon 11. The mutation c.1824C > T results in activation of the cryptic donor splice site, which leads to the synthesis of progerin protein lacking 50 amino acids. The accumulation of progerin is the reason for appearance of the phenotype. In this review, we discuss current knowledge on the molecular mechanisms underlying the development of HGPS and provide a critical analysis of current research trends in this field. We also discuss the mouse models available so far, the current status of treatment of the disease, and future prospects for the development of efficient therapies, including gene therapy for HGPS.

Origins of truncated supplementary capsid proteins in rAAV8 vectors produced with the baculovirus system

The ability to produce large quantities of recombinant Adeno-Associated Virus (rAAV) vectors is an important factor for the development of gene therapy-based medicine. The baculovirus/insect cell expression system is one of the major systems for large scale rAAV production. So far, most technological developments concerned the optimization of the AAV rep and cap genes in order to be expressed correctly in a heterologous system. However, the effect of the baculovirus infection on the production of rAAV has not been examined in detail. In this study we show that the baculoviral cathepsin (v-CATH) protease is active on several (but not all) rAAV serotypes, leading to a partial degradation of VP1/VP2 proteins. Subsequently, we identified the principal v-CATH cleavage site in the rAAV8 capsid proteins and demonstrated that the cleavage is highly specific. The proteolytic degradation of VP1/VP2 AAV capsid proteins reduces the infectivity of rAAV vectors but can be prevented by the use of a baculovirus vector with a deletion of the chiA/v-cath locus or by addition of the E64 protease inhibitor during production. Moreover, the codon optimization of AAV cap performed for several serotypes and originally aimed at the removal of potential alternative initiation codons, resulted in incorporation of additional forms of truncated VP1 into the rAAV capsids.

The Limb-Girdle Muscular Dystrophies: Is Treatment on the Horizon?

There has been an ever-expanding list of the Limb-Girdle Muscular Dystrophies (LGMD). There are currently 8 subtypes of autosomal dominant (AD) and 26 subtypes of autosomal recessive (AR) LGMD. Despite continued research efforts to conquer this group of genetic neuromuscular disease, patients continue to be treated symptomatically with the aim of prevention or addressing complications. Mouse models have been helpful in clarifying disease pathogenesis as well as strategizing pathways for treatment. Discoveries in translational research as well as molecular therapeutic approaches have kept clinicians optimistic that more promising clinical trials will lead the way to finding the cure for these devastating disorders. It is well known that the challenge for these rare diseases is the ability to assemble adequate numbers of patients for a clinically meaningful trial, but current efforts in developing patient registries have been encouraging. Natural history studies will be essential in establishing and interpreting the appropriate outcome measures for clinical trials. Nevertheless, animal studies continue to be key in providing proof of concept that will be necessary in moving research along. This review will briefly discuss each type of LGMD, highlighting their distinguishing features, then focus on research efforts that have been published in the literature for the past few years, many of which are still in the preclinical trial stage.

Can an in vivo imaging system be used to determine localization and biodistribution of AAV5-mediated gene expression following subretinal and intravitreal delivery in mice?

Recombinant adeno associated viruses (AAV) are the most commonly used vectors in animal model studies of gene therapy for retinal diseases. The ability of a vector to localize and remain in the target tissue, and in this manner to avoid off-target effects beyond the site of delivery, is critical to the efficacy and safety of the treatment. The in vivo imaging system (IVIS) is a non-invasive imaging tool used for detection and quantification of bioluminescence activity in rodents. Our aim was to investigate whether IVIS can detect localization and biodistribution of AAV5 vector in mice following subretinal (SR) and intravitreal (IVT) injections. AAV5 carrying firefly luciferase DNA under control of the ubiquitous cytomegalovirus (CMV) promoter was injected unilaterally IVT or SR (in the central or peripheral retina) of forty-one mice. Luciferase activity was tracked for up to 60 weeks in the longest surviving animals, using repeated (up to 12 times) IVIS bioluminescence imaging. Luciferase presence was also confirmed immunohistochemically (IHC) and by PCR in representative animals. In the SR group, IVIS readings demonstrated luciferase activity in all (32/32) eyes, and luciferase presence was confirmed by IHC (4/4 eyes) and PCR (12/12 eyes). In the IVT group, IVIS readings demonstrated luciferase activity in 7/9 eyes, and luciferase presence was confirmed by PCR in 5/5 eyes and by IHC (2/2 eyes). In two SR-injected animals (one each from the central and peripheral injection sites), PCR detected luciferase presence in the ipsilateral optic nerves, a finding that was not detected by IVIS or IHC. Our results show that when evaluating SR delivery, IVIS has a sensitivity and specificity of 100% compared with the gold standard PCR. When evaluating IVT delivery, IVIS has a sensitivity of 78% and specificity of 100%. These finding confirm the ability of IVIS to detect in-vivo localized expression of AAV following SR delivery in the retina up to 60 weeks post-treatment, using repeated imaging for longitudinal evaluation, without fading of the biological signal, thereby replacing the need for post mortem processing in order to confirm vector expression. However, IVIS is probably not sensitive enough, compared with genome detection, to demonstrate biodistribution to the optic nerve, as it could not detect luciferase activity in ipsilateral optic nerves following SR delivery in mice.

Muscle Wasting Diseases: Novel Targets and Treatments

Adequate skeletal muscle plasticity is an essential element for our well-being, and compromised muscle function can drastically affect quality of life, morbidity, and mortality. Surprisingly, however, skeletal muscle remains one of the most under-medicated organs. Interventions in muscle diseases are scarce, not only in neuromuscular dystrophies, but also in highly prevalent secondary wasting pathologies such as sarcopenia and cachexia. Even in other diseases that exhibit a well-established risk correlation of muscle dysfunction due to a sedentary lifestyle, such as type 2 diabetes or cardiovascular pathologies, current treatments are mostly targeted on non-muscle tissues. In recent years, a renewed focus on skeletal muscle has led to the discovery of various novel drug targets and the design of new pharmacological approaches. This review provides an overview of the current knowledge of the key mechanisms involved in muscle wasting conditions and novel pharmacological avenues that could ameliorate muscle diseases.

Genetics instability of wtAAV2 genome and AAV promoter activities in the Baculovirus/Sf9 cells system

The human Adeno-Associated Virus serotype 2 (wtAAV2) is a common non-pathological virus and its recombinant form (rAAV) is widely used as gene therapy vector. Although rAAVs are routinely produced in the Baculovirus/Sf9 cell system, wtAAV2 has never been studied in this context. We tried to produce wtAAV2 in the baculovirus/Sf9 cell system hypothesizing that the wtAAV2 may be considered as a normal recombinant AAV transgene. Through our attempts to produce wtAAV2 in Baculovirus/Sf9, we found that wtAAV2 p5 promoter, which controls the expression of large Rep proteins in mammalian cells, was active in this system. p5 promoter activity in the baculovirus/Sf9 cell system leads to the expression of Rep78 that finally excises wtAAV2 genome from the baculovirus genome during the earliest phases of baculovirus stock production. Via p5 promoter expression kinetics and strand specific RNA-Seq analysis of wtAAV2, rAAV and Rep2/Cap2 cassettes in the baculovirus context we could demonstrate that wtAAV2 native promoters, p5, p19 and p40 are all active in the context of the baculovirus system and lead to the expression of different proteins and peptides. In addition, this study has proven that the baculovirus brings at least some of the helper functions needed in the AAV replication/life cycle.

Molecular Therapies for Muscular Dystrophies

PURPOSE OF REVIEW

To construct a framework to understand the different molecular interventions for muscular dystrophy.

RECENT FINDINGS

The recent approval of antisense oligonucleotides treatment for Duchenne muscular dystrophy and spinal muscular atrophy and current clinical trials using recombinant adeno-associated virus for the treatment of those diseases suggests that we are at a tipping point where we are able to treat and potentially cure muscular dystrophies. Understanding the basic molecular pathogenesis of muscular dystrophies and the molecular biology of the treatment allows for critical evaluation of the proposed therapies.

Cre Recombinase Mediates the Removal of Bacterial Backbone to Efficiently Generate rSV40

Gene therapy has been shown to be a feasible approach to treat inherited disorders in vivo. Among the currently used viral vector systems, adeno-associated virus (AAV) vectors are the most advanced and have been applied in patients successfully. An important drawback of non-integrating AAV vectors is their loss of expression upon cell division, while repeating systemic administration lacks efficacy due to the induction of neutralizing antibodies. In addition, a significant percentage of the general population is not eligible for AAV-mediated gene therapy due to pre-existing immunity. Development of additional viral vectors may overcome this hurdle. Simian virus 40 (SV40)-derived vectors have been reported to transduce different tissues, including the liver, and prevalence of neutralizing antibodies in the general population is very low. This renders recombinant SV40 (rSV40) vector an interesting candidate for effective (re-)administration. Clinical use of SV40 vectors is in part hampered by less advanced production methods compared to AAVs. To optimize the production of rSV40 and make it better suitable for clinical practice, we developed a production system that relies on Cre recombinase-mediated removal of the bacterial plasmid backbone.

Impact of Inverted Terminal Repeat Integrity on rAAV8 Production Using the Baculovirus/Sf9 Cells System

Adeno-associated virus (AAV) inverted terminal repeats (ITRs) are key elements of AAV. These guanine-cytosine-rich structures are involved in the replication and encapsidation of the AAV genome, along with its integration in and excision from the host genome. These sequences are the only AAV-derived DNA sequences conserved in recombinant AAV (rAAV), as they allow its replication, encapsidation, and long-term maintenance and expression in target cells. Due to the original vector design, plasmids containing the gene of interest flanked by ITRs and used for rAAV production often present incomplete, truncated, or imperfect ITR sequences. For example, pSUB201 and its derivatives harbor a truncated (14 nt missing on the external part of the ITR), flop-orientated ITR plus 46 bp of non-ITR viral DNA at each end of the rAAV genome. It has been shown that rAAV genomes can be replicated, even with incomplete, truncated, or imperfect ITR sequences, leading to the production of rAAV vectors in transfection experiments. Nonetheless, it was hypothesized that unmodified wild-type (WT) ITR sequences could lead to a higher yield of rAAV, with less non-rAAV encapsidated DNA originating from the production cells and/or baculovirus shuttle vector genomes. This work studied the impact of imperfect ITRs on the level of encapsidated rAAV genomes and baculovirus-derived DNA sequences using the baculovirus/Sf9 cells production system. Replacement of truncated ITRs with WT and additional wtAAV2 sequences has an impact on the two major features of rAAV production: (1) a rise from 10% to 40% of full capsids obtained, and (2) up to a 10-fold reduction in non-rAAV encapsidated DNA. Furthermore, this study considered the impact on these major parameters of additional ITR elements and ITRs coupled with various regulatory elements of different origins. Implementation of the use of complete ITRs in the frame of the baculovirus-based rAAV expression system is one step that will be required to optimize the quality of rAAV-based gene therapy drugs.

Silencing Nfix rescues muscular dystrophy by delaying muscle regeneration

Muscular dystrophies are severe disorders due to mutations in structural genes, and are characterized by skeletal muscle wasting, compromised patient mobility, and respiratory functions. Although previous works suggested enhancing regeneration and muscle mass as therapeutic strategies, these led to no long-term benefits in humans. Mice lacking the transcription factor Nfix have delayed regeneration and a shift toward an oxidative fiber type. Here, we show that ablating or silencing the transcription factor Nfix ameliorates pathology in several forms of muscular dystrophy. Silencing Nfix in postnatal dystrophic mice, when the first signs of the disease already occurred, rescues the pathology and, conversely, Nfix overexpression in dystrophic muscles increases regeneration and markedly exacerbates the pathology. We therefore offer a proof of principle for a novel therapeutic approach for muscular dystrophies based on delaying muscle regeneration.

Strategies aimed at promoting muscle regeneration to treat muscular dystrophy have met with limited success. Here the authors show instead that delaying muscle regeneration, by ablation of the transcription factor Nfix, ameliorates muscular dystrophy in mice.

Class I-restricted T-cell responses to a polymorphic peptide in a gene therapy clinical trial for α-1-antitrypsin deficiency

Adeno-associated virus (AAV)-mediated gene therapy is currently being pursued as a treatment for the monogenic disorder α-1-antitrypsin (AAT) deficiency. Results from phase I and II studies have shown relatively stable and dose-dependent increases in transgene-derived wild-type AAT after local intramuscular vector administration. In this report we describe the appearance of transgene-specific T-cell responses in two subjects that were part of the phase II trial. The patient with the more robust T-cell response, which was associated with a reduction in transgene expression, was characterized more thoroughly in this study. We learned that the AAT-specific T cells in this patient were cytolytic in phenotype, mapped to a peptide in the endogenous mutant AAT protein that contained a common polymorphism not incorporated into the transgene, and were restricted by a rare HLA class I C alleles present only in this patient. These human studies illustrate the genetic influence of the endogenous gene and HLA haplotype on the outcome of gene therapy.

Circulating miRNAs are generic and versatile therapeutic monitoring biomarkers in muscular dystrophies

The development of medical approaches requires preclinical and clinical trials for assessment of therapeutic efficacy. Such evaluation entails the use of biomarkers, which provide information on the response to the therapeutic intervention. One newly-proposed class of biomarkers is the microRNA (miRNA) molecules. In muscular dystrophies (MD), the dysregulation of miRNAs was initially observed in muscle biopsy and later extended to plasma samples, suggesting that they may be of interest as biomarkers. First, we demonstrated that dystromiRs dysregulation occurs in MD with either preserved or disrupted expression of the dystrophin-associated glycoprotein complex, supporting the utilization of dystromiRs as generic biomarkers in MD. Then, we aimed at evaluation of the capacity of miRNAs as monitoring biomarkers for experimental therapeutic approach in MD. To this end, we took advantage of our previously characterized gene therapy approach in a mouse model for α-sarcoglycanopathy. We identified a dose-response correlation between the expression of miRNAs on both muscle tissue and blood serum and the therapeutic benefit as evaluated by a set of new and classically-used evaluation methods. This study supports the utility of profiling circulating miRNAs for the evaluation of therapeutic outcome in medical approaches for MD.

Manufacturing of recombinant adeno-associated viral vectors for clinical trials

The ability to elicit robust and long-term transgene expression in vivo together with minimal immunogenicity and little to no toxicity are only a few features that make recombinant adeno-associated virus (rAAV) vectors ideally suited for many gene therapy applications. Successful preclinical studies have encouraged the use of rAAV for therapeutic gene transfer to patients in the clinical setting. Nevertheless, the use of rAAV in clinical trials has underscored the need for production and purification systems capable of generating large amounts of highly pure rAAV particles. To date, generating vector quantities sufficient to meet the expanding clinical demand is still a hurdle when using current production systems. In this chapter, we will provide a description of the current methods to produce clinical grade of rAAV under current good manufacturing practice (cGMP) settings.

Where do we stand in trial readiness for autosomal recessive limb girdle muscular dystrophies?

Autosomal recessive limb girdle muscular dystrophies (LGMD2) are a group of genetically heterogeneous diseases that are typically characterised by progressive weakness and wasting of the shoulder and pelvic girdle muscles. Many of the more than 20 different conditions show overlapping clinical features with other forms of muscular dystrophy, congenital, myofibrillar or even distal myopathies and also with acquired muscle diseases. Although individually extremely rare, all types of LGMD2 together form an important differential diagnostic group among neuromuscular diseases. Despite improved diagnostics and pathomechanistic insight, a curative therapy is currently lacking for any of these diseases. Medical care consists of the symptomatic treatment of complications, aiming to improve life expectancy and quality of life. Besides well characterised pre-clinical tools like animal models and cell culture assays, the determinants of successful drug development programmes for rare diseases include a good understanding of the phenotype and natural history of the disease, the existence of clinically relevant outcome measures, guidance on care standards, up to date patient registries, and, ideally, biomarkers that can help assess disease severity or drug response. Strong patient organisations driving research and successful partnerships between academia, advocacy, industry and regulatory authorities can also help accelerate the elaboration of clinical trials. All these determinants constitute aspects of translational research efforts and influence patient access to therapies. Here we review the current status of determinants of successful drug development programmes for LGMD2, and the challenges of translating promising therapeutic strategies into effective and accessible treatments for patients.

Prospect of gene therapy for cardiomyopathy in hereditary muscular dystrophy

INTRODUCTION

Cardiac involvement is a common feature in muscular dystrophies. It presents as heart failure and/or arrhythmia. Traditionally, dystrophic cardiomyopathy is treated with symptom-relieving medications. Identification of disease-causing genes and investigation on pathogenic mechanisms have opened new opportunities to treat dystrophic cardiomyopathy with gene therapy. Replacing/repairing the mutated gene and/or targeting the pathogenic process/mechanisms using alternative genes may attenuate heart disease in muscular dystrophies.

AREAS COVERED

Duchenne muscular dystrophy is the most common muscular dystrophy. Duchenne cardiomyopathy has been the primary focus of ongoing dystrophic cardiomyopathy gene therapy studies. Here, we use Duchenne cardiomyopathy gene therapy to showcase recent developments and to outline the path forward. We also discuss gene therapy status for cardiomyopathy associated with limb-girdle and congenital muscular dystrophies, and myotonic dystrophy.

EXPERT OPINION

Gene therapy for dystrophic cardiomyopathy has taken a slow but steady path forward. Preclinical studies over the last decades have addressed many fundamental questions. Adeno-associated virus-mediated gene therapy has significantly improved the outcomes in rodent models of Duchenne and limb girdle muscular dystrophies. Validation of these encouraging results in large animal models will pave the way to future human trials.

Reengineering a transmembrane protein to treat muscular dystrophy using exon skipping

Exon skipping uses antisense oligonucleotides as a treatment for genetic diseases. The antisense oligonucleotides used for exon skipping are designed to bypass premature stop codons in the target RNA and restore reading frame disruption. Exon skipping is currently being tested in humans with dystrophin gene mutations who have Duchenne muscular dystrophy. For Duchenne muscular dystrophy, the rationale for exon skipping derived from observations in patients with naturally occurring dystrophin gene mutations that generated internally deleted but partially functional dystrophin proteins. We have now expanded the potential for exon skipping by testing whether an internal, in-frame truncation of a transmembrane protein γ-sarcoglycan is functional. We generated an internally truncated γ-sarcoglycan protein that we have termed Mini-Gamma by deleting a large portion of the extracellular domain. Mini-Gamma provided functional and pathological benefits to correct the loss of γ-sarcoglycan in a Drosophila model, in heterologous cell expression studies, and in transgenic mice lacking γ-sarcoglycan. We generated a cellular model of human muscle disease and showed that multiple exon skipping could be induced in RNA that encodes a mutant human γ-sarcoglycan. Since Mini-Gamma represents removal of 4 of the 7 coding exons in γ-sarcoglycan, this approach provides a viable strategy to treat the majority of patients with γ-sarcoglycan gene mutations.

Costamere proteins and their involvement in myopathic processes

Muscle fibres are very specialised cells with a complex structure that requires a high level of organisation of the constituent proteins. For muscle contraction to function properly, there is a need for not only sarcomeres, the contractile structures of the muscle fibre, but also costameres. These are supramolecular structures associated with the sarcolemma that allow muscle adhesion to the extracellular matrix. They are composed of protein complexes that interact and whose functions include maintaining cell structure and signal transduction mediated by their constituent proteins. It is important to improve our understanding of these structures, as mutations in various genes that code for costamere proteins cause many types of muscular dystrophy. In this review, we provide a description of costameres detailing each of their constituent proteins, such as dystrophin, dystrobrevin, syntrophin, sarcoglycans, dystroglycans, vinculin, talin, integrins, desmin, plectin, etc. We describe as well the diseases associated with deficiency thereof, providing a general overview of their importance.

Serum proteomic profiling reveals fragments of MYOM3 as potential biomarkers for monitoring the outcome of therapeutic interventions in muscular dystrophies

Therapy-responsive biomarkers are an important and unmet need in the muscular dystrophy field where new treatments are currently in clinical trials. By using a comprehensive high-resolution mass spectrometry approach and western blot validation, we found that two fragments of the myofibrillar structural protein myomesin-3 (MYOM3) are abnormally present in sera of Duchenne muscular dystrophy (DMD) patients, limb-girdle muscular dystrophy type 2D (LGMD2D) and their respective animal models. Levels of MYOM3 fragments were assayed in therapeutic model systems: (1) restoration of dystrophin expression by antisense oligonucleotide-mediated exon-skipping in mdx mice and (2) stable restoration of α-sarcoglycan expression in KO-SGCA mice by systemic injection of a viral vector. Following administration of the therapeutic agents MYOM3 was restored toward wild-type levels. In the LGMD model, where different doses of vector were used, MYOM3 restoration was dose-dependent. MYOM3 fragments showed lower inter-individual variability compared with the commonly used creatine kinase assay, and correlated better with the restoration of the dystrophin-associated protein complex and muscle force. These data suggest that the MYOM3 fragments hold promise for minimally invasive assessment of experimental therapies for DMD and other neuromuscular disorders.

Clinical and genetic spectrum in limb-girdle muscular dystrophy type 2E

OBJECTIVE

To determine the clinical spectrum of limb-girdle muscular dystrophy 2E (LGMD2E) and to investigate whether genetic or biochemical features can predict the phenotype of the disease.

METHODS

All LGMD2E patients followed in participating centers were included. A specific clinical protocol was created, including quantitative evaluation of motor, respiratory, and cardiac function. Phenotype was defined as severe or mild if the age at loss of ambulation occurred before or after 18 years. Molecular analysis of SGCB gene and biochemical features of muscle biopsies were reviewed.

RESULTS

Thirty-two patients were included (16 male, 16 female; age 7-67 years; 15 severe, 12 mild, and 5 unknown). Neurologic examination showed proximal muscle weakness in all patients, but distal involvement was also observed in patients with severe disease early in the disease course. Cardiac involvement was observed in 20 patients (63%) even before overt muscle involvement. Six patients had restrictive respiratory insufficiency requiring assisted ventilation (19%). Seventeen different mutations were identified, and 3 were recurrent. The c.377_384dup (13 alleles) was associated with the severe form, the c.-22_10dup (10) with the milder form, and the c.341C>T (9) with both. The entire sarcoglycan complex was undetectable by muscle immunohistochemistry or Western blot in 9/10 severe cases and reduced in 7/7 mild cases. The residual amount of sarcoglycan in muscle resulted a predictor of age at loss of ambulation.

CONCLUSIONS

This study expands the spectrum of phenotype in β-sarcoglycanopathy and provides strong evidence that severity of clinical involvement may be predicted by SGCB gene mutation and sarcoglycan protein expression.

Wrist flexion and extension torques measured by highly sensitive dynamometer in healthy subjects from 5 to 80 years

BACKGROUND

Wrist movements become impaired with disease progression in various neuromuscular disorders. With the development of new therapies, thorough measurement of muscle strength is crucial to document natural disease progression and to assess treatment efficacy. We developed a new dynamometer enabling wrist flexion and extension torque measurement with high sensitivity. The aims of the present study were to collect norms for healthy children and adults, to compute predictive equations, to assess the reliability of the measurements and to test the feasibility of using the device in patients with a neuromuscular disease.

METHODS

The peak isometric torque of wrist flexion and extension was measured with the MyoWrist dynamometer in 345 healthy subjects aged between 5 and 80 years old and in 9 patients with limb girdle muscle dystrophy type 2 C (LGMD2C) aged between 16 and 38 years old.

RESULTS

Predictive equations are proposed for the wrist flexion and extension strength in children and adults. Intra-rater and inter-rater reliability was good with ICCs higher than 0.9 for both wrist flexion and extension. However, retest values were significantly higher by 4% than test results. The dynamometer was applied with no difficulty to patients with LGMD2C and was sensitive enough to detect strength as weak as 0.82 N.m. From our models, we quantified the mean strength of wrist extension in LGMD2C patients to 39 ± 17% of their predicted values.

CONCLUSIONS

The MyoWrist dynamometer provides reliable and sensitive measurement of both wrist flexion and extension torques. However, a training session is recommended before starting a study as a small but significant learning effect was observed. Strength deficit can be quantified from predictive equations that were computed from norms of healthy children and adults.

The Skeletal Muscle Environment and Its Role in Immunity and Tolerance to AAV Vector-Mediated Gene Transfer

Since the early days of gene therapy, muscle has been one the most studied tissue targets for the correction of enzyme deficiencies and myopathies. Several preclinical and clinical studies have been conducted using adeno-associated virus (AAV) vectors. Exciting progress has been made in the gene delivery technologies, from the identification of novel AAV serotypes to the development of novel vector delivery techniques. In parallel, significant knowledge has been generated on the host immune system and its interaction with both the vector and the transgene at the muscle level. In particular, the role of underlying muscle inflammation, characteristic of several diseases affecting the muscle, has been defined in terms of its potential detrimental impact on gene transfer with AAV vectors. At the same time, feedback immunomodulatory mechanisms peculiar of skeletal muscle involving resident regulatory T cells have been identified, which seem to play an important role in maintaining, at least to some extent, muscle homeostasis during inflammation and regenerative processes. Devising strategies to tip this balance towards unresponsiveness may represent an avenue to improve the safety and efficacy of muscle gene transfer with AAV vectors.

Biotherapies of neuromuscular disorders

This review focuses on the most recent data on biotherapeutic approaches, using DNA, RNA, recombinant proteins, or cells as therapeutic tools or targets for the treatment of neuromuscular diseases. Many of these novel technologies have now reached the clinical stage and have or are about to move to the market. Others, like genome editing are still in an early stage but hold great promise.

Analysing regenerative potential in zebrafish models of congenital muscular dystrophy

The congenital muscular dystrophies (CMDs) are a clinically and genetically heterogeneous group of muscle disorders. Clinically hypotonia is present from birth, with progressive muscle weakness and wasting through development. For the most part, CMDs can mechanistically be attributed to failure of basement membrane protein laminin-α2 sufficiently binding with correctly glycosylated α-dystroglycan. The majority of CMDs therefore arise as the result of either a deficiency of laminin-α2 (MDC1A) or hypoglycosylation of α-dystroglycan (dystroglycanopathy). Here we consider whether by filling a regenerative medicine niche, the zebrafish model can address the present challenge of delivering novel therapeutic solutions for CMD. In the first instance the readiness and appropriateness of the zebrafish as a model organism for pioneering regenerative medicine therapies in CMD is analysed, in particular for MDC1A and the dystroglycanopathies. Despite the recent rapid progress made in gene editing technology, these approaches have yet to yield any novel zebrafish models of CMD. Currently the most genetically relevant zebrafish models to the field of CMD, have all been created by N-ethyl-N-nitrosourea (ENU) mutagenesis. Once genetically relevant models have been established the zebrafish has several important facets for investigating the mechanistic cause of CMD, including rapid ex vivo development, optical transparency up to the larval stages of development and relative ease in creating transgenic reporter lines. Together, these tools are well suited for use in live-imaging studies such as in vivo modelling of muscle fibre detachment. Secondly, the zebrafish's contribution to progress in effective treatment of CMD was analysed. Two approaches were identified in which zebrafish could potentially contribute to effective therapies. The first hinges on the augmentation of functional redundancy within the system, such as upregulating alternative laminin chains in the candyfloss fish, a model of MDC1A. Secondly high-throughput small molecule screens not only provide effective therapies, but also an alternative strategy for investigating CMD in zebrafish. In this instance insight into disease mechanism is derived in reverse. Zebrafish models are therefore clearly of critical importance in the advancement of regenerative medicine strategies in CMD. This article is part of a Directed Issue entitled: Regenerative Medicine: The challenge of translation.

Gene therapy prolongs survival and restores function in murine and canine models of myotubular myopathy

Loss-of-function mutations in the myotubularin gene (MTM1) cause X-linked myotubular myopathy (XLMTM), a fatal, congenital pediatric disease that affects the entire skeletal musculature. Systemic administration of a single dose of a recombinant serotype 8 adeno-associated virus (AAV8) vector expressing murine myotubularin to Mtm1-deficient knockout mice at the onset or at late stages of the disease resulted in robust improvement in motor activity and contractile force, corrected muscle pathology, and prolonged survival throughout a 6-month study. Similarly, single-dose intravascular delivery of a canine AAV8-MTM1 vector in XLMTM dogs markedly improved severe muscle weakness and respiratory impairment, and prolonged life span to more than 1 year in the absence of toxicity or a humoral or cell-mediated immune response. These results demonstrate the therapeutic efficacy of AAV-mediated gene therapy for myotubular myopathy in small- and large-animal models, and provide proof of concept for future clinical trials in XLMTM patients.

Gene-based therapies of neuromuscular disorders: an update and the pivotal role of patient organizations in their discovery and implementation

This review updates the state-of-the art accomplishments of the multifaceted gene-based therapies, which include DNA or RNA as either therapeutic tools or targets for the treatment of neuromuscular diseases. It also provides insights into the key role that patient organizations have played in research and development; in particular, by addressing bottlenecks and generating boundary conditions that have contributed to scientific breakthroughs, and the effectiveness of innovation processes. Several gene therapy methods have reached the clinical stage and are now addressing both specific and classical issues related to this novel technology. Not ready yet for clinical application, genome editing is at its infancy. More rapidly progressing, RNA-based therapeutics, and especially exon skipping, exon inclusion and stop codon readthrough strategies, are about to move to the market. Most importantly, patients were at the forefront of this discovery process, from basic knowledge to innovation and translational research in a rapidly growing field of unmet medical needs. In recent years, Duchenne muscular dystrophy was the fertile ground for new therapeutic concepts that have been extended to other neuromuscular disorders, such as spinal muscular atrophy, myotonic dystrophies or fascioscapulohumeral dystrophy. In line with their longstanding policy, patient organizations will keep working in a proactive manner to bring together all stakeholders with a view to working out truly therapeutic solutions over a long-term perspective.