AAV-mediated follistatin gene therapy improves functional outcomes in the TIC-DUX4 mouse model of FSHD

Have a Little Heart (or Not): Highly Minimized Skeletal Muscle Regulatory Cassettes with Low or No Activity in the Heart

Adeno-associated virus-mediated gene therapies for certain muscle disorders require regulatory cassettes that provide high-level, striated muscle-specific activity. However, cardiotoxicity has emerged as a serious concern in clinical trials for Duchenne muscular dystrophy and X-linked myotubular myopathy. While this may be caused by systemic inflammatory effects of the treatment, high transgene expression in the heart may also play a role. Thus, certain muscle disorders may require a modulated level of therapeutic expression in the heart, while others may not require any cardiac expression at all. Additionally, the size of some cargos requires regulatory cassettes to be small enough that large cDNAs and other therapeutic payloads can be accommodated. Thus, we have performed enhancer/promoter optimization to develop highly minimized regulatory cassettes that are active in skeletal muscles, with either low or no detectable activity in cardiac muscle. Our No-heart (NH) cassette is active in most skeletal muscles, but exhibits only very low activity in extensor digitorum longus (EDL), soleus, and diaphragm, and no activity in the heart. By contrast, our Have a Little Heart (HLH) cassette displays high activity in most skeletal muscles, comparable to the ∼800-bp CK8 cassette, with increased activity in EDL, soleus, and diaphragm, and low activity in the heart. Due to their small size, these cassettes can be used in therapeutic strategies with both flexible (e.g., antisense) and stringent (e.g., CRISPR/Cas or bicistronic) size limitations. Thus, our new cassettes may be useful for gene therapies of muscle disorders in which the need for low or almost no expression in cardiac muscle would outweigh the need for high levels of therapeutic product in certain skeletal muscles.

Challenges and Considerations of Preclinical Development for iPSC-Based Myogenic Cell Therapy

Cell therapies derived from induced pluripotent stem cells (iPSCs) offer a promising avenue in the field of regenerative medicine due to iPSCs' expandability, immune compatibility, and pluripotent potential. An increasing number of preclinical and clinical trials have been carried out, exploring the application of iPSC-based therapies for challenging diseases, such as muscular dystrophies. The unique syncytial nature of skeletal muscle allows stem/progenitor cells to integrate, forming new myonuclei and restoring the expression of genes affected by myopathies. This characteristic makes genome-editing techniques especially attractive in these therapies. With genetic modification and iPSC lineage specification methodologies, immune-compatible healthy iPSC-derived muscle cells can be manufactured to reverse the progression of muscle diseases or facilitate tissue regeneration. Despite this exciting advancement, much of the development of iPSC-based therapies for muscle diseases and tissue regeneration is limited to academic settings, with no successful clinical translation reported. The unknown differentiation process in vivo, potential tumorigenicity, and epigenetic abnormality of transplanted cells are preventing their clinical application. In this review, we give an overview on preclinical development of iPSC-derived myogenic cell transplantation therapies including processes related to iPSC-derived myogenic cells such as differentiation, scaling-up, delivery, and cGMP compliance. And we discuss the potential challenges of each step of clinical translation. Additionally, preclinical model systems for testing myogenic cells intended for clinical applications are described.

FSHD muscle shows perturbation in fibroadipogenic progenitor cells, mitochondrial function and alternative splicing independently of inflammation

Facioscapulohumeral muscular dystrophy (FSHD) is a prevalent, incurable myopathy. FSHD is highly heterogeneous, with patients following a variety of clinical trajectories, complicating clinical trials. Skeletal muscle in FSHD undergoes fibrosis and fatty replacement that can be accelerated by inflammation, adding to heterogeneity. Well controlled molecular studies are thus essential to both categorize FSHD patients into distinct subtypes and understand pathomechanisms. Here, we further analyzed RNA-sequencing data from 24 FSHD patients, each of whom donated a biopsy from both a non-inflamed (TIRM-) and inflamed (TIRM+) muscle, and 15 FSHD patients who donated peripheral blood mononucleated cells (PBMCs), alongside non-affected control individuals. Differential gene expression analysis identified suppression of mitochondrial biogenesis and up-regulation of fibroadipogenic progenitor (FAP) gene expression in FSHD muscle, which was particularly marked on inflamed samples. PBMCs demonstrated suppression of antigen presentation in FSHD. Gene expression deconvolution revealed FAP expansion as a consistent feature of FSHD muscle, via meta-analysis of 7 independent transcriptomic datasets. Clustering of muscle biopsies separated patients in an unbiased manner into clinically mild and severe subtypes, independently of known disease modifiers (age, sex, D4Z4 repeat length). Lastly, the first genome-wide analysis of alternative splicing in FSHD muscle revealed perturbation of autophagy, BMP2 and HMGB1 signalling. Overall, our findings reveal molecular subtypes of FSHD with clinical relevance and identify novel pathomechanisms for this highly heterogeneous condition.

A human immune/muscle xenograft model of FSHD muscle pathology

Background

Facioscapulohumeral muscular dystrophy (FSHD) disease progression is associated with muscle inflammation, although its role in FSHD muscle pathology is unknown.

Methods

We have developed a novel humanized mouse strain, NSG-SGM3-W41, that supports the co- engraftment of human hematopoietic stem cells (HSCs) and muscle myoblasts as an experimental model to investigate the role of innate immunity in FSHD muscle pathology.

Results

The NSG-SGM3-W41 mouse supports the selective expansion of human innate immune cell lineages following engraftment of human HSCs and the co-engraftment and differentiation of patient-derived FSHD or control muscle myoblasts. Immunohistological and NanoString RNA expression assays establish that muscle xenografts from three FSHD subjects were immunogenic compared to those from unaffected first-degree relatives. FSHD muscle xenografts preferentially accumulated human macrophages and B cells and expressed early complement genes of the classical and alternative pathways including complement factor C3 protein, which is a mediator of early complement function through opsonization to mark damaged cells for macrophage engulfment. FSHD muscle xenografts also underwent immune donor dependent muscle turnover as assayed by human spectrin β1 immunostaining of muscle fibers and by NanoString RNA expression assays of muscle differentiation genes.

Conclusions

The NSG-SGM3-W41 mouse provides an experimental model to investigate the role of innate immunity and complement in FSHD muscle pathology and to develop FSHD therapeutics targeting DUX4 and the innate immunity inflammatory responses.

Flavones provide resistance to DUX4-induced toxicity via an mTor-independent mechanism

Facioscapulohumeral muscular dystrophy (FSHD) is among the most common of the muscular dystrophies, affecting nearly 1 in 8000 individuals, and is a cause of profound disability. Genetically, FSHD is linked to the contraction and/or epigenetic de-repression of the D4Z4 repeat array on chromosome 4, thereby allowing expression of the DUX4 gene in skeletal muscle. If the DUX4 transcript incorporates a stabilizing polyadenylation site the myotoxic DUX4 protein will be synthesized, resulting in muscle wasting. The mechanism of toxicity remains unclear, as many DUX4-induced cytopathologies have been described, however cell death does primarily occur through caspase 3/7-dependent apoptosis. To date, most FSHD therapeutic development has focused on molecular methods targeting DUX4 expression or the DUX4 transcript, while therapies targeting processes downstream of DUX4 activity have received less attention. Several studies have demonstrated that inhibition of multiple signal transduction pathways can ameliorate DUX4-induced toxicity, and thus compounds targeting these pathways have the potential to be developed into FSHD therapeutics. To this end, we have screened a group of small molecules curated based on their reported activity in relevant pathways and/or structural relationships with known toxicity-modulating molecules. We have identified a panel of five compounds that function downstream of DUX4 activity to inhibit DUX4-induced toxicity. Unexpectedly, this effect was mediated through an mTor-independent mechanism that preserved expression of ULK1 and correlated with an increase in a marker of active cellular autophagy. This identifies these flavones as compounds of interest for therapeutic development, and potentially identifies the autophagy pathway as a target for therapeutics.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

Macrophages are essential for skeletal muscle homeostasis, but how their dysregulation contributes to the development of fibrosis in muscle disease remains unclear. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six clusters and unexpectedly found that none corresponded to traditional definitions of M1 or M2 macrophages. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 (gal-3) and osteopontin (Spp1). Spatial transcriptomics, computational inferences of intercellular communication, and in vitro assays indicated that macrophage-derived Spp1 regulates stromal progenitor differentiation. Gal-3+ macrophages were chronically activated in dystrophic muscle, and adoptive transfer assays showed that the gal-3+ phenotype was the dominant molecular program induced within the dystrophic milieu. Gal-3+ macrophages were also elevated in multiple human myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining their transcriptional programs and reveal Spp1 as a major regulator of macrophage and stromal progenitor interactions.

Human DUX4 and porcine DUXC activate similar early embryonic programs in pig muscle cells: implications for preclinical models of FSHD

Human DUX4 and its mouse ortholog Dux are normally expressed in the early embryo-the 4-cell or 2-cell cleavage stage embryo, respectively-and activate a portion of the first wave of zygotic gene expression. DUX4 is epigenetically suppressed in nearly all somatic tissue, whereas facioscapulohumeral dystrophy (FSHD)-causing mutations result in its aberrant expression in skeletal muscle, transcriptional activation of the early embryonic program and subsequent muscle pathology. Although DUX4 and Dux both activate an early totipotent transcriptional program, divergence of their DNA binding domains limits the use of DUX4 expressed in mice as a preclinical model for FSHD. In this study, we identify the porcine DUXC messenger ribonucleic acid expressed in early development and show that both pig DUXC and human DUX4 robustly activate a highly similar early embryonic program in pig muscle cells. These results support further investigation of pig preclinical models for FSHD.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

The monocytic/macrophage system is essential for skeletal muscle homeostasis, but its dysregulation contributes to the pathogenesis of muscle degenerative disorders. Despite our increasing knowledge of the role of macrophages in degenerative disease, it still remains unclear how macrophages contribute to muscle fibrosis. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six novel clusters. Unexpectedly, none corresponded to traditional definitions of M1 or M2 macrophage activation. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 and spp1. Spatial transcriptomics and computational inferences of intercellular communication indicated that spp1 regulates stromal progenitor and macrophage interactions during muscular dystrophy. Galectin-3 + macrophages were chronically activated in dystrophic muscle and adoptive transfer assays showed that the galectin-3 + phenotype was the dominant molecular program induced within the dystrophic milieu. Histological examination of human muscle biopsies revealed that galectin-3 + macrophages were also elevated in multiple myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining the transcriptional programs induced in muscle macrophages, and reveal spp1 as a major regulator of macrophage and stromal progenitor interactions.

Flavones provide resistance to DUX4-induced toxicity via an mTor-independent mechanism

Facioscapulohumeral muscular dystrophy (FSHD) is among the most common of the muscular dystrophies, affecting nearly 1 in 8000 individuals, and is a cause of profound disability. Genetically, FSHD is linked to the contraction and/or epigenetic de-repression of the D4Z4 repeat array on chromosome 4, thereby allowing expression of the DUX4 gene in skeletal muscle. If the DUX4 transcript incorporates a stabilizing polyadenylation site the myotoxic DUX4 protein will be synthesized, resulting in muscle wasting. The mechanism of toxicity remains unclear, as many DUX4-induced cytopathologies have been described, however cell death does primarily occur through caspase 3/7-dependent apoptosis. To date, most FSHD therapeutic development has focused on molecular methods targeting DUX4 expression or the DUX4 transcript, while therapies targeting processes downstream of DUX4 activity have received less attention. Several studies have demonstrated that inhibition of multiple signal transduction pathways can ameliorate DUX4-induced toxicity, and thus compounds targeting these pathways have the potential to be developed into FSHD therapeutics. To this end, we have screened a group of small molecules curated based on their reported activity in relevant pathways and/or structural relationships with known toxicity-modulating molecules. We have identified a panel of five compounds that function downstream of DUX4 activity to inhibit DUX4-induced toxicity. Unexpectedly, this effect was mediated through an mTor-independent mechanism that preserved expression of ULK1 and correlated with an increase in a marker of active cellular autophagy. This identifies these flavones as compounds of interest for therapeutic development, and potentially identifies the autophagy pathway as a target for therapeutics.

Modern Approaches to Mouse Genome Editing Using the CRISPR-Cas Toolbox and Their Applications in Functional Genomics and Translational Research

Mice have been used in biological research for over a century, and their immense contribution to scientific breakthroughs can be seen across all research disciplines, with some of the main beneficiaries being the fields of medicine and life sciences. Genetically engineered mouse models (GEMMs), along with other model organisms, are fundamentally important research tools frequently utilised to enhance our understanding of pathophysiology and biological mechanisms behind disease. In the 1980s, it became possible to precisely edit the mouse genome to create gene knockout and knock-in mice, although with low efficacy. Recent advances utilising CRISPR-Cas technologies have considerably improved our ability to do this with ease and precision, while also allowing the generation of desired genetic variants from single nucleotide substitutions to large insertions/deletions. It is now quick and relatively easy to genetically edit somatic cells which were previously more recalcitrant to traditional approaches. Further refinements have created a 'CRISPR toolkit' that has expanded the use of CRISPR-Cas beyond gene knock-ins and knockouts. In this chapter, we review some of the latest applications of CRISPR-Cas technologies in GEMMs, including nuclease-dead Cas9 systems for activation or repression of gene expression, base editing and prime editing. We also discuss improvements in Cas9 specificity, targeting efficacy and delivery methods in mice. Throughout, we provide examples wherein CRISPR-Cas technologies have been applied to target clinically relevant genes in preclinical GEMMs, both to generate humanised models and for experimental gene therapy research.

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.

Transplantation of PSC-derived myogenic progenitors counteracts disease phenotypes in FSHD mice

Facioscapulohumeral muscular dystrophy (FSHD) is a genetically dominant progressive myopathy caused by improper silencing of the DUX4 gene, leading to fibrosis, muscle atrophy, and fatty replacement. Approaches focused on muscle regeneration through the delivery of stem cells represent an attractive therapeutic option for muscular dystrophies. To investigate the potential for cell transplantation in FSHD, we have used the doxycycline-regulated iDUX4pA-HSA mouse model in which low-level DUX4 can be induced in skeletal muscle. We find that mouse pluripotent stem cell (PSC)-derived myogenic progenitors engraft in muscle actively undergoing DUX4-mediated degeneration. Donor-derived muscle tissue displayed reduced fibrosis and importantly, engrafted muscles showed improved contractile specific force compared to non-transplanted controls. These data demonstrate the feasibility of replacement of diseased muscle with PSC-derived myogenic progenitors in a mouse model for FSHD, and highlight the potential for the clinical benefit of such a cell therapy approach.

Long-Term Systemic Treatment of a Mouse Model Displaying Chronic FSHD-like Pathology with Antisense Therapeutics That Inhibit DUX4 Expression

Silencing the expression of the double homeobox 4 (DUX4) gene offers great potential for the treatment of facioscapulohumeral muscular dystrophy (FSHD). Several research groups have recently reported promising results using systemic antisense therapy in a transgenic small animal model of FSHD, the ACTA1-MCM/FLExDUX4 mouse model. However, the treatment was applied in non-DUX4-induced mice or shortly after DUX4 activation, which resulted in conditions that do not correctly represent the situation in a clinic. Here, we generated progressive FSHD-like pathology in ACTA1-MCM/FLExDUX4 mice and then treated the animals with vivoPMO-PACS4, an antisense compound that efficiently downregulates DUX4. To best mimic the translation of this treatment in clinical settings, the systemic antisense oligonucleotide administration was delayed to 3 weeks after the DUX4 activation so that the pathology was established at the time of the treatment. The chronic administration of vivoPMO-PACS4 for 8 weeks downregulated the DUX4 expression by 60%. Consequently, the treated mice showed an increase by 18% in body-wide muscle mass and 32% in muscle strength, and a reduction in both myofiber central nucleation and muscle fibrosis by up to 29% and 37%, respectively. Our results in a more suitable model of FSHD pathology confirm the efficacy of vivoPMO-PACS4 administration, and highlight the significant benefit provided by the long-term treatment of the disease.

Randomized phase 2 study of ACE-083, a muscle-promoting agent, in facioscapulohumeral muscular dystrophy

Introduction/Aims

Facioscapulohumeral muscular dystrophy (FSHD) is a slowly progressive muscular dystrophy without approved therapies. In this study we evaluated whether locally acting ACE‐083 could safely increase muscle volume and improve functional outcomes in adults with FSHD.

Methods

Participants were at least 18 years old and had FSHD1/FSHD2. Part 1 was open label, ascending dose, assessing safety and tolerability (primary objective). Part 2 was randomized, double‐blind for 6 months, evaluating ACE‐083240 mg/muscle vs placebo injected bilaterally every 3 weeks in the biceps brachii (BB) or tibialis anterior (TA) muscles, followed by 6 months of open label. Magnetic resonance imaging measures included total muscle volume (TMV; primary objective), fat fraction (FF), and contractile muscle volume (CMV). Functional measures included 6‐minute walk test, 10‐meter walk/run, and 4‐stair climb (TA group), and performance of upper limb midlevel/elbow score (BB group). Strength, patient‐reported outcomes (PROs), and safety were also evaluated.

Results

Parts 1 and 2 enrolled 37 and 58 participants, respectively. Among 55 participants evaluable in Part 2, the least‐squares mean (90% confidence interval, analysis of covariance) treatment difference for TMV was 16.4% (9.8%‐23.0%) in the BB group (P < .0001) and 9.5% (3.2%‐15.9%) in the TA group (P = .01). CMV increased significantly in the BB and TA groups and FF decreased in the TA group. There were no consistent improvements in functional or PRO measures in either group. The most common adverse events were mild or moderate injection‐site reactions.

Discussion

Significant increases in TMV with ACE‐083 vs placebo did not result in consistent functional or PRO improvements with up to 12 months of treatment.

Randomized Phase 2 Study of ACE-083 in Patients With Charcot-Marie-Tooth Disease

BACKGROUND AND OBJECTIVES

The goal of this work was to determine whether locally acting ACE-083 is safe and well tolerated and increases muscle volume, motor function, and quality of life (QoL) in adults with Charcot-Marie-Tooth disease (CMT) type 1.

METHODS

This phase 2 study enrolled adults with CMT1 or CMTX (N = 63). Part 1 was open label and evaluated the safety and tolerability of different dose levels of ACE-083 for use in part 2. Part 2 was a randomized, placebo-controlled, 6-month study of 240 mg/muscle ACE-083 injected bilaterally into the tibialis anterior muscle, followed by a 6-month, open-label extension in which all patients received ACE-083. Pharmacodynamic endpoints included total muscle volume (TMV; primary endpoint), contractile muscle volume (CMV), and fat fraction. Additional secondary endpoints included 6-minute walk test, 10-m walk/run, muscle strength, and QoL. Safety was assessed with treatment-emergent adverse events (TEAEs) and clinical laboratory tests.

RESULTS

In part 1 (n = 18), ACE-083 was generally safe and well tolerated at all dose levels, with no serious adverse events, TEAEs of grade 3 or greater, or death reported. In part 2 (n = 45 enrolled, n = 44 treated), there was significantly greater change in TMV with ACE-083 compared with placebo (least-squares mean difference 13.5%; p = 0.0096). There was significant difference between ACE-083 and placebo for CMV and change in ankle dorsiflexion strength. Fat fraction and all other functional outcomes were not significantly improved by ACE-083. Moderate to mild injection-site reactions were the most common TEAEs.

DISCUSSION

Despite significantly increased TMV and CMV, patients with CMT receiving ACE-083 in tibialis anterior muscles did not demonstrate greater functional improvement compared with those receiving placebo.

TRIAL REGISTRATION INFORMATION

Clinical Trials Registration: NCT03124459.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that intramuscular ACE-083 is safe and well tolerated and increases total muscle volume after 6 months of treatment in adults with CMT1 or CMTX.

New intranasal and injectable gene therapy for healthy life extension

SignificanceUsing CMV as a gene therapy vector we illustrated that CMV can be used therapeutically as a monthly inhaled or intraperitoneally delivered treatment for aging-associated decline. Exogenous telomerase reverse transcriptase or follistatin genes were safely and effectively delivered in a murine model. This treatment significantly improved biomarkers associated with healthy aging, and the mouse lifespan was increased up to 41% without an increased risk of cancer. The impact of this research on an aging population cannot be understated as the global aging-related noncommunicable disease burden quickly rises.

Persistent Fibroadipogenic Progenitor Expansion Following Transient DUX4 Expression Provokes a Profibrotic State in a Mouse Model for FSHD

FSHD is caused by loss of silencing of the DUX4 gene, but the DUX4 protein has not yet been directly detected immunohistologically in affected muscle, raising the possibility that DUX4 expression may occur at time points prior to obtaining adult biopsies for analysis, with consequent perturbations of muscle being responsible for disease progression. To test the extent to which muscle can regenerate following DUX4-mediated degeneration, we employed an animal model with reversible DUX4 expression, the iDUX4pA;HSA mouse. We find that muscle histology does recover substantially after DUX4 expression is switched off, with the extent of recovery correlating inversely with the duration of prior DUX4 expression. However, despite fairly normal muscle histology, and recovery of most cytological parameters, the fibroadipogenic progenitor compartment, which is significantly elevated during bouts of fiber-specific DUX4 expression, does not return to basal levels, even many weeks after a single burst of DUX4 expression. We find that muscle that has recovered from a DUX4 burst acquires a propensity for severe fibrosis, which can be revealed by subsequent cardiotoxin injuries. These results suggest that a past history of DUX4 expression leads to maintained pro-fibrotic alterations in the cellular physiology of muscle, with potential implications for therapeutic approaches.

Improving molecular and histopathology in diaphragm muscle of the double transgenic ACTA1-MCM/FLExDUX4 mouse model of FSHD with systemic antisense therapy

Facioscapulohumeral muscular dystrophy (FSHD) is a rare muscle dystrophy causing muscle weakness initially in the face, shoulders and upper arms, and extended to lower body muscles as the disease progresses. Respiratory restriction in FSHD is increasingly reported to be more common and severe than previously thought, with the involvement of diaphragm weakness in pulmonary insufficiency being under debate. As aberrant expression of the double homeobox 4 (DUX4) gene is the prime cause of FSHD, we and others have developed numerous strategies and reported promising results on downregulating DUX4 expression in both cellular and animal models of FSHD. However, the effect of DUX4 and anti-DUX4 approaches on diaphragm muscle has not been elucidated. Here we show that toxic DUX4 expression causes pathology that affects the diaphragm of ACTA1-MCM/FLExDUX4 mouse model of FSHD at both molecular and histological levels. Of importance, a systemic antisense treatment that suppresses DUX4 and target genes expression by 50% significantly improves muscle regeneration and muscle fibrosis, and prevents modification in myofiber type composition, supporting its development as a treatment for FSHD.

Meeting report: the 2021 FSHD International Research Congress

Facioscapulohumeral muscular dystrophy (FSHD) is the second most common genetic myopathy, characterized by slowly progressing and highly heterogeneous muscle wasting with a typical onset in the late teens/early adulthood [1]. Although the etiology of the disease for both FSHD type 1 and type 2 has been attributed to gain-of-toxic function stemming from aberrant DUX4 expression, the exact pathogenic mechanisms involved in muscle wasting have yet to be elucidated [2–4]. The 2021 FSHD International Research Congress, held virtually on June 24–25, convened over 350 researchers and clinicians to share the most recent advances in the understanding of the disease mechanism, discuss the proliferation of interventional strategies and refinement of clinical outcome measures, including results from the ReDUX4 trial, a phase 2b clinical trial of losmapimod in FSHD [NCT04003974].

Canine DUXC: Implications for DUX4 retrotransposition and preclinical models of FSHD

Mis-expression of DUX4 in skeletal muscle causes facioscapulohumeral muscular dystrophy (FSHD). Human DUX4 and mouse Dux are retrogenes derived from retrotransposition of the mRNA from the parental DUXC gene. Primates and rodents have lost the parental DUXC gene and it is unknown whether DUXC had a similar role in driving an early pluripotent transcriptional program. Dogs and other Laurasiatherians have retained DUXC, providing an opportunity to determine the functional similarity to the retrotransposed DUX4 and Dux. Here we identify expression of two isoforms of DUXC mRNA in canine testis tissues, one encoding the canonical double homeodomain protein (DUXC), similar to DUX4/Dux, and a second that includes an in-frame alternative exon that disrupts the conserved amino-acid sequence of the first homeodomain (DUXC-ALT). Expression of DUXC in canine cells induces a pluripotent program similar to DUX4 and Dux, and induces expression of a similar set of retrotransposons of the ERV/MaLR and LINE-1 families, as well as pericentromeric satellite repeats; whereas DUXC-ALT did not robustly activate gene expression in these assays. Important for preclinical models of FSHD, human DUX4 and canine DUXC show higher conservation of their homeodomains and corresponding binding motifs compared to the conservation between human DUX4 and mouse Dux, and human DUX4 activates a highly similar transcriptional program in canine cells. Together, these findings show that retrotransposition resulted in the loss of an alternatively spliced isoform and that DUXC containing mammals might be good candidates for certain pre-clinical models of FSHD.

Human miRNA miR-675 inhibits DUX4 expression and may be exploited as a potential treatment for Facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is a potentially devastating myopathy caused by de-repression of the DUX4 gene in skeletal muscles. Effective therapies will likely involve DUX4 inhibition. RNA interference (RNAi) is one powerful approach to inhibit DUX4, and we previously described a RNAi gene therapy to achieve DUX4 silencing in FSHD cells and mice using engineered microRNAs. Here we report a strategy to direct RNAi against DUX4 using the natural microRNA miR-675, which is derived from the lncRNA H19. Human miR-675 inhibits DUX4 expression and associated outcomes in FSHD cell models. In addition, miR-675 delivery using gene therapy protects muscles from DUX4-associated death in mice. Finally, we show that three known miR-675-upregulating small molecules inhibit DUX4 and DUX4-activated FSHD biomarkers in FSHD patient-derived myotubes. To our knowledge, this is the first study demonstrating the use of small molecules to suppress a dominant disease gene using an RNAi mechanism.

DUX4 Role in Normal Physiology and in FSHD Muscular Dystrophy

In the last decade, the sequence-specific transcription factor double homeobox 4 (DUX4) has gone from being an obscure entity to being a key factor in important physiological and pathological processes. We now know that expression of DUX4 is highly regulated and restricted to the early steps of embryonic development, where DUX4 is involved in transcriptional activation of the zygotic genome. While DUX4 is epigenetically silenced in most somatic tissues of healthy humans, its aberrant reactivation is associated with several diseases, including cancer, viral infection and facioscapulohumeral muscular dystrophy (FSHD). DUX4 is also translocated, giving rise to chimeric oncogenic proteins at the basis of sarcoma and leukemia forms. Hence, understanding how DUX4 is regulated and performs its activity could provide relevant information, not only to further our knowledge of human embryonic development regulation, but also to develop therapeutic approaches for the diseases associated with DUX4. Here, we summarize current knowledge on the cellular and molecular processes regulated by DUX4 with a special emphasis on FSHD muscular dystrophy.

Effect of Chinese herbal medicines on the overall survival of patients with muscular dystrophies in Taiwan

ETHNOPHARMACOLOGICAL RELEVANCE

Muscular dystrophies are a rare, severe, and genetically inherited group of disorders characterized by progressive loss of muscle fibers, leading to muscle weakness. The current treatment plan for muscular dystrophies includes the use of steroids to slow muscle deterioration by dampening the inflammatory response.

AIM OF THE STUDY

Chinese herbal medicine (CHM) has been offered as an adjunctive therapy in Taiwan's medical healthcare plan, making it possible to track CHM usage in patients with muscular dystrophic disease. Therefore, we explored the long-term effects of CHM use on the overall mortality of patients with muscular dystrophies.

MATERIALS AND METHODS

A total of 581 patients with muscular dystrophies were identified from the database of Registry for Catastrophic Illness Patients in Taiwan. Among them, 80 and 201 patients were CHM users and non-CHM users, respectively. Student's t-test, chi-squared test, Cox proportional hazard model, and Kaplan-Meier curve (log-rank test) were used for evaluation. Association rules and network analyses were performed to explore the combination of CHMs used in muscular dystrophies.

RESULTS

Compared to non-CHM users, there were more female patients, more comorbidities, including chronic pulmonary disease and peptic ulcer disease in the CHM user group. Patients with prednisolone usage exhibited a lower risk of overall mortality than those who did not, after adjusting for age, sex, use of CHM, and comorbidities. CHM users showed a lower risk of overall mortality after adjusting for age, sex, prednisolone use, and comorbidities. The cumulative incidence of the overall survival was significantly higher in CHM users. Association rule and network analysis showed that one main CHM cluster was commonly used to treat patients with muscular dystrophies in Taiwan. The cluster includes Yin-Qiao-San, Ban-Xia-Bai-Zhu-Tian-Ma-Tang, Zhi-Ke (Citrus aurantium L.), Yu-Xing-Cao (Houttuynia cordata Thunb.), Che-Qian-Zi (Plantago asiatica L.), and Da-Huang (Rheum palmatum L.).

CONCLUSIONS

Our data suggest that adjunctive therapy with CHM may help to reduce the overall mortality among patients with muscular dystrophies. The identification of the CHM cluster allows us to narrow down the key active compounds and may enable future therapeutic developments and clinical trial designs to improve overall survival in these patients.

Advances in research on pharmacotherapy of sarcopenia

Sarcopenia is a comprehensive degenerative disease with the progressive loss of skeletal muscle mass with age, accompanied by the loss of muscle strength and muscle dysfunction. As a new type of senile syndrome, sarcopenia seriously threatens the health of the elderly. The first-line treatment for sarcopenia is exercise and nutritional supplements. However, pharmacotherapy will provide more reliable and sustainable interventions in geriatric medicine. Clinical trials of new drugs targeting multiple molecules are ongoing. This article focuses on the latest progress in pharmacotherapeutic approaches of sarcopenia in recent years by comprehensively reviewing the clinical outcomes of the existing and emerging pharmacotherapies as well as the molecular mechanisms underlying their therapeutic benefits and side effects.

Identification of candidate miRNA biomarkers for facioscapulohumeral muscular dystrophy using DUX4-based mouse models

Facioscapulohumeral muscular dystrophy (FSHD) is caused by misexpression of DUX4 in skeletal myocytes. As DUX4 is the key therapeutic target in FSHD, surrogate biomarkers of DUX4 expression in skeletal muscle are critically needed for clinical trials. Although no natural animal models of FSHD exist, transgenic mice with inducible DUX4 expression in skeletal muscles rapidly develop myopathic phenotypes consistent with FSHD. Here, we established a new, more-accurate FSHD-like mouse model based on chronic DUX4 expression in a small fraction of skeletal myonuclei that develops pathology mimicking key aspects of FSHD across its lifespan. Utilizing this new aged mouse model and DUX4-inducible mouse models, we characterized the DUX4-related microRNA signatures in skeletal muscles, which represent potential biomarkers for FSHD. We found increased expression of miR-31-5p and miR-206 in muscles expressing different levels of DUX4 and displaying varying degrees of pathology. Importantly, miR-206 expression is significantly increased in serum samples from FSHD patients compared with healthy controls. Our data support miR-31-5p and miR-206 as new potential regulators of muscle pathology and miR-206 as a potential circulating biomarker for FSHD.

This article has an associated First Person interview with the first author of the paper.

Summary: Candidate miRNA biomarkers for facioscapulohumeral muscular dystrophy (FSHD) were identified using FSHD-like mouse models that present cumulative pathology from chronic expression of DUX4 in skeletal muscles and confirmed in FSHD patient serum.

Pathomechanisms and biomarkers in facioscapulohumeral muscular dystrophy: roles of DUX4 and PAX7

Facioscapulohumeral muscular dystrophy (FSHD) is characterised by progressive skeletal muscle weakness and wasting. FSHD is linked to epigenetic derepression of the subtelomeric D4Z4 macrosatellite at chromosome 4q35. Epigenetic derepression permits the distal-most D4Z4 unit to transcribe DUX4, with transcripts stabilised by splicing to a poly(A) signal on permissive 4qA haplotypes. The pioneer transcription factor DUX4 activates target genes that are proposed to drive FSHD pathology. While this toxic gain-of-function model is a satisfying "bottom-up" genotype-to-phenotype link, DUX4 is rarely detectable in muscle and DUX4 target gene expression is inconsistent in patients. A reliable biomarker for FSHD is suppression of a target gene score of PAX7, a master regulator of myogenesis. However, it is unclear how this "top-down" finding links to genomic changes that characterise FSHD and to DUX4. Here, we explore the roles and interactions of DUX4 and PAX7 in FSHD pathology and how the relationship between these two transcription factors deepens understanding via the immune system and muscle regeneration. Considering how FSHD pathomechanisms are represented by "DUX4opathy" models has implications for developing therapies and current clinical trials.

Follistatin-induced muscle hypertrophy in aged mice improves neuromuscular junction innervation and function

Sarcopenia, or age-related loss of muscle mass and strength, is an important contributor to loss of physical function in older adults. The pathogenesis of sarcopenia is likely multifactorial, but recently the role of neurological degeneration, such as motor unit loss, has received increased attention. Here, we investigated the longitudinal effects of muscle hypertrophy (via overexpression of human follistatin, a myostatin antagonist) on neuromuscular integrity in C57BL/6J mice between the ages of 24 and 27 months. Following follistatin overexpression (delivered via self-complementary adeno-associated virus subtype 9 injection), muscle weight and torque production were significantly improved. Follistatin treatment resulted in improvements of neuromuscular junction innervation and transmission but had no impact on age-related losses of motor units. These studies demonstrate that follistatin overexpression-induced muscle hypertrophy not only increased muscle weight and torque production but also countered age-related degeneration at the neuromuscular junction in mice.

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

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

The elusive promise of myostatin inhibition for muscular dystrophy

PURPOSE OF REVIEW

Recent terminations of clinical trials of myostatin inhibitors in muscular dystrophy have raised questions about the predictiveness of mouse models for this therapeutic strategy.

RECENT FINDINGS

A variety of myostatin inhibitors have been developed for preclinical and clinical studies. These inhibitors have ameliorated the phenotype of many but not all mouse models of muscular dystrophy. However, randomized double-blinded placebo controlled trials in both pediatric and adult muscular dystrophies have, as of yet, not demonstrated functional improvement.

SUMMARY

The present article will review the preclinical promise of myostatin inhibitors, the clinical trial experience to date of these inhibitors in muscular dystrophy, and the potential reasons for the lack of observed translation.

A stromal progenitor and ILC2 niche promotes muscle eosinophilia and fibrosis-associated gene expression

Despite the well-accepted view that chronic inflammation contributes to the pathogenesis of Duchenne muscular dystrophy (DMD), the function and regulation of eosinophils remain an unclear facet of type II innate immunity in dystrophic muscle. We report the observation that group 2 innate lymphoid cells (ILC2s) are present in skeletal muscle and are the principal regulators of muscle eosinophils during muscular dystrophy. Eosinophils were elevated in DMD patients and dystrophic mice along with interleukin (IL)-5, a major eosinophil survival factor that was predominantly expressed by muscle ILC2s. We also find that IL-33 was upregulated in dystrophic muscle and was predominantly produced by fibrogenic/adipogenic progenitors (FAPs). Exogenous IL-33 and IL-2 complex (IL-2c) expanded muscle ILC2s and eosinophils, decreased the cross-sectional area (CSA) of regenerating myofibers, and increased the expression of genes associated with muscle fibrosis. The deletion of ILC2s in dystrophic mice mitigated muscle eosinophilia and impaired the induction of IL-5 and fibrosis-associated genes. Our findings highlight a FAP/ILC2/eosinophil axis that promotes type II innate immunity, which influences the balance between regenerative and fibrotic responses during muscular dystrophy.

Immune cells that comprise type II innate immunity coalesce to regulate tissue repair and fibrosis. Kastenschmidt et al. report that ILC2s reside in skeletal muscle, are activated in muscular dystrophy, and promote muscle eosinophilia. Stromal progenitors expressed IL-33, which expanded ILC2s and promoted a transcriptional program associated with muscle fibrosis.

Facioscapulohumeral muscular dystrophy: genetics, gene activation and downstream signalling with regard to recent therapeutic approaches: an update

Whilst a disease-modifying treatment for Facioscapulohumeral muscular dystrophy (FSHD) does not exist currently, recent advances in complex molecular pathophysiology studies of FSHD have led to possible therapeutic approaches for its targeted treatment. Although the underlying genetics of FSHD have been researched extensively, there remains an incomplete understanding of the pathophysiology of FSHD in relation to the molecules leading to DUX4 gene activation and the downstream gene targets of DUX4 that cause its toxic effects. In the context of the local proximity of chromosome 4q to the nuclear envelope, a contraction of the D4Z4 macrosatellite induces lower methylation levels, enabling the ectopic expression of DUX4. This disrupts numerous signalling pathways that mostly result in cell death, detrimentally affecting skeletal muscle in affected individuals. In this regard different options are currently explored either to suppress the transcription of DUX4 gene, inhibiting DUX4 protein from its toxic effects, or to alleviate the symptoms triggered by its numerous targets.

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.

Current Therapeutic Approaches in FSHD

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common muscular dystrophies. Over the last decade, a consensus was reached regarding the underlying cause of FSHD allowing—for the first time—a targeted approach to treatment. FSHD is the result of a toxic gain-of-function from de-repression of the DUX4 gene, a gene not normally expressed in skeletal muscle. With a clear therapeutic target, there is increasing interest in drug development for FSHD, an interest buoyed by the recent therapeutic successes in other neuromuscular diseases. Herein, we review the underlying disease mechanism, potential therapeutic approaches as well as the state of trial readiness in the planning and execution of future clinical trials in FSHD.

Therapeutic Approaches in Facioscapulohumeral Muscular Dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common types of muscular dystrophy, affecting roughly one in 8000 individuals. The complex underlying genetics and poor mechanistic understanding has caused a bottleneck in therapeutic development. Until the discovery of DUX4 and its causal role in FSHD, most trials were untargeted with limited results. Emerging approaches can learn from these early trials to increase their chance of success. Here, we explore the evolution of FSHD clinical trials from nonspecific anabolic or anti-inflammatory/oxidant strategies to cutting-edge molecular therapies targeting DUX4, and we discuss the importance of clinical outcome measures. With combined advances across multiple facets of FSHD research, the field is now poised to accelerate the process of therapeutic discovery and testing.

Designed U7 snRNAs inhibit DUX4 expression and improve FSHD-associated outcomes in DUX4 overexpressing cells and FSHD patient myotubes

Facioscapulohumeral muscular dystrophy (FSHD) arises from epigenetic changes that de-repress the DUX4 gene in muscle. The full-length DUX4 protein causes cell death and muscle toxicity, and therefore we hypothesize that FSHD therapies should center on inhibiting full-length DUX4 expression. In this study, we developed a strategy to accomplish DUX4 inhibition using U7-small nuclear RNA (snRNA) antisense expression cassettes (called U7-asDUX4). These non-coding RNAs were designed to inhibit production or maturation of the full-length DUX4 pre-mRNA by masking the DUX4 start codon, splice sites, or polyadenylation signal. In so doing, U7-asDUX4 snRNAs operate similarly to antisense oligonucleotides. However, in contrast to oligonucleotides, which are limited by poor uptake in muscle and a requirement for lifelong repeated dosing, U7-asDUX4 snRNAs can be packaged within myotropic gene therapy vectors and may require only a single administration when delivered to post-mitotic cells in vivo. We tested several U7-asDUX4s that reduced DUX4 expression in vitro and improved DUX4-associated outcomes. Inhibition of DUX4 expression via U7-snRNAs could be a new prospective gene therapy approach for FSHD or be used in combination with other strategies, like RNAi therapy, to maximize DUX4 silencing in individuals with FSHD.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Cellular and animal models for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common forms of muscular dystrophy and presents with weakness of the facial, scapular and humeral muscles, which frequently progresses to the lower limbs and truncal areas, causing profound disability. Myopathy results from epigenetic de-repression of the D4Z4 microsatellite repeat array on chromosome 4, which allows misexpression of the developmentally regulated DUX4 gene. DUX4 is toxic when misexpressed in skeletal muscle and disrupts several cellular pathways, including myogenic differentiation and fusion, which likely underpins pathology. DUX4 and the D4Z4 array are strongly conserved only in primates, making FSHD modeling in non-primate animals difficult. Additionally, its cytotoxicity and unusual mosaic expression pattern further complicate the generation of in vitro and in vivo models of FSHD. However, the pressing need to develop systems to test therapeutic approaches has led to the creation of multiple engineered FSHD models. Owing to the complex genetic, epigenetic and molecular factors underlying FSHD, it is difficult to engineer a system that accurately recapitulates every aspect of the human disease. Nevertheless, the past several years have seen the development of many new disease models, each with their own associated strengths that emphasize different aspects of the disease. Here, we review the wide range of FSHD models, including several in vitro cellular models, and an array of transgenic and xenograft in vivo models, with particular attention to newly developed systems and how they are being used to deepen our understanding of FSHD pathology and to test the efficacy of drug candidates.

Dnmt3b regulates DUX4 expression in a tissue-dependent manner in transgenic D4Z4 mice

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is a skeletal muscle disorder that is caused by derepression of the transcription factor DUX4 in skeletal muscle cells. Apart from SMCHD1, DNMT3B was recently identified as a disease gene and disease modifier in FSHD. However, the exact role of DNMT3B at the D4Z4 repeat array remains unknown.

METHODS

To determine the role of Dnmt3b on DUX4 repression, hemizygous mice with a FSHD-sized D4Z4 repeat array (D4Z4-2.5 mice) were cross-bred with mice carrying an in-frame exon skipping mutation in Dnmt3b (Dnmt3bMommeD14 mice). Additionally, siRNA knockdowns of Dnmt3b were performed in mouse embryonic stem cells (mESCs) derived from the D4Z4-2.5 mouse model.

RESULTS

In mESCs derived from D4Z4-2.5 mice, Dnmt3b was enriched at the D4Z4 repeat array and DUX4 transcript levels were upregulated after a knockdown of Dnmt3b. In D4Z4-2.5/Dnmt3bMommeD14 mice, Dnmt3b protein levels were reduced; however, DUX4 RNA levels in skeletal muscles were not enhanced and no pathology was observed. Interestingly, D4Z4-2.5/Dnmt3bMommeD14 mice showed a loss of DNA methylation at the D4Z4 repeat array and significantly higher DUX4 transcript levels in secondary lymphoid organs. As these lymphoid organs seem to be more sensitive to epigenetic modifiers of the D4Z4 repeat array, different immune cell populations were quantified in the spleen and inguinal lymph nodes of D4Z4-2.5 mice crossed with Dnmt3bMommeD14 mice or Smchd1MommeD1 mice. Only in D4Z4-2.5/Smchd1MommeD1 mice the immune cell populations were disturbed.

CONCLUSIONS

Our data demonstrates that loss of Dnmt3b results in derepression of DUX4 in lymphoid tissues and mESCs but not in myogenic cells of D4Z4-2.5/Dnmt3bMommeD14 mice. In addition, the Smchd1MommeD1 variant seems to have a more potent role in DUX4 derepression. Our studies suggest that the immune system is particularly but differentially sensitive to D4Z4 chromatin modifiers which may provide a molecular basis for the yet underexplored immune involvement in FSHD.

The prospects of targeting DUX4 in facioscapulohumeral muscular dystrophy

PURPOSE OF REVIEW

Facioscapulohumeral muscular dystrophy (FSHD) is a neuromuscular disorder, which is caused by incomplete repression of the transcription factor double homeobox 4 (DUX4) in skeletal muscle. To date, there is no DUX4-targeting treatment to prevent or delay disease progression. In the present review, we summarize developments in therapeutic strategies with the focus on inhibiting DUX4 and DUX4 target gene expression.

RECENT FINDINGS

Different studies show that DUX4 and its target genes can be repressed with genetic therapies using diverse strategies. Additionally, different small compounds can reduce DUX4 and its target genes in vitro and in vivo.

SUMMARY

Most studies that show DUX4 repression by genetic therapies have only been tested in vitro. More efforts should be made to test them in vivo for clinical translation. Several compounds have been shown to prevent DUX4 and target gene expression in vitro and in vivo. However, their efficiency and specificity has not yet been shown. With emerging clinical trials, the clinical benefit from DUX4 repression in FSHD will likely soon become apparent.

Therapeutic Strategies Targeting DUX4 in FSHD

Facioscapulohumeral muscular dystrophy (FSHD) is a common muscle dystrophy typically affecting patients within their second decade. Patients initially exhibit asymmetric facial and humeral muscle damage, followed by lower body muscle involvement. FSHD is associated with a derepression of DUX4 gene encoded by the D4Z4 macrosatellite located on the subtelomeric part of chromosome 4. DUX4 is a highly regulated transcription factor and its expression in skeletal muscle contributes to multiple cellular toxicities and pathologies ultimately leading to muscle weakness and atrophy. Since the discovery of the FSHD candidate gene DUX4, many cell and animal models have been designed for therapeutic approaches and clinical trials. Today there is no treatment available for FSHD patients and therapeutic strategies targeting DUX4 toxicity in skeletal muscle are being actively investigated. In this review, we will discuss different research areas that are currently being considered to alter DUX4 expression and toxicity in muscle tissue and the cell and animal models designed to date.

Myostatin inhibition promotes fast fibre hypertrophy but causes loss of AMP-activated protein kinase signalling and poor exercise tolerance in a model of limb-girdle muscular dystrophy R1/2A

KEY POINTS

Limb-girdle muscular dystrophy R1 (LGMD R1) is caused by mutations in the CAPN3 gene and is characterized by progressive muscle loss, impaired mitochondrial function and reductions in the slow oxidative gene expression programme. Myostatin is a negative regulator of muscle growth, and its inhibition improves the phenotype in several muscle wasting disorders. The effect of genetic and pharmacological inhibition of myostatin signalling on the disease phenotype in a mouse model of LGMD R1 (CAPN3 knockout mouse-C3KO) was studied. Inhibition of myostatin signalling in C3KO muscles resulted in significant muscle hypertrophy; however, there were no improvements in muscle strength and exacerbation of exercise intolerance concomitant with further reduction of muscle oxidative capacity was observed. Inhibition of myostatin signalling is unlikely to be a valid therapeutic strategy for LGMD R1.

ABSTRACT

Limb-girdle muscular dystrophy R1 (LGMD R1) is caused by mutations in the CAPN3 gene and is characterized by progressive muscle loss, impaired mitochondrial function and reductions in the slow oxidative gene expression programme. There are currently no therapies available to patients. We sought to determine if induction of muscle growth, through myostatin inhibition, represents a viable therapeutic strategy for this disease. Myostatin is a negative regulator of muscle growth, and its inhibition improves the phenotype in several muscle wasting disorders. However, the effect of myostatin depends on the genetic and pathophysiological context and may not be efficacious in all contexts. We found that genetic inhibition of myostatin through overexpression of follistatin (an endogenous inhibitor of myostatin) in our LGMD R1 model (C3KO) resulted in 1.5- to 2-fold increase of muscle mass for the majority of limb muscles. However, muscle strength was not improved and exercise intolerance was exacerbated. Pharmacological inhibition of myostatin, using an anti-myostatin antibody, resulted in statistically significant increases in muscle mass; however, functional testing did not reveal changes in muscle strength nor endurance in treated C3KO mice. Histochemical and biochemical evaluation of follistatin overexpressing mice revealed a reduction in the percentage of oxidative fibres and decreased activation of AMP-activated protein kinase signalling in transgenics compared to C3KO muscles. Our data suggest that muscle hypertrophy, induced by myostatin inhibition, leads to loss of oxidative capacity, which further compromises metabolically impaired C3KO muscles and thus is unlikely to be a valid strategy for treatment of LGMD R1.

Induction of a local muscular dystrophy using electroporation in vivo: an easy tool for screening therapeutics

Intramuscular injection and electroporation of naked plasmid DNA (IMEP) has emerged as a potential alternative to viral vector injection for transgene expression into skeletal muscles. In this study, IMEP was used to express the DUX4 gene into mouse tibialis anterior muscle. DUX4 is normally expressed in germ cells and early embryo, and silenced in adult muscle cells where its pathological reactivation leads to Facioscapulohumeral muscular dystrophy. DUX4 encodes a potent transcription factor causing a large deregulation cascade. Its high toxicity but sporadic expression constitutes major issues for testing emerging therapeutics. The IMEP method appeared as a convenient technique to locally express DUX4 in mouse muscles. Histological analyses revealed well delineated muscle lesions 1-week after DUX4 IMEP. We have therefore developed a convenient outcome measure by quantification of the damaged muscle area using color thresholding. This method was used to characterize lesion distribution and to assess plasmid recirculation and dose–response. DUX4 expression and activity were confirmed at the mRNA and protein levels and through a quantification of target gene expression. Finally, this study gives a proof of concept of IMEP model usefulness for the rapid screening of therapeutic strategies, as demonstrated using antisense oligonucleotides against DUX4 mRNA.

Transcriptional and cytopathological hallmarks of FSHD in chronic DUX4-expressing mice

Facioscapulohumeral muscular dystrophy (FSHD) is caused by loss of repression of the DUX4 gene; however, the DUX4 protein is rare and difficult to detect in human muscle biopsies, and pathological mechanisms are obscure. FSHD is also a chronic disease that progresses slowly over decades. We used the sporadic, low-level, muscle-specific expression of DUX4 enabled by the iDUX4pA-HSA mouse to develop a chronic long-term muscle disease model. After 6 months of extremely low sporadic DUX4 expression, dystrophic muscle presented hallmarks of FSHD histopathology, including muscle degeneration, capillary loss, fibrosis, and atrophy. We investigated the transcriptional profile of whole muscle as well as endothelial cells and fibroadiopogenic progenitors (FAPs). Strikingly, differential gene expression profiles of both whole muscle and, to a lesser extent, FAPs, showed significant overlap with transcriptional profiles of MRI-guided human FSHD muscle biopsies. These results demonstrate a pathophysiological similarity between disease in muscles of iDUX4pA-HSA mice and humans with FSHD, solidifying the value of chronic rare DUX4 expression in mice for modeling pathological mechanisms in FSHD and highlighting the importance FAPs in this disease.

Transgenic mice expressing tunable levels of DUX4 develop characteristic facioscapulohumeral muscular dystrophy-like pathophysiology ranging in severity

BACKGROUND

All types of facioscapulohumeral muscular dystrophy (FSHD) are caused by the aberrant activation of the somatically silent DUX4 gene, the expression of which initiates a cascade of cellular events ultimately leading to FSHD pathophysiology. Typically, progressive skeletal muscle weakness becomes noticeable in the second or third decade of life, yet there are many individuals who are genetically FSHD but develop symptoms much later in life or remain relatively asymptomatic throughout their lives. Conversely, FSHD may clinically present prior to 5-10 years of age, ultimately manifesting as a severe early-onset form of the disease. These phenotypic differences are thought to be due to the timing and levels of DUX4 misexpression.

METHODS

FSHD is a dominant gain-of-function disease that is amenable to modeling by DUX4 overexpression. We have recently created a line of conditional DUX4 transgenic mice, FLExDUX4, that develop a myopathy upon induction of human DUX4-fl expression in skeletal muscle. Here, we use the FLExDUX4 mouse crossed with the skeletal muscle-specific and tamoxifen-inducible line ACTA1-MerCreMer to generate a highly versatile bi-transgenic mouse model with chronic, low-level DUX4-fl expression and cumulative mild FSHD-like pathology that can be reproducibly induced to develop more severe pathology via tamoxifen induction of DUX4-fl in skeletal muscles.

RESULTS

We identified conditions to generate FSHD-like models exhibiting reproducibly mild, moderate, or severe DUX4-dependent pathophysiology and characterized progression of pathology. We assayed DUX4-fl mRNA and protein levels, fitness, strength, global gene expression, and histopathology, all of which are consistent with an FSHD-like myopathic phenotype. Importantly, we identified sex-specific and muscle-specific differences that should be considered when using these models for preclinical studies.

CONCLUSIONS

The ACTA1-MCM;FLExDUX4 bi-transgenic mouse model has mild FSHD-like pathology and detectable muscle weakness. The onset and progression of more severe DUX4-dependent pathologies can be controlled via tamoxifen injection to increase the levels of mosaic DUX4-fl expression, providing consistent and readily screenable phenotypes for assessing therapies targeting DUX4-fl mRNA and/or protein and are useful to investigate certain conserved downstream FSHD-like pathophysiology. Overall, this model supports that DUX4 expression levels in skeletal muscle directly correlate with FSHD-like pathology by numerous metrics.

Mouse models for muscular dystrophies: an overview

Muscular dystrophies (MDs) encompass a wide variety of inherited disorders that are characterized by loss of muscle tissue associated with a progressive reduction in muscle function. With a cure lacking for MDs, preclinical developments of therapeutic approaches depend on well-characterized animal models that recapitulate the specific pathology in patients. The mouse is the most widely and extensively used model for MDs, and it has played a key role in our understanding of the molecular mechanisms underlying MD pathogenesis. This has enabled the development of therapeutic strategies. Owing to advancements in genetic engineering, a wide variety of mouse models are available for the majority of MDs. Here, we summarize the characteristics of the most commonly used mouse models for a subset of highly studied MDs, collated into a table. Together with references to key publications describing these models, this brief but detailed overview would be useful for those interested in, or working with, mouse models of MD.

CDR1as/miRNAs-related regulatory mechanisms in muscle development and diseases

Muscles are critical tissues for mammals due to their close association with movement and physiology. Myogenesis involves proliferation, differentiation, and fusion of myoblast, in which many well-known protein-coding genes, as well as linear non-coding RNAs such as microRNAs (miRNAs), are involved. Recently, circular RNAs (circRNAs) have attracted much attention since several circRNAs are known to play significant roles in muscle development and diseases through limited mechanisms, particularly through sponging miRNAs. Through advanced researches, increasing evidence suggests that Cerebellar Degeneration-Related protein 1 antisense (CDR1as) is an important circRNA that regulates the levels of mRNAs expression via competitively sponged miRNAs. Here, we reviewed the robust expression and base pairing relationships of CDR1as and several myogenic miRNAs, as well as these miRNAs and their targeted genes in muscles or some muscle-related diseases. These potential CDR1as/miRNAs/mRNA pathways will provide the basis for further research on the function of CDR1as in muscle development, and eventually extend the versatile roles of CDR1as in mammals.

Identification of the hyaluronic acid pathway as a therapeutic target for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is linked to epigenetic derepression of the germline/embryonic transcription factor DUX4 in skeletal muscle. However, the etiology of muscle pathology is not fully understood, as DUX4 misexpression is not tightly correlated with disease severity. Using a DUX4-inducible cell model, we show that multiple DUX4-induced molecular pathologies that have been observed in patient-derived disease models are mediated by the signaling molecule hyaluronic acid (HA), which accumulates following DUX4 induction. These pathologies include formation of RNA granules, FUS aggregation, DNA damage, caspase activation, and cell death. We also observe previously unidentified pathologies including mislocalization of mitochondria and the DUX4- and HA-binding protein C1QBP. These pathologies are prevented by 4-methylumbelliferone, an inhibitor of HA biosynthesis. Critically, 4-methylumbelliferone does not disrupt DUX4-C1QBP binding and has only a limited effect on DUX4 transcriptional activity, establishing that HA signaling has a central function in pathology and is a target for FSHD therapeutics.