Massive idiosyncratic exon skipping corrects the nonsense mutation in dystrophic mouse muscle and produces functional revertant fibers by clonal expansion

Evolving Therapeutic Options for the Treatment of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is the most common childhood form of muscular dystrophy. It is caused by mutations in the DMD gene, leading to reduced or absent expression of the dystrophin protein. Clinically, this results in loss of ambulation, cardiomyopathy, respiratory failure, and eventually death. In the past decades, the use of corticosteroids has slowed down the disease progression. More recently, the development of genetically mediated therapies has emerged as the most promising treatment for DMD. These strategies include exon skipping with antisense oligonucleotides, gene replacement therapy with adeno-associated virus, and gene editing with CRISPR (clustered regularly interspaced short palindromic repeats) technology. In this review, we highlight the most up-to-date therapeutic progresses in the field, with emphasis on past and recent experiences, as well as the latest clinical results of DMD micro-dystrophin gene therapy. Additionally, we discuss the lessons learned along the way and the challenges encountered, all of which have helped advance the field, with the potential to finally alleviate such a devastating disease.

The Adiponectin Receptor Agonist, ALY688: A Promising Therapeutic for Fibrosis in the Dystrophic Muscle

Duchenne muscular dystrophy (DMD) is one of the most devastating myopathies, where severe inflammation exacerbates disease progression. Previously, we demonstrated that adiponectin (ApN), a hormone with powerful pleiotropic effects, can efficiently improve the dystrophic phenotype. However, its practical therapeutic application is limited. In this study, we investigated ALY688, a small peptide ApN receptor agonist, as a potential novel treatment for DMD. Four-week-old mdx mice were subcutaneously treated for two months with ALY688 and then compared to untreated mdx and wild-type mice. In vivo and ex vivo tests were performed to assess muscle function and pathophysiology. Additionally, in vitro tests were conducted on human DMD myotubes. Our results showed that ALY688 significantly improved the physical performance of mice and exerted potent anti-inflammatory, anti-oxidative and anti-fibrotic actions on the dystrophic muscle. Additionally, ALY688 hampered myonecrosis, partly mediated by necroptosis, and enhanced the myogenic program. Some of these effects were also recapitulated in human DMD myotubes. ALY688's protective and beneficial properties were mainly mediated by the AMPK-PGC-1α axis, which led to suppression of NF-κβ and TGF-β. Our results demonstrate that an ApN mimic may be a promising and effective therapeutic prospect for a better management of DMD.

T Cell Responses to Dystrophin in a Natural History Study of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin, but many patients have rare revertant fibers that express dystrophin. The skeletal muscle pathology of DMD patients includes immune cell infiltration and inflammatory cascades. There are several strategies to restore dystrophin in skeletal muscles of patients, including exon skipping and gene therapy. There is some evidence that dystrophin restoration leads to a reduction in immune cells, but dystrophin epitopes expressed in revertant fibers or following genome editing, cell therapy, or microdystrophin delivery after adeno-associated viral gene therapy may elicit T cell production in patients. This may affect the efficacy of the therapeutic intervention, and potentially lead to serious adverse events. To confirm and extend previous studies, we performed annual enzyme- linked immunospot interferon-gamma assays on peripheral blood mononuclear cells from 77 pediatric boys with DMD recruited into a natural history study, 69 of whom (89.6%) were treated with corticosteroids. T cell responses to dystrophin were quantified using a total of 368 peptides spanning the entire dystrophin protein, organized into nine peptide pools. Peptide mapping pools were used to further localize the immune response in one positive patient. Six (7.8%) patients had a T cell-mediated immune response to dystrophin at at least one time point. All patients who had a positive result had been treated with corticosteroids, either prednisolone or prednisone. Our results show that ∼8% of DMD individuals in our cohort have a pre-existing T cell-mediated immune response to dystrophin, despite steroid treatment. Although these responses are relatively low level, this information should be considered a useful immunological baseline before undertaking clinical trials and future DMD studies. We further highlight the importance for a robust, reproducible standard operating procedure for collecting, storing, and shipping samples from multiple centers to minimize the number of inconclusive data.

Dystrophin myonuclear domain restoration governs treatment efficacy in dystrophic muscle

Dystrophin is essential for muscle health: its sarcolemmal absence causes the fatal, X-linked condition, Duchenne muscular dystrophy (DMD). However, its normal, spatial organization remains poorly understood, which hinders the interpretation of efficacy of its therapeutic restoration. Using female reporter mice heterozygous for fluorescently tagged dystrophin (Dmd^EGFP), we here reveal that dystrophin distribution is unexpectedly compartmentalized, being restricted to myonuclear-defined sarcolemmal territories extending ~80 µm, which we called "basal sarcolemmal dystrophin units (BSDUs)." These territories were further specialized at myotendinous junctions, where both Dmd transcripts and dystrophin protein were enriched. Genome-level correction in X-linked muscular dystrophy mice via CRISPR/Cas9 gene editing restored a mosaic of separated dystrophin domains, whereas transcript-level Dmd correction, following treatment with tricyclo-DNA antisense oligonucleotides, restored dystrophin initially at junctions before extending along the entire fiber-with levels ~2% sufficient to moderate the dystrophic process. We conclude that widespread restoration of fiber dystrophin is likely critical for therapeutic success in DMD, perhaps most importantly, at muscle-tendon junctions.

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

The skeletal muscle phenotype of the DE50-MD dog model of Duchenne muscular dystrophy

Background: Animal models of Duchenne muscular dystrophy (DMD) are essential to study disease progression and assess efficacy of therapeutic intervention, however dystrophic mice fail to display a clinically relevant phenotype, limiting translational utility. Dystrophin-deficient dogs exhibit disease similar to humans, making them increasingly important for late-stage preclinical evaluation of candidate therapeutics. The DE50-MD canine model of DMD carries a mutation within a human 'hotspot' region of the dystrophin gene, amenable to exon-skipping and gene editing strategies. As part of a large natural history study of disease progression, we have characterised the DE50-MD skeletal muscle phenotype to identify parameters that could serve as efficacy biomarkers in future preclinical trials. Methods: Vastus lateralis muscles were biopsied from a large cohort of DE50-MD dogs and healthy male littermates at 3-monthly intervals (3-18 months) for longitudinal analysis, with multiple muscles collected post-mortem to evaluate body-wide changes. Pathology was characterised quantitatively using histology and measurement of gene expression to determine statistical power and sample sizes appropriate for future work. Results: DE50-MD skeletal muscle exhibits widespread degeneration/regeneration, fibrosis, atrophy and inflammation. Degenerative/inflammatory changes peak during the first year of life, while fibrotic remodelling appears more gradual. Pathology is similar in most skeletal muscles, but in the diaphragm, fibrosis is more prominent, associated with fibre splitting and pathological hypertrophy. Picrosirius red and acid phosphatase staining represent useful quantitative histological biomarkers for fibrosis and inflammation respectively, while qPCR can be used to measure regeneration ( MYH3, MYH8), fibrosis ( COL1A1), inflammation ( SPP1), and stability of DE50-MD dp427 transcripts. Conclusion: The DE50-MD dog is a valuable model of DMD, with pathological features similar to young, ambulant human patients. Sample size and power calculations show that our panel of muscle biomarkers are of strong pre-clinical value, able to detect therapeutic improvements of even 25%, using trials with only six animals per group.

Current Status of Antisense Oligonucleotide-Based Therapy in Neuromuscular Disorders

Neuromuscular disorders include a wide range of diseases affecting the peripheral nervous system, which are primarily characterized by progressive muscle weakness and wasting. While there were no effective therapies until recently, several therapeutic approaches have advanced to clinical trials in the past few years. Among these, the antisense technology aiming at modifying RNA processing and function has remarkably progressed and a few antisense oligonucleotides (ASOs) have now been approved. Despite these recent clinical successes, several ASOs have also failed and clinical programs have been suspended, in most cases when the route of administration was systemic, highlighting the existing challenges notably with respect to effective ASO delivery. In this review we summarize the recent advances and current status of antisense based-therapies for neuromuscular disorders, using successful as well as unsuccessful examples to highlight the variability of outcomes depending on the target tissue and route of administration. We describe the different ASO-mediated therapeutic approaches, including splice-switching applications, steric-blocking strategies and targeted gene knock-down mediated by ribonuclease H recruitment. In this overview, we discuss the merits and challenges of the current ASO technology, and discuss the future of ASO development.

Revertant Phenomenon in DMD and LGMD2I and Its Therapeutic Implications: A Review of Study Under Mentorship of Terrence Partridge

This review recollects my initial research focus on revertant fibers (expressing dystrophin in the background of frame-shifting mutation) in Duchenne muscular dystrophy (DMD) muscles in Professor Terrence Partridge’s Muscle Cell Biology Laboratory in MRC Clinical Research Science Center, Harmmersmith Hospital, London, UK. Our data indicated that revertant fibers are most likely resulted from epigenetic random events which skip exon(s) flanking the mutated exon, leading to the restoration of the reading frame. Some of these events establish themselves as relatively permanent skipping patterns, a mechanism similar to multiple transcript species established in various cell types. With this hypothesis, antisense oligonucleotide-mediated exon skipping is likely to have a great chance to achieve restoration of therapeutic levels of dystrophin in DMD muscles. This leads to our first reports of local and systemic efficacy of antisense oligonucleotide-mediated exon skipping for DMD treatment. The experience under Terry’s mentorship shaped my thinking and led me to explore another revertant feature in the dystroglycanopathy caused by mutations in the Fukutin Related Protein (FKRP) gene which functions as a glycosyltransferase. Mutant FKRPs retain partial function and produce a fraction of normal to no detectable levels of laminin-binding α-dystroglycan (matriglycan) in most of the muscle fibers. Reversion to near normal levels of matriglycan expression in muscles with FKRP mutations depends on muscle regeneration and in muscles of neonate mice, suggesting that changes in metabolism and gene expression could be sufficient to compensate for the reduced function of mutant FKRP genes even those associated with severe congenital muscular dystrophy (CMD). This is now supported by our successful demonstration that supply of FKRP mutant mice with ribitol, a precursor for substrate of FKRP, is sufficient to restore the levels of matriglycan with therapeutic significance. Our data overall suggest that rare events of reversion in muscular dystrophy, and likely other diseases could provide unique insight for mechanisms and therapeutic exploitation.

Highly sensitive screening of antisense sequences for different types of DMD mutations in patients' urine-derived cells

Exon skipping using short antisense oligonucleotides (AONs) is a promising treatment for Duchenne muscular dystrophy (DMD). Several exon-skipping drugs, including viltolarsen (NS-065/NCNP-01), have been approved worldwide. Immortalized human skeletal muscle cell lines, such as rhabdomyosarcoma cells, are frequently used to screen efficient oligonucleotide sequences. However, rhabdomyosarcoma cells do not recapitulate DMD pathophysiology as they express endogenous dystrophin. To overcome this limitation, we recently established a direct human somatic cell reprogramming technology and successfully developed a cellular skeletal muscle DMD model by using myogenic differentiation 1 (MYOD1)-transduced urine-derived cells (MYOD1-UDCs). Here, we compared in vitro drug screening systems in MYOD1-UDCs and rhabdomyosarcoma cells. We collected UDCs from patients with DMD amenable to exon 51 skipping, and obtained MYOD1-UDCs. We then compared the efficiency of exon 51 skipping induced by various morpholino-based AONs, including eteplirsen in differentiated MYOD1-UDCs (UDC-myotubes) and rhabdomyosarcoma cells. Exon skipping was induced more efficiently in UDC-myotubes than in rhabdomyosarcoma cells even at a low AON concentration (1 μM). Furthermore, exon 51 skipping efficiency was higher in UDC-myotubes with a deletion of exons 49-50 than in those with a deletion of exons 48-50, suggesting that the skipping efficiency may vary depending on the DMD mutation pattern. An essential finding of this study is that the sequence of eteplirsen consistently leads to much lower efficiency than other sequences. These findings underscore the importance of AON sequence optimization by our cellular system, which enables highly sensitive screening of exon skipping drugs that target different types of DMD mutations.

Multiplex in situ hybridization within a single transcript: RNAscope reveals dystrophin mRNA dynamics

Dystrophin plays a vital role in maintaining muscle health, yet low mRNA expression, lengthy transcription time and the limitations of traditional in-situ hybridization (ISH) methodologies mean that the dynamics of dystrophin transcription remain poorly understood. RNAscope is highly sensitive ISH method that can be multiplexed, allowing detection of individual transcript molecules at sub-cellular resolution, with different target mRNAs assigned to distinct fluorophores. We instead multiplex within a single transcript, using probes targeted to the 5' and 3' regions of muscle dystrophin mRNA. Our approach shows this method can reveal transcriptional dynamics in health and disease, resolving both nascent myonuclear transcripts and exported mature mRNAs in quantitative fashion (with the latter absent in dystrophic muscle, yet restored following therapeutic intervention). We show that even in healthy muscle, immature dystrophin mRNA predominates (60-80% of total), with the surprising implication that the half-life of a mature transcript is markedly shorter than the time invested in transcription: at the transcript level, supply may exceed demand. Our findings provide unique spatiotemporal insight into the behaviour of this long transcript (with implications for therapeutic approaches), and further suggest this modified multiplex ISH approach is well-suited to long genes, offering a highly tractable means to reveal complex transcriptional dynamics.

Suppression of Nonsense Mutations by New Emerging Technologies

Nonsense mutations often result from single nucleotide substitutions that change a sense codon (coding for an amino acid) to a nonsense or premature termination codon (PTC) within the coding region of a gene. The impact of nonsense mutations is two-fold: (1) the PTC-containing mRNA is degraded by a surveillance pathway called nonsense-mediated mRNA decay (NMD) and (2) protein translation stops prematurely at the PTC codon, and thus no functional full-length protein is produced. As such, nonsense mutations result in a large number of human diseases. Nonsense suppression is a strategy that aims to correct the defects of hundreds of genetic disorders and reverse disease phenotypes and conditions. While most clinical trials have been performed with small molecules, there is an increasing need for sequence-specific repair approaches that are safer and adaptable to personalized medicine. Here, we discuss recent advances in both conventional strategies as well as new technologies. Several of these will soon be tested in clinical trials as nonsense therapies, even if they still have some limitations and challenges to overcome.

Report of a TREAT-NMD/World Duchenne Organisation Meeting on Dystrophin Quantification Methodology

Representatives of academia, patient organisations, industry and the United States Food and Drug Administration attended a workshop on dystrophin quantification methodology. The aims of the workshop were to provide an overview of methods used to quantify dystrophin levels in human skeletal muscle and their applicability to clinical trial samples, outline the gaps with regards to validating the methods for robust clinical applications prior to regulatory agency review, and to align future efforts towards further optimizing these methods. The workshop facilitated a constructive but also critical discussion on the potential and limitations of techniques currently used in the field of translational research (western blot and immunofluorescence analysis) and emerging techniques (mass spectrometry and capillary western immunoassay). Notably, all participants reported variation in dystrophin levels between muscle biopsies from different healthy individuals and agreed on the need for a common reference sample.

AAV9 Edits Muscle Stem Cells in Normal and Dystrophic Adult Mice

CRISPR editing of muscle stem cells (MuSCs) with adeno-associated virus serotype-9 (AAV9) holds promise for sustained gene repair therapy for muscular dystrophies. However, conflicting evidence exists on whether AAV9 transduces MuSCs. To rigorously address this question, we used a muscle graft model. The grafted muscle underwent complete necrosis before regenerating from its MuSCs. We injected AAV9.Cre into Ai14 mice. These mice express tdTomato upon Cre-mediated removal of a floxed stop codon. About 28%-47% and 24%-89% of Pax7+ MuSCs expressed tdTomato in pre-grafts and regenerated grafts (p > 0.05), respectively, suggesting AAV9 efficiently transduced MuSCs, and AAV9-edited MuSCs renewed successfully. Robust MuSC transduction was further confirmed by delivering AAV9.Cre to Pax7-ZsGreen-Ai14 mice in which Pax7+ MuSCs are genetically labeled by ZsGreen. Next, we co-injected AAV9.Cas9 and AAV9.gRNA to dystrophic mdx mice to repair the mutated dystrophin gene. CRISPR-treated and untreated muscles were grafted to immune-deficient, dystrophin-null NSG.mdx4cv mice. Grafts regenerated from CRISPR-treated muscle contained the edited genome and yielded 2.7-fold more dystrophin+ cells (p = 0.015). Importantly, increased dystrophin expression was not due to enhanced formation of revertant fibers or de novo transduction by residual CRISPR vectors in the graft. We conclude that AAV9 effectively transduces MuSCs. AAV9 CRISPR editing of MuSCs may provide enduring therapy.

Mutation-Based Therapeutic Strategies for Duchenne Muscular Dystrophy: From Genetic Diagnosis to Therapy

Duchenne and Becker muscular dystrophy (DMD/BMD) are X-linked muscle disorders caused by mutations of the DMD gene, which encodes the subsarcolemmal protein dystrophin. In DMD, dystrophin is not expressed due to a disruption in the reading frame of the DMD gene, resulting in a severe phenotype. Becker muscular dystrophy exhibits a milder phenotype, having mutations that maintain the reading frame and allow for the production of truncated dystrophin. To date, various therapeutic approaches for DMD have been extensively developed. However, the pathomechanism is quite complex despite it being a single gene disorder, and dystrophin is expressed not only in a large amount of skeletal muscle but also in cardiac, vascular, intestinal smooth muscle, and nervous system tissue. Thus, the most appropriate therapy would be complementation or restoration of dystrophin expression, such as gene therapy using viral vectors, readthrough therapy, or exon skipping therapy. Among them, exon skipping therapy with antisense oligonucleotides can restore the reading frame and yield the conversion of a severe phenotype to one that is mild. In this paper, I present the significance of molecular diagnosis and the development of mutation-based therapeutic strategies to complement or restore dystrophin expression.

PITX2 Enhances the Regenerative Potential of Dystrophic Skeletal Muscle Stem Cells

Duchenne muscular dystrophy (DMD), one of the most lethal genetic disorders, involves progressive muscle degeneration resulting from the absence of DYSTROPHIN. Lack of DYSTROPHIN expression in DMD has critical consequences in muscle satellite stem cells including a reduced capacity to generate myogenic precursors. Here, we demonstrate that the c-isoform of PITX2 transcription factor modifies the myogenic potential of dystrophic-deficient satellite cells. We further show that PITX2c enhances the regenerative capability of mouse DYSTROPHIN-deficient satellite cells by increasing cell proliferation and the number of myogenic committed cells, but importantly also increasing dystrophin-positive (revertant) myofibers by regulating miR-31. These PITX2-mediated effects finally lead to improved muscle function in dystrophic (DMD/mdx) mice. Our studies reveal a critical role for PITX2 in skeletal muscle repair and may help to develop therapeutic strategies for muscular disorders.

A novel high-throughput immunofluorescence analysis method for quantifying dystrophin intensity in entire transverse sections of Duchenne muscular dystrophy muscle biopsy samples

Clinical trials using strategies aimed at inducing dystrophin expression in Duchenne muscular dystrophy (DMD) are underway or at advanced planning stage, including splice switching antisense oligonucleotides (AON), drugs to induce read-through of nonsense mutations and viral mediated gene therapy. In all these strategies, different dystrophin proteins, often internally deleted, are produced, similar to those found in patients with the milder DMD allelic variant, Becker muscular dystrophy (BMD). The primary biological endpoint of these trials is to induce functional dystrophin expression. A reliable and reproducible method for quantification of dystrophin protein expression at the sarcolemma is crucial to monitor the biochemical outcome of such treatments. We developed a new high throughput semi quantitative fluorescent immunofluorescence method for quantifying dystrophin expression in transverse sections of skeletal muscle. This technique is completely operator independent as it based on an automated scanning system and an image processing script developed with Definiens software. We applied this new acquisition-analysis method to quantify dystrophin and sarcolemma-related proteins using paediatric control muscles from cases without a neuromuscular disorder as well as DMD and BMD samples. The image analysis script was instructed to recognize myofibres immunostained for spectrin or laminin while dystrophin was quantified in each identified myofibre (from 2,000 to over 20,000 fibres, depending on the size of the biopsy). We were able to simultaneously extrapolate relevant parameters such as mean sarcolemmal dystrophin, mean spectrin and laminin intensity, fibre area and diameter. In this way we assessed dystrophin production in each muscle fibre in samples of DMD, BMD and controls. This new method allows the unbiased quantification of dystrophin in every myofibre within a transverse muscle section and will be of help for translational research projects as a biological outcome in clinical trials in DMD and BMD.

Link between MHC Fiber Type and Restoration of Dystrophin Expression and Key Components of the DAPC by Tricyclo-DNA-Mediated Exon Skipping

Exon skipping mediated by tricyclo-DNA (tc-DNA) antisense oligonucleotides has been shown to induce significant levels of dystrophin restoration in mdx, a mouse model of Duchenne muscular dystrophy. This translates into significant improvement in key disease indicators in muscle, cardio-respiratory function, heart, and the CNS. Here we examine the relationship between muscle fiber type, based on myosin heavy chain (MHC) profile, and the ability of tc-DNA to restore not only dystrophin but also other members of the dystrophin-associated glycoprotein complex (DAPC). We first profiled this relationship in untreated mdx muscle, and we found that all fiber types support reversion events to a dystrophin-positive state, in an unbiased manner. Importantly, we show that only a small fraction of revertant fibers expressed other members of the DAPC. Immunoblot analysis of protein levels, however, revealed robust expression of these components, which failed to correctly localize to the sarcolemma. We then show that tc-DNA treatment leads to nearly all fibers expressing not only dystrophin but also other key components of the DAPC. Of significance, our work shows that MHC fiber type does not bias the expression of any of these important proteins. This work also highlights that the improved muscle physiology following tc-DNA treatment reported previously results from the complete restoration of the dystrophin complex in all MHCII fibers with equal efficiencies.

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

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

Proceedings of the 2017 National Toxicology Program Satellite Symposium

The 2017 annual National Toxicology Program Satellite Symposium, entitled "Pathology Potpourri," was held in Montreal, Quebec, Canada at the Society of Toxicologic Pathology's 36th annual meeting. The goal of this symposium was to present and discuss challenging diagnostic pathology and/or nomenclature issues. This article presents summaries of the speakers' talks along with select images that were used by the audience for voting and discussion. Various lesions and other topics covered during the symposium included renal papillary degeneration in perinatally exposed animals, an atriocaval mesothelioma, an unusual presentation of an alveolar-bronchiolar carcinoma, a paraganglioma of the organ of Zuckerkandl (also called an extra-adrenal pheochromocytoma), the use of human muscle samples to illustrate the challenges of manual scoring of fluorescent staining, intertubular spermatocytic seminomas, medical device pathology assessment and discussion of the approval process, collagen-induced arthritis, incisor denticles, ameloblast degeneration and poorly mineralized enamel matrix, connective tissue paragangliomas, microcystin-LR toxicity, perivascular mast cells in the forebrain thalamus unrelated to treatment, and 2 cases that provided a review of the International Harmonization of Nomenclature and Diagnostic Criteria (INHAND) bone nomenclature and recommended application of the terminology in routine nonclinical toxicity studies.

Exon Skipping Therapy

Exondys 51 is the first therapy for Duchenne muscular dystrophy (DMD) to have been granted accelerated approval by the FDA. Approval was granted based on using dystrophin expression as a surrogate marker. Exondys 51 targets DMD exon 51 for skipping to restore the reading frame for 13% of Duchenne patients.

Targeted RNA-Seq profiling of splicing pattern in the DMD gene: exons are mostly constitutively spliced in human skeletal muscle

We have analysed the splicing pattern of the human Duchenne Muscular Dystrophy (DMD) NB transcript in normal skeletal muscle. To achieve depth of coverage required for the analysis of this lowly expressed gene in muscle, we designed a targeted RNA-Seq procedure that combines amplification of the full-length 11.3 kb DMD cDNA sequence and 454 sequencing technology. A high and uniform coverage of the cDNA sequence was obtained that allowed to draw up a reliable inventory of the physiological alternative splicing events in the muscular DMD transcript. In contrast to previous assumptions, we evidenced that most of the 79 DMD exons are constitutively spliced in skeletal muscle. Only a limited number of 12 alternative splicing events were identified, all present at a very low level. These include previously known exon skipping events but also newly described pseudoexon inclusions and alternative 3′ splice sites, of which one is the first functional NAGNAG splice site reported in the DMD gene. This study provides the first RNA-Seq-based reference of DMD splicing pattern in skeletal muscle and reports on an experimental procedure well suited to detect condition-specific differences in this low abundance transcript that may prove useful for diagnostic, research or RNA-based therapeutic applications.

Isolation, Culture, and Immunostaining of Skeletal Muscle Myofibers from Wildtype and Nestin-GFP Mice as a Means to Analyze Satellite Cell

Multinucleated myofibers, the functional contractile units of adult skeletal muscle, harbor mononuclear Pax7+ myogenic progenitors on their surface between the myofiber basal lamina and plasmalemma. These progenitors, known as satellite cells, are the primary myogenic stem cells in adult muscle. This chapter describes our laboratory protocols for isolating, culturing, and immunostaining intact myofibers from mouse skeletal muscle as a means for studying satellite cell dynamics. The first protocol discusses myofiber isolation from the flexor digitorum brevis (FDB) muscle. These short myofibers are plated in dishes coated with PureCol collagen (formerly known as Vitrogen) and maintained in a mitogen-poor medium (± supplemental growth factors). Employing such conditions, satellite cells remain at the surface of the parent myofiber while synchronously undergoing a limited number of proliferative cycles and rapidly differentiate. The second protocol discusses the isolation of longer myofibers from the extensor digitorum longus (EDL) muscle. These EDL myofibers are routinely plated individually as adherent myofibers in wells coated with Matrigel and maintained in a mitogen-rich medium, conditions in which satellite cells migrate away from the parent myofiber, proliferate extensively, and generate numerous differentiating progeny. Alternatively, these EDL myofibers can be plated as non-adherent myofibers in uncoated wells and maintained in a mitogen-poor medium (± supplemental growth factors), conditions that retain satellite cell progeny at the myofiber niche similar to the FDB myofiber cultures. However, the adherent myofiber format is our preferred choice for monitoring satellite cells in freshly isolated (Time 0) myofibers. We conclude this chapter by promoting the Nestin-GFP transgenic mouse as an efficient tool for direct analysis of satellite cells in isolated myofibers. While satellite cells have been often detected by their expression of the Pax7 protein or the Myf5nLacZ knockin reporter (approaches that are also detailed herein), the Nestin-GFP reporter distinctively permits quantification of satellite cells in live myofibers, which enables linking initial Time 0 numbers and subsequent performance upon culturing. We additionally point out to the implementation of the Nestin-GFP transgene for monitoring other selective cell lineages as illustrated by GFP expression in capillaries, endothelial tubes and neuronal cells. Myofibers from other types of muscles, such as diaphragm, masseter, and extraocular, can also be isolated and analyzed using protocols described herein. Collectively, this chapter provides essential tools for studying satellite cells in their native position and their interplay with the parent myofiber.

Diseases of the skeletal muscle

After the advances created by the use of cryostat sections and histochemistry 60 years ago, muscle histopathology is now living a real renaissance. In the field of genetic neuromuscular disorders, muscle biopsy analysis is fundamental to address questions about pathogenicity and protein expression when new genes are discovered through next-generation sequencing approaches. Moreover, the identification of the same gene mutated in previously considered distinct histopathologic entities imposes a constant reassessment of morphologic boundaries in several groups of disorders. In other fields like the acquired inflammatory myopathies, histologic analysis nowadays helps to affirm a diagnosis, set up therapeutic strategies, and verify the success of immunosuppressive treatment. In this exciting scenario morphologists are definitely key figures in the neuromuscular field. The objective of this chapter is to give an overview on morphology of the most frequent and recently identified muscle conditions, stressing the importance that only a combined analysis of clinical findings, muscle histology, and specific ancillary investigations is effective in reaching a precise diagnosis and orienting therapy.

Impaired regenerative capacity and lower revertant fibre expansion in dystrophin-deficient mdx muscles on DBA/2 background

Duchenne muscular dystrophy, one of the most common lethal genetic disorders, is caused by mutations in the DMD gene and a lack of dystrophin protein. In most DMD patients and animal models, sporadic dystrophin-positive muscle fibres, called revertant fibres (RFs), are observed in otherwise dystrophin-negative backgrounds. RFs are thought to arise from skeletal muscle precursor cells and clonally expand with age due to the frequent regeneration of necrotic fibres. Here we examined the effects of genetic background on muscle regeneration and RF expansion by comparing dystrophin-deficient mdx mice on the C57BL/6 background (mdx-B6) with those on the DBA/2 background (mdx-DBA), which have a more severe phenotype. Interestingly, mdx-DBA muscles had significantly lower RF expansion than mdx-B6 in all age groups, including 2, 6, 12, and 18 months. The percentage of centrally nucleated fibres was also significantly lower in mdx-DBA mice compared to mdx-B6, indicating that less muscle regeneration occurs in mdx-DBA. Our study aligns with the model that RF expansion reflects the activity of precursor cells in skeletal muscles, and it serves as an index of muscle regeneration capacity.

Translational development of splice-modifying antisense oligomers

INTRODUCTION

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

Therapeutic Potential of Immunoproteasome Inhibition in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy is an inherited fatal genetic disease characterized by mutations in dystrophin gene, causing membrane fragility leading to myofiber necrosis and inflammatory cell recruitment in dystrophic muscles. The resulting environment enriched in proinflammatory cytokines, like IFN-γ and TNF-α, determines the transformation of myofiber constitutive proteasome into the immunoproteasome, a multisubunit complex involved in the activation of cell-mediate immunity. This event has a fundamental role in producing peptides for antigen presentation by MHC class I, for the immune response and also for cytokine production and T-cell differentiation. Here, we characterized for the first time the presence of T-lymphocytes activated against revertant dystrophin epitopes, in the animal model of Duchenne muscular dystrophy, the mdx mice. Moreover, we specifically blocked i-proteasome subunit LMP7, which was up-regulated in dystrophic skeletal muscles, and we demonstrated the rescue of the dystrophin expression and the amelioration of the dystrophic phenotype. The i-proteasome blocking lowered myofiber MHC class I expression and self-antigen presentation to T cells, thus reducing the specific antidystrophin T cell response, the muscular cell infiltrate, and proinflammatory cytokine production, together with muscle force recovery. We suggest that i-proteasome inhibition should be considered as new promising therapeutic approach for Duchenne muscular dystrophy pathology.

Inhibition of muscle fibrosis results in increases in both utrophin levels and the number of revertant myofibers in Duchenne muscular dystrophy

Duchenne Muscular Dystrophy is characterized by: near absence of dystrophin in skeletal muscles; low percentage of revertant myofibers; up-regulation of utrophin synthesis; and a high degree of muscle fibrosis. In patient quadriceps femoris biopsies (n = 6, ages between 3–9 years) an inverse correlation was observed between the levels of collagen type I – representing fibrosis – and the levels of utrophin. This correlation was independent of the patient's age and was observed in the entire muscle biopsy sections. In the mdx mice diaphragm (n = 6/group), inhibition of fibrosis by halofuginone resulted in increases in the levels of utrophin. The utrophin/fibrosis relationships were not limited to collagen type I, but also applied to other constituents of the fibrosis machinery. The inverse correlation was found also in old mdx mice with established fibrosis. In addition, inhibition of collagen type I levels was associated with increases in the numbers of revertant myofibers, both as single myofibers and in clusters in the diaphragm and the gastrocnemius.

In summary, our results demonstrate an inverse correlation between the level of muscle fibrosis and the level of utrophin and that of the number of revertant myofibers. These findings may reveal common links between the fibrotic and utrophin-synthesis pathways and offer new insights into the regulation of utrophin synthesis.

Myogenic Potential of Muscle Side and Main Population Cells after Intravenous Injection into Sub-lethally IrradiatedmdxMice

Muscle side population (SP) cells have demonstrated hematopoietic and myogenic activities in vivo upon intravenous (IV) injection into lethally irradiated mdx mice. In contrast, muscle main population (MP) cells were unable to rescue the bone marrow of lethally irradiated mice and, consequently, their in vivo myogenic potential could not be assessed using this method. In the current study, muscle SP or MP cells derived from syngeneic wild-type male mice were delivered to sub-lethally irradiated mdx female mice by single or serial IV injections. Recipient mice were euthanized 12 weeks after transplantation at which time the quadriceps and diaphragm muscles were analyzed for the presence of donor-derived cells. Mice injected with 104muscle SP cells or with 106MP cells appeared to have similar numbers of dystrophin-positive myofibers containing fused donor nuclei. Analysis of the remaining tissue via real-time quantitative PCR indicated that mice injected with muscle SP cells had a higher percentage of donor-derived Y-DNA in the quadriceps than mice injected with MP cells, suggesting that muscle SP cells may be enriched for progenitors able to engraft dystrophic skeletal muscles from the circulation. Although the overall engraftment did not reach therapeutically significant levels, these results indicate that further optimization of cell delivery techniques may lead to improved efficacy of cell-mediated therapy using muscle SP cells.

Recent advances in innovative therapeutic approaches for Duchenne muscular dystrophy: from discovery to clinical trials

Duchenne muscular dystrophy (DMD) is an X-linked progressive degenerative muscle disorder caused by the absence of dystrophin. There is no curative therapy, although innovative therapeutic approaches have been aggressively investigated over recent years. Currently, the international clinical trial registry platform for this disease has been constructed and clinical trials for innovative therapeutic approaches are underway. Among these, exon skipping and read-through of nonsense mutations are in the most advanced stages, with exon skipping theoretically applicable to a larger number of patients. To date, exon skipping that targets exons 51, 44, 45, and 53 is being globally investigated including in USA, EU, and Japan. The latest announcement from Japan was made, demonstrating successful dystrophin production in muscles of patients with DMD after treating with exon 53 skipping antisense oligonucleotides (ASOs). However, the innovative therapeutic approaches have demonstrated limited efficacy. To address this issue in exon skipping, studies to unveil the mechanism underlying gymnotic delivery of ASO uptake in living cells have been conducted in an effort to improve in vivo delivery. Further, establishing the infrastructures to integrate multi-institutional clinical trials are needed to facilitate the development of successful therapies for DMD, which ultimately is applicable to other myopathies and neurodegenerative diseases, including spinal muscular atrophy and motor neuron diseases.

Somatic mosaicism: on the road to cancer

Diversity is the basis of fitness selection. Although the genome of an individual is considered to be largely stable, there is theoretical and experimental evidence--both in model organisms and in humans--that genetic mosaicism is the rule rather than the exception. The continuous generation of cell variants, their interactions and selective pressures lead to life-long tissue dynamics. Individuals may thus enjoy 'clonal health', defined as a clonal composition that supports healthy morphology and physiology, or suffer from clonal configurations that promote disease, such as cancer. The contribution of mosaicism to these processes starts during embryonic development. In this Opinion article, we argue that the road to cancer might begin during these early stages.

Isolation of Mouse Periocular Tissue for Histological and Immunostaining Analyses of the Extraocular Muscles and Their Satellite Cells

The extraocular muscles (EOMs) comprise a group of highly specialized skeletal muscles controlling eye movements. Although a number of unique features of EOMs including their sparing in Duchenne muscular dystrophy have drawn a continuous interest, knowledge about these hard to reach muscles is still limited. The goal of this chapter is to provide detailed methods for the isolation and histological analysis of mouse EOMs. We first introduce in brief the basic anatomy and established nomenclature of the extraocular primary and accessory muscles. We then provide a detailed description with step-by-step images of our procedure for isolating (and subsequently cryosectioning) EOMs while preserving the integrity of their original structural organization. Next, we present several useful histological protocols frequently used by us, including: (1) a method for highlighting the general organization of periocular tissue, using the MyoD(Cre) × R26(mTmG) reporter mouse that elegantly distinguishes muscle (MyoD(Cre)-driven GFP(+)) from the non-myogenic constituents (Tomato(+)); (2) analysis by H&E staining, allowing for example, detection of the pathological features of the dystrophin-null phenotype in affected limb and diaphragm muscles that are absent in EOMs; (3) detection of the myogenic progenitors (i.e., satellite cells) in their native position underneath the myofiber basal lamina using Pax7/laminin double immunostaining. The EOM tissue harvesting procedure described here can also be adapted for isolating and studying satellite cells and other cell types. Overall, the methods described in this chapter should provide investigators the necessary tools for entering the EOM research field and contribute to a better understanding of this highly specialized muscle group and its complex micro-anatomy.

In vivo gene editing in dystrophic mouse muscle and muscle stem cells

Frame-disrupting mutations in the DMD gene, encoding dystrophin, compromise myofiber integrity and drive muscle deterioration in Duchenne muscular dystrophy (DMD). Removing one or more exons from the mutated transcript can produce an in-frame mRNA and a truncated, but still functional, protein. In this study, we developed and tested a direct gene-editing approach to induce exon deletion and recover dystrophin expression in the mdx mouse model of DMD. Delivery by adeno-associated virus (AAV) of clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 endonucleases coupled with paired guide RNAs flanking the mutated Dmd exon23 resulted in excision of intervening DNA and restored the Dmd reading frame in myofibers, cardiomyocytes, and muscle stem cells after local or systemic delivery. AAV-Dmd CRISPR treatment partially recovered muscle functional deficiencies and generated a pool of endogenously corrected myogenic precursors in mdx mouse muscle.

TNF-α-Induced microRNAs Control Dystrophin Expression in Becker Muscular Dystrophy

The amount and distribution of dystrophin protein in myofibers and muscle is highly variable in Becker muscular dystrophy and in exon-skipping trials for Duchenne muscular dystrophy. Here, we investigate a molecular basis for this variability. In muscle from Becker patients sharing the same exon 45-47 in-frame deletion, dystrophin levels negatively correlate with microRNAs predicted to target dystrophin. Seven microRNAs inhibit dystrophin expression in vitro, and three are validated in vivo (miR-146b/miR-374a/miR-31). microRNAs are expressed in dystrophic myofibers and increase with age and disease severity. In exon-skipping-treated mdx mice, microRNAs are significantly higher in muscles with low dystrophin rescue. TNF-α increases microRNA levels in vitro whereas NFκB inhibition blocks this in vitro and in vivo. Collectively, these data show that microRNAs contribute to variable dystrophin levels in muscular dystrophy. Our findings suggest a model where chronic inflammation in distinct microenvironments induces pathological microRNAs, initiating a self-sustaining feedback loop that exacerbates disease progression.

Aquapuncture Using Stem Cell Therapy to Treat Mdx Mice

Duchenne muscular dystrophy (DMD) occurs due to genetic mutations that lead to absence or decrease of dystrophin protein generating progressive muscle degeneration. Cell therapy using mesenchymal stem cell (MSC) has been described as a treatment to DMD. In this work, MSC derived from deciduous teeth, called stem cells from human exfoliated deciduous teeth (SHED), were injected in acupoint as an alternative therapy to minimize muscle degeneration in twenty-two mdx mice. The treatment occurred three times with intervals of 21 days, and animals were analyzed four times: seven days prior treatment (T-7); 10 days after first treatment (T10); 10 days after second treatment (T31); and 10 days after third treatment (T52). Animals were evaluated by wire test for estimate strength and blood was collected to perform a creatinine phosphokinase analysis. After euthanasia, cranial tibial muscles were collected and submitted to histological and immunohistochemistry analyses. Treated groups presented improvement of strength and reduced creatinine phosphokinase levels. Also, a slight dystrophin increase was observed in tibial cranial muscle when aquapuncture was associated SHED. All therapies have minimized muscle degeneration, but the association of aquapuncture with SHED appears to have better effect, reducing muscle damage, suggesting a therapeutic value.

The effect of the muscle environment on the regenerative capacity of human skeletal muscle stem cells

Background

Muscle stem cell transplantation is a possible treatment for muscular dystrophy. In addition to the intrinsic properties of the stem cells, the local and systemic environment plays an important role in determining the fate of the grafted cells. We therefore investigated the effect of modulating the host muscle environment in different ways (irradiation or cryoinjury or a combination of irradiation and cryoinjury) in two immunodeficient mouse strains (mdx nude and recombinase-activating gene (Rag)2-/γ chain-/C5-) on the regenerative capacity of two types of human skeletal muscle-derived stem cell (pericytes and CD133+ cells).

Methods

Human skeletal muscle-derived pericytes or CD133+ cells were transplanted into muscles of either mdx nude or recombinase-activating gene (Rag)2-/γ chain-/C5- host mice. Host muscles were modulated prior to donor cell transplantation by either irradiation, or cryoinjury, or a combination of irradiation and cryoinjury. Muscles were analysed four weeks after transplantation, by staining transverse cryostat sections of grafted muscles with antibodies to human lamin A/C, human spectrin, laminin and Pax 7. The number of nuclei and muscle fibres of donor origin and the number of satellite cells of both host and donor origin were quantified.

Results

Within both host strains transplanted intra-muscularly with both donor cell types, there were significantly more nuclei and muscle fibres of donor origin in host muscles that had been modulated by cryoinjury, or irradiation+cryoinjury, than by irradiation alone. Irradiation has no additive effects in further enhancing the transplantation efficiency than cryodamage. Donor pericytes did not give rise to satellite cells. However, using CD133+ cells as donor cells, there were significantly more nuclei, muscle fibres, as well as satellite cells of donor origin in Rag2-/γ chain-/C5- mice than mdx nude mice, when the muscles were injured by either cryodamage or irradiation+cryodamage.

Conclusions

Rag2-/γ chain-/C5- mice are a better recipient mouse strain than mdx nude mice for human muscle stem cell transplantation. Cryodamage of host muscle is the most effective method to enhance the transplantation efficiency of human skeletal muscle stem cells. This study highlights the importance of modulating the muscle environment in preclinical studies to optimise the efficacy of transplanted stem cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0036-8) contains supplementary material, which is available to authorized users.

What do mouse models of muscular dystrophy tell us about the DAPC and its components?

There are over 30 mouse models with mutations or inactivations in the dystrophin-associated protein complex. This complex is thought to play a crucial role in the functioning of muscle, as both a shock absorber and signalling centre, although its role in the pathogenesis of muscular dystrophy is not fully understood. The first mouse model of muscular dystrophy to be identified with a mutation in a component of the dystrophin-associated complex (dystrophin) was the mdx mouse in 1984. Here, we evaluate the key characteristics of the mdx in comparison with other mouse mutants with inactivations in DAPC components, along with key modifiers of the disease phenotype. By discussing the differences between the individual phenotypes, we show that the functioning of the DAPC and consequently its role in the pathogenesis is more complicated than perhaps currently appreciated.

Antisense mediated exon skipping therapy for duchenne muscular dystrophy (DMD)

Duchenne Muscular Dystrophy (DMD) is a lethal disease caused by mutations in the dystrophin gene (DMD) that result in the absence of essential muscle protein dystrophin. Among many different approaches for DMD treatment, exon skipping, mediated by antisense oligonucleotides, is one of the most promising methods for restoration of dystrophin expression. This approach has been tested extensively targeting different exons in numerous models both in vitro and in vivo. During the past 10 years, there has been a considerable progress by using DMD animal models involving three types of antisense oligonucleotides (2'-O-methyl phosphorothioate (2OME-PS), phosphorodiamidate morpholino oligomer (PMO)) and peptide nucleic acid (PNA).

Satellite cells from dystrophic muscle retain regenerative capacity

Duchenne muscular dystrophy is an inherited disorder that is characterized by progressive skeletal muscle weakness and wasting, with a failure of muscle maintenance/repair mediated by satellite cells (muscle stem cells). The function of skeletal muscle stem cells resident in dystrophic muscle may be perturbed by being in an increasing pathogenic environment, coupled with constant demands for repairing muscle. To investigate the contribution of satellite cell exhaustion to this process, we tested the functionality of satellite cells isolated from the mdx mouse model of Duchenne muscular dystrophy. We found that satellite cells derived from young mdx mice contributed efficiently to muscle regeneration within our in vivo mouse model. To then test the effects of long-term residence in a dystrophic environment, satellite cells were isolated from aged mdx muscle. Surprisingly, they were as functional as those derived from young or aged wild type donors. Removing satellite cells from a dystrophic milieu reveals that their regenerative capacity remains both intact and similar to satellite cells derived from healthy muscle, indicating that the host environment is critical for controlling satellite cell function.

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Grafted mdx satellite cells regenerate muscle as well as wild-type satellite cells.

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Aged mdx myofibers bear more satellite cells than aged wild type fibers.

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mdx satellite cells retain their ability to activate.

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Aged mdx satellite cells are robustly regenerative in vivo.

Extraocular muscle satellite cells are high performance myo-engines retaining efficient regenerative capacity in dystrophin deficiency

Extraocular muscles (EOMs) are highly specialized skeletal muscles that originate from the head mesoderm and control eye movements. EOMs are uniquely spared in Duchenne muscular dystrophy and animal models of dystrophin deficiency. Specific traits of myogenic progenitors may be determinants of this preferential sparing, but very little is known about the myogenic cells in this muscle group. While satellite cells (SCs) have long been recognized as the main source of myogenic cells in adult muscle, most of the knowledge about these cells comes from the prototypic limb muscles. In this study, we show that EOMs, regardless of their distinctive Pax3-negative lineage origin, harbor SCs that share a common signature (Pax7(+), Ki67(-), Nestin-GFP(+), Myf5(nLacZ+), MyoD-positive lineage origin) with their limb and diaphragm somite-derived counterparts, but are remarkably endowed with a high proliferative potential as revealed in cell culture assays. Specifically, we demonstrate that in adult as well as in aging mice, EOM SCs possess a superior expansion capacity, contributing significantly more proliferating, differentiating and renewal progeny than their limb and diaphragm counterparts. These robust growth and renewal properties are maintained by EOM SCs isolated from dystrophin-null (mdx) mice, while SCs from muscles affected by dystrophin deficiency (i.e., limb and diaphragm) expand poorly in vitro. EOM SCs also retain higher performance in cell transplantation assays in which donor cells were engrafted into host mdx limb muscle. Collectively, our study provides a comprehensive picture of EOM myogenic progenitors, showing that while these cells share common hallmarks with the prototypic SCs in somite-derived muscles, they distinctively feature robust growth and renewal capacities that warrant the title of high performance myo-engines and promote consideration of their properties for developing new approaches in cell-based therapy to combat skeletal muscle wasting.

Monoclonal antibodies for clinical trials of Duchenne muscular dystrophy therapy

Most pathogenic mutations in Duchenne and Becker muscular dystrophies involve deletion of single or multiple exons from the dystrophin gene, so exon-specific monoclonal antibodies (mAbs) can be used to distinguish normal and mutant dystrophin proteins. In Duchenne therapy trials, mAbs can be used to identify or rule out dystrophin-positive "revertant" fibres, which have an internally-deleted dystrophin protein and which occur naturally in some Duchenne patients. Using phage-displayed peptide libraries, we now describe the new mapping of the binding sites of five dystrophin mAbs to a few amino-acids within single exons. The phage display method also confirmed previous mapping of MANEX1A (exon 1) and MANDRA1 (exon 77) by other methods. Of the 79 dystrophin exons, mAbs are now available against single exons 1, 6, 8, 12, 13, 14, 17, 21, 26, 28, 38, 41, 43, 44, 45, 46, 47, 50, 51, 58, 59, 62, 63, 75 and 77. Many have been used in clinical trials, as well as for diagnosis and studies of dystrophin isoforms.

Triple trans-splicing adeno-associated virus vectors capable of transferring the coding sequence for full-length dystrophin protein into dystrophic mice

Recombinant adeno-associated virus (rAAV) vectors have been shown to permit very efficient widespread transgene expression in skeletal muscle after systemic delivery, making these increasingly attractive as vectors for Duchenne muscular dystrophy (DMD) gene therapy. DMD is a severe muscle-wasting disorder caused by DMD gene mutations leading to complete loss of dystrophin protein. One of the major issues associated with delivery of the DMD gene, as a therapeutic approach for DMD, is its large open reading frame (ORF; 11.1 kb). A series of truncated microdystrophin cDNAs (delivered via a single AAV) and minidystrophin cDNAs (delivered via dual-AAV trans-spliced/overlapping reconstitution) have thus been extensively tested in DMD animal models. However, critical rod and hinge domains of dystrophin required for interaction with components of the dystrophin-associated protein complex, such as neuronal nitric oxide synthase, syntrophin, and dystrobrevin, are missing; these dystrophin domains may still need to be incorporated to increase dystrophin functionality and stabilize membrane rigidity. Full-length DMD gene delivery using AAV vectors remains elusive because of the limited single-AAV packaging capacity (4.7 kb). Here we developed a novel method for the delivery of the full-length DMD coding sequence to skeletal muscles in dystrophic mdx mice using a triple-AAV trans-splicing vector system. We report for the first time that three independent AAV vectors carrying "in tandem" sequential exonic parts of the human DMD coding sequence enable the expression of the full-length protein as a result of trans-splicing events cojoining three vectors via their inverted terminal repeat sequences. This method of triple-AAV-mediated trans-splicing could be applicable to the delivery of any large therapeutic gene (≥11 kb ORF) into postmitotic tissues (muscles or neurons) for the treatment of various inherited metabolic and genetic diseases.

Mouse regenerating myofibers detected as false-positive donor myofibers with anti-human spectrin

Abstract Stem cell transplantation is being tested as a potential therapy for a number of diseases. Stem cells isolated directly from tissue specimens or generated via reprogramming of differentiated cells require rigorous testing for both safety and efficacy in preclinical models. The availability of mice with immune-deficient background that carry additional mutations in specific genes facilitates testing the efficacy of cell transplantation in disease models. The muscular dystrophies are a heterogeneous group of disorders, of which Duchenne muscular dystrophy is the most severe and common type. Cell-based therapy for muscular dystrophy has been under investigation for several decades, with a wide selection of cell types being studied, including tissue-specific stem cells and reprogrammed stem cells. Several immune-deficient mouse models of muscular dystrophy have been generated, in which human cells obtained from various sources are injected to assess their preclinical potential. After transplantation, the presence of engrafted human cells is detected via immunofluorescence staining, using antibodies that recognize human, but not mouse, proteins. Here we show that one antibody specific to human spectrin, which is commonly used to evaluate the efficacy of transplanted human cells in mouse muscle, detects myofibers in muscles of NOD/Rag1(null)mdx(5cv), NOD/LtSz-scid IL2Rγ(null) mice, or mdx nude mice, irrespective of whether they were injected with human cells. These "reactive" clusters are regenerating myofibers, which are normally present in dystrophic tissue and the spectrin antibody is likely recognizing utrophin, which contains spectrin-like repeats. Therefore, caution should be used in interpreting data based on detection of single human-specific proteins, and evaluation of human stem cell engraftment should be performed using multiple human-specific labeling strategies.

Ongoing therapeutic trials and outcome measures for Duchenne muscular dystrophy

Muscular dystrophy is a heterogeneous group of genetic disorders characterised by progressive muscle tissue degeneration. No effective treatment has been discovered for these diseases. Preclinical and clinical studies aimed at the development of new therapeutic approaches have been carried out, primarily in subjects affected with dystrophinopathies (Duchenne and Becker muscular dystrophy). In this review, we outline the current therapeutic approaches and past and ongoing clinical trials, highlighting both the advantages and limits of each one. The experimental designs of these trials were based on different rationales, including immunomodulation, readthrough strategies, exon skipping, gene therapy, and cell therapy. We also provide an overview of available outcome measures, focusing on their reliability in estimating meaningful clinical improvement in order to aid in the design of future trials. This perspective is extremely relevant to the field considering the recent development of novel therapeutic approaches that will result in an increasing number of clinical studies over the next few years.

Full-length dysferlin expression driven by engineered human dystrophic blood derived CD133+ stem cells

The protein dysferlin is abundantly expressed in skeletal and cardiac muscles, where its main function is membrane repair. Mutations in the dysferlin gene are involved in two autosomal recessive muscular dystrophies: Miyoshi myopathy and limb-girdle muscular dystrophy type 2B. Development of effective therapies remains a great challenge. Strategies to repair the dysferlin gene by skipping mutated exons, using antisense oligonucleotides (AONs), may be suitable only for a subset of mutations, while cell and gene therapy can be extended to all mutations. AON-treated blood-derived CD133+ stem cells isolated from patients with Miyoshi myopathy led to partial dysferlin reconstitution in vitro but failed to express dysferlin after intramuscular transplantation into scid/blAJ dysferlin null mice. We thus extended these experiments producing the full-length dysferlin mediated by a lentiviral vector in blood-derived CD133+ stem cells isolated from the same patients. Transplantation of engineered blood-derived CD133+ stem cells into scid/blAJ mice resulted in sufficient dysferlin expression to correct functional deficits in skeletal muscle membrane repair. Our data suggest for the first time that lentivirus-mediated delivery of full-length dysferlin in stem cells isolated from Miyoshi myopathy patients could represent an alternative therapeutic approach for treatment of dysferlinopathies.

Revertant fibers in the mdx murine model of Duchenne muscular dystrophy: an age- and muscle-related reappraisal

Muscles in Duchenne dystrophy patients are characterized by the absence of dystrophin, yet transverse sections show a small percentage of fibers (termed "revertant fibers") positive for dystrophin expression. This phenomenon, whose biological bases have not been fully elucidated, is present also in the murine and canine models of DMD and can confound the evaluation of therapeutic approaches. We analyzed 11 different muscles in a cohort of 40 mdx mice, the most commonly model used in pre-clinical studies, belonging to four age groups; such number of animals allowed us to perform solid ANOVA statistical analysis. We assessed the average number of dystrophin-positive fibers, both absolute and normalized for muscle size, and the correlation between their formation and the ageing process. Our results indicate that various muscles develop different numbers of revertant fibers, with different time trends; besides, they suggest that the biological mechanism(s) behind dystrophin re-expression might not be limited to the early development phases but could actually continue during adulthood. Importantly, such finding was seen also in cardiac muscle, a fact that does not fit into the current hypothesis of the clonal origin of "revertant" myonuclei from satellite cells. This work represents the largest, statistically significant analysis of revertant fibers in mdx mice so far, which can now be used as a reference point for improving the evaluation of therapeutic approaches for DMD. At the same time, it provides new clues about the formation of revertant fibers/cardiomyocytes in dystrophic skeletal and cardiac muscle.

Antisense therapy in neurology

Antisense therapy is an approach to fighting diseases using short DNA-like molecules called antisense oligonucleotides. Recently, antisense therapy has emerged as an exciting and promising strategy for the treatment of various neurodegenerative and neuromuscular disorders. Previous and ongoing pre-clinical and clinical trials have provided encouraging early results. Spinal muscular atrophy (SMA), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS), Duchenne muscular dystrophy (DMD), Fukuyama congenital muscular dystrophy (FCMD), dysferlinopathy (including limb-girdle muscular dystrophy 2B; LGMD2B, Miyoshi myopathy; MM, and distal myopathy with anterior tibial onset; DMAT), and myotonic dystrophy (DM) are all reported to be promising targets for antisense therapy. This paper focuses on the current progress of antisense therapies in neurology.