Initial pulmonary respiration causes massive diaphragm damage and hyper-CKemia in Duchenne muscular dystrophy dog

Trained immunity as a potential target for therapeutic immunomodulation in Duchenne muscular dystrophy

Dysregulated inflammation involving innate immune cells, particularly of the monocyte/macrophage lineage, is a key contributor to the pathogenesis of Duchenne muscular dystrophy (DMD). Trained immunity is an evolutionarily ancient protective mechanism against infection, in which epigenetic and metabolic alterations confer non-specific hyperresponsiveness of innate immune cells to various stimuli. Recent work in an animal model of DMD (mdx mice) has shown that macrophages exhibit cardinal features of trained immunity, including the presence of innate immune system "memory". The latter is reflected by epigenetic changes and durable transmissibility of the trained phenotype to healthy non-dystrophic mice by bone marrow transplantation. Mechanistically, it is suggested that a Toll-like receptor (TLR) 4-regulated, memory-like capacity of innate immunity is induced at the level of the bone marrow by factors released from the damaged muscles, leading to exaggerated upregulation of both pro- and anti-inflammatory genes. Here we propose a conceptual framework for the involvement of trained immunity in DMD pathogenesis and its potential to serve as a new therapeutic target.

Osteopontin and Ki-67 expression in World Health Organization graded canine meningioma

Osteopontin (OPN) is a matrix protein involved in tumour initiation and progression. In human meningioma, OPN has been correlated with World Health Organization (WHO) grade, brain invasion and recurrence. The aim of this study was to investigate OPN as a possible malignancy marker in canine meningioma by correlating its expression to WHO grade and proliferative activity as measured by the Ki-67 labelling index (LI). Thirty-five formalin-fixed, paraffin-embedded canine meningioma samples were classified according to the current human WHO classification. Evaluation of OPN expression was performed by immunohistochemical (IHC) labelling and calculation of the OPN intensity score (IS), OPN IHC score and Allred score. The scores were compared with WHO grades, Ki-67 LI, location and invasiveness. Nineteen meningiomas were graded as WHO grade I (54.3%), nine as grade II (25.7%) and seven as grade III (20.0%). Twenty-six tumours were located intracranially, four were retrobulbar and five were spinal meningiomas. In all specimens OPN expression was detected in moderate to high degrees. Neither the OPN scores nor the Ki-67 LIs were correlated with WHO grades. However, the OPN IS and OPN IHC score were significantly higher in WHO grade I samples compared with grade II samples (P <0.05). The OPN IS and OPN IHC score were significantly lower in meningioma samples that invaded surrounding tissues (P = 0.01 and 0.019, respectively). The results indicate a generally high expression of OPN in canine meningioma independent of WHO grade. Further research into the role of OPN as a possible therapeutic target or predictor of recurrence is warranted.

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.

Longitudinal assessment of blood-borne musculoskeletal disease biomarkers in the DE50-MD dog model of Duchenne muscular dystrophy

Background: Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease caused by mutations in the dystrophin gene. Due to their phenotypic similarity to human patients, large animal models are invaluable tools for pre-clinical trials. The DE50-MD dog is a relatively new model of DMD, and carries a therapeutically-tractable mutation lying within the hotspot for human patients, making it especially valuable. Prior to conducting therapeutic trials using this novel animal model, it is essential to establish a panel of viable biomarkers. Methods: We evaluated a panel of blood-borne biomarkers of musculoskeletal disease in the DE50-MD dog. Venous blood samples were obtained monthly throughout an 18-month study period in DE50-MD (N=18) and wild-type (WT) control (N=14) dogs. A panel of potential plasma/serum biomarkers of DMD was measured and their theoretical utility in future clinical trials determined using sample size calculations. Results: Compared to WT dogs, DE50-MD dogs had substantially higher circulating creatine kinase (CK) activities, myomesin-3 (MYOM3), and the dystromiRs miR-1, miR-133a and miR-206, but significantly lower serum myostatin concentrations. An age-associated pattern, similar to that observed in DMD patients, was seen for CK and MYOM3. Sample size calculations suggested that low cohort sizes (N≤3) could be used to detect up to a 50% improvement in DE50-MD results towards WT levels for each biomarker or a combination thereof (via principal component analysis); as few as N=3 animals should enable detection of a 25% improvement using a combined biomarker approach (alpha 0.05, power 0.8). Conclusions: We have established a panel of blood-borne biomarkers that could be used to monitor musculoskeletal disease or response to a therapeutic intervention in the DE50-MD dog using low numbers of animals. The blood biomarker profile closely mimics that of DMD patients, supporting the hypothesis that this DMD model would be suitable for use in pre-clinical trials.

Longitudinal assessment of blood-borne musculoskeletal disease biomarkers in the DE50-MD dog model of Duchenne muscular dystrophy

Background: Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease caused by mutations in the dystrophin gene. Due to their phenotypic similarity to human patients, large animal models are invaluable tools for pre-clinical trials. The DE50-MD dog is a relatively new model of DMD, and carries a therapeutically-tractable mutation lying within the hotspot for human patients, making it especially valuable. Prior to conducting therapeutic trials using this novel animal model, it is essential to establish a panel of viable biomarkers. Methods: We evaluated a panel of blood-borne biomarkers of musculoskeletal disease in the DE50-MD dog. Venous blood samples were obtained monthly throughout an 18-month study period in DE50-MD (N=18) and wild-type (WT) control (N=14) dogs. A panel of potential plasma/serum biomarkers of DMD was measured and their theoretical utility in future clinical trials determined using sample size calculations. Results: Compared to WT dogs, DE50-MD dogs had substantially higher circulating creatine kinase (CK) activities, myomesin-3 (MYOM3), and the dystromiRs miR-1, miR-133a and miR-206, but significantly lower serum myostatin concentrations. An age-associated pattern, similar to that observed in DMD patients, was seen for CK and MYOM3. Sample size calculations suggested that low cohort sizes (N≤3) could be used to detect up to a 50% improvement in DE50-MD results towards WT levels for each biomarker or a combination thereof (via principal component analysis); as few as N=3 animals should enable detection of a 25% improvement using a combined biomarker approach (alpha 0.05, power 0.8). Conclusions: We have established a panel of blood-borne biomarkers that could be used to monitor musculoskeletal disease or response to a therapeutic intervention in the DE50-MD dog using low numbers of animals. The blood biomarker profile closely mimics that of DMD patients, supporting the hypothesis that this DMD model would be suitable for use in pre-clinical trials.

Macrophage plasticity in Duchenne muscular dystrophy: a nexus of pathological remodelling with therapeutic implications

Duchenne muscular dystrophy (DMD) is characterized by chronic skeletal muscle necrosis, leading to muscle regeneration failure and fibrosis. Although macrophages (MPs) are normally essential for muscle regeneration, dysregulated MP function promotes pathological muscle remodelling. Infiltrating MPs can be predominantly pro-inflammatory (M1 biased), anti-inflammatory (M2 biased) or of a mixed phenotype and can originate from the adult bone marrow (monocyte dependent) or embryonic precursors (monocyte independent). In mdx mice (genetic model of DMD) lacking either Toll-like receptor (Tlr) 2 or Tlr4, it is found that MP infiltration of dystrophic muscle is significantly reduced and that the MP phenotype is shifted toward a more anti-inflammatory profile. This is accompanied by significant improvements in muscle histology and force production. Lack of the chemokine receptor CCR2, which impedes monocyte release from the bone marrow, leads to similar beneficial effects in mdx mice. Evidence was also found for Tlr4-regulated induction of trained innate immunity in MPs cultured from the bone marrow of mdx mice before their entry into the muscle. These MPs exhibit epigenetic and metabolic alterations, accompanied by non-specific hyper-responsiveness to multiple stimuli, which is manifested by potentiated upregulation of both pro- and anti-inflammatory genes. In summary, exaggerated recruitment of monocyte-derived MPs and signs of trained innate immunity at the level of the bone marrow are features of the immunophenotype associated with dystrophic muscle disease. These phenomena are regulated by Toll-like receptors that bind endogenous damage-associated molecular pattern (DAMP) molecules, suggesting that DAMP release from dystrophic muscles modulates MP plasticity at the bone marrow level through Toll-like receptor-driven mechanisms.

A Dystrophin Exon-52 Deleted Miniature Pig Model of Duchenne Muscular Dystrophy and Evaluation of Exon Skipping

Duchenne muscular dystrophy (DMD) is a lethal X-linked recessive disorder caused by mutations in the DMD gene and the subsequent lack of dystrophin protein. Recently, phosphorodiamidate morpholino oligomer (PMO)-antisense oligonucleotides (ASOs) targeting exon 51 or 53 to reestablish the DMD reading frame have received regulatory approval as commercially available drugs. However, their applicability and efficacy remain limited to particular patients. Large animal models and exon skipping evaluation are essential to facilitate ASO development together with a deeper understanding of dystrophinopathies. Using recombinant adeno-associated virus-mediated gene targeting and somatic cell nuclear transfer, we generated a Yucatan miniature pig model of DMD with an exon 52 deletion mutation equivalent to one of the most common mutations seen in patients. Exon 52-deleted mRNA expression and dystrophin deficiency were confirmed in the skeletal and cardiac muscles of DMD pigs. Accordingly, dystrophin-associated proteins failed to be recruited to the sarcolemma. The DMD pigs manifested early disease onset with severe bodywide skeletal muscle degeneration and with poor growth accompanied by a physical abnormality, but with no obvious cardiac phenotype. We also demonstrated that in primary DMD pig skeletal muscle cells, the genetically engineered exon-52 deleted pig DMD gene enables the evaluation of exon 51 or 53 skipping with PMO and its advanced technology, peptide-conjugated PMO. The results show that the DMD pigs developed here can be an appropriate large animal model for evaluating in vivo exon skipping efficacy.

Animal models for researching approaches to therapy of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a relatively widespread genetic disease which develops as a result of a mutation in the gene DMD encoding dystrophin. In this review, animal models of DMD are described. These models are used in preclinical studies to elucidate the pathogenesis of the disease or to develop effective treatments; each animal model has its own advantages and disadvantages. For instance, Caenorhabditis elegans, Drosophila melanogaster, and zebrafish (sapje) are suitable for large-scale chemical screening of large numbers of small molecules, but their disease phenotype differs from that of mammals. The use of larger animals is important for understanding of the potential efficacy of various treatments for DMD. While mdx mice have their advantages, they exhibit a milder disease phenotype compared to humans or dogs, making it difficult to evaluate the efficacy of new treatment for DMD. The disease in dogs and pigs is more severe and progresses faster than in mice, but it is more difficult to breed and obtain sufficient numbers of specimens in order to achieve statistically significant results. Moreover, working with large animals is also more labor-intensive. Therefore, when choosing the optimal animal model for research, it is worth considering all the goals and objectives.

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

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

"Betwixt Mine Eye and Heart a League Is Took": The Progress of Induced Pluripotent Stem-Cell-Based Models of Dystrophin-Associated Cardiomyopathy

The ultimate goal of precision disease modeling is to artificially recreate the disease of affected people in a highly controllable and adaptable external environment. This field has rapidly advanced which is evident from the application of patient-specific pluripotent stem-cell-derived precision therapies in numerous clinical trials aimed at a diverse set of diseases such as macular degeneration, heart disease, spinal cord injury, graft-versus-host disease, and muscular dystrophy. Despite the existence of semi-adequate treatments for tempering skeletal muscle degeneration in dystrophic patients, nonischemic cardiomyopathy remains one of the primary causes of death. Therefore, cardiovascular cells derived from muscular dystrophy patients' induced pluripotent stem cells are well suited to mimic dystrophin-associated cardiomyopathy and hold great promise for the development of future fully effective therapies. The purpose of this article is to convey the realities of employing precision disease models of dystrophin-associated cardiomyopathy. This is achieved by discussing, as suggested in the title echoing William Shakespeare's words, the settlements (or "leagues") made by researchers to manage the constraints ("betwixt mine eye and heart") distancing them from achieving a perfect precision disease model.

Duchenne Muscular Dystrophy: recent advances in protein biomarkers and the clinical application

INTRODUCTION

Early biomarker discovery studies have praised the value of their emerging results, predicting an unprecedented impact on health care. Biomarkers are expected to provide tests with increased specificity and sensitivity compared to existing measures, improve the decision-making process, and accelerate the development of therapies. For rare disorders, like Duchenne Muscular Dystrophy (DMD) such biomarkers can assist the development of therapies, therefore also helping to find a cure for the disease.

AREA COVERED

State-of-the-art technologies have been used to identify blood biomarkers for DMD and efforts have been coordinated to develop and promote translation of biomarkers for clinical practice. Biomarker translation to clinical practice is however, adjoined by challenges related to the complexity of the disease, involving numerous biological processes, and the limited sample resources. This review highlights the current progress on the development of biomarkers, describing the proteomics technologies used, the most promising findings and the challenges encountered.

EXPERT OPINION

Strategies for effective use of samples combined with orthogonal proteomics methods for protein quantification are essential for translating biomarkers to the patient's bed side. Progress is achieved only if strong evidence is provided that the biomarker constitutes a reliable indicator of the patient's health status for a specific context of use.

Sarcolemmal Complement Membrane Attack Complex Deposits During Acute Rejection of Myofibers in Nonhuman Primates

We have previously studied in nonhuman primates several aspects of the acute rejection of myofibers, including the histological characteristics, the mechanisms of myofiber elimination by the T cells, and the development of anti-donor antibodies. Here, we report the participation of the complement membrane attack complex (MAC) in this context. We used muscle sections of macaques from experiments of allogeneic muscle precursor cell transplantation with confirmed rejection of the graft-derived myofibers. Sections were stained with hematoxylin and eosin, alizarin red and for immunodetection of MAC, CD8, CD4, C3, C4d, and immunoglobulins. The prominent finding was the presence of sarcolemmal MAC (sMAC) deposits in biopsies with ongoing acute rejection or with recent acute rejection. The numbers of sMAC-positive myofibers were variable, being higher when there was an intense lymphocyte infiltration. Few sMAC-positive myofibers were necrotic or had evidence of sarcolemma permeation. The immunodetection of C3, C4d, and immunoglobulins did not provide significant elements. In conclusion, sMAC deposits were related to myofiber rejection. The fact that the vast majority of sMAC-positive myofibers had no signs of necrosis or sarcolemmal permeation suggests that MAC would not be harmful to myofibers by itself.

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

Duchenne muscular dystrophy (DMD) is caused by mutations in DMD, which codes for dystrophin. Because the progressive and irreversible degeneration of muscle occurs from childhood, earlier therapy is required to prevent dystrophic progression. Exon skipping by antisense oligonucleotides called phosphorodiamidate morpholino oligomers (PMOs), which restores the DMD reading frame and dystrophin expression, is a promising candidate for use in neonatal patients, yet the potential remains unclear. Here, we investigate the systemic efficacy and safety of early exon skipping in dystrophic dog neonates. Intravenous treatment of canine X-linked muscular dystrophy in Japan dogs with a 4-PMO cocktail resulted in ∼3%-27% in-frame exon 6-9 skipping and dystrophin restoration across skeletal muscles up to 14% of healthy levels. Histopathology was ameliorated with the reduction of fibrosis and/or necrosis area and centrally nucleated fibers, significantly in the diaphragm. Treatment induced cardiac multi-exon skipping, though dystrophin rescue was not detected. Functionally, treatment led to significant improvement in the standing test. Toxicity was not observed from blood tests. This is the first study to demonstrate successful multi-exon skipping treatment and significant functional improvement in dystrophic dogs. Early treatment was most beneficial for respiratory muscles, with implications for addressing pulmonary malfunction in patients.

The golden retriever model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by mutations in the DMD gene and loss of the protein dystrophin. The absence of dystrophin leads to myofiber membrane fragility and necrosis, with eventual muscle atrophy and contractures. Affected boys typically die in their second or third decade due to either respiratory failure or cardiomyopathy. Despite extensive attempts to develop definitive therapies for DMD, the standard of care remains prednisone, which has only palliative benefits. Animal models, mainly the mdx mouse and golden retriever muscular dystrophy (GRMD) dog, have played a key role in studies of DMD pathogenesis and treatment development. Because the GRMD clinical syndrome is more severe than in mice, better aligning with the progressive course of DMD, canine studies may translate better to humans. The original founder dog for all GRMD colonies worldwide was identified in the early 1980s before the discovery of the DMD gene and dystrophin. Accordingly, analogies to DMD were initially drawn based on similar clinical features, ranging from the X-linked pattern of inheritance to overlapping histopathologic lesions. Confirmation of genetic homology between DMD and GRMD came with identification of the underlying GRMD mutation, a single nucleotide change that leads to exon skipping and an out-of-frame DMD transcript. GRMD colonies have subsequently been established to conduct pathogenetic and preclinical treatment studies. Simultaneous with the onset of GRMD treatment trials, phenotypic biomarkers were developed, allowing definitive characterization of treatment effect. Importantly, GRMD studies have not always substantiated findings from mdx mice and have sometimes identified serious treatment side effects. While the GRMD model may be more clinically relevant than the mdx mouse, usage has been limited by practical considerations related to expense and the number of dogs available. This further complicates ongoing broader concerns about the poor rate of translation of animal model preclinical studies to humans with analogous diseases. Accordingly, in performing GRMD trials, special attention must be paid to experimental design to align with the approach used in DMD clinical trials. This review provides context for the GRMD model, beginning with its original description and extending to its use in preclinical trials.

Translating golden retriever muscular dystrophy microarray findings to novel biomarkers for cardiac/skeletal muscle function in Duchenne muscular dystrophy

BACKGROUND

In Duchenne muscular dystrophy (DMD), abnormal cardiac function is typically preceded by a decade of skeletal muscle disease. Molecular reasons for differences in onset and progression of these muscle groups are unknown. Human biomarkers are lacking.

METHODS

We analyzed cardiac and skeletal muscle microarrays from normal and golden retriever muscular dystrophy (GRMD) dogs (ages 6, 12, or 47+ mo) to gain insight into muscle dysfunction and to identify putative DMD biomarkers. These biomarkers were then measured using human DMD blood samples.

RESULTS

We identified GRMD candidate genes that might contribute to the disparity between cardiac and skeletal muscle disease, focusing on brain-derived neurotropic factor (BDNF) and osteopontin (OPN/SPP1, hereafter indicated as SPP1). BDNF was elevated in cardiac muscle of younger GRMD but was unaltered in skeletal muscle, while SPP1 was increased only in GRMD skeletal muscle. In human DMD, circulating levels of BDNF were inversely correlated with ventricular function and fibrosis, while SPP1 levels correlated with skeletal muscle function.

CONCLUSION

These results highlight gene expression patterns that could account for differences in cardiac and skeletal disease in GRMD. Most notably, animal model-derived data were translated to DMD and support use of BDNF and SPP1 as biomarkers for cardiac and skeletal muscle involvement, respectively.

Serum Osteopontin as a Novel Biomarker for Muscle Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy is a lethal X-linked muscle disorder. We have already reported that osteopontin (OPN), an inflammatory cytokine and myogenic factor, is expressed in the early dystrophic phase in canine X-linked muscular dystrophy in Japan, a dystrophic dog model. To further explore the possibility of OPN as a new biomarker for disease activity in Duchenne muscular dystrophy, we monitored serum OPN levels in dystrophic and wild-type dogs at different ages and compared the levels to other serum markers, such as serum creatine kinase, matrix metalloproteinase-9, and tissue inhibitor of metalloproteinase-1. Serum OPN levels in the dystrophic dogs were significantly elevated compared with those in wild-type dogs before and 1 hour after a cesarean section birth and at the age of 3 months. The serum OPN level was significantly correlated with the phenotypic severity of dystrophic dogs at the period corresponding to the onset of muscle weakness, whereas other serum markers including creatine kinase were not. Immunohistologically, OPN was up-regulated in infiltrating macrophages and developmental myosin heavy chain-positive regenerating muscle fibers in the dystrophic dogs, whereas serum OPN was highly elevated. OPN expression was also observed during the synergic muscle regeneration process induced by cardiotoxin injection. In conclusion, OPN is a promising biomarker for muscle regeneration in dystrophic dogs and can be applicable to boys with Duchenne muscular dystrophy.

Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder. It is caused by loss-of-function mutations in the dystrophin gene. Currently, there is no cure. A highly promising therapeutic strategy is to replace or repair the defective dystrophin gene by gene therapy. Numerous animal models of DMD have been developed over the last 30 years, ranging from invertebrate to large mammalian models. mdx mice are the most commonly employed models in DMD research and have been used to lay the groundwork for DMD gene therapy. After ~30 years of development, the field has reached the stage at which the results in mdx mice can be validated and scaled-up in symptomatic large animals. The canine DMD (cDMD) model will be excellent for these studies. In this article, we review the animal models for DMD, the pros and cons of each model system, and the history and progress of preclinical DMD gene therapy research in the animal models. We also discuss the current and emerging challenges in this field and ways to address these challenges using animal models, in particular cDMD dogs.

Post-transcriptional modulation of interleukin 8 by CNOT6L regulates skeletal muscle differentiation

CNOT6L is a deadenylase subunit belonging to the CCR4-NOT complex, a major deadenylase complex in eukaryotes involved at multiple levels in regulation of gene expression. While CNOT6L is expressed in skeletal muscle cells, its specific functions in this tissue are still largely unknown. Our previous work highlighted the functional of CNOT6L in skeletal muscle cell differentiation. To further explore how CNOT6L regulates myogenesis, we used here gene expression analysis to identify CNOT6L mRNA targets in human myoblasts. Among these novel targets, IL-8 (interleukin 8) mRNA was the most upregulated in CNOT6L knock-down (KD) cells. Biochemical approaches and poly (A) tail length assays showed that IL-8 mRNA is a direct target of CNOT6L, and further investigations by loss- and gain-of-function assays pointed out that IL-8 is an important effector of myogenesis. Therefore, we have characterized CNOT6L-IL-8 as a new signaling axis that regulates myogenesis.

Dystrophin-deficient large animal models: translational research and exon skipping

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disorder caused by mutations in the dystrophin gene. Affecting approximately 1 in 3,600-9337 boys, DMD patients exhibit progressive muscle degeneration leading to fatality as a result of heart or respiratory failure. Despite the severity and prevalence of the disease, there is no cure available. While murine models have been successfully used in illustrating the mechanisms of DMD, their utility in DMD research is limited due to their mild disease phenotypes such as lack of severe skeletal muscle and cardiac symptoms. To address the discrepancy between the severity of disease displayed by murine models and human DMD patients, dystrophin-deficient dog models with a splice site mutation in intron 6 were established. Examples of these are Golden Retriever muscular dystrophy and beagle-based Canine X-linked muscular dystrophy. These large animal models are widely employed in therapeutic DMD research due to their close resemblance to the severity of human patient symptoms. Recently, genetically tailored porcine models of DMD with deleted exon 52 were developed by our group and others, and can potentially act as a new large animal model. While therapeutic outcomes derived from these large animal models can be more reliably extrapolated to DMD patients, a comprehensive understanding of these models is still needed. This paper will discuss recent progress and future directions of DMD studies with large animal models such as canine and porcine models.

Differential roles of MMP-9 in early and late stages of dystrophic muscles in a mouse model of Duchenne muscular dystrophy

Matrix metalloprotease (MMP)-9 is an endopeptidase associated with the pathogenesis of Duchenne muscular dystrophy (DMD). The precise function of MMP-9 in DMD has not been elucidated to date. We investigated the effect of genetic ablation of MMP-9 in the mdx mouse model (mdx/Mmp9(-/-)). At the early disease stage, the muscles of mdx/Mmp9(-/-) mice showed reduced necrosis and neutrophil invasion, accompanied by down-regulation of chemokine MIP-2. In addition, muscle regeneration was enhanced, which coincided with increased macrophage infiltration and upregulation of MCP-1, and resulted in increased muscle strength. The mdx/Mmp9(-/-) mice also displayed accelerated upregulation of osteopontin expression in skeletal muscle at the acute onset phase of dystrophy. However, at a later disease stage, the mice exhibited muscle growth impairment through altered expression of myogenic factors, and increased fibroadipose tissue. These results showed that MMP-9 might have multiple functions during disease progression. Therapy targeting MMP-9 may improve muscle pathology and function at the early disease stage, but continuous inhibition of this protein may result in the accumulation of fibroadipose tissues and reduced muscle strength at the late disease stage.

Differential Gene Expression Profiling of Dystrophic Dog Muscle after MuStem Cell Transplantation

BACKGROUND

Several adult stem cell populations exhibit myogenic regenerative potential, thus representing attractive candidates for therapeutic approaches of neuromuscular diseases such as Duchenne Muscular Dystrophy (DMD). We have recently shown that systemic delivery of MuStem cells, skeletal muscle-resident stem cells isolated in healthy dog, generates the remodelling of muscle tissue and gives rise to striking clinical benefits in Golden Retriever Muscular Dystrophy (GRMD) dog. This global effect, which is observed in the clinically relevant DMD animal model, leads us to question here the molecular pathways that are impacted by MuStem cell transplantation. To address this issue, we compare the global gene expression profile between healthy, GRMD and MuStem cell treated GRMD dog muscle, four months after allogenic MuStem cell transplantation.

RESULTS

In the dystrophic context of the GRMD dog, disease-related deregulation is observed in the case of 282 genes related to various processes such as inflammatory response, regeneration, calcium ion binding, extracellular matrix organization, metabolism and apoptosis regulation. Importantly, we reveal the impact of MuStem cell transplantation on several molecular and cellular pathways based on a selection of 31 genes displaying signals specifically modulated by the treatment. Concomitant with a diffuse dystrophin expression, a histological remodelling and a stabilization of GRMD dog clinical status, we show that cell delivery is associated with an up-regulation of genes reflecting a sustained enhancement of muscle regeneration. We also identify a decreased mRNA expression of a set of genes having metabolic functions associated with lipid homeostasis and energy. Interestingly, ubiquitin-mediated protein degradation is highly enhanced in GRMD dog muscle after systemic delivery of MuStem cells.

CONCLUSIONS

Overall, our results provide the first high-throughput characterization of GRMD dog muscle and throw new light on the complex molecular/cellular effects associated with muscle repair and the clinical efficacy of MuStem cell-based therapy.

Eccentric muscle challenge shows osteopontin polymorphism modulation of muscle damage

A promoter polymorphism of the osteopontin (OPN) gene (rs28357094) has been associated with multiple inflammatory states, severity of Duchenne muscular dystrophy (DMD) and muscle size in healthy young adults. We sought to define the mechanism of action of the polymorphism, using allele-specific in vitro reporter assays in muscle cells, and a genotype-stratified intervention in healthy controls. In vitro reporter constructs showed the G allele to respond to estrogen treatment, whereas the T allele showed no transcriptional response. Young adult volunteers (n = 187) were enrolled into a baseline study, and subjects with specific rs28357094 genotypes enrolled into an eccentric muscle challenge intervention [n = 3 TT; n = 3 GG/GT (dominant inheritance model)]. Female volunteers carrying the G allele showed significantly greater inflammation and increased muscle volume change as determined by magnetic resonance imaging T1- and T2-weighted images after eccentric challenge, as well as greater decrement in biceps muscle force. Our data suggest a model where the G allele enables enhanced activities of upstream enhancer elements due to loss of Sp1 binding at the polymorphic site. This results in significantly greater expression of the pro-inflammatory OPN cytokine during tissue remodeling in response to challenge in G allele carriers, promoting muscle hypertrophy in normal females, but increased damage in DMD patients.

Conditional expression of TGF-β1 in skeletal muscles causes endomysial fibrosis and myofibers atrophy

To study the effects of transforming growth factor beta 1 (TGF-β1) on fibrosis and failure of regeneration of skeletal muscles, we generated a tet-repressible muscle-specific TGF-β1 transgenic mouse in which expression of TGF-β1 is controlled by oral doxycycline. The mice developed muscle weakness and atrophy after TGF-β1 over-expression. We defined the group of mice that showed phenotype within 2 weeks as early onset (EO) and the rest as late onset (LO), which allowed us to further examine phenotypic differences between the groups. While only mice in the EO group showed significant muscle weakness, pathological changes including endomysial fibrosis and smaller myofibers were observed in both groups at two weeks after the TGF-β1 was over-expressed. In addition, the size of the myofibers and collagen accumulation were significantly different between the two groups. The amount of latent and active TGF-β1 in the muscle and circulation were significantly higher in the EO group compared to the LO or control groups. The up-regulation of the latent TGF-β1 indicated that endogenous TGF-β1 was induced by the expression of the TGF-β1 transgene. Our studies showed that the primary effects of TGF-β1 over-expression in skeletal muscles are muscle wasting and endomysial fibrosis. In addition, the severity of the pathology is associated with the total amount of TGF-β1 and the expression of endogenous TGF-β1. The findings suggest that an auto-feedback loop of TGF-β1 may contribute to the severity of phenotypes.