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Neuromuscular disorders (NMDs) encompass a diverse group of genetic diseases characterized by loss of muscle functionality. Despite extensive efforts to develop therapies, no curative treatment exists for any of the NMDs. For multiple disorders, however, therapeutic strategies are currently being tested in clinical settings, and the first successful treatments have now entered clinical practice (e.g. spinraza for spinal muscular atrophy). Successful clinical translation depends on the quality and translatability of preclinical findings and on the predictive value of the experimental models used in their initial development. This Special Issue of Disease Models & Mechanisms has a particular focus on translational research for NMDs. The collection includes original research focusing on advances in the development of novel in vitro and in vivo models, broader understanding of disease pathology and progression, and approaches to modify the disease course in these models. We also present a series of special articles and reviews that highlight our understanding of cellular mechanisms, biomarkers to tract disease pathology, the diversity of mouse models for NMDs, the importance of high-quality preclinical studies and data validation, and the pitfalls of successfully moving a potential therapeutic strategy to the clinic. In this Editorial, we summarize the highlights of these articles and place their findings in the broader context of the NMD research field., (© 2020. Published by The Company of Biologists Ltd.)

Introduction: Neuromuscular disorders are diseases of the musculature and/or the nervous system, generally leading to loss of muscle function. They are a frequent cause of disability and treatment options are often only symptomatic. Interestingly, for a number of neuromuscular disorders the application of antisense oligonucleotides has therapeutic potential., Areas Covered: The authors describe how this approach is exploited for different neuromuscular diseases, focusing on literature published in the past 10 years. For each disease the opportunities of this approach, the state of the art, and current challenges are described., Expert Opinion: A lot of progress has been made in the development of antisense-mediated approaches during recent years and they may become clinically applicable in the near future.

Duchenne muscular dystrophy is a severe neuromuscular disorder that is caused by mutations in the DMD gene, resulting in a disruption of dystrophin production. Next to dystrophin expression in the muscle, different isoforms of the protein are also expressed in the brain and lack of these isoforms leads to cognitive and behavioral deficits in patients. It remains unclear how the loss of the shorter dystrophin isoform Dp140 affects these processes. Using a variety of behavioral tests, we found that mdx and mdx4cv mice (which lack Dp427 or Dp427 + Dp140, respectively) exhibit similar deficits in working memory, movement patterns and blood–brain barrier integrity. Neither model showed deficits in spatial learning and memory, learning flexibility, anxiety or spontaneous behavior, nor did we observe differences in aquaporin 4 and glial fibrillary acidic protein. These results indicate that in contrast to Dp427, Dp140 does not play a crucial role in processes of learning, memory and spontaneous behavior. [ABSTRACT FROM AUTHOR]

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder caused by loss of dystrophin. This lack also affects cardiac structure and function, and cardiovascular complications are a major cause of death in DMD. Newly developed therapies partially restore dystrophin expression. It is unclear whether this will be sufficient to prevent or ameliorate cardiac involvement in DMD. We here establish the cardiac electrophysiological and structural phenotype in young (2–3 months) and aged (6–13 months) dystrophin-deficient mdx mice expressing 100% human dystrophin (hDMD), 0% human dystrophin (hDMDdel52-null) or low levels (~ 5%) of human dystrophin (hDMDdel52-low). Compared to hDMD, young and aged hDMDdel52-null mice displayed conduction slowing and repolarisation abnormalities, while only aged hDMDdel52-null mice displayed increased myocardial fibrosis. Moreover, ventricular cardiomyocytes from young hDMDdel52-null animals displayed decreased sodium current and action potential (AP) upstroke velocity, and prolonged AP duration at 20% and 50% of repolarisation. Hence, cardiac electrical remodelling in hDMDdel52-null mice preceded development of structural alterations. In contrast to hDMDdel52-null, hDMDdel52-low mice showed similar electrophysiological and structural characteristics as hDMD, indicating prevention of the cardiac DMD phenotype by low levels of human dystrophin. Our findings are potentially relevant for the development of therapeutic strategies aimed at restoring dystrophin expression in DMD. [ABSTRACT FROM AUTHOR]

Duchenne muscular dystrophy (DMD) is a severe, progressive neuromuscular disorder caused by mutations in the DMD gene resulting in loss of functional dystrophin protein. The muscle dystrophin isoform is essential to protect muscles from contraction-induced damage. However, most dystrophin isoforms are expressed in the brain. In addition to progressive muscle weakness, many DMD patients therefore also exhibit intellectual and behavioral abnormalities. The most commonly used mouse model for DMD, the mdx mouse, lacks only the full-length dystrophin isoforms and has been extensively characterized for muscle pathology. In this study, we assessed behavioral effects of a lack of full-length dystrophins on spontaneous behavior, discrimination and reversal learning, anxiety, and short-term spatial memory and compared performance between male and female mdx mice. In contrast to our previous study using only female mdx mice, we could not reproduce the earlier observed reversal learning deficit. However, we did notice small differences in the number of visits made during the Y-maze and dark-light box. Results indicate that it is advisable to establish standard operating procedures specific to behavioral testing in mdx mice to allow the detection of the subtle phenotypic differences and to eliminate inter and intra laboratory variance. [ABSTRACT FROM AUTHOR]

Duchenne muscular dystrophy (DMD) is a severe, progressive neuromuscular disorder caused by reading frame disrupting mutations in the DMD gene leading to absence of functional dystrophin. Antisense oligonucleotide (AON)-mediated exon skipping is a therapeutic approach aimed at restoring the reading frame at the pre-mRNA level, allowing the production of internally truncated partly functional dystrophin proteins. AONs work in a sequence specific manner, which warrants generating humanized mouse models for preclinical tests. To address this, we previously generated the hDMDdel52/mdx mouse model using transcription activator like effector nuclease (TALEN) technology. This model contains mutated murine and human DMD genes, and therefore lacks mouse and human dystrophin resulting in a dystrophic phenotype. It allows preclinical evaluation of AONs inducing the skipping of human DMD exons 51 and 53 and resulting in restoration of dystrophin synthesis. Here, we have further characterized this model genetically and functionally. We discovered that the hDMD and hDMDdel52 transgene is present twice per locus, in a tail-to-tail-orientation. Long-read sequencing revealed a partial deletion of exon 52 (first 25 bp), and a 2.3 kb inversion in intron 51 in both copies. These new findings on the genomic make-up of the hDMD and hDMDdel52 transgene do not affect exon 51 and/or 53 skipping, but do underline the need for extensive genetic analysis of mice generated with genome editing techniques to elucidate additional genetic changes that might have occurred. The hDMDdel52/mdx mice were also evaluated functionally using kinematic gait analysis. This revealed a clear and highly significant difference in overall gait between hDMDdel52/mdx mice and C57BL6/J controls. The motor deficit detected in the model confirms its suitability for preclinical testing of exon skipping AONs for human DMD at both the functional and molecular level. [ABSTRACT FROM AUTHOR]

Dystrophin is a sub-sarcolemmal component of skeletal muscle fibres and is enriched at the postsynaptic membrane of the neuromuscular junction (NMJ). In the mdx mouse, dystrophin absence not only causes muscle damage but also mild synaptic dysfunctions and clear morphological aberrations at NMJs. In particular, reduction of postsynaptic sensitivity for the neurotransmitter acetylcholine and extra exhaustion of presynaptic acetylcholine release during intense synaptic activity exists. Current experimental therapeutic approaches in Duchenne muscular dystrophy aim to restore dystrophin expression. An important question is what dystrophin levels are needed to improve muscle function. Recent experimental and clinical studies suggested that levels as low as a few percent of normal can be beneficial. Similarly, it is of interest to know how dystrophin levels relate to NMJ function and morphology. We investigated NMJs of a series of mdx-Xist Δhs mice, which expressed dystrophin between ~2% and 19% of normal. Most functional and morphological NMJ parameters of these mice remained comparable to mdx . On the other hand, mdx +/- mice (expressing ~50% dystrophin) showed normal NMJ features. Thus, the minimal dystrophin level required for normal NMJ function and morphology lies between 19% and 50% of normal when expression of dystrophin is not uniform. [ABSTRACT FROM AUTHOR]