Genetic modifiers of Duchenne and facioscapulohumeral muscular dystrophies

In dystrophic mdx hindlimb muscles where fibrosis is limited, versican haploinsufficiency transiently improves contractile function without reducing inflammation

Versican is increased with inflammation and fibrosis, and is upregulated in Duchenne muscular dystrophy. In fibrotic diaphragm muscles from dystrophic mdx mice, genetic reduction of versican attenuated macrophage infiltration and improved contractile function. Versican is also implicated in myogenesis. Here, we investigated whether versican modulated mdx hindlimb muscle pathology, where inflammation and regeneration are increased but fibrosis is minimal. Immunohistochemistry and qRT-PCR were used to assess how fiber type and glucocorticoids (α-methylprednisolone) modify versican expression. To genetically reduce versican, female mdx and male versican haploinsufficient (hdf) mice were bred resulting in male mdx-hdf and mdx (control) pups. Versican expression, contractile function, and pathology were evaluated in hindlimb muscles. Versican immunoreactivity was greater in slow versus fast hindlimb muscles. Versican mRNA transcripts were reduced by α-methylprednisolone in soleus, but not in fast extensor digitorum longus, muscles. In juvenile (6-wk-old) mdx-hdf mice, versican expression was most robustly decreased in soleus muscles leading to improved force output and a modest reduction in fatiguability. These functional benefits were not accompanied by decreased inflammation. Muscle architecture, regeneration markers, and fiber type also did not differ between mdx-hdf mice and mdx littermates. Improvements in soleus contractile function were not retained in adult (20-wk-old) mdx-hdf mice. In conclusion, soleus muscles from juvenile mdx mice were most responsive to pharmacological or genetic approaches targeting versican; however, the benefits of versican reduction were limited due to low fibrosis. Preclinical matrix research in dystrophy should account for muscle phenotype (including age) and the interdependence between inflammation and fibrosis. NEW & NOTEWORTHY The proteoglycan versican is upregulated in muscular dystrophy. In fibrotic diaphragm muscles from mdx mice, versican reduction attenuated macrophage infiltration and improved performance. Here, in hindlimb muscles from 6- and 20-wk-old mdx mice, where pathology is mild, versican reduction did not decrease inflammation and contractile function improvements were limited to juvenile mice. In dystrophic mdx muscles, the association between versican and inflammation is mediated by fibrosis, demonstrating interdependence between the immune system and extracellular matrix.

French National Protocol for diagnosis and care of facioscapulohumeral muscular dystrophy (FSHD)

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common genetically inherited myopathies in adults. It is characterized by incomplete penetrance and variable expressivity. Typically, FSHD patients display asymmetric weakness of facial, scapular, and humeral muscles that may progress to other muscle groups, particularly the abdominal and lower limb muscles. Early-onset patients display more severe muscle weakness and atrophy, resulting in a higher frequency of associated skeletal abnormalities. In these patients, multisystem involvement, including respiratory, ocular, and auditory, is more frequent and severe and may include the central nervous system. Adult-onset FSHD patients may also display some degree of multisystem involvement which mainly remains subclinical. In 95% of cases, FSHD patients carry a pathogenic contraction of the D4Z4 repeat units (RUs) in the subtelomeric region of chromosome 4 (4q35), which leads to the expression of DUX4 retrogene, toxic for muscles (FSHD1). Five percent of patients display the same clinical phenotype in association with a mutation in the SMCHD1 gene located in chromosome 18, inducing epigenetic modifications of the 4q D4Z4 repeated region and expression of DUX4 retrogene. This review highlights the complexities and challenges of diagnosing and managing FSHD, underscoring the importance of standardized approaches for optimal patient outcomes. It emphasizes the critical role of multidisciplinary care in addressing the diverse manifestations of FSHD across different age groups, from skeletal abnormalities in early-onset cases to the often-subclinical multisystem involvement in adults. With no current cure, the focus on alleviating symptoms and slowing disease progression through coordinated care is paramount.

Evaluation of Neuromuscular Diseases and Complaints by Quantitative Muscle MRI

Background: Quantitative muscle MRI (qMRI) is a promising tool for evaluating and monitoring neuromuscular disorders (NMD). However, the application of different imaging protocols and processing pipelines restricts comparison between patient cohorts and disorders. In this qMRI study, we aim to compare dystrophic (limb-girdle muscular dystrophy), inflammatory (inclusion body myositis), and metabolic myopathy (Pompe disease) as well as patients with post-COVID-19 conditions suffering from myalgia to healthy controls. Methods: Ten subjects of each group underwent a 3T lower extremity muscle MRI, including a multi-echo, gradient-echo, Dixon-based sequence, a multi-echo, spin-echo (MESE) T2 mapping sequence, and a spin-echo EPI diffusion-weighted sequence. Furthermore, the following clinical assessments were performed: Quick Motor Function Measure, patient questionnaires for daily life activities, and 6-min walking distance. Results: Different involvement patterns of conspicuous qMRI parameters for different NMDs were observed. qMRI metrics correlated significantly with clinical assessments. Conclusions: qMRI metrics are suitable for evaluating patients with NMD since they show differences in muscular involvement in different NMDs and correlate with clinical assessments. Still, standardisation of acquisition and processing is needed for broad clinical use.

PAX7, a Key for Myogenesis Modulation in Muscular Dystrophies through Multiple Signaling Pathways: A Systematic Review

Muscular dystrophy is a heterogenous group of hereditary muscle disorders caused by mutations in the genes responsible for muscle development, and is generally defined by a disastrous progression of muscle wasting and massive loss in muscle regeneration. Pax7 is closely associated with myogenesis, which is governed by various signaling pathways throughout a lifetime and is frequently used as an indicator in muscle research. In this review, an extensive literature search adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was performed to identify research that examined signaling pathways in living models, while quantifying Pax7 expression in myogenesis. A total of 247 articles were retrieved from the Web of Science (WoS), PubMed and Scopus databases and were thoroughly examined and evaluated, resulting in 19 articles which met the inclusion criteria. Admittedly, we were only able to discuss the quantification of Pax7 carried out in research affecting various type of genes and signaling pathways, rather than the expression of Pax7 itself, due to the massive differences in approach, factor molecules and signaling pathways analyzed across the research. However, we highlighted the thorough evidence for the alteration of the muscle stem cell precursor Pax7 in multiple signaling pathways described in different living models, with an emphasis on the novel approach that could be taken in manipulating Pax7 expression itself in dystrophic muscle, towards the discovery of an effective treatment for muscular dystrophy. Therefore, we believe that this could be applied to the potential gap in muscle research that could be filled by tuning the well-established marker expression to improve dystrophic muscle.

Evaluating Genetic Modifiers of Duchenne Muscular Dystrophy Disease Progression Using Modeling and MRI

BACKGROUND AND OBJECTIVES

Duchenne muscular dystrophy (DMD) is a progressive muscle degenerative disorder with a well-characterized disease phenotype but considerable interindividual heterogeneity that is not well understood. The aim of this study was to evaluate the effects of dystrophin variations and genetic modifiers of DMD on rate and age of muscle replacement by fat.

METHODS

One hundred seventy-five corticosteroid treated participants from the ImagingDMD natural history study underwent repeated magnetic resonance spectroscopy (MRS) of the vastus lateralis (VL) and soleus (SOL) to determine muscle fat fraction (FF). MRS was performed annually in most instances; however, some individuals had additional visits at 3 or 6 monthss intervals. FF changes over time were modeled using nonlinear mixed effects to estimate disease trajectories based on the age that the VL or SOL reached half-maximum change in FF (mu) and the time required for FF change (sigma). Computed mu and sigma values were evaluated for dystrophin variations that have demonstrated the ability to lead to a mild phenotype as well as compared between different genetic polymorphism groups.

RESULTS

Participants with dystrophin gene deletions amenable to exon 8 skipping (n = 4) had minimal increases in SOL FF and had an increase in VL mu value by 4.4 years compared with a reference cohort (p = 0.039). Participants with nonsense variations within exons that may produce milder phenotypes (n = 11) also had minimal increases in SOL and VL FFs. No differences in estimated FF trajectories were seen for individuals amenable to exon 44 skipping (n = 10). Modeling of the SPP1, LTBP4, and thrombospondin-1 (THBS1) genetic modifiers did not result in significant differences in muscle FF trajectories between genotype groups (p > 0.05); however, trends were noted for the polymorphisms associated with long-range regulation of LTBP4 and THBS1 that deserve further follow-up.

DISCUSSION

The results of this study link the historically mild phenotypes seen in individuals amenable to exon 8 skipping and with certain nonsense variations with alterations in trajectories of lower extremity muscle replacement by fat.

Step Activity Monitoring in Boys with Duchenne Muscular Dystrophy and its Correlation with Magnetic Resonance Measures and Functional Performance

BACKGROUND

Muscles of boys with Duchenne muscular dystrophy (DMD) are progressively replaced by fatty fibrous tissues, and weakness leads to loss of ambulation (LoA). Step activity (SA) monitoring is a quantitative measure of real-world ambulatory function. The relationship between quality of muscle health and SA is unknown in DMD.

OBJECTIVE

To determine SA in steroid treated boys with DMD across various age groups, and to evaluate the association of SA with quality of muscle health and ambulatory function.

METHODS

Quality of muscle health was measured by magnetic resonance (MR) imaging transverse magnetization relaxation time constant (MRI-T2) and MR spectroscopy fat fraction (MRS-FF). SA was assessed via accelerometry, and functional abilities were assessed through clinical walking tests. Correlations between SA, MR, and functional measures were determined. A threshold value of SA was determined to predict the future LoA.

RESULTS

The greatest reduction in SA was observed in the 9- < 11years age group. SA correlated with all functional and MR measures.10m walk/run test had the highest correlation with SA. An increase in muscle MRI-T2 and MRS-FF was associated with a decline in SA. Two years prior to LoA, SA in boys with DMD was 32% lower than age matched boys with DMD who maintained ambulation for more than two-year period. SA monitoring can predict subsequent LoA in Duchenne, as a daily step count of 3200 at baseline was associated with LoA over the next two-years.

CONCLUSION

SA monitoring is a feasible and accessible tool to measure functional capacity in the real-world environment.

Lessons Learned from Discontinued Clinical Developments in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked condition caused by a deficiency of functional dystrophin protein. Patients experience progressive muscle weakness, cardiomyopathy and have a decreased life expectancy. Standards of care, including treatment with steroids, and multidisciplinary approaches have extended the life expectancy and improved the quality of life of patients. In the last 30 years, several compounds have been assessed in preclinical and clinical studies for their ability to restore functional dystrophin levels or to modify pathways involved in DMD pathophysiology. However, there is still an unmet need with regards to a disease-modifying treatment for DMD and the attrition rate between early-phase and late-phase clinical development remains high. Currently, there are 40 compounds in clinical development for DMD, including gene therapy and antisense oligonucleotides for exon skipping. Only five of them have received conditional approval in one jurisdiction subject to further proof of efficacy. In this review, we present data of another 16 compounds that failed to complete clinical development, despite positive results in early phases of development in some cases. We examine the reasons for the high attrition rate and we suggest solutions to avoid similar mistakes in the future.

Targeted Gene Delivery: Where to Land

Genome-editing technologies have the potential to correct most genetic defects involved in blood disorders. In contrast to mutation-specific editing, targeted gene insertion can correct most of the mutations affecting the same gene with a single therapeutic strategy (gene replacement) or provide novel functions to edited cells (gene addition). Targeting a selected genomic harbor can reduce insertional mutagenesis risk, while enabling the exploitation of endogenous promoters, or selected chromatin contexts, to achieve specific transgene expression levels/patterns and the modulation of disease-modifier genes. In this review, we will discuss targeted gene insertion and the advantages and limitations of different genomic harbors currently under investigation for various gene therapy applications.

Multiple Structural Microform Defects Suggest Role of Modifier Genes

ABSTRACT

One of the biggest challenges in clinical genetics is establishing associations between specific germline mutations and the resulting spectrum of phenotypes. The careful characterization of clinical presentations continues to be a tool for establishing these genotype phenotype correlations. The authors intend, by presenting a case study, proposing that the concomitant occurrence of a combinations of mild structural anomalies in the same individual may be due to changes in genes that can be linked by related pathways. A new born with cleft lip and palate was referred at the Cleft Lip and Palate Center. The anamnese was performed and collected data of familiar history, parental consanguinity, and information about pregnancy period. The careful characterization of clinical presentations and the genetic pathways was studied. It is possible that there is no single mutation that can be clearly identified as the etiology of the combination of the defects displayed in the present case.

PDE10A Inhibition Reduces the Manifestation of Pathology in DMD Zebrafish and Represses the Genetic Modifier PITPNA

Duchenne muscular dystrophy (DMD) is a severe genetic disorder caused by mutations in the DMD gene. Absence of dystrophin protein leads to progressive degradation of skeletal and cardiac function and leads to premature death. Over the years, zebrafish have been increasingly used for studying DMD and are a powerful tool for drug discovery and therapeutic development.

In our study, a birefringence screening assay led to identification of phosphodiesterase 10A (PDE10A) inhibitors that reduced the manifestation of dystrophic muscle phenotype in dystrophin-deficient sapje-like zebrafish larvae. PDE10A has been validated as a therapeutic target by pde10a morpholino-mediated reduction in muscle pathology and improvement in locomotion, muscle, and vascular function as well as long-term survival in sapje-like larvae. PDE10A inhibition in zebrafish and DMD patient-derived myoblasts were also associated with reduction of PITPNA expression that has been previously identified as a protective genetic modifier in two exceptional dystrophin-deficient golden retriever muscular dystrophy (GRMD) dogs that escaped the dystrophic phenotype. The combination of a phenotypic assay and relevant functional assessments in the sapje-like zebrafish enhances the potential for the prospective discovery of DMD therapeutics. Indeed, our results suggest a new application for a PDE10A inhibitor as a potential DMD therapeutic to be investigated in a mouse model of DMD.

Minimal Consequences of CMAH and DBA/2 Backgrounds on a FKRP Deficient Model

BACKGROUND

Muscular dystrophies (MD) are a large group of genetic diseases characterized by a progressive loss of muscle. The Latent TGFβ Binding Protein 4 (LTBP4) in the DBA/2 background and the Cytidine Monophosphate-sialic Acid Hydroxylase (CMAH) proteins were previously identified as genetic modifiers in severe MD.

OBJECTIVE

We investigated whether these modifiers could also influence a mild phenotype such as the one observed in a mouse model of Limb-Girdle MD2I (LGMD2I).

METHODS

The FKRPL276I mouse model was backcrossed onto the DBA/2 background, and in separate experiments the Cmah gene was inactivated in FKRPL276I mice by crossing with a Cmah-/- mouse and selecting the double-mutants. The mdx mouse was used as control for these two genome modifications. Consequences at the histological level as well as quantification of expression level by RT-qPCR of genes relevant for muscular dystrophy were then performed.

RESULTS

We observed minimal to no effect of the DBA/2 background on the mild FKRPL276I mouse phenotype, while this same background was previously shown to increase inflammation and fibrosis in the mdx mouse. Similarly, the Cmah-/- deletion had no observable effect on the FKRPL276I mouse phenotype whereas it was seen to increase features of regeneration in mdx mice.

CONCLUSIONS

These modifiers were not observed to impact the severity of the presentation of the mild FKRPL276I model. An interesting association of the CMAH modifier with the regeneration process in the mdx model was seen and sheds new light on the influence of this protein on the dystrophic phenotype.

Genetic modifiers and phenotypic variability in neuromuscular disorders

Neuromuscular disorders are mostly rare diseases with autosomal dominant, recessive, or X-linked inheritance. Interestingly, among patients carrying the same mutations, a range of phenotypic severity is reported. This phenotypic variability in neuromuscular disorders is still not fully understood. This review will focus on genetic modifiers and will briefly describe metabolic pathways, in which they are involved. Genetic modifiers are variants in the same or other genes that modulate the phenotype. Proteins encoded by genetic modifiers in neuromuscular diseases are taking part in different metabolic processes, most commonly in inflammation, growth and regeneration, endoplasmic reticulum metabolism, and cytoskeletal activities. Recent advances in omics technologies, development of computational algorithms, and establishing large international consortia intensified discovery sped up investigation of genetic modifiers. As more individuals affected by neuromuscular disorders are tested, it is often suggested that classic models of genetic causation cannot explain phenotypic variability. There is a growing interest in their discovery and identifying shared metabolic pathways can contribute to design targeted therapies. We provide an update on variants acting as genetic modifiers in neuromuscular disorders and strategies used for their discovery.

Desmin prevents muscle wasting, exaggerated weakness and fragility, and fatigue in dystrophic mdx mouse

KEY POINTS

Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix. Deletion of the desmin gene in mdx mice [double knockout (DKO) mice] induces marked muscle weakness and fatigue resistance compared to mdx mice. Muscle fragility (higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice. By contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy. Desmin cDNA transfer with adeno-associated virus in newborn mdx mice reduced muscle weakness. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic muscle.

ABSTRACT

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by dystrophin deficiency. Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix that contributes to muscle function. In the present study, we attempted to provide further insight into the roles of desmin, for which the expression is increased in the muscle from the mouse mdx DMD model. We show that a deletion of the desmin gene (Des) in mdx mice [double knockout (DKO) mice, mdx:desmin-/-] induces a marked muscle weakness; namely, a reduced absolute maximal force production and increased fatigue compared to that in mdx mice. Fragility (i.e. higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice, despite the promotion of supposedly less fragile muscle fibres in DKO mice, and this worsening of fragility was related to a decreased muscle excitability. Moreover, in contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy, as indicated by smaller and fewer fibres, with a reduced percentage of centronucleated fibres, potentially explaining the severe muscle weakness. Notably, Desmin cDNA transfer with adeno-associated virus in newborn mdx mice improved specific maximal force normalized to muscle weight. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic mdx mice, which differ, at least in part, from those observed in healthy muscle.

Can simple and low-cost motor function assessments help in the diagnostic suspicion of Duchenne muscular dystrophy?

OBJECTIVE

Duchenne muscular dystrophy, an X-linked genetic disease, leads to progressive muscle weakness mainly in the lower limbs. Motor function tests help to monitor disease progression. Can low-cost, simple assessments help in the diagnostic suspicion of Duchenne muscular dystrophy? The authors aim to define the sensitivity of time to rise from the floor, time to walk 10meters, and time to run 10meters, evaluating them as eventual diagnostic screening tools.

METHODS

This is an analytical, observational, retrospective (1998-2015), and prospective study (2015-2018). Cases were recruited from the database of the pediatric neurology department and the healthy, from child care consultations, with normal gait development (up to 15 months) and without other comorbidities (neuromuscular, pulmonary, heart diseases) from the same university hospital.

RESULTS

128 Duchenne muscular dystrophy patients and 344 healthy children were analyzed, equally distributed in age groups. In Duchenne muscular dystrophy, there is a progressive increase in the means of the times to perform the motor tests according to the age group, which accelerates very abruptly after 7 years of age. Healthy children acquire maximum motor capacity at 6 years and stabilize their times. The time to rise showed a p-value <0.05 and a strong association (effect size [ES] >0.8) in all age groups (except at 12 years), with time to walk 10 meters from 9 years, and with time to run 10 meters , from 5 years. The 100% sensitivity points were defined as follows: time to rise, at 2s; time to walk 10 meters, 5s; time to run 10 meters, 4s.

CONCLUSIONS

Time to rise is a useful and simple tool in the screening of neuromuscular disorders such as Duchenne muscular dystrophy, a previously incurable disease with new perspectives for treatment.

Molecular and Clinical Characteristics of a National Cohort of Paediatric Duchenne Muscular Dystrophy Patients in Norway

BACKGROUND

As new gene-related treatment options for Duchenne muscular dystrophy (DMD) are being developed, precise information about the patients' genetic diagnosis and knowledge about the diversities of natural history in DMD is vital.

OBJECTIVE

To obtain detailed insight into the genetic and clinical characteristics of paediatric DMD in Norway.

METHODS

94 boys with DMD, aged 0-18 years, were identified over a period of 3.5 years, yielding a national prevalence of 13.5×10-5 boys. 73 boys (78%) were recruited to full genetic and clinical or limited (genetic only) evaluation.

RESULTS

Molecular analysis disclosed 64% deletions, 18% duplications and 18% point mutations. The mean age of diagnosis was 3.9±2.0 years. 78% were treated with glucocorticoids from age 5.8±1.5 years. 23 boys (35%) had lost ambulation at an age of 10.7±2.0 years. 17% were treated for left ventricular dysfunction from age 12.1±3.0 years and 12% had received night-time non-invasive positive pressure ventilation from age 13.0±2.5 years.

CONCLUSIONS

The distribution of mutation types and sites was similar to previous studies but with more duplications and fewer point mutations. Any genotype-phenotype correlations were not uncovered. The boys were diagnosed early but there is still diagnostic delay among boys presenting with late motor development. Glucocorticoid treatment was widespread, especially among the younger boys. The clinical results of this comprehensive nationwide study highlight the large variability of disease progression in DMD.

Skeletal muscle in health and disease

Skeletal muscle fibres are multinucleated cells that contain postmitotic nuclei (i.e. they are no longer able to divide) and perform muscle contraction. They are formed by fusion of muscle precursor cells, and grow into elongating myofibres by the addition of further precursor cells, called satellite cells, which are also responsible for regeneration following injury. Skeletal muscle regeneration occurs in most muscular dystrophies in response to necrosis of muscle fibres. However, the complex environment within dystrophic skeletal muscle, which includes inflammatory cells, fibroblasts and fibro-adipogenic cells, together with the genetic background of the in vivo model and the muscle being studied, complicates the interpretation of laboratory studies on muscular dystrophies. Many genes are expressed in satellite cells and in other tissues, which makes it difficult to determine the molecular cause of various types of muscular dystrophies. Here, and in the accompanying poster, we discuss our current knowledge of the cellular mechanisms that govern the growth and regeneration of skeletal muscle, and highlight the defects in satellite cell function that give rise to muscular dystrophies.

Summary: The mechanisms of skeletal muscle development, growth and regeneration are described. We discuss whether these processes are dysregulated in inherited muscle diseases and identify pathways that may represent therapeutic targets.

Muscular Dystrophies

Muscular dystrophies represent a complex, varied, and important subset of neuromuscular disorders likely to require the care of a pulmonologist. The spectrum of conditions encapsulated by this subset ranges from severe and fatal congenital muscular dystrophies with onset in infancy to mild forms of limb and girdle weakness with onset in adulthood and minimal respiratory compromise. The list and classification of muscular dystrophies are undergoing near-constant revision, based largely on new insights from genetics and molecular medicine. The authors present an overview of the muscular dystrophies, including their basic features, common clinical phenotypes, and important facets of management.

Cardiopulmonary phenotypic discordance is common in Duchenne muscular dystrophy

OBJECTIVE

To determine the prevalence of discordant cardiopulmonary function among patients with Duchenne muscular dystrophy (DMD) in our clinic.

METHODS

Retrospective chart review from 1999 to 2017.

INCLUSION CRITERIA

DMD patients age ≥ 18 years, alive, with discordant cardiopulmonary function. No patients received glucocorticoid therapy. Discordant cardiopulmonary function was defined as either: good heart function (EF ≥ 40%) and bad lung function (FVC < 1 L) (Group A); or, bad heart function (EF < 40%) and good lung function (FVC ≥ 1 L) (Group B).

RESULTS

Among 74 eligible patients, 25 patients (34%) had discordant cardiopulmonary function (21 patients in Group A and 4 patients in Group B). Three dystrophin mutations were shared by >2 patients (nine patients with deletion of exon 44; three patients with deletion of exon 51; three patients with duplication of exon 2). Among the 15 patients with a shared genotype, eight patients (53%) had discordant cardiopulmonary function (five patients in group A, three patients in group B). Twenty-six patients had a deletion involving or distal to exon 45. Ten of these patients (38%) had discordant cardiopulmonary function (eight patients in Group A, two patients in Group B).

CONCLUSION

In our cohort of DMD patients, discordant cardiopulmonary function was common (present in one-third of our patients), and the dystrophin genotype did not reliably predict a patient's cardiopulmonary phenotype. If confirmed by larger, multi-center studies, our findings have significant implications for predicting patient prognosis, evaluating DMD therapies, and designing new DMD therapies.

Multiple Exon Skipping in the Duchenne Muscular Dystrophy Hot Spots: Prospects and Challenges

Duchenne muscular dystrophy (DMD), a fatal X-linked recessive disorder, is caused mostly by frame-disrupting, out-of-frame deletions in the dystrophin (DMD) gene. Antisense oligonucleotide-mediated exon skipping is a promising therapy for DMD. Exon skipping aims to convert out-of-frame mRNA to in-frame mRNA and induce the production of internally-deleted dystrophin as seen in the less severe Becker muscular dystrophy. Currently, multiple exon skipping has gained special interest as a new therapeutic modality for this approach. Previous retrospective database studies represented a potential therapeutic application of multiple exon skipping. Since then, public DMD databases have become more useful with an increase in patient registration and advances in molecular diagnosis. Here, we provide an update on DMD genotype-phenotype associations using a global DMD database and further provide the rationale for multiple exon skipping development, particularly for exons 45–55 skipping and an emerging therapeutic concept, exons 3–9 skipping. Importantly, this review highlights the potential of multiple exon skipping for enabling the production of functionally-corrected dystrophin and for treating symptomatic patients not only with out-of-frame deletions but also those with in-frame deletions. We will also discuss prospects and challenges in multiple exon skipping therapy, referring to recent progress in antisense chemistry and design, as well as disease models.

Exercise biology of neuromuscular disorders

Neuromuscular disorders (NMDs) are chronic conditions that affect the neuromuscular system. Many NMDs currently have no cure; however, as more effective therapies become available for NMD patients, these individuals will exhibit improved health and/or prolonged lifespans. As a result, persons with NMDs will likely desire to engage in a more diverse variety of activities of daily living, including increased physical activity or exercise. Therefore, there is a need to increase our knowledge of the effects of acute exercise and chronic training on the neuromuscular system in NMD contexts. Here, we discuss the disease mechanisms and exercise biology of Duchenne muscular dystrophy (DMD), spinal muscular atrophy (SMA), and myotonic dystrophy type 1 (DM1), which are among the most prevalent NMDs in children and adults. Evidence from clinical and preclinical studies are reviewed, with emphasis on the functional outcomes of exercise, as well as on the putative cellular mechanisms that drive exercise-induced remodelling of the neuromuscular system. Continued investigation of the molecular mechanisms of exercise adaptation in DMD, SMA, and DM1 will assist in enhancing our understanding of the biology of these most prevalent NMDs. This information may also be useful for guiding the development of novel therapeutic targets for future pursuit.

Myelin abnormality in Charcot-Marie-Tooth type 4J recapitulates features of acquired demyelination

OBJECTIVE

Charcot-Marie-Tooth type 4J (CMT4J) is a rare autosomal recessive neuropathy caused by mutations in FIG4 that result in loss of FIG4 protein. This study investigates the natural history and mechanisms of segmental demyelination in CMT4J.

METHODS

Over the past 9 years, we have enrolled and studied a cohort of 12 CMT4J patients, including 6 novel FIG4 mutations. We evaluated these patients and related mouse models using morphological, electrophysiological, and biochemical approaches.

RESULTS

We found sensory motor demyelinating polyneuropathy consistently in all patients. This underlying myelin pathology was associated with nonuniform slowing of conduction velocities, conduction block, and temporal dispersion on nerve conduction studies, which resemble those features in acquired demyelinating peripheral nerve diseases. Segmental demyelination was also confirmed in mice without Fig4 (Fig4-/- ). The demyelination was associated with an increase of Schwann cell dedifferentiation and macrophages in spinal roots where nerve-blood barriers are weak. Schwann cell dedifferentiation was induced by the increasing intracellular Ca2+ . Suppression of Ca2+ level by a chelator reduced dedifferentiation and demyelination of Schwann cells in vitro and in vivo. Interestingly, cell-specific knockout of Fig4 in mouse Schwann cells or neurons failed to cause segmental demyelination.

INTERPRETATION

Myelin change in CMT4J recapitulates the features of acquired demyelinating neuropathies. This pathology is not Schwann cell autonomous. Instead, it relates to systemic processes involving interactions of multiple cell types and abnormally elevated intracellular Ca2+ . Injection of a Ca2+ chelator into Fig4-/- mice improved segmental demyelination, thereby providing a therapeutic strategy against demyelination. Ann Neurol 2018;83:756-770.