Pathophysiological concepts in the congenital myopathies: blurring the boundaries, sharpening the focus

Congenital myopathies

The congenital myopathies are inherited muscle disorders characterized clinically by hypotonia and weakness, usually from birth, with a static or slowly progressive clinical course. Historically, the congenital myopathies have been classified according to major morphological features seen on muscle biopsy as nemaline myopathy, central core disease, centronuclear or myotubular myopathy, and congenital fiber type disproportion. However, in the past two decades, the genetic basis of these different forms of congenital myopathy has been further elucidated with the result being improved correlation with histological and genetic characteristics. However, these notions have been challenged for three reasons. First, many of the congenital myopathies can be caused by mutations in more than one gene that suggests an impact of genetic heterogeneity. Second, mutations in the same gene can cause different muscle pathologies. Third, the same genetic mutation may lead to different pathological features in members of the same family or in the same individual at different ages. This chapter provides a clinical overview of the congenital myopathies and a clinically useful guide to its genetic basis recognizing the increasing reliance of exome, subexome, and genome sequencing studies as first-line analysis in many patients.

Common data elements and minimum data sets in cerebral palsy: Start small to grow big

This commentary is on the original article by Wilson et al. on pages 1470–1476 of this issue.

Novel autosomal dominant TPM3 mutation causes a combined congenital fibre type disproportion-cap disease histological pattern

Tropomyosin 3 (TPM3) gene mutations associate with autosomal dominant and recessive nemaline myopathy 1 (NEM1), congenital fiber type disproportion myopathy (CFTD) and cap myopathy (CAPM1), and a combination of caps and nemaline bodies. We report on a 47-year-old man with polyglobulia, restricted vital capacity and mild apnea hypopnea syndrome, requiring noninvasive ventilation. Physical assessment revealed bilateral ptosis and facial paresis, with high arched palate and retrognathia; global hypotonia and diffuse axial weakness, including neck and upper and lower limb girdle and foot dorsiflexion weakness. Whole body MRI showed a diffuse fatty replacement with an unspecific pattern. A 122 gene NGS neuromuscular disorders panel revealed the heterozygous VUS c.709G>A (p.Glu237Lys) on exon 8 of TMP3. A deltoid muscle biopsy showed a novel histological pattern combining fiber type disproportion and caps. Our findings support the pathogenicity of the novel TPM3 variant and widen the phenotypic gamut of TMP3-related congenital myopathy.

Severe ACTA1-related nemaline myopathy: intranuclear rods, cytoplasmic bodies, and enlarged perinuclear space as characteristic pathological features on muscle biopsies

Nemaline myopathy (NM) is a muscle disorder with broad clinical and genetic heterogeneity. The clinical presentation of affected individuals ranges from severe perinatal muscle weakness to milder childhood-onset forms, and the disease course and prognosis depends on the gene and mutation type. To date, 14 causative genes have been identified, and ACTA1 accounts for more than half of the severe NM cases. ACTA1 encodes α-actin, one of the principal components of the contractile units in skeletal muscle. We established a homogenous cohort of ten unreported families with severe NM, and we provide clinical, genetic, histological, and ultrastructural data. The patients manifested antenatal or neonatal muscle weakness requiring permanent respiratory assistance, and most deceased within the first months of life. DNA sequencing identified known or novel ACTA1 mutations in all. Morphological analyses of the muscle biopsy specimens showed characteristic features of NM histopathology including cytoplasmic and intranuclear rods, cytoplasmic bodies, and major myofibrillar disorganization. We also detected structural anomalies of the perinuclear space, emphasizing a physiological contribution of skeletal muscle α-actin to nuclear shape. In-depth investigations of the nuclei confirmed an abnormal localization of lamin A/C, Nesprin-1, and Nesprin-2, forming the main constituents of the nuclear lamina and the LINC complex and ensuring nuclear envelope integrity. To validate the relevance of our findings, we examined muscle samples from three previously reported ACTA1 cases, and we identified the same set of structural aberrations. Moreover, we measured an increased expression of cardiac α-actin in the muscle samples from the patients with longer lifespan, indicating a potential compensatory effect. Overall, this study expands the genetic and morphological spectrum of severe ACTA1-related nemaline myopathy, improves molecular diagnosis, highlights the enlargement of the perinuclear space as an ultrastructural hallmark, and indicates a potential genotype/phenotype correlation.

Natural history study and statistical modelling of disease progression in a preclinical model of myotubular myopathy

Generating reliable preclinical data in animal models of disease is essential in therapy development. Here we perform statistical analysis and joint longitudinal-survival modelling of the progressive phenotype observed in Mtm1-/y knock-out mice, a faithful model for myotubular myopathy (XLMTM). Analysis of historical data was used to generate a model for phenotype progression, which was then confirmed with phenotypic data from a new colony of mice derived via in vitro fertilization in an independent animal house, highlighting the reproducibility of disease phenotype in Mtm1-/y mice. This combined data was then used to refine the phenotypic parameters analyzed in these mice, and improve the model generated for expected disease progression. The disease progression model was then used to test therapeutic efficacy of Dnm2 targeting. Dnm2 reduction by antisense oligonucleotides blocked or postponed disease development, and resulted in a significant dose-dependent improvement outside the expected disease progression in untreated Mtm1-/y mice. This provides an example of optimizing disease analysis and testing therapeutic efficacy in a preclinical model, that can be applied by scientists testing therapeutic approaches using neuromuscular disease models in different laboratories.

Phenotypic Variability of MEGF10 Variants Causing Congenital Myopathy: Report of Two Unrelated Patients from a Highly Consanguineous Population

Congenital myopathies are rare neuromuscular hereditary disorders that manifest at birth or during infancy and usually appear with muscle weakness and hypotonia. One of such disorders, early-onset myopathy, areflexia, respiratory distress, and dysphagia (EMARDD, OMIM: 614399, MIM: 612453), is a rare autosomal recessive disorder caused by biallelic mutations (at homozygous or compound heterozygous status) in MEGF10 (multiple epidermal growth factor-like domains protein family). Here, we report two unrelated patients, who were born to consanguineous parents, having two novel MEGF10 deleterious variants. Interestingly, the presence of MEGF10 associated EMARDD has not been reported in Saudi Arabia, a highly consanguineous population. Moreover, both variants lead to a different phenotypic onset of mild and severe types. Our work expands phenotypic features of the disease and provides an opportunity for genetic counseling to the inflicted families.

Multi-omics comparisons of different forms of centronuclear myopathies and the effects of several therapeutic strategies

Omics analyses are powerful methods to obtain an integrated view of complex biological processes, disease progression, or therapy efficiency. However, few studies have compared different disease forms and different therapy strategies to define the common molecular signatures representing the most significant implicated pathways. In this study, we used RNA sequencing and mass spectrometry to profile the transcriptomes and proteomes of mouse models for three forms of centronuclear myopathies (CNMs), untreated or treated with either a drug (tamoxifen), antisense oligonucleotides reducing the level of dynamin 2 (DNM2), or following modulation of DNM2 or amphiphysin 2 (BIN1) through genetic crosses. Unsupervised analysis and differential gene and protein expression were performed to retrieve CNM molecular signatures. Longitudinal studies before, at, and after disease onset highlighted potential disease causes and consequences. Main pathways in the common CNM disease signature include muscle contraction, regeneration and inflammation. The common therapy signature revealed novel potential therapeutic targets, including the calcium regulator sarcolipin. We identified several novel biomarkers validated in muscle and/or plasma through RNA quantification, western blotting, and enzyme-linked immunosorbent assay (ELISA) assays, including ANXA2 and IGFBP2. This study validates the concept of using multi-omics approaches to identify molecular signatures common to different disease forms and therapeutic strategies.

Early Findings in Neonatal Cases of RYR1-Related Congenital Myopathies

Ryanodine receptor type 1-related congenital myopathies are the most represented subgroup among congenital myopathies (CMs), typically presenting a central core or multiminicore muscle histopathology and high clinical heterogeneity. We evaluated a cohort of patients affected with Ryanodine receptor type 1-related congenital myopathy (RYR1-RCM), focusing on four patients who showed a severe congenital phenotype and underwent a comprehensive characterization at few months of life. To date there are few reports on precocious instrumental assessment. In two out of the four patients, a muscle biopsy was performed in the first days of life (day 5 and 37, respectively) and electron microscopy was carried out in two patients detecting typical features of congenital myopathy. Two patients underwent brain MRI in the first months of life (15 days and 2 months, respectively), one also a fetal brain MRI. In three children electromyography was performed in the first week of life and neurogenic signs were excluded. Muscle MRI obtained within the first years of life showed a typical pattern of RYR1-CM. The diagnosis was confirmed through genetic analysis in three out of four cases using Next Generation Sequencing (NGS) panels. The development of a correct and rapid diagnosis is a priority and may lead to prompt medical management and helps optimize inclusion in future clinical trials.

Facilitations and Hurdles of Genetic Testing in Neuromuscular Disorders

Neuromuscular disorders (NMDs) comprise a heterogeneous group of disorders that affect about one in every thousand individuals worldwide. The vast majority of NMDs has a genetic cause, with about 600 genes already identified. Application of genetic testing in NMDs can be useful for several reasons: correct diagnostic definition of a proband, extensive familial counselling to identify subjects at risk, and prenatal diagnosis to prevent the recurrence of the disease; furthermore, identification of specific genetic mutations still remains mandatory in some cases for clinical trial enrollment where new gene therapies are now approaching. Even though genetic analysis is catching on in the neuromuscular field, pitfalls and hurdles still remain and they should be taken into account by clinicians, as for example the use of next generation sequencing (NGS) where many single nucleotide variants of “unknown significance” can emerge, complicating the correct interpretation of genotype-phenotype relationship. Finally, when all efforts in terms of molecular analysis have been carried on, a portion of patients affected by NMDs still remain “not genetically defined”. In the present review we analyze the evolution of genetic techniques, from Sanger sequencing to NGS, and we discuss “facilitations and hurdles” of genetic testing which must always be balanced by clinicians, in order to ensure a correct diagnostic definition, but taking always into account the benefit that the patient could obtain especially in terms of “therapeutic offer”.

Facilitations and Hurdles of Genetic Testing in Neuromuscular Disorders

Neuromuscular disorders (NMDs) comprise a heterogeneous group of disorders that affect about one in every thousand individuals worldwide. The vast majority of NMDs has a genetic cause, with about 600 genes already identified. Application of genetic testing in NMDs can be useful for several reasons: correct diagnostic definition of a proband, extensive familial counselling to identify subjects at risk, and prenatal diagnosis to prevent the recurrence of the disease; furthermore, identification of specific genetic mutations still remains mandatory in some cases for clinical trial enrollment where new gene therapies are now approaching. Even though genetic analysis is catching on in the neuromuscular field, pitfalls and hurdles still remain and they should be taken into account by clinicians, as for example the use of next generation sequencing (NGS) where many single nucleotide variants of "unknown significance" can emerge, complicating the correct interpretation of genotype-phenotype relationship. Finally, when all efforts in terms of molecular analysis have been carried on, a portion of patients affected by NMDs still remain "not genetically defined". In the present review we analyze the evolution of genetic techniques, from Sanger sequencing to NGS, and we discuss "facilitations and hurdles" of genetic testing which must always be balanced by clinicians, in order to ensure a correct diagnostic definition, but taking always into account the benefit that the patient could obtain especially in terms of "therapeutic offer".

L-Carnitine ameliorates congenital myopathy in a tropomyosin 3 de novo mutation transgenic zebrafish

Background

Congenital myopathy (CM) is a group of clinically and genetically heterogeneous muscle disorders, characterized by muscle weakness and hypotonia from birth. Currently, no definite treatment exists for CM. A de novo mutation in Tropomyosin 3-TPM3(E151G) was identified from a boy diagnosed with CM, previously TPM3(E151A) was reported to cause CM. However, the role of TPM3(E151G) in CM is unknown.

Methods

Histopathological, swimming behavior, and muscle endurance were monitored in TPM3 wild-type and mutant transgenic fish, modelling CM. Gene expression profiling of muscle of the transgenic fish were studied through RNAseq, and mitochondria respiration was investigated.

Results

While TPM3(WT) and TPM3(E151A) fish show normal appearance, amazingly a few TPM3(E151G) fish display either no tail, a crooked body in both F0 and F1 adults. Using histochemical staining for the muscle biopsy, we found TPM3(E151G) displays congenital fiber type disproportion and TPM3(E151A) resembles nemaline myopathy. TPM3(E151G) transgenic fish dramatically swimming slower than those in TPM3(WT) and TPM3(E151A) fish measured by DanioVision and T-maze, and exhibit weaker muscle endurance by swimming tunnel instrument. Interestingly, l-carnitine treatment on TPM3(E151G) transgenic larvae significantly improves the muscle endurance by restoring the basal respiration and ATP levels in mitochondria. With RNAseq transcriptomic analysis of the expression profiling from the muscle specimens, it surprisingly discloses large downregulation of genes involved in pathways of sodium, potassium, and calcium channels, which can be rescued by l-carnitine treatment, fatty acid metabolism was differentially dysregulated in TPM3(E151G) fish and rescued by l-carnitine treatment.

Conclusions

These results demonstrate that TPM3(E151G) and TPM3(E151A) exhibit different pathogenicity, also have distinct gene regulatory profiles but the ion channels were downregulated in both mutants, and provides a potential mechanism of action of TPM3 pathophysiology. Our results shed a new light in the future development of potential treatment for TPM3-related CM.

Core myopathies - a short review

Congenital myopathies represent a clinically and genetically heterogeneous group of early-onset neuromuscular diseases with characteristic, but not always specific, histopathological features, often presenting with stable and/or slowly progressive truncal and proximal weakness. It is often not possible to have a diagnosis on clinical ground alone. Additional extraocular, respiratory, distal involvement, scoliosis, and distal laxity may provide clues. The "core myopathies" collectively represent the most common form of congenital myopathies, and the name pathologically corresponds to histochemical appearance of focally reduced oxidative enzyme activity and myofibrillar changes on ultrastructural studies. Because of the clinical, pathological, and molecular overlaps, central core disease and multiminicore disease will be discussed together.

Speech treatment in nemaline myopathy: A single-subject experimental study

PURPOSE

The objective of this work was to verify the efficacy of a treatment based on myofunctional therapy techniques which aimed to improve the tongue strength, precision, and speed of a ten-year-old girl with nemaline myopathy (NM) and the repercussions of this therapy on her speech intelligibility. NM is a rare congenital muscle disorder that causes extreme muscle weakness, especially in the face and neck, as well as severe dysarthria and dysphagia, although this does not affect the nervous system or cognitive development.

METHOD

This was a single-subject experimental study which used an interrupted pre- and post-treatment time-series design, and which applied autoregressive integrated moving-average predictive models and Holt exponential smoothing. During the treatment phases, the participant's tongue strength and the rate of speech diadochokinesia and voluntary lingual movements were estimated and the repercussions of the intervention in terms of speech intelligibility were ascertained via an experiment with 'naïve' judges.

RESULTS

The treatment produced a sustained and significant increase in the maximum strength of the patient's tongue, which increased from an initial 4 kPa to 11 kPa at the end of the treatment phase. However, this was far from the average 58 kPa for age- and sex-matched normative data. There were no significant changes either in the rates of voluntary lingual mobility or speech diadochokinesia. Speech intelligibility, as assessed by naïve judges, improved from 40 % in the pre-treatment phase to 67 % in the post-treatment phase.

CONCLUSIONS

NM and other rare primary muscle disorders allow us to estimate the effects of severe muscle weakness in people with dysarthria without cognitive impairment or alterations in central nervous system, peripheral nervous system or in gap junction. In this case, the treatment did not increase the patient's lingual and articulatory movement speed but did increase her tongue strength from 5 % to 10 % of the levels otherwise expected for her age and significantly improved the intelligibility of her speech and communication.

The Impact of Next-Generation Sequencing on the Diagnosis, Treatment, and Prevention of Hereditary Neuromuscular Disorders

The impact of high-throughput sequencing in genetic neuromuscular disorders cannot be overstated. The ability to rapidly and affordably sequence multiple genes simultaneously has enabled a second golden age of Mendelian disease gene discovery, with flow-on impacts for rapid genetic diagnosis, evidence-based treatment, tailored therapy development, carrier-screening, and prevention of disease recurrence in families. However, there are likely many more neuromuscular disease genes and mechanisms to be discovered. Many patients and families remain without a molecular diagnosis following targeted panel sequencing, clinical exome sequencing, or even genome sequencing. Here we review how massively parallel, or next-generation, sequencing has changed the field of genetic neuromuscular disorders, and anticipate future benefits of recent technological innovations such as RNA-seq implementation and detection of tandem repeat expansions from short-read sequencing.

Myostatin: a Circulating Biomarker Correlating with Disease in Myotubular Myopathy Mice and Patients

Myotubular myopathy, also called X-linked centronuclear myopathy (XL-CNM), is a severe congenital disease targeted for therapeutic trials. To date, biomarkers to monitor disease progression and therapy efficacy are lacking. The Mtm1 ^-/y mouse is a faithful model for XL-CNM, due to myotubularin 1 (MTM1) loss-of-function mutations. Using both an unbiased approach (RNA sequencing [RNA-seq]) and a directed approach (qRT-PCR and protein level), we identified decreased Mstn levels in Mtm1 ^-/y muscle, leading to low levels of myostatin in muscle and plasma. Myostatin (Mstn or growth differentiation factor 8 [Gdf8]) is a protein released by myocytes and inhibiting muscle growth and differentiation. Decreasing Dnm2 by genetic cross with Dnm2 ^+/- mice or by antisense oligonucleotides blocked or postponed disease progression and resulted in an increase in circulating myostatin. In addition, plasma myostatin levels inversely correlated with disease severity and with Dnm2 mRNA levels in muscles. Altered Mstn levels were associated with a generalized disruption of the myostatin pathway. Importantly, in two different forms of CNMs we identified reduced circulating myostatin levels in plasma from patients. This provides evidence of a blood-based biomarker that may be used to monitor disease state in XL-CNM mice and patients and supports monitoring circulating myostatin during clinical trials for myotubular myopathy.

Update on Congenital Myopathies in Adulthood

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

Congenital myopathies: an update

Congenital myopathies comprise a clinical, histopathological, and genetic heterogeneous group of rare hereditary muscle diseases that are defined by architectural abnormalities in the muscle fibres. They are subdivided by the predominant structural pathological change on muscle biopsy, resulting in five subgroups: (1) core myopathies; (2) nemaline myopathies; (3) centronuclear myopathies; (4) congenital fibre type disproportion myopathy; and (5) myosin storage myopathy. Besides the clinical features, muscle biopsy, muscle imaging, and genetic analyses are essential in the diagnosis of congenital myopathies. Using next-generation sequencing techniques, a large number of new genes are being identified as the cause of congenital myopathies as well as new mutations in known genes, broadening the phenotype-genotype spectrum of congenital myopathies. Management is performed by a multidisciplinary team specialized in neuromuscular disorders, where the (paediatric) neurologist has an essential role. To date, only supportive treatment is available, but novel pathomechanisms are being discovered and gene therapies are being explored. WHAT THIS PAPER ADDS: Many new genes are being identified in congenital myopathies, broadening the phenotype-genotype spectrum. Management is performed by a multidisciplinary team specialized in neuromuscular disorders.

ASC-1 Is a Cell Cycle Regulator Associated with Severe and Mild Forms of Myopathy

OBJECTIVE

Recently, the ASC-1 complex has been identified as a mechanistic link between amyotrophic lateral sclerosis and spinal muscular atrophy (SMA), and 3 mutations of the ASC-1 gene TRIP4 have been associated with SMA or congenital myopathy. Our goal was to define ASC-1 neuromuscular function and the phenotypical spectrum associated with TRIP4 mutations.

METHODS

Clinical, molecular, histological, and magnetic resonance imaging studies were made in 5 families with 7 novel TRIP4 mutations. Fluorescence activated cell sorting and Western blot were performed in patient-derived fibroblasts and muscles and in Trip4 knocked-down C2C12 cells.

RESULTS

All mutations caused ASC-1 protein depletion. The clinical phenotype was purely myopathic, ranging from lethal neonatal to mild ambulatory adult patients. It included early onset axial and proximal weakness, scoliosis, rigid spine, dysmorphic facies, cutaneous involvement, respiratory failure, and in the older cases, dilated cardiomyopathy. Muscle biopsies showed multiminicores, nemaline rods, cytoplasmic bodies, caps, central nuclei, rimmed fibers, and/or mild endomysial fibrosis. ASC-1 depletion in C2C12 and in patient-derived fibroblasts and muscles caused accelerated proliferation, altered expression of cell cycle proteins, and/or shortening of the G0/G1 cell cycle phase leading to cell size reduction.

INTERPRETATION

Our results expand the phenotypical and molecular spectrum of TRIP4-associated disease to include mild adult forms with or without cardiomyopathy, associate ASC-1 depletion with isolated primary muscle involvement, and establish TRIP4 as a causative gene for several congenital muscle diseases, including nemaline, core, centronuclear, and cytoplasmic-body myopathies. They also identify ASC-1 as a novel cell cycle regulator with a key role in cell proliferation, and underline transcriptional coregulation defects as a novel pathophysiological mechanism. ANN NEUROL 2020;87:217-232.

KLHL40 Mutation Associated with Severe Nemaline Myopathy, Fetal Akinesia, and Cleft Palate

The congenital myopathies are a heterogeneous group of inherited neuromuscular disorders characterized by early-onset muscular weakness, hypotonia, and developmental delay. The congenital myopathies are further classified into centranuclear myopathies, nemaline myopathies, core myopathies, and congenital fiber-type disproportion based on major pathological features found in muscle biopsies. There is no evidence at present to show that cleft palate is associated with severe nemaline myopathies and fetal akinesia. The results of our study suggest that KLHL40 mutation associated with Cleft palate, nemaline myopathy and fetal akinesia.

Novel mutations in MYBPC1 are associated with myogenic tremor and mild myopathy

OBJECTIVE

To define a distinct, dominantly inherited, mild skeletal myopathy associated with prominent and consistent tremor in two unrelated, three-generation families.

METHODS

Clinical evaluations as well as exome and panel sequencing analyses were performed in affected and nonaffected members of two families to identify genetic variants segregating with the phenotype. Histological assessment of a muscle biopsy specimen was performed in 1 patient, and quantitative tremor analysis was carried out in 2 patients. Molecular modeling studies and biochemical assays were performed for both mutations.

RESULTS

Two novel missense mutations in MYBPC1 (p.E248K in family 1 and p.Y247H in family 2) were identified and shown to segregate perfectly with the myopathy/tremor phenotype in the respective families. MYBPC1 encodes slow myosin binding protein-C (sMyBP-C), a modular sarcomeric protein playing structural and regulatory roles through its dynamic interaction with actin and myosin filaments. The Y247H and E248K mutations are located in the NH₂ -terminal M-motif of sMyBP-C. Both mutations result in markedly increased binding of the NH₂ terminus to myosin, possibly interfering with normal cross-bridge cycling as the first muscle-based step in tremor genesis. The clinical tremor features observed in all mutation carriers, together with the tremor physiology studies performed in family 2, suggest amplification by an additional central loop modulating the clinical tremor phenomenology.

INTERPRETATION

Here, we link two novel missense mutations in MYBPC1 with a dominant, mild skeletal myopathy invariably associated with a distinctive tremor. The molecular, genetic, and clinical studies are consistent with a unique sarcomeric origin of the tremor, which we classify as "myogenic tremor." ANN NEUROL 2019.

Bi-allelic mutations in MYL1 cause a severe congenital myopathy

Congenital myopathies are typically characterised by early onset hypotonia, weakness and hallmark features on biopsy. Despite the rapid pace of gene discovery, ∼50% of patients with a congenital myopathy remain without a genetic diagnosis following screening of known disease genes. We performed exome sequencing on two consanguineous probands diagnosed with a congenital myopathy and muscle biopsy showing selective atrophy/hypotrophy or absence of type II myofibres. We identified variants in the gene (MYL1) encoding the skeletal muscle fast-twitch specific myosin essential light chain (ELC) in both probands. A homozygous essential splice acceptor variant (c.479-2A > G, predicted to result in skipping of exon 5 was identified in Proband 1, and a homozygous missense substitution (c.488T>G, p.(Met163Arg)) was identified in Proband 2. Protein modelling of the p.(Met163Arg) substitution predicted it might impede intermolecular interactions that facilitate binding to the IQ domain of myosin heavy chain, thus likely impacting on the structure and functioning of the myosin motor. MYL1 was markedly reduced in skeletal muscle from both probands, suggesting that the missense substitution likely results in an unstable protein. Knock down of myl1 in zebrafish resulted in abnormal morphology, disrupted muscle structure and impaired touch-evoked escape responses, thus confirming that skeletal muscle fast-twitch specific myosin ELC is critical for myofibre development and function. Our data implicate MYL1 as a crucial protein for adequate skeletal muscle function and that MYL1 deficiency is associated with severe congenital myopathy.

The Use of Muscle Ultrasound in the Diagnosis and Differential Diagnosis of Congenital Disorders of Muscle in the Age of Next Generation Genetics

Congenital disorders of muscle most importantly encompass the congenital muscular dystrophies as well as the congenital myopathies. With the broader availability of next generation genetic testing there has been an expansion of phenotypes and genotypes, while the very large genes such as titin, nebulin, and RYR1 have also become accessible to complete sequencing. This development has had considerable diagnostic power while at the same time also creating challenges in the interpretation of the many variants of uncertain significance that will need a solid clinical plausibility test, based on "deep" phenotyping, taking into account clinical, extended clinical, histological, and physiological data. One tool in this context is imaging of skeletal muscle, including by ultrasound. Muscle ultrasound is a useful, noninvasive, child-friendly technique for visualizing normal and pathological skeletal muscle. By virtue of its different mode of image acquisition compared to muscle MRI, it allows for the assessment of different and often earlier changes, also circumventing the need for sedation. Herein we highlight the important role of muscle ultrasound as a diagnostic tool and an extension of the physical exam in the work-up of congenital onset muscle disease, presenting various relevant clinical scenarios. We show how muscle ultrasound can confirm or refute skeletal muscle involvement and yield information about the nature of the involvement (myopathic vs neurogenic). Muscle ultrasound can also guide the appropriate next diagnostic steps and recognize diagnostically important qualitative patterns to help confirm or refute genetic considerations raised by next generation sequencing. We illustrate specific muscle ultrasound involvement patterns, which constitute accessible diagnostic hints and show that muscle ultrasound, in conjunction with the clinical phenotype, the histological appearance of the muscle biopsy (when available), and the ascertained genotype, can be a very powerful tool in integrating all available information into a final accurate and precise diagnosis in the age of next generation sequencing.

Myopathology of Congenital Myopathies: Bridging the Old and the New

Congenital myopathies (CM) are a genetically heterogeneous group of neuromuscular disorders most commonly presenting with neonatal/childhood-onset hypotonia and muscle weakness, a relatively static or slowly progressive disease course, and originally classified into subcategories based on characteristic histopathologic findings in muscle biopsies. This enduring concept of disease definition and classification based on the clinicopathologic phenotype was pioneered in the premolecular era. Advances in molecular genetics have brought into focus the increased blurring of the original seemingly "watertight" categories through broadening of the clinical phenotypes in existing genes, and continuous identification of novel genetic backgrounds. This review summarizes the histopathologic landscape of the 4 "classical" subtypes of CM-nemaline myopathies, core myopathies, centronuclear myopathies, and congenital fiber type disproportion and some of the emerging and novel genetic diseases with a CM presentation.

Modulation of autophagy in human diseases strategies to foster strengths and circumvent weaknesses

Autophagy is central to the maintenance of intracellular homeostasis across species. Accordingly, autophagy disorders are linked to a variety of diseases from the embryonic stage until death, and the role of autophagy as a therapeutic target has been widely recognized. However, autophagy-associated therapy for human diseases is still in its infancy and is supported by limited evidence. In this review, we summarize the landscape of autophagy-associated diseases and current autophagy modulators. Furthermore, we investigate the existing autophagy-associated clinical trials, analyze the obstacles that limit their progress, offer tactics that may allow barriers to be overcome along the way and then discuss the therapeutic potential of autophagy modulators in clinical applications.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

Recent advances in understanding congenital myopathies

By definition, congenital myopathy typically presents with skeletal muscle weakness and hypotonia at birth. Traditionally, congenital myopathy subtypes have been predominantly distinguished on the basis of the pathological hallmarks present on skeletal muscle biopsies. Many genes cause congenital myopathies when mutated, and a burst of new causative genes have been identified because of advances in gene sequencing technology. Recent discoveries include extending the disease phenotypes associated with previously identified genes and determining that genes formerly known to cause only dominant disease can also cause recessive disease. The more recently identified congenital myopathy genes account for only a small proportion of patients. Thus, the congenital myopathy genes remaining to be discovered are predicted to be extremely rare causes of disease, which greatly hampers their identification. Significant progress in the provision of molecular diagnoses brings important information and value to patients and their families, such as possible disease prognosis, better disease management, and informed reproductive choice, including carrier screening of parents. Additionally, from accurate genetic knowledge, rational treatment options can be hypothesised and subsequently evaluated in vitro and in animal models. A wide range of potential congenital myopathy therapies have been investigated on the basis of improved understanding of disease pathomechanisms, and some therapies are in clinical trials. Although large hurdles remain, promise exists for translating treatment benefits from preclinical models to patients with congenital myopathy, including harnessing proven successes for other genetic diseases.

The genetics of congenital myopathies

Congenital myopathies are a clinically and genetically heterogeneous group of conditions that most commonly present at or around the time of birth with hypotonia, muscle weakness, and (often) respiratory distress. Historically, this group of disorders has been subclassified based on muscle histopathologic characteristics. There has been an explosion of gene discovery, and there are now at least 32 different genetic causes of disease. With this increased understanding of the genetic basis of disease has come the knowledge that the mutations in congenital myopathy genes can present with a wide variety of clinical phenotypes and can result in a broad spectrum of histopathologic findings on muscle biopsy. In addition, mutations in several genes can share the same histopathologic features. The identification of new genes and interpretation of different pathomechanisms at a molecular level have helped us to understand the clinical and histopathologic similarities that this group of disorders share. In this review, we highlight the genetic understanding for each subtype, its pathogenesis, and the future key issues in congenital myopathies.

Genetics of neuromuscular fetal akinesia in the genomics era

Fetal hypokinesia or akinesia encompasses a broad spectrum of disorders, united by impaired movement in utero. Often, the underlying aetiology is genetic in origin, affecting part of the neuromuscular system. The affordable and high-throughput nature of next-generation DNA sequencing has led to an explosion in disease gene discovery across rare diseases, including fetal akinesias. A genetic diagnosis has clinical utility as it may affect management and prognosis and informs recurrence risk, facilitating family planning decisions. More broadly, knowledge of disease genes increasingly allows population-based preconception carrier screening, which has reduced the incidence of recessive diseases in several populations. Despite gains in knowledge of the genetics of fetal akinesia, many families lack a genetic diagnosis. In this review, we describe the developments in Mendelian genetics of neuromuscular fetal akinesia in the genomics era. We examine genetic diagnoses with neuromuscular causes, specifically including the lower motor neuron, peripheral nerve, neuromuscular junction and muscle.

Characterization of congenital myopathies at a Korean neuromuscular center

INTRODUCTION

Congenital myopathies are muscle diseases characterized by specific histopathologic features, generalized hypotonia from birth, and perinatal complications, although some cases develop during childhood or, rarely, in adulthood. We undertook this study to characterize congenital myopathies among patients registered at our institution.

METHODS

Clinical, histopathologic, and genetic features were evaluated in 34 patients recruited for this study.

RESULTS

The majority of patients experienced a childhood onset, and no disease-related mortality was recorded during follow-up. Functional outcomes were no better for those with late-onset disease, indicating later disease progression can be significant. Nemaline myopathy was the most frequent pathology, followed by central core disease and centronuclear myopathy. Among the 18 (54.5%) genetically confirmed patients, NEB and RYR1 mutations were the most common, followed by DNM2 mutations.

DISCUSSION

This study shows features not previously reported and suggests that congenital myopathy should be considered an important issue among adult patients. Muscle Nerve 58: 235-244, 2018.

Gonorazky H, Cohn RD, Campbell C. Treating pediatric neuromuscular disorders: The future is now

Pediatric neuromuscular diseases encompass all disorders with onset in childhood and where the primary area of pathology is in the peripheral nervous system. These conditions are largely genetic in etiology, and only those with a genetic underpinning will be presented in this review. This includes disorders of the anterior horn cell (e.g., spinal muscular atrophy), peripheral nerve (e.g., Charcot-Marie-Tooth disease), the neuromuscular junction (e.g., congenital myasthenic syndrome), and the muscle (myopathies and muscular dystrophies). Historically, pediatric neuromuscular disorders have uniformly been considered to be without treatment possibilities and to have dire prognoses. This perception has gradually changed, starting in part with the discovery and widespread application of corticosteroids for Duchenne muscular dystrophy. At present, several exciting therapeutic avenues are under investigation for a range of conditions, offering the potential for significant improvements in patient morbidities and mortality and, in some cases, curative intervention. In this review, we will present the current state of treatment for the most common pediatric neuromuscular conditions, and detail the treatment strategies with the greatest potential for helping with these devastating diseases.

An Overview of Congenital Myopathies

PURPOSE OF REVIEW

This article uses a case-based approach to highlight the clinical features as well as recent advances in molecular genetics, muscle imaging, and pathophysiology of the congenital myopathies.

RECENT FINDINGS

Congenital myopathies refer to a heterogeneous group of genetic neuromuscular disorders characterized by early-onset muscle weakness, hypotonia, and developmental delay. Congenital myopathies are further classified into core myopathies, centronuclear myopathies, nemaline myopathies, and congenital fiber-type disproportion based on the key pathologic features found in muscle biopsies. Genotype and phenotype correlations are hampered by the diverse clinical variability of the genes responsible for congenital myopathies, ranging from a severe neonatal course with early death to mildly affected adults with late-onset disease. An increasing number of genes have been identified, which, in turn, are associated with overlapping morphologic changes in the myofibers. Precise genetic diagnosis has important implications for disease management, including family counseling; avoidance of anesthetic-related muscle injury for at-risk individuals; monitoring for potential cardiac, respiratory, or orthopedic complications; as well as for participation in clinical trials or potential genetic therapies.

SUMMARY

Collaboration with neuromuscular experts, geneticists, neuroradiologists, neuropathologists, and other specialists is needed to ensure accurate and timely diagnosis based on clinical and pathologic features. An integrated multidisciplinary model of care based on expert-guided standards will improve quality of care and optimize outcomes for patients and families with congenital myopathies.

The histone code reader Spin1 controls skeletal muscle development

While several studies correlated increased expression of the histone code reader Spin1 with tumor formation or growth, little is known about physiological functions of the protein. We generated Spin1^M5 mice with ablation of Spin1 in myoblast precursors using the Myf5-Cre deleter strain. Most Spin1^M5 mice die shortly after birth displaying severe sarcomere disorganization and necrosis. Surviving Spin1^M5 mice are growth-retarded and exhibit the most prominent defects in soleus, tibialis anterior, and diaphragm muscle. Transcriptome analyses of limb muscle at embryonic day (E) 15.5, E16.5, and at three weeks of age provided evidence for aberrant fetal myogenesis and identified deregulated skeletal muscle (SkM) functional networks. Determination of genome-wide chromatin occupancy in primary myoblast revealed direct Spin1 target genes and suggested that deregulated basic helix-loop-helix transcription factor networks account for developmental defects in Spin1^M5 fetuses. Furthermore, correlating histological and transcriptome analyses, we show that aberrant expression of titin-associated proteins, abnormal glycogen metabolism, and neuromuscular junction defects contribute to SkM pathology in Spin1^M5 mice. Together, we describe the first example of a histone code reader controlling SkM development in mice, which hints at Spin1 as a potential player in human SkM disease.

Congenital myopathies: clinical phenotypes and new diagnostic tools

Congenital myopathies are a group of genetic muscle disorders characterized clinically by hypotonia and weakness, usually from birth, and a static or slowly progressive clinical course. Historically, congenital myopathies have been classified on the basis of major morphological features seen on muscle biopsy. However, different genes have now been identified as associated with the various phenotypic and histological expressions of these disorders, and in recent years, because of their unexpectedly wide genetic and clinical heterogeneity, next-generation sequencing has increasingly been used for their diagnosis. We reviewed clinical and genetic forms of congenital myopathy and defined possible strategies to improve cost-effectiveness in histological and imaging diagnosis.

Amphiphysin (BIN1) negatively regulates dynamin 2 for normal muscle maturation

Regulation of skeletal muscle development and organization is a complex process that is not fully understood. Here, we focused on amphiphysin 2 (BIN1, also known as bridging integrator-1) and dynamin 2 (DNM2), two ubiquitous proteins implicated in membrane remodeling and mutated in centronuclear myopathies (CNMs). We generated Bin1-/- Dnm2+/- mice to decipher the physiological interplay between BIN1 and DNM2. While Bin1-/- mice die perinatally from a skeletal muscle defect, Bin1-/- Dnm2+/- mice survived at least 18 months, and had normal muscle force and intracellular organization of muscle fibers, supporting BIN1 as a negative regulator of DNM2. We next characterized muscle-specific isoforms of BIN1 and DNM2. While BIN1 colocalized with and partially inhibited DNM2 activity during muscle maturation, BIN1 had no effect on the isoform of DNM2 found in adult muscle. Together, these results indicate that BIN1 and DNM2 regulate muscle development and organization, function through a common pathway, and define BIN1 as a negative regulator of DNM2 in vitro and in vivo during muscle maturation. Our data suggest that DNM2 modulation has potential as a therapeutic approach for patients with CNM and BIN1 defects. As BIN1 is implicated in cancers, arrhythmia, and late-onset Alzheimer disease, these findings may trigger research directions and therapeutic development for these common diseases.

A defect in myoblast fusion underlies Carey-Fineman-Ziter syndrome

Multinucleate cellular syncytial formation is a hallmark of skeletal muscle differentiation. Myomaker, encoded by Mymk (Tmem8c), is a well-conserved plasma membrane protein required for myoblast fusion to form multinucleated myotubes in mouse, chick, and zebrafish. Here, we report that autosomal recessive mutations in MYMK (OMIM 615345) cause Carey-Fineman-Ziter syndrome in humans (CFZS; OMIM 254940) by reducing but not eliminating MYMK function. We characterize MYMK-CFZS as a congenital myopathy with marked facial weakness and additional clinical and pathologic features that distinguish it from other congenital neuromuscular syndromes. We show that a heterologous cell fusion assay in vitro and allelic complementation experiments in mymk knockdown and mymk^insT/insT zebrafish in vivo can differentiate between MYMK wild type, hypomorphic and null alleles. Collectively, these data establish that MYMK activity is necessary for normal muscle development and maintenance in humans, and expand the spectrum of congenital myopathies to include cell-cell fusion deficits.

Antisense oligonucleotide-mediated Dnm2 knockdown prevents and reverts myotubular myopathy in mice

Centronuclear myopathies (CNM) are non-dystrophic muscle diseases for which no effective therapy is currently available. The most severe form, X-linked CNM, is caused by myotubularin 1 (MTM1) loss-of-function mutations, while the main autosomal dominant form is due to dynamin2 (DNM2) mutations. We previously showed that genetic reduction of DNM2 expression in Mtm1 knockout (Mtm1KO) mice prevents development of muscle pathology. Here we show that systemic delivery of Dnm2 antisense oligonucleotides (ASOs) into Mtm1KO mice efficiently reduces DNM2 protein level in muscle and prevents the myopathy from developing. Moreover, systemic ASO injection into severely affected mice leads to reversal of muscle pathology within 2 weeks. Thus, ASO-mediated DNM2 knockdown can efficiently correct muscle defects due to loss of MTM1, providing an attractive therapeutic strategy for this disease.

Recessive MYPN mutations cause cap myopathy with occasional nemaline rods

Congenital myopathies are phenotypically and genetically heterogeneous. We describe homozygous truncating mutations in MYPN in 2 unrelated families with a slowly progressive congenital cap myopathy. MYPN encodes the Z-line protein myopalladin implicated in sarcomere integrity. Functional experiments demonstrate that the mutations lead to mRNA defects and to a strong reduction in full-length protein expression. Myopalladin signals accumulate in the caps together with alpha-actinin. Dominant MYPN mutations were previously reported in cardiomyopathies. Our data uncover that mutations in MYPN cause either a cardiac or a congenital skeletal muscle disorder through different modes of inheritance. Ann Neurol 2017;81:467-473.

Why Is there a Limit to the Changes in Myofilament Ca2+-Sensitivity Associated with Myopathy Causing Mutations?

Mutations in striated muscle contractile proteins have been found to be the cause of a number of inherited muscle diseases; in most cases the mechanism proposed for causing the disease is derangement of the thin filament-based Ca²⁺-regulatory system of the muscle. When considering the results of experiments reported over the last 15 years, one feature has been frequently noted, but rarely discussed: the magnitude of changes in myofilament Ca²⁺-sensitivity due to myopathy-causing mutations in skeletal or heart muscle seems to be always in the range 1.5-3x EC₅₀. Such consistency suggests it may be related to a fundamental property of muscle regulation; in this article we will investigate whether this observation is true and consider why this should be so. A literature search found 71 independent measurements of HCM mutation-induced change of EC₅₀ ranging from 1.15 to 3.8-fold with a mean of 1.87 ± 0.07 (sem). We also found 11 independent measurements of increased Ca²⁺-sensitivity due to mutations in skeletal muscle proteins ranging from 1.19 to 2.7-fold with a mean of 2.00 ± 0.16. Investigation of dilated cardiomyopathy-related mutations found 42 independent determinations with a range of EC₅₀ wt/mutant from 0.3 to 2.3. In addition we found 14 measurements of Ca²⁺-sensitivity changes due skeletal muscle myopathy mutations ranging from 0.39 to 0.63. Thus, our extensive literature search, although not necessarily complete, found that, indeed, the changes in myofilament Ca²⁺-sensitivity due to disease-causing mutations have a bimodal distribution and that the overall changes in Ca²⁺-sensitivity are quite small and do not extend beyond a three-fold increase or decrease in Ca²⁺-sensitivity. We discuss two mechanism that are not necessarily mutually exclusive. Firstly, it could be that the limit is set by the capabilities of the excitation-contraction machinery that supplies activating Ca²⁺ and that striated muscle cannot work in a way compatible with life outside these limits; or it may be due to a fundamental property of the troponin system and the permitted conformational transitions compatible with efficient regulation.

Muscle dysfunction caused by loss of Magel2 in a mouse model of Prader-Willi and Schaaf-Yang syndromes

Prader-Willi syndrome is characterized by severe hypotonia in infancy, with decreased lean mass and increased fat mass in childhood followed by severe hyperphagia and consequent obesity. Scoliosis and other orthopaedic manifestations of hypotonia are common in children with Prader-Willi syndrome and cause significant morbidity. The relationships among hypotonia, reduced muscle mass and scoliosis have been difficult to establish. Inactivating mutations in one Prader-Willi syndrome candidate gene, MAGEL2, cause a Prader-Willi-like syndrome called Schaaf-Yang syndrome, highlighting the importance of loss of MAGEL2 in Prader-Willi syndrome phenotypes. Gene-targeted mice lacking Magel2 have excess fat and decreased muscle, recapitulating altered body composition in Prader-Willi syndrome. We now demonstrate that Magel2 is expressed in the developing musculoskeletal system, and that loss of Magel2 causes muscle-related phenotypes in mice consistent with atrophy caused by altered autophagy. Magel2-null mice serve as a preclinical model for therapies targeting muscle structure and function in children lacking MAGEL2 diagnosed with Prader-Willi or Schaaf-Yang syndrome.