Congenital myopathies: an update

Biallelic variants in RYR1 and STAC3 are predominant causes of King-Denborough Syndrome in an African cohort

King-Denborough Syndrome (KDS) is a congenital myopathy (CM) characterised by myopathy, dysmorphic features and susceptibility to malignant hyperthermia. The objective of this study was to investigate the genotype-phenotype correlation in Black African patients presenting with CM, specifically those with KDS-like phenotypes, who remained undiagnosed for over 25 years. A cohort of 67 Black African patients with CM was studied, of whom 44 were clinically evaluated and diagnosed with KDS. Whole-exome sequencing (WES) was performed as part of an international genomics study (ICGNMD) to identify potential pathogenic mutations. Genomic assessments focused on identifying relevant genes, including RYR1 and STAC3, and establishing genotype-phenotype correlations. The study identified RYR1 and STAC3 mutations as the predominant genetic causes of KDS in this cohort, with mutations in both genes exhibiting autosomal recessive inheritance. While RYR1 has previously been linked to autosomal dominant mutations, STAC3, which was formerly associated exclusively with Native American Myopathy/Bailey-Bloch Myopathy, congenital hypotonia, and susceptibility to malignant hyperthermia, is now newly associated with CM-KDS in this study. This establishes the first genotype-phenotype correlation for 44 Black African individuals with KDS. This study marks a significant milestone in research on understudied African populations with CM, emphasising the lengthy diagnostic journey these patients endured. The findings highlight the pressing need for improved access to genomic medicine in underserved regions and underscore the importance of expanding research and diagnostic capabilities in Africa. This work contributes to the advancement of genetic medicine in underrepresented populations, facilitating better diagnostic and therapeutic outcomes.

The widening genetic and myopathologic spectrum of congenital myopathies (CMYOs): a narrative review

Congenital myopathies (CMYOs) represent a genetically and clinically heterogeneous group of disorders characterized by early-onset muscle weakness and distinct myopathologic features. The advent of next-generation sequencing (NGS) has accelerated the identification of causative genes, leading to the discovery of novel CMYOs and thereby challenging the traditional classification. In this comprehensive review, we focus on the clinical, myopathologic, molecular and pathophysiological features of 33 newly identified CMYOs.

Early life lipid overload in Native American Myopathy is phenocopied by stac3 knockout in zebrafish

Understanding the early stages of human congenital myopathies is critical for proposing strategies for improving musculoskeletal muscle performance, such as restoring the functional integrity of the cytoskeleton. SH3 and cysteine-rich domain 3 (STAC3) are proteins involved in nutrient regulation and are an essential component of the excitation-contraction (EC) coupling machinery for Ca2+ releasing. A mutation in STAC3 causes debilitating Native American Myopathy (NAM) in humans, while loss of this gene in mice and zebrafish (ZF) results in premature death. Clinically, NAM patients demonstrated increased lipids in skeletal muscle, but it is unclear if neutral lipids are associated with altered muscle function in NAM. Using a CRISPR/Cas9 induced stac3-/- knockout (KO) zebrafish model, we determined that loss of stac3 leads to delayed larval hatching which corresponds with muscle weakness and decreased whole-body Ca2+ level during early skeletal development. Specifically, we observed defects in the cytoskeleton in F-actin and slow muscle fibers at 5 and 7 days post-fertilizations (dpf). Myogenesis regulators such as myoD and myf5, mstnb were significantly altered in stac3-/- larvae. These muscle alterations were associated with elevated neutral lipid levels starting at 5 dpf and persisting beyond 7 dpf. Larva lacking stac3 had reduced viability with no larva knockouts surviving past 11 dpf. This data suggests that our stac3-/- zebrafish serve as an alternative model to study the diminished muscle function seen in NAM patients. The data gathered from this new model over time supports a mechanistic view of lipotoxicity as a critical part of the pathology of NAM and the associated loss of function in muscle.

A case of acute hypercapnic respiratory failure secondary to late onset nemaline rod myopathy: A multi-disciplinary approach

Background

Nemaline rod myopathy (NRM) is a rare muscle disorder defined by muscle weakness, respiratory insufficiency, and dysphagia. Respiratory muscle involvement can lead to acute hypercapnic respiratory failure, posing significant challenges in management.

Case presentation

Our patient is a 73-year-old male with a history of polymyositis, who presented with acute hypercapnic respiratory failure secondary to a suspected polymyositis flare. Despite initial management, the patient experienced complications, including dysphagia, thrombocytopenia, and altered mental status. Neurological consultations revealed conflicting opinions regarding the primary diagnosis, suggesting inclusion body myositis. The patient's condition continued to deteriorate, prompting discussions about prognosis and palliative care options. This case highlights the challenges in managing respiratory failure in patients with late-onset nemaline myopathy and the importance of multidisciplinary care in addressing complex medical needs.

Conclusion

This case emphasises the complexity of managing respiratory failure in patients with late-onset nemaline myopathy and the significance of adopting a multidisciplinary approach. Timely interventions, including respiratory support, dysphagia management, and palliative care discussions, are vital in optimizing patient care and quality of life. Further research is warranted to elucidate optimal management strategies and improve outcomes in this patient population.

Nutritional Strategies for Muscle Atrophy: Current Evidence and Underlying Mechanisms

Skeletal muscle can undergo detrimental changes in various diseases, leading to muscle dysfunction and atrophy, thus severely affecting people's lives. Along with exercise, there is a growing interest in the potential of nutritional support against muscle atrophy. This review provides a brief overview of the molecular mechanisms driving skeletal muscle atrophy and summarizes recent advances in nutritional interventions for preventing and treating muscle atrophy. The nutritional supplements include amino acids and their derivatives (such as leucine, β-hydroxy, β-methylbutyrate, and creatine), various antioxidant supplements (like Coenzyme Q10 and mitoquinone, resveratrol, curcumin, quercetin, Omega 3 fatty acids), minerals (such as magnesium and selenium), and vitamins (such as vitamin B, vitamin C, vitamin D, and vitamin E), as well as probiotics and prebiotics (like Lactobacillus, Bifidobacterium, and 1-kestose). Furthermore, the study discusses the impact of a combined approach involving nutritional support and physical therapy to prevent muscle atrophy, suggests appropriate multi-nutritional and multi-modal interventions based on individual conditions to optimize treatment outcomes, and enhances the recovery of muscle function for patients. By understanding the molecular mechanisms behind skeletal muscle atrophy and implementing appropriate interventions, it is possible to enhance the recovery of muscle function and improve patients' quality of life.

Unusual cause of muscle weakness, type II respiratory failure and pulmonary hypertension: a case report of ryanodine receptor type 1(RYR1)-related myopathy

BACKGROUND

Patients with congenital myopathies may experience respiratory involvement, resulting in restrictive ventilatory dysfunction and respiratory failure. Pulmonary hypertension (PH) associated with this condition has never been reported in congenital ryanodine receptor type 1(RYR1)-related myopathy.

CASE PRESENTATION

A 47-year-old woman was admitted with progressively exacerbated chest tightness and difficulty in neck flexion. She was born prematurely at week 28. Her bilateral lower extremities were edematous and muscle strength was grade IV-. Arterial blood gas analysis revealed hypoventilation syndrome and type II respiratory failure, while lung function test showed restrictive ventilation dysfunction, which were both worse in the supine position. PH was confirmed by right heart catheterization (RHC), without evidence of left heart disease, congenital heart disease, or pulmonary artery obstruction. Polysomnography indicated nocturnal hypoventilation. The ultrasound revealed reduced mobility of bilateral diaphragm. The level of creatine kinase was mildly elevated. Magnetic resonance imaging showed myositis of bilateral thigh muscle. Muscle biopsy of the left biceps brachii suggested muscle malnutrition and congenital muscle disease. Gene testing revealed a missense mutation in the RYR1 gene (exon33 c.C4816T). Finally, she was diagnosed with RYR1-related myopathy and received long-term non-invasive ventilation (NIV) treatment. Her symptoms and cardiopulmonary function have been greatly improved after 10 months.

CONCLUSIONS

We report a case of RYR1-related myopathy exhibiting hypoventilation syndrome, type II respiratory failure and PH associated with restrictive ventilator dysfunction. Pulmonologists should keep congenital myopathies in mind in the differential diagnosis of type II respiratory failure, especially in patients with short stature and muscle weakness.

Novel compound heterozygous mutations in SCN4A as a potential genetic cause contributing to myopathic manifestations: A case report and literature review

Background

SCN4A mutations account for a diverse array of clinical manifestations, encompassing periodic paralysis, myotonia, and newly recognized symptoms like classical congenital myopathy or congenital myasthenic syndromes. We describe the initial occurrence of myopathic features mimic with recessive classical CM in a Korean infant presenting with novel compound heterozygous SCN4A mutations. The infant exhibited profound hypotonia after birth, thereby expanding the spectrum of SCN4A-related channelopathy.

Methods

The genetic analyses comprised targeted exome sequencing, employing a Celemics G-Mendeliome DES Panel, along with Sanger sequencing.

Results

Considering the clinical manifestations observed in the proband, SCN4A variants emerged as the primary contenders for autosomal recessive (AR) congenital myopathy 22a, classic (#620351). Sanger sequencing validated the association of SCN4A variants with the phenotype, affirming the AR nature of the compound heterozygous variants in both the carrier mother (c.3533G > T/p.Gly1178Val) and the father (c.4216G > A/p.Ala1406Thr).

Conclusion

Our report emphasizes the association of novel compound heterozygous mutations in SCN4A with myopathic features resembling CM, as supporting by muscle biopsy. It is essential to note that pathogenic SCN4A LoF mutations are exceedingly rare. This study contributes to our understanding of SCN4A mutations and their role in myopathic features mimic with classical CM.

Phenotype-Genotype Correlation of a Cohort of Patients with Congenital Myopathy: A Single Centre Experience from India

Background

Congenital myopathies (CMs) are a diverse group of inherited muscle disorders with broad genotypic and phenotypic heterogeneity. While the literature on CM is available from European countries, comprehensive data from the Indian subcontinent is lacking.

Objectives

This study aims to describe the clinical and histopathological characteristics of a cohort of genetically confirmed CMs from India and attempts to do phenotype-genotype correlation.

Methods

A retrospective chart review of genetically confirmed CMs was evaluated between January 2016 and December 2020 at the neuromuscular clinic. The clinical, genetic, and follow-up data were recorded in a pre-structured proforma as per the medical records, and the data was analyzed.

Results

A total of 31(M: F = 14 : 17) unrelated patients were included. The median age at onset and duration of illness are 2.0(IQR:1-8) years and 6.0(IQR:3-10) years respectively. Clinical features observed were proximodistal weakness (54.8%), facial weakness (64.5%), and myopathic facies (54.8%), followed by ptosis (33.3%), and ophthalmoplegia (19.4%). Muscle histopathology was available in 38.7% of patients, and centronuclear myopathy was the most common histopathology finding. The pathogenic genetic variants were identified in RYR1 (29.0%), DNM2 (19.4%), SELENON (12.9%), KBTBD13 (9.7%), NEB (6.5%), and MYPN (6.5%) genes. Novel mutations were observed in 30.3% of the cohort. Follow-up details were available in 77.4% of children, and the median duration of follow-up and age at last follow-up was 4.5 (Range 0.5-11) years and 13 (Range 3-35) years, respectively. The majority were ambulant with minimal assistance at the last follow-up. Mortality was noted in 8.3% due to respiratory failure in Centronuclear myopathy 1 and congenital myopathy 3 with rigid spines (SELENON).

Conclusion

This study highlights the various phenotypes and patterns of genetic mutations in a cohort of pediatric patients with congenital myopathy from India. Centronuclear myopathy was the most common histological classification and the mutations in RYR1 followed by DNM2 gene were the common pathogenic variants identified. The majority were independent in their activities of daily living during the last follow-up, highlighting the fact that the disease has slow progression irrespective of the genotype.

Nemaline myopathy: reclassification of previously reported variants according to ACMG guidelines, and report of novel genetic variants

Nemaline myopathy (NM) is a heterogeneous genetic neuromuscular disorder characterized by rod bodies in muscle fibers resulting in multiple complications due to muscle weakness. NM patients and their families could benefit from genetic analysis for early diagnosis, carrier and prenatal testing; however, clinical classification of variants is subject to change as further information becomes available. Reclassification can significantly alter the clinical management of patients and their families. We used the newly published data and ACMG/AMP guidelines to reassess NM-associated variants previously reported by clinical laboratories (ClinVar). Our analyses on rare variants that were not canonical loss-of-function (LOF) resulted in the downgrading of ~29% (28/97) of variants from pathogenic or likely-pathogenic (P/LP) to variants of uncertain significance (VUS). In addition, we analyzed the splicing effect of variants identified in NM patients by clinical laboratories or research, using an accurate in silico prediction tool that applies a deep-learning network. We identified 55 rare variants that may impact splicing (cryptic splicing). We also analyzed six new NM families and identified eight variants in NEB and ACTA1, including three novel variants: homozygous pathogenic c.164A > G (p.Tyr55Cys), and homozygous likely pathogenic c.980T > C (p.Met327Thr) in ACTA1, and heterozygous VUS c.18694-3T > G in NEB. This study demonstrates the importance of reclassifying variants to facilitate more definitive "calls" on causality or no causality in clinical genetic testing of patients with NM. Reclassification of ~150 variants is now available for improved clinical management, risk counseling and screening of NM patients.

Successful heart transplant in a child with congenital core myopathy and delayed-onset restrictive cardiomyopathy due to recessive mutations in the titin (TTN) gene

BACKGROUND

Mutations in the TTN gene, encoding the muscle filament titin, are a major cause of inherited dilated cardiomyopathy. Early-onset skeletal muscle disorders due to recessive TTN mutations have recently been described, sometimes associated with cardiomyopathies.

CASE DESCRIPTION

We report the case of a boy with congenital core myopathy due to compound heterozygosity for TTN variants. He presented in infancy with rapidly evolving restrictive cardiomyopathy, requiring heart transplantation at the age of 5 years with favorable long-term cardiac and neuromuscular outcome.

CONCLUSION

Heart transplantation may have a role in selected patients with TTN-related congenital myopathy with disproportionally severe cardiac presentation compared to skeletal and respiratory muscle involvement.

Background and roles: myosin in autoimmune diseases

The myosin superfamily is a group of molecular motors. Autoimmune diseases are characterized by dysregulation or deficiency of the immune tolerance mechanism, resulting in an immune response to the human body itself. The link between myosin and autoimmune diseases is much more complex than scientists had hoped. Myosin itself immunization can induce experimental autoimmune diseases of animals, and myosins were abnormally expressed in a number of autoimmune diseases. Additionally, myosin takes part in the pathological process of multiple sclerosis, Alzheimer's disease, Parkinson's disease, autoimmune myocarditis, myositis, hemopathy, inclusion body diseases, etc. However, research on myosin and its involvement in the occurrence and development of diseases is still in its infancy, and the underlying pathological mechanisms are not well understood. We can reasonably predict that myosin might play a role in new treatments of autoimmune diseases.

X-Linked Myotubular Myopathy in a Female Patient with a Pathogenic Variant in the MTM1 Gene

X-linked centronuclear myopathy is caused by pathogenic variants in the MTM1 gene, which encodes myotubularin, a phosphatidylinositol 3-phosphate (PI3P) phosphatase. This form of congenital myopathy predominantly affects males. This study presents a case of X-linked myotubular myopathy in a female carrier of a pathogenic c.1261-10A>G variant in the MTM1 gene.

A review of major causative genes in congenital myopathies

In this review, we focus on congenital myopathies, which are a genetically heterogeneous group of hereditary muscle diseases with slow or minimal progression. They are mainly defined and classified according to pathological features, with the major subtypes being core myopathy (central core disease), nemaline myopathy, myotubular/centronuclear myopathy, and congenital fiber-type disproportion myopathy. Recent advances in molecular genetics, especially next-generation sequencing technology, have rapidly increased the number of known causative genes for congenital myopathies; however, most of the diseases related to the novel causative genes are extremely rare. There remains no cure for congenital myopathies. However, there have been recent promising findings that could inform the development of therapy for several types of congenital myopathies, including myotubular myopathy, which indicates the importance of prompt and correct diagnosis. This review discusses the major causative genes (NEB, ACTA1, ADSSL1, RYR1, SELENON, MTM1, DNM2, and TPM3) for each subtype of congenital myopathies and the relevant latest findings.

Therapeutic approaches in different congenital myopathies

Congenital myopathies are rare and severe genetic diseases affecting the skeletal muscle function in children and adults. They present a variable spectrum of phenotypes and a genetic heterogeneity. Subgroups are defined according to the clinical and histopathological features and encompass core myopathy, centronuclear myopathy, nemaline myopathy and other rare congenital myopathies. No approved treatment exists to date for any congenital myopathies. To tackle this important unmet need, an increased number of proof-of-concept studies recently assessed the therapeutic potential of various strategies, either pharmacological or genetic-based, aiming at counteracting muscle weakness or/and cure the pathology. Here, we list the implicated genes and cellular pathways, and review the therapeutic approaches preclinically tested and the ongoing/completed clinical trials for the different types of congenital myopathies.

Molecular signatures of inherited and acquired sporadic late onset nemaline myopathies

Acquired sporadic late onset nemaline myopathy (SLONM) and inherited nemaline myopathy (iNM) both feature accumulation of nemaline rods in muscle fibers. Unlike iNM, SLONM is amenable to therapy. The distinction between these disorders is therefore crucial when the diagnosis remains ambiguous after initial investigations. We sought to identify biomarkers facilitating this distinction and to investigate the pathophysiology of nemaline rod formation in these different disorders. Twenty-two muscle samples from patients affected by SLONM or iNM underwent quantitative histological analysis, laser capture microdissection for proteomic analysis of nemaline rod areas and rod-free areas, and transcriptomic analysis. In all iNM samples, nemaline rods were found in subsarcolemmal or central aggregates, whereas they were diffusely distributed within muscle fibers in most SLONM samples. In SLONM, muscle fibers harboring nemaline rods were smaller than those without rods. Necrotic fibers, increased endomysial connective tissue, and atrophic fibers filled with nemaline rods were more common in SLONM. Proteomic analysis detected differentially expressed proteins between nemaline rod areas and rod-free areas, as well as between SLONM and iNM. These differentially expressed proteins implicated immune, structural, metabolic, and cellular processes in disease pathophysiology. Notably, immunoglobulin overexpression with accumulation in nemaline rod areas was detected in SLONM. Transcriptomic analysis corroborated proteomic findings and further revealed substantial gene expression differences between SLONM and iNM. Overall, we identified unique pathological and molecular signatures associated with SLONM and iNM, suggesting distinct underlying pathophysiological mechanisms. These findings represent a step towards enhanced diagnostic tools and towards development of treatments for SLONM.

Progressive external ophthalmoplegia

Progressive external ophthalmoplegia (PEO), characterized by ptosis and impaired eye movements, is a clinical syndrome with an expanding number of etiologically distinct subtypes. Advances in molecular genetics have revealed numerous pathogenic causes of PEO, originally heralded in 1988 by the detection of single large-scale deletions of mitochondrial DNA (mtDNA) in skeletal muscle of people with PEO and Kearns-Sayre syndrome. Since then, multiple point variants of mtDNA and nuclear genes have been identified to cause mitochondrial PEO and PEO-plus syndromes, including mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and sensory ataxic neuropathy dysarthria ophthalmoplegia (SANDO). Intriguingly, many of those nuclear DNA pathogenic variants impair maintenance of the mitochondrial genome causing downstream mtDNA multiple deletions and depletion. In addition, numerous genetic causes of nonmitochondrial PEO have been identified.

Tissue Engineering Applied to Skeletal Muscle: Strategies and Perspectives

Muscle tissue is formed by elongated and contractile cells with specific morphofunctional characteristics. Thus, it is divided into three basic types: smooth muscle tissue, cardiac striated muscle tissue and skeletal striated muscle tissue. The striated skeletal muscle tissue presents high plasticity, regeneration and growth capacity due to the presence of satellite cells, quiescent myoblasts that are activated in case of injury to the tissue and originate new muscle fibers when they differentiate. In more severe deficiencies or injuries there is a loss of their regenerative capacity, thus compromising the body's functionality at different levels. Tissue engineering studies the development of biomaterials capable of stimulating the recovery of cellular activity in injured body tissues, as well as the activity of cells with muscle differentiation potential in injury repair. However, the need for three-dimensional re-assembly in a complex organization makes it difficult to mimic this tissue and fully regenerate it for the sake of precise and effective movements. Thus, this article aims to provide a narrative review of tissue engineering strategies applied to the regeneration of skeletal muscle, in a critical evaluation of research, whether aimed at injury or atrophies such as spinal muscular atrophy.

Cultured Neonatal Murine Primary Myotubes as a Model to Study Muscle Atrophy

Besides genetic disorders, skeletal muscle atrophy mainly occurs as a consequence of underlying conditions such as prolonged inactivity, aging, and metabolic diseases, ultimately contributing to the risk of disability. Disturbances in cellular and molecular mechanisms involved in proteolysis and protein synthesis underpin muscle fiber shrinkage and decreased muscle fiber diameter. Stress-induced primary myotube culture is an established model for studying muscle atrophy. An in vitro model is an essential criterion in establishing preliminary data in a cell-autonomous manner that can later be validated using in vivo models. Here, we describe protocols for the isolation, culture, and differentiation of primary murine myotubes and the induction of myotube atrophy using dexamethasone, a synthetic corticosteroid. We further elaborate the procedure to validate degenerative parameters, such as assessing muscle fiber diameter, expression of muscle atrophy genes, and protein synthesis status under dexamethasone treatment. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Isolation and culture of primary myoblasts from rat or mouse pups Support Protocol 1: Preparation of coated tissue culture ware Support Protocol 2: Subculture of myoblasts Basic Protocol 2: Induction and assessment of myotube atrophy.

A Splice Variant of the MYH7 Gene Is Causative in a Family with Isolated Left Ventricular Noncompaction Cardiomyopathy

Variants of the MYH7 gene have been associated with a number of primary cardiac conditions, including left ventricular noncompaction cardiomyopathy (LVNC). Most cases of MYH7-related diseases are associated with such variant types as missense substitutions and in-frame indels. Thus, truncating variants in MYH7 (MYH7tv) and associated mechanism of haploinsufficiency are usually considered not pathogenic in these disorders. However, recent large-scale studies demonstrated evidence of the significance of MYH7tv for LVNC and gave rise to an assumption that haploinsufficiency may be the causal mechanism for LVNC. In this article, we present a family with isolated LVNC and a heterozygous splice variant of the MYH7 gene, analyze possible consequences of this variant and conclude that not all variants that are predicted truncating really act through haploinsufficiency. This study can highlight the importance of a precise assessment of MYH7 splicing variants and their participation in the development of LVNC.

Novel Splicing Mutation in MTM1 Leading to Two Abnormal Transcripts Causes Severe Myotubular Myopathy

X-linked myotubular myopathy (XLMTM) is a severe form of centronuclear myopathy, characterized by generalized weakness and respiratory insufficiency, associated with pathogenic variants in the MTM1 gene. NGS targeted sequencing on the DNA of a three-month-old child affected by XLMTM identified the novel hemizygous MTM1 c.1261-5T>G intronic variant, which interferes with the normal splicing process, generating two different abnormal transcripts simultaneously expressed in the patient’s muscular cells. The first aberrant transcript, induced by the activation of a cryptic splice site in intron 11, includes four intronic nucleotides upstream of exon 12, resulting in a shift in the transcript reading frame and introducing a new premature stop codon in the catalytic domain of the protein (p.Arg421SerfsTer7). The second aberrant MTM1 transcript, due to the lack of recognition of the 3′ acceptor splice site of intron 11 from the spliceosome complex, leads to the complete skipping of exon 12. We expanded the genotypic spectrum of XLMTM underlying the importance of intron−exons boundaries sequencing in male patients affected by XLMTM.

Comparison of incorporation of wild type and mutated actins into sarcomeres in skeletal muscle cells: A fluorescence recovery after photobleaching study

The α-actin mutation G15R in the nucleotide-binding pocket of skeletal muscle, causes severe actin myopathy in human skeletal muscles. Expressed in cultured embryonic quail skeletal myotubes, YFP-G15R-α-actin incorporates in sarcomeres in a pattern indistinguishable from wildtype YFP-α-actin. However, patches of YFP-G15R-α-actin form, resembling those in patients. Analyses with FRAP of incorporation of YFP-G15R-α-actin showed major differences between fast-exchanging plus ends of overlapping actin filaments in Z-bands, versus slow exchanging ends of overlapping thin filaments in the middle of sarcomeres. Wildtype skeletal muscle YFP-α-actin shows a faster rate of incorporation at plus ends of F-actin than at their minus ends. Incorporation of YFP-G15R-α-actin molecules is reduced at plus ends, increased at minus ends. The same relationship of wildtype YFP-α-actin incorporation is seen in myofibrils treated with cytochalasin-D: decreased dynamics at plus ends, increased dynamics at minus ends, and F-actin aggregates. Speculation: imbalance of normal polarized assembly of F-actin creates excess monomers that form F-actin aggregates. Two other severe skeletal muscle YFP-α-actin mutations (H40Y and V163L) not in the nucleotide pocket do not affect actin dynamics, and lack F-actin aggregates. These results indicate that normal α-actin plus and minus end dynamics are needed to maintain actin filament stability, and avoid F-actin patches.

A case of giant dental calculus in a patient with centronuclear myopathy

INTRODUCTION

Centronuclear myopathy is a hereditary congenital muscle disease. It is characterized by generalized muscle hypotonia from early childhood, elongated cacial appearance, mandibular undergroth, and dental malposition. In this report, we discuss the clinical course and management of a patient with centronuclearmyopathy, who developed a giant dental calculus in the floor of the mouth and underwent surgical excision.

CASE REPORT

A 37-year-old Japanese man was referred to our hospital, and reported a swelling in the floor of the mouth. The patient affects centronuclear myopathy and has generalized muscle weakness. CT images showed a high-density area in the floor of the mouth measuring 35 × 28 × 20 mm. The lesion was clinically diagnosed as giant dental calculus, and surgically removed.

CONCLUSION

We have experienced a case of giant dental calculus in a patient with centronuclear myopathy. In dental treatment, we must consider generalized muscle weakness.

Clinical Characteristics and Neurologic Outcomes of X-Linked Myotubular Myopathy

Purpose: X-linked myotubular myopathy (XLMTM) is a rare condition of centronuclear myopathy caused by myotubularin 1 (MTM1) mutations. Patients with XLMTM show different neurodevelopmental outcomes after the neonatal period depending on age and acquired hypoxic damage. We aim to evaluate the clinical characteristics and neurodevelopmental outcomes of patients with XLMTM who were followed up at a single center. It is essential to understand the volume and conditions to prepare for being a candidate for new therapeutic strategies. Methods: Patients diagnosed with centronuclear myopathy by muscle pathology and MTM1 mutation analysis were included. We retrospectively investigated motor milestones, communication skills, and bulbar and respiratory function in the patients. The patients were categorized into two groups: with and without hypoxic insults (HI). Results: All 13 patients were severely affected by neonatal hypotonia and required respiratory support and a feeding tube during the neonatal period. The follow-up duration was 4.4 years (range, 0.3 to 8.9). In the non-HI group, developmental milestones were delayed but were slowly achieved. Some patients underwent training in oral feeding with thickened foods and weaning from ventilation. Patients with HI showed poor motor function catch-up and communication skills. Three deaths were associated with acute respiratory failure.Conclusion: Patients with XLMTM without HI can survive long-term with the slow achievement of motor milestones and bulbar and respiratory function. However, hypoxic brain damage following acute respiratory failure negatively influences their developmental potential or even lead to death. Therefore, parental education for proper respiratory management is necessary, especially for young children.

Muscle biopsy in myositis: What the rheumatologist needs to know

The appropriate analysis of skeletal muscle tissues is a key element in many diagnostic procedures and can deliver valuable information about the organ that is affected. Although arguably the frequency of muscle biopsy may be declining in certain domains where genetic analysis is now the first line of diagnostic evaluation, it still has an important role in assessment of patients with neuromuscular disorders such as congenital myopathies, muscular dystrophies, metabolic and inflammatory diseases. Here, we have comprehensively discussed the aspects of a modern and fruitful approach to muscle biopsy histopathological studies in rheumatological disorders. We have focussed on the neuromuscular involvement in myositis and its differential diagnoses in both adult and paediatric settings. We have also covered the clinical indications for the biopsy, technical aspects and practical points relevant for the rheumatologists. Finally, we have critically discussed the current and future opportunities that a muscle biopsy may offer and its limitations.

Correlation of Phenotype–Genotype and Protein Structure in RYR1-Related Myopathy

Introduction

Next generation sequencing results in an explosive identification of rare variants of RYR1, making the correlation between phenotype and genotype complicated. We analyzed the data of 33 patients with RYR1-related myopathy, attempting to elucidate correlations between phenotype, genotype, and protein structure of RyR1.

Methods

Clinical, histopathologic, and genetic data were evaluated, and variants were mapped to the cryo-EM RyR1 structure. The three-dimensional structure of the variant on RyR1 was analyzed.

Results

The clinical spectrum was highly variable regardless of the mode of inheritance. Recessive variations were associated with more severe feeding problems and respiratory insufficiency in infancy (p < 0.05). Forty pathogenic and likely pathogenic variations were identified, and 14 of them were novel. Missense was the most common variation type regardless of inheritance mode. Arginine (15/45) was the most frequently involved residue. All but one dominant variation clustered in Pore forming and pVSD domains, while recessive variations enriched in Bsol (7/25) and SPRYs (6/25) domains. Analysis of the spatial structure of variants showed that dominant variants may impact RyR1 mainly by breaking down hydrogen or electrovalent bonds (10/21); recessive variants located in different domains may impact the function of RyR1 through different pathways. Variants located in RyR1 coupling sites (PY1&2 and the outermost of Bsol) may cause the most severe clinical manifestation.

Conclusion

Clinical diversity of RYR1-related myopathy was impacted by the inheritance mode, variation type, and variant location. Dominant and recessive variants have different sensitive domains impacting the function of RyR1 through different pathways.

Autosomal dominantly inherited myopathy likely caused by the TNNT1 variant p.(Asp65Ala)

Nemaline myopathies (NEM) are genetically and clinically heterogenous. Biallelic or monoallelic variants in TNNT1, encoding slow skeletal troponin T1 (TnT1), cause NEM. We report a 2-year-old patient and his mother carrying the heterozygous TNNT1 variant c.194A>C/p.(Asp65Ala) that occurred de novo in the mother. Both had muscle hypotrophy and muscle weakness. Muscle pathology in the proband's mother revealed slow twitch type 1 fiber hypotrophy and fast twitch type 2 fiber hypertrophy that was confirmed by a reduced ratio of slow skeletal myosin to fast skeletal myosin type 2a. RT-PCR and immunoblotting data demonstrated increased levels of high-molecular-weight TnT1 isoforms in skeletal muscle of the proband's mother that were also observed in some controls. In an overexpression system, complex formation of TnT1-D65A with tropomyosin 3 (TPM3) was enhanced. The previously reported TnT1-E104V and TnT1-L96P mutants showed reduced or no co-immunoprecipitation with TPM3. Our studies support pathogenicity of the TNNT1 p.(Asp65Ala) variant. This article is protected by copyright. All rights reserved.

Clinical and genetic features of infancy-onset congenital myopathies from a Chinese paediatric centre

BACKGROUND

Congenital myopathies are a group of rare neuromuscular diseases characterized by specific histopathological features. The relationship between the pathologies and the genetic causes is complex, and the prevalence of myopathy-causing genes varies among patients from different ethnic groups. The aim of the present study was to characterize congenital myopathies with infancy onset among patients registered at our institution.

METHOD

This retrospective study enrolled 56 patients based on the pathological and/or genetic diagnosis. Clinical, histopathological and genetic features of the patients were analysed with long-term follow-up.

RESULTS

Twenty-six out of 43 patients who received next-generation sequencing had genetic confirmation, and RYR1 variations (12/26) were the most prevalent. Eighteen novel variations were identified in 6 disease-causing genes, including RYR1, NEB, TTN, TNNT1, DNM2 and ACTA1. Nemaline myopathy (17/55) was the most common histopathology. The onset ages ranged from birth to 1 year. Thirty-one patients were followed for 3.83 ± 3.05 years (ranging from 3 months to 11 years). No patient died before 1 year. Two patients died at 5 years and 8 years respectively. The motor abilities were stable or improved in 23 patients and deteriorated in 6 patients. Ten (10/31) patients developed respiratory involvement, and 9 patients (9/31) had mildly abnormal electrocardiograms and/or echocardiograms.

CONCLUSION

The severity of congenital myopathies in the neonatal/infantile period may vary in patients from different ethnic groups. More concern should be given to cardiac monitoring in patients with congenital myopathies even in those with static courses.

Ophthalmological Manifestations of Hereditary Myopathies

BACKGROUND

Myopathies are neuromuscular disorders of the skeletal muscles, in which the main symptom is muscle weakness due to muscle fiber dysfunction. Myopathies may be classified into two main categories: inherited and acquired. Hereditary myopathies are a heterogeneous group of diseases that include congenital myopathies, mitochondrial myopathies, myotonic syndromes, muscular dystrophies, and other myopathies.

PURPOSE

The objective of this paper is to review the ophthalmological findings and genetic patterns of hereditary myopathies.

METHODS

This review is based on articles obtained by a relevant search of the PubMed database.

CONCLUSION

Ophthalmoplegia with or without ptosis and pupil sparing appeared to be the most frequent ophthalmological manifestation of myopathies. The identification of the main ophthalmological features can help in the diagnosis and treatment of these muscular diseases.

Rare Genetic Syndromes and Oral Anomalies: A Review of the Literature and Case Series with a New Classification Proposal

Rare genetic syndromes, conditions with a global average prevalence of 40 cases/100,000 people, are associated with anatomical, physiological, and neurological anomalies that may affect different body districts, including the oral district. So far, no classification of oral abnormalities in rare genetic syndromes is present in the literature. The aim of this narrative review is to analyze literature on rare genetic syndromes affecting dento-oro-maxillofacial structures (teeth, maxillary bones, oral soft tissues, or mixed) and to propose a classification according to the detected oral abnormalities. In addition, five significant cases of rare genetic syndromes are presented. The Scale for the Assessment of Narrative Review Articles (SANRA) was followed for this review. From 674 papers obtained through PubMed search, 351 were selected. Sixty-two rare genetic syndromes involving oral manifestations were found and classified. The proposed classification aims to help the clinician to easily understand which dento-oro-maxillofacial findings might be expected in the presence of each rare genetic syndrome. This immediate framework may both help in the diagnosis of dento-oro-maxillofacial anomalies related to the underlying pathology as well as facilitate the drafting of treatment plans with the involvement of a multidisciplinary team.

A Systematic Review and Meta-Analysis of the Prevalence of Congenital Myopathy

Background: Congenital myopathy constitutes a heterogeneous group of orphan diseases that are mainly classified on the basis of muscle biopsy findings. This study aims to estimate the prevalence of congenital myopathy through a systematic review and meta-analysis of the literature.

Methods: The PubMed, MEDLINE, Web of Science, and Cochrane Library databases were searched for original research articles published in English prior to July 30, 2021. The quality of the included studies was assessed by a checklist adapted from STrengthening the Reporting of OBservational studies in Epidemiology (STROBE). To derive the pooled epidemiological prevalence estimates, a meta-analysis was performed using the random effects model. Heterogeneity was assessed using the Cochrane Q statistic as well as the I² statistic.

Results: A total of 11 studies were included in the systematic review and meta-analysis. Of the 11 studies included, 10 (90.9%) were considered medium-quality, one (9.1%) was considered low-quality, and no study was assessed as having a high overall quality. The pooled prevalence of congenital myopathy in the all-age population was 1.62 (95% CI, 1.13–2.11) per 100,000, while the prevalence in the child population was 2.76 (95% CI, 1.34–4.18) per 100,000. In the pediatric population, the prevalence among males was 2.92 (95% CI, −1.70 to 7.55) per 100,000, while the prevalence among females was 2.47 (95% CI, −1.67 to 6.61) per 100,000. The prevalence estimates of the all-age population per 100,000 were 0.20 (95% CI 0.10–0.35) for nemaline myopathy, 0.37 (95% CI 0.21–0.53) for core myopathy, 0.08 (95% CI −0.01 to 0.18) for centronuclear myopathy, 0.23 (95% CI 0.04–0.42) for congenital fiber-type disproportion myopathy, and 0.34 (95% CI, 0.24–0.44) for unspecified congenital myopathies. In addition, the prevalence estimates of the pediatric population per 100,000 were 0.22 (95% CI 0.03–0.40) for nemaline myopathy, 0.46 (95% CI 0.03–0.90) for core myopathy, 0.44 (95% CI 0.03–0.84) for centronuclear myopathy, 0.25 (95% CI −0.05 to 0.54) for congenital fiber-type disproportion myopathy, and 2.63 (95% CI 1.64–3.62) for unspecified congenital myopathies.

Conclusions: Accurate estimates of the prevalence of congenital myopathy are fundamental to supporting public health decision-making. The high heterogeneity and the lack of high-quality studies highlight the need to conduct higher-quality studies on orphan diseases.

A novel frameshift ACTN2 variant causes a rare adult-onset distal myopathy with multi-minicores

Introduction

Distal myopathies are a group of rare muscle disorders characterized by selective or predominant weakness in the feet and/or hands. In 2019, ACTN2 gene was firstly identified to be a cause of a new adult‐onset distal muscular dystrophy calling actininopathy and another distinctly different myopathy, named multiple structured core disease (MsCD). Thus, the various phenotypes and limited mutations in ACTN2‐related myopathy make the genotype‐phenotype correlation hard to understand.

Aims

To investigate the clinical features and histological findings in a Chinese family with distal myopathy. Whole exome sequencing and several functional studies were performed to explore the pathogenesis of the disease.

Results

We firstly identified a novel frameshift variant (c.2504delT, p.Phe835Serfs*66) within ACTN2 in a family including three patients. The patients exhibited adult‐onset distal myopathy with multi‐minicores, which, interestingly, was more like a combination of MsCD and actininopathy. Moreover, functional analysis using muscle samples revealed that the variant significantly increased the expression level of α‐actinin‐2 and resulted in abnormal Z‐line organization of muscle fiber. Vitro studies suggested aggregate formations might be involved in the pathogenesis of the disease.

Conclusion

Our results expanded the phenotypes of ACTN2‐related myopathy and provided helpful information to clarify the molecular mechanisms.

A Homozygous Deep Intronic Mutation Alters the Splicing of Nebulin Gene in a Patient With Nemaline Myopathy

Nemaline myopathy is a rare disorder affecting the muscle sarcomere. Mutations in nebulin gene (NEB) are known to be responsible for about 50% of nemaline myopathy cases. Nebulin is a giant protein which is formed integrally with the sarcomeric thin filament. This complex gene is under extensive alternative splicing giving rise to multiple isoforms. In this study, we report a 6-year-old boy presenting with general muscular weaknesses. Identification of rod-shaped structures in the patient' biopsy raised doubt about the presence of a nemaline myopathy. Next-generation sequencing was used to identify a causative mutation for the patient syndrome. A homozygous deep intronic substitution was found in the intron 144 of the NEB. The variant was predicted by in silico tools to create a new donor splice site. Molecular analysis has shown that the mutation could alter splicing events of the nebulin gene leading to a significant decrease of isoforms level. This change in the expression level of nebulin could give rise to functional consequences in the sarcomere. These results are consistent with the phenotypes observed in the patient. Such a discovery of variants in this gene will allow a better understanding of the involvement of nebulin in neuromuscular diseases and help find new treatments for the nemaline myopathy.

MR imaging of inherited myopathies: a review and proposal of imaging algorithms

PURPOSE OF REVIEW

The aims of this review are to discuss the imaging modalities used to assess muscle changes in myopathies, to provide an overview of the inherited myopathies focusing on their patterns of muscle involvement in magnetic resonance imaging (MR), and to propose up-to-date imaging-based diagnostic algorithms that can help in the diagnostic workup.

CONCLUSION

Familiarization with the most common and specific patterns of muscular involvement in inherited myopathies is very important for radiologists and neurologists, as imaging plays a significant role in diagnosis and follow-up of these patients.

KEY POINTS

• Imaging is an increasingly important tool for diagnosis and follow-up in the setting of inherited myopathies. • Knowledge of the most common imaging patterns of muscle involvement in inherited myopathies is valuable for both radiologists and neurologists. • In this review, we present imaging-based algorithms that can help in the diagnostic workup of myopathies.

Clinical and genetic characterization of PYROXD1-related myopathy patients from Turkey

Congenital myopathies (CMs) are a heterogeneous group of inherited muscle disorders characterized by muscle weakness at birth, while limb-girdle muscular dystrophies (LGMD) have a later onset and slower disease progression. Thus, detailed clinical phenotyping of genetically defined disease entities are required for the full understanding of genotype-phenotype correlations. A recently defined myopathic genetic disease entity is caused by bi-allelic variants in a gene coding for pyridine nucleotide-disulfide oxidoreductase domain 1 (PYROXD1) with unknown substrates. Here, we present three patients from two consanguineous Turkish families with mild LGMD, facial weakness, normal CK levels, and slow progress. Genomic analyses revealed a homozygous known pathogenic missense variant (c.464A>G, p.Asn155Ser) in family 1 with two affected females. In the affected male of family 2, we found this variant in a compound heterozygous state together with a novel frameshift variant (c.329_332delTCTG, p.Leu112Valfs*8), which is the second frameshift variant known so far in PYROXD1. We have been able to define a large homozygous region in family 1 sharing a common haplotype with family 2 in the critical region. Our data suggest that c.464A>G is a Turkish founder mutation. To gain deeper insights, we performed a systematic review of all published PYROXD1-related myopathy cases. Our analysis showed that the c.464A > G variant was found in 87% (20/23) of the patients and that it may cause either a childhood- or adult-onset phenotype, irrespective of its presence in a homozygous or compound heterozygous state. Interestingly, only four patients had elevated CK levels (up to 1000 U/L), and cardiac involvement was found in few compound heterozygous cases.

Evaluating next-generation sequencing in neuromuscular diseases with neonatal respiratory distress

With the exception of infantile spinal muscular atrophy (SMA) and congenital myotonic dystrophy 1 (DM1), congenital myopathies and muscular dystrophies with neonatal respiratory distress pose diagnostic challenges. Next-generation sequencing (NGS) provides hope for the diagnosis of these rare diseases. We evaluated the efficiency of next-generation sequencing (NGS) in ventilated newborns with peripheral hypotonia. We compared the results of our previous study in a cohort of 19 patients analysed by Sanger sequencing from 2007 to 2012, with a diagnostic yield of 26% (5/19), and those of a new retrospective study in 28 patients from 2007 to 2018 diagnosed using MyoPanel, a neuromuscular disease panel, with a diagnostic yield of 43% (12/28 patients). Pathogenic variants were found in five genes: ACTA1 (n = 4 patients), RYR1 (n = 2), CACNA1S (n = 1), NEB (n = 3), and MTM1 (n = 2). Myopanel increased the diagnosis of congenital neuromuscular diseases, but more than half the patients remained undiagnosed. Whole exome sequencing did not seem to fully respond to this diagnostic limitation. Therefore, explorations with whole genome sequencing will be the next step.

The Phenotype and Genotype of Congenital Myopathies Based on a Large Pediatric Cohort

BACKGROUND

Congenital myopathies (CMs) are a clinically and genetically heterogeneous group of hereditary muscular disorders. The distribution of genetic and histologic subtypes has been addressed in only a few cohorts, and the relationship between phenotypes and genotypes is only partially understood.

METHODS

This is a retrospective cross-sectional data collection study conducted at a single center. The clinical, histopathological, and molecular characterization of 104 patients with CM is reported.

RESULTS

The most common histopathological subtype was core myopathy (42%). Patients with severe endomysial fibrosis were more commonly unable to walk than patients with only a mild-grade endomysial fibrosis (56% vs 16%). Inability to walk was also more prevalent in patients with severe fatty replacement (44% vs 19%). The genetic etiology was more frequently identified among those patients with "specific" histologic findings (74% vs 62%). A definite molecular diagnosis was reached in 65 of 104 patients (62%), with RYR1 (24/104) and TTN (8/104) being the most frequent causative genes. Neonatal onset occurred in 56%. Independent ambulation was achieved by 74%. Patients who walked late were more likely to become wheelchair-dependent. Respiratory support was needed in one of three patients. Gastrostomy placement was required in 15%. Cardiac involvement was observed in 3%, scoliosis in 43%, and intellectual disability in 6%.

CONCLUSIONS

This study provides an updated picture of the clinical, histopathological, and molecular landscape of CMs. Independently of the causative gene, fibrosis and fatty replacement in muscle biopsy specimens are associated with clinical severity. Mutations in TTN are responsible for a higher proportion of cases than previously thought.

ALDH2 mutation promotes skeletal muscle atrophy in mice via accumulation of oxidative stress

Muscle atrophy is promoted by various factors including aging, immobilization, unloading and use of drugs such as steroids. However, genetic risk factors for muscle atrophy are less well known. Here, we show that a missense SNP in the ALDH2 gene, rs671 (ALDH2*2), a dominant negative mutation, promotes significant muscle atrophy in the ALDH2*2 mouse model, accompanied by decreased expression of anabolic and catabolic muscle factors and acquisition of a low turnover state. We also demonstrate that expression of LC3, which is require for auto-phagosome formation during autophagy, increases in ALDH2*2 mouse muscles. We show that 4-hydroxynonenal (4HNE), a peroxidated lipid-protein and oxidant, accumulates in ALDH2*2 mouse muscles. We have shown that the rs671 mutation is associated with increased serum levels of acetaldehyde, an alcohol metabolite. We show that expression of the atrogenes Atrogin1 and MuRF1 significantly increased in myogenic cells following acetaldehyde treatment, an outcome significantly inhibited in vitro by Trolox C, an anti-oxidant. Muscle atrophy in ALDH2*2 mice was also significantly rescued by dietary administration of the anti-oxidant vitamin E, which blocked 4HNE accumulation in muscle. Taken together, our data indicate that rs671 is a genetic risk factor for muscle atrophy, but that such atrophy can be rescued by vitamin E treatment.

MyoSight-semi-automated image analysis of skeletal muscle cross sections

BACKGROUND

Manual analysis of cross-sectional area, fiber-type distribution, and total and centralized nuclei in skeletal muscle cross sections is tedious and time consuming, necessitating an accurate, automated method of analysis. While several excellent programs are available, our analyses of skeletal muscle disease models suggest the need for additional features and flexibility to adequately describe disease pathology. We introduce a new semi-automated analysis program, MyoSight, which is designed to facilitate image analysis of skeletal muscle cross sections and provide additional flexibility in the analyses.

RESULTS

We describe staining and imaging methods that generate high-quality images of immunofluorescent-labelled cross sections from mouse skeletal muscle. Using these methods, we can analyze up to 5 different fluorophores in a single image, allowing simultaneous analyses of perinuclei, central nuclei, fiber size, and fiber-type distribution. MyoSight displays high reproducibility among users, and the data generated are in close agreement with data obtained from manual analyses of cross-sectional area (CSA), fiber number, fiber-type distribution, and number and localization of myonuclei. Furthermore, MyoSight clearly delineates changes in these parameters in muscle sections from a mouse model of Duchenne muscular dystrophy (mdx).

CONCLUSIONS

MyoSight is a new program based on an algorithm that can be optimized by the user to obtain highly accurate fiber size, fiber-type identification, and perinuclei and central nuclei per fiber measurements. MyoSight combines features available separately in other programs, is user friendly, and provides visual outputs that allow the user to confirm the accuracy of the analyses and correct any inaccuracies. We present MyoSight as a new program to facilitate the analyses of fiber type and CSA changes arising from injury, disease, exercise, and therapeutic interventions.

Precision Medicine in Rare Diseases

Rare diseases are those that have a low prevalence in the population (less than 5 individuals per 10,000 inhabitants). However, infrequent pathologies affect a large number of people, since according to the World Health Organization (WHO), there are about 7000 rare diseases that affect 7% of the world’s population. Many patients with rare diseases have suffered the consequences of what is called the diagnostic odyssey, that is, extensive and prolonged serial tests and clinical visits, sometimes for many years, all with the hope of identifying the etiology of their disease. For patients with rare diseases, obtaining the genetic diagnosis can mean the end of the diagnostic odyssey, and the beginning of another, the therapeutic odyssey. This scenario is especially challenging for the scientific community, since more than 90% of rare diseases do not currently have an effective treatment. This therapeutic failure in rare diseases means that new approaches are necessary. Our research group proposes that the use of precision or personalized medicine techniques can be an alternative to find potential therapies in these diseases. To this end, we propose that patients’ own cells can be used to carry out personalized pharmacological screening for the identification of potential treatments.

Whole-body muscle magnetic resonance imaging in patients with muscle symptoms: incidental findings and outcomes

BACKGROUND AND PURPOSE

Whole-body muscle magnetic resonance imaging (MRI) has become widely used for diagnostic workup in patients with muscle diseases. The prevalence of incidental findings in diagnostic whole-body muscle MRI is unknown. Here, the prevalence and outcomes of incidentalomas in whole-body muscle MRI in a large cohort of patients with muscle symptoms were studied.

METHODS

Two hundred and six patients who consulted at our neuromuscular clinic with muscle weakness and/or myalgia and/or increased serum creatine kinase and in whom a whole-body muscle MRI was performed between January 2016 and March 2020 were included.

RESULTS

Whole-body muscle MRI revealed at least one incidentaloma in 132 patients (64.1%), with mean age at MRI examination 50.4 years (19-74 years). Most of the incidental findings were benign. However, diagnostic examinations were indicated in 16.3% of the incidentalomas, treatment was needed in 4.7% and, in retrospect, symptoms related to the incidental findings were identified in 14.3%. Three malignant (glioblastoma multiforme, renal cell carcinoma and hepato-splenomegaly related to a lymphoma) and one precancerous (low-grade appendiceal mucinous neoplasm) incidental findings were identified. In one patient an abdominal aortic aneurysm was detected and in another a large cerebral arteriovenous malformation. In 1.2% of the incidentalomas, i.e. periventricular white matter lesions (LAMA2 mutations) and fibrous dysplasia (ANO5 mutations), an indirect link with the muscle disease could be established.

CONCLUSIONS

It is concluded that incidental findings in diagnostic whole-body muscle MRI in patients with muscle symptoms occur frequently. Most of them are benign, but in some timely detecting the incidentaloma leads to early treatment and can thus impact prognosis.

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.

Reissner fibre-induced urotensin signalling from cerebrospinal fluid-contacting neurons prevents scoliosis of the vertebrate spine

Reissner fibre (RF), discovered by the 19^th-century German anatomist Ernst Reissner, is a filamentous structure present in cerebrospinal fluid (CSF). RF forms by aggregation of a glycoprotein called SCO-spondin (Sspo), but its function has remained enigmatic. Recent studies have shown that zebrafish sspo mutants develop a curved embryonic body axis. Zebrafish embryos with impaired cilia motility also develop curved bodies, which arises from failure of expression of urotensin related peptide (urp) genes in CSF-contacting neurons (CSF-cNs), impairing downstream signalling in trunk muscles. Here, we show that sspo mutants can survive into adulthood, but display severe curvatures of the vertebral column, resembling the common human spine disorder idiopathic scoliosis (IS). sspo mutants also exhibit significant reduction of urp gene expression from CSF-cNs. Consistent with epinephrine in CSF being bound by RF and required for urp expression, treating sspo mutants with this catecholamine rescued expression of the urp genes and axial defects. More strikingly, providing Urp2, specifically in the CSF-cNs, rescued body curvature of sspo homozygotes during larval stages as well as in the adult. These findings bridge existing gaps in our knowledge between cilia motility, RF, Urp signalling and spine deformities, and suggest that targeting the Urotensin pathway could provide novel therapeutic avenues for IS.

Summary: Reissner fibre (RF) is a glycoprotein filament suspended in cerebrospinal fluid (CSF). We show that RF regulates zebrafish spine morphogenesis by regulating Urotensin signalling from CSF-contacting neurons of the spinal cord.

Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview

Muscular contraction is a fundamental phenomenon in all animals; without it life as we know it would be impossible. The basic mechanism in muscle, including heart muscle, involves the interaction of the protein filaments myosin and actin. Motility in all cells is also partly based on similar interactions of actin filaments with non-muscle myosins. Early studies of muscle contraction have informed later studies of these cellular actin-myosin systems. In muscles, projections on the myosin filaments, the so-called myosin heads or cross-bridges, interact with the nearby actin filaments and, in a mechanism powered by ATP-hydrolysis, they move the actin filaments past them in a kind of cyclic rowing action to produce the macroscopic muscular movements of which we are all aware. In this special issue the papers and reviews address different aspects of the actin-myosin interaction in muscle as studied by a plethora of complementary techniques. The present overview provides a brief and elementary introduction to muscle structure and function and the techniques used to study it. It goes on to give more detailed descriptions of what is known about muscle components and the cross-bridge cycle using structural biology techniques, particularly protein crystallography, electron microscopy and X-ray diffraction. It then has a quick look at muscle mechanics and it summarises what can be learnt about how muscle works based on the other studies covered in the different papers in the special issue. A picture emerges of the main molecular steps involved in the force-producing process; steps that are also likely to be seen in non-muscle myosin interactions with cellular actin filaments. Finally, the remarkable advances made in studying the effects of mutations in the contractile assembly in causing specific muscle diseases, particularly those in heart muscle, are outlined and discussed.