Congenital myopathies with secondary neuromuscular transmission defects; A case report and review of the literature

Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

KLHL40-Related Myopathy: A Systematic Review and Insight into a Follow-up Biomarker via a New Case Report

BACKGROUND

Mutations in the KLHL40 gene are a common cause of severe or even lethal nemaline myopathy. Some cases with mild forms have been described, although the cases are still anecdotal. The aim of this paper was to systematically review the cases described in the literature and to describe a 12-year clinical and imaging follow-up in an Italian patient with KLHL40- related myopathy in order to suggest possible follow-up measurements.

METHODS

Having searched through three electronic databases (PubMed, Scopus, and EBSCO), 18 articles describing 65 patients with homozygous or compound heterozygous KLHL40 mutations were selected. A patient with a KLHL40 homozygous mutation (c.1582G>A/p.E528K) was added and clinical and genetic data were collected.

RESULTS

The most common mutation identified in our systematic review was the (c.1516A>C) followed by the (c.1582G>A). In our review, 60% percent of the patients died within the first 4 years of life. Clinical features were similar across the sample. Unfortunately, however, there is no record of the natural history data in the surviving patients. The 12-year follow-up of our patient revealed a slow improvement in her clinical course, identifying muscle MRI as the only possible marker of disease progression.

CONCLUSIONS

Due to its clinical and genotype homogeneity, KLHL40-related myopathy may be a condition that would greatly benefit from the development of new gene therapies; muscle MRI could be a good biomarker to monitor disease progression.

Diagnostic yield of a practical electrodiagnostic protocol discriminating between different congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a group of heterogeneous diseases of the neuromuscular junction. We report electrodiagnostic testing (EDX) and genetic findings in a series of 120 CMS patients tested with a simple non-invasive EDX workup with surface recording of CMAPs and 3Hz repetitive nerve stimulation of accessory, radial and deep fibular nerves. Five ENMG phenotypes were retrieved based on the presence or not of R-CMAPs and the distribution pattern of decremental CMAP responses which significantly correlated with genetic findings (p <0.00001). R-CMAPs were found in all COLQ-mutated patients (CMS1A) and Slow Channel CMS (SCCMS) (CMS1B). CMS1A exhibited greater decrements in accessory nerve RNS than CMS1B. Patients without R-CMAPs were classified into CMS2A (DOK7-, MUSK-, GFPT1-, GMPPB-, TOR1AIP-mutated) when exhibiting predominant accessory nerve RNS decrements, CMS2B (CHRNE, CHRND, RAPSN) with predominant radial nerve RNS decrements, or CMS2C (AGRN) if there were predominant fibular decrements. Our algorithm may have a major impact on diagnostic and therapeutic monitoring in CMS patients, as well as for validation of the pathogenicity of genetic variants. It should also be part of the evaluation of unexplained muscle weakness or complex neuromuscular phenotypes.

Early clinical and pre-clinical therapy development in Nemaline myopathy

INTRODUCTION

Nemaline myopathies (NM) represent a group of clinically and genetically heterogeneous congenital muscle disorders with the common denominator of nemaline rods on muscle biopsy. NEB and ACTA1 are the most common causative genes. Currently, available treatments are supportive.

AREAS COVERED

We explored experimental treatments for NM, identifying at least eleven mainly pre-clinical approaches utilizing murine and/or human muscle cells. These approaches target either i) the causative gene or associated genes implicated in the same pathway; ii) pathophysiologically relevant biochemical mechanisms such as calcium/myosin regulation of muscle contraction; iii) myogenesis; iv) other therapies that improve or optimize muscle function more generally; v) and/or combinations of the above. The scope and efficiency of these attempts is diverse, ranging from gene-specific effects to those widely applicable to all NM-associated genes.

EXPERT OPINION

The wide range of experimental therapies currently under consideration for NM is promising. Potential translation into clinical use requires consideration of additional factors such as the potential muscle type specificity as well as the possibility of gene expression remodeling. Challenges in clinical translation include the rarity and heterogeneity of genotypes, phenotypes, and disease trajectories, as well as the lack of longitudinal natural history data and validated outcomes and biomarkers.

Novel copy number variation of COLQ gene in a Moroccan patient with congenital myasthenic syndrome: a case report and review of the literature

Background

Congenital myasthenic syndromes (CMSs) are rare genetic diseases due to abnormalities of the neuromuscular junction leading to permanent or transient muscle fatigability and weakness. To date, 32 genes were found to be involved in CMSs with autosomal dominant and/or recessive inheritance patterns. CMS with acetylcholinesterase deficiency, in particular, was determined to be due to biallelic mutations of COLQ gene with early-onset clinical signs. Here, we report clinical features and novel molecular findings of COLQ-related CMS in a Moroccan patient with a review of the literature for this rare form.

Case presentation

In this study, we report the case of a 28-month-old Moroccan female patient with hypotonia, associated to axial muscle weakness, global motor delay, bilateral ptosis, unilateral partial visual field deficiency with normal ocular motility, and fatigable muscle weakness. Clinical exome sequencing revealed a novel homozygous deletion of exon 13 in COLQ gene, NM_005677.4(COLQ):c.(814+1_815-1)_(954+1_955-1) del p.(Gly272Aspfs*11). This finding was subsequently confirmed by quantitative real-time PCR (qPCR) in the proband and her parents. In silico analysis of protein-protein interaction network by STRING tool revealed that 12 proteins are highly associated to COLQ with an elevated confidence score. Treatment with Salbutamol resulted in clear benefits and recovery.

Conclusions

This clinical observation illustrates the important place of next-generation sequencing in the precise molecular diagnosis of heterogeneous forms of CMS, the appropriate management and targeted treatment, and genetic counseling of families, with a better characterization of the mutational profile of this rare disease in the Moroccan population.

Congenital myasthenic syndromes: where do we go from here?

Congenital myasthenia syndromes are rare but often treatable conditions affecting neuromuscular transmission. They result from loss or impaired function of one of a number of proteins secondary to a genetic defect. An estimate of the prevalence in the UK gave 9.2 cases per million, however, this is likely an underestimate since the adoption of next generation sequencing for diagnosis away from specialist centres is enhancing the 'pick up' rate. Next generation sequencing has helped identify a series of novel genes that harbour mutations causative for congenital myasthenic syndrome that include not only genes that encode proteins specifically expressed at the neuromuscular junction but also those that are ubiquitously expressed. The list of genes harbouring disease-causing mutations for congenital myasthenic syndrome continues to expand and is now over 30, but with many of the newly identified genes it is increasingly being recognised that abnormal neuromuscular transmission is only one component of a multifaceted phenotype in which muscle, the central nervous system, and other organs may also be affected. Treatment can be tailored to the underlying molecular mechanism for impaired neuromuscular transmission but treating the more complex multifaceted disorders and will require development of new therapies.

The Relevance of Blepharoptosis in Diagnostic Suspicion of Myopathies

Blepharoptosis (ptosis) is classified, based on etiology, into mechanical, cerebral, neurogenic, neuromuscular, myogenic, and due to miscellaneous causes. Primary myopathic diseases are rare causes of blepharoptosis and many patients with myogenic ptosis undergo a series of extensive investigations before a myopathy is being considered. In this study, we report four patients with different myopathic disorders who had blepharoptosis as a presenting symptom of their disease. Moreover, we highlight frequent diagnostic errors and difficulties in patients with myopathies who present blepharoptosis. Lack of clear cut aggravation of symptoms by fatigue and response to cholinesterase inhibitors treatment, the association of proximal, distal or extraocular muscle weakness, and positive family history or evidence of a multi systemic disorder should prompt evaluation of an underlying myopathy.

A C. elegans genome-wide RNAi screen for altered levamisole sensitivity identifies genes required for muscle function

At the neuromuscular junction (NMJ), postsynaptic ionotropic acetylcholine receptors (AChRs) transduce a chemical signal released from a cholinergic motor neuron into an electrical signal to induce muscle contraction. To identify regulators of postsynaptic function, we conducted a genome-wide RNAi screen for genes required for proper response to levamisole, a pharmacological agonist of ionotropic L-AChRs at the Caenorhabditis elegans NMJ. A total of 117 gene knockdowns were found to cause levamisole hypersensitivity, while 18 resulted in levamisole resistance. Our screen identified conserved genes important for muscle function including some that are mutated in congenital myasthenic syndrome, congenital muscular dystrophy, congenital myopathy, myotonic dystrophy, and mitochondrial myopathy. Of the genes found in the screen, we further investigated those predicted to play a role in endocytosis of cell surface receptors. Loss of the Epsin homolog epn-1 caused levamisole hypersensitivity and had opposing effects on the levels of postsynaptic L-AChRs and GABA_A receptors, resulting in increased and decreased abundance, respectively. We also examined other genes that resulted in a levamisole-hypersensitive phenotype when knocked down including gas-1, which functions in Complex I of the mitochondrial electron transport chain. Consistent with altered ATP synthesis impacting levamisole response, treatment of wild-type animals with levamisole resulted in L-AChR–dependent depletion of ATP levels. These results suggest that the paralytic effects of levamisole ultimately lead to metabolic exhaustion.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

Molecular Mechanisms of Pathologies of Skeletal and Cardiac Muscles Caused by Point Mutations in the Tropomyosin Genes

The review is devoted to tropomyosin (Tpm) - actin-binding protein, which plays a crucial role in the regulation of contraction of skeletal and cardiac muscles. Special attention is paid to myopathies and cardiomyopathies - severe hereditary diseases of skeletal and cardiac muscles associated with point mutations in Tpm genes. The current views on the molecular mechanisms of these diseases and the effects of such mutations on the Tpm structure and functions are considered in detail. Besides, some part of the review is devoted to analysis of the properties of Tpm homodimers and heterodimers with myopathic substitutions of amino acid residues in only one of the two chains of the Tpm dimeric molecule.

Neuromuscular Diseases of the Newborn

The peripheral nervous system (PNS) is composed of motor neurons, nerve roots, plexuses, peripheral nerves (motor, sensory and autonomic), neuromuscular junction, and skeletal muscles. Disorders of the PNS in neonates most frequently cause weakness, hypotonia, and contractures, which may be generalized or focal. Since these findings may also occur with brain and spinal cord lesions, key features of the history and neurologic exam, together with diagnostic testing, are helpful in reaching a diagnosis. This review covers the diagnostic approach to PNS disorders in the neonate and includes a discussion of representative diseases of the motor neuron, brachial plexus, peripheral nerves, neuromuscular junction, and muscles. The importance of reaching a precise genetic diagnosis is highlighted with a discussion of current and emerging treatments for neonatal PNS diseases, particularly spinal muscular atrophy.

Trouble at the junction: When myopathy and myasthenia overlap

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

[Research advances in limb-girdle muscular dystrophy type 2Q]

Limb-girdle muscular dystrophy (LGMD) is a group of muscular dystrophies with predominantly proximal muscular weakness, and some genes associated with this disease have been identified at present. LGMD type 2Q (LGMD2Q) is a subtype of LGMD and is associated with PLEC gene mutation. Major phenotypes of PLEC gene mutation include epidermolysis bullosa with late-onset muscular dystrophy and epidermolysis bullosa with other lesions. LGMD2Q without skin lesions is rarely reported. This article reviews the pathogenic gene PLEC and clinical manifestations of LGMD2Q, so as to deepen the understanding of the pathogenic gene and phenotype of LGMD2Q.

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.

Congenital myasthenic syndromes

OBJECTIVES

Congenital myasthenic syndromes (CMSs) are a genotypically and phenotypically heterogeneous group of neuromuscular disorders, which have in common an impaired neuromuscular transmission. Since the field of CMSs is steadily expanding, the present review aimed at summarizing and discussing current knowledge and recent advances concerning the etiology, clinical presentation, diagnosis, and treatment of CMSs.

METHODS

Systematic literature review.

RESULTS

Currently, mutations in 32 genes are made responsible for autosomal dominant or autosomal recessive CMSs. These mutations concern 8 presynaptic, 4 synaptic, 15 post-synaptic, and 5 glycosilation proteins. These proteins function as ion-channels, enzymes, or structural, signalling, sensor, or transporter proteins. The most common causative genes are CHAT, COLQ, RAPSN, CHRNE, DOK7, and GFPT1. Phenotypically, these mutations manifest as abnormal fatigability or permanent or fluctuating weakness of extra-ocular, facial, bulbar, axial, respiratory, or limb muscles, hypotonia, or developmental delay. Cognitive disability, dysmorphism, neuropathy, or epilepsy are rare. Low- or high-frequency repetitive nerve stimulation may show an abnormal increment or decrement, and SF-EMG an increased jitter or blockings. Most CMSs respond favourably to acetylcholine-esterase inhibitors, 3,4-diamino-pyridine, salbutamol, albuterol, ephedrine, fluoxetine, or atracurium.

CONCLUSIONS

CMSs are an increasingly recognised group of genetically transmitted defects, which usually respond favorably to drugs enhancing the neuromuscular transmission. CMSs need to be differentiated from neuromuscular disorders due to muscle or nerve dysfunction.

The unfolding spectrum of inherited distal myopathies

Distal myopathies are a group of rare muscle diseases characterized by distal weakness at onset. Although acquired myopathies can occasionally present with distal weakness, the majority of distal myopathies have a genetic etiology. Their age of onset varies from early-childhood to late-adulthood while the predominant muscle weakness can affect calf, ankle dorsiflexor, or distal upper limb muscles. A spectrum of muscle pathological changes, varying from nonspecific myopathic changes to rimmed vacuoles to myofibrillar pathology to nuclei centralization, have been noted. Likewise, the underlying molecular defect is heterogeneous. In addition, there is emerging evidence that distal myopathies can result from defective proteins encoded by genes causative of neurogenic disorders, be manifestation of multisystem proteinopathies or the result of the altered interplay between different genes. In this review, we provide an overview on the clinical, electrophysiological, pathological, and molecular aspects of distal myopathies, focusing on the most recent developments in the field. Muscle Nerve 59:283-294, 2019.

Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers

Tropomyosin (Tpm) is an actin-binding protein that plays a vital role in the regulation of muscle contraction. Fast skeletal muscles express 2 Tpm isoforms, α (Tpm 1.1) and β (Tpm 2.2), resulting in the existence of 2 forms of dimeric Tpm molecule: αα-homodimer and αβ-heterodimer. ββ-Homodimer is unstable and absent in the native state, despite which most of the studies of myopathy-relating Tpm mutations have been performed on the ββ-homodimer. Here, we applied different methods to investigate the effects of myopathic mutations R133W and N202K in the β-chain of Tpm on properties of αβ-heterodimers and to compare them with the features of ββ-homodimers with the same mutations. The results show that properties of αβ-Tpm and ββ-Tpm with substitutions in the β-chain differ significantly, and this indicates that the effects of myopathic mutations in the Tpm β-chain should be studied on the Tpm αβ-heterodimer.-Bershitsky, S. Y., Logvinova, D. S., Shchepkin, D. V., Kopylova, G. V., Matyushenko, A. M. Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers.

A Rare Case of Severe Congenital RYR1-Associated Myopathy

Congenital myopathies are a group of rare inherited diseases, defined by hypotonia and muscle weakness. We report clinical and genetic characteristics of a male preterm newborn, whose phenotype was characterized by severe hypotonia and hyporeactivity, serious respiratory distress syndrome that required mechanical ventilation, clubfoot, and other dysmorphic features. The diagnostic procedure was completed with the complete exome sequencing of the proband and of his parents and his sister, which showed new mutations in the ryanodine receptor gene (RYR1), which maps to chromosome 19q13.2 and encodes the skeletal muscle isoform of a calcium-release channel in the sarcoplasmic reticulum (RyR1). This report confirms that early diagnosis and accurate study of genomic disorders are very important, enabling proper genetic counselling of the reproductive risk, as well as disease prognosis and patient management.

The Neuromuscular Junction and Wide Heterogeneity of Congenital Myasthenic Syndromes

Congenital myasthenic syndromes (CMS) are genetic disorders characterised by impaired neuromuscular transmission. This review provides an overview on CMS and highlights recent advances in the field, including novel CMS causative genes and improved therapeutic strategies. CMS due to mutations in SLC5A7 and SLC18A3, impairing the synthesis and recycling of acetylcholine, have recently been described. In addition, a novel group of CMS due to mutations in SNAP25B, SYT2, VAMP1, and UNC13A1 encoding molecules implicated in synaptic vesicles exocytosis has been characterised. The increasing number of presynaptic CMS exhibiting CNS manifestations along with neuromuscular weakness demonstrate that the myasthenia can be only a small part of a much more extensive disease phenotype. Moreover, the spectrum of glycosylation abnormalities has been increased with the report that GMPPB mutations can cause CMS, thus bridging myasthenic disorders with dystroglycanopathies. Finally, the discovery of COL13A1 mutations and laminin α5 deficiency has helped to draw attention to the role of extracellular matrix proteins for the formation and maintenance of muscle endplates. The benefit of β2-adrenergic agonists alone or combined with pyridostigmine or 3,4-Dyaminopiridine is increasingly being reported for different subtypes of CMS including AChR-deficiency and glycosylation abnormalities, thus expanding the therapeutic repertoire available.

Myopathology in congenital myopathies

Congenital myopathies are clinically and genetically a heterogeneous group of early onset neuromuscular disorders, characterized by hypotonia and muscle weakness. Clinical severity and age of onset are variable. Many patients are severely affected at birth while others have a milder, moderately progressive or nonprogressive phenotype. Respiratory weakness is a major clinical aspect that requires regular monitoring. Causative mutations in several genes have been identified that are inherited in a dominant, recessive or X-linked manner, or arise de novo. Muscle biopsies show characteristic pathological features such as nemaline rods/bodies, cores, central nuclei or caps. Small type 1 fibres expressing slow myosin are a common feature and may sometimes be the only abnormality. Small cores (minicores) devoid of mitochondria and areas showing variable myofibrillar disruption occur in several neuromuscular disorders including several forms of congenital myopathy. Muscle biopsies can also show more than one structural defect. There is considerable clinical, pathological and genetic overlap with mutations in one gene resulting in more than one pathological feature, and the same pathological feature being associated with defects in more than one gene. Increasing application of whole exome sequencing is broadening the clinical and pathological spectra in congenital myopathies, but pathology still has a role in clarifying the pathogenicity of gene variants as well as directing molecular analysis.

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.

Glu20Ter Variant in PLEC 1f Isoform Causes Limb-Girdle Muscle Dystrophy with Lung Injury

Plectinopathies are orphan diseases caused by PLEC gene mutations. PLEC is encoding the protein plectin, playing a role in linking cytoskeleton components in various tissues. In this study, we describe the clinical case of a 26-year-old patient with an early onset plectinopathy variant “limb-girdle muscle dystrophy type 2Q,” report histopathological and ultrastructural findings in m. vastus lateralis biopsy and a novel homozygous likely pathogenic variant (NM_201378.3:c.58G>T, NP_958780.1:p.Glu20Ter) in isoform 1f of the gene PLEC. The patient had an early childhood onset with retarded physical development, moderate weakness in pelvic girdle muscles, progressive weakening of limb-girdle muscles after the age of 21, pronounced atrophy of axial muscles, and hypertrophy of the gastrocnemius, deltoid, and triceps muscles, intermittent dyspnea, and no skin involvement. Findings included: non-infectious bronchiolitis and atelectasis signs, biopsy revealed myodystrophal pattern without macrophage infiltration, muscle fiber cytoskeleton disorganization resulted from the plectin loss, incomplete reparative rhabdomyogenesis, and moderate endomysial fibrosis. We have determined a novel likely pathogenic variant in PLEC 1f isoform that causes limb-girdle muscle dystrophy type 2Q and described the third case concerning an isolated myodystrophic phenotype of LGMD2Q with the likely pathogenic variant in PLEC 1f isoform. In addition, we have demonstrated the presence of severe lung injury in a patient and his siblings with the same myodystrophic phenotype and discussed the possible role of plectin deficiency in its pathogenesis.

Treatment of muscle weakness in neuromuscular disorders

INTRODUCTION

Weakness is one of the predominant clinical manifestations of neuromuscular disorders (NMDs), which strongly influences daily life, prognosis, and outcome of affected patients. One of the major therapeutic goals in NMD-patients is to completely resolve muscle weakness. Various treatment options are available and include physical therapy, electrotherapy, diet, drugs, avoidance or withdrawal of muscle-toxic and weakness-inducing agents, detoxification, stem-cell-therapy, plasma-exchange, respiratory therapy, or surgery. Most accessible to treatment is weakness from immune-mediated neuropathies, immune-mediated transmission-disorders, and idiopathic immune myopathies. Areas covered: This manuscript aims to summarize and discuss recent findings and future perspectives concerning the treatment of muscle weakness in NMDs. Data were obtained by a literature search in databases such as PubMed and Current-Contents. Expert commentary: Weakness is most easily treatable in acquired NMDs and in hereditary myopathies and neuropathies beneficial treatment options are also available. Research needs to be encouraged and intensified to further expand the spectrum of treatment options for weakness.

Anticholinesterase Therapy Worsening Head Drop and Limb Weakness Due to a Novel DOK7 Mutation

Dok-7 myasthenia is an autosomal recessive congenital myasthenic syndrome due to DOK7 mutations. Anticholinesterase therapy is ineffective and may worsen the weakness in patients with Dok-7 myasthenia or few other forms of congenital myasthenic syndromes. We describe a 31-year-old man previously diagnosed with seronegative myasthenia gravis. Repetitive stimulation of the right spinal accessory nerve showed 51% decrement. Needle electromyography revealed myopathic changes in clinically affected muscles. Muscle biopsy was normal. The patient was referred to us for worsening weakness after taking pyridostigmine. We searched for DOK7 mutations and identified compound heterozygous mutations of a common c.1124_1127dupTGCC mutation and a novel splice site mutation, c.772+2_+4delinsCCGGGCAGGCGGGCA. Discontinuation of pyridostigmine improved weakness. He further regained strength with oral albuterol therapy and decrement was reduced to 25%. Worsening of symptoms with anticholinesterase therapy in patients with "seronegative myasthenia gravis" should prompt clinicians to consider a possibility of congenital myasthenic syndromes to avoid unnecessary use of immunosuppressive therapy. Patients with Dok-7 myasthenia respond well to oral albuterol treatment.

Assessment of the functionality and stability of detergent purified nAChR from Torpedo using lipidic matrixes and macroscopic electrophysiology

In our previous study we examined the functionality and stability of nicotinic acetylcholine receptor (nAChR)-detergent complexes (nAChR-DCs) from affinity-purified Torpedo californica (Tc) using fluorescence recovery after photobleaching (FRAP) in Lipidic Cubic Phase (LCP) and planar lipid bilayer (PLB) recordings for phospholipid and cholesterol like detergents. In the present study we enhanced the functional characterization of nAChR-DCs by recording macroscopic ion channel currents in Xenopus oocytes using the two electrode voltage clamp (TEVC). The use of TEVC allows for the recording of macroscopic currents elicited by agonist activation of nAChR-DCs that assemble in the oocyte plasma membrane. Furthermore, we examined the stability of nAChR-DCs, which is obligatory for the nAChR crystallization, using a 30 day FRAP assay in LCP for each detergent. The present results indicate a marked difference in the fractional fluorescence recovery (ΔFFR) within the same detergent family during the 30 day period assayed. Within the cholesterol analog family, sodium cholate and CHAPSO displayed a minimum ΔFFR and a mobile fraction (MF) over 80%. In contrast, CHAPS and BigCHAP showed a marked decay in both the mobile fraction and diffusion coefficient. nAChR-DCs containing phospholipid analog detergents with an alkylphosphocholine (FC) and lysofoscholine (LFC) of 16 carbon chains (FC-16, LFC-16) were more effective in maintaining a mobile fraction of over 80% compared to their counterparts with shorter acyl chain (C12, C14). The significant differences in macroscopic current amplitudes, activation and desensitization rates among the different nAChR-DCs evaluated in the present study allow to dissect which detergent preserves both, agonist activation and ion channel function. Functionality assays using TEVC demonstrated that LFC16, LFC14, and cholate were the most effective detergents in preserving macroscopic ion channel function, however, the nAChR-cholate complex display a significant delay in the ACh-induce channel activation. In summary, these results suggest that the physical properties of the lipid analog detergents (headgroup and acyl chain length) are the most effective in maintaining both the stability and functionality of the nAChR in the detergent solubilized complex.

Congenital myasthenic syndrome due to mutation in CHRNE gene with clinical worsening and thymic hyperplasia attributed to association with autoimmune-myasthenia gravis

We report a patient with congenital myasthenic syndrome (CMS) due to mutation in CHRNE with symptoms since the age of 4; mild to moderate fatigable weakness involved mainly ocular, bulbar and limb muscles; functional impact of the disease in their development and physical activity was modest. By the age of 34, the patient experienced gradual worsening of fatigue with dyspnoea and pronounced limb weakness, requiring significant increase of pyridostigmine. Further, a remarkable and sustained clinical improvement followed thymectomy with hyperplastic thymus. Despite of the absence of detectable antibodies to acetyl-choline receptor (AChR) (including clustered-AChR), muscle-specific kinase and low-density lipoprotein receptor-related protein-4 antibodies in the serum obtained nine years after thymectomy, the clinical, genetic and histological features are in keeping with the extremely rare association of two rare neuromuscular junction disorders - CMS and myasthenia gravis (MG). The inexistence of other conditions that could potentially associate with thymic hyperplasia also supports the diagnosis of MG.

Inherited disorders of the neuromuscular junction: an update

Congenital myasthenic syndromes (CMSs) are a group of heterogeneous inherited disorders caused by mutations in genes affecting the function and structure of the neuromuscular junction. This review updates the reader on established and novel subtypes of congenital myasthenia, and the treatment strategies for these increasingly heterogeneous disorders. The discovery of mutations associated with the N-glycosylation pathway and in the family of serine peptidases has shown that causative genes encoding ubiquitously expressed molecules can produce defects at the human neuromuscular junction. By contrast, mutations in lipoprotein-like receptor 4 (LRP4), a long-time candidate gene for congenital myasthenia, and a novel phenotype of myasthenia with distal weakness and atrophy due to mutations in AGRN have now been described. In addition, a pathogenic splicing mutation in a nonfunctional exon of CHRNA1 has been reported emphasizing the importance of analysing nonfunctional exons in genetic analysis. The benefit of salbutamol and ephedrine alone or combined with pyridostigmine or 3,4-DAP is increasingly being reported for particular subtypes of CMS.