126th International Workshop: congenital myasthenic syndromes, 24-26 September 2004, Naarden, the Netherlands

Pediatric Neuromuscular Diseases

The diagnostic and referral workflow for children with neuromuscular disorders is evolving, particularly as newborn screening programs are expanding in tandem with novel therapeutic developments. However, for the children who present with symptoms and signs of potential neuromuscular disorders, anatomic localization, guided initially by careful history and physical examination, continues to be the cardinal initial step in the diagnostic evaluation. It is important to consider whether the localization is more likely to be in the lower motor neuron, peripheral nerve, neuromuscular junction, or muscle. After that, disease etiologies can be divided broadly into inherited versus acquired categories. Considerations of localization and etiologies will help generate a differential diagnosis, which in turn will guide diagnostic testing. Once a diagnosis is made, it is important to be aware of current treatment options, as a number of new therapies for some of these disorders have been approved in recent years. Families are also increasingly interested in clinical research, which may include natural history studies and interventional clinical trials. Such research has proliferated for rare neuromuscular diseases, leading to exciting advances in diagnostic and therapeutic technologies, promising dramatic changes in the landscape of these disorders in the years to come.

Congenital Myasthenic Syndromes in Turkey: Clinical and Molecular Characterization of 16 Cases With Three Novel Mutations

BACKGROUND

Congenital myasthenic syndromes (CMS) are composed of numerous hereditary disorders involving genetic mutations in proteins essential to the integrity of neuromuscular transmission. The symptoms of CMS vary according to the age at onset of symptoms, and the type and severity of muscle weakness. Effective treatment and genetic counseling depend upon the underlying pathogenic molecular mechanism and subtype of CMS.

METHODS

A retrospective and cross-sectional study was performed with 16 patients with a genetically confirmed diagnosis of CMS to share our experience with clinical symptoms, demographic data, genetic variants, and treatments applied.

RESULTS

Sixteen patients with a specific CMS genetic diagnosis (three novel mutations) were identified, including CHRNE (n = 7), DOK7 (n = 2), AGRN (n = 2), RAPSN (n = 1), CHRNA1 (n = 1), CHRNB1 (n = 1), CHAT (n = 1), and SCN4A (n = 1). Age at onset of symptoms ranged from the neonatal period to 12 years. Genetic diagnosis was confirmed between the ages of three months and 17 years. A significant delay was determined between the onset of symptoms and genetic diagnosis of the disease.

CONCLUSIONS

This study highlights the importance of genetic testing in CMS. Due to the rarity of CMS, more cases will be recognized and reported as the use of laboratory and genetic testing accelerates. We hope that our experience will grow and contribute further to the literature as clinical follow-up and treatment increase.

Clinicopathological-genetic features of congenital myasthenic syndrome from a Chinese neuromuscular centre

Congenital myasthenic syndrome (CMS) encompasses a heterogeneous group of inherited disorders affecting nerve transmission across the neuromuscular junction. The aim of this study was to characterize the clinical, physiological, pathohistological and genetic features of nine unrelated Chinese patients with CMS from a single neuromuscular centre. A total of nine patients aged from neonates to 34 years were enrolled who exhibited initial symptoms. Physical examinations revealed that all patients exhibited muscle weakness. Muscle biopsies demonstrated multiple myopathological changes, including increased fibre size variation, myofibrillar network disarray, necrosis, myofiber grouping, regeneration, fibre atrophy and angular fibres. Genetic testing revealed six different mutated genes, including AGRN (2/9), CHRNE (1/9), GFPT1 (1/9), GMPPB (1/9), PLEC (3/9) and SCN4A (1/9). In addition, patients exhibited differential responses to pharmacological treatment. Prompt utilization of genetic testing will identify novel variants and expand our understanding of the phenotype of this rare syndrome. Our findings contribute to the clinical, pathohistological and genetic spectrum of congenital myasthenic syndrome in China.

Congenital myasthenic syndrome in Turkey: clinical and genetic features in the long-term follow-up of patients

Congenital Myasthenic Syndromes (CMS) are rare disorders that occur as a result of defects in the structure and in the function of neuromuscular junctions. Molecular genetic diagnosis is important to select the most suitable therapeutic option and treatment. Eight patients with congenital myasthenic syndromes who presented to the Çukurova University Pediatric Neurology Department Outpatient Clinic between June 2015 and May 2018 were reviewed. Mutations in the acetylcholine receptor (subunits in epsilon) (CHRNE) in three and mutations in the collagenic tail of endplate acetylcholinesterase (COLQ) gene in five patients were identified; p.W148 mutation was detected to be homozygous in four, c.1169A > G novel mutation in COLQ gene was homozygous in one, c452_454delAGG mutation was homozygous in the other patient, IVS7 + 2T > C(c.802 + 2T > C) mutation was homozygous in a patient and compound heterozygous mutations of c.865C > T(p.Leu289Phe) and c.872C > G(p.A2916)(p.Arg291Gly) in the CHRNE gene in the last patient. The parents of all the evaluated patients were consanguineous. Ptosis, ophthalmoplegia, generalized hypotonia, bulbar weakness, and respiratory crisis were the main findings at the time of presentation. Pyridostigmine is the first-line drug therapy in primary AChR deficiency. Beta adrenergic agonists, ephedrine, and albuterol are the other treatment options for CMS subtypes caused by mutations in COLQ. This study points out the genetic and phenotypic features of CMS patients in the Turkish population and it also reports previously unreported mutations in the literature. CHRNE and COLQ gene mutations are common in the Turkish population. Patients can get serious benefits and recover after the treatment. The treatment should be planned according to genetic tests and clinical findings.

A case report of an intermediate phenotype between congenital myasthenic syndrome and D-2- and L-2-hydroxyglutaric aciduria due to novel SLC25A1 variants

Background

Variants in the SLC25A1 gene are associated with a severe neurometabolic disease, D-2- and L-2-hydroxyglutaric aciduria (D/L-2-HGA). A report in 2014 presented the first account of congenital myasthenic syndrome (CMS) with mild intellectual disability (ID) caused by SLC25A1. To date, only two missense variants in SLC25A1 have been linked to CMS.

Case presentations

A Chinese boy presented fatigable muscular weakness, myasthenic crisis, epilepsy and developmental delay along with mild elevation of urinary 2-ketoglutarate (2-KG) and lactic acid levels. He showed a partial response to pyridostigmine. Genetic analysis using trio whole-exome sequencing (WES), Sanger sequencing, and cosegregation analyses revealed two novel pathogenic variants of SLC25A1 (c.628C > T, p.R210X; c.145G > A, p.V49M).

Conclusions

We report a boy who carries novel compound heterozygous variants of SLC25A1 and presents a phenotype intermediate between CMS and D/L-2-HGA. This case expands the range of known phenotypes and genotypes associated with SLC25A1.

Calcium influx through muscle nAChR-channels: One route, multiple roles

Although Ca²⁺ influx through muscle nAChR-channels has been described over the past 40 years, its functions remain still poorly understood. In this review we suggest possible roles of Ca²⁺ entry at all stages of muscle development, summarizing the evidence present in literature. nAChRs are expressed in myoblasts prior to fusion, and can be activated in the absence of an ACh-releasing nerve terminal, with Ca²⁺ influx likely contributing to regulate cell fusion. Upon establishment of nerve-muscle contact, Ca²⁺ influx contributes to orchestrate the signaling required for the correct formation of the neuromuscular junction. Finally, in the mature synapse, Ca²⁺ entry through postsynaptic nAChR-channels - highly Ca²⁺ permeable, in particular in humans - acts on K⁺ and Na⁺ channels to shape endplate excitability. However, when genetic defects cause excessive channel activation, Ca²⁺ influx becomes toxic and causes endplate myopathy. Throughout the review, we highlight how Ricardo Miledi has contributed to construct this whole body of knowledge, from the initial description of Ca²⁺ permeability of endplate nAChR channels, to the rationale for the treatment of endplate excitotoxic damage under pathological conditions. This article is part of a Special Issue entitled: SI: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

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.

Italian recommendations for diagnosis and management of congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are genetic disorders due to mutations in genes encoding proteins involved in the neuromuscular junction structure and function. CMS usually present in young children, but perinatal and adult onset has been reported. Clinical presentation is highly heterogeneous, ranging from mild symptoms to severe manifestations, sometimes with life-threatening respiratory episodes, especially in the first decade of life. Although considered rare, CMS are probably underestimated due to diagnostic difficulties. Because of the several therapeutic opportunities, CMS should be always considered in the differential diagnosis of neuromuscular disorders. The Italian Network on CMS proposes here recommendations for proper CMS diagnosis and management, aiming to guide clinicians in their practical approach to CMS patients.

[Congenital myasthenic syndromes in adulthood : Challenging, rare but treatable]

The congenital myasthenic syndromes (CMS) represent a heterogeneous group of diseases with a broad spectrum of phenotypes. The common characteristic is an inherited genetic defect of the neuromuscular junction. Although in some patients the specific gene defect remains to be detected, the increasing identification of causative genes in recent years has already provided unique insights into the functionality of structural proteins at the neuromuscular junction. Neonatal and early childhood onset is observed in most CMS subtypes; however, late onset in adolescence or adulthood also occurs and establishing the diagnosis at these stages imposes particular challenges. To enable appropriate therapeutic interventions for an at least in principle treatable condition, determining the genetic cause is warranted. In this overview, the critical clinical and diagnostic features of the different CMS subtypes are presented and illustrated using typical cases. Furthermore, specific diagnostic clues are outlined. Finally, the overlap between CMS and muscular dystrophies is discussed. Illustrating characteristic patient examples, the essential clinical and additional diagnostic findings of various CMS subtypes and special diagnostic indications are presented.

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.

A common CHRNE mutation in Brazilian patients with congenital myasthenic syndrome

The most common causes of congenital myasthenic syndromes (CMS) are CHRNE mutations, and some pathogenic allelic variants in this gene are especially frequent in certain ethnic groups. In the southern region of Brazil, a study found the c.130dupG CHRNE mutation in up to 33% of families with CMS. Here, we aimed to verify the frequency of this mutation among individuals with CMS in a larger cohort of CMS patients from different areas of Brazil and to characterize clinical features of these patients. Eighty-four patients with CMS, from 72 families, were clinically evaluated and submitted to direct sequencing of the exon 2 of CHRNE. The c.130dupG mutation was found in 32 patients (23 families), with 26 patients (19 families, 26.3%) in homozygosis, confirming its high prevalence in different regions of Brazil. Among the homozygous patients, the following characteristics were frequent: onset of symptoms before 2 years of age (92.3%), little functional restriction (92.3%), fluctuating symptoms (100%), ocular muscle impairment (96.1%), ptosis (100%), limb weakness (88.4%), response to pyridostigmine (100%), facial involvement (77%), and bulbar symptoms (70.8%). The pretest probability of finding at least one allele harbouring the c.130dupG mutation was 38.1%. Selecting only patients with impaired eye movement together with limb weakness and improvement with pyridostigmine, the probability increases to 72.2%. This clinical pre-selection of patients is likely a useful tool for regions where CHRNE mutations have a founder effect. In conclusion, the CHRNE mutation c.130dupG leads to fairly benign natural course of the disease with relative homogeneity.

Sleep in infants with congenital myasthenic syndromes

BACKGROUND AND OBJECTIVES

Infants with congenital myasthenic syndrome (CMS) are at risk of brief resolved unexplained event (BRUE) and sleep-disordered breathing. The aim of the study was to explore sleep in infants with CMS with a particular focus on heart rate (HR) variability.

METHODS

Overnight polygraphy was performed and HR variations associated with respiratory events were analysed. Bradycardia and tachycardia were defined as a variation of HR of ±10 bpm from baseline and analysed as events/hour.

RESULTS

The data of 5 infants with CMS were analysed. Two patients had known mutations (COLQ and RAPSN). One patient had a tracheostomy. The apnoea-hypopnoea index (AHI) was abnormal in all the patients (range 2.8-47.7 events/h), with the highest AHI being observed in the 3 youngest infants. Nocturnal transcutaneous gas exchange was normal in all patients except the tracheostomised patient. Mean HR was 114 ± 23 bpm with a mean HR index of 4.5 ± 4.3 events/h. The amplitudes of HR variations (bradycardia or tachycardia) were around 15-20 bpm, regardless of the type of respiratory event, and comparable between patients. No correlations were found between HR indexes or variations and the type and mean duration of respiratory events. Ventilatory support was initiated in 3 infants immediately after the sleep study because of a high AHI and/or nocturnal hypoventilation.

CONCLUSIONS

All 5 infants had an abnormal AHI with younger infants having the highest AHI. Three infants required ventilatory support after the polygraphy, underlining its clinical usefulness. No significant abnormalities of HR were observed during the sleep studies.

A Missense Mutation in Epsilon-subunit of Acetylcholine Receptor Causing Autosomal Dominant Slow-channel Congenital Myasthenic Syndrome in a Chinese Family

BACKGROUND

Congenital myasthenic syndromes are a group of rare disorders that are clinically and genetically heterogeneous and caused by mutations in the genes encoding proteins of the neuromuscular junction. Here, we described a Chinese family that presented with phenotypes of classic slow-channel congenital myasthenic syndrome (SCCMS).

METHODS

Clinical characteristics and electrophysiological features of three patients from a Chinese family were examined, and next-generation sequencing followed by direct sequencing was carried out.

RESULTS

The patients revealed variability in clinical and electrophysiological features. However, weakness, scoliosis, and repetitive-compound muscle action potential were found in all affected members in the family. A heterozygous C>T missense mutation at nucleotide 865 in acetylcholine receptor epsilon-subunit (CHRNE) gene that causes a leucine-to-phenylalanine substitution at position 289 (L289F) was found.

CONCLUSIONS

We reported a SCCMS family of Chinese origin. In the family, classical clinical phenotype with phenotypic variability among different members was found. Genetic testing could help diagnose this rare disease.

Ephedrine for myasthenia gravis, neonatal myasthenia and the congenital myasthenic syndromes

BACKGROUND

Myasthenia is a condition in which neuromuscular transmission is affected by antibodies against neuromuscular junction components (autoimmune myasthenia gravis, MG; and neonatal myasthenia gravis, NMG) or by defects in genes for neuromuscular junction proteins (congenital myasthenic syndromes, CMSs). Clinically, some individuals seem to benefit from treatment with ephedrine, but its effects and adverse effects have not been systematically evaluated.

OBJECTIVES

To assess the effects and adverse effects of ephedrine in people with autoimmune MG, transient neonatal MG, and the congenital myasthenic syndromes.

SEARCH METHODS

On 17 November 2014, we searched the Cochrane Neuromuscular Disease Group Specialized Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE. We also searched reference lists of articles, conference proceedings of relevant conferences, and prospective trial registers. In addition, we contacted manufacturers and researchers in the field.

SELECTION CRITERIA

We considered randomised controlled trials (RCTs) and quasi-RCTs comparing ephedrine as a single or add-on treatment with any other active treatment, placebo, or no treatment in adults or children with autoimmune MG, NMG, or CMSs.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed study design and quality, and extracted data. We contacted study authors for additional information. We collected information on adverse effects from included articles, and contacted authors.

MAIN RESULTS

We found no RCTs or quasi-RCTs, and therefore could not establish the effect of ephedrine on MG, NMG and CMSs. We describe the results of 53 non-randomised studies narratively in the Discussion section, including observations of endurance, muscle strength and quality of life. Effects may differ depending on the type of myasthenia. Thirty-seven studies were in participants with CMS, five in participants with MG, and in 11 the precise form of myasthenia was unknown. We found no studies for NMG. Reported adverse effects included tachycardia, sleep disturbances, nervousness, and withdrawal symptoms.

AUTHORS' CONCLUSIONS

There was no evidence available from RCTs or quasi-RCTs, but some observations from non-randomised studies are available. There is a need for more evidence from suitable forms of prospective RCTs, such as series of n-of-one RCTs, that use appropriate and validated outcome measures.

Germline mutations in RYR1 are associated with foetal akinesia deformation sequence/lethal multiple pterygium syndrome

INTRODUCTION

Foetal akinesia deformation sequence syndrome (FADS) is a genetically heterogeneous disorder characterised by the combination of foetal akinesia and developmental defects which may include pterygia (joint webbing). Traditionally multiple pterygium syndrome (MPS) has been divided into two forms: prenatally lethal (LMPS) and non-lethal Escobar type (EVMPS) types. Interestingly, FADS, LMPS and EVMPS may be allelic e.g. each of these phenotypes may result from mutations in the foetal acetylcholine receptor gamma subunit gene (CHRNG). Many cases of FADS and MPS do not have a mutation in a known FADS/MPS gene and we undertook molecular genetic studies to identify novel causes of these phenotypes.

RESULTS

After mapping a novel locus for FADS/LMPS to chromosome 19, we identified a homozygous null mutation in the RYR1 gene in a consanguineous kindred with recurrent LMPS pregnancies. Resequencing of RYR1 in a cohort of 66 unrelated probands with FADS/LMPS/EVMPS (36 with FADS/LMPS and 30 with EVMPS) revealed two additional homozygous mutations (in frame deletions). The overall frequency of RYR1 mutations in probands with FADS/LMPS was 8.3%.

CONCLUSIONS

Our findings report, for the first time, a homozygous RYR1 null mutation and expand the range of RYR1-related phenotypes to include early lethal FADS/LMPS. We suggest that RYR1 mutation analysis should be performed in cases of severe FADS/LMPS even in the absence of specific histopathological indicators of RYR1-related disease.

Mutations in the Mitochondrial Citrate Carrier SLC25A1 are Associated with Impaired Neuromuscular Transmission

Background and Objective

Congenital myasthenic syndromes are rare inherited disorders characterized by fatigable weakness caused by malfunction of the neuromuscular junction. We performed whole exome sequencing to unravel the genetic aetiology in an English sib pair with clinical features suggestive of congenital myasthenia.

Methods

We used homozygosity mapping and whole exome sequencing to identify the candidate gene variants. Mutant protein expression and function were assessed in vitro and a knockdown zebrafish model was generated to assess neuromuscular junction development.

Results

We identified a novel homozygous missense mutation in the SLC25A1 gene, encoding the mitochondrial citrate carrier. Mutant SLC25A1 showed abnormal carrier function. SLC25A1 has recently been linked to a severe, often lethal clinical phenotype. Our patients had a milder phenotype presenting primarily as a neuromuscular (NMJ) junction defect. Of note, a previously reported patient with different compound heterozygous missense mutations of SLC25A1 has since been shown to suffer from a neuromuscular transmission defect. Using knockdown of SLC25A1 expression in zebrafish, we were able to mirror the human disease in terms of variable brain, eye and cardiac involvement. Importantly, we show clear abnormalities in the neuromuscular junction, regardless of the severity of the phenotype.

Conclusions

Based on the axonal outgrowth defects seen in SLC25A1 knockdown zebrafish, we hypothesize that the neuromuscular junction impairment may be related to pre-synaptic nerve terminal abnormalities. Our findings highlight the complex machinery required to ensure efficient neuromuscular function, beyond the proteomes exclusive to the neuromuscular synapse.

Congenital myasthenic syndromes: Clinical and molecular report on 7 Sicilian patients

Background:

Congenital myasthenic syndromes (CMS) are a heterogeneous group of diseases involving neuromuscular transmission. The classification of these syndromes is based on the localization of the defect (pre-synaptic, post-synaptic, and neuromuscular junction) and on the molecular analysis.

Aim:

To report on a series of 7 patients affected by post-synaptic CMS.

Patients and Methods:

We examined sex, familiarity, age of onset, clinical symptoms, and response to tensilon test, patellar and pupillary reflexes, presence of cranial nerve involvement, Gowers′ sign, presence of ptosis, grade of muscular weakness, and response to the treatment and gene deletions.

Results:

Ptosis, muscular hypotonia, and light variability in muscular weakness were the main clinical signs. Cholinergic receptor, nicotinic, epsilon (CHRNE) gene mutations were mainly reported.

Conclusions:

The study points out that the clinical and molecular pattern reported in our patients do not differentiate from the data reported in the literature. Treatment with pyridostigmine and modulation of the therapy allows a good quality of life.

Slow channel congenital myasthenic syndrome responsive to a combination of fluoxetine and salbutamol

INTRODUCTION

Slow channel congenital myasthenic syndrome is a dominant disorder characterized by prolonged acetylcholine receptor ion-channel activation.

METHODS

Molecular genetic techniques, electrophysiology, and binding studies in human embryonic kidney (HEK) 293 cells determined mutant function and expression levels. Patient response to treatment was measured by quantitative myasthenic gravis and Medical Research Council grade strength scores.

RESULTS

We report an unusual case due to heteroallelic mutations in CHRNE. The slow channel mutation, p.εS278del, is accompanied by a severe low-expression mutation, p.εR217L, on the second allele. Expression studies and cosegregation of p.εS278del with the disorder in the patient's offspring demonstrate robust expression of the p.εS278del mutation. The patient showed modest benefits from standard treatment with fluoxetine, but there was dramatic improvement when salbutamol was combined with fluoxetine.

CONCLUSIONS

This case suggests that salbutamol, which is beneficial in some other congenital myasthenic syndromes, might also be considered in addition to fluoxetine in slow channel syndrome.

Bulbar dysfunction: An early presentation of congenital myasthenic syndrome in three infants

Congenital myasthenic syndromes are a group of rare genetic disorders affecting neuromuscular transmission. A high index of suspicion is required as clinical manifestations can be variable and nonspecific. Clinical phenotype includes arthrogryposis, respiratory crises, facial deformities, and weakness. With the availability of molecular genetics, this group of conditions can now be more clearly delineated and targeted treatment can be initiated. We describe three children who presented with bulbar difficulties and had Cholinergic receptor, nicotinic, and epsilon or receptor associated protein of the synapse mutations.

Congenital myasthenic syndrome: a brief review

Congenital myasthenic syndromes comprise heterogeneous genetic diseases characterized by compromised neuromuscular transmission. Congenital myasthenic syndromes are classified as presynaptic, synaptic, or postsynaptic, depending on the primary defect's location within the neuromuscular junction. Presynaptic forms are the rarest, affecting an estimated 7-8% of patients; synaptic forms account for approximately 14-15% of patients; and the remaining 75-80% are attributable to postsynaptic defects. Clinical manifestations vary by congenital myasthenic syndrome subtype. Electrophysiologic, morphologic, and molecular descriptions of various forms of congenital myasthenic syndromes have led to an enhanced understanding of clinical manifestations and disease pathophysiology. Although congenital myasthenic syndromes are indicated by clinical manifestations, family history, electrophysiologic studies, and responses to acetylcholinesterase inhibitors, overlap in some presentations occurs. Therefore, genetic testing may be necessary to identify specific mutations in CHAT, COLQ, LAMB2, CHRNA, CHRNB, CHRND, CHRNE, CHRNG, RAPSN, DOK7, MUSK, AGRN, SCN4A, GFPT1, or PLEC1 genes. The identification of congenital myasthenic syndromes subtypes will prove important in the treatment of these patients. Different drugs may be beneficial, or should be avoided because they are ineffective or worsen some forms of congenital myasthenic syndromes. We explore the classification, clinical manifestations, electrophysiologic features, genetics, and treatment responses of each congenital myasthenic syndrome subtype.

Congenital myasthenic syndrome due to homozygous CHRNE mutations: report of patients in Arabia

We describe the clinical characteristics of 3 siblings from 1 family with congenital myasthenic syndrome due to homozygous mutations of the gene coding for the epsilon subunit of the acetylcholine receptor (CHRNE). Onset of symptoms occurred in the first few months of life with ptosis, restricted ocular motility, mild proximal weakness, and difficulty swallowing. Multiple hospital admissions were required due to recurrent pulmonary infections. There was no decremental conduction on repetitive nerve stimulation, but jitter was increased on single fiber electromyographic. Since early childhood, our patients have done well without pulmonary or bulbar symptoms and with partial improvement on pyridostigmine therapy. Response of ptosis to diagnostic ice pack test was striking. Although these siblings have a clinical history and examination findings typical of homozygous CHRNE mutations, the clinical presentation of congenital myasthenia subtypes is variable, and accurate genotyping is essential in choosing the appropriate treatment.

Clinical features in a series of fast channel congenital myasthenia syndrome

Fast channel congenital myasthenic syndromes are rare, but frequently result in severe weakness. We report a case of 12 fast channel patients to highlight clinical features and management difficulties. Patients were diagnosed through genetic screening and identification of mutations shown to cause fast channel syndrome. Data was obtained from clinical notes, history, examination and follow up. Patterns of muscle weakness involved limb, trunk, bulbar, respiratory, facial and extraocular muscles. Patients responded to treatment with anticholinesterase medication and 3,4-diaminopyridine. Fast channel syndrome contrasted with AChR deficiency in the occurrence of severe respiratory crises in infancy and childhood. The death of two children even when on treatment and the family histories of sibling deaths re-inforces the need for accurate genetic diagnosis, optimised pharmacological treatment and additional supportive measures to manage acute respiratory crises. Referral to a specialist paediatric respiratory centre and regular resuscitation training for parents are recommended.

A retrospective clinical study of the treatment of slow-channel congenital myasthenic syndrome

Slow-channel congenital myasthenic syndrome (CMS) is a rare subtype of CMS caused by dominant "gain of function" mutations in the acetylcholine receptor. Clinically, the cervical and forearm extensor muscles seem to be preferentially weaker; and conventional treatment with anticholinesterases fails to improve symptoms. In contrast, open channel blockers such as fluoxetine and quinidine have been shown to be of benefit. The objectives of our study were to provide further insight into the clinical features of slow-channel CMS and evaluate response to recommended therapy. We carried out a retrospective clinical follow up study of 15 slow-channel CMS patients referred to the Munich CMS Centre. Detailed clinical data were collected by clinicians involved in the care of each patient, with a particular focus on response and tolerability to recommended therapy. Patients varied widely as regard onset of symptoms, severity of disease and mutations involved. Patients received up to four different medications and some had none. Our results strengthen previous reported findings in terms of clinical phenotype variability and the poor response to pyridostigmine. Although treatment with fluoxetine was beneficial in most patients, a number of our patients suffered significant adverse effects that hindered optimum dose titration or led to treatment cessation. Slow-channel CMS is rare and exhibits distinct clinical and genetic characteristics. Our study suggests that fluoxetine, despite being effective in most patients, can be associated with significant side effects, thus reducing treatment effectiveness in clinical practice.

Genetic heterogeneity and pathophysiological mechanisms in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a rare heterogeneous group of inherited neuromuscular disorders associated with distinctive clinical, electrophysiological, ultrastructural and genetic abnormalities. These genetic defects either impair neuromuscular transmission directly or result in secondary impairments, which eventually compromise the safety margin of neuromuscular transmission. In this report we will explore the significant progress made in understanding the molecular pathogenesis of CMS, which is important for both patients and clinicians in terms of reaching a definite diagnosis and selecting the most appropriate treatment.

Modulation of the Ca(2+) permeability of human endplate acetylcholine receptor-channel

In slow-channel congenital myasthenic syndrome, point mutations of the endplate acetylcholine receptor (AChR) prolong channel openings, leading to excessive Ca(2+) entry with ensuing endplate degeneration and myasthenic symptoms. The Ca(2+) permeability of the human endplate AChR-channel is quite high, and is further increased by two slow-channel mutations in its ɛ subunit, worsening the pathological cascade. To gain further support to the hypothesis that the ɛ subunit plays a crucial role in controlling Ca(2+) permeability of endplate AChR-channel, in this work we measured the fractional Ca(2+) current (P(f), i.e., the percentage of the total current carried by Ca(2+) ions) of a panel of AChR carrying slow-channel mutations in the α, β and ɛ subunits detected in patients (α(N217K), α(S226Y), α(C418W), β(V266A), β(V266M), ɛ(I257F), ɛ(V265A) and ɛ(L269F)). We confirm that only mutations in the ɛ subunit altered Ca(2+) permeability of AChR-channels, with ɛ(L269F) increasing P(f) (10% vs. 7% of wild type AChR) and ɛ(I257F) decreasing it (to 4.6%). We also found that, for ɛ(L269F)-AChR, the Ca(2+) permeability and ACh-induced cell death can be normalized by clinically relevant concentrations of salbutamol or verapamil, providing the first evidence that the Ca(2+) permeability of AChR-channels can be modulated and this treatment may provide protection against excitotoxic insults.

Congenital muscular dystrophy, myasthenic symptoms and epidermolysis bullosa simplex (EBS) associated with mutations in the PLEC1 gene encoding plectin

Mutations in the PLEC1 gene encoding plectin have been reported in neonatal epidermolysis bullosa simplex with muscular dystrophy of later-onset (EBS-MD). A neuromuscular transmission defect has been reported in one previous patient. We report a boy presenting from birth with features of a congenital muscular dystrophy and late-onset myasthenic symptoms. Repetitive nerve stimulation showed significant decrement, and strength improved with pyridostigmine. Subtle blistering noticed only retrospectively prompted further genetic testing, revealing recessive PLEC1 mutations. We conclude that PLEC1 should be considered in the differential diagnosis of congenital muscular dystrophies and myasthenic syndromes, even in the absence of prominent skin involvement.

Long-term follow-up in patients with congenital myasthenic syndrome due to CHAT mutations

BACKGROUND

Congenital myasthenic syndromes (CMSs) are a group of clinically and genetically heterogeneous inherited disorders of the neuromuscular junction. Mutations in the acetylcholine transferase (CHAT) gene cause a pre-synaptic CMS, typically associated with episodic apnoea and worsening of myasthenic symptoms during crises caused by infections, fever or stress. Between crises symptoms may be mild and variable. Acetylcholinesterase - inhibitor therapy is reported to improve clinical symptoms and reduce crises.

PATIENTS AND METHODS

We present data on the long-term follow-up of 11 patients with a congenital myasthenic syndrome due to nine different CHAT mutations; ten of the patients have not been previously reported.

RESULTS AND CONCLUSIONS

Manifestation varied from the neonatal period to the age of two years, follow-up time from nine months to 12 years. This cohort of CHAT patients studied here enabled us to describe two distinct phenotypes: The neonatal-onset group suffers from apnoeic crises, respirator dependency and bulbar weakness. Apnoea should be carefully distinguished from seizures; a CMS should be taken into account early to start appropriate therapy. Infantile-onset patients show mild permanent weakness, but experience apnoeic crises and worsening which resolve with Acetylcholinesterase - inhibitor treatment. However, after several years of treatment proximal muscle strength may decrease and lead to wheelchair dependency despite the continuation of Acetylcholinesterase - inhibitor therapy.

What have we learned from the congenital myasthenic syndromes

The congenital myasthenic syndromes have now been traced to an array of molecular targets at the neuromuscular junction encoded by no fewer than 11 disease genes. The disease genes were identified by the candidate gene approach, using clues derived from clinical, electrophysiological, cytochemical, and ultrastructural features. For example, electrophysiologic studies in patients suffering from sudden episodes of apnea pointed to a defect in acetylcholine resynthesis and CHAT as the candidate gene (Ohno et al., Proc Natl Acad Sci USA 98:2017-2022, 2001); refractoriness to anticholinesterase medications and partial or complete absence of acetylcholinesterase (AChE) from the endplates (EPs) has pointed to one of the two genes (COLQ and ACHE ( T )) encoding AChE, though mutations were observed only in COLQ. After a series of patients carrying mutations in a disease gene have been identified, the emerging genotype-phenotype correlations provided clues for targeted mutation analysis in other patients. Mutations in EP-specific proteins also prompted expression studies that proved pathogenicity, highlighted important functional domains of the abnormal proteins, and pointed to rational therapy.

Myasthenic syndrome due to defects in rapsyn: Clinical and molecular findings in 39 patients

BACKGROUND

Pathogenic mutations in rapsyn result in endplate acetylcholine receptor (AChR) deficiency and are a common cause of postsynaptic congenital myasthenic syndromes.

METHODS

Clinical, electrophysiologic, pathologic, and molecular studies were done in 39 patients.

RESULTS

In all but one patient, the disease presented in the first 2 years of life. In 9 patients, the myasthenic symptoms included constant or episodic ophthalmoparesis, and 1 patient had a pure limb-girdle phenotype. More than one-half of the patients experienced intermittent exacerbations. Long-term follow-up was available in 25 patients after start of cholinergic therapy: 21 became stable or were improved and 2 of these became asymptomatic; 3 had a progressive course; and 1 died in infancy. In 7 patients who had endplate studies, the average counts of AChR per endplate and the synaptic response to ACh were less reduced than in patients harboring low AChR expressor mutations. Eight patients were homozygous and 23 heterozygous for the common p.N88K mutation. Six mutations, comprising 3 missense mutations, an in-frame deletion, a splice-site mutation, and a nonsense mutation, are novel. Homozygosity for p.N88K was associated with varying grades of severity. No genotype-phenotype correlations were observed except in 8 Near-Eastern patients homozygous for the promoter mutation (c.-38A>G), who had a mild course.

CONCLUSIONS

All but 1 patient presented early in life and most responded to cholinergic agonists. With early diagnosis and therapy, rapsyn deficiency has a benign course in most patients. There was no consistent phenotype-genotype correlation except for an E-box mutation associated with jaw deformities.

AChR deficiency due to epsilon-subunit mutations: two common mutations in the Netherlands

Congenital myasthenic syndromes are a clinically and genetically heterogeneous group of hereditary disorders affecting neuromuscular transmission. We have identified mutations within the acetylcholine receptor (AChR) ε-subunit gene underlying congenital myasthenic syndromes in nine patients (seven kinships) of Dutch origin. Previously reported mutations ε1369delG and εR311Q were found to be common; ε1369delG was present on at least one allele in seven of the nine patients, and εR311Q in six. Phenotypes ranged from relatively mild ptosis and external ophthalmoplegia to generalized myasthenia. The common occurrence of εR311Q and ε1369delG suggests a possible founder for each of these mutations originating in North Western Europe, possibly in Holland. Knowledge of the ethnic or geographic origin within Europe of AChR deficiency patients can help in targeting genetic screening and it may be possible to provide a rapid genetic diagnosis for patients of Dutch origin by screening first for εR311Q and ε1369delG.

Therapeutic strategies in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are classified in terms of the located defect: presynaptic, postsynaptic, and synaptic. They are inherited disorders caused by various genetic defects, all but the slow-channel CMS by recessive inheritance. To date, 10 different CMS are known and further CMS subtypes and their genetic cause may be disclosed by future investigations. Prognosis in CMS is variable and largely depends on the pathophysiological and genetic defect. Subtypes showing progression and life-threatening crises with apneas are generally less favorable than others. Therapeutic agents used in CMS depend on the underlying defect and include acetylcholinesterase inhibitor, 3,4-diaminopyridine, quinidine sulfate, fluoxetine, acetazolamide, and ephedrine. Although there are no double-blind, placebo-controlled clinical trials for CMS, several drugs have shown convincingly positive clinical effects. It is therefore necessary to start a rational therapy regime as early as possible. In most CMS, however, mild and severe clinical courses are reported, which makes assessment on an individual basis necessary. This review emphasizes therapeutic strategies in CMS.

Congenital myasthenic syndromes and the formation of the neuromuscular junction

The congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders affecting neuromuscular transmission. Underlying mutations have been identified in at least 11 different genes. The majority of CMS patients have disorders due to mutations in postsynaptic proteins. Initial studies focused on dysfunction of the acetylcholine receptor (AChR) itself as the major cause of CMS. However, it is becoming apparent that mutations of proteins involved in clustering the AChR and maintaining neuromuscular junction structure form important subgroups. Analysis of the mutations in the AChR-clustering protein, rapsyn, show diverse causes for defective AChR localization and suggest that the common mutation rapsyn-N88K results in AChR clusters that are less stable than those generated by wild-type rapsyn. More recently, mutations in the newly identified endplate protein Dok-7 have been shown to affect AChR clustering and the generation and maintenance of specialized structures at the endplate. Dok-7 binds MuSK and many of the mutations of DOK7 impair the MuSK signaling pathway. Components of this pathway will provide attractive gene candidates for additional forms of CMS. The phenotypic characteristics of the different CMS in which muscle groups may be differentially affected not only provide clues for targeted genetic screening, but also pose further intriguing questions about underlying molecular mechanisms.

Neurophysiological strategies for the diagnosis of disorders of the neuromuscular junction in children

The disorders of the neuromuscular junction seen in children, the congenital myasthenic syndromes and autoimmune myasthenia gravis, are very rare. Their clinical symptoms and signs may be variable, most notably in the neonate and infant. They should enter the differential diagnosis of many different clinical presentations, such as 'floppy infant' or attacks of episodic apnoea. Many molecular genetic abnormalities have been discovered in congenital myasthenic syndromes, but screening for these is impracticable without confirmation of a disorder of the neuromuscular junction. In this review, the different neurophysiological tests that can be used are discussed, with their merits and contraindications in children. After consideration of the acceptability of the different techniques, it has been found that stimulation single-fibre electromyography (stimSFEMG) of orbicularis oculi seems to be the best choice in children. In most instances it can be performed on the conscious child and it has a good specificity and high sensitivity. A protocol for using stimSFEMG is described.

The evaluation of the hypotonic infant

The pediatric neurologist is regularly asked to evaluate a hypotonic patient. This consultation request usually occurs in 2 different situations; the first is in the newborn period when the neurologist is asked to evaluate the "floppy infant," and the second is in the latter half of the first year of life and is usually accompanied by concern about the developmental progress of the infant and, in particular, the motor development of the infant. In this article, I will try to outline the factors related to the production of muscle tone in infants and children. The elements of the clinical evaluation of the hypotonic child including those clinical tests most helpful in the measurement of tone will be reviewed. A scheme for localizing the origin of the disturbance in muscle tone is presented, many of the known causes of the tone abnormalities are reviewed, and a rational approach to the diagnostic evaluation of these children is offered.

High throughput genetic analysis of congenital myasthenic syndromes using resequencing microarrays

Background

The use of resequencing microarrays for screening multiple, candidate disease loci is a promising alternative to conventional capillary sequencing. We describe the performance of a custom resequencing microarray for mutational analysis of Congenital Myasthenic Syndromes (CMSs), a group of disorders in which the normal process of neuromuscular transmission is impaired.

Methodology/Principal Findings

Our microarray was designed to assay the exons and flanking intronic regions of 8 genes linked to CMSs. A total of 31 microarrays were hybridized with genomic DNA from either individuals with known CMS mutations or from healthy controls. We estimated an overall microarray call rate of 93.61%, and we found the percentage agreement between the microarray and capillary sequencing techniques to be 99.95%. In addition, our microarray exhibited 100% specificity and 99.99% reproducibility. Finally, the microarray detected 22 out of the 23 known missense mutations, but it failed to detect all 7 known insertion and deletion (indels) mutations, indicating an overall sensitivity of 73.33% and a sensitivity with respect to missense mutations of 95.65%.

Conclusions/Significance

Overall, our microarray prototype exhibited strong performance and proved highly efficient for screening genes associated with CMSs. Until indels can be efficiently assayed with this technology, however, we recommend using resequencing microarrays for screening CMS mutations after common indels have been first assayed by capillary sequencing.

Congenital myasthenic syndromes: spotlight on genetic defects of neuromuscular transmission

The neuromuscular junction (NMJ) is a complex structure that efficiently communicates the electrical impulse from the motor neuron to the skeletal muscle to induce muscle contraction. Genetic and autoimmune disorders known to compromise neuromuscular transmission are providing further insights into the complexities of NMJ function. Congenital myasthenic syndromes (CMSs) are a genetically and phenotypically heterogeneous group of rare hereditary disorders affecting neuromuscular transmission. The understanding of the molecular basis of the different types of CMSs has evolved rapidly in recent years. Mutations were first identified in the subunits of the nicotinic acetylcholine receptor (AChR), but now mutations in ten different genes - encoding post-, pre- or synaptic proteins - are known to cause CMSs. Pathogenic mechanisms leading to an impaired neuromuscular transmission modify AChRs or endplate structure or lead to decreased acetylcholine synthesis and release. However, the genetic background of many CMS forms is still unresolved. A precise molecular classification of CMS type is of paramount importance for the diagnosis, counselling and therapy of a patient, as different drugs may be beneficial or deleterious depending on the molecular background of the particular CMS.