Current status of the congenital myasthenic syndromes

Clinical and Electrophysiologic Responses to Acetylcholinesterase Inhibitors in MuSK-Antibody-Positive Myasthenia Gravis: Evidence for Cholinergic Neuromuscular Hyperactivity

BACKGROUND AND PURPOSE

Patients with muscle-specific tyrosine kinase (MuSK) antibody (MuSK-Ab)-positive myasthenia gravis (MG) show distinct responses to acetylcholinesterase inhibitors (AChEIs). Although clinical responses to AChEIs in MuSK-Ab MG are reasonably well known, little is known about the electrophysiologic responses to AChEIs. We therefore investigated the clinical and electrophysiologic responses to AChEIs in MuSK-Ab-positive MG patients.

METHODS

We retrospectively reviewed the medical records and electrodiagnostic findings of 17 MG patients (10 MuSK-Ab-positive and 7 MuSK-Ab-negative patients) who underwent electrodiagnostic testing before and after a neostigmine test (NT).

RESULTS

The frequency of intolerance to pyridostigmine bromide (PB) was higher in MuSK-Ab-positive patients than in MuSK-Ab-negative patients (50% vs. 0%, respectively; p=0.044), while the maximum tolerable dose of PB was lower in the former (90 mg/day vs. 480 mg/day, p=0.023). The frequency of positive NT results was significantly lower in MuSK-Ab-positive patients than in MuSK-Ab-negative patients (40% vs. 100%, p=0.035), while the nicotinic side effects of neostigmine were more frequent in the former (80% vs. 14.3%, p=0.015). Repetitive compound muscle action potentials (R-CMAPs) developed more frequently after NT in MuSK-Ab-positive patients than in MuSK-Ab-negative patients (90% vs. 14.3%, p=0.004). The frequency of a high-frequency-stimulation-induced decrement-increment pattern (DIP) was higher in MuSK-Ab-positive patients than in MuSK-Ab-negative patients (100% vs. 17.7%, p=0.003).

CONCLUSIONS

These results suggest that MuSK-Ab-positive MG patients exhibit unique and hyperactive responses to AChEIs. Furthermore, R-CMAP and DIP development on a standard AChEI dose may be a distinct neurophysiologic feature indicative of MuSK-Ab-positive MG.

DPAGT1 myasthenia and myopathy: genetic, phenotypic, and expression studies

OBJECTIVE

To investigate patients with DPAGT1 (UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephosphotransferase 1)-associated myasthenic syndrome.

METHODS

We performed exome and Sanger sequencing, determined glycoprotein expression in patient muscles, assessed pathogenicity of the mutant proteins by examining their expression and enzymatic activity in transfected cells, evaluated structural changes in muscle and the neuromuscular junction, and examined electrophysiologic aspects of neuromuscular transmission in vitro.

RESULTS

Patients 1 and 2, 16 and 14 years of age, had progressive fatigable weakness since infancy and are intellectually disabled. Patient 3, a less severely affected brother of patient 1, also has autistic features. Each patient harbors 2 novel heteroallelic mutations in DPAGT1, an enzyme subserving protein N-glycosylation. Patients 1 and 3 harbor Met1Leu, which reduces protein expression, and His375Tyr, which decreases enzyme activity. Patient 2 carries Val264Met, which abolishes enzyme activity, and a synonymous Leu120Leu mutation that markedly augments exon skipping, resulting in some skipped and infrequent nonskipped alleles. Therefore, the nonskipped allele rescues the phenotype. Intracellular microelectrode studies indicate combined pre- and postsynaptic defects of neuromuscular transmission with evidence for somatic mosaicism in patient 2. Structural studies reveal hypoplastic endplates, fiber-type disproportion, tubular aggregates, and degeneration of muscle fiber organelles resulting in autophagocytosis.

CONCLUSIONS

DPAGT1 myasthenia affects multiple parameters of neuromuscular transmission, causes fiber-type disproportion and an autophagic myopathy, and can be associated with intellectual disability. We speculate that hypoglycosylation of synapse-specific proteins causes defects in central as well as motor synapses.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

Congenital myasthenic syndrome due to choline acetyltransferase mutations in infants: clinical suspicion and comprehensive electrophysiological assessment are important for early diagnosis

Congenital myasthenic syndromes are inherited disorders caused by various defects in neuromuscular transmission. Although the typical presentation is fatigable weakness with prominent cranial involvement, neonates can lack these hallmark manifestations, and in those with choline acetyltransferase gene mutations, basal electrophysiological testing can yield negative findings. The authors report the case of a male infant presenting at birth with oculomotor and bulbofacial weakness, hypotonia, clubfoot, and severe respiratory insufficiency. Electromyography showed myogenic signs, and basal repetitive nerve stimulation yielded negative findings. Since age 6 months, the infant had progressively improved, acquiring autonomous respiration. Prolonged subtetanic repetitive nerve stimulation disclosed a marked decremental response compatible with suspected congenital myasthenic syndrome with episodic apnea. Genetic testing identified 2 novel choline acetyltransferase mutations (R470X, F580C). Keeping a high clinical suspicion of this rare condition and undertaking early comprehensive electrophysiological assessments including prolonged repetitive nerve stimulation (10 Hz for 5 minutes) can expedite the diagnosis.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Channelopathies

Channelopathies are a heterogeneous group of disorders resulting from the dysfunction of ion channels located in the membranes of all cells and many cellular organelles. These include diseases of the nervous system (e.g., generalized epilepsy with febrile seizures plus, familial hemiplegic migraine, episodic ataxia, and hyperkalemic and hypokalemic periodic paralysis), the cardiovascular system (e.g., long QT syndrome, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia), the respiratory system (e.g., cystic fibrosis), the endocrine system (e.g., neonatal diabetes mellitus, familial hyperinsulinemic hypoglycemia, thyrotoxic hypokalemic periodic paralysis, and familial hyperaldosteronism), the urinary system (e.g., Bartter syndrome, nephrogenic diabetes insipidus, autosomal-dominant polycystic kidney disease, and hypomagnesemia with secondary hypocalcemia), and the immune system (e.g., myasthenia gravis, neuromyelitis optica, Isaac syndrome, and anti-NMDA [N-methyl-D-aspartate] receptor encephalitis). The field of channelopathies is expanding rapidly, as is the utility of molecular-genetic and electrophysiological studies. This review provides a brief overview and update of channelopathies, with a focus on recent advances in the pathophysiological mechanisms that may help clinicians better understand, diagnose, and develop treatments for these diseases.

[Beneficial effects of 3,4-diaminopyridine in a 26-year-old woman with DOK7 congenital myasthenic syndrome who was originally diagnosed with facioscapulohumeral dystrophy]

We report a 26-year-old woman who had respiratory dysfunction and muscle weakness at birth and was diagnosed with facioscapulohumeral dystrophy at the age of 5. The extent of muscle weakness fluctuated daily or weekly and deteriorated in menstrual periods. At the age of 12, she noted improvements in symptoms when taking procaterol hydrochloride and began to take it regularly. After that, her condition stabilized. At the age of 26, she visited our hospital presenting with ptosis, muscle weakness in the face, trunk, and proximal limbs, and easy fatigability. Serum CK was normal; anti-acetylcholine receptor and anti-muscle specific tyrosine kinase antibodies were negative. A repetitive stimulation test in the trapezius muscle showed a waning phenomenon. Gene analysis for congenital myasthenic syndrome (CMS) revealed a new mutation in the DOK7 gene; the diagnosis of CMS was confirmed. Her symptoms worsened with ambenonium chloride but improved with 3,4-diaminopyridine. Our findings suggest that daily or weekly fluctuation and worsening with a menses in muscle weakness is an important diagnostic feature of CMS.

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.

Molecular mechanisms of SH2- and PTB-domain-containing proteins in receptor tyrosine kinase signaling

Intracellular signaling is mediated by reversible posttranslational modifications (PTMs) that include phosphorylation, ubiquitination, and acetylation, among others. In response to extracellular stimuli such as growth factors, receptor tyrosine kinases (RTKs) typically dimerize and initiate signaling through phosphorylation of their cytoplasmic tails and downstream scaffolds. Signaling effectors are recruited to these phosphotyrosine (pTyr) sites primarily through Src homology 2 (SH2) domains and pTyr-binding (PTB) domains. This review describes how these conserved domains specifically recognize pTyr residues and play a major role in mediating precise downstream signaling events.

LRP4 third β-propeller domain mutations cause novel congenital myasthenia by compromising agrin-mediated MuSK signaling in a position-specific manner

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Using Sanger and exome sequencing in a CMS patient, we identified two heteroallelic mutations, p.Glu1233Lys and p.Arg1277His, in LRP4 coding for the postsynaptic low-density lipoprotein receptor-related protein 4. LRP4, expressed on the surface of the postsynaptic membrane of the neuromuscular junction, is a receptor for neurally secreted agrin, and LRP4 bound by agrin activates MuSK. Activated MuSK in concert with Dok-7 stimulates rapsyn to concentrate and anchor AChR on the postsynaptic membrane and interacts with other proteins implicated in the assembly and maintenance of the neuromuscular junction. LRP4 also functions as an inhibitor of Wnt/beta-catenin signaling. The identified mutations in LRP4 are located at the edge of its 3rd beta-propeller domain and decrease binding affinity of LRP4 for both MuSK and agrin. Mutations in the LRP4 3rd beta-propeller domain were previously reported to impair Wnt signaling and cause bone diseases including Cenani-Lenz syndactyly syndrome and sclerosteosis-2. By analyzing naturally occurring and artificially introduced mutations in the LRP4 3rd beta-propeller domain, we show that the edge of the domain regulates the MuSK signaling whereas its central cavity governs Wnt signaling. We conclude that LRP4 is a new CMS disease gene and that the 3rd beta propeller domain of LRP4 mediates the two signaling pathways in a position-specific manner.

HnRNP L and hnRNP LL antagonistically modulate PTB-mediated splicing suppression of CHRNA1 pre-mRNA

CHRNA1 gene, encoding the muscle nicotinic acetylcholine receptor alpha subunit, harbors an inframe exon P3A. Inclusion of exon P3A disables assembly of the acetylcholine receptor subunits. A single nucleotide mutation in exon P3A identified in congenital myasthenic syndrome causes exclusive inclusion of exon P3A. The mutation gains a de novo binding affinity for a splicing enhancing RNA-binding protein, hnRNP LL, and displaces binding of a splicing suppressing RNA-binding protein, hnRNP L. The hnRNP L binds to another splicing repressor PTB through the proline-rich region and promotes PTB binding to the polypyrimidine tract upstream of exon P3A, whereas hnRNP LL lacking the proline-rich region cannot bind to PTB. Interaction of hnRNP L with PTB inhibits association of U2AF(65) and U1 snRNP with the upstream and downstream of P3A, respectively, which causes a defect in exon P3A definition. HnRNP L and hnRNP LL thus antagonistically modulate PTB-mediated splicing suppression of exon P3A.

[Myasthenia, from the internist's point of view]

Myasthenia gravis is an autoimmune disease due to specific antibodies inducing a neuromuscular transmission defect causing muscle fatigability. If onset of the disease may be at any age, myasthenia gravis concerns mostly young adults, in majority females. The disease characteristic features are the following: ocular symptoms (ptosis or diplopia) as main initial manifestation, extension to other muscles in 80 % of the cases, variability of the deficit, effort induced worsening, successive periods of exacerbation during the disease course, severity depending on respiratory and swallowing impairment (if rapid worsening, a myasthenic crisis is to be suspected), association with thymoma in 20 % of patients and with other various autoimmune diseases, most commonly hyperthyroidism and Hashimoto's disease. Diagnosis relies on the clinical features, improvement with cholinesterase inhibitors, detection of specific autoantibodies (anti-AChR or anti-MuSK), and significant decrement evidenced by electrophysiological tests. The points concerning specifically the internist have been highlighted in this article: diagnostic traps, associated autoimmune diseases, including inflammatory myopathies that may mimic myasthenia gravis, adverse effects of medications commonly used in internal medicine, some of them inducing myasthenic syndromes. The treatment is well codified: the treatment is well codified: (1) respect of adverse drugs contra-indications, systematically use of cholinesterase inhibitors, (2) thymectomy if thymoma completed with radiotherapy if malignant, (3) corticosteroids or immunosuppressive agent in severe or disabling form, (4) intensive care unit monitoring, plasmapheresis or intravenous immunoglobulins for patients with myasthenic crisis.

Clinical features of congenital myasthenic syndrome due to mutations in DPAGT1

BACKGROUND

A newly defined congenital myasthenic syndrome (CMS) caused by DPAGT1 mutations has recently been reported. While many other CMS-associated proteins have discrete roles localised to the neuromuscular junction, DPAGT1 is ubiquitously expressed, modifying many proteins, and as such is an unexpected cause of isolated neuromuscular involvement.

METHODS

We present detailed clinical characteristics of five patients with CMS caused by DPAGT1 mutations.

RESULTS

Patients have prominent limb girdle weakness and minimal craniobulbar symptoms. Tubular aggregates on muscle biopsy are characteristic but may not be apparent on early biopsies. Typical myasthenic features such as pyridostigmine and 3, 4- diaminopyridine responsiveness, and decrement on repetitive nerve stimulation are present.

CONCLUSIONS

These patients mimic myopathic disorders and are likely to be under-diagnosed. The descriptions here should facilitate recognition of this disorder. In particular minimal craniobulbar involvement and tubular aggregates on muscle biopsy help to distinguish DPAGT1 CMS from the majority of other forms of CMS. Patients with DPAGT1 CMS share similar clinical features with patients who have CMS caused by mutations in GFPT1, another recently identified CMS subtype.

Salbutamol-responsive limb-girdle congenital myasthenic syndrome due to a novel missense mutation and heteroallelic deletion in MUSK

Congenital myasthenic syndromes (CMS) are clinically and genetically heterogeneous disorders characterized by a neuromuscular transmission defect. In recent years, causative mutations have been identified in atleast 15 genes encoding proteins of the neuromuscular junction. Mutations in MUSK are known as a very rare genetic cause of CMS and have been described in only three families, world-wide. Consequently, the knowledge about efficient drug therapy is very limited. We identified a novel missense mutation (p.Asp38Glu) heteroallelic to a genomic deletion affecting exons 2–3 of MUSK as cause of a limb-girdle CMS in two brothers of Turkish origin. Clinical symptoms included fatigable limb weakness from early childhood on. Upon diagnosis of a MUSK-related CMS at the age of 16 and 13 years, respectively, treatment with salbutamol was initiated leading to an impressive improvement of clinical symptoms, while treatment with esterase inhibitors did not show any benefit. Our findings highlight the importance of a molecular diagnosis in CMS and demonstrate considerable similarities between patients with MUSK and DOK7-related CMS in terms of clinical phenotype and treatment options.

Fluoxetine prevents acetylcholine-induced excitotoxicity blocking human endplate acetylcholine receptor

INTRODUCTION

Fluoxetine is an open channel blocker of fetal muscle acetylcholine (ACh) receptor (AChR) and slow-channel mutant AChRs. It is used commonly to treat patients with slow-channel congenital myasthenic syndromes. Fluoxetine effects on adult wild-type endplate AChR are less characterized, although muscle AChR isoforms are differentially modulated by some drugs.

METHODS

Excitotoxicity assays and patch clamp recordings were performed in human embryonic kidney 293 (HEK) cells expressing wild-type or slow-channel mutant human AChRs.

RESULTS

Fluoxetine (2-10 μM) abolished ACh-induced death and decreased ACh-activated whole-cell currents in cells expressing all AChR types. In outside-out patches, fluoxetine rapidly curtailed ACh evoked unitary activity and macroscopic currents. The effect was increased if fluoxetine was applied before ACh.

CONCLUSIONS

Fluoxetine is an open channel blocker, but it also affects AChR in the closed state. AChR blockade likely underlies the rescue of HEK cells from ACh-induced death.

GFPT1-myasthenia: clinical, structural, and electrophysiologic heterogeneity

OBJECTIVE

To identify patients with GFPT1-related limb-girdle myasthenia and analyze phenotypic consequences of the mutations.

METHODS

We performed genetic analysis, histochemical, immunoblot, and ultrastructural studies and in vitro electrophysiologic analysis of neuromuscular transmission.

RESULTS

We identified 16 recessive mutations in GFPT1 in 11 patients, of which 12 are novel. Ten patients had slowly progressive limb-girdle weakness responsive to cholinergic agonists with onset between infancy and age 19 years. One patient (no. 6) harbored a nonsense mutation and a second mutation that disrupts the muscle-specific GFPT1 exon. This patient never moved in utero, was apneic and arthrogrypotic at birth, and was bedfast, tube-fed, and barely responded to therapy at age 6 years. Histochemical studies in 9 of 11 patients showed tubular aggregates in 6 and rimmed vacuoles in 3. Microelectrode studies of intercostal muscle endplates in 5 patients indicated reduced synaptic response to acetylcholine in 3 and severely reduced quantal release in patient 6. Endplate acetylcholine receptor content was moderately reduced in only one patient. The synaptic contacts were small and single or grape-like, and quantitative electron microscopy revealed hypoplastic endplate regions. Numerous muscle fibers of patient 6 contained myriad dilated and degenerate vesicular profiles, autophagic vacuoles, and bizarre apoptotic nuclei. Glycoprotein expression in muscle was absent in patient 6 and reduced in 5 others.

CONCLUSIONS

GFPT1-myasthenia is more heterogeneous than previously reported. Different parameters of neuromuscular transmission are variably affected. When disruption of muscle-specific isoform determines the phenotype, this has devastating clinical, pathologic, and biochemical consequences.

Structure and superorganization of acetylcholine receptor-rapsyn complexes

The scaffolding protein at the neuromuscular junction, rapsyn, enables clustering of nicotinic acetylcholine receptors in high concentration and is critical for muscle function. Patients with insufficient receptor clustering suffer from muscle weakness. However, the detailed organization of the receptor-rapsyn network is poorly understood: it is unclear whether rapsyn first forms a wide meshwork to which receptors can subsequently dock or whether it only forms short bridges linking receptors together to make a large cluster. Furthermore, the number of rapsyn-binding sites per receptor (a heteropentamer) has been controversial. Here, we show by cryoelectron tomography and subtomogram averaging of Torpedo postsynaptic membrane that receptors are connected by up to three rapsyn bridges, the minimum number required to form a 2D network. Half of the receptors belong to rapsyn-connected groups comprising between two and fourteen receptors. Our results provide a structural basis for explaining the stability and low diffusion of receptors within clusters.

Clinical features in a large Iranian family with a limb-girdle congenital myasthenic syndrome due to a mutation in DPAGT1

Mutations in DPAGT1 are a newly recognised cause of congenital myasthenic syndrome. DPAGT1 encodes an early component of the N-linked glycosylation pathway. Initially mutations in DPAGT1 have been associated with the onset of the severe multisystem disorder - congenital disorder of glycosylation type 1J. However, recently it was established that certain mutations in this gene can cause symptoms restricted to muscle weakness resulting from defective neuromuscular transmission. We report four cases from a large Iranian pedigree with prominent limb-girdle weakness and minimal craniobulbar symptoms who harbour a novel mutation in DPAGT1, c.652C>T, p.Arg218Trp. This myasthenic syndrome may mimic myopathic disorders and is likely under-diagnosed.

Recurrent apparent life-threatening event as the first manifestation of congenital myasthenia

OBJECTIVE

To alert pediatricians about the importance of a careful investigation on recurrent apparent life-threatening events. Reports of the association of these events with congenital myasthenic syndromes were not found.

CASE DESCRIPTION

A seven-month-old infant with recurrent apparent life-threatening events was admitted for investigation. During hospital stay, she presented cyanosis and respiratory failure, requiring mechanical ventilation for three days. After clinical improvement, hypotheses of gastroesophageal reflux and pulmonary aspiration were ruled out. The presence of eyelid ptosis, general hypotonia and weak crying led to the suspicion of congenital myasthenia, which was confirmed. Treatment with oral piridostigmine led to neurological and nutritional normalization, without any other apparent life-threatening event during the next three years.

COMMENTS

The careful etiological investigation of apparent life-threatening events may lead to rare diagnosis that requires specific treatments, such as congenital myasthenia.

Quinine sulfate as a therapeutic option in a patient with slow channel congenital myasthenic syndrome

Slow channel congenital myasthenic syndrome is caused by a genetically determined kinetic anomaly of the acetylcholine receptor at the neuromuscular junction leading to its prolonged open state. Patients typically present with fatigability and static weakness of neck, hand and finger extensors. The open-channel blockers fluoxetine and quinidine have been used as standard treatment, although the former is limited by its side effects. We describe a patient with a novel "de novo" mutation in the α subunit of acetylcholine receptor with clinical and electrophysiological hallmarks of the disease. The patient showed marked treatment response to fluoxetine as well as quinine, a stereoisomer of quinidine, expanding the treatment options for this hereditary disorder.

A practical approach to molecular diagnostic testing in neuromuscular diseases

Molecular diagnosis is an important aspect in the care of patients with neuromuscular disorders. Because of the rapidly evolving nature of the field, the approach to obtaining a molecular diagnosis may be challenging. This article provides a general approach to molecular diagnostic testing while reviewing the principles of genetics and genetic disorders and the indications and limitations of testing methods in common hereditary neuromuscular disorders.

Structure and activation of MuSK, a receptor tyrosine kinase central to neuromuscular junction formation

MuSK (muscle-specific kinase) is a receptor tyrosine kinase that plays a central signaling role in the formation of neuromuscular junctions (NMJs). MuSK is activated in a complex spatio-temporal manner to cluster acetylcholine receptors on the postsynaptic (muscle) side of the synapse and to induce differentiation of the nerve terminal on the presynaptic side. The ligand for MuSK is LRP4 (low-density lipoprotein receptor-related protein-4), a transmembrane protein in muscle, whose binding affinity for MuSK is potentiated by agrin, a neuronally derived heparan-sulfate proteoglycan. In addition, Dok7, a cytoplasmic adaptor protein, is also required for MuSK activation in vivo. This review focuses on the physical interplay between these proteins and MuSK for activation and downstream signaling, which culminates in NMJ formation. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Congenital myasthenic syndromes due to mutations in ALG2 and ALG14

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed linkage analysis, whole-exome and whole-genome sequencing to determine the underlying defect in patients with an inherited limb-girdle pattern of myasthenic weakness. We identify ALG14 and ALG2 as novel genes in which mutations cause a congenital myasthenic syndrome. Through analogy with yeast, ALG14 is thought to form a multiglycosyltransferase complex with ALG13 and DPAGT1 that catalyses the first two committed steps of asparagine-linked protein glycosylation. We show that ALG14 is concentrated at the muscle motor endplates and small interfering RNA silencing of ALG14 results in reduced cell-surface expression of muscle acetylcholine receptor expressed in human embryonic kidney 293 cells. ALG2 is an alpha-1,3-mannosyltransferase that also catalyses early steps in the asparagine-linked glycosylation pathway. Mutations were identified in two kinships, with mutation ALG2p.Val68Gly found to severely reduce ALG2 expression both in patient muscle, and in cell cultures. Identification of DPAGT1, ALG14 and ALG2 mutations as a cause of congenital myasthenic syndrome underscores the importance of asparagine-linked protein glycosylation for proper functioning of the neuromuscular junction. These syndromes form part of the wider spectrum of congenital disorders of glycosylation caused by impaired asparagine-linked glycosylation. It is likely that further genes encoding components of this pathway will be associated with congenital myasthenic syndromes or impaired neuromuscular transmission as part of a more severe multisystem disorder. Our findings suggest that treatment with cholinesterase inhibitors may improve muscle function in many of the congenital disorders of glycosylation.

Synaptic basal lamina-associated congenital myasthenic syndromes

Proteins associated with the basal lamina (BL) participate in complex signal transduction processes that are essential for the development and maintenance of the neuromuscular junction (NMJ). Most important junctional BL proteins are collagens, such as collagen IV (α3-6), collagen XIII, and ColQ; laminins; nidogens; and heparan sulfate proteoglycans, such as perlecan and agrin. Mice lacking Colq (Colq(-/-)), laminin β2 (Lamb2(-/-)), or collagen XIII (Col13a1(-/-)) show immature nerve terminals enwrapped by Schwann cell projections that invaginate into the synaptic cleft and decrease contact surface for neurotransmission. Human mutations in COLQ, LAMB2, and AGRN cause congenital myasthenic syndromes (CMSs) owing to deficiency of ColQ, laminin-β2, and agrin, respectively. In these syndromes the NMJ ultrastructure shows striking resemblance to that of mice lacking the corresponding protein; furthermore, the extracellular localization of mutant proteins may provide favorable conditions for replacement strategies based on gene therapy and stem cells.

Synaptic dysfunction in congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are hereditary disorders of neuromuscular transmission characterized by fatigable muscle weakness. The number of cases recognized is increasing with improved diagnosis. To date we have identified over 300 different mutations present in over 350 unrelated kinships. The underlying genetic defects are diverse, involving a series of different genes with a variety of different phenotypes. The type of treatment and its effectiveness will depend on the underlying pathogenic mechanism. We aim to define the molecular mechanism for each mutation identified and feed this information back to the clinic as a basis to tailor patient treatment. Here, we describe some of the methods that can be used to define if a DNA sequence variant is pathogenic with reference to variants in DOK7. We highlight a new mechanism for disruption of AChR function, where a mutation in the AChR ɛ-subunit gene causes reduced ion channel conductance and discuss new methods for identifying gene mutations. The study of these disorders is proving highly informative for understanding the diverse molecular mechanisms that can underlie synaptic dysfunction.

Congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders caused by genetic defects affecting neuromuscular transmission and leading to muscle weakness accentuated by exertion. The characterization of CMS comprises two complementary steps: establishing the diagnosis and identifying the pathophysiological type of CMS. The combination of clinical, electrophysiological, and morphological studies allows the physician to refer a given CMS to mutation(s) in one of the 18 causative genes discovered to date and, in turn, to classify the CMS according to the location of the mutated proteins at the neuromuscular junction into presynaptic compartment, synaptic basal lamina, and postsynaptic compartment CMS. This complete characterization is essential for counseling and therapy of the patient, depending on the molecular background of the respective CMS. Despite comprehensive characterization, the phenotypic expression of one given gene involved is variable, and the etiology of many CMS remains to be discovered.

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.

Targeting the fetal acetylcholine receptor in rhabdomyosarcoma

INTRODUCTION

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence. Recent efforts to enhance overall survival of patients with clinically advanced RMS have failed and there is a demand for conceptually novel treatments. Immune therapeutic options targeting the fetal nicotinic acetylcholine receptor (fnAChR), which is broadly expressed on RMS, are novel approaches to overcome the therapeutic resistance of RMS. Expression of the fnAChR is restricted to developing fetal muscles, some apparently dispensable ocular muscle fibers and thymic myoid cells. Therefore, after-birth fnAChR is a tumor-associated and almost tumor-specific antigen on RMS cells.

AREAS COVERED

This review gives an overview on nAChR function and expression pattern in RMS tumor cells, and deals with the immunological significance of fnAChR-expressing cells, including the risk of anti-nAChR autoimmunity as a potential side effect of fnAChR-directed immunotherapies. The article also addresses the advantages and disadvantages of vaccination strategies, immunotoxins and chimeric T cells targeting the fnAChR.

EXPERT OPINION

Finally, we suggest technical and biological strategies to improve the available immunotherapeutic tools including increasing the in vivo expression of the target fnAChR on RMS cells.

Identification of DPAGT1 as a new gene in which mutations cause a congenital myasthenic syndrome

Congenital myasthenic syndromes (CMS) are a group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. This is a heterogenous group of disorders with 15 different genes implicated in the development of the disease. Using whole-exome sequencing we identified DPAGT1 as a new gene associated with CMS. DPAGT1 catalyses the first step of N-linked protein glycosylation. DPAGT1 patients are characterized by weakness of limb muscles, response to treatment with cholinesterase inhibitors, and the presence of tubular aggregates on muscle biopsy. We showed that DPAGT1 is required for glycosylation of acetylcholine receptor (AChR) subunits and efficient export of AChR to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1-associated CMS is reduced levels of AChRs at the endplate region. This finding demonstrates that impairment of the N-linked glycosylation pathway can lead to the development of CMS.

Current pharmacologic management in selected neuromuscular diseases

For generations, the neuromuscular disorder care community has focused on establishing the correct diagnosis and providing supportive care. As the pathophysiology and genetics of these conditions became better understood, novel treatments targeting the disease mechanism were developed. This has led to some significant disease-modifying and supportive treatments for several neuromuscular disorders. The current treatments for amyotrophic lateral sclerosis (ALS), neuromuscular junction disorders, inflammatory myopathies, and myotonia are reviewed. Additionally, investigational treatments for ALS, Duchenne muscular dystrophy, and spinal muscular atrophy are discussed.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

Myasthenic syndrome AChRα C-loop mutant disrupts initiation of channel gating

Congenital myasthenic syndromes (CMSs) are neuromuscular disorders that can be caused by defects in ace-tylcholine receptor (AChR) function. Disease-associated point mutants can reveal the unsuspected functional significance of mutated residues. We identified two pathogenic mutations in the extracellular domain of the AChR α subunit (AChRα) in a patient with myasthenic symptoms since birth: a V188M mutation in the C-loop and a heteroallelic G74C mutation in the main immunogenic region. The G74C mutation markedly reduced surface AChR expression in cultured cells, whereas the V188M mutant was expressed robustly but had severely impaired kinetics. Single-channel patch-clamp analysis indicated that V188M markedly decreased the apparent AChR channel opening rate and gating efficiency. Mutant cycle analysis of energetic coupling among conserved residues within or dispersed around the AChRα C-loop revealed that V188 is functionally linked to Y190 in the C-loop and to D200 in β-strand 10, which connects to the M1 transmembrane domain. Furthermore, V188M weakens inter-residue coupling of K145 in β-strand 7 with Y190 and with D200. Cumulatively, these results indicate that V188 of AChRα is part of an interdependent tetrad that contributes to rearrangement of the C-loop during the initial coupling of agonist binding to channel gating.

Pharmacogenomics: mapping monogenic mutations to direct therapy

The molecular mapping of mutations that underlie congenital disorders of monogenic origin can result in both a broader understanding of the molecular basis of the disorder and novel therapeutic insights. Indeed, genotyping patients and then replicating the behavior of the mutant gene products in well-defined biochemical or electrophysiological systems will allow tailoring of therapy to be mutation- and protein sequence-dependent. In this issue of the JCI, Shen and colleagues describe such an approach that identified novel mutations in the α subunit of the nicotinic receptor linked to myasthenia gravis.