Congenital myasthenic syndromes: an update

Rare disease research workflow using multilayer networks elucidates the molecular determinants of severity in Congenital Myasthenic Syndromes

Exploring the molecular basis of disease severity in rare disease scenarios is a challenging task provided the limitations on data availability. Causative genes have been described for Congenital Myasthenic Syndromes (CMS), a group of diverse minority neuromuscular junction (NMJ) disorders; yet a molecular explanation for the phenotypic severity differences remains unclear. Here, we present a workflow to explore the functional relationships between CMS causal genes and altered genes from each patient, based on multilayer network community detection analysis of complementary biomedical information provided by relevant data sources, namely protein-protein interactions, pathways and metabolomics. Our results show that CMS severity can be ascribed to the personalized impairment of extracellular matrix components and postsynaptic modulators of acetylcholine receptor (AChR) clustering. This work showcases how coupling multilayer network analysis with personalized -omics information provides molecular explanations to the varying severity of rare diseases; paving the way for sorting out similar cases in other rare diseases.

Dystrophic Myopathy of the Diaphragm with Recurrent Severe Respiratory Failure is Congenital Myasthenic Syndrome 11

We here present the case of a patient with a congenital myasthenic syndrome (CMS) due to pathogenic variants in the RAPSN gene. During childhood he experienced recurrent episodes of respiratory failure during respiratory infections. This and other cases were reported as isolated dystrophy of the diaphragmatic musculature. In adulthood, whole exome sequencing revealed two heterozygous pathogenic variants in the RAPSN gene. This led to the revision of the diagnosis to rapsyn CMS11 (OMIM:616326, MONDO:0014588). EMG, muscle ultrasound and the revision of muscle biopsies taken in childhood support this diagnosis. After the revision of the diagnosis, treatment with pyridostigmine was started. This resulted in a reduction of fatigability and an improvement in functional abilities and quality of life.

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

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

Case Report: A Novel AChR Epsilon Variant Causing a Clinically Discordant Salbutamol Responsive Congenital Myasthenic Syndrome in Two Egyptian Siblings

Congenital myasthenic syndromes (CMS) are inherited disorders that lead to abnormal neuromuscular transmission. Post-synaptic mutations are the main cause of CMS, particularly mutations in CHRNE. We report a novel homozygous CHRNE pathogenic variant in two Egyptian siblings showing a CMS. Interestingly, they showed different degrees of extraocular and skeletal muscle involvement; both presented only a partial response to cholinesterase inhibitors, and rapidly and substantially ameliorated after the addition of oral β2 adrenergic agonists. Here, we enlarge the genetic spectrum of CHRNE-related congenital myasthenic syndromes and highlight the importance of a β2 adrenergic agonists treatment.

Motoneuron-specific loss of VAChT mimics neuromuscular defects seen in congenital myasthenic syndrome

Motoneurons (MNs) control muscle activity by releasing the neurotransmitter acetylcholine (ACh) at the level of neuromuscular junctions. ACh is packaged into synaptic vesicles by the vesicular ACh transporter (VAChT), and disruptions in its release can impair muscle contraction, as seen in congenital myasthenic syndromes (CMS). Recently, VAChT gene mutations were identified in humans displaying varying degrees of myasthenia. Moreover, mice with a global deficiency in VAChT expression display several characteristics of CMS. Despite these findings, little is known about how a long-term decrease in VAChT expression in vivo affects MNs structure and function. Using Cre-loxP technology, we generated a mouse model where VAChT is deleted in select groups of MNs (mnVAChT-KD). Molecular analysis revealed that the VAChT deletion was specific to MNs and affected approximately 50% of its population in the brainstem and spinal cord, with alpha-MNs primarily targeted (70% in spinal cord). Within each animal, the cell body area of VAChT-deleted MNs was significantly smaller compared to MNs with VAChT preserved. Likewise, muscles innervated by VAChT-deleted MNs showed atrophy while muscles innervated by VAChT-containing neurons appeared normal. In addition, mnVAChT KD mice had decreased muscle strength, were hypoactive, leaner and exhibited kyphosis. This neuromuscular dysfunction was evident at 2 months of age and became progressively worse by 6 months. Treatment of mutants with a cholinesterase inhibitor was able to improve some of the motor deficits. As these observations mimic what is seen in CMS, this new line could be valuable for assessing the efficacy of potential CMS drugs.

Building neuromuscular junctions in vitro

The neuromuscular junction (NMJ) has been the model of choice to understand the principles of communication at chemical synapses. Following groundbreaking experiments carried out over 60 years ago, many studies have focused on the molecular mechanisms underlying the development and physiology of these synapses. This Review summarizes the progress made to date towards obtaining faithful models of NMJs in vitro We provide a historical approach discussing initial experiments investigating NMJ development and function from Xenopus to mice, the creation of chimeric co-cultures, in vivo approaches and co-culture methods from ex vivo and in vitro derived cells, as well as the most recent developments to generate human NMJs. We discuss the benefits of these techniques and the challenges to be addressed in the future for promoting our understanding of development and human disease.

Modeling the neuromuscular junction in vitro: an approach to study neuromuscular junction disorders

The neuromuscular junction (NMJ) is a specialized structure that works as an interface to translate the action potential of the presynaptic motor neuron (MN) in the contraction of the postsynaptic myofiber. The design of appropriate experimental models is essential to have efficient and reliable approaches to study NMJ development and function, but also to generate conditions that recapitulate distinct features of diseases. Initial studies relied on the use of tissue slices maintained under the same environment and in which single motor axons were difficult to trace. Later, MNs and muscle cells were obtained from primary cultures or differentiation of progenitors and cocultured as monolayers; however, the tissue architecture was lost. Current approaches include self-assembling 3D structures or the incorporation of biomaterials with cells to generate engineered tissues, although the incorporation of Schwann cells remains a challenge. Thus, numerous investigations have established different NMJ models, some of which are quite complex and challenging. Our review summarizes the in vitro models that have emerged in recent years to coculture MNs and skeletal muscle, trying to mimic the healthy and diseased NMJ. We expect our review may serve as a reference for choosing the appropriate experimental model for the required purposes of investigation.

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.

A Panel of Slow-Channel Syndrome Mice Reveals a Unique Locomotor Behavioral Signature

Muscle nicotinic acetylcholine receptor (nAChR) mutations can lead to altered channel kinetics and neuromuscular junction degeneration, a neurodegenerative disorder collectively known as slow-channel syndrome (SCS). A multivariate analysis using running wheels was used to generate activity profiles for a variety of SCS models, uncovering unique locomotor patterns for the different nAChR mutants. Particularly, the αL251T and ɛL269F mutations exhibit decreased event distance, duration, and velocity over a period of 24 hours. Our approach suggests a robust relationship between the pathophysiology of SCS and locomotor activity.

Congenital myasthenic syndrome due to rapsyn deficiency: A case report with a new mutation and compound heterozygosity

Introduction

The congenital myasthenic syndromes are a heterogeneous group of genetic disorders characterized by an abnormal synaptic transmission in the neuromuscular plate.

Report

We present a two-year-old patient, male, with hypotonia, palpebral ptosis, and proximal symmetric weakness with a neonatal onset that motivated several and prolonged hospitalizations for pneumonia and respiratory failure. From two years of age, the parents noticed that the facial and general weakness worsened in the afternoons and with repeated or prolonged physical activity. The physical examination showed palpebral ptosis, predominantly proximal weakness, and fatigability with sustained muscular effort. The electromyography showed a 27% decrement in the Compound Muscular Action Potential and the case-parents genetic study showed compound heterozygosity with the transmission of two different mutations in the rapsyn gene from both parents. The patient received pyridostigmine with great improvement, achieving optimal performance in school, sports, and daily life activities.

Conclusions

Weakness and fatigability with neonatal onset, mainly affecting the muscles with brain stem innervation and the decrement greater than 10 percent in the Compound Muscular Action Potential in the electromyographic studies, should make us suspect in a congenital myasthenic syndrome. We review the literature and key clinical points to establish a timely diagnosis and effective treatment in some of these syndromes.

Collagen XIII and Other ECM Components in the Assembly and Disease of the Neuromuscular Junction

Alongside playing structural roles, the extracellular matrix (ECM) acts as an interaction platform for cellular homeostasis, organ development, and maintenance. The necessity of the ECM is highlighted by the diverse, sometimes very serious diseases that stem from defects in its components. The neuromuscular junction (NMJ) is a large peripheral motor synapse differing from its central counterparts through the ECM included at the synaptic cleft. Such synaptic basal lamina (BL) is specialized to support NMJ establishment, differentiation, maturation, stabilization, and function and diverges in molecular composition from the extrasynaptic ECM. Mutations, toxins, and autoantibodies may compromise NMJ integrity and function, thereby leading to congenital myasthenic syndromes (CMSs), poisoning, and autoimmune diseases, respectively, and all these conditions may involve synaptic ECM molecules. With neurotransmission degraded or blocked, muscle function is impaired or even prevented. At worst, this can be fatal. The article reviews the synaptic BL composition required for assembly and function of the NMJ molecular machinery through the lens of studies primarily with mouse models but also with human patients. In-depth focus is given to collagen XIII, a postsynaptic-membrane-spanning but also shed ECM protein that in recent years has been revealed to be a significant component for the NMJ. Its deficiency in humans causes CMS, and autoantibodies against it have been recognized in autoimmune myasthenia gravis. Mouse models have exposed numerous details that appear to recapitulate human NMJ phenotypes relatively faithfully and thereby can be readily used to generate information necessary for understanding and ultimately treating human diseases. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc.

A new severe mutation in the SLC5A7 gene related to congenital myasthenic syndrome type 20

Congenital myasthenic syndromes are a group of genetically determined rare diseases resulting from ultrastructural alterations in synaptic proteins. Up to 32 genes are known to be involved in those syndromes and many mutations have been reported, of which less than 8% affect the presynaptic complex. One of these syndromes is caused by the impairment of the presynaptic sodium-dependent high-affinity choline transporter 1, as a result of a mutation of the SCL5A7 gene associated with congenital myasthenic syndrome type 20 (MIM # 617143). We present a new case of this syndrome, caused by a mutation not previously described. A full term infant presented with acute respiratory failure and generalized weakness. The genetic analysis revealed the patient to be compound heterozygous for a new mutation of the SCL5A7 gene. The genetic analysis of congenital myasthenic syndromes provide information on the ultrastructural underlying mechanisms, which is valuable for differential diagnosis and specific treatments.

Congenital myasthenic syndromes in Turkey: Clinical clues and prognosis with long term follow-up

Congenital myasthenic syndromes (CMS) are a group of hereditary disorders affecting the neuromuscular junction. Here, we present clinical, electrophysiological and genetic findings of 69 patients from 51 unrelated kinships from Turkey. Genetic tests of 60 patients were performed at Mayo Clinic. Median follow-up time was 9.8 years (range 1-22 years). The most common CMS was primary acetylcholine receptor (AChR) deficiency (31/51) and the most common mutations in AChR were c.1219 + 2T > G (12/51) and c.1327delG (6/51) in CHRNE. Four of our 5 kinships with AChE deficiency carried p.W148X that truncates the collagen domain of COLQ, and was previously reported only in patients from Turkey. These were followed by GFPT1 deficiency (4/51), DOK7 deficiency (3/51), slow channel CMS (3/51), fast channel CMS (3/51), choline acetyltransferase deficiency (1/51) and a CMS associated with desmin deficiency (1/51). Distribution of muscle weakness was sometimes useful in giving a clue to the CMS subtype. Presence of repetitive compound muscle action potentials pointed to AChE deficiency or slow channel CMS. Our experience confirms that one needs to be cautious using pyridostigmine, since it can worsen some types of CMS. Ephedrine/salbutamol were very effective in AChE and DOK7 deficiencies and were useful as adjuncts in other types of CMS. Long follow-up gave us a chance to assess progression of the disease, and to witness 12 mainly uneventful pregnancies in 8 patients. In this study, we describe some new phenotypes and detail the clinical features of the well-known CMS.

Vesicular acetylcholine transporter defect underlies devastating congenital myasthenia syndrome

OBJECTIVE

To identify the genetic basis of a recessive congenital neurologic syndrome characterized by severe hypotonia, arthrogryposis, and respiratory failure.

METHODS

Identification of the responsible gene by exome sequencing and assessment of the effect of the mutation on protein stability in transfected rat neuronal-like PC12^A123.7 cells.

RESULTS

Two brothers from a nonconsanguineous Yemeni Jewish family manifested at birth with severe hypotonia and arthrogryposis. The older brother died of respiratory failure at 5 days of age. The proband, now 4.5 years old, has been mechanically ventilated since birth with virtually no milestones achievement. Whole exome sequencing revealed homozygosity of SLC18A3 c.1078G>C, p.Gly360Arg in the affected brothers but not in other family members. SLC18A3 p.Gly360Arg is not reported in world populations but is present at a carrier frequency of 1:30 in healthy Yemeni Jews. SLC18A3 encodes the vesicular acetylcholine transporter (VAChT), which loads newly synthesized acetylcholine from the neuronal cytoplasm into synaptic vesicles. Mice that are VAChT-null have been shown to die at birth of respiratory failure. In human VAChT, residue 360 is located in a conserved region and substitution of arginine for glycine is predicted to disrupt proper protein folding and membrane embedding. Stable transfection of wild-type and mutant human VAChT into neuronal-like PC12^A123.7 cells revealed similar mRNA levels, but undetectable levels of the mutant protein, suggesting post-translational degradation of mutant VAChT.

CONCLUSION

Loss of function of VAChT underlies severe arthrogryposis and respiratory failure. While most congenital myasthenic syndromes are caused by defects in postsynaptic proteins, VAChT deficiency is a presynaptic myasthenic syndrome.

Neuromuscular junction disorders

Diseases of the neuromuscular junction comprise a wide range of disorders. Antibodies, genetic mutations, specific drugs or toxins interfere with the number or function of one of the essential proteins that control signaling between the presynaptic nerve ending and the postsynaptic muscle membrane. Acquired autoimmune disorders of the neuromuscular junction are the most common and are described here. In myasthenia gravis, antibodies to acetylcholine receptors or to proteins involved in receptor clustering, particularly muscle-specific kinase, cause direct loss of acetylcholine receptors or interfere with the agrin-induced acetylcholine receptor clustering necessary for efficient neurotransmission. In the Lambert-Eaton myasthenic syndrome (LEMS), loss of the presynaptic voltage-gated calcium channels results in reduced release of the acetylcholine transmitter. The conditions are generally recognizable clinically and the diagnosis confirmed by serologic testing and electromyography. Screening for thymomas in myasthenia or small cell cancer in LEMS is important. Fortunately, a wide range of symptomatic treatments, immunosuppressive drugs, or other immunomodulating therapies is available. Future research is directed to understanding the pathogenesis, discovering new antigens, and trying to develop disease-specific treatments.

Novel potent pyridoxine-based inhibitors of AChE and BChE, structural analogs of pyridostigmine, with improved in vivo safety profile

We report a novel class of carbamate-type ChE inhibitors, structural analogs of pyridostigmine. A small library of congeneric pyridoxine-based compounds was designed, synthesized and evaluated for AChE and BChE enzymes inhibition in vitro. The most active compounds have potent enzyme inhibiting activity with IC50 values in the range of 0.46-2.1μM (for AChE) and 0.59-8.1μM (for BChE), with moderate selectivity for AChE comparable with that of pyridostigmine and neostigmine. Acute toxicity studies using mice models demonstrated excellent safety profile of the obtained compounds with LD50 in the range of 22-326mg/kg, while pyridostigmine and neostigmine are much more toxic (LD50 3.3 and 0.51mg/kg, respectively). The obtained results pave the way to design of novel potent and safe cholinesterase inhibitors for symptomatic treatment of neuromuscular disorders.

Skeletal muscle sodium channelopathies

PURPOSE OF REVIEW

This is an update on skeletal muscle sodium channelopathies since knowledge in the field have dramatically increased in the past years.

RECENT FINDING

The relationship between two phenotypes and SCN4A has been confirmed with additional cases that remain extremely rare: severe neonatal episodic laryngospasm mimicking encephalopathy, which should be actively searched for since patients respond well to sodium channel blockers; congenital myasthenic syndromes, which have the particularity to be the first recessive Nav1.4 channelopathy. Deep DNA sequencing suggests the contribution of other ion channels in the clinical expressivity of sodium channelopathies, which may be one of the factors modulating the latter. The increased knowledge of channel molecular structure, the quantity of sodium channel blockers, and the availability of preclinical models would permit a most personalized choice of medication for patients suffering from these debilitating neuromuscular diseases.

SUMMARY

Advances in the understanding of the molecular structure of voltage-gated sodium channels, as well as availability of preclinical models, would lead to improved medical care of patients suffering from skeletal muscle, as well as other sodium channelopathies.

A recessive Nav1.4 mutation underlies congenital myasthenic syndrome with periodic paralysis

OBJECTIVE

To determine the molecular basis of a complex phenotype of congenital muscle weakness observed in an isolated but consanguineous patient.

METHODS

The proband was evaluated clinically and neurophysiologically over a period of 15 years. Genetic testing of candidate genes was performed. Functional characterization of the candidate mutation was done in mammalian cell background using whole cell patch clamp technique.

RESULTS

The proband had fatigable muscle weakness characteristic of congenital myasthenic syndrome with acute and reversible attacks of most severe muscle weakness as observed in periodic paralysis. We identified a novel homozygous SCN4A mutation (p.R1454W) linked to this recessively inherited phenotype. The p.R1454W substitution induced an important enhancement of fast and slow inactivation, a slower recovery for these inactivated states, and a frequency-dependent regulation of Nav1.4 channels in the heterologous expression system.

CONCLUSION

We identified a novel loss-of-function mutation of Nav1.4 that leads to a recessive phenotype combining clinical symptoms and signs of congenital myasthenic syndrome and periodic paralysis, probably by decreasing channel availability for muscle action potential genesis at the neuromuscular junction and propagation along the sarcolemma.

COLQ variant associated with Devon Rex and Sphynx feline hereditary myopathy

Some Devon Rex and Sphynx cats have a variably progressive myopathy characterized by appendicular and axial muscle weakness, megaesophagus, pharyngeal weakness and fatigability with exercise. Muscle biopsies from affected cats demonstrated variable pathological changes ranging from dystrophic features to minimal abnormalities. Affected cats have exacerbation of weakness following anticholinesterase dosing, a clue that there is an underlying congenital myasthenic syndrome (CMS). A genome-wide association study and whole-genome sequencing suggested a causal variant for this entity was a c.1190G>A variant causing a cysteine to tyrosine substitution (p.Cys397Tyr) within the C-terminal domain of collagen-like tail subunit (single strand of homotrimer) of asymmetric acetylcholinesterase (COLQ). Alpha-dystroglycan expression, which is associated with COLQ anchorage at the motor end-plate, has been shown to be deficient in affected cats. Eighteen affected cats were identified by genotyping, including cats from the original clinical descriptions in 1993 and subsequent publications. Eight Devon Rex and one Sphynx not associated with the study were identified as carriers, suggesting an allele frequency of ~2.0% in Devon Rex. Over 350 tested cats from other breeds did not have the variant. Characteristic clinical features and variant presence in all affected cats suggest a model for COLQ CMS. The association between the COLQ variant and this CMS affords clinicians the opportunity to confirm diagnosis via genetic testing and permits owners and breeders to identify carriers in the population. Moreover, accurate diagnosis increases available therapeutic options for affected cats based on an understanding of the pathophysiology and experience from human CMS associated with COLQ variants.

MuSK frizzled-like domain is critical for mammalian neuromuscular junction formation and maintenance

The muscle-specific kinase MuSK is one of the key molecules orchestrating neuromuscular junction (NMJ) formation. MuSK interacts with the Wnt morphogens, through its Frizzled-like domain (cysteine-rich domain [CRD]). Dysfunction of MuSK CRD in patients has been recently associated with the onset of myasthenia, common neuromuscular disorders mainly characterized by fatigable muscle weakness. However, the physiological role of Wnt-MuSK interaction in NMJ formation and function remains to be elucidated. Here, we demonstrate that the CRD deletion of MuSK in mice caused profound defects of both muscle prepatterning, the first step of NMJ formation, and synapse differentiation associated with a drastic deficit in AChR clusters and excessive growth of motor axons that bypass AChR clusters. Moreover, adult MuSKΔCRD mice developed signs of congenital myasthenia, including severe NMJs dismantlement, muscle weakness, and fatigability. We also report, for the first time, the beneficial effects of lithium chloride, a reversible inhibitor of the glycogen synthase kinase-3, that rescued NMJ defects in MuSKΔCRD mice and therefore constitutes a novel therapeutic reagent for the treatment of neuromuscular disorders linked to Wnt-MuSK signaling pathway deficiency. Together, our data reveal that MuSK CRD is critical for NMJ formation and plays an unsuspected role in NMJ maintenance in adulthood.

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.

A COLQ missense mutation in Labrador Retrievers having congenital myasthenic syndrome

Congenital myasthenic syndromes (CMSs) are heterogeneous neuromuscular disorders characterized by skeletal muscle weakness caused by disruption of signal transmission across the neuromuscular junction (NMJ). CMSs are rarely encountered in veterinary medicine, and causative mutations have only been identified in Old Danish Pointing Dogs and Brahman cattle to date. Herein, we characterize a novel CMS in 2 Labrador Retriever littermates with an early onset of marked generalized muscle weakness. Because the sire and dam share 2 recent common ancestors, CMS is likely the result of recessive alleles inherited identical by descent (IBD). Genome-wide SNP profiles generated from the Illumina HD array for 9 nuclear family members were used to determine genomic inheritance patterns in chromosomal regions encompassing 18 functional candidate genes. SNP haplotypes spanning 3 genes were consistent with autosomal recessive transmission, and microsatellite data showed that only the segment encompassing COLQ was inherited IBD. COLQ encodes the collagenous tail of acetylcholinesterase, the enzyme responsible for termination of signal transduction in the NMJ. Sequences from COLQ revealed a variant in exon 14 (c.1010T>C) that results in the substitution of a conserved amino acid (I337T) within the C-terminal domain. Both affected puppies were homozygous for this variant, and 16 relatives were heterozygous, while 288 unrelated Labrador Retrievers and 112 dogs of other breeds were wild-type. A recent study in which 2 human CMS patients were found to be homozygous for an identical COLQ mutation (c.1010T>C; I337T) provides further evidence that this mutation is pathogenic. This report describes the first COLQ mutation in canine CMS and demonstrates the utility of SNP profiles from nuclear family members for the identification of private mutations.

Clinical and molecular analysis of a novel COLQ missense mutation causing congenital myasthenic syndrome in a Syrian family

BACKGROUND

Congenital myasthenic syndromes with end-plate acetylcholinesterase deficiency are rare autosomal recessive disorders characterized by onset of the disease in early childhood, general weakness exacerbated by exertion, ophthalmoplegia, and refractoriness to anticholinesterase drugs. To date, all reported cases have been attributed to mutations in 18 genes including the COLQ gene that encodes a specific collagen that anchors acetylcholinesterase at the basal lamina of the neuromuscular junction. We identified a Syrian family with two children of consanguineous parents from two branches affected with congenital myasthenic syndrome with end-plate acetylcholinesterase deficiency.

METHOD

The absence of acetylcholinesterase antibodies was demonstrated biochemically. Consequently, all the coding regions, exon-intron boundaries, and the 5' and 3' untranslated regions of the COLQ gene were amplified and sequenced using the Sanger sequencing method.

RESULTS

We observed that the severity of the phenotype in the two affected children differed. One child had mild symptoms that included difficulties in gait and feeding with mild respiratory insufficiency. Her sibling died in the first months of life because of severe respiratory failure. The second patient had severe symptoms from birth and has been mechanically ventilated. DNA sequencing revealed a novel homozygous single nucleotide substitution mutation (c.1010T>C) in the COLQ gene in both patients. This substitution leads to a missense amino acid substitution at position 337 of the protein (p.Ile337Thr). This mutation is likely to impair ColQ's trimeric organization and therefore its anchoring within the synaptic basal lamina.

CONCLUSION

We identified the molecular cause underlying congenital myasthenic syndrome in two patients. The marked phenotypic variation suggests that other factors including modifier genes may affect the severity of this 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.