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Background Congenital myasthenic syndrome (CMS) is a clinically and genetically heterogeneous neuromuscular disorder characterized by muscle weakness and caused by mutations in more than 35 different genes. This condition should not be overlooked as a subset of patients with CMS are treatable. However, the diagnosis of CMS is often difficult due to the broad variability in disease severity and course. Case report A five-year-old boy without remarkable family history was born with marked general muscle hypotonia and weakness, respiratory insufficiency, anomalies, and multiple joint contractures. Congenital myopathy was suspected based upon type 1 fiber predominance on muscle biopsy. However, he was diagnosed with CMS at age 4 years when his ptosis and ophthalmoplegia were found to be improved by edrophonium chloride and repetitive nerve stimulation showed attenuation of compound muscle action potentials. An exome sequencing identified a compound heterozygous missense variant of c.737C > T (p.A246V) and a novel intronic insertion c.1166 + 4_1166 + 5insAAGCCCACCAC in RAPSN. RT-PCR analysis which showed the skipping of exon 7 in a skeletal muscle sample confirmed that the intronic insertion was pathogenic. His myasthenic symptoms were remarkably improved by pyridostigmine. Conclusion The patient’s diagnosis of CMS was confirmed by exome sequencing, and RT-PCR revealed that the skipping of exon 7 in RAPSN was caused by a novel intronic insertion. The genetic information uncovered in this case should therefore be added to the collection of tools for diagnosing and treating CMS.

Congenital myasthenic syndromes (CMS) comprise a heterogeneous group of genetic disorders of the neuromuscular junction. Next generation sequencing has been increasingly used for molecular diagnosis in CMS patients. This study aimed to identify the disease-causing variants in Thai patients. We recruited patients with a diagnosis of CMS based on clinical and electrophysiologic findings, and whole exome sequencing was performed. Thirteen patients aged from 2 to 54 years (median: 8 years) from 12 families were enrolled. Variants were identified in 9 of 13 patients (69%). Five novel variants and two previously reported variant were found in the COLQ, RAPSN and CHRND gene. The previously reported c.393+1G>A splice site variant in the COLQ gene was found in a majority of patients. Five patients harbor the homozygous splice site c.393+1G>A variant, and two patients carry compound heterozygous c.393+1G>A, c.718-1G>T, and c.393+1G>A, c.865G>T (p.Gly289Ter) variants. The novel variants were also found in RAPSN (p.Cys251del, p.Arg282Cys) and CHRND (p.Met481del). Molecular diagnosis in CMS patients can guide treatment decisions and may be life changing, especially in patients with COLQ mutations.

Aim Congenital myasthenic syndromes (CMS) are rare, genetically and phenotypically diverse disorders of neuromuscular transmission. Data on prevalence among children are scarce. Whole exome sequencing facilitated discovery of novel CMS mutations and enabled targeted treatment. Our aim was to identify the prevalence, genetic subtypes and clinical characteristics of CMS in pediatric population of Slovenia. Methods In this observational, national, cross-sectional study, medical records were retrospectively reviewed. Children with genetically confirmed CMS, referred over a 19 – year period (2000–2018) to the University Medical Centre, Ljubljana, Slovenia, were included in the study. Genetic and phenotypic characteristics were collected and prevalence of CMS in children was calculated. Results Eight children with a confirmed genetic mutation in 5 different genes (CHRNE, CHRND, RAPSN, CHAT, MUSK) causative of the CMS were identified. Calculated prevalence of genetically confirmed CMS was 22.2 cases per 1.000.000 children at the end of 2018. Interpretation The prevalence of genetically confirmed CMS in Slovenian children at the end of 2018 exceeds previously reported prevalence by more than two-fold, which suggests that prevalence in the literature is likely to be underestimated. Two extremely rarely detected mutations in MUSK and CHRND gene were detected and patient's clinical descriptions add important information on genotype-phenotype correlation.

Objectives To present phenotype features of a large cohort of congenital myasthenic syndromes (CMS) and correlate them with their molecular diagnosis. Methods Suspected CMS patients were divided into three groups: group A (limb, bulbar or axial weakness, with or without ocular impairment, and all the following: clinical fatigability, electrophysiology compatible with neuromuscular junction involvement and anticholinesterase agents response), group B (limb, bulbar or axial weakness, with or without ocular impairment, and at least one of additional characteristics noted in group A) and group C (pure ocular syndrome). Individual clinical findings and the clinical groups were compared between the group with a confirmed molecular diagnosis of CMS and the group without molecular diagnosis or with a non-CMS molecular diagnosis. Results Seventy-nine patients (68 families) were included in the cohort: 48 in group A, 23 in group B and 8 in group C. Fifty-one were considered confirmed CMS (30 CHRNE, 5 RAPSN, 4 COL13A1, 3 DOK7, 3 COLQ, 2 GFPT1, 1 CHAT, 1 SCN4A, 1 GMPPB, 1 CHRNA1), 7 probable CMS, 5 non-CMS and 16 unsolved. The chance of a confirmed molecular diagnosis of CMS was significantly higher for group A and lower for group C. Some individual clinical features, alterations on biopsy and electrophysiology enhanced specificity for CMS. Muscle imaging showed at least mild alterations in the majority of confirmed cases, with preferential involvement of soleus, especially in CHRNE CMS. Conclusions Stricter clinical criteria increase the chance of confirming a CMS diagnosis, but may lose sensitivity, especially for some specific genes.

Altered ubiquitin signaling and disrupted protein quality control have been implicated in the pathogenesis of PD. The aim of the study was to systematically examine the overlaps between E3 ubiquitin ligase genes and early onset PD (EOPD). A total of 695 EOPD patients were analyzed aggregate burden for rare variants (MAF

ObjectiveTo investigate the diagnostic challenges of congenital myasthenic syndromes (CMS) in adult neuromuscular practice.MethodsWe searched the Mayo Clinic database for patients with CMS diagnosed in adulthood in the neuromuscular clinic between 2000 and 2016. Clinical, laboratory, and electrodiagnostic data were reviewed.ResultsWe identified 34 patients with CMS, 30 of whom had a molecular diagnosis (14 DOK7, 6 RAPSN, 2 LRP4, 2 COLQ, 2 slow-channel syndrome, 1 primary acetylcholine receptor deficiency, 1 AGRN, 1 GFPT1, and 1 SCN4A). Ophthalmoparesis was often mild and present in 13 patients. Predominant limb-girdle weakness occurred in 19 patients. Two patients had only ptosis. Age at onset ranged from birth to 39 years (median 5 years). The median time from onset to diagnosis was 26 years (range 4–56 years). Thirteen patients had affected family members. Fatigable weakness was present when examined. Creatine kinase was elevated in 4 of 23 patients (range 1.2–4.2 times the upper limit of normal). Repetitive nerve stimulation revealed a decrement in 30 patients. Thirty-two patients were previously misdiagnosed with seronegative myasthenia gravis (n = 16), muscle diseases (n = 15), weakness of undetermined cause (n = 8), and others (n = 4). Fifteen patients received immunotherapy or thymectomy without benefits. Fourteen of the 25 patients receiving pyridostigmine did not improve or worsen.ConclusionMisdiagnosis occurred in 94% of the adult patients with CMS and causes a median diagnostic delay of nearly 3 decades from symptom onset. Seronegative myasthenia gravis and muscle diseases were the 2 most common misdiagnoses, which led to treatment delay and unnecessary exposure to immunotherapy, thymectomy, or muscle biopsy.

Background Congenital myasthenic syndrome with episodic apnoea (CMS-EA) is a rare but potentially treatable cause of apparent life-threatening events in infancy. The underlying mechanisms for sudden and recurrent episodes of respiratory arrest in these patients are unclear. Whilst CMS-EA is most commonly caused by mutations in CHAT, the list of associated genotypes is expanding. Methods We reviewed clinical information from 19 patients with CMS-EA, including patients with mutations in CHAT, SLC5A7 and RAPSN, and patients lacking a genetic diagnosis. Results Lack of genetic diagnosis was more common in CMS-EA than in CMS without EA (56% n = 18, compared to 7% n = 97). Most patients manifested intermittent apnoea in the first 4 months of life (74%, n = 14). A degree of clinical improvement with medication was observed in most patients (74%, n = 14), but the majority of cases also showed a tendency towards complete remission of apnoeic events with age (mean age of resolution 2 years 5 months). Signs of impaired neuromuscular transmission were detected on neurophysiology studies in 79% (n = 15) of cases, but in six cases, this was only apparent following specific neurophysiological testing protocols (prolonged high-frequency stimulation). Conclusions A relatively large proportion of CMS-EA remains genetically undiagnosed, which suggests the existence of novel causative CMS genes which remain uncharacterised. In light of the potential for recurrent life-threatening apnoeas in early life and the positive response to therapy, early diagnostic consideration of CMS-EA is critical, but without specific neurophysiology tests, it may go overlooked.

Introduction: Fluoxetine is a selective serotonin reuptake inhibitor and long-lived open channel blocker of the acetylcholine receptor often used in the treatment of slow-channel congenital myasthenic syndromes (CMS). Methods: We report a 42-year woman who had a history of episodic limb weakness that worsened after initiation of fluoxetine for treatment of depression. Genetic testing for CMS revealed a homozygous pathogenic mutation in the rapsyn (RAPSN) gene (p.Asn88Lys). Electrodiagnostic testing was performed prior to and 1 month after discontinuation of fluoxetine. Results: Two-Hz repetitive nerve stimulation of the fibular and spinal accessory nerves showed a baseline decrement of 36% and 14%, respectively. One month after discontinuing fluoxetine, the spinal accessory nerve decrement was no longer present, and the decrement in the fibular nerve was improved at 17%. Conclusion: This case demonstrates worsening of both clinical and electrophysiologic findings in a patient with CMS secondary to a RAPSN mutation treated with fluoxetine. This article is protected by copyright. All rights reserved.

Congenital myasthenic syndromes (CMS) are heterogeneous disorders caused by germline mutations in genes expressed at the neuromuscular junction. Mutations have been identified in 24 genes encoding acetylcholine receptor subunits (CHRNA1, CHRNB1, CHRND, CHRNE and CHRNG), skeletal muscle sodium channel (SCN4A), signaling molecules driving acetylcholine receptor subunits clustering (AGRN, LRP4, MUSK and DOK7), synaptic structural proteins (COLQ, LAMB2 and COL13A1), postsynaptic structural proteins (RAPSN and PLEC), presynaptic molecules (CHAT, SYT2), glycosylation enzymes (GFPT1, DPAGT1, ALG2, ALG14 and GMPPB), and other less characterized molecules (PREPL and SCL25A1). CMS are recessive disorders, except for slow channel CMS and synaptotagmin 2 (SYT2)-CMS. Onsets are largely less than 2 years, but adult-onset is not rare, especially in slow-channel CMS and limb-girdle type CMS caused by glycosylation defects and by DOK7 mutations. Clinical features include fatigable muscle weakness, amyotrophy and minor facial anomalies. Eye, facial and bulbar muscles are frequently affected, but sparing of these muscles is observed, especially in limb-girdle type CMS. Serum creatine kinase levels are frequently elevated in slow-channel CMS, glutamine-fructose-6-phosphate aminotransferase 1 (GFPT1)-CMS, and guanosine diphosphate mannose pyrophosphorylase B (GMPPB)-CMS. Electrophysiological findings supporting compromised neuromuscular signal transmission are a prerequisite for diagnosing CMS. Most CMS patients are likely to be underdiagnosed, and recognition of CMS in undiagnosed muscle weakness and/or amyotrophy is critical for diagnosing CMS.

INTRODUCTION Congenital myasthenic syndromes (CMS) usually present neonatally or in early childhood. When they present later, they may be mistaken for seronegative autoimmune myasthenia, and unnecessary immunosuppressive treatment may be administered. METHODS Patients who met criteria for seronegative generalized myasthenia without congenital or early childhood onset, but with an affected sibling were tested for CMS associated genes using exome and Sanger sequencing. RESULTS Four sibling pairs from nonconsanguineous families were identified. Three had mutations in the RAPSN gene, and 1 had a mutation in CHRNA1. One sibling of a pair with symptoms of fatigue but no convincing features of neuromuscular dysfunction tested negative on genetic studies. The definite CMS cases comprised 7 of 25 seronegative patients with definite generalized myasthenia in the clinic, and over half had been treated for autoimmune myasthenia. CONCLUSIONS CMS is probably underdiagnosed in seronegative myasthenic disorders and should be considered in the differential diagnosis. Muscle Nerve 54: 721-727, 2016.

Mutations in RAPSN are an important cause of congenital myasthenic syndrome (CMS), leading to endplate acetylcholine receptor deficiency. We present three RAPSN early-onset CMS patients (from a Brazilian cohort of 61 CMS patients). Patient 1 and patient 2 harbor the mutation p.N88K in homozygosity, while patient 3 harbors p.N88K in compound heterozygosity with another pathogenic variant (p.V165M; c.493G ≥ A). At onset, patient 3 presented with more severe symptoms compared to the other two, showing generalized weakness and repeated episodes of respiratory failure in the first years of life. During adolescence, she became gradually less symptomatic and does not require medication anymore, presenting better long-term outcomes than patients 1 and 2. This case series illustrates the variability of RAPSN early-onset CMS, with patient 3, despite severe onset, revealing an almost complete reversal of myasthenic symptoms, not limited to apneic episodes. Moreover, it suggests that RAPSN CMS may be underdiagnosed in non-European countries.

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.

Congenital myasthenic syndromes (CMS) are a heterogeneous group of genetic disorders, all of which impair neuromuscular transmission. Epidemiological data and frequencies of gene mutations are scarce in the literature. Here we describe the molecular genetic and clinical findings of sixty-four genetically confirmed CMS patients from Spain. Thirty-six mutations in the CHRNE, RAPSN, COLQ, GFPT1, DOK7, CHRNG, GMPPB, CHAT, CHRNA1, and CHRNB1 genes were identified in our patients, with five of them not reported so far.. These data provide an overview on the relative frequencies of the different CMS subtypes in a large Spanish population. CHRNE mutations are the most common cause of CMS in Spain, accounting for 27% of the total. The second most common are RAPSN mutations. We found a higher rate of GFPT1 mutations in comparison with other populations: Remarkably, several founder mutations made a large contribution to CMS in Spain: RAPSN c.264C > A (p.Asn88Lys), CHRNE c.130insG (Glu44Glyfs*3), CHRNE c.1353insG (p.Asn542Gluf*4), DOK7 c.1124_1127dup (p.Ala378Serfs*30), and particularly frequent in Spain in comparison with other populations, COLQ c.1289A > C (p.Tyr430Ser). Furthermore, we describe phenotypes and distinguishing clinical signs associated with the various CMS genes which might help to identify specific CMS subtypes to guide diagnosis and management. (C) 2017 Elsevier B.V. All rights reserved.

Congenital myasthenic syndromes are heterogeneous genetic disorders characterized by compromised neuromuscular transmission for which defects in multiple mechanisms are responsible. Clinical manifestations vary by syndrome subtype, but generally affected individuals present with fatigable muscle weakness, which can affect ocular, bulbar, limb and respiratory muscles. The onset of disease is usually, but not invariably, in infancy or childhood. The severity of disease varies greatly from death in early childhood to mild muscular weakness throughout life (Engel, 2012). To date, mutations in 16 different genes have been implicated in the development of congenital myasthenic syndromes. These genes encode the presynaptic protein (CHAT), synaptic components (COLQ, AGRN, LAMB2), and postsynaptic proteins (CHRNA1, CHRNB1, CHRNG, CHRND, CHRNE, RAPSN, MUSK, DOK7, GFPT1, SCN4A, PLEC and DPAGT1) (Chaouch et al., 2012; Engel, 2012; Lorenzoni et al., 2012). The functional properties of these genes have highlighted a number of mechanisms that involve the synthesis or packaging of acetylcholine load into synaptic vesicles, the Ca2p-dependent evoked release of acetylcholine from nerve terminals, and the efficiency of released load in generating a postsynaptic depolarization. The functional efficiency of neurotransmission depends on the endplate architecture, density and state of acetylcholinesterase (AChE) in the synaptic space, as well as the density, affinity for acetylcholine and kinetic properties of the acetylcholine receptor. However, for many disease-associated proteins, their role at the neuromuscular junction and contribution to the development of these syndromes remains unknown (Chaouch et al., 2012; Engel, 2012; Lorenzoni et al., 2012). Diagnostic precision is important in assessing the prognosis and managing these disorders and genetic testing is now available for a number of genes implicated in these disorders. The current technology is slow and expensive but research groups and some diagnostic laboratories are moving towards more efficient, cost-effective screening methods, such as next generation sequencing in targeted gene panels and subsequent exome sequencing in the negative families. There are still a number of genes to be identified as those identified to date account for

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. Three different aspects have been investigated by members of the national French CMS Network: the difficulties in making a proper diagnosis; the course and long-term prognosis; and the response to therapy, especially for CMS that do not respond to cholinesterase inhibitors. CMS diagnosis is late in most cases because of confusion with other entities such as: congenital myopathies, due to the frequent presentation in patients of myopathies such as permanent muscle weakness, atrophy and scoliosis, and the abnormalities of internal structure, diameter and distribution of fibers (type I predominance, type II atrophy) seen on biopsy; seronegative autoimmune myasthenia gravis, when CMS is of late onset; and metabolic myopathy, with the presence of lipidosis in muscle. The long-term prognosis of CMS was studied in a series of 79 patients recruited with the following gene mutations: CHRNA; CHRNE; DOK7; COLQ; RAPSN; AGRN; and MUSK. Disease-course patterns (progressive worsening, exacerbation, stability, improvement) could be variable throughout life in a given patient. DOK7 patients had the most severe disease course with progressive worsening: of the eight wheelchair-bound and ventilated patients, six had mutations of this gene. Pregnancy was a frequent cause of exacerbation. Anticholinesterase agents are the first-line therapy for CMS patients, except for cases of slow-channel CMS, COLQ and DOK7. In our experience, 3,4-DAP was a useful complement for several patients harboring CMS with AChR loss or RAPSN gene mutations. Ephedrine was given to 18 patients (eight DOK7, five COLQ, four AGRN and one RAPSN). Tolerability was good. Therapeutic responses were encouraging even in the most severely affected patients, particularly with DOK7 and COLQ. Salbutamol was a good alternative in one patient who was allergic to ephedrine.

Mutations in DOK7 have recently been shown to underlie a recessive congenital myasthenic syndrome (CMS) associated with small simplified neuromuscular junctions ('synaptopathy') but normal acetylcholine receptor and acetylcholinesterase function. We identified DOK7 mutations in 27 patients from 24 kinships. Mutation 1124_1127dupTGCC was common, present in 20 out of 24 kinships. All patients were found to have at least one allele with a frameshift mutation in DOK7 exon 7, suggesting that loss of function(s) associated with the C-terminal region of Dok-7 underlies this disorder. In 15 patients, we were able to study the clinical features in detail. Clinical onset was usually characterized by difficulty in walking developing after normal motor milestones. Proximal muscles were usually more affected than distal, leading to a 'limb-girdle' pattern of weakness; although ptosis was often present from an early age, eye movements were rarely involved. Patients did not show long-term benefit from anticholinesterase medication and sometimes worsened, and where tried responded to ephedrine. The phenotype can be distinguished from 'limb-girdle' myasthenia associated with tubular aggregates, where DOK7 mutations were not detected and patients respond to anticholinesterase treatments. CMS due to DOK7 mutations are common within our UK cohort and is likely to be under-diagnosed; recognition of the phenotype will help clinical diagnosis, targeted genetic screening and appropriate management.

The objective of the study was to evaluate the epidemiology of patients with congenital myasthenic syndrome (CMS) in Israel. Targeted mutation analysis was performed based on the clinical symptoms and electrophysiological findings for known CMS. Additional specific tests were performed in patients of Iranian and/or Iraqi Jewish origin. All medical records were reviewed and clinical data, genetic mutations and outcomes were recorded. Forty-five patients with genetic mutations in known CMS genes from 35 families were identified. Mutations in RAPSN were identified in 13 kinships in Israel. The most common mutation was c.-38A>G detected in 8 patients of Iranian and/or Iraqi Jewish origin. Four different recessive mutations in COLQ were identified in 11 kinships, 10 of which were of Muslim-Arab descent. Mutations in CHRNE were identified in 7 kinships. Less commonly detected mutations were in CHRND, CHAT, GFPT1 and DOK7. In conclusion, mutations in RAPSN and COLQ are the most common causes of CMS in our cohort. Specific mutations in COLQ, RAPSN, and CHRNE occur in specific ethnic populations and should be taken into account when the diagnosis of a CMS is suspected.

Background: Congenital myasthenic syndromes (CMS) form a heterogeneous group of disorders caused by defects of the neuromuscular transmission. The clinical picture of CMS varies widely between mild and life-limiting forms. Until now, data of long-term follow-up are rare. Patients and Methods: A retrospective analysis (clinical, genetic, and treatment data) of 31 patients with CMS was performed. Additionally, a standardized, not yet validated test to examine 21 patients has been used. Results: We identified 31 patients with 9 different genetic defects of CMS. We formed different phenotypic groups in consideration of clinical symptoms; 18/31 patients presented ocular problems; 21/31 patients had bulbar symptoms at the beginning. Transient respiratory problems were found in 19/31 patients at onset; 6 had persistent respiratory problems; 18/31 patients showed first symptoms of neonatal (CHRNE [3/7], RAPSN [7/8], CHAT [3/5], COLQ [2/3], CHRND [1/1], CHRNB1 [1/1] mutations and “fast-channel” syndrome [1/1]); 12/18 patients later (between 2 months and 6 years). Despite an early and appropriate treatment, following long-term problems were identified: 12/18 patients suffered from difficulties in eating, 11/31 used a wheelchair, scoliosis developed in 9/31 patients, and 5/31 patients needed respiratory support. Conclusion: In patients with CMS, there is no direct genotype-phenotype correlation; nonetheless, in this cohort we identified specific phenotypic groups. Not all patients with CMS benefit in the same way from treatment. Some patients developed irrespective of genetic background and despite an appropriate and early therapy long-term problems. The applied test highlighted problems in ocular, bulbar, cervical, and limb muscle additional to neurological examination.

Rapsyn (RAPSN) mutations are a common cause of postsynaptic congenital myasthenic syndromes. We present a comprehensive description of the clinical and molecular findings of ten patients with CMS due to mutations in RAPSN, mostly with a long-term follow-up. Two patients were homozygous and eight were heterozygous for the common p.Asn88Lys mutation. In three of the heterozygous patients we have identified three novel mutations (c.869T > C; p.Leu290Pro, c.1185delG; p.Thr396Profs*12, and c.358delC; p.G1n120Serfs*8). In our cohort, the RAPSN mutations lead to a relatively homogeneous phenotype, characterized by fluctuating ptosis, occasional bulbar symptoms, neck muscle weakness, and mild proximal muscle weakness with exacerbations precipitated by minor infections. Interestingly, episodic exacerbations continue to occur during adulthood. These were characterized by proximal limb girdle weakness and ptosis, and not so much by respiratory insufficiency after age 6. All patients presented during neonatal period and responded to cholinergic agonists. In most of the affected patients, additional use of 3,4-diaminopyridine resulted in significant clinical benefit. The disease course is stable except for intermittent worsening. (C) 2015 Elsevier B.V. All rights reserved.

Congenital myasthenic syndromes (CMS) are rare genetic disorders characterized by impaired neuromuscular transmission. They are caused by mutations in synaptic, presynaptic and post synaptic proteins. Rapsyn is a postsynaptic peripheral membrane protein that anchors the nicotinic acetylcholine receptor to the motor endplate. CMS patients of Iraqi and Persian Jewish origin, carry a common founder mutation in the E box of the RAPSN promoter region (− 38A-G) that causes impaired transcriptional activities of the promoter region. We describe a Persian Jewish family with two siblings affected with typical CMS, harboring the common heterozygous (− 38A-G) E-box mutation associated with a previously unreported heterozygous p.224 insT causing an insertion of Threonine in the TPR6 domain. To the best of our knowledge, this is the first mutation in the TPR6 domain and might give supportive evidence to the role of this domain in rapsyn self association and consequently co-clustering with AchR in the post synaptic membrane.

Introduction: Myasthenia gravis (MG) is an autoimmune disease. Patients without detectable antibodies against the nicotinic acetylcholine receptor or the muscle-specific tyrosine kinase are referred to as seronegative MG (SNMG). Because late-onset congenital myasthenic syndromes (CMSs) due to RAPSN or DOK7 mutations may be mistaken for SNMG, we investigated their frequency in a nationwide SNMG cohort. Methods: We performed sequencing of RAPSN and DOK7 in all Norwegian SNMG patients (n = 74) and 37 healthy controls, examining for the N88K and c.1124_1127dupTGCC mutations, respectively. Results: We found 1 patient homozygous for N88K and 2 carriers of the N88K mutation. Sequencing of DOK7 revealed no mutations. Conclusions: This study confirms that rapsn CMS can be mistaken for SNMG. In addition, the frequency of rapsn CMS in our nationwide SNMG cohort was found to be low. SNMG patients with an atypical clinical presentation and pediatric cases should be tested for the N88K mutation before initiation of immunosuppressive drug treatment or thymectomy. Muscle Nerve, 2011

A workshop on the respiratory management of congenital myasthenic syndromes (CMS) in childhood was held on 8 December 2009 at the UCL Institute of Neurology, UK. The workshop was sponsored by the Muscular Dystrophy Campaign and attended by 20 participants from 9 centres (8 in UK and 1 in Europe). The aim of the workshop was to review respiratory management of childhood CMS in major UK specialist centres and develop recommendations for standards of care to help anticipate and treat respiratory complications. The congenital myasthenic syndromes are a heterogeneous group of genetic disorders which result in impaired neuromuscular transmission and fatiguable weakness. Advances in molecular genetic diagnosis have lead to the identification of distinct genotypes [1] which are associated with particular patterns of respiratory decompensation. Many CMS children present with respiratory difficulties at birth and most are at risk of ventilatory failure with intercurrent respiratory illnesses. However, recurrent, life-threatening episodic apnoea in infancy and childhood (often precipitated by infection or even stress) is a particular feature of the presynaptic CMS caused by mutations in CHAT and postsynaptic CMS due to mutations in RAPSN. Other phenotypes, such as the synaptic CMS due to mutations in COLQ and the slow channel syndromes caused by mutations in the acetylcholine receptor subunits, cause an end plate myopathy which results in progressive respiratory muscle weakness and respiratory failure requiring long term ventilatory support. The fast channel syndromes are associated with recurrent and particularly severe respiratory crises in infancy and childhood. Other genotypes have diverse respiratory presentations, for example the post-synaptic CMS caused by mutations in DOK7 may be associated, in severe cases, with respiratory failure requiring ventilation at birth, or congenital stridor due to vocal cord palsy often necessitating tracheostomy, without any evidence of the limb gir

Objective To perform genetic testing of patients with congenital myasthenic syndromes (CMS) from the Southern Brazilian state of Parana. Patients and methods Twenty-five CMS patients from 18 independent families were included in the study. Known CMS genes were sequenced and restriction digest for the mutation RAPSN p.N88K was performed in all patients. Results We identified recessive mutations of CHRNE in ten families, mutations in DOK7 in three families and mutations in COLQ, CHRNA1 and CHRNB1 in one family each. The mutation CHRNE c.70insG was found in six families. We have repeatedly identified this mutation in patients from Spain and Portugal and haplotype studies indicate that CHRNE c.70insG derives from a common ancestor. Conclusions Recessive mutations in CHRNE are the major cause of CMS in Southern Brazil with a common mutation introduced by Hispanic settlers. The second most common cause is mutations in DOK7. The minimum prevalence of CMS in Parana is 0.18/100000.

The congenital myasthenic syndromes (CMS) are a diverse group of genetic disorders caused by abnormal signal transmission at the motor endplate, a special synaptic contact between motor axons and each skeletal muscle fibre. Most CMS stem from molecular defects in the muscle nicotinic acetylcholine receptor, but they can also be caused by mutations in presynaptic proteins, mutations in proteins associated with the synaptic basal lamina, defects in endplate development and maintenance, or defects in protein glycosylation. The specific diagnosis of some CMS can sometimes be reached by phenotypic clues pointing to the mutated gene. In the absence of such clues, exome sequencing is a useful technique for finding the disease gene. Greater understanding of the mechanisms of CMS have been obtained from structural and electrophysiological studies of the endplate, and from biochemical studies. Present therapies for the CMS include cholinergic agonists, long-lived open-channel blockers of the acetylcholine receptor ion channel, and adrenergic agonists. Although most CMS are treatable, caution should be exercised as some drugs that are beneficial in one syndrome can be detrimental in another.

The ENMC hosted a group of 18 experts on Congenital Myasthenic Syndromes (CMS). CMS are inherited disorders in which the safety margin of the neuromuscular transmission is compromised by one or more specific mechanism(s). CMS are caused by various genetic defects. The objectives of the workshop included progress in deciphering the molecular basis of CMS (sessions 1–4) and clinical conclusions for epidemiology, diagnosis and therapy (sessions 5–7). To date, genes known to cause CMS if mutated are the presynaptic choline acetyltransferase gene CHAT, the gene COLQ encoding the synaptic protein ColQ, the genes encoding the different subunits of the postsynaptic acetylcholine receptor (CHRNA1, CHRNB1, CHRND, CHRNE), the genes for the postsynaptic proteins rapsyn (RAPSN), muscle-specific receptor tyrosine kinase (MUSK) and the postsynaptic sodium channel (SCN4A). Since the last ENMC workshop on CMS in October 1999, four novel CMS genes have been identified, namely CHAT, RAPSN, SCN4A and MUSK. As a consequence, several new patients presenting with varying phenotypes of CMS have been described worldwide. In particular, mutations in RAPSN and CHAT turned out to be of high clinical relevance, on one hand because of their apparent worldwide frequency, on the other hand because of their specific clinical phenotype with the occurrence of sudden apneic episodes. Furthermore, considerable progress has been made using a variety of

The objective is mutation analysis of the RAPSN gene in a patient with sporadic congenital myasthenic syndrome (CMS). Mutations in various genes encoding proteins expressed at the neuromuscular junction may cause CMS. Most mutations affect the epsilon subunit gene of the acetylcholine receptor (AChR) leading to endplate AChR deficiency. Recently, mutations in the RAPSN gene have been identified in several CMS patients with AChR deficiency. In most patients, RAPSN N88K was identified, either homozygously or heteroallelic to a second missense mutation. A sporadic CMS patient from Germany was analyzed for RAPSN mutations by RFLP, long-range PCR and sequence analysis. Clinically, the patient presents with an early onset CMS, associated with arthrogryposis multiplex congenita, recurrent episodes of respiratory insufficiency provoked by infections, and a moderate general weakness, responsive to anticholinesterase treatment. The mutation RAPSN N88K was found heterozygously to a large deletion of about 4.5 kb disrupting the RAPSN gene. Interestingly, an Alu-mediated unequal homologous recombination may have caused the deletion. We hypothesize that numerous interspersed Alu elements may predispose the RAPSN locus for genetic rearrangements.

Background: Mutations in various genes of the neuromuscular junction may cause congenital myasthenic syndromes (CMS). Most mutations identified to date affect the e-subunit gene of the acetylcholine receptor (AChR), leading to end-plate AChR deficiency. Recently, three different mutations in the RAPSN gene have been identified in four CMS patients with AChR deficiency. Objective: To perform mutation analysis of the RAPSN gene in patients with sporadic or autosomal recessive CMS. Methods: One hundred twenty CMS patients from 110 unrelated families were analyzed for the RAPSN mutation N88K by restriction fragment length polymorphism and sequence analysis. Results: In 12 CMS patients from 10 independent families, RAPSN N88K was identified either homozygous or heteroallelic to another missense mutation. Symptoms usually started perinatally or in the first years of life. However, one patient did not show any myasthenic symptoms before the third decade. Clinical symptoms typically included bilateral ptosis, weakness of facial, bulbar, and limb muscles, and a favorable response to anticholinesterase treatment. Crisis-like exacerbations with respiratory insufficiency provoked by stress, fever, or infections in early childhood were frequent. All RAPSN N88K families originate from Central or Western European countries. Genotype analysis indicated that they derive from a common ancestor (founder). Conclusions: The RAPSN mutation N88K is a frequent cause of rapsyn-related CMS in European patients. In general, patients ( RAPSN N88K) were characterized by mild to moderate myasthenic symptoms with favorable response to anticholinesterase treatment. However, severity and onset of symptoms may vary to a great extent.

Introduction Congenital myasthenic syndromes (CMS) are rare genetic syndromes resulting from presynaptic, synaptic or postsynaptic defects of the neuromuscular junction. In 2006 mutations in DOK7, a gene encoding a muscle-intrinsic activator of MUSK, were proven a postsynaptic cause of CMS. Until today 65 DOK7 mutations have been described. Case report A 16 year old girl, daughter to healthy, non-consanguineous parents has been followed up at our clinic since birth. History includes decreased fetal movements, hypotonia at birth and slightly delayed motor milestones with sitting at 8 months and walking at 15 months. Neurological examination discloses facial and proximal muscle weakness, bulbar and respiratory weakness, ptosis, joint laxity, massive kyphoscoliosis, feeding difficulty and failure to thrive. The patient is currently ambulant, able to climb stairs and walk approximately 50 meters, before getting tired. Under nocturnal noninvasive ventilation (BiPAP) she maintains oxygen saturation levels >98%. Creatin kinase is 200 U/l (normal: 150 U/l). Electrophysiologic studies revealed normal NCV's and a mildly myopathic EMG. Repetitive nerve stimulation demonstrated mild decrement at infancy, but now is normal. Muscle biopsy showed predominance of type 1 fibers and decreased oxidative activity. Since the clinical features were consistent with CMS, molecular analysis of CHAT, RAPSN, CHRNE and DOK7 was undertaken, which in turn revealed a compound heterozygous mutation in the DOK7 gene. Conclusion To the best of our knowledge this is the first Greek patient with confirmed DOK7 myasthenic syndrome. One of the mutations, the c.472C>T (p.Arg158Trp) has not been previously described. Accurate diagnosis of DOK7 myasthenic syndromes bears important clinical implications, since ephedrine or salbutamol rather than acetylcholinesterase inhibitors are the treatment of choice.

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. © 2013 The Author (2013). Published by Oxford University Press on behalf of the Guarantors of Brain.

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 syndromes are rare genetic disorders compromising neuromuscular transmission. The defects are mainly mutations in the muscle acetylcholine receptor, or associated proteins rapsyn and Dok-7. We analyzed three unrelated Italian patients with typical clinical features of congenital myasthenic syndrome, who all benefitted from cholinesterase inhibitors. We found five mutations: a previously unreported homozygous alphaG378D mutation in the CHRNA1 gene, a previously unreported heterozygous epsilonY8X mutation associated with a known heterozygous epsilonM292del deletion in the CHRNE gene, and the common heterozygous N88K mutation associated with a previously unreported heterozygous IVS1 + 2TG splice site mutation in the RAPSN gene. All three patients had two mutant alleles; parents or offspring with a single mutated allele were asymptomatic, thus all mutations exerted their effects recessively. The previously unreported mutations are likely to reduce the number of AChRs at the motor endplate, although the alphaG378D mutation might produce a mild fast channel syndrome. The alphaG378D mutation was recessive, but recessive CHRNA1 mutations have rarely been reported previously, so studies on the effect of this mutation at the cellular level would be of interest.

Pathogenic mutations in rapsyn result in endplate acetylcholine receptor (AChR) deficiency and are a common cause of postsynaptic congenital myasthenic syndromes.Clinical, electrophysiologic, pathologic, and molecular studies were done in 39 patients.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.-38AG), who had a mild course.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.

International audience; While TPM2 mutations identified so far in muscular diseases were all associated with a dominant inheritance pattern, we report the identification of a homozygous null allele mutation in the TPM2 gene in a patient who presented with a recessive form of nemaline myopathy associated with a non-lethal multiple pterygium syndrome (Escobar-MPS MIM# 265000). The TPM2 mutation led to a complete absence of the skeletal muscle isoform of beta-tropomyosin not compensated by expression of other beta-tropomyosin isoforms. Escobar syndrome has been recently described as a prenatal form of myasthenia associated with recessive mutations in genes of the neuromuscular junction (CHRNG, CHRNA1, CHRND, RAPSN). This observation expands the cause of Escobar variant-MPS to a component of the contractile apparatus. This first report of the clinical expression of the complete absence of TPM2 in human indicated that TPM2 expression at the early period of prenatal life plays a major role for normal fetal movements.

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Between 1995 and 2005, defects in seven end-plate (EP)–associated proteins encoded by 10 different genes have been identified as molecular targets of the CMS.1 In 2006, Okada and coworkers identified Dok-7 as a muscle-intrinsic activator of MuSK required for synaptogenesis.2 Dok-7 harbors N-terminal pleckstrin homology (PH) and phosphotyrosine-binding (PTB) domains, and is strongly expressed at the postsynaptic region of skeletal muscle and in heart. Subsequently, mutations in DOK7 were shown to cause a CMS that preferentially involved proximal limb muscles.3–5 The majority of patients were heterozygous or homozygous for a 1124_1127dupTGCC mutation. Only this mutation was functionally characterized, and in two patients only a single mutation was detected.3 Studies in six patients eventually shown to carry DOK7 mutations demonstrated small EPs and simplified junctional folds, but the counts of the acetylcholine receptor (AChR) per EP were deemed appropriate for size of the EPs.6 The amplitude of the synaptic response to acetylcholine (ACh), reflected by the amplitude of the miniature EP potential (MEPPA) and EP potential (EPPA), and the number of quanta released by nerve impulse (m) were reduced, whereas the amplitude of the miniature EP current (MEPCA) was reported as normal. The impaired safety margin of neuromuscular transmission was attributed to the reduced EPPA.6 This article describes clinical features of 16 unrelated CMS patients with Dok-7 myasthenia. In 14 of these patients, we analyze parameters of neuromuscular transmission in vitro and examine 613 EP regions of 409 EPs by quantitative electron microscopy. We find that the structural changes and the electrophysiological alterations are more variable than previously reported. We identify mutations in DNA or complementary DNA (cDNA) in each patient, and evaluate most of these mutations by expression studies.

The Congenital Myasthenic Syndromes (CMS), a group of heterogeneous genetic disorders of neuromuscular transmission, are often misdiagnosed as congenital muscular dystrophy (CMD) or myopathies and present particular management problems. We present our experience of 46 children with CMS, referred to us between 1992–2007 with provisional diagnoses of congenital myopathy (22/46), CMS or limb-girdle myasthenia (9/46), central hypotonia or neurometabolic disease (5/46), myasthenia gravis (4/46), limb–girdle or congenital muscular dystrophy (4/46) and SMA (2/46). Diagnosis was often considerably delayed (up to 18y4 m), despite the early symptoms in most cases. Diagnostic clues in the neonates were feeding difficulties (29/46), hypotonia with or without limb weakness (21/46), ptosis (19/46), respiratory insufficiency (12/46), contractures (4/46) and stridor (6/46). Twenty–five children had delayed motor milestones. Fatigability developed in 43 and a variable degree of ptosis was eventually present in 40. Over the period of the study, the mainstay of EMG diagnosis evolved from repetitive nerve stimulation to stimulation single fibre EMG. The patients were studied by several different operators. 66 EMGs were performed in 40 children, 29 showed a neuromuscular junction abnormality, 7 were myopathic, 2 had possible neurogenic changes and 28 were normal or inconclusive. A repetitive CMAP was detected in only one of seven children with a COLQ mutation and neither of the two children with Slow Channel Syndrome mutations. Mutations have been identified so far in 32/46 children: 10 RAPSN, 7 COLQ, 6 CHRNE, 7 DOK7, 1 CHRNA1 and 1 CHAT. 24 of 25 muscle biopsies showed myopathic changes with fibre size variation; 14 had type-1 fibre predominance. Three cases showed small type-1 fibres resembling fibre type disproportion, and four showed core-like lesions. No specific myopathic features were associated with any of the genes. Twenty children responded to Pyridostigmine treatment alone, 11 to Pyridostigmine with either 3, 4 DAP or Ephedrine and five to Ephedrine alone. Twenty one children required acute or chronic respiratory support, with tracheostomy in 4 and nocturnal or emergency non-invasive ventilation in 9. Eight children had gastrostomy. Another 11 were underweight for height indicative of failure to thrive and required dietetic input. A high index of clinical suspicion, repeat EMG by an experienced electromyographer and, if necessary, a therapeutic trial of Pyridostigmine facilitates the diagnosis of CMS with subsequent molecular genetic confirmation. This guides rational therapy and multidisciplinary management, which may be crucial for survival, particularly in pedigrees where previous deaths have occurred in infancy.

Dok ('downstream-of-kinase') family of cytoplasmic proteins play a role in signalling downstream of receptor and non-receptor phosphotyrosine kinases. Recently, a skeletal muscle receptor tyrosine kinase (MuSK)-interacting cytoplasmic protein termed Dok-7 has been identified. Subsequently, we and others identified mutations in DOK7 as a cause of congenital myasthenic syndromes (CMS), providing evidence for a crucial role of Dok-7 in maintaining synaptic structure. Here we present clinical and molecular genetic data of 14 patients from 12 independent kinships with 13 different mutations in the DOK7 gene. The clinical picture of CMS with DOK7 mutations is highly variable. The age of onset may vary between birth and the third decade. However, most of the patients display a characteristic 'limb-girdle' pattern of weakness with a waddling gait and ptosis, but without ophthalmoparesis. Respiratory problems were frequent. Patients did not benefit from long-term therapy with esterase inhibitors; some of the patients even worsened. DOK7 mutations have emerged as one of the major genetic defects in CMS. The clinical picture differs significantly from CMS caused by mutations in other genes, such as the acetylcholine receptor (AChR) subunit genes. None of the patients with DOK7 mutations had tubular aggregates in the muscle biopsy, implying that 'limb-girdle myasthenia (LGM) with tubular aggregates' previously described in literature may be a pathogenic entity distinct from CMS caused by DOK7 mutations.

Objective Congenital myasthenic syndromes (CMS) are a rare heterogeneous group of inherited neuromuscular disorders associated with distinctive clinical, electrophysiological and genetic abnormalities. Mutations in various genes of the neuromuscular junction may cause presynaptic, synaptic or postsynaptic defects. In this study, the genetic and clinical characteristics of Israeli patients with CMS were evaluated. Methods Twentysix patients with CMS from 16 independent families in whom genetic mutation was detected were diagnosed in two Israeli medical centers. Genetic evaluation was initially performed depending on patients' clinical symptoms and known CMS genes were sequenced. Patients of Jewish Iranian or Iraqi ethnic origin were screened directly for a known 38 A-G mutation in RAPSYN. In two patients, whole exome sequencing was analyzed. All patients' medical records were reviewed and their clinical data, electrophysiological studies and outcome were recorded. Results The diagnosis of CMS was confirmed in 26 patients of 16 families. Among these patients, 8 patients of 4 families had recessive mutations in COLQ, eight patients of 5 families, mutations in CHRNE, and eight patients of 5 families, mutations in RAPSN. CHRND was diagnosed in 2 patients from one family. The mean age of onset of clinical symptoms was 6 months. Ptosis, ophthalmoplegia and feeding problems were the most common symptoms. Conclusions RAPSYN and CHRNE are the most common causes of CMS in our cohort. The second most common cause is COLQ. Specific mutations in RAPSYN, CHRNE and COLQ are detected in unique ethnic populations in Israel.

Congenital myasthenic syndromes are inherited disorders of neuromuscular transmission characterized by fatigable muscle weakness. Autosomal recessive acetylcholine receptor (AChR) deficiency syndromes, in which levels of this receptor at the neuromuscular junction are severely reduced, may be caused by mutations within genes encoding the AChR or the AChR-clustering protein, rapsyn. Most patients have mutations within the rapsyn coding region and are either homozygous for N88K or heteroallelic for N88K and a second mutation. In some cases the second allele carries a null mutation but in many the mutations are missense, and are located in different functional domains. Little is known about the functional effects of these mutations, but we hypothesize that they would have an effect on AChR clustering by a variety of mechanisms that might correlate with disease severity. Here we expressed RAPSN mutations A25V, N88K, R91L, L361R and K373del in TE671 cells and in rapsyn-/- myotubes to determine their pathogenic mechanisms. The A25Vmutation impaired colocalization of rapsyn with AChR and prevented agrin-induced AChR clusters in rapsyn-/- myotubes. In TE671 cells, R91L reduced the ability of rapsyn to self-associate, and K373del-rapsyn was significantly less stable than wild-type. The effects of mutations L361R and N88K were more subtle: in TE671 cells, in comparison with wild-type rapsyn, L361R-rapsyn showed reduced expression/stability, and both N88K-rapsyn and L361R-rapsyn showed significantly reduced co-localization with AChR. N88K-rapsyn and L361R-rapsyn could effectively mediate agrin-induced AChR clusters, but these were reduced in number and were less stable than with wild-type rapsyn. The disease severity of patients harbouring the compound allelic mutations was greater than that of patients with homozygous rapsyn mutation N88K, suggesting that the second mutant allele may largely determine severity. © The Author (2006). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved.

Objective: Congenital myasthenic syndromes (CMS) with underlying RAPSN mutations turned out to be of high clinical relevance due to their worldwide frequency. To date, all reported patients with CMS with sequence variations in the translated region of RAPSN carry the mutation N88K on at least one allele. The authors report two patients lacking the common N88K allele but harboring differing novel mutations of the RAPSN gene on both alleles: one patient is homozygous for a missense mutation (R164C); the second patient is compound heterozygous for a splice (IVS1-15C>A) and another missense mutation (L283P). Methods: The authors analyzed the RAPSN gene for sequence variations and carried out in vitro studies in order to delineate the potential pathogenicity of the three novel RAPSN mutations. Results: For the putative splice mutation (IVS1-15C>A), the authors constructed wild-type and mutated RAPSN minigenes for transfection and subsequent RNA analysis. The mutation generates a novel acceptor splice site leading to retention of 13 nucleotides of intron 1 in the mature mRNA and subsequently to a frameshift transcript. Cotransfection of wild-type AChR subunits with RAPSN- constructs carrying R164C and L283P indicate that both mutations diminish coclustering of AChR with rapsyn. Conclusions: Screening for the common mutation RAPSN N88K facilitates targeted genetic analysis in congenital myasthenic syndromes. However, absence of a N88K allele does not exclude underlying RAPSN mutations as cause of the congenital myasthenic syndromes. Sequencing of the entire gene may be considered in patients with joint contractures and respiratory problems even in the absence of the mutation N88K.

The Congenital Myasthenic Syndromes (CMS) are a group of rare genetic disorders affecting neuromuscular transmission and often presents within first year of life. A high index of suspicion is usually required as clinical manifestations can be variable and non specific. RAPSN mutations are likely to be one of the common causes for CMS in patients of Indo-European ethnic origin. Clinical phenotype includes arthrogryposis, respiratory crises, transient torticollis at birth, facial deformities and weakness. Chronic stridor as an early manifestation of CMS has not been described before.

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. Three different aspects have been investigated by the members of the French CMS network: the difficulties to make a proper diagnosis, the course and long term prognosis, and the response to therapy, especially for the CMS that do not respond to cholinesterase inhibitors. CMS diagnosis was late in most cases due to confusion with other entities: congenital myopathies, due to the frequent myopathic presentation of patients including permanent weakness, atrophy, scoliosis, and biopsy often showing abnormalities of internal structure, diameter and distribution of fibers (type I predominance, type II atrophy); seronegative autoimmune myasthenia gravis when CMS was late onset; metabolic myopathy, with presence of lipidosis in muscle. We studied long term prognosis of CMS in a series of 81 patients recruited with the following mutated genes: CHRNA, CHRNE, DOK7, COLQ, RAPSN, AGRN, MUSK. Course pattern (progressive worsening, exacerbation, stability, improvement) could be variable along the life in one single patient. DOK7 patients had the most severe course with progressive worsening: among the 8 wheel-chair bound and ventilated, 6 were mutated for this gene. Pregnancy was a frequent cause of exacerbation. Anticholinesterase agents are the first line therapy for CMS patients, except for Slow-Channel, COLQ and DOK7. In our experience, 3,4-DAP was a useful complement for several patients harboring CMS with AChR loss CMS or RAPSN gene mutations. Ephedrine was given to 18 patients (8 DOK7, 5 COLQ, 4 AGRN, 1 RAPSN). Tolerance was good. Therapeutic response was encouraging even in the most severely affected patients, particularly with DOK7 and COLQ. Salbutamol was a good alternative in one patient, allergic to Ephedrine.

We report the cases of 3 children with postsynaptic congenital myasthenic syndrome with acetylcholine receptor deficiency due to rapsyn deficiency. Symptoms began at the neonatal period with hypotonia, arthrogryposis, bulbar symptoms, and respiratory distress. Two of the 3 children needed tracheostomy and gastrostomy. Electromyograms showed a decremental response to repetitive stimulation. Muscle biopsies were normal or showed type I fiber preponderance. Genetic studies identified mutations in the rapsyn gene (RAPSN). The 3 patients were heterozygous for N88 K and a second mutation (either Y86X, 1083_1084 dupCT or IVS4-2 A > G). The patients responded favorably to anticholinesterase treatment, with a clear improvement of clinical symptoms, especially the bulbar symptoms of apneas and swallowing disturbances. This paper underlines the importance of anticholinesterase medication in patients with congenital myasthenic syndrome due to rapsyn deficiency.

We contrast the phenotypes associated with hereditary acetylcholine receptor deficiency arising from mutations in either the acetylcholine receptor epsilon subunit or the endplate acetylcholine receptor clustering protein rapsyn. Mutational screening was performed by amplification of promoter and coding regions by PCR and direct DNA sequencing. We identified mutations in 37 acetylcholine receptor deficiency patients; 18 had acetylcholine receptor-epsilon mutations, 19 had rapsyn mutations. Mutated acetylcholine receptor-epsilon associated with bulbar symptoms, ptosis and ophthalmoplegia at birth, and generalized weakness. Mutated rapsyn caused either an early onset (rapsyn-EO) or late onset (rapsyn-LO) phenotype. Rapsyn-EO associated with arthrogryposis and life-threatening exacerbations during early childhood. Rapsyn-LO presented with limb weakness in adolescence or adulthood resembling seronegative myasthenia gravis. Awareness of distinct phenotypic features of acetylcholine receptor deficiency resulting from acetylcholine receptor-epsilon or rapsyn mutations should facilitate targeted genetic diagnosis, avoid inappropriate immunological therapy and, in some infants, prompt the rapid introduction of treatment that could be life saving.

Rapsyn mutations in 16 unrelated patients with a congenital/hereditary myasthenic syndrome were identified, and a mutation (N88K) common to each of them was found. Two distinct phenotypes were noted: early and late onset. The former is frequently associated with arthrogryposis multiplex congenita and life-threatening crises. The late-onset phenotype developed in adolescence or adulthood and was initially mistaken for seronegative myasthenia gravis. Recognition of this late-onset phenotype should prevent inappropriate immunotherapy.

Rapsyn, a 43-kDa postsynaptic protein, is essential for anchoring and clustering acetylcholine receptors (AChRs) at the endplate (EP). Mutations in the rapsyn gene have been found to cause a postsynaptic congenital myasthenic syndrome (CMS). We detected six patients with CMS due to mutations in the rapsyn gene (RAPSN). In vitro studies performed in the anconeus muscle biopsies of four patients showed severe reduction of miniature EP potential amplitudes. Electron microscopy revealed various degrees of impaired development of postsynaptic membrane folds. All patients carried the N88K mutation. Three patients were homozygous for N88K and had less severe phenotypes and milder histopathologic abnormalities than the three patients who were heterozygous and carried a second mutation (either L14P, 46insC, or Y269X). Surprisingly, two N88K homozygous patients had one asymptomatic relative each who carried the same genotype, suggesting that additional genetic factors to RAPSN mutations are required for disease expression.

Rapsyn is essential for clustering the acetylcholine receptor at the postsynaptic membrane of the neuromuscular junction. Direct sequencing of RAPSN in two children with congenital myasthenic syndromes with no mutation in any of the AChR subunits identified two heterozygous recessive mutations in each: a previously characterized N88K mutation in both, and a second frameshifting mutation in Patient (Pt) 1 and a nonsense mutation in Pt 2. An intercostal muscle biopsy in Pt 1 revealed decreased AChRs per endplate and decreased amplitude of the miniature endplate potential, predicted consequences of rapsyn deficiency. Clinically, both children manifested with hypomotility in utero, fatigable ocular and limb weakness since birth, decreased strength during viral illness, decremental response on electromyography, and absence of AChR antibodies. Pt 1, however, had a more severe clinical course with recurrent episodes of respiratory failure, contractures, and craniofacial malformations. In both patients, treatment with pyridostigmine was of some benefit, but the addition of 3,4-diaminopyridine led to significant clinical improvement. Thus, rapsyn deficiency predicting similar consequences at the cellular level can result in phenotypes with marked differences in severity of symptoms, risk of respiratory failure, and presence of contractures and craniofacial malformations.

Objective: Mutation analysis of the acetylcholine receptor (AChR) e subunit gene in patients with sporadic or autosomal recessive congenital myasthenic syndromes (CMS). Background: The nicotinic AChR of skeletal muscle is a neurotransmitter-gated ion channel that mediates synaptic transmission at the vertebrate neuromuscular junction. Mutations in its gene may cause congenital myasthenic syndromes. A recently described mutation in exon 12 of the AChR e subunit (e1267delG) disrupts the cytoplasmic loop and the fourth transmembrane region (M4) of the AChR e subunit. Methods: Forty-three CMS patients from 35 nonrelated families were clinically classified as sporadic cases of CMS (group III according to European Neuromuscular Centre consensus) and were analyzed for e1267delG by PCR amplification and sequence analysis. Results: The authors report the complete genomic sequence and organization of the gene coding for the e subunit of the human AChR (accession number AF105999). Homozygous e1267delG was identified in 13 CMS patients from 11 independent families. All e1267delG families were of Gypsy or southeastern European origin. Genotype analysis indicated that they derive from a common ancestor (founder) causing CMS in the southeastern European Gypsy population. Phenotype analysis revealed a uniform pattern of clinical features including bilateral ptosis and mild to moderate fatigable weakness of ocular, facial, bulbar, and limb muscles. Conclusions: The mutation e1267delG might be frequent in European congenital myasthenic syndrome patients of Gypsy ethnic origin. In general, patients (e1267delG) were characterized by the onset of symptoms in early infancy, the presence of ophthalmoparesis, positive response to anticholinesterase treatment, and the benign natural course of the disease.