Postsynaptic Congenital Myasthenic Syndromes Research Articles

Congenital Myasthenic Syndromes

Very few areas of medical genetics have been so profoundly impacted by the advent of next- generation sequencing (NGS) as the field of congenital myasthenic syndromes (CMS). This is due to the formidable genetic heterogeneity of CMS, a dearth of diagnostic clinical clues of CMS types, and the imperative need to establish an accurate molecular diagnosis of CMS type before any medication is started. A molecular diagnosis of CMS is fundamental not only to provide an appropriate therapy, but more importantly, to avoid potential deleterious treatments. Thus, NGS has transformed the tedious and expensive task of searching for causative mutations in an ever-expanding list of genes linked to CMS into an effective, and relatively inexpensive process that can rapidly identify the variant of CMS in question. One of the consequences of this transformation is a paradigm shift in the clinical practice of CMS that no longer requires, with rare exceptions, the use of special muscle biopsies that enable the analysis of the function and ultrastructure of the neuromuscular junction to determine the type of CMS. Another technological advance of recent years is CRISPR/Cas9, which allows genome editing at the zygotic stage, thus greatly simplifying the generation of mouse models carrying the same human CMS mutations in orthologous mouse genes. This permits an in-depth analysis of the pathogenesis and treatments of CMS caused by specific gene mutations. In terms of therapy, in addition to the classical pharmacologic treatments of CMS, including pyridostigmine sulfate, albuterol and 3,4 diaminopyridine, AAV-based gene therapies are now at the preclinical stage for several types of CMS. In this brief review, CMS are classified in six major groups: (1). presynaptic CMS, (2) synaptic CMS, (3) postsynaptic CMS; 4. CMS affecting the agrin-signal transduction pathway, (5) CMS linked to disorders of glycosylation, and (6) CMS associated with abnormalities of the cytoskeleton.

Genetic Basis of Congenital Myasthenic Syndrome: A Review Study

The neuromuscular junction is a highly specialized cholinergic synapse, essential for initiating nerve-evoked muscle contractions by means of neuromuscular transmission. Loss or dysfunction of any component of this junction might affect synaptic performance. Congenital Myasthenic Syndromes (CMSs) are rare heterogeneous disorders of autosomal inheritance caused by genetic defects affecting neuromuscular transmission that results in skeletal muscle weakness and abnormal fatigability on exertion. The onset is usually from birth to childhood. CMSs are more uncommon than autoimmune myasthenia gravis. CMSs are classified based on their genetic and clinical presentations into presynaptic, synaptic basal lamina, and postsynaptic CMSs. To date, mutations in more than 25 genes have been implicated in the pathogenesis of CMSs. In this review article, different CMSs diagnostic procedures are investigated, and the genetic, clinical, and molecular aspects of CMSs are outlined.

Intrafamilial variation in clinical manifestations and response to salbutamol in siblings with congenital myasthenic syndrome caused by DOK7 mutations

The congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders arising from genetic defects in presynaptic, synaptic, and postsynaptic proteins of the neuromuscular junction (NMJ) resulting in variable and characteristically fatigable muscle weakness affecting limb, ocular, bulbar, trunk, and respiratory muscles from early life. DOK7 mutation resulting in synaptic and postsynaptic CMS clinically presents with limb-girdle myasthenia with sparing of facial and EOM. They characteristically worsen with conventional treatment and show excellent response to salbutamol/ ephedrine. Here we present a case highlighting a varied presentation beginning in late childhood and its evolution to reveal its congenital nature and subsequent management with salbutamol.

Electromyography: Beyond positive diagnosis of myastenic syndrome

Objectives Electromyography (EMG) plays a major role to confirm the presence of neuromuscular transmission disorders. Furthermore, it helps to recognize the presynaptic or postsynaptic nature of the disorder. Indeed, specialized neurophysiological tests and characteristic electrophysiological findings are widely useful for etiological diagnosis [1] , [2] . Results Here we report two cases, a 36 years-old woman and 37 years-old men who presented to our department for facial muscle weakness. The female presented for drooping of both eyelids with diplopia, which appears while reading. Neurological examination showed bilateral asymmetric ptosis and complete ophtalmoplegia. Pupillary light reflex was normal as well as the rest of the neurological examination. The neostigmine test was positive. Repetitive nerve stimulation (RNS) testat 3 Hz stimulation, showed a significant decremental response in both area and amplitude with repetitive compound muscle action potentials (R-CMAP). Muscle biopsy eliminated centro-nuclear myopathy. We then considered the diagnosis of postsynaptic congenital myasthenic syndrome. The man presented with a ten days history of facial muscle weakness, dysphonia and swallowing disorders without diurnal fluctuation. Neurological examination noted symmetric proximal limb weakness. Clinical picture was concordant with acute inflammatory demyelinating polyradiculoneuropathy. Lumbar puncture did not evidence albumino-cytologic dissociation. Electrodiagnostic studies demonstrated a reduction in the initial CMAP amplitude, a decremental response to slow stimulation rates (3 Hz), followed by incremental response after a tetanic stimulation consistant with Lambert Eaton Syndrome. Conclusion In general, full anamnestic data and careful clinical examination are sufficient to make the diagnosis of myastenic syndrome. Nevertheless, the lack of diurnal fluctuation and a clear pattern of muscle distribution can sometimes lead to misdiagnosing this disorder [2] , [3] . Moreover, myastenic syndrome with predominant facial muscle involvement share overlapping symptoms with other muscle disease like mitochondrial and centro-nuclear myopathy. These findings delineate the contribution of EMG to differential and etiologic diagnosis [1] .

Post-exercise facilitation in a post-synaptic congenital myasthenic syndrome (P2.123)

Post-exercise facilitation in a post-synaptic congenital myasthenic syndrome (P2.123)

Effect of 3,4-diaminopyridine at the murine neuromuscular junction.

We investigated the effects of 3,4-diaminopyridine (3,4-DAP) and its acetylated metabolite, N-(4-amino-pyridin-3-yl) acetamide (3-Ac), at the mammalian neuromuscular junction. Quantal release of acetylcholine was studied in diaphragm muscles of mice, using in vitro intracellular microelectrode recordings. Under conditions of low probability of release, 3,4-DAP produced a 1,000% increase in quantal release, but 3-Ac had no effect. Under conditions of normal probability of release, the effect of 3,4-DAP was modest and limited by concurrent depletion of synaptic vesicles, especially with high concentrations of 3,4-DAP and high frequencies of nerve stimulation. These findings predict 3,4-DAP is most effective in conditions with low probability of quantal release, such as Lambert-Eaton myasthenic syndrome. A beneficial effect is also expected in disorders of neuromuscular transmission in which the effect of 3,4-DAP on quantal release is not limited by depletion of synaptic vesicles, such as postsynaptic congenital myasthenic syndromes. Muscle Nerve, 2016 Muscle Nerve 55: 223-231, 2017.

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

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.Gln120Serfs*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.

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

Background:Congenital myasthenic syndromes (CMS) are a heterogeneous group of diseases involving neuromuscular transmission. The classification of these syndromes is based on the localization of the defect (pre-synaptic, post-synaptic, and neuromuscular junction) and on the molecular analysis.Aim:To report on a series of 7 patients affected by post-synaptic CMS.Patients and Methods:We examined sex, familiarity, age of onset, clinical symptoms, and response to tensilon test, patellar and pupillary reflexes, presence of cranial nerve involvement, Gowers′ sign, presence of ptosis, grade of muscular weakness, and response to the treatment and gene deletions.Results:Ptosis, muscular hypotonia, and light variability in muscular weakness were the main clinical signs. Cholinergic receptor, nicotinic, epsilon (CHRNE) gene mutations were mainly reported.Conclusions:The study points out that the clinical and molecular pattern reported in our patients do not differentiate from the data reported in the literature. Treatment with pyridostigmine and modulation of the therapy allows a good quality of life.

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

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.-38A>G), 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.

Targeting of the ETS Factor Gabpα Disrupts Neuromuscular Junction Synaptic Function

The GA-binding protein (GABP) transcription factor has been shown in vitro to regulate the expression of the neuromuscular proteins utrophin, acetylcholine esterase, and acetylcholine receptor subunits delta and epsilon through the N-box promoter motif (5'-CCGGAA-3'), but its in vivo function remains unknown. A single point mutation within the N-box of the gene encoding the acetylcholine receptor epsilon subunit has been identified in several patients suffering from postsynaptic congenital myasthenic syndrome, implicating the GA-binding protein in neuromuscular function and disease. Since conventional gene targeting results in an embryonic-lethal phenotype, we used conditional targeting to investigate the role of GABPalpha in neuromuscular junction and skeletal muscle development. The diaphragm and soleus muscles from mutant mice display alterations in morphology and distribution of acetylcholine receptor clusters at the neuromuscular junction and neurotransmission properties consistent with reduced receptor function. Furthermore, we confirmed decreased expression of the acetylcholine receptor epsilon subunit and increased expression of the gamma subunit in skeletal muscle tissues. Therefore, the GABP transcription factor aids in the structural formation and function of neuromuscular junctions by regulating the expression of postsynaptic genes.

Common founder effect of rapsyn N88K studied using intragenic markers.

Mutations in the human gene encoding rapsyn have been linked to a recessive form of postsynaptic congenital myasthenic syndrome due to deficient clustering of acetylcholine receptors at the endplate. All patients reported to date carry the N88K mutation, suggesting a possible common founder effect. To decrease the likelihood of a recombination event occurring within the span of neighboring microsatellite markers, we used seven intragenic single nucleotide polymorphisms (SNPs) spanning 8 kb to characterize the haplotype associated with N88K. In three affected N88K homozygous individuals, we identified a common haplotype present in all heterozygous carriers of N88K. Of note, in two asymptomatic N88K homozygous individuals, a second haplotype was present that differed at three SNP sites downstream from the N88K mutation. Our findings of a common haplotype associated with the N88K mutation support a founder effect. The discordant haplotype found in homozygous individuals suggests that recombination events may have occurred within the rapsyn gene and that this may have implications in the phenotypic expression of the disease.

Syndromes myasthéniques congénitaux: Expression phénotypique et caractérisation physiopathologique

Congenital Myasthenic Syndromes (CMS) are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. The twenty five past Years saw major advances in identifying different types of CMS due to abnormal presynaptic, synaptic, and postsynaptic proteins. CMS diagnosis requires two steps: 1) positive diagnosis supported by myasthenic signs beginning in neonatal period, efficacy of anticholinesterase medications, positive family history, negative tests for anti-acetylcholine receptor (AChR) antibodies, electromyographic studies (decremental response at low frequency, repetitive CMAP after one single stimulation); 2) pathophysiological characterisation of CMS implying specific studies: light and electron microscopic analysis of endplate (EP) morphology, estimation of the number of AChR per EP, acetylcholinesterase (AChE) expression, molecular genetic analysis. Most CMS are postsynaptic due to mutations in the AChR subunits genes that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh resulting respectively in Slow Channel Syndrome (characterized by a autosomal dominant transmission, repetitive CMAP, refractoriness to anticholinesterase medication) and fast channel, recessively transmitted. AChR deficiency without kinetic abnormalities is caused by recessive mutations in AChR genes (mostly epsilon subunit) or by primary rapsyn deficiency, a post synaptic protein involved in AChR concentration. Recently, mutations in SCN4A sodium channel have been reported in one patient. AChE deficiency is identified on the following data: recessive transmission, presence of repetitive CMAP, refractoriness to cholinesterase inhibitors, slow pupillary response to light and absent expression of the enzyme at EP. This synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) that anchors the catalytic subunits in the synaptic basal lamina. The most frequent presynaptic CMS is caused by mutations of choline acetyltransferase. Several CMS are still not characterized. Many EP molecules are potential etiological candidates. In these unidentified cases, other methods of investigations are required: linkage analysis, when sufficient number of informative relatives are available, microelectrophysiological studies performed in intercostal or anconeus muscles. Prognosis of CMS, depending on severity and evolution of symptoms, is difficult to assess, and it cannot not be simply derived from mutation identification. Most patients respond favourably to anticholinesterase medications or to 3,4 DAP which is effective not only in presynaptic but also in postsynaptic CMS. Specific therapies for slow channel CMS are quinidine and fluoxetine that normalize the prolonged opening episodes. Clinical benefits derived from the full characterisation of each case include genetic counselling and specific therapy.

Electrophysiological and morphological characterization of a case of autosomal recessive congenital myasthenic syndrome with acetylcholine receptor deficiency due to a N88K rapsyn homozygous mutation

Congenital myasthenic syndromes are rare heterogeneous hereditary disorders, which lead to defective neuromuscular transmission resulting in fatigable muscle weakness. Post-synaptic congenital myasthenic syndromes are caused by acetylcholine receptor kinetic abnormalities or by acetylcholine receptor deficiency. Most of the congenital myasthenic syndromes with acetylcholine receptor deficiency are due to mutations in acetylcholine receptor subunit genes. Some have recently been attributed to mutations in the rapsyn gene. Here, we report the case of a 28-year-old French congenital myasthenic syndrome patient who had mild diplopia and fatigability from the age of 5 years. His muscle biopsy revealed a marked reduction in rapsyn and acetylcholine receptor at neuromuscular junctions together with a simplification of the subneural apparatus structure. In this patient, we excluded mutations in the acetylcholine receptor subunit genes and identified the homozygous N88K rapsyn mutation, which has already been shown by cell expression to impair rapsyn and acetylcholine receptor aggregation at the neuromuscular junction. The detection of the N88K mutation at the heterozygous state in five of 300 unrelated control subjects shows that this mutation is not infrequent in the healthy population. Electrophysiological measurements on biopsied intercostal muscle from this patient showed that his rapsyn mutation-induced fatigable weakness is expressed not only in a diminution in acetylcholine receptor membrane density but also in a decline of endplate potentials evoked at low frequency.

Congenital myasthenic syndromes: multiple molecular targets at the neuromuscular junction.

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

Congenital myasthenic syndromes: progress over the past decade.

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic basal lamina, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A basal lamina CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.