Slow-channel Syndrome Research Articles

Innovative Therapeutic Approaches in Congenital Myasthenic Syndromes.

To provide real-word clinical follow-up data on patients carrying variations of congenital myasthenic syndromes (CMS) and who respond to some innovative drugs. Patients recruited from the Neurology Department of the Mustapha Bacha university hospital in Algiers. Treated with innovative drugs, they were monitored and their clinical progress was evaluated on the basis of clinical arguments suggestive of CMSs, but also para clinical arguments (electromyography and genetic study). Six patients carrying different mutations in different genes of CMSs were studied. They had different pathophysiologic profiles (slow or fast channel syndromes, low expressor of receptor). Their therapeutic management was based on innovative drugs, normally indicated in other, non-neurological pathologies. Their outcome was toward a clear clinical improvement. This work relates the interest of proposing treatments (outside of Pyridostigmine) in the management of CMSs. These therapies can greatly modify the prognosis of patients suffering from this orphan disease. This study provides Class IV evidence that for patients with congenital myasthenic syndromes, some innovative treatments are effective.

Congenital myasthenic syndromes: a retrospective natural history study of respiratory outcomes in a single centre.

Respiratory problems are a major cause of morbidity and mortality in patients with congenital myasthenic syndromes, a rare heterogeneous group of neuromuscular disorders caused by genetic defects impacting the structure and function of the neuromuscular junction. Recurrent, life-threatening episodic apnoea in early infancy and childhood and progressive respiratory failure requiring ventilation are features of certain genotypes of congenital myasthenic syndromes. Robb et al. published empirical guidance on respiratory management of the congenital myasthenic syndromes, but other than this workshop report, there are little published longitudinal natural history data on respiratory outcomes of these disorders. We report a retrospective, single-centre study on respiratory outcomes in a cohort of 40 well characterized genetically confirmed cases of congenital myasthenic syndromes, including 10 distinct subtypes (DOK7, COLQ, RAPSN, CHAT, CHRNA1, CHRNG, COL13A1, CHRNE, CHRNE fast channel syndrome and CHRNA1 slow channel syndrome), with many followed up over 20 years in our centre. A quantitative and longitudinal analysis of key spirometry and sleep study parameters, as well as a description of historical hospital admissions for respiratory decompensation, provides a snapshot of the respiratory trajectory of congenital myasthenic syndrome patients based on genotype.

A novel phenotype of AChR-deficiency syndrome with predominant facial and distal weakness resulting from the inclusion of an evolutionary alternatively-spliced exon in CHRNA1

Primary acetylcholine receptor deficiency is the most common subtype of congenital myasthenic syndrome, resulting in reduced amount of acetylcholine receptors expressed at the muscle endplate and impaired neuromuscular transmission. AChR deficiency is caused mainly by pathogenic variants in the ε-subunit of the acetylcholine receptor encoded by CHRNE, although pathogenic variants in other subunits are also seen. We report the clinical and molecular features of 13 patients from nine unrelated kinships with acetylcholine receptor deficiency harbouring the CHRNA1 variant NM_001039523.3:c.257G>A (p.Arg86His) in homozygosity or compound heterozygosity. This variant results in the inclusion of an alternatively-spliced evolutionary exon (P3A) that causes expression of a non-functional acetylcholine receptor α-subunit. We compare the clinical findings of this group to the other cases of acetylcholine receptor deficiency within our cohort. We report differences in phenotype, highlighting a predominant pattern of facial and distal weakness in adulthood, predominantly in the upper limbs, which is unusual for acetylcholine receptor deficiency syndromes, and more in keeping with slow-channel syndrome or distal myopathy. Finally, we stress the importance of including alternative exons in variant analysis to increase the probability of achieving a molecular diagnosis.

Pregnancy outcomes in patients with congenital myasthenic syndromes.

The congenital myasthenic syndromes (CMS) are a heterogeneous group of inherited disorders that affect neuromuscular junction transmission. Data on pregnancy outcomes in women with CMS are limited due to their infrequency. In this study we explored pregnancy with CMS in a large cohort of women attending a national specialty clinic in England. All women with CMS who had a documented pregnancy were invited to complete a questionnaire assessing clinical status during pregnancy and postpartum, pregnancy outcomes, fetal outcomes, and medication use during pregnancy. Among 16 women with CMS (acetylcholine receptor deficiency [CHRNE], slow channel syndrome [CHRNA1], DOK7, RAPSYN and glycosylation [DPAGT1 and GFPT1]), 27 pregnancies were recorded: 26 single pregnancies and 1 twin pregnancy. Symptom worsening was reported in 63% of pregnancies, but recovery to baseline function was seen in all but one patient. Miscarriage and cesarean section occurred in 31% and 33% of the women, respectively. Over half of the patients continued taking their medication during pregnancy, which included pyridostigmine (n=10), 3,4-diaminopyridine (n=9), ephedrine (n=3), salbutamol (n=3), and quinidine (n=1). No fetal malformations were recorded. Our results show that clinical worsening during pregnancy was common but rarely persistent. The majority of women with CMS can safely plan pregnancy, but close follow-up is required from their neurology and obstetric teams. Although we identified no safety concerns, continued medication use should be reviewed on a case-by-case basis.

Caspase-9: A Multimodal Therapeutic Target With Diverse Cellular Expression in Human Disease.

Caspase-9, a cysteine-aspartic protease known for its role as an initiator of intrinsic apoptosis, regulates physiological cell death and pathological tissue degeneration. Its nonapoptotic functions, including regulation of cellular differentiation/maturation, innate immunity, mitochondrial homeostasis, and autophagy, reveal a multimodal landscape of caspase-9 functions in health and disease. Recent work has demonstrated that caspase-9 can drive neurovascular injury through nonapoptotic endothelial cell dysfunction. CASP9 polymorphisms have been linked with various cancers, neurological disorders, autoimmune pathologies and lumbar disc disease. Clinical reports suggest alterations in caspase-9 expression, activity or function may be associated with acute and chronic neurodegeneration, retinal neuropathy, slow-channel myasthenic syndrome, lumbar disc disease, cardiomyopathies, atherosclerosis and autoimmune disease. Healthy tissues maintain caspase-9 activity at low basal levels, rendering supraphysiological caspase-9 activation a tractable target for therapeutic interventions. Strategies for selective inhibition of caspase-9 include dominant negative caspase-9 mutants and pharmacological inhibitors derived from the XIAP protein, whose Bir3 domain is an endogenous highly selective caspase-9 inhibitor. However, the mechanistic implications of caspase-9 expression and activation remain indeterminate in many pathologies. By assembling clinical reports of caspase-9 genetics, signaling and cellular localization in human tissues, this review identifies gaps between experimental and clinical studies on caspase-9, and presents opportunities for further investigations to examine the consequences of caspase activity in human disease.

Congenital myasthenic syndromes in adult neurology clinic: A long road to diagnosis and therapy.

To investigate the diagnostic challenges of congenital myasthenic syndromes (CMS) in adult neuromuscular practice. We 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. We 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. Misdiagnosis 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.

Beta-2 Adrenergic Receptor Agonists Enhance AChR Clustering in C2C12 Myotubes: Implications for Therapy of Myasthenic Disorders.

Background:Congenital myasthenic syndromes (CMS) are a group of inherited neuromuscular transmission disorders causing fatiguable muscle weakness. ADRB2 agonists have been observed to provide therapeutic benefit where destabilisation of NMJ structures is part of the underlying pathology, such as in DOK7, COLQ and MuSK CMS as well as in slow channel syndrome. However, very little is known about the molecular mechanisms underlying the effects of ADRB2 agonists in CMS.Objective:In vitro investigation into whether an ADRB2 agonist affects the AChR clustering pathway and has the potential to increase the number and stability of AChR clusters.Methods:Cultured C2C12 mouse myotubes overexpressing the common DOK7 frameshift mutation c.1124_1127dupTGCC were incubated with salbutamol sulphate and the effect on AChR cluster numbers were investigated. Moreover, agrin-induced AChR clusters in C2C12 WT cells were left to disperse after agrin-wash-off, and the effects of incubation with salbutamol sulphate on AChR cluster numbers were explored.Results:Salbutamol sulphate induced a significant increase in the number of AChR clusters formed on C2C12 cells overexpressing c.1124_1127dupTGCC. Furthermore, significantly more clusters remained in C2C12 WT myotubes incubated with salbutamol sulphate following agrin wash-off.Conclusions:The results suggest that ADRB2 agonists directly affect proteins located at the neuromuscular junction and exert a stabilising effect on AChR clusters.

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.

Mutations Causing Slow-Channel Myasthenia Reveal That a Valine Ring in the Channel Pore of Muscle AChR is Optimized for Stabilizing Channel Gating.

We identify two novel mutations in acetylcholine receptor (AChR) causing a slow-channel congenital myasthenia syndrome (CMS) in three unrelated patients (Pts). Pt 1 harbors a heterozygous βV266A mutation (p.Val289Ala) in the second transmembrane domain (M2) of the AChR β subunit (CHRNB1). Pts 2 and 3 carry the same mutation at an equivalent site in the ε subunit (CHRNE), εV265A (p.Val285Ala). The mutant residues are conserved across all AChR subunits of all species and are components of a valine ring in the channel pore, which is positioned four residues above the leucine ring. Both βV266A and εV265A reduce the amino acid size and lengthen the channel opening bursts by fourfold by enhancing gating efficiency by approximately 30-fold. Substitution of alanine for valine at the corresponding position in the δ and α subunit prolongs the burst duration four- and eightfold, respectively. Replacing valine at ε codon 265 either by a still smaller glycine or by a larger leucine also lengthens the burst duration. Our analysis reveals that each valine in the valine ring contributes to channel kinetics equally, and the valine ring has been optimized in the course of evolution to govern channel gating.

Congenital Myasthenic Syndrome with Severe Apnoes Requiring Intubation and Arthrogryposis: Therapeutic Consequence of Genetics

Congenital myasthenic syndromes (CMS) result from the failure to achieve muscle depolarization due to disorders in the structure and function of the neuromuscular synapse. Depending on the genetical disorder they can be divided into presynaptic, synaptic, and postsynaptic defects. The therapy depends on the structural defect of the neuromuscular junction and the genetic defect. There is no strong genotype-phenotype correlation, the clinical situation alone cannot determine the required therapy. Therefore, we prefer a fast genetic testing during hospital treatment. We describe a 7-month-old boy with a muscle hypotonia, severe contractions, and recurrent apnoeic crises as requiring ventilator support. The repetitive nerve stimulation showed a decrement of the muscle of 54% suspected for a CMS. Intravenous application of pyridostigmin caused a strong impairment of the situation by producing lots of secretion, tears, and arterial hypotonia with no improvement of the muscle strength. Suction of secretion, atropin intravenous, and oxygen via nasal tube was necessary. After the tensilon testing, we suspected a CMS due to a slow-channel syndrome or a mutation of the COLQ gene. The genetic testing was necessary for further investigations. The genetic testing showed a homozygote mutation of the CHRNA1 gene: c.199A>T;p.Asn67Tyr, which is first described in our patient. The CHRNA1 gene encodes for the α subunit of the acetylcholine receptor. Autosomal recessive mutations of the CHRNA1 gene are described just in few patients. The phenotype probably ranges from fetal akinesia to severe CMS. Severe apnoes and contractions in patients with CMS were a clinical clue for mutations of the RAPSN or CHAT gene so far. The genetic test ensures the diagnosis of a CMS, a slow channel syndrome or a mutation of the COLQ gene could be excluded. The therapy with pyridostimin could be continued. Therefore, we recommend a much faster genetic testing during the patient’s stay in hospital to be able to decide which therapy the patient need.

What's in the Literature?

The spectrum of congenital myasthenic syndromes continues to expand, and recent studies found mutations in enzymes that glycosylate peptides. Affected patients have early onset of a limb-girdle pattern of weakness, and the disorder is treatment responsive. Another article addressed quinine use for slow channel syndrome. There is an additional report on the use of rituximab in acquired myasthenia gravis, and the risk of developing cancer with azathioprine therapy for myasthenia gravis is also covered. Regarding amyotrophic lateral sclerosis, we review articles addressing the utility and safety profile of botulinum injections for sialorrhea and possible associations with autoimmune diseases. Moving to peripheral nerve, we address reports on small fiber neuropathy as a cause of muscle cramps, reexamine clinical and electrodiagnostic features of childhood Guillain–Barré syndrome, and review a recent article on an unusual syndrome of postirradiation lower motor neuron syndrome affecting the cauda equina. Finally, recent reports show that mutations in the skeletal muscle ryanodine receptor may be a common cause of rhabdomyolysis and exertional myalgias and another cause of axial myopathy. Advances in clinical and basic science concepts of myotonic dystrophy are also reviewed.

P.12.3 Do congenital myasthenic syndromes in childhood have a common face? Clinical profile of slow channel, CHRNE and RAPSYN mutations

Congenital myasthenic syndromes (CMSs) form a group of hereditary disorders, which are clinically and genetically heterogeneous. The neuromuscular transmission cascade is disrupted at presynaptic, synaptic or postsynaptic level, the latter is the most common. As a large part of CMSs have their onset in childhood, performing diagnostic studies leading to diagnosis can be very challenging. CMSs are highly treatable but treatment depends on the mutation type, so clinicians are urged to perform tailored genetic testing guided by clinical features. This systematic review of the literature shows the clinical neurological features of published cases with CMS due to a slow channel syndrome, RAPSYN deficiency or acetylcholine receptor (AChR) deficiency. English and Dutch case reports and series describing patients with a genetically proven CHRNE, RAPSYN or slow channel CMS from 1980 to 2012 were collected. Clinical features were evaluated. Findings were mostly in line with clinical features mentioned in previous literature. Minor differences were encountered. Very common were ocular, bulbar and limb weakness for AChR deficiency (N = 100). Cervical and distal weakness was often seen in slow channel syndromes (N = 51). Bulbar symptoms, arthrogryposis multiplex congenita, respiratory difficulties and the N88K founder mutation occurred frequently in the RAPSYN deficient patients (N = 98). Opposed to earlier reports no early- and late-onset phenotype was identified in the latter group and ophthalmoparesis was rare. In slow channel syndromes bulbar (25%) and ocular weakness (50%) was more common than previously reported. Ethnicity was not always in line with previous literature but could still be helpful in more targeted genetic testing. Overall we think that detailed descriptions of clinical features could guide the clinician in the diagnostic process.

P.12.2 Dominant distal myopathy due to slow channelopathy

Slow channel syndrome, first recognized by Engel in 1982, has distinct phenotypic features, dominant inheritance, selective weakness of cervical scapular and finger extensor weakness, ophtalmoparesis. Histochemical findings show type 1 fibers preponderance, small group of atrophic fibers of either fiber type, increased fiber size variability, tubular aggregates. We examined 3 biopsies in 3 patients (2 males – 55 years and 66 years, 1 female – 61 years) presenting a dominant distal myopathy with variable age at onset (from second to third decade). The patients had ophtalmoparesis, upper limb extensor, forearm and finger weakness and distal myopathy with a progressive slow course. The woman had also some difficulty in anterior tibial flexor muscle but also atrophy of forearms muscles. Electrophysiology on single stimulus of ulnar nerve showed repetitive CMAP. The genomic wide linkage analysis identified 3 regions co-segregating with the disease in chromosome 1, 13, 17. Next generation sequencing showed in all patients a known mutation in epsilon-subunit of AchR receptor and a mutation in phospholipase. Muscle showed mild changes consisting in atrophic fibers, mostly of type 2, some fiber type grouping, central nuclei and ring fibers. Such fibers are consistent with abnormal regeneration.We suggest that the occurrence of mutations involving both AchR receptor and phospholipase might concur in determining a new distal phenotype with permanent weakness. Our family was previously diagnosed as myotonic dystrophy or distal myopathy. The presence of the histochemical findings of both myopathy and neuropathy were present. We suggest the possibility of calcium overload due to prolonged open time of the AChR channel with a presynaptic component might determine a distal neuro-myopathy with slow evolution and muscle rearrangement due to a muscle fiber degeneration and functional denervation. The slow channel mutation should be searched in families with permanent distal weakness.

Selective inhibition of caspases in skeletal muscle reverses the apoptotic synaptic degeneration in slow-channel myasthenic syndrome

Slow-channel syndrome (SCS) is a congenital myasthenic disorder caused by point mutations in subunits of skeletal muscle acetylcholine receptor leading to Ca(2+) overload and degeneration of the postsynaptic membrane, nuclei and mitochondria of the neuromuscular junction (NMJ). In both SCS muscle biopsies and transgenic mouse models for SCS (mSCS), the endplate regions are shrunken, and there is evidence of DNA damage in the subsynaptic region. Activated caspase-9, -3 and -7 are intensely co-localized at the NMJ, and the Ca(2+)-activated protease, calpain, and the atypical cyclin-dependent kinase (Cdk5) are overactivated in mSCS. Thus, the true mediator(s) of the disease process is not clear. Here, we demonstrate that selective inhibition of effector caspases, caspase-3 and -7, or initiator caspase, caspase-9, in limb muscle in vivo by localized expression of recombinant inhibitor proteins dramatically decreases subsynaptic DNA damage, increases endplate area and improves ultrastructural abnormalities in SCS transgenic mice. Calpain and Cdk5 are not affected by this treatment. On the other hand, inhibition of Cdk5 by expression of a dominant-negative form of Cdk5 has no effect on the degeneration. Together with previous studies, these results indicate that focal activation of caspase activity at the NMJ is the principal pathological process responsible for the synaptic apoptosis in SCS. Thus, treatments that reduce muscle caspase activity are likely to be of benefit for SCS patients.

A mouse model of the slow channel myasthenic syndrome: Neuromuscular physiology and effects of ephedrine treatment

In the slow channel congenital myasthenic syndrome mutations in genes encoding the muscle acetylcholine receptor give rise to prolonged ion channel activations. The resulting cation overload in the postsynaptic region leads to damage of synaptic structures, impaired neuromuscular transmission and fatigable muscle weakness. Previously we identified and characterised in detail the properties of the slow channel syndrome mutation εL221F. Here, using this mutation, we generate a transgenic mouse model for the slow channel syndrome that expresses mutant human ε-subunits harbouring an EGFP tag within the M3–M4 cytoplasmic region, driven by a ~1500bp region of the CHRNB promoter. Fluorescent mutant acetylcholine receptors are assembled, cluster at the motor endplates and give rise to a disease model that mirrors the human condition. Mice demonstrate mild fatigable muscle weakness, prolonged endplate and miniature endplate potentials, and variable degeneration of the postsynaptic membrane. We use our model to investigate ephedrine as a potential treatment. Mice were assessed before and after six weeks on oral ephedrine (serum ephedrine concentration 89±3ng/ml) using an inverted screen test and in vivo electromyography. Treated mice demonstrated modest benefit for screen hang time, and in measures of compound muscle action potentials and mean jitter that did not reach statistical significance. Ephedrine and salbutamol show clear benefit when used in the treatment of DOK7 or COLQ congenital myasthenic syndromes. Our results highlight only a modest potential benefit of these β2-adrenergic receptor agonists for the treatment of the slow channel syndrome.

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

Slow channel congenital myasthenic syndrome is a dominant disorder characterized by prolonged acetylcholine receptor ion-channel activation. Molecular genetic techniques, electrophysiology, and binding studies in human embryonic kidney (HEK) 293 cells determined mutant function and expression levels. Patient response to treatment was measured by quantitative myasthenic gravis and Medical Research Council grade strength scores. We report an unusual case due to heteroallelic mutations in CHRNE. The slow channel mutation, p.εS278del, is accompanied by a severe low-expression mutation, p.εR217L, on the second allele. Expression studies and cosegregation of p.εS278del with the disorder in the patient's offspring demonstrate robust expression of the p.εS278del mutation. The patient showed modest benefits from standard treatment with fluoxetine, but there was dramatic improvement when salbutamol was combined with fluoxetine. This case suggests that salbutamol, which is beneficial in some other congenital myasthenic syndromes, might also be considered in addition to fluoxetine in slow channel syndrome.

Zebrafish model for congenital myasthenic syndrome reveals mechanisms causal to developmental recovery

Mutations in muscle ACh receptors cause slow-channel syndrome (SCS) and Escobar syndrome, two forms of congenital myasthenia. SCS is a dominant disorder with mutations reported for all receptor subunits except γ. Escobar syndrome is distinct, with mutations located exclusively in γ, and characterized by developmental improvement of muscle function. The zebrafish mutant line, twister, models SCS in terms of a dominant mutation in the α subunit (α(twi)) but shows the behavioral improvement associated with Escobar syndrome. Here, we present a unique electrophysiological study into developmental improvement for a myasthenic syndrome. The embryonic α(twi)βδγ receptor isoform produces slowly decaying synaptic currents typical of SCS that transit to a much faster decay upon the appearance of adult ε, despite the α(twi) mutation. Thus, the continued expression of α(twi) into adulthood is tolerated because of the ε expression and associated recovery, raising the likelihood of unappreciated myasthenic cases that benefit from the γ-ε switch.

Transgenic mouse model reveals an unsuspected role of the acetylcholine receptor in statin-induced neuromuscular adverse drug reactions

High cholesterol levels are an established risk factor for cardiovascular disease (CVD), the world's leading cause of death. Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (statins) are prescribed to lower serum cholesterol levels and reduce the risk of CVD. Despite the success of statins, many patients abandon treatment owing to neuromuscular adverse drug reactions (ADRs). Genome-wide association studies have identified the single-nucleotide polymorphism (SNP) rs4149056 in the SLCO1B1 gene as being associated with an increased risk for statin-induced ADRs. By studying slow-channel syndrome transgenic mouse models, we determined that statins trigger ADRs in mice expressing the mutant allele of the rs137852808 SNP in the nicotinic acetylcholine receptor (nAChR) α-subunit gene CHRNA1. Mice expressing this allele show a remarkable contamination of end-plates with caveolin-1 and develop early signs of neuromuscular degeneration upon statin treatment. This study demonstrates that genes coding for nAChR subunits may contain variants associated with statin-induced ADRs.

Acetylcholine Receptor Gating in a Zebrafish Model for Slow-Channel Syndrome

Slow-channel syndrome (SCS) is an autosomal-dominant disease resulting from mutations in muscle acetylcholine (ACh) receptor subunits. The associated fatigue and muscle degeneration are proposed to result from prolonged synaptic responses that overload intracellular calcium. Single-channel studies on reconstituted receptors bearing human mutations indicate that the prolonged responses result from an increase in receptor open duration and, in some cases, increased sensitivity to ACh. We show that both of these aberrant receptor properties are recapitulated in heterozygotic zebrafish bearing an L258P mutation in the α subunit, thus affording the unique opportunity to compare the single-channel properties of mutant receptors to the synaptic currents in vivo. Whole-cell recordings revealed synaptic currents that decayed along a multiexponential time course, reflecting receptors containing mixtures of wild-type and mutant α subunits. Treatment with quinidine, an open-channel blocker used to treat the human disorder, restored fast synaptic current kinetics and the ability to swim. Quinidine block also revealed that mutant receptors generate a large steady-state current in the absence of ACh. The spontaneous openings reflected a destabilization of the closed state, leading to an apparent increase in the sensitivity of these receptors to ACh. The effective block by quinidine on synaptic currents as well as nonliganded openings points to dual sources for the calcium-dependent myopathy in certain forms of SCS.

172 An unusual case of congenital myasthenic syndrome: the mechanism for treatment response

Congenital myasthenic syndromes (CMS) are a heterogenous group of inherited disorders that impair neuromuscular transmission. Mutations in the gene that codes for the epsilon subunuit (CHRNE) of the Acetylcholine receptor...