Congenital myasthenic syndrome caused by decreased agonist binding affinity due to a mutation in the acetylcholine receptor epsilon subunit

[Congenital myasthenic syndromes with kinetic abnormalities of the acetylcholine receptor]

Congenital myasthenic syndromes (CMS) are genetically and phenotypically very heterogeneous conditions resulting in a defect in the neuromuscular transmission. Post-synaptic forms are the most frequent CMSs, and acetyl choline receptor (low expressor) deficiency is the most commonly involved pathophysiological mechanism. CMS with kinetic abnormalities of the acetylcholine receptor (AChr) are much rarer and can give rise to potentially life-threatening phenotypes. Among them, two types have been described: the slow channel syndrome (SCS) and the fast channel syndrome (FCS). Diagnosis and therapeutic management of such entities are specific to each type. In this work, we will illustrate the phenotypic aspects of CMS with kinetic abnormalities of the AChR by a narrative review of three Algerian families.

Impaired gating of γ- and ε-AChR respectively causes Escobar syndrome and fast-channel myasthenia

OBJECTIVE

To dissect the kinetic defects of acetylcholine receptor (AChR) γ subunit variant in an incomplete form of the Escobar syndrome without pterygium and compare it with those of a variant of corresponding residue in the AChR ε subunit in a congenital myasthenic syndrome (CMS).

METHODS

Whole exome sequencing, α-bungarotoxin binding assay, single channel patch-clamp recordings, and maximum likelihood analysis of channel kinetics.

RESULTS

We identified compound heterozygous variants in AChR γ and ε subunits in three Escobar syndrome (1-3) and three CMS patients (4-6), respectively. Each Escobar syndrome patient carries γP121R along with γV221Afs*44 in patients 1 and 2, and γY63* in patient 3. Three CMS patients share εP121T along with εR20W, εG-8R, and εY15H in patients 4, 5, and 6, respectively. Surface expressions of γP121R- and εP121T-AChR were 80% and 138% of the corresponding wild-type AChR, whereas εR20W, εG-8R, and εY15H reduced receptor expression to 27%, 35%, and 30% of wild-type εAChR, respectively. γV221Afs*44 and γY63* are null variants. Thus, γP121R and εP121T determine the phenotype. γP121R and εP121T shorten channel opening burst duration to 28% and 18% of corresponding wild-type AChR by reducing the channel gating equilibrium constant 44- and 63-fold, respectively.

INTERPRETATION

Similar impairment of channel gating efficiency of a corresponding P121 residue in the acetylcholine-binding site of the AChR γ and ε subunits causes Escobar syndrome without pterygium and fast-channel CMS, respectively, suggesting that therapy for the fast-channel CMS will benefit Escobar syndrome.

Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders characterized by impaired neuromuscular signal transmission due to germline pathogenic variants in genes expressed at the neuromuscular junction (NMJ). A total of 35 genes have been reported in CMS (AGRN, ALG14, ALG2, CHAT, CHD8, CHRNA1, CHRNB1, CHRND, CHRNE, CHRNG, COL13A1, COLQ, DOK7, DPAGT1, GFPT1, GMPPB, LAMA5, LAMB2, LRP4, MUSK, MYO9A, PLEC, PREPL, PURA, RAPSN, RPH3A, SCN4A, SLC18A3, SLC25A1, SLC5A7, SNAP25, SYT2, TOR1AIP1, UNC13A, VAMP1). The 35 genes can be classified into 14 groups according to the pathomechanical, clinical, and therapeutic features of CMS patients. Measurement of compound muscle action potentials elicited by repetitive nerve stimulation is required to diagnose CMS. Clinical and electrophysiological features are not sufficient to identify a defective molecule, and genetic studies are always required for accurate diagnosis. From a pharmacological point of view, cholinesterase inhibitors are effective in most groups of CMS, but are contraindicated in some groups of CMS. Similarly, ephedrine, salbutamol (albuterol), amifampridine are effective in most but not all groups of CMS. This review extensively covers pathomechanical and clinical features of CMS by citing 442 relevant articles.

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.

Congenital myasthenic syndromes: where do we go from here?

Congenital myasthenia syndromes are rare but often treatable conditions affecting neuromuscular transmission. They result from loss or impaired function of one of a number of proteins secondary to a genetic defect. An estimate of the prevalence in the UK gave 9.2 cases per million, however, this is likely an underestimate since the adoption of next generation sequencing for diagnosis away from specialist centres is enhancing the 'pick up' rate. Next generation sequencing has helped identify a series of novel genes that harbour mutations causative for congenital myasthenic syndrome that include not only genes that encode proteins specifically expressed at the neuromuscular junction but also those that are ubiquitously expressed. The list of genes harbouring disease-causing mutations for congenital myasthenic syndrome continues to expand and is now over 30, but with many of the newly identified genes it is increasingly being recognised that abnormal neuromuscular transmission is only one component of a multifaceted phenotype in which muscle, the central nervous system, and other organs may also be affected. Treatment can be tailored to the underlying molecular mechanism for impaired neuromuscular transmission but treating the more complex multifaceted disorders and will require development of new therapies.

Zonisamide upregulates neuregulin-1 expression and enhances acetylcholine receptor clustering at the in vitro neuromuscular junction

Decreased acetylcholine receptor (AChR) clustering compromises signal transmission at the neuromuscular junction (NMJ) in myasthenia gravis, congenital myasthenic syndromes, and motor neuron diseases. Although the enhancement of AChR clustering at the NMJ is a promising therapeutic strategy for these maladies, no drug is currently available for this enhancement. We previously reported that zonisamide (ZNS), an anti-epileptic and anti-Parkinson's disease drug, enhances neurite elongation of the primary spinal motor neurons (SMNs). As nerve sprouting occurs to compensate for the loss of AChR clusters in human diseases, we examined the effects of ZNS on AChR clustering at the NMJ. To this end, we established a simple and quick co-culture system to reproducibly make in vitro NMJs using C2C12 myotubes and NSC34 motor neurons. ZNS at 1-20 μM enhanced the formation of AChR clusters dose-dependently in co-cultured C2C12 myotubes but not in agrin-treated single cultured C2C12 myotubes. We observed that molecules that conferred responsiveness to ZNS were not secreted into the co-culture medium. We found that 10 μM ZNS upregulated the expression of neuregulin-1 (Nrg1) in co-cultured cells but not in single cultured C2C12 myotubes or single cultured NSC34 motor neurons. In accordance with this observation, inhibition of the Nrg1/ErbB signaling pathways nullified the effect of 10 μM ZNS on the enhancement of AChR clustering in in vitro NMJs. Although agrin was not induced by 10 μM ZNS in co-cultured cells, anti-agrin antibody attenuated ZNS-mediated enhancement of AChR clustering. We conclude that ZNS enhances agrin-dependent AChR-clustering by upregulating the Nrg1/ErbB signaling pathways in the presence of NMJs.

Agonist efficiency from concentration-response curves: Structural implications and applications

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC₅₀) that indicates potency, a high-concentration asymptote that indicates efficacy, and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency (η), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that η can be calculated from EC₅₀ and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of η-values is bimodal with population means at 51% (including acetylcholine, nornicotine, and dimethylphenylpiperazinium) and 40% (including epibatidine, varenicline, and cytisine). The value of η is related inversely to the size of the agonist's headgroup, with high- versus low-efficiency ligands having an average volume of 70 vs. 102 Å³. Most binding site mutations have only a small effect on acetylcholine efficiency, except for αY190A (35%), αW149A (60%), and those at αG153 (42%). If η is known, the EC₅₀ and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC₅₀ of a CRC that has been normalized to 1. Given η, the level of constitutive activity can be estimated from a single CRC.

A novel fast-channel myasthenia caused by mutation in β subunit of AChR reveals subunit-specific contribution of the intracellular M1-M2 linker to channel gating

Genetic variants causing the fast-channel congenital myasthenic syndrome (CMS) have been identified in the α, δ, and ε but not the β subunit of acetylcholine receptor (AChR). A 16-year-old girl with severe myasthenia had low-amplitude and fast-decaying miniature endplate potentials. Mutation analysis revealed two heteroallelic variants in CHRNB1 encoding the AChR β subunit: a novel c.812C>T (p.P248L) variant in M1-M2 linker (p.P271L in HGVS nomenclature), and a ~430 bp deletion causing loss of exon 8 leading to frame-shift and a premature stop codon (p.G251Dfs*21). P248 is conserved in all β subunits of different species, but not in other AChR subunits. Measurements of radio-labeled α-bungarotoxin binding show that βP248L reduces AChR expression to 60% of wild-type. Patch clamp recordings of ACh-elicited single channel currents demonstrate that βP248L shortens channel opening bursts from 3.3 ms to 1.2 ms, and kinetic analyses predict that the decay of the synaptic response is accelerated 2.4-fold due to reduced probability of channel reopening. Substituting βP248 with threonine, alanine or glycine reduces the burst duration to 2.3, 1.7, and 1.5 ms, respectively. In non-β subunits, substituting leucine for residues corresponding to βP248 prolongs the burst duration to 4.5 ms in the α subunit, shortens it to 2.2 ms in the δ subunit, and has no effect in the ε subunit. Conversely, substituting proline for residues corresponding to βP248 prolongs the burst duration to 8.7 ms in the α subunit, to 4.6 ms in the δ subunit, but has no effect in the ε subunit. Thus, this fast channel CMS is caused by the dual defects of βP248L in reducing expression of the mutant receptor and accelerating the decay of the synaptic response. The results also reveal subunit-specific contributions of the M1-M2 linker to the durations of channel opening bursts.

Slow-channel myasthenia due to novel mutation in M2 domain of AChR delta subunit

OBJECTIVE

To characterize the molecular and phenotypic basis of a severe slow-channel congenital myasthenic syndrome (SCCMS).

METHODS

Intracellular and single-channel recordings from patient endplates; alpha-bungarotoxin binding studies; direct sequencing of AChR genes; microsatellite analysis; kinetic analysis of AChR activation; homology modeling of adult human AChR structure.

RESULTS

Among 24 variants reported to cause SCCMS only two appear in the AChR δ-subunit. We here report a 16-year-old patient harboring a novel δL273F mutation (δL294F in HGVS nomenclature) in the second transmembrane domain (M2) of the AChR δ subunit. Kinetic analyses with ACh and the weak agonist choline indicate that δL273F prolongs the channel opening bursts 9.4-fold due to a 75-fold increase in channel gating efficiency, whereas a previously identified εL269F mutation (εL289F in HGVS nomenclature) at an equivalent location in the AChR ε-subunit prolongs channel opening bursts 4.4-fold due to a 30-fold increase in gating efficiency. Structural modeling of AChR predicts that inter-helical hydrophobic interactions between the mutant residue in the δ and ε subunit and nearby M2 domain residues in neighboring α subunits contribute to structural stability of the open relative to the closed channel states.

INTERPRETATION

The greater increase in gating efficiency by δL273F than by εL269F explains why δL273F has more severe clinical effects. Both δL273F and εL269F impair channel gating by disrupting hydrophobic interactions with neighboring α-subunits. Differences in the extent of impairment of channel gating in δ and ε mutant receptors suggest unequal contributions of ε/α and δ/α subunit pairs to gating efficiency.

A 3D culture model of innervated human skeletal muscle enables studies of the adult neuromuscular junction

Two-dimensional (2D) human skeletal muscle fiber cultures are ill-equipped to support the contractile properties of maturing muscle fibers. This limits their application to the study of adult human neuromuscular junction (NMJ) development, a process requiring maturation of muscle fibers in the presence of motor neuron endplates. Here we describe a three-dimensional (3D) co-culture method whereby human muscle progenitors mixed with human pluripotent stem cell-derived motor neurons self-organize to form functional NMJ connections. Functional connectivity between motor neuron endplates and muscle fibers is confirmed with calcium imaging and electrophysiological recordings. Notably, we only observed epsilon acetylcholine receptor subunit protein upregulation and activity in 3D co-cultures. Further, 3D co-culture treatments with myasthenia gravis patient sera shows the ease of studying human disease with the system. Hence, this work offers a simple method to model and evaluate adult human NMJ de novo development or disease in culture.

A single molecular distance predicts agonist binding energy in nicotinic receptors

Agonists turn on receptors because they bind more strongly to active (R*) versus resting (R) conformations of their target sites. Here, to explore how agonists activate neuromuscular acetylcholine receptors, we built homology models of R and R* neurotransmitter binding sites, docked ligands to those sites, ran molecular dynamics simulations to relax ("equilibrate") the structures, measured binding site structural parameters, and correlated them with experimental agonist binding energies. Each binding pocket is a pyramid formed by five aromatic amino acids and covered partially by loop C. We found that in R* versus R, loop C is displaced outward, the pocket is smaller and skewed, the agonist orientation is reversed, and a key nitrogen atom in the agonist is closer to the pocket center (distance dx) and a tryptophan pair but farther from αY190. Of these differences, the change in dx shows the largest correlation with experimental binding energy and provides a good estimate of agonist affinity, efficacy, and efficiency. Indeed, concentration-response curves can be calculated from just dx values. The contraction and twist of the binding pocket upon activation resemble gating rearrangements of the extracellular domain of related receptors at a smaller scale.

Effects of Quinine, Quinidine and Chloroquine on Human Muscle Nicotinic Acetylcholine Receptors

The genus Cinchona is known for a range of alkaloids, such as quinine, quinidine, cinchonine, and cinchonidine. Cinchona bark has been used as an antimalarial agent for more than 400 years. Quinine was first isolated in 1820 and is still acknowledged in the therapy of chloroquine-resistant falciparum malaria; in lower dosage quinine has been used as treatment for leg cramps since the 1940s. Here we report the effects of the quinoline derivatives quinine, quinidine, and chloroquine on human adult and fetal muscle nicotinic acetylcholine receptors (nAChRs). It could be demonstrated that the compounds blocked acetylcholine (ACh)-evoked responses in Xenopus laevis oocytes expressing the adult nAChR composed of αβ𝜀δ subunits in a concentration-dependent manner, with a ranked potency of quinine (IC₅₀ = 1.70 μM), chloroquine (IC₅₀ = 2.22 μM) and quinidine (IC₅₀ = 3.96 μM). At the fetal nAChR composed of αβγδ subunits, the IC₅₀ for quinine was found to be 2.30 μM. The efficacy of the block by quinine was independent of the ACh concentration. Therefore, quinine is proposed to inhibit ACh-evoked currents in a non-competitive manner. The present results add to the pharmacological characterization of muscle nAChRs and indicate that quinine is effective at the muscular nAChRs close to therapeutic blood concentrations required for the therapy and prophylaxis of nocturnal leg cramps, suggesting that the clinically proven efficacy of quinine could be based on targeting nAChRs.

Congenital Myasthenic Syndromes in 2018

PURPOSE OF REVIEW

Summarize features of the currently recognized congenital myasthenic syndromes (CMS) with emphasis on novel findings identified in the past 6 years.

RECENT FINDINGS

Since the last review of the CMS in this journal in 2012, several novel CMS were identified. The identified disease proteins are SNAP25B, synaptotagmin 2, Munc13-1, synaptobrevin-1, GFPT1, DPAGT1, ALG2, ALG14, Agrin, GMPPB, LRP4, myosin 9A, collagen 13A1, the mitochondrial citrate carrier, PREPL, LAMA5, the vesicular ACh transporter, and the high-affinity presynaptic choline transporter. Exome sequencing has provided a powerful tool for identifying novel CMS. Identifying the disease genes is essential for determining optimal therapy. The landscape of the CMS is still unfolding.

The Neuromuscular Junction and Wide Heterogeneity of Congenital Myasthenic Syndromes

Congenital myasthenic syndromes (CMS) are genetic disorders characterised by impaired neuromuscular transmission. This review provides an overview on CMS and highlights recent advances in the field, including novel CMS causative genes and improved therapeutic strategies. CMS due to mutations in SLC5A7 and SLC18A3, impairing the synthesis and recycling of acetylcholine, have recently been described. In addition, a novel group of CMS due to mutations in SNAP25B, SYT2, VAMP1, and UNC13A1 encoding molecules implicated in synaptic vesicles exocytosis has been characterised. The increasing number of presynaptic CMS exhibiting CNS manifestations along with neuromuscular weakness demonstrate that the myasthenia can be only a small part of a much more extensive disease phenotype. Moreover, the spectrum of glycosylation abnormalities has been increased with the report that GMPPB mutations can cause CMS, thus bridging myasthenic disorders with dystroglycanopathies. Finally, the discovery of COL13A1 mutations and laminin α5 deficiency has helped to draw attention to the role of extracellular matrix proteins for the formation and maintenance of muscle endplates. The benefit of β2-adrenergic agonists alone or combined with pyridostigmine or 3,4-Dyaminopiridine is increasingly being reported for different subtypes of CMS including AChR-deficiency and glycosylation abnormalities, thus expanding the therapeutic repertoire available.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

A common CHRNE mutation in Brazilian patients with congenital myasthenic syndrome

The most common causes of congenital myasthenic syndromes (CMS) are CHRNE mutations, and some pathogenic allelic variants in this gene are especially frequent in certain ethnic groups. In the southern region of Brazil, a study found the c.130dupG CHRNE mutation in up to 33% of families with CMS. Here, we aimed to verify the frequency of this mutation among individuals with CMS in a larger cohort of CMS patients from different areas of Brazil and to characterize clinical features of these patients. Eighty-four patients with CMS, from 72 families, were clinically evaluated and submitted to direct sequencing of the exon 2 of CHRNE. The c.130dupG mutation was found in 32 patients (23 families), with 26 patients (19 families, 26.3%) in homozygosis, confirming its high prevalence in different regions of Brazil. Among the homozygous patients, the following characteristics were frequent: onset of symptoms before 2 years of age (92.3%), little functional restriction (92.3%), fluctuating symptoms (100%), ocular muscle impairment (96.1%), ptosis (100%), limb weakness (88.4%), response to pyridostigmine (100%), facial involvement (77%), and bulbar symptoms (70.8%). The pretest probability of finding at least one allele harbouring the c.130dupG mutation was 38.1%. Selecting only patients with impaired eye movement together with limb weakness and improvement with pyridostigmine, the probability increases to 72.2%. This clinical pre-selection of patients is likely a useful tool for regions where CHRNE mutations have a founder effect. In conclusion, the CHRNE mutation c.130dupG leads to fairly benign natural course of the disease with relative homogeneity.

Mutations causing congenital myasthenia reveal principal coupling pathway in the acetylcholine receptor ε-subunit

We identify 2 homozygous mutations in the ε-subunit of the muscle acetylcholine receptor (AChR) in 3 patients with severe congenital myasthenia: εR218W in the pre-M1 region in 2 patients and εE184K in the β8-β9 linker in 1 patient. Arg218 is conserved in all eukaryotic members of the Cys-loop receptor superfamily, while Glu184 is conserved in the α-, δ-, and ε-subunits of AChRs from all species. εR218W reduces channel gating efficiency 338-fold and AChR expression on the cell surface 5-fold, whereas εE184K reduces channel gating efficiency 11-fold but does not alter AChR cell surface expression. Determinations of the effective channel gating rate constants, combined with mutant cycle analyses, demonstrate strong energetic coupling between εR218 and εE184, and between εR218 and εE45 from the β1-β2 linker, as also observed for equivalent residues in the principal coupling pathway of the α-subunit. Thus, efficient and rapid gating of the AChR channel is achieved not only by coupling between conserved residues within the principal coupling pathway of the α-subunit, but also between corresponding residues in the ε-subunit.

Congenital myasthenic syndrome with episodic apnoea: clinical, neurophysiological and genetic features in the long-term follow-up of 19 patients

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.

Genetic basis and phenotypic features of congenital myasthenic syndromes

The congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. The disease proteins reside in the nerve terminal, the synaptic basal lamina, or in the postsynaptic region, or at multiple sites at the neuromuscular junction as well as in other tissues. Targeted mutation analysis by Sanger or exome sequencing has been facilitated by characteristic phenotypic features of some CMS. No fewer than 20 disease genes have been recognized to date. In one-half of the currently identified probands, the disease stems from mutations in genes encoding subunits of the muscle form of the acetylcholine receptor (CHRNA1, CHRNB, CHRNAD1, and CHRNE). In 10-14% of the probands the disease is caused by mutations in RAPSN, DOK 7, or COLQ, and in 5% by mutations in CHAT. Other less frequently identified disease genes include LAMB2, AGRN, LRP4, MUSK, GFPT1, DPAGT1, ALG2, and ALG 14 as well as SCN4A, PREPL, PLEC1, DNM2, and MTM1. Identification of the genetic basis of each CMS is important not only for genetic counseling and disease prevention but also for therapy, because therapeutic agents that benefit one type of CMS can be harmful in another.

New compound heterozygous variants of the cholinergic receptor nicotinic delta subunit gene in a Chinese male with congenital myasthenic syndrome: A case report

INTRODUCTION

Congenital myasthenic syndromes (CMS) are a group of genetic disorders that stem mostly from molecular defects in nicotinic acetylcholine receptors (AChRs). Defects in the cholinergic receptor nicotinic delta subunit (CHRND) gene can cause a series of myasthenic syndromes. Here, we report 2 new compound heterozygous variants of the CHRND gene in a Chinese male with CMS.

CASE PRESENTATION

A 43-year-old Chinese male presented with progressive muscle weakness, difficulty chewing, and an inability to lift his head from the time he was 8 years old. He was treated with pyridostigmine, which was partially effective. Two weeks prior, he was hospitalized for dyspnea. Upon examination, he was unable to drum his cheeks and exhibited fatigable muscle weakness and facial muscle atrophy. Sequencing of his exome revealed 2 previously unreported mutations in CHRND, c.59G>A (exon2) and c.423G>C (exon5).

CONCLUSIONS

We identified a new mutational site that contributes to the onset of CMS.

Genetics update: Monogenetics, polygene disorders and the quest for modifying genes

The genetic channelopathies are a broad collection of diseases. Many ion channel genes demonstrate wide phenotypic pleiotropy, but nonetheless concerted efforts have been made to characterise genotype-phenotype relationships. In this review we give an overview of the factors that influence genotype-phenotype relationships across this group of diseases as a whole, using specific individual channelopathies as examples. We suggest reasons for the limitations observed in these relationships. We discuss the role of ion channel variation in polygenic disease and highlight research that has contributed to unravelling the complex aetiological nature of these conditions. We focus specifically on the quest for modifying genes in inherited channelopathies, using the voltage-gated sodium channels as an example. Epilepsy related to genetic channelopathy is one area in which precision medicine is showing promise. We will discuss the successes and limitations of precision medicine in these conditions. This article is part of the Special Issue entitled 'Channelopathies.'

Agrin-LRP4-MuSK signaling as a therapeutic target for myasthenia gravis and other neuromuscular disorders

INTRODUCTION

Signal transduction at the neuromuscular junction (NMJ) is compromised in a diverse array of diseases including myasthenia gravis, Lambert-Eaton myasthenic syndrome, Isaacs' syndrome, congenital myasthenic syndromes, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. Except for sarcopenia, all are orphan diseases. In addition, the NMJ signal transduction is impaired by tetanus, botulinum, curare, α-bungarotoxin, conotoxins, organophosphate, sarin, VX, and soman to name a few. Areas covered: This review covers the agrin-LRP4-MuSK signaling pathway, which drives clustering of acetylcholine receptors (AChRs) and ensures efficient signal transduction at the NMJ. We also address diseases caused by autoantibodies against the NMJ molecules and by germline mutations in genes encoding the NMJ molecules. Expert opinion: Representative small compounds to treat the defective NMJ signal transduction are cholinesterase inhibitors, which exert their effects by increasing the amount of acetylcholine at the synaptic space. Another possible therapeutic strategy to enhance the NMJ signal transduction is to increase the number of AChRs, but no currently available drug has this functionality.

A mechanism for acetylcholine receptor gating based on structure, coupling, phi, and flip

Gupta et al. use single-channel electrophysiology to investigate the gating mechanism of acetylcholine receptor ion channels. They propose that channel opening starts at the M2–M3 linker and ligand-binding sites and proceeds through four brief intermediate conformations before ending with the collapse of a gate bubble.

Nicotinic acetylcholine receptors are allosteric proteins that generate membrane currents by isomerizing (“gating”) between resting and active conformations under the influence of neurotransmitters. Here, to explore the mechanisms that link the transmitter-binding sites (TBSs) with the distant gate, we use mutant cycle analyses to measure coupling between residue pairs, phi value analyses to sequence domain rearrangements, and current simulations to reproduce a microsecond shut component (“flip”) apparent in single-channel recordings. Significant interactions between amino acids separated by >15 Å are rare; an exception is between the αM2–M3 linkers and the TBSs that are ∼30 Å apart. Linker residues also make significant, local interactions within and between subunits. Phi value analyses indicate that without agonists, the linker is the first region in the protein to reach the gating transition state. Together, the phi pattern and flip component suggest that a complete, resting↔active allosteric transition involves passage through four brief intermediate states, with brief shut events arising from sojourns in all or a subset. We derive energy landscapes for gating with and without agonists, and propose a structure-based model in which resting→active starts with spontaneous rearrangements of the M2–M3 linkers and TBSs. These conformational changes stabilize a twisted extracellular domain to promote transmembrane helix tilting, gate dilation, and the formation of a “bubble” that collapses to initiate ion conduction. The energy landscapes suggest that twisting is the most energetically unfavorable step in the resting→active conformational change and that the rate-limiting step in the reverse process is bubble formation.

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.

REVIEW ■ : Molecular Basis of Congenital Myasthenic Syndromes: Mutations in the Acetylcholine Receptor

The congenital myasthenic syndromes include end-plate (EP) acetylcholinesterase deficiency, presynaptic abnormalities affecting the evoked release or size of transmitter quanta, and acetylcholine (ACh) receptor (AChR) channelopathies stemming from a kinetic abnormality and/or deficiency of AChR. A kinetic abnor mality predicts, and AChR deficiency may predict, one or more mutations in an AChR subunit gene. These clues have led to the identification of 53 mutations in different subunits of AChR in 55 kinships of the congenital myasthenic syndromes. The mutations either increase or decrease the response to ACh, produce AChR deficiency, or both. In the slow-channel syndromes, prolonged opening episodes of AChR cause cationic overloading of the EP and an EP myopathy; the mutations occur in different subunits and different domains of the subunits and have dominant positive effects. The M1 and M2 mutations slow channel closure, increase apparent affinity for ACh, and variably enhance desensitization, and the extracellular αG153S enhances affinity for ACh, promoting reopening of the diliganded receptor. In the low-affinity fast-channel syndrome, εP121L reduces affinity for ACh and reopening of the diliganded receptor, resulting in a de creased response to ACh and shorter burst durations. Severe EP AChR deficiency results from heterozy gous or homozygous mutations that terminate translation prematurely; these are concentrated in the ε subunit, probably because substitution of the fetal γ for the adult ε subunit can rescue the phenotype from fatal null mutations in ε. Variable AChR deficiency and variable functional abnormalities stem from hetero allelic nonsense and missense mutations in AChR subunit genes. NEUROSCIENTIST 4:185-194, 1998

Investigation of Congenital Myasthenia Reveals Functional Asymmetry of Invariant Acetylcholine Receptor (AChR) Cys-loop Aspartates

We identify two heteroallelic mutations in the acetylcholine receptor δ-subunit from a patient with severe myasthenic symptoms since birth: a novel δD140N mutation in the signature Cys-loop and a mutation in intron 7 of the δ-subunit gene that disrupts splicing of exon 8. The mutated Asp residue, which determines the disease phenotype, is conserved in all eukaryotic members of the Cys-loop receptor superfamily. Studies of the mutant acetylcholine receptor expressed in HEK 293 cells reveal that δD140N attenuates cell surface expression and apparent channel gating, predicting a reduced magnitude and an accelerated decay of the synaptic response, thus reducing the safety margin for neuromuscular transmission. Substituting Asn for Asp at equivalent positions in the α-, β-, and ϵ-subunits also suppresses apparent channel gating, but the suppression is much greater in the α-subunit. Mutant cycle analysis applied to single and pairwise mutations reveals that αAsp-138 is energetically coupled to αArg-209 in the neighboring pre-M1 domain. Our findings suggest that the conserved αAsp-138 and αArg-209 contribute to a principal pathway that functionally links the ligand binding and pore domains.

In vivo functional analysis of the Drosophila melanogaster nicotinic acetylcholine receptor Dα6 using the insecticide spinosad

The vinegar fly, Drosophila melanogaster, has been used to identify and manipulate insecticide resistance genes. The advancement of genome engineering technology and the increasing availability of pest genome sequences has increased the predictive and diagnostic capacity of the Drosophila model. The Drosophila model can be extended to investigate the basic biology of the interaction between insecticides and the proteins they target. Recently we have developed an in vivo system that permits the expression and study of key insecticide targets, the nicotinic acetylcholine receptors (nAChRs), in controlled genetic backgrounds. Here this system is used to study the interaction between the insecticide spinosad and a nAChR subunit, Dα6. Reciprocal chimeric subunits were created from Dα6 and Dα7, a subunit that does not respond to spinosad. Using the in vivo system, the Dα6/Dα7 chimeric subunits were tested for their capacity to respond to spinosad. Only the subunits containing the C-terminal region of Dα6 were able to respond to spinosad, thus confirming the importance this region for spinosad binding. A new incompletely dominant, spinosad resistance mechanism that may evolve in pest species is also examined. First generated using chemical mutagenesis, the Dα6(P146S) mutation was recreated using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 system, the first use of this technology to introduce a resistant mutation into a controlled genetic background. Both alleles present with the same incompletely dominant, spinosad resistance phenotype, proving the P146S replacement to be the causal mutation. The proximity of the P146S mutation to the conserved Cys-loop indicates that it may impair the gating of the receptor. The results of this study enhance the understanding of nAChR structure:function relationships.

Activation of endplate nicotinic acetylcholine receptors by agonists

The interaction of a small molecule made in one cell with a large receptor made in another is the signature event of cell signaling. Understanding the structure and energy changes associated with agonist activation is important for engineering drugs, receptors and synapses. The nicotinic acetylcholine receptor (AChR) is a ∼300kD ion channel that binds the neurotransmitter acetylcholine (ACh) and other cholinergic agonists to elicit electrical responses in the central and peripheral nervous systems. This mini-review is in two sections. First, general concepts of skeletal muscle AChR operation are discussed in terms of energy landscapes for conformational change. Second, adult vs. fetal AChRs are compared with regard to interaction energies between ACh and agonist-site side chains, measured by single-channel electrophysiology and molecular dynamics simulations. The five aromatic residues that form the core of each agonist binding site can be divided into two working groups, a triad (led by αY190) that behaves similarly at all sites and a coupled pair (led by γW55) that has a large influence on affinity only in fetal AChRs. Each endplate AChR has 5 homologous subunits, two of α(1) and one each of β, δ, and either γ (fetal) or ϵ (adult). These nicotinic AChRs have only 2 functional agonist binding sites located in the extracellular domain, at αδ and either αγ or αϵ subunit interfaces. The receptor undergoes a reversible, global isomerization between structures called C and O. The C shape does not conduct ions and has a relatively low affinity for ACh, whereas O conducts cations and has a higher affinity. When both agonist sites are empty (filled only with water) the probability of taking on the O conformation (PO) is low, <10(-6). When ACh molecules occupy the agonist sites the C→O opening rate constant and C↔O gating equilibrium constant increase dramatically. Following a pulse of ACh at the nerve-muscle synapse, the endplate current rises rapidly to reach a peak that corresponds to PO ∼0.96.

Congenital myasthenic syndromes: pathogenesis, diagnosis, and treatment

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.

Congenital myasthenic syndrome in Japan: ethnically unique mutations in muscle nicotinic acetylcholine receptor subunits

Congenital myasthenic syndromes (CMS) are caused by mutations in genes expressed at the neuromuscular junction. Most CMS patients have been reported in Western and Middle Eastern countries, and only four patients with COLQ mutations have been reported in Japan. We here report six mutations in acetylcholine receptor (AChR) subunit genes in five Japanese patients. Five mutations are novel, and one mutation is shared with a European American patient but with a different haplotype. Among the observed mutations, p.Thr284Pro (p.Thr264Pro according to the legacy annotation) in the epsilon subunit causes a slow-channel CMS. Five other mutations in the delta and epsilon subunits are splice site, frameshift, null, or missense mutations causing endplate AChR deficiency. We also found a heteroallelic p.Met465Thr in the beta subunit in another patient. p.Met465Thr, however, was likely to be polymorphism, because single channel recordings showed mild shortening of channel openings without affecting cell surface expression of AChR, and the minor allelic frequency of p.Met465Thr was 5.1% in the Japanese population. Lack of shared mutant alleles between the Japanese and the other patients suggests that most mutations described here are ethnically unique or de novo in each family.

Clinical Impact and Cost-Effectiveness of Whole Exome Sequencing as a Diagnostic Tool: A Pediatric Center's Experience

BACKGROUND

There are limited reports of the use of whole exome sequencing (WES) as a clinical diagnostic tool. Moreover, there are no reports addressing the cost burden associated with genetic tests performed prior to WES.

OBJECTIVE

We demonstrate the performance characteristics of WES in a pediatric setting by describing our patient cohort, calculating the diagnostic yield, and detailing the patients for whom clinical management was altered. Moreover, we examined the potential cost-effectiveness of WES by examining the cost burden of diagnostic workups.

METHODS

To determine the clinical utility of our hospital's clinical WES, we performed a retrospective review of the first 40 cases. We utilized dual bioinformatics analyses pipelines based on commercially available software and in-house tools.

RESULTS

Of the first 40 clinical cases, we identified genetic defects in 12 (30%) patients, of which 47% of the mutations were previously unreported in the literature. Among the 12 patients with positive findings, seven have autosomal dominant disease and five have autosomal recessive disease. Ninety percent of the cohort opted to receive secondary findings and of those, secondary medical actionable results were returned in three cases. Among these positive cases, there are a number of novel mutations that are being reported here. The diagnostic workup included a significant number of genetic tests with microarray and single-gene sequencing being the most popular tests. Significantly, genetic diagnosis from WES led to altered patient medical management in positive cases.

CONCLUSION

We demonstrate the clinical utility of WES by establishing the clinical diagnostic rate and its impact on medical management in a large pediatric center. The cost-effectiveness of WES was demonstrated by ending the diagnostic odyssey in positive cases. Also, in some cases it may be most cost-effective to directly perform WES. WES provides a unique glimpse into the complexity of genetic disorders.

A single mutation in the acetylcholine receptor δ-subunit causes distinct effects in two types of neuromuscular synapses

Mutations in AChR subunits, expressed as pentamers in neuromuscular junctions (NMJs), cause various types of congenital myasthenic syndromes. In AChR pentamers, the adult ε subunit gradually replaces the embryonic γ subunit as the animal develops. Because of this switch in subunit composition, mutations in specific subunits result in synaptic phenotypes that change with developmental age. However, a mutation in any AChR subunit is considered to affect the NMJs of all muscle fibers equally. Here, we report a zebrafish mutant of the AChR δ subunit that exhibits two distinct NMJ phenotypes specific to two muscle fiber types: slow or fast. Homozygous fish harboring a point mutation in the δ subunit form functional AChRs in slow muscles, whereas receptors in fast muscles are nonfunctional. To test the hypothesis that different subunit compositions in slow and fast muscles underlie distinct phenotypes, we examined the presence of ε/γ subunits in NMJs using specific antibodies. Both wild-type and mutant larvae lacked ε/γ subunits in slow muscle synapses. These findings in zebrafish suggest that some mutations in human congenital myasthenic syndromes may affect slow and fast muscle fibers differently.

Fast-channel congenital myasthenic syndrome with a novel acetylcholine receptor mutation at the α-ε subunit interface

Congenital myasthenic syndromes (CMS) result from the failure to achieve muscle depolarisation due to disorders in the structure and/or function of the neuromuscular synapse. Mutations of the nicotinic acetylcholine receptor (nAChR) form a major subset of CMS. We describe a patient who presented with recurrent apnoeic crises in the neonatal period requiring ventilator support. Electromyography revealed compound muscle action potential decrement upon repetitive stimulation. Sequencing of nAChR subunit genes revealed two missense mutations. One previously reported null mutation p.εTyr15His, and a second novel missense mutation, p.εThr38Lys, that is well expressed in mammalian cell culture and thus likely to exert its effect via alteration of ion channel kinetics. Functional analysis revealed abbreviated ion channel bursts characteristic of a fast channel CMS. The mutation p.εThr38Lys occurs at the interface between the α and ε subunits of the nAChR pentamer and leads to instability of the open channel. The effects of this mutation on channel function were investigated in relation to other fast channel mutants at an analogous subunit interface within the nAChR pentamer. Fast channel syndromes are frequently characterised by severe myasthenic weakness with apnoeic crises; knowledge of the underlying mutation and its functional consequences can be vital for appropriate therapy and patient management.

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

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

Nicotinic receptor transduction zone: invariant arginine couples to multiple electron-rich residues

Gating of the muscle-type acetylcholine receptor (AChR) channel depends on communication between the ACh-binding site and the remote ion channel. A key region for this communication is located within the structural transition zone between the ligand-binding and pore domains. Here, stemming from β-strand 10 of the binding domain, the invariant αArg209 lodges within the hydrophobic interior of the subunit and is essential for rapid and efficient channel gating. Previous charge-reversal experiments showed that the contribution of αArg209 to channel gating depends strongly on αGlu45, also within this region. Here we determine whether the contribution of αArg209 to channel gating depends on additional anionic or electron-rich residues in this region. Also, to reconcile diverging findings in the literature, we compare the dependence of αArg209 on αGlu45 in AChRs from different species, and compare the full agonist ACh with the weak agonist choline. Our findings reveal that the contribution of αArg209 to channel gating depends on additional nearby electron-rich residues, consistent with both electrostatic and steric contributions. Furthermore, αArg209 and αGlu45 show a strong interdependence in both human and mouse AChRs, whereas the functional consequences of the mutation αE45R depend on the agonist. The emerging picture shows a multifaceted network of interdependent residues that are required for communication between the ligand-binding and pore domains.

Function of interfacial prolines at the transmitter-binding sites of the neuromuscular acetylcholine receptor

The neuromuscular acetylcholine (ACh) receptor has two conserved prolines in loop D of the complementary subunit at each of its two transmitter-binding sites (α-ε and α-δ). We used single-channel electrophysiology to estimate the energy changes caused by mutations of these prolines with regard to unliganded gating (ΔG0) and the affinity change for ACh that increases the open channel probability (ΔGB). The effects of mutations of ProD2 (εPro-121/δPro-123) were greater than those of its neighbor (εPro-120/δPro-122) and were greater at α-ε versus α-δ. The main consequence of the congenital myasthenic syndrome mutation εProD2-L was to impair the establishment of a high affinity for ACh and thus make ΔGB less favorable. At both binding sites, most ProD2 mutations decreased constitutive activity (increased ΔG0). LRYHQG and RL substitutions reduced substantially the net binding energy (made ΔGB(ACh) less favorable) by ≥2 kcal/mol at α-ε and α-δ, respectively. Mutant cycle analyses were used to estimate energy coupling between the two ProD2 residues and between each ProD2 and glycine residues (αGly-147 and αGly-153) on the primary (α subunit) side of each binding pocket. The distant binding site prolines interact weakly. ProD2 interacts strongly with αGly-147 but only at α-ε and only when ACh is present. The results suggest that in the low to-high affinity change there is a concerted inter-subunit strain in the backbones at εProD2 and αGly-147. It is possible to engineer receptors having a single functional binding site by using a α-ε or α-δ ProD2-R knock-out mutation. In adult-type ACh receptors, the energy from the affinity change for ACh is approximately the same at the two binding sites (approximately -5 kcal/mol).

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

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

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

A novel congenital myasthenic syndrome due to decreased acetylcholine receptor ion-channel conductance

Muscle acetylcholine receptor ion channels mediate neurotransmission by depolarizing the postsynaptic membrane at the neuromuscular junction. Inherited disorders of neuromuscular transmission, termed congenital myasthenic syndromes, are commonly caused by mutations in genes encoding the five subunits of the acetylcholine receptor that severely reduce endplate acetylcholine receptor numbers and/or cause kinetic abnormalities of acetylcholine receptor function. We tracked the cause of the myasthenic disorder in a female with onset of first symptoms at birth, who displayed mildly progressive bulbar, respiratory and generalized limb weakness with ptosis and ophthalmoplegia. Direct DNA sequencing revealed heteroallelic mutations in exon 8 of the acetylcholine receptor ε-subunit gene. Two alleles were identified: one with the missense substitution p.εP282R, and the second with a deletion, c.798_800delCTT, which result in the loss of a single amino acid, residue F266, within the M2 transmembrane domain. When these acetylcholine receptor mutations were expressed in HEK 293 cells, the p.εP282R mutation caused severely reduced expression on the cell surface, whereas p.εΔF266 gave robust surface expression. Single-channel analysis for p.εΔF266 acetylcholine receptor channels showed the longest burst duration population was not different from wild-type acetylcholine receptor (4.39 ± 0.6 ms versus 4.68 ± 0.7 ms, n = 5 each) but that the amplitude of channel openings was reduced. Channel amplitudes at different holding potentials showed that single-channel conductance was significantly reduced in p.εΔF266 acetylcholine receptor channels (42.7 ± 1.4 pS, n = 8, compared with 70.9 ± 1.6 pS for wild-type, n = 6). Although a phenylalanine residue at this position within M2 is conserved throughout ligand-gated excitatory cys-loop channel subunits, deletion of equivalent residues in the other subunits of muscle acetylcholine receptor did not have equivalent effects. Modelling the impact of p.εΔF266 revealed only a minor alteration to channel structure. In this study we uncover the novel mechanism of reduced acetylcholine receptor channel conductance as an underlying cause of congenital myasthenic syndrome, with the 'low conductance' phenotype that results from the p.εΔF266 deletion mutation revealed by the coinheritance of the low-expressor mutation p.εP282R.

Current status of the congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Clinical, electrophysiologic, and morphologic studies have paved the way for detecting CMS-related mutations in proteins residing in the nerve terminal, the synaptic basal lamina, and in the postsynaptic region of the motor endplate. The disease proteins identified to date include choline acetyltransferase (ChAT), the endplate species of acetylcholinesterase (AChE), β2-laminin, the acetylcholine receptor (AChR), rapsyn, plectin, Na(v)1.4, the muscle specific protein kinase (MuSK), agrin, downstream of tyrosine kinase 7 (Dok-7), and glutamine-fructose-6-phosphate transaminase 1 (GFPT1). Myasthenic syndromes associated with centronuclear myopathies were recently recognized. Analysis of properties of expressed mutant proteins contributed to finding improved therapy for most CMS. Despite these advances, the molecular basis of some phenotypically characterized CMS remains elusive. Moreover, other types of CMS and disease genes likely exist and await discovery.

Clinical features in a series of fast channel congenital myasthenia syndrome

Fast channel congenital myasthenic syndromes are rare, but frequently result in severe weakness. We report a case of 12 fast channel patients to highlight clinical features and management difficulties. Patients were diagnosed through genetic screening and identification of mutations shown to cause fast channel syndrome. Data was obtained from clinical notes, history, examination and follow up. Patterns of muscle weakness involved limb, trunk, bulbar, respiratory, facial and extraocular muscles. Patients responded to treatment with anticholinesterase medication and 3,4-diaminopyridine. Fast channel syndrome contrasted with AChR deficiency in the occurrence of severe respiratory crises in infancy and childhood. The death of two children even when on treatment and the family histories of sibling deaths re-inforces the need for accurate genetic diagnosis, optimised pharmacological treatment and additional supportive measures to manage acute respiratory crises. Referral to a specialist paediatric respiratory centre and regular resuscitation training for parents are recommended.

Hexosamine biosynthetic pathway mutations cause neuromuscular transmission defect

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.

Novel mouse model reveals distinct activity-dependent and -independent contributions to synapse development

The balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholine receptor (AChR) aggregation, guide ingrowing axons and contribute to correct synaptic patterning are unknown. Synaptic activity is of central importance and to understand synaptogenesis, it is necessary to distinguish between activity-dependent and activity-independent processes. By engineering a mutated fetal AChR subunit, we used homologous recombination to develop a mouse line that expresses AChR with massively reduced open probability during embryonic development. Through histological and immunochemical methods as well as electrophysiological techniques, we observed that endplate anatomy and distribution are severely aberrant and innervation patterns are completely disrupted. Nonetheless, in the absence of activity AChRs form postsynaptic specializations attracting motor axons and permitting generation of multiple nerve/muscle contacts on individual fibers. This process is not restricted to a specialized central zone of the diaphragm and proceeds throughout embryonic development. Phenotypes can be attributed to separate activity-dependent and -independent pathways. The correct patterning of synaptic connections, prevention of multiple contacts and control of nerve growth require AChR-mediated activity. In contrast, myotube survival and acetylcholine-mediated dispersal of AChRs are maintained even in the absence of AChR-mediated activity. Because mouse models in which acetylcholine is entirely absent do not display similar effects, we conclude that acetylcholine binding to the AChR initiates activity-dependent and activity-independent pathways whereby the AChR modulates formation of the NMJ.

Localization by site-directed mutagenesis of a galantamine binding site on α7 nicotinic acetylcholine receptor extracellular domain

Galantamine is an approved drug treatment for Alzheimer's disease. Initially identified as a weak cholinesterase inhibitor, we have established that galantamine mainly acts as an 'allosterically potentiating ligand (APL)' of nicotinic acetylcholine receptors (nAChR). Meanwhile other 'positive allosteric modulators (PAM)' of nAChR channel activity have been discovered, and for one of them a binding site within the transmembrane domain has been proposed. Here we show, by performing site-directed mutagenesis studies of ectopically expressed chimeric chicken α7/mouse 5-hydroxytryptamine 3 receptor-channel complex, in combination with whole-cell current measurements, in the presence and absence of galantamine, that the APL binding site is different from the proposed PAM binding site. We demonstrate that residues T197, I196, and F198 of ß-strand 10 represent major elements of the galantamine binding site. Residue K123, earlier suggested as being 'close to' the APL binding site, is not part of this site but rather appears to play a role in coupling of agonist binding to channel opening and closing. Our data confirm our earlier results that the galantamine binding site is different from the ACh binding site. Both sites are in close proximity and hence may influence each other in a synergistic fashion. Other interesting areas identified in the present study are a 'hinge' region around and containing residues F122, K123, and K143 possibly being involved in relaying the signal of agonist binding to gating of the transmembrane channel, and a 'folding centre', with P119 as the dominating residue, that crucially positions the agonist binding site with respect to the hinge region.