Congenital myasthenic syndromes: genetic defects of the neuromuscular junction

Congenital Myasthenic Syndrome: Spectrum of Mutations in an Indian Cohort

OBJECTIVES

To investigate the mutational spectrum and genotype-phenotype correlation in Indian patients with congenital myasthenic syndrome (CMS), using next-generation sequencing of 5 genes.

METHODS

CHRNE, COLQ, DOK7, RAPSN, and GFPT1 were sequenced in 25 affected patients.

RESULTS

We found clinically significant variants in 18 patients, of which variants in CHRNE were the most common, and 9 were novel. A common pathogenic COLQ variant was also detected in 4 patients with isolated limb-girdle congenital myasthenia.

CONCLUSIONS

Targeted screening of 5 genes is an effective alternate test for CMS, and an affordable one even in a developing country such as India. In addition, we recommend that patients with isolated limb-girdle congenital myasthenia be screened initially for the common COLQ pathogenic variant. This study throws the first light on the genetic landscape of CMSs in India.

Truncating SLC5A7 mutations underlie a spectrum of dominant hereditary motor neuropathies

Objective

To identify the genetic cause of disease in 2 previously unreported families with forms of distal hereditary motor neuropathies (dHMNs).

Methods

The first family comprises individuals affected by dHMN type V, which lacks the cardinal clinical feature of vocal cord paralysis characteristic of dHMN-VII observed in the second family. Next-generation sequencing was performed on the proband of each family. Variants were annotated and filtered, initially focusing on genes associated with neuropathy. Candidate variants were further investigated and confirmed by dideoxy sequence analysis and cosegregation studies. Thorough patient phenotyping was completed, comprising clinical history, examination, and neurologic investigation.

Results

dHMNs are a heterogeneous group of peripheral motor neuron disorders characterized by length-dependent neuropathy and progressive distal limb muscle weakness and wasting. We previously reported a dominant-negative frameshift mutation located in the concluding exon of the SLC5A7 gene encoding the choline transporter (CHT), leading to protein truncation, as the likely cause of dominantly-inherited dHMN-VII in an extended UK family. In this study, our genetic studies identified distinct heterozygous frameshift mutations located in the last coding exon of SLC5A7, predicted to result in the truncation of the CHT C-terminus, as the likely cause of the condition in each family.

Conclusions

This study corroborates C-terminal CHT truncation as a cause of autosomal dominant dHMN, confirming upper limb predominating over lower limb involvement, and broadening the clinical spectrum arising from CHT malfunction.

Optimization of the choline transporter (CHT) inhibitor ML352: Development of VU6001221, an improved in vivo tool compound

This Letter describes the further lead optimization of the CHT inhibitor probe, ML352 (VU0476201), and the development of VU6001221, an improved in vivo tool. A multi-dimensional optimization effort encountered steep SAR, and ultimately, subtle tuning of the electronics of the central phenyl core provided VU6001221, a CHT inhibitor with comparable potency for choline uptake inhibition as ML352, yet improved PK and CNS penetration. Moreover, VU6001221 enabled evaluation, for the first time, of a CHT inhibitor in a standard preclinical rodent cognition model, novel object recognition (NOR). We observed VU6001221 to elicit a dose-responsive increase in NOR, raising the possibility of agonism of synaptic α7 nicotinic ACh receptors by elevated extracellular choline, that if confirmed would represent a novel molecular strategy to enhance cognition.

Synthesis and structure-activity relationships of a series of 4-methoxy-3-(piperidin-4-yl)oxy benzamides as novel inhibitors of the presynaptic choline transporter

The synthesis and SAR of 4-methoxy-3-(piperidin-4-yl) benzamides identified after a high-throughput screen of the MLPCN library is reported. SAR was explored around the 3-piperidine substituent as well as the amide functionality of the reported compounds. Starting from the initial lead compounds, 1-7, iterative medicinal chemistry efforts led to the identification of ML352 (10m). ML352 represents a potent and selective inhibitor of CHT based on a drug-like scaffold.

Regulation of nicotinic acetylcholine receptor turnover by MuRF1 connects muscle activity to endo/lysosomal and atrophy pathways

Muscle atrophy is a process of muscle wasting induced under a series of catabolic stress conditions, such as denervation, disuse, cancer cachexia, heart and renal failure, AIDS, and aging. Neuromuscular junctions (NMJs), the synapses between motor neurons and muscle fibers undergo major changes in atrophying muscles, ranging from mild morphological alterations to complete disintegration. In this study, we hypothesized that remodeling of NMJs and muscle atrophy could be linked together. To test this, we examined if a major atrophy-promoting E3 ubiquitin ligase, MuRF1, is involved in the maintenance of NMJs. Immunofluorescence revealed that MuRF1 is highly enriched close to the NMJ. Affinity precipitation and in vivo imaging showed that MuRF1 interacts in endocytic structures with both, acetylcholine receptor, the primary postsynaptic protein of the NMJ, as well as with Bif-1, an autophagy- and endocytosis-regulating factor. In vivo imaging, radio labeling, and weighing approaches demonstrated that metabolic destabilization of acetylcholine receptors and muscle atrophy induced by denervation were significantly rescued in MuRF1-KO animals. Notably, interaction with Bif-1, and the rescue of AChR lifetime and muscle atrophy were specific to MuRF1 but not MuRF2. Our data demonstrate an involvement of MuRF1 in membrane protein-turnover, including the degradation of AChRs at the NMJ under atrophying conditions where MuRF1 also interacts and associates with Bif-1.

Acetylcholine receptor pathway mutations explain various fetal akinesia deformation sequence disorders

Impaired fetal movement causes malformations, summarized as fetal akinesia deformation sequence (FADS), and is triggered by environmental and genetic factors. Acetylcholine receptor (AChR) components are suspects because mutations in the fetally expressed gamma subunit (CHRNG) of AChR were found in two FADS disorders, lethal multiple pterygium syndrome (LMPS) and Escobar syndrome. Other AChR subunits alpha1, beta1, and delta (CHRNA1, CHRNB1, CHRND) as well as receptor-associated protein of the synapse (RAPSN) previously revealed missense or compound nonsense-missense mutations in viable congenital myasthenic syndrome; lethality of homozygous null mutations was predicted but never shown. We provide the first report to our knowledge of homozygous nonsense mutations in CHRNA1 and CHRND and show that they were lethal, whereas novel recessive missense mutations in RAPSN caused a severe but not necessarily lethal phenotype. To elucidate disease-associated malformations such as frequent abortions, fetal edema, cystic hygroma, or cardiac defects, we studied Chrna1, Chrnb1, Chrnd, Chrng, and Rapsn in mouse embryos and found expression in skeletal muscles but also in early somite development. This indicates that early developmental defects might be due to somite expression in addition to solely muscle-specific effects. We conclude that complete or severe functional disruption of fetal AChR causes lethal multiple pterygium syndrome whereas milder alterations result in fetal hypokinesia with inborn contractures or a myasthenic syndrome later in life.

Contribution of valine 7' of TMD2 to gating of neuronal alpha3 receptor subtypes

The second transmembrane domain (TMD2) of the Cys-loop family of ligand-gated ion channels forms the channel pore. The functional role of the amino acid residues contributing to the channel pore in neuronal nicotinic alpha3 receptors is not well understood. We characterized the contribution of TMD2 position V7' to channel gating in neuronal nicotinic alpha3 receptors. Site-directed mutagenesis was used to substitute position alpha3 (V7') with four different amino acids (A, F, S, or Y) and coexpressed each mutant subunit with wild-type (WT) beta2 or beta4 subunits in Xenopus oocytes. Whole-cell voltage clamp experiments show that substitution for an alanine, serine, or phenylalanine decreased by 2.3-6.2-fold the ACh-EC(50) for alpha3beta2 and alpha3beta4 receptor subtypes. Interestingly, mutation V7'Y did not produce a significant change in ACh-EC(50) when coexpressed with the beta2 subunit but showed a significant approximately two-fold increase with beta4. Similar responses were obtained with nicotine as the agonist. The antagonist sensitivity of the mutant channels was assessed by using dihydro-beta-erythroidine (DHbetaE) and methyllycaconitine (MLA). The apparent potency of DHbetaE as an antagonist increased by approximately 3.7- and 11-fold for the alpha3beta2 V7'S and V7'F mutants, respectively, whereas no evident changes in antagonist potency were observed for the V7'A and V7'Y mutants. The V7'S and V7'F mutations increase MLA antagonist potency for the alpha3beta4 receptor by approximately 6.2- and approximately 9.3-fold, respectively. The V7'A mutation selectively increases the MLA antagonist potency for the alpha3beta4 receptor by approximately 18.7-fold. These results indicate that position V7' contributes to channel gating kinetics and pharmacology of the neuronal nicotinic alpha3 receptors.

A newly identified chromosomal microdeletion of the rapsyn gene causes a congenital myasthenic syndrome

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

Origin of acetylcholinesterase in the neuromuscular junction formed in the in vitro innervated human muscle

Synaptic basal lamina is interposed between the pre- and postsynaptic membrane of the neuromuscular junction (NMJ). This position permits deposition of basal lamina-bound NMJ components of both neuronal and muscle fibre origin. One such molecule is acetylcholinesterase (AChE). The origin of NMJ AChE has been investigated previously as the answer would elucidate the relative contributions of muscle fibers and motor neurons to NMJ formation. However, in the experimental models used in prior investigations either the neuronal or muscular components of the NMJs were removed, or the NMJs were poorly differentiated. Therefore, the question of AChE origin in the intact and functional NMJ remains open. Here, we have approached this question using an in vitro model in which motor neurons, growing from embryonic rat spinal cord explants, form well differentiated NMJs with cultured human myotubes. By immunocytochemical staining with species-specific anti-AChE antibodies, we are able to differentiate between human (muscular) and rat (neuronal) AChE at the NMJ. We observed strong signal at the NMJ after staining with human AChE antibodies, which suggests a significant muscular AChE contribution. However, a weaker, but still clearly recognizable signal is observed after staining with rat AChE antibodies, suggesting a smaller fraction of AChE was derived from motor neurons. This is the first report demonstrating that both motor neuron and myotube contribute synaptic AChE under conditions where they interact with each other in the formation of an intact and functional NMJ.

Structural and functional organization of synaptic acetylcholinesterase

The expression of the synaptic asymmetric form of the enzyme acetylcholinesterase (AChE) depends of two different genes: the gene that encodes for the catalytic subunit and the gene that encodes for the collagenic tail, ColQ. Asymmetric AChE is specifically localized to the basal lamina at the neuromuscular junction (NMJ). This highly organized distribution pattern suggests the existence of one or more specific binding sites in ColQ required for its anchorage to the synaptic basal lamina. Recent evidence support this notion: first, the presence of two heparin-binding domains in ColQ that interact with heparan sulfate proteoglycans (HSPGs) at the synaptic basal lamina; and second, a knockout mouse for perlecan, a HSPG concentrated in nerve-muscle contact, in which absence of asymmetric AChE at the NMJ is observed. The physiological importance of collagen-tailed AChE form in skeletal muscle has been illustrated by the identification of several mutations in the ColQ gene. These mutations determine end-plate acetylcholinesterase deficiency and induce one type of synaptic functional disorders observed in Congenital Myasthenic Syndromes (CMSs).

Clinical evaluation and management of myasthenia gravis

Myasthenia gravis (MG) is a syndrome of fluctuating skeletal muscle weakness that worsens with use and improves with rest. Eye, facial, oropharyngeal, axial, and limb muscles may be involved in varying combinations and degrees of severity. Its etiology is heterogeneous, divided initially between those rare congenital myasthenic syndromes, which are genetic, and the bulk of MG, which is acquired and autoimmune. The autoimmune conditions are divided in turn between those that possess measurable serum acetylcholine receptor (AChR) antibodies and a smaller group that does not. The latter group includes those MG patients who have serum antibodies to muscle-specific tyrosine kinase (MuSK). Therapeutic considerations differ for early-onset MG, late-onset MG, and MG associated with the presence of a thymoma. Most MG patients can be treated effectively, but there is still a need for more specific immunological approaches.

Clinical and neurophysiological characteristics of congenital myasthenic syndromes presenting in early infancy

The congenital myasthenic syndromes (CMS) constitute a group of genetic disorders, which affect neuromuscular transmission, presenting usually within the first years of life and with a clinical spectrum ranging from mild muscle weakness to severe disability with life-threatening episodes. We present clinical and neurophysiological data of 11 patients (four males, seven females) with CMS diagnosed during the last 5 years. Eight of the 11 patients presented immediately after birth and the remainder by 10 months of age; eight patients had contractures at birth and seven of them required assisted ventilation either immediately in the neonatal period, or at some point afterwards due to respiratory distress or recurrent apnoeas. Neurological signs at presentation were: in nine patients profound hypotonia, in five absent tendon reflexes, in seven ptosis and in eight bulbar signs. In six patients an edrophonium test was performed: only three of them had a positive response; however, eight out of 11 patients responded at least partially at some point in their illness to pyridostigmine. Diagnosis of CMS was confirmed either by demonstration of a decrement after repetitive nerve stimulation or by increased instability and jitter after stimulated single fibre EMG. In five patients, there was a positive family history with death of at least one previous sibling with an undiagnosed neuromuscular disorder. As regards final outcome, five patients died at ages ranging from 1 to 17 months, two patients are still ventilator-dependent at 3 and 5 months of life, respectively, and four patients still survive with either a mild or a moderate motor delay (follow-up range 8-38 months). None of the clinical or neurophysiological characteristics were correlated with outcome (Fisher's exact test). We conclude that a significant number of CMS patients may present in the neonatal period with a variable clinical expression and usually with a poor prognosis. The recognition of specific clinical constellations combined with a search for aetiology at a molecular level will enable the further characterisation of subgroups of CMS.

The C-terminal t peptide of acetylcholinesterase forms an α helix that supports homomeric and heteromeric interactions

Acetylcholinesterase subunits of type T (AChET) possess an alternatively spliced C-terminal peptide (t peptide) which endows them with amphiphilic properties, the capacity to form various homo-oligomers and to associate, as a tetramer, with anchoring proteins containing a proline rich attachment domain (PRAD). The t peptide contains seven conserved aromatic residues. By spectroscopic analyses of the synthetic peptides covering part or all of the t peptide of Torpedo AChET, we show that the region containing the aromatic residues adopts an alpha helical structure, which is favored in the presence of lipids and detergent micelles: these residues therefore form a hydrophobic cluster in a sector of the helix. We also analyzed the formation of disulfide bonds between two different AChET subunits, and between AChET subunits and a PRAD-containing protein [the N-terminal fragment of the ColQ protein (QN)] possessing two cysteines upstream or downstream of the PRAD. This shows that, in the complex formed by four T subunits with QN (T4-QN), the t peptides are not folded on themselves as hairpins but instead are all oriented in the same direction, antiparallel to that of the PRAD. The formation of disulfide bonds between various pairs of cysteines, introduced by mutagenesis at various positions in the t peptides, indicates that this complex possesses a surprising flexibility.

Structural abnormalities of the AChR caused by mutations underlying congenital myasthenic syndromes

The objective was to define the molecular mechanisms underlying congenital myasthenic syndromes (CMS) by studying mutations within genes encoding the acetylcholine receptor (AChR) and related proteins at the neuromuscular junction. It was found that mutations within muscle AChRs are the most common cause of CMS. The majority are located within the epsilon-subunit gene and result in AChR deficiency.

Channelopathies in pediatric neurology

Although the genetic neurologic channelopathies are uncommon, they serve as models that further understanding of disease mechanisms in paroxysmal disorders. Many other neurologic channelopathies likely will be identified in the future.

Trimerization domain of the collagen tail of acetylcholinesterase

In the collagen-tailed forms of cholinesterases, each subunit of a specific triple helical collagen, ColQ, may be attached through a proline-rich domain (PRAD) situated in its N-terminal noncollagenous region, to tetramers of acetylcholinesterase (AChE) or butyrylcholinesterase (BChE). This heteromeric assembly ensures the functional anchoring of AChE in extracellulare matrices, for example, at the neuromuscular junction. In this study, we analyzed the influence of deletions in the noncollagenous C-terminal region of ColQ on its capacity to form a triple helix. We show that an 80-residue segment located downstream of the collagenous regions contains the trimerization domain, that it can form trimers without the collagenous regions, and that a pair of cysteines located at the N-boundary of this domain facilitates oligomerization, although it is not absolutely required. We further show that AChE subunits can associate with nonhelical collagen ColQ monomers, forming ColQ-associated tetramers (G4-Q), which are secreted or are anchored at the cell surface when the C-terminal domain of ColQ is replaced by a GPI-addition signal.

Mutation causing severe myasthenia reveals functional asymmetry of AChR signature cystine loops in agonist binding and gating

We describe a highly disabling congenital myasthenic syndrome (CMS) associated with rapidly decaying, low-amplitude synaptic currents, and trace its cause to a valine to leucine mutation in the signature cystine loop (cys-loop) of the AChR alpha subunit. The recently solved crystal structure of an ACh-binding protein places the cys-loop at the junction between the extracellular ligand-binding and transmembrane domains where it may couple agonist binding to channel gating. We therefore analyzed the kinetics of ACh-induced single-channel currents to identify elementary steps in the receptor activation mechanism altered by the alphaV132L mutation. The analysis reveals that alphaV132L markedly impairs ACh binding to receptors in the resting closed state, decreasing binding affinity for the second binding step 30-fold, but attenuates gating efficiency only about twofold. By contrast, mutation of the equivalent valine residue in the delta subunit impairs channel gating approximately fourfold with little effect on ACh binding, while corresponding mutations in the beta and epsilon subunits are without effect. The unique functional contribution of the alpha subunit cys-loop likely owes to its direct connection via a beta strand to alphaW149 at the center of the ligand-binding domain. The overall findings reveal functional asymmetry between cys-loops of the different AChR subunits in contributing to ACh binding and channel gating.

Congenital myasthenic syndrome caused by low-expressor fast-channel AChR delta subunit mutation

OBJECTIVE

To determine the molecular basis of a disabling congenital myasthenic syndrome (CMS) observed in two related and one unrelated Arab kinship.

BACKGROUND

CMS can arise from defects in presynaptic, synaptic basal lamina-associated, or postsynaptic proteins. Most CMS are postsynaptic, and most reside in the AChR epsilon subunit; only two mutations have been reported in the AChR delta subunit to date.

METHODS

Cytochemistry, electron microscopy, alpha-bungarotoxin binding studies, microelectrode and patch-clamp recordings, mutation analysis, mutagenesis, and expression studies in human embryonic kidney cells were employed.

RESULTS

Endplate studies showed AChR deficiency, fast decaying, low-amplitude endplate currents, and abnormally brief channel opening events. Mutation analysis revealed a novel homozygous missense mutation (deltaP250Q) of the penultimate proline in the first transmembrane domain (TMD1) of the AChR delta subunit. Expression studies indicate that deltaP250Q (1) hinders delta/alpha subunit association during early AChR assembly; (2) hinders opening of the doubly occupied closed receptor (A(2)R); and (3) speeds the dissociation of acetylcholine from A(2)R. Mutagenesis studies indicate that deltaP250L also has fast-channel effects, whereas epsilon P245L and epsilon P245Q, identical mutations of the corresponding proline in the epsilon subunit, have mild slow-channel effects.

CONCLUSIONS

deltaP250Q represents the third mutation observed in the AChR delta subunit. The severe phenotype caused by deltaP250Q is attributed to endplate AChR deficiency, fast decay of the synaptic response, and lack of compensatory factors. That the penultimate prolines in TMD1 of the delta and epsilon subunits exert a reciprocal regulatory effect on the length of the channel opening bursts reveals an unexpected functional asymmetry between the two subunits.

Impairment of synaptic vesicle exocytosis and recycling during neuromuscular weakness produced in mice by 2,4-dithiobiuret

Chronic treatment of rodents with 2,4-dithiobiuret (DTB) induces a neuromuscular syndrome of flaccid muscle weakness that mimics signs seen in several human neuromuscular disorders such as congenital myasthenic syndromes, botulism, and neuroaxonal dystrophy. DTB-induced muscle weakness results from a reduction of acetylcholine (ACh) release by mechanisms that are not yet clear. The objective of this study was to determine if altered release of ACh during DTB-induced muscle weakness was due to impairments of synaptic vesicle exocytosis, endocytosis, or internal vesicular processing. We examined motor nerve terminals in the triangularis sterni muscles of DTB-treated mice at the onset of muscle weakness. Uptake of FM1-43, a fluorescent marker for endocytosis, was reduced to approximately 60% of normal after either high-frequency nerve stimulation or K(+) depolarization. Terminals ranged from those with nearly normal fluorescence ("bright terminals") to terminals that were poorly labeled ("dim terminals"). Ultrastructurally, the number of synaptic vesicles that were labeled with horseradish peroxidase (HRP) was also reduced by DTB to approximately 60%; labeling among terminals was similarly variable. A subset of DTB-treated terminals having abnormal tubulovesicular profiles in their centers did not respond to stimulation with increased uptake of HRP and may correspond to dim terminals. Two findings suggest that posttetanic "slow endocytosis" remained qualitatively normal: the rate of this type of endocytosis as measured with FM1-43 did not differ from normal, and HRP was observed in organelles associated with this pathway- coated vesicles, cisternae, as well as synaptic vesicles but not in the tubulovesicular profiles. In DTB-treated bright terminals, end-plate potential (EPP) amplitudes were decreased, and synaptic depression in response to 15-Hz stimulation was increased compared with those of untreated mice; in dim terminals, EPPs were not observed during block with D-tubocurarine. Nerve-stimulation-induced unloading of FM1-43 was slower and less complete than normal in bright terminals, did not occur in dim terminals, and was not enhanced by alpha-latrotoxin. Collectively, these results indicate that the size of the recycling vesicle pool is reduced in nerve terminals during DTB-induced muscle weakness. The mechanisms by which this reduction occurs are not certain, but accumulated evidence suggests that they may include defects in either or both exocytosis and internal vesicular processing.