Recent advances in congenital myasthenic syndromes

[Advances in the diagnosis and treatment of congenital myasthenic syndrome]

Congenital myasthenic syndrome (CMS) is a group of clinical and genetic heterogeneous diseases caused by impaired neuromuscular transmission due to genetic defects. At present, it has been reported that more than 30 genes can cause CMS. All CMS subtypes have the clinical features of fatigue and muscle weakness, but age of onset, symptoms, and treatment response vary with the molecular mechanisms underlying genetic defects. Pharmacotherapy and symptomatic/supportive treatment are the main methods for the treatment of CMS, and antisense oligonucleotide technology has been proven to be beneficial for CHRNA 1-related CMS in animals. Since CMS is a group of increasingly recognized clinical and genetic heterogeneous diseases, an understanding of the latest knowledge and research advances in its clinical features, genetic research, and treatment helps to give early diagnosis and treatment as well as gain a deeper understanding of the pathogenesis of CMS, so as to make new breakthroughs in the treatment of CMS.

InMeRF: prediction of pathogenicity of missense variants by individual modeling for each amino acid substitution

In predicting the pathogenicity of a nonsynonymous single-nucleotide variant (nsSNV), a radical change in amino acid properties is prone to be classified as being pathogenic. However, not all such nsSNVs are associated with human diseases. We generated random forest (RF) models individually for each amino acid substitution to differentiate pathogenic nsSNVs in the Human Gene Mutation Database and common nsSNVs in dbSNP. We named a set of our models ‘Individual Meta RF’ (InMeRF). Ten-fold cross-validation of InMeRF showed that the areas under the curves (AUCs) of receiver operating characteristic (ROC) and precision–recall curves were on average 0.941 and 0.957, respectively. To compare InMeRF with seven other tools, the eight tools were generated using the same training dataset, and were compared using the same three testing datasets. ROC-AUCs of InMeRF were ranked first in the eight tools. We applied InMeRF to 155 pathogenic and 125 common nsSNVs in seven major genes causing congenital myasthenic syndromes, as well as in VANGL1 causing spina bifida, and found that the sensitivity and specificity of InMeRF were 0.942 and 0.848, respectively. We made the InMeRF web service, and also made genome-wide InMeRF scores available online (https://www.med.nagoya-u.ac.jp/neurogenetics/InMeRF/).

A Novel c.973G>T Mutation in the ε-subunit of the Acetylcholine Receptor Causing Congenital Myasthenic Syndrome in an Iranian Family

Congenital myasthenic syndrome (CMS) constitutes a group of inherited disorders of neuromuscular junctions. The majority of postsynaptic syndromes result from mutations in the CHRNE gene that causes muscle nicotine acetylcholine deficiency. In this study, we report on a 2 and a half-year-old boy with normal developmental milestones and bilateral ptosis. Clinical courses, electrophysiological studies and molecular genetic analysis were assessed. Polymerase chain reaction (PCR) and direct DNA sequencing of the CHRNE gene were performed for the proband and all the family members. A novel homozygous missense mutation of c.973G>T was found in the CHRNE gene. Segregation studies were suggested to be the genetic cause of the disease. Using three in silico tools and the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) variant classification guidelines indicated that the novel variant c.973G>T was likely pathogenic. Our results recommended first screening of the CHRNE gene for pathogenic mutations in Iranian origin.

Congenital myasthenic syndromes

OBJECTIVES

Congenital myasthenic syndromes (CMSs) are a genotypically and phenotypically heterogeneous group of neuromuscular disorders, which have in common an impaired neuromuscular transmission. Since the field of CMSs is steadily expanding, the present review aimed at summarizing and discussing current knowledge and recent advances concerning the etiology, clinical presentation, diagnosis, and treatment of CMSs.

METHODS

Systematic literature review.

RESULTS

Currently, mutations in 32 genes are made responsible for autosomal dominant or autosomal recessive CMSs. These mutations concern 8 presynaptic, 4 synaptic, 15 post-synaptic, and 5 glycosilation proteins. These proteins function as ion-channels, enzymes, or structural, signalling, sensor, or transporter proteins. The most common causative genes are CHAT, COLQ, RAPSN, CHRNE, DOK7, and GFPT1. Phenotypically, these mutations manifest as abnormal fatigability or permanent or fluctuating weakness of extra-ocular, facial, bulbar, axial, respiratory, or limb muscles, hypotonia, or developmental delay. Cognitive disability, dysmorphism, neuropathy, or epilepsy are rare. Low- or high-frequency repetitive nerve stimulation may show an abnormal increment or decrement, and SF-EMG an increased jitter or blockings. Most CMSs respond favourably to acetylcholine-esterase inhibitors, 3,4-diamino-pyridine, salbutamol, albuterol, ephedrine, fluoxetine, or atracurium.

CONCLUSIONS

CMSs are an increasingly recognised group of genetically transmitted defects, which usually respond favorably to drugs enhancing the neuromuscular transmission. CMSs need to be differentiated from neuromuscular disorders due to muscle or nerve dysfunction.

Differential effects of spinal motor neuron-derived and skeletal muscle-derived Rspo2 on acetylcholine receptor clustering at the neuromuscular junction

We recently reported that R-spondin 2 (Rspo2), a secreted activator of Wnt/β-catenin signaling, promotes acetylcholine receptor (AChR) clustering and neuromuscular junction (NMJ) formation via its receptor, Lgr5. Rspo2 is expressed highly in spinal motor neurons (SMNs) and marginally in the skeletal muscle, but the origin of Rspo2 at the NMJ remains elusive. We rescued Rspo2-deficient (Rspo2-/-) mice by specifically expressing Rspo2 in the skeletal muscle and SMNs. SMN-specific Rspo2 mitigated or over-corrected abnormal features of the NMJs and AChR clusters observed in Rspo2-/- mice including (i) abnormal broadening of enlarged AChR clusters, (ii) three of six abnormal ultrastructural features, and (iii) abnormal expression of nine genes in SMNs and the diaphragm. In contrast, muscle-specific Rspo2 normalized all six abnormal ultrastructural features, but it had no effect on AChR clustering and NMJ formation at the light microscopy level or on abnormal gene expression in SMNs and the diaphragm. These results suggest that SMN-derived Rspo2 plays a major role in AChR clustering and NMJ formation in the postsynaptic region, and muscle-derived Rspo2 also plays a substantial role in juxtaposition of the active zones and synaptic folds.

Protein-anchoring therapy to target extracellular matrix proteins to their physiological destinations

Endplate acetylcholinesterase (AChE) deficiency is a form of congenital myasthenic syndrome (CMS) caused by mutations in COLQ, which encodes collagen Q (ColQ). ColQ is an extracellular matrix (ECM) protein that anchors AChE to the synaptic basal lamina. Biglycan, encoded by BGN, is another ECM protein that binds to the dystrophin-associated protein complex (DAPC) on skeletal muscle, which links the actin cytoskeleton and ECM proteins to stabilize the sarcolemma during repeated muscle contractions. Upregulation of biglycan stabilizes the DPAC. Gene therapy can potentially ameliorate any disease that can be recapitulated in cultured cells. However, the difficulty of tissue-specific and developmental stage-specific regulated expression of transgenes, as well as the difficulty of introducing a transgene into all cells in a specific tissue, prevents us from successfully applying gene therapy to many human diseases. In contrast to intracellular proteins, an ECM protein is anchored to the target tissue via its specific binding affinity for protein(s) expressed on the cell surface within the target tissue. Exploiting this unique feature of ECM proteins, we developed protein-anchoring therapy in which a transgene product expressed even in remote tissues can be delivered and anchored to a target tissue using specific binding signals. We demonstrate the application of protein-anchoring therapy to two disease models. First, intravenous administration of adeno-associated virus (AAV) serotype 8-COLQ to Colq-deficient mice, resulting in specific anchoring of ectopically expressed ColQ-AChE at the NMJ, markedly improved motor functions, synaptic transmission, and the ultrastructure of the neuromuscular junction (NMJ). In the second example, Mdx mice, a model for Duchenne muscular dystrophy, were intravenously injected with AAV8-BGN. The treatment ameliorated motor deficits, mitigated muscle histopathologies, decreased plasma creatine kinase activities, and upregulated expression of utrophin and DAPC component proteins. We propose that protein-anchoring therapy could be applied to hereditary/acquired defects in ECM and secreted proteins, as well as therapeutic overexpression of such factors.

SRSF1 suppresses selection of intron-distal 5' splice site of DOK7 intron 4 to generate functional full-length Dok-7 protein

Dok-7 is a non-catalytic adaptor protein that facilitates agrin-induced clustering of acetylcholine receptors (AChR) at the neuromuscular junction. Alternative selection of 5′ splice sites (SSs) of DOK7 intron 4 generates canonical and frame-shifted transcripts. We found that the canonical full-length Dok-7 enhanced AChR clustering, whereas the truncated Dok-7 did not. We identified a splicing cis-element close to the 3′ end of exon 4 by block-scanning mutagenesis. RNA affinity purification and mass spectrometry revealed that SRSF1 binds to the cis-element. Knocking down of SRSF1 enhanced selection of the intron-distal 5′ SS of DOK7 intron 4, whereas MS2-mediated artificial tethering of SRSF1 to the identified cis-element suppressed it. Isolation of an early spliceosomal complex revealed that SRSF1 inhibited association of U1 snRNP to the intron-distal 5′ SS, and rather enhanced association of U1 snRNP to the intron-proximal 5′ SS, which led to upregulation of the canonical DOK7 transcript. Integrated global analysis of CLIP-seq and RNA-seq also indicated that binding of SRSF1 immediately upstream to two competing 5′ SSs suppresses selection of the intron-distal 5′ SS in hundreds of human genes. We demonstrate that SRSF1 critically regulates alternative selection of adjacently placed 5′ SSs by modulating binding of U1 snRNP.

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.