Association of the gene encoding the δ-subunit of the muscle acetylcholine receptor (CHRND) with acquired autoimmune myasthenia gravis

Phenotypic variability in congenital myasthenic syndrome with GFPT1 mutation

BACKGROUND

Congenital myasthenic syndrome (CMS) is phenotypically and genetically different from myasthenia gravis. CMS can present in adolescents and can be treatable. Genetic testing is helpful in diagnosis, and guides therapy, alleviating the need of muscle biopsy. Also, Genetic diagnosis allows a diagnosis of certainty, especially if there is any doubt about a muscular pathology Henceforth, it is an important differential in those presenting with fixed or fluctuating weakness.

METHOD

Herein, we report two adolescent females with positive Glutamine-fructose-6-phosphate transaminase1( GFPT)mutation(c.322G > A p.Arg111His) with different phenotypic features. One of them presented with dysmorphic features, hyperextensible joints, features suggestive of metabolic myopathy on muscle biopsy and a strongly positive acetylcholine receptor (AChR) antibodies in serum. The second case presented with clinical features typical of congenital limb girdle myasthenic syndrome.

CONCLUSION

Our case had limb girdle weakness, dysmorphic features, uniquely positive AChR antibody, mitochondrial pathology on muscle biopsy and positive GFPT1 mutation. This phenotype has not been reported previously. Given the condition being potentially treatable, GFPT1 mutation subtype of CMS should be considered in differential diagnosis of limb girdle weakness phenotype even in the absence of family history.

Congenital myasthenic syndromes by Epsilon subunit mutations: Phenotypic profiles of 17 Algerian families

BACKGROUND

Congenital myasthenic syndromes (CMS) are a heterogeneous group of rare genetic disorders. The acetyl choline receptor contains five subunits, with a predominance of mutations affecting the epsilon subunit gene called cholinergic receptor nicotinic epsilon (CHRNE) gene.

OBJECTIVE

To study the clinical phenotype of 17 families with CHRNE gene mutations.

METHODS

We report a series of 17 families with 22 affected patients carrying different mutations encoding CHRNE proteins.

RESULTS

We studied their clinical and biological phenotypes, as well as their evolutionary profile and their response to the different therapies proposed. A phenotypic comparison was made between the families carrying the founding Maghrebian mutation and the other mutations found in this series.

CONCLUSION

The CHRNE gene mutations are the most frequent ones in CMS. The phenotypes reported in this study are heterogeneous, and can depend on the causative mutation.

Innovative Therapeutic Approaches in Congenital Myasthenic Syndromes

Background and Objectives

To provide real-word clinical follow-up data on patients carrying variations of congenital myasthenic syndromes (CMS) and who respond to some innovative drugs.

Methods

Patients recruited from the Neurology Department of the Mustapha Bacha university hospital in Algiers. Treated with innovative drugs, they were monitored and their clinical progress was evaluated on the basis of clinical arguments suggestive of CMSs, but also para clinical arguments (electromyography and genetic study).

Results

Six patients carrying different mutations in different genes of CMSs were studied. They had different pathophysiologic profiles (slow or fast channel syndromes, low expressor of receptor). Their therapeutic management was based on innovative drugs, normally indicated in other, non-neurological pathologies. Their outcome was toward a clear clinical improvement.

Discussion

This work relates the interest of proposing treatments (outside of Pyridostigmine) in the management of CMSs. These therapies can greatly modify the prognosis of patients suffering from this orphan disease.

Classification of Evidence

This study provides Class IV evidence that for patients with congenital myasthenic syndromes, some innovative treatments are effective.

A novel missense compound heterozygous variant in TLR1 gene is associated with susceptibility to rheumatoid arthritis - structural perspective and functional annotations

INTRODUCTION

Besides human leukocyte antigen (HLA-DRB1) locus, more than 100 loci across the genome have been identified and linked with the onset, expression and/or progression of rheumatoid arthritis (RA). However, there are still grey areas in our understanding of the key genetic contributors of the disease, particularly in familial cases.

METHODS

In the present study, we have performed the whole exome sequencing (WES) of RA patients from two consanguineous families of Pakistan in a quest to identify novel, high-impact, RA-susceptibility genetic variants.

RESULTS

Through stepwise filtering, around 17,000 variants (common in the affected members) were recognized, out of which 2651 were predicted to be deleterious. Of these, 196 had direct relevance to RA. When selected for homozygous recessive mode of inheritance, two novel pathogenic variants (c.1324T>C, p.Cys442→Arg442; c.2036T>C, p.Ile679→Thr679) in the TLR1 gene displayed the role of compound heterozygosity in modulating the phenotypic expression and penetrance of RA. The structural and functional consequences of the TLR1 missense single nucleotide mutations (Cys442→Arg442; Ile679→Thr679) were evaluated through molecular dynamic simulation (MDS) studies. Analysis showed domain's rigidification, conferring stability to mutant TLR1-TIR/TIRAP-TIR complex with concomitant increase in molecular interactions with pro-inflammatory cytokines, compared to the wild-type conformation. Gene co-expression network analysis highlighted interlinked partnering genes along with interleukin-6 production of TLR1 (corrected p-value 2.98e-4) and acetylcholine receptor activity of CHRNG (corrected p-value 6.12e-2) as highly enriched associated functions.

CONCLUSION

The results, validated through case-control study subjects, suggested that the variants identified through WES and confirmed through Sanger sequencing and MDS are the novel disease variants and are likely to confer RA-susceptibility, independently and/or in a family-specific context. Key Points • Exploration of population based/ethno-specific big data is imperative to identify novel causal variants of RA. • Two new deleterious missense mutations in mutational hotspot exon 4 of TLR1 gene have been identified in Pakistani RA patients. • MD simulation data provides evidence for domain's rigidification, conferring stability to mutant TLR1-TIR/TIRAP-TIR complex, with concomitant increase in production of pro-inflammatory cytokines, thus adding to the onset/erosive outcome of RA.

[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.

Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors

Congenital myasthenic syndrome (CMS) is a heterogeneous group of inherited disorder which does not associate with anti-acetylcholine receptor (AChR) antibody. The presence of AChR autoantibody is pathogenic and highly sensitive and specific for autoimmune myasthenia gravis (MG). We describe 2 children from unrelated families who presented with hypotonia, ptosis and fatigability in early infancy with anti-AChR antibodies detected via ELISA on 2 separate occasions in the sera. Both were treated as refractory autoimmune MG due to poor clinical response to acetylcholinesterase inhibitor and immunotherapy. In view of the atypical clinical features, genetic studies of CMS were performed and both were confirmed to have novel pathogenic mutations in the COLQ gene. To the best of our knowledge, the presence of anti-AChR antibody in COLQ-related CMS has never been reported in the literature. The clinical presentation of early onset phenotype, and refractoriness to acetylcholinesterase inhibitor and immunotherapy should prompt CMS as a differential diagnosis.

Identifying the culprits in neurological autoimmune diseases

The target organ of neurological autoimmune diseases (NADs) is the central or peripheral nervous system. Multiple sclerosis (MS) is the most common NAD, whereas Guillain-Barré syndrome (GBS), myasthenia gravis (MG), and neuromyelitis optica (NMO) are less common NADs, but the incidence of these diseases has increased exponentially in the last few years. The identification of a specific culprit in NADs is challenging since a myriad of triggering factors interplay with each other to cause an autoimmune response. Among the factors that have been associated with NADs are genetic susceptibility, epigenetic mechanisms, and environmental factors such as infection, microbiota, vitamins, etc. This review focuses on the most studied culprits as well as the mechanisms used by these to trigger NADs.

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Neurological autoimmune diseases are caused by a complex interaction between genes, environmental factors, and epigenetic deregulation.

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Infectious agents can cause an autoimmune reaction to myelin epitopes through molecular mimicry and/or bystander activation.

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Gut microbiota dysbiosis contributes to neurological autoimmune diseases.

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Smoking increases the risk of NADs through inflammatory signaling pathways, oxidative stress, and Th17 differentiation.

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Deficiency in vitamin D favors NAD development through direct damage to the central and peripheral nervous system.

Immunopathogenesis in Myasthenia Gravis and Neuromyelitis Optica

Myasthenia gravis (MG) and neuromyelitis optica (NMO) are autoimmune channelopathies of the peripheral neuromuscular junction (NMJ) and central nervous system (CNS) that are mainly mediated by humoral immunity against the acetylcholine receptor (AChR) and aquaporin-4 (AQP4), respectively. The diseases share some common features, including genetic predispositions, environmental factors, the breakdown of tolerance, the collaboration of T cells and B cells, imbalances in T helper 1 (Th1)/Th2/Th17/regulatory T cells, aberrant cytokine and antibody secretion, and complement system activation. However, some aspects of the immune mechanisms are unique. Both targets (AChR and AQP4) are expressed in the periphery and CNS, but MG mainly affects the NMJ in the periphery outside of CNS, whereas NMO preferentially involves the CNS. Inflammatory cells, including B cells and macrophages, often infiltrate the thymus but not the target—muscle in MG, whereas the infiltration of inflammatory cells, mainly polymorphonuclear leukocytes and macrophages, in NMO, is always observed in the target organ—the spinal cord. A review of the common and discrepant characteristics of these two autoimmune channelopathies may expand our understanding of the pathogenic mechanism of both disorders and assist in the development of proper treatments in the future.

Congenital myasthenic syndrome due to mutation in CHRNE gene with clinical worsening and thymic hyperplasia attributed to association with autoimmune-myasthenia gravis

We report a patient with congenital myasthenic syndrome (CMS) due to mutation in CHRNE with symptoms since the age of 4; mild to moderate fatigable weakness involved mainly ocular, bulbar and limb muscles; functional impact of the disease in their development and physical activity was modest. By the age of 34, the patient experienced gradual worsening of fatigue with dyspnoea and pronounced limb weakness, requiring significant increase of pyridostigmine. Further, a remarkable and sustained clinical improvement followed thymectomy with hyperplastic thymus. Despite of the absence of detectable antibodies to acetyl-choline receptor (AChR) (including clustered-AChR), muscle-specific kinase and low-density lipoprotein receptor-related protein-4 antibodies in the serum obtained nine years after thymectomy, the clinical, genetic and histological features are in keeping with the extremely rare association of two rare neuromuscular junction disorders - CMS and myasthenia gravis (MG). The inexistence of other conditions that could potentially associate with thymic hyperplasia also supports the diagnosis of MG.

Myasthenia Gravis: paradox versus paradigm in autoimmunity

Myasthenia Gravis (MG) is a paradigm of organ-specific autoimmune disease (AID). It is mediated by antibodies that target the neuromuscular junction. The purpose of this review is to place MG in the general context of autoimmunity, to summarize the common mechanisms between MG and other AIDs, and to describe the specific mechanisms of MG. We have chosen the most common organ-specific AIDs to compare with MG: type 1 diabetes mellitus (T1DM), autoimmune thyroid diseases (AITD), multiple sclerosis (MS), some systemic AIDs (systemic lupus erythematous (SLE), rheumatoid arthritis (RA), Sjogren's syndrome (SS)), as well as inflammatory diseases of the gut and liver (celiac disease (CeD), Crohn's disease (CD), and primary biliary cirrhosis (PBC)). Several features are similar between all AIDs, suggesting that common pathogenic mechanisms lead to their development. In this review, we address the predisposing factors (genetic, epigenetic, hormones, vitamin D, microbiota), the triggering components (infections, drugs) and their interactions with the immune system [1,2]. The dysregulation of the immune system is detailed and includes the role of B cells, Treg cells, Th17 and cytokines. We particularly focused on the role of TNF-α and interferon type I whose role in MG is very analogous to that in several other AIDS. The implication of AIRE, a key factor in central tolerance is also discussed. Finally, if MG is a prototype of AIDS, it has a clear specificity compared to the other AIDS, by the fact that the target organ, the muscle, is not the site of immune infiltration and B cell expansion, but exclusively that of antibody-mediated pathogenic mechanisms. By contrast, the thymus in the early onset subtype frequently undergoes tissue remodeling, resulting in the development of ectopic germinal centers surrounded by high endothelial venules (HEV), as observed in the target organs of many other AIDs.

A candidate gene for autoimmune myasthenia gravis

OBJECTIVE

We sought to identify a causative mutation in a previously reported kindred with parental consanguinity and 5 of 10 siblings with adult-onset autoimmune myasthenia gravis.

METHODS

We performed genome-wide homozygosity mapping, and sequenced all known genes in the one region of extended homozygosity. Quantitative and allele-specific reverse transcriptase PCR (RT-PCR) were performed on a candidate gene to determine the RNA expression level in affected siblings and controls and the relative abundance of the wild-type and mutant alleles in a heterozygote.

RESULTS

A region of shared homozygosity at chromosome 13q13.3-13q14.11 was found in 4 affected siblings and 1 unaffected sibling. A homozygous single nucleotide variant was found in the 3'-untranslated region of the ecto-NADH oxidase 1 gene (ENOX1). No other variants likely to be pathogenic were found in genes in this region or elsewhere. The ENOX1 sequence variant was not found in 764 controls. Quantitative RT-PCR showed that expression of ENOX1 decreased to about 20% of normal levels in lymphoblastoid cells from individuals homozygous for the variant and to about 50% in 2 unaffected heterozygotes. Allele-specific RT-PCR showed a 55%-60% reduction in the level of the variant transcript in heterozygous cells due to reduced mRNA stability.

CONCLUSION

These results indicate that this sequence variant in ENOX1 may contribute to the familial autoimmune myasthenia in these patients.

Genetic factors in autoimmune myasthenia gravis

Autoimmune myasthenia gravis (MG) is a multifactorial disease, markedly influenced by genetic factors, even though it shows limited heritability. The clinically typical form of autoimmune MG with thymus hyperplasia shows the most reproducible genetic associations, especially with the A1-B8-DR3 (8.1) haplotype of the major histocompatibility complex (MHC). However, because of strong linkage disequilibrium, the causative polymorphism in this region is not known yet. Increasing the density of genetic markers has nevertheless recently revealed the complex, but highly significant contribution of this essential genetic region in controlling the disease phenotype and the quantitative expression of serum autoantibodies. The advances of the human genome program, the development of genotyping and sequencing tools with increasing throughput, and the availability of powerful statistical methods now make feasible the dissection of a complex genetic region, such as the MHC and beyond, the systematic search throughout the genome for variants influencing disease predisposition. The identification of such functional variants should provide new clues to the pathogenesis of MG, as recently illustrated by the study of a promoter polymorphism of the CHRNA1 locus, influencing its thymic expression and central tolerance, or of a coding variant of the PTPN22 intracellular phosphatase.

CCR2-64I and CCR5Delta32 Polymorphisms in Korean Patients with Myasthenia Gravis

BACKGROUND AND PURPOSE

Chemokines participate in the regulation of immune and inflammatory responses by interacting with their receptors, which are primarily expressed on immune and inflammatory cells such as B- and T-lymphocytes and antigen-presenting cells. Chemokines and their receptors are therefore considered to mediate inflammation and tissue damage in autoimmune disorders. Chemokine receptor (CCR) genotypes were recently identified, and the importance of their genetic polymorphisms in some autoimmune and infectious disorders has been demonstrated. To define the roles of the polymorphism of the CCR2 gene at codon 64 (CCR2-64I) and the 32-bp deletion in the coding region of CCR5 (CCR5Delta32) in Korean patients with myasthenia gravis (MG), we compared these genotypes in MG cases and healthy controls and investigated the clinical features associated with these genotypes.

METHODS

One hundred and fifteen healthy controls (51 men and 64 women) and 109 MG patients (44 men and 65 women) from three University hospitals were included. We examined each patient for clinical features using electrophysiology tests, laboratory tests, and thymic pathology. The CCR2-64I and CCR5Delta32 polymorphisms were determined by the PCR-RFLP method.

RESULTS

We detected no difference in the frequencies of CCR2-64I polymorphism between MG patients and healthy controls. All of the MG patients and the healthy controls were homozygous for the wild-type CCR5 genotype. The results of electrophysiological tests and thymic pathologies were not influenced by the type of CCR2-64I polymorphism. However, the anti-acetylcholine-receptor (AChR) antibody titer was higher in the CCR2 G/G genotype (13.34+/-12.71 nmol/L) than in the CCR2 A/A genotype (5.83+/-2.56 nmol/L).

CONCLUSIONS

We found no evidence of an increased risk for MG associated with the CCR2-64I and CCR5Delta32 polymorphisms. However, the increased anti-AChR antibody titer in the patients with the CCR2 G/G genotype suggests that the CCR2 gene play a role in the pathophysiology of MG.

Myasthenia gravis induced in mice by immunization with the recombinant extracellular domain of rat muscle-specific kinase (MuSK)

Myasthenia gravis (MG) is mostly caused by anti-acetylcholine receptor (AChR) auto-antibodies (Abs). Such Abs are undetectable in 10-15% of MG patients, but many have anti-muscle-specific kinase (MuSK) Abs. We injected recombinant rat-MuSK extracellular domain in H-2(a), H-2(b), H-2(bm12) and H-2(d) mice. Certain strains exhibited exercise-induced fatigue, tremors, weight loss, and some died after 2-3 injections. Compound muscle action potentials showed decrement with low-frequency repetitive nerve stimulation. Miniature endplate potentials decreased, suggesting lower numbers of endplates functional AChRs. Myasthenic sera inhibited agrin-induced AChR aggregation in C2C12 myotubes.

CONCLUSION

Anti-MuSK Abs induce MG, which might also result from blocking the agrin-signaling pathway.

Association of the PTPN22*R620W polymorphism with autoimmune myasthenia gravis

OBJECTIVE

Our objective was to investigate a role of the intracellular tyrosine phosphatase PTPN22*R620W variant in autoimmune myasthenia gravis (MG), considering disease heterogeneity.

METHODS

We used a case-control design, comparing 470 patients and 296 controls, all French whites. Patients were categorized depending on the presence of a thymoma and serum anti-titin antibodies.

RESULTS

The 620W risk allele was increased in 293 nonthymoma patients without anti-titin antibodies (odds ratio, 1.97; 95% confidence interval, 1.32-2.97, p = 0.00059) but not in nonthymoma patients with anti-titin antibodies or in thymoma patients.

INTERPRETATION

Our genetic findings strengthen the concept that these groups of patients correspond to etiologically distinct disease entities.

Are MuSK antibodies the primary cause of myasthenic symptoms?

OBJECTIVE

To investigate the morphologic, electrophysiologic, and molecular correlates of muscle-specific tyrosine kinase-seropositive [MuSK(+)] myasthenia gravis (MG).

BACKGROUND

Anti-MuSK antibodies are detected in some of acetylcholine receptor-seronegative [AChR(-)] patients with MG with prominent facial, bulbar, and respiratory muscle involvement. The morphologic and electrophysiologic correlates of MuSK(+) MG have not been investigated to date.

METHODS

Immunohistochemistry, electron microscopy, and in vitro electrophysiology studies were performed on an intercostal muscle specimen of a patient with MuSK(+) MG and in control subjects. MUSK was directly sequenced, and the nucleotide changes were traced with allele-specific PCR in control subjects.

RESULTS

A man aged 34 years has had facial weakness since childhood and progressive bulbar and respiratory muscle weakness and intermittent diplopia since age 21 years. He has thin temporalis and masseter muscles, a high-arched palate, and an atrophic tongue. EMG shows a 36% decrement in facial muscles. His mother has similar facial features. His endplates (EPs) show no AChR or MuSK deficiency, but the amplitudes of the miniature EP potentials and currents are reduced to 35% and 55% of normal, respectively. EP ultrastructure is well preserved, but some junctional folds immunostain faintly for immunoglobulin G. Mutation analysis of MUSK reveals one rare and two common DNA polymorphisms.

CONCLUSIONS

1) The circulating anti-muscle-specific tyrosine kinase antibodies caused neither muscle-specific tyrosine kinase nor acetylcholine receptor deficiency at the endplates; 2) the reduced intercostal miniature endplate potential and current amplitudes were not accounted for by acetylcholine receptor deficiency; 3) the faint immunoglobulin G deposits at the endplates may or may not represent anti-muscle-specific tyrosine kinase antibodies; and 4) the anti-muscle-specific tyrosine kinase antibodies may not be the primary cause of myasthenic symptoms in this patient.