Late presentations of congenital myasthenic syndromes: How many do we miss?

The Importance of Early Treatment of Inherited Neuromuscular Conditions

There has been tremendous progress in treatment of neuromuscular diseases over the last 20 years, which has transformed the natural history of these severely debilitating conditions. Although the factors that determine the response to therapy are many and in some instance remain to be fully elucidated, early treatment clearly has a major impact on patient outcomes across a number of inherited neuromuscular conditions. To improve patient care and outcomes, clinicians should be aware of neuromuscular conditions that require prompt treatment initiation. This review describes data that underscore the importance of early treatment of children with inherited neuromuscular conditions with an emphasis on data resulting from newborn screening efforts.

[Myasthenia in adults, children, and adolescents: what's new?]

Therapy of myasthenia gravis (MG) is increasingly oriented to the patient's antibody status. In addition to symptomatic therapy, steroids, classic long-term immunosuppressive therapies and thymectomy are regularly used. In recent years, new therapeutic approaches have been developed that particularly benefit acetylcholine receptor (AChR) antibody (Abs) positive patients with highly active disease. While the C5 complement inhibitor eculizumab was reserved for treatment-refractory generalized courses of AChR-Abs positive MG, two new drugs, the neonatal Fc receptor inhibitor efgartigimod and the more advanced C5 complement inhibitor ravulizumab, have recently been approved as add-on therapy for AChR-Abs positive generalized MG (gMG). In highly active courses of MG with Abs against the muscle-specific receptor tyrosine kinase (MuSK), the use of rituximab should be considered early in the course of the disease. The efficacy of the new drugs in children and adolescents with juvenile MG (JMG) is currently being tested in clinical trials. The new guideline recommends the use of modern immunomodulators based on a step-by-step approach depending on disease activity. With the German Myasthenia Register (MyaReg), the changing therapeutic landscape and quality of life of patients with myasthenic syndromes can be assessed, thus providing real-world data on the care of MG patients. Despite treatment based on the previous guideline, many MG patients suffer considerable impairment to their quality of life. With the new immunomodulators, there is the possibility of early intensified immunotherapy, which, in contrast to long-term immunosuppressants, can lead to a rapid improvement in the course of the disease.

Two patients with congenital myasthenic syndrome caused by COLQ gene mutations and the consequent ColQ protein defect

Objective

To report two cases of congenital myasthenic syndromes (CMS) in a Chinese family with mutations in the COLQ gene and to prove the consequence defect of the ColQ protein.

Method

Clinical characteristics of the two children from the same family were described. Next-generation sequencing (NGS) and sanger sequencing was performed on the proband and family members. The consequence of the mutation was predicted by 3D protein structure prediction using I-TASSER. The wild type and mutant were transfected to 293T cells, and ColQ protein was detected by Western Blot.

Results

The diagnosis of CMS was based on a symptom combination of fatigable muscle weakness, ptosis, scoliosis, and hypotonia, aggravation of muscle weakness after the neostigmine test, and a 46% decrement in repetitive nerve stimulation. A muscle biopsy was performed on the proband, revealing mild variation in the myofiber size. NGS data revealed two compound heterozygous mutations at c.173delC (p.Pro58Hisfs*22) and c.C706T (p.R236X) in the COLQ gene, where the former was a novel mutation. A 3D structure prediction showed two truncated ColQ proteins with 78aa and 235aa, respectively. The truncated ColQ protein was proved in 293T cells transfected with c.173delC or c.C706T mutants by Western Blot.

Conclusions

The mutations of c.173delC and c.C706T in the COLQ gene led to truncated ColQ protein and contributed to the pathogenesis of CMS in this Chinese family.

Diagnostic yield of a practical electrodiagnostic protocol discriminating between different congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a group of heterogeneous diseases of the neuromuscular junction. We report electrodiagnostic testing (EDX) and genetic findings in a series of 120 CMS patients tested with a simple non-invasive EDX workup with surface recording of CMAPs and 3Hz repetitive nerve stimulation of accessory, radial and deep fibular nerves. Five ENMG phenotypes were retrieved based on the presence or not of R-CMAPs and the distribution pattern of decremental CMAP responses which significantly correlated with genetic findings (p <0.00001). R-CMAPs were found in all COLQ-mutated patients (CMS1A) and Slow Channel CMS (SCCMS) (CMS1B). CMS1A exhibited greater decrements in accessory nerve RNS than CMS1B. Patients without R-CMAPs were classified into CMS2A (DOK7-, MUSK-, GFPT1-, GMPPB-, TOR1AIP-mutated) when exhibiting predominant accessory nerve RNS decrements, CMS2B (CHRNE, CHRND, RAPSN) with predominant radial nerve RNS decrements, or CMS2C (AGRN) if there were predominant fibular decrements. Our algorithm may have a major impact on diagnostic and therapeutic monitoring in CMS patients, as well as for validation of the pathogenicity of genetic variants. It should also be part of the evaluation of unexplained muscle weakness or complex neuromuscular phenotypes.

Slow Channel Syndrome Revisited: 40 Years Clinical Follow-Up and Genetic Characterization of Two Cases

BACKGROUND

The slow channel syndrome is a rare hereditary disorder caused by a dominant gain-of-function variant in one of the subunits of the acetylcholine receptor at the neuromuscular junction. Patients typically experience axial, limb and particularly extensor finger muscle weakness.

OBJECTIVE

Age at diagnosis is variable and although the long-term prognosis is important for newly diagnosed patients, extensive follow-up studies are rare. We aim to provide answers and perspective for this patient group by presenting an elaborate description of the lifetime follow-up of two slow channel syndrome patients.

METHODS

We describe 40 years follow-up in two, genetically confirmed cases (CHRNA1; c.866G >  T p.(Ser289Ile)(legacy Ser269Ile) and CHRNE; c.721C >  T p.(Leu241Phe)(legacy Leu221Phe) variants).

RESULTS

We find that the disease course has a fluctuating pattern and is only mildly progressive. However, hormonal imbalances, (psychological) stress or excessive hot or cold environments are often aggravating factors. Quinidine and fluoxetine are helpful, but ephedrine and salbutamol may also improve symptoms.

CONCLUSION

Slow channel syndrome is mildly progressive with a fluctuating pattern. The observations reported here provide a lifespan perspective and answers to the most pressing questions about prognosis and treatment options for newly diagnosed patients.

Congenital myasthenic syndrome in a cohort of patients with 'double' seronegative myasthenia gravis

BACKGROUND

Congenital myasthenic syndromes (CMS) have some phenotypic overlap with seronegative myasthenia gravis (SNMG).

OBJECTIVE

The aim of this single center study was to assess the minimum occurrence of CMS misdiagnosed as double SNMG in a Brazilian cohort.

METHODS

The genetic analysis of the most common mutations in CHRNE, RAPSN, and DOK7 genes was used as the main screening tool.

RESULTS

We performed genetic analysis in 22 patients with a previous diagnosis of 'double' SNMG. In this study, one CMS patient was confirmed due to the presence of compound heterozygous variants in the CHRNE gene (c.130insG/p.Cys210Phe).

CONCLUSIONS

This study confirmed that CMS due to CHNRE mutations can be mistaken for SNMG. In addition, our study estimated the prevalence of misdiagnosed CMS to be 4.5% in 'double' SNMG patients of our center. Based on our findings, genetic screening could be helpful in the diagnostic workup of patients with 'double' SNMG in whom differential diagnosis is recommended.

Diagnosis of DOK7 congenital myasthenic syndrome during pregnancy: A case report and literature review

INTRODUCTION

Pregnancy among patients with congenital myasthenic syndrome (CMS) is a rare occurrence. Since most of the patients with CMS reach adulthood, questions regarding clinical outcome with pregnancy arise.

CASE REPORT

We describe a 38-year-old Portuguese female who presented in the second trimester of pregnancy with proximal fluctuating limb-girdle weakness, hyperlordosis, waddling gait, dysphagia, dysphonia and ptosis, with no ophthalmoparesis. Initial diagnosis of seronegative myasthenia, supported by neurophysiology findings, led to unsuccessful treatment with intravenous immunoglobulin, pyridostigmine, prednisolone and plasmapheresis, and the patient slowly progressed to a severe tetraparesis with facial and bulbar involvement. Genetic testing for CMS identified a novel compound heterozygous mutation (c.1124_1127dupTGCC and c.935_936del) in the DOK7 gene. Subsequent treatment with salbutamol resulted in substantial clinical benefit.

CONCLUSIONS

This case underlines the importance of considering the diagnosis of CMS in patients with fluctuating weakness during pregnancy. Patients of child-bearing potential diagnosed with CMS, particularly due to DOK7 mutations, should be counseled in advance and closely followed during pregnancy.

Novel compound heterozygous variants in the GFPT1 gene leading to rare limb-girdle congenital myasthenic syndrome with rimmed vacuoles

BACKGROUND

 Congenital myasthenic syndrome (CMS) is a heterogeneous group of rare disorders with impaired neuromuscular transmission caused by genetic defects, which is characterized by fatigable muscle weakness.

CASE PRESENTATION

 Herein, we report a case of limb-girdle CMS (LG-CMS) in a 15-year-old Chinese girl with limb weakness and mild ptosis. The patient presented with well-defined clinical manifestations, muscle imaging, and electrophysiological features associated with CMS. On muscle biopsy, in addition to tubular aggregates identified, an extremely unusual pathological change of rimmed vacuoles in muscle fibers was observed. Whole-exome sequencing disclosed two novel heterozygous variants (c.14 T>A and c.581 T>C) in the human glutamine-fructose-6-phosphate transaminase 1 (GFPT1) gene, leading to the substitutions of phenylalanine to tyrosine (p.F5Y) and serine (p.F194S), respectively. Both variants were predicted to be likely pathogenic by SIFT, Polyphen-2, and Mutation Taster. Treatments with pyridostigmine bromide and albuterol produced a dramatic improvement.

CONCLUSIONS

 Collectively, molecular genetic analysis and muscle biopsy play crucial roles in the diagnosis of GFPT1-related LG-CMS with rimmed vacuoles (a rare phenotype of CMS) and have important implications for treatment decision.

Italian recommendations for diagnosis and management of congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are genetic disorders due to mutations in genes encoding proteins involved in the neuromuscular junction structure and function. CMS usually present in young children, but perinatal and adult onset has been reported. Clinical presentation is highly heterogeneous, ranging from mild symptoms to severe manifestations, sometimes with life-threatening respiratory episodes, especially in the first decade of life. Although considered rare, CMS are probably underestimated due to diagnostic difficulties. Because of the several therapeutic opportunities, CMS should be always considered in the differential diagnosis of neuromuscular disorders. The Italian Network on CMS proposes here recommendations for proper CMS diagnosis and management, aiming to guide clinicians in their practical approach to CMS patients.

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

OBJECTIVE

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

METHODS

We searched the Mayo Clinic database for patients with CMS diagnosed in adulthood in the neuromuscular clinic between 2000 and 2016. Clinical, laboratory, and electrodiagnostic data were reviewed.

RESULTS

We identified 34 patients with CMS, 30 of whom had a molecular diagnosis (14 DOK7, 6 RAPSN, 2 LRP4, 2 COLQ, 2 slow-channel syndrome, 1 primary acetylcholine receptor deficiency, 1 AGRN, 1 GFPT1, and 1 SCN4A). Ophthalmoparesis was often mild and present in 13 patients. Predominant limb-girdle weakness occurred in 19 patients. Two patients had only ptosis. Age at onset ranged from birth to 39 years (median 5 years). The median time from onset to diagnosis was 26 years (range 4-56 years). Thirteen patients had affected family members. Fatigable weakness was present when examined. Creatine kinase was elevated in 4 of 23 patients (range 1.2-4.2 times the upper limit of normal). Repetitive nerve stimulation revealed a decrement in 30 patients. Thirty-two patients were previously misdiagnosed with seronegative myasthenia gravis (n = 16), muscle diseases (n = 15), weakness of undetermined cause (n = 8), and others (n = 4). Fifteen patients received immunotherapy or thymectomy without benefits. Fourteen of the 25 patients receiving pyridostigmine did not improve or worsen.

CONCLUSION

Misdiagnosis occurred in 94% of the adult patients with CMS and causes a median diagnostic delay of nearly 3 decades from symptom onset. Seronegative myasthenia gravis and muscle diseases were the 2 most common misdiagnoses, which led to treatment delay and unnecessary exposure to immunotherapy, thymectomy, or muscle biopsy.

Congenital myasthenic syndromes in Turkey: Clinical clues and prognosis with long term follow-up

Congenital myasthenic syndromes (CMS) are a group of hereditary disorders affecting the neuromuscular junction. Here, we present clinical, electrophysiological and genetic findings of 69 patients from 51 unrelated kinships from Turkey. Genetic tests of 60 patients were performed at Mayo Clinic. Median follow-up time was 9.8 years (range 1-22 years). The most common CMS was primary acetylcholine receptor (AChR) deficiency (31/51) and the most common mutations in AChR were c.1219 + 2T > G (12/51) and c.1327delG (6/51) in CHRNE. Four of our 5 kinships with AChE deficiency carried p.W148X that truncates the collagen domain of COLQ, and was previously reported only in patients from Turkey. These were followed by GFPT1 deficiency (4/51), DOK7 deficiency (3/51), slow channel CMS (3/51), fast channel CMS (3/51), choline acetyltransferase deficiency (1/51) and a CMS associated with desmin deficiency (1/51). Distribution of muscle weakness was sometimes useful in giving a clue to the CMS subtype. Presence of repetitive compound muscle action potentials pointed to AChE deficiency or slow channel CMS. Our experience confirms that one needs to be cautious using pyridostigmine, since it can worsen some types of CMS. Ephedrine/salbutamol were very effective in AChE and DOK7 deficiencies and were useful as adjuncts in other types of CMS. Long follow-up gave us a chance to assess progression of the disease, and to witness 12 mainly uneventful pregnancies in 8 patients. In this study, we describe some new phenotypes and detail the clinical features of the well-known CMS.

Gonorazky H, Cohn RD, Campbell C. Treating pediatric neuromuscular disorders: The future is now

Pediatric neuromuscular diseases encompass all disorders with onset in childhood and where the primary area of pathology is in the peripheral nervous system. These conditions are largely genetic in etiology, and only those with a genetic underpinning will be presented in this review. This includes disorders of the anterior horn cell (e.g., spinal muscular atrophy), peripheral nerve (e.g., Charcot-Marie-Tooth disease), the neuromuscular junction (e.g., congenital myasthenic syndrome), and the muscle (myopathies and muscular dystrophies). Historically, pediatric neuromuscular disorders have uniformly been considered to be without treatment possibilities and to have dire prognoses. This perception has gradually changed, starting in part with the discovery and widespread application of corticosteroids for Duchenne muscular dystrophy. At present, several exciting therapeutic avenues are under investigation for a range of conditions, offering the potential for significant improvements in patient morbidities and mortality and, in some cases, curative intervention. In this review, we will present the current state of treatment for the most common pediatric neuromuscular conditions, and detail the treatment strategies with the greatest potential for helping with these devastating diseases.

Congenital myasthenic syndromes: recent advances

PURPOSE OF REVIEW

Congenital myasthenic syndromes (CMS) are a group of heterogeneous inherited disorders caused by mutations in genes encoding proteins essential for the integrity of neuromuscular transmission. This review updates the reader on recent findings that have expanded the phenotypic spectrum and suggested improved treatment strategies.

RECENT FINDINGS

The use of next-generation sequencing is continuing to unearth new genes in which mutations can give rise to defective neuromuscular transmission. The defective transmission may be part of an overall more complex phenotype in which there may be muscle, central nervous system or other involvement. Notably, mutations in series of genes encoding presynaptic proteins are being identified. Further work on mutations found in the AGRN-MUSK acetylcholine receptor clustering pathway has helped characterize the role of LRP4 and broadened the phenotypic spectrum for AGRN mutations. Mutations in another extracellular matrix protein, collagen 13A1 and in GMPPB have also been found to cause a CMS. Finally, there are an increasing number of reports for the beneficial effects of treatment with β2-adrenergic receptor agonists.

SUMMARY

Recent studies of the CMS illustrate the increasing complexity of the genetics, pathophysiological mechanisms and the need to tailor therapy for the genetic disorders of the neuromuscular junction.

Molecular characterization of congenital myasthenic syndromes in Spain

Congenital myasthenic syndromes (CMS) are a heterogeneous group of genetic disorders, all of which impair neuromuscular transmission. Epidemiological data and frequencies of gene mutations are scarce in the literature. Here we describe the molecular genetic and clinical findings of sixty-four genetically confirmed CMS patients from Spain. Thirty-six mutations in the CHRNE, RAPSN, COLQ, GFPT1, DOK7, CHRNG, GMPPB, CHAT, CHRNA1, and CHRNB1 genes were identified in our patients, with five of them not reported so far. These data provide an overview on the relative frequencies of the different CMS subtypes in a large Spanish population. CHRNE mutations are the most common cause of CMS in Spain, accounting for 27% of the total. The second most common are RAPSN mutations. We found a higher rate of GFPT1 mutations in comparison with other populations. Remarkably, several founder mutations made a large contribution to CMS in Spain: RAPSN c.264C > A (p.Asn88Lys), CHRNE c.130insG (Glu44Glyfs*3), CHRNE c.1353insG (p.Asn542Gluf*4), DOK7 c.1124_1127dup (p.Ala378Serfs*30), and particularly frequent in Spain in comparison with other populations, COLQ c.1289A > C (p.Tyr430Ser). Furthermore, we describe phenotypes and distinguishing clinical signs associated with the various CMS genes which might help to identify specific CMS subtypes to guide diagnosis and management.

Intragenic DOK7 deletion detected by whole-genome sequencing in congenital myasthenic syndromes

OBJECTIVE

To identify the genetic cause in a patient affected by ptosis and exercise-induced muscle weakness and diagnosed with congenital myasthenic syndromes (CMS) using whole-genome sequencing (WGS).

METHODS

Candidate gene screening and WGS analysis were performed in the case. Allele-specific PCR was subsequently performed to confirm the copy number variation (CNV) that was suspected from the WGS results.

RESULTS

In addition to the previously reported frameshift mutation c.1124_1127dup, an intragenic 6,261 bp deletion spanning from the 5' untranslated region to intron 2 of the DOK7 gene was identified by WGS in the patient with CMS. The heterozygous deletion was suspected based on reduced coverage on WGS and confirmed by allele-specific PCR. The breakpoints had microhomology and an inverted repeat, which may have led to the development of the deletion during DNA replication.

CONCLUSIONS

We report a CMS case with identification of the breakpoints of the intragenic DOK7 deletion using WGS analysis. This case illustrates that CNVs undetected by Sanger sequencing may be identified by WGS and highlights their relevance in the molecular diagnosis of a treatable neurologic condition such as CMS.

What's in the Literature?

In this edition of this column, we review new studies concerning the pathophysiology, treatment, and outcomes of patients with necrotizing myopathy, genetic testing in congenital myopathies, and limb girdle muscular dystrophies, and the incidence of polyneuropathy in the myotonic dystrophies. Various studies in myasthenia gravis, including those concerning antibody testing, clinical features, and quality of life are also reviewed as are recent findings in congenital myasthenic syndromes. Finally, 2 studies concerning polyneuropathy are discussed, including one on the association of polyneuropathy in patients with the metabolic syndrome and one on laboratory testing in patients with otherwise idiopathic small fiber polyneuropathy.