GFPT1-myasthenia: clinical, structural, and electrophysiologic heterogeneity

Agenesis of Pectoralis Major Muscle in Late-Onset GFPT1-Related Congenital Myasthenic Syndrome: A Case Report

Objectives

The objective of this study was to expand the phenotypic spectrum of glutamine-fructose-6-phosphate transaminase 1 (GFPT1)-related congenital myasthenia syndrome (CMS).

Methods

A 61-year-old man with agenesis of the left pectoralis major muscle presented with progressive muscle weakness for a decade that transiently improved after exertion.

Results

His examination revealed proximal and distal muscle weakness in upper extremities and proximal muscle weakness in lower extremities. Muscle enzymes were elevated. An electromyogram revealed a myopathic pattern; however, a muscle biopsy of deltoid muscle and genetic testing for limb-girdle muscular dystrophies were nondiagnostic. A 3-Hz repetitive nerve stimulation of the spinal accessory nerve recording from trapezius muscle demonstrated a >20% drop in amplitude of the 5th compound motor action potential relative to 1st at both baseline and after 45-second exercise. Acetylcholine receptor binding, lipoprotein-related protein 4, muscle-specific kinase, and voltage-gated calcium channel P/Q antibodies were negative. Genetic testing targeting CMS revealed 2 likely pathogenic variants within GFPT1: novel c.7+2T>G (intron 1) that was predicted to result in a null allele and known c*22 C>A (exon 19) associated with reduced GFPT1 expression. His muscle strength dramatically improved after pyridostigmine initiation.

Discussion

In addition to other reported neurodevelopmental abnormalities, pectoralis major muscle agenesis (or Poland syndrome) may be a clinical manifestation of GFPT1-related CMS.

Splicing regulation of GFPT1 muscle-specific isoform and its roles in glucose metabolisms and neuromuscular junction

Glutamine:fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme of the hexosamine biosynthetic pathway (HBP). A 54-bp exon 9 of GFPT1 is specifically included in skeletal and cardiac muscles to generate a long isoform of GFPT1 (GFPT1-L). We showed that SRSF1 and Rbfox1/2 cooperatively enhance, and hnRNP H/F suppresses, the inclusion of human GFPT1 exon 9 by modulating recruitment of U1 snRNP. Knockout (KO) of GFPT1-L in skeletal muscle markedly increased the amounts of GFPT1 and UDP-HexNAc, which subsequently suppressed the glycolytic pathway. Aged KO mice showed impaired insulin-mediated glucose uptake, as well as muscle weakness and fatigue likely due to abnormal formation and maintenance of the neuromuscular junction. Taken together, GFPT1-L is likely to be acquired in evolution in mammalian striated muscles to attenuate the HBP for efficient glycolytic energy production, insulin-mediated glucose uptake, and the formation and maintenance of the neuromuscular junction.

The Hexosamine Biosynthesis Pathway: Regulation and Function

The hexosamine biosynthesis pathway (HBP) produces uridine diphosphate-N-acetyl glucosamine, UDP-GlcNAc, which is a key metabolite that is used for N- or O-linked glycosylation, a co- or post-translational modification, respectively, that modulates protein activity and expression. The production of hexosamines can occur via de novo or salvage mechanisms that are catalyzed by metabolic enzymes. Nutrients including glutamine, glucose, acetyl-CoA, and UTP are utilized by the HBP. Together with availability of these nutrients, signaling molecules that respond to environmental signals, such as mTOR, AMPK, and stress-regulated transcription factors, modulate the HBP. This review discusses the regulation of GFAT, the key enzyme of the de novo HBP, as well as other metabolic enzymes that catalyze the reactions to produce UDP-GlcNAc. We also examine the contribution of the salvage mechanisms in the HBP and how dietary supplementation of the salvage metabolites glucosamine and N-acetylglucosamine could reprogram metabolism and have therapeutic potential. We elaborate on how UDP-GlcNAc is utilized for N-glycosylation of membrane and secretory proteins and how the HBP is reprogrammed during nutrient fluctuations to maintain proteostasis. We also consider how O-GlcNAcylation is coupled to nutrient availability and how this modification modulates cell signaling. We summarize how deregulation of protein N-glycosylation and O-GlcNAcylation can lead to diseases including cancer, diabetes, immunodeficiencies, and congenital disorders of glycosylation. We review the current pharmacological strategies to inhibit GFAT and other enzymes involved in the HBP or glycosylation and how engineered prodrugs could have better therapeutic efficacy for the treatment of diseases related to HBP deregulation.

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.

Abnormal decrement on high-frequency repetitive nerve stimulation in congenital myasthenic syndrome with GFPT1 mutations and review of literature

Objectives

Congenital myasthenic syndrome (CMS) is a clinically and genetically heterogeneous group of inherited disorders characterized by neuromuscular junction defects. Mutations in GFPT1 have been shown to underlie CMS. An increasing number of patients with CMS due to mutations in GFPT1 have been reported. However, a comprehensive review of clinical and genetic analyses of GFPT-related CMS worldwide is lacking, especially, given that the common or hotspot mutations in GFPT1 have not been reported. Here, we described the clinical and genetic findings of three patients with GFPT1 mutations from southwestern China and reviewed the clinical and genetic features of patients with GFPT1-related CMS worldwide.

Methods

Clinical, laboratory, electrophysiological, myopathological, and genetic analyses of three patients with GFPT1-related CMS from southwestern China were conducted, and a review of previously published or reported cases about congenital myasthenic syndrome with GFPT1 mutations in the PubMed database was made.

Results

The clinical, laboratory, electrophysiological, and myopathological features by muscle biopsy of three patients with GFPT1-related CMS were consistent with those of previously reported patients with GFPT1 mutations. Additionally, an abnormal decrement in high-frequency RNS was found. Two different homozygous missense mutations (c.331C>T, p.R111C; c.44C>T, p.T15M) were detected by whole-exome sequencing (WES) or targeted neuromuscular disorder gene panels.

Conclusion

A distinct decremental response to high-frequency RNS was found in three patients with GFPT1-related CMS from southwestern China, which has never been reported thus far. In addition, the location and degree of tubular aggregates (TAs) seemed to be associated with the severity of clinical symptoms and serum creatine kinase levels, further expanding the phenotypic spectrum of GFPT1-related CMS. Lastly, some potential hotspot mutations in GFPT1 have been found in GFPT1-CMS worldwide.

GFPT1-Associated Congenital Myasthenic Syndrome Mimicking a Glycogen Storage Disease – Diagnostic Pitfalls in Myopathology Solved by Next-Generation-Sequencing

GFPT1-related congenital myasthenic syndrome (CMS) is characterized by progressive limb girdle weakness, and less prominent involvement of facial, bulbar, or respiratory muscles. While tubular aggregates in muscle biopsy are considered highly indicative in GFPT1-associated CMS, excessive glycogen storage has not been described. Here, we report on three affected siblings with limb-girdle myasthenia due to biallelic pathogenic variants in GFPT1: the previously reported missense variant c.41G >  A (p.Arg14Gln) and the novel truncating variant c.1265_1268del (p.Phe422TrpfsTer26). Patients showed progressive proximal atrophic muscular weakness with respiratory involvement, and a lethal disease course in adulthood. In the diagnostic workup at that time, muscle biopsy suggested a glycogen storage disease. Initially, Pompe disease was suspected. However, enzymatic activity of acid alpha-glucosidase was normal, and gene panel analysis including 38 genes associated with limb-girdle weakness (GAA included) remained unevocative. Hence, a non-specified glycogen storage myopathy was diagnosed. A decade later, the diagnosis of GFPT1-related CMS was established by genome sequencing. Myopathological reexamination showed pronounced glycogen accumulations, that were exclusively found in denervated muscle fibers. Only single fibers showed very small tubular aggregates, identified in evaluation of serial sections. This family demonstrates how diagnostic pitfalls can be addressed by an integrative approach including broad genetic analysis and re-evaluation of clinical as well as myopathological findings.

Diverse myopathological features in the congenital myasthenia syndrome with GFPT1 mutation

INTRODUCTION

Mutations in the GFPT1 gene are associated with a particular subtype of congenital myasthenia syndrome (CMS) called limb-girdle myasthenia with tubular aggregates. However, not all patients show tubular aggregates in muscle biopsy, suggesting the diversity of myopathology should be further investigated.

METHODS

In this study, we reported two unrelated patients clinically characterized by easy fatigability, limb-girdle muscle weakness, positive decrements of repetitive stimulation, and response to pyridostigmine. The routine examinations of myopathology were conducted. The causative gene was explored by whole-exome screening. In addition, we summarized all GFPT1-related CMS patients with muscle biopsy in the literature.

RESULTS

Pathogenic biallelic GFPT1 mutations were identified in the two patients. In patient one, muscle biopsy indicated vacuolar myopathic changes and atypical pathological changes of myofibrillar myopathy characterized by desmin deposits, Z-disc disorganization, and electronic dense granulofilamentous aggregation. In patient two, muscle biopsy showed typical myopathy with tubular aggregates. Among the 51 reported GFPT1-related CMS patients with muscle biopsy, most of them showed tubular aggregates myopathy, while rimmed vacuolar myopathy, autophagic vacuolar myopathy, mitochondria-like myopathy, neurogenic myopathy, and unspecific myopathic changes were also observed in some patients. These extra-synaptic pathological changes might be associated with GFPT1-deficiency hypoglycosylation and altered function of muscle-specific glycoproteins, as well as partly responsible for the permanent muscle weakness and resistance to acetylcholinesterase inhibitor therapy.

CONCLUSIONS

Most patients with GFPT1-related CMS had tubular aggregates in the muscle biopsy, but some patients could show great diversities of the pathological change. The myopathological findings might be a biomarker to predict the prognosis of the disease.

Cardiomyocyte protein O-GlcNAcylation is regulated by GFAT1 not GFAT2

In response to cardiac injury, increased activity of the hexosamine biosynthesis pathway (HBP) is linked with cytoprotective as well as adverse effects depending on the type and duration of injury. Glutamine-fructose amidotransferase (GFAT; gene name gfpt) is the rate-limiting enzyme that controls flux through HBP. Two protein isoforms exist in the heart called GFAT1 and GFAT2. There are conflicting data on the relative importance of GFAT1 and GFAT2 during stress-induced HBP responses in the heart. Using neonatal rat cardiac cell preparations, targeted knockdown of GFPT1 and GFPT2 were performed and HBP activity measured. Immunostaining with specific GFAT1 and GFAT2 antibodies was undertaken in neonatal rat cardiac preparations and murine cardiac tissues to characterise cell-specific expression. Publicly available human heart single cell sequencing data was interrogated to determine cell-type expression. Western blots for GFAT isoform protein expression were performed in human cardiomyocytes derived from induced pluripotent stem cells (iPSCs). GFPT1 but not GFPT2 knockdown resulted in a loss of stress-induced protein O-GlcNAcylation in neonatal cardiac cell preparations indicating reduced HBP activity. In rodent cells and tissue, immunostaining for GFAT1 identified expression in both cardiac myocytes and fibroblasts whereas immunostaining for GFAT2 was only identified in fibroblasts. Further corroboration of findings in human heart cells identified an enrichment of GFPT2 gene expression in cardiac fibroblasts but not ventricular myocytes whereas GFPT1 was expressed in both myocytes and fibroblasts. In human iPSC-derived cardiomyocytes, only GFAT1 protein was expressed with an absence of GFAT2. In conclusion, these results indicate that GFAT1 is the primary cardiomyocyte isoform and GFAT2 is only present in cardiac fibroblasts. Cell-specific isoform expression may have differing effects on cell function and should be considered when studying HBP and GFAT functions in the heart.

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.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

Congenital disorders of glycosylation: Still "hot" in 2020

BACKGROUND

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases caused by defects in the genes important for the process of protein and lipid glycosylation. With the ever growing number of the known subtypes and discoveries regarding the disease mechanisms and therapy development, it remains a very active field of study.

SCOPE OF REVIEW

This review brings an update on the CDG-related research since 2017, describing the novel gene defects, pathobiomechanisms, biomarkers and the patients' phenotypes. We also summarize the clinical guidelines for the most prevalent disorders and the current therapeutical options for the treatable CDG.

MAJOR CONCLUSIONS

In the majority of the 23 new CDG, neurological involvement is associated with other organ disease. Increasingly, different aspects of cellular metabolism (e.g., autophagy) are found to be perturbed in multiple CDG.

GENERAL SIGNIFICANCE

This work highlights the recent trends in the CDG field and comprehensively overviews the up-to-date clinical recommendations.

Congenital myasthenic syndrome caused by a frameshift insertion mutation in GFPT1

Objective

Description of a new variant of the glutamine-fructose-6-phosphate transaminase 1 (GFPT1) gene causing congenital myasthenic syndrome (CMS) in 3 children from 2 unrelated families.

Methods

Muscle biopsies, EMG, and whole-exome sequencing were performed.

Results

All 3 patients presented with congenital hypotonia, muscle weakness, respiratory insufficiency, head lag, areflexia, and gastrointestinal dysfunction. Genetic analysis identified a homozygous frameshift insertion in the GFPT1 gene (NM_001244710.1: c.686dupC; p.Arg230Ter) that was shared by all 3 patients. In one of the patients, inheritance of the variant was through uniparental disomy (UPD) with maternal origin. Repetitive nerve stimulation and single-fiber EMG was consistent with the clinical diagnosis of CMS with a postjunctional defect. Ultrastructural evaluation of the muscle biopsy from one of the patients showed extremely attenuated postsynaptic folds at neuromuscular junctions and extensive autophagic vacuolar pathology.

Conclusions

These results expand on the spectrum of known loss-of-function GFPT1 mutations in CMS12 and in one family demonstrate a novel mode of inheritance due to UPD.

Trouble at the junction: When myopathy and myasthenia overlap

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

Novel compound heterozygous GFPT1 mutations in a family with limb-girdle myasthenia with tubular aggregates

Limb-girdle myasthenia with tubular aggregates, a subtype of congenital myasthenic syndrome, is an extremely rare autosomal recessive genetic disease characterized by prominent limb-girdle weakness and good response to acetylcholinesterase inhibitor therapy. Herein, we reported two novel mutations of GFPT1 gene in a Chinese pedigree. Two siblings presented with fatigue, weakness of limb-girdle and decrement of the muscle action potential with repetitive nerve stimulation. Thus, myasthenia gravis was initially suspected, but anti-AChR antibodies were negative. Two novel missense mutations (p.Lys154Asn and p.Asn363Ser) in GFPT1 were identified through genetic testing conducted on 167 well-established genes associated with muscular diseases by targeted high throughput sequencing. Both mutations have not been recorded in the dsSNP database, Exome Aggregation Consortium database and 1000 Genomes Project database. The mutation sites were co-segregated with the phenotype and conserved between the different species. The mutations were not found in the 200 unrelated normal controls. Muscle biopsies revealed tubular aggregates, in accordance with previous reports with GFPT1 mutations. Subsequently, dramatic improvement in strength occurred following anti-cholinesterase therapy. Our study will be helpful for the diagnosis and treatment for Limb-girdle myasthenia with tubular aggregates.

Phenotype of a limb-girdle congenital myasthenic syndrome patient carrying a GFPT1 mutation

We report a 38-year-old woman who presented with mild proximal dominant muscle weakness and fatigability that fluctuated during menstruation and treatment with ephedrine-containing medication. The patient had been diagnosed with "congenital myopathy with tubular aggregates" by muscle biopsy at age 19. Her revised diagnosis was congenital myasthenic syndrome (CMS) caused by a mutation in GFPT1 (2p13.3 [MIM 610542], c.722_723insG homozygote, CMS-GFPT1) based on a screening gene analysis. Muscle CT revealed diffuse atrophy of proximal and axial muscles focused on the vastus lateralis, hamstrings, medial gastrocnemius and soleus muscles. Oral administration of pyridostigmine bromide clearly ameliorated weakness and fatigability. This is the first reported case of CMS-GFPT1 in Japan. Since CMS symptoms are reactive to treatment, it is important for clinicians to make an accurate diagnosis at an early stage to improve patient QOL. Tubular aggregates in muscle biopsy and day-to-day fluctuations are important features of the disorder. Quantitative muscle strength measurement was effective for evaluating treatment efficacy.

GFPT1 deficiency in muscle leads to myasthenia and myopathy in mice

Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway which yields precursors required for protein and lipid glycosylation. Mutations in GFPT1 and other genes downstream of this pathway cause congenital myasthenic syndrome (CMS) characterized by fatigable muscle weakness owing to impaired neurotransmission. The precise pathomechanisms at the neuromuscular junction (NMJ) owing to a deficiency in GFPT1 is yet to be discovered. One of the challenges we face is the viability of Gfpt1^−/− knockout mice. In this study, we use Cre/LoxP technology to generate a muscle-specific GFPT1 knockout mouse model, Gfpt1^tm1d/tm1d, characteristic of the human CMS phenotype. Our data suggest a critical role for muscle derived GFPT1 in the development of the NMJ, neurotransmission, skeletal muscle integrity and highlight that a deficiency in skeletal muscle alone is sufficient to cause morphological postsynaptic NMJ changes that are accompanied by presynaptic alterations despite the conservation of neuronal GFPT1 expression. In addition to the conventional morphological NMJ changes and fatigable muscle weakness, Gfpt1^tm1d/tm1d mice display a progressive myopathic phenotype with the presence of tubular aggregates in muscle, characteristic of the GFPT1-CMS phenotype. We further identify an upregulation of skeletal muscle proteins glypican-1, farnesyltransferase/geranylgeranyltransferase type-1 subunit α and muscle-specific kinase, which are known to be involved in the differentiation and maintenance of the NMJ. The Gfpt1^tm1d/tm1d model allows for further investigation of pathophysiological consequences on genes and pathways downstream of GFPT1 likely to involve misglycosylation or hypoglycosylation of NMJs and muscle targets.

Leukoencephalopathy due to variants in GFPT1-associated congenital myasthenic syndrome

OBJECTIVE

To determine the molecular etiology of disease in 4 individuals from 2 unrelated families who presented with proximal muscle weakness and features suggestive of mitochondrial disease.

METHODS

Clinical information and neuroimaging were reviewed. Genome sequencing was performed on affected individuals and biological parents.

RESULTS

All affected individuals presented with muscle weakness and difficulty walking. In one family, both children had neonatal respiratory distress while the other family had 2 children with episodic deteriorations. In each family, muscle biopsy demonstrated ragged red fibers. MRI was suggestive of a mitochondrial leukoencephalopathy, with extensive deep cerebral white matter T2 hyperintense signal and selective involvement of the middle blade of the corpus callosum. Through genome sequencing, homozygous GFPT1 missense variants were identified in the affected individuals of each family. The variants detected (p.Arg14Leu and p.Thr151Lys) are absent from population databases and predicted to be damaging by in silico prediction tools. Following the genetic diagnosis, nerve conduction studies were performed and demonstrated a decremental response to repetitive nerve stimulation, confirming the diagnosis of myasthenia. Treatment with pyridostigmine was started in one family with favorable response.

CONCLUSIONS

GFPT1 encodes a widely expressed protein that controls the flux of glucose into the hexosamine-biosynthesis pathway that produces precursors for glycosylation of proteins. GFPT1 variants and defects in other enzymes of this pathway have previously been associated with congenital myasthenia. These findings identify leukoencephalopathy as a previously unrecognized phenotype in GFPT1-related disease and suggest that mitochondrial dysfunction could contribute to this disorder.

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 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.

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.

Clinical and research strategies for limb-girdle congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are a group of rare disorders that cause fatigable muscle weakness due to defective signal transmission at the neuromuscular junction, a specialized synapse between peripheral motor neurons and their target muscle fibers. There are now over 30 causative genes that have been reported for CMS. Of these, there are 10 that are associated with a limb-girdle pattern of muscle weakness and are thus classed as LG-CMS. Next-generation sequencing and advanced methods of data sharing are likely to uncover further genes that are associated with similar clinical phenotypes, contributing to better diagnosis and effective treatment of LG-CMS patients. This review highlights clinical and pathological hallmarks of LG-CMS in relation to the underlying genetic defects and pathways. Tailored animal and cell models are essential to elucidate the exact function and pathomechanisms at the neuromuscular synapse that underlie LG-CMS. The integration of genomics and proteomics data derived from these models and patients reveals new and often unexpected insights that are relevant beyond the rare genetic disorder of LG-CMS and may extend to the functioning of mammalian synapses in health and disease more generally.

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.

Vesicular acetylcholine transporter defect underlies devastating congenital myasthenia syndrome

OBJECTIVE

To identify the genetic basis of a recessive congenital neurologic syndrome characterized by severe hypotonia, arthrogryposis, and respiratory failure.

METHODS

Identification of the responsible gene by exome sequencing and assessment of the effect of the mutation on protein stability in transfected rat neuronal-like PC12^A123.7 cells.

RESULTS

Two brothers from a nonconsanguineous Yemeni Jewish family manifested at birth with severe hypotonia and arthrogryposis. The older brother died of respiratory failure at 5 days of age. The proband, now 4.5 years old, has been mechanically ventilated since birth with virtually no milestones achievement. Whole exome sequencing revealed homozygosity of SLC18A3 c.1078G>C, p.Gly360Arg in the affected brothers but not in other family members. SLC18A3 p.Gly360Arg is not reported in world populations but is present at a carrier frequency of 1:30 in healthy Yemeni Jews. SLC18A3 encodes the vesicular acetylcholine transporter (VAChT), which loads newly synthesized acetylcholine from the neuronal cytoplasm into synaptic vesicles. Mice that are VAChT-null have been shown to die at birth of respiratory failure. In human VAChT, residue 360 is located in a conserved region and substitution of arginine for glycine is predicted to disrupt proper protein folding and membrane embedding. Stable transfection of wild-type and mutant human VAChT into neuronal-like PC12^A123.7 cells revealed similar mRNA levels, but undetectable levels of the mutant protein, suggesting post-translational degradation of mutant VAChT.

CONCLUSION

Loss of function of VAChT underlies severe arthrogryposis and respiratory failure. While most congenital myasthenic syndromes are caused by defects in postsynaptic proteins, VAChT deficiency is a presynaptic myasthenic syndrome.

Effective Treatment With Albuterol in DOK7 Congenital Myasthenic Syndrome in Children

BACKGROUND

Congenital myasthenic syndromes consist of rare disorders resulting from mutations in genes encoding for presynaptic, synaptic, and postsynaptic proteins that are involved in the signal transmission of the neuromuscular junction. They are characterized by fatigable weakness of the skeletal muscles with symptom onset from birth to early childhood. DOK7 (downstream of tyrosine kinase 7) congenital myasthenic syndrome was previously treated successfully with ephedrine and salbutamol; however, both are unavailable in the United States.

METHODS

Case report of a child with muscle weakness.

RESULTS

This report describes a boy who presented only with progressive limb-girdle muscle weakness since age 2 years. The muscle biopsy with extensive studies revealed no obvious etiologies. His muscle weakness rapidly worsened, requiring a wheelchair for daily activities. Expanded neuromuscular gene panel promptly led to the diagnosis of DOK7 congenital myasthenic syndrome, and his muscle strength dramatically and persistently improved in four weeks with albuterol treatment, allowing him to walk independently. In a brief literature review, 15 patients (five treated between ages 5 and 17 years) from the Mayo Clinic with DOK7 mutations were also successfully treated with albuterol.

CONCLUSION

DOK7 congenital myasthenic syndrome often presents with limb-girdle muscle weakness, which can become progressive without proper treatment. If muscle biopsy reveals no obvious etiology, an expanded neuromuscular gene panel may lead to a specific diagnosis of congenital myasthenic syndrome such as those due to DOK7 mutation. Albuterol is often used to treat bronchial asthma; however, it can also dramatically and persistently improve the muscle strength of DOK7 congenital myasthenic syndrome.

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Congenital Myasthenic Syndromes with Predominant Limb Girdle Weakness

Congenital myasthenic syndromes are a heterogeneous group of genetically determined disorders characterized by impaired neuromuscular transmission. They usually present from birth to childhood and are characterised by exercise induced weakness and fatigability. Genotype-phenotype correlations are difficult. However, in some patients particular phenotypic aspects may point towards a specific genetic defect. The absence of ptosis and ophthalmoparesis in patients with limb-girdle weakness makes the diagnosis of a neuromuscular transmission defect particularly challenging (LG-CMS). This is illustrated by a well-documented case published by Walton in 1956. The diagnosis of LG-CMS is secured by demonstrating a neuromuscular transmission defect with single fibre EMG or repetitive nerve stimulation, in the absence of auto-antibodies. Ultimately, a genetic test is required to identify the underlying cause and assure counselling and optimization of treatment. LG-CMS are inherited in autosomal recessive traits, and are often associated with mutations in DOK7 and GFPT1, and less frequently with mutations in COLQ, ALG2, ALG14 and DPAGT. Genetic characterization of CMS is of the upmost importance when choosing the adequate treatment. Some of the currently used drugs can either ameliorate or aggravate the symptoms depending on the underlying genetic defect. The drug most frequently used for the treatment of CMS is pyridostigmine an acetylcholinesterase inhibitor. However, pyridostigmine is not effective or is even detrimental in DOK7- and COLQ-related LG-CMS, while beta-adrenergic agonists (ephedrine, salbutamol) show some sustained benefit. Standard clinical trials may be difficult, but standardized follow-up of patients and international collaboration may help to improve the standards of care of these conditions.

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.

Inherited disorders of the neuromuscular junction: an update

Congenital myasthenic syndromes (CMSs) are a group of heterogeneous inherited disorders caused by mutations in genes affecting the function and structure of the neuromuscular junction. This review updates the reader on established and novel subtypes of congenital myasthenia, and the treatment strategies for these increasingly heterogeneous disorders. The discovery of mutations associated with the N-glycosylation pathway and in the family of serine peptidases has shown that causative genes encoding ubiquitously expressed molecules can produce defects at the human neuromuscular junction. By contrast, mutations in lipoprotein-like receptor 4 (LRP4), a long-time candidate gene for congenital myasthenia, and a novel phenotype of myasthenia with distal weakness and atrophy due to mutations in AGRN have now been described. In addition, a pathogenic splicing mutation in a nonfunctional exon of CHRNA1 has been reported emphasizing the importance of analysing nonfunctional exons in genetic analysis. The benefit of salbutamol and ephedrine alone or combined with pyridostigmine or 3,4-DAP is increasingly being reported for particular subtypes of CMS.

Endomicroscopy and electromyography of neuromuscular junctions in situ

Objective

Electromyography (EMG) is used routinely to diagnose neuromuscular dysfunction in a wide range of peripheral neuropathies, myopathies, and neuromuscular degenerative diseases including motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Definitive neurological diagnosis may also be indicated by the analysis of pathological neuromuscular innervation in motor-point biopsies. Our objective in this study was to preempt motor-point biopsy by combining live imaging with electrophysiological analysis of slow degeneration of neuromuscular junctions (NMJs) in vivo.

Methods

We combined conventional needle electromyography with fiber-optic confocal endomicroscopy (CEM), using an integrated hand-held, 1.5-mm-diameter probe. We utilized as a test bed, various axotomized muscles in the hind limbs of anaesthetized, double-homozygous thy1.2YFP16: Wld^S mice, which coexpress the Wallerian-degeneration Slow (Wld^S) protein and yellow fluorescent protein (YFP) in motor neurons. We also tested exogenous vital stains, including Alexa488-α-bungarotoxin; the styryl pyridinium dye 4-Di-2-Asp; and a GFP conjugate of botulinum toxin Type A heavy chain (GFP-HcBoNT/A).

Results

We show that an integrated EMG/CEM probe is effective in longitudinal evaluation of functional and morphological changes that take place over a 7-day period during axotomy-induced, slow neuromuscular synaptic degeneration. EMG amplitude declined in parallel with overt degeneration of motor nerve terminals. EMG/CEM was safe and effective when nerve terminals and motor endplates were selectively stained with vital dyes.

Interpretation

Our findings constitute proof-of-concept, based on live imaging in an animal model, that combining EMG/CEM may be useful as a minimally invasive precursor or alternative to motor-point biopsy in neurological diagnosis and for monitoring local administration of potential therapeutics.

DPAGT1 myasthenia and myopathy: genetic, phenotypic, and expression studies

OBJECTIVE

To investigate patients with DPAGT1 (UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephosphotransferase 1)-associated myasthenic syndrome.

METHODS

We performed exome and Sanger sequencing, determined glycoprotein expression in patient muscles, assessed pathogenicity of the mutant proteins by examining their expression and enzymatic activity in transfected cells, evaluated structural changes in muscle and the neuromuscular junction, and examined electrophysiologic aspects of neuromuscular transmission in vitro.

RESULTS

Patients 1 and 2, 16 and 14 years of age, had progressive fatigable weakness since infancy and are intellectually disabled. Patient 3, a less severely affected brother of patient 1, also has autistic features. Each patient harbors 2 novel heteroallelic mutations in DPAGT1, an enzyme subserving protein N-glycosylation. Patients 1 and 3 harbor Met1Leu, which reduces protein expression, and His375Tyr, which decreases enzyme activity. Patient 2 carries Val264Met, which abolishes enzyme activity, and a synonymous Leu120Leu mutation that markedly augments exon skipping, resulting in some skipped and infrequent nonskipped alleles. Therefore, the nonskipped allele rescues the phenotype. Intracellular microelectrode studies indicate combined pre- and postsynaptic defects of neuromuscular transmission with evidence for somatic mosaicism in patient 2. Structural studies reveal hypoplastic endplates, fiber-type disproportion, tubular aggregates, and degeneration of muscle fiber organelles resulting in autophagocytosis.

CONCLUSIONS

DPAGT1 myasthenia affects multiple parameters of neuromuscular transmission, causes fiber-type disproportion and an autophagic myopathy, and can be associated with intellectual disability. We speculate that hypoglycosylation of synapse-specific proteins causes defects in central as well as motor synapses.