Exome sequencing detection of two untranslated GFPT1 mutations in a family with limb-girdle myasthenia

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.

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.

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.

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.

Next-Generation Sequencing Identifies Extended HLA Class I and II Haplotypes Associated With Early-Onset and Late-Onset Myasthenia Gravis in Italian, Norwegian, and Swedish Populations

Genetic susceptibility to myasthenia gravis (MG) associates with specific HLA alleles and haplotypes at the class I and II regions in various populations. Previous studies have only examined alleles at a limited number of HLA loci that defined only broad serotypes or alleles defined at the protein sequence level. Consequently, genetic variants in noncoding and untranslated HLA gene segments have not been fully explored but could also be important determinants for MG. To gain further insight into the role of HLA in MG, we applied next-generation sequencing to analyze sequence variation at eleven HLA genes in early-onset (EO) and late-onset (LO) non-thymomatous MG patients positive for the acetylcholine receptor (AChR) antibodies and ethnically matched controls from Italy, Norway, and Sweden. For all three populations, alleles and haplotype blocks present on the ancestral haplotype AH8.1 were associated with risk in AChR-EOMG patients. HLA-B*08:01:01:01 was the dominant risk allele in Italians (OR = 3.28, P = 1.83E-05), Norwegians (OR = 3.52, P = 4.41E-16), and in Swedes HLA-B*08:01 was the primary risk allele (OR = 4.24, P <2.2E-16). Protective alleles and haplotype blocks were identified on the HLA-DRB7, and HLA-DRB13.1 class II haplotypes in Italians and Norwegians, whereas in Swedes HLA-DRB7 exhibited the main protective effect. For AChR-LOMG patients, the HLA-DRB15.1 haplotype and associated alleles were significantly associated with susceptibility in all groups. The HLA-DR13-HLA-DR-HLA-DQ haplotype was associated with protection in all AChR-LOMG groups. This study has confirmed and extended previous findings that the immunogenetic predisposition profiles for EOMG and LOMG are distinct. In addition, the results are consistent with a role for non-coding HLA genetic variants in the pathogenesis of MG.

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.

The changing paradigm of intron retention: regulation, ramifications and recipes

Intron retention (IR) is a form of alternative splicing that has long been neglected in mammalian systems although it has been studied for decades in non-mammalian species such as plants, fungi, insects and viruses. It was generally assumed that mis-splicing, leading to the retention of introns, would have no physiological consequence other than reducing gene expression by nonsense-mediated decay. Relatively recent landmark discoveries have highlighted the pivotal role that IR serves in normal and disease-related human biology. Significant technical hurdles have been overcome, thereby enabling the robust detection and quantification of IR. Still, relatively little is known about the cis- and trans-acting modulators controlling this phenomenon. The fate of an intron to be, or not to be, retained in the mature transcript is the direct result of the influence exerted by numerous intrinsic and extrinsic factors at multiple levels of regulation. These factors have altered current biological paradigms and provided unexpected insights into the transcriptional landscape. In this review, we discuss the regulators of IR and methods to identify them. Our focus is primarily on mammals, however, we broaden the scope to non-mammalian organisms in which IR has been shown to be biologically relevant.

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.

Congenital myasthenic syndromes

OBJECTIVES

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

METHODS

Systematic literature review.

RESULTS

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

CONCLUSIONS

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

Whole-exome sequencing reveals a rare interferon gamma receptor 1 mutation associated with myasthenia gravis

Our study is aimed to explore the underlying genetic basis of myasthenia gravis. We collected a Chinese pedigree with myasthenia gravis, and whole-exome sequencing was performed on the two affected siblings and their parents. The candidate pathogenic gene was identified by bioinformatics filtering, which was further verified by Sanger sequencing. The homozygous mutation c.G40A (p.V14M) in interferon gamma receptor 1was identified. Moreover, the mutation was also detected in 3 cases of 44 sporadic myasthenia gravis patients. The p.V14M substitution in interferon gamma receptor 1 may affect the signal peptide function and the translocation on cell membrane, which could disrupt the binding of the ligand of interferon gamma and antibody production, contributing to myasthenia gravis susceptibility. We discovered that a rare variant c.G40A in interferon gamma receptor 1 potentially contributes to the myasthenia gravis pathogenesis. Further functional studies are needed to confirm the effect of the interferon gamma receptor 1 on the myasthenia gravis phenotype.

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.