Limb-girdle Congenital Myasthenic Syndrome Research Articles

Pathogenic DPAGT1 variants in limb-girdle congenital myasthenic syndrome (LG-CMS) associated with tubular aggregates and ORAI1 hypoglycosylation.

Limb-girdle congenital myasthenic syndrome (LG-CMS) is a genetically heterogeneous disorder characterized by muscle weakness and fatigability. The LG-CMS gene DPAGT1 codes for an essential enzyme of the glycosylation pathway, a posttranslational modification mechanism shaping the structure and function of proteins. In DPAGT1-related LG-CMS, reduced glycosylation of the acetylcholine receptor (AChR) reduces its localization at the neuromuscular junction (NMJ), and results in diminished neuromuscular transmission. LG-CMS patients also show tubular aggregates on muscle biopsy, but the origin and potential contribution of the aggregates to disease development are not understood. Here, we describe two LG-CMS patients with the aim of providing a molecular diagnosis and to shed light on the pathways implicated in tubular aggregate formation. Following clinical examination of the patients, we performed next-generation sequencing (NGS) to identify the genetic causes, analysed the biopsies at the histological and ultrastructural levels, investigated the composition of the tubular aggregates, and performed experiments on protein glycosylation. We identified novel pathogenic DPAGT1 variants in both patients, and pyridostigmine treatment quantitatively improved muscle force and function. The tubular aggregates contained proteins of the sarcoplasmic reticulum (SR) and structurally conformed to the aggregates observed in tubular aggregate myopathy (TAM). TAM arises from overactivation of the plasma membrane calcium channel ORAI1, and functional studies on muscle extracts from our LG-CMS patients evidenced abnormal ORAI1 glycosylation. We expand the genetic variant spectrum of LG-CMS and provide a genotype/phenotype correlation for pathogenic DPAGT1 variants. The discovery of ORAI1 hypoglycosylation in our patients highlights a physiopathological link between LG-CMS and TAM.

Clinical and genetic characterisation of a large Indian congenital myasthenic syndrome cohort.

Congenital myasthenic syndromes (CMS) are a rare group of inherited disorders caused by gene defects associated with the neuromuscular junction and potentially treatable with commonly available medications such as acetylcholinesterase inhibitors and β2 adrenergic receptor agonists. In this study, we identified and genetically characterized the largest cohort of CMS patients from India to date. Genetic testing of clinically suspected patients evaluated in a South Indian hospital during the period 2014-19 was carried out by standard diagnostic gene panel testing or using a two-step method that included hotspot screening followed by whole-exome sequencing. In total, 156 genetically diagnosed patients (141 families) were characterized and the mutational spectrum and genotype-phenotype correlation described. Overall, 87 males and 69 females were evaluated, with the age of onset ranging from congenital to fourth decade (mean 6.6 ± 9.8 years). The mean age at diagnosis was 19 ± 12.8 (1-56 years), with a mean diagnostic delay of 12.5 ± 9.9 (0-49 years). Disease-causing variants in 17 CMS-associated genes were identified in 132 families (93.6%), while in nine families (6.4%), variants in genes not associated with CMS were found. Overall, postsynaptic defects were most common (62.4%), followed by glycosylation defects (21.3%), synaptic basal lamina genes (4.3%) and presynaptic defects (2.8%). Other genes found to cause neuromuscular junction defects (DES, TEFM) in our cohort accounted for 2.8%. Among the individual CMS genes, the most commonly affected gene was CHRNE (39.4%), followed by DOK7 (14.4%), DPAGT1 (9.8%), GFPT1 (7.6%), MUSK (6.1%), GMPPB (5.3%) and COLQ (4.5%). We identified 22 recurrent variants in this study, out of which eight were found to be geographically specific to the Indian subcontinent. Apart from the known common CHRNE variants p.E443Kfs*64 (11.4%) and DOK7 p.A378Sfs*30 (9.3%), we identified seven novel recurrent variants specific to this cohort, including DPAGT1 p.T380I and DES c.1023+5G>A, for which founder haplotypes are suspected. This study highlights the geographic differences in the frequencies of various causative CMS genes and underlines the increasing significance of glycosylation genes (DPAGT1, GFPT1 and GMPPB) as a cause of neuromuscular junction defects. Myopathy and muscular dystrophy genes such as GMPPB and DES, presenting as gradually progressive limb girdle CMS, expand the phenotypic spectrum. The novel genes MACF1 and TEFM identified in this cohort add to the expanding list of genes with new mechanisms causing neuromuscular junction defects.

A novel DOK7 mutation causing limb-girdle congenital myasthenic syndrome

We report a case series of 5 Latino patients with limb-girdle pattern weakness, four patients are sisters, with one patient unrelated. Repetitive nerve stimulation showed a significant decrement in all cases. Targeted genetic testing for congenital myasthenic syndromes demonstrated a known DOK7 pathogenic mutation in each case, and in all five cases also revealed a novel DOK7 missense mutation in exon 7 with c.94G>A; providing strong evidence this mutation is pathogenic. DOK7-related congenital myasthenic syndrome often lacks oculobulbar involvement, and may present with limb-girdle weakness, mimicking limb-girdle muscular dystrophy.

FP.21 Congenital myasthenic syndrome with Desmin aggregates: A novel association in recessive desminopathies due to a recurrent intronic DES mutation

Desminopathies (DES; OMIM: 125660) are highly variable presenting as cardiomyopathy, myofibrillar myopathy (MFM) or a combination of both. Dominant mutations are more common while a few severe cases of recessive myofibrillar myopathy were reported with complete loss of desmin. Here, we report three unsolved Limb girdle congenital myasthenic syndrome (LG-CMS) patients with a recurrent homozygous intronic DES mutation identified by systematic reanalysis through Solve-RD project. Three unrelated patients (M:F – 2:1) aged 16, 21 and 22 years presented with history of delayed motor milestones, childhood onset of progressive muscle involvement. Fatigable ptosis and limb weakness was reported in all three patients with gradually progressive speech and swallowing difficulty. Elevated serum creatine kinase was common finding and repetitive nerve stimulation showed decrement (>10%) in two patients. Clinical improvement of 25% with pyridostigmine and salbutamol was noted in one patient. Initial exome sequencing analysis did not reveal any significant variants in known CMS genes. On re-analysis, we identified a novel intronic homozygous substitution DES(NM_001927.4):c.1023+5G>A in all three patients. In-silico analysis predicted pathogenicity due to altered donor splice site. Muscle biopsy in one of the male patients showed mixed pattern with reduced sarcoplasmic Desmin staining along with Desmin positive aggregates. Neuromuscular junction involvement has been previously reported in severe desmin-null patients. The presence of a recessive CMS-MFM phenotype of moderate severity with desmin aggregates and absence of cardiac involvement in our patients expands the known spectrum of Desminopathies and reiterates the importance of early genetic diagnosis to identify treatable CMS phenotypes. Due to the presence of identical rare homozygous variant in three unrelated south Indian patients, we suspect a possibility of founder allele.

OTHER NMDs: EP.349 GFPT1-related myopathy overlapping with autophagic vacuolar myopathy. A case report

Autophagic vacuolar myopathies are a heterogeneous group of muscle diseases with a common denominator of altered autophagy and defective protein quality control. Pathogenic variants in GFPT1 cause limb-girdle congenital myasthenic syndrome. Muscle biopsies from affected patients usually show characteristic tubular aggregates, fiber disproportion, and in limited reports, autophagic vacuoles. A 27-year-old female born to non-consanguineous parents, presented with a progressive axial and limb-girdle weakness from 22 months of age, without cognitive impairment. She developed a progressive dorsolumbar scoliosis and restrictive respiratory pattern from 4 years of age. CK levels were normal and EMG without RNS revealed a myopathic pattern. The muscle biopsy showed variable fiber size and large numbers of fibres containing single or multiple cytoplasmic vacuoles containing cellular debris. Oxidative activity was increased in those fibers. Muscle MRI revealed generalized atrophy predominantly in axial and limb-girdle muscles. Trio-WES analysis failed to find pathogenic variants in genes related to autophagic vacuolar myopathy, but research-based reanalysis identified a homozygous pathogenic variant in GFPT1 (NM_001244710.1 c.686-2A>G). RNS revealed a mild and inconstant area decrement at trapezius muscle and SFEMG a pathological jitter with blocks at extensor digitorum communis, consistent with the diagnosis of congenital myasthenic syndrome. Treatment with pyridostigmine and 2,4-diaminopyrimidine was initiated with mild improvement. She is now able to walk independently indoors holding her head, without respiratory or oropharyngeal concerns. The initial diagnosis of autophagic vacuolar myopathy based on the presence of autophagic vacuoles delayed the genetic diagnosis and treatment in this case. We propose the inclusion of GFPT1 under the umbrella of autophagic vacuolar myopathies and encourage a deeper study of the GFPT1 function in autophagy.

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.

Mutations in GFPT1-related congenital myasthenic syndromes are associated with synaptic morphological defects and underlie a tubular aggregate myopathy with synaptopathy.

Mutations in GFPT1 (glutamine-fructose-6-phosphate transaminase 1), a gene encoding an enzyme involved in glycosylation of ubiquitous proteins, cause a limb-girdle congenital myasthenic syndrome (LG-CMS) with tubular aggregates (TAs) characterized predominantly by affection of the proximal skeletal muscles and presence of highly organized and remodeled sarcoplasmic tubules in patients' muscle biopsies. We report here the first long-term clinical follow-up of 11 French individuals suffering from LG-CMS with TAs due to GFPT1 mutations, of which nine are new. Our retrospective clinical evaluation stresses an evolution toward a myopathic weakness that occurs concomitantly to ineffectiveness of usual CMS treatments. Analysis of neuromuscular biopsies from three unrelated individuals demonstrates that the maintenance of neuromuscular junctions (NMJs) is dramatically impaired with loss of post-synaptic junctional folds and evidence of denervation-reinnervation processes affecting the three main NMJ components. Moreover, molecular analyses of the human muscle biopsies confirm glycosylation defects of proteins with reduced O-glycosylation and show reduced sialylation of transmembrane proteins in extra-junctional area. Altogether, these results pave the way for understanding the etiology of this rare neuromuscular disorder that may be considered as a "tubular aggregates myopathy with synaptopathy".

DPAGT1-CDG: Functional analysis of disease-causing pathogenic mutations and role of endoplasmic reticulum stress.

Pathogenic mutations in DPAGT1 are manifested as two possible phenotypes: congenital disorder of glycosylation DPAGT1-CDG (also known as CDG-Ij), and limb-girdle congenital myasthenic syndrome (CMS) with tubular aggregates. UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosamine phosphotransferase (GPT), the protein encoded by DPAGT1, is an endoplasmic reticulum (ER)-resident protein involved in an initial step in the N-glycosylation pathway. The aim of the present study was to examine the effect of six variants in DPAGT1 detected in patients with DPAGT1-CDG, and the role of endoplasmic reticulum stress, as part of the search for therapeutic strategies to use against DPAGT1-CDG. The effect of the six mutations, i.e., c.358C>A (p.Leu120Met), c.791T>G (p.Val264Gly), c.901C>T (p.Arg301Cys), c.902G>A (p.Arg301His), c.1154T>G (p.Leu385Arg), and of the novel mutation c.329T>C (p.Phe110Ser), were examined via the analysis of DPAGT1 transcriptional profiles and GTP levels in patient-derived fibroblasts. In addition, the transient expression of different mutations was analysed in COS-7 cells. The results obtained, together with those of bioinformatic studies, revealed these mutations to affect the splicing process, the stability of GTP, or the ability of this protein to correctly localise in the ER membrane. The unfolded protein response (UPR; the response to ER stress) was found not to be active in patient-derived fibroblasts, unlike that seen in cells from patients with PMM2-CDG or DPM1-CDG. Even so, the fibroblasts of patients with DPAGT1-CDG seemed to be more sensitive to the stressor tunicamycin. The present work improves our knowledge of DPAGT1-CDG and provides bases for developing tailored splicing and folding therapies.

Limb-girdle congenital myasthenic syndrome in a Chinese family with novel mutations in MUSK gene and literature review

ObjectivesTo describe the clinical and genetic features of a Chinese congenital myasthenic syndromes (CMS) patient with two novel missense mutations in muscle specific receptor tyrosine kinase (MUSK) gene and review 15 MUSK-related CMS patients from 8 countries. MethodsThe patient was a 30-year-old man with chronic progressively proximal limb weakness for 22 years and diagnosed as muscular dystrophy before. Serum creatine kinase (CK) was normal. Repetitive nerve stimulation (RNS) test showed decrements at low rate stimulation. Weakness became worse after conventional doses of pyridostigmine. Mild multiple atrophy of thigh and leg muscle was observed in MRI. Open muscle biopsy and genetic analysis were performed. One hundred healthy individuals were set for control. ResultsMuscle biopsy showed mild variation in fiber size. Two missense mutations in MUSK gene (p.P650T and p.I795S) were identified in the patient. The mutation of p.I795S was identified in his father and p.P650T in his mother. Both of them were not detected among the healthy controls and predicted to be damaging or disease causing by prediction tools. ConclusionIn this study, we identified a limb-girdle CMS (LG-CMS) patient carrying two novel heterozygous missense mutations in MUSK gene. CMS related genes should be analyzed in patients with limb-girdle weakness, normal CK, decrement of CMAP at RNS and mild change in muscle biopsy or MRI.

Global N-linked Glycosylation is Not Significantly Impaired in Myoblasts in Congenital Myasthenic Syndromes Caused by Defective Glutamine-Fructose-6-Phosphate Transaminase 1 (GFPT1).

Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the first enzyme of the hexosamine biosynthetic pathway. It transfers an amino group from glutamine to fructose-6-phosphate to yield glucosamine-6-phosphate, thus providing the precursor for uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) synthesis. UDP-GlcNAc is an essential substrate for all mammalian glycosylation biosynthetic pathways and N-glycan branching is especially sensitive to alterations in the concentration of this sugar nucleotide. It has been reported that GFPT1 mutations lead to a distinct sub-class of congenital myasthenic syndromes (CMS) termed “limb-girdle CMS with tubular aggregates”. CMS are hereditary neuromuscular transmission disorders in which neuromuscular junctions are impaired. To investigate whether alterations in protein glycosylation at the neuromuscular junction might be involved in this impairment, we have employed mass spectrometric strategies to study the N-glycomes of myoblasts and myotubes derived from two healthy controls, three GFPT1 patients, and four patients with other muscular diseases, namely CMS caused by mutations in DOK7, myopathy caused by mutations in MTND5, limb girdle muscular dystrophy type 2A (LGMD2A), and Pompe disease. A comparison of the relative abundances of bi-, tri-, and tetra-antennary N-glycans in each of the cell preparations revealed that all samples exhibited broadly similar levels of branching. Moreover, although some differences were observed in the relative abundances of some of the N-glycan constituents, these variations were modest and were not confined to the GFPT1 samples. Therefore, GFPT1 mutations in CMS patients do not appear to compromise global N-glycosylation in muscle cells.

A 3'-UTR mutation creates a microRNA target site in the GFPT1 gene of patients with congenital myasthenic syndrome.

Mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) cause the neuromuscular disorder limb-girdle congenital myasthenic syndrome (LG-CMS). One recurrent GFPT1 mutation detected in LG-CMS patients is a c.*22C>A transversion in the 3'-untranslated region (UTR). Because this variant does not alter the GFPT1 open reading frame, its pathogenic relevance has not yet been established. We found that GFPT1 protein levels were reduced in myoblast cells of the patients carrying this variant. In silico algorithms predicted that the mutation creates a microRNA target site for miR-206*. Investigation of the expression of this so far unrecognized microRNA confirmed that miR-206* (like its counterpart miR-206) is abundant in skeletal muscle. MiR-206* efficiently reduced the expression of reporter constructs containing the mutated 3'-UTR while no such effect was observed with reporter constructs containing the wild-type 3'-UTR or when a specific anti-miR-206* inhibitor was added. Moreover, anti-miR-206* inhibitor treatment substantially rescued GFPT1 expression levels in patient-derived myoblasts. Our data demonstrate that the c.*22C>A mutation in the GFPT1 gene leads to illegitimate binding of microRNA resulting in reduced protein expression. We confirm that c.*22C>A is a causative mutation and suggest that formation of microRNA target sites might be a relevant pathomechanism in Mendelian disorders. Variants in the 3'-UTRs should be considered in genetic diagnostic procedures.

G.P.141: Dpagt1 mutation: Limb-girdle congenital myasthenic syndrome due to glycosylation defect

Congenital myasthenic syndromes (CMSs) are heterogeneous genetic disorders that result from impaired signal transmission at the neuromuscular junction (NMJ). Congenital disorders of glycosylation (CDG) are a challenging group of diseases affecting different organs in particular the central nervous system and the skeletal muscle. Only recently CDG have been implicated in some forms of post-synaptic autosomal recessive CMS. The protein encoded by the DPAGT1 gene is an enzyme that catalyzes the first step in the dolichol-linked oligosaccharide pathway for glycoprotein biosynthesis. Mutations in DPAGT1 were described in patients with CMS and tubular aggregates on muscle biopsy. We report a 16 year old young man, first child of a consanguineous family, who presented with walking difficultywith onset by the age of 3 year. The motor milestones were slightly delayed. There was speech difficulty which fluctuated during the day. On examination, he had generalized muscle weakness with proximal groups being more affected, excessive lordosis, waddling gait, and contractures of the Achilles tendons. There was no bulbar or extra-ocular muscle involvement. He had a mild intellectual disability with an IQ score of 50–55. Tensilon test was positive. Electromyography revealed decrementing response to repetitive nerve stimulation. He responded well to pyridostigmine, 3,4-diaminopyridine and salbutamol treatment. We carried out whole exome sequencing and identified a novel homozygous DPAGT1 mutation (c.509A>T). The non-synonymous change (p.Tyr170Phe) is predicted to be deleterious by in silico analysis and affects an aminoacid residue that is highly conserved across species. In conclusion we report a novel mutation on the DPAGT1 gene responsible for a form of “limb-girdle CMS”. Comparing with the previously reported cases this family has mental retardation as an unusual feature and a good response to a combined treatment with pyridostigmine and 3,4-diaminopyridine.

Salbutamol-responsive limb-girdle congenital myasthenic syndrome due to a novel missense mutation and heteroallelic deletion in MUSK

Congenital myasthenic syndromes (CMS) are clinically and genetically heterogeneous disorders characterized by a neuromuscular transmission defect. In recent years, causative mutations have been identified in atleast 15 genes encoding proteins of the neuromuscular junction. Mutations in MUSK are known as a very rare genetic cause of CMS and have been described in only three families, world-wide. Consequently, the knowledge about efficient drug therapy is very limited. We identified a novel missense mutation (p.Asp38Glu) heteroallelic to a genomic deletion affecting exons 2–3 of MUSK as cause of a limb-girdle CMS in two brothers of Turkish origin. Clinical symptoms included fatigable limb weakness from early childhood on. Upon diagnosis of a MUSK-related CMS at the age of 16 and 13years, respectively, treatment with salbutamol was initiated leading to an impressive improvement of clinical symptoms, while treatment with esterase inhibitors did not show any benefit. Our findings highlight the importance of a molecular diagnosis in CMS and demonstrate considerable similarities between patients with MUSK and DOK7-related CMS in terms of clinical phenotype and treatment options.

Clinical features in a large Iranian family with a limb-girdle congenital myasthenic syndrome due to a mutation in DPAGT1

Mutations in DPAGT1 are a newly recognised cause of congenital myasthenic syndrome. DPAGT1 encodes an early component of the N-linked glycosylation pathway. Initially mutations in DPAGT1 have been associated with the onset of the severe multisystem disorder – congenital disorder of glycosylation type 1J. However, recently it was established that certain mutations in this gene can cause symptoms restricted to muscle weakness resulting from defective neuromuscular transmission. We report four cases from a large Iranian pedigree with prominent limb-girdle weakness and minimal craniobulbar symptoms who harbour a novel mutation in DPAGT1, c.652C>T, p.Arg218Trp. This myasthenic syndrome may mimic myopathic disorders and is likely under-diagnosed.

Mutations in GFPT1 that underlie limb-girdle congenital myasthenic syndrome result in reduced cell-surface expression of muscle AChR

Mutations in GFPT1 underlie a congenital myasthenic syndrome (CMS) characterized by a limb-girdle pattern of muscle weakness. Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is a key rate-limiting enzyme in the hexosamine biosynthetic pathway providing building blocks for the glycosylation of proteins and lipids. It is expressed ubiquitously and it is not readily apparent why mutations in this gene should cause a syndrome with symptoms restricted to muscle and, in particular, to the neuromuscular junction. Data from a muscle biopsy obtained from a patient with GFPT1 mutations indicated that there were reduced endplate acetylcholine receptors. We, therefore, further investigated the relationship between identified mutations in GFPT1 and expression of the muscle acetylcholine receptor. Cultured myotubes derived from two patients with GFPT1 mutations showed a significant reduction in cell-surface AChR expression (Pt1 P < 0.0001; Pt2 P = 0.0097). Inhibition of GFPT1 enzymatic activity or siRNA silencing of GFPT1 expression both resulted in reduced AChR cell-surface expression. Western blot and gene-silencing experiments indicate this is due to reduced steady-state levels of AChR α, δ, ε, but not β subunits rather than altered transcription of AChR-subunit RNA. Uridine diphospho-N-acetylglucosamine, a product of the hexosamine synthetic pathway, acts as a substrate at an early stage in the N-linked glycosylation pathway. Similarity between CMS due to GFPT1 mutations and CMS due to DPAGT1 mutations would suggest that reduced endplate AChR due to defective N-linked glycosylation is a primary disease mechanism in this disorder.

Mutations in DPAGT1 Cause a Limb-Girdle Congenital Myasthenic Syndrome with Tubular Aggregates

Congenital myasthenic syndromes are a heterogeneous group of inherited disorders that arise from impaired signal transmission at the neuromuscular synapse. They are characterized by fatigable muscle weakness. We performed whole-exome sequencing to determine the underlying defect in a group of individuals with an inherited limb-girdle pattern of myasthenic weakness. We identify DPAGT1 as a gene in which mutations cause a congenital myasthenic syndrome. We describe seven different mutations found in five individuals with DPAGT1 mutations. The affected individuals share a number of common clinical features, including involvement of proximal limb muscles, response to treatment with cholinesterase inhibitors and 3,4-diaminopyridine, and the presence of tubular aggregates in muscle biopsies. Analyses of motor endplates from two of the individuals demonstrate a severe reduction of endplate acetylcholine receptors. DPAGT1 is an essential enzyme catalyzing the first committed step of N-linked protein glycosylation. Our findings underscore the importance of N-linked protein glycosylation for proper functioning of the neuromuscular junction. Using the DPAGT1-specific inhibitor tunicamycin, we show that DPAGT1 is required for efficient glycosylation of acetylcholine-receptor subunits and for efficient export of acetylcholine receptors to the cell surface. We suggest that the primary pathogenic mechanism of DPAGT1 mutations is reduced levels of acetylcholine receptors at the endplate region. These individuals share clinical features similar to those of congenital myasthenic syndrome due to GFPT1 mutations, and their disorder might be part of a larger subgroup comprising the congenital myasthenic syndromes that result from defects in the N-linked glycosylation pathway and that manifest through impaired neuromuscular transmission.

1106 The clinical phenotypic spectrum of GFPT1 associated congenital myasthenic syndrome

We report on the clinical features of a distinct group of DOK 7 negative limb girdle congenital myasthenic syndrome (LG-CMS), the cause of which has only recently been unravelled. Congenital...

Congenital myasthenic syndrome with tubular aggregates caused by GFPT1 mutations

Congenital myasthenic syndrome (CMS) is a clinically and genetically heterogeneous group of inherited disorders of the neuromuscular junction. A difficult to diagnose subgroup of CMS is characterised by proximal muscle weakness and fatigue while ocular and facial involvement is only minimal. DOK7 mutations have been identified as causing the disorder in about half of the cases. More recently, using classical positional cloning, we have identified mutations in a previously unrecognised CMS gene, GFPT1, in a series of DOK7-negative cases. However, detailed description of clinical features of GFPT1 patients has not been reported yet. Here we describe the clinical picture of 24 limb-girdle CMS (LG-CMS) patients and pathological findings of 18 of them, all carrying GFPT1 mutations. Additional patients with CMS, but without tubular aggregates, and patients with non-fatigable weakness with tubular aggregates were also screened. In most patients with GFPT1 mutations, onset of the disease occurs in the first decade of life with characteristic limb-girdle weakness and fatigue. A common feature was beneficial and sustained response to acetylcholinesterase inhibitor treatment. Most of the patients who had a muscle biopsy showed tubular aggregates in myofibers. Analysis of endplate morphology in one of the patients revealed unspecific abnormalities. Our study delineates the phenotype of CMS associated with GFPT1 mutations and expands the understanding of neuromuscular junction disorders. As tubular aggregates in context of a neuromuscular transmission defect appear to be highly indicative, we suggest calling this condition congenital myasthenic syndrome with tubular aggregates (CMS-TA).

P26 Pyridostigmine-responsive limb-girdle congenital myasthenic syndrome with frequent tubular aggregates

P26 Pyridostigmine-responsive limb-girdle congenital myasthenic syndrome with frequent tubular aggregates

Hexosamine Biosynthetic Pathway Mutations Cause Neuromuscular Transmission Defect

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.