Hexosamine Biosynthetic Pathway Mutations Cause Neuromuscular Transmission Defect

Institute of Cell Biology, Eidgenössische Technische Hochschule (ETH) Zürich, Zürich, Switzerland.
The American Journal of Human Genetics (Impact Factor: 10.93). 02/2011; 88(2):162-72. DOI: 10.1016/j.ajhg.2011.01.008
Source: PubMed


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.

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Available from: Ariana Kariminejad
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    • "Knockdown of several CMS genes in zebrafish embryos has been recently used to demonstrate direct or indirect effects on the neuromuscular junction [35] [36]. Interestingly, SLC25A1 knockdown showed variable phenotype ranging from mild, moderate to severe; with confirmed aberrant NMJ development and function similar to what was seen in the 2 affected siblings. "
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    ABSTRACT: Background and Objective: Congenital myasthenic syndromes are rare inherited disorders characterized by fatigable weakness caused by malfunction of the neuromuscular junction. We performed whole exome sequencing to unravel the genetic aetiology in an English sib pair with clinical features suggestive of congenital myasthenia. Methods:We used homozygosity mapping and whole exome sequencing to identify the candidate gene variants. Mutant protein expression and function were assessed in vitro and a knockdown zebrafish model was generated to assess neuromuscular junction development. Results:We identified a novel homozygous missense mutation in the SLC25A1 gene, encoding the mitochondrial citrate carrier. Mutant SLC25A1 showed abnormal carrier function. SLC25A1 has recently been linked to a severe, often lethal clinical phenotype. Our patients had a milder phenotype presenting primarily as a neuromuscular (NMJ) junction defect. Of note, a previously reported patient with different compound heterozygous missense mutations of SLC25A1 has since been shown to suffer from a neuromuscular transmission defect. Using knockdown of SLC25A1 expression in zebrafish, we were able to mirror the human disease in terms of variable brain, eye and cardiac involvement. Importantly, we show clear abnormalities in the neuromuscular junction, regardless of the severity of the phenotype. Conclusions: Based on the axonal outgrowth defects seen in SLC25A1 knockdown zebrafish, we hypothesize that the neuromuscular junction impairment may be related to pre-synaptic nerve terminal abnormalities. Our findings highlight the complex machinery required to ensure efficient neuromuscular function, beyond the proteomes exclusive to the neuromuscular synapse.
    Full-text · Article · May 2014
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    • "No abnormal variants were detected in DOK7 or GFPT1 which give rise to CMS with a predominantly limb-girdle pattern of muscle weakness [7] [3]. However, subsequent bi-directional sanger sequence analysis of PCR-amplified exonic regions and their flanking sequences in DNA from the proband (Case 2) detected a previously unreported homozygous variant mutation in exon5 of DPAGT1 (RefSeq: NM_001382.3) "
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    ABSTRACT: 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.
    Full-text · Article · Apr 2013 · Neuromuscular Disorders
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    • "In addition to the mutations we have described for ALG2, ALG14 and DPAGT1, it is possible that the pathogenic mechanism that underlies congenital myasthenic syndromes due to GFPT1 mutations (Senderek et al., 2011) is owing to reduced availability of one key product of the hexosamine synthetic pathway, uridinediphopho-N-acetylglucosamine (UDP-GlcNAc), as a substrate for DPAGT1. The congenital myasthenic syndromes that result from mutations in these four genes have a similar phenotype of a limb-girdle pattern of muscle weakness with eye, facial and bulbar muscles largely spared, which contrasts with many myasthenic syndromes. "
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    ABSTRACT: 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 linkage analysis, whole-exome and whole-genome sequencing to determine the underlying defect in patients with an inherited limb-girdle pattern of myasthenic weakness. We identify ALG14 and ALG2 as novel genes in which mutations cause a congenital myasthenic syndrome. Through analogy with yeast, ALG14 is thought to form a multiglycosyltransferase complex with ALG13 and DPAGT1 that catalyses the first two committed steps of asparagine-linked protein glycosylation. We show that ALG14 is concentrated at the muscle motor endplates and small interfering RNA silencing of ALG14 results in reduced cell-surface expression of muscle acetylcholine receptor expressed in human embryonic kidney 293 cells. ALG2 is an alpha-1,3-mannosyltransferase that also catalyses early steps in the asparagine-linked glycosylation pathway. Mutations were identified in two kinships, with mutation ALG2p.Val68Gly found to severely reduce ALG2 expression both in patient muscle, and in cell cultures. Identification of DPAGT1, ALG14 and ALG2 mutations as a cause of congenital myasthenic syndrome underscores the importance of asparagine-linked protein glycosylation for proper functioning of the neuromuscular junction. These syndromes form part of the wider spectrum of congenital disorders of glycosylation caused by impaired asparagine-linked glycosylation. It is likely that further genes encoding components of this pathway will be associated with congenital myasthenic syndromes or impaired neuromuscular transmission as part of a more severe multisystem disorder. Our findings suggest that treatment with cholinesterase inhibitors may improve muscle function in many of the congenital disorders of glycosylation.
    Full-text · Article · Mar 2013 · Brain
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