Pre- and post-synaptic abnormalities associated with impaired neuromuscular transmission in a group of patients with ‘limb-girdle myasthenia’

School of Neurology, Neurobiology and Psychiatry, University of Newcastle upon Tyne, Bath, Newcastle upon Tyne, UK.
Brain (Impact Factor: 9.2). 09/2006; 129(Pt 8):2061-76. DOI: 10.1093/brain/awl200
Source: PubMed


The properties of neuromuscular junctions (NMJs) were studied in motor-point biopsy samples from eight patients with congenital myasthenic syndromes affecting primarily proximal limb muscles ['limb-girdle myasthenia' (LGM)]. All had moderate to severe weakness of the proximal muscles, without short-term clinical fatigability but with marked variation in strength over periods of weeks or months, with little or no facial weakness or ptosis and no ophthalmoplegia. Most had a characteristic gait and stance. All patients showed decrement of the compound muscle action potential (CMAP) on repetitive stimulation at 3 Hz, and increased jitter and blocking was detected by SFEMG, confirming the presence of impaired neuromuscular transmission. None of the patients had serum antibodies against acetylcholine receptors (AChRs). Two of the patients had similarly affected siblings. Intracellular recording from isolated nerve-muscle preparations revealed that the quantal content (the number of ACh quanta released per nerve impulse) was only approximately 50% of that in controls. However, the quantal size (amplitude of miniature end-plate currents) and the kinetic properties of synaptic potentials and currents were similar to control values. The area of synaptic contact and extent of post-synaptic folding were approximately 50% of control values. Thus, the quantal content per unit area of synaptic contact was normal. The number of AChRs per NMJ was also reduced to approximately 50% of normal, so the local AChR density was normal. Immunolabelling studies revealed qualitatively normal distributions and abundance of each of 14 proteins normally concentrated at the NMJ, including components of the basal lamina, post-synaptic membrane and post-synaptic cytoskeleton. DNA analysis failed to detect mutations in the genes encoding any of the following proteins: AChR subunits, rapsyn, ColQ, ChAT or muscle-specific kinase. Response of these patients to treatment was varied: few showed long-term improvement with pyridostigmine and some even deteriorated with treatments, while others had intolerable side-effects. Several patients showed improvement with 3,4-diaminopyridine, but this was generally only transient. Ephedrine was helpful in half of the patients. We conclude that impaired neuromuscular transmission in these LGM patients results from structural abnormalities of the NMJ, including reduced size and post-synaptic folding, rather from any abnormality in the immediate events of neuromuscular transmission.

Download full-text


Available from: Clarke R Slater, Jan 24, 2014
    • "This variable response to cholinesterase inhibitors has been previously reported in literature. Slater et al. had observed some patients with “limb girdle myasthenia” who showed initial motor strength improvement following initiation of pyridostigmine with subsequent secondary deterioration seen after a few days to weeks while on treatment.[4] A reduced quantal release of acetylcholine (ACh) in addition to altered structural integrity of the neuromuscular junction has been proposed to be the reason for impaired neuromuscular transmission in limb girdle myasthenia. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Congenital myasthenic syndromes (CMS) are frequently misdiagnosed due to their wide clinical heterogeneity. Molecular defects in various end-plate associated proteins are being identified. Better understanding of the molecular pathogenesis and genotype-phenotype correlations can help evolve newer therapeutic targets. We present a report of two siblings with familial limb girdle myasthenia who showed significant objective clinical improvement after initiation of terbutaline. The possible mechanism of action and utility of terbutaline in the setting of CMS are described. Terbutaline is a potential treatment option in certain subtypes of CMS refractory to conventional medicines. However, long-term follow-up is required to determine the overall efficacy and safety profile.
    No preview · Article · Apr 2013 · Annals of Indian Academy of Neurology
  • Source
    • "In addition, patients did not have prominent daytime-fatigable weakness, but rather experienced prolonged periods of exacerbations lasting weeks or months as described in other cases [2] [12]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: DOK7 mutations cause a congenital myasthenic syndrome (OMIM 254300) characterized by a "limb-girdle" phenotype. We identified 7 French-Canadian patients with a previously undiagnosed proximal myopathy. A genome wide scan was performed. Homozygosity mapping identified a locus on chromosome 4p16.2 containing DOK7. Sequencing of DOK7 revealed homozygous 1124_1127dupTGCC mutations in all individuals. SNP genotyping of 42kb surrounding DOK7 in our cohort and in 9 patients of various European origins demonstrated a shared haplotype suggesting a common ancestral European mutation. In our cohort, fatigability was not prominent; rather patients reported prolonged periods of increased weakness. Abnormalities on repetitive nerve stimulation and single fiber EMG were not invariably present. There was considerable intra-familial phenotypic variability, and we report an asymptomatic individual. DOK7 mutations should be considered in patients with early-onset myopathy, even in the absence of symptoms suggesting a possible myasthenia.
    Full-text · Article · Jul 2010 · Neuromuscular Disorders
  • Source
    • "The light-microscope studies which focused on the cholinesterase reactivity demonstrated that our patient had a reduction of the endplate size, a finding that has also been reported in patients with limb girdle myasthenia, and suggestive of incomplete development of endplates (29). Indeed, the electron microscopy studies revealed prominent simplification of the postsynaptic membranes, and the intracellular microelectrode studies showed reduction of amplitudes of miniature synaptic potentials and currents consistent with postsynaptic failure (30). "
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a severe congenital myasthenic syndrome (CMS) caused by two missense mutations in the gene encoding the muscle specific receptor tyrosine kinase (MUSK). The identified MUSK mutations M605I and A727V are both located in the kinase domain of MuSK. Intracellular microelectrode recordings and microscopy studies of the neuromuscular junction conducted in an anconeus muscle biopsy revealed decreased miniature endplate potential amplitudes, reduced endplate size and simplification of secondary synaptic folds, which were consistent with postsynaptic deficit. The study also showed a striking reduction of the endplate potential quantal content, consistent with additional presynaptic failure. Expression studies in MuSK deficient myotubes revealed that A727V, which is located within the catalytic loop of the enzyme, caused severe impairment of agrin-dependent MuSK phosphorylation, aggregation of acetylcholine receptors (AChRs) and interaction of MuSK with Dok-7, an essential intracellular binding protein of MuSK. In contrast, M605I, resulted in only moderate impairment of agrin-dependent MuSK phosphorylation, aggregation of AChRs and interaction of MuSK with Dok-7. There was no impairment of interaction of mutants with either the low-density lipoprotein receptor-related protein, Lrp4 (a co-receptor of agrin) or with the mammalian homolog of the Drosophila tumorous imaginal discs (Tid1). Our findings demonstrate that missense mutations in MUSK can result in a severe form of CMS and indicate that the inability of MuSK mutants to interact with Dok-7, but not with Lrp4 or Tid1, is a major determinant of the pathogenesis of the CMS caused by MUSK mutations.
    Full-text · Article · Apr 2010 · Human Molecular Genetics
Show more