Summary 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.
Different mechanisms have been proposed to explain the pathological basis of perifascicular muscle fibre atrophy in dermatomyositis. These include ischaemia due to immune-mediated microvascular ...injury, enhanced expression of type 1 interferon-induced gene transcripts in perifascicular capillaries and muscle fibres, and occlusion of larger perimysial blood vessels. Microvascular complement deposition is a feature of dermatomyositis pathology but the trigger for complement activation, the predominant complement pathway involved, or its role in the pathogenesis of the disease, has not been clearly defined. In the first step of this study we examined the density of capillaries and transverse vessels and searched for occlusion or depletion of larger perimysial blood vessels in 10 patients with dermatomyositis. This revealed an invariable association of perifascicular atrophy with capillary and transverse vessel depletion. The capillary and transverse vessel densities in non-atrophic fibre regions were not significantly different from those in muscle specimens of 10 age-matched controls. Next, in the same 10, as well as in 40 additional dermatomyositis patients, we searched for vascular deposits of IgG, IgM, and the C5b-9 complement membrane attack complex. Thirty-one of 50 dermatomyositis specimens contained C5b-9 reactive endomysial microvessels but none of these or other vessels reacted for IgG. Ten of 50 specimens harboured IgM-positive capillaries but only a few of these reacted for C5b-9. Finally, we analysed and compared different pathways of complement activation in dermatomyositis, lupus nephritis, and necrotic muscle fibres in Duchenne dystrophy. In lupus nephritis, C5-b9 deposits co-localized with IgG, IgM, C1q, and C4d, consistent with immune complex dependent activation of the classical complement pathway. In both dermatomyositis and Duchenne dystrophy, C5-b9 deposits co-localized with C1q and C4d and rarely with IgM indicating activation of the classical complement pathway. We conclude that: perifascicular atrophy in dermatomyositis is consistently associated with focal microvascular depletion, and that microvascular membrane attack complex deposits in dermatomyositis result from activation of the classical complement pathway triggered by direct binding of C1q to injured endothelial cells.
To identify and characterize the molecular basis of a syndrome associated with myasthenia, cortical hyperexcitability, cerebellar ataxia, and intellectual disability.
We performed in vitro ...microelectrode studies of neuromuscular transmission, performed exome and Sanger sequencing, and analyzed functional consequences of the identified mutation in expression studies.
Neuromuscular transmission at patient endplates was compromised by reduced evoked quantal release. Exome sequencing identified a dominant de novo variant, p.Ile67Asn, in SNAP25B, a SNARE protein essential for exocytosis of synaptic vesicles from nerve terminals and of dense-core vesicles from endocrine cells. Ca(2+)-triggered exocytosis is initiated when synaptobrevin attached to synaptic vesicles (v-SNARE) assembles with SNAP25B and syntaxin anchored in the presynaptic membrane (t-SNAREs) into an α-helical coiled-coil held together by hydrophobic interactions. Pathogenicity of the Ile67Asn mutation was confirmed by 2 measures. First, the Ca(2+) triggered fusion of liposomes incorporating v-SNARE with liposomes containing t-SNAREs was hindered when t-SNAREs harbored the mutant SNAP25B moiety. Second, depolarization of bovine chromaffin cells transfected with mutant SNAP25B or with mutant plus wild-type SNAP25B markedly reduced depolarization-evoked exocytosis compared with wild-type transfected cells.
Ile67Asn variant in SNAP25B is pathogenic because it inhibits synaptic vesicle exocytosis. We attribute the deleterious effects of the mutation to disruption of the hydrophobic α-helical coiled-coil structure of the SNARE complex by replacement of a highly hydrophobic isoleucine by a strongly hydrophilic asparagine.
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 (
). 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.
Introduction: Congenital myasthenic syndromes (CMS) are disabling but treatable disorders. Anticholinesterase therapy is effective in most of them, but is contraindicated in endplate (EP) ...acetylcholinesterase (AChE) deficiency, the slow‐channel syndrome, Dok‐7 myasthenia, and β2‐laminin deficiency, and is not useful in CMS due to defects in muscle‐specific kinase (MuSK), agrin, and plectin. EP AChE, Dok‐7, and β2‐laminin deficiencies respond favorably to ephedrine, but ephedrine can no longer be prescribed in the USA. Methods: We used albuterol, another sympathomimetic agent, to treat 3 patients with EP AChE deficiency and 15 with Dok‐7 myasthenia. Response to therapy was evaluated by a 9‐point questionnaire pertaining to activities of daily life. Results: Comparison of the pre‐ and posttreatment responses indicated a beneficial response to albuterol (P < 0.001) in both patient groups. The adverse effects of therapy were like those of ephedrine. Conclusion: Our observations should spur controlled, prospective clinical trials of albuterol in these as well as other CMS. Muscle Nerve, 2011
Myofibrillar myopathy (MFM) is a morphologically distinct disorder in which disintegration of the Z‐disk and then of the myofibrils is followed by abnormal accumulation of multiple proteins. ...Mutations in desmin, αB‐crystallin, and myotilin, all Z‐disk–related proteins, cause MFM in the minority of cases. ZASP (a Z‐band alternatively spliced PDZ motif‐containing protein) is another Z‐disk–associated protein, and targeted deletion of ZASP in mouse causes skeletal and cardiac myopathy. We therefore searched for mutations in ZASP in 54 MFM patients and detected 3 heterozygous missense mutations in 11. Their age at onset was 44 to 73 years. Dominant inheritance was apparent in seven patients, cardiac involvement in three, and signs of peripheral neuropathy in five. Most patients had proximal and distal weakness, but in six, the weakness was greater distally than proximally. Ten carried either of two mutations in exon 6 (A147T and A165V) at or within a motif important in linking ZASP to the Z‐disk; one carried a missense mutation in exon 9 (R268C). We conclude that (1) mutations in ZASP cause stereotyped MFM pathology; (2) cardiomyopathy, distal more than proximal weakness, and neuropathy are in the spectrum of zaspopathy; and (3) mutations in ZASP define a novel form of autosomal dominant muscular dystrophy in humans. Ann Neurol 2005;57:269–276
Abstract Myofibrillar myopathies represent a group of muscular dystrophies with a similar morphologic phenotype. They are characterized by a distinct pathologic pattern of myofibrillar dissolution ...associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins and sometimes congophilic material. The clinical features of myofibrillar myopathies are more variable. These include progressive muscle weakness, that often involves or begins in distal muscles but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. EMG of the affected muscles reveals myopathic motor unit potentials and abnormal irritability often with myotonic discharges. Rarely, neurogenic motor unit potentials or slow nerve conductions are present. The generic diagnosis of myofibrillar myopathies is based on muscle biopsy findings in frozen sections. To date, all myofibrillar myopathy mutations have been traced to Z-disk-associated proteins, namely, desmin, αB-crystallin, myotilin, ZASP, filamin C and Bag3. However, in the majority of the myofibrillar myopathy patients the disease gene awaits discovery.
Objective
Myofibrillar myopathies (MFMs) are morphologically distinct but genetically heterogeneous muscular dystrophies in which disintegration of Z disks and then of myofibrils is followed by ...ectopic accumulation of multiple proteins. Cardiomyopathy, neuropathy, and dominant inheritance are frequent associated features. Mutations in αB‐crystallin, desmin, myotilin, Zasp, or filamin‐C can cause MFMs and were detected in 32 of 85 patients of the Mayo MFM cohort. Bag3, another Z‐disk–associated protein, has antiapoptotic properties, and its targeted deletion in mice causes fulminant myopathy with early lethality. We therefore searched for mutations in BAG3 in 53 unrelated MFM patients.
Methods
We searched for mutations in BAG3 by direct sequencing. We analyzed structural changes in muscle by histochemistry, immunocytochemistry, and electron microscopy, examined mobility of the mutant Bag3 by nondenaturing electrophoresis, and searched for abnormal aggregation of the mutant protein in COS‐7 (SV‐40 transformed monkey kidney fibroblast‐7) cells.
Results
We identified a heterozygous p.Pro209Leu mutation in three patients. All presented in childhood, had progressive limb and axial muscle weakness, and experienced development of cardiomyopathy and severe respiratory insufficiency in their teens; two had rigid spines, and one a peripheral neuropathy. Electron microscopy showed disintegration of Z disks, extensive accumulation of granular debris and larger inclusions, and apoptosis of 8% of the nuclei. On nondenaturing electrophoresis of muscle extracts, the Bag3 complex migrated faster in patient than control extracts, and expression of FLAG‐labeled mutant and wild‐type Bag3 in COS cells showed abnormal aggregation of the mutant protein.
Interpretation
We conclude mutation in Bag3 defines a novel severe autosomal dominant childhood muscular dystrophy. Ann Neurol 2008
The term myofibrillar myopathy (MFM) is a noncommittal term for a pathologic pattern of myofibrillar dissolution associated with accumulation of myofibrillar degradation products and ectopic ...expression of multiple proteins. Ultrastructural studies implicate the Z-disk as the site of the initial pathologic change, and mutations in two Z-disk-related proteins, desmin and alphaB-crystallin, have been identified in a minority of patients with MFM. The authors' objective was to determine whether mutations in myotilin, a key Z-disk component and the disease protein in limb-girdle muscular dystrophy (LGMD) 1A, are another cause of MFM.
The authors used histochemical, immunocytochemical, ultrastructural, and mutation analysis.
The authors detected four missense mutations in 6 of 57 patients with MFM in the serine-rich exon 2 of MYOT, where the two previously identified LGMD1A mutations are located. Three mutations were novel, and one had been previously identified in LGMD1A. Each patient had evidence for neuropathy, and at least three kinships had associated cardiomyopathy. Distal weakness greater than proximal weakness was present in three patients. Except for minor differences, the morphologic features were similar to those in other patients with MFM.
1) Mutations in myotilin cause MFM; 2) exon 2 of MYOT is a hotspot for mutations; 3) peripheral neuropathy, cardiomyopathy, and distal weakness greater than proximal weakness are part of the spectrum of myotilinopathy; 4) not all cases of myotilinopathy have a limb-girdle phenotype; and 5) the molecular basis of the majority of MFM cases remains to be discovered.
To find determinants of the occurrence of repetitive compound muscle action potential (R-CMAP) and to assess the efficacy of channel blocker therapy in slow-channel congenital myasthenic syndrome ...(SCCMS).
Neurologic examination, EMG study, laboratory test, muscle biopsy, and next-generation and Sanger sequencing; literature review of reported patients with SCCMS, including EMG, kinetics of mutant acetylcholine receptors (AChRs), and response to therapy; and simulation of the decay phase of endplate potential (EPP) were performed.
Three newly characterized and 57 reported patients with SCCMS with mutations of AChR subunits were included. In patients with R-CMAP, the length of channel opening bursts of mutant AChR was increased 8.68 ± 2.82 (mean ± SD)-fold compared to wild-type; in patients without R-CMAP, the length was increased 3.84 ± 0.65-fold (95% confidence interval 3.18-6.50,
= 0.000014). The EPP amplitude after refractory period of action potential in muscle fiber is above the threshold in patients with R-CMAP but below the threshold in patients without R-CMAP. In patients with good results from channel blocker therapy, treatment was initiated 11.60 ± 5.17 years after onset of symptoms; in patients with no to moderate benefit from channel blocker therapy, treatment was initiated 30.70 ± 12.72 years after onset (95% confidence interval -28.57 to -9.63,
= 0.00089).
In SCCMS, the R-CMAP occurrence is related to the extent of prolongation of the opening episodes of mutant AChR channel. Channel blocker treatment is more effective the sooner it is started after the onset of symptoms.
This study provides Class IV evidence that channel blocker therapy in patients with SCCMS improves symptoms.
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