Congenital myasthenic syndromes. New insights from molecular genetic and patch-clamp studies

Congenital myasthenic syndrome in Turkey: clinical and genetic features in the long-term follow-up of patients

Congenital Myasthenic Syndromes (CMS) are rare disorders that occur as a result of defects in the structure and in the function of neuromuscular junctions. Molecular genetic diagnosis is important to select the most suitable therapeutic option and treatment. Eight patients with congenital myasthenic syndromes who presented to the Çukurova University Pediatric Neurology Department Outpatient Clinic between June 2015 and May 2018 were reviewed. Mutations in the acetylcholine receptor (subunits in epsilon) (CHRNE) in three and mutations in the collagenic tail of endplate acetylcholinesterase (COLQ) gene in five patients were identified; p.W148 mutation was detected to be homozygous in four, c.1169A > G novel mutation in COLQ gene was homozygous in one, c452_454delAGG mutation was homozygous in the other patient, IVS7 + 2T > C(c.802 + 2T > C) mutation was homozygous in a patient and compound heterozygous mutations of c.865C > T(p.Leu289Phe) and c.872C > G(p.A2916)(p.Arg291Gly) in the CHRNE gene in the last patient. The parents of all the evaluated patients were consanguineous. Ptosis, ophthalmoplegia, generalized hypotonia, bulbar weakness, and respiratory crisis were the main findings at the time of presentation. Pyridostigmine is the first-line drug therapy in primary AChR deficiency. Beta adrenergic agonists, ephedrine, and albuterol are the other treatment options for CMS subtypes caused by mutations in COLQ. This study points out the genetic and phenotypic features of CMS patients in the Turkish population and it also reports previously unreported mutations in the literature. CHRNE and COLQ gene mutations are common in the Turkish population. Patients can get serious benefits and recover after the treatment. The treatment should be planned according to genetic tests and clinical findings.

COLQ-mutant Congenital Myasthenic Syndrome with Microcephaly: A Unique Case with Literature Review

Congenital Myasthenic Syndrome (CMS) is a group of inherited neuromuscular junction disorders caused by defects in several genes. Clinical features include delayed motor milestones, recurrent respiratory illnesses and variable fatigable weakness. The central nervous system involvement is typically not part of the CMS. We report here a Saudi girl with genetically proven Collagen Like Tail Subunit Of Asymmetric Acetylcholinesterase (COLQ) mutation type CMS who has global developmental delay, microcephaly and respiratory failure. We have reviewed the literature regarding COLQ-type CMS and to the best of our knowledge this is the first ever reported association of congenital myasthenia syndrome with microcephaly.

Electrophysiological analysis of neuromuscular synaptic function in myasthenia gravis patients and animal models

Study of the electrophysiological function of the neuromuscular junction (NMJ) is instrumental in the understanding of the symptoms and pathophysiology of myasthenia gravis (MG), an autoimmune disorder characterized by fluctuating and fatigable muscle weakness. Most patients have autoantibodies to the acetylcholine receptor at the NMJ. However, in recent years autoantibodies to other crucial postsynaptic membrane proteins have been found in previously 'seronegative' MG patients. Electromyographical recording of compound and single-fibre muscle action potentials provides a crucial in vivo method to determine neuromuscular transmission failure while ex vivo (miniature) endplate potential recordings can reveal the precise synaptic impairment. Here we will review these electrophysiological methods used to assess NMJ function and discuss their application and typical results found in the diagnostic and experimental study of patients and animal models of the several forms of MG.

Selective inhibition of caspases in skeletal muscle reverses the apoptotic synaptic degeneration in slow-channel myasthenic syndrome

Slow-channel syndrome (SCS) is a congenital myasthenic disorder caused by point mutations in subunits of skeletal muscle acetylcholine receptor leading to Ca(2+) overload and degeneration of the postsynaptic membrane, nuclei and mitochondria of the neuromuscular junction (NMJ). In both SCS muscle biopsies and transgenic mouse models for SCS (mSCS), the endplate regions are shrunken, and there is evidence of DNA damage in the subsynaptic region. Activated caspase-9, -3 and -7 are intensely co-localized at the NMJ, and the Ca(2+)-activated protease, calpain, and the atypical cyclin-dependent kinase (Cdk5) are overactivated in mSCS. Thus, the true mediator(s) of the disease process is not clear. Here, we demonstrate that selective inhibition of effector caspases, caspase-3 and -7, or initiator caspase, caspase-9, in limb muscle in vivo by localized expression of recombinant inhibitor proteins dramatically decreases subsynaptic DNA damage, increases endplate area and improves ultrastructural abnormalities in SCS transgenic mice. Calpain and Cdk5 are not affected by this treatment. On the other hand, inhibition of Cdk5 by expression of a dominant-negative form of Cdk5 has no effect on the degeneration. Together with previous studies, these results indicate that focal activation of caspase activity at the NMJ is the principal pathological process responsible for the synaptic apoptosis in SCS. Thus, treatments that reduce muscle caspase activity are likely to be of benefit for SCS patients.

Skeletal muscle IP3R1 receptors amplify physiological and pathological synaptic calcium signals

Ca(2+) release from internal stores is critical for mediating both normal and pathological intracellular Ca(2+) signaling. Recent studies suggest that the inositol 1,4,5-triphosphate (IP(3)) receptor mediates Ca(2+) release from internal stores upon cholinergic activation of the neuromuscular junction (NMJ) in both physiological and pathological conditions. Here, we report that the type I IP(3) receptor (IP(3)R(1))-mediated Ca(2+) release plays a crucial role in synaptic gene expression, development, and neuromuscular transmission, as well as mediating degeneration during excessive cholinergic activation. We found that IP(3)R(1)-mediated Ca(2+) release plays a key role in early development of the NMJ, homeostatic regulation of neuromuscular transmission, and synaptic gene expression. Reducing IP(3)R(1)-mediated Ca(2+) release via siRNA knockdown or IP(3)R blockers in C2C12 cells decreased calpain activity and prevented agonist-induced acetylcholine receptor (AChR) cluster dispersal. In fully developed NMJ in adult muscle, IP(3)R(1) knockdown or blockade effectively increased synaptic strength at presynaptic and postsynaptic sites by increasing both quantal release and expression of AChR subunits and other NMJ-specific genes in a pattern resembling muscle denervation. Moreover, in two mouse models of cholinergic overactivity and NMJ Ca(2+) overload, anti-cholinesterase toxicity and the slow-channel myasthenic syndrome (SCS), IP(3)R(1) knockdown eliminated NMJ Ca(2+) overload, pathological activation of calpain and caspase proteases, and markers of DNA damage at subsynaptic nuclei, and improved both neuromuscular transmission and clinical measures of motor function. Thus, blockade or genetic silencing of muscle IP(3)R(1) may be an effective and well tolerated therapeutic strategy in SCS and other conditions of excitotoxicity or Ca(2+) overload.

Clinical features in a series of fast channel congenital myasthenia syndrome

Fast channel congenital myasthenic syndromes are rare, but frequently result in severe weakness. We report a case of 12 fast channel patients to highlight clinical features and management difficulties. Patients were diagnosed through genetic screening and identification of mutations shown to cause fast channel syndrome. Data was obtained from clinical notes, history, examination and follow up. Patterns of muscle weakness involved limb, trunk, bulbar, respiratory, facial and extraocular muscles. Patients responded to treatment with anticholinesterase medication and 3,4-diaminopyridine. Fast channel syndrome contrasted with AChR deficiency in the occurrence of severe respiratory crises in infancy and childhood. The death of two children even when on treatment and the family histories of sibling deaths re-inforces the need for accurate genetic diagnosis, optimised pharmacological treatment and additional supportive measures to manage acute respiratory crises. Referral to a specialist paediatric respiratory centre and regular resuscitation training for parents are recommended.

Neurophysiological strategies for the diagnosis of disorders of the neuromuscular junction in children

The disorders of the neuromuscular junction seen in children, the congenital myasthenic syndromes and autoimmune myasthenia gravis, are very rare. Their clinical symptoms and signs may be variable, most notably in the neonate and infant. They should enter the differential diagnosis of many different clinical presentations, such as 'floppy infant' or attacks of episodic apnoea. Many molecular genetic abnormalities have been discovered in congenital myasthenic syndromes, but screening for these is impracticable without confirmation of a disorder of the neuromuscular junction. In this review, the different neurophysiological tests that can be used are discussed, with their merits and contraindications in children. After consideration of the acceptability of the different techniques, it has been found that stimulation single-fibre electromyography (stimSFEMG) of orbicularis oculi seems to be the best choice in children. In most instances it can be performed on the conscious child and it has a good specificity and high sensitivity. A protocol for using stimSFEMG is described.

Activation of single nicotinic receptor channels from Caenorhabditis elegans muscle

Nicotinic acetylcholine receptors (nAChRs) are pentameric neurotransmitter-gated ion channels that mediate synaptic transmission throughout the nervous system in vertebrates and invertebrates. Caenorhabditis elegans is a nonmammalian model for the study of the nervous system and a model of parasitic nematodes. Nematode muscle nAChRs are of considerable interest because they are targets for anthelmintic drugs. We show single-channel activity of C. elegans muscle nAChRs for the first time. Our results reveal that in the L1 larval stage acetylcholine (ACh) activates mainly a levamisole-sensitive nAChR (L-AChR). A single population of 39 pS channels, which are 5-fold more sensitive to levamisole than ACh, is detected. In contrast to mammalian nAChRs, open durations are longer for levamisole than for ACh. Studies in mutant strains reveal that UNC-38, UNC-63, and UNC-29 subunits are assembled into a single L-AChR in the L1 stage and that these subunits are irreplaceable, suggesting that they are vital for receptor function throughout development. Recordings from a strain mutated in the LEV-1 subunit show a main population of channels with lower conductance (26 pS), prolonged open durations, and reduced sensitivity to levamisole. Thus, although LEV-1 is preferentially incorporated into native L-AChRs, receptors lacking this subunit can still function. No single-channel activity from levamisole-insensitive nAChRs is detected. Thus, during neuromuscular transmission in C. elegans, the majority of ACh-activated current flows through L-AChRs. This study contributes to the understanding of the molecular mechanisms underlying functional diversity of the nAChR family and offers an excellent strategy to test novel antiparasitic drugs.

Identification of pathogenic mutations in the human rapsyn gene

Rapsyn, a complex postsynaptic protein of the striated muscle, assembles acetylcholine receptors (AChR) at high density at the motor endplate (EP). Neuromuscular junctions of mice lacking rapsyn show no clusters of AChRs or other structural postsynaptic proteins such as beta-dystroglycan and utrophin. Humans with mutations in the rapsyn gene ( RAPSN) are affected with a postsynaptic form of congenital myasthenic syndrome (CMS) characterized by impairment of the morphologic development of the postsynaptic region. We have identified four patients from four different families with RAPSNmutations and CMS, confirmed in two cases by microelectrode and electron microscopy studies. The N88K mutation was present in all patients. One patient who was homozygous for N88K was only mildly affected, while the other three patients who were heterozygous for N88K and a second mutation (either L14P, 46insC, or Y269X) were severely affected. Mutations 46insC and Y269X predicts truncation of the protein. L14P predicts a conformational change at the N-terminus that may disrupt membrane association. N88K occurs within the putative leucine zipper motif potentially important for AChR clustering. These findings may explain the severe clinical involvement of compound heterozygous patients.

Nicotinic receptor M3 transmembrane domain: position 8' contributes to channel gating

The nicotinic acetylcholine receptor (nAChR) is a pentamer of homologous subunits with composition alpha(2)(beta)(epsilon)(delta) in adult muscle. Each subunit contains four transmembrane domains (M1-M4). Position 8' of the M3 domain is phenylalanine in all heteromeric alpha subunits, whereas it is a hydrophobic nonaromatic residue in non-alpha subunits. Given this peculiar conservation pattern, we studied its contribution to muscle nAChR activation by combining mutagenesis with single-channel kinetic analysis. Construction of nAChRs carrying different numbers of phenylalanine residues at 8' reveals that the mean open time decreases as a function of the number of phenylalanine residues. Thus, all subunits contribute through this position independently and additively to the channel closing rate. The impairment of channel opening increases when the number of phenylalanine residues at 8' increases from two (wild-type nAChR) to five. The gating equilibrium constant of the latter mutant nAChR is 13-fold lower than that of the wild-type nAChR. The replacement of (alpha)F8', (beta)L8', (delta)L8', and (epsilon)V8' by a series of hydrophobic amino acids reveals that the structural bases of the observed kinetic effects are nonequivalent among subunits. In the alpha subunit, hydrophobic amino acids at 8' lead to prolonged channel lifetimes, whereas they lead either to normal kinetics (delta and epsilon subunits) or impaired channel gating (beta subunit) in the non-alpha subunits. The overall results indicate that 8' positions of the M3 domains of all subunits contribute to channel gating.

Nicotinic receptor fourth transmembrane domain: hydrogen bonding by conserved threonine contributes to channel gating kinetics

The fourth transmembrane domain (M4) of the nicotinic acetylcholine receptor (AChR) contributes to the kinetics of activation, yet its close association with the lipid bilayer makes it the outermost of the transmembrane domains. To investigate mechanistic and structural contributions of M4 to AChR activation, we systematically mutated alphaT422, a conserved residue that has been labeled by hydrophobic probes, and evaluated changes in rate constants underlying ACh binding and channel gating steps. Aromatic and nonpolar mutations of alphaT422 selectively affect the channel gating step, slowing the rate of opening two- to sevenfold, and speeding the rate of closing four- to ninefold. Additionally, kinetic modeling shows a second doubly liganded open state for aromatic and nonpolar mutations. In contrast, serine and asparagine mutations of alphaT422 largely preserve the kinetics of the wild-type AChR. Thus, rapid and efficient gating of the AChR channel depends on a hydrogen bond involving the side chain at position 422 of the M4 transmembrane domain.

Ion channel genes and human neurological disease: recent progress, prospects, and challenges

What do epilepsy, migraine headache, deafness, episodic ataxia, periodic paralysis, malignant hyperthermia, and generalized myotonia have in common? These human neurological disorders can be caused by mutations in genes for ion channels. Many of the channel diseases are "paroxysmal disorders" whose principal symptoms occur intermittently in individuals who otherwise may be healthy and active. Some of the ion channels that cause human neurological disease are old acquaintances previously cloned and extensively studied by channel specialists. In other cases, however, disease-gene hunts have led the way to the identification of new channel genes. Progress in the study of ion channels has made it possible to analyze the effects of human neurological disease-causing channel mutations at the level of the single channel, the subcellular domain, the neuronal network, and the behaving organism.