Aug. 14, 2022
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Congenital myasthenic syndromes are inherited disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Because they are relatively infrequent, they often go undiagnosed or misdiagnosed. Some congenital myasthenic syndromes can be diagnosed by clinical clues; others require special laboratory studies that define parameters of neuromuscular transmission and analyze the structure of the endplate. The genetic basis of more than 30 congenital myasthenic disorders has now been identified, with four novel syndromes recognized since the last review. Most congenital myasthenic syndromes are treatable but therapy has to be tailored for the underlying molecular defect because therapies beneficial in some congenital myasthenic syndromes can be harmful in another type. This article provides an overview of the clinical aspects of the congenital myasthenic syndromes, describes their historical aspects and current classification, summarizes general and specific features of the different disorders, and considers their pathophysiology, pathogenesis, and prognosis.
• The congenital myasthenic syndromes are not uncommon but are commonly misdiagnosed.
• Identification of the clinical, physiologic, and molecular features of the congenital myasthenic syndromes is relevant to diagnosis, prevention, and therapy.
• To date, no fewer than 30 genetically distinct congenital myasthenic syndromes have been recognized. In the Mayo cohort of congenital myasthenic syndromes patients, molecular defects in AChR subunits, rapsyn, ColQ, Dok7, and ChAT account for 51%, 15%, 13%, 10%, and 5%, or a total of 93% of the congenital myasthenic syndromes.
• Cholinergic agonists benefit the congenital myasthenic syndromes caused by low-expressor and fast-channel mutations in AChR subunits, rapsyn, choline acetyltransferase, glutamine-fructose-6- phosphate transaminase 1 (GFPT1), and are of variable benefit in congenital myasthenic syndromes caused by mutations in DPAGT1.
• Cholinergic agonists can also improve the myasthenic features associated with some congenital myopathies.
• The slow-channel syndrome responds to the long-lived open-channel blockers of AChR, like fluoxetine, quinine, or quinidine.
• The congenital myasthenic syndromes caused by defects in Dok7, LRP4, and ColQ respond to ephedrine or albuterol. Albuterol is also beneficial as an adjuvant to cholinergic agonists in congenital myasthenic syndromes caused by defects in rapsyn and by low-expressor mutations in AChR subunits.
• Because medications that benefit one type of syndrome can worsen another type, a correct genetic diagnosis is essential before treatment is initiated.
• Most congenital myasthenic syndromes disease proteins reside in the nerve terminal, the synaptic space, or in the postsynaptic region, but some are also expressed in the central nervous system and other tissues.
The congenital myasthenic syndromes were described as early as 1937 (126) but received little attention until after the autoimmune origin of acquired myasthenia gravis was discovered. In the late 1970s and early 1980s, three different syndromes were delineated by clinical, electromyographic, conventional microelectrode, cytochemical, and ultrastructural criteria: congenital endplate acetylcholinesterase deficiency (40), the slow-channel myasthenic syndrome (41), and a disorder attributed to reduced synthesis or vesicular packaging of acetylcholine (39; 91). From the early 1990s, single-channel recordings have been used to analyze the kinetic properties of AChR channels at intercostal muscle endplates of congenital myasthenic syndrome patients (88). The data derived from a combination of the above studies enabled the candidate gene approach and led to discovery of mutations in endplate associated proteins, namely mutations in AChR that cause slow- and fast-channel syndromes or endplate AChR deficiency; in ColQ that result in endplate acetylcholinesterase deficiency; in choline acetyltransferase that impair ACh synthesis and cause frequent episodes of apnea; in rapsyn that impair anchoring of AChR in the postsynaptic membrane; and in Nav1.4, the voltage-gated sodium channel of skeletal muscle, that inhibit generation of the muscle action potential (45; 47).
Between 2005 and 2011, congenital myasthenic syndrome mutations were also observed in MuSK, which is required for postsynaptic development (27); in Dok-7 (14), a muscle intrinsic activator of MuSK required for maintaining the structural integrity of the neuromuscular junction (134); in agrin (64), an activator of LRP4; in laminin beta2 that alters the endplate geometry (79); in plectin, an intermediate filament linker essential for cytoskeletal support (133); and in GFPT1 (138).
Since 2012, an increasing number of congenital myasthenic syndromes disease genes and proteins were identified by whole-exome sequencing (150). The identified disease proteins include SNAP25B (144), synaptotagmin 2 (60; 157), Munc13-1 (46), and synaptobrevin-1 (Engel et al 2017; 129), all essential for synaptic vesicle exocytosis; DPAGT1 (15), ALG2, ALG14 (31), and GMPPB (16); LRP4 (104; 135); Myosin 9A (101); collagen 13A1 (73); the mitochondrial citrate carrier (26); PREPL (121); the vesicular ACh transporter (103); the high-affinity presynaptic choline transporter (11; 05); laminin 5A (77); and agrin (105; 155). A study describes clinical and genetic features of patients in Turkey with long-term follow-up (83).
The first manifestation of a congenital myasthenic syndrome can be decreased fetal motility in utero. Some patients harboring mutations in rapsyn (109; 23; 32) or in the AChR delta subunit (22; 94), or in the fetal AChR gamma subunit (63; 92) are born with multiple joint contractures. In the neonatal period, most patients are hypotonic with a feeble cry, poor suck, and symmetric eyelid ptosis; they may also have stridor from palsy of the vocal chords, choking spells, respiratory insufficiency, or episodes of apnea. Characteristically, the symptoms are worsened by crying or activity and increase by the end of the day. Motor milestones are delayed, and most patients never learn to run or climb stairs well. They fatigue abnormally on exertion and cannot keep up with their peers in sports. Many have impaired ocular ductions. Small muscle bulk and spinal deformities can become apparent in later life. The slow-channel syndrome and the syndromes caused by mutations in DOK7, GFPT1, and DPAGT1 progress slowly and can lead to severe disability in later life. Low-expressor mutations in non-epsilon AChR subunits are associated with a severe clinical phenotype. In contrast, the syndromes caused by most low-expressor mutations in the AChR epsilon subunit have a mild or static course because expression of the fetal gamma subunit partially substitutes for the defect in the epsilon subunit. The mildest congenital syndromes present with fatigable oculobulbar or limb muscle weakness later in life. The edrophonium or neostigmine test is positive except in endplate acetylcholinesterase deficiency and in some cases of the slow-channel syndrome and Dok-7 myasthenia. Table 1 lists generic and specific clinical features of the various congenital myasthenic syndromes.
• Fatigable weakness involving ocular, bulbar, and limb muscles since infancy or early childhood
Exceptions and caveats
• In some congenital myasthenic syndromes, the onset is delayed
Specific clinical features of the congenital myasthenic syndromes
• Repetitive compound muscle action potentials (CMAPs)
Endplate acetylcholinesterase deficiency
• Refractoriness to cholinesterase inhibitors
Slow-channel congenital myasthenic syndrome
• Repetitive CMAPs
Choline acetyltransferase, vesicular ACh transporter, and presynaptic high-affinity choline transporter deficiency syndromes
• Recurrent apneic episodes, spontaneous or with fever, vomiting, or excitement
• Multiple congenital joint contractures or dysmorphic features in one fourth of patients
• Selective wasting first of distal leg and then of distal arm muscles in some patients
• Proximal greater than distal limb and axial muscle weakness, mild facial weakness and ptosis, and normal ocular ductions in the majority
• Proximal and sometimes distal muscle weakness. Ptosis and respiratory weakness are uncommon.
DPAGT1, ALG2, ALG14, and GMPPB myasthenias
• Limb-girdle or diffuse weakness; ocular ductions spared
• Ataxia, intellectual disability, and cortical hyperexcitability
Synaptotagmin 2, Munc13-1, synaptobrevin-1, and laminin alpha 5 myasthenias
• Low amplitude evoked CMAP at rest that markedly facilitates on high-frequency stimulation, as in the Lambert-Eaton syndrome; hyporeflexia; possible motor neuronopathy in synaptotagmin 2 myasthenia
Laminin beta2 myasthenia
• Nephrotic syndrome, ocular abnormalities (Pierson syndrome)
Plectin deficiency myasthenia
• Epidermolysis bullosa simplex
PREPL deficiency myasthenia
• Most cases associated with cystinuria and growth hormone deficiency
Mitochondrial citrate carrier deficiency myasthenia
• Associated with variable developmental anomalies of the eye, brain, and heart
• High proportion of central myofiber nuclei as the predominant pathologic alteration
Laminin alpha 5 myasthenia
• Brain malformations, ocular defects, cardiomyopathy, skin abnormalities
Rabphilin 3a myasthenia
• Learning disabilities, recurrent abdominal pain, transient hyperglycemia, impaired coordination
There are no specific clues to the diagnosis of the fast-channel congenital myasthenic syndrome, primary endplate AChR deficiency, and most cases of rapsyn deficiency. A similar list of causative congenital myasthenic syndrome genes was reported from Spain (53).
The prognosis and complications vary with each type of congenital myasthenic syndrome, the nature of the mutation causing a given syndrome, and the age at which appropriate therapy is started. Also, the course of a congenital myasthenic syndrome represented by single or few patients cannot predict the prognosis in the yet unidentified patients. Therefore, the comments below consider the prognosis for only those congenital myasthenic syndromes represented by 10 or more reported patients.
Choline acetyltransferase deficiency. Untreated infants with disease can die suddenly from respiratory arrest. Patients harboring mutations near the active site tunnel of the enzyme on one allele and low expressor mutations on the second allele may be refractory to therapy. Less severely affected patients survive to adult life with appropriate therapy to prevent the respiratory crises. In some patients, the respiratory crises become less frequent or may disappear after the second decade of life.
Endplate acetylcholinesterase deficiency. In most patients, this disease results in severe muscle weakness and atrophy and respiratory insufficiency from early life. Spinal deformities often appear in the first decade and most untreated patients become wheelchair-dependent by the age of 10 years. Some patients with missense mutations in the C-terminal domain of ColQ have a milder course and better prognosis. The prognosis is worsened when patients not responding to anti-acetylcholinesterase drugs are treated with increasing doses of these drugs, which then cause excessive bronchial secretions and potentially fatal respiratory complications. Most patients show some response to ephedrine with or without improvement of the EMG decrement (19; 86). Ephedrine is no longer available in the United States; fortunately, albuterol is as effective as ephedrine and is now the preferred medication for this syndrome (71).
Slow-channel syndrome. This disease is always progressive unless it is treated. Patients with mutations in the extracellular domain of AChR, which increase the affinity for ACh, have a milder course than those with mutations in transmembrane domains of AChR, which enhance the efficiency channel gating to a pathologic extent. The least affected patients may only show weakness of cervical, scapular, and dorsal forearm muscles in the sixth decade of life, whereas severely affected patients become wheelchair-bound and respirator-dependent before the end of the first decade. Progressive scoliosis, which contributes to the respiratory insufficiency, and cervical spondylosis due to chronic head drop are common complications. Therapy with long-lived open-channel blockers of AChR such as quinine, quinidine, and fluoxetine improve the prognosis even in the most severely affected patient (55; 57; 30). Among 24 variants reported to cause slow-channel congenital myasthenic syndrome, only two appear in the AChR δ-subunit (140). The slow-channel syndrome responds to long-lived open-channel blockers of AChR, such as fluoxetine, quinine, or quinidine.
Fast-channel syndrome. The prognosis varies from mild focal weakness and a static course to extreme weakness of all voluntary muscles from birth. Mutations that reduce affinity for ACh or alter the fidelity of gating carry a worse prognosis (113; 142) than mutations in the transmembrane domains that affect gating (156). Combined therapy with 3,4-DAP and pyridostigmine is beneficial in most patients.
Primary AChR deficiency. Because limited expression of the fetal gamma subunit can partially compensate for defects in the adult epsilon subunit, patients with low-expressor mutations in the epsilon subunit have a better prognosis than patients harboring low-expressor mutations in both alleles of a nonepsilon subunit that lack compensatory subunits. Most patients with low-expressor epsilon subunit mutations show ptosis, limited ocular ductions, variable facial weakness, and mild fatigable weakness of cervical, limb, and axial muscles. The course is usually nonprogressive, and patients seldom develop respiratory insufficiency or significant spinal deformities, and most are improved by cholinergic agonists. Patients failing to respond to these medications have improved with additional therapy with albuterol (127; 125) or 3,4-diaminopyridine.
In contrast, patients with low expressor mutations in both alleles of non-epsilon subunits have severe generalized weakness, frequent choking spells and respiratory crises, and high mortality in early life. They respond variably to cholinergic agonists. Biallelic null mutations of non-epsilon subunits are lethal in embryonic life (84; 152; 153).
Rapsyn deficiency. The prognosis is variable. Approximately one fourth of the patients are born with multiple joint contractures, but these do not predict future disability. Few patients presenting in the second decade or later in life have only mild fatigable weakness (23). Importantly, intercurrent infections can worsen the symptoms and precipitate respiratory crises. No consistent genotype-phenotype correlations have emerged, but it has been suggested that the disease is more severe in patients harboring compound allelic mutations than in those homozygous for the common N88K mutation, suggesting that the second mutant allele may largely determine severity (32). Long-term follow-up of 25 patients treated with cholinergic therapy showed that 21 became stable or were clinically improved and two of these became asymptomatic; three had a progressive course; and one died in infancy (89).
Dok-7 myasthenia. The disease is generally progressive but the rate of progression and the extent of disability in adult life varies greatly. In the eight first reported patients, disability progressed from moderate to severe during the teens and twenties, but one patient developed respiratory failure after 23 years of age (146). The author has observed patients with a milder course who are still ambulatory in the fifth decade of life. No genotype-phenotype correlations have emerged to date. Importantly, cholinergic agonists, such as pyridostigmine or 3,4-diaminopyridine, can aggravate the disease, whereas ephedrine or other beta-adrenergic receptor agonists (134; 131) or albuterol (71) are of clear benefit.
GFPT1 myasthenia. Most patients present in the first decade. As in Dok-7 myasthenia, the weakness affects mainly the limb-girdle muscles. Only a few patients have respiratory complications (138). The course is relatively benign in most, but one patient observed by the author presented in infancy with severe facial, bulbar, respiratory, and limb-girdle weakness (137; 81). The disease responds to AChE inhibitors. Some patients derive additional benefit from albuterol and 3,4-DAP.
A 5-year-old boy was born by induction after 40 weeks of gestation. Fetal movements, which began at 16 weeks, were feeble. Immediately after birth, he had a weak cry and suck, was hypotonic, and required intermittent respiratory support over the next 11 days. During the first year of life, he continued to have intermittent respiratory difficulties, some associated with apnea that required resuscitation. Eyelid ptosis was noted during infancy. At 15 months of age, he underwent a fundus duplication procedure for gastrointestinal reflux, a tracheostomy, and a percutaneous gastrostomy. He had repeated episodes of pneumonia until the age of 3 years. He sat up at 7 months of age and learned to walk at 27 months but fell frequently. He fatigued easily, and his speech became slurred and his eyes diverged when he was tired. There was no history of a similarly affected family member.
At the age of 5 years, EMG studies revealed a borderline defect of neuromuscular transmission: stimulation of the facial and peroneal nerves at 2 Hz elicited a 10% decrement of the fifth compared to the first evoked CMAP. The decremental response worsened following exercise and was improved by edrophonium and diaminopyridine (3,4-DAP). Continuous 10 Hz stimulation of the peroneal nerve over 5 minutes decreased the initial CMAP amplitude by 80% to 90% and increased the CMAP decrement at 2 Hz to over 50%. An electrocardiogram suggested right axis deviation and right ventricular hypertrophy.
In vitro electrophysiologic studies of an intercostal muscle showed no defect in evoked quantal release or in the synaptic response to the released quanta in rested muscles. However, trains of stimuli at 10 Hz abolished the extracellularly recorded CMAP and the contractile response in less than 5 minutes. At the end of stimulation, the initial amplitude of the endplate potential was decreased by 80% and that of the miniature endplate potential by 50%. (In normal subjects a similar stimulation protocol decreases the endplate potential by less than 30% and does not significantly alter the miniature endplate potential amplitude.) Following stimulation, the observed electrophysiologic abnormalities disappeared slowly over 15 minutes. Patch-clamp studies of single AChR channels showed that the conductance and kinetic properties of the AChR channel were normal. The number of AChRs per endplate and the ultrastructure of the endplate was normal.
The stimulation-dependent decrease of the miniature endplate potential amplitude in this patient was consistent with a defect in the resynthesis and the vesicular packaging of ACh. Candidate proteins for this type of defect were the presynaptic high-affinity presynaptic choline transporter, choline acetyltransferase, the vesicular acetylcholine (ACh) transporter, or the vesicular proton pump. Direct sequencing of choline acetyltransferase (ChAT) revealed two heteroallelic missense mutations in the gene’s coding region (R482G and R560H). Neither mutation significantly altered choline acetyltransferase expression in fibroblasts, but biochemical analysis of the kinetic properties of bacterially expressed choline acetyltransferase harboring R482G or R560H reduced the catalytic efficiency of the enzyme to 28% and 2% of wild-type, respectively. The combined clinical, EMG, and laboratory findings established that the patient suffered from a presynaptic congenital myasthenic syndrome caused by a molecular defect in choline acetyltransferase.
The syndromes are caused by defects in presynaptic, synaptic basal lamina, and postsynaptic proteins as well as in proteins subserving glycosylation of proteins at the neuromuscular junction or in endplate development. Table 2 lists the congenital myasthenic syndromes in the Mayo cohort of 359 patients and indicates additional syndromes identified at other medical centers.
Index cases Mayo Clinic
• Choline acetyltransferase deficiency**
• Endplate AChE deficiency**
• Primary kinetic defect ± AChR deficiency**
Defects in endplate development and maintenance
• Agrin deficiency**
Defects in protein glycosylation
• GFPT1 myasthenia**
• PREPL deficiency myasthenia
** Genetic defect detected.
*** Mutations in ALG2 and ALG14 (31), the mitochondrial citrate carrier (26), the vesicular ACh transporter (103), the presynaptic high-affinity choline transporter (11), Myo9A (101), GMPPB (16), COL13A1 (73), and SYT2 (60), laminin alpha 5 (77), and rabphilin 3a (80) were identified at other medical centers.
In the Mayo cohort, the pure presynaptic syndromes were the least frequent, accounting for only 6% of the total. A presynaptic component consisting of reduced quantal release by nerve impulse is also present in endplate acetylcholinesterase deficiency (41), in the Dok-7 (14; 134) and GFPT1 (137) myasthenias, and in a centronuclear myopathy-associated myasthenia (72).
The postsynaptic syndromes and those associated with endplate development and maintenance account for 77% of the cases. Within this group, a kinetic abnormality or deficiency of AChR accounts for 51% of all cases.
Inheritance. Most congenital myasthenic syndromes are caused by loss of function mutations transmitted by autosomal recessive inheritance. Mutations that underlie the slow-channel syndrome and markedly prolong opening events of the AChR channel are caused by gain-of-function mutations transmitted by autosomal dominant inheritance (41; 43), but mutations that prolong the channel opening events to a lesser extent are transmitted by recessive inheritance (34). Defects in SNAP25B (144) and synaptotagmin 2 (60) are caused by dominant loss-of-function mutations. However, recessive mutations in synaptotagmin 2 have been reported to cause severe presynaptic congenital myasthenic syndrome (12; 36).
The safety margin of neuromuscular transmission. In each congenital myasthenic syndrome, the safety margin of neuromuscular transmission is compromised by one or more mechanisms. This safety margin is a function of the difference between the postsynaptic depolarization caused by the endplate potential (EPP) and the depolarization required to activate Nav1.4. All congenital myasthenic syndromes identified so far have been traced to one or more factors that render the EPP subthreshold for activating Nav1.4 or to a defect in Nav1.4 itself. The amplitude of the EPP depends on the number of quanta released by nerve impulse and the miniature endplate potential (MEPP) amplitude that represents the depolarization caused by a single quantum. The MEPP amplitude in turn depends on the number of ACh molecules per synaptic vesicle, the density and kinetic properties of acetylcholinesterase in the synaptic basal lamina, the density and properties of AChR on the postsynaptic membrane, the endplate geometry, and the input resistance of the muscle fiber. The endplate current is independent of input resistance but otherwise depends on the same factors as the EPP. Based on these principles, the factors governing the safety margin can be grouped into the following major categories: factors that affect the number of ACh molecules per synaptic vesicle; factors that affect quantal release mechanisms; the density and kinetic properties of acetylcholinesterase in the synaptic space; and factors that affect the efficacy of individual quanta (159). The efficacy of individual quanta depends on the endplate geometry, the packing density of AChRs on the postsynaptic membrane, the affinity of AChR for ACh, and the kinetic properties of the AChR channel.
Choline acetyltransferase deficiency. Mutations in choline acetyltransferase either reduce the expression or decrease the catalytic efficiency of the enzyme. The decreased rate of ACh resynthesis during physiologic activity progressively decreases the MEPP amplitude when neuronal impulse flow is increased. This disorder leaves no anatomic footprints (112).
Paucity of synaptic vesicles and reduced quantal release. In this congenital myasthenic syndrome, the number of quanta released by nerve impulse is decreased due to a decreased number of readily releasable quanta. Electron microscopy reveals that the decrease in quantal content of the EPP and number of readily releasable quanta is proportionate to a decreased density of synaptic vesicles in the nerve terminals (154). The disease gene remains unidentified.
SNAP25B-myasthenia. SNAP25 is one of the three SNARE proteins essential for synaptic vesicle exocytosis. A patient with cerebellar ataxia, intellectual disability, and cortical hyperexcitability harbored a dominant-negative mutation that was predicted to disrupt the hydrophobic coiled-coil configuration of the SNARE complex (144). Stimulation-evoked quantal release was markedly reduced at the neuromuscular junction. The myasthenia was improved by 3,4-diaminopyridine (144).
Synaptobrevin-1 myasthenia. Synaptobrevin-1 is another SNARE protein essential for exocytosis. It has different isoforms, and either the A or the D or both isoforms are predicted to be expressed at the motor endplate. A homozygous mutation in synaptobrevin-1 was identified in a girl who has been hypotonic and weak since infancy and expired at age 14 of respiratory failure. Repetitive stimulation studies indicated marked facilitation of the evoked compound muscle action potential at high frequencies of stimulation. The identified mutation elongates the C terminus of both isoforms; this hinders exocytosis by increasing the free energy required for fusing the synaptic vesicle with the presynaptic membrane (143).
Munc13-1 myasthenia. This was observed in a paralyzed microcephalic infant with cortical hyperexcitability who harbored a homozygous stop codon mutation in Munc13-1. Munc13-1 is targeted to active zones at all cholinergic neuromuscular synapses and to nearly all glutamatergic synapses in the brain and is required at both sites for docking and priming the synaptic vesicles for release. In the inactive state, Munc18 locks syntaxin, another SNARE protein, in a folded state.
Munc13 unlocks syntaxin by displacing Munc18; this allows syntaxin 1 to interact with synaptobrevin and SNAP25B to effect vesicle exocytosis upon Ca2+ entry into the nerve terminal. Absence of functioning Munc13-1 relegates syntaxin to a permanently locked state; this inhibits exocytosis, predisposes to seizures, and causes microcephaly by abrogating the role of syntaxin in developing mature neurons (46).
Defects in the vesicular ACh transporter and the high-affinity presynaptic choline transporter. Defects in the vesicular ACh transporter (103) and the high-affinity presynaptic choline transporter (11) hinder ACh resynthesis. The clinical features of generalized myasthenia associated with sudden episodes of apnea resemble those of choline acetyltransferase deficiency (112).
Synaptotagmin 2 myasthenia. Synaptotagmin 2 is another presynaptic protein. Its main function is to serve as a calcium sensor for exocytosis. Two kinships mutations in this gene caused a Lambert-Eaton syndrome-like disorder with lower limb predominant leg weakness, areflexia, and low amplitude compound muscle action potentials that were greatly facilitated by exercise, as well as a motor neuronopathy (60). Electrophysiologic testing and improvement of symptoms with 3,4 DAP were consistent with a presynaptic disorder (157). Recessive mutations in this gene also were reported in two infants with severe myasthenic symptoms (12).
Endplate acetylcholinesterase deficiency (AChE). The endplate species of acetylcholinesterase is composed of catalytic subunits encoded by AChET and structural subunits encoded by COLQ. No spontaneous mutations have been observed in AChET. ColQ, which anchors the complex in the synaptic basal lamina, is composed of three identical strands. The N terminal residues of each strand bind a catalytic homotetramer. Mutations in the N terminal region of ColQ prevent its association with the catalytic subunits; frameshift or nonsense mutations in collagenic midsection of ColQ produce an insertion incompetent single-stranded enzyme; and mutations of critical residues in the globular C-terminal region of ColQ prevent insertion of ColQ into the synaptic basal lamina (107; 68).
Absence of acetylcholinesterase from the endplate prolongs the lifetime of ACh in the synaptic cleft so that each ACh molecule can bind multiple times to AChRs before leaving the cleft by diffusion. This prolongs the duration of the MEPP and EPP, and when the EPP outlasts the absolute refractory period of the muscle fiber, it generates a second (or repetitive) muscle action potential. Cholinergic overactivity at the endplate results in cationic overloading of the postsynaptic region; that causes degeneration of the junctional folds with loss of AChR. The nerve terminals are abnormally small and often encased by Schwann cells, which reduce quantal content of the EPP due to a decrease in the number of readily releasable quanta; this might represent a compensatory mechanism to protect the postsynaptic region from overexposure to ACh. The safety margin of neuromuscular transmission is compromised by decrease in the number of readily releasable quanta, loss of AChR from degenerating junctional folds, altered endplate geometry, and desensitization of AChR from overexposure to ACh (40). A report documents a congenital myasthenic syndrome with novel pathogenic variants in COLQ associated with the presence of antibodies to acetylcholine receptors (148).
Laminin beta 2 myasthenia. Laminin beta 2, encoded by LAMB2, is a component of the basal lamina of different tissues, with high expression in kidney, eye, and the neuromuscular junction. At the synapse, it governs the appropriate alignment of the axon terminal with the postsynaptic region and, hence, pre- and postsynaptic trophic interactions. Defects in beta2 laminin result in Pierson syndrome with renal and ocular malformations. A patient carrying heteroallelic missense and frameshift mutations in LAMB2 had Pierson syndrome as well as severe ocular, respiratory, and proximal limb muscle weakness (79). The renal defect was corrected by renal transplant at 15 months of age. In vitro microelectrode studies revealed decreased quantal release by nerve impulse and a decreased MEPP amplitude. Electron microscopy showed abnormally small nerve terminals often encased by Schwann cells, accounting for the decreased quantal release by nerve impulse; the synaptic space was widened and the junctional folds were simplified, accounting for the decreased MEPP amplitude.
Laminin alpha 5 myasthenia. Laminin alpha 5, encoded by LAMA5, also a component of the synaptic basal lamina, is required for postsynaptic development and maturation. The single reported patient with a LAMA5 mutation was homozygous for an Ag2659Trp mutation. Expression studies in vitro showed that the mutation decreased cell adhesion and neurite outgrowth. Electron microscopy studies of patient endplates revealed that a decreased nerve terminal size would compromise quantal release by nerve impulse (77).
Rabphilin 3a myasthenia. Ultrastructural studies suggest the mutant rabphilin 3a disrupts synaptic vesicle release or recycling, or both (80).
COL13A1 myasthenia. COL13A1 encodes the alpha chain of atypical nonfibrillary collagen and the protein is localized at the motor endplate. Homozygous loss of function mutations were identified in the alpha chain COL13A1 in three patients in two kinships with limb, axial, and variable oculobulbar and respiratory muscle symptoms. The effects on endplate structure and parameters of neuromuscular transmission were not determined. Expression of one of the observed mutations in C2C12 cells reduced the length of AChR clusters (73). Previous studies revealed that collagen XIII is a muscle-derived factor promoting synaptic development and maturation (70). COL13A1 is concentrated on the junctional folds from where it is secreted in the synaptic space in an autocrine manner, and its main function is to ensure adhesion of the presynaptic and postsynaptic regions. In collagen 13A1-/- mice the nerve terminals are abnormally separated from the postsynaptic region and the nerve terminals are inappropriately enveloped by Schwann cells. In myotube cultures of the null mice, addition of shed exodomains of collagen XIII enhanced the maturation of AChR clusters. Synaptic transmission in the mice is compromised by low-amplitude synaptic potentials, decreased probability of spontaneous and evoked quantal release, and likely by decreased density of AChR on the junctional folds. Response to AChE inhibitor treatment was ineffective but albuterol proved beneficial (38).
Slow-channel syndromes. Pathogenic mutations residing in different subunits of the AChR prolong opening events of the AChR by increasing the rate at which the channel opens, or by decreasing the rate at which it closes, or by increasing the receptor’s affinity for ACh that allows it to reopen repeatedly during a single ACh occupancy (45; 145). The prolonged channel-opening events prolong the duration of the synaptic potentials, and as in endplate acetylcholinesterase deficiency, elicit a repetitive muscle action potential. The prolonged synaptic response also causes cationic overloading of the postsynaptic region and degeneration of the junctional folds that cause loss of AChR from the folds. The safety margin of neuromuscular transmission is compromised by loss of AChR, altered endplate geometry, and progressive depolarization block of the endplate at physiologic rates of stimulation when each consecutive EPP arises in wake of the preceding EPP before the postsynaptic membrane has become repolarized (41; 141).
Fast-channel syndromes. Pathogenic mutations residing in different subunits of the AChR curtail the duration of the channel opening events by decreasing the rate at which the channel opens or by increasing the rate at which it closes (156), or by decreasing affinity for ACh (113) or by altering the fidelity of channel openings, which typically become briefer than normal (90). The kinetic mutations are generally accompanied by a null mutation in the second allele so that the kinetic mutation dominates the phenotype. The structure of the endplate is normal; AChR expression is also normal (113; 142) unless the kinetic mutation also decreases AChR expression (156). The safety margin is compromised by a decreased probability of channel openings, which decreases the synaptic response to ACh, and by the accelerated decay of the synaptic response.
Primary endplate AChR deficiency. Endplate AChR deficiency results from frameshift, splice-site or nonsense mutations, chromosomal microdeletions, or missense mutations in the promoter region, the signal peptide region, or of residues essential for assembly of the pentameric receptor. Morphologic studies show an increased number of small endplate regions distributed over an increased span of the muscle fiber, and the distribution of AChRs on the junctional folds is patchy and attenuated (42; 110). The structural integrity of the junctional folds is preserved, but at some endplate regions the junctional folds are less complex so that the postsynaptic membrane is less folded. This in itself reduces the synaptic response by reducing the input resistance of postsynaptic membrane and, hence, the amplitude of the MEPP and EPP. The safety margin is compromised by the AChR deficiency and by simplification of the junctional folds.
Sodium channel myasthenia. The first observed patient had myasthenic symptoms as well as attacks of normokalemic periodic paralysis. Quantal release by nerve impulse and the amplitude of the EPP were normal, but suprathreshold EPPs failed to generate muscle fiber action potentials. SCN4A, the gene encoding Nav1.4, harbored 2 mutations (S246L in the S4/S5 linker in domain I and V1442E in S4/S5 linker in domain IV). Nav1.4 expression at patient endplates was normal, and the endplates showed no structural abnormality. Recombinant V1442E-sodium channels expressed in HEK cells showed marked enhancement of fast inactivation close to the resting potential and enhanced use-dependent inactivation on high-frequency stimulation; S246L had only minor kinetic effects. The safety margin in this syndrome is impaired because a large fraction of the Nav1.4 channels are inexcitable in the resting state (149). A second patient with similar clinical features harbored a homozygous R1457H mutation in the S4 segment of domain IV that causes reduced depolarization and recovery only after extended repolarization of Nav1.4 (06). A third patient also carried a homozygous R1454W substitution in the S4 segment of domain IV. This mutation enhances fast and slow inactivation and delays recovery from the inactive states (54).
Plectin deficiency associated myasthenia. Plectin is an intermediate filament-associated protein concentrated at sites of mechanical stress. In normal muscle, it is expressed at the surface membrane, the intermyofibrillar membranous network, the nuclear membrane, and the postsynaptic region of the endplate. Pathogenic mutations in plectin are associated with a simplex variety of epidermolysis bullosa, muscular dystrophy, and a myasthenic syndrome in some cases. In two patients investigated by the author, plectin was absent from the muscle; the myopathy was associated with dislocated muscle fiber organelles, structurally abnormal nuclei, focal plasma membrane defects, and focal calcium ingress into the muscle fibers. The neuromuscular junctions showed destruction of the junctional folds. Mutation analysis in each patient revealed two recessive nonsense or frameshift mutations in plectin. The mean MEPP amplitude was reduced to approximately 50% of normal (10; 133; 52).
PREPL deficiency associated myasthenia. This myasthenia is often associated with hypotonia-cystinuria syndrome caused by recessive deletions involving contiguous segments of SLC3A1 and PREPL genes. An infant harboring a paternally inherited nonsense mutation in PREPL and a maternally inherited deletion involving both PREPL and SLC3A1 had myasthenia and growth hormone deficiency but no cystinuria; therefore, the PREPL deficiency determined the phenotype. The patient responded to pyridostigmine before age one year and then improved spontaneously. She had large endplates expressing abundant AChR, small MEPPs, and reduced quantal release. PREPL is an effector of the clathrin associated adaptor protein 1 (AP-1) required for normal function of the vesicular ACh transporter. Hence, absence of PREPL is predicted to result in the reduced filling of the synaptic vesicles with ACh and result in a low MEPP amplitude. The cause of the reduced quantal release is not fully understood (121).
Mitochondrial citrate carrier (SLC25A1) deficiency. This enzyme mediates exchange of the mitochondrial citrate/isocitrate with cytosolic malate. The exported citrate is then converted to acetyl-CoA and oxaloacetate by ATP-citrate lyase. Dysfunction of SLC25A1 can interfere with brain, eye, and heart development and impairs neuromuscular transmission (08). One affected patient had obsessive-compulsive traits, hyperreflexia, and calf hypertrophy, and an affected sister had intellectual disability. A third patient had agenesis of the corpus callosum, optic atrophy, and seizures. Interestingly, in vitro endplate studies in one patient revealed no AChR deficiency. The MEPP amplitude and evoked quantal release were normal (26). In 2019, nine additional patients from three consanguineous Arab families were reported (01). The cause of the neuromuscular transmission defect remains unexplained.
MYO9A myasthenia. Recessive mutations in MYO9A were identified in one German patient harboring R1517H and R2283H, and in two Kurdish siblings with a homozygous D1698G mutation. MYO9A is expressed at the neuromuscular junction and plays a role in neuronal branching and axon guidance required for normal neuromuscular junction development. The German patient suffered from proximal and distal muscle weakness, ptosis, episodic apnea, and respiratory failure of neonatal onset. The Kurdish siblings had similar symptoms as well as difficulty chewing and swallowing, learning difficulties, and respiratory crises (101).
TOR1AIP1 myasthenia. A recessive homozygous frameshift mutation in TOR1AIP1 was described in two siblings with limb-girdle weakness and impaired neuromuscular transmission. TOR1AIP1 encodes the inner nuclear membrane protein lamin-associated protein 1. The disorder might be related to the destabilization of synaptic structure rather than a direct effect of components of synaptic transmission (33).
Myasthenic syndrome-associated with congenital myopathies. EMG findings consistent with impaired neuromuscular transmission and a beneficial effect of pyridostigmine have been documented in patients with centronuclear myopathies caused by mutations in amphiphysin (BIN1) (28), myotubularin (MTM1) (123), and dynamin 2 (DNM2) (51), in tropomyosin 3 (TPM3) (96), in the ryanodine receptor (RYR) (65), in centronuclear myopathy without known mutation (71), and in two cousins with a recessive type of desminopathy (37).
Prenatal congenital myasthenic syndromes with fetal akinesia and deformations. Fetal hypomotility can result in intrauterine growth retardation, multiple joint contractures, subcutaneous edema, pterygia (webbing of the neck, axilla, elbows, fingers, or popliteal fossa), lung hypoplasia, and other congenital malformations. The syndrome is often lethal; the nonlethal form is referred to as the Escobar syndrome. Fetal akinesia has many causes. Those due to defects in neuromuscular transmission include transplacental transfer from mother to fetus of anti-AChR antibodies that contain a high-titer of complement-fixing anti-gamma subunit specificities (120), mutations in the AChR gamma subunit, and biallelic null mutations in non-epsilon AChR subunits as well as in RAPSN and DOK7 (118).
The first recognized prenatal congenital myasthenic syndrome was caused by mutations in the fetal gamma subunit of AChR. In humans, gamma-AChR appears on myotubes around the ninth developmental week and becomes concentrated at nascent nerve-muscle junctions around the sixteenth developmental week. Subsequently, the gamma subunit is replaced by the adult epsilon subunit and is no longer present at fetal endplates after the thirty-first developmental week (61). Thus, pathogenic mutations of the gamma-subunit result in hypomotility in utero, mostly between the sixteenth and thirty-first developmental weeks. The clinical consequences at birth are multiple joint contractures, small muscle bulk, multiple pterygia, camptodactyly, rocker-bottom feet with prominent heels, characteristic facies with mild ptosis, and a small mouth with downturned corners. If the patient survives after birth, myasthenic symptoms are absent because by then the normal epsilon subunit is expressed at the endplates (63; 92).
Most mutations in AChR causing myasthenia reside in the epsilon subunit and are associated with compensatory expression of the fetal gamma subunit. This suggested that biallelic null mutations of nonepsilon subunits, for which there are no substituting subunits, would be embryonic lethal (42; 47). This notion has now been confirmed by reports of lethal fetal akinesia syndromes due to biallelic null mutations in the AChR alpha, beta, and delta subunits (84; 152), as well as in rapsyn (152) and Dok-7 (153).
Myasthenia due to defect in agrin. Agrin is a multidomain proteoglycan secreted into the synaptic basal lamina by the nerve terminal. It acts by phosphorylating and thereby activating MuSK by way of its receptor LRP4 (67; 160). Null variants in AGRN cause lethal fetal akinesia deformation (50). Two siblings with eyelid ptosis but normal ocular ductions and only mild weakness of the facial and hip-girdle muscles carried a homozygous missense mutation in AGRN at codon 1709 (p.Gly1709Arg) in the agrin A region. Light microscopic preparations showed newly formed, partially denervated and remodeled endplates, and electron microscopy revealed some abandoned and partially occupied postsynaptic regions. Aggregation of AChR at the endplates was not affected, but the number of AChRs per endplate was not determined (64). A severe myasthenic syndrome was observed in another patient, caused by two heteroallelic p.Q353X mutations that abolished agrin expression and by a p.V1727F mutation that disrupted the endplate architecture and restricted agrin-induced clustering of AChR (78). A third report describes five kinships with wasting affecting first the lower and later the upper limbs, fatty degeneration of the posterior leg muscles, sparing of the axial and cranial muscles, slow progression, and response to albuterol or ephedrine but not to pyridostigmine (98). A report documents that agrin variants affect clustering of acetylcholine receptors in a domain-specific manner (105).
Myasthenia due to defects in MuSK. MuSK under the influence of agrin and LRP4 plays a role in maturation and maintenance of the synapse and in directing rapsyn to concentrate AChR in the postsynaptic membrane (35; 59; 97; 25; 58). In a kinship, a brother and sister carried two heteroallelic mutations: c.220insC, which is a frame-shifting null mutation, and a missense mutation (V790M), which did not affect the catalytic kinase activity of MuSK but decreased its expression and stability, resulting in decreased agrin-dependent AChR aggregation. The endplates consisted of multiple small regions linked by nerve sprouts. AChR expression per endplate was reduced to approximately 45% of normal. When the missense mutation was overexpressed in mouse muscle by electroporation, it reduced synaptic AChR expression and resulted in aberrant axonal outgrowth similar to that observed in the patient. In vitro electrophysiologic recordings were unavailable. The safety margin in this congenital myasthenic syndrome is likely compromised by the AChR deficiency. Abnormally simple junctional folds that would further reduce the synaptic response to ACh could also attenuate the synaptic response to ACh (27). A homozygous missense mutation in MuSK (p.P344R) was reported by Mihaylova and colleagues (87), and a third patient was described with p.A727V and p.M605I mutations by Maselli and colleagues (76). The last reports also document reduction of the miniature endplate potential (MEPP) and miniature endplate current (MEPC) amplitudes to about 30% of normal, decrease of evoked quantal release to about 50% of normal, and simplification of the junctional folds. Two novel missense mutations in MUSK (p.C317R and p.A617V) were reported: p.C317R, located at the frizzled-like cysteine-rich domain of MuSK, disrupts an integral part of the MuSK architecture results in loss of MuSK phosphorylation and acetylcholine receptor (AChR) cluster formation, and p.A617V, located at the kinase domain of MuSK, enhances MuSK phosphorylation that causes anomalous AChR cluster formation (124). Finally, one MuSK deficient patient had late-onset limb-girdle weakness associated with vocal cord paralysis (117).
LRP4 myasthenia. The first reported patient was 17-year-old girl with moderately severe fatigable limb-girdle weakness, dysplastic synaptic contacts, borderline endplate AChR deficiency, but no demonstrable defect of neuromuscular transmission at intercostal muscle endplates (104). Subsequently, two sisters with moderately severe LRP4 myasthenia were found to have small, underdeveloped, and functionally abnormal endplates with reduced expression for AChR and AChE. Most endplates were poorly differentiated or had degenerating junctional folds, and some endplates were denuded of nerve terminals. The amplitude of the endplate potential (EPP), the miniature EPP, and the quantal content of the EPP were all markedly reduced. Expression studies revealed the mutant protein hinders LRP4 from binding, activating, and phosphorylating MuSK (135).
Rapsyn deficiency. Rapsyn under the influence of agrin, LRP4, and MuSK concentrates AChR on the terminal expansions of the postsynaptic junctional folds. Mutations in different domains of rapsyn cause endplate AChR deficiency (109; 111; 09; 93; 108; 32; 89). Most patients carry a common N88K mutation, but one patient harbored two heteroallelic mutations other than N88K (75). The morphologic features of the endplate and the factors that impair the safety margin are the same as in primary AChR deficiency.
DOK7 myasthenia. DOK7 is essential for activating MuSK and clustering AChR at the endplate (114). Twelve DOK-7 mutations were detected in 19 patients (14). Subsequently, mutations in 16 patients were reported by Selcen and colleagues (134), and 15 patients and five novel mutations were described (17). All had limb-girdle weakness with lesser face, jaw, or neck muscle weakness. A wide clinical spectrum in the course of the disease and the distribution of weakness has now been recognized (95; 116; 134). Seven biopsied patients had small endplates relative to muscle fiber size, and electron microscopy revealed simplified junctional folds (146). Subsequent ultrastructural and electrophysiology studies in patients investigated at the Mayo Clinic demonstrated ongoing destruction of existing endplates and attempts to form new endplates (134). The number of AChRs per endplate was smaller than normal but appropriate for the reduced size of the endplates. The mean MEPC and MEPP amplitudes were reduced to approximately two thirds of normal. The safety margin of neuromuscular transmission is likely impaired by a combination of factors operating to a different extent in different patients (134). A biallelic c.1263dupC in DOK7 variant was reported to cause fetal akinesia-deformation sequence (118), and phenotypic differences were noted in two unrelated patients harboring identical mutations in DOK7 (20).
GFPT1-myasthenia. GFPT1 controls the flux of glucose into the hexosamine pathway, and, thus, the formation of hexosamine products and the availability of precursors for N- and O-linked glycosylation of proteins. A defect in GFPT1 predicts hypoglycosylation and, hence, defective function of several endplate-associated proteins (138). Tubular aggregates in type 2 muscle fibers are a morphologic marker for the disease, but not all patients display it. Further studies of 11 patients revealed small synaptic contacts and hypoplastic endplate regions (137). One patient whose mutations abrogate expression of the muscle-specific exon of GFPT1 has had severe facial, bulbar, and respiratory muscle weakness and was quadriplegic since birth. She has an autophagic vacuolar myopathy, reduced evoked quantal release, and a low MEPP amplitude. Long-term studies of 11 GFPT1 deficient patients reveal impaired maintenance of the neuromuscular junctions, reduced O-glycosylation of proteins, and reduced sialylation of extrajunctional transmembrane proteins (13).
DPAGT1-myasthenia. DPAGT1catalyzes the first committed step of N-liked protein glycosylation, and its deficiency predicts failure of N-glycosylation of multiple proteins. The target proteins are distributed throughout the organism, but in five reported patients neuromuscular transmission was selectively affected (15). As in GFPT1 deficiency, muscle fibers harbor tubular aggregates and the weakness tends to spare the craniobulbar muscles. In two siblings and a third patient the myasthenia was associated with intellectual disability and variable response to pyridostigmine. Intercostal muscle studies showed fiber type disproportion, small tubular aggregates, and an autophagic vacuolar myopathy (136). Evoked quantal release, postsynaptic response to ACh, and endplate AChR content were all reduced to approximately 50% of normal. Immunoblots of muscle extracts revealed reduced to absent glycosylation of different proteins, including that of STIM1. STIM1 is an SR-associated calcium sensor that operates in concert with ORAI1 on the plasma membrane to homeostatically regulate the SR calcium content (147). Because mutations in STIM1 cause a tubular aggregate myopathy (21), hypoglycosylation of this protein is a likely cause of the tubular aggregates in disorders of muscle in N-glycosylation.
ALG2 and ALG14 myasthenias. ALG2 catalyzes the second and third committed steps of N-glycosylation. ALG14 forms a multiglycosyltransferase complex with ALG13 and DPAGT1 and catalyzes the first committed step of N-glycosylation. These defects are predicted to cause endplate AChR deficiency. The effects on endplate structure and parameters of neuromuscular transmission have not been determined (31). Five patients with identified variants in ALG14 had severe hypotonia, progressive cerebral atrophy, and epilepsy refractory to therapy. Three patients had congenital contractures. All patients died during the first year of life. Treatment with pyridostigmine was only temporally effective (132).
GMPPB myasthenia. GMPPB is an enzyme involved in the O-glycosylation of α-dystroglycan. Its mutations can cause muscular dystrophy with variable structural changes in brain and ocular abnormalities. Seven of 11 patients in three of six kinships harboring mutations in GMPBB also had myasthenic symptoms involving predominantly the limb-girdle muscles; muscle biopsies done in 6 of the 11 patients revealed dystrophic features (16; 102), and one patient had centronuclear myopathy (99).
Rabphilin 3a myasthenia. The identified mutations are postulated to impair one or more steps in synaptic vesicle recycling and thereby reduce the size of the available vesicle pool and the efficiency neuromuscular transmission (80).
There is no reliable information on the incidence and prevalence of congenital myasthenic syndromes. Congenital myasthenic syndrome patients are observed less frequently than patients with autoimmune myasthenia gravis and are often misdiagnosed or go undiagnosed for a number of reasons: they can closely mimic other disorders; few physicians are familiar with congenital myasthenic syndrome; and the diagnosis of some congenital myasthenic syndromes requires specialized methods of investigation currently available only at few medical centers. Some congenital myasthenic syndromes are endemic in the Near East where closed communities and consanguineous marriages are relatively frequent (106; 85).
The congenital myasthenic syndromes, like other inherited disorders, can be partially prevented by genetic counseling based on the knowledge of the pattern of inheritance and by prenatal diagnosis based on chorionic villus sampling at 9 to 12 weeks of gestation, provided that the mutations causing the congenital myasthenic syndrome have been identified.
Table 3 lists the disorders that should be considered in the differential diagnosis of the congenital myasthenic syndromes.
Neonatal period, infancy, childhood
• Spinal muscular atrophy
• Motor neuron disease
The diagnostic workup includes a careful clinical history; physical examination; electromyography studies; tests for circulating antibodies directed against AChR, MuSK, and the voltage-gated P/Q type calcium channel; and tests for botulism in some infants. The physical examination should include detailed manual muscle testing as well as tests for fatigable weakness, such as measuring the arm elevation time, the number of times a patient can rise from squatting or from a low stool, whether eyelid ptosis increases with sustained upward gaze, and the number of steps the patient can climb or the distance the patient can walk before having to rest. Respiratory muscle strength can be evaluated by measuring the maximal inspiratory and expiratory pressures and the vital capacity.
The EMG examination should include repetitive stimulation of nerves at 2 to 3 Hz in search of a greater than 10% decrement of the fifth compared to the first evoked CMAP in multiple muscles, and especially in muscles that are significantly weak. The repetitive CMAP typical in the slow-channel syndrome and endplate acetylcholinesterase deficiency is elicited by a single nerve stimulus. A decremental EMG response is frequently present in facial and trapezius muscles when it cannot be detected in other muscles. If repetitive nerve stimulation studies fail to show a decremental response, then single fiber EMG is needed to exclude a defect of neuromuscular transmission.
Table 1 lists generic and specific clinical features relevant to the diagnosis of congenital myasthenic syndromes. In some patients, however, no clinical clues point to a specific type of congenital myasthenic syndrome or for targeted mutation analysis. In these patients, one can search for mutations in known congenital myasthenic syndrome genes, first analyzing the ones harboring the most mutations (Table 2). Another approach is to search for known common mutations in a given gene (eg, p.N88K in RPSN or c.1124, or 1127dupTGCC in DOK7) or search for mutations frequently encountered in a given ethnic group (eg, c.1267delG in CHRNE in gypsies; or the c.1293insG founder mutation in CHRNE in North Africa) (122). In still other patients, in vitro analysis of parameters of neuromuscular transmission, patch-clamp recordings from single AChR channels, measurement of the number of AChRs per endplate, morphologic studies that include examination of the fine structure, and immunocytochemical properties of the neuromuscular junction are required to define the phenotype, which may then provide clues for a molecular diagnosis. A description of these specialized tests is beyond the scope of this clinical summary, but it is worth mentioning that such studies pave the way for discovery of yet unidentified disease genes and syndromes.
Sanger sequencing for known congenital myasthenic syndromes is now commercially available and facilitates diagnosis and management by neuromuscular specialists. It is best used in a targeted manner based on specific clinical features, as listed in Table 1, or beginning with the most frequently mutated genes, as shown in Table 1. However, this approach has a number of drawbacks:
• It is expensive, especially if used in a shotgun manner.
• It does not establish that recessive mutations are heteroallelic unless they are homozygous; therefore, DNA from both parents must also be analyzed.
• It will miss intronic mutations not close to exons.
• Evaluation of pathogenicity is based on software programs whose reliability is still debated.
• It does not provide information on kinetic consequences of mutations in AChR or ChAT, or on pathogenicity of mutations that render the disease protein structurally unstable.
• Negative results do not exclude the diagnosis of a congenital myasthenic syndrome because only previously identified disease genes are sequenced.
A better approach to mutation discovery is whole-exome sequencing that examines exons and nearby intronic regions. Kits available for this method capture only about 85% of the entire exome but read only 75% of the exome with more than 20x coverage. The enormous amounts of generated data need to be filtered against previously identified polymorphisms in dbSNP and scrutinized for mutations in genes encoding EP-related genes. The putative mutations must be confirmed by Sanger sequencing and the non-truncating mutations examined by expression studies. The cost of exome sequencing with the required bioinformatics analysis is still high and there are some pitfalls even in this approach:
• Disease-causing variants in noncoding regions and some large deletions or duplications, as well as pathogenic nucleotide repeats, are missed.
• Pathogenic variants causing rare diseases may have previously been entered in dbSNP.
• Synonymous variants in good candidate genes that might affect a splice enhancer/repressor are often filtered out.
• Pathogenic mutations may reside in novel genes whose function is yet unknown.
Exome sequencing is most efficient when large and/or multiple kinships are available for analysis. At the least, it should be done on the patient and both parents to assure that identified recessive mutations are biallelic. If whole-exome sequencing fails to reveal the disease-causing mutation, RNA sequencing and whole-genome sequencing may reveal disease-causing variants in noncoding regions or large deletions or duplications, as well as pathogenic nucleotide repeats.
Therapeutic agents. In general terms, the congenital myasthenic syndromes either decrease or increase the synaptic response to ACh. When a congenital myasthenic syndrome reduces the synaptic response, cholinesterase inhibitors, which increase the number of AChRs activated by each quantum, and 3,4-DAP, which increases the number of quanta released by nerve impulse, are used. However, in some congenital myasthenic syndromes caused by defects in MuSK, Dok7, AGRN, and LRP4, cholinergic agonists can be harmful, but these disorders often respond to ephedrine or albuterol. When the synaptic response is increased, as in the slow-channel syndromes, long-lived open-channel blockers of the AChR channel, quinidine and fluoxetine, are employed. That some agents are beneficial in one type of congenital myasthenic syndrome but harmful in others underlines the need for a specific diagnosis.
Anti-acetylcholinesterase drugs. These drugs increase the number of ACh that can diffuse across the synaptic space to bind to AChR and the number of times a single ACh molecule can bind to AChR molecules. These effects enhance the amplitude as well as the duration of the synaptic response and, hence, the safety margin of neuromuscular transmission. Anti-acetylcholinesterase medications are contraindicated in endplate acetylcholinesterase deficiency, Dok-7 myasthenia, laminin beta2 myasthenia, and the slow-channel syndrome, and can be harmful in the syndromes due to defects in MuSK and agrin.
Among the anti-acetylcholinesterase medications, pyridostigmine bromide is generally preferred to the shorter-acting neostigmine bromide. Pyridostigmine is available in 60 mg tablets, in 180 mg slow-release (“timespan”) tablets, and as a syrup containing 12 mg of the medication per ml. The drug acts within 45 minutes, and its effects last from 3 to 6 hours. It is less likely to cause muscarinic side effects and may be more effective in controlling bulbar weakness than neostigmine (115). The dose in adults may range from 30 mg taken every 6 hours to 90 mg taken every 3 hours. The timespan preparation is used at bedtime because its effects last through the night. The syrup is useful in children and in patients requiring nasogastric feeding. The total daily dose should not exceed 600 mg per day in adults and 7 mg/kg per day in children. Long-term pyridostigmine therapy can also cause degeneration of the junctional folds, but this adverse effect can be ameliorated by the addition of a β2-adrenergic receptor agonist like albuterol (151).
Neostigmine (prostigmine) bromide is available in 15 mg tablets, and one tablet is equivalent to a 60 mg tablet of pyridostigmine bromide. Orally administered neostigmine acts within 30 minutes, and its effects last for 2 to 3 hours. In adults, the dose may range from 7.5 mg to 30 mg taken every 3 to 4 hours. In infants and children, the recommended dose is 2 mg/kg per day. Muscarinic side effects, and especially abdominal cramps and diarrhea, are mitigated by concurrent administration of atropine or glycopyrrolate. Neostigmine methylsulfate is administered intramuscularly or intravenously, and one administered parenterally is equivalent to 15 mg given by mouth. For children, the recommended parenteral dose is 0.01 to 0.04 mg/kg every 2 to 3 hours. Intranasal therapy with 9 to 14 mg of neostigmine methylsulfate is also effective (139).
Ephedrine and albuterol. These adrenergic agents benefit endplate cholinesterase deficiency, Dok-7 myasthenia, LRP4 myasthenia, as well as agrin, MuSK, and laminin beta2 deficiencies, but their mode of action is unclear. Albuterol and ephedrine are also useful adjuvants in treating congenital myasthenic syndromes caused by low-expressor mutations of AChR subunits (127; 125). Albuterol also benefits the slow-channel syndrome as an adjuvant to fluoxetine (48). In adults, the dose of ephedrine is 25 to 50 mg two to three times daily; in children the dose is 3 mg/kg per day in divided doses. The dose for short-acting albuterol tablets for adults and children over 12 years of age is 2 to 4 mg three or four times daily and for children 6 to 12 years 2 mg to 4 mg three to four times daily. For extended-release tablets (marketed as VoSpire) for adults and patients over 12 years of age, the dose is 8 mg every 12 hours. The probable mode of action of these beta-2 adrenergic agonists is to enhance the expression and clustering of AChR on the postsynaptic membrane (29). The side effects include nervousness, insomnia, muscle cramps, palpitation, and hypertension.
3,4-Diaminopyridine. This medication prolongs the duration of the presynaptic action potential by blocking the outward potassium current (74; 128). This increases calcium entry into the nerve terminal when it is depolarized, which, in turn, increases quantal release. The recommended dose of 3,4-DAP is up to 1 mg/kg per day in divided doses. The drug is well tolerated, with generally only mild side effects. These include peripheral and perioral paresthesias, adrenergic side effects (palpitation, sleeplessness, ventricular extrasystoles), and cholinergic side effects (increased bronchial secretions, cough, and diarrhea). Higher doses are not recommended because of possible seizures (82; 130). A previous history of seizures or potentially epileptogenic activity in the electroencephalogram contraindicates use of the medication. The drug is not approved for clinical use by the U.S. Food and Drug Administration, but it can be obtained as an investigational drug and used under auspices of Institutional Review Boards. The drug is available by prescription in Canada, England, and some European countries.
Quinidine sulfate. This medication is a long-lived open-channel blocker of AChR (49). It is only used in the slow-channel congenital myasthenic syndrome and is contraindicated in all other types of congenital myasthenic syndrome, as well as in autoimmune myasthenia gravis. The initial dose is 200 mg three times daily for one week; the dose is then gradually increased to maintain a serum level of 1 to 2.5 µg/mL (3 to 7.5 µM/L). After a satisfactory serum level is established, equivalent doses of a slow-release form of quinidine can be employed. Children are treated with 15 to 60 mg/kg of quinidine sulfate in four to six divided doses per day (55). Untoward effects include gastrointestinal reactions with diarrhea and cinchonism. Hypersensitivity to the drug can result in drug fever, abnormal liver function tests, hemolytic anemia, agranulocytosis, thrombocytopenic purpura, and toxic drug rash. Quinidine also worsens atrioventricular conduction defects and can aggravate a prolonged QT interval, which predisposes to ventricular arrhythmias. Quinidine also inhibits cytochrome P450IIDA, which impairs the metabolism codeine, tricyclic antidepressants, other antiarrhythmic drugs, and digoxin. Administration of verapamil, cimetidine, and agents that alkalinize urine may augment the serum quinidine level, and quinidine potentiates the anticoagulant effects of warfarin (02).
Fluoxetine. This is the medication of choice for patients with slow-channel myasthenia. The dose in adults is gradually increased from 10 mg twice daily until a total daily dose of 80 to 100 mg is attained (57). Like quinidine, fluoxetine should not be used in any other congenital myasthenic syndrome or in autoimmune myasthenia gravis. The maximal dose of fluoxetine for children has not been established. Fluoxetine is a less effective long-lived open-channel blocker of the AChR than quinidine but is eliminated more slowly and, therefore, provides more evenly sustained blood levels. Adverse effects of fluoxetine include mild nausea, nervousness, insomnia, sexual dysfunction, and hyponatremia in the elderly (03). Fluoxetine has been reported to increase the risk of suicide-related behaviors in depressed children and adolescents (158; 07). Therefore, caution is required when the medication is used in children or adolescents and should not be used in those with signs of depression.
Management of the different types of congenital myasthenic syndrome.
Primary endplate AChR deficiency. Most patients respond favorably but incompletely to cholinesterase inhibitors. The additional use of 3,4-DAP results in further significant improvement in about one third of the cases (56). Albuterol may also benefit patients not responding well to cholinergic agonists (127; 125). External ocular muscle weakness is often refractory to therapy (04).
Slow-channel syndrome. Quinidine (55) or fluoxetine (57) are the medications of choice.
Fast-channel syndrome. Pyridostigmine in combination with 3,4-DAP are the medications of choice (113; 56; 142).
Endplate acetylcholinesterase deficiency. Acetylcholinesterase inhibitors should be avoided. Ephedrine sulfate (19) or albuterol are the medication of choice (71).
Rapsyn deficiency. Most patients respond well to pyridostigmine (23). Some patients derive additional benefit from the use of 3,4-DAP, ephedrine (09), or albuterol.
Choline acetyltransferase deficiency. Prophylactic use of pyridostigmine is indicated once the diagnosis is established. Because apneic attacks occur suddenly in infants and children, the parents should be provided with an inflatable rescue bag and a fitted mask; they should be instructed in the intramuscular injection of neostigmine methylsulfate (0.04 mg/kg in infants and children) and advised to install an apnea monitor in the home (44; 24).
GFPT1-myasthenia. This disorder responds to pyridostigmine. Albuterol and 3,4-DAP can confer additional benefit.
DPAGT1, ALG2, ALG14, and DMPBB myasthenias. The few patients reported to date responded alone or in combination to pyridostigmine, albuterol, or ephedrine, and 3,4-DAP (15; 31; 16).
Paucity of synaptic vesicles associated with reduced quantal release. The single reported patient responded partially to pyridostigmine (154).
Congenital agrin myasthenia. In the first reported kinship, the index patient failed to respond to a cholinesterase inhibitor and 3,4-diaminopyridine but responded partially to ephedrine (64). Another patient responded moderately to pyridostigmine, but not to 3,4-DAP (78). Patients presenting with wasting first of the limb muscles responded well to albuterol or ephedrine but not to pyridostigmine (98).
Congenital MuSK myasthenia. In one patient, pyridostigmine was ineffective, but 3,4-diaminopyride in combination with pyridostigmine had a beneficial effect (27). In another patient, pyridostigmine and 3,4-diaminopyridine led to gradual slow improvement (87). A third patient failed to respond to cholinergic agonists but had a moderate response to albuterol (76).
LRP4 myasthenia. The first observed patient was worsened by exposure to pyridostigmine (104). Two subsequently investigated patients were also worsened by pyridostigmine as well as by 3,4-diaminopyridine but were improved by albuterol (135).
Dok-7 myasthenia. Pyridostigmine should be avoided because it frequently worsens the disease and some patients deteriorate even after a few days of therapy. 3,4-DAP benefited three out of eight patients and worsened one patient. The treatment of choice is ephedrine (14; 146; 69) or albuterol (71).
Congenital myasthenic syndrome caused by plectin deficiency. One patient with this type of myasthenia failed to respond to pyridostigmine but was transiently improved by 3,4-DAP (10). A second patient was transiently improved by prednisone (133).
Sodium channel myasthenia. In the first reported patient, therapy with pyridostigmine improved the patient’s endurance; additional therapy with acetazolamide, which mitigates periodic paralysis due to mutations in Nav1.4, prevented further attacks of respiratory and bulbar weakness (149).
Congenital myasthenic syndrome caused by a defect in laminin beta2. The single reported patient was profoundly worsened by a cholinesterase inhibitor but was partially improved by ephedrine (79).
Synaptotagmin 2 myasthenia. This congenital myasthenic syndrome responds well to 3,4-DAP (157).
Synaptobrevin-1 myasthenia. The single identified patient responded slightly to pyridostigmine (143).
Endplate acetylcholinesterase deficiency. The prognosis is worsened when patients not responding to anti-acetylcholinesterase drugs are treated with increasing doses of these drugs, which then cause excessive bronchial secretions and potentially fatal respiratory complications. Some patients are improved subjectively and objectively by ephedrine with or without improvement of the EMG decrement (19; 86). Ephedrine, however, is no longer available in the United States; fortunately, albuterol is at least as, or even more, effective than ephedrine in this disease (71).
Slow-channel syndromes. Therapy with long-lived open-channel blockers of AChR such as quinidine and fluoxetine improve the prognosis even in the most severely affected patients (55; 57; 30).
Fast-channel syndromes. Therapy with pyridostigmine and 3,4-DAP mitigates the patients’ symptoms and improves the prognosis.
Primary AChR deficiency. Because limited expression of the fetal gamma subunit can at least partially compensate for defects in the adult epsilon subunit, patients with low-expressor mutations in this subunit have a better prognosis than patients who carry low expressor mutations in both alleles of a non-epsilon subunit for which there are no compensatory subunits. Most patients with low-expressor epsilon subunit mutations show ptosis, limited ocular ductions, variable facial weakness, and mild fatigable weakness of cervical, limb, and axial muscles, and most respond to pyridostigmine. Their course is usually not progressive, and they seldom develop respiratory insufficiency or significant spinal deformities; most are improved by cholinergic agonist. Patients failing to respond to these medications may be improved by therapy with albuterol (127; 125) and 3,4-diaminopyridine.
In contrast, patients with low expressor mutations in both alleles of non-epsilon subunits have severe generalized weakness, frequent choking spells and respiratory crises, and high mortality in early life. They respond variably to cholinergic agonists. Biallelic null mutations of non-epsilon subunits are lethal in embryonic life (84; 152; 153).
Rapsyn deficiency. Long-term follow-up of 25 patients treated with cholinergic therapy showed that 21 became stable or were clinically improved and two of these became asymptomatic; three had a progressive course; and one died in infancy. Therapy with 3,4-DAP or albuterol or both have and additional beneficial effect in some patients (89).
Dok-7 myasthenia. It is now clear that cholinergic agonists, such as pyridostigmine or 3,4-diaminopyridine, can aggravate the disease, whereas ephedrine (14; 134; 131) or albuterol (71) are beneficial.
Myasthenias due to defects in GFPT1, DPAGT1, ALG2, ALG14, and GMPBB. These congenital myasthenic syndromes respond to pyridostigmine and may derive additional benefit from albuterol and 3,4-DAP.
Plectinopathy myasthenia. There is no response to cholinergic agents. Prednisone therapy was of temporary benefit in one patient (133).
Synaptotagmin 2 myasthenia. Synaptotagmin 2 myasthenia responds well to 3,4-DAP (157).
Myo9A myasthenia. Myo9A myasthenia is improved pyridostigmine but worsened by fluoxetine and 3,4-DAP (101).
Vesicular ACh transporter deficiency. Vesicular ACh transporter deficiency is improved by pyridostigmine (103).
Presynaptic high-affinity choline transporter deficiency. Presynaptic high-affinity choline transporter deficiency was improved by pyridostigmine in four out of five patients (11).
Pregnancy may worsen the respiratory status of congenital myasthenic syndrome patients because upward displacement of the diaphragm reduces the lung vital capacity. Patients with significant fatigable weakness of the abdominal muscles may need assistance in the third stage of labor. Despite these potential problems, several of the author’s patients had tolerated their pregnancy well and had uneventful deliveries.
Pyridostigmine has been safely used in pregnant myasthenic patients for more than 50 years. A single report describes microcephaly in an infant whose mother had been exposed to more than four times the recommended dose of pyridoxine during gestation (100). The frequency of severe birth defects was not significantly different in 127 offspring of myasthenic mothers than in nearly 2 million offspring of non-myasthenic mothers, but not all myasthenic mothers may have been exposed to pyridostigmine during gestation (62).
Fluoxetine acts as a long-lived open-channel blocker of AChR, but it is also a serotonin reuptake inhibitor and is widely used as an antidepressant. Pooled data from multiple studies of obstetrical outcomes in women exposed to this group of drugs during pregnancy indicate that they do not increase the risk of major or minor malformations but significantly increase the risk of spontaneous abortions (119).
Quinidine has a long record of safety during pregnancy and is generally well tolerated (66), but, paradoxically, it was listed as a category D drug in the Swedish catalog of pharmaceutical agents (FASS) because reproduction-physiology studies suggested an increased incidence of fetal malformations (18).
The possible adverse effects of 3,4-DAP on the fetus during gestation have not been evaluated. Therefore, women are advised not to take 3,4-DAP when not practicing birth control or when pregnant.
There is no contraindication to the use of conventional anesthetic agents, but neuromuscular blocking agents should be avoided because these can provoke prolonged respiratory paralysis.
Work done in the author’s laboratory was supported by research grant NS6277 and NS109491 from the National Institutes of Health.
Emma Ciafaloni MD FAAN
Dr. Ciafaloni of the University of Rochester received personal compensation for serving on advisory boards and/or as a consultant for Alexion, Avexis, Biogen, PTC Therapeutics, Ra Pharma, Strongbridge Biopharma PLC, and Wave; and for serving on a speaker’s bureau for Biogen. Dr Ciafaloni also received research and/or grant support from Orphazyme, Santhera, and Sarepta.See Profile
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