New mutations in acetylcholine receptor subunit genes reveal heterogeneity in the slow-channel congenital myasthenic syndrome

Abstract

Mutations in genes encoding the epsilon, delta, beta and alpha subunits of the end plate acetylcholine (ACh) receptor (AChR) are described and functionally characterized in three slow-channel congenital myasthenic syndrome patients. All three had prolonged end plate currents and AChR channel opening episodes and an end plate myopathy with loss of AChR from degenerating junctional folds. Genetic analysis revealed heterozygous mutations: epsilon L269F and delta Q267E in Patient 1, beta V266M in Patient 2, and alpha N217K in Patient 3 that were not detected in 100 normal controls. Patients 1 and 2 have no similarly affected relatives; in Patient 3, the mutation cosegregates with the disease in three generations. epsilon L269F, delta Q267E and beta V266M occur in the second and alpha N217K in the first transmembrane domain of AChR subunits; all have been postulated to contribute to the lining of the upper half of the channel lumen and all but delta Q267E are positioned toward the channel lumen, and introduce an enlarged side chain. Expression studies in HEK cells indicate that all of the mutations express normal amounts of AChR. epsilon L269F, beta V266M, and alpha N217K slow the rate of channel closure in the presence of ACh and increase apparent affinity for ACh; epsilon L269F and alpha N217K enhance desensitization, and epsilon L269F and beta V266M cause pathologic channel openings in the absence of ACh, rendering the channel leaky, delta Q267E has none of these effects and is therefore a rare polymorphism or a benign mutation. The end plate myopathy stems from cationic overloading of the postsynaptic region. The safety margin of neuromuscular transmission is compromised by AChR loss from the junctional folds and by a depolarization block owing to temporal summation of prolonged end plate potentials at physiologic rates of stimulation.

The Therapy of Congenital Myasthenic Syndromes

“…If these tests point to a candidate gene, then molecular genetic analysis becomes feasible. If a mutation is discovered in the candidate gene, then expression studies with the recombinant mutant molecule can be used to confirm its pathogenicity, analyze the electrophysiologic 6 or biochemical 7 properties of the mutant protein, and test drugs that may mitigate the disease. [8][9][10] The candidate gene approach, based on clinical, morphologic, and electrophysiologic findings, has pointed to defects in choline acetyltransferase (ChAT), 7 AChE, 11 AChR, 12 rapsyn, 13 muscle spe-cific tyrosine kinase (MuSK), 14 and Dok-7 15 as causes of CMS.…”

Therapeutic Strategies in Congenital Myasthenic Syndromes

“…These are inherited disorders, caused by various genetic defects, and all but the slow-channel CMS by recessive inheritance. [1][2][3][4][5] To date, mutations in 10 different genes have been found to cause a CMS: CHAT, coding for the presynaptic choline acetyltransferase 6 ; COLQ, coding for the endplate acetylcholine esterase 7,8 ; CHRNA1, CHRNB1, CHRND, and CHRNE coding for four different AChR subunits 9,10 ; RAPSN, coding for the postsynaptic protein rapsyn 11 ; MUSK, coding for the muscle-specific kinase 12 ; DOK7, coding for the downstream of kinase 7 protein 13 ; and SCN4A, coding for the postsynaptic voltage-gated sodium channel Na v 1.4. 14…”

Sorting receptor Rer1 controls surface expression of muscle acetylcholine receptors by ER retention of unassembled α-subunits

“…1). AChRs are targets of autoimmune antibodies leading to Myasthenia gravis (2), and mutations in AChR subunits can lead to congenital myasthenic syndromes (3).…”