A β‐subunit mutation in the acetylcholine receptor channel gate causes severe slow‐channel syndrome

Abstract

Point mutations in the genes encoding the acetylcholine receptor (AChR) subunits have been recognized in some patients with slow-channel congenital myasthenic syndromes (CMS). Clinical, electrophysiological, and pathological differences between these patients may be due to the distinct effects of individual mutations. We report that a spontaneous mutation of the beta subunit that interrupts the leucine ring of the AChR channel gate causes an eightfold increase in channel open time and a severe CMS characterized by severe endplate myopathy and extensive remodeling of the postsynaptic membrane. The pronounced abnormalities in neuromuscular synaptic architecture and function, muscle fiber damage and weakness, resulting from a single point mutation are a dramatic example of a mutation having a dominant gain of function and of hereditary excitotoxicity.

Slow-Channel Myasthenic Syndrome Caused By Enhanced Activation, Desensitization, and Agonist Binding Affinity Attributable to Mutation in the M2 Domain of the Acetylcholine Receptor α Subunit

“…The SCCMS mutations discovered to date are in the extracellular, M1, or M2 domains of AChR subunits Sine et al, 1995;Engel et al, 1996b;Gomez et al, 1996). The M2 mutations identified previously, however, are in the outer helical region of M2 (⑀T264P, ␤V266M, and ⑀L269F), or at the central leucine ring (␤L262M) that may form the channel gate (Unwin, 1993), and the mutated residues are accessible to labeling reagents when converted to cysteine, predicting that they face the , n ϭ 1.0; carbamylcholine, wild type: K ov ϭ 1.6 ϫ 10…”

“…Expression of the ␣V249F AChR in HEK fibroblasts demonstrates increased channel openings in the absence of ACh, prolonged openings in its presence, enhanced steady-state desensitization, and nanomolar rather than micromolar affinity of one of the two binding sites in the resting activatable state. Thus, neuromuscular transmission is compromised because cationic overloading leads to degenerating junctional folds and loss of AChR, because an increased fraction of AChR is desensitized in the resting state, and because physiological rates of stimulation elicit additional desensitization and depolarization block of transmission.Key words: slow-channel congenital myasthenic syndrome; neuromuscular transmission; endplate myopathy; acetylcholine receptor ␣ subunit gene; mutation analysis; single-channel recording; desensitization; agonist binding affinity Recent studies of congenital myasthenic syndromes (CMS) revealed mutations in acetylcholine receptor (AChR) subunit genes that either reduce expression of AChR (Engel et al, 1996a) or alter its kinetic properties to decrease (Ohno et al, 1996) or increase Sine et al, 1995; Engel et al, 1996b;Gomez et al, 1996) the response to ACh. Mutations that increase response to ACh prolong elementary activation episodes, and these disorders are referred to as slow-channel C MS (SCCMS).…”

“…Key words: slow-channel congenital myasthenic syndrome; neuromuscular transmission; endplate myopathy; acetylcholine receptor ␣ subunit gene; mutation analysis; single-channel recording; desensitization; agonist binding affinity Recent studies of congenital myasthenic syndromes (CMS) revealed mutations in acetylcholine receptor (AChR) subunit genes that either reduce expression of AChR (Engel et al, 1996a) or alter its kinetic properties to decrease (Ohno et al, 1996) or increase Sine et al, 1995; Engel et al, 1996b;Gomez et al, 1996) the response to ACh. Mutations that increase response to ACh prolong elementary activation episodes, and these disorders are referred to as slow-channel C MS (SCCMS).…”

Calpain activation impairs neuromuscular transmission in a mouse model of the slow-channel myasthenic syndrome

“…The slow-channel congenital myasthenic syndrome (SCS) is a dominantly inherited neuromuscular disorder characterized by impaired neuromuscular transmission and degeneration of the neuromuscular junction (NMJ) (1)(2)(3). Patients with SCS manifest fatigability and progressive weakness of the skeletal muscles, with symptoms ranging from eye muscle and mild limb weakness to severe incapacitation and respiratory failure.…”

“…Ultrastructural study of muscle in SCS reveals a focal degenerative and remodeling process selectively localized to the NMJ termed endplate myopathy. This progressive, purely synaptic disease process, in which the cleft is widened and nerve terminals are shrunken, is characterized by expansion and degeneration of postsynaptic folds as well as degeneration of subsynaptic nuclei, mitochondria, and myofibrils (1)(2)(3)(4).…”