Molecular basis of congenital myasthenic syndromes: Mutations in the acetylcholine receptor

The congenital myasthenic syndromes include end-plate (EP) acetytcholinesterase deficiency, presynaptic abnormalities affecting the evoked release or size of transmitter quanta, and acetylcholine (ACh) receptor (AChR) channelopathies stemming from a kinetic abnormality and/or deficiency of AChR. A kinetic abnormality predicts, and AChR deficiency may predict, one or more mutations in an AChR subunit gene. These clues have led to the identification of 53 mutations in different subunits of AChR in 55 kinships of the congenital myasthenic syndromes. The mutations either increase or decrease the response to ACh, produce AChR deficiency, or both. In the slow-channel syndromes, prolonged opening episodes of AChR cause cationic overloading of the EP and an EP myopathy; the mutations occur in different subunits and different domains of the subunits and have dominant positive effects. The M1 and M2 mutations slow channel closure, increase apparent affinity for ACh, and variably enhance desensitization, and the extracellular alpha G153S enhances affinity for ACh, promoting reopening of the diliganded receptor. In the low-affinity fast-channel syndrome, epsilon P121L reduces affinity for ACh and reopening of the diliganded receptor, resulting in a decreased response to ACh and shorter burst durations. Severe EP AChR deficiency results from heterozygous or homozygous mutations that terminate translation prematurely; these are concentrated in the epsilon subunit, probably because substitution of the fetal gamma for the adult epsilon subunit can rescue the phenotype from fatal null mutations in epsilon. Variable AChR deficiency and variable functional abnormalities stem from heteroallelic nonsense and missense mutations in AChR subunit genes.