NEW MUTATIONS IN ACETYLCHOLINE-RECEPTOR SUBUNIT GENES REVEAL HETEROGENEITY IN THE SLOW-CHANNEL CONGENITAL MYASTHENIC SYNDROME

Mutations in genes encoding the epsilon, delta, beta and a subunits of the end plate acetylcholine (ACh) receptor (AChR) are described and f unctionally 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 heterozy gous 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 relat ives; in Patient 3, the mutation cosegregates with the disease in thre e generations, epsilon L269F, delta Q267E and beta V266M occur in the second and alpha N217K in the first transmembrane domain of AChR subun its; all have been postulated to contribute to the lining of the upper half of the channel lumen and all but delta Q267E are positioned towa rd the channel lumen, and introduce an enlarged side chain. Expression studies in HEK cells indicate that all of the mutations express norma l 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 desensitizatio n, and epsilon L269F and beta V266M cause pathologic channel openings in the absence of ACh, rendering the channel leaky. delta Q267E has no ne of these effects and is therefore a rare polymorphism or a benign m utation. 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 depolar ization block owing to temporal summation of prolonged end plate poten tials at physiologic rates of stimulation.