CONGENITAL MYASTHENIC SYNDROMES DUE TO HETEROALLELIC NONSENSE MISSENSE MUTATIONS IN THE ACETYLCHOLINE-RECEPTOR EPSILON-SUBUNIT GENE - IDENTIFICATION AND FUNCTIONAL-CHARACTERIZATION OF 6 NEW MUTATIONS/

We describe and functionally characterize six mutations of the acetylc holine receptor (AChR) epsilon subunit gene in three congenital myasth enic syndrome patients, Endplate studies demonstrated severe endplate AChR deficiency, dispersed endplate regions and well preserved junctio nal folds in all three patients. Electrophysiologic studies were consi stent with expression of the fetal gamma-AChR at the endplates in one patient, prolongation of some channel events in another and gamma-AChR expression as well as some shorter than normal channel events in stil l another, Genetic analysis revealed two recessive and heteroallelic e psilon subunit gene mutations in each patient, One mutation in each (e psilon C190T [epsilon R64X], epsilon 127ins5 and epsilon 553del7) gene rates a nonsense codon that predicts truncation of the epsilon subunit in its N-terminal, extracellular domain; and one mutation in each gen erates a missense codon (epsilon R147L, epsilon P245L and epsilon R311 W), None of the mutations was detected in 100 controls, Expression stu dies in HEK cells indicate that the three nonsense mutations are null mutations and that surface expression of AChRs harboring the missense mutations is significantly reduced, Kinetic analysis of AChRs harborin g the missense mutations show that epsilon R147L is kinetically benign , epsilon P245L prolongs burst open duration 2-fold by slowing the rat e of channel closing and epsilon R311W shortens burst duration 2-fold by slowing the rate of channel opening and speeding the rate of ACh di ssociation, The modest changes in activation kinetics are probably ove rshadowed by reduced expression of the missense mutations, The consequ ences of the endplate AChR deficiency are mitigated by persistent expr ession of gamma-AChR, changes in the release of transmitter quanta and appearance of multiple endplate regions on the muscle fiber.