Objective: To characterize the molecular basis of a novel fast-channel congenital myasthenic syndrome.
Methods: We used the candidate gene approach to identify the pathogenic mutation in the acetylcholine receptor (AChR) ε subunit, genetically engineered the mutant AChR into HEK cells, and evaluated the level of expression and kinetic properties of the mutant receptor.
Results: An 8-year-old boy born to consanguineous parents had severe myasthenic symptoms since birth. He is wheelchair bound and pyridostigmine therapy enables him to take only a few steps. Three similarly affected siblings died in infancy. He carries a homozygous p.W55R mutation at the α/ε subunit interface of the AChR agonist binding site. The mutant protein expresses well in HEK cells. Patch-clamp analysis of the mutant receptor expressed in HEK cells reveals 30-fold reduced apparent agonist affinity, 75-fold reduced apparent gating efficiency, and strikingly attenuated channel opening probability (P(open)) over a range agonist concentrations.
Conclusion: Introduction of a cationic Arg into the anionic environment of α/ε AChR binding site hinders stabilization of cationic ACh by aromatic residues and accounts for the markedly perturbed kinetic properties of the receptor. The very low P(open) explains the poor response to pyridostigmine and the high fatality of the disease.