Objective: To identify the genetic basis of a slow-channel myasthenic syndrome, characterize functional properties of the mutant receptor, and selectively silence the mutant allele.
Methods: We performed nutation analysis, cloning, and patch-clamp analysis of the functional properties of the mutant receptor; screening for a small interfering RNA with check plasmid; and assessed of the efficacy of small interfering RNA at the messenger RNA, protein, and functional levels.
Results: We traced the cause of a slow-channel myasthenic syndrome to a C418W mutation in the M4 domain of the acetylcholine receptor alpha subunit. The mutation is the first one to occur spontaneously in an M4 domain of the receptor, and it is positioned within a stripe of hydrophobic residues facing the lipid bilayer. Kinetic analysis shows that alphaC418W enhances the channel opening equilibrium constant 26-fold without altering agonist affinity. Using a check plasmid as a screening tool, we identified a small interfering RNA that markedly suppresses the mutant but not the wild-type allele at the messenger RNA, protein, and functional levels.
Interpretation: alphaC418W occurring in humans causes a slow-channel syndrome by enhancing the relative stability of the channel open state. Efficient and selective knockdown of the mutant allele holds promise of therapeutic gene silencing.