Characterization of a novel missense mutation E637K in the pore-S6 loop of HERG in a patient with long QT syndrome.

In a 32-year-old woman with marked QT prolongation (QTc=0.61 s) and repeated episodes of syncope, we identified a single pertinent base substitution (G to A at 1909) in HERG by genetic analysis. This novel missense mutation is predicted to cause an amino acid substitution of lysine for glutamic acid at position 637 (E637K) in the pore-S6 loop. Therefore, we investigated the role of a glutamic acid at the vicinity of the pore in HERG channels by mutating it to a lysine. We characterized the electrophysiological properties of the E637K mutation using a Xenopus oocyte heterologous expression system. Injection of the E637K mutant cRNA alone into Xenopus oocytes did not result in any expression of detectable currents. Coexpression of wild-type (WT) and E637K (E637K/WT) elicited only about 30% of the control peak tail current that was expected from expression of WT alone. Kinetic analyses revealed that E637K/WT decelerated the rate of channel activation and enhanced steady-state inactivation. Furthermore, the reversal potentials at low concentrations of K+ showed a positive shift in oocytes injected with E637K/WT compared with WT alone. These results indicated that the E637K mutation causes apparent dominant negative suppression of WT HERG channel function and suggest that E637 at the Pore-S6 is a crucial component of the activation and inactivation gate of HERG channels.