Trafficking and Gating Mechanisms of HERG1A C-Terminus (LQTS-2) Truncation Mutations on HERG1A-HERG1B Hetero-Multimeric Channel

Abstract

In heart, HERG1A-HERG1B subunits generate the rapid component of delayed rectifier potassium current, IKr, critical for normal repolarization. Congenital mutations in HERG1A lead to long QT syndrome-2 (LQTS-2), a condition that increases susceptibility to fatal arrhythmias. Many LQTS-2 mutations that are localized to HERG1A C-terminus generate truncated variants and their behavior when co-expressed with HERG1B and their (patho)physiological effects in heart are largely unknown. Therefore, we compared the physiological impact and mechanistic bases of two HERG1A C-terminus truncation mutations (G965X, R1014X). Bungarotoxin-binding optical assays for channel cell-surface expression and electrophysiological recordings were carried out in Human Embryonic Kidney cells. R1014X displayed normal surface density whereas, the G965X mutant, displayed a significant decrease in channel surface density, which was fully rescued by wild-type HERG1A-HERG1B subunits. Homo-multimeric (mutant+HERG1B), G965X and R1014X channel subunits yielded currents with severely reduced current amplitude. When co-expressed with wild-type HERG1A, both mutants exerted a dominant negative effect but to different extents: G965X current amplitude was partially rescued while R1014X current remained unchanged. Homo-multimeric mutant channels displayed a significant rightward shift in the activation curve which was partially (G965X), or not (R1014X) rescued with wild-type HERG1A. The data reveal these mutants exert a purely biophysical effect on hetero-multimeric (mutant HERG1A + wild-type HERG1A + HERG1B) channels. These mechanistic insights may enhance therapies for LQTS-2 mutations. Furthermore, we show for the first time that the R1014X shows normal surface expression, but is non-functional and exerts a strong dominant-negative effect on wild-type HERG1A channels which is in contrast to existing data that demonstrates that R1014X causes HERG1A channel dysfunction by defective trafficking. Finally, studies that explore the functional impact of these mutations in heart will provide novel information that will be more predictive of disease penetrance.