Characterization of the novel mutant A78T‐HERG from a long QT syndrome type 2 patient: Instability of the mutant protein and stabilization by heat shock factor 1

Background The human ether‐a‐go‐go‐related gene (HERG) encodes the α‐subunit of rapidly activating delayed‐rectifier potassium channels. Mutations in this gene cause long QT syndrome type 2 (LQT2). In most cases, mutations reduce the stability of the channel protein, which can be restored by heat shock (HS). Methods We identified the novel mutant A78T‐HERG in a patient with LQT2. The purpose of the current study was to characterize this mutant protein and test whether HS and heat shock factors (HSFs) could stabilize the mutant protein. A78T‐HERG and wild‐type HERG (WT‐HERG) were expressed in HEK293 cells and analyzed by immunoblotting, immunoprecipitation, immunofluorescence, and whole‐cell patch clamping. Results When expressed in HEK293 cells, WT‐HERG gave rise to immature and mature forms of the protein at 135 and 155 kDa, respectively. A78T‐HERG gave rise only to the immature form, which was heavily ubiquitinated. The proteasome inhibitor MG132 increased the expression of immature A78T‐HERG and increased both the immature and mature forms of WT‐HERG. WT‐HERG, but not A78T‐HERG, was expressed on the plasma membrane. In whole‐cell patch clamping experiments, depolarizing pulses evoked E4031‐sensitive HERG channel currents in cells transfected with WT‐HERG, but not in cells transfected with A78T‐HERG. The A78V mutant, but not A78G mutant, remained in the immature form similarly to A78T. Maturation of the A78T‐HERG protein was facilitated by HS, expression of HSF‐1, or exposure to geranyl geranyl acetone. Conclusions A78T‐HERG was characterized by protein instability and reduced expression on the plasma membrane. The stability of the mutant was partially restored by HSF‐1, indicating that HSF‐1 is a target for the treatment for LQT2 caused by the A78T mutation in HERG.