Question: 

The human ether-á-go-go-related geneHERG (also known as KCNH2 or Kv11.1) encodes the pore-forming a-subunit of the rapid delayed rectifier potassium current I_Kr. I_Kr plays a major role in the repolarization of cardiac action potentials. Loss-of-function mutations or blockade of the hERG potassium channel with drugs such as class III antiarrhythmics or antihistamines can induce Long-QT-syndromes (LQTS) which predispose patients to cardiac arrhythmias and sudden death. We have identified a so far uncharacterized dominant missense mutation in theHERG1 gene (N996I) in a patient with LQT. We intended to express the wildtype (WT) and mutated channel in HEK 293 cells and characterise the malfunction.

Methodology: 

HERG channel a-subunits exist as 2 isoforms (1a and 1b) that are identical except for structurally divergent N termini. Native cardiac I_Kr channels are tetraheteromers containing 2 of each a-subunit types. Both hERG1a and hERG1b subunits were cloned from a human heart cDNA library and the specific N996I mutation introduced by site directed mutagenesis. HEK 293 cells were transiently transfected with either mutated hERG1a or equal amounts of mutated hERG1a and wild type hERG1b cDNAs. The resulting potassium currents were characterized electrophysiolocically and compared to WT.

Results: 

WT and mutated hERG1a potassium current showed similar current densities. Coexpression of hERG1b increased the current density, but there was no difference between WT and mutated channels. Also the voltage-dependence of activation was unchanged with half-maximal activation constants between –27 mV and –29 mV for the different channel assemblies. The deactivation was faster in the mutated hERG1a/b channels with t_fast = 62 ms and t_slow = 480 ms versus t_fast = 90 ms and t_fast = 620 ms in WT channels, determined from the tail current at –40 mV after a 5-second pulse to + 50 mV. Similar results were found for the solely expressed hERG1a subunits. Steady-state inactivation of the tail current was shifted by 20 mV to the depolarizing direction in the mutated channel compared to the WT. Also the recovery from inactivation was different between WT and mutated channels.

Conclusion: 

Loss-of-function can be due to either defects in a) channel opening (gating), b) ion permeation or c) protein maturation and trafficking. For the N996I mutated channel a defect in ion permeation resulting in an accelerated inactivation appears to be the more likely than a defect in opening or maturation and trafficking.