hERG channel trafficking: novel targets in drug-induced long QT syndrome

The cardiac potassium channel hERG (human ether-a-go-go-related gene) encodes the α-subunit of the rapid delayed rectifier current IKr in the heart, which contributes to terminal repolarization in human cardiomyocytes. Direct block of hERG/IKr channels by a large number of therapeutic compounds produces acLQTS [acquired LQTS (long QT syndrome)] characterized by drug-induced QT prolongation and torsades de pointes arrhythmias. The cardiotoxicity associated with unintended hERG block has prompted pharmaceutical companies to screen developmental compounds for hERG blockade and made hERG a major target in drug safety programmes. More recently, a novel form of acLQTS has been discovered that may go undetected in most conventional safety assays. Several therapeutic compounds have been identified that reduce hERG/IKr currents not by direct block but by inhibition of hERG/IKr trafficking to the cell surface. Important examples are antineoplastic Hsp90 (heat-shock protein 90) inhibitors such as (i) geldanamycin, (ii) the leukaemia drug arsenic trioxide, (iii) the antiprotozoical pentamidine, (iv) probucol, a cholesterol-lowering drug, and (v) fluoxetine, a widely used antidepressant. Increased awareness of drug-induced hERG trafficking defects will help to further reduce the potentially lethal adverse cardiac events associated with acLQTS. The cardiac potassiumchannel hERG (humanether-a-go-gorelatedgene)encodesthe α-subunitoftherapiddelayedrectifiercurrentIKr intheheart,whichcontributesprominentlyto terminal repolarization in human ventricular myocytes. The strategic position occupied by hERG/IKr channels in cardiac repolarization was revealed first in patients when loss-offunction mutations in the hERG gene were discovered to cause inherited LQTS2 (long QT syndrome2) [1,2]. LQTS is characterized by a prolongation of the QT interval on the electrocardiogram, which is considered a surrogate marker for ventricular repolarization at the cellular level. As a direct consequence of abnormal repolarization in LQTS, patients may present with syncope, TdP (torsades de pointes) arrhythmias) or sudden cardiac death [3].