A structural basis for drug-induced long QT syndrome

Abstract

L ong QT syndrome (LQT) is an abnormality of cardiac muscle repolarization that predisposes affected individuals to a ventricular arrhythmia that can degenerate into ventricular fibrillation and cause sudden death (1). The cellular mechanism of the lengthened QT interval recorded on the body surface electrocardiogram is prolonged ventricular action potentials. Recent genetic discoveries have determined that the molecular mechanism of inherited LQT is mutations in one of several genes [e.g., HERG (2), KCNE2 (3)] that encode ion channel subunits important for normal repolarization of the ventricle. HERG encodes the pore-forming subunits of channels that conduct the rapid delayed rectifier K ϩ current (I Kr ; refs. 4 and 5). LQT also can be acquired as a side effect of treatment with commonly used medications, including some antiarrhythmic, antihistamine, antibiotic, psychoactive, and gastrointestinal prokinetic agents (6, 7). Although drug-induced LQT theoretically could result from reduction of any voltage-gated K ϩ current that contributes to ventricular repolarization, all known drugs with this side effect preferentially block I Kr (1,8,9). It is unclear why so many structurally diverse compounds block HERG channels, but this undesirable side effect now is recognized as a major hurdle in the development of new and safe drugs (9, 10). Previous studies have characterized single residues of HERG that when mutated altered the sensitivity of the channel to block by the methanesulfonanilide antiarrhythmic drug dofetilide. However, these residues are believed to alter drug binding by an allosteric effect related to the loss of inactivation gating (11). A more recent study found that mutation of a Phe located in the S6 domain of HERG decreased block by dofetilide and quinidine (12). However, it is unlikely that a single residue could confer drug sensitivity. Thus, despite the obvious clinical importance of HERG channel block to acquired LQT, the structural basis of the high-affinity drug binding site has not been adequately characterized. Materials and Methods Mutagenesis and Channel Expression in Oocytes.Mutations were introduced into the HERG K ϩ channel (13) by using sitedirected mutagenesis as described previously (14). Complementary RNAs for injection into oocytes were prepared with SP6 Cap-Scribe (Boehringer Mannheim) after linearization of the expression construct with EcoRI. Isolation and maintenance of Xenopus oocytes and cRNA injection were performed as described previously (14-16).Voltage Clamp. The two-microelectrode voltage clamp technique (17) was used to record membrane currents in oocytes 2-4 days after cRNA injection as previously described (18). To attenuate endogenous chloride currents, Cl Ϫ was replaced with Mes (2-[N-morpholino]ethanesulfonic acid) in the external solution that contained 96 mM NaMes͞2 mM KMes͞2 mM CaMes 2 ͞5 mM Hepes͞1 mM MgCl 2 adjusted to pH 7.6 with methane sulfonic acid. In some experiments, KMes was increased to 96 mM with a similar reduction in NaMes.MK-499 was supplied by Me...

Structural basis of action for a human ether-a-go-go-related gene 1 potassium channel activator

“…The binding site for structurally diverse blockers was previously localized to specific residues in S6 and the base of the pore helix that face toward the central cavity (29). F656 and Y652 in S6 are the most important residues for binding of blockers.…”

The Binding Site for Channel Blockers That Rescue Misprocessed Human Long QT Syndrome Type 2 ether-a-gogo-related Gene (HERG) Mutations

“…In our experiments we were guided by more recent insights into the structure of the methanesulfonanilide binding site of HERG K ϩ channels (19,20). We used the known structure-activity relationship of the noncardiac drug astemizole to compare the relative efficacies of channel block and channel rescue.…”

“…These blockers appear to interact with different portions of the extended binding site for methanesulfonanilides in the inner vestibule of HERG (19). We used E4031, a methanesulfonanilide drug in which the methanesulfonyl group is thought to bind to a pocket formed between the pore helix and the S6 helix and its aromatic piperidine ring to aromatic amino acid residues at Tyr-652 and Phe-656.…”

Induced pluripotent stem cells used to reveal drug actions in a long QT syndrome family with complex genetics

“…PCR products were purified after de novo mutation in the principal heart sodium channel (SCN5A) and a common polymorphism in KCNH2 that codes for hERG, a critical heart potassium channel known to contribute to most cases of drug-induced LQTS (Mitcheson et al, 2000). In a first study, we had characterized the biophysical properties of the mutant sodium channels alone expressed in HEK293 cells and found that the mutation resulted in defects in inactivation consistent with the LQT phenotype observed in the proband (Bankston et al, 2007b).…”