Novel Gain-of-Function Mechanism in K ⁺ Channel–Related Long-QT Syndrome:

Abstract

Abstract-The N629D mutation, adjacent to the GFG signature sequence of the HERG1 A K ϩ channel, causes long-QT syndrome (LQTS). Expression of N629D in Xenopus oocytes produces a rapidly activating, noninactivating current. N629D is nonselective among monovalent cations; permeation of K ϩ was similar to that of Na ϩ or Cs ϩ . During repolarization to potentials between Ϫ30 and Ϫ70 mV, N629D manifested an inward tail current, which was abolished by replacement of extracellular Na ϩ (Na ϩ e ) with extracellular N-methyl-D-glucamine (NMG e ). Because LQTS occurs in heterozygous patients, we coexpressed N629D and wild type (WT) at equimolar concentrations. Heteromultimer formation was demonstrated by analyzing the response to 0 [K ϩ ] e . The outward time-dependent current was nearly eliminated for WT at 0 [K ϩ ] e , whereas no reduction was observed for homomultimeric N629D or for the equimolar coexpressed current. To assess physiological significance, dofetilide-sensitive currents were recorded during application of simulated action potential clamps. During phase 3 repolarization, WT manifested outward currents, whereas homomultimeric N629D manifested inward depolarizing currents. During coexpression studies, variable phenotypes were observed ranging from a reduction in outward repolarizing current to net inward depolarizing current during phase 3. In summary, N629D replaces the WT outward repolarizing tail current with an inward depolarizing sodium current, which is expected to delay later stages of repolarization and contribute to arrhythmogenesis. Thus, the consequences of N629D resemble the pathophysiology seen in LQT3 Na ϩ channel mutations and may be considered the first LQTS K Key Words: long-QT syndrome Ⅲ HERG1 Ⅲ K ϩ channel Ⅲ gain of function F amilial long-QT syndrome (LQTS) results from defects in sodium and potassium ion channels that cause prolongation of cardiac repolarization and arrhythmias. 1 LQT2 is associated with mutations of the human ether-a-go-go-related gene, HERG1. [2][3][4][5][6][7][8][9][10] The HERG1 primary transcript is alternatively processed, giving rise to at least 3 functional mRNAs, HERG1 A, HERG1 AЈ, and HERG1 B, encoding proteins with distinct physiological properties. 11,12 There are several mechanisms by which individual mutations in HERG1 produce LQTS. 5-9 Some exert a dominant phenotype through loss of repolarizing current. These include mutations that either cause defects in intracellular transport or result in channels that do not open. Alternatively, V630L forms heterotetramers with wild type (WT) that have reduced open probability due to a negative shift in voltage dependence of inactivation. Abnormally fast deactivation mutations caused by mutations in the N terminus of HERG1 A result in reduction of outward current during phase 3 repolarization and thus LQTS. 10 The recently described N629D mutation 9 is of particular interest, as it alters the pore selectivity signature sequence from GFGN to GFGD. 13 In most K ϩ channels, including the extensively studied Shaker chann...

A structural basis for drug-induced long QT syndrome

“…2e). This finding was unexpected because previous studies concluded HERG channel inactivation was required for high-affinity drug block (11,12), suggesting that enhanced inactivation might increase drug potency.…”

“…2d). LeesMiller et al (12) also found that mutation of F656 (to Val) reduced the block of HERG by quinidine and dofetilide.…”

“…Access of MK-499 and related drugs to the binding site on HERG requires channel opening (32). Closure of the activation gate (deactivation) traps the molecule within the cavity (18), and inactivation can dramatically enhance binding affinity (11,12) of the drug to the site described here. Other voltage-gated K ϩ channels (Kv1-Kv4) contain a Pro-X-Pro sequence in the S6 domain that has been proposed to cause a sharp bend in the S6 helices (33) and may reduce the volume of the channel cavity.…”

“…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.…”

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

“…LQT2 mutations are, with only one exception, loss-of-function mutations that reduce the repolarizing cardiac potassium current I Kr thereby prolonging the cardiac action potential (3,4). When expressed in heterologous cells loss of function is caused either by mal-or nonfunctioning tetrameric channels inserted into the plasma membrane or by trafficking-deficient mutant channels retained in the endoplasmic reticulum (ER) (5)(6)(7)(8).…”