Mutations In KCNE1 Research Articles

KCNE1 Separates the Main Voltage Sensor Movement and Channel Opening in KCNQ1/KCNE1 Channels

The IKs channel is a slowly activating potassium channel that generates one of the potassium currents that limits the duration of the cardiac action potential. The IKs channel consists of 4 pore forming alpha subunits (KCNQ1) and 2-4 beta subunits (KCNE1). The four KCNQ1 subunits form functional potassium channels, but the KCNE1 beta subunit is necessary to recapitulate the slow activation kinetics of the IKs channel. The mechanism by which KCNE1 slows the kinetics of KCNQ1 channels is a matter of current controversy. Here, we use a combination of voltage clamp fluorometry (VCF) and gating current measurements to show that IKs channel activation occurs in two steps: (1) mutually independent voltage sensor movements in the four KCNQ1 subunits generate the main gating charge movement and underlie the delay in the activation time course of the KCNQ1/KCNE1 currents, (2) a slower and concerted conformational change of all four voltage sensors and the gate, which opens the KCNQ1/KCNE1 channel. Gating currents develop with a similar time and voltage dependence as the first fluorescence component, as if the first component reports on the main S4 charge movement. In contrast to other Kv channels, the voltage dependences of the main voltage sensor movement and channel opening are separated by over 100 mV. The two activation steps in KCNQ1/KCNE1 channels can be farther separated by a disease-causing mutation in KCNE1. We determine rates and voltage dependence of the gating transitions to construct a model for KCNQ1/KCNE1 channels. Our model is consistent with that KCNQ1/KCNE1 channel has a fast S4 movement at negative voltages that moves the majority of gating charge and a slower second conformational change at positive voltages that moves a smaller amount of gating charge and opens the gate.

Glycine/Serine Polymorphism at Position 38 Influences KCNE1 Subunit’s Modulatory Actions on Rapid and Slow Delayed Rectifier K<sup>+</sup> Currents

KCNE1 encodes a modulator of KCNH2 and KCNQ1 delayed rectifier K(+) current channels. KCNE1 mutations might cause long QT syndrome (LQTS) by impairing KCNE1 subunit's modulatory actions on these channels. There are major and minor polymorphismic KCNE1 variants whose 38(th) amino acids are glycine and serine [KCNE1(38G) and KCNE1(38S) subunits], respectively. Despite its frequent occurrence, the influence of this polymorphism on the K(+) channels' function is unclear. Patch-clamp recordings were obtained from human embryonic kidney -293T cells. KCNH2 channel current density in KCNE1(38S)-transfected cells was smaller than that in KCNE1(38G)-transfected cells by 34%. The voltage-sensitivity of the KCNQ1 channel current in KCNE1(38S)-transfected cells was lowered compared to that in KCNE1(38G)-transfected cells, with a +13mV shift in the half-maximal activation voltage. KCNH2 channel current density or KCNQ1 channel voltage-sensitivity was not different between KCNE1(38G)-transfected cells and cells transfected with both KCNE1(38G) and KCNE1(38S). Moreover, the KCNH2 channel current in KCNE1(38S)-transfected cells was more susceptible to E4031, a QT prolonging drug and a condition with hypokalemia, than that in KCNE1(38G)-transfected cells. Homozygous inheritance of KCNE1(38S) might cause a mild reduction of the delayed rectifier K(+) currents and might thereby increase an arrhythmogenic potential particularly in the presence of QT prolonging factors. By contrast, heterozygous inheritance of KCNE1(38G) and KCNE1(38S) might not affect the K(+) currents significantly. (Circ J 2014; 78: 610-618).

Paradigm shifts in the genetics of inherited arrhythmias: Using next-generation sequencing technologies to uncover hidden etiologies

Long QT syndrome (LQTS), a rare inheritable arrhythmia first described by Romano andWard et al. in the 1960s, is characterized by the prolongation of the QT interval on surface ECGs and an increased risk of potentially fatal ventricular arrhythmias, especially torsade de pointes [1]. In 1995, after years of extensive clinical investigation and linkage analysis, several research groups, including Keating et al., successfully identified three distinct LQTS phenotypes (LQT1, LQT2, and LQT3) associated with mutations in genes encoding plasma membrane ion channels (KCNQ1, KCNH2, and SCN5A, respectively) [2–4]. These seminal studies motivated further extensive genetic screening in LQTS patients using a candidate gene approach and functional analyses of the mutant genes. These efforts provided evidence that ion channel genes represent the genetic basis for several other arrhythmogenic syndromes that occur in the structurally intact heart, often referred to as idiopathic ventricular fibrillation. At present, 13 genes responsible for LQTS have been identified. Approximately 90% of the genotyped LQTS subjects belong to the three major subtypes (LQT1-3) [5] in which numbers of distinct genotype-specific clinical characteristics have been demonstrated, including T-wave morphology [6], triggers for cardiac events [7], response to the epinephrine provocation test [8] and drug therapy [9]. Genetic testing for known arrhythmia susceptibility genes has become the standard-of-care for a number of disorders, including LQTS. Considering the remarkable progress of research in this area, there is no doubt that 1995 was the year in which genetic technologies experienced a paradigm shift with respect to both the understanding and clinical management of inherited arrhythmias. However, it should be noted that despite the rapid progress in understanding the genetic basis, the etiology still remains unknown in approximately 20% of LQTS conditions [10]. Therefore, additional studies are needed to reveal the missing heritable factors associated with these syndromes. Traditionally, DNA sequence information has been elucidated using Sanger sequencing, but this method is limited by the amount of DNA that can be processed at a given time and by the read length (average 800 base pairs). Although the Human Genome Project completed the sequencing of the entire human genome in 2001 using Sanger sequencing, it took 13 years of effort at an estimated cost of $2.7 billion. Now, next-generation sequencing (NGS), a revolutionary new genetic technology, has enabled whole genomes to be sequenced over a period of a few days at projected costs of less than $10,000. This technology involves massively parallel sequencing of clonally amplified or single DNA molecules that are spatially separated in a flow cell. To date, the genes responsible for disease in more than 3000 Mendelian disorders have yet to be identified. NGS opens up exciting new possibilities of discovering the genes associated with these monogenic disorders. Classical strategies involve linkage analysis in families with known shared genetic heritage by identifying candidate genomic regions encompassing the gene

Regulation of Voltage Sensor Movement in KCNQ Channels by KCNE Beta Subunits

The KCNQ1 potassium channel plays widely different physiological roles different cell types. In the heart KCNQ1 forms part of the voltage-gated IKs channels that limit the duration of the cardiac action potential, whereas in epithelial cells KCNQ1 forms part of a voltage-independent K+ channel that is important for ion secretion. The different functions of KCNQ1 are mainly due to the co-assembly of KCNQ1 with different KCNE beta subunits. For example, the IKS channel consists of 4 α-subunits (KCNQ1) which assemble with 2 to 4 β subunits (KCNE1), whereas in epithelial cells KCNQ1 co-assembles with KCNE3 to form voltage-independent K+ channels. Mutations in either KCNQ1 or KCNE1 cause cardiac arrhythmia syndromes. Here we use Voltage clamp fluorometry (VCF) to directly study the effects of wild type and mutant KCNE subunits on the voltage sensor movement in KCNQ1 channel. We assess the voltage sensor movement (fluorescence) and channel opening (current), in order to understand the coupling between the KCNQ1 voltage sensor and channel gate in the presence of KCNE subunits. Our data shows that KCNE1 splits the voltage sensor movement in two separates phases: one at hyperpolarized potentials that moves the voltage sensor to an active state and a second at more depolarized potentials, which is tightly coupled to channel opening. Interestingly, VCF shows that KCNE3 locks the voltage sensor of KCNQ1, presumably in its activated state, thereby generating a voltage-independent K+ channel. Furthermore, arrhythmia-causing mutations in KCNE1 shift the voltage dependence of the two different voltage sensor movements, revealing some of the molecular mechanisms underlying these arrhythmia-causing mutations. Our data suggests a putative mechanism for how KCNE1 subunit exerts its effects on the voltage sensor movement during IKs channel activation.

Long QT Mutations and KCNE1 β-Subunit Modulate S4 Movement in KCNQ1 Channel

The KCNQ1 potassium channel is expressed in many different cell types and plays widely different physiological roles. For example, in the heart KCNQ1 forms part of the voltage-gated IKs channels that contributes to limiting the duration of the cardiac action potential. The different functions of the KCNQ1 channel are mainly due to the co-assembly of the KCNQ1 channel with different beta subunits from the KCNE family. The IKS channel consists of 4 α-subunits (KCNQ1) which assemble with 2 to 4 β subunits (KCNE1). Mutations in either KCNQ1 or KCNE1 subunit cause multiple cardiac arrhythmia syndromes such as long QT syndrome, short QT syndrome, and familial atrial fibrillation. Here we use Voltage clamp fluorometry (VCF) to directly study the effects of wild type, mutant KCNQ1 and KCNE1 subunits on the voltage sensor movement in KCNQ1 channel. We assess the voltage sensor movement (fluorescence) and channel opening (current), in order to understand the coupling between the KCNQ1 voltage sensor and channel gate in the presence of KCNE1. Our data show that KCNE1 splits the voltage sensor movement in two separates phases. An early phase (1) involving voltage sensor movements to its active state upon changes in the membrane potential occurs at hyperpolarized potentials. A second phase (2) that happens at much more depolarized potentials involves an additional voltage sensor movement which is tightly coupled to channel opening. Mutations in KCNE1 that cause arrhythmia shift the voltage dependence of the voltage sensor movements of either phases 1 or 2 (or both), revealing some of the molecular mechanisms underlying the pathophysiology of these arrhythmia-causing mutations. Our data suggests a putative mechanism for how KCNE1 exerts its effects on the voltage sensor movement during IKs channel activation.

Probing the Membrane Bound KCNE1 Protein with Solid State NMR Spectroscopy

KCNE1, also known as MinK, is a membrane protein that associates with the KCNQ1 channel protein to form a voltage-gated potassium channel. This ion channel is essential to the cardiac action potential that mediates heartbeat and is also critical for potassium ion homeostasis in the inner ear. Dominant mutations in KCNE1 lead to congenital long-QT syndrome and congenital deafness. KCNE1 has been over expressed in E. coli, purified into micelles using his-tag affinity chromatography, and reconstituted into POPC/POPG vesicles. 31P NMR powder spectra results confirm vesicle formation. Different KCNE1 mutants have been labeled using MTSL, one mutant outside the membrane and the other inside the membrane. By measuring 31P relaxation times of the lipids, we can determine the depth that at which KCNE1 is buried inside the vesicles. We also introduced a bicelle system to study the topology of uniform 15N labeled KCNE1 with respect to the lipid bilayer. By measuring the 15N NMR signal, we are able to figure out the structural topology of KCNE1 within the lipid bilayer.

A novel mutation in the transmembrane nonpore region of the KCNH2 gene causes severe clinical manifestations of long QT syndrome

Long QT syndrome (LQTS) is characterized by prolonged ventricular repolarization and variable clinical course with arrhythmia-related syncope and sudden death. Mutations in the nonpore region of the LQTS-associated KCNH2 gene (also known as hERG) are mostly associated with coassembly or trafficking abnormalities, resulting in haplotype insufficiency and milder clinical phenotypes compared with mutations in the pore domain. To investigate the effect of a nonpore mutation on the channel current, which was identified from an LQTS family with severe clinical phenotypes. Two members of a Japanese family with LQTS were searched for mutations in KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2, and KCNJ2 genes by using automated DNA sequencing. We characterized the electrophysiological properties and glycosylation pattern of the mutant channels by using patch clamp recording and Western blot analysis. In the LQTS patient with torsades de pointes and cardiopulmonary arrest, we identified the novel T473P mutation in the transmembrane nonpore region of KCNH2. The proband's father carried the same mutation and showed prolonged corrected QT interval and frequent torsades de pointes in the presence of hypokalemia following the administration of garenoxacin. Patch clamp analysis in heterologous cells showed that hERG T473P channels generated no current and exhibited a dominant negative effect when coexpressed with wild-type protein. Only incompletely glycosylated hERG T473P channels were observed by using Western blot analysis, suggesting impaired trafficking. These results demonstrated that a trafficking-deficient mutation in the transmembrane nonpore region of KCNH2 causes a dominant negative effect and a severe clinical course in affected patients.

How Does KCNE1 Regulate the Kv7.1 Potassium Channel? Model-Structure, Mutations, and Dynamics of the Kv7.1-KCNE1 Complex

The voltage-gated potassium channel Kv7.1 and its auxiliary subunit KCNE1 are expressed in the heart and give rise to the major repolarization current. The interaction of Kv7.1 with the single transmembrane helix of KCNE1 considerably slows channel activation and deactivation, raises single-channel conductance, and prevents slow voltage-dependent inactivation. We built a Kv7.1-KCNE1 model-structure. The model-structure agrees with previous disulfide mapping studies and enables us to derive molecular interpretations of electrophysiological recordings that we obtained for two KCNE1 mutations. An elastic network analysis of Kv7.1 fluctuations in the presence and absence of KCNE1 suggests a mechanistic perspective on the known effects of KCNE1 on Kv7.1 function: slow deactivation is attributed to the low mobility of the voltage-sensor domains upon KCNE1 binding, abolishment of voltage-dependent inactivation could result from decreased fluctuations in the external vestibule, and amalgamation of the fluctuations in the pore region is associated with enhanced ion conductivity.

Activation of Voltage Sensing Domains during KCNQ1 Channel Opening: Concerted or Independent?

In the heart, Iks current is a major contributor in limiting the duration of the action potential. The Iks channel consists of four α-subunits (KCNQ1) which assemble with β subunits (KCNE1). Mutations in either KCNQ1 or KCNE1 cause multiple cardiac arrhythmia syndromes such as LQT syndrome, short QT syndrome and familial atrial fibrillation. Iks channels, characterized mainly by its slow activation and deactivation kinetics and opening at depolarized voltages, differ from those of functional tetrameric voltage-gated KCNQ1 channels expressed alone: fast activating and deactivating kinetic and opening at less depolarized voltages. Understanding the channel structure-function relationship represents a valuable tool in predicting not only cardiac arrhythmia risks, but most importantly possible therapeutic solutions. We aim to unveil the molecular mechanism underlying channel opening in wild type (wt) KCNQ1 channel and its interaction with KCNE1, as well as the molecular mechanism underlying arrhythmia-inducing mutations. To date, two models have been proposed for KCNQ1 channel activation: 1) a cooperative S4 movement, in which the channel opening occurs after a coordinated S4 voltage-sensing domain movement and 2) S4 moves independently to each other concomitantly rendering a step-wise current activation. To test the validity of these models, we characterize the electrophysiological properties of natural occurring LQT syndrome mutations such as R231 in KCNQ1 channel using two electrode voltage clamp (TEVC) and voltage clamp fluorescence (VCF) techniques. R231A mutation has been shown to cause KCNQ1 channel constitutively activated, probably by keeping S4 voltage-sensing domain locked in the activated state. We performed TEVC and VCF measurements of tetrameric constructs of R231 combined with wt KCNQ1 channel to determine whether the S4 voltage-sensing domains in KCNQ1 move independently of each other. Our data will provide insight into the mechanism by which KCNQ1 channel operates.

The voltage-gated channel accessory protein KCNE2: multiple ion channel partners, multiple ways to long QT syndrome

The single-pass transmembrane protein KCNE2 or MIRP1 was once thought to be the missing accessory protein that combined with hERG to fully recapitulate the cardiac repolarising current IKr. As a result of this role, it was an easy next step to associate mutations in KCNE2 to long QT syndrome, in which there is delayed repolarisation of the heart. Since that time however, KCNE2 has been shown to modify the behaviour of several other channels and currents, and its role in the heart and in the aetiology of long QT syndrome has become less clear. In this article, we review the known interactions of the KCNE2 protein and the resulting functional effects, and the effects of mutations in KCNE2 and their clinical role.

N‐ and O‐linked glycosylation coordinate cell‐surface localization of a cardiac potassium channel

<i>N</i>‐ and <i>O</i>‐linked glycosylation coordinate cell‐surface localization of a cardiac potassium channel

Partial restoration of the long QT syndrome associated KCNQ1 A341V mutant by the KCNE1 β-subunit

Background The A341V mutation in the pore-forming KCNQ1 subunit of the slowly activating delayed-rectifier potassium current (IKs) underlies a common form of the long QT syndrome, and is associated with an unusually severe phenotype. However, there is controversy regarding the underlying mechanism responsible for the clinically observed phenotype. We investigated the biophysical characteristics of A341V in a cardiac environment by utilizing a cardiac cell line, and in particular the impact of the KCNE1 β-subunit. Methods Whole-cell current were recorded from transiently transfected HL-1 cells, a cardiac cell line. Mutant KCNQ1 and KCNE1 were constructed by site-directed mutagenesis. Results The A341V mutant resulted in a non-functional channel when expressed alone. When co-expressed with wild type KCNE1, A341V produced a slowly activating current, with a smaller current density, slower rates of activation, and a depolarized shift in its activation curve compared to the wild type KCNQ1 + KCNE1. Confocal microscopy confirmed the surface expression of GFP-tagged A341V, suggesting a functionally defective protein. A T58A mutation in KCNE1 abolished functional restoration of A341V. Under heterozygous conditions, the expression of A341V + KCNQ1 + KCNE1 reduced but did not abolish the electrophysiological changes observed in A341V + KCNE1. A dominant negative effect of A341V was also observed. Action potential simulations revealed that the A341V mutation is arrhythmogenic. Conclusions The KCNE1 β-subunit partially rescued the non-functional A341V mutant, with electrophysiological properties distinct from the wild type IKs. General significance The severity of the A341V phenotype may be due to a combination of a significant suppression of the IKs with altered biophysical characteristics.

Protein kinase C downregulates IKs by stimulating KCNQ1-KCNE1 potassium channel endocytosis

The slow-activating cardiac repolarization K(+) current (I(Ks)), generated by the KCNQ1-KCNE1 potassium channel complex, is controlled via sympathetic and parasympathetic regulation in vivo. Inherited KCNQ1 and KCNE1 mutations predispose to ventricular fibrillation and sudden death, often triggered by exercise or emotional stress. Protein kinase C (PKC), which is activated by α1 adrenergic receptor stimulation, is known to downregulate I(Ks) via phosphorylation of KCNE1 serine 102, but the underlying mechanism has remained enigmatic. We previously showed that KCNE1 mediates dynamin-dependent endocytosis of KCNQ1-KCNE1 complexes. This study sought to determine the potential role of endocytosis in I(Ks) downregulation by PKC. We utilized patch clamping and fluorescence microscopy to study Chinese hamster ovary (CHO) cells coexpressing KCNQ1, KCNE1, and wild-type or dominant-negative mutant (K44A) dynamin 2, and neonatal mouse ventricular myocytes. The PKC activator phorbol 12-myristate 13-acetate (PMA) decreased I(Ks) density by >60% (P < .05) when coexpressed with wild-type dynamin 2 in CHO cells, but had no effect when coexpressed with K44A-dynamin 2. Thus, functional dynamin was required for downregulation of I(Ks) by PKC activation. PMA increased KCNQ1-KCNE1 endocytosis in CHO cells expressing wild-type dynamin 2, but had no effect on KCNQ1-KCNE1 endocytosis in CHO cells expressing K44A-dynamin 2, determined using the Pearson correlation coefficient to quantify endosomal colocalization of KCNQ1 and KCNE1 with internalized fluorescent transferrin. KCNE1-S102A abolished the effect of PMA on I(Ks) currents and endocytosis. Importantly, PMA similarly stimulated endocytosis of endogenous KCNQ1 and KCNE1 in neonatal mouse myocytes. PKC activation downregulates I(Ks) by stimulating KCNQ1-KCNE1 channel endocytosis.

Screening for Copy Number Variation in Genes Associated With the Long QT Syndrome: Clinical Relevance

The aim of this study was to investigate, in a set of 93 mutation-negative long QT syndrome (LQTS) probands, the frequency of copy number variants (CNVs) in LQTS genes. LQTS is an inherited cardiac arrhythmia characterized by a prolonged heart rate-corrected QT (QTc) interval associated with sudden cardiac death. Recent studies suggested the involvement of duplications or deletions in the occurrence of LQTS. However, their frequency remains unknown in LQTS patients. Point mutations in KCNQ1, KCNH2, and SCN5A genes were excluded by denaturing high-performance liquid chromatography or direct sequencing. We applied Multiplex Ligation-dependent Probe Amplification (MLPA) to detect CNVs in exons of these 3 genes. Abnormal exon copy numbers were confirmed by quantitative multiplex PCR of short fluorescent fragment (QMPSF). Array-based comparative genomic hybridization (array CGH) analysis was performed using Agilent Human Genome 244K Microarrays to further map the genomic rearrangements. We identified 3 different deletions in 3 unrelated families: 1 in KCNQ1 and 2 involving KCNH2. We showed in the largest family that the deletion involving KCNH2 is fully penetrant and segregates with the long QT phenotype in 7 affected members. Our study demonstrates that CNVs in KCNQ1 and KCNH2 explain around 3% of LQTS in patients with no point mutation in these genes. This percentage is likely higher than the frequency of point mutations in ANKB, KCNE1, KCNE2, KCNJ2, CACNA1C, CAV3, SCN4B, AKAP9, and SNTA1 together. Thus, we propose that CNV screening in KCNQ1 and KCNH2 may be performed routinely in LQTS patients.

The cardiac I Ks channel, complex indeed

The cardiac IKs channel is a major repolarization current in the heart that responds rapidly and robustly to sympathetic nervous system stimulation to ensure adequate diastolic filling time in the face of accompanying accelerated heart rate. In cardiac myocytes, the IKs channel is a macromolecular complex composed of a pore-forming α (KCNQ1) subunit and modulatory β (KCNE1) subunit, as well as intercellular proteins critical for controlling the phosphorylation state of the complex (1). Although KCNQ1 alone assembles to form a voltage-gated potassium channel, the presence of KCNE1 is required to reproduce the kinetic properties of the native IKs channel, and KCNE1 coassembly is necessary to mediate the characteristic IKs response to sympathetic stimulation (2). The importance of the KCNE1 subunit is evidenced by its role in IKs channelopathies: mutation in either KCNQ1 or KCNE1 can underlie congenital long QT syndrome, and the presence of the β subunit is reported to be required for the gain-of-function phenotype in two KCNQ1 mutations implicated in congenital atrial fibrillation (3, 4). Among the questions surrounding the nature of the KCNQ1–KCNE1 association is that of the stoichiometry of the α and β subunits. Because of its homology with the remainder of the Kv channel superfamily, KCNQ1 (Kv7.1) is thought to form channels consisting of four α subunits (5). The number …

An Examination of KCNE1 Mutations and Common Variants in Chronic Tinnitus

Chronic tinnitus is a highly prevalent and often incapacitating condition frequently associated with sensorineural hearing loss. While its etiology remains incompletely understood there is a growing awareness of genetic factors that predispose to, or aggravate chronic tinnitus. Candidate genes for the disorder include KCNE1, a potassium channel subunit gene that has been implicated in maturation defects of central vestibular neurons, in Menière's disease, and in noise-induced hearing loss. 201 Caucasian outpatients with a diagnosis of chronic tinnitus were systematically screened for mutations in the KCNE1 open reading frame and in the adjacent sequence by direct sequencing. Allele frequencies were determined for 46 known variants, plus two novel KCNE1 mutations. These comprised one missense substitution (V47I) in the highly conserved region encoding the KCNE1 transmembrane domain, and one rare variant in the gene's 3'UTR. When genotypes were grouped assuming dominance of the minor alleles, no significant genotype or compound genotype effects were observed on tinnitus severity. The newly identified V47I substitution argues in favor of an enlarged spectrum of mutations in hearing disorders. However, with regard to allele frequencies in healthy control populations from earlier studies, more common KCNE1 variants are unlikely to play a major role in chronic tinnitus. Further investigations are invited to address variation in additional channel subunits as possible risk factors in tinnitus.

Jervell and Lange-Nielsen Syndrome: Novel Compound Heterozygous Mutations in theKCNQ1in a Korean Family

The Jervell and Lange-Nielsen syndrome (JLNS) is an autosomal recessive syndrome characterized by congenital deafness and cardiac phenotype (QT prolongation, ventricular arrhythmias, and sudden death). JLNS has been shown to occur due to homozygous mutation in KCNQ1 or KCNE1. There have been a few clinical case reports on JLNS in Korea; however, these were not confirmed by a genetic study. We identified compound heterozygous mutations in KCNQ1 in a 5-yr-old child with JLNS, who visited the hospital due to recurrent syncope and seizures and had congenital sensorineural deafness. His electrocardiogram revealed a markedly prolonged corrected QT interval with T wave alternans. The sequence analysis of the proband revealed the presence of novel compound heterozygous deletion/splicing error mutations (c.828-830 delCTC, p.S277del/c.921G>A, p.V307V). Each mutation in KCNQ1 was identified on the maternal and paternal side. With β-blocker therapy the patient has remained symptom-free for three and a half years.

Mechanisms of disease pathogenesis in long QT syndrome type 5

KCNE1 associates with the pore-forming alpha-subunit KCNQ1 to generate the slow (I(Ks)) current in cardiac myocytes. Mutations in either KCNQ1 or KCNE1 can alter the biophysical properties of I(Ks) and mutations in KCNE1 underlie cases of long QT syndrome type 5 (LQT5). We previously investigated a mutation in KCNE1, T58P/L59P, which causes severe attenuation of I(Ks). However, how T58P/L59P acts to disrupt I(Ks) has not been determined. In this study, we investigate and compare the effects of T58P/L59P with three other LQT5 mutations (G52R, S74L, and R98W) on the biophysical properties of the current, trafficking of KCNQ1, and assembly of the I(Ks) channel. G52R and T58P/L59P produce currents that lack the kinetic behavior of I(Ks). In contrast, S74L and R98W both produce I(Ks)-like currents but with rightward shifted voltage dependence of activation. All of the LQT5 mutants express protein robustly, and T58P/L59P and R98W cause modest, but significant, defects in the trafficking of KCNQ1. Despite defects in trafficking, in the presence of KCNQ1, T58P/L59P and the other LQT5 mutants are present at the plasma membrane. Interestingly, in comparison to KCNE1 and the other LQT5 mutants, T58P/L59P associates only weakly with KCNQ1. In conclusion, we identify the disease mechanisms for each mutation and reveal that T58P/L59P causes disease through a novel mechanism that involves defective I(Ks) complex assembly.

Empirical correlation of triggered activity and spatial and temporal re-entrant substrates with arrhythmogenicity in a murine model for Jervell and Lange-Nielsen syndrome

KCNE1 encodes the β-subunit of the slow component of the delayed rectifier K+ current. The Jervell and Lange-Nielsen syndrome is characterized by sensorineural deafness, prolonged QT intervals, and ventricular arrhythmogenicity. Loss-of-function mutations in KCNE1 are implicated in the JLN2 subtype. We recorded left ventricular epicardial and endocardial monophasic action potentials (MAPs) in intact, Langendorff-perfused mouse hearts. KCNE1−/− but not wild-type (WT) hearts showed not only triggered activity and spontaneous ventricular tachycardia (VT), but also VT provoked by programmed electrical stimulation. The presence or absence of VT was related to the following set of criteria for re-entrant excitation for the first time in KCNE1−/− hearts: Quantification of APD90, the MAP duration at 90% repolarization, demonstrated alterations in (1) the difference, ∆APD90, between endocardial and epicardial APD90 and (2) critical intervals for local re-excitation, given by differences between APD90 and ventricular effective refractory period, reflecting spatial re-entrant substrate. Temporal re-entrant substrate was reflected in (3) increased APD90 alternans, through a range of pacing rates, and (4) steeper epicardial and endocardial APD90 restitution curves determined with a dynamic pacing protocol. (5) Nicorandil (20 µM) rescued spontaneous and provoked arrhythmogenic phenomena in KCNE1−/− hearts. WTs remained nonarrhythmogenic. Nicorandil correspondingly restored parameters representing re-entrant criteria in KCNE1−/− hearts toward values found in untreated WTs. It shifted such values in WT hearts in similar directions. Together, these findings directly implicate triggered electrical activity and spatial and temporal re-entrant mechanisms in the arrhythmogenesis observed in KCNE1−/− hearts.

Genotyping Has a Minor Role in Selecting Therapy for Congenital Long-QT Syndromes at Present

Genotyping has led to enormous advances in understanding the phenotype and clinical course of the congenital long-QT syndromes (LQTS). In the most common LQTS forms, LQT1, LQT2, and LQT3, genotyping a clearly affected person in each family provides the ability to easily identify family members with reduced penetrance of the QTc and symptoms phenotypes, thus, providing also the opportunity to prevent sudden death and cardiac arrest by administering presymptomatic therapy. Response by Moss and Goldberg see p 227 The role of genotyping in selecting the therapy for LQTS is, on the other hand, unclear. Data from the literature and from the Salt Lake City LQTS database will be presented to show that genotyping has only a minor role in selecting therapy for LQTS. The discussion will primarily address the LQT1, LQT2, and LQT3 forms (caused by mutations of the LQT1, 2, 3, 5, and 6 genes)1 and the rare LQT4, LQT7, and LQT8 forms will be discussed briefly, and because so little is known about the new LQT9-11 forms they are not ready for a discussion of genotyping for therapy decisions. The pleural term “Long-QT syndromes” is indeed now very appropriate because of the diversity (both molecular and phenotype) of the conditions caused by the 11 accepted or proposed LQTS genes. The idea of genotype-based therapy has been discussed for some time, being encouraged by the experimental findings with mexiletine and flecainide in LQT3 and potassium supplementation in LQT2, plus other laboratory findings.2–8 The concept has received additional attention recently as some details outlining the complexity of the molecular substrate of LQTS are being reported.9–18 There are several concepts that are important for the discussion of this controversy. First, what we really want to accomplish by treatment is to prevent sudden death and aborted cardiac arrest. Most …