A recessive C-terminal Jervell and Lange-Nielsen mutation of the KCNQ1 channel impairs subunit assembly

A triple cysteine motif as major determinant of the modulation of neuronal KV7 channels by the paracetamol metabolite N-acetyl-p-benzo quinone imine

BACKGROUND AND PURPOSE

The analgesic action of paracetamol involves KV7 channels, and its metabolite N-acetyl-p-benzo quinone imine (NAPQI), a cysteine modifying reagent, was shown to increase currents through such channels in nociceptors. Modification of cysteine residues by N-ethylmaleimide, H2O2, or nitric oxide has been found to modulate currents through KV7 channels. The study aims to identify whether, and if so which, cysteine residues in neuronal KV7 channels might be responsible for the effects of NAPQI.

EXPERIMENTAL APPROACH

To address this question, we used a combination of perforated patch-clamp recordings, site-directed mutagenesis, and mass spectrometry applied to recombinant KV7.1 to KV7.5 channels.

KEY RESULTS

Currents through the cardiac subtype KV7.1 were reduced by NAPQI. Currents through all other subtypes were increased, either by an isolated shift of the channel voltage dependence to more negative values (KV7.3) or by such a shift combined with increased maximal current levels (KV7.2, KV7.4, KV7.5). A stretch of three cysteine residues in the S2-S3 linker region of KV7.2 was necessary and sufficient to mediate these effects.

CONCLUSION AND IMPLICATION

The paracetamol metabolite N-acetyl-p-benzo quinone imine (NAPQI) modifies cysteine residues of KV7 subunits and reinforces channel gating in homomeric and heteromeric KV7.2 to KV7.5, but not in KV7.1 channels. In KV7.2, a triple cysteine motif located within the S2-S3 linker region mediates this reinforcement that can be expected to reduce the excitability of nociceptors and to mediate antinociceptive actions of paracetamol.

Enhancing the interpretation of genetic observations in KCNQ1 in unselected populations: relevance to secondary findings

AIMS

Rare variants in the KCNQ1 gene are found in the healthy population to a much greater extent than the prevalence of Long QT Syndrome type 1 (LQTS1). This observation creates challenges in the interpretation of KCNQ1 rare variants that may be identified as secondary findings in whole exome sequencing.This study sought to identify missense variants within sub-domains of the KCNQ1-encoded Kv7.1 potassium channel that would be highly predictive of disease in the context of secondary findings.

METHODS AND RESULTS

We established a set of KCNQ1 variants reported in over 3700 patients with diagnosed or suspected LQTS sent for clinical genetic testing and compared the domain-specific location of identified variants to those observed in an unselected population of 140 000 individuals. We identified three regions that showed a significant enrichment of KCNQ1 variants associated with LQTS at an odds ratio (OR) >2: the pore region, and the adjacent 5th (S5) and 6th (S6) transmembrane (TM) regions. An additional segment within the carboxyl terminus of Kv7.1, conserved region 2 (CR2), also showed an increased OR of disease association. Furthermore, the TM spanning S5-Pore-S6 region correlated with a significant increase in cardiac events.

CONCLUSION

Rare missense variants with a clear phenotype of LQTS have a high likelihood to be present within the pore and adjacent TM segments (S5-Pore-S6) and a greater tendency to be present within CR2. This data will enhance interpretation of secondary findings within the KCNQ1 gene. Further, our data support a more severe phenotype in LQTS patients with variants within the S5-Pore-S6 region.

Computational Study on Effect of KCNQ1 P535T Mutation in a Cardiac Ventricular Tissue

Heart diseases such as arrhythmia are the main causes of sudden death. Arrhythmias are typically caused by mutations in specific genes, damage in the cardiac tissue, or due to some chemical exposure. Arrhythmias caused due to mutation is called inherited arrhythmia. Induced arrhythmias are caused due to tissue damage or chemical exposure. Mutations in genes that encode ion channels of the cardiac cells usually result in (dysfunction) improper functioning of the channel. Improper functioning of the ion channel may lead to major changes in the action potential (AP) of the cardiac cells. This further leads to distorted electrical activity of the heart. Distorted electrical activity will affect the ECG that results in arrhythmia. KCNQ1 P535T mutation is one such gene mutation that encodes the potassium ion channel (KV7.1) of the cardiac ventricular tissue. Its clinical significance is not known. This study aims to perform a simulation study on P535T mutation in the KCNQ1 gene that encodes the potassium ion channel KV7.1 in the ventricular tissue grid. The effect of P535T mutation on transmural tissue grids for three genotypes (wild type, heterozygous, and homozygous) of cells are studied and the generated pseudo-ECGs are compared. Results show the delayed repolarization in the cells of ventricular tissue grid. Slower propagation of action potential in the transmural tissue grid is observed in the mutated (heterozygous and homozygous) genotypes. Longer QT interval is also observed in the pseudo-ECG of heterozygous and homozygous genotype tissue grids. From the pseudo-ECGs, it is observed that KCNQ1 P535T mutation leads to Long QT Syndrome (LQTS) which may result in life-threatening arrhythmias, such as Torsade de Pointes (TdP), Jervell and Lange-Nielsen syndrome (JLNS), and Romano-Ward syndrome (RWS).

Kv7 Channels and Excitability Disorders

Kv7.1-Kv7.5 (KCNQ1-5) K⁺ channels are voltage-gated K⁺ channels with major roles in neurons, muscle cells and epithelia where they underlie physiologically important K⁺ currents, such as neuronal M current and cardiac I_Ks. Specific biophysical properties of Kv7 channels make them particularly well placed to control the activity of excitable cells. Indeed, these channels often work as 'excitability breaks' and are targeted by various hormones and modulators to regulate cellular activity outputs. Genetic deficiencies in all five KCNQ genes result in human excitability disorders, including epilepsy, arrhythmias, deafness and some others. Not surprisingly, this channel family attracts considerable attention as potential drug targets. Here we will review biophysical properties and tissue expression profile of Kv7 channels, discuss recent advances in the understanding of their structure as well as their role in various neurological, cardiovascular and other diseases and pathologies. We will also consider a scope for therapeutic targeting of Kv7 channels for treatment of the above health conditions.

Genetic intolerance analysis as a tool for protein science

Recent advances in whole genome and exome sequencing have dramatically increased the database of human gene variations. There are now enough sequenced human exomes and genomes to begin to identify gene variations that are notable because they are NOT observed in sequenced human genomes, apparently because they are subject to "purifying selection", exemplifying genetic intolerance. Such "dysprocreative" gene variations are embryonic lethal or prevent reproduction through any one of a number of possible mechanisms. Here we review an emerging quantitative approach, "Missense Tolerance Ratio" (MTR) analysis, that is used to assess protein-encoding gene (cDNA) sequence intolerance to missense mutations based on analysis of the >100 K and growing number of currently available human genome and exome sequences. This approach is already useful for analyzing intolerance to mutations in cDNA segments with a resolution on the order of 90 bases. Moreover, as the number of sequenced genomes/exomes increases by orders of magnitude it may eventually be possible to assess mutational tolerance in a statistically robust manner at or near single site resolution. Here we focus on how cDNA intolerance analysis complements other bioinformatic methods to illuminate structure-folding-function relationships for the encoded proteins. A set of disease-linked membrane proteins is employed to provide examples.

2019 HRS expert consensus statement on evaluation, risk stratification, and management of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an arrhythmogenic disorder of the myocardium not secondary to ischemic, hypertensive, or valvular heart disease. ACM incorporates a broad spectrum of genetic, systemic, infectious, and inflammatory disorders. This designation includes, but is not limited to, arrhythmogenic right/left ventricular cardiomyopathy, cardiac amyloidosis, sarcoidosis, Chagas disease, and left ventricular noncompaction. The ACM phenotype overlaps with other cardiomyopathies, particularly dilated cardiomyopathy with arrhythmia presentation that may be associated with ventricular dilatation and/or impaired systolic function. This expert consensus statement provides the clinician with guidance on evaluation and management of ACM and includes clinically relevant information on genetics and disease mechanisms. PICO questions were utilized to evaluate contemporary evidence and provide clinical guidance related to exercise in arrhythmogenic right ventricular cardiomyopathy. Recommendations were developed and approved by an expert writing group, after a systematic literature search with evidence tables, and discussion of their own clinical experience, to present the current knowledge in the field. Each recommendation is presented using the Class of Recommendation and Level of Evidence system formulated by the American College of Cardiology and the American Heart Association and is accompanied by references and explanatory text to provide essential context. The ongoing recognition of the genetic basis of ACM provides the opportunity to examine the diverse triggers and potential common pathway for the development of disease and arrhythmia.

Mutational and phenotypic spectra of KCNE1 deficiency in Jervell and Lange-Nielsen Syndrome and Romano-Ward Syndrome

KCNE1 encodes a regulatory subunit of the KCNQ1 potassium channel-complex. Both KCNE1 and KCNQ1 are necessary for normal hearing and cardiac ventricular repolarization. Recessive variants in these genes are associated with Jervell and Lange-Nielson syndrome (JLNS1 and JLNS2), a cardio-auditory syndrome characterized by congenital profound sensorineural deafness and a prolonged QT interval that can cause ventricular arrhythmias and sudden cardiac death. Some normal-hearing carriers of heterozygous missense variants of KCNE1 and KCNQ1 have prolonged QT intervals, a dominantly inherited phenotype designated Romano-Ward syndrome (RWS), which is also associated with arrhythmias and elevated risk of sudden death. Coassembly of certain mutant KCNE1 monomers with wild-type KCNQ1 subunits results in RWS by a dominant negative mechanism. This paper reviews variants of KCNE1 and their associated phenotypes, including biallelic truncating null variants of KCNE1 that have not been previously reported. We describe three homozygous nonsense mutations of KCNE1 segregating in families ascertained ostensibly for nonsyndromic deafness: c.50G>A (p.Trp17*), c.51G>A (p.Trp17*), and c.138C>A (p.Tyr46*). Some individuals carrying missense variants of KCNE1 have RWS. However, heterozygotes for loss-of-function variants of KCNE1 may have normal QT intervals while biallelic null alleles are associated with JLNS2, indicating a complex genotype-phenotype spectrum for KCNE1 variants.

Doxorubicin induces caspase-mediated proteolysis of KV7.1

K_v7.1 (KCNQ1) coassembles with KCNE1 to generate the cardiac I_Ks-channel. Gain- and loss-of-function mutations in KCNQ1 are associated with cardiac arrhthymias, highlighting the importance of modulating I_Ks activity for cardiac function. Here, we report proteolysis of K_v7.1 as an irreversible posttranslational modification. The identification of two C-terminal fragments of K_v7.1 led us to identify an aspartate critical for the generation of one of the fragments and caspases as responsible for mediating proteolysis. Activating caspases reduces K_v7.1/KCNE1 currents, which is abrogated in cells expressing caspase-resistant channels. Enhanced cleavage of K_v7.1 can be detected for the LQT mutation G460S, which is located adjacent to the cleavage site, whereas a calmodulin-binding-deficient mutation impairs cleavage. Application of apoptotic stimuli or doxorubicin-induced cardiotoxicity provokes caspase-mediated cleavage of endogenous I_Ks in human cardiomyocytes. In summary, caspases are novel regulatory components of I_Ks channels that may have important implications for the molecular mechanism of doxorubicin-induced cardiotoxicity.

Anne Strigli et al. report that the voltage-gated potassium channel K_v7.1 undergoes caspase-mediated proteolytic cleavage, which reduces its cardiac activity. Their findings implicate caspases as regulators of the I_Ks channel complex, which may have implications for understanding drug-induced cardiotoxicity.

Acute blockade of inner ear marginal and dark cell K+ secretion: Effects on gravity receptor function

Specific pharmacological blockade of KCNQ (Kv7) channels with XE991 rapidly (within 20 min) and profoundly alters inner ear gravity receptor responses to head motion (Lee et al., 2017). We hypothesized that these effects were attributable to the suppression of K⁺ secretion following blockade of KCNQ1-KCNE1 channels in vestibular dark cells and marginal cells. To test this hypothesis, K⁺ secretion was independently inhibited by blocking the Na⁺-K⁺-2Cl^- cotransporter (NKCC1, Slc12a2) rather than KCNQ1-KCNE1 channels. Acute blockade of NKCC1 with ethacrynic acid (40 mg/kg) eliminated auditory responses (ABRs) within approximately 70 min of injection, but had no effect on vestibular gravity receptor function (VsEPs) over a period of 2 h in the same animals. These findings show that, vestibular gravity receptors are highly resistant to acute disruption of endolymph secretion unlike the auditory system. Based on this we argue that acute suppression of K⁺ secretion alone does not likely account for the rapid profound effects of XE991 on gravity receptors. Instead the effects of XE991 likely require additional action at KCNQ channels located within the sensory epithelium itself.

The Role of KCNQ1 Mutations and Maternal Beta Blocker Use During Pregnancy in the Growth of Children With Long QT Syndrome

OBJECTIVE

Two missense mutations in KCNQ1, an imprinted gene that encodes the alpha subunit of the voltage-gated potassium channel Kv7.1, cause autosomal dominant growth hormone deficiency and maternally inherited gingival fibromatosis. We evaluated endocrine features, birth size, and subsequent somatic growth of patients with long QT syndrome 1 (LQT1) due to loss-of-function mutations in KCNQ1.

DESIGN

Medical records of 104 patients with LQT1 in a single tertiary care center between 1995 and 2015 were retrospectively reviewed.

METHODS

Clinical and endocrine data of the LQT1 patients were included in the analyses.

RESULTS

At birth, patients with a maternally inherited mutation (n = 52) were shorter than those with paternal inheritance of the mutation (n = 29) (birth length, -0.70 ± 1.1 SDS vs. -0.2 ± 1.0 SDS, P < 0.05). Further analyses showed, however, that only newborns (n = 19) of mothers who had received beta blockers during pregnancy were shorter and lighter at birth than those with paternal inheritance of the mutation (n = 29) (-0.89 ± 1.0 SDS vs. -0.20 ± 1.0 SDS, P < 0.05; and 3,173 ± 469 vs. 3,515 ± 466 g, P < 0.05). Maternal beta blocker treatment during the pregnancy was also associated with lower cord blood TSH levels (P = 0.011) and significant catch-up growth during the first year of life (Δ0.08 SDS/month, P = 0.004). Later, childhood growth of the patients was unremarkable.

CONCLUSION

Loss-of-function mutations in KCNQ1 are not associated with abnormalities in growth, whereas maternal beta blocker use during pregnancy seems to modify prenatal growth of LQT1 patients-a phenomenon followed by catch-up growth after birth.

Calmodulin confers calcium sensitivity to the stability of the distal intracellular assembly domain of Kv7.2 channels

Tetrameric coiled-coil structures are present in many ion channels, often adjacent to a calmodulin (CaM) binding site, although the relationship between the two is not completely understood. Here we examine the dynamic properties of the ABCD domain located in the intracellular C-terminus of tetrameric, voltage-dependent, potassium selective Kv7.2 channels. This domain encompasses the CaM binding site formed by helices A and B, followed by helix C, which is linked to the helix D coiled-coil. The data reveals that helix D stabilizes CaM binding, promoting trans-binding (CaM embracing neighboring subunits), and they suggest that the ABCD domain can be exchanged between subunits of the tetramer. Exchange is faster when mutations in AB weaken the CaM interaction. The exchange of ABCD domains is slower in the presence of Ca²⁺, indicating that CaM stabilization of the tetrameric assembly is enhanced when loaded with this cation. Our observations are consistent with a model that involves a dynamic mechanism of helix D assembly, which supports reciprocal allosteric coupling between the A-B module and the coiled-coil formed by the helix D. Thus, formation of the distal helix D tetramer influences CaM binding and CaM-dependent Kv7.2 properties, whereas reciprocally, CaM and Ca²⁺ influence the dynamic behavior of the helix D coiled-coil.

Molecular cloning and functional expression of the K+ channel KV7.1 and the regulatory subunit KCNE1 from equine myocardium

BACKGROUND

The voltage-gated K⁺-channel K_V7.1 and the subunit KCNE1, encoded by the KCNQ1 and KCNE1 genes, respectively, are responsible for termination of the cardiac action potential. In humans, mutations in these genes can predispose patients to arrhythmias and sudden cardiac death (SCD).

AIM

To characterize equine K_V7.1/KCNE1 currents and compare them to human K_V7.1/KCNE1 currents to determine whether K_V7.1/KCNE1 plays a similar role in equine and human hearts.

METHODS

mRNA encoding K_V7.1 and KCNE1 was isolated from equine hearts, sequenced, and cloned into expression vectors. The channel subunits were heterologously expressed in Xenopus laevis oocytes or CHO-K1 cells and characterized using voltage-clamp techniques.

RESULTS

Equine K_V7.1/KCNE1 expressed in CHO-K1 cells exhibited electrophysiological properties that are overall similar to the human orthologs; however, a slower deactivation was found which could result in more open channels at fast rates.

CONCLUSION

The results suggest that the equine K_V7.1/KCNE1 channel may be important for cardiac repolarization and this could indicate that horses are susceptible to SCD caused by mutations in KCNQ1 and KCNE1.

The impact of recent advances in genetics in understanding disease mechanisms underlying the long QT syndromes

Long QT syndrome refers to a characteristic abnormality of the electrocardiogram and it is associated with a form of ventricular tachycardia known as torsade-de-pointes and sudden arrhythmic death. It can occur as part of a hereditary syndrome or can be acquired usually because of drug administration. Here we review recent genetic, molecular and cellular discoveries and outline how they have furthered our understanding of this disease. Specifically we focus on compound mutations, genome wide association studies of QT interval, modifier genes and the therapeutic implications of this recent work.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Regulation of KCNQ/Kv7 family voltage-gated K+ channels by lipids

Many years of studies have established that lipids can impact membrane protein structure and function through bulk membrane effects, by direct but transient annular interactions with the bilayer-exposed surface of protein transmembrane domains, and by specific binding to protein sites. Here, we focus on how phosphatidylinositol 4,5-bisphosphate (PIP₂) and polyunsaturated fatty acids (PUFAs) impact ion channel function and how the structural details of the interactions of these lipids with ion channels are beginning to emerge. We focus on the Kv7 (KCNQ) subfamily of voltage-gated K⁺ channels, which are regulated by both PIP₂ and PUFAs and play a variety of important roles in human health and disease. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Molecular pathogenesis of long QT syndrome type 1

Long QT syndrome type 1 (LQT1) is a subtype of a congenital cardiac syndrome caused by mutation in the KCNQ1 gene, which encodes the α-subunit of the slow component of delayed rectifier K+ current (IKs) channel. Arrhythmias in LQT1 are characterized by prolongation of the QT interval on ECG, as well as the occurrence of life-threatening cardiac events, frequently triggered by adrenergic stimuli (e.g., physical or emotional stress). During the past two decades, much advancement has been made in understanding the molecular pathogenesis underlying LQT1. Uncovering the genotype-phenotype correlations in LQT1 is of clinical importance to better understand the gene-specific differences that may influence the propensity for developing life-threatening arrhythmias under specific conditions. Elucidation of these mechanisms will also help to improve the diagnosis and management of this cardiac disorder based on gene-specific considerations. This review describes the current medical consensus and recent developments regarding the molecular pathogenesis of LQT1 and provides a novel insight into the adrenergic regulation of this disease.

Cardiac Delayed Rectifier Potassium Channels in Health and Disease

Cardiac delayed rectifier potassium channels conduct outward potassium currents during the plateau phase of action potentials and play pivotal roles in cardiac repolarization. These include IKs, IKr and the atrial specific IKur channels. In this article, we will review their molecular identities and biophysical properties. Mutations in the genes encoding delayed rectifiers lead to loss- or gain-of-function phenotypes, disrupt normal cardiac repolarization and result in various cardiac rhythm disorders, including congenital Long QT Syndrome, Short QT Syndrome and familial atrial fibrillation. We will also discuss the prospect of using delayed rectifier channels as therapeutic targets to manage cardiac arrhythmia.

Identification of a key residue in Kv7.1 potassium channel essential for sensing external potassium ions

A glutamate at position 290 in the human K_v7.1 S5-pore linker is required for its inhibition by high concentrations of extracellular potassium.

K_v7.1 voltage-gated K⁺ (K_v) channels are present in the apical membranes of marginal cells of the stria vascularis of the inner ear, where they mediate K⁺ efflux into the scala media (cochlear duct) of the cochlea. As such, they are exposed to the K⁺-rich (∼150 mM of external K⁺ (K⁺_e)) environment of the endolymph. Previous studies have shown that K_v7.1 currents are substantially suppressed by high K⁺_e (independent of the effects of altering the electrochemical gradient). However, the molecular basis for this inhibition, which is believed to involve stabilization of an inactivated state, remains unclear. Using sequence alignment of S5-pore linkers of several K_v channels, we identified a key residue, E290, found in only a few K_v channels including K_v7.1. We used substituted cysteine accessibility methods and patch-clamp analysis to provide evidence that the ability of K_v7.1 to sense K⁺_e depends on E290, and that the charge at this position is essential for K_v7.1’s K⁺_e sensitivity. We propose that K_v7.1 may use this feedback mechanism to maintain the magnitude of the endocochlear potential, which boosts the driving force to generate the receptor potential of hair cells. The implications of our findings transcend the auditory system; mutations at this position also result in long QT syndrome in the heart.

Iron Overload Leading to Torsades de Pointes in β-Thalassemia and Long QT Syndrome

The authors present a unique case of torsades de pointes in a β-thalassemia patient with early iron overload in the absence of any structural abnormalities as seen in hemochromatosis. Genetic testing showed a novel KCNQ1 gene mutation 1591C>T [Gln531Ter(X)]. Testing of the gene mutation in Xenopus laevis oocytes showed loss of function of the IKs current. The authors hypothesize that iron overload combined with the KCNQ1 gene mutation leads to prolongation of QTc and torsades de pointes.

KCNQ potassium channels in sensory system and neural circuits

M channels, an important regulator of neural excitability, are composed of four subunits of the Kv7 (KCNQ) K(+) channel family. M channels were named as such because their activity was suppressed by stimulation of muscarinic acetylcholine receptors. These channels are of particular interest because they are activated at the subthreshold membrane potentials. Furthermore, neural KCNQ channels are drug targets for the treatments of epilepsy and a variety of neurological disorders, including chronic and neuropathic pain, deafness, and mental illness. This review will update readers on the roles of KCNQ channels in the sensory system and neural circuits as well as discuss their respective mechanisms and the implications for physiology and medicine. We will also consider future perspectives and the development of additional pharmacological models, such as seizure, stroke, pain and mental illness, which work in combination with drug-design targeting of KCNQ channels. These models will hopefully deepen our understanding of KCNQ channels and provide general therapeutic prospects of related channelopathies.

Structural basis of a Kv7.1 potassium channel gating module: studies of the intracellular c-terminal domain in complex with calmodulin

Kv7 channels tune neuronal and cardiomyocyte excitability. In addition to the channel membrane domain, they also have a unique intracellular C-terminal (CT) domain, bound constitutively to calmodulin (CaM). This CT domain regulates gating and tetramerization. We investigated the structure of the membrane proximal CT module in complex with CaM by X-ray crystallography. The results show how the CaM intimately hugs a two-helical bundle, explaining many channelopathic mutations. Structure-based mutagenesis of this module in the context of concatemeric tetramer channels and functional analysis along with in vitro data lead us to propose that one CaM binds to one individual protomer, without crosslinking subunits and that this configuration is required for proper channel expression and function. Molecular modeling of the CT/CaM complex in conjunction with small-angle X-ray scattering suggests that the membrane proximal region, having a rigid lever arm, is a critical gating regulator.

Uncoupling PIP2-calmodulin regulation of Kv7.2 channels by an assembly destabilizing epileptogenic mutation

We show that the combination of an intracellular bi-partite calmodulin (CaM)-binding site and a distant assembly region affect how an ion channel is regulated by a membrane lipid. Our data reveal that regulation by phosphatidylinositol(4,5)bisphosphate (PIP2) and stabilization of assembled Kv7.2 subunits by intracellular coiled-coil regions far from the membrane are coupled molecular processes. Live-cell fluorescence energy transfer measurements and direct binding studies indicate that remote coiled-coil formation creates conditions for different CaM interaction modes, each conferring different PIP2 dependency to Kv7.2 channels. Disruption of coiled-coil formation by epilepsy-causing mutation decreases apparent CaM-binding affinity and interrupts CaM influence on PIP2 sensitivity.

The KCNQ1 channel - remarkable flexibility in gating allows for functional versatility

The KCNQ1 channel (also called Kv7.1 or KvLQT1) belongs to the superfamily of voltage-gated K(+) (Kv) channels. KCNQ1 shares several general features with other Kv channels but also displays a fascinating flexibility in terms of the mechanism of channel gating, which allows KCNQ1 to play different physiological roles in different tissues. This flexibility allows KCNQ1 channels to function as voltage-independent channels in epithelial tissues, whereas KCNQ1 function as voltage-activated channels with very slow kinetics in cardiac tissues. This flexibility is in part provided by the association of KCNQ1 with different accessory KCNE β-subunits and different modulators, but also seems like an integral part of KCNQ1 itself. The aim of this review is to describe the main mechanisms underlying KCNQ1 flexibility.

Cellular mechanisms of mutations in Kv7.1: auditory functions in Jervell and Lange-Nielsen syndrome vs. Romano-Ward syndrome

As a result of cell-specific functions of voltage-activated K⁺ channels, such as Kv7.1, mutations in this channel produce profound cardiac and auditory defects. At the same time, the massive diversity of K⁺ channels allows for compensatory substitution of mutant channels by other functional channels of their type to minimize defective phenotypes. Kv7.1 represents a clear example of such functional dichotomy. While several point mutations in the channel result in a cardio-auditory syndrome called Jervell and Lange-Nielsen syndrome (JLNS), about 100-fold mutations result in long QT syndrome (LQTS) denoted as Romano–Ward syndrome (RWS), which has an intact auditory phenotype. To determine whether the cellular mechanisms for the diverse phenotypic outcome of Kv7.1 mutations, are dependent on the tissue-specific function of the channel and/or specialized functions of the channel, we made series of point mutations in hKv7.1 ascribed to JLNS and RWS. For JLNS mutations, all except W248F yielded non-functional channels when expressed alone. Although W248F at the end of the S4 domain yielded a functional current, it underwent marked inactivation at positive voltages, rendering the channel non-functional. We demonstrate that by definition, none of the JLNS mutants operated in a dominant negative (DN) fashion. Instead, the JLNS mutants have impaired membrane trafficking, trapped in the endoplasmic reticulum (ER) and Cis-Golgi. The RWS mutants exhibited varied functional phenotypes. However, they can be summed up as exhibiting DN effects. Phenotypic differences between JLNS and RWS may stem from tissue-specific functional requirements of cardiac vs. inner ear non-sensory cells.

Arrhythmogenic cardiomyopathy in a patient with a rare loss-of-function KCNQ1 mutation

BACKGROUND

Ventricular tachycardia (VT) is a common manifestation of advanced cardiomyopathies. In a subset of patients with dilated cardiomyopathy, VT is the initial and the cardinal manifestation of the disease. The molecular genetic basis of this subset of dilated cardiomyopathy is largely unknown.

METHODS AND RESULTS

We identified 10 patients with dilated cardiomyopathy who presented with VT and sequenced 14 common causal genes for cardiomyopathies and arrhythmias. Functional studies included cellular patch clamp, confocal microscopy, and immunoblotting. We identified nonsynonymous variants in 4 patients, including a rare missense p.R397Q mutation in the KCNQ1 gene in a 60-year-old man who presented with incessant VT and had mild cardiac dysfunction. The p.R397Q mutation was absent in an ethnically matched control group, affected a conserved amino acid, and was predicted by multiple algorithms to be pathogenic. Co-expression of the mutant KCNQ1 with its partner unit KCNE1 was associated with reduced tail current density of slowly activating delayed rectifier K(+) current (IKs). The mutation reduced membrane localization of the protein.

CONCLUSIONS

Dilated cardiomyopathy with an initial presentation of VT may be a forme fruste of arrhythmogenic cardiomyopathy caused by mutations in genes encoding the ion channels. The findings implicate KCNQ1 as a possible causal gene for arrhythmogenic cardiomyopathy.

Multiplex ligation-dependent probe amplification copy number variant analysis in patients with acquired long QT syndrome

AIMS

Thirteen genetic loci map to families with congenital long QT syndrome (cLQT) and multiple single nucleotide mutations have been functionally implicated in cLQT. Studies have investigated copy number variations (CNVs) in the cLQT genes to ascertain their involvement in cLQT. In these studies 3-12% of cLQT patients who were mutation negative by all other methods carried CNVs in cLQT genes. Prolongation of the QT interval can also be acquired after exposure to certain drugs [acquired LQT (aLQT)]. Single nucleotide mutations in cLQT genes have also been associated with and functionally implicated in aLQT, but to date no studies have explored CNVs as an additional susceptibility factor in aLQT. The aim of this study was to explore the contribution of CNVs in determining susceptibility to aLQT.

METHODS AND RESULTS

In this study we screened the commonest cLQT genes (KCNQ1; KCNH2; SCN5A; KCNE1, and KCNE2) in a general population of healthy volunteers and in a cohort of subjects presenting with aLQT for CNVs using the multiplex ligation-dependent probe amplification method. Copy number variants were detected and confirmed in 1 of 197 of the healthy volunteers and in 1 of 90 subjects with aLQT. The CNV in the aLQT subject was functionally characterized and demonstrated impaired channel function.

CONCLUSION

Copy number variation is a possible additional risk factor for aLQT and should be considered for incorporation into pharmacogenetic screening of LQTS genes in addition to mutation detection to improve the safety of medication administration.

Recessive cardiac phenotypes in induced pluripotent stem cell models of Jervell and Lange-Nielsen syndrome: disease mechanisms and pharmacological rescue

Jervell and Lange-Nielsen syndrome (JLNS) is one of the most severe life-threatening cardiac arrhythmias. Patients display delayed cardiac repolarization, associated high risk of sudden death due to ventricular tachycardia, and congenital bilateral deafness. In contrast to the autosomal dominant forms of long QT syndrome, JLNS is a recessive trait, resulting from homozygous (or compound heterozygous) mutations in KCNQ1 or KCNE1. These genes encode the α and β subunits, respectively, of the ion channel conducting the slow component of the delayed rectifier K(+) current, IKs. We used complementary approaches, reprogramming patient cells and genetic engineering, to generate human induced pluripotent stem cell (hiPSC) models of JLNS, covering splice site (c.478-2A>T) and missense (c.1781G>A) mutations, the two major classes of JLNS-causing defects in KCNQ1. Electrophysiological comparison of hiPSC-derived cardiomyocytes (CMs) from homozygous JLNS, heterozygous, and wild-type lines recapitulated the typical and severe features of JLNS, including pronounced action and field potential prolongation and severe reduction or absence of IKs. We show that this phenotype had distinct underlying molecular mechanisms in the two sets of cell lines: the previously unidentified c.478-2A>T mutation was amorphic and gave rise to a strictly recessive phenotype in JLNS-CMs, whereas the missense c.1781G>A lesion caused a gene dosage-dependent channel reduction at the cell membrane. Moreover, adrenergic stimulation caused action potential prolongation specifically in JLNS-CMs. Furthermore, sensitivity to proarrhythmic drugs was strongly enhanced in JLNS-CMs but could be pharmacologically corrected. Our data provide mechanistic insight into distinct classes of JLNS-causing mutations and demonstrate the potential of hiPSC-CMs in drug evaluation.

Cellular mechanisms underlying the increased disease severity seen for patients with long QT syndrome caused by compound mutations in KCNQ1

The KCNQ1 (potassium voltage-gated channel, KQT-like subfamily, member 1) gene encodes the Kv7.1 potassium channel which forms a complex with KCNE1 (potassium voltage-gated channel Isk-related family member 1) in the human heart to produce the repolarizing IKs (slow delayed rectifier potassium current). Mutations in KCNQ1 can perturb IKs function and cause LQT1 (long QT syndrome type 1). In LQT1, compound mutations are relatively common and are associated with increased disease severity. LQT1 compound mutations have been shown to increase channel dysfunction, but whether other disease mechanisms, such as defective channel trafficking, contribute to the increase in arrhythmic risk has not been determined. Using an imaging-based assay we investigated the effects of four compound heterozygous mutations (V310I/R594Q, A341V/P127T, T391I/Q530X and A525T/R518X), one homozygous mutation (W248F) and one novel compound heterozygous mutation (A178T/K422fs39X) (where fs denotes frameshift) on channel trafficking. By analysing the effects in the equivalent of a homozygous, heterozygous and compound heterozygous condition, we identify three different types of behaviour. A341V/P127T and W248F/W248F had no effect, whereas V310I/R594Q had a moderate, but not compound, effect on channel trafficking. In contrast, T391I/Q530X, A525T/R518X and A178T/K422fs39X severely disrupted channel trafficking when expressed in compound form. In conclusion, we have characterized the disease mechanisms for six LQT1 compound mutations and report that, for four of these, defective channel trafficking underlies the severe clinical phenotype.

Calmodulin orchestrates the heteromeric assembly and the trafficking of KCNQ2/3 (Kv7.2/3) channels in neurons

Mutations in KCNQ2 and KCNQ3 genes are responsible for benign familial neonatal seizures and epileptic encephalopathies. Some of these mutations have been shown to alter the binding of calmodulin (CaM) to specific C-terminal motifs of KCNQ subunits, known as the A and B helices. Here, we show that the mutation I342A in the A helix of KCNQ3 abolishes CaM interaction and strongly decreases the heteromeric association with KCNQ2. The assembly of KCNQ2 with KCNQ3 is essential for their expression at the axon initial segment (AIS). We find that the I342A mutation alters the targeting of KCNQ2/3 subunits at the AIS. However, the traffic of the mutant channels was rescued by provision of exogenous CaM. We show that CaM enhances the heteromeric association of KCNQ2/KCNQ3-I342A subunits by binding to their B helices in a calcium-dependent manner. To further assert the implication of CaM in channel assembly, we inserted a mutation in the second coil-coil domain of KCNQ2 (KCNQ2-L638P) to prevent its heteromerization with KCNQ3. We observe that the expression of a Ca(2+)-insensitive form of CaM favours the assembly of KCNQ3 with KCNQ2-L638P. Our data thus indicate that both apoCaM and Ca(2+)/CaM bind to the C-terminal domains of KCNQ2 and KCNQ3 subunits, and regulate their heteromeric assembly. Hence, CaM may control the composition and distribution of KCNQ channels in neurons.

Beckwith-Wiedemann syndrome and long QT syndrome due to familial-balanced translocation t(11;17)(p15.5;q21.3) involving the KCNQ1 gene

We report a child with Beckwith-Wiedemann syndrome (BWS) as the consequence of an apparently balanced, maternally inherited reciprocal translocation t(11;17)(p15.5;q21.3). His mother and aunt, who inherited the translocation from their father, did not have BWS. At birth, long QT syndrome (LQTS) was diagnosed in this child and, secondarily, among apparently healthy family members carrying the translocation. By FISH analysis, the breakpoint in 11p15.5 interrupts the KCNQ1 gene between exons 2 and 10 and causes a loss of methylation of the IC2 (and thus BWS) on the maternally inherited der(11) chromosome. To explain the presence of LQTS segregating with the t(11;17) translocation in this family, we hypothesize that the translocation that interrupts KCNQ1 allow translation of an abnormal short allele that interferes in a dominant negative way with the normal isoform 1 of KCNQ1 in the heart (where this allele is not subject to parental imprint). This appears to be the first report of BWS with congenital LQTS, which should be considered as a rare but serious complication to be searched systematically in patients with BWS due to 11p15 rearrangements.

Regions of KCNQ K(+) channels controlling functional expression

KCNQ1–5 α-subunits assemble to form K⁺ channels that play critical roles in the function of numerous tissues. The channels are tetramers of subunits containing six transmembrane domains. Each subunit consists of a pore region (S5-pore-S6) and a voltage-sensor domain (S1-S4). Despite similar structures, KCNQ2 and KCNQ3 homomers yield small current amplitudes compared to other KCNQ homomers and KCNQ2/3 heteromers. Two major mechanisms have been suggested as governing functional expression. The first involves control of channel trafficking to the plasma membrane by the distal part of the C-terminus, containing two coiled–coiled domains, required for channel trafficking and assembly. The proximal half of the C-terminus is the crucial region for channel modulation by signaling molecules such as calmodulin (CaM), which may mediate C- and N-terminal interactions. The N-terminus of KCNQ channels has also been postulated as critical for channel surface expression. The second mechanism suggests networks of interactions between the pore helix and the selectivity filter (SF), and between the pore helix and the S6 domain that govern KCNQ current amplitudes. Here, we summarize the role of these different regions in expression of functional KCNQ channels.

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Readthrough of long-QT syndrome type 1 nonsense mutations rescues function but alters the biophysical properties of the channel

The nonsense mutations R518X-KCNQ1 and Q530X-KCNQ1 cause LQT1 (long-QT syndrome type 1) and result in a complete loss of IKs channel function. In the present study we attempted to rescue the function of these mutants, in HEK (human embryonic kidney)-293 cells, by promoting readthrough of their PTCs (premature termination codons) using the pharmacological agents G-418, gentamicin and PTC124. Gentamicin and G-418 acted to promote full-length channel protein expression from R518X at 100 μM and from Q530X at 1 mM. In contrast, PTC124 did not, at any dose tested, induce readthrough of either mutant. G-418 (1 mM) treatment also acted to significantly (P<0.05) increase current density and peak-tail current density, at +80 mV for R518X, but not Q530X, to 58±11% and 82±17% of the wild-type level respectively. However, the biophysical properties of the currents produced from R518X, while similar, were not identical with wild-type as the voltage-dependence of activation was significantly (P<0.05) shifted by +25 mV. Overall, these findings indicate that although functional rescue of LQT1 nonsense mutations is possible, it is dependent on the degree of readthrough achieved and the effect on channel function of the amino acid substituted for the PTC. Such considerations will determine the success of future therapies.

Several polymorphisms of KCNQ1 gene are associated with plasma lipid levels in general Chinese populations

Background

Potassium voltage-gated channel, KQT-like subfamily, member 1 (KCNQ1) is thought to be an important candidate gene of diabetes. Several single nucleotide polymorphisms (SNPs) in a 40-kb linkage disequilibrium (LD) block in its intron 15 have been identified to be associated with diabetes in East Asian populations in recent genome-wide association studies. The aim of this study was to investigate whether KCNQ1 polymorphisms influence the levels of the metabolic phenotypes in general Chinese populations.

Methodology/Principal Findings

We investigated the associations of two SNPs (rs2237892 and rs2237895) in the aforementioned 40-kb LD block, a missense variant rs12720449 (P448R) in exon 10, and a synonymous variant rs1057128 (S546S) in exon 13 with metabolic phenotypes in a Uyghur population (n = 478) and replicated these associations in a Han population (n = 2,485). We found that rs2237892-T allele was significantly associated with decreased triglyceride levels (p_combined = 0.001). The minor G allele of the rs12720449, with sharp difference of the allelic frequency between European and East Asian populations (0.2% versus 14%, respectively), was associated with a lower triglyceride levels than G allele in Uyghur subjects (p = 0.004), in Han subjects (p = 0.052), and in subjects of meta-analysis (p_combined = 0.001). Moreover, the minor A allele of the rs1057128 was also associated with decreased triglyceride levels in meta-analysis (p_combined = 0.010).

Conclusions

To the best of our knowledge, this is the first report associating a missense mutation of KCNQ1, rs12720449, with triglyceride levels. Rs2237892, representing the 40-kb LD block, is also associated with triglyceride levels in Han population. Further studies are required to replicate these findings in other East Asian populations.

Potassium-channel mutations and cardiac arrhythmias--diagnosis and therapy

The coordinated generation and propagation of action potentials within cardiomyocytes creates the intrinsic electrical stimuli that are responsible for maintaining the electromechanical pump function of the human heart. The synchronous opening and closing of cardiac Na(+), Ca(2+), and K(+) channels corresponds with the activation and inactivation of inward depolarizing (Na(+) and Ca(2+)) and outward repolarizing (K(+)) currents that underlie the various phases of the cardiac action potential (resting, depolarization, plateau, and repolarization). Inherited mutations in pore-forming α subunits and accessory β subunits of cardiac K(+) channels can perturb the atrial and ventricular action potential and cause various cardiac arrhythmia syndromes, including long QT syndrome, short QT syndrome, Brugada syndrome, and familial atrial fibrillation. In this Review, we summarize the current understanding of the molecular and cellular mechanisms that underlie K(+)-channel-mediated arrhythmia syndromes. We also describe translational advances that have led to the emerging role of genetic testing and genotype-specific therapy in the diagnosis and clinical management of individuals who harbor pathogenic mutations in genes that encode α or β subunits of cardiac K(+) channels.

Cell excitability necessary for male mating behavior in Caenorhabditis elegans is coordinated by interactions between big current and ether-a-go-go family K(+) channels

Variations in K(+) channel composition allow for differences in cell excitability and, at an organismal level, provide flexibility to behavioral regulation. When the function of a K(+) channel is disrupted, the remaining K(+) channels might incompletely compensate, manifesting as abnormal organismal behavior. In this study, we explored how different K(+) channels interact to regulate the neuromuscular circuitry used by Caenorhabditis elegans males to protract their copulatory spicules from their tail and insert them into the hermaphrodite's vulva during mating. We determined that the big current K(+) channel (BK)/SLO-1 genetically interacts with ether-a-go-go (EAG)/EGL-2 and EAG-related gene/UNC-103 K(+) channels to control spicule protraction. Through rescue experiments, we show that specific slo-1 isoforms affect spicule protraction. Gene expression studies show that slo-1 and egl-2 expression can be upregulated in a calcium/calmodulin-dependent protein kinase II-dependent manner to compensate for the loss of unc-103 and conversely, unc-103 can partially compensate for the loss of SLO-1 function. In conclusion, an interaction between BK and EAG family K(+) channels produces the muscle excitability levels that regulate the timing of spicule protraction and the success of male mating behavior.

Distal end of carboxyl terminus is not essential for the assembly of rat Eag1 potassium channels

The assembly of four pore-forming α-subunits into tetramers is a prerequisite for the formation of functional K(+) channels. A short carboxyl assembly domain (CAD) in the distal end of the cytoplasmic carboxyl terminus has been implicated in the assembly of Eag α-subunits, a subfamily of the ether-à-go-go K(+) channel family. The precise role of CAD in the formation of Eag tetrameric channels, however, remains unclear. Moreover, it has not been determined whether other protein regions also contribute to the assembly of Eag subunits. We addressed these questions by studying the biophysical properties of a series of different rat Eag1 (rEag1) truncation mutants. Two truncation mutants without CAD (K848X and W823X) yielded functional phenotypes similar to those for wild-type (WT) rEag1 channels. Furthermore, nonfunctional rEag1 truncation mutants lacking the distal region of the carboxyl terminus displayed substantial dominant-negative effects on the functional expression of WT as well as K848X and W823X channels. Our co-immunoprecipitation studies further revealed that truncation mutants containing no CAD indeed displayed significant association with rEag1-WT subunits. Finally, surface biotinylation and protein glycosylation analyses demonstrated that progressive truncations of the carboxyl terminus resulted in aggravating disruptions of membrane trafficking and glycosylation of rEag1 proteins. Overall, our data suggest that the distal carboxyl terminus, including CAD, is dispensable for the assembly of rEag1 K(+) channels but may instead be essential for ensuring proper protein biosynthesis. We propose that the S6 segment and the proximal carboxyl terminus may constitute the principal subunit recognition site for the assembly of rEag1 channels.

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

KV7 channelopathies

KV7 voltage-gated potassium channels, encoded by the KCNQ gene family, have caught increasing interest of the scientific community for their important physiological roles, which are emphasized by the fact that four of the five so far identified members are related to different hereditary diseases. Furthermore, these channels prove to be attractive pharmacological targets for treating diseases characterized by membrane hyperexcitability. KV7 channels are expressed in brain, heart, thyroid gland, pancreas, inner ear, muscle, stomach, and intestines. They give rise to functionally important potassium currents, reduction of which results in pathologies such as long QT syndrome, diabetes, neonatal epilepsy, neuromyotonia, or progressive deafness. Here, we summarize some key traits of KV7 channels and review how their molecular deficiencies could explain diverse disease phenotypes. We also assess the therapeutic potential of KV7 channels; in particular, how the activation of KV7 channels by the compounds retigabine and R-L3 may be useful for treatment of epilepsy or cardiac arrhythmia.