Biophysical properties of 9 KCNQ1 mutations associated with long-QT syndrome

Role of Potassium Ion Channels in Epilepsy: Focus on Current Therapeutic Strategies

BACKGROUND

Epilepsy is one of the prevalent neurological disorders characterized by disrupted synchronization between inhibitory and excitatory neurons. Disturbed membrane potential due to abnormal regulation of neurotransmitters and ion transport across the neural cell membrane significantly contributes to the pathophysiology of epilepsy. Potassium ion channels (KCN) regulate the resting membrane potential and are involved in neuronal excitability. Genetic alterations in the potassium ion channels (KCN) have been reported to result in the enhancement of the release of neurotransmitters, the excitability of neurons, and abnormal rapid firing rate, which lead to epileptic phenotypes, making these ion channels a potential therapeutic target for epilepsy. The aim of this study is to explore the variations reported in different classes of potassium ion channels (KCN) in epilepsy patients, their functional evaluation, and therapeutic strategies to treat epilepsy targeting KCN.

METHODOLOGY

A review of all the relevant literature was carried out to compile this article.

RESULTS

A large number of variations have been reported in different genes encoding various classes of KCN. These genetic alterations in KCN have been shown to be responsible for disrupted firing properties of neurons. Antiepileptic drugs (AEDs) are the main therapeutic strategy to treat epilepsy. Some patients do not respond favorably to the AEDs treatment, resulting in pharmacoresistant epilepsy.

CONCLUSION

Further to address the challenges faced in treating epilepsy, recent approaches like optogenetics, chemogenetics, and genome editing, such as clustered regularly interspaced short palindromic repeats (CRISPR), are emerging as target-specific therapeutic strategies.

Long QT syndrome-associated calmodulin variants disrupt the activity of the slowly activating delayed rectifier potassium channel

Calmodulin (CaM) is a highly conserved mediator of calcium (Ca2+ )-dependent signalling and modulates various cardiac ion channels. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS). LQTS patients display prolonged ventricular recovery times (QT interval), increasing their risk of incurring life-threatening arrhythmic events. Loss-of-function mutations to Kv7.1 (which drives the slow delayed rectifier potassium current, IKs, a key ventricular repolarising current) are the largest contributor to congenital LQTS (>50% of cases). CaM modulates Kv7.1 to produce a Ca2+ -sensitive IKs, but little is known about the consequences of LQTS-associated CaM mutations on Kv7.1 function. Here, we present novel data characterising the biophysical and modulatory properties of three LQTS-associated CaM variants (D95V, N97I and D131H). We showed that mutations induced structural alterations in CaM and reduced affinity for Kv7.1, when compared with wild-type (WT). Using HEK293T cells expressing Kv7.1 channel subunits (KCNQ1/KCNE1) and patch-clamp electrophysiology, we demonstrated that LQTS-associated CaM variants reduced current density at systolic Ca2+ concentrations (1 μm), revealing a direct QT-prolonging modulatory effect. Our data highlight for the first time that LQTS-associated perturbations to CaM's structure impede complex formation with Kv7.1 and subsequently result in reduced IKs. This provides a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype. KEY POINTS: Calmodulin (CaM) is a ubiquitous, highly conserved calcium (Ca2+ ) sensor playing a key role in cardiac muscle contraction. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS), a life-threatening cardiac arrhythmia syndrome. LQTS-associated CaM variants (D95V, N97I and D131H) induced structural alterations, altered binding to Kv7.1 and reduced IKs. Our data provide a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype.

Pharmacological rescue of specific long QT variants of KCNQ1/KCNE1 channels

The congenital Long QT Syndrome (LQTS) is an inherited disorder in which cardiac ventricular repolarization is delayed and predisposes patients to cardiac arrhythmias and sudden cardiac death. LQT1 and LQT5 are LQTS variants caused by mutations in KCNQ1 or KCNE1 genes respectively. KCNQ1 and KCNE1 co-assemble to form critical I_KS potassium channels. Beta-blockers are the standard of care for the treatment of LQT1, however, doing so based on mechanisms other than correcting the loss-of-function of K⁺ channels. ML277 and R-L3 are compounds that enhance I_KS channels and slow channel deactivation in a manner that is dependent on the stoichiometry of KCNE1 subunits in the assembled channels. In this paper, we used expression of I_KS channels in Chinese hamster ovary (CHO) cells and Xenopus oocytes to study the potential of these two drugs (ML277 and R-L3) for the rescue of LQT1 and LQT5 mutant channels. We focused on the LQT1 mutation KCNQ1-S546L, and two LQT5 mutations, KCNE1-L51H and KCNE1-G52R. We found ML277 and R-L3 potentiated homozygote LQTS mutations in the I_KS complexes-KCNE1-G52R and KCNE1-L51H and in heterogeneous I_KS channel complexes which mimic heterogeneous expression of mutations in patients. ML277 and R-L3 increased the mutant I_KS current amplitude and slowed current deactivation, but not in wild type (WT) I_KS. We obtained similar results in the LQT1 mutant (KCNQ1 S546L/KCNE1) with ML277 and R-L3. ML277 and R-L3 had a similar effect on the LQT1 and LQT5 mutants, however, ML277 was more effective than R-L3 in this modulation. Importantly we found that not all LQT5 mutants expressed with KCNQ1 resulted in channels that are potentiated by these drugs as the KCNE1 mutant D76N inhibited drug action when expressed with KCNQ1. Thus, our work shows that by directly studying the treatment of LQT1 and LQT5 mutations with ML277 and R-L3, we will understand the potential utility of these activators as options in specific LQTS therapeutics.

Structural and electrophysiological basis for the modulation of KCNQ1 channel currents by ML277

The KCNQ1 ion channel plays critical physiological roles in electrical excitability and K⁺ recycling in organs including the heart, brain, and gut. Loss of function is relatively common and can cause sudden arrhythmic death, sudden infant death, epilepsy and deafness. Here, we report cryogenic electron microscopic (cryo-EM) structures of Xenopus KCNQ1 bound to Ca²⁺/Calmodulin, with and without the KCNQ1 channel activator, ML277. A single binding site for ML277 was identified, localized to a pocket lined by the S4-S5 linker, S5 and S6 helices of two separate subunits. Several pocket residues are not conserved in other KCNQ isoforms, explaining specificity. MD simulations and point mutations support this binding location for ML277 in open and closed channels and reveal that prevention of inactivation is an important component of the activator effect. Our work provides direction for therapeutic intervention targeting KCNQ1 loss of function pathologies including long QT interval syndrome and seizures.

KCNQ1 channels are active in heart, brain and gut. Functional loss causes epilepsy and sudden arrhythmic death. Here, authors describe a key activator drug binding site, explaining isoform and drug selectivity, and point the way for new drug design.

Molecular autopsy and subsequent functional analysis reveal de novo DSG2 mutation as cause of sudden death

Sudden cardiac death (SCD) is a common cause of death in young adults. In up to 80% of cases a genetic cause is suspected. Next-generation sequencing of candidate genes can reveal the cause of SCD, provide prognostic management, and facilitate pre-symptomatic testing and prevention in relatives. Here we present a proband who experienced SCD in his sleep for which molecular autopsy was performed. We performed a post-mortem genetic analysis of a 49-year-old male who died during sleep after competitive kayaking, using a Cardiomyopathy and Primary Arrhythmia next-generation sequencing panel, each containing 51 candidate genes. Autopsy was not performed. Genetic testing of the proband resulted in missense variants in KCNQ1 (c.1449C > A; p.(Asn483Lys)) and DSG2 (c.2979G > T; p.(Gln993His)), both absent from the gnomAD database. Familial segregation analysis showed de novo occurrence of the DSG2 variant and presence of the KCNQ1 variant in the proband's mother and daughter. KCNQ1 p.(Asn483Lys) was predicted to be pathogenic by MutationTaster. However, none of the KCNQ1 variant carrying family members showed long QTc on ECG or Holter. We further functionally analysed this variant using patch-clamp in a heterologous expression system (Chinese Hamster Ovary (CHO) cells) expressing the KCNQ1 mutant in combination with KCNE1 wild type protein and showed no significant changes in electrophysiological function of Kv7.1. Based on the above evidence, we concluded that the DSG2 p.(Gln993His) variant is the most likely cause of SCD in the presented case, and that there is insufficient evidence that the identified KCNQ1 p.(Asn483Lys) variant would confer risk for SCD in his mother and daughter. Fortunately, the DSG2 variant was not inherited by the proband's two children. This case report indicates the added value of molecular autopsy and the importance of subsequent functional study of variants to inform patients and family members about the risk of variants they might carry.

Disease-linked supertrafficking of a potassium channel

Gain-of-function (GOF) mutations in the voltage-gated potassium channel subfamily Q member 1 (KCNQ1) can induce cardiac arrhythmia. In this study, it was tested whether any of the known human GOF disease mutations in KCNQ1 act by increasing the amount of KCNQ1 that reaches the cell surface-"supertrafficking." Seven of the 15 GOF mutants tested were seen to surface traffic more efficiently than the WT channel. Among these, we found that the levels of R231C KCNQ1 in the plasma membrane were fivefold higher than the WT channel. This was shown to arise from the combined effects of enhanced efficiency of translocon-mediated membrane integration of the S4 voltage-sensor helix and from enhanced post-translational folding/trafficking related to the energetic linkage of C231 with the V129 and F166 side chains. Whole-cell electrophysiology recordings confirmed that R231C KCNQ1 in complex with the voltage-gated potassium channel-regulatory subfamily E member 1 not only exhibited constitutive conductance but also revealed that the single-channel activity of this mutant is only 20% that of WT. The GOF phenotype associated with R231C therefore reflects the effects of supertrafficking and constitutive channel activation, which together offset reduced channel activity. These investigations show that membrane protein supertrafficking can contribute to human disease.

Structural Basis for the Modulation of Human KCNQ4 by Small-Molecule Drugs

Among the five KCNQ channels, also known as the K_v7 voltage-gated potassium (K_v) channels, KCNQ2-KCNQ5 control neuronal excitability. Dysfunctions of KCNQ2-KCNQ5 are associated with neurological disorders such as epilepsy, deafness, and neuropathic pain. Here, we report the cryoelectron microscopy (cryo-EM) structures of human KCNQ4 and its complexes with the opener retigabine or the blocker linopirdine at overall resolutions of 2.5, 3.1, and 3.3 Å, respectively. In all structures, a phosphatidylinositol 4,5-bisphosphate (PIP₂) molecule inserts its head group into a cavity within each voltage-sensing domain (VSD), revealing an unobserved binding mode for PIP₂. Retigabine nestles in each fenestration, inducing local shifts. Instead of staying within the central pore, linopirdine resides in a cytosolic cavity underneath the inner gate. Electrophysiological analyses of various mutants corroborated the structural observations. Our studies reveal the molecular basis for the modulatory mechanism of neuronal KCNQ channels and provide a framework for structure-facilitated drug discovery targeting these important channels.

KCNQ1 and Long QT Syndrome in 1/45 Amish: The Road From Identification to Implementation of Culturally Appropriate Precision Medicine

Supplemental Digital Content is available in the text.

In population-based research exome sequencing, the path from variant discovery to return of results is not well established. Variants discovered by research exome sequencing have the potential to improve population health.

Physical and functional interaction sites in cytoplasmic domains of KCNQ1 and KCNE1 channel subunits

The cardiac potassium I_Ks current is carried by a channel complex formed from α-subunits encoded by KCNQ1 and β-subunits encoded by KCNE1. Deleterious mutations in either gene are associated with hereditary long QT syndrome. Interactions between the transmembrane domains of the α- and β-subunits determine the activation kinetics of I_Ks. A physical and functional interaction between COOH termini of the proteins has also been identified that impacts deactivation rate and voltage dependence of activation. We sought to explore the specific physical interactions between the COOH termini of the subunits that confer such control. Hydrogen/deuterium exchange coupled to mass spectrometry narrowed down the region of interaction to KCNQ1 residues 352-374 and KCNE1 residues 70-81, and provided evidence of secondary structure within these segments. Key mutations of residues in these regions tended to shift voltage dependence of activation toward more depolarizing voltages. Double-mutant cycle analysis then revealed energetic coupling between KCNQ1-I368 and KCNE1-D76 during channel activation. Our results suggest that the proximal COOH-terminal regions of KCNQ1 and KCNE1 participate in a physical and functional interaction during channel opening that is sensitive to perturbation and may explain the clustering of long QT mutations in the region.NEW & NOTEWORTHY Interacting ion channel subunits KCNQ1 and KCNE1 have received intense investigation due to their critical importance to human cardiovascular health. This work uses physical (hydrogen/deuterium exchange with mass spectrometry) and functional (double-mutant cycle analyses) studies to elucidate precise and important areas of interaction between the two proteins in an area that has eluded structural definition of the complex. It highlights the importance of pathogenic mutations in these regions.

Mechanisms of KCNQ1 channel dysfunction in long QT syndrome involving voltage sensor domain mutations

Mutations that induce loss of function (LOF) or dysfunction of the human KCNQ1 channel are responsible for susceptibility to a life-threatening heart rhythm disorder, the congenital long QT syndrome (LQTS). Hundreds of KCNQ1 mutations have been identified, but the molecular mechanisms responsible for impaired function are poorly understood. We investigated the impact of 51 KCNQ1 variants with mutations located within the voltage sensor domain (VSD), with an emphasis on elucidating effects on cell surface expression, protein folding, and structure. For each variant, the efficiency of trafficking to the plasma membrane, the impact of proteasome inhibition, and protein stability were assayed. The results of these experiments combined with channel functional data provided the basis for classifying each mutation into one of six mechanistic categories, highlighting heterogeneity in the mechanisms resulting in channel dysfunction or LOF. More than half of the KCNQ1 LOF mutations examined were seen to destabilize the structure of the VSD, generally accompanied by mistrafficking and degradation by the proteasome, an observation that underscores the growing appreciation that mutation-induced destabilization of membrane proteins may be a common human disease mechanism. Finally, we observed that five of the folding-defective LQTS mutant sites are located in the VSD S0 helix, where they interact with a number of other LOF mutation sites in other segments of the VSD. These observations reveal a critical role for the S0 helix as a central scaffold to help organize and stabilize the KCNQ1 VSD and, most likely, the corresponding domain of many other ion channels.

Compound heterozygous KCNQ1 mutations (A300T/P535T) in a child with sudden unexplained death: Insights into possible molecular mechanisms based on protein modeling

Sudden death in a child is a devastating event with important medical implications for surviving relatives. Because it may be the first manifestation of unknown inherited cardiac disease, molecular autopsy can be helpful to determine the cause of death and identify at risk family members. The aim of the study was to perform a molecular autopsy in a seven year-old girl with sudden unexplained death, to find evidence supporting the possible pathogenicity of mutations identified in inherited cardiac disease genes, and to clinically and genetically assess first-degree relatives. DNA from the index case was extracted from umbilical cord cells stored at birth, and DNA of first-degree relatives from blood samples. Targeted sequencing was performed using a Haloplex design including 81 cardiogenes. Possible functional consequences of the mutations were analyzed using protein modeling and structural mobility analyses. The child was compound heterozygous for KCNQ1 variants p.Ala300Thr and p.Pro535Thr. Ala300Thr is known to cause long QT syndrome in the homozygous state, while Pro535Thr is novel and of unknown clinical significance. The father and sibling were Ala300Thr heterozygous, and had normal QTc intervals at rest and during exercise. The asymptomatic mother was heterozygous for Pro535Thr, and showed borderline QTc at rest, but prolonged QTc during exercise. Protein modeling predicted that Ala300Thr alters the mobility profile of the Kv7.1 tetramer and Thr535 disrupts a calmodulin-binding site, probably causing co-assembly or trafficking defects of the mutant monomer. Altogether, the evidence strongly suggests that this child was affected with a recessive form of Romano Ward syndrome.

Identification of KCNQ1 compound heterozygous mutations in three Chinese families with Jervell and Lange-Nielsen Syndrome

CONCLUSION

Besides expanding the spectrum of KCNQ1 mutations causing Jervell and Lange-Nielsen Syndrome (JLNS), the results showed diversity of its phenotypes, and emphasized the importance of molecular genetic analysis in confirming clinical diagnosis and making diagnosis possible before the emergency symptoms for deaf individuals.

OBJECTIVES

This study aimed to investigate four patients from three Chinese families with congenital hearing loss clinically and genetically.

METHOD

Genetic analysis of previously reported deafness genes based on massively parallel sequencing was conducted in more than five thousand Chinese hearing loss patients. Detailed clinical features of the patients with compound heterozygous or homozygous mutations of KCNQ1 gene were collected and analyzed.

RESULTS

Compound mutations of KCNQ1 were found to be the genetic etiology of four patients from three families. Among the six KCNQ1 mutations, c.546C > A was identified as a novel mutation, c.965C > T had been reported in JLNS, while c.683 + 5G > A, c.1484_1485delCT, c.905C > T and c.1831G > A were previously reported in LQT1. In addition to congenital profound hearing loss in all subjects, two sibling subjects showed typical JLNS cardiac phenotype of prolonged QTc and recurrent syncopal episodes. One subject presented not only JLNS, but also iron-deficiency anemia and epilepsy. The other subject did not present any cardiac phenotype.

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.

Potassium Channels and Human Epileptic Phenotypes: An Updated Overview

Potassium (K⁺) channels are expressed in almost every cells and are ubiquitous in neuronal and glial cell membranes. These channels have been implicated in different disorders, in particular in epilepsy. K⁺ channel diversity depends on the presence in the human genome of a large number of genes either encoding pore-forming or accessory subunits. More than 80 genes encoding the K⁺ channels were cloned and they represent the largest group of ion channels regulating the electrical activity of cells in different tissues, including the brain. It is therefore not surprising that mutations in these genes lead to K⁺ channels dysfunctions linked to inherited epilepsy in humans and non-human model animals. This article reviews genetic and molecular progresses in exploring the pathogenesis of different human epilepsies, with special emphasis on the role of K⁺ channels in monogenic forms.

Stop-codon and C-terminal nonsense mutations are associated with a lower risk of cardiac events in patients with long QT syndrome type 1

BACKGROUND

In long QT syndrome type 1 (LQT1), the location and type of mutations have been shown to affect the clinical outcome. Although haploinsufficiency, including stop-codon and frameshift mutations, has been associated with a lower risk of cardiac events in patients with LQT1, nonsense mutations have been presumed functionally equivalent.

OBJECTIVE

The purpose of this study was to evaluate clinical differences between patients with nonsense mutations.

METHODS

The study sample comprised 1090 patients with genetically confirmed mutations. Patients were categorized into 5 groups, depending on mutation type and location: missense not located in the high-risk cytoplasmic loop (c-loop) (n = 698), which is used as reference; missense c-loop (n = 192); stop-codon (n = 67); frameshift (n = 39); and others (n = 94). The primary outcome was a composite end point of syncope, aborted cardiac arrest, and long QT syndrome-related death (cardiac events). Outcomes were evaluated using the multivariate Cox proportional hazards regression analysis. Standard patch clamp techniques were used.

RESULTS

Compared to patients with missense non-c-loop mutations, the risk of cardiac events was reduced significantly in patients with stop-codon mutations (hazard ratio [HR] 0.57; 95% confidence interval [CI] 0.34-0.96; P = .035), but not in patients with frameshift mutations (HR 1.01; 95% CI 0.58-1.77; P = .97). Our data suggest that currents of the most common stop-codon mutant channel (Q530X) were larger than those of haploinsufficient channels (wild type: 42 ± 6 pA/pF, n = 20; Q530X+wild type: 79 ± 14 pA/pF, n = 20; P < .05) and voltage dependence of activation was altered.

CONCLUSION

Stop-codon mutations are associated with a lower risk of cardiac events in patients with LQT1, while frameshift mutations are associated with the same risk as the majority of the missense mutations. Our data indicate functional differences between these previously considered equivalent mutation subtypes.

IKs Gain- and Loss-of-Function in Early-Onset Lone Atrial Fibrillation

INTRODUCTION

Atrial fibrillation (AF) is the most frequent cardiac arrhythmia. The potassium current IKs is essential for cardiac repolarization. Gain-of-function mutation in KCNQ1, the gene encoding the pore-forming α-subunit of the IKs channel (KV 7.1), was the first ion channel dysfunction to be associated with familial AF. We hypothesized that early-onset lone AF is associated with a high prevalence of mutations in KCNQ1.

METHODS AND RESULTS

We bidirectionally sequenced the entire coding sequence of KCNQ1 in 209 unrelated patients with early-onset lone AF (<40 years) and investigated the identified mutations functionally in a heterologous expression system. We found 4 nonsynonymous KCNQ1 mutations (A46T, R195W, A302V, and R670K) in 4 unrelated patients (38, 31, 39, and 36 years, respectively). None of the mutations were present in the control group (n = 416 alleles). No other mutations were found in genes previously associated with AF. The mutations A46T, R195W, and A302V have previously been associated with long-QT syndrome. In line with previous reports, we found A302V to display a pronounced loss-of-function of the IKs current, while the other mutants exhibited a gain-of-function phenotype.

CONCLUSIONS

Mutations in the IKs channel leading to gain-of-function have previously been described in familial AF, yet this is the first time a loss-of-function mutation in KCNQ1 is associated with early-onset lone AF. These findings suggest that both gain-of-function and loss-of-function of cardiac potassium currents enhance the susceptibility to AF.

Genetic and forensic implications in epilepsy and cardiac arrhythmias: a case series

Epilepsy affects approximately 3% of the world's population, and sudden death is a significant cause of death in this population. Sudden unexpected death in epilepsy (SUDEP) accounts for up to 17% of all these cases, which increases the rate of sudden death by 24-fold as compared to the general population. The underlying mechanisms are still not elucidated, but recent studies suggest the possibility that a common genetic channelopathy might contribute to both epilepsy and cardiac disease to increase the incidence of death via a lethal cardiac arrhythmia. We performed genetic testing in a large cohort of individuals with epilepsy and cardiac conduction disorders in order to identify genetic mutations that could play a role in the mechanism of sudden death. Putative pathogenic disease-causing mutations in genes encoding cardiac ion channel were detected in 24% of unrelated individuals with epilepsy. Segregation analysis through genetic screening of the available family members and functional studies are crucial tasks to understand and to prove the possible pathogenicity of the variant, but in our cohort, only two families were available. Despite further research should be performed to clarify the mechanism of coexistence of both clinical conditions, genetic analysis, applied also in post-mortem setting, could be very useful to identify genetic factors that predispose epileptic patients to sudden death, helping to prevent sudden death in patients with epilepsy.

Novel Kv7.1-phosphatidylinositol 4,5-bisphosphate interaction sites uncovered by charge neutralization scanning

Kv7.1 to Kv7.5 α-subunits belong to the family of voltage-gated potassium channels (Kv). Assembled with the β-subunit KCNE1, Kv7.1 conducts the slowly activating potassium current IKs, which is one of the major currents underlying repolarization of the cardiac action potential. A known regulator of Kv7 channels is the lipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 increases the macroscopic current amplitude by stabilizing the open conformation of 7.1/KCNE1 channels. However, knowledge about the exact nature of the interaction is incomplete. The aim of this study was the identification of the amino acids responsible for the interaction between Kv7.1 and PIP2. We generated 13 charge neutralizing point mutations at the intracellular membrane border and characterized them electrophysiologically in complex with KCNE1 under the influence of diC8-PIP2. Electrophysiological analysis of corresponding long QT syndrome mutants suggested impaired PIP2 regulation as the cause for channel dysfunction. To clarify the underlying structural mechanism of PIP2 binding, molecular dynamics simulations of Kv7.1/KCNE1 complexes containing two PIP2 molecules in each subunit at specific sites were performed. Here, we identified a subset of nine residues participating in the interaction of PIP2 and Kv7.1/KCNE1. These residues may form at least two binding pockets per subunit, leading to the stabilization of channel conformations upon PIP2 binding.

Familial Atrial Fibrillation Predicts Increased Risk of Mortality

Background—

Atrial fibrillation (AF) is a common arrhythmia. Several studies have shown association of genetic variants with AF and that familial AF increases the risk of AF. We have previously shown a substantial heritability of AF in a twin study. The objective of this study was to determine whether having a co-twin with AF influences mortality.

Methods and Results—

We identified all Danish twins with AF born during and after 1912 in the Danish Twin Registry, the National Patient Registry, and the Central Office of Civil Registration. For each twin, we randomly identified 4 twins without AF, matched on sex, zygosity, and age. We compared survival among the co-twins of the affected twins (co-cases, n=2164) and the co-twins of the unaffected twins (co-controls, n=8626). The co-cases showed increased death rates compared with the co-controls (hazard ratio, 1.20; 95% confidence interval, 1.11–1.30; P <0.0001), and this effect was more pronounced in monozygotic twins (hazard ratio, 1.30; 95% confidence interval, 1.09–1.55; P =0.003), compared with dizygotic same sex (hazard ratio, 1.16; 95% confidence interval, 1.04–1.29; P =0.006) and opposite sex twins (hazard ratio, 1.20; 95% confidence interval, 0.97–1.47; P =0.093).

Conclusions—

The mortality rate was 20% higher in twins who had a co-twin with AF than in twins without familial AF. This effect was almost doubled in monozygotic twins compared with dizygotic twins, suggesting the influence of genetic factors.

Dysfunctional potassium channel subunit interaction as a novel mechanism of long QT syndrome

BACKGROUND

The slowly-activating delayed rectifier current IKs contributes to repolarization of the cardiac action potential, and is composed of a pore-forming α-subunit, KCNQ1, and a modulatory β-subunit, KCNE1. Mutations in either subunit can cause long QT syndrome, a potentially fatal arrhythmic disorder. How KCNE1 exerts its extensive control over the kinetics of IKs remains unresolved

OBJECTIVE

To evaluate the impact of a novel KCNQ1 mutation on IKs channel gating and kinetics

METHODS

KCNQ1 mutations were expressed in Xenopus oocytes in the presence and absence of KCNE1. Voltage clamping and MODELLER software were used to characterize and model channel function. Mutant and wt genes were cloned into FLAG, Myc and HA expression vectors to achieve differential epitope tagging, and expressed in HEK293 cells for immunohistochemical localization and surface biotinylation assay.

RESULTS

We identified 2 adjacent mutations, S338F and F339S, in the KCNQ1 S6 domain in unrelated probands. The novel KCNQ1 S338F mutation segregated with prolonged QT interval and torsade de pointes; the second variant, F339S, was associated with fetal bradycardia and prolonged QT interval, but no other clinical events. S338F channels expressed in Xenopus oocytes had slightly increased peak conductance relative to wild type, with a more positive activation voltage. F339S channels conducted minimal current. Unexpectedly, S338F currents were abolished by co-expression with intact WT KCNE1 or its C-terminus (aa63-129), despite normal membrane trafficking and surface co-localization of KCNQ1 S338F and wt KCNE1. Structural modeling indicated that the S338F mutation specifically alters the interaction between the S6 domain of one KCNQ1 subunit and the S4-S5 linker of another, inhibiting voltage-induced movement synergistically with KCNE1 binding.

CONCLUSIONS

A novel KCNQ1 mutation specifically impaired channel function in the presence of KCNE1. Our structural model shows that this mutation effectively immobilizes voltage gating by an inhibitory interaction that is additive with that of KCNE1. Our findings illuminate a previously unreported mechanism for LQTS, and validate recent theoretical models of the closed state of the KCNQ1:KCNE1 complex.

An allosteric mechanism for drug block of the human cardiac potassium channel KCNQ1

The intracellular aspect of the sixth transmembrane segment within the ion-permeating pore is a common binding site for many voltage-gated ion channel blockers. However, the exact site(s) at which drugs bind remain controversial. We used extensive site-directed mutagenesis coupled with molecular modeling to examine mechanisms in drug block of the human cardiac potassium channel KCNQ1. A total of 48 amino acid residues in the S6 segment, S4-S5 linker, and the proximal C-terminus of the KCNQ1 channel were mutated individually to alanine; alanines were mutated to cysteines. Residues modulating drug block were identified when mutant channels displayed <50% block on exposure to drug concentrations that inhibited wild-type current by ≥90%. Homology modeling of the KCNQ1 channel based on the Kv1.2 structure unexpectedly predicted that the key residue modulating drug block (F351) faces away from the permeating pore. In the open-state channel model, F351 lines a pocket that also includes residues L251 and V254 in S4-S5 linker. Docking calculations indicated that this pocket is large enough to accommodate quinidine. To test this hypothesis, L251A and V254A mutants were generated that display a reduced sensitivity to blockage with quinidine. Thus, our data support a model in which open state block of this channel occurs not via binding to a site directly in the pore but rather by a novel allosteric mechanism: drug access to a side pocket generated in the open-state channel configuration and lined by S6 and S4-S5 residues.

In silico directed mutagenesis identifies the CD81/claudin-1 hepatitis C virus receptor interface

Hepatitis C virus (HCV) entry is dependent on host cell molecules tetraspanin CD81, scavenger receptor BI and tight junction proteins claudin-1 and occludin. We previously reported a role for CD81/claudin-1 receptor complexes in HCV entry; however, the molecular mechanism(s) driving association between the receptors is unknown. We explored the molecular interface between CD81 and claudin-1 using a combination of bioinformatic sequence-based modelling, site-directed mutagenesis and Fluorescent Resonance Energy Transfer (FRET) imaging methodologies. Structural modelling predicts the first extracellular loop of claudin-1 to have a flexible beta conformation and identifies a motif between amino acids 62-66 that interacts with CD81 residues T149, E152 and T153. FRET studies confirm a role for these CD81 residues in claudin-1 association and HCV infection. Importantly, mutation of these CD81 residues has minimal impact on protein conformation or HCV glycoprotein binding, highlighting a new functional domain of CD81 that is essential for virus entry.

Congenital long-QT syndrome in Addison's disease: a novel association

This report describes a teenager found to have both Addision's disease and long-QT syndrome type 1. This association is unique, but congenital long-QT channelopathies have been associated with other endocrinopathies. It remains to be seen whether genetic investigation should be performed for all patients with long-QTc's and endocrinopathies.

Fine architecture and mutation mapping of human brain inhibitory system ligand gated ion channels by high-throughput homology modeling

The common architecture of the brain inhibitory system ligand-gated ion-channels was examined at the level of each of the subunits and in their assembled pentameric arrangements. Structural modeling of the GABAA receptor, GlyR1, and the serotonin receptor, 5HTR3A, was carried out on a multi-homolog basis employing a high-throughput homology modeling pipeline. The locations of all the known mutations of each of the subunits of the receptor subfamily were mapped upon their computed structures and structural relationships between patterns of mutations in different subunits were identified, resulting in the zoning of mutations to four specific regions of the common subunit structure. These classifications may be of value in discerning probable molecular mechanisms and functional manifestations of emerging mutations and polymorphisms, providing the foundation for a family-specific predictive algorithm that may allow researchers to focus experimental effort on the most probable molecular indicators of compromised receptor function and disease mechanism.

Elevated serum gastrin levels in Jervell and Lange-Nielsen syndrome: a marker of severe KCNQ1 dysfunction?

BACKGROUND

The potassium channel I(Ks), which is encoded by the KCNQ1 gene, is expressed in organ systems including the inner ear, kidneys, lungs, intestine, and stomach in addition to the heart. Increasing evidence indicates that I(Ks) in the stomach plays an essential role in enabling gastric acid production. It is not known whether gastric acid production is disordered in patients with long QT type 1. Serum gastrin levels become elevated in subjects with disordered gastric acid production.

OBJECTIVE

The purpose of this study was to evaluate serum gastrin levels, as a surrogate for impaired gastric acid secretion, in patients with KCNQ1 mutations, and to see if gastrin levels correlate with severity of cardiac disease.

METHODS

Fasting serum gastrin levels were measured using a standardized immunometric technique in an index case and 12 subjects with known KCNQ1 mutations.

RESULTS

An adult female with Jervell and Lange-Nielsen syndrome (JLNS; with KCNQ1 nonsense mutations p.Arg518X and p.Arg190AlafsX95 ) presented with multiple gastric carcinoid tumors and grossly elevated serum gastrin levels (943-1,570 pmol/L; normal 6-55 pmol/L) and absent acid secretion. Gastrin levels in two girls with JLNS, unrelated to the index case (missense mutations p.Leu266Pro and Gly269Ser), also were high (305 and 229 pmol/L). Gastrin levels were normal in 10 KCNQ1 heterozygous single mutation carriers, even in those with severe long QT syndrome, including three heterozygous family members of the JLNS subjects.

CONCLUSION

JLNS may be associated with elevated gastrin levels, impaired acid secretion, and risk of gastric carcinoid tumors. Among KCNQ1 single mutation carriers, gastrin levels were normal and did not appear to be linked to the severity of clinical expression of long QT syndrome.

Characterization of a binding site for anionic phospholipids on KCNQ1

The KCNQ family of potassium channels underlie a repolarizing K(+) current in the heart and the M-current in neurones. The assembly of KCNQ1 with KCNE1 generates the delayed rectifier current I(Ks) in the heart. Characteristically these channels are regulated via G(q/11)-coupled receptors and the inhibition seen after phospholipase C activation is now thought to occur from membrane phosphatidylinositol (4,5)-bisphosphate (PIP(2)) depletion. It is not clear how KCNQ1 recognizes PIP(2) and specifically which residues in the channel complex are important. Using biochemical techniques we identify a cluster of basic residues namely, Lys-354, Lys-358, Arg-360, and Lys-362, in the proximal C terminus as being involved in binding anionic phospholipids. The mutation of specific residues in combination, to alanine leads to the loss of binding to phosphoinositides. Functionally, the introduction of these mutations into KCNQ1 leads to shifts in the voltage dependence of channel activation toward depolarized potentials and reductions in current density. Additionally, the biophysical effects of the charge neutralizing mutations, which disrupt phosphoinositide binding, mirror the effects we see on channel function when we deplete cellular PIP(2) levels through activation of a G(q/11)-coupled receptor. Conversely, the addition of diC8-PIP(2) to the wild-type channel, but not a PIP(2) binding-deficient mutant, acts to shift the voltage dependence of channel activation toward hyperpolarized potentials and increase current density. In conclusion, we use a combined biochemical and functional approach to identify a cluster of basic residues important for the binding and action of anionic phospholipids on the KCNQ1/KCNE1 complex.

Molecular genetics of long QT syndrome

Long QT syndrome (LQTS) is a cardiac disorder associated with sudden death especially in young, seemingly healthy individuals. It is characterised by abnormalities of the heart beat detected as lengthening of the QT interval during cardiac repolarisation. The incidence of LQTS is given as 1 in 2000 but this may be an underestimation as many cases go undiagnosed, due to the rarity of the condition and the wide spectrum of symptoms. Presently 12 genes associated with LQTS have been identified with differing signs and symptoms, depending on the locus involved. The majority of cases have mutations in the KCNQ1 (LQT1), KCNH2 (LQT2) and SCN5A (LQT3) genes. Genetic testing is increasingly used when a clearly affected proband has been identified, to determine the nature of the mutation in that family. Unfortunately tests on probands may be uninformative, especially if the defect does not lie in the set of genes which are routinely tested. Novel mutations in these known LQTS genes and additional candidate genes are still being discovered. The functional implications of these novel mutations need to be assessed before they can be accepted as being responsible for LQTS. Known epigenetic modification affecting KCNQ1 gene expression may also be involved in phenotypic variability of LQTS. Genetic diagnosis of LQTS is thus challenging. However, where a disease associated mutation is identified, molecular diagnosis can be important in guiding therapy, in family testing and in determining the cause of sudden cardiac death. New developments in technology and understanding offer increasing hope to families with this condition.