Trafficking-deficient long QT syndrome mutation KCNQ1-T587M confers severe clinical phenotype by impairment of KCNH2 membrane localization: evidence for clinically significant IKr-IKs alpha-subunit interaction

Clinical characterization of type 1 long QT syndrome caused by C-terminus Kv7.1 variants

BACKGROUND

Variants in the KCNQ1 gene, encoding the α-subunit of the slow component of delayed rectifier K+ channel Kv7.1, cause long QT syndrome (LQTS) type 1. The location of variants may be one of the factors in determining prognosis. However, detailed genotype-phenotype relationships associated with C-terminus variants remain unelucidated.

OBJECTIVE

We investigated the clinical characteristics and variant-specific arrhythmic risks in patients with LQTS carrying Kv7.1 C-terminus variants.

METHODS

The study comprises 202 consecutive patients with LQTS (98 probands and 104 family members) who carry a rare heterozygous variant in the Kv7.1 C-terminus. Their clinical characteristics and arrhythmic events were investigated.

RESULTS

We identified 36 unique C-terminus variants (25 missense and 11 non-missense). The p.R366W variant was identified in 8 families, and p.T587M was identified in 21 families in large numbers from northwestern Japan. As for the location of the variant, we found that the variants in highly conserved regions and nonhelical domains were associated with longer QTc intervals compared with the variants in other regions. Both p.R366W and p.T587M variants are located in the highly conserved and functionally pivotal regions close to helices A and D, which are associated with calmodulin binding and channel assembly (tetramerization), respectively. The probands carrying p.T587M and p.R366W variants had worse arrhythmia outcomes compared with those with other C-terminus variants. The haplotype analysis of p.T587M families was suggestive of a founder effect.

CONCLUSION

The arrhythmic risk of C-terminus variants in Kv7.1 in LQTS is not homogeneous, and locations of variants can be a determining factor for prognosis.

The electrophysiologic effects of KCNQ1 extend beyond expression of IKs: evidence from genetic and pharmacologic block

AIMS

While variants in KCNQ1 are the commonest cause of the congenital long QT syndrome, we and others find only a small IKs in cardiomyocytes from human-induced pluripotent stem cells (iPSC-CMs) or human ventricular myocytes.

METHODS AND RESULTS

We studied population control iPSC-CMs and iPSC-CMs from a patient with Jervell and Lange-Nielsen (JLN) syndrome due to compound heterozygous loss-of-function (LOF) KCNQ1 variants. We compared the effects of pharmacologic IKs block to those of genetic KCNQ1 ablation, using JLN cells, cells homozygous for the KCNQ1 LOF allele G643S, or siRNAs reducing KCNQ1 expression. We also studied the effects of two blockers of IKr, the other major cardiac repolarizing current, in the setting of pharmacologic or genetic ablation of KCNQ1: moxifloxacin, associated with a very low risk of drug-induced long QT, and dofetilide, a high-risk drug. In control cells, a small IKs was readily recorded but the pharmacologic IKs block produced no change in action potential duration at 90% repolarization (APD90). In contrast, in cells with genetic ablation of KCNQ1 (JLN), baseline APD90 was markedly prolonged compared with control cells (469 ± 20 vs. 310 ± 16 ms). JLN cells displayed increased sensitivity to acute IKr block: the concentration (μM) of moxifloxacin required to prolong APD90 100 msec was 237.4 [median, interquartile range (IQR) 100.6-391.6, n = 7] in population cells vs. 23.7 (17.3-28.7, n = 11) in JLN cells. In control cells, chronic moxifloxacin exposure (300 μM) mildly prolonged APD90 (10%) and increased IKs, while chronic exposure to dofetilide (5 nM) produced greater prolongation (67%) and no increase in IKs. However, in the siRNA-treated cells, moxifloxacin did not increase IKs and markedly prolonged APD90.

CONCLUSION

Our data strongly suggest that KCNQ1 expression modulates baseline cardiac repolarization, and the response to IKr block, through mechanisms beyond simply generating IKs.

Pueraria mirifica, an estrogenic tropical herb, unveiled the severity of Type 1 LQTS caused by KCNQ1-T587M

After taking an estrogen-containing supplement derived from a tropical plant Pueraria mirifica, a 24-year-old woman presented marked QT prolongation and repetitive torsade de pointes. The patient was found to carry a heterozygous KCNQ1-T587M mutation. This is the first report on Pueraria mirifica-related acquired long QT syndrome.

Modelling sudden cardiac death risks factors in patients with coronavirus disease of 2019: the hydroxychloroquine and azithromycin case

AIMS

Coronavirus disease of 2019 (COVID-19) has rapidly become a worldwide pandemic. Many clinical trials have been initiated to fight the disease. Among those, hydroxychloroquine and azithromycin had initially been suggested to improve clinical outcomes. Despite any demonstrated beneficial effects, they are still in use in some countries but have been reported to prolong the QT interval and induce life-threatening arrhythmia. Since a significant proportion of the world population may be treated with such COVID-19 therapies, evaluation of the arrhythmogenic risk of any candidate drug is needed.

METHODS AND RESULTS

Using the O'Hara-Rudy computer model of human ventricular wedge, we evaluate the arrhythmogenic potential of clinical factors that can further alter repolarization in COVID-19 patients in addition to hydroxychloroquine (HCQ) and azithromycin (AZM) such as tachycardia, hypokalaemia, and subclinical to mild long QT syndrome. Hydroxychloroquine and AZM drugs have little impact on QT duration and do not induce any substrate prone to arrhythmia in COVID-19 patients with normal cardiac repolarization reserve. Nevertheless, in every tested condition in which this reserve is reduced, the model predicts larger electrocardiogram impairments, as with dofetilide. In subclinical conditions, the model suggests that mexiletine limits the deleterious effects of AZM and HCQ.

CONCLUSION

By studying the HCQ and AZM co-administration case, we show that the easy-to-use O'Hara-Rudy model can be applied to assess the QT-prolongation potential of off-label drugs, beyond HCQ and AZM, in different conditions representative of COVID-19 patients and to evaluate the potential impact of additional drug used to limit the arrhythmogenic risk.

The phosphorylation state of both hERG and KvLQT1 mediates protein-protein interactions between these complementary cardiac potassium channel alpha subunits

KvLQT1 and hERG are the α-subunits of the voltage-gated K⁺ channels which carry the cardiac repolarizing currents IKs and IKr, respectively. These currents function in vivo with some redundancy to maintain appropriate action potential durations (APDs) in cardiomyocytes. As such, protein-protein interactions between hERG and KvLQT1 may be important in normal cardiac electrophysiology, as well as in arrhythmia and sudden cardiac death. Previous phenomenological observations of functional, mutual downregulation between these complementary repolarizing currents in transgenic rabbit models and human cell culture motivate our investigations into protein-protein interactions between hERG and KvLQT1. Previous data suggest that a dynamic, physical interaction between hERG and KvLQT1 modulates the respective currents. However, the mechanism by which hERG-KvLQT1 interactions are regulated is still poorly understood. Phosphorylation is proposed to play a role since modifying the phosphorylation state of each protein has been shown to alter channel kinetics, and both hERG and KvLQT1 are targets of the Ser/Thr protein kinase PKA, activated by elevated intracellular cAMP. In this work, quantitative apFRET analyses of phosphonull and phosphomimetic hERG and KvLQT1 mutants indicate that unphosphorylated hERG does not interact with KvLQT1, suggesting that hERG phosphorylation is important for wild-type proteins to interact. For proteins already potentially interacting, phosphorylation of KvLQT1 appears to be the driving factor abrogating hERG-KvLQT1 interaction. This work increases our knowledge about hERG-KvLQT1 interactions, which may contribute to the efforts to elucidate mechanisms that underlie many types of arrhythmias, and also further characterizes novel protein-protein interactions between two distinct potassium channel families.

Inter-Regulation of Kv4.3 and Voltage-Gated Sodium Channels Underlies Predisposition to Cardiac and Neuronal Channelopathies

Background: Genetic variants in voltage-gated sodium channels (Na_v) encoded by SCNXA genes, responsible for I_Na, and K_v4.3 channels encoded by KCND3, responsible for the transient outward current (I_to), contribute to the manifestation of both Brugada syndrome (BrS) and spinocerebellar ataxia (SCA19/22). We examined the hypothesis that K_v4.3 and Na_v variants regulate each other’s function, thus modulating I_Na/I_to balance in cardiomyocytes and I_Na/I_(A) balance in neurons. Methods: Bicistronic and other constructs were used to express WT or variant Na_v1.5 and K_v4.3 channels in HEK293 cells. I_Na and I_to were recorded. Results: SCN5A variants associated with BrS reduced I_Na, but increased I_to. Moreover, BrS and SCA19/22 KCND3 variants associated with a gain of function of I_to, significantly reduced I_Na, whereas the SCA19/22 KCND3 variants associated with a loss of function (LOF) of I_to significantly increased I_Na. Auxiliary subunits Na_vβ1, MiRP3 and KChIP2 also modulated I_Na/I_to balance. Co-immunoprecipitation and Duolink studies suggested that the two channels interact within the intracellular compartments and biotinylation showed that LOF SCN5A variants can increase K_v4.3 cell-surface expression. Conclusion: Na_v and K_v4.3 channels modulate each other’s function via trafficking and gating mechanisms, which have important implications for improved understanding of these allelic cardiac and neuronal syndromes.

Long QT Syndrome and Sinus Bradycardia-A Mini Review

Congenital long-QT syndrome (LQTS) is an inherited cardiac disorder characterized by the prolongation of ventricular repolarization, susceptibility to Torsades de Pointes (TdP), and a risk for sudden death. Various types of congenital LQTS exist, all due to specific defects in ion channel-related genes. Interestingly, almost all of the ion channels affected by the various types of LQTS gene mutations are also expressed in the human sinoatrial node (SAN). It is therefore not surprising that LQTS is frequently associated with a change in basal heart rate (HR). However, current data on how the LQTS-associated ion channel defects result in impaired human SAN pacemaker activity are limited. In this mini-review, we provide an overview of known LQTS mutations with effects on HR and the underlying changes in expression and kinetics of ion channels. Sinus bradycardia has been reported in relation to a large number of LQTS mutations. However, the occurrence of both QT prolongation and sinus bradycardia on a family basis is almost completely limited to LQTS types 3 and 4 (LQT3 and Ankyrin-B syndrome, respectively). Furthermore, a clear causative role of this sinus bradycardia in cardiac events seems reserved to mutations underlying LQT3.

A hERG mutation E1039X produced a synergistic lesion on IKs together with KCNQ1-R174C mutation in a LQTS family with three compound mutations

Congenital long QT syndrome (LQTS) caused by compound mutations is usually associated with more severe clinical phenotypes. We identified a LQTS family harboring three compound mutations in different genes (KCNQ1-R174C, hERG-E1039X and SCN5A-E428K). KCNQ1-R174C, hERG-E1039X and SCN5A-E428K mutations and/or relevant wild-type (WT) cDNAs were respectively expressed in mammalian cells. I_Ks-like, I_Kr-like, I_Na-like currents and the functional interaction between KCNQ1-R174C and hERG-E1039X channels were studied using patch-clamp and immunocytochemistry techniques. (1) Expression of KCNQ1-R174C alone showed no I_Ks. Co-expression of KCNQ1-WT + KCNQ1-R174C caused a loss-of-function in I_Ks and blunted the activation of I_Ks in response to isoproterenol. (2) Expression of hERG-E1039X alone and co-expression of hERG-WT + hERG-E1039X negatively shifted inactivation curves and decelerated the recovery time from inactivation. (3) Expression of SCN5A-E428K increased peak I_Na, but had no effect on late I_Na. (4) I_Ks and I_Kr interact, and hERG-E1039X caused a loss-of-function in I_Ks. (5) Immunocytochemical studies indicated that KCNQ1-R174C is trafficking defective and hERG-E1039X is defective in biosynthesis/degradation, but the abnormities were rescued by co-expression with WT. Thus, KCNQ1-R174C and hERG-E1039X disrupted I_Ks and I_Kr functions, respectively. The synergistic lesion, caused by KCNQ1-R174C and hERG-E1039X in I_Ks, is very likely why patients showed more severe phenotypes in the compound mutation case.

Ion Channel Trafficking: Control of Ion Channel Density as a Target for Arrhythmias?

The shape of the cardiac action potential (AP) is determined by the contributions of numerous ion channels. Any dysfunction in the proper function or expression of these ion channels can result in a change in effective refractory period (ERP) and lead to arrhythmia. The processes underlying the correct targeting of ion channels to the plasma membrane are complex, and have not been fully characterized in cardiac myocytes. Emerging evidence highlights ion channel trafficking as a potential causative factor in certain acquired and inherited arrhythmias, and therapies which target trafficking as opposed to pore block are starting to receive attention. In this review we present the current evidence for the mechanisms which underlie precise control of cardiac ion channel trafficking and targeting.

Glucose ingestion causes cardiac repolarization disturbances in type 1 long QT syndrome patients and healthy subjects

BACKGROUND

Both hypoglycemia and severe hyperglycemia constitute known risk factors for cardiac repolarization changes potentially leading to malignant arrhythmias. Patients with loss of function mutations in KCNQ1 are characterized by long QT syndrome (LQTS) and may be at increased risk for glucose-induced repolarization disturbances.

OBJECTIVE

The purpose of this study was to test the hypothesis that KCNQ1 LQTS patients are at particular risk for cardiac repolarization changes during the relative hyperglycemia that occurs after an oral glucose load.

METHODS

Fourteen KCNQ1 LQTS patients and 28 control participants matched for gender, body mass index, and age underwent a 3-hour oral 75-g glucose tolerance test with ECGs obtained at 7 time points. Fridericia corrected QT interval (QTcF), Bazett corrected QT interval (QTcB), and the Morphology Combination Score (MCS) were calculated.

RESULTS

QTc and MCS increased in both groups. MCS remained elevated until 150 minutes after glucose ingestion, and the maximal change from baseline was larger among KCNQ1 LQTS patients compared with control subjects (0.28 ± 0.27 vs 0.15 ± 0.13; P <.05).

CONCLUSION

Relative hyperglycemia induced by ingestion of 75-g glucose caused cardiac repolarization disturbances that were more severe in KCNQ1 LQTS patients compared with control subjects.

The interaction between delayed rectifier channel alpha-subunits does not involve hetero-tetramer formation

We have previously reported a physiologically relevant interaction between KCNQ1 (Q1) and KCNH2 (H2). While the H2 C-terminus has been suggested to play a role, so far, no more detailed information regarding the interaction site is available. The methods used in the study are cell culture, PCR for mutagenesis, patch clamp for ion current recordings, co-immunoprecipitation for determination of protein interaction. Co-expression of Q1 and H2 resulted in an increase of I H2 (tails after +50 mV; Q1 + H2, 36 ± 6 pA/pF; H2, 14 ± 2 pA/pF; n = 10; 12; P < 0.05). Upon expressing a non-conductive (dominant-negative) Q1-pore mutation (dnQ1), there was still an increase in I H2 (tails after +50 mV; H2 + dnQ1, 24 ± 4 pA/pF; n = 10; P < 0.05) making the pore region unlikely as an interaction site. Experiments using the KCNH2-pore blocking agent quinidine supported these findings. If Q1 and H2 formed hetero-tetramers, steric changes within the pore should change the quinidine half-inhibitory concentrations (IC50). However, I H2 sensitivity did not significantly change in the presence or absence of Q1 (IC50 341 ± 63 vs. 611 ± 293 nmol/L, respectively, P = n.s.), providing further evidence that the pore is not a likely H2-Q1 interaction site. To obtain further insights into the role of intra-cytoplasmic structures, we used both C- and N-terminally truncated mutant H2 proteins. Both H2 mutants co-immunoprecipitated with Q1, suggesting no specific role of C- or N-termini. Accordingly, rather than these, the transmembrane domains of the α-subunits appear relevant for the interaction. Our results largely exclude the formation of hetero-tetramers between H2 and Q1 comprising the pore region or H2 C- or N-termini.

Characterization of a novel KCNQ1 mutation for type 1 long QT syndrome and assessment of the therapeutic potential of a novel IKs activator using patient-specific induced pluripotent stem cell-derived cardiomyocytes

Introduction

Type 1 long QT syndrome (LQT1) is a common type of cardiac channelopathy associated with loss-of-function mutations of KCNQ1. Currently there is a lack of drugs that target the defected slowly activating delayed rectifier potassium channel (IKs). With LQT1 patient-specific human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (hiPSC-CMs), we tested the effects of a selective IKs activator ML277 on reversing the disease phenotypes.

Methods

A LQT1 family with a novel heterozygous exon 7 deletion in the KCNQ1 gene was identified. Dermal fibroblasts from the proband and her healthy father were reprogrammed to hiPSCs and subsequently differentiated into hiPSC-CMs.

Results

Compared with the control, LQT1 patient hiPSC-CMs showed reduced levels of wild type KCNQ1 mRNA accompanied by multiple exon skipping mRNAs and a ~50% reduction of the full length Kv7.1 protein. Patient hiPSC-CMs showed reduced IKs current (tail current density at 30 mV: 0.33 ± 0.02 vs. 0.92 ± 0.21, P < 0.05) and prolonged action potential duration (APD) (APD 50 and APD90: 603.9 ± 39.2 vs. 319.3 ± 13.8 ms, P < 0.005; and 671.0 ± 41.1 vs. 372.9 ± 14.2 ms, P < 0.005). ML277, a small molecule recently identified to selectively activate K_V7.1, reversed the decreased IKs and partially restored APDs in patient hiPSC-CMs.

Conclusions

From a LQT1 patient carrying a novel heterozygous exon7 deletion mutation of KCNQ1, we generated hiPSC-CMs that faithfully recapitulated the LQT1 phenotypes that are likely associated with haploinsufficiency and trafficking defect of KCNQ1/Kv7.1. The small molecule ML277 restored IKs function in hiPSC-CMs and could have therapeutic value for LQT1 patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0027-z) contains supplementary material, which is available to authorized users.

Elevated heart rate triggers action potential alternans and sudden death. translational study of a homozygous KCNH2 mutation

BACKGROUND

Long QT syndrome (LQTS) leads to arrhythmic events and increased risk for sudden cardiac death (SCD). Homozygous KCNH2 mutations underlying LQTS-2 have previously been termed "human HERG knockout" and typically express severe phenotypes. We studied genotype-phenotype correlations of an LQTS type 2 mutation identified in the homozygous index patient from a consanguineous Turkish family after his brother died suddenly during febrile illness.

METHODS AND RESULTS

Clinical work-up, DNA sequencing, mutagenesis, cell culture, patch-clamp, in silico mathematical modelling, protein biochemistry, confocal microscopy were performed. Genetic analysis revealed a homozygous C-terminal KCNH2 mutation (p.R835Q) in the index patient (QTc ∼506 ms with notched T waves). Parents were I° cousins - both heterozygous for the mutation and clinically unremarkable (QTc ∼447 ms, father and ∼396 ms, mother). Heterologous expression of KCNH2-R835Q showed mildly reduced current amplitudes. Biophysical properties of ionic currents were also only nominally changed with slight acceleration of deactivation and more negative V50 in R835Q-currents. Protein biochemistry and confocal microscopy revealed similar expression patterns and trafficking of WT and R835Q, even at elevated temperature. In silico analysis demonstrated mildly prolonged ventricular action potential duration (APD) compared to WT at a cycle length of 1000 ms. At a cycle length of 350 ms M-cell APD remained stable in WT, but displayed APD alternans in R835Q.

CONCLUSION

Kv11.1 channels affected by the C-terminal R835Q mutation display mildly modified biophysical properties, but leads to M-cell APD alternans with elevated heart rate and could precipitate SCD under specific clinical circumstances associated with high heart rates.

Long QT syndrome in South Africa: the results of comprehensive genetic screening

Congenital long QT syndrome (cLQTS) is a genetic disorder predisposing to ventricular arrhythmia, syncope and sudden death. Over 700 different cLQTS-causing mutations in 13 genes are known. The genetic spectrum of LQTS in 44 South African cLQTS patients (23 known to carry the South African founder mutation p.A341V in KCNQ1) was established by screening for mutations in the coding regions of KCNQ1, KCNH2, KCNE1, KCNE2 and SCN5A, the most frequently implicated cLQTS-causing genes (five-gene screening). Fourteen disease-causing mutations were identified, eight (including the founder mutation) in KCNQ1, five in KCNH2 and one in KCNE1. Two mutations were novel. Two double heterozygotes were found among the 23 families (8.5%) carrying the founder mutation. In conclusion, cLQTS in South Africa reflects both a strong founder effect and a genetic spectrum similar to that seen in other populations. Consequently, five-gene screening should be offered as a standard screening option, as is the case internationally. This will disclose compound and double heterozygotes. Fivegene screening will most likely be even more informative in other South African sub-populations with a greater genetic diversity.

A590T mutation in KCNQ1 C-terminal helix D decreases IKs channel trafficking and function but not Yotiao interaction

KCNQ1 encodes the α subunit of the voltage-gated channel that mediates the cardiac slow delayed rectifier K(+) current (IKs). Here, we report a KCNQ1 allele encoding an A590T mutation [KCNQ1(A590T)] found in a 39-year-old female with a mild QT prolongation. A590 is located in the C-terminal α helical region of KCNQ1 that mediates subunit tetramerization, membrane trafficking, and interaction with Yotiao. This interaction is known to be required for the proper modulation of IKs by cAMP. Since previous studies reported that mutations in the vicinity of A590 impair IKs channel surface expression and function, we examined whether and how the A590T mutation affects the IKs channel. Electrophysiological measurements in HEK-293T cells showed that the A590T mutation caused a reduction in IKs density and a right-shift of the current-voltage relation of channel activation. Immunocytochemical and immunoblot analyses showed the reduced cell surface expression of KCNQ1(A590T) subunit and its rescue by coexpression of the wild-type KCNQ1 [KCNQ1(WT)] subunit. Moreover, KCNQ1(A590T) subunit interacted with Yotiao and had a cAMP-responsiveness comparable to that of KCNQ1(WT) subunit. These findings indicate that the A590 of KCNQ1 subunit plays important roles in the maintenance of channel surface expression and function via a novel mechanism independent of interaction with Yotiao.

Block copolymer/DNA vaccination induces a strong allergen-specific local response in a mouse model of house dust mite asthma

Background

Allergic asthma is caused by abnormal immunoreactivity against allergens such as house dust mites among which Dermatophagoides farinae (Der f) is a common species. Currently, immunotherapy is based on allergen administration, which has variable effect from patient to patient and may cause serious side effects, principally the sustained risk of anaphylaxis. DNA vaccination is a promising approach by triggering a specific immune response with reduced allergenicity.

Objective

The aim of the study is to evaluate the effects of DNA immunization with Der f1 allergen specific DNA on allergic sensitization, inflammation and respiratory function in mice.

Methods

Mice were vaccinated 28 and 7 days before allergen exposure with a Der f1-encoding plasmid formulated with a block copolymer. Asthma was induced by skin sensitization followed by intra-nasal challenges with Der f extract. Total lung, broncho-alveolar lavage (BAL) and spleen cells were analyzed by flow cytometry for their surface antigen and cytokine expression. Splenocytes and lung cell IFN-γ production by CD8+ cells in response to Der f CMH1-restricted peptides was assessed by ELISPOT. IgE, IgG1 and IgG2a were measured in serum by ELISA. Specific bronchial hyperresponsiveness was assessed by direct resistance measurements.

Results

Compared to animals vaccinated with an irrelevant plasmid, pVAX-Der f1 vaccination induced an increase of B cells in BAL, and an elevation of IL-10 and IFN-γ but also of IL-4, IL-13 and IL-17 producing CD4+ lymphocytes in lungs and of IL-4 and IL-5 in spleen. In response to CD8-restricted peptides an increase of IFN-γ was observed among lung cells. IgG2a levels non-specifically increased following block copolymer/DNA vaccination although IgE, IgG1 levels and airways resistances were not impacted.

Conclusions & Clinical Relevance

DNA vaccination using a plasmid coding for Der f1 formulated with the block copolymer 704 induces a specific immune response in the model of asthma used herein.

KCNE genetics and pharmacogenomics in cardiac arrhythmias: much ado about nothing?

Voltage-gated ion channels respond to changes in membrane potential with conformational shifts that either facilitate or stem the movement of charged ions across the cell membrane. This controlled movement of ions is particularly important for the action potentials of excitable cells such as cardiac myocytes and therefore essential for timely beating of the heart. Inherited mutations in ion channel genes and in the genes encoding proteins that regulate them can cause lethal cardiac arrhythmias either by direct channel disruption or by altering interactions with therapeutic drugs, the best-understood example of both these scenarios being long QT syndrome (LQTS). Unsurprisingly, mutations in the genes encoding ion channel pore-forming α subunits underlie the large majority (~90%) of identified cases of inherited LQTS. Given that inherited LQTS is comparatively rare in itself (~0.04% of the US population), is pursuing study of the remaining known and unknown LQTS-associated genes subject to the law of diminishing returns? Here, with a particular focus on the KCNE family of single transmembrane domain K(+) channel ancillary subunits, the significance to cardiac pharmacogenetics of ion channel regulatory subunits is discussed.

Interactions between hERG and KCNQ1 α-subunits are mediated by their COOH termini and modulated by cAMP

KCNQ1 and hERG encode the voltage-gated potassium channel α-subunits of the cardiac repolarizing currents I(Ks) and I(Kr), respectively. These currents function in vivo with some redundancy to maintain appropriate action potential durations (APDs), and loss-of-function mutations in these channels manifest clinically as long QT syndrome, characterized by the prolongation of the QT interval, polymorphic ventricular tachycardia, and sudden cardiac death. Previous cellular electrophysiology experiments in transgenic rabbit cardiomyocytes and heterologous cell lines demonstrated functional downregulation of complementary repolarizing currents. Biochemical assays indicated direct, protein-protein interactions between KCNQ1 and hERG may underlie the interplay between I(Ks) and I(Kr). Our objective was to investigate hERG-KCNQ1 interactions in the intact cellular environment primarily through acceptor photobleach FRET (apFRET) experiments. We quantitatively assessed the extent of interactions based on fluorophore location and the potential regulation of interactions by physiologically relevant signals. apFRET experiments established specific hERG-KCNQ1 associations in both heterologous and primary cardiomyocytes. The largest FRET efficiency (E(f); 12.0 ± 5.2%) was seen between ion channels with GFP variants fused to the COOH termini. Acute treatment with forskolin + IBMX or a membrane-permeable cAMP analog significantly and specifically reduced the extent of hERG-KCNQ1 interactions (by 41 and 38%, respectively). Our results demonstrate direct interactions between KCNQ1 and hERG occur in both intact heterologous cells and primary cardiomyocytes and are mediated by their COOH termini. Furthermore, this interplay between channel proteins is regulated by intracellular cAMP.

Repolarization reserve determines drug responses in human pluripotent stem cell derived cardiomyocytes

Unexpected induction of arrhythmias in the heart is still one of the major risks of new drugs despite recent improvements in cardiac safety assays. Here we address this in a novel emerging assay system. Eleven reference compounds were administrated to spontaneously beating clusters of cardiomyocytes from human pluripotent stem cells (hPSC-CM) and the responses determined using multi-electrode arrays. Nine showed clear dose-dependence effects on field potential (FP) duration. Of these, the Ca(2+) channel blockers caused profound shortening of action potentials, whereas the classical hERG blockers, like dofetilide and d,l-sotalol, induced prolongation, as expected. Unexpectedly, two potent blockers of the slow component of the delayed rectifier potassium current (I(Ks)), HMR1556 and JNJ303, had only minor effects on the extracellular FP of wild-type hPSC-CM despite evidence of functional I(Ks) channels. These compounds were therefore re-evaluated under conditions that mimicked reduced "repolarization reserve," a parameter reflecting the capacity of cardiomyocytes to repolarize and a strong risk factor for the development of ventricular arrhythmias. Strikingly, in both pharmacological and genetic models of diminished repolarization reserve, HMR1556 and JNJ03 strongly increased the FP duration. These profound effects indicate that I(Ks) plays an important role in limiting action potential prolongation when repolarization reserve is attenuated. The findings have important clinical implications and indicate that enhanced sensitization to repolarization-prolonging compounds through pharmacotherapy or genetic predisposition should be taken into account when assessing drug safety.

Modulation of hERG potassium channel gating normalizes action potential duration prolonged by dysfunctional KCNQ1 potassium channel

Long QT syndrome (LQTS) is a genetic disease characterized by a prolonged QT interval in an electrocardiogram (ECG), leading to higher risk of sudden cardiac death. Among the 12 identified genes causal to heritable LQTS, ∼90% of affected individuals harbor mutations in either KCNQ1 or human ether-a-go-go related genes (hERG), which encode two repolarizing potassium currents known as I(Ks) and I(Kr). The ability to quantitatively assess contributions of different current components is therefore important for investigating disease phenotypes and testing effectiveness of pharmacological modulation. Here we report a quantitative analysis by simulating cardiac action potentials of cultured human cardiomyocytes to match the experimental waveforms of both healthy control and LQT syndrome type 1 (LQT1) action potentials. The quantitative evaluation suggests that elevation of I(Kr) by reducing voltage sensitivity of inactivation, not via slowing of deactivation, could more effectively restore normal QT duration if I(Ks) is reduced. Using a unique specific chemical activator for I(Kr) that has a primary effect of causing a right shift of V(1/2) for inactivation, we then examined the duration changes of autonomous action potentials from differentiated human cardiomyocytes. Indeed, this activator causes dose-dependent shortening of the action potential durations and is able to normalize action potentials of cells of patients with LQT1. In contrast, an I(Kr) chemical activator of primary effects in slowing channel deactivation was not effective in modulating action potential durations. Our studies provide both the theoretical basis and experimental support for compensatory normalization of action potential duration by a pharmacological agent.

DNA/amphiphilic block copolymer nanospheres reduce asthmatic response in a mouse model of allergic asthma

Asthma is a chronic, inflammatory, respiratory disease caused by an abnormal reactivity against allergens. The most promising treatments for asthma are based on specific immunotherapies, but they lack efficiency and can induce deleterious side effects. Among new modalities of immunotherapy currently in development, DNA vaccination presents a promising approach, as it enables targeted immunotherapy in association with reduced allergenicity. We have developed an innovative, DNA-based vaccine against Dermatophagoides farinae 1 allergen (Der f 1), one of the allergens most commonly encountered by asthma patients in Europe. Intramuscular administration of a Der f 1-encoding plasmid formulated with the block copolymer 704 in healthy mice induced a strong humoral and cellular response with a pro-helper T cell type 1 bias. Administration of the same formulation in asthmatic mice, according to an early vaccination protocol, led to a reduction of airway hyperresponsiveness and a significant decrease in the level of inflammatory cytokines in the bronchoalveolar lavage of Der f 1-vaccinated mice.

Interaction between the cardiac rapidly (IKr) and slowly (IKs) activating delayed rectifier potassium channels revealed by low K+-induced hERG endocytic degradation

Cardiac repolarization is controlled by the rapidly (I(Kr)) and slowly (I(Ks)) activating delayed rectifier potassium channels. The human ether-a-go-go-related gene (hERG) encodes I(Kr), whereas KCNQ1 and KCNE1 together encode I(Ks). Decreases in I(Kr) or I(Ks) cause long QT syndrome (LQTS), a cardiac disorder with a high risk of sudden death. A reduction in extracellular K(+) concentration ([K(+)](o)) induces LQTS and selectively causes endocytic degradation of mature hERG channels from the plasma membrane. In the present study, we investigated whether I(Ks) compensates for the reduced I(Kr) under low K(+) conditions. Our data show that when hERG and KCNQ1 were expressed separately in human embryonic kidney (HEK) cells, exposure to 0 mM K(+) for 6 h completely eliminated the mature hERG channel expression but had no effect on KCNQ1. When hERG and KCNQ1 were co-expressed, KCNQ1 significantly delayed 0 mM K(+)-induced hERG reduction. Also, hERG degradation led to a significant reduction in KCNQ1 in 0 mM K(+) conditions. An interaction between hERG and KCNQ1 was identified in hERG+KCNQ1-expressing HEK cells. Furthermore, KCNQ1 preferentially co-immunoprecipitated with mature hERG channels that are localized in the plasma membrane. Biophysical and pharmacological analyses indicate that although hERG and KCNQ1 closely interact with each other, they form distinct hERG and KCNQ1 channels. These data extend our understanding of delayed rectifier potassium channel trafficking and regulation, as well as the pathology of LQTS.

Phenotypic Manifestations of Mutations in Genes Encoding Subunits of Cardiac Potassium Channels

Since 1995, when a potassium channel gene, hERG (human ether-à-go-go-related gene), now referred to as KCNH2 , encoding the rapid component of cardiac delayed rectifier potassium channels was identified as being responsible for type 2 congenital long-QT syndrome, a number of potassium channel genes have been shown to cause different types of inherited cardiac arrhythmia syndromes. These include congenital long-QT syndrome, short-QT syndrome, Brugada syndrome, early repolarization syndrome, and familial atrial fibrillation. Genotype-phenotype correlations have been investigated in some inherited arrhythmia syndromes, and as a result, gene-specific risk stratification and gene-specific therapy and management have become available, particularly for patients with congenital long-QT syndrome. In this review article, the molecular structure and function of potassium channels, the clinical phenotype due to potassium channel gene mutations, including genotype-phenotype correlations, and the diverse mechanisms underlying the potassium channel gene–related diseases will be discussed.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

Trafficking-competent KCNQ1 variably influences the function of HERG long QT alleles

BACKGROUND

Mutations in the KCNQ1 and human ether-a-go-go-related gene (HERG) genes cause the long QT syndromes, LQTS1 and LQTS2, due to reductions in the cardiac repolarizing I(Ks) and I(Kr) currents, respectively. It was previously reported that KCNQ1 coexpression modulates HERG function by enhancing membrane expression of HERG, and that the 2 proteins coimmunoprecipitate, and colocalize in myocytes. In vivo studies in genetically modified rabbits also support a HERG-KCNQ1 interaction.

OBJECTIVE

We sought to determine whether KCNQ1 influences the current characteristics of HERG genetic variants.

METHODS

This study used expression of HERG and KCNQ1 wild-type (WT) and mutant channels in heterologous systems, combined with whole-cell patch clamp analysis and biochemistry.

RESULTS

Supporting the notion that KCNQ1 needs to be trafficking competent to influence HERG function, we found that although the tail current density of HERG expressed in Chinese Hamster Ovary (CHO) cells was approximately doubled by WT KCNQ1 coexpression, it was not altered in the presence of the trafficking-defective KCNQ1(T587M) variant. Activation and deactivation kinetics of HERG variants were not altered. The HERG(M124T) variant, previously shown to be mildly impaired functionally, was restored to WT levels by KCNQ1-WT but not KCNQ1(T587M) coexpression. The tail current densities of the severely trafficking-impaired HERG(G601S) and HERG(F805C) variants were only slightly improved by KCNQ1 coexpression. The trafficking competent but incompletely processed HERG(N598Q), and a mutation in the selectivity filter, HERG(G628S), were not improved by KCNQ1 coexpression.

CONCLUSION

These findings suggest a functional codependence of HERG on KCNQ1 during channel biogenesis. Moreover, KCNQ1 variably modulates LQTS2 mutations with distinct underlying pathologies.

Delayed rectifier K(+) currents and cardiac repolarization

The two components of the cardiac delayed rectifier current have been the subject of numerous studies since firstly described. This current controls the action potential duration and is highly regulated. After identification of the channel subunits underlying IKs, KCNQ1 associated with KCNE1, and IKr, HERG, their involvement in human cardiac channelopathies have provided various models allowing the description of the molecular mechanisms of the KCNQ1 and HERG channels trafficking, activity and regulation. More recently, studies have been focusing on the unveiling of different partners of the pore-forming proteins that contribute to their maturation, trafficking, activity and/or degradation, on one side, and on their respective expression in the heterogeneous cardiac tissue, on the other side. The aim of this review is to report and discuss the major works on IKs and IKr and the most recent ones that help to understand the precise function of these currents in the heart.