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The KCNQ1 gene encodes the α-subunit of the cardiac voltage-gated potassium (Kv) channel KCNQ1, also denoted as Kv7.1 or KvLQT1. The channel assembles with the ß-subunit KCNE1, also known as minK, to generate the slowly activating cardiac delayed rectifier current IKs, a key regulator of the heart rate dependent adaptation of the cardiac action potential duration (APD). Loss-of-function variants in KCNQ1 cause the congenital Long QT1 (LQT1) syndrome, characterized by delayed cardiac repolarization and a QT interval prolongation in the surface electrocardiogram (ECG). Autosomal dominant loss-of-function variants in KCNQ1 result in the LQT syndrome called Romano-Ward syndrome (RWS), while autosomal recessive variants affecting function, lead to Jervell and Lange-Nielsen syndrome (JLNS), associated with deafness. The aim of this study was the characterization of novel KCNQ1 variants identified in patients with RWS to widen the spectrum of known LQT1 variants, and improve the interpretation of the clinical relevance of variants in the KCNQ1 gene. We functionally characterized nine human KCNQ1 variants using the voltage-clamp technique in Xenopus laevis oocytes, from which we report seven novel variants. The functional data was taken as input to model surface ECGs, to subsequently compare the functional changes with the clinically observed QTc times, allowing a further interpretation of the severity of the different LQTS variants. We found that the electrophysiological properties of the variants correlate with the severity of the clinically diagnosed phenotype in most cases, however, not in all. Electrophysiological studies combined with in silico modelling approaches are valuable components for the interpretation of the pathogenicity of KCNQ1 variants, but assessing the clinical severity demands the consideration of other factors that are included, for example in the Schwartz score.

Aims Mutation type, location, dominant-negative I Ks reduction, and possibly loss of cyclic adenosine monophosphate (cAMP)-dependent I Ks stimulation via protein kinase A (PKA) influence the clinical severity of long QT syndrome type 1 (LQT1). Given the malignancy of KCNQ1-p.A341V, we assessed whether mutations neighbouring p.A341V in the S6 channel segment could also increase arrhythmic risk. Methods and results Clinical and genetic data were obtained from 1316 LQT1 patients [450 families, 166 unique KCNQ1 mutations, including 277 p.A341V-positive subjects, 139 patients with p.A341-neighbouring mutations (91 missense, 48 non-missense), and 900 other LQT1 subjects]. A first cardiac event represented the primary endpoint. S6 segment missense variant characteristics, particularly cAMP stimulation responses, were analysed by cellular electrophysiology. p.A341-neighbouring mutation carriers had a QTc shorter than p.A341V carriers (477 ± 33 vs. 490 ± 44 ms) but longer than the remaining LQT1 patient population (467 ± 41 ms) (P Conclusion KCNQ1 S6 segment mutations surrounding p.A341 increase arrhythmic risk. p.A341V-specific loss of PKA-dependent I Ks enhancement correlates with its phenotypic severity. Cellular studies providing further insights into I Ks-channel regulation and knowledge of structure-function relationships could improve risk stratification. These findings impact on clinical management.

There are limited data on the gene expression profiles of ion channels in the sinoatrial node (SAN) of dogs and monkeys. In this study, the messenger RNA (mRNA) expression profiles of various ion channels in the SAN of naive dogs and monkeys were examined using RNAscope®in situ hybridization and compared with those in the surrounding right atrium (RA) of each species. Regional-specific Cav1.3 and HCN4 expression was observed in the SAN of dogs and monkeys. Additionally, HCN1 in dogs was only expressed in the SAN. The expression profiles of Cav3.1 and Cav3.2 in the SAN and RA were completely different between dogs and monkeys. Dog hearts only expressed Cav3.2; however, Cav3.1 was detected only in monkeys, and the expression score in the SAN was slightly higher than that in the RA. Although Kir3.1 and NCX1 in dogs were equally expressed in both the SAN and RA, the expression scores of these genes in the SAN of monkeys were slightly higher than those in the RA. The Kir3.4 expression score in the SAN of dogs and monkeys was also slightly higher than that in the RA. The mRNA expression scores of Kv11.1/ERG and KvLQT1 were equally observed in both the SAN and RA of dogs and monkeys. HCN2 was not detected in dogs and monkeys. In summary, we used RNAscope to demonstrate the SAN-specific gene expression patterns of ion channels, which may be useful in explaining the effect of pacemaking and/or hemodynamic effects in nonclinical studies.

Hypertrophic cardiomyopathy (HCM) and Long QT Syndrome (LQTS) are inherited diseases characterized by a wide genetic heterogeneity. Based on the separate incidence of these pathologies and on the absence of linkage, the occurrence of both diseases in the same individual has an incidence of about 1/250000. We describe a rare case report of a 24 years-old patient with maternal familiarity for type 1 LQTS (mother carrier of KCNQ1 c.1781G>A) and paternal familiarity for HCM (father carrier of TNNI3 c.592C>G mutation) who inherited both gene mutations and was diagnosed with HCM and LQTS later in adolescence, after clinical and genetic evaluations.

Aims In long QT syndrome (LQTS) patients, modifier genes modulate the arrhythmic risk associated with a disease-causing mutation. Their recognition can improve risk stratification and clinical management, but their discovery represents a challenge. We tested whether a cellular-driven approach could help to identify new modifier genes and especially their mechanism of action. Methods and results We generated human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) from two patients carrying the same KCNQ1-Y111C mutation, but presenting opposite clinical phenotypes. We showed that the phenotype of the iPSC-CMs derived from the symptomatic patient is due to impaired trafficking and increased degradation of the mutant KCNQ1 and wild-type human ether-a-go-go-related gene. In the iPSC-CMs of the asymptomatic (AS) patient, the activity of an E3 ubiquitin-protein ligase (Nedd4L) involved in channel protein degradation was reduced and resulted in a decreased arrhythmogenic substrate. Two single-nucleotide variants (SNVs) on the Myotubularin-related protein 4 (MTMR4) gene, an interactor of Nedd4L, were identified by whole-exome sequencing as potential contributors to decreased Nedd4L activity. Correction of these SNVs by CRISPR/Cas9 unmasked the LQTS phenotype in AS cells. Importantly, the same MTMR4 variants were present in 77% of AS Y111C mutation carriers of a separate cohort. Thus, genetically mediated interference with Nedd4L activation seems associated with protective effects. Conclusion Our finding represents the first demonstration of the cellular mechanism of action of a protective modifier gene in LQTS. It provides new clues for advanced risk stratification and paves the way for the design of new therapies targeting this specific molecular pathway.

KCNQ1 is a pore-forming K+ channel subunit critically important to cardiac repolarization at high heart rates. (2R)-N-[4-(4-methoxyphenyl)-2-thiazolyl]-1-[(4-methylphenyl)sulfonyl]-2 piperidinecarboxamide, or ML277, is an activator of this channel that rescues function of pathophysiologically important mutant channel complexes in human induced pluripotent stem cell–derived cardiomyocytes, and that therefore may have therapeutic potential. Here we extend our understanding of ML277 actions through cell-attached single-channel recordings of wild-type and mutant KCNQ1 channels with voltage sensor domains fixed in resting, intermediate, and activated states. ML277 has profound effects on KCNQ1 single-channel kinetics, eliminating the flickering nature of the openings, converting them to discrete opening bursts, and increasing their amplitudes approximately threefold. KCNQ1 single-channel behavior after ML277 treatment most resembles IO state-locked channels (E160R/R231E) rather than AO state channels (E160R/R237E), suggesting that at least during ML277 treatment, KCNQ1 does not frequently visit the AO state. Introduction of KCNE1 subunits reduces the effectiveness of ML277, but some enhancement of single-channel openings is still observed.

Introduction: Human induced pluripotent stem cells (hiPSCs) harboring cardiac myosin heavy chain 6 promoter can differentiate into functional cardiomyocytes called “iCell cardiomyocytes” under blasticidin treatment condition. While iCell cardiomyocytes are expected to be used for predicting cardiotoxicity of drugs, their responses to antiarrhythmic agents remain to be elucidated. We first examined electrophysiological properties of iCell cardiomyocytes and mRNA levels of ion channels and Ca handling proteins, and then evaluated effects of class I antiarrhythmic agents on their Na+ and Ca2+ currents. Methods: iCell cardiomyocytes were cultured for 8–14 days (38–44 days after inducing their differentiation), according to the manufacturer's protocol. We determined their action potentials (APs) and sarcolemmal ionic currents using whole-cell patch clamp techniques, and also mRNA levels of ion channels and Ca handling proteins by RT-PCR. Effects of three class I antiarrhythmic agents, pirmenol, pilsicainide and mexiletine, on Na+ channel current (INa) and L-type Ca2+ channel current (ICaL) were evaluated by the whole-cell patch clamp. Results: iCell cardiomyocytes revealed sinoatrial node-type (18%), atrial-type (18%) and ventricular-type (64%) spontaneous APs. The maximum peak amplitudes of INa, ICaL, and rapidly-activating delayed-rectifier K+ channel current were −62.7 ± 13.7, −8.1 ± 0.7, and 3.0 ± 1.0 pA/pF, respectively. The hyperpolarization-activated cation channel and inward-rectifier K+ channel currents were observed, whereas the T-type Ca2+ channel or slowly-activating delayed-rectifier K+ channel current was not detectable. mRNAs of Nav1.5, Cav1.2, Kir2.1, HCN4, KvLQT1, hERG and SERCA2 were detected, while that of HCN1, minK or MiRP was not. The class Ia antiarrhythmic agent pirmenol and class Ic agent pilsicainide blocked INa in a concentration-dependent manner with IC50 of 0.87 ± 0.37 and 0.88 ± 0.16 μM, respectively; the class Ib agent mexiletine revealed weak INa block with a higher IC50 of 30.0 ± 3.0 μM. Pirmenol, pilsicainide and mexiletine blocked ICaL with IC50 of 2.00 ± 0.39, 7.7 ± 2.5 and 5.0 ± 0.1 μM, respectively. Conclusions: In iCell cardiomyocytes, INa was blocked by the class Ia and Ic antiarrhythmic agents and ICaL was blocked by all the class I agents within the ranges of clinical concentrations, suggesting their cardiotoxicity. Keywords: iCell cardiomyocytes, class I antiarrhythmic agents, Na+ channel current, L-type Ca2+ channel current

KCNQ1 (Kv7.1 or KvLQT1) is a potassium ion channel protein found in the heart, ear, and other tissues. In complex with the KCNE1 accessory protein, it plays a role during the repolarization phase of the cardiac action potential. Mutations in the channel have been associated with several diseases, including congenital deafness and long QT syndrome. Nuclear magnetic resonance (NMR) structural studies in detergent micelles and a cryo-electron microscopy structure of KCNQ1 from Xenopus laevis have shown that the voltage sensor domain (Q1-VSD) of the channel has four transmembrane helices, S1–S4, being overall structurally similar with other VSDs. In this study, we describe a reliable method for the reconstitution of Q1-VSD into (POPC/POPG) lipid bilayer vesicles. Site-directed spin labeling electron paramagnetic resonance spectroscopy was used to probe the structural dynamics and topology of several residues of Q1-VSD in POPC/POPG lipid bilayer vesicles. Several mutants were probed to determine their location and corresponding immersion depth (in angstroms) with respect to the membrane. The dynamics of the bilayer vesicles upon incorporation of Q1-VSD were studied using (31)P solid-state NMR spectroscopy by varying the protein:lipid molar ratios confirming the interaction of the protein with the bilayer vesicles. Circular dichroism spectroscopic data showed that the α-helical content of Q1-VSD is higher for the protein reconstituted in vesicles than in previous studies using DPC detergent micelles. This study provides insight into the structural topology and dynamics of Q1-VSD reconstituted in a lipid bilayer environment, forming the basis for more advanced structural and functional studies.

Funding Acknowledgements Type of funding sources: None. Background Short QT syndrome (SQTS) is a rare, autosomal dominant disease causing sudden cardiac death (SCD). Current guidelines recommend genetic testing. Associated variants in KCNQ1, KCNH2, KCNJ2 and SLC4A3 genes have been reported. Purpose We report a family with a variant in the SLC4A3 gene with several presentations of SCD and high clinical penetrance of SQTS. Methods We performed a post-mortem genetic testing in the index patient in whom prior ECG was available. Subsequently, clinical and electrophysiological work-up and cascade screening (CS) of the detected suspected variant was carried out in available relatives. Results The index patient had suffered a SCD at the age of 17 (figure, upper panel, arrow). A previously registered ECG showed a shortened QTc of 340ms (figure, lower panel). Autopsy revealed no structural heart disease. Post-mortem genetic testing revealed variants in the LDB3, MYH7 and a novel heterozygous missense variant, p.(Ser1039Arg) also in the SLC4A3 gene. Although predictive bioinformatic algorithms (AlignGVGD, SIFT, MutationTaster, Polyphen2) showed conflicting classifications, family history was notable for SCD without post-mortem genetic work-up in three second degree relatives (figure, upper panel, patients 207, 208 and 305, age of death 33, 25 and 33 years respectively). CS was performed in first and second degree relatives of the index patient and was highly suggestive for disease association of the variant in the SLC4A3 gene with co-segregation in all clinically affected family members. Only one patient with the variant had a normal QTc (figure, upper panel, patient 202) of 407ms, however this patient was on regular QT-prolonging medication (risperidone and loperamide). Conclusion Genetic testing revealed a novel in the SLC4A3 gene, which was recently implicated in the pathogenesis of the SQTS. Although predictive bioinformatic algorithms yielded conflicting results, CS of family members suggests a likely pathogenicity (class IV) of the variant. Further CS or functional tests are necessary to establish causality. Abstract Figure. ECG of index patient and family tree

Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministry of Health of the Czech Republic Introduction The complex diagnostic work up in SCA survivors often does not yield a concrete cardiological diagnosis. Moreover, there is conflicting evidence whether genetic testing could support or guide clinical diagnostics. Purpose To assess the molecular architecture of idiopathic ventricular fibrillation in cases without apparent evidence of specific structural or arrhythmic cardiac disease at initial diagnostic work up in a representative Czech cohort. Patients and Methods Between 2013 - 2020 we have ascertained 100 SCA survivors (54 M / 46 F; age range at cardiac arrest 5-69 years). Genetic counselling was followed by massively parallel DNA sequencing using custom-made panels comprising 100 cardiac conditions-related genes. Subsequently, thorough cardiological screening examinations in first degree relatives were carried out. Presence of pathogenic variants was validated by Sanger DNA sequencing and through family segregation analyses. Results Highly likely or certain molecular aetiology (i.e. based on the presence of Class 4 or 5 variants) was disclosed in 20/100 (20%) in PKP2 (3x), SCN5A (4x), RYR2 (3x), TTN (2x), PLN, FLNC, PRKAG2, KCNH2, KCNQ1, SLC4A, TNNT2, and DSP. Interestingly, the KCNE1 p.Asp85Asn (LQT 5 lite) variant, was detected in further 3/100 cases, representing a recognized risk factor for ventricular arrhythmias. Conclusions Genetic testing facilitates stratification of the cause of arrhythmia in a substantial portion of SCA survivors. The utility of positive outcomes of genetic testing was substantiated in 10/20 gene positive patients, where the genetic stratification led to diagnosis of concealed arrhythmogenic cardiomyopathy, whose extent of morphological changes was under the diagnostic sensibility of imaging modalities or ECG. Our results enable individualized care in SCA survivors and assure targeted preventive approaches in their relatives.

Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): Ministry of Education, Youth and Sports of the Czech Republic Introduction Loss-of-function variants of the KCNQ1 gene are associated with life-threatening arrhythmogenic long QT syndrome type 1 (LQT1). This gene encodes structure of the slow delayed rectifier potassium channel (IKs). Some functional characteristics of the C-terminal KCNQ1 variant c.1686G > C (p.R562S) have been recently described [1]. However, accumulation of the current under beta-adrenergic stimulation, essential for shortening the action potential duration during exercise, have not been tested. Purpose The aim of this study was to analyse clinical and genetic characteristics of the R562S variant in our patients and to investigate impact of the variant on IKs channel function with a special focus on reactivity of the channels on beta-adrenergic stimulation. Methods The clinical diagnosis was established according to ESC Guidelines including QTc analysis at rest and after exercise. The molecular genetics diagnostics followed according to current practices (the massive parallel sequencing since 2016). The biophysical analysis was performed on Chinese hamster ovary cells (CHO) by the whole cell patch clamp technique at 37 °C. CHO cells were transiently transfected with wild type (WT) and/or R562S human IKs channels (KCNQ1/KCNE1/Yotiao, 1:2:4). Cyclic adenosine monophosphate (cAMP, 200 µM) and okadaic acid (OA, 0.2 µM) in the pipette solution were used to simulate the beta-adrenergic stimulation. In the confocal microscopy experiments, expression of Yotiao was omitted and GFP-tagged KCNQ1 was used. Results The variant R562S-Kv7.1 has been identified in 7 heterozygous carriers from 3 putatively unrelated families in the Czech Republic. The genotype was associated with long QT syndrome phenotype (prolonged QTc, symptoms including syncopes and aborted cardiac arrest) in some of the carriers. The basic functional analysis proved that both homozygous and heterozygous R562S channels are expressed on the cell membrane (confocal microscopy) and carry IKs (whole cell patch clamp) which agrees with the recently published data on this variant. Importantly, reactivity on beta-adrenergic stimulation was absent in both homozygous and heterozygous R562S channels (n = 14 and 8, respectively), but present in the wild-type channels (increase by 51.4 ± 11.1 % at 120-s cAMP/OA diffusion; n = 12). Conclusions The R562S-Kv7.1 variant may be a founder LQT1 variant in our region which will be further investigated in the future. This variant impairs response of IKs channel to beta-adrenergic stimulation. Absence of this essential regulation may considerably aggravate the channel dysfunction and, thus, may result in life-threatening arrhythmias in R562S carriers during exercise.

Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): British Heart Foundation Phosphatidylinositol-4,5-biphosphate (PIP2) is implicated in the regulation and modulation of the IKS channel. The channel is formed at the plasma membrane by the co-assembly of KCNQ1 and KCNE1. Patients with Congenital Long QT 1(LQT1) syndrome are predisposed to Polymorphic VT due to mutations in KCNQ1, leading to impaired channel activity. We initially transfect Human Embryonic Kidney (HEK) cells with a mammalian vector expressing KCNQ1 gene tagged with green fluorescent protein, along with KCNE1 to form the wild type (WT) IKS channel. The cells were also transfected with a constitutively active PI(4)P 5-kinase(PIP5K), which converts the phospholipid Phosphatidylinositol-4-phosphate to PIP2, therefore increasing endogenous levels of PIP2. To ensure the enzyme remains localised at the plasma membrane we attached it to CFP-FKBP and we co-transfected the cells with Lyn11-FRB construct that tethers to the plasma membrane. When these cells were perfused with Rapamycin it induced chemical dimerization of CF-PIP5K to lyn11. We utilised an inactive PIP5K as a control. Mutants were created with a site directed mutagenesis kit. In the presence CF-PIP5K, whole cell voltage clamp recordings demonstrated a 2.5 fold statistically significant increase in WT channel activity (at +80mV,p We substituted serine with alanine at site 27 and 92(S27A/S92A) to generate a mutant known to disrupt cAMP mediated upregulation, there was a statistical 3.3 fold (80 + mV) increase in current density when co-expressed with CF-PIP5K. We then substituted serine with aspartic acid (S27D/S92D) to create a Phosphomimetic mutation, this mutant reproduces the effects of sympathetic mediated augmentation of IKS channel. In the presence of enhanced PIP2 levels, the S27D/S92D failed to demonstrate a statistical increase in current, implying the channel is at its maximum activity and hence we failed to observe any further modulation. We then proceeded to interrogate how PIP2 interacts with sympathetic signalling system. Pseudojanin(PJ) causes depletion of PIP2 hence perturbing channel activity. When PJ was expressed with KCNQ1 and KCNE1 we observed an 80% reduction in channel activity at +80mV(P Here we illustrate how increasing PIP2 levels can revive IKS channel activity in mutant genotype, therefore supporting evidence of its capabilities as a potential therapeutic tool. This modulation is independent of the PKA-cAMP system. Abstract Figure. Current Increment

Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI Background Advancing age has been identified as a strong predictor of post-coronary artery bypass grafting (CABG) atrial fibrillation (AF). However, the exact substrate underlying this increased aging-related propensity to post-CABG AF remains insufficiently understood. Purpose We aimed to assess whether a preexisting electrical, structural, autonomic, and/or molecular remodeling could explain the increased risk of elderly individuals for post-CABG AF occurrence. Methods Thirty-one consecutive patients free of AF history scheduled for an elective CABG procedure were prospectively evaluated. All patients underwent continuous ECG monitoring prior to and following the CABG procedure, up to the day of discharge. The occurrence of post-CABG AF was recorded. The right atrial appendage, normally considered "surgical waste", was collected and divided into 3 samples that were used for single cell action potential recordings using the microelectrode technique, for histological, and mRNA analysis. Electrical remodeling preexistent to CABG was assessed by P-wave analysis (i.e., amplitude, duration, fragmentation) using 12-lead surface ECG and Lewis leads recordings and atrial action potential analysis (i.e., amplitude, duration). Atrial structural remodeling was evaluated using echocardiographic (i.e., left atrial [LA] area and antero-posterior diameter) and histological (i.e., fibrosis and fat tissue infiltration) parameters. Autonomic remodeling was evaluated using heart rate variability analysis based on the pre-CABG Holter ECG recordings. Molecular remodeling was assessed by mRNA analysis of several genes previously incriminated in AF occurrence (i.e., CACNA1C, GJA5, KCNE2, KCNJ2, KCNQ1, and SCN5A). Results Post-CABG AF occurred in 11 (35.4%) patients. Patients with post-CABG AF were significantly older than their non-arrhythmic counterparts (65.2 ± 5.8 years vs. 58.0 ± 10.4 years; p = 0.04). Age above 59 years predicted post-CABG AF with 81.8% sensitivity and 57.9% specificity. All analyzed electrical and autonomic parameters were similar between patients above and below the age of 59 years (all p >0.05). However, patients >59 years had significantly higher LA area and diameter (both p< 0.01), more important atrial fatty infiltration (p = 0.02), and significantly lower mRNA expression of SCN5A (p = 0.01). Conclusion In the present cohort, post-CABG AF occurrence was 3.4-fold more common in patients >59 years than in their younger peers. Although there was no evidence of aging-related cellular electrical or autonomic remodeling, elderly individuals presented significantly more important LA dilation and atrial fatty infiltration, and significantly lower expression of SCN5A, encoding for Na+ channels. These changes could contribute to the increased aging-related propensity to post-CABG AF by altering intra-atrial conduction and promoting reentry.

Cardiac arrhythmias significantly contribute to cardiovascular morbidity and mortality. The rabbit heart serves as an accepted model system for studying cardiac cell excitation and arrhythmogenicity. Accordingly, primary cultures of adult rabbit ventricular cardiomyocytes serve as a preferable model to study molecular mechanisms of human cardiac excitation. However, the use of adult rabbit cardiomyocytes is often regarded as excessively costly. Therefore, we developed and characterized a novel low-cost rabbit cardiomyocyte model, namely, 3-week-old ventricular cardiomyocytes (3wRbCMs). Ventricular myocytes were isolated from whole ventricles of 3-week-old New Zealand White rabbits of both sexes by standard enzymatic techniques. Using wheat germ agglutinin, we found a clear T-tubule structure in acutely isolated 3wRbCMs. Cells were adenovirally infected (multiplicity of infection of 10) to express Green Fluorescent Protein (GFP) and cultured for 48 h. The cells showed action potential duration (APD90 = 253 ± 24 ms) and calcium transients similar to adult rabbit cardiomyocytes. Freshly isolated and 48-h-old-cultured cells expressed critical ion channel proteins: calcium voltage-gated channel subunit alpha1 C (Cavα1c), sodium voltage-gated channel alpha subunit 5 (Nav1.5), potassium voltage-gated channel subfamily D member 3 (Kv4.3), and subfamily A member 4 (Kv1.4), and also subfamily H member 2 (RERG. Kv11.1), KvLQT1 (K7.1) protein and inward-rectifier potassium channel (Kir2.1). The cells displayed an appropriate electrophysiological phenotype, including fast sodium current (INa), transient outward potassium current (Ito), L-type calcium channel peak current (ICa,L), rapid and slow components of the delayed rectifier potassium current (IKr and IKs), and inward rectifier (IK1). Although expression of the channel proteins and some currents decreased during the 48 h of culturing, we conclude that 3wRbCMs are a new, low-cost alternative to the adult-rabbit-cardiomyocytes system, which allows the investigation of molecular mechanisms of cardiac excitation on morphological, biochemical, genetic, physiological, and biophysical levels.

Aims NOS1AP single-nucleotide polymorphisms (SNPs) correlate with QT prolongation and cardiac sudden death in patients affected by long QT syndrome type 1 (LQT1). NOS1AP targets NOS1 to intracellular effectors. We hypothesize that NOS1AP SNPs cause NOS1 dysfunction and this may converge with prolonged action-potential duration (APD) to facilitate arrhythmias. Here we test (i) the effects of NOS1 inhibition and their interaction with prolonged APD in a guinea pig cardiomyocyte (GP-CMs) LQT1 model; (ii) whether pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from LQT1 patients differing for NOS1AP variants and mutation penetrance display a phenotype compatible with NOS1 deficiency. Methods and results In GP-CMs, NOS1 was inhibited by S-Methyl-L-thiocitrulline acetate (SMTC) or Vinyl-L-NIO hydrochloride (L-VNIO); LQT1 was mimicked by IKs blockade (JNJ303) and β-adrenergic stimulation (isoproterenol). hiPSC-CMs were obtained from symptomatic (S) and asymptomatic (AS) KCNQ1-A341V carriers, harbouring the minor and major alleles of NOS1AP SNPs (rs16847548 and rs4657139), respectively. In GP-CMs, NOS1 inhibition prolonged APD, enhanced ICaL and INaL, slowed Ca2+ decay, and induced delayed afterdepolarizations. Under action-potential clamp, switching to shorter APD suppressed ‘transient inward current’ events induced by NOS1 inhibition and reduced cytosolic Ca2+. In S (vs. AS) hiPSC-CMs, APD was longer and ICaL larger; NOS1AP and NOS1 expression and co-localization were decreased. Conclusion The minor NOS1AP alleles are associated with NOS1 loss of function. The latter likely contributes to APD prolongation in LQT1 and converges with it to perturb Ca2+ handling. This establishes a mechanistic link between NOS1AP SNPs and aggravation of the arrhythmia phenotype in prolonged repolarization syndromes.

The function of the sinoatrial node (SAN), the pacemaker of the heart, declines with age, resulting in increased incidence of sinoatrial node dysfunction (SND) in older adults. The present study assesses potential ionic mechanisms underlying age associated SND. Two group studies have identified complex and various changes in some of membrane ion channels in aged rat SAN, the first group (Aging Study-1) indicates a considerable changes of gene expression with up-regulation of mRNA in ion channels of Cav1.2, Cav1.3 and KvLQT1, Kv4.2, and the Ca2+handling proteins of SERCA2a, and down-regulation of Cav3.1, NCX, and HCN1 and the Ca2+-clock proteins of RYR2. The second group (Aging Study-2) suggests a different pattern of changes, including down regulation of Cav1.2, Cav1.3 and HCN4, and RYR2, and an increase of NCX and SERCA densities and proteins. Although both data sets shared a similar finding for some specific ion channels, such as down regulation of HCN4, NCX, and RYR2, there are contradictory changes for some other membrane ion channels, such as either up-regulation or down-regulation of Cav1.2, NCX and SERCA2a in aged rat SAN. The present study aims to test a hypothesis that age-related SND may arise from different ionic and molecular remodeling patterns. To test this hypothesis, a mathematical model of the electrical action potential of rat SAN myocytes was modified to simulate the functional impact of age-induced changes on membrane ion channels and intracellular Ca2+handling as observed in Aging Study-1 and Aging Study-2. The role and relative importance of each individually remodeled ion channels and Ca2+-handling in the two datasets were evaluated. It was shown that the age-induced changes in ion channels and Ca2+-handling, based on either Aging Study-1 or Aging Study-2, produced similar bradycardic effects as manifested by a marked reduction in the heart rate (HR) that matched experimental observations. Further analysis showed that although the SND arose from an integrated action of all remodeling of ion channels and Ca2+-handling in both studies, it was the change toICaLthat played the most important influence.

Background Identification of pts with left ventricular non-compaction cardiomyopathy (NCCM) at high risk of life-threatening arrhythmic events is focused on clinical and instrumental criteria. The genetic component is not taken into account when deciding on a strategy for preventive measures. Purpose To analyze life-threatening arrhythmic events in pts with the arrhythmic variant of NCCM and to identify genetic factors that may increase the risk of life-threatening arrhythmic events. Methods The study included 159 pts, among whom 58 had malignant ventricular arrhythmias (mean age 43.54±11.95 years, 34/24/58.6% male). The NCCM criteria were confirmed using Echocardiography and CMR. Mutation search was carried out by the next generation sequencing (NGS) in 21 pts with nonsustained ventricular tachycardia (NVT). Results During the follow-up period (median follow-up of 36 months), life-threatening arrhythmic events (documented sustained VT, SCD with successful resuscitation and CD implantation, SCD) were registered in 29 pts (50%), and 10 pts had CD implanted. As a result of sequencing, 23.8% (5 out of 21) of pts with NVT revealed rare genetic variants with uncertain clinical significance (VUS) in 3 genes encoding ion channel proteins: KCNQ1, KCNH2 (9.5%), RYR2 (9.5%). In 9 (42.9%) pts, 10 identified mutations were localized in the genes responsible for the synthesis of sarcomeric proteins: MYBPC3 (23.8%), MYH7 (23.8%). 12 genetic variants were found in overlapping phenotypes between NCCM, ARVC and DCM: PKP2, DSP, PLN, TRPM4 (14.3%), RBM20, LDB3 (9.5%), DPP6, DTNA, NEXN in 11 (52.4%) pts. In this group of pts, the most common (81.8% of cases) were life-threatening arrhythmic events that required surgical intervention: 6 pts were implanted with CD for primary prevention of SCD, two pts - CRT-D, two pts with systolic dysfunction were performed with OHT and one patient was implanted with the Accucinch transcatheter direct mitral valve annuloplasty system. In pts with mutations in sarcomeric protein genes, CD was implanted in 4.7% of cases, and in 14.3% of pts with mutations in ion channel genes (p Conclusion The presence of mutations in genes that control the functioning of ion channels in complex overlapping genotypes is an additional risk factor for ventricular arrhythmias in patients with NCCM. Determining the genetic component opens up new opportunities for personalizing the risk stratification of pts with NCCM. Funding Acknowledgement Type of funding source: None

Background Type 1 Long QT Syndrome (LQT1) is an arrhythmogenic disorder, caused by loss-of-function mutations on KCNQ1 gene, coding for Kv7.1 potassium channel. Although LQT1 is described as the most benign form of LQTS, patients still experience arrhythmic events and there is an unmet need for personalized risk stratification. Attempts have been made to correlate the location of mutations with outcome, but the results are unequivocal. Purpose We provide in the present study a new mutation site-specific risk profile obtained from a large cohort of LQT1 patients. Methods We gathered data on 963 patients with the diagnosis of LQT1 and divided the Kv7.1 channel into 5 functional regions: the N-terminus (NT), the voltage sensor (VS, including transmembrane segments S1 to S4), the cytoplasmic loops (CL), the pore (PO, including the transmembrane segments S5, S6 and the S5-S6 extracellular linker), the C-terminus (CT). Results We studied 963 LQT1 patients: 518 (54%) females; average age 20±17 years; mean QTc at baseline ECG 465±38ms. During a mean follow-up of 8±7 years, 172 (18%) patients experienced arrhythmic events: 31 (3%) experienced one or more cardiac arrests, while 141 (15%) experienced one or more syncopal spells. We identified 188 different variants in the KCNQ1 gene, with the following distribution: 15 (8%) in the NT, 33 (18%) in the VS, 27 (14%) in the CL, 43 (23%) in the PO, 70 (37%) in the CT. The frequency of pathogenic variants per number of amino acids (a.a.) was higher in the CL region, as compared to the other domains (1 mutation every 1.4 a.a.). The duration of QTc interval was significantly longer for patients with mutations in the PO region (473±40 ms) and in the CL region (468±38 ms) as compared to the other regions (p Importantly, in a multivariate analysis PO and CL regions were associated with a higher probability of experiencing arrhythmic events (OR 2.89, 95% CI 1.95–4.29, p=0.019 and OR 1.61, 95% CI 1.0–2.49, p=0.05, respectively. Figure) than the other regions. Interestingly, the risk was independent from QTc interval duration. Conclusions Mutations affecting the PO and the CL region of the Kv7.1. channel are associated with a higher probability of experiencing arrhythmic events. This finding is clinically relevant, because it will allow for a more personalized, mutation site-specific risk stratification. Mutation site and arrhythmic events Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): Italian Ministry of Research and University Dipartimenti di Eccellenza 2018–2022 grant to the Molecular Medicine Department (University of Pavia)

Background Left cardiac sympathetic denervation (LCSD) is an effective anti-fibrillatory, minimally invasive therapy for patients with either LQTS or CPVT. As a pre-ganglionic fiber resection, re-growth or re-innervation is not possible. However, if a suboptimal CSD was performed, chances for post-LCSD breakthrough cardiac events (BCEs) are increased. Purpose To examine the prevalence and outcomes of patients requiring a re-do LCSD. Methods We performed a retrospective review of the 251 LQTS and CPVT patients who underwent CSD (mostly LCSD) at our institution to identify the subset referred for additional CSD because of recurrent BCEs after their primary LCSD that was performed elsewhere. Clinical data on symptomatic status prior to diagnosis and BCEs after first surgery as well as the surgical reports were reviewed to determine the reasons for the repeat procedure. Results Among the 15 patients (6% overall, 6 female; 13 LQTS, 2 CPVT) referred for additional CSD, 3 patients (20%) had a re-do LCSD performed instead of a sequential RCSD because of incomplete resection with the primary LCSD. Patient 1 is a male with Jervell-and-Lange Nielsen Syndrome (JLNS; KCNQ1-K421fs9X; KCNQ1-N365H), who first experienced syncope with torsades at age 3 after which a primary bilateral CSD was performed elsewhere. He had 2 BCEs and at age 12 underwent re-do LCSD, where his left-sided T2-T4 sympathetic chain with remnants of T1 were found to be intact. Patient 2 is a male who had 3 cardiac events prior to diagnosis (genotype negative LQTS) and LCSD at age 3. Following 3 BCEs, he underwent re-do LCSD at age 5 where scarring of T3 and intact stellate ganglion were found. Patient 3 is an adult female with CPVT (RYR2-deletion exon 3) who had multiple syncopal events, out-of-hospital arrest (leading to ICD) and 2 ICD-storms before primary bilateral CSD. Re-do LCSD was performed after identifying an intact left stellate ganglion and T2. Conclusions Before performing a primary bilateral CSD, proceeding to a RCSD after post-LCSD BCEs, or doing a videoscopic LCSD in the first place, it must be recognized that LCSD's success hinges on fully resecting the lower half of the stellate ganglion and the sympathetic chain through T4. Anything less is a partial denervation and likely ineffective procedure. Funding Acknowledgement Type of funding source: None

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.

Funding Acknowledgements none Introduction Short QT syndrome (SQTS) is a rare, autosomal dominant disease causing ventricular fibrillation and sudden cardiac death. Genetic testing is recommended according to current guidelines. Mutations in KCNQ1, KCNH2, KCNJ2 and more recently SLC4A3 genes have been implicated in SQTS. These genes encode potassium channel subunits and a bicarbonate transporter regulating intracellular pH. A dominant mutation in this transporter can lead to increased intracellular pH and shortened action potential. Purpose We present a family with a short QT phenotype and recurrent syncope in whom a novel genetic variant was detected by whole-exome sequencing (WES), confirmed by cascade screening. Methods We performed a thorough work-up of the index patient including medical history, physical examination, 12-lead ECG, echocardiography, stress testing, coronary angiography, flecainide challenge, and genetic testing with NGS. QTc was determined using Bazett’s formula. CS of all 1° and two 2° relatives was performed. Results The ECG of the index patient showed a QTc of 340ms and characteristics compatible with a SQTS (figure). Clinical work-up was unremarkable. A first genetic search with next generation sequencing focusing on genes that have been previously involved in the pathogenesis of channelopathies detected a rare known heterozygous missense variant in the KCNH2 gene (Arg328Cys, frequency 0.053%), which was predicted to be pathogenic according to various prediction algorithms (Polyphen, SIFT, Align GVGD, mutation taster). ECG screening of all asymptomatic first-degree family members identified a SQT phenotype in the mother (QTc 355ms), but not in the father (QTc 380ms) and sister (410ms). The KCNH2 variant was found in the father and sister but not the affected mother, which excludes this variant as the causative mutation in this family. Therefore, reanalysis of WES data was performed and revealed a novel heterozygous missense variant p.(Arg370Cys) in the SLC4A3 gene, recently associated with SQTS. A mutation in this gene at the same position has been previously reported in SQTS. The p.(Arg370Cys) mutation was found in the mother but not in the unaffected father or sister. Furthermore the mutation was present in two affected maternal uncles (QTc 319ms and 342ms) supporting the assumption that this was the causative mutation in this family. Conclusions A novel genetic variant in the SLC4A3 gene leading to sQT phenotype could be detected using WES and cascade screening. Predictive bioinformatic algorithms to assess the pathogenicity of missense variants are of limited relevance, but genetic analysis of additional unaffected and affected family members may be instrumental to identify pathogenic DNA sequence variations. Abstract Figure. Pedigree and ECGs of the family

Kv7 potassium channels are strongly activated by a variety of small molecules with diverse mechanisms of action. Wang et al. investigate a compound that targets the voltage-sensing domain, ICA-069673, and demonstrate that four drug-sensitive subunits are required for maximal effect., KCNQ2-5 (Kv7.2–Kv7.5) channels are strongly influenced by an emerging class of small-molecule channel activators. Retigabine is the prototypical KCNQ activator that is thought to bind within the pore. It requires the presence of a Trp side chain that is conserved among retigabine-sensitive channels but absent in the retigabine-insensitive KCNQ1 subtype. Recent work has demonstrated that certain KCNQ openers are insensitive to mutations of this conserved Trp, and that their effects are instead abolished or attenuated by mutations in the voltage-sensing domain (VSD). In this study, we investigate the stoichiometry of a VSD-targeted KCNQ2 channel activator, ICA-069673, by forming concatenated channel constructs with varying numbers of drug-insensitive subunits. In homomeric WT KCNQ2 channels, ICA-069673 strongly stabilizes an activated channel conformation, which is reflected in the pronounced deceleration of deactivation and leftward shift of the conductance–voltage relationship. A full complement of four drug-sensitive subunits is required for maximal sensitivity to ICA-069673—even a single drug-insensitive subunit leads to significantly weakened effects. In a companion article (see Yau et al. in this issue), we demonstrate very different stoichiometry for the action of retigabine on KCNQ3, for which a single retigabine-sensitive subunit enables near-maximal effect. Together, these studies highlight fundamental differences in the site and mechanism of activation between retigabine and voltage sensor–targeted KCNQ openers.

Abuse of the common anti-diarrheal loperamide is associated with QT interval prolongation as well as development of the potentially fatal arrhythmia torsades de pointes. The mechanism underlying this cardiotoxicity is high affinity inhibition of the human ether-a-go-go-related gene (hERG) cardiac K+ channel. N-Desmethyl loperamide is the major metabolite of loperamide and is a close structural relative of the parent molecule. To date no information is available regarding the affinity of N-desmethyl loperamide for human cardiac ion channels. The effects of N-desmethyl loperamide on various cloned human cardiac ion channels including hERG, KvLQT1/mink and Nav1.5 were studied and compared to that of the parent. N-Desmethyl loperamide was a much weaker (7.5-fold) inhibitor of hERG compared to loperamide. However, given the higher plasma levels of the metabolite relative to the parent, it is likely that N-desmethyl loperamide can contribute, at least secondarily, to the cardiotoxicity observed with loperamide abuse. We used the recently solved cryo-EM structure of the hERG channel together with previously published inhibitors, to understand the basis of the interactions as well as the difference that a single methyl plays in the hERG channel blocking affinities of these two compounds.