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BackgroundDrug-induced QT prolongation (diLQT) is a feared side-effect as exposing susceptible individuals to fatal arrhythmias. The occurrence of diLQT is primarily attributed to unintended drug interactions with cardiac ion channels, notably the hERG channels that generate the repolarizing current (IKr) and thereby regulate the late repolarization phase. There is an important inter-individual susceptibility to develop diLQT which is of unknown origin but can be reproduced in patient-specific iPSC-derived cardiomyocytes (iPS-CMs).ObjectiveWe aimed to investigate the dynamics of hERG channels in response to sotalol and to identify regulators of the susceptibility to developing diLQT.MethodsWe measured electrophysiological activity and cellular distribution of hERG channels after hERG blocker treatment in iPS-CMs derived from patients with highest or lowest sensitivity (HS or LS) to sotalol administrationin vivo(i.e., based on the measure of the maximal change in QT interval 3 hours after administration). Specific small-interfering RNAs (siRNA) and CAVIN1-T2A-GFP adenovirus were used to manipulateCAVIN1expression.ResultsWhile HS and LS iPS-CMs showed similar electrophysiological characteristics at the baseline, the late repolarization phase was prolonged, and IKrsignificantly decreased after exposure of HS iPS-CMs to low sotalol concentrations. IKrreduction was caused by a rapid translocation of hERG channel from the plasma membrane to the cytoskeleton upon sotalol application. This phenomenon was suppressed by blocking active endocytosis using dynasore.CAVIN1, essential for caveolae biogenesis, was two-times more expressed in HS iPS-CMs and its knockdown using siRNA decreased their sensitivity to sotalol.CAVIN1overexpression in LS iPS-CMs using adenovirus showed reciprocal effects. Mechanistically, we found that treatment with sotalol promoted trafficking of the hERG channel from the plasma membrane to the cytoskeleton through caveolae and in a manner dependent on CAVIN1 expression.CAVIN1silencing reduced the number of caveolae at the membrane and abrogated the internalization of hERG channel in sotalol-treated HS iPS-CMs. CAVIN1 also controlled cardiomyocyte responses to other hERG blockers such as E4031, vandetanib, and clarithromycin.ConclusionsOur study identifies unbridled turnover of the potassium channel hERG as a mechanism supporting the inter-individual susceptibility underlying diLQT development and demonstrates how this phenomenon is finely tuned by CAVIN1.CLINICAL PERSPECTIVEWhat is new?-The inter-individual susceptibility underlying diLQT development involves unbridled turnover of cardiac ion channels from the plasma membrane.-This phenomenon is finely tuned by CAVIN1, a protein that is essential for essential for caveolae biogenesis.-Treatment with hERG blocker promoted trafficking of the hERG channel from the plasma membrane to the cytoskeleton through caveolae and in a manner dependent on CAVIN1 expression.What are the clinical implications?-While congenital long QT is primarily from a genetic origin, the development of drug-induced long QT involves differences in the trafficking machinery of cardiac ion channels.-The prediction of CAVIN1 expression levels could help preventing drug-induced cardiotoxicity.

Arrhythmias are a critical health burden, responsible for a significant proportion of sudden cardiac death. They arise from inherited or acquired abnormalities in cardiac ion channels or their associated regulatory proteins functioning within cardiomyocytes. In opposite with the classical in vitro models, human induced pluripotent stem cells–derived cardiomyocytes (hiPSC-CMs) offer a patient-specific model that recapitulates disease phenotype and mirrors the reaction to pharmacotherapy. Thus far, the hiPSC-CMs model has genuinely improved our knowledge concerning both, the pathogenicity of genetic variants in inherited arrhythmias and the toxicity of some factors implicated in acquired forms of arrhythmia. Besides, as these cells preserve patient genetic background, they enable the customization of therapies and open thereby the door for personalized medicine. In this chapter, we will outline studies that leveraged the hiPSC-CMs model to explore the mechanism underlying inherited or acquired arrhythmic phenotypes and will point out its sustained limits.

Aims: several inherited arrhythmic diseases have been linked to single gene mutations in cardiac ion channels and interacting proteins. However, the mechanisms underlying most arrhythmias, are thought to involve altered regulation of the expression of multiple effectors. In this study, we aimed to examine the role of a transcription factor (TF) belonging to the Iroquois homeobox family, IRX5, in cardiac electrical function. Methods and results: using human cardiac tissues, transcriptomic correlative analyses between IRX5 and genes involved in cardiac electrical activity showed that in human ventricular compartment, IRX5 expression strongly correlated to the expression of major actors of cardiac conduction, including the sodium channel, Nav1.5, and Connexin 40 (Cx40). We then generated human-induced pluripotent stem cells (hiPSCs) derived from two Hamamy syndrome-affected patients carrying distinct homozygous loss-of-function mutations in IRX5 gene. Cardiomyocytes derived from these hiPSCs showed impaired cardiac gene expression programme, including misregulation in the control of Nav1.5 and Cx40 expression. In accordance with the prolonged QRS interval observed in Hamamy syndrome patients, a slower ventricular action potential depolarization due to sodium current reduction was observed on electrophysiological analyses performed on patient-derived cardiomyocytes, confirming the functional role of IRX5 in electrical conduction. Finally, a cardiac TF complex was newly identified, composed by IRX5 and GATA4, in which IRX5 potentiated GATA4-induction of SCN5A expression. Conclusion: altogether, this work unveils a key role for IRX5 in the regulation of human ventricular depolarization and cardiac electrical conduction, providing therefore new insights into our understanding of cardiac diseases., National Research Agency; European Union (EU); Horizon 2020; Marie Curie Actions International Incoming Fellowship FP7-PEOPLE-2012-IIF; La Fédération Française de Cardiologie; Fondation LefoulonDelalande; Eiffel Scholarship Programme of Excellence (Campus France), Doctoral School of Science and Technology-Lebanese University and The Fondation Genavie

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Aims The Brugada syndrome (BrS) is an inherited cardiac disorder predisposing to ventricular arrhythmias. Despite considerable efforts, its genetic basis and cellular mechanisms remain largely unknown. The objective of this study was to identify a new susceptibility gene for BrS through familial investigation. Methods and results Whole-exome sequencing performed in a three-generation pedigree with five affected members allowed the identification of one rare non-synonymous substitution (p.R211H) in RRAD, the gene encoding the RAD GTPase, carried by all affected members of the family. Three additional rare missense variants were found in 3/186 unrelated index cases. We detected higher levels of RRAD transcripts in subepicardium than in subendocardium in human heart, and in the right ventricle outflow tract compared to the other cardiac compartments in mice. The p.R211H variant was then subjected to electrophysiological and structural investigations in human cardiomyocytes derived from induced pluripotent stem cells (iPSC-CMs). Cardiomyocytes derived from induced pluripotent stem cells from two affected family members exhibited reduced action potential upstroke velocity, prolonged action potentials and increased incidence of early afterdepolarizations, with decreased Na+ peak current amplitude and increased Na+ persistent current amplitude, as well as abnormal distribution of actin and less focal adhesions, compared with intra-familial control iPSC-CMs Insertion of p.R211H-RRAD variant in control iPSCs by genome editing confirmed these results. In addition, iPSC-CMs from affected patients exhibited a decreased L-type Ca2+ current amplitude. Conclusion This study identified a potential new BrS-susceptibility gene, RRAD. Cardiomyocytes derived from induced pluripotent stem cells expressing RRAD variant recapitulated single-cell electrophysiological features of BrS, including altered Na+ current, as well as cytoskeleton disturbances.

International audience; Patients with HIV present with a higher prevalence of QT prolongation, of which molecularbases are still not clear. Among HIV proteins, Tat serves as a transactivator that stimulatesviral genes expression and is required for efficient HIV replication. Tat is actively secretedinto the blood by infected T-cells and affects organs such as the heart. Tat has been shown toalter cardiac repolarization in animal models but how this is mediated and whether this is alsothe case in human cells is unknown. In the present study, we show that Tat transfection inheterologous expression systems led to a decrease in hERG (underlying cardiac IKr) andhuman KCNE1-KCNQ1 (underlying cardiac IKs) currents and to an acceleration of theirdeactivation. This is consistent with a decrease in available phosphatidylinositol-(4,5)-bisphosphate (PIP2). A mutant Tat, unable to bind PIP2, did not reproduce the observedeffects. In addition, WT-Tat had no effect on a mutant KCNQ1 which is PIP2-insensitive,further confirming the hypothesis. Twenty four-hour incubation of human induced pluripotentstem cells-derived cardiomyocytes with Wild-type Tat reduced IKr and accelerated itsdeactivation. Concordantly, this Tat incubation led to a prolongation of the action potential(AP) duration. Events of AP alternans were also recorded in the presence of Tat, and wereexacerbated at a low pacing cycle length. Altogether, these data obtained on human K+channels both in heterologous expression systems and in human cardiomyocytes stronglysuggest that Tat sequesters PIP2, leading to a reduction of IKr and IKs, and provide a molecularmechanism for QT prolongation in HIV-infected patients.Key