The long QT syndrome type 2-associated Kv11.1 R56Q+/- variant induces discordant alternans and premature activity in human stem cell-derived cardiomyocyte monolayers

Background Long QT Syndrome (LQTS) is associated with sudden cardiac arrest, but risk stratification of genetic variants can be challenging, especially when the functional testing suggests an uncertain phenotypic effect. Objective Here, we have studied arrhythmia mechanisms in CRISPR-edited human induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) monolayers harbouring the hERG channel variant, R56Q +/- , which, while associated with LQTS, has an uncertain pathogenicity despite extensive study. Rather than prolonging the action potential, this variant reduces protective hERG currents that resist premature depolarizations; however, the mechanism by which reduced protective currents produces arrhythmia is uncertain. Methods We investigate arrhythmia mechanisms in hERG R56Q +/ hiPSC-CMs using a combination of multi-electrode array (MEA) and optical mapping (OM) to report steady-state beat-to-beat membrane voltage and cytoplasmic calcium dynamics and their adaptations to imposed rate changes. Results Our findings demonstrate that, without prolonging the average steady-state action potential duration (APD), the hERG R56Q +/- introduces beat-to-beat heterogeneity in APD and spatially discordant alternans. This heterogeneous spatial repolarization was associated with a steeper electrical restitution relationship and an increased incidence of premature activity. These features would be expected to increase susceptibility to re-entrant arrhythmias. Conclusion These findings provide a plausible mechanism for sudden arrest risk in individuals harbouring hERG variants of uncertain significance with similar channel dysfunction and are important considerations for pharmacogenetic rescue strategies, which often result in hERG channels with compromised hERG protective currents.