Targeted activation of hERG channels rescues electrical instability induced by the hERG R56Q+/- Long QT Syndrome variant

Aims
Long QT Syndrome Type 2 (LQTS2) is associated with inherited variants in the cardiac hERG K+ channel. However, the pathogenicity of hERG channel gene variants is often uncertain. Using CRISPR-Cas9 gene-edited hiPSC-derived cardiomyocytes (hiPSC-CMs), we investigated the pathogenic mechanism underlying the LQTS-associated hERG R56Q variant, and its phenotypic rescue by the type 1 hERG activator, RPR260243.

Methods and Results
These approaches enable characterization of the unclear causative mechanism of arrhythmia in the R56Q variant (an N-terminal PAS domain mutation that primarily accelerates channel deactivation) and translational investigation of the potential for targeted pharmacologic manipulation of hERG deactivation. Using perforated patch clamp electrophysiology of single hiPSC-CMs, programmed electrical stimulation showed that the hERG R56Q variant does not significantly alter the mean APD90. However, the R56Q variant increases the beat-to-beat variability in APD90 during pacing at constant cycle lengths, enhances the variance of action potential duration (APD90) during rate transitions, and increases the incidence of 2:1 block. During paired S1-S2 stimulations measuring electrical restitution properties, the R56Q variant was also found to increase the variability in rise time and duration of the response to premature stimulations. Application of the hERG channel activator, RPR260243, reduces the APD variance in hERG R56Q hiPSC-CMs, reduces the variability in responses to premature stimulations, and increases the post-repolarization refractoriness.

Conclusion
Based on our findings, we propose that the hERG R56Q variant leads to heterogeneous APD dynamics, which could result in spatial dispersion of repolarization and increased risk for re-entry without significantly affecting the average APD90. Furthermore, our data highlight the antiarrhythmic potential of targeted slowing of hERG deactivation gating, which we demonstrate increases protection against premature action potentials and reduces electrical heterogeneity in hiPSC-CMs.