Abstract 4358248: Patient-specific iPSC-derived Cardiomyocytes Reveal Variable Phenotypic Severity in a <i> KCNH ₂ </i> p.Y427H Parent-Proband Trio

Background: Type 2 Long QT Syndrome (LQT 2 ), caused by KCNH 2 mutations, is associated with an increased risk of sudden cardiac death (SCD). Clinical severity varies considerably among carriers. We investigated a family harboring the KCNH 2 p.Y427H variant, comprising a symptomatic proband (nine cardiac events), an asymptomatic carrier mother, and a mutation-negative father. Hypothesis: We hypothesized that patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) can recapitulate the observed clinical phenotype discordance in vitro. Methods: Whole-exome sequencing identified the variant. Peripheral blood mononuclear cells (PBMCs) from all three individuals were reprogrammed into iPSCs and differentiated into cardiomyocytes. Electrophysiological assessment included patch-clamp recordings of I Kr and I Ca,L , measurements of action potential duration APD 90 and APD 50 under pacing, and evaluation of early afterdepolarizations (EADs) and triggered activities (TAs) under spontaneous conditions. Nifedipine, an L-type calcium channel blocker, was used to evaluate arrhythmia suppression. Results: I Kr was markedly reduced in the proband-derived iPSC-CMs (0.26 [0.19–0.29] pA/pF) and mother-derived iPSC-CMs (0.29 [0.27–0.30]) compared to the father-derived iPSC-CMs (1.65 [1.23–2.11], P = 0.001). APD 90 was significantly prolonged in the proband-derived iPSC-CMs (824 [691–893] ms) versus the mother-derived iPSC-CMs (559 [491–574] ms, P = 0.010) and father-derived iPSC-CMs (397 [331–403] ms, P &lt; 0.001). Proband-derived iPSC-CMs exhibited a higher incidence of spontaneous EADs and TAs. Notably, I Ca,L was upregulated in the proband-derived iPSC-CMs (–18.4 [–19.6 to –13.0] pA/pF) compared to the mother-derived iPSC-CMs (–7.23 [–9.5 to –5.9], P = 0.002) and father-derived iPSC-CMs (–7.49 [–8.66 to –5.91], P = 0.001). Nifedipine significantly suppressed EADs and TAs in the proband-derived iPSC-CMs. Conclusion: The KCNH 2 p.Y427H variant induces I Kr loss-of-function. Divergent calcium current responses among family members may reflect a compensatory mechanism modulating phenotypic severity. Patient-specific iPSC-CMs reproduce this variability, providing a platform for mechanistic insights, personalized risk assessment, and therapy in LQT 2 .