Electrophysiological basis and pharmacological responses in short QT syndrome type 5: insights from iPSC-derived cardiomyocytes

Background Short QT syndrome type 5 (SQTS5) caused by CACNB2 variants is a rare inherited arrhythmia associated with a high risk of sudden cardiac death, yet its electrophysiological mechanisms and therapeutic options remain poorly defined. Purpose The study aimed to delineate the arrhythmogenic mechanisms of SQTS5 and evaluate potential pharmacological interventions. Methods A human induced pluripotent stem cell (hiPSC) line carrying a CACNB2 mutation and a CRISPR/Cas9-corrected isogenic control cell line were differentiated into cardiomyocytes (hiPSC-CMs). Electrophysiological properties and pharmacological responses were assessed at single-cell and two-dimensional tissue levels using calcium imaging, patch-clamp, and optical mapping. Results At the single-cell level, SQTS5 cells exhibited shortened action potential duration (APD) accompanied by a substantial reduction in L-type calcium current (ICa-L) density, with increased arrhythmia-associated events (AAEs) and greater interbeat interval variability. Detailed biophysical assessment indicated that the loss of ICa-L function was primarily attributable to delayed activation, enhanced inactivation, and slowed recovery from inactivation. At the tissue level, SQTS5 cell sheets recapitulated disease phenotypes, including abbreviated APD, reduced excitation wavelength, and increased susceptibility to spontaneous rotor activity. Under proarrhythmic stimulation with carbachol plus epinephrine, SQTS5 cells showed further APD shortening, additional suppression of ICa-L, and a pronounced increase in AAEs and interbeat variability. Consistently, in cell sheets, the stimulation further reduced APD and excitation wavelength and significantly increased rotor inducibility, whereas isogenic controls were minimally affected at both levels. For drug testing, among clinically used antiarrhythmic agents, disopyramide significantly prolonged APD by modulating repolarizing potassium currents (IKr, IKs) and restoring ICa-L in a gene-dependent manner, resulting in reduced AAEs, improved interbeat stability, increased excitation wavelength, and suppression of reentrant arrhythmias. Although ibutilide prolonged APD by inhibiting IKr and Ito and activating late INa, it failed to restore ICa-L or reduce arrhythmic burden. Conclusions hiPSC-derived cardiomyocytes from an SQTS5 patient carrying a CACNB2 mutation exhibit a vulnerable electrophysiological state and heightened sensitivity to proarrhythmic stimulation. APD prolongation alone is insufficient to restore electrical stability; instead, therapeutic benefit is more likely achieved by targeting the underlying pathogenic current deficit, providing a preclinical rationale for genotype- and mechanism-guided pharmacotherapy.