Variable Expressivity in Long QT Syndrome Type 2 Rabbits Defines a Low Threshold for Therapeutic I_Kr Restoration

Long QT syndrome type 2 (LQT2) is caused by pathogenic variants in the hERG potassium channel, resulting in QT prolongation, polymorphic ventricular tachycardia (pVT), and sudden cardiac death. We previously developed a transgenic LQT2 rabbit model that recapitulates the human disease and segregates into two phenotypically distinct lines, LQT2A and LQT2G. Using optical mapping and cardiomyocyte patch clamp, we investigated the electrophysiological basis of variable disease expressivity and its potential future implications for gene therapy. LQT2G hearts were highly arrhythmogenic, with isoproterenol inducing frequent premature ventricular contractions and pVT, whereas LQT2A hearts required additional I_Kr blockade to reliably trigger arrhythmias. Patch clamp recordings revealed I_Kr expression in 21% of LQT2A cardiomyocytes at near-wild-type current density, compared with only 5.5% of LQT2G cells exhibiting low I_Kr. Computer simulations demonstrated that restoration of I_Kr in >23% of cardiomyocytes fully suppressed early afterdepolarizations through electrotonic coupling. To assess achievable transgene expression, we designed a novel minimally invasive gene delivery method for rabbits. To this end, neonatal wild-type rabbits received three different doses of AAV9 encoding the red-fluorescent reporter mKate2 via jugular vein injection, resulting in viral-load-dependent, regionally distributed mosaic cardiac expression reaching approximately 20%. However, high-dose intravenous AAV9 delivery was associated with dose-dependent hindlimb weakness and dorsal root ganglia pathology, indicating that this administration route is not suitable in rabbits despite achieving biologically sufficient levels of cardiac transduction. Our experimental observations and computational modeling identify ~20% as a plausible range for effective I_Kr restoration, generating a testable therapeutic hypothesis.