Gene therapy for short QT syndrome type 1 (SQT1): a proof-of-concept efficacy trial in transgenic SQT1 rabbits

Background Short QT syndrome type 1 (SQT1) is a genetic channelopathy caused by gain-of-function variants in the KCNH2-encoded Kv11.1 potassium channel, responsible for the repolarising IKr current. These variants lead to pathologically shortened cardiac repolarisation and QT intervals on 12-lead ECG, predisposing patients to life-threatening ventricular arrhythmias and sudden cardiac death. Purpose To investigate the therapeutic efficacy of our KCNH2-specific suppression-replacement gene therapy (KCNH2-SupRep) in transgenic SQT1 rabbits generated by cardiac-specific overexpression of KCNH2-N588K. Methods KCNH2-SupRep was developed by combining a custom-designed KCNH2 shRNA with its corresponding shRNA-immune KCNH2 cDNA, packaged into a single AAV9 vector and delivered directly into the aortic root via a Swan Ganz catheter (1E10 vg/kg). Therapeutic efficacy was evaluated in vivo by 12-lead ECG recordings pre- and three weeks post-therapy, ex vivo by optical voltage mapping, and at the cellular level by patch-clamp, calcium imaging, and qPCR in ventricular cardiomyocytes (VCMs) isolated from adult transgenic SQT1 and wild-type (WT) rabbits. Results In vivo, KCNH2-SupRep normalized the heart-rate corrected QT interval (QTc) in transgenic SQT1 rabbits towards WT levels (baseline-SQT1: 153±4ms vs SupRep-SQT1: 167±5ms, p=0.0002; Fig. 1A), without adversely impacting repolarisation heterogeneity. No such QTc changes were observed in sham-treated SQT1 rabbits (sham-SQT1) (baseline-SQT1: 155±4ms vs sham-SQT1: 146±5ms, p=0.1422). Ex vivo, at a 2.5Hz stimulation frequency, KCNH2-SupRep therapeutically prolonged the APD90 in SupRep-SQT1 hearts (176±4ms) near WT levels (179±4ms), compared to untreated (UT)-SQT1 (154±4), and resolved the pathologically increased apicobasal APD90 heterogeneity observed in UT-SQT1 hearts, supporting an anti-arrhythmic effect of KCNH2-SupRep (Fig. 1B). At the cellular level, at a 1Hz stimulation frequency, KCNH2-SupRep prolonged the APD90 in VCMs isolated from SupRep-SQT1 rabbits (363±11ms) closer to WT levels (404±14ms) as compared to UT- (314±12ms) and sham-SQT1 (322±11ms; Fig. 1C). Moreover, KCNH2-SupRep restored the cellular surrogate of the electro-mechanical window (cEMW, Ca2+ transient duration – APD) and normalized IKr in nearly 50% of VCMs (Fig. 1D), in line with a 50-60% suppression of the mutant KCNH2 transcript. This partial therapeutic effect on IKr may explain the incomplete APD90 normalization in individual VCMs. Nonetheless, both in vivo and ex vivo, this partial therapeutic effect was sufficient to completely restore the electrical properties. Conclusion This proof-of-concept study is the first to demonstrate the efficacy of gene therapy in treating SQT1 in a medium-sized animal model. KCNH2-SupRep rescued the disease phenotype in vivo, ex vivo and at cellular levels in transgenic SQT1 rabbits.Therapeutic efficacy of KCNH2-SupRep