Arrhythmia/Electrophysiology Ca -Stimulated Adenylyl Cyclase AC1 Generates Efficient Biological Pacing as Single Gene Therapy and in Combination With HCN2

Background—Biological pacing performed solely via HCN2 gene transfer in vivo results in relatively slow idioventricular rates and only moderate autonomic responsiveness. We induced biological pacing using the Ca -stimulated adenylyl cyclase AC1 gene expressed alone or in combination with HCN2 and compared outcomes with those with single-gene HCN2 transfer. Methods and Results—We implanted adenoviral HCN2, AC1, or HCN2/AC1 constructs into the left bundle branches of atrioventricular-blocked dogs. During steady-state gene expression (days 5–7), differences between AC1, HCN2/AC1, and HCN2 alone were evident in basal beating rate, escape time, and dependence on electronic backup pacing. In HCN2, AC1, and HCN2/AC1, these parameters were as follows: basal beating rate: 50 1.5, 60 5.0, and 129 28.9 bpm (P 0.05 for HCN2/AC1 versus HCN2 or AC1 alone), respectively; escape time: 2.4 0.2, 1.3 0.2, and 1.1 .0.4 seconds (P 0.05 for AC1 and HCN2/AC1 versus HCN2); and percent electronic beats: 34 8%, 2 1%, and 6 2% (P 0.05 for AC1 and HCN2/AC1 versus HCN2). Instantaneous (SD1) and long-term (SD2) heart rate variability and circadian rhythm analyzed via 24-hour Holter recordings showed a shift toward greater sensitivity to parasympathetic modulation in animals injected with AC1 and a high degree of sympathetic modulation in animals injected with HCN2/AC1. Conclusion—AC1 or HCN2/AC1 overexpression in left bundle branches provides highly efficient biological pacing and greater sensitivity to autonomic modulation than HCN2 alone. (Circulation. 2012;126:528-536.)