HCN2/SkM1 Gene Transfer Into Canine Left Bundle Branch Induces Stable, Autonomically Responsive Biological Pacing at Physiological Heart Rates

Abstract

Objectives This study sought to test the hypothesis that hyperpolarization-activated cyclic nucleotide–gated (HCN)–based biological pacing might be improved significantly by hyperpolarizing the action potential (AP) threshold via coexpression of the skeletal muscle sodium channel 1 (SkM1). Background Gene-based biological pacemakers display effective in vivo pacemaker function. However, approaches used to date have failed to manifest optimal pacemaker properties, defined as basal beating rates of 60 to 90 beats/min, a brisk autonomic response achieving maximal rates of 130 to 160 beats/min, and low to absent electronic backup pacing. Methods We implanted adenoviral SkM1, HCN2, or HCN2/SkM1 constructs into left bundle branches (LBB) or left ventricular (LV) epicardium of atrioventricular-blocked dogs. Results During stable peak gene expression on days 5 to 7, HCN2/SkM1 LBB-injected dogs showed highly stable in vivo pacemaker activity superior to SkM1 or HCN2 alone and superior to LV-implanted dogs with regard to beating rates (resting approximately 80 beats/min; maximum approximately 130 beats/min), no dependence on electronic backup pacing, and enhanced modulation of pacemaker function during circadian rhythm or epinephrine infusion. In vitro isolated LV of dogs overexpressing SkM1 manifested a significantly more negative AP threshold. Conclusions LBB-injected HCN2/SkM1 potentially provides a more clinically suitable biological pacemaker strategy than other reported constructs. This superiority is attributable to the more negative AP threshold and injection into the LBB.

Biological Pacemakers

“…The current report suggested that the expression of HCN2, rather than other isoforms such HCN4, along with SkM1 in the canine LBB, was sufficient to provide an improved automaticity and autonomic responsiveness similar to the highly expressing HCN4 in sinus node pacemaker cells (1). Although cardiac function in LBB-injected dogs appeared to be grossly improved, it remains unclear how exogenous HCN2/SkM1 expression in LBB-injected cells is sufficient to functionally compensate for the difference in anatomic structure and protein expression of LBB cells compared with sinus node pacemaker cardiomyocytes.…”

“…In this issue of the Journal, Boink et al (1) reported the study of biological pacemaker function in mongrel dogs that underwent radiofrequency ablation of the atrioventricular node and were then treated with gene transfer using the hyperpolarization-activated cyclic nucleotide-gated current channel 2 (HCN2) construct, the skeletal muscle sodium channel (SkM1) construct, or the dual (HCN2/SkM1) construct injected into the left bundle branch (LBB) or the left See page 1192 ventricular (LV) subepicardium. The researchers coexpressed SkM1 with HCN2 on the basis of the hypothesis that when HCN2 generated the inward current that would drive the membrane toward threshold, SkM1 would create greater availability of sodium channels during diastole because of a more favorable inactivation curve than the cardiac sodium channel, leading to a more negative threshold potential, improved pacemaker stability, and increased beating rates.…”

“…In summary, despite the many points that remain to be elucidated and the technical limitations to be addressed, the effort of Boink et al (1) along with other research groups to establish a biological alternative to electrical pacemakers is extremely significant for the possible clinical applications and the improvement in patient management. None of the clinical problems detract at all from the science but do demonstrate that we will have job security for quite some time to come.…”

Biological Pacemakers – A Review

“…A more favourable response was obtained by expressing HCN2 with SkM1. Expressing this combination in adenoviruses and injection in the left bundle branch in dogs indicated an adequate resting rate and good response to autonomic stimulation [50]. Most studies using gene transfer use Adenoviruses as vectors.…”

Biology of the Sinus Node and its Disease

“…In an attempt to reproduce the complexity of the SAN, groups are now either modulating multiple features simultaneously or using transcription factors. 83 Tbx18 and Tbx3 are prime candidates currently being studied in animals. 8,11 Overexpression of Tbx18 has had particular success, reprogramming ventricular cells into SAN-like cells leading to robust and autonomically sensitive ectopic pacing in large animals.…”