TBX18 overexpression enhances pacemaker function in a rat subsidiary atrial pacemaker model of sick sinus syndrome

Abstract

Key points The sinoatrial node (SAN) is the primary pacemaker of the heart. SAN dysfunction, or ‘sick sinus syndrome’, can cause excessively slow heart rates and pauses, leading to exercise limitation and syncope, currently treated by implantation of an electronic pacemaker.‘Biopacemaking’ utilises gene therapy to restore pacemaker activity by manipulating gene expression. Overexpressing the HCN pacemaker ion channel has been widely used with limited success.We utilised bradycardic rat subsidiary atrial pacemaker tissue to evaluate alternative gene targets: the Na+/Ca2+ exchanger NCX1, and the transcription factors TBX3 and TBX18 known to be involved in SAN embryonic development.TBX18 overexpression restored normal SAN function, as assessed by increased rate, improved heart rate stability and restoration of isoprenaline response. TBX3 and NCX1 were not effective in accelerating the rate of subsidiary atrial pacemaker tissue.Gene therapy targeting TBX18 could therefore have the potential to restore pacemaker function in human sick sinus syndrome obviating electronic pacemakers. AbstractThe sinoatrial node (SAN) is the primary pacemaker of the heart. Disease of the SAN, sick sinus syndrome, causes heart rate instability in the form of bradycardia and pauses, leading to exercise limitation and syncope. Biopacemaking aims to restore pacemaker activity by manipulating gene expression, and approaches utilising HCN channel overexpression have been widely used. We evaluated alternative gene targets for biopacemaking to restore normal SAN pacemaker physiology within bradycardic subsidiary atrial pacemaker (SAP) tissue, using the Na+/Ca2+ exchanger NCX1, and the transcription factors TBX3 and TBX18. TBX18 expression in SAP tissue restored normal SAN function, as assessed by increased rate (SAN 267.5 ± 13.6 bpm, SAP 144.1 ± 8.6 bpm, SAP‐TBX18 214.4 ± 14.4 bpm; P < 0.001), improved heart rate stability (standard deviation of RR intervals fell from 39.3 ± 7.2 ms to 6.9 ± 0.8 ms, P < 0.01; root mean square of successive differences of RR intervals fell from 41.7 ± 8.2 ms to 6.1 ± 1.2 ms, P < 0.01; standard deviation of points perpendicular to the line of identity of Poincaré plots (SD1) fell from 29.5 ± 5.8 ms to 7.9 ± 2.0 ms, P < 0.05) and restoration of isoprenaline response (increases in rates of SAN 65.5 ± 1.3%, SAP 28.4 ± 3.4% and SAP‐TBX18 103.3 ± 10.2%; P < 0.001). These changes were driven by a TBX18‐induced switch in the dominant HCN isoform in SAP tissue, with a significant upregulation of HCN2 (from 1.01 × 10−5 ± 2.2 × 10−6 to 2.8 × 10−5 ± 4.3 × 10−6 arbitrary units, P < 0.001). Biophysically detailed computer modelling incorporating isoform‐specific HCN channel electrophysiology confirmed that the measured changes in HCN abundance could account for the observed changes in beating rates. TBX3 and NCX1 were not effective in accelerating the rate of SAP tissue.

Genetic variants in incident SUDEP cases from a community-based prospective cohort with epilepsy

“…TBX18 is attractive target for biopace making as it is important in sinoatrial node development and so has the potential to have a broad effect on cardiac tissue phenotype 26. TBX18 had been associated with the generation of cardiac pacemaker cells and sick sinus syndrome 27 28. TBX18 could have an important role in restoring pacemaker function in human sick sinus syndrome (the most common bradycardia in humans and may increase the risk of sudden death) 28.…”

“…TBX18 had been associated with the generation of cardiac pacemaker cells and sick sinus syndrome 27 28. TBX18 could have an important role in restoring pacemaker function in human sick sinus syndrome (the most common bradycardia in humans and may increase the risk of sudden death) 28. KIF6 is ubiquitously expressed in coronary arteries and other vascular tissue and previously related with coronary heart disease.…”

Overexpression of the medium‑conductance calcium‑activated potassium channel (SK4) and the HCN2 channel to generate a biological pacemaker

“…electrophysiological data have shown that iK1 can promote an increase in MdP hyperpolarization, accelerate the four-phase depolarization slope and shorten the aPd, thus increasing the frequency of i(f)-induced automaticity (18). other studies indicated that biological pacemaker activity can also be successfully created by overexpression of T-box18 (TBX18) in vivo (22)(23)(24). The hypothesized mechanisms underlying this phenomenon are as follows: i) TBX18 can convert ventricular myocytes into sinoatrial node-like cells; and ii) TBX18 drives upregulation of Hcn4 or Hcn2, and downregulation of connexin 43, which increases automaticity and the conduction of impulse.…”

The Cardiac Pacemaker Story—Fundamental Role of the Na+/Ca2+ Exchanger in Spontaneous Automaticity

“…Wolf et al (2013) proposed a computational model of ankyrin-B syndrome SNCs in which alterations of I CaL and NCX as well as I NaK due to ankyrin-B dysfunction increase variability in SN automaticity (Wolf et al, 2013). Choudhury et al (2018) overexpressed NCX1 in bradycardic rat subsidiary atrial pacemaker tissue and found that it did not accelerate the rate of spontaneous depolarizations in contrast to overexpression of TBX18, which increased rate as well as stability and rescued isoproterenol response (Choudhury et al, 2018).…”