Stable In Vivo Ventricular Pacing Created By Tbx18-Induced Pacemaker Cells Upon Inhibition Of Epithelial-To-Mesenchymal Transformation In An Ambulatory Rat Model Of Complete Atrioventricular Block

Introduction: We have previously demonstrated conversion of ventricular cardiomyocytes to induce pacemaker cells (iPMs) by TBX18. However, the iPMs’ automaticity waned over time due to epithelial-to-mesenchymal transformation (EMT)-mediated cell detachment, collaterally activated by TBX18. We hypothesized that inhibitions of EMT with direct myocardial gene transfer of TBX18 can induce stable biological pacing in a rat model of complete atrioventricular block (CAVB). Methods: A rat model of CAVB was created by ablating the AV node area with a high-frequency electrosurgical generator. Upon CAVB, adenoviral gene transfer of human TBX18 was performed to the LV apex. In another group of TBX18-injected rats, an osmotic pump was implanted for peritoneal release of an inhibitor of EMT, A83. Control animals were implanted with an A83-osmotic pump, but injected with a GFP adenovirus. Animal were followed for 3 wks by telemetry and weekly ECG. Results: Immediately upon CAVB, control rats’ heart rate dropped from 352±40 to 102±38 bpm (n=15). The CAVB rats were sustained by slow junctional rhythm which persisted from wk1 (132±19) through wk4 (134±22). TBX18-injected rats’ mean ventricular rate increased from 165±46 (d1) to 186±35 bpm at wk1 but began wavering, 179±46 at wk2 and 167±44 at wk3. In contrast, TBX18-injected rats with EMT inhibition exhibited stable biological pacing at rates significantly higher than control and TBX18-only rats; 171±41 (d1), 157±35 (wk1), 176±54 (wk2), and 231±33 (wk3). TBX18-only group showed non-sustained ectopic beats that often competed with junctional escape beats. In contrast, TBX18-injected rats with EMT inhibition demonstrated highly-organized, sustained ventricular beats that often approach the sinus rate. With EMT inhibition, the TBX18-transduced myocardium was restricted to the injection site, similar to the GFP. Without EMT inhibition, however, TBX18-transduced myocardium appeared to spread away from the injection site. Conclusions: Stable, TBX18-mediated biological pacing in vivo can be achieved by simultaneous inhibition of EMT. The iPMs, with suppressed EMT, retain cell-cell coupling and synchronous pacing, and may allow the iPMs to better overcome the source-to-sink mismatch in ventricular myocardium.