(H-284) Overexpression of Tbx5 in Cardiac Nodal Cells Alters Conduction, Arrhythmogenesis, and Calcium Handling in the Right Atrium

Introduction:: T-box transcriptional activator (Tbx5) plays a critical role in the development of the cardiac conduction system and arrhythmias. Dominant mutations in Tbx5 have been associated with atrial fibrillation, the most common type of arrhythmia. To elucidate the role of
Tbx5 in atrial fibrillation pathogenesis, Tbx5 was over-expressed in the nodal cells (pacemaker cells) of mice hearts.

Materials and Methods:: Cardiac electrophysiology was assessed in the right atrium of mice with Tbx5 overexpression in nodal cells (TBX5CT+/- : HCN4-Cre+/-, n = 10) and control (TBX5CT+/+ : HCN4-Cre+/-, n = 9) littermates. Optical mapping was performed to record transmembrane potential and intracellular calcium transients from right atrial tissue preparations. Pseudo-electrocardiograms (ECG) were recorded to determine heart rhythm. The atrial preparations were superfused with Tyrode’s solution without (baseline) and with 800 uM acetylcholine (to provoke abnormal heart rhythm). During both these conditions, the atrial preparation was subjected to cardiac restitution and an arrhythmia inducibility protocol during baseline conditions. Action potential duration at 80% repolarization (APD80), calcium transient duration (CaTD), and transverse conduction velocity (CVT) were calculated in the right atria. From the pseudo-ECG recording, the presence of arrhythmias, atrial tachycardia and atrial fibrillation, and the duration of arrhythmia were assessed.

Results, Conclusions, and Discussions:: The overexpression of Tbx5 resulted in increased CaTD in the right atrium (47.18 ± 5.89 ms at cycle length = 100 ms) compared to control mice (37.42 ± 0.04 ms, p =0.049). APD80 was not significantly different between Tbx5 overexpressing (32.10 ± 7.43 ms at cycle length = 100 ms) and control mice (32.46 ± 10.03 ms, p = 0.94). Conduction block in the propagation of electrical impulses in the right atrium was observed in 3 of 10 Tbx5 overexpressing mice but not in control mice. Among Tbx5 overexpressing mice, those that developed conduction block had slower CVT (0.19 ± 0.04 m/s at cycle length = 100 ms) than those without conduction block (0.38 ± 0.04 m/s, p = 0.0001) or control mice (0.35 ± 0.04 m/s, p = 0.0002). Slow conduction creates an arrhythmogenic substrate and increases the risk of re-entrant arrhythmias. Atrial tachycardia was observed in the Tbx5 overexpressing mice with conduction block under baseline conditions. Acetylcholine treatment increased atrial tachycardia incidences in control mice (p = 0.013) but not Tbx5 overexpressing mice (p = 1.000). The overexpression of Tbx5 in nodal cells creates an arrhythmogenic substrate in the right atria through calcium mishandling, cardiac conduction slowing, and decreased sensitivity to acetylcholine-induced arrhythmias. A recent study using cap analysis of gene expression has shown that Tbx5 has differential, sex-specific expression in healthy and failing human atria. Future work will elucidate the roles of sex and overexpression of Tbx5 in atrial fibrillation pathogenesis.

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