Loss of the Tbx5 gene in the ventricular conduction system induces spontaneous ventricular arrhythmias

Abstract

The ventricular conduction system (VCS) and the working myocardium (WM) derive from the same myogenic progenitors during the embryonic development. The VCS development is regulated by numerous genes including the T-box transcription factor 5 (Tbx5). In human, Tbx5 mutations cause Holt-Oram syndrome, an autosomal dominant disease associated with heart abnormalities such as arrhythmias. In adult mice, the deletion of the Tbx5 gene provoked a slowing of ventricular conduction due to a loss of the gap junction Connexin 40 and dysfunction of cardiac channels. However, it is still unknown how Tbx5 deletion affects the development and the function of the VCS. The aim is to study the consequences of the Tbx5 deletion in embryonic progenitors of the ventricular conduction system on cardiac function in adult mice. Conditional deletion of Tbx5 alleles in mice was induced by expressing the Cre recombinase under the control of the Connexin 40 (Cx40) gene. We studied two groups of mutants: Tbx5 deletion at embryonic days E13-14 targeting both the ventricular conduction system and the working myocardium (“VCS + WM” group; n = 40) and mice deleted at E18-P2 targeting only the ventricular conduction system (“VCS” group; n = 17). Surface ECGs were achieved monthly until 3 months of age, when immunostaining was performed on the left VCS. Immunostaining showed a reduced expression of the two major connexines in the ventricle (Cx40 and Cx43) and a reduced number of conductive cells in both mutants, suggesting a defective maturation of the VCS (Fig. 1). Furthermore, the loss of Cx43 was correlated with that of Tbx5, reflecting a direct transcriptional effect. Surface ECG revealed a significant increase of the ventricular activation duration in both mutants (P < 0.01). Moreover, premature ventricular contractions (PVCs) and a left QRS axis deviation were observed in both mutants. Finally, these PVCs disappeared with age in both mutants under resting conditions, and are unmasked when the heart is challenged with a β-1 adrenergic agonist. Our results provide new insights into Tbx5 targets in the VCS. Tbx5 deletion impacts the maturation and the anatomy of the VCS as proved by the deviation of the ventricular electrical axis, which even occurs when the VCS is already formed. Tbx5 appears to have a direct impact on the Gja1 gene, encoding the gap junction Cx43, which is reduced at the junction between the VCS and the WM in both mutants and may be responsible for PVCs.