An atrial fibrillation-associated regulatory region modulates cardiac <i>Tbx5</i> levels and arrhythmia susceptibility.

Abstract

Heart development and rhythm control are highly Tbx5 dosage-sensitive. <i>TBX5</i> haploinsufficiency causes congenital conduction disorders, whereas increased expression levels of <i>TBX5</i> in human heart samples has been associated with atrial fibrillation (AF). We deleted the conserved mouse orthologues of two independent AF-associated genomic regions in the <i>Tbx5</i> locus, one intronic (RE(int)) and one downstream (RE(down)) of <i>Tbx5</i>. In both lines, we observed a modest (30%) increase of <i>Tbx5</i> in the postnatal atria. To gain insight into the effects of slight dosage increase in vivo, we investigated the atrial transcriptional, epigenetic and electrophysiological properties of both lines. Increased atrial <i>Tbx5</i> expression was associated with induction of genes involved in development, ion transport and conduction, with increased susceptibility to atrial arrhythmias, and increased action potential duration of atrial cardiomyocytes. We identified an AF-associated variant in the human RE(int) that increases its transcriptional activity. Expression of the AF-associated transcription factor <i>Prrx1</i> was induced in <i>Tbx5^RE(int)KO</i> cardiomyocytes. We found that some of the transcriptional and functional changes in the atria caused by increased <i>Tbx5</i> expression were normalized when reducing cardiac <i>Prrx1</i> expression in <i>Tbx5^RE(int)KO</i> mice, indicating an interaction between these two AF genes. We conclude that modest increases in expression of dose-dependent transcription factors, caused by common regulatory variants, significantly impact on the cardiac gene regulatory network and disease susceptibility.