Abstract EC.07: Tbx5 Maintains Atrial Identity By Regulating An Atrial Enhancer Network

Abstract

Atrial cardiomyocytes (aCMs) and ventricular cardiomyocytes (vCMs) are distinct cell types, but the mechanisms underlying these differences are not well understood. By selectively inactivating the transcription factor Tbx5 in the atrial working myocardium of the neonatal mouse heart, we show that it is required to maintain atrial identity. aCM inactivation of Tbx5 led to the downregulation of highly chamber specific genes including Myl7 and Nppa , and conversely, increased the expression of ventricular identity genes including Myl2 . Using concurrent single nucleus RNA- and ATAC- sequencing, we examined genomic accessibility changes underlying the atrial identity expression program, identifying 1846 genomic loci with greater accessibility in control vs KO aCMs. Nearly 70% of the control-enriched ATAC regions were bound by TBX5, demonstrating a novel role for TBX5 in maintaining atrial genomic accessibility. These regions were associated with genes that had higher expression in control aCMs compared to KO aCMs, suggesting they act as TBX5-dependent enhancers. This hypothesis was tested using HiChIP to analyze enhancer chromatin looping and we determined that 510 chromatin loops were sensitive to TBX5 dosage. Of the loops enriched in control aCMs, 73.7% were anchored in control-enriched ATAC regions. Together, these data demonstrate a role for TBX5 in maintaining atrial identity by supporting chamber specific chromatin confirmation required for aCM specificity.