Epigenetic Profiling of Cardiac Pacemaker Cells Reveals a Conserved Isl1 Enhancer That Regulates Sinoatrial Node Development and Function

Cardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1, have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified. We used ATAC-Seq on sorted neonatal mouse SAN to define regions of open chromatin in PCs and neighboring right atrial cardiomyocytes. PC-specific ATAC-seq peaks were located near established SAN genes and were enriched for distinct sets of transcription factor binding sites. In-vivo testing of a novel ATAC-seq peak at the Isl1 locus defined a regulatory element that was active specifically in the cardiac inflow at E8.5 and throughout later SAN development and maturation. Deletion of the enhancer uncovered a role for this cis-regulatory element in SAN development and function. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations. Overall design: Examination of regions of open chromatin in isolated RNA-seq validated cardiac SAN PCs and RACMs from neonatal mouse hearts (p0) using ATAC-seq.