Abstract 705: Synergistic Activation of the Cardiac Enhancer Landscape During Cardiac Reprogramming

Abstract

Direct cardiac reprogramming of fibroblasts to cardiomyocytes is an attractive therapeutic strategy to restore cardiac function following injury. The cardiac transcription factors Gata4, Mef2c, and Tbx5 are sufficient to directly reprogram fibroblasts to a cardiac fate and their cardiogenic activity is enhanced by the addition of Hand2 and Akt1. However, the mechanisms by which these transcription factors orchestrate this cell fate conversion remains elusive. To understand the mechanistic basis of cardiac reprogramming, we performed a genome-wide analysis of cardiogenic transcription factor binding sites and enhancer activation throughout cardiac reprogramming. We found that cardiogenic transcription factors act cooperatively by co-occupying regulatory elements enriched with Mef2 binding sites during cardiac reprogramming. Importantly, these transcription factors when overexpressed in isolation are incapable of activating reprogramming enhancer elements. Further, we discovered that the early reprogramming enhancer landscape most closely resembles that of the neonatal heart. Acquisition of enhancers associated with cardiac maturation occurred at later stages in reprogramming, with addition of Hand2 and Akt1 augmenting activation of this more mature enhancer landscape. Additionally, we constructed a cardiac reprogramming gene regulatory network which demonstrated downregulation of the EGF receptor signaling pathway. We found that inhibition of EGF receptor signaling augments reprogramming toward the cardiac phenotype. Our findings demonstrate that cardiac reprogramming is coordinated by synergistic transcriptional activation across a broad landscape of cardiac enhancers and define the epigenetic landscape of the cardiac phenotype.