PROJECT SUMMARY Cardiovascular disease remains the leading cause of death worldwide, necessitating continued research to develop novel therapeutic strategies. Historically, adult mammalian cardiomyocytes (CMs) were thought to be post-mitotic and therefore unable to regenerate the myocardium after injury. However, in recent years, scientists have shown that the adult mammalian CM is capable of a small amount of proliferation, though this competence is potentially restricted to a subset of cardiomyocytes. Patterson et. al demonstrated using the hybrid mouse diversity panel that having greater percentages of the rare mononuclear diploid cardiomyocyte (MNDCM) is associated with improved function, smaller scars, and enhanced CM proliferation after myocardial infarction. An accompanying genome-wide association analysis identified genetic loci associated with the frequency of the MNDCM population. One gene to come out of this screen was Runx1. Concurrently, RUNX1 captured the attention of cardiac regeneration researchers due to its increased presence in disease states, with some suggesting it may be a marker for dedifferentiation (fetal gene induction). CM-specific overexpression of Runx1 results in a doubling of the MNDCM population, thereby validating its influence on the population. Via multiple contexts including postnatal development and adult injury, knocking out Runx1 decreases DNA synthesis while overexpressing Runx1 increases DNA synthesis. Furthermore, an initial analysis of RNA sequencing data demonstrates that RUNX1 overexpression in a neonatal mouse upregulates known fetal CM genes and markers of CM cell cycle activity. These preliminary data are supported by the literature, which has shown in many other tissues that RUNX1, a transcription factor, directly regulates many genes associated with the cell cycle, indicating that RUNX1's role may be highly conserved across cell types. The central hypothesis of this study is that Runx1 regulates the CM response to heart failure via transcriptional induction of fetal genes and cell cycle activity. To test this idea the work proposed here will utilize both gain- and loss-of-function Runx1 mouse models temporally controlled by a CM-specific Cre. Aim 1 will investigate the effect of Runx1 on post-infarction outcomes and CM cell cycle. Aim 2 will assess transcriptional control of RUNX1 through two complementary genome-wide approaches: RNA sequencing and CUT&Tag. Results from this study will further advance the field's understanding of the genetic components involved during a cardiac injury and improve future treatment strategies.