The molecular determinants of atrial fibrillation - PROJECT SUMMARY Atrial Fibrillation (AF) is an arrhythmia characterized by the aberrant, unorganized initiation and propagation of electrical impulses across the atria. The most common serious arrhythmia, AF affected an estimated 33.5 million globally in 2010, with a lifetime risk of 1 in 3 individuals older than 55 years old. A greater understanding of the mechanisms of AF is required to design more effective treatment strategies. Genome-wide association studies have linked AF with over 143 genomic loci, including the transcription factor (TF) TBX5. In our previous study, we integrated single-nucleus RNA- and ATACsequencing (multiomics) of control and Tbx5 KO aCMs with TBX5 chromatin occupancy in aCMs to identify direct TBX5 targets that might contribute to AF. In the process, we uncovered a novel function for TBX5 in maintaining genomic accessibility at enhancer elements, through genomic recruitment of CHD4. Inactivating CHD4 resulted in spontaneous AF and increased AF vulnerability in mice, and downregulated genes enhanced by TBX5. This finding is links genomic organization and a novel gene-activating function of CHD4 to rhythm maintenance, although the mechanism underpinnings of CHD4-mediated gene activation remain elusive. To better understand gene expression changes central to AF, we performed multiomics on a second AF mouse model caused by Liver Kinase B1 (LKB1) inactivation in aCMs, a gene decreased in human AF patient aCMs, revealing 632 core atrial rhythm (AR) genes that are commonly downregulated in aCMs from both models. 61% of AR genes were adjacent to regions co-occupied by CHD4-TBX5, and include the Na+ channel SCN5A, regulators FGF12/13 and the gap junction channels Connexin 43 (GJA1) and Connexin 40 (GJA5). These data lead to our central hypothesis that CHD4 maintains rhythm homeostasis in healthy atria through co-activating enhancers bound by TBX5, promoting the expression of genes critical for normal aCM electrical function. To determine the mechanisms by which the CHD4-TBX5 complex promotes the atrial enhancer network, we will identify CHD4 interacting proteins required for its activator function, by examining proteins bound at genes differentially regulated by CHD4 in aCMs and ventricular CMs. Our preliminary data suggests that FGF12 overexpression is protective against AF in the Tbx5 atrial KO AF model. We will examine how FGF12 alters atrial composition and electrical function in the established mouse AF models. This proposal is significant because it will mechanistically characterize genes required for atrial function, determine how they are regulated, and test their potential as therapies in multiple models, laying the groundwork for the development of more effective treatment strategies.
