Cilia in Heart Development and Disease

Congenital Heart Disease (CHD) is the most common birth defect affecting 1% of all live born infants. While ~90% of patients with CHD survive into adulthood, there are many comorbidities that make CHD an increasingly significant public health problem. Genomic analyses of large cohorts of CHD patients have identified a significant genetic contribution to CHD, but the link between etiology and clinical outcome remains an important question. When I began my search for the cause of CHD, I identified the cilium as being central to left-right (LR) axis and cardiac development, and most recently, as part of the Pediatric Cardiac Genomics Consortium (PCGC), identified significant contributions from mutations affecting cilia and chromatin remodeling genes to human CHD. However, the question of how cilia dysfunction precisely influences CHD remains unanswered. I have assembled a group of co-investigators with expertise in mouse and zebrafish development, live-cell imaging, optogenetics and genomics to take a multi-pronged approach to understanding the central role of the cilium in heart development with the long-term goal of leveraging this data with ongoing genomic and clinical studies to improve clinical outcomes of CHD. First, we will resolve the long-standing question of how cilia instruct cardiac LR asymmetry. We will use single-cell RNAseq to define the cellular composition of the left-right organizer (LRO), a transient ciliated organ that is essential for instructing cardiac asymmetry. We will then establish the molecular mechanism linking cilia signaling at the LRO to cardiac LR development in mouse and zebrafish embryos. Together these experiments will uncover the mechanism by which an embryo determines LR asymmetry, and provide gene sets that will inform the search for human CHD candidate genes. Second, we will investigate the role of cilia in cardiac valve formation. We have found dynamic, flow-sensitive cilia in the presumptive atrio-ventricular valve region of the zebrafish heart, and will test the hypothesis that valve specification is driven by the mechanical forces occurring at interfaces between differentially contracting chambers, and that valve cilia are the mechanotransducers leading to changes in transcription of klf2/klf4 and downstream valve morphogenesis. Third, we will unravel the mechanism by which epigenetic factors influence cardiac development. The important role of chromatin remodeling genes in human CHD has raised the question whether any of these transcriptionally regulate cilia in heart development. We have already found that histone H2B monoubiquitination (H2BUb1) transcriptionally regulates cilia function at the LRO. We are now testing how H2BUb1 affects cardiac development in mouse embryos and human iPSC- derived cardiomyocytes through cilia-dependent and/or cilia-independent mechanism(s). Our long-term goal of translating the basic biologic and genomic data to clinical impact will be addressed by returning mechanistic and genetic data from our cilia work to the PCGC CHD genomics project so that the resulting discoveries lead to personalized medicine for patients with CHD. !