Molecular and cellular mechanisms in cardiac outflow tract formation and defects

Project Summary Cardiac outflow tract (OFT) defects have an estimated prevalence of 1-2 in 1,000 live births. The 22q11.2 deletion syndrome or 22q11.2DS is one of the most frequent genetic causes of cardiac OFT defects. A total of 60% of patients with 22q11.2DS have congenital heart disease that ranges from mild to severe including bicuspid aortic valve (BAV), isolated ventricular septal defects (VSDs) to tetralogy of Fallot (TOF) or persistent truncus arteriosus (PTA). These clinical findings suggest genetic modifiers may affect phenotypic expression . In this project, we propose to use the Lgdel/+ mouse model to understand the relationship between neural crest cells (NCCs) and adjacent endocardial cells (ECCs) in forming and remodeling of the cardiac OFT. Mesenchymal cells (MCs) derived from NCCs and ECCs occupy the distal and proximal OFT, respectively, and form a distinct OFT MC boundary during heart development. Proper deployment of MCs from the two lineages ensures correct position and formation of aorto-pulmonary-ventricular septum and semilunar valves to separate the heart outlet into the systemic and pulmonary circulation. The function of NCCs in OFT defects has been well studied with respect to 22q11.2DS, however, the role of ECCs in OFT malformations has not been investigated. We have begun to fill this knowledge gap by studying the Lgdel/+ mouse, which was generated by deleting one copy of the mouse syntenic region of human 22q11.2 containing 26 protein-coding genes (22q11.2DS genes). We found a spectrum of OFT defects ranging from isolated VSD to TOF. The structural defects are preceded by a disrupted OFT MC boundary, increased expression of Edn1 during endocardial-to-mesenchymal transformation (EMT), and decreased NOTCH1 signaling and Ctgf expression during post-EMT OFT remodeling. By single cell RNA sequencing (scRNA-seq), we identified Edn1 as part of a unique gene program operating in a subset of ECCs undergoing EMT. Based on these findings, we propose an overall hypothesis that 22q11.2DS genes control OFT development by regulating the function of ECCs and the cell-cell communications between MCs from ECC and NCC origins, via interacting with genes essential for OFT formation. We will test this hypothesis in three specific aims. Aim 1 will determine whether the 22q11.2DS genes regulate EMT through modulating the EMT gene program, and if Edn1 acts downstream of 22q11.2DS genes to regulate the process. Aim 2 will ascertain whether 22q11.2DS genes also regulate OFT remodeling through a cell-cell interaction network that patterns the OFT MC boundary, and if Ctgf functions as a hub gene required for the post-EMT OFT remodeling, downstream of 22q11.2DS genes. Aim 3 will define whether Notch1 haploinsufficiency can potentiate the 22q11.2DS OFT defects. At the completion of this study, we expect discoveries that will establish genetic, molecular, and cell crosstalk regulated by important syndromic and non-syndromic CHD genes essential for mouse OFT morphogenesis. The information will provide deeper understanding of heart developmental biology and inform the disease mechanism of OFT defects, with a broader implication in congenital heart disease.