Project Summary/Abstract Myxomatous degeneration leads to mitral valve prolapse, which occurs in almost 3% of the general population and 10% of the elderly, is a significant cause of morbidity and mortality. Additionally, early-onset of myxomatous mitral valve degeneration is associated with both syndromic and non-syndromic diseases, supporting there is an underlying genetic etiology. Despite the frequency of mitral valve diseases, the cellular, molecular, and genetic etiologies underlying myxomatous degeneration of the mitral valves remain poorly understood. Presently, valve reconstruction and replacement surgeries are the only therapies available for mitral valve diseases. Thus, in order to develop novel non-invasive pharmacological therapies that can effectively prevent and ameliorate mitral valve diseases, it is essential to understand conserved mechanisms the underlie the progression of valve diseases in vertebrates. The specific aims of this proposal are to interrogate the mechanisms by which loss of the Nr2f transcription factors can lead to the development of myxomatous valves in zebrafish and mice. Numerous studies have indicated that mutations in Nr2f genes in humans are associated with a spectrum of congenital heart defects, some of which are correlated with myxomatous valve degeneration. While requirements for Nr2f factors are well-established in heart development, previous work has not implicated Nr2f transcription factors in homeostasis of mature valves and myxomatous valve degeneration. Interestingly, the majority of previous genes associated with myxomatous valve degeneration are involved in the regulation of extracellular matrix, mechanotransduction, and cilia. Our preliminary analysis in adult zebrafish mutants called acorn worm (aco), which are deficient for Nr2f1a, show they develop myxomatous atrioventricular valves with all the hallmarks of myxomatous valves in mammals. Furthermore, we identify Nr2f proteins are expressed in previously unrecognized populations of cells within the atrioventricular valves. In Aim 1, we will use tissue-specific rescue and knockout approaches in zebrafish and mice to determine if valve endothelial cells require Nr2f to maintain valve homeostasis and prevent myxomatous generation. In Aim 2, we will employ pharmacological and genetic epistasis to decipher if RA and signals including Fibrillin 1, whose misexpression is associated with myxomatous degeneration in humans, function downstream of Nr2f1a to promote myxomatous atrioventricular valve degeneration. In Aim 3, we will use lineage tracing and ablation studies to determine if specific immune cells contribute to the progression of myxomatous atrioventricular valves in aco mutants. Our use of these unique mutants with complementary analysis in mice will dramatically improve our understanding of conserved mechanisms that can lead to the progression of myxomatous valve degeneration in vertebrates. Ultimately, our studies may provide the foundation ...