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Introduction: Imaging studies patients with mitral valve prolapse (MVP) have previously shown cross sectional evidence of left ventricular (LV) fibrosis as well as increased risk for sudden cardiac death. This, along with evidence for post-repair ventricular dysfunction in Mitral valve surgery patients support a link between fibrosis and MVP. The development of fibrosis as a result of MVP, however, remains poorly understood. Hypothesis: Regionalized LV Fibrosis in MVP patients is driven by increased mechanical tension through the chordae tendineae by billowing leaflets. Methods: We performed histopathologic analysis on cardiac biopsies of 6 patients who underwent surgical repair for MV prolapse. We additionally performed longitudinal histopathologic analysis on hearts harvested from a mouse modal of human non-syndromic MVP (Dzip1 S14R/+). To assess the cellular response to increased mechanical tension, pathological stretch assays on human cardiac fibroblasts were performed in tandem with RNA seq, as well ATP luminescence assays to assess ATP release from stretched cells. Results: Biopsies from peripapillary regions, inferobasal wall, LV septum, and LV apex demonstrated regionalized myocardial fibrosis that correlated with increased macrophages and myofibroblasts. Our mouse model demonstrated similar regionalized deposition of collagen that increased over time, as well as an increase in macrophages and myofibroblasts. Stretching of human cardiac fibroblasts caused significant changes in 232 transcripts (Bonferroni corrected P value Conclusions: A mouse model of human MVP phenocopied regionalized myocardial fibrosis seen in humans and will provide a useful tool to investigate mechanisms of myocardial fibrosis in MVP patients. Stretch initiates changes in pro-fibrotic gene transcription, and causes ATP release from human cardiac fibroblasts.

Background: Bicuspid aortic valve (BAV) disease is a congenital defect that affects 0.5% to 1.2% of the population and is associated with comorbidities including ascending aortic dilation and calcific aortic valve stenosis. To date, although a few causal genes have been identified, the genetic basis for the vast majority of BAV cases remains unknown, likely pointing to complex genetic heterogeneity underlying this phenotype. Identifying genetic pathways versus individual gene variants may provide an avenue for uncovering additional BAV causes and consequent comorbidities. Methods: We performed genome-wide association Discovery and Replication Studies using cohorts of 2131 patients with BAV and 2728 control patients, respectively, which identified primary cilia genes as associated with the BAV phenotype. Genome-wide association study hits were prioritized based on P value and validated through in vivo loss of function and rescue experiments, 3-dimensional immunohistochemistry, histology, and morphometric analyses during aortic valve morphogenesis and in aged animals in multiple species. Consequences of these genetic perturbations on cilia-dependent pathways were analyzed by Western and immunohistochemistry analyses, and assessment of aortic valve and cardiac function were determined by echocardiography. Results: Genome-wide association study hits revealed an association between BAV and genetic variation in human primary cilia. The most associated single-nucleotide polymorphisms were identified in or near genes that are important in regulating ciliogenesis through the exocyst, a shuttling complex that chaperones cilia cargo to the membrane. Genetic dismantling of the exocyst resulted in impaired ciliogenesis, disrupted ciliogenic signaling and a spectrum of cardiac defects in zebrafish, and aortic valve defects including BAV, valvular stenosis, and valvular calcification in murine models. Conclusions: These data support the exocyst as required for normal ciliogenesis during aortic valve morphogenesis and implicate disruption of ciliogenesis and its downstream pathways as contributory to BAV and associated comorbidities in humans.