Defining the role of Fzd6 in lymphatic network development and function - Project Summary The lymphatic system plays an important role in maintaining tissue-fluid homeostasis, the absorption of dietary fats, and the transport of immune cells throughout the body to combat infection. Central conducting lymphatic anomaly (CCLA) is a disorder resulting from lymphatic network conduction abnormalities which can result in the accumulation of fluid in tissues. Despite recent advances in patient sample collection and more thorough genetic testing, only about 40% of patients with CCLA have a defined genetic cause. Identification of the underlying disease mechanism in these patients has resulted in the targeted trial of specific pathway inhibitors including MEK, ERK, and PI3K. Elucidating novel genetic causes of CCLA will inform the development of new custom therapies. Exome sequencing in a cohort of five families with children who presented with non-immune fetal hydrops revealed FZD6 variants. FZD6 has therefore emerged as a potential novel cause of CCLA. FZD6 is a WNT receptor and a core member of the planar cell polarity (PCP) signaling pathway. In humans, loss of FZD6 function causes nail dysplasia, which is also present in the identified cohort. The core PCP genes Celsr1 and Vangl2 are shown to be involved in lymphatic valve formation in mice and defects in PCP signaling due to pathogenic variants in CELSR1 have been implicated in lymphatic malformations in humans. However, the role of FZD6 in lymphatic network development and function is unstudied. We hypothesize that FZD6 is important for lymphatic system development, and the identified variants result in CCLA by disrupting PCP signaling. Further, in addition to the role PCP plays during valve formation, we propose PCP has an earlier role in shaping the lymphatic network by regulating lymphatic endothelial cell dynamics. I will test these hypotheses using PCP loss of function mouse models. I will assess the morphology of the developing lymphatic vessels and valves in PCP mutant embryos compared to littermate controls in the dermis and mesentery (Aim 1). I will then assess the molecular function of FZD6 variants identified in patients. Subsequently, I will determine how the loss of PCP function affects lymphatic network uptake and conductance capabilities by injecting a tracer molecule (Aim 2). Finally, I will determine which lymphatic endothelial cellular behaviors are influenced by PCP signaling through a live-imaging approach (Aim 3). Completing these aims will identify a novel genetic driver of CCLA and inform targeted treatments to rescue the effects of FZD6 loss. Further, the results of this study will fill the gaps in our understanding of the role of PCP signaling in lymphatic network formation during development.
