Lyve1-Driven Nras(Q61R) Causes Edema, Enlarged Lymphatic Vessels, and Hepatic Vascular Defects in Embryonic Mice.

BACKGROUND: Kaposiform lymphangiomatosis (KLA) is a complex lymphatic anomaly associated with a somatic activating NRAS p.Q61R (NRAS(Q61R)) mutation. KLA is characterized by malformed lymphatic vessels that can lead to effusions and coagulopathy. The goal of this study was to generate an in vivo mouse model to determine if prenatal expression of the Nras(Q61R) mutation in lymphatic endothelial cells induces disease characteristics found in KLA patients. PROCEDURE: A Cre-loxP system was used to conditionally express Nras(Q61R) in cells expressing lymphatic vessel endothelial hyaluronan receptor 1 (Lyve1), a marker of lymphatic and other types of endothelial cells that starts being expressed at embryonic day (E) 7.5. Because pups did not survive birth, embryos were collected at E14.5, E15.5, and E18.5 for gross analysis, histology and immunostaining, and organ whole-mounts. RESULTS: Staining for NRAS(Q61R) demonstrated robust recombination in the Nras(Q61R) mutant embryos and localization of Nras(Q61R) at sites of vascular abnormalities. Nras(Q61R) mutant embryos had significant edema and dysmorphic jugular lymph sacs with abnormal Lyve1-positive cellular masses. The lymphatic vessel network in the back skin of the Nras(Q61R) mutant embryos had fewer branch points and increased vessel diameter. Nras(Q61R) mutant embryos had severe hepatic defects characterized by disordered and enlarged vessels. By E18.5, Nras(Q61R) mutant embryos were dead. CONCLUSIONS: Conditional expression of Nras(Q61R) in Lyve1-positive cells caused edema, abnormal lymphatic development, and hepatic vascular defects in mouse embryos. These findings further support the role of NRAS(Q61R) as a driver of the lymphatic overgrowth, vessel enlargement, and dysfunction in the pathophysiology of KLA.