The Noonan Syndrome Gene Lztr1 Controls Cardiovascular Function by Regulating Vesicular Trafficking

Rationale: Noonan syndrome (NS) is one of the most frequent genetic disorders. Bleeding problems are among the most common, yet poorly defined complications associated with NS. A lack of consensus on the management of bleeding complications in NS patients indicates an urgent need for new therapeutic approaches. Objective: Bleeding disorders have recently been described in NS patients harboring mutations of LZTR1, an adaptor for CULLIN3 ubiquitin ligase complex. Here, we assessed the pathobiology of LZTR1-mediated bleeding disorders.Methods and Results: Whole-body and vascular specific knockout of Lztr1 results in perinatal lethality due to cardiovascular dysfunction. Lztr1 deletion in blood vessels of adult mice leads to abnormal vascular leakage. We found that defective adherent and tight junctions in Lztr1-depleted endothelial cells are caused by dysregulation of vesicular trafficking. LZTR1 affects the dynamics of fusion and fission of recycling endosomes by controlling ubiquitination of the ESCRT-III component CHMP1B, whereas NS-associated LZTR1 mutations diminish CHMP1B ubiquitination. LZTR1-mediated dysregulation of CHMP1B ubiquitination triggers endosomal accumulation and subsequent activation of VEGFR2 and decreases blood levels of soluble VEGFR2 in Lztr1 haploinsufficient mice. Inhibition of VEGFR2 activity by cediranib rescues vascular abnormalities observed in Lztr1 knockout mice. Conclusions: Lztr1 deletion phenotypically overlaps with bleeding diathesis observed in NS patients. ELISA screening of soluble VEGFR2 in the blood of LZTR1-mutated NS patients may predict both the severity of NS phenotypes and potential responders to anti-VEGF therapy. VEGFR inhibitors could be beneficial for the treatment of bleeding disorders in NS patients.