A novel cause of type 1 von Willebrand disease: impaired exocytosis of Weibel-Palade bodies due to biallelic MADD variants.

The regulated secretion of von Willebrand factor (VWF) from Weibel-Palade bodies (WPBs) in endothelial cells is fundamental to hemostasis. This process relies on recruiting Rab GTPases and their effectors to the WPB membrane, with the guanine nucleotide exchange factor MAPK-activating death domain (MADD) playing a central role. Biallelic variants in MADD lead to a pleiotropic neurological and developmental disorder that can include bleeding abnormalities. This study investigates the impact of pathogenic MADD variants on VWF secretion using patient-derived endothelial cells. We isolated endothelial colony-forming cells (ECFCs) from 3 pediatric patients with biallelic MADD variants and unaffected heterozygous family members. All patients exhibited low VWF plasma levels (22-30 IU/dL). Proteomic analysis of patient-derived ECFCs revealed an absence of MADD peptides, reduced VWF, and downregulation of proteins involved in the exocytotic machinery, including Rab3D and the Rab3/27 effector Slp4-a. Functional assays demonstrated diminished Rab27A and Rab3D activity and their failure to localize to WPBs in patient cells. Biochemical and live-imaging studies showed that histamine-induced VWF and VWF propeptide secretion were significantly reduced in patient cells due to delayed and reduced degranulation of WPBs. Our findings demonstrate the critical role of MADD in maintaining the secretion competence of WPBs and the magnitude of VWF secretion by regulating the recruitment of the endothelial exocytotic machinery. This study highlights the in vivo significance of WPB exocytosis in maintaining plasma VWF levels and establishes MADD as the first causal gene for quantitative von Willebrand disease in patients without pathogenic VWF variants.