Defective VWF secretion due to expression of MYH9-RD E1841K mutant in endothelial cells disrupts hemostasis

Mutations in MYH9, the gene encoding the heavy chain of nonmuscle myosin IIa (NMII-A), cause MYH9-related disease (MYH9-RD), which is an autosomal-dominant thrombocytopenia with bleeding tendency. Previously, we showed that NMII-A in endothelial cells (ECs) is critical for hemostasis via regulating von Willebrand factor (VWF) release from Weibel-Palade bodies (WPBs). The aim of this study was to determine the role of the expression of MYH9 mutants in ECs in the pathogenesis of the MYH9-RD bleeding symptom. First, we expressed the 5 most common NMII-A mutants in ECs and found that E1841K mutant-expressing ECs secreted less VWF than the controls in response to a cyclic adenosine monophosphate (cAMP) signaling agonist. Then, we generated 2 knockin mouse lines, 1 with Myh9 E1841K in ECs and the other in megakaryocytes. Endothelium-specific E1841K mice exhibited impaired cAMP-induced VWF release and a prolonged bleeding time with normal platelets, whereas megakaryocyte-specific E1841K mice exhibited macrothrombocytopenia and a prolonged bleeding time with normal VWF release. Finally, we presented mechanistic findings that E1841K mutation not only interferes with S1943 phosphorylation and impairs the peripheral distribution of Rab27a-positive WPBs in Ecs under quiescent condition but also interferes with S1916 phosphorylation by disrupting the interaction with zyxin and CKIIα and reduces actin framework formation around WPBs and subsequent VWF secretion under the stimulation by a cAMP agonist. Altogether, our results suggest that impaired cAMP-induced endothelial VWF secretion by E1841K mutant expression may contribute to the MYH9-RD bleeding phenotype.