BLOC-2 subunit HPS6 deficiency affects the tubulation and secretion of von Willebrand factor from mouse endothelial cells

Hermansky-Pudlak syndrome (HPS) is a recessive disorder with bleeding diathesis, which has been linked to platelet granule defects. Both platelet granules and endothelial Weibel-Palade bodies (WPBs) are members of lysosome-related organelles (LROs) whose formation is regulated by HPS protein associated complexes such as BLOC (biogenesis of lysosome-related organelles complex) -1, -2, -3, AP-3 (adaptor protein complex-3) and HOPS (homotypic fusion and protein sorting complex). Von Willebrand factor (VWF) is critical to hemostasis, which is stored in a highly-multimerized form as tubules in the WPBs. In this study, we found the defective, but varying, release of VWF into plasma after desmopressin (DDAVP) stimulation in HPS1 (BLOC-3 subunit), HPS6 (BLOC-2 subunit), and HPS9 (BLOC-1 subunit) deficient mice. In particular, VWF tubulation, a critical step in VWF maturation, was impaired in HPS6 deficient WPBs. This likely reflects a defective endothelium, contributing to the bleeding tendency in HPS mice or patients. The differentially defective regulated release of VWF in these HPS mouse models suggests the need for precise HPS genotyping before DDAVP administration to HPS patients.