Venous thrombosis (VT) is the third leading cause of cardiovascular death in industrialized countries. Using a genome wide association study (GWAS) approach we have recently identified SLC44A2 as a new susceptibility gene for VT, the Arg154 isoform being associated with an increased risk. The SLC44A2 gene does not belong to the coagulation/fibrinolytic cascades and encodes choline transporter-like protein 2 (CTL2). Little is known about the function of CTL2. It has been associated with transfusion-related acute lung injury (TRALI), a process which involves activated neutrophils in a CTL2 isoform-dependent manner. In TRALI, alloantibodies react with the Arg154 (also called HNA-3a antigen), but not with the Gln154 (HNA-3b antigen). The direct binding of HNA-3a antibodies to HNA-3a+ neutrophils results in neutrophil activation and subsequent neutrophil-mediated destruction of pulmonary endothelial cells. The TRALI model, thought being different from VT physiopathology, paves the way to understand why SLCC44A2 plays a role in VT and why Arg154 isoform carriers have an increased risk of VT. The aim of the current project is to characterize the role of SLC44A2 in the physiopathology of VT using in-depth molecular functional studies. Given the increasing evidence of neutrophil involvement in VT physiopathology, we speculate that Arg154 isoform favors neutrophil adhesion and activation which in turn provides the initiating stimulus for VT development. To explain why the HNA-3a antigen associated with TRALI is also linked to an increased risk of VT, we made several assumptions based on 1) the receptor properties of CTL2 and 2) on the presence of CTL2 antibodies in blood circulation. We will characterize the interaction between CTL2 and von Willebrand factor (VWF), and how the polymorphism modulates this interaction. We will define the domain of interaction of VWF to CTL2 and elaborate potent modulating tools. For this, we will use human embryonic kidney cells (HEK-293 cell line) transfected to over-express either the CTL2/HNA-3a or the CTL2/HNA-3b isoform, in static and flow-based cell recruitment assays. Screening for anti-CTL2 autoantibodies will be performed in a large sample of individuals with VT from the EDITH case/control study. If present, antibodies from patients will be purified and they will be characterized. We will investigate the effects of the Arg154Gln polymorphism in vivo through work on SLC44A2 knock-out mice and the generation of knock-in mutant-mice. These humanized mice will be submitted to the most clinically relevant model of VT which consists in the partial ligation of the inferior vena cava. The development of the thrombus, the capacity of neutrophils to adhere to the venous endothelium and to become activated will be compared in the mice carrying either the CTL2/HNA-3a or CTL2/HNA-3b isoform. The capacity of NET release by activated neutrophils will also be compared. The present translational project is the transformation of omics data, obtained through the largest investigation to date of the influence of common genetic variations on VT risk by meta-analyzing 12 VT GWAS, into new ways of understanding, prediction (by identification and validation of new biomarkers) and treatment of patients with VT.