Lentiviral gene rescue of a Bernard-Soulier mouse model to study platelet glycoprotein Ibβ function

Lentiviral gene therapy reverts GPIX expression and phenotype in Bernard-Soulier syndrome type C

Bernard-Soulier syndrome (BSS) is a rare congenital disease characterized by macrothrombocytopenia and frequent bleeding. It is caused by pathogenic variants in three genes (GP1BA, GP1BB, or GP9) that encode for the GPIbα, GPIbβ, and GPIX subunits of the GPIb-V-IX complex, the main platelet surface receptor for von Willebrand factor, being essential for platelet adhesion and aggregation. According to the affected gene, we distinguish BSS type A1 (GP1BA), type B (GP1BB), or type C (GP9). Pathogenic variants in these genes cause absent, incomplete, or dysfunctional GPIb-V-IX receptor and, consequently, a hemorrhagic phenotype. Using gene-editing tools, we generated knockout (KO) human cellular models that helped us to better understand GPIb-V-IX complex assembly. Furthermore, we developed novel lentiviral vectors capable of correcting GPIX expression, localization, and functionality in human GP9-KO megakaryoblastic cell lines. Generated GP9-KO induced pluripotent stem cells produced platelets that recapitulated the BSS phenotype: absence of GPIX on the membrane surface and large size. Importantly, gene therapy tools reverted both characteristics. Finally, hematopoietic stem cells from two unrelated BSS type C patients were transduced with the gene therapy vectors and differentiated to produce GPIX-expressing megakaryocytes and platelets with a reduced size. These results demonstrate the potential of lentiviral-based gene therapy to rescue BSS type C.

Murine models of glycoprotein Ib-IX

The utility of mouse models to dissect the molecular basis of hemostasis and thrombosis is now well established. The anucleate properties of circulating blood platelet and their specialized release from mature megakaryocytes makes the use of in vivo models all the more informative and powerful. Indeed, they are powerful but there do exist limitations. Here, we review the contributions of mouse models to the pathogenesis of the Bernard-Soulier syndrome, their use in platelet-specific gene expression, the recent development of mice expressing both human GPIb-IX and human von Willebrand factor (VWF), and finally the use of GPIb-IX mouse models to examine the impact of platelet biology beyond clotting. The humanization of the receptor and ligand axis is likely to be a major advancement in the characterization of therapeutics in the complex pathogenesis that drives thrombosis. When appropriate, we highlight some limitations of each mouse model, but this is not to minimize the contributions these models to the field. Rather, the limitations are meant to provide context for any direct application to the important mechanisms supporting human primary hemostasis and thrombosis.

Why thromboembolism occurs in some patients with thrombocytopenia and treatment strategies

Platelets play such an important role in the process of thrombosis that patients with thrombocytopenia generally have an increased risk of bleeding. However, abnormal thrombotic events can sometimes occur in patients with thrombocytopenia, which is unusual and inexplicable. The treatments for thrombocytopenia and thromboembolism are usually contradictory. This review introduces the mechanisms of thromboembolism in patients with different types of thrombocytopenia and outlines treatment recommendations for the prevention and treatment of thrombosis. According to the cause of thrombocytopenia, this article addresses four etiologies, including inherited thrombocytopenia (Myh9-related disease, ANKRD26-associated thrombocytopenia, Glanzmann thrombasthenia, Bernard-Soulier syndrome), thrombotic microangiopathy (thrombotic thrombocytopenic purpura, atypical hemolytic uremic syndrome, hemolytic uremic syndrome, Hemolysis Elevated Liver enzymes and Low Platelets syndrome, disseminated intravascular coagulation), autoimmune-related thrombocytopenia (immune thrombocytopenic purpura, antiphospholipid syndrome, systemic lupus erythematosus), and acquired thrombocytopenia (Infection-induced thrombocytopenia and drug-induced thrombocytopenia, heparin-induced thrombocytopenia). We hope to provide more evidence for clinical applications and future research.

Microtubule plus-end tracking Adenopolyposis Coli negatively regulates proplatelet formation

Platelets are produced upon profound reorganization of mature megakaryocytes (MK) leading to proplatelet elongation and release into the blood stream, a process termed thrombopoiesis. This highly dynamic process requires microtubules (MT) reorganization by mechanisms that are still incompletely understood. Adenomatous polyposis coli (APC) is a microtubule plus-end tracking protein involved in the regulation of MT in a number of cell systems and its inactivation has been reported to alter hematopoiesis. The aim of our study was to investigate the role of APC in megakaryopoiesis and the final steps of platelet formation. Down-regulation of APC in cultured human MK by RNA interference increased endomitosis and the proportion of cells able to extend proplatelets (68.8% (shAPC1) and 52.5% (shAPC2) vs 28.1% in the control). Similarly an increased ploidy and amplification of the proplatelet network were observed in MK differentiated from Lin- cells of mice with APC-deficiency in the MK lineage. In accordance, these mice exhibited increased platelet counts when compared to wild type mice (1,323 ± 111 vs 919 ± 52 platelets/µL; n = 12 p 0.0033**). Their platelets had a normal size, ultrastructure and number of microtubules coils and their main functions were also preserved. Loss of APC resulted in lower levels of acetylated tubulin and decreased activation of the Wnt signaling pathway. Thus, APC appears as an important regulator of proplatelet formation and overall thrombopoiesis.