Lineage Restricted JAK2V617F Expression and Its Role in Thrombosis and Hemostasis; The Potential Involvement of Endothelial JAK2V617F in Clotting Abnormalities

Abstract 1737

Myeloproliferative disorders (MPDs) are a heterogeneous group of bone marrow disorders characterized by increases in one or more blood cell lineages. A single, somatic mutation in JAK2 (V617F) is responsible for many of the features of Philadelphia chromosome-negative MPDs (Polycythemia Vera, Essential Thrombocythemia and Primary Myelofibrosis). Clinically, the most common cause of death in these patients is arterial thrombosis; however some patients display a bleeding diathesis. Although the incidence and potential causes of dysfunctional hemostasis in patients with MPDs has been studied extensively, the critical regulating factors are unclear and therefore it has been difficult to develop an effective therapeutic regimen for these complications. As the formation of stable thrombi requires interactions between endothelial cells, platelets and leukocytes, we have recently generated mice that express human JAK2V617F in each of these cell lineages by crossing a JAK2V617F/Flip-Flop (FF1) mouse with mice expressing lineage-specific Cre recombinases. These crosses have generated the following mice; 1) Tie2-Cre/FF1, where JAK2V617F is expressed in all hematopoietic and endothelial cells, 2) Pf4-Cre/FF1, where JAK2V617F expression is limited to platelets, and 3) LysM-Cre/FF1, where JAK2V617F expression is limited to leukocytes.

Expression of human JAK2V617F was confirmed in megakaryocytes, platelets, leukocytes and endothelial cells (Tie2-Cre/FF1), megakaryocytes and platelets (Pf4-Cre/FF1) and leukocytes (LysM-Cre/FF1) by conventional and real-time PCR. Of the 3 mouse strains, only Tie2-Cre/FF1 exhibited a MPD phenotype. Platelet counts were significantly increased compared to Tie2-Cre controls (at 3 months, Tie2-Cre: 779 (±61)/ml;Tie2-Cre/FF1: 2943 (± 217)/ml) without significant increases in any other cells types. Tie2-Cre/FF1 mice also exhibit greatly increased number of CFU-MKs and bone marrow derived megakaryocytes. Therefore, Tie2-Cre/FF1 mouse exhibits an ET-like phenotype. Although circulating platelet counts did not increase in Pf4-Cre/FF1 mice, we did observe an increase in the number of CFU-MKs in colony assays.

Next we determined the roles of the lineage-restricted JAK2V617F expression on hemostasis in vitro and in vivo. Aggregometry on washed platelets showed no significant difference between any group and their controls in response to PAR4 (100–400mM), ADP (2–20mM) or collagen (1–10mg/ml). Additionally, we were unable to show a significant difference in GPIIbIIIa activation or surface expression of P-selectin in response to the same agonists. Despite no clear platelet abnormalities in any of the 3 mouse lineages, we identified significant hemostatic abnormalities in vivo in Tie2-Cre/FF1 mice. Tail bleeding time was significantly increased in Tie2-Cre/FF1 mice compared to Tie2-Cre controls (Tie2-Cre average, 2min 47secs; Tie2-Cre/FF1, 6mins 37secs) while Tie2-Cre/FF1 mice also exhibited an increased occurrence of re-bleeding compared to Tie2-Cre controls. Additionally, we performed FeCl₃ carotid artery occlusion assays to better determine in vivo thrombosis. We found that at 10% FeCl₃, Tie2-Cre control mice exhibited complete artery occlusion in approximate 6 min. In contrast, Tie2-Cre/FF1 mice failed to show any sign of arterial occlusion throughout the duration of the experiment (30 min). Given the significant increase in platelet numbers in Tie2-Cre/FF1 mice, we next determined if acquired von Willibrand Disease (VWD) could account for prolonged bleeding and reduced clotting; plasma vW Factor levels by ELISA were normal. In contrast to Tie2-Cre/FF1 mice, neither the PF4-Cre/FF1 or LysM-Cre/FF1 mice exhibit dysfunctional thrombosis.

These data provide compelling evidence that expression of JAK2V617F in cells other than just platelets or just leukocytes is necessary to generate the hemostatic abnormalities seen in patients with MPDs. Recent findings show that some patients express endothelial JAK2V617F and patients with ET exhibit increased numbers of circulating endothelial progenitors. Thus, our data is consistent with the hypothesis that expression of JAK2V617F in endothelial cells, in addition to hematopoietic cells results in the bleeding diathesis seen in patients with MPDs.