Absence of JAK2V617F mutated endothelial colony-forming cells in patients with JAK2V617F myeloproliferative neoplasms and splanchnic vein thrombosis

Abstract

Philadelphia (Ph)-negative myeloproliferative neoplasms (MPN) are acquired hematologic diseases with increased production of mature blood cells. They include polycythemia vera (PV), essential thrombocythemia (ET) and myelofibrosis (MF). The most frequent molecular abnormality found in Ph negative MPN is JAK2V617F, an activating mutation of JAK2 which is responsible for constitutive signaling of various cytokine receptors. Arterial and venous thromboses are the main complications of these diseases and are responsible for high rates of morbidity and mortality. Of note there is a disproportionate incidence of thrombosis at unusual sites including splanchnic vein thrombosis.1 Splanchnic vein thromboses (SVT) involve one or more abdominal veins, the two most frequent are Portal Vein Thrombosis (PVT) and Budd Chiari Syndrome (BCS). Pathophysiology of thrombosis in MPN is complex and involves abnormalities in blood cells, plasma factors, and endothelial cells (ECs). Several groups, using different techniques, have shown JAK2V617F expression in endothelial cells (Supplemental Fig. 1, http://links.lww.com/HS/A79). Using laser capture microdissection, JAK2V617F was demonstrated in ECs from hepatic venules in 2 of 3 patients with PV and BCS.2JAK2V617F endothelial cells were demonstrated in microdissected splenic capillaries and in ECs cultured from splenic vein in patients with myelofibrosis but without SVT.3 Although these teams performed experiments to ensure that the DNA they obtained originated from ECs, it is difficult to completely rule out a possible contamination by blood cells. Analysis of endothelial progenitor cells, specifically endothelial colony forming cells (ECFCs), is an alternative way to look for JAK2V617F ECs. Indeed, ECFCs are reported to be the only “true” endothelial progenitor cells, as they are the only ones able to generate blood vessels in vivo: they display clonogenic potential, endothelial but not myeloid cell surface markers, and pronounced postnatal vascularisation ability in vivo.4,5 ECFCs are a unique tool to investigate endothelial molecular dysfunction in disease, as they give access to endothelial cells from patients in a non-invasive way and a promising tool for vascular regenerative approaches and gene therapy.6 Yoder et al studied 11 JAK2V617F MPN patients and reported 3 JAK2V617F ECFCs derived from only 1 of 11 patients. Of note, this patient presented with thrombosis and later developed PV.4 In another study, the JAK2V617F mutation was not detected in any of 75 ECFCs obtained from 57 patients with JAK2V617F MPN but no thrombosis.7 Teofili et al reported JAK2V617F ECFCs in 5 of 22 MPN patients, all with thrombotic complications including 1 with BCS and 1 with PVT.8 Lastly, 4 of 5 JAK2V617F-positive patients with BCS but without overt MPN had JAK2V617F ECFCs cultured from the bone marrow.9 Taken together, these results suggest that the presence of JAK2V617F ECFCs in patients is associated with thrombosis, even in the absence of overt MPN. Our groups have previously demonstrated (a) that the presence of JAK2V617F in ECs modifies their phenotype and makes them prothrombotic,10 highlighting the importance of looking for JAK2V617F ECs in patients; (b) the importance of using correctly characterized ECFCs in investigating this.6 Confirming that JAK2V617F positive ECFCs are associated with previous thrombosis in MPN patients would suggest that ECFCs culture and JAK2V617F genotyping may be used as a marker of thrombotic risk in MPN patients, before they develop thrombosis.