Role of the JAK2V617F mutation in the pathophysiology of thrombosis in myeloproliferative diseases

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The sporadic mutation Val617Phe of the Janus kinase JAK2 (JAK2V617F) is associated with the Bcr/Abl-negative myeloproliferative disorders (MPDs): polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). These are clonal hematopoietic stem cell disorders characterized by the proliferation of particular hematopoietic lineages (erythroid in PV, megakaryocytic in ET, and granulocytic and megakaryocytic in PMF) without blockage in cell maturation. JAK2V617F is a gain of function mutation leading to growth factors hypersensitivity and has been detected in most PV patients and in ~60% of ET and PMF patients. Thrombosis reveals MPD in about 30% of patients and is a major cause of morbidity and mortality. Up to 40% of ET and PV patients develop thrombosis. Ischemic stroke is more frequent than acute coronary syndrome and peripheral artery disease. Incidence of vein thrombosis is also increased with a particularly high frequency of splanchnic vein thrombosis (i.e. of the digestive vascular bed). The mechanisms underlying the MPD thrombotic diathesis are still largely elusive. Impairment of platelet and endothelial cell properties is suspected but so far not demonstrated. Clinical studies identified advanced age, prior history of thrombosis, leukocytosis and the JAK2V617F status as risk factors for thrombosis in MPD patients. The direct assessment of the effect of JAK2V617F on platelet function in MPD patients is limited by the heterogeneous extension of the mutant clone and the frequent use of prophylactic treatment for thrombosis. An extensive study of endothelial cells in MPDs is difficult to achieve due to the scarcity of tissue samples. The recently described mouse models in which JAK2V617F expression is inducible and tissue-specific offer an attractive alternative to determine the impact of this mutation on haemostatic functions.The aim of our project is to elucidate the specific mechanisms by which JAK2V617F favours arterial, venous and microcirculatory thrombosis. The strategy is based on the use of cultured cells expressing JAK2V617F, of blood and vascular samples from patients with JAK2V617F and on several transgenic mouse models. Five tasks have been identified: (1) determine the cellular localization and phenotypic consequences of JAK2V167F in patients; (2) analyse functional consequences of JAK2V617F in vitro; (3) assess the functional consequences of JAK2V617F using different mouse models of thrombosis in vivo and determine the precise mechanisms involved; (4) evaluate the potential role of JAK2V617F in the activation of coagulation; (5) prepare future studies by determining the prevalence of the very recently discovered calreticulin (CALR) mutations in patients with thrombosis of hepatic or portal vein and development of new genetically modified mice harbouring this mutation. This could lead to the identification of new therapeutic approaches and of markers for thrombotic risk in MPD patients.