Molecular biomarkers of thrombosis in myeloproliferative neoplasms

• Polycythemia Vera (PV), Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF) are myeloproliferative neoplasms (MPN) and are characterized by mutually exclusive JAK2, CALR and MPL mutations
• Thrombotic events are very frequent and represent the main cause of morbidity and mortality in patients with MPN
• JAK2V617F mutation is the major contributory factor for the pathogenesis of thrombosis in MPN
• Pathogenesis of blood clotting activation in these diseases is multifactorial, and involves various abnormalities of hemostatic cells

ABSTRACTJAK2 mutations define polycythemia vera (PV), CALR and MPL mutations are specific to JAK2 unmutated essential thrombocythemia (ET) and primary myelofibrosis (PMF). We overviewed the current knowledge on the relationship between these phenotypic driver mutations and thrombotic complications that are major cause of morbidity and mortality in patients with myeloproliferative neoplasms (MPN) particularly PV and ET. The JAK2 mutation is found in 50-60% of patients with ET and PMF. The International Prognostic Score for Thrombosis in ET (IPSET-thrombosis) identified JAK2 mutation as an independent risk factor and a 3-tiered prognostic model was devised. IPSET-thrombosis model outperformed the 2-tiered conventional risk stratification that includes age and thrombotic history. PV is usually associated with a JAK2 mutation and studies looking at the role of JAK2V617F allele burden associated with thrombosis are so far inconclusive. In PMF, the rate of major thrombosis is around 2%pt-yr and JAK2 mutation emerged as an independent risk factor for these events. Calreticulin/MPL (CALR) is the second most frequent mutation and occurs in half of JAK2 and MPL wild-type patients with ET and PMF. Despite the fact that these mutations are associated with high platelet counts, the risk of thrombosis compared with JAK2 and MPL mutated cases is significantly lower. The role of MPL in the prediction of thrombosis is of difficult demonstration due to the low frequency in ET and PMF. Therefore, these epidemiologic studies pointed out the role of JAK2V617F mutation as a major contributory factor for the pathogenesis of thrombosis in MPN. Abnormalities of blood cells arising from the clonal proliferation of hematopoietic stem cells may explain the switch to a procoagulant phenotype.