Myeloproliferative Neoplasms

Polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) constitute theBCR-ABL1–negative myeloproliferative neoplasms and are characterized by mutually exclusive Janus kinase 2 (JAK2), calreticulin (CALR), and myeloproliferative leukemia virus oncogene (MPL) mutations; respective frequencies of these mutations are approximately 95%, 0%, and 0% in PV, 60%, 20%, and 3% in ET, and 60%, 25%, and 7% in PMF. These mutations might be accompanied by other mutations that are less specific to myeloproliferative neoplasms but are prognostically relevant, such as additional sex combs–like 1 (ASXL1). Characteristic bone marrow morphology is required for World Health Organization–compliant diagnosis, especially in distinguishing ET from prefibrotic PMF and masked PV. Survival is the longest in ET, although still inferior to that of the age- and sex-matched control population; median survivals for patients younger than 60 years are approximately 33 years for ET, 24 for PV, and 15 for PMF. Major disease complications include thrombosis and leukemic or fibrotic transformation. In PV and ET, risk factors for survival include older age, leukocytosis, and thrombosis, whereasJAK2mutation in ET is associated with increased risk of thrombosis. In PMF, type 1 or type 1–likeCALRmutations are associated with superior andASXL1with inferior survival. Prevention of thrombosis in PV is secured by phlebotomy (hematocrit target <45%) and in both PV and ET by low-dose aspirin therapy; high-risk patients derive additional antithrombotic benefit from cytoreductive therapy with hydroxyurea as first-line and interferon-alfa and busulfan as second-line drugs of choice. Although the JAK inhibitor ruxolitinib was recently approved for use in hydroxyurea-resistant PV, its role in routine clinical practice remains debatable. In myelofibrosis, stem cell transplant is the current treatment of choice for genetically or clinically high-risk disease; for all other patients requiring treatment, participation in clinical trials may be preferred because currently available drugs, including JAK inhibitors, are palliative and not shown to be disease modifying.