Polycythemia vera (PV),
essential thrombocythemia (ET), and
primary myelofibrosis (PMF) constitute the BCR-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, whereas JAK2 mutation in ET is associated with increased risk of
thrombosis. In PMF, type 1 or type 1-like CALR mutations are associated with superior and ASXL1 with 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.