Polycythemia Vera (PV)

JAK2 V617F–driven myeloproliferative neoplasm — diagnosis, risk stratification, phlebotomy, cytoreductive therapy with hydroxyurea and ruxolitinib, and pegylated interferon

Key Points

• Polycythemia vera is defined by clonal erythrocytosis driven by the JAK2 V617F point mutation in ~96% of cases (JAK2 exon 12 mutations in the remainder) — absolute erythrocytosis with suppressed erythropoietin is the diagnostic cornerstone.
• The 2022 WHO/ICC diagnostic criteria require hemoglobin >16.5 g/dL (men) or >16.0 g/dL (women) or hematocrit >49% (men) or >48% (women), bone marrow hypercellularity with trilineage myeloproliferation, and JAK2 mutation — erythropoietin level below normal LLN is a minor criterion.
• Thrombosis is the leading cause of morbidity and mortality — hematocrit control to <45% reduces thrombotic events by ~4-fold (CYTO-PV trial) and is the primary treatment goal for all patients.
• All patients require phlebotomy (hematocrit target <45%) and low-dose aspirin (81 mg/day); cytoreductive therapy is added for high-risk patients (age ≥60 or prior thrombosis) with first-line hydroxyurea or ropeginterferon alfa-2b (Besremi).
• Ruxolitinib (Jakafi), an oral JAK1/2 inhibitor, is FDA-approved for PV patients inadequate to or intolerant of hydroxyurea — it achieves hematocrit control and spleen volume reduction with durable responses.
• Risk of transformation to myelofibrosis (~10% at 10 years) and acute myeloid leukemia (~2–3% at 10 years) warrants lifelong monitoring; DNMT3A, TET2, ASXL1, SRSF2, and IDH1/2 co-mutations increase transformation risk.

Pathogenesis, Diagnosis, and WHO Criteria

**Molecular pathogenesis:** Polycythemia vera arises from a clonal hematopoietic stem cell that harbors activating mutations in JAK2, constitutively activating downstream JAK-STAT signaling pathways (particularly STAT3 and STAT5) without requiring cytokine input. The hallmark mutation — JAK2 V617F (a G→T substitution at nucleotide 1849 of JAK2 exon 14 on chromosome 9p24) — renders the kinase constitutively active, driving the erythropoiesis, leukocytosis, and thrombocytosis characteristic of PV. **Mutation frequencies:** JAK2 V617F: ~96% of PV; JAK2 exon 12 mutations: ~3% (associated with…

Risk Stratification and Thrombosis Prevention

**Risk stratification — the driver of treatment intensity:** The primary clinical concern in PV is thrombosis (arterial > venous) — not transformation. Risk stratification identifies patients who need cytoreductive therapy on top of phlebotomy and aspirin. **Traditional (PVSG/ELN 2018) risk groups:** - **Low risk:** Age 50%) correlates with higher thrombosis risk and more frequent transformation - Leukocytosis (WBC >10,000–11,000/μL) is an independent thrombosis risk factor — emerging evidence suggests this should drive more aggressive cytoreduction **Phlebotomy — the cornerstone:** The…

Cytoreductive Therapy — Hydroxyurea, Ropeginterferon, and Ruxolitinib

Cytoreductive therapy is indicated for **high-risk PV** (age ≥60 or prior thrombosis) and is considered for intermediate-risk patients with cardiovascular risk factors or inadequate hematocrit control requiring very frequent phlebotomies. **Hydroxyurea (hydroxycarbamide) — first-line:** Most widely used cytoreductive agent for high-risk PV. Inhibits ribonucleotide reductase, reducing proliferation of all three cell lines. Dose: start 15–20 mg/kg/day; titrate to achieve hematocrit <45% with WBC 4,000–10,000/μL and platelets <400,000/μL, while maintaining ANC ≥2,000/μL. Major trial: MPD-RC 112…

Monitoring, Transformation, and Special Situations

**Routine monitoring in PV:** - CBC with differential: every 3–6 months (more frequently during cytoreductive dose titration) - Hematocrit: at each visit — the primary therapeutic target - Bone marrow biopsy: not required for routine monitoring; indicated at diagnosis (major criterion) and if transformation is suspected (rising blasts, increasing fibrosis, new cytopenias) - JAK2 V617F allele burden: on ropeginterferon (molecular response monitoring); less relevant on HU or ruxolitinib - Molecular panel (NGS): at diagnosis to identify co-mutations (DNMT3A, TET2, ASXL1, SRSF2, EZH2, IDH1/2)…