Myeloproliferative Disorders

• Constitutive tyrosine kinase activation and JAK-STAT signaling: The pathophysiologic hallmark involves acquisition of mutations causing uncontrolled myeloid proliferation. BCR-ABL (Philadelphia chromosome, t(9;22)) generates a constitutively active fusion protein with aberrant tyrosine kinase activity, driving proliferation independent of normal hematopoietic growth factors. JAK2 V617F mutation (present in ~95% of PV, 50-60% of ET/PMF) and its upstream effector mutations (CALR exon 9 insertions/deletions, MPL W515L/K) similarly dysregulate the JAK-STAT and MAPK/PI3K pathways. This leads to erythropoietin and thrombopoietin hypersensitivity and bypassing of normal apoptotic controls, resulting in exponential clonal expansion.

• Altered bone marrow microenvironment and fibrosis: In PMF and blast-phase disease, aberrant megakaryocytes and granulocytes produce excessive transforming growth factor-beta (TGF-β), fibroblast growth factor (FGF), and angiopoietin-1, triggering secondary bone marrow fibrosis with replacement of hematopoietic tissue by fibrous tissue and osteosclerosis. This creates a microenvironmental feedback loop where abnormal stromal signaling further amplifies MPN clone dominance while paradoxically impairing normal hematopoiesis, leading to anemia and thrombocytopenia despite marrow hypercellularity.

• Loss of normal apoptotic and proliferative controls: MPNs demonstrate reduced apoptosis in CD34+ and mature myeloid cells due to upregulation of anti-apoptotic proteins (BCL-2, MCL-1) and downregulation of pro-apoptotic signals. Additionally, dysregulated expression of cell cycle checkpoint proteins and defective DNA damage responses accumulate secondary mutations (TP53, ASXL1, SRSF2, IDH1/2, EZH2), promoting clonal evolution and blast transformation. The combination of constitutive proliferative signals and impaired death pathways creates a "perfect storm" for malignant transformation.

• BCR-ABL fusion gene (Philadelphia chromosome): Results from reciprocal translocation t(9;22)(q34;q11) creating the pathognomonic BCR-ABL transcript. Present in essentially 100% of CML and approximately 5% of ALL and AML. Rarely acquired somatically in adults; can be constitutional or result from environmental exposures (though specific carcinogens are unclear).

• JAK2 V617F point mutation: Gain-of-function mutation in exon 14 of JAK2 tyrosine kinase gene. Found in ~95% of PV, 50-60% of ET and PMF, and rarely in other myeloid disorders. Homozygous in approximately 25-30% of patients. Can occur de novo or with predisposing germline polymorphisms (GGCC haplotype in TERT-JAK2 region increases acquisition risk).

• CALR and MPL mutations: Calreticulin (CALR) exon 9 insertions or deletions present in 20-30% of ET and PMF, predominantly in JAK2-negative patients. Myeloprolifetin (MPL) W515L or W515K mutations occur in 3-10% of ET and PMF. These mutations activate thrombopoietin signaling and JAK-STAT pathways. Mutually exclusive with each other and with JAK2 V617F, defining the "triple-negative" subset (5-10% of ET/PMF).

• Secondary driver mutations and clonal evolution: ASXL1, SRSF2, EZH2 mutations associated with increased risk of transformation and poor prognosis. TP53 mutations (<5% at diagnosis) mark high-risk disease. IDH1/2 mutations occur in 5-10% of advanced disease. These accumulate during disease progression, explaining age-dependent incidence and increased transformation risk with disease duration.

• Polycythemia vera: Plethoric facies, splenomegaly, thrombotic/hemorrhagic complications. Patients present with aquagenic pruritus (pathognomonic burning sensation after bathing due to mast cell degranulation from increased histamine), facial plethora, and elevated hematocrit (>49% in men, >48% in women). Splenomegaly occurs in 75% due to extramedullary hematopoiesis. Thrombotic complications (MI, CVA, splenic/mesenteric infarction, Budd-Chiari syndrome) and paradoxical bleeding (despite high platelet counts, qualitative platelet dysfunction) dominate morbidity.

• Essential thrombocythemia: Marked thrombocytosis with mild symptoms until thrombotic events occur. Platelets characteristically >450,000/μL (often >1,000,000/μL); many patients asymptomatic at diagnosis. Bleeding manifestations (mucosal, GI) occur paradoxically despite thrombocytosis due to von Willebrand factor depletion from high shear stress and platelet dysfunction. Thrombotic events (DVT, arterial thrombosis, stroke, portal vein thrombosis) may be initial presentation. Mild splenomegaly in 25-30%.

• Primary myelofibrosis: Constitutional symptoms, massive splenomegaly, severe anemia, leukoerythroblastosis. Presents with constitutional B symptoms (fever, night sweats, weight loss) in 25-40%, reflecting elevated inflammatory cytokine production (TNF-α, IL-1β, IL-6). Massive splenomegaly (80-90%) with hepatomegaly results from extramedullary hematopoiesis and sequestration. Leukoerythroblastic blood picture with teardrop-shaped RBCs (polychromasia from immature RBC release), circulating blasts, and early myeloid precursors. Severe symptomatic anemia requiring transfusions. Portal hypertension and variceal bleeding from splenic enlargement.

• Chronic myeloid leukemia: Chronic phase asymptomatic, accelerated phase with splenomegaly and constitutional symptoms, blast crisis with rapid progression. Chronic phase (90% at diagnosis): often incidental finding on routine labs with WBC >100,000/μL, predominantly mature neutrophils. Massive splenomegaly (90%) may cause left upper quadrant pain. Accelerated phase: anemia, thrombocytopenia, increased basophils/eosinophils (>15%), blasts 10-19%, or clonal cytogenetic evolution. Blast crisis (20% of patients): >20% blasts; 70% myeloid, 30% lymphoid; rapid deterioration with bleeding, infection, multiorgan failure.

• Bone marrow biopsy with reticulin/trichrome staining: Essential for morphologic assessment and grading of fibrosis. In early CML, marrow hypercellular with myeloid hyperplasia and left-shifted myeloid maturation (increased immature precursors); toxic granulations and Döhle bodies in neutrophils; mild megakaryocytic hyperplasia. In PV, hypercellular marrow with trilineage hyperplasia (erythroid predominant), increased megakaryocytes with hyperlobulated nuclei, and increased reticulin; no significant fibrosis. In ET, hypercellular with megakaryocytic hyperplasia and clustering (abnormal megakaryocytes with dense, hyperlobulated nuclei); normal erythropoiesis and granulopoiesis. In PMF, hypercellular (early) to hypocellular (late) with extensive reticulin and collagen fibrosis (MF-3 fibrosis grade), abnormal megakaryocytes with cloud-like chromatin and hyperlobulation, and osteosclerosis. Graded using EUTOS fibrosis scoring system (MF-0 to MF-3).

• Gross pathology appearance: Spleen: massive enlargement (can exceed 4 kg in PMF vs. 200-300g normal), dark red/mahogany color from congestion and extramedullary hematopoiesis, firm consistency from fibrosis in advanced PMF. Bone marrow: red, hyperplastic appearance in early disease; in PMF, tan-gray fibrotic tissue replacing normal marrow space with focal red islands of hematopoiesis.

• Laboratory values and diagnostic criteria (WHO 2016/2022):
• PV diagnosis requires: Elevated hemoglobin (>16.5 g/dL men, >16 g/dL women) OR hematocrit (>49% men, >48% women) OR increased RBC mass plus JAK2 V617F or CALR/MPL mutation (or subnormal serum erythropoietin); OR hemoglobin/hematocrit elevations plus bone marrow trilineage hyperplasia. Leukocyte alkaline phosphatase elevated (unlike CML where it's low). Uric acid elevated from increased cell turnover. LAP score elevated (distinguishes from CML).

• ET diagnosis requires: Sustained thrombocytosis ≥450,000/μL with absence of other causes, JAK2/CALR/MPL mutation OR bone marrow showing megakaryocytic hyperplasia with increased or abnormal megakaryocytes, and absence of BCR-ABL and PMF-fibrosis criteria. Bleeding time usually normal or prolonged (despite elevated count).

• PMF diagnosis requires: Leukoerythroblastosis with circulating blasts <20%, bone marrow fibrosis grade MF-2-3, plus either JAK2/CALR/MPL mutation OR presence of clonal marker (cytogenetic abnormality) OR marked elevation of serum LDH. Anemia (Hgb <10 g/dL) present in majority.

• CML diagnosis requires: BCR-ABL translocation (Philadelphia chromosome t(9;22)) detected by karyotype, FISH, or qPCR. WBC elevated (often >100,000/μL) with left shift to immature forms. Basophils and eosinophils characteristically increased (>5%). Leukocyte alkaline phosphatase low (distinctly different from reactive leukocytosis). Blast percentage <20% in chronic phase.

• Cytogenetic and molecular studies: Karyotype essential; t(9;22) present in 95% of CML (rare variant translocations ~5%). FISH or qPCR for BCR-ABL quantification (critical for monitoring TKI response; major molecular response = BCR-ABL/ABL ratio <0.1% on international scale). JAK2 V617F allele burden quantification; homozygous mutations associated with increased risk of thrombosis. CALR and MPL sequencing if JAK2-negative. Next-generation sequencing for secondary mutations (ASXL1, SRSF2, EZH2, TP53, IDH1/2) increasingly important for risk stratification and prediction of transformation.

• Peripheral blood smear morphology: CML: left-shift with full myeloid spectrum, toxic granulations, Döhle bodies, circulating myelocytes and metamyelocytes; increased basophils and eosinophils; mild anisocytosis. PV: polychromasia, round normocytic RBCs, slightly increased WBC, increased platelets. ET: marked thrombocytosis, platelets may form clumps ("giant platelets"), otherwise relatively normal RBCs and WBC. PMF: leukoerythroblastosis with WBC and RBC both elevated and left-shifted; marked poikilocytosis with characteristic teardrop-shaped RBCs (resulting from forced passage through fibrotic marrow), nucleated RBCs, immature myeloid forms, occasional blasts.

• CML: Tyrosine kinase inhibitors (TKIs) as first-line therapy; allogeneic stem cell transplantation for advanced disease or TKI resistance. Imatinib (Gleevec) 400 mg daily for chronic phase (induces complete cytogenetic response in 80-90% at 18 months; major molecular response in 70-80%), with dose escalation to 600-800 mg for resistance or accelerated phase. Second-generation TKIs (dasatinib, nilotinib, bosutinib) show superior early molecular response and are preferred for high-risk disease or BCR-ABL kinase domain mutations. Continuous monitoring with quantitative PCR (qPCR) of BCR-ABL transcript every 3 months initially, then quarterly; major molecular response (BCR-ABL/ABL <0.1%) correlates with improved long-term outcomes. Allogeneic hematopoietic stem cell transplantation (HSCT) reserved for accelerated phase, blast crisis, or those intolerant/resistant to TKIs; provides only curative option but with significant mortality (5-50% depending on age, donor, disease phase).

• PV and ET: Antiplatelet therapy (low-dose aspirin) for thromboprophylaxis; cytoreductive agents (hydroxyurea, peginterferon, ruxolitinib) for high-risk patients. Low-dose aspirin (75-100 mg daily) universally recommended for thrombotic prevention (reduces cardiovascular events by ~40% in PV). Hydroxyurea (0.5-2 g daily) first-line cytoreductive agent for high-risk disease (age >60 or prior thrombosis); reduces WBC, RBC, and platelet counts, though potential concern for leukemogenic transformation (0.5-2% at 10 years). Pegylated interferon-alpha increasingly used in younger patients (superior molecular responses, no teratogenicity concerns). Ruxolitinib (JAK1/2 inhibitor) increasingly recognized for ET/PV, especially in patients with splenomegaly or constitutional symptoms. Phlebotomy in PV to maintain target hematocrit <45%; target iron-replete patients. Allopurinol and hydration for hyperuricemia/gout prophylaxis.

• PMF: JAK inhibitors (ruxolitinib) for symptomatic disease; allogeneic HSCT only curative option for fit candidates. Ruxolitinib (Jakafi) approved for MF regardless of JAK2 status; dramatically reduces splenomegaly (median 35% reduction) and constitutional symptoms within 4-8 weeks; mechanism via JAK1/2 inhibition reducing inflammatory cytokine production. Fedratinib (TKI selective for JAK2/FLT3) alternative. Allogeneic HSCT only curative therapy but reserved for younger patients (<65-70 years, ECOG 0-1) with high-risk disease (circulating blasts, complex karyotype, unfavorable mutations) given significant transplant-related mortality (20-40%). Supportive care with transfusions, iron chelation, and G-CSF as needed. Lenalidomide or thalidomide with prednisone for isolated anemia in del(5q) or other favorable

Disease-related complications

• Arterial and venous thrombosis: the leading cause of death in PV and ET. Hyperviscosity, JAK2-mutant leukocyte–platelet activation, and endothelial adhesion drive stroke, MI, and DVT. Any new focal deficit or chest pain in an MPN patient is an emergency and is managed as standard ACS/stroke.
• **Splanchnic vein thrombosis and *Budd–Chiari syndrome***: emergency. New ascites, tender hepatomegaly, and RUQ pain; portal hypertension with hypersplenism and iron deficiency can mask the erythrocytosis, so counts may look normal.
• Acquired von Willebrand syndrome: extreme thrombocytosis adsorbs and clears high-molecular-weight vWF multimers → mucocutaneous bleeding despite a high platelet count. Signaled by low ristocetin cofactor activity; antiplatelet therapy worsens it.
• Blast-phase transformation (secondary AML): emergency. Suggested by new cytopenias, rapidly enlarging spleen, bone pain, and ≥20% blasts; prognosis is poor and TKI/JAK-inhibitor responses are lost.
• Post-PV/post-ET myelofibrosis: falling hematocrit with transfusion dependence, teardrop cells, and progressive splenomegaly.
• Splenic infarction and portal hypertension: sharp LUQ pain with a friction rub; variceal hemorrhage from extramedullary hematopoiesis.
• Hyperuricemia, gout, and tumor lysis with cytoreduction — NCCN myeloproliferative neoplasm guidance supports hydration and allopurinol.
• Leukostasis or priapism with very high WBC in CML: emergency requiring urgent cytoreduction/leukapheresis.

Treatment-related complications

• Hydroxyurea: macrocytosis without B12/folate deficiency, painful leg and oral ulcers, non-melanoma skin cancer, myelosuppression.
• Interferon alfa: flu-like syndrome, depression/suicidality, autoimmune thyroiditis; favored when pregnancy is a consideration.
• Ruxolitinib: dose-limiting anemia and thrombocytopenia, herpes zoster and latent TB/HBV reactivation; abrupt withdrawal can cause a cytokine rebound with fever and hemodynamic instability — taper.
• BCR-ABL TKIs: imatinib periorbital edema and cytopenias; dasatinib pleural effusion; nilotinib QT prolongation and peripheral arterial occlusive disease; ponatinib arterial thrombotic events (boxed warning). NCCN CML guidance advises baseline cardiovascular risk assessment and ECG monitoring where relevant.

• PV triad on the stem: aquagenic pruritus after a hot shower, erythromelalgia (burning, red hands relieved by aspirin), and facial plethora. Best next step is serum erythropoietin plus JAK2 V617F testing — a suppressed EPO with erythrocytosis is the discriminator.

• Low leukocyte alkaline phosphatase = CML; high LAP = leukemoid reaction or PV. The other CML giveaway is basophilia (plus eosinophilia), which is essentially never seen in a reactive leukocytosis. LAP is dated in practice but remains a favorite exam distinction.

• **Teardrop cells (dacrocytes) + leukoerythroblastosis + a *dry tap* on aspiration = primary myelofibrosis. The dry tap comes from collagen/reticulin fibrosis, so the next step is a core biopsy**, not a repeat aspirate.

• Budd–Chiari or unexplained splanchnic vein thrombosis in a young patient: test for JAK2 V617F even if the CBC is normal — portal hypertension, hypersplenism, and iron deficiency can mask polycythemia. This is the single association examiners test most.

• t(9;22) BCR-ABL1 is CML's defining lesion (p210 transcript); the same translocation in ALL (usually p190) confers worse prognosis. Treatment is a tyrosine kinase inhibitor (imatinib), monitored by quantitative BCR-ABL1 PCR per NCCN CML guidance.

• Phlebotomy to a hematocrit below 45% is the backbone of PV management (supported by the CYTO-PV trial) and is combined with low-dose aspirin; hydroxyurea is added for high-risk disease.

• Distractor to avoid — thrombocytosis ≠ thrombosis risk only. With platelets in the extreme range, suspect acquired von Willebrand syndrome and check ristocetin cofactor activity before starting aspirin.

• Distractor to avoid — secondary vs primary polycythemia. High EPO points to hypoxia (COPD, sleep apnea, high altitude, right-to-left shunt) or an EPO-secreting tumor (renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma); Gaisböck syndrome is relative, with a normal red cell mass. Likewise, reactive thrombocytosis (iron deficiency, inflammation, post-splenectomy with Howell-Jolly bodies) mimics ET.