What are myeloproliferative disorders?

What are Myeloproliferative Disorders?

Myeloproliferative neoplasms (MPNs) are a group of clonal hematopoietic stem cell disorders characterized by excessive production of one or more mature blood cell lineages in the bone marrow, arising from acquired genetic mutations that lead to uncontrolled proliferation of red blood cells, white blood cells, or platelets. 1, 2

Core Disease Categories

MPNs are broadly divided into two main groups 3:

Philadelphia Chromosome-Negative Classical MPNs

• Polycythemia vera (PV): Clonal erythrocytosis with elevated hemoglobin/hematocrit
• Essential thrombocythemia (ET): Clonal thrombocytosis without erythrocytosis or significant fibrosis
• Primary myelofibrosis (PMF): Megakaryocyte proliferation with bone marrow fibrosis 1, 2

Philadelphia Chromosome-Positive MPN

• Chronic myeloid leukemia (CML): Distinguished by BCR-ABL1 fusion from t(9;22) translocation 4, 3

Nonclassical MPNs (Rare Subtypes)

• Chronic neutrophilic leukemia (CNL)
• Chronic eosinophilic leukemia (CEL)
• Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions
• MPN not otherwise specified/unclassifiable 5, 3

Pathophysiology

The diseases originate from clonal expansion of abnormal hematopoietic stem cells with acquired driver mutations 1, 6. Key molecular features include:

• JAK2 V617F mutation: Present in >90% of PV cases and ~60% of ET/PMF cases 2
• CALR and MPL mutations: Found in JAK2-negative ET and PMF 7
• BCR-ABL1 fusion: Pathognomonic for CML 3

The mutations cause constitutive activation of the JAK-STAT signaling pathway, leading to uncontrolled myeloid proliferation independent of normal growth factor regulation 6.

Clinical Manifestations

Shared Features Across MPNs 1, 4:

• Bone marrow hypercellularity with atypical megakaryocytic hyperplasia and clustering
• Variable splenomegaly (from organomegaly due to extramedullary hematopoiesis)
• Leukocytosis and/or thrombocytosis
• Clonal cytogenetic abnormalities (in some cases)

Disease-Specific Presentations:

• PV: Elevated hematocrit requiring phlebotomy, pruritus (especially after warm baths), plethora, thrombotic complications 1
• ET: Marked thrombocytosis, microvascular symptoms (erythromelalgia, headaches), thrombohemorrhagic events 1
• PMF: Progressive bone marrow failure, constitutional symptoms (night sweats, weight loss, fatigue), massive splenomegaly, leukoerythroblastosis 2

Major Complications

PV and ET are characterized by significant thrombohemorrhagic complications and high risk of transformation to myelofibrosis and acute myeloid leukemia 1. The complications include:

• Thrombosis: Both arterial and venous, including unusual sites (splanchnic, cerebral) 2, 6
• Hemorrhage: Particularly with extreme thrombocytosis or acquired von Willebrand syndrome 8
• Disease transformation:
  • Post-PV/post-ET myelofibrosis
  • Acute myeloid leukemia (2-10% risk) 1, 6
• Hyperviscosity complications: In PV with uncontrolled hematocrit 9

Diagnostic Approach

The diagnosis requires integration of clinical, morphologic, and molecular features 1, 2:

Essential Testing:

• Complete blood count with differential
• Bone marrow aspirate and biopsy with morphologic assessment
• Cytogenetic analysis (karyotype)
• Molecular testing for driver mutations (JAK2, CALR, MPL for Ph-negative MPNs; BCR-ABL1 for CML) 3, 7

Key Diagnostic Criteria (WHO 2016) 7:

For PV: Requires hemoglobin >16.5 g/dL (men) or >16 g/dL (women), bone marrow trilineage myeloproliferation, and JAK2 mutation

For ET: Requires platelet count ≥450 × 10⁹/L, megakaryocyte proliferation without significant granulocytic/erythroid proliferation, JAK2/CALR/MPL mutation, and exclusion of other myeloid neoplasms

For PMF: Requires megakaryocyte proliferation with atypia, reticulin/collagen fibrosis (grade 2-3), JAK2/CALR/MPL mutation, and presence of anemia, splenomegaly, leukocytosis, or elevated LDH

Important Distinctions

CML must be distinguished from other MPNs by BCR-ABL1 testing, as it has unique treatment with tyrosine kinase inhibitors and dramatically different prognosis 4, 10. If BCR-ABL1 is negative in suspected CML, testing for JAK2 and other MPN-associated mutations is mandatory 3.

The term "chronic myeloproliferative disorder" historically included CML but is now reserved for Ph-negative MPNs (PV, ET, PMF) due to CML's distinct molecular pathogenesis and therapeutic response 4.

Myelodysplastic/Myeloproliferative Neoplasms (MDS/MPN)

A related category exists with overlapping features of both dysplasia and proliferation 3:

• Chronic myelomonocytic leukemia (CMML) - most common
• MDS/MPN with neutrophilia (atypical CML)
• MDS/MPN with SF3B1 mutation and thrombocytosis
• MDS/MPN not otherwise specified

CMML is characterized by persistent monocytosis with features of both myeloproliferative and myelodysplastic processes 11.

Clinical Significance

MPNs are chronic diseases with variable natural history ranging from indolent courses requiring minimal intervention to aggressive phenotypes with shortened survival 5, 6. The median age at diagnosis is approximately 60 years, with slight male predominance 4. Incidence of PV is approximately 2.3 per 100,000 population 4.

Allogeneic hematopoietic stem cell transplantation remains the only curative therapy but carries significant morbidity and mortality risks, reserved for patients with expected survival <5 years or high-risk disease features 2, 5.