What are Myeloproliferative Neoplasms?
Myeloproliferative neoplasms (MPNs) are a heterogeneous group of clonal hematopoietic stem cell disorders characterized by excessive proliferation of one or more myeloid cell lineages in the bone marrow, leading to increased peripheral blood cell counts. 1
Classification and Disease Categories
MPNs are broadly divided into two main groups according to the most recent WHO and ICC classifications 1:
Classic MPN Group:
- Chronic myeloid leukemia (CML) - characterized by BCR-ABL1 fusion
- Polycythemia vera (PV) - excessive red blood cell production
- Essential thrombocythemia (ET) - elevated platelet counts
- Primary myelofibrosis (PMF) - bone marrow fibrosis with extramedullary hematopoiesis
- Chronic neutrophilic leukemia
- Chronic eosinophilic leukemia
- Juvenile myelomonocytic leukemia
- MPN not otherwise specified (unclassifiable)
MDS/MPN Overlap Group:
This category includes disorders with both dysplastic and proliferative features 1, 2:
- Chronic myelomonocytic leukemia (CMML)
- Atypical CML (BCR-ABL1 negative)
- MDS/MPN with SF3B1 variant and thrombocytosis
- MDS/MPN with ring sideroblasts and thrombocytosis
- MDS/MPN not otherwise specified
Molecular Pathogenesis
The nomenclature was changed from "myeloproliferative disorders" to "myeloproliferative neoplasms" in 2008 to accurately reflect their neoplastic nature 3. This distinction is critical because these are true malignancies, not benign reactive conditions.
Key Driver Mutations:
The Philadelphia chromosome-negative MPNs (PV, ET, PMF) are driven by gain-of-function mutations that activate JAK-STAT signaling 4, 5:
- JAK2 V617F or JAK2 exon 12 mutations - most common in PV (>95%), also found in ET and PMF
- CALR mutations - found in ET and PMF patients without JAK2 mutations
- MPL mutations - less common, activate thrombopoietin receptor signaling
These mutations are mutually exclusive in most cases and arise decades before clinical disease manifests 4. Mutation analysis of JAK2, CALR, and MPL is now considered diagnostic and has been incorporated into WHO diagnostic criteria 6.
Clinical Significance and Natural History
MPNs are chronic diseases with potential for transformation to acute myeloid leukemia (AML) or progression to myelofibrosis, both associated with poor prognosis 4. The transformation risk varies by subtype:
- ET and low-risk PV: 5-15% risk of leukemic transformation over time
- Primary myelofibrosis: Higher risk of progression and transformation
- Secondary AML from MPN: Particularly resistant to standard chemotherapy with dismal outcomes 2
Key Clinical Features:
Thrombotic complications represent a major cause of morbidity and mortality in MPNs 4, 7. The thrombotic risk is driven not only by elevated blood counts but also by accelerated atherosclerosis and inflammatory mechanisms. This makes MPNs a model disease linking clonal hematopoiesis to cardiovascular disease 7.
Diagnostic Approach
The 2008 WHO classification (revised in 2016) integrates morphological, molecular, and clinical criteria 8, 9:
For PV:
- Hemoglobin >16.5 g/dL (men) or >16 g/dL (women)
- Bone marrow showing trilineage myeloproliferation
- Presence of JAK2 mutation (V617F or exon 12)
- Subnormal serum erythropoietin level
For ET:
- Platelet count ≥450 × 10⁹/L
- Bone marrow with megakaryocyte proliferation showing large, mature morphology
- Presence of JAK2, CALR, or MPL mutation
- Exclusion of other myeloid neoplasms
For PMF:
- Megakaryocyte proliferation with atypia
- Reticulin and/or collagen fibrosis (grade 2-3 in overt PMF)
- Presence of JAK2, CALR, or MPL mutation
- Clinical features: anemia, splenomegaly, leukoerythroblastosis, elevated LDH
For CML specifically, both G-banded chromosome analysis and quantitative RT-PCR for BCR-ABL1 fusion must be performed at diagnosis 1. If no BCR-ABL1 fusion is detected despite clinical suspicion, molecular testing for other MPN-associated mutations is mandatory 6.
Important Caveats
Cytogenetic studies are not essential for diagnosis of Philadelphia-negative MPNs but provide important prognostic information, particularly in primary myelofibrosis where karyotype is included in the DIPSSPlus prognostic scoring system 6.
The presence of MPN-associated mutations alone does not establish diagnosis - these must be interpreted in the context of clinical, laboratory, and morphologic findings to distinguish true MPN from clonal hematopoiesis of indeterminate potential (CHIP), which represents pre-malignant clonal expansion without overt disease 1, 7.