Causes of Essential Thrombocytosis
Essential thrombocytosis is caused by clonal genetic mutations affecting the JAK-STAT signaling pathway, with approximately 90% of cases harboring driver mutations in JAK2 (64%), CALR (23%), or MPL (4%) genes 1.
Primary (Clonal) Causes
The fundamental pathophysiology involves acquired somatic mutations that constitutively activate megakaryocyte proliferation:
Driver Mutations
- JAK2 V617F mutation: Present in approximately 64% of ET cases, this mutation leads to constitutive activation of the JAK-STAT pathway, preferentially stimulating megakaryopoiesis 1, 2
- CALR mutations: Found in 23% of patients, these mutations affect calcium signaling and megakaryocyte development 1
- MPL mutations: Present in 4% of cases, directly affecting the thrombopoietin receptor 1
- Triple-negative cases: Approximately 10% lack any of these driver mutations but still represent clonal disease 2
Additional Mutations
Beyond driver mutations, approximately 50% of patients harbor additional somatic mutations that may influence disease behavior 2:
- TET2 (9-11%)
- ASXL1 (7-20%)
- DNMT3A (7%)
- SF3B1 (5%)
Cytogenetic Abnormalities
Less than 10% of patients demonstrate chromosomal abnormalities including +9, 20q-, 13q-, del(5q), and unbalanced translocations between 1q and 7p 3, 2.
Secondary (Reactive) Causes to Exclude
Before diagnosing essential thrombocytosis, you must systematically exclude reactive causes of thrombocytosis 3, 4:
Common Reactive Causes
- Iron deficiency anemia (most common metabolic cause)
- Infection (acute or chronic)
- Inflammation (including connective tissue diseases like rheumatoid arthritis and systemic lupus erythematosus)
- Tissue injury (32.2% of secondary thrombocytosis cases) 5
- Post-splenectomy state
- Malignancy (metastatic cancer, lymphoproliferative disorders)
- Post-surgical state
Important Clinical Caveat
The presence of a condition associated with reactive thrombocytosis does not automatically exclude ET if diagnostic criteria are otherwise met 3, 4. You must still evaluate for clonal markers and bone marrow morphology.
Diagnostic Algorithm for Determining Cause
When encountering thrombocytosis (platelets ≥450 × 10⁹/L):
Confirm persistence: Repeat CBC to document sustained elevation 4
Screen for reactive causes:
- C-reactive protein (inflammation)
- Ferritin and iron studies (iron deficiency)
- Clinical assessment for infection, tissue injury, malignancy
- History of splenectomy
If no clear reactive cause identified, test for clonal disease:
- JAK2 V617F mutation testing (first-line)
- If JAK2 negative: CALR and MPL mutation testing
- Bone marrow biopsy showing megakaryocytic proliferation with enlarged, mature megakaryocytes with hyperlobulated nuclei 4
Exclude other myeloid neoplasms 4:
- Polycythemia vera (check hemoglobin/hematocrit, trial iron replacement if deficient)
- Primary myelofibrosis (assess for reticulin fibrosis, aberrant megakaryocyte morphology)
- CML (BCR-ABL testing)
- MDS (evaluate for dysplasia)
Pathophysiologic Mechanism
The mutations cause thrombocytosis through dysregulated thrombopoietin signaling. In heterozygous JAK2 V617F, the mutation preferentially stimulates megakaryopoiesis, manifesting as ET. Decreased MPL receptor expression shifts the balance toward increased megakaryocyte signaling and reduced thrombopoietin clearance by platelets, even without coding sequence mutations 6.
In 30-40% of ET cases, the disease-causing gene remains unidentified despite clear clonal disease 6, indicating our incomplete understanding of megakaryopoiesis regulation.