JAK2 Mutation and Splenomegaly in Myeloproliferative Neoplasms

Relationship Between JAK2 Mutation and Splenomegaly

The European Society for Medical Oncology notes that JAK2V617F mutation is found in approximately 95% of polycythemia vera (PV) cases and about 50-60% of essential thrombocythemia (ET) and primary myelofibrosis (PMF) cases, which commonly present with splenomegaly due to extramedullary hematopoiesis 3, 4, 5
Increasing splenomegaly is a diagnostic criterion for disease progression in both post-polycythemia vera myelofibrosis (PPV-MF) and post-essential thrombocythemia myelofibrosis (PET-MF), as indicated by the International Working Group for Myeloproliferative Neoplasms Research and Treatment 2

The American Society of Hematology recommends that splenomegaly be considered a sign of disease progression, with the appearance of a newly palpable splenomegaly or an increase in palpable splenomegaly of ≥5 cm from the left costal margin 2, 5
Symptomatic splenomegaly is a major indication for treatment in JAK2-mutated MPNs, according to the National Comprehensive Cancer Network 5

The European Society for Medical Oncology suggests that JAK inhibitors, such as ruxolitinib, are the current first-line therapy for symptomatic splenomegaly in patients with JAK2-mutated MPNs, with significant reduction in spleen volume in phase III clinical trials (COMFORT-I and COMFORT-II) 5
The American Society of Hematology notes that spleen reduction with JAK inhibitors is usually dramatic but is also drug- and dose-dependent, and that splenectomy may be considered for patients who do not respond to or cannot tolerate JAK inhibitors 5

Regular monitoring of spleen size is essential for patients with JAK2-mutated MPNs, as increasing splenomegaly may indicate disease progression from PV or ET to myelofibrosis, according to the International Working Group for Myeloproliferative Neoplasms Research and Treatment 2, 3

The European Society for Medical Oncology warns that not all patients with JAK2 mutations will develop splenomegaly, and that clinical manifestations vary based on the specific MPN subtype and disease stage 1, 3
The American Society of Hematology notes that JAK inhibitors may control splenomegaly but do not significantly decrease or eliminate the MPN clone in most patients, and that sudden withdrawal of JAK inhibitors can provoke a shock-like syndrome due to the re-emergence of suppressed inflammatory cytokines 5