Thrombopoietin receptor is required for the oncogenic function of CALR mutants

Myeloproliferative neoplasms (MPNs) are diseases characterized by the pathologic expansion of myeloid cells of the hematopoietic lineage. The three ‘classical' MPNs include polycythemia vera (PV, increase in erythrocytes), essential thrombocythemia (ET, increase in platelets) and primary myelofibrosis (PMF, usually elevated platelet counts associated with fibrotic deposition in the bone marrow).1 MPNs are essentially clonal diseases driven by somatic mutations in hematopoietic stem and progenitor cells. So far, three genes have been identified that can drive the disease phenotype when mutated.2 Activating mutations in Janus Kinase 2 (JAK2) and the thrombopoietin receptor (MPL) have been known for close to a decade and their mechanism of action has been extensively studied.3, 4, 5, 6, 7, 8 Recently, we and others identified somatic mutations in the CALR gene in 25–35% of ET and PMF patients.9, 10