Role of megakaryocytes in the pathophysiology of myeloproliferative neoplasms, and patterns of clonal evolution and phenotypic switch in these disorders. (A) Megakaryocyte carrying a somatic mutation in JAK2, MPL, or CALR, all representing gain-of-function mutations of thrombopoietin signaling through its receptor. How a mutant calreticulin involves gain of thrombopoietin signaling is currently under investigation, but the available evidence strongly supports this notion. Enhanced thrombopoietin signaling results in excessive platelet production and, likely, also in abnormal megakaryocyte contribution to the bone marrow matrix environment, leading to thrombocytosis initially and to bone marrow fibrosis in the long term. (B) Patterns of clonal evolution and phenotypic switch in different genotypic entities. Clonal evolution of JAK2 (V617F)-mutant myeloproliferative neoplasms is associated with acquired copy-neutral loss of heterozygosity of chromosome 9p, responsible for the transition from heterozygosity to homozygosity for JAK2 (V617F). This may involve a phenotypic switch from essential thrombocythemia to polycythemia vera, and in the long term, from this latter to myelofibrosis. Essential thrombocythemia may also progress directly to myelofibrosis without the intermediate stage of polycythemia vera. Clonal evolution of MPL-mutant myeloproliferative neoplasms is associated with acquired copy-neutral loss of heterozygosity of chromosome 1p, involving transition from heterozygosity to homozygosity for the MPL mutation. This molecular mechanism and/or cooperating mutations may be responsible for a phenotypic switch from essential thrombocythemia to myelofibrosis. Clonal evolution of CALR-mutant myeloproliferative neoplasms appears to be mainly associated with a progressive expansion of a mutant heterozygous clone that eventually becomes fully dominant in the bone marrow. Activation of megakaryocytes by mutant CALR and/or cooperating mutations may be responsible for a phenotypic switch from essential thrombocythemia to myelofibrosis. Within each genotypic entity, thrombocythemia is likely the initial phenotype. Transition from one phenotype to another may or may not occur, depending on several factors, and transition rates may vary considerably. According to the pathophysiologic model depicted here, myelofibrosis is a late stage in the evolution of the different myeloproliferative neoplasms.