© 2017 by The American Society of Hematology. Myeloproliferative neoplasms (MPNs) arise in the hematopoietic stem cell (HSC) compartment as a result of the acquisition of somatic mutations in a single HSC that provides a selective advantage to mutant HSC over normal HSC and promotes myeloid differentiation to engender a myeloproliferative phenotype. This population of somatically mutated HSC, which initiates and sustains MPNs, is termed MPN stemcells. In > 95%ofcases,mutationsthat drive the development of an MPN phenotype occur in a mutually exclusive manner in 1 of 3 genes: JAK2, CALR, or MPL. The thrombopoietin receptor, MPL, is the key cytokine receptor in MPN development, and these mutations all activate MPLJAK- STAT signaling in MPN stem cells. Despitecommonbiological features,MPNs display diverse disease phenotypes as a result of both constitutional and acquired factors that influence MPN stem cells, and likely alsoasa resultofheterogeneity in the HSC in which MPN-initiating mutations arise. As the MPN clone expands, it exerts cell-extrinsic effects on components of the bone marrow niche that can favor the survival and expansion of MPN stem cells over normal HSC, further sustaining and driving malignant hematopoiesis. Although developed as targeted therapies for MPNs, current JAK2 inhibitors do not preferentially targetMPNstemcells, and as a result, rarely inducemolecular remissions in MPN patients. As the understanding of the molecular mechanisms underlying the clonal dominance of MPN stem cells advances, this will help facilitate the development of therapies that preferentially target MPN stem cells over normal HSC.
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