Key Points• LCN2 acts to generate reactive oxygen species, leading to increased DNA strand breaks and apoptosis in normal CD34 1 cells.• LCN2 promotes the generation of osteoblasts but diminishes adipogenesis, resembling the composition of the MF marrow microenvironment.Myelofibrosis (MF) is characterized by cytopenias, constitutional symptoms, splenomegaly, and marrow histopathological abnormalities (fibrosis, increased microvessel density, and osteosclerosis). The microenvironmental abnormalities are likely a consequence of the elaboration of a variety of inflammatory cytokines generated by malignant megakaryocytes and monocytes. We observed that levels of a specific inflammatory cytokine, lipocalin-2 (LCN2), were elevated in the plasmas of patients with myeloproliferative neoplasms (MF > polycythemia vera or essential thrombocythemia) and that LCN2 was elaborated by MF myeloid cells. LCN2 generates increased reactive oxygen species, leading to increased DNA strand breaks and apoptosis of normal, but not MF, CD34 1 cells. Furthermore, incubation of marrow adherent cells or mesenchymal stem cells with LCN2 increased the generation of osteoblasts and fibroblasts, but not adipocytes. LCN2 priming of mesenchymal stem cells resulted in the upregulation of RUNX2 gene as well as other genes that are capable of further affecting osteoblastogenesis, angiogenesis, and the deposition of matrix proteins. These data indicate that LCN2 is an additional MF inflammatory cytokine that likely contributes to the creation of a cascade of events that results in not only a predominance of the MF clone but also a dysfunctional microenvironment. (Blood. 2015;126(8):972-982)

committed to multiple hematopoietic lineages. 3 In contrast, Cambier et al provided evidence in another PV patient who developed CML that the PV and CML originated from distinct clones. 4 Our findings indicate that both the JAK2V617F mutation and BCR-ABL1 can occur concurrently in both CFU-GM and BFU-E and that JAK2V617F occurs before the acquisition of BCR-ABL1.The clinical phenotype of myeloproliferative neoplasms frequently evolves over time. Although this progression can be enhanced by the use of chemotherapeutic agents, it represents the natural clonal evolution of these malignancies. The development of CML in PV occurs much less frequently than myelofibrosis or acute myeloid leukemia, but should be considered in patients with PV who develop extreme leukocytosis. The contribution of BCR-ABL1 to disease progression appears to be greater than that of JAK2V617F, because these patients display a clinical phenotype that is consistent with CML rather than PV. These clinical observations represent an example of clonal evolution in which the initial genetic event is a mutation leading to the activation of a tyrosine kinase (JAK2), which is then followed by either a second genetic event leading to the acquisition of a fusion protein resulting in activation of another protein kinase (BCR-ABL1) or by homologous recombination resulting in JAK2V617F-homozygous HPCs that lack BCR-ABL1. Subsequently, the JAK2V617F-heterozygous HPCs with BCR-ABL1 can also undergo homologous recombination and become homozygous for JAK2V617F.