In most patients with systemic mastocytosis (SM), including aggressive SM and mast cell leukemia (MCL), neoplastic cells express the oncogenic KIT mutation D816V. KIT D816V is associated with constitutive tyrosine kinase (TK) activity and thus represents an attractive drug target.

IntroductionMany cases of Philadelphia chromosome-negative myeloproliferative neoplasms (MPN) are characterized by an activating point mutation of JAK2 (JAK2 V617F ). It has been generally accepted that JAK2 V617F -positive cells outpace normal hematopoietic cells as a result of constitutively active growth factor signaling 1 ; however, failure of JAK2 V617F to confer a significant competitive advantage over normal hematopoiesis in 2 independent knock-in MPN models 2,3 suggests that additional factors may be required to promote expansion of JAK2 V617F -positive cells in patients.As MPN patients overproduce certain proinflammatory cytokines known to suppress normal hematopoiesis, 4 it is conceivable that JAK2 V617F may protect mutant stem cells and progenitors from the apoptotic cues induced by these cytokines.In this context, we recently observed that TNF␣ levels are elevated in mice with retrovirally induced JAK2 V617F MPN. 5 The physiologic effects of TNF␣ are complex and cell type-dependent, ranging from stimulation of proliferation to induction of apoptosis. 6 TNF␣ negatively regulates the expansion and self-renewal of pluripotent hematopoietic stem cells (HSCs) 7,8 and has inhibitory effects on normal as well as some leukemic human hematopoietic progenitor cells. [9][10][11] TNF␣'s involvement in the evolution of leukemia is not without precedent. Studies in Fanconi anemia (FA) have implicated TNF␣ hypersensitivity as a central mechanism of clonal evolution and progression to acute myeloid leukemia. In the FA Complementation Group C murine model (Fancc Ϫ/Ϫ ) TNF␣ induces bone marrow failure 12 and can promote the evolution of somatically mutated TNF␣-resistant preleukemic stem cell clones. 13 Taking into account TNF␣'s role in clonal evolution and that elevated TNF␣ levels are present in human MPN we hypothesized that JAK2 V617F induces TNF␣ expression and simultaneously confers TNF␣ resistance to MPN progenitor cells. Methods Isolation and culture of primary cellsBlood mononuclear cells (MNCs) were obtained from peripheral blood samples of patients with polycythemia vera (PV) and essential thromobocythemia (ET), myelofibrosis (MF), or normal volunteers. CD34 ϩ cells were obtained from bone marrow of normal, PV and ET patients or peripheral blood of MF patients. All patients gave their informed consent in accordance with the Declaration of Helsinki to participate in the study, which was approved by the Institutional Review Boards of Oregon Health & Science University (OHSU), Portland Veterans Affairs Medical Center, Cornell University, and Freiburg University.Submitted April 13, 2011; accepted August 7, 2011. Prepublished online as Blood First Edition paper, August 22, 2011; DOI 10.1182 DOI 10. /blood-2011 An Inside Blood analysis of this article appears at the front of this issue.The online version of this article contains a data supplement.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''adverti...