Myeloproliferative neoplasms (MPN) are clonal diseases of the hematopoietic stem cell (HSC) resulting in aberrant proliferation of erythroid, megakaryocytic and myeloid lineages. They represent clonal disorders of the hematopoietic stem cell with an inherent tendency towards leukemic transformation. MPN are subdivided into three disease entities: polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Somatic gain of function mutations in the genes JAK2, CALR, or MPL can be detected in blood cells of close to 90% of MPN patients. These gene mutations are known to act as drivers that can cause MPN phenotypes in patients and mouse models. Disease manifestation and prognosis is in part influenced by additional somatic mutation that synergize with the driver mutations, but alone do not cause disease. These additional mutations most frequently occur in genes encoding epigenetic regulators, such as TET2, DNMT3a or EZH2.Much focus is currently invested into studying disease progression and malignant transformation. However, at late stages most cancers are no longer curable. The discovery of clonal hematopoiesis of indeterminate potential (CHIP) that can be detected in a substantial proportion of healthy individuals with increasing age led to a major change of paradigm for understanding the pathogenesis of MPN, since mutations in JAK2, in particular JAK2-V617F, are among the most frequently found gene alterations in CHIP. The frequency of JAK2-V617F mutation in the normal population by far exceeds the incidence on MPN. This indicates that the acquisition of the JAK2-V617F mutation is not the rate limiting step, but rather there are factors that limit the expansion of the mutated HSCs. This research proposal focuses on identifying and understanding the early steps in disease initiation in MPN. The proposed experiments will address the following specific aims:1. To identify factors determining the early steps in MPN disease initiation: we will study the effects inflammation and of the immune surveillance in suppressing or promoting the early expansion of the mutant MPN HSCs and the role of modifier gene mutations that are frequently observed together with JAK2-V617F in patients with MPN. We will identify inherited predisposing gene mutations that favor MPN disease initiation.2. To study the mechanism of action of IFNa, currently the most effective drug in inducing remissions, on MPN stem cells and understand factors that determine the responsiveness of HSCs to IFNa.3. To examine genotype-phenotype correlations in MPN and define factors that determine ET versus PV phenotypes in MPN with mutated JAK2.Elucidating the underlying early events responsible for MPN disease initiation will be important to designing strategies for intervention. We expect that diagnosis of pre-clinical cones carrying driver gene mutations will become more frequent as the methodologies for sensitive detection of somatic mutation become commonplace. Questions whether to treat individuals with CHIP (e.g. with IFNa) or whether to inhibit inflammation will need to be answered. By advancing our understanding of the underlying early events in the initiation of MPN from mutant clones will likely lead to new targets for therapeutic intervention. Targeting mutant JAK2 expressing HSCs early in the evolution of MPN has the best chance to change the natural history of MPNs.