Decreased expression and impaired phosphorylation of ICSBP results in development of AML in vivo

Leukemia is a hematologic disorder affecting various cell lineages at different stages of maturation. The process of leukemogenesis is time consuming and additive, involving acquisition of multiple cytogenetic lesions and molecular aberrations. Hence, it confirms to the multi-hit hypothesis of carcinogenesis. Acute myeloid leukemia (AML) is one of the cancer forms affecting differentiation of cells involved in innate immune system of the myeloid lineage. Unfortunately, disease progression is still poorly understood because of the variety of targets affected. The development of AML requires at least genetic lesion from two different classes of mutation; one that dysregulates proliferation or apoptosis and another that causes differentiation block. Furthermore, there is great complexity to the genetic abnormalities in AML, which makes it difficult to determine the order of acquisition of the mutations, later ones being less likely to be pathogenesis driving, and their functional significance in leukemogenesis. Therefore, molecular mechanisms affecting the pathogenesis processes require considerable attention.;Although majority of AML occur "de novo", it can also arise as secondary AML and therapy-related AML. Secondary AML develops from clinical conditions such as MPD (myeloproliferative disorder) or MDS (myelodysplastic syndrome). CML, a form of MPD, is characterized by cytokine hypersensitivity and apoptosis resistance. Secondary mutations affecting the development of a differentiation block in myeloid cells is associated with progression of MPD to AML and portends poor prognosis. Identifying molecular markers of this transition may suggest targets for therapeutic intervention.;Interferon consensus sequence binding protein (ICSBP, also known as IRF8) is an interferon-regulatory transcription factor that functions as a leukemia tumor suppressor. In mice, ICSBP deficiency induces an MPD which resembles CML and progresses to AML over time. This suggests that ICSBP deficiency is sufficient for myeloproliferation, but additional genetic lesions are necessary for AML. Common lesions, such as mutations in SHP2 or Flt3, occurring in MPD are not sufficient for the onset of AML, which suggests that ICSBP must predispose to accumulation of such lesions. In addition, since activity of ICSBP is influenced by tyrosine phosphorylation state, we hypothesized that mutations in molecular pathways that regulate this process might synergize with ICSBP deficiency for progression to AML. Consistent with this, we found that constitutive activation of SHP2 protein tyrosine phosphatase synergized with ICSBP haplo-insufficiency to facilitate cytokine-induced myeloproliferation, apoptosis resistance, and rapid progression to AML in a murine bone marrow transplantation model. Constitutive SHP2 activation cooperated with ICSBP deficiency to increase the number of progenitors in the bone marrow and myeloid blasts in circulation, indicating a block in differentiation.;Mutations in signaling pathways affecting SHP2 function also influence myeloproliferation of ICSBP haplo-insufficient cells. ICSBP deficiency is associated with cytokine hypersensitivity, including responsiveness to proliferative cytokines such as Flt3 ligand (FL). Interestingly, SHP2 activity is increased by cytokine stimulation or constitutive activation of the Flt3 receptor tyrosine kinase. Hence, there are multiple mechanisms which can activate Shp2 in leukemia; PTPN11 mutations or mutations of activating cytokine receptors such as Flt3 or cKit. A leukemia associated mutation in one of these receptors would constitutively activate Shp2 in the absence of PTPN11 mutation. Since SHP2 activation and ICSBP deficiency may coexist in human myeloid malignancies, our studies have identified molecular mechanisms potentially involved in disease progression.