Title: The Role of PTPN11 in Myelofibrosis Project Summary/Abstract Myelofibrosis (MF) is the most severe form of myeloproliferative neoplasm (MPN), characterized by deposition of fibrous tissues in the bone marrow, abnormal megakaryopoiesis, and extramedullary hematopoiesis. The median survival for patients with MF is ~5 years. A somatic activating V617F mutation in the Janus Kinase 2 (JAK2V617F) has been found in 50-60% patients with MF. Additional mutations in the genes encoding thrombopoietin receptor (MPL) and calreticulin (CALR) have been found in MF but at a lower frequency than JAK2V617F. Ruxolitinib, a JAK1/JAK2 inhibitor, has been approved for treatment of MF. Although Ruxolitinib can reduce splenomegaly, it failed to produce disease remission or prevent fibrosis in patients with MPN/MF. It has already become clear that current chemotherapies including JAK2 inhibitors are not sufficient to cure MPN/MF. So, there is a critical need to identify new therapeutic target(s) and develop novel targeted therapies for MF. In preliminary studies, we have identified protein tyrosine phosphatase PTPN11 as a potential therapeutic target in MF. We have found that PTPN11 is constitutively phosphorylated in mouse and human MPN hematopoietic cells/progenitors. Knockdown of PTPN11 significantly inhibits proliferation of cells expressing JAK2V617F. In preliminary studies, using conditional PTPN11 knockout and JAK2V617F knock-in mice, we have observed that deletion of PTPN11 prevents the development of PV and markedly inhibits myelofibrosis. We also have observed that pharmacologic inhibition of PTPN11 significantly inhibits the growth of JAK2V617F-positive MPN cells and markedly reduces the bone marrow fibrosis in JAK2V617F knock-in mice. So, we hypothesize that PTPN11 may play an important role in the pathogenesis of MF, and inhibition of PTPN11 might be useful for treatment of MPN/MF. To test our hypothesis, we have proposed three specific aims. In Aim 1, we will investigate the role of PTPN11 in the pathogenesis of myelofibrosis (MF). In Aim 2, we will determine the efficacy of PTPN11 inhibition against MPN cells/progenitors and animal models of MPN/MF. In Aim 3, we will determine the mechanism by which PTPN11 depletion or inhibition blocks the development/ progression of MPN/MF. Our proposed studies will provide the first demonstration that PTPN11 plays an important role in the pathogenesis of MF and inhibition of PTPN11 is a novel therapeutic approach in MF. We will also have mechanistic understanding on how PTPN11 contributes to the development of MF. Results from these studies should lead to the design of a Phase I/II clinical trial involving PTPN11 inhibitor for treatment of MPN/MF.