Project Summary Patients with Noonan syndrome (NS), a developmental disorder characterized by congenital heart disease, dysmorphic facial structures, short stature, and mental retardation, are at an increased risk of developing leukemias, especially juvenile myelomonocytic leukemia (JMML), a fatal childhood myeloproliferative neoplasm (MPN). The mechanisms underlying the leukemogenesis are not completely understood. Lack of such knowledge impedes the development of therapeutic interventions for effectively controlling leukemic progression in NS and for improving treatment outcomes in NS-associated leukemias. Germline activating mutations of PTPN11 (encoding SHP2), a protein tyrosine phosphatase that we previously demonstrated is required for embryogenesis and hematopoietic cell development, are associated with more than 50% of patients with NS. Studies from our laboratory and others have established a cell-intrinsic role of PTPN11 mutations in NS and associated leukemias. Intriguingly, our most recent studies suggest that PTPN11 mutations in the bone marrow (BM) microenvironment can also induce a profound myeloid malignancy. The objective of the current project is to determine the cellular and molecular mechanisms by which PTPN11 mutated microenvironmental cells impact resident hematopoietic stem cells. The central hypothesis of the proposal is that germline PTPN11 mutations in the BM microenvironment greatly promote the leukemic progression in NS and cause stem cell transplantation failure. We plan to test this hypothesis and accomplish the objective of this application by pursuing the following aims. 1). Identify the cellular components and protein factors that mediate the leukemogenic effects of PTPN11 mutations in the BM microenvironment. 2). Determine the molecular mechanisms by which PTPN11 mutations change the activities of microenvironmental cells. 3). validate the pathogenic effects of PTPN11 mutations in NS patient-derived BM microenvironmental cells. This project will not only greatly advance our understanding of the pathogenesis of NS-associated leukemias, but also provide a molecular basis for the rational design of new therapeutics targeting the detrimental microenvironment in NS patients, which may eventually lead to significant improvements in controlling leukemic progression in NS and in stem cell transplantation therapy for NS-associated leukemias.