Understanding the molecular and biochemical basis of leukemic growth is essential for developing targeted therapeutics. However, with the notable exception of imatinib mesylate, limited progress has been made toward achieving the goal of developing effective and safe inhibitors of leukemia-specific molecules. The p21ras (Ras) family of signal switch molecules is deregulated in myeloid malignancies including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and myeloproliferative disorders (MPD). Studies of human leukemia samples have shown that NRAS and KRAS2 point mutations, the BCR-ABL fusion, PTPN11 mutations, FLT3 internal tandem duplications, and NF1 inactivation all deregulate Ras signaling. Our laboratory has generated strains of mice that develop MPD due to conditional expression of an oncogenic KrasG12D from the endogenous locus or inactivation of the Nf1 tumor suppressor gene. Neither strain spontaneously develops AML, suggesting that cooperating mutations are necessary for the progression from MPD to AML. My long-term career goal is to increase our current understanding of the molecular and biochemical mechanisms that underlie malignant growth, and to use these insights to develop more effective and less toxic therapies. Myeloid leukemia offers a tractable experimental system for addressing these questions. Moreover, new treatments are urgently needed for AML, MDS, and most types of MPD. I have generated mice that express oncogenic NrasG12D from its endogenous locus in hematopoietic cells, and have shown that this results in a distinctive MPD. I have also utilized retroviral insertional mutagenesis (RIM) as a general strategy for introducing mutations that cooperate with oncogenic Nras and Kras in leukemogenesis. The overall goal of this K08 application is to exploit these accurate models of human myeloid malignancies to address biologic and preclinical questions through three specific aims. These aims are: 1) To identify and characterize genes that cooperate with hyperactive Ras in myeloid leukemogenesis; 2) To characterize how NrasG12D expression alters Ras-regulated signaling networks and perturbs the growth of primary hematopoietic cells, and to investigate functional difference between oncogenic isoforms of Nras and Kras in leukemogenesis; 3) To investigate the effect of inhibiting MEK on leukemic growth and to identify genes that modulate sensitivity and resistance to targeted inhibitors in vivo. Acute myeloid leukemia arises in a variety of clinical settings and the treatment is largely unsatisfactory. The goal of this project is to use mouse strains that accurately model human disease to identify genes important for leukemia development and to test novel therapies that target and eradicate tumor cells more effectively.