PROJECT SUMMARY Acute myeloid leukemia (AML) is a genetically and cellularly heterogenous disease characterized by the expansion of hematopoietic cells across a range of cell states from stem-like cells to differentiated myeloid cells. The most mutated genes in AML are DNMT3A, NPM1 and the receptor tyrosine kinase FLT3. Despite early clinical responses, most patients relapse, and FLT3-mutant clones are not always eradicated. Our lab has developed genetically engineered mouse models of acute myeloid leukemia that are capable of activating mutations in Flt3 with Dre-recombinase, and then genetically reverting them with Cre-recombinase. We have used these models to benchmark Flt3 oncogene-addiction against best-in-class small molecule kinase inhibitors of FLT3, observing difference in disease remission and relapse. These studies have refined our interest on identifying which cells along the hematopoietic hierarchy are capable of driving relapse and which molecular pathways underlie their survival following chemical/genetic inhibition of FLT3. The major goal of this proposal is to understand the cellular mechanisms that maintain FLT3-mutant clone persistence during targeted therapy. Our preliminary data indicate that Flt3-inhibtion results in a profound differentiation response and induction of Type I Interferon signaling. We will complete integrative studies with human specimen and our innovative multi- recombinase mouse models of leukemia to derive clinically meaningful insights from mechanistic observations in model systems. In aim 1 we will determine which cells are capable of propagating leukemic disease and resolve cellular reservoirs of leukemic stem cell activity. We will perform these studies using genetically engineered mouse models, serial transplantation of purified cell populations, and functional cell ablation studies. We hypothesize that FLT3-inhibtion induced differentiation generates mature cells that are capable of reacquiring stem-like properties and