HoxA9 and Meis1 cooperate to cause acute myeloid leukemia (AML), and are downstream effectors of a group of human AML oncoproteins, including MLL fusions. HoxA9 controls a self-renewal/differentiation switch and Meis1 confers leukemic potential, which correlates with its ability to activate transcription of stemness genes including Sox4, FLT3, and CD34. The biochemical and genetic mechanisms by which HoxA9 enforces self- renewal and by which Meis1 confers leukemic potential are not as yet understood. In this proposal, we use novel cell systems derived in our laboratory to address the key biochemical and genetic questions underlying myeloid leukemogenesis by HoxA9 and Meis1. In Aim 1, we focus on how apical oncoproteins activate transcription of Meis1. We will modulate Meis1 transcription using wild-type and conditional forms of MLL-ENL and NUP98-NSD1, a novel histone H3 Lysine 36 methyltransferase AML oncoprotein we recently discovered that coactivates transcription of Meis1 and HoxA9. Aim 2: In three different cell models, we discovered that the stem cell gene Sox4 is co-activated by HoxA9 and Meis1. Sox4 induces AML in mice and is activated in many other forms of human cancer. We hypothesize that Sox4 is a downstream effecter of HoxA9/Meis1 leukemogenesis. In Aim 2, we will characterize the Sox4 promoter, focusing on how HoxA9 and Meis1 cooperate with each other and other cofactors to activate Sox4 transcription. We hypothesize that HoxA9 and Meis1 catalyze distinct epigenetic modifications that cooperate functionally in activation. We will also deduce the requirement of Sox4 in HoxA9/Meis1 leukemogenesis, identify its genetic targets, and begin to pursue its cofactor function in establishing the AML phenotype. Aim 3: Terminal differentiation genes are strongly repressed by HoxA9, yet they are robustly activated when conditional forms of HoxA9 are switched off. In Aim 3, we will focus on how HoxA9 maintains active repression of terminal differentiation genes. For Aims 1-3, we use a novel technology called Chromatin Immunoprecipitation DNA Selection Ligation (ChIP-DSL), which will produce a high-resolution map of the histone marks of activation and repression, and of the binding sites for oncoproteins, transcription factors, and polymerases over 20 to 40 kbp of genomic DNA surrounding the transcriptional initiation site of each promoter. Lentiviral reporter vectors will be used to locate minimal promoters within responsive regions indicated by ChIP-DSL, and classical molecular and biochemical approaches will be taken to identify the activation mechanisms. Aim 4: We have already defined an N-terminal domain in HoxA9 and a C-terminal domain in Meis1 that are both required to activate transcription of Sox4, FLT3, and CD34. Our hypothesis is that each domain recruits a different histone modifying activity that cooperates in gene activation. In Aim 4, we identify cofactors that bind these domains, verify their importance in gene activation and in leukemogenesis, and begin to investigate the epigenetic changes they catalyze. These activities and the biochemical surfaces that recruit these cofactors are novel drug targets in AML. Project Narrative: This proposal focuses on how HoxA9 and Meis1 cause acute myeloid leukemia (AML) in humans, a cancer of blood stem cells. We focus on 3 of the most important questions: 1) How do human cancer proteins activate expression of the Meis1 leukemia gene? 2) How do HoxA9 and Meis1 activate expression of the Sox4 leukemia gene? 3) How does HoxA9 prevent expression of maturation genes? Discovering mechanisms underlying these questions may lead to the development of new AML therapeutics.