PROJECT SUMMARY/ABSTRACT Chronic myelomonocytic leukemia (CMML) is a devastating cancer with an urgent and unmet need for effective therapies (median survival: ~28 months). Approximately 30% of CMML cases evolve to acute myeloid leukemia (AML) soon after their initial diagnosis, contributing to the poor prognosis of CMML patients. During the prior funding period, we collaborated with the International MDS/MPN Consortium and demonstrated that concurrent NRAS and ASXL1 mutations define a population of CMML patients with shorter leukemia-free survival than those with ASXL1 mutations only. Based on our human data, we characterized NrasG12D/+; Asxl1-/- (NA) mice that model CMML patients with concurrent NRAS and ASXL1 mutations. NA mice developed CMML with accelerated progression and in ~50% of these mice CMML transformed to AML (secondary AML, sAML). NA leukemia cells exhibited hyperactivation of MEK/ERK signaling and increased global level of H3K27Ac, a histone mark bound by bromodomain and extra-terminal domain (BET) proteins for gene transcriptional activation. Upregulation of AP-1 transcription factors (TFs) mediated the overexpression of PD-L1 and CD86, two inhibitory immune checkpoint ligands, and helped establish a suppressive immune microenvironment in NA-sAML recipients. Combined inhibition of MEK and pan-BET proteins led to downregulation of AP-1 TF expression, partial mitigation of the suppressive immune microenvironment, enhancement of CD8 T cell cytotoxicity, and prolonged survival in NA-sAML mice. Based on our preliminary results, we hypothesize that Asxl1-/- and oncogenic Nras cooperate to accelerate CMML and promote its transformation to AML via reprogramming the immune microenvironment, which includes but may not limit to T cells. Moreover, immunomodulatory agents may further improve the therapeutic benefits of combine MEK and BET inhibition through establishing durable anti-leukemia activities in immune cells. In this renewal application, we propose the following two aims to test our hypothesis: 1) To identify the molecular and cellular mechanisms underlying the dysregulation of immune microenvironment in in NA mice; and 2) determine whether immunomodulatory agents further improve the therapeutic effects of combined MEK and BET inhibition in NA mice. Together, our proposed studies will not only provide fundamental perspectives on disease mechanisms but also test hypotheses that could potentially lead directly to potent and efficacious novel therapies for treating CMML and transformed AML.