Project summary Mixed phenotype acute leukemia (MPAL) is a rare and aggressive leukemia with features of both acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). MPAL patients are treated with either AML or ALL regimens, and patient survival is significantly impacted by age. While the median survival of children with MPAL is about 139 months, the median survival of the adult patients is only 11 months. Large scale integrative genomic analyses of pediatric and adult MPAL have identified numerous genetic mutations. However, many mutations are commonly seen in both adult and pediatric MPAL, suggesting that additional unidentified factors other than genetic mutations may contribute to the poorer prognosis observed in adult patients with MPAL. Introduction of MPAL genetic mutations in mouse models rarely leads to MPAL, instead typically results in AML or ALL. As a result, it is difficult to study MPAL in vivo and understand how aging contributes to this disease. Recently, we have developed an innovative mouse model of MPAL after serial bone marrow transplantation, in which about 90% mice developed spontaneous B/myeloid MPAL, a major subtype of human MPAL. MPAL in our model is highly dependent on aging of bone marrow cells, as transplantation with young mouse bone marrow cells did not result in such a phenotype. Strikingly, we found that gene knockout of histone deacetylase Sirt1 substantially inhibited development of MPAL in our mouse model. Therefore, our preliminary studies simultaneously generated a new mouse model for B/M MPAL, and identified a novel role of Sirt1 in this malignancy. In this proposal, we will further characterize this mouse model and test the effect of targeting SIRT1 on B/M MPAL. In specific aim 1, we will characterize genome-wide mutations and epigenetic alterations in mouse B/M MPAL for comparison with adult human B/M MPAL. In specific aim 2, we will determine the effect of inhibiting Sirt1 and its metabolic pathways for hematopoietic stem cell aging on B/M MPAL in the mouse model and adult human B/M MPAL samples. Successful completion of this proposal will validate a novel mouse model of B/M MPAL for future mechanistic studies and testing therapeutic intervention, advance our understanding of aging effect on adult B/M MPAL, and identify new therapeutic targets to improve treatment of this devastating disease.