3126 – SINGLE CELL SEQUENCING OF MLL-REARRANGED LEUKEMIA REVEALS MECHANISMS OF LEUKEMIA INITIATING CELL PLASTICITY

Leukemia initiating cells (LICs) are the subset of leukemic cells that drive leukemia progression, resist therapy, and spark disease relapse. Although LICs have been previously thought to be rare and phenotypically primitive, recent data suggest that the LIC state may actually be heterogeneous. Here, we use single-cell transcriptomics combined with limiting dilution xenotransplantation to dissect the ontogeny of MLL-rearranged B-lymphoblastic leukemia (MLL-r B-ALL), an aggressive form of childhood leukemia. Compared to acute myeloid leukemia (AML), LICs are abundant in MLL-r B-ALL. Recapitulating the unique clinical behavior of this form of leukemia, MLL-r B-ALL cells undergo a B-lymphoid to myeloid lineage switch under chemotherapy pressure consistent with primitive, multipotent transcriptional programs in LICs. Although we can indeed identify rare, chemotherapy-resistant, primitive LICs, we also observe LICs emerging from more differentiated populations. These facultative LICs self-renew and possess the capability to replenish the full cellular diversity of MLL-r B-ALL, including the primitive populations that retain multipotency programs. In mechanistic studies, we find that the LICs that drive this bottom-up reconstitution of the leukemic cellular ontogeny bear signatures of MYC activation and oxidative phosphorylation. We confirm recruitment of these pathways in actively reconstituting, phenotypically differentiated LICs, and define a pathway by which MYC rewires metabolism in MLL-r B-ALL. We find that MYC is required for LIC plasticity in vitro and in vivo. We conclude that the high LIC content and dual lineage and LIC plasticities of MLL-r B-ALL contribute to its chemotherapy resistance and persistently poor outcomes. Leukemia initiating cells (LICs) are the subset of leukemic cells that drive leukemia progression, resist therapy, and spark disease relapse. Although LICs have been previously thought to be rare and phenotypically primitive, recent data suggest that the LIC state may actually be heterogeneous. Here, we use single-cell transcriptomics combined with limiting dilution xenotransplantation to dissect the ontogeny of MLL-rearranged B-lymphoblastic leukemia (MLL-r B-ALL), an aggressive form of childhood leukemia. Compared to acute myeloid leukemia (AML), LICs are abundant in MLL-r B-ALL. Recapitulating the unique clinical behavior of this form of leukemia, MLL-r B-ALL cells undergo a B-lymphoid to myeloid lineage switch under chemotherapy pressure consistent with primitive, multipotent transcriptional programs in LICs. Although we can indeed identify rare, chemotherapy-resistant, primitive LICs, we also observe LICs emerging from more differentiated populations. These facultative LICs self-renew and possess the capability to replenish the full cellular diversity of MLL-r B-ALL, including the primitive populations that retain multipotency programs. In mechanistic studies, we find that the LICs that drive this bottom-up reconstitution of the leukemic cellular ontogeny bear signatures of MYC activation and oxidative phosphorylation. We confirm recruitment of these pathways in actively reconstituting, phenotypically differentiated LICs, and define a pathway by which MYC rewires metabolism in MLL-r B-ALL. We find that MYC is required for LIC plasticity in vitro and in vivo. We conclude that the high LIC content and dual lineage and LIC plasticities of MLL-r B-ALL contribute to its chemotherapy resistance and persistently poor outcomes.