Mechanisms of Leukemia Stem Cell Plasticity Revealed By Single Cell Analysis

Leukemia stem cells (LSCs) are the subset of leukemic cells that drive leukemia progression, resist therapy, and remain latent to spark disease relapse. LSCs are quantified by efficiency of xenografting immunodeficient mice, a measurement that is predictive of leukemia outcome. Although LSCs have been previously thought to be rare and phenotypically primitive, recent data indicate that LSCs 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), LSCs 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 present in LSCs. Although we identify rare, chemotherapy-resistant, primitive LSCs, we also observe LSCs emerging from more differentiated populations. These facultative LSCs self-renew and possess the capability to replenish the full cellular diversity of MLL-r B-ALL. Using CITE-seq, we find that stem cell programs can be fully uncoupled from immunophenotype in MLL-r B-ALL. In mechanistic studies, we find that the phenotypically differentiated LSCs that drive 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 LSCs, and define a pathway by which MYC rewires metabolism in MLL-r B-ALL LSCs. We find that MYC is required for LSC plasticity in vitro and in vivo. Targeting oxidative metabolism impairs LSC engraftment, identifying a potential therapeutic intervention. We conclude that the high LSC content and dual lineage and LSC plasticities of MLL-r B-ALL contribute to its chemotherapy resistance and persistently poor outcomes.