Leukaemias and other cancers possess a rare population of cells capable of the limitless self-renewal necessary for cancer initiation and maintenance. Eradication of these cancer stem cells is probably a critical part of any successful anti-cancer therapy, and may explain why conventional cancer therapies are often effective in reducing tumour burden, but are only rarely curative. Given that both normal and cancer stem cells are capable of self-renewal, the extent to which cancer stem cells resemble normal tissue stem cells is a critical issue if targeted therapies are to be developed. However, it remains unclear whether cancer stem cells must be phenotypically similar to normal tissue stem cells or whether they can retain the identity of committed progenitors. Here we show that leukaemia stem cells (LSC) can maintain the global identity of the progenitor from which they arose while activating a limited stem-cell- or self-renewal-associated programme. We isolated LSC from leukaemias initiated in committed granulocyte macrophage progenitors through introduction of the MLL-AF9 fusion protein encoded by the t(9;11)(p22;q23). The LSC were capable of transferring leukaemia to secondary recipient mice when only four cells were transferred, and possessed an immunophenotype and global gene expression profile very similar to that of normal granulocyte macrophage progenitors. However, a subset of genes highly expressed in normal haematopoietic stem cells was re-activated in LSC. LSC can thus be generated from committed progenitors without widespread reprogramming of gene expression, and a leukaemia self-renewal-associated signature is activated in the process. Our findings define progression from normal progenitor to cancer stem cell, and suggest that targeting a self-renewal programme expressed in an abnormal context may be possible.

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Summary The histone 3 lysine 79 (H3K79) methyltransferase Dot1l has been implicated in the development of leukemias bearing translocations of the Mixed Lineage Leukemia (MLL) gene. We identified the MLL-fusion targets in an MLL-AF9 leukemia model, and conducted epigenetic profiling for H3K79me2, H3K4me3, H3K27me3 and H3K36me3 in hematopoietic progenitor and leukemia stem cells (LSC). We found abnormal profiles only for H3K79me2 on MLL-AF9 fusion target loci in LSC. Inactivation of Dot1l lead to down-regulation of direct MLL-AF9 targets and an MLL-translocation associated gene expression signature, while global gene expression remained largely unaffected. Suppression of MLL-translocation associated gene expression corresponded with dependence of MLL-AF9 leukemia on Dot1l in vivo. These data point to DOT1L as a potential therapeutic target in MLL-rearranged leukemia.

A subset of cells called leukemia stem cells (LSCs) possess limitless self-renewal and are responsible for maintenance of leukemia. Selective eradication of LSCs could offer significant therapeutic benefit and there is great interest in identifying the signaling pathways that control their development. Here, LSCs were studied in mouse models of acute myelogenous leukemia (AML) induced either by co-expression of the Hoxa9 and Meis1a oncogenes or the fusion oncoprotein MLL-AF9. We show that the Wnt/β-catenin signaling pathway is required for self-renewal of LSCs derived from either hematopoietic stem cells (HSC) or more differentiated granulocyte macrophage progenitors (GMP). Since the Wnt/β-catenin pathway is normally active in HSCs, but not in GMP, reactivation of β-catenin signaling is required for transformation of progenitor cells by certain oncogenes. β-catenin is not absolutely required for self-renewal of adult HSCs, thus targeting the Wnt/β-catenin pathway may represent a new therapeutic opportunity in AML.