Although the role of Hedgehog (Hh) signalling in embryonic pattern formation is well established 1 , its functions in adult tissue renewal and maintenance remain unclear, and the relationship of these functions to cancer development has not been determined. Here we show that the lossof Smoothened (Smo), an essential component of the Hh pathway 2 , impairs haematopoietic stem cell renewal and decreases induction of chronic myelogenous leukaemia (CML) by the BCR-ABL1 oncoprotein 3 . Loss of Smo causes depletion of CML stem cells-the cells that propagate the leukaemia-whereas constitutively active Smo augments CML stem cell number and accelerates disease. As a possible mechanism for Smo action, we show that the cell fate determinant Numb, which depletes CML stem cells, is increased in the absence of Smo activity. Furthermore, pharmacological inhibition of Hh signalling impairs not only the propagation of CML driven by wildtype BCR-ABL1, but also the growth of imatinib-resistant mouse and human CML. These data indicate that Hh pathway activity is required for maintenance of normal and neoplastic stem cells of the haematopoietic system and raise the possibility that the drug resistance and disease recurrence associated with imatinib treatment of CML 4,5 might be avoided by targeting this essential stem cell maintenance pathway.

Human acute myelogenous leukemia (AML) is thought to arise from a rare population of malignant stem cells. Cells of this nature, herein referred to as leuke-mic stem cells (LSCs), have been documented for nearly all AML subtypes and appear to fulfill the criteria for stem cells in that they are self-renewing and give rise to the cells found in many leukemic populations. Because these cells are likely to be critical for the genesis and perpetu-ation of leukemic disease, the present studies sought to characterize unique molecular properties of the LSC population , with particular emphasis on the transcription factor, nuclear factor-B (NF-B). Previous experiments have shown that unstimulated human CD34 progenitor cells do not express NF-B. In contrast , primary AML CD34 cells display readily detectable NF-B activity as assessed by electrophoretic mobility shift assay and gene expression studies. Furthermore , detailed analyses of enriched AML stem cells (CD34 /CD38 /CD123) indicate that NF-B is also active in the LSC population. Given the expression of NF-B in leukemic, but not normal primitive cells, the hypothesis that inhibition of NF-B might induce leukemia-specific apoptosis was tested by treating primary cells with the proteasome inhibitor MG-132, a well-known inhibitor of NF-B. Leukemic CD34 /CD38 cells displayed a rapid induction of cell death in response to MG-132, whereas normal CD34 /CD38 cells showed little if any effect. Taken together, these data indicate that primitive AML cells aberrantly express NF-B and that the presence of this factor may provide unique opportunities to preferentially ablate LSCs. (Blood. 2001;98: 2301-2307)