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Cell Stem Cell. 2011 Jun 3;8(6):649-62. doi: 10.1016/j.stem.2011.05.004.

Functional crosstalk between Bmi1 and MLL/Hoxa9 axis in establishment of normal hematopoietic and leukemic stem cells.

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Leukaemia and Stem Cell Biology Lab, Department of Haematological Medicine, King's College London, London SE5 9NU, UK.


Bmi1 is required for efficient self-renewal of hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs). In this study, we investigated whether leukemia-associated fusion proteins, which differ in their ability to activate Hox expression, could initiate leukemia in the absence of Bmi1. AML1-ETO and PLZF-RARα, which do not activate Hox, triggered senescence in Bmi1(-/-) cells. In contrast, MLL-AF9, which drives expression of Hoxa7 and Hoxa9, readily transformed Bmi1(-/-) cells. MLL-AF9 could not initiate leukemia in Bmi1(-/-)Hoxa9(-/-) mice, which have further compromised HSC functions. But either gene could restore the ability of MLL-AF9 to establish LSCs in the double null background. As reported for Bmi1, Hoxa9 regulates expression of p16(Ink4a)/p19(ARF) locus and could overcome senescence induced by AML1-ETO. Together, these results reveal an important functional interplay between MLL/Hox and Bmi1 in regulating cellular senescence for LSC development, suggesting that a synergistic targeting of both molecules is required to eradicate a broader spectrum of LSCs.

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