Research Report 2010 - MDC

URE ∆/∆ mice regularly developed aggressive hematopoieticmalignancies, such as acute myeloid leukemia, Tcell lymphoma and B1 cell chronic lymphoid leukemia.Results from the URE ∆/∆ animal model provided the firstdemonstration that interference with the fine-tunedregulation of a single transcription factor, through disruptionof a key cis-regulatory element, can be sufficientto initiate the formation of cancer stem cells andsubsequent tumor development.Epigenetic control of hematopoietic stem cellfunctionMethylation of CpG dinucleotides within the DNA is amajor epigenetic modification, which in mammals iscontrolled by at least 3 different DNA-methyltranferases(DNMTs): DNMT3a and -b for de novo methylationand DNMT1 for methylation maintenance. The impactof methylation on stem cell features has been studiedin embryonic stem (ES) cells, but little is known aboutits function in somatic stem cells. Recent advances inthe genome-wide mapping of DNA methylationdemonstrated that methylation of CpGs are dynamicepigenetic marks that undergo extensive changes duringcellular differentiation. However, whether and howthese changes are required for cell fate choice, in particularfor that of stem cells, remained unknown.Moreover, altered methylation is a hallmark of cancer,and drugs targeting methylating enzymes are used incancer therapy. The relationship between tumor-associatedalterations in methylation and cancer stem cellproperties was still elusive.We could show that alternative functional programs ofHSCs are governed by gradual differences in the methylationlevel (Bröske et al. in press). Constitutive methylationis essential for HSC self-renewal, but dispensablefor homing, cell cycle control and suppression of apoptosis.Remarkably, HSCs from mice with reduced DNMT1 activity fail to suppress key myeloerythroid regulatorsand as a consequence can differentiate into myeloerythroidbut not into lymphoid progeny. We revealed thata similar methylation dosage effect controls stem cellfunction in leukemia. Thus, our data identified DNAmethylation as an essential epigenetic mechanism toprotect stem cells from premature activation of predominantdifferentiation programs and suggest thatmethylation dynamics determines stem cell functionsin tissue homeostasis and cancer (Figure 1).Consequently, these results provide the hope thatdemethylating drugs may be instrumental to impairthe function of cancer stem cells in cancer therapy.Model of DNA methylation dosage effects on stem cell multipotency.In the presence of physiological DNMT1 concentrations (top panel),stem cells are competent of a diverse functional repertoire whichincludes self-renew, differentiation into myeloid or lymphoid progeny(MP, LP) and suppression of apoptosis. The molecular basis of thisdiversity lies in a tightly balanced expression of genes that control selfrenewalwith genes that prime for certain developmental fates.Reduction in DNMT1 levels (middle panel) narrows the functionaloptions of stem cells by lifting the suppression of a predominantmyeloerythroid gene program via insufficient promoter methylation.As a consequence, expression of self-renewal and lymphoid genesbecomes blocked. Finally, complete loss of DNMT1 (bottom panel)blocks the entire functional stem cell repertoire and causes the rapidelimination of the stem cell pool by cell-autonomous induction ofapoptosis. Methyl +++: high methylation level required, Methyl +: lowmethylation level required.Selected publicationsSteidl U, Rosenbauer F, Verhaak RGW, Gu X, Out HH, Bruns I, Steidl C,Costa DB, Klippel S, Wagner K, Aivado M, Kobbe G, Valk PJ, Passegué E,Libermann TA, Delwel R, and Tenen DG. (2006) Essential role of Jun familytranscription factors in PU.1-induced leukemic stem cells. Nature Genet38, 1269-1277.Scheller M, Huelsken J, Rosenbauer F, Taketo MM, Birchmeier W, Tenen DG,and Leutz A. (2006) Hematopoietic stem cell and multi lineage defects byβ-catenin activation. Nature Immunol 7, 1037-1047.Rosenbauer F, Owens BM, Yu L, Tumang JR, Steidl U, Kutok JL, Clayton LK,Wagner K, Scheller M, Iwasaki H, Liu C, Hackanson B, Akashi K, Leutz A,Rothstein TL, Plass C, and Tenen DG. (2006) Lymphoid cell growth andtransformation are suppressed by a key regulatory element of the geneencoding PU.1. Nature Genet 38, 27-37.Rosenbauer F and Tenen DG. (2007) Transcription factors in myeloid development:Balancing differentiation with transformation. Nature RevImmunol 7,105-117.Bröske AM, Vockentanz L, Kharazi S, Huska M, Mancini E, Scheller M, EnnsA, Prinz M, Jaenisch R, Nerlov C, Leutz A, Andrade-Navarro MA, JacobsenSEW, Rosenbauer F. (2009). DNA methylation protects hematopoieticstem cell multipotency from myeloerythroid restriction. Nature Geneticsin press.Cancer Research 95