Oncogene (2001) 20, 3100 ± 3109 ã 2001 Nature Publishing Group All rights reserved 0950 ± 9232/01 $15.00 www.nature.com/onc Avian erythroleukemia: a model for corepressor function in cancer Luc EG Rietveld1,2, Eric Caldenhoven1,2 and Hendrik G Stunnenberg*,1 1Department of Molecular Biology, NCMLS, Geert Grooteplein Zuid 26, PO Box 9101 6500 HB Nijmegen, The Netherlands Transcriptional regulation at the level of chromatin plays of chromatin, such as inhibitors of methylation or crucial roles during eukaryotic development and dier- histone acetylation (Cameron et al., 1999). Thus, entiation. A plethora of studies revealed that the altering the state of chromatin may be an important acetylation status of histones is controlled by multi- tool in a cure for cancer. protein complexes containing (de)acetylase activities. In A link between (de)acetylation of histones and the current model, histone deacetylases and acetyltrans- transcriptional regulation was proposed several decades ferases are recruited to chromatin by DNA-bound ago (Allfrey et al., 1964). The identi®cation of proteins repressors and activators, respectively. Shifting the that can modulate the acetylation state of histones balance between deacetylation, i.e. repressive chromatin provided further evidence for the link between and acetylation, i.e. active chromatin can lead to chromatin structure and gene regulation. The extent aberrant gene transcription and cancer. In human acute of histone acetylation is determined by the opposing promyelocytic leukemia (APL) and avian erythroleuke- action of histone acetyltransferases (HATs) and histone mia (AEL), chromosomal translocations and/or muta- deacetylases (HDACs) (reviewed by Kouzarides, 1999). tions in nuclear hormone receptors, RARa [NR1B1] and In a simplistic view, these enzymes are recruited to TRa [NR1A1], yielded oncoproteins that deregulate chromatin by DNA-bound transcription factors there- transcription and alter chromatin structure. The onco- by (locally) modifying the histone tails and conse- genic receptors are locked in their `o' mode thereby quently the chromatin structure. Transcriptional constitutively repressing transcription of genes that are regulators that recruit histone deacetylases may control critical for dierentiation of hematopoietic cells. AEL tumour development at dierent stages and can grossly involves an oncogenic version of the chicken TRa,v- be divided into three dierent categories. Firstly, ErbA. Apart from repression by v-ErbA via recruitment transcriptional repressors that control cell-cycle and of corepressor complexes, other repressors and corepres- proliferation such as Mad and Rb. Secondly, methyl- sors appear to be involved in repression of v-ErbA target binding proteins that mediate transcriptional silencing genes, such as carbonic anhydrase II (CAII). Reactiva- of genes which may be critical in the formation of tion of repressed genes in APL and AEL by chromatin certain cancers. Thirdly, transcriptional regulators such modifying agents such as inhibitors of histone deacety- as TR and RAR that control dierentiation and lase or of methylation provides new therapeutic strategies development (reviewed in Cress and Seto, 2000). in the treatment of acute myeloid leukemia. Oncogene This review focuses on the role of nuclear hormone (2001) 20, 3100 ± 3109. receptors (NR) in cancer. Aberrant nuclear receptor function due to a hormonal imbalance or receptor Keywords: Erythroleukemia; corepressor; v-ErbA; his- mutations has been strongly correlated with disease tone deacetylase; carbonic anhydrase II; methylation and cancer (reviewed in Forrest et al., 1996; Goldhirsch and Gelber, 1996). Thyroid hormone receptor (TR) and retinoic acid receptor (RAR) are both class II Introduction nuclear receptors which are ligand-controlled transcrip- tion factors that govern transcription of a multitude of The mechanisms by which oncoproteins deregulate target genes by recruiting multicomponent cofactor transcription of speci®c target genes during oncogenic complexes. In the absence of ligand, nuclear hormone transformation are still poorly understood. Mutations, receptors recruit corepressor complexes containing deletions and fusions resulting from chromosomal HDAC activity, whereas in the presence of ligand they translocations in cellular or viral (proto-)oncogenes recruit coactivator proteins with HAT activity and/or have altered their cognate protein functions, causing multicomponent coactivator complexes. In human aberrant gene regulation and inducing cancer. Recent promyelocytic leukemia (APL), chromosomal translo- studies show that reactivation of genes silenced in cations lead to fusion proteins involving RARa and cancer can be accomplished by treatment of trans- dierent fusion partners, such as PML, PLZF, NPM formed cells with compounds that aect the structure and NUMA (reviewed in Redner et al., 1999). APL- patients carrying the PML-RARa fusion can be cured by treatment with the RARa ligand all-trans retinoic *Correspondence: HG Stunnenberg acid (ATRA) which induces terminal dierentiation of 2These authors contributed equally APL blasts (Warrell et al., 1991, 1993). PLZF ± RARa Corepressors in erythroleukemia LEG Rietveld et al 3101 patients are not responsive to ATRA, suggesting that v-ErbA and transcriptional repression dierent or additional mechanisms are involved in PLZF ± RARa- compared to PML ± RARa-induced The v-ErbA oncogene is a viral variant of chicken TRa APL. Treatment with histone deacetylase inhibitors and was fused to the viral gag gene during the genesis such as trichostatin A (TSA) plus ATRA overcomes of AEV. The way in which v-ErbA contributes to the the maturation blockade in APL induced by PLZF ± leukemic phenotype has long been elusive. The role of RARa (reviewed in Redner et al., 1999), suggesting a numerous mutations occurring throughout the receptor link between chromatin modi®cations and cancer moiety have been analysed in great detail (Figure 1, development. In chicks, the avian erythroblastosis virus reviewed in Beug et al., 1994). Due to the mutations in (AEV) causes erythroleukemia (AEL). AEV encodes an the Zn-®nger, the v-ErbA DNA-binding speci®city and oncogenic variant of the TRa, v-ErbA, and the anity have been aected. Furthermore, the ability of mutated EGF-receptor v-ErbB. Due to a number of v-ErbA to dimerise with RXR is impaired. Several mutations throughout the ligand-binding domain, v- mutations and a small N-terminal deletion also aect ErbA is unable to respond to ligand. Restoration of the ability of v-ErbA to bind ligand and to activate thyroid hormone responsiveness in erythroleukemic transcription (Munoz et al., 1988; Zenke et al., 1990; cells by overexpression of a ligand-responsive thyroid Barettino et al., 1993). hormone receptor partially restores the ability of these A decade ago, it was demonstrated that v-ErbA cells to dierentiate (Disela et al., 1991). The inability constitutively inhibits T3-mediated activation of cog- of oncogenic receptors to eectively respond to nate target genes (Sap et al., 1989; Damm et al., 1989; physiological concentrations of ligand provides a Pain et al., 1990; Zenke et al., 1990; Baniahmad et al., molecular basis of oncogenesis. Recently, a plethora 1992). From these and many other studies, v-ErbA was of molecular studies on the modulation of chromatin initially postulated to merely antagonise TR function by NR have been published that provide new and through constitutive binding to TR responsive elements sometimes contradictory explanations for the mechan- (TRE), occluding the endogenous receptor (the `bind- isms causing APL and AEL. In this review, we will ing site occlusion' model). Another step was the focus on chromatin modifying complexes and their role identi®cation of target genes that were directly in transcriptional repression in AEV-induced leukemia. regulated by v-ErbA: the erythrocyte anion transport protein gene (Band 3), the d-aminolevulinate synthase gene (ALA-S), the lysozyme gene and the carbonic AEV and leukemia anhydrase II gene (CAII) (Zenke et al., 1988; Pain et al., 1990; Baniahmad et al., 1990). Notwithstanding the AEV is an acute leukemogenic retrovirus that causes identi®cation of target genes, progress in elucidating fatal erythroleukemia in young chicks. The target cell the mechanisms by which v-ErbA silences transcription for AEV is the committed erythroid progenitor remained slow. A further piece of the puzzle was (Samarut and Gazzolo, 1982). In culture-transformed provided by Damm and Evans (1993). The authors erythroblast cells called HD3 are arrested in their showed that the transformation-defective AEV-mutant ability to terminally dierentiate and show an td359 encodes a v-ErbA variant that failed to suppress increased rate of proliferation (Graf and Beug, 1983). basal transcription and exhibited an impaired ability to AEV-transformed HD3 cells provide an excellent antagonise TR. One of two mutations speci®c for the model system to investigate the molecular basis of td359 variant (Figure 1, Pro144 Arg in the `hinge' the disease. Besides the v-ErbA oncoprotein, the AEV region), abolished repressive functions but not hor- virus encodes a second oncoprotein, v-ErbB, a mutated mone binding and activation properties when intro- constitutively active transmembrane receptor for epi- duced in the context of wild type TR. Thus, active dermal growth factor (EGF) (Downward et al., 1984; repression in addition to TR occlusion appeared to be Sap et al., 1986). v-ErbA has no transformation