Balanced DNMT activity is most important for the prevention of cell transformation. The genetic disruption of two DNMTs, DNMT1 and DNMT3b, in a cancer cell line induces demethylation of all known hypermethylated tumour-suppressor genes (Rhee et al, 2002; Paz et al, 2003) and remarkably slow growth (Rhee et al, 2002). Somatic DNMT mutations have not so far been described in human tumours, although DNMT1 is located in 19p13.3, a region of common loss of heterozygosity in human tumours. DNMT3b germline mutation are responsible for the immunodeficiency centromeric instability-facial anomalies (ICF) syndrome, the cancer risk of which is not known, while no DNMT1 germline mutation in any genetic syndrome has so far been reported. Results obtained in mouse models again reflect the need for well-adjusted DNMT function to maintain cellular homeostasis: DNMT1 knockout mice are ‘protected’ against the development of colorectal adenomas when crossed with APC-deficient mice (Laird et al, 1995), but they are ‘prone’ to develop lymphomas in the context of mice susceptible to this type of neoplasia (Eden et al, 2003). These latter results may be explained if lymphomas rely predominantly on chromosomal instability dependent on genomic DNA hypomethylation, while colon tumours rely more on the CpG island methylation status of tumour-suppressor genes (Yamada et al, 2005). Something has to read the DNA methylation markers. The most likely candidates are transcriptional repressors that have an appetite for methylated CpGs. These are the methyl-CpG-binding domain proteins (MBDs) (Esteller, 2005a). MBDs are important ‘translators’ between DNA methylation and histone modifier genes that establish a transcriptionally inactive chromatin environment. This family of proteins consists of five well-characterised members (MeCP2, MBD1, MBD2, MBD3 and MBD4) (Esteller, 2005a). MBD proteins are associated with hypermethylated CpG island promo- ters of tumour-suppressor genes and their transcriptional silencing (Esteller, 2005a), showing remarkable specificity in vitro (Fraga et al, 2003) and in vivo (Ballestar et al, 2003; Klose et al, 2005). Individual loss of MeCP2, MBD1, MBD2, for which single knockout mice are viable, do not appear to affect tumour formation significantly (Esteller, 2005a), which suggests that the remaining MBDs may compensate for the function of the missing one. However, deficiency of MBD2 suppresses intestinal tumor- igenesis in an Mbd2-knockout mouse derived from a lineage with an autosomal-dominantly inherited predisposition to multiple intestinal neoplasia (Min) (Sansom et al, 2003). We can hypothesise that the absence of MBD2 produces a ‘leak’ in the CpG island hypermethylation silencing of tumour-suppressor genes, thereby partially aborting aberrant cancer growth. This may not be universal for all tumour types and in this regard deficiency of MBD2 does not enhance lymphomagenesis in p53- deficient mice (Sansom et al, 2005). Expression analysis of MBD proteins in tumours has revealed increased overall levels associated with enhanced proliferation (Esteller, 2005a). Mutations in MBDs do occur in sporadic tumours, albeit rarely (Bader et al, 2003). MBD4 is an exception and is frequently targeted by inactivating frameshift mutations in microsatellite-unstable tumours (Riccio et al, 1999). However, we should keep in mind that MBD4 is unusual: it has a glycosylase domain that removes thymidine from T:G mismatches. GENES MEDIATING THE DISRUPTION OF HISTONE MODIFICATIONS A first draft of an aberrant histone modification signature for human cancer has been produced (Fraga et al, 2005; Fraga and Esteller, 2005; Seligson et al, 2005). The next task is to identify the molecules involved in its establishment: histone acetyltransferases (HATs), histone methyltransferases (HMTs) and histone deacety- lases (HDACs). & 2006 Cancer Research UK Epigenetics M Esteller With respect to histone acetylation, we have found a loss of recruitment of a family of the specific K-16 HATs MOZ, MOF and MORF to DNA-repetitive sequences in cancer cells (Fraga et al, 2005). These HATs are already altered in leukaemias by the generation of fusion proteins such as MOZ-CBP and MORF-CBP that are also associated with significant global losses of acetylation of K16-H4 (Fraga et al, 2005). What makes the case even more interesting is that the other partner of the fusion protein generated is usually CBP or p300, two other HATs with numerous substrates, but that do not act on lysine 16 of H4 (Fraga and Esteller, 2005). A tumour-suppressor role for these global HATs has been strongly suggested from many sources: CBP, p300 and pCAF somatic mutations have been described in primary human tumours (Gayther et al, 2000; Ozdag et al, 2002; Ionov et al, 2004; Kishimoto et al, 2005); patients with Rubinstein – Taybi syndrome, caused by germline mutations in the CBP gene, have an increased tendency to develop tumours at an early age (Gibbons, 2005); and CBP heterozygous mice develop haematological tumours (Gibbons, 2005). In the case of HDAC, the deacetylation of K16-H4 seems to be particularly closely regulated: in yeast, Sir2 deacetylates this residue, and its human homologue, Sirtuin 1 (SIRT1), also deacetylates the tumour-suppressor protein p53, thereby establish- ing another link with cancer. We can view the picture from this angle and hypothesise that there is increased recruitment of SIRT1 to the K16-H4 position in repeat DNA sequences in the transformed cell. In this regard, overexpression of SIRT1 is observed in leukemia cells (Bradbury et al, 2005). For HDACs with a broader deacetylation specificity than SIRT1, such as HDAC1 and HDAC2, no somatic mutations in tumours have been described, but a dysregulated expression seems to be a common feature of human neoplasia (Gibbons, 2005). A similar scenario could be proposed for the trimethylation of lysine 20 of H4 (Fraga et al, 2005). This reaction is catalysed by the HMTs Suv4-20h1 and Suv4-20h2, in addition to PR/SET7-SET8 (Fraga and Esteller, 2005). These enzymes can be targets for disruption in cancer cells, as occurs with another HMT, MLL1, which is translocated to multiple partners in haematological malignancies (Fraga and Esteller, 2005). The loss of trimethylation of histone H4 in cancer cells might be a consequence of a deficiency of these enzymatic activities in tumours, and/or a lack of recruitment to heterochromatic regions, where most of the histones containing such modifications are localised. We should keep in mind that mice deficient in SUV39H1 HMTs (which target lysine 9 of histone H3) have increased chromosomal instability and tumour risk (Peters et al, 2001) and another K9-H3 histone methyltransferase, RIZ1, undergoes CpG island hypermethylation- associated silencing in many tumour types (Esteller, 2005a). Lack of HMTs specific to K20-H4 has not yet been described in tumours, but all these issues can now be addressed with new tools, such as specific antibodies against Suv4-20h (Fraga et al, 2005). GENES MEDIATING THE DISRUPTION OF CHROMATIN REMODELLING Polycomb-group (PcG) and trithorax-group (trxG) genes are epigenetic silencers and activators for gene transcription, respec- tively (Valk-Lingbeek et al, 2004; Raaphorst, 2005). Both are part of multitask protein complexes including HDAC and HMTs activities. In the case of the human homologue of trithorax, MLL, I have already mentioned the existence of translocations that generate transforming fusion proteins (Gibbons, 2005). For PcG in human there are two complexes, polycomb repressive complex 1 (PRC1) and 2 (PRC2), which contain the BMI1 and enhancer of zeste homologue 2 (EZH2) oncogenes, respectively (Valk-Lingbeek et al, 2004; Raaphorst, 2005). BMI1 downregulates the expression of tumour-suppressor genes, such as p16INK4a and p14ARF British Journal of Cancer (2006) 94(2), 179 – 183 181