182 Epigenetics M Esteller (Valk-Lingbeek et al, 2004), transgenic mice have a high predisposition to lymphomagenesis, subtypes of human lympho- mas harbour BMI1 gene amplification and overexpression and it induces cell immortalisation (Valk-Lingbeek et al, 2004; Raa- phorst, 2005). EZH2 undergoes gene amplification in several tumour types (Bracken et al, 2003), it is overexpressed in prostate and breast cancers, and its downregulation leads to growth inhibition (Raaphorst, 2005). Another powerful epigenetic repressor family of chromatin- remodelling factors is the Tudor domain ‘Royal Family’ (Hughes- Davies et al, 2003). Two of its members, HP1 and BS69, bind to EMSY. The latter gene is a critical component in breast cancer development in the BRCA2 pathway. Most importantly, EMSY behaves as an oncogene in mammary neoplasm, where it shows gene amplification (Hughes-Davies et al, 2003). The list of targeted epigenetic repressors also includes Breast Cancer Metastasis Suppressor 1 (BRMS1), which is able to inhibit metastasis without ‘touching’ tumorigenesis itself (Meehan and Welch, 2003). BRMS1 joins the mSin3a HDAC complexes to silence the targeted genes (Meehan and Welch, 2003). An opposing force to BRMS1 is the group of metastasis-associated genes (MTAs). MTA1, MTA2, and MTA3 are components of the nucleosome remodelling and deacetylation complex, as a part of the NuRD complex. Metastasis-associated gene 1 overexpression is associated with the most malignant behaviour in several human tumours. Metastasis- associated gene 3 has a predominant role in lymphoma and breast cancer, in the latter leading to aberrant expression of the transcriptional repressor Snail and loss of expression of the cell- adhesion molecule E-cadherin, an event associated with invasive growth of breast cancers (Fujita et al, 2003). The best-recognised chromatin remodelling ‘force’ is the SWI/ SNF family of ATP-hydrolysing enzymes (Gibbons, 2005). These are multiprotein complexes, as we have seen in the case of the PcG, but in this case with transcriptional activation properties. Among the many members of the SWI/SNF family, four are involved with cancer development: PASG/LSH, BRG1, HLTF and SNF5. Potential mutations of PASG/LSH have been identified in leukaemia (Lee et al, 2000); BRG1 genetic alterations occur in several tumour types and the reintroduction of the gene inhibits cell growth (Medina et al, 2005); helicase-like transcription factor (HLTF) undergoes methylation-associated silencing in different neoplasms (Esteller, 2005a); and SNF5 shows inactivating somatic and germline mutations in malignant rhabdoid tumours (Imbalzano and Jones, 2005). TARGETING EPIGENETIC GENES IN CANCER THERAPY The ‘holy grail’ of current research in cancer therapy is the design of specific drugs against molecular alterations found only in the transformed cell, such as mutations in oncogenes. Epigenetics can offer many new targets for this approach. However, only two types of epigenetic drugs, neither of which is very specific, have nowadays a real impact: DNA-demethylating agents and histone deacetylase inhibitors (HADCis) (Villar-Garea and Esteller, 2004; Esteller, 2005b). We have to be patient with other potential epigenetic drugs, such as histone acetyltransferase inhibitors, including anacardic acid, curcumin, and peptide CoA conjugates; in addition to undisclosed histone methyltransferase inhibitors or those HDACis that are specific for SIRT1 (class III HDAC), such as nicotinamide and splitomycin. REFERENCES Of the class of DNA-demethylating agents, the first drug used to inhibit DNA methylation was 5-azacytidine (Vidaza). This substance causes covalent arrest of DNMTs, resulting in cytotoxi- city. 5-Azacytidine was tested for its usefulness as an antileukaemic drug before its demethylating activity was known (Esteller, 2005b). The analogue 5-aza-20-deoxycytidine (Decitabine) is one of the most commonly used demethylating drugs in assays with cultured cells. Zebularine is another recently developed cytidine analogue (Yoo et al, 2004). It forms a covalent complex with DNA methyltransferases (Yoo et al, 2004). Furthermore, zebularine has also shown promising antitumoral effects in xenografts (Yoo et al, 2004) and thymic lymphomas (Herranz et al, 2005) in mice. Perhaps the most interesting feature of this DNA-demethylating agent is that it is chemically stable and of low toxicity (Yoo et al, 2004; Herranz et al, 2005), and can be taken orally. It is in the field of haematological malignancies that DNA-demethylating agents have had their greatest success so far, especially in high-risk myelodysplastic syndrome using 5-aza-2-deoxycytidine (Esteller, 2005b). In 2004, the FDA approved the use of 5-azacytidine (Vidaza) for the treatment of all myelodysplastic syndrome subtypes. On the other hand, naturally occurring and synthetic HDACis are also the focus of interest because of their great potential utility against cancer. Overall, HDACis manifest a wide range of activities against all HDACs. These compounds can be classified into the following groups according to their chemical nature: hydroxamic acids, such as trichostatin A, SAHA, PXD101 and NVP-LAQ-824; carboxylic acids, such as sodium valproate and butyrate; benzamides, such as MS-272 and others, including trapoxins and FK228 (Villar-Garea and Esteller, 2004). It is believed that the anticancer effects of HDACis are mediated by the reactivation of the expression of tumour-suppressor genes. However, the treat- ment of cancer cell lines with HDACis has pleiotropic effects inducing differentiation, cell-cycle arrest and apoptosis. In this regard, the observation that cancer cells have lost monoacetylated lysine 16 histone H4 (Fraga et al, 2005) implies a new molecular pathway that may explain the beneficial effects of HDAC inhibitors because these compounds may promote the restoration of normal histone H4 acetylation levels in the whole cell, restoring the normal chromatin status of repetitive DNA sequences (Fraga et al, 2005). It is clear from in vitro, preclinical studies and ongoing clinical trials that HDACis have enormous potential as anticancer drugs. In this regard, SAHA may soon be approved for the treatment of cutaneous lymphoma. CONCLUSIONS Cancer is a genetic and epigenetic disease. We cannot study and explore one field and ignore the other. The overall disruption of the epigenetic landscape is the most common feature of all human tumours. This includes global loss of genomic DNA methylation, local CpG island hypermethylation-associated gene silencing and a characteristic histone modification pattern. Stochastic and selec- tive forces drive the whole process, and in the same way as in the cell cycle, apoptosis and DNA repair genes are targeted, genes involved in DNA methylation, histone modification and chromatin remodelling also become disrupted. Some of these will act as oncogenes, others as tumour-suppressor genes. Some will be altered by genetic lesions, others by epigenetic lesions. Ultimately, we will have to find better drugs to combat both processes. Bader S, Walker M, McQueen HA, Sellar R, Oei E, Wopereis S, Zhu Y, Peter A, Bird AP, Harrison DJ (2003) MBD1, MBD2 and CGBP genes at chromosome 18q21 are infrequently mutated in human colon and lung cancers. Oncogene 22: 3506 – 3510 British Journal of Cancer (2006) 94(2), 179 – 183 & 2006 Cancer Research UK