REVIEWS gh it remains MYCtarget of the con oftiple-negative/b ATAD2 has a key role in tumorig their signalling netw can oc tion activities of drugg ngoractivat ind nd tra hecapreserdo r20 on of the key developmental his r in key dev I are associate ofgiven tissu 山as man ignancies nd m Thus,thes lead to the exp ngdcrcntiatio Inhibition of DOTILwasre om and t ng- tion in th ion of that inc ncer Stem cell bility of sel may be orch strated and alzedpmecusorcel nethylation activityoffering the pos of a p netic targeted the rapy was shown in eny)an arcinoma.In this tion protei ture I type sults in the exp TIP60:also kno a所cin ing pro 3BP1; oncogene and TP53BP1 act as tum 3RD2.BRD3.BRD4 and b targe in This ve suppressing tun oming the ger etic changes that driv dis rs,it is recognized that i s extremely chall hat lev eins are altered in clin the tran rodisease states,epec nd s r MYC whose pathological activatio is among synd e,developmental de and autism spectrum disorde n and the HAT CREB binding protein (CREBBP auses Rubinstein- Taybi syndrome he activity of the mYc Mein Ho of this dis onatal crehhe mi the mice reports ndicat hat M may be effectiv xhibit behavi pairme fhe and is ph har ical inhibition of one of its regulato art CREBBP might also be a key target for presen n the 4 binds cetylat s mer n epigenetic pr NATURE REVIEWSIDRUG DISCOVERY VOLUME 11 MAY 2012 387Stem cell An unspecialized precursor cell with the capacity to self-renew (continuously produce unaltered progeny) and to differentiate into more mature specialized cell types. Haploinsufficiency A disease mechanism in which one of two copies of a gene is mutated, resulting in insufficient activity of the gene products (typically a protein) to bring about a functional, wild-type condition. Brachydactyly mental retardation syndrome A disorder that presents with a range of features, including intellectual disabilities, developmental delays, behavioural abnormalities, sleep disturbance, craniofacial and skeletal abnormalities, and autism spectrum disorder. Presenilins A family of related multipass transmembrane proteins that function as part of the γ-secretase intramembrane protease complex. They were first identified in screens for mutations causing early-onset forms of familial Alzheimer’s disease. agents are non-selective within their target protein fami￾lies and have substantial side effects. Although it remains to be demonstrated in the clinic, agents that target spe￾cific HDACs with greater selectivity may be beneficial in certain cancers. For example, treatment of neuroblastoma cell lines with a selective inhibitor of HDAC8 mimicked genetic knockdown of HDAC8 as well as inhibiting cellular proliferation and triggering differentiation29,30. Second-generation HDAC inhibitors — several of which are more selective — are currently in clinical trials for multiple types of cancer (TABLE 2). Deregulation of epigenetic regulatory proteins and their signalling networks can occur via several mecha￾nisms, including direct inactivating or activating muta￾tions, gene amplification, indirect upregulation or inactivation of enzymes, and translocations that lead to the expression of gain-of-function fusion proteins that contain reader domains31. Well-known examples include overexpression of the key developmental histone lysine N-methyltransferase EZH2 in several types of leukaemia and in various solid tumours32. The gene encoding the protein methyltransferase MLL is also subject to many chromosomal translocations that lead to the expression of chimeric fusion proteins and inappropriate recruitment of other epigenetic factors such as the methyltransferase DOT1-like protein (DOT1L)33. Inhibition of DOT1L was recently shown to selectively kill cells and tumour xenografts that contained MLL trans￾locations34. EZH2 can be aberrantly upregulated by the overexpression of dominant mutations that increase its trimethylation activity, offering the possibility of selective therapy targeting the mutant protein35. A recent example of a potential epigenetic targeted therapy was shown in a model of midline carcinoma. In this cancer, carcino￾genesis is driven by chromosomal translocation, which results in the expression of a fusion protein containing the bromodomain of bromodomain-containing protein 4 (BRD4) or BRD3 and a testis-specific transcription factor (NUT) that drives carcinogenesis. A selective antagonist of the BET family of bromodomains (which includes BRD2, BRD3, BRD4 and bromodomain testis-specific protein (BRDT)) resulted in the selective killing of BRD4– NUT-positive midline carcinoma xenografts36. Modulation of epigenetic mechanisms also offers the potential for overcoming the genetic changes that drive cancer — especially oncoproteins that may not be drugga￾ble. For example, with the exception of nuclear hormone receptors, it is recognized that it is extremely challenging to inhibit most sequence-specific transcription factors using small molecules37. This includes the transcription factor MYC, whose pathological activation is among the most common genetic events observed in cancer genomes38. Although MYC was one of the first known and most common oncoproteins39, over 30 years of research have failed to identify compounds that can directly inhibit the activity of the MYC protein. However, several recent exciting reports indicate that MYC may be effectively inhibited in several haematological malignancies through pharmacological inhibition of one of its regulatory part￾ners, BRD4. BRD4 binds acetylated histones via its bro￾modomain and mediates chromatin-dependent signalling and transcription at MYC target loci40. Inhibition of the interaction between BRD4 and acetylated histones results in reduced levels of MYC target genes and inhibition of transcription of the MYC gene itself 41,42. Similarly, overexpression of the bromodomain-con￾taining nuclear cofactor ATPase AAA domain-containing protein 2 (ATAD2) is crucial for the proliferation and survival of triple-negative/basal-like breast cancer cells and controls the expression of the oncogene MYB43. The bromodomain of ATAD2 has a key role in tumorigenesis44. These results highlight the potential for targeting ‘undrug￾gable’ oncogenic transcription factors by inhibiting the catalytic or chromatin-interaction activities of druggable epigenetic cofactors that drive the expression of oncogenic transcription factors. There are numerous other cancer-linked alterations in the genes coding for (and the activity of) readers, writers and erasers of histone marks. Many of these alterations occur in key developmental genes and are associated with cancers that derive from stem cell-like early progeni￾tors of a given tissue type, such as many haematological malignancies45–48 and medulloblastoma49,50. Thus, these self-renewing cells may be locked in an epigenetic state that prevents them from undergoing differentiation. Inhibition of mutated epigenetic proteins or inhibition of the transcriptional programme of other oncogenic signal￾ling factors could be an attractive strategy for overcom￾ing the block to differentiation in these types of cancers. Similarly, the oxygen-independent glycolytic metabo￾lism that is observed in rapidly proliferating cancer cells (known as the Warburg effect) may be orchestrated and maintained by epigenetic signalling networks51. Genomic instability is also a hallmark of cancer, and inactivation of epigenetic proteins that contribute to DNA damage checkpoints (such as the HAT 60 kDa Tat￾interactive protein (TIP60; also known as KAT5)52 or the tumour protein p53 binding protein 1 (TP53BP1; a Tudor domain-containing protein)53 appears to contribute to oncogenesis. Although TIP60 and TP53BP1 act as tumour suppressors and are not likely to be therapeutic targets, the actions of these proteins underscore the extensive role of epigenetic proteins in oncogenesis, both positive (driving tumour growth) and negative (suppressing tumour growth). This dichotomy also raises important safety￾related issues for potential epigenetic therapy (see below). Neuropsychiatric disorders. Several studies have shown that levels of epigenetic proteins are altered in clinical neurodisease states, especially in intellectual disability syndromes. Haploinsufficiency of HDAC4 causes brachy￾dactyly mental retardation syndrome, developmental delays and behavioural problems54. Moreover, haploinsuffi￾ciency of the HAT CREB binding protein (CREBBP) causes Rubinstein–Taybi syndrome, a genetic disorder that results in cognitive dysfunction. In a mouse model of this disorder — neonatal Crebbp+/– mice — the mice exhibit behavioural impairments, and this phenotype can be reversed by inhibition of histone deacetylation55. CREBBP might also be a key target for presenilins in the regulation of memory formation and neuronal survival56. In addition, mutations in epigenetic proteins can result REVIEWS NATURE REVIEWS | DRUG DISCOVERY VOLUME 11 | MAY 2012 | 387 © 2012 Macmillan Publishers Limited. All rights reserved