Crudely speaking, full-blown cancer may be described as having progressed through two stages, initiation and progression. As we discuss below, changes in 'epigenetic modifications' can be linked to both of these stages. However, before describing specific examples, we will consider the mechanisms by which aberrant histone modification profiles, or indeed the dysregulated activity of the associated enzymes, may actually give rise to cancer. Current evidence indicates that this can occur via at least two mechanisms; (i) by altering gene expression programmes, including the aberrant regulation of oncogenes and/or tumour suppressors, and (ii) on a more global level, histone modifications may affect genome integrity and/or chromosome segregation. Although it is beyond the scope of this review to fully discuss all of these possibilities, we will provide a few relevant examples highlighting these mechanisms.
Mouse models are invaluable tools for determining whether a particular factor is capable of inducing or initiating tumourigenesis. A good example is provided by the analysis of the MOZ-TIF2 fusion that is associated with acute myeloid leukaemia (AML) 99, 100
. The MOZ protein is a HAT 101
and TIF2 is a nuclear receptor coactivator that binds another HAT, CBP 102
. When the MOZ-TIF2 fusion was transduced into normal committed murine haematopoietic progenitor cells, which lack self-renewal capacity, the fusion conferred the ability to self-renew in vitro
and resulted in AML in vivo103
. Thus, the fusion protein induces properties typical of leukaemic stem cells. Interestingly, the intrinsic HAT activity of MOZ is required for neither self-renewal nor leukaemic transformation, but its nucleosome-binding motif is essential for both 103, 104
. Importantly, the CBP interaction domain within TIF2 is also essential for both processes 103, 104
. Thus, it seems that both self-renewal and leukaemic transformation involve aberrant recruitment of CBP to MOZ nucleosome-binding sites. Consequently, the transforming ability of MOZ-TIF2 most likely involves an erroneous histone acetylation profile at MOZ-binding sites. These findings provide a clear indication that the dysregulated function of histone modifying enzymes can be linked to the initiation stage of cancer development.
An activating mutation within the non-receptor tyrosine kinase JAK2 is believed to be a cancer-inducing event leading to the development of several different haematological malignancies, but there were few insights into how this could occur 105, 106
. Recently however, JAK2 was identified as an H3 kinase, specifically phosphorylating H3Y41 in haematopoietic cells. JAK2-mediated phosphorylation of H3Y41 prevents HP1α from binding, via its chromoshadow domain, to this region of H3 and thereby relieves gene repression 14
. This antagonistic mechanism was shown to operate at the lmo2
gene, a key haematopoietic oncogene 14, 107, 108
In humans, extensive gene silencing caused by overexpression of EZH2 has been linked to the progression of multiple solid malignancies, including those of breast, bladder and prostate 109, 110, 111
. This process almost certainly involves widespread elevated levels of H3K27me3, the mark laid down by EZH2. However, it has also recently been reported that EZH2 is inactivated in numerous myeloid malignancies, suggesting that EZH2 is a tumour suppressor protein 112, 113
. This is clearly at odds to the situation in solid tumours where elevated EZH2 activity is consistent with an oncogenic function. One possible explanation for this apparent dichotomy is that the levels of H3K27me3 need to be carefully regulated in order to sustain cellular homeostasis. In other words, aberrant perturbation of the equilibrium controlling H3K27me3 (in either direction) may promote cancer development. In this regard, it is noteworthy that mutations in UTX (an H3K27me3 demethylase) have been identified in a variety of tumours 114
, supporting the notion that H3K27me3 levels are a critical parameter for determining cellular identity.
Finally, changes in histone modifications have been linked to genome instability, chromosome segregation defects and cancer. For example, homozygous null mutant embryos for the gene PR-Set7
(an H4K20me1 HMT) display early lethality due to cell-cycle defects, massive DNA damage and improper mitotic chromosome condensation 115
. Moreover, mice deficient for the SUV39 H3K9 methyltransferase demonstrate reduced levels of heterochromatic H3K9me2/3 and they have impaired genomic stability and show an increased risk of developing cancer 116
It now seems clear that aberrant histone modification profiles are intimately linked to cancer. Crucially, however, unlike DNA mutations, changes in the epigenome associated with cancer are potentially reversible, which opens up the possibility that 'epigenetic drugs' may have a powerful impact within the treatment regimes of various cancers. Indeed, HDAC inhibitors have been found to be particularly effective in inhibiting tumour growth, promoting apoptosis and inducing differentiation (reviewed in 117
), at least in part via the reactivation of certain tumour suppressor genes. Moreover, the Food and Drug Administration has recently approved them for therapeutic use against specific types of cancer, such as T-cell cutaneous lymphoma, and other compounds are presently in phase II and III clinical trials 118
Other histone-modifying enzyme inhibitors, such as HMT inhibitors, are presently in the developmental phase. But before we plunge head-first into a full discovery programme for other inhibitors, we should consider a number of important issues relevant to the development of such initiatives (see 118
for full discussion). First, we do not fully understand how HDAC inhibitors achieve their efficacy. Do they for instance exert their effects via modulating the acetylation of histone or non-histone substrates? Second, the majority of HDAC inhibitors are not enzyme-specific, that is, they inhibit a broad range of different HDAC enzymes. It is not known whether this promotes their efficacy or whether it would be therapeutically advantageous to develop inhibitors capable of targeting specific HDACs. Thus, when developing new inhibitors such as those targeting HMTs, we need to consider whether we should aim for enzyme-specific inhibitors, enzyme subfamily specific inhibitors, or similarly to the HDAC inhibitors, pan-inhibitors. Nevertheless, the fact that these drugs are safe and the fact that they work at all, given the broad target specificity, are extremely encouraging. So the truth is that even though there is still a lot to learn about chromatin as a target, 'epigenetic' drugs clearly show great promise.