1. health and fitness
2. disease
3. cancer

doi:10.1111/j.1440-1746.2010.06447.x
REVIEW
Epigenetic regulation of signaling pathways in cancer: Role
of the histone methyltransferase EZH2
jgh_6447
19..27
Daisy PF Tsang and Alfred SL Cheng
Institute of Digestive Disease and Department of Medicine and Therapeutics, the Chinese University of Hong Kong, China
Key words
cancer, epigenetic silencing, EZH2, histone
modifications, signaling pathways.
Accepted for publication 14 July 2010.
Correspondence
Professor Alfred S.L. Cheng, Institute of
Digestive Disease, the Chinese University of
Hong Kong, Shatin, NT, Hong Kong. Email:
alfredcheng@cuhk.edu.hk
There is no potential conflict of interest for
each of the authors.
Abstract
EZH2 is the histone H3 lysine 27 methyltransferase of polycomb-repressive complex 2. It
transcriptionally silences cohorts of developmental regulators in stem/progenitors and
cancer cells. EZH2 is essential in maintaining stem cell identity by globally repressing
differentiation programs. Analogously, it plays a key role in oncogenesis by targeting
signaling molecules that control cell differentiation. Emerging data indicate that EZH2
promotes cancer formation and progression through epigenetic activation of oncogenic
signaling cascades and inhibition of pro-differentiation pathways. Genome-wide mapping
analysis has been expanding the repertoire of target genes and the associated signaling
pathways regulated by EZH2. Better understanding of the molecular basis of such regulations in various cancer types will help establish EZH2-mediated epigenetic silencing as a
therapeutic target.
Introduction
Oncogenesis is a complex process associated with accumulation of
genetic and epigenetic defects that alter the transcriptional
program. High throughput ‘omics’ analysis of gene expression in
cancer patient cohorts has led to the identification of key genes and
signaling pathways that drive cancer progression.1 However, relatively little is known about the mechanisms that underlie deregulation of signaling pathways in cancers. A detailed understanding
of the regulation of the signal transduction pathways should shed
light on the development of novel targeted therapy.
Epigenetic aberrations are now established in tumor initiation,
promotion, and progression.2–4 The most well-characterized alteration is CpG DNA hypermethylation. This is driven by DNA
methyltransferase (DNMT)s. Methylation often accumulates in
promoter regions of tumor suppressor genes, thereby contributing
to loss of tumor suppressor pathways in most if not all human
cancers.5,6
In addition to DNA methylation, there is accumulating evidence
that histone modifications and their associated chromatinmodifying enzymes play causal roles in cancer development.2,6,7
Common histone modifications leading to gene silencing in cancer
include histone H3 lysine 9 methylation, deacetylation, and
histone H3 lysine 27 trimethylation (H3K27me3). These all contribute to the inaccessibility of the promoter elements to transcription factor binding.8,9 In this review we focus on one of the histone
methyltransferases (HMTs), called enhancer of zeste homolog
(EZH2), which is the catalytic subunit of Polycomb-repressive
complex (PRC)2 that methylates H3K27me3.10–12 Here we
describe the polycomb-mediated silencing processes and review
the molecular mechanisms of EZH2 functions in cancer cells,
especially recent findings that depict the signaling pathways
altered by EZH2 that drive oncogenesis. Clinical implications will
also be mentioned.
Polycomb-mediated epigenetic
silencing
Gene silencing by polycomb-mediated
H3K27me3
Polycomb group (PcG) proteins promote gene repression through
epigenetic modification of histones.13,14 PcGs are highly conserved, from Drosophila to human, and form distinct multimeric
complexes. Among the four core components of PRC2 (EZH2,
SUZ12, EED, and NURF55),15–18 EZH2 via the SET-domain catalyzes H3K27me3 necessary for PRC2-mediated gene repression.19,20 H3K27me3 then serves as the binding site of
chromodomain of PRC1, which contains BMI1, RING1, HPH,
and HPC subunits, to form heterochromatin structure.14,20,21 It is
believed that the binding of PRC1 prevents recruitment of transcriptional activation factors, thereby blocking initiation of transcription by RNA polymerase II.11,14,20,21 Thus, PRC1 and 2
cooperate to ‘initiate’ and ‘maintain’ the chromatin organization
for gene repression.
Polycomb-mediated H3K27me3 had previously been recognized in the transcriptional silencing of differentiation genes, e.g.
Hox transcription factors, and in the early steps of mammalian
X-chromosome inactivation.11,21 New polycomb proteins and their
functions have recently been discovered. For example, PHD finger
protein 1 in human or polycomblike in Drosophila has been found
to interact with EZH2 to stimulate PRC2 activity and generate
high levels of H3K27me3 in target genes.22 Through genome-wide
studies, transcription factors and signaling components with key
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EZH2-regulated signaling pathways
DPF Tsang and ASL Cheng
roles in cell fate decisions are now identified to be PRC2 targets in
a wide variety of organisms (see below). Interestingly, phosphorylation of EZH2 protein on serine 21 by activation of AKT signaling pathway can inhibit EZH2-mediated H3K27me3 and hence
release gene silencing.23 On the other hand, UTX and JMJD3 have
recently been discovered as H3K27 demethyltransferases via the
JmjC-domain.24 Thus, H3K27me3 is a reversible marker; it can be
dynamically regulated by site-specific histone methyltransferases
and demethylases.
Interaction between EZH2, DNMTs and HDACs
in gene silencing
Because Drosophila and Caenorhabditis elegans deploy
H3K27me3 but little or no DNA methylation in gene silencing,
H3K27me3 and DNA hypermethylation have been considered
as independent epigenetic systems. However, since the landmark
discovery that deciphered the physical and functional interaction
between human PRC2 subunits and DNMTs in silencing certain
target genes,25 new models have been evolved to illustrate
the H3K27-CpG methylation link in normal and cancer development; informative data have been based on chromatin immunoprecipitation (ChIP)-based analysis and bioinformatics database
mining.26–28 The results of these studies indicate that genes acquiring H3K27me3 by EZH2-containing PRC2 in stem cells during
normal development are predisposed for de novo DNA hypermethylation and conversion to long-term silencing in the presence of
oncogenic cues, such as inflammation.4,29 In other words, EZH2
pre-ordains certain genes to later become CpG hypermethylated
during cellular transformation.26–28
More recently, using genome-wide location analysis and
various DNA methylation analyses, we and others have established PRC2-mediated H3K27me3 as an epigenetic mark
pathogenically involved in cancer through silencing of tumorsuppressor genes.30–34 This process is mechanistically independent of DNA methylation because: (i) H3K27me3 targets
generally show no or low DNA methylation in their promoters,
and (ii) EZH2 inhibition reactivates hundreds of genes silenced
by H3K27me3 without affecting their DNA methylation
levels.31,32 Thus, while both H3K27me3 and DNA hypermethylation can be linked for transcriptional repression, not all genes
suppressed by polycomb-mediated methylation are necessarily
maintained by promoter DNA methylation. The reason why
DNA methylation in cancer affects some silenced H3K27me3
targets but not others is not known. One possibility is that it
might be related to tissue- and cancer-specific differences in the
HMT/DMNT machinery activation.21,31 Physical and functional
links between EZH2 and histone deacetylase (HDAC) are,
however, well-established.35,36 PRC2 can physically associate
with HDAC1 and HDAC2, which in turn can deacetylase
H3K27, H3K9, H3K14 or H4K811. PRC2-mediated repression of
gene activity involves histone deacetylation. HDAC inhibitors
like trichostatin A reactivate genes silenced by H3K27me3 in
cancer cells, and this effect is synergistically enhanced with
EZH2 inhibition.31,37 Collectively, these studies illustrate the
interplay between EZH2, DNMTs, and HDACs epigenetic
silencing pathways that contribute to aberrant gene expression in
cancer cells.
20
Altered expression and functions of
EZH2 in cancers
EZH2 overabundance correlates with tumor
aggressiveness and poor prognosis
EZH2 is generally not expressed in adult tissues. However, it is
overexpressed in a broad range of hematopoietic and solid human
malignancies (Table 1), where its overabundance is often associated with poor prognosis.30,41,42,48,50,57 Through gene expression
profiling, overexpression of EZH2 was first reported in hormonerefractory, metastatic prostate cancer.37 In this cancer, high EZH2
concentration is strongly correlated with tumor progression and
greater risk of recurrence after prostatectomy.37,58 A ‘Polycomb
repression signature’ composed of 14 direct H3K27me3 targets,
derived from a specific cohort of genes transcriptionally repressed
by EZH2 in metastatic prostate cancer,37 has been shown to predict
poor clinical outcome.57 In breast cancer, abnormally high levels of
EZH2 are likewise associated with tumor aggressiveness,
increased risk of metastasis and shorter patient survival.41,42,50 Subsequent studies have also described the prognostic value of EZH2
in cutaneous melanoma, Hodgkin’s lymphoma and cancers of the
bladder, colon, endometrium, and liver.44,46,48,50,51
Increased cell proliferation is commonly associated with EZH2
overexpression in cancers. Conversely, loss of EZH2 inhibits
growth of cancer cells.16,37,59,60 Concordantly, breast and colon
cancer patients with EZH2-positive tumors have much higher proliferative index (up to 15-fold) than that with EZH2-negative
Table 1 EZH2 overexpression and functions in human cancers. The
order of EZH2-overexpressing cancer types are listed according to their
time of discovery
Type of cancer
Functions
References
Prostate cancer
Cellular transformation
Proliferation
Invasion and metastasis
Cellular transformation
Proliferation
Invasion and metastasis
Proliferation
Proliferation
Anti-differentiation
Cellular transformation
Proliferation
Proliferation
Proliferation
Proliferation
Invasion and metastasis
Proliferation
Proliferation
Anti-differentiation
Proliferation
Anti-apoptosis
Proliferation
Invasion and metastasis
Proliferation
Invasion and metastasis
38–40
Breast carcinoma
Lymphomas
Myeloma
Bladder carcinoma
Colon cancer
Cutaneous melanoma
Hepatocellular carcinoma
Endometrial cancer
Lung cancer
Pancreatic cancer
Gastric cancer
Ewing’s sarcoma
41–43
44
45
46,47
48,49
50
51
50
52
53
54
55,56
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DPF Tsang and ASL Cheng
tumors.42,48 Moreover, in accordance with the coordinated action
of PRCs in gene silencing, co-expression of EZH2 and BMI-1 is
observed in B-cell non-Hodgkin lymphomas, lung and hepatocellular carcinoma,44,52,60 and correlated with histological severity.52,60
Taken together, these studies demonstrate the significant association of high EZH2 levels with aggressive forms of cancer and
emphasize EZH2 as a prognostic indicator of outcome in cancer
patients.
Functions of EZH2 in carcinogenesis
Many phenotypic characteristics are shared between stem cells
and tumor cells, such as high differentiation capacity and proliferation rates. These observations have given rise to the notion that
undifferentiated stem/progenitors or dedifferentiated precursor
cells may play a key role in carcinogenesis. Likewise, the role of
EZH2 in inhibiting differentiation and maintaining pluripotency in
embryonic stem (ES) cells61–63 suggests similar roles in oncogenesis.11,64,65 PRC2 subunits and the associated H3K27me3 have been
shown to occupy a large cohort of developmental regulators to
repress the differentiation program in ES cells.61,63 Intriguingly,
these genes are also co-occupied and silenced by Oct-4, Sox-2, and
Nanog, transcription factors that are essential in sustaining pluripotency and self-renewal of ES cells.61,63 Remarkably, ES cells and
poorly-differentiated human tumors are found to share gene
expression signatures defined in part by PRC2- and Oct-4/Sox-2/
Nanog-target genes.66
The anti-differentiation property of EZH2 is also manifested in
various cancer types (Table 1). In Ewing tumor, EZH2 is directly
induced by a fusion protein EWS-FLI-1, which plays a key role in
tumor pathogenesis.55,56 Knockdown of EZH2 suppresses development of Ewing tumors in association with upregulation of genes
involved in neuroectodermal/endothelial differentiation. These
findings seem to indicate a key role for EZH2 in maintaining an
undifferentiated phenotype in Ewing tumor.56 On the other hand,
EZH2 is crucial in regulating cell cycle via the retinoblastoma
pathway.11,12,67,68 Knockdown of EZH2 deregulates genes involved
in G2/M transition in a way that inhibits cell proliferation by
inducing G2/M arrest.43,67 The findings indicate that EZH2 overexpression results in decreased BRCA1 with high levels of Cdc2CyclinB1 complex, which drives mitosis and uncontrolled
proliferation.43 EZH2-containing PRC2 also transcriptionally
represses cell cycle suppressor p16 via H3K27me3.68,69 This in
vitro finding is consistent with the statistical inverse correlation
detected between EZH2 and p16 expression in cutaneous melanoma and endometrial carcinomas.50 Collectively, these findings
are consistent with the proposal that EZH2 promotes tumor growth
by inhibiting tumor differentiation and enhancing cell cycle
progression.
In addition, increased EZH2 expression promotes neoplastic
transformation of immortalized epithelial cells.39–41,56 In line with
the strong correlation between EZH2 expression and aggressive
tumor subgroups, EZH2 can increase the invasiveness and
metastatic potential of cancer cells.39,41,46,56 Furthermore, EZH2
has been recently shown to directly regulate apoptosis in cancer
cells.70 EZH2 antagonizes the pro-apoptotic activity of E2F1
by epigenetic repression of Bim expression, thus permitting
cancer cells with aberrant E2F1 activity to evade apoptosis.70
Based on its aberrant expression in tumor tissues and various
EZH2-regulated signaling pathways
pro-tumorigenic properties, EZH2 can now be considered as a
bona fide oncogene.
EZH2-mediated deregulation of
signaling pathways
Global scanning of PRC1 and PRC2 chromatin binding sites in
Drosophila has uncovered many transcriptional regulators and key
components of signal transduction pathways, including Wingless,
Hedgehog, Notch and Delta.71 Genome-wide searches of PRC2
target genes in mammalian cells including fibroblasts, ES and
cancer cells also revealed cohorts of signaling components that
control cell differentiation.19,61,63,72,73 Emerging data indicate that
EZH2 has a master regulatory function in controlling key signaling
pathways in cancers by transcriptional repression of signaling
molecules.
Wnt/b-catenin signaling
The canonical Wnt signaling pathway, which regulates the ability
of the b-catenin protein to drive activation of specific target genes,
is aberrantly activated in the development of various human
cancers. Gain-of-function mutations of the CTNNB1 gene (encoding b-catenin) and loss-of-function mutations of adenomatous
polyposis coli and AXIN genes have been suggested to be the
preferred routes to chronic Wnt signaling dysfunction in cancers.74,75 However, the occurrence of these mutations is much less
prevalent than the abnormal accumulation of b-catenin observed in
tumor tissues, e.g. HCC76. Recent studies indicate that histone
modifications might control key epigenetic regulators of the Wnt/
b-catenin signaling pathway in cancers. Jiang et al. have recently
found that transcriptional repression of DACT3, a Wnt antagonist
interacting with Dishevelled, leads to constitutive activation of
Wnt/b-catenin signaling in colorectal cancer.77
Unlike some Wnt signaling inhibitors that are silenced by
DNA methylation, e.g. SFRP genes,78 DACT3 repression in
cancer cells is associated with bivalent H3K27me3 and
H3K4me3 chromatin modifications.77 This finding supports our
previous observation that not all genes suppressed by polycombmediated methylation are necessarily maintained by promoter
DNA methylation.31 Pharmacological inhibition of histone
methylation and deacetylation robustly induces DACT3 expression. This inhibits Wnt/b-catenin signaling and causes dramatic
apoptosis of colon cancer cells.77 While the epigenetic regulator
driving DACT3 repression in colon cancer has not been defined,
we have recently used chromatin immunoprecipitation microarray (ChIP-Chip) analysis to uncover a panel of Wnt/b-catenin
signal antagonists whose promoters were concordantly occupied
by EZH2 and H3K27me3 in HCC cells (Cheng et al., manuscript
submitted). Further analyses illustrated that EZH2-mediated transcriptional repression of these Wnt pathway inhibitors allows
constitutive Wnt/b-catenin signaling, which likely plays a critical
role in EZH2-stimulated cellular proliferation (Cheng et al.,
manuscript submitted). Collectively, these studies provide
mechanistic and functional links between EZH2-mediated
H3K27me3 and Wnt/b-catenin signaling in the development of
colorectal and liver cancers.
EZH2 and b-catenin co-expression may cause uncontrolled
growth, placing large numbers of cells susceptible to secondary
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EZH2-regulated signaling pathways
DPF Tsang and ASL Cheng
events of neoplastic transformation. This notion is further supported by a recent transgenic study demonstrating that targeted
overexpression of EZH2 in the mammary gland induces b-catenin
nuclear accumulation and causes epithelial hyperplasia.79
In addition to the canonical Wnt pathway, a direct engagement
of EZH2 in noncanonical Wnt signaling has been recently elucidated.80 In this study, cigarette smoke was shown to activate
polycomb machinery to repress a Wnt signaling antagonist
DKK-1 in lung cancer cells.80 Specifically, exposure of tobacco
smoke condensate induced recruitment of EZH2, Suz12 and
Bmi-1 to the DKK-1 promoter, and caused remodeling of histone
marks (increased H3K27me3 and decreased H4K16Ac) in the
absence of DNA methylation.80 Repression of DKK-1 activates
Wnt receptor complex and JNK, a downstream effector of
noncanonical Wnt signaling, which coincides with enhanced
tumorigenicity of lung cancer cells in vivo.80 This study shows
that environmental insults can repress Wnt antagonist(s) via
polycomb-mediated epigenetic silencing, thereby modulating
noncanonical Wnt signaling and enhancing the malignant phenotype of cancer cells.
Ras and NF-kB signaling pathways
Hyperactivation of Ras effector pathways (the extracellular
signal-regulated kinase (ERK) and AKT kinase pathways)
are known to promote carcinogenesis including prostate.
Whereas AKT is activated by loss of the gene encoding phosphatase and tensin homolog (PTEN), the mechanism underlying
ERK activation was not known. A recent systematic screen of
the RasGAP family, which consists of the negative regulators of
Ras signaling, has identified DAB2IP as a new tumor suppressor
in prostate tumorigenesis, acting via inhibition of ERK and AKT
pathways.81 Moreover, DAB2IP negatively regulates NF-kB
signaling via its period-like domain. DAB2IP loss activates
NF-kB pathway leading to prostate cancer invasion.81 Intriguingly, EZH2 is shown to epigenetically silence DAB2IP and
activates Ras, ERK, AKT and NF-kB, whereas DAB2IP reconstitution substantially suppresses activation.81 This study provides
solid in vitro and in vivo data to prove a causal role for EZH2 in
concordant epigenetic regulation of two prominent oncogenic
pathways, thus establishing an oncogene-tumor suppressorsignaling cascade in the promotion of prostate cancer growth and
metastasis.
PTEN functions as a negative regulator of the PI3K/AKT
pathway via dephosphorylation of PtdIns(3,4,5)P(3) to regulate
cell cycle, proliferation, apoptosis, cell adhesion, and epithelialmesenchymal-transition (EMT) during embryonic development
and cancer progression.82 While PTEN loss-of-function is mainly
attributed to mutation, deletion, or promoter methylation in many
human cancers,82 a recent study has delineated an epigenetic
silencing mechanism of PTEN via polycomb-mediated
H3K27me3.83 PRC-1 and -2 subunits including Bmi-1, EZH2 and
Suz12 have been shown to associate with PTEN promoter and
downregulate PTEN expression through H3K27me3. Consequently, this activates the PI3K/AKT/GSK-3b pathway and
enhances the invasiveness of nasopharyngeal epithelial cells via
EMT83. This study provides mechanistic and functional links
between PRCs and PI3K/AKT signaling in nasopharyngeal cancer
progression and metastasis.
22
Beta-adrenergic receptor signaling
By integrating in vitro cell line data, in vivo tumor profiling and
ChIP-chip data, Yu et al. have identified adrenergic receptor beta
2 (ADRB2), a critical mediator of b-adrenergic signaling, as a key
target of EZH2 in prostate cancer.30 Stimulation of ADRB2 activates b-adrenergic signaling by raising intracellular cyclic adenosine monophosphate (AMP) levels; this inhibits cell proliferation.84
In prostate cancer, EZH2-mediated transcriptional repression of
ADRB2 not only promotes tumorigenesis but also enhances cancer
cell invasion, thus providing a functional link between polycomb
silencing and b-adrenergic signaling.30 Of potential clinical significance, administration of ADRB2 agonist significantly inhibited prostate tumorigenicity in a nude mice xenograft model.30 In
clinical specimens, low ADRB2 expression is associated with
poor prognosis of clinically localized prostate cancer.30 Collectively, these findings indicate that characterization of EZH2 direct
target genes in key signaling pathway may be useful for the identification of novel cancer biomarkers and potential therapeutic
targets.
Bone morphogenetic protein and
Notch signaling
Bone morphogenetic proteins (BMPs) mediate a wide variety of
biological responses that range from proliferation to differentiation to apoptosis, depending on developmental stage; dysregulation of the molecular effectors of BMP signaling may contribute to
cancer.85 While BMP2/4 exhibits a pro-differentiative effect on
neural stem cells, EZH2 expression impairs the BMP-dependent
astroglial differentiation program in a subset of gliobastoma
tumor-initiating cells.86 EZH2 binds to and represses BMP receptor 1B (BMPR1B), whose overexpression inhibits proliferation,
increases astroglial differentiation and decreases tumorigenicity.86
Knockdown of EZH2 significantly decreases the methylation
density of the BMPR1B promoter,86 an observation that concurs
with the notion that EZH2 recruits DMNTs for DNA methylation
in long-term epigenetic silencing.25 Intriguingly, BMPR1B is
also a PRC2 target gene transcriptionally repressed in human
ES cells.61 On this basis, it is hypothesized that, in a subset of
gliobastoma tumor-initiating cells, the reversible repression
program on the developmental regulators is replaced by permanent
silencing, locking the cell into a perpetual state of self-renewal.28,86
In summary, this study depicts a molecular mechanism of
astroglial tumorigenesis in which concurrent polycomb- and
DNA methylation-mediated transcriptional repression of BMP
effector desensitizes tumor-initiating cells to normal differentiation cues, thereby predisposing them to subsequent malignant
transformation.
Together with BMP and other stem cell pathways, Notch signaling is involved in both embryonic development and adult tissue
homeostasis. It exerts regulatory effects by controlling proliferation and differentiation of both embryonic and adult stem cells.87
While the relationship between EZH2 and Notch signaling in
cancer remains unclear, we have recently demonstrated epigenetic
repression of Notch-1 in primary satellite cells, the myogenic
progenitor cells, which may lead to inhibition of skeletal muscle
differentiation.88 The silencing cascade is initiated by tumor necrosis factor a, an important myogenic regulator, which stimulates the
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DPF Tsang and ASL Cheng
EZH2-regulated signaling pathways
EZH2
DACT3
Wnt/b -catenin
Cell proliferation
Anti-apoptosis
DKK1
Ras
Transformation
Cell proliferation
DAB2IP
NF-kB
Cell invasion
ADRB2
BMPRIB
b -adrenergic
Transformation
Cell invasion
NOTCH-1
BMP
Differentiation
NOTCH
Differentiation
Tumor initiation, growth and metastasis
Figure 1 EZH2-mediated epigenetic regulation of signaling pathways.
EZH2 activates oncogenic signaling cascades (green boxes) and inhibits
pro-differentiation pathways (red boxes) through epigenetic silencing of
the negative regulators and positive effectors (blue and purple ovals),
respectively, thereby establishing an oncogene-tumor suppressorsignaling paradigm in driving tumor initiation, growth and progression.
As DACT3 is repressed via H3K27me3, EZH2 is implicated to be its
epigenetic regulator.
recruitment of EZH2 for initiate repression via H3K27me3, followed by DNMT3b-mediated DNA methylation in the Notch-1
promoter.88 Interestingly, genome-wide mapping in Drosophila
has also revealed the binding of polycomb proteins in the Notch-1
homolog,70 further highlighting the significant role of polycomb in
Notch signaling. Since Notch-1 signaling has been implicated in
the induction of growth arrest and apoptosis in human cancers, e.g.
HCC89, the potential regulation of Notch-1 signaling by polycomb/
EZH2 in tumorigenesis warrants further investigation. Collectively, these studies establish causal roles for EZH2 in driving
cancer initiation, development and progression through epigenetic
activation of oncogenic signaling cascades, e.g. Wnt/b-catenin,
Ras, NF-kB, and inhibition of pro-differentiation pathways, e.g.
b-adrenergic, BMP, Notch (Fig. 1).
Therapeutic implications of
EZH2-mediated signaling deregulation
The silencing of important signaling components: the negative
regulators of oncogenic pathways, e.g. DACT3, DKK1, DAB2IP
and the positive effectors of anti-tumorigenic/pro-differentiation
pathways, e.g. ADRB2, BMPR1B, NOTCH-1 by polycomb/EZH2
have major therapeutic implications. These findings underscore
the utility of developing EZH2 inhibitors; such agents might suppress oncogenic pathways that have proven difficult to target
directly, in the treatment of cancers. For example, the complexity
of the Wnt/b-catenin pathway, which constitutes components at
different subcellular levels, renders it almost intractable to therapeutic intervention.90 Given the molecular diversity and cancerspecificity of EZH2-repressed Wnt/b-catenin signal antagonists
(Cheng et al., manuscript submitted),76,80 it might be prudent to
treat Wnt-addicted cancers by EZH2-targeted pharmacological32
or lentiviral RNA interference91 approach. 3-deazaneplanocin A
(DZNep), an S-adenosylhomocysteine hydrolase inhibitor which
depletes cellular levels of PRC2 subunits, blocks the associated
H3K27me3 and reactivates PRC2-silenced genes32 to induce
apoptosis in malignant but not in normal cells.32,92,93 In addition,
curcumin, isolated from turmeric spice, has recently been shown
to downregulate EZH2 via the mitogen-activated protein kinase
(MAPK) pathway, thereby identifying inhibition of polycomb
function as one of the major anti-carcinogenic mechanisms
of this well-known chemopreventive natural compound.94 It
is conceivable that blockage of PRC2 function might inhibit
multiple signaling pathways cooperatively to yield dramatic
anti-tumorigenic effects, as shown in animal studies.91,92,95
Furthermore, given EZH2 is highly expressed in advanced
cancers37,41,42,46 and is causal in driving metastasis,56,81 such
inhibitors may also have significant therapeutic effect on metastatic cancers for which no effective curative treatment is currently available.
Although polycomb-mediated epigenetic silencing has been
proposed as an attractive candidate for targeted therapy,11,31,32
caution should be taken when considering the newly-identified
tumor suppressive functions of polycomb proteins.96,97 For
example, the PRC1 components have been shown to repress mitogenic JAK-STAT pathway in Drosophila, suggesting that polycomb proteins can restrict growth directly by silencing mitogenic
signaling pathways.98 In addition, recurrent somatic mutation of
EZH2, which replaces a single tyrosine (Tyr641) in the SET
domain, has recently been found in a subset of diffuse large B-cell
lymphomas.99 This finding, together with the discovery of mutations in the H3K27me3 demethylase UTX in several cancer
types,100,101 infers that a precise balance of H3K27me3 is critical
for normal cell growth.102 It is therefore necessary to dissect the
oncogenic mechanisms that underlie both increased and decreased
H3K27me3 activities before testing EZH2-targeted therapy in
clinical trials.
Future directions and conclusions
It is clear that, in the post-genomic era, the ever-evolving genomic
technologies have permitted an unprecedented opportunity to
interrogate the binding patterns of chromatin-modifying enzymes
and the associated histone modifications on a genome-wide scale
and in an unbiased manner. Upcoming ChIP-chip or ChIP coupled
with massive parallel sequencing (ChIP-seq) data will likely
produce location analysis maps of polycomb proteins in a vast
variety of immortalized nontumorigenic normal and cancer cell
lines, as well as in primary malignant tissues. By comparing
these datasets, we will be able to determine the commonality of
binding sites for the same enzymes and histone marks among cells
of different origins. Similarly, we will gain knowledge about
overlap of binding sites of different enzymes and how they cooperate to produce overall epigenetic landscapes throughout the
genome of the same cell type. We predict that these approaches
will identify more direct polycomb/EZH2 targets involved in
signal transduction cascades. Further, the results may strengthen
support for the notion that polycomb-epigenetic silencing contributes to tumor formation and progression via deregulation of signaling pathways.
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EZH2-regulated signaling pathways
DPF Tsang and ASL Cheng
from the Food and Health Bureau, and the Direct Grant (Ref no.
2007.1.033) from the Chinese University of Hong Kong.
References
Figure 2 Novel approach for cancer therapy. In cancers, tumorsuppressor gene (TSG)s can be epigenetically silenced by DNA-,
histone-methylation or combined mechanisms. Combinatorial epigenetic therapies targeting these machineries should reactivate these
genes by chromatin remodeling and reverse the aberrant signaling
cascades leading to cancer remission. Black and white circles indicate
methylated and unmethylated CpG sites, respectively. Red and brown
lines indicate tails of methylated and unmethylated H3K27 resulting in
compacted and relaxed chromatin states, respectively.
The role of chromatin structure inevitably dictates transcriptional activity. However, how global patterns of epigenetic alterations form, and the interplay between different epigenetic
alterations remain unclear. Recent studies have begun to unravel
the role of specific transcription factors, e.g. FOXD3103 and
YY1104 in influencing epigenetic patterns during carcinogenesis.
Besides, the oncogenic viral transcriptional regulators, e.g. the X
protein of hepatitis B virus have been demonstrated to induce
regional promoter hypermethylation and global hypomethylation.105 Ongoing studies will examine its influence on histone
modifications, including H3K27me3. Intertwined histone
(H3K27)- and promoter-methylation events likely contribute to
tumor suppressor gene silencing and thereby aberrant signaling
(Fig. 2). By delineating the crosstalk between polycomb/EZH2
and other epigenetic silencing machineries in regulation of signaling pathways in cancer, combinatorial regimes of different
epigenetic-modulating drugs7,32,106 can be developed for novel,
effective cancer therapy, even at advanced stages. The goal of
epigenetic therapy is to achieve pharmacological reactivation of
abnormally silenced tumor-suppressor genes in cancer patients.
These pathways could potentially reverse the oncogenic signaling cascades and revive the pro-differentiation pathways leading
to cancer remission.
Acknowledgments
We thank members of our laboratory for discussions. Work in our
laboratory is partially supported by grants from the General
Research Fund (Ref no. 09/060/GRF and 462309) from the
Research Grants Council, the Research Fund for the Control of
Infectious Diseases (Ref nos. 08070172, 08070332 and 09080042)
24
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