Cancer Epigenetics

Department of Leukemia, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030, USA.
CA A Cancer Journal for Clinicians (Impact Factor: 115.84). 10/2010; 60(6):376-92. DOI: 10.3322/caac.20085
Source: PubMed
ABSTRACT
Epigenetics refers to stable alterations in gene expression with no underlying modifications in the genetic sequence and is best exemplified by differentiation, in which multiple cell types diverge physiologically despite a common genetic code. Interest in this area of science has grown over the past decades, especially since it was found to play a major role in physiologic phenomena such as embryogenesis, imprinting, and X chromosome inactivation, and in disease states such as cancer. The latter had been previously thought of as a disease with an exclusive genetic etiology. However, recent data have demonstrated that the complexity of human carcinogenesis cannot be accounted for by genetic alterations alone, but also involves epigenetic changes in processes such as DNA methylation, histone modifications, and microRNA expression. In turn, these molecular alterations lead to permanent changes in the expression of genes that regulate the neoplastic phenotype, such as cellular growth and invasiveness. Targeting epigenetic modifiers has been referred to as epigenetic therapy. The success of this approach in hematopoietic malignancies validates the importance of epigenetic alterations in cancer, not only at the therapeutic level but also with regard to prevention, diagnosis, risk stratification, and prognosis.

Full-text

Available from: Jean-Pierre Issa, Nov 24, 2014
Cancer Epigenetics
Rodolphe Taby, MD
1
; Jean-Pierre J. Issa, MD
2
Abstract
Epigenetics refers to stable alterations in gene expression with no underlying modifications in the genetic sequence and is
best exemplified by differentiation, in which multiple cell types diverge physiologically despite a common genetic code.
Interest in this area of science has grown over the past decades, especially since it was found to play a major role in
physiologic phenomena such as embryogenesis, imprinting, and X chromosome inactivation, and in disease states such as
cancer. The latter had been previously thought of as a disease with an exclusive genetic etiology. However, recent data
have demonstrated that the complexity of human carcinogenesis cannot be accounted for by genetic alterations alone,
but also involves epigenetic changes in processes such as DNA methylation, histone modifications, and microRNA
expression. In turn, these molecular alterations lead to permanent changes in the expression of genes that regulate the
neoplastic phenotype, such as cellular growth and invasiveness. Targeting epigenetic modifiers has been referred to as
epigenetic therapy. The success of this approach in hematopoietic malignancies validates the importance of epigenetic
alterations in cancer, not only at the therapeutic level but also with regard to prevention, diagnosis, risk stratification, and
prognosis. CA Cancer J Clin 2010;60:376-392.
©
2010 American Cancer Society, Inc.
Introduction
The term “epigenetics” refers to variability in gene expression, heritable through mitosis and potentially meiosis,
without any underlying modification in the actual genetic sequence. This alteration in gene expression plays a
fundamental role in several aspects of natural development, from embryogenesis, in which a resetting of the “epi-
genetic code” takes place in the very early moments after conception,
1
to the determination of cellular fate and its
commitment to a particular lineage. Epigenetics also play a fundamental role in biological diversity such as pheno-
typic variation among genetically identical individuals.
2
Indeed, epigenetic processes account fully for the differ-
ences between queen bees and worker bees in Apis mellifera species.
3
Several mechanisms fall under the banner of the
epigenetic machinery, the most studied of which are DNA methylation; histone modifications; and small, noncod-
ing RNAs. In this review, we will first describe the general mechanisms through which the epigenetic code is
established and then focus on the alterations of the epigenome taking place in cancer, with an emphasis on how
these aberrations can potentially be used in the clinical setting.
Epigenetic Mechanisms
DNA Methylation
DNA methylation is a covalent modification of the cytosine ring at the 5 position of a CpG dinucleotide, whereby
a methyl group is deposited on the carbon 5 of that ring using S-adenosyl methionine as a methyl donor. This
transfer of methyl group is a replication-dependent reaction catalyzed by DNA methyltransferases (DNMTs),
present at the replication fork during the S-phase.
4
CpG dinucleotides, the usual targets of DNA methylation in
1
Postdoctoral Fellow, Department of Leukemia, The University of Texas M. D. Anderson Cancer Center, Houston, TX;
2
Professor, Department of Leukemia,
The University of Texas M. D. Anderson Cancer Center, Houston, TX.
Corresponding author: Jean-Pierre J. Issa, MD, Department of Leukemia, The University of Texas M. D. Anderson Cancer Center, Unit 428, 1515 Holcombe Blvd., Houston,
TX 77030; jpissa@mdanderson.org
DISCLOSURES: Dr. Issa is an American Cancer Society Clinical Research professor supported by a generous gift from the F. M. Kirby Foundation. Dr. Issa has also received
research support from Celgene, Merck, and Eisai, and he has consultancies with Syndax and GlaxoSmithKline.
American Cancer Society, Inc. doi: 10.3322/caac.20085.
Available online at http://cajournal.org and http://cacancerjournal.org
Cancer Epigenetics
376 CA: A Cancer Journal for Clinicians
Page 1
mammals, are scattered throughout the genome and
present at a lower-than-expected abundance. This has
been explained over evolution by the spontaneous
deamination of the cytosine in the CpG site into a
thymine.
5
However, in certain areas of the genome, a
high concentration of these CpG dinucleotides is
found, and these are referred to as “CpG islands”
(CGIs).
6
These CGIs average 1000 base pairs and can
be found at the 5 promoter region of approximately
50% of genes. In a normal differentiated cell, CpG loci
disseminated across the genome are highly methyl-
ated, whereas most promoter CGIs are protected from
the spreading of methylation inside their boundaries.
6
DNA methylation at gene promoter CGIs has
been correlated with permanent expression silenc-
ing such as that noted in the inactive X chromosome in
women.
4
DNA methylation leads to silencing by di-
rect inhibition of transcription factor binding to their
relative sites and by recruitment of methyl-binding
domain proteins (MBDs).
4
These MBDs are present
in transcription corepressor complexes involving sev-
eral other members of the epigenetic machinery such
as histone deacetylases (HDAC) and histone methyl-
transferases, resulting in chromatin reconfiguration
and gene silencing.
7
One such MBD is MeCP2, the
deletion of which causes the neurodevelopmental dis-
order called Rett syndrome.
8
Throughout evolution,
DNA methylation has been used to silence the expres-
sion of endogenous repeats and infecting retrotrans-
posons, keeping them from disrupting normal gene
expression.
9
An overview of epigenetic regulation in
eukaryotic cells is presented in Figure 1. Other physi-
ological phenomena in which DNA methylation in
CGIs plays a fundamental role are X chromosome in-
activation,
10
genomic imprinting in which one allele is
expressed depending on its paternal or maternal ori-
gin,
11
and somatic tissue-specific repression of a set of
germ cell-specific genes.
12
Although DNA methylation patterns in adult cells
are relatively stable, important changes have been
FIGURE 1. Normal Transcriptional Regulation in Higher Eukaryotes Is Shown. DNA is packaged in nucleosomal building blocks in a way that determines its accessi-
bility to the nuclear environmentandtranscriptionalstatus. (Left) Transcriptionally active genes are marked by methylation-free promoters and an open, highly acetylated
chromatin configuration that allows access to transcription factors and polymerase II (pol II). (Right) Repetitive elements are silenced by high levels of DNA methylation,
specific histone lysine methylation, and a closed chromatin state. A switch from active to inactive chromatin characterizes some genes in cancer cells. HAT indicates
histone acetyltransferase; TF, transcription factor; RNA pol II, RNA pol II; DNMT1, DNA methyltransferase 1; HDAC, histone deacetylase; MBD, methyl-CpG binding
protein; HMT, histone methyltransferase; P, gene promoter; LINE 1, long interspersed nuclear element 1; SINE, short interspersed nuclear element; TD rep, tandem
repeats.
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described in aging tissues. A global decrease in
5-methylcytosine content was reported in cultured
human fibroblasts
13
and promoter-specific hyper-
methylation was observed in epithelial tissues.
14,15
Global profiling using methylated CGI amplifica-
tion in combination with microarray analysis dem-
onstrated several hundreds of gene promoters to
acquire methylation in aging mice intestinal muco-
sae whereas hundreds of others were found to have a
parallel loss of DNA methylation.
16
This linear
change of 5-methylcytosine content with aging has
a strong tissue specificity and has been shown to be
common across mammals. Indeed, both the amount
and pattern of DNA methylation have been found
to diverge between human monozygotic twins as
they age.
17
It is still not clear whether the accumu-
lation of these DNA methylation defects with time
is of a random or rather programmed nature, and
although their pathophysiologic consequences are
unknown, they have been proposed to play a role in
aging disorders, including cancer.
DNA methylation is catalyzed by a group of en-
zymes in mammals called DNMT1, DNMT3a, and
DNMT3b. DNMT1, known as the “maintenance
methyltransferase,” has been shown to have a 10-fold
preference for hemimethylated DNA (only 1 of the 2
DNA strands is methylated) compared with an un-
methylated strand, and is used mostly by the cell to
maintain the DNA methylation status in a stable fash-
ion through cell division.
18
DNMT3a and DNMT3b,
known as “de novo” methyltransferases, are used by
the mammalian cell to methylate previously unmethy-
lated DNA. It is worth mentioning that DNMT1
demonstrates far higher catalytic activity than
DNMT3a and DNMT3b,
19
and all 3 are involved in
important cellular functions such as differentiation.
20
The functional importance of these enzymes is high-
lighted by the fact that DNMT deletion is embryoni-
cally lethal in mice.
21
Post-Translational Histone Modifications
DNA is wrapped around histone proteins to form nu-
cleosomes, in a way that regulates accessibility of the
genetic sequence to the nuclear environment.
22
Each
nucleosome is comprised of a tetramer of 2 histone 2A
(H2A) and 2 histone 2B (H2B) molecules, flanked by
H3 and H4 dimers. H3 and H4 have N-terminal tails
that, in their deacetylated form, are positively charged,
leading to a closed and tight chromatin configuration
around the negatively charged deoxyribonucleic acid.
The addition of an acetyl group neutralizes the posi-
tive charge of the lysine residues in these N-terminal
tails, loosening up this tight bond between DNA and
histones, resulting in a more open chromatin configu-
ration accessible to being successfully transcribed.
23
Two consecutive nucleosomes are tied together by
linker histone H1. Recent studies have shown that the
abundance of these linker histones is tightly related to
chromatin configuration and might be altered in
cancer cells.
24
Histone modifications comprise a multitude of co-
valent reactions affecting the histone N-terminal tails,
and form a code that fine tunes the way DNA is
wrapped around these proteins. These post-translational
modifications include acetylation, methylation, phos-
phorylation, ubiquitination, sumoylation, and ADP ri-
bosylation.
22
These reactions occur in a very targeted and
amino acid-specific way, the most studied of which are
acetylation and methylation of specific lysine residues on
histones H3 and H4. Several enzymes catalyzing these
reactions, namely histone acetyltransferases (HAT),
HDAC, histone methyltransferases (HMT), and his-
tone demethylases (HDMT), have been identified.
These enzymes exert their function in the setting of ei-
ther transcriptional activator or repressor complexes, de-
pending on the specific substrate residue.
Histone acetylation status results from an intricate
cross-talk between HATs and HDACs. HATs are
separated according to their cellular location and func-
tion into 2 distinct groups: the cytoplasmic B-type
HATs and the nuclear A-type HATs.
25
The latter are
presumed to have more impact on gene transcription,
whereas cytoplasmic HATs can catalyze acetylation of
nonhistone proteins. The most studied HAT families
are GCN5-related N-acetyltransferase (GNAT),
MYST (MOZ, Ybf2/Sas3, Sas2, and Tip60), and
p300/CREB-binding protein (CBP), all of which are
associated with complexes such as GCN5, PCAF,
MOF, and p300/CBP, respectively. These complexes
interact with each other and, through both targeted
promoter-specific and nontargeted general acetylation
reactions, play significant roles in development, differ-
entiation, and cell cycle progression.
26
HDACs are a class of enzymes catalyzing the op-
posite action to HATs. They influence a myriad of
cellular processes including signal transduction, ap-
optosis, cell cycle regulation, and cell growth.
27
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378 CA: A Cancer Journal for Clinicians
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HDACs catalyze deacetylation of both histone and
nonhistone proteins and, similar to HATs, can be
either nuclear or cytoplasmic. This cytoplasmic deacety-
lase activity can lead to post-translational modifications
of transcription factors and chaperone proteins, and can
have major effects on several important pathways, such
as the NF-
B (nuclear factor kappa-light-chain-
enhancer of activated B cells) pathway,
28
the APE1-
Ref1 oxidative stress response pathway,
29
and the
phosphatase and tensin homolog (PTEN) phos-
phatase gene.
30
Similarly to HATs, HDACs exert
their catalytic activity through an association with
protein complexes, such as the sirtuin (silent mating
type information regulation 2 homolog) 1 (SIRT1)
protein deacetylase complex.
31
Histone methylation also plays a major role in gene
expression regulation.
32
Histone methylation is asso-
ciated with transcriptional repression or activation de-
pending on the specific amino acid affected. For
example, methylation of histone H3 lysines 4 and 36 is
associated with active gene expression, whereas meth-
ylation of histone H3 lysines 9 and 27 is associated
with gene silencing. Histone methylation is catalyzed
by a large number of enzymes, the majority of which
contain a specific protein module called SET
(su(var)3-9, enhancer-of-zeste, trithorax) domain.
33
Similar to acetylation/deacetylation, histone meth-
ylation is reversible and catalyzed by 2 families of
HDMTs, namely the lysine-specific demethylase 1
(LSD1) and the Jumonji domain-containing en-
zymes.
34,35
Histone methylases and HDMTs are
usually part of large protein complexes that regulate
gene transcription.
Histones can also be targeted by other post-
translational modifications such as phosphorylation,
ADP-ribosylation, and ubiquitination. These affect a
limited number of residues but could play an impor-
tant role in gene regulation. For example, serine 10
phosphorylation is inversely correlated with lysine
methylation, and this methylation/phosphorylation
module is conserved across different proteins.
36
Histone modifications (and DNA methylation)
ultimately affect gene expression in part by influ-
encing nucleosome positioning. Active genes dem-
onstrate a lack of nucleosomes at their transcription
start site, whereas epigenetically silenced genes have
a nucleosome positioned critically at the start of
transcription.
37
Thus, nucleosome positioning can
be involved in either the activation or repression of
gene transcription.
38
The Swi/Snf protein complexes
play a major role in this process.
39
Through their target-
ing to specific gene promoters, these complexes can acti-
vate or repress transcription via 3 biochemical processes:
nucleosome remodelling, nucleosome sliding, and octa-
mer transfer.
40
It is still unknown whether nucleosome
formation and positioning is mainly determined by un-
derlying proximal genetic sequences (“cis effect”) or by
other mechanisms operated by ATP-dependent nucleo-
some remodelling complexes in a sequence-independent
manner (“trans effect”). Recent studies have suggested
that the answer is more likely to be a mixture of the 2, in
some type of a nucleosome positioning code governing
histone-DNA interactions.
37
Noncoding RNAs
Small noncoding RNAs refer to a family of RNAs
that, by complementarity to the 3 untranslated region
of messenger RNAs, lead to their degradation and
subsequent inhibition of gene expression.
41
Part of
this family of noncoding RNAs are 20- to 22-
nucleotide microRNAs (miRNAs), resulting from the
sequential splicing of primary then pre-RNAs. These
oligonucleotides are first synthesized as long, noncod-
ing RNAs that are processed by the RNA cleaving
enzyme DROSHA in the nucleus, transported into
the cytoplasm in the form of short hairpin RNAs, and
further cleaved by the enzyme DICER into their final
configuration of double-stranded miRNAs.
41
miRNAs
are then incorporated in the RNA-induced silencing
complex and transported back in the nucleus, where
they exert their biological effect. Through Watson-
Crick base pairing, miRNAs bind to complementary
sequences of mRNAs and induce either degradation
or translational silencing of the target mRNAs.
41
It is
interesting to note that miRNAs are also themselves
epigenetically regulated at their promoter level, and
target many genes that play important roles in such
processes as cell cycle progression, apoptosis, and dif-
ferentiation.
42
A single miRNA can have hundreds of
target mRNAs, highlighting the implication of this
gene regulation system in cellular functions.
43
The
study of miRNAs has become the subject of intense
interest, especially after the discovery of the funda-
mental role of these small, noncoding RNAs in a myr-
iad of cellular and biological processes ranging from
development to disease states.
44
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Epigenetic Changes in Cancer
Cancer cells have genome-wide aberrations at the epige-
netic level, including global hypomethylation, promoter-
specific hypermethylation, histone deacetylation, global
down-regulation of miRNAs, and up-regulation of cer-
tain actors of the epigenetic machinery such as EZH2.
These aberrations confer a selective growth advantage to
neoplastic cells, leading to apoptotic deficiency, uninhib-
ited cellular proliferation, and tumorigenicity (Fig. 2). In
the following sections, we will describe these different
layers of epigenetic regulation and their aberrant func-
tioning in cancer cells.
DNA Methylation in Cancer
Tumorigenesis is a result of the activation of onco-
genic and/or inactivation of proapoptotic or tumor
suppressor pathways. Initially, these were believed to
result exclusively from genetic events such as muta-
tions, amplifications, gene rearrangements, or dele-
tions.
45
We now understand that DNA methylation is
an alternate way of silencing tumor suppressor genes,
in a manner equivalent to genetic mutations.
46
Examples
of this mechanism of tumorigenesis are numerous,
notably methylation of the mismatch repair gene hu-
man mutL homolog 1 (MLH1) in colorectal cancer,
the DNA repair gene O-6-methylguanine-DNA
methyltransferase (MGMT) in gliomas and colorectal
cancer, and the cell cycle regulator p16 (cyclin-
dependent kinase inhibitor 2A [CDKN2A]) in colo-
rectal and other malignancies.
47
A “cross-talk” has
been shown to exist between these mechanisms and
genetic ones in a cell. This is exemplified in colorectal
cancer, in which CGI promoter hypermethylation has
been shown to be present only in the wild-type allele
of silenced genes.
48
In addition, aberrant DNA meth-
ylation was more frequent than copy number changes
when studied on a whole-genome level in malignant
gliomas.
49
This is the case in colorectal cancer as well,
in which individual tumors are found to harbor more
hypermethylated genes than genetic mutations, and
within individual genes, hypermethylation was found
to be more frequent than genetic changes.
50
DNA
methylation effects on pathway alterations can be ei-
ther direct, by affecting promoters of tumor suppres-
sor genes, or indirect, by silencing known inhibitors
of oncogenes, such as the silencing of the secreted
FIGURE 2. Tumorigenic Mechanisms in Mammalian Cells Are Shown. Both genetic and epigenetic aberrations are involved in neoplastic transformation. These 2
alternate pathways of tumorigenesis are linked by an intricate cross-talk and can, either individually or in synergy, lead to the development of the malignant phenotype.
Cancer Epigenetics
380 CA: A Cancer Journal for Clinicians
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frizzled-related protein (SFRP) family of genes, lead-
ing to the activation of the Wnt pathway in colorectal
carcinogenesis.
51
Similar to mutations, silencing of tu-
mor suppressor genes confers a selective proliferative
advantage to corresponding cells, mediates invasive-
ness, and facilitates metastasis.
DNA hypermethylation is an early event in tumor-
igenesis, most likely playing a major role in tumor ini-
tiation and progression, and creating a fertile ground
for the accumulation of a multitude of simultaneous
genetic and epigenetic aberrations.
52
This is supported
by the finding of a “field defect,” in which normal
tissue adjacent to a tumor is found to harbor several
“epi-mutations” as well, most notably in colorectal
cancers
53
but also in gastric cancer and liver cancer.
Another example is MGMT hypermethylation, which
plays a direct role in the accumulation of G-to-A mu-
tations in the KRAS gene in colorectal tumors.
54
These data led to a new thinking regarding the mech-
anisms behind tumor initiation and progression, even
at the earliest stages of carcinogenesis.
Aberrant patterns of DNA methylation in cancer
have significant interneoplastic and interindividual
variability, accounting not only for tumor type speci-
ficity but also personal variability.
55
The latter is best
represented by the presence of a subgroup of patients
demonstrating high levels of simultaneous gene pro-
moter methylation, defining a phenomenon now
known as CGI methylator phenotype or CIMP.
56
The best studied subgroup of CIMP-positive patients
was described in colon cancer, in which these tumors
were reported to comprise 20% to 40% of cases and
were found to be associated with microsatellite insta-
bility (MSI), a defective human MutL homolog
(MLH1) function, a location mostly in the ascend-
ing colon, an older patient age, and female predom-
inance.
57
These CIMP-positive tumors often are
clinically distinct from those in the rest of the pa-
tient population for the tumor type in question,
which suggests that DNA methylation could be
used for personalized cancer treatment in the clini-
cal oncology setting.
On the other end of the spectrum, we find global
DNA hypomethylation, the first epigenetic alteration
noted in cancer cells.
58
In various cancers, 5 methyl-
cytosine content was found to decrease by an average
of 10%.
59
This affects both repetitive elements such as
LINE1 and Alu
60
and specific gene promoters.
61
One
potential consequence of profound hypomethylation
is genomic instability, predisposing patients to mu-
tations, deletions, amplifications, inversions, and
translocations.
62
This may occur in part through re-
activation of mobile elements. Indeed, hypomethy-
lation correlates with a higher rate of chromosomal
changes in patients with colon cancer
63
and is asso-
ciated with a poor prognosis.
64
Another potential
consequence of DNA hypomethylation is the reac-
tivation of normally silenced genes.
65
This could
lead to the disruption of normal gene expression
and potential activation of growth-promoting and
antiapoptotic pathways. Furthermore, promoter hy-
pomethylation can lead to reactivation of miRNAs
embedded in the coding regions of certain genes, re-
sulting in silencing or aberrant expression of the cor-
responding protein.
42
Hypomethylation by genetic
disruption of DNMT1 is protective against carcino-
genesis in some models,
66
but can also promote tumor
formation in others.
67
Histone Modifications in Cancer
There is limited information regarding global histone
modification profiling in cancer cell lines and primary
tumors. Recent studies have demonstrated a global
loss of histone H4 lysine 16 monoacetylation and his-
tone H4 lysine 20 trimethylation in cancer.
68
These
modifications were found to occur throughout the ge-
nome, specifically overlapping with areas of DNA hy-
pomethylation in repetitive sequences. Conversely,
loss of histone H3 lysine 9 acetylation and lysine 4
dimethylation or trimethylation and gain of histone
H3 lysine 9 dimethylation or trimethylation and lysine
27 trimethylation can be found at specific gene pro-
moters and can contribute to tumorigenesis by silenc-
ing critical tumor suppressor genes.
69
One interesting
observation is the correlation between genes that are
marked by DNA methylation in cancer and those
found to be bound to the repressive polycomb group
(PcG) proteins in embryonal cells.
70
These 2 groups
appear to overlap, implying that certain genes are
“poised” for silencing and “predetermined” to be the
target of specific repressive histone marks in cancer.
Unlike DNA methylation, in which a bona fide
DNA demethylase has not yet been identified, post-
translational histone modifications are well character-
ized as a 2-way street governed by a balance of catalytic
enzymes.
71
Shifting of this balance in cancer can occur
through altered expression or function of epigenetic
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modifiers, and this has been found to play a role in
both murine and human neoplasia. For example, the
histone methyltransferase PcG protein EZH2 cata-
lyzes H3K27 trimethylation.
72
Its overexpression was
found to promote tumor growth both in vitro and in
vivo,
73
and is present in several cancers in the clinical
setting, such as melanomas, lymphomas, and prostate
and breast cancers.
74
EZH2 has also been found to be
useful as a potential biomarker to distinguish aggres-
sive prostate and breast tumors from more indolent
ones.
73,75
In prostate cancer, EZH2 expression has
been correlated with aberrant H3K27 trimethylation
affecting potential tumor suppressor genes.
76
Re-
cently, mutations of EZH2 were found in lympho-
mas,
77
but their functional significance there remains
to be clarified. The H3K27 repressive methylation
mark can also be over-represented in cancer through
an alternative mechanism, inactivation of a specific
H3K27 demethylase, UTX.
78
The latter has been
shown to be somatically mutated in several tumor
types, such as multiple myeloma, esophageal squa-
mous cell carcinoma, and renal cell carcinoma.
78
Re-
introduction of UTX in cancer cells presenting with
an inactivating mutation of this gene led to a reversion
of the malignant phenotype.
78
Another histone meth-
ylase, multiple myeloma SET domain (MMSET), is
genetically altered by a common chromosomal trans-
location in multiple myeloma, resulting in altered ex-
pression of target genes.
79
In addition, the histone H3
lysine 9 methyltransferase SUV39H may play a role in
carcinogenic initiation and progression.
80
Its deletion
in mice was found to lead to chromosomal instability
and increased tumor formation.
80
Perhaps one of the
most relevant clinical entities highlighting the impor-
tance of HMTs in cancer is the 11q23 translocation in
leukemias.
81
These have rearrangements giving rise to
a multitude of fusion proteins involving the mixed lin-
eage leukemia (MLL1) H3 lysine 4 HMT. MLL1
fusion proteins act as constitutively active chimeric
transcription factors and lead to up-regulation of
downstream homeobox (HOX) genes and activation
of several leukemogenic pathways such as RAS and
fms-related tyrosine kinase 3 (FLT3). MLL leuke-
mias appear to have a unique transcriptional signa-
ture
82
and a poor prognosis overall.
81
In addition to alterations in histone methylases/
HDMTs in cancer, numerous changes in gene-specific
histone acetylation have also been described. These
can be primary or secondary to aberrant recruitment.
For example, the chimeric oncoprotein promyelocytic
leukemia-retinoic acid receptor
(PML-RAR
) pro-
duced by the t(15:17) translocation in acute promyelo-
cytic leukemia targets specific promoters through the
aberrant recruitment of HDACs and HMTs, leading to
silencing of gene expression.
83,84
In addition, DNA hy-
permethylation can lead to aberrant HDAC and HMT
recruitment to specific promoters.
4
Conversely, direct
primary changes in HATs/HDACs can also occur in
cancer. Several studies have demonstrated a direct effect
of p300/CBP HAT on cellular proliferation.
85
There is
an interesting interaction reported between p300/CBP
and the viral oncogenic protein E1A.
86
This associa-
tion disrupts the interaction between the p300/CBP
complex and other HATs, in turn leading to increased
tumorigenesis. This mimics the effect of E1A on the
retinoblastoma (Rb) tumor suppressor gene.
87
Muta-
tions of p300/CBP are also found in Rubinstein-
Taybi syndrome, a developmental disorder associated
with an increased risk of solid tumors, leukemias, and
lymphomas.
86
p300 mutations have also been noted in
several human malignancies, including glioblastomas
and breast and colorectal cancers.
86
One of the limitations of studying histone modifi-
cations in cancer is the requirement for a relatively
large number of fresh or fresh frozen cells. This has
limited the study of these modifications in clinical tis-
sue samples, although some data are beginning to ac-
cumulate in leukemias.
88
Advances in technology to
analyze histone modifications are needed to improve
our understanding of various tumors.
miRNAs in Cancer
The first studies that suggested a link between
miRNA deregulation and cancer were focusing on
observations made in Caenorhabditis elegans and later
in Drosophila, with the discovery of lin-4 and let-7
miRNAs in the former
89,90
and the Bantam miRNA
in the latter.
91
Knockout of lin-4 or let-7 in C. elegans
led to abnormal differentiation,
92
whereas Bantam up-
regulation in Drosophila led to cellular growth and
the inhibition of apoptosis.
93
Mice studies confirmed
the previous findings, and Dicer knockout led to a
defective miRNA production and impaired cellular
differentiation.
94
These observations suggested that
miRNAs might play a role in human neoplasia. In-
deed, microarray studies have shown that there are
global alterations in miRNA expression in cancer,
95
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382 CA: A Cancer Journal for Clinicians
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with many miRNAs that are down-regulated by ge-
netic or epigenetic events, and some that are up-
regulated. For example, the let-7 family of miRNAs is
aberrantly down-regulated in breast and lung tumors,
leading to RAS pathway oncogenic activation.
96
An-
other example is the down-regulation of miR-15 and
miR-16 in chronic lymphocytic leukemia (CLL) and
resultant activation of the BCL2 proto-oncogene.
97
Overexpressed miRNAs include the miR-17-92 clus-
ter, which plays a role in the development of lung and
breast cancers as well as chronic myeloid leukemia
through targeting of the transcription factor E2F1, a
major cell cycle regulator.
98
miR-17-92 cluster ampli-
fication has also been shown to frequently play a role
in the development of B-cell lymphoma.
99
Its overex-
pression led to increased disease aggressiveness in
mouse models.
100
This cluster of miRNAs has also
been shown to be activated by the oncogene c-myc,
101
highlighting its importance in tumorigenesis.
An interesting question relates to mechanisms of
miRNA deregulation in cancer. Many miRNAs are
transcriptionally regulated in a similar way as
protein-coding genes and can be overexpressed by
genetic mechanisms (eg, amplification) or sup-
pressed by genetic (eg, deletion) or epigenetic (eg,
hypermethylation) ones. Recently, DICER and
DROSHA expressions were also found to be altered
in some cancers.
102
Clinical Applications: Epigenetic Tumor
Markers
The rationale for the use of aberrant DNA methylation
of a particular gene or a set of selected genes for clinical
assessment comes from its frequency, stability, and vari-
ability between patients, which may indicate clinical use-
fulness. As mentioned earlier, DNA methylation is a
stable and clonally propagated mark. Furthermore,
DNA is less prone to degradation than RNA. Highly
sensitive and/or quantitative methylation detection tech-
niques are available, such as bisulfite pyrosequencing,
103
methylation-specific polymerase chain reaction,
104
or
bisulfite treatment combined with high-throughput
deep sequencing.
105
Moreover, aberrant methylation of
some gene promoters is more common and easier to de-
tect than the presence of mutations. This is especially
valuable if the cancer cell or the cancer cell-derived free
DNA is embedded in non-neoplastic cells or normal
DNA molecules. Examples illustrating the potential use
of epigenetic biomarkers in a clinical setting are de-
scribed below and in Table 1.
57,63,106-124
Aberrant DNA Methylation in Cancer Risk
Assessment and Prevention
There are 2 potential ways by which DNA methyl-
ation can be used for risk assessment: the detection of
constitutional aberrant DNA methylation and the de-
tection of acquired abnormalities that are harbingers
of cancer development. The first relates to the trans-
generational transmissibility of epigenetic alterations.
Although a resetting of epigenetic marks takes place in
the germline,
1
making the heritability of epigenetic
modifications between parents and their offspring
highly improbable, constitutional epigenetic alter-
ations are noted in certain individuals,
125
which could
be either inherited or an acquired germline defect.
The clinical entity that illustrates this clearly is the
autosomal dominant hereditary nonpolyposis colo-
rectal cancer (HNPCC) syndrome, in which affected
individuals are highly predisposed to developing colo-
rectal and endometrial cancers at a relatively young
age.
126
This syndrome is caused by defects in mis-
match repair, leading to MSI. Genes potentially in-
volved are MLH1, human mutS homolog 2 (MSH2),
MSH6, and postmeiotic segregation increased 2 (S.
cerevisiae) (PMS2). It is interesting to note that a few
individuals with HNPCC were described in whom no
sequence mutation was detected in any of these genes,
whereas MLH1 or MSH2 promoter methylation was
found to be present in normal tissues, including circu-
lating white blood cells.
127
In the case of MSH2, this
has been traced to a mutation in the tumor-associated
calcium signal transducer 1 (TACSTD1) gene imme-
diately adjacent to MSH2, leading to aberrant tran-
scription through its promoter and associated DNA
hypermethylation.
128
No such mutation was detected
for MLH1, which therefore appears to be a rare germ-
line defect that is occasionally inherited. Constitu-
tional epigenetic changes (epimutations) can also
result from genetic variations in the form of single nu-
cleotide polymorphisms occasionally occurring in
close proximity to a promoter and resulting in a pre-
disposition toward acquired DNA methylation. This
likely occurs via disruption of binding of transacting
protective proteins such as Sp1.
129
Thus, the transgen-
erational heritability of epigenetic modifications can
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result either from cis-acting events or from epigenetic
transmission per se, but a familial cancer predisposition
related exclusively to epigenetic phenomena appears to
be relatively rare.
The second approach to cancer risk assessment is
based on methylation studies of normal or preneoplas-
tic tissues to detect acquired epimutations. For exam-
ple, in lung cancer, methylation of the p16 gene was
found to be present in preneoplastic lesions in smokers
whereas no methylation was detected in never-
smokers. Hence, p16 methylation in conjunction with
other genes (such as p14, p15, E-cadherin, and RAS
association family 1A [RASSF1A]) has been proposed
as a biomarker to assess a patient’s risk for developing
lung cancer, and this is being tested by detecting
methylation in sputum.
118
Indeed, in one prospective
study of 98 cases and 92 matched controls, promoter
methylation of 14 genes in sputum was evaluated for
lung cancer risk assessment. Promoter hypermethyl-
ation of 6 genes was found to be associated with a
greater than 50% risk for subsequently developing
lung cancer. The concomitant hypermethylation of 3
or more of these 6 genes was associated with an odds
ratio of 6.5 for developing lung cancer, with a sensitivity
TABLE 1. Examples of Clinically Relevant Epigenetic Biomarkers
EPIGENETIC BIOMARKER CLINICAL RELEVANCE SUPPORTING LITERATURE SENSITIVITY/SPECIFICITY/OR/HR
Hypermethylation of
GSTP1
Diagnosis/early detection of
prostate cancer
Cairns 2001,
106
Lee
1994,
107
Eilers 2007
122
Sensitivity/specificity: 92%/86%
Hypermethylation of
DAPK
Association with early recurrence
and pathological stage in bladder
cancer
Jarmalaite 2008,
108
Catto
2005
109
OR, 2.2 (95% CI, 1.04-4.5)
Hypermethylation of
MGMT
Predictor of response to carmustine
and temozolomide in gliomas
Hegi 2005,
110
Esteller
2000
111
HR for death associated with nonmethylation,
9.5 (95% CI, 3.0-42.7)
HR for progression of disease associated with
nonmethylation, 10.8 (95% CI, 4.4-30.8)
CIMP Subtype classification, risk
stratification, and prognostic
relevance in colorectal cancer,
leukemias, MDS, etc.
Issa 2008,
63
Shen 2007,
57
Issa 2005,
112
Issa 2004,
113
Shen 2002,
114
Shen
2010
123
HR for overall survival in MDS patients, 1.68
(95% CI, 1.0-2.81)
HR for progression-free survival in MDS
patients, 1.95 (95% CI, 1.18-3.21)
CIMP Correlation with favorable
prognosis in gliomas
Noushmehr 2010
115
G-CIMP status as an independent predictor
of survival (
P
.01)
CIMP Determinant of poor prognosis in
neuroblastomas
Abe 2005
116
HR, 22.1 (95% CI, 5.3-93.4)
Promoter methylation of
p16
,
CDH13
,
RASSF1A
,
and
APC
Association with early recurrence
in stage I NSCLC
Brock 2008
117
OR of recurrent cancer, 25.25
Promoter methylation of
p16
and of
MGMT
-
RASSF1A
-
DAPK
-
PAX5
in plasma and sputum,
respectively
Association with smoking and lung
cancer risk
Belinsky 2005,
118
Belinsky
2006
124
OR for cancer development, 6.5
Sensitivity/specificity: 65%/65%
Quantitation of promoter
methylation of
p16
,
p14
ARF
,
MGMT
, and
GSTP1
Detection of bladder cancer in
urine sediment DNA
Hoque 2006
119
Sensitivity/specificity: 82%/96%
Global histone
modification profiling in
primary prostatectomy
tissue samples
Correlation with prognosis and risk
of recurrence in low-grade prostate
cancer
Seligson 2005
120
HR, 9.2 (95% CI, 1.02-82.2)
microRNA signature Association with clinical outcome
(event-free survival) in
cytogenetically normal AML
patients with high-risk molecular
features
Marcucci 2008
121
HR for an event, 1.8 (95% CI, 1.0-3.0)
OR indicates odds ratio; HR, hazard ratio; GSTP1, glutathione S-transferase-
; DAPK, death-associated protein kinase; 95% CI, 95% confidence interval; MGMT, O-6-
methylguanine-DNA methyltransferase; CIMP, CpG island methylator phenotype; MDS, myelodysplastic syndrome; G-CIMP, glioma CpG island methylator phenotype;
CDH13, cadherin 13, H-cadherin (heart); RASSF1A, RAS association family 1A; APC, adenomatous polyposis coli; NSCLC, non-small cell lung cancer; PAX5
, paired box
gene 5
; AML, acute myeloid leukemia.
Cancer Epigenetics
384 CA: A Cancer Journal for Clinicians
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and specificity in the range of 65%.
124
It is interesting
to note that hypermethylation of P16 and MGMT was
detectable in sputum years before the clinical occur-
rence of lung cancer.
130
Another example is found in
colorectal cancer patients, in whom loss of imprinting
(LOI) of insulin-like growth factor II (IGF II)was
found concurrently in cancer and adjacent normal
colorectal tissue. LOI of IGF II was also found in pe-
ripheral blood lymphocytes and its measurement was
found to be predictive of the risk of developing colon
cancer.
131
Also in the colon, age-related methylation
in normal tissues has been proposed to mark a field
defect associated with cancer risk, and measure-
ment of this field could be a useful biomarker.
52
These data are relevant to cancer prevention be-
cause DNA methylation can be reversed by drug
intervention. Therefore, its detection at a preneo-
plastic stage would open the door to cancer preven-
tion strategies, either passively through close
monitoring of the investigated tissue (serial
colonoscopies/bronchoscopies, imaging studies,
etc.) or actively by the use of hypomethylating drugs
and/or chromatin remodelling agents to try and re-
vert the premalignant phenotype.
Aberrant DNA Methylation as a Diagnostic
Tool
Aberrant methylation has been tested in the clinical
setting as a diagnostic biomarker in biopsy specimens
or in bodily fluids such as serum, sputum, bron-
choalveolar lavage, saliva, urine, pleural or perito-
neal effusions, and stool. For example, glutathione
S-transferase-
(GSTP1) promoter hypermethylation
was found in 100% of human prostatic carcinoma tis-
sue specimens in one study
107
and was able to detect
the presence of malignancy in biopsy samples in a
study of 86 patients in whom prostate cancer was sus-
pected, with a sensitivity and specificity of 92% and
86%, respectively, and positive and negative predictive
values of 82% and 94%, respectively.
122
Similarly, the
presence of vimentin methylation in stool samples was
found to have a 46% sensitivity (95% confidence inter-
val [95% CI], 35%-56%) and a 90% specificity (95%
CI, 85%-94%) in diagnosing colon cancer.
132
A po-
tential lack of specificity of single markers can be rem-
edied by the use of a panel of several aberrantly
methylated genes. For example, methylation of a
panel of 9 genes in urine sediment DNA from 175
patients and 94 controls was able to predict the presence
of bladder cancer with a sensitivity of 82% (95% CI,
75%-87%) and a specificity of 96% (95% CI,
90%-99%).
119
One limitation to the use of DNA
methylation as a biomarker for disease diagnosis and
assessment is the possibility that aberrant methylation
could originate from a precancerous lesion or reflect an
age-related phenomenon.
14
Indeed, most studies pub-
lished to date have suggested that this approach has a
low positive predictive value despite relatively good
sensitivity and specificity. More sensitive methods are
being developed to address this issue.
105
Aberrant DNA Methylation and Assessment of
Prognosis/Response to Therapeutics
Methylation patterns can be useful to assess clinical
outcomes or response to chemotherapeutic agents. In
general, high levels of DNA methylation are associ-
ated with a poor prognosis such as in lung cancer
117
or myelodysplastic syndrome (MDS).
123
In a study
of 51 cases with stage I non-small cell lung cancer
(NSCLC) who developed an early recurrence after
curative surgical resection and 116 controls who were
free of disease recurrence after surgery, the promoter
methylation status of 7 genes was investigated in tu-
mor and lymph node samples for its association with
NSCLC recurrence. Methylation of 4 of those genes
(P16, cadherin 13 [CDH13], RASSF1A, and adenoma-
tous polyposis coli [APC]) demonstrated an independent
association with tumor recurrence, with methylation of
P16 and CDH13 found to have an odds ratio of recurrent
cancer of 15.5 and 25.25, respectively, in the training and
combined training-validation cohorts. Similarly, MDS
patients with higher levels of methylation, as assessed by
studying a panel of 10 genes, were found to have a shorter
median overall survival (12.3 months vs 17.5 months,
respectively; P .04) and a shorter median progression-
free survival (6.4 months vs 14.9 months, respectively;
P .009) when compared with patients with lower lev-
els of methylation. However, in some instances, intense
hypermethylation defines a distinct subgroup of cancers
that may have a favorable prognosis. This is the case
in colon cancer, in which simultaneous methylation of
multiple genes termed CIMP is associated with
MLH1 methylation, which results in a favorable prog-
nosis.
56
CIMP has also been described recently in
glioblastoma multiforme, in which it also was found to
be associated with a better outcome; CIMP-positive
cases were significantly younger at the time of diagno-
sis (median age of 36 years vs 59 years, respectively),
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closely associated with IDH1 somatic mutations, and
had a significantly better survival (median survival of
150 weeks vs 42 weeks, respectively) compared with
CIMP-negative cases (P .0165).
115
Methylation can also be useful as a predictive biomar-
ker. For example, methylation of the MGMT DNA re-
pair gene reportedly correlates with a good response to
temozolomide and better overall clinical outcome in pa-
tients with glioblastoma multiforme.
110
Indeed, MGMT
promoter methylation, present in approximately 45% of
cases, was found to be correlated with a significant ben-
efit from temozolomide therapy (median survival of 21.7
months compared with 15.3 months without temozolo-
mide therapy; P .007). In patients without MGMT
methylation, the effects of temozolomide were less clear
(median survival of 12.7 months compared with 11.8
months without temozolomide therapy; P .06).
These data suggest that tumor methylation profiling
could be useful for risk stratification and making thera-
peutic decisions.
miRNAs in Cancer Diagnosis, Classification,
and Prognosis
miRNA profiling has been shown to be informative
both as a diagnostic tool and as a potential prognostic
biomarker.
95
For example, miRNA profiling was
shown to be useful in a series of tissue samples derived
from metastatic sites of unknown primary origins.
133
The prognostic significance of miRNAs in cancer is
currently being extensively studied. In CLL, the ex-
pression of a panel of 13 miRNAs was shown to cor-
relate with disease aggressiveness as reflected by the
time elapsed between diagnosis and first treatment.
134
However, this predictive ability has not been shown to
be independent from other CLL prognostic markers.
134
In lung cancer, miR-155 and let-7 miRNA levels were
found to be correlated with disease aggressiveness and
clinical outcome.
135
Higher let-7 levels were associ-
ated with a more indolent disease and better survival
after surgical resection. miR-155 has also been shown to
be of prognostic value in patients with diffuse large B-cell
lymphoma,
136
in whom it is present at significantly
higher levels in the activated B-cell phenotype than in
the germinal center phenotype. miRNA profiling
could also be useful in the future as part of a model
integrating multiple prognostic information.
121
Clinical Applications: Epigenetic
Therapy
With the understanding of the mechanisms underly-
ing the silencing of tumor suppressor genes in cancer
came the idea of pharmacologically relieving the in-
hibitory effects of DNA methylation and chromatin
remodelling on gene expression. There are 2 classes of
drugs that modify epigenetics and have been approved
by the US Food and Drug Administration (FDA) for
the treatment of cancer: DNA methylation inhibitors
and HDAC inhibitors (Table 2).
137-141
The available DNA methylation inhibitors are nu-
cleoside analogues that exert their demethylating ac-
tivity through the establishment of an irreversible
covalent bond with DNMTs after their incorporation
into DNA.
142
Hypomethylation requires that the cells
TABLE 2. US Food and Drug Administration-Approved Epigenetic-Acting Drugs
EPIGENETIC-ACTING DRUG CLINICAL INDICATION MAJOR DATA SUPPORTING LITERATURE
DNA methyltransferase inhibitors
5-azacytidine (azacitidine) Symptomatic MDS 16% overall response rate;
66% hematologic
improvement/transfusion
independence
Kaminskas 2005
137
Fenaux 2009
138
5-aza-2-deoxycytidine (decitabine) Intermediate and High-risk MDS 73% objective response
rate; 34% complete
response rate
Kantarjian 2007
139
Histone deacetylase inhibitors
Suberoylanilide hydroxamic acid (vorinostat) Progressive, persistent, or recurrent
cutaneous T-cell lymphoma
30% objective response rate Mann 2007
140
Romidepsin (depsipeptide) Progressive, persistent, or recurrent
cutaneous T-cell lymphoma
34% overall response rate;
6% complete response rate
Piekarz 2009
141
MDS indicates myelodysplastic syndrome.
Cancer Epigenetics
386 CA: A Cancer Journal for Clinicians
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be proliferating after DNMT inhibition. These DNA
methylation inhibitors were first introduced in the
clinic several decades ago. At that time, they were used
as cytotoxic chemotherapy at relatively high doses
143
and were found to be toxic (at these doses) without
great antitumor activity. In the past 10 to 15 years,
these drugs were reintroduced at lower doses that
promoted the hypomethylating effect, and results
of clinical trials indicated that repeated exposure
induced DNA demethylation accompanied by a
better antineoplastic effect than when used at higher
doses.
144
This led to the approval of 5-azacytidine
(azacitidine) in 2004
145
and 5-aza-2-deoxycytidine
(decitabine) in 2006
146
by the US FDA for the treat-
ment of patients with MDS. Azacitidine induced an
overall response rate in the range of 20% to 60% and
significantly improved survival compared with stan-
dard of care.
138
Decitabine induced a high response
rate at optimal doses
139
(complete response [CR]/
pathologic CR rate of 40%) and has been shown to
prolong survival when compared with historical con-
trols.
147
The major side effect with these drugs
is myelosuppression and the regimens used cur-
rently are well tolerated.
144
Some of the shortcom-
ings of these drugs are their relatively short half-life,
their instability in aqueous solutions, a lack of
specificity inherent to their mechanism of action,
and the fact that acquired resistance is nearly uni-
versal, without a clear mechanism. This has led to a
search for potentially different DNA methylation
inhibitors and several were identified such as the
cytidine analogue zebularine, the antiarrhythmic
procainamide (a weak inhibitor), and SGI-1027, a
drug that may inhibit DNA methylation without
requiring incorporation.
148
Another interesting class of epigenetically targeted
drugs are HDAC inhibitors.
149
HDAC inhibitors
were initially identified through differentiation
screens. These drugs target the catalytic domain of
HDACs, thus interfering with their substrate recog-
nition. Most HDAC inhibitors affect zinc-dependent
HDACs and are divided into several classes depend-
ing on their chemical nature. The ones described to
date comprise the short-chain fatty acids (such as so-
dium phenylbutyrate, sodium butyrate, and valproic
acid); the hydroxamic acids (such as trichostatin A,
vorinostat, and panobinostat); the cyclic peptides
(such as romidepsin); and the benzamides, com-
prised of MGCD-0103 and entinostat. In 2006,
suberoylanilide hydroxamic acid (vorinostat) was ap-
proved by the US FDA for the treatment of patients
with progressive, persistent, or recurrent cutaneous
T-cell lymphoma.
140
Recently, depsipeptide (ro-
midepsin) received FDA approval for use in the re-
fractory form of the same disease. Clinical trials of
these and other HDAC inhibitors in other malig-
nancies are currently ongoing. Early results sug-
gest activity in other lymphoid malignancies such
as Hodgkin lymphoma, but limited activity in
solid tumors.
150
Similar to DNA hypomethylating agents, HDAC
inhibitors suffer from non-gene selectivity. The exact
mechanism by which these drugs exert their gene
expression reactivating effect is still unclear. One
straightforward mechanism proposed is the induced
hyperacetylation of histone proteins, leading to an
open chromatin configuration and transcriptional ac-
tivation.
149
However, the mechanism of action of
these drugs is more complex because they are active
both in the nucleus and the cytoplasm, and HDACs
catalyze the deacetylation of both histone and nonhis-
tone proteins. In fact, HDAC inhibitors might very
well be exerting their antitumor activity through ap-
optosis or cellular differentiation induction by affect-
ing multiple cellular pathways, some transcriptionally
and some post-transcriptionally. Some of these path-
ways, along with the biological effects epigenetically
targeted drugs have on tumor cells, are shown in Fig-
ure 3. There is currently interest in developing drugs
that target other epigenetic pathways such as histone
methylases/HDMT, MBDs, and histone readers.
The lack of specificity of epigenetically targeted
drugs raises concerns about their use in clinical prac-
tice. Some of these concerns would be the reactivation
of normally silenced sequences (such as repetitive ele-
ments) or imprinted genes. This reactivation could
theoretically lead to allelic imbalance or genomic in-
stability, and other deleterious effects of retrotranspo-
son activation. To date, there are no data supporting
these concerns clinically, but it is possible that prob-
lems will emerge after several years of therapy. This
has led researchers in the field to try and develop new
compounds selectively targeting specific genes. One
example is the development of a methylated oligonu-
cleotide directed toward the 5 promoter region of the
insulin-like growth factor 2 growth-promoting gene,
subsequently leading to the methylation of this pro-
moter and transient silencing of the gene.
151
This line
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of research is still in its infancy and could face signifi-
cant problems in drug delivery.
It is important to mention that epigenetic drugs are
promising not only as single agents but also in combi-
nation with other epigenetically targeted drugs or with
conventional chemotherapy. Several studies, both in
vitro and in vivo, have demonstrated the synergistic
effect of sequentially administering DNMT inhibi-
tors (such as decitabine) and HDAC inhibitors (such
as vorinostat),
142
and this approach is currently being
tested in clinical trials. Furthermore, a synergistic ef-
fect was also found when combining epigenetic drugs
with conventional chemotherapy,
152
and trials are cur-
rently testing these combinations in the clinical setting
for several tumor types.
Several studies have tried to link DNA methylation
profiles at study entry with response to therapy. To
date, these studies of a limited number of genes have
been negative.
123
Entire epigenome studies of this is-
sue are currently ongoing. Studies also have tried to
correlate global hypomethylation, as assessed by the
methylation levels of LINE1 and Alus repetitive ele-
ments, at days 5 and 12 after decitabine therapy with
clinical response. Results were controversial. Indeed,
some studies found a trend toward a positive correla-
tion between global hypomethylation at day 5 and
clinical response
144
in patients with leukemia, whereas
other studies found an inverse correlation between lev-
els of hypomethylation at day 12 and achievement of
CR
153
in patients with chronic myelogenous leukemia
that was resistant to imatinib mesylate. The latter finding
was hypothesized to be due to a cell death mechanism of
response, with the resistant cells capable of sustaining
more hypomethylation.
In contrast to DNA methylation markers, gene
expression induction has been consistently linked to
subsequent response to decitabine. This has been
demonstrated for P15,
139
ER,
154
P53R2/ribonucleoside-
diphosphate reductase subunit M2 B (RRM2B),
155
and miR-29b.
156
Perhaps one of the major drawbacks of epigenetic
therapy is the presence of spontaneous and/or ac-
quired resistance to these drugs, both in vitro and in
vivo. Indeed, in a panel of cancer cell lines, resistance
to the hypomethylating agent decitabine was mani-
fested by a 1000-fold difference in the half maximal
FIGURE 3. Epigenetic Therapy Is Shown. The 2 main families of epigenetically acting drugs, DNA methyltransferase (DNMT) inhibitors and histone deacetylase
(HDAC) inhibitors, exert their antineoplastic effect via several mechanisms such as cell cycle arrest, apoptosis induction, and immune recognition. These effects eventu-
ally result in differentiation or cancer cell death.
Cancer Epigenetics
388 CA: A Cancer Journal for Clinicians
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(50%) inhibitory concentration (IC
50
) of this drug
among the cell lines tested.
157
Resistance mechanisms
were hypothesized to be related to variations in the
parameters affecting nucleoside analogue metabolism,
starting with transport inside the cell (human equili-
brative nucleoside transporter 1 [hENT1] and hENT2),
initial phosphorylation (deoxycytidine kinase [DCK] for
decitabine and uridine-cytidine kinase [UCK] for azaci-
tidine), and finally catabolism by the enzyme cytidine
deaminase (CDA). Indeed, in vitro studies demon-
strated that low levels of DCK and hENT1 and high
levels of CDA were correlated with resistance to hypo-
methylating agents. In fact, the observed cross-resistance
between decitabine and cytarabine (2 nucleoside ana-
logues sharing the same need for phosphorylation by
DCK for incorporation into the DNA) and the lack of
cross-resistance between decitabine and azacytidine in-
dicate that incorporation into the DNA plays a major
role in cancer cell resistance to nucleoside analogues, in-
cluding decitabine. These observations were found to be
relevant in vivo as well, because low levels of DCK/low
DCK activity were correlated with poor response to nu-
cleoside analogues in, for example, childhood acute lym-
phoblastic leukemias
158
and pancreatic cancer.
159
The full-scale implementation of these molecular
markers of sensitivity to hypomethylating agents in
the clinical setting faces several challenges. One is the
less-than-perfect correlation between the clinical ac-
tivity of these drugs and their hypomethylating effect.
One possibility is that beyond a certain threshold,
more hypomethylation does not correlate with a better
clinical outcome. In fact, there may be molecular bar-
riers downstream of hypomethylation that prevent ad-
equate gene reactivation. Another possibility is the
hypomethylation-independent mechanisms of anti-
neoplastic activity of decitabine and azacitidine. Both
drugs can induce DNA damage at relatively high
doses, and azacitidine also affects RNA methyl-
ation.
160
Both of these effects may also be involved in
clinical responses.
Conclusions
Understanding the complexity of the epigenome and
of all the actors involved in modulating its interactions
with genomic sequences is of fundamental importance
in health and disease. This understanding will allow us
to reach newer horizons in our search for the mecha-
nisms governing cellular fate. On the tumorigenic
spectrum, the time when we switch from untargeted
cytotoxicity to reversion of the malignant phenotype is
drawing near.
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Cancer Epigenetics
392 CA: A Cancer Journal for Clinicians
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  • Source
    • "The miRNAs specific binding to complementary sequences of mRNAs may either RISC induces mRNA degradation (perfect pairings) or the translation into protein is blocked (imperfect miRNA-mRNA target pairing) was defined by Watson-Crick base pairing between positions 2 to 8 from the 5'miRNA (also known as the seed) with the 3'untranslated region (UTR) of their target mRNAs (Ghildiyal and Zamore, 2009; Bartel, 2009). The formed miRNA with RISC are then transported back into the nucleus to exert its biological effect since a single miRNA is capable of targeting hundreds of mRNAs, which highlights the impact of gene regulation system in cellular functions (Taby and Issa, 2010). The complementary passenger strand (miRNA*), which was initially thought to be degraded and known as a nonfunctional bioproduct of miRNA biogenesis (Gregory et al., 2005), has recently become an interesting revelation "
    [Show abstract] [Hide abstract] ABSTRACT: MicroRNAs (miRNAs) are short non-coding RNAs of 20-24 nucleotides that play important roles in carcinogenesis. Accordingly, miRNAs control numerous cancer-relevant biological events such as cell proliferation, cell cycle control, metabolism and apoptosis. In this review, we summarize the current knowledge and concepts concerning the biogenesis of miRNAs, miRNA roles in cancer and their potential as biomarkers for cancer diagnosis and prognosis including the regulation of key cancer-related pathways, such as cell cycle control and miRNA dysregulation. Moreover, microRNA molecules are already receiving the attention of world researchers as therapeutic targets and agents. Therefore, in-depth knowledge of microRNAs has the potential not only to identify their roles in cancer, but also to exploit them as potential biomarkers for cancer diagnosis and identify therapeutic targets for new drug discovery.
    Full-text · Article · Oct 2014 · Asian Pacific journal of cancer prevention: APJCP
  • Source
    • "The Dark Side of Epigenetics: Carcinogenesis and Resistance While epigenetics is an exploitable anticancer mechanism, the plasticity of epigenetic changes, with subsequent molecular alterations that regulate the neoplastic phenotype, contributes to carcinogenesis, tumor promotion, chemoresistance, and radioresistance as much as or more than genetic variability [2]. In particular, the yin of epigenetic silencing of tumor suppressor genes is an important mechanism for carcinogenesis. "
    [Show abstract] [Hide abstract] ABSTRACT: In cancer chemotherapy, one axiom, which has practically solidified into dogma, is that acquired resistance to antitumor agents or regimens, nearly inevitable in all patients with metastatic disease, remains unalterable and irreversible, rendering therapeutic rechallenge futile. However, the introduction of epigenetic therapies, including histone deacetylase inhibitors (HDACis) and DNA methyltransferase inhibitors (DNMTIs), provides oncologists, like computer programmers, with new techniques to “overwrite” the modifiable software pattern of gene expression in tumors and challenge the “one and done” treatment prescription. Taking the epigenetic code-as-software analogy a step further, if chemoresistance is the product of multiple nongenetic alterations, which develop and accumulate over time in response to treatment, then the possibility to hack or tweak the operating system and fall back on a “system restore” or “undo” feature, like the arrow icon in the Windows XP toolbar, reconfiguring the tumor to its baseline nonresistant state, holds tremendous promise for turning advanced, metastatic cancer from a fatal disease into a chronic, livable condition. This review aims 1) to explore the potential mechanisms by which a group of small molecule agents including HDACis (entinostat and vorinostat), DNMTIs (decitabine and 5-azacytidine), and redox modulators (RRx-001) may reprogram the tumor microenvironment from a refractory to a nonrefractory state, 2) highlight some recent findings, and 3) discuss whether the current “once burned forever spurned” paradigm in the treatment of metastatic disease should be revised to promote active resensitization attempts with formerly failed chemotherapies.
    Full-text · Article · Oct 2014 · Translational oncology
  • Source
    • "Genome-wide association studies of bladder cancer identified single-nucleotide polymorphisms (SNPs) on chromosome 8q24, upstream of the MYC oncogene, on chromosome 3q28 near the TP63 tumor suppressor gene [5], and in the PSCA gene to be associated with bladder cancer risk [6]. DNA methylation is one of the most consistent epigenetic changes occurring in human cancers [7, 8]. And it is well established that aberrant hypermethylation of the promoter region of tumor suppressor genes is associated with transcriptional silencing, and that hypermethylation is an alternative mechanism of functional inactivation [9] Moreover, promoter hypermethylation of tumor-related gene has also been proposed as a novel biomarker for detecting cancer and predicting prognosis [10]. "
    [Show abstract] [Hide abstract] ABSTRACT: Bladder cancer is one of the most common cancers worldwide. Fibulin-1, a multi-functional extracellular matrix protein, has been demonstrated to be involved in many kinds of cancers, while its function in bladder cancer remains unclear. So here we investigated the expression and function of fibulin-1 in Bladder cancer. We used real-time PCR, Western blot analysis and immunohistochemistry to determine the expression of fibulin-1 in Bladder cancer cells and patient tissues respectively. Methylation-specific PCR and quantitative sequencing were used to examine the methylation status of FBLN1 gene promoter. Eukaryotic expression plasmid and lentiviral vector were used to overexpress fibulin-1 in Bladder cancer cells 5637, HT-1376 to investigate its function in vitro and in vivo. We identified that fibulin-1 was significantly down-regulated in bladder cancer, and its dysregulation was associated with non-muscle-invasive bladder cancer (NMIBC) grade and recurrence. The promoter region of FBLN1 was generally methylated in bladder cancer cell lines and tissues, further investigation in patient tissues showed that the methylation status was associated with the fibulin-1 expression. Overexpression of fibulin-1 significantly suppressed tumor growth, induced tumor cell apoptosis, decreased cell motility, and inhibited angiogenesis in cultured bladder cancer cells and xenograft tumor in nude mice. Altogether, our results indicated that fibulin-1 expression is associated with NMIBC grade and recurrence, it is epigenetically down-regulated and functions as a tumor suppressor gene and angiogenesis inhibitor in bladder cancer.
    Full-text · Article · Sep 2014 · BMC Cancer
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