REVIEW
Ali et al. Epigenetic Deregulations in Gastric Cancer
signatures are reversed in cancer cells.
3
The initial events
that are responsible for this abnormal epigenetic behavior
are still not fully understood.
1
Once this process is started,
irregular genetic expression of tumor suppressor and onco-
genes imbalance may contribute to cancer development.
5
Rapidly growing field of cancer epigenetics has revealed
the role of every component of epigenetic mechanism.
DNA methylation and covalent and non-covalent histone
modifications alter the chromatin structure that may affect
DNA exposure to transcription network molecules result-
ing in abnormal genetic expression. Non-coding RNAs,
especially microRNA regulate gene expression through
post-transcriptional silencing of target genes.
GC is the fourth most common cancer in the world and
the second most prevalent cause of cancer related death.
6
It may arise either from precursor lesions or de novo.
Gastric adenomas or flat dysplasia may lead to develop-
ment of GC.
3
There are 10% chances of malignant trans-
formation of gastric dysplasia/adenoma into GC.
3
GC,
like other cancers, is a complex disease. Multiple fac-
tors including bacterial infection, dietary habits, smok-
ing and genetic polymorphisms determine the risk of GC
development. Various genetic aberrations and epimutations
are witnessed during the initiation and progression of GC.
Abnormal functioning of many cellular processes such as
cell cycle, DNA repair, angiogenesis and growth factor/
receptors are involved.
2 78
Two-thirds of GC cases are
diagnosed at advanced stages, when surgery can be the only
option.
9
Current situation necessitates carrying out research
for better understanding of molecular events responsible for
the malignant transformation of normal cells.
GC is particularly an epigenetic phenomenon in which
more than 90% of the heritable alterations are of epige-
netic origin.
10
Epigenetic alterations have been acknowl-
edged as an important mechanism contributing to early
gastric carcinogenesis.
3
Many studies have characterized
epigenetic abnormalities in Intestinal metaplasia (IM)
and adenoma, which are precursors of GC. CpG island
hyper-methylation and repetitive DNA hypo-methylation
increase from the chronic gastritis to GC (during different
stages of gastric carcinogenesis). Interestingly, IM sam-
ples, which are epigenetically altered lesions, displayed
enhanced CpG island hyper-methylation and repetitive
Nongyue He
DNA hypo-methylation, regardless of Helicobacter pylori
(HP) infection status or association with GC.
11
2. DNA METHYLATION
CpG islands are 0.5–2 kb regions rich in cytosine-guanine
dinucleotides and are present in the 5
promoter region of
app. 40–50% of human genes. Methylation of cytosines
within CpG islands is associated with loss of gene expres-
sion by repression of transcription and is observed in
tumorigenesis, as well as under physical conditions such
as X-chromosome inactivation and aging.
3
DNA methy-
lation is a heritable and enzyme-induced modification in
humans.
2
DNA methylation inhibits transcription factor
binding, and thus, inhibits gene activation.
12
As compared
with normal cells, the malignant cells show major disrup-
tions in their DNA methylation pattern.
3
Cancer, particu-
larly GC (GC), is prevalently an epigenetic phenomenon
that is dependent on an altered DNA methylation pattern.
10
DNA methylation is involved in silencing gene expression
by establishing and maintaining a repressive status at gene
promoters.
13
Increased and decreased methylations at specific
sequences (hyper-methylation and hypo-methylation,
respectively) are characteristic of tumor DNA in com-
parison with normal DNA and promote carcinogenesis in
multiple ways including genomic instability.
14 15
Promoter
hyper-methylation is associated with a wide variety of
genes, including genes involved in cell cycle, cell growth
and proliferation, angiogenesis, apoptosis, DNA repair and
metastasis.
4 12 16–21
It usually results in silencing of tumor
suppressor genes providing free space for oncogenes to
regulate cellular functions without any check. Apparently,
hyper-methylation-associated gene silencing provides can-
cer cells with a growth advantage similar to deletions and
mutations.
Not all the DNA methylations are tumor specific. In
colon tissues, studies have shown DNA methylation in cer-
tain genes due to aging is classified as Type A methylation.
In contrast, tumor-specific methylation such as p16 and
hMLH1 is classified as Type C methylation.
3
Age-specific
promoter hyper-methylations may be related to increased
DNA damage and increased duration of carcinogen
J. Nanosci. Nanotechnol. 12, 1–12, 2012 3