Cancer is a major health problem facing the entire human population. It affects almost every race, gender,
or age in the social hierarchy. Many mechanisms have been proposed for the development of cancer;
either genetic or epigenetic. Over the years, a cumulative body of research indicated that epigenetic mutations
(epimutations) represent a chief player in the course of cancer, where it affects cell proliferation,
cancer initiation, progression, apoptosis, and metastasis. In this minireview we highlight the main mechanisms
of epigenetic-mediated cancer development with emphasis on the environmental pollution role
in this arena.
Cancer is a large group of more 100 different diseases that can arise anywhere in the
human body [1-3]. It involves uncontrolled cellular proliferation, with the potential to invade
or spread to other parts of the body [4]. Cancer is considered the second common leading
cause of death worldwide. This condition was responsible for about 9.6 million deaths in
2018, where about 1 in 6 deaths is due to cancer [5]. Cancer arises from accumulation of
genetic mutations and/or epigenetic mutations [6,7]. Several genes are involved in the
carcinogenesis process, and they are reported elsewhere. The most common causes of cancer
are epimutations, where environmental pollutions are the main players [8,9]. Epigenetics is a
kind of non-sequence dependent inheritance, where a change in the DNA methylation, histone
modification, among others, might cause cancer to develop [10-12]. Different mechanisms are
involved in epigenetic-mediated carcinogenesis, each of them was extensively studied during
the last four decades [6]. Interestingly, newly developed epigenetic-based cancer therapies
provide unique and validated approach to treat different types of cancers [13,14].
Outdoor air pollution is a global challenging life-threatening problem, where it is classified
as a class I human carcinogen [15,16]. A large number of reports shows that air pollution is
connected to increased risk of sever types of cancers including lung,
head and neck, and nasopharyngeal cancers [17,18]. Other many
diseases including respiratory diseases and heart disease are also
correlated with prolonged exposure to polluted air [19,20]. Sources
of air pollution include, but not limited to, industrial processes
car fumes, and the household combustion of solid fuel [21]. All
these sources contain specific chemicals that are known to be
carcinogenic to humans.
DNA adducts
The chemical substances in polluted air can cause DNA damage
(via adducts) [22]. These substances can trigger cancer through
induction of severe immune responses such as oxidative stress and
long-term inflammation in the upper aerodigestive tract [22, 23].
It has been reported by several research groups that DNA adducts
have been found in individuals living in polluted regions, and this
adducts might be associated with cancer development [24,25].
DNA isolated from individuals working in polluted environments
showed 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) and
bulky DNA adducts [26]. DNA adducts are covalent bonds occurs
due to interaction of cancer-triggering chemical substances such
as polycyclic aromatic hydrocarbons (PAHs) with DNA [27-29].
Some DNA adducts can be eliminated by specific repair proteins,
while others are persistent, and the latter group is the main
cause of pollution-related cancer initiation [30, 31]. Furthermore,
persistent DNA adducts can cause base-pair substitutions, deletions
and chromosomal rearrangements and other chromosomal
abnormalities [32,33].
DNA adducts, however, could be used as a reliable biomarker
of exposure to carcinogens. Several studies were carried out to
correlate DNA adducts with the occupation of individuals exposed.
Categories include police officers [23,34,35], school children [36],
bus and taxi drivers [24], and gasoline salesmen and roadside
residents [22]. Meanwhile, chemical substances such as aromatic
amines (AAs) and heterocyclic aromatic amines (HAAs) are
activated by cytochrome P450 oxidation forming N-hydroxylated
intermediates, which can react with DNA directly. The resultant
compounds are transformed into unstable esters that induce
genetic mutations [37]. Recently, several research groups developed
specific DNA biomarker to detect the mutagenic effect of prolonged
exposure to polluted air. The genes used in these studies involved
CDKN2, p53, KRAS, HPRT, DAPK, and RAR-β among others [38-41].
Protein adducts
Protein adducts could also be caused by air pollution, including
benzopyrene-hemoglobin adducts and 4-aminophenyl-hemoglobin
adducts in polluted areas [42]. The cancer-causing effects of
these adducts have yet to be investigated experimentally [43].
Hemoglobin adducts formed by 4-aminobiphenyl was reported in
a group of children living in polluted areas [44]. Furthermore, airborn
substances were detected in proteins of exposed individuals
in a suburban group. Higher levels of 2-amino-apidic semialdehyde
(AAS) was observed in plasma proteins and in hemoglobin
extracted from bus drivers, while malondialdehyde was observed
in their plasma, and PAH was detected in their albumin [45].
Unfortunately, no repair mechanisms are there to handle protein
adducts, and these adducts accumulate with chronic exposure to
triggering factors. For that reason, hemoglobin (Hb) and serum
albumin carcinogen adducts could be used straightforwardly as a
biomarker for AAs and HAAs exposure. Moreover, plasma levels
of p53 and p21WAF1 proteins were found to be associated with
exposure to PAHs and benzo[a]pyrene [46].
Epigenetics is a wide-angled mechanism by which almost
all biological processes are regulated. With regard to caner
development, epigenetic-mediated gene regulation is involved in
cancer initiation, progression, metastasis, and angiogenesis, along
with cancer stem cell induction as a primary step towards initiating
the cascade [47,48]. Players of epigenetic landscape could be
classified into different classes; on DNA level, histone level, miRNA
level, and imprinting.
Cytosine Methylation
Cytosine methylation is the most common epigenetic mark in
cancer [49-51]. Normally, cytosine (occurs in CpG dinucleotide)
is methylated in a high percentage of human genome sequences
to maintain chromosome stability, while those CpGs located
in promote regions, especially of tumor suppressor genes, are
normally hypomethylated [52,53]. These hypo- or un-methylated
promoter regions allow for transcription of TSGs that have crucial
roles in controlling cell proliferation. The methyl group in the 5mC
lies in the major groove of the double helix DNA and interfere with
binding of transcription factor, and hence avert gene expression
[54]. Upon adding the methyl group to the fifth carbon atom in
cytosine residue by the action of DNMT, methylated DNA-binding
proteins such as MECP2 and the MBD protein family were recruited
to bind to methylated cytosines and suppress transcription process
by hindering the attachment of transcription factors [55].
Histone deacetylase (HDAC) was also recruited to remove the
acetyl groups from the histone tails rendering it to closed structure
in this case, it is known as heterochromatin [56,57]. If these
actions took place in the promoter region, the no RNA polymerase
would be able to attach its specific site to start transcription, and
hence the corresponding gene in this case is said to be silenced.
Methylation always occurs in CpG dinucleotides either in regions
rich of these CpGs (CpG islands) or within the body of the gene
[58]. Hypermethylation of tumor suppressor genes is positively
correlated with the initiation of cancer in different organs [59].
Our studies indicated that global DNA methylation was increases
in traffic workers with exposure to car fumes in Cairo, Egypt [60].
This study indicated that tumor (transforming) growth factor (TGF)
was found to be hypermethylated in traffic workers exposed to car
fumes for a long time (more than 6 hours/day for at least 5 years).
Furthermore, P53 was also found by our group to be dysregulated in
people exposed to car fumes for a long time in a cohort population in
Egypt (unpublished data). Hypermethylation and/or dysregulation
of these genes might indicate a future cancer incidence in individual
with these recorded abnormalities. On the other hand, many drugs
have been designed to modulate the methylome of malignant cells
as a way of treatment. In our laboratory, we investigated different
kind of DNMT inhibitors such as procaine and cyclophosphamide
in different cancer cells including breast, colon, liver, cervical, lung,
and laryngeal cancer [61-64].
Repetitive sequences
Repetitive elements constitute a large percentage of the human
genome. It is normally hypermethylated, while in cancer, it becomes
hypomethylated [65]. Various types of repeats in human genome
including LINE-1, Alu, centromeric tandem repeats, and juxta
centromeric tandem repeats are considered the most frequently
studied repetitive sequences in cancer, where it was found to be
hypomethylated. In addition, hypomethylation of these repeats
allows it to jump to other genomic loci [66]. Global hypomethylation
of LINE-1 and Alu was associated with long term or short-term
exposure to AAP. Specifically, genome wide hypomethylation
of Alu and LINE-1 repeats might lead to repositioning of these
sequences, leading to insertional mutations and genomic instability
[67,68]. On the other hand, centromeric tandem repeats, and juxta
centromeric tandem repeats play a crucial role in maintaining DNA
wrapped within heterochromatin structure, where no transcription
is allowed at the point of sister chromatid association. This
heterochromatinization leads to chromosome stability. Therefore,
hypomethylation of these repeats allows for heterochromatin to
be transformed to more open form i.e., euchromatin, leading to
chromosome rearrangements, and hence, genomic instability [69].
Histone methylation
The histone methylation has been widely proven to regulate
transcription [70,71]. The methylation of histone tail residue is
associated with both activation and suppression of transcription
[72]. Histone methylation occurs in the residues of arginine
and lysine on the histone tails H3 and H4 proteins [47]. Lysine
methylation is stimulated by histone-lysine-N-methyltransferases
also known as K-methyltransferases and involves the transport of
methyl groups from the S-adenosyl methionine. EZH2 (Zeste 2) is
one of the main proteins involved in the control and differentiation
of stem cells, K-methyltransferase, which stimulates the
trimethylation of methyl H3K27 [71,73].
EZH2 is a member of the polycomb repressive complex 2, a
protein compound that contains both the K-methyltransferase
protein and the reader proteins that recognizes the H3K27me3
[74,75]. H3K27me3 is usually involved in silencing genes
associated with the evolution and differentiation of stem cells,
including the Hox genes [76,77]. However, in many cancers, EZH2
is expressed excessively at both transcriptional and protein levels.
Overexpression of EZH2 has been designated as vital in prostate
cancer, where an increase in EZH2 protein staining in the cell
nucleus was detected with a progression from benign to metastatic
disease [78,79]. Other studies have identified excessive expression
of EZH2 as a key feature of breast cancer, lymphoma and glaucoma,
among other cancers [80-82]. In cancer cells, H3K27me3 was also
shown to suppress gene expression independently of the DNA
methylation, whereas in normal cells, EZH2 was shown to control
DNA methylation through interaction with DNMTs [83]. In addition,
dysregulation has recently been described in other members of the
polycomb repressive complex, including proteins that interact with
polycomb repressive complex 2 after the modulation of H3K27me3
mark by EZH2 [84,85]. In contrast to H3K27me3-mediated
silencing of histone, histone methylation can also be a marker
associated with activation of transcription [86]. Currently, we
are using temozolomide (TMZ) as a histone methylation agent in
colorectal cancer cells to identify its interaction with other histone
tags (unpublished data). JMJD2C is K-demethylase which stimulates
the removal of methyl markers from H3K9, a histone marker that
is commonly associated with suppression of gene expression [87].
JMJD2C has been detected in many types of cancers, including
esophageal and breast cancers [88]. Lysine demethylase 1, a type
of K-demethylase, which targets H3K9 and H3K4, has recently
found to be overexpressed in ER- breast cancer caused by estrogen
receptors, mesothelioma, and bladder cancer [88].
Although more research is needed to understand the functional
consequences of dysregulation of histone, K-demethylases and
K-methyltransferases are important in the cancer-causing process
and represent new targets for treatment. Our current work involves
modulating H3K27me3 to control breast cancer. We are aiming to
demethylate this histone mark using demethylating agent such as
5-Aza cytidine.
Histone acetylation
Unlike histone methylation, acetylation of histone is sturdily
connected to transcription activation [89,90]. Acetylation of
histone occurs on the lysine residues and is believed to enhance
transcription by neutralizing the positively charged histones,
thereby reducing its interaction with the negatively charged DNA
[91]. The maintenance of histone marks is controlled by histone
acetyl transferase (HATs), also known as K-acetyltransferases, and
histone deacetylases [92]. HATs stimulate the addition of acetyl
groups to lysine using acetyl coenzyme A as a cofactor, thereby
transforming the chromatin structure to be euchromatin. On the
other hand, HDACs remove acetyl groups and induce a closed or
suppressive chromatin structure (heterochromatin) [93].
Three diverse families of HATs are known; the Gcn5 family, the
MYST family, and the p300/CBP family [94,95]. It turns out that
the HATs of each of these families play a role in causing cancer,
either from improper activation or suppression of the target gene
activity [96]. The Wnt signaling pathway, previously shown to
be dysregulated in some types of cancers, is now indicated to be
enhanced by HAT Gcn5 in breast cancer [97]. HDACs, in human, are a
group 18 enzymes that catalyze the removal of histone acetyl marks
and are involved in repressing the transcription process in several
cancer-related genes such as p53, PTEN, APC, and p21 [56,98].
Several cancer studies have indicated that histone deacetylation is
an early step in carcinogenesis process [99-101]. In rat model with
skin cancer, early loss of mono-acetylation of histone H4K16 was
detected, indicating the role of deacetylation in promoting cancer
[102].
Furthermore, histone deacetylation was also observed in
different malignant cell lines including breast cancer, colon cancer,
and lymphoma cells signifying that histone deacetylation is a
common event in cancer [103]. The deregulation of HDAC in cancer
cells offers a new target for chemotherapy i.e., HDAC inhibitors
(HDACi). HDAC inhibitors are widely used as a therapeutic option in
several diseases, including cancer. We extensively used Vorinostat
(one of the FDA approved HDACi) in different cell lines to identify
its role in controlling the progression of the disease [104].
miRNAs and cancer
MicroRNAs (miRNAs) are a type of small (20-24 nucleotides)
non-coding RNA molecules that play a central role in posttranscriptional
regulation of gene expression [105,106]. Since its
discovery in 1993, several studies have proven its role as oncogenes
(oncomiRs) or tumor suppressors (anti-oncomiRs), under certain
conditions in human cancers, including colorectal, liver, lung,
breast, and brain cancer [107]. In these cancers, it has been shown
that uncontrolled miRNAs affect the distinctive features of cancer,
including the maintenance of proliferative signals, the evasion of
growth inhibitors, resistance to cell death, invasion activation and
metastasis (metastamiRs), and angiogenesis (angiomiRs) [108].
Convincing evidence has shown that miRNA expression is
unregulated in human cancer through various mechanisms,
including amplification or deletion of miRNA genes, atypical
miRNA control, abnormal epigenetic changes and defects in the
miRNA biosynthesis pathway [105]. Furthermore, because of their
complicated role in cancer, miRNAs represent attractive candidates
for cancer treatment italic synthesizing either miRNA antagonists
or miRNA mimics to suppress or augment specific miRNA
expression, respectively.
Genomic imprinting
Genomic imprinting is type of inheritance that does not follow
the usual Mendelian type of inheritance [109]. It is an epigenetic
phenomenon that causes some genes to be differentially expressed
based on their parent of origin [110]. This process involves
DNA methylation and histone modification as epigenetic marks
without affecting the sequence of DNA. Nearly 100 genes have
been characterized so far, and a number of these genes have been
implicated in the development of tumors. Examples of human
imprinted genes are IGF2, IGF2R, PEG3, GNAS, and MEST [111,112].
The physiological role of many of the imprinted genes in controlling
cell proliferation suggesting their potential involvement in tumor
formation process [113]. Because most of the imprinted genes play
central roles in cellular growth, development, and metabolism,
the abnormal expression of these genes either due to genetic or
epigenetic mutations often causes human ailments, including
cancers [114]. For example, loss of imprinting (LOI) in IGF2 due to
abnormal methylation of differentially methylated regions has been
observed in many types of tumors [115].
Cancer is a disease that arise from the consecutive buildup of
genetic and epigenetic changes in cells. Notwithstanding powerful
studies, several questions remain unanswered about the exact role
of epigenetics in cancer initiation and progression. The proposed
mechanisms have accounted for a small percentage of the entire
story, and the rest is demanding further investigation. Being the
future field of studying cancer, epigenetic-based drugs (epidrugs)
were introduced. These drugs might replace the well-known
medications that target only symptoms, but not the core cause of
diseases.
Professor, Chief Doctor, Director of Department of Pediatric Surgery, Associate Director of Department of Surgery, Doctoral Supervisor Tongji hospital, Tongji medical college, Huazhong University of Science and Technology
Senior Research Engineer and Professor, Center for Refining and Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia
Interim Dean, College of Education and Health Sciences, Director of Biomechanics Laboratory, Sport Science Innovation Program, Bridgewater State University