ABSTRACT
“Epigenetics has always been all the weird and wonderful things that cannot be explained by genetics” a famous quote by Denise Barlow. Coining the term epigenetics is credited to Conrad Waddington who describes it as a “branch of biology which studies causal interactions between genes and their products, which bring the phenotype into being”. Epigenetics adds a new stratum of events between genes and their expression. It proposes a control system of “switches” that turn genes on or off causing heritable effects in humans. Thousands of DNA damaging events take place every day in our body but efficient DNA repair systems prevent that. Accumulation of DNA damage has been linked to cancer and genetic deficiencies in specific DNA repair genes are associated with tumor prone phenotypes. Epigenetic silencing of DNA repair genes may promote tumorigenesis. This review summarizes current knowledge of the epigenetic inactivation of DNA repair components in cancer.
InTRoduCTIon
Cancer is characterized by uncontrolled cell division and malignant growth. Cancer cells have a higher proliferation rate than their corresponding normal tissue and they often bypass apoptosis. Furthermore, they can acquire the capability to separate from their original tissue and can develop metastatically in other regions of the body.
The expression of genetic information within an individual cell dictates how that cell subsequently behaves. Events at the molecular level can influence the expression of certain genes and thereby adversely affect cellular functions to such a degree as to initiate a pathologic process. Cancer cells, for example, must undergo a number of molecular events that allow them to acquire several distinct pathologic behavioural properties. These properties result in deleterious clinical consequences for the host, but paradoxically empower that abnormal cell and its progeny with a survival advantage over normal cells.
Much of the focus of molecular biologic research has concentrated on investigating the role of genetic changes - that is, direct alterations of dnA base sequence through mutation, deletion or insertion
Promoter DNA Methylation of
DNA Repair Genes in Cancer
*1Shilpa V., 2Lakshmi Krishnamoorthy.
1Doctoral Scholar, Department of Biochemistry, Kidwai Memorial Institute of Oncology, Dr. M. H. Marigowda Road, Bangalore-560029. 2Professor and Head, Department of Biochemistry, Kidwai Memorial Institute of Oncology,
Dr. M. H. Marigowda Road, Bangalore-560029.
and their effect on subsequent gene expression and cell behaviour. Recently, alternative mechanisms of gene modulation have been observed that affect its expression and remain preserved after cell division without disrupting the actual sequence at all.
C. H. Waddington in 1939 coined the term “Epigenetics”, which he defined as “the causal interactions between genes and their products, which bring the phenotype into being”.1 Epigenetics, later was defined as heritable changes in gene expression that are not due to any alteration in the dnA sequence.2 It is increasingly apparent that, in human cancers, heritable losses of gene function may be mediated by epigenetic as well as by genetic abnormalities.3,4 The argument as to whether cancer is an epigenetic or a genetic disease, in fact emphasizes that synergy between two processes drives tumor progression from the earliest to latest stages. Inclusion of epigenetic events in our concepts of how tumors evolve heightens our need to understand the basic nature of chromatin changes that set heritable states of gene function. From a translational standpoint, it enriches the potential and suggests new targets to consider for cancer prevention and therapeutic strategies. The methylation of dnA is recognized as a key mechanism in the regulation of gene expression and evidence for its role in the development of a wide variety of cancers is rapidly accumulating.4
Research into dnA methylation has been progressing at a furious pace, despite uncertainty about its origin and physiological function. Consistent with a resurgence of interest in the idea that cancer is a disease of faulty development, there has been a revived quest in studies to uncover epigenetic processes involved in neoplastic development and progression.5, 6 Epigenetic information, after all, is essential for development and it is clear that cancer is ultimately a disease of aberrant gene expression.
Effective dnA repair is the backbone of cancer-free survival. Mutations in dnA repair genes such as base excision repair (BER), nucleotide excision repair (nER), mismatch repair (MMR), dnA crosslink repair, and several others is the cause of inherited cancer syndromes. As an alternative mechanism to genetic mutation, epigenetic gene inactivation can be brought about that either inactivates or reduces efficiency of dnA repair genes. In this review, we will discuss some examples of dnA repair mechanisms in cancer.
EpIgEnETIC FEATuRES oF noRMAl
CEll
Epigenetic mechanisms are used in many different ways to regulate gene expression. Epigenetic changes never involve a change in the primary dnA sequence or a change in base pairing but are reflected primarily in dnA cytosine modification patterns, histone post-translational modifications, or deposition of certain histone variants along specific gene sequences. These epigenetic modifications of genes are generally reversible, but can get transmitted to the daughter cells.7
one common and perhaps the most permanent and stable mechanism of epigenetic gene inactivation is the methylation of the 5-carbon of the dnA base cytosine in the 5’-Cpg-3’ dinucleotide sequence context of Cpg island or promoter regions. These methylation reactions carried out by dnA cytosine methyltransferases are a main component of the epigenetic regulatory mechanisms in mammals.8 Although most Cpg sites in the human genome are methylated, Cpg dense regions known as Cpg islands are typically unmethylated in normal tissue which spans the 5’ end of the regulatory region of many genes.
dnA methylation plays an essential role in normal development through its effects on gene imprinting, condensation of chromatin, stabilization of chromosomes, X-chromosome inactivation, tissue-specific silencing of gene expression and transcriptional silencing of repetitive elements. 5-Methylcytosine (m5C) was first found in dnA of higher eukaryotes by Hotchkiss in 1948. This epigenetic regulation also coordinates gene expression during cell differentiation in mammalian embryogenesis. 9, 10
The methylation of mammalian genomic dnA is catalyzed by dnA methyltransferases (dnMTs) that can be divided into maintenance and de novo dnMTs. A methyl (-CH3) group is covalently bonded to the 5-carbon on the cytosine base. This process is mediated by dnMTs. The methyl group is provided by S-adenosyl methionine (SAM), and this is converted to S-adenosyl homocysteine (SAH) in the process. This is recycled back to SAM in a folate and cobalamin dependant pathway (Fig. 1).11
last century. They were initially described in the dnA of the tubercle bacillus and subsequently were extracted from calf thymus, where they were known as epi-cytosine, producing a different chromotographic profile from normal cytosine.12, 13 Their purpose was not defined until relatively recently. It was speculated that they acted as a primitive host defense mechanism to silence dnA from viral organisms and provided an explanation for the latency of certain viral infections and how such agents can escape detection.14, 15
EpIgEnETIC CHAngES In CAnCER
promoter hypermethylation for down regulating genes has been known to play a critical role in tumorigenesis. genes that are unmethylated in normal tissues at all ages are also found to be hypermehtylated quite early in tumorigenesis. These early losses of cell cycle control, altered regulation of gene transcription factors, disruption of cell-cell and cell-substratum interaction and even multiple types of genetic instability are characteristic features of human cancer.
While promoter hypermethylation and associated gene silencing generally remain very stable in cancer cells, these changes, unlike mutations are potentially
reversible. Such epigenetic plasticity is an excellent candidate to mediate the dynamic heterogeneity of cell populations inherent to complex tumor traits such as metastasis. In this regard, most epithelial tumors are highly invasive, and the cells relaxed can form metastatic foci, which grow within visceral organs.
Two mechanisms have been proposed to account for transcriptional repression via dnA methylation. In the first mechanism, dnA methylation directly inhibits the binding of transcription factors (TFs) such as Ap-2, c-Myc/ Myn, E2F and nFkB to their binding sites within promoter sequence. In this mechanism, Cpg dinucleotides have to be present within the binding site of TFs, which are sensitive to methylation of Cpg dinucleotides (Fig. 2).16 The second mode of repression includes binding of proteins specific for m5Cpg dinucleotides to methylated dnA. Methylated dnA recruits m5Cpg-binding protein (MeCp) and m5Cpg-binding domain (MBd) proteins. MeCp1 and MeCp2 bind
Figure 1: The methylation cycle
Adapted from Ref 11. Methylation of cytosine catalysed by dnA methyltransferase, which uses methyl group from SAM.
Figure 2: Repression of transcription via CpG dinucleotide methylation.
specifically to methylated dnA in whole genome and form spatial obstacle that disable binding of TFs to promoter sequences (Fig. 2).16
Methylated cytosine has a greater propensity to undergo spontaneous deamination and the formation of thymine. If this occurs on a tumor suppressor gene, then a point mutation develops that can lead to uncontrolled cell proliferation (Fig. 3).11
In the human genome, the actual prevalence of Cpg dinucleotide pairs is only about 1% as opposed to the expected 6% (1/16). However, localized
high-density concentrations of Cpg repeat sequences between several hundred to a few thousand base pairs are noted to exist as islands in the promoter regions of many common genes, and in particular, genes associated with tumor suppression.17 These are normally unmethylated, but if these regions become methylated, failure to transcribe the downstream gene occurs, causing silencing of that gene (Fig. 4).11
What induces methylation to occur at previously unmethylated locations in incipient cancer cells or, indeed, whether methylation is the primary event responsible for genetic silencing or merely a secondary event is unknown. Theoretically, abnormal methylation patterns could arise as a result of an over active “methylating” factor or the loss of a “demethylating” factor. The observation that dnMTs levels are increased in cancers would tend to suggest this concept, but current investigations indicate that the elevation is likely to be a secondary effect of increased cell proliferation rather than a causal mechanism for the former.18, 19 It is now accepted that promoter methylation of genes involved in the control of cell proliferation results in their inactivation, and this is a fundamental event in the pathway to carcinogenesis.
Figure 3: Methylation precipitating a point mutation.
Adapted from Ref 11. Cytosine to thymine point mutation after deamination of methylated cytosine.
Figure 4: Mechanisms of carcinogenesis induced by methylation events.
HypERMETHylATIon oF dnA REpAIR
gEnES In CAnCER
Hypermethylation of the promoter Cpg islands can affect genes involved in the cell cycle, dnA repair, the metabolism of carcinogens, cell-cell interaction, apoptosis, and angiogenesis, all of which are involved in the development of cancer.20, 6 Hypermethylation occurs at different stages in the development of cancer and in different cellular networks, and it interacts
with genetic lesions (Table 1).21 Such interactions can be seen when hypermethylation inactivates the Cpg island of the promoter of the dnA repair genes hMLH1, BRCA1, MGMT.6, 22-24 In each case, silencing of the dnA repair gene blocks the repair of genetic mistakes, thereby opening the way to neoplastic transformation of the cell.
The profiles of hypermethylation of the Cpg islands in tumor suppressor genes are specific to the cancer
Table 1. Epigenetic Aberrations among Different Tumor Types.
Type of Cancer Epigenetic Disruption
Colon cancer
Cpg-island hypermethylation (hMLH1, p16INK4a, p14ARF, RARB2, SFRP1, and WRN), hypermethylation of miRnAs (miR-124a), global genomic hypomethylation, loss of im-printing of IGF2, mutations of histone modifiers (EP300 and HDAC2), diminished monoacetylated and trimethylated forms of histone H4
Breast cancer Cpg-island hypermethylation (BRCA1, E-cadherin, TMS1, and estrogen receptor), global genomic hypomethylation
Lung cancer Cpg-island hypermethylation (p16INK4a, DAPK, and RASSF1A), global genomic hy-pomethylation, genomic deletions of CBP and the chromatin- remodelling factor BRG1
Glioma Cpg-island hypermethylation (dnA-repair enzyme MGMT, EMP3, and THBS1)
Leukemia Cpg-island hypermethylation (p15INK4b, EXT1, and ID4), translocations of histone modifiers (CBP, MOZ, MORF, MLL1, MLL3, and NSD1)
Lymphoma Cpg-island hypermethylation (p16INK4a, p73, and dnA-repair enzyme MGMT), di-minished monoacetylated and trimethylated forms of histone H4
Bladder cancer Cpg-island hypermethylation (p16INK4a and TPEF/HPP1), hypermethylation of miR-nAs (miR-127), global genomic hypomethylation
Kidney cancer Cpg-island hypermethylation (VHL), loss of imprinting of IGF2, global genomic hy-pomethylation
Prostate cancer Cpg-island hypermethylation (GSTP1), gene amplification of polycomb histone methyl-transferase EZH2, aberrant modification pattern of histones H3 and H4
Esophageal cancer Cpg-island hypermethylation (p16INK4b and p14ARF), gene amplification of histone demethylase JMJD2C/GASC1 Stomach cancer Cpg-island hypermethylation
Stomach cancer Cpg-island hypermethylation (hMLH1 and p14ARF)
Liver cancer Cpg-island hypermethylation (SOCS1 and GSTP1), global genomic hypomethylation Ovarian cancer Cpg-island hypermethylation (BRCA1)
BRCA1 denotes breast-cancer susceptibility gene 1, BRG1 BRM/SWI2-related gene 1, CBP cyclic AMp response element –
binding protein (CREB) – binding protein, DAPK death-associated protein kinase, EMP3 epithelial membrane protein 3,
EP300 E1A binding protein p300, EXT1 exostosin 1, EZH2 enhancer of zeste drosophila homologue 2, GSTP1 glutathione
S-transferase 1, HDAC2 histone deacetylase 2, hMLH1 homologue of Mutl Escherichia coli, ID4 inhibitor of dnA binding 4,
IGF2 insulin-like growth factor 2, JMJD2C/GASC1 Jumonji domain-containing protein 2C, MGMT o6-methylguanine–
type (Fig. 5 and Table 1).21, 25, 26 Each tumor type can be assigned a specific, defining dnA “hypermethylome”. Such patterns of epigenetic inactivation occur not only in sporadic tumors but also in inherited cancer syndromes, in which hypermethylation can be the second lesion in Knudson’s two hit model of how cancer develops.27, 28 Recently devised epigenomic techniques have revealed maps of hypermethylation of the Cpg islands that suggest the occurrence of 100-400 hypermethylated Cpg islands in the promoter regions of a given tumor.
Epigenetic inactivation of dnA repair genes in cancer has been reported for several dnA repair pathways including BER, nER, MMR, amongst several others. Within one dnA repair pathway, specific genes are often preferentially methylated. The specificity by which epigenetic mechanism occurs is yet to be determined and the preference for specific genes is yet to be understood.
Epigenetic inactivation processes can result in an increase in genetic instability during tumorigenesis that
Figure 5: Hypermethylation profile of promoter region CpG island of tumor suppressor genes in human cancer.
Adapted from Ref 21. Four tumor cells are shown undergoing transcriptional silencing by dnA hypermethylation of the regulatory regions of tumor-suppressor genes. In colon cancer, entrance into the cell cycle occurs by means of p16INK4a methylation. In leukemia cells, p15INK4b methylation initiates proliferation. In breast-cancer cells, defects in dnA repair are related to methylation of BRCA1 and in glioma cells, methylation of o6-methylguanine–dnA methyltransferase (MGMT)
can be directly attributed to the deficiencies in dnA repair. Therefore, inactivation of dnA repair genes can be seen as an important event in cancer initiation and/or progression by reducing genomic stability leading to genetic aberrations at other important gene loci. Such a mechanism is proven for inactivation of MMR pathways in colorectal tumors but awaits direct confirmation for a number of other dnA repair genes that are found methylated in tumors. on the other hand, diminished dnA repair is expected to lead to reduced cell survival in general and additional events are likely occurring that enable a cell with reduced repair capacity to undergo uncontrolled proliferation instead of cell death (e.g. mutation in Tp53).
one of the most studied genes in ovarian cancer is BRCA1, due to its role in both inherited and sporadic forms of this disease.29 The clinical outcome for ovarian cancer patients having tumor-hypermethylated BRCA1 has recently been compared with patients with germline BRCA1 mutations or wild-type BRCA1.30 BRCA1 hypermethylation occurs in 10–15% of sporadic disease cases, associates strongly with loss of BRCA1 RnA and protein and significantly correlates with poor patient response.29, 31 These studies suggest that BRCA1 hypermethylation, which has been reported to be observed in ovarian cancer patient serum, may represent a minimally invasive approach for predicting patient response to standard therapies.32
Another example is the hypermethylation of the Cpg promoter region of the MMR gene MLH1 observed in a subgroup of human colorectal cancers that show microsatellite instability. Microsatellite sequences are polymorphic, short, repeating segments of dnA (about 1 to 4 base pairs) distributed across the genome. Alterations to their pattern frequently occur if there is a deficiency in the cells’ ability to repair defects in dnA. The methylation of MLH1 results in failure to produce a functional protein and impairs the ability of the cell to repair mismatches that occur in the genome during proliferation, resulting in an increased mutation rate, some 100 times greater than that in normal cells. Microsatellite instability is noted in approximately 13% of all sporadic cases of colorectal cancer and in nearly all patients with hereditary nonpolyposis colorectal cancer, which in turn is linked to mutations of the MMR genes hMLH1 and hMSH2. In a significant proportion of tumors positive for microsatellite instability, no mutational abnormality can be shown, but hypermethylation and loss of hMlH1 protein expression does occur.33
In prostate cancer, GSTP1 is observed to be silenced by promoter methylation.34-37 GSTP1 promoter methylation has been detected in cancerous as well as prostatic intraepithelial neoplasia (pIn) lesions, whereas it has been rarely detected in normal prostate tissues.38-40 Hypermethylation of GSTP1 was also found in a subset of proliferative inflammatory atrophy (pIA) lesions, which are believed to be precursors of tumors.41
The presence of aberrant Cpg island methylation alone does not signal the presence of an invasive cancer. This may be the case, but premalignant or precursor lesions on their way to full tumorigenesis can also carry this epigenetic culprit. In fact, this finding may be useful in early detection screenings, especially in those people with a high familial risk of developing cancer because they may have patterns of Cpg island hypermethylation similar to the sporadic cases.42
The MgMT protein (o6-methylguanine dnA methyltransferase) is directly responsible for repairing the addition of alkyl groups to the guanine (g) base of the dnA.43 This base is the preferred point of attack in the dnA for several alkylating chemotherapeutic drugs, such as BCnu [1, 3-bis(2-chloroethyl)-1-nitrosourea], ACnu [1-(4-amino-2-methyl-5-pyrimidinyl) methyl- 3-(2-chloroethyl)-3-nitrosourea], procarbazine, streptozotocin, or temozolamide. Thus tumours that have MgMT inactivation due to hypermethylation become sensitized to the action of alkylating agents since the repair mechanism is already impaired which leads to cell death.
Similar cases to that described for MgMT can be cited for other dnA repair and detoxifier genes that also undergo aberrant dnA methylation where the methylated status of the dnA repair gene renders host cell sensitive to chemotherapeutic drugs that would otherwise be chemoresistant. For example, the response to cisplatin and derivatives may be a direct function of the methylation state of the Cpg island of hMlH1, the response to adriamycin may be related to the methylation status of gSTp1 and the response to certain dnA damaging drugs could be a function of the state of BRCA1 hypermethylation.44-47
MMR dependent apoptotic response lead to increased resistance to cisplatin and carboplatin drugs that are the cornerstone amongst treatments for ovarian cancer and a wide variety of tumors.49, 50 Restoration of hMLH1 expression, either by gene transfer or by reversal of epigenetic silencing, leads to increased sensitivity to carboplatin and other cytotoxic agents.43, 51
The fact how Cpg islands become hypermethylated in some types of cancer but not in others is yet to be understood. Inactivation of a particular gene by methylation could give certain tumor types a growth advantage. Cpg islands can have a location within a particular nucleotide sequence that allows them to become hypermethylated, or they can be located in a chromosomal region that is prone to large-scale epigenetic dysregulation.51, 52 When combined with other profiling techniques, such as gene expression profiling, a patient’s dnA methylome may play a crucial role in the development of personalized medicine.
dIFFEREnCES BETWEEn loSS oF
gEnE FunCTIon vERSuS EpIgEnETIC
CHAngES
There are some fundamental differences between genetic and epigenetic silencing that are potentially very significant for tumor biology. First, when genetic events are responsible for disruption of both alleles in the classic two-hit paradigm for loss of tumor suppressor gene function, each event produces a fixed level for loss of gene dosage. First genetic hit may potentially result in phenotypically functional haplo- insufficiency states.53 The onset of selective cell advantage does not occur without the complete loss of gene dosage produced by the second hit. In contrast, the loss of gene transcription associated with aberrant promoter Cpg island methylation is mediated by the density of methylation within the region. loss in gene function in association with aberrant promoter methylation may manifest in a more subtle selective advantage than gene mutations during tumor progression.54
Second, while promoter hypermethylation and associated gene silencing generally remain very stable in cancer cells, these changes unlike mutations, are potentially reversible.55 Such epigenetic plasticity is an excellent candidate to mediate the dynamic heterogeneity of cell populations inherent to complex
tumor traits such as metastasis. In this regard, most epithelial tumors are highly invasive, and the cells released can form metastatic foci, which grow within the visceral organs.
The combination of genetic and epigenetic events in cancer now provides a mechanism for complete inactivation of both allelic locations. Classification of tumors into categories based on molecular characteristics and whether they display certain methylation patterns is possible. Tumours are termed either as displaying a Cpg island methylator phenotype (single gene methylation) or as Cpg island methylator phenotype positive – CIMp+ (multiple gene methylations). distinguishing between molecularly heterogenous groups of tumours with the same origin in a epigenetic manner may yield clinically useful parameters for prognosis.56
METHylATIon RElATEd
MuTATIonAl EvEnTS
Methylated cytosine residues show a high propensity to undergo deamination to form thymine.57 In this way, a C-to-T point mutational event occurs. Because thymine is a normal component of human dnA, this mutation may not be correctly recognized by the dnA repair mechanisms. Instead of repairing the mutated thymine, the complementary strand guanine may be substituted for adenine to form the normal T–A opposition. Hence, a G-to-A point mutation occurs.
The transformation of C- to- T can occur either through spontaneous deamination of 5mC or by an enzyme-mediated mechanism where methyltransferase binding results in deamination before the methyl transfer to form uracil, which is then substituted by thymine after two rounds of dnA replication. The major cause of the high mutation rate at Cpg dinucleotides is likely to be spontaneous deamination of 5mC.58
ClInICAl RElEvAnCE oF dnA
METHylATIon
the diagnosis of cancer, it can provide prognostic information about the cancer and it can offer a potential means for cancer therapy.
diagnosis of Early Cancers
one key to improving the clinical outcome in patients with cancer is the urgent need to diagnose the disease at its earliest possible stage, which translates into a survival benefit for the patient. If diagnosis is possible before extensive local invasion, lymph node spread, or disseminated disease, then the surgical resection can be less radical, with fewer complications and side effects. This forms the rationale for population screening and the surveillance of high-risk patients.
If methylation of gene promoter regions does prove to be a consistent and early event in the incipient cancer cells, perhaps with specific gene combinations for different cancers, a potential tool for diagnosing premalignant lesions could become available. This may provide the means for a more accurate screening and surveillance rationale by identifying higher-risk patients on a molecular basis. It would also provide justification for more definitive treatment of patients who have molecular but not yet all the typical pathologic or microscopic features associated with frank malignancy.
The position of Cpg island promoter methylation is constant within an individual gene. potentially then, for all patients, a single primer strategy can be used to detect tumor-specific methylation changes in a given gene by methylation-specific pCR procedures.59 Such assays could be applied to dnA obtained from distal sites such as serum, urine or sputum, even without knowing methylation status of the marker directly in primary tumor dnA. These characteristics of promoter hypermethylation renders this to be a valuable dnA marker for early tumour detection or identification of high risk individuals.
predicting outcome and Monitoring
progress
Staging of tumors based on the levels and pattern distribution of dnA methylation may provide a convenient way to assess a tumor’s biologic aggressiveness and to predict patient response. Methylation and inactivation of genes essential to
regulation of specific vital cell proliferation events would be associated with a particular behavioural trait of a tumor, such as the ability to form distant metastases.
The presence of free tumor dnA in the serum of patients has been recognized as a potential means of monitoring the efficacy of cancer therapy.60 Although genetic defects in the dnA specific to the tumor of origin can be identified and sometimes correlated with clinical parameters, the process is expensive and time-consuming and may not be a reliable reflection of the state of the disease.61-63 Abnormal gene promoter region methylation patterns within circulating serum tumor dnA from patients with breast, lung, liver and head and neck tumors have recently been identified.64-67 This provides a rapid, quantitative and less expensive biologic marker. Subsequent clinical correlation will determine whether this approach has the sensitivity to be a useful molecular serum marker. If so, methylation patterns of circulating dnA released by the tumor may provide a means for monitoring the progress of a tumor and its response to therapy.
Tailored Therapeutic options
Information about how a cancer develops through molecular events could allow a clinician to predict more accurately how such a cancer is likely to respond to specific chemotherapeutic agents. In this way, a regimen tailored to the individual patient and based on knowledge of the tumor’s chemosensitivity could be designed.
As our understanding of tumor biology increases, the genes involved in different intracellular biochemical reactions specific to individual cancers will be identified. The methylation profile of such genes could be used to predict the efficacy of therapy designed to interrupt these pathways. Reversal of abnormal methylation patterns would seem an attractive and logical therapeutic means of arresting cancer growth or spread or even obliterating a neoplasm. Several molecular study groups are investigating the transcriptional failure that accompanies dnA methylation and with this understanding will come several potential targets for engineering novel pharmacologic weapons against cancer.
agents such as 5-azacytidine was being tried as chemotherapy agents. Several years of study have documented some efficacy in treating hematopoietic malignancies, as shown by studies done by lubbert and Momparler et.al.68, 69
The recurrence of malignancy or the resistance to current therapies is one of the greatest concerns in the treatment of ovarian cancer. In particular, epigenetic inhibitors hold promise for overcoming chemoresistance in ovarian cancer through the restoration of drug response genes and pathways.70 It was shown that the dnMT inhibitor, decitabine, decreased cisplatin resistance in both ovarian cancer cells and a mouse xenograft through demethylation of the hMlH1 promoter.43 A combination of decitabine and belinostat treatments showed greater cisplatin sensitization of a platinum-resistant mouse xenograft than either single treatment alone.71 Based on the preclinical results of dnMT inhibitors or HdAC inhibitors, epigenetic drugs are undergoing clinical trial investigations for the treatment of recurrent resistant ovarian cancer.72
ConCluSIon
Cancer is a polygenetic disease, but it is also a polyepigenetic disease. We cannot understand the dynamics and plasticity of cancer cells if we do not invoke epigenetic changes. given the high
mortality rate for ovarian cancer patients due to difficulties in early detection and recurrence after current chemotherapy, the identification of promising therapeutic targets for molecular targeted therapy as well as the identification of relevant biomarkers for early detection is an immediate and crucial goal. growing evidence supports the importance of epigenetic changes in tumorigenesis as much as the classical and better known, genetic changes.
With the development of high-throughput genomic/ epigenomic approaches, it is now possible to identify the global differences between cancers and normal tissues. Thus, a comprehensive understanding of both the genomics and epigenomics of ovarian cancer through the use of integrated approaches should make it possible to identify epigenetically activated oncogenes. This will facilitate the identification of more significant biomarkers and promising novel therapeutic targets for ovarian cancer and consequently, will contribute to the improved detection of ovarian cancer and its treatment.
The profile of Cpg island hypermethylation is specific to the tumor type, opening the avenue for its use as a biomolecular marker of the disease. An issue strengthened by the development of automatic pCR-based technologies is the easy detection of cancer lesions. But more questions continue to arise: What is the real contribution of dnA hypermethylation to tumorigenesis? Why are some tumor suppressor genes
Figure 6: Diagnostic, Prognostic and Pharmacodynamic Biomarkers
more prone to be hypermethylated than others? Are there any genetic disruptions prompting some of the dnA methylation changes observed or is it is the other way around? Will we ever find/create a dnA demethylating agent specific for the hypermethylated tumor suppressor genes? With the realization of the Human Epigenome project, we stand a good chance at answering the above questions. Furthermore, an exhaustive annotation of all deoxycytosine and histone modifications throughout the human genome, could allow the establishment of epigenetic cancer diagnostic, prognostic and pharmacodynamic biomarkers (Fig 6).73-77 In summary, a greater understanding of the role of epigentics in cancer will allow for improved interventions against this devastating malignancy.
ACKnoWlEdgEMEnT
We thank Bhaskari J for her thoughtful review of the manuscript and suggestions.
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