1. Introduction
Ovarian cancer is the leading cause of gynecologiccancer death, while constituting only 3% of all female cancers(1). Although the exact cause ofovarian malignancies remains unknown, the fact that >50% ofdeaths occur in postmenopausal women aged 55–74 years, suggests ahormonal risk. Due to the lack of specific symptoms in early stage,70% of cases are not diagnosed until the cancer has reached anadvanced stage, FIGO Stages IIB to IV (spread of tumor within thepelvis or elsewhere in the abdomen) (2). Early detection of ovarian cancerreportedly increases the five-year survival rate by up to 92%;however, the actual overall five-year survival rate is only 15–45%(3). Despite advances in cancerresearch and treatment, these survival statistics have remainedlargely unchanged for many years. The lack of early detectionmarkers and the development of drug resistance followingchemotherapy, are the main obstacles to effective treatmentstrategies. A better understanding of the molecular pathogenesis ofovarian cancer is needed in order to develop new drug therapies ordiagnostic biomarkers and elucidate the role of environmentalexposures to the individual’s predisposition to the disease.
Ovarian epithelial carcinoma (OEC) is the mostcommon ovarian malignancy, with substantial histopathologicalheterogeneity. According to the 2003 World Health Organizationclassification scheme, the most common histologic subtype is serousovarian carcinoma (~60%), while other subtypes include endometrioid(10–20%), clear cell (10%), transitional (6%), mucinous (<5%),and undifferentiated (<1%) subtypes (4). The underlying genetic basis ofovarian cancer contributes to this heterogeneity. The majority ofOECs (90%) are sporadic, with the remaining OECs being inherited.Inherited ovarian cancers account for 5–10% of all ovarian cancersand are characterized by the development of highly aggressiveneoplasms at an earlier age of onset than their sporadiccounterparts (4). Mutations ofBRCA1 andBRCA2 tumor suppressor genes areresponsible for most hereditary ovarian cancers. The two genes areessential for DNA repair and play integral roles in genomicstability and integrity (5).
A number of studies (6–8) havereported the use of the candidate gene approach in the search forcommon risk variants associated with ovarian cancer. Identificationof common genetic susceptibility alleles may lead to a greaterunderstanding of disease etiology, potentially leading to geneticscreening approach that could be used to identify the proportion ofthe population that would benefit from screening. Genes have beenselected from relevant biological pathways, steroid hormonemetabolism, DNA repair, apoptosis and cell cycle control, as wellas known oncogenes and tumor suppressor genes. However, the genesthat participate in the development of ovarian cancer representonly a small portion of the ovarian cancer-associated genes, asmany of them are merely associated with ovarian cancer developmentbut do not contribute to its initiation and progression. Moreover,molecular pathways in different ovarian tumors may varysignificantly. Thus, genetic alterations alone cannot account forthe complexity of ovarian cancer. Since genetic factors are almostimpossible to reverse, the potential reversibility of epigeneticmechanisms makes them attractive candidates for the preventionand/or treatment of ovarian carcinoma (9–11).
Epigenetic mechanisms are heritable changes in geneexpression without altering the primary DNA sequence (12). Epigenetics involves the interplaybetween DNA methylation, histone modifications and expression ofnon-coding RNAs in the regulation of gene transcription (13). Increasing evidence has shown thatepigenetic alterations including DNA methylation play a significantrole in cancer, from the silencing of tumor suppressors to theactivation of oncogenes and the promotion of metastasis (14). DNA methylation is a key element intissue differentiation during early embryonic development. Thediversion of a normal cell cycle to those of a less differentiatedstatus comprises one of the initial steps of tumorigenesis(15). Aberrant DNA methylation isnow recognized as one of the most common molecular abnormalities incancer frequently associated with drug resistance (14).
DNA methylation comprises the best known epigeneticmechanism associated with gene expression. DNA methylation occurson the cytosine residues of CG (also designated as CpG)dinucleotides. Enzymes known as DNA methyltransferases (DNMTs)catalyse the addition of a methyl group to the cytosine ring toform methyl cytosine, employing S-adenosylmethionine as a methyldonor (16). In humans and othermammals, DNA modification occurs predominantly on cytosines thatprecede a guanosine in the DNA sequence (16). These dinucleotides can be clusteredin small stretches of DNA, termed CpG islands, which are oftenassociated with promoter regions. Most CpG sites outside the CpGislands are methylated, suggesting a role in the global maintenanceof the genome, while most CpG islands in gene promoters areunmethylated, which allows active gene transcription (16,17).Generally, when a given stretch of cytosines in a CpG islandlocated in the promoter region of a gene is methylated, that geneis silenced by methylation, and such a CpG island would be termed‘hypermethylated’. Conversely, when a given stretch of cytosines ina CpG island located in the promoter region of a gene is notmethylated, that gene is not silenced by methylation, and the CpGisland in this case would be ‘hypomethylated’ (18). Methylation of promoters inhibitstheir recognition by transcription factors and RNA polymerase, asmethylated cytosines preferentially bind to a protein known asmethyl cytosine binding protein, or MeCP. When a promoter regionnormally recognized by an activating transcription factor, ismethylated, its transcription is inhibited (19).
The DNA methylation profile of a tumor cell is areflection of its somatic lineage, environmental exposure andgenetic predisposition. The DNA methylation profile is thereforedistinct for each histological subtype, suggesting differenttumorigenic mechanisms. The detection of the epigenetic signatureof each cancer cell may be useful in the identification ofcandidate biomarkers for disease detection, classification andmonitoring and facilitate personalized cancer treatment.
2. DNA methylation in ovarian cancer
In ovarian cancer, in addition to othernon-gynaecological cancers, two opposite epigenetic phenomenaoccur: i) An overall global decrease in DNA methylation ofheterochromatin leading to demethylation of several oncogenes, ii)specific CpG island hypermethylation associated with the promotersof tumor suppressor genes (9,20–22)(Fig. 1). The aberrant methylationof CpG islands in gene promoters has been correlated with a loss ofgene expression, and it appears that DNA methylation provides analternative pathway to gene deletion or mutation for the loss oftumor suppressor gene (TSG) function (23). The epigenetic silencing of TSGinduces such mechanisms as uncontrolled cell division, the abilityto infiltrate surrounding tissues, metastasis, avoiding apoptosisor sustaining angiogenesis, all of which are responsible forpromoting tumor development. In ovarian cancer, a large number ofTSGs have been found to undergo hypermethylation (24–26).
One of the most studied genes in ovarian cancer isbreast cancer early onset gene 1 (BRCA1) gene, due to itsrole in inherited and sporadic forms of the disease (27,28).BRCA1 is important in maintaining genomic stability(29), and interacts with numerousproteins, forming complexes that are involved in recognizing andsubsequently repairing DNA. Evidence suggests that in cases ofsporadic ovarian cancer promoter hypermethylation, non-somaticmutation is the cause forBRCA1 inactivation (30). Aberrant methylation of the genepromoter may also serve as an alternative explanation for the lossof heterozygosity associated withBRCA1 deficiency inovarian carcinomas (31). Completeor partial inactivation of the BRCA1 gene through hypermethylationof its promoter has been reported in 15% of sporadic ovarian tumors(27,32). Hypermethylation leads to thesilencing of this gene in ovarian tumors and levels of methylationcorrelated with decreasedBRCA1 expression (33,34).Compared to stage I and healthy subjects, there were higherBRCA1 promoter methylation frequencies in stage II and IIIovarian cancers (34). In a seriescomparing the methylation status ofBRCA1 among tumorsamples obtained from patients with benign ovarian tumors,borderline tumors as well as carcinomas, promoter methylation wasdetected in 31% of carcinomas but in none of the benign orborderline tumors (35).Hypermethylation ofBRCA1 was detected at a significantlyhigher frequency in serous carcinomas than in tumors of the otherhistological types (36). Of note,methylation ofBRCA1, while frequent in sporadic ovariancancer, it has not been reported in the hereditary type of thedisease, nor in samples from women with a germ-lineBRCA1mutation (37,38).BRCA2 does not exhibit asimilar methylation profile in ovarian cancer (39). Findings of previous studies haveshown that methylated CpGs at theBRCA2 promoter were eitherabsent or at very low levels in tumor DNA compared to normaltissues (33).
A number of other classical TSGs have been found toundergo hypermethylation in cases of ovarian cancer. Tumorsuppressor genes involved in DNA mismatch repair (MMR) have adistinct carcinogenic mechanism in ovarian tumors. DNA MMR is anendogenous molecular mechanism that reverses replication errorsthat escape correcting by replicative DNA polymerases. InMMR-defective cells, both base-to-base mismatches andinsertion/deletion loops, are left uncorrected (40). This results in increasedspontaneous somatic mutations. This effect is particularly obviousin non-expressed sequences comprising multiple simple repeats(microsatellites), and the characteristic microsatelliteinstability (MSI) is diagnostic for MMR-defective tumors (41,42).Approximately 10% of ovarian cancers are related to this molecularpathway (43). Defective MMR isoften a consequence of germ-line mutations in thehMLH1,hMSH2,MGMT or, occasionally,MSH6 orPMS2 genes. Hypermethylation of theMLH1 geneaccompanied by loss of the gene expression has been reported in10–30% of ovarian malignancies, while in cases with acquiredresistance to platinum-based chemotherapy,hMLH1 promotermethylation has been identified in 56% of cases (44,45).The methylation frequency ofhMSH2 promoters has beenreported to be as high as 57% in ovarian cancers. Methylation ofhMSH2 correlated with histological grade and lymphatic metastasis.Additionally, the methylation rates of hMSH2 were significantlyhigher in endometrioid adenocarcinoma tissues compared to otherpathological types of the disease (44).
RAS association domain family protein 1a (RASSF1A)which is an inhibitor of the anaphase-promoting complex, togetherwithOPCML, are among the most frequently methylated genesin ovarian cancer (46,47). Genes involved in cell cyclepathways such as p16 and p15 have also been affected by alteredmethylation of their promoters (48). E-cadherin is a transmembraneglycoprotein that mediates calcium-dependent interactions betweenadjacent epithelial cells. It has been found that the risk ofE-cadherin hypermethylation was 1.347-fold among patients withovarian cancer than that among patients with benign ovarian lesions(48). Other genes involved incell adherence, such as H-cadherin and CDH1, have shown similarresults (49). HSulf-1, whichencodes an arylsulfatase that acts on cell surface heparin sulfateproteoglycans and inhibits growth factor signalling, was found tobe methylated in >50% of ovarian tumors and cell lines (50).
Methylation profiles of several genes belonging inthe family of the Homeobox (HOX) genes have also been investigatedin cases of ovarian carcinomas. Homeobox genes constitute a familyof transcription factors that function during embryonic developmentto control pattern formation, differentiation, and proliferation(51).HOX genes areexpressed in normal adult reproductive tissue where they areinvolved in regulating differentiation. Findings of previousstudies suggest that the abnormal expression of particularHOX genes is associated with ovarian cancers (52). Methylation of theHOXA9 genehas been observed in 95% of patients with high grade serous ovariancarcinoma (53). It has beensuggested that the methylation status ofHOXA9 andHOXAD11 genes may serve as potential diagnostic andprognostic biomarkers (53,54).
The majority of studies assessing the methylationstatus of TSGs have focused on single genes with varying reportedfrequencies in different tissues. Hypermethylation in ovariancancer, however, has been found to be associated with theinactivation of almost every pathway involved in ovarian cancerdevelopment, including DNA repair, cell cycle regulation,apoptosis, cell adherence and detoxification pathways (32,38,55–58).
In addition to the hypermethylation ofpromoter-associated CpG islands, global hypomethylation andspecific hypomethylation of protein expressed genes thatsubsequently become overexpressed plays a significant role inovarian cancer. Hypomethylation in the centromere and subtelomericregions is involved in the induction of genomic instability (GI),leading to chromosomal translocations and gene disruption throughthe reactivation of transposable elements (21). Decreased methylation of LINE-1elements is correlated with high grade, advanced stage and poorprognosis in ovarian cancer patients (59). Satellite DNA hypomethylation is anindependent marker of poor prognosis. Hypomethylation is increasedfrom non-neoplastic tissue toward ovarian cancer as well asadvanced grade and stage (60).
In addition to repetitive elements and DNAsatellites, a number of protein-coding genes are overexpressed inovarian cancer, in association with promoter hypomethylation.Several oncogenes have been reported to have an increasedepigenetically induced expression. Oncogenes such asCLDN4(encoding an integral component of tight junctions),MAL(mal, T-cell differentiation protein) andBORIS (brother ofthe regulator of imprinted sites) belong to a number of oncogenesthat contribute to drug resistance and are associated with overallprognosis of the disease (61–63).Upregulation, together with hypomethylation of the ABCG2 multidrugtransporter andTUBB3 genes, which is a determinant oftaxane resistance, have been observed in cases of advanced ovariancarcinoma with drug-acquired chemoresistance (64,65).Other cancer-associated genes includingMCJ (66,67)andSNGG (synucelin-γ), encoding an activator of the MAPKand Elk-1 signaling cascades (63,68),are upregulated in ovarian cancer in association with DNAhypomethylation.
3. Diagnosis
Since aberrant methylation is one of the earliestmolecular alterations during tumorigenesis, it has been suggestedas a promising strategy for the early detection of ovarian cancer.However, methylation of single genes may have limited value inclinical applications. At present, no single epigenetic biomarkeris able to accurately detect early ovarian cancer in either tissueor body fluids. Analysis of the methylation status of multiplegenes simultaneously in a blood-based assay may provide a moresensitive and specific method for the molecular classification andprognosis of ovarian cancer.
A genome-wide DNAm profiling of a large ovariancancer case control cohort demonstrated that active ovarian cancerhas a significant impact on the DNAm pattern in peripheral blood(69). A microarray-based analysison ovarian tumors identified 112 methylated loci prognostic forprogression-free survival in advanced ovarian cancer patients(70). The data suggested that ahigher degree of CpG island methylation is associated with earlydisease recurrence following chemotherapy (71). Promoter hypermethylation of atleast one of six genes (BRCA1,RASSF1A,APC,p14ARF,p16INK4A andDAPK) was observed in41/50 ovarian cancer serum specimens. Thus, hypermethylation ofcertain genes may present an early event in ovarian tumorigenesisthat can be detected in the serum DNA from patients withovary-confined (stage IA or B) tumors and in cytologically negativeperitoneal fluid (56). A recentstudy that used multiplex methylation-specific PCR to analyze themethylation status of cell-free serum DNA of seven candidate genes(APC,RASSF1A,CDH1,RUNX3,TFPI2,SFRP5 andOPCML), achieved asensitivity and specificity of 85.3 and 90.5%, respectively, instage I OEC. The detection rates were markedly higher compared witha single CA125, which produced a sensitivity of 56.1% at 64.15%specificity (72). Another studydemonstrated notable detection sensitivities and specificitiesusing a 10-gene panel in plasma (73).
The role of DNA methylation biomarkers in ovariancancer is promising. However, progression towards clinical practiceis hampered by the lack of detection techniques combining highaccuracy with low cost. The main obstacles that are to be overcomeare the standardization of analysis techniques and establishment ofreliable reference values.
4. Treatment
Chemoresistance
The current chemotherapy strategy in treatingovarian cancer patients involves a combination of a platinum- and ataxane-based therapy. While most ovarian cancer patients respondcompletely to chemotherapy, the majority of the initial responderseventually develop chemoresistance (74). In addition to mutations, DNAmethylation-induced silencing of various drug response genes andpathways also facilitates the development of ovarian tumor celldrug resistance (75). It wasshown that the silencing ofSFRP5, which is a Wntantagonist, by DNA hypermethylation was associated with platinumresistance of ovarian cancer (76). Similarly, hypermethylation ofseveral genes such ashMLH1, the argininebiosynthesis-related geneASS1, andESR2 (encodingthe ER-b) are involved in platinum resistance (77–79).Platinum resistance has also been correlated with stage-progressivehypermethylation of the Methylation Controlled DNAJ (MCJ)gene which resulted in loss of gene expression and correlated witha poor response to chemotherapy (67). DAPK, which is a gene involved inapoptosis, has also been shown to be silenced in drug-resistantcancer due to methylation (80).
In addition to the loss of expression due to DNAmethylation, it was shown that hypomethylation along with anincrease in expression of the myelin and lymphocyte protein(MAL) gene is associated with platinum resistance (62). Hypomethylation and upregulation oftheABCG2 multidrug transporter gene was also shown to occurduring chemoresistance in two ovarian carcinoma cell lines(81). Based on the association ofDNA methylation of specific genes with platinum sensitivity, it wasshown that the hypomethylation-mediated activation of the cellgrowth-promoting pathways, PI3K/Akt, TGF-β and cell cycleprogression, may contribute to cisplatin resistance in ovariancancer cells (82).
At present, only two biomarkers of protein origin(CA125 and HE4) are considered as indicators of response tochemotherapy. Epigenetic markers may supplement these proteinspossibly by increasing their sensitivity and specificity. DNAmethylation biomarkers in particular, have several advantages overother biomarkers such as proteins, gene expression and DNAmutations, since they are stable, can easily be distinguished, andcan be detected in specific DNA regions (CpG islands) (83). In the future, the overall DNAmethylation profile of the resected ovarian tumor may may be usedfor the development of individually tailored treatment regimens(84).
Epigenetic therapy
Unlike cancer-associated gene mutations, DNAmethylation and other epigenetic modifications are potentiallyreversible. This makes epigenetic agents attractive candidates fordisease prevention and resensitization to chemotherapeutic agents.Demethylation of tumor suppressor genes may have a positive effectin cancer progression, whereas the decrease of methylation ofoncogenes which reactivate these genes, may have an adverse effect.There are two types of DNA methylation inhibitors: nucleoside andnon-nucleoside analogues. Nucleoside analogues inhibit methylationwhen they are integrated into DNA and block the release of DNMTs byforming a covalent complex with these enzymes (85). They have been found to haveclinical activities especially on hematopoietic malignancies(86–88). These inhibitors have been used toinduce the re-expression of silenced TSGs caused byhypermethylation. Although aberrant promoter methylation iscorrected by DNA methylation inhibitors, when the drug is stopped,the aberrant methylation and gene silencing is re-established(16). Non-nucleoside analoguesare thus small molecular inhibitors that bind to the catalyticregion of DNMTs and suppress translation.
Azacytidine and decitabine are the first two DNMTinhibitors approved for the therapy of myelodysplastic syndromes(13,31). Decitabine, a potent methylationinhibitor, has been shown to cause demethylation in numerousovarian cell lines, reversing the silencing of several TSGs(89,90). Decitabine has also been reported todecrease cisplatin resistance in both ovarian cancer cells and amouse xenograft through demethylation of thehMLH1 promoter(91). Two clinical trials haveprovided evidence that azacytidine and decitabine are capable ofreversing platinum resistance in ovarian cancer patients (92,93).However, DNMT inhibitors may simultaneously cause widespreadgenomic hypomethylation that potentially leads to genomicinstability (94).
Histone deacetylation is a well-known epigeneticmechanism that also contributes to silencing of TSGs in cancer.While HDACIs and DNMTIs have demonstrated clinical activity assingle-agent therapies for hematopoietic malignancies, DNAmethylation and histone deacetylation often co-ordinately inhibitgene transcription, and restoration of the two silencing mechanismsmay be necessary for maximal gene derepression (13). Treatment with a DNMTI/HDACIcombination, in ovarian cancer cases, was synergistic forupregulation of the pro-apoptotic geneTMS1/ASC, incontrast to either agent alone (95). An earlier integrated microarrayanalysis demonstrated that a DNMTI/HDACI-combined treatment ofovarian cancer cells affects more genes that either agentindividually (96). Conventionalchemotherapy together with methylation inhibitors have also beenexamined in phase I/II clinical trials. Decitabine in combinationwith carboplatin demonstrated no significant improvement overplatinum alone in an ovarian cancer study (97). Another similar study that useslow-dose decitabine plus carboplatin resulted in more diseaseresponses and establishedin vivo biological activity inblood and tumor specimens of ovarian cancer patients (93). Carboplatin when combined with5-azacytidine also showed encouraging results (92).
5. Conclusion
Epigenetic alterations such as DNA methylation areclearly involved in ovarian cancer initiation and progression.Global DNA hypomethylation and localized hypermethylation ofspecific gene promoters contribute to genome instability andtranscriptional silencing of tumor suppressor genes, respectively.Early studies focused on the methylation patterns of single genesassociated with tumorigenesis. However, newer genome-wide methodshave identified a group of genes whose regulation is altered by DNAmethylation during ovarian cancer progression. The profiling of DNAmethylomes may provide new insight into the development ofbiomarkers with clinical value for cancer risk assessment, earlydetection, prevention and prognosis. Therapeutic agents that targetmethylation are already being tested for future use and have provenbeneficial in other types of malignancies. This is an exciting andrapidly evolving area of research in which investigations may leadto the possible detection of interindividual drug responsedifferences and their reversal.
References
1 | Hennessy BT, Coleman RL and Markman M:Ovarian cancer. Lancet. 374:1371–1382. 2009.View Article :Google Scholar :PubMed/NCBI |
2 | Jemal A, Siegel R, Ward E, Hao Y, Xu J andThun MJ: Cancer statistics, 2009. CA Cancer J Clin. 59:225–249.2009.View Article :Google Scholar |
3 | Bookman MA, Brady MF, McGuire WP, et al:Evaluation of new platinum-based treatment regimens inadvanced-stage ovarian cancer: a Phase III Trial of the GynecologicCancer Intergroup. J Clin Oncol. 27:1419–1425. 2009.View Article :Google Scholar :PubMed/NCBI |
4 | Claus EB, Schildkraut JM, Thompson WD andRisch NJ: The genetic attributable risk of breast and ovariancancer. Cancer. 77:2318–2324. 1996.View Article :Google Scholar :PubMed/NCBI |
5 | Antoniou A, Pharoah PD, Narod S, et al:Average risks of breast and ovarian cancer associated with BRCA1 orBRCA2 mutations detected in case Series unselected for familyhistory: a combined analysis of 22 studies. Am J Hum Genet.72:1117–1130. 2003.ViewArticle :Google Scholar |
6 | Tian C, Ambrosone CB, Darcy KM, Krivak TC,Armstrong DK, Bookman MA, Davis W, Zhao H, Moysich K, Gallion H andDeLoia JA: Common variants in ABCB1, ABCC2 and ABCG2 genes andclinical outcomes among women with advanced stage ovarian cancertreated with platinum and taxane-based chemotherapy: a GynecologicOncology Group study. Gynecol Oncol. 124:575–581. 2012.View Article :Google Scholar |
7 | Darcy KM, Brady WE, Blancato JK, DicksonRB, Hoskins WJ, McGuire WP and Birrer MJ: Prognostic relevance ofc-MYC gene amplification and polysomy for chromosome 8 insuboptimally-resected, advanced stage epithelial ovarian cancers: aGynecologic Oncology Group study. Gynecol Oncol. 114:472–479. 2009.View Article :Google Scholar |
8 | Tuefferd M, Couturier J, Penault-Llorca F,Vincent-Salomon A, Broët P, Guastalla JP, Allouache D, Combe M,Weber B, Pujade-Lauraine E and Camilleri-Broët S: HER2 status inovarian carcinomas: a multicenter GINECO study of 320 patients.PLoS One. 2:e11382007.View Article :Google Scholar :PubMed/NCBI |
9 | Baylin SB and Ohm JE: Epigenetic genesilencing in cancer - a mechanism for early oncogenic pathwayaddiction? Nat Rev Cancer. 6:107–116. 2006.View Article :Google Scholar :PubMed/NCBI |
10 | Ushijima T and Asada K: Aberrant DNAmethylation in contrast with mutations. Cancer Sci. 101:300–305.2010.View Article :Google Scholar :PubMed/NCBI |
11 | Yoo CB and Jones PA: Epigenetic therapy ofcancer: past, present and future. Nat Rev Drug Discov. 5:37–50.2006.ViewArticle :Google Scholar :PubMed/NCBI |
12 | Russo VEA, Riggs AD and Martienssen RA:Epigenetic mechanisms of gene regulation. Cold Spring HarborLaboratory Press; Plainview, NY: 1996 |
13 | Jones PA and Baylin SB: The epigenomics ofcancer. Cell. 128:683–692. 2007.View Article :Google Scholar :PubMed/NCBI |
14 | Wilting RH and Dannenberg JH: Epigeneticmechanisms in tumorigenesis, tumor cell heterogeneity and drugresistance. Drug Resist Updat. 15:21–38. 2012.View Article :Google Scholar :PubMed/NCBI |
15 | Koukoura O, Sifakis S and Spandidos DA:DNA methylation in the human placenta and fetal growth (Review).Mol Med Rep. 5:883–839. 2012.PubMed/NCBI |
16 | Herman JG and Baylin SB: Gene silencing incancer in association with promoter hypermethylation. N Engl J Med.349:2042–2054. 2003.View Article :Google Scholar :PubMed/NCBI |
17 | Weber M and Schubeler D: Genomic patternsof DNA methylation: targets and function of an epigenetic mark.Curr Opin Cell Biol. 19:273–280. 2007.View Article :Google Scholar :PubMed/NCBI |
18 | Bird AP and Wolffe AP: Methylation-inducedrepression - belts, braces, and chromatin. Cell. 99:451–454. 1999.View Article :Google Scholar :PubMed/NCBI |
19 | Costello JF and Plass C: Methylationmatters. J Med Genet. 38:285–303. 2001.View Article :Google Scholar :PubMed/NCBI |
20 | Catteau A, Harris WH, Xu CF and Solomon E:Methylation of the BRCA1 promoter region in sporadic breast andovarian cancer: correlation with disease characteristics. Oncogene.18:1957–1965. 1999.View Article :Google Scholar :PubMed/NCBI |
21 | Esteller M: Epigenetics in cancer. N EnglJ Med. 358:1148–1159. 2008.View Article :Google Scholar |
22 | Esteller M, Silva JM, Dominguez G, et al:Promoter hypermethylation and BRCA1 inactivation in sporadic breastand ovarian tumors. J Natl Cancer Inst. 92:564–569. 2000.View Article :Google Scholar :PubMed/NCBI |
23 | Baylin SB and Chen WY: Aberrant genesilencing in tumor progression: implications for control of cancer.Cold Spring Harb Symp Quant Biol. 70:427–433. 2005.View Article :Google Scholar :PubMed/NCBI |
24 | Horak P, Pils D, Haller G, et al:Contribution of epigenetic silencing of tumor necrosisfactor-related apoptosis inducing ligand receptor 1 (DR4) to TRAILresistance and ovarian cancer. Mol Cancer Res. 3:335–343. 2005.View Article :Google Scholar |
25 | Petrocca F, Iliopoulos D, Qin HR, et al:Alterations of the tumor suppressor gene ARLTS1 in ovarian cancer.Cancer Res. 66:10287–10291. 2006.View Article :Google Scholar :PubMed/NCBI |
26 | Yu Y, Fujii S, Yuan J, et al: Epigeneticregulation of ARHI in breast and ovarian cancer cells. Ann NY AcadSci. 983:268–277. 2003.View Article :Google Scholar :PubMed/NCBI |
27 | Baldwin RL, Nemeth E, Tran H, et al: BRCA1promoter region hypermethylation in ovarian carcinoma: apopulation-based study. Cancer Res. 60:5329–5333. 2000.PubMed/NCBI |
28 | Hilton JL, Geisler JP, Rathe JA,Hattermann-Zogg MA, DeYoung B and Buller RE: Inactivation of BRCA1and BRCA2 in ovarian cancer. J Natl Cancer Inst. 94:1396–1406.2002.View Article :Google Scholar :PubMed/NCBI |
29 | Li ML and Greenberg RA: Links betweengenome integrity and BRCA1 tumor suppression. Trends Biochem Sci.37:418–424. 2012.View Article :Google Scholar :PubMed/NCBI |
30 | McCoy ML, Mueller CR and Roskelley CD: Therole of the breast cancer susceptibility gene 1 (BRCA1) in sporadicepithelial ovarian cancer. Reprod Biol Endocrinol. 1:722003.View Article :Google Scholar :PubMed/NCBI |
31 | Rzepecka IK, Szafron L, Stys A, et al:High frequency of allelic loss at the BRCA1 locus in ovariancancers: clinicopathologic and molecular associations. CancerGenet. 205:94–100. 2012.View Article :Google Scholar :PubMed/NCBI |
32 | Strathdee G, Appleton K, Illand M, et al:Primary ovarian carcinomas display multiple methylator phenotypesinvolving known tumor suppressor genes. Am J Pathol. 158:1121–1127.2001.View Article :Google Scholar |
33 | Chan KY, Ozcelik H, Cheung AN, Ngan HY andKhoo US: Epigenetic factors controlling the BRCA1 and BRCA2 genesin sporadic ovarian cancer. Cancer Res. 62:4151–4156.2002.PubMed/NCBI |
34 | Wang YQ, Yan Q, Zhang JR, Li SD, Yang YXand Wan XP: Epigenetic inactivation of BRCA1 through promoterhypermethylation in ovarian cancer progression. J Obstet GynaecolRes. 39:549–554. 2013.View Article :Google Scholar :PubMed/NCBI |
35 | Wang C, Horiuchi A, Imai T, et al:Expression of BRCA1 protein in benign, borderline, and malignantepithelial ovarian neoplasms and its relationship to methylationand allelic loss of the BRCA1 gene. J Pathol. 202:215–223. 2004.View Article :Google Scholar :PubMed/NCBI |
36 | Yang HJ, Liu VW, Wang Y, Tsang PC and NganHY: Differential DNA methylation profiles in gynecological cancersand correlation with clinico-pathological data. BMC Cancer.6:2122006.View Article :Google Scholar :PubMed/NCBI |
37 | Bol GM, Suijkerbuijk KP, Bart J, Vooijs M,van der Wall E and van Diest PJ: Methylation profiles of hereditaryand sporadic ovarian cancer. Histopathology. 57:363–370. 2010.View Article :Google Scholar :PubMed/NCBI |
38 | Rathi A, Virmani AK, Schorge JO, et al:Methylation profiles of sporadic ovarian tumors and nonmalignantovaries from high-risk women. Clin Cancer Res. 8:3324–3331.2002.PubMed/NCBI |
39 | Kontorovich T, Cohen Y, Nir U and FriedmanE: Promoter methylation patterns of ATM, ATR, BRCA1, BRCA2 and p53as putative cancer risk modifiers in Jewish BRCA1/BRCA2 mutationcarriers. Breast Cancer Res Treat. 116:195–200. 2009.View Article :Google Scholar :PubMed/NCBI |
40 | Blasi MF, Ventura I, Aquilina G, et al: Ahuman cell-based assay to evaluate the effects of alterations inthe MLH1 mismatch repair gene. Cancer Res. 66:9036–9044. 2006.View Article :Google Scholar :PubMed/NCBI |
41 | Jiricny J and Nyström-Lahti M: Mismatchrepair defects in cancer. Curr Opin Genet Dev. 10:157–161. 2000.View Article :Google Scholar |
42 | Kunkel TA and Erie DA: DNA mismatchrepair. Annu Rev Biochem. 74:681–710. 2005.View Article :Google Scholar :PubMed/NCBI |
43 | Murphy MA and Wentzensen N: Frequency ofmismatch repair deficiency in ovarian cancer: a systematic review.This article is a US Government work and, as such, is in the publicdomain of the United States of America. Int J Cancer.129:1914–1922. 2011.View Article :Google Scholar |
44 | Zhang H, Zhang S, Cui J, Zhang A, Shen Land Yu H: Expression and promoter methylation status of mismatchrepair gene hMLH1 and hMSH2 in epithelial ovarian cancer. Aust N ZJ Obstet Gynaecol. 48:505–509. 2008.View Article :Google Scholar :PubMed/NCBI |
45 | Watanabe Y, Ueda H, Etoh T, et al: Achange in promoter methylation of hMLH1 is a cause of acquiredresistance to platinum-based chemotherapy in epithelial ovariancancer. Anticancer Res. 27:1449–1452. 2007. |
46 | Barton CA, Hacker NF, Clark SJ and O’BrienPM: DNA methylation changes in ovarian cancer: implications forearly diagnosis, prognosis and treatment. Gynecol Oncol.109:129–139. 2008.View Article :Google Scholar :PubMed/NCBI |
47 | Ozdemir F, Altinisik J, Karateke A,Coksuer H and Buyru N: Methylation of tumor suppressor genes inovarian cancer. Exp Ther Med. 4:1092–1096. 2012.PubMed/NCBI |
48 | Moselhy SS, Kumosani TA, Kamal IH, JalalJA, Abdul Jabaar HS and Dalol A: Hypermethylation of P15, P16, andE-cadherin genes in ovarian cancer. Toxicol Ind Health. Apr9–2013.(Epub ahead of print). |
49 | Dhillon VS, Young AR, Husain SA and AslamM: Promoter hypermethylation of MGMT, CDH1, RAR-beta and SYK tumoursuppressor genes in granulosa cell tumours (GCTs) of ovarianorigin. Br J Cancer. 90:874–881. 2004.View Article :Google Scholar :PubMed/NCBI |
50 | Staub J, Chien J, Pan Y, et al: Epigeneticsilencing of HSulf-1 in ovarian cancer: implications inchemoresistance. Oncogene. 26:4969–4978. 2007.View Article :Google Scholar :PubMed/NCBI |
51 | Samuel S and Naora H: Homeobox geneexpression in cancer: insights from developmental regulation andderegulation. Eur J Cancer. 41:2428–2437. 2005.View Article :Google Scholar :PubMed/NCBI |
52 | Kelly ZL, Michael A, Butler-Manuel S,Pandha HS and Morgan RG: HOX genes in ovarian cancer. J OvarianRes. 4:162011.View Article :Google Scholar |
53 | Montavon C, Gloss BS, Warton K, et al:Prognostic and diagnostic significance of DNA methylation patternsin high grade serous ovarian cancer. Gynecol Oncol. 124:582–588.2012.View Article :Google Scholar :PubMed/NCBI |
54 | Widschwendter M, Apostolidou S, Jones AA,et al: HOXA methylation in normal endometrium from premenopausalwomen is associated with the presence of ovarian cancer: a proof ofprinciple study. Int J Cancer. 125:2214–2218. 2009.View Article :Google Scholar :PubMed/NCBI |
55 | Swisher EM, Gonzalez RM, Taniguchi T, etal: Methylation and protein expression of DNA repair genes:association with chemotherapy exposure and survival in sporadicovarian and peritoneal carcinomas. Mol Cancer. 8:482009.View Article :Google Scholar |
56 | Ibanez de Caceres I, Battagli C, EstellerM, et al: Tumor cell-specific BRCA1 and RASSF1A hypermethylation inserum, plasma, and peritoneal fluid from ovarian cancer patients.Cancer Res. 64:6476–6481. 2004.PubMed/NCBI |
57 | Makarla PB, Saboorian MH, Ashfaq R, et al:Promoter hypermethylation profile of ovarian epithelial neoplasms.Clin Cancer Res. 11:5365–5369. 2005.View Article :Google Scholar :PubMed/NCBI |
58 | Tam KF, Liu VW, Liu SS, et al: Methylationprofile in benign, borderline and malignant ovarian tumors. JCancer Res Clin Oncol. 133:331–341. 2007.View Article :Google Scholar :PubMed/NCBI |
59 | Pattamadilok J, Huapai N, RattanatanyongP, et al: LINE-1 hypomethylation level as a potential prognosticfactor for epithelial ovarian cancer. Int J Gynecol Cancer.18:711–717. 2008.View Article :Google Scholar :PubMed/NCBI |
60 | Widschwendter M, Jiang G, Woods C, et al:DNA hypomethylation and ovarian cancer biology. Cancer Res.64:4472–4480. 2004.View Article :Google Scholar :PubMed/NCBI |
61 | Honda H, Pazin MJ, Ji H, Wernyj RP andMorin PJ: Crucial roles of Sp1 and epigenetic modifications in theregulation of the CLDN4 promoter in ovarian cancer cells. J BiolChem. 281:21433–21444. 2006.View Article :Google Scholar :PubMed/NCBI |
62 | Lee PS, Teaberry VS, Bland AE, et al:Elevated MAL expression is accompanied by promoter hypomethylationand platinum resistance in epithelial ovarian cancer. Int J Cancer.126:1378–1389. 2010.PubMed/NCBI |
63 | Woloszynska-Read A, James SR, Link PA, YuJ, Odunsi K and Karpf AR: DNA methylation-dependent regulation ofBORIS/CTCFL expression in ovarian cancer. Cancer Immun.7:212007.PubMed/NCBI |
64 | Izutsu N, Maesawa C, Shibazaki M, et al:Epigenetic modification is involved in aberrant expression of classIII β-tubulin, TUBB3, in ovarian cancer cells. Int J Oncol.32:1227–1235. 2008.PubMed/NCBI |
65 | Balch C, Matei DE, Huang TH and Nephew KP:Role of epigenomics in ovarian and endometrial cancers.Epigenomics. 2:419–447. 2010.View Article :Google Scholar :PubMed/NCBI |
66 | Strathdee G, Davies BR, Vass JK, SiddiquiN and Brown R: Cell type-specific methylation of an intronic CpGisland controls expression of the MCJ gene. Carcinogenesis.25:693–701. 2004.View Article :Google Scholar :PubMed/NCBI |
67 | Strathdee G, Vass JK, Oien KA, Siddiqui N,Curto-Garcia J and Brown R: Demethylation of the MCJ gene in stageIII/IV epithelial ovarian cancer and response to chemotherapy.Gynecol Oncol. 97:898–903. 2005.View Article :Google Scholar :PubMed/NCBI |
68 | Gupta A, Godwin AK, Vanderveer L, Lu A andLiu J: Hypomethylation of the synuclein gamma gene CpG islandpromotes its aberrant expression in breast carcinoma and ovariancarcinoma. Cancer Res. 63:664–673. 2003.PubMed/NCBI |
69 | Teschendorff AE, Menon U, Gentry-MaharajA, et al: An epigenetic signature in peripheral blood predictsactive ovarian cancer. PLoS One. 4:e82742009.View Article :Google Scholar :PubMed/NCBI |
70 | Wei SH, Balch C, Paik HH, et al:Prognostic DNA methylation biomarkers in ovarian cancer. ClinCancer Res. 12:2788–2794. 2006.View Article :Google Scholar :PubMed/NCBI |
71 | Wei SH, Chen CM, Strathdee G, et al:Methylation microarray analysis of late-stage ovarian carcinomasdistinguishes progression-free survival in patients and identifiescandidate epigenetic markers. Clin Cancer Res. 8:2246–2252.2002. |
72 | Zhang Q, Hu G, Yang Q, et al: A multiplexmethylation-specific PCR assay for the detection of early-stageovarian cancer using cell-free serum DNA. Gynecol Oncol.130:132–139. 2013.View Article :Google Scholar :PubMed/NCBI |
73 | Melnikov A, Zaborina O, Dhiman N,Prabhakar BS, Chakrabarty AM and Hendrickson W: Clinical andenvironmental isolates ofBurkholderia cepacia exhibitdifferential cytotoxicity towards macrophages and mast cells. MolMicrobiol. 36:1481–1493. 2000. |
74 | Ozols RF: Systemic therapy for ovariancancer: current status and new treatments. Semin Oncol. 33:S3–S11.2006.View Article :Google Scholar :PubMed/NCBI |
75 | Balch C, Huang TH, Brown R and Nephew KP:The epigenetics of ovarian cancer drug resistance andresensitization. Am J Obstet Gynecol. 191:1552–1572. 2004.View Article :Google Scholar :PubMed/NCBI |
76 | Su HY, Lai HC, Lin YW, et al: Epigeneticsilencing of SFRP5 is related to malignant phenotype andchemoresistance of ovarian cancer through Wnt signaling pathway.Int J Cancer. 127:555–567. 2010.View Article :Google Scholar :PubMed/NCBI |
77 | Nicholson LJ, Smith PR, Hiller L, et al:Epigenetic silencing of argininosuccinate synthetase confersresistance to platinum-induced cell death but collateralsensitivity to arginine auxotrophy in ovarian cancer. Int J Cancer.125:1454–1463. 2009.View Article :Google Scholar |
78 | Strathdee G, MacKean MJ, Illand M andBrown R: A role for methylation of the hMLH1 promoter in loss ofhMLH1 expression and drug resistance in ovarian cancer. Oncogene.18:2335–2341. 1999.View Article :Google Scholar :PubMed/NCBI |
79 | Yap OW, Bhat G, Liu L and Tollefsbol TO:Epigenetic modifications of the estrogen receptor beta gene inepithelial ovarian cancer cells. Anticancer Res. 29:139–144.2009.PubMed/NCBI |
80 | Lehmann U, Celikkaya G, Hasemeier B,Langer F and Kreipe H: Promoter hypermethylation of thedeath-associated protein kinase gene in breast cancer is associatedwith the invasive lobular subtype. Cancer Res. 62:6634–6638.2002.PubMed/NCBI |
81 | Curley MD, Therrien VA, Cummings CL, etal: CD133 expression defines a tumor initiating cell population inprimary human ovarian cancer. Stem Cells. 27:2875–2883.2009.PubMed/NCBI |
82 | Li M, Balch C, Montgomery JS, et al:Integrated analysis of DNA methylation and gene expression revealsspecific signaling pathways associated with platinum resistance inovarian cancer. BMC Med Genomics. 2:342009.View Article :Google Scholar :PubMed/NCBI |
83 | Laird PW: The power and the promise of DNAmethylation markers. Nat Rev Cancer. 3:253–266. 2003.View Article :Google Scholar :PubMed/NCBI |
84 | Ivanov M, Kacevska M and Ingelman-SundbergM: Epigenomics and interindividual differences in drug response.Clin Pharmacol Ther. 92:727–736. 2012.View Article :Google Scholar :PubMed/NCBI |
85 | Santi DV, Norment A and Garrett CE:Covalent bond formation between a DNA-cytosine methyltransferaseand DNA containing 5-azacytosine. Proc Natl Acad Sci USA.81:6993–6997. 1984.View Article :Google Scholar :PubMed/NCBI |
86 | Kaminskas E, Farrell A, Abraham S, et al:Approval summary: azacitidine for treatment of myelodysplasticsyndrome subtypes. Clin Cancer Res. 11:3604–3608. 2005.View Article :Google Scholar :PubMed/NCBI |
87 | Issa JP, Garcia-Manero G, Giles FJ, et al:Phase 1 study of low-dose prolonged exposure schedules of thehypomethylating agent 5-aza-2′-deoxycytidine (decitabine) inhematopoietic malignancies. Blood. 103:1635–1640. 2004.PubMed/NCBI |
88 | Issa JP, Gharibyan V, Cortes J, et al:Phase II study of low-dose decitabine in patients with chronicmyelogenous leukemia resistant to imatinib mesylate. J Clin Oncol.23:3948–3956. 2005.View Article :Google Scholar :PubMed/NCBI |
89 | Sasaki M, Kaneuchi M, Fujimoto S, Tanaka Yand Dahiya R: Hypermethylation can selectively silence multiplepromoters of steroid receptors in cancers. Mol Cell Endocrinol.202:201–207. 2003.View Article :Google Scholar :PubMed/NCBI |
90 | Takai N, Kawamata N, Walsh CS, et al:Discovery of epigenetically masked tumor suppressor genes inendometrial cancer. Mol Cancer Res. 3:261–269. 2005.View Article :Google Scholar :PubMed/NCBI |
91 | Plumb JA, Strathdee G, Sludden J, Kaye SBand Brown R: Reversal of drug resistance in human tumor xenograftsby 2′-deoxy-5-azacytidine-induced demethylation of the hMLH1 genepromoter. Cancer Res. 60:6039–6044. 2000. |
92 | Fu S, Hu W, Iyer R, et al: Phase 1b-2astudy to reverse platinum resistance through use of ahypomethylating agent, azacitidine, in patients withplatinum-resistant or platinum-refractory epithelial ovariancancer. Cancer. 117:1661–1669. 2011.View Article :Google Scholar |
93 | Fang F, Balch C, Schilder J, et al: Aphase 1 and pharmacodynamic study of decitabine in combination withcarboplatin in patients with recurrent, platinum-resistant,epithelial ovarian cancer. Cancer. 116:4043–4053. 2010.View Article :Google Scholar :PubMed/NCBI |
94 | Chen H, Hardy TM and Tollefsbol TO:Epigenomics of ovarian cancer and its chemoprevention. Front Genet.2:672011.View Article :Google Scholar :PubMed/NCBI |
95 | Terasawa K, Sagae S, Toyota M, et al:Epigenetic inactivation of TMS1/ASC in ovarian cancer. Clin CancerRes. 10:2000–2006. 2004.View Article :Google Scholar :PubMed/NCBI |
96 | Shi H, Wei SH, Leu YW, et al: Tripleanalysis of the cancer epigenome: an integrated microarray systemfor assessing gene expression, DNA methylation, and histoneacetylation. Cancer Res. 63:2164–2171. 2003.PubMed/NCBI |
97 | Appleton K, Mackay HJ, Judson I, et al:Phase I and pharmacodynamic trial of the DNA methyltransferaseinhibitor decitabine and carboplatin in solid tumors. J Clin Oncol.25:4603–4609. 2007.View Article :Google Scholar :PubMed/NCBI |
