**1. Introduction**

Among gynecological malignancies, ovarian cancer, a molecularly heterogeneous condition associated with poorest prognosis. The highest mortality rates are associated with ovarian cancer, reflecting the third most prevalent cancer in female carcinomas of the gynecologic system. While it accounts for just 3% of all female cancers, the worldwide annual prevalence of ovarian cancer is 220,000, with 21,750 reported new cases and 13,940 estimated deaths annually [1]. Specific diagnosis in more than 70% of OC cases is a potent factor for high fatality rates at an advanced disease stage and carries a poor prognosis with current therapies. In ovarian cancer, the median age of disease diagnosis is 60 years and its lifetime incidence is one in seventy with estimated lifetime mortality of one in ninety-five [2, 3]. Epithelial ovarian cancer (EOC) accounts for 90% of all types of OC cases distinguished at histopathological, clinical, and molecular levels by heterogeneity. The precise cause of the malignancy of the ovaries is still unclear. Significant risk factors associated with OC have been identified as a strong family history of either ovarian or breast cancer. More than one-fifth (approximately 23%) of ovarian carcinomas have inherited susceptibility and have BRCA1 and BRCA2 tumor suppressor gene mutations [4].

Rapid growth, unspecific clinical symptoms at the early stage of the disease, and the absence of earlier diagnosis methods make it challenging to diagnose promptly due to lack of effective screening. As a result, when the tumor has spread beyond the pelvis and is unlikely to be entirely removed by surgery, EOC is usually diagnosed at an advanced stage (FIGO III/IV). Long-term survival rates are poor (10–30 percent for women with disseminated malignancies. However, an ovarian cancer diagnosis at the localized level is considerable curable (over 95 percent five-year survival rate; [5]). Therefore, it is needed to explore cost effective and sensitive screening program for early detections and biomarkers to predict disease behaviors and responses to therapies. In identifying promising biomarkers of clinical utility for early diagnosis of OC, a better understanding of the EOC genome portrait would benefit.

Altered epigenetic states are closely associated with tumorigenesis of the ovaries. Epigenetics is characterized as a heritable alteration in gene expression without the DNA sequence itself being altered and involves DNA methylation, histone modification, nucleosome repositioning, and micro-RNAs (miRNAs) posttranscriptional gene regulation [6, 7]. Cancer vulnerability is inherited, but most of this inheritability remains unknown. Epigenetic changes in the parental germ line that do not require transmission of genetic variants from parent to offspring may mediate such missing heritability. DNA methylation, the addition of a methyl moiety to the cytosine-5 location within the sense of a CpG dinucleotide, mediated by DNA methyltransferases (DNMTs), is the most studied epigenetic shift [6]. While most CpG sites are methylated in the human genome, CpG-dense regions known as CpG islands (often gene-associated) are usually unmethylated in normal tissue. Also, histone proteins associated with DNA are subject to extensive modifications that mediate the assembly of chromatin that is transcriptionally permissive or restrictive (i.e., open or closed). DNA methylation and histone modifications are now recognized to be closely related [6]. The complete epigenetic state corresponding to a particular cell phenotype (e.g., DNA methylation, histone modification, and miRNA expression) is now referred to as the epigenome [8]. Though repressive epigenetic changes (including DNA methylation) control genes in normal tissues (e.g., imprinted genes and inactivation of female X-chromosomes), they are dramatically altered in cancer [6, 9]. Global DNA hypomethylation and localized hypermethylation of promoter-associated CpG islands occur primarily in cancer cells, with the latter acting as a replacement for point mutations or deletions to induce transcriptional silencing of tumor suppressor genes [6].

## **2. DNA methylation in ovarian cancer**

The substantial shortcomings of the therapies examined above in the treatment of ovarian cancer have set the stage for the use, either alone or in combination with other therapies, of novel epigenetic therapies to treat this disease. By adding a methyl group to stimulate regions of DNA to silence gene expression, the epigenetic alteration of DNA methylation controls gene expression. This mechanism is critical during sensitive cellular processes, such as embryonic development, inactivation of X-chromosomes, and genomic imprinting. The organ's normal development and maturation are determined by a precise balance of active and silenced genes [10]. On the other hand, cancer promotes global hypermethylation of CpG islands associated with promoters, which are typically unmethylated in normal tissue, silencing genes essential for cellular homeostases, such as genes suppress tumors. To promote tumorigenesis, aberrant DNA methylation and structural chromatin changes will alter gene

**55**

*Epigenetic Events in Ovarian Cancer*

*DOI: http://dx.doi.org/10.5772/intechopen.95472*

cancer is correlated with cancer cell growth inhibition [12].

pared to non-invasive cancers and benign cystadenomas [17].

inhibition and inhibits PI3K/Akt oncogenic signaling [20].

Chromatin modifying enzymes are altered in ovarian cancers beyond DNA methylation. High levels of H3K9 methyltransferase G9a, which adds histone methyl groups (H3K9) to promote the compact structure of chromatin and silence genes, have been associated with late-stage high-grade and serous ovarian cancer, as well as shorter survival in patients with ovarian cancer [18]. Genes marked by the chromatin modifications of activating H3K4me3 and silencing H3K27me3 are identified as "poised" or bivalent; these are not transcribed into embryonic stem cells but resolved as differentiated stem cells into active and transcribed (H3K4me3) or silenced and not transcribed (H3K27me3). In 499 high-grade serous ovarian cancers, compared to eight normal fallopian tube samples, these bivalent chromatin loci were silenced and included genes in the PI3K and TGF-beta signaling pathways. Stem-like cells of ovarian cancer and chemo-resistant cells of ovarian cancer have demonstrated increased silencing of these genes [19]. As previously mentioned, the gene encoding the ARID1A chromatin remodeler is mutated in over 50 percent of ovarian clear cell carcinomas. In a mouse model of ovarian cancer, Bitler et al. showed inhibiting EZH2 methyltransferase, which adds the H3K27me3 mark to silence gene expression, induced regression of ARID1A-mutated tumors. This occurred via PIK3IPI upregulation, an ARID1A and EZH2 target increased by EZH2

**3. Histone modification in ovarian cancer**

expression [11]. A repressive and tightly woven chromatin structure is caused by DNA methylation, which can minimize gene expression in DNA repair, apoptosis, differentiation, drug resistance, angiogenesis, and metastasis. In cancer, gene promoters' hypermethylation causes the genes involved in cell cycle control, including BRCA1, CDKN2A, RASSF1A, LOTI, DAPK, ICAM-l, PALB2, RAD51C, and BRIP1 to be downregulated. Therefore, substantial loss of CpG hypermethylation in ovarian

In ovarian cancers, hypermethylation of particular gene promoters has been established. Compared to non-neoplastic tissues, promoter hypermethylation of tumor suppressors BRCA1 and RASSF1A were significantly higher in ovarian cancers [13]. This hypermethylation silences expression to suppress BRCA1 activity, driving genomic instability in ovarian cancers, analogous to the mutations in BRCA1 discussed earlier. RASSF1A encodes a protein controlling the cell cycle; silencing this gene promotes cell-cycle progression and unregulated cell development. Compared to benign cases, tissues from patients with serous and non-serous EOC display significantly higher RASSF1A promoter methylation. [14]. In clear-cell ovarian cancer, hypermethylation is also observed. 22 separate CpG loci associated with nine genes (VWA1, FOXP1, FGFRL1, LINC00340, KCNH2, ANK1, ATXN2, NDRG21 and SLC16A11) were hypermethylated. Inversely associated with tumor gene expression, multiple loci methylation, most notably KCNH22 (HERG, a potassium channel). Loss of KCNH2 (HERG) expression by methylation may be a good prognostic marker, provided that overexpression of the Eag family members of the potassium channel promotes increased proliferation and results in poor prognosis [15]. However, superficial cell carcinomas also suppress methylation of the gene encoding the HNF1B transcription factor, while this gene is methylated in high-grade serous ovarian cancers [16]. In invasive carcinomas, Makarla et al. found hypermethylation of eight cancer-related genes (p16, RARβ, E-cadherin, H-cadherin, APC, GSTP1, MGMT, and RASSF1A) was significantly higher com-

### *Epigenetic Events in Ovarian Cancer DOI: http://dx.doi.org/10.5772/intechopen.95472*

*Ovarian Cancer - Updates in Tumour Biology and Therapeutics*

tional silencing of tumor suppressor genes [6].

**2. DNA methylation in ovarian cancer**

would benefit.

Rapid growth, unspecific clinical symptoms at the early stage of the disease, and the absence of earlier diagnosis methods make it challenging to diagnose promptly due to lack of effective screening. As a result, when the tumor has spread beyond the pelvis and is unlikely to be entirely removed by surgery, EOC is usually diagnosed at an advanced stage (FIGO III/IV). Long-term survival rates are poor (10–30 percent for women with disseminated malignancies. However, an ovarian cancer diagnosis at the localized level is considerable curable (over 95 percent five-year survival rate; [5]). Therefore, it is needed to explore cost effective and sensitive screening program for early detections and biomarkers to predict disease behaviors and responses to therapies. In identifying promising biomarkers of clinical utility for early diagnosis of OC, a better understanding of the EOC genome portrait

Altered epigenetic states are closely associated with tumorigenesis of the ovaries. Epigenetics is characterized as a heritable alteration in gene expression without the DNA sequence itself being altered and involves DNA methylation, histone modification, nucleosome repositioning, and micro-RNAs (miRNAs) posttranscriptional gene regulation [6, 7]. Cancer vulnerability is inherited, but most of this inheritability remains unknown. Epigenetic changes in the parental germ line that do not require transmission of genetic variants from parent to offspring may mediate such missing heritability. DNA methylation, the addition of a methyl moiety to the cytosine-5 location within the sense of a CpG dinucleotide, mediated by DNA methyltransferases (DNMTs), is the most studied epigenetic shift [6]. While most CpG sites are methylated in the human genome, CpG-dense regions known as CpG islands (often gene-associated) are usually unmethylated in normal tissue. Also, histone proteins associated with DNA are subject to extensive modifications that mediate the assembly of chromatin that is transcriptionally permissive or restrictive (i.e., open or closed). DNA methylation and histone modifications are now recognized to be closely related [6]. The complete epigenetic state corresponding to a particular cell phenotype (e.g., DNA methylation, histone modification, and miRNA expression) is now referred to as the epigenome [8]. Though repressive epigenetic changes (including DNA methylation) control genes in normal tissues (e.g., imprinted genes and inactivation of female X-chromosomes), they are dramatically altered in cancer [6, 9]. Global DNA hypomethylation and localized hypermethylation of promoter-associated CpG islands occur primarily in cancer cells, with the latter acting as a replacement for point mutations or deletions to induce transcrip-

The substantial shortcomings of the therapies examined above in the treatment

of ovarian cancer have set the stage for the use, either alone or in combination with other therapies, of novel epigenetic therapies to treat this disease. By adding a methyl group to stimulate regions of DNA to silence gene expression, the epigenetic alteration of DNA methylation controls gene expression. This mechanism is critical during sensitive cellular processes, such as embryonic development, inactivation of X-chromosomes, and genomic imprinting. The organ's normal development and maturation are determined by a precise balance of active and silenced genes [10]. On the other hand, cancer promotes global hypermethylation of CpG islands associated with promoters, which are typically unmethylated in normal tissue, silencing genes essential for cellular homeostases, such as genes suppress tumors. To promote tumorigenesis, aberrant DNA methylation and structural chromatin changes will alter gene

**54**

expression [11]. A repressive and tightly woven chromatin structure is caused by DNA methylation, which can minimize gene expression in DNA repair, apoptosis, differentiation, drug resistance, angiogenesis, and metastasis. In cancer, gene promoters' hypermethylation causes the genes involved in cell cycle control, including BRCA1, CDKN2A, RASSF1A, LOTI, DAPK, ICAM-l, PALB2, RAD51C, and BRIP1 to be downregulated. Therefore, substantial loss of CpG hypermethylation in ovarian cancer is correlated with cancer cell growth inhibition [12].

In ovarian cancers, hypermethylation of particular gene promoters has been established. Compared to non-neoplastic tissues, promoter hypermethylation of tumor suppressors BRCA1 and RASSF1A were significantly higher in ovarian cancers [13]. This hypermethylation silences expression to suppress BRCA1 activity, driving genomic instability in ovarian cancers, analogous to the mutations in BRCA1 discussed earlier. RASSF1A encodes a protein controlling the cell cycle; silencing this gene promotes cell-cycle progression and unregulated cell development. Compared to benign cases, tissues from patients with serous and non-serous EOC display significantly higher RASSF1A promoter methylation. [14]. In clear-cell ovarian cancer, hypermethylation is also observed. 22 separate CpG loci associated with nine genes (VWA1, FOXP1, FGFRL1, LINC00340, KCNH2, ANK1, ATXN2, NDRG21 and SLC16A11) were hypermethylated. Inversely associated with tumor gene expression, multiple loci methylation, most notably KCNH22 (HERG, a potassium channel). Loss of KCNH2 (HERG) expression by methylation may be a good prognostic marker, provided that overexpression of the Eag family members of the potassium channel promotes increased proliferation and results in poor prognosis [15]. However, superficial cell carcinomas also suppress methylation of the gene encoding the HNF1B transcription factor, while this gene is methylated in high-grade serous ovarian cancers [16]. In invasive carcinomas, Makarla et al. found hypermethylation of eight cancer-related genes (p16, RARβ, E-cadherin, H-cadherin, APC, GSTP1, MGMT, and RASSF1A) was significantly higher compared to non-invasive cancers and benign cystadenomas [17].
