Current data and future perspectives on DNA methylation in ovarian cancer (Review).

Ovarian cancer (OC) is one of the three most prevalent and lethal malignancies affecting the female reproductive organs, alongside endometrial and cervical cancers ( 1 ). Its incidence has been increasing worldwide, and it now has the second highest yearly incidence rate among cancers of the female reproductive system ( 2 , 3 ). Epithelial OC (EOC), which constitutes 85 to 90% of all ovarian tumors, is the most common subtype ( 4 ). The histological subtypes of EOC vary based on the tissue origin, as detailed in Table I . Malignant ovarian germ cell tumors and sex cord-stromal tumors, however, are relatively rare ( 4 ). OC is characterized by a combination of direct spread, intraabdominal seeding and lymphatic metastasis, with peritoneal metastases in the advanced stages of the disease being associated with high mortality rates and a poor patient prognosis ( 5 ). The primary treatments for patients with advanced-stage OC currently include surgical tumor removal and platinum-based combination chemotherapy ( 6 ). Despite advancements in treatment, the prognosis of patients with advanced-stage disease remains poor, and OC continues to have the highest mortality rate among all malignant gynecological malignancies ( 7 ). OC often remains asymptomatic in the early stages due to the covert growth of the ovary, which is the reason why numerous younger women do not experience symptoms from their ovarian tumors. The absence of reliable biomarkers for the early detection of OC often results in the disease progressing to a more difficult-to-treat late stage. At present, an increasing number of studies have indicated that epigenetic modifications play a pivotal role in tumor growth etiology, and variations in the epigenetic status are emerging as promising non-invasive biomarkers for the early diagnosis and monitoring of OC ( 8 - 10 ). DNA methylation, the most extensively studied and most well-characterized epigenetic modification, regulates gene expression by adding methyl groups to the promoter region of DNA ( 11 , 12 ). DNA methylation is a complex epigenetic modification mediated by a complex network of enzymes, cofactors, and regulatory proteins in a process that involves a variety of channels and receptors that facilitate the interaction between DNA methyltransferases and their targets. These include chromatin remodeling complexes, histone modifiers and transcription factors. In turn, matrix proteins provide the structural framework for enzymes and cofactors involved in methylation and thus play a key role in the process of DNA methylation ( 10 , 13 ). Unlike normal cells, tumor cells often display abnormal DNA methylation levels in specific regions of tumor-suppressor gene and/or oncogene promoters ( 14 , 15 ). This disruption of key biological processes, including cell proliferation, cell cycle regulation and apoptosis, due to the abnormal DNA methylation patterns of certain genes, has been found to be associated with the development of OC ( 16 , 17 ). Recent studies have also suggested that DNA methylation plays a role in OC cell metastasis ( 11 , 18 ). The current understanding posits that DNA methylation markers are crucial in the prevention, diagnosis and treatment of OC, and DNA methylation-related drugs have also exhibited efficacy in reducing or eliminating resistance to chemotherapy and molecular targeting in patients with OC ( 4 , 8 , 19 ). High-grade serous OC (HGSOC), the most prevalent subtype of OC, has the highest recurrence rate and the worst prognosis. It is widely acknowledged that the primary challenge in treating HGSOC is the acquired resistance to platinum-based drug therapy ( 20 , 21 ). The study by Feng et al ( 22 ) proposed that NCALD and LAMA3 could serve as novel markers for determining the sensitivity to chemotherapy in patients with HGSOC, and that hypermethylation and the low expression of NCALD and LAMA3 are linked to a poor progression-free survival. It is thus suggested that the methylation of gene promoter regions plays a crucial role in platinum resistance in patients with OC. The present review focuses on the roles of DNA methylation variations in tumor suppressor genes, oncogenes, signaling pathway genes and microRNAs (miRNAs/miRs) involved in the development of OC. Given the challenges posed by drug resistance and relapse mechanisms, which significantly affect the management and prognosis of this disease, the latest findings on the role of DNA methylation in the screening, diagnosis and the treatment of OC are also summarized. The present review comprehensively discusses the current evidence for the role of DNA methylation in both oncogenic and tumor suppressor pathways implicated in OC, in order to identify promising biomarkers or therapeutic targets.

Despite the notable advances made in the treatment of OC in recent years, the majority of patients with advanced-stage OC continue to experience recurrence and eventually succumb to chemoresistance. Tumorigenesis, progression and resistance to treatment are predominantly mediated by epigenetic regulation, particularly DNA methylation. The present review aimed to provide an overview of methylation-specific modifications of genes related to OC and their clinical applications, thereby emphasizing the significance of DNA methylation in OC. In general, tumor suppressor genes, such as BRCA1/2, p53, RASSF1A, CHD5, FBP1, ALDH1A2, FOXD3, IGFBP-3, ZNF671, SPARC and MGMT are often found to be underexpressed and hypermethylated in OC tissues ( Table II ). Conversely, oncogenes, such as HOXA9, CBX8, SLC6A12, AGR2 and GABRP exhibit a high expression and DNA hypomethylation ( Table III ). In addition to this, the study by Bauerschlag et al ( 190 ) discovered that the hypomethylation of genes such as growth regulating estrogen receptor binding 1, TGFB induced factor homeobox 1 and transducer of ERBB2, and the hypermethylation of genes such as transmembrane and coiled-coil domains 5, protein tyrosine phosphatase receptor type N and guanylate cyclase 2C, were associated with longer survival periods of patients with OC, suggesting potential prognostic value. The altered DNA methylation of some genes of the classical pathway can also have an impact on the development of OC ( Tables IV and V ). miRNAs play a more intricate role in the development of OC. Their expression may be downregulated due to gene hypermethylation, such as the expression of miR-152 and miR-148a ( 191 ), or they may be overexpressed due to gene hypomethylation, such as miR-21, miR-203 and miR-205 ( 166 ) ( Table VI ). Overall, miRNAs serve as target genes, and investigating whether they are regulated by DNA methylation contributes to the development, diagnosis, staging and treatment resistance of OC, and thus warrants further exploration of their potential clinical applications. Building on comprehensive clinical studies exploring the link between DNA methylation and OC, DNMTis have emerged as a promising therapeutic avenue in clinical settings. They play a pivotal role in overcoming chemoresistance and recurrence in OC. Current therapeutic strategies include the combined use of DNMTis with histone deacetylase inhibitors, DNMTis with PARP inhibitors, among others. These combinations could potentially open up new clinical trial opportunities for patients with advanced malignant ovarian tumors who are unresponsive to immunotherapy. Epigenetics, and in particular DNA methylation, is now providing novel and very promising techniques for the discovery of specific biomarkers and their subsequent screening. As previously described by Belsky et al ( 192 ), the DNA methylation of related genes can be used as a biomarker to predict the rate of aging. Previous studies on DNA methylation in OC have provided critical evidence for understanding ovarian tumorigenesis ( 61 - 64 ). These findings provide potential diagnostic biomarkers and therapeutic targets. However, numerous inconsistencies remain regarding the results of aberrant DNA methylation within these tumor suppressor genes in OC. When analyzing the possible reasons for these inconsistencies, the most significant reason is the sample size. Human samples vary greatly in terms of genetics, environment, lifestyle and individual differences. In a number of studies, the sample size is usually too small due to the huge variation in patients with OC. Future studies are urgently required to address these controversies by analyzing large sample sizes. In addition to this, factors such as the lack of functional studies, differences in the methods of DNA methylation detection used, and different promoter regions for DNA methylation detection may also contribute to the discrepancies. Based on the evidence provided in the present review, targeted DNA methylation inhibitors have promising applications in the treatment of OC. Therefore, it is evident that additional studies are warranted to bridge existing knowledge gaps and reconcile inconsistencies. In light of the known limitations, future research directions, including conducting larger multicenter studies, the development of animal models to determine causality, and the initiation of clinical trials involving methylating or demethylating drugs are proposed. In conclusion, the present review aimed to broaden the understanding of the role of DNA methylation in OC and determine its potential as a biomarker. This could also the focus for future research and further in-depth analysis of this disease. The ultimate goal is to facilitate early diagnosis and treatment, and to promptly address the pressing clinical issues of OC recurrence and chemoresistance.