Biomarkers of platinum resistance in ovarian cancer: what can we use to improve treatment

in Endocrine-Related Cancer
Authors:
Belinda van ZylHunter Medical Research Institute, Newcastle, New South Wales, Australia
School of Medicine and Public Health, Faculty of Health and Medicine, University of Newcastle, New South Wales, Australia

Search for other papers by Belinda van Zyl in
Current site
Google Scholar
PubMed
Close
,
Denise TangHunter Medical Research Institute, Newcastle, New South Wales, Australia
School of Biomedical Sciences and Pharmacy, Faculty of Health and Medicine, University of Newcastle, New South Wales, Australia

Search for other papers by Denise Tang in
Current site
Google Scholar
PubMed
Close
, and
Nikola A BowdenHunter Medical Research Institute, Newcastle, New South Wales, Australia
School of Medicine and Public Health, Faculty of Health and Medicine, University of Newcastle, New South Wales, Australia

Search for other papers by Nikola A Bowden in
Current site
Google Scholar
PubMed
Close

Correspondence should be addressed to N A Bowden: Nikola.Bowden@newcastle.edu.au
Free access

Ovarian cancer has poor survival rates due to a combination of diagnosis at advanced disease stages and disease recurrence as a result of platinum chemotherapy resistance. High-grade serous ovarian cancer (HGSOC), the most common ovarian cancer subtype, is conventionally treated with surgery and paclitaxel/carboplatin combination chemotherapy. Initial response rates are 60–80%, but eventually the majority of patients become platinum-resistant with subsequent relapses. Extensive research on individual biomarkers of platinum resistance has revealed many potential targets for the development new treatments. While this is ongoing, there are also epigenetic, DNA repair, genome and immune changes characterised in platinum-resistant HGSOC that can be targeted with current therapies. This review discusses biomarkers of platinum chemotherapy resistance in ovarian cancer with a focus on biomarkers that are targetable with alternative treatment combinations to those currently used. After decades of research focused on elucidating the biological cause of platinum resistance, future research needs to focus on using this knowledge to overcome resistance for patients with ovarian cancer.

Abstract

Ovarian cancer has poor survival rates due to a combination of diagnosis at advanced disease stages and disease recurrence as a result of platinum chemotherapy resistance. High-grade serous ovarian cancer (HGSOC), the most common ovarian cancer subtype, is conventionally treated with surgery and paclitaxel/carboplatin combination chemotherapy. Initial response rates are 60–80%, but eventually the majority of patients become platinum-resistant with subsequent relapses. Extensive research on individual biomarkers of platinum resistance has revealed many potential targets for the development new treatments. While this is ongoing, there are also epigenetic, DNA repair, genome and immune changes characterised in platinum-resistant HGSOC that can be targeted with current therapies. This review discusses biomarkers of platinum chemotherapy resistance in ovarian cancer with a focus on biomarkers that are targetable with alternative treatment combinations to those currently used. After decades of research focused on elucidating the biological cause of platinum resistance, future research needs to focus on using this knowledge to overcome resistance for patients with ovarian cancer.

Introduction

Ovarian cancer has an annual worldwide incidence of approximately 240,000 and annual mortality rate of 152,000 (Ervik et al. 2016). The high annual mortality rate is due to a combination of diagnosis at advanced disease stages and disease recurrence as a result of chemotherapy resistance. There are several subtypes of ovarian cancer with differing histology, anatomical origins and molecular profiles resulting in vastly different inherent sensitivities to chemotherapy. Herein, we review biomarkers of platinum chemotherapy resistance in ovarian cancer with a focus on biomarkers that are potentially targetable for future treatment combinations.

Ovarian cancer subtypes

There are 2 major subtypes of ovarian cancer that are determined by the tissue of origin and classified as either epithelial and non-epithelial. Non-epithelial ovarian cancers include sex cord stromal, germ cell and non-specified ovarian cancers. Epithelial ovarian cancers include transitional cell, mucinous, clear cell and serous ovarian cancer. The focus of this review is the epithelial ovarian cancer subtypes, in particular the most common, HGSOC.

Epithelial ovarian cancer

Epithelial ovarian cancer (EOC) is an umbrella term that covers a diverse groups of tumours that can be classified into different subtypes based on the 2 main pathways of tumorigenesis according to a unifying theory proposed by Kurman and Shih (Shih Ie & Kurman 2004, Kurman & Shih Ie 2010). Type 1 EOC are considered to be genetically stable, slow to develop and are usually contained within the ovary at presentation. Therefore, most are diagnosed at an early stage and respond well to mostly surgical treatment. They consist of low-grade serous, low-grade endometrioid, clear cell, mucinous and transitional or Brenner tumours that have developed from clearly recognised precursor lesions or borderline tumours (Kurman & Shih Ie 2010). Conversely, type 2 EOCs are highly invasive, grow quickly and are thus typically diagnosed at an advanced stage. These aggressive tumours consist mainly of high-grade serous, high-grade endometrioid, malignant mixed mesodermal tumours and undifferentiated carcinoma (Kurman & Shih Ie 2010).

Correct diagnosis of the subtype and stage of EOC is extremely important as each subtype responds differently to standard treatment options, and late-stage disease has poor survival rates. Despite these differences, almost all ovarian cancers are treated initially with surgical removal of tumour tissue termed ‘debulking’, followed by 6 courses of paclitaxel/carboplatin combination chemotherapy (in some cases with the addition of bevacizumab). However, upfront surgery is not appropriate for some patients, and neoadjuvant combination chemotherapy is administered prior to surgery. After debulking surgery, EOC is surgically staged based on the International Federation of Gynecologists and Obstetricians (FIGO) criteria (Prat et al. 2015). Most type 2 EOCs are diagnosed at stage IIIC or higher resulting in poor survival rates. Neoadjuvant chemotherapy regimes can affect the cellular architecture and morphological features of the tumour, thus making correct subtyping difficult if this treatment approach is used (McCluggage 2008).

Serous ovarian cancer

Serous ovarian cancer (SOC) is the most common subtype, accounting for ~70% of all ovarian cancers. SOC is not a single disease but is composed of high-grade serous ovarian cancer (HGSOC) and low-grade serous ovarian cancer (LGSOC). These are not two grades of the same neoplasm but rather distinctly different tumour types (O’Neill et al. 2005, McCluggage 2008) with vastly variable clincopathologic features and behaviours (Kurman 2013) derived from different pathogenetic pathways of formation (Malpica et al. 2007). HGSOC generally affects older women who present at a later FIGO stage. Even though there is an initial response to treatment, they become resistant over time and have an overall poorer prognosis. LGSOC is mainly diagnosed in younger women, is slow growing (McCluggage 2008), and more likely to be non-responsive to chemotherapy and ultimately has a better prognosis than HGSOC with a longer overall survival time (Ramalingam 2016).

HGSOC is characterised by a number of histological features that are not exclusive to the subtype, however are favoured by it. The cellular architecture is, predominantly and sometimes exclusively, papillary in nature across large sheets of cells and is associated with slit-like rather than round gland formations and psammoma bodies (Clements & Young 2008, Ramalingam 2016). For a diagnosis of HGSOC based on the two-tier grading system by Malpica and colleagues (Malpica et al. 2004), carcinomas must have nuclear atypia often in the form of multinucleated cells as well as >12 mitoses per 10 high-power fields. Solid variants do occur with minimal or no papillary or glandular differentiation making it difficult to determine morphologically if the tumour is HGSOC or an undifferentiated carcinoma. HGSOC is also characterised by p53 mutations (Vang et al. 2016), detected as aberrant p53 using immunohistochemistry (IHC) or by targeted next-generation sequencing (Cole et al. 2016).

There have been many theories on the origins of HGSOC. Originally, it was considered that HGSOC originates directly from the surface epithelium undergoing metaplastic changes. The ‘incessant ovulation’ hypothesis first proposed by Fathalla in 1971, described continual ovulation resulting in a repetitive cycle of damage and repair to the ovarian surface epithelium (OSE), leading to an increase in inflammation and hormonal level fluctuations resulting in oxidative DNA damage (Fathalla 1971). Humans are at an increased risk of this damage and repair cycle due to the high number of uninterrupted ovulation cycles compared to other animal models that have ‘rest periods’. However, there has been an increase in pharmacologically induced non-ovulatory rest periods since the introduction of the oral contraceptive pill (OCP).

The second theory was that it derives from cortical inclusion cysts (CIC) found within the ovary. These cysts are developed from the invagination of OSE that forms Müllerian type tissue and then is subjected to neoplastic transformation (Kurman 2013, Banet & Kurman 2015, Zeppernick et al. 2015). Although it has been speculated that HGSOC is of ovarian origin, there has been no definitive identification of a precursor legion. Therefore, a paradigm shift away from the ovary towards the epithelium of the fimbriated end of the fallopian tube developed (Kurman 2013, Zeppernick et al. 2015, Ramalingam 2016), wherein serous tubal intraepithelial carcinoma forms as a precursor to HGSOC. Although it is believed that 50–60% of HGSOC originate from the fallopian tube (Kroeger & Drapkin 2017), many are found to contain p53 mutations that are identical to those found in the corresponding serous tubal in situ carcinoma (STIC), thus suggesting a genetic connection between the tumour and the STIC (Kindelberger et al. 2007, Lee et al. 2007). This relationship has also been observed on the protein level, with HGSOC staining positive for PAX8, a Müllerian marker. However, staining negative for the mesothelial marker calretinin, indicating that HGSOC’s expression profile is closer to that of the fallopian tube than that of the surface epithelium of the ovary (Zeppernick et al. 2015). Although there are several possible pathways and varied conceivable originating sites, the exact cell of origin of HGSOC has not been fully elucidated and requires further investigation to allow for a better understanding of this disease.

Chemotherapy

Platinum chemotherapy

Platinum chemotherapy was accidentally discovered in 1965 when it was first observed that a platinum compound was inhibiting cell division in E. coli (Rosenberg et al. 1965). The compound was later named ‘cisplatin’ and its effect on the division of cancer cells was confirmed in animal studies in 1970 (Rosenberg & VanCamp 1970). Clinical trials began soon after in 1972, and in 1978, cisplatin was approved in the USA by the Federal Drug Administration (FDA) for the treatment of testicular, bladder and ovarian cancer. The discovery was a turning point for the treatment of advanced ovarian cancer.

There are now 5 platinum chemotherapy analogues approved for use in the treatment of cancer: cisplatin, carboplatin, oxaliplatin, nedaplatin and lobaplatin. The mechanism of action for the analogues used most commonly to treat ovarian cancer, cisplatin and carboplatin; is direct insertion of platinum into DNA to form crosslinks (Fig. 1). The resultant structural distortion of the DNA is either removed by specific DNA repair processes or it triggers a signalling cascade resulting in apoptosis.

Figure 1
Figure 1

Mechanism of action for platinum chemotherapy. Platinum binds to the N-7 position of adjacent guanine (G) bases resulting in interstrand or intrastrand crosslinks.

Citation: Endocrine-Related Cancer 25, 5; 10.1530/ERC-17-0336

Cisplatin or carboplatin monotherapy is used to treat some solid tumours, with testicular cancer obtaining cure rates of over 90% (Verhoeven et al. 2013). Platinum monotherapy is rarely used in HGSOC treatment (reviewed in Harter et al. 2010), it is only occasionally used for treatment of elderly patients where combination chemotherapy is not well tolerated.

Combination chemotherapy

HGSOC is most commonly treated with a combination of carboplatin and paclitaxel (Stuart et al. 2011), a tubulin target that blocks mitotic spindle assembly and halts cell division (Kampan et al. 2015). Many other agents are used in combination with platinum chemotherapies after relapse. These incude pegylated doxorubicin (TopII inhibitor, blocks replication) (Staropoli et al. 2014), gemcitabine (nucleoside analog, blocks DNA replication) (Pfisterer et al. 2006), trabectedin (transcription factor blocker) (D’Incalci & Galmarini 2010) and bevacizumab (angiogenesis inhibitor) (Oza et al. 2015).

Figure 2
Figure 2

Nucleotide Excision Repair and Homologous Recombination Pathways. (A) Nucleotide excision repair recognises platinum-induced interstrand and intrastrand crosslinks and a co-ordinated process of DNA unwinding, incision, excision and synthesis follows. The process can result in a DNA double-strand break, which is recognised by homologous recombination repair. (B) Homologous recombination repair recognises double-strand breaks and initiates a process of single strand DNA formation, coating, filament formation, strand invasion and a final step of DNA synthesis.

Citation: Endocrine-Related Cancer 25, 5; 10.1530/ERC-17-0336

Subtype-specific response to chemotherapy

Paclitaxel and carboplatin combination chemotherapy produces initial response rates in HGSOC of 60–80% (Selvakumaran et al. 2003), but eventually the majority of patients become platinum resistant with subsequent relapses. Clear cell, transitional cell, mucinous and LGSOC are predominantly resistant to platinum chemotherapy resulting in low-use of platinums in treatment regimes for these subtypes. The high level of resistance in these subtypes is problematic when assessing older studies that did not differentiate outcome analysis based on histological subtype. More recently, it has become standard practice to identify serous vs non-serous subtypes of ovarian cancer. In future studies, to assess new treatments or combinations, it would be ideal to segregate results of histological subtypes to ascertain accurate responses for each subtype.

Platinum chemotherapy resistance

Patients treated with platinum chemotherapy are categorized as either platinum sensitive or platinum resistant based on the amount of time from end of treatment to relapse, referred to as the platinum-free interval. The platinum-free interval is distinct from the progression-free interval (PFI) most commonly used to assess clinical trial outcomes (Davis et al. 2014). Davis et al. (2014) highlights this distinction in a 2014 review of platinum-resistant ovarian cancer; PFI is defined as the time from diagnosis to relapse, including the time undergoing first-line surgery and chemotherapy. The authors concluded that platinum-free interval is a more accurate way to categorize platinum response or sensitivity for ovarian cancer.

Platinum response is generally classified into refractory, resistant, partially-sensitive or sensitive. The Gynecologic Cancer InterGroup (GCOG) consensus statement recommended the following timelines for platinum response classifications: (1) Platinum-refractory: progression while receiving last line of platinum-based therapy or within 4 weeks of last platinum dose; (2) Platinum-resistant: progression-free interval since last line of platinum of less than 6 months; (3) Partially platinum sensitive: progression-free interval since last line of platinum of 6–12 months and (4) Platinum sensitive: progression-free interval since last line of platinum of more than 12 months (Stuart et al. 2011).

Platinum-sensitive ovarian cancer has a median survival of 2 years, with a range of 3 months to over 10 years. Platinum-resistant ovarian cancer has a median survival of 9–12 months and less than 15% respond to subsequent chemotherapy (Davis et al. 2014). Ultimately, almost all HGSOC patients become platinum resistant and succumb to the disease (Davis et al. 2014).

There has been a large body of research focused on identifying the mechanisms underlying HGSOC platinum resistance. The most studied mechanisms are within the cancer cells themselves, including p53 (Reles et al. 2001, Yang-Hartwich et al. 2015) and genomewide mutations (Patch et al. 2015), epigenetic changes (Wei et al. 2006, Vang et al. 2013, Chang et al. 2017) and dysfunctional DNA repair (Barakat et al. 2010). Possibly working together in concert, these genetic mechanisms lead to genomic instability that allows cancer cells to adapt and survive DNA damage caused by platinum chemotherapy. Although all these mechansisms have been associated with resistance, the exact mechanisms remain undefined.

Similarly the presence of cancer stem cells (CSCs) (Steg et al. 2012) and epithelial-to-mesenchymal transition (EMT) (Marchini et al. 2013, Chebouti et al. 2017) is associated with platinum resistance in HGSOC. Platinum chemotherapy is most effective on proliferating cells that make up the majority of rapidly growing cancer; therefore, it is hypothesized that populations of latent CSCs and mesenchymal-like cells are less likely to respond to platinum chemotherapy. In addition to changes to the genome and phenotype of HGSOC cancer cells, the tumour microenvironment, in particular, immune cell infiltration, angiogenesis and hypoxia have also been implicated in platinum chemoresistance.

To date, the complete set of mechanisms underlying platinum chemotherapy resistance and how they interact is not fully understood. The ultimate goal of establishing biomakers is to further this understanding and to assist clinicians and patients to make better informed treatment decisions. Some of the previously reported biomarkers have high potential for developing targeted therapies or for re-purposing non-traditional chemotherapies to improve treatment of platinum-resistant HGSOC. The main mechanisms of resistance and subtypes of biomarkers reported to date are reviewed in the following sections.

Mechanisms and biomarkers of resistance

Cancer stem cells

The cancer stem cells (CSCs) model of disease progression remains controversial as the process is still largely uncharacterized. CSCs are a relatively small subset of cancer cells that indefinitely self-renew, initiate and maintain tumour growth and may remain in quiescence for prolonged periods (Clevers 2011, Prasetyanti & Medema 2017). In HGSOC, they have been associated with platinum resistance and disease recurrence (Steg et al. 2012, Pylvas-Eerola et al. 2016). The mechanism of CSC associated platinum resistance is largely uncharacterised, but quiescence during chemotherapy remains the most likely mechanism. Quiescent ovarian CSCs are largely unaffected by chemotherapy as it relies on cell division to damage DNA and elicit an effect (Ottevanger 2017).

Markers of CSCs have been extensively studied in HGSOC, with ALDH and CD133 (Silva et al. 2011, Kryczek et al. 2012) being the most consistently replicated markers in both model systems and HGSOC tissue (Silva et al. 2011, Ruscito et al. 2017). Efforts have been made to identify CSCs markers for development of new treatments for HGSOC, from which the most promising so far is bone morphologenetic protein 2 (BMP2). BMP2 is upregulated in ovarian cancer cells (Le Page et al. 2006) and has been associated with poor prognosis (Le Page et al. 2009). An ovarian cancer cell hierarchical differentiation pattern in which BMP2 acts as a feedback mechanism promoting ovarian CSC expansion and suppressing progenitor proliferation was recently reported (Choi et al. 2015), but further studies to confirm this discovery in clinical populations is needed before directing new treatments towards this target. The evidence for HGSOC CSCs as targetable biomarkers of platinum resistance is compelling, but has yet to be translated into prognostic testing or development of targeted treatments.

Epithelial-mesenchymal transition

EMT is a process whereby cells undergo a series of changes that result in a transition from an epithelial cell phenotype to a mesenchymal cell phenotype (reviewed in Thiery & Sleeman 2006). The process is intricately linked to the presence of CSCs and many studies have focused on the role of CSCs in EMT resulting in cancer progression and treatment resistance. There is a substantial body of evidence that EMT is a vital component of cancer progression, particularly in HGSOCs (Takai et al. 2014). HGSOCs develop from epithelial cells but often display a mesenchymal phenotype, particularly if platinum resistant (Marchini et al. 2013, Chebouti et al. 2017).

Extensive molecular profiling of HGSOC has also identified a subgroup of HGSOC that exhibits a distinct mesenchymal gene expression profile (Yoshida et al. 2009, Cancer Genome Atlas Research 2011). Marchini and coworkers analysed gene expression profiles of 23 patient-matched treatment – naïve and platinum-resistant (after several lines of platinum therapy) HGSOC tumour samples. A resistance gene expression signature indicative of TGFβ-mediated EMT was identified and confirmed in a validation set of 52 EOCs (Marchini et al. 2013).

Despite the pivotal role EMT seems to play on HGSOC progression, development of therapeutics to specifically target and reverse EMT has proven difficult due to the complexity of the EMT process. Key components of the EMT process are also involved in apoptosis, metabolism, cell proliferation, angiogenesis and cell growth (Huang et al. 2012). PI3K-AKT-mTOR inhibitors are the most promising therapeutic targets for EMT reversal, but ascertaining if the disease control is a result of EMT reversal or suppression of the other processes previously mentioned will be difficult to achieve. Another approach to reversing EMT may be targeting the epigenetic alterations that drive the transition. These include HGSOC-specific microRNAs, DNA methylation and histone acetylation patterns.

Figure 3
Figure 3

The role of DNA repair deficiencies in high-grade serous ovarian cancer (HGSOC) platinum chemotherapy resistance. (A) NER-and HR-proficient (NER+ and HR+) HGSOC is hypothesised to be sensitive to platinum chemotherapy if apoptosis is greater than DNA repair. (B) NER-deficient (NER−) and HR+ HGSOC is hypothesised to be platinum resistant due to lack of platinum cross-link recognition by NER. (C) NER+ and HR-deficient (HR−) HGSOC is hypothesised to be platinum sensitive due to NER triggering apoptosis and DNA double-strand breaks that are not repaired by HR. (D) NER− and HR− HGSOC is hypothesised to be platinum resistant due to lack of NER triggered apoptosis and a high mutation load.

Citation: Endocrine-Related Cancer 25, 5; 10.1530/ERC-17-0336

miRNAs

MicroRNAs (miRNAs) are short (18–25 nucleotides) non-coding fragments of RNA that bind to and inhibit mRNA. There are over 1000 human miRNAs and most have been associated with regulation of mRNA in normal and disease processes. miRNAs can regulate multiple mRNAs and subesquent proteins that are pivotal for drug response, therefore, inhibiting specific miRNAs to overcome platinum resistance is appealing.

Several mechanisms to target miRNAs are currently in development for cancer treatment including expression vector ‘miRNA sponges’ (Ebert et al. 2007, Chen et al. 2014), antisense or mimic oligos (Trang et al. 2011) and small molecule inhibitors (SMIRs) (Watashi et al. 2010). SMIRs are the most promising therapeutic target for miRNAs, but significant barriers to delivery of these non-small-molecule agents and pharmacodynamic and pharmacokinetic properties are still major issues to overcome (Monroig Pdel et al. 2015). Several recent studies have focused on determining miRNAs involved in HGSOC platinum resistance (Table 1). The most promising targets to date are miR-622 (Choi et al. 2016), which targets the Ku pathway and downregulates NHEJ; miR-484 that targets VEGFB and VEGFR2 pathways and tumour vasculature (Vecchione et al. 2013); and a miRNA profile of 9 miRNAs that are involved in regulation of EMT and TGF/WNT signalling(Boac et al. 2016). Overexpression of miR-27a, miR-23a, miR-30c, Let-7g, miR-199a-3p (Eitan et al. 2009), miR-141-3p (Ying et al. 2015) and many others (reviewed in Yu et al. 2017) have also been associated with cisplatin resistance in HGSOC, therefore, determing which miRNAs are the best for miRNA targeted therapy development will be a challenge.

Table 1

miRNAs associated with HGSOC platinum resistance.

miRNA Action Effect on platinum chemotherapy Reference
Let-7b Overexpression in HGSOC Poor survival and resistance to chemotherapy Tang et al. (2014)
miR-9 Downregulates BRCA1High levels in SOC Sensitizes to cisplatinBetter response and longer PFS Sun et al. (2013)
miR-21 Over expression in HGSOC from the TGCAOver expression in A2780 cisplatin-resistant cells regulates Programmed cell death 4, c-IAP2 and NAV3 Shorter PFSCisplatin resistance Chan et al. (2014)Pink et al. (2015)
miR-27amiR-23amiR-30cLet-7gmiR-199a-3p Overexpression in stage I and stage III HGSOC Cisplatin resistance Eitan et al. (2009)
miR-141-3p Overexpression in OC cell lines Cisplatin resistance Ying et al. (2015)
miR-146a Overexpression in SOC omental lesions Cisplatin resistance Vang et al. (2013)
miR-150 Overexpression in SOC omental lesions Cisplatin resistance Vang et al. (2013)
miR-181a Over expression in SOCSuppresses Smad7 and mediates EMT Cisplatin resistance, shorter OS and PFS Pink et al. (2015)
miR-484 Low expression in SOC, targets VEGFB and VEGFR2 pathways and tumour vasculature Poor chemotherapy response (stable or progressive disease)Does not mediate chemoresistance in vitro Vecchione et al. (2013)
miR-622 Targets the Ku pathway and downregulates NHEJ Mediates chemoresistance Choi et al. (2016)
Profile of 9 miRNAs Regulation of EMT and TGF/WNT signaling Mediates chemoresistance Boac et al. (2016)

Biomarkers that can be targeted using existing therapeutics

DNA methylation

DNA methylation is a key epigenetic regulator of gene expression. Aberrant DNA methylation has long been recognized as a contributing factor to the development of cancer. DNA methylation biomarkers have proven to be robust prognostic biomarkers of HGSOC. Wei and coworkers reported a set of 112 methylated loci that predicted progression-free survival after platinum chemotherapy with 95% accuracy (Wei et al. 2006). Progression-free survival after platinum chemotherapy was significantly shorter for patients with higher levels of methylation, suggesting that CpG island methylation is a strong biomarker to target to overcome chemotherapy resistance (Wei et al. 2006). Similarly, the Cancer Genome Atlas Network (Cancer Genome Atlas Research 2011) reported 168 genes as epigenetically silenced in HGSOC due to elevated DNA methylation and reduced tumour expression. The consistent methylation profiles reported for HGSOC (Gloss & Samimi 2014) have been a target for testing new combination treatment regimes (Fig. 4).

Figure 4
Figure 4

Therapeutic targets: epigenetic and immune biomarkers of HGSOC platinum chemotherapy resistance. Potential nuclear therapeutic targets/biomakers for platinum-resistant HGSOC are histone deactylases (HDA), DNA methylation (mC) and high C(C>T)C mutation load across the genome. Potential cell surface for platinum-resistant HGSOC are tumour-specific antigens and PDL-1 (PD-1 on T-cells).

Citation: Endocrine-Related Cancer 25, 5; 10.1530/ERC-17-0336

Cancer therapeutics to inhibit DNA methyltransferases have been successfully developed and approved for treatment. In 2004, azacitidine (DNA and RNA DNMT inhibitor) was approved for treatment of myelodysplastic syndrome (MDS), followed by decitabine (DNA-specific DNMT inhibitor) approval in 2006. DNMT inhibitors are cytotoxic when given at higher doses, but Fang et al. (2010) were the first to assess decitabine at repeated low dose to reduce DNA methylation and re-instate cisplatin sensitivity in a Phase 1 clinical trial for HGSOC. The combination was well tolerated with minimal adverse events. The follow-up Phase 2 trial reported 12/17 platinum-resistant patients had a complete response, partial response or stable disease after repeated low-dose decitabine followed by carboplatin (Matei et al. 2011, 2012, Fang et al. 2014). This is in contrast to Glasspool et al. (2014) who reported that the addition of a single-dose of decitabine 7 days before carboplatin reduced the efficacy of carboplatin in patients with partially platinum-sensitive HGSOC (relapsed 6–12 months after previous platinum therapy). The study authors concluded that assessment of patient selection strategies, different treatment schedules and alternative DNMT inhibitors should be considered.

Azacitidine and carboplatin combination treatment for platinum-resistant HGSOC was assessed in a phase 1b/2a clinical trial (Fu et al. 2011). Thirty patients received azacitidine for 5 days and carboplatin on day 2 every 28 days. The overall response rate (ORR) was 13.8% (4/29; 95% CI, 10.1–17.5%): 1 patient achieved a clinical complete response, 3 patients achieved clinical partial response and 10 patients had stable disease, with a median survival of 14 months (Fu et al. 2011). The results of clinical trials that have combined demethylating agents and carboplatin indicate that repeated low-dose, patient selection or inclusion of additional agents is required to achieve clinical benefit from this combination, and it is a promising area for further studies.

Histone deacetylases

Histone acetylation is another component of epigenetic regualtion of gene expression. Histone deacetylases (HDAC) actively mediate the level of acetylation of histone structures, when high deacetylation is present the result is suppression of gene expression (reviewed in Ropero & Esteller 2007). Altered expression and mutations in HDACs have been reported in most cancers (Fraga et al. 2005), therefore, HDAC inhibitors were developed as promising cancer therapeutics.

Human HDACs are grouped into classes based on their homology to yeast HDACs. HDAC inhibitors that target the different structures of Class I, II and IV HDACs are made up of hydroxamic acids, carboxylic acids, benzamides, epoxides and cyclic peptides (Delcuve et al. 2012). The common mechanism of action for HDAC inhibitors is hyperacetylation of histones resulting in an open chromatin structure. The open chromatin structure is thought to allow better access for DNA-damaging therapies such as platinum chemotherapy, resulting in higher levels of apoptosis (Sato et al. 2006). DNA methyltransferase and HDAC inhibitors are synergistic in re-expression of epigenetically silenced genes (Cameron et al. 1999), therefore, combination therapies targeting epigenetic regulation of platinum resistance are of intense interest.

Falchook et al. (2013) assessed the safety and efficacy of combination DNMT inhibitor azacitidine, HDAC inhibitor valproic acid and carboplatin in a cohort of 32 patients with treatment-resistant solid tumours, including 10 platinum-resistant ovarian cancer patients. Among the patients with ovarian cancer, three (30%) achieved minor partial responses or stable disease lasting ≥4 months. Dose delays and reductions due to adverse events, including grade ≥3 fatigue and neutropenia in the majority of patients, made assessment of the combination difficult. The authors concluded that lower continuous treatment doses and patient selection by methylation status warrants further follow-up studies.

DNA repair

Homologous recombination repair

Homologous recombination repair (HR) repairs double-strand breaks that occur as a result of many DNA-damaging insults including ionizing radiation and chemotherapy (Powell & Kachnic 2003). The mechanistic role of HR in platinum chemotherapy response is to repair double-strand DNA breaks that occur at sites of platinum crosslinks during DNA replication. BRCA1 and BRCA2 are members of the (HR) repair pathway (Fig. 2) and have been associated with risk of developing HGSOC (reviewed in Powell & Kachnic 2003). The Cancer Genome Atlas network used integrated analysis of mRNA, miRNA, methylation and DNA copy number to determine that approximately 50% of HGSOC are HR deficient (Cancer Genome Atlas Research 2011), indicating that DNA repair deficiency is a key driver of HGSOC.

BRCA mutations

Germline mutations in BRCA1 and BRCA2 are the most established risk factor for HGSOC. The largest study of BRCA mutation incidence to date (n = 1001), reported 14.1% of ovarian cancer patients have a germline BRCA mutation, with the highest incidence of 22.6% in HGSOC patients (Alsop et al. 2012). HR deficiency resulting from BRCA mutations leads to an accumulation of double-strand breaks after platinum chemotherapy, which in turn causes increased apoptosis and platinum sensitivity. It is well established that BRCA mutation carriers with HGSOC are more sensitive to platinum chemotherapy regimes and have longer overall survival than non-carriers (Vencken et al. 2011, Alsop et al. 2012, Rudaitis et al. 2014). Initially sensitive to platinum, BRCA mutation carriers eventually become resistant (Alsop et al. 2012) and attempts to inhibit other components of the HR pathway for further treatment have proven successful for some HGSOC patients, in particular, PARP inhibitors (reviewed in Scott et al. 2015).

PARP inhibitors are a synthetically lethal therapeutic cancers with DNA repair defects, particularly BRCA1 or BRCA2 mutations. In HR-deficient tumours, PARP inhibition blocks a downstream DNA repair process, which triggers apoptosis. Olaparib, is the first PARP inhibitor to be approved in most countries as maintenance treatment for patients with platinum-sensitive, relapsed ovarian cancer and a germline or somatic BRCA1/2 mutation or as monotherapy for advanced ovarian cancer patients with a germline BRCA1/2 mutation (Pujade-Lauraine et al. 2017).

BRCAness

In addition to germline BCRA mutations, somatic BRCA mutations occur in HGSOCs. Moschetta and colleagues recently reviewed studies reporting somatic BRCA mutations and concluded 5–7% of HGSOC contain somatic BRCA mutations (Moschetta et al. 2016). Hypermethylation of the BRCA1 promoter results in reduced expression of BRCA1 resulting in a HR-deficient phenotype in approximately 11% of HGSOCs (Geisler et al. 2002, Patch et al. 2015). Similarly, amplification of EMSY, which encodes a BRCA2-binding partner, leads to impairment of BRCA2 function (Wilkerson et al. 2011). Collectively, somatic BRCA mutations, hypermethylation of BRCA1 promoter and EMSY amplification all result in HGSOC phenotypes and platinum chemotherapy response similar to germline BRCA mutation carriers and are referred to as HGSOC with ‘BRCAness’.

In recent times, the BRCA ‘wild-type’ HGSOC subtype has not received as much attention as the BRCA mutant/BRCAness subtype with HR deficiency. The BRCA ‘wild-type’ or HR proficient subtype are more likely to be platinum resistant; therefore, it is a strong clinical subgroup to target for further development of platinum resistance biomarkers.

Intricate biological processes such as DNA repair rarely work in isolation. Most often, many of the proteins associated with a particular pathway have overlapping roles in multiple pathways and each process interacts with other similar processes. DNA repair is no exception, of the 6 DNA repair pathways HR interacts most closely with nucleotide excision repair (NER), the process responsible for recognizing platinum-induced DNA crosslinks as bulky adducts before double-strand breaks occur.

Nucleotide excision repair

The NER pathway consists of approximately 30 proteins that remove helix-distorting lesions such as platinum chemotherapy crosslinks via a step-wise process: damage recognition, unwinding of the DNA locally around damage, incision of damaged DNA by endonucleases and DNA resynthesis and ligation (Costa et al. 2003) (Fig. 2). There are two branches of damage recognition that converge on a common repair pathway: transcription coupled repair (TCR) and global genome repair (GGR). TCR is linked to active gene transcription and is initiated when RNA polymerase is stalled at DNA damage during transcription. GGR however is not dependent on transcription and scans the entire genome including both active and silent genes and non-transcribed regions using DNA damage-binding proteins XPC and UV-DDB (DDB1 and DDB2) (Nouspikel 2009). Early studies found an association between higher expression of NER mRNA (Dabholkar et al. 1992, 1994) before treatment with platinum resistance in ovarian cancer. This suggested over-active NER repairs platinum-induced DNA crosslinks before double-strand breaks can occur, indicating that NER expression could be a predictive biomarker of platinum response. This area has received little interest in the last 10 years, possibly due to the seemingly opposite discovery that when quantified after platinum chemotherapy, low NER expression has been associated with platinum resistance (Stevens et al. 2005, Barakat et al. 2010).

Recognition of excessive DNA crosslinks by the GGR portion of the NER pathway triggers apoptosis rather than attempting to repair the damage (Stoyanova et al. 2009) (Fig. 3). Therefore, lack of induction of GGR after platinum chemotherapy results in a reduction of both cross-link repair by the NER pathway and apoptotic signalling, ultimately leading to limited or no response to platinum treatment. Lower levels of NER post-platinum chemotherapy have been confirmed in ovarian cancer studies including low DDB2 in platinum-resistant ovarian cancer cell lines (Barakat et al. 2010) and low XPA in platinum-resistant ovarian cancer tumours (Barakat et al. 2010). In addition, low NER and platinum resistance has been reported for several other cancer types including non-small-cell lung cancer, gastric cancer, colorectal cancer and melanoma (Bowden et al. 2010, 2013). NER proteins are a promising area in biomarker development for platinum resistance for use in real-time clinical settings after platinum treatment has concluded.

NER recognizes DNA crosslinks caused by platinum chemotherapy and converts the cross-link to a DNA double-strand break. HR is then required to repair the double-strand break to prevent apoptosis (Stergiou et al. 2011) (Fig. 3). For HGSOC with BRCA mutations or ‘BRCAness’ HR deficiency is already present. If deficient NER is also present, it may be the cause of the eventual platinum resistance seen in HR-deficient HGSOC. In addition, if deficient NER is not processing the DNA crosslinks into double-strand breaks, there is no requirement for HR to repair double-strand DNA breaks. Therefore, targeting HR deficiency with PARP inhibitors is ineffective, this requires further investigation but may be the underlying cause of PARPi failure in some patients (Fig. 3).

Platinum-resistant HGSOC with functional NER and HR deficiency is likely to be sensitive to trabectedin, a transcription factor inhibitor (Schoffski et al. 2011). Trabectedin shows decreased activity (2- to 8-fold) in NER-deficient cell lines, while cells deficient in HR are approximately 100 times more sensitive to the drug, indicating that trabectedin relies on DNA double-strand breaks (Herrero et al. 2006).

More studies into the role of NER and HR in HGSOC platinum resistance are required to understand the relationship and potentially develop ways to subtype HGSOC based on NER and HR proficiency.

Mutation load

The relationship between DNA repair dysfunction and increased mutation load across the cancer genome is well established (Alexandrov et al. 2013, Le et al. 2015). Ovarian cancer has a lower mutation load than cancer types such as melanoma and non-small-cell lung cancer, which have a high mutation load as a result of environmental and chemical carcinogen exposure (Alexandrov et al. 2013). The cancer types with high mutational load have historically been difficult to treat, but have shown exceptional response to immune checkpoint inhibitors (Le et al. 2015, Antonia et al. 2016, Ugurel et al. 2017). Patch and coworkers recently reported a significant C(C>T)C platinum chemotherapy imprint in the genome of platinum-resistant HGSOC (Patch et al. 2015). The platinum chemotherapy imprint is similar to the C>T UV-fingerprint consistently seen across the melanoma genome (Pleasance et al. 2010). Both platinum chemotherapy and UV-light DNA damage require NER and HR to either repair the damage or trigger apoptosis. If only one of the pathways is functional, it may compensate for the other when challenged with platinum chemotherapy. This further supports the need for both NER and HR to be further developed as dual biomarkers of platinum resistance in HGSOC.

The increase in mutation load in platinum-resistant HGSOC is likely to be a strong predictor of response to immune checkpoint inhibitors, due to the increase in cancer-specific antigens (Fig. 4). Therefore, platinum resistance itself is a potential biomarker for predicting HGSOC responders to checkpoint inhibition. However, it has become clear from extensive clinical trial follow-up in melanoma that mutation load alone is not the strongest predictor of durable response to checkpoint inhibition. The extent and subtype of tumour-infiltrating lymphocytes (TILs), immune cell subsets in peripheral blood and the extent of disease are also strong predictors of response.

Immune cell subsets

The recent advancement of immune checkpoint inhibitors such as anti-PD1 (pembrolizumab, nivolumab and avelumab) and anti-CTLA4 (ipilumimab) monoclonal antibodies has led to biomarker development in relation to TILs (Tumeh et al. 2014) and circulating immune cell subsets (Huang et al. 2017). Pretreatment tumour samples obtained from patients that responded to anti-PD1 immunotherapy had higher numbers of CD8+, PD-1 and PD-L1-expressing cells at the invasive tumour margin and inside tumours (Tumeh et al. 2014). Several studies have performed similar analysis in HGSOC and found higher levels of CD8+ TILs in stromal tissue were associated with better overall survival. Lo et al. (2017) reported increased densities of CD3+ and CD8+ and PD-1+ T-cells in HGSOC after platinum chemotherapy. However, the increase in these T-cell subtypes was dependent on presence before treatment, indicating that platinum chemotherapy can induce a desired immune response, but only if the required T-cells are already present in the tumour (Tumeh et al. 2014). It is feasible that TILs present in platinum-resistant HGSOC before and after treatment could be used to predict response to checkpoint inhibitors (Fig. 4).

Conclusion

There is a suite of approved cancer therapeutics, with established safety and toxicity profiles, that should be assessed in the immediate future based on biomarkers of platinum-resistant HGSOC (Table 2). Several early phase clinical trials using methylation, HDAC and immunotherapy agents have already reported promising results (Table 3). Retrospective analysis of specific biomarkers in patient cohorts that received these therapies may shine a light on why a good response occurred in only a subset of patients, which will inform a new round of trials with selected patient populations.

Table 2

Biomarkers for alternative treatment for platinum-resistant HGSOC.

Biomarkers of platinum-resistant HGSOC Alternative treatment Evidence of response to alternative treatment References
High mutation load Checkpoint inhibitor Immunotherapy (anti-CTLA4 and anti-PD1) MelanomaNSCLCsMismatch repair deficient colorectal cancer Takai et al. (2014), Tang et al. (2014), Thiery & Sleeman (2006)
CD8, PD-1 and PD-L1-expressing cells inside tumours Checkpoint inhibitor Immunotherapy (anti-CTLA4 and anti-PD1) Melanoma Tumeh et al. (2014)
Low or absent NER and BRCA mutation/BRCAness PARP inhibitors Ovarian cancer Reviewed in Reles et al. (2001)
Normal NER and and BRCA mutation/BRCAness Trabectidin Sarcoma Stoyanova et al. (2009)
Methylation marker panel Azacytidine and carboplatinAzacitidine, valporic acid (HDAC inhibitor) and carboplatin Ovarian cancerOvarian, prostate, cervical and colorectal cancer Nouspikel (2009) Prasetyanti & Medema (2017)
Table 3

Clinical trials using agents to target high-grade serous ovarian cancer platinum resistance biomarkers.

Agents Biomarker target Outcomes Reference
Decitabine, repeated low-dose followed by cisplatin DNA methylation Phase 1: Minimal adverse events Fang et al. (2010)
Decitabine, repeated low-dose followed by cisplatin DNA methylation Phase 2: 12/17 platinum-resistant patients had CR, PR or SD Matei et al. (2011, 2012)
Azacitdine and carboplatin DNA methylation Phase 1b/2a: ORR 4/29 patients (1 CR, 3 PR, 10 SD), median survival of 10 months Fu et al. (2011)
Azacitidine, valproic acid and carboplatin DNA methylation and histone deactylation Phase 2: 3/10 patients had PR or SD Falchook et al. (2013)
Avelumab (anti-PD-L1 immune checkpoint inhibitor) High mutation load (heavy platinum pretreatment) Phase 1b: 41/75 patients had PR or SD Disis et al. (2015)
Pembrolizumab (anti-PD-1 immune checkpoint inhibitor) Expression of PD-1 and High mutation load (heavy platinum pretreatment) Phase 1b: 9/26 patients has CR, PR or SD Varga et al. (2015)
Nivolumab (anti-PD-1 immune checkpoint inhibitor) High mutation load (heavy platinum pretreatment) Phase 1b: 9/20 CR, PR or SD Hamanishi et al. (2015)

Patient selection, dose selection, treatment timing and different combinations of the therapies listed in Table 2 will also be key to identifying effective treatments. Rather than focussing on single proteins, pathways or biological processes, as new therapeutics are developed the highly mutated, immunogenic and epigenetically altered platinum-resistant HGSOC phenotype should be exploited to overcome treatment resistance.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this review.

Funding

B v Z is supported by the Hunter Medical Research Institute Vanessa McGuigan Memorial Ovarian Cancer Project and N A B is supported by the Cancer Institute NSW and University of Newcastle, Australia.

Author contribution statement

B v Z and D T participated in writing the manuscript. N A B devised and oversaw the literature analysis and interpretation and writing the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors would like to acknowledge the Priority Research Centre for Cancer Research, Innovation and Translation, University of Newcastle, Australia who provided support for writing days to complete this review.

References

  • Alexandrov LB, Nik-Zainal S, Wedge DC, Aparicio SA, Behjati S, Biankin AV, Bignell GR, Bolli N, Borg A, Borresen-Dale AL, et al. 2013 Signatures of mutational processes in human cancer. Nature 500 415421. (https://doi.org/10.1038/nature12477)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Alsop K, Fereday S, Meldrum C, deFazio A, Emmanuel C, George J, Dobrovic A, Birrer MJ, Webb PM, Stewart C, et al. 2012 BRCA mutation frequency and patterns of treatment response in BRCA mutation-positive women with ovarian cancer: a report from the Australian Ovarian Cancer Study Group. Journal of Clinical Oncology 30 26542663. (https://doi.org/10.1200/JCO.2011.39.8545)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Antonia SJ, Lopez-Martin JA, Bendell J, Ott PA, Taylor M, Eder JP, Jager D, Pietanza MC, Le DT, de Braud F, et al. 2016 Nivolumab alone and nivolumab plus ipilimumab in recurrent small-cell lung cancer (CheckMate 032): a multicentre, open-label, phase 1/2 trial. Lancet Oncology 17 883895. (https://doi.org/10.1016/S1470-2045(16)30098-5)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Banet N & Kurman RJ 2015 Two types of ovarian cortical inclusion cysts: proposed origin and possible role in ovarian serous carcinogenesis. International Journal of Gynecological Pathology 34 38. (https://doi.org/10.1097/PGP.0000000000000120)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Barakat BM, Wang QE, Han C, Milum K, Yin DT, Zhao Q, Wani G, Arafa el SA, El-Mahdy MA & Wani AA 2010 Overexpression of DDB2 enhances the sensitivity of human ovarian cancer cells to cisplatin by augmenting cellular apoptosis. International Journal of Cancer 127 977988. (https://doi.org/10.1002/ijc.25112)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Boac BM, Xiong Y, Marchion DC, Abbasi F, Bush SH, Ramirez IJ, Khulpateea BR, Clair McClung E, Berry AL, Bou Zgheib N, et al. 2016 Micro-RNAs associated with the evolution of ovarian cancer cisplatin resistance. Gynecologic Oncology 140 259263. (https://doi.org/10.1016/j.ygyno.2015.12.026)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Bowden NA, Ashton KA, Avery-Kiejda KA, Zhang XD, Hersey P & Scott RJ 2010 Nucleotide excision repair gene expression after Cisplatin treatment in melanoma. Cancer Research 70 79187926. (https://doi.org/10.1158/0008-5472.CAN-10-0161)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Bowden NA, Ashton KA, Vilain RE, Avery-Kiejda KA, Davey RJ, Murray HC, Budden T, Braye SG, Zhang XD, Hersey P, et al. 2013 Regulators of global genome repair do not respond to DNA damaging therapy but correlate with survival in melanoma. PLoS ONE 8 e70424. (https://doi.org/10.1371/journal.pone.0070424)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Cameron EE, Bachman KE, Myohanen S, Herman JG & Baylin SB 1999 Synergy of demethylation and histone deacetylase inhibition in the re-expression of genes silenced in cancer. Nature Genetics 21 103107. (https://doi.org/10.1038/5047)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Cancer Genome Atlas Research Network 2011 Integrated genomic analyses of ovarian carcinoma. Nature 474 609615. (https://doi.org/10.1038/nature10166)

  • Chan JK, Blansit K, Kiet T, Sherman A, Wong G, Earle C & Bourguignon LY 2014 The inhibition of miR-21 promotes apoptosis and chemosensitivity in ovarian cancer. Gynecologic Oncology 132 739744. (https://doi.org/10.1016/j.ygyno.2014.01.034)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chang PY, Liao YP, Wang HC, Chen YC, Huang RL, Wang YC, Yuan CC & Lai HC 2017 An epigenetic signature of adhesion molecules predicts poor prognosis of ovarian cancer patients. Oncotarget 8 5343253449. (https://doi.org/10.18632/oncotarget.18515)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chebouti I, Kasimir-Bauer S, Buderath P, Wimberger P, Hauch S, Kimmig R & Kuhlmann JD 2017 EMT-like circulating tumor cells in ovarian cancer patients are enriched by platinum-based chemotherapy. Oncotarget 8 4882048831. (https://doi.org/10.18632/oncotarget.16179)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chen L, Zhang K, Shi Z, Zhang A, Jia Z, Wang G, Pu P, Kang C & Han L 2014 A lentivirus-mediated miR-23b sponge diminishes the malignant phenotype of glioma cells in vitro and in vivo. Oncology Reports 31 15731580. (https://doi.org/10.3892/or.2014.3012)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Choi YJ, Ingram PN, Yang K, Coffman L, Iyengar M, Bai S, Thomas DG, Yoon E & Buckanovich RJ 2015 Identifying an ovarian cancer cell hierarchy regulated by bone morphogenetic protein 2. PNAS 112 E6882E6888. (https://doi.org/10.1073/pnas.1507899112)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Choi YE, Meghani K, Brault ME, Leclerc L, He YJ, Day TA, Elias KM, Drapkin R, Weinstock DM, Dao F, et al. 2016 Platinum and PARP inhibitor resistance due to overexpression of microRNA-622 in BRCA1-mutant ovarian cancer. Cell Reports 14 429439. (https://doi.org/10.1016/j.celrep.2015.12.046)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Clements PB & Young RH 2008 Atlas of Gynecologic Surgical Pathology. Philadelphia, PA, USA: Saunders Elsevier.

  • Clevers H 2011 The cancer stem cell: premises, promises and challenges. Nature Medicine 17 313319. (https://doi.org/10.1038/nm.2304)

  • Cole AJ, Dwight T, Gill AJ, Dickson KA, Zhu Y, Clarkson A, Gard GB, Maidens J, Valmadre S, Clifton-Bligh R, et al. 2016 Assessing mutant p53 in primary high-grade serous ovarian cancer using immunohistochemistry and massively parallel sequencing. Scientific Reports 6 26191. (https://doi.org/10.1038/srep26191)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Costa RMA, Chiganças V, da Silva Galhardo R, Carvalho H & Menck CFM 2003 The eukaryotic nucleotide excision repair pathway. Biochimie 85 10831099. (https://doi.org/10.1016/j.biochi.2003.10.017)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • D’Incalci M & Galmarini CM 2010 A review of trabectedin (ET-743): a unique mechanism of action. Molecular Cancer Therapeutics 9 21572163. (https://doi.org/10.1158/1535-7163.MCT-10-0263)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Dabholkar M, Bostick-Bruton F, Weber C, Bohr VA, Egwuagu C & Reed E 1992 ERCC1 and ERCC2 expression in malignant tissues from ovarian cancer patients. Journal of the National Cancer Institute 84 15121517. (https://doi.org/10.1093/jnci/84.19.1512)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Dabholkar M, Vionnet J, Bostick-Bruton F, Yu JJ & Reed E 1994 Messenger RNA levels of XPAC and ERCC1 in ovarian cancer tissue correlate with response to platinum-based chemotherapy. Journal of Clinical Investigation 94 703708. (https://doi.org/10.1172/JCI117388)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Davidson B, Holth A, Hellesylt E, Tan TZ, Huang RY, Trope C, Nesland JM & Thiery JP 2015 The clinical role of epithelial-mesenchymal transition and stem cell markers in advanced-stage ovarian serous carcinoma effusions. Human Pathology 46 18. (https://doi.org/10.1016/j.humpath.2014.10.004)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Davis A, Tinker AV & Friedlander M 2014 “Platinum resistant” ovarian cancer: what is it, who to treat and how to measure benefit? Gynecologic Oncology 133 624631. (https://doi.org/10.1016/j.ygyno.2014.02.038)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Delcuve GP, Khan DH & Davie JR 2012 Roles of histone deacetylases in epigenetic regulation: emerging paradigms from studies with inhibitors. Clinical Epigenetics 4 5. (https://doi.org/10.1186/1868-7083-4-5)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Disis ML, Patel MR, Pant S, Infante JR, Lockhart AC, Kelly K, Beck JT, Gordon MS, Weiss GJ, Ejadi S, et al. 2015 Avelumab (MSB0010718C), an anti-PD-L1 antibody, in patients with previously treated, recurrent or refractory ovarian cancer: a phase Ib, open-label expansion trial. American Society of Clinical Oncology 33 (15 Suppl) 5509. (https://doi.org/10.1200/jco.2015.33.15_suppl.5509)

    • Search Google Scholar
    • Export Citation
  • Ebert MS, Neilson JR & Sharp PA 2007 MicroRNA sponges: competitive inhibitors of small RNAs in mammalian cells. Nature Methods 4 721726. (https://doi.org/10.1038/nmeth1079)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Eitan R, Kushnir M, Lithwick-Yanai G, David MB, Hoshen M, Glezerman M, Hod M, Sabah G, Rosenwald S & Levavi H 2009 Tumor microRNA expression patterns associated with resistance to platinum based chemotherapy and survival in ovarian cancer patients. Gynecologic Oncology 114 253259. (https://doi.org/10.1016/j.ygyno.2009.04.024)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Ervik M, Lam F, Ferlay J, Mery L, Soerjomataram I & Bray F 2016 Cancer today. In Cancer Today. Ed IAfRo Cancer. Lyon, France: International Agency for Research on Cancer.

    • Search Google Scholar
    • Export Citation
  • Falchook GS, Fu S, Naing A, Hong DS, Hu W, Moulder S, Wheler JJ, Sood AK, Bustinza-Linares E, Parkhurst KL, et al. 2013 Methylation and histone deacetylase inhibition in combination with platinum treatment in patients with advanced malignancies. Investigational New Drugs 31 11921200. (https://doi.org/10.1007/s10637-013-0003-3)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Fang F, Balch C, Schilder J, Breen T, Zhang S, Shen C, Li L, Kulesavage C, Snyder AJ, Nephew KP, et al. 2010 A phase 1 and pharmacodynamic study of decitabine in combination with carboplatin in patients with recurrent, platinum-resistant, epithelial ovarian cancer. Cancer 116 40434053. (https://doi.org/10.1002/cncr.25204)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Fang F, Zuo Q, Pilrose J, Wang Y, Shen C, Li M, Wulfridge P, Matei D & Nephew KP 2014 Decitabine reactivated pathways in platinum resistant ovarian cancer. Oncotarget 5 35793589.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Fathalla MF 1971 Incessant ovulation – a factor in ovarian neoplasia? Lancet 2 163. (https://doi.org/10.1016/S0140-6736(71)92335-X)

  • Fraga MF, Ballestar E, Villar-Garea A, Boix-Chornet M, Espada J, Schotta G, Bonaldi T, Haydon C, Ropero S, Petrie K, et al. 2005 Loss of acetylation at Lys16 and trimethylation at Lys20 of histone H4 is a common hallmark of human cancer. Nature Genetics 37 391400. (https://doi.org/10.1038/ng1531)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Fu S, Hu W, Iyer R, Kavanagh JJ, Coleman RL, Levenback CF, Sood AK, Wolf JK, Gershenson DM, Markman M, et al. 2011 Phase 1b-2a study to reverse platinum resistance through use of a hypomethylating agent, azacitidine, in patients with platinum-resistant or platinum-refractory epithelial ovarian cancer. Cancer 117 16611669. (https://doi.org/10.1002/cncr.25701)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Geisler JP, Hatterman-Zogg MA, Rathe JA & Buller RE 2002 Frequency of BRCA1 dysfunction in ovarian cancer. Journal of the National Cancer Institute 94 6167. (https://doi.org/10.1093/jnci/94.1.61)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Glasspool RM, Brown R, Gore ME, Rustin GJ, McNeish IA, Wilson RH, Pledge S, Paul J, Mackean M, Hall GD, et al. 2014 A randomised, phase II trial of the DNA-hypomethylating agent 5-aza-2′-deoxycytidine (decitabine) in combination with carboplatin vs carboplatin alone in patients with recurrent, partially platinum-sensitive ovarian cancer. British Journal of Cancer 110 19231929. (https://doi.org/10.1038/bjc.2014.116)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Gloss BS & Samimi G 2014 Epigenetic biomarkers in epithelial ovarian cancer. Cancer Letters 342 257263. (https://doi.org/10.1016/j.canlet.2011.12.036)

  • Hamanishi J, Mandai M, Ikeda T, Minami M, Kawaguchi A, Murayama T, Kanai M, Mori Y, Matsumoto S, Chikuma S, et al. 2015 Safety and antitumor activity of Anti-PD-1 antibody, nivolumab, in patients with platinum-resistant ovarian cancer. Journal of Clinical Oncology 33 40154022. (https://doi.org/10.1200/JCO.2015.62.3397)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Harter P, Hilpert F, Mahner S, Heitz F, Pfisterer J & du Bois A 2010 Systemic therapy in recurrent ovarian cancer: current treatment options and new drugs. Expert Review of Anticancer Therapy 10 8188. (https://doi.org/10.1586/era.09.165)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Herrero AB, Martin-Castellanos C, Marco E, Gago F & Moreno S 2006 Cross-talk between nucleotide excision and homologous recombination DNA repair pathways in the mechanism of action of antitumor trabectedin. Cancer Research 66 81558162. (https://doi.org/10.1158/0008-5472.CAN-06-0179)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Huang RY, Chung VY & Thiery JP 2012 Targeting pathways contributing to epithelial-mesenchymal transition (EMT) in epithelial ovarian cancer. Current Drug Targets 13 16491653. (https://doi.org/10.2174/138945012803530044)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Huang AC, Postow MA, Orlowski RJ, Mick R, Bengsch B, Manne S, Xu W, Harmon S, Giles JR, Wenz B, et al. 2017 T-cell invigoration to tumour burden ratio associated with anti-PD-1 response. Nature 545 6065. (https://doi.org/10.1038/nature22079)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Kampan NC, Madondo MT, McNally OM, Quinn M & Plebanski M 2015 Paclitaxel and its evolving role in the management of ovarian cancer. BioMed Research International 2015 413076. (https://doi.org/10.1155/2015/413076)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Kindelberger DW, Lee Y, Miron A, Hirsch MS, Feltmate C, Medeiros F, Callahan MJ, Garner EO, Gordon RW, Birch C, et al. 2007 Intraepithelial carcinoma of the fimbria and pelvic serous carcinoma: evidence for a causal relationship. American Journal of Surgical Pathology 31 161169. (https://doi.org/10.1097/01.pas.0000213335.40358.47)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kroeger PT Jr & Drapkin R 2017 Pathogenesis and heterogeneity of ovarian cancer. Current Opinion in Obstetrics and Gynecology 29 2634. (https://doi.org/10.1097/GCO.0000000000000340)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kryczek I, Liu S, Roh M, Vatan L, Szeliga W, Wei S, Banerjee M, Mao Y, Kotarski J, Wicha MS, et al. 2012 Expression of aldehyde dehydrogenase and CD133 defines ovarian cancer stem cells. International Journal of Cancer 130 2939. (https://doi.org/10.1002/ijc.25967)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Kurman RJ 2013 Origin and molecular pathogenesis of ovarian high-grade serous carcinoma. Annals of Oncology 24 (Supplement 10) x16x21. (https://doi.org/10.1093/annonc/mdt463)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kurman RJ & ShihIe M 2010 The origin and pathogenesis of epithelial ovarian cancer: a proposed unifying theory. American Journal of Surgical Pathology 34 433443. (https://doi.org/10.1097/PAS.0b013e3181cf3d79)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Le DT, Uram JN, Wang H, Bartlett BR, Kemberling H, Eyring AD, Skora AD, Luber BS, Azad NS, Laheru D, et al. 2015 PD-1 blockade in tumors with mismatch-repair deficiency. New England Journal of Medicine 372 25092520. (https://doi.org/10.1056/NEJMoa1500596)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Le Page C, Ouellet V, Madore J, Ren F, Hudson TJ, Tonin PN, Provencher DM & Mes-Masson AM 2006 Gene expression profiling of primary cultures of ovarian epithelial cells identifies novel molecular classifiers of ovarian cancer. British Journal of Cancer 94 436445. (https://doi.org/10.1038/sj.bjc.6602933)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Le Page C, Puiffe ML, Meunier L, Zietarska M, de Ladurantaye M, Tonin PN, Provencher D & Mes-Masson AM 2009 BMP-2 signaling in ovarian cancer and its association with poor prognosis. Journal of Ovarian Research 2 4. (https://doi.org/10.1186/1757-2215-2-4)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Lee Y, Miron A, Drapkin R, Nucci MR, Medeiros F, Saleemuddin A, Garber J, Birch C, Mou H, Gordon RW, et al. 2007 A candidate precursor to serous carcinoma that originates in the distal fallopian tube. Journal of Pathology 211 2635. (https://doi.org/10.1002/path.2091)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lo CS, Sanii S, Kroeger DR, Milne K, Talhouk A, Chiu DS, Rahimi K, Shaw PA, Clarke BA & Nelson BH 2017 Neoadjuvant chemotherapy of ovarian cancer results in three patterns of tumor-infiltrating lymphocyte response with distinct implications for immunotherapy. Clinical Cancer Research 23 925934.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Malpica A, Deavers MT, Lu K, Bodurka DC, Atkinson EN, Gershenson DM & Silva EG 2004 Grading ovarian serous carcinoma using a two-tier system. American Journal of Surgical Pathology 28 496504. (https://doi.org/10.1097/00000478-200404000-00009)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Malpica A, Deavers MT, Tornos C, Kurman RJ, Soslow R, Seidman JD, Munsell MF, Gaertner E, Frishberg D & Silva EG 2007 Interobserver and intraobserver variability of a two-tier system for grading ovarian serous carcinoma. American Journal of Surgical Pathology 31 11681174. (https://doi.org/10.1097/PAS.0b013e31803199b0)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Marchini S, Fruscio R, Clivio L, Beltrame L, Porcu L, Fuso Nerini I, Cavalieri D, Chiorino G, Cattoretti G, Mangioni C, et al. 2013 Resistance to platinum-based chemotherapy is associated with epithelial to mesenchymal transition in epithelial ovarian cancer. European Journal of Cancer 49 520530. (https://doi.org/10.1016/j.ejca.2012.06.026)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Matei D, Shen C, Fang F, Schilder J, Li M, Arnold A, Zeng Y, Pilrose J, Kulesavage C, Balch C, et al. 2011 A phase II study of decitabine and carboplatin in recurrent platinum (Pt)-resistant ovarian cancer (OC). Journal of Clinical Oncology 29 21972205.

    • Search Google Scholar
    • Export Citation
  • Matei D, Fang F, Shen C, Schilder J, Arnold A, Zeng Y, Berry WA, Huang T & Nephew KP 2012 Epigenetic resensitization to platinum in ovarian cancer. Cancer Research 72 21972205. (https://doi.org/10.1158/0008-5472.CAN-11-3909)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • McCluggage WG 2008 My approach to and thoughts on the typing of ovarian carcinomas. Journal of Clinical Pathology 61 152163. (https://doi.org/10.1136/jcp.2007.049478)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Monroig Pdel C, Chen L, Zhang S & Calin GA 2015 Small molecule compounds targeting miRNAs for cancer therapy. Advanced Drug Delivery Reviews 81 104116. (https://doi.org/10.1016/j.addr.2014.09.002)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Moschetta M, George A, Kaye SB & Banerjee S 2016 BRCA somatic mutations and epigenetic BRCA modifications in serous ovarian cancer. Annals of Oncology 27 14491455. (https://doi.org/10.1093/annonc/mdw142)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Nouspikel T 2009 DNA repair in mammalian cells: nucleotide excision repair: variations on versatility. Cellular and Molecular Life Sciences 66 9941009. (https://doi.org/10.1007/s00018-009-8737-y)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • O’Neill CJ, Deavers MT, Malpica A, Foster H & McCluggage WG 2005 An immunohistochemical comparison between low-grade and high-grade ovarian serous carcinomas: significantly higher expression of p53, MIB1, BCL2, HER-2/neu, and C-KIT in high-grade neoplasms. American Journal of Surgical Pathology 29 10341041.

    • Search Google Scholar
    • Export Citation
  • Ottevanger PB 2017 Ovarian cancer stem cells more questions than answers. Seminars in Cancer Biology 44 6771. (https://doi.org/10.1016/j.semcancer.2017.04.009)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Oza AM, Cook AD, Pfisterer J, Embleton A, Ledermann JA, Pujade-Lauraine E, Kristensen G, Carey MS, Beale P, Cervantes A, et al. 2015 Standard chemotherapy with or without bevacizumab for women with newly diagnosed ovarian cancer (ICON7): overall survival results of a phase 3 randomised trial. Lancet Oncology 16 928936. (https://doi.org/10.1016/S1470-2045(15)00086-8)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Patch AM, Christie EL, Etemadmoghadam D, Garsed DW, George J, Fereday S, Nones K, Cowin P, Alsop K, Bailey PJ, et al. 2015 Whole-genome characterization of chemoresistant ovarian cancer. Nature 521 489494. (https://doi.org/10.1038/nature14410)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Pfisterer J, Plante M, Vergote I, du Bois A, Hirte H, Lacave AJ, Wagner U, Stahle A, Stuart G, Kimmig R, et al. 2006 Gemcitabine plus carboplatin compared with carboplatin in patients with platinum-sensitive recurrent ovarian cancer: an intergroup trial of the AGO-OVAR, the NCIC CTG, and the EORTC GCG. Journal of Clinical Oncology 24 46994707. (https://doi.org/10.1200/JCO.2006.06.0913)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pink RC, Samuel P, Massa D, Caley DP, Brooks SA & Carter DR 2015 The passenger strand, miR-21-3p, plays a role in mediating cisplatin resistance in ovarian cancer cells. Gynecologic Oncology 137 143151. (https://doi.org/10.1016/j.ygyno.2014.12.042)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pleasance ED, Cheetham RK, Stephens PJ, McBride DJ, Humphray SJ, Greenman CD, Varela I, Lin ML, Ordonez GR, Bignell GR, et al. 2010 A comprehensive catalogue of somatic mutations from a human cancer genome. Nature 463 191196. (https://doi.org/10.1038/nature08658)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Powell SN & Kachnic LA 2003 Roles of BRCA1 and BRCA2 in homologous recombination, DNA replication fidelity and the cellular response to ionizing radiation. Oncogene 22 57845791. (https://doi.org/10.1038/sj.onc.1206678)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Prasetyanti PR & Medema JP 2017 Intra-tumor heterogeneity from a cancer stem cell perspective. Molecular Cancer 16 41. (https://doi.org/10.1186/s12943-017-0600-4)

  • Prat J, Belhadj H, Berek J, Bermudez A, Bhatla N, Cain J, Denny L, Fujiwara K, Hacker N, Avall-Lundqvist E, et al. 2015 Abridged republication of FIGO’s staging classification for cancer of the ovary, fallopian tube, and peritoneum. European Journal of Gynaecological Oncology 36 367369. (https://doi.org/10.3802/jgo.2015.26.2.87)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Pujade-Lauraine E, Ledermann JA, Selle F, Gebski V, Penson RT, Oza AM, Korach J, Huzarski T, Poveda A, Pignata S, et al. 2017 Olaparib tablets as maintenance therapy in patients with platinum-sensitive, relapsed ovarian cancer and a BRCA1/2 mutation (SOLO2/ENGOT-Ov21): a double-blind, randomised, placebo-controlled, phase 3 trial. Lancet Oncology 18 12741284. (https://doi.org/10.1016/S1470-2045(17)30469-2)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pylvas-Eerola M, Liakka A, Puistola U, Koivunen J & Karihtala P 2016 Cancer stem cell properties as factors predictive of chemoresistance in neoadjuvantly-treated patients with ovarian cancer. Anticancer Research 36 34253431.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Ramalingam P 2016 Morphologic, immunophenotypic, and molecular features of epithelial ovarian cancer. Oncology 30 166176.

  • Reles A, Wen WH, Schmider A, Gee C, Runnebaum IB, Kilian U, Jones LA, El-Naggar A, Minguillon C, Schonborn I, et al. 2001 Correlation of p53 mutations with resistance to platinum-based chemotherapy and shortened survival in ovarian cancer. Clinical Cancer Research 7 29842997.

    • Search Google Scholar
    • Export Citation
  • Ropero S & Esteller M 2007 The role of histone deacetylases (HDACs) in human cancer. Molecular Oncology 1 1925. (https://doi.org/10.1016/j.molonc.2007.01.001)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Rosenberg B & VanCamp L 1970 The successful regression of large solid sarcoma 180 tumors by platinum compounds. Cancer Research 30 17991802.

  • Rosenberg B, Vancamp L & Krigas T 1965 Inhibition of cell division in Escherichia Coli by electrolysis products from a platinum electrode. Nature 205 698699. (https://doi.org/10.1038/205698a0)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Rudaitis V, Zvirblis T, Kanopiene D, Janulynaite D, Griskevicius L & Janavicius R 2014 BRCA1/2 mutation status is an independent factor of improved survival for advanced (stage III–IV) ovarian cancer. International Journal of Gynecological Cancer 24 13951400. (https://doi.org/10.1097/IGC.0000000000000247)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ruscito I, Cacsire Castillo-Tong D, Vergote I, Ignat I, Stanske M, Vanderstichele A, Ganapathi RN, Glajzer J, Kulbe H, Trillsch F, et al. 2017 Exploring the clonal evolution of CD133/aldehyde-dehydrogenase-1 (ALDH1)-positive cancer stem-like cells from primary to recurrent high-grade serous ovarian cancer (HGSOC). A study of the Ovarian Cancer Therapy-Innovative Models Prolong Survival (OCTIPS) Consortium. European Journal of Cancer 79 214225. (https://doi.org/10.1016/j.ejca.2017.04.016)

    • Search Google Scholar
    • Export Citation
  • Sato T, Suzuki M, Sato Y, Echigo S & Rikiishi H 2006 Sequence-dependent interaction between cisplatin and histone deacetylase inhibitors in human oral squamous cell carcinoma cells. International Journal of Oncology 28 12331241.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Schoffski P, Taron M, Jimeno J, Grosso F, Sanfilipio R, Casali PG, Le Cesne A, Jones RL, Blay JY, Poveda A, et al. 2011 Predictive impact of DNA repair functionality on clinical outcome of advanced sarcoma patients treated with trabectedin: a retrospective multicentric study. European Journal of Cancer 47 10061012. (https://doi.org/10.1016/j.ejca.2011.01.016)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Scott CL, Swisher EM & Kaufmann SH 2015 Poly (ADP-ribose) polymerase inhibitors: recent advances and future development. Journal of Clinical Oncology 33 13971406. (https://doi.org/10.1200/JCO.2014.58.8848)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Selvakumaran M, Pisarcik DA, Bao R, Yeung AT & Hamilton TC 2003 Enhanced cisplatin cytotoxicity by disturbing the nucleotide excision repair pathway in ovarian cancer cell lines. Cancer Research 63 13111316.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Shih Ie M & Kurman RJ 2004 Ovarian tumorigenesis: a proposed model based on morphological and molecular genetic analysis. American Journal of Pathology 164 15111518. (https://doi.org/10.1016/S0002-9440(10)63708-X)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Silva IA, Bai S, McLean K, Yang K, Griffith K, Thomas D, Ginestier C, Johnston C, Kueck A, Reynolds RK, et al. 2011 Aldehyde dehydrogenase in combination with CD133 defines angiogenic ovarian cancer stem cells that portend poor patient survival. Cancer Research 71 39914001. (https://doi.org/10.1158/0008-5472.CAN-10-3175)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Staropoli N, Ciliberto D, Botta C, Fiorillo L, Grimaldi A, Lama S, Caraglia M, Salvino A, Tassone P & Tagliaferri P 2014 Pegylated liposomal doxorubicin in the management of ovarian cancer: a systematic review and metaanalysis of randomized trials. Cancer Biology and Therapy 15 707720. (https://doi.org/10.4161/cbt.28557)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Steg AD, Bevis KS, Katre AA, Ziebarth A, Dobbin ZC, Alvarez RD, Zhang K, Conner M & Landen CN 2012 Stem cell pathways contribute to clinical chemoresistance in ovarian cancer. Clinical Cancer Research 18 869881. (https://doi.org/10.1158/1078-0432.CCR-11-2188)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Stergiou L, Eberhard R, Doukoumetzidis K & Hengartner MO 2011 NER and HR pathways act sequentially to promote UV-C-induced germ cell apoptosis in Caenorhabditis elegans. Cell Death and Differentiation 18 897906. (https://doi.org/10.1038/cdd.2010.158)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Stevens EV, Raffeld M, Espina V, Kristensen GB, Trope CG, Kohn EC & Davidson B 2005 Expression of xeroderma pigmentosum A protein predicts improved outcome in metastatic ovarian carcinoma. Cancer 103 23132319. (https://doi.org/10.1002/cncr.21031)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Stoyanova T, Roy N, Kopanja D, Bagchi S & Raychaudhuri P 2009 DDB2 decides cell fate following DNA damage. PNAS 106 1069010695. (https://doi.org/10.1073/pnas.0812254106)

  • Stuart GC, Kitchener H, Bacon M, duBois A, Friedlander M, Ledermann J, Marth C, Thigpen T, Trimble E, Participants of 4th Ovarian Cancer Consensus C, et al. 2011 2010 Gynecologic Cancer InterGroup (GCIG) consensus statement on clinical trials in ovarian cancer: report from the Fourth Ovarian Cancer Consensus Conference. International Journal of Gynecological Cancer 21 750755. (https://doi.org/10.1097/IGC.0b013e31821b2568)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Sun C, Li N, Yang Z, Zhou B, He Y, Weng D, Fang Y, Wu P, Chen P, Yang X, et al. 2013 miR-9 regulation of BRCA1 and ovarian cancer sensitivity to cisplatin and PARP inhibition. Journal of the National Cancer Institute 105 17501758. (https://doi.org/10.1093/jnci/djt302)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Takai M, Terai Y, Kawaguchi H, Ashihara K, Fujiwara S, Tanaka T, Tsunetoh S, Tanaka Y, Sasaki H, Kanemura M, et al. 2014 The EMT (epithelial-mesenchymal-transition)-related protein expression indicates the metastatic status and prognosis in patients with ovarian cancer. Journal of Ovarian Research 7 76. (https://doi.org/10.1186/1757-2215-7-76)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Tang Z, Ow GS, Thiery JP, Ivshina AV & Kuznetsov VA 2014 Meta-analysis of transcriptome reveals let-7b as an unfavorable prognostic biomarker and predicts molecular and clinical subclasses in high-grade serous ovarian carcinoma. International Journal of Cancer 134 306318. (https://doi.org/10.1002/ijc.28371)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Thiery JP & Sleeman JP 2006 Complex networks orchestrate epithelial-mesenchymal transitions. Nature Reviews Molecular Cell Biology 7 131142. (https://doi.org/10.1038/nrm1835)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Trang P, Wiggins JF, Daige CL, Cho C, Omotola M, Brown D, Weidhaas JB, Bader AG & Slack FJ 2011 Systemic delivery of tumor suppressor microRNA mimics using a neutral lipid emulsion inhibits lung tumors in mice. Molecular Therapy 19 11161122. (https://doi.org/10.1038/mt.2011.48)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Tumeh PC, Harview CL, Yearley JH, Shintaku IP, Taylor EJ, Robert L, Chmielowski B, Spasic M, Henry G, Ciobanu V, et al. 2014 PD-1 blockade induces responses by inhibiting adaptive immune resistance. Nature 515 568571. (https://doi.org/10.1038/nature13954)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Ugurel S, Rohmel J, Ascierto PA, Flaherty KT, Grob JJ, Hauschild A, Larkin J, Long GV, Lorigan P, McArthur GA, et al. 2017 Survival of patients with advanced metastatic melanoma: the impact of novel therapies-update 2017. European Journal of Cancer 83 247257. (https://doi.org/10.1016/j.ejca.2017.06.028)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Vang S, Wu HT, Fischer A, Miller DH, MacLaughlan S, Douglass E, Comisar L, Steinhoff M, Collins C, Smith PJ, et al. 2013 Identification of ovarian cancer metastatic miRNAs. PLoS ONE 8 e58226. (https://doi.org/10.1371/journal.pone.0058226)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Vang R, Levine DA, Soslow RA, Zaloudek C, Shih IeM & Kurman RJ 2016 Molecular alterations of TP53 are a defining feature of ovarian high-grade serous carcinoma: a rereview of cases lacking TP53 mutations in The Cancer Genome Atlas Ovarian Study. International Journal of Gynecological Pathology 35 4855. (https://doi.org/10.1097/PGP.0000000000000207)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Varga A, Piha-Paul SA, Ott PA, Mehnert JM, Berton-Rigaud D, Johnson EA, Cheng JD, Yuan S, Rubin EH & Matei DE 2015 Antitumor activity and safety of pembrolizumab in patients (pts) with PD-L1 positive advanced ovarian cancer: Interim results from a phase Ib study. American Society for Clinical Oncology 33 (15 Suppl) 5510. (https://doi.org/10.1200/jco.2015.33.15_suppl.5510)

    • Search Google Scholar
    • Export Citation
  • Vecchione A, Belletti B, Lovat F, Volinia S, Chiappetta G, Giglio S, Sonego M, Cirombella R, Onesti EC, Pellegrini P, et al. 2013 A microRNA signature defines chemoresistance in ovarian cancer through modulation of angiogenesis. PNAS 110 98459850. (https://doi.org/10.1073/pnas.1305472110)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Vencken PM, Kriege M, Hoogwerf D, Beugelink S, van der Burg ME, Hooning MJ, Berns EM, Jager A, Collee M, Burger CW, et al. 2011 Chemosensitivity and outcome of BRCA1- and BRCA2-associated ovarian cancer patients after first-line chemotherapy compared with sporadic ovarian cancer patients. Annals of Oncology 22 13461352. (https://doi.org/10.1093/annonc/mdq628)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Verhoeven RH, Gondos A, Janssen-Heijnen ML, Saum KU, Brewster DH, Holleczek B, Crocetti E, Rosso S, Hakulinen T, Aareleid T, et al. 2013 Testicular cancer in Europe and the USA: survival still rising among older patients. Annals of Oncology 24 508513. (https://doi.org/10.1093/annonc/mds460)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Watashi K, Yeung ML, Starost MF, Hosmane RS & Jeang KT 2010 Identification of small molecules that suppress microRNA function and reverse tumorigenesis. Journal of Biological Chemistry 285 2470724716. (https://doi.org/10.1074/jbc.M109.062976)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wei SH, Balch C, Paik HH, Kim YS, Baldwin RL, Liyanarachchi S, Li L, Wang Z, Wan JC, Davuluri RV, et al. 2006 Prognostic DNA methylation biomarkers in ovarian cancer. Clinical Cancer Research 12 27882794. (https://doi.org/10.1158/1078-0432.CCR-05-1551)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wilkerson PM, Dedes KJ, Wetterskog D, Mackay A, Lambros MB, Mansour M, Frankum J, Lord CJ, Natrajan R, Ashworth A, et al. 2011 Functional characterization of EMSY gene amplification in human cancers. Journal of Pathology 225 2942. (https://doi.org/10.1002/path.2944)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yang-Hartwich Y, Soteras MG, Lin ZP, Holmberg J, Sumi N, Craveiro V, Liang M, Romanoff E, Bingham J, Garofalo F, et al. 2015 p53 protein aggregation promotes platinum resistance in ovarian cancer. Oncogene 34 36053616. (https://doi.org/10.1038/onc.2014.296)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Ying HC, Xu HY, Lv J, Ying TS & Yang Q 2015 MicroRNA signatures of platinum-resistance in ovarian cancer. European Journal of Gynaecological Oncology 36 1620.

  • Yoshida S, Furukawa N, Haruta S, Tanase Y, Kanayama S, Noguchi T, Sakata M, Yamada Y, Oi H & Kobayashi H 2009 Expression profiles of genes involved in poor prognosis of epithelial ovarian carcinoma: a review. International Journal of Gynecological Cancer 19 992997. (https://doi.org/10.1111/IGC.0b013e3181aaa93a)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yu X, Zheng H, Chan MT & Wu WK 2017 Modulation of chemoresponsiveness to platinum-based agents by microRNAs in cancer. American Journal of Cancer Research 7 17691778.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Zeppernick F, Meinhold-Heerlein I & ShihIe M 2015 Precursors of ovarian cancer in the fallopian tube: serous tubal intraepithelial carcinoma – an update. Journal of Obstetrics and Gynaecology Research 41 611. (https://doi.org/10.1111/jog.12550)

    • Crossref
    • Search Google Scholar
    • Export Citation

 

  • Collapse
  • Expand

Society for Endocrinology logo

  • View in gallery

    Mechanism of action for platinum chemotherapy. Platinum binds to the N-7 position of adjacent guanine (G) bases resulting in interstrand or intrastrand crosslinks.

  • View in gallery

    Nucleotide Excision Repair and Homologous Recombination Pathways. (A) Nucleotide excision repair recognises platinum-induced interstrand and intrastrand crosslinks and a co-ordinated process of DNA unwinding, incision, excision and synthesis follows. The process can result in a DNA double-strand break, which is recognised by homologous recombination repair. (B) Homologous recombination repair recognises double-strand breaks and initiates a process of single strand DNA formation, coating, filament formation, strand invasion and a final step of DNA synthesis.

  • View in gallery

    The role of DNA repair deficiencies in high-grade serous ovarian cancer (HGSOC) platinum chemotherapy resistance. (A) NER-and HR-proficient (NER+ and HR+) HGSOC is hypothesised to be sensitive to platinum chemotherapy if apoptosis is greater than DNA repair. (B) NER-deficient (NER−) and HR+ HGSOC is hypothesised to be platinum resistant due to lack of platinum cross-link recognition by NER. (C) NER+ and HR-deficient (HR−) HGSOC is hypothesised to be platinum sensitive due to NER triggering apoptosis and DNA double-strand breaks that are not repaired by HR. (D) NER− and HR− HGSOC is hypothesised to be platinum resistant due to lack of NER triggered apoptosis and a high mutation load.

  • View in gallery

    Therapeutic targets: epigenetic and immune biomarkers of HGSOC platinum chemotherapy resistance. Potential nuclear therapeutic targets/biomakers for platinum-resistant HGSOC are histone deactylases (HDA), DNA methylation (mC) and high C(C>T)C mutation load across the genome. Potential cell surface for platinum-resistant HGSOC are tumour-specific antigens and PDL-1 (PD-1 on T-cells).

  • Alexandrov LB, Nik-Zainal S, Wedge DC, Aparicio SA, Behjati S, Biankin AV, Bignell GR, Bolli N, Borg A, Borresen-Dale AL, et al. 2013 Signatures of mutational processes in human cancer. Nature 500 415421. (https://doi.org/10.1038/nature12477)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Alsop K, Fereday S, Meldrum C, deFazio A, Emmanuel C, George J, Dobrovic A, Birrer MJ, Webb PM, Stewart C, et al. 2012 BRCA mutation frequency and patterns of treatment response in BRCA mutation-positive women with ovarian cancer: a report from the Australian Ovarian Cancer Study Group. Journal of Clinical Oncology 30 26542663. (https://doi.org/10.1200/JCO.2011.39.8545)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Antonia SJ, Lopez-Martin JA, Bendell J, Ott PA, Taylor M, Eder JP, Jager D, Pietanza MC, Le DT, de Braud F, et al. 2016 Nivolumab alone and nivolumab plus ipilimumab in recurrent small-cell lung cancer (CheckMate 032): a multicentre, open-label, phase 1/2 trial. Lancet Oncology 17 883895. (https://doi.org/10.1016/S1470-2045(16)30098-5)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Banet N & Kurman RJ 2015 Two types of ovarian cortical inclusion cysts: proposed origin and possible role in ovarian serous carcinogenesis. International Journal of Gynecological Pathology 34 38. (https://doi.org/10.1097/PGP.0000000000000120)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Barakat BM, Wang QE, Han C, Milum K, Yin DT, Zhao Q, Wani G, Arafa el SA, El-Mahdy MA & Wani AA 2010 Overexpression of DDB2 enhances the sensitivity of human ovarian cancer cells to cisplatin by augmenting cellular apoptosis. International Journal of Cancer 127 977988. (https://doi.org/10.1002/ijc.25112)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Boac BM, Xiong Y, Marchion DC, Abbasi F, Bush SH, Ramirez IJ, Khulpateea BR, Clair McClung E, Berry AL, Bou Zgheib N, et al. 2016 Micro-RNAs associated with the evolution of ovarian cancer cisplatin resistance. Gynecologic Oncology 140 259263. (https://doi.org/10.1016/j.ygyno.2015.12.026)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Bowden NA, Ashton KA, Avery-Kiejda KA, Zhang XD, Hersey P & Scott RJ 2010 Nucleotide excision repair gene expression after Cisplatin treatment in melanoma. Cancer Research 70 79187926. (https://doi.org/10.1158/0008-5472.CAN-10-0161)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Bowden NA, Ashton KA, Vilain RE, Avery-Kiejda KA, Davey RJ, Murray HC, Budden T, Braye SG, Zhang XD, Hersey P, et al. 2013 Regulators of global genome repair do not respond to DNA damaging therapy but correlate with survival in melanoma. PLoS ONE 8 e70424. (https://doi.org/10.1371/journal.pone.0070424)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Cameron EE, Bachman KE, Myohanen S, Herman JG & Baylin SB 1999 Synergy of demethylation and histone deacetylase inhibition in the re-expression of genes silenced in cancer. Nature Genetics 21 103107. (https://doi.org/10.1038/5047)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Cancer Genome Atlas Research Network 2011 Integrated genomic analyses of ovarian carcinoma. Nature 474 609615. (https://doi.org/10.1038/nature10166)

  • Chan JK, Blansit K, Kiet T, Sherman A, Wong G, Earle C & Bourguignon LY 2014 The inhibition of miR-21 promotes apoptosis and chemosensitivity in ovarian cancer. Gynecologic Oncology 132 739744. (https://doi.org/10.1016/j.ygyno.2014.01.034)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chang PY, Liao YP, Wang HC, Chen YC, Huang RL, Wang YC, Yuan CC & Lai HC 2017 An epigenetic signature of adhesion molecules predicts poor prognosis of ovarian cancer patients. Oncotarget 8 5343253449. (https://doi.org/10.18632/oncotarget.18515)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chebouti I, Kasimir-Bauer S, Buderath P, Wimberger P, Hauch S, Kimmig R & Kuhlmann JD 2017 EMT-like circulating tumor cells in ovarian cancer patients are enriched by platinum-based chemotherapy. Oncotarget 8 4882048831. (https://doi.org/10.18632/oncotarget.16179)

    • PubMed
    • Search Google Scholar
    • Export Citation
  • Chen L, Zhang K, Shi Z, Zhang A, Jia Z, Wang G, Pu P, Kang C & Han L 2014 A lentivirus-mediated miR-23b sponge diminishes the malignant phenotype of glioma cells in vitro and in vivo. Oncology Reports 31 15731580. (https://doi.org/10.3892/or.2014.3012)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Choi YJ, Ingram PN, Yang K, Coffman L, Iyengar M, Bai S, Thomas DG, Yoon E & Buckanovich RJ 2015 Identifying an ovarian cancer cell hierarchy regulated by bone morphogenetic protein 2. PNAS 112 E6882E6888. (https://doi.org/10.1073/pnas.1507899112)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Choi YE, Meghani K, Brault ME, Leclerc L, He YJ, Day TA, Elias KM, Drapkin R, Weinstock DM, Dao F, et al. 2016 Platinum and PARP inhibitor resistance due to overexpression of microRNA-622 in BRCA1-mutant ovarian cancer. Cell Reports 14 429439. (https://doi.org/10.1016/j.celrep.2015.12.046)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Clements PB & Young RH 2008 Atlas of Gynecologic Surgical Pathology. Philadelphia, PA, USA: Saunders Elsevier.

  • Clevers H 2011 The cancer stem cell: premises, promises and challenges. Nature Medicine 17 313319. (https://doi.org/10.1038/nm.2304)

  • Cole AJ, Dwight T, Gill AJ, Dickson KA, Zhu Y, Clarkson A, Gard GB, Maidens J, Valmadre S, Clifton-Bligh R, et al. 2016 Assessing mutant p53 in primary high-grade serous ovarian cancer using immunohistochemistry and massively parallel sequencing. Scientific Reports 6 26191. (https://doi.org/10.1038/srep26191)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Costa RMA, Chiganças V, da Silva Galhardo R, Carvalho H & Menck CFM 2003 The eukaryotic nucleotide excision repair pathway. Biochimie 85 10831099. (https://doi.org/10.1016/j.biochi.2003.10.017)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • D’Incalci M & Galmarini CM 2010 A review of trabectedin (ET-743): a unique mechanism of action. Molecular Cancer Therapeutics 9 21572163. (https://doi.org/10.1158/1535-7163.MCT-10-0263)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Dabholkar M, Bostick-Bruton F, Weber C, Bohr VA, Egwuagu C & Reed E 1992 ERCC1 and ERCC2 expression in malignant tissues from ovarian cancer patients. Journal of the National Cancer Institute 84 15121517. (https://doi.org/10.1093/jnci/84.19.1512)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Dabholkar M, Vionnet J, Bostick-Bruton F, Yu JJ & Reed E 1994 Messenger RNA levels of XPAC and ERCC1 in ovarian cancer tissue correlate with response to platinum-based chemotherapy. Journal of Clinical Investigation 94 703708. (https://doi.org/10.1172/JCI117388)

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Davidson B, Holth A, Hellesylt E, Tan TZ, Huang RY, Trope C, Nesland JM & Thiery JP 2015 The clinical role of epithelial-mesenchymal transition and stem cell markers in advanced-stage ovarian serous carcinoma effusions. Human Pathology 46 18. (https://doi.org/10.1016/j.humpath.2014.10.004)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Davis A, Tinker AV & Friedlander M 2014 “Platinum resistant” ovarian cancer: what is it, who to treat and how to measure benefit? Gynecologic Oncology 133 624631. (https://doi.org/10.1016/j.ygyno.2014.02.038)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Delcuve GP, Khan DH & Davie JR 2012 Roles of histone deacetylases in epigenetic regulation: emerging paradigms from studies with inhibitors. Clinical Epigenetics 4 5. (https://doi.org/10.1186/1868-7083-4-5)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • Disis ML, Patel MR, Pant S, Infante JR, Lockhart AC, Kelly K, Beck JT, Gordon MS, Weiss GJ, Ejadi S, et al. 2015 Avelumab (MSB0010718C), an anti-PD-L1 antibody, in patients with previously treated, recurrent or refractory ovarian cancer: a phase Ib, open-label expansion trial. American Society of Clinical Oncology 33 (15 Suppl) 5509. (https://doi.org/10.1200/jco.2015.33.15_suppl.5509)

    • Search Google Scholar
    • Export Citation
  • Ebert MS, Neilson JR & Sharp PA 2007 MicroRNA sponges: competitive inhibitors of small RNAs in mammalian cells. Nature Methods 4 721726. (https://doi.org/10.1038/nmeth1079)

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation