A Succinct Molecular Profile of High-Grade Ovarian Cancer

Abstract

Several studies have been carried out to determine the complexity of ovarian cancer as a disease with multiple distinct types that presents with symptoms similar to those in other gynaecological, gastrointestinal and genitourinary diseases. The malignant variants of common epithelial and germ cell tumours constitute the bulk of ovarian tumours and are classified histologically based on the presumed tissue of origin. Molecular diagnosis is now aiding in the early detection and treatment of ovarian cancer even before metastasis sets in. Thus studying the molecular profiles of each type is key to understanding the origin and pathogenesis as well as genetic aberrations and mutations involved in the development of the disease. Ovarian cancers originate either from the ovary or fallopian tube and are found majorly to harbour mutations in PTEN, KRAS, BRAF, BRCA1, BRCA2 and TP53, with TP53 mutations being the most frequent. Genetic testing for ovarian cancers involves testing for the aforementioned genes, and in the nearest future, an advanced method that would detect these genes in blood and uterine lavage is expected. There is an urgent need for further studies on the detailed mechanisms underlying the roles of mutant TP53 in ovarian cancer development and its potential role in therapeutic interventions.

1. Introduction

Ovarian Cancer is the eighth most common cancer among women worldwide, with an incidence of about 239,000 new cases and 152,000 deaths annually [1]. It is a complex disease with multiple distinct molecular and histologic types, each with different aetiology, risk factors, prognosis and response to treatment. Several factors make ovarian cancer treatment difficult, even though most patients experience or present symptoms, but such symptoms tend to overlap or mimic other symptoms presented in other gynaecological, genitourinary and gastrointestinal diseases. Therefore, early diagnosis is rarely achieved and as such, it is mostly carried out after metastasis [2].

Almost all malignant and benign ovarian tumours are of epithelial, stromal or germ cell origin, with those of epithelial origin accounting for more than 90% [3]. Malignant ovarian cancers also referred to as carcinomas are classified into five main histologic types (histotypes) namely: endometroid, mucinous, clear cell, low-grade serous and high-grade serous ovarian carcinoma (Table 1). The pathogenesis and origin of ovarian cancer are not well understood. However, most tumours appear to originate from other parts of the reproductive system and affect the ovary secondarily [4].

Histologic Type	Features
High-grade Serous Ovarian carcinoma	Tumour cells are atypical, with large irregular nuclei Papillary growth pattern Highly proliferative and aggressive Targeted genes: BRCA1 and BRCA2, and TP53
Low-grade Serous Ovarian carcinoma	Tumour cells possess small uniform nuclei Micro-papillary growth pattern Low proliferative and less aggressive Targeted genes: KRAS, NRAS, BRAF, and PIK3CA
Endometrioid	Shows cystic and solid patterns Usually associated with endometriosis High grade have similar profile with HGSOC Targeted genes: ARID1A, POLE, PIK3CA, and PTEN
Mucinous	Large tumour cells filled with mucus-like substance Early diagnosis Targeted genes: PIK3CA, HER2,and KRAS
Clear cell	Cells with clear cytoplasm containing glycogen Papillary, solid, tubulo-cystic or mixed patterns of growth Usually associated with endometriosis Early diagnosis Targeted genes: ARID1A, PTEN, and PIK3CA,

Table 1.

Salient features of histological subtypes of epithelial ovarian carcinomas.

One of the most common genetic abnormality seen in ovarian cancer is mutation and loss of TP53 function, including DNA copy number abnormalities which affects cell proliferation and apoptosis [2].

Despite the continuous effort to develop early screening strategies, only a negligible fraction of ovarian cancers are diagnosed while they are localized to the ovaries. Diagnosis is mostly made after the disease has spread to the pelvic organs, abdomen, and/or beyond the peritoneal cavity, and this makes treatment difficult.

The standard treatment of ovarian cancer is platinum and taxane-based combination chemotherapy after cytoreductive surgery to remove a significant bulk of the tumour. Despite being considered chemosensitive, the majority of the patients subjected to cytoreductive surgery and combination therapy will need second-line chemotherapy due to tumour recurrence within 2 years [5].

2. Description of molecular profiles

Recently, ovarian cancers have been classified into type 1 and type 2 tumours. Type 1 constitutes of low-grade tumours (mucinous, endometrioid, low-grade serous and clear cell types) that harbour mutations in PTEN, KRAS and BRAF with microsatellite instability and are thought to originate from the ovary. While type 2 tumours are high-grade serous and carcinosarcomas that originate from the fallopian tube and have mutations in BRCA1, BRCA2 and TP53 [6, 7, 8].

Whole-exome and Whole-genome sequencing studies of ovarian cancer have not only revealed its genetic heterogeneity, but have identified the genomic effect of aberrant DNA damage and repair processes in endometrioid, high-grade serous, and clear cell ovarian cancers [9].

3. Histopathology of ovarian cancer

Ovarian tumours have different histologic features and patterns due to the different tissues found within the ovary. With increased histologic examinations over the years, the classification of ovarian tumours has evolved, which is mostly based on the presumed tissue of origin, and includes common epithelial tumours, lipoid cell tumours, gonadoblastoma, sex cord-stromal tumours, germ cell tumours, soft tissue tumours not specific to the ovary, metastatic tumours and unclassified tumours [10]. The malignant variants of the common epithelial tumours and germ cell tumours will be discussed briefly.

3.1 Common epithelial Tumours

Most benign and malignant ovarian tumours belong to this group and are derived from the common celomic epithelium on the surface of the ovary, which is also derived from the split lateral mesoderm and which also infolds to form the Mullerian duct. The Mullerian duct forms the endocervix, uterine corpus and uterine tube, thus explaining the different epithelial patterns (serous, mucinous, clear cell and endometrioid) in this group of tumours. Each of these patterns includes a completely benign (partly cystic, regular lining cells often covering stromal projections and with gland-like spaces) type and an adenocarcinoma (with invasive features that are either well differentiated with gland-like spaces or poorly differentiated sheets). Between these two patterns is an intermediate pattern termed carcinoma of low malignant potential that exhibits cellular stratification with variable mitotic activity and clear atypia but without stromal invasion [10].

Histologic diagnosis of an ovarian tumour should be done on the primary tumour, not on the histologic pattern of metastasis because not all peritoneal lesions are metastases even when there is a confirmed primary ovarian carcinoma [11].

One of the problems associated with adenocarcinoma of low malignant potential and sometimes with low-grade adenocarcinoma is the evaluation of glandular structures in lymph nodes to determine if such are metastases or simple benign glandular inclusions. It is also vital to ascertain the significance of glandular inclusions in lymph nodes after surgical removal because the epithelium in benign inclusion is tubal with cilia, shows a simple papillary pattern and has peripheral gland-like spaces that may extend around lymphoid follicles and sometimes extends into a follicle, but without stromal response, while adenocarcinoma usually invades a follicle and often shows a desmoplastic response. Mitoses are rarely seen in benign inclusions but present in adenocarcinoma [10].

Invasion of stroma by cribriform-like tumour composed of strands of infiltrating malignant cells is a feature of mucinous adenocarcinoma. Mucinous tumours might either be of the endocervical or intestinal type, the former is characterized by cells with basal nuclei and the latter is characterized by goblet cells, sometimes argentaffin cells and rarely Paneth cells [10].

Clear cell carcinoma of the ovary usually arises from the ovarian surface epithelium, and sometimes from endometriosis and is said to represent a separate clinicopathologic entity. This carcinoma has been reported to exist in three architectural patterns viz.: solid, papillary and tubulocystic [12, 13]. Clear cell tumours are generally considered malignant due to failure to recognize benign or borderline types. Recent studies have found benign, borderline and micro-invasive tumours, with borderline tumours having 1–3 layers of clear, eosinophilic or hobnail cells, while the micro-invasive tumours showed evidence of focal stromal reaction with rare mitoses [14].

Endometrioid adenocarcinoma however is less common than the mucinous or serous type and has a histologic pattern similar to that of carcinoma of the endometrium that ranges from a well-differentiated glandular pattern to poorly differentiated adenocarcinoma with very few glands.

3.2 Malignant germ cell tumours

These are derived from the primitive germ cells found between the junction of the hindgut and yolk sac. They later migrate through the mesentery to the posterior wall of the embryo, just beneath the celomic epithelium. In the process of their migration, some germ cells become arrested or extend beyond their usual position to form extragonadal germ cell tumours that are histologically similar to those in the ovary. Most types of germ cell tumours occur in pure form, with few occurring in mixed form. Thus, multiple sections must be examined to make a definitive diagnosis.

Malignant germ tumours include dysgerminoma, endodermal sinus tumour, immature teratoma and mixed germ cell tumour. Dysgerminoma is similar to seminoma of the testis and is derived from undifferentiated germ cells, and consists of strands, sheets, and groups of cells that are large and uniform in size with a central nucleus and varying mitotic activity. Lymphoid follicles with germinal centres are sometimes present with some showing granulomatous reaction [10].

Endodermal sinus tumours exhibit a central vascular strand with thin walls covered by a single layer of epithelial-like cells of hobnail pattern. Another common feature seen is a meshwork pattern with round hyaline globules that reacts to a-fetoprotein (AFP) in addition to other multiple pattern such as glandular, alveolar, hepatoid and myxomatous [10].

Teratomas are considered to arise from a single germ cell following the first meiotic division, with each of the three germ cell layers represented and consisting of both mature and immature elements. The histologic grade of immature teratomas is based on the presence of neuroepithelial components, the quantity of the neuroepithelial component and the degree of immaturity. Mature teratomas on the other hand are either cystic, constituting 99% or solid accounting for the remaining 1%. Solid mature teratoma usually constitutes tissues from all three cell layers with a well-differentiated glial component. The benign cystic teratoma that shows squamous epithelial involvement, and mesodermal and endodermal differentiation is usually benign, only a minute percentage undergo malignant progression to form squamous cell carcinoma [10, 15].

Mixed germ cell tumours include gonadoblastoma which is usually benign but sometimes associated with endodermal sinus tumour, embryonal carcinoma, choriocarcinoma or dysgerminoma.

4. Endometrioid ovarian cancer

Endometrioid ovarian carcinoma (EOVC) is an uncommon subtype of epithelial ovarian carcinoma (EOC) that constitute approximately 10% of all ovarian carcinomas. EOVC tends to present at a younger age and earlier stage, are associated with endometriosis, frequent CTNNB1 and PTEN mutations and a higher frequency of microsatellite instability. Also, both the molecular and histologic makeup of EOVC is analogous to that of endometrioid endometrial carcinoma [6, 16].

5. High-grade serous ovarian carcinoma

5.1 Origin and epidemiology of high-grade serous ovarian cancer

EOC genomic predisposition is now recognized in about 15% of affected women, where Breast cancer susceptibility genes BRCA1 and BRCA2 were identified as the main causative agents of hereditary EOC. Different forms of mutations in these genes and other double-strand DNA break repair genes are mainly associated with susceptibility to HGSOC [1].

High-grade serous ovarian cancer (HGSOC) is the most common form of EOC, accounting for about 75% of all EOC (Figure 1). It has been found to originate from the fallopian tube epithelium due to its link with IGF-1R/AKT pathway, which is activated by follicular fluid [17]. However, the molecular basis of how it is transferred to the ovaries is yet to be understood. A recent study revealed that follicular fluid plays a vital role in events leading to the development and intraperitoneal metastasis of HGCOS, by supporting migration, proliferation, invasion, anchorage, adhesion and anoikis insensitivity [18].

Figure 1.

Histologic distribution of ovarian cancer.

5.2 Hereditary susceptibility

A study revealed that 15–20% of HGSOC patients have germline BRCA1 and/or BRCA2 mutations, which necessitates conducting germline testing on first-degree relatives to identify carriers [19]. Furthermore, it has been found that by the age of 80, the cumulative risk of EOC is about 44% and 17% in BRCA1 and BRCA2 mutation carriers respectively [20]. Therefore, such carriers are recommended to have a prophylactic risk-reduction surgery after childbearing, when the risk begins to increase. Apart from the above-mentioned genes, other genes with moderate penetrance include RAD51C, RAD51D, and BR1P1, which cumulatively are responsible for about 5% of EOC. Thus, genetic testing for HGSOC includes BRCA1, BRCA2, RAD51C, RAD51D, BR1P1 and in the nearest future TP53 detection in blood and uterine lavage [1, 21].

5.3 Pathology/molecular abnormality

The growth pattern of HGSOC is diverse, encompassing glandular, solid, large papillae with high mitotic rate and frequent necrosis [21]. HGSOC is characterized by recurrent mutations in RB1, CDK12, BRCA1, BRCA2, NF1 and TP53, and also frequent DNA losses and gains. This makes it chromosomally unstable with the potential of developing acquired chemoresistance [22, 23]. Thus, the homozygous and heterozygous loss is an important mechanism in tumour suppressor genes inactivation [1]. Also, studies have shown that homologous recombination is defective in almost half of HGSOC, and this deficiency is a key determinant of platinum sensitivity and treatment with Poly ADP-ribose polymerase inhibitors (PARPi) [22, 24]. HGSOC can be molecularly stratified into four different prognostic subtypes namely: mesenchymal-C1, immune-C2, differentiated-C4, and proliferative-C5, and into seven copy-number signatures [22, 25, 26, 27].

6. Recurrent clear cell ovarian carcinoma

Clear cell ovarian cancers (CCOC) are a subtype of EOC with very distinct biology to HGSOC. They exhibit a very poor prognosis and a low response to platinum-based chemotherapy. This subtype is molecularly heterogeneous for point mutations, gene copy number, and alterations in the PI3K/AKT/mTOR pathway. Thus, these could affect response to targeted therapy [28, 29]. Histologically, patients with CCOC must be correctly diagnosed because HGSOC with clear cell features can easily be misdiagnosed as CCOC [30]. This might also result in the decreased or failed response to treatment in such CCOC patients misdiagnosed with HGSOC.

A tumour profiling study to identify potential druggable targets in CCOC was carried out by employing protein expression by immunohistochemistry (IHC), next-generation sequencing (NGS) and gene amplification by fluorescent in situ hybridization (FISH). On the basis of IHC, this study revealed an 80.8% RRM1 loss, 79.6% ERCC1 loss, 56.4% MGMT loss, 50.8% TS loss and a 62.6% TOP2A overexpression [31]. The NGS identified 50.5%, 18.1%, and 12.4% mutations in PIK3CA, TP53, and KRAS respectively. Of which TP53 mutations were observed on exons 4 to 8, while most PIK3CA mutations occurred on exons 9 and 20. For FISH analyses, HER2 was amplified in 9.3% of pure and 3.8% of mixed CCOC samples, while cMET, was amplified in 3.2% of pure CCOC and none was amplified in mixed CCOC. Mutations in ATM and APC were also observed only in pure CCOC tumours [31].

Even though CCOCs are mostly chemoresistant, few patients do respond to platinum chemotherapy. Different mechanisms of CCOC chemoresistance have been reported including increased drug detoxification, increased DNA repair, decreased drug detoxification, abnormal growth factor signalling and cell cycle control. Loss of ARID 1A expression and alterations in the PI3K/AKT/mTOR pathway may also contribute to the chemoresistance in CCOC [32, 33, 34]. The PI3K/AKT/mTOR is a key mediator of oncogenic signalling, which may be overactive due to PTEN loss. The PI3K pathway is a complex signalling network that coordinates signals from other membrane receptors such as MET [35].

It is important to note that the signalling pathway of the receptor tyrosine kinase MET and its ligand hepatocyte growth factor (HGF) is crucial for cell motility, growth and survival, and is functionally linked to the VEGF signalling pathway [31]. Therefore, research into the kinase inhibitor agents that target MET, VEGF receptor 2 and other tyrosine kinases are urgently needed.

7. TP53: function and the consequence of its mutations in ovarian cancers

The TP53 gene is a tumour suppressor gene located on the short arm of chromosome 17 and contains 11 exons that encode for 53 kDa phosphoprotein (TP53 protein), a transcription factor of genes responsible for cell cycle arrest and apoptosis. It is a nuclear transcriptional factor that upon binding to the nucleic acid component of the cell, it facilitates the regulation of several cellular processes through the control of several expression genes to maintain overall genome integrity and homeostasis [36].

Following deoxyribonucleic acid (DNA) damage, the TP53 gene initiates the activation of DNA repair proteins by arresting cell growth by holding the cell cycle at the G1/S transitioning phase. This allows DNA repairs and the initiation of apoptosis on cells with irreparable DNA damage [37]. The activation of TP53 function has been associated with numerous carcinogenesis-inducing stimuli which induce DNA damage such as Gamma or UV irradiation, nucleolar or ribosomal stress, hypoxia, inappropriate activation of proto-oncogenes and mitogenic signalling among others [38, 39, 40]. Once initiated, the TP53 through the promotion of expression of the necessary genes responsible for cellular damage regulatory activities, where appropriate initiates cell cycle arrest, cellular senescence and differentiation, and cell death [41, 42]. For example, upon DNA damage, the TP53 protein binds to the damaged DNA and stimulates another cell cycle regulatory gene (CDKN1A) to produce p21 protein which interacts and forms a complex with cyclin-dependent kinase 2 (CDK2), a cell division-stimulating protein [43]. The formed CDKN1A-CDK2 complex arrests the affected cell and stopped its progression past the G1- phase of the cell cycle and induces cellular senescence [41, 42, 44]. This TP53- dependent blocking of cellular proliferation contributes to the prevention of cell transformation and tumour progression by triggering programmed cell death either by apoptosis or ferroptosis [36, 45]. However, an aberration in the TP53 gene might result in the cessation of its cell cycle regulation and promotes carcinogenesis [46]. Therefore, these anti-tumour functions of TP53 on DNA-damaged cells could be utilized for the development of anticancer drugs and appropriate management strategies.

TP53 is one of the most frequently mutated genes in human cancer with more than 50% of human cancer types associated with its mutations [47]. This is because of its essential role in DNA damage-induced cellular regulation and tumour suppression. There are over 36,000 TP53 mutations identified of which approximately 80% of them are missense mutations with amino acid substitution [47]. According to IARC TP53 Database, 6.5% of the identified TP53 mutations have been reported to be associated with ovarian cancer of which approximately 70% of them are of the missense mutation subtypes while others include point and null mutations. Many of the missense mutations occurred at specific residues in the DNA binding domain which suggests a feature of selectivity peculiar to these mutants (http://www-TP53.iarc.fr/).

The mechanism underlying the development and progression of ovarian cancers as it relates to TP53 mutation has been extensively studied. However, it is not well understood and researchers have suggested possible ways of its action. For example, one mechanism explored by researchers is the gain of function property. Mutant TP53 acquires a “gain of function” property that favours ovarian cancer progressive activities that may manifest as acquired resistance to chemotherapy, enhanced invasiveness which positively increased metastatic capabilities and down-regulation of certain metabolic pathways among others [48]. The gain of function property of the mutant TP53 can be observed in the abrogation of function upon interaction with its family members such as p63 and p73. They both can form complexes with the Wild-type TP53 and serve the tumour suppressor functions in cells [49]. However, mutant TP53 with a gain of function property has been reported to form a complex with phosphorylated p63 which prevents the Wild-type p63 natural function of tumour suppression, and at the same time induces the activation of certain oncogenic genes such as Cyclin G2 and Dicer [50, 51, 52]. Similarly, a study reported that mutant TP53 directly binds to Wild-type p73 and as a result, it prevents the inactivation of PDGFβ- the natural function of the p73- which subsequently favours invasiveness and metastasis [53]. Another possible mechanism of mutant TP53-induced ovarian carcinogenesis may be associated with protein aggregation [54]. This is because the TP53 mutants especially of the missense subtype category have been reported to induce structural changes which potentially expose adhesion molecules that can co-aggregate with the Wild-type TP53 or any of its family members causing trans- or cis-DN effects on the Wild-types TP53 and its analogues [54, 55, 56]. This can explain the reason certain ovarian cancers present with an aggregation phenotype and as such, they are considered aggregation-associated diseases by some scholars. In light of the aforementioned possible mechanisms by which mutant TP53 aid in the development and progression of ovarian cancer, and the near 100% prevalence of this mutation in the high-grade serous ovarian cancer- the most prevalent type of ovarian cancer- type, it can be deduced that the TP53 mutation in ovarian cancers presents with an opportunity worthy of exploring in therapeutic interventions and inhibition studies.

8. Conclusion

With Ovarian cancer being the 8th most common cancer among women globally, and one of the most complex diseases with multiple types each having distinct aetiology, risk factors and distinct response to treatment, tremendous progress has been so far recorded in understanding the molecular profiles of each type as well as the role played by TP53 mutation in the development and progression of ovarian cancers. Also, understanding epidemiology, histopathology, and hereditary susceptibility are of equal importance.

Therefore, further molecular and biochemical studies that will explain detailed mechanisms underlying the role of the mutant TP53 in ovarian cancer development and progression, especially the high-grade serous ovarian carcinoma are recommended. More so, further studies on the TP53 mutation types will favour the development of the right therapeutic interventions for ovarian cancers.

Conflict of interest

The authors declare that they have no competing interests.