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Introduction

Ovarian cancer is the fifth-most common cause of cancer death in women in the USA and the other developed coun­tries. According to the American Cancer Society ovarian can­cer was responsible for 14 240 cancer-related deaths in 2016 and estimates that there will be diagnosed 22 440 new cas­es of ovarian cancer in the US in 2017⁽¹⁾. The age-standard­ized incidence has been estimated at 9.4 per 100 000 popu­lation in more developed areas, and 5 per 100 000 population in less developed areas⁽²⁾. Slovakia has one of the highest in­cidence rate among the European countries. In 2010, there were 511 registered cases of ovarian cancer what represents standardized incidence of 11.3 per 100 000 population. Out of registered cases, 277 women died from ovarian cancer in 2010⁽³⁾.

Relatively poor prognosis and high death rate is linked to the late presentation in most cases. The symptoms of ovar­ian cancer are often subtle, painless, and may mimic much more common disorders such as dyspepsia, irritable bowel syndrome, menstruation and menopause⁽⁴⁾. Symptoms com­monly associated with ovarian cancer involve abdominal dis­tension, abdominal or pelvic bloating, abdominal mass, loss of appetite and abdominal or pelvic pain, then diarrhea, iso­lated abdominal pain, weight loss, change in bowel habits, constipation, urinary frequency or urgency, dyspepsia, and abnormal vaginal bleeding⁽⁴⁾.

There are several accepted risk factors that increase the probability of developing ovarian cancer, including increasing age (over 55), nulliparity, early menarche or late menopause (all associated with number of ovulation cycles), familial history of ovarian and other types of cancer, overweight⁽⁵⁶⁾. Contrary, there are some positive factors that lower the risk of ovarian cancer, including full-term pregnancies, breast feeding, oral contraceptives⁽¹⁶⁾. The role of diet in the devel­opment of ovarian cancer is not clearly understood and re­sults are often controversial^.

Classification of ovarian cancer

Ovarian cancer is highly heterogeneous disease with dif­ferent histological subtypes. Primary ovarian tumors fall in­to four main groups according to the type of cells from which cancer can develop: epithelial, sex cord-stromal, germ cell, and mixed-cell type (Figure 1).

Epithelial ovarian carcinoma is the most common form and comprises approximately 90% of all cases⁽⁶⁾. Histologi­cal subtypes of epithelial ovarian cancer include serous, en­dometrioid, clear cell, mucinous, malignant Brenner tumors, and mixed-cell type tumors, which are not longer included in the 2014 WHO classification.

Epithelial ovarian cancer may be grouped based on their clinicopathologic features and genetic profile into two types (Figure 2). Type I involves slowly developing tumors including low-grade serous carcinoma, endometrioid carcinoma, clear­cell carcinoma, mucinous carcinoma then very rare malignant Brenner tumors, and seromucinous carcinoma. Most type I tu­mors are believed to arise from endometriosis or borderline serous tumors. This group of cancer accounts for about 25% of ovarian malignancies and cause 10% deaths. Type II com­prised more aggressive tumors including the high-grade se­rous carcinomas, high-grade endometroid, and rare undiffer­entiated carcinomas and carcinosarcomas. Tumors of this type are believed to arise from fallopian tube. Type II accounts for 75% of ovarian cancers and cause 90% of deaths⁽⁸⁹⁾.

Ovarian sex cord-stromal tumors are formed by cells that normally give rise to cells surrounding the oocytes and contain the cells that produce ovarian hormones⁽¹⁰⁾. These tumors are presented in younger patients as a low-grade disease and are usually not aggressive. Surgery is a key treatment with gen­erally favorable prognosis. They accounts for about 7% of all primary ovarian cancer⁽¹⁰⁾. The World Health Organization re­classified this group ovarian cancer into pure stromal tumors, pure sex cord tumors, and mixed sex cord-stromal tumors⁽¹¹⁾.

Germ cell tumors of the ovary are uncommon tumors de­veloping from the ovarian germ cells. These tumors are fre­quently unilateral, and despite they are aggressive, they are generally curable and treatable if found early⁽¹²⁾. Germ cell tumors account for about 20-25% of ovarian neoplasms, but only 5% are malignant. Group of germ cell tumors includes, in order of frequency: dysgerminomas, immature teratomas, yolk sack tumors, and mixed germ cell tumors. Less com­mon germ cell tumors include embryonal carcinomas, chori­ocarcinomas, and malignant struma ovarii tumors⁽¹³⁾.

Molecular background of ovarian cancer

Although majority of ovarian cancers are sporadic, fami­ly history is one of the strongest risk factors. Approximately 10% of ovarian cancers are hereditary and, of those, 90% cas­es are closely associated with germline mutations in BRCA1 (17q21.31) and BRCA2 (13q13.1) genes⁽¹⁴⁾. Pal et al., 2005
performed full genes sequencing of BRCA1 and BRCA2 genes in cohort of 232 women with epithelial ovarian carcinomas ⁽¹⁵⁾. Invasive ovarian carcinoma was diagnosed in 209 women, 32 women (15.3%) had mutations in BRCA1 or BRCA2 genes. Of these, 20 women had mutations in BRCA1, 12 in BRCA2 gene, respectively. Variants of uncertain significance were detected in 8.2% of women with invasive ovarian carcinoma. No mu­tations were identified in women with borderline or invasive mucinous tumors. Among the BRCA mutation-positive wom­en, 63% had serous tumors. A family history of breast and/or ovarian carcinomas was reported in 65% of relatives of BRCA1 carriers, 75% BRCA2 carriers, and 43.5% non BRCA1/2 carriers, respectively⁽¹⁵⁾. Interestingly, BRCA2 mutations show clearly genotype-phenotype correlation. Central part of the gene (nt 3035-6629 in exon 11) is named ovarian cluster region and are associated with increased risk of ovarian, rather than breast cancer. Studies on BRCA1 mutations are less conclusive⁽¹⁶⁾.

Some other malignant syndromes may develop ovarian cancer as one of their symptoms. Hereditary non-polyposis colorectal cancer (HNPCC, or Lynch syndrome) exhibits ovar­ian cancer as an extracolonic manifestation of Lynch syn­drome II subtype. Lynch syndrome is caused by mutations in mismatch repair (MMR) genes, including MSH2, MLH1, MLH6, PMS1, and PMS2. Women with a germline mutation in one of the MMR genes MLH1, MSH2 or MSH6 have 6-8% life time risk of ovarian cancer⁽¹⁷⁾. Grindedal et al.,⁽¹⁸⁾ performed a retrospective study of 144 women with ovarian cancer due to mutations in mentioned genes. They found 51 women (35.4%) had a mutation in MLH1, 78 (54.2%) had a mutation in MSH2, and 15 (10.4%) had a mutation in MSH6. They also found that overall survival of women with mutation in MMR genes is better than those with BRCA1/2 mutation⁽¹⁸⁾. Ovar­ian cancers associated with Lynch syndrome are predomi­nantly clear-cell or endometrioid in histology⁽¹⁹⁾.

Peutz-Jeghers syndrome (PJS) is an autosomal domi­nant inherited disorder which is characterized by interstinal hamartomatous polyps, mucocutaneous melanin deposition, and increased risk of cancer, including ovarian and breast. In most PJS patients germ line inactivating mutations of the STK11/LKB1 gene (19p13.3) has been indentified⁽²⁰⁾. Women with PJS have an increased incidence of rare sex cord tumor with annular tubules (SCTAT). SCTAT accounted for 1.4% of ovarian sex cord-stromal tumors and are all benign⁽²¹⁾.

To screen other germ line mutations in 12 tumor suppres­sion genes, Walsh et al.⁽²²⁾ used targeted capture and mas­sively parallel genomic sequencing. Study was conducted on

360 women with ovarian carcinomas. 24% carried germ-line loss-of-function mutations: 18% in BRCA1 or BRCA2 and 6% in BARD1, BRIP1, CHEK2, MRE11A, MSH6, NBN, PALB2, RAD 50, RAD51C, and TP53⁽²²⁾. However, the risk associated with majority of these genes is not well understood yet.

Epithelial ovarian tumors are heterogeneous in the term of histological features and molecular profiling. Each subtype is characterized by the presence of certain gene mutations. The Cancer Genome Atlas Research Network⁽²³⁾ reported that high­grade serous ovarian cancer (or type II) is characterized by TP53 mutations in almost all tumors (96% of 489 high-grade serous ovarian adenocarcinomas). On the other hand, TP53 is rarely affected in type I tumors, except mucinous one. These tumors are characterized by mutations in regulators of the mi­togen-activated protein kinase (MAPK) pathway (e.g. KRAS or BRAF), as well as a number of other genomic variants, includ­ing PIK3CA, PTEN, ARID1A, CTNNB1, CDKN2A⁽²⁴⁾. These evi­dence underline the prediction that low grade serous carcino­ma is unlikely to be a precursor lesion for high grade serous carcinoma but rather they are two separated entities.

The array comparative genomic hybridization is a meth­od for genome-wide screening for genomic abnormalities. Caserta et al.⁽²⁵⁾ performed BAC array CGH in ovarian adeno­carcinoma in 10 patients with familial history⁽²⁵⁾. The most common findings were loss of 6q (4 cases with mosaic loss of 6q), 9p (4 cases), 10q (3 cases), 21q (3 cases), 22q (4 cases); and gain of 8q and 9q (8 cases) and 12p (1 case). They observed two cases with monosomy X, and two cases with a micro deletion of 17p terminal. Some cases showed genomic profile with total or mosaic segmental gain on chro­mosomes 2p, 3q, 4q, 7q and 13q⁽²⁵⁾.

Gunn et al.⁽²⁶⁾ used array CGH to identify somatic chromo­somal aberrations in 90 clinically relevant genes ini 8 ovarian tumors. They found one patient was positive for HER2 gene amplification, four positive for CCNE1 gene amplification and four for MYC gene amplification. They detected high lev­el amplification of KRAS in one patient, as well as in CCND2. Other two high-risk patients had amplification of CCND3, and PLAG1 genes. In patients with high level CCNE1 amplifica­tion was observed co-amplification of CCND1, CCND3, MYC and ETV6 genes; high level amplification of CCND3 tumors was accompanied with co-amplification of MYC and AKT2 genes; and high level of MYC tumors showed co-amplifica­tion of JAK2, FGFR3, and MYB genes. One tumor with high level MYC gene amplification also showed bi-allelic PTEN gene deletion. Eight tumors were positive for amplification of the chromosome 3q26 region which contains MECOM, SnoN/SKiL, and ECT2 genes. All these genes were showed to be implicated in ovarian pathogenesis in high grade se­rous carcinomas. Interestingly, no TP53 aberration has been detected which points to the fact that primary mechanism of TP53 lost of function is caused by point mutations⁽²⁶⁾.

Ovarian cancer screening and prevention

High mortality rate of ovarian cancer is mainly associat­ed with lack of key signs of disease and absence of reliable screening tests⁽²⁴⁾. Currently, the most widely used screening methods are transvaginal ultrasound and serum cancer an­tigen CA-125 but the efficacy of this approach is disputable. Partridge with co-workers estimated the risk of ovarian malig­nancy among asymptomatic women with abnormal transvagi­nal ultrasound and CA-125. They concluded that screening for ovarian cancer using simultaneous CA-125 and transvaginal ultrasound did not reduce ovarian cancer mortality and was associated with high false-positive tests⁽²⁷⁾. Falls positive re­sults may lead to unwanted surgery which is associated with its own risks and its benefit may be questionable.

To prevent development of ovarian cancer prophylactic surgical removal of the ovary may be considered in some cases. Bilateral salpingo-oophorectomy can be recommend­ed for women with identified BRCA1 or BRCA2 gene muta­tions and strong family history of ovarian cancer associated syndromes, for women who are 35-40 years of age and have completed childbearing. However, this intervention is link to physical and psychosocial issues⁽²⁸⁾.

Conclusion

Ovarian cancer is highly heterogeneous disease charac­terized by several histological subtypes, which are, in turn, characterized by their own molecular profile. Introduction of the genome-wide technologies such as aCGH or next-gene sequencing brought new insights into understanding the fun­damental of disease, classification, prognostics and treat­ment. Searching for specific changes on the gene level can help to predict the course of disease and find the suitable tar­geted treatment for women.

Acknowledgements

This work was created with the support of the OP Resear­ch and Development for the project: „Completion of technical infrastructure in the field of research of diagnostic procedu­res and methods of early diagnosis of the most common on­cological women diseases“ ITMS 26210120026, co-financed by the European Regional Development Fund.

Poďakovanie

Tento článok vznikol vďaka podpore v rámci OP Výskum a vývoj pre projekt: Dobudovanie technickej infraštruktúry v oblasti výskumu diagnostických postupov a metód v rám­ci včasnej diagnostiky najčastejších onkologických ochorení u žien, ITMS 26210120026, spolufinancovaný zo zdrojov Eu­rópskeho fondu regionálneho rozvoja.

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