Clinical potential of mucins in diagnosis, prognosis, and therapy of ovarian cancer.

Ovarian cancer is the sixth most common cancer worldwide and the seventh leading cause of cancer-related deaths in women. 1 One in 57 women in developed countries will develop ovarian cancer at some point in her lifetime. Incidence of ovarian cancer is highest in white and Hawaiian women, and lowest in Native American women. 2 According to epidemiological studies, family history is the strongest risk factor for the development of ovarian cancer, which further depends on the number of affected first and second-degree relatives. 3 Among the genetic risk factors associated with hereditary ovarian cancers, mutations in BRCA1 and BRCA2 are thought to be particularly important. 4 Age is another common risk factor for ovarian cancer. The incidence of ovarian cancer is increased after menopause, and advanced age (early 60s) at first diagnosis contributes to the high mortality associated with the disease. The ovaries of postmenopausal women typically become folded, resulting in the formation of deep clefts and small inclusion cysts lined with ovarian surface epithelial cells. 3 , 5 These changes provide an environment for the development of ovarian cancer. 6 Nulliparity, use of fertility drugs, and endocrine disorders are other risk factors for the disease. 7 The risk of ovarian cancer also increases with the number of ovulatory events. Repetitive wounding and inflammation of the ovarian surface epithelium associated with ovulation might contribute to increased cell proliferation and mutation rates, which ultimately result in tumour formation. 6 , 8 By contrast, multiparity, use of oral contraceptives, pregnancy, and lactation are associated with a lower risk of ovarian cancer because of the decreased number of ovulation cycles. 3 , 9 Most (90%) ovarian cancers are epithelial in origin and, hence, are referred as epithelial ovarian cancers. The remaining ovarian tumours are gonadal-stromal (6%), germ cell (3%), and metastatic (1%) tumours. 10 The ovarian surface epithelium, which is the outermost cell layer of the normal ovary ( figure 1 ), has no unique features or known major functions. Therefore, early cellular and molecular changes and minor anomalies remain undetected in epithelial ovarian cancers. 11 Nonetheless, aberrant fimbrial epithelium with hyperplasia and a p53 signature is a plausible precursor lesion for many advanced serous ovarian cancers. 12 Because of the anatomic location of the ovaries and the lack of early symptoms, clinical differentiatiation between benign, borderline, and malignant tumours is difficult. On the basis of epithelial characteristics, epithelial ovarian cancer is classified into five histological phenotypes: serous tumours (fallopian-tube-like epithelium), endometrioid proliferative (endometrium-like epithelium), mucinous tumours (endocervix or colonic epithelium), clear cell carcinoma (epithelium of the gestational endometrium), and transitional or Brenner tumours (epithelium of the urinogenital tract). Additionally, on the basis of disease advancement, ovarian tumours are subdivided into four progression stages (I–IV) according to the extent of metastasis. 3 , 10 Because of the lack of adequate screening tools, epithelial ovarian cancer is diagnosed, in most cases, at stages III or IV, when it has already metastasised to the peritoneum or distant sites. Mucins are high-molecular-weight glycoproteins widely expressed by epithelial cells of the gastrointestinal, respiratory, and urinogenital tracts that have multiple implications in cancer development. 13 – 15 Because most ovarian cancers are of epithelial origin, mucins are attractive diagnostic and therapeutic targets. In fact, CA125, which is used in the diagnosis of epithelial ovarian cancer, is the mucin MUC16. 15 Research suggests clinical importance of mucins in various cancers. We review the current knowledge on mucins in epithelial ovarian cancer and discuss their potential role and clinical usefulness in the diagnosis, prognosis, and treatment of this disease.

The development of tumour vaccines for therapy and prophylaxis after primary therapy has recently received substantial attention. Newer therapeutic vaccines are being developed with the intentions of treating primary tumours and preventing their recurrence. Tumour vaccines can be categorised into three types: antigen-based, antibody-based, and cell-based ( figure 3 ). Antigen-based vaccines are targeted against a specific tumour antigen or a protein that is significantly overexpressed on tumour cells. In this approach, either the DNA that encodes the tumour antigen or the synthetic peptide is introduced into the patient. The DNA is taken up by the dendritic cells, processed, and presented on the surface as an MHC–antigen complex to activate cytotoxic T cells. The antibody-based vaccine approach uses monoclonal antibodies that can trigger potent antibody-dependent cellular cytotoxicity and T-cell response. Furthermore, antibodies against tumour antigens can induce anti-idiotype antibodies that mimic the epitopes in tumour antigens and can trigger potent antitumour response. For the cell-based vaccines, either tumour cells (containing tumour antigens) taken from the same patient (autologous) or a different patient (allogeneic) or dendritic cells (from the patient) loaded with tumour antigens are reintroduced into the patient to stimulate the immune system. Several vaccine formulations, including peptide-based vaccines (small peptide sequences derived from tumour antigens), virus augmented vaccines, and carbohydrate or glycolipid vaccines are in clinical trials. 77 Therefore, another possible approach involves the use of mucins to develop cancer vaccines to boost immune responses in ovarian cancer. 78 The induction of potential antimucin responses is particularly useful in targeting tumours that overexpress mucin. To generate efficient antigen-based vaccines, immune adjuvants or carrier proteins, such as Bacille Calmette-Guerin and keyhole limpet haemocyanin are generally conjugated to the mucin peptides. In a pilot study, a heptavalent vaccine containing MUC1 peptide conjugated with keyhole limpet haemocyanin and various mucin-derived carbohydrate epitopes, in combination with QS21 (a Quillaja saponaria saponin used as an immunological adjuvant), safely induced antibody response in patients with ovarian cancer. 63 Preclinical research has also shown efficacy of DNA vaccines and has explained the immunological mechanism of their action. 79 DNA vaccines are easier to construct than peptide-based vaccines and can generate long-lasting immune responses. MUC1 has been used as a cancer DNA vaccine in the mouse model and produces long-term growth suppression of tumours. 80 In a recent phase I clinical trial, autologous dendritic cells pulsed with mannan–MUC1 fusion protein were used to immunise and treat patients with advanced malignant disease, including ovarian carcinoma. 64 Mannan-conjugated MUC1 is rapidly targeted to mannose receptors on dendritic cells, endocytosed, processed, and presented on the dendritic-cell surface with MHC class I molecules. In a recent study, dendritic cells expressing a combined PADRE/MUC4-derived polyepitope DNA vaccine also induced multiple cytotoxic T-cell responses. 81 Because MUC4 overexpression is present in most epithelial ovarian cancer, this strategy can be tested in preclinical and clinical trials. The potential of MUC16 and MUC1 to generate antibody-based vaccines has been assessed in several clinical trials ( table 2 ). Although repeated intradermal doses of a MUC1 antibody, HMFG1, was safe and tolerable, therapeutically relevant titres of MUC1 antibodies were not produced in a recent study. 62 By contrast, oregovomab, a modified murine antibody against MUC16, induced both humoral and cellular responses in several clinical trials in patients with ovarian cancer. 69 Survival was increased in patients that mounted T-cell responses towards MUC16. 82 However, in a randomised, placebo-controlled study, oregovomab treatment lengthened the time to relapse in a small subpopulation only. 70 An alternative approach to generate an immune response against MUC16 involves the use of anti-idiotype antibodies. ACA125 (abagovomab) is an anti-idiotypic antibody generated against the MUC16 antibody, OC125, which mimics the antigenic epitope of MUC16 and serves as a surrogate when given to patients. In phase I or II clinical trial, patients treated with ACA125 developed anti-anti-idiotypic antibodies (Ab3) and MUC16 antibodies, and the Ab3-positive patients survived for longer than others. 71 In a recent phase I trial, long vaccination schedules (nine vs six injections) were more effective in generating humoral and cellular responses. 83 To improve the potency of the anti-idiotypic vaccination, preclinical studies have examined the usefulness of a fusion protein of ACA125 with interleukin 6. ACA125–interleukin 6 fusion protein resulted in an increased induction of Ab3 by stimulating ACA125-specific B cells. 84

Most patients with early-stage epithelial ovarian cancer are asymptomatic or present vague symptoms, including abdominal fullness, dyspepsia, bloating, pelvic pain, and early satiety. 7 , 16 , 17 Additionally, ascites, pleural effusions, and an umbilical mass known as a Sister Mary Joseph nodule, may be evident. For non-invasive diagnosis, transvaginal sonography of the pelvis is the preferred screening method. Serum concentrations of MUC16 are high in more than 80% of patients with advanced epithelial ovarian cancer, and this measurement is routinely used in diagnosis. 17 However, transvaginal sonography and measurement of MUC16 both have limited specificity. Moreover, the concentration of MUC16 is raised in only a few patients with early disease. The limitations of current diagnostic methods have prompted researchers to investigate new markers of early disease to screen patients efficiently and monitor disease progression. An exploratory or complete surgical intervention (laparotomy) is part of primary therapy, which facilitates precise staging and histopathological classification of the disease and tumour debulking. 17 , 18 After surgery, systemic chemotherapy is commonly used to eradicate residual disease. Postoperative chemotherapy typically includes taxane and platinum-based adjuvant treatments, which act by different mechanisms. Taxanes, such as, paclitaxel and docetaxel, bind to tubulin polymers leading to their stabilisation; whereas the platinum analogues (cisplatin and carboplatin) form intrastrand cross-links with DNA and are the most active agents for the treatment of this disease. 17 , 19 Combination chemotherapy with both taxane and platinum analogues has also been investigated, and data from randomised trials have led to use of combined carboplatin and paclitaxel as standard first-line-treatment. 20 , 21 Some evidence supports the use of radiation-based therapy, particularly for chemo-resistant ovarian cancer; 22 , 23 however, radiotherapy remains controversial for advanced epithelial ovarian cancer because of toxic effects. 24

Diagnosis, treatment, and management of ovarian cancer are challenging. In dealing with the epithelial ovarian cancer, we need to achieve earlier detection and improved therapeutic outcomes. Great progress has been made in recent years in aetiology, clinical pathology, and treatment. Mucins are important biomolecules for cellular homoeostasis and protection of epithelial surfaces. These molecules respond to physiological and immunological changes in expression and protein modification. These changes in cancer have multiple implications and clinical significance. Changes to the expression of mucins in ovarian cancer might be exploited in diagnosis, prognosis, and treatment. MUC16 is already being used in the clinics as the only serum marker for ovarian cancer. Other differentially expressed mucins can also be used for the same purpose or combined with MUC16 to improve sensitivity and specificity of the assay. Data about the functional role of mucins (MUC1 and MUC4 in particular) suggest important roles in the pathobiology of cancer, and hence, their clinical significance needs to be investigated in ovarian cancer. Nonetheless, preliminary expression data suggest that mucins can be exploited for immunolocalisation and immunotherapy. Mucins can also be used for the targeted delivery of pharmaceutical agents. Vaccines targeting ovarian tumour-associated mucins are in development and might help eliminate non-resectable peritoneal metastases after surgical removal of primary ovarian tumours. New approaches targeting mucins in ovarian cancer can be tested for the development of peptide-based or DNA-based vaccines against a broad range of epitopes to overcome tumour heterogeneity. As we improve our understanding of the molecular mechanisms of the biological action of mucins and their aberrant regulation, new interventions can be developed for ovarian and other cancers. Progress in mucin biology, antibody engineering, biophysics, and computational technology will likely improve disease outcomes in ovarian carcinoma and cancer in general.