Ovarian cancer has a high mortality rate because it is often only diagnosed when it is in an advanced stage. Considerable effort is underway to identify screening and diagnostic strategies that improve the likelihood of early diagnosis. This article describes standard and novel serum biomarkers that play an important role in this effort, as well as implementation of symptoms reporting in ovarian cancer screening. 
by Dr Kimberly Lowe

The epidemiology of ovarian cancer
Every year more than 200,000 women worldwide are diagnosed with ovarian cancer [1].  The disease, which includes malignancies found in the ovary, peritoneum and fallopian tubes, has a higher fatality-to-case ratio than any other gynaecological malignancy [2]. The high mortality rate is attributed to the fact that most women are not diagnosed until the disease has become metastatic [3]. Only 30% of women who receive a diagnosis of advanced-stage ovarian cancer will live five years past their diagnosis date [4]. On the contrary, women who are diagnosed with early-stage disease benefit from a 70–90% five-year survival rate [4]. As a result, there is substantial scientific and clinical effort underway to identify screening modalities that will increase the number of women who are diagnosed with early-stage disease.

The incidence of ovarian cancer is highest among Caucasian women living in Northern Europe and the United States; however, this disease is present among women of all races and ethnic backgrounds. Only 5–10% of ovarian cancer cases are familial [2]. The aetiology for the remaining sporadic cases is poorly understood and few risk factors have been identified. Women tend to be considered at high-risk for ovarian cancer if they have a family history of two or more ovarian or breast cancers among their first or second-degree relatives, if they are of Ashkenazi Jewish ethnicity, or if their probability of carrying a BRCA I or BRAC II mutation is greater than 20% as determined by the BRCAPRO 95% posterior probability interval [5, 6]. There is evidence to suggest that parous women and those who use oral contraceptives are at lower risk for ovarian cancer.  This may be related to the observed association between the number of ovulatory cycles a woman experiences in her lifetime and the risk of this disease, such that women who have interruption in their menstrual cycle for longer periods of time are at lower risk for ovarian cancer.  

Serum biomarkers for ovarian cancer diagnosis
Biomarkers that can be measured in the blood are a valuable tool for disease diagnosis because they provide a screening modality that is low-cost, and can easily be applied to large numbers of people. This is especially true for ovarian cancer because of the rarity and anatomical location of this disease. CA125 is currently the most widely used serum biomarker for ovarian cancer [7]. The molecule is a high-molecular-weight glycoprotein that is elevated in the serum of 75–90% of all patients with ovarian cancer [2]. CA125 is an extremely valuable biomarker for ovarian cancer because it can be used as a diagnostic tool to identify incident cases, to assess response to treatment in prevalent cases and to monitor recurrence [8–10]. Unfortunately, CA125 is associated with a high false-positive rate among women with benign gynaecological conditions and it has poor sensitivity for early-stage disease [7, 11]. As a result, there is an urgent need to identify new biomarkers that can replace or complement CA125.

The use of proteomic and genomic technologies is becoming more common in the field of biomarker discovery and has led to the identification of many promising biomarkers for ovarian cancer [8]. Candidate biomarkers must be able to effectively discriminate women who have ovarian cancer from women who do not have cancer, with good sensitivity at high levels of specificity. Mesothelin [7, 12–14] and human epididymis protein 4 (HE4) [15, 16] are two of the most promising biomarkers emerging in this field because of their ability to improve early detection. The diagnostic performance of these biomarkers has been well studied and they consistently complement the performance of CA125.

Mesothelin is a 40-kDa polypeptide that is attached to the surface of mesothelial lining cells.  It is believed that mesothelin plays a role in cell-adhesion, as well as cellular recognition and signalling. Mesothelin is overexpressed in pancreatic adenocarcinoma, mesothelioma, and ovarian cancer [17]. Ovarian cancer cells express a higher level of mesothelin than healthy cells, and soluble mesothelin has been found in the blood of 40–67% of ovarian cancer patients [18].  Research results have shown that the sensitivity of mesothelin to differentiate ovarian cancer patients from healthy women is 38% at 95% specificity and 27% at 98% specificity in the general population [19]. The diagnostic accuracy of mesothelin has also been shown to be equivalent regardless of a woman’s risk status for ovarian cancer [Table 1] [6]. In addition, mesothelin has shown substantial promise as a complementary diagnostic tool with CA125. At 98% specificity, the sensitivity of CA125 alone was 78.8%, but increased to 86.5% when CA125 was measured as a composite  marker with mesothelin [20].

HE4 is an 11-kDa protein found on human chromosome 20q12-13.1, and is a precursor to the epididymal secretory protein E4 [6]. While the function of HE4 is unknown, like mesothelin, it is expressed on ovarian cancer cells and is also elevated in the serum of ovarian cancer patients [15, 21]. The sensitivity of HE4 has been shown to be 62% at 95% specificity and 55% at 98% specificity in the general population [19].  Reported area under the curve (AUC) values for discriminating ovarian cancer cases from healthy women have been greater than 0.80 [6, 19]. In addition, HE4 is especially promising as a marker for early detection in that it has shown potential in differentiating women with ovarian cancer from women with benign ovarian conditions. The sensitivity of HE4 at 95% specificity to differentiate ovarian cancer patients from women with benign ovarian conditions has been shown to be 61.76% for women with average ovarian cancer risk and 75.61% for women who are at high-risk for this disease [Table 1] [6].  Such levels of diagnostic accuracy rival those of CA125.  HE4 has also been shown to complement CA125. Immunochemical analysis has shown that 32% of ovarian cancers that do not express CA125 will express HE4 [13].

Symptoms as a biomarker for ovarian cancer diagnosis
Historically referred to as the “silent killer,” ovarian cancer was believed to have no symptoms at all until the disease had progressed to advanced stages. However, there is emerging evidence to suggest that women with ovarian cancer do experience symptoms [22–27], even when the disease is in an early stage [28–30]. A preliminary study of 1,700 women in the United States and Canada reported that 95% of women with ovarian cancer reported symptoms prior to diagnosis, including abdominal symptoms (77%), gastrointestinal symptoms (70%), pain (58%), constitutional symptoms (50%), urinary symptoms (34%) and pelvic symptoms (26%) [28]. In contrast to what is published in most medical textbooks, 89% of the women in this study who were diagnosed with early-stage ovarian cancer experienced symptoms prior to their diagnosis [28]. These symptoms are likely to cause women to seek medical care, which may allow an opportunity for the early detection of ovarian cancer if it is present [31].

An ovarian cancer Symptom Index was developed in 2007 based on the results of a case-control study, in which the frequency, severity, and duration of symptoms were compared in 149 women with ovarian cancer and 233 controls [29]. The Symptoms Index is considered to be positive if a woman reports that any of the following symptoms are new to her within the last year and occur more than 12 times per month: pelvic or abdominal pain, feeling full quickly, inability to eat normally, abdominal bloating, or increased abdominal size [29].  Using these criteria, the Symptoms Index was found to have a sensitivity of 56.7% for early-stage disease and 79.5% for late-stage disease [29]. Additional research conducted by Anderson et al. [32] has shown that a multimodal screening strategy including both CA125 and the Symptoms Index has greater sensitivity for detecting ovarian cancer than CA125 alone, especially among early-stage cancers that are commonly missed by CA125 [7]. In this study, the sensitivity of a composite biomarker that included CA125 and the Symptoms Index to detect early stage disease was 80.6%, which was substantially higher than the sensitivity for either screening modality alone [Figure 1] [32].

Conclusions
Early detection is the key to improved survival among women with ovarian cancer. CA125 is the current gold standard for ovarian cancer diagnosis; however, its limitations have prompted the need to discover biomarkers that have good sensitivity for early-stage disease and the ability to differentiate women with ovarian cancer from women with benign ovarian conditions. Although biomarkers are most commonly thought of as diagnostic tools that can be measured in the blood, using symptoms as a diagnostic biomarker for ovarian cancer is also showing great promise.   
 
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The author
Kimberly Lowe, PhD, MHS
Affiliate Staff Scientist,
Fred Hutchinson Cancer Research Center,
Seattle, WA, USA
Senior Scientist,
Exponent Health Sciences, Seattle, WA, USA