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EN2: a novel prostate cancer biomarker Extensive efforts to identify a clinically useful biomarker for the diagnosis of prostate cancer have resulted in important insights into the biology of the disease, but no new test has been approved by regulatory authorities. The unmet need has also shifted to identifying biomarkers that not only diagnose prostate cancer but also indicate whether the patient has ‘significant’ disease. EN2 is a homeobox-containing transcription factor secreted specifically by prostate cancers into urine, where it can be detected by a simple ELISA assay. A number of studies have demonstrated the enormous potential of EN2 to address this unmet need and provide the urologist with a simple, cheap and efficient prostate cancer biomarker. KEYWORDS: biomarker n engrailed-2 n prostate cancer n significant disease

Prostate cancer is the most common malignancy of men in the USA, and approximately one in six men will be diagnosed with prostate cancer dur‑ ing their lifetime [1]. It is the second most com‑ mon cause of cancer death with a 5‑year survival of 80%. The observation that this figure drops to approximately 30% in those with metastatic disease at diagnosis [101] highlights one of the key characteristics of this malignancy: that early diagnosis of limited-stage disease is associated with a radically different clinical picture and therefore justifies the enormous investment in defining new early diagnostic biomarkers. The complexity of prostate cancer is further exempli‑ fied by the fact that small-volume, low-grade dis‑ ease does not need immediate treatment and can be monitored, with intervention initiated only upon signs of progression. The factors deter‑ mining whether a cancer is ‘significant’, that is, needs immediate treatment, versus one that can be managed by active surveillance, are still controversial and no uniform criteria is accepted by all urologists [2]. Biomarkers that could help define significant versus nonsignificant disease would have enormous potential.

Risk factors & conventional biomarkers of prostate cancer A large number of dietary and lifestyle asso‑ ciations have been linked to prostate cancer development and its natural history, but to date no single or combined factors have suffi‑ cient predictive power to recommend a specific intervention, for example, dietary manipulation (reviewed in [3]). An increased risk of prostate cancer has been linked to a positive family history, and the presence of single-nucleotide

polymorphisms. Mutations in BRCA1 and BRCA2, the DNA repair genes, are also asso‑ ciated with a 3.5- and 8.6-fold increased risk of prostate cancer, respectively, with a younger age at onset and more aggressive disease [4–7]. How‑ ever, this knowledge has only resulted in more frequent testing with a conventional biomarker PSA, with its known limitations. PSA is a serum-based biomarker for prostate cancer originally approved by the US FDA in 1986 as a monitoring tool for prostate cancer post-treatment. It has been used in conjunc‑ tion with clinical history and examination for diagnosis of prostate cancer for over 20 years (reviewed in [8]). Sensitivity (or the true posi‑ tive rate) measures the proportion of actual positives that are correctly identified as such. For prostate cancer this is histological confir‑ mation of cancer following an elevated serum PSA. Specificity measures the proportion of negatives that are correctly identified as such (and is sometimes called the true negative rate). Despite a number of high-profile studies, PSA is not recommended for use as a screening bio‑ marker owing to its low sensitivity and speci‑ ficity [102]; PSA was not recommended by the USA preventative services owing to insufficient evidence regarding the balance between harm and benefit. A false positive (or precisely a false positive error) occurs when an abnormally high PSA is not confirmed by a histological diag‑ nosis and may be associated with benign con‑ ditions, such as benign prostatic hyperplasia, infection and inflammation. The false-positive value in general decreases the sensitivity of the test. Conversely, some aggressive prostate can‑ cers do not produce PSA. There is an ongoing

10.2217/BMM.13.115 © 2013 Future Medicine Ltd

Biomarkers Med. (2013) 7(6), 893–901

Sophie E McGrath1, Agnieszka Michael1, Richard Morgan1 & Hardev Pandha*1 Faculty of Health & Medical Sciences, University of Surrey, Guildford, GU2 7WG, UK *Author for correspondence: Tel.: +44 1483688602 Fax: +44 1483688558 [email protected] 1

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McGrath, Michael, Morgan & Pandha

debate about the optimal total PSA cutoff value to separate low- and high-grade cancer. No sin‑ gle total PSA cutoff separates men at high risk for prostate cancer from men at low risk, nor men affected with high-grade from low-grade disease [9]. At a total PSA cutoff of ≥4 ng/ml a significant number of prostate cancers still remain undetected. Reciprocally, as demonstrated in the PCPT study, men with low total PSA levels (≤0.5, 0.6–1.0, 1.1–2.0, 2.1–3.0 and 3.1–4.0 ng/ml) still resulted in detection of prostate cancer in 6.6, 10.1, 17.0, 23.9 and 26.9%, respectively [9,10]. Twenty-five percent of these tumors also had a Gleason score ≥7. Clearly setting a PSA ‘cutoff’ level of 4 ng/ml will still result in some cancers evading detection; however, to date there is no evidence that lowering the PSA threshold below 4 ng/ml improves overall survival [9]. This has prompted investigators to study other molecular isoforms of PSA, as well as free:total PSA, PSA velocity, PSA doubling time and PSA density [9,11]. A meta-ana ­lysis of the percentage of free PSA (%free PSA), which is free:total PSA × 100, demonstrated an improved diagnostic performance of %free PSA when using a total PSA range of 4–10 ng/ml compared with the 2–4‑ng/ml range [12]. The sensitivity was 95% in both groups; however, the specificity was 18% in the 4–10‑ng/ml range, dropping to 6% in the 2–4‑ng/ml range. The free PSA test may be unreliable in clinical practice; however, as it is unstable at 4°C and room temperature [13], and can be influenced by prostate volume including benign enlargement [14], the change in total PSA concentration over time can be recorded as PSA velocity where the change is recorded per year, or PSA doubling time, which involves a specific value increase [9]. Patients with benign prostatic hypertrophy demonstrated a linear increase in total PSA levels over time, in contrast to prostate cancer patients where an initial linear increase was followed by an exponential rise approximately 5 years before cancer detection [15]. However, further investiga‑ tion of PSA velocity showed a high specificity, but a sensitivity greatly dependent on the total PSA value, reading as low as 11% when total PSA is

EN2: a novel prostate cancer biomarker.

Extensive efforts to identify a clinically useful biomarker for the diagnosis of prostate cancer have resulted in important insights into the biology ...
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