Although the use of serum prostate-specific antigen (PSA) to screen for prostate cancer is widespread clinically (1
), PSA has several limitations as an early detection biomarker. PSA is highly specific for tissue of prostatic origin, but is not cancer-specific. Moreover, serum levels are frequently elevated in benign conditions. Currently, whereas most men undergo needle biopsy when levels of PSA are more than 4.0 ng/ml, less than half of these biopsies result in a diagnosis of prostate cancer (2
). PSA also has sensitivity limitations, as shown by the Prostate Cancer Prevention Trial (PCPT), which demonstrated that 15% of men with PSA of 0 to 4.0 ng/ml have prostate cancer, of which 15% have high Gleason grade disease (4
). Despite the development of multivariate models, such as the PCPT risk calculator that incorporates PSA and other clinical factors in an attempt to provide an individual risk estimate (6
), men are commonly referred for biopsy in the United States on the basis of their serum PSA concentration alone.
Moreover, screening with PSA has probably led to the overdiagnosis of prostate cancer—an estimated 23 to 44% of all screening-detected cancers would never have caused symptoms (7
)—and overtreatment. Two randomized trials evaluating the effect of PSA screening on prostate cancer mortality showed that during the first decade of follow-up, PSA screening has a modest effect on prostate cancer mortality, with substantial risks of negative biopsy and over diagnosis and overtreatment of indolent cancer (cancer that would not cause symptoms in a lifetime) (3
). Together, these results highlight the limitations of the current serum PSA–based paradigm of prostate cancer early detection. Recognizing these limitations, several groups, including the United States Preventative Task Force, the American Cancer Society, and the American Urological Association, have advocated for individualized decision making between a patient and his physician regarding PSA screening and/or proceeding to biopsy (9
). Biomarkers to assist this process, however, are lacking.
Several modifications of serum PSA, including free PSA, rate of PSA change (PSA velocity), various PSA isoforms, and related proteins, have been proposed as prostate cancer biomarkers that can be used to help PSA-screened men make more informed decisions about proceeding to biopsy (10
). These strategies, however, rely on surrogate biomarkers that are tissue-specific and not intrinsically cancer-specific. An alternative approach is to develop clinically robust assays for cancer-specific biomarkers that have been identified through genomic and transcriptomic studies (11
Recently, chromosomal rearrangements were identified in prostate cancer that fuse the 5′ untranslated region of the androgen-regulated gene transmembrane protease, serine 2 (TMPRSS2) with v-etserythroblastosis virus E26 oncogene homolog (avian) (ERG) or ets variant 1 (ETV1); ERG and ETV1 are both members of the erythroblastosis virus E26 transformation-specific (ETS) transcription factor family (12
). Subsequent studies confirmed ETS gene fusions in about 50% of PSA screened prostate cancers (13
). Fusions between TMPRSS2 and ERG, which result in a truncated ERG protein product, represent about 90% of all ETS gene fusions (13
). Fusion of TMPRSS2 and ERG loci at the chromosomal level [as detected by fluorescence in situ hybridization (FISH)] and subsequent overexpression of the TMPRSS2:ERG transcript and truncated ERG protein product are essentially 100% specific for the presence of prostate cancer in tissue-based studies (13
). Additionally, multiple studies have demonstrated that TMPRSS2:ERG fusions are only detectable in about 15% of high-grade prostatic intraepithelial neoplasia (PIN) lesions, invariably adjacent to fusion-positive cancer (16
). In vitro and in vivo functional studies have also demonstrated a functional role for TMPRSS2:ERG fusions in prostate cancer oncogenesis (13
). Together, TMPRSS2:ERG gene fusions are highly specific biomarkers that define a distinct molecular subtype of prostate cancer.
The protein product of the TMPRSS2:ERG fusion is neither chimeric nor known to be secreted, which precludes the possibility of antibody-based detection in serum (as for PSA). However, a clinical grade, urine-based assay for the noncoding transcript prostate cancer antigen 3 (PCA3) [a prostate-specific gene overexpressed in greater than 95% of prostate cancers (21
)] has been developed and has proven useful as an adjunct to serum PSA for prostate cancer detection (22
). In addition, research-grade, reverse transcription–polymerase chain reaction (RT-PCR)– based assays have shown that TMPRSS2:ERG mRNA is indeed detectable in urine (24
To translate these findings to clinical practice, we have developed a clinical-grade, transcription-mediated amplification (TMA) assay for quantifying TMPRSS2:ERG mRNA, which is normalized to the amount of PSA mRNA by TMA (which controls for the abundance of prostate cells and prostate mRNA) to generate a TMPRSS2:ERG “score.” The assay is based on the same technology as the PCA3 assay. We tested prospectively collected, post–digital rectal exam (DRE) urine from men presenting for biopsy and/or prostatectomy. We then correlated urine TMPRSS2:ERG levels with clinicopathologic features, including indicators of clinically significant cancer. We also measured PCA3 in the same urine specimens and report the combined performance of TMPRSS2:ERG and PCA3 for prostate cancer risk stratification of PSA-screened men. This work represents an initial step in using a panel of cancer-specific biomarkers for early detection of prostate cancer.