ERG gene rearrangements are common in prostatic small cell carcinomas

Clinical potential of the ERG oncoprotein in prostate cancer

Oncogenic activation of ERG resulting from gene fusion is present in over half of all patients with prostate cancer in Western countries. Although the underlying genetic mechanisms have been extensively studied, evaluation of the ERG oncoprotein--the translational product of ERG gene fusions--has just begun. The robust correlation between ERG oncoprotein detection and gene fusion status enables rapid characterization of this protein in large patient cohorts. Recent studies have focused on characterizing the ERG oncoprotein and determining its potential role in the diagnosis and biological stratification of prostate cancer.

ERG protein expression and genomic rearrangement status in primary and metastatic prostate cancer--a comparative study of two monoclonal antibodies

BACKGROUND

Overexpression of the ERG protein is highly prevalent in prostate cancer (PCa) and commonly results from gene fusions involving the ERG gene. Recently, N-terminal epitope-targeted mouse and a C-terminal epitope-targeted rabbit monoclonal anti-ERG antibody (ERG-MAbs) have been introduced for the detection of the ERG protein. Independent studies reported that immunohistochemistry (IHC) with both ERG-MAbs highly correlates with the underlying ERG gene rearrangement status. However, comparative studies of both antibodies are lacking. Here, we are among the first to compare the mouse ERG-MAb with the rabbit ERG-MAb for their concordance on the same PCa cohort. Furthermore, we assessed whether the ERG protein expression is conserved in lymph node and distant PCa metastases.

METHODS

We evaluated tissue microarrays of 278 specimens containing 265 localized PCa, 29 lymph node, 30 distant metastases and 13 normal prostatic tissues. We correlated ERG protein expression with ERG rearrangement status using an ERG break-apart fluorescence in-situ hybridization assay and IHC of both ERG-MAbs.

RESULTS

ERG expression and ERG rearrangement status were highly concordant regardless of whether the mouse or rabbit ERG-MAb was used (97.8% versus 98.6%, respectively). Of interest, both ERG antibodies reliably detected the ERG expression in lymph node and distant PCa metastases, of which a subset underwent decalcification. Lymphocytes only revealed immunoreactivity using the rabbit ERG-MAb. If ERG protein expression was present in localized PCa, we observed the same pattern in the corresponding lymph node metastases.

CONCLUSIONS

By demonstrating a broad applicability of IHC to study ERG protein expression using either antibody, this study adds an important step toward a facilitated routine clinical application. Further, we demonstrate that the clonal nature of the ERG rearrangement is not restricted to the genomic level, but proceeds in the proteome. Together, our results simplify future efforts to further eliucidate the biological role of ERG in PCa.

Review of small cell carcinomas of the prostate

Small cell carcinoma of the prostate is a rare neoplasm, with only a few series hitherto reported. A little less than half of the cases are associated with conventional acinar adenocarcinoma, which are usually high grade. Although consensus has not been reached, the majority of patients with small cell neuroendocrine carcinoma of the prostate have advanced disease at diagnosis and disproportionally low PSA levels compared to patients with conventional acinar adenocarcinoma. Treatment consists mainly of chemotherapy associated with surgery. Radiation therapy is reserved for selected cases. This study reviews the most up-to-date information on small cell carcinomas of the prostate.