Prognostic relevance of Bcl-2 overexpression in surgically treated prostate cancer is not caused by increased copy number or translocation of the gene

Abstract

Background: Overexpression of anti-apoptotic Bcl-2 plays a role in prostate cancer progression, particularly in transformation to androgen-independent disease. Androgen-independent prostate cancers have been shown to harbor Bcl-2 gene copy number gains frequently suggesting that this genetic alteration might play a role in Bcl-2 overexpression. The relation of Bcl-2 overexpression and copy number gains or translocation of the BCL-2 gene in prostate cancer under hormone-naïve conditions is unknown.

Methods: Prostate cancers of 3,261 hormone-naïve patients undergoing radical prostatectomy were arrayed in a TMA with one tissue core (diameter 0.6 mm) per tumor. Bcl-2 immunohistochemistry, analyzed for Bcl-2 expression level (negative, low, and high), was correlated with clinical, histopathological and molecular (Ki67, p53) tumor features, and biochemical failure. Cancers with high-level Bcl-2 expression were evaluated for genetic aberrations by fluorescence in situ hybridization (FISH).

Results: Bcl-2 expression was significantly up-regulated in tumors with aggressive phenotype as indicated by high Gleason score (P < 0.0001), advanced stage (P < 0.0001), and high proliferation index (P = 0.0114). The different Bcl-2 expression levels translated into significantly different survival curves showing better outcome for patients with lower Bcl-2 levels. The prognostic information obtained from the anti-apoptotic Bcl-2 was independent from the proliferation index (Ki67) of the cancer. FISH analysis detected no copy number gains or translocation of the Bcl-2 gene.

Conclusion: Bcl-2 overexpression in prostate cancers under hormone-naïve conditions is not associated with increased copy numbers of the gene. This suggests that these frequently detected genetic alterations in androgen-independent tumors occur late in prostate cancer progression.