Detection of low level HER-2/neu gene amplification in prostate cancer by fluorescence in situ hybridization

PURPOSE Although expression of the HER-2/neu oncogene has been correlated with tumo r progression in prostate cancer, the biologic significance of detecting HE R-2/neu gene amplification by fluorescence in situ hybridization (FISH) or evidence for protein overexpression by immunohistochemistry (IHC) remains u nclear. In this study, we directly compared HER-2/neu FISH and IHC to deter mine which may be more predictive of the response to trastuzumab. PATIENTS AND METHODS Forty patients with prostate cancer were analyzed for gene amplification by FISH performed with HER-2/neu and chromosome 17 (CEP 17) DNA probes (Vysis ). Protein expression was examined by immunofluorescence and by IHC using t he DAKO HercepTest antibody protocol and a monoclonal antibody to Her-2/neu on archival paraffin sections, The patients included 30 men with primary t umors that were treated with radical prostatectomy. Of these, 15 demonstrat ed subsequent disease progression within 3 years. Five patients with prosta tic intraepithelial neoplasia were tested, as were five with metastatic dis ease whose samples were obtained before androgen ablation therapy. RESULTS None of the 30 primary prostate cancer specimens showed overexpression for HER-2/neu by immunofluorescence or by IHC with the DAKO protocol. One sampl e showed 3+ membrane expression with the monoclonal antibody. In contrast, low copy number gene amplifications (3-8 HER-2/neu signals/nucleus) were de tected in 16 of 30 samples (53%) by FISH. Most amplified cells were diploid for CEP 17, demonstrating that amplification was not due to total cell ane uploidy. FISH and IHC determined that prostatic intraepithelial neoplasia s amples were normal. Four of five (80%) metastatic samples were amplified fo r HER-2/neu by FISH. Nearly 70% of metastatic cancer cells among all five s pecimens demonstrated aneuploidy. A single lymph node metastasis showed 3membrane staining by IHC (DAKO). CONCLUSIONS In contrast to breast cancer, FISH detects HER-2/neu amplification in a sub stantial proportion of prostate cancers that do not overexpress HER-2/neu b y IHC. Although the biologic significance of this finding is uncertain, it has implications for the direction of current and planned clinical trials o f trastuzumab in advanced prostate cancer, Including determination of patie nt eligibility.