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<b>Purpose</b>: The aim of our study was to observe the associations between the ETS-related gene (<i>ERG)</i> and the phosphatase and tensin homolog gene (<i>PTEN</i>) immunoexpression in prostate cancer and related lesions and highlight the clinical significance of these findings. <b>Methods</b>: We evaluated the immunohistochemical expression of <i>ERG</i> and <i>PTEN</i> in a series of 151 invasive prostate adenocarcinomas, including low-grade (Gleason grade pattern 3) and high-grade (Gleason grade patterns 4, 5) morphological patterns which corresponded to 45.5% and 54.4% of the cases, respectively. Additionally, we evaluated the immunoexpression of the two markers both in foci of high-grade prostatic intraepithelial neoplasia (HGPIN), as a precursor lesion of cancer, and in foci of intraductal carcinoma of the prostate (IDCP). Finally, to ensure the malignant nature of the prostate glands examined, we employed p63 and alpha-methylacyl-CoA racemase (<i>AMACR)</i> expression. <b>Results</b>: We found that <i>PTEN</i> loss was observed in 50.7%, and <i>ERG</i> positivity was detected in 41.8% of our cancerous samples. In HGPIN, <i>PTEN</i> loss appeared to be linked with a high-grade adjacent invasive carcinoma component which also displayed <i>PTEN</i> loss. As far as IDCP is concerned, <i>ERG</i> immunonegativity was correlated with adjacent high-grade invasive cancer, which was also <i>ERG</i> immunonegative. <b>Conclusions</b>: Our findings suggest that the clonal expansion of invasive cancer appears to be associated with distinct immunophenotypic cellular alterations of both early and late cancer-related histological lesions. Patients with <i>PTEN</i> loss in HGPIN in prostate biopsies should be closely monitored due to the increased likelihood of having an associated invasive high-grade carcinoma that may have not been sampled. Given the clinical significance that derives from <i>PTEN</i> expression in HGPIN lesions, we suggest the routine use of <i>PTEN</i> immunohistochemistry in prostate cancer biopsies in which HGPIN is the only finding.