ERG expression in mucinous prostatic adenocarcinoma and prostatic adenocarcinoma with mucinous features: comparison with conventional prostatic adenocarcinoma.

TMPRSS2-ERG is the most common gene fusion in conventional prostatic adenocarcinoma (PCa), identified in about 40% to 70% of cases. Mucinous PCa and PCa with mucinous features are rare subtypes of prostate cancer, and ERG expression in these subtypes has not been well characterized in a large series. A search was made through the surgical pathology and expert consultation files of 2 major academic institutions for cases of mucinous PCa and PCa with mucinous features. The former were obtained from radical prostatectomy cases and the latter from radical prostatectomy cases, transurethral resection of the prostate, and prostate needle core biopsies. A tissue microarray composed of additional cases of mucinous PCa was also included in the study. Immunohistochemical stains for ERG were performed on all the cases. A total of 51 cases of mucinous PCa and PCa with mucinous features were identified. Twenty-five of 51 (47%) cases were positive for ERG expression, including 10/24 (42%) radical prostatectomy specimens, 7/14 (50%) biopsies, 2/4 (50%) transurethral resection of the prostate specimens, 6/9 (67%) from a tissue microarray. This is the largest study to date specifically characterizing ERG expression in mucinous PCa and PCa with mucinous features, with emphasis on comparison with adjacent conventional PCa. ERG is expressed in almost 50% of cases of mucinous PCa and PCa with mucinous features, similar to rates of expression in conventional PCa. This study strongly suggests that these rare subtypes of PCa are clonally related to conventional PCa.