Assessing the order of critical alterations in prostate cancer development and progression by IHC: further evidence that PTEN loss occurs subsequent to ERG gene fusion

BACKGROUND: ERG rearrangements and PTEN loss are two of the most common genetic alterations in prostate cancer. However, there is still significant controversy regarding the order of events of these two changes during the carcinogenic process. We used IHC to determine ERG and PTEN status and calculated the fraction of cases with homogeneous/heterogeneous ERG and PTEN staining in a given tumor. METHODS AND RESULTS: Using a single standard tissue section from the index tumor from radical prostatectomies (N= 77), enriched for relatively high grade and stage tumors, we examined ERG and PTEN status by IHC. We determined whether ERG or PTEN staining was homogeneous (all tumor cells staining positive) or heterogeneous (focal tumor cell staining) in a given tumor focus. 57% (N=44/77) of tumor foci showed ERG positivity, with 93% of these (N=41/44) cases showing homogeneous ERG staining in which all tumor cells stained positively. 53% (N=41/77) of tumor foci showed PTEN loss, and of these, 66% (N=27/41) showed heterogeneous PTEN loss. In ERG homogeneously positive cases, any PTEN loss occurred in 56 % (N=23/41) of cases, and of these, 65% (N=15/23) showed heterogeneous loss. In ERG negative tumors, 51.5% (N=17/33) showed PTEN loss, and of these, 64.7% (N=11/17) showed heterogeneous PTEN loss. In a subset of cases, genomic deletions of PTEN were verified by FISH in regions with PTEN protein loss as compared to regions with intact PTEN protein, which did not show PTEN genomic loss. CONCLUSIONS: These results support the concept that PTEN loss tends to occur as a subclonal event within a given established prostatic carcinoma clone after ERG gene fusion. The combination of ERG and PTEN IHC staining can be used as a simple test to ascertain PTEN and ERG gene rearrangement status within a given prostate cancer in either a research or clinical setting.