Increased gene copy number of ERG on chromosome 21 but not TMPRSS2-ERG fusion predicts outcome in prostatic adenocarcinomas

Abstract

The role of TMPRSS2-ERG gene fusion in prostate cancer prognostication remains controversial. We evaluated the prognostic role of TMPRSS2-ERG fusion using fluorescence in situ hybridization analysis in a case-control study nested in The Johns Hopkins retropubic radical prostatectomy cohort. In all, 10 tissue microarrays containing paired tumors and normal tissues obtained from 172 cases (recurrence) and 172 controls (non-recurrence) matched on pathological grade, stage, race/ethnicity, and age at the time of surgery were analyzed. All radical prostatectomies were performed at our institution between 1993 and 2004. Recurrence was defined as biochemical recurrence, development of clinical evidence of metastasis, or death from prostate carcinoma. Each tissue microarray spot was scored for the presence of TMPRSS2-ERG gene fusion and for ERG gene copy number gains. The odds ratio of recurrence and 95% confidence intervals were estimated from conditional logistic regression. Although the percentage of cases with fusion was slightly lower in cases than in controls (50 vs 57%), the difference was not statistically significant (P=0.20). The presence of fusion due to either deletion or split event was not associated with recurrence. Similarly, the presence of duplicated ERG deletion, duplicated ERG split, or ERG gene copy number gain with a single ERG fusion was not associated with recurrence. ERG gene polysomy without fusion was significantly associated with recurrence (odds ratio 2.0, 95% confidence interval 1.17-3.42). In summary, TMPRSS2-ERG fusion was not prognostic for recurrence after retropubic radical prostatectomy for clinically localized prostate cancer, although men with ERG gene copy number gain without fusion were twice more likely to recur.

Figure 1

Detection of TMPRSS2–ERG fusion events by FISH. (a) No fusion events: Two intact ERG alleles are seen in the nuclei of benign prostatic epithelial cells. Two sets of juxtaposed red and green signals with occasional yellow overlap are noted in each nucleus (yellow box). (b) TMPRSS2–ERG fusion by split: One intact ERG allele and a second rearranged ERG allele in prostate adenocarcinoma (right box) and adjacent high-grade PIN epithelial cell (left box). The rearranged ERG alleles show split of the red and green signals indicative of an ERG fusion by translocation. (c) TMPRSS2–ERG fusion by deletion: One intact ERG allele and a second rearranged ERG allele in prostate adenocarcinoma. The rearranged ERG alleles show the absence of a green signal with a remaining red signal seen indicative of ERG fusion through deletion. (d) ERG gene copy number gain without fusion: increased ERG gene copy number in prostate adenocarcinoma (red box). Three sets of juxtaposed red–green signals are seen compared with background nuclei each showing only two sets of juxtaposed red–green signals.

Figure 2

Evaluation of chromosome 21 numerical alterations. (a) Overview of BAC clones used in the study. Probe set 1 flanking the ERG genomic locus (RP11-476D17, RP11-95I21) is the set used to determine ERG gene rearrangement status in all cases in the current study. Probe set 2 was only used in a subset of cases to further assess chromosome 21 numerical alterations by targeting centromeric (RP11-22D1) and telomeric (RP11-35C4) regions on the long arm of chromosome 21. (b) Prostate adenocarcinoma showing no evidence of numerical chromosome 21 alteration using probe set 2. (c) Prostate adenocarcinoma showing chromosome 21 long-arm copy number gains as indicated by the presence of more than two red and/or more than two green signals per nucleus (white arrows). (d) Comparison of ERG gene copy number gain detected using probe set 1 and chromosome 21 numerical alteration assessed using probe set 2 in a subset of two tissue microarrays (60 cases) from our study. A high concordance rate (24/26; 93%) is found between the two sets of probes supporting that ERG gene copy number gain are a reflection of chromosome 21 copy number gains.

Figure 2

Evaluation of chromosome 21 numerical alterations. (a) Overview of BAC clones used in the study. Probe set 1 flanking the ERG genomic locus (RP11-476D17, RP11-95I21) is the set used to determine ERG gene rearrangement status in all cases in the current study. Probe set 2 was only used in a subset of cases to further assess chromosome 21 numerical alterations by targeting centromeric (RP11-22D1) and telomeric (RP11-35C4) regions on the long arm of chromosome 21. (b) Prostate adenocarcinoma showing no evidence of numerical chromosome 21 alteration using probe set 2. (c) Prostate adenocarcinoma showing chromosome 21 long-arm copy number gains as indicated by the presence of more than two red and/or more than two green signals per nucleus (white arrows). (d) Comparison of ERG gene copy number gain detected using probe set 1 and chromosome 21 numerical alteration assessed using probe set 2 in a subset of two tissue microarrays (60 cases) from our study. A high concordance rate (24/26; 93%) is found between the two sets of probes supporting that ERG gene copy number gain are a reflection of chromosome 21 copy number gains.