ERG-TMPRSS2 rearrangement is shared by concurrent prostatic adenocarcinoma and prostatic small cell carcinoma and absent in small cell carcinoma of the urinary bladder: evidence supporting monoclonal origin

Abstract

Prostatic carcinoma is a heterogeneous disease with frequent multifocality and variability in morphology. Particularly, prostatic small cell carcinoma is a rare variant with aggressive behavior. Distinction between small cell carcinoma of the prostate and urinary bladder may be challenging, especially in small biopsy specimens without associated prostatic adenocarcinoma or urothelial carcinoma. Recently, gene fusions between ETS genes, particularly ETS-related gene (ERG), and transmembrane protease, serine 2 (TMPRSS2) have been identified as a frequent event in prostate cancer. Thus, molecular methods may be helpful in determining the primary site of small cell carcinoma. Thirty cases of prostatic small cell carcinoma from the authors' archives were studied, among which 13 had concurrent prostatic adenocarcinoma. Tricolor fluorescence in situ hybridization (FISH) was performed on formalin-fixed paraffin-embedded tissue sections with a probe cocktail for 3'/5' ERG and TMPRSS2. Cases of small cell carcinoma of the bladder and conventional prostatic adenocarcinoma (25 each) were also tested as controls. ERG gene alterations were found only in prostate malignancies and not in benign prostatic tissue or bladder small cell carcinoma. TMPRSS2-ERG gene fusion was found in 47% (14/30) of prostatic small cell carcinoma. Of cases with concurrent prostatic adenocarcinoma, 85% (11/13) had identical findings in both components. In 20% of rearranged cases, the ERG abnormality was associated with 5' ERG deletion. In 17% (5/30) of cases, gain of the 21q22 locus was present. Two cases showed discordant aberrations in the small cell carcinoma and adenocarcinoma, one with deletion of 5' ERG and one with gain of chromosome 21q, both in only the adenocarcinoma component. Small cell carcinoma of the prostate demonstrates TMPRSS2-ERG rearrangement with comparable frequency to prostatic adenocarcinoma. In cases with concurrent adenocarcinoma and small cell carcinoma, the majority showed identical abnormalities in both components, indicating a likely common clonal origin. Discordant alterations were present in rare cases, suggesting that acquisition of additional genetic changes in multifocal tumors may be responsible for disease progression to a more aggressive phenotype. TMPRSS2-ERG fusion is absent in bladder small cell carcinoma, supporting the utility of FISH in distinguishing prostate from bladder primary tumors and identifying metastatic small cell carcinoma of unknown origin.

Figure 1

Schematic representation of ERG rearrangement identification by tricolor FISH. The red probe hybridizes to the 3′ sequence of ERG, the green probe hybridizes to the 5′ sequence of ERG, and the aqua probe hybridizes to TMPRSS2. (a) Wild-type (normal) findings at the 21q22 locus show a triplet of overlapping red and green signals with aqua signals spaced at a variable distance from the red–green pair. (b) Cells with ERG–TMPRSS2 rearrangement exhibit splitting (separation) of the red–green signal pair, accompanied by a fused red–aqua signal for one allele. (c) A subset of cases with rearrangement showed loss of the corresponding green signal for one allele, indicating 5′ ERG deletion.

Figure 2

Morphology and ERG gene rearrangement by FISH in small cell carcinoma and adenocarcinoma of the prostate. (a) Low magnification shows admixed small cell carcinoma and concurrent prostatic acinar adenocarcinoma. (b) The wild-type (normal) pattern of ERG demonstrates proximate or fused ERG 3′ (red) and ERG 5′ (green) signals with TMPRSS2 (aqua) either adjacent to or slightly separated from the red–green signal pair. (c) The small cell carcinoma cells are arranged in cords, nests, or sheets, with the nucleus showing prominent hyperchromasia, nuclear molding, small punctate nucleoli, and brisk mitotic activity. (e) In contrast, the typical prostatic adenocarcinoma component retains the characteristic small, round glands. ERG gene rearrangements were detectable by FISH in both small cell (d) and adenocarcinoma components (f). In the typical rearrangement, the green signal (ERG 5′, thin arrows) is separated from the red signal with red and aqua signals approximated (ERG 3′–TMPRSS2 fusion, thick arrows (d, f)). In other cases, rearrangement was associated with a red–aqua signal doublet, with loss of the corresponding green signal (g). A subset of cases with and without rearrangement demonstrated copy number gain at the 21q22 locus (h).

Figure 2

Morphology and ERG gene rearrangement by FISH in small cell carcinoma and adenocarcinoma of the prostate. (a) Low magnification shows admixed small cell carcinoma and concurrent prostatic acinar adenocarcinoma. (b) The wild-type (normal) pattern of ERG demonstrates proximate or fused ERG 3′ (red) and ERG 5′ (green) signals with TMPRSS2 (aqua) either adjacent to or slightly separated from the red–green signal pair. (c) The small cell carcinoma cells are arranged in cords, nests, or sheets, with the nucleus showing prominent hyperchromasia, nuclear molding, small punctate nucleoli, and brisk mitotic activity. (e) In contrast, the typical prostatic adenocarcinoma component retains the characteristic small, round glands. ERG gene rearrangements were detectable by FISH in both small cell (d) and adenocarcinoma components (f). In the typical rearrangement, the green signal (ERG 5′, thin arrows) is separated from the red signal with red and aqua signals approximated (ERG 3′–TMPRSS2 fusion, thick arrows (d, f)). In other cases, rearrangement was associated with a red–aqua signal doublet, with loss of the corresponding green signal (g). A subset of cases with and without rearrangement demonstrated copy number gain at the 21q22 locus (h).