Characterization of ETS gene aberrations in select histologic variants of prostate carcinoma

Abstract

Histologic variants of prostate carcinoma account for 5-10% of the disease and are typically seen in association with conventional acinar carcinoma. These variants often differ from the latter in clinical, immunophenotypic, and biologic potential. Recently, recurrent gene fusions between the androgen-regulated gene TMPRSS2 and the ETS transcription factors ERG, ETV1, ETV4, or ETV5 have been identified in a majority of conventional prostate carcinomas. However, the frequency and significance of this critical molecular event is unknown in the histologic variants of prostate carcinoma. Here, we used break-apart fluorescence in situ hybridization to assess TMPRSS2 and ETS aberrations in a series of select histologic variants: foamy gland carcinoma (N=17), ductal adenocarcinoma (N=18), mucinous carcinoma (N=18), and small cell carcinoma (N=7). A histologic variation of acinar adenocarcinoma, demonstrating glomeruloid morphology (N=9), was also investigated. Overall, 55% of histologic variant or variation morphologies demonstrated ETS aberrations (ERG in 54% and ETV1 in 1%). TMPRSS2:ERG fusion was identified in 83% (15/18), 71% (5/7), 50% (9/18), 33% (3/9), and 29% (5/17) of mucinous, small cell, ductal, glomeruloid, and foamy gland prostate carcinomas, respectively. Previously, we reported that 100% of androgen-independent metastatic prostate carcinomas harboring TMPRSS2:ERG gene fusion were associated with interstitial deletion (Edel). Interestingly, ERG rearrangement in small cell carcinomas occurred exclusively through Edel, supporting the notion that TMPRSS2:ERG with Edel is an aggressive molecular subtype. SPINK1, a biomarker expressed exclusively in a subset of ETS negative prostate carcinomas, was expressed in 6% of ETS negative histologic variants, specifically in ductal adenocarcinoma. Notably, 88% (43/49) variant morphologies in this cohort showed concordance of TMPRSS2:ERG fusion with associated conventional acinar type, suggesting that variant morphology is clonally related to the latter. Overall, our data provide insight into the origin, molecular mechanism, and phenotypic association of ETS fusions in histologic variants of prostate carcinoma.

Figure 1. H&E staining and corresponding FISH images of ERG rearrangement in different histologic variants and glomeruloid histologic variation of acinar adenocarcinoma

Representative morphological images of four histologic variants (foamy gland, mucinous, ductal, and small cell) as well as prostate carcinoma with glomerulation features were shown at lower magnification (left row) and high magnification (middle row). ERG break-apart FISH assay was performed and corresponding images were shown at right row. The rectangular boxes show magnified images illustrating the ERG rearrangement pattern. ERG rearrangement negative case was indicated by two pairs of colocalized green and red signals. ERG rearrangement positive (with deletion) case showed loss of one green labeled probe 5′ to ERG. ERG rearrangement positive (translocation) case showed one pair of split 5′ (green) and 3′ (red) signals.

Figure 2. Summary matrix of TMPRSS2 and ETS aberrations in different histologic variants, glomeruloid histologic variation of acinar adenocarcinoma, and associated conventional prostate carcinoma

Prostate carcinoma patients of histologic variants or variation with associated conventional prostate carcinoma component were shown in the summary matrix. Patient case numbers are incidated on the left of the map. Each column represents one case; each row represents FISH evaluation for TMPRSS2 or ETS aberration at each tumor focus. TMPRSS2:ETS gene rearrangement status of cases with two tumor foci, histologic counterpart and associated conventional prostate carcinoma were shown. Color legend signifies respective aberrations or availability.

Figure 3. Representative example of ductal adenocarcinoma case

H&E sections with two adjacent tumor foci of carcinoma showing concordant ERG rearrangement. (A, B), ductal adenocarcioma component (Grey) (A, C), adjacent conventional acinar tumor foci (Blue). (B&C insert), One yellow signal and individual red color indicating that fusion through deletion has occurred.

Figure 4. SPINK1 protein expression exclusively in ductal adenocarcinoma of prostate by immunohistochemistry

SPINK1 protein expression was evaluated in this cohort using immunohistochemistry. Histology of one ductal adenocarcinoma case is shown in A (H&E, ×100) and B, yellow boxed area in A (H&E, × 200), demonstrating complex branching and papillary cribriform structure. Strong cytoplasmic staining of SPINK1 was observed in ductal adenocarcinoma focus in C. D, FISH image of the green-boxed area in B, displaying absence of ERG rearrangement, indicated by two pairs of colocalized red and green signals.