Gene Fusion Characterization of Rare Aggressive Prostate Cancer Variants ‐ Adenosquamous Carcinoma, Pleomorphic Giant Cell Carcinoma, and Sarcomatoid Carcinoma: An Analysis of 19 Cases

Abstract

Aims We evaluated the molecular underpinnings of rare aggressive prostate cancer variants adenosquamous, pleomorphic giant cell, and sarcomatoid carcinomas. Methods and Results We retrieved 19 tumors with one or more variant(s) and performed ERG immunohistochemistry, a next‐generation sequencing assay targeting recurrent gene fusions, and fluorescence in situ hybridization (FISH) for ERG and BRAF. Divergent differentiation included: sarcomatoid (n=10), adenosquamous (n=7), and pleomorphic giant cell carcinoma (n=7). Five patients had more than one variant. Four had variants only in metastases. ERG rearrangement was detected in 9 (47%, 7 via sequencing, showing TMPRSS2‐ERG and one GRHL2‐ERG fusion, and 2 via FISH, showing rearrangement via deletion). Of these, ERG immunohistochemistry was positive in the adenocarcinoma for 8/9 (89%) but only 5/9 (56%, typically decreased) in the variant. One patient had false‐positive ERG immunohistochemistry in the sarcomatoid component despite negative FISH. Two (11%) harbored BRAF fusions (FAM131A‐BRAF and SND1‐BRAF). Conclusions ERG gene fusions are present in these rare prostate cancer variants with a close frequency to conventional prostate cancer (9/19, 47%). ERG immunohistochemistry usually detects rearrangement in the adenocarcinoma but is less sensitive for the variant histology with weak to negative staining. Adenosquamous and sarcomatoid variants particularly can occur together. Molecular assessment may be an additional tool in select cases to confirm prostatic origin of unusual tumors. The presence of 2 BRAF gene rearrangements suggests that this gene fusion may be enriched in this setting, as RAF kinase fusions have been previously reported in 1‐2% of prostate cancers.