Abstract 2565: Detection of ETV1 expression in human prostate tissue specimens using a novel and highly specific rabbit monoclonal antibody

Introduction: ETV1 is frequently involved in genomic fusions and translocation events that lead to its overexpression in multiple cancers. These events occur in approximately 5% of prostate cancers, which are mutually exclusive from tumors harboring TMPRSS2-ERG fusion or PTEN deletion. Studies suggest a correlation between strong ETV1 protein expression and poor outcome in prostate cancer. ETV1 has been reported to synergistically cooperate with KIT as a lineage survival factor in gastrointestinal stromal tumors. The expression of ETV1 in a subset of sarcomas that harbor CICrearrangements or CIC-DUX4 gene fusions presents a reliable molecular signature for the diagnosis of this cancer. Our understanding of the role that ETV1 plays in the activation of prostate cancer has been limited by the lack of ETV1 specific antibodies. Methods: A novel ETV1 monoclonal antibody (MAb) was raised by immunization of ETV1 peptides in rabbit followed by screening of hybridomas by ELISA and immunoblot assays. Further screening using exogenously expressed ETV1, ETV4, ETV5, ERG, SPDEF, and FLI1 proteins identified the clone with the most reactive MAb. Purified MAb was used to evaluate ETV1 expression on a tissue micro-array (TMA) constructed from radical prostatectomies of 50 African American (AA) and 50 Caucasian American (CA) patients by immunohistochemistry (IHC). Key residues required for Mab binding were mapped by ELISA using overlapping peptides and alanine scanning. Results: IHC evaluation using the ETV1 specific rabbit Mab on a prostate cancer TMA derived from 100 patients identified five ETV1 positive cases, of whom four were CA. The index tumors for these five ETV1 cases were ERG negative. One patient harbored both ERG positive and ERG negative tumor foci, and as expected, the ETV1 positive tumor focus was ERG negative, and vice versa. Conclusions: We developed a novel rabbit monoclonal ETV1 antibody that is suitable for IHC assay in human prostate tissue. An evaluation of prostate cancer specimens confirmed the reported frequency of ETV1 alteration. Further evaluation using tissue specimens from larger cohorts to establish the sensitivity and specificity of this antibody and validate the utility of ETV1 detection in improving the diagnosis and stratification of prostate and other cancers are in progress. Citation Format: Shyh-Han Tan, Denise Young, Sally Elsamanoudi, Jacob Kagan, Sudhir Srivastava, Albert Dobi, Gyorgy Petrovics, Isabell A. Sesterhenn, Gregory T. Chesnut. Detection of ETV1 expression in human prostate tissue specimens using a novel and highly specific rabbit monoclonal antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2565.