Abstract 3437: Cistromically-dominant class2 mutants of FOXA1 de-repress WNT signaling to drive prostate cancer metastasis
Authors: Parolia Abhijit , Cieslik Marcin , Carson Sandra , Chu Shih-Chun , Xiao Lanbo , Ouchi Takahiro , Jaber Mustapha , Zhang Yuping , Vats Pankaj , Cao Xuhong , Chinnaiyan Arul 
Published: September 2020

Abstract
Forkhead box A1 (FOXA1) is a pioneer transcription factor that is essential for the normal development of several endoderm-derived organs, including the prostate gland. FOXA1 independently binds to and de-compacts condensed chromatin to reveal the binding sites of partnering nuclear hormone receptors. In prostate luminal epithelial cells, FOXA1 delimits tissue-specific enhancers that are transcriptionally activated by the androgen receptor (AR). Thus, it is intriguing that FOXA1 is frequently mutated in hormone-receptor-driven prostate, breast, bladder and salivary-gland tumors. In a recent study, leveraging an aggregate cohort of 1,546 prostate cancers, we classified FOXA1 alterations into three novel structural classes that diverge in clinical incidence and genetic co-alteration profiles, with a collective prevalence of 35%. Class1 mutations comprise of missense and in-frame indels within the Wing2 secondary structure and originate in early prostate cancer without other driver alterations in genes such as ETS or SPOP. Contrarily, class2 mutations comprise of frameshift mutations which truncate the C-terminal end of the protein and are clonally acquired in the metastatic disease. Finally, class-3 genomic rearrangements comprise of duplications and translocations that reposition a conserved enhancer element to drive overexpression of FOXA1 or other oncogenes. Here, we mechanistically expand upon the neomorphic class2 mutants and show that they centrally enable prostate cancer metastasis. Remarkably, we found truncated class2 mutants to entirely displace their wild-type counterpart from the chromatin - a characteristic we termed as “cistromic dominance” - due to higher DNA affinity. Through proteomic assays, we identified the C-terminal regulatory domain of wild-type FOXA1 to interact with and recruit a bonafide transcriptional WNT-repressor called TLE3 to the chromatin. Notably, class2 mutants had lost this interaction and dominantly displace TLE3 from chromatin to de-repress WNT signaling. Accordingly, we found CRISPR-engineered prostate cancer cells harboring class2 FOXA1 mutations to show higher invasive and metastatic ability both in vitro and in vivo. Also, the increase in the metastatic ability of class2-mutant cells was completely reversed upon treatment with a WNT inhibitor. Overall, our study reaffirms the central role of FOXA1 in mediating AR-driven oncogenesis and provides mechanistic insights into how distinct classes of FOXA1 alterations drive initiation or metastatic progression of prostate cancer. These results reveal a novel FOXA1-TLE3-WNT signaling axis and rationalize its therapeutic co-inhibition in advanced prostate cancer.
Citation Format: Abhijit Parolia, Marcin Cieslik, Sandra Carson, Shih-Chun Chu, Lanbo Xiao, Takahiro Ouchi, Mustapha Jaber, Yuping Zhang, Pankaj Vats, Xuhong Cao, Arul Chinnaiyan. Cistromically-dominant class2 mutants of FOXA1 de-repress WNT signaling to drive prostate cancer metastasis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3437.