Abstract 1433: FOXA1 alterations distinctively drive prostate tumorigenesis or therapy resistance in mice

Abstract Androgen receptor (AR) signaling is critical for survival of prostate cancer (PCa) cells, thus making androgen deprivation therapy (ADT; e.g., castration) as the mainstay for treatment. Notably, oncogenic functions of AR rely on chromatin-binding regulatory proteins, which includes a pioneer transcription factor called FOXA1. FOXA1 de-compacts chromatin to enable AR’s binding to the DNA and expression of it's target genes. Recently, our lab found FOXA1 alterations to recur within three distinct structural classes in over 35% of metastatic castration resistant PCa (mCRPC) in Caucasian men. Subsequent studies found FOXA1 mutations to be prevalent in over 40% of primary PCa in Chinese men, positioning FOXA1 as a principal oncogene in this disease. Yet, hitherto, no studies have defined the pathobiology of FOXA1 alterations in mouse models. Here, we have developed the first-in-field transgenic mice with conditional overexpression of FOXA1 mutants in the prostate luminal epithelia. We found truncal FOXA1 Class1 mutants to initiate luminal hyperplasia in a monogenic model, or hyperproliferative prostate adenocarcinoma in a compound Trp53-deficient genetic background. Mechanistically, Class1 mutants concurrently upregulate the AR and mTORC1/2 pathways to drive transformation, with this being the first report of FOXA1-driven PCa formation in mice. In contrast, FOXA1 Class2 mutants—which are acquired in mCRPC—do not drive prostate luminal transformation. Instead, single-cell multi-omics (RNA+ATAC) profiling of Class2-mutant mouse prostate tissue uncovered extensive transcriptional remodeling of epithelial cells into a luminal stem-like cell fate, leading to a dramatic 15-20-fold expansion of the progenitor population vs control tissue. These luminal stem cells are similar to the Club/Hillock cells detected in the human prostate that have been implicated in driving resistance to ADT. Consistently, we found Class2-mutant prostates to show minimal atrophy upon castration, with immunohistological assessment uncovering a higher density of Ki67+ luminal epithelial cells relative to wild type and Class1 tissues. Class2-mutant organoids also showed higher subcutaneous grafting ability in mice in limiting dilution assays. Mechanistically, we found the cistromically-dominant Class2 mutants to pioneer over 40,000 neo-enhancer elements harboring motifs of stemness-associated transcription factors, which in turn instruct the ADT-resistant luminal progenitor gene program. Altogether, findings from our mouse models uncover FOXA1’s versatility as a driver oncogene that, depending on the mutation type, either activates enhancer-wired luminal tumorigenesis (Class1-initiating event) or therapy resistance-associated stemness (Class2-promoting alteration) gene programs in the mouse prostate tissue. Citation Format: Sanjana Eyunni, Abhijit Parolia, Eleanor Young, James Matthew George, Rahul Mannan, Sandra E. Carson, Yuping Zhang, Jean Tien, Mustapha Jaber, Jie Luo, Matthew Pang, Rohit Mehra, Xuhong Cao, Fengyun Su, Rui Wang, Marcin Cieslik, Dong Kee Lee, Jianming Xu, Arul M. Chinnaiyan. FOXA1 alterations distinctively drive prostate tumorigenesis or therapy resistance in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1433.