Abstract 4497: Distinct structural classes of activating FOXA1 alterations in prostate cancer progression

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 is frequently mutated in the hormone-receptor driven prostate, breast, bladder, and salivary gland tumors. In prostate luminal epithelial cells, wild-type FOXA1 delimits tissue-specific enhancers that are transcriptionally activated by AR, and extensively reprograms AR-activity in the transformed prostate epithelia. However, how FOXA1 alterations affect cancer development is unclear, with FOXA1 previously ascribed both tumor suppressive and oncogenic roles. In this study, we assemble an aggregate cohort of 1546 prostate cancers (PCa) and, for the first time, show that FOXA1 alterations fall into three distinct structural classes that diverge in clinical incidence, genetic co-alteration profiles, and oncogenic gain-of-functions. Notably, we find the three classes of FOXA1 alterations to collectively recur at a frequency of 35% in metastatic PCa. Class1 activating mutations originate in early PCa without concurrent ETS or SPOP alterations, selectively recur within the Wing2-region of the DNA-binding Forkhead domain (FKHD), confer enhanced chromatin mobility and binding frequency, and strongly trans-activate a luminal androgen receptor (AR) program of prostate oncogenesis. By contrast, class2 activating mutations are acquired in metastatic PCa, truncate the C-terminal regulatory domain of FOXA1, confer chromatin-binding dominance by increasing DNA affinity, and aberrantly activate the WNT/β-Catenin pathway to enable PCa metastasis. Finally, class3 genomic rearrangements are comprised of tandem duplications and translocations within the highly-syntenic FOXA1 locus. These structural variations amplify or reposition a conserved enhancer element, which we named FOXA1 Mastermind (FOXMIND), to drive overexpression of FOXA1 or other oncogenes, respectively. In summary, our study reaffirms the central role of FOXA1 in mediating AR-driven oncogenesis, and provides mechanistic insights into how different classes of FOXA1 alterations uniquely promote PCa initiation and/or metastatic progression. Furthermore, these results have direct implications in understanding the biology of other hormone-receptor driven cancers where similar FOXA1 alterations are found, and rationalize therapeutic co-targeting of oncogenic FOXA1-activity to extort more potent and durable disease remissions. Citation Format: Abhijit Parolia, Marcin Cieslik, Shih-Chun Chu, Lanbo Xiao, Takahiro Ouchi, Yuping Zhang, Xiaoju Wang, Pankaj Vats, Xuhong Cao, Fengyun Su, Rui Wang, Felix Feng, Yi-Mi Wu, Robert Lonigro, Dan R. Robinson, Arul M. Chinnaiyan. Distinct structural classes of activating FOXA1 alterations in prostate cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4497.