Divergently evolved FOXA1 mutations drive prostate tumorigenesis or therapy-resistant intra-luminal plasticity [Single-cell Multiome]

FOXA1 is recurrently altered in hormone-driven malignancies, with activating hotspot mutations detected in 10-40% of primary prostate cancers based on ethnicity. Yet, the tumorigenic potential and pathobiology of FOXA1 remain unexplored in vivo. Here, we generated and characterized knock-in mouse models harboring FOXA1 mutant transgenes that represent distinct classes of activating mutations. Our findings reveal that FOXA1 class 1 mutations (i.e., wing 2 alterations) in a Trp53-null background, drive high-grade, invasive luminal adenocarcinoma with full penetrance by 40 weeks of age. These hyperproliferative lesions retain luminal characteristics, gain expression of NSD2, and regress upon androgen withdrawal—mirroring features of primary human prostate adenocarcinomas. Mechanistically, class 1 mutants aberrantly activate mTORC1/2 signaling and reprogram androgen receptor (AR) activity by expanding the enhancer landscape at non-canonical chimeric AR-half elements enriched in patient tumors. In contrast, FOXA1 class 2 mutations (i.e., C-terminal truncations) do not drive transformation but induce intra-luminal plasticity in androgen-intact normal tissues, which is otherwise triggered in response to castration. Class 2 mutants lead to a 10- to 20-fold expansion of Ar+/Ck8+ luminal epithelia expressing stem/progenitor markers such as Trop2, Ck4, and Psca. Mechanistically, class 2 mutants maintain AR activity while de-compacting over 40,000 new cis-regulatory elements bound by Klf5 and AP-1 transcription factors to activate stemness and WNT gene programs. Class 2-mutant-expressing prostate epithelial cells resist castration-induced atrophy, remain proliferative in androgen-deprived conditions, and form organoids and allografts at higher frequencies. Additionally, metastatic prostate tumors from patients harboring class 2 mutations show elevated KLF5 expression and stemness signatures. Collectively, our data establish FOXA1 as a critical oncogene in both primary and advanced AR-dependent prostate cancers, revealing its functional versatility in driving either tumor initiation or therapy resistance depending on the mutation class.