Abstract LB312: FOXA1 mutations co-opt nascent transcription factor networks in partnership with androgen receptor to enhance prostate tumorigenicity

Abstract Gain of function mutations in the pioneer transcription factor FOXA1 are found in 10-40% of human prostate cancers. These mutations rewire global chromatin accessibility, driving prostate cell growth, yet the underlying molecular details behind this transformation remain elusive. To investigate this further, we leveraged single nucleus multiome sequencing of primary mouse organoids following inducible expression of FOXA1 alleles with clinically relevant missense, in/del or truncating mutations in the Wing2 region of the forkhead binding domain. While each FOXA1 mutant induces distinct transcriptomic and DNA accessibility features, all shared one compelling feature, aberrant epithelial lineage gene expression, with a marked shift toward a secretory luminal fate (L1), mirroring what is seen in FOXA1-mutant human prostate cancers. A standout mutation was a truncation of FOXA1 downstream of the Wing2 domain (G275X) that caused an 11-fold surge in the frequency of L1 cells, coupled with enrichment of a composite androgen receptor-FOXA1 hybrid motif and cooperativity with the Oct family transcription factor POU2F1. G275X triggered this L1 state transition within 24 hours, a dramatic acceleration compared to the 5-day timeline of wildtype FOXA1 while exhibiting a shorter chromatin residence time in single molecule tracking assays.In vivo studies revealed that these Foxa1 mutants enhance prostate tumorigenicity when combined with Trp53/Pten loss, producing tumors with secretory luminal cell histology. Thus, Wing2 domain mutations in FOXA1 confer gain of function pioneering activity, resulting in co-option of AR to establish transcription factor networks (Pou2f1/Oct1) that collectively enhance tumorigenicity. These findings illuminate FOXA1 as a master regulator in prostate cancer and opens new avenues for targeted therapeutic interventions. Citation Format: Erik Ladewig, Abbas Nazir, Tyler Park, Vinson B. Fan, Robert Tijian, Christina Leslie, Charles Sawyers. FOXA1 mutations co-opt nascent transcription factor networks in partnership with androgen receptor to enhance prostate tumorigenicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB312.