PRC2/FOXO1-Mediated Repression Determines Interchangeability of ETS Oncogenes in Prostate Cancer and Ewing Sarcoma.

Genes encoding ETS family transcription factors are altered by chromosomal rearrangement in 60-70% of prostate cancers and nearly all Ewing sarcomas. Ewing sarcoma rearrangements result in chimeric fusion of ETS proteins to the RNA-binding protein EWSR1. Prostate cancer rearrangements result in aberrant expression of ETS proteins such as ETV1, ETV4, ETV5 or ERG that can interact with wild-type EWSR1, suggesting common mechanisms between these diseases. Here, we find that ETV1, ETV4, and ETV5 can phenocopy EWSR1::FLI1 in Ewing sarcoma cell lines. However, rescue of EWSR1::FLI1 knockdown by ERG requires an ERG mutant that disrupts interaction with PRC2. This suggests that EWSR1::ERG fusions that drive Ewing sarcoma avoid PRC2 interactions. We then identify an endogenous PRC2/FOXO1 complex and demonstrate that FOXO1 bridges the ERG/PRC2 interaction. AKT-mediated degradation of FOXO1 and subsequent loss of the ERG/PRC2 interaction provides a mechanism for ERG synergy with PTEN deletion in prostate cancer. Implications: These findings indicate that ETS transcription factors that drive prostate cancer and Ewing sarcoma utilize similar mechanisms and thus could be targeted by similar therapeutic approaches.