Epigenetic De-repression of PROX1 Promotes Neuroendocrine Prostate Cancer Progression

Abstract Histologic transformation of prostate cancer from adenocarcinoma to neuroendocrine prostate cancer (NEPC) is associated with aggressive disease and poor prognosis. This lineage transition is accompanied by Polycomb complex 2 (PRC2)-mediated epigenetic de-repression of cell-fate determining transcription factors, including prospero-homeobox 1 (PROX1). Here, we sought to functionally characterize the role of PROX1 in NEPC. An unbiased CRISPR screen in two NEPC patient-derived organoid models demonstrated high cellular dependency for PROX1. Knockout of PROX1 impeded tumor growth in NEPC models and overexpression of PROX1 promoted tumor growth and spontaneous metastasis in prostate adenocarcinoma. Transcriptomic and cistromic analyses across castration resistant adenocarcinoma and neuroendocrine models pointed to PROX1-mediated regulation of neuroendocrine-lineage transcriptional programs. Immunoprecipitation followed by mass spectrometry identified three phosphorylated sites in the DNA-binding domain of PROX1 that are critical for its stability and function. CHEK1 and CDK2 were predicted to be upstream kinases that phosphorylate PROX1, and treatment with a CHEK1 or CDK2 inhibitor reduced NEPC viability. Together, these results substantiate the role of PROX1 in NEPC and identify PROX1 phosphorylation in the DNA binding domain, which might represent a therapeutic target in NEPC.