Combined BET bromodomain and DNA methyltransferase inhibition targets critical survival pathways in transdifferentiated prostate cancer.

Lineage plasticity, or transdifferentiation, is increasingly recognized as a resistance mechanism to androgen receptor (AR) inhibition in prostate cancer. Lineage plasticity is characterized by loss of AR signaling and epithelial differentiation, along with activation of stemness-associated pathways, epithelial-mesenchymal transition, or alternative differentiation programs such as neuroendocrine prostate cancer (NEPC). Loss of the tumor suppressors TP53 and RB1 is common in tumors exhibiting lineage plasticity; however, the mechanisms by which TP53/RB1 loss promotes this phenotype remain poorly understood, and effective treatments are limited. Using multiomic profiling of TP53/RB1-loss prostate cancer models, we identified alterations in chromatin accessibility, DNA methylation, and gene expression associated with lineage plasticity. Importantly, many pathways activated upon TP53/RB1 loss could be blocked through BET bromodomain inhibition. TP53/RB1-deficient cells also harbored widespread DNA methylation changes that silenced pathways linked with restraining lineage plasticity. Combined BET bromodomain and DNA methyltransferase (DNMT) inhibition was more effective than single-agent treatment in suppressing growth of TP53/RB1-loss models exhibiting a stem-like or NEPC program. This was partly explained by abrogation of discrete lineage plasticity pathways modulated by each agent. Altogether, our work suggests combined BET bromodomain and DNMT inhibition is a promising therapeutic approach for prostate tumors exhibiting lineage plasticity.