Upstream and downstream targeting of the SPOP ubiquitin signaling pathway in prostate cancer

PROJECT SUMMARY / ABSTRACT Prostate cancer (PCa) is a clinically and molecular heterogenous disease, with distinct subtypes. These subtypes are associated with characteristic genomic alterations, gene expression profiles, and treatment responses, implying truly distinct biology. We have defined one such molecular subclass of PCa characterized by recurrent missense mutations in the ubiquitin ligase SPOP -- representing about 10-15% of PCa, in both primary and metastatic disease. These SPOP mutant cancers display distinct biology and response to therapies. However, other key players in the SPOP pathway influencing its action in PCa, and the broader ability to specifically target this axis for patient benefit, remain incompletely understood. Preliminary data generated by our multidisciplinary, collaborative group have defined novel elements of the SPOP signaling pathway, including the upstream regulator G3BP1, and suggest that the subclass of PCa defined by deregulation of SPOP signaling are preferentially targetable with novel therapeutic interventions. The overall objective of this proposal is to define the mechanistic, biological, and therapeutic consequences of alterations to the SPOP signaling pathway, including its upstream regulator G3BP1. Using novel models and human prostate cancer samples, our preliminary data demonstrate that a novel endogenous inhibitor of the SPOP ubiquitin ligase, G3BP1, can potentially phenocopy the oncogenic effects of SPOP mutation, and that modulation of this effect with small molecule inhibitors can be a viable therapeutic strategy. Furthermore, we show that modulating SPOP activity reprograms androgen receptor (AR) function through key downstream substrates, altering chromatin accessibility and transcription driven by AR and making these cancers highly reliant on AR activity. This project will elucidate the molecular details underlying these phenomena through the following Aims: 1) define the role of G3BP1-driving prostate tumorigenesis in model systems and human prostate cancer, and the ability to target this upstream axis. 2) establish the therapeutic potential of targeting the SPOP axis by modulating targetable downstream signaling nodes. To accomplish this, we will leverage unique, biologically and clinically relevant model systems, novel small molecule inhibitors, innovative approaches to modulating ubiquitin ligase signaling, and data from human prostate cancer samples. This project will define the critical dependencies in specific subtypes of prostate cancer and broader applicability to treatment response, and provide the foundation for precision clinical trials and novel strategies to attack this subclass of cancer.