Abstract 4174: SPOP mutation drives tumorigenesis in mouse prostate - a novel model of ETS negative prostate cancer

Background: Recurrent mutations in SPOP are the most common point mutations in prostate cancer, occurring in about 10% of cases across multiple independent cohorts. F133V is the most frequently (50%) mutated residue. SPOP mutation defines a distinct molecular subclass of prostate cancer, universally negative for ETS rearrangements. No available prostate cancer cell lines harbor endogenous SPOP mutations, making model systems a critical need. Here, we describe a new mouse model with conditional transgenic expression of SPOP-F133V in the mouse prostate. Experimental Design: We generated a conditional mouse with the SPOP-F133V transgene knocked in to the Rosa26 locus (R26 F133V ); these mice were crossed with Pb-Cre4 mice to express SPOP-F133V specifically in the prostate. Since PTEN deletion is known to drive prostate cancer progression, we crossed Pb-Cre4; R26 F133V mice with Pten f/f mice to study the impact of SPOP mutation in the background of conditional heterozygous ( f/+ ) and homozygous ( f/f ) loss of Pten. Organoid lines were derived from transgenic mouse prostates and infected with inducible Cre-ERT2 to serve as in vitro platforms, 2D as well as 3D, for additional studies. RNA-Seq was performed on independently induced samples. Results: Pb-Cre4;R26 F133V mouse prostates showed a minimal histological phenotype, with rare cytological changes of atypical nuclei (p f/+ , SPOP-F133V expression resulted HG-PIN in 80% (n = 6) of mice, age 6 month, compared to only 20% (n = 8) in control mice. In addition, the HG-PIN in R26 F133V ;Pten f/+ mice showed a distinct phenotype with strong nuclear atypia absent in controls. In the background of Pten f/f , SPOP-F133V leads to poorly differentiated, invasive cancer (n = 8 out of 9) compared to control mice (Pten f/f ), which displayed only HG-PIN (n = 6). Organoids with expression of SPOP-F133V showed increased proliferation and increased ki67 staining. To define signaling pathways controlled by mutant SPOP in the prostate, we performed RNA-seq on mouse organoids expressing SPOP-F133V and controls. We interrogated the gene space nominated by the mouse F133V mutation to the human prostate cancer TCGA transcriptome. Unsupervised clustering demonstrated a highly significant enrichment of ETS negative human prostate cancer (p −16 ), supporting the relevance of our transgenic models to human prostate cancer. Summary: Mutation in SPOP causes an early onset of HG-PIN in the prostate of Pten f/+ mice, and progression to poorly differentiated invasive cancer in Pten f/f mice. HG-PIN shows a very distinct histological phenotype with strong nuclear atypia. Gene expression in murine prostate organoids expressing SPOP-F133V strongly correlates with human tumors, providing relevance for this novel mouse model in defining the biology and therapeutic vulnerability of this subclass of prostate cancer. Citation Format: Mirjam Blattner, Deli Liu, Dennis Huang, Dong Gao, Andrea Sboner, Yu Chen, Mark A. Rubin, Christopher Barbieri. SPOP mutation drives tumorigenesis in mouse prostate - a novel model of ETS negative prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4174.