Cooperativity of TMPRSS2-ERG fusion with p53 inactivation in prostate cancer pathogenesis

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PROJECT SUMMARY Prostate cancer (PCa) is the second leading cause of cancer death in American men. The TMPRSS2-ETS related gene (ERG) fusion (termed T2-ERG), juxtaposing the strong androgen-responsive TMPRSS2 gene promoter with the coding region of the putative oncogene ERG, occurs in approximately 50% of all PCa in patients. It has been inferred that overexpressed truncated T2-ERG is a key player in PCa pathogenesis. Notably, a number of studies show that overexpression of T2-ERG protein alone is insufficient to trigger formation of PCa in mice unless at very advanced age (> 24 months), implying the requirement of additional lesions in T2-ERG overexpression-induced PCa pathogenesis. By analyzing the NGS data from > 1500 cases of PCa patient specimens, we showed that T2-ERG fusion and p53 inactivation (loss of p53 tumor suppressor function due to TP53 gene deletion and/or mutation) co-occurred in both primary and metastatic PCa, supporting the notion that T2-ERG fusion and TP53 inactivation cooperate to drive PCa pathogenesis and progression. We demonstrated that prostate-specific T2-ERG transgene and Trp53 knockout (T2-ERG+/p53-) not only largely enhanced ERG target gene expression, but also induced high-grade prostatic intraepithelial neoplasia (HGPIN) and cancerous lesions in mice at 12-15 months of age. Mechanistically, we identified LSD1 as a direct binding partner of ERG protein and showed that cyclin-dependent kinase-2 (CDK2) phosphorylates LSD1 at threonine 59 (T59-p), which disrupts LSD1 interaction with HP1γ, an epigenetic `reader' of H3K9me2/3 and induces demethylation of these transcription repressive chromatin marks at ERG target gene loci. Based on these novel preliminary data, we hypothesize that in p53-proficient cells the oncogenic potential of T2-ERG is constrained under a transcription repressive chromatin state through association with the LSD1-HP1γ complex. However, p53 inactivation induces loss or downregulated expression of p21WAF1, aberrant activation of CDKs and accelerated cell cycle progression, which in turn triggers LSD1 T59-p-mediated disassociation of HP1γ from LSD1, HP1γ eviction from chromatin due to accelerated cell cycle progression-associated H3S10 phosphorylation (H3S10-p), thereby promoting H3K9me demethylation at ERG target gene loci, oncogenic reprogramming of ERG transcriptome, and PCa pathogenesis and progression. To test this hypothesis, we will determine the molecular basis and regulation of LSD1 phosphorylation-mediated inhibition of LSD1-HP1γ interaction in T2-ERG+/p53- PCa cells (Aim 1), define the mechanism and extent to which LSD1 enzymatic activity and T59 phosphorylation regulates ERG transcriptional program in T2-ERG+/p53- PCa cells (Aim 2), and determine the clinical significance of the interplay between ERG and p53 signaling and anti-cancer efficacy of targeting LSD1 and ERG pathways in T2-ERG+/p53- PCa (Aim 3). Findings from the proposed studies will shed new light on molecular mechanisms of PCa pathogenesis and could lead to development of novel therapeutics for the treatment of T2-ERG+/p53- PCa.