Introduction and Objectives: The oncogenic activation of ERG in prostate cancer has been well established in recent years. Towards understanding the functional role of ERG, our laboratory previously identified two major types of ERG splice variants in human prostate tumors: Type I, which encodes a full length prototypical ERG protein, and Type II, which encodes a truncated protein lacking the ETS DNA-binding domain. Analysis of clinico-pathologic data from patients and ratios of Type I/Type II ERG revealed that an increased ratio correlated with a higher Gleason sum and poorly differentiated phenotype. However, the functional significance of the ERG splice variants is not clearly understood. Studying the function and interaction of the individual splice variants can further enhance our understanding of ERG, and advance efforts towards its clinical utility in the treatment of prostate cancer.

Methods: Exogenous Type II ERG was expressed in TMPRSS2-ERG fusion harboring VCaP cells and endogenous levels of Type I ERG and C-MYC proteins were evaluated. Type I and Type II ERG were also co-transfected in various cell lines to examine interaction among splice variants and regulatory efficiency with luciferase reporter constructs. In addition, quantitative RNA expression data of C-MYC and Type I/Type II ERG ratio were assessed by correlation analysis in patient samples.

Results: Expression of exogenous Type II ERG resulted in a decrease in endogenous Type I ERG protein expression in VCaP cells in a dose dependent manner, with a concomitant decrease in C-MYC. This decrease was not observed in the other cell lines. Decreased regulatory efficiency of Type I ERG, by addition of Type II ERG, was also observed. Patient data analysis also revealed a positive correlation between C-MYC expression and increased Type I/Type II ratio.

Conclusions: Decreased Type I/Type II ratios in VCaP cells showed decreases in C-MYC, suggesting that modulation of Type II ERG alters Type I ERG and thus its regulation of C-MYC. This interference was not observed in other cell lines, indicating the interaction is context dependent. While Type I ERG positively regulated downstream targets, addition of Type II ERG decreased their expression, implying an interaction between splice variants. Analysis of patient data affirmed in vitro results, thus we propose that modulation of Type I/Type II ratio may regulate downstream targets including oncogenic C-MYC in prostate cancer, providing a strong rationale for the biomarker and therapeutic utility of ERG splice variants.

Funding source: DoD/CDMRP (PC073614) grant to S.S.

Citation Format: Anshu Rastogi, Shyh-Han Tan, Gyorgy Petrovics, Yongmei Chen, Taduru Sreenath, David G. McLeod, Shiv Srivastava, Albert Dobi. Functional analysis of ERG splice variants and their effect on c-myc expression in prostate cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3004. doi:10.1158/1538-7445.AM2013-3004

Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.