Abstract B124: ERG mediated transcriptional regulation of DLX1 homeobox gene represents a novel mechanism underlying prostate cancer progression
Authors: Goel Sakshi , Bhatia Vipul , Palecha Mahendra Shyamlal , Carskadon Shannon , Palanisamy Nallasivam , Ateeq Bushra 
Published: March 2020

Abstract
Homeobox genes are the master regulatory transcription factors involved in the embryonic development and their deregulation in homeostatic biological processes can lead to cancer initiation and progression. A member of Distal less homeobox gene family (DLX), namely DLX1 plays pivotal role in the craniofacial, jaw and GABAergic (gamma aminobutyric acid) interneuron development. Higher expression levels of DLX1 has been associated with primary and metastatic prostate tumor. Moreover, DLX1 and HOXC6 are well-established diagnostic biomarkers for the early detection of prostate cancer (PCa). However, the mechanism involved in DLX1 up-regulation and functional significance in metastatic castration-resistant prostate cancer (mCRPC) progression remains unexplored. Here, we show that benign prostate epithelial cells acquire several oncogenic properties upon ectopic overexpression of DLX1. Similarly, CRISPR/Cas9 mediated DLX1 knockout in castrate-resistant 22RV1 cells show reduced proliferation, invasion, foci formation in cell-based assays, and a remarkable reduction (~80%) in tumor burden in mouse xenograft model, confirming its role in tumor progression. Moreover, our global gene expression profiling data established the role of DLX1 in regulating critical cancer hallmarks such as epithelial-to-mesenchymal transition (EMT), apoptosis and cell cycle regulation. We also demonstrate positive association between DLX1 and the most recurrent TMPRSS2-ERG gene fusion in many independent PCa cohorts, wherein, ~96% of the TMPRSS2-ERG fusion positive patients also harbour higher level of DLX1. Our ChIP-qPCR and luciferase-reporter assay data show that ERG occupies the DLX1 promoter region, and orchestrate its transcriptional control. Nevertheless, a subset of ERG-fusion negative patients also exhibits a higher DLX1 expression, where androgen receptor (AR) and AR variant 7 (AR-V7) plays critical role in transcriptional regulation of DLX1 in an ERG-independent manner. Since, Bromodomain and extra-terminal domain inhibitors (BETi) are known to inhibit the transcriptional activity of AR, AR-V7 and ERG, we examined the effect of BETi on DLX1. Interestingly, a significant decrease in the expression of DLX1 in BETi treated PCa cell lines with a concomitant loss of their oncogenic potential was observed. Taken together, our findings reveal that DLX1 serves as an oncogene thereby regulating several oncogenic properties associated with PCa progression. We discovered a novel transcriptional regulatory network involving ERG dependent up-regulation of DLX1 in TMPRSS2-ERG fusion positive PCa, while AR and AR-V7 governed expression in ERG-independent manner in CRPC cells. Conclusively, our findings suggest that BET domain inhibitors can be used as an adjuvant therapy for DLX1-positive subset of metastatic castration resistant prostate cancer patients.
Citation Format: Sakshi Goel, Vipul Bhatia, Mahendra Shyamlal Palecha, Shannon Carskadon, Nallasivam Palanisamy, Bushra Ateeq. ERG mediated transcriptional regulation of DLX1 homeobox gene represents a novel mechanism underlying prostate cancer progression [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B124. doi:10.1158/1535-7163.TARG-19-B124