Abstract 5244: BET inhibition decreases the expression of DNA repair enzymes in pancreatic cancer
Authors: Miller Aubrey Lynn , Fehling Samuel C. , Garcia Patrick L. , Yoon Karina J. 
Published: August 2019

Pancreatic ductal adenocarcinoma (PDAC), the most common type of pancreatic cancer (PC), is now the third leading cause of cancer related deaths in the US. This is a highly aggressive disease in that 80% of patients present with locally advanced or metastatic disease and their only treatment option is systemic chemotherapy. Ultimately patient tumors develop resistance to therapy and resulting in a median survival of ~6 months. Therefore, there is an imperative need to identify therapies that provide a more durable response for this patient population.

The family of bromodomain and extraterminal domain (BET) proteins has recently become a target of interest for the treatment of PDAC. The BET proteins (BRD2, BRD3, BRD4, and BRDT) function to regulate transcription by recruiting positive transcriptional activators to the promoters of genes. Our lab has shown that inhibition of BET bromodomain function using the BET inhibitor JQ1 suppresses PDAC tumor growth in vivo and inhibits cell viability in vitro. Importantly, PDAC cells and tumors exposed to JQ1 show evidence of DNA damage. We hypothesized that because of the role BET proteins are known to play in regulating gene expression, that the observed JQ1-induced DNA damage may result from JQ1 inhibiting the expression of DNA repair genes whose expression are dependent on BET protein transcriptional complexes. qRT-PCR and immunoblot analysis demonstrated that treatment of the pancreatic cancer cell line BxPC3 with JQ1 inhibited the expression of two double strand break repair proteins, Ku80 and RAD51, both of which have been shown to be overexpressed in cancer including PDAC. Although it has been established that JQ1 co-occupies 99% of the genomic loci that BRD4 is known to bind to, specific gene products whose expression is dependent on BRD4 in PDAC have not been thoroughly investigated.

The goal of the current study was to determine if the expression of DNA repair genes Ku80 and RAD51 were specific gene targets of BRD4 transcriptional complexes. We exposed BxPC3 cells to JQ1 for 48 hours and assessed changes in the association of BRD4 with the promoter loci of Ku80 and RAD51 using chromatin immunoprecipitation (ChIP) assays. qRT-PCR of the ChIP demonstrated that treatment with JQ1 significantly (p&lt;0.05) decreased the association of BRD4 with the promoters of both genes. To further confirm that these gene products were dependent on BRD4 for their expression, we down-regulated BRD4 using shRNA and evaluated the effect on the expression of Ku80 and RAD51. Immunoblot revealed that by reducing the expression of BRD4 by 53% in BxPC3 cells, the expression of Ku80 and RAD51 were also decreased by 35% and 93% respectively. We conclude that the expression of Ku80 and RAD51 are dependent on BRD4 transcriptional complexes and are gene targets that contribute to the anti-tumor efficacy of JQ1 in PDAC.

Citation Format: Aubrey Lynn Miller, Samuel C. Fehling, Patrick L. Garcia, Karina J. Yoon. BET inhibition decreases the expression of DNA repair enzymes in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5244.