Abstract 578: Loss of MED12 predicts for a DNA damage repair deficiency phenotype and activates immune signalling via the STING pathway
Authors: Orafidiya Folake A. , Davidson Catherine , Wilkinson Richard D. , Walker Steve M. , Knight Laura A. , Harkin Paul D. , McCabe Nuala , Kennedy Richard D. 
Published: August 2018

Abstract
Introduction: Genome integrity is maintained by a number of pathways which recognize and repair both exogenous and endogenous DNA damage. Deficiency in these pathways results in mutations that accelerate tumorigenesis. A subgroup of cancers such as those with mutations in the BRCA/ Fanconi anemia (FA) pathway have been reported to benefit from DNA damaging chemotherapy. Previously, we identified a molecular subgroup in breast cancer characterised by upregulation of immune genes. This group referred to as the DNA damage repair deficient (DDRD) group also showed enhanced response to DNA damaging chemotherapy. Here, we identify the same subgroup in prostate cancer and investigate the mutations associated with the subgroup in prostate cancer. Experimental procedures: Unsupervised hierarchical clustering in primary prostate cancer identified a subset of patients which demonstrated activation of immune signalling genes and enrichment for BRCA mutations, representing the DDRD group. Using data from The Cancer Genome Atlas (TCGA), we show that mutation of the MED12 gene is associated with the DDRD group. In order to study the role of MED12 in prostate cancer, isogenic cell lines with knockdown of MED12 were generated using RNAi technology. We evaluated the expression of immune genes in these cell lines using RT-qPCR. Immunofluorescence, DNA-RNA immunoprecipitation and western blot analyses were also used in the characterization of these cell lines. Results: Our data reveals that the loss of MED12 leads to a significant upregulation of the immune genes associated with DNA repair deficiency; CXCL10, CCL5, MX1 and the immune check-pointing gene PD-L1 (p&lt;0.05). Accumulation of γH2AX foci was identified in cells depleted in MED12 (p&lt;0.001). A significant increase in DNA-RNA hybrids (Rloops) was also observed in MED12- deficient cells (p&lt;0.0001) indicating an endogenous source of genomic instability. Furthermore, we show the presence of cytosolic nucleic acid which triggers immune response via the STING/TBK1/IRF3 pathway which is characteristic of the DDRD-like phenotype in MED12-deficient cells.Conclusion: Mutations in the MED12 gene have previously been reported in 5 % of prostate cancer. Our findings demonstrate that the loss of MED12 in prostate cancer results in a DDRD-like phenotype. Upon further characterization, MED12 may be used as a predictor of response to stratify patients for chemotherapy or immune checkpoint targeted therapy in prostate cancer
Citation Format: Folake A. Orafidiya, Catherine Davidson, Richard D. Wilkinson, Steve M. Walker, Laura A. Knight, Paul D. Harkin, Nuala McCabe, Richard D. Kennedy. Loss of MED12 predicts for a DNA damage repair deficiency phenotype and activates immune signalling via the STING pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 578.