CSIG-24. EPIGENETIC SUPPRESSION ALLOWS GBM TO MAINTAIN P53 WILD-TYPE STATUS

Abstract Introduction Glioblastoma (GBM) is the most common primary brain malignancy in adults. The vast majority of GBM cases maintain wild-type status of p53, the protein considered to be the most critical tumor suppressor. How GBM displays such a malignant phenotype despite retaining normal p53 protein is unknown. We also aim to translate these molecular findings into therapeutics via a randomized clinical trial. METHODS A variety of molecular biological techniques were employed. These include CRISPR-CAS, next generation sequencing, stereotactic mouse brain injections, and histological analyses of human GBM tumors. Approval for a small, single site, blinded, randomized clinical trial for newly diagnosed GBM is underway. Overall survival, recurrence-free survival, immunohistochemical analyses will be assessed. RESULTS Human p53 wild-type GBM cell lines express Brd at higher levels compared to p53 mutant cell (~10 fold, p < 0.05). Cell viability assays show that deletion of Brd leads to a ~50% decrease (p < 0.05) in cell survival. Critically, this effect is abrogated upon deletion of p53 itself, showing that Brd functions through the inhibition of p53’s tumor suppressive effects. Furthermore, immunoprecipitation of Brd shows that it is a member of a complex of proteins that bind to p53 target genes. The importance of these findings are corroborated in vivo, as mice stereotactically injected with human GBM cells that lack Brd survive ~2-fold longer (p < 0.05) than mice that retain Brd. We have begun the process to start clinical trials with an already FDA-approved drug (currently used for the treatment of lymphoma and sarcoma) that blocks the Brd pathway. CONCLUSION Brd is the key suppressor of p53 target genes in GBM. Brd binds to a multimeric protein complex, which inhibits p53 target genes. Thus, the tumor suppressive effects of p53 are prevented in GBM. Inhibition of this pathway in GBM patients is underway.