The Role of BCL6 in Glioblastoma

Glioblastoma (GBM) is the most common and most deadly brain tumour to occur in adults. Initially patients respond to radiation and chemotherapy, which primarily work by causing large amounts of DNA damage, leading to cell death. However, this process does not happen effectively in GBM and understanding how these cells resist cell death in response to therapy is key to improving the efficacy of treatment. BCL6 is a transcription factor that stops cell death in response to DNA damage, primarily through repressing transcription of DNA damage response genes. Recent work in our lab has shown BCL6 to be present in untreated GBM tumours and up-regulated in GBM cells treated with chemotherapy or radiation, and inhibition of BCL6 leads to a profound loss in proliferative activity. These results indicate that BCL6 may be used as a mechanism of therapy resistance in GBM cells. The objective of this study was to establish a role for BCL6 in GBM cells using luciferase reporter assays, electrophoretic mobility shift assays (EMSA), quantitative chromatin immunoprecipitation (qChIP) and targeted inhibition of BCL6 with subsequent transcriptional analysis by RNA sequencing. We observed that BCL6 appeared to be a transcriptional activator in GBM, as shown by increased luciferase activity in GBM cells treated with radiation. EMSA experiments revealed that overexpressed BCL6 formed complexes with co-repressors, but endogenous BCL6 did not. qChIP experiments revealed that BCL6 was not bound to tradtional BCL6 target genes. Analysis of transcriptional profiles has identified a unique subset of genes which are downregulated when BCL6 is inhibited and upregulated in response to chemotherapy, and these genes were related to cell survival. These changes indicate that these genes may be regulated by BCL6 in chemotherapy treated cells. Together, these results illustrate that BCL6 appears to have an active and unique function in GBM cells, and reinforces this transcription factors position as an attractive therapeutic target in GBM