CBIO-24GLIOBLASTOMA MULTIFORME CELLS ARE ADDICTED TO THE ONCOGENIC SURVIVAL FACTOR BCL6

Abstract

The reluctance of GBM cells to apoptose is the primary hurdle in drug and radiation-mediated killing, which suggests an overriding antiapoptotic mechanism in GBM mediates survival. A prime candidate for that survival factor is BCL6, a zinc finger transcription factor that prevents apoptosis in response to DNA damage. In maturation of normal lymphocytes, BCL6 expression allows cells to survive DNA breakage and recombination, the process used to generate genetic diversity in antibody and immune receptor genes. However, ectopic BCL6 expression in lymphoma, leukemia and breast cancer similarly bypasses death in response to DNA damage induced by therapy. BCL6 transcript was elevated in glioma compared to normal brain tissue, and expression increased with grade. Consistent with this, BCL6 protein was variably expressed in >60% of GBM specimens examined. We examined BCL6 expression and activity in a panel of GBM cell lines. A low basal level of BCL6 protein was present under normal growth conditions, and consistent with induction by DNA damage, was substantially increased by the genotoxic drugs doxorubicin, temozolomide and ionising radiation, both in vitro and in an intra-cranial tumour model. Co-immunoprecipitation showed that BCL6 was associated with its co-repressors SMRT and NCOR in GBM cell lines, and a BCL6 reporter assay indicated that BCL6 had transcriptional repression activity. Inhibition of BCL6 activity by either siRNA, a specific peptide mimetic inhibitor, or by over-expression of dominant negative BCL6 notably decreased viability, and reduced the clonogenic potential, of GBM cells. Induction of BCL6 in the presence of inhibitors restored the clonogenic activity. Finally, GBM cells with a BCL6 somatic knock-out were non-viable, dying within 48 hours of genome editing. Together, these data imply that BCL6 is essential for on-going survival of GBM cells in vitro, and that DNA damage induced by therapy upregulates the oncogene, further facilitating the addiction.