EXTH-34. ACETYLATION SUPPRESSION MEETS ONCOLYTIC VIRUSES FOR THE TREATMENT OF PEDIATRIC DIFFUSE MIDLINE GLIOMA

Abstract Diffuse midline glioma (DMG) remains a devastating pediatric brain tumor without effective treatment. In a recent clinical trial, we demonstrated the feasibility and efficacy of Delta-24-RGD oncolytic virus combined with radiation therapy in patients with newly diagnosed diffuse intrinsic pontine gliomas (NCT03178032). Because the adenoviral protein E1A binds to P300 and redirects acetylation upon virus infection, we hypothesized that combining oncolytic adenoviruses with P300 inhibitors will cause a remarkable shifting in acetylation and the post-translational landscape of DMG, ultimately enhancing the anti-glioma effect of oncolytic viruses. Western blot analyses showed a longitudinal reduction in H3K27ac (89%) and H3K18ac (99%) in a panel of human (TP54 and SF8628) and murine (24B_1 and 26C_7) DMG cells after Delta-24-RGD infection. In addition, using histone fractionation by high-performance liquid chromatography, we detected an increase in total H3 upon adenovirus infection. Mass spectrometry of histones in the human DIPG cell line, TP54 using middle-down approach illustrated post-translational changes across histones, including hypoacetylation of the H3.2K14 mark. To further modify histone acetylation in DMG during Delta-24-RGD infection, we combined virotherapy with C646, a selective and competitive histone acetyltransferase inhibitor of the E1A-binding p300/CBP complex of proteins. In vitro, the combination of Delta-24-RGD with C646 synergistically increased cell death in a panel of human and murine DIPG cell lines (zip-scores ranging from 6.3-11.3 with zip-scores above 10 being indicative of synergy). Ongoing experiments using a panel of adenoviruses expressing E1A with defective P300 binding or expressing a dominant-negative form of P300 or wild-type P300 are designed to test the extent to which E1A binding of P300 drives hypoacetylation in DMG upon virus infection. In addition, we are performing in vivo experiments to establish the anti-cancer effect of the combination of Delta-24-RGD and P300 inhibitors in clinically relevant animal models of DMG, with a feasible translation to the clinic.