PDTM-34. RADIOSENSITIZATION BY BRD4 INHIBITION IN DIFFUSE INTRINSIC PONTINE GLIOMA

Abstract

Abstract Diffuse intrinsic pontine glioma (DIPG), the most frequent brainstem tumor in pediatrics, is one of the devastating childhood cancers, and virtually all patients die within two years after diagnosis. Since these tumors occur in the brainstem which is vital area, there are no surgical options for providing relief to patients, and conventional chemotherapy as well as radiation therapy provides palliative relief at best. DIPG shows increased H3 K27 acetylation (H3K27ac), which binds to BET bromodomain protein 4 (BRD4) and they are strongly associated with active transcription. Here we tested the hypothesis that BRD4 inhibition by JQ1 enhances radiation-induced DNA damage, making it a potential radiosensitizer in the treatment of DIPG. We evaluated the effects of JQ1 on genes expression using RNA sequence. Radiation-induced DNA double-strand break (DSB) repair was analyzed by immunocytochemistry and western blotting of DSB markers γH2AX and 53BP1, and DSB repair markers pRad50 and Rad51. Clonogenic survival assay was used to determine the effect of JQ1 on radiation response of DIPG cells. In vivo response to radiation monotherapy and combination therapy of RT and JQ1 were evaluated in patient-derived DIPG xenografts. JQ1 significantly reduced the expression of DNA DSB repair genes in DIPG cells. JQ1 sustained high levels of γH2AX and 53BP1 and reduced the levels of pRad50 and Rad51 in irradiated DIPG cells. JQ1 reduced clonogenic survival and enhanced radiation effect in DIPG cells. In vivo studies revealed increased survival of animals treated with combination therapy of RT and JQ1 in compared to either monotherapy. Together, these results highlight JQ1 as a potential radiosensitizer and provide a rationale for developing combination therapy with radiation in the treatment of DIPG.