Abstract 2819: Targeting histone chaperone FACT complex sensitizes medulloblastoma cells to chemo and radiation therapy
Authors: Ray Sutapa , Song Heyu , Roychoudhury Shrabasti , Biswas Pranjal , Ray Sutapa , Bhakat Kishor 
Published: August 2018

Medulloblastoma (MB) is the most common malignant brain tumor in children. Post-surgical radiotherapy in combination with adjuvant chemotherapy are considered as standard treatment of care for MB. However, this long-term treatment often leads to harmful neurologic effects to the developing brain in children. Therefore, identification of novel therapeutic target to increase efficacy and reduce toxicity in MB represent an unmet clinical need.

Apurinic/apyrimidinic (AP)-endonuclease (APE1) plays a central role in the base excision repair (BER) pathway by repairing AP sites and single-strand breaks, the downstream cytotoxic BER intermediate that occurs following radiation and chemotherapy. Elevated levels of APE1 or its activity has been shown to be associated with response to radiation and chemotherapy in MB. However, how APE1 repairs AP sites in the context of nucleosome in cells and how APE1-dependnt BER pathway can be manipulated to increase the therapeutic efficacy of chemo and radiation therapy remain an open question. We have recently shown that APE1 is acetylated (AcAPE1) at AP site damage in chromatin by p300 and that acetylation enhances AP-endonuclease activity of APE1. Here, we show that APE1 interacts with the nucleosome remodeling histone chaperone Facilitates of Chromatin Transcription (FACT) complex (a heterodimer of SSRP1 and SPT16 proteins) and that FACT complex is required for binding and acetylation of APE1 at AP site damages in highly aggressive MB cell lines HD-MB03 and ONS-76. Furthermore, we have mapped genome-wide binding of AcAPE1 in HDMB cells and our ChIP-seq data analysis revealed ~68154 (p&lt;.001) AcAPE1 binding predominantly located in the transcribed gene regions corresponding to 16370 genes. We aligned ChIP-Seq with RNA-Seq of HD-MB03 cells and found a significant (p&lt;0.0001) correlation of AcAPE1 binding with gene expression. Furthermore, our data show that inhibition of FACT with small molecule inhibitor CBL0137/curaxin, reduces SSRP1 and SPT16 levels from soluble fraction and trap them in chromatin. We found that CBL0137/curaxin treatment results in inhibition of APE1 acetylation, and that MB cells cannot repair DNA damages in the presence of FACT inhibitor. FACT has been shown to play an important role in removal of cisplatin-induced DNA adducts via nucleotide excision repair pathway (NER). Of importance, downregulation of APE1 levels or inhibition of FACT with CBL0137/curaxin sensitizes a panel of highly aggressive MB cell lines to cisplatin and alkylating drug temozolomide.

Together, our study suggest that co-suppression of both BER and NER pathways via targeting FACT complex can greatly sensitize MB tumors to chemo and radiation therapy. A major implication of our studies is that combination of curaxin with low dose of cisplatin may have considerable therapeutic potential to improve efficacy and reduce cytotoxic side effects of cisplatin to MB patients.

Citation Format: Sutapa Ray, Heyu Song, Shrabasti Roychoudhury, Pranjal Biswas, Sutapa Ray, Kishor Bhakat. Targeting histone chaperone FACT complex sensitizes medulloblastoma cells to chemo and radiation therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2819.