LAUSR

The Sonic hedgehog (Shh)-subgroup of the pediatric brain tumor medulloblastoma (MB) has the highest frequency of post-radiotherapy local recurrence, which is fatal. Recently, there have been major advances in characterizing the genomic divergence during MB tumor recurrence (1) and the role of spatial heterogeneity in MB (2). However, there is no evidence that demonstrates actionable targets in the Shh-pathway for prevention of tumor recurrence post-radiation. Previously we have shown that Shh-target Y-box binding protein 1 (YB1) drives MB cell proliferation (3) and controls the expression of IGF2, which contributes to radiation resistance in MB (4). Stem-like tumor cells occupying the peri-vascular niche (PVN) are proposed to drive MB recurrence through their radiation-resistant properties (4, 5). In the SmoA1 mouse model for Shh MB, we observe that these cells feature elevated levels of the oncogene YB1. Ectopic expression of YB1 in this model significantly reduced survival, while phosphomutant YB1 inhibited tumor formation. Using mouse organotypic MB slice cultures we found that YB1 is essential for expansion of the cancer stem-like cell population from the PVN post-radiation. We also demonstrate localization of endogenous phosphorylated YB1 to sites of DNA damage post-radiation, and we show that ectopic expression of YB1 in primary MB cell (MBC) cultures drives rapid DNA repair in a PARP-dependent manner. Mechanistically, we show that inhibiting YB-1 phosphorylation using the flavonoid Fisetin disrupts the physical interaction of YB1 and PARP, thus, derailing the DNA repair process post-radiation in MBCs. Subsequently, using a novel ex vivo assay of tumor Neurosphere-implantation on a cerebellar Brain Slice (NoBS) culture, we demonstrate the effectiveness of Fisetin in preventing the survival of cancer stem-like cells in the MB tumor microenvironment. Our findings demonstrate a novel role for YB1 in promoting Shh MB tumor growth, and they reveal a critical requirement for YB1-phosphorylation in driving radiation resistance through PARP-mediated DNA repair, enabling escape from DNA damage-induced apoptosis. Moreover, our results imply that YB1 phosphorylation inhibitors can be used to enhance radiation responsiveness, thereby reducing incidence of medulloblastoma recurrence. Reference: 1. Morrissy AS, Garzia L, Shih DJ, Zuyderduyn S, Huang X, et al. 2016. Nature 529: 351-72. 2. Morrissy AS, Cavalli FMG, Remke M, Ramaswamy V, Shih DJH, et al. 2017. Nat Genet 49: 780-83. 3. Dey A, Robitaille M, Remke M, Maier C, Malhotra A, et al. 2016. Oncogene 35: 4256-684. 4. Fernandez LA, Squatrito M, Northcott P, Awan A, Holland EC, et al. 2012. Oncogene 31: 1923-375. 5. Hambardzumyan D, Becher OJ, Rosenblum MK, Pandolfi PP, Manova-Todorova K, Holland EC. 2008. Genes Dev 22: 436-48 Citation Format: Abhinav Dey, Anshu Malhotra, Anna Kenney. YB1-phosphorylation mediates YB1-PARP interaction to regulate DNA-repair post-radiation in Shh medulloblastoma [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 4125.