Abstract 5720: Niraparib enhances radiosensitivity of glioblastoma cells with proficient BRCA1/2 through proliferation-related nucleolar protein DDX21

Abstract Background Intrinsic resistance to irradiation is one of important factors leading to recurrence of glioblastoma (GBM) patients underwent standard treatment. The radiosensitization by various poly-(ADP-ribose)-polymerase (PARP) inhibitors have been intensively investigated in many solid tumor models especially harboring deficient BRCA1/2. However, the impact of a PARP1/2 inhibitor niraparib on radiosensitivity of glioblastoma cell lines with proficient BRCA1/2 and underling mechanisms has yet not been fully elucidated. Methods GDSC dataset with IC50 of PARP inhibitors and transcriptome profiles as well as NCI-60 cell lines with known survival fraction at 2 Gy (SF2) were used to deduce potential pathways by which PARP inhibitors suppress cell viability and confer radiosensitization through linear regression and reactome gene set enrichment analysis. Two GBM cell lines with proficient BRCA1/2, A172 and U-87-MG were used to evaluate IC50 and radiation dose enhancement factor DEF37 of niraparib by CCK8 experiment and clonogenicity assay. Changes in the distribution of cell cycle and DDX21 protein expression were analyzed with flow cytometry and western blot respectively. The involvement of DDX21 in the mechanism mediating radiosensization of niraparib was performed with small inference RNA experiment. Results Pathways relevant to ribosome biosynthesis and functions such as eukaryotic translation initiation, rRNA processing was found to be responsible for cytotoxicity of niraparib in 519 tumor cell lines. Moreover, mRNA expression of PARP1/2, genes participated in ribosome biosynthesis and homologous recombination (HR) were all significantly negatively associated with SF2 in 44 NCI-60 cell lines. The IC50 of niraparib in A172 and U-87-MG cell lines were 10.77±3.31 and 32.37±2.84 μM respectively. The DEF37 was established as 1.99 at 348 nM, 2.17 at 1044 nM for A172 cell line and 1.10 at 1056 nM, 1.44 at 3169 nM for U-87-MG cell line, respectively. Treatment with niraparib alone and in the combination with radiation (4 Gy) resulted in significant increase in fraction of G2 phase in both cell lines. Treatment with niraparib alone did not change protein expression level of DDX21 in both cell lines. However, knockdown of DDX21 with siRNA significantly reduced proliferation in U-87-MG cells. Niraparib at 1056 nM further decreased clonogenic number in the combination with 4 Gy radiation in U-87-MG cells compared with those treated with radiation alone. Importantly, knockdown of DDX21 resulted in significant increasing in clonogenic number of U-87-MG cells treated with radiation alone or the combination of niraparib and radiation compared with control cells. Conclusions Niraparib has radiosensitization effect for GBM cells with proficient BRCA1/2 at low concentration. DDX21 may be involved in the radiosensitization mechanism of niraparib. Citation Format: Jia Luo, He Xiao, Qian Chen, Yanlan Li, Chuan Chen, Mingying Geng. Niraparib enhances radiosensitivity of glioblastoma cells with proficient BRCA1/2 through proliferation-related nucleolar protein DDX21 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5720.