Abstract 5574: Retinoic acid as a therapeutic adjuvant in low-grade gliomas

Introduction: Malignant gliomas are incurable brain tumors and the most common primary brain tumors in adults. Mutations in isocitrate dehydrogenase (IDH) are enriched in a subset of malignant low-grade gliomas (MLGGs) that arise as lower grade neoplasms and steadily progress to more aggressive and universally fatal glioblastomas (GBMs) usually within 5-10 years after initial diagnosis. Gliomas with IDH1/2 mutations are characterized by hypermethylation of multiple genes, including immune genes, thereby rendering them highly immune evasive. Retinol binding protein 1 (RBP1) gene, that encodes a protein involved in retinoic acid (RA) generation, is significantly hyper-methylated in IDH mutant cells. Since RA is instrumental in the induction of multiple immune genes including immune-activating ligands such as NKG2D ligands (NKG2D-L), we hypothesized that treatment of IDH mutant cells with RA will increase the expression of NKG2D-L on the tumor cells, leading to increased recognition and lysis by immune cells. In addition, we hypothesized that RBP1 methylation was crucial to development of the IDH mutant immune-evasive phenotype. Methods: Primary glioma cells (IDH WT and mutant) were treated with 1μM RA for 48 hours. The cells were assessed for changes in expression of NKG2D-L and other type-1 immune response genes. Effect of RA treatment on NK killing and immune cell chemotaxis were evaluated. We also assessed the impact of RBP1 knock-down in IDH WT cells. Finally, we evaluated the efficacy of RA as a therapeutic modality for low-grade gliomas in a murine model. Results: RA-treated IDH mutant gliomas had a higher expression of NKG2D-L and a higher sensitivity to killing by NK cells, when compared with untreated IDH mutant cells. RA treatment of IDH mutant cells also induced increased chemotaxis of NKs and CD8+ T cells in a CCL2-dependant manner. In addition, RA demonstrated superior anti-tumor efficacy in vivo, compared with controls. Finally, we demonstrated that RBP1 knock-down in IDH WT cells was sufficient to replicate the immune evasive phenotype observed in IDH mutant cells. Conclusions: RA treatment reverses the immune evasion in IDH mutant cells primarily by inducing the upregulation of NKG2D-L and by mediating increased NK and CD8+ T cell chemotaxis via CCL2 induction. Additionally, we established that RBP1 gene silencing is crucial for the development of the immune evasive IDH mutant phenotype. Note: This abstract was not presented at the meeting. Citation Format: Aparna Rao, Xiaoran Zhang, Jason Kim, Alexander Pomerantz, Youngmi Kim, Eric Holland, Nduka Amankulor. Retinoic acid as a therapeutic adjuvant in low-grade gliomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5574. doi:10.1158/1538-7445.AM2017-5574