Abstract 170: Glioblastoma patient-derived organoids as a model for discovering novel markers of therapy resistance in the context of tumor microenvironment: potential role of melanoma antigens
Authors: Breznik Barbara , Majc Bernarda , Habič Anamarija , Krapež Gloria , Porčnik Andrej , Mlakar Jernej , Turnšek Tamara Lah , Novak Metka , Tacer Klementina Fon 
Published: June 2023

Abstract
Introduction: Glioblastoma is resistant to standard treatment, leading to tumor recurrence and early death of patients. Using organoid models, we aim to explore the mechanisms of resistance to cancer therapies and identify novel therapeutic vulnerabilities, including stress protecting melanoma-associated antigens (MAGEs). MAGEs are normally restricted to expression in testis but are abnormally activated in cancer and are associated with therapy resistance.
Methods: Organoids have been established from fresh tumor biopsies and are stored in Gliobank, a Slovenian collection of patient tumor samples with corresponding clinical data. Organoids were first characterized and compared with the corresponding parental tumor tissue by immunofluorescence and qPCR of selected markers. Organoids were then treated with a combination of irradiation, the chemotherapeutic agent temozolomide and/or a chemokine receptor antagonist, and response to therapy was assessed by immunofluorescence, qPCR, cell viability and invasion assays, multiplex ELISA and proximity ligation assay.
Results: Organoids recapitulated the gene expression profile of parental glioblastoma tissues, including expression of genes related to cancer stem cells, DNA damage response, cell cycle progression, epithelial- to- mesenchymal transition, and cytokine signaling. In addition, the organoids preserved the cellular composition of the parental glioblastoma tissues, consisting of glioblastoma (stem) cells, macrophages, microglia, lymphocytes, and endothelial cells. Standard treatment decreased viability and invasion of organoids in only 2 of 6 patients, suggesting that organoids recapitulate tumor therapy resistance. The increased expression of MDM2 and CDKN1A in organoids after treatment with irradiation and temozolomide suggests that the p53 pathway and DNA damage response mechanisms may contribute to the therapy resistance. Next, we detected high abundance of secreted cytokines in the culture medium of the organoids, including CXCL12. However, treatment of the organoids with the CXCR4 antagonist Plerixafor, which blocked the interactions between CXCL12 and CXCR4 in the organoids, had no effect on organoid viability and invasion, suggesting resistance of glioblastoma to this immunotherapy. Since high expression of several type I MAGEs correlates with poor prognosis of glioma patients based on TCGA database analysis, we investigate the role of MAGEs in therapy resistance of glioblastoma organoids.
Conclusions: Patient-derived tumor organoids provide a valuable tool to 1) identify novel therapeutic vulnerabilities in the context of the tumor microenvironment, 2) evaluate the effect of novel candidate treatments including immunotherapy, and 3) discover novel markers of therapy resistance.
Citation Format: Barbara Breznik, Bernarda Majc, Anamarija Habič, Gloria Krapež, Andrej Porčnik, Jernej Mlakar, Tamara Lah Turnšek, Metka Novak, Klementina Fon Tacer. Glioblastoma patient-derived organoids as a model for discovering novel markers of therapy resistance in the context of tumor microenvironment: potential role of melanoma antigens [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 170.