P01.12.b Combined Effects of Ire1 Inhibition and Temozolomide in Glioblastoma Mouse Models: Potential Mechanisms of Action

Abstract BACKGROUND Glioblastoma (GB), the most frequent and aggressive primary brain tumor in adult, remains an incurable disease. New efficient therapy is required to improve patient care. We showed that coupling temozolomide (TMZ) treatment with a pharmacological inhibition of IRE1, one of the mediators of the Unfolded Protein Response, is a promising therapeutic approach as it leads to a significant survival benefit in a GB preclinical mouse model. The mechanisms underlying this survival benefit are however unknown and need further investigation. MATERIAL AND METHODS We performed a multi-omics integrative analysis (RNA-seq, WES) on two preclinical syngeneic mouse models (GL261 cells in C57BL/6 mice), testing two pharmacological inhibitors of IRE1: MKC8866 targeting its RNase activity and B4-7 targeting its kinase activity. Several treatment regimens were investigated: no treatment, a TMZ-based protocol, IRE1 inhibitor, and a combination of TMZ and IRE1 inhibitor. We performed multi-omics data analysis from nucleic acid extracted at sacrifice for the different treatment conditions. We used the nf-core/sarek workflow to detect variants. Mutational signatures were identified with sigProfiler. RNA-seq data was analyzed with nf-core/rnaseq workflow, DESeq2 and WGCNA R packages. RESULTS For both inhibitors, we confirmed the survival benefit with the combined treatment compared to TMZ only. Tumors treated with TMZ only had an increased Tumor Mutational Burden (TMB), inducing mostly CT transitions. The association of TMZ with an IRE1 inhibitor led to an even higher TMB composed of TMZ-induced mutations. The mutational profiles of the tumors treated by an IRE1 inhibitor (combined with TMZ or not) were partially but specifically associated with a defective DNA mismatch repair mechanism. Transcriptome analysis highlighted a co-expression module up-regulated in the combination regimen and enriched in genes involved in the DNA repair mechanism. These results suggest a relationship between IRE1 activity and DNA repair mechanisms. CONCLUSION Using preclinical models of GB and drugs targeting different activities of IRE1, we show that IRE1 inhibition combined with TMZ leads to enhanced mouse survival. The analysis of tumor exome and transcriptome suggests a link between IRE1 and the DNA repair system. These results reinforce the idea that IRE1 inhibition could be of strong therapeutic interest.