TARGETING METABOLIC PLASTICITY OF GLIOBLASTOMA TUMOR INITIATING CELLS TO RESTORE SENSITIVITY TO LSD1-DIRECTED THERAPY

Despite all progresses, we are still far away to reach effective treatments for patients with glioblastoma (GBM). It remains up to date an unmet clinical need. GBM malignant traits are sustained by the existence of tumor-initiating cells (TICs) that are able to adapt and survive in the different GBM microenvironments. TICs plasticity is under the control of epigenetic modifications that, among others, regulate metabolic genes' expression. We have recently shown that Lysine Specific Demethylase 1 (LSD1) genetic and pharmacological inhibition employing the selective LSD1 inhibitor (LSD1i) impairs TICs ability to restore homeostasis after ER stress or nutrient starvation by hindering a proper Activating transcription factor 4 (ATF4)-dependent integrated stress response (ISR). In line with the GBM intertumor heterogeneity, a cohort of patient-derived TICs displayed resistance toward LSD1i. LSD1i reduces the growth, survival, stem cell-like traits and in vivo tumorigenicity of LSD1i-sensitive cells, but it exerts no effects in LSD1-resistant cells. Indeed, we measured no modifications in in vitro proliferation, even upon exposure to a prolonged treatment, survival, self-renewal and in vivo tumorigenicity. Our data demonstrate that LSD1i provokes a dysregulation of ER homeostasis leading to physical and metabolic rearrangements of mitochondria in LSD1isensitive cells. This activates the pro-apoptotic pathway thus leading to cell death. None of this is observed in LSD1i-resistant TICs. LSD1i non-responsive cells display metabolic flexibility which allow them to face LSD1i-dependent perturbations, thus efficiently restoring cellular homeostasis after stress. By performing a high-throughput synthetic lethal shRNA screening, we discovered that Post-GPI Attachment to Proteins 1 (PGAP1), a protein involved in the quality check of proteins’ folding in the ER, is a mediator of such resistance. Targeting PGAP1 in LSD1-resistant TICs sensitizes refractory cells toward LSD1i treatment reshaping their metabolic features. This project pioneers a new approach to precision medicine by exploiting metabolic weaknesses as points of vulnerability to sensitize LSD1i-resistant patients. Unveiling novel aspects of the epigenetic-metabolic interplay that sustains both GBM-TICs plasticity and tumorigenicity will be of further translational impact.