TMIC-15. HYPERACTIVATING THE HIPPO PATHWAY EFFECTOR TAZ DIFFERENTIALLY DISTORTS THE TUMOR MICROENVIRONMENT, PROMOTES TUMOR-ASSOCIATED NEUTROPHIL INFILTRATION, AND PHENOCOPIES MESENCHYMAL-GLIOBLASTOMA

Abstract Glioblastoma (GBM), the deadliest and most common adult brain malignancy, is molecularly and clinically heterogeneous. The most common subtype (both primary and recurrent), mesenchymal (MES)-GBM, has the worst prognosis and highest treatment resistance. MES-GBM exhibits hyperactive transcriptional coactivator with PDZ-binding motif (TAZ), a Hippo tumor suppressive pathway effector whose expression in GBMs predicts short survival. Yet, how Hippo-TAZ dysregulation might drive GBM MES transition remains elusive, precluding subtype-specific treatments. Tumor evolution requires signaling dysregulation and co-opting the tumor microenvironment (TME). Understanding GBM heterogeneity was recently complicated by the notion that subtypes vary in TME immune composition. The MES-GBM TME is differentially-distorted in silico, with more tumor-associated macrophages/microglia (TAMs) and neutrophils (TANs). Yet, how TAZ hyperactivity, MES transition, and GBM TME distortion interrelate and impact tumor progression remains unknown. We suspected that TME distortion facilitates immune evasion, MES transition, and tumor progression, worsening treatment responses. To test this, we devised an orthotopic xenograft mouse model phenotypically and histopathologically recapitulating human MES-GBM by expressing constitutively-active TAZ (TAZ4SA) in human GBM cells lacking MES signatures (GBM4SA). GBM4SA mice lived significantly shorter compared to mice with GBM expressing vector (GBMvector) or mutant TAZ unable to bind its effector, TEAD (GBM4SA-S51A). Moreover, more myeloid cells infiltrate the GBM4SA TME than the GBMvector or GBM4SA-S51A TMEs. While most myeloid cells infiltrating the GBMvector and GBM4SA-S51A TMEs were TAMs, most infiltrating the GBM4SA TME were TANs, suggesting TAZ hyperactivation differentially distorts the TME. Next, to delineate the roles of TANs in GBM4SA tumor progression, mice were depleted of neutrophils by administering Ly6G antibody. Serial blood smears and flow cytometry revealed effective depletion was achieved. We are currently investigating the impact of systemic neutrophil depletion on GBM mesenchymal transition and tumor progression in hopes of informing future GBM clinical management and novel TME-targeted immunotherapies.