ATPS-92Bmi1 IDENTIFIES TREATMENT-REFRACTORY STEM CELLS IN HUMAN GLIOBLASTOMA

Abstract

PURPOSE: Glioblastoma (GBM) is a fatal primary adult brain tumor, characterized by extensive cellular heterogeneity. Even with surgery, chemotherapy with temozolomide (TMZ), and radiation, tumor re-growth and patient relapse are inevitable. Brain tumor initiating cells (BTICs), a rare subset of GBM cells with stem cell properties, were shown to be both chemo- and radio-resistant. We hypothesize that these treatment-resistant BTICs cause tumor relapse and a subset of genes regulating BTIC self-renewal drives GBM recurrence. METHODS: Using patient-derived primary GBM samples, we designed an in vitro model of tumor recurrence by treating cells with TMZ and radiation. We then adapted the existing treatment protocol for adults with primary GBM for treatment of immunocompromised mice engrafted with GFP+ human GBM cells. Post-chemoradiotherapy, GFP+ tumor cells were recovered from mouse brains and profiled for self-renewal, proliferation and mRNA expression of important stem cell genes. Using in vitro and in vivo loss-of-function experiments, we investigated the regulatory functions of key BTIC genes in tumor formation. RESULTS: GBM cells showed an increase in Bmi1 levels post-chemoradiotherapy in vitro, suggesting the presence of a treatment-refractory BTIC population. GFP+ cells extracted from chemoradiotherapy treated human tumor xenografts showed increased self-renewal and elevated BTIC marker expression. Although treated mice responded to therapy with decreased tumor size, we observed tumor relapse post-chemoradiotherapy with increased Bmi1 expression. Knockdown of Bmi1 diminished self-renewal and proliferation of GBM cells and delayed tumorigenesis in xenografted mice, highlighting a critical role for Bmi1 in tumor initiation and maintenance. CONCLUSION: Our data confirms the existence of a rare treatment-refractory BTICs population that escapes therapy, and drives tumor relapse and recurrence with enhanced self-renewal capacity. Our human BTIC in vitro assays and human-mouse BTIC xenograft model provide fundamental tools to characterize the functional relevance and of key stem cell self-renewal genes in GBM recurrence.