PS2 - 171 Bmi1 Identifies Treatment-Refractory Stem Cells in Human Glioblastoma

Abstract

Glioblastoma (GBM) is an aggressive brain tumor that is resistant to conventional radiation and cytotoxic chemotherapies. We hypothesize that brain tumor initiating cells (BTICs), a subpopulation of treatment-resistant cells with stem cell properties cause tumor relapse and a subset of neural stem cell genes regulate BTIC self-renewal, driving GBM recurrence. We adapted the existing treatment protocol for adults with primary GBM for in vivo treatment of immunocompromised mice engrafted with GBMs. Post-chemoradiotherapy, the recovered GFP+GBMs were profiled for self-renewal and expression of critical stem cell genes. Using invitro and invivo gain-of-function/loss-of-function experiments, we investigated the regulatory functions of Bmi1 in primary neural stem & progenitor cells (NSPCs) and GBM tumor formation. Finally, global RNA-Seq profiling was performed to understand the consequences of Bmi1 dysregulation on target gene expression. GBM cells showed an increase in Bmi1 levels post-chemoradiotherapy, suggesting the presence of a treatment-refractory BTICs. GFP+cells extracted from treated xenografts had higher self-renewal and BTIC marker expression. Although treated mice responded to therapy, we observed tumor relapse with increased Bmi1 expression. Knockdown of Bmi1 diminished self-renewal and proliferation of GBM cells and delayed tumorigenesis, highlighting a critical role for Bmi1 in tumor maintenance. Conversely, over-expressing Bmi1 in NSPCs failed to initiate tumor formation in vivo. Using high-throughput sequencing data, we generated a map of signaling pathways dysregulated in GBM that may lead to tumor recurrence. Our data confirms the existence of a rare treatment-refractory BTIC population with enhanced self-renewal capacity that escapes therapy and drives tumor relapse.