Abstract 3342: Truncated glioma-associated oncogene homolog 1 (tGLI1) is a novel mediator of mesenchymal subtype of glioblastoma and a novel transcriptional activator of CD44

Abstract

Abstract The subpopulation of stem-like cells, or glioma stem cells (GSCs), is a key driver of tumor initiation, recurrence, and chemoresistance. Recently, patient-derived GSC lines were identified to contain two mutually exclusive subtypes termed proneural and mesenchymal. Mesenchymal GBM and GSCs were found to be more aggressive and radioresistant; however, the molecular pathways supporting mesenchymal GBM and mesenchymal GSCs are not well understood. To address this knowledge gap, we examined truncated glioma-associated oncogene homolog 1 (tGLI1) for its role in mesenchymal subtypes of GBM and GSCs. Discovered in our laboratory, tGLI1 is a novel alternatively spliced, gain-of-function variant of GLI1 zinc-finger transcription factor with an in-frame deletion of 41 codons retains all of the known functional domains of GLI1, but gains the ability to activate genes not regulated by GLI1. In this study, we report for the first time that tGLI1 is a tumor-specific transcription factor that facilitates GBM growth, enriches in the mesenchymal subtype of GBM and GSCs, and promotes mesenchymal GSCs via transcriptionally activating CD44 expression. Using an orthotopic GBM xenograft mouse model, we observed that tGLI1-overexpressing tumors grew more aggressively, with increased proliferation and angiogenesis, compared to control and GLI1-overexpressing xenografts. IHC analysis using tGLI1- and GLI1-selective antibodies we developed showed that tGLI1 was significantly expressed in GBM specimens but undetectable in normal brains whereas GLI1 was expressed in both tissues. Datamining further showed that tGLI1 activation signature (tGAS) was correlated with glioma grade, tumor angiogenesis, and poor overall survival, and that GBMs with high tGAS were enriched with mesenchymal GBM and GSC gene signatures. Neurospheres contained increased levels of tGLI1, but not GLI1, compared to the monolayer culture. Mesenchymal GSCs expressed more tGLI1 than proneural GSCs. Ectopic tGLI1 expression significantly enhanced the ability of GSCs to yield neurospheres in vitro and form aggressive tumors in mouse brains. Finally, we examined the ability of tGLI1 to regulate stemness genes and found that tGLI1 binds to and transactivates the promoter of the CD44 gene, a marker and mediator for mesenchymal GSCs. Collectively, these findings advanced our understanding of GBM biology by establishing tGLI1 as a novel transcriptional activator of CD44 and a novel mediator of mesenchymal GBM and GSCs. Citation Format: Tadas K. Rimkus. Truncated glioma-associated oncogene homolog 1 (tGLI1) is a novel mediator of mesenchymal subtype of glioblastoma and a novel transcriptional activator of CD44 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3342.