Abstract 425: Novel transgenic mouse model of tGLI1 for in vivo studies of breast cancer and glioblastoma

Truncated glioma-associated oncogene homolog 1 (tGLI1) was first identified in 2009 as a gain-of-function isoform of the GLI1 transcription factor. tGLI1 isoform lacks exon 3 and part of exon 4, but retains all functional GLI1 domains to promote expression of GLI1 target genes. Since its discovery, we have shown that tGLI1 expression is unique to tumor tissue of breast and brain cancers. In glioblastoma (GBM), we show that tGLI1 is more potent in driving an aggressive phenotype compared to GLI1. We observed increased growth rate and vascularity in tGLI1-positive GBM xenografts along with increased tubule formation by surrounding vascular endothelial cells. We also found that tGLI1 expression in GBM promotes expression of heparanase, VEGF-A, VEGF-C, and TEM7, all of which can mediate GBM angiogenesis and vascularity. Most recently, we show that tGLI1 is enriched in mesenchymal GBM and that tGLI1 drives formation of glioma stem cells. Similarly, we have shown that tGLI1 drives breast cancer brain metastasis (BCBM) and supports radioresistance of breast cancer stem cells (CSCs). We found that tGLI1-expressing breast cancer cells secrete more VEGF-A and exhibit increased cell motility, invasion, and anchorage-independent growth. Additionally we observed that tGLI1 expression promotes breast CSCs through upregulation of stemness genes such as Nanog, Sox2, CD44, and Oct4, ultimately activating astrocytes. tGLI1-mediated astrocyte activation renders the brain more susceptible to and supportive of breast cancer cell invasion. Indeed, we show that tGLI1 loss suppresses breast CSCs and reduces BCBM in vitro, providing a mechanism for tGLI1-dependent breast cancer brain metastasis. While we have shown the oncogenic role of tGLI1 in vitro, no studies show whether tGLI1 is adequate for cell transformation or tumor initiation and progression in vivo. Thus, we generated the first conditional tGLI1 knock-in (KI) mouse model to study the role of tGLI1 in tumor initiation, progression, and metastasis. Our tGLI1-KI model was generated such that the human tGLI1 gene was inserted in an inverted orientation and flanked with loxP sites, yielding no tGLI1 product. Cre-mediated recombination reorients tGLI1 gene, allowing for its transcription. Cre-mediated knock-in of tGLI1 also induces concomitant luciferase gene transcription to allow for imaging of animals. To model GBM tumorigenesis, we crossed tGLI1-KI mice to the GFAP-cre model, inducing tGLI1 expression in astrocytes. We will also cross tGLI1-KI mice with MMTV-cre, inducing tGLI1-KI in mammary epithelial cells to model breast tumorigenesis. Our transgenic tGLI1-KI mouse is the first to model the oncogenic impact of tGLI1 in vivo. We anticipate our tGLI1-KI mouse model to expand our understanding of tGLI1-driven tumorigenesis and support therapeutic development in GBM and breast cancer. Citation Format: Angelina T. Regua, Dongqin Zhu, Austin Arrigo, Daniel Doheny, Hui-Wen Lo. Novel transgenic mouse model of tGLI1 for in vivo studies of breast cancer and glioblastoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 425.