Abstract 414: Identification and functional characterization of a novel tumor-specific gain-of-function GLI1 splice variant that promotes cancer migration and invasion

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC The family of GLI zinc-finger transcription factors regulates the expression of genes involved in many important cellular processes, notably, embryonal development and cellular differentiation. The glioma-associated oncogene homolog 1 (GLI1) isoform, in particular, has attracted much attention because of its frequent activation in many human cancers and its interactions with other signaling pathways. Unlike other components (PTCH and SMO) of the Hedgehog signaling pathway, mutations have never been reported in GLI1 and other genetic alterations, such as gene amplification, are extremely rare. To date, only two human GLI1 isoforms have been identified with one as the full-length wild-type GLI1, identified in 1987, and the other, an N-terminal truncated GLI1 that lacks 128 amino acids as a weaker GLI1 variant, identified in 2008. Here, we report the identification of a novel truncated GLI1 splice variant, tGLI1, with an in-frame deletion of 123 bases (41 codons) spanning the entire exon 3 and part of exon 4 of the GLI1 gene. Expression of tGLI1 is undetectable in normal cells but is high in glioblastoma multiforme (GBM) and other cancer cells. Although tGLI1 undergoes nuclear translocalization and transactivates GLI1-binding sites similar to GLI1, unlike GLI1, it is associated with increased motility and invasiveness of GBM cells. Using microarray analysis, we found over 100 genes to be differentially expressed in tGLI1- compared to GLI1-expressing GBM cells, although both cell types expressed equal levels of known GLI1-regulated genes, such as, PTCH1. Specifically, expression of 75 genes was significantly higher and 26 genes lower in tGLI1-expressing GBM cells compared to those with the wild-type GLI1. We further showed that one of the tGLI1 upregulated genes, CD24, an invasion-associated gene to be required for the migratory and invasive phenotype of GBM cells. Expression of tGLI1, but not GLI1, correlates with CD24 levels in GBM cell lines and xenografts. The tGLI1-expressing GBM xenografts are more invasive than those with the wild-type counterpart. Furthermore, the tGLI1 variant, but not GLI1, binds to the CD24 promoter with high affinity. Furthermore, promoter deletional analysis identified a 50-bp region within the CD24 promoter to be required for tGLI1-mediated transactivation. Importantly, tGLI1-expressing GBM xenografts are significantly more invasive than those carrying GLI1. Ongoing studies are investigating the role that tGLI1 may play in tumorigenesis and other malignant phenotypes. Together, these data provide conclusive evidence for a novel tumor-specific, gain-of-function GLI1 splice variant that promotes migration and invasiveness of GBM cells and open up a new research paradigm on the role of the GLI1 pathway in malignancy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 414.