Abstract 5218: A c-Myc driven threshold model of tumor initiating in glioblastoma
Authors: Kozono David , Nitta Masayuki , Sampetrean Oltea , Ng Kimberly , Merzon Dmitry , Kushwaha Deepa , Zhu Kaya , Ligon Keith , Zhu Shan , Saya Hediyuki , Chen Clark C. 
Published: August 2012

Abstract
Recent evidence suggests that glioblastoma is driven by a subset of Tumor Initiating (TI) cells characterized by their capacity to form tumors in xenograft models and self-renew in vitro. These TI cells share many properties of neural stem/progenitor cells, including the expression of certain cell surface markers. With serial passage, many cells lose their capacity to TI. The transition between TI proficient and deficient states remains poorly understood. There are two theoretic models for the maintenance of TI states. In the “elite” model, TI activity is restricted to a predetermined subpopulation of cells. The alternative “stochastic” model suggests that any tumor cell has a finite chance of acquiring TI capacity through random fluctuations in cell physiology. To distinguish between these two models, we established culturing conditions capable of sustaining a single cell line in both low and high TI states. For both primary and long-term passaged glioblastoma lines, culturing under sphere forming conditions lead to increased in vivo tumorigenicity relative to serum based conditions. Further, cells cultured under sphere forming conditions exhibited increased expression of neural stem cell markers, including Nestin, Musashi, Olig2, and Sox2. Moreover, the ability of individual subclones to form xenografts closely correlated with their ability to grow under sphere forming conditions. In this context, the “elite” model predicts that only a subset of subclones (derived from a single line) cultured under serum conditions would possess high TI capacity and grow under sphere forming conditions. Moreover, individual cells derived from the same subclone would exhibit similar capacity for growth under sphere forming conditions. Surprisingly, single cells derived from a single glioblastoma subclone exhibited a wide range of growth capacity under sphere forming conditions, suggesting the TI state is in part driven by a stochastic process. On the other hand, only a subset of subclones from a single glioblastoma line displayed capacity for growth under sphere forming conditions, as predicted by the elite model. Transcriptome profiling of the different subclones revealed a gene signature associated with TI capacity. Analysis of this signature showed enrichment for genes regulated by c-Myc. Indeed, clones with increased TI capacity tend to harbor increased c-Myc expression. Additionally, over-expression of c-myc increased the TI capacity of glioblastoma cells in xenograft models and led to the formation of malignant brain tumor in an Ink4a/ARF null transgenic murine model. Finally, analysis of The Cancer Genome Atlas Project (TCGA) glioblastoma database revealed c-Myc over-expression in glioblastoma cells, particularly in the pro-neural and mesenchymal subtypes. Our results are most consistent with a threshold model in which TI states in glioblastomas are driven by expression levels of critical factors such as c-Myc.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5218. doi:1538-7445.AM2012-5218