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Dissecting Glioma Genesis through a Glial-Specific Mouse Model

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Project Summary During the last funding cycle, we used a glial-specific transgenic mouse model, RCAS-tv-a, to functionally determine the oncogenic role of the insulin-like growth factor binding protein 2 (IGFBP2), which was identified through our genomic profiling studies as a marker for high-grade gliomas. We have found that IGFBP2 is a novel oncogene, which, when combined with K-ras or platelet-derived growth factor beta (PDGFB), leads to development and progression of two major types of diffuse glioma (astrocytoma and oligodendroglioma) (published in Dunlap et al., PNAS, 2007). Our in vivo and ex vivo studies revealed that IGFBP2 activates the Akt pathway, and inhibition of Akt by a pharmacological inhibitor for Akt blocks IGFBP2-mediated cell viability. We also examined PDGFB-driven glioma development in an INK4a/ARF null background. These studies revealed that the combination of INK4a/ARF loss and PDGFB over-expression was sufficient to generate high-grade gliomas, even in the absence of co-delivered IGFBP2. Examination of these tumors revealed an up-regulation of endogenous IGFBP2, suggesting that INK4a/ARF may be a negative regulator of IGFBP2. These results were confirmed by identifying a similar correlation between INK4a/ARF and IGFBP2 in 17 human glioma samples. In a similar manner, a recent study by Charles Sawyer's group identified an association of IGFBP2 over-expression with PTEN mutation and PI3K/Akt activation. Thus, IGFBP2 over- expression and loss of tumor suppressor function, such as that of PTEN and INK4a/ARF, may exert a combined effect on the induction of the most advanced stage of glioma. We hypothesize that IGFBP2 in combination with PTEN loss activates Akt pathways in the development and progression of gliomas, and that IGFBP2 is important for glioma maintenance. For these studies, we will use our established RCAS system and a new Tet-inducible RCAS system. Our specific aims are the following: 1) To determine the role of IGFBP2 over-expression in the maintenance of high-grade glioma, 2) To determine the role of Akt activation in IGFBP2- mediated glioma progression, 3) To examine the cooperation between oncogene over-expression and PTEN inactivation in IGFBP2-mediated glioma genesis and in the development of GBM, and 4) To examine the effects of IGFBP2 expression on Akt pathway therapeutics.
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