Screening for Candidate Brain Tumor Genes : Identifying Genes that Cooperate with Platelet-Derived Growth Factor in Glioma Development and Progression

Abstract: Malignant primary brain tumors, gliomas, often overexpress both platelet-derived growth factor (PDGF) ligands and receptors providing an autocrine and/or paracrine boost to tumor growth. Glioblastoma multiforme (GBM) is the most frequent glioma. Its aggressive and infiltrative growth renders it extremely difficult to treat. Median survival after diagnosis is currently only 14 months. The present thesis describes the use of retroviral tagging to identify candidate cancer-causing genes that cooperate with PDGF in brain tumor formation. Newborn mice were injected intracerebrally with a Moloney murine leukemia retrovirus carrying the sis/PDGF-B oncogene and a replication competent helper virus. Brain tumors with many characteristics of human glioblastomas developed after 13-42 weeks. Analysis of proviral integrations in the brain tumors identified almost 70 common insertion sites (CISs). These CISs were named brain tumor loci and harbored known but also putative novel cancer-causing genes.An array with over 15000 unique cDNAs was used to screen for differentially expressed genes in the mouse brain tumors compared to normal brain. Known tumor genes and markers of immature cells were upregulated in the tumors. Short latency tumors were further distinguished as fast growing and GBM-like. Long latency tumors resembled slow-growing oligodendrogliomas and contained significantly less integrations as compared to short latency tumors.The gene Prkg2, encoding the cGMP-dependent protein kinase II, was targeted by insertions in two brain tumors. Overexpression of Prkg2 in human glioma cell lines led to a reduction in colony formation, cell proliferation and migration. A glioma cell line expressing markers of immature stem cells showed loss of cell adhesion, G1 cell cycle arrest and decreased activation of the survival signaling protein Akt upon stimulation with a cGMP analog that activates the Prkg2 protein. The present thesis shows that proviral tagging may be a useful tool in the search for candidate glioma genes.