Abstract
BACKGROUND: Platelet-derived growth factors (PDGFs) are produced and secreted by human glioblastoma cells, and the PDGFR-α gene is amplified in a fraction of both low- and high-grade gliomas. Glioma involves interactions between the malignant cells and their microenvironment. We have therefore studied glioma formation in two PDGF-dependent brain tumor models where PDGF is expressed in vivo in astrocytic cells under control of the GFAP-promoter (Hede et al. 2009; Nazarenko et al 2011). PDGF-AL transgenic mice develop a characteristic, lethal phenotype due to highly proliferating oligodendrocyte progenitor cells that spread diffusely throughout the brain. A few of the PDGF-AL mice present with advanced lesions similar to human anaplastic oligoastrocytoma. PDGF-B transgenic mice do not develop brain tumors and are phenotypically normal, but when crossed with p53 null mice, they develop brain tumors at a high rate. The tumors occur between 2-6 months, are similar to human glioblastoma, and consistent with previous findings from human brain tumors Pdgfr-α is localized on glial tumor cells, while Pdgfr-β is on the vasculature. Thus, PDGF-AL and –B differ in their capacity to induce brain tumors. METHODS: Efforts to target the PDGF pathway for treatment of glioma have not been successful. In search for new therapies, we need more knowledge on the early stage of glioma development. Therefore we have used the PDGF-B transgenic mice in crossings with p53null to study the pre-neoplastic stage. Immuno-histochemical stainings and immuno-blot analyses were used to detect changes in histone methylation and the Luminometric Methylation Assay (LUMA) to monitor global DNA methylation in the brain. RESULTS: The hGFAPpPDGFB/Trp53 null (B + p53-/-) mice develop glioblastoma-like brain tumors at a high frequency in adulthood. The present work reveals epigenetic changes in the mouse brain tumors as well as in neural stem/progenitor cells during the histologically normal pre-neoplastic stage of the brain. DNA hypomethylation and elevated Histone3 Lysine9 di-methylation (H3K9Me2) were detected in the tumors. During the pre-neoplastic stage, global DNA hypomethylation was also observed in samples of the adult frontal brain lateral ventricular wall, but could not be observed in whole brain samples. Elevated H3K9Me2 levels were observed in neurosphere cultures from the B + p53-/- pre-neoplastic brain but not in wild type (WT) neurospheres using immuno-blot analysis. The H3K9Me2 level was also higher in brain tumor spheres compared to neurospheres derived from the same mouse. CONCLUSIONS: We propose that epigenetically disturbed neural stem/progenitor cells are the early targets of neoplastic transformation in the brain. SECONDARY CATEGORY: n/a.