Glioma is the most frequent primary tumor of the central nervous system. By using the RCAS/tv-a mouse glioma model, we have studied mechanisms controlling glioma development and the effect of cell of origin on these processes. SOX5 was identified as a brain tumor locus in a retroviral insertional mutagenesis screen of PDGF-B induced mouse gliomas. Here we found that SOX5 could suppress PDGFB-induced glioma development particularly in Ink4a-/- mice. Analysis of putative PDGF-B signaling pathways revealed that the underlying mechanism could involve the activation of AKT and p27, which caused an acute cellular senescence. When cultured in a highly selective serum free medium, glioma-initiating cells could be isolated from mouse GBMs and their self-renewal and proliferation was independent on exogenous EGF and FGF2. Addition of serum into the medium induced aberrant differentiation that was reversible. Specific depletion of viral PDGF-B demonstrated that PDGF-B was necessary for stemness and tumorigenicity of GICs by preventing them to differentiate. Subsequently, by applying the same culture conditions, GICs of APC, NSC and OPC origins were isolated from mouse GBMs. GICs derived from NSCs exhibited higher self-renewal, faster proliferation and more potent tumorigenicity than those of APC or OPC origin. Furthermore, addition of 5% serum significantly inhibited the proliferation of APC- and OPC-derived GICs, but did not in NSC-derived GICs. Transcriptome analysis revealed that GICs of the same cell of origin displayed distinct expression profiles. In the last study, we showed that OPCs could serve as the origin for astrocytic glioma. Results from immunostainings revealed that these tumors might belong to a different molecular subtype than the oligodendroglial tumors induced in OPCs. We also found differences in tumorigenic potential between OPCs in neonatal and adult mice, which suggest that developmental age of the cell of origin is important for its susceptibility to oncogenic transformation.

Malignant glioma is a highly lethal and destructive disease with no proper cure. We have investigated some of the hallmarks of cancer in connection to glioma and found ways to disrupt these and prevent tumor growth. The work is done within the context of a glioma subtype distinguished by activation of PDGF signaling termed the proneural subtype. In two of the studies we have investigated mechanisms regulating the glioma cells themselves, and in the other two we have focused on the tumor stroma. In the first study, glioma-initiating cells were isolated in defined serum free culture medium from PDGF-B driven murine glioma and shown to be independent of EGF and FGF2 for self-renewal and proliferation. When cultured in serum the GICs displayed an aberrant differentiation pattern that was reversible. Specific depletion of the transduced PDGF-B caused a loss of self-renewal and tumorigenicity and induced oligodendrocyte differentiation. The transcription factor S-SOX5 has previously been shown to have a tumor suppressive effect on PDGF-B induced murine glioma, and to induce cellular senescence in PDGF-B stimulated cells in vitro. We found that S-SOX5 had a negative effect on proliferation of newly established human glioma cells cultured under stem cell conditions. We also revealed a connection between alterations causing up-regulation of SOX5 with the proneural subgroup and a tendency towards co-occurrence with PDGFRA alterations. Angiogenesis, the formation of new blood vessels from existing ones, is an important hallmark for glioma malignancy. We found that the anti-angiogenic protein HRG had a negative effect on glioma progression in PDGF-B induced experimental tumors and that HRG was able to completely prevent formation of glioblastomas. Subsequently it was shown that HRG could skew pro-tumorigenic tumor associated macrophages into an anti-tumorigenic phenotype. Stromal cells had not previously been fully investigated in gliomas. We observed a correlation between tumor malignancy and increased numbers of tumor-associated macrophages as well as pericytes in PDGF-B induced gliomas. There was also a correlation between tumor grade and vessel functionality that had not previously been shown. Our results offer further understanding of gliomagenesis and present possible future therapies.