Molecular determinants of glioma subsets with distinct histology or sensitivity to signal transduction inhibitors

Abstract

Gliomas are brain tumors that currently are treated with surgery, radiation and chemotherapy. Less than 7.5% of patients with GBM survive more than 2 years after diagnosis. The conclusion, from several decades of intense research on glioma biology, is that gliomas are tumors with high genetic, morphological and clinical variation, which makes them hard to diagnose and treat. Predicative markers are therefore highly warranted. In this thesis we identified new molecular characteristics of glioma subsets with distinct histology or sensitivity to signal transduction inhibitors.

Grade II gliomas are histologically diverse and are commonly divided into astrocytomas, oligodendrogliomas and oligoastrocytomas. We have identified rPTPb/z as a novel marker for grade II oligodendrogliomas, which is the type with better survival and response to chemotherapy.

Glioblastomas are currently subdivided into primary and secondary glioblastomas, which follow different biological as well as clinical routes. Current knowledge about glioma genetics suggests the need for a refined sub-categorization. We have defined a high grade glioma subtype which is sensitive to PDGFR inhibitor (imatinib, Glivec). This subtype expresses high levels of PDGFR-a and CXCL12. Potentially this subtype constitutes a group of tumors which would best respond to imatinib treatment.

IGF-1R signaling has been shown to be important for glioma growth. NVP-AEW541 is a novel small-molecule inhibitor of IGF-1R. Analysis of the effects of NVP-AEW541 on a panel of high grade glioma cell cultures revealed differences in sensitivity. PIK3CA, PTEN and AKT status was analyzed to investigate their effect on NVP-AEW541 sensitivity. 3/4 high-sensitive cultures demonstrated serum-dependent AKT phosphorylation, whereas AKT phosphorylation was only affected in 1/5 cultures with low sensitivity. Exon 9 mutations in PIK3CA were found in 2/2 sensitive and in 3/5 low sensitive. PTEN expression varied more than ten-fold between cultures, in a manner not associated with drug sensitivity. Interestingly down-regulation of PTEN decreased sensitivity to NVP-AEW541.

CD133-expressing tumor cells with stem cell characteristics have been identified and isolated from several malignancies including glioblastomas. These are suggested to be responsible for the tumor formation. Two novel CD133+ glioma cell subsets were identified. These subsets differed with regard to marker gene expression, ability to grow as neurospheres and in sensitivity to tyrosine kinase inhibitors. Furthermore, genes defining these two subsets were found to be co-expressed in glioblastoma tissue.

These studies have thus generated a series of novel observations relevant for the continued rational development of new treatment strategies, and improved diagnosis, of different types of gliomas. Furthermore, the identification of two novel types of tentative brain cancer stem cells should possibly aid in the ultimate identification of the cell types that undergo the initial transformation in glioma formation.