Studies on the Roles of PDGFRA and EGFR in the Classification and Identification of Therapeutic Targets for Human Gliomas

Sammanfattning: Glioma is the most common type of primary tumor in the adult central nervous system (CNS). However, the current classification of gliomas is highly subjective and even inaccurate in some cases, which leads to clinical confusion and hinders the development of targeted therapies. EGFR and PDGFRA play crucial roles in glia development and glioma pathogenesis. In this thesis we aim to establish a glial genesis-guided molecular classification scheme for gliomas based on the genes co-expressed with EGFR or PDGFRA and to clarify the clinical relevance and the mechanism of PDGFRA expression in different glioma subtypes. We also aim to investigate the role of cell surface PDGFRA expression in regulating glioma cell proliferation. In order to establish a glial genesis-guided classification scheme, we identified 69 genes co-expressed with EGFR (EM) or PDGFRA (PM) as classifiers. Using these 69 classifiers, gliomas are clarified into EM (highly expressing EM genes), PM (highly expressing PM genes), and EMlowPMlow (lowly expressing both EM and PM genes) subtypes in a morphology-independent manner. Our results showed that besides their distinct patterns of genomic alterations, EM gliomas were associated with higher age at diagnosis, poorer prognosis, stronger expression of neural stem cell genes and astrogenesis genes, while PM and EMlowPMlow gliomas were associated with younger age at diagnosis and better prognosis. PM gliomas were enriched in the expression of oligodendrogenesis genes, whereas EMlowPMlow gliomas were enriched in the signatures of mature neurons and oligodendrocytes. These findings constitute a framework for improving molecular diagnosis and identifying therapeutic targets to combat gliomas. To investigate the clinical relevance of PDGFRA in gliomas, the clinical outcomes of gliomas with the top 25% of PDGFRA expression levels (PDGFRA-high) were compared with the gliomas with lowest 25% of PDGFRA expression levels (PDGFRA-low). We found that PDGFRA-high gliomas contained nearly all morphological subtypes, which was associated with frequent IDH1 mutation, 1p LOH, 19q LOH, less EGFR amplification, younger age at disease onset and better survival compared to PDGFRA-low gliomas. We also found that the PDGFRA expression can be induced and maintained by fibroblast growth factor 2 (FGF2) in primary glioma cells. FGF2-dependent maintenance of PDGFRA expression was concordant with the maintenance of a subset of gliogenic genes and higher rates of cell proliferation. Our findings suggest a role of FGF2 in regulating PDGFRA expression in the subset of gliomas. To investigate the role of cell surface expression of PDGFRA in regulating cell proliferation, we compared the growth rate of primary glioma cells having high cell surface PDGFRA expression level with the glioma cells having low cell surface PDGFRA expression level. We demonstrated that glioma cell proliferation correlated with the extent of surface expression of PDGFRA in both glioma cell lines and their corresponding tumor samples. We also found that MEK inhibitor U0126 treatment can decrease the surface PDGFRA expression and result in deviation of PDGFRA from endosomal trafficking and recycling compartment to the Golgi network in a reversible, dose- and time-dependent manner without affecting total PDGFRA expression. U0126 mediated down-regulation of PDGFRA surface expression correlated with diminished cell proliferation. Our findings suggested that the trafficking of PDGFRA in glioma cells is regulated by ERK activity and can potentially be manipulated to combat glioma growth.