The 18kDa translocator protein in human glioma

Abstract

Abstract The translocator protein (TSPO) is a mitochondrial protein with five transmembrane domains. It is involved in many biological functions including cholesterol transport, cell proliferation and apoptosis. TSPO is expressed at a low level in normal brain tissues but its expression is up-regulated in brain tumours and many inflammatory diseases. A positive correlation has been identified between TSPO and proliferation in brain tumours suggesting a possible role for TSPO in tumour progression. Vascular endothelial cells and glial cells have been shown to express TSPO in many neuropathological conditions, including tumours of the central nervous system (CNS). Therefore, studying the contribution of neurovascular unit (NVU) cellular components to TSPO expression warrants further research. Here, TSPO expression was examined for several cell types of the NVU via immunohistochemistry (IHC) and immunofluorescence (IF). TSPO was found to be expressed by astrocytes, macrophages and endothelial cells in normal condition. Additionally, TSPO expression levels in glioma associated macrophages/microglia (GAMMs) and vascular tissues in oligodendroglioma has not been previously studied. This perhaps represents a missed opportunity to utilize TSPO expression as a diagnostic marker or predictor of tumour behaviour. Therefore, tissue microarray (TMA) and Mass Cytometry have been used to examine the vascular, GAMMs and neoplastic TSPO expression in oligodendrogliomas. It was noted in TMA analysis that neoplastic cell contribution to overall TSPO expression levels was greater compared to the vascular (CD31) and inflammatory cell (GAMMs) components. Next, tissue from fifty oligodendrogliomas was examined by IHC to discern whether TSPO expression varied significantly between low (WHO II) and high grade (WHO III) tumours. TSPO expression was significantly increased in high-grade oligodendrogliomas compared to low-grade. Furthermore, TSPO was found to be a predictor of prognosis for oligodendroglioma patients. High TSPO expression levels correlated with poor outcome in terms of disease progression and mean survival time. Furthermore, the regulation of TSPO expression is very poorly understood. This thesis has studied the epigenetic regulatory mechanism that might control TSPO expression in oligodendroglioma. For that purpose, DNA was isolated from thirty-eight cases of oligodendrogliomas and the methylation status of the TSPO gene and its promoter examined by using the Infinium methylation EPIC beadChip. No difference within the promoter methylation state was seen between samples showing high or low TSPO expression. As an alternative mechanism, RNA sequencing for oligodendroglioma frozen tissues RNA samples was done to identify possible candidate’s genes which might regulate TSPO expression. In conclusion, this thesis has identified TSPO as a possible biomarker of oligodendroglioma progression and patient’s survival. It provided a wide coverage of the possible cellular components that contributed to TSPO expression in NVU and oligodendrogliomas. Moreover, TSPO expression was not found to be regulated by promoter methylation suggesting other alternative mechanism which might be involved.