Abstract
Diffuse Low-Grade Gliomas (DLGG) are WHO grade II glial tumors affecting younger adults. They are characterized as silent, slow growing tumors with fewer mitotic activities. However, they diffuse and invade the healthy brain via blood vessels and nerve fibers. These, over a period of years develop to malignant Glioblastoma, aggressive brain tumors where patients have an average medial survival of 12-15 months after diagnosis. Ill-defined phenotypic and cellular diversity of DLGG poses serious limitation to treatment and prevention at the early stage.In my PhD thesis, I aimed to address this limitation by characterizing the cellular heterogeneity in IDH1-mutated DLGG. By performing immunofluorescence analysis on grade II astrocytoma and oligodendroglioma, I have identified two largely non-overlapping cellular subpopulations expressing SOX9 and OLIG1 transcription factors, which represent astrocyte-like and oligodendrocyte-like cells, respectively. Upon further investigation, I have shown that these subpopulations express distinct molecular markers. Sox9 cells are associated with APOE, KCNN3, CRYAB and ID4, while Olig1 cells showed strong correlation with the expression of PDGFRA, SOX8, MASH1, and SOX4. In addition, the sox9 cells show a particular activation of signaling pathways including Notch, BMP and their downstream targets.To ascertain the role of Notch signaling in regulating the formation of these tumoral subpopulations, I used magnetic sorting of tumor cells from freshly resected glioma samples and overexpressed Notch Intracellular Domain (NICD), an active form of Notch. Increased Notch activation resulted in an upregulation of Sox9- and downregulation of Olig1-associated cell markers. I have then extended these analyses on one anaplastic IDH1 mutated patient derived cell line which reproduced similar gene expression profile confirming the robustness of the role of Notch signaling in regulating the plasticity of the cells. Parallel experiments performed by activation of Bone Morphogenetic Protein (BMP) signaling on IDH1 mutated cell line did not show a prominent effect on the plasticity. Nevertheless, BMP signal activation highly upregulated CRYAB, a SOX9 related marker and downregulated OLIG1 and OLIG2.In conclusion, I have identified two non-overlapping tumor subpopulations in diffuse low-grade gliomas and demonstrated the deterministic role of Notch signaling pathway in their formation. I believe that these findings would aid in better understanding tumoral heterogeneity in DLGG and be extended in designing new therapeutic strategies against these tumors.