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The malignant primary brain tumor glioblastoma has a dismal prognosis and is distinguished by its heterogeneous character. Current treatment with surgical resection, radiotherapy and adjuvant chemotherapy with the alkylating agent temozolomide does not provide a cure, but simply prolongs survival by a few months. Since the tumors recur, cells remaining after treatment can act as cancer stem cells and are able to reform the tumor. 

This thesis provides insights into glioblastoma heterogeneity and how dominant transcriptional programs have a substantial impact on glioblastoma cell responses to altered levels of the intrinsic proteins BMP4 and SOX2. SOX2 has a role as a stem cell transcription factor in the normal nervous system and in glioblastoma, while BMP4 acts as a cue for astrocytic differentiation during normal nervous system development. As a response to BMP4, we find a wide spectrum of growth-inhibition across 40 human glioblastoma cell lines and correlate the extent of the response with baseline gene expression in the cells. We discover a connection between high SOX2 expression and a more pronounced growth-inhibitory response and establish a causative relationship between SOX2 downregulation and reduced proliferation in BMP4-responsive cell lines. We also find how BMP4 can induce a senescence-like phenotype in glioblastoma and connect it to a mesenchymal phenotype on a proneural-mesenchymal scale by investigating clonally derived cultures from the same tumor. Through elimination of senescent cells by senolytic treatment and generation p21-knockout cells we also establish a p21-dependence for BMP4-induced senescence.

Studies on cellular organization identify a hierarchical cell-state pattern which the cells move through during culture and show that external perturbations (here by BMP4 and temozolomide) alter this hierarchy, demonstrating a substantial cellular plasticity.

Also, we establish a strategy to eradicate endogenous SOX2 with the inducible exogenous SOX2-system present, demonstrating that SOX2 is not an essential transcription factor in all glioblastomas. 

In summary, this thesis highlights several aspects of inter- and intratumoral heterogeneity as well as cellular plasticity, providing valuable insights that could help guide the glioblastoma community in the pursuit of more effective therapies against glioblastoma.