Glioblastoma (GBM) is one of the deadliest types of human cancer. Despite a very aggressive treatment regime – including resection of the tumor, radiation and chemotherapy – its estimated recurrence rate is more than 90%. Recurrence is mostly caused by the regrowth of highly invasive cells spreading from the tumor bulk, which are not removed by resection. To develop an effective therapeutic approach, we need to better understand the underlying molecular mechanism of radiation resistance and tumor spreading in GBM. Radioresistance in GBM is attributed to glioma stem cells (GSCs), a fraction of perivascular, self-renewing, multipotent and tumor-initiating cells. Growing evidence highlights the perivascular space as a niche for GSC survival, resistance to therapy, progression and dissemination. The unknown factor is the dynamics of GSCs, how they end up in the vascular niche and how this impacts on radioresistance. My overall hypothesis is that GSCs reach the perivascular niche through vessel co-option - the directional migration of tumor cells towards vessels - and that targeting vessel co-option has the potential to radiosensitize GBM. With this project, we aim to uncover the exact molecular and cellular connections among vessel co-option, GSCs, the vascular niche and radioresistance. Using multiple strategies, such as multiphoton intravital microscopy, orthotopic models of GBM, organotypic cultures, screenings and survival studies, we will investigate and mechanistically change the dynamics of GSC and differentiated GBM cells in order to understand the role of their interaction with brain vessels and whether this confers resistance to radiotherapy. These studies will provide clinically relevant insights into the involvement of GSCs, the vascular niche and vessel co-option in the resistance of GBM to therapy. Since all GBM patients receive radiotherapy, many would benefit from therapeutic strategies aimed at increasing its efficacy.

Stem cells guard tissue integrity by renewing the pool of differentiated cells, and must therefore be shielded from threats such as transposable elements or viruses. In that line, stem cells can largely resist viral infections, but they do so without deploying the antiviral pathways at play in differentiated cells. So, what guards the guardians? I recently discovered a stem cell-specific immune pathway, driven by a protein termed aviD (antiviral Dicer), which mounts an antiviral RNA interference (RNAi) response and thwarts RNA virus infection. This illustrates that stem cells implement specific defence mechanisms that are poorly understood, or unknown. My proposal aims at bridging the current knowledge gap in stem cell immunity. We will do so by investigating the role of the aviD pathway in vivo using an aviD knock-out mouse, as well as by unravelling the molecular regulation of the pathway (Aim 1). Stem cell’s demise can additionally be provoked by the expression of transposable elements (TEs). Converging evidence suggests that aviD and the RNAi pathway may play a second defensive role in shielding uninfected stem cells from TE expression, which we will explore in Aim 2. Because cancer is characterised by a stemness transcriptional program, aviD may similarly control TEs in tumour cells. We will study aviD expression in various tumour models and assess its role in controlling TEs. This is of special interest because awakening TE expression in cancer is being leveraged by new therapeutic approaches in multiple malignancies. Similarly, we will explore if inhibiting aviD and the RNAi pathway could have an antitumour effect (Aim 2). Finally, I hypothesize that stem cells are protected by additional, unknown antiviral pathways, which we aim to identify and characterise using new genome-scale approaches (Aim 3). Overall, this proposal aims at unveiling new biology in stem cell immunity as well as developing clinical applications in the field of antitumour therapy.