Thesis title: Unraveling the molecular mechanisms driving brain tumorigenesis to unlock innovative therapies
Brain tumors remain among the most challenging malignancies to treat due to high heterogeneity, aggressive growth, immunosuppressive microenvironment, and resistance to therapy. Despite advances in clinical care, long-term survival remains poor, underscoring the need for innovative therapeutic strategies. Recent progress in elucidating the genetic basis of central nervous system tumors has shifted classification toward molecular features, accompanied by an increasing number of clinical trials targeting genetic and epigenetic alterations as well as immune modulation. Within this context, my PhD research focused on two of the most lethal brain tumors, glioblastoma (GB) and medulloblastoma (MB), with the goal of identifying novel molecular targets that could be exploited therapeutically. In GB, we identified MEX3A, an RNA-binding protein and E3 ubiquitin ligase, as a key regulator of tumor growth. MEX3A is highly upregulated in GB and inversely correlates with RIG-I, a cytosolic RNA sensor activating type I interferon (IFN) signaling and apoptosis. We revealed that MEX3A promotes RIG-I ubiquitin-dependent degradation, whereas its silencing restores RIG-I levels, suppresses GB proliferation, and prolongs survival in orthotopic xenograft models. Given the role of RIG-I in immune activation and tumor suppression, we explored whether the stimulation of RIG-I could provide therapeutic benefit. We tested RNA agonists of RIG-I and found that M8 (5′pppRNA) potently activates IFN signaling, apoptosis, and tumor suppression. Notably, the combination of MEX3A silencing with M8 achieved stronger anti-tumor effects in GB cells than either intervention alone. Together, these results establish the MEX3A/RIG-I axis as a promising therapeutic target in GB supporting the development of RNA-based interventions to potentiate anti- tumor responses. In parallel, we investigated the molecular mechanisms underlying MB tumorigenesis, focusing on the Sonic Hedgehog (SHH) subgroup, the most prevalent and genetically defined. In this context, we uncovered a novel role for formins, main regulators of cytoskeletal dynamics, with INF2 acting as a negative regulator of SHH signaling by counteracting mDia2-mediated activation of Gli1 transcription. Notably, INF2 expression increases during late cerebellar development, coinciding with SHH pathway being switched off, whereas its genetic inhibition enhances Gli1 expression and proliferation of granule neuron progenitors (GNPs), the cells of origin of SHH-MB. Consistently, INF2 was downregulated in SHH-MB samples, and its overexpression reduced proliferation and invasion of primary tumor cells. These findings highlight formins as novel regulators of SHH-MB biology and relevant therapeutic candidates. Collectively, our studies underscore novel molecular vulnerabilities in GB and MB, providing a strong rationale for innovative therapeutic strategies that could ultimately transform the treatment landscape of these devastating brain tumors.