Titolo della tesi: PLK1-NOTCH1 dynamics control stress-specific NOTCH1 requirement in the DNA damage response of T-ALL
Acute T-cell lymphoblastic leukemia (T-ALL) is a tumor of hematopoietic origin manifested as diffuse bone marrow infiltration of immature T lymphoblasts. The disease is heterogeneous and characterized by a large set of chromosomal and genetic alterations that deregulate the growth of maturing thymocytes. The onset of this neoplasm is linked to transformation events that occur in critical phases of the intrathymic differentiation process of T cells due to the deregulated expression and/or activity of different pathways involved in thymocyte development. Indeed, T-ALL is characterized by specific gene alterations, such as: chromosomal aberrations, altered gene expression profiles, gene mutations, rearrangement of immunoglobulin (Ig) and T cell receptor (TCR) genes. Mutations have been characterized in several genes, among them there is NOTCH1: the central role in T-ALL make it a potentially important therapeutic target in this disease. Much of the current effort targeting NOTCH1 in T-ALL aims to block Notch signaling with small molecule inhibitors of the γ secretase complex. GSIs reduce levels of intracellular Notch1 and transcriptional down-regulation of its target genes in T-ALL. Early observations of in vitro resistance to GSI treatment suggest that T-ALL cells can become resistance to NOTCH inhibition. Our results suggest a complex network in cancer cells, where Notch1 signaling is both upregulated and downregulated during the DNA damage-induced G2 checkpoint. Notably, in T-ALL cells, we observed that Notch1 expression is sustained, allowing some cells to survive checkpoint mediated arrest and evade apoptosis. Remarkably, we found that while there is variability in the intrinsic ability of T-ALL cells to exhibit resistance to Notch1 inhibition under basal conditions, exposure to DNA damage triggers a fascinating adaptation at the G2 checkpoint, where NOTCH1 plays a crucial role in promoting resistance to chemotherapeutic agents, regardless of the cells' initial sensitivity to its inhibition. Since NOTCH1 is a known transcriptional regulator, it was evaluated whether cells subjected to damage can survive the stress conditions through transcriptional reprogramming of genes involved in G2 checkpoint and repair processes (DDR). Although the exact mechanisms by which NOTCH1 regulates survival after induction of genotoxic damage are not currently understood, it is conceivable that there is a molecular switch that, by acting on Notch1 signaling, allows tumor cells to adapt to chemotherapy-induced genotoxic stress conditions and, as a result, develop a phenotype resistant to therapy. To develop targeted therapy, especially in relapsed and refractory cases of T-ALL, investigations were performed to understand the possible role of NOTCH1 in the G2 checkpoint during the response to genotoxic stress. These findings highlight a critical temporal window during which Notch1 signaling plays a pivotal role in the acquisition of therapy resistance, offering new opportunities to identify strategies to overcome this phenomenon.