Functional analysis of the tumor suppressor gene p53 in the process of a symmetric cell division

Abstract

The process of asymmetric cell division (ACD) is a key and highly conserved mechanism to balance the expansion of stem/progenitor cells and the generation of cell diversity during development. Alterations in the ACD process can disrupt this delicate balance, triggering failures in cell identity and cell overgrowth, even tumor formation. Drosophila melanogaster neural stem cells, known as neuroblasts (NBs), divide asymmetrically giving rise to two different daughter cells: one of them retains the self-renewal capacity of the stem cell (the NB), while the other daughter cell is committed to neural differentiation. These NBs have been employed for decades as an excellent model system to study the ACD process. Remarkably, over the past years, it has been determined that genes that regulate ACD can behave as tumor suppressors. Likewise, genes initially discovered as tumor suppressors have been shown a posteriori to modulate ACD. Thus, the main objective of this PhD thesis was to get deep insight into the potential function of known tumor suppressors as novel ACD regulators. Specifically, we aimed to analyze the function of the tumor suppressor gene p53 in ACD, as p53 is a crucial tumor suppressor mutated in most human cancers. Intriguingly, in addition to the classical functions of p53 as inductor of cell cycle arrest and apoptosis in cells under stress conditions, novel functions for p53 in unstressed cells have been more recently unveiled, such as the regulation of the mode of stem cell division. In this work, we have found that Drosophila p53 does have an impact on ACD. The absence of p53 affected the correct asymmetric division of embryonic neural progenitor lineages. Additionally, p53 was required for the correct localization in mitotic NBs of the ACD regulators Numb, Pins and Brat, critical in the ACD process for the correct identity of the daughter cells. Moreover, we observed that p53 transcriptionally activates the ACD regulators Numb, Brat and Traf4. Recent work in the lab has shown that p53, in fact, directly binds the regulatory regions of all these three genes. Interestingly, human and mouse homologs of Drosophila brat and Traf4 (TRIM32 and TRAF4, respectively) were recently identified in a meta-analysis of transcriptomic and ChIP-seq datasets as conserved targets of p53. Lastly, we found that the lack of p53 did not cause tumor-like overgrowth in larval brain NB lineages, result that can be explained by the high redundancy in ACD regulation.