MDM2 as a chromatin modifier

Since its discovery about 40 years ago, the transcription factor p53 has turned into the most extensively studied protein in the context of human cancers due to its essential role in promoting tumor suppression. P53 regulates central processes such as the induction of cell cycle arrest, senescence, or apoptosis in response to cellular stresses, thus preventing tumorigenesis in mammals. Being the major negative regulator of p53, the E3 ubiquitin ligase MDM2 is as much of interest to cancer researchers as p53 itself. Down to the present day, its major function is assigned to antagonizing p53. However, evidence of additional, p53- independent, functions is accumulating. One of these functions is the role of MDM2 as a p53-independent regulator of transcription. Previous studies have proven that MDM2 can interact with the general transcription machinery. Additionally, it acts as a chromatin-modifying co-factor promoting the formation of the repressive histone modifications H3K27me3 and H2AK119ub1 by Polycomb repressor complexes. However, since most of these studies were either conducted in vitro or in the absence of p53 in vivo, a comprehensive analysis of the MDM2 chromatin association in the presence of its major interaction partner p53 is still missing. In this thesis, we have investigated the global chromatin-binding pattern of endogenous MDM2 protein in various cell systems with diverse p53 status. Strikingly, comparative analyses of MDM2 binding sites identified in p53 wild-type, deleted and mutated systems revealed that MDM2 associates with more than 50 % of all CpG islands identified in human cells. This targeted binding of MDM2 to CpG islands is mediated through its direct interaction with the histone demethylase and CpG island-binding protein KDM2B, a known component of a variant Polycomb repressor complex. Preliminary results addressing the function of this KDM2B-directed chromatin recruitment of MDM2 indicate that both proteins cooperate in the repression of CpG island-associated genes, potentially through affecting the recruitment of RNA Polymerase II to those sites. This hypothesis is further strengthened by gene expression studies conducted in p53 mutated cells. In these studies, we found that MDM2 and Polycomb repressor complexes cooperatively repress target genes of the inducible TNF signaling pathway. Since CpG islands associate with the transcriptional start sites of about 50-60 % of all human genes, it is highly possible that this newly identified MDM2-KDM2B axis is central to the regulation of a multitude of physiological processes in the cell.2022-06-2