Exploring the role of the USP7 deubiquitinase in cell cycle control and cancer therapy

Abstract

The classical model of the cell cycle consists of a sequence of ordered events leading to the duplication of DNA and ultimately cell division. To prevent genomic instability and cell death, DNA replication and mitosis need to be properly coordinated. However, how cells sense that DNA replication is finished, and how and when mitosis is triggered are yet not fully understood. During the last years, many papers have reported that posttranslational modifications such as ubiquitination, can regulate all these processes. In addition, we recently described that the inhibition of the USP7 deubiquitinase, leads to a premature termination of DNA replication and DNA damage for reasons that remained unknown. During this thesis, we have used this tool to investigate how or if DNA replication termination is the actual signal for mitotic entry, ensuring that chromosome segregation is not initiated before the entire genome has been replicated. Our results suggest that USP7 inhibition leads to the ubiquitination of several replication factors and their displacement from replisomes, while at the same time triggers a premature activation of the mitotic kinases (e.g. CDK1) throughout the cell cycle. Mechanistically, we found that USP7 inhibitors phenocopy the effects of the phosphatase PP2A inhibition. This results in premature chromosome condensation and cell death, which is not mediated by P53. In contrast, the toxicity of USP7 inhibitors can be counteracted by CDK1 inhibitors or PP2A activators, both of which limit the activation of the mitotic program. Our results indicate that the anticancer effects of USP7 inhibitors are not solely dependent on P53 as previously thought, but also related to a premature activation of the mitotic machinery in cells that have not terminated DNA replication. Besides its interest for cancer therapy, our work suggests the existence of a ubiquitin-based signaling code that coordinates DNA replication termination and mitotic entry