Abstract
Cellular DNA consistently suffers damage from various endogenous and exogenous sources. DNA Damage Response (DDR) serves as an integrated cellular network to sense cellular stress and activate pathways responsible for arresting cell cycle progression, stimulating DNA damage repair, and initiating apoptosis. Efficient DDR protects cells from genomic instability while defective or deficient DDR can allow unrepaired lesions to cause permanent and catastrophic mutations in the genome, which may cause disease conditions such as cancer and can reduce efficacy of genotoxic anti-cancer therapy. Thus, the proper regulation of DDR mechanisms is crucial for organismal health and viability. Ubiquitination has been long known to provide one such major way of regulating DDR by controlling DDR protein activity and stability. Deubiquitination, the process of reversing or modulating ubiquitin signals, has more recently come to the forefront of DDR research as an important new angle in ubiquitin-mediated regulation of DDR. Our laboratory was the first to identify the novel deubiquitinase USP24 as a regulator of DDR proteins. This dissertation seeks to characterize the role USP24 plays in modulating DDR by testing how USP24 affects DDR sub-pathways, cell cycle checkpoints and apoptosis, and how USP24 affects genome stability. We found that USP24 deubiquitinates and stabilizes p53, thereby promoting p53-dependent apoptotic response to UV irradiation. Using both a previously established, as well a novel dual-color reporter method to study mutagenesis, we also show that USP24 maintains genome stability. We further discovered that USP24 depletion increases cell proliferation, decreases the inactivating phosphorylation of CDK1 (Y15), and promotes faster progression through G2/M phase. Interestingly, we have gathered evidence that USP24 affects G2/M checkpoint regulation through both p53-dependent and p53-independent mechanisms. Finally, we also found that USP24 may contribute to regulating transcriptional stability in the DNA Damage Response and regulate physiological sensitivity to genotoxic agents. These studies indicate that USP24 plays an important role in activating DDR mechanisms thereby assisting in tumor suppression and maintaining organismal health.