Role of USP24 in Modulation of DNA Damage Response

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.</p

Role of deubiquitinases in DNA damage response

DNA damage response (DDR) serves as an integrated cellular network to detect cellular stress and react by activating pathways responsible for halting cell cycle progression, stimulating DNA damage repair, and initiating apoptosis. Efficient DDR protects cells from genomic instability while defective DDR can allow DNA lesions to go unrepaired, causing permanent mutations that will affect future generations of cells and possibly cause disease conditions such as cancer. Therefore, DDR mechanisms must be tightly regulated in order to ensure organismal health and viability. One major way of DDR regulation is ubiquitination, which has been long known to control DDR protein localization, activity, and stability. The reversal of this process, deubiquitination, has more recently come to the forefront of DDR research as an important new angle in ubiquitin-mediated regulation of DDR. As such, deubiquitinases have emerged as key factors in DDR. Importantly, deubiquitinases are attractive small-molecule drug targets due to their well-defined catalytic residues that provide a promising avenue for developing new cancer therapeutics. This review focuses on the emerging roles of deubiquitinases in various DNA repair pathways

A Rapid and Precise Mutation-Activated Fluorescence Reporter for Analyzing Acute Mutagenesis Frequency

Mutagenesis reporters are critical for quantifying genome stability. However, current methods rely on cell survival/death to report mutation, which takes weeks and prevents evaluation of acute or time-dependent changes. Existing methods also have other limitations, such as cell type restrictions. Using our discovery that mCherryFP fluorescence depends on residue Trp98, we replaced this codon with a stop codon to generate a mutation biosensor (termed CherryOFF), with a green fluorescence protein (GFP) as an internal control. We found that the red fluorescence of this biosensor is activated by a specific A/T-G/C nucleotide transition. Compared with the established hypoxanthine phosphoribosyl transferase assay, our reporter has similar or better ability to detect changes of mutation frequency induced by physical/chemical mutagens or manipulation of mutation-related genes. Furthermore, CherryOFF-GFP can report mutagenesis independently of cell-death events, can be adapted to many cell types, and can generate readouts within 1 day for the measurement of acute or time-dependent events

34 Genes Induced by UV Radiation in a BRG1-Dependent Manner.

<p>34 Genes Induced by UV Radiation in a BRG1-Dependent Manner.</p

BRG1 controls ATF3 expression under UV conditions.

<p>(<b>A</b>) RT-PCR analysis of ATF3 gene expression in SW13+vector and SW13+BRG1 cells. Both cells were collected 6 hours after UV treatment. RNA was purified and RT-PCR products were analyzed on agarose gel. (<b>B</b>) Binding of BRG1 to ATF3 gene promoter region. 293T cells were UV irradiated and incubated for 6 hours followed by Chromatin Immunoprecipitation (ChIP) assay with anti-BRG1 antibody.</p