Understanding Replication Stress Response in Mammalian Cells

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Project Summary In each cell cycle, DNA replication machinery encounters replication fork barriers including DNA lesions, secondary structure-forming repetitive sequences, and transcriptional machinery. Oncogenic transformation also perturbs normal replication and results in replication fork dysfunction commonly referred to as replication stress. Response to replication stress is an essential aspect of the DNA damage response in cells, and the consequences of inappropriate response results in genome instability and cancer. We have recently identified a novel regulatory pathway that is required for the protection of stalled replication forks and recovery from replication stress. We showed that the mammalian replisome contains a previously unidentified and completely unstudied protein, RTF2 (Replication Termination Factor 2), which must be removed for proper response to replication stress. We showed that RTF2 is removed from stalled forks in a process that is dependent on the proteasomal shuttle proteins DDI1 and DDI2, which interact with RTF2 and the proteasome. Persistence of RTF2 at stalled forks resulted in replication fork restart defects, hyperactivation of the DNA damage signaling, accumulation of single stranded DNA, sensitivity to replication drugs including hydroxyurea and aphidicolin, and chromosome instability. Our results establish that removal of RTF2 is necessary for cells to manage replication stress and maintain genome integrity. The first goal of the proposed studies is to fully understand how RTF2 functions during DNA replication. To this end, we will fully characterize replication without RTF2, using a conditional knockout mouse and cell model, and identify the mechanism of how RTF2 regulates DNA replication during unperturbed conditions. The second goal is to determine how RTF2 is itself regulated under replication stress and why it needs to be removed from the replisome. RTF2 ubiquitination is necessary for interaction with DDI1/2, thus we will identify the regulatory network of this ubiquitination and subsequent removal of RTF2 from the replication fork. The final goal in this project, is to leverage the idea that the removal of proteins during DNA damage response is as equally important as recruitment of DNA repair proteins to sites of DNA damage. Most published studies have concentrated on proteins traveling or recruited to sites of DNA damage. However, our work on DDIs and RTF2 suggests a large component of the DNA damage response network is missing, i.e. proteins that must be removed from the sites of damage to allow for proper DNA damage response and repair. In order to identify other proteins removed during replication stress, we will use an approach similar to the one we used for our DDI studies and detect proteins inappropriately enriched at stressed replication forks using a recently-developed technique, Isolation of Proteins On Nascent DNA (iPOND). We envision that our studies will identify yet unknown regulatory networks essential during the DNA damage response that prevents development of cancer-causing genome instability.