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Efficient S phase entry is essential for development, tissue repair, and immune defences. However, hyperactive or expedited S phase entry causes replication stress, DNA damage and oncogenesis, highlighting the need for strict regulation. Recent paradigm shifts and conflicting reports demonstrate the requirement for a discussion of the G1/S transition literature. Here, we review the recent studies, and propose a unified model for the S phase entry decision. In this model, competition between mitogen and DNA damage signalling over the course of the mother cell cycle constitutes the predominant control mechanism for S phase entry of daughter cells. Mitogens and DNA damage have distinct sensing periods, giving rise to three Commitment Points for S phase entry (CP1-3). S phase entry is mitogen-independent in the daughter G1 phase, but remains sensitive to DNA damage, such as single strand breaks, the most frequently-occurring lesions that uniquely threaten DNA replication. To control CP1-3, dedicated hubs integrate the antagonistic mitogenic and DNA damage signals, regulating the stoichiometric cyclin: CDK inhibitor ratio for ultrasensitive control of CDK4/6 and CDK2. This unified model for the G1/S cell cycle transition combines the findings of decades of study, and provides an updated foundation for cell cycle research., (© The Author(s) 2020. Published by Oxford University Press on behalf of Nucleic Acids Research.)

Multiple DNA repair pathways may be involved in the removal of the same DNA lesion caused by endogenous or exogenous agents. Although distinct DNA repair machinery fulfill overlapping roles in the repair of DNA lesions, the mechanisms coordinating different pathways have not been investigated in detail. Here, we show that Ku70, a core protein of nonhomologous end-joining (NHEJ) repair pathway, can directly interact with DNA polymerase-β (Pol-β), a central player in the DNA base excision repair (BER), and this physical complex not only promotes the polymerase activity of Pol-β and BER efficiency but also enhances the classic NHEJ repair. Moreover, we find that DNA damages caused by methyl methanesulfonate (MMS) or etoposide promote the formation of Ku70-Pol-β complexes at the repair foci. Furthermore, suppression of endogenous Ku70 expression by small interfering RNA reduces BER efficiency and leads to higher sensitivity to MMS and accumulation of the DNA strand breaks. Similarly, Pol-β knockdown impairs total-NHEJ capacity but only has a slight influence on alternative NHEJ. These results suggest that Pol-β and Ku70 coordinate 2-way crosstalk between the BER and NHEJ pathways.-Xia, W., Ci, S., Li, M., Wang, M., Dianov, G. L., Ma, Z., Li, L., Hua, K., Alagamuthu, K. K., Qing, L., Luo, L., Edick, A. M., Liu, L., Hu, Z., He, L., Pan, F., Guo, Z. Two-way crosstalk between BER and c-NHEJ repair pathway is mediated by Pol-β and Ku70.

The ARF (Alternative Reading Frame) protein is encoded in the Ink4a locus of human chromosome 9 that is frequently mutated in cancer cells. It was recently demonstrated that ARF is induced in response to DNA damage and inhibits, by direct interaction, the E3 ubiquitin ligase Mule that regulates p53 protein levels. Mule inhibition leads to p53 accumulation and activates cellular DNA damage responses. Mule has also recently been identified as a major E3 ubiquitin ligase involved in the regulation of DNA base excision repair. In this review, we will summarise the major properties of Mule and ARF and their roles in the coordination of DNA repair and DNA replication., (Copyright © 2011 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

Werner Syndrome is a premature aging disorder characterized by genomic instability, elevated recombination, and replication defects. It has been hypothesized that defective processing of certain replication fork structures by WRN may contribute to genomic instability. Fluorescence resonance energy transfer (FRET) analyses show that WRN and Flap Endonuclease-1 (FEN-1) form a complex in vivo that colocalizes in foci associated with arrested replication forks. WRN effectively stimulates FEN-1 cleavage of branch-migrating double-flap structures that are the physiological substrates of FEN-1 during replication. Biochemical analyses demonstrate that WRN helicase unwinds the chicken-foot HJ intermediate associated with a regressed replication fork and stimulates FEN-1 to cleave the unwound product in a structure-dependent manner. These results provide evidence for an interaction between WRN and FEN-1 in vivo and suggest that these proteins function together to process DNA structures associated with the replication fork.

Cyclins are central engines of cell cycle progression in conjunction with cyclin‐dependent kinases (CDKs). Among the different cyclins controlling cell cycle progression, cyclin F does not partner with a CDK, but instead forms via its F‐box domain an SCF (Skp1‐Cul1‐F‐box)‐type E3 ubiquitin ligase module. Although various substrates of cyclin F have been identified, the vulnerabilities of cells lacking cyclin F are not known. Thus, we assessed viability of cells lacking cyclin F upon challenging them with more than 180 different kinase inhibitors. The screen revealed a striking synthetic lethality between Chk1 inhibition and cyclin F loss. Chk1 inhibition in cells lacking cyclin F leads to DNA replication catastrophe. Replication catastrophe depends on accumulation of the transcription factor E2F1 in cyclin F‐depleted cells. We find that SCF‐cyclin F controls E2F1 ubiquitylation and degradation during the G2/M phase of the cell cycle and upon challenging cells with Chk1 inhibitors. Thus, Cyclin F restricts E2F1 activity during the cell cycle and upon checkpoint inhibition to prevent DNA replication stress. Our findings pave the way for patient selection in the clinical use of checkpoint inhibitors. Synopsis: A kinase inhibitor screen for synthetic lethality in cells lacking the ubiquitin ligase adaptor Cyclin F leads to the identification of E2F1 as new SCF‐Cyclin F substrate, whose unrestricted activity promotes catastrophic replication stress upon checkpoint suppression with Chk1 inhibitors. A screen to identify vulnerabilities of cells lacking cyclin F identifies Chk1 inhibitors as primary hits.Cyclin F loss predisposes cells to the toxic effect of Chk1i by inducing DNA replication catastrophe.Aberrant accumulation of E2F1 upon cyclin F loss promotes DNA damage upon Chk1 inhibition.Lack of cyclin F prevents degradation of E2F1 in G2/M and upon Chk1 inhibitors.A non‐degradable E2F1 lacking a CY motif induces cell death and DNA replication stress upon Chk1 inhibition. [ABSTRACT FROM AUTHOR]

DNA single-strand breaks (SSBs) arise as a consequence of spontaneous DNA instability and are also formed as DNA repair intermediates. Their repair is critical because they otherwise terminate gene transcription and generate toxic DNA double-strand breaks (DSBs) on replication. To prevent the formation of DSBs, SSB repair must be completed before DNA replication. To accomplish this, cells should be able to detect unrepaired SSBs, and then delay cell cycle progression to allow more time for repair; however, to date there is no evidence supporting the coordination of SSB repair and replication in human cells. Here we report that ataxiatelangiectasia mutated kinase (ATM) plays a major role in restricting the replication of SSB-containing DNA and thus prevents DSB formation. We show that ATM is activated by SSBs and coordinates their repair with DNA replication. SSB-mediated ATM activation is followed by a G1 cell cycle delay that allows more time for repair and thus prevents the replication of damaged DNA and DSB accrual. These findings establish an unanticipated role for ATM in the signaling of DNA SSBs and provide important insight into the molecular defects leading to genetic instability in patients with ataxia-telangiectasia. [ABSTRACT FROM AUTHOR]