UV-triggered p21 degradation facilitates damaged-DNA replication and preserves genomic stability

The levels of p53 govern the hierarchy of DNA damage tolerance pathway usage

It is well-established that, through canonical functions in transcription and DNA repair, the tumor suppressor p53 plays a central role in safeguarding cells from the consequences of DNA damage. Recent data retrieved in tumor and stem cells demonstrated that p53 also carries out non-canonical functions when interacting with the translesion synthesis (TLS) polymerase iota (POLι) at DNA replication forks. This protein complex triggers a DNA damage tolerance (DDT) mechanism controlling the DNA replication rate. Given that the levels of p53 trigger non-binary rheostat-like functions in response to stress or during differentiation, we explore the relevance of the p53 levels for its DDT functions at the fork. We show that subtle changes in p53 levels modulate the contribution of some DDT factors including POLι, POLη, POLζ, REV1, PCNA, PRIMPOL, HLTF and ZRANB3 to the DNA replication rate. Our results suggest that the levels of p53 are central to coordinate the balance between DDT pathways including (i) fork-deceleration by the ZRANB3-mediated fork reversal factor, (ii) POLι-p53-mediated fork-slowing, (iii) POLι- and POLη-mediated TLS and (iv) PRIMPOL-mediated fork-acceleration. Collectively, our study reveals the relevance of p53 protein levels for the DDT pathway choice in replicating cells.

Human CST complex restricts excessive PrimPol repriming upon UV induced replication stress by suppressing p21

DNA replication stress, caused by various endogenous and exogenous agents, halt or stall DNA replication progression. Cells have developed diverse mechanisms to tolerate and overcome replication stress, enabling them to continue replication. One effective strategy to overcome stalled replication involves skipping the DNA lesion using a specialized polymerase known as PrimPol, which reinitiates DNA synthesis downstream of the damage. However, the mechanism regulating PrimPol repriming is largely unclear. In this study, we observe that knockdown of STN1 or CTC1, components of the CTC1/STN1/TEN1 complex, leads to enhanced replication progression following UV exposure. We find that such increased replication is dependent on PrimPol, and PrimPol recruitment to stalled forks increases upon CST depletion. Moreover, we find that p21 is upregulated in STN1-depleted cells in a p53-independent manner, and p21 depletion restores normal replication rates caused by STN1 deficiency. We identify that p21 interacts with PrimPol, and STN1 depletion stimulates p21-PrimPol interaction and facilitates PrimPol recruitment to stalled forks. Our findings reveal a previously undescribed interplay between CST, PrimPol and p21 in promoting repriming in response to stalled replication, and shed light on the regulation of PrimPol repriming at stalled forks.

Correction: Chromosomal Integrity after UV Irradiation Requires FANCD2-Mediated Repair of Double Strand Breaks

[This corrects the article DOI: 10.1371/journal.pgen.1005792.].

Polymerase iota (Pol ι) prevents PrimPol-mediated nascent DNA synthesis and chromosome instability

Recent studies have described a DNA damage tolerance pathway choice that involves a competition between PrimPol-mediated repriming and fork reversal. Screening different translesion DNA synthesis (TLS) polymerases by the use of tools for their depletion, we identified a unique role of Pol ι in regulating such a pathway choice. Pol ι deficiency unleashes PrimPol-dependent repriming, which accelerates DNA replication in a pathway that is epistatic with ZRANB3 knockdown. In Pol ι-depleted cells, the excess participation of PrimPol in nascent DNA elongation reduces replication stress signals, but thereby also checkpoint activation in S phase, triggering chromosome instability in M phase. This TLS-independent function of Pol ι requires its PCNA-interacting but not its polymerase domain. Our findings unravel an unanticipated role of Pol ι in protecting the genome stability of cells from detrimental changes in DNA replication dynamics caused by PrimPol.

Revisiting the Function of p21CDKN1A in DNA Repair: The Influence of Protein Interactions and Stability

The p21CDKN1A protein is an important player in the maintenance of genome stability through its function as a cyclin-dependent kinase inhibitor, leading to cell-cycle arrest after genotoxic damage. In the DNA damage response, p21 interacts with specific proteins to integrate cell-cycle arrest with processes such as transcription, apoptosis, DNA repair, and cell motility. By associating with Proliferating Cell Nuclear Antigen (PCNA), the master of DNA replication, p21 is able to inhibit DNA synthesis. However, to avoid conflicts with this process, p21 protein levels are finely regulated by pathways of proteasomal degradation during the S phase, and in all the phases of the cell cycle, after DNA damage. Several lines of evidence have indicated that p21 is required for the efficient repair of different types of genotoxic lesions and, more recently, that p21 regulates DNA replication fork speed. Therefore, whether p21 is an inhibitor, or rather a regulator, of DNA replication and repair needs to be re-evaluated in light of these findings. In this review, we will discuss the lines of evidence describing how p21 is involved in DNA repair and will focus on the influence of protein interactions and p21 stability on the efficiency of DNA repair mechanisms.

Characterization of Molecular Subtypes in Head and Neck Squamous Cell Carcinoma With Distinct Prognosis and Treatment Responsiveness

Head and neck squamous cell carcinoma (HNSCC) is one of the most aggressive malignancies with complex phenotypic, etiological, biological, and clinical heterogeneities. Previous studies have proposed different clinically relevant subtypes of HNSCC, but little is known about its corresponding prognosis or suitable treatment strategy. Here, we identified 101 core genes from three prognostic pathways, including mTORC1 signaling, unfold protein response, and UV response UP, in 124 pairs of tumor and matched normal tissues of HNSCC. Moreover, we identified three robust subtypes associated with distinct molecular characteristics and clinical outcomes using consensus clustering based on the gene expression profiles of 944 HNSCC patients from four independent datasets. We then integrated the genomic information of The Cancer Genome Atlas (TCGA) HNSCC cohort to comprehensively evaluate the molecular features of different subtypes and screen for potentially effective therapeutic agents. Cluster 1 had more arrested oncogenic signaling, the highest immune cell infiltration, the highest immunotherapy and chemotherapeutic responsiveness, and the best prognosis. By contrast, Cluster 3 showed more activated oncogenic signaling, the lowest immune cell infiltration, the lowest immunotherapy and chemotherapy responsiveness, and the worst prognosis. Our findings corroborate the molecular diversity of HNSCC tumors and provide a novel classification strategy that may guide for prognosis and treatment allocation.

Impact of the interplay between stemness features, p53 and pol iota on replication pathway choices

Using human embryonic, adult and cancer stem cells/stem cell-like cells (SCs), we demonstrate that DNA replication speed differs in SCs and their differentiated counterparts. While SCs decelerate DNA replication, differentiated cells synthesize DNA faster and accumulate DNA damage. Notably, both replication phenotypes depend on p53 and polymerase iota (POLι). By exploring protein interactions and newly synthesized DNA, we show that SCs promote complex formation of p53 and POLι at replication sites. Intriguingly, in SCs the translocase ZRANB3 is recruited to POLι and required for slow-down of DNA replication. The known role of ZRANB3 in fork reversal suggests that the p53–POLι complex mediates slow but safe bypass of replication barriers in SCs. In differentiated cells, POLι localizes more transiently to sites of DNA synthesis and no longer interacts with p53 facilitating fast POLι-dependent DNA replication. In this alternative scenario, POLι associates with the p53 target p21, which antagonizes PCNA poly-ubiquitination and, thereby potentially disfavors the recruitment of translocases. Altogether, we provide evidence for diametrically opposed DNA replication phenotypes in SCs and their differentiated counterparts putting DNA replication-based strategies in the spotlight for the creation of therapeutic opportunities targeting SCs.

Inverse and reciprocal regulation of p53/p21 and Bmi-1 modulates vasculogenic differentiation of dental pulp stem cells

Dental pulp stem cells (DPSC) are capable of differentiating into vascular endothelial cells. Although the capacity of vascular endothelial growth factor (VEGF) to induce endothelial differentiation of stem cells is well established, mechanisms that maintain stemness and prevent vasculogenic differentiation remain unclear. Here, we tested the hypothesis that p53 signaling through p21 and Bmi-1 maintains stemness and inhibits vasculogenic differentiation. To address this hypothesis, we used primary human DPSC from permanent teeth and Stem cells from Human Exfoliated Deciduous (SHED) teeth as models of postnatal mesenchymal stem cells. DPSC seeded in biodegradable scaffolds and transplanted into immunodeficient mice generated mature human blood vessels invested with smooth muscle actin-positive mural cells. Knockdown of p53 was sufficient to induce vasculogenic differentiation of DPSC (without vasculogenic differentiation medium containing VEGF), as shown by increased expression of endothelial markers (VEGFR2, Tie-2, CD31, VE-cadherin), increased capillary sprouting in vitro; and increased DPSC-derived blood vessel density in vivo. Conversely, induction of p53 expression with small molecule inhibitors of the p53-MDM2 binding (MI-773, APG-115) was sufficient to inhibit VEGF-induced vasculogenic differentiation. Considering that p21 is a major downstream effector of p53, we knocked down p21 in DPSC and observed an increase in capillary sprouting that mimicked results observed when p53 was knocked down. Stabilization of ubiquitin activity was sufficient to induce p53 and p21 expression and reduce capillary sprouting. Interestingly, we observed an inverse and reciprocal correlation between p53/p21 and the expression of Bmi-1, a major regulator of stem cell self-renewal. Further, direct inhibition of Bmi-1 with PTC-209 resulted in blockade of capillary-like sprout formation. Collectively, these data demonstrate that p53/p21 functions through Bmi-1 to prevent the vasculogenic differentiation of DPSC.

Regulation of DNA Replication Licensing and Re-Replication by Cdt1

In eukaryotic cells, DNA replication licensing is precisely regulated to ensure that the initiation of genomic DNA replication in S phase occurs once and only once for each mitotic cell division. A key regulatory mechanism by which DNA re-replication is suppressed is the S phase-dependent proteolysis of Cdt1, an essential replication protein for licensing DNA replication origins by loading the Mcm2-7 replication helicase for DNA duplication in S phase. Cdt1 degradation is mediated by CRL4^Cdt2 ubiquitin E3 ligase, which further requires Cdt1 binding to proliferating cell nuclear antigen (PCNA) through a PIP box domain in Cdt1 during DNA synthesis. Recent studies found that Cdt2, the specific subunit of CRL4^Cdt2 ubiquitin E3 ligase that targets Cdt1 for degradation, also contains an evolutionarily conserved PIP box-like domain that mediates the interaction with PCNA. These findings suggest that the initiation and elongation of DNA replication or DNA damage-induced repair synthesis provide a novel mechanism by which Cdt1 and CRL4^Cdt2 are both recruited onto the trimeric PCNA clamp encircling the replicating DNA strands to promote the interaction between Cdt1 and CRL4^Cdt2. The proximity of PCNA-bound Cdt1 to CRL4^Cdt2 facilitates the destruction of Cdt1 in response to DNA damage or after DNA replication initiation to prevent DNA re-replication in the cell cycle. CRL4^Cdt2 ubiquitin E3 ligase may also regulate the degradation of other PIP box-containing proteins, such as CDK inhibitor p21 and histone methylase Set8, to regulate DNA replication licensing, cell cycle progression, DNA repair, and genome stability by directly interacting with PCNA during DNA replication and repair synthesis.

Targeting Non-Oncogene Addiction for Cancer Therapy

While Next-Generation Sequencing (NGS) and technological advances have been useful in identifying genetic profiles of tumorigenesis, novel target proteins and various clinical biomarkers, cancer continues to be a major global health threat. DNA replication, DNA damage response (DDR) and repair, and cell cycle regulation continue to be essential systems in targeted cancer therapies. Although many genes involved in DDR are known to be tumor suppressor genes, cancer cells are often dependent and addicted to these genes, making them excellent therapeutic targets. In this review, genes implicated in DNA replication, DDR, DNA repair, cell cycle regulation are discussed with reference to peptide or small molecule inhibitors which may prove therapeutic in cancer patients. Additionally, the potential of utilizing novel synthetic lethal genes in these pathways is examined, providing possible new targets for future therapeutics. Specifically, we evaluate the potential of TONSL as a novel gene for targeted therapy. Although it is a scaffold protein with no known enzymatic activity, the strategy used for developing PCNA inhibitors can also be utilized to target TONSL. This review summarizes current knowledge on non-oncogene addiction, and the utilization of synthetic lethality for developing novel inhibitors targeting non-oncogenic addiction for cancer therapy.

Regulation of translesion DNA synthesis in mammalian cells

The genomes of all living cells are under endogenous and exogenous attacks every day, causing diverse genomic lesions. Most of the lesions can be timely repaired by multiple DNA repair pathways. However, some may persist during S-phase, block DNA replication, and challenge genome integrity. Eukaryotic cells have evolved DNA damage tolerance (DDT) to mitigate the lethal effects of arrested DNA replication without prior removal of the offending DNA damage. As one important mode of DDT, translesion DNA synthesis (TLS) utilizes multiple low-fidelity DNA polymerases to incorporate nucleotides opposite DNA lesions to maintain genome integrity. Three different mechanisms have been proposed to regulate the polymerase switching between high-fidelity DNA polymerases in the replicative machinery and one or more specialized enzymes. Additionally, it is known that proliferating cell nuclear antigen (PCNA) mono-ubiquitination is essential for optimal TLS. Given its error-prone property, TLS is closely associated with spontaneous and drug-induced mutations in cells, which can potentially lead to tumorigenesis and chemotherapy resistance. Therefore, TLS process must be tightly modulated to avoid unwanted mutagenesis. In this review, we will focus on polymerase switching and PCNA mono-ubiquitination, the two key events in TLS pathway in mammalian cells, and summarize current understandings of regulation of TLS process at the levels of protein-protein interactions, post-translational modifications as well as transcription and noncoding RNAs. Environ. Mol. Mutagen. 61:680-692, 2020. © 2020 Wiley Periodicals, Inc.

CDK-Independent and PCNA-Dependent Functions of p21 in DNA Replication

p21^Waf/CIP1 is a small unstructured protein that binds and inactivates cyclin-dependent kinases (CDKs). To this end, p21 levels increase following the activation of the p53 tumor suppressor. CDK inhibition by p21 triggers cell-cycle arrest in the G1 and G2 phases of the cell cycle. In the absence of exogenous insults causing replication stress, only residual p21 levels are prevalent that are insufficient to inhibit CDKs. However, research from different laboratories has demonstrated that these residual p21 levels in the S phase control DNA replication speed and origin firing to preserve genomic stability. Such an S-phase function of p21 depends fully on its ability to displace partners from chromatin-bound proliferating cell nuclear antigen (PCNA). Vice versa, PCNA also regulates p21 by preventing its upregulation in the S phase, even in the context of robust p21 induction by irradiation. Such a tight regulation of p21 in the S phase unveils the potential that CDK-independent functions of p21 may have for the improvement of cancer treatments.

Stress-induced changes in CARF expression determine cell fate to death, survival, or malignant transformation

CARF (Collaborator of ARF) was discovered as an ARF-interacting protein that activated ARF-p53-p21WAF1 signaling involved in cellular response to a variety of stresses, including oxidative, genotoxic, oncogenic, or telomere deprotection stresses, leading to senescence, growth arrest, or apoptosis. Of note, whereas suppression of CARF was lethal, its enrichment was associated with increased proliferation and malignant transformation of cells. These reports have predicted that CARF could serve as a multi-stress marker with a predictive value for cell fates. Here, we recruited various in vitro stress models and examined their effect on CARF expression using human normal fibroblasts. We demonstrate that CARF levels in stress and post-stress conditions could predict the fate of cells towards either death or enhanced proliferation and malignant transformation. We provide extensive molecular evidence that (i) CARF expression changes in response to stress, (ii) it modulates cell death or survival signaling and determines the fate of cells, and (iii) it may serve as a predictive measure of cellular response to stress and an important marker for biosafety.

Multiple interaction nodes define the postreplication repair response to UV-induced DNA damage that is defective in melanomas and correlated with UV signature mutation load

Ultraviolet radiation‐induced DNA mutations are a primary environmental driver of melanoma. The reason for this very high level of unrepaired DNA lesions leading to these mutations is still poorly understood. The primary DNA repair mechanism for UV‐induced lesions, that is, the nucleotide excision repair pathway, appears intact in most melanomas. We have previously reported a postreplication repair mechanism that is commonly defective in melanoma cell lines. Here we have used a genome‐wide approach to identify the components of this postreplication repair mechanism. We have used differential transcript polysome loading to identify transcripts that are associated with UV response, and then functionally assessed these to identify novel components of this repair and cell cycle checkpoint network. We have identified multiple interaction nodes, including global genomic nucleotide excision repair and homologous recombination repair, and previously unexpected MASTL pathway, as components of the response. Finally, we have used bioinformatics to assess the contribution of dysregulated expression of these pathways to the UV signature mutation load of a large melanoma cohort. We show that dysregulation of the pathway, especially the DNA damage repair components, are significant contributors to UV mutation load, and that dysregulation of the MASTL pathway appears to be a significant contributor to high UV signature mutation load.

RNA-splicing factor SART3 regulates translesion DNA synthesis

Translesion DNA synthesis (TLS) is one mode of DNA damage tolerance that uses specialized DNA polymerases to replicate damaged DNA. DNA polymerase η (Polη) is well known to facilitate TLS across ultraviolet (UV) irradiation and mutations in POLH are implicated in skin carcinogenesis. However, the basis for recruitment of Polη to stalled replication forks is not completely understood. In this study, we used an affinity purification approach to isolate a Polη-containing complex and have identified SART3, a pre-mRNA splicing factor, as a critical regulator to modulate the recruitment of Polη and its partner RAD18 after UV exposure. We show that SART3 interacts with Polη and RAD18 via its C-terminus. Moreover, SART3 can form homodimers to promote the Polη/RAD18 interaction and PCNA monoubiquitination, a key event in TLS. Depletion of SART3 also impairs UV-induced single-stranded DNA (ssDNA) generation and RPA focus formation, resulting in an impaired Polη recruitment and a higher mutation frequency and hypersensitivity after UV treatment. Notably, we found that several SART3 missense mutations in cancer samples lessen its stimulatory effect on PCNA monoubiquitination. Collectively, our findings establish SART3 as a novel Polη/RAD18 association regulator that protects cells from UV-induced DNA damage, which functions in a RNA binding-independent fashion.

Polo-like Kinase 1 Inhibition as a Therapeutic Approach to Selectively Target BRCA1-Deficient Cancer Cells by Synthetic Lethality Induction

PURPOSE

BRCA1 and BRCA2 deficiencies are widespread drivers of human cancers that await the development of targeted therapies. We aimed to identify novel synthetic lethal relationships with therapeutic potential using BRCA-deficient isogenic backgrounds.

EXPERIMENTAL DESIGN

We developed a phenotypic screening technology to simultaneously search for synthetic lethal (SL) interactions in BRCA1- and BRCA2-deficient contexts. For validation, we developed chimeric spheroids and a dual-tumor xenograft model that allowed the confirmation of SL induction with the concomitant evaluation of undesired cytotoxicity on BRCA-proficient cells. To extend our results using clinical data, we performed retrospective analysis on The Cancer Genome Atlas (TCGA) breast cancer database.

RESULTS

The screening of a kinase inhibitors library revealed that Polo-like kinase 1 (PLK1) inhibition triggers strong SL induction in BRCA1-deficient cells. Mechanistically, we found no connection between the SL induced by PLK1 inhibition and PARP inhibitors. Instead, we uncovered that BRCA1 downregulation and PLK1 inhibition lead to aberrant mitotic phenotypes with altered centrosomal duplication and cytokinesis, which severely reduced the clonogenic potential of these cells. The penetrance of PLK1/BRCA1 SL interaction was validated using several isogenic and nonisogenic cellular models, chimeric spheroids, and mice xenografts. Moreover, bioinformatic analysis revealed high-PLK1 expression in BRCA1-deficient tumors, a phenotype that was consistently recapitulated by inducing BRCA1 deficiency in multiple cell lines as well as in BRCA1-mutant cells.

CONCLUSIONS

We uncovered an unforeseen addiction of BRCA1-deficient cancer cells to PLK1 expression, which provides a new means to exploit the therapeutic potential of PLK1 inhibitors in clinical trials, by generating stratification schemes that consider this molecular trait in patient cohorts.

AKT inhibition impairs PCNA ubiquitylation and triggers synthetic lethality in homologous recombination-deficient cells submitted to replication stress

Translesion DNA synthesis (TLS) and homologous recombination (HR) cooperate during S-phase to safeguard replication forks integrity. Thus, the inhibition of TLS becomes a promising point of therapeutic intervention in HR-deficient cancers, where TLS impairment might trigger synthetic lethality (SL). The main limitation to test this hypothesis is the current lack of selective pharmacological inhibitors of TLS. Herein, we developed a miniaturized screening assay to identify inhibitors of PCNA ubiquitylation, a key post-translational modification required for efficient TLS activation. After screening a library of 627 kinase inhibitors, we found that targeting the pro-survival kinase AKT leads to strong impairment of PCNA ubiquitylation. Mechanistically, we found that AKT-mediated modulation of Proliferating Cell Nuclear Antigen (PCNA) ubiquitylation after UV requires the upstream activity of DNA PKcs, without affecting PCNA ubiquitylation levels in unperturbed cells. Moreover, we confirmed that persistent AKT inhibition blocks the recruitment of TLS polymerases to sites of DNA damage and impairs DNA replication forks processivity after UV irradiation, leading to increased DNA replication stress and cell death. Remarkably, when we compared the differential survival of HR-proficient vs HR-deficient cells, we found that the combination of UV irradiation and AKT inhibition leads to robust SL induction in HR-deficient cells. We link this phenotype to AKT ability to inhibit PCNA ubiquitylation, since the targeted knockdown of PCNA E3-ligase (RAD18) and a non-ubiquitylable (PCNA K164R) knock-in model recapitulate the observed SL induction. Collectively, this work identifies AKT as a novel regulator of PCNA ubiquitylation and provides the proof-of-concept of inhibiting TLS as a therapeutic approach to selectively kill HR-deficient cells submitted to replication stress.

Direct binding of Cdt2 to PCNA is important for targeting the CRL4Cdt2 E3 ligase activity to Cdt1

The C-terminal end of Cdt2 contains a PIP box for binding to PCNA to promote CRL4^Cdt2 function, creating a new paradigm where the substrate receptor and substrates bind to a common multivalent docking platform for ubiquitination.

The CRL4^Cdt2 ubiquitin ligase complex is an essential regulator of cell-cycle progression and genome stability, ubiquitinating substrates such as p21, Set8, and Cdt1, via a display of substrate degrons on proliferating cell nuclear antigens (PCNAs). Here, we examine the hierarchy of the ligase and substrate recruitment kinetics onto PCNA at sites of DNA replication. We demonstrate that the C-terminal end of Cdt2 bears a PCNA interaction protein motif (PIP box, Cdt2^PIP), which is necessary and sufficient for the binding of Cdt2 to PCNA. Cdt2^PIP binds PCNA directly with high affinity, two orders of magnitude tighter than the PIP box of Cdt1. X-ray crystallographic structures of PCNA bound to Cdt2^PIP and Cdt1^PIP show that the peptides occupy all three binding sites of the trimeric PCNA ring. Mutating Cdt2^PIP weakens the interaction with PCNA, rendering CRL4^Cdt2 less effective in Cdt1 ubiquitination and leading to defects in Cdt1 degradation. The molecular mechanism we present suggests a new paradigm for bringing substrates to the CRL4-type ligase, where the substrate receptor and substrates bind to a common multivalent docking platform to enable subsequent ubiquitination.

Chromatin modifiers Mdm2 and RNF2 prevent RNA:DNA hybrids that impair DNA replication

The p53-Mdm2 system is key to tumor suppression. We have recently reported that p53 as well as Mdm2 are capable of supporting DNA replication fork progression. On the other hand, we found that Mdm2 is a modifier of chromatin, modulating polycomb repressor complex (PRC)-driven histone modifications. Here we show that, similar to Mdm2 knockdown, the depletion of PRC members impairs DNA synthesis, as determined in fiber assays. In particular, the ubiquitin ligase and PRC1 component RNF2/Ring1B is required to support DNA replication, similar to Mdm2. Moreover, the Ring finger domain of Mdm2 is not only essential for its ubiquitin ligase activity, but also for proper DNA replication. Strikingly, Mdm2 overexpression can rescue RNF2 depletion with regard to DNA replication fork progression, and vice versa, strongly suggesting that the two ubiquitin ligases perform overlapping functions in this context. H2A overexpression also rescues fork progression upon depletion of Mdm2 or RNF2, but only when the ubiquitination sites K118/K119 are present. Depleting the H2A deubiquitinating enzyme BAP1 reduces the fork rate, suggesting that both ubiquitination and deubiquitination of H2A are required to support fork progression. The depletion of Mdm2 elicits the accumulation of RNA/DNA hybrids, suggesting R-loop formation as a mechanism of impaired DNA replication. Accordingly, RNase H overexpression or the inhibition of the transcription elongation kinase CDK9 each rescues DNA replication upon depletion of Mdm2 or RNF2. Taken together, our results suggest that chromatin modification by Mdm2 and PRC1 ensures smooth DNA replication through the avoidance of R-loop formation.

The Tip of an Iceberg: Replication-Associated Functions of the Tumor Suppressor p53

The tumor suppressor p53 is a transcriptional factor broadly mutated in cancer. Most inactivating and gain of function mutations disrupt the sequence-specific DNA binding domain, which activates target genes. This is perhaps the main reason why most research has focused on the relevance of such transcriptional activity for the prevention or elimination of cancer cells. Notwithstanding, transcriptional regulation may not be the only mechanism underlying its role in tumor suppression and therapeutic responses. In the past, a direct role of p53 in DNA repair transactions that include the regulation of homologous recombination has been suggested. More recently, the localization of p53 at replication forks has been demonstrated and the effect of p53 on nascent DNA elongation has been explored. While some data sets indicate that the regulation of ongoing replication forks by p53 may be mediated by p53 targets such as MDM2 (murine double minute 2) and polymerase (POL) eta other evidences demonstrate that p53 is capable of controlling DNA replication by directly interacting with the replisome and altering its composition. In addition to discussing such findings, this review will also analyze the impact that p53-mediated control of ongoing DNA replication has on treatment responses and tumor suppressor abilities of this important anti-oncogene.

p53 Activity Results in DNA Replication Fork Processivity

p53 induces cell death upon DNA damage, but this may not confer all of its tumor suppressor activity. We report that p53 activation enhances the processivity of DNA replication, as monitored by multi-label fiber assays, whereas removal of p53 reduces fork progression. This is observed in tumor-derived U2OS cells but also in murine embryonic fibroblasts with heterozygous or homozygous p53 deletion and in freshly isolated thymocytes from mice with differential p53 status. Mdm2, a p53-inducible gene product, similarly supports DNA replication even in p53-deficient cells, suggesting that sustained Mdm2-expression is at least one of the mechanisms allowing p53 to prevent replicative stress. Thus, p53 helps to protect the genome during S phase, by preventing the occurrence of stalled or collapsed replication forks. These results expand p53's tumor-suppressive functions, adding to the ex-post model (elimination of damaged cells) an ex-ante activity; i.e., the prevention of DNA damage during replication.

P53 tumor suppressor is required for efficient execution of the death program following treatment with a cytotoxic limonoid obtained from Melia azedarach

This work examines the antitumor activity of an isomeric mixture (1), composed of the limonoids meliartenin and its interchangeable isomer 12-hydroxyamoorastatin. The results obtained showed that 1 displayed outstanding cytotoxic activity against CCRF-CEM, K562, A549 and HCT116 cells, with a highly selective effect on the latter, with an IC₅₀ value of 0.2 μM. Based on this finding, HCT116 cells were selected to study the mechanism of action of 1. Cell cycle analysis revealed that 1 induced sustained arrest in the S-phase, which was followed by the triggering of apoptotic cell death and reduced clonogenic capacity. This cytotoxicity was seen to be preceded by the upregulation of the tumor suppressor p53 and its target effector p21. In addition, it was found that p53 expression was required for efficient cell death induction, and thus that the toxicity of 1 relies mainly on p53-dependent mechanisms. Taken together, these findings position 1 as a potent antitumor agent, with potential for the development of novel chemotherapeutic drugs based on the induction of S-phase arrest.

p21: A Two-Faced Genome Guardian

Upon DNA damage or other stressors, the tumor suppressor p53 is activated, leading to transient expression of the cyclin-dependent kinase inhibitor (CKI) p21. This either triggers momentary G1 cell cycle arrest or leads to a chronic state of senescence or apoptosis, a form of genome guardianship. In the clinic, the presence of p21 has been considered an indicator of wildtype p53 activity. However, recent evidence suggests that p21 also acts as an oncogenic factor in a p53-deficient environment. Here, we discuss the controversial aspects of the two-faced involvement of p21 in cancer and speculate on how this new information may increase our understanding of its role in cancer pathogenesis. Prevailing notions indicate that p21 might also act as antiapoptotic agent, which may have relevant implications for future therapeutic strategies.

Mismatch repair proteins recruited to ultraviolet light-damaged sites lead to degradation of licensing factor Cdt1 in the G1 phase

Cdt1 is rapidly degraded by CRL4^Cdt2 E3 ubiquitin ligase after UV (UV) irradiation. Previous reports revealed that the nucleotide excision repair (NER) pathway is responsible for the rapid Cdt1-proteolysis. Here, we show that mismatch repair (MMR) proteins are also involved in the degradation of Cdt1 after UV irradiation in the G1 phase. First, compared with the rapid (within ∼15 min) degradation of Cdt1 in normal fibroblasts, Cdt1 remained stable for ∼30 min in NER-deficient XP-A cells, but was degraded within ∼60 min. The delayed degradation was also dependent on PCNA and CRL4^Cdt2. The MMR proteins Msh2 and Msh6 were recruited to the UV-damaged sites of XP-A cells in the G1 phase. Depletion of these factors with small interfering RNAs prevented Cdt1 degradation in XP-A cells. Similar to the findings in XP-A cells, depletion of XPA delayed Cdt1 degradation in normal fibroblasts and U2OS cells, and co-depletion of Msh6 further prevented Cdt1 degradation. Furthermore, depletion of Msh6 alone delayed Cdt1 degradation in both cell types. When Cdt1 degradation was attenuated by high Cdt1 expression, repair synthesis at the damaged sites was inhibited. Our findings demonstrate that UV irradiation induces multiple repair pathways that activate CRL4^Cdt2 to degrade its target proteins in the G1 phase of the cell cycle, leading to efficient repair of DNA damage.

Maintenance of Genome Integrity: How Mammalian Cells Orchestrate Genome Duplication by Coordinating Replicative and Specialized DNA Polymerases

Precise duplication of the human genome is challenging due to both its size and sequence complexity. DNA polymerase errors made during replication, repair or recombination are central to creating mutations that drive cancer and aging. Here, we address the regulation of human DNA polymerases, specifically how human cells orchestrate DNA polymerases in the face of stress to complete replication and maintain genome stability. DNA polymerases of the B-family are uniquely adept at accurate genome replication, but there are numerous situations in which one or more additional DNA polymerases are required to complete genome replication. Polymerases of the Y-family have been extensively studied in the bypass of DNA lesions; however, recent research has revealed that these polymerases play important roles in normal human physiology. Replication stress is widely cited as contributing to genome instability, and is caused by conditions leading to slowed or stalled DNA replication. Common Fragile Sites epitomize “difficult to replicate” genome regions that are particularly vulnerable to replication stress, and are associated with DNA breakage and structural variation. In this review, we summarize the roles of both the replicative and Y-family polymerases in human cells, and focus on how these activities are regulated during normal and perturbed genome replication.

Cyclin Kinase-independent role of p21CDKN1A in the promotion of nascent DNA elongation in unstressed cells

The levels of the cyclin-dependent kinase (CDK) inhibitor p21 are low in S phase and insufficient to inhibit CDKs. We show here that endogenous p21, instead of being residual, it is functional and necessary to preserve the genomic stability of unstressed cells. p21depletion slows down nascent DNA elongation, triggers permanent replication defects and promotes the instability of hard-to-replicate genomic regions, namely common fragile sites (CFS). The p21’s PCNA interacting region (PIR), and not its CDK binding domain, is needed to prevent the replication defects and the genomic instability caused by p21 depletion. The alternative polymerase kappa is accountable for such defects as they were not observed after simultaneous depletion of both p21 and polymerase kappa. Hence, in CDK-independent manner, endogenous p21 prevents a type of genomic instability which is not triggered by endogenous DNA lesions but by a dysregulation in the DNA polymerase choice during genomic DNA synthesis.

DOI:http://dx.doi.org/10.7554/eLife.18020.001

Cancer develops when cells in the body mutate in ways that allow them to rapidly grow and divide. To protect cells from becoming cancerous, various molecules act like guardians to prevent cells from dividing when their DNA is damaged, or if they are short of energy. Other guardian molecules monitor the DNA copying process to ensure that the newly-made DNA is as identical as possible to the original DNA template.

A protein called p21 belongs to the first group of guardian molecules: DNA damage triggers the production of p21, which prevents the cell from copying its DNA. This role relies on a section of the protein called the CDK binding domain. Cells that have already started to copy their genetic material also have low levels of p21.

Mansilla et al. used human cells to investigate whether p21 is also involved in the process of copying DNA. The experiments show that the low levels of p21 act to increase the speed at which the DNA is copied. This activity helps to ensure that all of the cell’s DNA is copied within the time available, including sections of DNA that are harder to copy because they are more fragile and prone to damage. This newly identified role does not involve the CDK binding domain, but instead requires a different section of the p21 protein known as the PCNA interacting region.

Mansilla et al. propose that p21 plays a dual role in protecting us from developing cancer. The PCNA interacting region is also found in other proteins that are involved in copying DNA. Therefore, a future challenge is to find out how these proteins interact with each other to ensure that cells accurately copy their DNA in a timely fashion.

DOI:http://dx.doi.org/10.7554/eLife.18020.002

SOX9 is targeted for proteasomal degradation by the E3 ligase FBW7 in response to DNA damage

SOX9 encodes a transcription factor that governs cell fate specification throughout development and tissue homeostasis. Elevated SOX9 is implicated in the genesis and progression of human tumors by increasing cell proliferation and epithelial-mesenchymal transition. We found that in response to UV irradiation or genotoxic chemotherapeutics, SOX9 is actively degraded in various cancer types and in normal epithelial cells, through a pathway independent of p53, ATM, ATR and DNA-PK. SOX9 is phosphorylated by GSK3β, facilitating the binding of SOX9 to the F-box protein FBW7α, an E3 ligase that functions in the DNA damage response pathway. The binding of FBW7α to the SOX9 K2 domain at T236-T240 targets SOX9 for subsequent ubiquitination and proteasomal destruction. Exogenous overexpression of SOX9 after genotoxic stress increases cell survival. Our findings reveal a novel regulatory mechanism for SOX9 stability and uncover a unique function of SOX9 in the cellular response to DNA damage. This new mechanism underlying a FBW7-SOX9 axis in cancer could have implications in therapy resistance.

Rad51 recombinase prevents Mre11 nuclease-dependent degradation and excessive PrimPol-mediated elongation of nascent DNA after UV irradiation

After UV irradiation, DNA polymerases specialized in translesion DNA synthesis (TLS) aid DNA replication. However, it is unclear whether other mechanisms also facilitate the elongation of UV-damaged DNA. We wondered if Rad51 recombinase (Rad51), a factor that escorts replication forks, aids replication across UV lesions. We found that depletion of Rad51 impairs S-phase progression and increases cell death after UV irradiation. Interestingly, Rad51 and the TLS polymerase polη modulate the elongation of nascent DNA in different ways, suggesting that DNA elongation after UV irradiation does not exclusively rely on TLS events. In particular, Rad51 protects the DNA synthesized immediately before UV irradiation from degradation and avoids excessive elongation of nascent DNA after UV irradiation. In Rad51-depleted samples, the degradation of DNA was limited to the first minutes after UV irradiation and required the exonuclease activity of the double strand break repair nuclease (Mre11). The persistent dysregulation of nascent DNA elongation after Rad51 knockdown required Mre11, but not its exonuclease activity, and PrimPol, a DNA polymerase with primase activity. By showing a crucial contribution of Rad51 to the synthesis of nascent DNA, our results reveal an unanticipated complexity in the regulation of DNA elongation across UV-damaged templates.

Effective intra-S checkpoint responses to UVC in primary human melanocytes and melanoma cell lines

The objective of this study was to assess potential functional attenuation or inactivation of the intra-S checkpoint during melanoma development. Proliferating cultures of skin melanocytes, fibroblasts, and melanoma cell lines were exposed to increasing fluences of UVC and intra-S checkpoint responses were quantified. Melanocytes displayed stereotypic intra-S checkpoint responses to UVC qualitatively and quantitatively equivalent to those previously demonstrated in skin fibroblasts. In comparison with fibroblasts, primary melanocytes displayed reduced UVC-induced inhibition of DNA strand growth and enhanced degradation of p21Waf1 after UVC, suggestive of enhanced bypass of UVC-induced DNA photoproducts. All nine melanoma cell lines examined, including those with activating mutations in BRAF or NRAS oncogenes, also displayed proficiency in activation of the intra-S checkpoint in response to UVC irradiation. The results indicate that bypass of oncogene-induced senescence during melanoma development was not associated with inactivation of the intra-S checkpoint response to UVC-induced DNA replication stress.

Expression of the p12 subunit of human DNA polymerase δ (Pol δ), CDK inhibitor p21(WAF1), Cdt1, cyclin A, PCNA and Ki-67 in relation to DNA replication in individual cells

We recently reported that the p12 subunit of human DNA polymerase δ (Pol δ4) is degraded by CRL4(Cdt2) which regulates the licensing factor Cdt1 and p21(WAF1) during the G1 to S transition. Presently, we performed multiparameter laser scanning cytometric analyses of changes in levels of p12, Cdt1 and p21(WAF1), detected immunocytochemically in individual cells, vis-à-vis the initiation and completion of DNA replication. The latter was assessed by pulse-labeling A549 cells with the DNA precursor ethynyl-2'-deoxyribose (EdU). The loss of p12 preceded the initiation of DNA replication and essentially all cells incorporating EdU were p12 negative. Completion of DNA replication and transition to G2 phase coincided with the re-appearance and rapid rise of p12 levels. Similar to p12 a decline of p21(WAF1) and Cdt1 was seen at the end of G1 phase and all DNA replicating cells were p21(WAF1) and Cdt1 negative. The loss of p21(WAF1) preceded that of Cdt1 and p12 and the disappearance of the latter coincided with the onset of DNA replication. Loss of p12 leads to conversion of Pol δ4 to its trimeric form, Pol δ3, so that the results provide strong support to the notion that Pol δ3 is engaged in DNA replication during unperturbed progression through the S phase of cell cycle. Also assessed was a correlation between EdU incorporation, likely reflecting the rate of DNA replication in individual cells, and the level of expression of positive biomarkers of replication cyclin A, PCNA and Ki-67 in these cells. Of interest was the observation of stronger correlation between EdU incorporation and expression of PCNA (r = 0.73) than expression of cyclin A (r = 0.47) or Ki-67 (r = 0.47).

Control of Trx1 redox state modulates protection against methyl methanesulfonate-induced DNA damage via stabilization of p21

Thioredoxin 1 (Trx1) is known to play an important role in protecting against cell death. However, the mechanism for control of Trx1 in cell death resulting from DNA damage has not been fully investigated. In this study, we used the DNA-damaging agent methyl methanesulfonate (MMS) to investigate the protective effects of Trx1 against DNA damage and cell death in HEK293 cells. We found that MMS application caused dose-dependent changes in the Trx1 redox state determined by redox western blotting. At lower concentrations, both reduced and oxidized Trx1 were observed, whereas the reduced band was fully oxidized at the higher concentration. Trx1 overexpression and small interfering RNA knockdown in cells revealed that reduced Trx1 after exposure to lower doses of MMS attenuated DNA damage, assessed by comet assay, and level of the DNA-damage marker histone γ-H2AX, possibly through scavenging intracellular ROS and an increase in p21 protein level via enhancing its stability. However, oxidized Trx1 lost its protective ability to DNA damage in response to higher concentration of MMS. Corresponding to the redox state control of Trx1, cell death induced by different dose of MMS was also found, by inhibiting phosphorylations of p38 and 4E-BP1. These results indicate that reduced Trx1 plays important protective roles against MMS-induced DNA damage and cell death, suggesting that cell protection is regulated by the intracellular redox state. Control of the redox state of Trx1 and its regulating proteins may offer a novel therapeutic strategy for the control of cancer.

The identification of translesion DNA synthesis regulators: Inhibitors in the spotlight

Over the past half-century, we have become increasingly aware of the ubiquity of DNA damage. Under the constant exposure to exogenous and endogenous genomic stress, cells must attempt to replicate damaged DNA. The encounter of replication forks with DNA lesions triggers several cellular responses, including the activation of translesion DNA synthesis (TLS), which largely depends upon specialized DNA polymerases with flexible active sites capable of accommodating bulky DNA lesions. A detrimental aspect of TLS is its intrinsic mutagenic nature, and thus the activity of the TLS polymerases must ideally be restricted to synthesis on damaged DNA templates. Despite their potential clinical importance in chemotherapy, TLS inhibitors have been difficult to identify since a direct assay designed to quantify genomic TLS events is still unavailable. Herein we discuss the methods that have been used to validate TLS inhibitors such as USP1, p21 and Spartan, highlighting research that has revealed their contribution to the control of DNA synthesis on damaged and undamaged templates.

Regulation of Rad6/Rad18 Activity During DNA Damage Tolerance

Replicative polymerases (pols) cannot accommodate damaged template bases, and these pols stall when such offenses are encountered during S phase. Rather than repairing the damaged base, replication past it may proceed via one of two DNA damage tolerance (DDT) pathways, allowing replicative DNA synthesis to resume. In translesion DNA synthesis (TLS), a specialized TLS pol is recruited to catalyze stable, yet often erroneous, nucleotide incorporation opposite damaged template bases. In template switching, the newly synthesized sister strand is used as a damage-free template to synthesize past the lesion. In eukaryotes, both pathways are regulated by the conjugation of ubiquitin to the PCNA sliding clamp by distinct E2/E3 pairs. Whereas monoubiquitination by Rad6/Rad18 mediates TLS, extension of this ubiquitin to a polyubiquitin chain by Ubc13-Mms2/Rad5 routes DDT to the template switching pathway. In this review, we focus on the monoubiquitination of PCNA by Rad6/Rad18 and summarize the current knowledge of how this process is regulated.

Identification of novel DNA-damage tolerance genes reveals regulation of translesion DNA synthesis by nucleophosmin

Cells cope with replication-blocking lesions via translesion DNA synthesis (TLS). TLS is carried out by low-fidelity DNA polymerases that replicate across lesions, thereby preventing genome instability at the cost of increased point mutations. Here we perform a two-stage siRNA-based functional screen for mammalian TLS genes and identify 17 validated TLS genes. One of the genes, NPM1, is frequently mutated in acute myeloid leukaemia (AML). We show that NPM1 (nucleophosmin) regulates TLS via interaction with the catalytic core of DNA polymerase-η (polη), and that NPM1 deficiency causes a TLS defect due to proteasomal degradation of polη. Moreover, the prevalent NPM1c+ mutation that causes NPM1 mislocalization in ~30% of AML patients results in excessive degradation of polη. These results establish the role of NPM1 as a key TLS regulator, and suggest a mechanism for the better prognosis of AML patients carrying mutations in NPM1.

Cells cope with replication-blocking DNA lesions by translesion DNA synthesis (TLS) polymerases, including polη. Here, the authors show that NPM1, a gene frequently mutated in acute myeloid leukaemia, protects polη from proteasomal degradation, and that NPM1 deficiency causes a TLS defect.

The fork and the kinase: a DNA replication tale from a CHK1 perspective

Replication fork progression is being continuously hampered by exogenously introduced and naturally occurring DNA lesions and other physical obstacles. Checkpoint kinase 1 (Chk1) is activated at replication forks that encounter damaged DNA. Subsequently, Chk1 inhibits the initiation of new replication factories and stimulates the firing of dormant origins (those in the vicinity of stalled forks). Chk1 also avoids fork collapse into DSBs (double strand breaks) and promotes fork elongation. At the molecular level, the current model considers stalled forks as the site of Chk1 activation and the nucleoplasm as the location where Chk1 phosphorylates target proteins. This model certainly serves to explain how Chk1 modulates origin firing, but how Chk1 controls the fate of stalled forks is less clear. Interestingly, recent reports demonstrating that Chk1 phosphorylates chromatin-bound proteins and even holds kinase-independent functions might shed light on how Chk1 contributes to the elongation of damaged DNA. Indeed, such findings have unveiled a puzzling connection between Chk1 and DNA lesion bypass, which might be central to promoting fork elongation and checkpoint attenuation. In summary, Chk1 is a multifaceted and versatile signaling factor that acts at ongoing forks and replication origins to determine the extent and quality of the cellular response to replication stress.

A mouse model uncovers LKB1 as an UVB-induced DNA damage sensor mediating CDKN1A (p21WAF1/CIP1) degradation

Exposure to ultraviolet (UV) radiation from sunlight accounts for 90% of the symptoms of premature skin aging and skin cancer. The tumor suppressor serine-threonine kinase LKB1 is mutated in Peutz-Jeghers syndrome and in a spectrum of epithelial cancers whose etiology suggests a cooperation with environmental insults. Here we analyzed the role of LKB1 in a UV-dependent mouse skin cancer model and show that LKB1 haploinsufficiency is enough to impede UVB-induced DNA damage repair, contributing to tumor development driven by aberrant growth factor signaling. We demonstrate that LKB1 and its downstream kinase NUAK1 bind to CDKN1A. In response to UVB irradiation, LKB1 together with NUAK1 phosphorylates CDKN1A regulating the DNA damage response. Upon UVB treatment, LKB1 or NUAK1 deficiency results in CDKN1A accumulation, impaired DNA repair and resistance to apoptosis. Importantly, analysis of human tumor samples suggests that LKB1 mutational status could be a prognostic risk factor for UV-induced skin cancer. Altogether, our results identify LKB1 as a DNA damage sensor protein regulating skin UV-induced DNA damage response.

Environmental insults are directly involved in cancer development. In particular, Ultraviolet (UV) radiation has been associated to the acquisition of different types skin cancer and premature skin aging. UV radiation causes modifications in the genetic material of cells (DNA) that if not repaired properly will lead to a mutated DNA (mutated genes) which might trigger the development of cancer. Understanding the molecular basis of the UV-induced DNA damage response is important to elucidate the mechanisms of skin homeostasis and tumorigenesis. Here we provide a UVB-induced skin cancer animal model showing that LKB1 tumor suppressor is also a DNA damage sensor. Importantly, the data suggest that reduced amounts of LKB1 protein in skin could be a risk factor for UV-induced skin carcinogenesis in humans.

The cell death and DNA damages caused by the Tet-On regulating HSV-tk/GCV suicide gene system in MCF-7 cells

Ganciclovir (GCV) affects the molecular mechanism of cell death and DNA damage by the rAAV (recombinant adeno-associated virus)-mediated Tet-On/HSV-tk/GCV suicide gene system in human breast cancer cell line MCF-7. A rAAV/TRE/Tet-On/HSV-tk combining a Tet-On regulating system and a suicide gene HSV-tk was used to transfect human breast cancer cell line MCF-7, and therapeutic effects on this system were studied. Afterwards, we used RT-PCR, western blotting, and a modified comet-assay to explore the potential mechanism of the HSV-tk/GCV suicide gene system in breast cancer treatments. MTT assay has shown that the cell number of GCV+rAAV+Dox group was significantly decreased compared with that of other groups after treatment and flow cytometric analysis detected that there was a substantial increase of S phase cells in this group, which means the HSV-tk/GCV suicide gene system probably works on the cell cycle. RT-PCR detected the expression level of p21 increased and PCNA had an opposite trend. Western blotting detected the protein expression of p21 and p53 increased and PCNA, CDK1, cyclin B decreased in GCV+rAAV+Dox group. The modified comet-assay shown that the very small extra fragments generated by the GCV+rAAV+Dox group treatment are visible as a small cloud extending from the comet in the direction of electrophoresis. The therapeutic mechanism of the HSV-tk/GCV suicide gene system on human breast cancer cell line MCF-7 is probably by upregulating the expression of p21 through a p53-dependent DNA damage signalling pathway, leading the decrease of protein expression of PCNA, cyclin B, CDK1 in MCF-7 cells and promoting the cell cycle arrest at G1/S phase. In summary, the HSV-tk/GCV suicide gene system arouses the death of MCF-7 cells from blocking the cell cycle and DNA damage.

Thymine DNA glycosylase is a CRL4Cdt2 substrate

The E3 ubiquitin ligase CRL4(Cdt2) targets proteins for destruction in S phase and after DNA damage by coupling ubiquitylation to DNA-bound proliferating cell nuclear antigen (PCNA). Coupling to PCNA involves a PCNA-interacting peptide (PIP) degron motif in the substrate that recruits CRL4(Cdt2) while binding to PCNA. In vertebrates, CRL4(Cdt2) promotes degradation of proteins whose presence in S phase is deleterious, including Cdt1, Set8, and p21. Here, we show that CRL4(Cdt2) targets thymine DNA glycosylase (TDG), a base excision repair enzyme that is involved in DNA demethylation. TDG contains a conserved and nearly perfect match to the PIP degron consensus. TDG is ubiquitylated and destroyed in a PCNA-, Cdt2-, and PIP degron-dependent manner during DNA repair in Xenopus egg extract. The protein can also be destroyed during DNA replication in this system. During Xenopus development, TDG first accumulates during gastrulation, and its expression is down-regulated by CRL4(Cdt2). Our results expand the group of vertebrate CRL4(Cdt2) substrates to include a bona fide DNA repair enzyme.

PIP degron proteins, substrates of CRL4Cdt2, and not PIP boxes, interfere with DNA polymerase η and κ focus formation on UV damage

Proliferating cell nuclear antigen (PCNA) is a well-known scaffold for many DNA replication and repair proteins, but how the switch between partners is regulated is currently unclear. Interaction with PCNA occurs via a domain known as a PCNA-Interacting Protein motif (PIP box). More recently, an additional specialized PIP box has been described, the « PIP degron », that targets PCNA-interacting proteins for proteasomal degradation via the E3 ubiquitin ligase CRL4^Cdt2. Here we provide evidence that CRL4^Cdt2-dependent degradation of PIP degron proteins plays a role in the switch of PCNA partners during the DNA damage response by facilitating accumulation of translesion synthesis DNA polymerases into nuclear foci. We show that expression of a nondegradable PIP degron (Cdt1) impairs both Pol η and Pol κ focus formation on ultraviolet irradiation and reduces cell viability, while canonical PIP box-containing proteins have no effect. Furthermore, we identify PIP degron-containing peptides from several substrates of CRL4^Cdt2 as efficient inhibitors of Pol η foci formation. By site-directed mutagenesis we show that inhibition depends on a conserved threonine residue that confers high affinity for PCNA-binding. Altogether these findings reveal an important regulative role for the CRL4^Cdt2 pathway in the switch of PCNA partners on DNA damage.