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

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.

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.

Current state of knowledge of human DNA polymerase eta protein structure and disease-causing mutations

POLη, encoded by the POLH gene, is a crucial protein for replicating damaged DNA and the most studied specialized translesion synthesis polymerases. Mutations in POLη are associated with cancer and the human syndrome xeroderma pigmentosum variant, which is characterized by extreme photosensitivity and an increased likelihood of developing skin cancers. The myriad of structural information about POLη is vast, covering dozens of different mutants, numerous crucial residues, domains, and posttranslational modifications that are essential for protein function within cells. Since POLη is key vital enzyme for cell survival, and mutations in this protein are related to aggressive diseases, understanding its structure is crucial for biomedical sciences, primarily due to its similarities with other Y-family polymerases and its potential as a targeted therapy-drug for tumors. This work provides an up-to-date review on structural aspects of the human POLη: from basic knowledge about critical residues and protein domains to its mutant variants, posttranslational modifications, and our current understanding of therapeutic molecules that target POLη. Thus, this review provides lessons about POLη's structure and gathers critical discussions and hypotheses that may contribute to understanding this protein's vital roles within the cells.

Mechanisms and Regulation of DNA-Protein Crosslink Repair During DNA Replication by SPRTN Protease

DNA-protein crosslinks (DPCs) are deleterious DNA lesions that occur when proteins are covalently crosslinked to the DNA by the action of variety of agents like reactive oxygen species, aldehydes and metabolites, radiation, and chemotherapeutic drugs. Unrepaired DPCs are blockades to all DNA metabolic processes. Specifically, during DNA replication, replication forks stall at DPCs and are vulnerable to fork collapse, causing DNA breakage leading to genome instability and cancer. Replication-coupled DPC repair involves DPC degradation by proteases such as SPRTN or the proteasome and the subsequent removal of DNA-peptide adducts by nucleases and canonical DNA repair pathways. SPRTN is a DNA-dependent metalloprotease that cleaves DPC substrates in a sequence-independent manner and is also required for translesion DNA synthesis following DPC degradation. Biallelic mutations in SPRTN cause Ruijs-Aalfs (RJALS) syndrome, characterized by hepatocellular carcinoma and segmental progeria, indicating the critical role for SPRTN and DPC repair pathway in genome maintenance. In this review, we will discuss the mechanism of replication-coupled DPC repair, regulation of SPRTN function and its implications in human disease and cancer.

Determination of DNA lesion bypass using a ChIP-based assay

DNA lesion bypass facilitates DNA synthesis across bulky DNA lesions, playing a critical role in DNA damage tolerance and cell survival after DNA damage. Assessing lesion bypass efficiency in the cell is important to better understanding of the mechanism of carcinogenesis and chemoresistance. Here we developed a chromatin immunoprecipitation (ChIP)-based method to measure lesion bypass activity across cisplatin-induced intrastrand crosslinks in cancer cells. DNA lesion bypass enables the replication to continue in the presence of replication blocks. Thus, the successful lesion bypass should result in the coexistence of DNA lesions and the newly synthesized DNA fragment opposite to this lesion. Using ChIP, we precipitated the cisplatin-induced intrastrand crosslinks, and quantitated the precipitated newly synthesized DNA that was labeled with BrdU. We validated this method on ovarian cancer cells with inhibited TLS activity. We then applied this method to show that ovarian cancer stem cells exhibit high lesion bypass activity relative to bulk cancer cells from the same cell line. In conclusion, this novel ChIP-based lesion bypass assay can detect the extent to which cisplatin-induced DNA lesions are bypassed in live cells. Our study may be applied more broadly to the study of other DNA lesions, as specific antibodies to these specific lesions are available.

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.

Structural Chromosome Instability: Types, Origins, Consequences, and Therapeutic Opportunities

Chromosomal instability (CIN) refers to an increased rate of acquisition of numerical and structural changes in chromosomes and is considered an enabling characteristic of tumors. Given its role as a facilitator of genomic changes, CIN is increasingly being considered as a possible therapeutic target, raising the question of which variables may convert CIN into an ally instead of an enemy during cancer treatment. This review discusses the origins of structural chromosome abnormalities and the cellular mechanisms that prevent and resolve them, as well as how different CIN phenotypes relate to each other. We discuss the possible fates of cells containing structural CIN, focusing on how a few cell duplication cycles suffice to induce profound CIN-mediated genome alterations. Because such alterations can promote tumor adaptation to treatment, we discuss currently proposed strategies to either avoid CIN or enhance CIN to a level that is no longer compatible with cell survival.

Multiple biochemical properties of the p53 molecule contribute to activation of polymerase iota-dependent DNA damage tolerance

We have previously reported that p53 decelerates nascent DNA elongation in complex with the translesion synthesis (TLS) polymerase ι (POLι) which triggers a homology-directed DNA damage tolerance (DDT) pathway to bypass obstacles during DNA replication. Here, we demonstrate that this DDT pathway relies on multiple p53 activities, which can be disrupted by TP53 mutations including those frequently found in cancer tissues. We show that the p53-mediated DDT pathway depends on its oligomerization domain (OD), while its regulatory C-terminus is not involved. Mutation of residues S315 and D48/D49, which abrogate p53 interactions with the DNA repair and replication proteins topoisomerase I and RPA, respectively, and residues L22/W23, which disrupt formation of p53-POLι complexes, all prevent this DDT pathway. Our results demonstrate that the p53-mediated DDT requires the formation of a DNA binding-proficient p53 tetramer, recruitment of such tetramer to RPA-coated forks and p53 complex formation with POLι. Importantly, our mutational analysis demonstrates that transcriptional transactivation is dispensable for the POLι-mediated DDT pathway, which we show protects against DNA replication damage from endogenous and exogenous sources.

ELM-the eukaryotic linear motif resource in 2020

The eukaryotic linear motif (ELM) resource is a repository of manually curated experimentally validated short linear motifs (SLiMs). Since the initial release almost 20 years ago, ELM has become an indispensable resource for the molecular biology community for investigating functional regions in many proteins. In this update, we have added 21 novel motif classes, made major revisions to 12 motif classes and added >400 new instances mostly focused on DNA damage, the cytoskeleton, SH2-binding phosphotyrosine motifs and motif mimicry by pathogenic bacterial effector proteins. The current release of the ELM database contains 289 motif classes and 3523 individual protein motif instances manually curated from 3467 scientific publications. ELM is available at: http://elm.eu.org.

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.

Unscheduled MRE11 activity triggers cell death but not chromosome instability in polymerase eta-depleted cells subjected to UV irradiation

The elimination of DNA polymerase eta (pol η) causes discontinuous DNA elongation and fork stalling in UV-irradiated cells. Such alterations in DNA replication are followed by S-phase arrest, DNA double-strand break (DSB) accumulation, and cell death. However, their molecular triggers and the relative timing of these events have not been fully elucidated. Here, we report that DSBs accumulate relatively early after UV irradiation in pol η-depleted cells. Despite the availability of repair pathways, DSBs persist and chromosome instability (CIN) is not detectable. Later on cells with pan-nuclear γH2AX and massive exposure of template single-stranded DNA (ssDNA), which indicate severe replication stress, accumulate and such events are followed by cell death. Reinforcing the causal link between the accumulation of pan-nuclear ssDNA/γH2AX signals and cell death, downregulation of RPA increased both replication stress and the cell death of pol η-deficient cells. Remarkably, DSBs, pan-nuclear ssDNA/γH2AX, S-phase arrest, and cell death are all attenuated by MRE11 nuclease knockdown. Such results suggest that unscheduled MRE11-dependent activities at replicating DNA selectively trigger cell death, but not CIN. Together these results show that pol η-depletion promotes a type of cell death that may be attractive as a therapeutic tool because of the lack of CIN.

Regulation of the error-prone DNA polymerase Polκ by oncogenic signaling and its contribution to drug resistance

The DNA polymerase Polκ plays a key role in translesion synthesis, an error-prone replication mechanism. Polκ is overexpressed in various tumor types. Here, we found that melanoma and lung and breast cancer cells experiencing stress from oncogene inhibition up-regulated the expression of Polκ and shifted its localization from the cytoplasm to the nucleus. This effect was phenocopied by inhibition of the kinase mTOR, by induction of ER stress, or by glucose deprivation. In unstressed cells, Polκ is continually transported out of the nucleus by exportin-1. Inhibiting exportin-1 or overexpressing Polκ increased the abundance of nuclear-localized Polκ, particularly in response to the BRAF^V600E-targeted inhibitor vemurafenib, which decreased the cytotoxicity of the drug in BRAF^V600E melanoma cells. These observations were analogous to how Escherichia coli encountering cell stress and nutrient deprivation can up-regulate and activate DinB/pol IV, the bacterial ortholog of Polκ, to induce mutagenesis that enables stress tolerance or escape. However, we found that the increased expression of Polκ was not excessively mutagenic, indicating that noncatalytic or other functions of Polκ could mediate its role in stress responses in mammalian cells. Repressing the expression or nuclear localization of Polκ might prevent drug resistance in some cancer cells.

The crosstalk between trace elements with DNA damage response, repair, and oxidative stress in cancer

DNA damage response (DDR) is a regulatory system responsible for maintaining genome integrity and stability, which can sense and transduce DNA damage signals. The severity of damage appears to determine DDRs, which can include damage repair, cell-cycle arrest, and apoptosis. Furthermore, defective components in DNA damage and repair machinery are an underlying cause for the development and progression of various types of cancers. Increasing evidence indicates that there is an association between trace elements and DDR/repair mechanisms. In fact, trace elements seem to affect mediators of DDR. Besides, it has been revealed that oxidative stress (OS) and trace elements are associated with cancer development. In this review, we discuss the role of some critical trace elements in the risk of cancer. In addition, we provide a brief introduction on DDR and OS in cancer. Finally, we will further review the interactions between some important trace elements including selenium, zinc, chromium, cadmium, and arsenic, and DDR, and OS in cancer.

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.

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.

Beyond interstrand crosslinks repair: contribution of FANCD2 and other Fanconi Anemia proteins to the replication of DNA

Biallelic mutations of FANCD2 and other components of the Fanconi Anemia (FA) pathway cause a disease characterized by bone marrow failure, cancer predisposition and a striking sensitivity to agents that induce crosslinks between the two complementary DNA strands (inter-strand crosslinks-ICL). Such genotoxins were used to characterize the contribution of the FA pathway to the genomic stability of cells, thus unravelling the biological relevance of ICL repair in the context of the disease. Notwithstanding this, whether the defect in ICL repair as the sole trigger for the multiple physiological alterations observed in FA patients is still under investigation. Remarkably, ICL-independent functions of FANCD2 and other components of the FA pathway were recently reported. FANCD2 contributes to the processing of very challenging double strand ends (DSEs: one ended Double Strand Breaks -DSBs- created during DNA replication). Other ICL-independent functions of FANCD2 include prevention of DNA breakage at stalled replication forks and facilitation of chromosome segregation at the end of M phase. The current understanding of replication-associated functions of FANCD2 and its relevance for the survival of genomically stable cells is herein discussed.

Epstein-Barr Virus Hijacks DNA Damage Response Transducers to Orchestrate Its Life Cycle

The Epstein–Barr virus (EBV) is a ubiquitous virus that infects most of the human population. EBV infection is associated with multiple human cancers, including Burkitt’s lymphoma, Hodgkin’s lymphoma, a subset of gastric carcinomas, and almost all undifferentiated non-keratinizing nasopharyngeal carcinoma. Intensive research has shown that EBV triggers a DNA damage response (DDR) during primary infection and lytic reactivation. The EBV-encoded viral proteins have been implicated in deregulating the DDR signaling pathways. The consequences of DDR inactivation lead to genomic instability and promote cellular transformation. This review summarizes the current understanding of the relationship between EBV infection and the DDR transducers, including ATM (ataxia telangiectasia mutated), ATR (ATM and Rad3-related), and DNA-PK (DNA-dependent protein kinase), and discusses how EBV manipulates the DDR signaling pathways to complete the replication process of viral DNA during lytic reactivation.

ACCIDENTAL DUPLICATION: Beyond interstrand crosslinks repair: Contribution of FANCD2 and other Fanconi Anemia proteins to the replication of DNA

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/ 10.1016/j.mrfmmm.2017.09.006. This duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

The NuRD complex-mediated p21 suppression facilitates chemoresistance in BRCA-proficient breast cancer

The Mi-2/nucleosome remodeling and deacetylase (NuRD) complex play a role in silencing gene expression. CHD4, the core component of the NuRD complex, which cooperates with histone deacetylase in reducing tumor suppressor genes (TSGs). To dissect the mechanisms underlying cancer promotion, we clarify the role of CHD4 in cyclin-dependent kinase inhibitor protein p21. Here, our data indicates that CHD4 deficiency impairs the recruitments of HDAC1 to the p21 promoter. ~ 300bp proximal promoter region is responsible for CHD4-HDAC1 axis-mediated p21 transcriptional activity. For identifying the role of anti-cancer drug response, knockdown of p21 overcomes cisplatin and poly-(ADP-ribose) polymerase (PARP) inhibitor-mediated growth suppression in CHD4-depleted cells. Consistent with in vitro data, tissue of patients and bioinformatics approach also showed positive correlation between CHD4 and p21. Overall, our findings not only identify that CHD4 deficiency preferentially impairs cell survival via increasing the level of p21, but also establishes targeting CHD4 as a potential therapeutic implication in BRCA-proficient breast cancer treatment.

Role of specialized DNA polymerases in the limitation of replicative stress and DNA damage transmission

Replication stress is a strong and early driving force for genomic instability and tumor development. Beside replicative DNA polymerases, an emerging group of specialized DNA polymerases is involved in the technical assistance of the replication machinery in order to prevent replicative stress and its deleterious consequences. During S-phase, altered progression of the replication fork by endogenous or exogenous impediments induces replicative stress, causing cells to reach mitosis with genomic regions not fully duplicated. Recently, specific mechanisms to resolve replication intermediates during mitosis with the aim of limiting DNA damage transmission to daughter cells have been identified. In this review, we detail the two major actions of specialized DNA polymerases that limit DNA damage transmission: the prevention of replicative stress by non-B DNA replication and the recovery of stalled replication forks.

DNA Fiber Analysis: Mind the Gap!

Understanding the mechanisms of replication stress response following genotoxic stress induction is rapidly emerging as a central theme in cell survival and human disease. The DNA fiber assay is one of the most powerful tools to study alterations in replication fork dynamics genome-wide at single-molecule resolution. This approach relies on the ability of many organisms to incorporate thymidine analogs into replicating DNA and is widely used to study how genotoxic agents perturb DNA replication. Here, we review different approaches available to prepare DNA fibers and discuss important limitations of each approach. We also review how DNA fiber analysis can be used to shed light upon several replication parameters including fork progression, restart, termination, and new origin firing. Next, we discuss a modified DNA fiber protocol to monitor the presence of single-stranded DNA (ssDNA) gaps on ongoing replication forks. ssDNA gaps are very common intermediates of several replication stress response mechanisms, but they cannot be detected by standard DNA fiber approaches due to the resolution limits of this technique. We discuss a novel strategy that relies on the use of an ssDNA-specific endonuclease to nick the ssDNA gaps and generate shorter DNA fibers that can be used as readout for the presence of ssDNA gaps. Finally, we describe a follow-up DNA fiber approach that can be used to study how ssDNA gaps are repaired postreplicatively.

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

Association Between Single Nucleotide Polymorphisms in DNA Polymerase Kappa Gene and Breast Cancer Risk in Chinese Han Population: A STROBE-Compliant Observational Study

DNA polymerases are responsible for ensuring stability of the genome and avoiding genotoxicity caused by a variety of factors during DNA replication. Consequently, these proteins have been associated with an increased cancer risk. DNA polymerase kappa (POLK) is a specialized DNA polymerase involved in translesion DNA synthesis (TLS) that allows DNA synthesis over the damaged DNA. Recently, some studies investigated relationships between POLK polymorphisms and cancer risk, but the role of POLK genetic variants in breast cancer (BC) remains to be defined. In this study, we aimed to evaluate the effects of POLK polymorphisms on BC risk.We used the Sequenom MassARRAY method to genotype 3 single nucleotide polymorphisms (SNPs) in POLK (rs3213801, rs10077427, and rs5744533), in order to determine the genotypes of 560 BC patients and 583 controls. The association of genotypes and BC was assessed by computing the odds ratio (OR) and 95% confidence intervals (95% CIs) from logistic regression analyses.We found a statistically significant difference between patient and control groups in the POLK rs10077427 genotypic groups, excluding the recessive model. A positive correlation was also found between positive progesterone receptor (PR) status, higher Ki67 index, and rs10077427 polymorphism. For rs5744533 polymorphism, the codominant, dominant, and allele models frequencies were significantly higher in BC patients compared to healthy controls. Furthermore, our results indicated that rs5744533 SNP has a protective role in the postmenopausal women. However, we failed to find any associations between rs3213801 polymorphism and susceptibility to BC.Our results indicate that POLK polymorphisms may influence the risk of developing BC, and, because of this, may serve as a prognostic biomarker among Chinese women.

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

Fanconi Anemia (FA) is a rare autosomal recessive disorder characterized by hypersensitivity to inter-strand crosslinks (ICLs). FANCD2, a central factor of the FA pathway, is essential for the repair of double strand breaks (DSBs) generated during fork collapse at ICLs. While lesions different from ICLs can also trigger fork collapse, the contribution of FANCD2 to the resolution of replication-coupled DSBs generated independently from ICLs is unknown. Intriguingly, FANCD2 is readily activated after UV irradiation, a DNA-damaging agent that generates predominantly intra-strand crosslinks but not ICLs. Hence, UV irradiation is an ideal tool to explore the contribution of FANCD2 to the DNA damage response triggered by DNA lesions other than ICL repair. Here we show that, in contrast to ICL-causing agents, UV radiation compromises cell survival independently from FANCD2. In agreement, FANCD2 depletion does not increase the amount of DSBs generated during the replication of UV-damaged DNA and is dispensable for UV-induced checkpoint activation. Remarkably however, FANCD2 protects UV-dependent, replication-coupled DSBs from aberrant processing by non-homologous end joining, preventing the accumulation of micronuclei and chromatid aberrations including non-homologous chromatid exchanges. Hence, while dispensable for cell survival, FANCD2 selectively safeguards chromosomal stability after UV-triggered replication stress.