DNA damage and decisions: CtIP coordinates DNA repair and cell cycle checkpoints

Drivers of centrosome abnormalities: Senescence progression and tumor immune escape

Centrosome abnormalities are a distinguishing feature of cancer and play a role in the aging process. Cancer cells may evade the immune system by activating immune checkpoints, altering their surrounding microenvironment, abnormalities in antigen presentation and recognition, and metabolic reprogramming to inhibit T-cell activity, allowing cancer cells to survive and spread within the host. When the centrosomes are abnormally shaped or numbered, mitotic errors can occur, cellular senescence occurs, cell death occurs, genomic instability occurs, and aneuploidy forms, resulting in diseases such as cancer. The present study is exploring the strategy of research progress in which centrosome abnormalities contribute to the aging process in various different ways as well as fuel immune escape from cancer cells, providing a new direction for cancer immunotherapy.

Targeting DNA Damage Repair to Enhance Antitumor Immunity in Radiotherapy: Mechanisms and Opportunities

Radiotherapy is a standard cancer treatment that involves the induction of DNA damage. DNA damage repair (DDR) pathways maintain genomic integrity and make tumors resistant to radiotherapy and certain chemotherapies. In turn, DDR dysfunction results in cumulative DNA damage, leading to increased sensitivity for antitumor treatment. Moreover, radiotherapy has been shown to trigger antitumor immunity. Currently, immunotherapy has become a new and widely used standard strategy for treating a broad spectrum of tumor types. Notably, recent studies have demonstrated that DDR pathways play important roles in driving the response to immunotherapy. Herein, we review and discuss how DDR affects antitumor immunity induced by radiotherapy. Furthermore, we summarize the development of strategies for combining DDR inhibitors with radiotherapy and/or immunotherapy to enhance their efficacy against cancers.

CtIP regulates G2/M transition and bipolar spindle assembly during mouse oocyte meiosis

CtBP-interacting protein (CtIP) is known for its multifaceted roles in DNA repair and genomic stability, directing the homologous recombination-mediated DNA double-stranded break repair pathway via DNA end resection, an essential error-free repair process vital for genome stability. Mammalian oocytes are highly prone to DNA damage accumulation due to prolonged G2/prophase arrest. Here, we explore the functions of CtIP in meiotic cell cycle regulation via a mouse oocyte model. Depletion of CtIP by siRNA injection results in delayed germinal vesicle breakdown and failed polar body extrusion. Mechanistically, CtIP deficiency increases DNA damage and decreases the expression and nuclear entry of CCNB1, resulting in marked impairment of meiotic resumption, which can be rescued by exogenous CCNB1 overexpression. Furthermore, depletion of CtIP disrupts microtubule-organizing centers coalescence at spindle poles as indicated by failed accumulation of γ-tubulin, p-Aurora kinase A, Kif2A, and TPX2, leading to abnormal spindle assembly and prometaphase arrest. These results provide valuable insights into the important roles of CtIP in the G2/M checkpoint and spindle assembly in mouse oocyte meiotic cell cycle regulation.

DNA binding and bridging by human CtIP in the healthy and diseased states

The human DNA repair factor CtIP helps to initiate the resection of double-stranded DNA breaks for repair by homologous recombination, in part through its ability to bind and bridge DNA molecules. However, CtIP is a natively disordered protein that bears no apparent similarity to other DNA-binding proteins and so the structural basis for these activities remains unclear. In this work, we have used bulk DNA binding, single molecule tracking, and DNA bridging assays to study wild-type and variant CtIP proteins to better define the DNA binding domains and the effects of mutations associated with inherited human disease. Our work identifies a monomeric DNA-binding domain in the C-terminal region of CtIP. CtIP binds non-specifically to DNA and can diffuse over thousands of nucleotides. CtIP-mediated bridging of distant DNA segments is observed in single-molecule magnetic tweezers experiments. However, we show that binding alone is insufficient for DNA bridging, which also requires tetramerization via the N-terminal domain. Variant CtIP proteins associated with Seckel and Jawad syndromes display impaired DNA binding and bridging activities. The significance of these findings in the context of facilitating DNA break repair is discussed.

Modelling DNA damage-repair and beyond

The paper presents a review of mechanistic modelling studies of DNA damage and DNA repair, and consequences to follow in mammalian cell nucleus. We hypothesize DNA deletions are consequences of repair of double strand breaks leading to the modifications of genome that play crucial role in long term development of genetic inheritance and diseases. The aim of the paper is to review formation mechanisms underlying naturally occurring DNA deletions in the human genome and their potential relevance for bridging the gap between induced DNA double strand breaks and deletions in damaged human genome from endogenous and exogenous events. The model of the cell nucleus presented enables simulation of DNA damage at molecular level identifying the spectrum of damage induced in all chromosomal territories and loops. Our mechanistic modelling of DNA repair for double stand breaks (DSB), single strand breaks (SSB) and base damage (BD), shows the complexity of DNA damage is responsible for the longer repair times and the reason for the biphasic feature of mammalian cells repair curves. In the absence of experimentally determined data, the mechanistic model of repair predicts the in vivo rate constants for the proteins involved in the repair of DSB, SSB, and of BD.

Increasing DNA damage sensitivity through corylin-mediated inhibition of homologous recombination

BACKGROUND

DNA repair allows the survival of cancer cells. Therefore, the development of DNA repair inhibitors is a critical need for sensitizing cancers to chemoradiation. Sae2CtIP has specific functions in initiating DNA end resection, as well as coordinating cell cycle checkpoints, and it also greatly interacts with the DDR at different levels.

RESULTS

In this study, we demonstrated that corylin, a potential sensitizer, causes deficiencies in DNA repair and DNA damage checkpoints in yeast cells. More specifically, corylin increases DNA damage sensitivity through the Sae2-dependent pathway and impairs the activation of Mec1-Ddc2, Rad53-p and γ-H2A. In breast cancer cells, corylin increases apoptosis and reduces proliferation following Dox treatment by inhibiting CtIP. Xenograft assays showed that treatment with corylin combined with Dox significantly reduced tumor growth in vivo.

CONCLUSIONS

Our findings herein delineate the mechanisms of action of corylin in regulating DNA repair and indicate that corylin has potential long-term clinical utility as a DDR inhibitor.

New Discoveries on Protein Recruitment and Regulation during the Early Stages of the DNA Damage Response Pathways

Maintaining genomic stability and properly repairing damaged DNA is essential to staying healthy and preserving cellular homeostasis. The five major pathways involved in repairing eukaryotic DNA include base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), non-homologous end joining (NHEJ), and homologous recombination (HR). When these pathways do not properly repair damaged DNA, genomic stability is compromised and can contribute to diseases such as cancer. It is essential that the causes of DNA damage and the consequent repair pathways are fully understood, yet the initial recruitment and regulation of DNA damage response proteins remains unclear. In this review, the causes of DNA damage, the various mechanisms of DNA damage repair, and the current research regarding the early steps of each major pathway were investigated.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Development and validation of a novel DNA damage repair-related long non-coding RNA signature in predicting prognosis, immunity, and drug sensitivity in uterine corpus endometrial carcinoma

Background

DNA damage response (DDR) confer resistance to chemoradiotherapy in cancer cells. However, the role of DDR-related lncRNAs (DRLs) in uterine corpus endometrial carcinoma (UCEC) is poorly understood. In this study, we aimed to identify a DRL-related prognostic signature that could guide the clinical treatment of UCEC.

Methods

We extracted transcriptome and clinical data of patients with UCEC from The Cancer Genome Atlas (TCGA) database and identified DRLs using Spearman correlation analysis. Univariate and multivariate Cox analyses were used to determine candidate prognostic DRLs. The samples were randomly divided into training and test cohorts in a 1:1 ratio. A DRL-related risk signature was constructed from the training cohort data using the least absolute shrinkage and selection operator (LASSO) algorithm, and validated using the test and entire cohorts. Subsequently, a prognostic nomogram was developed using a multivariate Cox regression analysis. The functional annotation, immune microenvironment, tumor mutation burden (TMB), immune checkpoint blockade (ICB) efficacy, and drug sensitivity were also comprehensively analyzed between different risk groups. Finally, the function of AC019069.1 was validated in vitro.

Results

A novel risk signature was developed based on nine DRLs. The risk score efficiently predicted the prognosis of patients with UCEC. Based on the median risk score, two subgroups were identified. The DDR-related pathways were upregulated in the high-risk group. Additionally, high-risk patients have low immune activity, poor response to ICB, and weak sensitivity to chemotherapeutic agents, possibly because of the proficient DDR system. Finally, we demonstrated AC019069.1 could promote cell proliferation, decrease apoptosis and maintain genome stability of UCEC cells.

Conclusions

The developed DRL-related signature can predict the prognosis, immune microenvironment, immunotherapy, and chemoradiotherapy responsiveness of UCEC. Our study also revealed the potential value of DDR-targeted therapy in treating high-risk patients with UCEC.

Cross-talk between DNA damage response and the central carbon metabolic network underlies selective vulnerability of Purkinje neurons in ataxia-telangiectasia

Cerebellar ataxia is often the first and irreversible outcome in the disease of ataxia-telangiectasia (A-T), as a consequence of selective cerebellar Purkinje neuronal degeneration. A-T is an autosomal recessive disorder resulting from the loss-of-function mutations of the ataxia-telangiectasia-mutated ATM gene. Over years of research, it now becomes clear that functional ATM-a serine/threonine kinase protein product of the ATM gene-plays critical roles in regulating both cellular DNA damage response and central carbon metabolic network in multiple subcellular locations. The key question arises is how cerebellar Purkinje neurons become selectively vulnerable when all other cell types in the brain are suffering from the very same defects in ATM function. This review intended to comprehensively elaborate the unexpected linkages between these two seemingly independent cellular functions and the regulatory roles of ATM involved, their integrated impacts on both physical and functional properties, hence the introduction of selective vulnerability to Purkinje neurons in the disease will be addressed.

Ubiquitination Links DNA Damage and Repair Signaling to Cancer Metabolism

Changes in the DNA damage response (DDR) and cellular metabolism are two important factors that allow cancer cells to proliferate. DDR is a set of events in which DNA damage is recognized, DNA repair factors are recruited to the site of damage, the lesion is repaired, and cellular responses associated with the damage are processed. In cancer, DDR is commonly dysregulated, and the enzymes associated with DDR are prone to changes in ubiquitination. Additionally, cellular metabolism, especially glycolysis, is upregulated in cancer cells, and enzymes in this metabolic pathway are modulated by ubiquitination. The ubiquitin-proteasome system (UPS), particularly E3 ligases, act as a bridge between cellular metabolism and DDR since they regulate the enzymes associated with the two processes. Hence, the E3 ligases with high substrate specificity are considered potential therapeutic targets for treating cancer. A number of small molecule inhibitors designed to target different components of the UPS have been developed, and several have been tested in clinical trials for human use. In this review, we discuss the role of ubiquitination on overall cellular metabolism and DDR and confirm the link between them through the E3 ligases NEDD4, APC/C^CDH1, FBXW7, and Pellino1. In addition, we present an overview of the clinically important small molecule inhibitors and implications for their practical use.

Multifaceted Influence of Histone Deacetylases on DNA Damage Repair: Implications for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most commonly diagnosed cancers and a leading cause of cancer-related mortality worldwide, but its pathogenesis remains largely unknown. Nevertheless, genomic instability has been recognized as one of the facilitating characteristics of cancer hallmarks that expedites the acquisition of genetic diversity. Genomic instability is associated with a greater tendency to accumulate DNA damage and tumor-specific DNA repair defects, which gives rise to gene mutations and chromosomal damage and causes oncogenic transformation and tumor progression. Histone deacetylases (HDACs) have been shown to impair a variety of cellular processes of genome stability, including the regulation of DNA damage and repair, reactive oxygen species generation and elimination, and progression to mitosis. In this review, we provide an overview of the role of HDAC in the different aspects of DNA repair and genome instability in HCC as well as the current progress on the development of HDAC-specific inhibitors as new cancer therapies.

SIAH2 regulates DNA end resection and replication fork recovery by promoting CtIP ubiquitination

Human CtIP maintains genomic integrity primarily by promoting 5′ DNA end resection, an initial step of the homologous recombination (HR). A few mechanisms have been suggested as to how CtIP recruitment to damage sites is controlled, but it is likely that we do not yet have full understanding of the process. Here, we provide evidence that CtIP recruitment and functioning are controlled by the SIAH2 E3 ubiquitin ligase. We found that SIAH2 interacts and ubiquitinates CtIP at its N-terminal lysine residues. Mutating the key CtIP lysine residues impaired CtIP recruitment to DSBs and stalled replication forks, DSB end resection, overall HR repair capacity of cells, and recovery of stalled replication forks, suggesting that the SIAH2-induced ubiquitination is important for relocating CtIP to sites of damage. Depleting SIAH2 consistently phenocopied these results. Overall, our work suggests that SIAH2 is a new regulator of CtIP and HR repair, and emphasizes that SIAH2-mediated recruitment of the CtIP is an important step for CtIP’s function during HR repair.

CtIP Regulates Mitotic Spindle Assembly by Modulating the TPX2-Aurora A Signaling Axis

CtBP-interacting protein (CtIP) plays a critical role in controlling the homologous recombination-mediated DNA double-stranded break (DSB) repair pathway through DNA end resection, and recent studies suggest that it also plays a role in mitosis. However, the mechanism by which CtIP contributes to mitosis regulation remains elusive. Here, we show that depletion of CtIP leads to a delay in anaphase progression resulting in misaligned chromosomes, an aberrant number of centrosomes, and defects in chromosome segregation. Additionally, we demonstrate that CtIP binds and colocalizes with Targeting protein for Xklp2 (TPX2) during mitosis to regulate the recruitment of TPX2 to the spindle poles. Furthermore, depletion of CtIP resulted in both a lower concentration of Aurora A, its downstream target, and very low microtubule intensity at the spindle poles, suggesting an important role for the CtIP-TPX2-Auroa A complex in microtubule dynamics at the centrosomal spindles. Our findings reveal a novel function of CtIP in regulating spindle dynamics through interactions with TPX2 and indicate that CtIP is involved in the proper execution of the mitotic program, where deregulation may lead to chromosomal instability.

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Polymerase ζ is Involved in Mitochondrial DNA Maintenance Processes in Concert with APE1 Activity

Mitochondrial DNA (mtDNA) damaged by reactive oxygen species (ROS) triggers so far poorly understood processes of mtDNA maintenance that are coordinated by a complex interplay among DNA repair, DNA degradation, and DNA replication. This study was designed to identify the proteins involved in mtDNA maintenance by applying a special long-range PCR, reflecting mtDNA integrity in the minor arc. A siRNA screening of literature-based candidates was performed under conditions of enforced oxidative phosphorylation revealing the functional group of polymerases and therein polymerase ζ (POLZ) as top hits. Thus, POLZ knockdown caused mtDNA accumulation, which required the activity of the base excision repair (BER) nuclease APE1, and was followed by compensatory mtDNA replication determined by the single-cell mitochondrial in situ hybridization protocol (mTRIP). Quenching reactive oxygen species (ROS) in mitochondria unveiled an additional, ROS-independent involvement of POLZ in the formation of a typical deletion in the minor arc region. Together with data demonstrating the localization of POLZ in mitochondria, we suggest that POLZ plays a significant role in mtDNA turnover, particularly under conditions of oxidative stress.

BMI-1 regulates DNA end resection and homologous recombination repair

BMI-1 is an essential regulator of transcriptional silencing during development. Recently, the role of BMI-1 in the DNA damage response has gained much attention, but the exact mechanism of how BMI-1 participates in the process is unclear. Here, we establish a role for BMI-1 in the repair of DNA double-strand breaks by homologous recombination (HR), where it promotes DNA end resection. Mechanistically, BMI-1 mediates DNA end resection by facilitating the recruitment of CtIP, thus allowing RPA and RAD51 accumulation at DNA damage sites. Interestingly, treatment with transcription inhibitors rescues the DNA end resection defects of BMI-1-depleted cells, suggesting BMI-1-dependent transcriptional silencing mediates DNA end resection. Moreover, we find that H2A ubiquitylation at K119 (H2AK119ub) promotes end resection. Taken together, our results identify BMI-1-mediated transcriptional silencing and promotion of H2AK119ub deposition as essential regulators of DNA end resection and thus the progression of HR.

Interactions between miRNAs and Double-Strand Breaks DNA Repair Genes, Pursuing a Fine-Tuning of Repair

The repair of DNA damage is a crucial process for the correct maintenance of genetic information, thus, allowing the proper functioning of cells. Among the different types of lesions occurring in DNA, double-strand breaks (DSBs) are considered the most harmful type of lesion, which can result in significant loss of genetic information, leading to diseases, such as cancer. DSB repair occurs through two main mechanisms, called non-homologous end joining (NHEJ) and homologous recombination repair (HRR). There is evidence showing that miRNAs play an important role in the regulation of genes acting in NHEJ and HRR mechanisms, either through direct complementary binding to mRNA targets, thus, repressing translation, or by targeting other genes involved in the transcription and activity of DSB repair genes. Therefore, alteration of miRNA expression has an impact on the ability of cells to repair DSBs, which, in turn, affects cancer therapy sensitivity. This latter gives account of the importance of miRNAs as regulators of NHEJ and HRR and places them as a promising target to improve cancer therapy. Here, we review recent reports demonstrating an association between miRNAs and genes involved in NHEJ and HRR. We employed the Web of Science search query TS (“gene official symbol/gene aliases*” AND “miRNA/microRNA/miR-”) and focused on articles published in the last decade, between 2010 and 2021. We also performed a data analysis to represent miRNA–mRNA validated interactions from TarBase v.8, in order to offer an updated overview about the role of miRNAs as regulators of DSB repair.

The UBE2D ubiquitin conjugating enzymes: Potential regulatory hubs in development, disease and evolution

Ubiquitination of cellular proteins plays critical roles in key signalling pathways and in the regulation of protein turnover in eukaryotic cells. E2 ubiquitin conjugating enzymes function as essential intermediates in ubiquitination reactions by acting as ubiquitin donors for the E3 ubiquitin ligase enzymes that confer substrate specificity. The members of the UBE2D family of E2 enzymes are involved in regulating signalling cascades through ubiquitination of target proteins that include receptor tyrosine kinases (RTKs) and components of the Hedgehog, TGFβ and NFκB pathways. UBE2D enzymes also function in transcriptional control by acting as donors for ubiquitination of histone tails by the Polycomb protein Ring1B and the DNA methylation regulator UHRF1 as well as having roles in DNA repair and regulation of the level of the tumour suppressor p53. Here we review the functional roles and mechanisms of regulation of the UBE2D proteins including recent evidence that regulation of the level of UBE2D3 is critical for controlling ubiquitination of specific targets during development. Cellular levels of UBE2D3 have been shown to be regulated by phosphorylation, which affects folding of the protein, reducing its stability. Specific variations in the otherwise highly conserved UBE2D3 protein sequence in amniotes and in a subgroup of teleost fishes, the Acanthomorpha, suggest that the enzyme has had important roles during vertebrate evolution.

A non-canonical, interferon-independent signaling activity of cGAMP triggers DNA damage response signaling

Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP), produced by cyclic GMP-AMP synthase (cGAS), stimulates the production of type I interferons (IFN). Here we show that cGAMP activates DNA damage response (DDR) signaling independently of its canonical IFN pathways. Loss of cGAS dampens DDR signaling induced by genotoxic insults. Mechanistically, cGAS activates DDR in a STING-TBK1-dependent manner, wherein TBK1 stimulates the autophosphorylation of the DDR kinase ATM, with the consequent activation of the CHK2-p53-p21 signal transduction pathway and the induction of G1 cell cycle arrest. Despite its stimulatory activity on ATM, cGAMP suppresses homology-directed repair (HDR) through the inhibition of polyADP-ribosylation (PARylation), in which cGAMP reduces cellular levels of NAD+; meanwhile, restoring NAD+ levels abrogates cGAMP-mediated suppression of PARylation and HDR. Finally, we show that cGAMP also activates DDR signaling in invertebrate species lacking IFN (Crassostrea virginica and Nematostella vectensis), suggesting that the genome surveillance mechanism of cGAS predates metazoan interferon-based immunity.

ZGRF1 promotes end resection of DNA homologous recombination via forming complex with BRCA1/EXO1

To maintain genomic stability, the mammalian cells has evolved a coordinated response to DNA damage, including activation of DNA repair and cell cycle checkpoint processes. Exonuclease 1 (EXO1)-dependent excision of DNA ends is important for the initiation of homologous recombination (HR) repair of DNA breaks, which is thought to play a key role in activating the ATR-CHK1 pathway to induce G2/M cell cycle arrest. But the mechanism is still not fully understood. Here, we report that ZGRF1 forms complexes with EXO1 as well as other repair proteins and promotes DNA repair through HR. ZGRF1 is recruited to DNA damage sites in a MDC1-RNF8-BRCA1 dependent manner. Furthermore, ZGRF1 is important for the recruitment of RPA2 to DNA damage sites and the following ATR-CHK1 mediated G2/M checkpoint in response to irradiation. ZGRF1 null cells show increased sensitivity to many DNA-damaging agents, especially PARPi and irradiation. Collectively,our findings identify ZGRF1 as a novel regulator of DNA end resection and G2/M checkpoint. ZGRF1 is a potential target of radiation and PARPi cancer therapy.

DNA damage-induced phosphorylation of CtIP at a conserved ATM/ATR site T855 promotes lymphomagenesis in mice

CtIP is a DNA end resection factor widely implicated in alternative end-joining (A-EJ)-mediated translocations in cell-based reporter systems. To address the physiological role of CtIP, an essential gene, in translocation-mediated lymphomagenesis, we introduced the T855A mutation at murine CtIP to nonhomologous end-joining and Tp53 double-deficient mice that routinely succumbed to lymphomas carrying A-EJ-mediated IgH-Myc translocations. T855 of CtIP is phosphorylated by ATM or ATR kinases upon DNA damage to promote end resection. Here, we reported that the T855A mutation of CtIP compromised the neonatal development of Xrcc4-/-Tp53-/- mice and the IgH-Myc translocation-driven lymphomagenesis in DNA-PKcs-/-Tp53-/- mice. Mechanistically, the T855A mutation limits DNA end resection length without affecting hairpin opening, translocation frequency, or fork stability. Meanwhile, after radiation, CtIP-T855A mutant cells showed a consistent decreased Chk1 phosphorylation and defects in the G2/M cell cycle checkpoint. Consistent with the role of T855A mutation in lymphomagenesis beyond translocation, the CtIP-T855A mutation also delays splenomegaly in λ-Myc mice. Collectively, our study revealed a role of CtIP-T855 phosphorylation in lymphomagenesis beyond A-EJ-mediated chromosomal translocation.

Almost famous: Human adenoviruses (and what they have taught us about cancer)

Papillomaviruses, polyomaviruses and adenoviruses are collectively categorized as the small DNA tumour viruses. Notably, human adenoviruses were the first human viruses demonstrated to be able to cause cancer, albeit in non-human animal models. Despite their long history, no human adenovirus is a known causative agent of human cancers, unlike a subset of their more famous cousins, including human papillomaviruses and human Merkel cell polyomavirus. Nevertheless, seminal research using human adenoviruses has been highly informative in understanding the basics of cell cycle control, gene expression, apoptosis and cell differentiation. This review highlights the contributions of human adenovirus research in advancing our knowledge of the molecular basis of cancer.

WEE1 inhibition reverses trastuzumab resistance in HER2-positive cancers

BACKGROUND

To date, many efforts have been made to understand the resistance mechanism of trastuzumab in human epidermal growth factor receptor 2 (HER2)-positive breast and gastric cancer. However, there is still a huge unmet medical need for patients with trastuzumab resistance.

METHODS

In our study, we generated four trastuzumab-resistant (HR) cancer cell lines from ERBB2-amplified gastric and biliary tract cancer cell lines (SNU-216, NCI-N87, SNU-2670, and SNU-2773).

RESULTS

Here, we found higher PD-L1 expression in trastuzumab-resistant (HR) HER2-positive cancer cells than in parental cells, and blocking PD-L1 reversed the resistance to trastuzumab in HR cells. Trastuzumab upregulated PD-L1 expression via NF-κB activation in both parental and HR cells, however, led to DNA damage only in parental cells. The WEE1 inhibitor adavosertib, which downregulates PD-L1 expression, enhanced trastuzumab efficacy by blocking BRCA1-CMTM6-PD-L1 signals and the HER2-CDCP-1-SRC axis. Additionally, the levels of galectin-9, CD163, FoxP3, and CTLA-4 were diminished by blocking WEE1 in the presence of human PBMCs in vitro.

CONCLUSION

Taken together, the strategy of co-targeting HER2 and WEE1 could overcome resistance to trastuzumab in HER2-positive cancers, supporting further clinical development in HER2-positive cancer patients.

Evaluation of DNA double-strand break repair capacity in human cells: Critical overview of current functional methods

DNA double-strand breaks (DSBs) are highly deleterious lesions, responsible for mutagenesis, chromosomal translocation or cell death. DSB repair (DSBR) is therefore a critical part of the DNA damage response (DDR) to restore molecular and genomic integrity. In humans, this process is achieved through different pathways with various outcomes. The balance between DSB repair activities varies depending on cell types, tissues or individuals. Over the years, several methods have been developed to study variations in DSBR capacity. Here, we mainly focus on functional techniques, which provide dynamic information regarding global DSB repair proficiency or the activity of specific pathways. These methods rely on two kinds of approaches. Indirect techniques, such as pulse field gel electrophoresis (PFGE), the comet assay and immunofluorescence (IF), measure DSB repair capacity by quantifying the time-dependent decrease in DSB levels after exposure to a DNA-damaging agent. On the other hand, cell-free assays and reporter-based methods directly track the repair of an artificial DNA substrate. Each approach has intrinsic advantages and limitations and despite considerable efforts, there is currently no ideal method to quantify DSBR capacity. All techniques provide different information and can be regarded as complementary, but some studies report conflicting results. Parameters such as the type of biological material, the required equipment or the cost of analysis may also limit available options. Improving currently available methods measuring DSBR capacity would be a major step forward and we present direct applications in mechanistic studies, drug development, human biomonitoring and personalized medicine, where DSBR analysis may improve the identification of patients eligible for chemo- and radiotherapy.

Structural basis for the coiled-coil architecture of human CtIP

The DNA repair factor CtIP has a critical function in double-strand break (DSB) repair by homologous recombination, promoting the assembly of the repair apparatus at DNA ends and participating in DNA-end resection. However, the molecular mechanisms of CtIP function in DSB repair remain unclear. Here, we present an atomic model for the three-dimensional architecture of human CtIP, derived from a multi-disciplinary approach that includes X-ray crystallography, small-angle X-ray scattering (SAXS) and diffracted X-ray tracking (DXT). Our data show that CtIP adopts an extended dimer-of-dimers structure, in agreement with a role in bridging distant sites on chromosomal DNA during the recombinational repair. The zinc-binding motif in the CtIP N-terminus alters dynamically the coiled-coil structure, with functional implications for the long-range interactions of CtIP with DNA. Our results provide a structural basis for the three-dimensional arrangement of chains in the CtIP tetramer, a key aspect of CtIP function in DNA DSB repair.

Hereditary Prostate Cancer: Genes Related, Target Therapy and Prevention

Prostate cancer (PCa) is globally the second most diagnosed cancer type and the most common cause of cancer-related deaths in men. Family history of PCa, hereditary breast and ovarian cancer (HBOC) and Lynch syndromes (LS), are among the most important risk factors compared to age, race, ethnicity and environmental factors for PCa development. Hereditary prostate cancer (HPCa) has the highest heritability of any major cancer in men. The proportion of PCa attributable to hereditary factors has been estimated in the range of 5–15%. To date, the genes more consistently associated to HPCa susceptibility include mismatch repair (MMR) genes (MLH1, MSH2, MSH6, and PMS2) and homologous recombination genes (BRCA1/2, ATM, PALB2, CHEK2). Additional genes are also recommended to be integrated into specific research, including HOXB13, BRP1 and NSB1. Importantly, BRCA1/BRCA2 and ATM mutated patients potentially benefit from Poly (ADP-ribose) polymerase PARP inhibitors, through a mechanism of synthetic lethality, causing selective tumor cell cytotoxicity in cell lines. Moreover, the detection of germline alterations in MMR genes has therapeutic implications, as it may help to predict immunotherapy benefits. Here, we discuss the current knowledge of the genetic basis for inherited predisposition to PCa, the potential target therapy, and the role of active surveillance as a management strategy for patients with low-risk PCa. Finally, the current PCa guideline recommendations are reviewed.

Germline Mutations in Other Homologous Recombination Repair-Related Genes Than BRCA1/2: Predictive or Prognostic Factors?

The homologous recombination repair (HRR) pathway repairs double-strand DNA breaks, mostly by BRCA1 and BRCA2, although other proteins such as ATM, CHEK2, and PALB2 are also involved. BRCA1/2 germline mutations are targeted by PARP inhibitors. The aim of this commentary is to explore whether germline mutations in HRR-related genes other than BRCA1/2 have to be considered as prognostic factors or predictive to therapies by discussing the results of two articles published in December 2020. The TBCRC 048 trial published by Tung et al. showed an impressive objective response rate to olaparib in metastatic breast cancer patients with germline PALB2 mutation compared to germline ATM and CHEK2 mutation carriers. Additionally, Yadav et al. observed a significantly longer overall survival in pancreatic adenocarcinoma patients with germline HRR mutations compared to non-carriers. In our opinion, assuming that PALB2 is a high-penetrant gene with a key role in the HRR system, PALB2 mutations are predictive factors for response to treatment. Moreover, germline mutations in the ATM gene provide a better outcome in pancreatic adenocarcinoma, being more often associated to wild-type KRAS. In conclusion, sequencing of HRR-related genes other than BRCA1/2 should be routinely offered as part of a biological characterization of pancreatic and breast cancers.

K3326X and Other C-Terminal BRCA2 Variants Implicated in Hereditary Cancer Syndromes: A Review

Whole genome analysis and the search for mutations in germline and tumor DNAs is becoming a major tool in the evaluation of risk as well as the management of hereditary cancer syndromes. Because of the identification of cancer predisposition gene panels, thousands of such variants have been catalogued yet many remain unclassified, presenting a clinical challenge for the management of hereditary cancer syndromes. Although algorithms exist to estimate the likelihood of a variant being deleterious, these tools are rarely used for clinical decision-making. Here, we review the progress in classifying K3326X, a rare truncating variant on the C-terminus of BRCA2 and review recent literature on other novel single nucleotide polymorphisms, SNPs, on the C-terminus of the protein, defined in this review as the portion after the final BRC repeat (amino acids 2058-3418).

Kinetics model of DNA double-strand break repair in eukaryotes

This manuscript outlines the kinetics of two main repair pathways of DNA double-strand break (DSB) in eukaryotes: non-homologous end joining (NHEJ) and homologous recombination repair (HRR). In this review, we discuss the precise study of recruitment kinetics of repair proteins based on the latest technologies in the past two decades. Then we simulate the theoretical description of the DNA repair process by mathematical models. In our study, the consecutive reactions chain (CRC) model and continuous-time random walk (CTRW) model have been unified by us, so that we can obtain the function of the number of intermediates with time in the same framework of equations, overcome the incompatibility between the two models. On this basis, we propose a data fitting workflow using these both models. Finally, we give an overview of different real-time quantitative methods and the new mechanism complexity that can be found from the corresponding dynamic models.

LncRNA H19 Suppresses Osteosarcomagenesis by Regulating snoRNAs and DNA Repair Protein Complexes

Osteosarcoma is one of the most frequent common primary malignant tumors in childhood and adolescence. Long non-coding RNAs (lncRNAs) have been reported to regulate the initiation and progression of tumors. However, the exact molecular mechanisms involving lncRNA in osteosarcomagenesis remain largely unknown. Li-Fraumeni syndrome (LFS) is a familial cancer syndrome caused by germline p53 mutation. We investigated the tumor suppressor function of lncRNA H19 in LFS-associated osteosarcoma. Analyzing H19-induced transcriptome alterations in LFS induced pluripotent stem cell (iPSC)-derived osteoblasts, we unexpectedly discovered a large group of snoRNAs whose expression was significantly affected by H19. We identified SNORA7A among the H19-suppressed snoRNAs. SNORA7A restoration impairs H19-mediated osteogenesis and tumor suppression, indicating an oncogenic role of SNORA7A. TCGA analysis indicated that SNORA7A expression is associated with activation of oncogenic signaling and poor survival in cancer patients. Using an optimized streptavidin-binding RNA aptamer designed from H19 lncRNA, we revealed that H19-tethered protein complexes include proteins critical for DNA damage response and repair, confirming H19's tumor suppressor role. In summary, our findings demonstrate a critical role of H19-modulated SNORA7A expression in LFS-associated osteosarcomas.

CSN7B defines a variant COP9 signalosome complex with distinct function in DNA damage response

Mammalian COP9 signalosome (CSN) exists as two variant complexes containing either CSN7A or CSN7B paralogs of unknown functional specialization. Constructing knockout cells, we found that CSN7A and CSN7B have overlapping functions in the deneddylation of cullin-RING ubiquitin ligases. Nevertheless, CSN^CSN7B has a unique function in DNA double-strand break (DSB) sensing, being selectively required for ataxia telangiectasia mutated (ATM)-dependent formation of NBS1^S343p and γH2AX as well as DNA-damage-induced apoptosis triggered by mitomycin C and ionizing radiation. Live-cell microscopy revealed rapid recruitment of CSN7B but not CSN7A to DSBs. Resistance of CSN7B knockout cells to DNA damage is explained by the failure to deneddylate an upstream DSB signaling component, causing a switch in DNA repair pathway choice with increased utilization of non-homologous end joining over homologous recombination. In mice, CSN7B knockout tumors are resistant to DNA-damage-inducing chemotherapy, thus providing an explanation for the poor prognosis of tumors with low CSN7B expression.

A Paradigm Revolution or Just Better Resolution-Will Newly Emerging Superresolution Techniques Identify Chromatin Architecture as a Key Factor in Radiation-Induced DNA Damage and Repair Regulation?

DNA double-strand breaks (DSBs) have been recognized as the most serious lesions in irradiated cells. While several biochemical pathways capable of repairing these lesions have been identified, the mechanisms by which cells select a specific pathway for activation at a given DSB site remain poorly understood. Our knowledge of DSB induction and repair has increased dramatically since the discovery of ionizing radiation-induced foci (IRIFs), initiating the possibility of spatiotemporally monitoring the assembly and disassembly of repair complexes in single cells. IRIF exploration revealed that all post-irradiation processes-DSB formation, repair and misrepair-are strongly dependent on the characteristics of DSB damage and the microarchitecture of the whole affected chromatin domain in addition to the cell status. The microscale features of IRIFs, such as their morphology, mobility, spatiotemporal distribution, and persistence kinetics, have been linked to repair mechanisms. However, the influence of various biochemical and structural factors and their specific combinations on IRIF architecture remains unknown, as does the hierarchy of these factors in the decision-making process for a particular repair mechanism at each individual DSB site. New insights into the relationship between the physical properties of the incident radiation, chromatin architecture, IRIF architecture, and DSB repair mechanisms and repair efficiency are expected from recent developments in optical superresolution microscopy (nanoscopy) techniques that have shifted our ability to analyze chromatin and IRIF architectures towards the nanoscale. In the present review, we discuss this relationship, attempt to correlate still rather isolated nanoscale studies with already better-understood aspects of DSB repair at the microscale, and consider whether newly emerging "correlated multiscale structuromics" can revolutionarily enhance our knowledge in this field.

Mutation Spectra of the MRN (MRE11, RAD50, NBS1/NBN) Break Sensor in Cancer Cells

Simple Summary

A DNA double strand break cuts a chromosome in two and is one of the most dangerous forms of DNA damage. Improper repair can lead to various chromosomal re-arrangements that have been detected in almost all cancer cells. A complex of three proteins (MRE11, RAD50, NBS1 or NBN) detects chromosome breaks and orchestrates repair processes. Mutations in these “break sensor” genes have been described in a multitude of cancers. Here, we provide a comprehensive analysis of reported mutations from data deposited on the Catalogue of Somatic Mutations in Cancer (COSMIC) archive. We also undertake an evolutionary analysis of these genes with the aim to understand whether these mutations preferentially accumulate in conserved residues. Interestingly, we find that mutations are overrepresented in evolutionarily conserved residues of RAD50 and NBS1/NBN but not MRE11.

Abstract

The MRN complex (MRE11, RAD50, NBS1/NBN) is a DNA double strand break sensor in eukaryotes. The complex directly participates in, or coordinates, several activities at the break such as DNA resection, activation of the DNA damage checkpoint, chromatin remodeling and recruitment of the repair machinery. Mutations in components of the MRN complex have been described in cancer cells for several decades. Using the Catalogue of Somatic Mutations in Cancer (COSMIC) database, we characterized all the reported MRN mutations. This analysis revealed several hotspot frameshift mutations in all three genes that introduce premature stop codons and truncate large regions of the C-termini. We also found through evolutionary analyses that COSMIC mutations are enriched in conserved residues of NBS1/NBN and RAD50 but not in MRE11. Given that all three genes are important to carcinogenesis, we propose these differential enrichment patterns may reflect a more severe pleiotropic role for MRE11.

Immunomodulatory Effects of Radiotherapy

Radiation therapy (RT), an integral component of curative treatment for many malignancies, can be administered via an increasing array of techniques. In this review, we summarize the properties and application of different types of RT, specifically, conventional therapy with x-rays, stereotactic body RT, and proton and carbon particle therapies. We highlight how low-linear energy transfer (LET) radiation induces simple DNA lesions that are efficiently repaired by cells, whereas high-LET radiation causes complex DNA lesions that are difficult to repair and that ultimately enhance cancer cell killing. Additionally, we discuss the immunogenicity of radiation-induced tumor death, elucidate the molecular mechanisms by which radiation mounts innate and adaptive immune responses and explore strategies by which we can increase the efficacy of these mechanisms. Understanding the mechanisms by which RT modulates immune signaling and the key players involved in modulating the RT-mediated immune response will help to improve therapeutic efficacy and to identify novel immunomodulatory drugs that will benefit cancer patients undergoing targeted RT.

USP52 regulates DNA end resection and chemosensitivity through removing inhibitory ubiquitination from CtIP

Human C-terminal binding protein (CtBP)–interacting protein (CtIP) is a central regulator to initiate DNA end resection and homologous recombination (HR). Several studies have shown that post-translational modifications control the activity or expression of CtIP. However, it remains unclear whether and how cells restrain CtIP activity in unstressed cells and activate CtIP when needed. Here, we identify that USP52 directly interacts with and deubiquitinates CtIP, thereby promoting DNA end resection and HR. Mechanistically, USP52 removes the ubiquitination of CtIP to facilitate the phosphorylation and activation of CtIP at Thr-847. In addition, USP52 is phosphorylated by ATM at Ser-1003 after DNA damage, which enhances the catalytic activity of USP52. Furthermore, depletion of USP52 sensitizes cells to PARP inhibition in a CtIP-dependent manner in vitro and in vivo. Collectively, our findings reveal the key role of USP52 and the regulatory complexity of CtIP deubiquitination in DNA repair.

C-terminal binding protein (CtBP) interacting protein (CtIP) is a fundamental factor for the initiation of DNA end resection to initiate DNA repair. Here the authors reveal mechanistic insights into the regulation of CtIP via the deubiquitinase USP52.

Polymerase δ promotes chromosomal rearrangements and imprecise double-strand break repair

Recent studies have implicated DNA polymerases θ (Pol θ) and β (Pol β) as mediators of alternative nonhomologous end-joining (Alt-NHEJ) events, including chromosomal translocations. Here we identify subunits of the replicative DNA polymerase δ (Pol δ) as promoters of Alt-NHEJ that results in more extensive intrachromosomal mutations at a single double-strand break (DSB) and more frequent translocations between two DSBs. Depletion of the Pol δ accessory subunit POLD2 destabilizes the complex, resulting in degradation of both POLD1 and POLD3 in human cells. POLD2 depletion markedly reduces the frequency of translocations with sequence modifications but does not affect the frequency of translocations with exact joins. Using separation-of-function mutants, we show that both the DNA synthesis and exonuclease activities of the POLD1 subunit contribute to translocations. As described in yeast and unlike Pol θ, Pol δ also promotes homology-directed repair. Codepletion of POLD2 with 53BP1 nearly eliminates translocations. POLD1 and POLD2 each colocalize with phosphorylated H2AX at ionizing radiation-induced DSBs but not with 53BP1. Codepletion of POLD2 with either ligase 3 (LIG3) or ligase 4 (LIG4) does not further reduce translocation frequency compared to POLD2 depletion alone. Together, these data support a model in which Pol δ promotes Alt-NHEJ in human cells at DSBs, including translocations.

Human CtIP: A 'double agent' in DNA repair and tumorigenesis

Human CtIP was originally identified as an interactor of the retinoblastoma protein and BRCA1, two bona fide tumour suppressors frequently mutated in cancer. CtIP is renowned for its role in the resection of DNA double-strand breaks (DSBs) during homologous recombination, a largely error-free DNA repair pathway crucial in maintaining genome integrity. However, CtIP-dependent DNA end resection is equally accountable for alternative end-joining, a mutagenic DSB repair mechanism implicated in oncogenic chromosomal translocations. In addition, CtIP contributes to transcriptional regulation of G1/S transition, DNA damage checkpoint signalling, and replication fork protection pathways. In this review, we present a perspective on the current state of knowledge regarding the tumour-suppressive and oncogenic properties of CtIP and provide an overview of their relevance for cancer development, progression, and therapy.

Regulation of the Human Papillomavirus Life Cycle by DNA Damage Repair Pathways and Epigenetic Factors

Human papillomaviruses are the causative agents of cervical and other anogenital cancers along with approximately 60% of oropharyngeal cancers. These small double-stranded DNA viruses infect stratified epithelia and link their productive life cycles to differentiation. HPV proteins target cellular factors, such as those involved in DNA damage repair, as well as epigenetic control of host and viral transcription to regulate the productive life cycle. HPVs constitutively activate the ATM and ATR DNA repair pathways and preferentially recruit these proteins to viral genomes to facilitate productive viral replication. In addition, the sirtuin deacetylases along with histone acetyltransferases, including Tip60, are targeted in HPV infections to regulate viral transcription and replication. These pathways provide potential targets for drug therapy to treat HPV-induced disease.

BRCA1 and S phase DNA repair pathways restrict LINE-1 retrotransposition in human cells

Long interspersed element-1 (LINE-1 or L1) is the only autonomous retrotransposon active in human cells. Different host factors have been shown to influence L1 mobility however, systematic analyses of these factors are limited. Here, we developed a high-throughput microscopy-based retrotransposition assay that identified the Double-Stranded Break (DSB) repair and Fanconi Anemia factors active in the S/G2 phase as potent inhibitors and regulators of L1 activity. In particular BRCA1, an E3 ubiquitin ligase with a key role in several DNA repair pathways, directly affects L1 retrotransposition frequency and structure and also plays a distinct role in controlling L1 ORF2 protein translation through L1 mRNA binding. These results suggest the existence of a “battleground” at the DNA replication fork between HR factors and L1 retrotransposons, and revealing a potential role for L1 in the genotypic evolution of tumors characterized by BRCA1 and HR repair deficiencies.

Interplay between BRCA1 and GADD45A and Its Potential for Nucleotide Excision Repair in Breast Cancer Pathogenesis

A fraction of breast cancer cases are associated with mutations in the BRCA1 (BRCA1 DNA repair associated, breast cancer type 1 susceptibility protein) gene, whose mutated product may disrupt the repair of DNA double-strand breaks as BRCA1 is directly involved in the homologous recombination repair of such DNA damage. However, BRCA1 can stimulate nucleotide excision repair (NER), the most versatile system of DNA repair processing a broad spectrum of substrates and playing an important role in the maintenance of genome stability. NER removes carcinogenic adducts of diol-epoxy derivatives of benzo[α]pyrene that may play a role in breast cancer pathogenesis as their accumulation is observed in breast cancer patients. NER deficiency was postulated to be intrinsic in stage I of sporadic breast cancer. BRCA1 also interacts with GADD45A (growth arrest and DNA damage-inducible protein GADD45 alpha) that may target NER machinery to actively demethylate genome sites in order to change the expression of genes that may be important in breast cancer. Therefore, the interaction between BRCA1 and GADD45 may play a role in breast cancer pathogenesis through the stimulation of NER, increasing the genomic stability, removing carcinogenic adducts, and the local active demethylation of genes important for cancer transformation.

"An End to a Means": How DNA-End Structure Shapes the Double-Strand Break Repair Process

Endogenously-arising DNA double-strand breaks (DSBs) rarely harbor canonical 5′-phosphate, 3′-hydroxyl moieties at the ends, which are, regardless of the pathway used, ultimately required for their repair. Cells are therefore endowed with a wide variety of enzymes that can deal with these chemical and structural variations and guarantee the formation of ligatable termini. An important distinction is whether the ends are directly “unblocked” by specific enzymatic activities without affecting the integrity of the DNA molecule and its sequence, or whether they are “processed” by unspecific nucleases that remove nucleotides from the termini. DNA end structure and configuration, therefore, shape the repair process, its requirements, and, importantly, its final outcome. Thus, the molecular mechanisms that coordinate and integrate the cellular response to blocked DSBs, although still largely unexplored, can be particularly relevant for maintaining genome integrity and avoiding malignant transformation and cancer.

CtIP is essential for telomere replication

The maintenance of telomere length is critical to longevity and survival. Specifically, the failure to properly replicate, resect, and/or form appropriate telomeric structures drives telomere shortening and, in turn, genomic instability. The endonuclease CtIP is a DNA repair protein that is well-known to promote genome stability through the resection of endogenous DNA double-stranded breaks. Here, we describe a novel role for CtIP. We show that in the absence of CtIP, human telomeres shorten rapidly to non-viable lengths. This telomere dysfunction results in an accumulation of fusions, breaks, and frank telomere loss. Additionally, CtIP suppresses the generation of circular, extrachromosomal telomeric DNA. These latter structures appear to arise from arrested DNA replication forks that accumulate in the absence of CtIP. Hence, CtIP is required for faithful replication through telomeres via its roles at stalled replication tracts. Our findings demonstrate a new role for CtIP as a protector of human telomere integrity.

APE2 promotes DNA damage response pathway from a single-strand break

As the most common type of DNA damage, DNA single-strand breaks (SSBs) are primarily repaired by the SSB repair mechanism. If not repaired properly or promptly, unrepaired SSBs lead to genome stability and have been implicated in cancer and neurodegenerative diseases. However, it remains unknown how unrepaired SSBs are recognized by DNA damage response (DDR) pathway, largely because of the lack of a feasible experimental system. Here, we demonstrate evidence showing that an ATR-dependent checkpoint signaling is activated by a defined plasmid-based site-specific SSB structure in Xenopus HSS (high-speed supernatant) system. Notably, the distinct SSB signaling requires APE2 and canonical checkpoint proteins, including ATR, ATRIP, TopBP1, Rad9 and Claspin. Importantly, the SSB-induced ATR DDR is essential for SSB repair. We and others show that APE2 interacts with PCNA via its PIP box and preferentially interacts with ssDNA via its C-terminus Zf–GRF domain, a conserved motif found in >100 proteins involved in DNA/RNA metabolism. Here, we identify a novel mode of APE2–PCNA interaction via APE2 Zf–GRF and PCNA C-terminus. Mechanistically, the APE2 Zf–GRF–PCNA interaction facilitates 3′-5′ SSB end resection, checkpoint protein complex assembly, and SSB-induced DDR pathway. Together, we propose that APE2 promotes ATR–Chk1 DDR pathway from a single-strand break.

Exploiting DNA repair defects in colorectal cancer

Colorectal cancer (CRC) is the third leading cause of cancer-related deaths worldwide. Therapies that take advantage of defects in DNA repair pathways have been explored in the context of breast, ovarian, and other tumor types, but not yet systematically in CRC. At present, only immune checkpoint blockade therapies have been FDA approved for use in mismatch repair-deficient colorectal tumors. Here, we discuss how systematic identification of alterations in DNA repair genes could provide new therapeutic opportunities for CRCs. Analysis of The Cancer Genome Atlas Colon Adenocarcinoma (TCGA-COAD) and Rectal Adenocarcinoma (TCGA-READ) PanCancer Atlas datasets identified 141 (out of 528) cases with putative driver mutations in 29 genes associated with DNA damage response and repair, including the mismatch repair and homologous recombination pathways. Genetic defects in these pathways might confer repair-deficient characteristics, such as genomic instability in the absence of homologous recombination, which can be exploited. For example, inhibitors of poly(ADP)-ribose polymerase are effectively used to treat cancers that carry mutations in BRCA1 and/or BRCA2 and have shown promising results in CRC preclinical studies. HR deficiency can also occur in cells with no detectable BRCA1/BRCA2 mutations but exhibiting BRCA-like phenotypes. DNA repair-targeting therapies, such as ATR and CHK1 inhibitors (which are most effective against cancers carrying ATM mutations), can be used in combination with current genotoxic chemotherapies in CRCs to further improve therapy response. Finally, therapies that target alternative DNA repair mechanisms, such as thiopurines, also have the potential to confer increased sensitivity to current chemotherapy regimens, thus expanding the spectrum of therapy options and potentially improving clinical outcomes for CRC patients.

MRN (MRE11-RAD50-NBS1) Complex in Human Cancer and Prognostic Implications in Colorectal Cancer

The MRE11-RAD50-NBS1 (MRN) complex has been studied in multiple cancers. The identification of MRN complex mutations in mismatch repair (MMR)-defective cancers has sparked interest in its role in colorectal cancer (CRC). To date, there is evidence indicating a relationship of MRN expression with reduced progression-free survival, although the significance of the MRN complex in the clinical setting remains controversial. In this review, we present an overview of the function of the MRN complex, its role in cancer progression, and current evidence in colorectal cancer. The evidence indicates that the MRN complex has potential utilisation as a biomarker and as a putative treatment target to improve outcomes of colorectal cancer.

Homologous Recombination-Mediated DNA Repair and Implications for Clinical Treatment of Repair Defective Cancers

Double-strand DNA breaks (DSBs) are generated by ionizing radiation and as intermediates during the processing of DNA, such as repair of interstrand cross-links and collapsed replication forks. These potentially deleterious DSBs are repaired primarily by the homologous recombination (HR) and nonhomologous end joining (NHEJ) DNA repair pathways. HR utilizes a homologous template to accurately restore damaged DNA, whereas NHEJ utilizes microhomology to join breaks in close proximity. The pathway available for DSB repair is dependent upon the cell cycle stage; for example, HR primarily functions during the S/G2 stages while NHEJ can repair DSBs at any cell cycle stage. Posttranslational modifications (PTMs) promote activity of specific pathways and subpathways through enzyme activation and precisely timed protein recruitment and degradation. This chapter provides an overview of PTMs occurring during DSB repair. In addition, clinical phenotypes associated with HR-defective cancers, such as mutational signatures used to predict response to poly(ADP-ribose) polymerase inhibitors, are discussed. Understanding these processes will provide insight into mechanisms of genome maintenance and likely identify targets and new avenues for therapeutic interventions.

Repair of protein-linked DNA double strand breaks: Using the adenovirus genome as a model substrate in cell-based assays

The DNA double strand breaks (DSBs) created during meiotic recombination and during some types of chemotherapy contain protein covalently attached to their 5' termini. Removal of the end-blocking protein is a prerequisite to DSB processing by non-homologous end-joining or homologous recombination. One mechanism for removing the protein involves CtIP-stimulated Mre11-catalyzed nicking of the protein-linked strand distal to the DSB terminus, releasing the end-blocking protein while it remains covalently attached to an oligonucleotide. Much of what is known about this repair process has recently been deciphered through in vitro reconstitution studies. We present here a novel model system based on adenovirus (Ad), which contains the Ad terminal protein covalently linked to the 5' terminus of its dsDNA genome, for studying the repair of 5' protein-linked DSBs in vivo. It was previously shown that the genome of Ad mutants that lack early region 4 (E4) can be joined into concatemers in vivo, suggesting that the Ad terminal protein had been removed from the genome termini prior to ligation. Here we show that during infection with the E4-deleted Ad mutant dl1004, the Ad terminal protein is removed in a manner that recapitulates removal of end-blocking proteins from cellular DSBs. In addition to displaying a dependence on CtIP, and Mre11 acting as the endonuclease, the protein-linked oligonucleotides that are released from the viral genome are similar in size to the oligos that remain attached to Spo11 and Top2 after they are removed from the 5' termini of DSBs during meiotic recombination and etoposide chemotherapy, respectively. The single nucleotide resolution that is possible with this assay, combined with the single sequence context in which the lesion is presented, make it a useful tool for further refining our mechanistic understanding of how blocking proteins are removed from the 5' termini of DSBs.

CtIP promotes G2/M arrest in etoposide-treated HCT116 cells in a p53-independent manner

Acquired resistance to cytotoxic antineoplastic agents is a major clinical challenge in tumor therapy; however, the mechanisms involved are still poorly understood. In this study, we show that knockdown of CtIP, a corepressor of CtBP, promotes cell proliferation and alleviates G2/M phase arrest in etoposide (Eto)-treated HCT116 cells. Although the expression of p21 and growth arrest and DNA damage inducible α (GADD45a), which are important targets of p53, was downregulated in CtIP-deficient HCT116 cells, p53 deletion did not affect G2/M arrest after Eto treatment. In addition, the phosphorylation levels of Ser317 and Ser345 in Chk1 and of Ser216 in CDC25C were lower in CtIP-deficient HCT116 cells than in control cells after Eto treatment. Our results indicate that CtIP may enhance cell sensitivity to Eto by promoting G2/M phase arrest, mainly through the ATR-Chk1-CDC25C pathway rather than the p53-p21/GADD45a pathway. The expression of CtIP may be a useful biomarker for predicting the drug sensitivity of colorectal cancer cells.