An extra allele of Chk1 limits oncogene-induced replicative stress and promotes transformation

Tolerance of Oncogene-Induced Replication Stress: A Fuel for Genomic Instability

Activation of oncogenes disturbs a wide variety of cellular processes and induces physiological dysregulation of DNA replication, widely referred to as replication stress (RS). Oncogene-induced RS can cause replication forks to stall or collapse, thereby leading to DNA damage. While the DNA damage response (DDR) can provoke an anti-tumor barrier to prevent the development of cancer, a small subset of cells triggers replication stress tolerance (RST), allowing precancerous cells to survive, thereby promoting clonal expansion and genomic instability (GIN). Genomic instability (GIN) is a hallmark of cancer, driving genetic alterations ranging from nucleotide changes to aneuploidy. These alterations increase the probability of oncogenic events and create a heterogeneous cell population with an enhanced ability to evolve. This review explores how major oncogenes such as RAS, cyclin E, and MYC induce RS through diverse mechanisms. Additionally, we delve into the strategies employed by normal and cancer cells to tolerate RS and promote GIN. Understanding the intricate relationship between oncogene activation, RS, and GIN is crucial to better understand how cancer cells emerge and to develop potential cancer therapies that target these vulnerabilities.

Elimusertib has Antitumor Activity in Preclinical Patient-Derived Pediatric Solid Tumor Models

The small-molecule inhibitor of ataxia telangiectasia and Rad3-related protein (ATR), elimusertib, is currently being tested clinically in various cancer entities in adults and children. Its preclinical antitumor activity in pediatric malignancies, however, is largely unknown. We here assessed the preclinical activity of elimusertib in 38 cell lines and 32 patient-derived xenograft (PDX) models derived from common pediatric solid tumor entities. Detailed in vitro and in vivo molecular characterization of the treated models enabled the evaluation of response biomarkers. Pronounced objective response rates were observed for elimusertib monotherapy in PDX, when treated with a regimen currently used in clinical trials. Strikingly, elimusertib showed stronger antitumor effects than some standard-of-care chemotherapies, particularly in alveolar rhabdomysarcoma PDX. Thus, elimusertib has strong preclinical antitumor activity in pediatric solid tumor models, which may translate to clinically meaningful responses in patients.

HJURP is recruited to double-strand break sites and facilitates DNA repair by promoting chromatin reorganization

HJURP is overexpressed in several cancer types and strongly correlates with patient survival. However, the mechanistic basis underlying the association of HJURP with cancer aggressiveness is not well understood. HJURP promotes the loading of the histone H3 variant, CENP-A, at the centromeric chromatin, epigenetically defining the centromeres and supporting proper chromosome segregation. In addition, HJURP is associated with DNA repair but its function in this process is still scarcely explored. Here, we demonstrate that HJURP is recruited to DSBs through a mechanism requiring chromatin PARylation and promotes epigenetic alterations that favor the execution of DNA repair. Incorporation of HJURP at DSBs promotes turnover of H3K9me3 and HP1, facilitating DNA damage signaling and DSB repair. Moreover, HJURP overexpression in glioma cell lines also affected global structure of heterochromatin independently of DNA damage induction, promoting genome-wide reorganization and assisting DNA damage response. HJURP overexpression therefore extensively alters DNA damage signaling and DSB repair, and also increases radioresistance of glioma cells. Importantly, HJURP expression levels in tumors are also associated with poor response of patients to radiation. Thus, our results enlarge the understanding of HJURP involvement in DNA repair and highlight it as a promising target for the development of adjuvant therapies that sensitize tumor cells to irradiation.

Identification of naturally occurring compounds as alternatives to radiation therapy for treatment of small cell lung cancer

Radiation resistance is one of the major problems in the treatment of small cell lung cancer (SCLC). Most of these patients are given radiation as first-line treatment and it was observed that the initial response in these patients is very good. However, they show relapse in a few months which is also associated with resistance to treatment. Thus, targeting the mechanism by which these cells develop resistance could be an important strategy to improve the survival chances of these patients. From the RNA-Seq data analysis, it was identified that CHEK1 gene was overexpressed. Chk1 protein which is encoded by the CHEK1 gene is an important protein that is involved in radiation resistance in SCLC. It is known to favour the cells to deal with replicative stress. CHEK1 is the major cause for developing radiation resistance in SCLC. Thus, natural compounds that could also serve as potential inhibitors for Chk1 were explored. Accordingly; the compounds were screened based on ADME, docking and MM-GBSA scores. MD simulations were performed for the selected protein-ligand complexes and the results were compared to the co-crystallised ligand, 3-(indol-2-yl)indazole. The results showed that compound INC000033832986 could be a natural alternative to the commercial ligand for the prevention of SCLC.Communicated by Ramaswamy H. Sarma.

Interactions between the DNA Damage Response and the Telomere Complex in Carcinogenesis: A Hypothesis

Contrary to what was once thought, direct cancer originating from normal stem cells seems to be extremely rare. This is consistent with a preneoplastic period of telomere length reduction/damage in committed cells that becomes stabilized in transformation. Multiple observations suggest that telomere damage is an obligatory step preceding its stabilization. During tissue turnover, the telomeres of cells undergoing differentiation can be damaged as a consequence of defective DNA repair caused by endogenous or exogenous agents. This may result in the emergence of new mechanism of telomere maintenance which is the final outcome of DNA damage and the initial signal that triggers malignant transformation. Instead, transformation of stem cells is directly induced by primary derangement of telomere maintenance mechanisms. The newly modified telomere complex may promote survival of cancer stem cells, independently of telomere maintenance. An inherent resistance of stem cells to transformation may be linked to specific, robust mechanisms that help maintain telomere integrity.

Clinical evidence for a role of E2F1-induced replication stress in modulating tumor mutational burden and immune microenvironment

DNA replication stress (RS) is frequently induced by oncogene activation and is believed to promote tumorigenesis. However, clinical evidence for the role of oncogene-induced RS in tumorigenesis remains scarce, and the mechanisms by which RS promotes cancer development remain incompletely understood. By performing a series of bioinformatic analyses on the oncogene E2F1, other RS-inducing factors, and replication fork processing factors in TCGA cancer database using previously established tools, we show that hyperactivity of E2F1 likely promotes the expression of several of these factors in virtually all types of cancer to induce RS and cytosolic self-DNA production. In addition, the expression of these factors positively correlates with that of ATR and Chk1 that govern the cellular response to RS, the tumor mutational load, and tumor infiltration of immune-suppressive CD4+Th2 cells and myeloid-derived suppressor cells (MDSCs). Consistently, high expression of these factors is associated with poor patient survival. Our study provides new insights into the role of E2F1-induced RS in tumorigenesis and suggests therapeutic approaches for E2F1-overexpressing cancers by targeting genomic instability, cytosolic self-DNA and the tumor immune microenvironment.

TUG1-mediated R-loop resolution at microsatellite loci as a prerequisite for cancer cell proliferation

Oncogene-induced DNA replication stress (RS) and consequent pathogenic R-loop formation are known to impede S phase progression. Nonetheless, cancer cells continuously proliferate under such high-stressed conditions through incompletely understood mechanisms. Here, we report taurine upregulated gene 1 (TUG1) long noncoding RNA (lncRNA), which is highly expressed in many types of cancers, as an important regulator of intrinsic R-loop in cancer cells. Under RS conditions, TUG1 is rapidly upregulated via activation of the ATR-CHK1 signaling pathway, interacts with RPA and DHX9, and engages in resolving R-loops at certain loci, particularly at the CA repeat microsatellite loci. Depletion of TUG1 leads to overabundant R-loops and enhanced RS, leading to substantial inhibition of tumor growth. Our data reveal a role of TUG1 as molecule important for resolving R-loop accumulation in cancer cells and suggest targeting TUG1 as a potent therapeutic approach for cancer treatment.

An ATR-PrimPol pathway confers tolerance to oncogenic KRAS-induced and heterochromatin-associated replication stress

Activation of the KRAS oncogene is a source of replication stress, but how this stress is generated and how it is tolerated by cancer cells remain poorly understood. Here we show that induction of KRASG12V expression in untransformed cells triggers H3K27me3 and HP1-associated chromatin compaction in an RNA transcription dependent manner, resulting in replication fork slowing and cell death. Furthermore, elevated ATR expression is necessary and sufficient for tolerance of KRASG12V-induced replication stress to expand replication stress-tolerant cells (RSTCs). PrimPol is phosphorylated at Ser255, a potential Chk1 substrate site, under KRASG12V-induced replication stress and promotes repriming to maintain fork progression and cell survival in an ATR/Chk1-dependent manner. However, ssDNA gaps are generated at heterochromatin by PrimPol-dependent repriming, leading to genomic instability. These results reveal a role of ATR-PrimPol in enabling precancerous cells to survive KRAS-induced replication stress and expand clonally with accumulation of genomic instability.

Molecular Biology and Therapeutic Targets of Primitive Tracheal Tumors: Focus on Tumors Derived by Salivary Glands and Squamous Cell Carcinoma

Primary tracheal tumors are rare, constituting approximately 0.1-0.4% of malignant diseases. Squamous cell carcinoma (SCC) and adenoid cystic carcinoma (ACC) account for about two-thirds of these tumors. Despite most primary tracheal cancers being eligible for surgery and/or radiotherapy, unresectable, recurrent and metastatic tumors may require systemic treatments. Unfortunately, the poor response to available chemotherapy as well as the lack of other real therapeutic alternatives affects the quality of life and outcome of patients suffering from more advanced disease. In this condition, target therapy against driver mutations could constitute an alternative to chemotherapy, and may help in disease control. The past two decades have seen extraordinary progress in developing novel target treatment options, shifting the treatment paradigm for several cancers such as lung cancer. The improvement of knowledge regarding the genetic and biological alterations, of major primary tracheal tumors, has opened up new treatment perspectives, suggesting the possible role of biological targeted therapies for the treatment of these rare tumors. The purpose of this review is to outline the state of knowledge regarding the molecular biology, and the preliminary data on target treatments of the main primary tracheal tumors, focusing on salivary-gland-derived cancers and squamous cell carcinoma.

Mechanisms used by cancer cells to tolerate drug-induced replication stress

Activation of oncogenes in cancer cells forces cell proliferation, leading to DNA replication stress (RS). As a consequence, cancer cells heavily rely on the intra S-phase checkpoint for survival. This fundamental principle formed the basis for the development of inhibitors against key players of the intra S-phase checkpoint, ATR and CHK1. These drugs are often combined with chemotherapeutic drugs that interfere with DNA replication to exacerbate RS and exhaust the intra S-phase checkpoint in cancer cells. However, drug resistance impedes efficient clinical use, suggesting that some cancer cells tolerate severe RS. In this review, we describe how an increased nucleotide pool, boosted stabilization and repair of stalled forks and firing of dormant origins fortify the RS response in cancer cells. Notably, the vast majority of the genes that confer RS tolerance are regulated by the E2F and NRF2 transcription factors. These transcriptional programs are frequently activated in cancer cells, allowing simultaneous activation of multiple tolerance avenues. We propose that the E2F and NRF2 transcriptional programs can be used as biomarker to select patients for treatment with RS-inducing drugs and as novel targets to kill RS-tolerant cancer cells. Together, this review aims to provide a framework to maximally exploit RS as an Achilles' heel of cancer cells.

Oncogenic RAS sensitizes cells to drug-induced replication stress via transcriptional silencing of P53

Cancer cells often experience high basal levels of DNA replication stress (RS), for example due to hyperactivation of oncoproteins like MYC or RAS. Therefore, cancer cells are considered to be sensitive to drugs that exacerbate the level of RS or block the intra S-phase checkpoint. Consequently, RS-inducing drugs including ATR and CHK1 inhibitors are used or evaluated as anti-cancer therapies. However, drug resistance and lack of biomarkers predicting therapeutic efficacy limit efficient use. This raises the question what determines sensitivity of individual cancer cells to RS. Here, we report that oncogenic RAS does not only enhance the sensitivity to ATR/CHK1 inhibitors by directly causing RS. Instead, we observed that HRAS^G12V dampens the activation of the P53-dependent transcriptional response to drug-induced RS, which in turn confers sensitivity to RS. We demonstrate that inducible expression of HRAS^G12V sensitized cells to ATR and CHK1 inhibitors. Using RNA-sequencing of FACS-sorted cells we discovered that P53 signaling is the sole transcriptional response to RS. However, oncogenic RAS attenuates the transcription of P53 and TGF-β pathway components which consequently dampens P53 target gene expression. Accordingly, live cell imaging showed that HRAS^G12V exacerbates RS in S/G2-phase, which could be rescued by stabilization of P53. Thus, our results demonstrate that transcriptional control of P53 target genes is the prime determinant in the response to ATR/CHK1 inhibitors and show that hyperactivation of the MAPK pathway impedes this response. Our findings suggest that the level of oncogenic MAPK signaling could predict sensitivity to intra-S-phase checkpoint inhibition in cancers with intact P53.

Reciprocal regulation of p21 and Chk1 controls the Cyclin D1-RB pathway to mediate senescence onset after G2 arrest

Senescence is an irreversible proliferation withdrawal that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 and RB family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the CDK inhibitor p21 and Chk1 controls D-type cyclin-CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting Cyclin D1-CDK2/CDK4. The resulting G2 exit, which precedes appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and DNA damage foci reduction. In p53/RB-proficient cancer cells, compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely Cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to Cyclin D1 and Cyclin E1-CDK complexes and down-regulating CDK6, whereas Chk2 knockdown enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting DNA damage-induced senescence onset in G2.

cGAS/STING cross-talks with cell cycle and potentiates cancer immunotherapy

The correct duplication and transfer of genetic material to daughter cells is the major event of cell division. Dysfunction of DNA replication or chromosome segregation presents challenges in cancer initiation and development as well as opportunities for cancer treatment. Cyclic GMP-AMP synthase (cGAS) of the innate immune system detects cytoplasmic DNA and mediates downstream immune responses through the molecule stimulator of interferon genes (STING). However, how cytosolic DNA sensor cGAS participates in guaranteeing accurate cell division and preventing tumorigenesis is still unclear. Recent evidence indicates malfunction of cGAS/STING pathway in cancer progression. Cell cycle-targeted therapy synergizes with immunotherapy via cGAS/STING activation, leading to promising therapeutic benefit. Here, we review the interactions between cell cycle regulation and cGAS/STING signaling, thus enabling us to understand the role of cGAS/STING in cancer initiation, development, and treatment.

An mRNA expression-based signature for oncogene-induced replication-stress

Oncogene-induced replication stress characterizes many aggressive cancers. Several treatments are being developed that target replication stress, however, identification of tumors with high levels of replication stress remains challenging. We describe a gene expression signature of oncogene-induced replication stress. A panel of triple-negative breast cancer (TNBC) and non-transformed cell lines were engineered to overexpress CDC25A, CCNE1 or MYC, which resulted in slower replication kinetics. RNA sequencing analysis revealed a set of 52 commonly upregulated genes. In parallel, mRNA expression analysis of patient-derived tumor samples (TCGA, n = 10,592) also revealed differential gene expression in tumors with amplification of oncogenes that trigger replication stress (CDC25A, CCNE1, MYC, CCND1, MYB, MOS, KRAS, ERBB2, and E2F1). Upon integration, we identified a six-gene signature of oncogene-induced replication stress (NAT10, DDX27, ZNF48, C8ORF33, MOCS3, and MPP6). Immunohistochemical analysis of NAT10 in breast cancer samples (n = 330) showed strong correlation with expression of phospho-RPA (R = 0.451, p = 1.82 × 10-20) and γH2AX (R = 0.304, p = 2.95 × 10-9). Finally, we applied our oncogene-induced replication stress signature to patient samples from TCGA (n = 8,862) and GEO (n = 13,912) to define the levels of replication stress across 27 tumor subtypes, identifying diffuse large B cell lymphoma, ovarian cancer, TNBC and colorectal carcinoma as cancer subtypes with high levels of oncogene-induced replication stress.

A catalytic-independent function of human DNA polymerase Kappa controls the stability and abundance of the Checkpoint Kinase 1

DNA polymerase kappa (Pol κ) has been well documented thus far for its specialized DNA synthesis activity during translesion replication, progression of replication forks through regions difficult to replicate, restart of stalled forks and replication checkpoint efficiency. Pol κ is also required for the stabilization of stalled forks although the mechanisms are poorly understood. Here we unveiled an unexpected role for Pol κ in controlling the stability and abundance of Chk1, an important actor for the replication checkpoint and fork stabilization. We found that loss of Pol κ decreased the Chk1 protein level in the nucleus of four human cell lines. Pol κ and not the other Y-family polymerase members is required to maintain the Chk1 protein pool all along the cell cycle. We showed that Pol κ depletion affected the protein stability of Chk1 and protected it from proteasome degradation. Importantly, we also observed that the fork restart defects observed in Pol κ-depleted cells could be overcome by the re-expression of Chk1. Strikingly, this new function of Pol κ does not require its catalytic activity. We propose that Pol κ could contribute to the protection of stalled forks through Chk1 stability.

Genetic insights into biological mechanisms governing human ovarian ageing

Reproductive longevity is essential for fertility and influences healthy ageing in women1,2, but insights into its underlying biological mechanisms and treatments to preserve it are limited. Here we identify 290 genetic determinants of ovarian ageing, assessed using normal variation in age at natural menopause (ANM) in about 200,000 women of European ancestry. These common alleles were associated with clinical extremes of ANM; women in the top 1% of genetic susceptibility have an equivalent risk of premature ovarian insufficiency to those carrying monogenic FMR1 premutations3. The identified loci implicate a broad range of DNA damage response (DDR) processes and include loss-of-function variants in key DDR-associated genes. Integration with experimental models demonstrates that these DDR processes act across the life-course to shape the ovarian reserve and its rate of depletion. Furthermore, we demonstrate that experimental manipulation of DDR pathways highlighted by human genetics increases fertility and extends reproductive life in mice. Causal inference analyses using the identified genetic variants indicate that extending reproductive life in women improves bone health and reduces risk of type 2 diabetes, but increases the risk of hormone-sensitive cancers. These findings provide insight into the mechanisms that govern ovarian ageing, when they act, and how they might be targeted by therapeutic approaches to extend fertility and prevent disease.

Synergistic targeting of CHK1 and mTOR in MYC-driven tumors

Deregulation of v-myc avian myelocytomatosis viral oncogene homolog (MYC) occurs in a broad range of human cancers and often predicts poor prognosis and resistance to therapy. However, directly targeting oncogenic MYC remains unsuccessful, and indirectly inhibiting MYC emerges as a promising approach. Checkpoint kinase 1 (CHK1) is a protein kinase that coordinates the G2/M cell cycle checkpoint and protects cancer cells from excessive replicative stress. Using c-MYC-mediated T-cell acute lymphoblastic leukemia (T-acute lymphoblastic leukemia) and N-MYC-driven neuroblastoma as model systems, we reveal that both c-MYC and N-MYC directly bind to the CHK1 locus and activate its transcription. CHIR-124, a selective CHK1 inhibitor, impairs cell viability and induces remarkable synergistic lethality with mTOR inhibitor rapamycin in MYC-overexpressing cells. Mechanistically, rapamycin inactivates carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase (CAD), the essential enzyme for the first three steps of de novo pyrimidine synthesis, and deteriorates CHIR-124-induced replicative stress. We further demonstrate that dual treatments impede T-acute lymphoblastic leukemia and neuroblastoma progression in vivo. These results suggest simultaneous targeting of CHK1 and mTOR as a novel and powerful co-treatment modality for MYC-mediated tumors.

The phosphorylation of CHK1 at Ser345 regulates the phenotypic switching of vascular smooth muscle cells both in vitro and in vivo

BACKGROUND AND AIMS

DNA damage and repair have been shown to be associated with carotid artery restenosis and atherosclerosis. The proliferation and migration of vascular smooth muscle cells (VSMCs) is the main cause of artery stenosis. This study aims to define the relationship between DNA damage and VSMCs proliferation.

METHODS

A rat carotid artery injury model was established, and human and rat VSMCs cultured in vitro. H₂O₂ was used to induce DNA damage in vitro. The selected CHK1 inhibitor, LY2603618, was used to inhibit CHK1 phosphorylation both in vivo and in vitro. γH2AX, αSMA and phosphorylated CHK1 were detected both in rat carotid artery and cultured VSMCs from different groups. Hyperplasia ratio of rat carotid artery intimal was measured.

RESULTS

DNA double-strand breaks occur in the rat carotid artery after injury. DNA damage induces CHK1 phosphorylation and down-regulates αSMA expression in VSMCs both in vitro and in vivo. The inhibition of CHK1 phosphorylation rescues αSMA expression in VSMCs both in vitro and in vivo, and rat carotid intimal hyperplasia after injury was suppressed.

CONCLUSIONS

Our data demonstrated that phosphorylation of CHK1 under DNA damage stress modulates VSMCs phenotypic switching. CHK1 inhibition may be a potential therapeutic strategy for intima hyperplasia treatment.

A Genome-Wide Pooled shRNA Screen Identifies PPP2R2A as a Predictive Biomarker for the Response to ATR and CHK1 Inhibitors

There is currently a lack of precise predictive biomarkers for patient selection in clinical trials of inhibitors targeting replication stress (RS) response proteins ATR and CHK1. The objective of this study was to identify novel predictive biomarkers for the response to these agents in treating non-small cell lung cancer (NSCLC). A genome-wide loss-of-function screen revealed that tumor suppressor PPP2R2A, a B regulatory subunit of protein phosphatase 2 (PP2A), determines sensitivity to CHK1 inhibition. A synthetic lethal interaction between PPP2R2A deficiency and ATR or CHK1 inhibition was observed in NSCLC in vitro and in vivo and was independent of p53 status. ATR and CHK1 inhibition resulted in significantly increased levels of RS and altered replication dynamics, particularly in PPP2R2A-deficient NSCLC cells. Mechanistically, PPP2R2A negatively regulated translation of oncogene c-Myc protein. c-Myc activity was required for PPP2R2A deficiency-induced alterations of replication initiation/RS and sensitivity to ATR/CHK1 inhibitors. We conclude that PPP2R2A deficiency elevates RS by upregulating c-Myc activity, rendering cells reliant on the ATR/CHK1 axis for survival. Our studies show a novel synthetic lethal interaction and identify PPP2R2A as a potential new predictive biomarker for patient stratification in the clinical use of ATR and CHK1 inhibitors. SIGNIFICANCE: This study reveals new approaches to specifically target PPP2R2A-deficient lung cancer cells and provides a novel biomarker that will significantly improve treatment outcome with ATR and CHK1 inhibitors.

Small molecule inhibitors and a kinase-dead expressing mouse model demonstrate that the kinase activity of Chk1 is essential for mouse embryos and cancer cells

The study use small molecule inhibitors and a kinase-dead expressing mouse model to demonstrate that the kinase activity of Chk1 is essential for mouse embryos and cancer cells.

Chk1 kinase is downstream of the ATR kinase in the sensing of improper replication. Previous cell culture studies have demonstrated that Chk1 is essential for replication. Indeed, Chk1 inhibitors are efficacious against tumors with high-level replication stress such as Myc-induced lymphoma cells. Treatment with Chk1 inhibitors also combines well with certain chemotherapeutic drugs, and effects associate with the induction of DNA damage and reduction of Chk1 protein levels. Most studies of Chk1 function have relied on the use of inhibitors. Whether or not a mouse or cancer cells could survive if a kinase-dead form of Chk1 is expressed has not been investigated before. Here, we generate a mouse model that expresses a kinase-dead (D130A) allele in the mouse germ line. We find that this mouse is overtly normal and does not have problems with erythropoiesis with aging as previously been shown for a mouse expressing one null allele. However, similar to a null allele, homozygous kinase-dead mice cannot be generated, and timed pregnancies of heterozygous mice suggest lethality of homozygous blastocysts at around the time of implantation. By breeding the kinase-dead Chk1 mouse with a conditional allele, we are able to demonstrate that expression of only one kinase-dead allele, but no wild-type allele, of Chek1 is lethal for Myc-induced cancer cells. Finally, treatment of melanoma cells with tumor-infiltrating T cells or CAR-T cells is effective even if Chk1 is inhibited, suggesting that Chk1 inhibitors can be safely administered in patients where immunotherapy is an essential component of the arsenal against cancer.

Direct regulation of Chk1 protein stability by E3 ubiquitin ligase HUWE1

The HECT E3 ubiquitin ligase HUWE1 is required for a wide array of important functions in cell biology. Although HUWE1 is known to play a role in DNA damage signaling, the mechanism(s) that underlie this function remain elusive. HUWE1 regulates effectors of DNA replication and genotoxic stress tolerance. However, the loss of HUWE1 can also result in the accrual of significant endogenous DNA damage due to insufficient remediation of replication stress induced by an overabundance of key substrates. We discovered that HUWE1 depletion leads to a significant increase in levels of the single-strand break effector kinase Chk1, independent of the DNA damage response, activation of apical DNA damage repair (DDR) signaling kinases (ATM and ATR), and the tumor suppressor p53. We also identified multiple lysine residues on Chk1 that are polyubiquitinated by HUWE1 in vitro, many of which are within the kinase domain. HUWE1 knockdown also markedly prolonged the protein half-life of Chk1 in steady-state conditions and resulted in greater stabilization of Chk1 protein than depletion of Cul4A, an E3 ubiquitin ligase previously described to control Chk1 abundance. Moreover, prolonged replication stress induced by hydroxyurea or camptothecin resulted in a reduction of Chk1 protein levels, which was rescued by HUWE1 knockdown. Our study indicates that HUWE1 plays a significant role in the regulation of the DDR signaling pathway by directly modulating the abundance of Chk1 protein.

Supraphysiological protection from replication stress does not extend mammalian lifespan

Replication Stress (RS) is a type of DNA damage generated at the replication fork, characterized by single-stranded DNA (ssDNA) accumulation, and which can be caused by a variety of factors. Previous studies have reported elevated RS levels in aged cells. In addition, mouse models with a deficient RS response show accelerated aging. However, the relevance of endogenous or physiological RS, compared to other sources of genomic instability, for the normal onset of aging is unknown. We have performed long term survival studies of transgenic mice with extra copies of the Chk1 and/or Rrm2 genes, which we previously showed extend the lifespan of a progeroid ATR-hypomorphic model suffering from high levels of RS. In contrast to their effect in the context of progeria, the lifespan of Chk1, Rrm2 and Chk1/Rrm2 transgenic mice was similar to WT littermates in physiological settings. Most mice studied died due to tumors -mainly lymphomas- irrespective of their genetic background. Interestingly, a higher but not statistically significant percentage of transgenic mice developed tumors compared to WT mice. Our results indicate that supraphysiological protection from RS does not extend lifespan, indicating that RS may not be a relevant source of genomic instability on the onset of normal aging.

Expression and prognostic significance of the P53-related DNA damage repair proteins checkpoint kinase 1 (CHK1) and growth arrest and DNA-damage-inducible 45 alpha (GADD45A) in human oral squamous cell carcinoma

DNA damage repair is a key factor in the maintenance of cell genome stability, plays an important role in the regulation of tumour evolution, and can affect the prognosis of cancer patients. This study aimed to detect the protein expression of the DNA damage repair protein P53 and its upstream and downstream regulators, CHK1, GADD45A, and MDM2, in oral squamous cell carcinoma (OSCC), in order to analyse the association between the expression of these proteins and overall survival, and to assess their prognostic implications for OSCC patients. The expression of the above proteins was detected by immunohistochemistry in 80 human OSCC tissue samples and in non-cancerous tissue samples. Compared to that in the non-cancerous tissue, the expression of CHK1, GADD45A, and MDM2 in OSCC tissue was significantly increased. The protein expression of the tumour suppressor gene P53 was also increased. Patients with high CHK1 and MDM2 expression levels had a reduced survival time and a poor prognosis, whereas patients with high GADD45A expression levels had a good prognosis. Our results indicate that high CHK1 expression is an independent risk factor for poor OSCC prognosis, and that CHK1 may be a potential target for OSCC clinical treatment.

Regulation of CHK1 by the Ubiquitin-Proteasome System

The checkpoint kinase 1 (CHK1) is a master regulator of genome integrity in vertebrate cells. Despite its important cell cycle functions, its regulation is still incompletely understood. Cassidy et al. provide novel insights on the regulation of the CHK1 abundance by the HECT E3 ligase HUWE1 during unperturbed cell cycle as well as in response to replicative stress. These results may help us to apprehend the underlying mechanism of tumorigenesis.

Overexpression of Claspin and Timeless protects cancer cells from replication stress in a checkpoint-independent manner

Oncogene-induced replication stress (RS) promotes cancer development but also impedes tumor growth by activating anti-cancer barriers. To determine how cancer cells adapt to RS, we have monitored the expression of different components of the ATR-CHK1 pathway in primary tumor samples. We show that unlike upstream components of the pathway, the checkpoint mediators Claspin and Timeless are overexpressed in a coordinated manner. Remarkably, reducing the levels of Claspin and Timeless in HCT116 cells to pretumoral levels impeded fork progression without affecting checkpoint signaling. These data indicate that high level of Claspin and Timeless increase RS tolerance by protecting replication forks in cancer cells. Moreover, we report that primary fibroblasts adapt to oncogene-induced RS by spontaneously overexpressing Claspin and Timeless, independently of ATR signaling. Altogether, these data indicate that enhanced levels of Claspin and Timeless represent a gain of function that protects cancer cells from of oncogene-induced RS in a checkpoint-independent manner.

Investigating the role of Sirtuins in cell reprogramming

Cell reprogramming has been considered a powerful technique in the regenerative medicine field. In addition to diverse its strengths, cell reprogramming technology also has several drawbacks generated during the process of reprogramming. Telomere shortening caused by the cell reprogramming process impedes the efficiency of cell reprogramming. Transcription factors used for reprogramming alter genomic contents and result in genetic mutations. Additionally, defective mitochondria functioning such as excessive mitochondrial fission leads to the limitation of pluripotency and ultimately reduces the efficiency of reprogramming. These problems including genomic instability and impaired mitochondrial dynamics should be resolved to apply cell reprograming in clinical research and to address efficiency and safety concerns. Sirtuin (NAD+-dependent histone deacetylase) has been known to control the chromatin state of the telomere and influence mitochondria function in cells. Recently, several studies reported that Sirtuins could control for genomic instability in cell reprogramming. Here, we review recent findings regarding the role of Sirtuins in cell reprogramming. And we propose that the manipulation of Sirtuins may improve defects that result from the steps of cell reprogramming.

Checkpoint Kinase 1 Expression Predicts Poor Prognosis in Nigerian Breast Cancer Patients

BACKGROUND

Checkpoint kinase 1 (CHEK1), a DNA damage sensor and cell death pathway stimulator, is regarded as an oncogene in tumours, where its activities are considered essential for tumourigenesis and the survival of cancer cells treated with chemotherapy and radiotherapy. In breast cancer, CHEK1 expression has been associated with an aggressive tumour phenotype, the triple-negative breast cancer subtype, an aberrant response to tamoxifen, and poor prognosis. However, the relevance of CHEK1 expression has, hitherto, not been investigated in an indigenous African population. We therefore aimed to investigate the clinicopathological, biological, and prognostic significance of CHEK1 expression in a cohort of Nigerian breast cancer cases.

MATERIAL AND METHODS

Tissue microarrays of 207 Nigerian breast cancer cases were tested for CHEK1 expression using immunohistochemistry. The clinicopathological, molecular, and prognostic characteristics of CHEK1-positive tumours were determined using the Chi-squared test and Kaplan-Meier and Cox regression analyses in SPSS Version 16.

RESULTS

Nuclear expression of CHEK1 was present in 61% of breast tumours and was associated with tumour size, triple-negative cancer, basal-like phenotype, the epithelial-mesenchymal transition, p53 over-expression, DNA homologous repair pathway dysfunction, and poor prognosis.

CONCLUSIONS

The rate expression of CHEK1 is high in Nigerian breast cancer cases and is associated with an aggressive phenotype and poor prognosis.

Targeting ATR in cancer

The chemical treatment of cancer started with the realization that DNA damaging agents such as mustard gas present notable antitumoural properties. Consequently, early drug development focused on genotoxic chemicals, some of which are still widely used in the clinic. However, the efficacy of such therapies is often limited by the side effects of these drugs on healthy cells. A refinement to this approach is to use compounds that can exploit the presence of DNA damage in cancer cells. Given that replication stress (RS) is a major source of genomic instability in cancer, targeting the RS-response kinase ataxia telangiectasia and Rad3-related protein (ATR) has emerged as a promising alternative. With ATR inhibitors now entering clinical trials, we here revisit the biology behind this strategy and discuss potential biomarkers that could be used for a better selection of patients who respond to therapy.

Evaluation of CHK1 activation in vulvar squamous cell carcinoma and its potential as a therapeutic target in vitro

CHK1 is an important regulator of the cell cycle and DNA damage response, and its altered expression has been identified in various tumors. Chk1 inhibitors are currently being evaluated as monotherapy and as potentiators of chemotherapy in clinical settings. However, to our knowledge, no previous study has investigated either the activation status or the therapeutic potential of CHK1 targeting in vulvar cancer. Therefore, we examined the expression status of activated CHK1 forms pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 in 294 vulvar squamous cell carcinomas (VSCC) using immunohistochemistry and analyzed their relationships with various clinicopathological variables and clinical outcome. To aid translation of preclinical studies, we also assessed cell sensitivity to the Chk1 inhibition in two vulvar cancer cell lines. Compared to the levels of pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 in normal vulvar squamous epithelium, high nuclear pCHK1^Ser345 expression was found in 57% of vulvar carcinomas, whereas low nuclear pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 expressions were observed in 58%, 64%, and 40% of the cases, respectively. Low levels of pCHK1^Ser317 and pCHK1^Ser280 in the nucleus correlated significantly with advanced tumor behaviors and aggressive features. None of pCHK1^Ser345, pCHK1^Ser317, pCHK1^Ser296, and pCHK1^Ser280 forms were identified as prognostic factors. In vitro inhibition of CHK1 by small molecular inhibitors or siRNA reduced viability by inducing DNA damage and apoptosis of vulvar cancer cell lines. In summary, we conclude that cellular functions regulated by CHK1 are phosphorylation/localization‐dependent and deregulation of CHK1 function occurs in VSCC and might contribute to tumorigenesis. Targeting CHK1 might represent as a useful antitumor strategy for the subgroup of VSCC harboring p53 mutations.

Exploring the links between cancer and placenta development

The development of metastatic cancer is a multistage process, which often requires decades to complete. Impairments in DNA damage control and DNA repair in cancer cell precursors generate genetically heterogeneous cell populations. However, despite heterogeneity most solid cancers have stereotypical behaviours, including invasiveness and suppression of immune responses that can be unleashed with immunotherapy targeting lymphocyte checkpoints. The mechanisms leading to the acquisition of stereotypical properties remain poorly understood. Reactivation of embryonic development processes in cells with unstable genomes might contribute to tumour expansion and metastasis formation. However, it is unclear whether these events are linked to immune response modulation. Tumours and embryos have non-self-components and need to avoid immune responses in their microenvironment. In mammalian embryos, neo-antigens are of paternal origin, while in tumour cells DNA mismatch repair and replication defects generate them. Inactivation of the maternal immune response towards the embryo, which occurs at the placental-maternal interface, is key to ensuring embryonic development. This regulation is accomplished by the trophoblast, which mimics several malignant cell features, including the ability to invade normal tissues and to avoid host immune responses, often adopting the same cancer immunoediting strategies. A better understanding as to whether and how genotoxic stress promotes cancer development through reactivation of programmes occurring during early stages of mammalian placentation could help to clarify resistance to drugs targeting immune checkpoint and DNA damage responses and to develop new therapeutic strategies to eradicate cancer.

Mechanisms of Oncogene-Induced Replication Stress: Jigsaw Falling into Place

Oncogene activation disturbs cellular processes and accommodates a complex landscape of changes in the genome that contribute to genomic instability, which accelerates mutation rates and promotes tumorigenesis. Part of this cellular turmoil involves deregulation of physiologic DNA replication, widely described as replication stress. Oncogene-induced replication stress is an early driver of genomic instability and is attributed to a plethora of factors, most notably aberrant origin firing, replication-transcription collisions, reactive oxygen species, and defective nucleotide metabolism.Significance: Replication stress is a fundamental step and an early driver of tumorigenesis and has been associated with many activated oncogenes. Deciphering the mechanisms that contribute to the replication stress response may provide new avenues for targeted cancer treatment. In this review, we discuss the latest findings on the DNA replication stress response and examine the various mechanisms through which activated oncogenes induce replication stress. Cancer Discov; 8(5); 537-55. ©2018 AACR.

ERF deletion rescues RAS deficiency in mouse embryonic stem cells

Mayor-Ruiz et al. show that deletion of ERF rescues the proliferative defects of RAS-devoid mESCs and restores their capacity to differentiate.

MEK inhibition in combination with a glycogen synthase kinase-3β (GSK3β) inhibitor, referred as the 2i condition, favors pluripotency in embryonic stem cells (ESCs). However, the mechanisms by which the 2i condition limits ESC differentiation and whether RAS proteins are involved in this phenomenon remain poorly understood. Here we show that RAS nullyzygosity reduces the growth of mouse ESCs (mESCs) and prohibits their differentiation. Upon RAS deficiency or MEK inhibition, ERF (E twenty-six 2 [Ets2]-repressive factor), a transcriptional repressor from the ETS domain family, translocates to the nucleus, where it binds to the enhancers of pluripotency factors and key RAS targets. Remarkably, deletion of Erf rescues the proliferative defects of RAS-devoid mESCs and restores their capacity to differentiate. Furthermore, we show that Erf loss enables the development of RAS nullyzygous teratomas. In summary, this work reveals an essential role for RAS proteins in pluripotency and identifies ERF as a key mediator of the response to RAS/MEK/ERK inhibition in mESCs.

Replication stress in hematopoietic stem cells in mouse and man

Life-long blood regeneration relies on a rare population of self-renewing hematopoietic stem cells (HSCs). These cells' nearly unlimited self-renewal potential and lifetime persistence in the body signifies the need for tight control of their genome integrity. Their quiescent state, tightly linked with low metabolic activity, is one of the main strategies employed by HSCs to preserve an intact genome. On the other hand, HSCs need to be able to quickly respond to increased blood demands and rapidly increase their cellular output in order to fight infection-associated inflammation or extensive blood loss. This increase in proliferation rate, however, comes at the price of exposing HSCs to DNA damage inevitably associated with the process of DNA replication. Any interference with normal replication fork progression leads to a specialized molecular response termed replication stress (RS). Importantly, increased levels of RS are a hallmark feature of aged HSCs, where an accumulating body of evidence points to causative relationships between RS and the aging-associated impairment of the blood system's functional capacity. In this review, we present an overview of RS in HSCs focusing on its causes and consequences for the blood system of mice and men.

Efficacy of ATR inhibitors as single agents in Ewing sarcoma

Ewing sarcomas (ES) are pediatric bone tumors that arise from a driver translocation, most frequently EWS/FLI1. Current ES treatment involves DNA damaging agents, yet the basis for the sensitivity to these therapies remains unknown. Oncogene-induced replication stress (RS) is a known source of endogenous DNA damage in cancer, which is suppressed by ATR and CHK1 kinases. We here show that ES suffer from high endogenous levels of RS, rendering them particularly dependent on the ATR pathway. Accordingly, two independent ATR inhibitors show in vitro toxicity in ES cell lines as well as in vivo efficacy in ES xenografts as single agents. Expression of EWS/FLI1 or EWS/ERG oncogenic translocations sensitizes non-ES cells to ATR inhibitors. Our data shed light onto the sensitivity of ES to genotoxic agents, and identify ATR inhibitors as a potential therapy for Ewing Sarcomas.

RNF126 as a Biomarker of a Poor Prognosis in Invasive Breast Cancer and CHEK1 Inhibitor Efficacy in Breast Cancer Cells

Purpose: (i) To investigate the expression of the E3 ligase, RNF126, in human invasive breast cancer and its links with breast cancer outcomes; and (ii) to test the hypothesis that RNF126 determines the efficacy of inhibitors targeting the cell-cycle checkpoint kinase, CHEK1.Experimental Design: A retrospective analysis by immunohistochemistry (IHC) compared RNF126 staining in 110 invasive breast cancer and 78 paired adjacent normal tissues with clinicopathologic data. Whether RNF126 controls CHEK1 expression was determined by chromatin immunoprecipitation and a CHEK1 promoter driven luciferase reporter. Staining for these two proteins by IHC using tissue microarrays was also conducted. Cell killing/replication stress induced by CHEK1 inhibition was evaluated in cells, with or without RNF126 knockdown, by MTT/colony formation, replication stress biomarker immunostaining and DNA fiber assays.Results: RNF126 protein expression was elevated in breast cancer tissue samples. RNF126 was associated with a poor clinical outcome after multivariate analysis and was an independent predictor. RNF126 promotes CHEK1 transcript expression. Critically, a strong correlation between RNF126 and CHEK1 proteins was identified in breast cancer tissue and cell lines. The inhibition of CHEK1 induced a greater cell killing and a higher level of replication stress in breast cancer cells expressing RNF126 compared to RNF126 depleted cells.Conclusions: RNF126 protein is highly expressed in invasive breast cancer tissue. The high expression of RNF126 is an independent predictor of a poor prognosis in invasive breast cancer and is considered a potential biomarker of a cancer's responsiveness to CHEK1 inhibitors. CHEK1 inhibition targets breast cancer cells expressing higher levels of RNF126 by enhancing replication stress. Clin Cancer Res; 24(7); 1629-43. ©2018 AACR.

Targeting radioresistant breast cancer cells by single agent CHK1 inhibitor via enhancing replication stress

Radiotherapy (RT) remains a standard therapeutic modality for breast cancer patients. However, intrinsic or acquired resistance limits the efficacy of RT. Here, we demonstrate that CHK1 inhibitor AZD7762 alone significantly inhibited the growth of radioresistant breast cancer cells (RBCC). Given the critical role of ATR/CHK1 signaling in suppressing oncogene-induced replication stress (RS), we hypothesize that CHK1 inhibition leads to the specific killing for RBCC due to its abrogation in the suppression of RS induced by oncogenes. In agreement, the expression of oncogenes c-Myc/CDC25A/c-Src/H-ras/E2F1 and DNA damage response (DDR) proteins ATR/CHK1/BRCA1/CtIP were elevated in RBCC. AZD7762 exposure led to significantly higher levels of RS in RBCC, compared to the parental cells. The mechanisms by which CHK1 inhibition led to specific increase of RS in RBCC were related to the interruptions in the replication fork dynamics and the homologous recombination (HR). In summary, RBCC activate oncogenic pathways and thus depend upon mechanisms controlled by CHK1 signaling to maintain RS under control for survival. Our study provided the first example where upregulating RS by CHK1 inhibitor contributes to the specific killing of RBCC, and highlight the importance of the CHK1 as a potential target for treatment of radioresistant cancer cells.

Checkpoint kinase 1 is essential for normal B cell development and lymphomagenesis

Checkpoint kinase 1 (CHK1) is critical for intrinsic cell cycle control and coordination of cell cycle progression in response to DNA damage. Despite its essential function, CHK1 has been identified as a target to kill cancer cells and studies using Chk1 haploinsufficient mice initially suggested a role as tumor suppressor. Here, we report on the key role of CHK1 in normal B-cell development, lymphomagenesis and cell survival. Chemical CHK1 inhibition induces BCL2-regulated apoptosis in primary as well as malignant B-cells and CHK1 expression levels control the timing of lymphomagenesis in mice. Moreover, total ablation of Chk1 in B-cells arrests their development at the pro-B cell stage, a block that, surprisingly, cannot be overcome by inhibition of mitochondrial apoptosis, as cell cycle arrest is initiated as an alternative fate to limit the spread of damaged DNA. Our findings define CHK1 as essential in B-cell development and potent target to treat blood cancer.

Targeting ATR/CHK1 pathway in acute myeloid leukemia to overcome chemoresistance

Resistance of acute myeloid leukemia to current therapies leads to frequent relapses. Identification of molecular mechanisms involved in chemoresistance constitutes a key challenge to define new therapeutic concepts. Here, we show that the ATR/CHK1 pathway, essential in maintaining genomic stability, is involved in resistance and proliferation characteristics of leukemic cells.

ATR/CHK1 inhibitors and cancer therapy

The cell cycle checkpoint proteins ataxia-telangiectasia-mutated-and-Rad3-related kinase (ATR) and its major downstream effector checkpoint kinase 1 (CHK1) prevent the entry of cells with damaged or incompletely replicated DNA into mitosis when the cells are challenged by DNA damaging agents, such as radiation therapy (RT) or chemotherapeutic drugs, that are the major modalities to treat cancer. This regulation is particularly evident in cells with a defective G1 checkpoint, a common feature of cancer cells, due to p53 mutations. In addition, ATR and/or CHK1 suppress replication stress (RS) by inhibiting excess origin firing, particularly in cells with activated oncogenes. Those functions of ATR/CHK1 make them ideal therapeutic targets. ATR/CHK1 inhibitors have been developed and are currently used either as single agents or paired with radiotherapy or a variety of genotoxic chemotherapies in preclinical and clinical studies. Here, we review the status of the development of ATR and CHK1 inhibitors. We also discuss the potential mechanisms by which ATR and CHK1 inhibition induces cell killing in the presence or absence of exogenous DNA damaging agents, such as RT and chemotherapeutic agents. Lastly, we discuss synthetic lethality interactions between the inhibition of ATR/CHK1 and defects in other DNA damage response (DDR) pathways/genes.

The Role of the Transcriptional Response to DNA Replication Stress

During DNA replication many factors can result in DNA replication stress. The DNA replication stress checkpoint prevents the accumulation of replication stress-induced DNA damage and the potential ensuing genome instability. A critical role for post-translational modifications, such as phosphorylation, in the replication stress checkpoint response has been well established. However, recent work has revealed an important role for transcription in the cellular response to DNA replication stress. In this review, we will provide an overview of current knowledge of the cellular response to DNA replication stress with a specific focus on the DNA replication stress checkpoint transcriptional response and its role in the prevention of replication stress-induced DNA damage.

TTF-1/NKX2-1 binds to DDB1 and confers replication stress resistance to lung adenocarcinomas

TTF-1, also known as NKX2-1, is a transcription factor that has indispensable roles in both lung development and physiology. We and others have reported that TTF-1 frequently exhibits high expression with increased copy number in lung adenocarcinomas, and also has a role as a lineage-survival oncogene through transcriptional activation of crucial target genes including ROR1 and LMO3. In the present study, we employed a global proteomic search for proteins that interact with TTF-1 in order to provide a more comprehensive picture of this still enigmatic lineage-survival oncogene. Our results unexpectedly revealed a function independent of its transcriptional activity, as TTF-1 was found to interact with DDB1 and block its binding to CHK1, which in turn attenuated ubiquitylation and subsequent degradation of CHK1. Furthermore, TTF-1 overexpression conferred resistance to cellular conditions under DNA replication stress (RS) and prevented an increase in consequential DNA double-strand breaks, as reflected by attenuated induction of pCHK2 and γH2AX. Our findings suggest that the novel non-transcriptional function of TTF-1 identified in this study may contribute to lung adenocarcinoma development by conferring tolerance to DNA RS, which is known to be inherently elicited by activation of various oncogenes.

Cell cycle proteins as promising targets in cancer therapy

Cancer is characterized by uncontrolled tumour cell proliferation resulting from aberrant activity of various cell cycle proteins. Therefore, cell cycle regulators are considered attractive targets in cancer therapy. Intriguingly, animal models demonstrate that some of these proteins are not essential for proliferation of non-transformed cells and development of most tissues. By contrast, many cancers are uniquely dependent on these proteins and hence are selectively sensitive to their inhibition. After decades of research on the physiological functions of cell cycle proteins and their relevance for cancer, this knowledge recently translated into the first approved cancer therapeutic targeting of a direct regulator of the cell cycle. In this Review, we focus on proteins that directly regulate cell cycle progression (such as cyclin-dependent kinases (CDKs)), as well as checkpoint kinases, Aurora kinases and Polo-like kinases (PLKs). We discuss the role of cell cycle proteins in cancer, the rationale for targeting them in cancer treatment and results of clinical trials, as well as the future therapeutic potential of various cell cycle inhibitors.

Excess Polθ functions in response to replicative stress in homologous recombination-proficient cancer cells

DNA polymerase theta (Polθ) is a specialized A-family DNA polymerase that functions in processes such as translesion synthesis (TLS), DNA double-strand break repair and DNA replication timing. Overexpression of POLQ, the gene encoding Polθ, is a prognostic marker for an adverse outcome in a wide range of human cancers. While increased Polθ dosage was recently suggested to promote survival of homologous recombination (HR)-deficient cancer cells, it remains unclear whether POLQ overexpression could be also beneficial to HR-proficient cancer cells. By performing a short interfering (si)RNA screen in which genes encoding druggable proteins were knocked down in Polθ-overexpressing cells as a means to uncover genetic vulnerabilities associated with POLQ overexpression, we could not identify genes that were essential for viability in Polθ-overexpressing cells in normal growth conditions. We also showed that, upon external DNA replication stress, Polθ expression promotes cell survival and limits genetic instability. Finally, we report that POLQ expression correlates with the expression of a set of HR genes in breast, lung and colorectal cancers. Collectively, our data suggest that Polθ upregulation, besides its importance for survival of HR-deficient cancer cells, may be crucial also for HR-proficient cells to better tolerate DNA replication stress, as part of a global gene deregulation response, including HR genes.

Summary: Our work suggests that Polθ upregulation may be crucial for homologous recombination (HR)-proficient cells to better tolerate DNA replication stress, as part of a global gene deregulation response, including HR genes.

Targeting the kinase activities of ATR and ATM exhibits antitumoral activity in mouse models of MLL-rearranged AML

Among the various subtypes of acute myeloid leukemia (AML), those with chromosomal rearrangements of the MLL oncogene (AML-MLL) have a poor prognosis. AML-MLL tumor cells are resistant to current genotoxic therapies because of an attenuated response by p53, a protein that induces cell cycle arrest and apoptosis in response to DNA damage. In addition to chemicals that damage DNA, efforts have focused on targeting DNA repair enzymes as a general chemotherapeutic approach to cancer treatment. Here, we found that inhibition of the kinase ATR, which is the primary sensor of DNA replication stress, induced chromosomal breakage and death of mouse AML(MLL) cells (with an MLL-ENL fusion and a constitutively active N-RAS independently of p53. Moreover, ATR inhibition as a single agent exhibited antitumoral activity, both reducing tumor burden after establishment and preventing tumors from growing, in an immunocompetent allograft mouse model of AML(MLL) and in xenografts of a human AML-MLL cell line. We also found that inhibition of ATM, a kinase that senses DNA double-strand breaks, also promoted the survival of the AML(MLL) mice. Collectively, these data indicated that ATR or ATM inhibition represent potential therapeutic strategies for the treatment of AML, especially MLL-driven leukemias.

CHK1 as a therapeutic target to bypass chemoresistance in AML

The nucleoside analog cytarabine, an inhibitor of DNA replication fork progression that results in DNA damage, is currently used in the treatment of acute myeloid leukemia (AML). We explored the prognostic value of the expression of 72 genes involved in various aspects of DNA replication in a set of 198 AML patients treated by cytarabine-based chemotherapy. We unveiled that high expression of the DNA replication checkpoint gene CHEK1 is a prognostic marker associated with shorter overall, event-free, and relapse-free survivals and determined that the expression of CHEK1 can predict more frequent and earlier postremission relapse. CHEK1 encodes checkpoint kinase 1 (CHK1), which is activated by the kinase ATR when DNA replication is impaired by DNA damage. High abundance of CHK1 in AML patient cells correlated with higher clonogenic ability and more efficient DNA replication fork progression upon cytarabine treatment. Exposing the patient cells with the high abundance of CHK1 to SCH900776, an inhibitor of the kinase activity of CHK1, reduced clonogenic ability and progression of DNA replication in the presence of cytarabine. These results indicated that some AML cells rely on an efficient CHK1-mediated replication stress response for viability and that therapeutic strategies that inhibit CHK1 could extend current cytarabine-based treatments and overcome drug resistance. Furthermore, monitoring CHEK1 expression could be used both as a predictor of outcome and as a marker to select AML patients for CHK1 inhibitor treatments.

The DNA damage response molecule MCPH1 in brain development and beyond

Microcephalin (MCPH1) is identified as being responsible for the neurodevelopmental disorder primary microcephaly type 1, which is characterized by a smaller-than-normal brain size and mental retardation. MCPH1 has originally been identified as an important regulator of telomere integrity and of cell cycle control. Genetic and cellular studies show that MCPH1 controls neurogenesis by coordinating the cell cycle and the centrosome cycle and thereby regulating the division mode of neuroprogenitors to prevent the exhaustion of the progenitor pool and thereby microcephaly. In addition to its role in neurogenesis, MCPH1 plays a role in gonad development. MCPH1 also functions as a tumor suppressor in several human cancers as well as in mouse models. Here, we review the role of MCPH1 in DNA damage response, cell cycle control, chromosome condensation and chromatin remodeling. We also summarize the studies on the biological functions of MCPH1 in brain size determination and in pathologies, including infertility and cancer.

USP7 is a SUMO deubiquitinase essential for DNA replication

Post-translational modification of proteins by ubiquitin (Ub) and Ub-like modifiers regulates DNA replication. We have previously shown that chromatin around replisomes is rich in SUMO and poor in Ub, whereas mature chromatin exhibits an opposite pattern. How this SUMO-rich, Ub-poor environment is maintained at sites of DNA replication in mammalian cells remains unexplored. Here we identify USP7 as a replisome-enriched SUMO deubiquitinase that is essential for DNA replication. By acting on SUMO and SUMOylated proteins, USP7 counteracts their ubiquitination. Inhibition or genetic deletion of USP7 leads to the accumulation of Ub on SUMOylated proteins, which are displaced away from replisomes. Our findings provide a model explaining the differential accumulation of SUMO and Ub at replication forks and identify an essential role of USP7 in DNA replication that should be considered in the development of USP7 inhibitors as anticancer agents.

New origin firing is inhibited by APC/CCdh1 activation in S-phase after severe replication stress

Defects in DNA replication and repair are known to promote genomic instability, a hallmark of cancer cells. Thus, eukaryotic cells have developed complex mechanisms to ensure accurate duplication of their genomes. While DNA damage response has been extensively studied in tumour cells, the pathways implicated in the response to replication stress are less well understood especially in non-transformed cells. Here we show that in non-transformed cells, APC/C(Cdh1) is activated upon severe replication stress. Activation of APC/C(Cdh1) prevents new origin firing and induces permanent arrest in S-phase. Moreover, Rad51-mediated homologous recombination is also impaired under these conditions. APC/C(Cdh1) activation in S-phase occurs after replication forks have been processed into double strand breaks. Remarkably, this activation, which correlates with decreased Emi1 levels, is not prevented by ATR/ATM inhibition, but it is abrogated in cells depleted of p53 or p21. Importantly, we found that the lack of APC/C(Cdh1) activity correlated with an increase in genomic instability. Taken together, our results define a new APC/C(Cdh1) function that prevents cell cycle resumption after prolonged replication stress by inhibiting origin firing, which may act as an additional mechanism in safeguarding genome integrity.

DNA replication stress: from molecular mechanisms to human disease

The genome of proliferating cells must be precisely duplicated in each cell division cycle. Chromosomal replication entails risks such as the possibility of introducing breaks and/or mutations in the genome. Hence, DNA replication requires the coordinated action of multiple proteins and regulatory factors, whose deregulation causes severe developmental diseases and predisposes to cancer. In recent years, the concept of "replicative stress" (RS) has attracted much attention as it impinges directly on genomic stability and offers a promising new avenue to design anticancer therapies. In this review, we summarize recent progress in three areas: (1) endogenous and exogenous factors that contribute to RS, (2) molecular mechanisms that mediate the cellular responses to RS, and (3) the large list of diseases that are directly or indirectly linked to RS.

Genomic stability during cellular reprogramming: Mission impossible?

The generation of induced pluripotent stem cells (iPSCs) from adult somatic cells is one of the most exciting discoveries in recent biomedical research. It holds tremendous potential in drug discovery and regenerative medicine. However, a series of reports highlighting genomic instability in iPSCs raises concerns about their clinical application. Although the mechanisms cause genomic instability during cellular reprogramming are largely unknown, several potential sources have been suggested. This review summarizes current knowledge on this active research field and discusses the latest efforts to alleviate the genomic insults during cellular reprogramming to generate iPSCs with enhanced quality and safety.