Claspin, a regulator of Chk1 in DNA replication stress pathway

Unlocking the genetic tapestry of autoimmune diseases: Unveiling common genes across multiple conditions

OBJECTIVES

This study aimed to unravel the complexities of autoimmune diseases by conducting a comprehensive analysis of gene expression data across 10 conditions, including systemic lupus erythematosus (SLE), psoriasis, Sjögren's syndrome, sclerosis, immune-associated diseases, osteoarthritis, cystic fibrosis, inflammatory bowel disease (IBD), type 1 diabetes, and Guillain-Barré syndrome.

METHODS

Gene expression profiles were rigorously examined to identify both upregulated and downregulated genes specific to each autoimmune disease. The study employed visual representation techniques such as heatmaps, volcano plots, and contour-MA plots to provide an intuitive understanding of the complex gene expression patterns in these conditions.

RESULTS

Distinct gene expression profiles for each autoimmune condition were uncovered, with psoriasis and osteoarthritis standing out due to a multitude of both upregulated and downregulated genes, indicating intricate molecular interplays in these disorders. Notably, common upregulated and downregulated genes were identified across various autoimmune conditions, with genes like SELENBP1, MMP9, BNC1, and COL1A1 emerging as pivotal players.

CONCLUSION

This research contributes valuable insights into the molecular signatures of autoimmune diseases, highlighting the unique gene expression patterns characterizing each condition. The identification of common genes shared among different autoimmune conditions, and their potential role in mitigating the risk of rare diseases in patients with more prevalent conditions, underscores the growing significance of genetics in healthcare and the promising future of personalized medicine.

Association of SNPs in the PAI1 Gene with Disease Recurrence and Clinical Outcome in Bladder Cancer

PURPOSE

Bladder cancer (BCa) is one of the most common cancer types worldwide and is characterized by a high rate of recurrence. In previous studies, we and others have described the functional influence of plasminogen activator inhibitor-1 (PAI1) in bladder cancer development. While polymorphisms in PAI1 have been associated with increased risk and worsened prognosis in some cancers, the mutational status of PAI1 in human bladder tumors has not been well defined.

METHODS

In this study, we evaluated the mutational status of PAI1 in a series of independent cohorts, comprised of a total of 660 subjects.

RESULTS

Sequencing analyses identified two clinically relevant 3' untranslated region (UTR) single nucleotide polymorphisms (SNPs) in PAI1 (rs7242; rs1050813). Somatic SNP rs7242 was present in human BCa cohorts (overall incidence of 72%; 62% in Caucasians and 72% in Asians). In contrast, the overall incidence of germline SNP rs1050813 was 18% (39% in Caucasians and 6% in Asians). Furthermore, Caucasian patients with at least one of the described SNPs had worse recurrence-free survival and overall survival (p = 0.03 and p = 0.03, respectively). In vitro functional studies demonstrated that SNP rs7242 increased the anti-apoptotic effect of PAI1, and SNP rs1050813 was related to a loss of contact inhibition associated with cellular proliferation when compared to wild type.

CONCLUSION

Further investigation of the prevalence and potential downstream influence of these SNPs in bladder cancer is warranted.

Damsel in distress calling on her knights: Illuminating the pioneering role of E3 ubiquitin ligases in guarding the genome integrity

The maintenance of genomic integrity is of utmost importance for the organisms to survive and to accurately inherit traits to their progenies. Any kind of DNA damage either due to defect in DNA duplication and/ or uncontrolled cell division or intracellular insults or environment radiation can result in gene mutation, chromosomal aberration and ultimately genomic instability, which may cause several diseases including cancers. Therefore, cells have evolved machineries for the surveillance of genomic integrity. Enormous exciting studies in the past indicate that ubiquitination (a posttranslational modification of proteins) plays a crucial role in maintaining the genomic integrity by diverse ways. In fact, various E3 ubiquitin ligases catalyse ubiquitination of key proteins to control their central role during cell cycle, DNA damage response (DDR) and DNA repair. Some E3 ligases promote genomic instability while others prevent it, deregulation of both of which leads to several malignancies. In this review, we consolidate the recent findings wherein the role of ubiquitination in conferring genome integrity is highlighted. We also discuss the latest discoveries on the mechanisms utilized by various E3 ligases to preserve genomic stability, with a focus on their actions during cell cycle progression and different types of DNA damage response as well as repair pathways.

Clinicopathological significance of claspin overexpression and its efficacy as a novel biomarker for the diagnosis of urothelial carcinoma

We previously reported that claspin is a key regulator in the progression of gastric cancer and renal cell carcinoma. However, the clinicopathological significance of claspin in urothelial carcinoma (UC) has not been investigated. We analyzed the expression and distribution of claspin in UC cases by immunohistochemistry. In the non-neoplastic urothelium, the expression of claspin was either weak or absent, whereas UC tissues showed nuclear staining. The expression of claspin was detected in 58 (42%) of a total of 138 upper tract UC cases treated by radical nephroureterectomy without neoadjuvant chemotherapy. Claspin-positive UC cases were associated with nodular/flat morphology, variant histology, high tumor grade, high pathological T grade, and lymphatic and venous invasion. The expression of claspin was significantly associated with decreased progression-free survival and cancer-specific survival. In addition, claspin was co-expressed with Ki-67, PD-L1, HER2, EGFR, and p53 in consecutive tumor sections of UC. An immunohistochemical analysis of claspin in biopsy specimens revealed that strong to moderate claspin staining was more frequently observed in carcinoma in situ in comparison to dysplasia or the benign urothelium. Furthermore, immunocytochemistry for claspin on urine cytology slides demonstrated that the proportion of claspin-positive cells was significantly greater in high-grade UC than in benign cases. These results suggest that claspin may be a novel prognostic marker and a possible therapeutic target molecule for UC. Moreover, claspin could be a useful diagnostic biomarker of urothelial neoplasia.

Overexpression of claspin promotes docetaxel resistance and is associated with prostate-specific antigen recurrence in prostate cancer

Although docetaxel (DTX) confers significant survival benefits in patients with castration‐resistant prostate cancer (CRPC), resistance to DTX inevitably occurs. Therefore, clarifying the mechanisms of DTX resistance may improve survival in patients with CRPC. Claspin plays a pivotal role in DNA replication stress and damage responses and is an essential regulator for the S‐phase checkpoint. CLSPN is an oncogenic gene that contributes to tumor proliferation in several human solid tumors. However, the clinical significance of claspin in prostate cancer (PCa) has not been examined. The present study aimed to elucidate the role of claspin and its relationship with DTX resistance in PCa. We immunohistochemically analyzed the expression of claspin in 89 PCa cases, of which 31 (35%) were positive for claspin. Claspin‐positive cases were associated with higher Gleason score, venous invasion, and perineural invasion. Kaplan–Meier analysis showed that high claspin expression was related to poor prostate‐specific antigen (PSA) relapse‐free prognosis. In a public database, high CLSPN expression was associated with poor PSA relapse‐free prognosis, Gleason score, T stage, lymph node metastasis, CRPC, and metastatic PCa. Claspin knockdown by siRNA decreased cell proliferation, upregulated DTX sensitivity, and suppressed the expression of Akt, Erk1/2, and CHK1 phosphorylation in DU145 and PC3 cell lines. Furthermore, claspin expression was much more upregulated in DTX‐resistant DU145 (DU145‐DR) than in parental DU145 cells. Claspin knockdown significantly upregulated the sensitivity to DTX in DU145‐DR cells. These results suggest that claspin plays an important role in PCa tumor progression and DTX resistance.

Signaling pathways involved in cell cycle arrest during the DNA breaks

Our genome bears tens of thousands of harms and devastations per day; In this regard, numerous sophisticated and complicated mechanisms are embedded by our cells in furtherance of remitting an unchanged and stable genome to their next generation. These mechanisms, that are collectively called DDR, have the duty of detecting the lesions and repairing them. it's necessary for the viability of any living cell that sustain the integrity and stability of its genetic content and this highlights the role of mediators that transduce the signals of DNA damage to the cell cycle in order to prevent the replication of a defective DNA. In this paper, we review the signaling pathways that lie between these processes and define how different ingredients of DDR are also able to affect the checkpoint signaling.

Implications of CLSPN Variants in Cellular Function and Susceptibility to Cancer

Claspin is a multifunctional protein that participates in physiological processes essential for cell homeostasis that are often defective in cancer, namely due to genetic changes. It is conceivable that Claspin gene (CLSPN) alterations may contribute to cancer development. Therefore, CLSPN germline alterations were characterized in sporadic and familial breast cancer and glioma samples, as well as in six cancer cell lines. Their association to cancer susceptibility and functional impact were investigated. Eight variants were identified (c.-68C>T, c.17G>A, c.1574A>G, c.2230T>C, c.2028+16G>A, c.3595-3597del, and c.3839C>T). CLSPN c.1574A>G (p.Asn525Ser) was significantly associated with breast cancer and was shown to cause partial exon skipping and decreased Claspin expression and Chk1 activation in a minigene splicing assay and in signalling experiments, respectively. CLSPN c.2028+16G>A was significantly associated with familial breast cancer and glioma, whereas c.2230T>C (p.Ser744Pro), was exclusively detected in breast cancer and glioma patients, but not in healthy controls. The remaining variants lacked a significant association with cancer. Nevertheless, the c.-68C>T promoter variant increased transcriptional activity in a luciferase assay. In conclusion, some of the CLSPN variants identified in the present study appear to modulate Claspin’s function by altering CLSPN transcription and RNA processing, as well as Chk1 activation.

Claspin overexpression is associated with high-grade histology and poor prognosis in renal cell carcinoma

Renal cell carcinoma (RCC) is one of the most common human cancers. We previously reported that claspin is a key regulator in the progression of gastric cancer, and it likely plays an important role in cancer stem cells of gastric cancer. However, the significance of claspin in RCC has not been examined. First, we analyzed the expression and distribution of claspin in 95 RCC cases by immunohistochemistry. In the nonneoplastic kidney, the staining of claspin was either weak or absent, whereas RCC tissue showed nuclear staining. In total, claspin expression was detected in 45 (47%) of 95 RCC cases. The claspin staining appeared relatively stronger in high nuclear grade RCC than in low nuclear grade RCC. Claspin-positive RCC cases were associated with higher T grade, tumor stage, nuclear grade, vein invasion, and poorer prognosis. CLSPN siRNA treatment decreased RCC cell proliferation. The levels of phosphorylated Erk and Akt were lower in CLSPN siRNA-transfected RCC cells than in control cells. In addition, claspin was coexpressed with CD44, epidermal growth factor receptor, p53, and programmed death ligand-1. These results suggest that claspin plays an important role in tumor progression in RCC and might be a prognostic marker and novel therapeutic target molecule.

Inhibition of CHK 1 (Checkpoint Kinase 1) Elicits Therapeutic Effects in Pulmonary Arterial Hypertension

OBJECTIVE

Pulmonary arterial hypertension (PAH) is a debilitating disease associated with progressive vascular remodeling of distal pulmonary arteries leading to elevation of pulmonary artery pressure, right ventricular hypertrophy, and death. Although presenting high levels of DNA damage that normally jeopardize their viability, pulmonary artery smooth muscle cells (PASMCs) from patients with PAH exhibit a cancer-like proproliferative and apoptosis-resistant phenotype accounting for vascular lumen obliteration. In cancer cells, overexpression of the serine/threonine-protein kinase CHK1 (checkpoint kinase 1) is exploited to counteract the excess of DNA damage insults they are exposed to. This study aimed to determine whether PAH-PASMCs have developed an orchestrated response mediated by CHK1 to overcome DNA damage, allowing cell survival and proliferation. Approach and Results: We demonstrated that CHK1 expression is markedly increased in isolated PASMCs and distal PAs from patients with PAH compared with controls, as well as in multiple complementary animal models recapitulating the disease, including monocrotaline rats and the simian immunodeficiency virus-infected macaques. Using a pharmacological and molecular loss of function approach, we showed that CHK1 promotes PAH-PASMCs proliferation and resistance to apoptosis. In addition, we found that inhibition of CHK1 induces downregulation of the DNA repair protein RAD 51 and severe DNA damage. In vivo, we provided evidence that pharmacological inhibition of CHK1 significantly reduces vascular remodeling and improves hemodynamic parameters in 2 experimental rat models of PAH.

CONCLUSIONS

Our results show that CHK1 exerts a proproliferative function in PAH-PASMCs by mitigating DNA damage and suggest that CHK1 inhibition may, therefore, represent an attractive therapeutic option for patients with PAH.

Mrc1 and Rad9 cooperate to regulate initiation and elongation of DNA replication in response to DNA damage

The S-phase checkpoint maintains the integrity of the genome in response to DNA replication stress. In budding yeast, this pathway is initiated by Mec1 and is amplified through the activation of Rad53 by two checkpoint mediators: Mrc1 promotes Rad53 activation at stalled forks, and Rad9 is a general mediator of the DNA damage response. Here, we have investigated the interplay between Mrc1 and Rad9 in response to DNA damage and found that they control DNA replication through two distinct but complementary mechanisms. Mrc1 rapidly activates Rad53 at stalled forks and represses late-firing origins but is unable to maintain this repression over time. Rad9 takes over Mrc1 to maintain a continuous checkpoint signaling. Importantly, the Rad9-mediated activation of Rad53 slows down fork progression, supporting the view that the S-phase checkpoint controls both the initiation and the elongation of DNA replication in response to DNA damage. Together, these data indicate that Mrc1 and Rad9 play distinct functions that are important to ensure an optimal completion of S phase under replication stress conditions.

Clinicopathological significance of claspin overexpression and its association with spheroid formation in gastric cancer

Gastric cancer (GC) is one of the leading causes of cancer-related death worldwide. Spheroid colony formation is a useful method to identify cancer stem cells (CSCs). The aim of this study was to identify a novel prognostic marker or therapeutic target for GC using a method to identify CSCs. We analyzed the microarray data in spheroid body-forming and parental cells and focused on the CLSPN gene because it is overexpressed in the spheroid body-forming cells in both the GC cell lines MKN-45 and MKN-74. Quantitative reverse-transcription polymerase chain reaction analysis revealed that CLSPN messenger RNA expression was up-regulated in GC cell lines MKN-45, MKN-74, and TMK-1. Immunohistochemistry of claspin showed that 94 (47%) of 203 GC cases were positive. Claspin-positive GC cases were associated with higher T and N grades, tumor stage, lymphatic invasion, and poor prognosis. In addition, claspin expression was coexpressed with CD44, human epidermal growth factor receptor type 2, and p53. CLSPN small interfering RNA treatment decreased GC cell proliferation and invasion. These results indicate that the expression of claspin might be a key regulator in the progression of GC and might play an important role in CSCs of GC.

The Augmented R-Loop Is a Unifying Mechanism for Myelodysplastic Syndromes Induced by High-Risk Splicing Factor Mutations

Mutations in several general pre-mRNA splicing factors have been linked to myelodysplastic syndromes (MDSs) and solid tumors. These mutations have generally been assumed to cause disease by the resultant splicing defects, but different mutations appear to induce distinct splicing defects, raising the possibility that an alternative common mechanism is involved. Here we report a chain of events triggered by multiple splicing factor mutations, especially high-risk alleles in SRSF2 and U2AF1, including elevated R-loops, replication stress, and activation of the ataxia telangiectasia and Rad3-related protein (ATR)-Chk1 pathway. We further demonstrate that enhanced R-loops, opposite to the expectation from gained RNA binding with mutant SRSF2, result from impaired transcription pause release because the mutant protein loses its ability to extract the RNA polymerase II (Pol II) C-terminal domain (CTD) kinase-the positive transcription elongation factor complex (P-TEFb)-from the 7SK complex. Enhanced R-loops are linked to compromised proliferation of bone-marrow-derived blood progenitors, which can be partially rescued by RNase H overexpression, suggesting a direct contribution of augmented R-loops to the MDS phenotype.

The E1B19K-deleted oncolytic adenovirus mutant AdΔ19K sensitizes pancreatic cancer cells to drug-induced DNA-damage by down-regulating Claspin and Mre11

Adenovirus-mediated sensitization of cancer cells to cytotoxic drugs depends on simultaneous interactions of early viral genes with cell death and survival pathways. It is unclear what cellular factors mediate these interactions in the presence of DNA-damaging drugs. We found that adenovirus prevents Chk1-mediated checkpoint activation through inactivation of Mre11 and downregulation of the pChk1 adaptor-protein, Claspin, in cells with high levels of DNA-damage induced by the cytotoxic drugs gemcitabine and irinotecan. The mechanisms for Claspin downregulation involve decreased transcription and increased degradation, further attenuating pChk1-mediated signalling. Live cell imaging demonstrated that low doses of gemcitabine caused multiple mitotic aberrations including multipolar spindles, micro- and multi-nucleation and cytokinesis failure. A mutant virus with the anti-apoptotic E1B19K-gene deleted (AdΔ19K) further enhanced cell killing, Claspin downregulation, and potentiated drug-induced DNA damage and mitotic aberrations. Decreased Claspin expression and inactivation of Mre11 contributed to the enhanced cell killing in combination with DNA-damaging drugs. These results reveal novel mechanisms that are utilised by adenovirus to ensure completion of its life cycle in the presence of cellular DNA damage. Taken together, our findings reveal novel cellular targets that may be exploited when developing improved anti-cancer therapeutics.

Human CST Facilitates Genome-wide RAD51 Recruitment to GC-Rich Repetitive Sequences in Response to Replication Stress

The telomeric CTC1/STN1/TEN1 (CST) complex has been implicated in promoting replication recovery under replication stress at genomic regions, yet its precise role is unclear. Here, we report that STN1 is enriched at GC-rich repetitive sequences genome-wide in response to hydroxyurea (HU)-induced replication stress. STN1 deficiency exacerbates the fragility of these sequences under replication stress, resulting in chromosome fragmentation. We find that upon fork stalling, CST proteins form distinct nuclear foci that colocalize with RAD51. Furthermore, replication stress induces physical association of CST with RAD51 in an ATR-dependent manner. Strikingly, CST deficiency diminishes HU-induced RAD51 foci formation and reduces RAD51 recruitment to telomeres and non-telomeric GC-rich fragile sequences. Collectively, our findings establish that CST promotes RAD51 recruitment to GC-rich repetitive sequences in response to replication stress to facilitate replication restart, thereby providing insights into the mechanism underlying genome stability maintenance.

Ataxin-3 promotes genome integrity by stabilizing Chk1

The Chk1 protein is essential for genome integrity maintenance and cell survival in eukaryotic cells. After prolonged replication stress, Chk1 can be targeted for proteasomal degradation to terminate checkpoint signaling after DNA repair finishes. To ensure proper activation of DNA damage checkpoint and DNA repair signaling, a steady-state level of Chk1 needs to be retained under physiological conditions. Here, we report a dynamic signaling pathway that tightly regulates Chk1 stability. Under unperturbed conditions and upon DNA damage, ataxin-3 (ATX3) interacts with Chk1 and protects it from DDB1/CUL4A- and FBXO6/CUL1-mediated polyubiquitination and subsequent degradation, thereby promoting DNA repair and checkpoint signaling. Under prolonged replication stress, ATX3 dissociates from Chk1, concomitant with a stronger binding between Chk1 and its E3 ligase, which causes Chk1 proteasomal degradation. ATX3 deficiency results in pronounced reduction of Chk1 abundance, compromised DNA damage response, G2/M checkpoint defect and decreased cell survival after replication stress, which can all be rescued by ectopic expression of ATX3. Taken together, these findings reveal ATX3 to be a novel deubiquitinase of Chk1, providing a new mechanism of Chk1 stabilization in genome integrity maintenance.

Cellular responses to replication stress: Implications in cancer biology and therapy

DNA replication is essential for cell proliferation. Any obstacles during replication cause replication stress, which may lead to genomic instability and cancer formation. In this review, we summarize the physiological DNA replication process and the normal cellular response to replication stress. We also outline specialized therapies in clinical trials based on current knowledge and future perspectives in the field.

Oxidative stress preferentially induces a subtype of micronuclei and mediates the genomic instability caused by p53 dysfunction

Reactive oxygen species (ROS) are known to cause many types of DNA lesions that could be converted into cancer-promoting genetic alterations. Evidence showed that tumor suppressor p53 plays an important role in regulating the generation of cellular ROS, either by reducing oxidative stress under physiological and mildly stressed conditions, or by promoting oxidative stress under highly stressed conditions. In this report we characterized the effect of oxidative stress on the induction of micronuclei, especially the subclass marked by pan-staining of γ-H2AX or MN-γ-H2AX (+). We found that MN-γ-H2AX (+) were more responsive to hydrogen peroxide (H2O2) than the MN-γ-H2AX (−). In human and mouse cells that are deficient in p53, the frequency of MN-γ-H2AX (+) is significantly elevated, but can be attenuated by antioxidant N-acetylcysteine (NAC). Depletion of p53-regulated antioxidant gene SESN1 by RNA interference also resulted in an elevation of MN-γ-H2AX (+). Furthermore, we found that in cells that were depleted of p400 by RNAi, and therefore were experiencing increased ROS, the frequency of MN-γ-H2AX (+), but not that of MN-γ-H2AX (−), was significantly induced. We further demonstrated that the induction of MN-γ-H2AX (+) by replication stress can also be attenuated by NAC, and that H2O2 also leads to increased phosphorylation of Chk1 and Rad17 that mimics replication stress, suggesting that replication stress and oxidative stress are intertwined and may reinforce each other in driving genomic instability. Our findings illustrate the importance of p53-regulated redox level in the maintenance of genomic stability.

Molecularly targeted therapies in non-small-cell lung cancer annual update 2014

There have been significant advances in the understanding of the biology and treatment of non-small-cell lung cancer (NSCLC) during the past few years. A number of molecularly targeted agents are in the clinic or in development for patients with advanced NSCLC. We are beginning to understand the mechanisms of acquired resistance after exposure to tyrosine kinase inhibitors in patients with oncogene addicted NSCLC. The advent of next-generation sequencing has enabled to study comprehensively genomic alterations in lung cancer. Finally, early results from immune checkpoint inhibitors are very encouraging. This review summarizes recent advances in the area of cancer genomics, targeted therapies, and immunotherapy.

Novel insights into Chk1 regulation by phosphorylation

Checkpoint kinase 1 (Chk1) is a conserved protein kinase central to the cell-cycle checkpoint during DNA damage response (DDR). Until recently, ATR, a protein kinase activated in response to DNA damage or stalled replication, has been considered as the sole regulator of Chk1. Recent progress, however, has led to the identification of additional protein kinases involved in Chk1 phosphorylation, affecting the subcellular localization and binding partners of Chk1. In fact, spatio-temporal regulation of Chk1 is of critical importance not only in the DDR but also in normal cell-cycle progression. In due course, many potent inhibitors targeted to Chk1 have been developed as anticancer agents and some of these inhibitors are currently in clinical trials. In this review, we summarize the current knowledge of Chk1 regulation by phosphorylation.

DNA-PKcs is required to maintain stability of Chk1 and Claspin for optimal replication stress response

The ataxia telangiectasia mutated and Rad3-related (ATR)-checkpoint kinase 1 (Chk1) axis is the major signaling pathway activated in response to replication stress and is essential for the intra-S checkpoint. ATR phosphorylates and activates a number of molecules to coordinate cell cycle progression. Chk1 is the major effector downstream from ATR and plays a critical role in intra-S checkpoint on replication stress. Activation of Chk1 kinase also requires its association with Claspin, an adaptor protein essential for Chk1 protein stability, recruitment and ATR-dependent Chk1 phosphorylation. We have previously reported that, on replication stress, the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is rapidly phosphorylated by ATR at the stalled replication forks and is required for cellular resistance to replication stresses although the impact of DNA-PKcs onto the ATR signaling pathway remains elusive. Here we report that ATR-dependent Chk1 phosphorylation and Chk1 signaling are compromised in the absence of DNA-PKcs. Our investigation reveals that DNA-PKcs is required to maintain Chk1–Claspin complex stability and transcriptional regulation of Claspin expression. The impaired Chk1 activity results in a defective intra-S checkpoint response in DNA-PKcs–deficient cells. Taken together, these results suggest that DNA-PKcs, in addition to its direct role in DNA damage repair, facilitates ATR-Chk1 signaling pathway in response to replication stress.

Replicating damaged DNA in eukaryotes

DNA damage is one of many possible perturbations that challenge the mechanisms that preserve genetic stability during the copying of the eukaryotic genome in S phase. This short review provides, in the first part, a general introduction to the topic and an overview of checkpoint responses. In the second part, the mechanisms of error-free tolerance in response to fork-arresting DNA damage will be discussed in some detail.

Regulation of Rad17 protein turnover unveils an impact of Rad17-APC cascade in breast carcinogenesis and treatment

Aberrant regulation of DNA damage checkpoint function leads to genome instability that in turn can predispose cellular tissues to become cancerous. Previous works from us and others demonstrated the role of Rad17 in either activation or termination of DNA damage checkpoint function. In the current study, we have revealed the unexpected accumulation of Rad17 in various types of breast cancer cell lines as well as human breast cancer tissues. We observed that Rad17 protein turnover rate in breast epithelial cells is much faster than in breast cancer cells, where the turnover of Rad17 is regulated by the Cdh1/APC pathway. We further observed that Rad17-mediated checkpoint function is modulated by proteolysis. Stabilization of Rad17 disrupts cellular response to chemotherapeutic drug-induced DNA damage and enhances cellular transformation. In addition, manipulation of Rad17 by RNA interference or stabilization of Rad17 significantly sensitize breast cancer cell to various chemotherapeutic drugs. Our present results indicate the manipulation of Rad17 proteolysis could be a valuable approach to sensitize breast cancer cell to the chemotherapeutic treatment despite of the critical role in governing DNA damage response and cellular recovery from genotoxic stress.

Molecular and genomic approach for understanding the gene-environment interaction between Nrf2 deficiency and carcinogenic nickel-induced DNA damage

Nickel (II) is a toxic and carcinogenic metal which induces a redox imbalance following oxidative stress. Nuclear factor erythroid-2 related factor 2 (Nrf2) is a redox factor that regulates oxidation/reduction status and consequently mediates cytoprotective responses against exposure to environmental toxicants. In this study, we investigated the protective roles of the Nrf2 gene against oxidative stress and DNA damage induced by nickel at sub-lethal doses. Under nickel exposure conditions, we detected significantly increased intracellular ROS generation, in addition to higher amounts of DNA damage using comet assay and γ-H2AX immunofluorescence staining in Nrf2 lacking cells, as compared to Nrf2 wild-type cells. In addition, we attempted to identify potential nickel and Nrf2-responsive targets and the relevant pathway. The genomic expression data were analyzed using microarray for the selection of synergistic effect-related genes by Nrf2 knockdown under nickel treatment. In particular, altered expressions of 6 upregulated genes (CAV1, FOSL2, MICA, PIM2, RUNX1 and SLC7A6) and 4 downregulated genes (APLP1, CLSPN, PCAF and PRAME) were confirmed by qRT-PCR. Additionally, using bioinformatics tool, we found that these genes functioned principally in a variety of molecular processes, including oxidative stress response, necrosis, DNA repair and cell survival. Thus, we describe the potential biomarkers regarded as molecular candidates for Nrf2-related cellular protection against nickel exposure. In conclusion, these findings indicate that Nrf2 is an important factor with a protective role in the suppression of mutagenicity and carcinogenicity by environmental nickel exposure in terms of gene-environment interaction.

Drosophila Claspin is required for the G2 arrest that is induced by DNA replication stress but not by DNA double-strand breaks

ATR and Chk1 are protein kinases that perform major roles in the DNA replication checkpoint that delays entry into mitosis in response to DNA replication stress by hydroxyurea (HU) treatment. They are also activated by ionizing radiation (IR) that induces DNA double-strand breaks. Studies in human tissue culture and Xenopus egg extracts identified Claspin as a mediator that increased the activity of ATR toward Chk1. Because the in vivo functions of Claspin are not known, we generated Drosophila lines that each contained a mutated Claspin gene. Similar to the Drosophila mei-41/ATR and grp/Chk1 mutants, embryos of the Claspin mutant showed defects in checkpoint activation, which normally occurs in early embryogenesis in response to incomplete DNA replication. Additionally, Claspin mutant larvae were defective in G2 arrest after HU treatment; however, the defects were less severe than those of the mei-41/ATR and grp/Chk1 mutants. In contrast, IR-induced G2 arrest, which was severely defective in mei-41/ATR and grp/Chk1 mutants, occurred normally in the Claspin mutant. We also found that Claspin was phosphorylated in response to HU and IR treatment and a hyperphosphorylated form of Claspin was generated only after HU treatment in mei-41/ATR-dependent and tefu/ATM-independent way. In summary, our data suggest that Drosophila Claspin is required for the G2 arrest that is induced by DNA replication stress but not by DNA double-strand breaks, and this difference is probably due to distinct phosphorylation statuses.

Claspin as a biomarker of human papillomavirus-related high grade lesions of uterine cervix

BACKGROUND

Claspin is a nuclear protein involved in DNA replication and damage response and is a key mediator for the S-phase checkpoint. Claspin expression is significantly high in several human solid tumors. Furthermore, high levels of claspin have been found in cervical cancer cell lines. Nevertheless, no data are available regarding claspin expression in cervical tissues.

METHODS

In order to investigate whether claspin immunoreactivity is related to the lesion severity and High-Risk (HR) HPV infection, we analyzed claspin expression by immunohistochemistry in a series of cervical biopsies which represent the steps occurring during cervical carcinogenesis (normal tissues, Cervical Intraepithelial Neoplasias 1, 2 and 3, Squamous Cell Carcinomas). All patients also had a cervico-vaginal sample for HPV testing, collected immediately before the colposcopy-guided biopsy. The HR-HPV DNA detection was performed by the HR-HPV Hybrid Capture 2 test. HPV genotyping was performed using the Linear Array HPV Genotyping Test.

RESULTS

Our results evidenced a constant and significant increase of the rate of claspin positivity from the normal tissues to carcinomas (pχ2(trend) < 0.0001). In fact, the normal tissues displayed either no or faint claspin immunoreactivity, whereas a moderate/high positivity was observed in 16% of the CIN1, 76% of the CIN2, 87.5% of the CIN3 and 93.3% of the cancers. Moreover, we found a statistically significant correlation between claspin expression and HR-HPV infection (pχ2 < 0.0001), irrespective of the genotype. Finally, we demonstrated the feasibility of claspin immunostaining in cervical cytology.

CONCLUSIONS

Our findings indicate that in vivo claspin expression is significantly related to HR-HPV infection and lesion grade both in histological and cytological samples. Therefore, the analysis of claspin expression could be clinically relevant in the diagnosis of HPV-related cervical lesions, in particular when applied to cervico-vaginal cytology. Moreover, giving information on the proliferation rate of each lesion, claspin immunostaining may contribute to the evaluation of progression risk, thus being helpful in patient management. Nevertheless, only large prospective studies may clarify the true clinical usefulness of claspin expression in distinguishing lesions with different progression potential.

Novel regulation of checkpoint kinase 1: Is checkpoint kinase 1 a good candidate for anti-cancer therapy?

DNA-damaging strategies, such as radiotherapy and the majority of chemotherapeutic therapies, are the most frequently used non-surgical anti-cancer therapies for human cancers. These therapies activate DNA damage/replication checkpoints, which induce cell-cycle arrest to provide the time needed to repair DNA damage. Due to genetic defect(s) in the ATM (ataxia-telangiectasia mutated)-Chk2-p53 pathway, an ATR (ATM- and Rad3-related)-Chk1-Cdc25 route is the sole checkpoint pathway in a majority of cancer cells. Chk1 inhibitors are expected to selectively induce the mitotic cell death (mitotic catastrophe) of cancer cells. However, recent new findings have pointed out that Chk1 is essential for the maintenance of genome integrity even during unperturbed cell-cycle progression, which is controlled by a variety of protein kinases. These observations have raised concerns about a possible risk of Chk1 inhibitors on the clinics. In this review, we summarize recent advances in Chk1 regulation by phosphorylation, and discuss Chk1 as a molecular target for cancer therapeutics.

Neurospora mrc1 homologue is involved in replication stability and is required for normal cell growth and chromosome integrity in mus-9 and mus-21 mutants

Stalled replication forks easily collapse and such structures can induce DNA strand breaks or toxic recombination products. Therefore, factors involved in stabilization of replication should be important for genome integrity. In our previous study, loss of both ATM and ATR homologues was shown to cause growth defects and chromosome instability in Neurospora crassa. To elucidate the relationships between these defects and replication instability, we focused on one of the viable replication factors, mrc1. We identified an mrc1 homologue from the N. crassa genome database. The mrc1 disruptant was sensitive to the replication inhibitor hydroxyurea (HU) and delayed restart of the cell cycle from HU treatment. Importantly, HU treatment induced histone H2A phosphorylation in the mrc1 mutant but not in the wild type. Furthermore, the HU-induced H2A phosphorylation was completely dependent on the ATM homologue mus-21, and dysfunction of mus-21 increased HU sensitivity of the mrc1 mutant. These results indicate that Neurospora mrc1 is important for stabilization of replication forks and that loss of mrc1 causes activation of the DNA damage checkpoint. Unexpectedly, loss of mrc1 did not affect cell growth, but the deletion of mrc1 reduced hyphal growth speed and conidia viability in the mus-9 and mus-21 mutants. The mrc1 mus-9 and mrc1 mus-21 double mutants also showed accumulation of micronuclei, which is a typical marker of chromosome instability. These results imply that activation of the checkpoint pathway can protect cells from instability of DNA replication caused by loss of mrc1.

Protein degradation in DNA damage response

DNA damage is a major threat to genome integrity. To reduce its deleterious effects, cells have developed coordinated responses, collectively referred to as the "DNA damage response" pathway (DDR). In multicellular organisms, the DDR pathway has a critical role in preventing tumorigenesis, which accounts for the wide use of drugs targeting DDR factors in anti-cancer therapy. Post-translational modifications such as phosphorylation, ubiquitylation, acetylation, sumoylation are integral part of the DDR pathway. Ubiquitylation of DDR-related factors has recently emerged both as a switch initiating signaling cascades and as a proteolytic signal coordinating recruitment and disassembly of those proteins. In this review we will present evidence supporting an increasingly important role for the ubiquitin-proteasome-mediated degradation in regulating DDR at different levels.

Nuclear Chk1 prevents premature mitotic entry

Chk1 inhibits the premature activation of the cyclin-B1-Cdk1. However, it remains controversial whether Chk1 inhibits Cdk1 in the centrosome or in the nucleus before the G2-M transition. In this study, we examined the specificity of the mouse monoclonal anti-Chk1 antibody DCS-310, with which the centrosome was stained. Conditional Chk1 knockout in mouse embryonic fibroblasts reduced nuclear but not centrosomal staining with DCS-310. In Chk1(+/myc) human colon adenocarcinoma (DLD-1) cells, Chk1 was detected in the nucleus but not in the centrosome using an anti-Myc antibody. Through the combination of protein array and RNAi technologies, we identified Ccdc-151 as a protein that crossreacted with DCS-310 on the centrosome. Mitotic entry was delayed by expression of the Chk1 mutant that localized in the nucleus, although forced immobilization of Chk1 to the centrosome had little impact on the timing of mitotic entry. These results suggest that nuclear but not centrosomal Chk1 contributes to correct timing of mitotic entry.

Timeless functions independently of the Tim-Tipin complex to promote sister chromatid cohesion in normal human fibroblasts

The Timeless-Tipin complex and Claspin are mediators of the ATR-dependent activation of Chk1 in the intra-S checkpoint response to stalled DNA replication forks. Tim-Tipin and Claspin also contribute to sister chromatid cohesion (SCC) in various organisms, likely through a replication-coupled process. Some models of the establishment of SCC posit that interactions between cohesin rings and replisomes could result in physiological replication stress requiring fork stabilization. The contributions of Timeless, Tipin, Claspin, Chk1 and ATR to SCC were investigated in genetically stable, human diploid fibroblast cell lines. Whereas Timeless, Tipin and Claspin showed similar contributions to UVC-induced activation of Chk1, siRNA-mediated knockdown of Timeless induced a 100-fold increase in sister chromatid discohesion, whereas the inductive effects of knocking down Tipin, Claspin and ATR were 4-20-fold. Knockdown of Chk1 did not significantly affect SCC. Consistent findings were obtained in two independently derived human diploid fibroblast lines and support a conclusion that SCC in human cells is strongly dependent on Timeless but independent of Chk1. Furthermore, the 10-fold difference in discohesion observed when depleting Timeless versus Tipin indicates that Timeless has a function in SCC that is independent of the Tim-Tipin complex, even though the abundance of Timeless is reduced when Tipin is targeted for depletion. A better understanding of how Timeless, Tipin and Claspin promote SCC will elucidate non-checkpoint functions of these proteins at DNA replication forks and inform models of the establishment of SCC.

Timeless links replication termination to mitotic kinase activation

The mechanisms that coordinate the termination of DNA replication with progression through mitosis are not completely understood. The human Timeless protein (Tim) associates with S phase replication checkpoint proteins Claspin and Tipin, and plays an important role in maintaining replication fork stability at physical barriers, like centromeres, telomeres and ribosomal DNA repeats, as well as at termination sites. We show here that human Tim can be isolated in a complex with mitotic entry kinases CDK1, Auroras A and B, and Polo-like kinase (Plk1). Plk1 bound Tim directly and colocalized with Tim at a subset of mitotic structures in M phase. Tim depletion caused multiple mitotic defects, including the loss of sister-chromatid cohesion, loss of mitotic spindle architecture, and a failure to exit mitosis. Tim depletion caused a delay in mitotic kinase activity in vivo and in vitro, as well as a reduction in global histone H3 S10 phosphorylation during G2/M phase. Tim was also required for the recruitment of Plk1 to centromeric DNA and formation of catenated DNA structures at human centromere alpha satellite repeats. Taken together, these findings suggest that Tim coordinates mitotic kinase activation with termination of DNA replication.

Understanding the limitations of radiation-induced cell cycle checkpoints

The DNA damage response pathways involve processes of double-strand break (DSB) repair and cell cycle checkpoint control to prevent or limit entry into S phase or mitosis in the presence of unrepaired damage. Checkpoints can function to permanently remove damaged cells from the actively proliferating population but can also halt the cell cycle temporarily to provide time for the repair of DSBs. Although efficient in their ability to limit genomic instability, checkpoints are not foolproof but carry inherent limitations. Recent work has demonstrated that the G1/S checkpoint is slowly activated and allows cells to enter S phase in the presence of unrepaired DSBs for about 4–6 h post irradiation. During this time, only a slowing but not abolition of S-phase entry is observed. The G2/M checkpoint, in contrast, is quickly activated but only responds to a level of 10–20 DSBs such that cells with a low number of DSBs do not initiate the checkpoint or terminate arrest before repair is complete. Here, we discuss the limitations of these checkpoints in the context of the current knowledge of the factors involved. We suggest that the time needed to fully activate G1/S arrest reflects the existence of a restriction point in G1-phase progression. This point has previously been defined as the point when mitogen starvation fails to prevent cells from entering S phase. However, cells that passed the restriction point can respond to DSBs, albeit with reduced efficiency.

Hsk1 kinase and Cdc45 regulate replication stress-induced checkpoint responses in fission yeast

In fission yeast, replication fork arrest activates the replication checkpoint effector kinase Cds1(Chk2/Rad53) through the Rad3(ATR/Mec1)-Mrc1(Claspin) pathway. Hsk1, the Cdc7 homologue of fission yeast required for efficient initiation of DNA replication, is also required for Cds1 activation. Hsk1 kinase activity is required for induction and maintenance of Mrc1 hyperphosphorylation, which is induced by replication fork block and mediated by Rad3. Rad3 kinase activity does not change in an hsk1 temperature-sensitive mutant, and Hsk1 kinase activity is not affected by rad3 mutation. Hsk1 kinase vigorously phosphorylates Mrc1 in vitro, predominantly at non-SQ/TQ sites, but this phosphorylation does not seem to affect the Rad3 action on Mrc1. Interestingly, the replication stress-induced activation of Cds1 and hyperphosphorylation of Mrc1 is almost completely abrogated in an initiation-defective mutant of cdc45, but not in an mcm2 or polε mutant. The results suggest that Hsk1-mediated loading of Cdc45 onto replication origins may play important roles in replication stress-induced checkpoint.

Genomic instability and cancer: lessons learned from human papillomaviruses

High-risk HPV E6 and E7 oncoproteins cooperate to subvert critical host cell cycle checkpoint control mechanisms in order to promote viral genome replication. This results not only in aberrant proliferation but also in host cellular changes that can promote genomic instability. The HPV-16 E7 oncoprotein was found to induce centrosome abnormalities thereby disrupting mitotic fidelity and increasing the risk for chromosome missegregation and aneuploidy. In addition, expression of the high-risk HPV E7 oncoprotein stimulates DNA replication stress as a potential source of DNA breakage and structural chromosomal instability. Proliferation of genomically unstable cells is sustained by several mechanisms including the accelerated degradation of claspin by HPV-16 E7 and the degradation of p53 by the high-risk HPV E6 oncoprotein. These results highlight the oncogenic potential of aberrant proliferation and opens new avenues for prevention of malignant progression, not only in HPV-associated cervical cancer but also in non-virally associated malignancies with disrupted cell cycle checkpoint control mechanisms.

APC/C-Cdh1: from cell cycle to cellular differentiation and genomic integrity

Anaphase-promoting complex/cyclosome (APC/C) is a multifunctional ubiquitin-protein ligase that targets various substrates for proteolysis inside and outside of the cell cycle. The activation of APC/C is dependent on two WD-40 domain proteins, Cdc20 and Cdh1. While APC/Cdc20 principally regulates mitotic progression, APC/Cdh1 shows a broad spectrum of substrates in and beyond cell cycle. In the past several years, numerous biochemical and mouse genetic studies have greatly attracted our attention to the emerging role of APC/Cdh1 in genomic integrity, cellular differentiation and human diseases. This review will aim to summarize the recently expanded understanding of APC/Cdh1 in regulating biological function and how its dysfunction may lead to diseases.

Chk1 suppressed cell death

The role of Chk1 in the cellular response to DNA replication stress is well established. However recent work indicates a novel role for Chk1 in the suppression of apoptosis following the disruption of DNA replication or DNA damage. This review will consider these findings in the context of known pathways of Chk1 signalling and potential applications of therapies that target Chk1.

Proteolysis of Rad17 by Cdh1/APC regulates checkpoint termination and recovery from genotoxic stress

Recent studies have shown a critical function for the ubiquitin-proteasome system (UPS) in regulating the signalling network for DNA damage responses and DNA repair. To search for new UPS targets in the DNA damage signalling pathway, we have carried out a non-biased assay to identify fast-turnover proteins induced by various types of genotoxic stress. This endeavour led to the identification of Rad17 as a protein exhibiting a distinctive pattern of upregulation followed by subsequent degradation after exposure to UV radiation in human primary cells. Our characterization showed that UV-induced Rad17 oscillation is mediated by Cdh1/APC, a ubiquitin-protein ligase. Studies using a degradation-resistant Rad17 mutant demonstrated that Rad17 stabilization prevents the termination of checkpoint signalling, which in turn attenuates the cellular re-entry into cell-cycle progression. The findings provide an insight into how the proteolysis of Rad17 by Cdh1/APC regulates the termination of checkpoint signalling and the recovery from genotoxic stress.

Human papillomavirus 16 E7 oncoprotein attenuates DNA damage checkpoint control by increasing the proteolytic turnover of claspin

The human papillomavirus (HPV) 16 E7 oncoprotein has been reported previously to stimulate DNA damage and to activate host cell DNA damage checkpoints. How HPV-16 E7 maintains proliferation despite activated DNA damage checkpoints is incompletely understood. Here, we provide evidence that cells expressing the HPV-16 E7 oncoprotein can enter mitosis in the presence of DNA damage. We show that this activity of HPV-16 E7 involves attenuation of DNA damage checkpoint control by accelerating the proteolytic turnover of claspin. Claspin mediates the activation of CHK1 by ATR in response to replication stress, and its degradation plays a critical role in DNA damage checkpoint recovery. Expression of a nondegradable mutant of claspin was shown to inhibit mitotic entry in HPV-16 E7-expressing cells. Multiple components of the SCF(beta-TrCP)-based claspin degradation machinery were found deregulated in the presence of HPV-16 E7, including cullin 1, beta-TrCP, Aurora A, and Polo-like kinase-1 (PLK1). In contrast, no difference in the expression level of the claspin deubiquitinating enzyme USP7 was detected. Levels of Aurora A and PLK1 as well as phosphorylated PLK1 at threonine 210, a prerequisite for DNA damage checkpoint recovery, remained detectable following replication stress in HPV-16 E7-expressing cells but not in control cells. In summary, our results suggest that the HPV-16 E7 oncoprotein alleviates DNA damage checkpoint responses and promotes mitotic entry by accelerating claspin degradation through a mechanism that involves deregulation of components of the SCF(beta-TrCP)-based claspin degradation machinery.

Family cancer syndromes: inherited deficiencies in systems for the maintenance of genomic integrity

Familial cancer syndromes have revealed important fundamental features regarding how all cancers arise through destabilization of the genome, such that somatic evolution can select for the disruption of critical cellular coordinating and regulatory features. The authors examine those cellular genes and systems whose normal role is to preserve genomic integrity and relate them to the genetic foundations of heritable cancers. By examining how these cellular systems normally function, how family cancer genes are able to affect the process of tumor progression can be learned. In so doing, a clearer picture of how sporadic cancers arise is additionally gained.

Checkpoint regulation of DNA replication

We discuss the mechanisms regulating entry into and progression through S phase in eukaryotic cells. Methods to study the G1/S transition are briefly reviewed and an overview of G1/S-checkpoints is given, with particular emphasis on fission yeast. Thereafter we discuss different aspects of the intra-S checkpoint and introduce the main molecular players and mechanisms.

Sonic Hedgehog signaling impairs ionizing radiation-induced checkpoint activation and induces genomic instability

The Sonic Hedgehog (Shh) pathway plays important roles in embryogenesis, stem cell maintenance, tissue repair, and tumorigenesis. Haploinsufficiency of Patched-1, a gene that encodes a repressor of the Shh pathway, dysregulates the Shh pathway and increases genomic instability and the development of spontaneous and ionizing radiation (IR)–induced tumors by an unknown mechanism. Here we show that Ptc1^+/− mice have a defect in the IR-induced activation of the ATR–Chk1 checkpoint signaling pathway. Likewise, transient expression of Gli1, a downstream target of Shh signaling, disrupts Chk1 activation in human cells by preventing the interaction of Chk1 with Claspin, a Chk1 adaptor protein that is required for Chk1 activation. These results suggest that inappropriate Shh pathway activation promotes tumorigenesis by disabling a key signaling pathway that helps maintain genomic stability and inhibits tumorigenesis.

A conserved proliferating cell nuclear antigen-interacting protein sequence in Chk1 is required for checkpoint function

Human checkpoint kinase 1 (Chk1) is an essential kinase required for cell cycle checkpoints and for coordination of DNA synthesis. To gain insight into the mechanisms by which Chk1 carries out these functions, we used mass spectrometry to identify previously uncharacterized interacting partners of Chk1. We describe a novel interaction between Chk1 and proliferating cell nuclear antigen (PCNA), an essential component of the replication machinery. Binding between Chk1 and PCNA was reduced in the presence of hydroxyurea, suggesting that the interaction is regulated by replication stress. A highly conserved PCNA-interacting protein (PIP) box motif was identified in Chk1. The intact PIP box is required for efficient DNA damage-induced phosphorylation and release of activated Chk1 from chromatin. We find that the PIP box of Chk1 is crucial for Chk1-mediated S-M and G(2)-M checkpoint responses. In addition, we show that mutations in the PIP box of Chk1 lead to decreased rates of replication fork progression and increased aberrant replication. These findings suggest an additional mechanism by which essential components of the DNA replication machinery interact with the replication checkpoint apparatus.

Germline alterations in the CLSPN gene in breast cancer families

About 5-10% of breast cancer is thought to be due to an inherited disease predisposition. Currently known genes account for less than half of the hereditary cases. Claspin, a tumor suppressor protein encoded by the CLSPN gene, is involved in monitoring of replication and sensoring of DNA damage and cooperates with CHK1 and BRCA1. Association with certain cell proliferation stimulatory features has also been described. Many previously identified susceptibility factors act in similar functional pathways as claspin, suggesting possible involvement of CLSPN in heritable breast cancer susceptibility. Here we have screened affected index cases from 125 Finnish cancer families for germline defects in CLSPN using conformation sensitive gel electrophoresis (CSGE) and direct sequencing. Altogether seven different sequence changes were observed, but none of them appeared to associate with breast cancer susceptibility. To our knowledge, this is the first study reporting the mutation screening of the CLSPN gene in familial breast cancer cases.

Integrating S-phase checkpoint signaling with trans-lesion synthesis of bulky DNA adducts

Bulky adducts are DNA lesions generated in response to environmental agents including benzo[a]pyrene (a combustion product) and solar ultraviolet radiation. Error-prone replication of adducted DNA can cause mutations, which may result in cancer. To minimize the detrimental effects of bulky adducts and other DNA lesions, S-phase checkpoint mechanisms sense DNA damage and integrate DNA repair with ongoing DNA replication. The essential protein kinase Chk1 mediates the S-phase checkpoint, inhibiting initiation of new DNA synthesis and promoting stabilization and recovery of stalled replication forks. Here we review the mechanisms by which Chk1 is activated in response to bulky adducts and potential mechanisms by which Chk1 signaling inhibits the initiation stage of DNA synthesis. Additionally, we discuss mechanisms by which Chk1 signaling facilitates bypass of bulky lesions by specialized Y-family DNA polymerases, thereby attenuating checkpoint signaling and allowing resumption of normal cell cycle progression.

Loss of CHK1 function impedes DNA damage-induced FANCD2 monoubiquitination but normalizes the abnormal G2 arrest in Fanconi anemia

Fanconi anemia (FA) is a cancer-prone hereditary disease resulting from mutations in one of the 13 genes defining the FANC/BRCA pathway. This pathway is involved in the cellular resistance to DNA-cross-linking agents. How the FANC/BRCA pathway is activated and why its deficiency leads to the accumulation of FA cells with a 4N DNA content are still poorly answered questions. We investigated the involvement of ATR pathway members in these processes. We show here that RAD9 and RAD17 are required for DNA interstrand cross-link (ICL) resistance and for the optimal activation of FANCD2. Moreover, we demonstrate that CHK1 and its interacting partner CLASPIN that act downstream in the ATR pathway are required for both FANCD2 monoubiquitination and assembling in subnuclear foci in response to DNA damage. Paradoxically, in the absence of any genotoxic stress, CHK1 or CLASPIN depletion results in an increased basal level of FANCD2 monoubiquitination and focalization. We also demonstrate that the ICL-induced accumulation of FA cells in late S/G2 phase is dependent on ATR and CHK1. In agreement with this, CHK1 phosphorylation is enhanced in FA cells, and chemical inhibition of the ATR/CHK1 axis in FA lymphoblasts decreases their sensitivity to mitomycin C. In conclusion, this work describes a complex crosstalk between CHK1 and the FANC/BRCA pathway: CHK1 activates this pathway through FANCD2 monoubiquitination, whereas FA deficiency leads to a CHK1-dependent G2 accumulation, raising the possibility that the FANC/BRCA pathway downregulates CHK1 activation.

The Mre11-Rad50-Nbs1 complex acts both upstream and downstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to regulate the S-phase checkpoint following UV treatment

The Mre11-Rad50-Nbs1 (MRN) complex is required for mediating the S-phase checkpoint following UV treatment, but the underlying mechanism is not clear. Here we demonstrate that at least two mechanisms are involved in regulating the S-phase checkpoint in an MRN-dependent manner following UV treatment. First, when replication forks are stalled, MRN is required upstream of ataxia telangiectasia mutated and Rad3-related protein (ATR) to facilitate ATR activation in a substrate and dosage-dependent manner. In particular, MRN is required for ATR-directed phosphorylation of RPA2, a critical event in mediating the S-phase checkpoint following UV treatment. Second, MRN is a downstream substrate of ATR. Nbs1 is phosphorylated by ATR at Ser-343 when replication forks are stalled, and this phosphorylation event is also important for down-regulating DNA replication following UV treatment. Moreover, we demonstrate that MRN and ATR/ATR-interacting protein (TRIP) interact with each other, and the forkhead-associated/breast cancer C-terminal domains (FHA/BRCT) of Nbs1 play a significant role in mediating this interaction. Mutations in the FHA/BRCT domains do not prevent ATR activation but specifically impair ATR-mediated Nbs1 phosphorylation at Ser-343, which results in a defect in the S-phase checkpoint. These data suggest that MRN plays critical roles both upstream and downstream of ATR to regulate the S-phase checkpoint when replication forks are stalled.

Targeting checkpoint kinase 1 in cancer therapeutics

Progression through the cell cycle is monitored by surveillance mechanisms known as cell cycle checkpoints. Our knowledge of the biochemical nature of checkpoint regulation during an unperturbed cell cycle and following DNA damage has expanded tremendously over the past decade. We now know that dysfunction in cell cycle checkpoints leads to genomic instability and contributes to tumor progression, and most agents used for cancer therapy, such as cytotoxic chemotherapy and ionizing radiation, also activate cell cycle checkpoints. Understanding how checkpoints are regulated is therefore important from the points of view of both tumorigenesis and cancer treatment. In this review, we present an overview of the molecular hierarchy of the checkpoint signaling network and the emerging role of checkpoint targets, especially checkpoint kinase 1, in cancer therapy. Further, we discuss the results of recent clinical trials involving the nonspecific checkpoint kinase 1 inhibitor, UCN-01, and the challenges we face with this new therapeutic approach.

Phosphatase type 2A-dependent and -independent pathways for ATR phosphorylation of Chk1

ATM and Rad3-related (ATR) is a regulatory kinase that, when activated by hydroxyurea, UV, or human immunodeficiency virus-1 Vpr, causes cell cycle arrest through Chk1-Ser(345) phosphorylation. We demonstrate here that of these three agents only Vpr requires protein phosphatase type 2A (PP2A) to activate ATR for Chk1-Ser(345) phosphorylation. A requirement for PP2A by Vpr was first shown with the PP2A-specific inhibitor okadaic acid, which reduced Vpr-induced G(2) arrest and Cdk1-Tyr(15) phosphorylation. Using small interference RNA to down-regulate specific subunits of PP2A indicated that the catalytic beta-isoform PP2A(Cbeta) and the A regulatory alpha-isoform PP2A(Aalpha) are involved in the G(2) induction, and these downregulations decreased the Vpr-induced, ATR-dependent phosphorylations of Cdk1-Tyr(15) and Chk1-Ser(345). In contrast, the same down-regulations had no effect on hydroxyurea- or UV-activated ATR-dependent Chk1-Ser(345) phosphorylation. Vpr and hydroxyurea/UV all induce ATR-mediated gammaH2AX-Ser(139) phosphorylation and foci formation, but down-regulation of PP2A(Aalpha) or PP2A(Cbeta) did not decrease gammaH2AX-Ser(139) phosphorylation by any of these agents or foci formation by Vpr. Conversely, H2AX down-regulation had little effect on PP2A(Aalpha/Cbeta)-mediated G(2) arrest and Chk1-Ser(345) phosphorylation by Vpr. The expression of vpr increases the amount and phosphorylation of Claspin, an activator of Chk1 phosphorylation. Down-regulation of either PP2A(Cbeta) or PP2A(Aalpha) had little effect on Claspin phosphorylation, but the amount of Claspin was reduced. Claspin may then be one of the phosphoproteins through which PP2A(Aalpha/Cbeta) affects Chk1 phosphorylation when ATR is activated by human immunodeficiency virus-1 Vpr.