Cdc7-dependent and -independent phosphorylation of Claspin in the induction of the DNA replication checkpoint

DBF4, not DRF1, is the crucial regulator of CDC7 kinase at replication forks

CDC7 kinase is crucial for DNA replication initiation and is involved in fork processing and replication stress response. Human CDC7 requires the binding of either DBF4 or DRF1 for its activity. However, it is unclear whether the two regulatory subunits target CDC7 to a specific set of substrates, thus having different biological functions, or if they act redundantly. Using genome editing technology, we generated isogenic cell lines deficient in either DBF4 or DRF1: these cells are viable but present signs of genomic instability, indicating that both can independently support CDC7 for bulk DNA replication. Nonetheless, DBF4-deficient cells show altered replication efficiency, partial deficiency in MCM helicase phosphorylation, and alterations in the replication timing of discrete genomic regions. Notably, we find that CDC7 function at replication forks is entirely dependent on DBF4 and not on DRF1. Thus, DBF4 is the primary regulator of CDC7 activity, mediating most of its functions in unperturbed DNA replication and upon replication interference.

CDC7 inhibition drives an inflammatory response and a p53-dependent senescent-like state in breast epithelial cells

Drugs that block DNA replication prevent cell proliferation, which may result in anticancer activity. The latter is dependent on the drug's mode of action as well as on cell type-dependent responses to treatment. The inhibition of Cell division cycle 7-related protein kinase (CDC7), a key regulator of DNA replication, decreases the efficiency of origin firing and hampers the restarting of paused replication forks. Here, we show that upon prolonged CDC7 inhibition, breast-derived MCF10A cells progressively withdraw from the cell cycle and enter a reversible senescent-like state. This is characterised by the rewiring of the transcriptional programme with the induction of cytokine and chemokine expression and correlates with the accumulation of Cyclic GMP-AMP synthase (cGAS)-positive micronuclei. Importantly, cell fate depends on Cellular tumour antigen p53 (p53) function as cells no longer enter senescence but are funnelled into apoptosis upon p53 knockout. This work uncovers key features of the secondary response to CDC7 inhibitors, which could aid the development of these compounds as anticancer drugs.

PTBP1 enforces ATR-CHK1 signaling determining the potency of CDC7 inhibitors

CDC7 kinase is crucial for DNA replication initiation and fork processing. CDC7 inhibition mildly activates the ATR pathway, which further limits origin firing; however, to date the relationship between CDC7 and ATR remains controversial. We show that CDC7 and ATR inhibitors are either synergistic or antagonistic depending on the degree of inhibition of each individual kinase. We find that Polypyrimidine Tract Binding Protein 1 (PTBP1) is important for ATR activity in response to CDC7 inhibition and genotoxic agents. Compromised PTBP1 expression makes cells defective in RPA recruitment, genomically unstable, and resistant to CDC7 inhibitors. PTBP1 deficiency affects the expression and splicing of many genes indicating a multifactorial impact on drug response. We find that an exon skipping event in RAD51AP1 contributes to checkpoint deficiency in PTBP1-deficient cells. These results identify PTBP1 as a key factor in replication stress response and define how ATR activity modulates the activity of CDC7 inhibitors.

Dbf4-Cdc7 (DDK) Inhibitor PHA-767491 Displays Potent Anti-Proliferative Effects via Crosstalk with the CDK2-RB-E2F Pathway

Precise regulation of DNA replication complex assembly requires cyclin-dependent kinase (CDK) and Dbf4-dependent kinase (DDK) activities to activate the replicative helicase complex and initiate DNA replication. Chemical probes have been essential in the molecular analysis of DDK-mediated regulation of MCM2-7 activation and the initiation phase of DNA replication. Here, the inhibitory activity of two distinct DDK inhibitor chemotypes, PHA-767491 and XL-413, were assessed in cell-free and cell-based proliferation assays. PHA-767491 and XL-413 show distinct effects at the level of cellular proliferation, initiation of DNA replication and replisome activity. XL-413 and PHA-767491 both reduce DDK-specific phosphorylation of MCM2 but show differential potency in prevention of S-phase entry. DNA combing and DNA replication assays show that PHA-767491 is a potent inhibitor of the initiation phase of DNA replication but XL413 has weak activity. Importantly, PHA-767491 decreased E2F-mediated transcription of the G1/S regulators cyclin A2, cyclin E1 and cyclin E2, and this effect was independent of CDK9 inhibition. Significantly, the enhanced inhibitory profile of PHA-767491 is mediated by potent inhibition of both DDK and the CDK2-Rb-E2F transcriptional network, that provides the molecular basis for its increased anti-proliferative effects in RB⁺ cancer cell lines.

DDK: The Outsourced Kinase of Chromosome Maintenance

The maintenance of genomic stability during the mitotic cell-cycle not only demands that the DNA is duplicated and repaired with high fidelity, but that following DNA replication the chromatin composition is perpetuated and that the duplicated chromatids remain tethered until their anaphase segregation. The coordination of these processes during S phase is achieved by both cyclin-dependent kinase, CDK, and Dbf4-dependent kinase, DDK. CDK orchestrates the activation of DDK at the G1-to-S transition, acting as the ‘global’ regulator of S phase and cell-cycle progression, whilst ‘local’ control of the initiation of DNA replication and repair and their coordination with the re-formation of local chromatin environments and the establishment of chromatid cohesion are delegated to DDK. Here, we discuss the regulation and the multiple roles of DDK in ensuring chromosome maintenance. Regulation of replication initiation by DDK has long been known to involve phosphorylation of MCM2-7 subunits, but more recent results have indicated that Treslin:MTBP might also be important substrates. Molecular mechanisms by which DDK regulates replisome stability and replicated chromatid cohesion are less well understood, though important new insights have been reported recently. We discuss how the ‘outsourcing’ of activities required for chromosome maintenance to DDK allows CDK to maintain outright control of S phase progression and the cell-cycle phase transitions whilst permitting ongoing chromatin replication and cohesion establishment to be completed and achieved faithfully.

Essential role of CK2α for the interaction and stability of replication fork factors during DNA synthesis and activation of the S-phase checkpoint

The ataxia telangiectasia mutated and Rad3-related (ATR)-CHK1 pathway is the major signalling cascade activated in response to DNA replication stress. This pathway is associated with the core of the DNA replication machinery comprising CDC45, the replicative MCM2-7 hexamer, GINS (altogether forming the CMG complex), primase-polymerase (POLε, -α, and -δ) complex, and additional fork protection factors such as AND-1, CLASPIN (CLSPN), and TIMELESS/TIPIN. In this study, we report that functional protein kinase CK2α is critical for preserving replisome integrity and for mounting S-phase checkpoint signalling. We find that CDC45, CLSPN and MCM7 are novel CK2α interacting partners and these interactions are particularly important for maintenance of stable MCM7-CDC45, ATRIP-ATR-MCM7, and ATR-CLSPN protein complexes. Consistently, cells depleted of CK2α and treated with hydroxyurea display compromised replisome integrity, reduced chromatin binding of checkpoint mediator CLSPN, attenuated ATR-mediated S-phase checkpoint and delayed recovery of stalled forks. In further support of this, differential gene expression analysis by RNA-sequencing revealed that down-regulation of CK2α accompanies global shutdown of genes that are implicated in the S-phase checkpoint. These findings add to our understanding of the molecular mechanisms involved in DNA replication by showing that the protein kinase CK2α is essential for maintaining the stability of the replisome machinery and for optimizing ATR-CHK1 signalling activation upon replication stress.

Multiomic analysis identifies CPT1A as a potential therapeutic target in platinum-refractory, high-grade serous ovarian cancer

Resistance to platinum compounds is a major determinant of patient survival in high-grade serous ovarian cancer (HGSOC). To understand mechanisms of platinum resistance and identify potential therapeutic targets in resistant HGSOC, we generated a data resource composed of dynamic (±carboplatin) protein, post-translational modification, and RNA sequencing (RNA-seq) profiles from intra-patient cell line pairs derived from 3 HGSOC patients before and after acquiring platinum resistance. These profiles reveal extensive responses to carboplatin that differ between sensitive and resistant cells. Higher fatty acid oxidation (FAO) pathway expression is associated with platinum resistance, and both pharmacologic inhibition and CRISPR knockout of carnitine palmitoyltransferase 1A (CPT1A), which represents a rate limiting step of FAO, sensitize HGSOC cells to platinum. The results are further validated in patient-derived xenograft models, indicating that CPT1A is a candidate therapeutic target to overcome platinum resistance. All multiomic data can be queried via an intuitive gene-query user interface (https://sites.google.com/view/ptrc-cell-line).

CDC7 kinase promotes MRE11 fork processing, modulating fork speed and chromosomal breakage

The CDC7 kinase is essential for the activation of DNA replication origins and has been implicated in the replication stress response. Using a highly specific chemical inhibitor and a chemical genetic approach, we now show that CDC7 activity is required to coordinate multiple MRE11‐dependent processes occurring at replication forks, independently from its role in origin firing. CDC7 localizes at replication forks and, similarly to MRE11, mediates active slowing of fork progression upon mild topoisomerase inhibition. Both proteins are also retained on stalled forks, where they promote fork processing and restart. Moreover, MRE11 phosphorylation and localization at replication factories are progressively lost upon CDC7 inhibition. Finally, CDC7 activity at reversed forks is required for their pathological MRE11‐dependent degradation in BRCA2‐deficient cells. Thus, upon replication interference CDC7 is a key regulator of fork progression, processing and integrity. These results highlight a dual role for CDC7 in replication, modulating both initiation and elongation steps of DNA synthesis, and identify a key intervention point for anticancer therapies exploiting replication interference.

Cdc7 activates replication checkpoint by phosphorylating the Chk1-binding domain of Claspin in human cells

Replication checkpoint is essential for maintaining genome integrity in response to various replication stresses as well as during the normal growth. The evolutionally conserved ATR-Claspin-Chk1 pathway is induced during replication checkpoint activation. Cdc7 kinase, required for initiation of DNA replication at replication origins, has been implicated in checkpoint activation but how it is involved in this pathway has not been known. Here, we show that Cdc7 is required for Claspin-Chk1 interaction in human cancer cells by phosphorylating CKBD (Chk1-binding-domain) of Claspin. The residual Chk1 activation in Cdc7-depleted cells is lost upon further depletion of casein kinase1 (CK1γ1), previously reported to phosphorylate CKBD. Thus, Cdc7, in conjunction with CK1γ1, facilitates the interaction between Claspin and Chk1 through phosphorylating CKBD. We also show that, whereas Cdc7 is predominantly responsible for CKBD phosphorylation in cancer cells, CK1γ1 plays a major role in non-cancer cells, providing rationale for targeting Cdc7 for cancer cell-specific cell killing.

Cdc7 kinase stimulates Aurora B kinase in M-phase

The conserved serine-threonine kinase, Cdc7, plays a crucial role in initiation of DNA replication by facilitating the assembly of an initiation complex. Cdc7 is expressed at a high level and exhibits significant kinase activity not only during S-phase but also during G2/M-phases. A conserved mitotic kinase, Aurora B, is activated during M-phase by association with INCENP, forming the chromosome passenger complex with Borealin and Survivin. We show that Cdc7 phosphorylates and stimulates Aurora B kinase activity in vitro. We identified threonine-236 as a critical phosphorylation site on Aurora B that could be a target of Cdc7 or could be an autophosphorylation site stimulated by Cdc7-mediated phosphorylation elsewhere. We found that threonines at both 232 (that has been identified as an autophosphorylation site) and 236 are essential for the kinase activity of Aurora B. Cdc7 down regulation or inhibition reduced Aurora B activity in vivo and led to retarded M-phase progression. SAC imposed by paclitaxel was dramatically reversed by Cdc7 inhibition, similar to the effect of Aurora B inhibition under the similar situation. Our data show that Cdc7 contributes to M-phase progression and to spindle assembly checkpoint most likely through Aurora B activation.

A Dual Inhibitor of Cdc7/Cdk9 Potently Suppresses T Cell Activation

T cell activation is mediated by signaling pathways originating from the T cell receptor (TCR). Propagation of signals downstream of the TCR involves a cascade of numerous kinases, some of which have yet to be identified. Through a screening strategy that we have previously introduced, PHA-767491, an inhibitor of the kinases Cdc7 and Cdk9, was identified to impede TCR signaling. PHA-767491 suppressed several T cell activation phenomena, including the expression of activation markers, proliferation, and effector functions. We also observed a defect in TCR signaling pathways upon PHA-767491 treatment. Inhibition of Cdc7/Cdk9 impairs T cell responses, which could potentially be detrimental for the immune response to tumors, and also compromises the ability to resist infections. The Cdc7/Cdk9 inhibitor is a strong candidate as a cancer therapeutic, but its effect on the immune system poses a problem for clinical applications.

DDK Has a Primary Role in Processing Stalled Replication Forks to Initiate Downstream Checkpoint Signaling

CDC7-DBF4 kinase (DDK) initiates DNA replication in eukaryotes by activating the replicative MCM helicase. DDK has diverse and apparently conflicting roles in the replication checkpoint response in various organisms, but the underlying mechanisms are far from settled. We show that human DDK promotes limited resection of newly synthesized DNA at stalled replication forks or sites of DNA damage to initiate replication checkpoint signaling. DDK is also required for efficient fork restart and G2/M cell cycle arrest. DDK exhibits genetic interactions with the ssDNA exonuclease EXO1 and phosphorylates EXO1 in vitro. EXO1 is also required for nascent strand degradation following exposure to HU, so DDK might regulate EXO1 directly. Lastly, sublethal DDK inhibition causes various mitotic abnormalities, which is consistent with a checkpoint deficiency. In summary, DDK has a primary and previously undescribed role in the replication checkpoint to promote ssDNA accumulation at stalled forks, which is required to initiate a robust checkpoint response and cell cycle arrest to maintain genome integrity.

Control of DNA Replication Initiation by Ubiquitin

Eukaryotic cells divide by accomplishing a program of events in which the replication of the genome is a fundamental part. To ensure all cells have an accurate copy of the genome, DNA replication occurs only once per cell cycle and is controlled by numerous pathways. A key step in this process is the initiation of DNA replication in which certain regions of DNA are marked as competent to replicate. Moreover, initiation of DNA replication needs to be coordinated with other cell cycle processes. At the molecular level, initiation of DNA replication relies, among other mechanisms, upon post-translational modifications, including the conjugation and hydrolysis of ubiquitin. An example is the precise control of the levels of the DNA replication initiation protein Cdt1 and its inhibitor Geminin by ubiquitin-mediated proteasomal degradation. This control ensures that DNA replication occurs with the right timing during the cell cycle, thereby avoiding re-replication events. Here, we review the events that involve ubiquitin signalling during DNA replication initiation, and how they are linked to human disease.

Claspin functions in cell homeostasis-A link to cancer?

Cancer remains one of the leading causes of mortality worldwide. Most cancers present high degrees of genomic instability. DNA damage and replication checkpoints function as barriers to halt cell cycle progression until damage is resolved, preventing the perpetuation of errors. Activation of these checkpoints is critically dependent on Claspin, an adaptor protein that mediates the phosphorylation of the effector kinase Chk1 by ATR. However, Claspin also performs other roles related to the protection and maintenance of cell and genome integrity. For instance, following DNA damage and checkpoint activation, Claspin bridges checkpoint responses to DNA repair or to apoptosis. During DNA replication, Claspin acts a sensor and couples DNA unwinding to strand polymerization, and may also indirectly regulate replication initiation at firing origins. As Claspin participates in several processes that are vital to maintenance of cell homeostasis, its function is tightly regulated at multiple levels. Nevertheless, little is known about its role in cancer. Accumulating evidence suggests that Claspin inactivation could be an essential event during carcinogenesis, indicating that Claspin may function as a tumour suppressor. In this review, we will examine the functions of Claspin and how its deregulation may contribute to cancer initiation and progression. To conclude, we will discuss means by which Claspin can be targeted for cancer therapy.

DNA Replication Dynamics and Cellular Responses to ATP Competitive CDC7 Kinase Inhibitors

The CDC7 kinase, by phosphorylating the MCM DNA helicase, is a key switch for DNA replication initiation. ATP competitive CDC7 inhibitors are being developed as potential anticancer agents; however how human cells respond to the selective pharmacological inhibition of this kinase is controversial and not understood. Here we have characterized the mode of action of the two widely used CDC7 inhibitors, PHA-767491 and XL-413, which have become important tool compounds to explore the kinase's cellular functions. We have used a chemical genetics approach to further characterize pharmacological CDC7 inhibition and CRISPR/CAS9 technology to assess the requirement for kinase activity for cell proliferation. We show that, in human breast cells, CDC7 is essential and that CDC7 kinase activity is formally required for proliferation. However, full and sustained inhibition of the kinase, which is required to block the cell-cycle progression with ATP competitor compounds, is problematic to achieve. We establish that MCM2 phosphorylation is highly sensitive to CDC7 inhibition and, as a biomarker, it lacks in dynamic range since it is easily lost at concentrations of inhibitors that only mildly affect DNA synthesis. Furthermore, we find that the cellular effects of selective CDC7 inhibitors can be altered by the concomitant inhibition of cell-cycle and transcriptional CDKs. This work shows that DNA replication and cell proliferation can occur with reduced CDC7 activity for at least 5 days and that the bulk of DNA synthesis is not tightly coupled to MCM2 phosphorylation and provides guidance for the development of next generation CDC7 inhibitors.

DDK Promotes Tumor Chemoresistance and Survival via Multiple Pathways

DBF4-dependent kinase (DDK) is a two-subunit kinase required for initiating DNA replication at individual origins and is composed of CDC7 kinase and its regulatory subunit DBF4. Both subunits are highly expressed in many diverse tumor cell lines and primary tumors, and this is correlated with poor prognosis. Inhibiting DDK causes apoptosis of tumor cells, but not normal cells, through a largely unknown mechanism. Firstly, to understand why DDK is often overexpressed in tumors, we identified gene expression signatures that correlate with DDK high- and DDK low-expressing lung adenocarcinomas. We found that increased DDK expression is highly correlated with inactivation of RB1-E2F and p53 tumor suppressor pathways. Both CDC7 and DBF4 promoters bind E2F, suggesting that increased E2F activity in RB1 mutant cancers promotes increased DDK expression. Surprisingly, increased DDK expression levels are also correlated with both increased chemoresistance and genome-wide mutation frequencies. Our data further suggest that high DDK levels directly promote elevated mutation frequencies. Secondly, we performed an RNAi screen to investigate how DDK inhibition causes apoptosis of tumor cells. We identified 23 kinases and phosphatases required for apoptosis when DDK is inhibited. These hits include checkpoint genes, G2/M cell cycle regulators, and known tumor suppressors leading to the hypothesis that inhibiting mitotic progression can protect against DDKi-induced apoptosis. Characterization of one novel hit, the LATS2 tumor suppressor, suggests that it promotes apoptosis independently of the upstream MST1/2 kinases in the Hippo signaling pathway.

p53 controls CDC7 levels to reinforce G1 cell cycle arrest upon genotoxic stress

DNA replication initiation is a key event in the cell cycle, which is dependent on 2 kinases - CDK2 and CDC7. Here we report a novel mechanism in which p53 induces G1 checkpoint and cell cycle arrest by downregulating CDC7 kinase in response to genotoxic stress. We demonstrate that p53 controls CDC7 stability post-transcriptionally via miR-192/215 and post-translationally via Fbxw7β E3 ubiquitin ligase. The p53-dependent pathway of CDC7 downregulation is interlinked with the p53-p21-CDK2 pathway, as p21-mediated inhibition of CDK2-dependent phosphorylation of CDC7 on Thr376 is required for GSK3ß-phosphorylation and Fbxw7ß-dependent degradation of CDC7. Notably, sustained oncogenic high levels of active CDC7 exert a negative feedback onto p53, leading to unrestrained S-phase progression and accumulation of DNA damage. Thus, p53-dependent control of CDC7 levels is essential for blocking G1/S cell-cycle transition upon genotoxic stress, thereby safeguarding the genome from instability and thus representing a novel general stress response.

DDK dependent regulation of TOP2A at centromeres revealed by a chemical genetics approach

In eukaryotic cells the CDC7/DBF4 kinase, also known as DBF4-dependent kinase (DDK), is required for the firing of DNA replication origins. CDC7 is also involved in replication stress responses and its depletion sensitises cells to drugs that affect fork progression, including Topoisomerase 2 poisons. Although CDC7 is an important regulator of cell division, relatively few substrates and bona-fide CDC7 phosphorylation sites have been identified to date in human cells. In this study, we have generated an active recombinant CDC7/DBF4 kinase that can utilize bulky ATP analogues. By performing in vitro kinase assays using benzyl-thio-ATP, we have identified TOP2A as a primary CDC7 substrate in nuclear extracts, and serine 1213 and serine 1525 as in vitro phosphorylation sites. We show that CDC7/DBF4 and TOP2A interact in cells, that this interaction mainly occurs early in S-phase, and that it is compromised after treatment with CDC7 inhibitors. We further provide evidence that human DBF4 localises at centromeres, to which TOP2A is progressively recruited during S-phase. Importantly, we found that CDC7/DBF4 down-regulation, as well S1213A/S1525A TOP2A mutations can advance the timing of centromeric TOP2A recruitment in S-phase. Our results indicate that TOP2A is a novel DDK target and have important implications for centromere biology.

Claspin recruits Cdc7 kinase for initiation of DNA replication in human cells

Claspin transmits replication stress signal from ATR to Chk1 effector kinase as a mediator. It also plays a role in efficient replication fork progression during normal growth. Here we have generated conditional knockout of Claspin and show that Claspin knockout mice are dead by E12.5 and Claspin knockout mouse embryonic fibroblast (MEF) cells show defect in S phase. Using the mutant cell lines, we report the crucial roles of the acidic patch (AP) near the C terminus of Claspin in initiation of DNA replication. Cdc7 kinase binds to AP and this binding is required for phosphorylation of Mcm. AP is involved also in intramolecular interaction with a N-terminal segment, masking the DNA-binding domain and a newly identified PIP motif, and Cdc7-mediated phosphorylation reduces the intramolecular interaction. Our results suggest a new role of Claspin in initiation of DNA replication during normal S phase through the recruitment of Cdc7 that facilitates phosphorylation of Mcm proteins.

Claspin mediates the transmission of a replication-stress signal from ATR to Chk1 and is necessary for efficient fork progression. Here the authors demonstrate that the C-terminal acidic patch is important for this role due to its interaction with Cdc7.

The Deubiquitinase USP9X Maintains DNA Replication Fork Stability and DNA Damage Checkpoint Responses by Regulating CLASPIN during S-Phase

Coordination of the multiple processes underlying DNA replication is key for maintaining genome stability and preventing tumorigenesis. CLASPIN, a critical player in replication fork stabilization and checkpoint responses, must be tightly regulated during the cell cycle to prevent the accumulation of DNA damage. In this study, we used a quantitative proteomics approach and identified USP9X as a novel CLASPIN-interacting protein. USP9X is a deubiquitinase involved in multiple signaling and survival pathways whose tumor suppressor or oncogenic activity is highly context dependent. We found that USP9X regulated the expression and stability of CLASPIN in an S-phase-specific manner. USP9X depletion profoundly impairs the progression of DNA replication forks, causing unscheduled termination events with a frequency similar to CLASPIN depletion, resulting in excessive endogenous DNA damage. Importantly, restoration of CLASPIN expression in USP9X-depleted cells partially suppressed the accumulation of DNA damage. Furthermore, USP9X depletion compromised CHK1 activation in response to hydroxyurea and UV, thus promoting hypersensitivity to drug-induced replication stress. Taken together, our results reveal a novel role for USP9X in the maintenance of genomic stability during DNA replication and provide potential mechanistic insights into its tumor suppressor role in certain malignancies. Cancer Res; 76(8); 2384-93. ©2016 AACR.

A high through-put screen for small molecules modulating MCM2 phosphorylation identifies Ryuvidine as an inducer of the DNA damage response

DNA replication is an essential process for cell division and as such it is a process that is directly targeted by several anticancer drugs. CDC7 plays an essential role in the activation of replication origins and has recently been proposed as a novel target for drug discovery. The MCM DNA helicase complex (MCM2-7) is a key target of the CDC7 kinase, and MCM phosphorylation status at specific sites is a reliable biomarker of CDC7 cellular activity. In this work we describe a cell-based assay that utilizes the "In Cell Western Technique" (ICW) to identify compounds that affect cellular CDC7 activity. By screening a library of approved drugs and kinase inhibitors we found several compounds that can affect CDC7-dependent phosphorylation of MCM2 in HeLa cells. Among these, Mitoxantrone, a topoisomerase inhibitor, and Ryuvidine, previously described as a CDK4 inhibitor, cause a reduction in phosphorylated MCM2 levels and a sudden blockade of DNA synthesis that is accompanied by an ATM-dependent checkpoint response. This study sheds light on the previously observed cytotoxity of Ryuvidine, strongly suggesting that it is related to its effect of causing DNA damage.

Regulation and roles of Cdc7 kinase under replication stress

Cdc7 (cell division cycle 7) kinase together with its activation subunit ASK (also known as Dbf4) play pivotal roles in DNA replication and contribute also to other aspects of DNA metabolism such as DNA repair and recombination. While the biological significance of Cdc7 is widely appreciated, the molecular mechanisms through which Cdc7 kinase regulates these various DNA transactions remain largely obscure, including the role of Cdc7-ASK/Dbf4 under replication stress, a condition associated with diverse (patho)physiological scenarios. In this review, we first highlight the recent findings on a novel pathway that regulates the stability of the human Cdc7-ASK/Dbf4 complex under replication stress, its interplay with ATR-Chk1 signaling, and significance in the RAD18-dependent DNA damage bypass pathway. We also consider Cdc7 function in a broader context, considering both physiological conditions and pathologies associated with enhanced replication stress, particularly oncogenic transformation and tumorigenesis. Furthermore, we integrate the emerging evidence and propose a concept of Cdc7-ASK/Dbf4 contributing to genome integrity maintenance, through interplay with RAD18 that can serve as a molecular switch to dictate DNA repair pathway choice. Finally, we discuss the possibility of targeting Cdc7, particularly in the context of the Cdc7/RAD18-dependent translesion synthesis, as a potential innovative strategy for treatment of cancer.

Replisome components--post-translational modifications and their effects

The process of DNA replication is highly regulated, but at the same time very dynamic. Once S-phase is initiated and replication elongation is occurring, the cells are committed to complete replication in order to ensure genome stability and survival. Many pathways exist to resolve situations where normal replisome progression is not possible. It is becoming more and more evident that post-translational modifications of replisome components play a key role in regulating these pathways which ensure fork progression. Here we review the known modifications of the progressing replisome and how these modifications are thought to affect DNA replication in unperturbed and perturbed S-phases.

Cell cycle-dependent formation of Cdc45-Claspin complexes in human cells is compromized by UV-mediated DNA damage

The replication factor Cdc45 has essential functions in the initiation and elongation steps of eukaryotic DNA replication and plays an important role in the intra-S-phase checkpoint. Its interactions with other replication proteins during the cell cycle and after intra-S-phase checkpoint activation are only partially characterized. In the present study, we show that the C terminal part of Cdc45 may mediate its interactions with Claspin. The interactions of human Cdc45 with the three replication factors Claspin, replication protein A and DNA polymerase δ are maximal during the S phase. Following UVC-induced DNA damage, Cdc45-Claspin complex formation is reduced, whereas the binding of Cdc45 to replication protein A is not affected. We also show that treatment of cells with UCN-01 and phosphatidylinositol 3-kinase-like kinase inhibitors does not rescue the UV-induced destabilization of Cdc45-Claspin interactions, suggesting that the loss of the interaction between Cdc45 and Claspin occurs upstream of ataxia telangiectasia and Rad 3-related activation in the intra-S-phase checkpoint.

MicroRNA-29a regulates the benzo[a]pyrene dihydrodiol epoxide-induced DNA damage response through Cdc7 kinase in lung cancer cells

Cdc7 kinase is a key regulator of DNA replication and has an important role in the cellular DNA damage response by controlling checkpoint signaling and cell survival. Yet, how the activity of Cdc7 kinase is regulated is poorly understood. In silico analysis identified microRNA-29 (miR-29)-binding sites in the 3'-untranslated region (UTR) of both Cdc7 and its activating subunit Dbf4. We show that miR-29a binds to Cdc7 and Dbf4 3'-UTRs and regulates kinase levels. We find that in response to DNA damage, upregulation of Cdc7 kinase correlates with a downregulation in miR-29a. Enforced miR-29a expression prevents the accumulation of Cdc7 in response to the environmental genotoxin, benzo[a]pyrene dihydrodiol epoxide (BPDE) present in cigarette smoke, resulting in aberrant checkpoint signaling and increased cell lethality. As BPDE sensitivity was rescued by overexpression of miRNA-resistant Cdc7/Dbf4, we propose that Cdc7 kinase is an important target of miR-29a in determining cell survival from genotoxic stress caused by this environmental toxin.