Identification of Mcm2 Phosphorylation Sites by S-phase-regulating Kinases

Selective interactions at pre-replication complexes categorize baseline and dormant origins

DNA synthesis in metazoans initiates within a select group of replication origins (baseline origins), whereas other (dormant) origins do not initiate replication despite recruiting apparently indistinguishable pre-replication complexes. Dormant origins are activated as backups when DNA synthesis stalls, allowing for complete genome duplication, yet it is unclear how cells selectively differentiate between baseline and dormant origins. We report here that during unperturbed cell proliferation, dormant origins selectively bind phosphorylated RecQL4 (pRecQL4), a member of the RecQ helicase family mutated in Rothmund-Thomson, RAPADILINO and Baller-Gerold syndromes. Origin-bound pRecQL4 prevents the binding of an essential replication initiation complex, MTBP-TICRR/TRESLIN, to dormant origins, thus restricting replication initiation to baseline origins. When cells encounter replication stress, pRecQL4 is required for the dissociation of the MTBP-TICRR/TRESLIN complex from chromatin, which, in turn, facilitates the subsequent redistribution of MTBP-TICRR/TRESLIN to both baseline and dormant origins and allows recovery from replication inhibition. Thus, the interactions between the MTBP-TICRR/TRESLIN complex and pRecQL4 at replication origins are critical for replication origin choice and facilitate recovery from replication stress.

SETD3-mediated histidine methylation of MCM7 regulates DNA replication by facilitating chromatin loading of MCM

The minichromosome maintenance complex (MCM) DNA helicase is an important replicative factor during DNA replication. The proper chromatin loading of MCM is a key step to ensure replication initiation during S phase. Because replication initiation is regulated by multiple biological cues, additional changes to MCM may provide better understanding towards this event. Here, we report that histidine methyltransferase SETD3 promotes DNA replication in a manner dependent on enzymatic activity. Nascent-strand sequencing (NS-seq) shows that SETD3 regulates replication initiation, as depletion of SETD3 attenuates early replication origins firing. Biochemical studies reveal that SETD3 binds MCM mainly during S phase, which is required for the CDT1-mediated chromatin loading of MCM. This MCM loading relies on histidine-459 methylation (H459me) on MCM7 which is catalyzed by SETD3. Impairment of H459 methylation attenuates DNA synthesis and chromatin loading of MCM. Furthermore, we show that CDK2 phosphorylates SETD3 at Serine-21 during the G1/S phase, which is required for DNA replication and cell cycle progression. These findings demonstrate a novel mechanism by which SETD3 methylates MCM to regulate replication initiation.

Identification of MCM2-Interacting Proteins Associated with Replication Initiation Using APEX2-Based Proximity Labeling Technology

DNA replication is a crucial biological process that ensures the accurate transmission of genetic information, underpinning the growth, development, and reproduction of organisms. Abnormalities in DNA replication are a primary source of genomic instability and tumorigenesis. During DNA replication, the assembly of the pre-RC at the G1-G1/S transition is a crucial licensing step that ensures the successful initiation of replication. Although many pre-replication complex (pre-RC) proteins have been identified, technical limitations hinder the detection of transiently interacting proteins. The APEX system employs peroxidase-mediated rapid labeling with high catalytic efficiency, enabling protein labeling within one minute and detection of transient protein interactions. MCM2 is a key component of the eukaryotic replication initiation complex, which is essential for DNA replication. In this study, we fused MCM2 with enhanced APEX2 to perform in situ biotinylation. By combining this approach with mass spectrometry, we identified proteins proximal to the replication initiation complex in synchronized mouse ESCs and NIH/3T3. Through a comparison of the results from both cell types, we identified some candidate proteins. Interactions between MCM2 and the candidate proteins CD2BP2, VRK1, and GTSE1 were confirmed by bimolecular fluorescence complementation. This research establishes a basis for further study of the component proteins of the conserved DNA replication initiation complex and the transient regulatory network involving its proximal proteins.

Minichromosome Maintenance Proteins: From DNA Replication to the DNA Damage Response

The DNA replication machinery is highly conserved from bacteria to eukaryotic cells. Faithful DNA replication is vital for cells to transmit accurate genetic information to the next generation. However, both internal and external DNA damages threaten the intricate DNA replication process, leading to the activation of the DNA damage response (DDR) system. Dysfunctional DNA replication and DDR are a source of genomic instability, causing heritable mutations that drive cancer evolutions. The family of minichromosome maintenance (MCM) proteins plays an important role not only in DNA replication but also in DDR. Here, we will review the current strides of MCM proteins in these integrated processes as well as the acetylation/deacetylation of MCM proteins and the value of MCMs as biomarkers in cancer.

Histone variant macroH2A1 regulates synchronous firing of replication origins in the inactive X chromosome

MacroH2A has been linked to transcriptional silencing, cell identity, and is a hallmark of the inactive X chromosome (Xi). However, it remains unclear whether macroH2A plays a role in DNA replication. Using knockdown/knockout cells for each macroH2A isoform, we show that macroH2A-containing nucleosomes slow down replication progression rate in the Xi reflecting the higher nucleosome stability. Moreover, macroH2A1, but not macroH2A2, regulates the number of nano replication foci in the Xi, and macroH2A1 downregulation increases DNA loop sizes corresponding to replicons. This relates to macroH2A1 regulating replicative helicase loading during G1 by interacting with it. We mapped this interaction to a phenylalanine in macroH2A1 that is not conserved in macroH2A2 and the C-terminus of Mcm3 helicase subunit. We propose that macroH2A1 enhances the licensing of pre-replication complexes via DNA helicase interaction and loading onto the Xi.

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.

Improved detection of DNA replication fork-associated proteins

Innovative methods to retrieve proteins associated with actively replicating DNA have provided a glimpse into the molecular dynamics of replication fork stalling. We report that a combination of density-based replisome enrichment by isolating proteins on nascent DNA (iPOND2) and label-free quantitative mass spectrometry (iPOND2-DRIPPER) substantially increases both replication factor yields and the dynamic range of protein quantification. Replication protein abundance in retrieved nascent DNA is elevated up to 300-fold over post-replicative controls, and recruitment of replication stress factors upon fork stalling is observed at similar levels. The increased sensitivity of iPOND2-DRIPPER permits direct measurement of ubiquitination events without intervening retrieval of diglycine tryptic fragments of ubiquitin. Using this approach, we find that stalled replisomes stimulate the recruitment of a diverse cohort of DNA repair factors, including those associated with poly-K63-ubiquitination. Finally, we uncover the temporally controlled association of stalled replisomes with nuclear pore complex components and nuclear cytoskeleton networks.

Dbf4-dependent kinase promotes cell cycle controlled resection of DNA double-strand breaks and repair by homologous recombination

DNA double-strand breaks (DSBs) can be repaired by several pathways. In eukaryotes, DSB repair pathway choice occurs at the level of DNA end resection and is controlled by the cell cycle. Upon cell cycle-dependent activation, cyclin-dependent kinases (CDKs) phosphorylate resection proteins and thereby stimulate end resection and repair by homologous recombination (HR). However, inability of CDK phospho-mimetic mutants to bypass this cell cycle regulation, suggests that additional cell cycle regulators may be important. Here, we identify Dbf4-dependent kinase (DDK) as a second major cell cycle regulator of DNA end resection. Using inducible genetic and chemical inhibition of DDK in budding yeast and human cells, we show that end resection and HR require activation by DDK. Mechanistically, DDK phosphorylates at least two resection nucleases in budding yeast: the Mre11 activator Sae2, which promotes resection initiation, as well as the Dna2 nuclease, which promotes resection elongation. Notably, synthetic activation of DDK allows limited resection and HR in G1 cells, suggesting that DDK is a key component of DSB repair pathway selection.

Replisome loading reduces chromatin motion independent of DNA synthesis

Chromatin has been shown to undergo diffusional motion, which is affected during gene transcription by RNA polymerase activity. However, the relationship between chromatin mobility and other genomic processes remains unclear. Hence, we set out to label the DNA directly in a sequence unbiased manner and followed labeled chromatin dynamics in interphase human cells expressing GFP-tagged proliferating cell nuclear antigen (PCNA), a cell cycle marker and core component of the DNA replication machinery. We detected decreased chromatin mobility during the S-phase compared to G1 and G2 phases in tumor as well as normal diploid cells using automated particle tracking. To gain insight into the dynamical organization of the genome during DNA replication, we determined labeled chromatin domain sizes and analyzed their motion in replicating cells. By correlating chromatin mobility proximal to the active sites of DNA synthesis, we showed that chromatin motion was locally constrained at the sites of DNA replication. Furthermore, inhibiting DNA synthesis led to increased loading of DNA polymerases. This was accompanied by accumulation of the single-stranded DNA binding protein on the chromatin and activation of DNA helicases further restricting local chromatin motion. We, therefore, propose that it is the loading of replisomes but not their catalytic activity that reduces the dynamics of replicating chromatin segments in the S-phase as well as their accessibility and probability of interactions with other genomic regions.

Discovery and Characterization of ZL-2201, a Potent, Highly Selective, and Orally Bioavailable Small-molecule DNA-PK Inhibitor

DNA-dependent protein kinase (DNA-PK), a driver of the non-homologous end-joining (NHEJ) DNA damage response pathway, plays an instrumental role in repairing double-strand breaks (DSB) induced by DNA-damaging poisons. We evaluate ZL-2201, an orally bioavailable, highly potent, and selective pharmacologic inhibitor of DNA-PK activity, for the treatment of human cancerous malignancies. ZL-2201 demonstrated greater selectivity for DNA-PK and effectively inhibited DNA-PK autophosphorylation in a concentration- and time-dependent manner. Initial data suggested a potential correlation between ataxia-telangiectasia mutated (ATM) deficiency and ZL-2201 sensitivity. More so, ZL-2201 showed strong synergy with topoisomerase II inhibitors independent of ATM status in vitro. In vivo oral administration of ZL-2201 demonstrated dose-dependent antitumor activity in the NCI-H1703 xenograft model and significantly enhanced the activity of approved DNA-damaging agents in A549 and FaDu models. From a phosphoproteomic mass spectrometry screen, we identified and validated that ZL-2201 and PRKDC siRNA decreased Ser108 phosphorylation of MCM2, a key DNA replication factor. Collectively, we have characterized a potent and selective DNA-PK inhibitor with promising monotherapy and combinatory therapeutic potential with approved DNA-damaging agents. More importantly, we identified phospho-MCM2 (Ser108) as a potential proximal biomarker of DNA-PK inhibition that warrants further preclinical and clinical evaluation.

Significance

ZL-2201, a potent and selective DNA-PK inhibitor, can target tumor models in combination with DNA DSB-inducing agents such as radiation or doxorubicin, with potential to improve recurrent therapies in the clinic.

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.

MCM2 in human cancer: functions, mechanisms, and clinical significance

Background

Aberrant DNA replication is the main source of genomic instability that leads to tumorigenesis and progression. MCM2, a core subunit of eukaryotic helicase, plays a vital role in DNA replication. The dysfunction of MCM2 results in the occurrence and progression of multiple cancers through impairing DNA replication and cell proliferation.

Conclusions

MCM2 is a vital regulator in DNA replication. The overexpression of MCM2 was detected in multiple types of cancers, and the dysfunction of MCM2 was correlated with the progression and poor prognoses of malignant tumors. According to the altered expression of MCM2 and its correlation with clinicopathological features of cancer patients, MCM2 was thought to be a sensitive biomarker for cancer diagnosis, prognosis, and chemotherapy response. The anti-tumor effect induced by MCM2 inhibition implies the potential of MCM2 to be a novel therapeutic target for cancer treatment. Since DNA replication stress, which may stimulate anti-tumor immunity, frequently occurs in MCM2 deficient cells, it also proposes the possibility that MCM2 targeting improves the effect of tumor immunotherapy.

Quantitative Phosphoproteomics and Acetylomics of Safranal Anticancer Effects in Triple-Negative Breast Cancer Cells

Safranal, as an aroma in saffron, is one of the cytotoxic compounds in saffron that causes cell death in triple-negative breast cancer cells. Our recent research reported the anti-cancer effects of safranal, which further demonstrated its impact on protein translation, mitochondrial dysfunction, and DNA fragmentation. To better understand the underlying mechanisms, we identified acetylated and phosphorylated peptides in safranal-treated cancer cells. We conducted a comprehensive phosphoproteomics and acetylomics analysis of safranal-treated MDA-MB-231 cells by using a combination of TMT labeling and enrichment methods including titanium dioxide and immunoprecipitation. We provide a wide range of phosphoproteome regulation in different signaling pathways that are disrupted by safranal treatment. Safranal influences the phosphorylation level on proteins involved in DNA replication and repair, translation, and EGFR activation/accumulation, which can lead the cells into apoptosis. Safranal causes DNA damage which is followed by the activation of cell cycle checkpoints for DNA repair. Over time, checkpoints and DNA repair are inhibited and cells are under a mitotic catastrophe. Moreover, safranal prevents repair by the hypo-acetylation of H4 and facilitates the transcription of proapoptotic genes by hyper-acetylation of H3, which push the cells to the brink of death.

Arbidol inhibits human esophageal squamous cell carcinoma growth in vitro and in vivo through suppressing ataxia telangiectasia and Rad3-related protein kinase

Human esophageal cancer has a global impact on human health due to its high incidence and mortality. Therefore, there is an urgent need to develop new drugs to treat or prevent the prominent pathological subtype of esophageal cancer, esophageal squamous cell carcinoma (ESCC). Based upon the screening of drugs approved by the Food and Drug Administration, we discovered that Arbidol could effectively inhibit the proliferation of human ESCC in vitro. Next, we conducted a series of cell-based assays and found that Arbidol treatment inhibited the proliferation and colony formation ability of ESCC cells and promoted G1-phase cell cycle arrest. Phosphoproteomics experiments, in vitro kinase assays and pull-down assays were subsequently performed in order to identify the underlying growth inhibitory mechanism. We verified that Arbidol is a potential ataxia telangiectasia and Rad3-related (ATR) inhibitor via binding to ATR kinase to reduce the phosphorylation and activation of minichromosome maintenance protein 2 at Ser108. Finally, we demonstrated Arbidol had the inhibitory effect of ESCC in vivo by a patient-derived xenograft model. All together, Arbidol inhibits the proliferation of ESCC in vitro and in vivo through the DNA replication pathway and is associated with the cell cycle.

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.

The structural basis of Cdc7-Dbf4 kinase dependent targeting and phosphorylation of the MCM2-7 double hexamer

The controlled assembly of replication forks is critical for genome stability. The Dbf4-dependent Cdc7 kinase (DDK) initiates replisome assembly by phosphorylating the MCM2-7 replicative helicase at the N-terminal tails of Mcm2, Mcm4 and Mcm6. At present, it remains poorly understood how DDK docks onto the helicase and how the kinase targets distal Mcm subunits for phosphorylation. Using cryo-electron microscopy and biochemical analysis we discovered that an interaction between the HBRCT domain of Dbf4 with Mcm2 serves as an anchoring point, which supports binding of DDK across the MCM2-7 double-hexamer interface and phosphorylation of Mcm4 on the opposite hexamer. Moreover, a rotation of DDK along its anchoring point allows phosphorylation of Mcm2 and Mcm6. In summary, our work provides fundamental insights into DDK structure, control and selective activation of the MCM2-7 helicase during DNA replication. Importantly, these insights can be exploited for development of novel DDK inhibitors.

Convergence of SIRT1 and ATR signaling to modulate replication origin dormancy

During routine genome duplication, many potential replication origins remain inactive or 'dormant'. Such origin dormancy is achieved, in part, by an interaction with the metabolic sensor SIRT1 deacetylase. We report here that dormant origins are a group of consistent, pre-determined genomic sequences that are distinguished from baseline (i.e. ordinarily active) origins by their preferential association with two phospho-isoforms of the helicase component MCM2. During normal unperturbed cell growth, baseline origins, but not dormant origins, associate with a form of MCM2 that is phosphorylated by DBF4-dependent kinase (DDK) on serine 139 (pS139-MCM2). This association facilitates the initiation of DNA replication from baseline origins. Concomitantly, SIRT1 inhibits Ataxia Telangiectasia and Rad3-related (ATR)-kinase-mediated phosphorylation of MCM2 on serine 108 (pS108-MCM2) by deacetylating the ATR-interacting protein DNA topoisomerase II binding protein 1 (TOPBP1), thereby preventing ATR recruitment to chromatin. In cells devoid of SIRT1 activity, or challenged by replication stress, this inhibition is circumvented, enabling ATR-mediated S108-MCM2 phosphorylation. In turn, pS108-MCM2 enables DDK-mediated phosphorylation on S139-MCM2 and facilitates replication initiation at dormant origins. These observations suggest that replication origin dormancy and activation are regulated by distinct post-translational MCM modifications that reflect a balance between SIRT1 activity and ATR signaling.

Salt-Inducible Kinase 1 is a potential therapeutic target in Desmoplastic Small Round Cell Tumor

Desmoplastic Small Round Cell Tumor (DSRCT) is a rare and aggressive malignant cancer caused by a chromosomal translocation t(11;22)(p13;q12) that produces an oncogenic transcription factor, EWSR1-WT1. EWSR1-WT1 is essential for the initiation and progression of DSRCT. However, the precise mechanism by which EWSR1-WT1 drives DSRCT oncogenesis remains unresolved. Through our integrative gene expression analysis, we identified Salt Inducible Kinase 1 (SIK1) as a direct target of EWSR1-WT1. SIK1 as a member of the AMPK related kinase is involved in many biological processes. We showed that depletion of SIK1 causes inhibition of tumor cell growth, similar to the growth inhibition observed when EWSR1-WT1 is depleted. We further showed that silencing SIK1 leads to cessation of DNA replication in DSRCT cells and inhibition of tumor growth in vivo. Lastly, combined inhibition of SIK1 and CHEK1with small molecule inhibitors, YKL-05-099 and prexasertib, respectively, showed enhanced cytotoxicity in DSRCT cells compared to inhibition of either kinases alone. This work identified SIK1 as a new potential therapeutic target in DSRCT and the efficacy of SIK1 inhibition may be improved when combined with other intervention strategies.

Structural mechanism for the selective phosphorylation of DNA-loaded MCM double hexamers by the Dbf4-dependent kinase

Loading of the eukaryotic replicative helicase onto replication origins involves two MCM hexamers forming a double hexamer (DH) around duplex DNA. During S phase, helicase activation requires MCM phosphorylation by Dbf4-dependent kinase (DDK), comprising Cdc7 and Dbf4. DDK selectively phosphorylates loaded DHs, but how such fidelity is achieved is unknown. Here, we determine the cryogenic electron microscopy structure of Saccharomyces cerevisiae DDK in the act of phosphorylating a DH. DDK docks onto one MCM ring and phosphorylates the opposed ring. Truncation of the Dbf4 docking domain abrogates DH phosphorylation, yet Cdc7 kinase activity is unaffected. Late origin firing is blocked in response to DNA damage via Dbf4 phosphorylation by the Rad53 checkpoint kinase. DDK phosphorylation by Rad53 impairs DH phosphorylation by blockage of DDK binding to DHs, and also interferes with the Cdc7 active site. Our results explain the structural basis and regulation of the selective phosphorylation of DNA-loaded MCM DHs, which supports bidirectional replication.

Chromatin loading of MCM hexamers is associated with di-/tri-methylation of histone H4K20 toward S phase entry

DNA replication is a key step in initiating cell proliferation. Loading hexameric complexes of minichromosome maintenance (MCM) helicase onto DNA replication origins during the G1 phase is essential for initiating DNA replication. Here, we examined MCM hexamer states during the cell cycle in human hTERT-RPE1 cells using multicolor immunofluorescence-based, single-cell plot analysis, and biochemical size fractionation. Experiments involving cell-cycle arrest at the G1 phase and release from the arrest revealed that a double MCM hexamer was formed via a single hexamer during G1 progression. A single MCM hexamer was recruited to chromatin in the early G1 phase. Another single hexamer was recruited to form a double hexamer in the late G1 phase. We further examined relationship between the MCM hexamer states and the methylation levels at lysine 20 of histone H4 (H4K20) and found that the double MCM hexamer state was correlated with di/trimethyl-H4K20 (H4K20me2/3). Inhibiting the conversion from monomethyl-H4K20 (H4K20me1) to H4K20me2/3 retained the cells in the single MCM hexamer state. Non-proliferative cells, including confluent cells or Cdk4/6 inhibitor-treated cells, also remained halted in the single MCM hexamer state. We propose that the single MCM hexamer state is a halting step in the determination of cell cycle progression.

Characterization of subcellular localization of eukaryotic clamp loader/unloader and its regulatory mechanism

Proliferating cell nuclear antigen (PCNA) plays a critical role as a processivity clamp for eukaryotic DNA polymerases and a binding platform for many DNA replication and repair proteins. The enzymatic activities of PCNA loading and unloading have been studied extensively in vitro. However, the subcellular locations of PCNA loaders, replication complex C (RFC) and CTF18-RFC-like-complex (RLC), and PCNA unloader ATAD5-RLC remain elusive, and the role of their subunits RFC2-5 is unknown. Here we used protein fractionation to determine the subcellular localization of RFC and RLCs and affinity purification to find molecular requirements for the newly defined location. All RFC/RLC proteins were detected in the nuclease-resistant pellet fraction. RFC1 and ATAD5 were not detected in the non-ionic detergent-soluble and nuclease-susceptible chromatin fractions, independent of cell cycle or exogenous DNA damage. We found that small RFC proteins contribute to maintaining protein levels of the RFC/RLCs. RFC1, ATAD5, and RFC4 co-immunoprecipitated with lamina-associated polypeptide 2 (LAP2) α which regulates intranuclear lamin A/C. LAP2α knockout consistently reduced detection of RFC/RLCs in the pellet fraction, while marginally affecting total protein levels. Our findings strongly suggest that PCNA-mediated DNA transaction occurs through regulatory machinery associated with nuclear structures, such as the nuclear matrix.

Cdc7-mediated phosphorylation of Cse4 regulates high-fidelity chromosome segregation in budding yeast

Faithful chromosome segregation maintains chromosomal stability as errors in this process contribute to chromosomal instability (CIN), which has been observed in many diseases including cancer. Epigenetic regulation of kinetochore proteins such as Cse4 (CENP-A in humans) plays a critical role in high-fidelity chromosome segregation. Here we show that Cse4 is a substrate of evolutionarily conserved Cdc7 kinase, and that Cdc7-mediated phosphorylation of Cse4 prevents CIN. We determined that Cdc7 phosphorylates Cse4 in vitro and interacts with Cse4 in vivo in a cell cycle-dependent manner. Cdc7 is required for kinetochore integrity as reduced levels of CEN-associated Cse4, a faster exchange of Cse4 at the metaphase kinetochores, and defects in chromosome segregation, are observed in a cdc7-7 strain. Phosphorylation of Cse4 by Cdc7 is important for cell survival as constitutive association of a kinase-dead variant of Cdc7 (cdc7-kd) with Cse4 at the kinetochore leads to growth defects. Moreover, phospho-deficient mutations of Cse4 for consensus Cdc7 target sites contribute to CIN phenotype. In summary, our results have defined a role for Cdc7-mediated phosphorylation of Cse4 in faithful chromosome segregation.

Discovery of AS-0141, a Potent and Selective Inhibitor of CDC7 Kinase for the Treatment of Solid Cancers

CDC7, a serine-threonine kinase, plays conserved and important roles in regulation of DNA replication and has been recognized as a potential anticancer target. We report here the optimization of a series of furanone analogues starting from compound 1 with a focus on ADME properties suitable for clinical development. By replacing the 2-chlorobenzene moiety in 1 with various aliphatic groups, we identified compound 24 as a potent CDC7 inhibitor with excellent kinase selectivity and favorable oral bioavailability in multiple species. Oral administration of 24 demonstrated robust in vivo antitumor efficacy in a colorectal cancer xenograft model. Compound 24 (AS-0141) is currently in phase I clinical trials for the treatment of solid cancers.

REV1-Polζ maintains the viability of homologous recombination-deficient cancer cells through mutagenic repair of PRIMPOL-dependent ssDNA gaps

BRCA1/2 mutant tumor cells display an elevated mutation burden, the etiology of which remains unclear. Here, we report that these cells accumulate ssDNA gaps and spontaneous mutations during unperturbed DNA replication due to repriming by the DNA primase-polymerase PRIMPOL. Gap accumulation requires the DNA glycosylase SMUG1 and is exacerbated by depletion of the translesion synthesis (TLS) factor RAD18 or inhibition of the error-prone TLS polymerase complex REV1-Polζ by the small molecule JH-RE-06. JH-RE-06 treatment of BRCA1/2-deficient cells results in reduced mutation rates and PRIMPOL- and SMUG1-dependent loss of viability. Through cellular and animal studies, we demonstrate that JH-RE-06 is preferentially toxic toward HR-deficient cancer cells. Furthermore, JH-RE-06 remains effective toward PARP inhibitor (PARPi)-resistant BRCA1 mutant cells and displays additive toxicity with crosslinking agents or PARPi. Collectively, these studies identify a protective and mutagenic role for REV1-Polζ in BRCA1/2 mutant cells and provide the rationale for using REV1-Polζ inhibitors to treat BRCA1/2 mutant tumors.

Proteogenomic characterization of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer with poor patient survival. Toward understanding the underlying molecular alterations that drive PDAC oncogenesis, we conducted comprehensive proteogenomic analysis of 140 pancreatic cancers, 67 normal adjacent tissues, and 9 normal pancreatic ductal tissues. Proteomic, phosphoproteomic, and glycoproteomic analyses were used to characterize proteins and their modifications. In addition, whole-genome sequencing, whole-exome sequencing, methylation, RNA sequencing (RNA-seq), and microRNA sequencing (miRNA-seq) were performed on the same tissues to facilitate an integrated proteogenomic analysis and determine the impact of genomic alterations on protein expression, signaling pathways, and post-translational modifications. To ensure robust downstream analyses, tumor neoplastic cellularity was assessed via multiple orthogonal strategies using molecular features and verified via pathological estimation of tumor cellularity based on histological review. This integrated proteogenomic characterization of PDAC will serve as a valuable resource for the community, paving the way for early detection and identification of novel therapeutic targets.

Frame-shift mediated reduction of gain-of-function p53 R273H and deletion of the R273H C-terminus in breast cancer cells result in replication-stress sensitivity

We recently documented that gain-of-function (GOF) mutant p53 (mtp53) R273H in triple negative breast cancer (TNBC) cells interacts with replicating DNA and PARP1. The missense R273H GOF mtp53 has a mutated central DNA binding domain that renders it unable to bind specifically to DNA, but maintains the capacity to interact tightly with chromatin. Both the C-terminal domain (CTD) and oligomerization domain (OD) of GOF mtp53 proteins are intact and it is unclear whether these regions of mtp53 are responsible for chromatin-based DNA replication activities. We generated MDA-MB-468 cells with CRISPR-Cas9 edited versions of the CTD and OD regions of mtp53 R273H. These included a frame-shift mtp53 R273Hfs387, which depleted mtp53 protein expression; mtp53 R273HΔ381-388, which had a small deletion within the CTD; and mtp53 R273HΔ347-393, which had both the OD and CTD regions truncated. The mtp53 R273HΔ347-393 existed exclusively as monomers and disrupted the chromatin interaction of mtp53 R273H. The CRISPR variants proliferated more slowly than the parental cells and mt53 R273Hfs387 showed the most extreme phenotype. We uncovered that after thymidine-induced G1/S synchronization, but not hydroxyurea or aphidicholin, R273Hfs387 cells displayed impairment of S-phase progression while both R273HΔ347-393 and R273HΔ381-388 displayed only moderate impairment. Moreover, reduced chromatin interaction of MCM2 and PCNA in mtp53 depleted R273Hfs387 cells post thymidine-synchronization revealed delayed kinetics of replisome assembly underscoring the slow S-phase progression. Taken together our findings show that the CTD and OD domains of mtp53 R273H play critical roles in mutant p53 GOF that pertain to processes associated with DNA replication.

TRPS1 drives heterochromatic origin refiring and cancer genome evolution

Exploitation of naturally occurring genetic mutations could empower the discovery of novel aspects of established cancer genes. We report here that TRPS1, a gene linked to the tricho-rhino-phalangeal syndrome (TRPS) and recently identified as a potential breast cancer driver, promotes breast carcinogenesis through regulating replication. Epigenomic decomposition of TRPS1 landscape reveals nearly half of H3K9me3-marked heterochromatic origins are occupied by TRPS1, where it encourages the chromatin loading of APC/C, resulting in uncontrolled origin refiring. TRPS1 binds to the genome through its atypical H3K9me3 reading via GATA and IKAROS domains, while TRPS-related mutations affect its chromatin binding, replication boosting, and tumorigenicity. Concordantly, overexpression of wild-type but not TRPS-associated mutants of TRPS1 is sufficient to drive cancer genome amplifications, which experience an extrachromosomal route and dynamically evolve to confer therapeutic resistance. Together, these results uncover a critical function of TRPS1 in driving heterochromatin origin firing and breast cancer genome evolution.

MTBP phosphorylation controls DNA replication origin firing

Faithful genome duplication requires regulation of origin firing to determine loci, timing and efficiency of replisome generation. Established kinase targets for eukaryotic origin firing regulation are the Mcm2-7 helicase, Sld3/Treslin/TICRR and Sld2/RecQL4. We report that metazoan Sld7, MTBP (Mdm2 binding protein), is targeted by at least three kinase pathways. MTBP was phosphorylated at CDK consensus sites by cell cycle cyclin-dependent kinases (CDK) and Cdk8/19-cyclin C. Phospho-mimetic MTBP CDK site mutants, but not non-phosphorylatable mutants, promoted origin firing in human cells. MTBP was also phosphorylated at DNA damage checkpoint kinase consensus sites. Phospho-mimetic mutations at these sites inhibited MTBP’s origin firing capability. Whilst expressing a non-phospho MTBP mutant was insufficient to relieve the suppression of origin firing upon DNA damage, the mutant induced a genome-wide increase of origin firing in unperturbed cells. Our work establishes MTBP as a regulation platform of metazoan origin firing.

The CRK2-CYC13 complex functions as an S-phase cyclin-dependent kinase to promote DNA replication in Trypanosoma brucei

BACKGROUND

Faithful DNA replication is essential to maintain genomic stability in all living organisms, and the regulatory pathway for DNA replication initiation is conserved from yeast to humans. The evolutionarily ancient human parasite Trypanosoma brucei, however, lacks many of the conserved DNA replication factors and may employ unusual mechanisms for DNA replication. Neither the S-phase cyclin-dependent kinase (CDK) nor the regulatory pathway governing DNA replication has been previously identified in T. brucei.

RESULTS

Here we report that CRK2 (Cdc2-related kinase 2) complexes with CYC13 (Cyclin13) and functions as an S-phase CDK to promote DNA replication in T. brucei. We further show that CRK2 phosphorylates Mcm3, a subunit of the Mcm2-7 sub-complex of the Cdc45-Mcm2-7-GINS complex, and demonstrate that Mcm3 phosphorylation by CRK2 facilitates interaction with Sld5, a subunit of the GINS sub-complex of the Cdc45-Mcm2-7-GINS complex.

CONCLUSIONS

These results identify the CRK2-CYC13 complex as an S-phase regulator in T. brucei and reveal its role in regulating DNA replication through promoting the assembly of the Cdc45-Mcm2-7-GINS complex.

Human DDK rescues stalled forks and counteracts checkpoint inhibition at unfired origins to complete DNA replication

Eukaryotic genomes replicate via spatially and temporally regulated origin firing. Cyclin-dependent kinase (CDK) and Dbf4-dependent kinase (DDK) promote origin firing, whereas the S phase checkpoint limits firing to prevent nucleotide and RPA exhaustion. We used chemical genetics to interrogate human DDK with maximum precision, dissect its relationship with the S phase checkpoint, and identify DDK substrates. We show that DDK inhibition (DDKi) leads to graded suppression of origin firing and fork arrest. S phase checkpoint inhibition rescued origin firing in DDKi cells and DDK-depleted Xenopus egg extracts. DDKi also impairs RPA loading, nascent-strand protection, and fork restart. Via quantitative phosphoproteomics, we identify the BRCA1-associated (BRCA1-A) complex subunit MERIT40 and the cohesin accessory subunit PDS5B as DDK effectors in fork protection and restart. Phosphorylation neutralizes autoinhibition mediated by intrinsically disordered regions in both substrates. Our results reveal mechanisms through which DDK controls the duplication of large vertebrate genomes.

Distinct Hepatic PKA and CDK Signaling Pathways Control Activity-Independent Pyruvate Kinase Phosphorylation and Hepatic Glucose Production

Pyruvate kinase is an important enzyme in glycolysis and a key metabolic control point. We recently observed a pyruvate kinase liver isoform (PKL) phosphorylation site at S113 that correlates with insulin resistance in rats on a 3 day high-fat diet (HFD) and suggests additional control points for PKL activity. However, in contrast to the classical model of PKL regulation, neither authentically phosphorylated PKL at S12 nor S113 alone is sufficient to alter enzyme kinetics or structure. Instead, we show that cyclin-dependent kinases (CDKs) are activated by the HFD and responsible for PKL phosphorylation at position S113 in addition to other targets. These CDKs control PKL nuclear retention, alter cytosolic PKL activity, and ultimately influence glucose production. These results change our view of PKL regulation and highlight a previously unrecognized pathway of hepatic CDK activity and metabolic control points that may be important in insulin resistance and type 2 diabetes.

Structural Basis for the Activation and Target Site Specificity of CDC7 Kinase

CDC7 is an essential Ser/Thr kinase that acts upon the replicative helicase throughout the S phase of the cell cycle and is activated by DBF4. Here, we present crystal structures of a highly active human CDC7-DBF4 construct. The structures reveal a zinc-finger domain at the end of the kinase insert 2 that pins the CDC7 activation loop to motif M of DBF4 and the C lobe of CDC7. These interactions lead to ordering of the substrate-binding platform and full opening of the kinase active site. In a co-crystal structure with a mimic of MCM2 Ser40 phosphorylation target, the invariant CDC7 residues Arg373 and Arg380 engage phospho-Ser41 at substrate P+1 position, explaining the selectivity of the S-phase kinase for Ser/Thr residues followed by a pre-phosphorylated or an acidic residue. Our results clarify the role of DBF4 in activation of CDC7 and elucidate the structural basis for recognition of its preferred substrates.

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DBF4 activates CDC7 kinase via a two-step mechanism

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Zinc-finger domain in CDC7 KI2 interacts with DBF4 motif M

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Invariant CDC7 residues Arg373 and Arg380 engage P+1 substrate site

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.

Gain-of-Function Mutant p53 R273H Interacts with Replicating DNA and PARP1 in Breast Cancer

Over 80% of triple-negative breast cancers (TNBC) express mutant p53 (mtp53) and some contain oncogenic gain-of-function (GOF) p53. We previously reported that GOF mtp53 R273H upregulates the chromatin association of mini chromosome maintenance (MCM) proteins MCM2-7 and PARP and named this the mtp53-PARP-MCM axis. In this study, we dissected the function and association between mtp53 and PARP using a number of different cell lines, patient-derived xenografts (PDX), tissue microarrays (TMA), and The Cancer Genome Atlas (TCGA) database. Endogenous mtp53 R273H and exogenously expressed R273H and R248W bound to nascent 5-ethynyl-2´-deoxyuridine-labeled replicating DNA. Increased mtp53 R273H enhanced the association of mtp53 and PARP on replicating DNA. Blocking poly-ADP-ribose gylcohydrolase also enhanced this association. Moreover, mtp53 R273H expression enhanced overall MCM2 levels, promoted cell proliferation, and improved the synergistic cytotoxicity of treatment with the alkylating agent temozolomide in combination with the PARP inhibitor (PARPi) talazoparib. Staining of p53 and PARP1 in breast cancer TMAs and comparison with the TCGA database indicated a higher double-positive signal in basal-like breast cancer than in luminal A or luminal B subtypes. Higher PARP1 protein levels and PAR proteins were detected in mtp53 R273H than in wild-type p53-expressing PDX samples. These results indicate that mtp53 R273H and PARP1 interact with replicating DNA and should be considered as dual biomarkers for identifying breast cancers that may respond to combination PARPi treatments. SIGNIFICANCE: p53 gain-of-function mutant 273H and PARP1 interact with replication forks and could serve as potential biomarkers for breast cancer sensitivity to PARP inhibitors. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/3/394/F1.large.jpg.

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.

Phenotypic proteomic profiling identifies a landscape of targets for circadian clock-modulating compounds

This study provides comprehensive insights into the mechanism of action and cellular effects of circadian period–modulating compounds, which is critical for clearly defining molecular targets to modulate daily rhythms for therapeutic benefit.

Determining the exact targets and mechanisms of action of drug molecules that modulate circadian rhythms is critical to develop novel compounds to treat clock-related disorders. Here, we have used phenotypic proteomic profiling (PPP) to systematically determine molecular targets of four circadian period–lengthening compounds in human cells. We demonstrate that the compounds cause similar changes in phosphorylation and activity of several proteins and kinases involved in vital pathways, including MAPK, NGF, B-cell receptor, AMP-activated protein kinases (AMPKs), and mTOR signaling. Kinome profiling further indicated inhibition of CKId, ERK1/2, CDK2/7, TNIK, and MST4 kinases as a common mechanism of action for these clock-modulating compounds. Pharmacological or genetic inhibition of several convergent kinases lengthened circadian period, establishing them as novel circadian targets. Finally, thermal stability profiling revealed binding of the compounds to clock regulatory kinases, signaling molecules, and ubiquitination proteins. Thus, phenotypic proteomic profiling defines novel clock effectors that could directly inform precise therapeutic targeting of the circadian system in humans.

Phosphoproteomic analysis sheds light on intracellular signaling cascades triggered by Formyl-Peptide Receptor 2

Formyl peptide receptors (FPRs) belong to the family of seven transmembrane Gi-protein coupled receptors (GPCR). FPR2 is considered the most promiscuous member of this family since it recognizes a wide variety of ligands. It plays a crucial role in several physio-pathological processes and different studies highlighted the correlation between its expression and the higher propensity to invasion and metastasis of some cancers. FPR2 stimulation by its synthetic agonist WKYMVm triggers multiple phosphorylations of intracellular signaling molecules, such as ERKs, PKC, PKB, p38MAPK, PI3K, PLC, and of non-signaling proteins, such as p47^phox and p67^phox which are involved in NADPH oxidase-dependent ROS generation. Biological effects of FPR2 stimulation include intracellular Ca²⁺ mobilization, cellular proliferation and migration, and wound healing. A systematic analysis of the phosphoproteome in FPR2-stimulated cells has not been yet reported. Herein, we describe a large-scale phosphoproteomic study in WKYMVm-stimulated CaLu-6 cells. By using high resolution MS/MS we identified 290 differentially phosphorylated proteins and 53 unique phosphopeptides mapping on 40 proteins. Phosphorylations on five selected phospho-proteins were further validated by western blotting, confirming their dependence on FPR2 stimulation. Interconnection between some of the signalling readout identified was also evaluated. Furthermore, we show that FPR2 stimulation with two anti-inflammatory agonists induces the phosphorylation of selected differentially phosphorylated proteins, suggesting their role in the resolution of inflammation. These data provide a promising resource for further studies on new signaling networks triggered by FPR2 and on novel molecular drug targets for human diseases.

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.

Molecular mechanism and potential target indication of TAK-931, a novel CDC7-selective inhibitor

Replication stress (RS) is a cancer hallmark; chemotherapeutic drugs targeting RS are widely used as treatments for various cancers. To develop next-generation RS-inducing anticancer drugs, cell division cycle 7 (CDC7) has recently attracted attention as a target. We have developed an oral CDC7-selective inhibitor, TAK-931, as a candidate clinical anticancer drug. TAK-931 induced S phase delay and RS. TAK-931-induced RS caused mitotic aberrations through centrosome dysregulation and chromosome missegregation, resulting in irreversible antiproliferative effects in cancer cells. TAK-931 exhibited significant antiproliferative activity in preclinical animal models. Furthermore, in indication-seeking studies using large-scale cell panel data, TAK-931 exhibited higher antiproliferative activities in RAS-mutant versus RAS-wild-type cells; this finding was confirmed in pancreatic patient-derived xenografts. Comparison analysis of cell panel data also demonstrated a unique efficacy spectrum for TAK-931 compared with currently used chemotherapeutic drugs. Our findings help to elucidate the molecular mechanisms for TAK-931 and identify potential target indications.

Post-Translational Modifications of the Mini-Chromosome Maintenance Proteins in DNA Replication

The eukaryotic mini-chromosome maintenance (MCM) complex, composed of MCM proteins 2-7, is the core component of the replisome that acts as the DNA replicative helicase to unwind duplex DNA and initiate DNA replication. MCM10 tightly binds the cell division control protein 45 homolog (CDC45)/MCM2-7/ DNA replication complex Go-Ichi-Ni-San (GINS) (CMG) complex that stimulates CMG helicase activity. The MCM8-MCM9 complex may have a non-essential role in activating the pre-replicative complex in the gap 1 (G1) phase by recruiting cell division cycle 6 (CDC6) to the origin recognition complex (ORC). Each MCM subunit has a distinct function achieved by differential post-translational modifications (PTMs) in both DNA replication process and response to replication stress. Such PTMs include phosphorylation, ubiquitination, small ubiquitin-like modifier (SUMO)ylation, O-N-acetyl-D-glucosamine (GlcNAc)ylation, and acetylation. These PTMs have an important role in controlling replication progress and genome stability. Because MCM proteins are associated with various human diseases, they are regarded as potential targets for therapeutic development. In this review, we summarize the different PTMs of the MCM proteins, their involvement in DNA replication and disease development, and the potential therapeutic implications.

Differential Sensitivity to CDK2 Inhibition Discriminates the Molecular Mechanisms of CHK1 Inhibitors as Monotherapy or in Combination with the Topoisomerase I Inhibitor SN38

DNA damage activates checkpoints to arrest cell cycle progression in S and G2 phases, thereby providing time for repair and recovery. The combination of DNA-damaging agents and inhibitors of CHK1 (CHK1i) is an emerging strategy for sensitizing cancer cells. CHK1i induce replication on damaged DNA and mitosis before repair is complete, and this occurs in a majority of cell lines. However, ∼15% of cancer cell lines are hypersensitive to single-agent CHK1i. As both abrogation of S phase arrest and single-agent activity depend on CDK2, this study resolved how activation of CDK2 can be essential for both replication and cytotoxicity. S phase arrest was induced with the topoisomerase I inhibitor SN38; the addition of CHK1i rapidly activated CDK2, inducing S phase progression that was inhibited by the CDK2 inhibitor CVT-313. In contrast, DNA damage and cytotoxicity induced by single-agent CHK1i in hypersensitive cell lines were also inhibited by CVT-313 but at 20-fold lower concentrations. The differential sensitivity to CVT-313 is explained by different activity thresholds required for phosphorylation of CDK2 substrates. While the critical CDK2 substrates are not yet defined, we conclude that hypersensitivity to single-agent CHK1i depends on phosphorylation of substrates that require high CDK2 activity levels. Surprisingly, CHK1i did not increase SN38-mediated cytotoxicity. In contrast, while inhibition of WEE1 also abrogated S phase arrest, it more directly activated CDK1, induced premature mitosis, and enhanced cytotoxicity. Hence, while high activity of CDK2 is critical for cytotoxicity of single-agent CHK1i, CDK1 is additionally required for sensitivity to the drug combination.

Meiosis-specific prophase-like pathway controls cleavage-independent release of cohesin by Wapl phosphorylation

Sister chromatid cohesion on chromosome arms is essential for the segregation of homologous chromosomes during meiosis I while it is dispensable for sister chromatid separation during mitosis. It was assumed that, unlike the situation in mitosis, chromosome arms retain cohesion prior to onset of anaphase-I. Paradoxically, reduced immunostaining signals of meiosis-specific cohesin, including the kleisin Rec8, were observed on chromosomes during late prophase-I of budding yeast. This decrease is seen in the absence of Rec8 cleavage and depends on condensin-mediated recruitment of Polo-like kinase (PLK/Cdc5). In this study, we confirmed that this release indeed accompanies the dissociation of acetylated Smc3 as well as Rec8 from meiotic chromosomes during late prophase-I. This release requires, in addition to PLK, the cohesin regulator, Wapl (Rad61/Wpl1 in yeast), and Dbf4-dependent Cdc7 kinase (DDK). Meiosis-specific phosphorylation of Rad61/Wpl1 and Rec8 by PLK and DDK collaboratively promote this release. This process is similar to the vertebrate “prophase” pathway for cohesin release during G2 phase and pro-metaphase. In yeast, meiotic cohesin release coincides with PLK-dependent compaction of chromosomes in late meiotic prophase-I. We suggest that yeast uses this highly regulated cleavage-independent pathway to remove cohesin during late prophase-I to facilitate morphogenesis of condensed metaphase-I chromosomes.

In meiosis the life and health of future generations is decided upon. Any failure in chromosome segregation has a detrimental impact. Therefore, it is currently believed that the physical connections between homologous chromosomes are maintained by meiotic cohesin with exceptional stability. Indeed, it was shown that cohesive cohesin does not show an appreciable turnover during long periods in oocyte development. In this context, it was long assumed but not properly investigated, that the prophase pathway for cohesin release would be specific to mitosis and would be safely suppressed during meiosis so as not to endanger essential connections between chromosomes. However, a previous study on budding yeast meiosis suggests the presence of cleavage-independent pathway of cohesin release during late prophase-I. In the work presented here we confirmed that the prophase pathway is not suppressed during meiosis, at least in budding yeast and showed that this cleavage-independent release is regulated by meiosis-specific phosphorylation of two cohesin subunits, Rec8 and Rad61(Wapl) by two cell-cycle regulators, PLK and DDK. Our results suggest that late meiotic prophase-I actively controls cohesin dynamics on meiotic chromosomes for chromosome segregation.

Inhibition of checkpoint kinase 1 following gemcitabine-mediated S phase arrest results in CDC7- and CDK2-dependent replication catastrophe

Combining DNA-damaging drugs with DNA checkpoint inhibitors is an emerging strategy to manage cancer. Checkpoint kinase 1 inhibitors (CHK1is) sensitize most cancer cell lines to DNA-damaging drugs and also elicit single-agent cytotoxicity in 15% of cell lines. Consequently, combination therapy may be effective in a broader patient population. Here, we characterized the molecular mechanism of sensitization to gemcitabine by the CHK1i MK8776. Brief gemcitabine incubation irreversibly inhibited ribonucleotide reductase, depleting dNTPs, resulting in durable S phase arrest. Addition of CHK1i 18 h after gemcitabine elicited cell division cycle 7 (CDC7)- and cyclin-dependent kinase 2 (CDK2)-dependent reactivation of the replicative helicase, but did not reinitiate DNA synthesis due to continued lack of dNTPs. Helicase reactivation generated extensive single-strand (ss)DNA that exceeded the protective capacity of the ssDNA-binding protein, replication protein A. The subsequent cleavage of unprotected ssDNA has been termed replication catastrophe. This mechanism did not occur with concurrent CHK1i plus gemcitabine treatment, providing support for delayed administration of CHK1i in patients. Alternative mechanisms of CHK1i-mediated sensitization to gemcitabine have been proposed, but their role was ruled out; these mechanisms include premature mitosis, inhibition of homologous recombination, and activation of double-strand break repair nuclease (MRE11). In contrast, single-agent activity of CHK1i was MRE11-dependent and was prevented by lower concentrations of a CDK2 inhibitor. Hence, both pathways require CDK2 but appear to depend on different CDK2 substrates. We conclude that a small-molecule inhibitor of CHK1 can elicit at least two distinct, context-dependent mechanisms of cytotoxicity in cancer cells.

O-GlcNAc transferase associates with the MCM2-7 complex and its silencing destabilizes MCM-MCM interactions

O-GlcNAcylation of proteins is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). The homeostasis of O-GlcNAc cycling is regulated during cell cycle progression and is essential for proper cellular division. We previously reported the O-GlcNAcylation of the minichromosome maintenance proteins MCM2, MCM3, MCM6 and MCM7. These proteins belong to the MCM2-7 complex which is crucial for the initiation of DNA replication through its DNA helicase activity. Here we show that the six subunits of MCM2-7 are O-GlcNAcylated and that O-GlcNAcylation of MCM proteins mainly occurs in the chromatin-bound fraction of synchronized human cells. Moreover, we identify stable interaction between OGT and several MCM subunits. We also show that down-regulation of OGT decreases the chromatin binding of MCM2, MCM6 and MCM7 without affecting their steady-state level. Finally, OGT silencing or OGA inhibition destabilizes MCM2/6 and MCM4/7 interactions in the chromatin-enriched fraction. In conclusion, OGT is a new partner of the MCM2-7 complex and O-GlcNAcylation homeostasis might regulate MCM2-7 complex by regulating the chromatin loading of MCM6 and MCM7 and stabilizing MCM/MCM interactions.

Role of MCM2-7 protein phosphorylation in human cancer cells

A heterohexameric complex composed of minichromosome maintenance protein 2–7 (MCM2–7), which acts as a key replicative enzyme in eukaryotes, is crucial for initiating DNA synthesis only once per cell cycle. The MCM complex remains inactive through the G1 phase, until the S phase, when it is activated to initiate replication. During the transition from the G1 to S phase, the MCM undergoes multisite phosphorylation, an important change that promotes subsequent assembly of other replisome members. Phosphorylation is crucial for the regulation of MCM activity and function. MCMs can be phosphorylated by multiple kinases and these phosphorylation events are involved not only in DNA replication but also cell cycle progression and checkpoint response. Dysfunctional phosphorylation of MCMs appears to correlate with the occurrence and development of cancers. In this review, we summarize the currently available data regarding the regulatory mechanisms and functional consequences of MCM phosphorylation and seek the probability that protein kinase inhibitor can be used therapeutically to target MCM phosphorylation in cancer.

Suppression of targeting of Dbf4-dependent kinase to pre-replicative complex in G0 nuclei

Intact G0 nuclei isolated from quiescent cells are not capable of DNA replication in interphase Xenopus egg extracts, which allow efficient replication of permeabilized G0 nuclei. Previous studies have shown multiple control mechanisms for maintaining the quiescent state, but DNA replication inhibition of intact G0 nuclei in the extracts remains poorly understood. Here, we showed that pre-RC is assembled on chromatin, but its activation is inhibited after incubating G0 nuclei isolated from quiescent NIH3T3 cells in the extracts. Concomitant with the inhibition of replication, Mcm4 phosphorylation mediated by Dbf4-dependent kinase (DDK) as well as chromatin binding of DDK is suppressed in G0 nuclei without affecting the nuclear transport of DDK. We further found that the nuclear extracts of G0 but not proliferating cells inhibit the binding of recombinant DDK to pre-RC assembled plasmids. In addition, we observed rapid activation of checkpoint kinases after incubating G0 nuclei in the egg extracts. However, specific inhibitors of ATR/ATM are unable to promote DNA replication in G0 nuclei in the egg extracts. We suggest that a novel inhibitory mechanism is functional to prevent the targeting of DDK to pre-RC in G0 nuclei, thereby suppressing DNA replication in Xenopus egg extracts.