CDC6: from DNA replication to cell cycle checkpoints and oncogenesis

Activation of the p38 MAPK/Akt/ERK1/2 signal pathways is required for the protein stabilization of CDC6 and cyclin D1 in low-dose arsenite-induced cell proliferation

Arsenic trioxide (ATO) is a first-line anti-cancer agent for acute promyelocytic leukemia, and induces apoptosis in other solid cancer cell lines including breast cancer cells. However, as with arsenites found in drinking water and used as raw materials for wood preservatives, insecticides, and herbicides, low doses of ATO can induce carcinogenesis after long-term exposure. At 24 h after exposure, ATO (0.01-1 µM) significantly increased cell proliferation and promoted cell cycle progression from the G1 to S/G2 phases in the non-tumorigenic MCF10A breast epithelial cell line. The expression of 14 out of 96 cell-cycle-associated genes significantly increased, and seven of these genes including cell division cycle 6 (CDC6) and cyclin D1 (CCND1) were closely related to cell cycle progression from G1 to S phase. Low-dose ATO steadily increased gene transcript and protein levels of both CDC6 and cyclin D1 in a dose- and time-dependent manner. Low-dose ATO produced reactive oxygen species (ROS), and activated the p38 MAPK, Akt, and ERK1/2 pathways at different time points within 60 min. Small molecular inhibitors and siRNAs inhibiting the activation of p38 MAPK, Akt, and ERK1/2 decreased the ATO-increased expression of CDC6 protein. Inhibiting the activation of Akt and ERK1/2, but not p38 MAPK, decreased the ATO-induced expression of cyclin D1 protein. This study reports for the first time that p38 MAPK/Akt/ERK1/2 activation is required for the protein stabilization of CDC6 in addition to cyclin D1 in ATO-induced cell proliferation and cell cycle modulation from G1 to S phase.

Association between the CDC6 G1321A polymorphism and the risk of cervical cancer

INTRODUCTION

Cell division cycle protein 6 (CDC6) plays critical roles in DNA replication and carcinogenesis. The biological significance of the CDC6 G1321A polymorphism (V441I, rs13706) on cervical carcinogenesis is still unknown. Here, we examined the potential influence of this polymorphism on cell proliferation and the individual's susceptibility to cervical cancer.

METHODS

We genotyped the CDC6 G1321A polymorphism in 87 cervical cancer cases and 110 healthy female subjects. Unconditional logistic regression analysis was used to estimate the association between the genotypes and the risk of cervical cancer. The BrdU incorporation assay was applied to analyze the effect of this polymorphism on cell proliferation.

RESULTS

Compared with the GG homozygotes, the cervical cancer risk was significantly reduced in the individuals with the heterozygous AG genotype (odds ratio [OR], 0.53; 95% confidence interval [CI], 0.28-0.98; P = 0.042) or the homozygous AA genotype (OR, 0.29; 95% CI, 0.09-0.89; P = 0.030). Further stratified analyses showed that the decreased risk of cervical cancer was more evident among younger subjects (≤44 years old) with the AG or AA genotypes (OR, 0.44; 95% CI, 0.21-0.92; P = 0.029 and OR, 0.12; 95% CI, 0.03-0.61; P = 0.010, respectively). The BrdU incorporation assay showed that 293T cells transfected with CDC6-441I (1321A) had a lower proliferation rate in comparison with those transfected with CDC6-441V (1321G), although the difference did not reach statistical significance at the 0.05 level.

CONCLUSIONS

The CDC6 G1321A polymorphism may contribute to the risk of cervical cancer. Further studies with more subjects and in diverse ethnic populations are necessary to confirm the general validity of our findings.

Cohesin organizes chromatin loops at DNA replication factories

Genomic DNA is packed in chromatin fibers organized in higher-order structures within the interphase nucleus. One level of organization involves the formation of chromatin loops that may provide a favorable environment to processes such as DNA replication, transcription, and repair. However, little is known about the mechanistic basis of this structuration. Here we demonstrate that cohesin participates in the spatial organization of DNA replication factories in human cells. Cohesin is enriched at replication origins and interacts with prereplication complex proteins. Down-regulation of cohesin slows down S-phase progression by limiting the number of active origins and increasing the length of chromatin loops that correspond with replicon units. These results give a new dimension to the role of cohesin in the architectural organization of interphase chromatin, by showing its participation in DNA replication.

DnaA, ORC, and Cdc6: similarity beyond the domains of life and diversity

To initiate chromosomal DNA replication, specific proteins bind to the replication origin region and form multimeric and dynamic complexes. Bacterial DnaA, the eukaryotic origin recognition complex (ORC), and Cdc6 proteins, most of which include an AAA+(-like) motif, play crucial roles in replication initiation. The importance of ATP binding and hydrolysis in these proteins has recently become recognized. ATP binding of Escherichia coli DnaA is required for the formation of the activated form of a DnaA multimer on the replication origin. The ATP-DnaA multimer can unwind duplex DNA in an origin-dependent manner, which is supported by various specific functions of several AAA+ motifs. DnaA-ATP hydrolysis is stimulated after initiation, repressing extra initiations, and sustaining once-per-cell cycle replication. ATP binding of ORC and Cdc6 in Saccharomyces cerevisiae is required for heteromultimeric complex formation and specific DNA binding. ATP hydrolysis of these proteins is important for the efficient loading of the minichromosome maintenance protein complex, a component of the putative replicative helicase. In this review, we discuss the roles of DnaA, ORC, and Cdc6 in replication initiation and its regulation. We also summarize the functional features of the AAA+ domains of these proteins, and the functional divergence of ORC in chromosomal dynamics.

Primase directs the release of DnaC from DnaB

An AAA+ ATPase, DnaC, delivers DnaB helicase at the E. coli chromosomal origin by a poorly understood process. This report shows that mutant proteins bearing alanine substitutions for two conserved arginines in a motif named box VII are defective in DNA replication, but this deficiency does not arise from impaired interactions with ATP, DnaB, or single-stranded DNA. Despite their ability to deliver DnaB to the chromosomal origin to form the prepriming complex, this intermediate is inactive. Quantitative analysis of the prepriming complex suggests that the DnaB-DnaC complex contains three DnaC monomers per DnaB hexamer and that the interaction of primase with DnaB and primer formation triggers the release of DnaC, but not the mutants, from DnaB. The interaction of primase with DnaB and the release of DnaC mark discrete events in the transition from initiation to the elongation stage of DNA replication.

Cdk2 is required for p53-independent G2/M checkpoint control

The activation of phase-specific cyclin-dependent kinases (Cdks) is associated with ordered cell cycle transitions. Among the mammalian Cdks, only Cdk1 is essential for somatic cell proliferation. Cdk1 can apparently substitute for Cdk2, Cdk4, and Cdk6, which are individually dispensable in mice. It is unclear if all functions of non-essential Cdks are fully redundant with Cdk1. Using a genetic approach, we show that Cdk2, the S-phase Cdk, uniquely controls the G₂/M checkpoint that prevents cells with damaged DNA from initiating mitosis. CDK2-nullizygous human cells exposed to ionizing radiation failed to exclude Cdk1 from the nucleus and exhibited a marked defect in G₂/M arrest that was unmasked by the disruption of P53. The DNA replication licensing protein Cdc6, which is normally stabilized by Cdk2, was physically associated with the checkpoint regulator ATR and was required for efficient ATR-Chk1-Cdc25A signaling. These findings demonstrate that Cdk2 maintains a balance of S-phase regulatory proteins and thereby coordinates subsequent p53-independent G₂/M checkpoint activation.

Metazoan cells contain multiple Cdks that regulate cell cycle progression. Recent studies have shown that mouse cells can grow normally with just Cdk1. The roles of the non-essential Cdks remain a fundamental question. In this study, we describe the generation and detailed characterization of CDK2-knockout human somatic cells. Our study demonstrates that Cdk2 is required for robust DNA damage checkpoint signaling. Loss of Cdk2 caused a marked deficiency in the G₂/M arrest—a basic response to DNA damage—in cells that were also nullizygous for P53. We propose that the multiple Cdks present in metazoan cells provide a mechanism by which the cell cycle can be efficiently halted after DNA damage. Significantly, this study reveals a heretofore unrecognized dependence for Cdk2 in p53-deficient cancer cells.

DNA replication licensing control and rereplication prevention

Eukaryotic DNA replication is tightly restricted to only once per cell cycle in order to maintain genome stability. Cells use multiple mechanisms to control the assembly of the prereplication complex (pre-RC), a process known as replication licensing. This review focuses on the regulation of replication licensing by posttranslational modifications of the licensing factors, including phosphorylation, ubiquitylation and acetylation. These modifications are critical in establishing the pre-RC complexes as well as preventing rereplication in each cell cycle. The relationship between rereplication and diseases, including cancer and virus infection, is discussed as well.

Therapeutic efficacy of the pan-cdk inhibitor PHA-793887 in vitro and in vivo in engraftment and high-burden leukemia models

OBJECTIVE

The aim of the work was to determine and characterize, in vitro and in vivo, the therapeutic activity of PHA-793887, a new potent pan-cdk inhibitor, in the context of hematopoietic neoplasms.

MATERIALS AND METHODS

Thirteen leukemic cell lines bearing different cytogenetic abnormalities and normal hematopoietic cells were used in cytotoxicity and colony assays. The drug activity at the molecular level was analyzed by Western blotting. PHA-793887 was also tested in vivo in several leukemia xenograft models.

RESULTS

PHA-793887 was cytotoxic for leukemic cell lines in vitro, with IC(50) ranging from 0.3 to 7 microM (mean: 2.9 microM), regardless of any specific chromosomal aberration. At these doses, the drug was not cytotoxic for normal unstimulated peripheral blood mononuclear cells or CD34(+) hematopoietic stem cells. Interestingly, in colony assays PHA-793887 showed very high activity against leukemia cell lines, with an IC(50) <0.1 microM (mean: 0.08 microM), indicating that it has efficient and prolonged antiproliferative activity. PHA-793887 induced cell-cycle arrest, inhibited Rb and nucleophosmin phosphorylation, and modulated cyclin E and cdc6 expression at low doses (0.2-1 microM) and induced apoptosis at the highest dose (5 microM). It was also effective in vivo in both subcutaneous xenograft and primary leukemic disseminated models that better mimic naturally occurring human disease. Interestingly, in one disseminated model derived from a relapsed Philadelphia-positive acute lymphoid leukemia patient, PHA-793887 showed strong therapeutic activity also when treatment was started after establishment of high disease burden.

CONCLUSIONS

We conclude that PHA-793887 has promising therapeutic activity against acute leukemias in vitro and in vivo.

Modulation of gene expression in endothelial cells by hyperlipaemic postprandial serum from healthy volunteers

A single high-fat challenge induces plasmatic pro-inflammatory and pro-oxidative responses in the postprandial state, even in healthy men. This period is also associated with vascular endothelial dysfunction, which is an early event in the development of cardiovascular diseases. However, knowledge about the mechanisms involved in postprandial hyperlipaemia-induced endothelial dysfunction is sparse. An objective of our study was to characterize the behaviour and gene expression of vascular endothelial cells exposed to postprandial hyperlipaemic sera. Human umbilical vein endothelial cells (HUVECs) were cultured in media containing 10% serum from healthy men withdrawn either before or 4 h after a high-fat challenge. Endothelial cell proliferation, adhesion and migration were then assessed. The transcriptomic profiles of endothelial cells exposed to pre and postprandial sera were also compared. Exposure to postprandial hyperlipaemic sera significantly decreased HUVEC proliferation when compared to preprandial serum (P < 0.0001), while no changes in migration or endothelial/monocyte interactions were observed. The transcriptomic analysis revealed changes in the expression of 675 genes, of which 431 have a known function. Among them, a set of differentially expressed genes was linked to cell cycle regulation and apoptosis and are regulated in favour of cell cycle arrest or death. This result was confirmed by measuring the induction of apoptosis after postprandial sera exposure (P = 0.011). Taken together, the transcriptomic results and pathway analysis showed that postprandial serum promotes apoptosis in HUVECs, potentially through the activation of the p53 network. We conclude that upon postprandial serum exposure, vascular endothelial cells transcriptionally regulate genes involved in the control of cell cycle and death to favour growth arrest and apoptosis. These findings support the hypothesis that postprandial hyperlipaemia is associated with vascular dysfunction and offer new insights into the mechanisms involved.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12263-010-0166-x) contains supplementary material, which is available to authorized users.

Cyclin E is stabilized in response to replication fork barriers leading to prolonged S phase arrest

Cyclin E is a regulator of cyclin-dependent protein kinases (Cdks) and is involved in mediating the cell cycle transition from G(1) to S phase. Here, we describe a novel function for cyclin E in the long term maintenance of checkpoint arrest in response to replication barriers. Exposure of cells to mitomycin C or UV irradiation, but not ionizing radiation, induces stabilization of cyclin E. Stabilization of cyclin E reduces the activity of Cdk2-cyclin A, resulting in a slowing of S phase progression and arrest. In addition, cyclin E is shown to be required for stabilization of Cdc6, which is required for activation of Chk1 and the replication checkpoint pathway. Furthermore, the stabilization of cyclin E in response to replication fork barriers depends on ATR, but not Nbs1 or Chk1. These results indicate that in addition to its well studied role in promoting cell cycle progression, cyclin E also has a role in regulating cell cycle arrest in response to DNA damage.

Activation of Cdc6 by MyoD is associated with the expansion of quiescent myogenic satellite cells

Cdc6, which alters chromatin ultrastructure to allow DNA replication in muscle stem cells transitioning out of quiescence, is identified as a target of the MyoD transcription factor.

MyoD is a transcriptional factor that is required for the differentiation of muscle stem cells (satellite cells). In this study, we describe a previously unknown function for MyoD in regulating a gene (Cdc6) that is vital to endowing chromatin with the capability of replicating DNA. In C2C12 and primary mouse myoblasts, we show that MyoD can occupy an E-box within the promoter of Cdc6 and that this association, along with E2F3a, is required for its activity. MyoD and Cdc6 are both expressed after quiescent C2C12 myoblasts or satellite cells in association with myofibers are stimulated for growth, but MyoD appears at least 2–3 h earlier than Cdc6. Finally, knockdown of MyoD impairs the ability of C2C12 cells to express Cdc6 after leaving quiescence, and as a result, they cannot fully progress into S phase. Our results define a mechanism by which MyoD helps myogenic satellite cells to enter into the first round of DNA replication after transitioning out of quiescence.

Cyclin A2-cyclin-dependent kinase 2 cooperates with the PLK1-SCFbeta-TrCP1-EMI1-anaphase-promoting complex/cyclosome axis to promote genome reduplication in the absence of mitosis

Limiting genome replication to once per cell cycle is vital for maintaining genome stability. Inhibition of cyclin-dependent kinase 1 (CDK1) with the specific inhibitor RO3306 is sufficient to trigger multiple rounds of genome reduplication. We demonstrated that although anaphase-promoting complex/cyclosome (APC/C) remained inactive during the initial G(2) arrest, it was activated upon prolonged inhibition of CDK1. Using cellular biosensors and live-cell imaging, we provide direct evidence that genome reduplication was associated with oscillation of APC/C activity and nuclear-cytoplasmic shuttling of CDC6 even in the absence of mitosis at the single-cell level. Genome reduplication was abolished by ectopic expression of EMI1 or depletion of CDC20 or CDH1, suggesting the critical role of the EMI1-APC/C axis. In support of this, degradation of EMI1 itself and genome reduplication were delayed after downregulation of PLK1 and beta-TrCP1. In the absence of CDK1 activity, activation of APC/C and genome reduplication was dependent on cyclin A2 and CDK2. Genome reduplication was then promoted by a combination of APC/C-dependent destruction of geminin (thus releasing CDT1), accumulation of cyclin E2-CDK2, and CDC6. Collectively, these results underscore the crucial role of cyclin A2-CDK2 in regulating the PLK1-SCF(beta-TrCP1)-EMI1-APC/C axis and CDC6 to trigger genome reduplication after the activity of CDK1 is suppressed.

Molecular profiling of breast cancer cell lines defines relevant tumor models and provides a resource for cancer gene discovery

Background

Breast cancer cell lines have been used widely to investigate breast cancer pathobiology and new therapies. Breast cancer is a molecularly heterogeneous disease, and it is important to understand how well and which cell lines best model that diversity. In particular, microarray studies have identified molecular subtypes–luminal A, luminal B, ERBB2-associated, basal-like and normal-like–with characteristic gene-expression patterns and underlying DNA copy number alterations (CNAs). Here, we studied a collection of breast cancer cell lines to catalog molecular profiles and to assess their relation to breast cancer subtypes.

Methods

Whole-genome DNA microarrays were used to profile gene expression and CNAs in a collection of 52 widely-used breast cancer cell lines, and comparisons were made to existing profiles of primary breast tumors. Hierarchical clustering was used to identify gene-expression subtypes, and Gene Set Enrichment Analysis (GSEA) to discover biological features of those subtypes. Genomic and transcriptional profiles were integrated to discover within high-amplitude CNAs candidate cancer genes with coordinately altered gene copy number and expression.

Findings

Transcriptional profiling of breast cancer cell lines identified one luminal and two basal-like (A and B) subtypes. Luminal lines displayed an estrogen receptor (ER) signature and resembled luminal-A/B tumors, basal-A lines were associated with ETS-pathway and BRCA1 signatures and resembled basal-like tumors, and basal-B lines displayed mesenchymal and stem/progenitor-cell characteristics. Compared to tumors, cell lines exhibited similar patterns of CNA, but an overall higher complexity of CNA (genetically simple luminal-A tumors were not represented), and only partial conservation of subtype-specific CNAs. We identified 80 high-level DNA amplifications and 13 multi-copy deletions, and the resident genes with concomitantly altered gene-expression, highlighting known and novel candidate breast cancer genes.

Conclusions

Overall, breast cancer cell lines were genetically more complex than tumors, but retained expression patterns with relevance to the luminal-basal subtype distinction. The compendium of molecular profiles defines cell lines suitable for investigations of subtype-specific pathobiology, cancer stem cell biology, biomarkers and therapies, and provides a resource for discovery of new breast cancer genes.

Alterations of ubiquitin ligases in human cancer and their association with the natural history of the tumor

Protein ubiquitination is critical for many cellular processes, through its ability to regulate protein degradation and various signaling mechanisms. In the ubiquitin (Ub) system, substrate specificity is achieved through the E3 family of Ub ligases. Because alterations of the ubiquitination machinery have been reported in human cancers, the selective interference with Ub ligases might represent a powerful therapeutic tool. Here, we report the first wide survey of misregulation of Ub ligases in cancer. We analysed 82 Ub ligases in nine types of cancer by in situ hybridization on tissue microarrays. We found 27 instances in which an Ub ligase was altered in a given type of tumor, when compared with normal tissues: 21 cases of overexpression and 6 cases of underexpression. We further analysed selected Ub ligases in large cohorts of breast and non-small-cell lung carcinomas. In five, of six, of these extended analyses (HUWE1, CCNB1IP1, SIAH1 and SIAH2 in breast cancer and CCNB1IP1 in lung cancer), we found that the levels of Ub ligases correlated significantly with relevant prognostic factors, and with clinical outcome. Our findings show that the alteration of Ub ligases is a frequent event in cancer and identify candidate targets for molecular therapies.

Nuclear transport of protein TTC4 depends on the cell cycle

TTC4 (tetratricopeptide repeat domain protein 4) is a putative tumor suppressor involved in the transformation of melanocytes. At present, the relationships between TTC4 and DNA replication proteins are largely unknown, as are the tissue distribution and subcellular localization of TTC4. Using reverse transcription with the polymerase chain reaction, we have observed that the murine TTC4 gene is ubiquitously expressed. Analysis of the TTC4 subcellular localization has shown that, upon overexpression, TTC4 localizes to the cytoplasm. Interestingly, co-expression with a known protein interaction partner, hampin/MSL1, results in the nuclear translocation of the TTC4 protein. The subcellular localization of endogenous TTC4 depends, however, on the cell cycle: it is mostly nuclear in the G1 and S phases and is evenly distributed between the nucleus and cytoplasm in G2. The nuclear transport of TTC4 is apparently a complex process dependent on interactions with other proteins during the progression of the cell cycle. Thus, the dynamic character of the nuclear accumulation of TTC4 might be a potential link with regard to its function in tumor suppression.

Oligonucleotide microarray analysis of apoptosis induced by 15-methoxypinusolidic acid in microglial BV2 cells

BACKGROUND AND PURPOSE

We conducted a genome wide gene expression analysis to explore the biological aspects of 15-methoxypinusolidic acid (15-MPA) isolated from Biota orientalis and tried to confirm the suitability of 15-MPA as a therapeutic candidate for CNS injuries focusing on microglia.

EXPERIMENTAL APPROACH

Murine microglial BV2 cells were treated with 15-MPA, and their transcriptome was analysed by using oligonucleotide microarrays. Genes differentially expressed upon 15-MPA treatment were selected for RT-PCR (reverse transcription-polymerase chain reaction) analysis to confirm the gene expression. Inhibition of cell proliferation and induction of apoptosis by 15-MPA were examined by bromodeoxyuridine assay, Western blot analysis of poly-ADP-ribose polymerase and flow cytometry.

KEY RESULTS

A total of 514 genes were differentially expressed by 15-MPA treatment. Biological pathway analysis revealed that 15-MPA induced significant changes in expression of genes in the cell cycle pathway. Genes involved in growth arrest and DNA damage [gadd45alpha, gadd45gamma and ddit3 (DNA damage-inducible transcript 3)] and cyclin-dependent kinase inhibitor (cdkn2b) were up-regulated, whereas genes involved in cell cycle progression (ccnd1, ccnd3 and ccne1), DNA replication (mcm4, orc1l and cdc6) and cell proliferation (fos and jun) were down-regulated. RT-PCR analysis for representative genes confirmed the expression levels. 15-MPA significantly reduced bromodeoxyuridine incorporation, increased poly-ADP-ribose polymerase cleavage and the number of apoptotic cells, indicating that 15-MPA induces apoptosis in BV2 cells.

CONCLUSION AND IMPLICATIONS

15-MPA induced apoptosis in murine microglial cells, presumably via inhibition of the cell cycle progression. As microglial activation is detrimental in CNS injuries, these data suggest a strong therapeutic potential of 15-MPA.

Microarray analysis of RNA extracted from formalin-fixed, paraffin-embedded and matched fresh-frozen ovarian adenocarcinomas

Background

Gene expression profiling of formalin-fixed, paraffin-embedded (FFPE) samples represents a valuable approach for advancing oncology diagnostics and enhancing retrospective clinical studies; however, at present, this methodology still requires optimization and thus has not been extensively used. Here, we utilized thorough quality control methods to assess RNA extracted from FFPE samples and then compared it to RNA extracted from matched fresh-frozen (FF) counterparts. We preformed genome-wide expression profiling of FF and FFPE ovarian serous adenocarcinoma sample pairs and compared their gene signatures to normal ovary samples.

Methods

RNA from FFPE samples was extracted using two different methods, Ambion and Agencourt, and its quality was determined by profiling starting total RNA on Bioanalyzer and by amplifying increasing size fragments of beta actin (ACTB) and claudin 3 (CLDN3) by reverse-transcriptase polymerase chain reaction. Five matched FF and FFPE ovarian serous adenocarcinoma samples, as well as a set of normal ovary samples, were profiled using whole genome Agilent microarrays. Reproducibility of the FF and FFPE replicates was measured using Pearson correlation, whereas comparison between the FF and FFPE samples was done using a Z-score analysis.

Results

Data analysis showed high reproducibility of expression within each FF and FFPE method, whereas matched FF and FFPE pairs demonstrated lower similarity, emphasizing an inherent difference between the two sample types. Z-score analysis of matched FF and FFPE samples revealed good concordance of top 100 differentially expressed genes with the highest correlation of 0.84. Genes characteristic of ovarian serous adenocarcinoma, including a well known marker CLDN3, as well as potentially some novel markers, were identified by comparing gene expression profiles of ovarian adenocarcinoma to those of normal ovary.

Conclusion

Conclusively, we showed that systematic assessment of FFPE samples at the RNA level is essential for obtaining good quality gene expression microarray data. We also demonstrated that profiling of not only FF but also of FFPE samples can be successfully used to identify differentially expressed genes characteristic of ovarian carcinoma.

Association analysis between the Cdc6 G1321A polymorphism and the risk for non-Hodgkin lymphoma and hepatocellular carcinoma

Cdc6 play crucial roles in DNA replication and carcinogenesis. The biological significance of the Cdc6 G1321A polymorphism (V441I, rs13706) is still not elucidated. Here we examined the influence of this polymorphism on the function of Cdc6 and the individual's susceptibility to non-Hodgkin lymphoma (NHL) and hepatocellular carcinoma (HCC). Unconditional logistic regression analysis revealed that the risk for NHL was significantly reduced in both AG heterozygotes [odds ratio (OR)=0.67, P=0.019] and AA homozygotes (OR=0.54, P=0.026), compared with GG homozygotes. Further stratification by subtypes of NHL showed that the AG as well as combined AA/AG genotypes were associated with decreased risk for B-cell-NHL (OR=0.62, P=0.011 and OR=0.61, P=0.006, respectively), especially for diffuse large B-cell lymphoma (DLBCL, OR=0.63, P=0.025 and OR=0.62, P=0.012, respectively). In addition, male individuals with the AA genotype displayed borderline significantly reduced risk for HCC (OR=0.48, P=0.054). Interestingly, the G1321A polymorphism did not affect caspase-mediated cleavage of Cdc6 during etoposide-induced apoptosis, but it was predicted to alter the secondary structure of Cdc6 mRNA. Our data provide the first evidence that the Cdc6 G1321A polymorphism is associated with decreased risk of cancer. Further studies are necessary to confirm the general validity of our findings.

Beyond heterochromatin: SIR2 inhibits the initiation of DNA replication

Over the last decade, data have accumulated that support a role for chromatin structure in regulating the initiation of DNA replication and its timing during S-phase. However, the mechanisms underlying how chromatin structure influences replication initiation are not always understood. For example, in Drosophila histone acetylation at the ACE3 and Ori-beta sequences near one of the amplified chorion loci is correlated with ORC (origin recognition complex) binding and re-replication of this locus. Whether histone acetylation promotes ORC binding or some later step in replication is not known. In yeast, hypo-acetylated heterochromatin and telomeric regions replicate late in S-phase but the mechanisms that restrict the initiation of replication at these loci are not fully understood. Nonetheless, it seems likely that histone acetylation and other types of histone modification will significantly impact DNA replication. A recent study published in Molecular Cell reveals a role for the conserved NAD(+)-dependent histone deacetylase, Sir2, in inhibiting the assembly of the multiprotein complex necessary for the selection and activation of yeast replication origins. Here, we highlight key conclusions from this study, place them in perspective with earlier work, and outline important future questions.

Replication licensing and cancer--a fatal entanglement?

Correct regulation of the replication licensing system ensures that chromosomal DNA is precisely duplicated in each cell division cycle. Licensing proteins are inappropriately expressed at an early stage of tumorigenesis in a wide variety of cancers. Here we discuss evidence that misregulation of replication licensing is a consequence of oncogene-induced cell proliferation. This misregulation can cause either under- or over-replication of chromosomal DNA, and could explain the genetic instability commonly seen in cancer cells.

A novel functional polymorphism in the Cdc6 promoter is associated with the risk for hepatocellular carcinoma

Cdc6 is essential for DNA replication and its deregulation is involved in carcinogenesis. To date, the biological significance of the polymorphism in Cdc6 promoter is still unknown. In this study, we aimed to evaluate the influence of the Cdc6 -515A>G polymorphism (rs4134994) on the individual's susceptibility to cancer and on the function of Cdc6. The Cdc6 -515A>G polymorphism was genotyped in 387 hepatocellular carcinoma (HCC) and 389 age- and sex-matched healthy subjects. The association between the genotypes and the risk for HCC was then estimated by unconditional logistic regression analysis with adjustment for age, sex and HBV status. Compared with the AA homozygotes, the homozygous GG genotype (adjusted OR=0.36, 95% confidence interval (CI)=0.18-0.72, P=0.004) or the combined AG/GG genotypes (adjusted OR=0.56, 95% CI=0.36-0.86, P=0.008) were statistically significantly associated with the reduced risk for HCC. Moreover, the analysis using luciferase reporter system showed that the G-allelic Cdc6 promoter displayed a decreased transcriptional activity compared with the A-allelic one. These results indicate that the individuals with G allele may have reduced Cdc6 expression and are therefore in reduced risk for HCC. Further investigation using electrophoretic mobility shift assay (EMSA) revealed that the G allele had a stronger binding strength to nuclear protein(s) which might function as negative regulator(s) for Cdc6 transcription. Our findings suggest that the -515A>G polymorphism may affect the Cdc6 promoter binding affinity with nuclear protein(s) and in turn the Cdc6 expression, which consequently modulates the individual's susceptibility to HCC.