DNA replication origin interference increases the spacing between initiation events in human cells

Epigenetic landscape for initiation of DNA replication

The key genetic process of DNA replication is initiated at specific sites referred to as replication origins. In eukaryotes, origins of DNA replication are not specified by a defined nucleotide sequence. Recent studies have shown that the structural context and topology of DNA sequence, chromatin features, and its transcriptional activity play an important role in origin choice. During differentiation and development, significant changes in chromatin organization and transcription occur, influencing origin activity and choice. In the last few years, a number of different genome-wide studies have broadened the understanding of replication origin regulation. In this review, we discuss the epigenetic factors and mechanisms that modulate origin choice and firing.

General and specific replication profiles are detected in normal human cells by genome-wide and single-locus molecular combing

Mammalian genomes are replicated under a flexible program, with random use of origins and variable fork rates, and many details of the process must be still unraveled. Molecular combing provides a set of direct data regarding the replication profile of eukaryotic cells: fork rates; organization of the replication clusters; proportion of unidirectional forks; and fork dynamics. In this study the replication profiles of different primary and immortalized non-cancer human cells (lymphocytes, lymphoblastoid cells, fibroblasts) were evaluated at the whole-genome level or within reference genomic regions harboring coding genes. It emerged that these different cell types are characterized by specific replication profiles. In primary fibroblasts, a remarkable fraction of the mammalian genome was found to be replicated by unidirectional forks, and interestingly, the proportion of unidirectional forks further increased in the replicating genome along the population divisions. A second difference concerned in the proportion of paused replication forks, again more frequent in primary fibroblasts than in PBL/lymphoblastoid cells. We concluded that these patterns, whose relevance could escape when genomic methods are applied, represent normal replication features. In single-locus analyses, unidirectional and paused replication forks were highly represented in all genomic regions considered with respect to the average estimates referring to the whole-genome. In addition, fork rates were significantly lower than whole-genome estimates. Instead, when considering the specificities of each genomic region investigated (early to late replication, normal or fragile site) no further differentiating features of replication profiles were detected. These data, representing the integration of genome-wide and single-locus analyses, highlight a large heterogeneity of replication profiles among cell types and within the genome, which should be considered for the correct use of replication datasets.

DNA replication origins

The onset of genomic DNA synthesis requires precise interactions of specialized initiator proteins with DNA at sites where the replication machinery can be loaded. These sites, defined as replication origins, are found at a few unique locations in all of the prokaryotic chromosomes examined so far. However, replication origins are dispersed among tens of thousands of loci in metazoan chromosomes, thereby raising questions regarding the role of specific nucleotide sequences and chromatin environment in origin selection and the mechanisms used by initiators to recognize replication origins. Close examination of bacterial and archaeal replication origins reveals an array of DNA sequence motifs that position individual initiator protein molecules and promote initiator oligomerization on origin DNA. Conversely, the need for specific recognition sequences in eukaryotic replication origins is relaxed. In fact, the primary rule for origin selection appears to be flexibility, a feature that is modulated either by structural elements or by epigenetic mechanisms at least partly linked to the organization of the genome for gene expression.

Time to be versatile: regulation of the replication timing program in budding yeast

Eukaryotic replication origins are activated at different times during the S phase of the cell cycle, following a temporal program that is stably transmitted to daughter cells. Although the mechanisms that control initiation at the level of individual origins are now well understood, much less is known on how cells coordinate replication at hundreds of origins distributed on the chromosomes. In this review, we discuss recent advances shedding new light on how this complex process is regulated in the budding yeast Saccharomyces cerevisiae. The picture that emerges from these studies is that replication timing is regulated in cis by mechanisms modulating the chromatin structure and the subnuclear organization of origins. These mechanisms do not affect the licensing of replication origins but determine their ability to compete for limiting initiation factors, which are recycled from early to late origins throughout the length of the S phase.

DNA replication timing

Patterns of replication within eukaryotic genomes correlate with gene expression, chromatin structure, and genome evolution. Recent advances in genome-scale mapping of replication kinetics have allowed these correlations to be explored in many species, cell types, and growth conditions, and these large data sets have allowed quantitative and computational analyses. One striking new correlation to emerge from these analyses is between replication timing and the three-dimensional structure of chromosomes. This correlation, which is significantly stronger than with any single histone modification or chromosome-binding protein, suggests that replication timing is controlled at the level of chromosomal domains. This conclusion dovetails with parallel work on the heterogeneity of origin firing and the competition between origins for limiting activators to suggest a model in which the stochastic probability of individual origin firing is modulated by chromosomal domain structure to produce patterns of replication. Whether these patterns have inherent biological functions or simply reflect higher-order genome structure is an open question.

Ku protein levels, localization and association to replication origins in different stages of breast tumor progression

Human origins of DNA replication are specific sequences within the genome whereby DNA replication is initiated. A select group of proteins, known as the pre-replication (pre-RC) complex, in whose formation the Ku protein (Ku70/Ku86) was shown to play a role, bind to replication origins to initiate DNA replication. In this study, we have examined the involvement of Ku in breast tumorigenesis and tumor progression and found that the Ku protein expression levels in human breast metastatic (MCF10AC1a) cells were higher in the chromatin fraction compared to hyperplastic (MCF10AT) and normal (MCF10A) human breast cells, but remained constant in both the nuclear and cytoplasmic fractions. In contrast, in human intestinal cells, the Ku expression level was relatively constant for all cell fractions. Nascent DNA abundance and chromatin association of Ku70/86 revealed that the c-myc origin activity in MCF10AC1a is 2.5 to 5-fold higher than in MCF10AT and MCF10A, respectively, and Ku was bound to the c-myc origin more abundantly in MCF10AC1a, by approximately 1.5 to 4.2-fold higher than in MCF10AT and MCF10A, respectively. In contrast, similar nascent DNA abundance and chromatin association was found for all cell lines for the lamin B2 origin, associated with the constitutively active housekeeping lamin B2 gene. Electrophoretic mobility shift assays (EMSAs) performed on the nuclear extracts (NEs) of the three cell types revealed the presence of protein-DNA replication complexes on both the c-myc and lamin B2 origins, but an increase in binding activity was observed from normal, to transformed, to cancer cells for the c-myc origin, whereas no such difference was seen for the lamin B2 origin. Overall, the results suggest that increased Ku chromatin association, beyond wild type levels, alters cellular processes, which have been implicated in tumorigenesis.

Cdc45 is a critical effector of myc-dependent DNA replication stress

c-Myc oncogenic activity is thought to be mediated in part by its ability to generate DNA replication stress and subsequent genomic instability when deregulated. Previous studies have demonstrated a nontranscriptional role for c-Myc in regulating DNA replication. Here, we analyze the mechanisms by which c-Myc deregulation generates DNA replication stress. We find that overexpression of c-Myc alters the spatiotemporal program of replication initiation by increasing the density of early-replicating origins. We further show that c-Myc deregulation results in elevated replication-fork stalling or collapse and subsequent DNA damage. Notably, these phenotypes are independent of RNA transcription. Finally, we demonstrate that overexpression of Cdc45 recapitulates all c-Myc-induced replication and damage phenotypes and that Cdc45 and GINS function downstream of Myc.

Back to the origin: reconsidering replication, transcription, epigenetics, and cell cycle control

In bacteria, replication is a carefully orchestrated event that unfolds the same way for each bacterium and each cell division. The process of DNA replication in bacteria optimizes cell growth and coordinates high levels of simultaneous replication and transcription. In metazoans, the organization of replication is more enigmatic. The lack of a specific sequence that defines origins of replication has, until recently, severely limited our ability to define the organizing principles of DNA replication. This question is of particular importance as emerging data suggest that replication stress is an important contributor to inherited genetic damage and the genomic instability in tumors. We consider here the replication program in several different organisms including recent genome-wide analyses of replication origins in humans. We review recent studies on the role of cytosine methylation in replication origins, the role of transcriptional looping and gene gating in DNA replication, and the role of chromatin's 3-dimensional structure in DNA replication. We use these new findings to consider several questions surrounding DNA replication in metazoans: How are origins selected? What is the relationship between replication and transcription? How do checkpoints inhibit origin firing? Why are there early and late firing origins? We then discuss whether oncogenes promote cancer through a role in DNA replication and whether errors in DNA replication are important contributors to the genomic alterations and gene fusion events observed in cancer. We conclude with some important areas for future experimentation.

Trapping DNA replication origins from the human genome

Synthesis of chromosomal DNA is initiated from multiple origins of replication in higher eukaryotes; however, little is known about these origins’ structures. We isolated the origin-derived nascent DNAs from a human repair-deficient cell line by blocking the replication forks near the origins using two different origin-trapping methods (i.e., UV- or chemical crosslinker-treatment and cell synchronization in early S phase using DNA replication inhibitors). Single-stranded DNAs (of 0.5–3 kb) that accumulated after such treatments were labeled with bromodeoxyuridine (BrdU). BrdU-labeled DNA was immunopurified after fractionation by alkaline sucrose density gradient centrifugation and cloned by complementary-strand synthesis and PCR amplification. Competitive PCR revealed an increased abundance of DNA derived from known replication origins (c-myc and lamin B2 genes) in the nascent DNA fractions from the UV-treated or crosslinked cells. Nucleotide sequences of 85 and 208 kb were obtained from the two libraries (I and II) prepared from the UV-treated log-phase cells and early S phase arrested cells, respectively. The libraries differed from each other in their G+C composition and replication-related motif contents, suggesting that differences existed between the origin fragments isolated by the two different origin-trapping methods. The replication activities for seven out of 12 putative origin loci from the early-S phase cells were shown by competitive PCR. We mapped 117 (library I) and 172 (library II) putative origin loci to the human genome; approximately 60% and 50% of these loci were assigned to the G-band and intragenic regions, respectively. Analyses of the flanking sequences of the mapped loci suggested that the putative origin loci tended to associate with genes (including conserved sites) and DNase I hypersensitive sites; however, poor correlations were found between such loci and the CpG islands, transcription start sites, and K27-acetylated histone H3 peaks.

Diminished origin-licensing capacity specifically sensitizes tumor cells to replication stress

Previous studies have shown that dormant licensed replication origins can be exploited to enhance recovery from replication stress. Since tumor cells express high levels of origin-licensing proteins, we examined whether depletion of such factors might specifically sensitize tumor versus nontumor cells. Consistent with previous findings, we observed that three tumor-derived cell lines overexpress ORC1, a licensing component, compared with four nontumor cell lines and that a greater level of ORC1 was required to maintain viability in the tumor cells. We determined siRNA-mediated knockdown conditions for each line that maximally reduced ORC1 but did not impact upon viability, which we considered would optimally deplete dormant origins. ORC1 depletion hypersensitized the tumor-derived cells to hydroxyurea and H202 but did not affect the sensitivity of the nontumor lines. Similar results were observed following depletion of ORC6 or CDC6. Furthermore, codepletion of p53 and ORC1 modestly impaired viability of 1BR3hTERT nontumor fibroblasts and more dramatically caused hypersensitivity to hydroxyurea. Finally, overexpression of the c-Myc oncogene combined with ORC1 depletion in nontumor BJhTERT cells diminished viability. Collectively, these findings suggest that tumor cells may have a reliance on origin-licensing capacity, suggesting that licensing factors could represent a target for drug-based cancer therapy.

Replication dynamics: biases and robustness of DNA fiber analysis

The factors that govern replication programs are still poorly identified in metazoans, especially in mammalian cells. Thanks to molecular combing, the dynamics of DNA replication can be assessed at the genome-scale level from the cumulative analysis of single DNA fibers. This technique notably enables measurement of replication fork speed and fork asymmetry and that of distances separating either initiation or termination events. The results presented here aim to evaluate requirements critical to accurate measurement of replication parameters by molecular combing. We show that sample size, fiber length and DNA counterstaining are crucial to gain robust information concerning replication dynamics. Our results thus provide a methodological frame to investigate the DNA replication program through molecular combing analyses.

Use of the DNA fiber spreading technique to detect the effects of mutant p53 on DNA replication

DNA replication involves a coordinated progression through S phase, and disruption of these regulated steps may cause gene abnormalities, which may lead to cancer. Different stages of DNA replication can be detected immunofluorescently that would indicate how replication is progressing in a cell population or under specific conditions. We describe a method for labeling replicating DNA with two nucleotide analogs, and then detecting the sequential patterns of incorporation using fluorescently labeled antibodies on DNA spread onto a glass slide. Quantification of the different types of replication patterns produced by this method reveals how replication is achieved under different conditions by the predominance and lengths of elongating replication forks progressing from single or clustered origins, as well as the sites of termination from two converging forks.

[Regulation of DNA replication timing]

Although distinct chromatin types have been long known to replicate at different timepoints of S phase, fine replication control has only recently become considered as an epigenetic phenomenon. It is now clear that in course of differentiation significant changes in genome replication timing occur, and these changes are intimately linked with the changes in transcriptional activity and nuclear architecture. Temporally coordinate replication is organized spatially into discrete units having specific chromosomal organization and function. Even though the functional aspects of such tight control of replication timing remain to be explored, one can confidently consider the replication program as yet another fundamental feature characteristic of the given differentiation state. The present review touches upon the molecular mechanisms of spatial and temporal control of replication timing, involving individual replication origins as well as large chromatin domains.

Differential chromatin structure encompassing replication origins in transformed and normal cells

This study examines the chromatin structure encompassing replication origins in transformed and normal cells. Analysis of the global levels of histone H3 acetylated at K9&14 (open chromatin) and histone H3 trimethylated at K9 (closed chromatin) revealed a higher ratio of open to closed chromatin in the transformed cells. Also, the trithorax and polycomb group proteins, Brg-1 and Bmi-1, respectively, were overexpressed and more abundantly bound to chromatin in the transformed cells. Quantitative comparative analyses of episomal and in situ chromosomal replication origin activity as well as chromatin immunoprecipitation (ChIP) assays, using specific antibodies targeting members of the pre-replication complex (pre-RC) as well as open/closed chromatin markers encompassing both episomal and chromosomal origins, revealed that episomal origins had similar levels of in vivo activity, nascent DNA abundance, pre-RC protein association, and elevated open chromatin structure at the origin in both cell types. In contrast, the chromosomal origins corresponding to 20mer1, 20mer2, and c-myc displayed a 2- to 3-fold higher activity and pre-RC protein abundance as well as higher ratios of open to closed chromatin and of Brg-1 to Bmi-1 in the transformed cells, whereas the origin associated with the housekeeping lamin B2 gene exhibited similar levels of activity, pre-RC protein abundance, and higher ratios of open to closed chromatin and of Brg-1 to Bmi-1 in both cell types. Nucleosomal positioning analysis, using an MNase-Southern blot assay, showed that all the origin regions examined were situated within regions of inconsistently positioned nucleosomes, with the nucleosomes being spaced farther apart from each other prior to the onset of S phase in both cell types. Overall, the results indicate that cellular transformation is associated with differential epigenetic regulation, whereby chromatin structure is more open, rendering replication origins more accessible to initiator proteins, thus allowing increased origin activity.

Comparative analysis of pre-replication complex proteins in transformed and normal cells

This study examines the abundance of the major protein constituents of the pre-replication complex (pre-RC), both genome-wide and in association with specific replication origins, namely the lamin B2, c-myc, 20mer1, and 20mer2 origins. Several pre-RC protein components, namely ORC1-6, Cdc6, Cdt1, MCM4, MCM7, as well as additional replication proteins, such as Ku70/86, 14-3-3, Cdc45, and PCNA, were comparatively and quantitatively analyzed in both transformed and normal cells. The results show that these proteins are overexpressed and more abundantly bound to chromatin in the transformed compared to normal cells. Interestingly, the 20mer1, 20mer2, and c-myc origins exhibited a two- to threefold greater origin activity and a two- to threefold greater in vivo association of the pre-RC proteins with these origins in the transformed cells, whereas the origin associated with the housekeeping lamin B2 gene exhibited both similar levels of activity and in vivo association of these pre-RC proteins in both cell types. Overall, the results indicate that cellular transformation is associated with an overexpression and increased chromatin association of the pre-RC proteins. This study is significant, because it represents the most systematic comprehensive analysis done to date, using multiple replication proteins and different replication origins in both normal and transformed cell lines.

Single-molecule analysis of DNA replication in Xenopus egg extracts

The recent advent in single-molecule imaging and manipulation methods has made a significant impact on the understanding of molecular mechanisms underlying many essential cellular processes. Single-molecule techniques such as electron microscopy and DNA fiber assays have been employed to study the duplication of genome in eukaryotes. Here, we describe a single-molecule assay that allows replication of DNA attached to the functionalized surface of a microfluidic flow cell in a soluble Xenopus leavis egg extract replication system and subsequent visualization of replication products via fluorescence microscopy. We also explain a method for detection of replication proteins, through fluorescently labeled antibodies, on partially replicated DNA immobilized at both ends to the surface.

Analysis of DNA replication profiles in budding yeast and mammalian cells using DNA combing

DNA combing is a powerful method developed by Bensimon and colleagues to stretch DNA molecules on silanized glass coverslips. This technique provides a unique way to monitor the activation of replication origins and the progression of replication forks at the level of single DNA molecules, after incorporation of thymidine analogs, such as 5-bromo-2'-deoxyuridine (BrdU), 5-iodo-2'-deoxyuridine (IdU) and 5-chloro-2'-deoxyuridine (CldU) in newly-synthesized DNA. Unlike microarray-based approaches, this assay gives access to the variability of replication profiles in individual cells. It can also be used to monitor the effect of DNA lesions on fork progression, arrest and restart. In this review, we propose standard DNA combing methods to analyze DNA replication in budding yeast and in human cells. We also show that 5-ethynyl-2'-deoxyuridine (EdU) can be used as a good alternative to BrdU for DNA combing analysis, as unlike halogenated nucleotides, it can be detected without prior denaturation of DNA.

Replication timing and its emergence from stochastic processes

The temporal organization of DNA replication has puzzled cell biologists since before the mechanism of replication was understood. The realization that replication timing correlates with important features, such as transcription, chromatin structure and genome evolution, and is misregulated in cancer and aging has only deepened the fascination. Many ideas about replication timing have been proposed, but most have been short on mechanistic detail. However, recent work has begun to elucidate basic principles of replication timing. In particular, mathematical modeling of replication kinetics in several systems has shown that the reproducible replication timing patterns seen in population studies can be explained by stochastic origin firing at the single-cell level. This work suggests that replication timing need not be controlled by a hierarchical mechanism that imposes replication timing from a central regulator, but instead results from simple rules that affect individual origins.

A diagnostic genetic test for the physical mapping of germline rearrangements in the susceptibility breast cancer genes BRCA1 and BRCA2

The BRCA1 and BRCA2 genes are involved in breast and ovarian cancer susceptibility. About 2 to 4% of breast cancer patients with positive family history, negative for point mutations, can be expected to carry large rearrangements in one of these two genes. We developed a novel diagnostic genetic test for the physical mapping of large rearrangements, based on molecular combing (MC), a FISH-based technique for direct visualization of single DNA molecules at high resolution. We designed specific Genomic Morse Codes (GMCs), covering the exons, the noncoding regions, and large genomic portions flanking both genes. We validated our approach by testing 10 index cases with positive family history of breast cancer and 50 negative controls. Large rearrangements, corresponding to deletions and duplications with sizes ranging from 3 to 40 kb, were detected and characterized on both genes, including four novel mutations. The nature of all the identified mutations was confirmed by high-resolution array comparative genomic hybridization (aCGH) and breakpoints characterized by sequencing. The developed GMCs allowed to localize several tandem repeat duplications on both genes. We propose the developed genetic test as a valuable tool to screen large rearrangements in BRCA1 and BRCA2 to be combined in clinical settings with an assay capable of detecting small mutations.

Modeling inhomogeneous DNA replication kinetics

In eukaryotic organisms, DNA replication is initiated at a series of chromosomal locations called origins, where replication forks are assembled proceeding bidirectionally to replicate the genome. The distribution and firing rate of these origins, in conjunction with the velocity at which forks progress, dictate the program of the replication process. Previous attempts at modeling DNA replication in eukaryotes have focused on cases where the firing rate and the velocity of replication forks are homogeneous, or uniform, across the genome. However, it is now known that there are large variations in origin activity along the genome and variations in fork velocities can also take place. Here, we generalize previous approaches to modeling replication, to allow for arbitrary spatial variation of initiation rates and fork velocities. We derive rate equations for left- and right-moving forks and for replication probability over time that can be solved numerically to obtain the mean-field replication program. This method accurately reproduces the results of DNA replication simulation. We also successfully adapted our approach to the inverse problem of fitting measurements of DNA replication performed on single DNA molecules. Since such measurements are performed on specified portion of the genome, the examined DNA molecules may be replicated by forks that originate either within the studied molecule or outside of it. This problem was solved by using an effective flux of incoming replication forks at the model boundaries to represent the origin activity outside the studied region. Using this approach, we show that reliable inferences can be made about the replication of specific portions of the genome even if the amount of data that can be obtained from single-molecule experiments is generally limited.

DNA replication fading as proliferating cells advance in their commitment to terminal differentiation

Terminal differentiation is the process by which cycling cells stop proliferating to start new specific functions. It involves dramatic changes in chromatin organization as well as gene expression. In the present report we used cell flow cytometry and genome wide DNA combing to investigate DNA replication during murine erythroleukemia-induced terminal cell differentiation. The results obtained indicated that the rate of replication fork movement slows down and the inter-origin distance becomes shorter during the precommitment and commitment periods before cells stop proliferating and accumulate in G1. We propose this is a general feature caused by the progressive heterochromatinization that characterizes terminal cell differentiation.

Evidence for sequential and increasing activation of replication origins along replication timing gradients in the human genome

Genome-wide replication timing studies have suggested that mammalian chromosomes consist of megabase-scale domains of coordinated origin firing separated by large originless transition regions. Here, we report a quantitative genome-wide analysis of DNA replication kinetics in several human cell types that contradicts this view. DNA combing in HeLa cells sorted into four temporal compartments of S phase shows that replication origins are spaced at 40 kb intervals and fire as small clusters whose synchrony increases during S phase and that replication fork velocity (mean 0.7 kb/min, maximum 2.0 kb/min) remains constant and narrowly distributed through S phase. However, multi-scale analysis of a genome-wide replication timing profile shows a broad distribution of replication timing gradients with practically no regions larger than 100 kb replicating at less than 2 kb/min. Therefore, HeLa cells lack large regions of unidirectional fork progression. Temporal transition regions are replicated by sequential activation of origins at a rate that increases during S phase and replication timing gradients are set by the delay and the spacing between successive origin firings rather than by the velocity of single forks. Activation of internal origins in a specific temporal transition region is directly demonstrated by DNA combing of the IGH locus in HeLa cells. Analysis of published origin maps in HeLa cells and published replication timing and DNA combing data in several other cell types corroborate these findings, with the interesting exception of embryonic stem cells where regions of unidirectional fork progression seem more abundant. These results can be explained if origins fire independently of each other but under the control of long-range chromatin structure, or if replication forks progressing from early origins stimulate initiation in nearby unreplicated DNA. These findings shed a new light on the replication timing program of mammalian genomes and provide a general model for their replication kinetics.

Eukaryotic chromosomes replicate from multiple replication origins that fire at different times in S phase. The mechanisms that specify origin position and firing time and coordinate origins to ensure complete genome duplication are unclear. Previous studies proposed either that origins are arranged in temporally coordinated groups or fire independently of each other in a stochastic manner. Here, we have performed a quantitative analysis of human genome replication kinetics using a combination of DNA combing, which reveals local patterns of origin firing and replication fork progression on single DNA molecules, and massive sequencing of newly replicated DNA, which reveals the population-averaged replication timing profile of the entire genome. We show that origins are activated synchronously in large regions of uniform replication timing but more gradually in temporal transition regions and that the rate of origin firing increases as replication progresses. Large regions of unidirectional fork progression are abundant in embryonic stem cells but rare in differentiated cells. We propose a model in which replication forks progressing from early origins stimulate initiation in nearby unreplicated DNA in a manner that explains the shape of the replication timing profile. These results provide a fundamental insight into the temporal regulation of mammalian genome replication.

R-loop-mediated genomic instability is caused by impairment of replication fork progression

Transcriptional R loops are anomalous RNA:DNA hybrids that have been detected in organisms from bacteria to humans. These structures have been shown in eukaryotes to result in DNA damage and rearrangements; however, the mechanisms underlying these effects have remained largely unknown. To investigate this, we first show that R-loop formation induces chromosomal DNA rearrangements and recombination in Escherichia coli, just as it does in eukaryotes. More importantly, we then show that R-loop formation causes DNA replication fork stalling, and that this in fact underlies the effects of R loops on genomic stability. Strikingly, we found that attenuation of replication strongly suppresses R-loop-mediated DNA rearrangements in both E. coli and HeLa cells. Our findings thus provide a direct demonstration that R-loop formation impairs DNA replication and that this is responsible for the deleterious effects of R loops on genome stability from bacteria to humans.

Common fragile sites: mechanisms of instability revisited

Common fragile sites (CFSs) are large chromosomal regions prone to breakage upon replication stress that are considered a driving force of oncogenesis. CFSs were long believed to contain sequences blocking fork progression, thus impeding replication completion and leading to DNA breaks upon chromosome condensation. However, recent studies show that delayed completion of DNA replication instead depends on a regional paucity in initiation events. Because the distribution and the timing of these events are cell type dependent, different chromosomal regions can be committed to fragility in different cell types. These new data reveal the epigenetic nature of CFSs and open the way to a reevaluation of the role played by these sites in the formation of chromosome rearrangements found in tumors from different tissues.

How dormant origins promote complete genome replication

Many replication origins that are licensed by loading MCM2-7 complexes in G1 are not normally used. Activation of these dormant origins during S phase provides a first line of defence for the genome if replication is inhibited. When replication forks fail, dormant origins are activated within regions of the genome currently engaged in replication. At the same time, DNA damage-response kinases activated by the stalled forks preferentially suppress the assembly of new replication factories, thereby ensuring that chromosomal regions experiencing replicative stress complete synthesis before new regions of the genome are replicated. Mice expressing reduced levels of MCM2-7 have fewer dormant origins, are cancer-prone and are genetically unstable, demonstrating the importance of dormant origins for preserving genome integrity. We review the function of dormant origins, the molecular mechanism of their regulation and their physiological implications.

Replication origins and timing of temporal replication in budding yeast: how to solve the conundrum?

Similarly to metazoans, the budding yeast Saccharomyces cereviasiae replicates its genome with a defined timing. In this organism, well-defined, site-specific origins, are efficient and fire in almost every round of DNA replication. However, this strategy is neither conserved in the fission yeast Saccharomyces pombe, nor in Xenopus or Drosophila embryos, nor in higher eukaryotes, in which DNA replication initiates asynchronously throughout S phase at random sites. Temporal and spatial controls can contribute to the timing of replication such as Cdk activity, origin localization, epigenetic status or gene expression. However, a debate is going on to answer the question how individual origins are selected to fire in budding yeast. Two opposing theories were proposed: the "replicon paradigm" or "temporal program" vs. the "stochastic firing". Recent data support the temporal regulation of origin activation, clustering origins into temporal blocks of early and late replication. Contrarily, strong evidences suggest that stochastic processes acting on origins can generate the observed kinetics of replication without requiring a temporal order. In mammalian cells, a spatiotemporal model that accounts for a partially deterministic and partially stochastic order of DNA replication has been proposed. Is this strategy the solution to reconcile the conundrum of having both organized replication timing and stochastic origin firing also for budding yeast? In this review we discuss this possibility in the light of our recent study on the origin activation, suggesting that there might be a stochastic component in the temporal activation of the replication origins, especially under perturbed conditions.

Genome-scale analysis of metazoan replication origins reveals their organization in specific but flexible sites defined by conserved features

In metazoans, thousands of DNA replication origins (Oris) are activated at each cell cycle. Their genomic organization and their genetic nature remain elusive. Here, we characterized Oris by nascent strand (NS) purification and a genome-wide analysis in Drosophila and mouse cells. We show that in both species most CpG islands (CGI) contain Oris, although methylation is nearly absent in Drosophila, indicating that this epigenetic mark is not crucial for defining the activated origin. Initiation of DNA synthesis starts at the borders of CGI, resulting in a striking bimodal distribution of NS, suggestive of a dual initiation event. Oris contain a unique nucleotide skew around NS peaks, characterized by G/T and C/A overrepresentation at the 5' and 3' of Ori sites, respectively. Repeated GC-rich elements were detected, which are good predictors of Oris, suggesting that common sequence features are part of metazoan Oris. In the heterochromatic chromosome 4 of Drosophila, Oris correlated with HP1 binding sites. At the chromosome level, regions rich in Oris are early replicating, whereas Ori-poor regions are late replicating. The genome-wide analysis was coupled with a DNA combing analysis to unravel the organization of Oris. The results indicate that Oris are in a large excess, but their activation does not occur at random. They are organized in groups of site-specific but flexible origins that define replicons, where a single origin is activated in each replicon. This organization provides both site specificity and Ori firing flexibility in each replicon, allowing possible adaptation to environmental cues and cell fates.

Failure of origin activation in response to fork stalling leads to chromosomal instability at fragile sites

Perturbed DNA replication in early stages of cancer development induces chromosomal instability preferentially at fragile sites. However, the molecular basis for this instability is unknown. Here, we show that even under normal growth conditions, replication fork progression along the fragile site, FRA16C, is slow and forks frequently stall at AT-rich sequences, leading to activation of additional origins to enable replication completion. Under mild replication stress, the frequency of stalling at AT-rich sequences is further increased. Strikingly, unlike in the entire genome, in the FRA16C region additional origins are not activated, suggesting that all potential origins are already activated under normal conditions. Thus, the basis for FRA16C fragility is replication fork stalling at AT-rich sequences and inability to activate additional origins under replication stress. Our results provide a mechanism explaining the replication stress sensitivity of fragile sites and thus, the basis for genomic instability during early stages of cancer development.

Nucleotide deficiency promotes genomic instability in early stages of cancer development

Chromosomal instability in early cancer stages is caused by stress on DNA replication. The molecular basis for replication perturbation in this context is currently unknown. We studied the replication dynamics in cells in which a regulator of S phase entry and cell proliferation, the Rb-E2F pathway, is aberrantly activated. Aberrant activation of this pathway by HPV-16 E6/E7 or cyclin E oncogenes significantly decreased the cellular nucleotide levels in the newly transformed cells. Exogenously supplied nucleosides rescued the replication stress and DNA damage and dramatically decreased oncogene-induced transformation. Increased transcription of nucleotide biosynthesis genes, mediated by expressing the transcription factor c-myc, increased the nucleotide pool and also rescued the replication-induced DNA damage. Our results suggest a model for early oncogenesis in which uncoordinated activation of factors regulating cell proliferation leads to insufficient nucleotides that fail to support normal replication and genome stability.

Preferential localization of human origins of DNA replication at the 5'-ends of expressed genes and at evolutionarily conserved DNA sequences

BACKGROUND

Replication of mammalian genomes requires the activation of thousands of origins which are both spatially and temporally regulated by as yet unknown mechanisms. At the most fundamental level, our knowledge about the distribution pattern of origins in each of the chromosomes, among different cell types, and whether the physiological state of the cells alters this distribution is at present very limited.

METHODOLOGY/PRINCIPAL FINDINGS

We have used standard λ-exonuclease resistant nascent DNA preparations in the size range of 0.7-1.5 kb obtained from the breast cancer cell line MCF-7 hybridized to a custom tiling array containing 50-60 nt probes evenly distributed among genic and non-genic regions covering about 1% of the human genome. A similar DNA preparation was used for high-throughput DNA sequencing. Array experiments were also performed with DNA obtained from BT-474 and H520 cell lines. By determining the sites showing nascent DNA enrichment, we have localized several thousand origins of DNA replication. Our major findings are: (a) both array and DNA sequencing assay methods produced essentially the same origin distribution profile; (b) origin distribution is largely conserved (>70%) in all cell lines tested; (c) origins are enriched at the 5'ends of expressed genes and at evolutionarily conserved intergenic sequences; and (d) ChIP on chip experiments in MCF-7 showed an enrichment of H3K4Me3 and RNA Polymerase II chromatin binding sites at origins of DNA replication.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that the program for origin activation is largely conserved among different cell types. Also, our work supports recent studies connecting transcription initiation with replication, and in addition suggests that evolutionarily conserved intergenic sequences have the potential to participate in origin selection. Overall, our observations suggest that replication origin selection is a stochastic process significantly dependent upon local accessibility to replication factors.

Mrc1 marks early-firing origins and coordinates timing and efficiency of initiation in fission yeast

How early- and late-firing origins are selected on eukaryotic chromosomes is largely unknown. Here, we show that Mrc1, a conserved factor required for stabilization of stalled replication forks, selectively binds to the early-firing origins in a manner independent of Cdc45 and Hsk1 kinase in the fission yeast Schizosaccharomyces pombe. In mrc1Δ cells (and in swi1Δ cells to some extent), efficiency of firing is stimulated, and its timing is advanced selectively at those origins that are normally bound by Mrc1. In contrast, the late or inefficient origins which are not bound by Mrc1 are not activated in mrc1Δ cells. The enhanced firing and precocious Cdc45 loading at Mrc1-bound early-firing origins are not observed in a checkpoint mutant of mrc1, suggesting that non-checkpoint function is involved in maintaining the normal program of early-firing origins. We propose that prefiring binding of Mrc1 is an important marker of early-firing origins which are precociously activated by the absence of this protein.

Pre-replication complex proteins assemble at regions of low nucleosome occupancy within the Chinese hamster dihydrofolate reductase initiation zone

Genome-scale mapping of pre-replication complex proteins has not been reported in mammalian cells. Poor enrichment of these proteins at specific sites may be due to dispersed binding, poor epitope availability or cell cycle stage-specific binding. Here, we have mapped sites of biotin-tagged ORC and MCM protein binding in G1-synchronized populations of Chinese hamster cells harboring amplified copies of the dihydrofolate reductase (DHFR) locus, using avidin-affinity purification of biotinylated chromatin followed by high-density microarray analysis across the DHFR locus. We have identified several sites of significant enrichment for both complexes distributed throughout the previously identified initiation zone. Analysis of the frequency of initiations across stretched DNA fibers from the DHFR locus confirmed a broad zone of de-localized initiation activity surrounding the sites of ORC and MCM enrichment. Mapping positions of mononucleosomal DNA empirically and computing nucleosome-positioning information in silico revealed that ORC and MCM map to regions of low measured and predicted nucleosome occupancy. Our results demonstrate that specific sites of ORC and MCM enrichment can be detected within a mammalian intitiation zone, and suggest that initiation zones may be regions of generally low nucleosome occupancy where flexible nucleosome positioning permits flexible pre-RC assembly sites.

Evaluating genome-scale approaches to eukaryotic DNA replication

Mechanisms regulating where and when eukaryotic DNA replication initiates remain a mystery. Recently, genome-scale methods have been brought to bear on this problem. The identification of replication origins and their associated proteins in yeasts is a well-integrated investigative tool, but corresponding data sets from multicellular organisms are scarce. By contrast, standardized protocols for evaluating replication timing have generated informative data sets for most eukaryotic systems. Here, I summarize the genome-scale methods that are most frequently used to analyse replication in eukaryotes, the kinds of questions each method can address and the technical hurdles that must be overcome to gain a complete understanding of the nature of eukaryotic replication origins.

Replication dynamics at common fragile site FRA6E

The replication dynamics at common fragile site FRA6E has been evaluated by molecular combing and interphase fluorescent in situ hybridisation (FISH) in primary human lymphocytes cultured under normal or aphidicolin-induced stress conditions. FRA6E is one of the most frequently expressed common fragile sites of the human genome. It harbours several genes, PARK2 being regarded as the most relevant one. According to the results obtained from interphase FISH analysis, FRA6E can be considered a mid-late-replicating sequence characterised by heterogeneous replication timing. Molecular combing did not reveal specific replication parameters at the fragile site: fork rates were highly comparable to those detected at an early replicating locus (LMNB2) used as control and in very good agreement with the whole-genome data obtained in parallel. The same indication applied to the density of initiation zones, the inter-origin distances from adjacent ongoing forks, the frequencies of unidirectional forks, fork arrest events and asynchronous forks. Interestingly, PARK2 appeared embedded in an early/late replication transition zone, corresponding to intron 8 (162 kb) and to the fragility core of FRA6E. In cells exposed to aphidicolin, few forks progressing at a rather slow rate were observed, the majority of them being unidirectional, but again a specific response of the fragile site was not observed. In summary, at FRA6E the replication process is not impaired per se, but chromosome breakages occur preferentially at an early/late replication transition zone. Aphidicolin might increase the occurrence of breakage events at FRA6E by prolonging the time interval separating the replication of early and late replication domains. These results may be of general significance to address the problem of fragile site instability.

BRG1 co-localizes with DNA replication factors and is required for efficient replication fork progression

For DNA replication to occur, chromatin must be remodeled. Yet, we know very little about which proteins alter nucleosome occupancy at origins and replication forks and for what aspects of replication they are required. Here, we demonstrate that the BRG1 catalytic subunit of mammalian SWI/SNF-related complexes co-localizes with origin recognition complexes, GINS complexes, and proliferating cell nuclear antigen at sites of DNA replication on extended chromatin fibers. The specific pattern of BRG1 occupancy suggests it does not participate in origin selection but is involved in the firing of origins and the process of replication elongation. This latter function is confirmed by the fact that Brg1 mutant mouse embryos and RNAi knockdown cells exhibit a 50% reduction in replication fork progression rates, which is associated with decreased cell proliferation. This novel function of BRG1 is consistent with its requirement during embryogenesis and its role as a tumor suppressor to maintain genome stability and prevent cancer.

Eukaryotic chromosome DNA replication: where, when, and how?

DNA replication is central to cell proliferation. Studies in the past six decades since the proposal of a semiconservative mode of DNA replication have confirmed the high degree of conservation of the basic machinery of DNA replication from prokaryotes to eukaryotes. However, the need for replication of a substantially longer segment of DNA in coordination with various internal and external signals in eukaryotic cells has led to more complex and versatile regulatory strategies. The replication program in higher eukaryotes is under a dynamic and plastic regulation within a single cell, or within the cell population, or during development. We review here various regulatory mechanisms that control the replication program in eukaryotes and discuss future directions in this dynamic field.

Defining replication origin efficiency using DNA fiber assays

The timely duplication of eukaryotic genomes depends on the coordinated activation of thousands of replication origins distributed along the chromosomes. Origin activation follows a temporal program that is imposed by the chromosomal context and is under the control of S-phase checkpoints. Although the general mechanisms regulating DNA replication are now well-understood at the level of individual origins, little is known on the coordination of thousands of initiation events at a genome-wide level. Recent studies using DNA combing and other single-molecule assays have shown that eukaryotic genomes contain a large excess of replication origins. Most of these origins remain "dormant" in normal growth conditions but are activated when fork progression is impeded. In this review, we discuss how DNA fiber technologies have changed our view of eukaryotic replication programs and how origin redundancy contributes to the maintenance of genome integrity in eukaryotic cells.

S phase progression in human cells is dictated by the genetic continuity of DNA foci

DNA synthesis must be performed with extreme precision to maintain genomic integrity. In mammalian cells, different genomic regions are replicated at defined times, perhaps to preserve epigenetic information and cell differentiation status. However, the molecular principles that define this S phase program are unknown. By analyzing replication foci within discrete chromosome territories during interphase, we show that foci which are active during consecutive intervals of S phase are maintained as spatially adjacent neighbors throughout the cell cycle. Using extended DNA fibers, we demonstrate that this spatial continuity of replication foci correlates with the genetic continuity of adjacent replicon clusters along chromosomes. Finally, we used bioinformatic tools to compare the structure of DNA foci with DNA domains that are seen to replicate during discrete time intervals of S phase using genome-wide strategies. Data presented show that a major mechanism of S phase progression involves the sequential synthesis of regions of the genome because of their genetic continuity along the chromosomal fiber.

Increased origin activity in transformed versus normal cells: identification of novel protein players involved in DNA replication and cellular transformation

Using libraries of replication origins generated previously, we identified three clones that supported the autonomous replication of their respective plasmids in transformed, but not in normal cells. Assessment of their in vivo replication activity by in situ chromosomal DNA replication assays revealed that the chromosomal loci corresponding to these clones coincided with chromosomal replication origins in all cell lines, which were more active by 2-3-fold in the transformed by comparison to the normal cells. Evaluation of pre-replication complex (pre-RC) protein abundance at these origins in transformed and normal cells by chromatin immunoprecipitation assays, using anti-ORC2, -cdc6 and -cdt1 antibodies, showed that they were bound by these pre-RC proteins in all cell lines, but a 2-3-fold higher abundance was observed in the transformed by comparison to the normal cells. Electrophoretic mobility shift assays (EMSAs) performed on the most efficiently replicating clone, using nuclear extracts from the transformed and normal cells, revealed the presence of a DNA replication complex in transformed cells, which was barely detectable in normal cells. Subsequent supershift EMSAs suggested the presence of transformation-specific complexes. Mass spectrometric analysis of these complexes revealed potential new protein players involved in DNA replication that appear to correlate with cellular transformation.

Role of yeast Rad5 and its human orthologs, HLTF and SHPRH in DNA damage tolerance

In the yeast Saccharomyces cerevisiae, the Rad6-Rad18 DNA damage tolerance pathway constitutes a major defense system against replication fork blocking DNA lesions. The Rad6-Rad18 ubiquitin-conjugating/ligase complex governs error-free and error-prone translesion synthesis by specialized DNA polymerases, as well as an error-free Rad5-dependent postreplicative repair pathway. For facilitating replication through DNA lesions, translesion synthesis polymerases copy directly from the damaged template, while the Rad5-dependent damage tolerance pathway obtains information from the newly synthesized strand of the undamaged sister duplex. Although genetic data demonstrate the importance of the Rad5-dependent pathway in tolerating DNA damages, there has been little understanding of its mechanism. Also, the conservation of the yeast Rad5-dependent pathway in higher order eukaryotic cells remained uncertain for a long time. Here we summarize findings published in recent years regarding the role of Rad5 in promoting error-free replication of damaged DNA, and we also discuss results obtained with its human orthologs, HLTF and SHPRH.

Temporal differences in DNA replication during the S phase using single fiber analysis of normal human fibroblasts and glioblastoma T98G cells

We have recently shown that replication forks pause near origins in normal human fibroblasts (NHF1-hTERT) but not glioblastoma T98G cells. This observation led us to question whether other differences in the replication program may exist between these cell types that may relate to their genetic integrity. To identify differences, we detected immunoflourescently the sequential incorporation of the nucleotide analogs IdU and CldU into replicating DNA at the start of every hour of a synchronized S phase. We then characterized the patterns of labeled replicating DNA tracks and quantified the percentages and lengths of the tracks found at these hourly intervals. From the directionality of labeling in single extended replicating DNA fibers, tracks were categorized as single bidirectional origins, unidirectional elongations, clusters of origins firing in tandem, or merging forks (terminations). Our analysis showed that the start of S phase is enriched in single bidirectional origins in NHF1-hTERT cells, followed by an increase in clustering during mid S phase and an increase in merging forks during late S phase. Early S phase in T98G cells also largely consisted of single bidirectional origin initiations; however, an increase in clustering was delayed until an hour later, and clusters were shorter in mid/late S phase than in NHF1-hTERT cells. The spike in merging forks also did not occur until an hour later in T98G cells. Our observations suggest models to explain the temporal replication of single and clustered origins, and suggest differences in the replication program in a normal and cancer cell line.

S-phase progression in mammalian cells: modelling the influence of nuclear organization

The control of DNA replication is of fundamental importance as cell proliferation demands that identical copies of the genetic material are passed to the two daughter cells that form during mitosis. These genetic copies are generated in the preceding S phase, where the entire DNA complement of the mother cell must be copied exactly once. As part of this process, it is known that different regions of mammalian genomes are replicated at specific times of a temporally defined replication programme. The key feature of this programme is that active genes in euchromatin are replicated before inactive ones in heterochromatin. This separation of S phase into periods where different classes of chromatin are duplicated is important in maintaining changes in gene expression that define individual cell types. Recent attempts to understand the structure of the S-phase timing programme have focused on the use of genome-wide strategies that inevitably use DNA isolated from large cell populations for analysis. However, this approach provides a composite view of events that occur within a population without knowledge of the cell-to-cell variability across the population. In this review, we attempt to combine information generated using genome-wide and single cell strategies in order to develop a coherent molecular understanding of S-phase progression. During this integration, we have explored how available information can be introduced into a modelling environment that best describes S-phase progression in mammalian cells.

Reconciling stochastic origin firing with defined replication timing

Eukaryotic chromosomes replicate with defined timing patterns. However, the mechanism that regulates the timing of replication is unknown. In particular, there is an apparent conflict between population experiments, which show defined average replication times, and single-molecule experiments, which show that origins fire stochastically. Here, we provide a simple simulation that demonstrates that stochastic origin firing can produce defined average patterns of replication firing if two criteria are met. The first is that origins must have different relative firing probabilities, with origins that have relatively high firing probability being likely to fire in early S phase and origins with relatively low firing probability being unlikely to fire in early S phase. The second is that the firing probability of all origins must increase during S phase to ensure that origins with relatively low firing probability, which are unlikely to fire in early S phase, become likely to fire in late S phase. In addition, we propose biochemically plausible mechanisms for these criteria and point out how stochastic and defined origin firing can be experimentally distinguished in population experiments.

DNA replication and the GINS complex: localization on extended chromatin fibers

Background

The GINS complex is thought to be essential for the processes of initiation and elongation of DNA replication. This complex contains four subunits, one of which (Psf1) is proposed to bind to both chromatin and DNA replication-associated proteins. To date there have been no microscopic analyses to evaluate the chromatin distribution of this complex. Here, we show the organization of GINS complexes on extended chromatin fibers in relation to sites of DNA replication and replication-associated proteins.

Results

Using immunofluorescence microscopy we were able to visualize ORC1, ORC2, PCNA, and GINS complex proteins Psf1 and Psf2 bound to extended chromatin fibers. We were also able to detect these proteins concurrently with the visualization of tracks of recently replicated DNA where EdU, a thymidine analog, was incorporated. This allowed us to assess the chromatin association of proteins of interest in relation to the process of DNA replication. ORC and GINS proteins were found on chromatin fibers before replication could be detected. These proteins were also associated with newly replicated DNA in bead-like structures. Additionally, GINS proteins co-localized with PCNA at sites of active replication.

Conclusion

In agreement with its proposed role in the initiation of DNA replication, GINS proteins associated with chromatin near sites of ORC binding that were devoid of EdU (absence of DNA replication). The association of GINS proteins with PCNA was consistent with a role in the process of elongation. Additionally, the large size of our chromatin fibers (up to approximately 7 Mb) allowed for a more expansive analysis of the distance between active replicons than previously reported.

Separate domains of Rev1 mediate two modes of DNA damage bypass in mammalian cells

The Y family DNA polymerase Rev1 has been proposed to play a regulatory role in the replication of damaged templates. To elucidate the mechanism by which Rev1 promotes DNA damage bypass, we have analyzed the progression of replication on UV light-damaged DNA in mouse embryonic fibroblasts that contain a defined deletion in the N-terminal BRCT domain of Rev1 or that are deficient for Rev1. We provide evidence that Rev1 plays a coordinating role in two modes of DNA damage bypass, i.e., an early and a late pathway. The cells carrying the deletion in the BRCT domain are deficient for the early pathway, reflecting a role of the BRCT domain of Rev1 in mutagenic translesion synthesis. Rev1-deficient cells display a defect in both modes of DNA damage bypass. Despite the persistent defect in the late replicational bypass of fork-blocking (6-4)pyrimidine-pyrimidone photoproducts, overall replication is not strongly affected by Rev1 deficiency. This results in almost completely replicated templates that contain gaps encompassing the photoproducts. These gaps are inducers of DNA damage signaling leading to an irreversible G(2) arrest. Our results corroborate a model in which Rev1-mediated DNA damage bypass at postreplicative gaps quenches irreversible DNA damage responses.