Why are we where we are? Understanding replication origins and initiation sites in eukaryotes using ChIP-approaches

Temporal myc dynamics permit mitotic bypass, promoting polyploid phenotypes

High Myc phenotypes are extensively documented in the hyperproliferative cell cycle of cancer cells, as well as non-proliferative endoreplication cycles engaged during normal development and stress response. Notably, endoreplication in cancer produces chemotherapy resistant polyploid cells, necessitating a clearer understanding of altered cell cycle regulation that uncouples DNA replication and mitotic cell division. The c-Myc oncogene is a well-established transcriptional regulator of cell cycle progression and has been extensively published as an essential driver of the G1/S transition. Beyond S phase, Myc transcriptionally activates the proteins that drive mitotic entry. Sustained activation of Myc through the cell cycle transcriptionally couples DNA replication and mitotic cell division. Based on the literature in this field, we propose a new model of temporal regulation of Myc activity that serves to either couple or uncouple these two processes, determining cell cycle fate - proliferation or polyploidy. The mitotic cell cycle requires two pulses of Myc activity - the first driving the G1/S transition and the second driving the G2/M transition. During mitosis, Myc activity must be silenced to achieve high-fidelity division. Absence of the second activity pulse during G2 results in the downregulation of the proteins essential for mitotic entry and permits premature activation of APC/C, inducing mitotic bypass. A subsequent rise of Myc activity following mitotic bypass permits genome re-replication, driving polyploid phenotypes. This model serves to provide a new level of understanding to the global regulation of S phase-mitosis coupling, as well as a new lens to view low Myc phenotypes.

Genome-Wide Mapping of Autonomously Replicating Sequences in the Marine Diatom Phaeodactylum tricornutum

Autonomously replicating sequences (ARSs) are important accessories in episomal vectors that allow them to be replicated and stably maintained within transformants. Despite their importance, no information on ARSs in diatoms has been reported. Therefore, we attempted to identify ARS candidates in the model diatom, Phaeodactylum tricornutum, via chromatin immunoprecipitation sequencing. In this study, subunits of the origin recognition complex (ORC), ORC2 and ORC4, were used to screen for ARS candidates. ORC2 and ORC4 bound to 355 sites on the P. tricornutum genome, of which 69 were constantly screened after multiple attempts. The screened ARS candidates had an AT-richness of approximately 50% (44.39-52.92%) and did not have conserved sequences. In addition, ARS candidates were distributed randomly but had a dense distribution pattern at several sites. Their positions tended to overlap with those of the genetic region (73.91%). Compared to the ARSs of several other eukaryotic organisms, the characteristics of the screened ARS candidates are complex. Thus, our findings suggest that the diatom has a distinct and unique native ARSs.

Temporal association of ORCA/LRWD1 to late-firing origins during G1 dictates heterochromatin replication and organization

DNA replication requires the recruitment of a pre-replication complex facilitated by Origin Recognition Complex (ORC) onto the chromatin during G1 phase of the cell cycle. The ORC-associated protein (ORCA/LRWD1) stabilizes ORC on chromatin. Here, we evaluated the genome-wide distribution of ORCA using ChIP-seq during specific time points of G1. ORCA binding sites on the G1 chromatin are dynamic and temporally regulated. ORCA association to specific genomic sites decreases as the cells progressed towards S-phase. The majority of the ORCA-bound sites represent replication origins that also associate with the repressive chromatin marks H3K9me3 and methylated-CpGs, consistent with ORCA-bound origins initiating DNA replication late in S-phase. Further, ORCA directly associates with the repressive marks and interacts with the enzymes that catalyze these marks. Regions that associate with both ORCA and H3K9me3, exhibit diminished H3K9 methylation in ORCA-depleted cells, suggesting a role for ORCA in recruiting the H3K9me3 mark at certain genomic loci. Similarly, DNA methylation is altered at ORCA-occupied sites in cells lacking ORCA. Furthermore, repressive chromatin marks influence ORCA's binding on chromatin. We propose that ORCA coordinates with the histone and DNA methylation machinery to establish a repressive chromatin environment at a subset of origins, which primes them for late replication.

Order from clutter: selective interactions at mammalian replication origins

Mammalian chromosome duplication progresses in a precise order and is subject to constraints that are often relaxed in developmental disorders and malignancies. Molecular information about the regulation of DNA replication at the chromatin level is lacking because protein complexes that initiate replication seem to bind chromatin indiscriminately. High-throughput sequencing and mathematical modelling have yielded detailed genome-wide replication initiation maps. Combining these maps and models with functional genetic analyses suggests that distinct DNA-protein interactions at subgroups of replication initiation sites (replication origins) modulate the ubiquitous replication machinery and supports an emerging model that delineates how indiscriminate DNA-binding patterns translate into a consistent, organized replication programme.

A replicator-specific binding protein essential for site-specific initiation of DNA replication in mammalian cells

Mammalian chromosome replication starts from distinct sites; however, the principles governing initiation site selection are unclear because proteins essential for DNA replication do not exhibit sequence-specific DNA binding. Here we identify a replication-initiation determinant (RepID) protein that binds a subset of replication-initiation sites. A large fraction of RepID-binding sites share a common G-rich motif and exhibit elevated replication initiation. RepID is required for initiation of DNA replication from RepID-bound replication origins, including the origin at the human beta-globin (HBB) locus. At HBB, RepID is involved in an interaction between the replication origin (Rep-P) and the locus control region. RepID-depleted murine embryonic fibroblasts exhibit abnormal replication fork progression and fewer replication-initiation events. These observations are consistent with a model, suggesting that RepID facilitates replication initiation at a distinct group of human replication origins.

Origins of mammalian DNA replication are poorly characterised because they lack an Identifiable consensus sequence. Here the authors identify RepID, a protein that binds to a subset of G-rich replication origins and facilitates initiation from those origins.

Replication landscape of the human genome

Despite intense investigation, human replication origins and termini remain elusive. Existing data have shown strong discrepancies. Here we sequenced highly purified Okazaki fragments from two cell types and, for the first time, quantitated replication fork directionality and delineated initiation and termination zones genome-wide. Replication initiates stochastically, primarily within non-transcribed, broad (up to 150 kb) zones that often abut transcribed genes, and terminates dispersively between them. Replication fork progression is significantly co-oriented with the transcription. Initiation and termination zones are frequently contiguous, sometimes separated by regions of unidirectional replication. Initiation zones are enriched in open chromatin and enhancer marks, even when not flanked by genes, and often border ‘topologically associating domains' (TADs). Initiation zones are enriched in origin recognition complex (ORC)-binding sites and better align to origins previously mapped using bubble-trap than λ-exonuclease. This novel panorama of replication reveals how chromatin and transcription modulate the initiation process to create cell-type-specific replication programs.

The physical origin and termination sites of DNA replication in human cells have remained elusive. Here the authors use Okazaki fragment sequencing to reveal global replication patterns and show how chromatin and transcription modulate the process.

The epigenetic regulation of autonomous replicons

The discovery of autonomous replicating sequences (ARSs) in Saccharomyces cerevisiae in 1979 was considered a milestone in unraveling the regulation of replication in eukaryotic cells. However, shortly afterwards it became obvious that in Saccharomyces pombe and all other higher organisms ARSs were not sufficient to initiate independent replication. Understanding the mechanisms of replication is a major challenge in modern cell biology and is also a prerequisite to developing application-oriented autonomous replicons for gene therapeutic treatments. This review will focus on the development of non-viral episomal vectors, their use in gene therapeutic applications and our current knowledge about their epigenetic regulation.

The hunt for origins of DNA replication in multicellular eukaryotes

Origins of DNA replication (ORIs) occur at defined regions in the genome. Although DNA sequence defines the position of ORIs in budding yeast, the factors for ORI specification remain elusive in metazoa. Several methods have been used recently to map ORIs in metazoan genomes with the hope that features for ORI specification might emerge. These methods are reviewed here with analysis of their advantages and shortcomings. The various factors that may influence ORI selection for initiation of DNA replication are discussed.

Recent advances in the genome-wide study of DNA replication origins in yeast

DNA replication, one of the central events in the cell cycle, is the basis of biological inheritance. In order to be duplicated, a DNA double helix must be opened at defined sites, which are called DNA replication origins (ORIs). Unlike in bacteria, where replication initiates from a single replication origin, multiple origins are utilized in the eukaryotic genomes. Among them, the ORIs in budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe have been best characterized. In recent years, advances in DNA microarray and next-generation sequencing technologies have increased the number of yeast species involved in ORIs research dramatically. The ORIs in some non-conventional yeast species such as Kluyveromyces lactis and Pichia pastoris have also been genome-widely identified. Relevant databases of replication origins in yeast were constructed, then the comparative genomic analysis can be carried out. Here, we review several experimental approaches that have been used to map replication origins in yeast and some of the available web resources related to yeast ORIs. We also discuss the sequence characteristics and chromosome structures of ORIs in the four yeast species, which can be utilized to improve yeast replication origins prediction.

Basal level of FANCD2 monoubiquitination is required for the maintenance of a sufficient number of licensed-replication origins to fire at a normal rate

Normal DNA replication starts following the stepwise recruitment of replication initiators to assemble Mini-chromosome Maintenance (MCM) 2-7 protein complexes at an adequate amount of DNA replication origins. Under normal conditions, the monoubiquitination of Fanconi Anemia (FA) group D2 protein (FANCD2) occurs in each S-phase of cell cycle, which is the basal level of FANCD2 monoubiquitination. However, little is known regarding the roles of this basal level of monoubiquitinated FANCD2. Here we show that monoubiquitinated FANCD2 in each S-phase of normal cell cycle is essential for replication origins to fire at a normal rate. We found that the basal level of the monoubiquitinated FANCD2 can interact with replication origins as well as mini-chromosome maintenance protein 3 (MCM3) in an S-phase specific manner to secure an enough number of the licensed-origins to fire. Non-monoubiquitinated FANCD2 or mutant MCM3 lacking AA 477-480 responsible for interacting with FANCD2 can lead to an insufficient amount of licensed origins to fire and, thereby, enlarged intervals between the fired origins. Our results demonstrate that the monoubiquitinated FANCD2 in each S-phase of normal cell cycle is required to maintain an enough number of licensed origins to initiate the normal DNA replication. This finding is the first to provide insights into how FANCD2 functions under normal condition of cell cycle to maintain genome stability, as well as resulting implications in the strategic improvement for the fight against human cancer.

Best practices for mapping replication origins in eukaryotic chromosomes

Understanding the regulatory principles ensuring complete DNA replication in each cell division is critical for deciphering the mechanisms that maintain genomic stability. Recent advances in genome sequencing technology facilitated complete mapping of DNA replication sites and helped move the field from observing replication patterns at a handful of single loci to analyzing replication patterns genome-wide. These advances address issues, such as the relationship between replication initiation events, transcription, and chromatin modifications, and identify potential replication origin consensus sequences. This unit summarizes the technological and fundamental aspects of replication profiling and briefly discusses novel insights emerging from mining large datasets, published in the last 3 years, and also describes DNA replication dynamics on a whole-genome scale.

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.

Genome-wide mapping of human DNA-replication origins: levels of transcription at ORC1 sites regulate origin selection and replication timing

We report the genome-wide mapping of ORC1 binding sites in mammals, by chromatin immunoprecipitation and parallel sequencing (ChIP-seq). ORC1 binding sites in HeLa cells were validated as active DNA replication origins (ORIs) using Repli-seq, a method that allows identification of ORI-containing regions by parallel sequencing of temporally ordered replicating DNA. ORC1 sites were universally associated with transcription start sites (TSSs) of coding or noncoding RNAs (ncRNAs). Transcription levels at the ORC1 sites directly correlated with replication timing, suggesting the existence of two classes of ORIs: those associated with moderate/high transcription levels (≥1 RNA copy/cell), firing in early S and mapping to the TSSs of coding RNAs; and those associated with low transcription levels (<1 RNA copy/cell), firing throughout the entire S and mapping to TSSs of ncRNAs. These findings are compatible with a scenario whereby TSS expression levels influence the efficiency of ORC1 recruitment at G₁ and the probability of firing during S.

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.

Open chromatin structures regulate the efficiencies of pre-RC formation and replication initiation in Epstein-Barr virus

Studies of EBV replication origins demonstrate an excess of pre-replication complexes that are formed at flexible MNase-sensitive sites in the genome.

Whether or not metazoan replication initiates at random or specific but flexible sites is an unsolved question. The lack of sequence specificity in origin recognition complex (ORC) DNA binding complicates genome-scale chromatin immunoprecipitation (ChIP)-based studies. Epstein-Barr virus (EBV) persists as chromatinized minichromosomes that are replicated by the host replication machinery. We used EBV to investigate the link between zones of pre-replication complex (pre-RC) assembly, replication initiation, and micrococcal nuclease (MNase) sensitivity at different cell cycle stages in a genome-wide fashion. The dyad symmetry element (DS) of EBV’s latent origin, a well-established and very efficient pre-RC assembly region, served as an internal control. We identified 64 pre-RC zones that correlate spatially with 57 short nascent strand (SNS) zones. MNase experiments revealed that pre-RC and SNS zones were linked to regions of increased MNase sensitivity, which is a marker of origin strength. Interestingly, although spatially correlated, pre-RC and SNS zones were characterized by different features. We propose that pre-RCs are formed at flexible but distinct sites, from which only a few are activated per single genome and cell cycle.

Human telomeres replicate using chromosome-specific, rather than universal, replication programs

Human telomere replication initiates either from within telomere repeats or from within the subtelomere using a chromosome-specific replication program that appears conserved between different cell types.

Telomeric and adjacent subtelomeric heterochromatin pose significant challenges to the DNA replication machinery. Little is known about how replication progresses through these regions in human cells. Using single molecule analysis of replicated DNA (SMARD), we delineate the replication programs—i.e., origin distribution, termination site location, and fork rate and direction—of specific telomeres/subtelomeres of individual human chromosomes in two embryonic stem (ES) cell lines and two primary somatic cell types. We observe that replication can initiate within human telomere repeats but was most frequently accomplished by replisomes originating in the subtelomere. No major delay or pausing in fork progression was detected that might lead to telomere/subtelomere fragility. In addition, telomeres from different chromosomes from the same cell type displayed chromosome-specific replication programs rather than a universal program. Importantly, although there was some variation in the replication program of the same telomere in different cell types, the basic features of the program of a specific chromosome end appear to be conserved.

Different roles of the human Orc6 protein in the replication initiation process

In eukaryotes, binding of the six-subunit origin recognition complex (ORC) to DNA provides an interactive platform for the sequential assembly of pre-replicative complexes. This process licenses replication origins competent for the subsequent initiation step. Here, we analyze the contribution of human Orc6, the smallest subunit of ORC, to DNA binding and pre-replicative complex formation. We show that Orc6 not only interacts with Orc1-Orc5 but also with the initiation factor Cdc6. Biochemical and imaging experiments reveal that this interaction is required for licensing DNA replication competent. Furthermore, we demonstrate that Orc6 contributes to the interaction of ORC with the chaperone protein HMGA1a (high mobility group protein A1a). Binding of human ORC to replication origins is not specified at the level of DNA sequence and the functional organization of origins is poorly understood. We have identified HMGA1a as one factor that might direct ORC to AT-rich heterochromatic regions. The systematic analysis of the interaction between ORC and HMGA1a revealed that Orc6 interacts with the acidic C-terminus of HMGA1a and also with its AT-hooks. Both domains support autonomous replication if targeted to DNA templates. As such, Orc6 functions at different stages of the replication initiation process. Orc6 can interact with ORC chaperone proteins such as HMGA1a to facilitate chromatin binding of ORC and is also an essential factor for pre-RC formation.

Genome-wide mapping of Arabidopsis thaliana origins of DNA replication and their associated epigenetic marks

Genome integrity requires faithful chromosome duplication. Origins of replication, the genomic sites at which DNA replication initiates, are scattered throughout the genome. Their mapping at a genomic scale in multicellular organisms has been challenging. In this study we profiled origins in Arabidopsis thaliana by high-throughput sequencing of newly synthesized DNA and identified ~1,500 putative origins genome-wide. This was supported by chromatin immunoprecipitation and microarray (ChIP-chip) experiments to identify ORC1- and CDC6-binding sites. We validated origin activity independently by measuring the abundance of nascent DNA strands. The midpoints of most A. thaliana origin regions are preferentially located within the 5' half of genes, enriched in G+C, histone H2A.Z, H3K4me2, H3K4me3 and H4K5ac, and depleted in H3K4me1 and H3K9me2. Our data help clarify the epigenetic specification of DNA replication origins in A. thaliana and have implications for other eukaryotes.

Genomic approaches to the initiation of DNA replication and chromatin structure reveal a complex relationship

The mechanisms regulating the coordinate activation of tens of thousands of replication origins in multicellular organisms remain poorly explored. Recent advances in genomics have provided valuable information about the sites at which DNA replication is initiated and the selection mechanisms of specific sites in both yeast and vertebrates. Studies in yeast have advanced to the point that it is now possible to develop convincing models for origin selection. A general model has emerged, but yeast data have also revealed an unsuspected diversity of strategies for origin positioning. We focus here on the ways in which chromatin structure may affect the formation of pre-replication complexes, a prerequisite for origin activation. We also discuss the need to exercise caution when trying to extrapolate yeast models directly to more complex vertebrate genomes.

The dyad symmetry element of Epstein-Barr virus is a dominant but dispensable replication origin

OriP, the latent origin of Epstein-Barr virus (EBV), consists of two essential elements: the dyad symmetry (DS) and the family of repeats (FR). The function of these elements has been predominantly analyzed in plasmids transfected into transformed cells. Here, we examined the molecular functions of DS in its native genomic context and at an ectopic position in the mini-EBV episome. Mini-EBV plasmids contain 41% of the EBV genome including all information required for the proliferation of human B cells. Both FR and DS function independently of their genomic context. We show that DS is the most active origin of replication present in the mini-EBV genome regardless of its location, and it is characterized by the binding of the origin recognition complex (ORC) allowing subsequent replication initiation. Surprisingly, the integrity of oriP is not required for the formation of the pre-replicative complex (pre-RC) at or near DS. In addition we show that initiation events occurring at sites other than the DS are also limited to once per cell cycle and that they are ORC-dependent. The deletion of DS increases initiation from alternative origins, which are normally used very infrequently in the mini-EBV genome. The sequence-independent distribution of ORC-binding, pre-RC-assembly, and initiation patterns indicates that a large number of silent origins are present in the mini-EBV genome. We conclude that, in mini-EBV genomes lacking the DS element, the absence of a strong ORC binding site results in an increase of ORC binding at dispersed sites.

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.

Cytometry of chromatin bound Mcm6 and PCNA identifies two states in G1 that are separated functionally by the G1 restriction point

BACKGROUND

Cytometric measurements of DNA content and chromatin-bound Mcm2 have demonstrated bimodal patterns of expression in G1. These patterns, the replication licensing function of Mcm proteins, and a correlation between Mcm loading and cell cycle commitment for cells re-entering the cell cycle, led us to test the idea that cells expressing a defined high level of chromatin-bound Mcm6 in G1 are committed--i.e., past the G1 restriction point. We developed a cell-based assay for tightly-bound PCNA (PCNA*) and Mcm6 (Mcm6*), DNA content, and a mitotic marker to clearly define G1, S, G2, and M phases of the cell cycle. hTERT-BJ1, hTERT-RPE-1, and Molt4 cells were extracted with Triton X-100 followed by methanol fixation, stained with antibodies and DAPI, then measured by cytometry.

RESULTS

Bivariate analysis of cytometric data demonstrated complex patterns with distinct clustering for all combinations of the 4 variables. In G1, cells clustered in two groups characterized by low and high Mcm6* expression. Serum starvation and release experiments showed that residence in the high group was in late G1, just prior to S phase. Kinetic experiments, employing serum withdrawal, and stathmokinetic analysis with aphidicolin, mimosine or nocodazole demonstrated that cells with high levels of Mcm6* cycled with the committed phases of the cell cycle (S, G2, and M).

CONCLUSIONS

A multivariate assay for Mcm6*, PCNA*, DNA content, and a mitotic marker provides analysis capable of estimating the fraction of pre and post-restriction point G1 cells and supports the idea that there are at least two states in G1 defined by levels of chromatin bound Mcm proteins.