Genome-Wide Mapping of Human DNA Replication by Optical Replication Mapping Supports a Stochastic Model of Eukaryotic Replication

AbstractDNA replication is regulated by the location and timing of replication initiation. Therefore, much effort has been invested in identifying and analyzing the sites of human replication initiation. However, the heterogeneous nature of eukaryotic replication kinetics and the low efficiency of individual initiation site utilization in metazoans has made mapping the location and timing of replication initiation in human cells difficult. A potential solution to the problem of human replication mapping is single-molecule analysis. However, current approaches do not provide the throughput required for genome-wide experiments. To address this challenge, we have developed Optical Replication Mapping (ORM), a high-throughput single-molecule approach to map newly replicated DNA, and used it to map early initiation events in human cells. The single-molecule nature of our data, and a total of more than 2000-fold coverage of the human genome on 27 million fibers averaging ~300 kb in length, allow us to identify initiation sites and their firing probability with high confidence. In particular, for the first time, we are able to measure genome-wide the absolute efficiency of human replication initiation. We find that the distribution of human replication initiation is consistent with inefficient, stochastic initiation of heterogeneously distributed potential initiation complexes enriched in accessible chromatin. In particular, we find sites of human replication initiation are not confined to well-defined replication origins but are instead distributed across broad initiation zones consisting of many initiation sites. Furthermore, we find no correlation of initiation events between neighboring initiation zones. Although most early initiation events occur in early-replicating regions of the genome, a significant number occur in late-replicating regions. The fact that initiation sites in typically late-replicating regions have some probability of firing in early S phase suggests that the major difference between initiation events in early and late replicating regions is their intrinsic probability of firing, as opposed to a qualitative difference in their firing-time distributions. Moreover, modeling of replication kinetics demonstrates that measuring the efficiency of initiation-zone firing in early S phase suffices to predict the average firing time of such initiation zones throughout S phase, further suggesting that the differences between the firing times of early and late initiation zones are quantitative, rather than qualitative. These observations are consistent with stochastic models of initiation-timing regulation and suggest that stochastic regulation of replication kinetics is a fundamental feature of eukaryotic replication, conserved from yeast to humans.

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High-throughput analysis of DNA replication in single human cells reveals constrained variability in the location and timing of replication initiation

10.1101/2021.05.14.443897 ◽

2021 ◽

Author(s):

Dashiell J Massey ◽

Amnon Koren

Keyword(s):

Dna Replication ◽

Replication Origin ◽

Replication Timing ◽

S Phase ◽

Human Cells ◽

Replication Initiation ◽

High Throughput Analysis ◽

Timing Variability ◽

Fixed Set ◽

Fixed Order

DNA replication occurs throughout the S phase of the cell cycle, initiating from replication origin loci that fire at different times. Debate remains about whether origins are a fixed set of loci used across all cells or a loose agglomeration of potential origins used stochastically in individual cells, and about how consistent their firing time during S phase is across cells. Here, we develop an approach for profiling DNA replication in single human cells and apply it to 2,305 replicating cells spanning the entire S phase. The resolution and scale of the data enabled us to specifically analyze initiation sites and show that these sites have confined locations that are consistently used among individual cells. Further, we find that initiation sites are activated in a similar, albeit not fixed, order across cells. Taken together, our results suggest that replication timing variability is constrained both spatially and temporally, and that the degree of variation is consistent across human cell lines.

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Genome-Wide Identification of Early-Firing Human Replication Origins by Optical Replication Mapping

10.1101/214841 ◽

2017 ◽

Cited By ~ 11

Author(s):

Kyle Klein ◽

Weitao Wang ◽

Tyler Borrman ◽

Saki Chan ◽

Denghong Zhang ◽

...

Keyword(s):

Single Molecule ◽

Replication Timing ◽

S Phase ◽

Replication Initiation ◽

Replication Origins ◽

Origin Firing ◽

Efficient Replication ◽

Mapping Technology ◽

Low Efficiency ◽

Early Replication

AbstractThe timing of DNA replication is largely regulated by the location and timing of replication origin firing. Therefore, much effort has been invested in identifying and analyzing human replication origins. However, the heterogeneous nature of eukaryotic replication kinetics and the low efficiency of individual origins in metazoans has made mapping the location and timing of replication initiation in human cells difficult. We have mapped early-firing origins in HeLa cells using Optical Replication Mapping, a high-throughput single-molecule approach based on Bionano Genomics genomic mapping technology. The single-molecule nature and 290-fold coverage of our dataset allowed us to identify origins that fire with as little as 1% efficiency. We find sites of human replication initiation in early S phase are not confined to well-defined efficient replication origins, but are instead distributed across broad initiation zones consisting of many inefficient origins. These early-firing initiation zones co-localize with initiation zones inferred from Okazaki-fragment-mapping analysis and are enriched in ORC1 binding sites. Although most early-firing origins fire in early-replication regions of the genome, a significant number fire in late-replicating regions, suggesting that the major difference between origins in early and late replicating regions is their probability of firing in early S-phase, as opposed to qualitative differences in their firing-time distributions. This observation is consistent with stochastic models of origin timing regulation, which explain the regulation of replication timing in yeast.

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Regulation of DNA Replication Licensing and Re-Replication by Cdt1

International Journal of Molecular Sciences ◽

10.3390/ijms22105195 ◽

2021 ◽

Vol 22 (10) ◽

pp. 5195

Author(s):

Hui Zhang

Keyword(s):

Cell Cycle ◽

Dna Damage ◽

Dna Replication ◽

Genome Stability ◽

E3 Ligase ◽

S Phase ◽

Replication Initiation ◽

Nuclear Antigen ◽

Repair Synthesis ◽

Ubiquitin E3 Ligase

In eukaryotic cells, DNA replication licensing is precisely regulated to ensure that the initiation of genomic DNA replication in S phase occurs once and only once for each mitotic cell division. A key regulatory mechanism by which DNA re-replication is suppressed is the S phase-dependent proteolysis of Cdt1, an essential replication protein for licensing DNA replication origins by loading the Mcm2-7 replication helicase for DNA duplication in S phase. Cdt1 degradation is mediated by CRL4Cdt2 ubiquitin E3 ligase, which further requires Cdt1 binding to proliferating cell nuclear antigen (PCNA) through a PIP box domain in Cdt1 during DNA synthesis. Recent studies found that Cdt2, the specific subunit of CRL4Cdt2 ubiquitin E3 ligase that targets Cdt1 for degradation, also contains an evolutionarily conserved PIP box-like domain that mediates the interaction with PCNA. These findings suggest that the initiation and elongation of DNA replication or DNA damage-induced repair synthesis provide a novel mechanism by which Cdt1 and CRL4Cdt2 are both recruited onto the trimeric PCNA clamp encircling the replicating DNA strands to promote the interaction between Cdt1 and CRL4Cdt2. The proximity of PCNA-bound Cdt1 to CRL4Cdt2 facilitates the destruction of Cdt1 in response to DNA damage or after DNA replication initiation to prevent DNA re-replication in the cell cycle. CRL4Cdt2 ubiquitin E3 ligase may also regulate the degradation of other PIP box-containing proteins, such as CDK inhibitor p21 and histone methylase Set8, to regulate DNA replication licensing, cell cycle progression, DNA repair, and genome stability by directly interacting with PCNA during DNA replication and repair synthesis.

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Developmental differences in genome replication program and origin activation

Nucleic Acids Research ◽

10.1093/nar/gkaa1124 ◽

2020 ◽

Vol 48 (22) ◽

pp. 12751-12777

Author(s):

Cathia Rausch ◽

Patrick Weber ◽

Paulina Prorok ◽

David Hörl ◽

Andreas Maiser ◽

...

Keyword(s):

Dna Replication ◽

Single Molecule ◽

Embryonic Stem ◽

S Phase ◽

Developmental Differences ◽

Centromeric Heterochromatin ◽

Genome Replication ◽

Origin Activation ◽

Spatio Temporal ◽

Mes Cells

Abstract To ensure error-free duplication of all (epi)genetic information once per cell cycle, DNA replication follows a cell type and developmental stage specific spatio-temporal program. Here, we analyze the spatio-temporal DNA replication progression in (un)differentiated mouse embryonic stem (mES) cells. Whereas telomeres replicate throughout S-phase, we observe mid S-phase replication of (peri)centromeric heterochromatin in mES cells, which switches to late S-phase replication upon differentiation. This replication timing reversal correlates with and depends on an increase in condensation and a decrease in acetylation of chromatin. We further find synchronous duplication of the Y chromosome, marking the end of S-phase, irrespectively of the pluripotency state. Using a combination of single-molecule and super-resolution microscopy, we measure molecular properties of the mES cell replicon, the number of replication foci active in parallel and their spatial clustering. We conclude that each replication nanofocus in mES cells corresponds to an individual replicon, with up to one quarter representing unidirectional forks. Furthermore, with molecular combing and genome-wide origin mapping analyses, we find that mES cells activate twice as many origins spaced at half the distance than somatic cells. Altogether, our results highlight fundamental developmental differences on progression of genome replication and origin activation in pluripotent cells.

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DNA Replication Control DuringDrosophilaDevelopment: Insights into the Onset of S Phase, Replication Initiation, and Fork Progression

Genetics ◽

10.1534/genetics.115.186627 ◽

2017 ◽

Vol 207 (1) ◽

pp. 29-47 ◽

Cited By ~ 18

Author(s):

Brian L. Hua ◽

Terry L. Orr-Weaver

Keyword(s):

Dna Replication ◽

S Phase ◽

Replication Initiation ◽

Replication Control

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Disruption of origin chromatin structure by helicase activation in the absence of DNA replication

10.1101/2021.03.17.435814 ◽

2021 ◽

Author(s):

Rachel A. Hoffman ◽

David M. MacAlpine

Keyword(s):

Dna Replication ◽

S Phase ◽

Replication Initiation ◽

Micrococcal Nuclease ◽

Sequence Context ◽

Α Subunit ◽

Chromatin Dynamics ◽

Conditional Mutant ◽

Local Sequence ◽

Efficient Replication

Prior to initiation of DNA replication, the eukaryotic helicase, Mcm2-7, must be activated to unwind DNA at replication start sites in early S-phase. To study helicase activation within origin chromatin, we constructed a conditional mutant of the polymerase α subunit Cdc17 (or Pol1) to prevent priming and block replication. Recovery of these cells at permissive conditions resulted in the generation of unreplicated gaps at origins, likely due to helicase activation prior to replication initiation. We used micrococcal nuclease (MNase)-based chromatin occupancy profiling under restrictive conditions to study chromatin dynamics associated with helicase activation. Helicase activation in the absence of DNA replication resulted in the disruption and disorganization of chromatin which extends up to one kilobase from early, efficient replication origins. The CMG holo-helicase complex also moves the same distance out from the origin, producing single-stranded DNA that activates the intra-S-phase checkpoint. Loss of the checkpoint did not regulate the progression and stalling of the CMG complex, but rather resulted in the disruption of chromatin at both early and late origins. Finally, we found that the local sequence context regulates helicase progression in the absence of DNA replication, suggesting that the helicase is intrinsically less processive when uncoupled from replication.

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DNA Replication Initiation Studied at the Single Molecule Level

Biophysical Journal ◽

10.1016/j.bpj.2012.11.445 ◽

2013 ◽

Vol 104 (2) ◽

pp. 74a

Author(s):

Hsin-Mei Cheng ◽

Philip Gröger ◽

Andreas Hartmann ◽

Elena M. Seco ◽

Silvia Ayora ◽

...

Keyword(s):

Dna Replication ◽

Single Molecule ◽

Replication Initiation ◽

Single Molecule Level ◽

Dna Replication Initiation

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Dynamics of DNA replication in a eukaryotic cell

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1818680116 ◽

2019 ◽

Vol 116 (11) ◽

pp. 4973-4982 ◽

Cited By ~ 18

Author(s):

Thomas Kelly ◽

A. John Callegari

Keyword(s):

Dna Replication ◽

Single Molecule ◽

Origin Recognition Complex ◽

Complex Dynamics ◽

Replication Timing ◽

Eukaryotic Cell ◽

Replication Initiation ◽

Integrative Model ◽

Genomic Locus ◽

Two Factors

Each genomic locus in a eukaryotic cell has a distinct average time of replication during S phase that depends on the spatial and temporal pattern of replication initiation events. Replication timing can affect genomic integrity because late replication is associated with an increased mutation rate. For most eukaryotes, the features of the genome that specify the location and timing of initiation events are unknown. To investigate these features for the fission yeast, Schizosaccharomyces pombe, we developed an integrative model to analyze large single-molecule and global genomic datasets. The model provides an accurate description of the complex dynamics of S. pombe DNA replication at high resolution. We present evidence that there are many more potential initiation sites in the S. pombe genome than previously identified and that the distribution of these sites is primarily determined by two factors: the sequence preferences of the origin recognition complex (ORC), and the interference of transcription with the assembly or stability of prereplication complexes (pre-RCs). We suggest that in addition to directly interfering with initiation, transcription has driven the evolution of the binding properties of ORC in S. pombe and other eukaryotic species to target pre-RC assembly to regions of the genome that are less likely to be transcribed.

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Dbf4-Dependent Kinase (DDK)-Mediated Proteolysis of CENP-A Prevents Mislocalization of CENP-A in Saccharomyces cerevisiae

G3 Genes|Genome|Genetics ◽

10.1534/g3.120.401131 ◽

2020 ◽

Vol 10 (6) ◽

pp. 2057-2068 ◽

Cited By ~ 3

Author(s):

Jessica R. Eisenstatt ◽

Lars Boeckmann ◽

Wei-Chun Au ◽

Valerie Garcia ◽

Levi Bursch ◽

...

Keyword(s):

Dna Replication ◽

Replication Initiation ◽

Centromeric Chromatin ◽

Dna Replication Initiation ◽

Link Type ◽

Genome Wide ◽

A Genome ◽

Evolutionarily Conserved ◽

Histone H3 Variant

The evolutionarily conserved centromeric histone H3 variant (Cse4 in budding yeast, CENP-A in humans) is essential for faithful chromosome segregation. Mislocalization of CENP-A to non-centromeric chromatin contributes to chromosomal instability (CIN) in yeast, fly, and human cells and CENP-A is highly expressed and mislocalized in cancers. Defining mechanisms that prevent mislocalization of CENP-A is an area of active investigation. Ubiquitin-mediated proteolysis of overexpressed Cse4 (GALCSE4) by E3 ubiquitin ligases such as Psh1 prevents mislocalization of Cse4, and psh1Δ strains display synthetic dosage lethality (SDL) with GALCSE4. We previously performed a genome-wide screen and identified five alleles of CDC7 and DBF4 that encode the Dbf4-dependent kinase (DDK) complex, which regulates DNA replication initiation, among the top twelve hits that displayed SDL with GALCSE4. We determined that cdc7-7 strains exhibit defects in ubiquitin-mediated proteolysis of Cse4 and show mislocalization of Cse4. Mutation of MCM5 (mcm5-bob1) bypasses the requirement of Cdc7 for replication initiation and rescues replication defects in a cdc7-7 strain. We determined that mcm5-bob1 does not rescue the SDL and defects in proteolysis of GALCSE4 in a cdc7-7 strain, suggesting a DNA replication-independent role for Cdc7 in Cse4 proteolysis. The SDL phenotype, defects in ubiquitin-mediated proteolysis, and the mislocalization pattern of Cse4 in a cdc7-7 psh1Δ strain were similar to that of cdc7-7 and psh1Δ strains, suggesting that Cdc7 regulates Cse4 in a pathway that overlaps with Psh1. Our results define a DNA replication initiation-independent role of DDK as a regulator of Psh1-mediated proteolysis of Cse4 to prevent mislocalization of Cse4.

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