Chii Mei Lin

Genome-wide depletion of replication initiation events in highly transcribed regions

This report investigates the mechanisms by which mammalian cells coordinate DNA replication with transcription and chromatin assembly. In yeast, DNA replication initiates within nucleosome-free regions, but studies in mammalian cells have not revealed a similar relationship. Here, we have used genome-wide massively parallel sequencing to map replication initiation events, thereby creating a database of all replication initiation sites within nonrepetitive DNA in two human cell lines. Mining this database revealed that genomic regions transcribed at moderate levels were generally associated with high replication initiation frequency. In genomic regions with high rates of transcription, very few replication initiation events were detected. High-resolution mapping of replication initiation sites showed that replication initiation events were absent from transcription start sites but were highly enriched in adjacent, downstream sequences. Methylation of CpG sequences strongly affected the location of replication initiation events, whereas histone modifications had minimal effects. These observations suggest that high levels of transcription interfere with formation of pre-replication protein complexes. Data presented here identify replication initiation sites throughout the genome, providing a foundation for further analyses of DNA-replication dynamics and cell-cycle progression.

DNA methylation supports intrinsic epigenetic memory in mammalian cells.

We have investigated the role of DNA methylation in the initiation and maintenance of silenced chromatin in somatic mammalian cells. We found that a mutated transgene, in which all the CpG dinucleotides have been eliminated, underwent transcriptional silencing to the same extent as the unmodified transgene. These observations demonstrate that DNA methylation is not required for silencing. The silenced CpG-free transgene exhibited all the features of heterochromatin, including silencing of transcriptional activity, delayed DNA replication, lack of histone H3 and H4 acetylation, lack of H3-K4 methylation, and enrichment in tri-methyl-H3-K9. In contrast, when we tested for transgene reactivation using a Cre recombinase-mediated inversion assay, we observed a marked difference between a CpG-free and an unmodified transgene: the CpG-free transgene resumed transcription and did not exhibit markers of heterochromatin whereas the unmodified transgene remained silenced. These data indicate that methylation of CpG residues conferred epigenetic memory in this system. These results also suggest that replication delay, lack of histone H3 and H4 acetylation, H3-K4 methylation, and enrichment in tri-methyl-H3-K9 are not sufficient to confer epigenetic memory. We propose that DNA methylation within transgenes serves as an intrinsic epigenetic memory to permanently silence transgenes and prevent their reactivation.

Dynamic alterations of replication timing in mammalian cells.

BACKGROUND The eukaryotic genome is divided into distinct replication timing domains, which are activated during S phase in a strictly conserved order. Cellular differentiation can alter replication timing in some loci, but recent experiments yielded conflicting data regarding the relationship between gene expression and replication timing. The genetic and epigenetic determinants of replication timing in mammalian cells have yet to be elucidated. RESULTS We developed a mammalian experimental system in which the timing of DNA replication can be altered in a controlled manner. This system utilizes sequences from the human beta-globin locus that exhibit orientation-dependent transcriptional silencing when inserted into the murine genome. We found that before insertion, the murine target site replicated late during S phase. After insertion, replication timing depended on the orientation of the transgene. In a transcription-permissive orientation, the transgene and flanking sequences replicated early. In the reverse (silencing-prone) orientation, these sequences replicated late. Early replication correlated with histone modifications of the transgene chromatin but could be observed in the absence of the beta-globin promoter. Importantly, the replication timing switch did not require a replication origin within the transgene. CONCLUSIONS Transgene insertions into mammalian heterochromatin can alter the timing of DNA replication at the insertion site. This differentiation-independent replication timing switch did not necessitate insertion of an active promoter or a replication origin. These observations suggest that the timing of DNA replication can be manipulated by changes in DNA sequence, but that the determinants of replication timing are distinct from the sequences that specify replication initiation sites.

The Human β-Globin Replication Initiation Region Consists of Two Modular Independent Replicators

ABSTRACT Previous studies have shown that mammalian cells contain replicator sequences, which can determine where DNA replication initiates. However, the specific sequences that confer replicator activity were not identified. Here we report a detailed analysis of replicator sequences that dictate initiation of DNA replication from the human β-globin locus. This analysis suggests that the β-globin replication initiation region contains two adjacent, redundant replicators. Each replicator was capable of initiating DNA replication independently at ectopic sites. Within each of these two replicators, we identified short, discrete, nonredundant sequences, which cooperatively determine replicator activity. Experiments with somatic cell hybrids further demonstrated that the requirements for initiation at ectopic sites were similar to the requirements for initiation within native human chromosomes. The replicator clustering and redundancy exemplified in the human β-globin locus may account for the extreme difficulty in identifying replicator sequences in mammalian cells and suggest that mammalian replication initiation sites may be determined by cooperative sequence modules.

Preventing gene silencing with human replicators

Transcriptional silencing, one of the major impediments to gene therapy in humans, is often accompanied by replication during late S-phase. We report that transcriptional silencing and late replication were prevented by DNA sequences that can initiate DNA replication (replicators). When replicators were included in silencing-prone transgenes, they did not undergo transcriptional silencing, replicated early and maintained histone acetylation patterns characteristic of euchromatin. A mutant replicator, which could not initiate replication, could not prevent gene silencing and replicated late when included in identical transgenes and inserted at identical locations. These observations suggest that replicators introduce epigenetic chromatin changes that facilitate initiation of DNA replication and affect gene silencing. Inclusion of functional replicators in gene therapy vectors may provide a tool for stabilizing gene expression patterns.

The Human β-Globin Locus Control Region Can Silence as Well as Activate Gene Expression

ABSTRACT Using recombinase-mediated cassette exchange to test multiple transgenes at the same site of integration, we demonstrate a novel chromatin context-dependent silencer activity of the β-globin locus control region (LCR). This silencer activity requires DNase I hypersensitive sites HS2 and HS3 but not HS4. After silencing, the silenced cassettes adopt a typical closed chromatin conformation (histone H3 and H4 deacetylation, histone H3-K4 methylation, DNA methylation, and replication in late S phase). In the absence of the LCR at the same site of integration, the chromatin remains decondensed. We demonstrate that the LCR is necessary but not sufficient to trigger these chromatin changes. We also provide evidence that this novel silencing activity is caused by transcriptional interference triggered by activation of transcription in the flanking sequences by the LCR.

Allele-Specific Genome-wide Profiling in Human Primary Erythroblasts Reveal Replication Program Organization

We have developed a new approach to characterize allele-specific timing of DNA replication genome-wide in human primary basophilic erythroblasts. We show that the two chromosome homologs replicate at the same time in about 88% of the genome and that large structural variants are preferentially associated with asynchronous replication. We identified about 600 megabase-sized asynchronously replicated domains in two tested individuals. The longest asynchronously replicated domains are enriched in imprinted genes suggesting that structural variants and parental imprinting are two causes of replication asynchrony in the human genome. Biased chromosome X inactivation in one of the two individuals tested was another source of detectable replication asynchrony. Analysis of high-resolution TimEX profiles revealed small variations termed timing ripples, which were undetected in previous, lower resolution analyses. Timing ripples reflect highly reproducible, variations of the timing of replication in the 100 kb-range that exist within the well-characterized megabase-sized replication timing domains. These ripples correspond to clusters of origins of replication that we detected using novel nascent strands DNA profiling methods. Analysis of the distribution of replication origins revealed dramatic differences in initiation of replication frequencies during S phase and a strong association, in both synchronous and asynchronous regions, between origins of replication and three genomic features: G-quadruplexes, CpG Islands and transcription start sites. The frequency of initiation in asynchronous regions was similar in the two homologs. Asynchronous regions were richer in origins of replication than synchronous regions.

Prevention of transcriptional silencing by a replicator-binding complex consisting of SWI/SNF, MeCP1 and hnRNP C1/C2

ABSTRACT Transcriptional silencing selectively impedes gene expression. Silencing is often accompanied by replication delay and can be prevented by replicator sequences. Here we report a replicator-binding protein complex involved in the prevention of transcriptional silencing. The protein complex interacts with an essential asymmetric region within the human β-globin Rep-P replicator and includes hnRNP C1/C2, SWI/SNF complex, and MeCP1, which are members of the locus control region (LCR)-associated remodeling complex (LARC). Interaction between LARC and Rep-P prevented transcriptional silencing and replication delay. Transgenes that did not contain the asymmetric LARC-binding region of Rep-P replicated late and exhibited stable silencing that could not be affected by a DNA methylation inhibitor. In contrast, transgenes that contain a mutation of the asymmetric region of Rep-P that could not bind LARC exhibited a silent state that could transiently be reactivated by DNA demethylation. The effect of DNA demethylation was transient, and prolonged exposure to a methylation inhibitor induced distinct, stable, methylation-independent silencing. These observations suggest that the interaction of LARC complex with replicators plays a role in preventing gene silencing and provides support for a novel, epigenetic mechanism of resistance to methylation inhibitors.

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.

Distinct epigenetic features of differentiation-regulated replication origins

BackgroundEukaryotic genome duplication starts at discrete sequences (replication origins) that coordinate cell cycle progression, ensure genomic stability and modulate gene expression. Origins share some sequence features, but their activity also responds to changes in transcription and cellular differentiation status.ResultsTo identify chromatin states and histone modifications that locally mark replication origins, we profiled origin distributions in eight human cell lines representing embryonic and differentiated cell types. Consistent with a role of chromatin structure in determining origin activity, we found that cancer and non-cancer cells of similar lineages exhibited highly similar replication origin distributions. Surprisingly, our study revealed that DNase hypersensitivity, which often correlates with early replication at large-scale chromatin domains, did not emerge as a strong local determinant of origin activity. Instead, we found that two distinct sets of chromatin modifications exhibited strong local associations with two discrete groups of replication origins. The first origin group consisted of about 40,000 regions that actively initiated replication in all cell types and preferentially colocalized with unmethylated CpGs and with the euchromatin markers, H3K4me3 and H3K9Ac. The second group included origins that were consistently active in cells of a single type or lineage and preferentially colocalized with the heterochromatin marker, H3K9me3. Shared origins replicated throughout the S-phase of the cell cycle, whereas cell-type-specific origins preferentially replicated during late S-phase.ConclusionsThese observations are in line with the hypothesis that differentiation-associated changes in chromatin and gene expression affect the activation of specific replication origins.