Replication timing of the human X-inactivation center (XIC) region: correlation with chromosome bands

Replication of ribosomal DNA in Arabidopsis occurs both inside and outside the nucleolus during S phase progression

Ribosomal RNA genes (rDNA) have been used as valuable experimental systems in numerous studies. Here, we focus on elucidating the spatiotemporal organisation of rDNA replication in Arabidopsis thaliana To determine the subnuclear distribution of rDNA and the progression of its replication during the S phase, we apply 5-ethynyl-2'-deoxyuridine (EdU) labelling, fluorescence-activated cell sorting, fluorescence in situ hybridization and structured illumination microscopy. We show that rDNA is replicated inside and outside the nucleolus, where active transcription occurs at the same time. Nascent rDNA shows a maximum of nucleolar associations during early S phase. In addition to EdU patterns typical for early or late S phase, we describe two intermediate EdU profiles characteristic for mid S phase. Moreover, the use of lines containing mutations in the chromatin assembly factor-1 gene fas1 and wild-type progeny of fas1xfas2 crosses depleted of inactive copies allows for selective observation of the replication pattern of active rDNA. High-resolution data are presented, revealing the culmination of replication in the mid S phase in the nucleolus and its vicinity. Taken together, our results provide a detailed snapshot of replication of active and inactive rDNA during S phase progression.

EnHERV: Enrichment analysis of specific human endogenous retrovirus patterns and their neighboring genes

Human endogenous retroviruses (HERVs) are flanked by long terminal repeats (LTRs), which contain the regulation part of the retrovirus. Remaining HERVs constitute 7% to 8% of the present day human genome, and most have been identified as solo LTRs. The HERV sequences have been associated with several molecular functions as well as certain diseases in human, but their roles in human diseases are yet to be established. We designed EnHERV to make accessible the identified endogenous retrovirus repetitive sequences from Repbase Update (a database of eukaryotic repetitive elements) that are present in the human genome. Defragmentation process was done to improve the RepeatMasker annotation output. The defragmented elements were used as core database in EnHERV. EnHERV is available at http://sysbio.chula.ac.th/enherv and can be searched using either gene lists of user interest or HERV characteristics. Besides the search function, EnHERV also provides an enrichment analysis function that allows users to perform enrichment analysis between selected HERV characteristics and user-input gene lists, especially genes with the expression profile of a certain disease. EnHERV will facilitate exploratory studies of specific HERV characteristics that control gene expression patterns related to various disease conditions. Here we analyzed 25 selected HERV groups/names from all four HERV superfamilies, using the sense and anti-sense directions of the HERV and gene expression profiles from 49 specific tissue and disease conditions. We found that intragenic HERVs were associated with down-regulated genes in most cancer conditions and in psoriatic skin tissues and associated with up-regulated genes in immune cells particularly from systemic lupus erythematosus (SLE) patients. EnHERV allowed the analysis of how different types of LTRs were differentially associated with specific gene expression profiles in particular disease conditions for further studies into their mechanisms and functions.

Cell line differences in replication timing of human glutamate receptor genes and other large genes associated with neural disease

There is considerable current interest in the function of epigenetic mechanisms in neuroplasticity with regard to learning and memory formation and to a range of neural diseases. Previously, we described replication timing on human chromosome 21q in the THP-1 human cell line (2n = 46, XY) and showed that several genes associated with neural diseases, such as the neuronal glutamate receptor subunit GluR-5 (GRIK1) and amyloid precursor protein (APP), were located in regions where replication timing transitioned from early to late S phase. Here, we compared replication timing of all known human glutamate receptor genes (26 genes in total) and APP in 6 different human cell lines including human neuron-related cell lines. Replication timings were obtained by integrating our previously reported data with new data generated here and information from the online database ReplicationDomain. We found that many of the glutamate receptor genes were clearly located in replication timing transition zones in neural precursor cells, but this relationship was less clear in embryonic stem cells before neural differentiation; in the latter, the genes were often located in later replication timing zones that displayed DNA hypermethylation. Analysis of selected large glutamate receptor genes (> 200 kb), and of APP, showed that their precise replication timing patterns differed among the cell lines. We propose that the transition zones of DNA replication timing are altered by epigenetic mechanisms, and that these changes may affect the neuroplasticity that is important to memory and learning, and may also have a role in the development of neural diseases.

A genetic program theory of aging using an RNA population model

Aging is a common characteristic of multicellular eukaryotes. Copious hypotheses have been proposed to explain the mechanisms of aging, but no single theory is generally acceptable. In this article, we refine the RNA population gene activating model (Lv et al., 2003) based on existing reports as well as on our own latest findings. We propose the RNA population model as a genetic theory of aging. The new model can also be applied to differentiation and tumorigenesis and could explain the biological significance of non-coding DNA, RNA, and repetitive sequence DNA. We provide evidence from the literature as well as from our own findings for the roles of repetitive sequences in gene activation. In addition, we predict several phenomena related to aging and differentiation based on this model.

[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.

R/G-band boundaries: genomic instability and human disease

The human genome is composed of large-scale compartmentalized structures resulting from variations in the amount of guanine and cytosine residues (GC%) and in the timing of DNA replication. These compartmentalized structures are related to the light- and dark-staining bands along chromosomes after the appropriate staining. Here we describe our current understanding of the biological importance of the boundaries between these light and dark bands (the so-called R/G boundaries). These R/G boundaries were identified following integration of information obtained from analyses of chromosome bands and genome sequences. This review also discusses the potential medical significance of these chromosomal regions for conditions related to genomic instability, such as cancer and neural disease. We propose that R/G-chromosomal boundaries, which correspond to regions showing a switch in replication timing from early to late S phase (early/late-switch regions) and of transition in GC%, have an extremely low number of replication origins and more non-B-form DNA structures than other genomic regions. Further, we suggest that genes located at R/G boundaries and which contain such DNA sequences have an increased risk of genetic instability and of being associated with human diseases. Finally, we propose strategies for genome and epigenome analyses based on R/G boundaries.

Regulation of DNA replication timing on human chromosome by a cell-type specific DNA binding protein SATB1

BACKGROUND

Replication timing of metazoan DNA during S-phase may be determined by many factors including chromosome structures, nuclear positioning, patterns of histone modifications, and transcriptional activity. It may be determined by Mb-domain structures, termed as "replication domains", and recent findings indicate that replication timing is under developmental and cell type-specific regulation.

METHODOLOGY/PRINCIPAL FINDINGS

We examined replication timing on the human 5q23/31 3.5-Mb segment in T cells and non-T cells. We used two independent methods to determine replication timing. One is quantification of nascent replicating DNA in cell cycle-fractionated stage-specific S phase populations. The other is FISH analyses of replication foci. Although the locations of early- and late-replicating domains were common between the two cell lines, the timing transition region (TTR) between early and late domains were offset by 200-kb. We show that Special AT-rich sequence Binding protein 1 (SATB1), specifically expressed in T-cells, binds to the early domain immediately adjacent to TTR and delays the replication timing of the TTR. Measurement of the chromosome copy number along the TTR during synchronized S phase suggests that the fork movement may be slowed down by SATB1.

CONCLUSIONS

Our results reveal a novel role of SATB1 in cell type-specific regulation of replication timing along the chromosome.

Methylation of DNA in cancer

Epigenetic mechanisms are essential for normal development and maintenance of tissue-specific gene expression patterns in mammals. Disruption of epigenetic processes can lead to altered gene function and malignant cellular transformation. Global changes in the epigenetic landscape are a hallmark of cancer. Methylation of cytosine bases in DNA provides a layer of epigenetic control in many eukaryotes that has important implications for normal biology and disease. DNA methylation is a crucial epigenetic modification of the genome that is involved in regulating many cellular processes. These include embryonic development, transcription, chromatin structure, X-chromosome inactivation, genomic imprinting, and chromosome stability. Consistent with these important roles, a growing number of human diseases including cancer have been found to be associated with aberrant DNA methylation. Recent advancements in the rapidly evolving field of cancer epigenetics have described extensive reprogramming of every component of the epigenetic machinery in cancer, such as DNA demethylation. Hypomethylation of the genome largely affects the intergenic and intronic regions of the DNA, particularly repeat sequences and transposable elements, and it is believed to result in chromosomal instability and increased mutation events. Therefore, we propose that R/G-chromosome band boundaries, which correspond with the early/late-switch regions of replication timing and a transition in relative GC content, correspond to "unstable" genomic regions in which concentrated occurrences of repetitive sequences and transposable elements including LINE and Alu elements are hypomethylated during tumorigenesis. In this review, we discuss the current understanding of alterations in DNA methylation composing the epigenetic landscape that occurs in cancer compared with normal cells, the roles of these changes in cancer initiation and progression, and the potential use of this knowledge in designing more effective treatment strategies.

Prediction of replication time zones at single nucleotide resolution in the human genome

The human genome is structured at multiple levels: it is organized into a series of replication time zones, and meanwhile it is composed of isochores. Accumulating evidence suggests a match between these two genome features. Based on newly developed software GC-Profile, we obtained a complete coverage of the human genome by 3198 isochores with boundaries at single nucleotide resolution. Interestingly, the experimentally confirmed replication timing sites in the regions of 1p36.1, 6p21.32, 17q11.2 and 22q12.1 nearly all coincide with the determined isochore boundaries. The precise boundaries of the 3198 isochores are available via the website: http://tubic.tju.edu.cn/isomap/.

Genome-wide sequence and functional analysis of early replicating DNA in normal human fibroblasts

BACKGROUND

The replication of mammalian genomic DNA during the S phase is a highly coordinated process that occurs in a programmed manner. Recent studies have begun to elucidate the pattern of replication timing on a genomic scale. Using a combination of experimental and computational techniques, we identified a genome-wide set of the earliest replicating sequences. This was accomplished by first creating a cosmid library containing DNA enriched in sequences that replicate early in the S phase of normal human fibroblasts. Clone ends were then sequenced and aligned to the human genome.

RESULTS

By clustering adjacent or overlapping early replicating clones, we identified 1759 "islands" averaging 100 kb in length, allowing us to perform the most detailed analysis to date of DNA characteristics and genes contained within early replicating DNA. Islands are enriched in open chromatin, transcription related elements, and Alu repetitive elements, with an underrepresentation of LINE elements. In addition, we found a paucity of LTR retroposons, DNA transposon sequences, and an enrichment in all classes of tandem repeats, except for dinucleotides.

CONCLUSION

An analysis of genes associated with islands revealed that nearly half of all genes in the WNT family, and a number of genes in the base excision repair pathway, including four of ten DNA glycosylases, were associated with island sequences. Also, we found an overrepresentation of members of apoptosis-associated genes in very early replicating sequences from both fibroblast and lymphoblastoid cells. These data suggest that there is a temporal pattern of replication for some functionally related genes.

Chromosome organization and chromatin modification: influence on genome function and evolution

Histone modifications of nucleosomes distinguish euchromatic from heterochromatic chromatin states, distinguish gene regulation in eukaryotes from that of prokaryotes, and appear to allow eukaryotes to focus recombination events on regions of highest gene concentrations. Four additional epigenetic mechanisms that regulate commitment of cell lineages to their differentiated states are involved in the inheritance of differentiated states, e.g., DNA methylation, RNA interference, gene repositioning between interphase compartments, and gene replication time. The number of additional mechanisms used increases with the taxon's somatic complexity. The ability of siRNA transcribed from one locus to target, in trans, RNAi-associated nucleation of heterochromatin in distal, but complementary, loci seems central to orchestration of chromatin states along chromosomes. Most genes are inactive when heterochromatic. However, genes within beta-heterochromatin actually require the heterochromatic state for their activity, a property that uniquely positions such genes as sources of siRNA to target heterochromatinization of both the source locus and distal loci. Vertebrate chromosomes are organized into permanent structures that, during S-phase, regulate simultaneous firing of replicon clusters. The late replicating clusters, seen as G-bands during metaphase and as meiotic chromomeres during meiosis, epitomize an ontological utilization of all five self-reinforcing epigenetic mechanisms to regulate the reversible chromatin state called facultative (conditional) heterochromatin. Alternating euchromatin/heterochromatin domains separated by band boundaries, and interphase repositioning of G-band genes during ontological commitment can impose constraints on both meiotic interactions and mammalian karyotype evolution.

Amplicons on human chromosome 11q are located in the early/late-switch regions of replication timing

Amplicons are frequently found in human tumor genomes, but the mechanism of their generation is still poorly understood. We previously measured the replication timing of the genes along the entire length of human chromosomes 11q and 21q and found that many "disease-related" genes are located in timing-transition regions. In this study, further scrutiny of the updated replication-timing map of human chromosome 11q revealed that both amplicons on human chromosomal bands 11q13 and 11q22 are located in the early/late-switch regions of replication timing in two human cell lines (THP-1 and Jurkat). Moreover, examination of synteny in the human and mouse genomes revealed that synteny breakage in both genomes occurred primarily at the early/late-switch regions of replication timing that we had identified. In conclusion, we found that the early/late-switch regions of replication timing coincided with "unstable" regions of the genome.

Transitions in replication timing in a 340 kb region of human chromosomal R-Band 1p36.1

DNA replication is initiated within a few chromosomal bands as normal human fibroblasts enter the S phase. In the present study, we determined the timing of replication of sequences along a 340 kb region in one of these bands, 1p36.13, an R band on chromosome 1. Within this region, we identified a segment of DNA (approximately 140 kb) that is replicated in the first hour of the S phase and is flanked by segments replicated 1-2 h later. Using a quantitative PCR-based assay to measure sequence abundance in size-fractionated (900-1,700 nt) nascent DNA, we mapped two functional origins of replication separated by 54 kb and firing 1 h apart. One origin was found to be functional during the first hour of S and was located within a CpG island associated with a predicted gene of unknown function (Genscan NT_004610.2). The second origin was activated in the second hour of S and was mapped to a CpG island near the promoter of the aldehyde dehydrogenase 4A1 (ALDH4A1) gene. At the opposite end of the early replicating segment, a more gradual change in replication timing was observed within the span of approximately 100 kb. These data suggest that DNA replication in adjacent segments of band 1p36.13 is organized differently, perhaps in terms of replicon number and length, or rate of fork progression. In the transition areas that mark the boundaries between different temporal domains, the replication forks initiated in the early replicated region are likely to pause or delay progression before replication of the 340 kb contig is completed.

Structural heterogeneity of pyrimidine/purine-biased DNA sequence analyzed by atomic force microscopy

We report here the direct evidence for the formation of alternative DNA structures in a plasmid DNA, termed pTIR10, containing a 0.23-kb pyrimidine/purine-biased (Pyr/Pur) stretch isolated from the rat genome. Long Pyr/Pur sequences are abundant in eukaryotic genomes, and they may modulate the biological activity of genes and genomes via formation of various types of triplex-related structures. The plasmid DNA in sodium acetate buffer (pH 4.35) was deposited on APS-modified mica, and after drying it was imaged with an atomic force microscope in air. Various types of thick protrusions have been observed on pTIR10 DNA. Structural parameters (width and height) of DNA molecules suggest that the alternative structures observed here are variations on the theme of an intramolecular triplex. The biological relevance of the structural features within Pyr/Pur stretches is discussed.

Functional complementation of a genetic deficiency with human artificial chromosomes

We have shown functional complementation of a genetic deficiency in human cultured cells, using artificial chromosomes derived from cloned human genomic fragments. A 404-kb human-artificial-chromosome (HAC) vector, consisting of 220 kb of alphoid DNA from the centromere of chromosome 17, human telomeres, and the hypoxanthine guanine phosphoribosyltransferase (HPRT) genomic locus, was transferred to HPRT-deficient HT1080 fibrosarcoma cells. We generated several cell lines with low-copy-number, megabase-sized HACs containing a functional centromere and one or possibly several copies of the HPRT1 gene complementing the metabolic deficiency. The HACs consisted of alternating alphoid and nonalphoid DNA segments derived only from the input DNA (within the sensitivity limits of FISH detection), and the largest continuous alphoid segment was 158-250 kb. The study of both the structure and mitotic stability of these HACs offers insights into the mechanisms of centromere formation in synthetic chromosomes and will further the development of this human-gene-transfer technology.

X-chromosome inactivation: counting, choice and initiation

In many sexually dimorphic species, a mechanism is required to ensure equivalent levels of gene expression from the sex chromosomes. In mammals, such dosage compensation is achieved by X-chromosome inactivation, a process that presents a unique medley of biological puzzles: how to silence one but not the other X chromosome in the same nucleus; how to count the number of X's and keep only one active; how to choose which X chromosome is inactivated; and how to establish this silent state rapidly and efficiently during early development. The key to most of these puzzles lies in a unique locus, the X-inactivation centre and a remarkable RNA--Xist--that it encodes.