Extensive Heterogeneity and Intrinsic Variation in Spatial Genome Organization.

Several general principles of global 3D genome organization have recently been established, including non-random positioning of chromosomes and genes in the cell nucleus, distinct chromatin compartments, and topologically associating domains (TADs). However, the extent and nature of cell-to-cell and cell-intrinsic variability in genome architecture are still poorly characterized. Here, we systematically probe heterogeneity in genome organization. High-throughput optical mapping of several hundred intra-chromosomal interactions in individual human fibroblasts demonstrates low association frequencies, which are determined by genomic distance, higher-order chromatin architecture, and chromatin environment. The structure of TADs is variable between individual cells, and inter-TAD associations are common. Furthermore, single-cell analysis reveals independent behavior of individual alleles in single nuclei. Our observations reveal extensive variability and heterogeneity in genome organization at the level of individual alleles and demonstrate the coexistence of a broad spectrum of genome configurations in a cell population.

Targeted Degradation of CTCF Decouples Local Insulation of Chromosome Domains from Genomic Compartmentalization.

The molecular mechanisms underlying folding of mammalian chromosomes remain poorly understood. The transcription factor CTCF is a candidate regulator of chromosomal structure. Using the auxin-inducible degron system in mouse embryonic stem cells, we show that CTCF is absolutely and dose-dependently required for looping between CTCF target sites and insulation of topologically associating domains (TADs). Restoring CTCF reinstates proper architecture on altered chromosomes, indicating a powerful instructive function for CTCF in chromatin folding. CTCF remains essential for TAD organization in non-dividing cells. Surprisingly, active and inactive genome compartments remain properly segregated upon CTCF depletion, revealing that compartmentalization of mammalian chromosomes emerges independently of proper insulation of TADs. Furthermore, our data support that CTCF mediates transcriptional insulator function through enhancer blocking but not as a direct barrier to heterochromatin spreading. Beyond defining the functions of CTCF in chromosome folding, these results provide new fundamental insights into the rules governing mammalian genome organization.

Clustering of strong replicators associated with active promoters is sufficient to establish an early-replicating domain.

Vertebrate genomes replicate according to a precise temporal program strongly correlated with their organization into A/B compartments. Until now, the molecular mechanisms underlying the establishment of early-replicating domains remain largely unknown. We defined two minimal cis-element modules containing a strong replication origin and chromatin modifier binding sites capable of shifting a targeted mid-late-replicating region for earlier replication. The two origins overlap with a constitutive or a silent tissue-specific promoter. When inserted side-by-side, these modules advance replication timing over a 250 kb region through the cooperation with one endogenous origin located 30 kb away. Moreover, when inserted at two chromosomal sites separated by 30 kb, these two modules come into close physical proximity and form an early-replicating domain establishing more contacts with active A compartments. The synergy depends on the presence of the active promoter/origin. Our results show that clustering of strong origins located at active promoters can establish early-replicating domains.

Promoters are key organizers of the duplication of vertebrate genomes.

In vertebrates, single cell analyses of replication timing patterns brought to light a very well controlled program suggesting a tight regulation on initiation sites. Mapping of replication origins with different methods has revealed discrete preferential sites, enriched in promoters and potential G-quadruplex motifs, which can aggregate into initiation zones spanning several tens of kilobases (kb). Another characteristic of replication origins is a nucleosome-free region (NFR). A modified yeast strain containing a humanized origin recognition complex (ORC) fires new origins at NFRs revealing their regulatory role. In cooperation with NFRs, the histone variant H2A.Z facilitates ORC loading through di-methylation of lysine 20 of histone H4. Recent studies using genome editing methods show that efficient initiation sites associated with transcriptional activity can synergize over several tens of kb by establishing physical contacts and lead to the formation of early domains of DNA replication demonstrating a co-regulation between replication initiation and transcription.

G-Quadruplexes in DNA Replication: A Problem or a Necessity?

DNA replication is a highly regulated process that ensures the correct duplication of the genome at each cell cycle. A precise cell type-specific temporal program controls the duplication of complex vertebrate genomes in an orderly manner. This program is based on the regulation of both replication origin firing and replication fork progression. G-quadruplexes (G4s), DNA secondary structures displaying noncanonical Watson-Crick base pairing, have recently emerged as key controllers of genome duplication. Here we discuss the various means by which G4s affect this fundamental cellular process.

G4 motifs affect origin positioning and efficiency in two vertebrate replicators.

DNA replication ensures the accurate duplication of the genome at each cell cycle. It begins at specific sites called replication origins. Genome-wide studies in vertebrates have recently identified a consensus G-rich motif potentially able to form G-quadruplexes (G4) in most replication origins. However, there is no experimental evidence to demonstrate that G4 are actually required for replication initiation. We show here, with two model origins, that G4 motifs are required for replication initiation. Two G4 motifs cooperate in one of our model origins. The other contains only one critical G4, and its orientation determines the precise position of the replication start site. Point mutations affecting the stability of this G4 in vitro also impair origin function. Finally, this G4 is not sufficient for origin activity and must cooperate with a 200-bp cis-regulatory element. In conclusion, our study strongly supports the predicted essential role of G4 in replication initiation.

Dimeric G-quadruplex motifs-induced NFRs determine strong replication origins in vertebrates.

Replication of vertebrate genomes is tightly regulated to ensure accurate duplication, but our understanding of the interplay between genetic and epigenetic factors in this regulation remains incomplete. Here, we investigated the involvement of three elements enriched at gene promoters and replication origins: guanine-rich motifs potentially forming G-quadruplexes (pG4s), nucleosome-free regions (NFRs), and the histone variant H2A.Z, in the firing of origins of replication in vertebrates. We show that two pG4s on the same DNA strand (dimeric pG4s) are sufficient to induce the assembly of an efficient minimal replication origin without inducing transcription in avian DT40 cells. Dimeric pG4s in replication origins are associated with formation of an NFR next to precisely-positioned nucleosomes enriched in H2A.Z on this minimal origin and genome-wide. Thus, our data suggest that dimeric pG4s are important for the organization and duplication of vertebrate genomes. It supports the hypothesis that a nucleosome close to an NFR is a shared signal for the formation of replication origins in eukaryotes.

A cohesin traffic pattern genetically linked to gene regulation.

Cohesin-mediated loop extrusion has been shown to be blocked at specific cis-elements, including CTCF sites, producing patterns of loops and domain boundaries along chromosomes. Here we explore such cis-elements, and their role in gene regulation. We find that transcription termination sites of active genes form cohesin- and RNA polymerase II-dependent domain boundaries that do not accumulate cohesin. At these sites, cohesin is first stalled and then rapidly unloaded. Start sites of transcriptionally active genes form cohesin-bound boundaries, as shown before, but are cohesin-independent. Together with cohesin loading, possibly at enhancers, these sites create a pattern of cohesin traffic that guides enhancer-promoter interactions. Disrupting this traffic pattern, by removing CTCF, renders cells sensitive to knockout of genes involved in transcription initiation, such as the SAGA complexes, and RNA processing such DEAD/H-Box RNA helicases. Without CTCF, these factors are less efficiently recruited to active promoters.

Cohesin mutations alter DNA damage repair and chromatin structure and create therapeutic vulnerabilities in MDS/AML.

The cohesin complex plays an essential role in chromosome maintenance and transcriptional regulation. Recurrent somatic mutations in the cohesin complex are frequent genetic drivers in cancer, including myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). Here, using genetic dependency screens of stromal antigen 2-mutant (STAG2-mutant) AML, we identified DNA damage repair and replication as genetic dependencies in cohesin-mutant cells. We demonstrated increased levels of DNA damage and sensitivity of cohesin-mutant cells to poly(ADP-ribose) polymerase (PARP) inhibition. We developed a mouse model of MDS in which Stag2 mutations arose as clonal secondary lesions in the background of clonal hematopoiesis driven by tet methylcytosine dioxygenase 2 (Tet2) mutations and demonstrated selective depletion of cohesin-mutant cells with PARP inhibition in vivo. Finally, we demonstrated a shift from STAG2- to STAG1-containing cohesin complexes in cohesin-mutant cells, which was associated with longer DNA loop extrusion, more intermixing of chromatin compartments, and increased interaction with PARP and replication protein A complex. Our findings inform the biology and therapeutic opportunities for cohesin-mutant malignancies.

A chromosome folding intermediate at the condensin-to-cohesin transition during telophase.

Chromosome folding is modulated as cells progress through the cell cycle. During mitosis, condensins fold chromosomes into helical loop arrays. In interphase, the cohesin complex generates loops and topologically associating domains (TADs), while a separate process of compartmentalization drives segregation of active and inactive chromatin. We used synchronized cell cultures to determine how the mitotic chromosome conformation transforms into the interphase state. Using high-throughput chromosome conformation capture (Hi-C) analysis, chromatin binding assays and immunofluorescence, we show that, by telophase, condensin-mediated loops are lost and a transient folding intermediate is formed that is devoid of most loops. By cytokinesis, cohesin-mediated CTCF-CTCF loops and the positions of TADs emerge. Compartment boundaries are also established early, but long-range compartmentalization is a slow process and proceeds for hours after cells enter G1. Our results reveal the kinetics and order of events by which the interphase chromosome state is formed and identify telophase as a critical transition between condensin- and cohesin-driven chromosome folding.

[G-quadruplex: key controllers of human genome duplication]. / G-quadruplex : acteurs majeurs de la duplication du génome humain.

The correct duplication of the human genome is under the control of a spatiotemporal program that determines where and when replication forks start. This regulation thus mainly operates on replication start sites named replication origins. During the S-phase, about 50 000 origins fire in one human cell. However, the normal or perturbed progression of replication forks also strongly impacts on replication. Recently, several studies have put forward the role of a noncanonical DNA structure, the G-quadruplex, in the control of genome duplication. In this review, we describe the major impact of this structure on starting points and on the progression of replication forks.

TAD disruption as oncogenic driver.

Topologically Associating Domains (TADs) are conserved during evolution and play roles in guiding and constraining long-range regulation of gene expression. Disruption of TAD boundaries results in aberrant gene expression by exposing genes to inappropriate regulatory elements. Recent studies have shown that TAD disruption is often found in cancer cells and contributes to oncogenesis through two mechanisms. One mechanism locally disrupts domains by deleting or mutating a TAD boundary leading to fusion of the two adjacent TADs. The other mechanism involves genomic rearrangements that break up TADs and creates new ones without directly affecting TAD boundaries. Understanding the mechanisms by which TADs form and control long-range chromatin interactions will therefore not only provide insights into the mechanism of gene regulation in general, but will also reveal how genomic rearrangements and mutations in cancer genomes can lead to misregulation of oncogenes and tumor suppressors.

Activation of proto-oncogenes by disruption of chromosome neighborhoods.

Oncogenes are activated through well-known chromosomal alterations such as gene fusion, translocation, and focal amplification. In light of recent evidence that the control of key genes depends on chromosome structures called insulated neighborhoods, we investigated whether proto-oncogenes occur within these structures and whether oncogene activation can occur via disruption of insulated neighborhood boundaries in cancer cells. We mapped insulated neighborhoods in T cell acute lymphoblastic leukemia (T-ALL) and found that tumor cell genomes contain recurrent microdeletions that eliminate the boundary sites of insulated neighborhoods containing prominent T-ALL proto-oncogenes. Perturbation of such boundaries in nonmalignant cells was sufficient to activate proto-oncogenes. Mutations affecting chromosome neighborhood boundaries were found in many types of cancer. Thus, oncogene activation can occur via genetic alterations that disrupt insulated neighborhoods in malignant cells.