Tom Sexton

Three-Dimensional Folding and Functional Organization Principles of the Drosophila Genome

Chromosomes are the physical realization of genetic information and thus form the basis for its readout and propagation. Here we present a high-resolution chromosomal contact map derived from a modified genome-wide chromosome conformation capture approach applied to Drosophila embryonic nuclei. The data show that the entire genome is linearly partitioned into well-demarcated physical domains that overlap extensively with active and repressive epigenetic marks. Chromosomal contacts are hierarchically organized between domains. Global modeling of contact density and clustering of domains show that inactive domains are condensed and confined to their chromosomal territories, whereas active domains reach out of the territory to form remote intra- and interchromosomal contacts. Moreover, we systematically identify specific long-range intrachromosomal contacts between Polycomb-repressed domains. Together, these observations allow for quantitative prediction of the Drosophila chromosomal contact map, laying the foundation for detailed studies of chromosome structure and function in a genetically tractable system.

Preferential associations between co-regulated genes reveal a transcriptional interactome in erythroid cells

The discovery of interchromosomal interactions in higher eukaryotes points to a functional interplay between genome architecture and gene expression, challenging the view of transcription as a one-dimensional process. However, the extent of interchromosomal interactions and the underlying mechanisms are unknown. Here we present the first genome-wide analysis of transcriptional interactions using the mouse globin genes in erythroid tissues. Our results show that the active globin genes associate with hundreds of other transcribed genes, revealing extensive and preferential intra- and interchromosomal transcription interactomes. We show that the transcription factor Klf1 mediates preferential co-associations of Klf1-regulated genes at a limited number of specialized transcription factories. Our results establish a new gene expression paradigm, implying that active co-regulated genes and their regulatory factors cooperate to create specialized nuclear hot spots optimized for efficient and coordinated transcriptional control.

Cohesins form chromosomal cis-interactions at the developmentally regulated IFNG locus

Cohesin-mediated sister chromatid cohesion is essential for chromosome segregation and post-replicative DNA repair. In addition, evidence from model organisms and from human genetics suggests that cohesin is involved in the control of gene expression. This non-canonical role has recently been rationalized by the findings that mammalian cohesin complexes are recruited to a subset of DNase I hypersensitive sites and to conserved noncoding sequences by the DNA-binding protein CTCF. CTCF functions at insulators (which control interactions between enhancers and promoters) and at boundary elements (which demarcate regions of distinct chromatin structure), and cohesin contributes to its enhancer-blocking activity. The underlying mechanisms remain unknown, and the full spectrum of cohesin functions remains to be determined. Here we show that cohesin forms the topological and mechanistic basis for cell-type-specific long-range chromosomal interactions in cis at the developmentally regulated cytokine locus IFNG. Hence, the ability of cohesin to constrain chromosome topology is used not only for the purpose of sister chromatid cohesion, but also to dynamically define the spatial conformation of specific loci. This new aspect of cohesin function is probably important for normal development and disease.

Gene regulation through nuclear organization

The nucleus is a highly heterogeneous structure, containing various landmarks such as the nuclear envelope and regions of euchromatin or dense heterochromatin. At a morphological level, regions of the genome that are permissive or repressive to gene expression have been associated with these architectural features. However, gene position within the nucleus can be both a cause and a consequence of transcriptional regulation. New results indicate that the spatial distribution of genes within the nucleus contributes to transcriptional control. In some cases, position seems to ensure maximal expression of a gene. In others, it ensures a heritable state of repression or correlates with a developmentally determined program of tissue-specific gene expression. In this review, we highlight mechanistic links between gene position, repression and transcription. Recent findings suggest that architectural features have multiple functions that depend upon organization into dedicated subcompartments enriched for distinct enzymatic machinery.

The Role of Chromosome Domains in Shaping the Functional Genome

The genome must be highly compacted to fit within eukaryotic nuclei but must be accessible to the transcriptional machinery to allow appropriate expression of genes in different cell types and throughout developmental pathways. A growing body of work has shown that the genome, analogously to proteins, forms an ordered, hierarchical structure that closely correlates and may even be causally linked with regulation of functions such as transcription. This review describes our current understanding of how these functional genomic secondary and tertiary structures form a blueprint for global nuclear architecture and the potential they hold for understanding and manipulating genomic regulation.

Genomic interactions: Chromatin loops and gene meeting points in transcriptional regulation

The chromosome conformation capture (3C) technique and its genome-wide applications (4C) have identified a plethora of distal DNA sequences that are frequently in close spatial proximity. In many cases, these have been correlated with transcriptional regulation of the interacting genes, but the functional significance of many of the extreme long-range and interchromosomal interactions remains unclear. This review summarises our current understanding of how chromatin conformation can impinge on gene expression, the major questions that need to be addressed to understand this more fully, and how these questions may be answered in the near future.

A chromatin insulator driving three-dimensional Polycomb response element (PRE) contacts and Polycomb association with the chromatin fiber

Regulation of gene expression involves long-distance communication between regulatory elements and target promoters, but how this is achieved remains unknown. Insulator elements have been proposed to modulate the communication between regulatory elements and promoters due to their ability to insulate genes from regulatory elements or to take part in long-distance interactions. Using a high-resolution chromatin conformation capture (H3C) method, we show that the Drosophila gypsy insulator behaves as a conformational chromatin border that is able to prohibit contacts between a Polycomb response element (PRE) and a distal promoter. On the other hand, two spaced gypsy elements form a chromatin loop that is able to bring an upstream PRE in contact with a downstream gene to mediate its repression. Chromatin immunoprecipitation (ChIP) profiles of the Polycomb protein and its associated H3K27me3 histone mark reflect this insulator-dependent chromatin conformation, suggesting that Polycomb action at a distance can be organized by local chromatin topology.

Cooperativity, Specificity, and Evolutionary Stability of Polycomb Targeting in Drosophila

Metazoan genomes are partitioned into modular chromosomal domains containing active or repressive chromatin. In flies, Polycomb group (PcG) response elements (PREs) recruit PHO and other DNA-binding factors and act as nucleation sites for the formation of Polycomb repressive domains. The sequence specificity of PREs is not well understood. Here, we use comparative epigenomics and transgenic assays to show that Drosophila domain organization and PRE specification are evolutionarily conserved despite significant cis-element divergence within Polycomb domains, whereas cis-element evolution is strongly correlated with transcription factor binding divergence outside of Polycomb domains. Cooperative interactions of PcG complexes and their recruiting factor PHO stabilize PHO recruitment to low-specificity sequences. Consistently, PHO recruitment to sites within Polycomb domains is stabilized by PRC1. These data suggest that cooperative rather than hierarchical interactions among low-affinity sequences, DNA-binding factors, and the Polycomb machinery are giving rise to specific and strongly conserved 3D structures in Drosophila.

Polycomb: a paradigm for genome organization from one to three dimensions

Polycomb group proteins are important transcriptional repressors in developmental control, both stably silencing genes out of their appropriate lineage, and conferring dynamic regulation of genes whose expression changes in response to developmental cues. Polycomb is a key organizer of the linear epigenome, forming distinct chromatin domains of associated histone modifications, and fine-tuning the activities of genetic elements. Polycomb also modulates three-dimensional genome architecture by the formation of regulatory chromatin loops and coalescing target genes at discrete nuclear foci. Recent studies suggest that the linear epigenetic domains and chromosome architecture are intimately linked and the developmental plasticity of these scales of chromosome organization is beginning to be explored.

Sensitive detection of chromatin coassociations using enhanced chromosome conformation capture on chip

Chromosome conformation capture (3C) is a powerful technique for analyzing spatial chromatin organization in vivo. Technical variants of the assay (4C) allow the systematic detection of genome-wide coassociations with bait sequences of interest, enabling the nuclear environments of specific genes to be probed. We describe enhanced 4C (e4C, enhanced chromosome conformation capture on chip), a technique incorporating additional enrichment steps for bait-specific sequences, and thus improving sensitivity in the detection of weaker, distal chromatin coassociations. In brief, e4C entails the fixation, restriction digestion and ligation steps of conventional 3C, with an optional chromatin immunoprecipitation (ChIP) step to select for subsets of chromatin coassociations, followed by bait enrichment by biotinylated primer extension and pull-down, adapter ligation and PCR amplification. Chromatin coassociations with the bait sequence can then be assessed by hybridizing e4C products to microarrays or sequencing. The e4C procedure takes approximately 1 week to go from tissue to DNA ready for microarray hybridization.