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TADs emerge as a functionally, but not structurally privileged scale in the hierarchical folding of chromosomes

ABSTRACT: Understanding how regulatory sequences interact in the context of chromosomal architecture is a central challenge in biology. Chromosome conformation capture revealed that mammalian chromosomes possess a rich hierarchy of structural layers, from multi-megabase compartments to sub-megabase topologically associating domains (TADs), and further down to sub-TAD loop domains. TADs appear to act as regulatory microenvironments by constraining and segregating regulatory interactions across discrete chromosomal regions. However, it is unclear whether other (or all) folding layers share similar properties, or rather TADs constitute a privileged folding scale with maximal impact on the organization of regulatory interactions. Here we present a novel parameter-free algorithm (CaTCH) that identifies hierarchical trees of chromosomal domains in Hi-C maps, stratified through their reciprocal physical insulation which is a simple and biologically relevant property. By applying CaTCH to published Hi-C datasets, we show that previously reported folding layers appear at different insulation levels. We demonstrate that although no structurally privileged folding level exists, TADs emerge as a functionally privileged scale defined by maximal enrichment of CTCF at boundaries, and maximal cell-type conservation. By measuring transcriptional output in embryonic stem cells and neural precursor cells, we show that TADs also maximize the likelihood that genes in a domain are co-regulated during differentiation. Finally, we observe that regulatory sequences occur at genomic locations corresponding to optimized mutual interactions at the scale of TADs. Our analysis thus suggests that the architectural functionality of TADs arises from the interplay between their ability to partition interactions and the genomic position of regulatory sequences.

ORGANISM(S): Mus musculus

PROVIDER: GSE84724 | GEO | 2016/12/12

SECONDARY ACCESSION(S): PRJNA330948

REPOSITORIES: GEO

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Project description:The three-dimensional organization of chromosomes is tightly related to their biological function. Both imaging and chromosome conformation capture studies have revealed several layers of organization, including segregation into active and inactive compartments at the megabase scale, and partitioning into domains (TADs) and associated loops at the sub-megabase scale. Yet, it remains unclear how these layers of genome organization form, interact with one another, and contribute to or result from genome activities. TADs seem to have critical roles in regulating gene expression by promoting or preventing interactions between promoters and distant cis-acting regulatory elements, and different architectural proteins, including cohesin, have been proposed to play central roles in their formation. But so far, experimental depletions of these proteins have resulted in marginal changes in chromosome organization. Here, we show that deletion of the cohesin-loading factor, Nipbl, leads to loss of chromosome-associated cohesin and results in dramatic genome reorganization. TADs and associated loops vanish globally, even in the absence of transcriptional changes. In contrast, segregation into compartments is preserved and even reinforced. Strikingly, the disappearance of TADs unmasks a finer compartment structure that accurately reflects the underlying epigenetic landscape. These observations demonstrate that the 3D organization of the genome results from the independent action of two distinct mechanisms: 1) cohesin-independent segregation of the genome into fine-scale compartment regions, defined by the underlying chromatin state; and 2) cohesin-dependent formation of TADs possibly by the recently proposed loop extrusion mechanism, enabling long-range and target-specific activity of promiscuous enhancers. The interplay between these mechanisms creates an architecture that is more complex than a simple structural hierarchy and can be central to guiding normal development.

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TADs emerge as a functionally, but not structurally privileged scale in the hierarchical folding of chromosomes

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