Andrea M. Chiariello

Hierarchical folding and reorganization of chromosomes are linked to transcriptional changes in cellular differentiation

Mammalian chromosomes fold into arrays of megabase‐sized topologically associating domains (TADs), which are arranged into compartments spanning multiple megabases of genomic DNA. TADs have internal substructures that are often cell type specific, but their higher‐order organization remains elusive. Here, we investigate TAD higher‐order interactions with Hi‐C through neuronal differentiation and show that they form a hierarchy of domains‐within‐domains (metaTADs) extending across genomic scales up to the range of entire chromosomes. We find that TAD interactions are well captured by tree‐like, hierarchical structures irrespective of cell type. metaTAD tree structures correlate with genetic, epigenomic and expression features, and structural tree rearrangements during differentiation are linked to transcriptional state changes. Using polymer modelling, we demonstrate that hierarchical folding promotes efficient chromatin packaging without the loss of contact specificity, highlighting a role far beyond the simple need for packing efficiency.

Polymer physics of chromosome large-scale 3D organisation

Chromosomes have a complex architecture in the cell nucleus, which serves vital functional purposes, yet its structure and folding mechanisms remain still incompletely understood. Here we show that genome-wide chromatin architecture data, as mapped by Hi-C methods across mammalian cell types and chromosomes, are well described by classical scaling concepts of polymer physics, from the sub-Mb to chromosomal scales. Chromatin is a complex mixture of different regions, folded in the conformational classes predicted by polymer thermodynamics. The contact matrix of the Sox9 locus, a region linked to severe human congenital diseases, is derived with high accuracy in mESCs and its molecular determinants identified by the theory; Sox9 self-assembles hierarchically in higher-order domains, involving abundant many-body contacts. Our approach is also applied to the Bmp7 locus. Finally, the model predictions on the effects of mutations on folding are tested against available data on a deletion in the Xist locus. Our results can help progressing new diagnostic tools for diseases linked to chromatin misfolding.

Active and poised promoter states drive folding of the extended HoxB locus in mouse embryonic stem cells

Gene expression states influence the 3D conformation of the genome through poorly understood mechanisms. Here, we investigate the conformation of the murine HoxB locus, a gene-dense genomic region containing closely spaced genes with distinct activation states in mouse embryonic stem (ES) cells. To predict possible folding scenarios, we performed computer simulations of polymer models informed with different chromatin occupancy features that define promoter activation states or binding sites for the transcription factor CTCF. Single-cell imaging of the locus folding was performed to test model predictions. While CTCF occupancy alone fails to predict the in vivo folding at genomic length scale of 10 kb, we found that homotypic interactions between active and Polycomb-repressed promoters co-occurring in the same DNA fiber fully explain the HoxB folding patterns imaged in single cells. We identify state-dependent promoter interactions as major drivers of chromatin folding in gene-dense regions.

Predicting chromatin architecture from models of polymer physics

We review the picture of chromatin large-scale 3D organization emerging from the analysis of Hi-C data and polymer modeling. In higher mammals, Hi-C contact maps reveal a complex higher-order organization, extending from the sub-Mb to chromosomal scales, hierarchically folded in a structure of domains-within-domains (metaTADs). The domain folding hierarchy is partially conserved throughout differentiation, and deeply correlated to epigenomic features. Rearrangements in the metaTAD topology relate to gene expression modifications: in particular, in neuronal differentiation models, topologically associated domains (TADs) tend to have coherent expression changes within architecturally conserved metaTAD niches. To identify the nature of architectural domains and their molecular determinants within a principled approach, we discuss models based on polymer physics. We show that basic concepts of interacting polymer physics explain chromatin spatial organization across chromosomal scales and cell types. The 3D structure of genomic loci can be derived with high accuracy and its molecular determinants identified by crossing information with epigenomic databases. In particular, we illustrate the case of the Sox9 locus, linked to human congenital disorders. The model in-silico predictions on the effects of genomic rearrangements are confirmed by available 5C data. That can help establishing new diagnostic tools for diseases linked to chromatin mis-folding, such as congenital disorders and cancer.

Polymer physics predicts the effects of structural variants on chromatin architecture

Structural variants (SVs) can result in changes in gene expression due to abnormal chromatin folding and cause disease. However, the prediction of such effects remains a challenge. Here we present a polymer-physics-based approach (PRISMR) to model 3D chromatin folding and to predict enhancer–promoter contacts. PRISMR predicts higher-order chromatin structure from genome-wide chromosome conformation capture (Hi-C) data. Using the EPHA4 locus as a model, the effects of pathogenic SVs are predicted in silico and compared to Hi-C data generated from mouse limb buds and patient-derived fibroblasts. PRISMR deconvolves the folding complexity of the EPHA4 locus and identifies SV-induced ectopic contacts and alterations of 3D genome organization in homozygous or heterozygous states. We show that SVs can reconfigure topologically associating domains, thereby producing extensive rewiring of regulatory interactions and causing disease by gene misexpression. PRISMR can be used to predict interactions in silico, thereby providing a tool for analyzing the disease-causing potential of SVs.The authors present a polymer-physics-based approach (PRISMR) to model 3D chromatin folding and to predict enhancer–promoter contacts. PRISMR correctly predicts ectopic contacts induced by pathogenic SVs at the mouse Epha4 locus.

Polymer models of the organization of chromosomes in the nucleus of cells

Understanding the mechanisms that control the organization of chromosomes in the space of the nucleus of cells, and its contribution to gene regulation, is a key open issue in molecular biology. New technologies have shown that chromosomes have a complex 3D organization, which dynamically changes across organisms and cell types. To understand such complex behaviors, quantitative models from polymer physics have been developed, to find the principles of chromosome folding, their origin and function. Here, we provide a short review of recent progress in such an important research field where Physical and Life Sciences meet.

Polymer Physics of the Large-Scale Structure of Chromatin

We summarize the picture emerging from recently proposed models of polymer physics describing the general features of chromatin large scale spatial architecture, as revealed by microscopy and Hi-C experiments.

Dynamic 3D chromatin architecture contributes to enhancer specificity and limb morphogenesis

The regulatory specificity of enhancers and their interaction with gene promoters is thought to be controlled by their sequence and the binding of transcription factors. By studying Pitx1, a regulator of hindlimb development, we show that dynamic changes in chromatin conformation can restrict the activity of enhancers. Inconsistent with its hindlimb-restricted expression, Pitx1 is controlled by an enhancer (Pen) that shows activity in forelimbs and hindlimbs. By Capture Hi-C and three-dimensional modeling of the locus, we demonstrate that forelimbs and hindlimbs have fundamentally different chromatin configurations, whereby Pen and Pitx1 interact in hindlimbs and are physically separated in forelimbs. Structural variants can convert the inactive into the active conformation, thereby inducing Pitx1 misexpression in forelimbs, causing partial arm-to-leg transformation in mice and humans. Thus, tissue-specific three-dimensional chromatin conformation can contribute to enhancer activity and specificity in vivo and its disturbance can result in gene misexpression and disease.A Pitx1 enhancer shows activity in forelimbs and hindlimbs but only interacts with Pitx1 in hindlimbs because of its three-dimensional configuration. Structural variants that affect three-dimensional conformation induce Pitx1 expression in forelimbs and cause partial arm-to-leg transformation in mice and humans.

Single-allele chromatin interactions identify regulatory hubs in dynamic compartmentalized domains

The promoters of mammalian genes are commonly regulated by multiple distal enhancers, which physically interact within discrete chromatin domains. How such domains form and how the regulatory elements within them interact in single cells is not understood. To address this we developed Tri-C, a new chromosome conformation capture (3C) approach, to characterize concurrent chromatin interactions at individual alleles. Analysis by Tri-C identifies heterogeneous patterns of single-allele interactions between CTCF boundary elements, indicating that the formation of chromatin domains likely results from a dynamic process. Within these domains, we observe specific higher-order structures that involve simultaneous interactions between multiple enhancers and promoters. Such regulatory hubs provide a structural basis for understanding how multiple cis-regulatory elements act together to establish robust regulation of gene expression.Tri-C is a new 3C approach to identify concurrent chromatin interactions at individual alleles. The authors observe specific higher-order structures involving simultaneous interactions between multiple enhancers and promoters, called regulatory hubs.

Molecular Dynamics simulations of the Strings and Binders Switch model of chromatin

In recent years interest has grown on the applications of polymer physics to model chromatin folding in order to try to make sense of the complexity of experimental data emerging from new technologies such as Hi-C or GAM, in a principled way. Here we review the methods employed to efficiently implement Molecular Dynamics computer simulations of polymer models, focusing in particular on the String&Binders Switch (SBS) model. The constant improvement of such methods and computer power is returning increasingly more accurate insights on the structure and molecular mechanisms underlying the spatial organization of chromosomes in the cell nucleus. We aim to provide an account of the state of the art of computational techniques employed in this type of investigations and to review recent applications of such methods to the description of real genomic loci, such as the Sox9 locus in mESC.