Investigating the three-dimensional architecture of genomes by polymer physics

In mammalian cell nuclei, chromatin has a spatial organization that is strictly related to cellular biological functions, such as regulation of gene transcription and expression. However, still today, chromatin structure is currently poorly understood despite being subjected to intense investigation. Recent findings have revealed that chromatin has a complex, hierarchical organization spanning from the sub-Mb scale up to the entire chromosome length. To shed light on this intricate pattern of interactions revealed by experimental data, polymer physics models have been introduced. In this work, we focused on the “String&Binders Switch” (SBS) model, where non-random chromatin conformations are established through specific interaction of chromatin with diffusing DNA-binding molecules, driving folding by formation of loops. The SBS model recapitulates several features of chromatin organization, such as the large-scale average behavior of experimental data, the mechanisms underlying the self-assembly of TADs and the hierarchical organization of genome, as emerging from experimental data. Moreover, by the SBS model, it is possible to reconstruct the 3D architecture of real genomic regions with high accuracy, without any a-priori knowledge of the molecular factors responsible for chromatin folding. Importantly, our polymer models are able to predict the effects of structural variants in the genomic sequence on the 3D architecture, with a very good accuracy. In this scenario, our polymer modeling methods emerge as a powerful approach to predict pathogenic effects, facilitating the interpretation and diagnosis of this type of genomic rearrangements

Polymer models of the hierarchical folding of the Hox-B chromosomal locus

As revealed by novel technologies, chromosomes in the nucleus of mammalian cells have a complex spatial organization that serves vital functional purposes. Here we use models from polymer physics to identify the mechanisms that control their three-dimensional spatial organization. In particular, we investigate a model of the Hox-B locus, an important genomic region involved in embryo development, to expose the principles regulating chromatin folding and its complex behaviors in mouse embryonic stem cells. We reconstruct with high accuracy the pairwise contact matrix of the Hox-B locus as derived by Hi-C experiments and investigate its hierarchical folding dynamics. We trace back the observed behaviors to general scaling properties of polymer physics

The scaling features of the 3D organization of chromosomes are highlighted by a transformation a la Kadanoff of Hi-C data

Technologies such as Hi-C and GAM have revealed that chromosomes are not randomly folded into the nucleus of cells, but are composed by a sequence of contact domains (TADs), each typically 0.5 Mb long. However, the larger scale organization of the genome remains still not well understood. To investigate the scaling behaviour of chromosome folding, here we apply an approach à la Kadanoff, inspired by the Renormalization Group theory, to Hi-C interaction data, across different cell types and chromosomes. We find that the genome is characterized by complex scaling features, where the average size of contact domains exhibits a power-law behaviour with the rescaling level. That is compatible with the existence of contact domains extending across length scales up to chromosomal sizes. The scaling exponent is statistically indistinguishable among the different murine cell types analysed. These results point toward a scenario of a universal higher-order spatial architecture of the genome, which could reflect fundamental, organizational principles

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

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

A Polymer Physics Investigation of the Architecture of the Murine Orthologue of the 7q11.23 Human Locus

In the last decade, the developments of novel technologies, such as Hi-C or GAM methods, allowed to discover that chromosomes in the nucleus of mammalian cells have a complex spatial organization, encompassing the functional contacts between genes and regulators. In this work, we review recent progresses in chromosome modeling based on polymer physics to understand chromatin structure and folding mechanisms. As an example, we derive in mouse embryonic stem cells the full 3D structure of the Bmp7 locus, a genomic region that plays a key role in osteoblastic differentiation. Next, as an application to Neuroscience, we present the first 3D model for the mouse orthologoue of the Williams–Beuren syndrome 7q11.23 human locus. Deletions and duplications of the 7q11.23 region generate neurodevelopmental disorders with multi-system involvement and variable expressivity, and with autism. Understanding the impact of such mutations on the rewiring of the interactions of genes and regulators could be a new key to make sense of their related diseases, with potential applications in biomedicine

A polymer physics investigation of the architecture of the murine orthologue of the 7q11.23 human locus

In the last decade, the developments of novel technologies, such as Hi-C or GAM methods, allowed to discover that chromosomes in the nucleus of mammalian cells have a complex spatial organization, encompassing the functional contacts between genes and regulators. In this work, we review recent progresses in chromosome modeling based on polymer physics to understand chromatin structure and folding mechanisms. As an example, we derive in mouse embryonic stem cells the full 3D structure of the Bmp7 locus, a genomic region that plays a key role in osteoblastic differentiation. Next, as an application to Neuroscience, we present the first 3D model for the mouse orthologoue of the Williams-Beuren syndrome 7q11.23 human locus. Deletions and duplications of the 7q11.23 region generate neurodevelopmental disorders with multi-system involvement and variable expressivity, and with autism. Understanding the impact of such mutations on the rewiring of the interactions of genes and regulators could be a new key to make sense of their related diseases, with potential applications in biomedicine