A 3D Map of the Human Genome at Kilobase Resolution Reveals Principles of Chromatin Looping

Organisms require an appropriate balance of stability and reversibility in gene expression programmes to maintain cell identity or to enable responses to stimuli; epigenetic regulation is integral to this dynamic control. Post-translational modification of histones by methylation is an important and widespread type of chromatin modification that is known to influence biological processes in the context of development and cellular responses. To evaluate how histone methylation contributes to stable or reversible control, we provide a broad overview of how histone methylation is regulated and leads to biological outcomes. The importance of appropriately maintaining or reprogramming histone methylation is illustrated by its links to disease and ageing and possibly to transmission of traits across generations.

/pdf/histone-methylation-a-dynamic-mark-in-health-disease-and-53e9mgdnmh.pdf

Histone methylation: a dynamic mark in health, disease and inheritance

Higher-order chromatin structure is emerging as an important regulator of gene expression. Although dynamic chromatin structures have been identified in the genome, the full scope of chromatin dynamics during mammalian development and lineage specification remains to be determined. By mapping genome-wide chromatin interactions in human embryonic stem (ES) cells and four human ES-cell-derived lineages, we uncover extensive chromatin reorganization during lineage specification. We observe that although self-associating chromatin domains are stable during differentiation, chromatin interactions both within and between domains change in a striking manner, altering 36% of active and inactive chromosomal compartments throughout the genome. By integrating chromatin interaction maps with haplotype-resolved epigenome and transcriptome data sets, we find widespread allelic bias in gene expression correlated with allele-biased chromatin states of linked promoters and distal enhancers. Our results therefore provide a global view of chromatin dynamics and a resource for studying long-range control of gene expression in distinct human cell lineages.

/pdf/chromatin-architecture-reorganization-during-stem-cell-1ydmwhf4h4.pdf

Chromatin architecture reorganization during stem cell differentiation

Large-scale chromosome structure and spatial nuclear arrangement have been linked to control of gene expression and DNA replication and repair Genomic techniques based on chromosome conformation capture (3C) assess contacts for millions of loci simultaneously, but do so by averaging chromosome conformations from millions of nuclei Here we introduce single-cell Hi-C, combined with genome-wide statistical analysis and structural modelling of single-copy X chromosomes, to show that individual chromosomes maintain domain organization at the megabase scale, but show variable cell-to-cell chromosome structures at larger scales Despite this structural stochasticity, localization of active gene domains to boundaries of chromosome territories is a hallmark of chromosomal conformation Single-cell Hi-C data bridge current gaps between genomics and microscopy studies of chromosomes, demonstrating how modular organization underlies dynamic chromosome structure, and how this structure is probabilistically linked with genome activity patterns

Single-cell Hi-C reveals cell-to-cell variability in chromosome structure

Cells are faced with the task of folding thousands of different polypeptides into a wide range of conformations. For many proteins, the folding process requires the action of molecular chaperones. In the cytosol of prokaryotic and eukaryotic cells, molecular chaperones of different structural classes form a network of pathways that can handle substrate polypeptides from the point of initial synthesis on ribosomes to the final stages of folding.

Pathways of chaperone-mediated protein folding in the cytosol

https://www.research.ed.ac.uk/portal/files/10739509/Chromatin_motion_is_constrained_by_association_with_nuclear_compartments_in_human_cells.pdf

Chromatin motion is constrained by association with nuclear compartments in human cells.

The spatial organisation of the genome in the nucleus has a role in the regulation of gene expression. In vertebrates, chromosomal regions with low gene-density are located close to the nuclear periphery. Correlations have also been made between the transcriptional state of some genes and their location near the nuclear periphery. However, a crucial issue is whether this level of nuclear organisation directly affects gene function, rather than merely reflecting it. To directly investigate whether proximity to the nuclear periphery can influence gene expression in mammalian cells, here we relocate specific human chromosomes to the nuclear periphery by tethering them to a protein of the inner nuclear membrane. We show that this can reversibly suppress the expression of some endogenous human genes located near the tethering sites, and even genes further away. However, the expression of many other genes is not detectably reduced and we show that location at the nuclear periphery is not incompatible with active transcription. The dampening of gene expression around the nuclear periphery is dependent on the activity of histone deacetylases. Our data show that the radial position within the nucleus can influence the expression of some, but not all, genes. This is compatible with the suggestion that re-localisation of genes relative to the peripheral zone of the nucleus could be used by metazoans to modulate the expression of selected genes during development and differentiation.

/pdf/recruitment-to-the-nuclear-periphery-can-alter-expression-of-1bpghmpzph.pdf

Recruitment to the nuclear periphery can alter expression of genes in human cells.

https://www.cell.com/current-biology/pdf/S0960-9822(10)00054-0.pdf

Methylation of H3K4 Is Required for Inheritance of Active Transcriptional States

Bursts and pulses: insights from single cell studies into transcriptional mechanisms.

Despite the distinctive structure of mitotic chromosomes, it has not been possible to visualise individual chromosomes in living interphase cells, where chromosomes spend over 90% of their time. Studies of interphase chromosome structure and dynamics use fluorescence in-situ hybridisation (FISH) on fixed cells, which potentially damages structure and loses dynamic information. We have developed a new methodology, involving photoactivation of labelled histone H3 at mitosis, to visualise individual and specific human chromosomes in living interphase cells. Our data revealed bulk chromosome volume and morphology are established rapidly after mitosis, changing only incrementally after the first hour of G1. This contrasted with the behaviour of specific loci on labelled chromosomes, which showed more progressive reorganisation, and revealed that “looping out” of chromatin from chromosome territories is a dynamic state. We measured considerable heterogeneity in chromosome decondensation, even between sister chromatids, which may reflect local structural impediments to decondensation and could potentially amplify transcriptional noise. Chromosome structure showed tremendous resistance to inhibitors of transcription, histone deacetylation and chromatin remodelling. Together, these data indicate steric constraints determine structure, rather than innate chromosome architecture or function-driven anchoring, with interphase chromatin organisation governed primarily by opposition between needs for decondensation and the space available for this to happen.

/pdf/stable-morphology-but-dynamic-internal-reorganisation-of-3yabla7up5.pdf

Jonathan R. Chubb

Papers

Chromatin motion is constrained by association with nuclear compartments in human cells.

Recruitment to the nuclear periphery can alter expression of genes in human cells.

Methylation of H3K4 Is Required for Inheritance of Active Transcriptional States

Bursts and pulses: insights from single cell studies into transcriptional mechanisms.

Stable Morphology, but Dynamic Internal Reorganisation, of Interphase Human Chromosomes in Living Cells