Transcriptional Regulation: a Genomic Overview

The availability of the Arabidopsis thaliana genome sequence allows a comprehensive analysis of transcriptional regulation in plants using novel genomic approaches and methodologies. Such a genomic view of transcription first necessitates the compilation of lists of elements. Transcription factors are the most numerous of the different types of proteins involved in transcription in eukaryotes, and the Arabidopsis genome codes for more than 1,500 of them, or approximately 6% of its total number of genes. A genome-wide comparison of transcription factors across the three eukaryotic kingdoms reveals the evolutionary generation of diversity in the components of the regulatory machinery of transcription. However, as illustrated by Arabidopsis, transcription in plants follows similar basic principles and logic to those in animals and fungi. A global view and understanding of transcription at a cellular and organismal level requires the characterization of the Arabidopsis transcriptome and promoterome, as well as of the interactome, the localizome, and the phenome of the proteins involved in transcription

Coordinated actions of microRNAs with other epigenetic factors regulate skeletal muscle development and adaptation

Epigenetics plays a pivotal role in regulating gene expression in development, in response to cellular stress or in disease states, in virtually all cell types. MicroRNAs (miRNAs) are short, non-coding RNA molecules that mediate RNA silencing and regulate gene expression. miRNAs were discovered in 1993 and have been extensively studied ever since. They can be expressed in a tissue-specific manner and play a crucial role in tissue development and many biological processes. miRNAs are responsible for changes in the cell epigenome because of their ability to modulate gene expression post-transcriptionally. Recently, numerous studies have shown that miRNAs and other epigenetic factors can regulate each other or cooperate in regulating several biological processes. On the one hand, the expression of some miRNAs is silenced by DNA methylation, and histone modifications have been demonstrated to modulate miRNA expression in many cell types or disease states. On the other hand, miRNAs can directly target epigenetic factors, such as DNA methyltransferases or histone deacetylases, thus regulating chromatin structure. Moreover, several studies have reported coordinated actions between miRNAs and other epigenetic mechanisms to reinforce the regulation of gene expression. This paper reviews multiple interactions between miRNAs and epigenetic factors in skeletal muscle development and in response to stimuli or disease

The bromodomain-containing protein Ibd1 links multiple chromatin related protein complexes to highly expressed genes in Tetrahymena thermophila

Background: The chromatin remodelers of the SWI/SNF family are critical transcriptional regulators. Recognition of lysine acetylation through a bromodomain (BRD) component is key to SWI/SNF function; in most eukaryotes, this function is attributed to SNF2/Brg1. Results: Using affinity purification coupled to mass spectrometry (AP-MS) we identified members of a SWI/SNF complex (SWI/SNFTt) in Tetrahymena thermophila. SWI/SNFTt is composed of 11 proteins, Snf5Tt, Swi1Tt, Swi3Tt, Snf12Tt, Brg1Tt, two proteins with potential chromatin interacting domains and four proteins without orthologs to SWI/SNF proteins in yeast or mammals. SWI/SNFTt subunits localize exclusively to the transcriptionally active macronucleus (MAC) during growth and development, consistent with a role in transcription. While Tetrahymena Brg1 does not contain a BRD, our AP-MS results identified a BRD-containing SWI/SNFTt component, Ibd1 that associates with SWI/SNFTt during growth but not development. AP-MS analysis of epitope-tagged Ibd1 revealed it to be a subunit of several additional protein complexes, including putative SWRTt, and SAGATt complexes as well as a putative H3K4-specific histone methyl transferase complex. Recombinant Ibd1 recognizes acetyl-lysine marks on histones correlated with active transcription. Consistent with our AP-MS and histone array data suggesting a role in regulation of gene expression, ChIP-Seq analysis of Ibd1 indicated that it primarily binds near promoters and within gene bodies of highly expressed genes during growth. Conclusions: Our results suggest that through recognizing specific histones marks, Ibd1 targets active chromatin regions of highly expressed genes in Tetrahymena where it subsequently might coordinate the recruitment of several chromatin remodeling complexes to regulate the transcriptional landscape of vegetatively growing Tetrahymena cells.Comment: Published on BMC Epigenetics & Chromati

Gene regulation and epigenotype in Friedreich's ataxia

Friedreich??????s ataxia (FRDA) is known to be provoked by an abnormal GAA-repeat expansion located in the first intron of the FXN gene. As a result of the GAA expansion, patients exhibit low levels of FXN mRNA, leading to FRDA. Here, via chromatin immunoprecipitation (ChIP), the presence of a RNA pol II transcriptional pausing site at exon 1 of the FXN gene was demonstrated. At this site, FRDA EBVcell lines exhibited elevated levels of the negative elongation factor NELF-E depending on the presence of a GAA repeat expansion compared to controls. This site may represent a rate-limiting step for FXN transcription and consequently provide a means to modify transcription levels in FRDA. Moreover, RNA pol II pausing site binding factors, such as NELF-E, were influenced by Nicotinamide treatment, a HDAC class III inhibitor. Therefore, factors sensitive to chromatin changes may influence the regulation of RNA pol II pausing and also balance otherwise positive chromatin changes. This new finding could explain the relatively minor effects of different drug approaches to up-regulate this gene. Furthermore, CTCF and the histone demethylase LSD1 were also found to be located at the FXN pausing site. Results suggest a function for LSD1 in demethylating H3K4me2 at the pausing site and potentially also in demethylating H3K9me3 in the case of frequently transcribed expanded GAA repeats. Therefore, LSD1 might play a crucial role in preventing heterochromatinisation of a euchromatic gene. Using primary transcript RNA-FISH, a delay in RNA pol II release from the pausing site and furthermore a dramatic loss of RNA pol II elongation in the presence of expanded GAA repeats was seen. The identified and characterised transcriptional pausing site at FXN is likely to play a repressive role and participates in the pathogenesis of FRDA.Imperial Users onl

Identification and characterization of cis-regulatory elements that target Polycomb in the mouse genome

Multicellular organisms consist of numerous cell types, each serving a specific function. Remarkably, almost all cells within an organism contain the same genetic information. Nevertheless, each cell type interprets this information differently, resulting in cell type specific gene expression patterns. These expression patterns define cellular function and are acquired upon lineage commitment of a pluripotent cell. Once acquired, these patterns can be stably maintained throughout subsequent cell divisions. For example, upon differentiation of a stem cell pluripotency-associated genes need to be silenced, while lineage-specific genes have to be activated. The maintenance and propagation of these expression patterns is thought to be mediated at least in part via the posttranslational modification of chromatin components. These covalent modifications are deposited by specialized enzymes that modify specific histone. However, while many of the enzymes responsible for establishing these marks have been identified, how they are targeted to specific loci remains unclear. Polycomb-group (PcG) proteins represent key regulators of gene expression, especially during early development where they play key roles in the stable repression of developmental regulators. They form several complexes that mediate the modification of distinct histones. For example, the PRC2 complex mediates trimethylation of histone H3 at lysine 27 (H3K27me3), a chromatin mark essential for proper development of both flies and mammals. However, despite the importance of this modification, it remains elusive how H3K27me3 is targeted to specific loci. In Drosophila melanogaster, it has been demonstrated that transcription factors (TFs) play a major role in guiding PcG complexes to specific DNA elements, termed Polycomb responsive elements (PREs). Efforts to identify similar DNA elements in mammals have proven less successful, with only a handful of PREs known today. Furthermore, it is unclear whether the correlation between TF binding and PcG recruitment observed in D. melanogaster is indeed reflecting a direct physical interaction or rather an indirect crosstalk involving other factors. In this study, we aimed to investigate the mechanisms that facilitate PRC2 recruitment and deposition of its associated H3K27me3 mark in mammals. We hypothesized that recruitment of PcG complexes to specific loci is encoded within the target DNA sequence either in the form of TF binding sites or other sequence queues. To test this, we employed a reductionist approach and inserted a set of endogenous PRC2 targets in mouse embryonic stem (ES) cells into a defined ectopic locus. We then examined whether these ectopically inserted DNA sequences could recapitulate the H3K27me3 levels observed at endogenous loci. Indeed, all of the tested elements were able to reconstitute endogenous PRC2 and H3K27me3 patterns. Further dissection of these elements revealed that DNA sequences rich in CpG dinucleotides and as short as 220 bp are sufficient to establish an H3K27me3 domain. Furthermore, we found that cell-type specific recruitment is determined by the transcriptional state of the target locus. In particular, transcriptional activity regulated by TF binding to a proximal cis-regulatory element can efficiently block the acquisition of H3K27me3. Finally, by systematically mutating the identified recruiter elements we demonstrate that DNA methylation directly prevents the recruitment of H3K27me3 to the underlying DNA sequence. Taken together, we propose a model whereby PRC2 recruitment and H3K27me3 deposition defines a default chromatin signature at transcriptionally inactive and unmethylated genomic regions. Furthermore, we show that TFs are involved in the recruitment of PRC2 by controlling the transcriptional activity of the target locus. This study therefore provides novel insights into the relationship between different gene regulatory mechanisms and broadens our understanding of the crosstalk between TFs and epigenetic modifications

KRAB–Zinc Finger Proteins and KAP1 Can Mediate Long-Range Transcriptional Repression through Heterochromatin Spreading

Krüppel-associated box domain-zinc finger proteins (KRAB–ZFPs) are tetrapod-specific transcriptional repressors encoded in the hundreds by the human genome. In order to explore their as yet ill-defined impact on gene expression, we developed an ectopic repressor assay, allowing the study of KRAB–mediated transcriptional regulation at hundreds of different transcriptional units. By targeting a drug-controllable KRAB–containing repressor to gene-trapping lentiviral vectors, we demonstrate that KRAB and its corepressor KAP1 can silence promoters located several tens of kilobases (kb) away from their DNA binding sites, with an efficiency which is generally higher for promoters located within 15 kb or less. Silenced promoters exhibit a loss of histone H3-acetylation, an increase in H3 lysine 9 trimethylation (H3K9me3), and a drop in RNA Pol II recruitment, consistent with a block of transcriptional initiation following the establishment of silencing marks. Furthermore, we reveal that KRAB–mediated repression is established by the long-range spreading of H3K9me3 and heterochromatin protein 1 β (HP1β) between the repressor binding site and the promoter. We confirm the biological relevance of this phenomenon by documenting KAP1–dependent transcriptional repression at an endogenous KRAB–ZFP gene cluster, where KAP1 binds to the 3′ end of genes and mediates propagation of H3K9me3 and HP1β towards their 5′ end. Together, our data support a model in which KRAB/KAP1 recruitment induces long-range repression through the spread of heterochromatin. This finding not only suggests auto-regulatory mechanisms in the control of KRAB–ZFP gene clusters, but also provides important cues for interpreting future genome-wide DNA binding data of KRAB–ZFPs and KAP1

Studies on genetic and epigenetic regulation of gene expression dynamics

The information required to build an organism is contained in its genome and the first biochemical process that activates the genetic information stored in DNA is transcription. Cell type specific gene expression shapes cellular functional diversity and dysregulation of transcription is a central tenet of human disease. Therefore, understanding transcriptional regulation is central to understanding biology in health and disease. Transcription is a dynamic process, occurring in discrete bursts of activity that can be characterized by two kinetic parameters; burst frequency describing how often genes burst and burst size describing how many transcripts are generated in each burst. Genes are under strict regulatory control by distinct sequences in the genome as well as epigenetic modifications. To properly study how genetic and epigenetic factors affect transcription, it needs to be treated as the dynamic cellular process it is. In this thesis, I present the development of methods that allow identification of newly induced gene expression over short timescales, as well as inference of kinetic parameters describing how frequently genes burst and how many transcripts each burst give rise to. The work is presented through four papers: In paper I, I describe the development of a novel method for profiling newly transcribed RNA molecules. We use this method to show that therapeutic compounds affecting different epigenetic enzymes elicit distinct, compound specific responses mediated by different sets of transcription factors already after one hour of treatment that can only be detected when measuring newly transcribed RNA. The goal of paper II is to determine how genetic variation shapes transcriptional bursting. To this end, we infer transcriptome-wide burst kinetics parameters from genetically distinct donors and find variation that selectively affects burst sizes and frequencies. Paper III describes a method for inferring transcriptional kinetics transcriptome-wide using single-cell RNA-sequencing. We use this method to describe how the regulation of transcriptional bursting is encoded in the genome. Our findings show that gene specific burst sizes are dependent on core promoter architecture and that enhancers affect burst frequencies. Furthermore, cell type specific differential gene expression is regulated by cell type specific burst frequencies. Lastly, Paper IV shows how transcription shapes cell types. We collect data on cellular morphologies, electrophysiological characteristics, and measure gene expression in the same neurons collected from the mouse motor cortex. Our findings show that cells belonging to the same, distinct transcriptomic families have distinct and non-overlapping morpho-electric characteristics. Within families, there is continuous and correlated variation in all modalities, challenging the notion of cell types as discrete entities