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Key Points Enhancers are genetic elements that have major roles in determining cell type-specific gene expression patterns and responses to internal and external signals. Mammalian genomes contain millions of enhancers, but only a subset of them are selected and activated in each cell type in the body. Enhancer selection involves collaborative interactions between 'pioneer' or lineage-determining transcription factors. Such factors are able to prime enhancers in a cell type-specific manner by binding to closely spaced recognition motifs. Signal-dependent transcription factors, such as nuclear receptors and nuclear factor-κB (NF-κB), primarily bind to regions of the genome that are primed in a cell type-specific manner by lineage-determining transcription factors. This enables such broadly expressed, signal-dependent transcription factors to regulate gene expression in a cell type-specific manner. A proportion of the genome contains a very high density of marks of active enhancers. Such enhancer-rich genomic regions, which are called super-enhancers, are different in each cell type and regulate genes required for establishing cell identity and function. An understanding of the mechanisms underlying the cell type-specific selection and function of enhancers will improve our understanding of the effects of natural genetic variation on complex phenotypes and diseases. Many gene expression patterns are dictated by enhancers. Mammalian genomes contain millions of potential enhancers, but only a small subset of them is active in any cell type. Emerging data uncover how cell type-specific enhancer function is established, including the involvement of higher-order genomic organization in the process. The human body contains several hundred cell types, all of which share the same genome. In metazoans, much of the regulatory code that drives cell type-specific gene expression is located in distal elements called enhancers. Although mammalian genomes contain millions of potential enhancers, only a small subset of them is active in a given cell type. Cell type-specific enhancer selection involves the binding of lineage-determining transcription factors that prime enhancers. Signal-dependent transcription factors bind to primed enhancers, which enables these broadly expressed factors to regulate gene expression in a cell type-specific manner. The expression of genes that specify cell type identity and function is associated with densely spaced clusters of active enhancers known as super-enhancers. The functions of enhancers and super-enhancers are influenced by, and affect, higher-order genomic organization.

Accurate control of gene expression is essential for normal development and dysregulation of transcription underpins cancer onset and progression. Similar to cell cycle regulation, RNA polymerase II-driven transcription can be considered as a unidirectional multistep cycle, with thousands of unique transcription cycles occurring in concert within each cell. Each transcription cycle comprises recruitment, initiation, pausing, elongation, termination and recycling stages that are tightly controlled by the coordinated action of transcriptional cyclin-dependent kinases and their cognate cyclins as well as the opposing activity of transcriptional phosphatases. Oncogenic dysregulation of transcription can entail defective control of gene expression, either at select loci or more globally, impacting a large proportion of the genome. The resultant dependency on the core-transcriptional machinery is believed to render ‘transcriptionally addicted’ cancers sensitive to perturbation of transcription. Based on these findings, small molecules targeting transcriptional cyclin-dependent kinases and associated proteins hold promise for the treatment of cancer. Here, we utilize the transcription cycles concept to explain how dysregulation of these finely tuned gene expression processes may drive tumorigenesis and how therapeutically beneficial responses may arise from global or selective transcriptional perturbation. This conceptual framework helps to explain tumour-selective transcriptional dependencies and facilitates the rational design of combination therapies.This Review discusses the concept of transcription cycles in cancer, providing a framework for our understanding of dysregulated transcription in cancer and therapeutic targeting of dysregulated transcription cycles.

Enhancer-mediated gene activation generally requires physical proximity between enhancers and their target gene promoters. However, the molecular mechanisms by which interactions between enhancers and promoters are formed are not well understood. Here, we investigate the function of the Mediator complex in the regulation of enhancer-promoter interactions, by combining rapid protein depletion and high-resolution MNase-based chromosome conformation capture approaches. We show that depletion of Mediator leads to reduced enhancer-promoter interaction frequencies, which are associated with a strong decrease in gene expression. In addition, we find increased interactions between CTCF-binding sites upon Mediator depletion. These changes in chromatin architecture are associated with a redistribution of the Cohesin complex on chromatin and a reduction in Cohesin occupancy at enhancers. Together, our results indicate that the Mediator and Cohesin complexes contribute to enhancer-promoter interactions and provide insights into the molecular mechanisms by which communication between enhancers and promoters is regulated. Here, the authors map chromatin conformation at high resolution after rapid Mediator depletion to show that its loss reduces the frequency of enhancer-promoter interactions and associated gene expression, with a corresponding redistribution of Cohesin.

Cyclin-dependent kinase 12 (CDK12) modulates transcription elongation by phosphorylating the carboxy-terminal domain of RNA polymerase II and selectively affects the expression of genes involved in the DNA damage response (DDR) and mRNA processing. Yet, the mechanisms underlying such selectivity remain unclear. Here we show that CDK12 inhibition in cancer cells lacking CDK12 mutations results in gene length-dependent elongation defects, inducing premature cleavage and polyadenylation (PCPA) and loss of expression of long (>45 kb) genes, a substantial proportion of which participate in the DDR. This early termination phenotype correlates with an increased number of intronic polyadenylation sites, a feature especially prominent among DDR genes. Phosphoproteomic analysis indicated that CDK12 directly phosphorylates pre-mRNA processing factors, including those regulating PCPA. These results support a model in which DDR genes are uniquely susceptible to CDK12 inhibition primarily due to their relatively longer lengths and lower ratios of U1 snRNP binding to intronic polyadenylation sites. Cdk12 is primarily involved in the regulation of DNA damage response (DDR) gene transcription as well as mRNA processing. Here, the authors demonstrate that CDK12 suppresses intronic polyadenylation, and that inhibition of this kinase primarily affects the expression of long genes with higher numbers of polyA sites, features common to many DDR genes.

Cancer is a disease of uncontrollably reproducing cells. It is governed by biochemical pathways that have escaped the regulatory bounds of normal homeostatic balance. This balance is maintained through precise spatiotemporal regulation of these pathways. The formation of biomolecular condensates via liquid–liquid phase separation (LLPS) has recently emerged as a widespread mechanism underlying the spatiotemporal coordination of biological activities in cells. Biomolecular condensates are widely observed to directly regulate key cellular processes involved in cancer cell pathology, and the dysregulation of LLPS is increasingly implicated as a previously hidden driver of oncogenic activity. In this Perspective, we discuss how LLPS shapes the biochemical landscape of cancer cells.Liquid–liquid phase separation (LLPS) has been revealed as a widespread mechanism underlying the spatiotemporal coordination of biological activities in cells. This Perspective discusses how LLPS shapes the biochemical landscape of cancer cells, providing insight into emerging findings of dysregulated LLPS promoting cancer pathology.

The carboxy-terminal domain (CTD) of the RNA polymerase II (RNAP II) subunit POLR2A is a platform for modifications specifying the recruitment of factors that regulate transcription, mRNA processing, and chromatin remodelling. Here we show that a CTD arginine residue (R1810 in human) that is conserved across vertebrates is symmetrically dimethylated (me2s). This R1810me2s modification requires protein arginine methyltransferase 5 (PRMT5) and recruits the Tudor domain of the survival of motor neuron (SMN, also known as GEMIN1) protein, which is mutated in spinal muscular atrophy. SMN interacts with senataxin, which is sometimes mutated in ataxia oculomotor apraxia type 2 and amyotrophic lateral sclerosis. Because POLR2A R1810me2s and SMN, like senataxin, are required for resolving RNA–DNA hybrids created by RNA polymerase II that form R-loops in transcription termination regions, we propose that R1810me2s, SMN, and senataxin are components of an R-loop resolution pathway. Defects in this pathway can influence transcription termination and may contribute to neurodegenerative disorders. Symmetric dimethylation of the human RNA polymerase II C-terminal domain residue R1810 by the protein arginine methyltransferase 5 (PRMT5) directly recruits the protein survival of motor neuron (SMN) and indirectly recruits the helicase senataxin to resolve R-loops and promote transcription termination. Control of transcription termination The repeating sequence of the C-terminal domain of RNA polymerase II is a favoured target of many modification enzymes. In this study, Jack Greenblatt and colleagues identify and characterize a symmetrical dimethylation modification of an arginine residue, R1810, in the the C-terminal domain. The R1810me2s modification is made by PRMT5, which interacts with SMN (survival of motor neuron) protein, and indirectly with senataxin proteins; mutations in each of these proteins are found in neurodegenerative diseases. The authors propose that the R1810me2s modification of RNA Pol II, together with the activity of SMN and senataxin, is part of a pathway for resolution of transcription-associated R-loops that, if absent, affects gene expression by disrupting transcription termination.

5-methylcytosine （m5C） is a post-transcriptional RNA modification identified in both stable and highly abundant tRNAs and rRNAs, and in mRNAs. However, its regulatory role in mRNA metabolism is still largely unknown. Here, we reveal that m5C modification is enriched in CG-rich regions and in regions immediately downstream of trans- lation initiation sites and has conserved, tissue-specific and dynamic features across mammalian transcriptomes. Moreover, m5C formation in mRNAs is mainly catalyzed by the RNA methyltransferase NSUN2, and m5C is specif- ically recognized by the mRNA export adaptor ALYREF as shown by in vitro and in vivo studies. NSUN2 modulates ALYREF＇s nuclear-cytoplasmic shuttling, RNA-binding affinity and associated mRNA export. Dysregulation of AL- YREF-mediated mRNA export upon NSUN2 depletion could be restored by reconstitution of wild-type but not meth- yltransferase-defective NSUN2. Our study provides comprehensive m5C profiles of mammalian transcriptomes and suggests an essential role for m5C modification in mRNA export and post-transcriptional regulation.

Long non-coding RNAs (lncRNAs) outnumber protein-coding transcripts, but their functions remain largely unknown. In this Review, we discuss the emerging roles of lncRNAs in the control of gene transcription. Some of the best characterized lncRNAs have essential transcription cis-regulatory functions that cannot be easily accomplished by DNA-interacting transcription factors, such as XIST, which controls X-chromosome inactivation, or imprinted lncRNAs that direct allele-specific repression. A growing number of lncRNA transcription units, including CHASERR, PVT1 and HASTER (also known as HNF1A-AS1) act as transcription-stabilizing elements that fine-tune the activity of dosage-sensitive genes that encode transcription factors. Genetic experiments have shown that defects in such transcription stabilizers often cause severe phenotypes. Other lncRNAs, such as lincRNA-p21 (also known as Trp53cor1) and Maenli (Gm29348) contribute to local activation of gene transcription, whereas distinct lncRNAs influence gene transcription in trans. We discuss findings of lncRNAs that elicit a function through either activation of their transcription, transcript elongation and processing or the lncRNA molecule itself. We also discuss emerging evidence of lncRNA involvement in human diseases, and their potential as therapeutic targets.This Review discusses the emerging roles of long non-coding RNAs (lncRNAs) in the regulation of transcription, for example by controlling the expression of transcription factors. Some lncRNA loci function in trans, but most function in cis, through their own transcription or through the lncRNA transcripts themselves.

The Mediator complex, which in humans is 1.4 MDa in size and includes 26 subunits, controls many aspects of RNA polymerase II (Pol II) function. Apart from its size, a defining feature of Mediator is its intrinsic disorder and conformational flexibility, which contributes to its ability to undergo phase separation and to interact with a myriad of regulatory factors. In this Review, we discuss Mediator structure and function, with emphasis on recent cryogenic electron microscopy data of the 4.0-MDa transcription preinitiation complex. We further discuss how Mediator and sequence-specific DNA-binding transcription factors enable enhancer-dependent regulation of Pol II function at distal gene promoters, through the formation of molecular condensates (or transcription hubs) and chromatin loops. Mediator regulation of Pol II reinitiation is also discussed, in the context of transcription bursting. We propose a working model for Mediator function that combines experimental results and theoretical considerations related to enhancer–promoter interactions, which reconciles contradictory data regarding whether enhancer–promoter communication is direct or indirect. We conclude with a discussion of Mediator’s potential as a therapeutic target and of future research directions.The Mediator complex is an important regulator of RNA polymerase II. This Review discusses recent structural insights into Mediator function and proposes a model that reconciles contradictory data on whether enhancer–promoter communication during transcription is direct or indirect.