At any given time, only some of the in our DNA are turned on, or “active,” while others are turned off. Making sure only the correct genes are turned on in the correct cells is critical for our health. Scientists aim to understand how in cells coordinate to turn a gene on, which could help identify strategies for treating improper regulation of gene activity. Two -funded studies recently published in the journal Science explore the protein interactions that help turn a gene on. Both studies found that proteins cluster together at regions in the DNA called enhancers and interact to trigger gene activation.
In one study, Dr. Ibrahim Cisse and a team of researchers used live-cell super-resolution microscopy to view single molecules of proteins in mouse cells. They were able to view the interactions of a protein complex called Mediator, which helps kick-start , and RNA polymerase II, the protein that carries out transcription by copying DNA into . They found that both Mediator and RNA polymerase II group into stable clusters forming liquid-like droplets, a process known as phase-separation. Protein interactions were found to be brief, with proteins able to move in and out of the droplets, and droplets able to fuse together. They propose that Mediator droplets cluster at enhancers and fuse with RNA polymerase II droplets, allowing interactions between Mediator and RNA polymerase II that spur transcription to turn on genes.
In another study, led by Dr. Robert Tijan and Dr. Xavier Darzacq, the research team used live-cell single-molecule imaging to explore how proteins called transcription factors bind to the DNA enhancer and interact to initiate gene activation. They found that transcription factors also form high-concentration clusters that localize at the enhancer to stabilize DNA binding, recruit RNA polymerase II, and activate transcription. The interactions between transcription factors and RNA Polymerase II were rapid, reversible, and selective, making them a potential class of drug targets for regulating the process of gene activation.
. Cho, WK, Spille, JH, Hecht, M, Lee, C, Li, C, Grube, V, and Cisse, II. Science. 361, 412-415. 2018 July 27.
. Chong, S, Dugast-Darzacq, C, Liu, Z, Dong, P, Dailey, GM, Cattoglio, C, Heckert, A, Banala, S, Lavis, L, Darzacq, X, and Tijan, R. Science. 361 (6400). 2018 July 27.
In the News:
It may take a village (of proteins) to turn on genes, Science News
Genes are the segments of our DNA that code for that determine our traits. Over 90% of our DNA does not encode genes and was long considered “junk” DNA that had no known purpose. We now know that much of this DNA does have a purpose, for example, some of this non-coding DNA contains enhancers- regions of DNA that help “turn on” genes to ultimately produce proteins. The timing of turning a gene on is very important for normal development and issues with timing can lead to development of disease. Enhancers are typically located far away from the gene they turn on, and how the enhancers find their target genes within the of the cell and how they interact with gene- coding regions to result in protein production is not well understood. In a recent study by -funded investigator Dr. Thomas Gregor and his research team, a live imaging approach was used to track the position of an enhancer and its target gene in developing fly embryos, while also monitoring gene activity. Using this technique, they were able to observe the moment when a gene was turned on. The results showed that close proximity between the enhancer and target gene was required not only to turn the gene on, but also to keep the gene active. When the enhancer disconnected from the target gene, the gene turned off. They also found that when the gene was turned on, the structure formed by the enhancer and target gene became more compact, and the results suggest changes in the 3D DNA arrangement improve the stability of this structure, allowing the gene to remain active. The results of this study improve our understanding of how gene activity is regulated and may help provide insight into how improper regulation of gene activity leads to developmental defects and disease.
Reference:. Chen, H, Levo, M, Barinov, L, Fujioka, M, Jaynes, JB, and Gregor, T. Nature Genetics. 2018.
In the news:
, Princeton University
, Genetic Engineering & Biotechnology News
The 3D organization of DNA in the nucleus of a cell plays an important role in determining which genes are turned on in that cell. This has important implications for human health, as problems with DNA organization are linked to human diseases such as cancer and early aging. Understanding how the DNA is organized in healthy cells is a critical step in identifying targets and developing treatments for abnormal nuclear organization. A current method for mapping 3D genome organization uses a technique called “proximity ligation” in which regions of DNA that are very close together, or “touching,” are linked together and then sequenced to determine where these DNA “touches” occur. This technique mostly identifies interactions of DNA regions within the same chromosome. However, imaging of the genome using microscopy techniques has shown that there are interactions between chromosomes and that these interactions tend to occur at discrete regions of the nucleus known as nuclear bodies. This indicates limitations of proximity ligation techniques in identifying interactions between chromosomes that occur over longer-range distances. In addition, both proximity ligation and microscopy techniques are limited to measuring simultaneous contacts between a small number of DNA regions, making it difficult to develop a comprehensive model of global genome organization.
A recent study led by NIH Common Fund 4D Nucleome Program-funded investigator Dr. Mitchell Guttman, developed a new technique for detecting simultaneous genome-wide interactions within the nucleus, called Split-Pool Recognition of Interactions by Tag Extension (SPRITE). SPRITE works by linking interacting DNA, RNA, and proteins in cells, isolating the nuclei, fragmenting the chromatin, “barcoding” interacting molecules within a complex, and sequencing and matching the areas with identical “barcodes” to identify interacting regions. Unlike proximity ligation and microscopy techniques, SPRITE is not limited in the number of simultaneous DNA interactions that it can identify. Using SPRITE, they were able to detect interactions that occur across larger distances than those found by proximity ligation techniques. They found two “hubs” of interactions between chromosomes, both associated with nuclear bodies: an inactive gene-poor hub that organizes around the nucleolus and an active gene-rich hub that organizes around regions called “nuclear speckles.” Using the SPRITE results, they created a global model of 3D genome organization, in which nuclear bodies act as inter-chromosomal hubs that shape the 3D packaging of DNA in the nucleus.
Reference: Higher-order inter-chromosomal hubs shape three-dimensional genome organization in the nucleus. Quinodoz, SA, Ollikainen, N, Tabak, B, Palla, A, Schmidt, JM, Detmar, E, Lai, MM, Shishkin, AA, Bhat, P, Takei, Y, Trinh, V, Aznauryan, E, Russell, P, Cheng, C, Jovanovic, M, Chow, A, Cai, L, McDonel, P, Garber, M, and Guttman, M. Cell. 2018.
In the News: Biologists Create 3D Maps of DNA Within the Innermost Parts of a Cell, SciTech Daily
As a cell moves through the cell cycle, the shape of the DNA in the nucleus changes dramatically, from a ball of DNA during normal cell activities to distinct X-shaped chromosomes as the cell prepares to divide. This change in shape is important for the orderly passage of one copy of the DNA to each new cell. Before the cells begin to divide, the DNA compacts into the dense X-shaped chromosomes that are made up of consecutive DNA loops. How the compaction of loops occurs is not well understood. One of the goals of the 4D Nucleome program is to determine how the structure of DNA in the nucleus of a cell changes over time.
In a recent study in Science, a team led by 4DN-funded researchers Dr. Leonid Mirny and Dr. Job Dekker combined techniques that are used to determine where regions of the genome “touch” with imaging and modeling techniques at one-minute time intervals to investigate how DNA rearrangement occurs before cell division. Based on their results, they propose a model in which cells use a protein called “condensin” to drive the compaction of DNA. Condensin proteins create DNA loops by pushing DNA through their ring-like structures. In this model, condensin I creates wide loops in the DNA that are then split into smaller loops by condensin II. The loops twist around a condesin scaffold in a structure resembling a spiral staircase, creating a condensed helix of consecutive DNA loops that makes up the X-structure of the chromosomes and results in formation of compact units of DNA that can be easily divided between new cells. Understanding how the organization of DNA changes throughout the cell cycle is critical for determining how problems with cell cycle-associated DNA rearrangement (such as chromosome breakage) lead to human diseases such as cancer.
Reference: A pathway for mitotic chromosome formation. Gibcus, JH, Samejima, K, Goloborodko, A, Samejima, I, Naumova, N, Nuebler, J, Kanemaki, MT, Xie, L, Paulson, JR, Earnshaw, WC, Mirny, LA, Dekker, J. Science. 2018 Jan 18. doi: 10.1126/science.aao6135.
In the news:
Packing a Genome, Step-by-Step, Howard Hughes Medical Institute
How Cells Pack Tangled DNA Into Neat Chromosomes, Quanta Magazine
Loop Loss in the Human Genome
DNA is organized in the small nucleus of a cell in the form of a DNA-protein complex called chromatin. The protein “cohesin" helps maintain DNA organization by tethering two regions of DNA on the same chromosome to form loops. The loops have long been thought to regulate which genes are turned on by controlling the distance between DNA promoters and enhancers. Promoters are regions of DNA that generally occur before a gene and serve as a landing space for the molecular machinery needed to activate the gene. Enhancers are regions of DNA that can increase gene activity when in close contact with the target gene’s promoter.
In one study, a group of researchers led by 4D Nucleome program-funded investigator Erez Lieberman Aiden used a technique called chromosome conformation capture (Hi-C) to map the formation of DNA loops throughout the genome at 20-minute intervals during the loss and recovery of cohesin. They found that cohesin removal led to the loss of loops. However, this surprisingly had modest effects on gene activity, with only a few genes experiencing significant changes in activity. They also observed formation of a separate group of cohesin-independent loops and links between different chromosomes. The results suggest that cohesin-dependent loops play only a modest role in regulating interactions between promoters and enhancers. Based on these results, the team proposes a revised model in which a combination of cohesin-dependent and -independent loops regulate gene activity.
In another study led by 4DN-funded researcher Leonid Mirny and Transformative Collaborative Project Awardee Francois Spitz, the protein responsible for loading cohesin onto chromatin, Nipbl, was deleted in a mouse model. The resulting changes in chromatin organization were then identified using a Hi-C technique. Nipbl deletion led to significant changes in chromatin organization, including loss of cohesin-dependent loops and enhancement of compartments made up of chromatin regions with similar activity. The loss of cohesin-dependent loops allowed formation of smaller chromatin compartments with fewer contacts between active and inactive chromatin regions. The results contradict a model of chromatin organization in which DNA loops combine to form larger compartments. Instead, the authors propose a revised model in which genomic compartment formation is interrupted by cohesin-dependent loops that can bring regions of different chromatin activities together to drive gene activity. Although these studies, used different approaches, they led to similar conclusions. Understanding how the 3-dimensional structure of chromatin is controlled over time and affects gene activity can lead to better treatment of human diseases linked to abnormal chromatin organization.
Cohesin Loss Eliminates All Loop Domains. Rao, SSP, Huang, S, St Hilaire, BG, Engreitz, JM, Perez, EM, Kieffer-Kwon, K, Sanborn, AL, Johnstone, SE, Bascom, GD, Bochkov, ID, Huang, X, Shamim, MS, Shin, J, Turner, D, Ye, Z, Omer, AD, Robinson, JT, Schlick, T, Bernstein, BE, Casellas, R, Lander, ES, and Lieberman Aiden, E. Cell (171), 305-320. 2017 October 5.
Two independent modes of chromatin organization revealed by cohesin removal. Schwarzer, W, Abdennum, n, Goloborodko, A, Pekowska, A, Fudenberg, G, Moe-Mie, Y, Fonesca, NA, Huber, W, Haering, CH, Mirny, L, and Spitz, F. Nature (551), 51-56. 2017 November 2.
In the News: Watch the human genome fold itself in four dimensions, Science News
Dynamic DNA Loops Affect How Cells Become Specialized
Genomic DNA is packaged and organized in the tiny nucleus of the cell as chromatin (a complex of DNA and histone proteins). The 3-dimensional organization of chromatin in the nucleus affects which genes are expressed and at what times. A complex network of chromatin loops is involved in coordinating changes in transcription during cell development. Chromatin loops can bring enhancers (sections of DNA that promote transcription when bound by proteins called transcription factors) closer to their target genes in the genome. How chromatin architecture changes as cells differentiate into specialized cell types and how these changes affect cell-type-specific gene expression and cellular function are not well understood.
A team of researchers, including 4DN program-funded researcher Erez Lieberman Aiden, used a technique called in situ chromosome conformation capture (Hi-C) to create high-resolution genome-wide looping maps to compare the chromatin structure of cells before and after differentiation to become specialized immune cells (macrophages). Hi-C is a method of detecting frequencies of contact between all mappable regions of the human genome. Following differentiation, they found genes at loops that were newly formed (“gained loops”) or newly activated by changes in chromatin architecture (“activated loops”) have increased expression. The gained and activated loops form multi-loop activation “hubs” that create long-range interactions between active enhancers and promoters and have increased binding of transcriptional regulators, thus facilitating transcription. The multi-loop hubs occur at genes known to play a role in macrophage development and function, indicating a role in regulating gene transcription during cell differentiation. This study could have broader implications for how chromosome organization instructs transcription in other cellular contexts and throughout human development.
Reference: Static and Dynamic DNA Loops form AP-1-Bound Activation Hubs during Macrophage Development. Phanstiel, DH, Van Bortle, K, Spacek, D, Hess, GT, Shamim, MS, Machol, I, Love, MI, Lieberman Aiden, E, Bassik, MC, Snyder, MP. Molecular Cell. 2017 September 21. 67(6): 1037-1048.
The 4D Nucleome Project
The 4D Nucleome Program aims to develop and apply approaches to map the structure and dynamics of the human and mouse genomes in space and time with the goal of understanding how the nucleus is organized and functions. The program will develop and benchmark robust experimental and computational approaches for measuring genome conformation and nuclear organization, and illuminate how they contribute to gene regulation and other genome functions. These efforts will lead to new insights into how the genome is organized, maintained, expressed and replicated, in both normal and disease states.
Reference: The 4D Nucleome Project. Dekker J, Belmont A, Guttman M, Leshyk V, Lis J, Lomvardas S, Mirny L, O’Shea C, Park P, Ren B, Ritland Politz C, Shendure J, Zhong S, & the 4D Nucleome
Network. Nature. 2017 September 14.
Visualizing the Nucleus in 3D
How DNA is packaged in the nucleus determines how it is used and how genes are expressed. DNA is condensed and packaged as chromatin (a complex of DNA and proteins called histones), which constantly changes as genes are expressed. Understanding chromatin packaging may reveal the structural code for how genes are turned on or off in human health and disease. For example, understanding chromatin packaging could be used to make cancer cells with abnormally structured chromatin “remember” how to be healthier through repackaging chromatin. Toward understanding chromatin packaging in the nucleus, 4D Nucleome (4DN) program grantee Dr. Clodagh O’Shea collaborated with fellow 4DN grantee Dr. Mark Ellisman to develop a new approach to visualize chromatin in 3D space. This method, called ChromET, combines electron microscopy tomography (EMT) and a labeling method that enhances the visualization of DNA in human cell lines. An electron microscope uses a beam of electrons to create an image of a sample and is capable of seeing much smaller objects than a traditional light microscope. The 4DN researchers used ChromET to show that chromatin is flexibly disordered and packed together at different concentrations in the nucleus. This is different from the textbook model of rigid higher-order chromatin folding. This new model of diverse chromatin structures – able to bend at various lengths and achieve different packing concentrations – is important because it provides an explanation for how different parts of the genome could be fine-tuned to make different structures, at different times, with different functions. This research brings us one step closer to discovering how the structural code of our genomes could be used to advance medical care.
Reference: ChromEMT: Visualizing 3D chromatin structure and compaction of the human genome in interphase and mitotic cells. Ou, HD, Phan S, Deerinck TJ, Thor A, Ellisman, MH, O’Shea CC. Science. 2017 July 28.
By studying cell-to-cell variability in dividing cells, a research team led by 4D Nucleome (4DN) grantees Drs. Peter Fraser and Amos Tanay revealed that genes don't have a fixed location in the cell, but are constantly moving. The cell cycle is a series of events during which a cell grows, replicates its chromosomes, and then divides. Chromosome conformation capture is a technique used to detect physical interactions between different segments of chromosomes. It can be used to investigate the 3D folding of chromosomes, and to identify where genes on those chromosomes are positioned in the nucleus. Most chromosome conformation capture experiments are done using millions of cells in different phases of the cell cycle, all mixed together. In this study, researchers used chromosome conformation capture and statistical analysis to look at the 3D folding of chromosomes in thousands of individual mouse embryonic stem cells, all separated from one another. Surprisingly, they found that genes don’t have a fixed location in the cell nucleus – rather, genes are continuously moving and change their positions as they progress through different stages of the cell cycle. This is important because changes in gene location can result in changes in gene activity, which can influence human health and disease. Studying chromosomal organization could illuminate how changes in the location of a gene can affect normal development as well as various diseases.
Reference: Cell-cycle dynamics of chromosomal organization at single-cell resolution. Nagano T, Lubling Y, Várnai C, Dudley C, Leung W, Baran Y, Mendelson Cohen N, Wingett S, Fraser P, Tanay A. Nature. 2017 July 6. 547(7661):61-67.
4D Nucleome grantees have combined imaging and genomic methods to discover new insights about certain proteins that impact gene expression and nuclear organization. Chromatin is a substance found within our chromosomes that consists of DNA and protein. The function of chromatin is to efficiently package over 6 feet of DNA into a remarkably small volume – the nucleus of a cell – and protect the DNA structure and sequence. Formation of chromatin structures and nuclear organization are not random, it turns out that chromatin is organized in chromosomal “neighborhoods” within the nucleus. Packaging of DNA into chromatin in these neighborhoods can control gene expression and DNA replication. Yet, the principles behind genome organization in the nucleus are not well understood. Understanding nuclear architecture is important because abnormalities in genome organization are associated with diseases, including cancer and premature aging. One way to study genome organization is through chromosome conformation capture, a technique that is used to detect physical interactions between different segments of the genome. Another method is fluorescent in situ hybridization (FISH), which researchers use to visualize and map the cell’s genetic material through imaging. Although both provide a wealth of information, bringing these approaches together (correlating imaging technologies with genomics and sequencing) will allow researchers to sequence information that they see.
Toward this aim, 4D Nucleome grantees Drs. Arjun Raj, Jennifer E. Phillips-Cremins, and Gerd A. Blobel have used FISH imaging and chromosome capture genomic methods to discover new insights about bromodomain and extraterminal motif (BET) proteins, which help regulate gene expression by controlling chromatin and nuclear organization. Although there are several different types of BET proteins, the role of each type and how they interact with chromatin is not well understood. The study of BET proteins is important because BET inhibitors are a class of drugs with anti-cancer, immunosuppressive, and other effects in clinical trials. In this study, they found that a certain type of BET protein contributes to the formation of boundaries, which help chromatin form chromosomal neighborhoods. This raises the possibility that BET inhibitors can influence gene expression by disrupting boundary function. Using imaging and genomic sequencing methods to gain a clearer understanding of BET proteins will help researchers understand how BET inhibitors work, bringing us one step closer to treating disease.
Reference: The BET Protein BRD2 Cooperates with CTCF to Enforce Transcriptional and Architectural Boundaries. Hsu SC, Gilgenast TG, Bartman CR, Edwards CR, Stonestrom AJ, Huang P, Emerson DJ, Evans P, Werner MT, Keller CA, Giardine B, Hardison RC, Raj A, Phillips-Cremins JE, Blobel GA. Molecular Cell. 2017 April 6. 66(1):102-116.
Researchers from the Common Fund’s 4DN Program have developed a new technology to peer inside the cell at the earliest stages of development, revealing important differences in how the DNA from the mother and the father are organized and potentially providing clues about how the multitude of cell types in an organism can arise from a single cell. During fertilization, maternal and paternal DNA coexist in the zygote, which later develops into an embryo following the instructions encoded in the DNA. Understanding how DNA from both parents is organized in the zygote is important because 3D organization of DNA is one way that a cell can control gene expression. Tight regulation of gene expression in these early developmental stages is key to tissue, organ, and organism development. One way to study genome organization is through chromosome conformation capture (Hi-C), a technique that is used to detect physical interactions between sequences of the genome. However, using this method to study the nuclear organization of a single zygote or embryo is challenging because it usually requires hundreds, thousands, or millions of cells.
To overcome this challenge, 4D Nucleome grantee Dr. Leonid Mirny, and collaborators, have developed a new form of Hi-C technology to analyze cells at a single cell level - a significant technological advancement in this field. Using this method, they found that nuclear architecture is uniquely reorganized during the oocyte to zygote transition in mice and that maternal and paternal DNA are packaged differently in mouse zygotes. The oocyte to zygote transition refers to a critical period of development, during which fundamental changes in nuclear function take place as the egg and sperm give rise to an embryonic genome. This exciting knowledge of the zygotic “ground state” will contribute to our understanding of how cells give rise to an astounding number of different cell types that make up an organism.
In the News: Mom’s and Dad’s Genes Packed Differently in Fertilized Egg, Genetic Engineering & Biotechnology News. In the beginning there was order, Nature.
Reference: Single-nucleus Hi-C reveals unique chromatin reorganization at oocyte-to-zygote transition. Flyamer IM, Gassler J, Imakaev M, Brandão HB, Ulianov SV, Abdennur N, Razin SV, Mirny LA, Tachibana-Konwalski K. Nature. 2017 April 6. 544(7648):110-114.
4D Nucleome grantee Dr. Clifford Brangwynne and collaborators have developed a new tool that uses light to manipulate matter inside living cells. Called optoDroplet, this tool helps explain the physics and chemistry behind how cells assemble a mysterious structure called a membraneless organelle. An organelle is a specialized part of a cell having some specific function. For example, the nucleus is an organelle that holds most of the cell’s genetic information. Organelles like the nucleus are walled off from the rest of the cell by a membrane. The cell also uses membraneless organelles that resemble liquid droplets and exhibit dynamic behavior, such as rapid assembly and disassembly of protein building blocks that make up the organelle. When these mechanisms go awry, aggregates of the protein building blocks can form. Protein aggregation is associated with a number of diseases, including amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease) and Alzheimer's disease. Understanding the process by which proteins condense into these droplet-like, membraneless organelles may be used to develop of interventions and treatments for diseases connected with protein aggregation. To better understand this process, Dr. Brangwynne’s group developed optoDroplet. This new tool relies on optogenetics, which involves proteins whose behavior can be altered by exposure to light. Using mouse and human cells, researchers showed that they could create membraneless organelles by switching on the light-activated proteins. They were also able to use this tool to generate protein aggregates, similar to those found in some diseases. The optoDroplet system will help researchers understand the basic mechanisms that underlie self-assembly of membraneless organelles in healthy living cells and may reveal how cells become diseased when this process goes awry.
Reference: Spatiotemporal Control of Intracellular Phase Transitions Using Light-Activated optoDroplets. Shin Y, Berry J, Pannucci N, Haataja MP, Toettcher JE, Brangwynne CP. Cell. 2017, January 12.
X-inactivation is the process by which one of the two X chromosomes present in female mammals is inactivated. This prevents them from having twice as many X chromosomes gene products as males, who only have a single X chromosome. A Barr body is the structure inside of the nucleus that consists of the inactive X chromosome. Although the Barr body appears to be a condensed blob under a microscope, a new study from 4D Nucleome grantee Job Dekker, Ph.D. and collaborators reveals a highly elaborate structure. Using a variety of methods, including chromosome conformation capture technologies and mouse models, they found that the inactive X chromosome is actually composed of two distinct lobes of inactive DNA. They also found that these lobes were separated by highly repetitive segments of DNA called “microsatellite repeats”. When these microsatellite repeats were removed, the bi-lobed chromosome structure vanished. In another finding of this study, a limited number of active genes in these lobes were separated by topologically associated domains (TADs), which are regions of the genome where DNA interactions frequently occur. This finding is significant because it suggests that TADs may organize gene expression in the inactive X chromosome, at least in neural progenitor cells.
“This is the most detailed molecular view we’ve been able to obtain of the DNA inside the Barr body,” said Dekker. “Under a microscope, the inactive X chromosome is very different than other chromosomes; it looks like a condensed, undefined, inactive ‘blob.’ Our study, using a range of experimental approaches including imaging and genomic methods, describes something else entirely: a highly organized and elaborate structure, rich in features that may silence or activate genes all along the chromosome.”
In the news: Read the press release from the University of Massachusetts Medical School here.
Reference: Structural organization of the inactive X chromosome in the mouse. Giorgetti L, Lajoie BR, Carter AC, Attia M, Zhan Y, Xu J, Chen CJ, Kaplan N, Chang HY, Heard E, Dekker J. Nature. 2016 July 28.
Transcription occurs when a particular segment of DNA is expressed into RNA. This process appears to take place in intermittent bursts and has been observed in organisms ranging from bacteria to humans. Transcriptional bursting is a term that describes this highly variable occurrence. Enhancers are short regions of DNA that can significantly increase the likelihood that a gene will be transcribed. There are hundreds of thousands of enhancers in the human genome, many of which precisely regulate patterns of gene expression that are required for the differentiation and growth of cells and tissues. To better understand the relationship between transcriptional bursts and enhancers, Dr. Michael Levine, a 4D Nucleome grantee, used quantitative analysis and live-imaging methods in Drosophila embryos in real time. Using this model organism, they report that enhancers regulate the frequency of transcriptional bursts and that strong enhancers produce more bursts than weak ones. They also report that shared enhancers can drive coordinated bursting of two different reporter genes, suggesting the importance of chromosome architecture in control of gene expression.
Reference: Enhancer Controls of Transcriptional Bursting. Fukaya T, Lim B, Levine M. Cell. 2016 June 9.
Keeping Chromosomes in the Loop
Chromosomes are compacted over 100-fold to form highly condensed structures during cell division, yet the active process of chromosome compaction into loops is not well understood. 4D Nucleome grantee Dr. Leonid Mirny, a professor of physics in MIT’s Institute for Medical Engineering and Sciences, and collaborators have developed model that explains how chromosomes condense into these compact structures. Using computer simulations of chromosomes, the team investigated whether loops could produce the compact chromosomes seen in dividing cells. Their work, reported in three papers published in Cell Reports, eLife, and Biophysical Journal, suggests that chromosome organization relies on proteins that act as molecular motors that pull strands of DNA into increasingly larger loops. The authors report that proteins thought to hold DNA together, cohesion and condensin, can also transform loosely tangled chromosomes into a series of small loops that condense each chromosome. This allows compacted chromosomes to remain segregated from one another. A similar model explains how chromosomes are organized when cells are not dividing, suggesting that this loop extrusion model helps to control which genes are expressed. “Nobody has ever directly observed this mechanism of loop extrusion. If it exists, it will solve lots of problems,” says Dr. Leonid Mirny. “We will know how chromosomes condense, how they segregate, how genes talk to enhancers. Lots of things can be solved by this mechanism.”
Compaction and segregation of sister chromatids via active loop extrusion. Goloborodko A, Imakaev MV, Marko JF, Mirny L. Elife. 2016 May 18.
Chromosome Compaction by Active Loop Extrusion. Goloborodko A, Marko JF, Mirny LA. Biophysical Journal. 2016 May 24.
Formation of Chromosomal Domains by Loop Extrusion. Fudenberg G, Imakaev M, Lu C, Goloborodko A, Abdennur N, Mirny LA. Cell Reports. 2016 May 31.
CRISPRainbow: Observing the Genome in Living Color
4D Nucleome investigators Drs. Thoru Pederson and David Grunwald, and colleagues, have developed a new technology called CRISPRainbow. While most researchers use CRISPR for editing genomes, Pederson, Grunwald and colleagues used CRISPR/Cas9 technology to tag specific locations of the genome, label these locations with fluorescent proteins, and track them in real time under a microscope. Existing technologies are only capable of following three genomic locations at a time in living cells, but CRISPRainbow allows researchers to tag and track up to seven different genomic locations in live cells. "Computers cooperating with spectral filters in the microscope read out combinations of colors and display them as a color that you request," explains Thoru Pederson, Ph.D. "For example, red and green can be yellow. Using the three primary colors and this approach, which is called computational coloring, we can generate an additional three colors.” A seventh label, white, is accomplished by combining all three primary colors. Fluorescent labels such as these are important for studying chromosome dynamics and movements of the genome, which may have important biological consequences.
In the news: Read about this study in Genetic Engineering & Biotechnology News.
Reference: Multiplexed labeling of genomic loci with dCas9 and engineered sgRNAs using CRISPRainbow. Ma H, Tu LC, Naseri A, Huisman M, Zhang S, Grunwald D, Pederson T. Nature Biotechnology. 2016 April 18.
Understanding Chromosome Structure and Cancer
DNA is not randomly arranged in the nucleus. Instead, nuclear organization is tightly controlled. For example, insulated neighborhoods are loops of DNA that function to maintain normal expression of genes within and outside of the loop. Defects in nuclear organization and the folding of the human genome have been linked to cancer. In a new study, 4D Nucleome investigator Dr. Job Dekker, Ph.D., Howard Hughes Medical Institute Investigator, University of Massachusetts Medical School, and collaborators investigated the causal relationship between chromosome structure and oncogene activation. Oncogenes are genes that have the potential to cause cancer, when activated through dysregulation of gene expression. Using DNA sequences from tumors and targeted mutations in cancer cell lines, they show that disruption of insulated neighborhoods can activate oncogenes. This suggests that disruption of chromatin architecture is causally linked to the formation of tumors. This work represents a step toward understanding 3D chromosome structure and the authors conclude that “understanding these regulatory processes may provide new approaches to therapeutics that have on impact an aberrant chromosome structure”.
Reference: Activation of proto-oncogenes by disruption of chromosome neighborhoods. Hnisz D, Weintraub AS, Day DS, Valton AL, Bak RO, Li CH, Goldmann J, Lajoie BR, Fan ZP, Sigova AA, Reddy J, Borges-Rivera D, Lee TI, Jaenisch R, Porteus MH, Dekker J, Young RA. Science. (6280) 1454 – 8.
Chromatin is a complex of genomic DNA and proteins that make up the chromosomes within the nucleus of a cell. The organization of genomic material into chromatin is presumed to play an important role in regulating expression of genes. However, the precise relationship between spatial genome organization and expression of resident genes in health and disease remains unclear. Toward understanding 3D genome architecture and its relationship to gene regulation, 4D Nucleome researcher Dr. Yijun Ruan, Ph.D., a Jackson Laboratory Professor, Florine Deschenes Roux Chair and Director of Genome Sciences, and his team worked with international collaborators to identify a framework in which genes are organized and transcribed at the chromosomal level.
For these studies, the authors used advanced 3D genome mapping technologies and simulation, as well as super-resolution microscopy. These models revealed higher order chromosome folding and specific chromatin interactions, mediated by the chromatin proteins CTCF and cohesin. These chromatin structures suggest a “barrier” between genes being actively transcribed and those that are not. Importantly, these studies further uncovered potential mechanistic links between genetic mutations associated with specific disease and chromatin topology.
“The significance of this paper lies in our advanced 3D genome mapping strategy,” Dr. Ruan said, “which allowed us to reveal, for the first time, the higher-order and detailed topological structures of the human genome mediated by CTCF and cohesin, and the relation to gene transcription regulation carried out by RNA polymerase II. This publication is also timely adding new excitement to the recently initiated 4D Nucleome program by NIH." For additional information, read The Jackson Laboratory news release.
Reference: CTCF-Mediated Human 3D Genome Architecture Reveals Chromatin Topology for Transcription. Tang Z, Luo OJ, Li X, Zheng M, Zhu JJ, Szalaj P, Trzaskoma P, Magalska A, Wlodarczyk J, Ruszczycki B, Michalski P, Piecuch E, Wang P, Wang D, Tian SZ, Penrad-Mobayed M, Sachs LM, Ruan X, Wei CL, Liu ET, Wilczynski GM, Plewczynski D, Li G, Ruan Y. Cell. (7) 11611 – 27.
This page last reviewed on August 24, 2018