Imperial College London

Professor Dame Amanda Fisher

Faculty of MedicineInstitute of Clinical Sciences

Head of the Institute of Clinical Sciences







Ms Alessandra Lisini +44 (0)20 3313 8236




CRB (Clinical Research Building)Hammersmith Campus





Publication Type

131 results found

Robles-Rebollo I, Cuartero S, Canellas-Socias A, Wells S, Karimi MM, Mereu E, Chivu AG, Heyn H, Whilding C, Dormann D, Marguerat S, Rioja I, Prinjha RK, Stumpf MPH, Fisher AG, Merkenschlager Met al., 2022, Cohesin couples transcriptional bursting probabilities of inducible enhancers and promoters., Nat Commun, Vol: 13

Innate immune responses rely on inducible gene expression programmes which, in contrast to steady-state transcription, are highly dependent on cohesin. Here we address transcriptional parameters underlying this cohesin-dependence by single-molecule RNA-FISH and single-cell RNA-sequencing. We show that inducible innate immune genes are regulated predominantly by an increase in the probability of active transcription, and that probabilities of enhancer and promoter transcription are coordinated. Cohesin has no major impact on the fraction of transcribed inducible enhancers, or the number of mature mRNAs produced per transcribing cell. Cohesin is, however, required for coupling the probabilities of enhancer and promoter transcription. Enhancer-promoter coupling may not be explained by spatial proximity alone, and at the model locus Il12b can be disrupted by selective inhibition of the cohesinopathy-associated BET bromodomain BD2. Our data identify discrete steps in enhancer-mediated inducible gene expression that differ in cohesin-dependence, and suggest that cohesin and BD2 may act on shared pathways.

Journal article

Djeghloul D, Dimond A, Kramer H, Brown K, Patel B, Wang Y-F, Futschik ME, Whilding C, Montoya A, Veland N, Cheriyamkunnel S, Montavon T, Jenuwein T, Merkenschlager M, Fisher AGet al., 2022, Loss of H3K9 tri-methylation alters chromosome compaction and transcription factor retention during mitosis, Publisher: BioArxiv

Recent studies have shown that repressive chromatin machinery, including DNA methyltransferases (DNMTs) and Polycomb Repressor Complexes (PRCs), bind to chromosomes throughout mitosis and their depletion results in increased chromosome size. Here we show that enzymes that catalyse H3K9 methylation, such as Suv39h1, Suv39h2, G9a and Glp, are also retained on mitotic chromosomes. Surprisingly however, mutants lacking H3K9me3 have unusually small and compact mitotic chromosomes that are associated with increased H3S10ph and H3K27me3 levels. Chromosome size and centromere compaction in these mutants were rescued by providing exogenous Suv39h1, or inhibiting Ezh2 activity. Quantitative proteomic comparisons of native mitotic chromosomes isolated from wildtype versus Suv39h1/Suv39h2 double-null ESCs revealed that H3K9me3 was essential for the efficient retention of bookmarking factors such as Esrrb. These results highlight an unexpected role for repressive heterochromatin domains in preserving transcription factor binding through mitosis, and underscore the importance of H3K9me3 for sustaining chromosome architecture and epigenetic memory during cell division.

Working paper

Brown KE, Fisher AG, 2021, Reprogramming lineage identity through cell-cell fusion, CURRENT OPINION IN GENETICS & DEVELOPMENT, Vol: 70, Pages: 15-23, ISSN: 0959-437X

Journal article

Weiss FD, Calderon L, Wang Y-F, Georgieva R, Guo Y, Cvetesic N, Kaur M, Dharmalingam G, Krantz ID, Lenhard B, Fisher A, Merkenschlager Met al., 2021, Neuronal genes deregulated in Cornelia de Lange Syndrome respond to removal and reexpression of cohesin, Nature Communications, Vol: 12, ISSN: 2041-1723

Cornelia de Lange Syndrome (CdLS) is a human developmental disorder caused by mutations that compromise the function of cohesin, a major regulator of 3D genome organization. Cognitive impairment is a universal and as yet unexplained feature of CdLS. We characterize the transcriptional profile of cortical neurons from CdLS patients and find deregulation of hundreds of genes enriched for neuronal functions related to synaptic transmission, signalling processes, learning and behaviour. Inducible proteolytic cleavage of cohesin disrupts 3D genome organization and transcriptional control in post-mitotic cortical mouse neurons, demonstrating that cohesin is continuously required for neuronal gene expression. The genes affected by acute depletion of cohesin belong to similar gene ontology classes and show significant numerical overlap with genes deregulated in CdLS. Interestingly, reconstitution of cohesin function largely rescues altered gene expression, including the expression of genes deregulated in CdLS.

Journal article

Karimi MM, Guo Y, Cui X, Pallikonda HA, Horkova V, Dore MH, Wang Y-F, Ruiz Gil S, Rodriguez-Eteban G, Robles Rebollo I, Bruno L, Georgieva R, Patel B, Elliott J, Dauphars D, Krangel MS, Lenhard B, Heyn H, Fisher AG, Stepanek O, Merkenschlager Met al., 2021, The order and logic of CD4 CD8 lineage choice and differentiation in mouse thymus, Nature Communications, Vol: 12, ISSN: 2041-1723

CD4 and CD8 mark helper and cytotoxic T cell lineages, respectively, and serve as coreceptors for MHC-restricted TCR recognition. How coreceptor expression is matched with TCR specificity is central to understanding CD4/CD8 lineage choice, but visualising coreceptor gene activity in individual selection intermediates has been technically challenging. It therefore remains unclear whether the sequence of coreceptor gene expression in selection intermediates follows a stereotypic pattern, or is responsive to signaling. Here we use single cell RNA sequencing (scRNA-seq) to classify mouse thymocyte selection intermediates by coreceptor gene expression. In the unperturbed thymus, Cd4+Cd8a- selection intermediates appear before Cd4-Cd8a+ selection intermediates, but the timing of these subsets is flexible according to the strength of TCR signals. Our data show that selection intermediates discriminate MHC class prior to the loss of coreceptor expression and suggest a model where signal strength informs the timing of coreceptor gene activity and ultimately CD4/CD8 lineage choice.

Journal article

Dimond A, Van de Pette M, Fisher AG, 2020, Illuminating epigenetics and inheritance in the immune system with bioluminescence, Trends in Immunology, Vol: 41, Pages: 994-1005, ISSN: 0167-5699

The remarkable process of light emission by living organisms has fascinated mankind for thousands of years. A recent expansion in the repertoire of catalytic luciferase enzymes, coupled with the discovery of the genes and pathways that encode different luciferin substrates, means that bioluminescence imaging (BLI) is set to revolutionize longitudinal and dynamic studies of gene control within biomedicine, including the regulation of immune responses. In this review article, we summarize recent advances in bioluminescence-based imaging approaches that promise to enlighten our understanding of in vivo gene and epigenetic control within the immune system.

Journal article

Djeghloul D, Patel B, Kramer H, Dimond A, Whilding C, Brown KE, Kohler A-C, Feytout A, Veland N, Elliott J, Bharat T, Tarafder A, Loewe J, Ng B, Guy J, Huseyin M, Klose RJ, Merkenschlager M, Fisher AGet al., 2020, Identifying proteins bound to native mitotic ESC chromosomes reveals chromatin repressors are important for compaction, Nature Communications, Vol: 11, Pages: 1-15, ISSN: 2041-1723

Epigenetic information is transmitted from mother to daughter cells through mitosis. Here, to identify factors that might play a role in conveying epigenetic memory through cell division, we report on the isolation of unfixed, native chromosomes from metaphase-arrested cells using flow cytometry and perform LC-MS/MS to identify chromosome-bound proteins. A quantitative proteomic comparison between metaphase-arrested cell lysates and chromosome-sorted samples reveals a cohort of proteins that were significantly enriched on mitotic ESC chromosomes. These include pluripotency-associated transcription factors, repressive chromatin-modifiers such as PRC2 and DNA methyl-transferases, and proteins governing chromosome architecture. Deletion of PRC2, Dnmt1/3a/3b or Mecp2 in ESCs leads to an increase in the size of individual mitotic chromosomes, consistent with de-condensation. Similar results were obtained by the experimental cleavage of cohesin. Thus, we identify chromosome-bound factors in pluripotent stem cells during mitosis and reveal that PRC2, DNA methylation and Mecp2 are required to maintain chromosome compaction.

Journal article

Bruno L, Ramlall V, Studer RA, Sauer S, Bradley D, Dharmalingam G, Carroll T, Ghoneim M, Chopin M, Nutt SL, Elderkin S, Rueda DS, Fisher AG, Siggers T, Beltrao P, Merkenschlager Met al., 2019, Selective deployment of transcription factor paralogs with submaximal strength facilitates gene regulation in the immune system, Nature Immunology, Vol: 20, Pages: 1372-1380, ISSN: 1529-2908

In multicellular organisms, duplicated genes can diverge through tissue-specific gene expression patterns, as exemplified by highly regulated expression of Runx transcription factor paralogs with apparent functional redundancy. Here we asked what cell type-specific biologies might be supported by the selective expression of Runx paralogs during Langerhans cell and inducible regulatory T cell differentiation. We uncovered functional non-equivalence between Runx paralogs. Selective expression of native paralogs allowed integration of transcription factor activity with extrinsic signals, while non-native paralogs enforced differentiation even in the absence of exogenous inducers. DNA-binding affinity was controlled by divergent amino acids within the otherwise highly conserved RUNT domain, and evolutionary reconstruction suggested convergence of RUNT domain residues towards sub-maximal strength. Hence, the selective expression of gene duplicates in specialized cell types can synergize with the acquisition of functional differences to enable appropriate gene expression, lineage choice and differentiation in the mammalian immune system.

Journal article

Ferreirós-Vidal I, Carroll T, Zhang T, Lagani V, Ramirez RN, Ing-Simmons E, Garcia A, Cooper L, Liang Z, Papoutsoglou G, Dharmalingam G, Guo Y, Tarazona S, Fernandes SJ, Noori P, SIlberberg G, Fisher AG, Tsamardinos I, Mortazavi A, Lenhard B, Conesa A, Tegner J, Merkenschlager M, Gomez-Cabrero Det al., 2019, Feedforward regulation of Myc coordinates lineage-specific with housekeeping gene expression during B cell progenitor cell differentiation, PLoS Biology, Vol: 17, ISSN: 1544-9173

The differentiation of self-renewingprogenitor cells requires not only the regulation of lineage-and developmental stage-specific genes, but also the coordinated adaptation of housekeeping functionsfrom a metabolically active, proliferative state towards quiescence. How metabolic and cell cycle states are coordinated with the regulation of cell type-specific genes is an important question, as dissociation between differentiation, cell cycle, and metabolic states is a hallmark of cancer. Here we use a model system to systematically identify key transcriptional regulators of Ikaros-dependent B cell progenitor differentiation. We find that the coordinated regulation of housekeeping functions and tissue-specific gene expressionrequires afeedforward circuit whereby Ikarosdownregulates the expression of Myc. Our findings show how coordination between differentiation and housekeeping statescan be achieved by interconnected regulators. Similar principles likely coordinate differentiation and housekeeping functions during progenitor cell differentiation in other cell lineages.

Journal article

Calderon L, Weiss FD, Carroll T, Irvine EE, Dharmalingam G, Tossell K, De Paola V, Whilding C, Ungless MA, Withers DJ, Fisher AG, Merkenschlager Met al., 2019, Cohesin is continuously required to sustain neuronal gene expression, 29th Mammalian Genetics and Development Workshop of the Genetics-Society, Publisher: HINDAWI LTD, ISSN: 0016-6723

Conference paper

Millership S, Tunster SJ, Van de Pette M, Choudhury A, Irvine E, Christian M, Fisher AG, John RM, Scott J, Withers DJet al., 2018, Neuronatin deletion causes postnatal growth restriction and adult obesity in 129S2/Sv mice, Molecular Metabolism, Vol: 18, Pages: 97-106, ISSN: 2212-8778

ObjectiveImprinted genes are crucial for the growth and development of fetal and juvenile mammals. Altered imprinted gene dosage causes a variety of human disorders, with growth and development during these crucial early stages strongly linked with future metabolic health in adulthood. Neuronatin (Nnat) is a paternally expressed imprinted gene found in neuroendocrine systems and white adipose tissue and is regulated by the diet and leptin. Neuronatin expression is downregulated in obese children and has been associated with stochastic obesity in C57BL/6 mice. However, our recent studies of Nnat null mice on this genetic background failed to display any body weight or feeding phenotypes but revealed a defect in glucose-stimulated insulin secretion due to the ability of neuronatin to potentiate signal peptidase cleavage of preproinsulin. Nnat deficiency in beta cells therefore caused a lack of appropriate storage and secretion of mature insulin.MethodsTo further explore the potential role of Nnat in the regulation of body weight and adiposity, we studied classical imprinting-related phenotypes such as placental, fetal, and postnatal growth trajectory patterns that may impact upon subsequent adult metabolic phenotypes.ResultsHere we find that, in contrast to the lack of any body weight or feeding phenotypes on the C57BL/6J background, deletion of Nnat in mice on 129S2/Sv background causes a postnatal growth restriction with reduced adipose tissue accumulation, followed by catch up growth after weaning. This was in the absence of any effect on fetal growth or placental development. In adult 129S2/Sv mice, Nnat deletion was associated with hyperphagia, reduced energy expenditure, and partial leptin resistance. Lack of neuronatin also potentiated obesity caused by either aging or high fat diet feeding.ConclusionsThe imprinted gene Nnat plays a key role in postnatal growth, adult energy homeostasis, and the pathogenesis of obesity via catch up growth effects, but this role

Journal article

Merkenschlager M, Cuartero S, Weiss F, Dharmalingam G, Guo Y, Ing-Simmons E, Masella S, Robles-Rebollo I, Xiao X, Barozzi I, Djeghloul D, Amano M, Niskanen H, Petretto E, Dowell R, Tachibana K, Kaikkonen M, Nasmyth K, Lenhard B, Natoli G, Fisher Aet al., 2018, Control of inducible gene expression links cohesin to hematopoietic progenitor self-renewal and differentiation, Nature Immunology, Vol: 19, Pages: 932-941, ISSN: 1529-2908

Cohesin is important for 3D genome organization. Nevertheless, even the complete removal of cohesin has surprisingly little impact on steady-state gene transcription and enhancer activity. Here we show that cohesin is required for the core transcriptional response of primary macrophages to microbial signals, and for inducible enhancer activity that underpins inflammatory gene expression. Consistent with a role for inflammatory signals in promoting myeloid differentiation of hematopoietic stem and progenitor cells (HPSCs), cohesin mutations in HSPCs led to reduced inflammatory gene expression and increased resistance to differentiation-inducing inflammatory stimuli. These findings uncover an unexpected dependence of inducible gene expression on cohesin, link cohesin with myeloid differentiation, and may help explain the prevalence of cohesin mutations in human acute myeloid leukemia.

Journal article

Cantone I, Fisher AG, 2017, Human X chromosome inactivation and reactivation: implications for cell reprogramming and disease, Philosophical Transactions of the Royal Society B: Biological Sciences, Vol: 372, ISSN: 1471-2970

X chromosome inactivation (XCI) is an exemplar of epigenetic regulation that is set up as pluripotent cells differentiate. Once established, XCI is stably propagated, but can be reversed in vivo or by pluripotent reprogramming in vitro. Although reprogramming provides a useful model for inactive X (Xi) reactivation in mouse, the relative instability andheterogeneity of human ESCs and iPSCs, hampers comparable progress in human. Here we review studies aimed at reactivating the human Xi using different reprogramming strategies. We outline our recent results using mouse ESCs to reprogram female human fibroblasts by cell-cell fusion. We show that pluripotent reprogramming induces widespread and rapid chromatin remodelling in which the human Xi loses XIST and H3K27m3 enrichment and selected Xi genes become reactivated, ahead of mitotic division. Using RNA sequencing to map the extent of human Xi reactivation, and chromatin modifying drugs to potentiate reactivation, we outline how this approach could be used to better design strategies to reexpress human X-linked loci. As cell fusion induces the expression of human pluripotency genes that represent both the 'primed' and 'naïve' states, this approach may also offer a fresh opportunity to segregate human pluripotent states with distinct Xi expression profiles, using single-cell-based approaches.

Journal article

Fisher CL, Marks H, Cho LT-Y, Andrews R, Wormald S, Carroll T, Iyer V, Tate P, Rosen B, Stunnenberg HG, Fisher AG, Skarnes WCet al., 2017, An efficient method for generation of bi-allelic null mutant mouse embryonic stem cells and its application for investigating epigenetic modifiers., Nucleic Acids Research, Vol: 45, Pages: e174-e174, ISSN: 0305-1048

Mouse embryonic stem (ES) cells are a popular model system to study biological processes, though uncovering recessive phenotypes requires inactivating both alleles. Building upon resources from the International Knockout Mouse Consortium (IKMC), we developed a targeting vector for second allele inactivation in conditional-ready IKMC 'knockout-first' ES cell lines. We applied our technology to several epigenetic regulators, recovering bi-allelic targeted clones with a high efficiency of 60% and used Flp recombinase to restore expression in two null cell lines to demonstrate how our system confirms causality through mutant phenotype reversion. We designed our strategy to select against re-targeting the 'knockout-first' allele and identify essential genes in ES cells, including the histone methyltransferase Setdb1. For confirmation, we exploited the flexibility of our system, enabling tamoxifen inducible conditional gene ablation while controlling for genetic background and tamoxifen effects. Setdb1 ablated ES cells exhibit severe growth inhibition, which is not rescued by exogenous Nanog expression or culturing in naive pluripotency '2i' media, suggesting that the self-renewal defect is mediated through pluripotency network independent pathways. Our strategy to generate null mutant mouse ES cells is applicable to thousands of genes and repurposes existing IKMC Intermediate Vectors.

Journal article

Fisher AG, Stumpf MPH, Merkenschlager M, 2017, Reconciling Epigenetic Memory and Transcriptional Responsiveness, CELL SYSTEMS, Vol: 4, Pages: 373-374, ISSN: 2405-4712

The molecular basis of cellular memory is important but poorly understood. Using estimates of histone dynamics, Martin Howard and colleagues construct a mathematical model that helps to explain both the stability and flexibility of Polycomb-mediated gene regulation in cellular memory.

Journal article

Liang Z, Brown KE, Carroll T, Taylor B, Vidal IF, Hendrich B, Rueda D, Fisher AG, Merkenschlager Met al., 2017, A high-resolution map of transcriptional repression, ELIFE, Vol: 6, ISSN: 2050-084X

Turning genes on and off is essential for development and homeostasis, yet little is known about the sequence and causal role of chromatin state changes during the repression of active genes. This is surprising, as defective gene silencing underlies developmental abnormalities and disease. Here we delineate the sequence and functional contribution of transcriptional repression mechanisms at high temporal resolution. Inducible entry of the NuRD-interacting transcriptional regulator Ikaros into mouse pre-B cell nuclei triggered immediate binding to target gene promoters. Rapid RNAP2 eviction, transcriptional shutdown, nucleosome invasion, and reduced transcriptional activator binding required chromatin remodeling by NuRD-associated Mi2beta/CHD4, but were independent of HDAC activity. Histone deacetylation occurred after transcriptional repression. Nevertheless, HDAC activity contributed to stable gene silencing. Hence, high resolution mapping of transcriptional repression reveals complex and interdependent mechanisms that underpin rapid transitions between transcriptional states, and elucidates the temporal order, functional role and mechanistic separation of NuRD-associated enzymatic activities.

Journal article

Van de Pette M, Abbas A, Feytout A, McNamara G, Bruno L, To WK, Dimond A, Sardini A, Webster Z, McGinty J, Paul EJ, Ungless MA, French PMW, Withers DJ, Uren A, Ferguson-Smith AC, Merkenschlager M, John RM, Fisher AGet al., 2017, Visualizing changes in Cdkn1c expression links early life adversity to imprint mis-regulation in adults, Cell Reports, Vol: 31, Pages: 1090-1099, ISSN: 2211-1247

Imprinted genes are regulated according to parental origin and can influence embryonic growth and metabolism and confer disease susceptibility.Here we designed sensitive allele-specific reporters to non-invasively monitor imprinted Cdkn1cexpression in mice and showed that expression was modulated by environmental factors encounteredin utero.Acute exposure to chromatin modifyingdrugs resulted in de-repression of paternally inherited (silent) Cdkn1calleles in embryos that was temporary and resolved after birth.In contrast, deprivation of maternal dietary proteinin uteroprovoked permanent de-repression of imprinted Cdkn1cexpression that was sustained into adulthood and occurred through a folate-dependent mechanism of DNA methylation loss.Given the function of imprinted genes in regulating behavior and metabolic processes in adults, these results establish imprinting deregulation as a credible mechanism linking early life adversity to later-life outcomes.Furthermore,Cdkn1c-luciferasemice offer non-invasivetools to identify factors that disrupt epigenetic processes and strategies to limit their long-term impact.

Journal article

Cantone I, Dharmalingam G, Chan YW, Kohler AC, Lenhard B, Merkenschlager M, Fisher AGet al., 2017, Allele-specific analysis of cell fusion-mediated pluripotent reprograming reveals distinct and predictive susceptibilities of human X-linked genes to reactivation, Genome Biology, Vol: 18, ISSN: 1474-760X

BackgroundInactivation of one X chromosome is established early in female mammalian development and can be reversed in vivo and in vitro when pluripotency factors are re-expressed. The extent of reactivation along the inactive X chromosome (Xi) and the determinants of locus susceptibility are, however, poorly understood. Here we use cell fusion-mediated pluripotent reprograming to study human Xi reactivation and allele-specific single nucleotide polymorphisms (SNPs) to identify reactivated loci.ResultsWe show that a subset of human Xi genes is rapidly reactivated upon re-expression of the pluripotency network. These genes lie within the most evolutionary recent segments of the human X chromosome that are depleted of LINE1 and enriched for SINE elements, predicted to impair XIST spreading. Interestingly, this cadre of genes displays stochastic Xi expression in human fibroblasts ahead of reprograming. This stochastic variability is evident between clones, by RNA-sequencing, and at the single-cell level, by RNA-FISH, and is not attributable to differences in repressive histone H3K9me3 or H3K27me3 levels. Treatment with the DNA demethylating agent 5-deoxy-azacytidine does not increase Xi expression ahead of reprograming, but instead reveals a second cadre of genes that only become susceptible to reactivation upon induction of pluripotency.ConclusionsCollectively, these data not only underscore the multiple pathways that contribute to maintaining silencing along the human Xi chromosome but also suggest that transcriptional stochasticity among human cells could be useful for predicting and engineering epigenetic strategies to achieve locus-specific or domain-specific human Xi gene reactivation.

Journal article

Fisher AG, 2016, Ordered chromatin changes and human X chromosome reactivation by cell fusion-mediated pluripotent reprogramming, Nature Communications, Vol: 7, ISSN: 2041-1723

Erasure of epigenetic memory is required to convert somatic cells towards pluripotency. Reactivation of the inactive X chromosome (Xi) has been used to model epigenetic reprogramming in mouse, but human studies are hampered by Xi epigenetic instability and difficulties in tracking partially reprogrammed iPSCs. Here we use cell fusion to examine the earliest events in the reprogramming-induced Xi reactivation of human female fibroblasts. We show that a rapid and widespread loss of Xi-associated H3K27me3 and XIST occurs in fused cells and precedes the bi-allelic expression of selected Xi-genes by many heterokaryons (30–50%). After cell division, RNA-FISH and RNA-seq analyses confirm that Xi reactivation remains partial and that induction of human pluripotency-specific XACT transcripts is rare (1%). These data effectively separate pre- and post-mitotic events in reprogramming-induced Xi reactivation and reveal a complex hierarchy of epigenetic changes that are required to reactivate the genes on the human Xi chromosome.

Journal article

Graham B, Marcais A, Dharmalingam G, Carroll T, Kanellopoulou C, Graumann J, Nesterova TB, Bermange A, Brazauskas P, Xella B, Kriaucionis S, Higgs DR, Brockdorff N, Mann M, Fisher AG, Merkenschlager Met al., 2016, MicroRNAs of the miR-290-295 Family Maintain Bivalency in Mouse Embryonic Stem Cells., Stem Cell Reports, Vol: 6, Pages: 635-642, ISSN: 2213-6711

Numerous developmentally regulated genes in mouse embryonic stem cells (ESCs) are marked by both active (H3K4me3)- and polycomb group (PcG)-mediated repressive (H3K27me3) histone modifications. This bivalent state is thought to be important for transcriptional poising, but the mechanisms that regulate bivalent genes and the bivalent state remain incompletely understood. Examining the contribution of microRNAs (miRNAs) to the regulation of bivalent genes, we found that the miRNA biogenesis enzyme DICER was required for the binding of the PRC2 core components EZH2 and SUZ12, and for the presence of the PRC2-mediated histone modification H3K27me3 at many bivalent genes. Genes that lost bivalency were preferentially upregulated at the mRNA and protein levels. Finally, reconstituting Dicer-deficient ESCs with ESC miRNAs restored bivalent gene repression and PRC2 binding at formerly bivalent genes. Therefore, miRNAs regulate bivalent genes and the bivalent state itself.

Journal article

Chiu FWY, Bagci H, Fisher AG, deMello AJ, Elvira KSet al., 2016, A microfluidic toolbox for cell fusion, Journal of Chemical Technology and Biotechnology, Vol: 91, Pages: 16-24, ISSN: 1097-4660

Cellular fusion is a key process in many fields ranging from historical gene mapping studies and monoclonal antibody production, through to cell reprogramming. Traditional methodologies for cell fusion rely on the random pairing of different cell types and generally result in low and variable fusion efficiencies. These approaches become particularly limiting where substantial numbers of bespoke one-to-one fusions are required, for example, for in-depth studies of nuclear reprogramming mechanisms. In recent years, microfluidic technologies have proven valuable in creating platforms where the manipulation of single cells is highly efficient, rapid and controllable. These technologies also allow the integration of different experimental steps and characterisation processes into a single platform. Although the application of microfluidic methodologies to cell fusion studies is promising, current technologies that rely on static trapping are limited both in terms of the overall number of fused cells produced and their experimental accessibility. Here we review some of the most exciting breakthroughs in core microfluidic technologies that will allow the creation of integrated platforms for controlled cell fusion at high throughput. © 2015 Society of Chemical Industry.

Journal article

Malinowski AR, Fisher AG, 2016, Reprogramming of Somatic Cells Towards Pluripotency by Cell Fusion., Pages: 289-299

Pluripotent reprogramming can be dominantly induced in a somatic nucleus upon fusion with a pluripotent cell such as embryonic stem (ES) cell. Cell fusion between ES cells and somatic cells results in the formation of heterokaryons, in which the somatic nuclei begin to acquire features of the pluripotent partner. The generation of interspecies heterokaryons between mouse ES- and human somatic cells allows an experimenter to distinguish the nuclear events occurring specifically within the reprogrammed nucleus. Therefore, cell fusion provides a simple and rapid approach to look at the early nuclear events underlying pluripotent reprogramming. Here, we describe a polyethylene glycol (PEG)-mediated cell fusion protocol to generate interspecies heterokaryons and intraspecies hybrids between ES cells and B lymphocytes or fibroblasts.

Book chapter

Gupta P, Lavagnolli T, Mira-Bontenbal H, Fisher AG, Merkenschlager Met al., 2015, Cohesin's role in pluripotency and reprogramming., Cell Cycle, Vol: 15, Pages: 324-330, ISSN: 1551-4005

Cohesin is required for ES cell self-renewal and iPS-mediated reprogramming of somatic cells. This may indicate a special role for cohesin in the regulation of pluripotency genes, perhaps by mediating long-range chromosomal interactions between gene regulatory elements. However, cohesin is also essential for genome integrity, and its depletion from cycling cells induces DNA damage responses. Hence, the failure of cohesin-depleted cells to establish or maintain pluripotency gene expression could be explained by a loss of long-range interactions or by DNA damage responses that undermine pluripotency gene expression. In recent work we began to disentangle these possibilities by analyzing reprogramming in the absence of cell division. These experiments showed that cohesin was not specifically required for reprogramming, and that the expression of most pluripotency genes was maintained when ES cells were acutely depleted of cohesin. Here we take this analysis to its logical conclusion by demonstrating that deliberately inflicted DNA damage - and the DNA damage that results from proliferation in the absence of cohesin - can directly interfere with pluripotency and reprogramming. The role of cohesin in pluripotency and reprogramming may therefore be best explained by essential cohesin functions in the cell cycle.

Journal article

Landeira D, Bagci H, Malinowski AR, Brown KE, Soza-Ried J, Feytout A, Webster Z, Ndjetehe E, Cantone I, Asenjo HG, Brockdorff N, Carroll T, Merkenschlager M, Fisher AGet al., 2015, Jarid2 coordinates nanog expression and PCP/Wnt signaling required for efficient ESC differentiation and early embryo development, Cell Reports, Vol: 12, Pages: 573-586, ISSN: 2211-1247

Jarid2 is part of the Polycomb Repressor complex 2 (PRC2) responsible for genome-wide H3K27me3 deposition. Unlike other PRC2-deficient embryonic stem cells (ESCs), however, Jarid2-deficient ESCs show a severe differentiation block, altered colony morphology, and distinctive patterns of deregulated gene expression. Here, we show that Jarid2−/− ESCs express constitutively high levels of Nanog but reduced PCP signaling components Wnt9a, Prickle1, and Fzd2 and lowered β-catenin activity. Depletion of Wnt9a/Prickle1/Fzd2 from wild-type ESCs or overexpression of Nanog largely phenocopies these cellular defects. Co-culture of Jarid2−/− with wild-type ESCs restores variable Nanog expression and β-catenin activity and can partially rescue the differentiation block of mutant cells. In addition, we show that ESCs lacking Jarid2 or Wnt9a/Prickle1/Fzd2 or overexpressing Nanog induce multiple ICM formation when injected into normal E3.5 blastocysts. These data describe a previously unrecognized role for Jarid2 in regulating a core pluripotency and Wnt/PCP signaling circuit that is important for ESC differentiation and for pre-implantation development.

Journal article

Blevins R, Bruno L, Carroll T, Elliott J, Marcais A, Loh C, Hertweck A, Krek A, Rajewsky N, Chen C-Z, Fisher AG, Merkenschlager Met al., 2015, microRNAs regulate cell-to-cell variability of endogenous target gene expression in developing mouse thymocytes, Plos Genetics, Vol: 11, Pages: 1-19, ISSN: 1553-7404

The development and homeostasis of multicellular organisms relies on gene regulationwithin individual constituent cells. Gene regulatory circuits that increase the robustness ofgene expression frequently incorporate microRNAs as post-transcriptional regulators.Computational approaches, synthetic gene circuits and observations in model organismspredict that the co-regulation of microRNAs and their target mRNAs can reduce cell-to-cellvariability in the expression of target genes. However, whether microRNAs directly regulatevariability of endogenous gene expression remains to be tested in mammalian cells. Herewe use quantitative flow cytometry to show that microRNAs impact on cell-to-cell variabilityof protein expression in developing mouse thymocytes. We find two distinct mechanismsthat control variation in the activation-induced expression of the microRNA target CD69.First, the expression of miR-17 and miR-20a, two members of the miR-17-92 cluster, is coregulatedwith the target mRNA Cd69 to form an activation-induced incoherent feed-forwardloop. Another microRNA, miR-181a, acts at least in part upstream of the target mRNA Cd69to modulate cellular responses to activation. The ability of microRNAs to render gene expressionmore uniform across mammalian cell populations may be important for normal developmentand for disease.

Journal article

Ing-Simmons E, Seitan VC, Faure AJ, Flicek P, Carroll T, Dekker J, Fisher AG, Lenhard B, Merkenschlager Met al., 2015, Spatial enhancer clustering and regulation of enhancer-proximal genes by cohesin, Genome Research, Vol: 25, Pages: 504-513, ISSN: 1054-9803

In addition to mediating sister chromatid cohesion during the cell cycle, the cohesin complex associates with CTCF and with active gene regulatory elements to form long-range interactions between its binding sites. Genome-wide chromosome conformation capture had shown that cohesin's main role in interphase genome organization is in mediating interactions within architectural chromosome compartments, rather than specifying compartments per se. However, it remains unclear how cohesin-mediated interactions contribute to the regulation of gene expression. We have found that the binding of CTCF and cohesin is highly enriched at enhancers and in particular at enhancer arrays or “super-enhancers” in mouse thymocytes. Using local and global chromosome conformation capture, we demonstrate that enhancer elements associate not just in linear sequence, but also in 3D, and that spatial enhancer clustering is facilitated by cohesin. The conditional deletion of cohesin from noncycling thymocytes preserved enhancer position, H3K27ac, H4K4me1, and enhancer transcription, but weakened interactions between enhancers. Interestingly, ∼50% of deregulated genes reside in the vicinity of enhancer elements, suggesting that cohesin regulates gene expression through spatial clustering of enhancer elements. We propose a model for cohesin-dependent gene regulation in which spatial clustering of enhancer elements acts as a unified mechanism for both enhancer-promoter “connections” and “insulation.”

Journal article

Lavagnolli T, Gupta P, Hörmanseder E, Mira-Bontenbal H, Dharmalingam G, Carroll T, Gurdon JB, Fisher AG, Merkenschlager Met al., 2015, Initiation and maintenance of pluripotency gene expression in the absence of cohesin., Genes Dev, Vol: 29, Pages: 23-38

Cohesin is implicated in establishing and maintaining pluripotency. Whether this is because of essential cohesin functions in the cell cycle or in gene regulation is unknown. Here we tested cohesin's contribution to reprogramming in systems that reactivate the expression of pluripotency genes in the absence of proliferation (embryonic stem [ES] cell heterokaryons) or DNA replication (nuclear transfer). Contrary to expectations, cohesin depletion enhanced the ability of ES cells to initiate somatic cell reprogramming in heterokaryons. This was explained by increased c-Myc (Myc) expression in cohesin-depleted ES cells, which promoted DNA replication-dependent reprogramming of somatic fusion partners. In contrast, cohesin-depleted somatic cells were poorly reprogrammed in heterokaryons, due in part to defective DNA replication. Pluripotency gene induction was rescued by Myc, which restored DNA replication, and by nuclear transfer, where reprogramming does not require DNA replication. These results redefine cohesin's role in pluripotency and reveal a novel function for Myc in promoting the replication-dependent reprogramming of somatic nuclei.

Journal article

Marcais A, Blevins R, Graumann J, Feytout A, Dharmalingam G, Carroll T, Amado IF, Bruno L, Lee K, Walzer T, Mann M, Freitas AA, Boothby M, Fisher AG, Merkenschlager Met al., 2014, microRNA-mediated regulation of mTOR complex components facilitates discrimination between activation and anergy in CD4 T cells, Journal of Experimental Medicine, Vol: 211, Pages: 2281-2295, ISSN: 0022-1007

T cell receptor (TCR) signals can elicit full activation with acquisition of effector functions or a state of anergy. Here, we ask whether microRNAs affect the interpretation of TCR signaling. We find that Dicer-deficient CD4 T cells fail to correctly discriminate between activating and anergy-inducing stimuli and produce IL-2 in the absence of co-stimulation. Excess IL-2 production by Dicer-deficient CD4 T cells was sufficient to override anergy induction in WT T cells and to restore inducible Foxp3 expression in Il2-deficient CD4 T cells. Phosphorylation of Akt on S473 and of S6 ribosomal protein was increased and sustained in Dicer-deficient CD4 T cells, indicating elevated mTOR activity. The mTOR components Mtor and Rictor were posttranscriptionally deregulated, and the microRNAs Let-7 and miR-16 targeted the Mtor and Rictor mRNAs. Remarkably, returning Mtor and Rictor to normal levels by deleting one allele of Mtor and one allele of Rictor was sufficient to reduce Akt S473 phosphorylation and to reduce co-stimulation–independent IL-2 production in Dicer-deficient CD4 T cells. These results show that microRNAs regulate the expression of mTOR components in T cells, and that this regulation is critical for the modulation of mTOR activity. Hence, microRNAs contribute to the discrimination between T cell activation and anergy.

Journal article

Piccolo FM, Fisher AG, 2014, Getting rid of DNA methylation, TRENDS IN CELL BIOLOGY, Vol: 24, Pages: 136-143, ISSN: 0962-8924

Journal article

Seitan VC, Faure AJ, Zhan Y, McCord RP, Lajoie BR, Ing-Simmons E, Lenhard B, Giorgetti L, Heard E, Fisher AG, Flicek P, Dekker J, Merkenschlager Met al., 2013, Cohesin-based chromatin interactions enable regulated gene expression within preexisting architectural compartments, GENOME RESEARCH, Vol: 23, Pages: 2066-2077, ISSN: 1088-9051

Journal article

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