Publications by authors named "Ikhlak Ahmed"

23 Publications

  • Page 1 of 1

Regulation of Circular RNA CircNFATC3 in Cancer Cells Alters Proliferation, Migration, and Oxidative Phosphorylation.

Front Cell Dev Biol 2021 19;9:595156. Epub 2021 Mar 19.

Weill Cornell Medicine-Qatar, Doha, Qatar.

Circular RNAs were once considered artifacts of transcriptome sequencing but have recently been identified as functionally relevant in different types of cancer. Although there is still no clear main function of circRNAs, several studies have revealed that circRNAs are expressed in various eukaryotic organisms in a regulated manner often independent of their parental linear isoforms demonstrating conservation across species. circNFATC3, an abundant and uncharacterized circular RNA of exon 2 and 3 from , was identified in transcriptomic data of solid tumors. Here we show that circNFATC3 gain- and loss-of-function experiments using RNAi-mediated circRNA silencing and circular mini vector-mediated overexpression of circularized constructs in breast and ovarian cancer cell lines affect molecular phenotypes. The knockdown of circNFATC3 induces a reduction in cell proliferation, invasion, migration, and oxidative phosphorylation. Gain-of-function of circNFATC3 in MDA-MB-231 and SK-OV-3 cells show a significant increase in cell proliferation, migration, and respiration. The above results suggest that circNFATC3 is a functionally relevant circular RNA in breast and ovarian cancer.
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http://dx.doi.org/10.3389/fcell.2021.595156DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8017239PMC
March 2021

Insights Into the Role of CircRNAs: Biogenesis, Characterization, Functional, and Clinical Impact in Human Malignancies.

Front Cell Dev Biol 2021 5;9:617281. Epub 2021 Feb 5.

Functional and Molecular Imaging Laboratory, Cancer Research Department, Sidra Medicine, Doha, Qatar.

Circular RNAs (circRNAs) are an evolutionarily conserved novel class of non-coding endogenous RNAs (ncRNAs) found in the eukaryotic transcriptome, originally believed to be aberrant RNA splicing by-products with decreased functionality. However, recent advances in high-throughput genomic technology have allowed circRNAs to be characterized in detail and revealed their role in controlling various biological and molecular processes, the most essential being gene regulation. Because of the structural stability, high expression, availability of microRNA (miRNA) binding sites and tissue-specific expression, circRNAs have become hot topic of research in RNA biology. Compared to the linear RNA, circRNAs are produced differentially by backsplicing exons or lariat introns from a pre-messenger RNA (mRNA) forming a covalently closed loop structure missing 3' poly-(A) tail or 5' cap, rendering them immune to exonuclease-mediated degradation. Emerging research has identified multifaceted roles of circRNAs as miRNA and RNA binding protein (RBP) sponges and transcription, translation, and splicing event regulators. CircRNAs have been involved in many human illnesses, including cancer and neurodegenerative disorders such as Alzheimer's and Parkinson's disease, due to their aberrant expression in different pathological conditions. The functional versatility exhibited by circRNAs enables them to serve as potential diagnostic or predictive biomarkers for various diseases. This review discusses the properties, characterization, profiling, and the diverse molecular mechanisms of circRNAs and their use as potential therapeutic targets in different human malignancies.
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http://dx.doi.org/10.3389/fcell.2021.617281DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7894079PMC
February 2021

Role of non-coding RNA networks in leukemia progression, metastasis and drug resistance.

Mol Cancer 2020 03 12;19(1):57. Epub 2020 Mar 12.

Translational Research Institute, Academic Health System, Hamad Medical Corporation, P.O. Box 3050, Doha, Qatar.

Early-stage detection of leukemia is a critical determinant for successful treatment of the disease and can increase the survival rate of leukemia patients. The factors limiting the current screening approaches to leukemia include low sensitivity and specificity, high costs, and a low participation rate. An approach based on novel and innovative biomarkers with high accuracy from peripheral blood offers a comfortable and appealing alternative to patients, potentially leading to a higher participation rate.Recently, non-coding RNAs due to their involvement in vital oncogenic processes such as differentiation, proliferation, migration, angiogenesis and apoptosis have attracted much attention as potential diagnostic and prognostic biomarkers in leukemia. Emerging lines of evidence have shown that the mutational spectrum and dysregulated expression of non-coding RNA genes are closely associated with the development and progression of various cancers, including leukemia. In this review, we highlight the expression and functional roles of different types of non-coding RNAs in leukemia and discuss their potential clinical applications as diagnostic or prognostic biomarkers and therapeutic targets.
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http://dx.doi.org/10.1186/s12943-020-01175-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7069174PMC
March 2020

Identification of human genetic variants controlling circular RNA expression.

RNA 2019 12 13;25(12):1765-1778. Epub 2019 Sep 13.

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, Doha, Qatar.

Circular RNAs (circRNAs) are abundant in eukaryotic transcriptomes and have been linked to various human disorders. However, understanding genetic control of circular RNA expression is in the early stages. Here we present the first integrated analysis of circRNAs and genome sequence variation from lymphoblastoid cell lines of the 1000 Genomes Project. We identified thousands of circRNAs in the RNA-seq data and show their association with local single-nucleotide polymorphic sites, referred to as circQTLs, which influence the circRNA transcript abundance. Strikingly, we found that circQTLs exist independently of eQTLs with most circQTLs having no effect on mRNA expression. Only a fraction of the polymorphic sites are shared and linked to both circRNA and mRNA expression with mostly similar effects on circular and linear RNA. A shared intronic QTL, rs55928920, of HMSD gene drives the circular and linear expression in opposite directions, potentially modulating circRNA levels at the expense of mRNA. Finally, circQTLs and eQTLs are largely independent and exist in separate linkage disequilibrium (LD) blocks with circQTLs highly enriched for functional genomic elements and regulatory regions. This study reveals a previously uncharacterized role of DNA sequence variation in human circular RNA regulation.
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http://dx.doi.org/10.1261/rna.071654.119DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6859849PMC
December 2019

Cell Type-Specific TGF-β Mediated EMT in 3D and 2D Models and Its Reversal by TGF-β Receptor Kinase Inhibitor in Ovarian Cancer Cell Lines.

Int J Mol Sci 2019 Jul 22;20(14). Epub 2019 Jul 22.

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, Education City, Qatar Foundation, Doha P.O. Box No. 24144, Qatar.

Transcriptome profiling of 3D models compared to 2D models in various cancer cell lines shows differential expression of TGF-β-mediated and cell adhesion pathways. Presence of TGF-β in these cell lines shows an increased invasion potential which is specific to cell type. In the present study, we identified exogenous addition of TGF-β can induce Epithelial to Mesenchymal Transition (EMT) in a few cancer cell lines. RNA sequencing and real time PCR were carried out in different ovarian cancer cell lines to identify molecular profiling and metabolic profiling. Since EMT induction by TGF-β is cell-type specific, we decided to select two promising ovarian cancer cell lines as model systems to study EMT. TGF-β modulation in EMT and cancer invasion were successfully depicted in both 2D and 3D models of SKOV3 and CAOV3 cell lines. Functional evaluation in 3D and 2D models demonstrates that the addition of the exogenous TGF-β can induce EMT and invasion in cancer cells by turning them into aggressive phenotypes. TGF-β receptor kinase I inhibitor (LY364947) can revert the TGF-β effect in these cells. In a nutshell, TGF-β can induce EMT and migration, increase aggressiveness, increase cell survival, alter cell characteristics, remodel the Extracellular Matrix (ECM) and increase cell metabolism favorable for tumor invasion and metastasis. We concluded that transcriptomic and phenotypic effect of TGF-β and its inhibitor is cell-type specific and not cancer specific.
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http://dx.doi.org/10.3390/ijms20143568DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6678358PMC
July 2019

Silencing of ANKRD12 circRNA induces molecular and functional changes associated with invasive phenotypes.

BMC Cancer 2019 Jun 11;19(1):565. Epub 2019 Jun 11.

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, Education City, Qatar Foundation, P.O. Box No, 24144, Doha, Qatar.

Background: Circular RNAs (circRNAs) that form through non-canonical backsplicing events of pre-mRNA transcripts are evolutionarily conserved and abundantly expressed across species. However, the functional relevance of circRNAs remains a topic of debate.

Methods: We identified one of the highly expressed circRNA (circANKRD12) in cancer cell lines and characterized it validated it by Sanger sequencing, Real-Time PCR. siRNA mediated silencing of the circular junction of circANKRD12 was followed by RNA Seq analysis of circANKRD12 silenced cells and control cells to identify the differentially regulated genes. A series of cell biology and molecular biology techniques (MTS assay, Migration analysis, 3D organotypic models, Real-Time PCR, Cell cycle analysis, Western blot analysis, and Seahorse Oxygen Consumption Rate analysis) were performed to elucidate the function, and underlying mechanisms involved in circANKRD12 silenced breast and ovarian cancer cells.

Results: In this study, we identified and characterized a circular RNA derived from Exon 2 and Exon 8 of the ANKRD12 gene, termed here as circANKRD12. We show that this circRNA is abundantly expressed in breast and ovarian cancers. The circANKRD12 is RNase R resistant and predominantly localized in the cytoplasm in contrast to its source mRNA. We confirmed the expression of this circRNA across a variety of cancer cell lines and provided evidence for its functional relevance through downstream regulation of several tumor invasion genes. Silencing of circANKRD12 induces a strong phenotypic change by significantly regulating cell cycle, increasing invasion and migration and altering the metabolism in cancer cells. These results reveal the functional significance of circANKRD12 and provide evidence of a regulatory role for this circRNA in cancer progression.

Conclusions: Our study demonstrates the functional relevance of circANKRD12 in various cancer cell types and, based on its expression pattern, has the potential to become a new clinical biomarker.
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http://dx.doi.org/10.1186/s12885-019-5723-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6558796PMC
June 2019

High-resolution protein-protein interaction mapping using all--all sequencing (AVA-Seq).

J Biol Chem 2019 07 10;294(30):11549-11558. Epub 2019 Jun 10.

Department of Genetic Medicine, Weill Cornell Medicine in Qatar, Doha 24144, Qatar

Two-hybrid systems can be used for investigating protein-protein interactions and may provide important information about gene products with unknown function. Despite their success in mapping protein interactions, two-hybrid systems have remained mostly untouched by improvements in next-generation DNA sequencing. The two-hybrid systems rely on one--all methods in which each bait is sequentially screened against an entire library. Here, we developed a screening method that joins both bait and prey as a convergent fusion into one bacterial plasmid vector that can then be amplified and paired-end sequencing by next-generation sequencing (NGS). Our method enables all--all sequencing (AVA-Seq) and utilizes NGS to remove multiple bottlenecks of the two-hybrid system. AVA-Seq allows for high-resolution protein-protein interaction mapping of a small set of proteins and has the potential for lower-resolution mapping of entire proteomes. Features of the system include ORF selection to improve efficiency, high bacterial transformation efficiency, a convergent fusion vector to allow paired-end sequencing of interactors, and the use of protein fragments rather than full-length proteins to better resolve specific protein contact points. We demonstrate the system's strengths and limitations on a set of proteins known to interact in humans and provide a framework for future large-scale projects.
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http://dx.doi.org/10.1074/jbc.RA119.008792DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6663860PMC
July 2019

Author Correction: Genus-wide sequencing support a two-locus model for sex-determination in Phoenix.

Nat Commun 2018 12 3;9(1):5219. Epub 2018 Dec 3.

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, PO Box 24144, Doha, Qatar.

In the original version of this Article, the affiliation of the first author, Maria F. Torres, 'Department of Biological Sciences, University of Cincinnati, Cincinnati, 45221, OH, USA' was incorrectly assigned as a present address and should have been listed as a full affiliation. This error has been corrected in both the PDF and HTML versions of the Article.
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http://dx.doi.org/10.1038/s41467-018-07742-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6277403PMC
December 2018

Genus-wide sequencing supports a two-locus model for sex-determination in Phoenix.

Nat Commun 2018 09 28;9(1):3969. Epub 2018 Sep 28.

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, PO Box 24144, Doha, Qatar.

The date palm tree is a commercially important member of the genus Phoenix whose 14 species are dioecious with separate male and female individuals. To identify sex determining genes we sequenced the genomes of 15 female and 13 male Phoenix trees representing all 14 species. We identified male-specific sequences and extended them using phased single-molecule sequencing or BAC clones. We observed that only four genes contained sequences conserved in all analyzed Phoenix males. Most of these sequences showed similarity to a single genomic locus in the closely related monoecious oil palm. CYP703 and GPAT3, two single copy genes present in males and critical for male flower development in other monocots, were absent in females. A LOG-like gene appears translocated into the Y-linked region and is suggested to play a role in suppressing female flowers. Our data are consistent with a two-mutation model for the evolution of dioecy in Phoenix.
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http://dx.doi.org/10.1038/s41467-018-06375-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6162277PMC
September 2018

DET1-mediated degradation of a SAGA-like deubiquitination module controls H2Bub homeostasis.

Elife 2018 09 7;7. Epub 2018 Sep 7.

Institut de biologie de l'Ecole normale supérieure (IBENS), Ecole normale supérieure, CNRS, INSERM, Université PSL, Paris, France.

DE-ETIOLATED 1 (DET1) is an evolutionarily conserved component of the ubiquitination machinery that mediates the destabilization of key regulators of cell differentiation and proliferation in multicellular organisms. In this study, we provide evidence from Arabidopsis that DET1 is essential for the regulation of histone H2B monoubiquitination (H2Bub) over most genes by controlling the stability of a deubiquitination module (DUBm). In contrast with yeast and metazoan DUB modules that are associated with the large SAGA complex, the Arabidopsis DUBm only comprises three proteins (hereafter named SGF11, ENY2 and UBP22) and appears to act independently as a major H2Bub deubiquitinase activity. Our study further unveils that DET1-DDB1-Associated-1 (DDA1) protein interacts with SGF11 , linking the DET1 complex to light-dependent ubiquitin-mediated proteolytic degradation of the DUBm. Collectively, these findings uncover a signaling path controlling DUBm availability, potentially adjusting H2Bub turnover capacity to the cell transcriptional status.
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http://dx.doi.org/10.7554/eLife.37892DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6128693PMC
September 2018

DNA DAMAGE BINDING PROTEIN2 Shapes the DNA Methylation Landscape.

Plant Cell 2016 Sep 16;28(9):2043-2059. Epub 2016 Aug 16.

Institut de Biologie Moléculaire de Plantes du CNRS, UPR 2357, F-67084 Strasbourg, France

In eukaryotes, DNA repair pathways help to maintain genome integrity and epigenomic patterns. However, the factors at the nexus of DNA repair and chromatin modification/remodeling remain poorly characterized. Here, we uncover a previously unrecognized interplay between the DNA repair factor DNA DAMAGE BINDING PROTEIN2 (DDB2) and the DNA methylation machinery in Loss-of-function mutation in DDB2 leads to genome-wide DNA methylation alterations. Genetic and biochemical evidence indicate that at many repeat loci, DDB2 influences de novo DNA methylation by interacting with ARGONAUTE4 and by controlling the local abundance of 24-nucleotide short interfering RNAs (siRNAs). We also show that DDB2 regulates active DNA demethylation mediated by REPRESSOR OF SILENCING1 and DEMETER LIKE3. Together, these findings reveal a role for the DNA repair factor DDB2 in shaping the Arabidopsis DNA methylation landscape in the absence of applied genotoxic stress.
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http://dx.doi.org/10.1105/tpc.16.00474DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5059809PMC
September 2016

Altered expression pattern of circular RNAs in primary and metastatic sites of epithelial ovarian carcinoma.

Oncotarget 2016 Jun;7(24):36366-36381

Department of Genetic Medicine, Weill Cornell Medicine-Qatar, Education City, Ar-Rayyan, Qatar.

Recently, a class of endogenous species of RNA called circular RNA (circRNA) has been shown to regulate gene expression in mammals and their role in cellular function is just beginning to be understood. To investigate the role of circRNAs in ovarian cancer, we performed paired-end RNA sequencing of primary sites, peritoneal and lymph node metastases from three patients with stage IIIC ovarian cancer. We developed an in-house computational pipeline to identify and characterize the circRNA expression from paired-end RNA-Seq libraries. This pipeline revealed thousands of circular isoforms in Epithelial Ovarian Carcinoma (EOC). These circRNAs are enriched for potentially effective miRNA seed matches. A significantly larger number of circRNAs are differentially expressed between tumor sites than mRNAs. Circular and linear expression exhibits an inverse trend for many cancer related pathways and signaling pathways like NFkB, PI3k/AKT and TGF-β typically activated for mRNA in metastases are inhibited for circRNA expression. Further, circRNAs show a more robust expression pattern across patients than mRNA forms indicating their suitability as biomarkers in highly heterogeneous cancer transcriptomes. The consistency of circular RNA expression may offer new candidates for cancer treatment and prognosis.
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http://dx.doi.org/10.18632/oncotarget.8917DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5095006PMC
June 2016

Ensembl regulation resources.

Database (Oxford) 2016 17;2016. Epub 2016 Feb 17.

European Molecular Biology Laboratory, European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, UK Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK.

New experimental techniques in epigenomics allow researchers to assay a diversity of highly dynamic features such as histone marks, DNA modifications or chromatin structure. The study of their fluctuations should provide insights into gene expression regulation, cell differentiation and disease. The Ensembl project collects and maintains the Ensembl regulation data resources on epigenetic marks, transcription factor binding and DNA methylation for human and mouse, as well as microarray probe mappings and annotations for a variety of chordate genomes. From this data, we produce a functional annotation of the regulatory elements along the human and mouse genomes with plans to expand to other species as data becomes available. Starting from well-studied cell lines, we will progressively expand our library of measurements to a greater variety of samples. Ensembl's regulation resources provide a central and easy-to-query repository for reference epigenomes. As with all Ensembl data, it is freely available at http://www.ensembl.org, from the Perl and REST APIs and from the public Ensembl MySQL database server at ensembldb.ensembl.org. Database URL: http://www.ensembl.org.
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http://dx.doi.org/10.1093/database/bav119DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4756621PMC
October 2016

K-core decomposition of a protein domain co-occurrence network reveals lower cancer mutation rates for interior cores.

J Clin Bioinforma 2015 3;5. Epub 2015 Mar 3.

Department of Genetic Medicine, Weill Cornell Medical College, New York, NY USA ; Genomic Core, Weill Cornell Medical College in Qatar, Qatar Foundation, Doha, 24144 Qatar.

Background: Network biology currently focuses primarily on metabolic pathways, gene regulatory, and protein-protein interaction networks. While these approaches have yielded critical information, alternative methods to network analysis will offer new perspectives on biological information. A little explored area is the interactions between domains that can be captured using domain co-occurrence networks (DCN). A DCN can be used to study the function and interaction of proteins by representing protein domains and their co-existence in genes and by mapping cancer mutations to the individual protein domains to identify signals.

Results: The domain co-occurrence network was constructed for the human proteome based on PFAM domains in proteins. Highly connected domains in the central cores were identified using the k-core decomposition technique. Here we show that these domains were found to be more evolutionarily conserved than the peripheral domains. The somatic mutations for ovarian, breast and prostate cancer diseases were obtained from the TCGA database. We mapped the somatic mutations to the individual protein domains and the local false discovery rate was used to identify significantly mutated domains in each cancer type. Significantly mutated domains were found to be enriched in cancer disease pathways. However, we found that the inner cores of the DCN did not contain any of the significantly mutated domains. We observed that the inner core protein domains are highly conserved and these domains co-exist in large numbers with other protein domains.

Conclusion: Mutations and domain co-occurrence networks provide a framework for understanding hierarchal designs in protein function from a network perspective. This study provides evidence that a majority of protein domains in the inner core of the DCN have a lower mutation frequency and that protein domains present in the peripheral regions of the k-core contribute more heavily to the disease. These findings may contribute further to drug development.
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http://dx.doi.org/10.1186/s13336-015-0016-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4357223PMC
March 2015

Insights into the role of DNA methylation in diatoms by genome-wide profiling in Phaeodactylum tricornutum.

Nat Commun 2013 ;4:2091

Environmental and Evolutionary Genomics Section, Institut de Biologie de l'École Normale Supérieure, CNRS UMR 8197 INSERM U1024, 46 rue d'Ulm, 75005 Paris, France.

DNA cytosine methylation is a widely conserved epigenetic mark in eukaryotes that appears to have critical roles in the regulation of genome structure and transcription. Genome-wide methylation maps have so far only been established from the supergroups Archaeplastida and Unikont. Here we report the first whole-genome methylome from a stramenopile, the marine model diatom Phaeodactylum tricornutum. Around 6% of the genome is intermittently methylated in a mosaic pattern. We find extensive methylation in transposable elements. We also detect methylation in over 320 genes. Extensive gene methylation correlates strongly with transcriptional silencing and differential expression under specific conditions. By contrast, we find that genes with partial methylation tend to be constitutively expressed. These patterns contrast with those found previously in other eukaryotes. By going beyond plants, animals and fungi, this stramenopile methylome adds significantly to our understanding of the evolution of DNA methylation in eukaryotes.
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http://dx.doi.org/10.1038/ncomms3091DOI Listing
December 2013

Ensembl 2013.

Nucleic Acids Res 2013 Jan 30;41(Database issue):D48-55. Epub 2012 Nov 30.

European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton Cambridge CB10 1SD, UK.

The Ensembl project (http://www.ensembl.org) provides genome information for sequenced chordate genomes with a particular focus on human, mouse, zebrafish and rat. Our resources include evidenced-based gene sets for all supported species; large-scale whole genome multiple species alignments across vertebrates and clade-specific alignments for eutherian mammals, primates, birds and fish; variation data resources for 17 species and regulation annotations based on ENCODE and other data sets. Ensembl data are accessible through the genome browser at http://www.ensembl.org and through other tools and programmatic interfaces.
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http://dx.doi.org/10.1093/nar/gks1236DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3531136PMC
January 2013

Histone H2B monoubiquitination facilitates the rapid modulation of gene expression during Arabidopsis photomorphogenesis.

PLoS Genet 2012 19;8(7):e1002825. Epub 2012 Jul 19.

Ecole Normale Supérieure, Institut de Biologie de l'ENS, IBENS, Paris, France.

Profiling of DNA and histone modifications has recently allowed the establishment of reference epigenomes from several model organisms. This identified a major chromatin state for active genes that contains monoubiquitinated H2B (H2Bub), a mark linked to transcription elongation. However, assessment of dynamic chromatin changes during the reprogramming of gene expression in response to extrinsic or developmental signals has been more difficult. Here we used the major developmental switch that Arabidopsis thaliana plants undergo upon their initial perception of light, known as photomorphogenesis, as a paradigm to assess spatial and temporal dynamics of monoubiquitinated H2B (H2Bub) and its impact on transcriptional responses. The process involves rapid and extensive transcriptional reprogramming and represents a developmental window well suited to studying cell division-independent chromatin changes. Genome-wide H2Bub distribution was determined together with transcriptome profiles at three time points during early photomorphogenesis. This revealed de novo marking of 177 genes upon the first hour of illumination, illustrating the dynamic nature of H2Bub enrichment in a genomic context. Gene upregulation was associated with H2Bub enrichment, while H2Bub levels generally remained stable during gene downregulation. We further report that H2Bub influences the modulation of gene expression, as both gene up- and downregulation were globally weaker in hub1 mutant plants that lack H2Bub. H2Bub-dependent regulation notably impacted genes with fast and transient light induction, and several circadian clock components whose mRNA levels are tightly regulated by sharp oscillations. Based on these findings, we propose that H2B monoubiquitination is part of a transcription-coupled, chromatin-based mechanism to rapidly modulate gene expression.
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http://dx.doi.org/10.1371/journal.pgen.1002825DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3400566PMC
January 2013

Genome-wide evidence for local DNA methylation spreading from small RNA-targeted sequences in Arabidopsis.

Nucleic Acids Res 2011 Sep 17;39(16):6919-31. Epub 2011 May 17.

Institut de Biologie de l'Ecole Normale Supérieure (IBENS), Centre National de la Recherche Scientifique (CNRS) UMR8197 - Institut National de la Santé et de la Recherche Médicale (INSERM) U1024, 46 rue d'Ulm, 75230 Paris cedex 05, France.

Transposable elements (TEs) and their relics play major roles in genome evolution. However, mobilization of TEs is usually deleterious and strongly repressed. In plants and mammals, this repression is typically associated with DNA methylation, but the relationship between this epigenetic mark and TE sequences has not been investigated systematically. Here, we present an improved annotation of TE sequences and use it to analyze genome-wide DNA methylation maps obtained at single-nucleotide resolution in Arabidopsis. We show that although the majority of TE sequences are methylated, ∼26% are not. Moreover, a significant fraction of TE sequences densely methylated at CG, CHG and CHH sites (where H = A, T or C) have no or few matching small interfering RNA (siRNAs) and are therefore unlikely to be targeted by the RNA-directed DNA methylation (RdDM) machinery. We provide evidence that these TE sequences acquire DNA methylation through spreading from adjacent siRNA-targeted regions. Further, we show that although both methylated and unmethylated TE sequences located in euchromatin tend to be more abundant closer to genes, this trend is least pronounced for methylated, siRNA-targeted TE sequences located 5' to genes. Based on these and other findings, we propose that spreading of DNA methylation through promoter regions explains at least in part the negative impact of siRNA-targeted TE sequences on neighboring gene expression.
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http://dx.doi.org/10.1093/nar/gkr324DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3167636PMC
September 2011

Integrative epigenomic mapping defines four main chromatin states in Arabidopsis.

EMBO J 2011 May 12;30(10):1928-38. Epub 2011 Apr 12.

Institut de Biologie de l'Ecole Normale Supérieure, Centre National de la Recherche Scientifique (CNRS) UMR8197, Institut National de la Santé et de la Recherche Médicale (INSERM) U1024, Paris, France.

Post-translational modification of histones and DNA methylation are important components of chromatin-level control of genome activity in eukaryotes. However, principles governing the combinatorial association of chromatin marks along the genome remain poorly understood. Here, we have generated epigenomic maps for eight histone modifications (H3K4me2 and 3, H3K27me1 and 2, H3K36me3, H3K56ac, H4K20me1 and H2Bub) in the model plant Arabidopsis and we have combined these maps with others, produced under identical conditions, for H3K9me2, H3K9me3, H3K27me3 and DNA methylation. Integrative analysis indicates that these 12 chromatin marks, which collectively cover ∼90% of the genome, are present at any given position in a very limited number of combinations. Moreover, we show that the distribution of the 12 marks along the genomic sequence defines four main chromatin states, which preferentially index active genes, repressed genes, silent repeat elements and intergenic regions. Given the compact nature of the Arabidopsis genome, these four indexing states typically translate into short chromatin domains interspersed with each other. This first combinatorial view of the Arabidopsis epigenome points to simple principles of organization as in metazoans and provides a framework for further studies of chromatin-based regulatory mechanisms in plants.
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http://dx.doi.org/10.1038/emboj.2011.103DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3098477PMC
May 2011

Integrative transcript and metabolite analysis of nutritionally enhanced DE-ETIOLATED1 downregulated tomato fruit.

Plant Cell 2010 Apr 30;22(4):1190-215. Epub 2010 Apr 30.

Centre for Systems and Synthetic Biology, University of London, Egham, Surrey TW20 0EX, United Kingdom.

Fruit-specific downregulation of the DE-ETIOLATED1 (DET1) gene product results in tomato fruits (Solanum lycopersicum) containing enhanced nutritional antioxidants, with no detrimental effects on yield. In an attempt to further our understanding of how modulation of this gene leads to improved quality traits, detailed targeted and multilevel omic characterization has been performed. Metabolite profiling revealed quantitative increases in carotenoid, tocopherol, phenylpropanoids, flavonoids, and anthocyanidins. Qualitative differences could also be identified within the phenolics, including unique formation in fruit pericarp tissues. These changes resulted in increased total antioxidant content both in the polar and nonpolar fractions. Increased transcription of key biosynthetic genes is a likely mechanism producing elevated phenolic-based metabolites. By contrast, high levels of isoprenoids do not appear to result from transcriptional regulation but are more likely related to plastid-based parameters, such as increased plastid volume per cell. Parallel metabolomic and transcriptomic analyses reveal the widespread effects of DET1 downregulation on diverse sectors of metabolism and sites of synthesis. Correlation analysis of transcripts and metabolites independently indicated strong coresponses within and between related pathways/processes. Interestingly, despite the fact that secondary metabolites were the most severely affected in ripe tomato fruit, our integrative analyses suggest that the coordinated activation of core metabolic processes in cell types amenable to plastid biogenesis is the main effect of DET1 loss of function.
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http://dx.doi.org/10.1105/tpc.110.073866DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2879742PMC
April 2010

Mapping human genetic diversity in Asia.

Science 2009 Dec;326(5959):1541-5

Asia harbors substantial cultural and linguistic diversity, but the geographic structure of genetic variation across the continent remains enigmatic. Here we report a large-scale survey of autosomal variation from a broad geographic sample of Asian human populations. Our results show that genetic ancestry is strongly correlated with linguistic affiliations as well as geography. Most populations show relatedness within ethnic/linguistic groups, despite prevalent gene flow among populations. More than 90% of East Asian (EA) haplotypes could be found in either Southeast Asian (SEA) or Central-South Asian (CSA) populations and show clinal structure with haplotype diversity decreasing from south to north. Furthermore, 50% of EA haplotypes were found in SEA only and 5% were found in CSA only, indicating that SEA was a major geographic source of EA populations.
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http://dx.doi.org/10.1126/science.1177074DOI Listing
December 2009

Utilizing linkage disequilibrium information from Indian Genome Variation Database for mapping mutations: SCA12 case study.

J Genet 2009 Apr;88(1):55-60

Functional Genomics Unit, Institute of Genomics and Integrative Biology (CSIR), Mall Road, New Delhi, 110 007, India.

Stratification in heterogeneous populations poses an enormous challenge in linkage disequilibrium (LD) based identification of causal loci using surrogate markers. In this study, we demonstrate the enormous potential of endogamous Indian populations for mapping mutations in candidate genes using minimal SNPs, mainly due to larger regions of LD. We show this by a case study of the PPP2R2B gene (approximately 400 kb) that harbours a CAG repeat, expansion of which has been implicated in spinocerebellar ataxia type 12 (SCA12). Using LD information derived from Indian Genome Variation database (IGVdb) on populations which share similar ethnic and linguistic backgrounds as the SCA12 study population, we could map the causal loci using a minimal set of three SNPs, without the generation of additional basal data from the ethnically matched population. We could also demonstrate transferability of tagSNPs from a related HapMap population for mapping the mutation.
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http://dx.doi.org/10.1007/s12041-009-0007-0DOI Listing
April 2009