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      Transposable Elements in Human Cancer: Causes and Consequences of Deregulation

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          Abstract

          Transposable elements (TEs) comprise nearly half of the human genome and play an essential role in the maintenance of genomic stability, chromosomal architecture, and transcriptional regulation. TEs are repetitive sequences consisting of RNA transposons, DNA transposons, and endogenous retroviruses that can invade the human genome with a substantial contribution in human evolution and genomic diversity. TEs are therefore firmly regulated from early embryonic development and during the entire course of human life by epigenetic mechanisms, in particular DNA methylation and histone modifications. The deregulation of TEs has been reported in some developmental diseases, as well as for different types of human cancers. To date, the role of TEs, the mechanisms underlying TE reactivation, and the interplay with DNA methylation in human cancers remain largely unexplained. We reviewed the loss of epigenetic regulation and subsequent genomic instability, chromosomal aberrations, transcriptional deregulation, oncogenic activation, and aberrations of non-coding RNAs as the potential mechanisms underlying TE deregulation in human cancers.

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          Most cited references171

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          Natural RNA circles function as efficient microRNA sponges.

          MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression that act by direct base pairing to target sites within untranslated regions of messenger RNAs. Recently, miRNA activity has been shown to be affected by the presence of miRNA sponge transcripts, the so-called competing endogenous RNA in humans and target mimicry in plants. We previously identified a highly expressed circular RNA (circRNA) in human and mouse brain. Here we show that this circRNA acts as a miR-7 sponge; we term this circular transcript ciRS-7 (circular RNA sponge for miR-7). ciRS-7 contains more than 70 selectively conserved miRNA target sites, and it is highly and widely associated with Argonaute (AGO) proteins in a miR-7-dependent manner. Although the circRNA is completely resistant to miRNA-mediated target destabilization, it strongly suppresses miR-7 activity, resulting in increased levels of miR-7 targets. In the mouse brain, we observe overlapping co-expression of ciRS-7 and miR-7, particularly in neocortical and hippocampal neurons, suggesting a high degree of endogenous interaction. We further show that the testis-specific circRNA, sex-determining region Y (Sry), serves as a miR-138 sponge, suggesting that miRNA sponge effects achieved by circRNA formation are a general phenomenon. This study serves as the first, to our knowledge, functional analysis of a naturally expressed circRNA.
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            Topological Domains in Mammalian Genomes Identified by Analysis of Chromatin Interactions

            The spatial organization of the genome is intimately linked to its biological function, yet our understanding of higher order genomic structure is coarse, fragmented and incomplete. In the nucleus of eukaryotic cells, interphase chromosomes occupy distinct chromosome territories (CT), and numerous models have been proposed for how chromosomes fold within CTs 1 . These models, however, provide only few mechanistic details about the relationship between higher order chromatin structure and genome function. Recent advances in genomic technologies have led to rapid revolutions in the study of 3D genome organization. In particular, Hi-C has been introduced as a method for identifying higher order chromatin interactions genome wide 2 . In the present study, we investigated the 3D organization of the human and mouse genomes in embryonic stem cells and terminally differentiated cell types at unprecedented resolution. We identify large, megabase-sized local chromatin interaction domains, which we term “topological domains”, as a pervasive structural feature of the genome organization. These domains correlate with regions of the genome that constrain the spread of heterochromatin. The domains are stable across different cell types and highly conserved across species, suggesting that topological domains are an inherent property of mammalian genomes. Lastly, we find that the boundaries of topological domains are enriched for the insulator binding protein CTCF, housekeeping genes, tRNAs, and SINE retrotransposons, suggesting that these factors may play a role in establishing the topological domain structure of the genome.
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              Complementary sequence-mediated exon circularization.

              Exon circularization has been identified from many loci in mammals, but the detailed mechanism of its biogenesis has remained elusive. By using genome-wide approaches and circular RNA recapitulation, we demonstrate that exon circularization is dependent on flanking intronic complementary sequences. Such sequences and their distribution exhibit rapid evolutionary changes, showing that exon circularization is evolutionarily dynamic. Strikingly, exon circularization efficiency can be regulated by competition between RNA pairing across flanking introns or within individual introns. Importantly, alternative formation of inverted repeated Alu pairs and the competition between them can lead to alternative circularization, resulting in multiple circular RNA transcripts produced from a single gene. Collectively, exon circularization mediated by complementary sequences in human introns and the potential to generate alternative circularization products extend the complexity of mammalian posttranscriptional regulation. Copyright © 2014 Elsevier Inc. All rights reserved.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                MDPI
                1422-0067
                04 May 2017
                May 2017
                : 18
                : 5
                : 974
                Affiliations
                [1 ]Division of Surgical Oncology, Department of Surgery Faculty of Medicine, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
                [2 ]Institute of Pathology, Medizinische Hochschule Hannover, Hannover 30625, Germany
                [3 ]PILAR (Philippine and Indonesian Scholar) Research and Education, 20 Station Road, Cambridge CB1 2JD, UK; w.wulaningsih@ 123456ucl.ac.uk
                [4 ]MRC (Medical Research Council) Unit for Lifelong Health and Ageing, University College London, London WC1B 5JU, UK
                [5 ]Division of Haematology/Oncology, Faculty of Medicine Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
                Author notes
                [* ]Correspondence: sl.anwar@ 123456ugm.ac.id (S.L.A.); Lehmann.Ulrich@ 123456MH-Hannover.de (U.L.); Tel.: +62-274-581333 (S.L.A.); +49-511-532-4501 (U.L.); Fax: +62-274-581333 (S.L.A.); +49-511-532-5799 (U.L.)
                Article
                ijms-18-00974
                10.3390/ijms18050974
                5454887
                28471386
                bfdff3e2-ecfb-4639-bd30-feed560c30dd
                © 2017 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 21 March 2017
                : 29 April 2017
                Categories
                Review

                Molecular biology
                transposable elements,epigenetics,cancer,genomic instability,non-coding rnas
                Molecular biology
                transposable elements, epigenetics, cancer, genomic instability, non-coding rnas

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