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Modeling Diseases of Noncoding Unstable Repeat Expansions Using Mutant Pluripotent Stem Cells

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Modeling Diseases of Noncoding Unstable Repeat Expansions Using Mutant Pluripotent Stem Cells Submit a Manuscript: http://www.wjgnet.com/esps/ World J Stem Cells 2015 June 26; 7(5): 823-838 Help Desk: http://www.wjgnet.com/esps/helpdesk.aspx ISSN 1948-0210 (online) DOI: 10.4252/wjsc.v7.i5.823 © 2015 Baishideng Publishing Group Inc. All rights reserved. REVIEW Modeling diseases of noncoding unstable repeat expansions using mutant pluripotent stem cells Shira Yanovsky-Dagan, Hagar Mor-Shaked, Rachel Eiges Shira Yanovsky-Dagan, Hagar Mor-Shaked, Rachel Eiges, transmitted across generations. Nearly all are inherited Stem Cell Research Laboratory, Medical Genetics Institute, as autosomal dominant conditions and are typically Shaare Zedek Medical Center affiliated with the Hebrew associated with anticipation. Pathologic unstable University School of Medicine, Jerusalem 91031, Israel repeat expansions can be classified according to their length, repeat sequence, gene location and underlying Author contributions: Yanovsky-Dagan S and Mor-Shaked pathologic mechanisms. This review summarizes H equally contributed to the preparation of the review; Eiges R the current contribution of mutant pluripotent stem designed the aim of the editorial and wrote the manuscript. cells (diseased human embryonic stem cells and Conflict-of-interest: The authors declare no conflicts of interest. patient-derived induced pluripotent stem cells) to the research of unstable repeat pathologies by focusing Open-Access: This article is an open-access article which was on particularly large unstable noncoding expansions. selected by an in-house editor and fully peer-reviewed by external Among this class of disorders are Fragile X syndrome reviewers. It is distributed in accordance with the Creative and Fragile X-associated tremor/ataxia syndrome, Commons Attribution Non Commercial (CC BY-NC 4.0) license, myotonic dystrophy type 1 and myotonic dystrophy which permits others to distribute, remix, adapt, build upon this type 2, Friedreich ataxia and C9 related amyotrophic work non-commercially, and license their derivative works on lateral sclerosis and/or frontotemporal dementia, different terms, provided the original work is properly cited and Facioscapulohumeral Muscular Dystrophy and potentially the use is non-commercial. See: http://creativecommons.org/ more. Common features that are typical to this subclass licenses/by-nc/4.0/ of conditions are RNA toxic gain-of-function, epigenetic loss-of-function, toxic repeat-associated non-ATG Correspondence to: Rachel Eiges, PhD, Stem Cell Research Laboratory, Medical Genetics Institute, Shaare Zedek Medical translation and somatic instability. For each mechanism Center affiliated with the Hebrew University School of Medicine, we summarize the currently available stem cell based 12 Shmu’el Bait Street, PO Box 3235, Jerusalem 91031, models, highlight how they contributed to better Israel. [email protected] understanding of the related mechanism, and discuss Telephone: +972-2-6555129 how they may be utilized in future investigations. Fax: +972-2-6666135 Key words: Unstable repeat associated disorders; Received: December 5, 2014 Human embryonic stem cells; Patient-derived induced Peer-review started: December 6, 2014 pluripotent stem cells; Disease modeling; Epigenetics; First decision: February 7, 2015 repeat-associated non-ATG translation; RNA toxicity; Revised: March 5, 2015 Repeat somatic instability Accepted: April 1, 2015 Article in press: April 7, 2015 © The Author(s) 2015. Published by Baishideng Publishing Published online: June 26, 2015 Group Inc. All rights reserved. Core tip: This review summarizes the current con- tribution of mutant pluripotent stem cells (diseased Abstract HESCs and patient-derived induced pluripotent stem Pathogenic mutations involving DNA repeat expansions cells) to the research of unstable repeat pathologies are responsible for over 20 different neuronal and by focusing on particularly large unstable noncoding neuromuscular diseases. All result from expanded tracts expansions. It demonstrates their importance as an of repetitive DNA sequences (mostly microsatellites) unlimited cell source for generating rarely available that become unstable beyond a critical length when impaired cells in culture, and as a model system for WJSC|www.wjgnet.com 823 June 26, 2015|Volume 7|Issue 5| Yanovsky-Dagan S et al . Modeling noncoding unstable repeat expansion diseases exploring the mechanisms that are involved with this [amyotrophic lateral sclerosis and/or frontotemporal class of mutations. For each mechanism we describe dementia (ALS­FTD); caused by a GGGGCC[11] the currently available stem cell based models, highlight expansion in intron 1 of the C9orf72 gene[12]], and how they contributed to better understanding of the [Facioscapulohumeral Muscular Dystrophy (FSHD); related mechanism, and discuss how they may be caused by a contraction of the D4Z4 macrosatellite utilized in future investigations. repeat in sub­telomeres of chromosome 4q35]. The outcome of the expansion mutation may be different depending on its gene location and length. Yanovsky-Dagan S, Mor-Shaked H, Eiges R. Modeling diseases Unlike small expansions, which commonly result of noncoding unstable repeat expansions using mutant pluripotent in alterations in protein function, large noncoding stem cells. World J Stem Cells 2015; 7(5): 823-838 Available expansions introduce further complexity because they from: URL: http://www.wjgnet.com/1948-0210/full/v7/i5/823. can lead to either loss­of­function, RNA gain­of­function, htm DOI: http://dx.doi.org/10.4252/wjsc.v7.i5.823 toxic protein gain­of­function, or to a combination of all these pathogenic mechanisms in unison[13]. In addition and in contrast to small expansions, large noncoding expansions frequently coincide with marked INTRODUCTION changes in repeat tract length between and within Unstable repeat expansions, termed also dynamic tissues of affected individuals[8,14,15]. This phenomenon, mutations, are the cause of over 20 different neur­ termed somatic repeat instability, results in mosaicism odevelopmental, neurodegenerative and neuromuscular for expansion size and occasionally correlates with heritable conditions that result from a change in the disease age of onset and severity. Modeling dynamic number of repeat units in coding genes when ordered mutations, specifically large expansions, in mice can be in tandem (for comprehensive review see[1]). Repeat particularly challenging due to the difficulty in artificially instabilities change across generations, and may vary inducing and stably maintaining very large repeat between cells within affected individuals. Nearly all are expansions (especially CG­rich) in vitro and in vivo. In inherited as autosomal dominant conditions and are addition, despite the similarities between mouse and typically associated with anticipation, meaning that human, there are still major differences between the the disease symptoms tend to be more severe and two species, leading to dissimilarities in biochemical occur earlier in age with successive generations. Most pathways and phenotypes. These crucial discrepancies unstable repeat associated disorders are caused by emphasize the need for complement model systems the addition of repeat units (mostly tri­nucleotides) that will reproducibly copy the underlying mechanisms that, above a certain threshold, lead to misregulation and clinical phenotypes in human. of genes and/or to their RNA/protein products. Disease Mutant human pluripotent stem cells, embryonic associated unstable repeat expansions can be classified [human embryonic stem cells (HESCs)] or artificially into two groups according to the length and gene induced [induced pluripotent stem cells (iPS cells)], may location of the expansion. One class of disorders results provide a great opportunity to study unstable repeat from small repeat copy changes (few to tens of units) expansions by complementing existing models[16]. They that usually reside in exon coding regions of genes. are expected to be especially useful in the study of Among them are the ployglutamine CAG expansion heritable conditions where the animal model fails to fully disorders like Huntington disease, Huntington’s disease recapitulate the phenotype of the disease in human, or like 2 (HDL2), Dentatorubral­pallidoluysian atrophy when disease relevant cell cultures from patients are (DRPLA), Spinal and bulbar muscular atrophy (SBMA) unavailable. The many advantages to the utilization and spinocerebellar ataxia (SCA) 1, 2, 3, 6, 7 and 17, of these cells for biomedical research include the fact which lead to protein gain­of­function alterations. A that they are human derived, untransformed cells with second class of unstable repeat expansion disorders an unlimited self­renewing capacity. In addition, they results from large expansions, ranging from hundreds have the potential to differentiate into a wide range to thousands of copies. Large pathogenic repeat of cell types in culture, and recapitulate early human expansions are typically located in noncoding regions embryo development while they differentiate in vitro. including promoters, introns and untranslated regions Moreover, as mutant pluripotent stem cells can be (UTRs) of genes[2]. Among disorders in this class are derived directly from genetically affected embryos that Fragile X syndrome (FXS) and [Fragile X­associated are obtained through preimplantation genetic diagnosis tremor/ataxia syndrome (FXTAS); both FXS and FXTAS (PGD) procedures (HESC)[17], or by reprogramming are caused by large expansion of a
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