1
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Shankar VG, Klann E. Size matters: Fighting repeat expansion size in fragile X syndrome using antisense oligonucleotides. Proc Natl Acad Sci U S A 2023; 120:e2309678120. [PMID: 37440569 PMCID: PMC10372623 DOI: 10.1073/pnas.2309678120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/15/2023] Open
Affiliation(s)
| | - Eric Klann
- Center for Neural Science, New York University, New York, NY10003
- New York University Neuroscience Institute, New York University Grossman School of Medicine, New York, NY10016
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2
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Guo P, Han D. Targeting Pathogenic DNA and RNA Repeats: A Conceptual Therapeutic Way for Repeat Expansion Diseases. Chemistry 2022; 28:e202201749. [PMID: 35727679 DOI: 10.1002/chem.202201749] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2022] [Indexed: 11/06/2022]
Abstract
Expansions of short tandem repeats (STRs) in the human genome cause nearly 50 neurodegenerative diseases, which are mostly inheritable, nonpreventable and incurable, posing as a huge threat to human health. Non-B DNAs formed by STRs are thought to be structural intermediates that can cause repeat expansions. The subsequent transcripts harboring expanded RNA repeats can further induce cellular toxicity through forming specific structures. Direct targeting of these pathogenic DNA and RNA repeats has emerged as a new potential therapeutic strategy to cure repeat expansion diseases. In this conceptual review, we first introduce the roles of DNA and RNA structures in the genetic instabilities and pathomechanisms of repeat expansion diseases, then describe structural features of DNA and RNA repeats with a focus on the tertiary structures determined by X-ray crystallography and solution nuclear magnetic resonance spectroscopy, and finally discuss recent progress and perspectives of developing chemical tools that target pathogenic DNA and RNA repeats for curing repeat expansion diseases.
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Affiliation(s)
- Pei Guo
- The Cancer Hospital of the University of Chinese Academy of Sciences, Zhejiang Cancer Hospital), Hangzhou Institute of Medicine (HIM), Chinese Academy of Sciences, Hangzhou, Zhejiang, 310022, P. R. China
| | - Da Han
- The Cancer Hospital of the University of Chinese Academy of Sciences, Zhejiang Cancer Hospital), Hangzhou Institute of Medicine (HIM), Chinese Academy of Sciences, Hangzhou, Zhejiang, 310022, P. R. China.,Institute of Molecular Medicine, Shanghai Key Laboratory for Nucleic Acid Chemistry and Nanomedicine, State Key Laboratory of Oncogenes and Related Genes, Renji Hospital, School of Medicine Shanghai Jiao Tong University, Shanghai, 200127, P. R. China
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3
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Rodriguez CM, Wright SE, Kearse MG, Haenfler JM, Flores BN, Liu Y, Ifrim MF, Glineburg MR, Krans A, Jafar-Nejad P, Sutton MA, Bassell GJ, Parent JM, Rigo F, Barmada SJ, Todd PK. A native function for RAN translation and CGG repeats in regulating fragile X protein synthesis. Nat Neurosci 2020; 23:386-397. [PMID: 32066985 PMCID: PMC7668390 DOI: 10.1038/s41593-020-0590-1] [Citation(s) in RCA: 41] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2018] [Accepted: 01/10/2020] [Indexed: 02/07/2023]
Abstract
Repeat-associated non-AUG-initiated translation of expanded CGG repeats (CGG RAN) from the FMR1 5'-leader produces toxic proteins that contribute to neurodegeneration in fragile X-associated tremor/ataxia syndrome. Here we describe how unexpanded CGG repeats and their translation play conserved roles in regulating fragile X protein (FMRP) synthesis. In neurons, CGG RAN acts as an inhibitory upstream open reading frame to suppress basal FMRP production. Activation of mGluR5 receptors enhances FMRP synthesis. This enhancement requires both the CGG repeat and CGG RAN initiation sites. Using non-cleaving antisense oligonucleotides (ASOs), we selectively blocked CGG RAN. This ASO blockade enhanced endogenous FMRP expression in human neurons. In human and rodent neurons, CGG RAN-blocking ASOs suppressed repeat toxicity and prolonged survival. These findings delineate a native function for CGG repeats and RAN translation in regulating basal and activity-dependent FMRP synthesis, and they demonstrate the therapeutic potential of modulating CGG RAN translation in fragile X-associated disorders.
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Affiliation(s)
- Caitlin M Rodriguez
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- Neuroscience Graduate Program, University of Michigan, Ann Arbor, MI, USA
| | - Shannon E Wright
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- Neuroscience Graduate Program, University of Michigan, Ann Arbor, MI, USA
| | - Michael G Kearse
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- Department of Biological Chemistry and Pharmacology, Center for RNA Biology, Ohio State University, Columbus, OH, USA
| | - Jill M Haenfler
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
| | - Brittany N Flores
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- Cellular and Molecular Biology Graduate Program, University of Michigan, Ann Arbor, MI, USA
| | - Yu Liu
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
| | - Marius F Ifrim
- Department of Cell Biology, Emory University School of Medicine, Atlanta, GA, USA
| | - Mary R Glineburg
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
| | - Amy Krans
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- VA Ann Arbor Healthcare System, Ann Arbor, MI, USA
| | | | - Michael A Sutton
- Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USA
| | - Gary J Bassell
- Department of Cell Biology, Emory University School of Medicine, Atlanta, GA, USA
| | - Jack M Parent
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
- VA Ann Arbor Healthcare System, Ann Arbor, MI, USA
| | - Frank Rigo
- Ionis Pharmaceuticals, Carlsbad, CA, USA
| | - Sami J Barmada
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA
| | - Peter K Todd
- Department of Neurology, University of Michigan, Ann Arbor, MI, USA.
- VA Ann Arbor Healthcare System, Ann Arbor, MI, USA.
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4
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Klusek J, Fairchild AJ, Roberts JE. Vagal Tone as a Putative Mechanism for Pragmatic Competence: An Investigation of Carriers of the FMR1 Premutation. J Autism Dev Disord 2019; 49:197-208. [PMID: 30097759 DOI: 10.1007/s10803-018-3714-7] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
Abstract
Pragmatic language skills exist across a continuum in typical and clinical populations, and are impaired in many neurodevelopmental disorders, most notably autism. The mechanisms underlying pragmatic impairment are poorly understood, although theory suggests dampened vagal tone plays a role. This study investigated the FMR1 premutation as a genetic model that may lend insight into the relationship between vagal function and pragmatic ability. Participants included 38 women with the FMR1 premutation and 23 controls. Vagal tone accounted for significant variance in pragmatics across both groups and statistically mediated the effect of FMR1 premutation status on pragmatic ability. Results support vagal tone as a biophysiological correlate of pragmatic ability, which informs potential mechanistic underpinnings and could have implications for targeted treatment.
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Affiliation(s)
- Jessica Klusek
- Department of Communication Sciences and Disorders, University of South Carolina, Keenan Building, 1229 Marion Street, Columbia, SC, 29201, USA.
| | - Amanda J Fairchild
- Department of Psychology, Barnwell College, University of South Carolina, 1512 Pendleton Street, Columbia, SC, 29208, USA
| | - Jane E Roberts
- Department of Psychology, Barnwell College, University of South Carolina, 1512 Pendleton Street, Columbia, SC, 29208, USA
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5
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hnRNP Q Regulates Internal Ribosome Entry Site-Mediated fmr1 Translation in Neurons. Mol Cell Biol 2019; 39:MCB.00371-18. [PMID: 30478144 DOI: 10.1128/mcb.00371-18] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2018] [Accepted: 11/15/2018] [Indexed: 01/05/2023] Open
Abstract
Fragile X syndrome (FXS) caused by loss of fragile X mental retardation protein (FMRP), is the most common cause of inherited intellectual disability. Numerous studies show that FMRP is an RNA binding protein that regulates translation of its binding targets and plays key roles in neuronal functions. However, the regulatory mechanism for FMRP expression is incompletely understood. Conflicting results regarding internal ribosome entry site (IRES)-mediated fmr1 translation have been reported. Here, we unambiguously demonstrate that the fmr1 gene, which encodes FMRP, exploits both IRES-mediated translation and canonical cap-dependent translation. Furthermore, we find that heterogeneous nuclear ribonucleoprotein Q (hnRNP Q) acts as an IRES-transacting factor (ITAF) for IRES-mediated fmr1 translation in neurons. We also show that semaphorin 3A (Sema3A)-induced axonal growth cone collapse is due to upregulation of hnRNP Q and subsequent IRES-mediated expression of FMRP. These data elucidate the regulatory mechanism of FMRP expression and its role in axonal growth cone collapse.
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6
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Sarnicola C, Farooq AV, Colby K. Fuchs Endothelial Corneal Dystrophy: Update on Pathogenesis and Future Directions. Eye Contact Lens 2019; 45:1-10. [DOI: 10.1097/icl.0000000000000469] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
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7
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Klusek J, Porter A, Abbeduto L, Adayev T, Tassone F, Mailick MR, Glicksman A, Tonnsen BL, Roberts JE. Curvilinear Association Between Language Disfluency and FMR1 CGG Repeat Size Across the Normal, Intermediate, and Premutation Range. Front Genet 2018; 9:344. [PMID: 30197656 PMCID: PMC6118037 DOI: 10.3389/fgene.2018.00344] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2018] [Accepted: 08/09/2018] [Indexed: 12/18/2022] Open
Abstract
Historically, investigations of FMR1 have focused almost exclusively on the clinical effects of CGG expansion within the categories of the premutation (55-200 CGG repeats) and fragile X syndrome (>200 CGG repeats). However, emerging evidence suggests that CGG-dependent phenotypes may occur across allele sizes traditionally considered within the "normal" range. This study adopted an individual-differences approach to determine the association between language production ability and CGG repeat length across the full range of normal, intermediate, and premutation alleles. Participants included 61 adult women with CGG repeats within the premutation (n = 37), intermediate (i.e., 41-54 repeats; n = 2), or normal (i.e., 6-40 repeats; n = 22) ranges. All participants were the biological mothers of a child with a developmental disorder, to control for the potential effects of parenting stress. Language samples were collected and the frequency of language disfluencies (i.e., interruptions in the flow of speech) served as an index of language production skills. Verbal inhibition skills, measured with the Hayling Sentence Completion Test, were also measured and examined as a correlate of language disfluency, consistent with theoretical work linking language disfluency with inhibitory deficits (i.e., the Inhibition Deficit Hypothesis). Blood samples were collected to determine FMR1 CGG repeat size. A general linear model tested CGG repeat size of the larger allele (allele-2) as the primary predictor of language disfluency, covarying for education level, IQ, age, and CGG repeats on the other allele. A robust curvilinear association between CGG length and language disfluency was detected, where low-normal (∼ <25 repeats) and mid-premutation alleles (∼90-110 repeats) were linked with higher rates of disfluency. Disfluency was not associated with inhibition deficits, which challenges prior theoretical work and suggests that a primary language deficit could account for elevated language disfluency in FMR1-associated conditions. Findings suggest CGG-dependent variation in language production ability, which was evident across individuals with and without CGG expansions on FMR1.
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Affiliation(s)
- Jessica Klusek
- Department of Communication Sciences and Disorders, University of South Carolina, Columbia, SC, United States
| | - Anna Porter
- Department of Psychology, University of South Carolina, Columbia, SC, United States
| | - Leonard Abbeduto
- Department of Psychiatry and Behavioral Sciences, University of California, Davis, Sacramento, CA, United States
- MIND Institute, University of California, Davis, Sacramento, CA, United States
| | - Tatyana Adayev
- Department of Human Genetics, New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, United States
| | - Flora Tassone
- MIND Institute, University of California, Davis, Sacramento, CA, United States
- Department of Biochemistry and Molecular Medicine, University of California, Davis, Sacramento, CA, United States
| | - Marsha R. Mailick
- Waisman Center, University of Wisconsin-Madison, Madison, WI, United States
| | - Anne Glicksman
- Department of Human Genetics, New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, United States
| | - Bridgette L. Tonnsen
- Department of Psychological Sciences, Purdue University, Lafayette, IN, United States
| | - Jane E. Roberts
- Department of Psychology, University of South Carolina, Columbia, SC, United States
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8
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Green KM, Linsalata AE, Todd PK. RAN translation-What makes it run? Brain Res 2016; 1647:30-42. [PMID: 27060770 DOI: 10.1016/j.brainres.2016.04.003] [Citation(s) in RCA: 72] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2016] [Revised: 03/24/2016] [Accepted: 04/01/2016] [Indexed: 12/14/2022]
Abstract
Nucleotide-repeat expansions underlie a heterogeneous group of neurodegenerative and neuromuscular disorders for which there are currently no effective therapies. Recently, it was discovered that such repetitive RNA motifs can support translation initiation in the absence of an AUG start codon across a wide variety of sequence contexts, and that the products of these atypical translation initiation events contribute to neuronal toxicity. This review examines what we currently know and do not know about repeat associated non-AUG (RAN) translation in the context of established canonical and non-canonical mechanisms of translation initiation. We highlight recent findings related to RAN translation in three repeat expansion disorders: CGG repeats in fragile X-associated tremor ataxia syndrome (FXTAS), GGGGCC repeats in C9orf72 associated amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and CAG repeats in Huntington disease. These studies suggest that mechanistic differences may exist for RAN translation dependent on repeat type, repeat reading frame, and the surrounding sequence context, but that for at least some repeats, RAN translation retains a dependence on some of the canonical translational initiation machinery. This article is part of a Special Issue entitled SI:RNA Metabolism in Disease.
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Affiliation(s)
- Katelyn M Green
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI, United States; Program in Cellular and Molecular Biology, University of Michigan Medical School, Ann Arbor, MI, United States
| | - Alexander E Linsalata
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI, United States; Program in Cellular and Molecular Biology, University of Michigan Medical School, Ann Arbor, MI, United States
| | - Peter K Todd
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI, United States; Program in Cellular and Molecular Biology, University of Michigan Medical School, Ann Arbor, MI, United States; Veterans Affairs Medical Center, Ann Arbor, MI, United States.
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9
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Kearse MG, Green KM, Krans A, Rodriguez CM, Linsalata AE, Goldstrohm AC, Todd PK. CGG Repeat-Associated Non-AUG Translation Utilizes a Cap-Dependent Scanning Mechanism of Initiation to Produce Toxic Proteins. Mol Cell 2016; 62:314-322. [PMID: 27041225 DOI: 10.1016/j.molcel.2016.02.034] [Citation(s) in RCA: 125] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2015] [Revised: 01/25/2016] [Accepted: 02/26/2016] [Indexed: 02/03/2023]
Abstract
Repeat-associated non-AUG (RAN) translation produces toxic polypeptides from nucleotide repeat expansions in the absence of an AUG start codon and contributes to neurodegenerative disorders such as ALS and fragile X-associated tremor/ataxia syndrome. How RAN translation occurs is unknown. Here we define the critical sequence and initiation factors that mediate CGG repeat RAN translation in the 5' leader of fragile X mRNA, FMR1. Our results reveal that CGG RAN translation is 30%-40% as efficient as AUG-initiated translation, is m(7)G cap and eIF4E dependent, requires the eIF4A helicase, and is strongly influenced by repeat length. However, it displays a dichotomous requirement for initiation site selection between reading frames, with initiation in the +1 frame, but not the +2 frame, occurring at near-cognate start codons upstream of the repeat. These data support a model in which RAN translation at CGG repeats uses cap-dependent ribosomal scanning, yet bypasses normal requirements for start codon selection.
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Affiliation(s)
- Michael G Kearse
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI 48109, USA
| | - Katelyn M Green
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Cellular and Molecular Biology Program, University of Michigan Medical School, Ann Arbor, MI 48109, USA
| | - Amy Krans
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Veterans Affairs Medical Center, Ann Arbor, MI 48105, USA
| | - Caitlin M Rodriguez
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Neuroscience Graduate Program, University of Michigan Medical School, Ann Arbor, MI 48109, USA
| | - Alexander E Linsalata
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Cellular and Molecular Biology Program, University of Michigan Medical School, Ann Arbor, MI 48109, USA
| | - Aaron C Goldstrohm
- Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI 48109, USA
| | - Peter K Todd
- Department of Neurology, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Cellular and Molecular Biology Program, University of Michigan Medical School, Ann Arbor, MI 48109, USA; Veterans Affairs Medical Center, Ann Arbor, MI 48105, USA; Neuroscience Graduate Program, University of Michigan Medical School, Ann Arbor, MI 48109, USA.
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10
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Renoux AJ, Carducci NM, Ahmady AA, Todd PK. Fragile X mental retardation protein expression in Alzheimer's disease. Front Genet 2014; 5:360. [PMID: 25452762 PMCID: PMC4233940 DOI: 10.3389/fgene.2014.00360] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2014] [Accepted: 09/27/2014] [Indexed: 12/31/2022] Open
Abstract
The FMR1 protein product, FMRP, is an mRNA binding protein associated with translational inhibition of target transcripts. One FMRP target is the amyloid precursor protein (APP) mRNA, and APP levels are elevated in Fmr1 KO mice. Given that elevated APP protein expression can elicit Alzheimer's disease (AD) in patients and model systems, we evaluated whether FMRP expression might be altered in Alzheimer's autopsy brain samples and mouse models compared to controls. In a double transgenic mouse model of AD (APP/PS1), we found no difference in FMRP expression in aged AD model mice compared to littermate controls. FMRP expression was also similar in AD and control patient frontal cortex and cerebellum samples. Fragile X-associated tremor/ataxia syndrome (FXTAS) is an age-related neurodegenerative disorder caused by expanded CGG repeats in the 5' untranslated region of the FMR1 gene. Patients experience cognitive impairment and dementia in addition to motor symptoms. In parallel studies, we measured FMRP expression in cortex and cerebellum from three FXTAS patients and found reduced expression compared to both controls and Alzheimer's patient brains, consistent with animal models. We also find increased APP levels in cerebellar, but not cortical, samples of FXTAS patients compared to controls. Taken together, these data suggest that a decrease in FMRP expression is unlikely to be a primary contributor to Alzheimer's disease pathogenesis.
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Affiliation(s)
- Abigail J Renoux
- Department of Molecular and Integrative Physiology, University of Michigan , Ann Arbor, MI, USA ; Department of Neurology, University of Michigan , Ann Arbor, MI, USA
| | | | - Arya A Ahmady
- Department of Neurology, University of Michigan , Ann Arbor, MI, USA
| | - Peter K Todd
- Department of Neurology, University of Michigan , Ann Arbor, MI, USA
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11
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Kearse MG, Todd PK. Repeat-associated non-AUG translation and its impact in neurodegenerative disease. Neurotherapeutics 2014; 11:721-31. [PMID: 25005000 PMCID: PMC4391382 DOI: 10.1007/s13311-014-0292-z] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
Nucleotide repeat expansions underlie numerous human neurological disorders. Repeats can trigger toxicity through multiple pathogenic mechanisms, including RNA gain-of-function, protein gain-of-function, and protein loss-of-function pathways. Traditionally, inference of the underlying pathogenic mechanism derives from the repeat location, with dominantly inherited repeats within transcribed noncoding sequences eliciting toxicity predominantly as RNA via sequestration of specific RNA binding proteins. However, recent findings question this assumption and suggest that repeats outside of annotated open reading frames may also trigger toxicity through a novel form of protein translational initiation known as repeat-associated non-AUG (RAN) translation. To date, RAN translation has been implicated in 4 nucleotide repeat expansion disorders: spinocerebellar ataxia type 8; myotonic dystrophy type 1 with CTG•CAG repeats; C9orf72 amyotrophic lateral sclerosis/frontotemporal dementia with GGGGCC•GGCCCC repeats; and fragile X-associated tremor/ataxia syndrome with CGG repeats. RAN translation contributes to hallmark pathological characteristics in these disorders by producing homopolymeric or dipeptide repeat proteins. Here, we review what is known about RAN translation, with an emphasis on how differences in both repeat sequence and context may confer different requirements for unconventional initiation. We then discuss how this new mechanism of translational initiation might function in normal physiology and lay out a roadmap for addressing the numerous questions that remain.
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Affiliation(s)
- Michael G. Kearse
- />Department of Neurology, University of Michigan Medical School, 4005 BSRB, 109 Zina Pitcher Place, Ann Arbor, MI 48109-2200 USA
| | - Peter K. Todd
- />Department of Neurology, University of Michigan Medical School, 4005 BSRB, 109 Zina Pitcher Place, Ann Arbor, MI 48109-2200 USA
- />Veterans Affairs Medical Center, Ann Arbor, MI 48105 USA
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12
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Pretto D, Yrigollen CM, Tang HT, Williamson J, Espinal G, Iwahashi CK, Durbin-Johnson B, Hagerman RJ, Hagerman PJ, Tassone F. Clinical and molecular implications of mosaicism in FMR1 full mutations. Front Genet 2014; 5:318. [PMID: 25278957 PMCID: PMC4166380 DOI: 10.3389/fgene.2014.00318] [Citation(s) in RCA: 73] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2014] [Accepted: 08/25/2014] [Indexed: 12/27/2022] Open
Abstract
Expansions of more than 200 CGG repeats (full mutation) in the FMR1 gene give rise to fragile X syndrome (FXS) through a process that generally involves hypermethylation of the FMR1 promoter region and gene silencing, resulting in absence of expression of the encoded protein, FMRP. However, mosaicism with alleles differing in size and extent of methylation often exist within or between tissues of individuals with FXS. In the current work, CGG-repeat lengths and methylation status were assessed for eighteen individuals with FXS, including 13 mosaics, for which peripheral blood cells (PBMCs) and primary fibroblast cells were available. Our results show that for both PBMCs and fibroblasts, FMR1 mRNA and FMRP expression are directly correlated with the percent of methylation of the FMR1 allele. In addition, Full Scale IQ scores were inversely correlated with the percent methylation and positively correlated with higher FMRP expression. These latter results point toward a positive impact on cognition for full mutation mosaics with lower methylation compared to individuals with fully methylated, full mutation alleles. However, we did not observe a significant reduction in the number of seizures, nor in the severity of hyperactivity or autism spectrum disorder, among individuals with mosaic genotypes in the presentation of FXS. These observations suggest that low, but non-zero expression of FMRP may be sufficient to positively impact cognitive function in individuals with FXS, with methylation mosaicism (lowered methylation fraction) contributing to a more positive clinical outcome.
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Affiliation(s)
- Dalyir Pretto
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - Carolyn M Yrigollen
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - Hiu-Tung Tang
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - John Williamson
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - Glenda Espinal
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - Chris K Iwahashi
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA
| | - Blythe Durbin-Johnson
- Department of Public Health Sciences, School of Medicine, University of California at Davis Davis, CA, USA
| | - Randi J Hagerman
- Department of Pediatrics, School of Medicine, University of California at Davis Davis, CA, USA ; MIND Institute, UC Davis Medical Center Sacramento, CA, USA
| | - Paul J Hagerman
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA ; Department of Pediatrics, School of Medicine, University of California at Davis Davis, CA, USA
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California at Davis Davis, CA, USA ; Department of Pediatrics, School of Medicine, University of California at Davis Davis, CA, USA
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13
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Juang BT, Ludwig AL, Benedetti KL, Gu C, Collins K, Morales C, Asundi A, Wittmann T, L'Etoile N, Hagerman PJ. Expression of an expanded CGG-repeat RNA in a single pair of primary sensory neurons impairs olfactory adaptation in Caenorhabditis elegans. Hum Mol Genet 2014; 23:4945-59. [PMID: 24821701 PMCID: PMC4140470 DOI: 10.1093/hmg/ddu210] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Fragile X-associated tremor/ataxia syndrome (FXTAS) is a severe neurodegenerative disorder that affects carriers of premutation CGG-repeat expansion alleles of the fragile X mental retardation 1 (FMR1) gene; current evidence supports a causal role of the expanded CGG repeat within the FMR1 mRNA in the pathogenesis of FXTAS. Though the mRNA has been observed to induce cellular toxicity in FXTAS, the mechanisms are unclear. One common neurophysiological characteristic of FXTAS patients is their inability to properly attenuate their response to an auditory stimulus upon receipt of a small pre-stimulus. Therefore, to gain genetic and cell biological insight into FXTAS, we examined the effect of expanded CGG repeats on the plasticity of the olfactory response of the genetically tractable nematode, Caenorhabditis elegans (C. elegans). While C. elegans is innately attracted to odors, this response can be downregulated if the odor is paired with starvation. We found that expressing expanded CGG repeats in olfactory neurons interfered with this plasticity without affecting either the innate odor-seeking response or the olfactory neuronal morphology. Interrogation of three RNA regulatory pathways indicated that the expanded CGG repeats act via the C. elegans microRNA (miRNA)-specific Argonaute ALG-2 to diminish olfactory plasticity. This observation suggests that the miRNA-Argonaute pathway may play a pathogenic role in subverting neuronal function in FXTAS.
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Affiliation(s)
- Bi-Tzen Juang
- Department of Biological Science and Technology, National Chiao Tung University, Hsinchu 300, Taiwan
| | - Anna L Ludwig
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA 95616, USA
| | - Kelli L Benedetti
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Chen Gu
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Kimberly Collins
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Christopher Morales
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Aarati Asundi
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Torsten Wittmann
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Noelle L'Etoile
- Department of Cell and Tissue Biology, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Paul J Hagerman
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA 95616, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA 95817, USA
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14
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Mohan A, Goodwin M, Swanson MS. RNA-protein interactions in unstable microsatellite diseases. Brain Res 2014; 1584:3-14. [PMID: 24709120 DOI: 10.1016/j.brainres.2014.03.039] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2014] [Revised: 03/25/2014] [Accepted: 03/26/2014] [Indexed: 12/14/2022]
Abstract
A novel RNA-mediated disease mechanism has emerged from studies on dominantly inherited neurological disorders caused by unstable microsatellite expansions in non-coding regions of the genome. These non-coding tandem repeat expansions trigger the production of unusual RNAs that gain a toxic function, which involves the formation of RNA repeat structures that interact with, and alter the activities of, various factors required for normal RNA processing as well as additional cellular functions. In this review, we explore the deleterious effects of toxic RNA expression and discuss the various model systems currently available for studying RNA gain-of-function in neurologic diseases. Common themes, including bidirectional transcription and repeat-associated non-ATG (RAN) translation, have recently emerged from expansion disease studies. These and other discoveries have highlighted the need for further investigations designed to provide the additional mechanistic insights essential for future therapeutic development.
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Affiliation(s)
- Apoorva Mohan
- Department of Molecular Genetics and Microbiology, Center for NeuroGenetics and the Genetics Institute, University of Florida, College of Medicine, Cancer Genetics Research Complex, 2033 Mowry Road, Gainesville, FL 32610-3610, USA
| | - Marianne Goodwin
- Department of Molecular Genetics and Microbiology, Center for NeuroGenetics and the Genetics Institute, University of Florida, College of Medicine, Cancer Genetics Research Complex, 2033 Mowry Road, Gainesville, FL 32610-3610, USA
| | - Maurice S Swanson
- Department of Molecular Genetics and Microbiology, Center for NeuroGenetics and the Genetics Institute, University of Florida, College of Medicine, Cancer Genetics Research Complex, 2033 Mowry Road, Gainesville, FL 32610-3610, USA.
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15
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Ludwig AL, Espinal GM, Pretto DI, Jamal AL, Arque G, Tassone F, Berman RF, Hagerman PJ. CNS expression of murine fragile X protein (FMRP) as a function of CGG-repeat size. Hum Mol Genet 2014; 23:3228-38. [PMID: 24463622 DOI: 10.1093/hmg/ddu032] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Large expansions of a CGG-repeat element (>200 repeats; full mutation) in the fragile X mental retardation 1 (FMR1) gene cause fragile X syndrome (FXS), the leading single-gene form of intellectual disability and of autism spectrum disorder. Smaller expansions (55-200 CGG repeats; premutation) result in the neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). Whereas FXS is caused by gene silencing and insufficient FMR1 protein (FMRP), FXTAS is thought to be caused by 'toxicity' of expanded-CGG-repeat mRNA. However, as FMRP expression levels decrease with increasing CGG-repeat length, lowered protein may contribute to premutation-associated clinical involvement. To address this issue, we measured brain Fmr1 mRNA and FMRP levels as a function of CGG-repeat length in a congenic (CGG-repeat knock-in) mouse model using 57 wild-type and 97 expanded-CGG-repeat mice carrying up to ~250 CGG repeats. While Fmr1 message levels increased with repeat length, FMRP levels trended downward over the same range, subject to significant inter-subject variation. Human comparisons of protein levels in the frontal cortex of 7 normal and 17 FXTAS individuals revealed that the mild FMRP decrease in mice mirrored the more limited data for FMRP expression in the human samples. In addition, FMRP expression levels varied in a subset of mice across the cerebellum, frontal cortex, and hippocampus, as well as at different ages. These results provide a foundation for understanding both the CGG-repeat-dependence of FMRP expression and for interpreting clinical phenotypes in premutation carriers in terms of the balance between elevated mRNA and lowered FMRP expression levels.
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Affiliation(s)
- Anna Lisa Ludwig
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA
| | - Glenda M Espinal
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA
| | - Dalyir I Pretto
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and
| | - Amanda L Jamal
- Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA
| | - Gloria Arque
- Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and
| | - Robert F Berman
- MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA
| | - Paul J Hagerman
- Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and
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16
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Todd PK, Oh SY, Krans A, He F, Sellier C, Frazer M, Renoux AJ, Chen KC, Scaglione KM, Basrur V, Elenitoba-Johnson K, Vonsattel JP, Louis ED, Sutton MA, Taylor JP, Mills RE, Charlet-Berguerand N, Paulson HL. CGG repeat-associated translation mediates neurodegeneration in fragile X tremor ataxia syndrome. Neuron 2013; 78:440-55. [PMID: 23602499 DOI: 10.1016/j.neuron.2013.03.026] [Citation(s) in RCA: 361] [Impact Index Per Article: 32.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/26/2013] [Indexed: 01/18/2023]
Abstract
Fragile X-associated tremor ataxia syndrome (FXTAS) results from a CGG repeat expansion in the 5' UTR of FMR1. This repeat is thought to elicit toxicity as RNA, yet disease brains contain ubiquitin-positive neuronal inclusions, a pathologic hallmark of protein-mediated neurodegeneration. We explain this paradox by demonstrating that CGG repeats trigger repeat-associated non-AUG-initiated (RAN) translation of a cryptic polyglycine-containing protein, FMRpolyG. FMRpolyG accumulates in ubiquitin-positive inclusions in Drosophila, cell culture, mouse disease models, and FXTAS patient brains. CGG RAN translation occurs in at least two of three possible reading frames at repeat sizes ranging from normal (25) to pathogenic (90), but inclusion formation only occurs with expanded repeats. In Drosophila, CGG repeat toxicity is suppressed by eliminating RAN translation and enhanced by increased polyglycine protein production. These studies expand the growing list of nucleotide repeat disorders in which RAN translation occurs and provide evidence that RAN translation contributes to neurodegeneration.
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Affiliation(s)
- Peter K Todd
- Department of Neurology, University of Michigan, Ann Arbor, MI 48109, USA.
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17
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Iliff AJ, Renoux AJ, Krans A, Usdin K, Sutton MA, Todd PK. Impaired activity-dependent FMRP translation and enhanced mGluR-dependent LTD in Fragile X premutation mice. Hum Mol Genet 2012; 22:1180-92. [PMID: 23250915 DOI: 10.1093/hmg/dds525] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Fragile X premutation-associated disorders, including Fragile X-associated Tremor Ataxia Syndrome, result from unmethylated CGG repeat expansions in the 5' untranslated region (UTR) of the FMR1 gene. Premutation-sized repeats increase FMR1 transcription but impair rapid translation of the Fragile X mental retardation protein (FMRP), which is absent in Fragile X Syndrome (FXS). Normally, FMRP binds to RNA and regulates metabotropic glutamate receptor (mGluR)-mediated synaptic translation, allowing for dendritic synthesis of several proteins. FMRP itself is also synthesized at synapses in response to mGluR activation. However, the role of activity-dependent translation of FMRP in synaptic plasticity and Fragile X-premutation-associated disorders is unknown. To investigate this question, we utilized a CGG knock-in mouse model of the Fragile X premutation with 120-150 CGG repeats in the mouse Fmr1 5' UTR. These mice exhibit increased Fmr1 mRNA production but impaired FMRP translational efficiency, leading to a modest reduction in basal FMRP expression. Cultured hippocampal neurons and synaptoneurosomes derived from CGG KI mice demonstrate impaired FMRP translation in response to the group I mGluR agonist 3,5-dihydroxyphenylglycine. Electrophysiological analysis reveals enhanced mGluR-mediated long-term depression (mGluR-LTD) at CA3-CA1 synapses in acute hippocampal slices prepared from CGG KI mice relative to wild-type littermates, similar to Fmr1 knockout mice. However, unlike mGluR-LTD in mice completely lacking FMRP, mGluR-LTD in CGG knock-in mice remains dependent on new protein synthesis. These studies demonstrate partially overlapping synaptic plasticity phenotypes in mouse models of FXS and Fragile X premutation disorders and support a role for activity-dependent synthesis of FMRP in enduring forms of synaptic plasticity.
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Affiliation(s)
- Adam J Iliff
- Neuroscience Graduate Program, University of Michigan, Ann Arbor, MI 48109, USA
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18
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Paek KY, Park SM, Hong KY, Jang SK. Cap-dependent translation without base-by-base scanning of an messenger ribonucleic acid. Nucleic Acids Res 2012; 40:7541-51. [PMID: 22638585 PMCID: PMC3424581 DOI: 10.1093/nar/gks471] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
‘Ribosome scanning’ is the generally accepted mechanism for explaining how a ribosome finds an initiation codon located far removed from the ribosome recruiting site (cap structure). However, the molecular characteristics of ribosome scanning along 5′ untranslated regions (UTRs) remain obscure. Herein, using a rabbit reticulocyte lysate (RRL) system and artificial ribonucleic acid (RNA) constructs composed of a capped leader RNA and an uncapped reporter RNA annealed through a double-stranded RNA (dsRNA) bridge, we show that the ribosome can efficiently bypass a stable, dsRNA region without melting the structure. The insertion of an upstream open reading frame in the capped leader RNA impaired the translation of reporter RNA, indicating that a ribosome associated with the 5′-end explores the regions upstream of the dsRNA bridge in search of the initiation codon. These data indicate that a ribosome may skip part(s) of an messenger RNA 5′UTR without thoroughly scanning it.
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Affiliation(s)
- Ki Young Paek
- PBC, Department of Life Science, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, Republic of Korea
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Why is vision impaired in fragile X premutation carriers? The role of fragile X mental retardation protein and potential FMR1 mRNA toxicity. Neuroscience 2012; 206:183-9. [PMID: 22266345 DOI: 10.1016/j.neuroscience.2012.01.005] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2011] [Revised: 01/02/2012] [Accepted: 01/04/2012] [Indexed: 11/20/2022]
Abstract
Dysfunctions of the geniculo-striatal magnocellular (M) visual pathway and its cortical recipients have been documented in fragile X syndrome and in FMR1 premutation carriers. However, the mechanism of this impairment is less clear. To elucidate this issue, we completed the measurement of visual functions at different stages of information processing: low-level mechanisms (contrast sensitivity biasing information processing toward the M and parvocellular [P] pathways), primary visual cortex (motion-defined and static Vernier threshold), and higher-level form and motion processing (coherence thresholds). Results revealed that FMR1 premutation carriers, relative to non-carrier controls, exhibited lower contrast sensitivity for M pathway-biased stimuli, higher Vernier threshold for motion-defined stimuli, and higher global motion coherence threshold. Although both elevated FMR1 mRNA and reduced fragile X mental retardation protein (FMRP) levels were associated with impaired visual functions, regression analysis indicated that FMRP was the primary factor. In premutation carriers, a toxic gain-of-function of elevated FMR1 mRNA has been suggested, whereas reduced FMRP is linked to neurodevelopmental aspects. Here, we showed that FMRP may the primary factor associated with visual dysfunctions.
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Abstract
Fragile X syndrome (FXS) is characterized by moderate to severe intellectual disability, which is accompanied by macroorchidism and distinct facial morphology. FXS is caused by the expansion of the CGG trinucleotide repeat in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. The syndrome has been studied in ethnically diverse populations around the world and has been extensively characterized in several populations. Similar to other trinucleotide expansion disorders, the gene-specific instability of FMR1 is not accompanied by genomic instability. Currently we do not have a comprehensive understanding of the molecular underpinnings of gene-specific instability associated with tandem repeats. Molecular evidence from in vitro experiments and animal models supports several pathways for gene-specific trinucleotide repeat expansion. However, whether the mechanisms reported from other systems contribute to trinucleotide repeat expansion in humans is not clear. To understand how repeat instability in humans could occur, the CGG repeat expansion is explored through molecular analysis and population studies which characterized CGG repeat alleles of FMR1. Finally, the review discusses the relevance of these studies in understanding the mechanism of trinucleotide repeat expansion in FXS.
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Affiliation(s)
- Emmanuel Peprah
- Center for Research on Genomics and Global Health, National Human Genome Research Institute, National Institute of Health, Bethesda, MD 20892, USA.
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21
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Renoux AJ, Todd PK. Neurodegeneration the RNA way. Prog Neurobiol 2011; 97:173-89. [PMID: 22079416 DOI: 10.1016/j.pneurobio.2011.10.006] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2011] [Revised: 10/06/2011] [Accepted: 10/27/2011] [Indexed: 02/09/2023]
Abstract
The expression, processing, transport and activities of both coding and non-coding RNAs play critical roles in normal neuronal function and differentiation. Over the past decade, these same pathways have come under scrutiny as potential contributors to neurodegenerative disease. Here we focus broadly on the roles of RNA and RNA processing in neurodegeneration. We first discuss a set of "RNAopathies", where non-coding repeat expansions drive pathogenesis through a surprisingly diverse set of mechanisms. We next explore an emerging class of "RNA binding proteinopathies" where redistribution and aggregation of the RNA binding proteins TDP-43 or FUS contribute to a potentially broad range of neurodegenerative disorders. Lastly, we delve into the potential contributions of alterations in both short and long non-coding RNAs to neurodegenerative illness.
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Affiliation(s)
- Abigail J Renoux
- Department of Molecular and Integrative Physiology, University of Michigan, 4005 BSRB, 109 Zina Pitcher Place, Ann Arbor, MI 48109, USA
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Decreased fragile X mental retardation protein expression underlies amygdala dysfunction in carriers of the fragile X premutation. Biol Psychiatry 2011; 70:859-65. [PMID: 21783174 PMCID: PMC3191264 DOI: 10.1016/j.biopsych.2011.05.033] [Citation(s) in RCA: 85] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/12/2010] [Revised: 04/15/2011] [Accepted: 05/27/2011] [Indexed: 11/22/2022]
Abstract
BACKGROUND The fragile X premutation provides a unique opportunity for the study of genetic and brain mechanisms of behavior and cognition in the context of neurodevelopment and neurodegeneration. Although the neurodegenerative phenotype, fragile X-associated tremor/ataxia syndrome, is well described, evidence of a causal link between the premutation and psychiatric disorder earlier in life, clear delineation of a behavioral/cognitive phenotype, and characterization of the physiological basis of observed symptoms have been elusive. METHODS We completed functional magnetic resonance imaging targeting the amygdala with an emotion-matching task and concurrent infrared eye tracking, FMR1 molecular genetic testing, and neuropsychological assessment in 23 men with the premutation (mean age = 32.9 years) and 25 male control subjects (mean age = 30.1 years). RESULTS Premutation carriers had significantly smaller left and right amygdala volume and reduced right amygdala activation during the task relative to control subjects. Although both elevated FMR1 messenger RNA and reduced fragile X mental retardation protein (FMRP) were associated with the reduced activation, multiple regression analysis suggested that reduced FMRP is the primary factor. Premutation carriers also had higher ratings of autism spectrum symptoms than control subjects, which were associated with the reduced amygdala response. CONCLUSIONS Although prior studies have emphasized a toxic gain-of-function effect of elevated messenger RNA associated with the premutation, the current results point to the role of reduced FMRP in alterations of brain activity and behavior.
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Repeat associated non-ATG translation initiation: one DNA, two transcripts, seven reading frames, potentially nine toxic entities! PLoS Genet 2011; 7:e1002018. [PMID: 21423665 PMCID: PMC3053344 DOI: 10.1371/journal.pgen.1002018] [Citation(s) in RCA: 89] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Diseases associated with unstable repetitive elements in the DNA, RNA, and amino acids have consistently revealed scientific surprises. Most diseases are caused by expansions of trinucleotide repeats, which ultimately lead to diseases like Huntington's disease, myotonic dystrophy, fragile X syndrome, and a series of spinocerebellar ataxias. These repeat mutations are dynamic, changing through generations and within an individual, and the repeats can be bi-directionally transcribed. Unsuspected modes of pathogenesis involve aberrant loss of protein expression; aberrant over-expression of non-mutant proteins; toxic-gain-of-protein function through expanded polyglutamine tracts that are encoded by expanded CAG tracts; and RNA-toxic-gain-of-function caused by transcripts harboring expanded CUG, CAG, or CGG tracts. A recent advance reveals that RNA transcripts with expanded CAG repeats can be translated in the complete absence of a starting ATG, and this Repeat Associated Non-ATG translation (RAN-translation) occurs across expanded CAG repeats in all reading frames (CAG, AGC, and GCA) to produce homopolymeric proteins of long polyglutamine, polyserine, and polyalanine tracts. Expanded CTG tracts expressing CUG transcripts also show RAN-translation occurring in all three frames (CUG, UGC, and GCU), to produce polyleucine, polycysteine, and polyalanine. These RAN-translation products can be toxic. Thus, one unstable (CAG)•(CTG) DNA can produce two expanded repeat transcripts and homopolymeric proteins with reading frames (the AUG-directed polyGln and six RAN-translation proteins), yielding a total of potentially nine toxic entities. The occurrence of RAN-translation in patient tissues expands our horizons of modes of disease pathogenesis. Moreover, since RAN-translation counters the canonical requirements of translation initiation, many new questions are now posed that must be addressed. This review covers RAN-translation and some of the pertinent questions.
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