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Rajan-Babu IS, Dolzhenko E, Eberle MA, Friedman JM. Sequence composition changes in short tandem repeats: heterogeneity, detection, mechanisms and clinical implications. Nat Rev Genet 2024; 25:476-499. [PMID: 38467784 DOI: 10.1038/s41576-024-00696-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 01/19/2024] [Indexed: 03/13/2024]
Abstract
Short tandem repeats (STRs) are a class of repetitive elements, composed of tandem arrays of 1-6 base pair sequence motifs, that comprise a substantial fraction of the human genome. STR expansions can cause a wide range of neurological and neuromuscular conditions, known as repeat expansion disorders, whose age of onset, severity, penetrance and/or clinical phenotype are influenced by the length of the repeats and their sequence composition. The presence of non-canonical motifs, depending on the type, frequency and position within the repeat tract, can alter clinical outcomes by modifying somatic and intergenerational repeat stability, gene expression and mutant transcript-mediated and/or protein-mediated toxicities. Here, we review the diverse structural conformations of repeat expansions, technological advances for the characterization of changes in sequence composition, their clinical correlations and the impact on disease mechanisms.
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Affiliation(s)
- Indhu-Shree Rajan-Babu
- Department of Medical Genetics, The University of British Columbia, and Children's & Women's Hospital, Vancouver, British Columbia, Canada.
| | | | | | - Jan M Friedman
- Department of Medical Genetics, The University of British Columbia, and Children's & Women's Hospital, Vancouver, British Columbia, Canada
- BC Children's Hospital Research Institute, Vancouver, British Columbia, Canada
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2
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Rodrigues B, Sousa V, Yrigollen CM, Tassone F, Villate O, Allen EG, Glicksman A, Tortora N, Nolin SL, Nogueira AJA, Jorge P. FMR1 allelic complexity in premutation carriers provides no evidence for a correlation with age at amenorrhea. Reprod Biol Endocrinol 2024; 22:71. [PMID: 38907244 PMCID: PMC11191145 DOI: 10.1186/s12958-024-01227-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/14/2023] [Accepted: 05/09/2024] [Indexed: 06/23/2024] Open
Abstract
BACKGROUND Premutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene, defined as between 55 and 200 CGGs, have been implicated in fragile X-associated primary ovarian insufficiency (FXPOI). Only 20% of female premutation carriers develop early ovulatory dysfunction, the reason for this incomplete penetrance is unknown. This study validated the mathematical model in premutation alleles, after assigning each allele a score representing allelic complexity. Subsequently, allelic scores were used to investigate the impact of allele complexity on age at amenorrhea for 58 premutation cases (116 alleles) previously published. METHODS The allelic score was determined using a formula previously described by our group. The impact of each allelic score on age at amenorrhea was analyzed using Pearson's test and a contour plot generated to visualize the effect. RESULTS Correlation of allelic score revealed two distinct complexity behaviors in premutation alleles. No significant correlation was observed between the allelic score of premutation alleles and age at amenorrhea. The same lack of significant correlation was observed regarding normal-sized alleles, despite a nearly significant trend. CONCLUSIONS Our results suggest that the use of allelic scores combination have the potential to explain female infertility, namely the development of FXPOI, or ovarian dysfunction, despite the lack of correlation with age at amenorrhea. Such a finding is of great clinical significance for early identification of females at risk of ovulatory dysfunction, enhancement of fertility preservation techniques, and increasing the probability for a successful pregnancy in females with premutations. Additional investigation is necessary to validate this hypothesis.
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Affiliation(s)
- Bárbara Rodrigues
- Molecular Genetics Laboratory, Laboratory Genetics Service, Genetics and Pathology Clinic, Unidade Local de Saúde de Santo António (ULSSA), Porto, Portugal
- UMIB - Unit for Multidisciplinary Research in Biomedicine, ICBAS - School of Medicine and Biomedical Sciences, UPorto - University of Porto, Porto, Portugal
- ITR - Laboratory for Integrative and Translational Research in Population Health, Porto, Portugal
| | - Vanessa Sousa
- Molecular Genetics Laboratory, Laboratory Genetics Service, Genetics and Pathology Clinic, Unidade Local de Saúde de Santo António (ULSSA), Porto, Portugal
- UMIB - Unit for Multidisciplinary Research in Biomedicine, ICBAS - School of Medicine and Biomedical Sciences, UPorto - University of Porto, Porto, Portugal
- ITR - Laboratory for Integrative and Translational Research in Population Health, Porto, Portugal
- Cytogenetics Laboratory, Department of Microscopy, ICBAS - School of Medicine and Biomedical Sciences, UPorto - University of Porto, Porto, Portugal
| | - Carolyn M Yrigollen
- Raymond G. Perelman Center for Cellular and Molecular Therapeutics, Children's Hospital of Philadelphia Research Institute, Philadelphia, PA, USA
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California, Davis, Stockton Blvd, USA
- MIND Institute, School of Medicine, University of California, Davis, CA, USA
| | - Olatz Villate
- Pediatric Oncology Group, Biocruces Bizkaia Health Research Institute, Barakaldo, Biscay, Basque Country, Spain
| | - Emily G Allen
- Department of Human Genetics, Emory University School of Medicine, Atlanta, GA, USA
| | - Anne Glicksman
- New York State Institute for Basic Research in Developmental Disabilities, New York, NY, USA
| | - Nicole Tortora
- New York State Institute for Basic Research in Developmental Disabilities, New York, NY, USA
| | - Sarah L Nolin
- New York State Institute for Basic Research in Developmental Disabilities, New York, NY, USA
| | - António J A Nogueira
- CESAM - Center for Environmental and Marine Studies, Department of Biology, University of Aveiro, Aveiro, Portugal
| | - Paula Jorge
- Molecular Genetics Laboratory, Laboratory Genetics Service, Genetics and Pathology Clinic, Unidade Local de Saúde de Santo António (ULSSA), Porto, Portugal.
- UMIB - Unit for Multidisciplinary Research in Biomedicine, ICBAS - School of Medicine and Biomedical Sciences, UPorto - University of Porto, Porto, Portugal.
- ITR - Laboratory for Integrative and Translational Research in Population Health, Porto, Portugal.
- Cytogenetics Laboratory, Department of Microscopy, ICBAS - School of Medicine and Biomedical Sciences, UPorto - University of Porto, Porto, Portugal.
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Shen YI, Cheng KC, Wei YJ, Lee IR. Structural Dynamics Role of AGG Interruptions in Inhibition CGG Repeat Expansion Associated with Fragile X Syndrome. ACS Chem Neurosci 2024; 15:230-235. [PMID: 38133821 DOI: 10.1021/acschemneuro.3c00712] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2023] Open
Abstract
Abnormal expansion of trinucleotide CGG repeats is responsible for Fragile X syndrome. AGG interruptions in CGG repeat tracts were found in most healthy individuals, suggesting a crucial role in preventing disease-prone repeat expansion. Previous biophysics studies emphasize a difference in the secondary structure affected by AGG interruptions. However, the mechanism of how AGG interruptions impede repeat expansion remains elusive. We utilized single-molecule fluorescence resonance energy transfer spectroscopy to investigate the structural dynamics of CGG repeats and their AGG-interrupted variants. Tandem CGG repeats fold into a stem-loop hairpin structure with the capability to undergo a conformational rearrangement to modulate the length of the overhang. However, this conformational rearrangement is much more retarded when two AGG interruptions are present. Considering the significance of hairpin slippage in repeat expansion, we present a molecular basis suggesting that the internal loop created by two AGG interruptions acts as a barrier, obstructing the hairpin slippage reconfiguration. This impediment potentially plays a crucial role in curbing abnormal expansion, thereby contributing to the genomic stability.
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Affiliation(s)
- Yang-I Shen
- Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
| | - Kai-Chun Cheng
- Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
| | - Yu-Jie Wei
- Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
| | - I-Ren Lee
- Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
- Genomics Research Center, Academia Sinica, Taipei 115, Taiwan
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Shieh M, Amkraut K, Spiridigliozzi GA, Adayev T, Nicholson K, McConkie‐Rosell A, McDonald M, Pennington M, Sebastian S, Lachiewicz AM. High performing male with fragile X syndrome with an unmethylated FMR1 full mutation: The relevance of clinical and genetic correlations. Clin Case Rep 2023; 11:e7371. [PMID: 37361657 PMCID: PMC10290186 DOI: 10.1002/ccr3.7371] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2023] [Revised: 04/28/2023] [Accepted: 05/08/2023] [Indexed: 06/28/2023] Open
Abstract
A high performing male with an unmethylated full mutation in the fragile X messenger ribonucleoprotein 1 (FMR1) gene surpassed our expectations into young adulthood. Although initial genetic findings helped make a correct fragile X syndrome (FXS) determination, the report was insufficient. Ten years later, we repeated and conducted additional genetic and clinical studies to determine whether more information could assist with treatment and counseling. The genetic findings were very consistent with his high functioning and would have enabled us to be more confident about a good developmental outcome had they been available previously. As FXS enters the mainstream of well-understood genetic disorders and technological advancements improve genetic tests, it should be clearer to clinical providers what a full FXS assessment could include to provide high quality information for care. For individuals with FXS who are high functioning, their families and clinical professionals would benefit from knowing more genetic findings, including, most importantly, methylation status, but also the FMR1 protein (FMRP) level and mRNA level. While we now know that obtaining only the CGG repeat number is not always adequate to inform accurate clinical care, future studies are likely to show the benefit of studying other biomarkers, such as mRNA levels.
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Affiliation(s)
- Meg Shieh
- Department of ChemistryBrown UniversityProvidenceRhode IslandUSA
- Department of PediatricsDuke University Health SystemDurhamNorth CarolinaUSA
| | - Keren Amkraut
- Department of PediatricsDuke University Health SystemDurhamNorth CarolinaUSA
| | - Gail A. Spiridigliozzi
- Department of PediatricsDuke University Health SystemDurhamNorth CarolinaUSA
- Department of Psychiatry and Behavioral SciencesDuke University Medical CenterDurhamNorth CarolinaUSA
| | - Tatyana Adayev
- Department of Human GeneticsNew York State Institute for Basic Research in Developmental DisabilitiesNew YorkNew YorkUSA
| | - Kaylea Nicholson
- Department of Communication SciencesDuke University Health CenterDurhamNorth CarolinaUSA
| | | | - Marie McDonald
- Department of PediatricsDuke University Health SystemDurhamNorth CarolinaUSA
| | | | - Siby Sebastian
- Department of PathologyDuke University Health SystemDurhamNorth CarolinaUSA
| | - Ave M. Lachiewicz
- Department of PediatricsDuke University Health SystemDurhamNorth CarolinaUSA
- Department of Psychiatry and Behavioral SciencesDuke University Medical CenterDurhamNorth CarolinaUSA
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Teng Y, Zhu M, Qiu Z. G-Quadruplexes in Repeat Expansion Disorders. Int J Mol Sci 2023; 24:ijms24032375. [PMID: 36768697 PMCID: PMC9916761 DOI: 10.3390/ijms24032375] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2022] [Revised: 01/19/2023] [Accepted: 01/23/2023] [Indexed: 01/27/2023] Open
Abstract
The repeat expansions are the main genetic cause of various neurodegeneration diseases. More than ten kinds of repeat sequences with different lengths, locations, and structures have been confirmed in the past two decades. G-rich repeat sequences, such as CGG and GGGGCC, are reported to form functional G-quadruplexes, participating in many important bioprocesses. In this review, we conducted an overview concerning the contribution of G-quadruplex in repeat expansion disorders and summarized related mechanisms in current pathological studies, including the increasing genetic instabilities in replication and transcription, the toxic RNA foci formed in neurons, and the loss/gain function of proteins and peptides. Furthermore, novel strategies targeting G-quadruplex repeats were developed based on the understanding of disease mechanism. Small molecules and proteins binding to G-quadruplex in repeat expansions were investigated to protect neurons from dysfunction and delay the progression of neurodegeneration. In addition, the effects of environment on the stability of G-quadruplex were discussed, which might be critical factors in the pathological study of repeat expansion disorders.
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Abstract
Roughly 3% of the human genome consists of microsatellites or tracts of short tandem repeats (STRs). These STRs are often unstable, undergoing high-frequency expansions (increases) or contractions (decreases) in the number of repeat units. Some microsatellite instability (MSI) is seen at multiple STRs within a single cell and is associated with certain types of cancer. A second form of MSI is characterised by expansion of a single gene-specific STR and such expansions are responsible for a group of 40+ human genetic disorders known as the repeat expansion diseases (REDs). While the mismatch repair (MMR) pathway prevents genome-wide MSI, emerging evidence suggests that some MMR factors are directly involved in generating expansions in the REDs. Thus, MMR suppresses some forms of expansion while some MMR factors promote expansion in other contexts. This review will cover what is known about the paradoxical effect of MMR on microsatellite expansion in mammalian cells.
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7
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Hwang YH, Hayward BE, Zafarullah M, Kumar J, Durbin Johnson B, Holmans P, Usdin K, Tassone F. Both cis and trans-acting genetic factors drive somatic instability in female carriers of the FMR1 premutation. Sci Rep 2022; 12:10419. [PMID: 35729184 PMCID: PMC9213438 DOI: 10.1038/s41598-022-14183-0] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2022] [Accepted: 05/18/2022] [Indexed: 11/30/2022] Open
Abstract
The fragile X mental retardation (FMR1) gene contains an expansion-prone CGG repeat within its 5' UTR. Alleles with 55-200 repeats are known as premutation (PM) alleles and confer risk for one or more of the FMR1 premutation (PM) disorders that include Fragile X-associated Tremor/Ataxia Syndrome (FXTAS), Fragile X-associated Primary Ovarian Insufficiency (FXPOI), and Fragile X-Associated Neuropsychiatric Disorders (FXAND). PM alleles expand on intergenerational transmission, with the children of PM mothers being at risk of inheriting alleles with > 200 CGG repeats (full mutation FM) alleles) and thus developing Fragile X Syndrome (FXS). PM alleles can be somatically unstable. This can lead to individuals being mosaic for multiple size alleles. Here, we describe a detailed evaluation of somatic mosaicism in a large cohort of female PM carriers and show that 94% display some evidence of somatic instability with the presence of a series of expanded alleles that differ from the next allele by a single repeat unit. Using two different metrics for instability that we have developed, we show that, as with intergenerational instability, there is a direct relationship between the extent of somatic expansion and the number of CGG repeats in the originally inherited allele and an inverse relationship with the number of AGG interruptions. Expansions are progressive as evidenced by a positive correlation with age and by examination of blood samples from the same individual taken at different time points. Our data also suggests the existence of other genetic or environmental factors that affect the extent of somatic expansion. Importantly, the analysis of candidate single nucleotide polymorphisms (SNPs) suggests that two DNA repair factors, FAN1 and MSH3, may be modifiers of somatic expansion risk in the PM population as observed in other repeat expansion disorders.
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Affiliation(s)
- Ye Hyun Hwang
- Department of Biochemistry and Molecular Medicine, University of California Davis, School of Medicine, Sacramento, CA, 95817, USA
| | - Bruce Eliot Hayward
- Laboratory of Molecular and Cellular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, USA
| | - Marwa Zafarullah
- Department of Biochemistry and Molecular Medicine, University of California Davis, School of Medicine, Sacramento, CA, 95817, USA
| | - Jay Kumar
- Department of Biochemistry and Molecular Medicine, University of California Davis, School of Medicine, Sacramento, CA, 95817, USA
| | - Blythe Durbin Johnson
- Department of Public Health Sciences, University of California, Davis, School of Medicine, Sacramento, CA, 95817, USA
| | - Peter Holmans
- Medical Research Council Centre for Neuropsychiatric Genetics and Genomics, Division of Psychological Medicine and Clinical Neurology, School of Medicine, Cardiff University, Cardiff, UK
| | - Karen Usdin
- Laboratory of Molecular and Cellular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, USA.
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, University of California Davis, School of Medicine, Sacramento, CA, 95817, USA. .,MIND Institute, University of California Davis Medical Center, Sacramento, CA, 95817, USA.
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Rajan-Babu IS, Lian M, Chong SS. Triplet-Primed PCR Assays for Accurate Screening of FMR1 CGG Repeat Expansion and Genotype Verification. Curr Protoc 2022; 2:e427. [PMID: 35609145 DOI: 10.1002/cpz1.427] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Fragile X syndrome and other fragile X-associated disorders are caused by the full-mutation (>200 copies) and premutation (55 to 200 copies) expansion, respectively, of the CGG short tandem repeat in the fragile X messenger ribonucleoprotein 1 (FMR1) gene. Clinical diagnostic laboratories use Southern blot analysis and polymerase chain reaction (PCR)-based tests to detect and/or size the FMR1 CGG repeats. The development of sensitive and high-throughput triplet-primed PCR (TP-PCR) assays has diminished the need to subject all samples to Southern blot analysis, which is both labor- and time-intensive. In this article, we describe two direct TP-PCR (dTP-PCR) assays for the detection of FMR1 CGG repeat expansions. We outline a protocol that is based on melting curve analysis of dTP-PCR amplicons for a rapid and cost-effective first-tier screening and identification of individuals with premutation and full-mutation expansions. We also describe a protocol that employs capillary electrophoresis to resolve the dTP-PCR amplicon fragments and to estimate the repeat sizes of normal (5 to 44 copies), intermediate (45 to 54 copies), and premutation alleles, as well as to detect full mutations and determine the structure of the FMR1 alleles. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Direct triplet-primed PCR master mix preparation and amplification of the FMR1 CGG repeat locus for melting curve analysis Basic Protocol 2: Melting curve analysis of direct triplet-primed PCR amplicons on the Rotor-Gene Q MD × 5plex high-resolution melt platform Alternate Protocol: Melting curve analysis of direct triplet-primed PCR amplicons on the LightCycler 480 system Basic Protocol 3: Generation of direct triplet-primed PCR melting curve analysis profiles Basic Protocol 4: Direct triplet-primed PCR master mix preparation and amplification of the FMR1 CGG repeat locus for capillary electrophoresis Basic Protocol 5: Generation of control FMR1 plasmids for direct triplet-primed PCR melting curve analysis Basic Protocol 6: Sanger sequencing assay to verify FMR1 CGG repeat size and structure of plasmid DNA controls.
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Affiliation(s)
- Indhu-Shree Rajan-Babu
- Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore.,Department of Medical Genetics, University of British Columbia, Vancouver, British Columbia, Canada.,Children's & Women's Hospital, Vancouver, British Columbia, Canada
| | - Mulias Lian
- Khoo Teck Puat-National University Children's Medical Institute, National University Hospital, Singapore, Singapore
| | - Samuel S Chong
- Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore.,Department of Obstetrics and Gynecology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore.,Department of Laboratory Medicine, National University Hospital, Singapore, Singapore
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9
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Rajan-Babu IS, Phang GP, Law HY, Lee CG, Chong SS. High-Throughput Methylation-Specific Triplet-Primed PCR and Melting Curve Analysis for Selective and Reliable Identification of Actionable FMR1 Genotypes. J Mol Diagn 2022; 24:241-252. [PMID: 35038595 DOI: 10.1016/j.jmoldx.2021.11.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2021] [Revised: 10/26/2021] [Accepted: 11/23/2021] [Indexed: 11/17/2022] Open
Abstract
Methylated FMR1 full-mutation expansions cause fragile X syndrome. FMR1 premutation carriers are susceptible to other late-onset conditions, and women with premutation are at risk of transmitting a fully expanded FMR1 allele to offspring. Identification of individuals with actionable FMR1 genotypes (full-mutation males and females, and premutation females at risk for primary ovarian insufficiency and/or having fragile X-affected offspring) can enable timely access to intervention services and genetic counseling. This study presents a rapid, first-tier test based on melting curve analysis of methylation-specific triplet-primed PCR amplicons (msTP-PCR MCA) for concurrent detection of FMR1 CGG-repeat expansions and their methylation status. The msTP-PCR MCA assay was optimized on 20 fragile X reference samples, and its performance was evaluated on 111 peripheral blood-derived DNA samples from patients who have undergone prior molecular testing with PCR and/or Southern blot analysis. The msTP-PCR MCA assay detected all samples with a methylated FMR1 CGG-repeat expansion, and had sensitivity, specificity, positive predictive value, and negative predictive values of 100%, 92.06%, 91.1%, and 100%, respectively. The msTP-PCR MCA assay identified premutation/full-mutation mosaicism down to 1%, detected skewed inactivation in females with FMR1 expansions, and enabled selective identification of all individuals with an actionable FMR1 genotype. The msTP-PCR MCA assay may aid in fragile X screening of at-risk populations and newborns and voluntary carrier screening of women of reproductive age.
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Affiliation(s)
- Indhu-Shree Rajan-Babu
- Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore; Department of Medical Genetics, University of British Columbia, and Children's and Women's Hospital, Vancouver, British Columbia, Canada.
| | - Gui-Ping Phang
- Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
| | - Hai-Yang Law
- Department of Pediatric Medicine, KK Women's and Children's Hospital, Singapore
| | - Caroline G Lee
- Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore; Division of Cellular and Molecular Research, Humphrey Oei Institute of Cancer Research, National Cancer Centre Singapore, Singapore; Cancer and Stem Cell Biology Program, Duke-NUS Medical School, Singapore
| | - Samuel S Chong
- Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore; Department of Obstetrics and Gynecology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore; Department of Laboratory Medicine, National University Hospital, Singapore.
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Gall-Duncan T, Sato N, Yuen RKC, Pearson CE. Advancing genomic technologies and clinical awareness accelerates discovery of disease-associated tandem repeat sequences. Genome Res 2022; 32:1-27. [PMID: 34965938 PMCID: PMC8744678 DOI: 10.1101/gr.269530.120] [Citation(s) in RCA: 26] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2020] [Accepted: 11/29/2021] [Indexed: 11/25/2022]
Abstract
Expansions of gene-specific DNA tandem repeats (TRs), first described in 1991 as a disease-causing mutation in humans, are now known to cause >60 phenotypes, not just disease, and not only in humans. TRs are a common form of genetic variation with biological consequences, observed, so far, in humans, dogs, plants, oysters, and yeast. Repeat diseases show atypical clinical features, genetic anticipation, and multiple and partially penetrant phenotypes among family members. Discovery of disease-causing repeat expansion loci accelerated through technological advances in DNA sequencing and computational analyses. Between 2019 and 2021, 17 new disease-causing TR expansions were reported, totaling 63 TR loci (>69 diseases), with a likelihood of more discoveries, and in more organisms. Recent and historical lessons reveal that properly assessed clinical presentations, coupled with genetic and biological awareness, can guide discovery of disease-causing unstable TRs. We highlight critical but underrecognized aspects of TR mutations. Repeat motifs may not be present in current reference genomes but will be in forthcoming gapless long-read references. Repeat motif size can be a single nucleotide to kilobases/unit. At a given locus, repeat motif sequence purity can vary with consequence. Pathogenic repeats can be "insertions" within nonpathogenic TRs. Expansions, contractions, and somatic length variations of TRs can have clinical/biological consequences. TR instabilities occur in humans and other organisms. TRs can be epigenetically modified and/or chromosomal fragile sites. We discuss the expanding field of disease-associated TR instabilities, highlighting prospects, clinical and genetic clues, tools, and challenges for further discoveries of disease-causing TR instabilities and understanding their biological and pathological impacts-a vista that is about to expand.
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Affiliation(s)
- Terence Gall-Duncan
- Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Nozomu Sato
- Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada
| | - Ryan K C Yuen
- Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Christopher E Pearson
- Program of Genetics and Genome Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1L7, Canada
- Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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Hayward BE, Usdin K. Mechanisms of Genome Instability in the Fragile X-Related Disorders. Genes (Basel) 2021; 12:genes12101633. [PMID: 34681027 PMCID: PMC8536109 DOI: 10.3390/genes12101633] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2021] [Revised: 10/12/2021] [Accepted: 10/14/2021] [Indexed: 12/17/2022] Open
Abstract
The Fragile X-related disorders (FXDs), which include the intellectual disability fragile X syndrome (FXS), are disorders caused by expansion of a CGG-repeat tract in the 5′ UTR of the X-linked FMR1 gene. These disorders are named for FRAXA, the folate-sensitive fragile site that localizes with the CGG-repeat in individuals with FXS. Two pathological FMR1 allele size classes are distinguished. Premutation (PM) alleles have 54–200 repeats and confer the risk of fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X-associated primary ovarian insufficiency (FXPOI). PM alleles are prone to both somatic and germline expansion, with female PM carriers being at risk of having a child with >200+ repeats. Inheritance of such full mutation (FM) alleles causes FXS. Contractions of PM and FM alleles can also occur. As a result, many carriers are mosaic for different sized alleles, with the clinical presentation depending on the proportions of these alleles in affected tissues. Furthermore, it has become apparent that the chromosomal fragility of FXS individuals reflects an underlying problem that can lead to chromosomal numerical and structural abnormalities. Thus, large numbers of CGG-repeats in the FMR1 gene predisposes individuals to multiple forms of genome instability. This review will discuss our current understanding of these processes.
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Laboratory testing for fragile X, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG). Genet Med 2021; 23:799-812. [PMID: 33795824 DOI: 10.1038/s41436-021-01115-y] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2021] [Revised: 02/01/2021] [Accepted: 02/02/2021] [Indexed: 11/08/2022] Open
Abstract
Molecular genetic testing of the FMR1 gene is commonly performed in clinical laboratories. Pathogenic variants in the FMR1 gene are associated with fragile X syndrome, fragile X-associated tremor ataxia syndrome (FXTAS), and fragile X-associated primary ovarian insufficiency (FXPOI). This document provides updated information regarding FMR1 pathogenic variants, including prevalence, genotype-phenotype correlations, and variant nomenclature. Methodological considerations are provided for Southern blot analysis and polymerase chain reaction (PCR) amplification of FMR1, including triplet repeat-primed and methylation-specific PCR.The American College of Medical Genetics and Genomics (ACMG) Laboratory Quality Assurance Committee has the mission of maintaining high technical standards for the performance and interpretation of genetic tests. In part, this is accomplished by the publication of the document ACMG Technical Standards for Clinical Genetics Laboratories, which is now maintained online ( http://www.acmg.net ). This subcommittee also reviews the outcome of national proficiency testing in the genetics area and may choose to focus on specific diseases or methodologies in response to those results. Accordingly, the subcommittee selected fragile X syndrome to be the first topic in a series of supplemental sections, recognizing that it is one of the most frequently ordered genetic tests and that it has many alternative methods with different strengths and weaknesses. This document is the fourth update to the original standards and guidelines for fragile X testing that were published in 2001, with revisions in 2005 and 2013, respectively.This versionClarifies the clinical features associated with different FMRI variants (Section 2.3)Discusses important reporting considerations (Section 3.3.1.3)Provides updates on technology (Section 4.1).
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Berry-Kravis E, Zhou L, Jackson J, Tassone F. Diagnostic profile of the AmplideX Fragile X Dx and Carrier Screen Kit for diagnosis and screening of fragile X syndrome and other FMR1-related disorders. Expert Rev Mol Diagn 2021; 21:255-267. [PMID: 33666525 DOI: 10.1080/14737159.2021.1899812] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
Introduction: In 2009, a novel, CGG repeat primed FMR1 PCR assay was designed with primers flanking the triplet repeat region, as well as a third chimeric primer complementary to the (CGG)n repeat, that was capable of amplifying alleles throughout the repeat range. This assay for the first time allowed consistent detection of large full mutation alleles with PCR, resolution of heterozygosity in females and mapping of AGG interspersions.Areas Covered: The AmplideX Fragile X Dx and Carrier Screen Kit (Asuragen, Inc.) represents a refined assay that underwent validation with sensitivity analyses for FDA approval. Single-site precision, analytical sensitivity and specificity, limit of detection and diagnostic performance were assessed in comparison to reference methods at three independent sites. Single-site precision across all genotype categories showed 100% agreement at 20 ng input across multiple operators, days, instruments and kit lots. Compared to Southern Blot analysis, the overall percent agreement was over 98% for all expanded alleles.Expert Opinion: Limitations include no methylation assessment and hard to see full mutation peaks in some mosaic samples, but overall the assay is considered a highly accurate and time-efficient assay for FMR1 allele size determination.
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Affiliation(s)
- Elizabeth Berry-Kravis
- Department of Pediatrics, Rush University Medical Center, Chicago, IL, USA.,Department of Neurological Sciences, Rush University Medical Center, Chicago, IL, USA.,Department of Biochemistry, Rush University Medical Center, Chicago, IL, USA
| | - Lili Zhou
- Department of Pediatrics, Rush University Medical Center, Chicago, IL, USA.,Department of Pathology, Rush University Medical Center, Chicago, IL, USA
| | - Jonathan Jackson
- Department of Pediatrics, Rush University Medical Center, Chicago, IL, USA
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, University of California Davis, School of Medicine, Sacramento, CA, USA.,MIND Institute, University of California Davis Medical Center, Sacramento, CA, USA
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14
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Podder A, Lee HJ, Kim BH. Fluorescent Nucleic Acid Systems for Biosensors. BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 2021. [DOI: 10.1246/bcsj.20200351] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Affiliation(s)
- Arup Podder
- Department of Chemistry, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
| | - Ha Jung Lee
- Department of Chemistry, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
| | - Byeang Hyean Kim
- Department of Chemistry, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
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15
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Mangin A, de Pontual L, Tsai YC, Monteil L, Nizon M, Boisseau P, Mercier S, Ziegle J, Harting J, Heiner C, Gourdon G, Tomé S. Robust Detection of Somatic Mosaicism and Repeat Interruptions by Long-Read Targeted Sequencing in Myotonic Dystrophy Type 1. Int J Mol Sci 2021; 22:2616. [PMID: 33807660 PMCID: PMC7962047 DOI: 10.3390/ijms22052616] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2021] [Revised: 02/26/2021] [Accepted: 02/27/2021] [Indexed: 02/07/2023] Open
Abstract
Myotonic dystrophy type 1 (DM1) is the most complex and variable trinucleotide repeat disorder caused by an unstable CTG repeat expansion, reaching up to 4000 CTG in the most severe cases. The genetic and clinical variability of DM1 depend on the sex and age of the transmitting parent, but also on the CTG repeat number, presence of repeat interruptions and/or on the degree of somatic instability. Currently, it is difficult to simultaneously and accurately determine these contributing factors in DM1 patients due to the limitations of gold standard methods used in molecular diagnostics and research laboratories. Our study showed the efficiency of the latest PacBio long-read sequencing technology to sequence large CTG trinucleotides, detect multiple and single repeat interruptions and estimate the levels of somatic mosaicism in DM1 patients carrying complex CTG repeat expansions inaccessible to most methods. Using this innovative approach, we revealed the existence of de novo CCG interruptions associated with CTG stabilization/contraction across generations in a new DM1 family. We also demonstrated that our method is suitable to sequence the DM1 locus and measure somatic mosaicism in DM1 families carrying more than 1000 pure CTG repeats. Better characterization of expanded alleles in DM1 patients can significantly improve prognosis and genetic counseling, not only in DM1 but also for other tandem DNA repeat disorders.
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Affiliation(s)
- Antoine Mangin
- Centre de Recherche en Myologie, Inserm, Institut de Myologie, Sorbonne Université, F-75013 Paris, France; (A.M.); (L.d.P.); (G.G.)
- Dementia Research Institute, Cardiff University, Cardiff CF10 3AT, UK
| | - Laure de Pontual
- Centre de Recherche en Myologie, Inserm, Institut de Myologie, Sorbonne Université, F-75013 Paris, France; (A.M.); (L.d.P.); (G.G.)
| | - Yu-Chih Tsai
- Pacific Biosciences, Menlo Park, CA 94025, USA; (Y.-C.T.); (J.Z.); (J.H.); (C.H.)
| | - Laetitia Monteil
- Genetics Department of the Hospital of Toulouse, F-31059 Toulouse, France;
| | - Mathilde Nizon
- CHU de Nantes, Service de Génétique Médicale, Laboratoire de Génétique Moléculaire, F-44000 Nantes, France; (M.N.); (P.B.)
| | - Pierre Boisseau
- CHU de Nantes, Service de Génétique Médicale, Laboratoire de Génétique Moléculaire, F-44000 Nantes, France; (M.N.); (P.B.)
| | - Sandra Mercier
- CHU Nantes, Service de Génétique Médicale, Centre de Référence des Maladies Neuromusculaires AOC, F-44000 Nantes, France;
| | - Janet Ziegle
- Pacific Biosciences, Menlo Park, CA 94025, USA; (Y.-C.T.); (J.Z.); (J.H.); (C.H.)
| | - John Harting
- Pacific Biosciences, Menlo Park, CA 94025, USA; (Y.-C.T.); (J.Z.); (J.H.); (C.H.)
| | - Cheryl Heiner
- Pacific Biosciences, Menlo Park, CA 94025, USA; (Y.-C.T.); (J.Z.); (J.H.); (C.H.)
| | - Geneviève Gourdon
- Centre de Recherche en Myologie, Inserm, Institut de Myologie, Sorbonne Université, F-75013 Paris, France; (A.M.); (L.d.P.); (G.G.)
| | - Stéphanie Tomé
- Centre de Recherche en Myologie, Inserm, Institut de Myologie, Sorbonne Université, F-75013 Paris, France; (A.M.); (L.d.P.); (G.G.)
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16
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Abstract
Neurodevelopmental disorders are the most prevalent chronic medical conditions encountered in pediatric primary care. In addition to identifying appropriate descriptive diagnoses and guiding families to evidence-based treatments and supports, comprehensive care for individuals with neurodevelopmental disorders includes a search for an underlying etiologic diagnosis, primarily through a genetic evaluation. Identification of an underlying genetic etiology can inform prognosis, clarify recurrence risk, shape clinical management, and direct patients and families to condition-specific resources and supports. Here we review the utility of genetic testing in patients with neurodevelopmental disorders and describe the three major testing modalities and their yields - chromosomal microarray, exome sequencing (with/without copy number variant calling), and FMR1 CGG repeat analysis for fragile X syndrome. Given the diagnostic yield of genetic testing and the potential for clinical and personal utility, there is consensus that genetic testing should be offered to all patients with global developmental delay, intellectual disability, and/or autism spectrum disorder. Despite this recommendation, data suggest that a minority of children with autism spectrum disorder and intellectual disability have undergone genetic testing. To address this gap in care, we describe a structured but flexible approach to facilitate integration of genetic testing into clinical practice across pediatric specialties and discuss future considerations for genetic testing in neurodevelopmental disorders to prepare pediatric providers to care for patients with such diagnoses today and tomorrow.
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Affiliation(s)
- Juliann M. Savatt
- Autism & Developmental Medicine Institute, Geisinger, Danville, PA, United States
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17
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Sihombing NRB, Winarni TI, Utari A, van Bokhoven H, Hagerman RJ, Faradz SMH. Surveillance and prevalence of fragile X syndrome in Indonesia. Intractable Rare Dis Res 2021; 10:11-16. [PMID: 33614370 PMCID: PMC7882090 DOI: 10.5582/irdr.2020.03101] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
Fragile X syndrome (FXS) is the most prevalent inherited cause of intellectual disability (ID) and autism spectrum disorder (ASD). Many studies have been conducted over the years, however, in Indonesia there is relatively less knowledge on the prevalence of FXS. We reviewed all studies involving FXS screening and cascade testing of the high-risk population in Indonesia for two decades, to elucidate the prevalence, as well as explore the presence of genetic clusters of FXS in Indonesia. The prevalence of FXS in the ID population of Indonesia ranged between 0.9-1.9%, while in the ASD population, the percentage was higher (6.15%). A screening and cascade testing conducted in a small village on Java Island showed a high prevalence of 45% in the ID population, suggesting a genetic cluster. The common ancestry of all affected individuals was suggestive of a founder effect in the region. Routine screening and subsequent cascade testing are essential, especially in cases of ID and ASD of unknown etiology in Indonesia.
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Affiliation(s)
- Nydia Rena Benita Sihombing
- Division of Human Genetics, Center for Biomedical Research (CEBIOR), Faculty of Medicine, Diponegoro University/Diponegoro National Hospital, Semarang, Indonesia
| | - Tri Indah Winarni
- Division of Human Genetics, Center for Biomedical Research (CEBIOR), Faculty of Medicine, Diponegoro University/Diponegoro National Hospital, Semarang, Indonesia
| | - Agustini Utari
- Division of Human Genetics, Center for Biomedical Research (CEBIOR), Faculty of Medicine, Diponegoro University/Diponegoro National Hospital, Semarang, Indonesia
- Department of Pediatrics, Faculty of Medicine, Diponegoro University, Semarang, Indonesia
| | - Hans van Bokhoven
- Department of Human Genetics, Donders Institute for Brain, Cognition, and Behaviour, Radboud University Medical Center, Nijmegen, The Netherlands
| | - Randi J Hagerman
- MIND Institute, UC Davis Health, University of California, Davis, California, USA
| | - Sultana MH Faradz
- Division of Human Genetics, Center for Biomedical Research (CEBIOR), Faculty of Medicine, Diponegoro University/Diponegoro National Hospital, Semarang, Indonesia
- Address correspondence to:Sultana MH Faradz, Center for Biomedical Research (CEBIOR), Faculty of Medicine, Diponegoro University, Jl. Prof. Soedarto SH no. 1, Semarang (50275), Central-Java, Indonesia. E-mail:
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18
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Rodrigues B, Vale-Fernandes E, Maia N, Santos F, Marques I, Santos R, Nogueira AJA, Jorge P. Development and Validation of a Mathematical Model to Predict the Complexity of FMR1 Allele Combinations. Front Genet 2020; 11:557147. [PMID: 33281866 PMCID: PMC7691586 DOI: 10.3389/fgene.2020.557147] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2020] [Accepted: 10/13/2020] [Indexed: 12/04/2022] Open
Abstract
The polymorphic trinucleotide repetitive region in the FMR1 gene 5'UTR contains AGG interspersions, particularly in normal-sized alleles (CGG < 45). In this range repetitive stretches are typically interrupted once or twice, although alleles without or with three or more AGG interspersions can also be observed. AGG interspersions together with the total length of the repetitive region confer stability and hinder expansion to pathogenic ranges: either premutation (55 < CGG < 200) or full mutation (CGG > 200). The AGG interspersions have long been identified as one of the most important features of FMR1 repeat stability, being particularly important to determine expansion risk estimates in female premutation carriers. We sought to compute the combined AGG interspersion numbers and patterns, aiming to define FMR1 repetitive tract complexity combinations. A mathematical model, the first to compute this cumulative effect, was developed and validated using data from 131 young and healthy females. Plotting of their allelic complexity enabled the identification of two statistically distinct groups - equivalent and dissimilar allelic combinations. The outcome, a numerical parameter designated allelic score, depicts the repeat substructure of each allele, measuring the allelic complexity of the FMR1 gene including the AGGs burden, thus allowing new behavioral scrutiny of normal-sized alleles in females.
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Affiliation(s)
- Bárbara Rodrigues
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
| | - Emídio Vale-Fernandes
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
- Centre for Medically Assisted Procreation/Public Gamete Bank, Centro Materno-Infantil do Norte Dr. Albino Aroso (CMIN), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
| | - Nuno Maia
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
| | - Flávia Santos
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
| | - Isabel Marques
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
| | - Rosário Santos
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
| | - António J. A. Nogueira
- Center for Environmental and Marine Studies (CESAM), Department of Biology, University of Aveiro, Aveiro, Portugal
| | - Paula Jorge
- Molecular Genetics Unit, Centro de Genética Médica Dr. Jacinto Magalhães (CGMJM), Centro Hospitalar Universitário do Porto (CHUP), Porto, Portugal
- Unit for Multidisciplinary Research in Biomedicine (UMIB), Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Porto, Portugal
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19
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Budimirovic DB, Schlageter A, Filipovic-Sadic S, Protic DD, Bram E, Mahone EM, Nicholson K, Culp K, Javanmardi K, Kemppainen J, Hadd A, Sharp K, Adayev T, LaFauci G, Dobkin C, Zhou L, Brown WT, Berry-Kravis E, Kaufmann WE, Latham GJ. A Genotype-Phenotype Study of High-Resolution FMR1 Nucleic Acid and Protein Analyses in Fragile X Patients with Neurobehavioral Assessments. Brain Sci 2020; 10:E694. [PMID: 33008014 PMCID: PMC7601415 DOI: 10.3390/brainsci10100694] [Citation(s) in RCA: 47] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2020] [Revised: 09/25/2020] [Accepted: 09/25/2020] [Indexed: 01/04/2023] Open
Abstract
Fragile X syndrome (FXS) is caused by silencing of the FMR1 gene, which encodes a protein with a critical role in synaptic plasticity. The molecular abnormality underlying FMR1 silencing, CGG repeat expansion, is well characterized; however, delineation of the pathway from DNA to RNA to protein using biosamples from well characterized patients with FXS is limited. Since FXS is a common and prototypical genetic disorder associated with intellectual disability (ID) and autism spectrum disorder (ASD), a comprehensive assessment of the FMR1 DNA-RNA-protein pathway and its correlations with the neurobehavioral phenotype is a priority. We applied nine sensitive and quantitative assays evaluating FMR1 DNA, RNA, and FMRP parameters to a reference set of cell lines representing the range of FMR1 expansions. We then used the most informative of these assays on blood and buccal specimens from cohorts of patients with different FMR1 expansions, with emphasis on those with FXS (N = 42 total, N = 31 with FMRP measurements). The group with FMRP data was also evaluated comprehensively in terms of its neurobehavioral profile, which allowed molecular-neurobehavioral correlations. FMR1 CGG repeat expansions, methylation levels, and FMRP levels, in both cell lines and blood samples, were consistent with findings of previous FMR1 genomic and protein studies. They also demonstrated a high level of agreement between blood and buccal specimens. These assays further corroborated previous reports of the relatively high prevalence of methylation mosaicism (slightly over 50% of the samples). Molecular-neurobehavioral correlations confirmed the inverse relationship between overall severity of the FXS phenotype and decrease in FMRP levels (N = 26 males, mean 4.2 ± 3.3 pg FMRP/ng genomic DNA). Other intriguing findings included a significant relationship between the diagnosis of FXS with ASD and two-fold lower levels of FMRP (mean 2.8 ± 1.3 pg FMRP/ng genomic DNA, p = 0.04), in particular observed in younger age- and IQ-adjusted males (mean age 6.9 ± 0.9 years with mean 3.2 ± 1.2 pg FMRP/ng genomic DNA, 57% with severe ASD), compared to FXS without ASD. Those with severe ID had even lower FMRP levels independent of ASD status in the male-only subset. The results underscore the link between FMR1 expansion, gene methylation, and FMRP deficit. The association between FMRP deficiency and overall severity of the neurobehavioral phenotype invites follow up studies in larger patient cohorts. They would be valuable to confirm and potentially extend our initial findings of the relationship between ASD and other neurobehavioral features and the magnitude of FMRP deficit. Molecular profiling of individuals with FXS may have important implications in research and clinical practice.
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Affiliation(s)
- Dejan B. Budimirovic
- Departments of Psychiatry and Neurogenetics, Fragile X Clinic, Kennedy Krieger Institute, Baltimore, MD 21205, USA;
- Department of Psychiatry & Behavioral Sciences-Child Psychiatry, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA
| | - Annette Schlageter
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Stela Filipovic-Sadic
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Dragana D. Protic
- Departments of Psychiatry and Neurogenetics, Fragile X Clinic, Kennedy Krieger Institute, Baltimore, MD 21205, USA;
- School of Medicine, University of Belgrade, 11000 Belgrade, Serbia
| | - Eran Bram
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - E. Mark Mahone
- Department of Neuropsychology, Kennedy Krieger Institute, Baltimore, MD 21205, USA;
| | - Kimberly Nicholson
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Kristen Culp
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Kamyab Javanmardi
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Jon Kemppainen
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Andrew Hadd
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
| | - Kevin Sharp
- Department of Pediatrics, Rush University Medical Center, Chicago, IL 60612, USA; (K.S.); (L.Z.); (E.B.-K.)
| | - Tatyana Adayev
- Institute for Basic Research in Developmental Disabilities, Staten Island, NY 10314, USA; (T.A.); (G.L.); (C.D.); (W.T.B.)
| | - Giuseppe LaFauci
- Institute for Basic Research in Developmental Disabilities, Staten Island, NY 10314, USA; (T.A.); (G.L.); (C.D.); (W.T.B.)
| | - Carl Dobkin
- Institute for Basic Research in Developmental Disabilities, Staten Island, NY 10314, USA; (T.A.); (G.L.); (C.D.); (W.T.B.)
| | - Lili Zhou
- Department of Pediatrics, Rush University Medical Center, Chicago, IL 60612, USA; (K.S.); (L.Z.); (E.B.-K.)
| | - William Ted Brown
- Institute for Basic Research in Developmental Disabilities, Staten Island, NY 10314, USA; (T.A.); (G.L.); (C.D.); (W.T.B.)
| | - Elizabeth Berry-Kravis
- Department of Pediatrics, Rush University Medical Center, Chicago, IL 60612, USA; (K.S.); (L.Z.); (E.B.-K.)
- Departments of Pediatrics, Neurological Sciences and Biochemistry, Rush University Medical Center, Chicago, IL 60612, USA
| | - Walter E. Kaufmann
- Department of Human Genetics, Emory University School of Medicine, Atlanta, GA 30322, USA;
- Boston Children’s Hospital, Harvard Medical School, Boston, MA 02115, USA
| | - Gary J. Latham
- Asuragen, Inc., Austin, TX 78744, USA; (A.S.); (S.F.-S.); (E.B.); (K.N.); (K.C.); (K.J.); (J.K.); (A.H.)
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Johansen Taber K, Lim-Harashima J, Naemi H, Goldberg J. Fragile X syndrome carrier screening accompanied by genetic consultation has clinical utility in populations beyond those recommended by guidelines. Mol Genet Genomic Med 2019; 7:e1024. [PMID: 31694075 PMCID: PMC6900367 DOI: 10.1002/mgg3.1024] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2019] [Revised: 08/30/2019] [Accepted: 10/01/2019] [Indexed: 01/12/2023] Open
Abstract
Background Fragile X syndrome (FXS) is the most common inherited form of intellectual disability. Many providers offer preconception or prenatal FXS carrier screening. However, guidelines recommend screening only for those with a family history or undergoing fertility evaluation. Wider screening has been resisted because of concerns about patient understanding of FXS‐associated inheritance patterns and phenotypes. Additionally, the clinical utility has been questioned. Methods We addressed these concerns by analyzing reproductive decision‐making and pregnancy management informed by post‐test genetic consultation among 122 FMR1 premutation carriers identified by expanded carrier screening. Results Sixty‐three percent of those screened met guidelines screening criteria; the remaining 37% did not. Ninety‐eight percent had undergone post‐test genetic consultation. Of respondents screened preconceptionally, 74% reported planning or pursuing actions to reduce the risk of an affected pregnancy; the extent to which couples planned/pursued these actions was not significantly different between those meeting either screening criterion (76%) versus those meeting neither criterion (55%). Of respondents screened prenatally, 41% pursued prenatal diagnostic testing; the extent to which couples pursued prenatal diagnosis was not significantly different between those who met either screening criterion (37%) versus those who met neither criterion (31%). Conclusion These results support the expansion of FXS screening criteria in guidelines.
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Affiliation(s)
| | | | | | - Jim Goldberg
- Myriad Women's Health, South San Francisco, CA, USA
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21
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Manor E, Gonen R, Sarussi B, Keidar-Friedman D, Kumar J, Tang HT, Tassone F. The role of AGG interruptions in the FMR1 gene stability: A survey in ethnic groups with low and high rate of consanguinity. Mol Genet Genomic Med 2019; 7:e00946. [PMID: 31453660 PMCID: PMC6785435 DOI: 10.1002/mgg3.946] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2019] [Revised: 07/21/2019] [Accepted: 07/26/2019] [Indexed: 11/07/2022] Open
Abstract
Background The prevalence and the role of AGG interruptions within the FMR1 gene in the normal population is unknown. In this study, we investigated the frequent of AGG loss, in one or two alleles within the normal population. The role of AGG in the FMR1 stability has been assessed by correlating AGG loss to the prevalence of premutation/full mutation in two ethnic groups differing in their consanguinity rate: high versus low consanguinity rate (HCR vs. LCR). Methods The CGG repeat allele size and AGG presence were measured in 6,865 and 6,204 females belonging to the LCR (5%) and HCR (>45%) groups, respectively, by Tripled‐Primed‐PCR technique. Results A lower prevalence of the premutation was observed in the HCR (1:158) as compared to the LCR group (1:128). No full mutation was found in the HCR females while in the LCR group the prevalence found was 1:1,149. Homozygosity rate was higher in the HCR population compared to the LCR group.The overall AGG loss was higher in the HCR population than in the LCR and increased with increased CGG repeat number in both ethnic groups. Conclusions Although we observed a significantly higher rate of homozygosity and AGG loss in the HCR group, this did not affect the prevalence of the premutation and full mutation in this population. Their prevalence was significantly lower than in the LCR population. Finally, we discuss whether the loss of AGG could be also a polymorphic event but not only a stabilizing factor.
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Affiliation(s)
- Esther Manor
- Faculty of Health Science, Ben-Gurion University of the Negev Genetic Institute, Soroka University Medical Center, Beer Sheva, Israel
| | | | | | | | - Jay Kumar
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California Davis, Sacramento, USA
| | - Hiu-Tung Tang
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California Davis, Sacramento, USA
| | - Flora Tassone
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California Davis, Sacramento, USA.,MIND Institute, Medical Center, University of California Davis, Sacramento, USA
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22
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Hayward B, Loutaev I, Ding X, Nolin SL, Thurm A, Usdin K, Smith CB. Fragile X syndrome in a male with methylated premutation alleles and no detectable methylated full mutation alleles. Am J Med Genet A 2019; 179:2132-2137. [DOI: 10.1002/ajmg.a.61286] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2019] [Revised: 06/14/2019] [Accepted: 06/23/2019] [Indexed: 12/17/2022]
Affiliation(s)
- Bruce Hayward
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular BiologyNational Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health Bethesda Maryland
| | - Inna Loutaev
- Section on Neuroadaptation and Protein MetabolismNational Institute of Mental Health, National Institutes of Health Bethesda Maryland
| | - Xiaohua Ding
- Molecular Diagnostic LaboratoryNew York State Institute for Basic Research in Developmental Disabilities Staten Island New York
| | - Sarah L. Nolin
- Molecular Diagnostic LaboratoryNew York State Institute for Basic Research in Developmental Disabilities Staten Island New York
| | - Audrey Thurm
- Office of the Clinical DirectorNational Institute of Mental Health, National Institutes of Health Bethesda Maryland
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular BiologyNational Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health Bethesda Maryland
| | - Carolyn B. Smith
- Section on Neuroadaptation and Protein MetabolismNational Institute of Mental Health, National Institutes of Health Bethesda Maryland
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Hayward BE, Usdin K. Assays for Determining Repeat Number, Methylation Status, and AGG Interruptions in the Fragile X-Related Disorders. Methods Mol Biol 2019; 1942:49-59. [PMID: 30900174 DOI: 10.1007/978-1-4939-9080-1_4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Knowledge of the CGG•CCG-repeat number, AGG interruption status, and the extent of DNA methylation of the FMR1 gene are vital for both diagnosis of the fragile X-related disorders and for basic research into disease mechanisms. We describe here assays that we use in our laboratory to assess these parameters. Our assays are PCR-based and include one for repeat size that can also be used to assess the extent of methylation and a related assay that allows the AGG interruption pattern to be reliably determined even in women. A second more quantitative methylation assay is also described. We also describe our method for cloning of repeats to generate the reference standards necessary for the accurate determination of repeat number and AGG interruption status.
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Affiliation(s)
- Bruce E Hayward
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, USA
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, USA.
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FERREIRA JORGEF, BATISTA JACQUELINES, FANTIN CLEITON. Screening for FMR1 expanded alleles in patients with Autism Spectrum Disorders in Manaus, Northern Brazil. AN ACAD BRAS CIENC 2019; 91:e20180882. [DOI: 10.1590/0001-3765201920180882] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2018] [Accepted: 10/04/2018] [Indexed: 11/22/2022] Open
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25
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Domniz N, Ries-Levavi L, Cohen Y, Marom-Haham L, Berkenstadt M, Pras E, Glicksman A, Tortora N, Latham GJ, Hadd AG, Nolin SL, Elizur SE. Absence of AGG Interruptions Is a Risk Factor for Full Mutation Expansion Among Israeli FMR1 Premutation Carriers. Front Genet 2018; 9:606. [PMID: 30619448 PMCID: PMC6300753 DOI: 10.3389/fgene.2018.00606] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2018] [Accepted: 11/19/2018] [Indexed: 11/13/2022] Open
Abstract
Introduction: Fragile X syndrome (FXS) is a common form of X-linked intellectual and developmental disability with a prevalence of 1/4000-5000 in males and 1/6000-8000 in females. Most cases of the syndrome result from expansion of a premutation (55-200 CGGs) to a full mutation (>200 CGGs) repeat located in the 5' untranslated region of the fragile X mental retardation (FMR1) gene. The risk for full mutation expansions increases dramatically with increasing numbers of CGG repeats. Recent studies, however, revealed AGG interruptions within the repeat area function as a "protective factor" decreasing the risk of intergenerational expansion. Materials and Methods: This study was conducted to validate the relevance of AGG analysis for the ethnically diverse Israeli population. To increase the accuracy of our results, we combined results from Israel with those from the New York State Institute for Basic Research in Developmental Disabilities (IBR). To the best of our knowledge this is the largest cohort of different ethnicities to examine risks of unstable transmissions and full mutation expansions among FMR1 premutation carriers. Results: The combined data included 1471 transmissions of maternal premutation alleles: 369 (25.1%) stable and 1,102 (74.9%) unstable transmissions. Full mutation expansions were identified in 20.6% (303/1471) of transmissions. A total of 97.4% (388/397) of transmissions from alleles with no AGGs were unstable, 79.6% (513/644) in alleles with 1 AGG and 46.7% (201/430) in alleles with 2 or more AGGs. The same trend was seen with full mutation expansions where 40% (159/397) of alleles with no AGGs expanded to a full mutation, 20.2% (130/644) for alleles with 1 AGG and only 3.2% (14/430) in alleles with 2 AGGs or more. None of the alleles with 3 or more AGGs expanded to full mutations. Conclusion: We recommend that risk estimates for FMR1 premutation carriers be based on AGG interruptions as well as repeat size in Israel and worldwide.
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Affiliation(s)
- Noam Domniz
- IVF Unit, Sheba Medical Center, Tel Hashomer, Israel.,Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
| | - Liat Ries-Levavi
- Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.,Danek Genetic Institute, Sheba Medical Center, Tel Hashomer, Israel
| | - Yoram Cohen
- IVF Unit, Sheba Medical Center, Tel Hashomer, Israel.,Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
| | - Lilach Marom-Haham
- IVF Unit, Sheba Medical Center, Tel Hashomer, Israel.,Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
| | - Michal Berkenstadt
- Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.,Danek Genetic Institute, Sheba Medical Center, Tel Hashomer, Israel
| | - Elon Pras
- Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.,Danek Genetic Institute, Sheba Medical Center, Tel Hashomer, Israel
| | - Anne Glicksman
- New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, United States
| | - Nicole Tortora
- New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, United States
| | | | | | - Sarah L Nolin
- New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, United States
| | - Shai E Elizur
- IVF Unit, Sheba Medical Center, Tel Hashomer, Israel.,Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
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26
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Fink DA, Nelson LM, Pyeritz R, Johnson J, Sherman SL, Cohen Y, Elizur SE. Fragile X Associated Primary Ovarian Insufficiency (FXPOI): Case Report and Literature Review. Front Genet 2018; 9:529. [PMID: 30542367 PMCID: PMC6278244 DOI: 10.3389/fgene.2018.00529] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2018] [Accepted: 10/22/2018] [Indexed: 11/29/2022] Open
Abstract
Abnormalities in the X-linked FMR1 gene are associated with a constellation of disorders, which have broad and profound implications for the person first diagnosed, and extended family members of all ages. The rare and pleiotropic nature of the associated disorders, both common and not, place great burdens on (1) the affected families, (2) their care providers and clinicians, and (3) investigators striving to conduct research on the conditions. Fragile X syndrome, occurring more severely in males, is the leading genetic cause of intellectual disability. Fragile X associated tremor and ataxia syndrome (FXTAS) is a neurodegenerative disorder seen more often in older men. Fragile X associated primary ovarian insufficiency (FXPOI) is a chronic disorder characterized by oligo/amenorrhea and hypergonadotropic hypogonadism before age 40 years. There may be significant morbidity due to: (1) depression and anxiety related to the loss of reproductive hormones and infertility; (2) reduced bone mineral density; and (3) increased risk of cardiovascular disease related to estrogen deficiency. Here we report the case of a young woman who never established regular menses and yet experienced a 5-year diagnostic odyssey before establishing a diagnosis of FXPOI despite a known family history of fragile X syndrome and early menopause. Also, despite having clearly documented FXPOI the woman conceived spontaneously and delivered two healthy children. We review the pathophysiology and management of FXPOI. As a rare disease, the diagnosis of FXPOI presents special challenges. Connecting patients and community health providers with investigators who have the requisite knowledge and expertise about the FMR1 gene and FXPOI would facilitate both patient care and research. There is a need for an international natural history study on FXPOI. The effort should be coordinated by a global virtual center, which takes full advantage of mobile device communication systems.
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Affiliation(s)
- Dorothy A Fink
- Hospital for Special Surgery, New York, NY, United States
| | | | - Reed Pyeritz
- Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
| | - Josh Johnson
- University of Colorado, Denver, CO, United States
| | | | - Yoram Cohen
- Sheba Medical Center, Tel Hashomer and Tel Aviv University, Tel Aviv, Israel
| | - Shai E Elizur
- Sheba Medical Center, Tel Hashomer and Tel Aviv University, Tel Aviv, Israel
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Dean DD, Agarwal S, Kapoor D, Singh K, Vati C. Molecular Characterization of FMR1 Gene by TP-PCR in Women of Reproductive Age and Women with Premature Ovarian Insufficiency. Mol Diagn Ther 2017; 22:91-100. [PMID: 29188551 DOI: 10.1007/s40291-017-0305-9] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
BACKGROUND Fragile X syndrome is caused by CGG repeat expansion mutation in the FMR1 gene. Normal alleles have 5-44 CGG repeats with AGG interruptions. The expanded gray zone (GZ) (45-54 repeats) and premutation (PM) (55-200 repeats) alleles are often uninterrupted and are unstably inherited in subsequent generations. The prevalence of PM and GZ carriers is high in the female population, at 1/66 and 1/113, respectively, and PM is associated with fertility problems in 20% of cases. OBJECTIVE Our objective was to molecularly characterize CGG repeats and AGG interruption sequences in the FMR1 gene in women of reproductive age and in women with premature ovarian insufficiency (POI). MATERIALS AND METHODS We conducted molecular analysis of the FMR1 gene from 300 women of reproductive age and 140 women with POI using triplet primed-polymerase chain reaction. This enabled us to identify carriers and to document CGG repeat size and the AGG interruption pattern. RESULTS In women of reproductive age, 1.7% were GZ carriers and 0.3% were PM carriers; in women with POI, 3.6% were GZ carriers and 2.14% were PM carriers. The frequency of GZ and PM carriers did not significantly differ between the cohorts (Fisher's exact test: p < 2.23 for GZ vs. control and p < 0.101 for PM vs. control). Carriers received genetic counselling; family screening identified an additional seven carriers. CONCLUSION We documented preliminary data on the prevalence of GZ and PM carriers among the studied cohorts. The identification of PM carriers among women with POI serves a dual purpose of recognizing a cause for ovarian dysfunction and enabling genetic counselling, which will help carriers when making reproductive decisions.
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Affiliation(s)
- Deepika Delsa Dean
- Department of Genetics, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, 226014, India
| | - Sarita Agarwal
- Department of Genetics, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, 226014, India.
| | - Deepa Kapoor
- Department of Obstetrics and Gynecology, General Hospital, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, 226014, India
| | - Kuldeep Singh
- Department of Pediatrics, AIIMS, Jodhpur, Rajasthan, India
| | - Chandra Vati
- Department of Obstetrics and Gynecology, Krishna Medical Centre, Lucknow, India
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28
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Hayward BE, Kumari D, Usdin K. Recent advances in assays for the fragile X-related disorders. Hum Genet 2017; 136:1313-1327. [PMID: 28866801 DOI: 10.1007/s00439-017-1840-5] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2017] [Accepted: 08/24/2017] [Indexed: 12/17/2022]
Abstract
The fragile X-related disorders are a group of three clinical conditions resulting from the instability of a CGG-repeat tract at the 5' end of the FMR1 transcript. Fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X-associated primary ovarian insufficiency (FXPOI) are disorders seen in carriers of FMR1 alleles with 55-200 repeats. Female carriers of these premutation (PM) alleles are also at risk of having a child who has an FMR1 allele with >200 repeats. Most of these full mutation (FM) alleles are epigenetically silenced resulting in a deficit of the FMR1 gene product, FMRP. This results in fragile X Syndrome (FXS), the most common heritable cause of intellectual disability and autism. The diagnosis and study of these disorders is challenging, in part because the detection of alleles with large repeat numbers has, until recently, been either time-consuming or unreliable. This problem is compounded by the mosaicism for repeat length and/or DNA methylation that is frequently seen in PM and FM carriers. Furthermore, since AGG interruptions in the repeat tract affect the risk that a FM allele will be maternally transmitted, the ability to accurately detect these interruptions in female PM carriers is an additional challenge that must be met. This review will discuss some of the pros and cons of some recently described assays for these disorders, including those that detect FMRP levels directly, as well as emerging technologies that promise to improve the diagnosis of these conditions and to be useful in both basic and translational research settings.
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Affiliation(s)
- Bruce E Hayward
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Building 8, Room 2A19, National Institutes of Health, 8 Center Drive MSC 0830, Bethesda, MD, 20892, USA
| | - Daman Kumari
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Building 8, Room 2A19, National Institutes of Health, 8 Center Drive MSC 0830, Bethesda, MD, 20892, USA
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Building 8, Room 2A19, National Institutes of Health, 8 Center Drive MSC 0830, Bethesda, MD, 20892, USA.
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29
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Hayward BE, Usdin K. Improved Assays for AGG Interruptions in Fragile X Premutation Carriers. J Mol Diagn 2017; 19:828-835. [PMID: 28818679 DOI: 10.1016/j.jmoldx.2017.06.008] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2017] [Revised: 06/05/2017] [Accepted: 06/21/2017] [Indexed: 10/19/2022] Open
Abstract
The learning disability fragile X syndrome results from the presence of >200 CGG/CCG repeats in exon 1 of the X-linked gene FMR1. Such alleles arise by expansion from maternally transmitted FMR1 premutation alleles, alleles having 55 to 200 repeats. Expansion risk is directly related to maternal repeat number. However, AGG interruptions to the repeat tract are important modifiers of expansion risk. Thus, the ability to identify such interruptions is crucial for the appropriate genetic counseling of females who are premutation carriers. First-generation triplet-primed PCR assays allow these interruptions to be detected. However, because the triplet primer used has multiple binding sites in the repeat tract, interpreting the results is not straightforward and it is not always possible to unambiguously determine the AGG-interruption status in females because of the difficulties associated with the presence of a second X chromosome. Interpretation is further complicated by any repeat size mosaicism that may be present. We have developed second-generation PCR assays that prime specifically at the interruptions. These assays are simpler to interpret and better able to evaluate this important determinant of expansion risk in females even in those with a mixture of premutation allele sizes.
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Affiliation(s)
- Bruce E Hayward
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.
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30
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Alfaro Arenas R, Rosell Andreo J, Heine Suñer D. Fragile X syndrome screening in pregnant women and women planning pregnancy shows a remarkably high FMR1 premutation prevalence in the Balearic Islands. Am J Med Genet B Neuropsychiatr Genet 2016; 171:1023-1031. [PMID: 27333191 DOI: 10.1002/ajmg.b.32470] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Accepted: 06/01/2016] [Indexed: 12/23/2022]
Abstract
There are no reported studies to determine incidence of Fragile X Syndrome (FXS) in women within the Spanish population. For this reason, together with the high incidence of FXS in the general population, the exclusively maternal expansion, the familial and social impact of the syndrome, and the ease of use and level of detection of current PCR-based techniques, we have conducted a population-based screening pilot program of which we present here the molecular results. We typed prospectively 3,413 pregnant and 318 non-pregnant women and found a prevalence of premutation (PM) carriers of 1 in 106, which is the highest described to date in any population. We also found 230 different alleles of which the most frequent are 10A9A9 (38.4%), 9A9A9 (15.1%), and 10A9 (10.5%). Furthermore, alleles with 0 AGG interruptions or with a pure (uninterrupted) CGG repeat run larger than 34 (presumably more unstable), were more frequent among PM alleles compared to normal alleles. Theà unexpected high frequency of expanded PM alleles in females in the general population makes a very compelling argument for the need for prenatal or preconceptional FXS screening in our community. Furthermore, we find FMR1 triplet primed PCR (TP-PCR) confidently and precisely determines sizes for both alleles of the CGG repeat in women and offers AGG information which greatly improves CGG expansion risk estimations for genetic counselling. Thus, TP-PCR is an informative, efficient and robust method for FXS screening in the female population. © 2016 Wiley Periodicals, Inc.
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Affiliation(s)
- Ramona Alfaro Arenas
- Secció Genética Hospital, Universitario Son Espases, Palma de Mallorca, Balearic Islands, Spain.,Institut d'Investigacions Sanitaries de Palma IdISPa, Palma de Mallorca, Balearic Islands, Spain
| | - Jordi Rosell Andreo
- Secció Genética Hospital, Universitario Son Espases, Palma de Mallorca, Balearic Islands, Spain.,Institut d'Investigacions Sanitaries de Palma IdISPa, Palma de Mallorca, Balearic Islands, Spain.,Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), Palma de Mallorca, Spain
| | - Dami Heine Suñer
- Secció Genética Hospital, Universitario Son Espases, Palma de Mallorca, Balearic Islands, Spain.,Institut d'Investigacions Sanitaries de Palma IdISPa, Palma de Mallorca, Balearic Islands, Spain.,Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), Palma de Mallorca, Spain
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31
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Finucane B, Lincoln S, Bailey L, Martin CL. Prognostic dilemmas and genetic counseling for prenatally detected fragile X gene expansions. Prenat Diagn 2016; 37:37-42. [PMID: 27862088 DOI: 10.1002/pd.4963] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2016] [Revised: 10/28/2016] [Accepted: 11/01/2016] [Indexed: 12/17/2022]
Abstract
With widespread adoption of fragile X carrier screening in pregnant women, the number of expectant couples receiving news of an unanticipated Fragile X Mental Retardation 1 (FMR1) gene expansion has increased. The most common abnormal result from maternal FMR1 testing involves an intermediate allele, also known as a gray zone result, which requires genetic counseling but poses minimal risk for an adverse developmental outcome. By contrast, the finding of a maternal FMR1 premutation or full mutation during pregnancy has important implications for the woman herself, her unborn child, and her extended family. These multiple levels of impact, in addition to the complex inheritance pattern and variable expressivity of fragile X-associated disorders, cause significant stress for newly identified expectant couples and pose unique challenges for genetic counselors in the prenatal setting. © 2016 John Wiley & Sons, Ltd.
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Affiliation(s)
- Brenda Finucane
- Autism & Developmental Medicine Institute, Geisinger Health System, Lewisburg, PA, USA
| | - Sharyn Lincoln
- Division of Genetics and Genomics, Boston Children's Hospital, Boston, MA, USA
| | - Lindsay Bailey
- Genomic Medicine Institute, Geisinger Health System, Danville, PA, USA
| | - Christa Lese Martin
- Autism & Developmental Medicine Institute, Geisinger Health System, Lewisburg, PA, USA
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32
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Molecular Correlates and Recent Advancements in the Diagnosis and Screening of FMR1-Related Disorders. Genes (Basel) 2016; 7:genes7100087. [PMID: 27754417 PMCID: PMC5083926 DOI: 10.3390/genes7100087] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2016] [Revised: 10/06/2016] [Accepted: 10/08/2016] [Indexed: 12/12/2022] Open
Abstract
Fragile X syndrome (FXS) is the most common monogenic cause of intellectual disability and autism. Molecular diagnostic testing of FXS and related disorders (fragile X-associated primary ovarian insufficiency (FXPOI) and fragile X-associated tremor/ataxia syndrome (FXTAS)) relies on a combination of polymerase chain reaction (PCR) and Southern blot (SB) for the fragile X mental retardation 1 (FMR1) CGG-repeat expansion and methylation analyses. Recent advancements in PCR-based technologies have enabled the characterization of the complete spectrum of CGG-repeat mutation, with or without methylation assessment, and, as a result, have reduced our reliance on the labor- and time-intensive SB, which is the gold standard FXS diagnostic test. The newer and more robust triplet-primed PCR or TP-PCR assays allow the mapping of AGG interruptions and enable the predictive analysis of the risks of unstable CGG expansion during mother-to-child transmission. In this review, we have summarized the correlation between several molecular elements, including CGG-repeat size, methylation, mosaicism and skewed X-chromosome inactivation, and the extent of clinical involvement in patients with FMR1-related disorders, and reviewed key developments in PCR-based methodologies for the molecular diagnosis of FXS, FXTAS and FXPOI, and large-scale (CGG)n expansion screening in newborns, women of reproductive age and high-risk populations.
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Towards a Better Molecular Diagnosis of FMR1-Related Disorders-A Multiyear Experience from a Reference Lab. Genes (Basel) 2016; 7:genes7090059. [PMID: 27598204 PMCID: PMC5042390 DOI: 10.3390/genes7090059] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2016] [Revised: 08/03/2016] [Accepted: 08/19/2016] [Indexed: 11/29/2022] Open
Abstract
The article summarizes over 20 years of experience of a reference lab in fragile X mental retardation 1 gene (FMR1) molecular analysis in the molecular diagnosis of fragile X spectrum disorders. This includes fragile X syndrome (FXS), fragile X-associated primary ovarian insufficiency (FXPOI) and fragile X-associated tremor/ataxia syndrome (FXTAS), which are three different clinical conditions with the same molecular background. They are all associated with an expansion of CGG repeats in the 5′UTR of FMR1 gene. Until 2016, the FMR1 gene was tested in 9185 individuals with the pre-screening PCR, supplemented with Southern blot analysis and/or Triplet Repeat Primed PCR based method. This approach allowed us to confirm the diagnosis of FXS, FXPOI FXTAS in 636/9131 (6.96%), 4/43 (9.3%) and 3/11 (27.3%) of the studied cases, respectively. Moreover, the FXS carrier status was established in 389 individuals. The technical aspect of the molecular analysis is very important in diagnosis of FXS-related disorders. The new methods were subsequently implemented in our laboratory. This allowed the significance of the Southern blot technique to be decreased until its complete withdrawal. Our experience points out the necessity of implementation of the GeneScan based methods to simplify the testing procedure as well as to obtain more information for the patient, especially if TP-PCR based methods are used.
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Hayward BE, Zhou Y, Kumari D, Usdin K. A Set of Assays for the Comprehensive Analysis of FMR1 Alleles in the Fragile X-Related Disorders. J Mol Diagn 2016; 18:762-774. [PMID: 27528259 DOI: 10.1016/j.jmoldx.2016.06.001] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2015] [Revised: 05/24/2016] [Accepted: 06/01/2016] [Indexed: 12/16/2022] Open
Abstract
The diagnosis and study of the fragile X-related disorders is complicated by the difficulty of amplifying the long CGG/CCG-repeat tracts that are responsible for disease pathology, the potential presence of AGG interruptions within the repeat tract that can ameliorate expansion risk, the occurrence of variable DNA methylation that modulates disease severity, and the high frequency of mosaicism for both repeat number and methylation status. These factors complicate patient risk assessment. In addition, the variability in these parameters that is seen when patient cells are grown in culture requires their frequent monitoring to ensure reproducible results in a research setting. Many existing assays have the limited ability to amplify long alleles, particularly in a mixture of different allele sizes. Others are better at this, but are too expensive for routine use in most laboratories or for newborn screening programs and use reagents that are proprietary. We describe herein a set of assays to routinely evaluate all of these important parameters in a time- and cost-effective way.
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Affiliation(s)
- Bruce E Hayward
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Yifan Zhou
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Daman Kumari
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.
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35
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González-Pérez J, Izquierdo-Álvarez S, Fuertes-Rodrigo C, Monge-Galindo L, Peña-Segura JL, López-Pisón FJ. [Triplet expansion cytosine-guanine-guanine: Three cases of OMIM syndrome in the same family]. Med Clin (Barc) 2016; 146:311-5. [PMID: 26776484 DOI: 10.1016/j.medcli.2015.11.022] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2015] [Revised: 11/18/2015] [Accepted: 11/19/2015] [Indexed: 11/25/2022]
Abstract
INTRODUCTION The dynamic increase in the number of triplet repeats of cytosine-guanine-guanine (CGG) in the FMR1 gene mutation is responsible for three OMIM syndromes with a distinct clinical phenotype: Fragile X syndrome (FXS) and two pathologies in adult carriers of the premutation (55-200 CGG repeats): Primary ovarian insufficiency (FXPOI) and tremor-ataxia syndrome (FXTAS) associated with FXS. CLINICAL OBSERVATION AND METHODS CGG mutation dynamics of the FMR1 gene were studied in DNA samples from peripheral blood from the index case and other relatives of first, second and third degree by TP-PCR, and the percentage methylation. RESULTS Diagnosis of FXS was confirmed in three patients (21.4%), eight patients (57.1%) were confirmed in the premutation range transmitters, one male patient with full mutation/permutation mosaicism (7.1%) and two patients (14.3%) with normal study. Of the eight permutated patients, three had FXPOI and one male patient had FXTAS. DISCUSSION Our study suggests the importance of making an early diagnosis of SXF in order to carry out a family study and genetic counselling, which allow the identification of new cases or premutated patients with FMR1 gene- associated syndromes (FXTAS, FXPOI).
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Affiliation(s)
- Jesús González-Pérez
- Unidad de Neuropediatría, Servicio de Pediatría, Hospital Universitario Miguel Servet, Zaragoza, Spain.
| | - Silvia Izquierdo-Álvarez
- Sección de Genética Clínica y Reproducción Asistida, Servicio de Bioquímica Clínica, Hospital Universitario Miguel Servet, Zaragoza, Spain
| | - Cristina Fuertes-Rodrigo
- Unidad de Neuropediatría, Servicio de Pediatría, Hospital Universitario Miguel Servet, Zaragoza, Spain
| | - Lorena Monge-Galindo
- Unidad de Neuropediatría, Servicio de Pediatría, Hospital Universitario Miguel Servet, Zaragoza, Spain
| | - José Luis Peña-Segura
- Unidad de Neuropediatría, Servicio de Pediatría, Hospital Universitario Miguel Servet, Zaragoza, Spain
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36
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Unique AGG Interruption in the CGG Repeats of the FMR1 Gene Exclusively Found in Asians Linked to a Specific SNP Haplotype. GENETICS RESEARCH INTERNATIONAL 2016; 2016:8319287. [PMID: 27042357 PMCID: PMC4793144 DOI: 10.1155/2016/8319287] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/27/2015] [Accepted: 01/31/2016] [Indexed: 11/17/2022]
Abstract
Fragile X syndrome (FXS) is the most common inherited intellectual disability. It is caused by the occurrence of more than 200 pure CGG repeats in the FMR1 gene. Normal individuals have 6-54 CGG repeats with two or more stabilizing AGG interruptions occurring once every 9- or 10-CGG-repeat blocks in various populations. However, the unique (CGG)6AGG pattern, designated as 6A, has been exclusively reported in Asians. To examine the genetic background of AGG interruptions in the CGG repeats of the FMR1 gene, we studied 8 SNPs near the CGG repeats in 176 unrelated Thai males with 19-56 CGG repeats. Of these 176 samples, we identified AGG interruption patterns from 95 samples using direct DNA sequencing. We found that the common CGG repeat groups (29, 30, and 36) were associated with 3 common haplotypes, GCGGATAA (Hap A), TTCATCGC (Hap C), and GCCGTTAA (Hap B), respectively. The configurations of 9A9A9, 10A9A9, and 9A9A6A9 were commonly found in chromosomes with 29, 30, and 36 CGG repeats, respectively. Almost all chromosomes with Hap B (22/23) carried at least one 6A pattern, suggesting that the 6A pattern is linked to Hap B and may have originally occurred in the ancestors of Asian populations.
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Zhao XN, Kumari D, Gupta S, Wu D, Evanitsky M, Yang W, Usdin K. Mutsβ generates both expansions and contractions in a mouse model of the Fragile X-associated disorders. Hum Mol Genet 2015; 24:7087-96. [PMID: 26420841 DOI: 10.1093/hmg/ddv408] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2015] [Accepted: 09/22/2015] [Indexed: 11/13/2022] Open
Abstract
Fragile X-associated disorders are Repeat Expansion Diseases that result from expansion of a CGG/CCG-repeat in the FMR1 gene. Contractions of the repeat tract also occur, albeit at lower frequency. However, these contractions can potentially modulate disease symptoms or generate an allele with repeat numbers in the normal range. Little is known about the expansion mechanism and even less about contractions. We have previously demonstrated that the mismatch repair (MMR) protein MSH2 is required for expansions in a mouse model of these disorders. Here, we show that MSH3, the MSH2-binding partner in the MutSβ complex, is required for 98% of germ line expansions and all somatic expansions in this model. In addition, we provide evidence for two different contraction mechanisms that operate in the mouse model, a MutSβ-independent one that generates small contractions and a MutSβ-dependent one that generates larger ones. We also show that MutSβ complexes formed with the repeats have altered kinetics of ATP hydrolysis relative to complexes with bona fide MMR substrates and that MutSβ increases the stability of the CCG-hairpins at physiological temperatures. These data may have important implications for our understanding of the mechanism(s) of repeat instability and for the role of MMR proteins in this process.
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Affiliation(s)
- Xiao-Nan Zhao
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology
| | - Daman Kumari
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology
| | - Shikha Gupta
- Section on Structure and Mechanisms of DNA repair, replication and recombination, Laboratory of Molecular Biology and
| | - Di Wu
- Section on Physical Biochemistry, Laboratory of Biochemistry and Genetics, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0830, USA
| | - Maya Evanitsky
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology
| | - Wei Yang
- Section on Structure and Mechanisms of DNA repair, replication and recombination, Laboratory of Molecular Biology and
| | - Karen Usdin
- Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology,
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38
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Simplified strategy for rapid first-line screening of fragile X syndrome: closed-tube triplet-primed PCR and amplicon melt peak analysis. Expert Rev Mol Med 2015; 17:e7. [PMID: 25936533 PMCID: PMC4836207 DOI: 10.1017/erm.2015.5] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Premutation and full-mutation hyperexpansion of CGG-triplets in the X-linked Fragile X Mental Retardation 1 (FMR1) gene have been implicated in fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X syndrome (FXS), respectively. The currently available molecular diagnostic tests are either costly or labour-intensive, which prohibits their application as a first-line FMR1 test in large-scale population-based screening programs. In this study, we demonstrate the utility of a simplified closed-tube strategy for rapid first-line screening of FXS based on melt peak temperature (Tm) analysis of direct triplet-primed polymerase chain reaction amplicons (dTP-PCR MCA). In addition, we also evaluated the correlation between Tm and CGG-repeat size based on capillary electrophoresis (CE) of dTP-PCR amplicons. The assays were initially tested on 29 FMR1 reference DNA samples, followed by a blinded validation on 107 previously characterised patient DNA samples. The dTP-PCR MCA produced distinct melt profiles of higher Tm for samples carrying an expanded allele. Among the samples tested, we also observed a good correlation between Tm and CGG-repeat size. In the blinded validation study, dTP-PCR MCA accurately classified all normal and expansion carriers, and the FMR1 genotypic classification of all samples was completely concordant with the previously determined genotypes as well as the dTP-PCR CE results. This simple and cost-effective MCA-based assay may be useful as a first-line FXS screening tool that could rapidly screen out the large majority of unaffected individuals, thus minimising the number of samples that need to be analysed by Southern blot analysis.
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39
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Abstract
DNA repair normally protects the genome against mutations that threaten genome integrity and thus cell viability. However, growing evidence suggests that in the case of the Repeat Expansion Diseases, disorders that result from an increase in the size of a disease-specific microsatellite, the disease-causing mutation is actually the result of aberrant DNA repair. A variety of proteins from different DNA repair pathways have thus far been implicated in this process. This review will summarize recent findings from patients and from mouse models of these diseases that shed light on how these pathways may interact to cause repeat expansion.
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Affiliation(s)
- Xiao-Nan Zhao
- Section on Genomic Structure and Function Laboratory of Cell and Molecular Biology National Institute of Diabetes, Digestive and Kidney Diseases National Institutes of Health, Bethesda, MD 20892-0830, USA
| | - Karen Usdin
- Section on Genomic Structure and Function Laboratory of Cell and Molecular Biology National Institute of Diabetes, Digestive and Kidney Diseases National Institutes of Health, Bethesda, MD 20892-0830, USA.
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