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da Silva CP, Camuzi D, Reis AHDO, Gonçalves AP, Dos Santos JM, Machado FB, Medina-Acosta E, Soares-Lima SC, Santos-Rebouças CB. Identification of a novel epigenetic marker for typical and mosaic presentations of Fragile X syndrome. Expert Rev Mol Diagn 2023; 23:1273-1281. [PMID: 37970883 DOI: 10.1080/14737159.2023.2284782] [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] [Received: 09/05/2023] [Accepted: 11/07/2023] [Indexed: 11/19/2023]
Abstract
BACKGROUND Fragile X syndrome (FXS) is primarily due to CGG repeat expansions in the FMR1 gene. FMR1 alleles are classified as normal (N), intermediate (I), premutation (PM), and full mutation (FM). FXS patients often carry an FM, but size mosaicism can occur. Additionally, loss of methylation boundary upstream of repeats results in de novo methylation spreading to FMR1 promoter in FXS patients. RESEARCH DESIGN AND METHODS This pilot study investigated the methylation boundary and adjacent regions in 66 males with typical and atypical FXS aged 1 to 30 years (10.86 ± 6.48 years). AmplideX FMR1 mPCR kit was used to discriminate allele profiles and methylation levels. CpG sites were assessed by pyrosequencing. RESULTS 40 out of 66 FXS patients (60.6%) showed an exclusive FM (n = 40), whereas the remaining (n = 26) exhibited size mosaicism [10 PM_FM (15.15%); 10 N_FM (15.15%); 2 N_PM_FM (3%)]. Four patients (6.1%) had deletions near repeats. Noteworthy, a CpG within FMR1 intron 2 displayed hypomethylation in FXS patients and hypermethylation in controls, demonstrating remarkable specificity, sensitivity, and accuracy when a methylation threshold of 69.5% was applied. CONCLUSIONS Since intragenic methylation is pivotal in gene regulation, the intronic CpG might be a novel epigenetic biomarker for FXS diagnosis.
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Affiliation(s)
- Camilla Pereira da Silva
- Department of Genetics, Institute of Biology Roberto Alcantara Gomes, State University of Rio de Janeiro, Rio de Janeiro, Brazil
| | - Diego Camuzi
- Molecular Carcinogenesis Program, Brazilian National Cancer Institute, Rio de Janeiro, Brazil
| | - Adriana Helena de Oliveira Reis
- Department of Genetics, Institute of Biology Roberto Alcantara Gomes, State University of Rio de Janeiro, Rio de Janeiro, Brazil
| | - Andressa Pereira Gonçalves
- Department of Genetics, Institute of Biology Roberto Alcantara Gomes, State University of Rio de Janeiro, Rio de Janeiro, Brazil
| | - Jussara Mendonça Dos Santos
- Department of Genetics, Institute of Biology Roberto Alcantara Gomes, State University of Rio de Janeiro, Rio de Janeiro, Brazil
| | - Filipe Brum Machado
- Department of Biological Sciences, Minas Gerais State University, Minas Gerais, Brazil
| | - Enrique Medina-Acosta
- Biotechnology Laboratory, Molecular Diagnostic, and Research Center, State University of the North Fluminense Darcy Ribeiro, Campos dos Goytacazes, Brazil
| | | | - Cíntia Barros Santos-Rebouças
- Department of Genetics, Institute of Biology Roberto Alcantara Gomes, State University of Rio de Janeiro, Rio de Janeiro, Brazil
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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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Khristich AN, Mirkin SM. On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability. J Biol Chem 2020; 295:4134-4170. [PMID: 32060097 PMCID: PMC7105313 DOI: 10.1074/jbc.rev119.007678] [Citation(s) in RCA: 161] [Impact Index Per Article: 40.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Expansions of simple tandem repeats are responsible for almost 50 human diseases, the majority of which are severe, degenerative, and not currently treatable or preventable. In this review, we first describe the molecular mechanisms of repeat-induced toxicity, which is the connecting link between repeat expansions and pathology. We then survey alternative DNA structures that are formed by expandable repeats and review the evidence that formation of these structures is at the core of repeat instability. Next, we describe the consequences of the presence of long structure-forming repeats at the molecular level: somatic and intergenerational instability, fragility, and repeat-induced mutagenesis. We discuss the reasons for gender bias in intergenerational repeat instability and the tissue specificity of somatic repeat instability. We also review the known pathways in which DNA replication, transcription, DNA repair, and chromatin state interact and thereby promote repeat instability. We then discuss possible reasons for the persistence of disease-causing DNA repeats in the genome. We describe evidence suggesting that these repeats are a payoff for the advantages of having abundant simple-sequence repeats for eukaryotic genome function and evolvability. Finally, we discuss two unresolved fundamental questions: (i) why does repeat behavior differ between model systems and human pedigrees, and (ii) can we use current knowledge on repeat instability mechanisms to cure repeat expansion diseases?
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Affiliation(s)
| | - Sergei M Mirkin
- Department of Biology, Tufts University, Medford, Massachusetts 02155.
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4
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Reversion to Normal of FMR1 Expanded Alleles: A Rare Event in Two Independent Fragile X Syndrome Families. Genes (Basel) 2020; 11:genes11030248. [PMID: 32111011 PMCID: PMC7140891 DOI: 10.3390/genes11030248] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2020] [Revised: 02/12/2020] [Accepted: 02/24/2020] [Indexed: 01/12/2023] Open
Abstract
Fragile X syndrome (FXS) is mostly due to the expansion and subsequent methylation of a polymorphic CGG repeat in the 5’ UTR of the FMR1 gene. Full mutation alleles (FM) have more than 200 repeats and result in FMR1 gene silencing and FXS. FMs arise from maternal premutations (PM) that have 56–200 CGGs; contractions of a maternal PM or FM are rare. Here, we describe two unaffected boys in two independent FXS families who inherited a non-mosaic allele in the normal and intermediate range, respectively, from their mothers who are carriers of an expanded CGG allele. The first boy inherited a 51 CGG allele (without AGG interruptions) from his mother, who carries a PM allele with 72 CGGs. The other boy inherited from his FM mother an unusual allele with 19 CGGs resulting from a deletion, removing 85 bp upstream of the CGG repeat. Given that transcription of the deleted allele was found to be preserved, we assume that the binding sites for FMR1 transcription factors are excluded from the deletion. Such unusual cases resulting in non-mosaic reduction of maternal CGG expansions may help to clarify the molecular mechanisms underlying the instability of the FMR1 gene.
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Bacrot S, Monnot S, Haddad G, Barcia G, Rachid M, Boisson M, Pasquier N, Rondeau S, Munnich A, Steffann J, Bonnefont JP, Raynaud M. Prenatal diagnosis of fragile X syndrome: Small meiotic recombination events at the FMR1 locus. Prenat Diagn 2019; 39:388-393. [PMID: 30779209 DOI: 10.1002/pd.5439] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2018] [Revised: 02/11/2019] [Accepted: 02/16/2019] [Indexed: 11/11/2022]
Affiliation(s)
- Séverine Bacrot
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Sophie Monnot
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Georges Haddad
- Service de gynécologie obstétrique, Centre hospitalier de Blois, Blois, France
| | - Giulia Barcia
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Myriam Rachid
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Marie Boisson
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Nathalie Pasquier
- Service de Génétique, CHRU de Tours, UMR 1253, iBrain, Université de Tours, Inserm, Tours, France
| | - Sophie Rondeau
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Arnold Munnich
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Julie Steffann
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Jean-Paul Bonnefont
- Université Paris Descartes - Sorbonne Paris Cite, Institut Imagine UMR1163, Fédération de Génétique médicale, Hôpital universitaire Necker-Enfants Malades, AP-HP, Paris, France
| | - Martine Raynaud
- Service de Génétique, CHRU de Tours, UMR 1253, iBrain, Université de Tours, Inserm, Tours, France
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Fernández E, Gennaro E, Pirozzi F, Baldo C, Forzano F, Turolla L, Faravelli F, Gastaldo D, Coviello D, Grasso M, Bagni C. FXS-Like Phenotype in Two Unrelated Patients Carrying a Methylated Premutation of the FMR1 Gene. Front Genet 2018; 9:442. [PMID: 30450110 PMCID: PMC6224343 DOI: 10.3389/fgene.2018.00442] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2018] [Accepted: 09/14/2018] [Indexed: 12/31/2022] Open
Abstract
Fragile X syndrome (FXS) is mostly caused by two distinct events that occur in the FMR1 gene (Xq27.3): an expansion above 200 repeats of a CGG triplet located in the 5′UTR of the gene, and methylation of the cytosines located in the CpG islands upstream of the CGG repeats. Here, we describe two unrelated families with one FXS child and another sibling presenting mild intellectual disability and behavioral features evocative of FXS. Genetic characterization of the undiagnosed sibling revealed mosaicism in both the CGG expansion size and the methylation levels in the different tissues analyzed. This report shows that in the same family, two siblings carrying different CGG repeats, one in the full-mutation range and the other in the premutation range, present methylation mosaicism and consequent decreased FMRP production leading to FXS and FXS-like features, respectively. Decreased FMRP levels, more than the number of repeats seem to correlate with the severity of FXS clinical phenotypes.
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Affiliation(s)
- Esperanza Fernández
- Center for Human Genetics, KU Leuven, Leuven, Belgium.,VIB & KU Leuven Center for Brain & Disease Research, Leuven, Belgium
| | - Elena Gennaro
- Laboratorio di Genetica Umana, Ospedali Galliera, Genoa, Italy
| | - Filomena Pirozzi
- Center for Human Genetics, KU Leuven, Leuven, Belgium.,VIB & KU Leuven Center for Brain & Disease Research, Leuven, Belgium
| | - Chiara Baldo
- Laboratorio di Genetica Umana, Ospedali Galliera, Genoa, Italy
| | - Francesca Forzano
- Clinical Genetics Department, Borough Wing Guy's Hospital, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom.,S.S.D. Genetica Medica, Ospedali Galliera, Genoa, Italy
| | - Licia Turolla
- U.O.S. Genetica Medica, Azienda ULSS 2, Treviso, Italy
| | - Francesca Faravelli
- Clinical Genetics Department, Great Ormond Street Hospital, London, United Kingdom
| | - Denise Gastaldo
- Department of Fundamental Neurosciences, University of Lausanne, Lausanne, Switzerland
| | | | - Marina Grasso
- Laboratorio di Genetica Umana, Ospedali Galliera, Genoa, Italy
| | - Claudia Bagni
- Center for Human Genetics, KU Leuven, Leuven, Belgium.,VIB & KU Leuven Center for Brain & Disease Research, Leuven, Belgium.,Department of Fundamental Neurosciences, University of Lausanne, Lausanne, Switzerland.,Department of Biomedicine and Prevention, University of Rome Tor Vergata, Rome, Italy
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7
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Prenatal Diagnosis of Fragile X Syndrome in a Twin Pregnancy Complicated by a Complete Retraction. Genes (Basel) 2018; 9:genes9060287. [PMID: 29880767 PMCID: PMC6027392 DOI: 10.3390/genes9060287] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2018] [Revised: 05/31/2018] [Accepted: 06/04/2018] [Indexed: 01/09/2023] Open
Abstract
Fragile X syndrome (FXS) is usually associated with a CGG repeat expansion >200 repeats within the FMR1 gene, known as a full mutation (FM). FM alleles produce abnormal methylation of the FMR1 promoter with reduction or silencing of FMR1 gene expression. Furthermore, premutation (PM: 55–199 CGGs) and full mutation alleles usually expand in size when maternally transmitted to progeny. This study describes a PM allele carried by the mother decreasing to a normal sized allele in a male from a dichorionic diamniotic (DCDA) twin pregnancy, with the female twin inheriting FM (200–790 CGGs), PM (130 CGGs) and normal-sized (39 CGGs) alleles. Further evidence of instability of the maternal PM allele was shown by a male proband (older brother) mosaic for PM (CGG 78 and 150 CGGs) and FM (200–813 CGGs), and a high level of FMR1 promoter methylation, between 50 and 70%, in multiple tissues. The fully-retracted, normal-sized allele was identified by PCR CGG sizing in the male twin, with no evidence of a FM allele identified using Southern blot analysis in multiple tissues collected postnatally and prenatally. Consistent with this, prenatal PCR sizing (35 CGGs) showed inconsistent inheritance of the maternal normal allele (30 CGGs), with single-nucleotide polymorphism (SNP) linkage analysis confirming that the abnormal FMR1 chromosome had been inherited from the mother’s PM chromosome. Importantly, the male twin showed no significant hypermethylation of the FMR1 promoter in all pre and postnatal tissues tested, as well as normal levels of FMR1 mRNA in blood. In summary, this report demonstrates the first postnatal follow up of a prenatal case in which FMR1 mRNA levels were approaching normal, with normal levels of FMR1 promoter methylation and normal CGG size in multiple pre and postnatally collected tissues.
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