Fragile X Syndrome: Introduction

Epigenetic insights into Fragile X Syndrome

Fragile X Syndrome (FXS) is a genetic neurodevelopmental disorder closely associated with intellectual disability and autism spectrum disorders. The core of the disease lies in the abnormal expansion of the CGG trinucleotide repeat sequence at the 5'end of the FMR1 gene. When the repetition exceeds 200 times, it causes the silencing of the FMR1 gene, leading to the absence of the encoded Fragile X mental retardation protein 1 (FMRP). Although the detailed mechanism by which the CGG repeat expansion triggers gene silencing is yet to be fully elucidated, it is known that this process does not alter the promoter region or the coding sequence of the FMR1 gene. This discovery provides a scientific basis for the potential reversal of FMR1 gene silencing through interventional approaches, thereby improving the symptoms of FXS. Epigenetics, a mechanism of genetic regulation that does not depend on changes in the DNA sequence, has become a new focus in FXS research by modulating gene expression in a reversible manner. The latest progress in molecular genetics has revealed that epigenetics plays a key role in the pathogenesis and pathophysiological processes of FXS. This article compiles the existing research findings on the role of epigenetics in Fragile X Syndrome (FXS) with the aim of deepening the understanding of the pathogenesis of FXS to identify potential targets for new therapeutic strategies.

Case report: genetic analysis of a novel frameshift mutation in FMR1 gene in a Chinese family

Fragile X syndrome (FXS) [OMIM 300624] is a common X-linked inherited syndrome with an incidence only second to that of trisomy 21. More than 95% of fragile X syndrome is caused by reduced or absent fragile X intellectual disability protein 1 (FMRP) synthesis due to dynamic mutation expansion of the CGG triplet repeat in the 5'UTR and abnormal methylation of the FMR1 (fragile X messenger ribonucleoprotein 1) gene [OMIM 309550]. Less than 5% of cases are caused by abnormal function of the FMRP due to point mutations or deletions in the FMR1 gene. In a proband with clinical suspicion of FXS and no CGG duplication, we found the presence of c.585_586del (p.Lys195AsnfsTer8) in exon 7 of the FMR1 gene using whole exome sequencing (WES). This variant resulted in frameshift and a premature stop codon after 8 aberrant amino acids. This variant is a novel pathogenic mutation, as determined by pedigree analysis, which has not been reported in any database or literature.

Narrative Review: Update on the Molecular Diagnosis of Fragile X Syndrome

The diagnosis and management of fragile X syndrome (FXS) have significantly improved in the last three decades, although the current diagnostic techniques are not yet able to precisely identify the number of repeats, methylation status, level of mosaicism, and/or the presence of AGG interruptions. A high number of repeats (>200) in the fragile X messenger ribonucleoprotein 1 gene (FMR1) results in hypermethylation of promoter and gene silencing. The actual molecular diagnosis is performed using a Southern blot, TP-PCR (Triplet-Repeat PCR), MS-PCR (Methylation-Specific PCR), and MS-MLPA (Methylation-Specific MLPA) with some limitations, with multiple assays being necessary to completely characterise a patient with FXS. The actual gold standard diagnosis uses Southern blot; however, it cannot accurately characterise all cases. Optical genome mapping is a new technology that has also been developed to approach the diagnosis of fragile X syndrome. Long-range sequencing represented by PacBio and Oxford Nanopore has the potential to replace the actual diagnosis and offers a complete characterization of molecular profiles in a single test. The new technologies have improved the diagnosis of fragile X syndrome and revealed unknown aberrations, but they are a long way from being used routinely in clinical practice.

On Terms: What's in a Name? Intellectual Disability and "Condition," "Disorder," "Syndrome," "Disease," and "Disability"

Individuals living with intellectual disability can have multiple co-occurring medical conditions including associated genetic diagnoses. The number of genetic etiologies associated with ID is growing, with some quite new and rare, and others more common and associated with what is often considered a syndrome. In the context of genetic etiologies and associated medical comorbidities, appropriate use of descriptive terminology warrants clarification. Using accurate nomenclature is essential for descriptions, especially as terms are used across both research and clinical reports. Here we discuss several terms that may be confused with each other, including "condition," "disorder," "syndrome," "disease," and "disability." Our goal is to shed light on the meanings of the five descriptors and their appropriate use in the ID population, especially in relation to those who have a genetic diagnosis.

Pilot study of socio-emotional factors and adaptive behavior in young females with fragile X syndrome

Girls with Fragile-X-Syndrome (FXS) present high levels of social anxiety, social avoidance, extreme shyness, tendency to social isolation, poor eye contact, learning difficulties, and depression. The aims of the present study, which is based on a group of young females with FXS are: 1) to analyze the possible associations between emotion recognition, theory of mind, and social anxiety, and adaptive behavior, and emotional state; 2) to study the relationship between intelligence quotient (IQ) and adaptive behavior; and 3) to assess whether social anxiety is more prevalent in girls with FXS. The study has 40 female participants aged between 7 and 16 years (26 positive full mutation FXS and 14 as a control group). A neuropsychological assessment was conducted using the following tests: WISC-V, NEPSY-II, SENA, ADHD Rating Scale, BAS, and ABAS-II. In comparison with the control group, the group with FXS presented a greater association between IQ and self-direction ability, and between emotion recognition and leadership. The FXS group presented higher levels of social anxiety and shyness. In the group of girls with FXS, IQ may have prognostic value for both self-direction ability and social adaptation level.

Language in young females with fragile X syndrome: Influence on the neurocognitive profile and adaptive behavior

Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability. The objective of this research is to analyze the relationship between linguistic functions and performance of the following neuropsychological functions: executive, quantitative reasoning, social perception, behavior, social skills, and adaptive behavior. A neuropsychological and behavioral evaluations were carried out with a group of 26 girls with FXS, and 14 girls without FXS as a control group, using standardized tests. The two groups were homogeneous in age and IQ. Significant differences were found between groups in the relationship between some language processes: inhibition, auditory working memory, cognitive flexibility, level of social adaptation, self-direction, conceptual adaptation, academic skills, leadership ability, theory of mind, and arithmetic. In the group of girls with FXS, it was found that different aspects of language influence some of the executive functions evaluated, in addition to some specific aspects of social perception, adaptive behavior, and quantitative reasoning, in different ways. Future research should incorporate the study of the influence of other cognitive variables such as visual perception and executive function on behavioral, social, and adaptive aspects to know the real influence of all the cognitive variables on the behavior of girls with FXS.