CGG Triplet Research Articles

Ovarian reserve in patients with FMR1 gene premutation and the role of fertility preservation

IntroductionWomen with premutation (PM) of the FMR1 gene may suffer from reduced ovarian reserve or even premature ovarian insufficiency (POI). We studied hormonal and ultrasound ovarian reserve, fertility and fertility preservation outcomes in these patients. Patients and methodRetrospective cohort study of 63 female FMR1 premutation carriers. ResultsSixty-three female patients bearing an FMR1 premutation were included. Median age was 30 years [26.5–35]. Median number of CGG triplets was 83 [77.2–92]. Before diagnosis of PM, 19 women (30%) had had in all 35 pregnancies, resulting in 20 births, including 7 affected children. After diagnosis of PM, 17 women (26.1%) had in all 23 pregnancies, at a median age of 34.5 years [32.2–36.0]: 2 after pre-implantation genetic diagnosis, 3 after oocyte donation, 18 spontaneously, and 5 ending in medical termination for fragile X syndrome. Thirty-three patients (52.4%) had POI diagnosis (median age, 30 years [27–34]) with median FSH level 84 IU/L [50.5–110] and median AMH level 0.08ng/mL [0.01–0.19]. After POI diagnosis, 8 women had in all 9 pregnancies: 3 following oocyte donation, and 6 spontaneous in 5 women (15.1%). Eight of the 9 pregnancies resulted in a live birth (including 2 affected children) and 1 in medical termination for trisomy 13. The median age of the 30 patients without POI was 31 years [25.2–35.0].Thirteen women (20.6%) underwent fertility preservation, at a median age of 29 years [24–33]: FSH 7.7 IU/L [6.8–9.9], AMH 1.1ng/mL [0.95–2.1], antral follicle count 9.5 [7.7–14.7]. A median 15 oocytes [10–26] were cryopreserved in a median 2 cycles [1–3]. At the time of writing, no oocytes had yet been thawed for in-vitro fertilization. ConclusionsThis study shows the importance of early fertility preservation after diagnosis of FMR1 premutation in women, due to early deterioration of ovarian reserve. Genetic counseling is essential in these patients, as spontaneous pregnancies are not uncommon, even in cases of impaired ovarian reserve, and can lead to birth of affected children.

Conformational and dynamic properties of the KH1 domain of FMRP and its fragile X syndrome linked G266E variant

The Fragile X messenger ribonucleoprotein (FMRP) is a multi-domain protein involved in interactions with various macromolecules, including proteins and coding/non-coding RNAs. The three KH domains (KH0, KH1 and KH2) within FMRP are recognized for their roles in mRNA binding. In the context of Fragile X syndrome (FXS), over-and-above CGG triplet repeats expansion, three specific point mutations have been identified, each affecting one of the three KH domains (R138QKH0, G266EKH1, and I304NKH2) resulting in the expression of non-functional FMRP. This study aims to elucidate the molecular mechanism underlying the loss of function associated with the G266EKH1 pathological variant. We investigate the conformational and dynamic properties of the isolated KH1 domain and the two KH1 site-directed mutants G266EKH1 and G266AKH1. Employing a combined in vitro and in silico approach, we reveal that the G266EKH1 variant lacks the characteristic features of a folded domain. This observation provides an explanation for functional impairment observed in FMRP carrying the G266E mutation within the KH1 domain, as it renders the domain unable to fold properly. Molecular Dynamics simulations suggest a pivotal role for residue 266 in regulating the structural stability of the KH domains, primarily through stabilizing the α-helices of the domain. Overall, these findings enhance our comprehension of the molecular basis for the dysfunction associated with the G266EKH1 variant in FMRP.

Fragile X Syndrome in children.

Fragile X syndrome is caused by the expansion of CGG triplets in the FMR1 gene, which generates epigenetic changes that silence its expression. The absence of the protein coded by this gene, FMRP, causes cellular dysfunction, leading to impaired brain development and functional abnormalities. The physical and neurologic manifestations of the disease appear early in life and may suggest the diagnosis. However, it must be confirmed by molecular tests. It affects multiple areas of daily living and greatly burdens the affected individuals and their families. Fragile X syndrome is the most common monogenic cause of intellectual disability and autism spectrum disorder; the diagnosis should be suspected in every patient with neurodevelopmental delay. Early interventions could improve the functional prognosis of patients with Fragile X syndrome, significantly impacting their quality of life and daily functioning. Therefore, healthcare for children with Fragile X syndrome should include a multidisciplinary approach.

Fragile X syndrome: clinical presentation, pathology and treatment

Fragile X syndrome is the monogenetic condition that produces more cases of autism and intellectual disability. The repetition of CGG triplets (> 200) and their methylation entail the silencing of the FMR1 gene. The FMRP protein (product of the FMR1 gene) interacts with ribosomes by controlling the translation of specific messengers, and its loss causes alterations in synaptic connectivity. Screening for fragile X syndrome is performed by polymerase chain reaction. Current recommendation of the American Academy of Pediatrics is to test individuals with intellectual disability, global developmental retardation or with a family history of presence of the mutation or pre-mutation. Hispanic countries such as Colombia, Chile and Spain report high prevalence of fragile X syndrome and have created fragile X national associations or corporations that seek to bring patients closer to available diagnostic and treatment networks.

8. Optical genome mapping analysis of FMR1 expansions in fragile X syndrome

Fragile X syndrome (FXS) is associated with cognitive impairment and intellectual disability. Nearly all cases of FXS are associated with an expansion of a CGG triplet repeat in the FMR1 gene. Phenotype severity is often correlated with the expansion size; thus, accurate sizing of the CCG repeat array is crucial. The repetitive nature of these regions presents difficulties: 1. PCR is unable to traverse through long repeats; 2. sequencing has limited read lengths; 3. Southern blot may be inaccurate, time consuming, and expensive. Optical genome mapping (OGM) has the potential to address some of these shortcomings. OGM utilizes images of ultra-long DNA molecules, labeled at specific sequence motifs linearized in nanochannel arrays and can be used for structural and copy number variation calling and absence of heterozygosity analysis. Bionano Genomics has developed a targeted analysis workflow for analysis of FMR1. To evaluate the capability of measuring repeat arrays consistent with normal (<45 repeats), premutation (55-200 repeats), and full mutation (>200 repeats), we analyzed the FMR1 repeats in 75 samples. We also analyzed the repeat region on 20 phenotypically normal control subjects. We observed alleles consistent with annotation across the entire range of repeat counts. Analytical sensitivity was measured at 97% with 100% PPV for expansions >200 repeats. The largest expansion we detected was ∼1000 repeats. In the control samples, we measured repeat counts below the full mutation cutoff in all cases. Repeatability studies were carried out to show analytical consistency and to establish minimum molecule quality requirements. EnFocus™ analysis report provides pass/fail for QC metrics for input data quality as well as analytic measurement quality using internal control regions of the genome on each autosome. OGM performance for FMR1 repeat lengths analyzed here show a much higher dynamic range compared to PCR and higher precision compared to Southern blot.

Potential Drug Therapy For Fragile X Tremor/Ataxia Syndrome

Fragile X‐associated tremor/ataxia syndrome (FXTAS) is a late‐onset neurological disorder that affects carriers of the alleles for the Fragile X gene, Fragile X Mental Retardation (FMR1) due to the CGG triplet repeat expansion within the 5’UTR of FMR1. Clinical symptoms that are likely to occur are progressive ataxia, intention tremor, intranuclear inclusions, and Parkinsonian‐like motor symptoms. The goal of this research is to assess the biological efficacy of Curcumin, Muscadine Grape Seed Extract (MSKE) on the neuroprotective capacity with respect to mitochondrial damage, and their ability to restore mitochondrial health in patient with FXTAS. To establish mitochondrial dysfunction, normal human cell lines and human‐induced pluripotent cells were treated with multiple concentrations of glucose/ glucose oxidase (GluOx) at 2,12,and 24 hour time points to induce varying intensities of oxidative stress. The degrees of oxidative stress were measured by apoptosis and mitochondrial reactive oxygen species (ROS) production. Curcumin and MSKE, compounds effective against oxidative damage in mitochondria, were used to rescue glucose oxidase induced oxidative damage in both cell lines. To test the ability of these drugs to restore mitochondrial health, cell viability and cellular superoxide production were assessed by propidium iodide and the MitoSox fluorescence assay, respectively. We anticipated that GluOx at varying concentrations and time points would proportionally increase levels of apoptosis and mitochondrial ROS, reflective of mitochondrial damage, with the most severe dysfunction occurring at a dose of 25 nM and the longest duration of 24‐hr exposure. Administration of MSKE in concentrations ranging from 10‐8 to 10‐5 M in half log increments, did not reverse the oxidative defects induced in the cell lines. However, curcumin concentrations increased cell viability at the 2, 12, and 24 hour time period. Results indicate that the research design should be modified by increasing concentration of both glucose and MSKE to provide a reliable test of the hypothesis. Although, the research design will be modified to better assessed the hypothesis, the overall research is translational in that it lends flexibility to testing therapeutics in neuronal FXTAS models and expands the discovery of mitochondrial markers for the syndrome.

Clinical and Molecular Correlates of Abnormal Changes in the Cerebellum and Globus Pallidus in Fragile X Premutation.

BackgroundFragile X premutation carriers (55–200 CGG triplets) may develop a progressive neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS), after the age of 50. The neuroradiologic markers of FXTAS are hyperintense T2-signals in the middle cerebellar peduncle—the MCP sign. We recently noticed abnormal T2-signals in the globus pallidus in male premutation carriers and controls but the prevalence and clinical significance were unknown.MethodsWe estimated the prevalence of the MCP sign and pallidal T2-abnormalities in 230 male premutation carriers and 144 controls (aged 8–86), and examined the associations with FXTAS symptoms, CGG repeat length, and iron content in the cerebellar dentate nucleus and globus pallidus.ResultsAmong participants aged ≥45 years (175 premutation carriers and 82 controls), MCP sign was observed only in premutation carriers (52 vs. 0%) whereas the prevalence of pallidal T2-abnormalities approached significance in premutation carriers compared with controls after age-adjustment (25.1 vs. 13.4%, p = 0.069). MCP sign was associated with impaired motor and executive functioning, and the additional presence of pallidal T2-abnormalities was associated with greater impaired executive functioning. Among premutation carriers, significant iron accumulation was observed in the dentate nucleus, and neither pallidal or MCP T2-abnormalities affected measures of the dentate nucleus. While the MCP sign was associated with CGG repeat length >75 and dentate nucleus volume correlated negatively with CGG repeat length, pallidal T2-abnormalities did not correlate with CGG repeat length. However, pallidal signal changes were associated with age-related accelerated iron depletion and variability and having both MCP and pallidal signs further increased iron variability in the globus pallidus.ConclusionsOnly the MCP sign, not pallidal abnormalities, revealed independent associations with motor and cognitive impairment; however, the occurrence of combined MCP and pallidal T2-abnormalities may present a risk for greater cognitive impairment and increased iron variability in the globus pallidus.

Recurrent missense variant in the nuclear export signal of FMR1 associated with FXS-like phenotype including intellectual disability, ASD, facial abnormalities

Fragile X syndrome (FXS; MIM 300624) is an X-linked genetic disorder characterized by physical abnormalities associated with intellectual disability and a wide spectrum of neurological and psychiatric impairments. FXS occurs more frequently in males, 1 in 5000 males and 1 in 8000 females accounting for 1–2% of overall intellectual disability (ID). In more than 99% of patients, FXS results from expansions of a CGG triplet repeat (>200 in male) of the FMR1 gene. In the last years an increasing number, albeit still limited, of FXS subjects carrying FMR1 mutations including deletions, splicing errors, missense, and nonsense variants was reported. Nevertheless, the studies concerning the functional consequences of mutations in the FMR1 gene are rare so far and, therefore, we do not have sufficient knowledge regarding the genotype/phenotype correlation. We report a child carrying a hemizygous missense FMR1 (NM_002024.5:c.1325G > A p.Arg442Gln) variant, maternally inherited, associated with facial abnormalities, developmental delay, and social and communication deficits assessed with formal neuropsychological tests. The study contributes to highlighting the clinical differences between the CGG triplet repeat dependent phenotype and FMR1variant dependent phenotype and it also confirms the pathogenicity of the variant being reported for the second time in the literature.

The cellular functions of G-quadruplex in neurological diseases

G-quadruplex (G4) is a unique nucleic acid structure that formed when a four-stranded structure is produced within a single-stranded guanine-rich sequence. Four guanine molecules form a square planar arrangement, termed G-quartet, which are stacked on top of each other to form the G4 structure in DNA (G4DNA) and in RNA (G4RNA). Recent studies have revealed that G4DNA and G4RNA are folded in cells, which suggested their biological and pharmacological significance in DNA replication, transcription, epigenetic modification, and RNA metabolism. So far, we have reported the following; 1) G4 is a target of cognitive function therapy for ATR-X intellectual disability syndrome, in which mutations are found in a G4 binding protein ATRX. 2) G4 is formed in heterochromatin depending on neuronal development. 3) G4 promotes prionoids in a CGG triplet repeat disease, Fragile X-associated tremor/ataxia syndrome (FXTAS). 4) 5-aminolevulinic acid is a potential candidate drug for treating some neurological diseases through the G4 binding ability. In this review, we summarized the significant roles of G4 in neurological diseases.

Altered expression of fragile X mental retardation-1 (FMR1) in the thymus in autoimmune myasthenia gravis

Predisposition to autoimmunity and inflammatory disorders is observed in patients with fragile X-associated syndromes. These patients have increased numbers of CGG triplets in the 5’ UTR region of FMR1 (Fragile X Mental Retardation 1) gene, that affects its expression. FMR1 is decreased in the thymus of myasthenia gravis (MG) patients, a prototypical autoimmune disease. We thus analyzed the number of CGG triplets in FMR1 in MG, and explored the regulatory mechanisms affecting thymic FMR1 expression. We measured the number of CGGs using thymic DNA from MG and controls, but no abnormalities in CGGs were found in MG that could explain thymic decrease of FMR1. We next analyzed by RT-PCR the expression of FMR1 and its transcription factors in thymic samples, and in thymic epithelial cell cultures in response to inflammatory stimuli. In control thymuses, FMR1 expression was higher in males than females, and correlated with CTCF (CCCTC-binding factor) expression. In MG thymuses, decreased expression of FMR1 was correlated with both CTCF and MAX (Myc-associated factor X) expression. Changes in FMR1 expression were supported by western blot analyses for FMRP. In addition, we demonstrated that FMR1, CTCF and MAX expression in thymic epithelial cells was also sensitive to inflammatory signals. Our results suggest that FMR1 could play a central role in the thymus and autoimmunity. First, in relation with the higher susceptibility of females to autoimmune diseases. Second, due to the modulation of its expression by inflammatory signals that are known to be altered in MG thymuses.

OBSERVED OUTCOMES OF FMR1 PGT-M ANALYSIS WITH INCORPORATION OF CGG REPEAT EXPANSION CAN LEAD TO EXTRA EMBRYOS SUITABLE FOR TRANSFER

Highlight utility of triplet repeat expansion for the identification of embryos with the abnormal FMR1 allele that may be considered for transfer following preimplantation genetic testing (PGT) FMR1-related disorders, which includes Fragile X syndrome. Trophectoderm biopsies from US-based IVF clinics were sent to CooperGenomics for PGT-M for FMR1-related disorders between January 2018 and February 2021. Samples underwent linkage analysis via Karyomapping (lllumina, USA) and triplet repeat size determination. When requested, screening for aneuploidy (PGT-A) via next generation sequencing (NGS) was completed in conjunction with PGT-M. Patients with FMR1 expansions on both alleles and those with a full expansion (>200 repeats) on one allele were removed from the study population. A total of 1130 blastocysts from 318 IVF cycles underwent triplet repeat size determination and linkage analysis (average 3.55 embryos per cycle). The average age of patient at time of egg retrieval was 34.5 years. 51.4% (581/1130) of embryos were suitable to transfer based on presence of the normal FMR1 allele via linkage analysis. When factoring in PGT-A, 27.7% of the study population were euploid embryos with the normal allele. 48.6% (549/1130) of embryos tested had the abnormal FMR1 allele detected via linkage analysis. Of embryos inheriting the abnormal allele, 30.6% of embryos had experienced an expansion to yield an abnormal allele with >200 repeats (168/549). This expansion places these embryos at increased risk of expressing a FMR1-related disorder. Of these higher risk embryos, 55.4% were male. Importantly, 381 blastocyst samples (69.4%) inheriting the abnormal allele did not experience clinically significant expansion, with repeats remaining <200. 53% (202) of these lower risk embryos further screened as euploid by PGT-A, yielding an additional 17.9% (202/1130) embryos that could be considered for transfer. 49.25% (99/201) were female euploid carriers and 50.74% were male euploid carriers (102/201). Preimplantation testing for monogenic diseases (PGT-M) is classically considered a binary linkage-based test for the determination of the allele present in an at-risk embryo. For triplet repeat conditions like FMR1-related disorders, quantifying the length of the CGG triplet repeat tract inside the FMR1 gene modifies classification of disease risk in the embryo. This results in more embryos available for transfer, which is an important factor to consider in a population where ovarian failure is a common factor associated with carrier status. Knowledge of embryo repeat size is an intangible resource for genetic counseling about which embryos can be considered for transfer. Providing linkage + triplet repeat analysis is a diagnostic advantage for more complete classification of embryo status.

Mosaicism in Fragile X syndrome: A family case series.

Fragile X syndrome (FXS) has a classic phenotype, however its expression can be variable among full mutation males. This is secondary to variable methylation mosaicisms and the number of CGG triplet repeats in the non-coding region of the Fragile X Mental Retardation 1 (FMR1) gene, producing a variable expression of the Fragile X Mental Retardation Protein (FMRP). Here we report a family with several individuals affected by FXS: a boy with a hypermethylated FMR1 mutation and a classic phenotype; a man with an FMR1 gene mosaicism in the range of premutation (PM) and full mutation (FM), who has a mild phenotype due to which FXS was initially disregarded; and the cases of four women with a FM and mosaicism. This report highlights the importance of DNA molecular testing for the diagnosis of FXS in patients with developmental delay, intellectual disability and/or autism due to the variable phenotype that occurs in individuals with FMR1 mosaicisms.

CGG repeat RNA G-quadruplexes interact with FMRpolyG to cause neuronal dysfunction in fragile X-related tremor/ataxia syndrome.

Fragile X-related tremor/ataxia syndrome (FXTAS) is a neurodegenerative disease caused by CGG triplet repeat expansions in FMR1, which elicit repeat-associated non-AUG (RAN) translation and produce the toxic protein FMRpolyG. We show that FMRpolyG interacts with pathogenic CGG repeat-derived RNA G-quadruplexes (CGG-G4RNA), propagates cell to cell, and induces neuronal dysfunction. The FMRpolyG polyglycine domain has a prion-like property, preferentially binding to CGG-G4RNA. Treatment with 5-aminolevulinic acid, which is metabolized to protoporphyrin IX, inhibited RAN translation of FMRpolyG and CGG-G4RNA-induced FMRpolyG aggregation, ameliorating aberrant synaptic plasticity and behavior in FXTAS model mice. Thus, we present a novel therapeutic strategy to target G4RNA prionoids.

Association of Allelic Variants of the Reelin Gene with Autistic Spectrum Disorder: A Systematic Review and Meta-Analysis of Candidate Gene Association Studies.

Autistic spectrum disorder (ASD) is a complex neurodevelopmental disability with a genetic basis, and several studies have suggested a potential role of the reelin gene (RELN) in ASD susceptibility. Accordingly, genetic association studies have explored this potential association, but the results have been controversial thus far. For this reason, we assessed the association of four genetic variants of RELN (the 5′UTR CGG triplet repeat and polymorphisms rs736707, rs362691, and rs2229864) with ASD by means of a systematic review and meta-analysis. We retrieved studies comparing the distribution of the above-mentioned genetic variants between ASD patients and healthy controls. A meta-analysis was conducted using a random effects model, and calculations of the odds ratios (ORs) and confidence intervals (CIs) were performed. A sensitivity analysis and tests to determine the heterogeneity of the results were also performed. Eleven previous studies fulfilled the inclusion criteria and analyzed the association of the above-mentioned genetic variants and ASD. We did not find any significant association between the allele or genotype frequencies of the analyzed polymorphisms and ASD, and large heterogeneity was found for the rs736707 polymorphism. Moreover, no significant differences were found between the 5′UTR triplet repeat and this disorder. In light of current evidence, no single genetic variant within this gene is clearly associated with the development of ASD, and ethnic differences may explain part of the observed heterogeneity. Larger studies among different ethnic groups are needed to establish the role of specific genetic variants within RELN in the etiology of this disorder.

Characterization of the Metabolic, Clinical and Neuropsychological Phenotype of Female Carriers of the Premutation in the X-Linked FMR1 Gene.

The X-linked FMR1 premutation (PM) is characterized by a 55–200 CGG triplet expansion in the 5′-untranslated region (UTR). Carriers of the PM were originally thought to be asymptomatic; however, they may present general neuropsychiatric manifestations including learning disabilities, depression and anxiety, among others. With age, both sexes may also develop the neurodegenerative disease fragile X-associated tremor/ataxia syndrome (FXTAS). Among carriers, females are at higher risk for developing immune disorders, hypertension, seizures, endocrine disorders and chronic pain, among others. Some female carriers younger than 40 years old may develop fragile X-associated primary ovarian insufficiency (FXPOI). To date, no studies have addressed the metabolic footprint – that includes mitochondrial metabolism – of female carriers and its link to clinical/cognitive manifestations. To this end, we performed a comprehensive biochemical assessment of 42 female carriers (24–70 years old) compared to sex-matched non-carriers. By applying a multivariable correlation matrix, a generalized bioenergetics impairment was correlated with diagnoses of the PM, FXTAS and its severity, FXPOI and anxiety. Intellectual deficits were strongly correlated with both mitochondrial dysfunction and with CGG repeat length. A combined multi-omics approach identified a down-regulation of RNA and mRNA metabolism, translation, carbon and protein metabolism, unfolded protein response, and up-regulation of glycolysis and antioxidant response. The suboptimal activation of the unfolded protein response (UPR) and endoplasmic-reticulum-associated protein degradation (ERAD) response challenges and further compromises the PM genetic background to withstand other, more severe forms of stress. Mechanistically, some of the deficits were linked to an altered protein expression due to decreased protein translation, but others seemed secondary to oxidative stress originated from the accumulation of either toxic mRNA or RAN-derived protein products or as a result of a direct toxicity of accumulated metabolites from deficiencies in critical enzymes.

FMRP ribonucleoprotein complexes and RNA homeostasis.

The Fragile Mental Retardation 1 gene (FMR1), at Xq27.3, encodes the fragile mental retardation protein (FMRP), and displays in its 5'-untranslated region a series of polymorphic CGG triplet repeats that may undergo dynamic mutation. Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability among men, and is most frequently due to FMR1 full mutation and consequent transcription repression. FMR1 premutations may associate with at least two other clinical conditions, named fragile X-associated primary ovarian insufficiency (FXPOI) and tremor and ataxia syndrome (FXTAS). While FXPOI and FXTAS appear to be mediated by FMR1 mRNA accumulation, relative reduction of FMRP, and triplet repeat translation, FXS is due to the lack of the RNA-binding protein FMRP. Besides its function as mRNA translation repressor in neuronal and stem/progenitor cells, RNA editing roles have been assigned to FMRP. In this review, we provide a brief description of FMR1 transcribed microsatellite and associated clinical disorders, and discuss FMRP molecular roles in ribonucleoprotein complex assembly and trafficking, as well as aspects of RNA homeostasis affected in FXS cells.

Fragile X syndrome: clinical presentation, pathology and treatment.

Fragile X syndrome is the monogenetic condition that produces more cases of autism and intellectual disability. The repetition of CGG triplets (> 200) and their methylation entail the silencing of the FMR1 gene. The FMRP protein (product of the FMR1 gene) interacts with ribosomes by controlling the translation of specific messengers, and its loss causes alterations in synaptic connectivity. Screening for fragile X syndrome is performed by polymerase chain reaction. Current recommendation of the American Academy of Pediatrics is to test individuals with intellectual disability, global developmental retardation or with a family history of presence of the mutation or premutation. Hispanic countries such as Colombia, Chile and Spain report high prevalence of fragile X syndrome and have created fragile X national associations or corporations that seek to bring patients closer to available diagnostic and treatment networks.

Fragile X associated tremor/ataxia syndrome: its clinical presentation, pathology, and treatment

The fragile X associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disease associated with the repetition of CGG triplets (55-200 CGG repetitions) in the FMR1 gene. The premutation of the FMR1 gene, contrasting with the full mutation (more than 200 CGG repetitions), presents an increased production of messenger and a similar or slightly decreased production of FMRP protein. FXTAS affects 40% of men and 16% of women carriers of the premutation. It presents with a wide constellation of neurological signs such as intention tremor, cerebellar ataxia, parkinsonism, executive function deficits, peripheral neuropathy and cognitive decline leading to dementia among others. In this review, we present what is currently known about the molecular mechanism, the radiological findings and the pathology, as well as the complexity of the diagnosis and management of FXTAS.

Increased Calcium Influx through L-type Calcium Channels in Human and Mouse Neural Progenitors Lacking Fragile X Mental Retardation Protein.

SummaryThe absence of FMR1 protein (FMRP) causes fragile X syndrome (FXS) and disturbed FMRP function is implicated in several forms of human psychopathology. We show that intracellular calcium responses to depolarization are augmented in neural progenitors derived from human induced pluripotent stem cells and mouse brain with FXS. Increased calcium influx via nifedipine-sensitive voltage-gated calcium (Cav) channels contributes to the exaggerated responses to depolarization and type 1 metabotropic glutamate receptor activation. The ratio of L-type/T-type Cav channel expression is increased in FXS progenitors and correlates with enhanced progenitor differentiation to glutamate-responsive cells. Genetic reduction of brain-derived neurotrophic factor in FXS mouse progenitors diminishes the expression of Cav channels and activity-dependent responses, which are associated with increased phosphorylation of the phospholipase C-γ1 site within TrkB receptors and changes of differentiating progenitor subpopulations. Our results show developmental effects of increased calcium influx via L-type Cav channels in FXS neural progenitors.

FXS-Like Phenotype in Two Unrelated Patients Carrying a Methylated Premutation of the FMR1 Gene

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.