The intermediate alleles of the fragile X CGG repeat in patients with mental retardation

We have analysed the size of the non-expanded FRAXA CGG repeat in 385 male patients affected by mental retardation and in 182 unrelated normal chromosomes as control. The results show that intermediate alleles with more than 40 repeats were not significantly more frequent in patients than in controls. These data do not corroborate previous findings supporting the idea that intermediate alleles may have a deleterious effect on mental retardation.     

Size bias of fragile X premutation alleles in late-onset movement disorders

Background

Fragile X‐associated tremor/ataxia syndrome (FXTAS), caused by premutation expansions (55–200 CGG repeats) of the FMR1 gene, shares clinical features with other movement disorders, particularly in the domains of gait ataxia, intention tremor and parkinsonism. However, the prevalence of FXTAS within other diagnostic categories is not well defined.

Methods

A meta‐analysis was conducted of all published (n = 14) genetic screens for expanded FMR1 alleles to assess the prevalence and CGG‐repeat size bias of FMR1 premutation alleles in those populations.

Results

In men with late‐onset cerebellar ataxia, the prevalence of premutation alleles (1.5%; 16/1049) was 13 times greater than expected based on its prevalence in the general population (2%; 16/818 for age of onset >50 years; odds ratio 12.4; 95% confidence interval 1.6 to 93.5). Meta‐analysis of CGG‐repeat data for screened patients with premutation alleles shows a shift to larger repeat size than in the general population (p<0.001). 86% (19/22) of premutation alleles were larger than 70 repeats in the patients screened, whereas only approximately 22% of premutation alleles are larger than 70 repeats in the general population.

Conclusions

Expanded FMR1 alleles contribute to cases of late‐onset sporadic cerebellar ataxia, suggesting that FMR1 genetic testing should be carried out in such cases. The biased distribution of FMR1 allele sizes has substantial implications for genetic counselling of carriers with smaller alleles who are at a low risk of developing FXTAS, and suggests that the estimated prevalence of FXTAS among men >50 years of age in the general population may be two to threefold lower than the initial figure of 1 in 3000.     

A premutation that generates a defect at crossing over explains the inheritance of fragile X mental retardation

The view that the Martin-Bell syndrome (X-linked mental retardation with fragile site at Xq27/8) is inherited in a regular X-linked fashion is becoming untenable with the increasing number of reports of transmission through phenotypically normal males. Analysis of the published pedigrees containing such males shows that their heterozygous daughters are never mentally retarded, and have either no fragile site or very few indeed. By contrast, in the next generation, a third of the female heterozygotes are mentally subnormal with an average of 29% fragile sites. These data suggest a premutation that generates the definitive mutation only when transmitted by a female. We propose an inherited sub-microscopic chromosome rearrangement involving the Xq27/8 region that causes no ill effect per se, but generates a significant genetic imbalance when involved in a recombination event with the other X chromosome. This hypothesis explains many of the puzzling genetic aspects of the Martin-Bell syndrome, but it also complicates the interpretation of linkage analysis with genetic markers.     

Anti-Mullerian hormone indicates early ovarian decline in fragile X mental retardation (FMR1) premutation carriers: a preliminary study

BACKGROUND: Women who carry the fragile X mental retardation (FMR1) premutationare at risk for fragile X-associated primary ovarian insufficiency.Past studies have shown that carriers who are still cyclinghave increased levels FSH compared with non-carriers. As anti-Mullerianhormone (AMH) has been shown as an excellent marker of ovariandecline, we examined AMH levels among premutation carriers tocharacterize their ovarian function. METHODS: We determined the level of FSH and AMH in serum samples collectedduring early follicular phase from women who carried longerFMR1 repeat alleles (defined as 70 repeats, n = 40) and thosewith shorter repeat alleles (<70 repeats, n = 75), identifiedby DNA analysis. Comparisons were made stratified by age andcarrier status. RESULTS: For all age groups, AMH levels were significantly lower amonglonger repeat allele carriers compared to shorter repeat allelecarriers (P = 0.002, 0.006 and 0.020 for women ages 18–30,31–40 and 41–50 years, respectively). In contrast,increased FSH indicative of early ovarian decline was only evidentfor longer repeat allele carriers aged 31–40 years (P= 0.089, 0.001 and 0.261 for women ages 18–30, 31–40and 41–50 years, respectively). CONCLUSIONS: These preliminary data suggest that AMH levels indicate earlyovarian decline among women with longer FMR1 repeat alleles;moreover, AMH appears to be a better marker than FSH in identifyingthis early decline.     

氯化锂改善脆性X染色体智力低下基因1敲除小鼠的旷场行为

目的 观察氯化锂是否可以改善脆性X染色体智力低下基因1(FMR1)敲除小鼠的旷场异常行为及探讨其可能机制.方法 4周龄FMR1基因敲除小鼠(KO)和野生型小鼠(WT)各90只,按随机数字法各分为对照组(生理盐水)、氯化锂30、60、90、120、200 mg/kg组共6组,每组15只.氯化锂组小鼠连续5d腹腔注射氯化锂.观察对照组和氯化锂组KO与WT小鼠总运动长度、总跨格次数、中央区活动时间以及中央区进入次数等旷场实验行为的差异.采用免疫印迹观察氯化锂对KO与WT小鼠海马和皮层的糖原合酶激酶(GSK)3β和磷酸化(P)-GSK3β的影响.结果 对照组中KO小鼠的总运动长度、总跨格次数、中央区活动时间、中央区进入次数均比WT小鼠多(均P＜0.05).与对照组比较,氯化锂5个剂量组KO小鼠旷场实验总运动长度、总跨格次数、中央区活动时间、中央区进入次数均有明显减少(均P＜0.05);而WT小鼠用药后总运动长度和中央区活动时间在氯化锂90、120、200 mg/kg剂量组低于对照组(均P＜0.05),总跨格次数在氯化锂达到200 mg/kg剂量时才较对照组减少[(75.73±5.12)次比(125.73±9.24)次,P＜0.05],中央区进入次数则在氯化锂5个剂量组均低于对照组(均P＜0.05).对照组KO小鼠皮层和海马的GSK3β表达量与WT小鼠比较差异没有统计学意义;而P-GSK3β表达量比WT小鼠少(均P＜0.05).氯化锂组KO小鼠皮层和海马GSK3β表达与对照组比较差异没有统计学意义,而皮层P-GSK3β的表达在氯化锂120、200 mg/kg组高于对照组(均P＜0.05),海马P-GSK3β的表达在氯化锂30 ～ 200 mg/kg 5个剂量组均高于对照组(均P＜0.05).氯化锂组WT小鼠皮层和海马GSK3β和P-GSK3β的表达与对照组比较差异均没有统计学意义.使用相同剂量氯化锂的KO小鼠皮层和海马P-GSK3β表达比WT小鼠少(均P＜0.05),而GSK3β表达差异没有统计学意义.结论 氯化锂可以改善FMR1基因敲除小鼠的旷场异常行为,其机制与氯化锂导致的P-GSK3β的表达增加有关.     

Arithmetic difficulties in females with the fragile X premutation

Females with the fragile X full mutation have been reported to have difficulty learning mathematics. Women with the fragile X premutation often give a history of mathematics difficulties in themselves especially with higher level math. In order to evaluate whether women with the premutation have difficulty with math, we asked women with both the fragile X premutation and full mutation to complete the Wide Range Achievement Test-3. For the group of 39 women with the fragile X premutation, the median standard score on the Arithmetic portion was 93, which was significantly lower (P = 0.001) than the median of the standardized norm of 100. Only nine of the women had Arithmetic scores at or above the 50th centile, while over half of the women had standard scores at or above the 50th centile in Reading and Spelling. The eight women with the full mutation also had lower Arithmetic scores than Reading and Spelling scores. These data suggest that mathematics may be an area of relative weakness for the women with the premutation as well as the full mutation. This possibility should be evaluated further by using other measures. This information is important both for counseling purposes and to understand whether a mathematics deficit is evidence of low expression of the FMR1 gene in the premutation state.     

Effect of X inactivation on fragile X frequency and mental retardation

The probability of a heterozygote being affected was estimated from the distribution of frequencies of early-replicating fragile X [fra(X)] chromosome in normal and mentally retarded heterozygotes, taking into account the prior probabilities of 0.35 for mental retardation and 0.65 for normality. The estimated probability of a heterozygote with 100% early-replicating fra(X) being mentally retarded was 78%, which coincides with the value of penetrance in males. Therefore, the manifestation of retardation in females seems to differ from that in males due solely to X inactivation. The frequencies of early-replicating fra(X) were significantly increased among the heterozygotes with the highest frequencies of fra(X) both in the normal group and in the mentally retarded. The mean frequencies of early-replicating fra(X) were 0.42 and 0.68 for normal and mentally retarded heterozygotes, respectively. Considering the overall frequency of retarded heterozygotes as 0.35, the mean frequency of early-replicating fra(X) obtained for all heterozygotes was 0.51, which is in accordance with the hypothesis of random X inactivation. Thus the fragile site appears to have equal chances of being detected when located either on the early- or on the late-replicating X. This leads to the conclusion that the frequency of the fragile site is a consequence of the proportion of cells with the active Martin-Bell syndrome (MBS) gene and not the result of a better visualization of the site on the early-replicating X.     

Prevalence of the fragile X syndrome in Yugoslav patients with non-specific mental retardation

Mutations at two fragile sites, FRAXA and FRAXE, loci are caused by an expansion of a CGG/GCC trinucleotide repeat and are characterized by mental retardation. Here we report molecular screening survey of 97 unrelated individuals diagnosed with non-specific mental retardation (MR), which produced positive test for FRAXA in two boys and none positive for the FRAXE mutation. In addition, we studied allelic frequency distribution for the FRAXA locus in this group of mentally retarded patients, as well as in the 99 healthy subjects of Yugoslav population. The distribution of FMR1 CGG repeat size in both groups was similar: the most common allele contained 29 repeats (32.86% in the healthy population and 54.54% in MR population), followed by the allele with 28 CGG repeats (21.43% in the healthy and 12.2% in MR population). Premutation alleles with more than 45 repeats were not found in control nor in the MR group.     

Effect of the fragile X status categories and the fragile X mental retardation protein levels on executive functioning in males and females with fragile X

The effects of a fragile X disorder on executive function impairment were assessed in 144 extended families, which included individuals with fragile X premutation and full mutation and their relatives without fragile X. A modification of the maximum-likelihood estimators for pedigree data, as well as ordinal logistic regression, were used in data analysis. The most outstanding deficit, occurring especially in males, involved impaired capacity to use an intention to regulate purposeful behavior. This deficit occurred independently of general cognitive impairment but was related to depletion of fragile X mental retardation 1 gene protein product. The other executive function deficits were accounted for by the general cognitive impairment. Possible mechanisms of the effect of fragile X premutation on impairments of executive functioning are considered.     

International workshop on the fragile X and X-linked mental retardation

The erythrocyte osmotic fragility was evaluated on 19 unmedicated subjects with Huntington's disease and 42 individuals at 50% risk, 27 children at 25% risk, and a group of 60 hematologically normal control persons. Five older subjects at 50% risk for Huntington's disease as well as 6 Alzheimer's disease individuals were also evaluated for comparison. The osmotic fragility of fresh and 24-hour incubated red cells was analyzed and a fragility index calculated for each individual. The fragility index for the Huntington's disease group was statistically lower than that of the control group (P < .001) suggesting that the Huntington's disease erythrocytes had a reduced osmotic fragility. In the 50% risk group, 45% of the subjects demonstrated decreased osmotic fragility and 55% had normal fragility. For those subjects in the 25% risk group, 22.2% had decreased fragility and 77.8% had normal fragility. Twenty-seven offspring were evaluated of the 14 persons at 50% risk for Huntington's disease with children; eight of the 14 individuals at 50% risk showed normal fragility and all 16 of their children showed fragility indices with the normal range. The remaining six persons at 50% risk for Huntington's disease had increased erythrocyte fragility and out of their 11 children, five showed normal fragility and six had decreased fragility. These data support the hypothesis of reduced erythrocyte osmotic fragility in individuals affected with and at risk for Huntington disease, and demonstrate the need of further study of the erythrocyte in this complex behavioral genetic disease.     

Linking the fragile X mental retardation protein to the lipoxygenase pathway

Fragile X mental retardation is caused by the absence of the FMRP (fragile X mental retardation protein) a RNA-binding protein encoded by the Fmr1 gene. Despite the large number of studies about this syndrome, it is still unclear how the absence of FMRP affects the physiology of the nervous system. It has been reported however that the brain of the Fmr1-KO mouse shows altered membrane protein and lipid oxidation. There is also indirect evidence that FMRP may be involved in a negative feedback mechanism with metabotropic glutamate receptors (mGluRs). In this article, we will discuss several lines of evidences which tend to prove that the lipoxygenase pathway might be the missing link between FMRP and mGluRs.     

Frequency and stability of the fragile X premutation

Although considered the most common heritable cause of neurodevelopmentaldisability, precise prevalence figures for the FMR1 mutationin the general population are lacking. Since no fragile X premutationalleles have yet been observed to originate from FMR1 alleleswithin the normal size range, there is also little Informationavailable about the origin of the fragile X premutation andmechanisms leading to instability of the FMR1 trinucleotiderepeat region. In this study, 977 genetically unrelated individualsfrom families unselected for mental retardation or fragile Xwere analyzed with Southern blot analysis for the presence ofFMR1 mutations. A subgroup of subjects with evidence of a largeCGG repeat number, and any available relatives, were furtherstudied with PCR to investigate the stability of the trinucleotiderepeat segment of FMR1. One subject had a 75 repeat length whichwas unstable (increased In size) when passed to subsequent generations.This Includes one male descendent who had a premutatlon/fullmutation mosaic pattern. Two other alleles with     

Neurobehavioral phenotype in carriers of the fragile X premutation

There have been contradictory findings in the fragile X (fraX) literature about possible neurocognitive and psychological symptoms due to the fraX premutation (pM). The purpose of the present study was to investigate the relationship between CGG repeat length and neurobehavioral functioning in carriers of the fraX pM. Eighty‐five female carriers of the pM with allele sizes ranging from 59–166 were administered a comprehensive IQ test (WAIS‐III) and completed a questionnaire designed to measure psychopathology (Symptom Checklist (SCL)‐90‐R). No relationship between allele size and cognition was identified. A significant negative relationship between allele size and age was found, as well as a positive relationship between allele size and depression. Follow‐up analyses separating small and large allele sizes (below and above 100 CGG repeats) indicated that individuals with larger allele sizes scored significantly higher on the Interpersonal Sensitivity and Depression subscales of the SCL‐90‐R. Despite the limitation of few individuals with high CGG repeat lengths, our findings suggest that females with larger premutated alleles (≥ 100 repeats) display some clinical manifestations of fraX syndrome. © 2001 Wiley‐Liss, Inc.