Cognitive and behavioral performance among FMR1 high‐repeat allele carriers surveyed from special education classes

The fragile X syndrome is caused by an unstable CGG repeat sequence in the 5′ untranslated region of the X‐linked, FMR1 gene. When the number of repeats exceeds 200, the region is hypermethylated and the gene is silenced. The lack of the protein produced by the FMR1 gene, FMRP, causes the fragile X syndrome. Recent evidence suggests that FMR1 alleles with unmethylated long repeat tracks (40–200 repeats) may cause a specific somatic phenotype in women, premature ovarian failure, and may cause variation in the levels of FMR1 mRNA and FMRP. Because FMR1 is known to be involved in the regulation of subset of genes expressed in the brain, we investigated the variation in cognitive and/or behavioral performance among carriers of high repeat alleles. Specifically, we administered cognitive, behavioral, and adaptive performance tests to children identified with high repeat alleles who attended special education classes in Atlanta, Georgia public schools and to those with < 40 repeats drawn from the same population. Overall, we found no significant effect of repeat size and the psychometric measures in our test battery after adjustment for multiple comparisons. All scales were found to be within 1 SD standard deviation of the mean. We did find an intriguing, albeit marginally statistically significant, association in the cognitive profile among males and not females, consistent with an X‐linked effect. After adjusting for the overall cognitive abilities score, Verbal Ability scores decreased and Nonverbal Reasoning scores increased with repeat number to a greater extent in males than females. Spatial Ability scores were not associated with repeat number. © 2002 Wiley‐Liss, Inc.     

Psychological status in female carriers of premutation FMR1 allele showing a complex relationship with the size of CGG expansion

We utilized a sample of 299 adult females aged between 19 and 86 years, carrying fragile X mental retardation (FMR1) alleles with small CCG expansions ranging from 50 to 141 repeats to analyse the relationships between psychological symptoms as assessed by the Symptom Checklist‐90‐Revised (SCL‐90‐R) and the size of the CGG repeat in the FMR1 gene. There were highly significant (negative) correlations between the size of the CGG repeat and a great majority of SCL‐90‐R subscale scores and all the global indices, suggesting that carriers of premutations in the mid‐size CGG repeat range may be at greatest risk for the development of psychiatric disorder.     

Social anxiety and autism spectrum traits among adult FMR1 premutation carriers

Behavioral symptoms and traits have been proposed as early markers in neurodegenerative diseases. The aim of this study was to evaluate social anxiety and autism in FMR1 premutation carriers using the Social Phobia Inventory (SPIN) and the Autism‐Spectrum Quotient (AQ) questionnaires. Fifty‐nine premutation carriers were compared with 50 controls. The SPIN test showed statistically significant differences between female but not male carriers. The AQ questionnaire found statistically significant differences between premutation carriers and controls in the total AQ as well as in the social skills and attention switching subdomains. A gender effect was only observed for the social skills subdomain. Spearman's correlation analysis revealed a moderately positive correlation with the total AQ scores as well as the social skills and communication subdomains. Our results show that fragile X‐associated tremor/ataxia syndrome (FXTAS) patients have higher AQ scores. Moreover, this is the first study to find statistically significant differences between FXTAS and no‐FXTAS premutation carriers in the communication and the imagination subdomains, suggesting that FXTAS patients present a broader autistic phenotype than premutation carriers without FXTAS. Based on our results, a wide range of behavioral/psychiatric traits should be included within the broader phenotypic presentation of individuals with the FMR1 premutation.     

CGG repeat in the FMR1 gene: size matters

The FMR1 gene contains a CGG repeat present in the 5'-untranslated region which can be unstable upon transmission to the next generation. The repeat is up to 55 CGGs long in the normal population. In patients with fragile X syndrome (FXS), a repeat length exceeding 200 CGGs (full mutation: FM) generally leads to methylation of the repeat and the promoter region, which is accompanied by silencing of the FMR1 gene. The absence of FMR1 protein, FMRP, seen in FM is the cause of the mental retardation in patients with FXS. The premutation (PM) is defined as 55-200 CGGs. Female PM carriers are at risk of developing primary ovarian insufficiency. Elderly PM carriers might develop a progressive neurodegenerative disorder called fragile X-associated tremor/ataxia syndrome (FXTAS). Although arising from the mutations in the same gene, distinct mechanisms lead to FXS (absence of FMRP), FXTAS (toxic RNA gain-of-function) and FXPOI. The pathogenic mechanisms thought to underlie these disorders are discussed. This review gives insight on the implications of all possible repeat length categories seen in fragile X families.     

A family with two female siblings with compound heterozygous FMR1 premutation alleles

Premutation alleles (55–200 CGG repeats) of the fragile X mental retardation (FMR1) gene have been linked to various types of clinical involvement ranging from mood and anxiety disorders to immunological disorders and executive function deficits. Carrier females typically have a premutation allele and a normal allele (<55 CGG repeats). Although rare, seven cases of females that carry two expanded alleles (compound heterozygous premutation) have been reported. Here, we report on four members of a family including two compound heterozygous premutation sisters with similar CGG allele sizes, affected with different levels of clinical severity.     

Fragile X syndrome: An overview and update of the FMR1 gene

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability and the leading form of the monogenic cause of autism. Fragile X mental retardation type 1 (FMR1) gene premutation is the first single‐gene cause of primary ovarian failure (Fragile X‐associated primary ovarian insufficiency [FXPOI]) and one of the most common causes of ataxia (fragile X‐associated tremor/ataxia syndrome [FXTAS]), multiple additional phenotypes such as fibromyalgia, hypothyroidism, migraine headaches, sleep disturbances, sleep apnea, restless legs syndrome, central pain syndrome, neuropathy and neuropsychiatric alterations has been described. Clinical involvement in men and women carrying the FMR1 premutation currently constitutes a real health problem in the society that should be taken into account. It is important to highlight that while in FXS there is a loss‐of‐function of the FMR1 gene, in premutation associated disorders there is a gain of FMR1 mRNA function. To date, the tremendous progress achieved in the understanding of the pathophysiology of FXS, has led to the development of several targeted therapies aimed at preventing or improving the neurological manifestations of the disease. This review is an update of the diseases associated with the FMR1 gene.     

Allele distribution at the FMR1 locus in the general Chinese population

Fragile X syndrome is an important disease of hereditary mental retardation. Its prevalence in the Chinese population is not clear. We amplified FMR1 CGG repeats from male newborns' blood spots. Approximately 45% of the males had 28 CGG repeats and another 19% had 29 repeats. Besides this major peak, there was a second peak at 34 and 35 repeats. From the 1000 males studied, 3 were found to have repeat numbers in the high borderline range (each with 50, 52 and 53 repeats). This result provides a low but significant risk of fragile X syndrome in the Chinese population.     

Small CGG repeat expansion alleles of FMR1 gene are associated with parkinsonism

Fragile X-associated tremor/ataxia syndrome (FXTAS) affects older males carrying premutation, that is, expansions of the CGG repeat (in the 55–200 range), in the FMR1 gene. The neurological changes are linked to the excessive FMR1 messenger RNA (mRNA), becoming toxic through a 'gain-of-function'. Because elevated levels of this mRNA are also found in carriers of the smaller expansion (grey zone) alleles, ranging from 40 to 54 CGGs, we tested for a possible role of these alleles in the origin of movement disorders associated with tremor.
We screened 228 Australian males affected with idiopathic Parkinson's disease and other causes of parkinsonism recruited from Victoria and Tasmania for premutation and grey zone alleles. The frequencies of either of these alleles were compared with the frequencies in a population-based sample of 578 Guthrie spots from consecutive Tasmanian male newborns (controls). There was a significant excess of premutation carriers (Fisher's exact test p = 0.006). There was also a more than twofold increase in grey zone carriers in the combined sample of the Victorian and Tasmanian cases, with odds ratio (OR ) = 2.36, and 95% confidence intervals (CI): 1.20–4.63, as well as in Tasmanian cases only (OR = 2.33, 95% CI: 1.06–5.13), compared with controls. The results suggest that the FMR1 grey zone alleles, as well as premutation alleles, might contribute to the aetiology of disorders associated with parkinsonism.     

Paternal retraction of a fragile X allele to normal size,showing normal function over two generations

The FMR1 premutation (PM:55-199 CGG) is associated with fragile X-associated tremor/ataxia syndrome (FXTAS) and when maternally transmitted is at risk of expansion to a hypermethylated full mutation (FM: ≥ 200 CGG) that causes fragile X syndrome (FXS). We describe a maternally transmitted PM (77 CGG) that was passed to a son (103 CGG), and to a daughter (220–1822 CGG), who were affected with FXTAS and FXS, respectively. The male with the PM showed low-level mosaicism for normal size of 30 and 37 CGG. This male had two offspring: one female mosaic for PM and FM (56, 157, >200 CGG) and another with only a 37 CGG allele detected in multiple tissues, neither with a clinical phenotype. The female with the 37 CGG allele showed normal levels of FMR1 methylation and mRNA and passed this 37 CGG allele to one of her daughters, who was also unaffected. These findings show that post-zygotic paternal retraction can lead to low-level mosaicism for normal size alleles, with these normal alleles being functional when passed over two generations.     

Tissue-specific methylation differences in a fragile X premutation carrier

Methylation of a premutation was found in a small percentage of blood cells in a male premutation carrier for the FMR1 mutation. To investigate the inter-tissue heterogeneity and possible clinical implications of this finding, fibroblast cells from the subject were also studied. Although the premutation size was found to be the same in leukocytes and fibroblasts, the methylation pattern was different. In cultured fibroblasts, the premutation was completely unmethylated, as is typical of premutations, whereas methylation of the premutation was detected in a small percentage of lymphocytes. However, the change in methylation did not affect the FMR1 protein (FMRP) expression, as immunocytochemical analysis of FMRP performed on cultured skin fibroblasts and a blood smear revealed normal levels of expression in both tissues.     

Expansions and contractions of the FMR1 CGG repeat in 5,508 transmissions of normal,intermediate, and premutation alleles

Instability of the FMR1 repeat, commonly observed in transmissions of premutation alleles (55–200 repeats), is influenced by the size of the repeat, its internal structure and the sex of the transmitting parent. We assessed these three factors in unstable transmissions of 14/3,335 normal (~5 to 44 repeats), 54/293 intermediate (45–54 repeats), and 1561/1,880 premutation alleles. While most unstable transmissions led to expansions, contractions to smaller repeats were observed in all size classes. For normal alleles, instability was more frequent in paternal transmissions and in alleles with long 3′ uninterrupted repeat lengths. For premutation alleles, contractions also occurred more often in paternal than maternal transmissions and the frequency of paternal contractions increased linearly with repeat size. All paternal premutation allele contractions were transmitted as premutation alleles, but maternal premutation allele contractions were transmitted as premutation, intermediate, or normal alleles. The eight losses of AGG interruptions in the FMR1 repeat occurred exclusively in contractions of maternal premutation alleles. We propose a refined model of FMR1 repeat progression from normal to premutation size and suggest that most normal alleles without AGG interruptions are derived from contractions of maternal premutation alleles.     

不同民族人群FMR1基因CGG重复序列检测与分析

目的 检测和分析脆性X综合征致病基因FMR1 CGG重复序列在汉族和壮族人群中的多态性分布。方法 采用PCR扩增技术和聚丙烯酰胺凝胶电泳技术对1060例汉族人（男280人，女780人）和283例壮族人（男85人，女198人）FMR1基因CGG重复序列进行分析，并用Southern blot技术对结果进行了验证。结果 汉族人群中共检测到33种等位基因，其中CGG重复序列范围为6～43，壮族人群共检测到27种等位基因，CGG重复序列范围为6～57，两类人群中最大频率等位基因分别为28和29。结论 应用PCR扩增技术进行了脆性X综合征大面积筛查，中国汉族和壮族人群中FMR1基因CGG重复序列变异分布略有差异。     

Frequency of FMR1 premutation carriers and rate of expansion to full mutation in a retrospective diagnostic FMR1 Korean sample

Detection of female premutation (PM) carriers of fragile X syndrome may be important in that a PM allele from the mother can expand to a full mutation (FM) when transmitted to the fetus. Although the PM carrier frequency might be different in varying populations, there is a little data on the Korean population. Furthermore, the risks of expansion to FM have not been studied in Korean PM carriers. In this retrospective study, we estimated the female PM carrier frequency and the risks of expansion to FM in Korean diagnostic samples collected for FMR1 gene testing. Of 10,241 pre‐conceptional or pregnant women, 13 PM [1 in 788; 95% confidence interval (CI), 1/1 250–1/455] and 75 intermediate allele carriers (1 in 137; 95% CI, 1/172–1/110) were identified. In 26 prenatal diagnoses cases, the PM allele was transmitted to the fetus in 13 pregnancies (50%), and five of these expanded to FM. All of the maternal alleles exceeding 70 repeats expanded to FM. In conclusion, the PM frequency in Korean diagnostic samples was lower than that reported in Western populations, while the risk for FM expansion in alleles exceeding 70 repeats might be higher than expected based upon previous reports.     

Clinical phenotypes of a juvenile sibling pair carrying the fragile X premutation

Individuals with alleles containing 55-200 CGG repeats in the fragile X mental retardation (FMR1) gene are premutation carriers. The premutation allele has been shown to lead to a number of types of clinical involvement, including shyness, anxiety, social deficits, attention deficit hyperactivity disorder (ADHD), and executive function deficits. Some of these problems could be due to mild deficits of the fragile X protein (FMRP) and a possible developmental effect of the elevated FMR1 mRNA observed in carriers. In addition, two abnormal phenotypes specific to the premutation have been described. Primary ovarian insufficiency (FXPOI), defined by cessation of menses prior to age 40, occurs in 20% of females with the premutation. The other phenotype, fragile X-associated tremor/ataxia syndrome (FXTAS), affects some older adult premutation carriers. Premutation females typically have one expanded allele (≥55 CGG repeats) and one normal allele (≤54 CGG repeats). This study describes the cognitive, behavioral, and molecular profile of a female with two alleles in the premutation range (60 and 67 CGG repeats) in comparison to her brother with a similar premutation size (65 CGG repeats). Both exhibited high IQ scores, anxiety, and some physical features associated with fragile X syndrome. This comparison allows us to examine the effect of the premutation in this male-female pair while controlling for environmental and background genetic factors.     

Penetrance of FMR1 premutation associated pathologies in fragile X syndrome families

Within the past few years, there has been a significant change in identifying and characterizing the FMR1 premutation associated phenotypes. The premutation has been associated with elevated FMR1 mRNA levels and slight to moderate reductions in FMRP levels. Furthermore, it has been established that ∼20% of female premutation carriers present primary ovarian insufficiency (POI) and that fragile X-associated tremor/ataxia syndrome (FXTAS) occurs in one-third of all male premutation carriers older than 50 years. Besides POI and FXTAS, new disorders have recently been described among individuals (especially females) with the FMR1 premutation. Those pathologies include thyroid disease, hypertension, seizures, peripheral neuropathy, and fibromyalgia. However there are few reports related to FXTAS penetrance among female premutation carriers or regarding these disorders recently associated to the FMR1 premutation. Therefore, we have evaluated 398 fragile X syndrome (FXS) families in an attempt to provide an estimation of the premutation associated phenotypes penetrance. Our results show that signs of FXTAS are detected in 16.5% of female premutation carriers and in 45.5% of premutated males older than 50 years. Furthermore, among females with the FMR1 premutation, penetrance of POI, thyroid disease and chronic muscle pain is 18.6, 15.9 and 24.4%, respectively. The knowledge of this data might be useful for accurate genetic counselling as well as for a better characterization of the clinical phenotypes of FMR1 premutation carriers.