A new insight into fragile X syndrome among Basque population

Fragile X syndrome and fragile X-associated tremor ataxia syndrome

Fragile X-associated disorders encompass several conditions, which are caused by expansion mutations in the fragile X mental retardation 1 (FMR1) gene. Fragile X syndrome is the most common inherited etiology of intellectual disability and results from a full mutation or >200 CGG repeats in FMR1. It is associated with developmental delay, autism spectrum disorder, and seizures. Fragile X-associated tremor/ataxia syndrome is a progressive neurodegenerative disease that occurs in premutation carriers of 55-200 CGG repeats in FMR1 and is characterized by kinetic tremor, gait ataxia, parkinsonism, executive dysfunction, and neuropathy. Fragile X-associated primary ovarian insufficiency also occurs in premutation carrier women and manifests with infertility and early menopause. The diseases constituting fragile X-associated disorders differ mechanistically, due to the distinct molecular properties of premutation versus full mutations. Fragile X syndrome occurs when there is a lack of fragile X mental retardation protein (FMRP) due to FMR1 methylation and silencing. In fragile X-associated tremor ataxia syndrome, a toxic gain of function is postulated with the production of excess CGG repeat-containing FMR1 mRNA, abnormal translation of the repeat sequence leading to production of polyglycine, polyalanine, and other polypeptides and to outright deficits in translation leading to reduced FMRP at larger premutation sizes. The changes in underlying brain chemistry due to FMR1 mutations have led to therapeutic studies in these disorders, with some progress being made in fragile X syndrome. This paper also summarizes indications for testing, genetic counseling issues, and what the future holds for these disorders.

Fragile X spectrum disorders

The fragile X mental retardation 1 gene (FMR1), which codes for the fragile X mental retardation 1 protein (FMRP), is located at Xp27.3. The normal allele of the FMR1 gene typically has 5 to 40 CGG repeats in the 5' untranslated region; abnormal alleles of dynamic mutations include the full mutation (> 200 CGG repeats), premutation (55-200 CGG repeats) and the gray zone mutation (45-54 CGG repeats). Premutation carriers are common in the general population with approximately 1 in 130-250 females and 1 in 250-810 males, whereas the full mutation and Fragile X syndrome (FXS) occur in approximately 1 in 4000 to 1 in 7000. FMR1 mutations account for a variety of phenotypes including the most common monogenetic cause of inherited intellectual disability (ID) and autism (FXS), the most common genetic form of ovarian failure, the fragile X-associated primary ovarian insufficiency (FXPOI, premutation); and fragile X-associated tremor/ataxia syndrome (FXTAS, premutation). The premutation can also cause developmental problems including ASD and ADHD especially in boys and psychopathology including anxiety and depression in children and adults. Some premutation carriers can have a deficit of FMRP and some unmethylated full mutation individuals can have elevated FMR1 mRNA that is considered a premutation problem. Therefore the term "Fragile X Spectrum Disorder" (FXSD) should be used to include the wide range of overlapping phenotypes observed in affected individuals with FMR1 mutations. In this review we focus on the phenotypes and genotypes of children with FXSD.

Distribution of AGG interruption patterns within nine world populations

The CGG trinucleotide repeat within the FMR1 gene is associated with multiple clinical disorders, including fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X syndrome. Differences in the distribution and prevalence of CGG repeat length and of AGG interruption patterns have been reported among different populations and ethnicities. In this study we characterized the AGG interruption patterns within 3,065 normal CGG repeat alleles from nine world populations including Australia, Chile, United Arab Emirates, Guatemala, Indonesia, Italy, Mexico, Spain, and United States. Additionally, we compared these populations with those previously reported, and summarized the similarities and differences. We observed significant differences in AGG interruption patterns. Frequencies of longer alleles, longer uninterrupted CGG repeat segments and alleles with greater than 2 AGG interruptions varied between cohorts. The prevalence of fragile X syndrome and FMR1 associated disorders in various populations is thought to be affected by the total length of the CGG repeat and may also be influenced by the AGG distribution pattern. Thus, the results of this study may be important in considering the risk of fragile X-related conditions in various populations.

In the Gray Zone in the Fragile X Gene: What are the Key Unanswered Clinical and Biological Questions?

Smaller expansions (41–54 CGG repeats) in the fragile X mental retardation 1 (FMR1) gene are termed “gray zone” alleles. Only recently has interest in these expansions increased due to reporting of phenotypes unique to gray zone carriers or similar to those seen in individuals with larger expansions. As minimal research has focused on gray zone expansions, this paper asks several questions related to this topic. These include the following: What is the definition of the gray zone? Is there a risk of developing neurological signs in these carriers? Are there secondary gene effects that impact gray zone alleles or a biologic advantage to carrying these repeats? How do we counsel patients with gray zone expansions? The answers to these questions will help to determine the significance of these expansions and provide needed information to the research community and clinicians.

Study of FMR1 gene association with ovarian dysfunction in a sample from the Basque Country

Premature ovarian failure (POF) is defined as cessation of menses before the age of 40. The most significant single gene associated with POF is the Fragile X Mental Retardation 1 gene (FMR1). In the present work we screened women with fertility problems from the Basque Country in order to determine, whether in these women, FMR1 CGG repeat size in the intermediate and premutation range was associated with their pathology, and whether intermediate and premutation carriers had endocrine signs of diminished ovarian function, using the most established measure of ovarian reserve, the gonadotropin FSH. A patient sample of 41 women with ovarian insufficiency and a control sample of 32 women with no fertility problems from the Basque Country were examined. The patient sample was classified into three categories according to the results of the retrospective assessment of their ovarian function. In group 2 of patients, women with irregular cycles, reduced fecundity and FSH levels ≥ 10IU/l, there is a significant increase in the number of intermediate and premutation FMR1 alleles (35-54 CGG repeats). In group 3 of patients, women with amenorrhea for at least four consecutive months and FSH levels ≥ 10IU/l, a significant increase in the number of intermediate FMR1 alleles (35-54 CGG repeats) was found in patients compared with controls. In this group all the patients had a serum concentration > 40 IU/l. The results suggest that in the analysed Basque sample the FMR1 gene has a role in the aetiology of POF. However, elevated FSH levels are more related to the menstrual cycle pattern than to the CGG repeat size.

Prevalence of CGG expansions of the FMR1 gene in a US population-based sample

The primary goal of this study was to calculate the prevalence of the premutation of the FMR1 gene and of the "gray zone" using a population-based sample of older adults in Wisconsin (n = 6,747 samples screened). Compared with past research, prevalence was relatively high (1 in 151 females and 1 in 468 males for the premutation and 1 in 35 females and 1 in 42 males for the gray zone as defined by 45-54 CGG repeats). A secondary study goal was to describe characteristics of individuals found to have the premutation (n = 30, 7 males and 23 females). We found that premutation carriers had a significantly higher rate of divorce than controls, as well as higher rates of symptoms that might be indicative of fragile X-associated tremor ataxia syndrome (FXTAS; numbness, dizziness/faintness) and fragile X primary ovarian insufficiency (FXPOI; age at last menstrual period). Although not statistically significant, premutation carriers were twice as likely to have a child with disability.

Single nucleotide polymorphism and FMR1 CGG repeat instability in two Basque valleys

Fragile X Syndrome (FXS, MIM 309550) is mainly due to the expansion of a CGG trinucleotide repeat sequence, found in the 5' untranslated region of the FMR1 gene. Some studies suggest that stable markers, such as single nucleotide polymorphisms (SNPs) and the study of populations with genetic identity, could provide a distinct advance to investigate the origin of CGG repeat instability. In this study, seven SNPs (WEX28 rs17312728:G>T, WEX70 rs45631657:C>T, WEX1 rs10521868:A>C, ATL1 rs4949:A>G, FMRb rs25707:A>G, WEX17 rs12010481:C>T and WEX10 ss71651741:C>T) have been analyzed in two Basque valleys (Markina and Arratia). We examined the association between these SNPs and the CGG repeat size, the AGG interruption pattern and two microsatellite markers (FRAXAC1 and DXS548). The results suggest that in both valleys WEX28-T, WEX70-C, WEX1-C, ATL1-G, and WEX10-C are preferably associated with cis-acting sequences directly influencing instability. But comparison of the two valleys reveals also important differences with respect to: (1) frequency and structure of "susceptible" alleles and (2) association between "susceptible" alleles and STR and SNP haplotypes. These results may indicate that, in Arratia, SNP status does not identify a pool of susceptible alleles, as it does in Markina. In Arratia valley, the SNP haplotype association reveals also a potential new "protective" factor.

Fragile X CGG repeat variation in Tamil Nadu, South India: a comparison of radioactive and methylation-specific polymerase chain reaction in CGG repeat sizing

Fragile X syndrome is the most frequent hereditary cause of mental retardation after Down syndrome. Expansion of CGG repeats in the 5' UTR of the fragile X mental retardation gene 1 (FMR1) causes gene inactivation in most of the cases. The FMR1 gene is classified into normal 5-44; gray zone 45-54; premutation 55 to <200; and full mutation ≥ 00 repeats. Precise sizing of FMR1 alleles is important to understand their variation, predisposition, and for genetic counseling. Meta-analysis reveals prevalence of premutation carriers as 1 in 259. No such reports are available in India. About 705 women from Tamil Nadu, South India, were screened for the FMR1 allelic variation by using radioactive polymerase chain reaction-polyacrylamide gel electrophoresis (PAGE) analysis. The women who were homozygous by radioactive polymerase chain reaction (rPCR) were reanalyzed by methylation-specific polymerase chain reaction (Ms-PCR) and GeneScan analysis. The techniques were validated and compared to arrive at a correction factor. Among 122 genotypes, 35 repeat variants ranging in size from 16 to 57 were observed. The most common repeat is 30 followed by 29. One in 353 women carried the premutation. No full mutations were observed. Screening populations with low frequency of premutations may not be applicable. Ms-PCR is more suitable for routine screening and clinical testing compared with rPCR-PAGE analysis.

Analysis of the Fragile X Trinucleotide Repeat in Basques: Association of Premutation and Intermediate Sizes, Anchoring AGGs and Linked Microsatellites with Unstable Alleles

Fragile X Syndrome (FXS) is associated with an unstable CGG repeat sequence in the 5’ untranslated region in the first exon of the FMR1 gene which resides at chromosome position Xq27.3 and is coincident with the fragile site FRAXA. The CGG sequence is polymorphic with respect to size and purity of the repeat. Interpopulation variation in the polymorphism of the FMR1 gene and consequently, in the predisposition to FXS due to the prevalence of certain unstable alleles has been observed. Spanish Basque population is distributed among narrow valleys in northeastern Spain with little migration between them until recently. This characteristic may have had an effect on allelic frequency distributions. We had previously reported preliminary data on the existence of FMR1 allele differences between two Basque valleys (Markina and Arratia). In the present work we extended the study to Uribe, Gernika, Durango, Goierri and Larraun, another five isolated valleys enclosing the whole area within the Spanish Basque region. We analyzed the prevalence of FMR1 premutated and intermediate/grey zone alleles. With the aim to complete the previous investigation about the stability of the Fragile X CGG repeat in Basque valleys, we also analyzed the existence of potentially unstable alleles, not only in relation with size and purity of CGG repeat but also in relation with DXS548 and FRAXAC1 haplotypes implicated in repeat instability. The data show that differences in allele frequencies as well as in the distribution of the mutational pathways previously identified are present among Basques. The data also suggest that compared with the analyzed Basque valleys, Gernika had increased frequency of susceptibility to instability alleles, although the prevalence of premutation and intermediate/grey zone alleles in all the analyzed valleys was lower than that reported in Caucasian populations.

Inbreeding coefficients for X-linked and autosomal genes in consanguineous marriages in Spanish populations: the case of Guipúzcoa (Basque Country)

Inbreeding patterns over the past two centuries have been studied more extensively in Spain and Italy than anywhere else in Europe. Consanguinity studies in mainland Spain have shown that populations settled along the Cantabrian cornice share inbreeding patterns that distinguish them from other populations further south. A visual representation of spatial variations of two key inbreeding variables is presented here for the first time via contour maps. This paper also analyzes time trends of mean inbreeding coefficients for X-linked (F(x)) and autosomal genes (F) (1862-1995) together with variations in F(x)/F ratios in Guipúzcoa, the most autochthonous Spanish Basque province. Because close cousin marriages are a mark of identity of the study population, we evaluated the contribution of uncle-niece/aunt-nephew (M12) and first cousin (M22) marriages to F(x) and F values and compared the frequencies of M12 and M22 pedigree subtypes and their corresponding F(x)/F ratios to those found in other Spanish populations. The mean Fx and F inbreeding levels in Guipúzcoa for the 134-year period analyzed were 1.51 x 10(-3) and 1.04 x 10(-3), respectively, and the F(x)/F ratio was seen to be very stable over time. Our findings show that major similarities exist for close consanguineous marriage subtypes between Basque and non-Basque Spanish populations, despite significant geographic variability in terms of first cousin pedigrees. The distortion seems to be caused by Guipúzcoa. The F(x)/F ratios for first cousins in Spanish populations were higher than expected (1.25), with values ranging from 1.34 to 1.48. The findings of the present study may be useful for advancing knowledge on the effects of the interaction between biology and culture and for exploring associations between mating patterns and the prevalence of certain diseases.

AGG interspersion analysis of the FMR1 CGG repeats in mental retardation of unspecific cause

OBJECTIVES

To study the AGG interspersion pattern in mentally retarded patients of unspecified cause.

METHODS

FMR1 CGG substructure in 104 normal and 232 mentally retarded (MR) males was determined by CGG repeat and AGG interspersion analyses. Genomic DNA of the study subjects was obtained for PCR and Southern hybridization analyses.

RESULTS

All study subjects had less than 53 CGG repeats and none had fragile X syndrome of mental retardation. There was a significant difference (P < 0.006) in the AGG interspersion pattern. MR males had (1) more variable internal substructures, (2) proportionally less 2 and 3 AGG but more 0 and 1 AGG, less (CGG)(9)AGG(CGG)(9)AGG(CGG)(9) but more (CGG)(9)AGG(CGG)(19) alleles and (3) a longer pure 3' CGG repeat.

CONCLUSIONS

Our results suggest that the MR alleles have a lesser number of interspersed AGG and a longer pure 3' CGG repeat than the normal population. They are thus more prone to instability and expansion to long repeat lengths as in the fragile X syndrome of mental retardation.