Original articleFragile X carrier screening and FMR1 allele distribution in the Japanese population
Introduction
Fragile X syndrome (FXS) has been reported to be the common cause of inherited mental retardation [1]. Clinically, these patients exhibit mental retardation, macroorchidism, large ears and long faces. In most cases, the mental retardation is moderate-to-severe, with frequent occurrences of autistic-like behaviors. Approximately 30% of the individuals with FXS are classified as being within the autistic spectrum [2]. While several reports have suggested there is an association between the FXS and autism, as of yet, no strong evidence has been found that confirms a link to autism [3].
FXS is caused by an expansion of the CGG repeat, which is located in the 5′-untranslated region (5′-UTR) of the first exon of the fragile X mental retardation 1 gene (FMR1) at the chromosomal locus Xq27.3 [4]. The number of CGG repeats is highly polymorphic, ranging from 6 to 50 triplets in normal individuals. The expansions with more than 200 repeats, are observed in fragile X syndrome and are named the full mutation. Full mutation results in hypermethylation of the CpG island within the FMR1 promoter region along with transcriptional silence of the gene. When a premutation (55–200 repeats) is maternally transmitted it can expand to a full mutation. It has been reported that the larger repeats carry greater risks of expansion than the smaller repeats [5]. The intermediate allele (between 40 and 54 repeats) have been shown to be slightly unstable upon transmission [5], [6]. A full mutation in a proband were expanded from an intermediate allele over a span of two generations [7]. The intermediate alleles have been termed ‘gray zone’ alleles [8] and the larger the size the greater the increase in the instability. The American College of Medical Genetics has recommended that intermediate alleles be considered as a possible risk factor for repeat expansion [9]. At the present time, the frequency of the intermediate alleles in the Japanese population remains unknown.
To determine the prevalence of FXS, a previous study examined patients with mental retardation for the full mutation and initially estimated the rate to be 1 for every 4000–6000 males, although this appeared to vary from group to group [10]. More recently, in order to determine more accurate estimations, several studies were performed in the general population and results indicated that 1 out of 113–441 females and 1 out of 813–1674 males were carriers with the premutation alleles [11], [12], [13], [14].
In another study that screened for the full mutation, it was found that there was a lower prevalence of the mutation in Japanese than in other populations [15]. In previous screenings among the normal Japanese population, no premutation allele were found in two different studies, one that examined 824 X chromosomes [16] and one that examined 826 X chromosomes [17]. These results were lower than that observed in Caucasians. Based on these findings, it appears that the prevalence of FXS and allele distribution in Japanese is different from other populations.
In this study, we focused on the CGG repeat length for use in both detecting the intermediate and premutation alleles among the general population. Furthermore, we also analyzed the length of the CGG repeats and their potential involvement for autism in Japanese.
Section snippets
Samples
A total of 946 normal Japanese samples (576 males and 370 females) collected by the Pharma SNP consortium (PSC) were analyzed [18], [19], [20]. PSC control population represents those who voluntarily took part in the project in response to public internet invitation for collecting healthy control population against major illness such as diabetes, hypertension, dementia, cancer, or allergic diseases. Although the socioeconomical and educational condition cannot be specified, they represent
FMR1 allele frequency in the general population
For the FMR1 (CGG)n allelic expansion, we analyzed 946 normal Japanese samples (576 males and 370 females). A total of 1161 alleles, (513 male and 324 female samples), were considered appropriate for amplification of the CGG repeat region of the FMR1 gene. We could not amplify the repeat region from 155 alleles, (63 males and 46 females). Results indicated there were no carriers with an allele for full mutation or premutation. As seen in Table 1 and Fig. 1, all of the detected alleles were
Discussion
In this study, we used the DNA samples from collected by the Pharma SNP consortium (PSC), a DNA Bank. The samples were kept relatively longer than the usual DNA testing. The amplification of the CGG repeat region is sometimes difficult if the sample is not fresh. Only the limited DNA samples were available and we could not amplify the repeat region from 155 alleles.
In previously reported studies, the CGG repeat allele frequencies differed ethnically [16], [22] from the data reported by Arinami
Acknowledgement
We are grateful thank to Professor Mitsuo Oshimura (Department of Biomedical science, Institute of Regenerative Medicine and Biofunction, Graduate School of Medical Science, Tottori University) for having helpful discussion. This work was supported by grants from the Ministry of Health, Labour and Welfare of Japan.
References (29)
- et al.
FMR1 and the fragile X syndrome: human genome epidemiology review
Genet Med
(2001) - et al.
Variation of the CGG repeat at the fragile X site results in genetic instability: resolution of the Sherman paradox
Cell
(1991) - et al.
Expansion of the fragile X CGG repeat in females with premutation or intermediate alleles
Am J Hum Genet
(2003) - et al.
Paternally transmitted FMR1 alleles are less stable than maternally transmitted alleles in the common and intermediate size range
Am J Hum Genet
(2002) - et al.
Technical standards and guidelines for fragile X: the first of a series of disease-specific supplements to the Standards and Guidelines for Clinical Genetics Laboratories of the American College of Medical Genetics
Genet Med
(2001) - et al.
Fragile-X carrier screening and the prevalence of premutation and full-mutation carriers in Israel
Am J Hum Genet
(2001) - et al.
Non-radioactive DNA diagnosis for the fragile X syndrome in mentally retarded Japanese males
Brain Dev
(1995) - et al.
Large-scale single-nucleotide polymorphism (SNP) and haplotype analyses, using dense SNP maps, of 199 drug-related genes in 752 subjects: the analysis of the association between uncommon SNPs within haplotype blocks and the haplotypes constructed with haplotype-tagging SNPs
Am J Hum Genet
(2004) - et al.
Genetic diversity at the FMR1 locus in Mexican population
Arch Med Res
(2005) - et al.
The behavioral phenotype in fragile X: symptoms of autism in very young children with fragile X syndrome, idiopathic autism, and other developmental disorders
J Dev Behav Pediatr
(2001)
The genetics of autism
Pediatrics
Expansion to full mutation of a FMR1 intermediate allele over two generations
Eur J Hum Genet
Familial transmission of the FMR1 CGG repeat
Am J Hum Genet
Prevalence of the fragile X syndrome in African-Americans
Am J Med Genet
Cited by (48)
Association analysis of FMR1 genetic variants and primary ovarian insufficiency in South Indian women with a novel approach of CGG repeats classification
2020, European Journal of Medical GeneticsEpidemiology of fragile X syndrome
2017, Fragile X Syndrome: From Genetics to Targeted TreatmentDistribution of the FMR1 gene in females by race/ethnicity: women with diminished ovarian reserve versus women with normal fertility (SWAN study)
2017, Fertility and SterilityCitation Excerpt :A power analysis was performed assuming initial estimates of n = 110 cases and n = 680 controls and using published female CGG repeat distributions. The reference distribution was defined as a weighted average of the Streuli et al. (8), Bretherick et al. (21), and Otsuka et al. (22) populations' FMR1 distributions. These sample size estimates, which were exceeded for this study for both cases and controls, provided 96% power to detect a statistically significant difference (alpha = 0.05) in the underlying CGG repeat distributions.
Ethnic effect on FMR1 carrier rate and AGG repeat interruptions among Ashkenazi women
2014, Genetics in MedicineSimplified strategy for rapid first-line screening of fragile X syndrome: Closed-tube triplet-primed PCR and amplicon melt peak analysis
2015, Expert Reviews in Molecular MedicineScreening for Fragile X Syndrome Among Filipino Children with Autism Spectrum Disorder
2023, Journal of Autism and Developmental Disorders