Elsevier

Brain and Development

Volume 32, Issue 2, February 2010, Pages 110-114
Brain and Development

Original article
Fragile X carrier screening and FMR1 allele distribution in the Japanese population

https://doi.org/10.1016/j.braindev.2008.12.015Get rights and content

Abstract

Fragile X syndrome (FXS), which is the most common form of familial mental retardation, is caused by the expansion of the CGG repeat in the FMR1 gene on the X chromosome. Previous studies have suggested that as compared to other populations, Japanese have a lower prevalence of FXS. In addition, in the normal population, there are no carriers who have the premutation allele. We analyzed a total of 946 normal Japanese (576 males and 370 females) and attempted to estimate the frequency of the FMR1 allele. Within this population, we found that 1,155 alleles were in the normal range (less than 40 CGG repeats) and had a modal number of 27 repeats (35.75%). No carriers with premutations (55–200 CGG repeats) were observed in this normal population. We also identified six intermediate-sized alleles (40–54 CGG repeats), with a reported incidence of 1 in 103 males and 1 in 324 females. However, this allele frequency was different from that previously reported for the Japanese population. Since data from previous studies has suggested that FXS might possibly be associated with the genetic mechanism of autism, we also analyzed the length of the CGG repeats in 109 autistic patients. In all cases the CGG repeat numbers were within the normal range (16–36 repeats) and no individuals presented with expanded premutation or intermediate alleles. This finding indicates that the length of the CGG repeat within the FMR1 is unlikely to be responsible for autism in Japanese.

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)

  • R. Muhle et al.

    The genetics of autism

    Pediatrics

    (2004)
  • A. Terracciano et al.

    Expansion to full mutation of a FMR1 intermediate allele over two generations

    Eur J Hum Genet

    (2004)
  • S.L. Nolin et al.

    Familial transmission of the FMR1 CGG repeat

    Am J Hum Genet

    (1996)
  • D.C. Crawford et al.

    Prevalence of the fragile X syndrome in African-Americans

    Am J Med Genet

    (2002)
  • Cited by (48)

    • Epidemiology of fragile X syndrome

      2017, Fragile X Syndrome: From Genetics to Targeted Treatment
    • Distribution of the FMR1 gene in females by race/ethnicity: women with diminished ovarian reserve versus women with normal fertility (SWAN study)

      2017, Fertility and Sterility
      Citation 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.

    View all citing articles on Scopus
    View full text