Distribution of AGG interruption patterns within nine world populations

FMR1 allelic complexity in premutation carriers provides no evidence for a correlation with age at amenorrhea

BACKGROUND

Premutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene, defined as between 55 and 200 CGGs, have been implicated in fragile X-associated primary ovarian insufficiency (FXPOI). Only 20% of female premutation carriers develop early ovulatory dysfunction, the reason for this incomplete penetrance is unknown. This study validated the mathematical model in premutation alleles, after assigning each allele a score representing allelic complexity. Subsequently, allelic scores were used to investigate the impact of allele complexity on age at amenorrhea for 58 premutation cases (116 alleles) previously published.

METHODS

The allelic score was determined using a formula previously described by our group. The impact of each allelic score on age at amenorrhea was analyzed using Pearson's test and a contour plot generated to visualize the effect.

RESULTS

Correlation of allelic score revealed two distinct complexity behaviors in premutation alleles. No significant correlation was observed between the allelic score of premutation alleles and age at amenorrhea. The same lack of significant correlation was observed regarding normal-sized alleles, despite a nearly significant trend.

CONCLUSIONS

Our results suggest that the use of allelic scores combination have the potential to explain female infertility, namely the development of FXPOI, or ovarian dysfunction, despite the lack of correlation with age at amenorrhea. Such a finding is of great clinical significance for early identification of females at risk of ovulatory dysfunction, enhancement of fertility preservation techniques, and increasing the probability for a successful pregnancy in females with premutations. Additional investigation is necessary to validate this hypothesis.

Fragile X syndrome in the largest world clustering. I. Genetic epidemiology and founder effect outline

The FMR1 5' regulation gene region harbors a CGG trinucleotide repeat expansion (CGG-TRE) that causes Fragile X syndrome (FXS) when it expands to more than 200 repetitions. Ricaurte is a small village in southwestern Colombia, with an FXS prevalence of 1 in 38 men and 1 in 100 women (~100 times higher than the worldwide reported prevalence), defining Ricaurte as the largest FXS cluster in the world. In the present study, using next-generation sequencing of whole exome capture, we genotype 55 individuals from Ricaurte (49 with either full mutation or with premutation), four individuals from neighboring villages (with either the full mutation or with the premutation), and one unaffected woman, native of Ricaurte, who did not belong to any of the affected families. With advanced clustering and haplotype reconstruction, we modeled a common haplotype of 33 SNPs spanning 83,567,899 bp and harboring the FMR1 gene. This reconstructed haplotype was found in all the men from Ricaurte who carried the expansion, demonstrating that the genetic conglomerate of FXS in this population is due to a founder effect. The definition of this founder effect and its population outlining will allow a better prediction, follow-up, precise and personalized characterization of epidemiological parameters, better knowledge of the disease's natural history, and confident improvement of the clinical attention, life quality, and health interventions for this community.

Refining reproductive risk for FMR1 premutation carriers in the general obstetric population

Female FMR1 premutation (FMR1 PM) carriers for fragile X syndrome (FXS) are at risk to have a child with FXS based on their CGG repeat size and AGG interruption number. Studies examining this risk in unselected populations of female PM carriers are lacking. This retrospective cohort study analyzed carrier status, CGG repeat length, AGG interruption result, and reproductive risk refinement in a population of female patients who underwent routine carrier screening for FXS. A total of 1536 PM carriers (0.43%) were identified, 95% of whom had between 55 and 90 CGG repeats. A number of 1334 carriers underwent AGG interruption testing. The majority had at least one AGG interruption and received a lower reproductive risk for FXS following AGG interruption testing (89% and 85%, respectively) as compared to their risk calculated based on CGG repeat size alone. The average change in risk across the population following AGG interruption testing was -3.4%, with a range from -50.8% to 48.9%. This article describes the range of CGG repeats and AGG interruptions in an unselected population of female PM carriers and suggests that most carriers would benefit from AGG interruption testing to refine their reproductive risk of having a child with FXS.

Diagnostic profile of the AmplideX Fragile X Dx and Carrier Screen Kit for diagnosis and screening of fragile X syndrome and other FMR1-related disorders

Introduction: In 2009, a novel, CGG repeat primed FMR1 PCR assay was designed with primers flanking the triplet repeat region, as well as a third chimeric primer complementary to the (CGG)n repeat, that was capable of amplifying alleles throughout the repeat range. This assay for the first time allowed consistent detection of large full mutation alleles with PCR, resolution of heterozygosity in females and mapping of AGG interspersions.Areas Covered: The AmplideX Fragile X Dx and Carrier Screen Kit (Asuragen, Inc.) represents a refined assay that underwent validation with sensitivity analyses for FDA approval. Single-site precision, analytical sensitivity and specificity, limit of detection and diagnostic performance were assessed in comparison to reference methods at three independent sites. Single-site precision across all genotype categories showed 100% agreement at 20 ng input across multiple operators, days, instruments and kit lots. Compared to Southern Blot analysis, the overall percent agreement was over 98% for all expanded alleles.Expert Opinion: Limitations include no methylation assessment and hard to see full mutation peaks in some mosaic samples, but overall the assay is considered a highly accurate and time-efficient assay for FMR1 allele size determination.

Development and Validation of a Mathematical Model to Predict the Complexity of FMR1 Allele Combinations

The polymorphic trinucleotide repetitive region in the FMR1 gene 5'UTR contains AGG interspersions, particularly in normal-sized alleles (CGG < 45). In this range repetitive stretches are typically interrupted once or twice, although alleles without or with three or more AGG interspersions can also be observed. AGG interspersions together with the total length of the repetitive region confer stability and hinder expansion to pathogenic ranges: either premutation (55 < CGG < 200) or full mutation (CGG > 200). The AGG interspersions have long been identified as one of the most important features of FMR1 repeat stability, being particularly important to determine expansion risk estimates in female premutation carriers. We sought to compute the combined AGG interspersion numbers and patterns, aiming to define FMR1 repetitive tract complexity combinations. A mathematical model, the first to compute this cumulative effect, was developed and validated using data from 131 young and healthy females. Plotting of their allelic complexity enabled the identification of two statistically distinct groups - equivalent and dissimilar allelic combinations. The outcome, a numerical parameter designated allelic score, depicts the repeat substructure of each allele, measuring the allelic complexity of the FMR1 gene including the AGGs burden, thus allowing new behavioral scrutiny of normal-sized alleles in females.

Premature ovarian failure and tissue engineering

Premature ovarian failure (POF) usually happens former to the age of 40 and affects the female physiological state premenopausal period. In this condition, ovaries stop working long before the expected menopausal time. Of diagnostic symptoms of the disease, one can mention amenorrhea and hypoestrogenism. The cause of POF in most cases is idiopathic; however, cancer therapy may also cause POF. Commonly utilized therapies such as hormone therapy, in-vitro activation, and regenerative medicine are the most well-known treatments for POF. Hence, these therapies may be associated with some complications. The aim of the present study is to discuss the beneficial effects of tissue engineering for fertility rehabilitation in patients with POF as a newly emerging therapy.

The role of AGG interruptions in the FMR1 gene stability: A survey in ethnic groups with low and high rate of consanguinity

Background

The prevalence and the role of AGG interruptions within the FMR1 gene in the normal population is unknown. In this study, we investigated the frequent of AGG loss, in one or two alleles within the normal population. The role of AGG in the FMR1 stability has been assessed by correlating AGG loss to the prevalence of premutation/full mutation in two ethnic groups differing in their consanguinity rate: high versus low consanguinity rate (HCR vs. LCR).

Methods

The CGG repeat allele size and AGG presence were measured in 6,865 and 6,204 females belonging to the LCR (5%) and HCR (>45%) groups, respectively, by Tripled‐Primed‐PCR technique.

Results

A lower prevalence of the premutation was observed in the HCR (1:158) as compared to the LCR group (1:128). No full mutation was found in the HCR females while in the LCR group the prevalence found was 1:1,149. Homozygosity rate was higher in the HCR population compared to the LCR group.The overall AGG loss was higher in the HCR population than in the LCR and increased with increased CGG repeat number in both ethnic groups.

Conclusions

Although we observed a significantly higher rate of homozygosity and AGG loss in the HCR group, this did not affect the prevalence of the premutation and full mutation in this population. Their prevalence was significantly lower than in the LCR population. Finally, we discuss whether the loss of AGG could be also a polymorphic event but not only a stabilizing factor.

Mutational mechanism for DAB1 (ATTTC)n insertion in SCA37: ATTTT repeat lengthening and nucleotide substitution

Dynamic mutations by microsatellite instability are the molecular basis of a growing number of neuromuscular and neurodegenerative diseases. Repetitive stretches in the human genome may drive pathogenicity, either by expansion above a given threshold, or by insertion of abnormal tracts in nonpathogenic polymorphic repetitive regions, as is the case in spinocerebellar ataxia type 37 (SCA37). We have recently established that this neurodegenerative disease is caused by an (ATTTC)_n insertion within an (ATTTT)_n in a noncoding region of DAB1. We now investigated the mutational mechanism that originated the (ATTTC)_n insertion within an ancestral (ATTTT)_n . Approximately 3% of nonpathogenic (ATTTT)_n alleles are interspersed by AT-rich motifs, contrarily to mutant alleles that are composed of pure (ATTTT)_n and (ATTTC)_n stretches. Haplotype studies in unaffected chromosomes suggested that the primary mutational mechanism, leading to the (ATTTC)_n insertion, was likely one or more T>C substitutions in an (ATTTT)_n pure allele of approximately 200 repeats. Then, the (ATTTC)_n expanded in size, originating a deleterious allele in DAB1 that leads to SCA37. This is likely the mutational mechanism in three similar (TTTCA)_n insertions responsible for familial myoclonic epilepsy. Because (ATTTT)_n tracts are frequent in the human genome, many loci could be at risk for this mutational process.

FXS-Like Phenotype in Two Unrelated Patients Carrying a Methylated Premutation of the FMR1 Gene

Fragile X syndrome (FXS) is mostly caused by two distinct events that occur in the FMR1 gene (Xq27.3): an expansion above 200 repeats of a CGG triplet located in the 5′UTR of the gene, and methylation of the cytosines located in the CpG islands upstream of the CGG repeats. Here, we describe two unrelated families with one FXS child and another sibling presenting mild intellectual disability and behavioral features evocative of FXS. Genetic characterization of the undiagnosed sibling revealed mosaicism in both the CGG expansion size and the methylation levels in the different tissues analyzed. This report shows that in the same family, two siblings carrying different CGG repeats, one in the full-mutation range and the other in the premutation range, present methylation mosaicism and consequent decreased FMRP production leading to FXS and FXS-like features, respectively. Decreased FMRP levels, more than the number of repeats seem to correlate with the severity of FXS clinical phenotypes.

Detecting AGG Interruptions in Females With a FMR1 Premutation by Long-Read Single-Molecule Sequencing: A 1 Year Clinical Experience

The fragile X syndrome arises from the FMR1 CGG expansion of a premutation (55–200 repeats) to a full mutation allele (>200 repeats) and is the most frequent cause of inherited X-linked intellectual disability. The risk for a premutation to expand to a full mutation allele depends on the repeat length and AGG triplets interrupting this repeat. In genetic counseling it is important to have information on both these parameters to provide an accurate risk estimate to women carrying a premutation allele and weighing up having children. For example, in case of a small risk a woman might opt for a natural pregnancy followed up by prenatal diagnosis while she might choose for preimplantation genetic diagnosis (PGD) if the risk is high. Unfortunately, the detection of AGG interruptions was previously hampered by technical difficulties complicating their use in diagnostics. Therefore we recently developed, validated and implemented a new methodology which uses long-read single-molecule sequencing to identify AGG interruptions in females with a FMR1 premutation. Here we report on the assets of AGG interruption detection by sequencing and the impact of implementing the assay on genetic counseling.

Prenatal Diagnosis of Fragile X: Can a Full Mutation Allele in the FMR1 Gene Contract to a Normal Size?

Here we describe a case of a prenatal diagnosis of a male fetus that inherited the unstable allele from his full mutation mosaic mother (29, 160, >200 CGG repeats) reduced to a normal size range (19 CGG repeats). Haplotype analysis showed that the fetus 19 CGG repeats allele derived from the maternal unstable allele which was inherited from his maternal grandmother. No size mosaicism was detected by testing the DNA from in vitro cultured samples, including seventh passage culture as well as from two amniocentesis samples. Sequence analysis confirmed that the allele was 19 CGG repeats long. Methylation assay showed no methylation. Although none of the techniques used in this study can provide with absolute certainty the diagnosis, the results strongly indicate the presence in the fetus of an allele with a CGG repeat number in the normal range. Because this is a prenatal diagnosis case, the crucial question is whether the 19 CGG allele derived from the maternal unstable expanded allele, which contracted to the normal range, became a normal stable allele or a normal unstable allele which could expand in the next generation. It is also possible that allele size mosaicism of the FMR1 gene that went undetected exists. Because this is a prenatal diagnosis case, we cannot with certainty exclude the presence of an undetected expanded allele of the FMR1 gene, in addition to the 19 CGG allele derived from an unstable expanded allele, which contracted to the normal range.

A general description for Chinese medicine in treating premature ovarian failure

Premature ovarian failure (POF) is a kind of gynecological disease that causes amenorrhea, infertility, menopause and urogenital symptoms. Currently hormone replacement therapy (HRT) is the most popular choice for women with POF to get rid of menopausal syndrome. However, as the popularization of Chinese herbs made Chinese medicine (CM) shine new lights, physicians are able to treat POF with both meno-herbs and integrated therapy. HRT has its own indications and contraindications. For example, unexplained vaginal bleeding, acute liver damage, liver dysfunction, vascular embolization, and breast cancer are all contraindications of HRT, and CM is taken by more physicians as an adjuvant therapy. This review, including a range of common Chinese herbs and formulations according to the existing literature, provides a general description of CM treating POF from the aspects of mechanisms and clinical application. It also highlights acupuncture as a unique physiotherapy for POF. Although the validity of CM has been supported by the evidence of many preclinical trials, clinical trials and meta-analysis, the adverse events with CM therapy still exist and no guarantee has been made for its safety. This review concludes the updated information for CM treating POF contributing to further studies.

[Triplet expansion cytosine-guanine-guanine: Three cases of OMIM syndrome in the same family]

INTRODUCTION

The dynamic increase in the number of triplet repeats of cytosine-guanine-guanine (CGG) in the FMR1 gene mutation is responsible for three OMIM syndromes with a distinct clinical phenotype: Fragile X syndrome (FXS) and two pathologies in adult carriers of the premutation (55-200 CGG repeats): Primary ovarian insufficiency (FXPOI) and tremor-ataxia syndrome (FXTAS) associated with FXS.

CLINICAL OBSERVATION AND METHODS

CGG mutation dynamics of the FMR1 gene were studied in DNA samples from peripheral blood from the index case and other relatives of first, second and third degree by TP-PCR, and the percentage methylation.

RESULTS

Diagnosis of FXS was confirmed in three patients (21.4%), eight patients (57.1%) were confirmed in the premutation range transmitters, one male patient with full mutation/permutation mosaicism (7.1%) and two patients (14.3%) with normal study. Of the eight permutated patients, three had FXPOI and one male patient had FXTAS.

DISCUSSION

Our study suggests the importance of making an early diagnosis of SXF in order to carry out a family study and genetic counselling, which allow the identification of new cases or premutated patients with FMR1 gene- associated syndromes (FXTAS, FXPOI).

Unique AGG Interruption in the CGG Repeats of the FMR1 Gene Exclusively Found in Asians Linked to a Specific SNP Haplotype

Fragile X syndrome (FXS) is the most common inherited intellectual disability. It is caused by the occurrence of more than 200 pure CGG repeats in the FMR1 gene. Normal individuals have 6-54 CGG repeats with two or more stabilizing AGG interruptions occurring once every 9- or 10-CGG-repeat blocks in various populations. However, the unique (CGG)6AGG pattern, designated as 6A, has been exclusively reported in Asians. To examine the genetic background of AGG interruptions in the CGG repeats of the FMR1 gene, we studied 8 SNPs near the CGG repeats in 176 unrelated Thai males with 19-56 CGG repeats. Of these 176 samples, we identified AGG interruption patterns from 95 samples using direct DNA sequencing. We found that the common CGG repeat groups (29, 30, and 36) were associated with 3 common haplotypes, GCGGATAA (Hap A), TTCATCGC (Hap C), and GCCGTTAA (Hap B), respectively. The configurations of 9A9A9, 10A9A9, and 9A9A6A9 were commonly found in chromosomes with 29, 30, and 36 CGG repeats, respectively. Almost all chromosomes with Hap B (22/23) carried at least one 6A pattern, suggesting that the 6A pattern is linked to Hap B and may have originally occurred in the ancestors of Asian populations.

FMR1 CGG Repeats: Reference Levels and Race-Ethnic Variation in Women With Normal Fertility (Study of Women's Health Across the Nation)

FMR1 premutation carriers (55-199 CGG repeats), and potentially women with high normal (35-44) or low normal (<28) CGG repeats, are at risk of premature ovarian aging. The scarcity of population data on CGG repeats <45 CGG, and variation in race-ethnicity, makes it difficult to determine true associations. DNA was analyzed for FMR1 CGG repeat lengths from 803 women (386 caucasians, 219 African Americans, 102 Japanese, and 96 Chinese) from the US-based Study of Women's Health Across the Nation (SWAN). Participants had ≥1 menses in the 3 months before enrollment, ≥1 pregnancy, no history of infertility or hormonal therapy, and menopause ≥46 years. Statistical analyses used Fisher exact tests. Among these women with normal reproductive histories, significant FMR1 repeat length differences were found across race-ethnicity for both the longer (P = .0002) and the shorter (P < .0001) alleles. The trinucleotide length variance was greater for non-Asian than Asian women (P < .0001), despite identical median values. Our data indicate that short allele lengths <25 CGG on one or both alleles are more common in non-Asian than Asian women. We confirm the minor allele in the 35 to 39 CGG range among Asians as reported previously. Only 2 (0.3%) premutation carriers were identified. These data demonstrate that FMR1 distributions do vary by race-ethnicity, even within the "normal" range. This study indicates the need to control for race-ethnicity in FMR1 ovarian aging research and provides race-ethnic population data for females separated by allele.