Paternal transmission of fragile X syndrome

Contribution of DNA/RNA Structures Formed by Expanded CGG/CCG Repeats Within the FMR1 Locus in the Pathogenesis of Fragile X-Associated Disorders

Repeat expansion disorders (REDs) encompass over 50 inherited neurological disorders and are characterized by the expansion of short tandem nucleotide repeats beyond a specific repeat length. Particularly intriguing among these are multiple fragile X-associated disorders (FXds), which arise from an expansion of CGG repeats in the 5' untranslated region of the FMR1 gene. Despite arising from repeat expansions in the same gene, the clinical manifestations of FXds vary widely, encompassing developmental delays, parkinsonism, dementia, and an increased risk of infertility. FXds also exhibit molecular mechanisms observed in other REDs, that is, gene- and protein-loss-of-function and RNA- and protein-gain-of-function. The heterogeneity of phenotypes and pathomechanisms in FXds results from the different lengths of the CGG tract. As the number of repeats increases, the structures formed by RNA and DNA fragments containing CGG repeats change significantly, contributing to the diversity of FXd phenotypes and mechanisms. In this review, we discuss the role of RNA and DNA structures formed by expanded CGG repeats in driving FXd pathogenesis and how the genetic instability of CGG repeats is mediated by the complex interplay between transcription, DNA replication, and repair. We also discuss therapeutic strategies, including small molecules, antisense oligonucleotides, and CRISPR-Cas systems, that target toxic RNA and DNA involved in the development of FXds.

Mechanisms of the FMR1 Repeat Instability: How Does the CGG Sequence Expand?

A dynamic mutation in exon 1 of the FMR1 gene causes Fragile X-related Disorders (FXDs), due to the expansion of an unstable CGG repeat sequence. Based on the CGG sequence size, two types of FMR1 alleles are possible: “premutation” (PM, with 56-200 CGGs) and “full mutation” (FM, with >200 triplets). Premutated females are at risk of transmitting a FM allele that, when methylated, epigenetically silences FMR1 and causes Fragile X syndrome (FXS), a very common form of inherited intellectual disability (ID). Expansions events of the CGG sequence are predominant over contractions and are responsible for meiotic and mitotic instability. The CGG repeat usually includes one or more AGG interspersed triplets that influence allele stability and the risk of transmitting FM to children through maternal meiosis. A unique mechanism responsible for repeat instability has not been identified, but several processes are under investigations using cellular and animal models. The formation of unusual secondary DNA structures at the expanded repeats are likely to occur and contribute to the CGG expansion. This review will focus on the current knowledge about CGG repeat instability addressing the CGG sequence expands.

Fragile X premutation carrier screening in Pakistani preconception women in primary care consultation

PURPOSE

Women of reproductive age who carry fragile X premutation (PM) alleles have 56 to 200 CGG repeats in the 5'-untranslated region of FMR1 gene are at increased risk for producing children with intellectual disabilities (ID) or autism spectrum disorders (ASD) due to expansion of PM alleles to full mutation alleles (> 200 repeats) during maternal transmission.

METHODS

In present study fragile X PM carrier screening was performed in total 808 women who were consulting primary health care centers for preconception care in Khyber Pakhtunkhwa region of Pakistan between April, 2018 and December, 2020. Polymerase chain reaction (PCR) was performed for detection of PM carrier women and the CGG repeats number was confirmed by Southern blotting and capillary electrophoresis.

RESULTS

The prevalence rate for PM carriers among preconception women was found to be 0.7% that was contributed by 0.5% women in risk group (RG1) with family history of ID and 0.2% in risk group 2 (RG2) with family history of ASD. PM carrier women had at least one affected child or sibling. In addition, the preconception women with FMR1 PM alleles were found to be at increased risk for primary ovary insufficiency (RG1: P = 0.0265, RG2: P = 0.0389), postpartum depression (RG1: P = 0.0240, RG2: P = 0.0501) and neuropsychiatric disorders (RG1: P = 0.0389, RG2: P = 0.0432).

CONCLUSIONS

Current study provides first evidence of fragile X PM carrier screening in Pakistani preconception women in primary care consultation. Findings of current study may help to improve preconception care and to reduce burden of fragile X associated disorders in our population.

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.

Laboratory testing for fragile X, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG)

Molecular genetic testing of the FMR1 gene is commonly performed in clinical laboratories. Pathogenic variants in the FMR1 gene are associated with fragile X syndrome, fragile X-associated tremor ataxia syndrome (FXTAS), and fragile X-associated primary ovarian insufficiency (FXPOI). This document provides updated information regarding FMR1 pathogenic variants, including prevalence, genotype-phenotype correlations, and variant nomenclature. Methodological considerations are provided for Southern blot analysis and polymerase chain reaction (PCR) amplification of FMR1, including triplet repeat-primed and methylation-specific PCR.The American College of Medical Genetics and Genomics (ACMG) Laboratory Quality Assurance Committee has the mission of maintaining high technical standards for the performance and interpretation of genetic tests. In part, this is accomplished by the publication of the document ACMG Technical Standards for Clinical Genetics Laboratories, which is now maintained online ( http://www.acmg.net ). This subcommittee also reviews the outcome of national proficiency testing in the genetics area and may choose to focus on specific diseases or methodologies in response to those results. Accordingly, the subcommittee selected fragile X syndrome to be the first topic in a series of supplemental sections, recognizing that it is one of the most frequently ordered genetic tests and that it has many alternative methods with different strengths and weaknesses. This document is the fourth update to the original standards and guidelines for fragile X testing that were published in 2001, with revisions in 2005 and 2013, respectively.This versionClarifies the clinical features associated with different FMRI variants (Section 2.3)Discusses important reporting considerations (Section 3.3.1.3)Provides updates on technology (Section 4.1).

DNA Methylation at Birth Predicts Intellectual Functioning and Autism Features in Children with Fragile X Syndrome

Fragile X syndrome (FXS) is a leading single-gene cause of intellectual disability (ID) with autism features. This study analysed diagnostic and prognostic utility of the Fragile X-Related Epigenetic Element 2 DNA methylation (FREE2m) assessed by Methylation Specific-Quantitative Melt Analysis and the EpiTYPER system, in retrospectively retrieved newborn blood spots (NBS) and newly created dried blood spots (DBS) from 65 children with FXS (~2–17 years). A further 168 NBS from infants from the general population were used to establish control reference ranges, in both sexes. FREE2m analysis showed sensitivity and specificity approaching 100%. In FXS males, NBS FREE2m strongly correlated with intellectual functioning and autism features, however associations were not as strong for FXS females. Fragile X mental retardation 1 gene (FMR1) mRNA levels in blood were correlated with FREE2m in both NBS and DBS, for both sexes. In females, DNAm was significantly increased at birth with a decrease in childhood. The findings support the use of FREE2m analysis in newborns for screening, diagnostic and prognostic testing in FXS.

Sexually dimorphic patterns in electroencephalography power spectrum and autism-related behaviors in a rat model of fragile X syndrome

Fragile X syndrome (FXS), a neurodevelopmental disorder with autistic features, is caused by the loss of the fragile X mental retardation protein. Sex-specific differences in the clinical profile have been observed in FXS patients, but few studies have directly compared males and females in rodent models of FXS. To address this, we performed electroencephalography (EEG) recordings and a battery of autism-related behavioral tasks on juvenile and young adult Fmr1 knockout (KO) rats. EEG analysis demonstrated that compared to wild-type, male Fmr1 KO rats showed an increase in gamma frequency band power in the frontal cortex during the sleep-like immobile state, and both male and female KO rats failed to show an increase in delta frequency power in the sleep-like state, as observed in wild-type rats. Previous studies of EEG profiles in FXS subjects also reported abnormally increased gamma frequency band power, highlighting this parameter as a potential translatable biomarker. Both male and female Fmr1 KO rats displayed reduced exploratory behaviors in the center zone of the open field test, and increased distance travelled in an analysis of 24-h home cage activity, an effect that was more prominent during the nocturnal phase. Reduced wins against wild-type opponents in the tube test of social dominance was seen in both sexes. In contrast, increased repetitive behaviors in the wood chew test was observed in male but not female KO rats, while increased freezing in a fear conditioning test was observed only in the female KO rats. Our findings highlight sex differences between male and female Fmr1 KO rats, and indicate that the rat model of FXS could be a useful tool for the development of new therapeutics for treating this debilitating neurodevelopmental disorder.

Mystery of Expansion: DNA Metabolism and Unstable Repeats

The mammalian genome mostly contains repeated sequences. Some of these repeats are in the regulatory elements of genes, and their instability, particularly the propensity to change the repeat unit number, is responsible for 36 well-known neurodegenerative human disorders. The mechanism of repeat expansion has been an unsolved question for more than 20 years. There are a few hypotheses describing models of mutation development. Every hypothesis is based on assumptions about unusual secondary structures that violate DNA metabolism processes in the cell. Some models are based on replication errors, and other models are based on mismatch repair or base excision repair errors. Additionally, it has been shown that epigenetic regulation of gene expression can influence the probability and frequency of expansion. In this review, we consider the molecular bases of repeat expansion disorders and discuss possible mechanisms of repeat expansion during cell metabolism.

Methylated premutation of the FMR1 gene in three sisters: correlating CGG expansion and epigenetic inactivation

Fragile X syndrome (FXS) is a very frequent cause of inherited intellectual disability (ID) and autism. Most FXS patients have an expansion over 200 repeats of (CGG)_n sequence ("full mutation" (FM)) located in the 5'UTR of the FMR1 gene, resulting in local DNA methylation (methylated "full mutation" (MFM)) and epigenetic silencing. The absence of the FMRP protein is responsible for the clinical phenotype of FXS. FM arises from a smaller maternal allele with 56-200 CGG repeats ("premutation" (PM)) during maternal meiosis. Carriers of PM alleles, which are typically unmethylated, can manifest other clinical features (primary ovarian insufficiency (POI) or FXS-associated tremor-ataxia syndrome (FXTAS)), known as fragile X-related disorders. In FXS families, rare males who have inherited an unmethylated "full mutation" (UFM) have been described. These individuals produce enough FMRP to allow normal intellectual functioning. Here we report the rare case of three sisters with a completely methylated PM of around 140 CGGs and detail their neuropsychological function. X inactivation analysis confirmed that the three sisters have a random inactivation of the X chromosome, suggesting that the PM allele is always methylated also when residing on the active X. We propose that in exceptional cases, just as the FM may be unmethylated, also a PM allele may be fully methylated. To our knowledge, females with a methylated PM allele and a mild impairment have reported only once. The study of these atypical individuals demonstrates that the size of the CGG expansion is not as tightly coupled to methylation as previously thought.

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.

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.

Absence of AGG Interruptions Is a Risk Factor for Full Mutation Expansion Among Israeli FMR1 Premutation Carriers

Introduction: Fragile X syndrome (FXS) is a common form of X-linked intellectual and developmental disability with a prevalence of 1/4000-5000 in males and 1/6000-8000 in females. Most cases of the syndrome result from expansion of a premutation (55-200 CGGs) to a full mutation (>200 CGGs) repeat located in the 5' untranslated region of the fragile X mental retardation (FMR1) gene. The risk for full mutation expansions increases dramatically with increasing numbers of CGG repeats. Recent studies, however, revealed AGG interruptions within the repeat area function as a "protective factor" decreasing the risk of intergenerational expansion. Materials and Methods: This study was conducted to validate the relevance of AGG analysis for the ethnically diverse Israeli population. To increase the accuracy of our results, we combined results from Israel with those from the New York State Institute for Basic Research in Developmental Disabilities (IBR). To the best of our knowledge this is the largest cohort of different ethnicities to examine risks of unstable transmissions and full mutation expansions among FMR1 premutation carriers. Results: The combined data included 1471 transmissions of maternal premutation alleles: 369 (25.1%) stable and 1,102 (74.9%) unstable transmissions. Full mutation expansions were identified in 20.6% (303/1471) of transmissions. A total of 97.4% (388/397) of transmissions from alleles with no AGGs were unstable, 79.6% (513/644) in alleles with 1 AGG and 46.7% (201/430) in alleles with 2 or more AGGs. The same trend was seen with full mutation expansions where 40% (159/397) of alleles with no AGGs expanded to a full mutation, 20.2% (130/644) for alleles with 1 AGG and only 3.2% (14/430) in alleles with 2 AGGs or more. None of the alleles with 3 or more AGGs expanded to full mutations. Conclusion: We recommend that risk estimates for FMR1 premutation carriers be based on AGG interruptions as well as repeat size in Israel and worldwide.

Fragile X Associated Primary Ovarian Insufficiency (FXPOI): Case Report and Literature Review

Abnormalities in the X-linked FMR1 gene are associated with a constellation of disorders, which have broad and profound implications for the person first diagnosed, and extended family members of all ages. The rare and pleiotropic nature of the associated disorders, both common and not, place great burdens on (1) the affected families, (2) their care providers and clinicians, and (3) investigators striving to conduct research on the conditions. Fragile X syndrome, occurring more severely in males, is the leading genetic cause of intellectual disability. Fragile X associated tremor and ataxia syndrome (FXTAS) is a neurodegenerative disorder seen more often in older men. Fragile X associated primary ovarian insufficiency (FXPOI) is a chronic disorder characterized by oligo/amenorrhea and hypergonadotropic hypogonadism before age 40 years. There may be significant morbidity due to: (1) depression and anxiety related to the loss of reproductive hormones and infertility; (2) reduced bone mineral density; and (3) increased risk of cardiovascular disease related to estrogen deficiency. Here we report the case of a young woman who never established regular menses and yet experienced a 5-year diagnostic odyssey before establishing a diagnosis of FXPOI despite a known family history of fragile X syndrome and early menopause. Also, despite having clearly documented FXPOI the woman conceived spontaneously and delivered two healthy children. We review the pathophysiology and management of FXPOI. As a rare disease, the diagnosis of FXPOI presents special challenges. Connecting patients and community health providers with investigators who have the requisite knowledge and expertise about the FMR1 gene and FXPOI would facilitate both patient care and research. There is a need for an international natural history study on FXPOI. The effort should be coordinated by a global virtual center, which takes full advantage of mobile device communication systems.

Fragile X-Associated Diminished Ovarian Reserve and Primary Ovarian Insufficiency from Molecular Mechanisms to Clinical Manifestations

Fragile X syndrome (FXS), is caused by a loss-of-function mutation in the FMR1 gene located on the X-chromosome, which leads to the most common cause of inherited intellectual disability in males and the leading single-gene defect associated with autism. A full mutation (FM) is represented by more than 200 CGG repeats within the FMR1 gene, resulting in FXS. A FM is inherited from women carrying a FM or a premutation (PM; 55–200 CGG repeats) allele. PM is associated with phenotypes distinct from those associated with FM. Some manifestations of the PM are unique; fragile-X-associated tremor/ataxia syndrome (FXTAS), and fragile-X-associated primary ovarian insufficiency (FXPOI), while others tend to be non-specific such as intellectual disability. In addition, women carrying a PM may suffer from subfertility or infertility. There is a need to elucidate whether the impairment of ovarian function found in PM carriers arises during the primordial germ cell (PGC) development stage, or due to a rapidly diminishing oocyte pool throughout life or even both. Due to the possibility of expansion into a FM in the next generation, and other ramifications, carrying a PM can have an enormous impact on one’s life; therefore, preconception counseling for couples carrying the PM is of paramount importance. In this review, we will elaborate on the clinical manifestations in female PM carriers and propose the definition of fragile-X-associated diminished ovarian reserve (FXDOR), then we will review recent scientific findings regarding possible mechanisms leading to FXDOR and FXPOI. Lastly, we will discuss counseling, preventative measures and interventions available for women carrying a PM regarding different aspects of their reproductive life, fertility treatment, pregnancy, prenatal testing, contraception and fertility preservation options.

Paternal transmission of a FMR1 full mutation allele

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability (ID) and autism. In most of cases, the molecular basis of this syndrome is a CGG repeat expansion in the 5' untranslated region of the FMR1 gene. It is inherited as an X linked dominant trait, with a reduced penetrance (80% for males and 30% for females). Full mutation (FM) expansion from premutated alleles (PM) is only acquired via maternal meiosis, while paternal transmission always remains in the PM range. We present a 16-year-old girl with a mild fragile X syndrome phenotype. FMR1 gene study showed that the patient inherited a mosaic premutation-full mutation with an unmethylated uninterrupted allele (175, >200 CGG) from her father. The father showed an 88 CGG uninterrupted unmethylated allele in blood and sperm cells. To our knowledge, this is the first case of a FMR1 mosaic premutation-full mutation allele inherited from a PM father. In our opinion, the most likely explanation could be a postzygotic somatic expansion. We can conclude that in rare cases of a child with a full mutation whose mother does not carry a premutation, the possibility of paternal transmission should be considered.

EMQN best practice guidelines for the molecular genetic testing and reporting of fragile X syndrome and other fragile X-associated disorders

Different mutations occurring in the unstable CGG repeat in 5' untranslated region of FMR1 gene are responsible for three fragile X-associated disorders. An expansion of over ∼200 CGG repeats when associated with abnormal methylation and inactivation of the promoter is the mutation termed ‘full mutation' and is responsible for fragile X syndrome (FXS), a neurodevelopmental disorder described as the most common cause of inherited intellectual impairment. The term ‘abnormal methylation' is used here to distinguish the DNA methylation induced by the expanded repeat from the ‘normal methylation' occurring on the inactive X chromosomes in females with normal, premutation, and full mutation alleles. All male and roughly half of the female full mutation carriers have FXS. Another anomaly termed ‘premutation' is characterized by the presence of 55 to ∼200 CGGs without abnormal methylation, and is the cause of two other diseases with incomplete penetrance. One is fragile X-associated primary ovarian insufficiency (FXPOI), which is characterized by a large spectrum of ovarian dysfunction phenotypes and possible early menopause as the end stage. The other is fragile X-associated tremor/ataxia syndrome (FXTAS), which is a late onset neurodegenerative disorder affecting males and females. Because of the particular pattern and transmission of the CGG repeat, appropriate molecular testing and reporting is very important for the optimal genetic counselling in the three fragile X-associated disorders. Here, we describe best practice guidelines for genetic analysis and reporting in FXS, FXPOI, and FXTAS, including carrier and prenatal testing.

The FMR1 gene, infertility, and reproductive decision-making: a review

The strongest association between FMR1 and the ovary in humans is the increased risk of premature ovarian failure (POF) in women who carry the premutation level of CGG repeats (55–199 CGGs). Research on the FMR1 gene has extended to other endpoints of relevance in the OB/GYN setting for women, including infertility and ovarian hormones. After reviewing the nomenclature changes that have occurred in recent years, this article reviews the evidence linking the length of the FMR1 repeat length to fertility and ovarian hormones (follicle stimulating hormone and anti-mullerian hormone as the primary methods to assess ovarian reserve in clinical settings). The literature is inconsistent on the association between the FMR1 trinucleotide repeat length and infertility. Elevated levels of follicle stimulating hormone have been found in women who carry the premutation; however the literature on the relationship between anti-mullerian hormone and the CGG repeat length are too disparate in design to make a summary statement. This article considers the implications of two transgenic mouse models (FXPM 130R and YAC90R) for theories on pathogenesis related to ovarian endpoints. Given the current screening/testing recommendations for reproductive age females and the variability of screening protocols in clinics, future research is recommended on pretest and posttest genetic counseling needs. Future research is also needed on ovarian health measurements across a range of CGG repeat lengths in order to interpret FMR1 test results in reproductive age women; the inconsistencies in the literature make it quite challenging to advise women on their risks related to FMR1 repeat length.

Ethnic effect on FMR1 carrier rate and AGG repeat interruptions among Ashkenazi women

PURPOSE

Fragile X syndrome, a common cause of intellectual disability, is usually caused by CGG trinucleotide expansion in the FMR1 gene. CGG repeat size correlates with expansion risk. Premutation alleles (55-200 repeats) may expand to full mutations in female meiosis. Interspersed AGG repeats decrease allele instability and expansion risk. The carrier rate and stability of FMR1 alleles were evaluated in large cohorts of Ashkenazi and non-Ashkenazi women.

METHODS

A total of 4,344 Ashkenazi and 4,985 non-Ashkenazi cases were analyzed using Southern blotting and polymerase chain reaction between 2004 and 2011. In addition, AGG interruptions were evaluated in 326 Ashkenazi and 298 non-Ashkenazi women who were recruited during 2011.

RESULTS

Both groups had major peaks of 30 and 29 repeats. Ashkenazi women had a higher frequency of 30 repeats and a lower frequency of other peaks (P < 0.0001). A higher rate of premutations in the 55-59 repeats range (1:114 vs. 1:277) was detected among the Ashkenazi women. Loss of AGG interruptions (<2) was significantly less common among Ashkenazi women (9 vs. 19.5% for non-Ashkenazi women, P = 0.0002).

CONCLUSION

Ashkenazi women have a high fragile X syndrome carrier rate and mostly lower-range premutations, and carry a low risk for expansion to a full mutation. Normal-sized alleles in Ashkenazi women have higher average number of AGG interruptions that may increase stability. These factors may decrease the risk for fragile X syndrome offspring among Ashkenazi women.

Longitudinal interviews of couples diagnosed with diminished ovarian reserve undergoing fragile X premutation testing

About 10 % of infertile/subfertile women are diagnosed with diminished ovarian reserve (DOR), of which < 5 % will become pregnant spontaneously. Fragile X (FMR1) genetic testing may provide a reason for her early ovarian aging and/or have reproductive implications. Seven women with DOR (genetic study subset) and the male partners of six of these women were separately interviewed about the experience of being asked to undergo this unanticipated genetic test. Three interviews were conducted (before, within 1 week after, and 3 months after learning the test results). None of the participants carried the FMR1 premutation (largest FMR1 allele 27-50 CGG repeats). For women, their pregnancy-seeking journey was long and exhausting. Women understood the reproductive implications of carrying the FMR1 premutation, and hoped for a negative result. Being offered a genetic test caused women to pause and re-think their future reproductive plans. Husbands viewed the infertility journey as filled with unknowns, of which the genetic test results would be one more puzzle piece. The expense of fertility testing/treatment was mentioned by both spouses, though more notably by husbands. The introduction of a possible genetic cause of infertility, with additional potential health consequences for future biological children, caused women to re-think their quest for pregnancy. In contrast, the genetic test was viewed as an additional source of information for their husbands as opposed to raising concern regarding potential reproductive ramifications.

ACMG Standards and Guidelines for fragile X testing: a revision to the disease-specific supplements to the Standards and Guidelines for Clinical Genetics Laboratories of the American College of Medical Genetics and Genomics

Molecular genetic testing of the FMR1 gene is commonly performed in clinical laboratories. Mutations in the FMR1 gene are associated with fragile X syndrome, fragile X tremor ataxia syndrome, and premature ovarian insufficiency. This document provides updated information regarding FMR1 gene mutations, including prevalence, genotype-phenotype correlation, and mutation nomenclature. Methodological considerations are provided for Southern blot analysis and polymerase chain reaction amplification of the FMR1 gene, including triplet repeat-primed and methylation-specific polymerase chain reaction. In addition to report elements, examples of laboratory reports for various genotypes are also included.

DNA repair/replication transcripts are down regulated in patients with Fragile X Syndrome

BACKGROUND

Fragile X Syndrome (FXS) and its associated disorders are caused by the expansion of the CGG repeat in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene, with disease classification based on the number of CGG repeats. The mechanisms of repeat expansion are dependent on the presence of cis elements and the absence of trans factors both of which are not mutually exclusive and contribute to repeat instability. Expansions associated with trans factors are due to the haploinsuffient or reduced expression of several DNA repair/metabolizing proteins. The reduction of expression in trans factors has been primarily conducted in animal models without substantial examination of many of these expansion mechanisms and trans factors in humans.

RESULTS

To understand the trans factors and pathways associated with trinucleotide repeat expansion we have analyzed two microarray datasets which characterized the transcript expression in patients with FXS and in controls.

CONCLUSION

We observed significant down regulation of DNA damage/repair pathway transcripts. This observation was consistent in both datasets, which used different populations. Within these datasets, several transcripts overlapped in the direction of association and fold change. Further characterization of these genes will be critical to understand their role in trinucleotide repeat instability in FXS.

Fragile X syndrome: the FMR1 CGG repeat distribution among world populations

Fragile X syndrome (FXS) is characterized by moderate to severe intellectual disability, which is accompanied by macroorchidism and distinct facial morphology. FXS is caused by the expansion of the CGG trinucleotide repeat in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. The syndrome has been studied in ethnically diverse populations around the world and has been extensively characterized in several populations. Similar to other trinucleotide expansion disorders, the gene-specific instability of FMR1 is not accompanied by genomic instability. Currently we do not have a comprehensive understanding of the molecular underpinnings of gene-specific instability associated with tandem repeats. Molecular evidence from in vitro experiments and animal models supports several pathways for gene-specific trinucleotide repeat expansion. However, whether the mechanisms reported from other systems contribute to trinucleotide repeat expansion in humans is not clear. To understand how repeat instability in humans could occur, the CGG repeat expansion is explored through molecular analysis and population studies which characterized CGG repeat alleles of FMR1. Finally, the review discusses the relevance of these studies in understanding the mechanism of trinucleotide repeat expansion in FXS.

Instability and chromatin structure of expanded trinucleotide repeats

Trinucleotide repeat expansion underlies at least 17 neurological diseases. In affected individuals, the expanded locus is characterized by dramatic changes in chromatin structure and in repeat tract length. Interestingly, recent studies show that several chromatin modifiers, including a histone acetyltransferase, a DNA methyltransferase and the chromatin insulator CTCF can modulate repeat instability. Here, we propose that the unusual chromatin structure of expanded repeats directly impacts their instability. We discuss several potential models for how this might occur, including a role for DNA repair-dependent epigenetic reprogramming in increasing repeat instability, and the capacity of epigenetic marks to alter sense and antisense transcription, thereby affecting repeat instability.

The Italian External Quality Assessment scheme for fragile x syndrome: the results of a 5-year survey

The Italian External Quality Assessment scheme for fragile X syndrome started in 2001 as an activity funded by the National Health System and coordinated by the National Institute of Public Health. The aim of this work is to present the data of 5 years (2001--2004 and 2006) of survey. The External Quality Assessment scheme was designed to cover the following points: (a) genotyping and (b) interpretation and reporting of results. Overall, the scheme covered about 65% of all Italian public laboratories. The average reporting of results was 91.6%, with an overall success rate of 76%. The rate of diagnostic errors observed was on average 5%. Inaccuracy in sizing of CGG repeats of normal and premutated alleles was reported. During the survey the proportion of laboratories using a Southern blotting, polymerase chain reaction, and ABI sizing kit in combination rose from 36.8% to 70.6%. The reports from laboratories showed incompleteness and considerable variations in expected outcomes. For this reason, in 2004 a model for written reports was introduced. In conclusion, these data underscore the need to participate in External Quality Assessment schemes as an educational resource to ensure quality in molecular genetic testing.

A girl with fragile X premutation from sperm donation

We present a girl with the fragile X premutation who obtained the premutation allele from donated sperm. Our patient has clinical characteristics of fragile X syndrome including emotional problems and neuropsychological difficulties presenting as learning disabilities. She is also at high risk for premature ovarian failure and low risk for the fragile X-associated tremor ataxia (FXTAS). We suggest fragile X DNA screening in gamete donor candidates to decrease the chance of fragile X involvement in their offspring.

Erectile dysfunction in Fragile X patients

AIM

To report the clinical experience during collecting sperm samples in the Fragile X syndrome (FXS) male patients.

METHODS

Two different polymerase chain reaction (PCR) based methods were used for the molecular diagnosis of FXS. Sperm collection was done mostly according to the laboratory manual of the World Health Organization.

RESULTS

We failed to collect sperm samples from five Fragile X subjects aged 18-60 years as a result of an unexpected erectile dysfunction (ED). Multiple examinations of the same subject at different times, and of different subjects from different provinces examined by different physicians, showed the same result consistently in all the five subjects.

CONCLUSION

Erectile reflex is an instinctive response in all healthy males. The absence of erection can be caused by hormonal, physical or neuronal malfunction. As hormonal profiles were reported to be generally normal in Fragile X men, we propose that an unknown physical factor or the neuronal circuit, or both, underlying the erection is compromised. The finding of this symptom in Fragile X patients may help better understand the clinical spectrum and pathogeneses of the disease.

Genetic counseling for fragile x syndrome: updated recommendations of the national society of genetic counselors

These recommendations describe the minimum standard criteria for genetic counseling and testing of individuals and families with fragile X syndrome, as well as carriers and potential carriers of a fragile X mutation. The original guidelines (published in 2000) have been revised, replacing a stratified pre- and full mutation model of fragile X syndrome with one based on a continuum of gene effects across the full spectrum of FMR1 CGG trinucleotide repeat expansion. This document reviews the molecular genetics of fragile X syndrome, clinical phenotype (including the spectrum of premature ovarian failure and fragile X-associated tremor-ataxia syndrome), indications for genetic testing and interpretation of results, risks of transmission, family planning options, psychosocial issues, and references for professional and patient resources. These recommendations are the opinions of a multicenter working group of genetic counselors with expertise in fragile X syndrome genetic counseling, and they are based on clinical experience, review of pertinent English language articles, and reports of expert committees. These recommendations should not be construed as dictating an exclusive course of management, nor does use of such recommendations guarantee a particular outcome. The professional judgment of a health care provider, familiar with the facts and circumstances of a specific case, will always supersede these recommendations.

Fragile X syndrome: diagnostic and carrier testing

This guideline is designed primarily as an educational resource for medical geneticists and other health care providers to help them provide quality medical genetic services. Adherence to this guideline does not necessarily assure a successful medical outcome. This guideline should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, the geneticist should apply his or her own professional judgment to the specific clinical circumstances presented by the individual patient or specimen. It may be prudent, however, to document in the patient's record the rationale for any significant deviation from this guideline.