CGG Repeat Length Research Articles

Is Low FMR1 CGG Repeat Length in Males Correlated with Family History of BRCA-Associated Cancers? An Exploratory Analysis of Medical Records.

The FMR1 gene has been studied extensively with regard to expansions and premutations, but much less research has focused on potential effects of low CGG repeat length. Previous studies have demonstrated that BRCA1/2 positive women are more likely to have an FMR1 genotype with one low CGG allele, and that women with both FMR1 alleles in the low CGG repeat range are more likely to have had breast cancer compared to women with normal numbers of CGG repeats. However, there has been no research as to whether low CGG repeat length impacts cancer risks in men. Therefore, this study aimed to examine cancer incidence and related risk factors in men with low CGG repeat length in the FMR1 gene. We utilized subject data from the Marshfield Personalized Medicine Research Project to compare cancer-related diagnoses between 878 males with low CGG repeat length (< 24 repeats) and 368 male controls with CGG repeats in the normal range (24 to 40 repeats). We utilized ICD-9 codes to examine various cancer diagnoses, family histories of cancer, other non-malignant neoplasms, cancer surveillance, and genetic susceptibility. Men with low CGG repeats were identified to have significantly higher rates of family history of any cancer type (p=0.011), family history of any BRCA-associated cancer (p=0.002), and specifically, family history of prostate cancer (p=0.007). The mean number of BRCA-associated cancer diagnoses (breast, prostate, pancreatic, and melanoma) per individual in the low CGG group was slightly higher than that of the control group, with this difference trending toward significance (p=0.091). Additionally, men with low CGG repeats had significantly higher rates of connective/soft tissue neoplasms (p=0.026). Additional research is needed to replicate the observations reported in this preliminary exploratory study, particularly including verification of ICD-9 codes and family history by a genetic counselor.

Association of skewed X-chromosome inactivation with FMR1 CGG repeat length and anti-Mullerian hormone levels: a cohort study

BackgroundPremutation range CGGn repeats of the FMR1 gene denote risk toward primary ovarian insufficiency (POI), also called premature ovarian failure (POF). This prospective cohort study was undertaken to determine if X-chromosome inactivation skew (sXCI) is associated with variations in FMR1 CGG repeat length and, if so, is also associated with age adjusted antimüllerian hormone (AMH) levels as an indicator of functional ovarian reserve (FOR).MethodsDNA samples of 58 women were analyzed for methylation status and confirmation of CGGn repeat length. Based on previously described FMR1 genotypes, there were 18 women with norm FMR1 (both alleles in range of CGGn=26–34), and 40 women who had at least one allele at CGGn<26 or CGG>34 (not-norm FMR1). As part of a routine evaluation of ovarian reserve, patients at our fertility center have their serum AMH assessed at first visit. Regression models were used to test the association of ovarian reserve, as indicated by serum AMH, with sXCI.ResultssXCI was significantly lower among infertility patients with norm FMR1 (6.5 ± 11.1, median and IQR) compared to those with not-norm FMR1 (12.0 ± 14.6, P = 0.005), though among young oocyte donors the opposite effect was observed. Women age >30 to 38 years old demonstrated greater ovarian reserve in the presence of lower sXCI as evidenced by significantly higher AMH levels (GLM sXCI_10%, f = 11.27; P = 0.004).ConclusionsTogether these findings suggest that FMR1 CGG repeat length may have a role in determining X-chromosome inactivation which could represent a possible mechanism for previously observed association of low age adjusted ovarian reserve with FMR1 variations in repeat length. Further, larger, investigations will be required to test this hypothesis.

Distribution of the FMR1 gene in females by race/ethnicity: women with diminished ovarian reserve versus women with normal fertility (SWAN study)

To study whether reported, but inconsistent, associations between the FMR1 CGG repeat lengths in the intermediate, high normal, or low normal range differentiate women diagnosed with diminished ovarian reserve (DOR) from population controls and whether associations vary by race/ethnic group. Case-control study. Academic and private fertility clinics. DOR cases (n = 129; 95 Whites, 22 Asian, 12 other) from five U.S. fertility clinics were clinically diagnosed, with regular menses and no fragile X syndrome family history. Normal fertility controls (n = 803; 386 Whites, 219 African-Americans, 102 Japanese, 96 Chinese) from the United States-based SWAN Study had one or more menstrual period in the 3months pre-enrollment, one or more pregnancy, no history of infertility or hormone therapy, and menopause ≥46years. Previously, the SWAN Chinese and Japanese groups had similar FMR1 CGG repeat lengths, thus they were combined. None. FMR1 CGG repeat lengths. Median CGG repeats were nearly identical by case/control group. DOR cases had fewer CGG repeats in the shorter FMR1 allele than controls among Whites, but this was not significant among Asians. White cases had fewer CGG repeats in the shorter allele than Asian cases. No significant differences were found in the high normal/intermediate range between cases and controls or by race/ethnic group within cases in the longer allele. This study refutes prior reports of an association between DOR and high normal/intermediate repeats and confirms an association between DOR and low normal repeats in Whites.

Cerebellar volume mediates the relationship between FMR1 mRNA levels and voluntary step initiation in males with the premutation

Recent evidence indicates that adults with a premutation (PM: 55–199 CGG repeats) expansion in the fragile X mental retardation 1 (FMR1) gene show postural control deficits that may reflect disruption to cerebellar motor regions. Less is known about the influence of reduced cerebellar volume and structural changes, and increase in CGG repeat and FMR1 mRNA levels on the attentional demands of step initiation in PM males. We investigated the effects of a concurrent cognitive task on choice stepping reaction time (CSRT) and explored the associations between CSRT performance, cerebellar volume, CGG size, and FMR1 mRNA levels in blood in PM males. We examined 19 PM males (ages 28–75) and 23 matched controls (CGG <44; ages 26–77), who performed a verbal fluency task during CSRT performance and single-task stepping without a secondary cognitive task. Our results provide preliminary evidence that smaller cerebellar volume (β = −2.73, p = 0.002) and increasing CGG repeat length (β = 1.69, p = 0.003) were associated with greater dual-task step initiation times in PM males, but not in controls. There was evidence of a mediating effect of cerebellar volume on the relationship between FMR1 mRNA levels and single-task CSRT performance in PM males (estimate coefficient = 8.69, standard error = 4.42, p = 0.049). These findings suggest increasing CGG repeat and FMR1 mRNA levels have neurotoxic effects on cerebellar regions underlying anticipatory postural responses during stepping. Cerebellar postural changes may be predictive of the increased risk of falls in older PM males.

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.

Defining the Performance Parameters of a Rapid Screening Tool for FMR1 CGG-Repeat Expansions Based on Direct Triplet-Primed PCR and Melt Curve Analysis

Population-based screening for CGG-repeat expansions in the fragile X mental retardation 1 (FMR1) gene that cause fragile X syndrome can now be performed more cost-effectively and simply by combining direct triplet-primed PCR (dTP-PCR) with melting curve analysis (MCA). We have now performed a detailed technical validation to define the operational parameters for achieving robust and reliable performance of the FMR1 dTP-PCR MCA assay. We compared the assay's performance on 2 real-time PCR platforms and determined its analytic sensitivity and specificity. We also assessed the assay's performance on DNA isolated from different sources, the effect of differences in CGG-repeat length and AGG-interruption pattern on melt peak temperature (Tm), and the effect of common substances found in DNA solutions on Tms. The assay performed well in distinguishing normal from expansion-carrying samples. The assay had detection sensitivity down to 1 ng and an analytical specificity beyond 150 ng. In addition to peripheral blood DNA, analysis could also be performed on DNA from saliva, buccal swabs, and dried blood spots. Salt increased Tms, glycogen contamination had minimal effect, whereas AGG interruptions lowered Tms. The FMR1 dTP-PCR MCA screening assay is highly sensitive and specific, performs well using DNA from different sources, and is robust and reproducible when reagent concentrations are maintained across all tested samples.

Prevalence and predictors of subjective memory complaints in adult male carriers of the FMR1 premutation

Objective: To examine the prevalence and predictors of subjective memory complaints among a cohort of male FMR1 premutation (PM) carriers with and without fragile X-associated tremor ataxia syndrome (FXTAS). Method: Twenty-two PM males (ages 26–80, 7 with FXTAS) and 24 matched controls with normal FMR1 alleles (ages 26–77) completed cross-sectional assessments of subjective memory complaints (memory complaints questionnaire, MAC-Q), objective memory function (Logical Memory subtest from the Wechsler Memory Scale, third edition), and psychiatric symptoms (Depression, Anxiety, and Stress Scales; the Structured Clinical Interview for DSM-IV-TR Axis I Disorders). Results: Although a greater proportion of PM males (36%) endorsed subjective memory complaints compared to controls (21%), formal statistical comparisons failed to reach significance. Multiple linear regression analyses revealed that subjective memory complaints were not associated with objective memory performance, but rather were predicted by elevated psychiatric symptoms. The relationship between psychiatric symptoms and subjective complaints found in the PM group was not statistically different to that found in the control group. There were no significant relationships between FMR1 molecular measures (CGG repeat length, FMR1 mRNA level) and measures of subjective memory complaints, objective memory performance, or psychiatric symptoms. Conclusions: In keeping with findings from the general population, this study suggests that subjective ratings of memory performance in PM males are associated with underlying psychological factors rather than cross-sectional objective memory function. However, future longitudinal studies are required to determine whether subjective memory complaints may predict changes in objective memory function over time.

Broad autism spectrum and obsessive–compulsive symptoms in adults with the fragile X premutation

Objective: Clinical observations and a limited number of research studies provide evidence that the fragile X premutation may confer risk for autism, executive dysfunction, and psychopathology. The link to autism spectrum symptoms and social cognition deficits with the premutation remains uncertain, and thus was the focus of the present investigation.Method: Our sample included 131 individuals, 42 men/22 women with the FMR1 premutation (mean age = 31.83 ± 8.59 years) with a normal neurological exam, and 48 men/19 women healthy age-matched controls (mean age = 29.48 ± 7.29 years). Individuals completed a comprehensive neuropsychological battery with additional assessments for social cognition, broad autism spectrum, and obsessive–compulsive (OC) symptoms.Results: Premutation carriers self-reported higher rates of autism-related symptoms (Autism Quotient; p = .001). Among males only, premutation carriers showed more atypical social interaction (p < .001) and stereotyped behavior (p = .014) during standardized clinical examination on the Autism Diagnostic Observation Schedule (ADOS) relative to controls. Female premutation carriers reported significantly higher rates of OC symptoms compared to control females (p = .012). Molecular measures defining the expanded premutation (FMR1 CGG repeat length and/or mRNA) were significantly associated with a measure of theory of mind (Reading the Mind in the Eyes Task).Conclusions: The results of this study indicate a higher rate of broad autism spectrum symptoms in some males with the premutation and provide evidence for an obsessive–compulsive subtype in female premutation carriers.

Correlation of normal-range FMR1 repeat length or genotypes and reproductive parameters.

This study aims to ascertain whether the length of normal-ranged CGG repeats on the FMR1 gene correlates with abnormal reproductive parameters. We performed a retrospective, cross-sectional study of all FMR1 carrier screening performed as part of routine care at a large university-based fertility center from January 2011 to March 2014. Correlations were performed between normal-range FMR1 length and baseline serum anti-Müllerian hormone (AMH), cycle day 3 follicle stimulating hormone (FSH), ovarian volumes (OV), antral follicle counts (AFC), and incidence of diminished ovarian reserve (DOR), while controlling for the effect of age. Six hundred three FMR1 screening results were collected. One subject was found to be a pre-mutation carrier and was excluded from the study. Baseline serum AMH, cycle day 3 FSH, OV, and AFC data were collected for the 602 subjects with normal-ranged CGG repeats. No significant difference in median age was noted amongst any of the FMR1 repeat genotypes. No significant correlation or association was found between any allele length or genotype, with any of the reproductive parameters or with incidence of DOR at any age (p > 0.05). However, subjects who were less than 35years old with low/low genotype were significantly more likely to have below average AMH levels compared to those with normal/normal genotype (RR 3.82; 95% CI 1.38-10.56). This large study did not demonstrate any substantial association between normal-range FMR1 repeat lengths and reproductive parameters.

Is FMR1 CGG repeat length a predictor of in vitro fertilization stimulation response or outcome?

To study a broad range of FMR1 CGG repeat lengths and assisted reproduction technology (ART) outcomes. Retrospective cohort study. Private ART practice. Fresh autologous ART stimulation cycles. None. Oocyte yield, live birth. We screened 14,088 fresh autologous ART cycles from 2012 to 2015, of which 4,690 cycles in 3,290 patients met the inclusion criteria. The FMR1 repeat length was statistically significantly but weakly associated with oocyte yield and other markers of ovarian response. The receiver operating characteristic curve analysis suggested extremely limited predictive ability. Moreover, the FMR1 repeat length was not statistically significantly associated with outcomes in multivariable models, including other markers of ovarian reserve. The FMR1 repeat length was not associated with embryo quality or live birth. Only patient age had a strong ability to predict live birth. The FMR1 repeat length is associated with ART response, but only weakly. It provides no incremental predictive ability beyond the conventionally used predictors, including patient age, antimüllerian hormone concentration, antral follicle count, and follicle-stimulating hormone level. These data suggest a possible role of the FMR1 repeat length within the normal range in ovarian response but demonstrate no clinically relevant indication for testing FMR1 as a predictor of ART outcomes.

X-inactivation in the clinical phenotype of fragile X premutation carrier sisters.

Objective:The purpose of this study is to describe a case series of 4 sisters with discordant clinical phenotypes associated with fragile X–associated tremor/ataxia syndrome (FXTAS) that may be explained by varying CGG repeat sizes and activation ratios (ARs) (the ratio of cells carrying the normal fragile X mental retardation 1 [FMR1] allele on the active X chromosome).Methods:Four sisters with premutation size FMR1 gene repeats underwent detailed clinical characterization. CGG repeat length was determined by PCR, and AR was determined using a newly developed commercial methylation PCR assay and was compared with the results from Southern blot with densitometric image analysis.Results:Sister 1 had the largest CGG expansion (82) and the lowest AR (12%), with the most severe clinical presentation. Sister 2 had a lower CGG expansion (70) and an AR of 10% but had a milder clinical presentation.Sister 3 had a similar CGG expansion (79) but a slightly higher AR of 15% and less neurologic involvement. Sister 4 had a similar CGG expansion size of 80 but had the largest AR (40%) and was the only sister not to be affected by FXTAS or have any neurologic signs on examination.Conclusions:These results suggest that premutation carrier women who have higher ARs may be less likely to show manifestations of FXTAS. If larger studies show similar patterns, AR data could potentially be beneficial to supplement CGG repeat size when counseling premutation carrier women in the clinic.

Distribution of the FMR1 gene in females by race-ethnicity: women with diminished ovarian reserve versus women with normal fertility (SWAN study)

Distribution of the FMR1 gene in females by race-ethnicity: women with diminished ovarian reserve versus women with normal fertility (SWAN study)

FMR1 CGG repeat length and ART outcomes

Previous work with fragile X syndrome suggests that women with FMR1 premutation (CGG repeats of 55-200) have higher FSH and lower AMH. Reproductive consequences of intermediate CGG repeat size (45-54) are unclear and studies have not analyzed IVF outcomes. Low repeats (<26) are associated with poorer IVF outcomes. Our objective was to analyze a broad range of CGG repeat lengths and ART outcomes in an IVF population.

Preliminary evidence of an effect of cerebellar volume on postural sway in FMR1 premutation males.

Recent evidence suggests that early changes in postural control may be discernible among females with premutation expansions (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene at risk of developing fragile X-associated tremor ataxia syndrome (FXTAS). Cerebellar dysfunction is well described in males and females with FXTAS, yet the interrelationships between cerebellar volume, CGG repeat length, FMR1 messenger RNA (mRNA) levels and changes in postural control remain unknown. This study examined postural sway during standing in a cohort of 22 males with the FMR1 premutation (ages 26-80) and 24 matched controls (ages 26-77). The influence of cerebellar volume, CGG repeat length and FMR1 mRNA levels on postural sway was explored using multiple linear regression. The results provide preliminary evidence that increasing CGG repeat length and decreasing cerebellar volume were associated with greater postural sway among premutation males. The relationship between CGG repeat length and postural sway was mediated by a negative association between CGG repeat size and cerebellar volume. While FMR1 mRNA levels were significantly elevated in the premutation group and correlated with CGG repeat length, FMR1 mRNA levels were not significantly associated with postural sway scores. These findings show for the first time that greater postural sway among males with the FMR1 premutation may reflect CGG repeat-mediated disruption in vulnerable cerebellar circuits implicated in postural control. However, longitudinal studies in larger samples are required to confirm whether the relationships between cerebellar volume, CGG repeat length and postural sway indicate greater risk for neurological decline.

Distribution of fragile X mental retardation 1 CGG repeat and flanking haplotypes in a large Chinese population.

Fragile X syndrome is mainly caused by a CGG repeat expansion within the 5′ UTR of the fragile X mental retardation 1 (FMR1) gene. Previous analyses of the FMR1 CGG repeat patterns and flanking haplotypes in Caucasians and African Americans have identified several factors that may influence repeat instability. However, the CGG repeat patterns and distribution for FRAXAC2 have not yet been investigated in mainland Chinese. We surveyed the CGG repeat lengths in 1113 Han Chinese (534 males and 579 females), and the CGG repeat patterns of 534 males were determined by sequence analysis. We also explored the flanking haplotypes (DXS548-FRAXAC1-FRAXAC2) in 566 unaffected and 28 unrelated fragile X Chinese males. The most frequent alleles for DXS548 and FRAXAC1 were identical between our Chinese population and other Asian populations. We identified several low-abundance alleles for DXS548 and FRAXAC1 not found in previous studies in mainland Chinese and Taiwanese cohorts. The most frequent allele was (CGG)29 followed by (CGG)30, and the most frequent patterns were 9 + 9 + 9, 10 + 9 + 9, and 9 + 9 + 6 + 9, similar to those in Singaporeans. We identified only one premutation female carrier with 89 CGG repeats in the 1113 Han Chinese. A few associations between the CGG repeat patterns and flanking haplotypes were determined in this study. In general, the Chinese population had a smaller number of alleles and lower expected heterozygosity for all three STR markers and FRAXA locus when compared with Caucasians and African Americans. We identified a novel haplotype 7-3-5 + that is significantly associated with the full mutation.

The Significance of Fragile X Mental Retardation Gene 1 CGG Repeat Sizes in the Normal and Intermediate Range in Women With Primary Ovarian Insufficiency

Primary ovarian insufficiency (POI) refers to the cessation of menses before the age of 40 years and occurs in 1% to 2% of women. The specific cause of POI is unknown in the majority of cases. Known causes include gene mutations of fragile X mental retardation 1 (FMR1), a gene located on the X chromosome. This gene codes for a protein essential for normal cognitive development and female reproductive function. FMR1 contains a CCG trinucleotide segment in an untranslated region of its DNA that is repeated less than 45 times in normal individuals (normal range of repeats). Mutation of FMR1 results in abnormal expansion of an unstable CGG triplet, leading to impaired cognitive and reproductive function. Between 55 and 200 CCG repeats is called premutation; affected individuals with premutation are at risk for POI and further expansion of repeats in subsequent generations. People with 45 to 54 CCG repeats (intermediate range of repeats) are considered to be at borderline risk for POI and further expansion of repeats. This case-control study was designed to determine whether the risk of POI is associated with CGG repeat length of normal and intermediate range (up to 55 repeats) in a large cohort of Dutch women with idiopathic POI. The second aim of the study was to examine a possible association between the number of CGG repeats and age at first manifestation of POI as a measurement for the severity of POI. The study population was composed of 375 well-phenotyped Dutch women diagnosed with POI; control subjects were 3368 women with natural menopause at 40 years or older. The FMR1 CGG repeat number was determined by polymerase chain reaction amplification of DNA extracted from blood samples drawn from each patient. Fisher exact test was used to assess the prevalence of intermediate-size CGG repeats in POI cases and control subjects. Analysis of variance tested differences in mean CGG repeat lengths on alleles 1 and 2 between POI cases and control subjects. Allele 1 has the lowest triple repeat number, and allele 2 has the highest triple repeat number. There was no significant difference between POI cases and control subjects in the frequency of intermediate-size CGG repeats on allele 2 (POI 2.7 vs control subjects 3.8%; the odds ratio was 0.72, with a 95% confidence interval of 0.38–1.39; P = 0.38). Linear regression analysis showed no association between the number of CGG repeats and age at first POI manifestation (β = 20.019, P = 0.72). These data suggest that intermediate-size CGG repeats are not associated with risk of POI. Therefore, evaluation of normal and intermediate FMR1 repeat size appears to be of little or no value in the diagnostic workup of women affected by POI or in the prognostic assessment of women at risk of developing POI. Reasons for caution in interpreting these results are as follows: (1) FMR1 CGG repeat lengths in POI cases and control subjects were genotyped in 2 different laboratories; technical differences could have affected the results; (2) distributions of CGG repeats could vary among ethnic populations; (3) women with primary amenorrhea (n = 17) were included in the POI group. The last 2 possible limitations were unlikely because exclusion of data from nonwhite women or women with primary amenorrhea did not affect the results.

35.: Case report: Concurrent occurrence of Fragile X syndrome and Fragile X associated tremor ataxia syndrome due to CGG repeat and methylation mosaicism

Fragile X syndrome (FXS) is a genetic disorder resulting from CGG trinucleotide repeat lengths greater than 200 and subsequent methylation of the FMR1 gene on the X chromosome. Fragile X associated tremor/ataxia syndrome (FXTAS) is a recently described syndrome of tremor and ataxia in a relative of a FXS patient with clinical signs not seen in typical FXS patients. This syndrome is typically seen in patients over the age of 50 and is thought to be a result of neuronal toxicity from excess mRNA production due to CGG repeat lengths of 55 to 200. We present a 34-year-old gentleman with a previous diagnosis of FXS presenting with phenotypic features of FXTAS including cerebellar ataxia. Genetic testing with methylation assay revealed that he is a FXS and FXTAS mosaic with methylated CGG repeat lengths of 110 and 540 contributing to FXS and unmethylated CGG repeat lengths of 90 and 600 contributing to cerebellar ataxia and diagnosis of FXTAS. Despite the close genotypic relationship between FXS an FXTAS there are distinct phenotypic differences attributable to different methylation state. To our knowledge this is the first case to be reported of FXS/FXTAS mosaicism and adds to the increasing understanding of the role played by methylation in determining phenotypic expression of genetic disorders, in particular the importance of methylation mosaicism. We suggest that in a FXS patient presenting with cerebellar ataxia or tremor without an apparent cause, further genetic testing with methylation analysis should be considered in the diagnostic process.

Curvilinear association of CGG repeats and age at menopause in women with FMR1 premutation expansions.

In a sample of post-menopausal premutation carrier mothers of children with the full mutation of fragile X syndrome (n = 88), this study examined the co-occurrence of the reproductive and psychiatric phenotypes associated with FMR1 premutations. Mean age at menopause was 43.1 years, and 35.2% of premutation carriers reported cessation of menses prior to age 40 (premature ovarian failure), but only 18% of carriers had been medically diagnosed by a physician as having Fragile X-associated Primary Ovarian Insufficiency. There was a significant curvilinear association between CGG repeat length and age at menopause, with women who had mid-range repeats having the earliest menopause, similar to the pattern that has been found for the psychiatric phenotype of the FMR1 premutation.

Clinical and molecular implications of mosaicism in FMR1 full mutations.

Expansions of more than 200 CGG repeats (full mutation) in the FMR1 gene give rise to fragile X syndrome (FXS) through a process that generally involves hypermethylation of the FMR1 promoter region and gene silencing, resulting in absence of expression of the encoded protein, FMRP. However, mosaicism with alleles differing in size and extent of methylation often exist within or between tissues of individuals with FXS. In the current work, CGG-repeat lengths and methylation status were assessed for eighteen individuals with FXS, including 13 mosaics, for which peripheral blood cells (PBMCs) and primary fibroblast cells were available. Our results show that for both PBMCs and fibroblasts, FMR1 mRNA and FMRP expression are directly correlated with the percent of methylation of the FMR1 allele. In addition, Full Scale IQ scores were inversely correlated with the percent methylation and positively correlated with higher FMRP expression. These latter results point toward a positive impact on cognition for full mutation mosaics with lower methylation compared to individuals with fully methylated, full mutation alleles. However, we did not observe a significant reduction in the number of seizures, nor in the severity of hyperactivity or autism spectrum disorder, among individuals with mosaic genotypes in the presentation of FXS. These observations suggest that low, but non-zero expression of FMRP may be sufficient to positively impact cognitive function in individuals with FXS, with methylation mosaicism (lowered methylation fraction) contributing to a more positive clinical outcome.

Fragile X full mutation expansions are inhibited by one or more AGG interruptions in premutation carriers.

Fragile X CGG repeat alleles often contain one or more AGG interruptions that influence allele stability and risk of a full mutation transmission from parent to child. We have examined transmissions of maternal and paternal alleles with 45-90 repeats to quantify the effect of AGG interruptions on fragile X repeat instability. A novel FMR1 polymerase chain reaction assay was used to determine CGG repeat length and AGG interruptions for 1,040 alleles from 705 families. We grouped transmissions into nine categories of five repeats by parental size and found that in every size category, alleles with no AGGs had the greatest risk for instability. For maternal alleles <75 repeats, 89% (24/27) that expanded to a full mutation had no AGGs. Two contractions in maternal transmission were accompanied by loss of AGGs, suggesting a mechanism for generating alleles that lack AGG interruptions. Maternal age was examined as a factor in full mutation expansions using prenatal samples to minimize ascertainment bias, and a possible effect was observed though it was not statistically significant (P = 0.06). These results strengthen the association of AGG repeats with CGG repeat stability and provide more accurate risk estimates of full mutation expansions for women with 45-90 repeat alleles.