Targeted long-read sequencing captures CRISPR editing and AAV integration outcomes in brain.

Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing is an emerging therapeutic modality that shows promise in Huntington's disease and spinocerebellar ataxia (SCA) mouse models. However, advancing CRISPR-based therapies requires methods to fully define in vivo editing outcomes. Here, we use polymerase-free, targeted long-read nanopore sequencing and evaluate single- and dual-gRNA AAV-CRISPR editing of human ATXN2 in transgenic mouse models of SCA type 2 (SCA2). Unbiased high sequencing coverage showed 10%-25% editing. Along with intended edits there was AAV integration, 1%-2% of which contained the entire AAV genome and were largely unmethylated. More than 150 kb deletions at target loci and rearrangements of the transgenic allele (1%) were also found. In contrast, PCR-based nanopore sequencing showed bias for partial AAV fragments and inverted terminal repeats (ITRs) and failed to detect full-length AAV. Cumulatively this work defines the spectrum of outcomes of CRISPR editing in mouse brain after AAV gene transfer using an unbiased long-read sequencing approach.

Toxicity after AAV delivery of RNAi expression constructs into nonhuman primate brain.

RNA interference (RNAi) for spinocerebellar ataxia type 1 can prevent and reverse behavioral deficits and neuropathological readouts in mouse models, with safety and benefit lasting over many months. The RNAi trigger, expressed from adeno-associated virus vectors (AAV.miS1), also corrected misregulated microRNAs (miRNA) such as miR150. Subsequently, we showed that the delivery method was scalable, and that AAV.miS1 was safe in short-term pilot nonhuman primate (NHP) studies. To advance the technology to patients, investigational new drug (IND)-enabling studies in NHPs were initiated. After AAV.miS1 delivery to deep cerebellar nuclei, we unexpectedly observed cerebellar toxicity. Both small-RNA-seq and studies using AAVs devoid of miRNAs showed that this was not a result of saturation of the endogenous miRNA processing machinery. RNA-seq together with sequencing of the AAV product showed that, despite limited amounts of cross-packaged material, there was substantial inverted terminal repeat (ITR) promoter activity that correlated with neuropathologies. ITR promoter activity was reduced by altering the miS1 expression context. The surprising contrast between our rodent and NHP findings highlight the need for extended safety studies in multiple species when assessing new therapeutics for human application.

CRISPR to the Rescue: Advances in Gene Editing for the FMR1 Gene.

Gene-editing using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is promising as a potential therapeutic strategy for many genetic disorders. CRISPR-based therapies are already being assessed in clinical trials, and evaluation of this technology in Fragile X syndrome has been performed by a number of groups. The findings from these studies and the advancement of CRISPR-based technologies are insightful as the field continues towards treatments and cures of Fragile X-Associated Disorders (FXADs). In this review, we summarize reports using CRISPR-editing strategies to target Fragile X syndrome (FXS) molecular dysregulation, and highlight how differences in FXS and Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) might alter treatment strategies for each syndrome. We discuss the various modifications and evolutions of the CRISPR toolkit that expand its therapeutic potential, and other considerations for moving these strategies from bench to bedside. The rapidly growing field of CRISPR therapeutics is providing a myriad of approaches to target a gene, pathway, or transcript for modification. As cures for FXADs have remained elusive, CRISPR opens new avenues to pursue.

Molecular biomarkers predictive of sertraline treatment response in young children with fragile X syndrome.

OBJECTIVES: Several neurotransmitters involved in brain development are altered in fragile X syndrome (FXS), the most common monogenic cause of autism spectrum disorder (ASD). Serotonin plays a vital role in synaptogenesis and postnatal brain development. Deficits in serotonin synthesis and abnormal neurogenesis were shown in young children with autism, suggesting that treating within the first years of life with a selective serotonin reuptake inhibitor might be the most effective time. In this study we aimed to identify molecular biomarkers involved in the serotonergic pathway that could predict the response to sertraline treatment in young children with FXS. METHODS: Genotypes were determined for several genes involved in serotonergic pathway in 51 children with FXS, ages 24-72months. Correlations between genotypes and deviations from baseline in primary and secondary outcome measures were modeled using linear regression models. RESULTS: A significant association was observed between a BDNF polymorphism and improvements for several clinical measures, including the Clinical Global Impression scale (P=0.008) and the cognitive T score (P=0.017) in those treated with sertraline compared to those in the placebo group. Additionally, polymorphisms in the MAOA, Cytochrome P450 2C19 and 2D6, and in the 5-HTTLPR gene showed a significant correlation with some of the secondary measures included in this study. CONCLUSION: This study shows that polymorphisms of genes involved in the serotonergic pathway could play a potential role in predicting response to sertraline treatment in young children with FXS. Larger studies are warranted to confirm these initial findings.

Genome-Wide Association and Exome Sequencing Study of Language Disorder in an Isolated Population.

BACKGROUND AND OBJECTIVE: Developmental language disorder (DLD) is a highly prevalent neurodevelopmental disorder associated with negative outcomes in different domains; the etiology of DLD is unknown. To investigate the genetic underpinnings of DLD, we performed genome-wide association and whole exome sequencing studies in a geographically isolated population with a substantially elevated prevalence of the disorder (ie, the AZ sample). METHODS: DNA samples were collected from 359 individuals for the genome-wide association study and from 12 severely affected individuals for whole exome sequencing. Multifaceted phenotypes, representing major domains of expressive language functioning, were derived from collected speech samples. RESULTS: Gene-based analyses revealed a significant association between SETBP1 and complexity of linguistic output (P = 5.47 × 10(-7)). The analysis of exome variants revealed coding sequence variants in 14 genes, most of which play a role in neural development. Targeted enrichment analysis implicated myocyte enhancer factor-2 (MEF2)-regulated genes in DLD in the AZ population. The main findings were successfully replicated in an independent cohort of children at risk for related disorders (n = 372). CONCLUSIONS: MEF2-regulated pathways were identified as potential candidate pathways in the etiology of DLD. Several genes (including the candidate SETBP1 and other MEF2-related genes) seem to jointly influence certain, but not all, facets of the DLD phenotype. Even when genetic and environmental diversity is reduced, DLD is best conceptualized as etiologically complex. Future research should establish whether the signals detected in the AZ population can be replicated in other samples and languages and provide further characterization of the identified pathway.

Clinical and Molecular Assessment in a Female with Fragile X Syndrome and Tuberous Sclerosis.

OBJECTIVE: Fragile X syndrome (FXS) and tuberous sclerosis (TSC) are genetic disorders that result in intellectual disability and an increased prevalence of autism spectrum disorders (ASD). While the clinical presentation of each disorder is distinct, the molecular causes are linked to a disruption in the mTORC1 (mammalian Target of Rapamycin Complex 1) and ERK1/2 (Extracellular signal-Regulated Kinase) signaling pathways. METHODS: We assessed the clinical and molecular characteristics of an individual seen at the UC Davis MIND Institute with a diagnosis of FXS and TSC. Clinical evaluation of physical, behavioral, and cognitive impairments were performed. Additionally, total and phosphorylated proteins along the mTORC1 and ERK1/2 pathways were measured in primary fibroblast cell lines from the proband. RESULTS: In this case the phenotypic effects that result in a human with both FXS and TSC are shown to be severe. Changes in mTORC1 and ERK1/2 signaling proteins and global protein synthesis were not found to be noticeably different between four cohorts (typically developing, FMR1 full mutation, FMR1 full mutation and TSC1 loss of function mutation, and TSC1 loss of function mutation); however cohort sizes prevented stringent comparisons. CONCLUSION: It has previously been suggested that disruption of the mTORC1 pathway was reciprocal in TSC and FXS double knock-out mouse models so that the regulation of these pathways were more similar to wild-type mice compared to mice harboring a Fmr1-/y or Tsc2-/+ mutation alone. However, in this first reported case of a human with a diagnosis of both FXS and TSC, substantial clinical impairments, as a result of these two disorders were observed. Differences in the mTORC and ERK1/2 pathways were not clearly established when compared between individuals with either disorder, or both.

Differential increases of specific FMR1 mRNA isoforms in premutation carriers.

BACKGROUND: Over 40% of male and ∼16% of female carriers of a premutation FMR1 allele (55-200 CGG repeats) will develop fragile X-associated tremor/ataxia syndrome, an adult onset neurodegenerative disorder, while about 20% of female carriers will develop fragile X-associated primary ovarian insufficiency. Marked elevation in FMR1 mRNA transcript levels has been observed with premutation alleles, and RNA toxicity due to increased mRNA levels is the leading molecular mechanism proposed for these disorders. However, although the FMR1 gene undergoes alternative splicing, it is unknown whether all or only some of the isoforms are overexpressed in premutation carriers and which isoforms may contribute to the premutation pathology. METHODS: To address this question, we have applied a long-read sequencing approach using single-molecule real-time (SMRT) sequencing and qRT-PCR. RESULTS: Our SMRT sequencing analysis performed on peripheral blood mononuclear cells, fibroblasts and brain tissue samples derived from premutation carriers and controls revealed the existence of 16 isoforms of 24 predicted variants. Although the relative abundance of all mRNA isoforms was significantly increased in the premutation group, as expected based on the bulk increase in mRNA levels, there was a disproportionate (fourfold to sixfold) increase, relative to the overall increase in mRNA, in the abundance of isoforms spliced at both exons 12 and 14, specifically Iso10 and Iso10b, containing the complete exon 15 and differing only in splicing in exon 17. CONCLUSIONS: These findings suggest that RNA toxicity may arise from a relative increase of all FMR1 mRNA isoforms. Interestingly, the Iso10 and Iso10b mRNA isoforms, lacking the C-terminal functional sites for fragile X mental retardation protein function, are the most increased in premutation carriers relative to normal, suggesting a functional relevance in the pathology of FMR1-associated disorders.

Screening newborn blood spots for 22q11.2 deletion syndrome using multiplex droplet digital PCR.

BACKGROUND: The diagnosis of 22q11 deletion syndrome (22q11DS) is often delayed or missed due to the wide spectrum of clinical involvement ranging from mild to severe, often life-threatening conditions. A delayed diagnosis can lead to life-long health issues that could be ameliorated with early intervention and treatment. Owing to the high impact of 22q11DS on public health, propositions have been made to include 22q11DS in newborn screening panels; however, the method of choice for detecting 22q11DS, fluorescent in situ hybridization, requires specialized equipment and is cumbersome for most laboratories to implement as part of their routine screening. We sought to develop a new genetic screen for 22q11DS that is rapid, cost-effective, and easily used by laboratories currently performing newborn screening. METHODS: We evaluated the accuracy of multiplex droplet digital PCR (ddPCR) in the detection of copy number of 22q11DS by screening samples from 26 patients with 22q11DS blindly intermixed with 1096 blood spot cards from the general population (total n = 1122). RESULTS: Multiplex ddPCR correctly identified all 22q11DS samples and distinguished between 1.5- and 3-Mb deletions, suggesting the approach is sensitive and specific for the detection of 22q11DS. CONCLUSIONS: These data demonstrate the utility of multiplex ddPCR for large-scale population-based studies that screen for 22q11DS. The use of samples from blood spot cards suggests that this approach has promise for newborn screening of 22q11DS, and potentially for other microdeletion syndromes, for which early detection can positively impact clinical outcome for those affected.

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.

AGG interruptions and maternal age affect FMR1 CGG repeat allele stability during transmission.

BACKGROUND: The presence of AGG interruptions in the CGG repeat locus of the fragile X mental retardation 1 (FMR1) gene decreases the instability of the allele during transmission from parent to child, and decreases the risk of expansion of a premutation allele to a full mutation allele (the predominant cause of fragile X syndrome) during maternal transmission. METHODS: To strengthen recent findings on the utility of AGG interruptions in predicting instability or expansion to a full mutation of FMR1 CGG repeat alleles, we assessed the outcomes of 108 intermediate (also named gray zone) and 710 premutation alleles that were transmitted from parent to child, and collected from four international clinical sites. We have used the results to revise our initial model that predicted the risk of a maternal premutation allele expanding to a full mutation during transmission and to test the effect of AGG interruptions on the magnitude of expanded allele instability of intermediate or premutation alleles that did not expand to a full mutation. RESULTS: Consistent with previous studies, the number of AGG triplets that interrupts the CGG repeat locus was found to influence the risk of allele instability, including expansion to a full mutation. The total length of the CGG repeat allele remains the best predictor of instability or expansion to a full mutation, but the number of AGG interruptions and, to a much lesser degree, maternal age are also factors when considering the risk of transmission of the premutation allele to a full mutation. CONCLUSIONS: Our findings demonstrate that a model with total CGG length, number of AGG interruptions, and maternal age is recommended for calculating the risk of expansion to a full mutation during maternal transmission. Taken together, the results of this study provide relevant information for the genetic counseling of female premutation carriers, and improve the current predictive models which calculate risk of expansion to a full mutation using only total CGG repeat length.

Distribution of AGG interruption patterns within nine world populations.

The CGG trinucleotide repeat within the FMR1 gene is associated with multiple clinical disorders, including fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X syndrome. Differences in the distribution and prevalence of CGG repeat length and of AGG interruption patterns have been reported among different populations and ethnicities. In this study we characterized the AGG interruption patterns within 3,065 normal CGG repeat alleles from nine world populations including Australia, Chile, United Arab Emirates, Guatemala, Indonesia, Italy, Mexico, Spain, and United States. Additionally, we compared these populations with those previously reported, and summarized the similarities and differences. We observed significant differences in AGG interruption patterns. Frequencies of longer alleles, longer uninterrupted CGG repeat segments and alleles with greater than 2 AGG interruptions varied between cohorts. The prevalence of fragile X syndrome and FMR1 associated disorders in various populations is thought to be affected by the total length of the CGG repeat and may also be influenced by the AGG distribution pattern. Thus, the results of this study may be important in considering the risk of fragile X-related conditions in various populations.

Schooling and variation in the COMT gene: the devil is in the details.

BACKGROUND: Schooling is considered one of the major contributors to the development of intelligence within societies and individuals. Genetic variation might modulate the impact of schooling and explain, at least partially, the presence of individual differences in classrooms. METHOD: We studied a sample of 1,502 children (mean age = 11.7 years) from Zambia. Approximately 57% of these children were enrolled in school, and the rest were not. To quantify genetic variation, we investigated a number of common polymorphisms in the catechol-O-methyltransferase (COMT) gene that controls the production of the protein thought to account for >60% of the dopamine degradation in the prefrontal cortex. RESULTS: Haplotype analyses generated results ranging from the presence to absence of significant interactions between a number of COMT haplotypes and indicators of schooling (i.e., in- vs. out-of-school and grade completed) in the prediction of nonverbal intelligence, depending on the parameter specification. However, an investigation of the distribution of corresponding p-values suggested that these positive results were false. CONCLUSIONS: Convincing evidence that the variation in the COMT gene is associated with individual differences in nonverbal intelligence either directly or through interactions with schooling was not found. p-values produced by the method of testing for haplotype effects employed here may be sensitive to parameter settings, invalid under default settings, and should be checked for validity through simulation.

Gene variants associated with antisocial behaviour: a latent variable approach.

OBJECTIVE: The aim of this study was to determine if a latent variable approach might be useful in identifying shared variance across genetic risk alleles that is associated with antisocial behaviour at age 15 years. METHODS: Using a conventional latent variable approach, we derived an antisocial phenotype in 328 adolescents utilizing data from a 15-year follow-up of a randomized trial of a prenatal and infancy nurse-home visitation programme in Elmira, New York. We then investigated, via a novel latent variable approach, 450 informative genetic polymorphisms in 71 genes previously associated with antisocial behaviour, drug use, affiliative behaviours and stress response in 241 consenting individuals for whom DNA was available. Haplotype and Pathway analyses were also performed. RESULTS: Eight single-nucleotide polymorphisms (SNPs) from eight genes contributed to the latent genetic variable that in turn accounted for 16.0% of the variance within the latent antisocial phenotype. The number of risk alleles was linearly related to the latent antisocial variable scores. Haplotypes that included the putative risk alleles for all eight genes were also associated with higher latent antisocial variable scores. In addition, 33 SNPs from 63 of the remaining genes were also significant when added to the final model. Many of these genes interact on a molecular level, forming molecular networks. The results support a role for genes related to dopamine, norepinephrine, serotonin, glutamate, opioid and cholinergic signalling as well as stress response pathways in mediating susceptibility to antisocial behaviour. CONCLUSIONS: This preliminary study supports use of relevant behavioural indicators and latent variable approaches to study the potential 'co-action' of gene variants associated with antisocial behaviour. It also underscores the cumulative relevance of common genetic variants for understanding the aetiology of complex behaviour. If replicated in future studies, this approach may allow the identification of a 'shared' variance across genetic risk alleles associated with complex neuropsychiatric dimensional phenotypes using relatively small numbers of well-characterized research participants.

Transmission of an FMR1 premutation allele in a large family identified through newborn screening: the role of AGG interruptions.

The CGG repeat within the premutation range in the fragile X mental retardation 1 (FMR1) gene can lead to neurodegenerative disorders and intellectual disabilities. An increase in size upon the transmission from parent to child is more likely to occur for larger alleles and without AGG interruptions. We describe the molecular structure and the transmission of an FMR1 premutation allele in a multigenerational family, identified through newborn screening for fragile X syndrome. Transmission of the premutation allele was traced through five generations in 14 of the 23 individuals who were genotyped through cascade testing. Allele size instability during transmission was observed, but no expansions to a full mutation were detected. Clinical and molecular characterizations of the participants lead to the diagnosis of fragile X-associated tremor ataxia syndrome in one subject identified as a premutation carrier. A gradual small increase in the size of the premutation allele was observed during transmission through five generations. The relative stability is likely due to the presence of two AGGs within the allele. The detection of AGG interruptions within the premutation alleles is important in genetic counseling, to better predict the risk of expansion during transmission from a premutation to a full-mutation allele.

Fragile X AGG analysis provides new risk predictions for 45-69 repeat alleles.

We investigated the effect of AGG interruptions on fragile X repeat instability upon transmission of fragile X intermediate and small premutation alleles with 45-69 CGG repeats. The FMR1 repeat structure was determined for 375 mothers, 48 fathers, and 538 offspring (457 maternal and 81 paternal transmissions) using a novel PCR assay to determine repeat length and AGG interruptions. The number of AGG interruptions and the length of uninterrupted CGG repeats at the 3' end were correlated with repeat instability on transmission. Maternal alleles with no AGGs conferred the greatest risk for unstable transmissions. All nine full mutation expansions were inherited from maternal alleles with no AGGs. Furthermore, the magnitude of repeat expansion was larger for alleles lacking AGG interruptions. Transmissions from paternal alleles with no AGGs also exhibited greater instability than those with one or more AGGs. Our results demonstrate that characterization of the AGG structure within the FMR1 repeat allows more accurate risk estimates of repeat instability and expansion to full mutations for intermediate and small premutation alleles.

The COMT Val/Met polymorphism is associated with reading-related skills and consistent patterns of functional neural activation.

In both children and adults there is large variability in reading skill, with approximately 5-10% of individuals characterized as having reading disability; these individuals struggle to learn to read despite adequate intelligence and opportunity. Although it is well established that a substantial portion of this variability is attributed to the genetic differences between individuals, specifics of the connections between reading and the genome are not understood. This article presents data that suggest that variation in the COMT gene, which has previously been associated with variation in higher-order cognition, is associated with reading and reading-related skills, at the level of both brain and behavior. In particular, we found that the COMT Val/Met polymorphism at rs4680, which results in the substitution of the ancestral Valine (Val) by Methionine (Met), was associated with better performance on a number of critical reading measures and with patterns of functional neural activation that have been linked to better readers. We argue that this polymorphism, known for its broad effects on cognition, may modulate (likely through frontal lobe function) reading skill.

Comparison of whole-genome DNA methylation patterns in whole blood, saliva, and lymphoblastoid cell lines.

Epigenetic mechanisms, including DNA methylation, that underlie neuropsychiatric conditions have become a promising area of research. Most commonly used DNA sources in such studies are peripheral (whole) blood (WB), saliva (SL), and lymphoblastoid cell lines (LCLs); thus, the question of the consistency of DNA methylation patterns in those cells is of particular interest. To investigate this question we performed comparative analyses of methylation patterns in WB, SL, and LCLs derived from the same individuals, using Illumina HumanMethylation27 BeadChip arrays. Our results showed that DNA methylation patterns in SL are relatively consistent with those in WB, whereas the patterns in LCLs are similarly distinct from both WB and SL. The results indicated that due to multiple random and directed changes in DNA methylation throughout cell culturing, LCLs are not a reliable source of DNA for epigenetic studies and should be used with caution when investigating epigenetic mechanisms underlying biological processes.

AGG interruptions within the maternal FMR1 gene reduce the risk of offspring with fragile X syndrome.

PURPOSE: The ability to accurately predict the likelihood of expansion of the CGG repeats in the FMR1 gene to a full mutation is of critical importance for genetic counseling of women who are carriers of premutation alleles (55-200 CGG repeats) and who are weighing the risk of having a child with fragile X syndrome. The presence of AGG interruptions within the CGG repeat tract is thought to decrease the likelihood of expansion to a full mutation during transmission, thereby reducing risk, although their contribution has not been quantified. METHODS: We retrospectively analyzed 267 premutation alleles for number and position of AGG interruptions, length of pure CGG repeats, and CGG repeat lengths present in the offspring of the maternal transmissions. In addition, we determined the haplotypes of four markers flanking the 5'-UTR locus in the premutation mothers. RESULTS: We found that the presence of AGG interruptions significantly increased genetic stability, whereas specific haplotypes had a marginal association with transmission instability. CONCLUSION: The presence of AGG interruptions reduced the risk of transmission of a full mutation for all maternal (premutation) repeat lengths below ~100 CGG repeats, with a differential risk (0 vs. 2 AGG) exceeding 60% for alleles in the 70- to 80-CGG repeat range.

A balanced t(10;15) translocation in a male patient with developmental language disorder.

We report the clinical and cytogenetic findings on a male child with developmental language disorder, no physical abnormalities, and a balanced t(10;15)(q24.1;q21.1) translocation. As the child's parents are unavailable for investigations, it is unclear whether the translocation is inherited or de novo. Fluorescence in situ hybridization (FISH) analyses were carried out using specific RP11-BAC clones mapping near 15q21.1 and 10q24.1 to refine the location of the breakpoints. The breakpoint on 15q21.1 interrupts the SEMA6D gene and the breakpoint on 10q24.1 is located between the ENTPD1 and CCNJ genes. The SEMA6D gene was further investigated in samples of individuals with developmental language disorders and controls; this investigation offered further evidence of the involvement of SEMA6D with developmental language disorders.

The role of AGG interruptions in the transcription of FMR1 premutation alleles.

Fragile X associated disorders are caused by a premutation allele in the fragile X mental retardation 1 gene (FMR1) and are hypothesized to result from the toxic effect of elevated levels of expanded FMR1 transcripts. Increased levels of FMR1 mRNA have indeed been reported in premutation carriers; however the mechanism by which expanded alleles lead to elevated levels of FMR1 mRNA in premutation carriers is unknown. Within the CGG repeat tract AGG interruptions are found, generally 1-3 present in normal/intermediate alleles (6-54 CGG repeats) and usually 0-1 in premutation alleles (55-200 CGG repeats). They are present at specific locations, generally occurring after 9 or 10 uninterrupted CGG repeats [(CGG)(9)AGG(CGG)(9)AGG(CGG)(n)]. We evaluated both the number of AGG interruptions and the resulting length of the uninterrupted 3' CGG repeat pure tract in premutation alleles derived from two large cohorts of male and female carriers to determine whether the presence of AGG interruptions or the length of a pure stretch of CGG repeats influence the levels of FMR1 mRNA in blood. Our findings indicate that neither the number of AGG interruptions, nor their position along the CGG tract have a significant affect on mRNA levels in premutation carriers. We also, as expected based on previous findings, observed a highly significant correlation between CGG repeat number (as both total length and length of pure CGG stretch) and FMR1 mRNA expression levels, in both males and females. Importantly, we did not observe any significant difference in FMR1 mRNA levels in premutation carriers based on age.