DNA diagnosis of the fragile X syndrome in a series of 236 mentally retarded subjects and evidence for a reversal of mutation in the FMR-1 gene

A Systematic Review of Fragile X-Associated Neuropsychiatric Disorders

OBJECTIVE

Fragile X premutation carriers are reported to have increased neuropsychiatric problems, and thus the term fragile X-associated neuropsychiatric disorders (FXAND) has been proposed. Unfortunately, published prevalence estimates of these phenomena are inconsistent. This systematic review clarified this issue by reviewing both fragile X premutation prevalence in patients with neurodevelopmental disorders and psychiatric disorder prevalence in premutation carriers without fragile X-associated tremor/ataxia syndrome (FXTAS). Average prevalence was derived from studies that used semistructured clinical interviews, diagnostic criteria, and validated rating scales.

METHODS

Forty-six studies were reviewed. The rate of fragile X premutation in neurodevelopmental disorders was assessed from five studies. Probands with neurodevelopmental disorders were more likely than those in the general population to be premutation carriers. The rate of psychiatric disorders in premutation carriers was assessed from five studies for neurodevelopmental, 13 studies for mood, 12 studies for anxiety, and two studies for psychotic disorders. The phenotype and sex distribution among premutation carriers were similar to those with fragile X syndrome.

RESULTS

Compared to control group and general population estimates, the most prevalent psychiatric disorders were neurodevelopmental disorders, anxiety disorders, and bipolar II disorder. Psychiatric disorders were also more common in males. Most studies relied only on past medical history to define the prevalence of psychiatric disorders, yielding variability in results.

CONCLUSIONS

Future studies are needed to avoid bias by identifying cohorts from population-based sampling, to describe cohort demographic characteristics to elucidate differences in age and sex, and to prioritize the use of validated psychiatric assessment methods.

Glycogen Synthase Kinase-3 Inhibitors: Preclinical and Clinical Focus on CNS-A Decade Onward

The protein kinase, GSK-3, participates in diverse biological processes and is now recognized a promising drug discovery target in treating multiple pathological conditions. Over the last decade, a range of newly developed GSK-3 inhibitors of diverse chemotypes and inhibition modes has been developed. Even more conspicuous is the dramatic increase in the indications that were tested from mood and behavior disorders, autism and cognitive disabilities, to neurodegeneration, brain injury and pain. Indeed, clinical and pre-clinical studies were largely expanded uncovering new mechanisms and novel insights into the contribution of GSK-3 to neurodegeneration and central nerve system (CNS)-related disorders. In this review we summarize new developments in the field and describe the use of GSK-3 inhibitors in the variety of CNS disorders. This remarkable volume of information being generated undoubtedly reflects the great interest, as well as the intense hope, in developing potent and safe GSK-3 inhibitors in clinical practice.

Sensory Processing Phenotypes in Fragile X Syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder that causes intellectual disability. It is a leading known genetic cause of autism. In addition to cognitive, social, and communication deficits, humans with FXS demonstrate abnormal sensory processing including sensory hypersensitivity. Sensory hypersensitivity commonly manifests as auditory, tactile, or visual defensiveness or avoidance. Clinical, behavioral, and electrophysiological studies consistently show auditory hypersensitivity, impaired habituation to repeated sounds, and reduced auditory attention in humans with FXS. Children with FXS also exhibit significant visuospatial impairments. Studies in infants and toddlers with FXS have documented impairments in processing texture-defined motion stimuli, temporal flicker, perceiving ordinal numerical sequence, and the ability to maintain the identity of dynamic object information during occlusion. Consistent with the observations in humans with FXS, fragile X mental retardation 1 ( Fmr1) gene knockout (KO) rodent models of FXS also show seizures, abnormal visual-evoked responses, auditory hypersensitivity, and abnormal processing at multiple levels of the auditory system, including altered acoustic startle responses. Among other sensory symptoms, individuals with FXS exhibit tactile defensiveness. Fmr1 KO mice also show impaired encoding of tactile stimulation frequency and larger size of receptive fields in the somatosensory cortex. Since sensory deficits are relatively more tractable from circuit mechanisms and developmental perspectives than more complex social behaviors, the focus of this review is on clinical, functional, and structural studies that outline the auditory, visual, and somatosensory processing deficits in FXS. The similarities in sensory phenotypes between humans with FXS and animal models suggest a likely conservation of basic sensory processing circuits across species and may provide a translational platform to not just develop biomarkers but also to understand underlying mechanisms. We argue that preclinical studies in animal models of FXS can facilitate the ongoing search for new therapeutic approaches in FXS by understanding mechanisms of basic sensory processing circuits and behaviors that are conserved across species.

Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.

Identification of Males with Cryptic Fragile X Alleles by Methylation-Specific Quantitative Melt Analysis

BACKGROUND

FMR1 full mutations (FMs) (CGG expansion >200) in males mosaic for a normal (<45 CGG) or gray-zone (GZ) (45–54 CGG) allele can be missed with the standard 2-step fragile X syndrome (FXS) testing protocols, largely because the first-line PCR tests showing a normal or GZ allele are not reflexed to the second-line test that can detect FM.

METHODS

We used methylation-specific quantitative melt analysis (MS-QMA) to determine the prevalence of cryptic FM alleles in 2 independent cohorts of male patients (994 from Chile and 2392 from Australia) referred for FXS testing from 2006 to 2013. All MS-QMA–positive cases were retested with commercial triplet primed PCR, methylation-sensitive Southern blot, and a methylation-specific EpiTYPER-based test.

RESULTS

All 38 FMs detected with the standard 2-step protocol were detected with MS-QMA. However, MS-QMA identified methylation mosaicism in an additional 15% and 11% of patients in the Chilean and Australian cohorts, respectively, suggesting the presence of a cryptic FM. Of these additional patients, 57% were confirmed to carry cryptic expanded alleles in blood, buccal mucosa, or saliva samples. Further confirmation was provided by identifying premutation (CGG 55–199) alleles in mothers of probands with methylation-sensitive Southern blot. Neurocognitive assessments showed that low-level mosaicism for cryptic FM alleles was associated with cognitive impairment or autism.

CONCLUSIONS

A substantial number of mosaic FM males who have cognitive impairment or autism are not diagnosed with the currently recommended 2-step testing protocol and can be identified with MS-QMA as a first-line test.

Think about it: FMR1 gene mosaicism

Fragile X syndrome (FXS) is one of the most frequent causes of mental retardation, intellectual disability, and autism. Most cases are the result of an expansion of the CGG trinucleotide repeat in the 5' untranslated region of the FMR1 gene and the subsequent functional loss of the related protein. We describe the case of a 4-year-old boy who clinically presents mild psychomotor delay without any major clinical dysmorphisms. Molecular analysis of the FMR1 gene showed mosaicism in terms of size and methylation, with one normal and 1 fully mutated allele, which is very rare in this syndrome. Physicians should therefore consider a diagnosis of FXS even if the patient's phenotype is mild. Although rare, diagnosing this condition has important consequences for the patient's rehabilitation and the family planning of parents and relatives.

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.

Fragile X mosaic male full mutation/normal allele detected by PCR/MS-MLPA

We report on a fragile X mosaic male full mutation/normal allele detected by PCR and methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA). This combined analysis provides a diagnostic approach for fragile X syndrome (FXS). The method assesses the presence of expansion (full mutation), the CpG methylation status and could determine copy number changes (large deletions/duplications) along the FMR1 and FMR2 (fragile X mental retardation) genes. The method avoids detection of premutations, which makes it applicable for newborn screening. It can also be used in clarification of mosaic cases. The PCR results in our patient showed one normal allele; three repeats larger than his mother's one. The MS-MLPA showed hypermethylated full mutation pattern in the proband. Both results are compatible with FXS mosaic case full mutation/normal allele. The patient demonstrates atypical mild clinical manifestation of the disease, which correlates to the presence of a normal size allele in the patient's cells.

Mosaic FMR1 deletion causes fragile X syndrome and can lead to molecular misdiagnosis: a case report and review of the literature

The most common cause of fragile X syndrome is expansion of a CGG trinucleotide repeat in the 5'UTR of FMR1. This expansion leads to transcriptional silencing of the gene. However, other mutational mechanisms, such as deletions of FMR1, also cause fragile X syndrome. The result is the same for both the expansion mediated silencing and deletion, absence of the gene product, FMRP. We report here on an 11-year-old boy with a cognitive and behavioral profile with features compatible with, but not specific to, fragile X syndrome. A mosaic deletion of 1,013,395 bp was found using high-density X chromosome microarray analysis followed by sequencing of the deletion breakpoints. We review the literature of FMR1 deletions and present this case in the context of other FMR1 deletions having mental retardation that may or may not have the classic fragile X phenotype.

Epidemiology and Etiology of Mental Retardation

Mental retardation (MR) is a manifestation of a heterogeneous set of impairments and conditions that result in cognitive limitation. It is a condition of medical, educational, and social importance. Physicians identify profound, severe, and moderate MR but rarely diagnose mild MR unless it is associated with a genetic or medical syndrome. From a medical perspective, the quest for etiology and the possibility of medical or surgical intervention to minimize deterioration are paramount. Educators, on the other hand are less concerned with causation than with academic achievement and school success. The majority of cases of mild MR is identified in school settings. Finally, the public uses the label to describe poor adaptive skills. Adults with MR who hold jobs, live independently, and participate in society are not always described as having MR. Thus some individuals characterized in childhood or adolescence as having mild MR become indistinguishable from the general population in adulthood.

Cytogenetic abnormalities and fragile-X syndrome in Autism Spectrum Disorder

BACKGROUND

Autism is a behavioral disorder with impaired social interaction, communication, and repetitive and stereotypic behaviors. About 5-10 % of individuals with autism have 'secondary' autism in which an environmental agent, chromosome abnormality, or single gene disorder can be identified. Ninety percent have idiopathic autism and a major gene has not yet been identified. We have assessed the incidence of chromosome abnormalities and Fragile X syndrome in a population of autistic patients referred to our laboratory.

METHODS

Data was analyzed from 433 patients with autistic traits tested using chromosome analysis and/or fluorescence in situ hybridization (FISH) and/or molecular testing for fragile X syndrome by Southern and PCR methods.

RESULTS

The median age was 4 years. Sex ratio was 4.5 males to 1 female [354:79]. A chromosome (cs) abnormality was found in 14/421 [3.33 %] cases. The aberrations were: 4/14 [28%] supernumerary markers; 4/14 [28%] deletions; 1/14 [7%] duplication; 3/14 [21%] inversions; 2/14 [14%] translocations. FISH was performed on 23 cases for reasons other than to characterize a previously identified cytogenetic abnormality. All 23 cases were negative. Fragile-X testing by Southern blots and PCR analysis found 7/316 [2.2 %] with an abnormal result. The mutations detected were: a full mutation (fM) and abnormal methylation in 3 [43 %], mosaic mutations with partial methylation of variable clinical significance in 3 [43%] and a permutation carrier [14%]. The frequency of chromosome and fragile-X abnormalities appears to be within the range in reported surveys (cs 4.8-1.7%, FRAX 2-4%). Limitations of our retrospective study include paucity of behavioral diagnostic information, and a specific clinical criterion for testing.

CONCLUSIONS

Twenty-eight percent of chromosome abnormalities detected in our study were subtle; therefore a high resolution cytogenetic study with a scrutiny of 15q11.2q13, 2q37 and Xp23.3 region should be standard practice when the indication is autism. The higher incidence of mosaic fragile-X mutations with partial methylation compared to FRAXA positive population [50% vs 15-40%] suggests that faint bands and variations in the Southern band pattern may occur in autistic patients.

FMR1 Gene Deletion/Reversion: A Pitfall of Fragile X Carrier Testing

The Fragile X syndrome is, in the majority of cases, caused by CGG trinucleotide amplification within the FMR1 gene. The syndrome is rarely caused by point mutations or deletions. Here we describe a family with 2 sons and 1 daughter affected by Fragile X syndrome and 2 unaffected daughters whose carrier status was unknown prior to this study. Analysis of DNA from each of the 2 daughters revealed two alleles in the normal size range. However, 1 daughter carried one allele of 10 CGG repeats that was not present in either the mother or the father. No evidence for mosaicism could be detected. Haplotype analysis of flanking polymorphic markers revealed that the 10 CGG allele was derived from the mutated allele inherited from the mother. Thus, this case most likely represents an additional case of a reverse mutation from a premutation allele in a female to a normal-sized allele in the offspring. It remains unclear how frequently such reversion events occur. The observation has important consequences for genetic testing, because many laboratories prescreen for the Fragile X syndrome by determining the length of the CGG repeat using PCR. If this shows alleles in the normal size range, a diagnosis of Fragile X syndrome is considered to be excluded. Because the routine PCR and/or Southern blot analyses alone may yield false-negative results in cases of a regression of the number of CGG repeats, we strongly recommend the inclusion of fragment length or haplotype analysis when determining the carrier status within Fragile X syndrome families.

Molecular screening of FRAXA and FRAXE in Indian patients with unexplained mental retardation

Fragile-X mental retardation is the commonest form of inherited mental retardation. We have studied 146 Indian patients (174 X chromosomes) with unexplained mental retardation by molecular methods. All study subjects were unrelated. Three of the 118 males were found to have the FMR1 full mutation. None of the patients tested were positive for the FMR2 full mutation. The Fragile X prevalence was 2.5% among males, which is lower than previously reported in Indian mentally retarded patients. Screening for Fragile X among patients with nonspecific mental retardation is important, even if there is no family history of mental retardation or typical behavioral or physical features associated with the Fragile-X phenotype. Identification of positive cases is also very important for the families, because of the high recurrence risk of the disease. Large multicenter screening programs with uniform criteria would be worthwhile to determine the prevalence of Fragile-X mental retardation in the Indian population.

FMR1 and the fragile X syndrome: human genome epidemiology review

The fragile X syndrome, an X-linked dominant disorder with reduced penetrance, is one of the most common forms of inherited mental retardation. The cognitive, behavioral, and physical phenotype varies by sex, with males being more severely affected because of the X-linked inheritance of the mutation. The disorder-causing mutation is the amplification of a CGG repeat in the 5' untranslated region of FMR1 located at Xq27.3. The fragile X CGG repeat has four forms: common (6-40 repeats), intermediate (41-60 repeats), premutation (61-200 repeats), and full mutation (>200-230 repeats). Population-based studies suggest that the prevalence of the full mutation, the disorder-causing form of the repeat, ranges from 1/3,717 to 1/8,918 Caucasian males in the general population. The full mutation is also found in other racial/ethnic populations; however, few population-based studies exist for these populations. No population-based studies exist for the full mutation in a general female population. In contrast, several large, population-based studies exist for the premutation or carrier form of the disorder, with prevalence estimates ranging from 1/246 to 1/468 Caucasian females in the general population. For Caucasian males, the prevalence of the premutation is approximately 1/1,000. Like the full mutation, little information exists for the premutation in other populations. Although no effective cure or treatment exists for the fragile X syndrome, all persons affected with the syndrome are eligible for early intervention services. The relatively high prevalence of the premutation and full mutation genotypes coupled with technological advances in genetic testing make the fragile X syndrome amenable to screening. The timing as well as benefits and harms associated with the different screening strategies are the subject of current research and discussion.

The fragile X syndrome

The fragile X syndrome is characterised by mental retardation, behavioural features, and physical features, such as a long face with large protruding ears and macro-orchidism. In 1991, after identification of the fragile X mental retardation (FMR1) gene, the cytogenetic marker (a fragile site at Xq27.3) became replaced by molecular diagnosis. The fragile X syndrome was one of the first examples of a "novel" class of disorders caused by a trinucleotide repeat expansion. In the normal population, the CGG repeat varies from six to 54 units. Affected subjects have expanded CGG repeats (>200) in the first exon of the FMR1 gene (the full mutation). Phenotypically normal carriers of the fragile X syndrome have a repeat in the 43 to 200 range (the premutation). The cloning of the FMR1 gene led to the characterisation of its protein product FMRP, encouraged further clinical studies, and opened up the possibility of more accurate family studies and fragile X screening programmes.

Screening and diagnosis for the fragile X syndrome among the mentally retarded: an epidemiological and psychological survey. Collaborative Fragile X Study Group

The fragile X syndrome is an X-linked mental retardation disorder caused by an expanded CGG repeat in the first exon of the fragile X mental retardation (FMR1) gene. Its frequency, X-linked inheritance, and consequences for relatives all prompt for diagnosis of this disorder on a large scale in all affected individuals. A screening for the fragile X syndrome has been conducted in a representative sample of 3,352 individuals in schools and institutes for the mentally retarded in the southwestern Netherlands, by use of a brief physical examination and the DNA test. The attitudes and reactions of (non)consenting parents/guardians were studied by (pre- and posttest) questionnaires. A total of 2,189 individuals (65%) were eligible for testing, since they had no valid diagnosis, cerebral palsy, or a previous test for the FMR1 gene mutation. Seventy percent (1,531/2,189) of the parents/guardians consented to testing. Besides 32 previously diagnosed fragile X patients, 11 new patients (9 males and 2 females) were diagnosed. Scoring of physical features was effective in preselection, especially for males (sensitivity .91 and specificity .92). Major motives to participate in the screening were the wish to obtain a diagnosis (82%), the hereditary implications (80%), and the support of research into mental retardation (81%). Thirty-four percent of the parents/guardians will seek additional diagnostic workup after exclusion of the fragile X syndrome. The prevalence of the fragile X syndrome was estimated at 1/ 6,045 for males (95% confidence interval 1/9,981-1/ 3,851). On the basis of the actual number of diagnosed cases in the Netherlands, it is estimated that >50% of the fragile X cases are undiagnosed at present.

The number of CGG repeats of the FMR1 locus in premutated and fully mutated heterozygotes and their offspring: implications for the origin of mosaicism

The size of the CGG repeat of the FMR1 gene was investigated with probe StB12.3 in 154 transmissions to the offspring of heterozygotes for the premutation and the full mutation. Among the 135 offspring of premutated heterozygotes there were three decreases in size of the repeats: in two of these cases a full mutation was present along with the decreased premutation, and in a third mosaic (46,fra(X)(q27.3),Y), a normal allele was observed. In the 19 offspring of fully mutated females with no detected mosaicism, there were three mosaics and three individuals who had full mutations that included a number of repeats smaller than those present in their mothers. Among the 32 offspring who received a premutation from their premutated mothers, 27 alleles were increased in size and 5 remained unaltered. Among 11 mosaic offspring of premutated mothers, the premutation increased in 4, decreased in 3, and was unchanged in 4. In contrast to the trend of an increasing premutation size in the non-mosaic offspring, the premutation present in mosaics can be smaller, larger, or of unaltered size with approximately equal frequencies. These data suggest that the premutations present in mosaics result from mitotic instability of the inherited full mutations. This is further supported by the finding of a mosaic male with a normal sized allele.

The fragile X phenotype in a mosaic male with a deletion showing expression of the FMR1 protein in 28% of the cells

The instability of the CGG repeat region of FMR1 is not restricted to the CGG repeat but expands to flanking sequences as well. A mosaic fragile X male is reported with a deletion of part of the CGG repeat and 30 bp immediately 3' of the repeat, thus confirming the presence of a hotspot for deletions in the CGG region of FMR1. The deletion, detected in 28% of his lymphocytes, did not impair the transcription and translation of FMR1, suggesting that regulatory elements are not present in the deleted region. The patient has the characteristic fragile X phenotype and assuming that the mosaic pattern detected in the lymphocytes reflects the mosaic pattern in brain, 28% expression of FMRP may not be sufficient for normal cognitive functioning.

Molecular and cytogenetic analysis of the fragile X syndrome in a series of 453 mentally retarded subjects: a study of 87 families

We report on a series of 453 mentally retarded subjects investigated for fragile X syndrome from 1982 to July 1995. The 22% rate of efficiency of FRAX positivity indicated a significant preselection by the clinicians. However, this rate dropped to 11% in the last year. Since 1992, Southern blot analysis was extended to include family members of the 87 positive subjects, for a total of 442 individuals examined with the probe StB12.3. In addition to premutated (118), fully mutated (148), and pre/full mutation mosaic subjects (27), 14 atypical cases were found. Some of these cases are described in more detail. In particular, we report on the hybridization and polymerase chain reaction data of 2 fragile X subjects with full mutation and a 2.8-kb allele and 1 with full mutation and a 2.4-kb allele. An intellectually normal male with 18% of fraXq27.3 and an unmethylated full mutation is also described. Finally, a mentally retarded child with only a lower allele of 2.7 kb is presented.

A PCR-based test suitable for screening for fragile X syndrome among mentally retarded males

Ever since the identification of the genetic cause of fragile X syndrome as the expansion of an unstable trinucleotide sequence, several diagnostic strategies have evolved from molecular studies. However, we still lack a simple test suitable for population screening. We have therefore developed a nonisotopic polymerase chain reaction (PCR)-based technique for the identification of fragile X full mutations among men, with easy visualization of the PCR products on silver-stained polyacrylamide gels. The technique consists of PCR amplification with primers that flank the trinucleotide repeats, with a product of 557 bp for the (CGG)29 allele. Conditions were established such that full mutations failed to amplify and were thus identified with 98% sensitivity compared with Southern blot analysis. To produce an indispensable internal control we added to the reaction a third primer, internal to this fragment, allowing the multiplex amplification of a monomorphic band corresponding to a CG-rich stretch 147 bp upstream of the polymorphic region. In trials involving 41 patients and 74 controls, the PCR-based test here described showed specificity of more than 98.6%, accuracy of 99% and a sensitivity of 98%. Thus, although not suitable for medical diagnosis, it constitutes a useful tool for screening for the fragile X syndrome in populations of mentally retarded males.

Mosaicism for the fragile X syndrome full mutation and deletions within the CGG repeat of the FMR1 gene

The main mutation responsible for the fragile X syndrome is the expansion of an untranslated CGG repeat in the first exon of the FMR1 gene, associated with the hypermethylation of the proximal CpG island and the CGG repeat region, and repression of transcription of FMR1. Fragile X syndrome mosaicism has been described as the coexistence of the full mutation and the permutation. We present here two cases of mosaicism for the full mutation in the FMR1 gene and deletions involving the CGG repeat region. In one case the deletion removed 113 bp proximal to the CGG repeat and part of the repeat itself, leaving 30 pure repeats, and representing 17% of lymphocytes of the patient. The 5' breakpoint of this deletion falls outside the putative hotspot for deletions in the CGG region of FMR1. In the second case the deleted region only involved the CGG sequence (leaving 15 pure repeats), with normal sequences flanking the repeat; this deleted ("normal") FMR1 was estimated to be in about 31% of blood lymphocytes. This second case can be considered a true regression of the CGG FMR1 expansion to a normal sized allele, although in mosaic form.

DNA testing for fragile X syndrome in schools for learning difficulties

Fragile X syndrome is the most common inherited cause of mental retardation. Early diagnosis is important not only for appropriate management of individuals but also to identify carriers who are unaware of their high risk of having an affected child. The disorder is associated with a cytogenetically visible fragile site (FRAXA) at Xq27.3, caused by amplification of a (CGG)n repeat sequence within the gene at this locus designated FMR1. Clinical and molecular studies have been undertaken to screen for fragile X syndrome in 154 children with moderate and severe learning difficulties of previously unknown origin. Southern blot analysis of peripheral blood showed the characteristic abnormally large (CGG)n repeat sequence associated with fragile X syndrome in four of the 154 children. The findings were confirmed by cytogenetic observation of the fragile site and by further molecular studies. The families of the affected children were offered genetic counselling and DNA tests to determine their carrier status. These findings show that there are still unrecognised cases of fragile X syndrome. Given the difficulty of making a clinical diagnosis and the implications for families when the diagnosis is missed, screening in high risk populations may be justified. The issues involved in screening all children in special schools for fragile X syndrome are discussed.