A rapid, non-radioactive screening test for fragile X mutations at the FRAXA and FRAXE loci

Simultaneous Screening of the FRAXA and FRAXE Loci for Rapid Detection of FMR1 CGG and/or AFF2 CCG Repeat Expansions by Triplet-Primed PCR

Moderate to hyper-expansion of trinucleotide repeats at the FRAXA and FRAXE fragile sites, with or without concurrent hypermethylation, has been associated with intellectual disability and other conditions. Unlike molecular diagnosis of FMR1 CGG repeat expansions in FRAXA, current detection of AFF2 CCG repeat expansions in FRAXE relies on low-throughput and otherwise inefficient techniques combining Southern blot analysis and PCR. A novel triplet-primed PCR assay was developed for simultaneous screening for trinucleotide repeat expansions at the FRAXA and FRAXE fragile sites, and was validated using archived clinical samples of known FMR1 and AFF2 genotypes. Population samples and FRAXE-affected samples were sequenced for the evaluation of variations in the AFF2 CCG repeat structure. The duplex assay accurately identified expansions at the FMR1 and AFF2 trinucleotide repeat loci. On Sanger sequencing of the AFF2 CCG repeat, the single-nucleotide polymorphism variant rs868914124(C) that effectively adds two CCG repeats at the 5'-end, was enriched in the Malay population and with short repeats (<11 CCGs), and was present in all six expanded AFF2 alleles of this study. All expanded AFF2 alleles contained multiple non-CCG interruptions toward the 5'-end of the repeat. A sensitive, robust, and rapid assay has been developed for the simultaneous detection of expansion mutations at the FMR1 and AFF2 trinucleotide repeat loci, simplifying screening for FRAXA- and FRAXE-associated disorders.

Genetic causes of intellectual disability in a birth cohort: a population-based study

Intellectual disability affects approximately 1–3% of the population and can be caused by genetic and environmental factors. Although many studies have investigated the etiology of intellectual disability in different populations, few studies have been performed in middle‐income countries. The present study estimated the prevalence of genetic causes related to intellectual disability in a cohort of children from a city in south Brazil who were followed from birth. Children who showed poor performance in development and intelligence tests at the ages of 2 and 4 were included. Out of 4,231 liveborns enrolled in the cohort, 214 children fulfilled the inclusion criteria. A diagnosis was established in approximately 90% of the children evaluated. Genetic causes were determined in 31 of the children and 19 cases remained unexplained even after extensive investigation. The overall prevalence of intellectual disability in this cohort due to genetic causes was 0.82%. Because this study was nested in a cohort, there were a large number of variables related to early childhood and the likelihood of information bias was minimized by collecting information with a short recall time. This study was not influenced by selection bias, allowing identification of intellectual disability and estimation of the prevalence of genetic causes in this population, thereby increasing the possibility of providing appropriate management and/or genetic counseling. © 2015 The Authors. American Journal of Medical Genetics Part A Published by Wiley Periodicals, Inc.

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

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

Capturing the fragile X premutation phenotypes: a collaborative effort across multiple cohorts

OBJECTIVE

To capture the neuropsychological profile among male carriers of the FMR1 premutation allele (55-200 CGG repeats) who do not meet diagnostic criteria for the late-onset fragile X-associated tremor/ataxia syndrome, FXTAS.

METHOD

We have initiated a multicenter collaboration that includes 3 independent cohorts, totaling 100 carriers of the premutation and 216 noncarriers. The initial focus of this collaboration has been on executive function. Four executive function scores are shared among the 3 cohorts (Controlled Oral Word Association Test, Stroop Color-Word Test, and Wechsler backward digit span and letter-number sequencing) whereas additional executive function scores are available for specific cohorts (Behavior Dyscontrol Scale, Hayling Sentence Completion Test Part B, and Wisconsin Card Sorting Test). Raw scores were analyzed by using statistical models that adjust for cohort-specific effects as well as age and education.

RESULTS

Carriers scored significantly lower compared to noncarriers on the Stroop Color-Word Test (p = .01), Hayling Sentence Completion Test Part B (p < .01), and Behavioral Dyscontrol Scale (p = .03), with the Hayling displaying a significant age-related decline (p = .01), as assessed by an age and repeat length-group interaction. Follow-up analysis of the collective data did not identify any specific age groups or repeat length ranges (i.e., low premutation = 55-70 repeats, midpremutation = 71-100 repeats, high premutation = 101-199 repeats) that were associated with an increased risk of executive function deficits.

CONCLUSIONS

Preliminary analyses do not indicate global executive function impairment among male carriers without FXTAS compared to noncarriers. However, impairment in inhibitory capacity may be present among a subset of carriers, though the risk factors for this group do not appear to be related to age or repeat length.

Distribution of CGG/GCC repeats at the FMR1 and FMR2 genes in an Indian population with mental retardation of unknown etiology

AIMS

Fragile X syndrome is one of the X-linked disorders associated with moderate to severe mental retardation. Fragile X A syndrome (FRAXA) and fragile X E syndrome (FRAXE) are caused by trinucleotide repeat expansion of CGG and GCC repeats at the 5' untranslated region of the FMR1 and FMR2 genes, respectively. The present study was undertaken to identify the repeat polymorphism and to estimate the risk of transmission in Andhra Pradesh and surrounding states of South India.

RESULTS

The FRAXA and FRAXE allelic polymorphisms were studied by radioactive polymerase chain reaction that revealed 25 FRAXA among 344 X-chromosomes and 20 FRAXE allelic variants among 212 X-chromosomes in our population. The most frequent FRAXA allele size was of 29 CGG repeats (27.5%) followed by allele sizes of 28 (20.8%) and 31 (7.2%), and that of FRAXE was 15 GCC repeats (24.0%) followed by allele containing 18 repeats (18.4%) and 16 repeats (11.3%).

CONCLUSIONS

CGG/GCC repeat polymorphism at the FMR1 and FMR2 loci observed in this study demonstrated a racial and ethnic variation among the populations.

Molecular diagnosis of Fragile X syndrome

Fragile X syndrome, the most prevalent inherited cause of mental retardation, is related to hyperexpansion of a polymorphic CGG repeat of the FMR1 gene. Expansion of 55-200 repeats are called premutations and characterize carriers who usually have no mental impairment. The disease causing full mutations exceed 200 CGG repeats, are hypermethylated and lead to transcriptional silencing of the gene and absence of the Fragile X mental retardation protein (FMRP). Diagnostic approaches involve molecular and immunocytochemical techniques. Southern blot, which allows mutations to be detected and methylation status to be determined in a single test, remains the procedure of choice for most laboratories. Modifications of PCR methods, including methylation specific PCR, are also proposed but their implementation is still in question because of inherent difficulties to amplify CGG repeats, distinguish between mosaic patterns and interpret results in female individuals. The FMRP antibody test is also suitable for large population screening and elucidation of Fragile X syndrome cases with no CGG expansion, but it is not widely applied. In search for novel diagnostic approaches, use of PCR as a first prescreening test followed by Southern blot is considered the most reliable procedure.

Lifespan changes in working memory in fragile X premutation males

Fragile X syndrome is the world's most common hereditary cause of developmental delay in males and is now well characterized at the biological, brain and cognitive levels. The disorder is caused by the silencing of a single gene on the X chromosome, the FMR1 gene. The premutation (carrier) status, however, is less well documented but has an emerging literature that highlights a more subtle profile of executive cognitive deficiencies that mirror those reported in fully affected males. Rarely, however, has the issue of age-related declines in cognitive performance in premutation males been addressed. In the present study, we focus specifically on the cognitive domain of working memory and its subcomponents (verbal, spatial and central executive memory) and explore performance across a broad sample of premutation males aged 18-69 years matched on age and IQ to unaffected comparison males. We further tease apart the premutation status into those males with symptoms of the newly identified neurodegenerative disorder, the fragile X-associated tremor/ataxia syndrome (FXTAS) and those males currently symptom-free. Our findings indicate a specific vulnerability in premutation males on tasks that require simultaneous manipulation and storage of new information, so-called executive control of memory. Furthermore, this vulnerability appears to exist regardless of the presence of FXTAS symptoms. Males with FXTAS symptoms demonstrated a more general impairment encompassing phonological working memory in addition to central executive working memory. Among asymptomatic premutation males, we observed the novel finding of a relationship between increased CGG repeat size and impairment to central executive working memory.

Impact of the Fragile X mental retardation 1 (FMR1) gene premutation on neuropsychiatric functioning in adult males without fragile X-associated Tremor/Ataxia syndrome: a controlled study

Fragile X Syndrome is the most common heritable form of mental retardation caused by silencing of the FMR1 gene, which arises from intergenerational trinucleotide repeat expansion leading to full mutation. An intermediary carrier condition, known as the premutation, is characterized by expansion up to 200 repeats without concomitant gene silencing. This prevalent allelic variant was initially thought to be free of phenotypic effects. However, recent reports have identified a degenerative disease, Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) in older men as well as premature ovarian failure in women. Previously reports are inconsistent regarding the neuropsychiatric phenotype associated with premutation due to small sample sizes, ascertainment bias, lack of adequate control groups, administration of measures with poor psychometric properties, and the confounding effects of FXTAS. We addressed these problems by conducting a controlled study of male carriers (n = 40) of the premutation without manifest symptoms of FXTAS, comparing their responses on specific, reliable, and valid measures of neuropsychiatric functioning to those of individuals with shared family environment (n = 22) and non-carrier comparison males (n = 43). Multivariate analyses revealed that the premutation confers significant risk for working memory difficulties, an associated feature of Attention-Deficit Disorder. Furthermore, both the family controls and men with premutation exhibited higher rates of Alcohol Abuse as compared to non-carrier control men. These findings highlight the importance of recognizing the distinct phenotypic outcomes that characterize the Fragile X premutation and the subtle risk factors that can act as precursors to more significant psychiatric impairment.

Age-dependent cognitive changes in carriers of the fragile X syndrome

Fragile X syndrome is a neurodevelopmental disorder that is caused by the silencing of a single gene on the X chromosome, the fragile X mental retardation 1 (FMR1) gene. Affected individuals display a unique neurocognitive phenotype that includes significant impairment in inhibitory control, selective attention, working memory, and visual-spatial cognition. In contrast, little is known about the trajectory and specificity of any cognitive impairment associated with the fragile X premutation (i.e., "carrier status") or its relationship with the recently identified neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). In the present study, we evaluated a broad sample of 40 premutation males (PM) aged 18-69 years matched on age and IQ to 67 unaffected comparison males (NC). Performance was compared across a range of cognitive domains known to be impaired in fragile X syndrome (i.e., "full mutation"). Tremor was also assessed using a self-report neurological questionnaire. PM displayed statistically significant deficits in their ability to inhibit prepotent responses, differentiating them from NC from age 30 onwards. With increasing age, the two groups follow different trajectories, with PM developing progressively more severe problems in inhibitory control. This deficit also has a strong co-occurrence in males displaying FXTAS-related symptomatology (p<.001). Selective attention was also impaired in PM but did not show any disproportionate aging effect. No other cognitive deficits were observed. We conclude that an inhibitory deficit and its impact across the lifespan are specifically associated with the fragile X premutation status, and may be a precursor for development of a more severe form of cognitive impairment or dementia, which has been reported in patients with the diagnosis of FXTAS.

Consensus characterization of 16 FMR1 reference materials: a consortium study

Fragile X syndrome, which is caused by expansion of a (CGG)(n) repeat in the FMR1 gene, occurs in approximately 1:3500 males and causes mental retardation/behavioral problems. Smaller (CGG)(n) repeat expansions in FMR1, premutations, are associated with premature ovarian failure and fragile X-associated tremor/ataxia syndrome. An FMR1-sizing assay is technically challenging because of high GC content of the (CGG)(n) repeat, the size limitations of conventional PCR, and a lack of reference materials available for test development/validation and routine quality control. The Centers for Disease Control and Prevention and the Association for Molecular Pathology, together with the genetic testing community, have addressed the need for characterized fragile X mutation reference materials by developing characterized DNA samples from 16 cell lines with repeat lengths representing important phenotypic classes and diagnostic cutoffs. The alleles in these materials were characterized by consensus analysis in nine clinical laboratories. The information generated from this study is available on the Centers for Disease Control and Prevention and Coriell Cell Repositories websites. DNA purified from these cell lines is available to the genetics community through the Coriell Cell Repositories. The public availability of these reference materials should help support accurate clinical fragile X syndrome testing.

PCR approach for detection of Fragile X syndrome and Huntington disease based on modified DNA: limits and utility

A group of mutations characterized by trinucleotide repeat expansion causes human diseases such as the Fragile X syndrome, Huntington disease (HD), and myotonic dystrophy. Methods based on PCR amplification of the CGG and CAG repeats region could facilitate the development of a rapid screening assay; unfortunately, amplification across CGG and CAG repeats can be inefficient and unreliable due to the G + C base composition. The utility of the PCR on modified DNA for amplification of the CGG and CAG repeats at the Fragile X syndrome and HD has been reported. In the present study, we analyzed the utility of PCR on modified DNA as a rapid screening method for diagnosis of patients with Fragile X syndrome and HD. A comparative analysis realized with 38 Fragile X and 29 HD patients showed that the molecular diagnosis by simple PCR on modified DNA has a sensitivity and specificity of 100% in Fragile X patients and 94.1% and 91.6% in HD patients. The results achieved from the statistical analysis allowed us to conclude that the amplification by simple PCR on modified DNA is a reliable and useful method for the molecular diagnosis of the Fragile X syndrome, but not for the HD.

The emerging fragile X premutation phenotype: Evidence from the domain of social cognition

Fragile X syndrome is a neurodevelopmental disorder that is caused by large methylated expansions of a CGG repeat (>200) region upstream of the FMR1 gene that results in the lack of expression of the fragile X mental retardation protein (FMRP). Affected individuals display a neurobehavioral phenotype that includes a significant impairment in social cognition alongside deficits in attentional control, inhibition and working memory. In contrast, relatively little is known about the trajectory and specificity of any cognitive impairment associated with the fragile X premutation ("carrier-status") (approximately 55-200 repeats). Here, we focus on one aspect of cognition that has been well documented in the fragile X full mutation, namely social cognition. The results suggest that premutation males display a pattern of deficit similar in profile, albeit milder in presentation, to that of the full mutation. However, little evidence emerged for a correlation between CGG repeat length and severity of phenotypic outcomes. The findings are discussed in the context of functional neuroimaging and brain-behaviour-molecular correlates. We speculate that the deficiencies in social cognition are attributable to impairment of neural pathways modulated by the cerebellum.

Mental retardation

Mental retardation (MR) occurs in 2-3% of the general population. Prevalence of milder MR is seven to ten times more than severe MR. Cause of severe MR can be determined in 60-70% of cases, as compared to mild MR where 35-55% remain idiopathic. The diagnostic process is aided considerably if the timing of a developmental insult can be determined: prenatal, periatal, postnatal (not mutually exclusive). History plays a pivotal role in approaching a diagnosis. After clinical evaluation one should be able to assess whether the disorder is static or progressive; approximate developmental quotient; possible timing of insult and possible underlying genetic etiology. Investigations should be based on history and physical examination. The important category of tests include: thyroid function tests, cytogenetic studies, metabolic work-up, fragile-X screening, radiological investigations, electrophysiological studies and specific tests according to the suspected diagnosis. Having an etiological explanation aids in the development of a specific treatment plan; helps families understand prognosis and recurrence risk and on the community level assists in the development of preventive strategies.

A new PCR assay useful for screening of FRAXE/FMR2 mental impairment among males

FRAXE (FMR2) is a fragile site associated with mental impairment located in Xq28, 600 kb distal to FRAXA (FMR1), the fragile X syndrome fragile site. The FRAXE mutation is an expansion of a CCG repeat that results in methylation of a nearby CpG island. FRAXE alleles could be divided into four categories: normal (6-30 CCG repeats), intermediate (31-60 CCG repeats), premutation (61-200 CCG repeats), and full mutation (over 200 repeats). We have developed a non-isotopic polymerase chain reaction (PCR)-based assay for the identification of FRAXE full mutation alleles among mentally impaired men. In this novel PCR test for the FRAXE locus, we used three primers to permit an amplification of a 223 bp monomorphic internal control fragment in addition to the amplification of a 419 bp (CCG)(16) FRAXE locus band. A linear series of 93 male patients referred for FRAXE testing but found to be negative for the (CCG)(n) expansion in the FMR2 gene by Southern blotting analysis were retested by our PCR technique. In addition, we analyzed two positive controls consisting of a FRAXE fully mutated male and one male with a Xq terminal deletion. The developed PCR test showed accuracy of 100% in the normal individuals retested by PCR analysis, as well as in the two positive control samples utilized, in which the strategy of multiplex amplification worked as expected. Although not suitable for medical diagnosis of females and mosaics, it constitutes an important strategy for PCR typing and for FRAXE population screening.

An effective strategy of using molecular testing to screen mentally retarded individuals for fragile X syndrome

Fragile X syndrome (FXS) is the most common form of familial mental retardation (MR). It is caused by the expansion of the CGG repeat in the FMR1 gene on the X chromosome. To date, FXS is not treatable, but can be prevented by prenatal genetic examination. Identifying women who carry a full mutation or premutation FMR1 gene is thus very important, and can be done by tracing family members of FXS subjects. However, most of the FXS subjects in Taiwan as well as those in many other countries have not been identified. In this study the authors attempt to develop reliable and inexpensive tests suitable for a large-scale screen of subjects with MR for FXS. Together with their previous study, a total of 311 male and 160 female subjects with MR were screened with nonradioactive Southern blot assay using mixed deoxyribonucleic acid from three subjects of the same sex. From these subjects, nine male subjects and one female FXS subject were diagnosed. All male subjects were also screened with nonradioactive polymerase chain reaction (PCR). These nine male FXS subjects were also detected on the basis of PCR amplification failure. No false-negative results were discerned. The PCR procedure was simplified further by combining it with an analysis of a blood spot on filter paper, which is a much simpler and cheaper method for sample collection and DNA preparation. This method was then used to screen 104 boys with MR. Two of them were suspected, and later confirmed with Southern blot assay, as subjects with FXS. This study suggests that simple PCR combined with blood spot analysis could be a reliable, inexpensive test that is feasible for a large-scale screening of male subjects with MR for FXS. However, Southern blot assay with mixed deoxyribonucleic acid is appropriate for screening female subjects. Based on this strategy, most FXS subjects could be identified easily for further management.

FRAXA and FRAXE prevalence in patients with nonspecific mental retardation in the Hellenic population

Mutations at FRAXA and FRAXE loci are due to expansions of a CGG trinucleotide repeat and are characterized by mental retardation. Here we report a pilot screening survey by means of cytogenetic and molecular methods of 433 unrelated retarded individuals and their parents of Hellenic origin coming from various parts of Greece and Cyprus. The purpose of the study was to estimate the frequency of FRAXA mutation in individuals with nonspecific mental retardation without family history and phenotypic stigmata in the Hellenic population. Five FRAXA-positive children (1.15%) were identified, of whom four were found to carry a full mutation and one a premutation. Furthermore we present preliminary data on a screening of FRAXE mutation frequency. We screened 257 male patients with nonspecific mental retardation, finding none positive for FRAXE mutation.

Implication of screening for FMR1 and FMR2 gene mutation in individuals with nonspecific mental retardation in Taiwan

Fragile X syndrome (FXS) is the most common form of familial mental retardation (MR), attributable to (CGG)n expansion in the FMR1 gene. FRAXE is less frequent, associated with a similar mutation of the FMR2 gene. This study attempted to ascertain the prevalence of both disorders in Taiwan, as well as to develop a method to effectively find carriers. A total of 321 patients with nonspecific MR were screened for the FMR1 and FMR2 mutation. Four of 206 boys and men (1.9%) and 1 in 115 girls and women (0.9%) were identified as having FXS. All four FXS boys or men could be identified by Southern blot analysis, as well as by a simple nonradioactive polymerase chain reaction analysis. None of the 206 boys or men had FMR2 full mutation. This confirmed the low incidence of FRAXE in Chinese. FXS appears to be more prevalent among patients with mild MR, because 4 of the 5 patients with FXS were from the 115 with mild MR (3.48%) and only 1 was from the other 206 with severe MR (0.49%). All five FXS cases were maternally inherited. Other family members were resistant to further searching for carriers. It is worth noting that none of these mothers had a discernible premarital family history of MR. Thus the negative family history could not preclude the possibility that a woman was a carrier. To identify female carriers of childbearing age, beyond the scope of family history, is thus worthy of further exploration. Screening men for carriers using this inexpensive method is probably feasible, even though normal transmitting men have no immediate risk of producing a child with the disease. Female carriers can then be effectively identified from these normal transmitting men and can take all preventive measures.

Molecular diagnosis of the fragile X and FRAXE syndromes in patients with mental retardation of unknown cause in Mexico

The fragile X syndrome (Fra-X) is the most common cause of inherited mental retardation with X-linked semi-dominant inheritance. The prevalence of Fra-X in the Mexican population is unknown. The aim of this population screening study was to determine if Fra-X or FRAXE mutations are the cause of a number of cases of mental retardation in a sample of Mexican children with mental retardation of unknown cause (MRUC) and to stress the importance of performing molecular analysis of the FMR-1 gene in all patients with MRUC. We report here the direct analysis of CGG and GCC repeats within the FMR-1 and FMR-2 genes, respectively, in 62 unrelated patients with MRUC. Two male index cases had the CGG expansion, although they did not express the Xq27.3 fragile site cytogenetically. Fra-X diagnosis was highly suspected on a clinical basis in one of the patients, but not in the other. Both mothers were found to be premutation carriers. The molecular studies of FMR-1 showed that the proportion of MRUC patients with Fra-X is 3.2%. This frequency was not significantly different to that reported in most populations. As reported in other series, no patients with FRAXE were found in our sample. Our findings confirm that the molecular analysis of the FMR-1 gene is necessary in MRUC patients to achieve unequivocal diagnosis of fragile X syndrome, carrier premutation detection and for accurate genetic counseling.

Methylation analysis of the fragile X syndrome by PCR

The fragile X syndrome is predominantly caused by a large expansion of a CGG trinucleotide repeat in the promoter region of the FMR1 gene, which is associated with methylation and downregulation of transcription. The molecular diagnosis of this disorder is based on repeat size and methylation analysis of the FMR1 gene usually by Southern blot analysis. We describe a PCR-based method for the analysis of methylation of the FMR1 gene, which involves bisulfite treatment of DNA prior to amplification. Fifty-two normal and 48 affected, premutation, or mosaic males were analyzed in a blinded study by this method. A prospective study of 30 males suspected of fragile X was also performed. Amplification specific for the methylated FMR1 sequence was readily observed in all individuals with a full mutation, whereas all normal and premutation individuals showed only amplification-specific for the unmethylated sequence, thus, allowing affected and unaffected males to be distinguished. A full mutation in the presence of mosaicism was also detectable by this method. Methylation-specific PCR appears to be a rapid and reliable tool for the diagnosis of fragile X males.

Mosaicism for the full mutation and a microdeletion involving the CGG repeat and flanking sequences in the FMR1 gene in eight fragile X patients

The molecular mechanism of the fragile X syndrome is based on the expansion of an unstable CGG repeat in the 5' untranslated region of the FMR1 gene in most patients. This expansion is associated with an abnormal DNA methylation leading to the absence of production of FMR1 protein (FMRP). Such expansion apparently predisposes the repeat and flanking regions to further instability that may lead to mosaic conditions with a full mutation and a premutation or, rarely, with normal or reduced alleles that can sometimes be transcriptionally active. In this study we describe eight unrelated fragile X patients who are mosaic for both a full mutation and an allele of normal (four cases) or reduced size (four cases). Sequencing analysis of the deletion breakpoints in 6 patients demonstrated an internal deletion confined to the CGG repeat in four of them, which represents the most likely explanation for the regression of the full mutation to a normal sized allele. In two patients with a reduced allele, the deletion encompassed the entire CGG repeat and part of the flanking regions. Analysis of FMRP by Western blot was performed in one of the mosaics with a normal sized allele and in three of those with a reduced allele. In the first patient's lymphocytes FMRP was detected, whereas in the three other patients the deletion is likely to impair transcription as no FMRP was present in their lymphocytes.

Expand Long PCR for fragile X mutation detection

Fragile X mutation detection by DNA analysis enables accurate diagnosis of the fragile X syndrome. The mutation involves the expansion of CGG repeats in the FMR1 gene and has been primarily detected by the Southern blotting method. In this study we present a novel, efficient and reliable PCR protocol that is more convenient for routine diagnosis of the fragile X syndrome. This method is based on the use of the Expand Long PCR System, which enables the amplification of normal, premutated and full-mutated alleles, and therefore provides complete CGG repeat analysis of the FMR1 gene. Normal alleles were easily detected by ethidium bromide staining of the agarose gels, suggesting that this assay could be used as a screening test for a large number of referrals. The amplified premutations and full mutations were identified by hybridization with a digoxigenin-labeled 5'-(CGG)5-3' probe, followed by chemiluminescent detection. The accuracy of our Expand Long PCR protocol was confirmed by Southern blot analysis, illustrating that the Expand Long PCR results concur with those of Southern blotting. In this paper we propose a new strategy for molecular diagnosis of the fragile X syndrome in which our Expand Long PCR assay is used as the first screening test for fragile X mutation detection.

Cognitive, behavioral, and neuroanatomical assessment of two unrelated male children expressing FRAXE

Standardized cognitive, behavioral, and neuroanatomical data are presented on 2 unrelated boys with the FRAXE (FMR2) GCC expansion mutation. In the context of normal IQ, both boys had a history of developmental delay, including significant problems with communication, attention, and overactivity. Additionally, one child was diagnosed with autistic disorder. Data from these 2 cases are compared to analogous information from previous reports about individuals with the FRAXE or FRAXA (FMR1) mutation. These comparisons support the idea that FRAXE is associated with nonspecific developmental delay and possibly high-functioning autism.

Genetic influences on mild mental retardation: concepts, findings and research implications

It has long been known that mild mental retardation (meaning that associated with an IQ in the 50–69 range) shows a strong tendency to run in families and that there is a much increased recurrence risk if either a parent or a sib has mental retardation. For example, Reed & Reed (1965) found that, having had one retarded child, the chance of a further retarded child was 6% if both parents and their sibs were ‘normal’, 13% if both parents were normal but one had a retarded sib, 20% if one parent was retarded, and 42% if both were retarded. A somewhat similar gradient was found in the more recent study by Bundey, Thake & Todd (1989). The appreciation that mild mental retardation was so strongly familial had led Lewis (1933) some years earlier to refer to it as ’subcultural‘ (meaning that it was a normal variation), in order to differentiate it from severe mental retardation, which was viewed as ‘pathological’. Penrose (1938, 1963) made the same classificatory distinction and the utility of a two-group approach became generally accepted (Burack, 1990).

Prenatal diagnosis and carrier screening for fragile X by PCR

During the past three years, we have conducted fragile X DNA studies for carrier screening and prenatal diagnosis using a previously described PCR protocol that accurately resolves normal FMR1 alleles and premutations and detects most full mutations [Brown et al., JAMA 270:1569-1575, 1996]. A total of 344 pregnant women with a family history of mental retardation of unknown cause were screened and 6 fragile X carriers were identified: two had full mutations, and four had premutations. The mentally retarded relatives of two other women were found to be fragile X positive although the women themselves were not carriers. In all, 6 carriers and 8 fragile X families were identified by this screening. We have also screened 40 pregnant women who were members of previously identified fragile X families, but whose carrier status was unknown. Ten were found to be carriers and were offered prenatal diagnosis. Prospective prenatal testing of 84 carrier women correctly detected 31 fetal samples (19 females, 12 males) with full mutations and 6 with premutations (2 females, 4 males). No false positives but one false negative occurred early on due to undetected maternal cell contamination. In addition, screening of 806 males with developmental delays of unknown cause gave positive results in 33 (4.1%). Potential problems and pitfalls of direct DNA testing are discussed. Because of the proven success of fragile X screening with direct molecular analysis, screening of all undiagnosed individuals with mental retardation and at risk pregnant women should now be considered. The identification of fragile X carriers and prenatal diagnosis of their pregnancies should significantly reduce the prevalence of this syndrome.

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.

Uniform amplification of a mixture of deoxyribonucleic acids with varying GC content

A PCR method for uniform amplification of a mixture of DNA templates differing in GC content is described using the two enzyme approach (Klentaq1 and Pfu DNA polymerase) and a combination of DMSO and betaine. This method was applied to amplify the CGG repeat region from the fragile X region.

A study of FRAXE in mentally retarded individuals referred for fragile X syndrome (FRAXA) testing in the United Kingdom

The folate-sensitive fragile site FRAXE is located in proximal Xq28 of the human X chromosome and lies approximately 600 kb distal to the fragile X syndrome (FRAXA) fragile site at Xq27.3. The cytogenetic expression of FRAXE is thought to be associated with mental handicap, but this is usually mild compared to that of the more common fragile X syndrome that is associated with the expression of the FRAXA fragile site. The exact incidence of FRAXE mental retardation is uncertain. We describe here the results of a U.K. survey designed to assess the frequency of FRAXE in a population of individuals referred for fragile X syndrome testing and found to be negative for expansion events at the FRAXA locus. No FRAXE expansion events were found in 362 cytogenetically negative males studied, and one expansion event was identified in a sample of 534 males for whom cytogenetic analyses were either unrecorded or not performed. Further FRAXE expansion events were detected in two related females known to be cytogenetically positive for a fragile site in Xq27.3-28. To gain insight into the FRAXE phenotype, the clinical details of the identified FRAXE male plus three other FRAXE individuals identified through previous referrals for fragile X syndrome testing are presented. For the population studied, we conclude that FRAXE mental retardation is a relatively rare but significant form of mental retardation for which genetic diagnosis would be appropriate.

Diagnosis and prevention of fragile-X syndrome. From the family study to the population screening programme: eighteen years of activity

Fragile-X syndrome, which derives its name from the expression of a fragile site (FRAXA) at Xq27.3 associated with the phenotype, has achieved distinction as the most common inherited cause of mental retardation. It is the first disorder shown to be due to dynamic mutation in heritable instable DNA.

In 1991 the mutation responsible for Fragile-X syndrome was delineated as an expansion of the trinucleotide (CGG) sequence within an evolutionarily conserved gene, at the position of the fragile-X site.

The DNA of the promoter in the 5' UTR region of FMR-1 gene becomes abnormally methylated when the CGG sequence exceeds approximately 230 repeats, resulting in the transcriptional suppression of FMR-1. Based on the length of CGG repeat in the FMR-1 gene, the alleles are usually classified as normal, premutation or full mutation. CGG instability correlates with the length of repeats and number of AGGs within the FMR-1 CGG tract. In a minority of cases the Fragile-X syndrome may be due to deletion, or to point mutation in the FMR-1 gene.

Combined molecular and cytogenetic analysis for the rapid diagnosis of fragile X syndrome

The fragile X mutation is the result of an abnormal expansion of a CGG repeat sequence in the FMR-1 gene. Molecular techniques enable the detection of the mutation and also of the exact length of this DNA sequence, allowing the classification of the tested subjects as normal, carrier or affected. We propose a protocol of analysis that combines a method of non-radioactive PCR, Southern blotting and cytogenetic testing. This protocol can be used for screening programme of selected groups of mentally retarded individuals and for prevention studies in families at risk.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.