The frequency of the fragile X chromosome among schoolchildren in Coventry

Epigenetic Regulations in Neuropsychiatric Disorders

Precise genetic and epigenetic spatiotemporal regulation of gene expression is critical for proper brain development, function and circuitry formation in the mammalian central nervous system. Neuronal differentiation processes are tightly regulated by epigenetic mechanisms including DNA methylation, histone modifications, chromatin remodelers and non-coding RNAs. Dysregulation of any of these pathways is detrimental to normal neuronal development and functions, which can result in devastating neuropsychiatric disorders, such as depression, schizophrenia and autism spectrum disorders. In this review, we focus on the current understanding of epigenetic regulations in brain development and functions, as well as their implications in neuropsychiatric disorders.

Incidence of Fragile X syndrome in Ireland

Described as the commonest single gene cause of learning disability internationally, the incidence of Fragile X syndrome (FXS) has never previously been determined in Ireland. The aim of this work was to determine the observed incidence of FXS in the island of Ireland; the Republic of Ireland (ROI) and Northern Ireland (NI) separately and combined. Ascertainment was achieved for a cross-sectional study by a retrospective, clinical and laboratory database review of positive FXS cases, born in either ROI or NI, between years 2000-2009 inclusive. The observed incidence of FXS per 10,000 live births in the island of Ireland in males was 0.94 (95%CI: 0.75-1.13) or ∼1:10,600 and in females was 0.23 (95%CI: 0.14-0.32) or ∼1:43,000. Comparable testing rates for FXS are present in ROI and NI, with on average 1.48% (1.30% in ROI, 1.96% in NI) of live male births and 0.4% (0.35% in ROI, 0.55% in NI) of live female births undergoing analysis which is comparable to other centres internationally. This study demonstrates the observed incidence of FXS in the island of Ireland is (i) approximately half the estimated worldwide incidence in males and is not explained by low levels of testing, and (ii) approximately one quarter the estimated worldwide incidence in females which may be explained by low levels of testing. © 2017 Wiley Periodicals, Inc.

Development of Genetic Testing for Fragile X Syndrome and Associated Disorders, and Estimates of the Prevalence of FMR1 Expansion Mutations

The identification of a trinucleotide (CGG) expansion as the chief mechanism of mutation in Fragile X syndrome in 1991 heralded a new chapter in molecular diagnostic genetics and generated a new perspective on mutational mechanisms in human genetic disease, which rapidly became a central paradigm (“dynamic mutation”) as more and more of the common hereditary neurodevelopmental disorders were ascribed to this novel class of mutation. The progressive expansion of a CGG repeat in the FMR1 gene from “premutation” to “full mutation” provided an explanation for the “Sherman paradox,” just as similar expansion mechanisms in other genes explained the phenomenon of “anticipation” in their pathogenesis. Later, FMR1 premutations were unexpectedly found associated with two other distinct phenotypes: primary ovarian insufficiency and tremor-ataxia syndrome. This review will provide a historical perspective on procedures for testing and reporting of Fragile X syndrome and associated disorders, and the population genetics of FMR1 expansions, including estimates of prevalence and the influence of AGG interspersions on the rate and probability of expansion.

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.

Dopamine Release and Uptake Impairments and Behavioral Alterations Observed in Mice that Model Fragile X Mental Retardation Syndrome

In this study we evaluated the relationship between amphetamine-induced behavioral alterations and dopamine release and uptake characteristics in Fmr1 knockout (Fmr1 KO) mice, which model fragile X syndrome. The behavioral analyses, obtained at millisecond temporal resolution and 2 mm spatial resolution using a force-plate actometer, revealed that Fmr1 KO mice express a lower degree of focused stereotypy compared to wild type (WT) control mice after injection with 10 mg/kg (ip) amphetamine. To identify potentially related neurochemical mechanisms underlying this phenomenon, we measured electrically-evoked dopamine release and uptake using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in striatal brain slices. At 10 weeks of age, dopamine release per pulse, which is dopamine release corrected for differences in uptake, was unchanged. However, at 15 (the age of behavioral testing) and 20 weeks of age, dopamine per pulse and the maximum rate of dopamine uptake was diminished in Fmr1 KO mice compared to WT mice. Dopamine uptake measurements, obtained at different amphetamine concentrations, indicated that dopamine transporters in both genotypes have equal affinities for amphetamine. Moreover, dopamine release measurements from slices treated with quinpirole, a D2-family receptor agonist, rule out enhanced D2 autoreceptor sensitivity as a mechanism of release inhibition. However, dopamine release, uncorrected for uptake and normalized against the corresponding pre-drug release peaks, increased in Fmr1 KO mice, but not in WT mice. Collectively, these data are consistent with a scenario in which a decrease in extracellular dopamine levels in the striatum result in diminished expression of focused stereotypy in Fmr1 KO mice.

Incidence of fragile X syndrome by newborn screening for methylated FMR1 DNA

Fragile X syndrome (FXS) results from a CGG-repeat expansion that triggers hypermethylation and silencing of the FMR1 gene. FXS is referred to as the most common form of inherited intellectual disability, yet its true incidence has never been measured directly by large population screening. Here, we developed an inexpensive and high-throughput assay to quantitatively assess FMR1 methylation in DNA isolated from the dried blood spots of 36,124 deidentified newborn males. This assay displays 100% specificity and 100% sensitivity for detecting FMR1 methylation, successfully distinguishing normal males from males with full-mutation FXS. Furthermore, the assay can detect excess FMR1 methylation in 82% of females with full mutations, although the methylation did not correlate with intellectual disability. With amelogenin PCR used for detecting the presence of a Y chromosome, this assay can also detect males with Klinefelter syndrome (KS) (47, XXY). We identified 64 males with FMR1 methylation and, after confirmatory testing, found seven to have full-mutation FXS and 57 to have KS. Because the precise incidence of KS is known, we used our observed KS incidence as a sentinel to assess ascertainment quality and showed that our KS incidence of 1 in 633 newborn males was not significantly different from the literature incidence of 1 in 576 (p = 0.79). The seven FXS males revealed an FXS incidence in males of 1 in 5161 (95% confidence interval of 1 in 10,653-1 in 2500), consistent with some earlier indirect estimates. Given the trials now underway for possible FXS treatments, this method could be used in newborn or infant screening as a way of ensuring early interventions for FXS.

Incidence of Fragile X in 5,000 Consecutive Newborn Males

Fragile X syndrome (FXS) is the commonest cause of inherited mental retardation in males. Even though this affirmation is repeated in virtually all papers referring to FXS, the precise frequency of this syndrome in the general population is unknown. We present a general population screening analyzing an anonymous series of 5,000 consecutive newborn males from the neonatal screening program of the population of Catalonia in Spain. The aim of the study is to determine the incidence of FXS via a simple and economical methodology based on the nonamplification of the fragment containing the CGG repeats of the FRAXA locus in the samples carrying alleles over 52 repeats. From the initial 5,000 samples, 4,920 were in the normal range, 15 gave rise to bands with more than 52 repeats (11 corresponded to intermediate alleles and four premutated alleles). After further studies, two samples were considered to be carriers of full mutations. According to these results, the incidence of FXS affected newborn males is 1 in 2,466, and 1 in 1,233 males is a carrier of the premutation. We can deduce that 1 in 8,333 is an affected female with clinical manifestations and 1 in 411 will be a premutation carrier woman. Upon reviewing the literature, there seems to be variability in the frequencies found by the different groups. Therefore, given that our study is limited to the Catalan population in Spain, these results should be taken as valid for the Catalan region and should only be extrapolated to other populations with caution.

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.

A risk-factor model of epistatic interaction, focusing on autism

Research to date on the genetics of autism has not uncovered a major susceptibility locus and indications are that a number of genes, perhaps as many as 15-20, may play detectable but minor roles in the etiology of the condition. To cope with this situation, a risk-factor model based on standard epidemiologic designs is proposed. The model supposes that adding a factor to a fixed set of existing factors always increases the total risk. Thus, according to the model genetic contributions cumulate but are not necessarily additive. A threshold, hence, epistasis is required. The model is applied to several conditions in which the risk of autism is elevated, some genetic (fragile X, tuberous sclerosis) and some exogenous (rubella and thalidomide embryopathies). Male gender is discussed as a risk factor. This approach is contrasted primarily with Gillberg and Coleman's view of autism as "a syndrome or series of syndromes caused by many different separate individual diseases." The principal point of difference is whether the effects of different causes cumulate or do not cumulate. In the present approach they do, in Gillberg and Coleman's they do not.

Sixty years of X-linked mental retardation: a historical footnote

X-linked mental retardation (XLMR) is a most exciting field of modern medical genetics. It made spectacular advances over the last twenty years, after the advent of molecular genetics. The discovery of the FMR1 gene unraveled the cause of the most common form of heritable mental retardation and provided the prototype of dynamic mutations. New genes continue to be mapped to the X chromosome and more and more are being cloned and characterized, clarifying the nosology of XLMR and, more importantly, adding to our understanding of the mechanisms of intellectual development, normal and abnormal. Looking back to a more or less recent past may provide clues for future discoveries.

The genetics of autism

OBJECTIVE

To review systematically the empirical evidence for the involvement of genetic risk factors in infantile autism.

METHOD

We aimed at including all relevant papers written in English. We conducted a Medline search in September 2000. In addition we searched the reference lists of related papers.

RESULTS

A relatively small number of reports including family and twin studies, comorbidity, cytogenetic and molecular genetic studies were reviewed.

CONCLUSION

As well family, twin, cytogenetic and molecular genetic studies supported the importance of genetic risk factors in infantile autism. In most individual cases probably at least a few gene variants simultaneously determine the genetic risk. Presently the most interesting chromosome regions concerning the aetiology of autism are chromosomes 7q31-35, 15q11-13 and 16p13.3 which have been suggested by different lines of genetic research.

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.

FRAXA and FRAXE: the results of a five year survey

We report the results of a five year survey of FRAXA and FRAXE mutations among boys aged 5 to 18 with special educational needs (SEN) related to learning disability. We tested their mothers using the X chromosome not transmitted to the son as a control chromosome, and the X chromosome inherited by the son to provide information on stability of transmission. We tested 3738 boys and 2968 mothers and found 20 FRAXA and one FRAXE full mutations among the boys and none among the mothers. This gives an estimated prevalence of full mutations in males of 1 in 5530 for FRAXA and 1 in 23 423 for FRAXE. We found an excess of intermediate and premutation alleles for both FRAXA and FRAXE. For FRAXA this was significant at the 0.001 level but the excess for FRAXE was significant only at the 0.03 level. We conclude that the excess of intermediate and premutation sized alleles for FRAXA may well be a contributing factor to the boys' mental impairment, while that for FRAXE may be a chance finding. We studied approximately 3000 transmissions from mother to son and found five instabilities of FRAXA in the common or intermediate range and three instabilities of FRAXE in the intermediate range. Thus instabilities in trinucleotide repeat size for FRAXA and FRAXE are rare, especially among alleles in the common size range.

Cytogenetic diagnosis of fragile X syndrome: study of 305 suspected cases in Saudi Arabia

BACKGROUND

Fragile X syndrome is the most common cause of inherited mental retardation. Patients with fragile X syndrome show variable mental disability, typical long and narrow facial appearance with large ears and prominent fontanelle and frequent macro-orchidism. It is generally associated with a fragile site at Xq 27.3, which can be observed in the metaphase chromosome following selective culture conditions. At the molecular level, the fragile X syndrome is associated with an amplification of CGG repeat sequence of the FMR1 gene. The prevalence estimates are reported as one per 1500 males and one per 2500 females. Estimated prevalence rates of fragile X syndrome in different ethnic groups range from 0.4-0.8 per 1000 in males and 0.2-0.6 per 1000 in females. In this study, we have determined the frequency of fragile X-positive cases in 305 preselected patients.

MATERIALS AND METHODS

Three hundred and five Saudi patients with mental retardation/developmental delay/clinical suspicion of fragile X syndrome were screened for fragile X chromosome by cytogenetic methods. The majority of patients (95.59%) screened were under the age of 20 years.

RESULTS

Two hundred and ninety-nine patients (98.03%) were in the category of mild to moderate mental retardation. Twenty-four males (7.86%) and two females (0.65%) were found to express fragile X site at q27.3. The frequency of fragile X-positive cells in males ranged between 7% and 58% (mean 26+/-13.11), while in the females it was between 14% and 21% (mean 12.5+/-35), respectively.

CONCLUSION

The frequency of fragile X positive cases found in this study is similar to other reports of fragile X syndrome in preselected patients.

Fragile X syndrome

Fragile X syndrome is the most common familial form of mental retardation. This X-linked disorder affects one in every 1000 males and one in every 2000 females. The female carrier rate in the general population is estimated to be 1/600. A fragile site at the distal long arm of the X chromosome (Xq 27.3) is the hallmark cytogenetic feature of the syndrome. Clinical features include physical as well as cognitive and neuropsychological deficits. Although fragile X syndrome follows an X-linked pattern of inheritance (which explains the predominance of affected males), females can also be affected. Many inconsistencies exist between the genetic inheritance pattern of fragile X and traditional Mendelian inheritance tenets of most X-linked diseases. Due to recent molecular advances, our understanding of the perplexing genetic issues surrounding fragile X syndrome has grown and diagnostic techniques have become both reliable and readily available.

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.

Fragile-X syndrome and myotonic dystrophy: parallels and paradoxes

Fragile-X syndrome and myotonic dystrophy are caused by triplet repeat expansions embedded in CpG islands in the transcribed non-coding regions of the FMR1 and the DMPK genes, respectively. Although initial reports emphasized differences in the mechanisms by which the expanded triplet repeats caused these diseases, results published in the past year highlight remarkable parallels in the likely molecular etiologies. At both loci, expansion is associated with altered chromatin, aberrant methylation, and suppressed expression of the adjacent FMR1 and DMAHP genes, implicating epigenetic mediation of these genetic diseases.

Fragile X syndrome is less common than previously estimated

In 1986, a population study of school children in the city of Coventry gave an overall prevalence in males and females for fragile X syndrome of 1/952. The 29 children diagnosed as having fragile X syndrome in this study have been re-evaluated with molecular diagnostic techniques. Eighteen of the original 29 children have been found not to have the expansion of the FMR1 gene associated with fragile X syndrome. Revised prevalence figures have been calculated giving rise to an overall prevalence figure of 1/2720 (range 1/2198-1/3089). If the four children lost to follow up are also assumed not to have the fragile X syndrome, the revised prevalence figure was 1/5714 (range 1/4762-1/6349). Clinical review of boys with severe mental retardation from this and a subsidiary study show that the clinical features of head circumference greater than the 50th centile, testicular volume greater than the 50th centile, and IQ between 35 and 70 remain helpful in distinguishing boys with fragile X syndrome from those who have non-specific mental retardation.

Predisposition to the fragile X syndrome in Jews of Tunisian descent is due to the absence of AGG interruptions on a rare Mediterranean haplotype

We have studied the ethnic distribution of the fragile X syndrome in Israel and have found that 36/136 (26.5%) of apparently unrelated pedigrees were of Tunisian Jewish descent. The Tunisian Jews, however, constitute only 2%-3% of the general Israeli population, identifying the first ethnic group significantly (P < .001) predisposed to the development of this disease. Associated with this increase in disease prevalence, we have found an unusually high incidence of FMR1 CGG repeats devoid of AGG interruptions among the normal Tunisian Jewish population (30/150, or 20.0%). Furthermore, the proportion of these alleles beyond the FMR1 CGG repeat instability threshold (>35 repeats) (8/150, or 5.3%) was significantly greater (P < .04) than that proportion found among non-Tunisian Jewish controls in Israel (1/136). Haplotype analysis has indicated that these large uninterrupted CGG repeat alleles are present on a previously unreported (DXS548-FRAXAC1-FRAXAC2) haplotype that accounts for all observed cases of disease among Tunisian Jewish X chromosomes. The high prevalence of disease among Tunisian Jews, we suggest, is due to a founder effect of this rare haplotype, which is completely devoid of AGG interruptions in the Jewish population of Tunisia.

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).

Prevalence of fragile X syndrome

The much-quoted prevalence figure of 1:1,000 males for fragile X syndrome is an overestimate in a mixed ethnic population. A reexamination of the individuals from whom those data were derived using molecular diagnostic techniques demonstrates a more realistic figure of 1:4,000 males.

Genetic variation and evolutionary stability of the FMR1 CGG repeat in six closed human populations

In an attempt to understand the allelic diversity and mutability of the human FMR1 CGG repeat, we have analyzed the AGG substructure of this locus within six genetically-closed populations (Mbuti pygmy, Baka pygmy, R. surui, Karitiana, Mayan, and Hutterite). Most alleles (61/92 or 66%) possessed two AGG interspersions occurring with a periodicity of one AGG every nine or ten CGG repeats, indicating that this pattern is highly conserved in all human populations. significant differences in allele distribution were observed among the populations for rare variants possessing fewer or more AGG interruptions than the canonical FMR1 CGG repeat sequence. Comparisons of expected heterozygosity of the FMR1 CGG repeat locus with 30 other microsatellite loci, demonstrated remarkably similar levels of polymorphism within each population, suggesting that most FMR1 CGG repeat alleles mutate at rates indistinguishable from other microsatellite loci. A single allele (1 out of 92) was identified with a large uninterrupted tract of pure repeats (42 pure CGG triplets). Retrospective pedigree analysis indicated that this allele had been transmitted unstably. Although such alleles mutate rapidly and likely represent evolving premutations, our analysis suggests that in spite of the estimated frequency of their occurrence, these unstable alleles do not significantly alter the expected heterozygosity of the FMR1 CGG repeat in the human population.

Prevalence of fragile X syndrome

The much-quoted prevalence figure of 1:1,000 males for fragile X syndrome is an overestimate in a mixed ethnic population. A reexamination of the individuals from whom those data were derived using molecular diagnostic techniques demonstrates a more realistic figure of 1:4,000 males.

Fragile X syndrome among children with mental retardation

Prospective screening for fragile X syndrome was carried out among 1,111 patients with mental retardation who attended the Genetic clinic. Using defined clinical criteria, 55 patients were selected for cytogenetic studies to detect folate sensitive fragile sites. Twenty patients were diagnosed to have the fragile X syndrome. The prevalence of fragile X (A) syndrome was 18 per 1,000 patients of both sexes with mental retardation, 2.8% among male patients with mental retardation, and 5.8% among subjects with nonspecific mental retardation.

FMR1 fully expanded mutation with minimal methylation in a high functioning fragile X male

Cytogenetic and molecular genetic analysis of a peripheral blood sample from a 31 year old, non-mentally retarded male with a family history of fragile X syndrome showed unexpected results. Nine percent of cells evaluated cytogenetically expressed a fragile X chromosome and molecular examination of the FMR1 gene showed a highly unusual pattern defined as a minimally methylated fully expanded mutation. This case illustrates the need to recognise exceptional variations of fragile X syndrome mutations.

Mental status of females with an FMR1 gene full mutation

The cloning of the FMR1 gene enables molecular diagnosis in patients and in carriers (male and female) of this X-linked mental retardation disorder. Unlike most X-linked disorders, a considerable proportion of the female carriers of a full mutation of the FMR1 gene is affected. In this study, the intelligence quotients (IQs) were ascertained by the Wechsler Adult Intelligence Scale in 33 adult females with a full mutation, with 28 first-degree adult female relatives (mainly sisters) without a full mutation as controls. Seventy-one percent of the females with a full mutation had IQ scores below 85. In paired analysis, no significant correlation could be detected between the IQs of the females with a full mutation and those of their first-degree female relatives, reflecting a dominant effect of the FMR1 gene full mutation in the mental development of females. Considering females with a full mutation only, we observed a significant relation between the proportion of normal FMR1 alleles on the active X chromosome and IQ. We present a model to explain this relationship.

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.

Health status of Victorian special school children

OBJECTIVE

This study sought to determine the health status and health needs of a sample of students attending special schools for the intellectually disabled in Victoria, Australia.

METHODOLOGY

Two hundred and forty-nine students not previously seen by a Community Child Health Medical Officer (CCHMO) were assessed at school. Data on student, parent and staff needs were obtained through personal interviews and documented on a standard questionnaire. Health status was documented using data obtained from parents and teachers as well as the clinical assessment.

RESULTS

Comparison of the number of problems reported by parents with the number confirmed at examination showed significant underreporting of vision, hearing and general medical problems. However, behaviour problems were nearly all reported. Many students had multiple problems with 63% having 2-4 problems and 11% having 5-8 problems. Ninety-nine (40%) of the 249 children seen had newly detected problems; vision (24), hearing (24) and obesity (9) were the most common. Two hundred and forty-four (98%) had known problems and 27% of these had insufficient information available from parents or staff to completely ascertain their health status. In 115 cases the primary problem was intellectual impairment of unknown cause. Down's syndrome was the next most common underlying diagnosis (30) followed by autism (24), epilepsy (21) and cerebral palsy (15). The most common secondary diagnoses were asthma (16), congenital heart defects (12), seizures (8) and skin problems (8). Many students required referral for further management both for newly detected problems (64%) and known problems (18%). Parents required counselling and/or discussion on a number of issues for both newly detected problems (66%) and known problems (39%); when counseling had taken place parent and staff concerns had reduced significantly by the time of the follow-up assessment.

CONCLUSIONS

This study demonstrates that in those students with known intellectual impairment there were many with other unrecognized health problems and unmet needs. These findings have implications for health services provided to children attending special schools.

A population-based study on the causes of mild and severe mental retardation

The causes of mental retardation (MR) were studied as part of a multidisciplinary epidemiological case-control study in 151 mentally retarded patients identified by screening four age cohorts (12,882 children) at 8-9 years of age in the province of Kuopio, Finland. The causes of MR in 77 severely retarded (SD < or = -3 SD) and 74 mildly retarded (-2 > SD > -3) children were divided into pre-, peri-, postnatal and unknown groups according to the probable time of onset. The causes were pre-, peri-, postnatal and unknown in 60%, 9%, 8% and 23%, and 22%, 1%, 3% and 74%, in the two populations, respectively. Genetic causes were found in 28% of all 151 cases; the three most common subgroups were trisomy 21, fragile X syndrome and aspartylglycosaminuria (13%, 4% and 2% respectively). The study design used provided reliable information on the causes of MR and also demonstrated those forms of genetic metabolic diseases typical of Finnish inheritance.

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.

Molecular analysis and test of linkage between the FMR-1 gene and infantile autism in multiplex families

Approximately 2%-5% of autistic children show cytogenetic evidence of the fragile X syndrome. This report tests whether infantile autism in multiplex autism families arises from an unusual manifestion of the fragile X syndrome. This could arise either by expansion of the (CGG)n trinucleotide repeat in FMR-1 or from a mutation elsewhere in the gene. We studied 35 families that met stringent criteria for multiplex autism. Amplification of the trinucleotide repeat and analysis of methylation status were performed in 79 autistic children and in 31 of their unaffected siblings, by Southern blot analysis. No examples of amplified repeats were seen in the autistic or control children or in their parents or grandparents. We next examined the hypothesis that there was a mutation elsewhere in the FMR-1 gene, by linkage analysis in 32 of these families. We tested four different dominant models and a recessive model. Linkage to FMR-1 could be excluded (lod score between -24 and -62) in all models by using probes DXS548, FRAXAC1, and FRAXAC2 and the CGG repeat itself. Tests for heterogeneity in this sample were negative, and the occurrence of positive lod scores in this data set could be attributed to chance. Analysis of the data by the affected-sib method also did not show evidence for linkage of any marker to autism. These results enable us to reject the hypothesis that multiplex autism arises from expansion of the (CGG)n trinucleotide repeat in FMR-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Social cognition skills among females with fragile X

Emotion perception and perspective-taking skills were examined among women with or without the fragile X gene. The performance of 56 control women was compared to the performance of 46 women who were carriers of the fragile X gene. Twenty-seven of the carrier women had 0-1% cytogenetic expression and did not appear affected by the gene, whereas the remaining 19 women had > or = 2% cytogenetic expression and did appear affected by the gene. The emotion perception task employed was one for which deficits have been reported among individuals with autism. The results show that performance on this emotion-perception test and the perspective-taking measure was significantly related to full-scale IQ scores, but not to fragile X group status when effects of IQ were removed. Thus the results do not support the hypothesis that perspective-taking or emotion perception deficits are a component of the fragile X phenotype in females and represent an important differentiation between fragile X and autism.

Evaluation of school children at high risk for fragile X syndrome utilizing buccal cell FMR-1 testing

We describe a pilot project utilizing saliva to identify the FMR-1 mutation in high-risk special education students from four public school districts in Colorado. The program included presentations to special education teachers regarding fragile X syndrome, parental consent for testing, completion of a behavior checklist by the teachers, identification of special education students at high risk for fragile X syndrome, subsequent brief examination of face and hands, collection of a saliva sample by either Gatorade swish or brushing of the inside of the cheek, and analysis for the FMR-1 mutation by PCR. Equivocal samples were studied by direct DNA testing using Southern blot analysis, and abnormal results were confirmed by a blood analysis for the FMR-1 mutation. Mutant individuals received genetic counseling and medical and educational assessments to optimize treatment and intervention. This pilot project was met with enthusiasm by the schools. Of the first 439 students evaluated, 68% were male with an average age of 7.75 years; 13% were mentally retarded or autistic. Most students referred for the evaluation were learning disabled (51%) and/or had an Attention Deficit Hyperactivity Disorder (ADHD) (35%). The overall prevalence of the FMR-1 mutation was 5 of 439 or 1.1%. This relatively low yield is probably due to the high number of non-retarded but learning disabled students tested. Of the mentally retarded patients tested, 3.5% were positive for the FMR-1 mutation; however, of the non-retarded or learning disabled patients, only 0.79% were FMR-1 positive.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevalence of fra(X) in the county of Funen in Denmark is lower than expected

Population based cytogenetic fra(X) surveys have not previously been reported from Denmark. In the present study we present an estimate of fra(X) based on 1) information from the Danish Central Cytogenetic Registry of diagnosed fra(X) males of all age groups in all Denmark in the period 1980-1992 and 2) a systematic cytogenetic fra(X) survey of 175 of 8-10-year-old children with special educational needs resident in a defined, demographically representative area of Denmark (the county of Funen). The study was performed in 1988-90 before the cloning of the FMR-1 gene. In the county of Funen there were 7,837 male children in the age group of 8-10 years. In the cytogenetic survey of learning disabled children, no fra(X) positive was diagnosed. There were 99 registered males with fra(X) in all Denmark, equivalent to a prevalence of 0.04 per 1,000 males (confidence interval 0.032:1,000-0.048:1,000). Molecular fra(X) surveys of different, large populations are needed in order to estimate the frequency of fra(X) and clarify whether significant differences in prevalence exist in different populations.

Cognitive functioning and information processing of adult mentally retarded men with fragile-X syndrome

The present study describes cognitive skills and information processing strategies of mentally retarded fra(X) men. Fifty-eight fra(X) positive and 58 fra(X) negative adults, matched on sex, chronological age, length of institutionalisation, general cognitive level, and living conditions, were evaluated with the Bayley or McCarthy Scales of Mental Abilities. Mental ages were mostly situated in the severe mental handicap category and were found to be negatively influenced by chronological age. A relative strength in perceptual performance and non-verbal reasoning and a deficit in sequential information processing turned out to be typical of all mentally retarded subjects, irrespective of fra(X) or control status. Fra(X) adults could be significantly differentiated from control persons on the ground of a higher level of acquired knowledge because of better vocabulary and verbal-expressive skills. On the other hand, they were less able to imitate non-verbal patterns, had more difficulty with visual-motor integration and co-ordination, and applied less efficient general mental processing skills in solving new problems. The memory profile of fra(X) adults was strongly determined by the meaning and the complexity of the information that has to be reproduced. In this article the profile of cognitive strengths and weaknesses in the fra(X) group will be discussed and some general advice for training is suggested.

Cytogenetic Study of Mentally Retarded Children in Taipei

560 blood samples collected from mentally retarded children in Taipei were karyotypically analyzed for the incidence of fragile X and other chromosome abnormalities. The fragile site at Xq27.3 was observed in 18 patients (3.21%), 11 males and 7 females, out of the 560 blood cultures using M medium. Down syndrome (6.25%), 24 males and 11 females, was the other major category of abnormality. Other abnormalities, including inversion, translocation, deletion, duplication, ring as well as an extra marker chromosome were observed. The overall incidence of chromosomal abnormalities in these children was 14.82%. Copyright 1994 S. Karger AG, Basel

Fragile-X syndrome in North East Essex: towards systematic screening: clinical selection

A systematic screening for fragile-X syndrome, using various clinical criteria to preselect for cytogenetic testing, was performed throughout the North East Essex Health District on 1100 people attending three different local services for people with learning disability. The selection procedure used varied from a gestalt impression to head, ear and testis measurement depending on the setting. Fifty-nine males and five females who met the selection criteria went on to have chromosome studies. Of these, 23 males and one female were positive (more than 4% positive cells). They came from 19 families. Whilst the true prevalence of fragile-X syndrome is not known in the district, at a minimum, it contributed 3.2% of the institutionalized males (health authority care), 4.4% of the boys and 2.1% of the girls attending special schools for severe learning disability, 7.9% of the boys attending schools for mild learning disability (Local Education Authority), and 3.5% of men attending the two adult training centres within the district (social services). These figures compare well with the yield from reported surveys in which all individuals without a known diagnosis were tested cytogenetically.

Where have all the fragile X boys gone?

A four-year retrospective survey of individuals referred for fragile X testing to South East Thames Regional Genetics Service was carried out to determine the accuracy of clinical diagnosis of fragile X syndrome among routine referrals for cytogenetic confirmation. 680 individuals from 565 pedigrees were tested for fragile X. Five affected males were identified in previously unknown families and 17 new pedigrees were diagnosed. Using the accepted prevalence data, a total of 80 affected males would have been expected in this period. The most likely explanation for the low diagnosis rate is failure of referral of affected males.

Frequent small amplifications in the FMR-1 gene in fra(X) families: limits to the diagnosis of 'premutations'

In five of 40 fra(X) families reinvestigated using the new intragenic probe StB12.3, small amplifications of the DNA fragment appeared unexpectedly in addition to the mutations found in the probands. This suggests that enlargements of the FMR-1 gene detectable by Southern blotting using this probe must be present at an appreciable frequency in the general population. A proportion of these may be classifiable as 'premutations', or precursors of the much amplified, hypermethylated, and somatically unstable fragment associated with the fragile X syndrome, while others will merely represent stable polymorphisms in fragment length. Hence, accurate diagnosis of some fra(X) carriers will depend upon a more precise measurement of insert size than is currently provided by the newly available molecular probes.

Specific frontal lobe deficits among women with the fragile X gene

The neurocognitive phenotype of fragile X and its relation to cytogenetic expression were examined among 10 fragile X women with > or = 2% expression, 10 0% obligate carriers, and 10 controls. Measures were obtained for intellectual ability, achievement, and verbal, nonverbal, memory, and frontal lobe functions. Results show that no group demonstrated deficits on verbal, nonverbal, or memory measures. In contrast, when controlling for effects of IQ, the expressing fragile X women exhibited (1) deficits on measures of frontal lobe functioning, and (2) enhanced performance on verbal, but not figural, memory. Frontal lobe deficits may account for behavioral and cognitive manifestations of fragile X.

A reinvestigation of thirty three fragile(X) families using probe StB12.3

We have reinvestigated 33 fragile X families using probe StB12.3. In 31 families the affected individual showed an insert while in 2 families no insert was detected. The insert fell into two size categories: small (less than 0.5 kb); and large (greater than 0.6 kb) accompanied by methylation of an EagI site. All individuals of either sex having a small insert were fra(X) negative and intellectually normal, while all males having a large insert were fra(X) positive and intellectually impaired. Females having a large insert were either fra(X) positive or negative and either intellectually normal or impaired. No new mutation was found. All daughters of males with a small insert had a small insert; females with a large insert produced males and females who had a large insert, while females with a small insert had offspring with either a large or a small insert. However, females with a small insert tended to fall into one of two categories: either they had only children with a small insert or only children with a large insert, there being only one exception to this rule. We found four unexpected small inserts, two in unrelated spouses and two in female carriers who proved to be compound heterozygotes, indicating that they had inherited an insert from both their parents. These observations suggest that individuals with a small insert must be not uncommon in the general population.

Methylation and mutation patterns in the fragile X syndrome

Chromosomes carrying the mutation causing the fragile X [fra(X)] syndrome have been shown to have an unstable DNA sequence close to or within the fragile site. The length variation is located within a DNA fragment containing a CGG trinucleotide repeat which is unstable in both mitosis and meiosis. We have used the probe StB12.3 from the region to analyze the mutations and the methylation patterns in 21 families segregating for the fra(X) syndrome. Among 40 fra(X) males all showed an abnormal pattern. The normal 2.8 kb band was absent in 36 individuals and replaced by a heterogeneous smear of larger size. The remaining four were shown to be "mosaics" with the presence of both mutated, unmethylated and mutated, methylated fragments. We found four normal transmitting males, one which was a great-grandson of another normal transmitting male indicating that the pre-mutation can remain stable through two meioses in the female. In nine fra(X) positive females the abnormal pattern consisted of a smear, usually seen in affected males, in addition to the normal bands. Five of these females were mentally normal. Of clinical importance is the prediction of mental impairment in females. We suggest that this is not made by the detection of the full mutation alone, but rather by the degree of methylation of the normal X chromosome. Our results suggest that difference of clinical expression in monozygotic twins may be correlated with difference in methylation pattern. Six out of 33 fra(X) negative females at risk were diagnosed as carriers. Our observations indicate that molecular heterogeneity is responsible for variable expression of the fra(X) syndrome in both males and females.

Analysis of mutations at the fragile X locus using the DNA probe Ox1.9

In this study, 40 families segregating for fragile X [fra (X)] syndrome were examined for the presence of a mutation within the FMR-1 gene. Using the DNA probe Ox1.9, both carriers and affected individuals were found to contain an insertion/amplification-type of mutation with somatic instability. Variability in the size of the mutation, which ranged from less than 0.2 kb to approximately 13 kb, was observed both between individuals (even from the same family) and within individuals, who showed a smear rather than a discrete band(s) on Southern blot analysis. Transmission of the mutation by males resulted in little change of its size, while transmission by females usually resulted in an increase in size. Correlations were observed between the size of inserted/amplified DNA and the level of chromosome fragility and the presence or absence of mental impairment. Overall, a mutation was detected in 66 of 67 (99%) clinically affected males, in 12 of 13 (92%) transmitting males and in 95 of 112 (85%) carrier females. Equivocal results were obtained in 12 (11%) of the carrier females. No mutation was detected in 58 females and 33 males predicted to be normal by linkage, or in one female and 36 normal control males. These results strongly suggest that the mutation detected by Ox1.9 is closely associated with the cytogenetic and clinical expression of fra (X) syndrome. Additionally, the use of this probe along with other probe/enzyme combinations should provide a sensitive clinical assay for the detection of carriers of fra (X) syndrome.