Reversal of fragile X phenotypes by manipulation of AβPP/Aβ levels in Fmr1KO mice

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading known genetic cause of autism. Fragile X mental retardation protein (FMRP), which is absent or expressed at substantially reduced levels in FXS, binds to and controls the postsynaptic translation of amyloid β-protein precursor (AβPP) mRNA. Cleavage of AβPP can produce β-amyloid (Aβ), a 39-43 amino acid peptide mis-expressed in Alzheimer's disease (AD) and Down syndrome (DS). Aβ is over-expressed in the brain of Fmr1(KO) mice, suggesting a pathogenic role in FXS. To determine if genetic reduction of AβPP/Aβ rescues characteristic FXS phenotypes, we assessed audiogenic seizures (AGS), anxiety, the ratio of mature versus immature dendritic spines and metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD) in Fmr1(KO) mice after removal of one App allele. All of these phenotypes were partially or completely reverted to normal. Plasma Aβ(1-42) was significantly reduced in full-mutation FXS males compared to age-matched controls while cortical and hippocampal levels were somewhat increased, suggesting that Aβ is sequestered in the brain. Evolving therapies directed at reducing Aβ in AD may be applicable to FXS and Aβ may serve as a plasma-based biomarker to facilitate disease diagnosis or assess therapeutic efficacy.

[Screening for carriers of the fragile X syndrome; ethical exploration]

Large-scale population screening for carriers of fragile X syndrome is premature. Notably the limited possibilities to detect carriers (of an instable premutation) in the general population with certainty, poses some ethical dilemmas. The possible psychological consequences of this are unknown. A pilot study can only be started if the added value of population screening as opposed to cascade screening is plausible, and the requirement of proportionality is satisfied. If this is the case, preconceptional screening is preferable to prenatal screening. Information should be provided in a process-like manner as this fits in best with the decision-making process that possibly follows participation in a population screening for carriers of fragile X syndrome. Further research into the ethics of cascade screening is desirable. This screening can be carried out on a complementary basis to population screening.

[Prevention of fragile X syndrome by prenatal genetic diagnosis: advantages and controversial aspects]

INTRODUCTION

Fragile X syndrome is the most common cause of hereditary mental retardation. Since the molecular mechanism causing it (anomalous expansion of the CGG triplet in the FMR1 gene and hypermethylation of its CpG island) was identified exactly ten years ago, it has been possible to give families in whom the syndrome is transmitted completely reliable prenatal genetic diagnosis of this.

OBJECTIVE

To report and discuss our experience in this field from 1994 to the present time.

PATIENTS AND METHODS

During this period we performed 15 prenatal diagnoses: 14 in samples of chorionic villi from 13 pregnancies (one a twin pregnancy) and 1 using amniotic fluid. In all cases we used Southern blot molecular techniques with the StB12.3 probe, the PCR of CGG triplet and DXS548 in some cases.

RESULTS

Nine fetuses were normal. Of the other six foetuses, three had full mutation, one had deletion of the FMR1 gene, another was premutated and another had an allele in the grey zone (50 repetitions).

CONCLUSIONS

Molecular prenatal diagnosis of SXF is fast and 100% reliable, although from the technical point of view it is complicated and requires use of various molecular techniques. From the clinical point of view, the low rate of mutations found assures offspring, although molecular studies do not predict mental 'status' in either girls with complete mutation or children with permutation.

[Genetic diagnosis of IVF embryos: preliminary results from 'preimplantation genetic diagnoses' in the Netherlands]

Preimplantation genetic diagnosis (PGD) is a very early form of genetic testing. It involves testing one or two cells taken from a recent embryo of eight cells produced by in vitro fertilization, and selective transfer of genetically normal embryos. So far in the Academic Hospital Maastricht, the Netherlands, 20 couples have undergone PGD, resulting in 6 ongoing pregnancies (one twin pregnancy). In three women the indications for PGD were: cystic fibrosis, sex-linked Pelizaeus-Merzbacher disease and chromosomal translocation, respectively. In the Netherlands PGD is only allowed if there is a high risk of a serious genetic disease. PGD can be carried out in Maastricht for: cystic fibrosis, sex-linked diseases, chromosomal abnormalities, fragile X syndrome, spinal muscular atrophy and myotonic dystrophy. The advantage of PGD is that it excludes the necessity of a therapeutic abortion. Disadvantages ages are the requirement of in vitro fertilization, which has only a 15-20% pregnancy rate, and the experimental nature of the PGD procedure. To date, about 200 children have been born worldwide following PGD.

[Genetics of Fragile X syndrome and its prevention]

The fragile X syndrome is the most common inherited form of mental retardation. Its prevalence is estimated to be one in 1000-4000 males and one in 2000-6000 females, depending of the region. A large canadian population study in Quebec has shown a frequency of 1/260 carrier women. The fragile X syndrome was the first disease shown to be associated with "dynamic mutations", caused by an amplification of an unstable DNA sequence transmitted from generations to generations till a pathologic expression: mental retardation. The prevention is possible by a specific DNA analysis of patients and male and female carriers. It is possible to detect the mutation or the premutation in pregnant women and to propose a prenatal diagnosis by molecular study on chorionic villi samples or cultivated amniocytes.

The social dynamics of genetic testing: the case of Fragile-X

This article considers a program to screen school children for Fragile-X Syndrome as a way to explore several features of the growing practice of genetic testing in American society. These include the common practice of predictive testing in nonclinical settings; the economic, entrepreneurial, and policy interests that are driving the development of genetic screening programs; and the public support for genetic testing even when there are no effective therapeutic interventions. Drawing from research on popular images of genetics, I argue that cultural beliefs and expectations, widely conveyed through popular narratives, are encouraging the search for diagnostic information and enhancing the appeal of genetic explanations for a growing range of conditions.

Slight instability of a FMR-1 allele over three generations in a family from the general population

We report on a family segregating a FMR-1 allele within the "grey zone" of triplet repeat length (n = 51). The allele showed a 1-unit increment when transmitted through a female meiosis and a 1-unit increment when transmitted through a male of the next generation. At the following generation, a pregnant woman had amniocentesis performed. The latter showed she transmitted the allele unchanged (n = 53) to her male fetus. This family was not ascertained through an affected subject, and there was no family history of mental retardation. Thus our observation reflects the natural history of an unstable allele in the general population. Systematic analysis of such alleles may help refine our understanding of the grey zone of triplet repeat length.

Informed choice in fragile X syndrome and its effects on prevalence

We present the effect of case finding, cascade testing, and counselling for fragile X syndrome in a population of 6.5 million over a decade. Carrier females made informed choices that resulted in a 10-fold decrease in the prevalence of affected males in their offspring.

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.

Single-cell analysis of unstable genes

PURPOSE

We have developed sensitive diagnostic procedures for studies on the normal and mutant alleles of the triplet repeat genes associated with myotonic dystrophy and fragile X in single human somatic cells, gametes and embryos.

METHODS

Polymerase chain reaction (PCR) assays for the normal alleles of the myotonic dystrophy and fragile X loci have been refined to the sensitivity of the single cell. In addition, we have developed a simple PCR-based technique, termed ¿Repeat Primer PCR', which can detect the full fragile X expansion in small samples of buccal cells.

CONCLUSIONS

The assay for the triplet repeat sequence in the myotonic dystrophy locus could not be used to study stability since we observed additional PCR products derived from in vitro expansion of the triplet repeat sequence during the PCR reaction itself. The implications of in vitro expansion and allele drop-out for studies on the timing of the expansion in development and preimplantation diagnosis of triplet repeat diseases are discussed. The development of a new PCR procedure to identify the expanded alleles of the fragile X locus could prove invaluable for monitoring the timing of repeat expansion in early embryonic development. Triplet repeat polymorphisms provide a means of identifying the maternally and paternally-derived alleles of the myotonic dystrophy gene. Using single cell reverse transcriptase PCR analysis, we have monitored the onset of the myotonic dystrophy gene transcription in early preimplantation embryos. Transcripts from the paternally-inherited allele of the myotonic dystrophy gene are already detectable in the 1-cell stage human embryo.

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

Screening of referrals for the mutations associated with the fragile X syndrome constitutes a significant workload in many genetics laboratories. Since the great majority of these referrals will be negative, there is a need for a rapid and inexpensive screening test. We have developed an assay which allows simultaneous amplification of the triplet repeat sequences at the FRAXA and FRAXE loci by polymerase chain reaction, and detection of the products on non-denaturing gels stained with ethidium bromide. Alleles of normal size are detected, leaving a small minority of samples to be tested by Southern blotting. A PCR based assay for detection of methylation at the CpG island upstream of the FMR-1 gene has also been devised.

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.

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.

The fragile X syndrome: implications of molecular genetics for the clinical syndrome

The fragile X syndrome of mental retardation is one of the most common genetic diseases. Characterization of the mutations involved has greatly improved our knowledge of the transmission of fragile X syndrome and new DNA-based diagnostics tools significantly outperform cytogenetic testing both for establishing the diagnosis and for determining carrier status. Fragile X mutations consist of an expansion of a CGG trinucleotide repeat localized in a gene (FMR-1) that is abnormally methylated in all affected individuals. They are classified as premutations (asymptomatic) and full mutations (associated with the disease). Several different DNA analysis protocols are used for fragile X genotyping but only a few have been tested on large samples of individuals. There are several clinical indications for direct DNA genotyping for fragile X including mental retardation, learning disability or hyperactivity in children with or without a family history of mental retardation, the establishment of carrier diagnosis in fragile X families and prenatal screening of children from carrier women.

A statewide public and professional education program on fragile X syndrome

Fragile X syndrome is reported to be the most common inherited cause of mental retardation known, but the majority of affected individuals are as yet undiagnosed. The project described in this paper was developed to increase the public and professional awareness of fragile X syndrome in the state of New Jersey. As a result there were increased efforts at diagnostic screening, provision of client and family support services, and prevention. This educational program proved to be a cost-effective method for increasing community awareness of a genetic disease on a statewide level.

Population screening for fragile X

A screening programme to detect fragile X syndrome has been operating in New South Wales, Australia, since 1984. The aim of this programme is to find previously unidentified individuals with the syndrome so that their extended families can be properly informed of the risks before making decisions about childbearing. 14,225 individuals attending adult and child facilities for the intellectually handicapped have been screened, of whom 8172 have been offered testing for the fragile X syndrome with a 79% uptake of the service. 253 probands were found, and in the extended families 818 females at 25-100% risk of being carriers were interviewed and counselled. Continuing contact was maintained and prenatal diagnosis was offered. The effect of the programme was assessed in a subgroup of 90 individuals, most of whom were appreciative of the service and felt that they had been adequately informed. The influence of knowing the diagnosis and its genetic implications were also assessed, the main consequences being a 26% reduction in births and a 61% uptake of prenatal diagnosis. Improved techniques for diagnosis of fragile X have benefited the families identified and counselled, suggesting that systematic screening for fragile X should be an essential component of community genetic services.

New York State screening program for fragile X syndrome: a progress report

New York State has established a program to screen post-pubertal mentally retarded males for the fragile X [fra(X)] syndrome. The goal of the program is to identify affected males and inform their families of the diagnosis. Females in these families who are at risk for inheriting the mutation will then be able to determine their carrier status and consider that information in making reproductive decisions. Males were evaluated for 10 features of the syndrome by physicians and nurses throughout the state; cytogenetic analysis was carried out on a subset of this population. A total of 1332 males has been screened and chromosome studies have been completed for 489. Forty-three (9%) were positive for fra(X), and an additional 11 other chromosome abnormalities were identified. The 43 patients belonged to 38 families. Of the 24 families who were informed of the diagnosis, 12 consulted genetic counseling centers for follow-up studies and 12 did not.

Fragile X screening program in a Spanish region

In a Spanish region with a population of one million, we screened 371 mentally retarded males, who had no previous diagnosis for fragile X [fra(X))] syndrome. Fifty-three of the 371 males were fra(X) positive. Of these 44 of 362 or 12.1% were unrelated. Family studies identified a large number of obligate carriers and women at risk for being carriers who were given genetic counseling including prenatal diagnostic information. Considering the age of the carriers and the fertility rate, 23 affected males could be born to these women. The prevention potential of this program suggests that it is highly cost-effective.

Expression of the fragile-X in the "premutated"/"non-imprinted" state

Data about the expression of the fragile site at Xq27.3 from 74 daughters of normal transmitting males (NTMs) were collected from 7 different genetic centers. The majority (85.1%) of these obligate female carriers did not show any cytogenetic expression of fra-X. The remaining 14.9% of these females had frequencies below 3%. In cases with a frequency below 3% of fra-X, a "premutated"/"non imprinted" state of a female carrier should be considered. The results of this collaborative study are in accordance with data from DNA studies taking the premutation model into account.

A 15-item checklist for screening mentally retarded males for the fragile X syndrome

A 15-item checklist, including physical and behavioral features frequently observed in fragile X syndrome, was used in a prospective study of 188 mentally retarded males in order to identify males at risk for this syndrome. Of the 188 males, 19 were found to have the fragile X syndrome, while the remaining 169 males had no recognizable cause of their mental retardation, including normal chromosomes. Significant differences (p less than 0.01) were found between mentally retarded males with and without the fragile X syndrome with increased hyperactivity; shorter attention span; more tactile defensiveness, hand-flapping, perseverative speech, and hyperextensibility; large ears and testes; higher frequency of simian creases or Sydney lines and plantar creases; and more positive family histories of mental retardation in the fragile X syndrome males. Multiple regression and discriminant analyses of the 188 males indicated several physical features were useful predictors for inclusion in the fragile X syndrome group. An overall correct classification rate of 93% was achieved based on 6 variables (plantar crease, simian crease, hyperflexibility, large testes, large ears, and a positive family history of mental retardation) that were entered into the discriminant equation. Therefore, our experience with a 15-item checklist suggests the potential of screening for the fragile X syndrome in mentally retarded males and that 6 of the 15 variables were particularly good predictors of this syndrome.

Fragile X screening program in New York State

Most fragile X [fra(X)] males in New York State have not been identified. Hence, a large number of female relatives are unaware of their risks for having an affected child. A program was established in New York State in 1987 to screen for the fra(X) syndrome in mentally retarded males with living relatives. The goal of the program is to identify affected males and inform their families about the diagnosis. In this way relatives would be able to assess their risks for having a fra(X) male. In order to identify the males a screening form was developed to assess 10 features which included physical characteristics, behavior, and family history. Males who exhibited at least 5 of these manifestations were selected for cytogenetic analysis. Any male who had macroorchidism or a family history of mental retardation was also included. A total of 995 males have been screened of which 352 (35%) were selected for cytogenetic analyses. Seventeen (10.5%) of the 161 completed studies were positive for fra(X). A large number of possible female carriers were identified in the families of the propositi. This program identifies fra(X) males in a population of the mentally retarded for whom there had been no previous diagnosis. By using a two-step procedure, it is possible to screen a large population of the mentally retarded for fra(X) without testing each male cytogenetically.

Cytogenetic survey for autistic fragile X carriers in a mental retardation center

A cytogenetic survey of 67 individuals previously identified as having mental retardation and autistic behaviors revealed 1 person (1.5%) with the fragile X chromosome (fra[X]) and 3 (4.5%) with autosome abnormalities. This low prevalence of fra(X) indicates that most persons with fra(X) in this mental retardation center did not have autistic behaviors severe enough to be identified as a secondary psychiatric diagnosis. The presence of other chromosomal abnormalities is consistent with the known causal heterogeneity of autism in mental retardation populations.

Screening for the fragile X: how many cells should we analyse?

Since nationwide screening for the fragile X would involve the analysis of thousands of individuals within a short period of time, the number of cells, N, which should be analysed is of fundamental importance. When selecting N, the crucial parameter should be the degree of expression of the fragile site in the affected individuals in the population to be screened. However, this degree of expression is not known, and for routine diagnostic purposes, N = 100 has been accepted by many centers. By taking data from two large series of affected males/females with a known degree of expression (one series from New South Wales, one from Belgium), we have estimated the fraction of affected males/females which would have been missed if the two series were rescreened with the analysis of less than 100 cells. Assuming that the degree of expression within these two series is similar to the degree of expression in all affected individuals within the two populations, the results indicate that a reduction of N in a screening program, say from 100 to 50 cells, would reduce the detection rate between 1 and 5%. The reduction would be greater in females than in males, and greater in the Belgian than in the Australian population.

Fragile X syndrome: incidence, clinical and cytogenetic findings in the black and white populations of South Carolina

Individuals in South Carolina with the Fragile X [fra(X)] or Martin-Bell syndrome have been ascertained by referral for evaluation of facial abnormalities, macroorchidism or mental deficit; by screening patients in residential and day programs for the mentally retarded; and by family follow up after an index case has been identified. Between 1982 and 1987, 100 positive fra(X) males were diagnosed. Of these, 35 were residents of residential facilities for the mentally retarded representing 2.5% of the population of institutionalized males. Another 23 were found in community day programs for the mentally retarded. Of these 58 cases, 28 (48%) were ascertained by screening for the craniofacial characteristics of the Martin-Bell syndrome, namely long face, midface hypoplasia, prominent forehead, large mandible and large simple pinnae. Although this screening procedure proved to be productive, it was found that the craniofacial traits of long face, midface hypoplasia, large jaw and simple pinnae were found less frequently in black fra(X) positive males and in prepubertal boys of both races.

Screening developmentally disabled male populations for fragile X: the effect of sample size

The fra(X) or Martin-Bell syndrome is the most common cause of inherited mental retardation (MR) in males. It is also associated with a variety of unusual behavioral and developmental disorders. Recent studies found great variability in the estimated strength of association between "autism" and the fra(X) syndrome, but not between MR and fra(X). We examined 31 studies which investigated the association of fra(X) syndrome with either MR or "autism" and found that the conclusion of those researchers could be significantly affected by sample size. Different behavioral and cytogenetic protocols will also influence the strength of association between fra(X) and autism.

Preventive screening for the fragile X syndrome

In an Australian population of 1.2 million, we screened 1977 intellectually handicapped persons, who were identified through the public schools and sheltered workshops, for the X-linked semidominant fragile X syndrome. We excluded 527 because they had another known diagnosis. The remaining 1450 were offered chromosomal analysis. Of the 1117 who consented (77 percent), an additional 196 were excluded, and among the 921 who were tested cytogenetically, 40 probands were found. Prevalence rates for persons with an intellectual handicap and the fragile X syndrome in the public school population were 1:2610 for males and 1:4221 for females. Family studies identified 84 women who were either obligate carriers or at high risk of being carriers, who were under the age of 35 and had no children. These women were given genetic counseling, and the availability of antenatal diagnosis was explained to them. If each of these 84 women had two children, 27 of their sons would have an intellectual handicap. We recommend cytogenetic screening for the fragile X syndrome in all currently identified intellectually handicapped people, followed by routine screening of children newly identified as intellectually handicapped in the school system.