Screening for fragile X syndrome: information needs for health planners

Neuropathology of FMR1-premutation carriers presenting with dementia and neuropsychiatric symptoms

CGG repeat expansions within the premutation range (55–200) of the FMR1 gene can lead to Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders. These CGG repeats are translated into a toxic polyglycine-containing protein, FMRpolyG. Pathology of Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders comprises FMRpolyG- and p62-positive intranuclear inclusions. Diagnosing a FMR1-premutation carrier remains challenging, as the clinical features overlap with other neurodegenerative diseases. Here, we describe two male cases with Fragile X-associated neuropsychiatric disorders-related symptoms and mild movement disturbances and novel pathological features that can attribute to the variable phenotype. Macroscopically, both donors did not show characteristic white matter lesions on MRI; however, vascular infarcts in cortical- and sub-cortical regions were identified. Immunohistochemistry analyses revealed a high number of FMRpolyG intranuclear inclusions throughout the brain, which were also positive for p62. Importantly, we identified a novel pathological vascular phenotype with inclusions present in pericytes and endothelial cells. Although these results need to be confirmed in more cases, we propose that these vascular lesions in the brain could contribute to the complex symptomology of FMR1-premutation carriers. Overall, our report suggests that Fragile X-associated tremor/ataxia syndrome and Fragile X-associated neuropsychiatric disorders may present diverse clinical involvements resembling other types of dementia, and in the absence of genetic testing, FMRpolyG can be used post-mortem to identify premutation carriers.

Fragile X CGG repeat variation in Tamil Nadu, South India: a comparison of radioactive and methylation-specific polymerase chain reaction in CGG repeat sizing

Fragile X syndrome is the most frequent hereditary cause of mental retardation after Down syndrome. Expansion of CGG repeats in the 5' UTR of the fragile X mental retardation gene 1 (FMR1) causes gene inactivation in most of the cases. The FMR1 gene is classified into normal 5-44; gray zone 45-54; premutation 55 to <200; and full mutation ≥ 00 repeats. Precise sizing of FMR1 alleles is important to understand their variation, predisposition, and for genetic counseling. Meta-analysis reveals prevalence of premutation carriers as 1 in 259. No such reports are available in India. About 705 women from Tamil Nadu, South India, were screened for the FMR1 allelic variation by using radioactive polymerase chain reaction-polyacrylamide gel electrophoresis (PAGE) analysis. The women who were homozygous by radioactive polymerase chain reaction (rPCR) were reanalyzed by methylation-specific polymerase chain reaction (Ms-PCR) and GeneScan analysis. The techniques were validated and compared to arrive at a correction factor. Among 122 genotypes, 35 repeat variants ranging in size from 16 to 57 were observed. The most common repeat is 30 followed by 29. One in 353 women carried the premutation. No full mutations were observed. Screening populations with low frequency of premutations may not be applicable. Ms-PCR is more suitable for routine screening and clinical testing compared with rPCR-PAGE analysis.

Population screening for reproductive risk for single gene disorders in Australia: now and the future

Abstract As the results of the Human Genome Project are realized, it has become technically possible to identify carriers of numerous autosomal and X-linked recessive disorders. Couples at risk of having a child with one of these conditions have a number of reproductive options to avoid having a child with the condition should they wish. In Australia the haemoglobinopathies are the only group of conditions for which population screening is widely offered and which is government funded. In some Australian states there are also population screening programs for cystic fibrosis and autosomal recessive conditions more common in Ashkenazi Jewish individuals which are generally offered on a user pays basis. It is predicted that as consumer demand increases and testing becomes cheaper, that many people planning or in the early stages of pregnancy will have carrier screening for multiple genetic conditions. This will have significant implications for genetic counseling, laboratory and prenatal testing resources. In addition such screening raises a number of ethical issues including the value of lives of those born with genetic conditions for which screening is available.

A model for offering carrier screening for fragile X syndrome to nonpregnant women: results from a pilot study

PURPOSE

To develop a model of offering population carrier screening for fragile X syndrome to nonpregnant women in primary care, using a program evaluation framework.

METHODS

A three-phase approach included: (I) needs assessment exploring staff and client attitudes, and informing development of educational materials, questionnaires and protocols; (II) offering screening to women, with questionnaires at baseline (Q1) and another (Q2) 1-month later; (III) genetic counseling for test-positive women and interviews with a subgroup of participants.

RESULTS

Of 338 volunteering for Phase II, 94% completed Q1, 59% completed Q2, and 20% (N = 65) chose testing revealing one premutation carrier and three gray zone results; 31 women were interviewed. Tested women had more positive attitudes toward screening (Q1: P < 0.001; Q2: P < 0.001) compared with untested, although there was no significant difference in mean knowledge scores or anxiety. Women generally supported being offered prepregnancy screening; however, reasons against being tested included: not currently planning a family; perceiving benefits of screening as unimportant; and having to return for testing.

CONCLUSION

This is the first prospective study exploring informed decision-making for fragile X syndrome carrier screening, using a thorough process of consultation, with no apparent harms identified. It provides a model for development of future genetic screening programs.

Recommendations from Multi-disciplinary Focus Groups on Cascade Testing and Genetic Counseling for Fragile X-associated Disorders

The purpose of this paper is to report the outcome of a collaborative project between the Fragile X Research and Treatment Center at the Medical Investigation of Neurodevelopmental Disorders (M.I.N.D.) Institute at the University of California at Davis, the National Fragile X Foundation (NFXF), and the Centers for Disease Control and Prevention (CDC). The objective of this collaboration was to develop and disseminate protocols for genetic counseling and cascade testing for the multiple disorders associated with the fragile X mental retardation 1 (FMR1) mutation. Over the last several years, there has been increasing insight into the phenotypic range associated with both the premutation and the full mutation of the FMR1 gene. To help develop recommendations related to screening for fragile X-associated disorders, four, two day advisory focus group meetings were conducted, each with a different theme. The four themes were: (1) fragile X-associated tremor/ataxia syndrome (FXTAS); (2) premature ovarian failure (POF) and reproductive endocrinology; (3) psychiatric, behavioral and psychological issues; and (4) population screening and related ethical issues.

Preconceptional and prenatal screening for fragile X syndrome: Experience with 40 000 tests

OBJECTIVES

To determine the carrier frequency of fragile X syndrome, and the rate of expansion from premutation (PM) carrier to full mutation (FM) fetus.

METHODS

Results were analyzed on women with no family history of fragile X syndrome, or who were PM/FM carriers, who were tested between January 1994 and June 2004. PM was defined 55-199 repeats, FM above 200.

RESULTS

Out of 40 079 women screened, 5 FM and 255 PM carriers were detected. There was no significant difference in carrier frequency between those with versus those without family history of mental retardation or developmental abnormalities: 1 in 128 (28/3596) versus 1 in 157 (232/36 483). However, the median of repeats differed significantly: 58 and 66 repeats, respectively, (P < 0.0001). Invasive prenatal diagnosis was carried out in 370 pregnancies (7 FM and 363 PM). Thirty FM fetuses were detected. There was a lower expansion rate in cases without a family history: 10% (17/169 PMs) compared to 50% (11/22 PMs) in those with a history, but this could be accounted for by the difference in allele size.

CONCLUSION

There is now sufficient information on screening parameters and prenatal diagnosis of fragile X syndrome to offer testing to women of reproductive age.

Population genetic screening programmes: principles, techniques, practices, and policies

This paper examines the professional and scientific views on the principles, techniques, practices, and policies that impact on the population genetic screening programmes in Europe. This paper focuses on the issues surrounding potential screening programmes, which require further discussion before their introduction. It aims to increase, among the health-care professions and health policy-makers, awareness of the potential screening programmes as an issue of increasing concern to public health. The methods comprised primarily the review of the existing professional guidelines, regulatory frameworks and other documents related to population genetic screening programmes in Europe. Then, the questions that need debate, in regard to different types of genetic screening before and after birth, were examined. Screening for conditions such as cystic fibrosis, Duchenne muscular dystrophy, familial hypercholesterolemia, fragile X syndrome, hemochromatosis, and cancer susceptibility was discussed. Special issues related to genetic screening were also examined, such as informed consent, family aspects, commercialization, the players on the scene and monitoring genetic screening programmes. Afterwards, these questions were debated by 51 experts from 15 European countries during an international workshop organized by the European Society of Human Genetics Public and Professional Policy Committee in Amsterdam, The Netherlands, 19-20, November, 1999. Arguments for and against starting screening programmes have been put forward. It has been questioned whether genetic screening differs from other types of screening and testing in terms of ethical issues. The general impression on the future of genetic screening is that one wants to 'proceed with caution', with more active impetus from the side of patients' organizations and more reluctance from the policy-makers. The latter try to obviate the potential problems about the abortion and eugenics issues that might be perceived as a greater problem than it is in reality. However, it seems important to maintain a balance between a 'professional duty of care' and 'personal autonomy'.

Newborn screening for fragile X syndrome

Newborn screening for fragile X syndrome (FXS) is technically possible, and in the relatively near future accurate and inexpensive screening technologies are likely to be available. When that happens, will America's public health system adopt newborn screening for fragile X syndrome? This article addresses this issue by first placing screening for FXS in the context of the history and current status of newborn screening policy and practice. Lack of a proven medical treatment may stand as a barrier to newborn screening, but strong arguments can be made that early intervention provides important services for identified newborns and their families. Furthermore, other arguments could be used to justify newborn screening, including informed reproductive risk, medically necessary information, and consumer demand. Fragile X syndrome is offered as a prototype for many of the issues that will face society as more genetic disorders are discovered and new technologies for screening are developed.

Fragile X and other trinucleotide repeat diseases

Hereditary unstable DNA is composed of strings of trinucleotide repeats, in which three nucleotides are repeated over and over (ie CAGCAGCAGCAG). These repeats are found in several sites within genes; depending on their location, the number of triplet repeats in a string can change as it is passed on to offspring. When the number of repeats increases to a critical size, it can have a variety of affects on gene function. The repeats may cause a loss in gene function (as in Fragile X) or may result in the gain of a new, abnormal protein and thus a new function (as in myotonic dystrophy and Huntington disease). Although a variety of trinucleotide repeat diseases have been reported and merit consideration, this discussion will focus primarily on Fragile X syndrome, myotonic dystrophy, and Huntington disease.

Pre-conception and antenatal screening for the fragile site on the X-chromosome

BACKGROUND

Fragile X is the most common cause of mental retardation after Down syndrome. It is the commonest inherited cause of mental retardation, and results from a dynamic mutation in a gene on the long arm of the X chromosome. Various strategies are used for prenatal screening.

OBJECTIVES

To determine whether pre-conceptual or antenatal screening for Fragile X carrier status in apparently low risk women confers any additional benefit over the existing practice of offering testing to women thought to be at increased risk.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group trials register (November 2001), the Cochrane Controlled Trials Register (The Cochrane Library Issue 3, 2001), MEDLINE (1980 to 2001), and reference lists of articles.

SELECTION CRITERIA

Randomised clinical trials comparing women being tested regardless of family history (intervention group) with women tested only when there is a family history of either fragile X and/or other undiagnosed mental illness/impairment (control group).

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed trial quality and extracted data.

MAIN RESULTS

No trials were included.

REVIEWER'S CONCLUSIONS

No information is available from randomised trials to indicate whether routine pre-conceptual or antenatal screening for fragile X carrier status confers any benefit over testing women thought to be at increased risk.

Fragile-X carrier screening and the prevalence of premutation and full-mutation carriers in Israel

Fragile-X syndrome is caused by an unstable CGG trinucleotide repeat in the FMR1 gene at Xq27. Intermediate alleles (51-200 repeats) can undergo expansion to the full mutation on transmission from mother to offspring. To evaluate the effectiveness of a fragile-X carrier-screening program, we tested 14,334 Israeli women of child-bearing age for fragile-X carrier status between 1992 and 2000. These women were either preconceptional or pregnant and had no family history of mental retardation. All those found to be carriers of premutation or full-mutation alleles were offered genetic counseling and also prenatal diagnosis, if applicable. We identified 207 carriers of an allele with >50 repeats, representing a prevalence of 1:69. There were 127 carriers with >54 repeats, representing a prevalence of 1:113. Three asymptomatic women carried the fully mutated allele. Among the premutation and full-mutation carriers, 177 prenatal diagnoses were performed. Expansion occurred in 30 fetuses, 5 of which had an expansion to the full mutation. On the basis of these results, the expected number of avoided patients born to women identified as carriers, the cost of the test in this study (U.S. $100), and the cost of lifetime care for a mentally retarded person (>$350,000), screening was calculated to be cost-effective. Because of the high prevalence of fragile-X premutation or full-mutation alleles, even in the general population, and because of the cost-effectiveness of the program, we recommend that screening to identify female carriers should be carried out on a wide scale.

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.

Screening for fragile X syndrome in women of reproductive age

We conducted a prospective intervention study of screening for fragile X syndrome in the general population. Antenatal and preconceptional screening were carried out in 9459 women aged between 19 and 44 with no known family history of fragile X syndrome. 80% were tested antenatally. 134 carriers were detected (a frequency of 1 in 70); 130 had a premutation (PM) and 4 had a full mutation (FM). Prenatal diagnosis was carried out in 108 concurrent or subsequent pregnancies among carriers involving 111 fetuses. Nine had an FM, a rate of 1 in 12; two of the affected embryos received the FM directly from the mother and in seven it was the result of expansion from a PM. In all cases with an FM the pregnancy was terminated. In PM carriers there was evidence of a selection against the mutated chromosome with a segregation ratio of 0.40. Owing to the high rate of premutated chromosomes in our population we conclude that screening for fragile X syndrome among women of reproductive age should be more widely available.

A five-year experience with fragile X screening of high-risk gravid women

OBJECTIVE

We sought to compare our 5-year program of fragile X screening of high-risk gravid women with our program of fragile X testing of affected individuals (probands).

STUDY DESIGN

All women referred to the prenatal genetics clinic from 1994 to 1998 who had a family history of unspecified mental retardation or learning or behavioral disorders (known fragile X families excluded) were offered fragile X screening. Results were compared with those of probands with the same diagnoses who underwent fragile X testing during the same time period.

RESULTS

We counseled 12,349 prenatal patients from 1994-1998, of whom 263 (2.1%) had a positive family history and underwent fragile X screening. No mutations or premutations were identified. In contrast, 31 (1.9%) of 1637 affected probands who underwent fragile X testing during the same time period had positive results, which was a significant difference (0/263 vs 31/1637; P <.05).

CONCLUSIONS

Testing the affected proband is superior to screening the pregnant relative of the proband for identification of families at risk for fragile X syndrome.

Improved fluorescent PCR-based assay for sizing CGG repeats at the FRAXA locus

Fragile X syndrome is the most frequent heritable genetic disease involving mental retardation and is usually caused by an expanded CGG repeat in the first exon of the FMR1 gene. Therefore, searching for CGG expansion at the FRAXA locus among the mentally retarded has become a routine investigation in neuro-paediatric practice. Consequently, we have developed a fluorescent PCR-based assay for sizing repeats as an alternative to laborious and time-consuming Southern blot. The procedure utilises a reverse fluorescent labelled primer, and the Expand Long Template PCR system (Roche) with addition of dimethylsulfoxide and 7-deaza-dGTP It allows precise determination of the CGG repeat number in males and females for alleles from normal to premutation size range and detection of full mutations in males. We believe that this PCR protocol, allowing a high sample throughput, is useful for first-line screening among mentally retarded males, possibly complemented by Southern blot analysis to assess the methylation status of large mutated alleles.

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.