Reverse mutations in the fragile X syndrome

Genetic Counseling for Fragile X Syndrome: Recommendations of the National Society of Genetic Counselors

The National Society of Genetic Counselors' (NSGC) recommendations for fragile X syndrome (FXS) genetic counseling are intended to assist health care professionals who provide genetic counseling for individuals and families in whom the diagnosis of FXS is strongly suspected or has been made. The recommendations are the opinions of genetic counselors with expertise in FXS counseling and are based on clinical experience, a review of pertinent English language medical articles, and reports of expert committees. These recommendations should not be construed as dictating an exclusive course of management, nor does use of such recommendations guarantee a particular outcome. These recommendations do not displace a health care provider's professional judgment based on the clinical circumstances of a particular client.

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

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

The FMR1 gene, infertility, and reproductive decision-making: a review

The strongest association between FMR1 and the ovary in humans is the increased risk of premature ovarian failure (POF) in women who carry the premutation level of CGG repeats (55–199 CGGs). Research on the FMR1 gene has extended to other endpoints of relevance in the OB/GYN setting for women, including infertility and ovarian hormones. After reviewing the nomenclature changes that have occurred in recent years, this article reviews the evidence linking the length of the FMR1 repeat length to fertility and ovarian hormones (follicle stimulating hormone and anti-mullerian hormone as the primary methods to assess ovarian reserve in clinical settings). The literature is inconsistent on the association between the FMR1 trinucleotide repeat length and infertility. Elevated levels of follicle stimulating hormone have been found in women who carry the premutation; however the literature on the relationship between anti-mullerian hormone and the CGG repeat length are too disparate in design to make a summary statement. This article considers the implications of two transgenic mouse models (FXPM 130R and YAC90R) for theories on pathogenesis related to ovarian endpoints. Given the current screening/testing recommendations for reproductive age females and the variability of screening protocols in clinics, future research is recommended on pretest and posttest genetic counseling needs. Future research is also needed on ovarian health measurements across a range of CGG repeat lengths in order to interpret FMR1 test results in reproductive age women; the inconsistencies in the literature make it quite challenging to advise women on their risks related to FMR1 repeat length.

Maternal FMR1 premutation allele expansion and contraction in fraternal twins

Fragile X syndrome results from an expansion of the CGG trinucleotide repeat in the 5' untranslated region of the Fragile X Mental Retardation 1 (FMR1) gene. Expansion of a maternal premutation allele is the mechanism by which a full mutation allele arises; contraction of a maternal premutation allele is rare. Here we report on both an expansion and contraction of a maternal FMR1 premutation allele in fraternal twins. The propositus was the product of a 29-week gestation twin pregnancy and was referred for FMR1 testing due to developmental delay. A FMR1 full mutation with complete methylation was observed on Southern blot analysis. Evaluation of the maternal FMR1 gene by PCR revealed a normal and premutation allele with CGG repeat numbers of 30 and 93, respectively. Subsequent FMR1 testing on the twin sister of the propositus detected CGG repeat numbers of 30 and 54. The FMR1 CGG repeat number of the reproductive partner was 30. The FMR1 CGG repeat 30 allele in the twin sister was determined to be of paternal origin and the FMR1 allele with a CGG repeat number of 54 was of maternal origin. This observation is particularly interesting not only because of the concomitant donation of a FMR1 expanded and contracted premutation allele in a twin pregnancy but also because of the significant degree of contraction (39 repeats) of the maternal premutation allele.

Longitudinal interviews of couples diagnosed with diminished ovarian reserve undergoing fragile X premutation testing

About 10 % of infertile/subfertile women are diagnosed with diminished ovarian reserve (DOR), of which < 5 % will become pregnant spontaneously. Fragile X (FMR1) genetic testing may provide a reason for her early ovarian aging and/or have reproductive implications. Seven women with DOR (genetic study subset) and the male partners of six of these women were separately interviewed about the experience of being asked to undergo this unanticipated genetic test. Three interviews were conducted (before, within 1 week after, and 3 months after learning the test results). None of the participants carried the FMR1 premutation (largest FMR1 allele 27-50 CGG repeats). For women, their pregnancy-seeking journey was long and exhausting. Women understood the reproductive implications of carrying the FMR1 premutation, and hoped for a negative result. Being offered a genetic test caused women to pause and re-think their future reproductive plans. Husbands viewed the infertility journey as filled with unknowns, of which the genetic test results would be one more puzzle piece. The expense of fertility testing/treatment was mentioned by both spouses, though more notably by husbands. The introduction of a possible genetic cause of infertility, with additional potential health consequences for future biological children, caused women to re-think their quest for pregnancy. In contrast, the genetic test was viewed as an additional source of information for their husbands as opposed to raising concern regarding potential reproductive ramifications.

Newborn screening and cascade testing for FMR1 mutations

We describe an ongoing pilot project in which newborn screening (NBS) for FMR1 mutations and subsequent cascade testing are performed by the MIND Institute at the University of California, Davis Medical Center (UCDMC). To date, out of 3,042 newborns initially screened, 44 extended family members have been screened by cascade testing of extended family members once a newborn is identified. Fourteen newborns (7 males and 7 females) and 27 extended family members (5 males and 22 females) have been identified with FMR1 mutations. Three family histories are discussed in detail, each demonstrating some benefits and risks of NBS and cascade testing for FMR1 mutations in extended family members. While we acknowledge inherent risks, we propose that with genetic counseling, clinical follow-up of identified individuals and cascade testing, NBS has significant benefits. Treatment for individuals in the extended family who would otherwise not have received treatment can be beneficial. In addition, knowledge of carrier status can lead to lifestyle changes and prophylactic interventions that are likely to reduce the risk of late onset neurological or psychiatric problems in carriers. Also with identification of carrier family members through NBS, reproductive choices become available to those who would not have known that they were at risk to have offspring with fragile X syndrome.

Trinucleotide repeat instability: genetic features and molecular mechanisms

Trinucleotide repeat expansions are an important cause of inherited neurodegenerative disease. The expanded repeats are unstable, changing in size when transmitted from parents to offspring (intergenerational instability, "meiotic instability") and often showing size variation within the tissues of an affected individual (somatic mosaicism, "mitotic instability"). Repeat instability is a clinically important phenomenon, as increasing repeat lengths correlate with an earlier age of onset and a more severe disease phenotype. The tendency of expanded trinucleotide repeats to increase in length during their transmission from parent to offspring in these diseases provides a molecular explanation for anticipation (increasing disease severity in successive affected generations). In this review, I explore the genetic and molecular basis of trinucleotide repeat instability. Studies of patients and families with trinucleotide repeat disorders have revealed a number of factors that determine the rate and magnitude of trinucleotide repeat change. Analysis of trinucleotide repeat instability in bacteria, yeast, and mice has yielded additional insights. Despite these advances, the pathways and mechanisms underlying trinucleotide repeat instability in humans remain largely unknown. There are many reasons to suspect that this uniquely human phenomenon will significantly impact upon our understanding of development, differentiation and neurobiology.

Genetic counseling for fragile x syndrome: updated recommendations of the national society of genetic counselors

These recommendations describe the minimum standard criteria for genetic counseling and testing of individuals and families with fragile X syndrome, as well as carriers and potential carriers of a fragile X mutation. The original guidelines (published in 2000) have been revised, replacing a stratified pre- and full mutation model of fragile X syndrome with one based on a continuum of gene effects across the full spectrum of FMR1 CGG trinucleotide repeat expansion. This document reviews the molecular genetics of fragile X syndrome, clinical phenotype (including the spectrum of premature ovarian failure and fragile X-associated tremor-ataxia syndrome), indications for genetic testing and interpretation of results, risks of transmission, family planning options, psychosocial issues, and references for professional and patient resources. These recommendations are the opinions of a multicenter working group of genetic counselors with expertise in fragile X syndrome genetic counseling, and they are based on clinical experience, review of pertinent English language articles, and reports of expert committees. These recommendations should not be construed as dictating an exclusive course of management, nor does use of such recommendations guarantee a particular outcome. The professional judgment of a health care provider, familiar with the facts and circumstances of a specific case, will always supersede these recommendations.

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

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

The contribution of cis-elements to disease-associated repeat instability: clinical and experimental evidence

Alterations in the length (instability) of gene-specific microsatellites and minisatellites are associated with at least 35 human diseases. This review will discuss the various cis-elements that contribute to repeat instability, primarily through examination of the most abundant disease-associated repetitive element, trinucleotide repeats. For the purpose of this review, we define cis-elements to include the sequence of the repeat units, the length and purity of the repeat tracts, the sequences flanking the repeat, as well as the surrounding epigenetic environment, including DNA methylation and chromatin structure. Gender-, tissue-, developmental- and locus-specific cis-elements in conjunction with trans-factors may facilitate instability through the processes of DNA replication, repair and/or recombination. Here we review the available human data that supports the involvement of cis-elements in repeat instability with limited reference to model systems. In diverse tissues at different developmental times and at specific loci, repetitive elements display variable levels of instability, suggesting vastly different mechanisms may be responsible for repeat instability amongst the disease loci and between various tissues.

Molecular mechanisms of TRS instability

To date several neurodegenerative disorders including myotonic dystrophy, Huntington's disease, Kennedy's disease, fragile X syndrome, spinocerebellar ataxias or Friedreich's ataxia have been linked to the expanding trinucleotide sequences. Although phenotypic features vary among these debilitating diseases, the structural abnormalities of the triplet repeat containing DNA sequences is the primary cause for all of these disorders. Expansions of the CAG repeat within coding regions of miscellaneous genes result in the synthesis of aberrant proteins containing enormously long polyglutamine stretches. Such proteins acquire toxic functions and/or may direct cells into the apoptotic cycle. On the other hand, massive expansions of various triplet repeats (i.e., CTG/CAG, CGG/CCG/, GAA/TTC) inside the noncoding regions lead to the silencing of transcription and therefore affect expression of the adjacent genes. The repetitive character of TRS allows stretches of such tracts to form slipped-stranded structures, self-complementary hairpins, triplexes or more complex configurations called "sticky DNA", which are not equally processed by some cellular mechanisms, as compared to random DNA. It is likely that the instability of the short TRS (below the threshold level) occurs due to the SILC pathway, which is driven by the DNA slippage. Accumulation of the short expansions leads to the disease premutation state where the MLC pathway becomes predominant. Independent of which mechanism is involved in the MLC pathway (replication, transcription, repair or recombination) the process of complementary strand synthesis is crucial for the TRS instability. Generally, dependent on the location of the tract which has higher potential to form secondary DNA structure, further processing of such tract may result in expansions (secondary structure formed at the newly synthesized strand) or deletions (structure present on the template strand). Analyses of molecular mechanisms of the TRS genetic instability using bacteria, yeast, cell lines and transgenic animals as models allowed the scientists to better understand the role of some major cellular processes in the development of neurodegenerative disorders in humans. However, it is necessary to remember that most of these investigations were focused on the involvement of each particular process separately. Much less of this work though was dedicated to the search for the interactions between such cellular systems that in effect could result in different rate of TRS expansions. Thus, more intensive studies are necessary in order to fully understand the phenomenon ofthe dynamic mutations leading to the human hereditary neurodegenerative diseases.

Expansion of the fragile X CGG repeat in females with premutation or intermediate alleles

The CGG repeat in the 5' untranslated region of the fragile X mental retardation 1 gene (FMR1) exhibits remarkable instability upon transmission from mothers with premutation alleles. A collaboration of 13 laboratories in eight countries was established to examine four issues concerning FMR1 CGG-repeat instability among females with premutation (approximately 55-200 repeats) and intermediate (approximately 46-60 repeats) alleles. Our central findings were as follows: (1) The smallest premutation alleles that expanded to a full mutation (>200 repeats) in one generation contained 59 repeats; sequence analysis of the 59-repeat alleles from these two females revealed no AGG interruptions within the FMR1 CGG repeat. (2) When we corrected for ascertainment and recalculated the risks of expansion to a full mutation, we found that the risks for premutation alleles with <100 repeats were lower than those previously published. (3) When we examined the possible influence of sex of offspring on transmission of a full mutation-by analysis of 567 prenatal fragile X studies of 448 mothers with premutation and full-mutation alleles-we found no significant differences in the proportion of full-mutation alleles in male or female fetuses. (4) When we examined 136 transmissions of intermediate alleles from 92 mothers with no family history of fragile X, we found that, in contrast to the instability observed in families with fragile X, most (99/136 [72.8%]) transmissions of intermediate alleles were stable. The unstable transmissions (37/136 [27.2%]) in these families included both expansions and contractions in repeat size. The instability increased with the larger intermediate alleles (19% for 49-54 repeats, 30.9% for 55-59, and 80% for 60-65 repeats). These studies should allow improved risk assessments for genetic counseling of women with premutation or intermediate-size alleles.

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.

Gene conversion (recombination) mediates expansions of CTG[middle dot]CAG repeats

Genetic recombination is a robust mechanism for expanding CTG.CAG triplet repeats involved in the etiology of hereditary neurological diseases (Jakupciak, J. P., and Wells, R. D. (1999) J. Biol. Chem. 274, 23468-23479). This two-plasmid recombination system in Escherichia coli with derivatives of pUC19 and pACYC184 was used to investigate the effect of triplet repeat orientation on recombination and extent of expansions; tracts of 36, 50, 80, and 36, 100, and 175 repeats in length, respectively, in all possible permutations of length and in both orientations (relative to the unidirectional replication origins) revealed little or no effect of orientation of expansions. The extent of expansions was generally severalfold the length of the progenitor tract and frequently exceeded the combined length of the two tracts in the cotransformed plasmids. Expansions were much more frequent than deletions. Repeat tracts bearing two G-to-A interruptions (polymorphisms) within either 171- or 219-base pair tracts substantially reduced the expansions compared with uninterrupted repeat tracts of similar lengths. Gene conversion, rather than crossing over, was the recombination mechanism. Prior studies showed that DNA replication, repair, and tandem duplication also mediated genetic instabilities of the triplet repeat sequence. However, gene conversion (recombinational repair) is by far the most powerful expansion mechanism. Thus, we propose that gene conversion is the likely expansion mechanism for myotonic dystrophy, spinocerebellar ataxia type 8, and fragile X syndrome.

Genetic instabilities in (CTG.CAG) repeats occur by recombination

The expansion of triplet repeat sequences (TRS) associated with hereditary neurological diseases is believed from prior studies to be due to DNA replication. This report demonstrates that the expansion of (CTG.CAG)(n) in vivo also occurs by homologous recombination as shown by biochemical and genetic studies. A two-plasmid recombination system was established in Escherichia coli with derivatives of pUC19 (harboring the ampicillin resistance gene) and pACYC184 (harboring the tetracycline resistance gene). The derivatives contained various triplet repeat inserts ((CTG.CAG), (CGG.CCG), (GAA.TTC), (GTC.GAC), and (GTG.CAC)) of different lengths, orientations, and extents of interruptions and a control non-repetitive sequence. The availability of the two drug resistance genes and of several unique restriction sites on the plasmids enabled rigorous genetic and biochemical analyses. The requirements for recombination at the TRS include repeat lengths >30, the presence of CTG.CAG on both plasmids, and recA and recBC. Sequence analyses on a number of DNA products isolated from individual colonies directly demonstrated the crossing-over and expansion of the homologous CTG.CAG regions. Furthermore, inversion products of the type [(CTG)(13)(CAG)(67)].[(CTG)(67)(CAG)(13)] were isolated as the apparent result of "illegitimate" recombination events on intrahelical pseudoknots. This work establishes the relationships between CTG.CAG sequences, multiple fold expansions, genetic recombination, formation of new recombinant DNA products, and the presence of both drug resistance genes. Thus, if these reactions occur in humans, unequal crossing-over or gene conversion may also contribute to the expansions responsible for anticipation associated with several hereditary neurological syndromes.

Possible founder effects for FRAXE alleles

To determine if FRAXE alleles may have haplotype associations with nearby microsatellites, we analyzed 149 unrelated control Caucasian X chromosomes for FRAXE GCC alleles along with five nearby microsatellites. The microsatellites included three that are new; GT25, CA4, and CA5 located approximately 24, approximately 48, and approximately 50 kb proximal to the FRAXE GCC repeat, and two that were identified previously: DXS8091 and DXS1691, located approximately 90 and approximately 5 kb distal. No significant correlations between haplotypes for the proximal microsatellites were found. Significant correlations of FRAXE GCC repeats and distal microsatellite allele sizes, DXS8091 (r = 0.24) and DXS1691 (r = -0.40), were found. One haplotype, 18-19 of DXS8091-DXS1691, was present on 57% of chromosomes with > or =22 FRAXE repeats but present on only 10% with <22 repeats. We conclude that this distal haplotype association likely reflects a FRAXE allele founder effect. The lack of association or founder effects seen for the three newly identified proximal markers, located within 50 kb of FRAXE GCC, may reflect an unusually high rate of mutation for these microsatellites or a higher rate of recombination in the proximal region.

Nucleotide excision repair affects the stability of long transcribed (CTG*CAG) tracts in an orientation-dependent manner in Escherichia coli

The influence of nucleotide excision repair (NER), the principal in vivo repair system for DNA damages, was investigated in Escherichia coli with uvrA, uvrB and uvrAuvrB mutants with the triplet repeat sequences (TRS) involved in myotonic dystrophy, the fragile X syndrome and Friedreich's ataxia. (CTG*CAG)175was more stable when the (CTG) strand was transcribed than when the (CAG) strand was transcribed in the alternate orientation. A lack of the UvrA protein dramatically increases the instability of this TRS in vivo as compared with the stability of the same sequence in uvrB mutant, which produces an intact UvrA protein. We propose that transcription transiently dissociates the triplet repeat complementary strands enabling the non-transcribed strand to fold into a hairpin conformation which is then sufficiently stable that replication bypasses the hairpin to give large deletions. If the TRS was not transcribed, fewer deletions were observed. Alternatively, in the uvrA-mutant, the hairpins existing on the lagging strand will suffer bypass DNA synthesis to generate deleted molecules. Hence, NER, functionally similar in both prokaryotes and eukaryotes, is an important factor in the genetic instabilities of long transcribed TRS implicated in human hereditary neuro-logical diseases.

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.

Prenatal diagnosis of the fragile X syndrome: loss of mutation owing to a double recombinant or gene conversion event at the FMR1 locus

The fragile X syndrome, an X linked mental retardation syndrome, is caused by an expanded CGG repeat in the first exon of the FMR1 gene. In patients with an expanded repeat the FMR1 promoter is methylated and, consequently, the gene is silenced and no FMR1 protein (FMRP) is produced, thus leading to the clinical phenotype. Here we describe a prenatal diagnosis performed in a female from a fragile X family carrying a large premutation. In chorionic villus DNA of the male fetus the normal maternal CGG allele and a normal pattern on Southern blot analysis were found in combination with the FRAXAC2 and DXS297 allele of the maternal at risk haplotype. A second chorionic villus sampling was performed giving identical results on DNA analysis and, in addition, expression of FMRP was shown by immunohistochemistry. We concluded that the male fetus was not affected with the fragile X syndrome. Subsequent detailed haplotype analysis showed a complex recombination pattern resembling either gene conversion or a double crossover within a 20 kb genomic region.

Deletion of all CGG repeats plus flanking sequences in FMR1 does not abolish gene expression

The fragile X syndrome is due to the new class of dynamic mutations. It is associated with an expansion of a trinucleotide repeat (CGG) in exon 1 of the fragile X mental retardation gene 1 gene (FMR1). Here we present a fragile X family with an unique female patient who was rendered hemizygous for the FRAXA locus due to a large deletion of one X chromosome. In addition, the other X had a microdeletion in FMR1. PCR and sequence analysis revealed that the microdeletion included all CGG repeats plus 97 bp of flanking sequences, leaving transcription start site and translation start site intact. Despite this total lack of CGG repeats in the FMR1 gene, Western blot analysis showed expression of FMRP, and the patient's phenotype was essentially normal. X-inactivation studies of the androgen-receptor (AR) locus and haplotype determination of microsatellite markers gave evidence that the deletion probably originated from regression of a fully mutated FMR1 gene. Although the minimal number of CGG repeats hitherto reported in FRAXA is six, and at least four other genes associated with CGG repeats are known, suggesting an as yet unknown function of these repeats, our study clearly demonstrates that the absence of CGG repeats does not abolish expression of the FMR1 gene in lymphoblastoid cells.

Prenatal diagnosis and carrier screening for fragile X by PCR

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