Fragile X syndrome without CCG amplification has an FMR1 deletion

Butyrate and acetyl-carnitine inhibit the cytogenetic expression of the fragile X in vitro

Cytogenetic expression of the fragile site at Xq27.3 is only found in those patients who have a full mutation of the FMR1 gene, i.e., a large amplification of the CGG repeat. However, an expansion of this repeat, although necessary, does not seem to be sufficient to cause expression of fra(X)(q27.3). Other factors are clearly needed, e.g., thymidylate stress. Little or no attention has been paid to the possible role of histones in the expression of the fragile sites, in spite of their structural and regulatory role in the chromatin complex. Histones can be modified by treating intact cells in vitro with butyrate, a substance that causes histone acetylation. The purpose of the present work is to test the effect of butyrate and of the acetylating compound acetyl-L-carnitine on the expression of fra(X)(q27.3) by treating peripheral lymphocytes of fragile X syndrome patients with these substances in vitro. We show that this treatment causes a significant inhibition of fra(X)(q27.3) expression.

No apparent involvement of the FMR1 gene in five patients with phenotypic manifestations of the fragile X syndrome

Most fragile X patients have a significant increase in the number of CGG repeats in the FMR1 gene. Two patients were described with a deletion and one patient with a point mutation in the FMR1 gene. We describe 5 patients with a fragile X or Martin-Bell phenotype. Two brothers were discordant for the region containing the FMR1 gene; if there is a common cause for the mental retardation this is not located in the FMR1 gene. In the other 3 patients the expression of the FMR1 gene was found to be normal and no abnormalities were noted in the FMR1 mRNA. No amplification was found in the GCC repeat which is associated with the fragile site FRAXE. We conclude that the Martin-Bell phenotype can also be caused by mutations outside the FMR1 gene.

Fragile X phenotype in a patient with a large de novo deletion in Xq27-q28

A 2-year-old boy with manifestations of the fragile X syndrome was found to have a cytogenetically visible deletion of Xq27-q28 including deletion of FMR-1. Molecular analysis of the patient was recently described in Tarleton et al. [1993: Hum Mol Genet 2(11): 1973-1974] and the deletion was estimated to be at least 3 megabases (Mb). His mother had 2 FMR-1 alleles with normal numbers of CGG repeats, 20 and 32, respectively. Thus, the deletion occurred as a de novo event. The patient does not appear to have clinical or laboratory findings other than those typically associated with fragile X syndrome, suggesting that the deletion does not remove other contiguous genes. This report describes the phenotype of the patient, including psychological studies.

Male with typical fragile X phenotype is deleted for part of the FMR1 gene and for about 100 kb of upstream region

We report on a patient with moderate mental retardation and a typical fragile X phenotype, with no family history and no fragile X site on cytogenetic analysis. The patient was found to have a deletion encompassing part of the FMR1 gene and a 70-100 kb region upstream of the FMR1 promotor region. This deletion is smaller than those previously reported and confirms that FMR1 is the major and probably the only gene implicated in the phenotype of the fragile X syndrome.

DNA diagnosis of the fragile X syndrome in a series of 236 mentally retarded subjects and evidence for a reversal of mutation in the FMR-1 gene

The cloning of the FMR-1 gene and the identification of an expanded CGG repeat in DNA of fragile X patients has made reliable DNA diagnosis feasible. Southern blotting and PCR assays of the CGG repeat in an unselected series of 236 mentally retarded subjects resulted in the identification of 10 new fragile X families. Reevaluation of previously assessed fragile X families resulted in the first observation of the presence of a reversal of mutation in the FMR-1 gene.

Mosaicism in fragile X affected males

Fragile X affected males have an expansion of a CGG repeat and a hypermethylated CpG island 5' to the FMR-1 gene. Mosaic males with both a premutation and full mutation have been noted among the affected individuals. Such mosaic males are most easily identified by the presence of a methylated restriction fragment characteristic of the full mutation and an additional unmethylated fragment in the premutation range in Southern analyses with EcoR I and the methylation-sensitive enzyme Eag I and a probe such as StB12.3. We analyzed a group of affected fragile X males by Southern blotting and found 41% (61/148) to be mosaic. The 148 individuals were divided between 36 pairs of brothers and 76 unrelated males. Little difference in the number of mosaics was seen between the brothers and the unrelated males nor was the expected distribution of mosaicism in brother pairs different from observed. Thus, these data do not suggest a familial basis for mosaicism. Our observation that 41% affected fragile X males were mosaic is significantly higher than previous reports. The difference is likely due to technical modifications which permitted the identification of faint premutation bands in some patients. The high percentage of affected males with mosaicism seen here suggests that the occurrence of such individuals may be a much more frequent event than presently recognized.

Comparison between the cytogenetic test for fragile X and the molecular analysis of the FMR-1 gene in Japanese mentally retarded individuals

The prevalence of the fragile X syndrome has been estimated by the results of population studies in which the disease was mostly diagnosed by cytogenetic examinations. To examine the reliability of the cytogenetic analysis for the estimation of the prevalence of the fragile X syndrome, the CGG repeat in the FMR-1 gene was assayed by Southern blot hybridization and polymerase chain reaction (PCR) in an institutionalized group of mentally retarded individuals consisting of 305 males and 129 females. The data thus obtained were compared with the cytogenetic data. The DNA analysis detected 7 full mutations among the alleles of the 296 unrelated males and 2 full mutations among the alleles of the 129 unrelated females. These findings were consistent with the cytogenetic data. No premutation was found in 554 X chromosomes in the unrelated mentally retarded patients nor 826 X chromosomes in unrelated control individuals. The distribution of the CGG repeat number in the normal range was not significantly different between the mentally retarded individuals and the control individuals. These data suggest that the estimates of the prevalence of the fragile X syndrome based on cytogenetic data in the population studies are almost reliable. Based on the finding that no premutations were found in this study, a small difference in the prevalence of the fragile X syndrome between Caucasians and Japanese is suggested.

Unstable trinucleotide repeats and the diagnosis of neurodegenerative disease

Human genes containing unstable triplet repeats are associated with several neuropsychiatric diseases. These diseases all show a marked variation in clinical symptoms and genetic anticipation. The molecular understanding of these diseases provides the diagnostic laboratory with the capability to test directly whether an individual's DNA contains the disease-causing mutation, either as a confirmation of a clinical diagnosis or presymptomatically.

Prenatal diagnosis of fragile X syndrome: management of the male fetus with a premutation

Direct detection of the fragile X mutation by DNA analysis has greatly simplified prenatal diagnosis of this disease. However, women carrying a fragile X premutation may pass their expanded trinucleotide repeat to sons without expansion to a full mutation. Such sons are predicted to be intellectually normal. In this situation, the accuracy with which the fetal status can be inferred from analysis of chorionic villus sample (CVS) DNA is unclear. We describe such a case, in which it was felt necessary to proceed to fetal blood sampling despite technically unambiguous DNA results from the CVS. The lack of prospective data means that this dilemma may be expected to recur over the next few years when performing prenatal diagnosis on fragile X premutation carriers.

Essential role for KH domains in RNA binding: impaired RNA binding by a mutation in the KH domain of FMR1 that causes fragile X syndrome

The KH domain is an evolutionarily conserved sequence motif present in many RNA-binding proteins, including the pre-mRNA-binding (hnRNP) K protein and the fragile X mental retardation gene product (FMR1). We assessed the role of KH domains in RNA binding by mutagenesis of KH domains in hnRNP K and FMR1. Conserved residues of all three hnRNP K KH domains are required for its wild-type RNA binding. Interestingly, while fragile X syndrome is usually caused by lack of FMR1 expression, a previously reported mutation in a highly conserved residue of one of its two KH domains (Ile-304-->Asn) also results in mental retardation. We found that the binding of this mutant protein to RNA is severely impaired. These results demonstrate an essential role for KH domains in RNA binding. Furthermore, they strengthen the connection between fragile X syndrome and loss of the RNA binding activity of FMR1.

Myotonic dystrophy with no trinucleotide repeat expansion

We report 3 patients from 2 families with myotonic dystrophy who do not show an abnormal expansion of CTG trinucleotide repeats within the myotonic dystrophy gene. Characteristic features of myotonic dystrophy in these patients were frontal balding, cataracts, cardiac conduction abnormalities, and testicular atrophy with myotonia and muscle weakness. Results of muscle histopathology were consistent with myotonic dystrophy. Genetic analysis of leukocyte and muscle DNA showed a normal number of CTG repeats. The demonstration of normal CTG repeat number for the myotonic dystrophy gene does not exclude the diagnosis of myotonic dystrophy.

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.

Molecular analysis of mutations in the gene FMR-1 segregating in fragile X families

Molecular genetic analysis of the transmission of mutations in 73 families with fragile X (one of the largest samples evaluated so far) has confirmed previous hypotheses that the fragile X syndrome results from two consecutive mutational steps, designated "premutation" and "full fragile X mutation". These mutations give rise to expansions of restriction fragments, most probably by amplification of the FMR-1 CGG repeat. Premutations are identified by small expansions that apparently have no effect on either the clinical or the cellular phenotype. Full mutations are reflected by large expansions and hypermethylation of the expanded gene region. All males showing large expansions were affected. Individuals with full mutations also expressed the fragile X, with only one exception. An affected "mosaic" male, showing a predominance of premutated fragments in his leukocytes, was shown to be fragile-X-negative on different occasions. About 50% of heterozygotes with full mutations were reported by clinicians to be mentally retarded. Conversion of the premutation to the full mutation may occur at oogenesis, as previously suggested, or after formation of a zygote at an early transitional stage in development when the CGG repeat behaves as a mitotically unstable element on maternally derived/imprinted X chromosomes carrying a premutation of sufficient repeat length.

FMR1 protein: conserved RNP family domains and selective RNA binding

Fragile X syndrome is the result of transcriptional suppression of the gene FMR1 as a result of a trinucleotide repeat expansion mutation. The normal function of the FMR1 protein (FMRP) and the mechanism by which its absence leads to mental retardation are unknown. Ribonucleoprotein particle (RNP) domains were identified within FMRP, and RNA was shown to bind in stoichiometric ratios, which suggests that there are two RNA binding sites per FMRP molecule. FMRP was able to bind to its own message with high affinity (dissociation constant = 5.7 nM) and interacted with approximately 4 percent of human fetal brain messages. The absence of the normal interaction of FMRP with a subset of RNA molecules might result in the pleiotropic phenotype associated with fragile X syndrome.

Molecular analysis of a ring chromosome X in a family with fragile X syndrome

The phenotypically normal sister of a patient affected by fragile X syndrome was referred for genetic counselling and was found to carry a mosaic karyotype 46,X,r(X)/45,X. Because the probability of the simultaneous chance occurrence of fragile X syndrome and a ring chromosome X in the same family is very low, we postulated that the breakpoint of the ring chromosome X originated in the cytogenetic break in Xq27.3 responsible for fragile X syndrome. In order to determine the relative positions of the breakpoint on the ring chromosome X and the (CGG)n unstable sequence responsible for the fragile X mutation, we used molecular markers to analyse the telomeric regions of chromosome X in this family. The results showed that the ring chromosome X was the maternal fragile X chromosome and that the telomeric deletion on the long arm encompassed the (CGG)n sequence. This suggests that the cytogenetic break in Xq27.3 is distinct from the unstable (CGG)n sequence, or that the break followed by the end-to-end fusion creating the ring chromosome was not completely conservative. Analysis of DNA markers on the short arm of chromosome X evidenced a deletion of a large part of the pseudoautosomal region, allowing us to position the genes involved in stature and in some syndromes associated with telomeric deletions of Xp on the proximal side of the pseudoautosomal region.

Trinucleotide repeat expansions in neurological disease

During the past year, new examples of human neurological disease have been discovered that have an unprecedented type of mutation as their cause: the remarkable expansion of trinucleotide repeats. These triplet repeats are normally polymorphic and exonic, though not always coding. In disease states they become markedly unstable and may expand moderately or by thousands of repeats in a single generation, influencing gene expression, message stability or protein structure.

The FMR-1 protein is cytoplasmic, most abundant in neurons and appears normal in carriers of a fragile X premutation

Fragile X mental retardation syndrome is caused by the unstable expansion of a CGG repeat in the FMR-1 gene. In patients with a full mutation, abnormal methylation results in suppression of FMR-1 transcription. FMR-1 is expressed in many tissues but its function is unknown. We have raised monoclonal antibodies specific for the FMR-1 protein. They detect 4-5 protein bands which appear identical in cells of normal males and of males carrying a premutation, but are absent in affected males with a full mutation. Immunohistochemistry shows a cytoplasmic localization of FMR-1. The highest levels were observed in neurons, while glial cells contain very low levels. In epithelial tissues, levels of FMR-1 were higher in dividing layers. In adult testis, FMR-1 was detected only in spermatogonia. FMR-1 was not detected in dermis and cardiac muscle except under pathological conditions.

The protein product of the fragile X gene, FMR1, has characteristics of an RNA-binding protein

Fragile X syndrome is one of the most common human genetic diseases and the most common cause of hereditary mental retardation. The gene that causes fragile X syndrome, FMR1, was recently identified and sequenced and found to encode a putative protein of unknown function. Here we report that FMR1 contains two types of sequence motifs recently found in RNA-binding proteins: an RGG box and two heterogeneous nuclear RNP K homology domains. We also demonstrate that FMR1 binds RNA in vitro. Using antibodies to FMR1, we detect its expression in divergent organisms and in cells of unaffected humans, but fragile X-affected patients express little or no FMR1. These findings demonstrate that FMR1 expression is directly correlated with the fragile X syndrome and suggest that anti-FMR1 antibodies will be important for diagnosis of fragile X syndrome. Furthermore, the RNA binding activity of FMR1 opens the way to understanding the function of FMR1.

Genes with triplet repeats: candidate mediators of neuropsychiatric disorders

Recently a new form of human mutation-expansion of trinucleotide repeats-has been found to cause the diseases of fragile X syndrome, spinal and bulbar muscular atrophy, myotonic dystrophy and, most recently, Huntington's disease. We review the emerging data on the genetics and neurobiology of these disorders. Three are characterized by unusual patterns of inheritance, in particular, genetic 'anticipation', in which the severity of the disorder increases and the age of onset decreases in successive generations of a pedigree. Several idiopathic neuropsychiatric disorders have features of inheritance consistent with anticipation. In bipolar affective disorder, there is evidence for both earlier age of onset and more severe illness in the second generation of a subset of unilineal pedigrees. There is also the suggestion of anticipation in some forms of schizophrenia, spinocerebellar atrophy and autism. Triplet repeats are present in additional known genes, both in coding regions and untranslated regions. Furthermore, many novel genes with triplet repeats are expressed in the human brain, and these are candidates to cause some forms of these neuropsychiatric disorders.

Direct molecular diagnosis of myotonic dystrophy

Myotonic dystrophy (DM) arises from an unstable trinucleotide (CTGn) repeat sequence within the DM locus at 19q13.3. Twenty-three myotonic dystrophy families containing 205 persons with no symptoms, minimal manifestations, classic DM or congenital DM were investigated to validate the application of the pM10M6 probe to direct molecular diagnosis. Affected family members had been diagnosed clinically and the unaffected family members had been assigned carrier probabilities close to either zero or 100%, using closely linked flanking markers. Southern analysis identified all 89 DM gene carriers as having expansions of the unstable element. PstI detected all small expansions of the repeat sequence as easily seen discrete bands; but large expansions were usually seen as diffuse smears, sometimes difficult to distinguish from lane background. EcoRI concentrated these diffuse smears, associated with somatic instability, into discrete bands which were easy to detect; but it did not resolve the smaller expansions present in 9 (10%) of the DM carriers. It is essential that PstI and EcoRI gels are run in parallel to detect all DM gene carriers. The extent of expansion of CTG correlated with age of onset and disease severity. Biopsies of various fetal tissues from two terminated pregnancies confirmed the diagnosis obtained by CVS and revealed no heterogeneity between tissues at this developmental stage. Further expansion occurred during the culture of CVS cells, indicating that direct prenatal diagnosis needs to be carried out on CVS tissue rather than on cultured cells. The intergenerational change of the repeat sequence from DM parent to DM offspring showed a significant parental sex difference for those parents with large expansions. Contraction of the unstable element was observed in the three males carrying the largest expansions and could explain why congenital DM is exclusively of maternal origin.

The full mutation in the FMR-1 gene of male fragile X patients is absent in their sperm

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG)n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene (FMR-1) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG)n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR-1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR-1 does not occur in meiosis but in a postzygotic stage.

Nucleus basalis magnocellularis and hippocampus are the major sites of FMR-1 expression in the human fetal brain

The expression of the FMR-1 gene, which is implicated in fragile-X syndrome was investigated in human fetuses by in situ hybridization. In 8 and 9 week-old fetuses, FMR-1 mRNAs are expressed in proliferating and migrating cells of the nervous system, in the retina, and in several non-nervous tissues. In the brain of 25 week-old fetuses, FMR-1 mRNAs are produced in all nearly differenciated structures, with the highest level in cholinergic neurons of the nucleus basalis magnocellularis and in pyramidal neurons of hippocampus. The early transcription of FMR-1 gene and the distribution of FMR-1 mRNAs in human fetuses suggest that alterations of FMR-1 gene expression may contribute to the pathogenesis of fragile-X syndrome and especially the mental retardation.

Guidelines for the diagnosis of fragile X syndrome. National Fragile X Foundation

Direct DNA analysis of the fragile X mutation has become available with the isolation of DNA probes that detect the unstable DNA sequence containing the CGG repeat. We present the various alternatives of combinations of probes and enzymes that can be used for the diagnosis of fragile X syndrome. An overview is given of all the different available probes. A different protocol is presented for postnatal and prenatal diagnosis of fragile X syndrome. This includes Southern blot analysis as well as direct analysis of the CGG repeat by PCR amplification. We discuss the role of constitutional cytogenetic analysis in the diagnosis of mentally retarded subjects and cytogenetic analysis for the diagnosis of fragile X syndrome.

Molecular genetic advances in fragile X syndrome

The fragile X syndrome is recognized as the most common heritable condition resulting in mental retardation. The disabilities are substantial, and therefore early detection is mandatory to assist with reproductive counseling of families in which the fragile X syndrome has occurred. Highly accurate, direct DNA diagnostic testing can now be performed to diagnose the fragile X syndrome without the involvement of individual family members, as was the situation with the use of DNA linkage analysis. Such testing is rapidly becoming a standard diagnostic tool for screening of individuals with suspected fragile X syndrome, of potential unaffected carriers, and of patients with undefined mental retardation. Fragile X testing should be considered for all children with developmental delay of unknown cause. Autistic children will occasionally be found to have mutations in FMR-1. Detection of affected individuals will allow early intervention for these individuals and will assist families with their reproductive decisions (including prevention) in subsequent offspring. An understanding of the molecular genetics of fragile X syndrome has resulted in the resolution of the Sherman paradox and is the first molecular characterization of a chromosomal fragile site, a finding that almost certainly will be important in understanding the cause of chromosomal rearrangements involving fragile sites. In addition, molecular details of the fragile X mutations have yielded insight into "heritable unstable elements," of which the fragile X chromosome is one of the first characterized examples. Thus a similar molecular mechanism involving a trinucleotide repeat may explain the genetics of myotonic dystrophy and spinal-bulbar muscular atrophy (Kennedy disease); it seems reasonable to assume that other genetic diseases also may result from disruption of genes by inherited unstable elements.

Tissue specific expression of FMR-1 provides evidence for a functional role in fragile X syndrome

We have performed mRNA in situ hybridization studies and northern blot analysis in the mouse and human, respectively, to determine the normal gene expression patterns of FMR-1. Expression in the adult mouse was localized to several regions of the brain and the tubules of the testes, which are two of the major organs affected in fragile X syndrome. Universal and very strong expression was observed in early mouse embryos, with differentially decreasing expression during subsequent stages of embryonic development. The early embryonic onset and tissue specificity of FMR-1 gene expression is consistent with involvement in the fragile X phenotype, and also suggests additional organ systems in which clinical manifestations of reduced FMR-1 gene expression may occur.

A point mutation in the FMR-1 gene associated with fragile X mental retardation

The vast majority of patients with fragile X syndrome show a folate-sensitive fragile site at Xq27.3 (FRAXA) at the cytogenetic level, and both amplification of the (CGG)n repeat and hypermethylation of the CpG island in the 5' fragile X gene (FMR-1) at the molecular level. We have studied the FMR-1 gene of a patient with the fragile X phenotype but without cytogenetic expression of FRAXA, a (CGG)n repeat of normal length and an unmethylated CpG island. We find a single point mutation in FMR-1 resulting in an lle367Asn substitution. This de novo mutation is absent in the patient's family and in 130 control X chromosomes, suggesting that the mutation causes the clinical abnormalities. Our results suggest that mutations in FMR-1 are directly responsible for fragile X syndrome, irrespective of possible secondary effects caused by FRAXA.