FMR2 expression in families with FRAXE mental retardation

FAN1 exo- not endo-nuclease pausing on disease-associated slipped-DNA repeats: A mechanism of repeat instability

Ongoing inchworm-like CAG and CGG repeat expansions in brains, arising by aberrant processing of slipped DNAs, may drive Huntington's disease, fragile X syndrome, and autism. FAN1 nuclease modifies hyper-expansion rates by unknown means. We show that FAN1, through iterative cycles, binds, dimerizes, and cleaves slipped DNAs, yielding striking exo-nuclease pauses along slip-outs: 5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'. CAG excision is slower than CTG and requires intra-strand A·A and T·T mismatches. Fully paired hairpins arrested excision, whereas disease-delaying CAA interruptions further slowed excision. Endo-nucleolytic cleavage is insensitive to slip-outs. Rare FAN1 variants are found in individuals with autism with CGG/CCG expansions, and CGG/CCG slip-outs show exo-nuclease pauses. The slip-out-specific ligand, naphthyridine-azaquinolone, which induces contractions of expanded repeats in vivo, requires FAN1 for its effect, and protects slip-outs from FAN1 exo-, but not endo-, nucleolytic digestion. FAN1's inchworm pausing of slip-out excision rates is well suited to modify inchworm expansion rates, which modify disease onset and progression.

Simultaneous Screening of the FRAXA and FRAXE Loci for Rapid Detection of FMR1 CGG and/or AFF2 CCG Repeat Expansions by Triplet-Primed PCR

Moderate to hyper-expansion of trinucleotide repeats at the FRAXA and FRAXE fragile sites, with or without concurrent hypermethylation, has been associated with intellectual disability and other conditions. Unlike molecular diagnosis of FMR1 CGG repeat expansions in FRAXA, current detection of AFF2 CCG repeat expansions in FRAXE relies on low-throughput and otherwise inefficient techniques combining Southern blot analysis and PCR. A novel triplet-primed PCR assay was developed for simultaneous screening for trinucleotide repeat expansions at the FRAXA and FRAXE fragile sites, and was validated using archived clinical samples of known FMR1 and AFF2 genotypes. Population samples and FRAXE-affected samples were sequenced for the evaluation of variations in the AFF2 CCG repeat structure. The duplex assay accurately identified expansions at the FMR1 and AFF2 trinucleotide repeat loci. On Sanger sequencing of the AFF2 CCG repeat, the single-nucleotide polymorphism variant rs868914124(C) that effectively adds two CCG repeats at the 5'-end, was enriched in the Malay population and with short repeats (<11 CCGs), and was present in all six expanded AFF2 alleles of this study. All expanded AFF2 alleles contained multiple non-CCG interruptions toward the 5'-end of the repeat. A sensitive, robust, and rapid assay has been developed for the simultaneous detection of expansion mutations at the FMR1 and AFF2 trinucleotide repeat loci, simplifying screening for FRAXA- and FRAXE-associated disorders.

AFF1 inhibits adipogenic differentiation via targeting TGM2 transcription

OBJECTIVES

AF4/FMR2 family member 1 (AFF1), known as a central scaffolding protein of super elongation complex (SEC), regulates gene transcription. We previously reported that AFF1 inhibited osteogenic differentiation of human mesenchymal stromal/stem cells (hMSCs). However, its role in adipogenic differentiation has not been elucidated.

MATERIALS AND METHODS

hMSCs and 3T3-L1 pre-adipocytes were cultured and induced for adipogenic differentiation. Small interfering RNAs (siRNAs) were applied to deplete AFF1 while lentiviruses expressing HA-Aff1 were used for overexpression. Oil Red O staining, triglyceride (TAG) quantification, quantitative real-time PCR (qPCR), Western blot analysis, immunofluorescence staining, RNA sequencing (RNA-seq) analysis and ChIP-qPCR were performed. To evaluate the adipogenesis in vivo, BALB/c nude mice were subcutaneously injected with Aff1-overexpressed 3T3-L1 pre-adipocytes.

RESULTS

AFF1 depletion leads to an enhanced adipogenesis in both hMSCs and 3T3-L1 pre-adipocytes. Overexpression of Aff1 in 3T3-L1 cells results in the reduction of adipogenic differentiation and less adipose tissue formation in vivo. Mechanistically, AFF1 binds to the promoter region of Tgm2 gene and regulates its transcription. Overexpression of Tgm2 largely rescues adipogenic differentiation of Aff1-deficient cells.

CONCLUSIONS

Our data indicate that AFF1 inhibits adipogenic differentiation by regulating the transcription of TGM2.

AFF1 and AFF4 differentially regulate the osteogenic differentiation of human MSCs

AFF1 and AFF4 belong to the AFF (AF4/FMR2) family of proteins, which function as scaffolding proteins linking two different transcription elongation factors, positive elongation factor b (P-TEFb) and ELL1/2, in super elongation complexes (SECs). Both AFF1 and AFF4 regulate gene transcription through elongation and chromatin remodeling. However, their function in the osteogenic differentiation of mesenchymal stem cells (MSCs) is unknown. In this study, we show that small interfering RNA (siRNA)-mediated depletion of AFF1 in human MSCs leads to increased alkaline phosphatase (ALP) activity, enhanced mineralization and upregulated expression of osteogenic-related genes. On the contrary, depletion of AFF4 significantly inhibits the osteogenic potential of MSCs. In addition, we confirm that overexpression of AFF1 and AFF4 differentially affects osteogenic differentiation in vitro and MSC-mediated bone formation in vivo. Mechanistically, we find that AFF1 regulates the expression of DKK1 via binding to its promoter region. Depletion of DKK1 in HA-AFF1-overexpressing MSCs abrogates the impairment of osteogenic differentiation. Moreover, we detect that AFF4 is enriched in the promoter region of ID1. AFF4 knockdown blunts the BRE luciferase activity, SP7 expression and ALP activity induced by BMP2 treatment. In conclusion, our data indicate that AFF1 and AFF4 differentially regulate the osteogenic differentiation of human MSCs.

Distribution of FMR1 and FMR2 Repeats in Argentinean Patients with Primary Ovarian Insufficiency

The premutation state of FMR1 (Fragile X Mental Retardation 1) has been associated with primary ovarian insufficiency (POI), and is the most common known genetic cause for 46,XX patients. Nevertheless, very few studies have analyzed its frequency in Latin American populations. Additionally, a relationship between alleles carrying a cryptic microdeletion in the 5’UTR of FMR2 and the onset of POI has only been studied in one population. Our aim was to analyze the incidence of FMR1 premutations and putative microdeletions in exon 1 of FMR2 in a cohort of Argentinean women with POI. We studied 133 patients and 84 controls. Fluorescent PCR was performed, and the FMR2 exon 1 was further sequenced in samples presenting less than 11 repeats. We found the frequency of FMR1 premutations to be 6.7% and 2.9% for familial and sporadic patients, respectively. Among controls, 1/84 women presented a premutation. In addition, although we did not find microdeletions in FMR2, we observed a change (T >C) adjacent to the repeats in two sisters with POI. Given the repetitive nature of the sequence involved, we could not ascertain whether this represents a single nucleotide polymorphism (SNP) or a deletion. Therefore, a relationship between FMR2 and POI could not be established for our population.

Autism spectrum disorder: FRAXE mutation, a rare etiology

Autism spectrum disorder (ASD) is characterized by impaired social interaction and communication, restricted interests and repetitive behaviors. Fragile X E is associated with X-linked non-specific mild intellectual disability (ID) and with behavioral problems. Most of the known genetic causes of ASD are also causes of ID, implying that these two identities share common genetic bases. We present a child with an ASD with a normal range of intelligence quotient, that later evolved to compulsive behavior. FRAXE locus analysis by polymerase chain reaction revealed a complete mutation of the FMR 2 gene. This report stresses the importance of clinicians being aware of the association between a full mutation of FMR2 and ASD associated with compulsive behavior despite normal intellectual level.

FRA2A is a CGG repeat expansion associated with silencing of AFF3

Folate-sensitive fragile sites (FSFS) are a rare cytogenetically visible subset of dynamic mutations. Of the eight molecularly characterized FSFS, four are associated with intellectual disability (ID). Cytogenetic expression results from CGG tri-nucleotide-repeat expansion mutation associated with local CpG hypermethylation and transcriptional silencing. The best studied is the FRAXA site in the FMR1 gene, where large expansions cause fragile X syndrome, the most common inherited ID syndrome. Here we studied three families with FRA2A expression at 2q11 associated with a wide spectrum of neurodevelopmental phenotypes. We identified a polymorphic CGG repeat in a conserved, brain-active alternative promoter of the AFF3 gene, an autosomal homolog of the X-linked AFF2/FMR2 gene: Expansion of the AFF2 CGG repeat causes FRAXE ID. We found that FRA2A-expressing individuals have mosaic expansions of the AFF3 CGG repeat in the range of several hundred repeat units. Moreover, bisulfite sequencing and pyrosequencing both suggest AFF3 promoter hypermethylation. cSNP-analysis demonstrates monoallelic expression of the AFF3 gene in FRA2A carriers thus predicting that FRA2A expression results in functional haploinsufficiency for AFF3 at least in a subset of tissues. By whole-mount in situ hybridization the mouse AFF3 ortholog shows strong regional expression in the developing brain, somites and limb buds in 9.5–12.5dpc mouse embryos. Our data suggest that there may be an association between FRA2A and a delay in the acquisition of motor and language skills in the families studied here. However, additional cases are required to firmly establish a causal relationship.

Some human genetic diseases are caused by dynamic mutations, or expansions of a short repeated sequence in the genome that can be unstably passed on from generation to generation. A subset of these dynamic mutations known as fragile sites can be seen as a break or gap on the chromosome when cells are cultured under specific conditions. To date eight folate-sensitive fragile sites (FSFS) have been characterized, and all are due to CGG-repeat expansions within the 5′ UTR or promoter region of the respective gene. When the repeat expands in size, it becomes hypermethylated and the adjacent gene or genes are transcriptionally silenced. For at least four of the eight known fragile sites this silencing of the associated gene(s) lead to intellectual disability syndromes such as fragile X. In this work we describe molecular characterization of an autosomal FSFS called FRA2A on chromosome 2. As the molecular cause of FRA2A, we identify an expansion of a CGG repeat which subsequently results in silencing of the neighbouring gene AFF3. This gene is one of the four autosomal paralogss of the AFF2/FMR2 gene which, when mutated, is the cause of the FRAXE syndrome. We find that FRA2A expression is associated with highly variable developmental anomalies in the three FRA2A families studied.

Epigenetics in nucleotide repeat expansion disorders

Over the past 20 years, nucleotide repeat expansion disorders have informed our broader understanding of neurodevelopmental and neurodegenerative disease. This is especially true with regard to the contributions of epigenetic mechanisms to neurologic disease pathogenesis. In this review, the authors describe a few of the myriad ways in which epigenetic processes underlie aspects of repeat expansion disorder pathophysiology and discuss how therapies targeted at epigenetic modulation hold promise for many of these disorders.

Intellectual disabilities, neuronal posttranscriptional RNA metabolism, and RNA-binding proteins: three actors for a complex scenario

Intellectual disability (ID) is the most frequent cause of serious handicap in children and young adults and interests 2-3% of worldwide population, representing a serious problem from the medical, social, and economic points of view. The causes are very heterogeneous. Genes involved in ID have various functions altering different pathways important in neuronal function. Regulation of mRNA metabolism is particularly important in neurons for synaptic structure and function. Here, we review ID due to alteration of mRNA metabolism. Functional absence of some RNA-binding proteins--namely, FMRP, FMR2P, PQBP1, UFP3B, VCX-A--causes different forms of ID. These proteins are involved in different steps of RNA metabolism and, even if a detailed analysis of their RNA targets has been performed so far only for FMRP, it appears clear that they modulate some aspects (translation, stability, transport, and sublocalization) of a subset of RNAs coding for proteins, whose function must be relevant for neurons. Two other proteins, DYRK1A and CDKL5, involved in Down syndrome and Rett syndrome, respectively, have been shown to have an impact on splicing efficiency of specific mRNAs. Both proteins are kinases and their effect is indirect. Interestingly, both are localized in nuclear speckles, the nuclear domains where splicing factors are assembled, stocked, and recycled and influence their biogenesis and/or their organization.

Microdeletion of Xq28 involving the AFF2 (FMR2) gene in two unrelated males with developmental delay

Fragile X E (FRAXE) is an X-linked form of intellectual disability characterized by mild to moderate cognitive impairment, speech delay, hyperactivity, and autistic behavior. The folate-sensitive fragile site FRAXE is located in Xq28 approximately 600 kb distal to the fragile X syndrome fragile site (FRAXA) and harbors an unstable GCC (CCG) triplet repeat adjacent to a CpG island in the 5' untranslated region of the AFF2 (FMR2) gene. The disorder results from amplification and methylation of the GCC repeat and resultant silencing of AFF2. Although chromosome abnormalities that disrupt AFF2 have been reported in two individuals with mild-moderate intellectual disability, microdeletions of Xq28 that delete only AFF2 have not been described as a potential cause of FRAXE-intellectual disability. We performed clinical and molecular characterization of two males with 240 and 499 kb deletions, respectively, at Xq28, both of which encompassed only one gene, AFF2. The 240 kb deletion in Patient 1 was intragenic and lead to the loss of 5' exons 2-4 of AFF2; the 499 kb deletion in Patient 2 removed the 5' exons 1-2 of AFF2 including approximately 350 kb upstream of the gene. Both individuals had developmental and speech delay, and one had mild dysmorphism. We predict disruption of AFF2 in these two patients is likely the cause of their overlapping phenotypes.

Fine-scale survey of X chromosome copy number variants and indels underlying intellectual disability

Copy number variants and indels in 251 families with evidence of X-linked intellectual disability (XLID) were investigated by array comparative genomic hybridization on a high-density oligonucleotide X chromosome array platform. We identified pathogenic copy number variants in 10% of families, with mutations ranging from 2 kb to 11 Mb in size. The challenge of assessing causality was facilitated by prior knowledge of XLID-associated genes and the ability to test for cosegregation of variants with disease through extended pedigrees. Fine-scale analysis of rare variants in XLID families leads us to propose four additional genes, PTCHD1, WDR13, FAAH2, and GSPT2, as candidates for XLID causation and the identification of further deletions and duplications affecting X chromosome genes but without apparent disease consequences. Breakpoints of pathogenic variants were characterized to provide insight into the underlying mutational mechanisms and indicated a predominance of mitotic rather than meiotic events. By effectively bridging the gap between karyotype-level investigations and X chromosome exon resequencing, this study informs discussion of alternative mutational mechanisms, such as noncoding variants and non-X-linked disease, which might explain the shortfall of mutation yield in the well-characterized International Genetics of Learning Disability (IGOLD) cohort, where currently disease remains unexplained in two-thirds of families.

AF4 is a critical regulator of the IGF-1 signaling pathway during Purkinje cell development

Deregulation of the insulin-like growth factor 1 (IGF-1) signaling pathway is a recurrent finding in mouse models and human patients with cerebellar ataxia and thus represents a common pathological cascade in neuronal cell death that may be targeted for therapy. We have previously identified a point mutation in AF4, a transcription cofactor of RNA polymerase II elongation and chromatin remodeling, that causes progressive and highly specific Purkinje cell (PC) death in the ataxic mouse mutant robotic, leading to the accumulation of AF4 in PCs. Here we confirm that the spatiotemporal pattern of PC degeneration in the robotic cerebellum correlates with the specific profile of AF4 upregulation. To identify the underlying molecular pathways, we performed microarray gene expression analysis of PCs obtained by laser capture microdissection (LCM) at the onset of degeneration. Igf-1 was significantly downregulated in robotic PCs compared with wild-type controls before and throughout the degenerative process. Consistently, we observed a decrease in the activation of downstream signaling molecules including type 1 IGF receptor (IGF-1R) and the extracellular signal-regulated kinase (ERK) 1 and ERK2. Chromatin immunoprecipitation confirmed that Igf-1 is a direct and the first validated target of the AF4 transcriptional regulatory complex, and treatment of presymptomatic robotic mice with IGF-1 indeed markedly delayed the progression of PC death. This study demonstrates that small changes in the levels of a single transcriptional cofactor can deleteriously affect normal cerebellum function and opens new avenues of research for the manipulation of the IGF-1 pathway in the treatment of cerebellar ataxia in humans.

Instability and chromatin structure of expanded trinucleotide repeats

Trinucleotide repeat expansion underlies at least 17 neurological diseases. In affected individuals, the expanded locus is characterized by dramatic changes in chromatin structure and in repeat tract length. Interestingly, recent studies show that several chromatin modifiers, including a histone acetyltransferase, a DNA methyltransferase and the chromatin insulator CTCF can modulate repeat instability. Here, we propose that the unusual chromatin structure of expanded repeats directly impacts their instability. We discuss several potential models for how this might occur, including a role for DNA repair-dependent epigenetic reprogramming in increasing repeat instability, and the capacity of epigenetic marks to alter sense and antisense transcription, thereby affecting repeat instability.

CGG-repeat expansion in the DIP2B gene is associated with the fragile site FRA12A on chromosome 12q13.1

A high level of cytogenetic expression of the rare folate-sensitive fragile site FRA12A is significantly associated with mental retardation. Here, we identify an elongated polymorphic CGG repeat as the molecular basis of FRA12A. This repeat is in the 5' untranslated region of the gene DIP2B, which encodes a protein with a DMAP1-binding domain, which suggests a role in DNA methylation machinery. DIP2B mRNA levels were halved in two subjects with FRA12A with mental retardation in whom the repeat expansion was methylated. In two individuals without mental retardation but with an expanded and methylated repeat, DIP2B expression was reduced to approximately two-thirds of the values observed in controls. Interestingly, a carrier of an unmethylated CGG-repeat expansion showed increased levels of DIP2B mRNA, which suggests that the repeat elongation increases gene expression, as previously described for the fragile X-associated tremor/ataxia syndrome. These data suggest that deficiency of DIP2B, a brain-expressed gene, may mediate the neurocognitive problems associated with FRA12A.

FRAXE intermediate alleles are associated with Parkinson’s disease

There is evidence that male subjects with a clinical picture of action tremor, Parkinsonism, and cerebellar ataxia may have Fragile X premutations (FRAXA). We analyzed FRAXA and FRAXE triplet repeats in 203 male subjects with Parkinson's disease (PD) and 370 healthy controls. No full mutations or premutations at the FRAXA and FRAXE loci were found in the subjects with PD or in the controls. FRAXA allele distribution was similar in patients and controls. FRAXE intermediate alleles (31-60 repeats CCG) were found in 13 of 203 (6.4%) subjects with PD and in only one of the 370 (0.27%) healthy controls (P < 0.001), thus indicating that these relatively large alleles may be associated with PD.

X-linked mild non-syndromic mental retardation with neuropsychiatric problems and the missense mutation A365E in PAK3

We describe a family of 19 males in five generations with mild to borderline non-syndromic X-linked mental retardation (MRX). There were no clinical manifestations in the affected males other than mental impairment and relatively long ears, with neuropsychiatric problems in some cases. Linkage analysis carried out on part of the pedigree using 34 markers spanning the X chromosome localized the gene between DXS454 and DXS1001 in Xq23. The maximum two-point lod score was 3.21 at DXS1059. PAK3 is a known MRX gene mapping to the same region. The affected males and obligate carrier females were found to have a missense mutation c.1094C > A in exon 10 causing an A365E substitution in a highly conserved region of the protein. The C to A base change abolishes a PvuII restriction enzyme site providing the basis for a simple test, if required, for carrier detection and prenatal diagnosis in the extended family.

Cytogenetic analysis of obsessive-compulsive disorder (OCD): identification of a FRAXE fragile site

Obsessive-compulsive disorder (OCD) is a chronic psychiatric disease characterized by recurrent obsessions, compulsions, or both. The prevalence rate of OCD is 2.1% in the general population. Here we report cytogenetic analysis of 26 patients affected with OCD. In one male patient (OCD-K33), we identified a fragile X chromosome by cytogenetic analysis with 21% of cells demonstrating a fragile site at Xq27-q28. Polymerase chain reaction (PCR) and Southern blot analysis demonstrated that the molecular basis of the OCD-K33 fragile X chromosome was expansion of the CCG repeat at FRAXE. The number of the expanded repeats was estimated to be more than 300 copies, qualifying it as a full FRAXE mutation. Further analysis of the family members of OCD-K33 revealed another member with a full FRAXE mutation (630-1,200 copies of the CCG repeat), who had the clinical phenotype of speech impairment, and two other members with normal phenotypes and no FRAXE expansion. The two FRAXE expansions lead to complete methylation at the CCG repeat. The co-segregation of the full FRAXE mutation with apparent neurologic disorders in the same family provides further support to the notion that FRAXE is a genetic neurologic condition. Our findings expand the spectrum of clinical phenotypes associated with FRAXE mutations.

Clinical, cytogenetic, and molecular description of a FRAXE French family

BACKGROUND

FRAXE is a second locus associated with X chromosome fragility. Similar to FRAXA, the common mutation is a GCC expansion located in the 5' untranslated region, leading to the hypermethylation of the region and to the subsequent inactivation of specific genes (FMR1 and FMR2, respectively). Unlike FRAXA, FRAXE has a rare occurrence and is less currently studied in routine analyses. The phenotype associated with FRAXE is usually considered as mild or moderate mental retardation, with incomplete penetrance. However, phenotype/genotype relations have been less characterized.

OBJECTIVE

We report a French family with three members affected with mental retardation, including a female suffering from West syndrome, and two mentally retarded males.

METHODS

After exclusion of the FRAXA expansion by Southern blot analysis, we performed a karyotype using folate-thymidine-deficient medium and a southern blot to search for FRAXE expansion.

RESULTS

All three mentally retarded patients had a number of repeats over 800 GCC and expressed more than 20% of fragile sites in their leukocytes. Another carrier female with a full expansion had a subnormal mental impairment.

CONCLUSIONS

Clinical features and both the cytogenetic and molecular findings seem to correlate in this family. We discuss the bias encountered when studying such families and some of the mechanisms that may explain part of the clinical variability.

X-linked mental retardation: vanishing boundaries between non-specific (MRX) and syndromic (MRXS) forms

This review covers the history and nosology of X-linked mental retardation (XLMR) in which the following, largely clinically based, subclassification was used: fragile X syndrome (FRAXA), syndromic forms (MRXS) and non-specific forms (MRX). After the discovery of the FMR2 gene at the FRAXE site, 10 MRX genes have been identified in the last 6 years. A short description is given of the strategies used to identify the genes that cause mental retardation (MR). Furthermore, their potential functions and the association with MR will be discussed. It is emphasized that mutations in several of these MR genes can result in non-specific, as well as in syndromic forms of XLMR. Present findings stress the importance of accurate clinical evaluation. Most considerably, genotype-phenotype correlation studies of affected individuals in XLMR families with MRX gene mutations are necessary to define the criteria of MRX vs MRXS subclassification.

Genetic effects on human cognition: lessons from the study of mental retardation syndromes

The molecular basis of human cognition is still poorly understood, but recent advances in finding genetic mutations that result in cognitive impairment may provide insights into the neurobiology of cognitive function. Here we review the progress that has been made so far and assess what has been learnt from this work on the relation between genes and cognitive processes. We review evidence that the pathway from genetic lesion to cognitive impairment can be dissected, that some genetic effects on cognition are relatively direct and we argue that the study of mental retardation syndromes is giving us new clues about the biological bases of cognition.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

Genes for cognitive function: developments on the X

Developments in human genome research enabled the first steps toward a molecular understanding of cognitive function. That there are numerous genes on the X chromosome affecting intelligence at the lower end of the cognitive range is no longer in doubt. Naturally occurring mutations have so far led to the identification of seven genes accounting for a small proportion of familial nonspecific X-linked mental retardation. These new data indicate that normal expression of many more X-linked and autosomal genes contribute to cognitive function. The emerging knowledge implicating genes in intracellular signaling pathways provides the insight to identify as candidates other X-linked and autosomal genes regulating the normal development of cognitive function. Recent advances in unravelling the underlying molecular complexity have been spectacular but represent only the beginning, and new technologies will need to be introduced to complete the picture.

Prevalence and phenotype consequence of FRAXA and FRAXE alleles in a large, ethnically diverse, special education-needs population

We conducted a large population-based survey of fragile X (FRAXA) syndrome in ethnically diverse metropolitan Atlanta. The eligible study population consisted of public school children, aged 7-10 years, in special education-needs (SEN) classes. The purpose of the study was to estimate the prevalence among whites and, for the first time, African Americans, among a non-clinically referred population. At present, 5 males with FRAXA syndrome (4 whites and 1 African American), among 1,979 tested males, and no females, among 872 tested females, were identified. All males with FRAXA syndrome were mentally retarded and had been diagnosed previously. The prevalence for FRAXA syndrome was estimated to be 1/3,460 (confidence interval [CI] 1/7,143-1/1,742) for the general white male population and 1/4, 048 (CI 1/16,260-1/1,244) for the general African American male population. We also compared the frequency of intermediate and premutation FRAXA alleles (41-199 repeats) and fragile XE syndrome alleles (31-199 repeats) in the SEN population with that in a control population, to determine if there was a possible phenotype consequence of such high-repeat alleles, as has been reported previously. No difference was observed between our case and control populations, and no difference was observed between populations when the probands were grouped by a rough estimate of IQ based on class placement. These results suggest that there is no phenotype consequence of larger alleles that would cause carriers to be placed in an SEN class.

DNA diagnosis of FRAXA and FRAXE in Chinese children with neurodevelopmental disorders and fragile X syndrome

Fragile X (FraX) syndrome is the most common cause of inherited mental retardation. To see whether FRAXA or FRAXE can account for the etiology of some unexplained neurodevelopmental disorders in children, we screened for trinucleotide repeat expansion in a consecutive cohort of 73 Chinese children and their mothers seen in 1995 (group 1) referred for developmental assessment due to developmental delay, language delay, attention deficit hyperactivity disorder, autistic spectrum disorder, mental retardation and/or learning disability. We also screened DNA samples of all five previously diagnosed cytogenetically-positive FraX boys, their mothers and sisters (group 2). A control group of unrelated teenagers and adults were recruited from the community (group 3). In group 1, 3 families (2 mothers and a mother and her son) were found to carry a small premutation allele at FRAXA (premutation frequency = 2%, 3/153 independent X chromosomes), but none had any expansion at FRAXE. In group 2, all 5 FraX boys had full mutation at FRAXA and normal repeat length at FRAXE. In group 3, 1 male has a premutation allele out of 18 males and 59 females tested (premutation frequency of control = 0.7%, 1 out of 136 X chromosomes). For FRAXE screening in group 3, 2 females were carriers (1.5%, 2 out of 136 X chromosomes). Thus, FRAXA and FRAXE cannot account for the etiology of neurodevelopmental disorders in our cohort of Chinese children, and the prevalence of FRAXE mutation in normal Chinese population appears to be higher than reported in the Caucasians.

Gene structure and subcellular localization of FMR2, a member of a new family of putative transcription activators

FMR2 is the gene associated with FRAXE mental retardation. It is expressed as an 8.7-kb transcript in placenta and adult brain. A fetal-specific FMR2 transcript of approximately 12 kb was detected in fetal brain and at a lower level in fetal lung and kidney. FMR2 is a large gene composed of 22 exons spanning at least 500 kb on Xq28. Alternative splicing involving exons 2, 3, 5, 7, and 21 was not tissue specific as tested on mRNA from human fetal and infant brain. FMR2 is translated into a 1311-amino-acid nuclear protein with putative transcription transactivation potential. Subcellular localization studies with green fluorescent protein as a reporter show that both nuclear addresses found in the FMR2 sequence are functional and direct the FMR2 protein into the nucleus. FMR2 together with AF4 and LAF4 forms a new family of nuclear proteins with DNA-binding capacity and transcription transactivation potential. BLAST searches of the dbEST database revealed the presence of at least two other groups of nonoverlapping ESTs showing high similarity to the FMR2-related family of proteins. One of them, represented by the EST W26686, maps to chromosome 5q31. Amino acid similarity among the proteins encoded by members of the gene family is high in the NH2 terminus, low in the middle, and high again in the COOH end. Available information from members of the family shows that genomic organization is conserved. This FMR2-related gene family encodes nuclear proteins with involvement in mental retardation (FMR2), cancer (AF4), and lymphocyte differentiation (LAF4) or with unknown function (EST W26686 and/or AA025630).