The prevalence of the FMR1 and FMR2 mutations among preschool children with language delay

Etiological heterogeneity in autism spectrum disorders: more than 100 genetic and genomic disorders and still counting

There is increasing evidence that autism spectrum disorders (ASDs) can arise from rare highly penetrant mutations and genomic imbalances. The rare nature of these variants, and the often differing orbits of clinical and research geneticists, can make it difficult to fully appreciate the extent to which we have made progress in understanding the genetic etiology of autism. In fact, there is a persistent view in the autism research community that there are only a modest number of autism loci known. We carried out an exhaustive review of the clinical genetics and research genetics literature in an attempt to collate all genes and recurrent genomic imbalances that have been implicated in the etiology of ASD. We provide data on 103 disease genes and 44 genomic loci reported in subjects with ASD or autistic behavior. These genes and loci have all been causally implicated in intellectual disability, indicating that these two neurodevelopmental disorders share common genetic bases. A genetic overlap between ASD and epilepsy is also apparent in many cases. Taken together, these findings clearly show that autism is not a single clinical entity but a behavioral manifestation of tens or perhaps hundreds of genetic and genomic disorders. Increased recognition of the etiological heterogeneity of ASD will greatly expand the number of target genes for neurobiological investigations and thereby provide additional avenues for the development of pathway-based pharmacotherapy. Finally, the data provide strong support for high-resolution DNA microarrays as well as whole-exome and whole-genome sequencing as critical approaches for identifying the genetic causes of ASDs.

Lack of FMR3 expression in a male with non-syndromic mental retardation and a microdeletion immediately distal to FRAXE CCG repeat

FRAXE fragile site associated mental retardation (FRAXE MR) belongs to a group of non-syndromic X-linked mental retardation. Two genes, FMR2 and FMR3 (likely a non-coding RNA) are transcribed from the FRAXE CpG island in the opposite directions. While the contribution of the FMR2 gene to FRAXE MR has been demonstrated, the role of the FMR3 gene is not known. We have screened 441 Brazilian mentally handicapped males for CCG repeat expansions in the FMR2 gene and identified a boy with a mutation (c.-414_-357del58) immediately distal to the FRAXE CCG repeat. We have established a skin fibroblast cell line from this patient and tested expression of both FMR2 and FMR3 genes. Reverse transcriptase PCR studies on the FMR2 and FMR3 genes showed that only the FMR3 gene transcription was abolished, suggesting a possible causal relationship between the lack of FMR3 expression and mental retardation in this patient. In the literature, there have been few deletions described near the FRAXE CCG repeat, but none was followed with expression studies. This is the first study showing missing expression in the FMR3 gene with normal FMR2 transcription leading to FRAXE mutation-likely phenotype. The FMR3 gene is likely a non-coding RNA gene. So far all individuals with FRAXE CCG repeat expansions and cytogenetically detectable FRAXE fragile site have both FMR2 and FMR3 gene expression abolished. Although the function of the FMR3 gene is not known, our present study together with previous studies on FRAXE MR suggest that it may play role in the processes underpinning normal learning and memory.

Laboratorial diagnosis of fragile-X syndrome: experience in a sample of individuals with pervasive developmental disorders

Fragile X syndrome is a frequent genetic disease associated to developmental disorders, including learning disability, mental retardation, behavioral problems and pervasive developmental disorders (autism and related conditions). We studied a sample of 82 individuals (69 males and 13 females) presenting with pervasive developmental disorders using three techniques for the diagnosis of fragile X syndrome (FXS). Cytogenetic analysis detected the fragile site in four males, but only one showed a consistent positive rate. Molecular study based on the PCR technique was inconclusive for most females (92.3%), which where latter submitted to Southern blotting analysis, and for one male (1.4%), excluding the FRAXA mutation in the remaining male individuals (98.6%). Molecular tests using the Southern blotting technique confirmed only one positive case (1.2%) in a male subject. These results showed that Southern blotting analysis of the FRAXA mutation has the best sensitivity and specificity for the diagnosis of FXS but also validated the PCR technique as a confinable screening test.

X linked mental retardation: a clinical guide

Mental retardation is more common in males than females in the population, assumed to be due to mutations on the X chromosome. The prevalence of the 24 genes identified to date is low and less common than expansions in FMR1, which cause Fragile X syndrome. Systematic screening of all other X linked genes in X linked families with mental retardation is currently not feasible in a clinical setting. The phenotypes of genes causing syndromic and non-syndromic mental retardation (NLGN3, NLGN4, RPS6KA3(RSK2), OPHN1, ATRX, SLC6A8, ARX, SYN1, AGTR2, MECP2, PQBP1, SMCX, and SLC16A2) are first discussed, as these may be the focus of more targeted mutation analysis. Secondly, the relative prevalence of genes causing only non-syndromic mental retardation (IL1RAPL1, TM4SF2, ZNF41, FTSJ1, DLG3, FACL4, PAK3, ARHGEF6, FMR2, and GDI) is summarised. Thirdly, the problem of recurrence risk where a molecular genetics diagnosis has not been made and what proportion of the male excess of mental retardation is due to monogenic disorders of the X chromosome are discussed.

FMR1 alleles in Tasmania: a screening study of the special educational needs population

The distribution of fragile X mental retardation-1 (FMR1) allele categories, classified by the number of CGG repeats, in the population of Tasmania was investigated in 1253 males with special educational needs (SEN). The frequencies of these FMR1 categories were compared with those seen in controls as represented by 578 consecutive male births. The initial screening was based on polymerase chain reaction analysis of dried blood spots. Inconclusive results were verified by Southern analysis of a venous blood sample. The frequencies of common FMR1 alleles in both samples, and of grey zone alleles in the controls, were similar to those in other Caucasian populations. Consistent with earlier reports, we found some (although insignificant) increase of grey zone alleles in SEN subjects compared with controls. The frequencies of predisposing flanking haplotypes among grey zone males FMR1 alleles were similar to those seen in other Caucasian SEN samples. Contrary to expectation, given the normal frequency of grey zone alleles, no premutation (PM) or full mutation (FM) allele was detected in either sample, with only 15 fragile X families diagnosed through routine clinical admissions registered in Tasmania up to 2002. An explanation of this discrepancy could be that the C19th founders of Tasmania carried few PM or FM alleles. The eight to ten generations since white settlement of Tasmania has been insufficient time for susceptible grey zone alleles to evolve into the larger expansions.

Molecular screening of FRAXA and FRAXE in Indian patients with unexplained mental retardation

Fragile-X mental retardation is the commonest form of inherited mental retardation. We have studied 146 Indian patients (174 X chromosomes) with unexplained mental retardation by molecular methods. All study subjects were unrelated. Three of the 118 males were found to have the FMR1 full mutation. None of the patients tested were positive for the FMR2 full mutation. The Fragile X prevalence was 2.5% among males, which is lower than previously reported in Indian mentally retarded patients. Screening for Fragile X among patients with nonspecific mental retardation is important, even if there is no family history of mental retardation or typical behavioral or physical features associated with the Fragile-X phenotype. Identification of positive cases is also very important for the families, because of the high recurrence risk of the disease. Large multicenter screening programs with uniform criteria would be worthwhile to determine the prevalence of Fragile-X mental retardation in the Indian population.

A developmental approach to understanding Fragile X syndrome in females

The psychological phenotype of females with fragile X syndrome (FraX) is discussed, focusing primarily on empirical findings over the past decade and on studies of probands with the full mutation (FM). A developmental approach is used to help characterize specific patterns of cognitive, neuropsychological, social, emotional, and behavioral functioning across the lifespan of females with FraX. Approximately half of females with the syndrome present with cognitive abilities that fall in the borderline to mentally retarded range, and the remaining females with average intellectual functioning may experience relative deficits in math achievement and problems with attention and executive functioning. Reports of socio-emotional functioning are somewhat inconsistent, due in part, perhaps, to methodological differences in study design. To date, much of what we understand about the psychological phenotype of FraX is based on cross-sectional studies of girls and women with the disorder. Symptoms associated with shyness, and social anxiety and avoidance have been reported in some school-age, adolescent, and adult females with FraX. Only recently have efforts begun to identify the developmental trajectory of FraX in infants and toddlers. There is a void of information specific to these developmental periods. Identifying key deficits in cognitive and socio-emotional functioning has important implications for early detection and intervention for girls with FraX. Directions for future research are discussed.

No widespread psychological effect of the fragile X premutation in childhood: evidence from a preliminary controlled study

This study was designed to examine the effect of the fragile X premutation (pM) on cognitive function and behavior. Participants included 14 children (7 males, 7 females) with the fragile X pM and 14 children without the fragile X pM (and without the fragile X full mutation [fM]), each of whom was matched by age and gender with one of the participants from the pM group. The children ranged in age from 3 years, 1 month, to 17 years, 11 months. Participants were individually administered measures of intellectual functioning, academic achievement, and visual motor integration. Parent rating scales of problem behaviors were completed. Group differences were examined using nonparametric statistics. No statistically significant differences were found between the premutation and nonpremutation groups. The results from this study are consistent with the hypothesis that the premutation does not, in general, have an effect on a child's development. However, this does not preclude cases where specific factors may lead to a specific phenotype.

[Fragile X syndrome confirmed by molecular analysis: a case-control study with pre and post-puberal patients]

The fragile X syndrome (FRAXA) is the most common cause of inherited mental retardation. However, it has been frequently underdiagnosed in pediatric population. The characterization of the most significant pre and post-puberal clinical features observed among patients that are positive for the FMR-1 mutation, is useful as a screening tool for ordering the DNA test. Therefore, a screening program for FRAXA has been conducted in a sample of 104 mentally retarded individuals (92 males and 12 females), comprehending familial history and physical examination in order to determine the clinical characteristics. The molecular test for the disease was performed in all individuals. Seventeen patients (14 males) were positive for the FMR-1 mutation. Familial mental retardation and poor eye contact were the most common clinical findings with statistical significance (p<0.05) in FRAXA pre and post-puberal patients. The post-puberal patients presented, as opposed to the control group, large ears, broad forehead and macroorchidism.

FMR1 and the fragile X syndrome: human genome epidemiology review

The fragile X syndrome, an X-linked dominant disorder with reduced penetrance, is one of the most common forms of inherited mental retardation. The cognitive, behavioral, and physical phenotype varies by sex, with males being more severely affected because of the X-linked inheritance of the mutation. The disorder-causing mutation is the amplification of a CGG repeat in the 5' untranslated region of FMR1 located at Xq27.3. The fragile X CGG repeat has four forms: common (6-40 repeats), intermediate (41-60 repeats), premutation (61-200 repeats), and full mutation (>200-230 repeats). Population-based studies suggest that the prevalence of the full mutation, the disorder-causing form of the repeat, ranges from 1/3,717 to 1/8,918 Caucasian males in the general population. The full mutation is also found in other racial/ethnic populations; however, few population-based studies exist for these populations. No population-based studies exist for the full mutation in a general female population. In contrast, several large, population-based studies exist for the premutation or carrier form of the disorder, with prevalence estimates ranging from 1/246 to 1/468 Caucasian females in the general population. For Caucasian males, the prevalence of the premutation is approximately 1/1,000. Like the full mutation, little information exists for the premutation in other populations. Although no effective cure or treatment exists for the fragile X syndrome, all persons affected with the syndrome are eligible for early intervention services. The relatively high prevalence of the premutation and full mutation genotypes coupled with technological advances in genetic testing make the fragile X syndrome amenable to screening. The timing as well as benefits and harms associated with the different screening strategies are the subject of current research and discussion.

Advances in research on the fragile X syndrome

Fragile X syndrome is a neurodevelopmental disorder that results from a single gene mutation on the X chromosome. The purpose of this review is to summarize key advances made in understanding the fragile X premutation gene seen in carriers and the full mutation gene seen in persons with the syndrome. DNA testing has replaced cytogenetic testing as the primary method for identification of fragile X, although the efficacy of protein level screening is being explored. The premutation is associated with no effects, although there is evidence of physical effects-primarily premature menopause and mild outward features of the fragile X syndrome-among premutation carriers. There is much controversy regarding premutation effects on psychological development. The few experimental studies carried out to date do not suggest noticeable or significant effects. One challenge in addressing this controversy is the sometimes ambiguous differentiation between premutation and full mutation genes. There is a well-established yet highly variable phenotype of the full mutation. Research from this decade has helped to address specific aspects of this phenotype, including the early course of its development in males, the influence of home and family environments, the nature of social difficulties and autistic features seen in boys and girls with fragile X, and the potential role of hyperarousal or hyper-reactivity. Studies in these areas, and on the role of FMR protein, will contribute towards ongoing advances in our understanding of fragile X syndrome and its mechanisms. The variability in physical, social, and cognitive features, as described in this review, is one that prohibits clear-cut screening guidelines designed to avoid high rates of both false positives and false negatives. Results from recent studies indicate the need to consider behavioral features in selecting candidates for fragile X screening. MRDD Research Reviews 2000;6:96-106.