A Mouse Model of the Human Fragile X Syndrome I304N Mutation

The mental retardation, autistic features, and behavioral abnormalities characteristic of the Fragile X mental retardation syndrome result from the loss of function of the RNA–binding protein FMRP. The disease is usually caused by a triplet repeat expansion in the 5′UTR of the FMR1 gene. This leads to loss of function through transcriptional gene silencing, pointing to a key function for FMRP, but precluding genetic identification of critical activities within the protein. Moreover, antisense transcripts (FMR4, ASFMR1) in the same locus have been reported to be silenced by the repeat expansion. Missense mutations offer one means of confirming a central role for FMRP in the disease, but to date, only a single such patient has been described. This patient harbors an isoleucine to asparagine mutation (I304N) in the second FMRP KH-type RNA–binding domain, however, this single case report was complicated because the patient harbored a superimposed familial liver disease. To address these issues, we have generated a new Fragile X Syndrome mouse model in which the endogenous Fmr1 gene harbors the I304N mutation. These mice phenocopy the symptoms of Fragile X Syndrome in the existing Fmr1–null mouse, as assessed by testicular size, behavioral phenotyping, and electrophysiological assays of synaptic plasticity. I304N FMRP retains some functions, but has specifically lost RNA binding and polyribosome association; moreover, levels of the mutant protein are markedly reduced in the brain specifically at a time when synapses are forming postnatally. These data suggest that loss of FMRP function, particularly in KH2-mediated RNA binding and in synaptic plasticity, play critical roles in pathogenesis of the Fragile X Syndrome and establish a new model for studying the disorder

A nonsense mutation in FMR1 causing fragile X syndrome

Fragile X syndrome is a common cause of inherited intellectual disability. It is caused by lack of the FMR1 gene product FMRP. The most frequent cause is the expansion of a CGG repeat located in the 5′UTR of FMR1. Alleles with 200 or more repeats become hypermethylated and transcriptionally silent. Only few patients with intragenic point mutations in FMR1 have been reported and, currently, routine analysis of patients referred for fragile X syndrome includes solely analysis for repeat expansion and methylation status. We identified a substitution in exon 2 of FMR1, c.80C>A, causing a nonsense mutation p.Ser27X, in a patient with classical clinical symptoms of fragile X syndrome. The mother who carried the mutation in heterozygous form presented with mild intellectual impairment. We conclude that further studies including western blot and DNA sequence analysis of the FMR1 gene should be performed in patients with typical symptoms of fragile X syndrome in whom no CGG repeat expansion is detected

Positional cloning of a gene involved in hereditary multiple exostoses

Hereditary multiple exostosis (EXT) is an autosomal dominant condition mainly characterized by the presence of multiple exostoses on the long bones, These exostoses are benign cartilaginous tumors (enchondromata). Three different EXT loci on chromosomes 8q (EXT1), 11p(EXT2) and 19p(EXT3) have been reported, and recently the EXT1 gene was identified by positional cloning. To isolate the EXT2 gene, we constructed a contig of yeast artificial chromosomes (YAC) and P1 clones covering the complete EXT2 candidate region on chromosome 11p11-p12, One of the transcribed sequences isolated from this region corresponds to a novel gene with homology to the EXT1 gene, and harbours inactivating mutations in different patients with hereditary multiple exostoses, This indicates that this gene is the EXT2 gene, EXT2 has an open reading frame encoding 718 amino acids with an overall homology of 30.9% with EXT1, suggesting that a family of related genes might be responsible for the development of EXT

High positive predictive value (PPV) of cell-free DNA (cfDNA) testing in a clinical study of 10,000 consecutive pregnancies

Background: Cell-free DNA (cfDNA) analysis in maternal blood for the detection of fetal Down syndrome is gradually replacing first trimester screening. We present here a large clinical series of 10,000 consecutive pregnancies. Objectives: To study the reliability of cell-free DNA (cfDNA) analysis in maternal blood for the detection of fetal trisomy 21, 18 and 13 in a clinical setting in 10,000 consecutive pregnancies with variable risk. cfDNA testing has been evaluated in an increasing number of pregnancies mainly at high risk for fetal trisomy, and some studies have suggested that its positive predictive value (PPV) might be lower in low-risk populations. Study design: CfDNA testing using the Harmony™ Prenatal Test was performed in 10,000 consecutive pregnancies with high or low a-priori risk for fetal trisomy 21, 18 and 13. Results: In 147 (1.47%) of the 10,000 pregnancies a high-risk cfDNA testing result indicated trisomy 21 (n=121), trisomy 18 (n=15) or trisomy 13 (n=11). It failed to detect 5 trisomies (2 trisomies 21, 2 trisomies 18, and 1 trisomy 13). Five false-positive results were recorded (4 in the high and 1 in the low risk population). The overall positive predictive value (PPV) was 96%, with a PPV of 96% in the high-risk (>1/200) population and 97% in the low risk (<1/200) population. Conclusions: In this large clinical series of 10,000 consecutive pregnancies, cfDNA testing proved very reliable in detecting fetal trisomy 21, 18 and 13, with a very high PPV both in high and low risk populations