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Longitudinal neuroimaging investigation of fragile X syndrome (FXS), the most common cause of inherited intellectual disability and autism, provides an opportunity to study the influence of a specific genetic factor on neurodevelopment in the living human brain. We examined voxel-wise gray and white matter volumes (GMV, WMV) over a 2-year period in 1- to 3-year-old boys with FXS (n = 41) and compared these findings to age- and developmentally matched controls (n = 28). We found enlarged GMV in the caudate, thalamus, and fusiform gyri and reduced GMV in the cerebellar vermis in FXS at both timepoints, suggesting early, possibly prenatal, genetically mediated alterations in neurodevelopment. In contrast, regions in which initial GMV was similar, followed by an altered growth trajectory leading to increased size in FXS, such as the orbital gyri, basal forebrain, and thalamus, suggests delayed or otherwise disrupted synaptic pruning occurring postnatally. WMV of striatal-prefrontal regions was greater in FXS compared with controls, and group differences became more exaggerated over time, indicating the possibility that such WM abnormalities are the result of primary FMRP-deficiency-related axonal pathology, as opposed to secondary connectional dysregulation between morphologically atypical brain structures. Our results indicate that structural abnormalities of different brain regions in FXS evolve differently over time reflecting time-dependent effects of FMRP deficiency and provide insight into their neuropathologic underpinnings. The creation of an early and accurate human brain phenotype for FXS in humans will significantly improve our capability to detect whether new disease-specific treatments can 'rescue' the FXS phenotype in affected individuals. early childhood | longitudinal | MRI | voxel-based morphometry doi: 10.1073/pnas.1002762107

Fragile X syndrome (FXS) has so far resisted efforts to define the basic cellular defects caused by the absence of a single protein, fragile X mental retardation protein (FMRP), because the patients have a wide variety of symptoms of varying severity. Immature-appearing dendritic spines on neurons found in FXS patients and fmr1-KO mice suggest a role for FMRP in modulating production of synaptic structural proteins. We isolated cortical synaptoneurosomes from WT and KO mice and studied MAPK pathway activation after group I metabotropic glutamate receptor (mGluR) stimulation. Here, we show that ERK in KO synaptoneurosomes is rapidly dephosphorylated upon mGluR1/5 stimulation, whereas it is phosphorylated in WT mice, suggesting that aberrant activation of phosphatases occurs in KO synapses in response to synaptic stimulation. In KO synapses, protein phosphatase 2A (PP2A) is overactivated after mGluR1 stimulation, and tyrosine phosphatase is overactivated after mGluR5 stimulation, causing the rapid deactivation of ERK. ERK activation can be restored in KO by pretreatment with phosphatase blockers; blocking of PP2A by okadaic acid could successfully restore normal ERK activation in KO synaptoneurosomes. We propose that overactivation of phosphatases in synapses may be a key deficit in FXS, which affects synaptic translation, transcription, and synaptic receptor regulation. dephosphoryiation | metabotropic glutamate receptors | phosphorylation | protein translation | synaptoneurosomes

Mutations in the human methyl-CpG-binding protein gene MECP2 cause the neurological disorder Rett syndrome and some cases of X-linked mental retardation (XLMR). We report that MeCP2 interacts with ATRX, a SWI2/SNF2 DNA helicase/ATPase that is mutated in ATRX syndrome ([alpha]-thalassemia/mental retardation, X-linked). MeCP2 can recruit the helicase domain of ATRX to heterochromatic foci in living mouse cells in a DNA methylation-dependent manner. Also, ATRX localization is disrupted in neurons of Mecp2-null mice. Point mutations within the methylated DNA-binding domain of MeCP2 that cause Rett syndrome or X-linked mental retardation inhibit its interaction with ATRX in vitro and its localization in vivo without affecting methyl-CpG binding. We propose that disruption of the MeCP2-ATRX interaction leads to pathological changes that contribute to mental retardation. DNA methylation | Rett syndrome | X-linked mental retardation

Fragile X syndrome is a common heritable form of mental retardation in humans. Recent neuroanatomical studies indicate an apparent immature appearance of neurons in fragile X syndrome patients and fragile X mental retardation protein (FMRP)-knockout mice, an animal model of this condition. In this work, we investigated possible alterations in synaptic plasticity in the neocortex of FMRP-knockout mice. Extracellular field potentials were recorded from the deep-layer visual neocortex. Long-term potentiation (LTP) was severely attenuated in brain slices from knockout mice relative to that observed in slices from wild-type mice. Considering that neocortical LTP can involve both NMDA receptor-dependent and -independent mechanisms, we attempted to distinguish the nature of LTP attenuated in the knockout condition. In slices from wild-type mice, LTP was partially attenuated by the NMDA receptor antagonist 3-[([+ or -])-2-carboxypiperazin-4-yl]-propyl-1-phosphate (CPP); however, the general metabotropic glutamate receptor (mGluR) antagonist [alpha]-methyl-4-carboxyphenylglycine (MCPG) strongly attenuated LTP, resulting in a response indistinguishable from that observed in slices from knockout mice. The selective mGluRs antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) attenuated LTP to a similar degree as did MCPG in wild-type slices, but MPEP did not alter the reduced potentiation in knockout slices. Our results suggest that LTP in layer V visual neocortex depends primarily on mGlu[R.sub.5] activation. Our data also indicate that mGlu[R.sub.5]mediated synaptic plasticity is absent in the neocortex of FMRP-knockout mice. Such an alteration may contribute to the cognitive and learning deficits exhibited in these mice as well as in fragile X syndrome. fragile X mental retardation protein | long-term potentiation | synaptic plasticity | visual cortex

We have analyzed the effects of gene activation on chromatin conformation throughout an ~170-kb region comprising the human fragile X locus, which includes a single expressed gene, FMR1 (fragile X mental retardation 1). We have applied three approaches: (17 chromosome conformation capture, which assesses relative interaction frequencies of chromatin segments; (ii) an extension of this approach that identifies domains whose conformation differs from the average, which we developed and named chromosome conformation profiling; and (iii) ChIP analysis of histone modifications. We find that, in normal cells where FMR1 is active, the FMR1 promoter is at the center of a large (~50 kb) domain of reduced intersegment interactions. In contrast, in fragile X cells where FMR1 is inactive, chromatin conformation is uniform across the entire region. We also find that histone modifications that are characteristic of active genes occur tightly localized around the FMR1 promoter in normal cells and are absent in fragile X cells. Therefore, the expression-correlated change in conformation affects a significantly larger domain than that marked by histone modifications. Domain-wide changes in interaction probability could reflect increased chromatin expansion and may also be related to an altered spatial disposition that results in increased intermingling with unrelated loci. The described approaches are widely applicable to the study of conformational changes of any locus of interest. chromatin domain | chromosome conformation capture | transcription | fragile X locus

Fragile X syndrome, the most frequent form of hereditary mental retardation, is due to a mutation of the fragile X mental retardation 1 (FMR1) gene on the X chromosome. Like fragile X patients, FMR1-knockout (FMR1-KO) mice lack the normal fragile X mental retardation protein (FMRP) and show both cognitive alterations and an immature neuronal morphology. We reared FMR1-KO mice in a C57BL/6 background in enriched environmental conditions to examine the possibility that experience-dependent stimulation alleviates their behavioral and neuronal abnormalities. FMR1-KO mice kept in standard cages were hyperactive, displayed an altered pattern of open field exploration, and did not show habituation. Quantitative morphological analyses revealed a reduction in basal dendrite length and branching together with more immature-appearing spines along apical dendrites of layer five pyramidal neurons in the visual cortex. Enrichment largely rescued these behavioral and neuronal abnormalities while increasing [alpha]-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit 1 (GluR1) levels in both genotypes. Enrichment did not, however, affect FMRP levels in the WT mice. These data suggest that FMRP-independent pathways activating glutamatergic signaling are preserved in FMR1-KO mice and that they can be elicited by environmental stimulation. fragile X mental retardation protein | mental retardation | FMR1 gene | AMPA receptor | dendritic spines

Cytosine methylation is an epigenetic mechanism in eukaryotes that is often associated with stable transcriptional silencing, such as in X-chromosome inactivation and genomic imprinting. Aberrant methylation patterns occur in several inherited human diseases and in many cancers. To understand how methylated and unmethylated states of cytosine residues are transmitted during DNA replication, we develop a population-epigenetic model of DNA methylation dynamics. The model is informed by our observation that de novo methylation can occur on the daughter strand while leaving the opposing cytosine unmethylated, as revealed by the patterns of methylation on the two complementary strands of individual DNA molecules. Under our model, we can infer site-specific rates of both maintenance and de novo methylation, values that determine the fidelity of methylation inheritance, from double-stranded methylation data. This approach can be used for populations of cells obtained from individuals without the need for cell culture. We use our method to infer cytosine methylation rates at several sites within the promoter of the human gene FMR1. bisulfite genomic sequencing | epigenetic fidelity | fragile X syndrome | mathematical modeling | population epigenetics

Fragile X syndrome results from the transcriptional silencing of a gene, Fmr1, that codes for an mRNA-binding protein (fragile X mental retardation protein, FMRP) present in neuronal dendrites. FMRP can act as a translational suppressor, and its own translation in dendrites is regulated by group I metabotropic glutamate receptors (mGluRs). Multiple lines of evidence suggest that mGluR-induced translation is exaggerated in Fragile X syndrome because of a lack of translational inhibition normally provided by FMRP. We characterized the role of FMRP in the regulation of mRNA granules, which sediment as a heavy peak after polysomes on sucrose gradients. In WT mouse brain, FMRP distributed with polysomes and granules. EM and biochemical analyses suggested that the granule fraction itself contained clusters of polysomes. In Fmr1 knockout brain, we observed a significant decrease in the amount of mRNA granules relative to WT mice. This difference appeared to be due to a role of FMRP in regulating the activation of granules during mGluR-induced translation; in vivo administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine increased granule content in Fmr1 knockout mouse brain to levels comparable with those seen in WT brain. In accord with a role of mGluR5 in the regulation of ongoing translation in vivo, we observed that the phosphorylation of several initiation factors in response to application of the mGluR1/5 agonist S-3,5-dihydroxyphenylglycine in vitro was blocked by methyl-6-(phenylethynyl)pyridine. Together, these data suggest that although large, polysome-containing granules can form in the absence of FMRP, their use in response to mGluR-induced translation is exaggerated. fragile X syndrome | translation | 2-methyl-6-(phenylethynyl)pyridine

Fragile X syndrome is caused by the absence of the fragile X mental retardation protein (FMRP). This RNA-binding protein is widely expressed in human and mouse tissues, and it is particularly abundant in the brain because of its high expression in neurons, where it localizes in the cell body and in granules throughout dendrites. Although FMRP is thought to regulate trafficking of repressed mRNA complexes and to influence local protein synthesis in synapses, it is not known whether it has additional functions in the control of translation in the cell body. Here, we have used recently developed approaches to investigate whether FMRP is associated with the translation apparatus. We demonstrate that, in the brain, FMRP is present in actively translating polyribosomes, and we show that this association is acutely sensitive to the type of detergent required to release polyribosomes from membranous structures. In addition, proteomic analyses of purified brain polyribosomes reveal the presence of several RNA-binding proteins that, similarly to FMRP, have been previously localized in neuronal granules. Our findings highlight the complex roles of FMRP both in actively translating polyribosomes and in repressed trafficking ribonucleoparticle granules.

Fragile X syndrome (fraX) is the most common known cause of inherited developmental disability, fraX is associated with a CGG expansion in the FMR1 gene on the long arm of the X chromosome. Behavioral deficits, including problems with impulse control and distractibility, are common in fraX. We used functional brain imaging with a Go/NoGo task to examine the neural substrates of response inhibition in females with fraX (ages 10-22) and age- and gender-matched typically developing subjects. Although subjects with fraX had significantly lower IQ scores, as a group their performance on the Go/NoGo task was equivalent to that of the typically developing group. However, females with fraX showed abnormal activation patterns in several cortical and subcortical regions, with significantly reduced activation in the supplementary motor area, anterior cingulate and midcingulate cortex, basal ganglia, and hippocampus. An important finding of our study is that neural responses in the right ventrolateral prefrontal cortex (PFC) and the left and right striatum were correlated with the level of FMR1 gene expression. Our findings support the hypothesis that frontostriatal regions typically associated with response inhibition are dysfunctional in females with fraX. In addition to task-related activation deficits, reduced levels of 'deactivation' were observed in the ventromedial PFC, and, furthermore, these reductions were correlated with the level of FMR1 gene expression. The ventromedial PFC is a key node in a 'default mode' network that monitors mental and physiological states; we suggest that self-monitoring processes may be aberrant in fraX. genetic | Go/NoGo | response inhibition | prefrontal cortex | basal ganglia

In humans, failure to express the fragile X mental retardation protein (FMRP) gives rise to fragile X syndrome, the most common form of inherited mental retardation. A fragile X knockout (fmr1 KO) mouse has been described that has some of the characteristics of patients with fragile X syndrome, including immature dendritic spines and subtle behavioral deficits. In our behavioral studies, fmr1 KO mice exhibited hyperactivity and a higher rate of entrance into the center of an open field compared with controls, suggesting decreased levels of anxiety. Our finding of impaired performance of fmr1 KO mice on a passive avoidance task is suggestive of a deficit in learning and memory. In an effort to understand what brain regions are involved in the behavioral abnormalities, we applied the [[sup.14]C]deoxyglucose method for the determination of cerebral metabolic rates for glucose (CM[R.sub.glc]). We measured CM[R.sub.glc] in 38 regions in adult male fmr1 KO and WT littermates. We found CM[R.sub.glc] was higher in all 38 regions in fmr1 KO mice, and in 26 of the regions, differences were statistically significant. Differences in CM[R.sub.glc] ranged from 12% to 46%, and the greatest differences occurred in regions of the limbic system and primary sensory and posterior parietal cortical areas. Regions most affected are consistent with behavioral deficiencies and regions in which FMRP expression is highest. Higher CM[R.sub.glc] in fragile X mice may be a function of abnormalities found in dendritic spines.

This review discusses genetics and molecular aspects of the fragile X syndrome, which represents one of the common forms of inherited mental retardation.

Byline: E. Yu. Siyanova (1), S. M. Mirkin (1) Keywords: trinucleotide repeat; expansion; replication; recombination; gene expression; genome instability; genome polymorphism; non-Mendelian inheritance; genetic anticipation Abstract: This review describes a novel type of genome instability, expansion of trinucleotide repeats. Originally discovered in 1991 upon cloning the gene responsible for the fragile X syndrome, it has proved to be a general phenomenon responsible for a growing number of human neurological disorders. Besides apparent medical importance, the discovery of trinucleotide repeat expansion unraveled a fundamental problem of human genetics: a non-Mendelian type of inheritance called anticipation. Understanding the mechanisms of repeat expansion and the molecular pathways leading from these expansions to human diseases became a formidable task for modern biology and one of its spectacular achievements. Here we discuss the major breakthroughs in this field made during the last decade, with an emphasis on molecular models of repeat expansion. Author Affiliation: (1) Department of Molecular Genetics, University of Illinois at Chicago, Chicago, IL, 60607, USA Article History: Registration Date: 08/10/2004

Primary X-linked metal retardation (MRX) is a heterogeneous condition with affected patients having no distinctive clinical features in common except cognitive impairment. Many genetic defects linked to MRX can involve ten different genes. A new gene on the long arm of the X-chromosome has been characterized and identified in unrelated individuals of mutations predicted to cause loss of function. It is highly expressed in fetal brain, encoding a ologophrenin-1 protein containing a Rho-GTPase-activating protein domain. This demonstrates a link between cognitive impairment and defective signalling pathways that rely on Ras-like GTPase.

The genetic basis of inherited diseases is explained by other processes besides traditional Mendelian heredity. Recent research shows that mitochondrial DNA inheritance, genetic mosaicism, imprinting, and uniparental disomy, are also involved in disease transmission from parents to offspring. The gap or break in an X chromosome called fragile x causes inherited mental retardation.

The gap or break in the X chromosome is called the fragile X syndrome. This gap at location xq27 causes inherited mental retardation in about 4% of all cases. Scientists have discovered the fragile X mental retardation gene, expressed in the brain, the code for which is practically in the same region as the fragile X location. The current explanation is that imprinting may cause inactivation of this gene leading to mental retardation.