The role of fragile X mental retardation protein in major mental disorders

Why is vision impaired in fragile X premutation carriers? The role of fragile X mental retardation protein and potential FMR1 mRNA toxicity

Dysfunctions of the geniculo-striatal magnocellular (M) visual pathway and its cortical recipients have been documented in fragile X syndrome and in FMR1 premutation carriers. However, the mechanism of this impairment is less clear. To elucidate this issue, we completed the measurement of visual functions at different stages of information processing: low-level mechanisms (contrast sensitivity biasing information processing toward the M and parvocellular [P] pathways), primary visual cortex (motion-defined and static Vernier threshold), and higher-level form and motion processing (coherence thresholds). Results revealed that FMR1 premutation carriers, relative to non-carrier controls, exhibited lower contrast sensitivity for M pathway-biased stimuli, higher Vernier threshold for motion-defined stimuli, and higher global motion coherence threshold. Although both elevated FMR1 mRNA and reduced fragile X mental retardation protein (FMRP) levels were associated with impaired visual functions, regression analysis indicated that FMRP was the primary factor. In premutation carriers, a toxic gain-of-function of elevated FMR1 mRNA has been suggested, whereas reduced FMRP is linked to neurodevelopmental aspects. Here, we showed that FMRP may the primary factor associated with visual dysfunctions.

Imaging-genetics in autism spectrum disorder: advances, translational impact, and future directions

Autism Spectrum Disorder (ASD) refers to a group of heterogeneous neurodevelopmental disorders that are unified by impairments in reciprocal social communication and a pattern of inflexible behaviors. Recent genetic advances have resolved some of the complexity of the genetic architecture underlying ASD by identifying several genetic variants that contribute to the disorder. Different etiological pathways associated with ASD may converge through effects on common molecular mechanisms, such as synaptogenesis, neuronal motility, and axonal guidance. Recently, with more sophisticated techniques, neuroimaging, and neuropathological studies have provided some consistency of evidence that altered structure, activity, and connectivity within complex neural networks is present in ASD, compared to typically developing children. The imaging-genetics approach promises to help bridge the gap between genetic variation, resultant biological effects on the brain, and production of complex neuropsychiatric symptoms. Here, we review recent findings from the developing field of imaging-genetics applied to ASD. Studies to date have indicated that relevant risk genes are associated with alterations in circuits that mediate socio-emotional, visuo-spatial, and language processing. Longitudinal studies ideally focused on early development, in conjunction with investigation for gene-gene, and gene-environment interactions may move the promise of imaging-genetics in ASD closer to the clinical domain.

Fragile X syndrome and targeted treatment trials

Work in recent years has revealed an abundance of possible new treatment targets for fragile X syndrome (FXS). The use of animal models, including the fragile X knockout mouse which manifests a phenotype very similar to FXS in humans, has resulted in great strides in this direction of research. The lack of Fragile X Mental Retardation Protein (FMRP) in FXS causes dysregulation and usually overexpression of a number of its target genes, which can cause imbalances of neurotransmission and deficits in synaptic plasticity. The use of metabotropic glutamate receptor (mGluR) blockers and gamma amino-butyric acid (GABA) agonists have been shown to be efficacious in reversing cellular and behavioral phenotypes, and restoring proper brain connectivity in the mouse and fly models. Proposed new pharmacological treatments and educational interventions are discussed in this chapter. In combination, these various targeted treatments show promising preliminary results in mitigating or even reversing the neurobiological abnormalities caused by loss of FMRP, with possible translational applications to other neurodevelopmental disorders including autism.

Increased prevalence of seizures in boys who were probands with the FMR1 premutation and co-morbid autism spectrum disorder

Seizures are a common co-occurring condition in those with fragile X syndrome (FXS), and in those with idiopathic autism spectrum disorder (ASD). Seizures are also associated with ASD in those with FXS. However, little is known about the rate of seizures and how commonly these problems co-occur with ASD in boys with the FMR1 premutation. We, therefore, determined the prevalence of seizures and ASD in boys with the FMR1 premutation compared with their sibling counterparts and population prevalence estimates. Fifty premutation boys who presented as clinical probands (N = 25), or non-probands (identified by cascade testing after the proband was found) (N = 25), and 32 non-carrier controls were enrolled. History of seizures was documented and ASD was diagnosed by standardized measures followed by a team consensus of ASD diagnosis. Seizures (28%) and ASD (68%) were more prevalent in probands compared with non-probands (0 and 28%), controls (0 and 0%), and population estimates (1 and 1.7%). Seizures occurred more frequently in those with the premutation and co-morbid ASD particularly in probands compared with those with the premutation alone (25 vs. 3.85%, p = 0.045). Although cognitive and adaptive functioning in non-probands were similar to controls, non-probands were more likely to meet the diagnosis of ASD than controls (28 vs. 0%, p < 0.0001). In conclusion, seizures were relatively more common in premutation carriers who presented clinically as probands of the family and seizures were commonly associated with ASD in these boys. Therefore, boys with the premutation, particularly if they are probands should be assessed carefully for both ASD and seizures.

A novel assay for evaluating fragile X locus repeats

We have developed a novel fragile X locus repeat assay that is a simple and high-throughput method that, with clinical validation, may be suitable for screening. It uses amplification of the FMR1 trinucleotide repeat region, followed by a hybridization assay to quantify the number of repeats in the amplicons. To our knowledge, this is the first repeat-counting assay that uses fluorescent signals rather than electrophoresis or mass spectrometry as the signaling mechanism. We also report the development of a simple microfluidic electrophoresis reflex test that uses the same amplicons and reduces the need for Southern blots to differentiate homozygous female normal samples from full mutations. The new assay, which is based on a suspension-array hybridization method, was tested on a series of male and female reference samples spanning the range from normal to full mutations. It was also tested on DNA from 1008 dried blood spot samples from pregnant women in their first trimester. The hybridization assay identified 51 of those as potentially expanded alleles of ≥45 repeats or as intermediate or higher in FMR1 repeat classification. Of these screen-positive samples, eight were confirmed by microfluidic electrophoresis as premutations consisting of ≥55 repeats. The FMR1 repeat assay is straightforward to run in high throughput, and the results are in the form of numerical ratios for ease of initial interpretation.

Above genetics: lessons from cerebral development in autism

While a distinct minicolumnar phenotype seems to be an underlying factor in a significant portion of cases of autism, great attention is being paid not only to genetics but to epigenetic factors which may lead to development of the conditions. Here we discuss the indivisible role the molecular environment plays in cellular function, particularly the pivotal position which the transcription factor and adhesion molecule, β-catenin, occupies in cellular growth. In addition, the learning environment is not only integral to postnatal plasticity, but the prenatal environment plays a vital role during corticogenesis, neuritogenesis, and synaptogenesis as well. To illustrate these points in the case of autism, we review important findings in genetics studies (e.g., PTEN, TSC1/2, FMRP, MeCP2, Neurexin-Neuroligin) and known epigenetic factors (e.g., valproic acid, estrogen, immune system, ultrasound) which may predispose towards the minicolumnar and connectivity patterns seen in the conditions, showing how one-gene mutational syndromes and exposure to certain CNS teratogens may ultimately lead to comparable phenotypes. This in turn may shed greater light on how environment and complex genetics combinatorially give rise to a heterogenetic group of conditions such as autism.

Dysregulation of fragile × mental retardation protein and metabotropic glutamate receptor 5 in superior frontal cortex of individuals with autism: a postmortem brain study

Background

Fragile X syndrome is caused by loss of function of the fragile X mental retardation 1 (FMR1) gene and shares multiple phenotypes with autism. We have previously found reduced expression of the protein product of FMR1 (FMRP) in vermis of adults with autism.

Methods

In the current study, we have investigated levels of FMRP in the superior frontal cortex of people with autism and matched controls using Western blot analysis. Because FMRP regulates the translation of multiple genes, we also measured protein levels for downstream molecules metabotropic glutamate receptor 5 (mGluR5) and γ-aminobutyric acid (GABA) A receptor β3 (GABRβ3), as well as glial fibrillary acidic protein (GFAP).

Results

We observed significantly reduced levels of protein for FMRP in adults with autism, significantly increased levels of protein for mGluR5 in children with autism and significantly increased levels of GFAP in adults and children with autism. We found no change in expression of GABRβ3. Our results for FMRP, mGluR5 and GFAP confirm our previous work in the cerebellar vermis of people with autism.

Conclusion

These changes may be responsible for cognitive deficits and seizure disorder in people with autism.

Neuropathologic features in the hippocampus and cerebellum of three older men with fragile X syndrome

Background

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability, and is the most common single-gene disorder known to be associated with autism. Despite recent advances in functional neuroimaging and our understanding of the molecular pathogenesis, only limited neuropathologic information on FXS is available.

Methods

Neuropathologic examinations were performed on post-mortem brain tissue from three older men (aged 57, 64 and 78 years) who had received a clinical or genetic diagnosis of FXS. In each case, physical and cognitive features were typical of FXS, and one man was also diagnosed with autism. Guided by reports of clinical and neuroimaging abnormalities of the limbic system and cerebellum of individuals with FXS, the current analysis focused on neuropathologic features present in the hippocampus and the cerebellar vermis.

Results

Histologic and immunologic staining revealed abnormalities in both the hippocampus and cerebellar vermis. Focal thickening of hippocampal CA1 and irregularities in the appearance of the dentate gyrus were identified. All lobules of the cerebellar vermis and the lateral cortex of the posterior lobe of the cerebellum had decreased numbers of Purkinje cells, which were occasionally misplaced, and often lacked proper orientation. There were mild, albeit excessive, undulations of the internal granular cell layer, with patchy foliar white matter axonal and astrocytic abnormalities. Quantitative analysis documented panfoliar atrophy of both the anterior and posterior lobes of the vermis, with preferential atrophy of the posterior lobule (VI to VII) compared with age-matched normal controls.

Conclusions

Significant morphologic changes in the hippocampus and cerebellum in three adult men with FXS were identified. This pattern of pathologic features supports the idea that primary defects in neuronal migration, neurogenesis and aging may underlie the neuropathology reported in FXS.

Maternal genetic mutations as gestational and early life influences in producing psychiatric disease-like phenotypes in mice

Risk factors for psychiatric disorders have traditionally been classified as genetic or environmental. Risk (candidate) genes, although typically possessing small effects, represent a clear starting point to elucidate downstream cellular/molecular pathways of disease. Environmental effects, especially during development, can also lead to altered behavior and increased risk for disease. An important environmental factor is the mother, demonstrated by the negative effects elicited by maternal gestational stress and altered maternal care. These maternal effects can also have a genetic basis (e.g., maternal genetic variability and mutations). The focus of this review is "maternal genotype effects" that influence the emotional development of the offspring resulting in life-long psychiatric disease-like phenotypes. We have recently found that genetic inactivation of the serotonin 1A receptor (5-HT1AR) and the fmr1 gene (encoding the fragile X mental retardation protein) in mouse dams results in psychiatric disease-like phenotypes in their genetically unaffected offspring. 5-HT1AR deficiency in dams results in anxiety and increased stress responsiveness in their offspring. Offspring of 5-HT1AR deficient dams display altered development of the hippocampus, which could be linked to their anxiety-like phenotype. Maternal inactivation of fmr1, like its inactivation in the offspring, results in a hyperactivity-like condition and is associated with receptor alterations in the striatum. These data indicate a high sensitivity of the offspring to maternal mutations and suggest that maternal genotype effects can increase the impact of genetic risk factors in a population by increasing the risk of the genetically normal offspring as well as by enhancing the effects of offspring mutations.