Advances in clinical and molecular understanding of the FMR1 premutation and fragile X-associated tremor/ataxia syndrome

Neuropsychiatry of fragile X-premutation carriers with and without fragile X-associated tremor-ataxia syndrome: implications for neuropsychology

OBJECTIVES

Clinical neuropsychologists benefit from clinical currency in recently ascertained neuropsychiatric illness, such as fragile X premutation (FXPM) disorders. The author reviewed the clinical literature through 2016 for neuropsychiatric phenotypes in FXPM disorders, including patients with fragile X-associated tremor/ataxia syndrome (FXTAS).

METHODS

A PubMed search using the search terms 'Fragile X,' 'Premutation,' 'Carriers,' 'Psychiatric,' 'Dementia,' 'Mood,' and 'Anxiety' for citations in the clinical literature through 2016 was reviewed for studies specifically examining the neuropsychiatric phenotype in FXPM patients. The relevant articles were classified according to specific neuropsychiatric syndromes, including child onset, adult onset with and without FXTAS, as well as common systemic comorbidities in FXPM patients.

RESULTS

Eighty-six articles were reviewed for the neuropsychiatric and other phenotypes in FXPM patients. The neuropsychiatric phenotype in FXPM patients is distinct from that of full mutation (Fragile X Syndrome) patients. FXTAS is associated with a specific cortical-subcortical major or mild neurocognitive disorder (NCD).

CONCLUSIONS

FXPM patients are at risk for neuropsychiatric illness. In addition, FXPM patients are at risk for other systemic conditions that should raise suspicion for FXPM-associated illnesses. Clinicians should consider a diagnosis of FXPM-associated neuropsychiatric illness when patients with specific clinical scenarios are encountered; especially in patient pedigrees consistent with a typical (often multigenerational) presentation of fragile X-associated conditions, confirmatory genetic testing should be considered. Clinical management should take into account the psychological challenges of a multigenerational genetic neuropsychiatric illness with a variable CNS and systemic clinical phenotype.

Altered redox mitochondrial biology in the neurodegenerative disorder fragile X-tremor/ataxia syndrome: use of antioxidants in precision medicine

A 55-200 expansion of the CGG nucleotide repeat in the 5'-UTR of the fragile X mental retardation 1 gene (FMR1) is the hallmark of the triplet nucleotide disease known as the "premutation" as opposed to those with >200 repeats, known as the full mutation or fragile X syndrome. Originally, premutation carriers were thought to be free of phenotypic traits; however, some are diagnosed with emotional and neurocognitive issues and, later in life, with the neurodegenerative disease fragile X-associated tremor/ataxia syndrome (FXTAS). Considering that mitochondrial dysfunction has been observed in fibroblasts and post-mortem brain samples from carriers of the premutation, we hypothesized that mitochondrial dysfunction-derived ROS may result in cumulative oxidative-nitrative damage. Fibroblasts from premutation carriers (n=31, all FXTAS-free except 8), compared to age- and sex-matched controls (n=25), showed increased mitochondrial ROS production, impaired Complex I activity, lower expression of MIA40 (rate-limiting step of the redox-regulated mitochondrial-disulfide-relay-system), increased mtDNA deletions, and increased biomarkers of lipid and protein oxidative-nitrative damage. Most of the outcomes were more pronounced in FXTAS-affected individuals. Significant recovery of mitochondrial mass and/or function was obtained with superoxide or hydroxyl radicals' scavengers, a glutathione peroxidase analog, or by overexpressing MIA40. The effects of ethanol (a hydroxyl radical scavenger) were deleterious, while others (by N-acetyl-cysteine, quercetin and epigallocatechin-3-gallate) were outcome- and/or carrier-specifics. The use of antioxidants in the context of precision medicine is discussed with the goal of improving mitochondrial function in carriers with the potential of decreasing the morbidity and/or delaying FXTAS onset.

What has been learned from mouse models of the Fragile X Premutation and Fragile X-associated tremor/ataxia syndrome?

OBJECTIVE

To describe in this review how research using mouse models developed to study the Fragile X premutation (PM) and Fragile X-associated tremor/ataxia syndrome (FXTAS) have contributed to understanding these disorders. PM carriers bear an expanded CGG trinucleotide repeat on the Fragile X Mental Retardation 1 (FMR1) gene, and are at risk for developing the late onset neurodegenerative disorder FXTAS.

CONCLUSIONS

Much has been learned about these genetic disorders from the development and study of mouse models. This includes new insights into the early cellular and molecular events that occur in PM carriers and in FXTAS, the presence of multiorgan pathology beyond the CNS, immunological dysregulation, unexpected synthesis of a potentially toxic peptide in FXTAS (i.e., FMRpolyG), and evidence that the disease process may be halted or reversed by appropriate molecular therapies given early in the course of disease.

FMR1 gene mutations in patients with fragile X syndrome and obligate carriers: 30 years of experience in Chile

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability (ID) and co-morbid autism. It is caused by an amplification of the CGG repeat (>200), which is known as the full mutation, within the 5'UTR of the FMR1 gene. Expansions between 55-200 CGG repeats are termed premutation and are associated with a greater risk for fragile X-associated tremor/ataxia syndrome and fragile X-associated premature ovarian insufficiency. Intermediate alleles, also called the grey zone, include approximately 45-54 repeats and are considered borderline. Individuals with less than 45 repeats have a normal FMR1 gene. We report the occurrence of CGG expansions of the FMR1 gene in Chile among patients with ID and families with a known history of FXS. Here, we present a retrospective review conducted on 2321 cases (2202 probands and 119 relatives) referred for FXS diagnosis and cascade screening at the Institute of Nutrition and Food Technology (INTA), University of Chile. Samples were analysed using traditional cytogenetic methods and/or PCR. Southern blot was used to confirm the diagnosis. Overall frequency of FMR1 expansions observed among probands was 194 (8·8%), the average age of diagnosis was 8·8 ± 5·4 years. Of 119 family members studied, 72 (60%) were diagnosed with a CGG expansion. Our results indicated that the prevalence of CGG expansions of the FMR1 gene among probands is relatively higher than other populations. The average age of diagnosis is also higher than reference values. PCR and Southern blot represent a reliable molecular technique in the diagnosis of FXS.

Fragile X-associated tremor/ataxia syndrome - features, mechanisms and management

Many physicians are unaware of the many phenotypes associated with the fragile X premutation, an expansion in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene that consists of 55-200 CGG repeats. The most severe of these phenotypes is fragile X-associated tremor/ataxia syndrome (FXTAS), which occurs in the majority of ageing male premutation carriers but in fewer than 20% of ageing women with the premutation. The prevalence of the premutation is 1 in 150-300 females, and 1 in 400-850 males, so physicians are likely to see people affected by FXTAS. Fragile X DNA testing is broadly available in the Western world. The clinical phenotype of FXTAS at presentation can vary and includes intention tremor, cerebellar ataxia, neuropathic pain, memory and/or executive function deficits, parkinsonian features, and psychological disorders, such as depression, anxiety and/or apathy. FXTAS causes brain atrophy and white matter disease, usually in the middle cerebellar peduncles, the periventricular area, and the splenium and/or genu of the corpus callosum. Here, we review the complexities involved in the clinical management of FXTAS and consider how targeted treatment for these clinical features of FXTAS will result from advances in our understanding of the molecular mechanisms that underlie this neurodegenerative disorder. Such targeted approaches should also be more broadly applicable to earlier forms of clinical involvement among premutation carriers.

Warburg effect linked to cognitive-executive deficits in FMR1 premutation

A 55-200 CGG repeat expansion in the 5'-UTR of the fragile X mental retardation 1 (FMR1) gene is known as a premutation. Some carriers are affected by the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS), primary ovarian insufficiency, and neurobehavioral impairments. Based on the mitochondrial dysfunction observed in fibroblasts and brain samples from carriers, as well as in neurons and brains from a mouse model of the premutation, we evaluated the presence of the Warburg effect in peripheral blood mononuclear cells (PBMCs) from 30 premutation carriers with either a rebalance of the metabolism [increasing glycolysis while decreasing oxidative phosphorylation (oxphos)] or a metabolic amplification (increasing glycolysis while maintaining/increasing oxphos). Deficits in oxphos-more pronounced in FXTAS-affected subjects-were accompanied by a shift toward glycolysis, suggesting increased glycolysis despite aerobic conditions. Differential proteomics extended these findings, unveiling a decreased antioxidant response, translation, and disrupted extracellular matrix and cytoskeleton organization with activation of prosenescence pathways. Lower bioenergetics segregated with increased incidence of low executive function, tremors, below-average IQ, and FXTAS. The combination of functional and proteomic data unveiled new mechanisms related to energy production in the premutation, showing the potential of being applicable to other psychiatric disorders to identify endophenotype-specific responses relevant to neurobiology.-Napoli, E., Song, G., Schneider, A., Hagerman, R., Eldeeb, M. A. A. A., Azarang, A., Tassone, F., Giulivi, C. Warburg effect linked to cognitive-executive deficits in FMR1 premutation.

Augmented noncanonical BMP type II receptor signaling mediates the synaptic abnormality of fragile X syndrome

Epigenetic silencing of fragile X mental retardation 1 (FMR1) causes fragile X syndrome (FXS), a common inherited form of intellectual disability and autism. FXS correlates with abnormal synapse and dendritic spine development, but the molecular link between the absence of the FMR1 product FMRP, an RNA binding protein, and the neuropathology is unclear. We found that the messenger RNA encoding bone morphogenetic protein type II receptor (BMPR2) is a target of FMRP. Depletion of FMRP increased BMPR2 abundance, especially that of the full-length isoform that bound and activated LIM domain kinase 1 (LIMK1), a component of the noncanonical BMP signal transduction pathway that stimulates actin reorganization to promote neurite outgrowth and synapse formation. Heterozygosity for BMPR2 rescued the morphological abnormalities in neurons both in Drosophila and in mouse models of FXS, as did the postnatal pharmacological inhibition of LIMK1 activity. Compared with postmortem prefrontal cortex tissue from healthy subjects, the amount of full-length BMPR2 and of a marker of LIMK1 activity was increased in this brain region from FXS patients. These findings suggest that increased BMPR2 signal transduction is linked to FXS and that the BMPR2-LIMK1 pathway is a putative therapeutic target in patients with FXS and possibly other forms of autism.

Robust Machine Learning-Based Correction on Automatic Segmentation of the Cerebellum and Brainstem

Automated segmentation is a useful method for studying large brain structures such as the cerebellum and brainstem. However, automated segmentation may lead to inaccuracy and/or undesirable boundary. The goal of the present study was to investigate whether SegAdapter, a machine learning-based method, is useful for automatically correcting large segmentation errors and disagreement in anatomical definition. We further assessed the robustness of the method in handling size of training set, differences in head coil usage, and amount of brain atrophy. High resolution T1-weighted images were acquired from 30 healthy controls scanned with either an 8-channel or 32-channel head coil. Ten patients, who suffered from brain atrophy because of fragile X-associated tremor/ataxia syndrome, were scanned using the 32-channel head coil. The initial segmentations of the cerebellum and brainstem were generated automatically using Freesurfer. Subsequently, Freesurfer's segmentations were both manually corrected to serve as the gold standard and automatically corrected by SegAdapter. Using only 5 scans in the training set, spatial overlap with manual segmentation in Dice coefficient improved significantly from 0.956 (for Freesurfer segmentation) to 0.978 (for SegAdapter-corrected segmentation) for the cerebellum and from 0.821 to 0.954 for the brainstem. Reducing the training set size to 2 scans only decreased the Dice coefficient ≤0.002 for the cerebellum and ≤ 0.005 for the brainstem compared to the use of training set size of 5 scans in corrective learning. The method was also robust in handling differences between the training set and the test set in head coil usage and the amount of brain atrophy, which reduced spatial overlap only by <0.01. These results suggest that the combination of automated segmentation and corrective learning provides a valuable method for accurate and efficient segmentation of the cerebellum and brainstem, particularly in large-scale neuroimaging studies, and potentially for segmenting other neural regions as well.

Fragile X mental retardation protein controls ion channel expression and activity

Fragile X-associated disorders are a family of genetic conditions resulting from the partial or complete loss of fragile X mental retardation protein (FMRP). Among these disorders is fragile X syndrome, the most common cause of inherited intellectual disability and autism. FMRP is an RNA-binding protein involved in the control of local translation, which has pleiotropic effects, in particular on synaptic function. Analysis of the brain FMRP transcriptome has revealed hundreds of potential mRNA targets encoding postsynaptic and presynaptic proteins, including a number of ion channels. FMRP has been confirmed to bind voltage-gated potassium channels (K_v 3.1 and K_v 4.2) mRNAs and regulates their expression in somatodendritic compartments of neurons. Recent studies have uncovered a number of additional roles for FMRP besides RNA regulation. FMRP was shown to directly interact with, and modulate, a number of ion channel complexes. The sodium-activated potassium (Slack) channel was the first ion channel shown to directly interact with FMRP; this interaction alters the single-channel properties of the Slack channel. FMRP was also shown to interact with the auxiliary β4 subunit of the calcium-activated potassium (BK) channel; this interaction increases calcium-dependent activation of the BK channel. More recently, FMRP was shown to directly interact with the voltage-gated calcium channel, Ca_v 2.2, and reduce its trafficking to the plasma membrane. Studies performed on animal models of fragile X syndrome have revealed links between modifications of ion channel activity and changes in neuronal excitability, suggesting that these modifications could contribute to the phenotypes observed in patients with fragile X-associated disorders.

Premutation in the Fragile X Mental Retardation 1 (FMR1) Gene Affects Maternal Zn-milk and Perinatal Brain Bioenergetics and Scaffolding

Fragile X premutation alleles have 55–200 CGG repeats in the 5′ UTR of the FMR1 gene. Altered zinc (Zn) homeostasis has been reported in fibroblasts from >60 years old premutation carriers, in which Zn supplementation significantly restored Zn-dependent mitochondrial protein import/processing and function. Given that mitochondria play a critical role in synaptic transmission, brain function, and cognition, we tested FMRP protein expression, brain bioenergetics, and expression of the Zn-dependent synaptic scaffolding protein SH3 and multiple ankyrin repeat domains 3 (Shank3) in a knock-in (KI) premutation mouse model with 180 CGG repeats. Mitochondrial outcomes correlated with FMRP protein expression (but not FMR1 gene expression) in KI mice and human fibroblasts from carriers of the pre- and full-mutation. Significant deficits in brain bioenergetics, Zn levels, and Shank3 protein expression were observed in the Zn-rich regions KI hippocampus and cerebellum at PND21, with some of these effects lasting into adulthood (PND210). A strong genotype × age interaction was observed for most of the outcomes tested in hippocampus and cerebellum, whereas in cortex, age played a major role. Given that the most significant effects were observed at the end of the lactation period, we hypothesized that KI milk might have a role at compounding the deleterious effects on the FMR1 genetic background. A higher gene expression of ZnT4 and ZnT6, Zn transporters abundant in brain and lactating mammary glands, was observed in the latter tissue of KI dams. A cross-fostering experiment allowed improving cortex bioenergetics in KI pups nursing on WT milk. Conversely, WT pups nursing on KI milk showed deficits in hippocampus and cerebellum bioenergetics. A highly significant milk type × genotype interaction was observed for all three-brain regions, being cortex the most influenced. Finally, lower milk-Zn levels were recorded in milk from lactating women carrying the premutation as well as other Zn-related outcomes (Zn-dependent alkaline phosphatase activity and lactose biosynthesis—whose limiting step is the Zn-dependent β-1,4-galactosyltransferase). In premutation carriers, altered Zn homeostasis, brain bioenergetics and Shank3 levels could be compounded by Zn-deficient milk, increasing the risk of developing emotional and neurological/cognitive problems and/or FXTAS later in life.

Improving Health Education for Women Who Carry an FMR1 Premutation

Women who carry an FMR1 (i.e., fragile X) premutation have specific health risks over their lifetime. However, little is known about their experience understanding these risks and navigating their health needs. The aim of this study was to use qualitative analysis to uncover both barriers and facilitators to personal healthcare using a framework of the Health Belief Model. Five focus groups were conducted with a total of 20 women who carry the FMR1 premutation using a semi-structured discussion guide. All sessions were transcribed verbatim and independently coded by two researchers. The coders used a deductive - inductive approach to determine the prominent themes related to the participants' experiences seeking healthcare for premutation-related conditions. Salient barriers to personal healthcare included difficult clinical translation of research findings, lack of knowledge among healthcare providers and among the women themselves, different priorities, and shortage of premutation-specific support and targeted educational materials. Facilitators included family members, national and community support organizations, research studies, compassionate physicians, and other premutation carriers. Addressing barriers to personal healthcare through up-to-date educational materials can help diminish misperceptions regarding health risks. Targeted educational materials will aid in information sharing and awareness for women who carry the FMR1 premutation and their physicians.

Granulosa cell and oocyte mitochondrial abnormalities in a mouse model of fragile X primary ovarian insufficiency

STUDY HYPOTHESIS

We hypothesized that the mitochondria of granulosa cells (GC) and/or oocytes might be abnormal in a mouse model of fragile X premutation (FXPM).

STUDY FINDING

Mice heterozygous and homozygous for the FXPM have increased death (atresia) of large ovarian follicles, fewer corpora lutea with a gene dosage effect manifesting in decreased litter size(s). Furthermore, granulosa cells (GC) and oocytes of FXPM mice have decreased mitochondrial content, structurally abnormal mitochondria, and reduced expression of critical mitochondrial genes. Because this mouse allele produces the mutant Fragile X mental retardation 1 (Fmr1) transcript and reduced levels of wild-type (WT) Fmr1 protein (FMRP), but does not produce a Repeat Associated Non-ATG Translation (RAN)-translation product, our data lend support to the idea that Fmr1 mRNA with large numbers of CGG-repeats is intrinsically deleterious in the ovary.

WHAT IS KNOWN ALREADY

Mitochondrial dysfunction has been detected in somatic cells of human and mouse FX PM carriers and mitochondria are essential for oogenesis and ovarian follicle development, FX-associated primary ovarian insufficiency (FXPOI) is seen in women with FXPM alleles. These alleles have 55-200 CGG repeats in the 5' UTR of an X-linked gene known as FMR1. The molecular basis of the pathology seen in this disorder is unclear but is thought to involve either some deleterious consequence of overexpression of RNA with long CGG-repeat tracts or of the generation of a repeat-associated non-AUG translation (RAN translation) product that is toxic.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Analysis of ovarian function in a knock-in FXPM mouse model carrying 130 CGG repeats was performed as follows on WT, PM/+, and PM/PM genotypes. Histomorphometric assessment of follicle and corpora lutea numbers in ovaries from 8-month-old mice was executed, along with litter size analysis. Mitochondrial DNA copy number was quantified in oocytes and GC using quantitative PCR, and cumulus granulosa mitochondrial content was measured by flow cytometric analysis after staining of cells with Mitotracker dye. Transmission electron micrographs were prepared of GC within small growing follicles and mitochondrial architecture was compared. Quantitative RT-PCR analysis of key genes involved in mitochondrial structure and recycling was performed.

MAIN RESULTS AND THE ROLE OF CHANCE

A defect was found in follicle survival at the large antral stage in PM/+ and PM/PM mice. Litter size was significantly decreased in PM/PM mice, and corpora lutea were significantly reduced in mice of both mutant genotypes. Mitochondrial DNA copy number was significantly decreased in GC and metaphase II eggs in mutants. Flow cytometric analysis revealed that PM/+ and PM/PM animals lack the cumulus GC that harbor the greatest mitochondrial content as found in wild-type animals. Electron microscopic evaluation of GC of small growing follicles revealed mitochondrial structural abnormalities, including disorganized and vacuolar cristae. Finally, aberrant mitochondrial gene expression was detected. Mitofusin 2 (Mfn2) and Optic atrophy 1 (Opa1), genes involved in mitochondrial fusion and structure, respectively, were significantly decreased in whole ovaries of both mutant genotypes. Mitochondrial fission factor 1 (Mff1) was significantly decreased in PM/+ and PM/PM GC and eggs compared with wild-type controls.

LIMITATIONS, REASONS FOR CAUTION

Data from the mouse model used for these studies should be viewed with some caution when considering parallels to the human FXPOI condition.

WIDER IMPLICATIONS OF THE FINDINGS

Our data lend support to the idea that Fmr1 mRNA with large numbers of CGG-repeats is intrinsically deleterious in the ovary. FXPM disease states, including FXPOI, may share mitochondrial dysfunction as a common underlying mechanism.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

Studies were supported by NIH R21 071873 (J.J./G.H), The Albert McKern Fund for Perinatal Research (J.J.), NIH Intramural Funds (K.U.), and a TUBITAK Research Fellowship Award (B.U.). No conflict(s) of interest or competing interest(s) are noted.

Memantine Improves Attentional Processes in Fragile X-Associated Tremor/Ataxia Syndrome: Electrophysiological Evidence from a Randomized Controlled Trial

Progressive cognitive deficits are common in patients with fragile X-associated tremor/ataxia syndrome (FXTAS), with no targeted treatment yet established. In this substudy of the first randomized controlled trial for FXTAS, we examined the effects of NMDA antagonist memantine on attention and working memory. Data were analyzed for patients (24 in each arm) who completed both the primary memantine trial and two EEG recordings (at baseline and follow-up) using an auditory “oddball” task. Results demonstrated significantly improved attention/working memory performance after one year only for the memantine group. The event-related potential P2 amplitude elicited by non-targets was significantly enhanced in the treated group, indicating memantine-associated improvement in attentional processes at the stimulus identification/discrimination level. P2 amplitude increase was positively correlated with improvement on the behavioral measure of attention/working memory during target detection. Analysis also revealed that memantine treatment normalized the P2 habituation effect at the follow-up visit. These findings indicate that memantine may benefit attentional processes that represent fundamental components of executive function/dysfunction, thought to comprise the core cognitive deficit in FXTAS. The results provide evidence of target engagement of memantine, as well as therapeutically relevant information that could further the development of specific cognitive or disease-modifying therapies for FXTAS.

X-inactivation in the clinical phenotype of fragile X premutation carrier sisters

Objective:

The purpose of this study is to describe a case series of 4 sisters with discordant clinical phenotypes associated with fragile X–associated tremor/ataxia syndrome (FXTAS) that may be explained by varying CGG repeat sizes and activation ratios (ARs) (the ratio of cells carrying the normal fragile X mental retardation 1 [FMR1] allele on the active X chromosome).

Methods:

Four sisters with premutation size FMR1 gene repeats underwent detailed clinical characterization. CGG repeat length was determined by PCR, and AR was determined using a newly developed commercial methylation PCR assay and was compared with the results from Southern blot with densitometric image analysis.

Results:

Sister 1 had the largest CGG expansion (82) and the lowest AR (12%), with the most severe clinical presentation. Sister 2 had a lower CGG expansion (70) and an AR of 10% but had a milder clinical presentation.Sister 3 had a similar CGG expansion (79) but a slightly higher AR of 15% and less neurologic involvement. Sister 4 had a similar CGG expansion size of 80 but had the largest AR (40%) and was the only sister not to be affected by FXTAS or have any neurologic signs on examination.

Conclusions:

These results suggest that premutation carrier women who have higher ARs may be less likely to show manifestations of FXTAS. If larger studies show similar patterns, AR data could potentially be beneficial to supplement CGG repeat size when counseling premutation carrier women in the clinic.

Matrix metalloproteinase-9 deletion rescues auditory evoked potential habituation deficit in a mouse model of Fragile X Syndrome

Sensory processing deficits are common in autism spectrum disorders, but the underlying mechanisms are unclear. Fragile X Syndrome (FXS) is a leading genetic cause of intellectual disability and autism. Electrophysiological responses in humans with FXS show reduced habituation with sound repetition and this deficit may underlie auditory hypersensitivity in FXS. Our previous study in Fmr1 knockout (KO) mice revealed an unusually long state of increased sound-driven excitability in auditory cortical neurons suggesting that cortical responses to repeated sounds may exhibit abnormal habituation as in humans with FXS. Here, we tested this prediction by comparing cortical event related potentials (ERP) recorded from wildtype (WT) and Fmr1 KO mice. We report a repetition-rate dependent reduction in habituation of N1 amplitude in Fmr1 KO mice and show that matrix metalloproteinase-9 (MMP-9), one of the known FMRP targets, contributes to the reduced ERP habituation. Our studies demonstrate a significant up-regulation of MMP-9 levels in the auditory cortex of adult Fmr1 KO mice, whereas a genetic deletion of Mmp-9 reverses ERP habituation deficits in Fmr1 KO mice. Although the N1 amplitude of Mmp-9/Fmr1 DKO recordings was larger than WT and KO recordings, the habituation of ERPs in Mmp-9/Fmr1 DKO mice is similar to WT mice implicating MMP-9 as a potential target for reversing sensory processing deficits in FXS. Together these data establish ERP habituation as a translation relevant, physiological pre-clinical marker of auditory processing deficits in FXS and suggest that abnormal MMP-9 regulation is a mechanism underlying auditory hypersensitivity in FXS.

SIGNIFICANCE

Fragile X Syndrome (FXS) is the leading known genetic cause of autism spectrum disorders. Individuals with FXS show symptoms of auditory hypersensitivity. These symptoms may arise due to sustained neural responses to repeated sounds, but the underlying mechanisms remain unclear. For the first time, this study shows deficits in habituation of neural responses to repeated sounds in the Fmr1 KO mice as seen in humans with FXS. We also report an abnormally high level of matrix metalloprotease-9 (MMP-9) in the auditory cortex of Fmr1 KO mice and that deletion of Mmp-9 from Fmr1 KO mice reverses habituation deficits. These data provide a translation relevant electrophysiological biomarker for sensory deficits in FXS and implicate MMP-9 as a target for drug discovery.

Molecular Pathological Classification of Neurodegenerative Diseases: Turning towards Precision Medicine

Neurodegenerative diseases (NDDs) are characterized by selective dysfunction and loss of neurons associated with pathologically altered proteins that deposit in the human brain but also in peripheral organs. These proteins and their biochemical modifications can be potentially targeted for therapy or used as biomarkers. Despite a plethora of modifications demonstrated for different neurodegeneration-related proteins, such as amyloid-β, prion protein, tau, α-synuclein, TAR DNA-binding protein 43 (TDP-43), or fused in sarcoma protein (FUS), molecular classification of NDDs relies on detailed morphological evaluation of protein deposits, their distribution in the brain, and their correlation to clinical symptoms together with specific genetic alterations. A further facet of the neuropathology-based classification is the fact that many protein deposits show a hierarchical involvement of brain regions. This has been shown for Alzheimer and Parkinson disease and some forms of tauopathies and TDP-43 proteinopathies. The present paper aims to summarize current molecular classification of NDDs, focusing on the most relevant biochemical and morphological aspects. Since the combination of proteinopathies is frequent, definition of novel clusters of patients with NDDs needs to be considered in the era of precision medicine. Optimally, neuropathological categorizing of NDDs should be translated into in vivo detectable biomarkers to support better prediction of prognosis and stratification of patients for therapy trials.

Identification of Males with Cryptic Fragile X Alleles by Methylation-Specific Quantitative Melt Analysis

BACKGROUND

FMR1 full mutations (FMs) (CGG expansion &gt;200) in males mosaic for a normal (&lt;45 CGG) or gray-zone (GZ) (45–54 CGG) allele can be missed with the standard 2-step fragile X syndrome (FXS) testing protocols, largely because the first-line PCR tests showing a normal or GZ allele are not reflexed to the second-line test that can detect FM.

METHODS

We used methylation-specific quantitative melt analysis (MS-QMA) to determine the prevalence of cryptic FM alleles in 2 independent cohorts of male patients (994 from Chile and 2392 from Australia) referred for FXS testing from 2006 to 2013. All MS-QMA–positive cases were retested with commercial triplet primed PCR, methylation-sensitive Southern blot, and a methylation-specific EpiTYPER-based test.

RESULTS

All 38 FMs detected with the standard 2-step protocol were detected with MS-QMA. However, MS-QMA identified methylation mosaicism in an additional 15% and 11% of patients in the Chilean and Australian cohorts, respectively, suggesting the presence of a cryptic FM. Of these additional patients, 57% were confirmed to carry cryptic expanded alleles in blood, buccal mucosa, or saliva samples. Further confirmation was provided by identifying premutation (CGG 55–199) alleles in mothers of probands with methylation-sensitive Southern blot. Neurocognitive assessments showed that low-level mosaicism for cryptic FM alleles was associated with cognitive impairment or autism.

CONCLUSIONS

A substantial number of mosaic FM males who have cognitive impairment or autism are not diagnosed with the currently recommended 2-step testing protocol and can be identified with MS-QMA as a first-line test.

Single-locus enrichment without amplification for sequencing and direct detection of epigenetic modifications

A gene-level targeted enrichment method for direct detection of epigenetic modifications is described. The approach is demonstrated on the CGG-repeat region of the FMR1 gene, for which large repeat expansions, hitherto refractory to sequencing, are known to cause fragile X syndrome. In addition to achieving a single-locus enrichment of nearly 700,000-fold, the elimination of all amplification steps removes PCR-induced bias in the repeat count and preserves the native epigenetic modifications of the DNA. In conjunction with the single-molecule real-time sequencing approach, this enrichment method enables direct readout of the methylation status and the CGG repeat number of the FMR1 allele(s) for a clonally derived cell line. The current method avoids potential biases introduced through chemical modification and/or amplification methods for indirect detection of CpG methylation events.

Phenobarbital use and neurological problems in FMR1 premutation carriers

Fragile X Syndrome (FXS) is a neurodevelopmental disorder caused by a CGG expansion in the FMR1 gene located at Xq27.3. Patients with the premutation in FMR1 present specific clinical problems associated with the number of CGG repeats (55-200 CGG repeats). Premutation carriers have elevated FMR1 mRNA expression levels, which have been associated with neurotoxicity potentially causing neurodevelopmental problems or neurological problems associated with aging. However, cognitive impairments or neurological problems may also be related to increased vulnerability of premutation carriers to neurotoxicants, including phenobarbital. Here we present a study of three sisters with the premutation who were exposed differentially to phenobarbital therapy throughout their lives, allowing us to compare the neurological effects of this drug in these patients.

Epigenetics and Triplet-Repeat Neurological Diseases

The term "junk DNA" has been reconsidered following the delineation of the functional significance of repetitive DNA regions. Typically associated with centromeres and telomeres, DNA repeats are found in nearly all organisms throughout their genomes. Repetitive regions are frequently heterochromatinized resulting in silencing of intrinsic and nearby genes. However, this is not a uniform rule, with several genes known to require such an environment to permit transcription. Repetitive regions frequently exist as dinucleotide, trinucleotide, and tetranucleotide repeats. The association between repetitive regions and disease was emphasized following the discovery of abnormal trinucleotide repeats underlying spinal and bulbar muscular atrophy (Kennedy's disease) and fragile X syndrome of mental retardation (FRAXA) in 1991. In this review, we provide a brief overview of epigenetic mechanisms and then focus on several diseases caused by DNA triplet-repeat expansions, which exhibit diverse epigenetic effects. It is clear that the emerging field of epigenetics is already generating novel potential therapeutic avenues for this group of largely incurable diseases.

Identification of a male with fragile X syndrome through newborn screening

A pilot newborn screening (NBS) study for fragile X syndrome was recently conducted at the University of California, Davis Medical Center. The screening study identified a case of a male with the full mutation completely methylated and no detectable expression of the fragile X mental retardation-1 (FMR1) gene. The patient was initially seen in clinic at the MIND Institute, for medical follow-up and a genetic counseling session at the chronological age of 3 months. Since then, he has been seen in clinic every six months for follow up, medical examination and developmental assessments. Longitudinally administered developmental testing of the infant has revealed persistent delays in development, consistent with fragile X syndrome. Cascade testing revealed that the patient's mother and two siblings also have the full mutation. The patient has been receiving speech and language therapy, combined with physical and occupational therapies on a weekly basis since the age of one year. He is currently being treated with 2.5 mg of sertraline, which has been demonstrated to be helpful for improving language in young children with the syndrome.

Advanced technologies for the molecular diagnosis of fragile X syndrome

Fragile X syndrome (FXS), a trinucleotide repeat disorder, is the most common heritable form of cognitive impairment. Since the discovery of the FMR1 gene in 1991, great strides have been made in the field of molecular diagnosis for FXS. Cytogenetic analysis, which was the method of diagnosis in the early 1990, was replaced by Southern blot and PCR analysis albeit with some limitations. In the past few years many PCR-based methodologies, able to amplify large full mutation expanded alleles, with or without methylation, have been proposed. Reviewed here are the advantages, disadvantages and limitations of the most recent developments in the field of FXS diagnosis.

Induced expression of expanded CGG RNA causes mitochondrial dysfunction in vivo

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder affecting carriers of premutation forms of the FMR1 gene, resulting in a progressive development of tremor, ataxia and neuropsychological problems. The disease is caused by an expanded CGG repeat in the FMR1 gene, leading to an RNA gain-of-function toxicity mechanism. In order to study the pathogenesis of FXTAS, new inducible transgenic mouse models have been developed that expresses either 11CGGs or 90CGGs at the RNA level under control of a Tet-On promoter. When bred to an hnRNP-rtTA driver line, doxycycline (dox) induced expression of the transgene could be found in almost all tissues. Dox exposure resulted in loss of weight and death within 5 d for the 90CGG RNA expressing mice. Immunohistochemical examination of tissues of these mice revealed steatosis and apoptosis in the liver. Decreased expression of GPX1 and increased expression of cytochrome C is found. These effects were not seen in mice expressing a normal sized 11CGG repeat. In conclusion, we were able to show in vivo that expression of an expanded CGG-repeat rather than overexpression of a normal CGG-repeat causes pathology. In addition, we have shown that expanded CGG RNA expression can cause mitochondrial dysfunction by regulating expression levels of several markers. Although FTXAS patients do not display liver abnormalities, our findings contribute to understanding of the molecular mechanisms underlying toxicity of CGG repeat RNA expression in an animal model. In addition, the dox inducible mouse lines offer new opportunities to study therapeutic interventions for FXTAS.

Anxiety disorders in fragile X premutation carriers: Preliminary characterization of probands and non-probands

A very high proportion of individuals with fragile X syndrome (FXS) (FMR1 full mutation, > 200 CGG repeats) experience clinically significant anxiety. Recent evidence suggests that adult fragile X premutation carriers (55-200 CGG repeats) also are at risk for anxiety disorders, and they demonstrate limbic system alterations mediated by FMRP and/or elevated FMR1 mRNA that may explain this heightened risk. However, less is known about psychiatric symptoms including anxiety among children and adolescents with the premutation. We completed structured DSM-IV based diagnostic interviews focused on current anxiety in 35 children, adolescents or young adults with the premutation (ages 5-23 years, M = 11.3 ± 4.3; 27 male; 20 probands and 15 non-probands) and 31 controls (ages 5-18 years, M = 9.9 ± 3.6; 22 males). Among premutation carriers, 70.6% met criteria for at least one anxiety disorder (most frequently generalized anxiety disorder, specific phobia, social phobia, or obsessive compulsive disorder), compared to 22.6% of controls and 9.8% of the general population in this age range. Premutation carriers with intellectual disability, male gender, and proband status were associated with the highest rates of anxiety disorders. However, non-probands did have higher rates of having any anxiety disorder (40.0%) compared to general population norms. Although the results implicate anxiety as a target of screening and intervention among youth with the premutation, larger studies of unselected samples from the population of premutation carriers are needed to confirm and specify the degree and extent of psychiatric disorders in this condition.

Maternal Consequences of the Detection of Fragile X Carriers in Newborn Screening

OBJECTIVES

The possibility of newborn screening for fragile X syndrome is complicated by the potential for identifying premutation carriers. Although knowing the child's carrier status has potential benefits, the possibility of late-onset disorders in carrier children and their parents raises concerns about whether such information would be distressing to parents and potentially more harmful than helpful. This study sought to answer this question by offering voluntary fragile X screening to new parents and returning results for both the full mutation and premutation FMR1 gene expansions. We tested the assumption that such information could lead to adverse mental health outcomes or decision regret. We also wanted to know if child age and spousal support were associated with the outcomes of interest.

METHODS

Eighteen mothers of screen-positive infants with the premutation and 15 comparison mothers completed a battery of assessments of maternal anxiety, postpartum depression, stress, family quality of life, decision regret, and spousal support. The study was longitudinal, with an average of 3 assessments per mother.

RESULTS

The premutation group was not statistically different from the comparison group on measures of anxiety, depression, stress, or quality of life. A subset of mothers experienced clinically significant anxiety and decision regret, but factors associated with these outcomes could not be identified. Greater spousal support was generally associated with more positive outcomes.

CONCLUSIONS

Although we did not find evidence of significant adverse events, disclosure of newborn carrier status remains an important consideration in newborn screening policy.

Psychosis and catatonia in fragile X: Case report and literature review

Fragile X mental retardation 1 (FMR1) premutation associated phenotypes have been explored extensively since the molecular mechanism emerged involving elevated FMR1 messenger ribonucleic acid (mRNA) levels. Lowered fragile X mental retardation protein (FMRP) can also occur which may have an additive effect to the high levels of mRNA leading to neurodevelopmental problems and psychopathology. This paper was aimed to review psychosis and catatonia in premutation carriers, express the role of elevated FMR1 mRNA and lowered FMRP in the phenotype of carriers and present a case of psychosis and catatonia in a carrier. This case also demonstrates additional genetic and environmental factors which may also affect the phenotype. We review the literature and report an exemplary case of a 25 year old male premutation carrier with elevated FMR1 mRNA, low FMRP, a cytochrome P450 family 2 subfamily D polypeptide 6 (CYP2D6)*2xN mutation and a perinatal insult. This patient developed an autism spectrum disorder, psychosis, catatonia with subsequent cognitive decline after electro-convulsive therapy (ECT) for his catatonia. He had a premutation of 72 CGG repeat in FMR1, FMR1 mRNA level that was over 2.4 times normal and FMRP level at 18% of normal, and additionally, a CYP2D6 allelic variant which leads to ultrarapid metabolism (UM) of medication. There is an overlapping pathophysiological mechanism of catatonia and fragile X-associated premutation phenotypes including autism and psychosis. This case demonstrates the shared phenotype and the overlap of the pathophysiological mechanisms that can influence the intervention. Multiple genetic and environmental hits can lead to more significant involvement in premutation carriers.

The Use of Induced Pluripotent Stem Cell Technology to Advance Autism Research and Treatment

Autism spectrum disorders (ASDs) are a heterogeneous group of neurodevelopmental disorders sharing a core set of symptoms, including impaired social interaction, language deficits, and repetitive behaviors. While ASDs are highly heritable and demonstrate a clear genetic component, the cellular and molecular mechanisms driving ASD etiology remain undefined. The unavailability of live patient-specific neurons has contributed to the difficulty in studying ASD pathophysiology. The recent advent of induced pluripotent stem cells (iPSCs) has provided the ability to generate patient-specific human neurons from somatic cells. The iPSC field has quickly grown, as researchers have demonstrated the utility of this technology to model several diseases, especially neurologic disorders. Here, we review the current literature around using iPSCs to model ASDs, and discuss the notable findings, and the promise and limitations of this technology. The recent report of a nonsyndromic ASD iPSC model and several previous ASD models demonstrating similar results points to the ability of iPSC to reveal potential novel biomarkers and therapeutics.

White matter disease and cognitive impairment in FMR1 premutation carriers

OBJECTIVE

This cross-sectional, observational study examined the role of white matter involvement in the cognitive impairment of individuals with the fragile X mental retardation 1 (FMR1) premutation.

METHODS

Eight asymptomatic premutation carriers, 5 participants with fragile X tremor/ataxia syndrome (FXTAS), and 7 noncarrier controls were studied. The mean age of the asymptomatic premutation carriers, participants with FXTAS, and noncarrier controls was 60, 71, and 67 years, respectively. Magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) were used to examine the middle cerebellar peduncles (MCP) and the genu and splenium of the corpus callosum in relation to executive function and processing speed. MRS measures were N-acetyl aspartate/creatine (NAA/Cr) and choline/creatine, and fractional anisotropy (FA) was used for DTI. Executive function was assessed with the Behavioral Dyscontrol Scale and the Controlled Oral Word Association Test (COWAT), and processing speed with the Symbol Digit Modalities Test.

RESULTS

Among all 13 FMR1 premutation carriers, significant correlations were found between N-acetyl aspartate/creatine and choline/creatine in the MCP and COWAT scores, and between FA in the genu and performance on the Behavioral Dyscontrol Scale, COWAT, and Symbol Digit Modalities Test; a correlation was also found between FA in the splenium and COWAT performance. In all regions studied, participants with FXTAS had the lowest mean FA.

CONCLUSION

Microstructural white matter disease as determined by MRS and DTI correlated with executive dysfunction and slowed processing speed in these FMR1 premutation carriers. Neuroimaging abnormalities in the genu and MCP suggest that disruption of white matter within frontocerebellar networks has an important role in the cognitive impairment associated with the FMR1 premutation.

White matter disease and cognitive impairment in FMR1 premutation carriers

OBJECTIVE

This cross-sectional, observational study examined the role of white matter involvement in the cognitive impairment of individuals with the fragile X mental retardation 1 (FMR1) premutation.

METHODS

Eight asymptomatic premutation carriers, 5 participants with fragile X tremor/ataxia syndrome (FXTAS), and 7 noncarrier controls were studied. The mean age of the asymptomatic premutation carriers, participants with FXTAS, and noncarrier controls was 60, 71, and 67 years, respectively. Magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) were used to examine the middle cerebellar peduncles (MCP) and the genu and splenium of the corpus callosum in relation to executive function and processing speed. MRS measures were N-acetyl aspartate/creatine (NAA/Cr) and choline/creatine, and fractional anisotropy (FA) was used for DTI. Executive function was assessed with the Behavioral Dyscontrol Scale and the Controlled Oral Word Association Test (COWAT), and processing speed with the Symbol Digit Modalities Test.

RESULTS

Among all 13 FMR1 premutation carriers, significant correlations were found between N-acetyl aspartate/creatine and choline/creatine in the MCP and COWAT scores, and between FA in the genu and performance on the Behavioral Dyscontrol Scale, COWAT, and Symbol Digit Modalities Test; a correlation was also found between FA in the splenium and COWAT performance. In all regions studied, participants with FXTAS had the lowest mean FA.

CONCLUSION

Microstructural white matter disease as determined by MRS and DTI correlated with executive dysfunction and slowed processing speed in these FMR1 premutation carriers. Neuroimaging abnormalities in the genu and MCP suggest that disruption of white matter within frontocerebellar networks has an important role in the cognitive impairment associated with the FMR1 premutation.

Methadone use in a male with the FMRI premutation and FXTAS

The fragile X-associated tremor ataxia syndrome (FXTAS) is caused by the premutation in FMR1 gene. Recent reports of environmental toxins appear to worsen the progression of FXTAS. Here we present a case of male adult with FXTAS and a long history of methadone use. The patient shows a faster progression in both symptoms of disease and MRI changes compared to what is typically seen in FXTAS. There has been no research regarding the role of narcotics in onset, progression, and severity of FXTAS symptoms. However, research has shown that narcotics can have a negative impact on several neurodegenerative diseases, and we hypothesize that in this particular case, methadone may have contributed to a faster progression of FXTAS as well as exacerbating white matter disease through RNA toxicity seen in premutation carriers.

Modeling neurodevelopmental disorders using human pluripotent stem cells

Neurodevelopmental disorders (NDs) are impairments that affect the development and growth of the brain and the central nervous system during embryonic and early postnatal life. Genetically manipulated animals have contributed greatly to the advancement of ND research, but many of them differ considerably from the human phenotype. Cellular in vitro models are also valuable, but the availability of human neuronal cells is limited and their lifespan in culture is short. Human pluripotent stem cells (hPSCs), including embryonic stem cells and induced pluripotent stem cells, comprise a powerful tool for studying developmentally regulated diseases, including NDs. We reviewed all recent studies in which hPSCs were used as in vitro models for diseases and syndromes characterized by impairment of neurogenesis or synaptogenesis leading to intellectual disability and delayed neurodevelopment. We analyzed their methodology and results, focusing on the data obtained following in vitro neural differentiation and gene expression and profiling of the derived neurons. Electrophysiological recording of action potentials, synaptic currents and response to neurotransmitters is pivotal for validation of the neuronal fate as well as for assessing phenotypic dysfunctions linked to the disease in question. We therefore focused on the studies which included electrophysiological recordings on the in vitro-derived neurons. Finally, we addressed specific issues that are critical for the advancement of this area of research, specifically in providing a reliable human pre-clinical research model and drug screening platform.

Molecular diagnosis of Fragile X syndrome in subjects with intellectual disability of unknown origin: implications of its prevalence in regional Pakistan

Fragile-X syndrome (FXS) is the most common form of inherited intellectual disability (ID) and affects 0.7–3.0% of intellectually compromised population of unknown etiology worldwide. It is mostly caused by repeat expansion mutations in the FMR1 at chromosome Xq27.3. The present study aimed to develop molecular diagnostic tools for a better detection of FXS, to assess implementation of diagnostic protocols in a developing country and to estimate the prevalence of FXS in a cohort of intellectually disabled subjects from Pakistan. From a large pool of individuals with below normal IQ range, 395 subjects with intellectual disability of unknown etiology belonging to different regions of the country were recruited. Conventional-PCR, modified-PCR and Southern blot analysis methods were employed for the detection of CGG repeat polymorphisms in the FMR1 gene. Initial screening with conventional-PCR identified 13 suspected patients. Subsequent investigations through modified PCR and Southern blot analyses confirmed the presence of the FMR1 mutation, suggesting a prevalence of 3.5% and 2.8% (mean 3.3%) among the male and female ID patients, respectively. These diagnostic methods were further customized with the in-house conditions to offer robust screening of referral patients/families for diagnostics and genetic counseling. Prescreening and early diagnosis are crucial for designing a prudent strategy for the management of subjects with ID. Outcome of the study recommends health practitioners for implementation of molecular based FXS diagnosis in routine clinical practice to give a better care for patients similar to the ones included in the study.

Fragile X premutation carriers: A systematic review of neuroimaging findings

BACKGROUND

Expansion of the CGG repeat region of the FMR1 gene from less than 45 repeats to between 55 and 200 repeats is known as the fragile X premutation. Carriers of the fragile X premutation may develop a neurodegenerative disease called fragile X-associated tremor/ataxia syndrome (FXTAS). Recent evidence suggests that premutation carriers experience other psychiatric difficulties throughout their lifespan.

METHODS

Medline, EMBASE and PsychINFO were searched for all appropriate English language studies published between January 1990 and December 2013. 419 potentially relevant articles were identified and screened. 19 articles were included in the analysis.

RESULTS

We discuss key structural magnetic resonance imaging (MRI) findings such as the MCP sign and white matter atrophy. Additionally, we discuss how functional MRI results have progressed our knowledge of how FXTAS may manifest, including reduced brain activation during social and memory tasks in multiple regions.

LIMITATIONS

This systematic review may have been limited by the search for articles on just 3 scientific databases. Differing techniques and methods of analyses between research groups and primary research articles may have caused differences in results between studies.

CONCLUSION

Current MRI studies into the fragile X premutation have been important in the diagnosis of FXTAS and identifying potential pathophysiological mechanisms. Associations with blood based measures have also demonstrated that neurodevelopmental and neurodegenerative aspects of the fragile X premutation could be functionally and pathologically separate. Larger longitudinal studies will be required to investigate these conclusions.

Fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that affects some but not all carriers of small, noncoding CGG-repeat expansions (55-200 repeats; premutation) within the fragile X gene (FMR1). Principal features of FXTAS include intention tremor, cerebellar ataxia, Parkinsonism, memory and executive function deficits, autonomic dysfunction, brain atrophy with white matter disease, and cognitive decline. Although FXTAS was originally considered to be confined to the premutation range, rare individuals with a gray zone (45-54 repeats) or an unmethylated full mutation (>200 repeats) allele have now been described, the constant feature of the disorder remaining the requirement for FMR1 expression, in contradistinction to the gene silencing mechanism of fragile X syndrome. Although transcriptional activity is required for FXTAS pathogenesis, the specific trigger(s) for FXTAS pathogenesis remains elusive, highlighting the need for more research in this area. This need is underscored by recent neuroimaging findings of changes in the central nervous system that consistently appear well before the onset of clinical symptoms, thus creating an opportunity to delay or prevent the appearance of FXTAS.

Dysregulated iron metabolism in the choroid plexus in fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder associated with premutation alleles of the FMR1 gene that is characterized by progressive action tremor, gait ataxia, and cognitive decline. Recent studies of mitochondrial dysfunction in FXTAS have suggested that iron dysregulation may be one component of disease pathogenesis. We tested the hypothesis that iron dysregulation is part of the pathogenic process in FXTAS. We analyzed postmortem choroid plexus from FXTAS and control subjects, and found that in FXTAS iron accumulated in the stroma, transferrin levels were decreased in the epithelial cells, and transferrin receptor 1 distribution was shifted from the basolateral membrane (control) to a predominantly intracellular location (FXTAS). In addition, ferroportin and ceruloplasmin were markedly decreased within the epithelial cells. These alterations have implications not only for understanding the pathophysiology of FXTAS, but also for the development of new clinical treatments that may incorporate selective iron chelation.

Psychological status in female carriers of premutation FMR1 allele showing a complex relationship with the size of CGG expansion

We utilized a sample of 299 adult females aged between 19 and 86 years, carrying fragile X mental retardation (FMR1) alleles with small CCG expansions ranging from 50 to 141 repeats to analyse the relationships between psychological symptoms as assessed by the Symptom Checklist-90-Revised (SCL-90-R) and the size of the CGG repeat in the FMR1 gene. There were highly significant (negative) correlations between the size of the CGG repeat and a great majority of SCL-90-R subscale scores and all the global indices, suggesting that carriers of premutations in the mid-size CGG repeat range may be at greatest risk for the development of psychiatric disorder.

Functional roles of alternative splicing factors in human disease

Alternative splicing (AS) is an important mechanism used to generate greater transcriptomic and proteomic diversity from a finite genome. Nearly all human gene transcripts are alternatively spliced and can produce protein isoforms with divergent and even antagonistic properties that impact cell functions. Many AS events are tightly regulated in a cell-type or tissue-specific manner, and at different developmental stages. AS is regulated by RNA-binding proteins, including cell- or tissue-specific splicing factors. In the past few years, technological advances have defined genome-wide programs of AS regulated by increasing numbers of splicing factors. These splicing regulatory networks (SRNs) consist of transcripts that encode proteins that function in coordinated and related processes that impact the development and phenotypes of different cell types. As such, it is increasingly recognized that disruption of normal programs of splicing regulated by different splicing factors can lead to human diseases. We will summarize examples of diseases in which altered expression or function of splicing regulatory proteins has been implicated in human disease pathophysiology. As the role of AS continues to be unveiled in human disease and disease risk, it is hoped that further investigations into the functions of numerous splicing factors and their regulated targets will enable the development of novel therapies that are directed at specific AS events as well as the biological pathways they impact. WIREs RNA 2015, 6:311–326. doi: 10.1002/wrna.1276

For further resources related to this article, please visit the http://wires.wiley.com/remdoi.cgi?doi=10.1002/wrna.1276WIREs website.

Conflict of interest: The authors have declared no conflicts of interest for this article.

RNA toxicity and missplicing in the common eye disease fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) is an inherited degenerative disease that affects the internal endothelial cell monolayer of the cornea and can result in corneal edema and vision loss in severe cases. FECD affects ∼5% of middle-aged Caucasians in the United States and accounts for >14,000 corneal transplantations annually. Among the several genes and loci associated with FECD, the strongest association is with an intronic (CTG·CAG)_n trinucleotide repeat expansion in the TCF4 gene, which is found in the majority of affected patients. Corneal endothelial cells from FECD patients harbor a poly(CUG)_n RNA that can be visualized as RNA foci containing this condensed RNA and associated proteins. Similar to myotonic dystrophy type 1, the poly(CUG)_n RNA co-localizes with and sequesters the mRNA-splicing factor MBNL1, leading to missplicing of essential MBNL1-regulated mRNAs. Such foci and missplicing are not observed in similar cells from FECD patients who lack the repeat expansion. RNA-Seq splicing data from the corneal endothelia of FECD patients and controls reveal hundreds of differential alternative splicing events. These include events previously characterized in the context of myotonic dystrophy type 1 and epithelial-to-mesenchymal transition, as well as splicing changes in genes related to proposed mechanisms of FECD pathogenesis. We report the first instance of RNA toxicity and missplicing in a common non-neurological/neuromuscular disease associated with a repeat expansion. The FECD patient population with this (CTG·CAG)_n trinucleotide repeat expansion exceeds that of the combined number of patients in all other microsatellite expansion disorders.

Clinical, molecular, and pharmacological aspects of FMR1 related disorders

BACKGROUND

Fragile X syndrome, the most common inherited cause of intellectual disability, is associated with a broad spectrum of disorders across different generations of a single family. This study reviews the clinical manifestations of fragile X-associated disorders as well as the spectrum of mutations of the fragile X mental retardation 1 gene (FMR1) and the neurobiology of the fragile X mental retardation protein (FMRP), and also provides an overview of the potential therapeutic targets and genetic counselling.

DEVELOPMENT

This disorder is caused by expansion of the CGG repeat (>200 repeats) in the 5 prime untranslated region of FMR1, resulting in a deficit or absence of FMRP. FMRP is an RNA-binding protein that regulates the translation of several genes that are important in synaptic plasticity and dendritic maturation. It is believed that CGG repeat expansions in the premutation range (55 to 200 repeats) elicit an increase in mRNA levels of FMR1, which may cause neuronal toxicity. These changes manifest clinically as developmental problems such as autism and learning disabilities as well as neurodegenerative diseases including fragile X-associated tremor/ataxia syndrome (FXTAS).

CONCLUSIONS

Advances in identifying the molecular basis of fragile X syndrome may help us understand the causes of neuropsychiatric disorders, and they will probably contribute to development of new and specific treatments.

Pharmacogenetic inhibition of eIF4E-dependent Mmp9 mRNA translation reverses fragile X syndrome-like phenotypes

Fragile X syndrome (FXS) is the leading genetic cause of autism. Mutations in Fmr1 (fragile X mental retardation 1 gene) engender exaggerated translation resulting in dendritic spine dysmorphogenesis, synaptic plasticity alterations, and behavioral deficits in mice, which are reminiscent of FXS phenotypes. Using postmortem brains from FXS patients and Fmr1 knockout mice (Fmr1(-/y)), we show that phosphorylation of the mRNA 5' cap binding protein, eukaryotic initiation factor 4E (eIF4E), is elevated concomitant with increased expression of matrix metalloproteinase 9 (MMP-9) protein. Genetic or pharmacological reduction of eIF4E phosphorylation rescued core behavioral deficits, synaptic plasticity alterations, and dendritic spine morphology defects via reducing exaggerated translation of Mmp9 mRNA in Fmr1(-/y) mice, whereas MMP-9 overexpression produced several FXS-like phenotypes. These results uncover a mechanism of regulation of synaptic function by translational control of Mmp-9 in FXS, which opens the possibility of new treatment avenues for the diverse neurological and psychiatric aspects of FXS.

Relevance of corpus callosum splenium versus middle cerebellar peduncle hyperintensity for FXTAS diagnosis in clinical practice

Fragile X-associated tremor ataxia syndrome (FXTAS) is caused by FMR1 premutation. The features include ataxia, action tremor and middle cerebellar peduncle (MCP) hyperintensity, the latter being the only major radiological criterion in the diagnosis of definite FXTAS until very recently. The importance of corpus callosum splenium (CCS) hyperintensity was recently reported and this sign is now considered as an additional major radiological diagnostic criterion in the diagnosis of FXTAS. However, little is known about its relevance for the diagnosis of FXTAS in clinical practice. We report a practical justification of the relevance of CCS hyperintensity in parallel with MCP hyperintensity for the diagnosis of FXTAS. Clinical and radiological study of 22 FMR1 premutation carriers with neurological signs that may be encountered in FXTAS compared to series of patients with essential tremor, multiple system atrophy of cerebellar type, Parkinson's disease, Alzheimer's disease and stroke. Among the 22 patients with FMR1 premutation [17 men, 5 women; mean age, 63 ± 7.5 (46-84)], 14 were diagnosed with definite FXTAS with the initial criteria. Considering CCS hyperintensity as a new major radiological criterion permitted the diagnosis of definite FXTAS in 3 additional patients. Overall CCS proved as frequent as MCP hyperintensity (64 versus 64 %), while 23 % of patients had CCS but not MCP hyperintensity, 14 % of patients had CCS hyperintensity but neither MCP, nor brainstem hyperintensity. In contrast with CCS hyperintensity, MCP hyperintensity proved less frequent in women than in men. CCS and MCP hyperintensity were more frequent in FXTAS than in the other neurodegenerative disorders. The combination of CCS and MCP hyperintensity was specific of FXTAS. We confirmed the relevance of CCS hyperintensity in FXTAS and we clarified its interest compared to MCP hyperintensity. Our results support the inclusion of CCS hyperintensity in the diagnostic criteria as a new major radiological criterion.

Immune mediated disorders in women with a fragile X expansion and FXTAS

Premutation alleles in fragile X mental retardation 1 (FMR1) can cause the late-onset neurodegenerative disorder, fragile X-associated tremor ataxia syndrome (FXTAS) and/or the fragile X-associated primary ovarian insufficiency in approximately 20% of heterozygotes. Heterozygotes of the FMR1 premutation have a higher incidence of immune mediated disorders such as autoimmune thyroid disorder, especially when accompanied by FXTAS motor signs. We describe the time course of symptoms of immune mediated disorders and the subsequent development of FXTAS in four women with an FMR1 CGG expansion, including three with the premutation and one with a gray zone expansion. These patients developed an immune mediated disorder followed by neurological symptoms that become consistent with FXTAS. In all patients we observed a pattern involving an initial appearance of disease symptoms-often after a period of heightened stress (depression, anxiety, divorce, general surgery) followed by the onset of tremor and/or ataxia. Immune mediated diseases are associated with the manifestations of FXTAS temporally, although further studies are needed to clarify this association. If a cause and effect relationship can be established, treatment of pre-existing immune mediated disorders may benefit patients with pathogenic FMR1 mutations.