Argonaute-2-dependent rescue of a Drosophila model of FXTAS by FRAXE premutation repeat

Non-canonical DNA/RNA structures associated with the pathogenesis of Fragile X-associated tremor/ataxia syndrome and Fragile X syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X syndrome (FXS) are primary examples of fragile X-related disorders (FXDs) caused by abnormal expansion of CGG repeats above a certain threshold in the 5′-untranslated region of the fragile X mental retardation (FMR1) gene. Both diseases have distinct clinical manifestations and molecular pathogenesis. FXTAS is a late-adult-onset neurodegenerative disorder caused by a premutation (PM) allele (CGG expansion of 55–200 repeats), resulting in FMR1 gene hyperexpression. On the other hand, FXS is a neurodevelopmental disorder that results from a full mutation (FM) allele (CGG expansions of ≥200 repeats) leading to heterochromatization and transcriptional silencing of the FMR1 gene. The main challenge is to determine how CGG repeat expansion affects the fundamentally distinct nature of FMR1 expression in FM and PM ranges. Abnormal CGG repeat expansions form a variety of non-canonical DNA and RNA structures that can disrupt various cellular processes and cause distinct effects in PM and FM alleles. Here, we review these structures and how they are related to underlying mutations and disease pathology in FXS and FXTAS. Finally, as new CGG expansions within the genome have been identified, it will be interesting to determine their implications in disease pathology and treatment.

Secondary structural choice of DNA and RNA associated with CGG/CCG trinucleotide repeat expansion rationalizes the RNA misprocessing in FXTAS

CGG tandem repeat expansion in the 5'-untranslated region of the fragile X mental retardation-1 (FMR1) gene leads to unusual nucleic acid conformations, hence causing genetic instabilities. We show that the number of G…G (in CGG repeat) or C…C (in CCG repeat) mismatches (other than A…T, T…A, C…G and G…C canonical base pairs) dictates the secondary structural choice of the sense and antisense strands of the FMR1 gene and their corresponding transcripts in fragile X-associated tremor/ataxia syndrome (FXTAS). The circular dichroism (CD) spectra and electrophoretic mobility shift assay (EMSA) reveal that CGG DNA (sense strand of the FMR1 gene) and its transcript favor a quadruplex structure. CD, EMSA and molecular dynamics (MD) simulations also show that more than four C…C mismatches cannot be accommodated in the RNA duplex consisting of the CCG repeat (antisense transcript); instead, it favors an i-motif conformational intermediate. Such a preference for unusual secondary structures provides a convincing justification for the RNA foci formation due to the sequestration of RNA-binding proteins to the bidirectional transcripts and the repeat-associated non-AUG translation that are observed in FXTAS. The results presented here also suggest that small molecule modulators that can destabilize FMR1 CGG DNA and RNA quadruplex structures could be promising candidates for treating FXTAS.

The Molecular Function of PURA and Its Implications in Neurological Diseases

In recent years, genome-wide analyses of patients have resulted in the identification of a number of neurodevelopmental disorders. Several of them are caused by mutations in genes that encode for RNA-binding proteins. One of these genes is PURA, for which in 2014 mutations have been shown to cause the neurodevelopmental disorder PURA syndrome. Besides intellectual disability (ID), patients develop a variety of symptoms, including hypotonia, metabolic abnormalities as well as epileptic seizures. This review aims to provide a comprehensive assessment of research of the last 30 years on PURA and its recently discovered involvement in neuropathological abnormalities. Being a DNA- and RNA-binding protein, PURA has been implicated in transcriptional control as well as in cytoplasmic RNA localization. Molecular interactions are described and rated according to their validation state as physiological targets. This information will be put into perspective with available structural and biophysical insights on PURA’s molecular functions. Two different knock-out mouse models have been reported with partially contradicting observations. They are compared and put into context with cell biological observations and patient-derived information. In addition to PURA syndrome, the PURA protein has been found in pathological, RNA-containing foci of patients with the RNA-repeat expansion diseases such as fragile X-associated tremor ataxia syndrome (FXTAS) and amyotrophic lateral sclerosis (ALS)/fronto-temporal dementia (FTD) spectrum disorder. We discuss the potential role of PURA in these neurodegenerative disorders and existing evidence that PURA might act as a neuroprotective factor. In summary, this review aims at informing researchers as well as clinicians on our current knowledge of PURA’s molecular and cellular functions as well as its implications in very different neuronal disorders.

Antisense Transcription across Nucleotide Repeat Expansions in Neurodegenerative and Neuromuscular Diseases: Progress and Mysteries

Unstable repeat expansions and insertions cause more than 30 neurodegenerative and neuromuscular diseases. Remarkably, bidirectional transcription of repeat expansions has been identified in at least 14 of these diseases. More remarkably, a growing number of studies has been showing that both sense and antisense repeat RNAs are able to dysregulate important cellular pathways, contributing together to the observed clinical phenotype. Notably, antisense repeat RNAs from spinocerebellar ataxia type 7, myotonic dystrophy type 1, Huntington's disease and frontotemporal dementia/amyotrophic lateral sclerosis associated genes have been implicated in transcriptional regulation of sense gene expression, acting either at a transcriptional or posttranscriptional level. The recent evidence that antisense repeat RNAs could modulate gene expression broadens our understanding of the pathogenic pathways and adds more complexity to the development of therapeutic strategies for these disorders. In this review, we cover the amazing progress made in the understanding of the pathogenic mechanisms associated with repeat expansion neurodegenerative and neuromuscular diseases with a focus on the impact of antisense repeat transcription in the development of efficient therapies.

Repeat Expansion Disease Models

Repeat expansion disorders are a group of inherited neuromuscular diseases, which are caused by expansion mutations of repeat sequences in the disease-causing genes. Repeat expansion disorders include a class of diseases caused by repeat expansions in the coding region of the genes, producing mutant proteins with amino acid repeats, mostly the polyglutamine (polyQ) diseases, and another class of diseases caused by repeat expansions in the noncoding regions, producing aberrant RNA with expanded repeats, which are called noncoding repeat expansion diseases. A variety of Drosophila disease models have been established for both types of diseases, and they have made significant contributions toward elucidating the molecular mechanisms of and developing therapies for these neuromuscular diseases.

RNA toxicity and foci formation in microsatellite expansion diseases

More than 30 incurable neurological and neuromuscular diseases are caused by simple microsatellite expansions consisted of 3-6 nucleotides. These repeats can occur in non-coding regions and often result in a dominantly inherited disease phenotype that is characteristic of a toxic RNA gain-of-function. The expanded RNA adopts unusual secondary structures, sequesters various RNA binding proteins to form insoluble nuclear foci, and causes cellular defects at a multisystem level. Nuclear foci are dynamic in size, shape and colocalization of RNA binding proteins in different expansion diseases and tissue types. This review sets to provide new insights into the disease mechanisms of RNA toxicity and foci modulation, in light of recent advancement on bi-directional transcription, antisense RNA, repeat-associated non-ATG translation and beyond.

Repeat-associated non-AUG translation from antisense CCG repeats in fragile X tremor/ataxia syndrome

OBJECTIVE

Repeat-associated non-AUG (RAN) translation drives production of toxic proteins from pathogenic repeat sequences in multiple untreatable neurodegenerative disorders. Fragile X-associated tremor/ataxia syndrome (FXTAS) is one such condition, resulting from a CGG trinucleotide repeat expansion in the 5' leader sequence of the FMR1 gene. RAN proteins from the CGG repeat accumulate in ubiquitinated inclusions in FXTAS patient brains and elicit toxicity. In addition to the CGG repeat, an antisense mRNA containing a CCG repeat is also transcribed from the FMR1 locus. We evaluated whether this antisense CCG repeat supports RAN translation and contributes to pathology in FXTAS patients.

METHODS

We generated a series of CCG RAN translation-specific reporters and utilized them to measure RAN translation from CCG repeats in multiple reading frames in transfected cells. We also developed antibodies against predicted CCG RAN proteins and used immunohistochemistry and immunofluorescence on FXTAS patient tissues to measure their accumulation and distribution.

RESULTS

RAN translation from CCG repeats is supported in all 3 potential reading frames, generating polyproline, polyarginine, and polyalanine proteins, respectively. Their production occurs whether or not the natural AUG start upstream of the repeat in the proline reading frame is present. All 3 frames show greater translation at larger repeat sizes. Antibodies targeted to the antisense FMR polyproline and polyalanine proteins selectively stain nuclear and cytoplasmic aggregates in FXTAS patients and colocalize with ubiquitinated neuronal inclusions.

INTERPRETATION

RAN translation from antisense CCG repeats generates novel proteins that accumulate in ubiquitinated inclusions in FXTAS patients. Ann Neurol 2016;80:871-881.

DNA secondary structure at chromosomal fragile sites in human disease

DNA has the ability to form a variety of secondary structures that can interfere with normal cellular processes, and many of these structures have been associated with neurological diseases and cancer. Secondary structure-forming sequences are often found at chromosomal fragile sites, which are hotspots for sister chromatid exchange, chromosomal translocations, and deletions. Structures formed at fragile sites can lead to instability by disrupting normal cellular processes such as DNA replication and transcription. The instability caused by disruption of replication and transcription can lead to DNA breakage, resulting in gene rearrangements and deletions that cause disease. In this review, we discuss the role of DNA secondary structure at fragile sites in human disease.

Bioinformatics tools for achieving better gene silencing in plants

RNA interference (RNAi) is one of the most popular and effective molecular technologies for knocking down the expression of an individual gene of interest in living organisms. Yet the technology still faces the major issue of nonspecific gene silencing, which can compromise gene functional characterization and the interpretation of phenotypes associated with individual gene knockdown. Designing an effective and target-specific small interfering RNA (siRNA) for induction of RNAi is therefore the major challenge in RNAi-based gene silencing. A 'good' siRNA molecule must possess three key features: (a) the ability to specifically silence an individual gene of interest, (b) little or no effect on the expressions of unintended siRNA gene targets (off-target genes), and (c) no cell toxicity. Although several siRNA design and analysis algorithms have been developed, only a few of them are specifically focused on gene silencing in plants. Furthermore, current algorithms lack a comprehensive consideration of siRNA specificity, efficacy, and nontoxicity in siRNA design, mainly due to lack of integration of all known rules that govern different steps in the RNAi pathway. In this review, we first describe popular RNAi methods that have been used for gene silencing in plants and their serious limitations regarding gene-silencing potency and specificity. We then present novel, rationale-based strategies in combination with computational and experimental approaches to induce potent, specific, and nontoxic gene silencing in plants.

Distinct roles for Toll and autophagy pathways in double-stranded RNA toxicity in a Drosophila model of expanded repeat neurodegenerative diseases

Dominantly inherited expanded repeat neurodegenerative diseases are caused by the expansion of variable copy number tandem repeat sequences in otherwise unrelated genes. Some repeats encode polyglutamine that is thought to be toxic; however, other repeats do not encode polyglutamine indicating either multiple pathogenic pathways or an alternative common toxic agent. As these diseases share numerous clinical features and expanded repeat RNA is a common intermediary, RNA-based pathogenesis has been proposed, based on its toxicity in animal models. In Drosophila, double-stranded (rCAG.rCUG∼100) RNA toxicity is Dicer dependent and generates single-stranded (rCAG)7, an entity also detected in affected Huntington's Disease (HD) brains. We demonstrate that Drosophila rCAG.rCUG∼100 RNA toxicity perturbs several pathways including innate immunity, consistent with the observation in HD that immune activation precedes neuronal toxicity. Our results show that Drosophila rCAG.rCUG∼100 RNA toxicity is dependent upon Toll signaling and sensitive to autophagy, further implicating innate immune activation. In exhibiting molecular and cellular hallmarks of HD, double-stranded RNA-mediated activation of innate immunity is, therefore, a candidate pathway for this group of human genetic diseases.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

RNA-binding proteins in microsatellite expansion disorders: mediators of RNA toxicity

Although protein-mediated toxicity in neurological disease has been extensively characterized, RNA-mediated toxicity is an emerging mechanism of pathogenesis. In microsatellite expansion disorders, expansion of repeated sequences in noncoding regions gives rise to RNA that produces a toxic gain of function, while expansions in coding regions can disrupt protein function as well as produce toxic RNA. The toxic RNA typically aggregates into nuclear foci and contributes to disease pathogenesis. In many cases, toxicity of the RNA is caused by the disrupted functions of RNA-binding proteins. We will discuss evidence for RNA-mediated toxicity in microsatellite expansion disorders. Different microsatellite expansion disorders are linked with alterations in the same as well as disease-specific RNA-binding proteins. Recent studies have shown that microsatellite expansions can encode multiple repeat-containing toxic RNAs through bidirectional transcription and protein species through repeat-associated non-ATG translation. We will discuss approaches that have characterized the toxic contributions of these various factors.

Crystallographic characterization of CCG repeats

CCG repeats are highly over-represented in exons of the human genome. Usually they are located in the 5′ UTR but are also abundant in translated sequences. The CCG repeats are associated with three tri-nucleotide repeat disorders: Huntington’s disease, myotonic dystrophy type 1 and chromosome X-linked mental retardation (FRAXE). In this study, we present two crystal structures containing double-stranded CCG repeats: one of an RNA in the native form, and one containing LNA nucleotides. Both duplexes form A-helices but with strands slipped in the 5′ (native structure) or the 3′ direction (LNA-containing structure). As a result, one of two expected C-C pairs is eliminated from the duplex. Each of the three observed C-C pairs interacts differently, forming either one weak H-bond or none. LNA nucleotides have no apparent effect on the helical parameters but the base stacking is increased compared to the native duplex and the distribution of electrostatic potential in the major groove is changed. The CCG crystal structures explain the thermodynamic fragility of CCG runs and throw light on the observation that the MBNL1 protein recognises CCG runs, as well as CUG and CAG, but not the relatively stable CGG repeats.

Neurodegeneration the RNA way

The expression, processing, transport and activities of both coding and non-coding RNAs play critical roles in normal neuronal function and differentiation. Over the past decade, these same pathways have come under scrutiny as potential contributors to neurodegenerative disease. Here we focus broadly on the roles of RNA and RNA processing in neurodegeneration. We first discuss a set of "RNAopathies", where non-coding repeat expansions drive pathogenesis through a surprisingly diverse set of mechanisms. We next explore an emerging class of "RNA binding proteinopathies" where redistribution and aggregation of the RNA binding proteins TDP-43 or FUS contribute to a potentially broad range of neurodegenerative disorders. Lastly, we delve into the potential contributions of alterations in both short and long non-coding RNAs to neurodegenerative illness.

Double-stranded RNA is pathogenic in Drosophila models of expanded repeat neurodegenerative diseases

The pathogenic agent responsible for the expanded repeat diseases, a group of neurodegenerative diseases that includes Huntington's disease is not yet fully understood. Expanded polyglutamine (polyQ) is thought to be the toxic agent in certain cases, however, not all expanded repeat disease genes can encode a polyQ sequence. Since a repeat-containing RNA intermediary is common to all of these diseases, hairpin-forming single-stranded RNA has been investigated as a potential common pathogenic agent. More recently, it has become apparent that most of the expanded repeat disease loci have transcription occurring from both strands, raising the possibility that the complementary repeat RNAs could form a double-stranded structure. In our investigation using Drosophila models of these diseases, we identified a fortuitous integration event that models bidirectional repeat RNA transcription with the resultant flies exhibiting inducible pathology. We therefore established further lines of Drosophila expressing independent complementary repeat RNAs and found that these are toxic. The Dicer pathway is essential for this toxicity and in neuronal cells accounts for metabolism of the high copy number (CAG.CUG)(100) double-stranded RNAs down to (CAG)(7) single-stranded small RNAs. We also observe significant changes to the microRNA profile in neurons. These data identify a novel pathway through which double-stranded repeat RNA is toxic and capable of eliciting symptoms common to neurodegenerative human diseases resulting from dominantly inherited expanded repeats.

Triplet repeat-derived siRNAs enhance RNA-mediated toxicity in a Drosophila model for myotonic dystrophy

More than 20 human neurological and neurodegenerative diseases are caused by simple DNA repeat expansions; among these, non-coding CTG repeat expansions are the basis of myotonic dystrophy (DM1). Recent work, however, has also revealed that many human genes have anti-sense transcripts, raising the possibility that human trinucleotide expansion diseases may be comprised of pathogenic activities due both to a sense expanded-repeat transcript and to an anti-sense expanded-repeat transcript. We established a Drosophila model for DM1 and tested the role of interactions between expanded CTG transcripts and expanded CAG repeat transcripts. These studies revealed dramatically enhanced toxicity in flies co-expressing CTG with CAG expanded repeats. Expression of the two transcripts led to novel pathogenesis with the generation of dcr-2 and ago2-dependent 21-nt triplet repeat-derived siRNAs. These small RNAs targeted the expression of CAG-containing genes, such as Ataxin-2 and TATA binding protein (TBP), which bear long CAG repeats in both fly and man. These findings indicate that the generation of triplet repeat-derived siRNAs may dramatically enhance toxicity in human repeat expansion diseases in which anti-sense transcription occurs.

Histone deacetylases suppress CGG repeat-induced neurodegeneration via transcriptional silencing in models of fragile X tremor ataxia syndrome

Fragile X Tremor Ataxia Syndrome (FXTAS) is a common inherited neurodegenerative disorder caused by expansion of a CGG trinucleotide repeat in the 5'UTR of the fragile X syndrome (FXS) gene, FMR1. The expanded CGG repeat is thought to induce toxicity as RNA, and in FXTAS patients mRNA levels for FMR1 are markedly increased. Despite the critical role of FMR1 mRNA in disease pathogenesis, the basis for the increase in FMR1 mRNA expression is unknown. Here we show that overexpressing any of three histone deacetylases (HDACs 3, 6, or 11) suppresses CGG repeat-induced neurodegeneration in a Drosophila model of FXTAS. This suppression results from selective transcriptional repression of the CGG repeat-containing transgene. These findings led us to evaluate the acetylation state of histones at the human FMR1 locus. In patient-derived lymphoblasts and fibroblasts, we determined by chromatin immunoprecipitation that there is increased acetylation of histones at the FMR1 locus in pre-mutation carriers compared to control or FXS derived cell lines. These epigenetic changes correlate with elevated FMR1 mRNA expression in pre-mutation cell lines. Consistent with this finding, histone acetyltransferase (HAT) inhibitors repress FMR1 mRNA expression to control levels in pre-mutation carrier cell lines and extend lifespan in CGG repeat-expressing Drosophila. These findings support a disease model whereby the CGG repeat expansion in FXTAS promotes chromatin remodeling in cis, which in turn increases expression of the toxic FMR1 mRNA. Moreover, these results provide proof of principle that HAT inhibitors or HDAC activators might be used to selectively repress transcription at the FMR1 locus.

Fragile X and autism: Intertwined at the molecular level leading to targeted treatments

Fragile X syndrome (FXS) is caused by an expanded CGG repeat (> 200 repeats) in the 5' untranslated portion of the fragile mental retardation 1 gene (FMR1), leading to deficiency or absence of the FMR1 protein (FMRP). FMRP is an RNA carrier protein that controls the translation of several other genes that regulate synaptic development and plasticity. Autism occurs in approximately 30% of FXS cases, and pervasive developmental disorder, not otherwise specified (PDD-NOS) occurs in an additional 30% of cases. Premutation repeat expansions (55 to 200 CGG repeats) may also give rise to autism spectrum disorders (ASD), including both autism and PDD-NOS, through a different molecular mechanism that involves a direct toxic effect of the expanded CGG repeat FMR1 mRNA. RNA toxicity can also lead to aging effects including tremor, ataxia and cognitive decline, termed fragile X-associated tremor ataxia syndrome (FXTAS), in premutation carriers in late life. In studies of mice bearing premutation expansions, there is evidence of early postnatal neuronal cell toxicity, presenting as reduced cell longevity, decreased dendritic arborization and altered synaptic morphology. There is also evidence of mitochondrial dysfunction in premutation carriers. Many of the problems with cellular dysregulation in both premutation and full mutation neurons also parallel the cellular abnormalities that have been documented in autism without fragile X mutations. Research regarding dysregulation of neurotransmitter systems in FXS, including the metabotropic glutamate receptor (mGluR)1/5 pathway and γ aminobutyric acid (GABA)A pathways, have led to new targeted treatments for FXS. Preliminary evidence suggests that these new targeted treatments will also be beneficial in non-fragile X forms of autism.

RNA-mediated neurodegeneration in repeat expansion disorders

Most neurodegenerative disorders are thought to result primarily from the accumulation of misfolded proteins, which interfere with protein homeostasis in neurons. For a subset of diseases, however, noncoding regions of RNAs assume a primary toxic gain-of-function, leading to degeneration in many tissues, including the nervous system. Here we review a series of proposed mechanisms by which noncoding repeat expansions give rise to nervous system degeneration and dysfunction. These mechanisms include transcriptional alterations and the generation of antisense transcripts, sequestration of mRNA-associated protein complexes that lead to aberrant mRNA splicing and processing, and alterations in cellular processes, including activation of abnormal signaling cascades and failure of protein quality control pathways. We place these potential mechanisms in the context of known RNA-mediated disorders, including the myotonic dystrophies and fragile X tremor ataxia syndrome, and discuss recent results suggesting that mRNA toxicity may also play a role in some presumably protein-mediated neurodegenerative disorders. Lastly, we comment on recent progress in therapeutic development for these RNA-dominant diseases.

Advances in understanding the molecular basis of FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder among carriers of premutation expansions (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. The clinical features of FXTAS, as well as other forms of clinical involvement in carriers without FXTAS, are thought to arise from a toxic gain of function of transcriptionally active FMR1 containing expanded CGG repeats. Although the precise mechanisms involved in rCGG toxicity are unknown, here we discuss the latest advances and models that contribute to the understanding of the molecular basis of FXTAS, and the emerging view of FXTAS as the end-stage of a process that begins in early development.

Evidence for RNA-mediated toxicity in the fragile X-associated tremor/ataxia syndrome

Fragile X premutation carriers are at risk for developing a late-onset, progressive neurodegenerative disorder termed fragile X-associated tremor/ataxia syndrome (FXTAS). A growing body of evidence suggests the characteristic excess CGG repeat containing FMR1 mRNA observed in premutation carriers is pathogenic and leads to clinical features of FXTAS. The current model suggests premutation mRNA transcripts can induce the formation of intranuclear inclusions by the sequestration of RNA-binding proteins and other proteins. The sequestered proteins are prevented from performing their normal functions, which is thought to lead to the neuropathology-observed FXTAS. This paper discusses the existing evidence that microsatellite expansions at the level of RNA play a role in the disease pathogenesis of FXTAS and some of the approaches that may uncover downstream effects of expanded riboCGG expression.

The FMR1 gene and fragile X-associated tremor/ataxia syndrome

The CGG-repeat present in the 5'UTR of the FMR1 gene is unstable upon transmission to the next generation. The repeat is up to 55 CGGs long in the normal population. In fragile X patients, a repeat length exceeding 200 CGGs (full mutation: FM) generally leads to methylation of the repeat and the promoter region, which is accompanied by silencing of the FMR1 gene. The gene product FMRP is involved in regulation of transport and translation of certain mRNA in the dendrite, thereby affecting synaptic plasticity. This is central to learning and memory processes. The absence of FMRP seen in FM is the cause of the mental retardation seen in fragile X patients. The premutation (PM) is defined as 55-200 CGGs. Female PM carriers are at risk of developing primary ovarian insufficiency. Recently it was discovered that elderly PM carriers might develop a progressive neurodegenerative disorder called fragile X-associated tremor/ataxia syndrome. Although arising from the mutations in the same gene, distinct mechanisms lead to fragile X syndrome (absence of FMRP) and FXTAS (toxic RNA gain of function). The pathogenic mechanisms thought to underlie these disorders are discussed, with a specific emphasis on FXTAS. This review gives insight on the implications of all possible repeat length categories seen in fragile X families.

Ectopic expression of CGG containing mRNA is neurotoxic in mammals

Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) is a progressive neurodegenerative disorder that has been diagnosed in a substantial fraction of older male fragile X premutation carriers. Patients affected by FXTAS have elevated levels of ribo-rCGG repeat containing FMR1 mRNA with normal to slightly reduced levels of FMRP in blood leukocytes. Coupled with the absence of FXTAS in fragile X syndrome patients, this suggests premutation-sized elongated rCGG repeats in the FMR1 transcript rather than alterations in the levels of FMRP are responsible for the FXTAS pathology. Mice expressing rCGG in the context of Fmr1 or the enhanced green fluorescent protein specifically in Purkinje neurons were generated to segregate the effects of rCGG from alterations in Fmr1 and to provide evidence that rCGG is necessary and sufficient to cause pathology similar to human FXTAS. The models exhibit the presence of intranuclear inclusions in Purkinje neurons, Purkinje neuron cell death and behavioral deficits. These results demonstrate that rCGG expressed in Purkinje neurons outside the context of Fmr1 mRNA can result in neuronal pathology in a mammalian system and demonstrate that expanded CGG repeats in RNA are the likely cause of the neurodegeneration in FXTAS.

Role of non-coding RNAs in neurodegeneration and stress response in Drosophila

The inherent limitations of genetic analysis in humans and other mammals as well as striking conservation of most genes controlling nervous system functioning in flies and mammals made Drosophila an attractive model to investigate various aspects of brain diseases. Since RNA research has made great progress in recent years here we present an overview of studies demonstrating the role of various non-coding RNAs in neurodegeneration and stress response in Drosophila as a model organism. We put special emphasis on the role of non-coding micro RNAs, hsr-omega transcripts, and artificial small highly structured RNAs as triggers of neuropathology including aggregates formation, cognitive abnormalities and other symptoms. Cellular stress is a conspicuous feature of many neurodegenerative diseases and the production of specialized proteins protects the nerve cells against aggregates formation. Therefore, herein we describe some data implicating various classes of non-coding RNAs in stress response in Drosophila. All these findings highlight Drosophila as an important model system to investigate various brain diseases potentially mediated by some non-coding RNAs including polyglutamine diseases, Alzheimer's disease, Huntigton's disease, and many others.

An update on inherited ataxias

This article provides an overview of recent advances in the field of inherited ataxias. In the past few years, new causative mutations that broaden the diagnostic spectrum of ataxias have been described. In addition, important advances have unveiled the molecular pathology of these disorders, resulting in a classification based on the pathogenetic pathways rather than clinical or genetic features. As concepts of treatment principles emerge, debate continues as to whether such concepts might be applicable to more than one genetically defined disorder or whether each ataxia disorder requires its own unique therapeutic approach. New clinical assessment instruments have been developed that will facilitate future interventional trials. A recent phase 2 clinical trial suggested a positive effect of high-dose idebenone in Friedreich's ataxia, raising hopes that a treatment option will soon be available for this disorder.

The biological effects of simple tandem repeats: lessons from the repeat expansion diseases

Tandem repeats are common features of both prokaryote and eukaryote genomes, where they can be found not only in intergenic regions but also in both the noncoding and coding regions of a variety of different genes. The repeat expansion diseases are a group of human genetic disorders caused by long and highly polymorphic tandem repeats. These disorders provide many examples of the effects that such repeats can have on many biological processes. While repeats in the coding sequence can result in the generation of toxic or malfunctioning proteins, noncoding repeats can also have significant effects including the generation of chromosome fragility, the silencing of the genes in which they are located, the modulation of transcription and translation, and the sequestering of proteins involved in processes such as splicing and cell architecture.

FXTAS: a bad RNA and a hope for a cure

BACKGROUND

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a newly identified neurodegenerative disorder due to intermediate expansion of trinucleotide CGG repeats (55 - 200 repeats) in the 5' untranslated region (UTR) of the Fragile X mental retardation 1 (FMR1) gene. FXTAS is now considered to be one of the most common inherited neurodegenerative disorders in males.

OBJECTIVE

To examine the future of potential therapies for this late-onset disease.

METHODS

Examination of relevent literature.

RESULTS/CONCLUSIONS

Accumulating evidence indicates that overproduced riboCGG repeats in the 5' UTR of FMR1 mRNA are toxic. Recently, proteins that bind specifically to rCGG repeats were identified. Progress in understanding the molecular pathogenesis of FXTAS, plus the availability of different animal models are discussed.

An antisense transcript spanning the CGG repeat region of FMR1 is upregulated in premutation carriers but silenced in full mutation individuals

Expansion of the polymorphic CGG repeats within the 5'-UTR of the FMR1 gene is associated with variable transcriptional regulation of FMR1. Here we report a novel gene, ASFMR1, overlapping the CGG repeat region of FMR1 and transcribed in the antisense orientation. The ASFMR1 transcript is spliced, polyadenylated and exported to the cytoplasm. Similar to FMR1, ASFMR1 is upregulated in individuals with premutation alleles and is not expressed from full mutation alleles. Moreover, it exhibits premutation-specific alternative splicing. Taken together, these observations suggest that in addition to FMR1, ASFMR1 may contribute to the variable phenotypes associated with the CGG repeat expansion.