Bidirectional expression of CUG and CAG expansion transcripts and intranuclear polyglutamine inclusions in spinocerebellar ataxia type 8

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.

Repeat associated non-ATG translation initiation: one DNA, two transcripts, seven reading frames, potentially nine toxic entities!

Diseases associated with unstable repetitive elements in the DNA, RNA, and amino acids have consistently revealed scientific surprises. Most diseases are caused by expansions of trinucleotide repeats, which ultimately lead to diseases like Huntington's disease, myotonic dystrophy, fragile X syndrome, and a series of spinocerebellar ataxias. These repeat mutations are dynamic, changing through generations and within an individual, and the repeats can be bi-directionally transcribed. Unsuspected modes of pathogenesis involve aberrant loss of protein expression; aberrant over-expression of non-mutant proteins; toxic-gain-of-protein function through expanded polyglutamine tracts that are encoded by expanded CAG tracts; and RNA-toxic-gain-of-function caused by transcripts harboring expanded CUG, CAG, or CGG tracts. A recent advance reveals that RNA transcripts with expanded CAG repeats can be translated in the complete absence of a starting ATG, and this Repeat Associated Non-ATG translation (RAN-translation) occurs across expanded CAG repeats in all reading frames (CAG, AGC, and GCA) to produce homopolymeric proteins of long polyglutamine, polyserine, and polyalanine tracts. Expanded CTG tracts expressing CUG transcripts also show RAN-translation occurring in all three frames (CUG, UGC, and GCU), to produce polyleucine, polycysteine, and polyalanine. These RAN-translation products can be toxic. Thus, one unstable (CAG)•(CTG) DNA can produce two expanded repeat transcripts and homopolymeric proteins with reading frames (the AUG-directed polyGln and six RAN-translation proteins), yielding a total of potentially nine toxic entities. The occurrence of RAN-translation in patient tissues expands our horizons of modes of disease pathogenesis. Moreover, since RAN-translation counters the canonical requirements of translation initiation, many new questions are now posed that must be addressed. This review covers RAN-translation and some of the pertinent questions.

Transcription-induced DNA toxicity at trinucleotide repeats: double bubble is trouble

Trinucleotide repeats (TNR) are a blessing and a curse. In coding regions, where they are enriched, short repeats offer the potential for continuous, rapid length variation with linked incremental changes in the activity of the encoded protein, a valuable source of variation for evolution. But at the upper end of these benign and beneficial lengths, trinucleotide repeats become very unstable, with a dangerous bias toward continual expansion, which can lead to neurological diseases in humans. The mechanisms of expansion are varied and the links to disease are complex. Where they have been delineated, however, they have often revealed unexpected, fundamental aspects of the underlying cell biology. Nowhere is this more apparent than in recent studies, which indicate that expanded CAG repeats can form toxic sites in the genome, which can, upon interaction with normal components of DNA metabolism, trigger cell death. Here we discuss the phenomenon of TNR-induced DNA toxicity, with special emphasis on the role of transcription. Transcription-induced DNA toxicity may have profound biological consequences, with particular relevance to repeat-associated neurodegenerative diseases.

Parasagittally aligned, mGluR1-dependent patches are evoked at long latencies by parallel fiber stimulation in the mouse cerebellar cortex in vivo

The parallel fibers (PFs) in the cerebellar cortex extend several millimeters along a folium in the mediolateral direction. The PFs are orthogonal to and cross several parasagittal zones defined by the olivocerebellar and corticonuclear pathways and the expression of molecular markers on Purkinje cells (PCs). The functions of these two organizations remain unclear, including whether the bands respond similarly or differentially to PF input. By using flavoprotein imaging in the anesthetized mouse in vivo, this study demonstrates that high-frequency PF stimulation, which activates a beamlike response at short latency, also evokes patches of activation at long latencies. These patches consist of increased fluorescence along the beam at latencies of 20-25 s with peak activation at 35 s. The long-latency patches are completely blocked by the type 1 metabotropic glutamate receptor (mGluR(1)) antagonist LY367385. Conversely, the AMPA and NMDA glutamate receptor antagonists DNQX and APV have little effect. Organized in parasagittal bands, the long-latency patches align with zebrin II-positive PC stripes. Additional Ca(2+) imaging demonstrates that the patches reflect increases in intracellular Ca(2+). Both the PLCβ inhibitor U73122 and the ryanodine receptor inhibitor ryanodine completely block the long-latency patches, indicating that the patches are due to Ca(2+) release from intracellular stores. Robust, mGluR(1)-dependent long-term potentiation (LTP) of the patches is induced using a high-frequency PF stimulation conditioning paradigm that generates LTP of PF-PC synapses. Therefore, the parasagittal bands, as defined by the molecular compartmentalization of PCs, respond differentially to PF inputs via mGluR(1)-mediated release of internal Ca(2+).

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Developments in RNA splicing and disease

Pre-mRNA processing, including 5'-end capping, splicing, editing, and polyadenylation, consists of a series of orchestrated and primarily cotranscriptional steps that ensure both the high fidelity and extreme diversity characteristic of eukaryotic gene expression. Alternative splicing and editing allow relatively small genomes to encode vast proteomic arrays while alternative 3'-end formation enables variations in mRNA localization, translation, and stability. Of course, this mechanistic complexity comes at a high price. Mutations in the myriad of RNA sequence elements that regulate mRNA biogenesis, as well as the trans-acting factors that act upon these sequences, underlie a number of human diseases. In this review, we focus on one of these key RNA processing steps, splicing, to highlight recent studies that describe both conventional and novel pathogenic mechanisms that underlie muscle and neurological diseases.

Modeling human trinucleotide repeat diseases in Drosophila

Drosophila is a powerful model system to study human trinucleotide repeat diseases. Findings in Drosophila models highlighted importance of host proteins, chaperons, and protein clearance pathways in polyglutamine diseases as well as that of RNA-binding proteins in noncoding repeat RNA toxicity diseases. Recent novel aspects revealed in Drosophila models include pleiotropic Ataxin 2 interactions, antisense transcription in trinucleotide repeat diseases, contribution of CAG RNA in polyglutamine diseases, and the role of RNA foci in CUG expansion diseases. Drosophila models have been also used for repeat stability studies.

Non-ATG-initiated translation directed by microsatellite expansions

Trinucleotide expansions cause disease by both protein- and RNA-mediated mechanisms. Unexpectedly, we discovered that CAG expansion constructs express homopolymeric polyglutamine, polyalanine, and polyserine proteins in the absence of an ATG start codon. This repeat-associated non-ATG translation (RAN translation) occurs across long, hairpin-forming repeats in transfected cells or when expansion constructs are integrated into the genome in lentiviral-transduced cells and brains. Additionally, we show that RAN translation across human spinocerebellar ataxia type 8 (SCA8) and myotonic dystrophy type 1 (DM1) CAG expansion transcripts results in the accumulation of SCA8 polyalanine and DM1 polyglutamine expansion proteins in previously established SCA8 and DM1 mouse models and human tissue. These results have implications for understanding fundamental mechanisms of gene expression. Moreover, these toxic, unexpected, homopolymeric proteins now should be considered in pathogenic models of microsatellite disorders.

Bidirectional transcription stimulates expansion and contraction of expanded (CTG)*(CAG) repeats

More than 12 neurogenetic disorders are caused by unstable expansions of (CTG)•(CAG) repeats. The expanded repeats are unstable in germline and somatic cells, with potential consequences for disease severity. Previous studies have shown that contractions of (CAG)(95) are more frequent when the repeat tract is transcribed. Here we determined whether transcription can promote repeat expansion, using (CTG)•(CAG) repeat tracts in the size range that is typical for myotonic dystrophy type 1. We derived normal human fibroblasts having single-copy genomic integrations of 800 CTG repeats. The repeat tract showed modest instability when it was not transcribed, yielding an estimated mutation rate of 0.28% per generation. Instability was enhanced several-fold by transcription in the forward or reverse transcription, and 30-fold by bidirectional transcription, yielding many expansions and contractions of more than 200 repeats. These results suggest that convergent bidirectional transcription, which has been reported at several disease loci, could contribute to somatic instability of highly expanded (CTG)•(CAG) repeats.

Determinants of R-loop formation at convergent bidirectionally transcribed trinucleotide repeats

R-loops have been described at immunoglobulin class switch sequences, prokaryotic and mitochondrial replication origins, and disease-associated (CAG)n and (GAA)n trinucleotide repeats. The determinants of trinucleotide R-loop formation are unclear. Trinucleotide repeat expansions cause diseases including DM1 (CTG)n, SCA1 (CAG)n, FRAXA (CGG)n, FRAXE (CCG)n and FRDA (GAA)n. Bidirectional convergent transcription across these disease repeats can occur. We find R-loops formed when CTG or CGG and their complementary strands CAG or CCG were transcribed; GAA transcription, but not TTC, yielded R-loops. R-loop formation was sensitive to DNA supercoiling, repeat length, insensitive to repeat interruptions, and formed by extension of RNA:DNA hybrids in the RNA polymerase. R-loops arose by transcription in one direction followed by transcription in the opposite direction, and during simultaneous convergent bidirectional transcription of the same repeat forming double R-loop structures. Since each transcribed disease repeat formed R-loops suggests they may have biological functions.

Convergent transcription through a long CAG tract destabilizes repeats and induces apoptosis

Short repetitive sequences are common in the human genome, and many fall within transcription units. We have previously shown that transcription through CAG repeat tracts destabilizes them in a way that depends on transcription-coupled nucleotide excision repair and mismatch repair. Recent observations that antisense transcription accompanies sense transcription in many human genes led us to test the effects of antisense transcription on triplet repeat instability in human cells. Here, we report that simultaneous sense and antisense transcription (convergent transcription) initiated from two inducible promoters flanking a CAG95 tract in a nonessential gene enhances repeat instability synergistically, arrests the cell cycle, and causes massive cell death via apoptosis. Using chemical inhibitors and small interfering RNA (siRNA) knockdowns, we identified the ATR (ataxia-telangiectasia mutated [ATM] and Rad3 related) signaling pathway as a key mediator of this cellular response. RNA polymerase II, replication protein A (RPA), and components of the ATR signaling pathway accumulate at convergently transcribed repeat tracts, accompanied by phosphorylation of ATR, CHK1, and p53. Cell death depends on simultaneous sense and antisense transcription and is proportional to their relative levels, it requires the presence of the repeat tract, and it occurs in both proliferating and nonproliferating cells. Convergent transcription through a CAG repeat represents a novel mechanism for triggering a cellular stress response, one that is initiated by events at a single locus in the genome and resembles the response to DNA damage.

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.

A variant of the KLK4 gene is expressed as a cis sense-antisense chimeric transcript in prostate cancer cells

In humans, more than 30,000 chimeric transcripts originating from 23,686 genes have been identified. The mechanisms and association of chimeric transcripts arising from chromosomal rearrangements with cancer are well established, but much remains unknown regarding the biogenesis and importance of other chimeric transcripts that arise from nongenomic alterations. Recently, a SLC45A3-ELK4 chimera has been shown to be androgen-regulated, and is overexpressed in metastatic or high-grade prostate tumors relative to local prostate cancers. Here, we characterize the expression of a KLK4 cis sense-antisense chimeric transcript, and show other examples in prostate cancer. Using non-protein-coding microarray analyses, we initially identified an androgen-regulated antisense transcript within the 3' untranslated region of the KLK4 gene in LNCaP cells. The KLK4 cis-NAT was validated by strand-specific linker-mediated RT-PCR and Northern blotting. Characterization of the KLK4 cis-NAT by 5' and 3' rapid amplification of cDNA ends (RACE) revealed that this transcript forms multiple fusions with the KLK4 sense transcript. Lack of KLK4 antisense promoter activity using reporter assays suggests that these transcripts are unlikely to arise from a trans-splicing mechanism. 5' RACE and analyses of deep sequencing data from LNCaP cells treated +/-androgens revealed six high-confidence sense-antisense chimeras of which three were supported by the cDNA databases. In this study, we have shown complex gene expression at the KLK4 locus that might be a hallmark of cis sense-antisense chimeric transcription.

Roles of trinucleotide-repeat RNA in neurological disease and degeneration

A large number of human diseases are caused by expansion of repeat sequences - typically trinucleotide repeats - within the respective disease genes. The abnormally expanded sequence can lead to a variety of effects on the gene: sometimes the gene is silenced, but in many cases the expanded repeat sequences confer toxicity to the mRNA and, in the case of polyglutamine diseases, to the encoded protein. This article highlights mechanisms by which the mRNAs with abnormally expanded repeats can confer toxicity leading to neuronal dysfunction and loss. Greater understanding of these mechanisms will provide the foundation for therapeutic advances for this set of human disorders.

Long non-coding RNAs in nervous system function and disease

Central nervous system (CNS) development, homeostasis, stress responses, and plasticity are all mediated by epigenetic mechanisms that modulate gene expression and promote selective deployment of functional gene networks in response to complex profiles of interoceptive and environmental signals. Thus, not surprisingly, disruptions of these epigenetic processes are implicated in the pathogenesis of a spectrum of neurological and psychiatric diseases. Epigenetic mechanisms involve chromatin remodeling by relatively generic complexes that catalyze DNA methylation and various types of histone modifications. There is increasing evidence that these complexes are directed to their sites of action by long non-protein-coding RNAs (lncRNAs), of which there are tens if not hundreds of thousands specified in the genome. LncRNAs are transcribed in complex intergenic, overlapping and antisense patterns relative to adjacent protein-coding genes, suggesting that many lncRNAs regulate the expression of these genes. LncRNAs also participate in a wide array of subcellular processes, including the formation and function of cellular organelles. Most lncRNAs are transcribed in a developmentally regulated and cell type specific manner, particularly in the CNS, wherein over half of all lncRNAs are expressed. While the numerous biological functions of lncRNAs are yet to be characterized fully, a number of recent studies suggest that lnRNAs are important for mediating cell identity. This function seems to be especially important for generating the enormous array of regional neuronal and glial cell subtypes that are present in the CNS. Further studies have also begun to elucidate additional roles played by lncRNAs in CNS processes, including homeostasis, stress responses and plasticity. Herein, we review emerging evidence that highlights the expression and function of lncRNAs in the CNS and suggests that lncRNA deregulation is an important factor in various CNS pathologies including neurodevelopmental, neurodegenerative and neuroimmunological disorders, primary brain tumors, and psychiatric diseases.

Partners in crime: bidirectional transcription in unstable microsatellite disease

Nearly two decades have passed since the discovery that the expansion of microsatellite trinucleotide repeats is responsible for a prominent class of neurological disorders, including Huntington disease and fragile X syndrome. These hereditary diseases are characterized by genetic anticipation or the intergenerational increase in disease severity accompanied by a decrease in age-of-onset. The revelation that the variable expansion of simple sequence repeats accounted for anticipation spawned a number of pathogenesis models and a flurry of studies designed to reveal the molecular events affected by these expansions. This work led to our current understanding that expansions in protein-coding regions result in extended homopolymeric amino acid tracts, often polyglutamine or polyQ, and deleterious protein gain-of-function effects. In contrast, expansions in noncoding regions cause RNA-mediated toxicity. However, the realization that the transcriptome is considerably more complex than previously imagined, as well as the emerging regulatory importance of antisense RNAs, has blurred this distinction. In this review, we summarize evidence for bidirectional transcription of microsatellite disease genes and discuss recent suggestions that some repeat expansions produce variable levels of both toxic RNAs and proteins that influence cell viability, disease penetrance and pathological severity.

Repeat expansion disease: progress and puzzles in disease pathogenesis

Repeat expansion mutations cause at least 22 inherited neurological diseases. The complexity of repeat disease genetics and pathobiology has revealed unexpected shared themes and mechanistic pathways among the diseases, such as RNA toxicity. Also, investigation of the polyglutamine diseases has identified post-translational modification as a key step in the pathogenic cascade and has shown that the autophagy pathway has an important role in the degradation of misfolded proteins--two themes that are likely to be relevant to the entire neurodegeneration field. Insights from repeat disease research are catalysing new lines of study that should not only elucidate molecular mechanisms of disease but also highlight opportunities for therapeutic intervention for these currently untreatable disorders.

RNA-targeted splice-correction therapy for neuromuscular disease

Splice-modulation therapy, whereby molecular manipulation of premessenger RNA splicing is engineered to yield genetic correction, is a promising novel therapy for genetic diseases of muscle and nerve-the prototypical example being Duchenne muscular dystrophy. Duchenne muscular dystrophy is the most common childhood genetic disease, affecting one in 3500 newborn boys, causing progressive muscle weakness, heart and respiratory failure and premature death. No cure exists for this disease and a number of promising new molecular therapies are being intensively studied. Duchenne muscular dystrophy arises due to mutations that disrupt the open-reading-frame in the DMD gene leading to the absence of the essential muscle protein dystrophin. Of all novel molecular interventions currently being investigated for Duchenne muscular dystrophy, perhaps the most promising method aiming to restore dystrophin expression to diseased cells is known as 'exon skipping' or splice-modulation, whereby antisense oligonucleotides eliminate the deleterious effects of DMD mutations by modulating dystrophin pre-messenger RNA splicing, such that functional dystrophin protein is produced. Recently this method was shown to be promising and safe in clinical trials both in The Netherlands and the UK. These trials studied direct antisense oligonucleotide injections into single peripheral lower limb muscles, whereas a viable therapy will need antisense oligonucleotides to be delivered systemically to all muscles, most critically to the heart, and ultimately to all other affected tissues including brain. There has also been considerable progress in understanding how such splice-correction methods could be applied to the treatment of related neuromuscular diseases, including spinal muscular atrophy and myotonic dystrophy, where defects of splicing or alternative splicing are closely related to the disease mechanism.

Recent advances in the manipulation of murine gene expression and its utility for the study of human neurological disease

Transgenic mouse models have vastly contributed to our knowledge of the genetic and molecular pathways underlying the pathogenesis of neurological disorders that affect millions of people worldwide. Not only have they allowed the generation of disease models mimicking the human pathological state but they have also permitted the exploration of the pathological role of specific genes through the generation of knock-out and knock-in models. Classical constitutive transgenic mice have several limitations however, due to behavioral adaptation process occurring and conditional mouse models are time-consuming and often lack extensive spatial or temporal control of gene manipulation. These limitations could be overcome by means of innovative methods that are now available such as RNAi, viral vectors and large cloning DNA vectors. These tools have been extensively used for the generation of mouse models and are characterized by the superior control of transgene expression that has been proven invaluable in the assessment of novel treatments for neurological diseases and to further investigate the molecular processes underlying the etiopathology of neurological disorders. Furthermore, in association with classical transgenic mouse models, they have allowed the validation of innovative therapeutic strategies for the treatment of human neurological disorders. This review describes how these tools have overcome the limitations of classical transgenic mouse models and how they have been of value for the study of human neurological diseases.

The spinocerebellar ataxias

Slowly progressive ataxia accompanied by cerebellar degeneration is often genetic in origin. The past 15 years have witnessed a revolution in our understanding of the causes of dominantly inherited ataxias, now known as the spinocerebellar ataxias (SCAs). Nearly 30 distinct genetic causes of SCA are known, numbered chronologically in order of discovery. All SCAs display classic cerebellar signs, and many display disabling noncerebellar features, most commonly brainstem dysfunction. Eye movement abnormalities are common, reflecting cerebellar and brainstem degeneration. Visual loss from retinal degeneration is rare in SCA, occurring most commonly and profoundly in SCA7. Although the SCAs are relentlessly progressive and currently untreatable, recent scientific advances have begun to shed light on various disease mechanisms that may lead to preventive therapies.

Animal models of human cerebellar ataxias: a cornerstone for the therapies of the twenty-first century

Cerebellar ataxias represent a group of disabling neurological disorders. Our understanding of the pathogenesis of cerebellar ataxias is continuously expanding. A considerable number of laboratory animals with neurological mutations have been reported and numerous relevant animal models mimicking the phenotype of cerebellar ataxias are becoming available. These models greatly help dissecting the numerous mechanisms of cerebellar dysfunction, a major step for the assessment of therapeutics targeting a given deleterious pathway and for the screening of old or newly synthesized chemical compounds. Nevertheless, differences between animal models and human disorders should not be overlooked and difficulties in terms of characterization should not be occulted. The identification of the mutations of many hereditary ataxias, the development of valuable animal models, and the recent identifications of the molecular mechanisms underlying cerebellar disorders represent a combination of key factors for the development of anti-ataxic innovative therapies. It is anticipated that the twenty-first century will be the century of effective therapies in the field of cerebellar ataxias. The animal models are a cornerstone to reach this goal.

Spinocerebellar ataxia type 31 is associated with "inserted" penta-nucleotide repeats containing (TGGAA)n

Spinocerebellar ataxia type 31 (SCA31) is an adult-onset autosomal-dominant neurodegenerative disorder showing progressive cerebellar ataxia mainly affecting Purkinje cells. The SCA31 critical region was tracked down to a 900 kb interval in chromosome 16q22.1, where the disease shows a strong founder effect. By performing comprehensive Southern blot analysis and BAC- and fosmid-based sequencing, we isolated two genetic changes segregating with SCA31. One was a single-nucleotide change in an intron of the thymidine kinase 2 gene (TK2). However, this did not appear to affect splicing or expression patterns. The other was an insertion, from 2.5-3.8 kb long, consisting of complex penta-nucleotide repeats including a long (TGGAA)n stretch. In controls, shorter (1.5-2.0 kb) insertions lacking (TGGAA)n were found only rarely. The SCA31 repeat insertion's length inversely correlated with patient age of onset, and an expansion was documented in a single family showing anticipation. The repeat insertion was located in introns of TK2 and BEAN (brain expressed, associated with Nedd4) expressed in the brain and formed RNA foci in the nuclei of patients' Purkinje cells. An electrophoretic mobility-shift assay showed that essential splicing factors, serine/arginine-rich splicing factors SFRS1 and SFRS9, bind to (UGGAA)n in vitro. Because (TGGAA)n is a characteristic sequence of paracentromeric heterochromatin, we speculate that the insertion might have originated from heterochromatin. SCA31 is important because it exemplifies human diseases associated with "inserted" microsatellite repeats that can expand through transmission. Our finding suggests that the ectopic microsatellite repeat, when transcribed, might cause a disease involving the essential splicing factors.

Machado-Joseph disease/SCA3 and myotonic dystrophy type 1 in a single patient

We report here, for the first time, the case of a 41-year-old man with both Machado-Joseph disease (MJD)/spinocerebellar ataxia type 3 (SCA3) and myotonic dystrophy type 1. The patient noted dysarthria at 14 years of age and unsteady gait at 30 years of age. Similar sized expansions of the CAG trinucleotide repeats in one allele of the ataxin-3 (ATXN3) gene were found in both the patient and his father, although in the other allele the length of the CAG repeats was shorter in the father compared with the patient. In the dystrophia myotonica protein kinase (DMPK) gene the CTG repeats were much more expanded in the patient compared with his father. Thus it is possible that, in the farther, the short CAG repeat in the non-expanded ATXN3 allele delayed the onset of cerebellar symptoms, and/or that the expanded CTG repeat in the DMPK gene in the patient accelerated the pathogenesis of MJD/SCA3.

Altered RNA splicing contributes to skeletal muscle pathology in Kennedy disease knock-in mice

Here, we used a mouse model of Kennedy disease, a degenerative disorder caused by an expanded CAG repeat in the androgen receptor (AR) gene, to explore pathways leading to cellular dysfunction. We demonstrate that male mice containing a targeted Ar allele with 113 CAG repeats (AR113Q mice) exhibit hormone- and glutamine length-dependent missplicing of Clcn1 RNA in skeletal muscle. Changes in RNA splicing are associated with increased expression of the RNA-binding protein CUGBP1. Furthermore, we show that skeletal muscle denervation in the absence of a repeat expansion leads to increased CUGBP1 expression. However, this induction of CUGBP1 is not sufficient to alter Clcn1 RNA splicing, indicating that changes mediated by both denervation and AR113Q toxicity contribute to altered RNA processing. To test this notion directly, we exogenously expressed the AR in vitro and observed hormone-dependent changes in the splicing of pre-mRNAs from a human cardiac troponin T minigene. These effects were notably similar to changes mediated by RNA with expanded CUG tracts, but not CAG tracts, highlighting unanticipated similarities between CAG and CUG repeat diseases. The expanded glutamine AR also altered hormone-dependent splicing of a calcitonin/calcitonin gene-related peptide minigene, suggesting that toxicity of the mutant protein additionally affects RNA processing pathways that are distinct from those regulated by CUGBP1. Our studies demonstrate the occurrence of hormone-dependent alterations in RNA splicing in Kennedy disease models, and they indicate that these changes are mediated by both the cell-autonomous effects of the expanded glutamine AR protein and by alterations in skeletal muscle that are secondary to denervation.

RNA gain-of-function in spinocerebellar ataxia type 8

Microsatellite expansions cause a number of dominantly-inherited neurological diseases. Expansions in coding-regions cause protein gain-of-function effects, while non-coding expansions produce toxic RNAs that alter RNA splicing activities of MBNL and CELF proteins. Bi-directional expression of the spinocerebellar ataxia type 8 (SCA8) CTG CAG expansion produces CUG expansion RNAs (CUG^exp) from the ATXN8OS gene and a nearly pure polyglutamine expansion protein encoded by ATXN8 CAG^exp transcripts expressed in the opposite direction. Here, we present three lines of evidence that RNA gain-of-function plays a significant role in SCA8: 1) CUG^exp transcripts accumulate as ribonuclear inclusions that co-localize with MBNL1 in selected neurons in the brain; 2) loss of Mbnl1 enhances motor deficits in SCA8 mice; 3) SCA8 CUG^exp transcripts trigger splicing changes and increased expression of the CUGBP1-MBNL1 regulated CNS target, GABA-A transporter 4 (GAT4/Gabt4). In vivo optical imaging studies in SCA8 mice confirm that Gabt4 upregulation is associated with the predicted loss of GABAergic inhibition within the granular cell layer. These data demonstrate that CUG^exp transcripts dysregulate MBNL/CELF regulated pathways in the brain and provide mechanistic insight into the CNS effects of other CUG^exp disorders. Moreover, our demonstration that relatively short CUG^exp transcripts cause RNA gain-of-function effects and the growing number of antisense transcripts recently reported in mammalian genomes suggest unrecognized toxic RNAs contribute to the pathophysiology of polyglutamine CAG CTG disorders.

We describe several lines of evidence that RNA gain-of-function effects play a significant role in spinocerebellar ataxia type 8 (SCA8) and has broader implications for understanding the CNS effects of other trinucleotide expansion disorders including myotonic dystrophy type 1, Huntington disease like-2, and spinocerebellar ataxia type 7. The SCA8 mutation is bidirectionally transcribed resulting in the expression of CUG^exp transcripts from ATXN8OS and CAG^exp transcripts and polyglutamine protein from the overlapping ATXN8 gene. These data suggest that SCA8 pathogenesis involves toxic gain-of-function effects at the RNA (CUG^exp) and/or protein (PolyQ) levels. We present three lines of evidence that CUG^exp transcripts play a significant role in SCA8: 1) CUG^exp transcripts accumulate as ribonuclear inclusions that co-localize with MBNL1 in selected neurons; 2) loss of Mbnl1 enhances motor deficits in SCA8 mice; 3) SCA8 CUG^exp transcripts trigger alternative splicing changes and increased expression of the CUGBP1-MBNL1 regulated CNS target, GABA-A transporter 4 (GAT4/Gabt4) which is associated with the predicted loss of GABAergic inhibition within the granular cell layer in SCA8 mice. Additionally, alternative splicing changes and GAT4 upregulation are induced by CUG^exp but not CAG^exp transcripts. From a therapeutic viewpoint, it is promising that this change is reversed in cells overexpressing MBNL1.

Pathogenic RNAs in microsatellite expansion disease

The expansion of unstable microsatellites is the cause of a number of inherited neuromuscular and neurological disorders. While these expanded repeats can be located in either the coding or non-coding regions of genes, toxic RNA transcripts have been primarily implicated in the pathogenesis of non-coding expansion diseases. In this review, we briefly summarize studies which support this RNA-mediated toxicity model for several neurologic disorders and highlight how pathogenic RNAs might negatively impact nervous system functions. However, it is important to note that the distinction between coding versus non-coding regions has become muddled by recent observations that the transcribed portion of the genome or transcriptome is considerably larger than previously appreciated. Thus, we also explore the possibility that a combination of protein and RNA gain-of-function events underlie some microsatellite expansion diseases.

Reversal of RNA dominance by displacement of protein sequestered on triplet repeat RNA

Genomic expansions of simple tandem repeats can give rise to toxic RNAs that contain expanded repeats. In myotonic dystrophy, the expression of expanded CUG repeats (CUGexp) causes abnormal regulation of alternative splicing and neuromuscular dysfunction. We used a transgenic mouse model to show that derangements of myotonic dystrophy are reversed by a morpholino antisense oligonucleotide, CAG25, that binds to CUGexp RNA and blocks its interaction with muscleblind-like 1 (MBNL1), a CUGexp-binding protein. CAG25 disperses nuclear foci of CUGexp RNA and reduces the overall burden of this toxic RNA. As MBNL1 is released from sequestration, the defect of alternative splicing regulation is corrected, thereby restoring ion channel function. These findings suggest an alternative use of antisense methods, to inhibit deleterious interactions of proteins with pathogenic RNAs.

The pathogenic mechanisms of polyglutamine diseases and current therapeutic strategies

Expansion of CAG trinucleotide repeat within the coding region of several genes results in the production of proteins with expanded polyglutamine (PolyQ) stretch. The expression of these pathogenic proteins leads to PolyQ diseases, such as Huntington's disease or several types of spinocerebellar ataxias. This family of neurodegenerative disorders is characterized by constant progression of the symptoms and molecularly, by the accumulation of mutant proteins inside neurons causing their dysfunction and eventually death. So far, no effective therapy actually preventing the physical and/or mental decline has been developed. Experimental therapeutic strategies either target the levels or processing of mutant proteins in an attempt to prevent cellular deterioration, or they are aimed at the downstream pathologic effects to reverse or ameliorate the caused damages. Certain pathomechanistic aspects of PolyQ disorders are discussed here. Relevance of disease models and recent knowledge of therapeutic possibilities is reviewed and updated.

Spinocerebellar ataxia 8: variable phenotype and unique pathogenesis

Spinocerebellar ataxia 8 (SCA8), a triplet repeat expansion disorder, is genetically distinct from the other inherited ataxias, but its unusually variable phenotype can make its diagnosis difficult. In this review we describe 3 new cases of genetically verified SCA8 to highlight the broad clinical spectrum of symptoms observed with this disorder and to draw attention to the features of myoclonus and migraine headaches, which in the context of cerebellar ataxia warrants the clinician to consider SCA8 as a potential diagnosis. We also address the controversy surrounding the genetic testing approach for diagnosing SCA8. Finally, we evaluate the evidence that SCA8 may affect calcium channel function and that the presentation of episodic ataxia and migraines suggests a clinical and pathogenic overlap of SCA8 with the channelopathies.

Instability and chromatin structure of expanded trinucleotide repeats

Trinucleotide repeat expansion underlies at least 17 neurological diseases. In affected individuals, the expanded locus is characterized by dramatic changes in chromatin structure and in repeat tract length. Interestingly, recent studies show that several chromatin modifiers, including a histone acetyltransferase, a DNA methyltransferase and the chromatin insulator CTCF can modulate repeat instability. Here, we propose that the unusual chromatin structure of expanded repeats directly impacts their instability. We discuss several potential models for how this might occur, including a role for DNA repair-dependent epigenetic reprogramming in increasing repeat instability, and the capacity of epigenetic marks to alter sense and antisense transcription, thereby affecting repeat instability.

Bidirectional expression of the SCA8 expansion mutation: one mutation, two genes

Spinocerebellar ataxia type 8 (SCA8) is a dominantly inherited, slowly progressive neurodegenerative disorder caused by a CTG.CAG repeat expansion located on chromosome 13q21. The expansion mutation was isolated directly from the DNA of a single patient using RAPID cloning and subsequently shown to co-segregate with disease in additional ataxia families including a seven-generation kindred (the MN-A family). The size-dependent penetrance of the repeat found in the large MN-A kindred makes it appear as though some parts of the family have a dominant disorder while other parts of this same family have recessive or sporadic forms of ataxia. While the linkage and size-dependent penetrance of the SCA8 CTG.CAG expansion in the MN-A family argue that the SCA8 expansion causes ataxia, the reduced penetrance in other SCA8 families and the discovery of expansions in the general population have led to a controversy surrounding whether or not the SCA8 expansion is pathogenic. A recently reported mouse model in which SCA8 BAC-expansion but not BAC-control lines develop a progressive neurological phenotype now demonstrates the pathogenicity of the (CTG.CAG)(n) expansion. These mice show a loss of cerebellar GABAergic inhibition and, similar to human patients, have 1C2-positive intranuclear inclusions in Purkinje cells and other neurons. Additional studies demonstrate that the SCA8 expansion is expressed in both directions (CUG and CAG) and that a novel gene expressed in the CAG direction encodes a pure polyglutamine expansion protein (ataxin 8, ATXN8). Moreover, the expression of non-coding (CUG)(n) expansion transcripts (ataxin 8 opposite strand, ATXN8OS) and the discovery of intranuclear polyglutamine inclusions suggest SCA8 pathogenesis may involve toxic gain-of-function mechanisms at both the protein and RNA levels. Our data, combined with the recently reported antisense transcripts spanning the DM1 repeat expansion in the CAG direction and the growing number of reports of antisense transcripts expressed throughout the mammalian genome, raises the possibility that bidirectional expression across pathogenic microsatellite expansions may occur in other expansion disorders, and that potential pathogenic effects of mutations expressed from both strands should be considered.

Transcription destabilizes triplet repeats

Triplet repeat expansion is the molecular basis for several human diseases. Intensive studies using systems in bacteria, yeast, flies, mammalian cells, and mice have provided important insights into the molecular processes that are responsible for mediating repeat instability. The age-dependent, ongoing repeat instability in somatic tissues, especially in terminally differentiated neurons, strongly suggests a robust role for pathways that are independent of DNA replication. Several genetic studies have indicated that transcription can play a critical role in repeat instability, potentially providing a basis for the instability observed in neurons. Transcription-induced repeat instability can be modulated by several DNA repair proteins, including those involved in mismatch repair (MMR) and transcription-coupled nucleotide excision repair (TC-NER). Though the mechanism is unclear, it is likely that transcription facilitates the formation of repeat-specific secondary structures, which act as intermediates to trigger DNA repair, eventually leading to changes in the length of the repeat tract. In addition, other processes associated with transcription can also modulate repeat instability, as shown in a variety of different systems. Overall, the mechanisms underlying repeat instability in humans are unexpectedly complicated. Because repeat-disease genes are widely expressed, transcription undoubtedly contributes to the repeat instability observed in many diseases, but it may be especially important in nondividing cells. Transcription-induced instability is likely to involve an extensive interplay not only of the core transcription machinery and DNA repair proteins, but also of proteins involved in chromatin remodeling, regulation of supercoiling, and removal of stalled RNA polymerases, as well as local DNA sequence effects.

Emerging pathogenic pathways in the spinocerebellar ataxias

The spinocerebellar ataxias (SCAs) are diseases characterized by neurodegeneration of the spinocerebellum. To date, 28 autosomal dominant SCAs have been described and seventeen causative genes identified. These genes play a role in a broad range of cellular processes. Recent studies focused on the wild type and pathogenic functions of these genes implicate both gene expression and glutamate-dependent and calcium-dependent neuronal signaling as important pathways leading to cerebellar dysfunction. Understanding how these genes cause disease will allow a deeper understanding of the cerebellum in particular as well as neurodegenerative disease in general.

Evolution and functions of long noncoding RNAs

RNA is not only a messenger operating between DNA and protein. Transcription of essentially the entire eukaryotic genome generates a myriad of non-protein-coding RNA species that show complex overlapping patterns of expression and regulation. Although long noncoding RNAs (lncRNAs) are among the least well-understood of these transcript species, they cannot all be dismissed as merely transcriptional "noise." Here, we review the evolution of lncRNAs and their roles in transcriptional regulation, epigenetic gene regulation, and disease.

SCA8 repeat expansion: large CTA/CTG repeat alleles in neurological disorders and functional implications

Spinocerebellar ataxia type 8 (SCA8) involves bidirectional expression of CUG (ATXN8OS) and CAG (ATXN8) expansion transcripts. The pathogenesis of SCA8 is complex and the spectrum of clinical presentations is broad. In the present study, we assessed the SCA8 repeat size ranges in Taiwanese Parkinson's disease, Alzheimer's disease and atypical parkinsonism and investigated the genetic variation modulating ATXN8 expression. Thirteen large SCA8 alleles and a novel ATXN8 -62 G/A promoter SNP were found. There is a significant difference in the proportion of the individuals carrying SCA8 larger alleles in atypical parkinsonism (P = 0.044) as compared to that in the control subjects. In lymphoblastoid cells carrying SCA8 large alleles, treatment of MG-132 or staurosporine significantly increases the cell death or caspase 3 activity. Although expressed at low steady-state, ATXN8 expression level is significantly higher (P = 0.012) in cells with SCA8 large alleles than that of the control cells. The ATXN8 transcriptional activity was significantly higher in the luciferase reporter construct containing the -62G allele than that containing the -62A allele in both neuroblastoma and embryonic kidney cells. Therefore, our preliminary results suggest that ATXN8 gene -62 G/A polymorphism may be functional in modulating ATXN8 expression.

Spinocerebellar ataxia type 8 larger triplet expansion alters histone modification and induces RNA foci

BACKGROUND

Spinocerebellar ataxia type 8 (SCA8) involves the expression of an expanded CTG/CAG combined repeats (CR) from opposite strands producing CUG expansion transcripts (ataxin 8 opposite strand, ATXN8OS) and a polyglutamine expansion protein (ataxin 8, ATXN8). The pathogenesis of SCA8 is complex and the spectrum of clinical presentations is broad.

RESULTS

Using stably induced cell models expressing 0, 23, 88 and 157 CR, we study the role of ATXN8OS transcripts in SCA8 pathogenesis. In the absence of doxycycline, the stable ATXN8OS CR cell lines exhibit low levels of ATXN8OS expression and a repeat length-related increase in staurosporine sensitivity and in the number of annexin positive cells. A repeat length-dependent repression of ATXN8OS expression was also notable. Addition of doxycycline leads to 25 approximately 50 times more ATXN8OS RNA expression with a repeat length-dependent increase in fold of ATXN8OS RNA induction. ChIP-PCR assay using anti-dimethyl-histone H3-K9 and anti-acetyl-histone H3-K14 antibodies revealed increased H3-K9 dimethylation and reduced H3-K14 acetylation around the ATXN8OS cDNA gene in 157 CR line. The repeat length-dependent increase in induction fold is probably due to the increased RNA stability as demonstrated by monitoring ATXN8OS RNA decay in cells treated with the transcriptional inhibitor, actinomycin D. In cells stably expressing ATXN8OS, RNA FISH experiments further revealed ribonuclear foci formation in cells carrying expanded 88 and 157 CR.

CONCLUSION

The present study demonstrates that the expanded CUG-repeat tracts are toxic to human cells and may affect ATXN8OS RNA expression and stability through epigenetic and post-transcriptional mechanisms.

Mechanisms of RNA-mediated disease

Recent mapping of functional sequence elements in the human genome has led to the realization that transcription is pervasive and that noncoding RNAs compose a significant portion of the transcriptome. Some dominantly inherited neurological disorders are associated with the expansion of microsatellite repeats in noncoding regions that result in the synthesis of pathogenic RNAs. Here, we review RNA gain-of-function mechanisms underlying three of these microsatellite expansion disorders to illustrate how some mutant RNAs cause disease.

Neurodegeneration: RNA turns number one suspect in polyglutamine diseases

Polyglutamine expansion diseases are triggered by the accumulation of toxic proteins. A new study reports that RNA molecules containing long CAG repeats can also be toxic to neurons and may play a significant role in pathogenesis.

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.

SCA8 mRNA expression suggests an antisense regulation of KLHL1 and correlates to SCA8 pathology

An increasing number of inherited neurodegenerative diseases are known to be caused by the expansion of unstable trinucleotide repeat tracts. Spinocerebellar ataxia type 8 (SCA8) has been identified as being partly caused by a CTG expansion in an untranslated, endogenous antisense RNA that overlaps the Kelch-like 1 (KLHL1) gene. Clinically, SCA8 patients show similar features to those with the other SCAs, including limb and truncal ataxia, ataxic dysarthria and horizontal nystagmus, all of which are signs of dysfunction of the cerebellar system. However, allele sizes within the SCA8 proposed pathogenic range have been reported in patients with ataxia of unknown etiology, in individuals from pedigrees with other SCA or Friedreich's ataxia, and in patients with Alzheimer's disease, schizophrenia or parkinsonism. These observations suggest that mutation of the SCA8 locus might affect neurons other than the cerebellum. Antisense transcripts are known to regulate complementary sense transcripts and are involved in several biologic functions, such as development, adaptive response, and viral infection. In order to test whether SCA8 affects the KLHL1 expression by antisense RNA in brain cells, we examined the expression pattern of KLHL1 and SCA8 in human tissues and in mouse brain regions. SCA8 expression was colocalized with KLHL1 transcript in many brain regions whose functions are correlated to the clinical symptoms of SCA8 patients. These findings lead to the hypothesis of a possible relevance that SCA8 transcript downregulates KLHL1 expression through an antisense mechanism, which then leads to SCA8 neuropathogenesis.

A comparison of huntington disease and huntington disease-like 2 neuropathology

Huntington disease-like 2 (HDL2) is an autosomal dominant disorder characterized by adult-onset, progressive motor abnormalities, psychiatric disturbances, and dementia ending in premature death. Clinically, it most closely resembles Huntington disease (HD), although a subset of affected individuals have parkinsonian features. Here, we systematically compare 5 HDL2 and 5 HD brains with the hypothesis that, reflecting the clinical presentation, the neuropathology of the 2 diseases would be similar. Gross and microscopic examination revealed prominent striatal neuron loss and astrocytic gliosis in a dorsal to ventral gradient in each disorder and cortical atrophy. Nuclear protein aggregates were as common in HDL2 as in HD, and the ultrastructural features of HDL2 and HD aggregates were similar. Electron microscopy also revealed degenerating neurons, some with evidence of autophagy, in both HDL2 and HD. Small ribonuclear foci, previously associated with potentially neurotoxic RNA transcripts in HDL2, rarely colocalized with protein aggregates in HDL2 brain, although the protein aggregates were stained by anti-TATA-box binding protein antibodies. Overall, the neuropathologic features of HDL2 and HD are very similar but not identical, suggesting that the pathogenetic mechanisms of the 2 diseases may partially overlap.

Mutagenic roles of DNA "repair" proteins in antibody diversity and disease-associated trinucleotide repeat instability

While DNA repair proteins are generally thought to maintain the integrity of the whole genome by correctly repairing mutagenic DNA intermediates, there are cases where DNA "repair" proteins are involved in causing mutations instead. For instance, somatic hypermutation (SHM) and class switch recombination (CSR) require the contribution of various DNA repair proteins, including UNG, MSH2 and MSH6 to mutate certain regions of immunoglobulin genes in order to generate antibodies of increased antigen affinity and altered effector functions. Another instance where "repair" proteins drive mutations is the instability of gene-specific trinucleotide repeats (TNR), the causative mutations of numerous diseases including Fragile X mental retardation syndrome (FRAXA), Huntington's disease (HD), myotonic dystrophy (DM1) and several spinocerebellar ataxias (SCAs) all of which arise via various modes of pathogenesis. These healthy and deleterious mutations that are induced by repair proteins are distinct from the genome-wide mutations that arise in the absence of repair proteins: they occur at specific loci, are sensitive to cis-elements (sequence context and/or epigenetic marks) and transcription, occur in specific tissues during distinct developmental windows, and are age-dependent. Here we review and compare the mutagenic role of DNA "repair" proteins in the processes of SHM, CSR and TNR instability.

RNA toxicity is a component of ataxin-3 degeneration in Drosophila

Polyglutamine (polyQ) diseases are a class of dominantly inherited neurodegenerative disorders caused by the expansion of a CAG repeat encoding glutamine within the coding region of the respective genes. The molecular and cellular pathways underlying polyQ-induced neurodegeneration are the focus of much research, and it is widely considered that toxic activities of the protein, resulting from the abnormally long polyQ tract, cause pathogenesis. Here we provide evidence for a pathogenic role of the CAG repeat RNA in polyQ toxicity using Drosophila. In a Drosophila screen for modifiers of polyQ degeneration induced by the spinocerebellar ataxia type 3 (SCA3) protein ataxin-3, we isolated an upregulation allele of muscleblind (mbl), a gene implicated in the RNA toxicity of CUG expansion diseases. Further analysis indicated that there may be a toxic role of the RNA in polyQ-induced degeneration. We tested the role of the RNA by altering the CAG repeat sequence to an interrupted CAACAG repeat within the polyQ-encoding region; this dramatically mitigated toxicity. In addition, expression of an untranslated CAG repeat of pathogenic length conferred neuronal degeneration. These studies reveal a role for the RNA in polyQ toxicity, highlighting common components in RNA-based and polyQ-protein-based trinucleotide repeat expansion diseases.

A subgenomic segment of Theiler's murine encephalomyelitis virus RNA causes demyelination

The DA strain of Theiler's murine encephalomyelitis virus (TMEV) causes a persistent central nervous system (CNS) infection of mice with a restricted virus gene expression and induces an inflammatory demyelinating disease that is thought to be immune mediated and a model of multiple sclerosis (MS). The relative contribution of virus vis-à-vis the immune system in the pathogenesis of DA-induced white matter disease remains unclear, as is also true in MS. To clarify the pathogenesis of DA-induced demyelination, we used Cre/loxP technology to generate a transgenic mouse that has tamoxifen (Tm)-inducible expression of a subgenomic segment of DA RNA in oligodendrocytes and Schwann cells. Tm-treated young transgenic mice developed progressive weakness leading to death, with abnormalities of oligodendrocytes and Schwann cells and demyelination, but without inflammation, demonstrating that DA virus can play a direct pathogenic role in demyelination. Tm treatment of mice at a later age resulted in milder disease, with evidence of peripheral nerve remyelination and focal fur depigmentation; surviving weak mice had persistent expression of the recombined transgene in the CNS, suggesting that the DA subgenomic segment can cause cellular dysfunction but not death, possibly similar to the situation seen during DA virus persistence. These studies demonstrate that DA RNA or a DA protein(s) is toxic to myelin-synthesizing cells. This Cre/loxP transgenic system allows for spatially and temporally controlled expression of the viral transgene and is valuable for clarifying nonimmune (and immune) mechanisms of demyelination induced by TMEV as well as other viruses.