Peter K. Todd

Autophagy and the ubiquitin-proteasome system: collaborators in neuroprotection

Protein degradation is an essential cellular function that, when dysregulated or impaired, can lead to a wide variety of disease states. The two major intracellular protein degradation systems are the ubiquitin-proteasome system (UPS) and autophagy, a catabolic process that involves delivery of cellular components to the lysosome for degradation. While the UPS has garnered much attention as it relates to neurodegenerative disease, important links between autophagy and neurodegeneration have also become evident. Furthermore, recent studies have revealed interaction between the UPS and autophagy, suggesting a coordinated and complementary relationship between these degradation systems that becomes critical in times of cellular stress. Here we describe autophagy and review evidence implicating this system as an important player in the pathogenesis of neurodegenerative disease. We discuss the role of autophagy in neurodegeneration and review its neuroprotective functions as revealed by experimental manipulation in disease models. Finally, we explore potential parallels and connections between autophagy and the UPS, highlighting their collaborative roles in protecting against neurodegenerative disease.

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. ANN NEUROL 2010;67:291–300

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.

Translation of Expanded CGG Repeats into FMRpolyG Is Pathogenic and May Contribute to Fragile X Tremor Ataxia Syndrome

Summary Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder caused by a limited expansion of CGG repeats in the 5′ UTR of FMR1. Two mechanisms are proposed to cause FXTAS: RNA gain-of-function, where CGG RNA sequesters specific proteins, and translation of CGG repeats into a polyglycine-containing protein, FMRpolyG. Here we developed transgenic mice expressing CGG repeat RNA with or without FMRpolyG. Expression of FMRpolyG is pathogenic, while the sole expression of CGG RNA is not. FMRpolyG interacts with the nuclear lamina protein LAP2β and disorganizes the nuclear lamina architecture in neurons differentiated from FXTAS iPS cells. Finally, expression of LAP2β rescues neuronal death induced by FMRpolyG. Overall, these results suggest that translation of expanded CGG repeats into FMRpolyG alters nuclear lamina architecture and drives pathogenesis in FXTAS.

RAN translation—What makes it run?

Nucleotide-repeat expansions underlie a heterogeneous group of neurodegenerative and neuromuscular disorders for which there are currently no effective therapies. Recently, it was discovered that such repetitive RNA motifs can support translation initiation in the absence of an AUG start codon across a wide variety of sequence contexts, and that the products of these atypical translation initiation events contribute to neuronal toxicity. This review examines what we currently know and do not know about repeat associated non-AUG (RAN) translation in the context of established canonical and non-canonical mechanisms of translation initiation. We highlight recent findings related to RAN translation in three repeat expansion disorders: CGG repeats in fragile X-associated tremor ataxia syndrome (FXTAS), GGGGCC repeats in C9orf72 associated amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and CAG repeats in Huntington disease. These studies suggest that mechanistic differences may exist for RAN translation dependent on repeat type, repeat reading frame, and the surrounding sequence context, but that for at least some repeats, RAN translation retains a dependence on some of the canonical translational initiation machinery. This article is part of a Special Issue entitled SI:RNA Metabolism in Disease.

Repeat-Associated Non-AUG Translation and Its Impact in Neurodegenerative Disease

Nucleotide repeat expansions underlie numerous human neurological disorders. Repeats can trigger toxicity through multiple pathogenic mechanisms, including RNA gain-of-function, protein gain-of-function, and protein loss-of-function pathways. Traditionally, inference of the underlying pathogenic mechanism derives from the repeat location, with dominantly inherited repeats within transcribed noncoding sequences eliciting toxicity predominantly as RNA via sequestration of specific RNA binding proteins. However, recent findings question this assumption and suggest that repeats outside of annotated open reading frames may also trigger toxicity through a novel form of protein translational initiation known as repeat-associated non-AUG (RAN) translation. To date, RAN translation has been implicated in 4 nucleotide repeat expansion disorders: spinocerebellar ataxia type 8; myotonic dystrophy type 1 with CTG•CAG repeats; C9orf72 amyotrophic lateral sclerosis/frontotemporal dementia with GGGGCC•GGCCCC repeats; and fragile X-associated tremor/ataxia syndrome with CGG repeats. RAN translation contributes to hallmark pathological characteristics in these disorders by producing homopolymeric or dipeptide repeat proteins. Here, we review what is known about RAN translation, with an emphasis on how differences in both repeat sequence and context may confer different requirements for unconventional initiation. We then discuss how this new mechanism of translational initiation might function in normal physiology and lay out a roadmap for addressing the numerous questions that remain.

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

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.

JC Polyomavirus Granule Cell Neuronopathy in a Patient Treated With Rituximab

IMPORTANCE Progressive multifocal leukoencephalopathy results from lytic infection of the glia by the JC polyomavirus (JCV); JCV granule cell neuronopathy is caused by infection with a mutated form of JCV, leading to a shift in viral tropism from the glia to cerebellar granule cells. This shift results in a clinical syndrome dominated by progressive cerebellar dysfunction that might elude standard diagnostic workup strategies for ataxia. OBSERVATIONS We present the case report of a patient receiving long-term rituximab therapy who developed progressive cerebellar ataxia and marked isolated cerebellar degeneration. This syndrome resulted from JCV granule cell neuronopathy associated with a novel JCV mutation. CONCLUSIONS AND RELEVANCE New onset or worsening of isolated cerebellar ataxia in patients being treated with rituximab or natalizumab warrants early assessment for JCV infection.

Transcriptional changes and developmental abnormalities in a zebrafish model of myotonic dystrophy type 1

Myotonic dystrophy type I (DM1) is a multi-system, autosomal dominant disorder caused by expansion of a CTG repeat sequence in the 3′UTR of the DMPK gene. The size of the repeat sequence correlates with age at onset and disease severity, with large repeats leading to congenital forms of DM1 associated with hypotonia and intellectual disability. In models of adult DM1, expanded CUG repeats lead to an RNA toxic gain of function, mediated at least in part by sequestering specific RNA splicing proteins, most notably muscleblind-related (MBNL) proteins. However, the impact of CUG RNA repeat expression on early developmental processes is not well understood. To better understand early developmental processes in DM1, we utilized the zebrafish, Danio rerio, as a model system. Direct injection of (CUG)91 repeat-containing mRNA into single-cell embryos induces toxicity in the nervous system and muscle during early development. These effects manifest as abnormal morphology, behavioral abnormalities and broad transcriptional changes, as shown by cDNA microarray analysis. Co-injection of zebrafish mbnl2 RNA suppresses (CUG)91 RNA toxicity and reverses the associated behavioral and transcriptional abnormalities. Taken together, these findings suggest that early expression of exogenously transcribed CUG repeat RNA can disrupt normal muscle and nervous system development and provides a new model for DM1 research that is amenable to small-molecule therapeutic development.

RAN translation from antisense CCG repeats in Fragile X Tremor/Ataxia Syndrome

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.