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

Pathobiology of the autophagy-lysosomal pathway in the Huntington's disease brain

Accumulated levels of mutant huntingtin protein (mHTT) and its fragments are considered contributors to the pathogenesis of Huntington's disease (HD). Although lowering mHTT by stimulating autophagy has been considered a possible therapeutic strategy, the role and competence of autophagy-lysosomal pathway (ALP) during HD progression in the human disease remains largely unknown. Here, we used multiplex confocal and ultrastructural immunocytochemical analyses of ALP functional markers in relation to mHTT aggresome pathology in striatum and the less affected cortex of HD brains staged from HD2 to HD4 by Vonsattel neuropathological criteria compared to controls. Immunolabeling revealed the localization of HTT/mHTT in ALP vesicular compartments labeled by autophagy-related adaptor proteins p62/SQSTM1 and ubiquitin, and cathepsin D (CTSD) as well as HTT-positive inclusions. Although comparatively normal at HD2, neurons at later HD stages exhibited progressive enlargement and clustering of CTSD-immunoreactive autolysosomes/lysosomes and, ultrastructurally, autophagic vacuole/lipofuscin granules accumulated progressively, more prominently in striatum than cortex. These changes were accompanied by rises in levels of HTT/mHTT and p62/SQSTM1, particularly their fragments, in striatum but not in the cortex, and by increases of LAMP1 and LAMP2 RNA and LAMP1 protein. Importantly, no blockage in autophagosome formation and autophagosome-lysosome fusion was detected, thus pinpointing autophagy substrate clearance deficits as a basis for autophagic flux declines. The findings collectively suggest that upregulated lysosomal biogenesis and preserved proteolysis maintain autophagic clearance in early-stage HD, but failure at advanced stages contributes to progressive HTT build-up and potential neurotoxicity. These findings support the prospect that ALP stimulation applied at early disease stages, when clearance machinery is fully competent, may have therapeutic benefits in HD patients.

Postmortem neuropathology in early Huntington disease

Two aspects of the neuropathology of early Huntington disease (HD) are examined. Neurons of the neostriatum are counted to determine relative loss in striosomes versus matrix at early stages, including for the first time in preclinical cases. An immunohistochemical procedure is described that tentatively distinguishes early HD from HD mimic disorders in postmortem brains. Counts of striatal projection neurons (SPNs) in striosomes defined by calbindin immunohistochemistry versus counts in the surrounding matrix are reported for 8 Vonsattel grade 0 (including 5 premanifest), 8 grade 1, 2 grade 2 HD, and for 8 control postmortem brains. Mean counts of striosome and matrix SPNs were significantly lower in premanifest grade 0 versus controls, with striosome counts significantly lower than matrix. In 8 grade 1 and 2 grade 2 brains, no striosomes with higher SPN counts than in the surrounding matrix were observed. Comparing dorsal versus ventral neostriatum, SPNs in dorsal striosomes and matrix declined more than ventral, making clear the importance of the dorsoventral site of tissue selection for research studies. A characteristic pattern of expanded polyglutamine-immunopositive inclusions was seen in all HD cases. Inclusions were always present in some SPNs and some pontine nucleus neurons and were absent in Purkinje cells, which showed no obvious cell loss.

Huntington disease-like 2: insight into neurodegeneration from an African disease

Huntington disease (HD)-like 2 (HDL2) is a rare genetic disease caused by an expanded trinucleotide repeat in the JPH3 gene (encoding junctophilin 3) that shows remarkable clinical similarity to HD. To date, HDL2 has been reported only in patients with definite or probable African ancestry. A single haplotype background is shared by patients with HDL2 from different populations, supporting a common African origin for the expansion mutation. Nevertheless, outside South Africa, reports of patients with HDL2 in Africa are scarce, probably owing to limited clinical services across the continent. Systematic comparisons of HDL2 and HD have revealed closely overlapping motor, cognitive and psychiatric features and similar patterns of cerebral and striatal atrophy. The pathogenesis of HDL2 remains unclear but it is proposed to occur through several mechanisms, including loss of protein function and RNA and/or protein toxicity. This Review summarizes our current knowledge of this African-specific HD phenocopy and highlights key areas of overlap between HDL2 and HD. Given the aforementioned similarities in clinical phenotype and pathology, an improved understanding of HDL2 could provide novel insights into HD and other neurodegenerative and/or trinucleotide repeat expansion disorders.

The Role of Junctophilin Proteins in Cellular Function

Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a joining region connecting these to another set of 2 MORN motifs, a putative alpha-helical region, a divergent region exhibiting low homology between JPH isoforms, and a carboxy-terminal transmembrane region anchoring into the ER/SR membrane. JPH isoforms play essential roles in developing and maintaining subcellular membrane junctions. Conversely, inherited mutations in JPH2 cause hypertrophic or dilated cardiomyopathy, while trinucleotide expansions in the JPH3 gene cause Huntington Disease-Like 2. Loss of JPH1 protein levels can cause skeletal myopathy, while loss of cardiac JPH2 levels causes heart failure and atrial fibrillation, among other disease. This review will provide a comprehensive overview of the JPH gene family, phylogeny, and evolutionary analysis of JPH genes and other MORN domain proteins. JPH biogenesis, membrane tethering, and binding partners will be discussed, as well as functional roles of JPH isoforms in excitable cells. Finally, potential roles of JPH isoform deficits in human disease pathogenesis will be reviewed.

Huntington's disease: lessons from prion disorders

Decades of research on the prion protein and its associated diseases have caused a paradigm shift in our understanding of infectious agents. More recent years have been marked by a surge of studies supporting the application of these findings to a broad array of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Here, we present evidence to suggest that Huntington's disease, a monogenic disorder of the central nervous system, shares features with prion disorders and that, it too, may be governed by similar mechanisms. We further posit that these similarities could suggest that, like other common neurodegenerative disorders, sporadic forms of Huntington's disease may exist.

The Neuropsychiatry of Huntington Disease-Like 2: A Comparison with Huntington's Disease

BACKGROUND

Huntington Disease-Like 2 (HDL2) is a rare autosomal dominant disorder caused by an abnormal CAG/CTG triplet repeat expansion on chromosome 16q24. The symptoms of progressive decline in motor, cognitive and psychiatric functioning are similar to those of Huntington's disease (HD). The psychiatric features of the HDL2 have been poorly characterized.

OBJECTIVE

To describe the neuropsychiatric features of HDL2 and compare them with those of HD.

METHODS

A blinded cross-sectional design was used to compare the behavioural component of the Unified Huntington's Disease Rating Scale (UHDRS) in participants with HDL2 (n = 15) and HD (n = 13) with African ancestry.

RESULTS

HDL2 patients presented with psychiatric symptoms involving mood disturbances and behavioural changes that were not significantly different from those in the HD group. Duration of disease and motor performance correlated (p < 0.001) with the Functional Capacity score and the Independence score of the UHDRS. HD patients reported movement dysfunction as the first symptom more frequently than HDL2 Patients (p < 0.001).

CONCLUSION

The psychiatric phenotype of HDL2 is similar to that of HD and linked to motor decline and disease duration. Psychiatric symptoms seem more severe for HDL2 patients in the early stages of the disease.

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.

Autophagy in Neurodegenerative Diseases: A Hunter for Aggregates

Cells have developed elaborate quality-control mechanisms for proteins and organelles to maintain cellular homeostasis. Such quality-control mechanisms are maintained by conformational folding via molecular chaperones and by degradation through the ubiquitin-proteasome or autophagy-lysosome system. Accumulating evidence suggests that impaired autophagy contributes to the accumulation of intracellular inclusion bodies consisting of misfolded proteins, which is a hallmark of most neurodegenerative diseases. In addition, genetic mutations in core autophagy-related genes have been reported to be linked to neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. Conversely, the pathogenic proteins, such as amyloid β and α-synuclein, are detrimental to the autophagy pathway. Here, we review the recent advances in understanding the relationship between autophagic defects and the pathogenesis of neurodegenerative diseases and suggest autophagy induction as a promising strategy for the treatment of these conditions.

Pre-clinical Mouse Models of Neurodegenerative Lysosomal Storage Diseases

There are over 50 lysosomal hydrolase deficiencies, many of which cause neurodegeneration, cognitive decline and death. In recent years, a number of broad innovative therapies have been proposed and investigated for lysosomal storage diseases (LSDs), such as enzyme replacement, substrate reduction, pharmacologic chaperones, stem cell transplantation, and various forms of gene therapy. Murine models that accurately reflect the phenotypes observed in human LSDs are critical for the development, assessment and implementation of novel translational therapies. The goal of this review is to summarize the neurodegenerative murine LSD models available that recapitulate human disease, and the pre-clinical studies previously conducted. We also describe some limitations and difficulties in working with mouse models of neurodegenerative LSDs.

A comparison between the neurocognitive profile of Huntington Disease-Like 2 and Huntington Disease: Exploring the presence of double dissociations

Huntington Disease Like-2 (HDL2) is a rare autosomal dominant genetic disease caused by a mutation in the JPH3 gene. HDL2 is the Huntington Disease (HD) phenocopy that has the greatest clinical resemblance to HD. Both are characterized by movement, psychiatric and cognitive dysfunction, which progress to dementia. The present study compared the neuropsychological profile of HDL2 with that of HD. Using a Single Case-Control Methodology in Neuropsychology, three HDL2 and seven matched HD patients were assessed with a comprehensive neuropsychological battery and compared to matched control samples, considering age, years of education, type of school (public/government) and language (all bi/multilingual). Potential double dissociations were explored by using Crawford, Garthwaite, and Wood's Inferential Methods for Comparing the Scores of Two Single-Cases in Case-Control Designs. Double dissociation between HDL2 and HD were identified in three tests, namely Letter Number Sequencing, Rey Auditory Learning Test Delayed and Recognition Trials. These dissociations possible are due to methodological limitations.

RNA toxicity in non-coding repeat expansion disorders

Several neurodegenerative disorders like amyotrophic lateral sclerosis (ALS) and spinocerebellar ataxia (SCA) are caused by non-coding nucleotide repeat expansions. Different pathogenic mechanisms may underlie these non-coding repeat expansion disorders. While gain-of-function mechanisms, such as toxicity associated with expression of repeat RNA or toxicity associated with repeat-associated non-ATG (RAN) products, are most frequently connected with these disorders, loss-of-function mechanisms have also been implicated. We review the different pathways that have been linked to non-coding repeat expansion disorders such as C9ORF72-linked ALS/frontotemporal dementia (FTD), myotonic dystrophy, fragile X tremor/ataxia syndrome (FXTAS), SCA, and Huntington's disease-like 2. We discuss modes of RNA toxicity focusing on the identity and the interacting partners of the toxic RNA species. Using the C9ORF72 ALS/FTD paradigm, we further explore the efforts and different methods used to disentangle RNA vs. RAN toxicity. Overall, we conclude that there is ample evidence for a role of RNA toxicity in non-coding repeat expansion diseases.

Emerging differences between Huntington's disease-like 2 and Huntington's disease: A comparison using MRI brain volumetry

Huntington's Disease-Like 2 (HDL2), caused by a CTG/CAG expansion in JPH3 on chromosome 16q24, is the most common Huntington's Disease (HD) phenocopy in populations with African ancestry. Qualitatively, brain MRIs of HDL2 patients have been indistinguishable from HD. To determine brain regions most affected in HDL2 a cross-sectional study using MRI brain volumetry was undertaken to compare the brains of nine HDL2, 11 HD and nine age matched control participants. Participants were ascertained from the region in South Africa with the world's highest HDL2 incidence. The HDL2 and HD patient groups showed no significant differences with respect to mean age at MRI, disease duration, abnormal triplet repeat length, or age at disease onset. Overall, intracerebral volumes were smaller in both affected groups compared to the control group. Comparing the HDL2 and HD groups across multiple covariates, cortical and subcortical volumes were similar with the exception that the HDL2 thalamic volumes were smaller. Consistent with other similarities between the two diseases, these results indicate a pattern of neurodegeneration in HDL2 that is remarkably similar to HD. However smaller thalamic volumes in HDL2 raises intriguing questions into the pathogenesis of both disorders, and how these volumetric differences relate to their respective phenotypes.

A role for autophagy in Huntington's disease

The lysosome-mediated degradation pathway known as macroautophagy is the most versatile means through which cells can eliminate and recycle unwanted materials. Through both selective and non-selective means, macroautophagy can degrade a wide range of cargoes from bulk cytosol to organelles and aggregated proteins. Although studies of disorders such as Parkinson's disease and Amyotrophic Lateral Sclerosis suggest that autophagic and lysosomal dysfunction directly contributes to disease, this had not been the case for the polyglutamine disorder Huntington's disease (HD), for which there was little indication of a disruption in the autophagic-lysosomal system. This supported the possibility of targeting autophagy as a much needed therapeutic approach to combat this disease. Possibly challenging this view, however, are a recent set of studies suggesting that the protein affected in Huntington's disease, huntingtin, might mechanistically contribute to macroautophagy. In this review, we will explore how autophagy might impact or be impacted by HD pathogenesis, and whether a therapeutic approach centering on autophagy may be possible for this yet incurable disease.

mTOR-Related Brain Dysfunctions in Neuropsychiatric Disorders

The mammalian target of rapamycin (mTOR) is an ubiquitously expressed serine-threonine kinase, which senses and integrates several intracellular and environmental cues to orchestrate major processes such as cell growth and metabolism. Altered mTOR signalling is associated with brain malformation and neurological disorders. Emerging evidence indicates that even subtle defects in the mTOR pathway may produce severe effects, which are evident as neurological and psychiatric disorders. On the other hand, administration of mTOR inhibitors may be beneficial for a variety of neuropsychiatric alterations encompassing neurodegeneration, brain tumors, brain ischemia, epilepsy, autism, mood disorders, drugs of abuse, and schizophrenia. mTOR has been widely implicated in synaptic plasticity and autophagy activation. This review addresses the role of mTOR-dependent autophagy dysfunction in a variety of neuropsychiatric disorders, to focus mainly on psychiatric syndromes including schizophrenia and drug addiction. For instance, amphetamines-induced addiction fairly overlaps with some neuropsychiatric disorders including neurodegeneration and schizophrenia. For this reason, in the present review, a special emphasis is placed on the role of mTOR on methamphetamine-induced brain alterations.

Pathogenic insights from Huntington's disease-like 2 and other Huntington's disease genocopies

PURPOSE OF REVIEW

Huntington's disease-like 2 (HDL2) is a rare, progressive, autosomal dominant neurodegenerative disorder that genetically, clinically, and pathologically closely resembles Huntington's disease. We review HDL2 pathogenic mechanisms and examine the implications of these mechanisms for Huntington's disease and related diseases.

RECENT FINDINGS

HDL2 is caused by a CTG/CAG repeat expansion in junctophilin-3. Available data from cell and animal models and human brain suggest that HDL2 is a complex disease in which transcripts and proteins expressed bidirectionally from the junctophilin-3 locus contribute to pathogenesis through both gain-and loss-of-function mechanisms. Recent advances indicate that the pathogenesis of Huntington's disease is equally complex, despite the emphasis on toxic gain-of-function properties of the mutant huntingtin protein.

SUMMARY

Studies examining in parallel the genetic, clinical, neuropathological, and mechanistic similarities between Huntington's disease and HDL2 have begun to identify points of convergence between the pathogenic pathways of the two diseases. Comparisons to other diseases that are phenotypically or genetically related to Huntington's disease and HDL2 will likely reveal additional common pathways. The ultimate goal is to identify shared therapeutic targets and eventually develop therapies that may, at least in part, be effective across multiple similar rare diseases, an essential approach given the scarcity of resources for basic and translational research.

Trinucleotide repeat disorders

Trinucleotide repeat disorders comprise a variable group of inherited neurodegenerative diseases, with a large range in prevalence figures. There is a broad range in clinical presentations, but many of these diseases lead to some form of ataxia or other movement disorders, which are frequently combined with cognitive or psychiatric disturbances. This group can be divided into CAG- versus non-CAG-repeat diseases. Apart from spinocerebellar ataxia type 6 and 12 (SCA6 and SCA12), these CAG-repeat diseases, as well as Huntington disease-like 2 (HDL2) and SCA8, can be neuropathologically identified using 1C2 polyglutamine antibodies. In fragile X-associated tremor and ataxia, SCA6 and SCA12 ubiquitin/p62-positive and 1C2-negative inclusion bodies can be observed. In the other diseases proteinaceous inclusions are not found. For definite diagnosis genetic analysis is necessary.

Protein sequestration as a normal function of long noncoding RNAs and a pathogenic mechanism of RNAs containing nucleotide repeat expansions

An emerging class of long noncoding RNAs (lncRNAs) function as decoy molecules that bind and sequester proteins thereby inhibiting their normal functions. Titration of proteins by lncRNAs has wide-ranging effects affecting nearly all steps in gene expression. While decoy lncRNAs play a role in normal physiology, RNAs expressed from alleles containing nucleotide repeat expansions can be pathogenic due to protein sequestration resulting in disruption of normal functions. This review focuses on commonalities between decoy lncRNAs that regulate gene expression by competitive inhibition of protein function through sequestration and specific examples of nucleotide repeat expansion disorders mediated by toxic RNA that sequesters RNA-binding proteins and impedes their normal functions. Understanding how noncoding RNAs compete with various RNA and DNA molecules for binding of regulatory proteins will provide insight into how similar mechanisms contribute to disease pathogenesis.

A Systematic Review of the Huntington Disease-Like 2 Phenotype

BACKGROUND

Huntington Disease-like 2 (HDL2) is a neurodegenerative disorder similar to Huntington Disease (HD) in its clinical phenotype, genetic characteristics, neuropathology and longitudinal progression. Proposed specific differences include an exclusive African ancestry, lack of eye movement abnormalities, increased Parkinsonism, and acanthocytes in HDL2.

OBJECTIVE

The objective was to determine the similarities and differences between HD and HDL2 by establishing the clinical phenotype of HDL2 with the published cases.

METHODS

A literature review of all clinically described cases of HDL2 until the end of 2016 was performed and a descriptive analysis was carried out.

RESULTS

Sixty-nine new cases were described between 2001 and 2016. All cases had likely African ancestry, and most were found in South Africa and the USA. Many features were found to be similar to HD, including a strong negative correlation between repeat length and age of onset. Chorea was noted in 48/57 cases (84%). Dementia was reported in 74% patients, and Parkinsonism in 37%. Psychiatric features were reported in 44 out of 47 cases. Patients with chorea had lower expanded repeat lengths compared to patients without chorea. Eye movements were described in 19 cases, 8 were abnormal. Acanthocytes were detected in 4 of the 13 patients tested. Nineteen out of 20 MRIs were reported as abnormal with findings similar to HD.

CONCLUSION

This review clarifies some aspects of the HDL2 phenotype and highlights others which require further investigation. Features that are unique to HDL2 have been documented in a minority of subjects and require prospective validation.

Quantitative Proteomic Analysis Reveals Similarities between Huntington's Disease (HD) and Huntington's Disease-Like 2 (HDL2) Human Brains

The pathogenesis of HD and HDL2, similar progressive neurodegenerative disorders caused by expansion mutations, remains incompletely understood. No systematic quantitative proteomics studies, assessing global changes in HD or HDL2 human brain, were reported. To address this deficit, we used a stable isotope labeling-based approach to quantify the changes in protein abundances in the cortex of 12 HD and 12 control cases and, separately, of 6 HDL2 and 6 control cases. The quality of the tissues was assessed to minimize variability due to post mortem autolysis. We applied a robust median sweep algorithm to quantify protein abundance and performed statistical inference using moderated test statistics. 1211 proteins showed statistically significant fold changes between HD and control tissues; the differences in selected proteins were verified by Western blotting. Differentially abundant proteins were enriched in cellular pathways previously implicated in HD, including Rho-mediated, actin cytoskeleton and integrin signaling, mitochondrial dysfunction, endocytosis, axonal guidance, DNA/RNA processing, and protein transport. The abundance of 717 proteins significantly differed between control and HDL2 brain. Comparative analysis of the disease-associated changes in the HD and HDL2 proteomes revealed that similar pathways were altered, suggesting the commonality of pathogenesis between the two disorders.

A Drosophila model of Huntington disease-like 2 exhibits nuclear toxicity and distinct pathogenic mechanisms from Huntington disease

Huntington disease-like 2 (HDL2) and Huntington disease (HD) are adult-onset neurodegenerative diseases characterized by movement disorders, psychiatric disturbances and cognitive decline. Brain tissue from HD and HDL2 patients shows degeneration of the striatum and ubiquitinated inclusions immunoreactive for polyglutamine (polyQ) antibodies. Despite these similarities, the diseases result from different genetic mutations. HD is caused by a CAG repeat expansion in the huntingtin (HTT) gene, while HDL2 results from an expansion at the junctophilin 3 (JPH3) locus. Recent evidence indicates that the HDL2 expansion may give rise to a toxic polyQ protein translated from an antisense mRNA derived from the JPH3 locus. To investigate this hypothesis, we generated and characterized a Drosophila HDL2 model and compared it with a previously established HD model. We find that neuronal expression of HDL2-Q15 is not toxic, while the expression of an expanded HDL2-Q138 protein is lethal. HDL2-Q138 forms large nuclear aggregates, with only smaller puncta observed in the cytoplasm. This is in contrast to what is observed in a Drosophila model of HD, where polyQ aggregates localize exclusively to the cytoplasm. Altering localization of HLD2 with the addition of a nuclear localization or nuclear export sequence demonstrates that nuclear accumulation is required for toxicity in the Drosophila HDL2 model. Directing HDL2-Q138 to the nucleus exacerbates toxicity in multiple tissue types, while confining HDL2-Q138 to the cytoplasm restores viability to control levels. We conclude that while HD and HDL2 have similar clinical profiles, distinct pathogenic mechanisms are likely to drive toxicity in Drosophila models of these disorders.

RNA FISH for detecting expanded repeats in human diseases

RNA fluorescence in situ hybridization (FISH) is a widely used technique for detecting transcripts in fixed cells and tissues. Many variants of RNA FISH have been proposed to increase signal strength, resolution and target specificity. The current variants of this technique facilitate the detection of the subcellular localization of transcripts at a single molecule level. Among the applications of RNA FISH are studies on nuclear RNA foci in diseases resulting from the expansion of tri-, tetra-, penta- and hexanucleotide repeats present in different single genes. The partial or complete retention of mutant transcripts forming RNA aggregates within the nucleoplasm has been shown in multiple cellular disease models and in the tissues of patients affected with these atypical mutations. Relevant diseases include, among others, myotonic dystrophy type 1 (DM1) with CUG repeats, Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3) with CAG repeats, fragile X-associated tremor/ataxia syndrome (FXTAS) with CGG repeats, myotonic dystrophy type 2 (DM2) with CCUG repeats, amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) with GGGGCC repeats and spinocerebellar ataxia type 32 (SCA32) with GGCCUG. In this article, we summarize the results obtained with FISH to examine RNA nuclear inclusions. We provide a detailed protocol for detecting RNAs containing expanded CAG and CUG repeats in different cellular models, including fibroblasts, lymphoblasts, induced pluripotent stem cells and murine and human neuronal progenitors. We also present the results of the first single-molecule FISH application in a cellular model of polyglutamine disease.

Junctophilin 3 (JPH3) expansion mutations causing Huntington disease like 2 (HDL2) are common in South African patients with African ancestry and a Huntington disease phenotype

Huntington disease (HD) is a progressive autosomal dominant neurodegenerative disorder, characterized by abnormal movements, cognitive decline, and psychiatric symptoms, caused by a CAG repeat expansion in the huntingtin (HTT) gene on chromosome 4p. A CAG/CTG repeat expansion in the junctophilin-3 (JPH3) gene on chromosome 16q24.2 causes a Huntington disease-like phenotype (HDL2). All patients to date with HDL2 have some African ancestry. The present study aimed to characterize the genetic basis of the Huntington disease phenotype in South Africans and to investigate the possible origin of the JPH3 mutation. In a sample of unrelated South African individuals referred for diagnostic HD testing, 62% (106/171) of white patients compared to only 36% (47/130) of black patients had an expansion in HTT. However, 15% (20/130) of black South African patients and no white patients (0/171) had an expansion in JPH3, confirming the diagnosis of Huntington disease like 2 (HDL2). Individuals with HDL2 share many clinical features with individuals with HD and are clinically indistinguishable in many cases, although the average age of onset and diagnosis in HDL2 is 5 years later than HD and individual clinical features may be more prominent. HDL2 mutations contribute significantly to the HD phenotype in South Africans with African ancestry. JPH3 haplotype studies in 31 families, mainly from South Africa and North America, provide evidence for a founder mutation and support a common African origin for all HDL2 patients. Molecular testing in individuals with an HD phenotype and African ancestry should include testing routinely for JPH3 mutations.

Nuclear retention of full-length HTT RNA is mediated by splicing factors MBNL1 and U2AF65

Huntington's disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Recent evidence suggests that HD is a consequence of multimodal, non-mutually exclusive mechanisms of pathogenesis that involve both HTT protein- and HTT RNA-triggered mechanisms. Here we provide further evidence for the role of expanded HTT (expHTT) RNA in HD by demonstrating that a fragment of expHTT is cytotoxic in the absence of any translation and that the extent of cytotoxicity is similar to the cytotoxicity of an expHTT protein fragment encoded by a transcript of similar length and with a similar repeat size. In addition, full-length (FL) expHTT is retained in the nucleus. Overexpression of the splicing factor muscleblind-like 1 (MBNL1) increases nuclear retention of expHTT and decreases the expression of expHTT protein in the cytosol. The splicing and nuclear export factor U2AF65 has the opposite effect, decreasing expHTT nuclear retention and increasing expression of expHTT protein. This suggests that MBNL1 and U2AF65 play a role in nuclear export of expHTT RNA.

Untangling the Thorns: Advances in the Neuroacanthocytosis Syndromes

There have been significant advances in neuroacanthocytosis (NA) syndromes in the past 20 years, however, confusion still exists regarding the precise nature of these disorders and the correct nomenclature. This article seeks to clarify these issues and to summarise the recent literature in the field. The four key NA syndromes are described here-chorea-acanthocytosis, McLeod syndrome, Huntington's disease-like 2, and pantothenate kinase- associated neurodegeneration. In the first two, acanthocytosis is a frequent, although not invariable, finding; in the second two, it occurs in approximately 10% of patients. Degeneration affecting the basal ganglia is the key neuropathologic finding, thus the clinical presentations can be remarkably similar. The characteristic phenotype comprises a variety of movement disorders, including chorea, dystonia, and parkinsonism, and also psychiatric and cognitive symptoms attributable to basal ganglia dysfunction. The age of onset, inheritance patterns, and ethnic background differ in each condition, providing diagnostic clues. Other investigations, including routine blood testing and neuroimaging can be informative. Genetic diagnosis, if available, provides a definitive diagnosis, and is important for genetic counseling, and hopefully molecular therapies in the future. In this article I provide a historical perspective on each NA syndrome. The first 3 disorders, chorea-acanthocytosis, McLeod syndrome, Huntington's disease-like 2, are discussed in detail, with a comprehensive review of the literature to date for each, while pantothenate kinase-associated neurodegeneration is presented in summary, as this disorder has recently been reviewed in this journal. Therapy for all of these diseases is, at present, purely symptomatic.

Autophagy and neurodegeneration

Most neurodegenerative diseases that afflict humans are associated with the intracytoplasmic deposition of aggregate-prone proteins in neurons. Autophagy is a powerful process for removing such proteins. In this Review, we consider how certain neurodegenerative diseases may be associated with impaired autophagy and how this may affect pathology. We also discuss how autophagy induction may be a plausible therapeutic strategy for some conditions and review studies in various models that support this hypothesis. Finally, we briefly describe some of the signaling pathways that may be amenable to therapeutic targeting for these goals.

Consensus paper: pathological mechanisms underlying neurodegeneration in spinocerebellar ataxias

Intensive scientific research devoted in the recent years to understand the molecular mechanisms or neurodegeneration in spinocerebellar ataxias (SCAs) are identifying new pathways and targets providing new insights and a better understanding of the molecular pathogenesis in these diseases. In this consensus manuscript, the authors discuss their current views on the identified molecular processes causing or modulating the neurodegenerative phenotype in spinocerebellar ataxias with the common opinion of translating the new knowledge acquired into candidate targets for therapy. The following topics are discussed: transcription dysregulation, protein aggregation, autophagy, ion channels, the role of mitochondria, RNA toxicity, modulators of neurodegeneration and current therapeutic approaches. Overall point of consensus includes the common vision of neurodegeneration in SCAs as a multifactorial, progressive and reversible process, at least in early stages. Specific points of consensus include the role of the dysregulation of protein folding, transcription, bioenergetics, calcium handling and eventual cell death with apoptotic features of neurons during SCA disease progression. Unresolved questions include how the dysregulation of these pathways triggers the onset of symptoms and mediates disease progression since this understanding may allow effective treatments of SCAs within the window of reversibility to prevent early neuronal damage. Common opinions also include the need for clinical detection of early neuronal dysfunction, for more basic research to decipher the early neurodegenerative process in SCAs in order to give rise to new concepts for treatment strategies and for the translation of the results to preclinical studies and, thereafter, in clinical practice.

Biophysical underpinnings of the repeat length dependence of polyglutamine amyloid formation

There are now 10 expanded CAG repeat diseases in which both disease risk and age of onset are strongly dependent on the repeat length of the polyglutamine (polyQ) sequence in the disease protein. Large, polyQ-rich inclusions in patient brains and in cell and animal models are consistent with the involvement of polyQ aggregation in the disease mechanism. This possibility is reinforced by studies showing strong repeat length dependence to the aggregation process, qualitatively mirroring the repeat length dependence of disease risk. Our understanding of the underlying biophysical principles that mediate the repeat length dependence of aggregation, however, is far from complete. A previous study of simple polyQ peptides showed that N*, the size of the critical nucleus that controls onset of aggregation, decreases from unfavorable tetramer to favorable monomer over the range Q23 to Q26. These data, however, do not explain why, for all peptides exhibiting N* ∼ 1, spontaneous aggregation rates continue to increase with increasing repeat length. Here we describe a novel kinetics analyses that maps out the nonlinear dependence with repeat length of a nucleation efficiency term that is likely related to aspects of nucleus structure. This trend accounts for why nucleus size increases to tetrameric at repeat lengths of Q23 or below. Intriguingly, both aggregation and age of onset trend with repeat length in similar ways, exhibiting large changes per added Gln at low repeat lengths and small changes per added Gln at relatively long repeat lengths. Fibril stability also increases with repeat length in a nonlinear fashion.

Huntington's disease and Huntington's disease-like syndromes: an overview

PURPOSE OF REVIEW

The differential diagnosis of chorea syndromes may be complex and includes various genetic disorders such as Huntington's disease and mimicking disorders called Huntington's disease-like (HDL) phenotypes. To familiarize clinicians with these (in some cases very rare) conditions we will summarize the main characteristics.

RECENT FINDINGS

HDL disorders are rare and account for about 1% of cases presenting with a Huntington's disease phenotype. They share overlapping clinical features, so making the diagnosis purely on clinical grounds may be challenging, however presence of certain characteristics may be a clue (e.g. prominent orofacial involvement in neuroferritinopathy etc.), Information of ethnic descent will also guide genetic work-up [HDL2 in Black Africans; dentatorubral-pallidoluysian atrophy (DRPLA) in Japanese etc.], Huntington's disease, the classical HDL disorders (except HDL3) and DRPLA are repeat disorders with anticipation effect and age-dependent phenotype in some, but genetic underpinnings may be more complicated in the other chorea syndromes.

SUMMARY

With advances in genetics more and more rare diseases are disentangled, allowing molecular diagnoses in a growing number of choreic patients. Hopefully, with better understanding of their pathophysiology we are moving towards mechanistic therapies.

RNA-binding protein misregulation in microsatellite expansion disorders

RNA-binding proteins (RBPs) play pivotal roles in multiple cellular pathways from transcription to RNA turnover by interacting with RNA sequence and/or structural elements to form distinct RNA-protein complexes. Since these complexes are required for the normal regulation of gene expression, mutations that alter RBP functions may result in a cascade of deleterious events that lead to severe disease. Here, we focus on a group of hereditary disorders, the microsatellite expansion diseases, which alter RBP activities and result in abnormal neurological and neuromuscular phenotypes. While many of these diseases are classified as adult-onset disorders, mounting evidence indicates that disruption of normal RNA-protein interaction networks during embryogenesis modifies developmental pathways, which ultimately leads to disease manifestations later in life. Efforts to understand the molecular basis of these disorders has already uncovered novel pathogenic mechanisms, including RNA toxicity and repeat-associated non-ATG (RAN) translation, and current studies suggest that additional surprising insights into cellular regulatory pathways will emerge in the future.

The neuropsychiatry of hyperkinetic movement disorders: insights from neuroimaging into the neural circuit bases of dysfunction

BACKGROUND

Movement disorders, particularly those associated with basal ganglia disease, have a high rate of comorbid neuropsychiatric illness.

METHODS

We consider the pathophysiological basis of the comorbidity between movement disorders and neuropsychiatric illness by 1) reviewing the epidemiology of neuropsychiatric illness in a range of hyperkinetic movement disorders, and 2) correlating findings to evidence from studies that have utilized modern neuroimaging techniques to investigate these disorders. In addition to diseases classically associated with basal ganglia pathology, such as Huntington disease, Wilson disease, the neuroacanthocytoses, and diseases of brain iron accumulation, we include diseases associated with pathology of subcortical white matter tracts, brain stem nuclei, and the cerebellum, such as metachromatic leukodystrophy, dentatorubropallidoluysian atrophy, and the spinocerebellar ataxias.

CONCLUSIONS

Neuropsychiatric symptoms are integral to a thorough phenomenological account of hyperkinetic movement disorders. Drawing on modern theories of cortico-subcortical circuits, we argue that these disorders can be conceptualized as disorders of complex subcortical networks with distinct functional architectures. Damage to any component of these complex information-processing networks can have variable and often profound consequences for the function of more remote neural structures, creating a diverse but nonetheless rational pattern of clinical symptomatology.

Suppressing aberrant GluN3A expression rescues synaptic and behavioral impairments in Huntington's disease models

Huntington's disease is caused by an expanded polyglutamine repeat in the huntingtin protein (HTT), but the pathophysiological sequence of events that trigger synaptic failure and neuronal loss are not fully understood. Alterations in N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs) have been implicated. Yet, it remains unclear how the HTT mutation affects NMDAR function, and direct evidence for a causative role is missing. Here we show that mutant HTT redirects an intracellular store of juvenile NMDARs containing GluN3A subunits to the surface of striatal neurons by sequestering and disrupting the subcellular localization of the endocytic adaptor PACSIN1, which is specific for GluN3A. Overexpressing GluN3A in wild-type mouse striatum mimicked the synapse loss observed in Huntington's disease mouse models, whereas genetic deletion of GluN3A prevented synapse degeneration, ameliorated motor and cognitive decline and reduced striatal atrophy and neuronal loss in the YAC128 Huntington's disease mouse model. Furthermore, GluN3A deletion corrected the abnormally enhanced NMDAR currents, which have been linked to cell death in Huntington's disease and other neurodegenerative conditions. Our findings reveal an early pathogenic role of GluN3A dysregulation in Huntington's disease and suggest that therapies targeting GluN3A or pathogenic HTT-PACSIN1 interactions might prevent or delay disease progression.

Role of autophagy in prion protein-induced neurodegenerative diseases

Prion diseases, characterized by spongiform degeneration and the accumulation of misfolded and aggregated PrP(Sc) in the central nervous system, are one of fatal neurodegenerative and infectious disorders of humans and animals. In earlier studies, autophagy vacuoles in neurons were frequently observed in neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases as well as prion diseases. Autophagy is a highly conserved homeostatic process by which several cytoplasmic components (proteins or organelles) are sequestered in a double-membrane-bound vesicle termed 'autophagosome' and degraded upon their fusion with lysosome. The pathway of intercellular self-digestion at basal physiological levels is indispensable for maintaining the healthy status of tissues and organs. In case of prion infection, increasing evidence indicates that autophagy has a crucial ability of eliminating pathological PrP(Sc) accumulated within neurons. In contrast, autophagy dysfunction in affected neurons may contribute to the formation of spongiform changes. In this review, we summarized recent findings about the effect of mammalian autophagy in neurodegenerative disorders, particularly in prion diseases. We also summarized the therapeutic potential of some small molecules (such as lithium, rapamycin, Sirtuin 1 and resveratrol) targets to mitigate such diseases on brain function. Furthermore, we discussed the controversial role of autophagy, whether it mediates neuronal toxicity or serves a protective function in neurodegenerative disorders.

Bidirectional transcription of trinucleotide repeats: roles for excision repair

Genomic instability at repetitive DNA regions in cells of the nervous system leads to a number of neurodegenerative and neuromuscular diseases, including those with an expanded trinucleotide repeat (TNR) tract at or nearby an expressed gene. Expansion causes disease when a particular base sequence is repeated beyond the normal range, interfering with the expression or properties of a gene product. Disease severity and onset depend on the number of repeats. As the length of the repeat tract grows, so does the size of the successive expansions and the likelihood of another unstable event. In fragile X syndrome, for example, CGG repeat instability and pathogenesis are not typically observed below tracts of roughly 50 repeats, but occur frequently at or above 55 repeats, and are virtually certain above 100-300 repeats. Recent evidence points to bidirectional transcription as a new aspect of TNR instability and pathophysiology. Bidirectional transcription of TNR genes produces novel proteins and/or regulatory RNAs that influence both toxicity and epigenetic changes in TNR promoters. Bidirectional transcription of the TNR tract appears to influence aspects of its stability, gene processing, splicing, gene silencing, and chemical modification of DNAs. Paradoxically, however, some of the same effects are observed on both the expanded TNR gene and on its normal gene counterpart. In this review, we discuss the possible normal and abnormal effects of bidirectional transcription on trinucleotide repeat instability, the role of DNA repair in causing, preventing, or maintaining methylation, and chromatin environment of TNR genes.

Autophagy and neuronal cell death in neurological disorders

Autophagy is implicated in the pathogenesis of major neurodegenerative disorders although concepts about how it influences these diseases are still evolving. Once proposed to be mainly an alternative cell death pathway, autophagy is now widely viewed as both a vital homeostatic mechanism in healthy cells and as an important cytoprotective response mobilized in the face of aging- and disease-related metabolic challenges. In Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis, and other diseases, impairment at different stages of autophagy leads to the buildup of pathogenic proteins and damaged organelles, while defeating autophagy's crucial prosurvival and antiapoptotic effects on neurons. The differences in the location of defects within the autophagy pathway and their molecular basis influence the pattern and pace of neuronal cell death in the various neurological disorders. Future therapeutic strategies for these disorders will be guided in part by understanding the manifold impact of autophagy disruption on neurodegenerative diseases.

Striatal neuronal loss correlates with clinical motor impairment in Huntington's disease

Huntington's disease (HD) is characterized clinically by chorea, motor impairment, psychiatric manifestations, and dementia. Atrophy of the striatum is the neuropathological hallmark of HD, and previous studies have suggested that striatal atrophy correlates more closely with motor impairment than with chorea. Motor impairment, as measured by motor impairment score, correlates with functional disability in HD patients, but chorea does not. In this study, we investigated the relation between neuronal loss and these motor features. We conducted neuropathological and stereologic assessments of neurons in putamen and subthalamic nuclei in HD patients and age-matched controls. In putamen, we estimated the total number and volume of medium spiny neurons labeled with dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP-32). In subthalamic nuclei, we estimated the total number of neurons on hematoxylin & eosin/luxol fast blue stains. In putamen of HD, immunohistochemistry showed DARPP-32 neuronal atrophy with extensive disruption of neurites and neuropil; stereologic studies found significant decreases in both the number and size of DARPP-32 neurons; we also detected a significant reduction of overall putamen volume in HD patients, compared to controls. In subthalamic nuclei, there was a mild, but significant, neuronal loss in the HD group. The loss of neurons in putamen and subthalamic nuclei as well as putaminal atrophy were significantly correlated with severity of motor impairment, but not with chorea. Our findings suggest that neuronal loss and atrophy in striatum and neuronal loss in subthalamic nuclei contribute specifically to the motor impairment of HD, but not to chorea.

Huntington disease-like 2 (HDL2) in Venezuela: frequency and ethnic origin

Huntington disease (HD) phenotypes without a HTT mutation are known as HD-like (HDL) syndromes and are caused by mutations in other loci. HDL2, almost indistinguishable from HD, is due to expansions in the Junctophilin 3 locus (JPH3) with a worldwide Sub-Saharan ethnic origin. Sixteen independent patients with involuntary movements, psychiatric disturbances and ataxia not having a HTT mutation were searched for loci PRNP (prion protein, HDL1), JPH3 (HDL2), ATN1 (dentatorubral-pallidoluysian atrophy), ATX2 (spinocerebellar ataxia 2) ATXN3 (spinocerebellar ataxia 3), and TBP (spinocerebellar ataxia 17=HDL4). Markers Duffy, Kell, Diego, D9S1120, plus six JPH3 intragenic single-nucleotide polymorphisms were tested to ascertain ethnic origin. Four unrelated choreic patients had an expanded allele at JPH3. Three of them carried the African marker Duffy null. All four families carried with the mutation the same haplotype most frequent in African populations; Amerindian alleles D9D1120*9 and Diego A; or Kell allele K were absent. HDL2 in Venezuela had a low, but higher relative frequency (2.6%) than that in other Caucasoid populations. It should be searched first in choreic patients not having HTT mutations. The most likely remote ethnic origin for all detected families was African.

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.

Brain, blood, and iron: perspectives on the roles of erythrocytes and iron in neurodegeneration

The terms "neuroacanthocytosis" (NA) and "neurodegeneration with brain iron accumulation" (NBIA) both refer to groups of genetically heterogeneous disorders, classified together due to similarities of their phenotypic or pathological findings. Even collectively, the disorders that comprise these sets are exceedingly rare and challenging to study. The NBIA disorders are defined by their appearance on brain magnetic resonance imaging, with iron deposition in the basal ganglia. Clinical features vary, but most include a movement disorder. New causative genes are being rapidly identified; however, the mechanisms by which mutations cause iron accumulation and neurodegeneration are not well understood. NA syndromes are also characterized by a progressive movement disorder, accompanied by cognitive and psychiatric features, resulting from mutations in a number of genes whose roles are also basically unknown. An overlapping feature of the two groups, NBIA and NA, is the occurrence of acanthocytes, spiky red cells with a poorly-understood membrane dysfunction. In this review we summarise recent developments in this field, specifically insights into cellular mechanisms and from animal models. Cell membrane research may shed light upon the significance of the erythrocyte abnormality, and upon possible connections between the two sets of disorders. Shared pathophysiologic mechanisms may lead to progress in the understanding of other types of neurodegeneration.

JPH3 repeat expansions cause a progressive akinetic-rigid syndrome with severe dementia and putaminal rim in a five-generation African-American family

We report the clinical, neuropsychological, genetic, and radiological features of a large five-generation African-American kindred from the southern USA presenting with a progressive akinetic-rigid syndrome and severe dementia, but clinically insignificant chorea, due to mutations in junctophillin 3 (JPH3). Overt disease onset was in the mid-20s to late 30s with cognitive decline, REM sleep disturbance, or psychiatric features, followed by development of a levodopa-unresponsive akinetic-rigid motor syndrome. Dystonia and myoclonus were present in some subjects. A bedridden, nonverbal severely akinetic-rigid state developed within 10 to 15 years after onset. CTG repeat expansions ranged from 47 to 53. Imaging revealed generalized cerebral atrophy with severe striatal involvement and putaminal rim hyperintensity. Analysis of our kindred indicates that JPH3 mutations should be considered in the differential diagnosis of early-onset dementia and hypokinetic-rigid syndromes in individuals of African descent. Moreover, chorea may not be overtly manifest at presentation or during significant parts of the disease course.

Loss of junctophilin-3 contributes to Huntington disease-like 2 pathogenesis

OBJECTIVE

Huntington disease-like 2 (HDL2) is a progressive, late onset autosomal dominant neurodegenerative disorder, with remarkable similarities to Huntington disease (HD). HDL2 is caused by a CTG/CAG repeat expansion. In the CTG orientation, the repeat is located within the alternatively spliced exon 2A of junctophilin-3 (JPH3), potentially encoding polyleucine and polyalanine, whereas on the strand antisense to JPH3, the repeat is in frame to encode polyglutamine. The JPH3 protein product serves to stabilize junctional membrane complexes and regulate neuronal calcium flux. We have previously demonstrated the potential pathogenic properties of JPH3 transcripts containing expanded CUG repeats. The aim of this study was to test the possibility that loss of JPH3 expression or expanded amino acid tracts also contribute to HDL2 pathogenesis.

METHODS

Transcripts from the HDL2 locus, and their protein products, were examined in HDL2, HD, and control frontal cortex. The effect of loss of Jph3 was examined in mice with partial or complete loss of Jph3.

RESULTS

Bidirectional transcription occurs at the HDL2 locus, although expression of antisense transcripts with expanded CAG repeats is limited. Protein products with expanded amino acid tracts were not detected in HDL2 brain. However, JPH3 transcripts and full-length JPH3 protein are decreased in HDL2 brain, and Jph3 hemizygous and null mice exhibit abnormal motor function.

INTERPRETATION

Our results suggest that the pathogenic mechanism of HDL2 is multifactorial, involving both a toxic gain of function of JPH3 RNA and a toxic loss of JPH3 expression.

A pathogenic mechanism in Huntington's disease involves small CAG-repeated RNAs with neurotoxic activity

Huntington's disease (HD) is an autosomal dominantly inherited disorder caused by the expansion of CAG repeats in the Huntingtin (HTT) gene. The abnormally extended polyglutamine in the HTT protein encoded by the CAG repeats has toxic effects. Here, we provide evidence to support that the mutant HTT CAG repeats interfere with cell viability at the RNA level. In human neuronal cells, expanded HTT exon-1 mRNA with CAG repeat lengths above the threshold for complete penetrance (40 or greater) induced cell death and increased levels of small CAG-repeated RNAs (sCAGs), of ≈21 nucleotides in a Dicer-dependent manner. The severity of the toxic effect of HTT mRNA and sCAG generation correlated with CAG expansion length. Small RNAs obtained from cells expressing mutant HTT and from HD human brains significantly decreased neuronal viability, in an Ago2-dependent mechanism. In both cases, the use of anti-miRs specific for sCAGs efficiently blocked the toxic effect, supporting a key role of sCAGs in HTT-mediated toxicity. Luciferase-reporter assays showed that expanded HTT silences the expression of CTG-containing genes that are down-regulated in HD. These results suggest a possible link between HD and sCAG expression with an aberrant activation of the siRNA/miRNA gene silencing machinery, which may trigger a detrimental response. The identification of the specific cellular processes affected by sCAGs may provide insights into the pathogenic mechanisms underlying HD, offering opportunities to develop new therapeutic approaches.

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormal CAG expansion in the Huntingtin gene (HTT), resulting in an expanded polyglutamine track in the HTT protein. Longer CAG expansions correlate with an earlier more severe manifestation of the disease that produces choreic movement, behavioural and psychiatric disturbances, and dementia. Although the causative gene is widely expressed, neuropathology is characterized by striatal and cortical atrophy. HTT interacts with proteins involved in transcription, cell signaling, and transport. The pathogenic role of mutant HTT is not fully understood. This study shows that CAG expanded HTT RNA also contributes to neuronal toxicity. Mutant HTT RNA gives rise to small CAG-repeated RNAs (sCAGs) with neurotoxic activity. These short RNAs interfere with cell functions by silencing the expression of genes that are fully or partially complementary, through a mechanism similar to that of microRNAs. These findings suggest that a small RNA–dependent mechanism may contribute to HD neuronal cell loss. The exhaustive identification of the target genes modulated by sCAGs may lead to a better understanding of HD pathology, allowing the development of new therapeutic strategies.

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.

A natural antisense transcript at the Huntington's disease repeat locus regulates HTT expression

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in exon 1 of huntingtin (HTT). Relatively little attention has been directed to the genomic features of the antisense strand at the HD locus, though the presence of a transcript from this strand has been suggested by a survey of the entire transcriptome and the existence of several EST tags. In this study, we identified huntingtin antisense (HTTAS), a natural antisense transcript at the HD repeat locus that contain the repeat tract. HTTAS is 5' capped, poly (A) tailed and contains three exons, alternatively spliced into HTTAS_v1 (exons 1 and 3) and HTTAS_v2 (exons 2 and 3). Exon 1 includes the repeat. HTTAS_v1 has a weak promoter, and is expressed at low levels in multiple tissue types and throughout the brain. Reporter assays indicate that while efficient promoter activity requires a short repeat, repeat expansion reduces promoter efficiency. Consistent with the reporter assays, levels of HTTAS_v1 are reduced in human HD frontal cortex. In cell systems, overexpression of HTTAS_v1 specifically reduces endogenous HTT transcript levels, while siRNA knockdown of HTTAS_v1 increases HTT transcript levels. Minigene constructs of the HD locus confirm the regulatory effect of HTTAS_v1 on HTT, and demonstrate that the effect is dependent on repeat length and is at least partially Dicer dependent. Together, these findings provide strong evidence for the existence of a gene antisense to HTT, with properties that include regulation of HTT expression.

Are polyglutamine diseases expanding?

It remains a matter of speculation as to whether the sense CUG-containing RNA and/or the antisense CAG-encoding polyglutamine peptide serves as the pathogenic moiety in Huntington's disease like-2 (HDL2). In this issue of Neuron, Wilburn et al. show that in a HDL2 mouse model, the polyglutamine peptide drives disease progression.