Somatic mosaicism in the central nervous system in spinocerebellar ataxia type 1 and Machado-Joseph disease

Stool is a sensitive and noninvasive source of DNA for monitoring expansion in repeat expansion disease mouse models

Repeat expansion diseases are a large group of human genetic disorders caused by expansion of a specific short tandem repeat tract. Expansion in somatic cells affects age of onset and disease severity in some of these disorders. However, alleles in DNA derived from blood, a commonly used source of DNA, usually show much less expansion than disease-relevant cells in the central nervous system in both humans and mouse models. Here we examined the extent of expansion in different DNA sources from mouse models of the fragile X-related disorders, Huntington's disease, spinocerebellar ataxia type 1 and spinocerebellar ataxia type 2. We found that DNA isolated from stool is a much better indicator of somatic expansion than DNA from blood. As stool is a sensitive and noninvasive source of DNA, it can be useful for studies of factors affecting the risk of expansion, or the monitoring of treatments aimed at reducing expansion in preclinical trials, as it would allow expansions to be examined longitudinally in the same animal and allow significant changes in expansion to be observed much earlier than is possible with other DNA sources.

Summary: Stool is a readily available, noninvasive and sensitive source of DNA for monitoring repeat expansion in mouse models of four different repeat expansion diseases.

Mutant Ataxin-3-Containing Aggregates (MATAGGs) in Spinocerebellar Ataxia Type 3: Dynamics of the Disorder

Spinocerebellar ataxia type 3 (SCA3) is the most common type of SCA worldwide caused by abnormal polyglutamine expansion in the coding region of the ataxin-3 gene. Ataxin-3 is a multi-faceted protein involved in various cellular processes such as deubiquitination, cytoskeletal organisation, and transcriptional regulation. The presence of an expanded poly(Q) stretch leads to altered processing and misfolding of the protein culminating in the production of insoluble protein aggregates in the cell. Various post-translational modifications affect ataxin-3 fibrillation and aggregation. This review provides an exhaustive assessment of the various pathogenic mechanisms undertaken by the mutant ataxin-3-containing aggregates (MATAGGs) for disease induction and neurodegeneration. This includes in-depth discussion on MATAGG dynamics including their formation, role in neuronal pathogenesis, and the debate over the toxic v/s protective nature of the MATAGGs in disease progression. Additionally, the currently available therapeutic strategies against SCA3 have been reviewed. The shift in the focus of such strategies, from targeting the steps that lead to or reduce aggregate formation to targeting the expression of mutant ataxin-3 itself via RNA-based therapeutics, has also been presented. We also discuss the intriguing promise that various growth and neurotrophic factors, especially the insulin pathway, hold in the modulation of SCA3 progression. These emerging areas show the newer directions through which SCA3 can be targeted including various preclinical and clinical trials. All these advances made in the last three decades since the discovery of the ataxin-3 gene have been critically reviewed here.

The Contribution of Somatic Expansion of the CAG Repeat to Symptomatic Development in Huntington's Disease: A Historical Perspective

The discovery in the early 1990s of the expansion of unstable simple sequence repeats as the causative mutation for a number of inherited human disorders, including Huntington’s disease (HD), opened up a new era of human genetics and provided explanations for some old problems. In particular, an inverse association between the number of repeats inherited and age at onset, and unprecedented levels of germline instability, biased toward further expansion, provided an explanation for the wide symptomatic variability and anticipation observed in HD and many of these disorders. The repeats were also revealed to be somatically unstable in a process that is expansion-biased, age-dependent and tissue-specific, features that are now increasingly recognised as contributory to the age-dependence, progressive nature and tissue specificity of the symptoms of HD, and at least some related disorders. With much of the data deriving from affected individuals, and model systems, somatic expansions have been revealed to arise in a cell division-independent manner in critical target tissues via a mechanism involving key components of the DNA mismatch repair pathway. These insights have opened new approaches to thinking about how the disease could be treated by suppressing somatic expansion and revealed novel protein targets for intervention. Exciting times lie ahead in turning these insights into novel therapies for HD and related disorders.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

DNA Repair in Huntington's Disease and Spinocerebellar Ataxias: Somatic Instability and Alternative Hypotheses

The use of genome wide association studies (GWAS) in Huntington's disease (HD) research, driven by unbiased human data analysis, has transformed the focus of new targets that could affect age at onset. While there is a significant depth of information on DNA damage repair, with many drugs and drug targets, most of this development has taken place in the context of cancer therapy. DNA damage repair in neurons does not rely on DNA replication correction mechanisms. However, there is a strong connection between DNA repair and neuronal metabolism, mediated by nucleotide salvaging and the poly ADP-ribose (PAR) response, and this connection has been implicated in other age-onset neurodegenerative diseases. Validation of leads including the mismatch repair protein MSH3, and interstrand cross-link repair protein FAN1, suggest the mechanism is driven by somatic CAG instability, which is supported by the protective effect of CAA substitutions in the CAG tract. We currently do not understand: how somatic instability is triggered; the state of DNA damage within expanding alleles in the brain; whether this damage induces mismatch repair and interstrand cross-link pathways; whether instability mediates toxicity, and how this relates to human ageing. We discuss DNA damage pathways uncovered by HD GWAS, known roles of other polyglutamine disease proteins in DNA damage repair, and a panel of hypotheses for pathogenic mechanisms.

Phenotypic variance in monozygotic twins with SCA3

BACKGROUND

Spinocerebellar ataxia type 3 (SCA3) is a hereditary neurodegenerative disorder with high clinical heterogeneity. Twin study is valuable to estimate the contributions of gene and/or environment to phenotypic variance. However, SCA3 twins were extremely sparse and rarely reported.

METHODS

A pair of monozygotic twins with SCA3 was assessed using well-acknowledged scales. Genetic modifiers and methylation levels were determined by Sanger sequencing and pyrosequencing.

RESULTS

Sharing identical CAG repeat lengths, the twins presented with similar symptoms, whereas, the younger sister had an earlier age at onset of two years. The occurrence time and severity of constipation, blepharospasm and fasciculation were markedly different between the twins. Notable methylation level differences of several CpG sites existed between the twins.

CONCLUSIONS

It is the first time to report SCA3 monozygotic twin worldwide. The role of epigenetic factors in the phenotype variance deserved more attention. The DNA methylation may influence the phenotypic variance by altering the occurrence time and severity of symptoms, indicating its potential in alleviating the disease.

Discovery of Therapeutic Approaches for Polyglutamine Diseases: A Summary of Recent Efforts

Polyglutamine (PolyQ) diseases are a group of neurodegenerative disorders caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the coding region of specific genes. This leads to the production of pathogenic proteins containing critically expanded tracts of glutamines. Although polyQ diseases are individually rare, the fact that these nine diseases are irreversibly progressive over 10 to 30 years, severely impairing and ultimately fatal, usually implicating the full-time patient support by a caregiver for long time periods, makes their economic and social impact quite significant. This has led several researchers worldwide to investigate the pathogenic mechanism(s) and therapeutic strategies for polyQ diseases. Although research in the field has grown notably in the last decades, we are still far from having an effective treatment to offer patients, and the decision of which compounds should be translated to the clinics may be very challenging. In this review, we provide a comprehensive and critical overview of the most recent drug discovery efforts in the field of polyQ diseases, including the most relevant findings emerging from two different types of approaches-hypothesis-based candidate molecule testing and hypothesis-free unbiased drug screenings. We hereby summarize and reflect on the preclinical studies as well as all the clinical trials performed to date, aiming to provide a useful framework for increasingly successful future drug discovery and development efforts.

Spinocerebellar ataxia type 3/Machado-Joseph disease: segregation patterns and factors influencing instability of expanded CAG transmissions

Controversies about Mendelian segregation and CAG expansion (CAGexp) instabilities during meiosis in spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) need clarification. Additional evidence about these issues was obtained from the cohort of all SCA3/MJD individuals living in South Brazil. A survey was carried out to update information registered since 2001. Deaths were checked with the Public Information System, and data was made anonymous. Anticipation and delta-CAGexp from parent-offspring pairs, and delta-CAGexp between siblings were obtained. One hundred and fifty-nine families (94% of the entire registry) were retrieved, comprising 3725 living individuals as of 2015, 625 of these being symptomatic. Minimal prevalence was 6:100,000. Carriers of a CAGexp represented 65.6% of sibs in the genotyped offspring (p < 0.001). Median instability was larger among paternal than maternal transmissions, and instabilities correlated with anticipation (r = 0.38; p = 0.001). Age of the parent correlated to delta-CAGexp among 115 direct parent-offspring CAGexp transmissions (ρ = 0.23, p = 0.014). In 98 additional kindreds, the delta-CAGexp between 269 siblings correlated with their delta-of-age (ρ = 0.27, p < 0.0001). SCA3/MJD was associated with a segregation distortion favoring the expanded allele in our cohort. Instability of expansion during meiosis was weakly influenced by the age of the transmitting parent at the time of conception.

Unstable repeat expansions in neurodegenerative diseases: nucleocytoplasmic transport emerges on the scene

An astonishing number of neurological diseases result from expansion of unstable repetitive sequences causing alterations in key neuronal processes. Some are progressive late-onset conditions related to aging, such as the spinocerebellar ataxias. In several of these pathologies, the expanded repeat is transcribed, producing an expanded RNA repeat that causes neurodegeneration by a complex mechanism, comprising 3 main pathways. These include (1) accumulation in the nucleus of RNA foci, resulting from sequestration of RNA-binding proteins functioning in important neuronal cascades; (2) decrease in availability of RNA-binding proteins, such as splicing factors, causing alternative splicing misregulation with imbalance in the expression ratio of neuronal isoforms; and (3) generation of neurotoxic peptides, produced from repeat-associated non-ATG-initiated translation across the RNA repeat, in all reading frames. Recently, 2 pathologies characterized by impaired motor function, cognitive decline, or/and degeneration of motor neurons have been found that have broaden our understanding of these diseases. Moreover, the finding of compromised nucleocytoplasmic transport opens new avenues for research. This review will cover the amazing progress regarding these conditions.

Expression levels of DNA replication and repair genes predict regional somatic repeat instability in the brain but are not altered by polyglutamine disease protein expression or age

Expansion of CAG/CTG trinucleotide repeats causes numerous inherited neurological disorders, including Huntington's disease (HD), several spinocerebellar ataxias and myotonic dystrophy type 1. Expanded repeats are genetically unstable with a propensity to further expand when transmitted from parents to offspring. For many alleles with expanded repeats, extensive somatic mosaicism has been documented. For CAG repeat diseases, dramatic instability has been documented in the striatum, with larger expansions noted with advancing age. In contrast, only modest instability occurs in the cerebellum. Using microarray expression analysis, we sought to identify the genetic basis of these regional instability differences by comparing gene expression in the striatum and cerebellum of aged wild-type C57BL/6J mice. We identified eight candidate genes enriched in cerebellum, and validated four--Pcna, Rpa1, Msh6 and Fen1--along with a highly associated interactor, Lig1. We also explored whether expression levels of mismatch repair (MMR) proteins are altered in a line of HD transgenic mice, R6/2, that is known to show pronounced regional repeat instability. Compared with wild-type littermates, MMR expression levels were not significantly altered in R6/2 mice regardless of age. Interestingly, expression levels of these candidates were significantly increased in the cerebellum of control and HD human samples in comparison to striatum. Together, our data suggest that elevated expression levels of DNA replication and repair proteins in cerebellum may act as a safeguard against repeat instability, and may account for the dramatically reduced somatic instability present in this brain region, compared with the marked instability observed in the striatum.

Association between repeat sizes and clinical and pathological characteristics in carriers of C9ORF72 repeat expansions (Xpansize-72): a cross-sectional cohort study

BACKGROUND

Hexanucleotide repeat expansions in chromosome 9 open reading frame 72 (C9ORF72) are the most common known genetic cause of frontotemporal dementia (FTD) and motor neuron disease (MND). We assessed whether expansion size is associated with disease severity or phenotype.

METHODS

We did a cross-sectional Southern blot characterisation study (Xpansize-72) in a cohort of individuals with FTD, MND, both these diseases, or no clinical phenotype. All participants had GGGGCC repeat expansions in C9ORF72, and high quality DNA was available from one or more of the frontal cortex, cerebellum, or blood. We used Southern blotting techniques and densitometry to estimate the repeat size of the most abundant expansion species. We compared repeat sizes between different tissues using Wilcoxon rank sum and Wilcoxon signed rank tests, and between disease subgroups using Kruskal-Wallis rank sum tests. We assessed the association of repeat size with age at onset and age at collection using a Spearman's test of correlation, and assessed the association between repeat size and survival after disease onset using Cox proportional hazards regression models.

FINDINGS

We included 84 individuals with C9ORF72 expansions: 35 had FTD, 16 had FTD and MND, 30 had MND, and three had no clinical phenotype. We focused our analysis on three major tissue subgroups: frontal cortex (available from 41 patients [21 with FTD, 11 with FTD and MND, and nine with MND]), cerebellum (40 patients [20 with FTD, 12 with FTD and MND, and eight with MND]), and blood (47 patients [15 with FTD, nine with FTD and MND, and 23 with MND] and three carriers who had no clinical phenotype). Repeat lengths in the cerebellum were smaller (median 12·3 kb [about 1667 repeat units], IQR 11·1-14·3) than those in the frontal cortex (33·8 kb [about 5250 repeat units], 23·5-44·9; p<0·0001) and those in blood (18·6 kb [about 2717 repeat units], 13·9-28·1; p=0·0002). Within these tissues, we detected no difference in repeat length between disease subgroups (cerebellum p=0·96, frontal cortex p=0·27, blood p=0·10). In the frontal cortex of patients with FTD, repeat length correlated with age at onset (r=0·63; p=0·003) and age at sample collection (r=0·58; p=0·006); we did not detect such a correlation in samples from the cerebellum or blood. When assessing cerebellum samples from the overall cohort, survival after disease onset was 4·8 years (IQR 3·0-7·4) in the group with expansions greater than 1467 repeat units (the 25th percentile of repeat lengths) versus 7·4 years (6·3-10·9) in the group with smaller expansions (HR 3·27, 95% CI 1·34-7·95; p=0·009).

INTERPRETATION

We detected substantial variation in repeat sizes between samples from the cerebellum, frontal cortex, and blood, and longer repeat sizes in the cerebellum seem to be associated with a survival disadvantage. Our findings indicate that expansion size does affect disease severity, which--if replicated in other cohorts--could be relevant for genetic counselling.

FUNDING

The ALS Therapy Alliance, the National Institute of Neurological Disorders and Stroke, the National Institute on Aging, the Arizona Department of Health Services, the Arizona Biomedical Research Commission, and the Michael J Fox Foundation for Parkinson's Research.

Abnormal base excision repair at trinucleotide repeats associated with diseases: a tissue-selective mechanism

More than fifteen genetic diseases, including Huntington’s disease, myotonic dystrophy 1, fragile X syndrome and Friedreich ataxia, are caused by the aberrant expansion of a trinucleotide repeat. The mutation is unstable and further expands in specific cells or tissues with time, which can accelerate disease progression. DNA damage and base excision repair (BER) are involved in repeat instability and might contribute to the tissue selectivity of the process. In this review, we will discuss the mechanisms of trinucleotide repeat instability, focusing more specifically on the role of BER.

Somatic instability of CTG repeats in the cerebellum of myotonic dystrophy type 1

INTRODUCTION

We statistically analyzed somatic instability of the CTG expansion in the central nervous system and visceral organs in 7 patients with myotonic dystrophy type 1 and also report intracerebellar instability in 2 patients.

METHODS

CTG repeat expansion was estimated in the samples from autopsied brains and visceral organs by Southern blot analysis. Pathological study was performed. Samples were taken from several sites in the cerebellum to examine intracerebellar instability.

RESULTS

The CTG repeat expansion was shortest in the cerebellar cortex among all tissues examined. With regard to the intracerebellar difference, the shortest expansion was seen in the cortices of the hemisphere and vermis, whereas it was moderate in the dentate nucleus and longest in the white matter of the hemisphere and middle cerebellar peduncle.

CONCLUSIONS

The shortest expansion might be attributable to packed granule cells in the cerebellar cortex. Further analysis of cell-specific methylation states might elucidate the enigma of somatic instability.

Peripheral nerve involvement in hereditary cerebellar and multisystem degenerative disorders

Hereditary ataxias (HA) encompass an increasing number of degenerative disorders characterized by progressive cerebellar ataxia usually accompanied by extracerebellar semeiology including peripheral nerve involvement. Classically, HA were classified according to their pathological hallmark comprising three main forms: (1) spinal form predominantly with degeneration of spinocerebellar tracts, posterior columns, and pyramidal tracts (Friedreich's ataxia, FA); (2) olivopontocerebellar atrophy (OPCA); and (3) cortical cerebellar atrophy (CCA). In the 1980s Harding proposed a clinico-genetic classification based upon age of onset, modality of transmission, and clinical semeiology. The main categories in this classification were as follows: (1) early onset cerebellar ataxia (EOCA) with age of onset below 25 years and usually with autosomal recessive (AR) transmission (this group encompasses FA and syndromes different from FA); (2) autosomal dominant cerebellar ataxia (ADCA) with adult onset and with either cerebellar-plus syndrome or pure cerebellar semeiology; and (3) idiopathic late onset onset cerebellar ataxia (ILOCA). With the advent of molecular genetics, the nosology of HA has been in a state of constant flux. At present EOCA comprises at least 17 genotypes (designated with the acronym of ARCA derived from AR cerebellar ataxia), whereas under the umbrella of ADCA 30 genotypes have been reported. In this chapter we will review peripheral nerve involvement in classical pathological entities (OPCA and CCA), ARCA, ADCA, and ILOCA paying special attention to the most prevalent syndromes in each category. As a general rule, nerve involvement is relatively common in any form of ataxia except ILOCA, the most common pattern being either sensory or sensorimotor neuronopathy with a dying-back process. An exception to this rule is AR spastic ataxia of Charlevoix-Saguenay where nerve conduction studies show the characteristic pattern of intermediate neuropathy implying that sacsin mutation causes both axonal and Schwann cell dysfunction.

Transcription elongation and tissue-specific somatic CAG instability

The expansion of CAG/CTG repeats is responsible for many diseases, including Huntington's disease (HD) and myotonic dystrophy 1. CAG/CTG expansions are unstable in selective somatic tissues, which accelerates disease progression. The mechanisms underlying repeat instability are complex, and it remains unclear whether chromatin structure and/or transcription contribute to somatic CAG/CTG instability in vivo. To address these issues, we investigated the relationship between CAG instability, chromatin structure, and transcription at the HD locus using the R6/1 and R6/2 HD transgenic mouse lines. These mice express a similar transgene, albeit integrated at a different site, and recapitulate HD tissue-specific instability. We show that instability rates are increased in R6/2 tissues as compared to R6/1 matched-samples. High transgene expression levels and chromatin accessibility correlated with the increased CAG instability of R6/2 mice. Transgene mRNA and H3K4 trimethylation at the HD locus were increased, whereas H3K9 dimethylation was reduced in R6/2 tissues relative to R6/1 matched-tissues. However, the levels of transgene expression and these specific histone marks were similar in the striatum and cerebellum, two tissues showing very different CAG instability levels, irrespective of mouse line. Interestingly, the levels of elongating RNA Pol II at the HD locus, but not the initiating form of RNA Pol II, were tissue-specific and correlated with CAG instability levels. Similarly, H3K36 trimethylation, a mark associated with transcription elongation, was specifically increased at the HD locus in the striatum and not in the cerebellum. Together, our data support the view that transcription modulates somatic CAG instability in vivo. More specifically, our results suggest for the first time that transcription elongation is regulated in a tissue-dependent manner, contributing to tissue-selective CAG instability.

A review of the inherited ataxias: recent advances in genetic, clinical and neuropathologic aspects

Inherited ataxias are a heterogeneous group of disorders characterized by autosomal dominant and recessive inheritance. Recent advances in genetic research have resulted in an improved comprehension of their clinical presentation. Autosomal dominant cerebellar ataxias (ADCAs) include spinocerebellar ataxias (SCAs) and dentatorubral-pallidoluysian atrophy (DRPLA); six of these have been found to be trinucleotide repeat disorders. Episodic ataxia, types 1 and 2, are at present recognized to be channelopathies, caused by point mutations. Friedreich's ataxia (FA) which is an autosomal recessive disorder, resulting from a a unique trinucleotide repeat, is now recognized to have a wide age of onset and clinical spectrum. Ataxia-telangiectasia (AT), also an autosomal recessive cerebellar ataxia, is characterized by immunodeficiency. In this article, the genetic and clinical characteristics of these diseases are reviewed in detail.

The nucleotide sequence, DNA damage location, and protein stoichiometry influence the base excision repair outcome at CAG/CTG repeats

Expansion of CAG/CTG repeats is the underlying cause of >14 genetic disorders, including Huntington's disease (HD) and myotonic dystrophy. The mutational process is ongoing, with increases in repeat size enhancing the toxicity of the expansion in specific tissues. In many repeat diseases, the repeats exhibit high instability in the striatum, whereas instability is minimal in the cerebellum. We provide molecular insights into how base excision repair (BER) protein stoichiometry may contribute to the tissue-selective instability of CAG/CTG repeats by using specific repair assays. Oligonucleotide substrates with an abasic site were mixed with either reconstituted BER protein stoichiometries mimicking the levels present in HD mouse striatum or cerebellum, or with protein extracts prepared from HD mouse striatum or cerebellum. In both cases, the repair efficiency at CAG/CTG repeats and at control DNA sequences was markedly reduced under the striatal conditions, likely because of the lower level of APE1, FEN1, and LIG1. Damage located toward the 5' end of the repeat tract was poorly repaired, with the accumulation of incompletely processed intermediates as compared to an AP lesion in the center or at the 3' end of the repeats or within control sequences. Moreover, repair of lesions at the 5' end of CAG or CTG repeats involved multinucleotide synthesis, particularly at the cerebellar stoichiometry, suggesting that long-patch BER processes lesions at sequences susceptible to hairpin formation. Our results show that the BER stoichiometry, nucleotide sequence, and DNA damage position modulate repair outcome and suggest that a suboptimal long-patch BER activity promotes CAG/CTG repeat instability.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

Clinical evaluation of oropharyngeal dysphagia in Machado-Joseph disease

CONTEXT: In Machado-Joseph disease, poor posture, dystonia and peripheral neuropathy are extremely predisposing to oropharyngeal dysphagia, which is more commonly associated with muscular dystrophy. OBJECTIVE: To evaluate the clinical characteristics of oropharyngeal dysphagia in Machado-Joseph disease patients. METHOD: Forty individuals participated in this study, including 20 with no clinical complaints and 20 dysphagic patients with Machado-Joseph disease of clinical type 1, who were all similar in terms of gender distribution, average age, and cognitive function. The medical history of each patient was reviewed and each subject underwent a clinical evaluation of deglutition. At the end, the profile of dysphagia in patients with Machado-Joseph disease was classified according to the Severity Scale of Dysphagia, as described by O'Neil and collaborators. RESULTS: Comparison between dysphagic patients and controls did not reveal many significant differences with respect to the clinical evaluation of the oral phase of deglutition, since afflicted patients only demonstrated deficits related to the protrusion, retraction and tonus of the tongue. However, several significant differences were observed with respect to the pharyngeal phase. Dysphagic patients presented pharyngeal stasis during deglutition of liquids and solids, accompanied by coughing and/or choking as well as penetration and/or aspiration; these signs were absent in the controls. CONCLUSIONS: Oropharyngeal dysphagia is part of the Machado-Joseph disease since the first neurological manifestations. There is greater involvement of the pharyngeal phase, in relation to oral phase of the deglutition. The dysphagia of these patients is classified between mild and moderate.

Differential vulnerability of neurons in Huntington's disease: the role of cell type-specific features

Abnormal expansion of a polyglutamine tract in huntingtin (Htt) protein results in Huntington's disease (HD), an autosomal dominant neurodegenerative disorder involving progressive loss of motor and cognitive function. Contrasting with the ubiquitous tissue expression of polyglutamine-expanded Htt, HD pathology is characterized by the increased vulnerability of specific neuronal populations within the striatum and the cerebral cortex. Morphological, biochemical, and functional characteristics of neurons affected in HD that might render these cells more vulnerable to the toxic effects of polyglutamine-Htt are covered in this review. The differential vulnerability of neurons observed in HD is discussed in the context of various major pathogenic mechanisms proposed to date, and in line with evidence showing a 'dying-back' pattern of degeneration in affected neuronal populations.

The (CAG)n tract of Machado-Joseph Disease gene (ATXN3): a comparison between DNA and mRNA in patients and controls

Machado-Joseph disease (MJD) is an autosomal dominant neurodegenerative disorder of late onset (occurring at a mean age of 40.2 years). The clinical manifestation of MJD is dependent on the presence of an expansion of the (CAG)(n) motif within exon 10 of the ATXN3 gene, located at 14q32.1. The variance in onset of MJD is only partially correlated (approximately 50-80%) with the extension of the CAG tract in genomic DNA (gDNA). The main aim of this work was to determine whether there are discrepancies in the size of the (CAG)(n) tract between gDNA and mRNA, and to establish whether there is a better association between age at onset and repeat size at the mRNA level. We typed gDNA and cDNA samples for the (CAG)(n) tract totalizing 108 wild-type and 52 expanded ATXN3 alleles. In wild-type alleles no differences were found between gDNA and cDNA. In expanded alleles, the CAG repeat size in gDNA was not always directly transcribed into the mRNA; on average there were differences of +1 repeat at the cDNA level. The slight discrepancies obtained were insufficient to cause significant differences in the distribution of the expanded alleles, and therefore no improvement in onset variance explanation was obtained with mRNA.

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.

Clinical aspects of CAG repeat diseases

Seven neurodegenerative disorders are known to be caused by unstable expansions of the trinucleotide CAG within human genes, and more will be discovered in the coming years. These disorders share some clinical similarities, as well as some differences, which are summarized here. These diseases have unusual clinical genetic properties related to the dynamic nature of CAG repeat expansions, including instability of the repeat expansion in meiosis, particularly male meiosis; a strong correlation between onset age and size of the repeat expansion; anticipation (earlier disease onset in succeeding generations); new mutations arising from unstable, mutable alleles with a high-normal CAG repeat number; and reduced penetrance for alleles in the low-affected range. Much more remains to be learned about the molecular biology and clinical pathophysiology of this new class of genetic diseases.

Trinucleotide repeat instability: genetic features and molecular mechanisms

Trinucleotide repeat expansions are an important cause of inherited neurodegenerative disease. The expanded repeats are unstable, changing in size when transmitted from parents to offspring (intergenerational instability, "meiotic instability") and often showing size variation within the tissues of an affected individual (somatic mosaicism, "mitotic instability"). Repeat instability is a clinically important phenomenon, as increasing repeat lengths correlate with an earlier age of onset and a more severe disease phenotype. The tendency of expanded trinucleotide repeats to increase in length during their transmission from parent to offspring in these diseases provides a molecular explanation for anticipation (increasing disease severity in successive affected generations). In this review, I explore the genetic and molecular basis of trinucleotide repeat instability. Studies of patients and families with trinucleotide repeat disorders have revealed a number of factors that determine the rate and magnitude of trinucleotide repeat change. Analysis of trinucleotide repeat instability in bacteria, yeast, and mice has yielded additional insights. Despite these advances, the pathways and mechanisms underlying trinucleotide repeat instability in humans remain largely unknown. There are many reasons to suspect that this uniquely human phenomenon will significantly impact upon our understanding of development, differentiation and neurobiology.

Repeat instability: mechanisms of dynamic mutations

Disease-causing repeat instability is an important and unique form of mutation that is linked to more than 40 neurological, neurodegenerative and neuromuscular disorders. DNA repeat expansion mutations are dynamic and ongoing within tissues and across generations. The patterns of inherited and tissue-specific instability are determined by both gene-specific cis-elements and trans-acting DNA metabolic proteins. Repeat instability probably involves the formation of unusual DNA structures during DNA replication, repair and recombination. Experimental advances towards explaining the mechanisms of repeat instability have broadened our understanding of this mutational process. They have revealed surprising ways in which metabolic pathways can drive or protect from repeat instability.

Intranuclear inclusions, neuronal loss and CAG mosaicism in two patients with Machado-Joseph disease

The presence of neuronal intranuclear inclusions (NIIs) and neuronal mosaicism has been described in some autosomal dominant spinocerebellar ataxias (SCA), but their implication in neurodegenerative mechanisms still remains unclear.

OBJECTIVE

To investigate the correlation between neuronal loss and NIIs, and the size of CAG triplet expansion in selected areas of the CNS in two SCA3 patients.

MATERIAL AND METHODS

Postmortem neuropathological study was carried out, and the regional distribution of neuronal loss was compared with NIIs. CAG expansion was analysed by PCR amplification in the same regions.

RESULTS

Marked neuronal loss was seen in the anterior horn of the spinal cord, pontine nuclei and motor nuclei of the brain stem. Moderate neurone loss was found in the locus ceruleus, colliculus and substantia nigra. Loss of granule and Purkinje cells was found in the cerebellum, mainly in the vermis. NIIs were present in neurones of the involved nuclei of the anterior horn of the spinal cord, medulla oblongata and pons, but not in the locus ceruleus, substantia nigra and cerebellum. A few NIIs were found in the striatum. The number of CAG repeats was 27/70 in the first patient and 21/74 in the second patient. The variation of the expanded allele size among different cerebral areas was +/-1-3 CAG repeats.

CONCLUSION

The partial correlation between neuronal loss and NIIs suggests that other factors distinct from NII formation may be involved in the neuronal death. Moreover, the low degree of mosaicism between regions without neuronal loss and regions with marked neuronal loss points to the existence of selective cellular vulnerability to the genetic defect.

Somatic mosaicism of the CAG repeat expansion in spinocerebellar ataxia type 3/Machado-Joseph disease

An expanded and unstable CAG repeat in the coding region of the MJD1 gene is the mutation responsible for spinocerebellar ataxia 3/Machado-Joseph disease. In order to determine whether there was a higher degree of instability in affected regions, the size of the expanded CAG repeat was analyzed in different regions of the central nervous system, in two unrelated SCA3/MJD patients. The degree of somatic mosaicism was quantified and compared to that in a SCA1 patient. Instability of the expanded CAG repeat was observed in peripheral tissues as well as in CNS of the three patients, but there was no correlation between the degree of mosaicism and the selective vulnerability of CNS structures. As in the other diseases caused by expanded CAG repeats, a lower degree of mosaicism was found in the cerebellar cortex of both SCA1 and SCA3/MJD patients, probably reflecting specific properties of this structure. In SCA3/MJD, the degree of mosaicism seemed to correlate with age at death rather than with the size of the expanded CAG repeat. Finally, somatic instability was more pronounced in SCA1 than in SCA3/MJD patients.

Immunogenicity of P/Q-type calcium channel in small cell lung cancer: investigation of alpha1 subunit polyglutamine expansion

The ectopic expression of neuronal P/Q-type voltage-gated calcium channels in small cell lung carcinoma (SCLC) is thought to induce antisynaptic autoimmunity in the paraneoplastic Lambert-Eaton myasthenic syndrome. The gene CACNL1A4, encoding the principal (alpha1A) subunit of this calcium channel, is mutated in several inherited neurological disorders. One of these disorders (spinocerebellar ataxia, type 6, or SCA-6) involves the expansion of a trinucleotide (CAG) repeat unit. We hypothesized that a somatic CAG repeat instability of this gene in neoplastic cells might generate a non-self epitope capable of initiating autoimmunity to P/Q-type calcium channels. We therefore analyzed the CACNL1A4 gene in SCLC lines established from metastases derived from seven individual patients (four associated with Lambert-Eaton myasthenic syndrome, one associated with myasthenia gravis, and two not associated with neurological autoimmunity). We compared their CAG repeat numbers (determined by polymerase chain reaction (PCR) amplification followed by separation of products on a 6% polyacrylamide/8M urea gel) to published norms and to DNA from a patient with SCA-6. The number of CAG repeats in SCLC DNA fell within a normal range whether or not the neoplasm was complicated by neurological autoimmunity. Therefore, it is unlikely that somatically unstable CAG repeat units in the gene encoding the P/Q-type voltage-gated calcium channel account for this tumor protein's immunogenicity in the Lambert-Eaton myasthenic syndrome.

Tissue-specific somatic mosaicism in spinal and bulbar muscular atrophy is dependent on CAG-repeat length and androgen receptor--gene expression level

The factors influencing the tissue-specific pattern of somatic mosaicism in CAG-repeat diseases have not yet been fully resolved. We performed a detailed analysis of the degree of somatic mosaicism in various tissues from 20 patients with spinal and bulbar muscular atrophy (SBMA), including 4 who were deceased. The most outstanding feature was the prominent somatic mosaicism observed in the cardiac and skeletal muscles, composed predominantly of postmitotic cells, and in the skin, prostate, and testis. The CNS tissues, liver, and spleen showed the least mosaicism. The tissue distribution of somatic mosaicism in patients with SBMA was markedly different from that in patients with Huntington disease (HD) and from that in patients with dentatorubral-pallidoluysian atrophy (DRPLA). The degree of somatic mosaicism correlated with the CAG-repeat number but not with age at examination. Furthermore, tissues with a higher mosaicism level corresponded well to those with a higher expression level of androgen receptor protein. The tissue-specific pattern of somatic mosaicism related not only to cell composition with different cell turnover rates but to repeat size and gene expression levels, and postnatal cell division is unlikely to be a major cause of somatic mosaicism probably because of the relative stability of CAG repeat in SBMA.

The complex clinical and genetic classification of inherited ataxias. I. Dominant ataxias

The clinical classification of autosomal dominant cerebellar ataxias (ADCAs) is intricate due to the variable and unpredictable association of signs and symptoms of central nervous system (CNS) and peripheral nervous system (PNS) deterioration during the life of a patient. However, for many purposes, particularly patient management, clinical systematics is the most useful method for labelling patients; in some instances there is no basis for any more fundamental classification of phenotypes. On the other hand, recent molecular-genetic approaches to dominant ataxias have had a profound impact in nosology, diagnostic procedures and the management of patients, since they are based on the fact that all mendelian neurological diseases can be precisely classified according to the locus involved as well as the particular mutant allele at that locus. Therefore, a clinical and genetic classification of dominant ataxias is herewith proposed as the best nosographical choice. Clinical, neuropathological, genetic, and pathogenetic aspects of ADCAs are reviewed and discussed to help the clinical neurologist guide diagnostic procedures and manage ataxic patients.

A mild case of Friedreich ataxia: lymphocyte and sural nerve analysis for GAA repeat length reveals somatic mosaicism

Friedreich ataxia (FRDA) is an autosomal recessive, neurodegenerative disease, characterized by progressive gait and limb ataxia, dysarthria, lower-limb areflexia, Babinski sign, loss of position and vibration senses, cardiomyopathy, and carbohydrate intolerance. It is the most common inherited ataxia, and is associated with a GAA triplet repeat expansion in the first intron of the X25 gene on the long arm of chromosome 9. We present a case whose clinical diagnosis was initially confounded by the mildness of the ataxic phenotype and a family history of multiple sclerosis. Evaluation of the X25 gene revealed that the patient was homozygous for the GAA triplet repeat expansion, pathognomonic of FRDA. Investigation of her sural nerve biopsy revealed a significantly smaller expansion size, constituting the first direct demonstration of somatic mosaicism involving the nervous system in FRDA. We speculate that a similar contraction in pathologically affected tissues could be the molecular basis for the mildness of the ataxia.

Somatic mosaicism of the expanded CAG trinucleotide repeat in mRNAs for the responsible gene of Machado-Joseph disease (MJD), dentatorubral-pallidoluysian atrophy (DRPLA), and spinal and bulbar muscular atrophy (SBMA)

The CAG trinucleotide repeats in mRNAs for the responsible genes of Machado-Joseph disease (MJD), dentatorubral-pallidoluysian atrophy (DRPLA), and X-linked spinal and bulbal muscular atrophy (SBMA) were examined in various neural and nonneural tissues of affected individuals. The tissue-specific variation of expanded CAG repeat alleles were apparent for mRNAs of all three genes. The expanded CAG repeats of the mRNA were shorter in the cerebellum than in other regions of the central nervous system in DRPLA and MJD, but not in SBMA, and were longer in the liver and colon in MJD. Transcripts of the responsible genes with expanded CAG repeats were detected in all tissues studied, and the tissue-specific variation in the CAG repeat size of the mRNA did not correlate with the tissue-specific severity of pathological involvement in these diseases.

Molecular genetics of the hereditary ataxias

One of us (MP) learned about the mapping of Huntington disease gene to chromosome 4 from the late Dr. Anita Harding. She got the news over the phone from her London office during a visit to Italy for a meeting on hereditary ataxias. In Britain, they receive Nature at least a week earlier than us. Dr. Harding was very excited, and she immediately said that that was the way to go if we wanted to understand the causes of hereditary ataxias, classify these diseases in a rational way, and eventually find a treatment. At that time, the challenge seemed, and indeed was, formidable. No clue was then available about the genetic basis of what Dr. Harding aptly called "hereditary ataxias of unknown cause," their classification was confused and controversial, and all attempts to find specific biochemical abnormalities had failed. Fourteen years later, the success of the molecular genetic studies is astounding. The defective genes have been identified for Friedreich ataxia, the major recessive "hereditary ataxia of unknown cause," and for five dominantly inherited "hereditary ataxias of unknown cause." Three more dominant ataxia genes have been mapped. The molecular pathogenesis of the dominant ataxias begins to be unraveled and animal models have been and are being developed. Information is also quickly accumulating about the defective protein in Friedreich ataxia. Direct molecular diagnosis is now possible. Classification has been revolutionized. Diagnostic criteria are being redefined in the light of the molecular discoveries. The goal of this review, dedicated to the memory of the late Dr. Harding, is to offer a concise summary of current knowledge about the molecular genetics of some of the hereditary ataxias that used to be classified as of "unknown cause."

Molecular characteristics of Machado-Joseph disease mutation in 25 newly described Brazilian families

Machado-Joseph disease (MJD) is a form of autosomal dominant spinocerebellar ataxia first described in North-American patients originating from the Portuguese islands of the Azores. Clinically this disorder is characterized by late onset progressive ataxia with associated features, such as: ophthalmoplegia, pyramidal and extrapyramidal signs and distal muscular atrophies. The causative mutation is an expansion of a CAG repeat in the coding region of the MJD1 gene. We have identified 25 unrelated families segregating the MJD mutation during a large collaborative study of spinocerebellar ataxias in Brazil. In the present study a total of 62 family members were genotyped for the CAG repeat in the MJD1 gene, as well as 63 non-MJD individuals (126 normal chromosomes), used as normal controls. We observed a wide gap between the size range of the normal and expanded CAG repeats: the normal allele had from 12 to 33 CAGs (mean = 23 CAGs), whereas the expanded alleles ranged from 66 to 78 CAGs (mean = 71.5 CAGs). There were no differences in CAG tract length according to gender of affected individuals or transmitting parent. We observed a significant negative correlation between age at onset of the disease and length of the CAG tract in the expended allele (r = -0.6, P = 0.00006); however, the size of the expanded CAG repeat could explain only about 40% of the variability in age at onset (r2 = 0.4). There was instability of the expanded CAG tract during transmission from parent to offspring, both expansions and contractions were observed; however, there was an overall tendency for expansion, with a mean increase of +2.4 CAGs. The tendency for expansion appeared to the greater in paternal (mean increase of +3.5 CAGs) than in maternal transmissions (mean increase of +1.3 CAGs). Anticipation was observed in all transmissions in which ages at onset for parent and offspring were known; however, anticipation was not always associated with an increase in the expanded CAG repeat length. Our results indicate that the molecular diagnosis of MJD can be confirmed or excluded in all suspected individuals, since alleles of intermediary size were not observed.

The neuropathology of CAG repeat diseases: review and update of genetic and molecular features

Classification of inherited neurodegenerative diseases is increasingly based on their genetic features, which supplement, clarify, and sometimes replace the older clinical and pathologic schemata. This change has been particularly rapid and impressive for the CAG repeat disorders. In Huntington's disease, X-linked spinobulbar muscular atrophy, dentatorubropallidoluysian atrophy, and a series of autosomal dominant cerebellar atrophies, genetic advances have resolved many nosologic issues, and opened new avenues for exploration of pathogenesis. In this review, we summarize classic and current concepts in neuropathology of these CAG repeat diseases.