Autosomal dominant cerebellar ataxias: clinical features, genetics, and pathogenesis

Prevalence of hereditary ataxia and spastic paraplegia in southeast Norway: a population-based study

A population-based, cross-sectional study was performed in southeast Norway, between January 2002 and February 2008, to identify subjects with hereditary ataxia and hereditary spastic paraplegia, and to estimate the prevalence of these disorders. Patients were recruited through colleagues, families, searches in computerized hospital archives and the National Patients' Association for Hereditary Ataxia and Spastic Paraplegia. Strict criteria were used for inclusion of familial and isolated subjects. A project neurologist examined all index subjects and clinical and genetic data were registered. The source population on January 1, 2008 was 2.63 million and the prevalence day was set as February 1, 2008. One hundred seventy-one subjects from 87 unrelated families with hereditary ataxia and 194 subjects from 65 unrelated families with hereditary spastic paraplegia were included. The total prevalence was estimated at 13.9/100 000. Hereditary ataxia prevalence in the region was estimated at 6.5/100 000: 4.2/100 000 for autosomal-dominant and 2.3/100 000 for autosomal recessive, 0.15/100 000 for Friedreich's ataxia and 0.4/100 000 for ataxia telangiectasia. Hereditary spastic paraplegia prevalence was 7.4/100 000: 5.5/100 000 for autosomal dominant-hereditary spastic paraplegia, 0.6/100 000 for autosomal recessive-hereditary spastic paraplegia and 1.3/100 000 for isolated subjects. Marked differences were found in the frequencies of hereditary ataxia subtypes compared with other countries, while those of the most common autosomal dominant-hereditary spastic paraplegia genotypes, SPG4, SPG3 and SPG31, were similar to those previously reported. Clear variations between age groups and counties were observed, but no gender differences. Mean age on prevalence day was 48 years, mean age at onset was 24 years. We present the largest population study performed on hereditary ataxia and hereditary spastic paraplegia prevalence and report a higher prevalence than expected. Better inclusion criteria and multiple search strategies may explain the observed differences.

Frequencies of spinocerebellar ataxia subtypes in Thailand: window to the population history?

Spinocerebellar ataxias (SCAs) are a heterogeneous group of disorders with almost 30 subtypes. The prevalence and relative frequency of each subtype vary among different populations. In this article, we report the relative frequency of six SCA subtypes in the Thai population and attempt to explain the observed pattern when compared with other populations in this region. We searched for SCA type 1, SCA2, SCA3, SCA6, SCA7 and dentatorubral-pallidoluysian atrophy mutations using GeneScan analysis in 340 patients from 182 families, in which at least one person had a clinical diagnosis of SCA. We analyzed the relative frequencies of SCA subtypes on a family basis, and compared these with the data in the Chinese and Indian populations. SCA3 was found in 19.2% of the patients (Agresti-Coull 95% confidence interval: 14.1-25.6%), SCA1 in 11.5% (7.6-17.1%) and SCA2 in 10.4% (6.7-15.8%). SCA6 was found in three families, with a relative frequency of 1.6% (0.3-5.0%). Compared with the related populations, the Thai SCA3 frequency was less than that of the Chinese, whereas it was higher than that in most of the Indian studies. The reverse is true for the SCA1/SCA2 frequency. A similar study in Singapore, where there was a clear history of population admixture, also showed the frequencies between those of the Chinese and the Indian populations. Although SCA3 was the most common identifiable SCA subtype in Thailand, SCA1 and SCA2 were also relatively common. Our results also supported some degree of admixture with the Indians in the Thai population and justify further study in the area.

Therapeutic prospects for spinocerebellar ataxia type 2 and 3

Spinocerebellar ataxia type 2 (SCA2) and type 3 (SCA3) are autosomal-dominant neurodegenerative disorders. SCA2 primarily affects cerebellar Purkinje neurons. SCA3 primarily affects dentate and pontine nuclei and substantia nigra. Both disorders belong to a class of polyglutamine (polyQ) expansion disorders. SCA2 is caused by a polyQ expansion in the amino-terminal region of a cytosolic protein ataxin-2 (Atxn2). SCA3 is caused by a polyQ expansion in the carboxy-terminal portion of a cytosolic protein ataxin-3 (Atxn3). Both disorders are found worldwide, but SCA2 is common among people of Cuban decent and SCA3 is common among people of Portuguese decent. No effective treatment exist for SCA2, SCA3 or any other polyQ-expansion disorder. Based on anecdotal evidence, a number of small scale clinical trials have been attempted previously for SCA2 and SCA3. These trials were underpowered and did not yield any promising results so far. A number of pathogenic mechanisms have been proposed to explain neuronal dysfunction and degeneration in SCA2 and SCA3. Knockdown of mutant Atxn2 and Atxn3 protein by RNAi or similar approach is most promising avenue of therapeutic development in the long term, but translation of this approach to clinic faces very serious technical challenges. Recent preclinical studies in SCA2 and SCA3 genetic mouse model suggested that abnormal neuronal calcium (Ca²⁺) signaling may play an important role in SCA2 and SCA3 pathology. These studies also suggested that dantrolene and other Ca²⁺ signaling inhibitors and stabilizers may have a therapeutic value for treatment of SCA2 and SCA3. Controlled clinical evaluation of dantrolene, memantine, riluzole, dihydropyridines, CoQ10, creatine or other Ca²⁺ blockers and stabilizers in SCA2 and SCA3 patients is necessary to test clinical importance of these ideas. The EUROSCA consortium provides a potential framework for such clinical evaluation.

The wobbly child: an approach to inherited ataxias

Genetic causes of ataxia are numerous. These disorders often present in the pediatric population, and finding a precise diagnosis can be quite challenging. Recent advances in molecular diagnosis make it difficult for the clinician to determine what investigations to undertake and in which order. This article presents 3 cases of pediatric onset ataxia with a genetic basis that will help to formulate and show a practical approach to this important clinical problem.

FGF14 regulates the intrinsic excitability of cerebellar Purkinje neurons

A missense mutation in the fibroblast growth factor 14 (FGF14) gene underlies SCA27, an autosomal dominant spinocerebellar ataxia in humans. Mice with a targeted disruption of the Fgf14 locus (Fgf14(-/-)) develop ataxia resembling human SCA27. We tested the hypothesis that loss of FGF14 affects the firing properties of Purkinje neurons, which play an important role in motor control and coordination. Current clamp recordings from Purkinje neurons in cerebellar slices revealed attenuated spontaneous firing in Fgf14(-/-) neurons. Unlike in the wild type animals, more than 80% of Fgf14(-/-) Purkinje neurons were quiescent and failed to fire repetitively in response to depolarizing current injections. Immunohistochemical examination revealed reduced expression of Nav1.6 protein in Fgf14(-/-) Purkinje neurons. Together, these observations suggest that FGF14 is required for normal Nav1.6 expression in Purkinje neurons, and that the loss of FGF14 impairs spontaneous and repetitive firing in Purkinje neurons by altering the expression of Nav1.6 channels.

A duplication at chromosome 11q12.2-11q12.3 is associated with spinocerebellar ataxia type 20

Spinocerebellar ataxia type 20 (SCA20) has been linked to chromosome 11q12, but the underlying genetic defect has yet to be identified. We applied single-nucleotide polymorphism genotyping to detect structural alterations in the genomic DNA of patients with SCA20. We found a 260 kb duplication within the previously linked SCA20 region, which was confirmed by quantitative polymerase chain reaction and fiber fluorescence in situ hybridization, the latter also showing its direct orientation. The duplication spans 10 known and 2 unknown genes, and is present in all affected individuals in the single reported SCA20 pedigree. While the mechanism whereby this duplication may be pathogenic remains to be established, we speculate that the critical gene within the duplicated segment may be DAGLA, the product of which is normally present at the base of Purkinje cell dendritic spines and contributes to the modulation of parallel fiber-Purkinje cell synapses.

Brain white matter damage in SCA1 and SCA2. An in vivo study using voxel-based morphometry, histogram analysis of mean diffusivity and tract-based spatial statistics

BACKGROUND AND PURPOSE

Neurodegeneration in spinocerebellar ataxia type 1(SCA1) and 2(SCA2) is associated with white matter(WM) damage. Voxel-Based Morphometry(VBM), histogram analysis of mean diffusivity(MD) and Tract-Based Spatial Statistics(TBSS) enable an in vivo quantitative analysis of WM volume and structure. We assessed with these 3 techniques the whole brain WM damage in SCA1 and SCA2.

PATIENTS AND METHODS

Ten patients with SCA1, 10 patients with SCA2 and 10 controls underwent MRI with acquisition of T1-weighted and diffusion tensor images. The results were correlated with severity of clinical deficit.

RESULTS

VBM showed atrophy of the brainstem and cerebellar WM without significant differences between SCA1 and SCA2. Focal atrophy of the cerebral subcortical WM was also present. Histogram analysis revealed increased MD in the brainstem and cerebellum in patients with SCA1 and SCA2 which in SCA2 was more pronounced and combined with mild increase of the MD in the cerebral hemispheres in SCA2. In SCA1 and SCA2 TBSS revealed decreased fractional anisotropy(FA) in the inferior, middle and superior cerebellar peduncles, pontine transverse fibres, medial and lateral lemnisci, spinothalamic tracts, corticospinal tracts and corpus callosum. The extent of tract changes was greater in SCA2 patients who also showed decreased FA in the short intracerebellar tracts. In both diseases VBM, histogram and TBSS results correlated with clinical severity.

CONCLUSIONS

Brain WM damage featuring a pontocerebeellar atrophy is similar in SCA1 and SCA2 but more pronounced in SCA2. In both diseases it correlates with severity of the clinical deficit.

New insights into the pathoanatomy of spinocerebellar ataxia type 3 (Machado-Joseph disease)

PURPOSE OF REVIEW

This review summarizes recent neuropathological findings in spinocerebellar ataxia type 3 and discusses their relevance for clinical neurology.

RECENT FINDINGS

The extent of the spinocerebellar ataxia type 3 related central nervous neurodegenerative changes has been recently systematically investigated in a series of pathoanatomical studies. These studies showed that the extent of the central nervous degenerative changes of spinocerebellar ataxia type 3 has been underestimated so far. The newly described pattern of central nervous neurodegeneration includes the visual, auditory, vestibular, somatosensory, ingestion-related, dopaminergic and cholinergic systems. These pathological findings were correlated with clinical findings and explain a variety of the spinocerebellar ataxia type 3 symptoms observed in clinical practice.

SUMMARY

Systematic pathoanatomical analysis of spinocerebellar ataxia type 3 brains helps to understand the structural basis of this neurodegenerative disease and offers explanations for a variety of disease symptoms. This better understanding of the neuropathology of the condition has implications for the treatment of spinocerebellar ataxia type 3 patients and represents a basis for further biochemical and molecular biological studies aimed at deciphering the pathomechanisms of this progressive ataxic disorder.

Prolonged cortical silent period but normal sensorimotor plasticity in spinocerebellar ataxia 6

Spinocerebellar ataxia 6 (SCA6) is a hereditary disease characterized by a trinucleotide repeat expansion in the CACNA1A gene and late-onset bilateral cerebellar atrophy. It is unclear if there is significant pathology outside of the cerebellum. We used transcranial magnetic stimulation to assess sensorimotor cortical circuits and cortical plasticity in 8 SCA6 patients and 8 age-matched controls. Behavioral performance was assessed using a rhythmic tapping task. Neurophysiological measures of SCA6 patients showed a prolonged cortical silent period (CSP) but normal MEP recruitment curve, short-latency afferent inhibition, long-latency afferent inhibition and ipsilateral silent period. Paired-associative stimulation induction also increased motor-evoked potentials normally. SCA6 patients had greater variability with cued rhythmic tapping than normals and deteriorated when the cue was removed; in comparison, normal subjects had similar variability between cued and uncued rhythmic tapping. Analysis using a Wing-Kristofferson timing model indicated that both clock variance and motor delay variance were abnormal. Conclusion. In SCA6, the circuits for sensorimotor integration and the mechanisms for LTP-like plasticity in the sensorimotor cortex are unimpaired. A prolonged CSP in SCA6 just like in other cerebellar atrophies would suggest that this neurophysiological change typifies cerebellar dysfunction.

Application of microarray outlier detection methodology to psychiatric research

BACKGROUND

Most microarray data processing methods negate extreme expression values or alter them so that they do not lie outside the mean level of variation of the system. While microarrays generate a substantial amount of false positive and spurious results, some of the extreme expression values may be valid and could represent true biological findings.

METHODS

We propose a simple method to screen brain microarray data to detect individual differences across a psychiatric sample set. We demonstrate in two different samples how this method can be applied.

RESULTS

This method targets high-throughput technology to psychiatric research on a subject-specific basis.

CONCLUSION

Assessing microarray data for both mean group effects and individual effects can lead to more robust findings in psychiatric genetics.

The involvement of DNA-damage and -repair defects in neurological dysfunction

A genetic link between defects in DNA repair and neurological abnormalities has been well established through studies of inherited disorders such as ataxia telangiectasia and xeroderma pigmentosum. In this review, we present a comprehensive summary of the major types of DNA damage, the molecular pathways that function in their repair, and the connection between defective DNA-repair responses and specific neurological disease. Particular attention is given to describing the nature of the repair defect and its relationship to the manifestation of the associated neurological dysfunction. Finally, the review touches upon the role of oxidative stress, a leading precursor to DNA damage, in the development of certain neurodegenerative pathologies, such as Alzheimer's and Parkinson's.

Spinocerebellar ataxia type 7 mimicking Kearns–Sayre syndrome: A clinical diagnosis is desirable

Spinocerebellar ataxias are a group of autosomal dominant cerebellar degenerative disorders, which are characterized by clinical and genetic variability. Spinocerebellar ataxia type 7 (SCA7) is less variable in clinical presentation than other SCAs. We present a pediatric patient with 13 and 70 trinucleotide CAG repeats within SCA7 gene and no family history, whose presentation mimicked Kearns-Sayre syndrome (KSS). We review the differential diagnosis of cerebellar ataxia with vision loss secondary to retinal pigmentary dystrophy. This paper supports concept of a desirable clinical diagnosis to avoid multiple genetic or invasive testing in children with neurodegenerative disorders.

Early onset autosomal dominant spinocerebellar ataxia with miosis: four cases

Previously, at least 29 different forms of autosomal dominant spinocerebellar ataxias (SCAs) have been described. We describe a family with four members through three generations with autosomal dominant ataxia in combination with miosis and hyperreflexia. This family's ataxia does not match any of the previously described SCAs and is probably a novel form of SCA. To continue with the search for the genetic background of this disease, more cases are needed.

Mutations in TTBK2, encoding a kinase implicated in tau phosphorylation, segregate with spinocerebellar ataxia type 11

The microtubule-associated protein tau (encoded by MAPT) and several tau kinases have been implicated in neurodegeneration, but only MAPT has a proven role in disease. We identified mutations in the gene encoding tau tubulin kinase 2 (TTBK2) as the cause of spinocerebellar ataxia type 11. Affected brain tissue showed substantial cerebellar degeneration and tau deposition. These data suggest that TTBK2 is important in the tau cascade and in spinocerebellar degeneration.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Diffusion tensor imaging of spinocerebellar ataxias types 1 and 2

BACKGROUND AND PURPOSE

Structural MR imaging does not enable reliable differentiation of spinocerebellar ataxia (SCA) types 1 and 2 (SCA1 and SCA2), and imaging may be normal during the first years after the onset of symptoms. We aimed at determining whether measurements of the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) may enable their differentiation.

MATERIALS AND METHODS

We enrolled 14 patients with SCA1, 11 with SCA2, and 9 age-matched controls. Diffusion tensor imaging (DTI) was performed on a 1.5T scanner, with b = 1000s/mm2 and 12 directions. ADC and FA were measured by means of regions of interest, positioned in the corticospinal tract at the level of the cerebral peduncle and at the level of the pons, in the transverse pontine fibers, in the superior and middle cerebellar peduncle, and in the hemispheric cerebellar white matter.

RESULTS

With respect to controls, the ADC was significantly elevated in the middle cerebellar peduncle and in hemispheric white matter in SCA1, and in all regions under consideration in SCA2. It was significantly higher in SCA2 than in SCA1 in all regions under consideration. With respect to controls, the FA was significantly reduced in all regions under consideration in SCA1 and in SCA2. It was significantly lower in SCA2 than in SCA1 in the transverse pontine fibers and in the corticospinal tract at the level of the cerebral peduncle. Correlations with clinical scores were found.

CONCLUSIONS

DTI did not enable differentiation between SCA1 and SCA2. However, strongly significant differences between the 2 subtypes and with respect to controls and correlations with clinical scores were found.

Inclusion formation by ataxins -1, -2, -3, and -7

The authors studied inclusion formation in vitro using transiently transfected PC12 cells, with epitope-tagged and untagged full-length and truncated wild-type and expanded ataxins -1, -2, -3, and -7. At 72 hours, no inclusions were seen with wild-type full-length or truncated ataxins -2, -3, or -7, and only one with ataxin-1. Truncation abolished nuclear localization of ataxins -1 and -7, and allowed nuclear entry of ataxin-2. Of the expanded ataxins, only -1 and -2 formed inclusions, and those of ataxin-2 were rare and exclusively cytoplasmic. Truncation resulted in inclusion formation by ataxins -3 and -7, increased ataxin-1 inclusions, and enabled formation of nuclear ataxin-2 inclusions. There was no recruitment of wild-type ataxin-1 to expanded ataxin-1 inclusions.

A universal mechanism ties genotype to phenotype in trinucleotide diseases

Trinucleotide hereditary diseases such as Huntington disease and Friedreich ataxia are cureless diseases associated with inheriting an abnormally large number of DNA trinucleotide repeats in a gene. The genes associated with different diseases are unrelated and harbor a trinucleotide repeat in different functional regions; therefore, it is striking that many of these diseases have similar correlations between their genotype, namely the number of inherited repeats and age of onset and progression phenotype. These correlations remain unexplained despite more than a decade of research. Although mechanisms have been proposed for several trinucleotide diseases, none of the proposals, being disease-specific, can account for the commonalities among these diseases. Here, we propose a universal mechanism in which length-dependent somatic repeat expansion occurs during the patient's lifetime toward a pathological threshold. Our mechanism uniformly explains for the first time to our knowledge the genotype–phenotype correlations common to trinucleotide disease and is well-supported by both experimental and clinical data. In addition, mathematical analysis of the mechanism provides simple explanations to a wide range of phenomena such as the exponential decrease of the age-of-onset curve, similar onset but faster progression in patients with Huntington disease with homozygous versus heterozygous mutation, and correlation of age of onset with length of the short allele but not with the long allele in Friedreich ataxia. If our proposed universal mechanism proves to be the core component of the actual mechanisms of specific trinucleotide diseases, it would open the search for a uniform treatment for all these diseases, possibly by delaying the somatic expansion process.

Trinucleotide diseases are a broad family of hereditary diseases characterized genetically by an expanded DNA region consisting of a repeated three-letter code. Patients inheriting such an abnormal DNA region experience sudden disease onset at an age that inversely depends on the size of the expanded region, followed by inevitable and highly predictable suffering and death. Despite more than a decade of research, the underlying mechanism of these diseases remains an enigma. Although the genes implicated with the various trinucleotide diseases are unrelated, and the defects in these genes occur in different parts of the DNA coding for the gene, the diseases' shared characteristics suggest a common mechanism underlies their root cause. We suggest a mechanism that uniformly explains how the inherited DNA repeats genetically encode the time of onset and the rate of progression of trinucleotide diseases. It suggests the disease manifests and progresses through the further expansion of the inherited abnormally expanded DNA region. It explains the clinical data of many diseases in this family, including previously unexplained onset-related phenomena. It also predicts that a general therapy for these diseases would be a drug or procedure that successfully interferes with the ongoing expansion of the disease trinucleotide repeat.

Genetic analysis of SCA 2 and 3 repeat expansions in essential tremor and atypical Parkinsonism

Anecdotal reports suggest that patients with spinocerebellar ataxia (SCA 2) patients can present with postural tremor with ataxia. We determined the prevalence of SCA2 and SCA3 mutations in a cohort of ET and atypical Parkinsonism patients. A total of 277 subjects comprising of 177 ET and 100 atypical Parkinsonism were examined. We identified one positive case of SCA3 among those who were diagnosed with ET, yielding a prevalence of 0.5%, but a zero prevalence among our atypical Parkinsonism patients. No study subjects carried an abnormal SCA2 repeat expansion. Our study highlights that SCA3 can present initially with ET symptoms, expanding the spectrum of genetic diseases that can be associated with ET-like phenotype. Routine screening for SCA2 and SCA3 in ET and atypical Parkinsonism patients may not be cost effective. However, in the long-term follow-up of patients who present with an ET phenotype, clinicians should be vigilant for other neurological signs, which may be point to an alternate diagnosis.

Survival estimates for patients with Machado-Joseph disease (SCA3)

Machado-Joseph disease (MJD), one of the most prevalent autosomal dominant cerebellar ataxias, is a neurodegenerative disease that starts during adulthood, with patients showing difficulties in gait, later becoming bedridden, and ultimately presenting premature death. There is, however, scarce data quantifying disease impact on patient survival. We investigated the overall survival of a large series of MJD patients and compared it with the survival of their asymptomatic relatives. A total of 412 affected and 413 unaffected individuals were ascertained from a consecutive sample of 82 families with a molecular diagnosis of MJD. Estimated mean survival time was 63.96 years [95% confidence interval (CI), 62.09-65.83] for the affected group and 78.61 years (95% CI, 74.75-82.47) for the unaffected group (p < 0.001). For a subset of 366 patients, mean age at onset was 36.37 years (95% CI, 35.21-37.53) and survival after disease onset was estimated as 21.18 years. Early onset and large CAG length predicted shorter overall survival times. This study presents quantitative data on the impact of MJD on overall survival, a phenomenon that is related to CAG length, age at onset, and year of birth.

Huntingtin has a membrane association signal that can modulate huntingtin aggregation, nuclear entry and toxicity

Huntington's disease is caused by an expanded polyglutamine tract in huntingtin protein, leading to accumulation of huntingtin in the nuclei of striatal neurons. The 18 amino-acid amino-terminus of huntingtin is an amphipathic alpha helical membrane-binding domain that can reversibly target to vesicles and the endoplasmic reticulum (ER). The association of huntingtin to the ER is affected by ER stress. A single point mutation in huntingtin 1-18 predicted to disrupt this helical structure displayed striking phenotypes of complete inhibition of polyglutamine-mediated aggregation, increased huntingtin nuclear accumulation and greatly increased mutant huntingtin toxicity in a striatal-derived mouse cell line. Huntingtin vesicular interaction mediated by 1-18 is specific to late endosomes and autophagic vesicles. We propose that huntingtin has a normal biological function as an ER-associated protein that can translocate to the nucleus and back out in response to ER stress or other events. The increased nuclear entry of mutant huntingtin due to loss of ER-targeting results in increased toxicity.

Machado-Joseph disease enhances genetic fitness: a comparison between affected and unaffected women and between MJD and the general population

BACKGROUND

Machado-Joseph disease (MJD SCA3), a spinocerebellar ataxia related to expansion of a CAG tract, has already been related to anticipation and meiotic drift. However, fitness of MJD carriers has been little studied.

OBJECTIVE

To analyze genetic fitness of MJD patients, comparing them to their unaffected relatives and to the general population (GP) of origin.

SUBJECTS AND METHODS

182 informants, belonging to 82 MJD families, agreed to participate in the study. Informants supplied data about 828 MJD patients. Number of children (NC), gender, age, school attainment, menarche and menopause were compared between general and emeritus (older than 45 years of age or deceased) groups.

RESULTS

Mean NC of the GP and of MJD patients were respectively 1.90 and 2.93+/-2.3 (p = 0.0037). Comparisons within families also showed differences: the mean NC of unaffected and affected emeritus MJD women were, respectively, 2.68 and 3.89 (p = 0.0037). Affected MJD women had earlier mean ages at the delivery of their first child and menopause (p < 0.011 and 0.07, respectively). Among affected women those who did not have children had larger CAG tracts than those who had children (p < 0.05).

CONCLUSION

MJD enhances the fitness of its carriers, and this phenomenon seems to have a biological basis.

Nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3: in vivo evidence

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat in the MJD1 gene resulting in an expanded polyglutamine repeat in the ataxin-3 protein. To study the course of the disease, we generated transgenic mice for SCA3 using full-length ataxin-3 constructs containing 15, 70, or 148 CAG repeats, respectively. Control mice (15 CAGs) were phenotypically normal and had no neuropathological findings. However, mice transgenic for ataxin-3 with expanded polyglutamine repeats were severely affected by a strong neurological phenotype with tremor, behavioral deficits, strongly reduced motor and exploratory activity, a hunchback, and premature death at 3 to 6 months of age. Neuropathological examination by immunohistochemical staining revealed ubiquitin- and ataxin-3-positive intranuclear inclusion bodies in a multitude of neurons. Directing ataxin-3 with 148 CAGs to the nucleus revealed an even more pronounced phenotype with more inclusions and earlier death, whereas mice transgenic with the same construct but attached to a nuclear export signal developed a milder phenotype with less inclusions. These studies indicate that nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3 in vivo.

Distinct distribution of autosomal dominant spinocerebellar ataxia in the Mexican population

Dominant ataxias show wide geographic variation. We analyzed 108 dominant families and 123 sporadic ataxia patients from Mexico for mutations causing SCA1-3, 6-8, 10, 12, 17 and DRPLA. Only 18.5% of dominant families remained undiagnosed; SCA2 accounted for half (45.4%), followed by SCA10 (13.9%), SCA3 (12%), SCA7 (7.4%), and SCA17 (2.8%). None had SCA1, 6, 8, 12 or DRPLA. Among sporadic cases, 6 had SCA2 (4.9%), and 2 had SCA17 (1.6%). In the SCA2 patients we identified 6 individuals with the rare (CAG)(33) allele, 2 of whom showed early onset ataxia. The distribution of dominant ataxia mutations in Mexicans is distinct from other populations.

Redefining the disease locus of 16q22.1-linked autosomal dominant cerebellar ataxia

The 16q22.1-linked autosomal dominant cerebellar ataxia (16q-ADCA; Online Mendelian Inheritance in Man [OMIN] #117210) is one of the most common ADCAs in Japan. Previously, we had reported that the patients share a common haplotype by founder effect and that a C-to-T substitution (-16C>T) in the puratrophin-1 gene was strongly associated with the disease. However, recently, an exceptional patient without the substitution was reported, indicating that a true pathogenic mutation might be present elsewhere. In this study, we clarified the disease locus more definitely by the haplotype analysis of families showing pure cerebellar ataxia. In addition to microsatellite markers, the single nucleotide polymorphisms (SNPs) that we identified on the disease chromosome were examined to confirm the borders of the disease locus. The analysis of 64 families with the -16C>T substitution in the puratrophin-1 gene revealed one family showing an ancestral recombination event between SNP04 and SNP05 on the disease chromosome. The analysis of 22 families without identifiable genetic mutations revealed another family carrying the common haplotype centromeric to the puratrophin-1 gene, but lacking the -16C>T substitution in this gene. We concluded that the disease locus of 16q-ADCA was definitely confined to a 900-kb genomic region between the SNP04 and the -16C>T substitution in the puratrophin-1 gene in 16q22.1.

Are interrupted SCA2 CAG repeat expansions responsible for parkinsonism?

BACKGROUND

Autosomal dominant parkinsonism (ADP) is caused in a large percentage of familial and sporadic cases by mutations in the LRRK2 gene, particularly G2019S. It is also caused by mutations in genes associated with autosomal dominant cerebellar ataxia (ADCA), notably CAG/CAA repeat expansions in SCA2.

METHODS

We screened 164 families with ADP for expansions in the SCA2, 3, and 17 genes and for the G2019S mutation in LRRK2. The SCA2 CAG/CAA repeat expansion was sequenced to determine its structure. The phenotypes of patients with ADP caused by the SCA2, LRRK2, and unknown mutations were compared, as well as those of SCA2 patients with interrupted or uninterrupted expansions of the same size.

RESULTS

Three French ADP families had SCA2 mutations. The expansions ranged from 37 to 39 repeats and were interrupted and stable upon transmission. All patients (n = 9) had levodopa-responsive parkinsonism without cerebellar signs. They had significantly more symmetric signs and less rigidity than ADP caused by the G2019S mutation in LRRK2 or by unknown mutations. Interestingly, two sisters carrying both the SCA2 and the G2019S LRRK2 mutations had markedly earlier onset than their mother with only SCA2. In contrast, similar-sized but uninterrupted repeats were associated with ADCA in which cerebellar ataxia was constant and associated only rarely with one or more mild parkinsonian signs.

CONCLUSION

These results suggest that the configuration of the SCA2 CAG/CAA repeat expansions plays an important role in phenotype variability. Uninterrupted SCA2 repeat expansions found in families with autosomal dominant cerebellar ataxia result in somatic mosaicism and produce large hairpin RNAs, which may interact with double-stranded RNA-binding proteins. These characteristics are modified by interruption of the SCA2 repeat expansion as found in families with autosomal dominant parkinsonism.

Une crampe de l’écrivain associée à une ataxie spinocérébelleuse de type 7

Spinocerebellar ataxia type 7 (SCA 7) is a rare autosomal dominant neurodegenerative disorder (ADCA) caused by expansion of a highly unstable CAG repeat. Clinical features including progressive cerebellar, retinal degeneration and pyramidal signs. We report a patient with SCA 7 diagnosis revealed by progressive cerebellar ataxia and writer's cramp.