Autosomal dominant cerebellar ataxias: polyglutamine expansions and beyond

Hereditäre Ataxien

Hereditäre Ataxien stellen aufgrund der Vielfalt der möglichen genetischen Ursachen eine große diagnostische Herausforderung für die medizinische Genetik dar. Dieses Problem wird dadurch verstärkt, dass zwar die Zahl der neu identifizierten Gene in den letzten 3 Jahren durch neue Sequenziertechnologien rasant zugenommen hat, häufig jedoch nur wenige Familien weltweit Mutationen in diesen Genen aufweisen, d. h. sie extrem selten sind. Der vorliegende Artikel gibt eine Übersicht über dominante und rezessive Ataxien und berücksichtigt dabei auch die neu identifizierten Ataxie-Gene. Um den Anforderungen einer praktisch-orientierten genetischen Diagnostik gerecht zu werden, versuchen wir dabei auch, Häufigkeitseinschätzungen der betroffenen Genorte zu geben und – sofern möglich – phänotypische Eigenschaften und Biomarker zu definieren, die eine genetische Diagnostik erfolgversprechend leiten können, insbesondere bei rezessiven Ataxien. Diese diagnostischen Indikatoren werden in Form von diagnostischen Pfaden zusammengefasst, die eine Orientierung bei der mehrstufigen genetischen Diagnostik dominanter und rezessiver Ataxien geben sollen. Aufgrund der Vielzahl der Genkandidaten und des großen phänotypischen Überlappungsbereichs wird es in den meisten Fällen jedoch am zeiteffizientesten und kostengünstigsten sein, Panel-Untersuchungen mittels Next-Generation-Sequencing-Technologien durchzuführen.

Silencing mutant ATXN3 expression resolves molecular phenotypes in SCA3 transgenic mice

Spinocerebellar ataxia type 3 (SCA3) is a neurodegenerative disease caused by a polyglutamine expansion in the deubiquitinating enzyme, Ataxin-3. Currently, there are no effective treatments for this fatal disorder but studies support the hypothesis that reducing mutant Ataxin-3 protein levels might reverse or halt the progression of disease in SCA3. Here, we sought to modulate ATXN3 expression in vivo using RNA interference. We developed artificial microRNA mimics targeting the 3'-untranslated region (3'UTR) of human ATXN3 and then used recombinant adeno-associated virus to deliver them to the cerebellum of transgenic mice expressing the full human disease gene (SCA3/MJD84.2 mice). Anti-ATXN3 microRNA mimics effectively suppressed human ATXN3 expression in SCA3/MJD84.2 mice. Short-term treatment cleared the abnormal nuclear accumulation of mutant Ataxin-3 throughout the transduced SCA3/MJD84.2 cerebellum. Analysis also revealed changes in the steady-state levels of specific microRNAs in the cerebellum of SCA3/MJD84.2 mice, a previously uncharacterized molecular phenotype of SCA3 that appears to be dependent on mutant Ataxin-3 expression. Our findings support the preclinical development of molecular therapies aimed at halting the expression of ATXN3 as a viable approach to SCA3 and point to microRNA deregulation as a potential surrogate marker of SCA3 pathogenesis.

Genome-wide expression profiling and functional characterization of SCA28 lymphoblastoid cell lines reveal impairment in cell growth and activation of apoptotic pathways

Background

SCA28 is an autosomal dominant ataxia associated with AFG3L2 gene mutations. We performed a whole genome expression profiling using lymphoblastoid cell lines (LCLs) from four SCA28 patients and six unrelated healthy controls matched for sex and age.

Methods

Gene expression was evaluated with the Affymetrix GeneChip Human Genome U133A 2.0 Arrays and data were validated by real-time PCR.

Results

We found 66 genes whose expression was statistically different in SCA28 LCLs, 35 of which were up-regulated and 31 down-regulated. The differentially expressed genes were clustered in five functional categories: (1) regulation of cell proliferation; (2) regulation of programmed cell death; (3) response to oxidative stress; (4) cell adhesion, and (5) chemical homeostasis. To validate these data, we performed functional experiments that proved an impaired SCA28 LCLs growth compared to controls (p < 0.005), an increased number of cells in the G0/G1 phase (p < 0.001), and an increased mortality because of apoptosis (p < 0.05). We also showed that respiratory chain activity and reactive oxygen species levels was not altered, although lipid peroxidation in SCA28 LCLs was increased in basal conditions (p < 0.05). We did not detect mitochondrial DNA large deletions. An increase of TFAM, a crucial protein for mtDNA maintenance, and of DRP1, a key regulator of mitochondrial dynamic mechanism, suggested an alteration of fission/fusion pathways.

Conclusions

Whole genome expression profiling, performed on SCA28 LCLs, allowed us to identify five altered functional categories that characterize the SCA28 LCLs phenotype, the first reported in human cells to our knowledge.

Utilization of genetic testing prior to subspecialist referral for cerebellar ataxia

OBJECTIVE

To evaluate the utilization of laboratory testing in the diagnosis of cerebellar ataxia, including the completeness of initial standard testing for acquired causes, the early use of genetic testing, and associated clinical and nonclinical factors, among a cohort referred for subspecialty consultation.

METHODS

Data were abstracted from records of 95 consecutive ataxia patients referred to one neurogenetics subspecialist from 2006-2010 and linked to publicly available data on characteristics of referral clinicians. Multivariable logistic and linear regression models were used to analyze unique associations of clinical and nonclinical factors with laboratory investigation of acquired causes and with early genetic testing prior to referral.

RESULTS

At referral, 27 of 95 patients lacked evidence of any of 14 laboratory studies suggested for initial work-up of an acquired cause for ataxia (average number of tests=4.5). In contrast, 92% of patients had undergone brain magnetic resonance imaging prior to referral. Overall, 41.1% (n=39) had genetic testing prior to referral; there was no association between family history of ataxia and obtaining genetic testing prior to referral (p=0.39). The level of early genetic testing was 31.6%, primarily due to genetic testing despite an incomplete laboratory evaluation for acquired causes and no family history. A positive family history was consistently associated with less extensive laboratory testing (p=0.004), and referral by a neurologist was associated with higher levels of early genetic testing.

CONCLUSIONS

Among consecutive referrals to a single center, a substantial proportion of sporadic cases had genetic testing without evidence of a work-up for acquired causes. Better strategies to guide decision making and subspecialty referrals in rare neurologic disorders are needed, given the cost and consequences of genetic testing.

Biological and clinical characteristics of individuals at risk for spinocerebellar ataxia types 1, 2, 3, and 6 in the longitudinal RISCA study: analysis of baseline data

BACKGROUND

Spinocerebellar ataxias (SCAs) are autosomal, dominantly inherited, fully penetrant neurodegenerative diseases. Our aim was to study the preclinical stage of the most common SCAs: SCA1, SCA2, SCA3, and SCA6.

METHODS

Between Sept 13, 2008, and Dec 1, 2011, offspring or siblings of patients with SCA1, SCA2, SCA3, or SCA6 were enrolled into a prospective, longitudinal observational study at 14 European centres. To be eligible for inclusion in our study, individuals had to have no ataxia and be aged 18-50 years if directly related to individuals with SCA1, SCA2, or SCA3, or 35-70 years if directly related to individuals with SCA6. We did anonymous genetic testing to identify mutation carriers. We assessed participants with clinical scales, questionnaires, and performance-based coordination tests. In eight of the 14 centres, participants underwent MRI. We analysed relations between outcome variables and time from onset (defined as the difference between present age and estimated age at ataxia onset). This study is registered with ClinicalTrials.gov, number NCT01037777.

FINDINGS

276 participants met inclusion criteria and agreed to participate, of whom 12 (4%) were excluded from final analysis because DNA samples were missing or genotyping failed. Estimated time from onset was -9 years (IQR -13 to -6) in 50 carriers of the SCA1 mutation, -12 years (-15 to -9) in 31 SCA2 mutation carriers, -8 years (-11 to -6) in 26 SCA3 mutation carriers, and -18 years (-22 to -16) in 16 SCA6 mutation carriers. Compared with non-carriers of each mutation, SCA1 mutation carriers had higher median scores on the scale for the assessment and rating of ataxia (SARA; 0·5 [IQR 0-1·0] vs 0 [0-0]; p=0·0052), as did SCA2 mutation carriers (0·5 [0-2·0] vs 0 [0-0·5]; p=0·0037). SCA2 mutation carriers had lower SCA functional index scores than did non-carriers (-0·43 [-0·91 to -0·07] vs 0·09 [-0·30 to 0·56]; p=0·0007). SCA2 mutation carriers had worse composite cerebellar functional scores than did their non-carrier counterparts (0·915 [0·861-0·959] vs 0·849 [0·764-0·886]; p=0·0039). All other differences between carriers and non-carriers were non-significant. In SCA1 and SCA2 mutation carriers, SARA scores were increased in participants who were closer to the estimated age at onset (SCA1: r=0·36, p=0·0112; SCA2: r=0·50, p=0·0038). 83 individuals (30%) underwent MRI. Voxel-based morphometry showed grey-matter loss in the brainstem and cerebellum in SCA1 and SCA2 mutation carriers, and normalised brainstem volume was lower in SCA2 mutation carriers (median 0·015, range 0·012-0·016) than in non-carriers (0·019, 0·017-0·021; p=0·0107).

INTERPRETATION

Preclinical SCA1 and SCA2 mutation carriers seem to have mild coordination deficits and abnormalities in the brain that are more common in carriers who are closer to the estimated onset of ataxia. Individuals in this early disease stage could be targeted in future preventive trials.

FUNDING

ERA-Net E-Rare and Polish Ministry of Science and Higher Education.

How do C9ORF72 repeat expansions cause amyotrophic lateral sclerosis and frontotemporal dementia: can we learn from other noncoding repeat expansion disorders?

PURPOSE OF REVIEW

The aim of this review is to describe disease mechanisms by which chromosome 9 open reading frame 72 (C9ORF72) repeat expansions could lead to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and to discuss these diseases in relation to other noncoding repeat expansion disorders.

RECENT FINDINGS

ALS and FTD are complex neurodegenerative disorders with a considerable clinical and pathological overlap, and this overlap is further substantiated by the recent discovery of C9ORF72 repeat expansions. These repeat expansions are currently the most important genetic cause of familial ALS and FTD, accounting for approximately 34.2 and 25.9% of the cases. Clinical phenotypes associated with these repeat expansions are highly variable, and combinations with mutations in other ALS-associated and/or FTD-associated genes may contribute to this pleiotropy. It is challenging, however, to diagnose patients with C9ORF72 expansions, not only because of large repeat sizes, but also due to somatic heterogeneity. Most other noncoding repeat expansion disorders share an RNA gain-of-function disease mechanism, a mechanism that could underlie the development of ALS and/or FTD as well.

SUMMARY

The discovery of C9ORF72 repeat expansions provides novel insights into the pathogenesis of ALS and FTD and highlights the importance of noncoding repeat expansions and RNA toxicity in neurodegenerative diseases.

Autosomal dominant hereditary ataxia in Sri Lanka

Background

Spinocerebellar ataxias (SCA) are a group of hereditary neurodegenerative disorders. Prevalence of SCA subtypes differ worldwide. Autosomal dominant ataxias are the commonest types of inherited ataxias seen in Sri Lanka. The aim of the study is to determine the genetic etiology of patients with autosomal dominant ataxia in Sri Lanka and to describe the clinical features of each genetic subtype.

Methods

Thirty four patients with autosomal dominant ataxia were recruited. For every patient the following was done: recording of clinical details and genotyping for SCA 1, 2, 3, 6, 7, 8, 12, and 17.

Results

Sixty one per cent of the subjects were identified as SCA1. One subject had SCA2, 12 remain unidentified. Mean age at onset was 34.8 ± 10years for SCA1 and 32.7 ± 9.8 for non SCA1. 76% of SCA1 patients and 50% of non SCA1 were using walking aids. Quantification of symptoms and signs were similar in the SCA1 and non SCA1 groups. Clinical depression was evidenced in 68.4% of SCA1 and 75% non SCA-1 patients. Mean CAG repeat length in SCA1 patients was 52.0 ± 3.8, with greater anticipation seen with paternal inheritance.

Conclusion

SCA1 was the predominant subtype and showed similar phenotype to previous reports. However, disease severity was higher and depression more prevalent in this population than previously described.

Brain changes associated with postural training in patients with cerebellar degeneration: a voxel-based morphometry study

Recent research indicates that physiotherapy can improve motor performance of patients with cerebellar degeneration. Given the known contributions of the cerebellum to motor learning, it remains unclear whether such observable changes in performance are mediated by the cerebellum or cerebral brain areas involved in motor control and learning. The current study addressed this question by assessing the increase in gray matter volume due to sensorimotor training in cerebellar patients using voxel-based morphometry. Nineteen human subjects with pure cerebellar degeneration and matched healthy controls were trained for 2 weeks on a balance task. Postural and clinical assessments along with structural magnetic resonance imaging were performed pretraining and post-training. The main findings were as follows. First, training enhanced balance performance in cerebellar patients. Second, in contrast to controls patients revealed significantly more post-training gray matter volume in the dorsal premotor cortex. Third, training-related increase in gray matter volume was observed within the cerebellum and was more pronounced in controls than in patients. However, statistically cerebellar changes were at the trend level and thus require additional, independent confirmation. We conclude that sensorimotor training of patients with cerebellar neurodegeneration induces gray matter changes primarily within nonaffected neocortical regions of the cerebellar-cortical loop. Residual function of the cerebellum appears to be exploited suggesting either a recovery from degeneration or intact processes of cerebellar plasticity in the remaining healthy tissue.

Ocular motor characteristics of different subtypes of spinocerebellar ataxia: distinguishing features

Because of frequent involvement of the cerebellum and brainstem, ocular motor abnormalities are key features of spinocerebellar ataxias and may aid in differential diagnosis. Our objective for this study was to distinguish the subtypes by ophthalmologic features after head-shaking and positional maneuvers, which are not yet recognized as differential diagnostic tools in most common forms of spinocerebellar ataxias. Of the 302 patients with a diagnosis of cerebellar ataxia in 3 Korean University Hospitals from June 2011 to June 2012, 48 patients with spinocerebellar ataxia types 1, 2, 3, 6, 7, or 8 or with undetermined spinocerebellar ataxias were enrolled. All patients underwent a video-oculographic recording of fixation abnormalities, gaze-evoked nystagmus, positional and head-shaking nystagmus, and dysmetric saccades. Logistic regression analysis controlling for disease duration revealed that spontaneous and positional downbeat nystagmus and perverted head-shaking nystagmus were strong predictors for spinocerebellar ataxia 6, whereas saccadic intrusions and oscillations were identified as positive indicators of spinocerebellar ataxia 3. In contrast, the presence of gaze-evoked nystagmus and dysmetric saccades was a negative predictor of spinocerebellar ataxia 2. Positional maneuvers and horizontal head shaking occasionally induced or augmented saccadic intrusions/oscillations in patients with spinocerebellar ataxia types 1, 2, and 3 and undetermined spinocerebellar ataxia. The results indicated that perverted head-shaking nystagmus may be the most sensitive parameter for SCA6, whereas saccadic intrusions/oscillations are the most sensitive for spinocerebellar ataxia 3. In contrast, a paucity of gaze-evoked nystagmus and dysmetric saccades is more indicative of spinocerebellar ataxia 2. Head-shaking and positional maneuvers aid in defining ocular motor characteristics in spinocerebellar ataxias. © 2013 Movement Disorder Society.

[Have centers of rare neurological diseases changed their practices and management of the hereditary cerebellar ataxias?]

The classification and management of hereditary cerebellar ataxias have been considerably changed by advances made in the field of genetics. Given the numerous genes implicated in the disorders, genetic analysis, which alone can confirm the diagnosis, needs to be based on phenotypically precise studies. Diagnostic algorithms including both recessive and dominant forms of ataxia have been proposed. The range of disease effects has been further expanded in the light of evidence of ataxias associated with permutations of the Fragile X gene, and ataxias linked to mutations of the nuclear genes coding for structural proteins of mitochondrial DNA. In the field of therapeutics, several studies are currently ongoing for Friedreich's ataxia.

Identification and characterization of novel PDYN mutations in dominant cerebellar ataxia cases

We have recently identified missense mutations in prodynorphin (PDYN), the precursor to dynorphin opioid peptides, as the cause for spinocerebellar ataxia (SCA23) in Dutch ataxia cases. We report a screen of PDYN for mutations in 371 cerebellar ataxia cases, which had a positive family history; most are of French origin. Sequencing revealed three novel putative missense mutations and one heterozygous two-base pair deletion in four independent SCA patients. These variants were absent in 400 matched controls and are located in the highly conserved dynorphin domain. To resolve the pathogenicity of the heterozygous variants, we assessed the peptide production of the mutant PDYN proteins. Two missense mutations raised dynorphin peptide levels, the two-base pair deletion terminated dynorphin synthesis, and one missense mutation did not affect PDYN processing. Given the outcome of our functional analysis, we may have identified at least two novel PDYN mutations in a French and a Moroccan SCA patient. Our data corroborates recent work that also showed that PDYN mutations only account for a small percentage (~0.1 %) of European SCA cases.

Mutations in potassium channel kcnd3 cause spinocerebellar ataxia type 19

OBJECTIVE

To identify the causative gene for the neurodegenerative disorder spinocerebellar ataxia type 19 (SCA19) located on chromosomal region 1p21-q21.

METHODS

Exome sequencing was used to identify the causal mutation in a large SCA19 family. We then screened 230 ataxia families for mutations located in the same gene (KCND3, also known as Kv4.3) using high-resolution melting. SCA19 brain autopsy material was evaluated, and in vitro experiments using ectopic expression of wild-type and mutant Kv4.3 were used to study protein localization, stability, and channel activity by patch-clamping.

RESULTS

We detected a T352P mutation in the third extracellular loop of the voltage-gated potassium channel KCND3 that cosegregated with the disease phenotype in our original family. We identified 2 more novel missense mutations in the channel pore (M373I) and the S6 transmembrane domain (S390N) in 2 other ataxia families. T352P cerebellar autopsy material showed severe Purkinje cell degeneration, with abnormal intracellular accumulation and reduced protein levels of Kv4.3 in their soma. Ectopic expression of all mutant proteins in HeLa cells revealed retention in the endoplasmic reticulum and enhanced protein instability, in contrast to wild-type Kv4.3 that was localized on the plasma membrane. The regulatory β subunit Kv channel interacting protein 2 was able to rescue the membrane localization and the stability of 2 of the 3 mutant Kv4.3 complexes. However, this either did not restore the channel function of the membrane-located mutant Kv4.3 complexes or restored it only partially.

INTERPRETATION

KCND3 mutations cause SCA19 by impaired protein maturation and/or reduced channel function.

Mutations in KCND3 cause spinocerebellar ataxia type 22

OBJECTIVE

To identify the causative gene in spinocerebellar ataxia (SCA) 22, an autosomal dominant cerebellar ataxia mapped to chromosome 1p21-q23.

METHODS

We previously characterized a large Chinese family with progressive ataxia designated SCA22, which overlaps with the locus of SCA19. The disease locus in a French family and an Ashkenazi Jewish American family was also mapped to this region. Members from all 3 families were enrolled. Whole exome sequencing was performed to identify candidate mutations, which were narrowed by linkage analysis and confirmed by Sanger sequencing and cosegregation analyses. Mutational analyses were also performed in 105 Chinese and 55 Japanese families with cerebellar ataxia. Mutant gene products were examined in a heterologous expression system to address the changes in protein localization and electrophysiological functions.

RESULTS

We identified heterozygous mutations in the voltage-gated potassium channel Kv4.3-encoding gene KCND3: an in-frame 3-nucleotide deletion c.679_681delTTC p.F227del in both the Chinese and French pedigrees, and a missense mutation c.1034G>T p.G345V in the Ashkenazi Jewish family. Direct sequencing of KCND3 further identified 3 mutations, c.1034G>T p.G345V, c.1013T>C p.V338E, and c.1130C>T p.T377M, in 3 Japanese kindreds. Immunofluorescence analyses revealed that the mutant p.F227del Kv4.3 subunits were retained in the cytoplasm, consistent with the lack of A-type K(+) channel conductance in whole cell patch-clamp recordings.

INTERPRETATION

Our data identify the cause of SCA19/22 in patients of diverse ethnic origins as mutations in KCND3. These findings further emphasize the important role of ion channels as key regulators of neuronal excitability in the pathogenesis of cerebellar degeneration.

Clinical features, neurogenetics and neuropathology of the polyglutamine spinocerebellar ataxias type 1, 2, 3, 6 and 7

The spinocerebellar ataxias type 1 (SCA1), 2 (SCA2), 3 (SCA3), 6 (SCA6) and 7 (SCA7) are genetically defined autosomal dominantly inherited progressive cerebellar ataxias (ADCAs). They belong to the group of CAG-repeat or polyglutamine diseases and share pathologically expanded and meiotically unstable glutamine-encoding CAG-repeats at distinct gene loci encoding elongated polyglutamine stretches in the disease proteins. In recent years, progress has been made in the understanding of the pathogenesis of these currently incurable diseases: Identification of underlying genetic mechanisms made it possible to classify the different ADCAs and to define their clinical and pathological features. Furthermore, advances in molecular biology yielded new insights into the physiological and pathophysiological role of the gene products of SCA1, SCA2, SCA3, SCA6 and SCA7 (i.e. ataxin-1, ataxin-2, ataxin-3, α-1A subunit of the P/Q type voltage-dependent calcium channel, ataxin-7). In the present review we summarize our current knowledge about the polyglutamine ataxias SCA1, SCA2, SCA3, SCA6 and SCA7 and compare their clinical and electrophysiological features, genetic and molecular biological background, as well as their brain pathologies. Furthermore, we provide an overview of the structure, interactions and functions of the different disease proteins. On the basis of these comprehensive data, similarities, differences and possible disease mechanisms are discussed.

Genotype-specific patterns of atrophy progression are more sensitive than clinical decline in SCA1, SCA3 and SCA6

Spinocerebellar ataxias are dominantly inherited disorders that are associated with progressive brain degeneration, mainly affecting the cerebellum and brainstem. As part of the multicentre European integrated project on spinocerebellar ataxias study, 37 patients with spinocerebellar ataxia-1, 19 with spinocerebellar ataxia-3 and seven with spinocerebellar ataxia-6 were clinically examined and underwent magnetic resonance imaging at baseline and after a 2-year follow-up. All patients were compared with age-matched and gender-matched healthy control subjects. Magnetic resonance imaging analysis included three-dimensional volumetry and observer-independent longitudinal voxel-based morphometry. Volumetry revealed loss of brainstem, cerebellar and basal ganglia volume in all genotypes. Most sensitive to change was the pontine volume in spinocerebellar ataxia-1, striatal volume in spinocerebellar ataxia-3 and caudate volume in spinocerebellar ataxia-6. Sensitivity to change, as measured by standard response mean, of the respective MRI measures was greater than that of the most sensitive clinical measure, the Scale for the Assessment and Rating of Ataxia. Longitudinal voxel-based morphometry revealed greatest grey matter loss in the cerebellum and brainstem in spinocerebellar ataxia-1, in the putamen and pallidum in spinocerebellar ataxia-3 and in the cerebellum, thalamus, putamen and pallidum in spinocerebellar ataxia-6. There was a mild correlation between CAG repeat length and volume loss of the bilateral cerebellum and the pons in spinocerebellar ataxia-1. Quantitative volumetry and voxel-based morphometry imaging demonstrated genotype-specific patterns of atrophy progression in spinocerebellar ataxias-1, 3 and 6, and they showed a high sensitivity to detect change that was superior to clinical scales. These structural magnetic resonance imaging findings have the potential to serve as surrogate markers, which might help to delineate quantifiable endpoints and non-invasive methods for rapid and reliable data acquisition, encouraging their use in clinical trials.

[The genetics of spinocerebellar ataxias]

Spinocerebellar ataxias are genetically heterogeneous autosomal dominant ataxia disorders. To date more than 30 different subtypes are known. In Germany particularly SCA1, SCA2, SCA3 and SCA6 are prevalent, as well as the less frequent subtypes SCA5, SCA14, SCA15, SCA17 and SCA28. Genetic causes range from coding repeat expansions (polyglutamine diseases), to non-coding expansions as well as conventional mutations. In some subtypes the genetic background is currently unknown. Age of onset, typical clinical findings and geographic distribution may help to reach a correct diagnosis; however a definitive diagnosis requires molecular genetic testing.

Altered Purkinje cell miRNA expression and SCA1 pathogenesis

Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited neurodegenerative disorder caused by polyglutamine repeat expansions in Ataxin-1. Recent evidence supports a role for microRNAs (miRNAs) deregulation in SCA1 pathogenesis. However, the extent to which miRNAs may modulate the onset, progression or severity of SCA1 remains largely unknown. In this study, we used a mouse model of SCA1 to determine if miRNAs are misregulated in pre- and post-symptomatic SCA1 cerebellum. We found a significant alteration in the steady-state levels of numerous miRNAs prior to and following phenotypic onset. In addition, we provide evidence that increased miR-150 levels in SCA1 Purkinje neurons may modulate disease pathogenesis by targeting the expression of Rgs8 and Vegfa.

Sodium valproate alleviates neurodegeneration in SCA3/MJD via suppressing apoptosis and rescuing the hypoacetylation levels of histone H3 and H4

Spinocerebellar ataxia type 3 (SCA3) also known as Machado-Joseph Disease (MJD), is one of nine polyglutamine (polyQ) diseases caused by a CAG-trinucelotide repeat expansion within the coding sequence of the ATXN3 gene. There are no disease-modifying treatments for polyQ diseases. Recent studies suggest that an imbalance in histone acetylation may be a key process leading to transcriptional dysregulation in polyQ diseases. Because of this possible imbalance, the application of histone deacetylase (HDAC) inhibitors may be feasible for the treatment of polyQ diseases. To further explore the therapeutic potential of HDAC inhibitors, we constructed two independent preclinical trials with valproic acid (VPA), a promising therapeutic HDAC inhibitor, in both Drosophila and cell SCA3 models. We demonstrated that prolonged use of VPA at specific dose partly prevented eye depigmentation, alleviated climbing disability, and extended the average lifespan of SCA3/MJD transgenic Drosophila. We found that VPA could both increase the acetylation levels of histone H3 and histone H4 and reduce the early apoptotic rate of cells without inhibiting the aggregation of mutant ataxin-3 proteins in MJDtr-Q68- expressing cells. These results collectively support the premise that VPA is a promising therapeutic agent for the treatment of SCA3 and other polyQ diseases.

Spinocerebellar ataxia type 13 is an uncommon SCA subtype in the Chinese Han population

The spinocerebellar ataxias (SCAs) are a clinically and genetically heterogeneous group of neurodegenerative disorders, among which SCA subtype 13 (SCA13) was found associated with mutations in the KCNC3 gene. Among 522 Chinese Han SCA patients (including familial and sporadic) we have collected since 1995, approximately 40% of them have not yet been assigned genotype. To investigate the mutation frequency of KCNC3 in SCA patients from mainland Chinese Han population, we analyzed the KCNC3 gene in 201 unrelated patients diagnosed with dominantly inherited cerebellar ataxia using the denaturing high-performance liquid chromatography (DHPLC) method. All analyzed samples displayed the normal elution profile, which denoted that no disease-related mutation was identified, suggesting that SCA13 be a rare form of SCA in mainland China.

Spinocerebellar ataxia types 1, 2, 3 and 6: the clinical spectrum of ataxia and morphometric brainstem and cerebellar findings

To assess the clinical spectrum of ataxia and cerebellar oculomotor deficits in the most common spinocerebellar ataxias (SCAs), we analysed the baseline data of the EUROSCA natural history study, a multicentric cohort study of 526 patients with either spinocerebellar ataxia type 1, 2, 3 or 6. To quantify ataxia symptoms, we used the Scale for the Assessment and Rating of Ataxia (SARA). The presence of cerebellar oculomotor signs was assessed using the Inventory of Non-Ataxia Symptoms (INAS). In a subgroup of patients, in which magnetic resonance images (MRIs) were available, we correlated MRI morphometric measures with clinical signs on an exploratory basis. The SARA subscores posture and gait (items 1-3), speech (item 4) and the limb kinetic subscore (items 5-8) did not differ between the genotypes. The scores of SARA item 3 (sitting), 5 (finger chase) and 6 (nose-finger test) differed between the subtypes whereas the scores of the remaining items were not different. In SCA1, ataxia symptoms were correlated with brainstem atrophy and in SCA3 with both brainstem and cerebellar atrophy. Cerebellar oculomotor deficits were most frequent in SCA6 followed by SCA3, whereas these abnormalities were less frequent in SCA1 and SCA2. Our data suggest that vestibulocerebellar, spinocerebellar and pontocerebellar circuits in SCA1, SCA2, SCA3 and SCA6 are functionally impaired to almost the same degree, but at different anatomical levels. The seemingly low prevalence of cerebellar oculomotor deficits in SCA1 and SCA2 is most probably related to the defective saccadic system in these disorders.

Spinocerebellar ataxia type 35 (SCA35)-associated transglutaminase 6 mutants sensitize cells to apoptosis

Spinocerebellar ataxia type 35 (SCA35) is an autosomal dominant neurodegenerative disorder. In our previous study, using exome sequencing and linkage analysis, two missense mutations of the transglutaminase 6 (TGM6) gene were identified as causative for SCA35. TGM6 encodes transglutaminase 6 (TG6), a member of the transglutaminase family of enzymes that catalyze the formation of a covalent bond between a free amine group and the γ-carboxamide group of protein- or peptide-bound glutamine. However, the precise role of TG6 in contributing to SCA35 remains unclear. In this study, we analyzed the subcellular distribution, expression and in vitro activity of two missense mutations of TG6 (D327G, L517W) and found that both mutants exhibited decreased transglutaminase activity and stability. Furthermore, overexpressing the TG6 mutants sensitized cells to staurosporine-induced apoptosis by increasing the activity of caspases. We propose that the pro-apoptotic role of these mutants might underlie the pathogenesis of SCA35.

Prospective analysis of falls in dominant ataxias

In a previous retrospective study, we demonstrated that falls are common and often injurious in dominant spinocerebellar ataxias (SCAs) and that nonataxia features play an important role in these falls. Retrospective surveys are plagued by recall bias for the presence and details of prior falls. We therefore sought to corroborate and extend these retrospective findings by means of a prospective extension of this fall study. 113 patients with SCA1, SCA2, SCA3 or SCA6, recruited from the EuroSCA natural history study, were asked to keep a fall diary in between their annual visits to the participating centres. Additionally, patients completed a detailed questionnaire about the first three falls, to identify specific fall circumstances. Relevant disease characteristics were retrieved from the EuroSCA registry. 84.1% of patients reported at least one fall during a time period of 12 months. Fall-related injuries were common and their frequency increased with that of falls. The presence of nonataxia symptoms was associated with a higher fall frequency. This study confirms that falls are a frequent and serious complication of SCA, and that the presence of nonataxia symptoms is an important etiological factor in its occurrence.

Acoustic impairment is a distinguishable clinical feature of Asidan/SCA36

OBJECTIVE

To investigate acoustic function of Asidan/spinocerebellar ataxia type 36 (SCA36) in which sensorineural hearing loss may be found as one of extracerebellar symptom that can be a distinguishable feature from other degenerative ataxias.

METHODS

Acoustic function in the groups of normal control (n=31), Asidan/SCA36 (n=13), cortical cerebellar atrophy (CCA, n=28), multiple system atrophy of cerebellar predominance (MSA-C, n=48), SCA31 (n=4), and other forms of SCAs (n=14) was evaluated by pure tone average (PTA) calculated by the results of audiogram and brainstem auditory evoked potentials (BAEPs).

RESULTS

PTA was significantly decreased in Asidan/SCA36 in comparison to normal control and other ataxic groups, but not significant within other ataxic groups and normal control. In comparison to other groups, Asidan/SCA36 showed a constant depression at 7 different frequencies in audiogram, especially at 4000 and 8000 Hz. BAEPs in 2 Asidan/SCA36 cases suggested possible involvement in the inner ear or the peripheral part of the auditory system. PTA in Asidan/SCA36 cases significantly correlated with their severity of ataxia.

CONCLUSIONS

In addition to signs for motor neuron involvement, acoustic impairment in Asidan/SCA36 is another characteristic clinical feature that is distinguishable from other forms of SCAs.

Large-scale screen for modifiers of ataxin-3-derived polyglutamine-induced toxicity in Drosophila

Polyglutamine (polyQ) diseases represent a neuropathologically heterogeneous group of disorders. The common theme of these disorders is an elongated polyQ tract in otherwise unrelated proteins. So far, only symptomatic treatment can be applied to patients suffering from polyQ diseases. Despite extensive research, the molecular mechanisms underlying polyQ-induced toxicity are largely unknown. To gain insight into polyQ pathology, we performed a large-scale RNAi screen in Drosophila to identify modifiers of toxicity induced by expression of truncated Ataxin-3 containing a disease-causing polyQ expansion. We identified various unknown modifiers of polyQ toxicity. Large-scale analysis indicated a dissociation of polyQ aggregation and toxicity.

The frequency of spinocerebellar ataxia type 23 in a UK population

Spinocerebellar ataxias (SCA) are a genetically heterogeneous group of neurodegenerative diseases characterised by progressive cerebellar ataxia, dysarthria and oculomotor abnormalities. Recently the prodynorphin (PDYN) gene was identified as the cause of SCA23 in four Dutch families displaying progressive gait and limb ataxia. In this study we aimed to assess the frequency of PDYN gene defects and extend the phenotype of SCA23 patients in a UK ataxia series and also in patients from Greece, Egypt and India. We sequenced the coding and flanking intronic regions of the PDYN gene in a total of 852 ataxia patients, of which 356 were sporadic with no family history, 320 had a positive family history, and 176 probands had a positive family history and at least one family member had also been investigated. We also analysed 190 patients with multiple-system atrophy with cerebellar features (MSA-C), a phenocopy of SCA23. We identified a novel putative pathogenic heterozygous missense variant in the PDYN gene in an early onset SCA patient with an unknown family history. This variant was not present in 570 matched British controls. This is the first study to screen for SCA23 in UK patients and confirms that PDYN mutations are a very rare cause of spinocerebellar ataxia, accounting for ~ 0.1 % of ataxia cases but perhaps with a higher frequency in pure cerebellar ataxia. Given the rarity of PDYN mutations, front-line diagnostic evaluation of UK familial and early onset pure spinocerebellar ataxia patients should focus on other known ataxia genes.

Spinocerebellar ataxia type 2: clinical presentation, molecular mechanisms, and therapeutic perspectives

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant genetic disease characterized by cerebellar dysfunction associated with slow saccades, early hyporeflexia, severe tremor of postural or action type, peripheral neuropathy, cognitive disorders, and other multisystemic features. SCA2, one of the most common ataxias worldwide, is caused by the expansion of a CAG triplet repeat located in the N-terminal coding region of the ATXN2 gene, which results in the incorporation of a segment of polyglutamines in the mutant protein, being longer expansions associated with earlier onset and more sever disease in subsequent generations. In this review, we offer a detailed description of the clinical manifestations of SCA2 and compile the experimental evidence showing the participation of ataxin-2 in crucial cellular processes, including messenger RNA maturation and translation, and endocytosis. In addition, we discuss in the light of present data the potential molecular mechanisms underlying SCA2 pathogenesis. The mutant protein exhibits a toxic gain of function that is mainly attributed to the generation of neuronal inclusions of phosphorylated and/or proteolytic cleaved mutant ataxin-2, which might alter normal ataxin-2 function, leading to cell dysfunction and death of target cells. In the final part of this review, we discuss the perspectives of development of therapeutic strategies for SCA2. Based on previous experience with other polyglutamine disorders and considering the molecular basis of SCA2 pathogenesis, a nuclei-acid-based strategy focused on the specific silencing of the dominant disease allele that preserves the expression of the wild-type allele is highly desirable and might prevent toxic neurodegenerative sequelae.

Infantile childhood onset of spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is a late-onset autosomal dominant cerebellar ataxia caused by triplet CAG/CTG expansion in the ATX2 gene. The initial symptoms usually appear when subjects are in their 30s.Pediatric onset is less common and usually associated with larger triplet expansions. We here report the case of a 1-year-old girl who presented with facial dysmorphism,dystonic features, developmental delay, and retinitis pigmentosa.She was diagnosed as carrying an expanded CAG/CTG tract (92 repeats) before a molecular diagnosis of SCA2 was made in her father. Facial dysmorphism associated with developmental delay and retinitis pigmentosa in early childhood should prompt a careful family investigation for ataxia and study of ATX2.

VAMP1 mutation causes dominant hereditary spastic ataxia in Newfoundland families

Our group previously described and mapped to chromosomal region 12p13 a form of dominantly inherited hereditary spastic ataxia (HSA) in three large Newfoundland (Canada) families. This report identifies vesicle-associated membrane protein 1 (VAMP1), which encodes a critical protein for synaptic exocytosis, as the responsible gene. In total, 50 affected individuals from these families and three independent probands from Ontario (Canada) share the disease phenotype together with a disruptive VAMP1 mutation that affects a critical donor site for the splicing of VAMP1 isoforms. This mutation leads to the loss of the only VAMP1 isoform (VAMP1A) expressed in the nervous system, thus highlighting an association between the well-studied VAMP1 and a neurological disorder. Given the variable phenotype seen in the affected individuals examined here, we believe that VAMP1 should be tested for mutations in patients with either ataxia or spastic paraplegia.

Recent advances in the genetics of cerebellar ataxias

The hereditary cerebellar ataxias are a clinically and genetically heterogeneous group of disorders that primarily affect the cerebellum; often there are additional features such as neuropathy, cognitive decline, or maculopathy that help define the clinical subtype of ataxia. They are commonly classified according to their mode of inheritance into autosomal dominant, autosomal recessive, X-linked, and mitochondrial forms. Great advances have been made in understanding the genetics of cerebellar ataxias in the last 15 years. At least 36 different forms of ADCA are known, 20 autosomal-recessive, two X-linked, and several forms of ataxia associated with mitochondrial defects are known to date. However, in about 40 % of suspected genetically determined ataxia cases, the underlying genetic defect remains undetermined. Although the majority of disease genes have been found in the last two decades, over the last 2 years the genetics has undergone a methodological revolution. New DNA sequencing technologies are enabling us to investigate the whole or large targeted proportions of the genome in a rapid, affordable, and comprehensive way. Exome and targeted sequencing has recently identified four new genes causing ataxia: TGM6, ANO10, SYT14, and rundataxin. This approach is likely to continue to discover new ataxia genes and make screening of existing genes more effective. Translating the genetic findings into isolated and overlapping disease pathways will help stratify patient groups and identify therapeutic targets for ataxia that have so far remained undiscovered.

Mouse models of polyglutamine diseases in therapeutic approaches: review and data table. Part II

Mouse models of human diseases are created both to understand the pathogenesis of the disorders and to find successful therapies for them. This work is the second part in a series of reviews of mouse models of polyglutamine (polyQ) hereditary disorders and focuses on in vivo experimental therapeutic approaches. Like part I of the polyQ mouse model review, this work is supplemented with a table that contains data from experimental studies of therapeutic approaches in polyQ mouse models. The aim of this review was to characterize the benefits and outcomes of various therapeutic strategies in mouse models. We examine whether the therapeutic strategies are specific to a single disease or are applicable to more than one polyQ disorder in mouse models. In addition, we discuss the suitability of mouse models in therapeutic approaches. Although the majority of therapeutic studies were performed in mouse models of Huntington disease, similar strategies were also used in other disease models.

Childhood cerebellar ataxia

Childhood presentations of ataxia, an impairment of balance and coordination caused by damage to or dysfunction of the cerebellum, can often be challenging to diagnose. Presentations tend to be clinically heterogeneous, but key considerations may vary based on the child's age at onset, the course of illness, and subtle differences in phenotype. Systematic investigation is recommended for efficient diagnosis. In this review, we outline common etiologies and describe a comprehensive approach to the evaluation of both acquired and genetic cerebellar ataxia in children.

A 1 Mb de novo deletion within 11q13.1q13.2 in a boy with mild intellectual disability and minor dysmorphic features

We report a 11 year old male patient ascertained for mild intellectual disability and minor dysmorphic features, carrying a 1 Mb de novo deletion on chromosome 11q13.1q13.2 detected by aCGH. This is the first report of a deletion in this region in a patient presenting with intellectual impairment and mild dysmorphic traits. The 1 Mb deleted area encompasses 47 RefSeq genes, including Cornichon homologue 2 (CNIH2), Cofilin-1 (CFL1) and neuronal PAS domain-containing protein 4 (NPAS4), which are highly expressed in the central nervous system. Knockout of the CNIH2 and CFL1 orthologues in animals results in migration disturbances, while low or no expression of Npas4 in mice results in impairment of memory and learning. These three genes have previously been suggested as candidate genes for neurological disorders.

New insights into behaviour using mouse ENU mutagenesis

Identifying genes involved in behavioural disorders in man is a challenge as the cause is often multigenic and the phenotype is modulated by environmental cues. Mouse mutants are a valuable tool for identifying novel pathways underlying specific neurological phenotypes and exploring the influence both genetic and non-genetic factors. Many human variants causing behavioural disorders are not gene deletions but changes in levels of expression or activity of a gene product; consequently, large-scale mouse ENU mutagenesis has the advantage over the study of null mutants in that it generates a range of point mutations that frequently mirror the subtlety and heterogeneity of human genetic lesions. ENU mutants have provided novel and clinically relevant functional information on genes that influence many aspects of mammalian behaviour, from neuropsychiatric endophenotypes to circadian rhythms. This review will highlight some of the most important findings that have been made using this method in several key areas of neurological disease research.

Computational prediction of the polyQ and CAG repeat spinocerebellar ataxia network based on sequence identity to untranslated regions

Computational prediction of biological networks would be a tremendous asset to systems biology and personalized medicine. In this paper, we use a moving window bioinformatic screen to identify transcripts with partial identity to the 5' and 3'UTRs of the polyQ spinocerebellar ataxia (SCA) genes ATXN1, ATXN2, ATXN3, ATXN7, TBP and CACNA1A and the CAG repeat expansion gene PPP2R2B. We find that the bioinformatic screen enriches for transcripts that encode proteins that interact and that have functions relevant to polyQ SCA. Transcription control and RNA binding are the primary functional groups represented in the proteins from the combined screens. The insulin growth factor pathway, the WNT pathway, long term potentiation, melanogenesis and ATM mediated DNA repair pathways were identified as important pathways. UGUUU repeats were identified as an abundant motif in the SCA network and PAXIP1, CELF2, CREBBP, EBF1, PLEKHG4, SRSF4, C5orf42, NFIA, STK24, and YWHAG were identified as statistically significant proteins in the polyQ and PPP2R2B network.

The clinical characteristics of spinocerebellar ataxia 36: a study of 2121 Japanese ataxia patients

Spinocerebellar ataxia 36 is caused by the expansion of the intronic GGCCTG hexanucleotide repeat in NOP56. The original article describing this condition demonstrated that patients with spinocerebellar ataxia 36 present with tongue atrophy, a finding that had not been seen in previous types of spinocerebellar ataxias. A total of 2121 patients with clinically diagnosed spinocerebellar ataxia participated in the study. We screened our patient samples for spinocerebellar ataxia 36 using the repeat-primed polymerase chain reaction method and also determined the clinical features of spinocerebellar ataxia 36. Of the ataxia cases examined, 12 were identified as spinocerebellar ataxia 36. Of these, 7 cases (6 families) were autosomal dominant, 4 cases (three families) had a positive family history but were not autosomal dominant, and 1 case was sporadic. The average age of onset was 51.7 years, and disease progression was slow. The main symptoms and signs of disease included ataxia, dysarthria, and hyperreflexia. Approximately half the affected patients demonstrated nystagmus, bulging eyes, and a positive pathological reflex, although dysphagia, tongue atrophy, and hearing loss were rare. Moreover, the observed atrophy of the cerebellum and brain stem was not severe. The patients identified in this study were concentrated in western Japan. The frequency of spinocerebellar ataxia 36 was approximately 1.2% in the autosomal dominant group, and the age of onset for this condition was later in comparison with other spinocerebellar ataxia subtypes.

Brain pathology of spinocerebellar ataxias

The autosomal dominant cerebellar ataxias (ADCAs) represent a heterogeneous group of neurodegenerative diseases with progressive ataxia and cerebellar degeneration. The current classification of this disease group is based on the underlying genetic defects and their typical disease courses. According to this categorization, ADCAs are divided into the spinocerebellar ataxias (SCAs) with a progressive disease course, and the episodic ataxias (EA) with episodic occurrences of ataxia. The prominent disease symptoms of the currently known and genetically defined 31 SCA types result from damage to the cerebellum and interconnected brain grays and are often accompanied by more specific extra-cerebellar symptoms. In the present review, we report the genetic and clinical background of the known SCAs and present the state of neuropathological investigations of brain tissue from SCA patients in the final disease stages. Recent findings show that the brain is commonly seriously affected in the polyglutamine SCAs (i.e. SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17) and that the patterns of brain damage in these diseases overlap considerably in patients suffering from advanced disease stages. In the more rarely occurring non-polyglutamine SCAs, post-mortem neuropathological data currently are scanty and investigations have been primarily performed in vivo by means of MRI brain imaging. Only a minority of SCAs exhibit symptoms and degenerative patterns allowing for a clear and unambiguous diagnosis of the disease, e.g. retinal degeneration in SCA7, tau aggregation in SCA11, dentate calcification in SCA20, protein depositions in the Purkinje cell layer in SCA31, azoospermia in SCA32, and neurocutaneous phenotype in SCA34. The disease proteins of polyglutamine ataxias and some non-polyglutamine ataxias aggregate as cytoplasmic or intranuclear inclusions and serve as morphological markers. Although inclusions may impair axonal transport, bind transcription factors, and block protein quality control, detailed molecular and pathogenetic consequences remain to be determined.

A SEL1L mutation links a canine progressive early-onset cerebellar ataxia to the endoplasmic reticulum-associated protein degradation (ERAD) machinery

Inherited ataxias are characterized by degeneration of the cerebellar structures, which results in progressive motor incoordination. Hereditary ataxias occur in many species, including humans and dogs. Several mutations have been found in humans, but the genetic background has remained elusive in dogs. The Finnish Hound suffers from an early-onset progressive cerebellar ataxia. We have performed clinical, pathological, and genetic studies to describe the disease phenotype and to identify its genetic cause. Neurological examinations on ten affected dogs revealed rapidly progressing generalized cerebellar ataxia, tremors, and failure to thrive. Clinical signs were present by the age of 3 months, and cerebellar shrinkage was detectable through MRI. Pathological and histological examinations indicated cerebellum-restricted neurodegeneration. Marked loss of Purkinje cells was detected in the cerebellar cortex with secondary changes in other cortical layers. A genome-wide association study in a cohort of 31 dogs mapped the ataxia gene to a 1.5 Mb locus on canine chromosome 8 (p_raw = 1.1×10⁻⁷, p_genome = 7.5×10⁻⁴). Sequencing of a functional candidate gene, sel-1 suppressor of lin-12-like (SEL1L), revealed a homozygous missense mutation, c.1972T>C; p.Ser658Pro, in a highly conserved protein domain. The mutation segregated fully in the recessive pedigree, and a 10% carrier frequency was indicated in a population cohort. SEL1L is a component of the endoplasmic reticulum (ER)–associated protein degradation (ERAD) machinery and has not been previously associated to inherited ataxias. Dysfunctional protein degradation is known to cause ER stress, and we found a significant increase in expression of nine ER stress responsive genes in the cerebellar cortex of affected dogs, supporting the pathogenicity of the mutation. Our study describes the first early-onset neurodegenerative ataxia mutation in dogs, establishes an ERAD–mediated neurodegenerative disease model, and proposes SEL1L as a new candidate gene in progressive childhood ataxias. Furthermore, our results have enabled the development of a genetic test for breeders.

Hereditary ataxias are a heterogeneous group of rare disorders characterized by progressive cerebellar neurodegeneration. Several causative mutations have been identified in various forms of human ataxias. In addition to humans, inherited ataxias have been described in several other species, including the domestic dog. In this study, we have studied the clinical and genetic properties of cerebellar ataxia in the Finnish Hound dog breed. The breed suffers from a progressive ataxia that has an early onset before the age of 3 months. Affected puppies have difficulties in coordinating their movements and balance, and have to be euthanized due to rapidly worsening symptoms. Our pedigree analysis suggested an autosomal recessive mode of inheritance, which was confirmed by identifying a homozygous mutation in the SEL1L gene through genome-wide association and linkage analyses. The SEL1L protein functions in a protein quality control pathway that targets misfolded proteins to degradation in the endoplasmic reticulum. Mutations in the SEL1L gene have not been previously found in ataxias. Our study indicates SEL1L as a novel candidate gene for human childhood ataxias, establishes a large animal model to investigate mechanisms of cerebellar neurodegeneration, and enables carrier screening for breeding purposes.

Exome sequencing in an SCA14 family demonstrates its utility in diagnosing heterogeneous diseases

OBJECTIVE

Genetic heterogeneity is common in many neurologic disorders. This is particularly true for the hereditary ataxias where at least 36 disease genes or loci have been described for spinocerebellar ataxia and over 100 genes for neurologic disorders that present primarily with ataxia. Traditional genetic testing of a large number of candidate genes delays diagnosis and is expensive. In contrast, recently developed genomic techniques, such as exome sequencing that targets only the coding portion of the genome, offer an alternative strategy to rapidly sequence all genes in a comprehensive manner. Here we describe the use of exome sequencing to investigate a large, 5-generational British kindred with an autosomal dominant, progressive cerebellar ataxia in which conventional genetic testing had not revealed a causal etiology.

METHODS

Twenty family members were seen and examined; 2 affected individuals were clinically investigated in detail without a genetic or acquired cause being identified. Exome sequencing was performed in one patient where coverage was comprehensive across the known ataxia genes, excluding the known repeat loci which should be examined using conventional analysis.

RESULTS

A novel p.Arg26Gly change in the PRKCG gene, mutated in SCA14, was identified. This variant was confirmed using Sanger sequencing and showed segregation with disease in the entire family.

CONCLUSIONS

This work demonstrates the utility of exome sequencing to rapidly screen heterogeneous genetic disorders such as the ataxias. Exome sequencing is more comprehensive, faster, and significantly cheaper than conventional Sanger sequencing, and thus represents a superior diagnostic screening tool in clinical practice.