Spinocerebellar ataxia type 2 has multiple ancestral origins.

INTRODUCTION: Spinocerebellar ataxia type 2 (SCA2) is a dominant neurodegenerative disorder due to expansions of a CAG repeat tract (CAGexp) at the ATXN2 gene. Previous studies found only one ancestral haplotype worldwide, with a C allele at rs695871. This homogeneity was unexpected, given the severe anticipations related to SCA2. We aimed to describe informative ancestral haplotypes found in South American SCA2 families. METHODS: Seventy-seven SCA2 index cases were recruited from Brazil, Peru, and Uruguay; 263 normal chromosomes were used as controls. The SNPs rs9300319, rs3809274, rs695871, rs1236900 and rs593226, and the STRs D12S1329, D12S1333, D12S1672 and D12S1332, were used to reconstruct haplotypes. RESULTS: Eleven ancestral haplotypes were found in SCA2 families. The most frequent ones were A-G-C-C-C (46.7 % of families), G-C-C-C-C (24.6 %) and A-C-C-C-C (10.3 %) and their mean (sd) CAGexp were 41.68 (3.55), 40.42 (4.11) and 45.67 (9.70) (p = 0.055), respectively. In contrast, the mean (sd) CAG lengths at normal alleles grouped per haplotypes G-C-G-A-T, A-G-C-C-C and G-C-C-C-C were 22.97 (3.93), 23.85 (3.59), and 30.81 (4.27) (p < 0.001), respectively. The other SCA2 haplotypes were rare: among them, a G-C-G-A-T lineage was found, evidencing a G allele in rs695871. CONCLUSION: We identified several distinct ancestral haplotypes in SCA2 families, including an unexpected lineage with a G allele at rs695871, a variation never found in hundreds of SCA2 patients studied worldwide. SCA2 has multiple origins in South America, and more studies should be done in other regions of the world.

Identification of the ataxin-1 interaction network and its impact on spinocerebellar ataxia type 1.

BACKGROUND: Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by a polyglutamine expansion in the ataxin-1 protein. The pathogenic mechanism resulting in SCA1 is still unclear. Protein-protein interactions affect the function and stability of ataxin-1. METHODS: Wild-type and mutant ataxin-1 were expressed in HEK-293T cells. The levels of expression were assessed using real-time polymerase chain reaction (PCR) and Western blots. Co-immunoprecipitation was done in HEK-293T cells expressing exogenous wild-type and mutant ataxin-1 using anti-Flag antibody following by tandem affinity purification in order to study protein-protein interactions. The candidate interacting proteins were validated by immunoprecipitation. Chromatin immunoprecipitation and high-throughput sequencing and RNA immunoprecipitation and high-throughput sequencing were performed using HEK-293T cells expressing wild-type or mutant ataxin-1. RESULTS: In this study using HEK-293T cells, we found that wild-type ataxin-1 interacted with MCM2, GNAS, and TMEM206, while mutant ataxin-1 lost its interaction with MCM2, GNAS, and TMEM206. Two ataxin-1 binding targets containing the core GGAG or AAAT were identified in HEK-293T cells using ChIP-seq. Gene Ontology analysis of the top ataxin-1 binding genes identified SLC6A15, NTF3, KCNC3, and DNAJC6 as functional genes in neurons in vitro. Ataxin-1 also was identified as an RNA-binding protein in HEK-293T cells using RIP-seq, but the polyglutamine expansion in the ataxin-1 had no direct effects on the RNA-binding activity of ataxin-1. CONCLUSIONS: An expanded polyglutamine tract in ataxin-1 might interfere with protein-protein or protein-DNA interactions but had little effect on protein-RNA interactions. This study suggested that the dysfunction of protein-protein or protein-DNA interactions is involved in the pathogenesis of SCA1.

Intercellular Propagation and Aggregate Seeding of Mutant Ataxin-1.

Intercellular propagation of aggregated protein inclusions along actin-based tunneling nanotubes (TNTs) has been reported as a means of pathogenic spread in Alzheimer's, Parkinson's, and Huntington's diseases. Propagation of oligomeric-structured polyglutamine-expanded ataxin-1 (Atxn1[154Q]) has been reported in the cerebellum of a Spinocerebellar ataxia type 1 (SCA1) knock-in mouse to correlate with disease propagation. In this study, we investigated whether a physiologically relevant polyglutamine-expanded ATXN1 protein (ATXN1[82Q]) could propagate intercellularly. Using a cerebellar-derived live cell model, we observed ATXN1 aggregates form in the nucleus, subsequently form in the cytoplasm, and finally, propagate to neighboring cells along actin-based intercellular connections. Additionally, we observed the facilitation of aggregate-resistant proteins into aggregates given the presence of aggregation-prone proteins within cells. Taken together, our results support a pathogenic role of intercellular propagation of polyglutamine-expanded ATXN1 inclusions.

Genetic etiology of a Chinese ataxia cohort: Expanding the mutational spectrum of hereditary ataxias.

INTRODUCTION: Hereditary ataxias demonstrate a high degree of clinical and genetic heterogeneity. Understanding the genetic etiology of hereditary ataxias is crucial for genetic counseling and clinical management. METHODS: The clinical and genetic data of patients with familial or sporadic ataxias who referred to our tertiary medical center were retrospectively analyzed. Probands in this study underwent SCA repeat expansion panel firstly to screen for repeat expansion SCAs; those with negative results had NGS-targeted panels or WES testing to detect conventional mutations. RESULTS: A total of 223 patients were enrolled from 206 families. 5 kinds of coexisting SCA repeat expansions were observed (SCA3/SCA17, SCA3/SCA8, SCA2/SCA8, SCA3/SCA12 and SCA8/SCA12) in 12 patients from 8 families, among which SCA2/SCA8, SCA8/SCA12 and SCA3/SCA12 were reported for the first time. The coexistence of expanded SCA3 with SCA17 alleles was the most common in our study. NGS identified pathogenic/likely pathogenic variants in 12 ataxia causative genes in 13 probands. Spastic paraplegia ataxia was the most common diagnosis. Six novel mutations were detected in five ataxia-related genes. CONCLUSION: Coexistence may not specific to a certain SCA subtype and the frequency might have been underestimated before. SCA repeat expansion panel should be considered in patients with overlapping SCA features. In addition, our study broadened the conventional mutation spectrum in ataxia-related genes. These results facilitate a better understanding of the genetic basis for hereditary ataxias.

Toxicity of pathogenic ataxin-2 in Drosophila shows dependence on a pure CAG repeat sequence.

Spinocerebellar ataxia type 2 is a polyglutamine (polyQ) disease associated with an expanded polyQ domain within the protein product of the ATXN2 gene. Interestingly, polyQ repeat expansions in ATXN2 are also associated with amyotrophic lateral sclerosis (ALS) and parkinsonism depending upon the length of the polyQ repeat expansion. The sequence encoding the polyQ repeat also varies with disease presentation: a pure CAG repeat is associated with SCA2, whereas the CAG repeat in ALS and parkinsonism is typically interrupted with the glutamine encoding CAA codon. Here, we asked if the purity of the CAG sequence encoding the polyQ repeat in ATXN2 could impact the toxicity of the ataxin-2 protein in vivo in Drosophila. We found that ataxin-2 encoded by a pure CAG repeat conferred toxicity in the retina and nervous system, whereas ataxin-2 encoded by a CAA-interrupted repeat or CAA-only repeat failed to confer toxicity, despite expression of the protein at similar levels. Furthermore, the CAG-encoded ataxin-2 protein aggregated in the fly eye, while ataxin-2 encoded by either a CAA/G or CAA repeat remained diffuse. The toxicity of the CAG-encoded ataxin-2 protein was also sensitive to the translation factor eIF4H, a known modifier of the toxic GGGGCC repeat in flies. These data indicate that ataxin-2 encoded by a pure CAG versus interrupted CAA/G polyQ repeat domain is associated with differential toxicity, indicating that mechanisms associated with the purity of the sequence of the polyQ domain contribute to disease.

Mechanisms of repeat-associated non-AUG translation in neurological microsatellite expansion disorders.

Repeat-associated non-AUG (RAN) translation was discovered in 2011 in spinocerebellar ataxia type 8 (SCA8) and myotonic dystrophy type 1 (DM1). This non-canonical form of translation occurs in all reading frames from both coding and non-coding regions of sense and antisense transcripts carrying expansions of trinucleotide to hexanucleotide repeat sequences. RAN translation has since been reported in 7 of the 53 known microsatellite expansion disorders which mainly present with neurodegenerative features. RAN translation leads to the biosynthesis of low-complexity polymeric repeat proteins with aggregating and cytotoxic properties. However, the molecular mechanisms and protein factors involved in assembling functional ribosomes in absence of canonical AUG start codons remain poorly characterised while secondary repeat RNA structures play key roles in initiating RAN translation. Here, we briefly review the repeat expansion disorders, their complex pathogenesis and the mechanisms of physiological translation initiation together with the known factors involved in RAN translation. Finally, we discuss research challenges surrounding the understanding of pathogenesis and future directions that may provide opportunities for the development of novel therapeutic approaches for this group of incurable neurodegenerative diseases.

Mutational mechanism for DAB1 (ATTTC)n insertion in SCA37: ATTTT repeat lengthening and nucleotide substitution.

Dynamic mutations by microsatellite instability are the molecular basis of a growing number of neuromuscular and neurodegenerative diseases. Repetitive stretches in the human genome may drive pathogenicity, either by expansion above a given threshold, or by insertion of abnormal tracts in nonpathogenic polymorphic repetitive regions, as is the case in spinocerebellar ataxia type 37 (SCA37). We have recently established that this neurodegenerative disease is caused by an (ATTTC)n insertion within an (ATTTT)n in a noncoding region of DAB1. We now investigated the mutational mechanism that originated the (ATTTC)n insertion within an ancestral (ATTTT)n . Approximately 3% of nonpathogenic (ATTTT)n alleles are interspersed by AT-rich motifs, contrarily to mutant alleles that are composed of pure (ATTTT)n and (ATTTC)n stretches. Haplotype studies in unaffected chromosomes suggested that the primary mutational mechanism, leading to the (ATTTC)n insertion, was likely one or more T>C substitutions in an (ATTTT)n pure allele of approximately 200 repeats. Then, the (ATTTC)n expanded in size, originating a deleterious allele in DAB1 that leads to SCA37. This is likely the mutational mechanism in three similar (TTTCA)n insertions responsible for familial myoclonic epilepsy. Because (ATTTT)n tracts are frequent in the human genome, many loci could be at risk for this mutational process.

Spinocerebellar ataxias.

There are over 40 autosomal dominant spinocerebellar ataxias (SCAs) now identified. In this chapter we delineate the phenotypes of SCAs 1-44 and dentatorubral-pallidoluysian atrophy (DRPLA) and highlight the clinical and genetic features of the well characterised SCAs in detail in the main section of the chapter, along with their frequency and age at onset. We have included a section on the key phenotypic features of rare spinocerebellar ataxias and discuss rare and unusual presentations and genetic mechanisms of the ataxias and show differences between adult and paediatric presentations. We look at unusual mechanisms where knowledge is evolving in some dominant ataxias. For ease of reference we have tabulated historical aspects of the ataxias, major neurological diagnostic features, ataxias with predominant paediatric and infantile onset and list recognisable nerve conduction features. We comment on the anti-sense ataxia gene mechanisms and we discuss potential developments including exome sequencing and potential therapeutic options. A gene table listing all of the identified SCAs and DRPLA is also included with key references and gene locations and symbols with OMIM reference numbers for further reading.

Serum neurofilament light is increased in multiple system atrophy of cerebellar type and in repeat-expansion spinocerebellar ataxias: a pilot study.

Blood biomarkers in degenerative ataxias are still largely missing. Here, we aimed to provide piloting proof-of-concept that serum Neurofilament light (NfL) could offer a promising peripheral blood biomarker in degenerative ataxias. Specifically, as a marker of neuronal damage, NfL might (1) help to differentiate multiple system atrophy of cerebellar type (MSA-C) from sporadic adult-onset ataxia (SAOA), and (2) show increases in repeat-expansion spinocerebellar ataxias (SCAs) which might be amenable to treatment in the future. To explore these two hypotheses, we measured serum NfL levels by single-molecule array (Simoa) technique in 115 subjects, comprising patients with MSA-C (n = 25), SAOA (n = 25), the most frequent repeat-expansion SCAs (SCA 1, 2, 3 and 6) (n = 20), and age-matched controls (n = 45). Compared to controls, NfL was significantly increased in MSA-C, with levels significantly higher than in SAOA (AUC = 0.74 (0.59-0.89), mean and 95% confidence interval, p = .004). NfL was also significantly increased in SCA patients as compared to controls (AUC = 0.91 (0.81-1.00), p < .001), including NfL increases in SCA1 and SCA3. These findings provide first proof-of-concept that NfL might provide a promising peripheral biomarker in degenerative ataxias, e.g. supporting the differentiation of MSA-C from SAOA, and indicating neuronal damage in repeat-expansion SCAs.

Analysis of (CAG)n expansion in ATXN1, ATXN2 and ATXN3 in Chinese patients with multiple system atrophy.

Multiple system atrophy (MSA) is a complex and multifactorial neurodegenerative disease, and its pathogenesis remains uncertain. Patients with MSA or spinocerebellar ataxia (SCA) show overlapping clinical phenotypes. Previous studies have reported that intermediate or long CAG expansions in SCA genes have been associated with other neurodegenerative disease. In this study, we screened for the number of CAG repeats in ATXN1, 2 and 3 in 200 patients with MSA and 314 healthy controls to evaluate possible associations between (CAG)n in these three polyQ-related genes and MSA. Our findings indicated that longer repeat lengths in ATXN2 were associated with increased risk for MSA in Chinese individuals. No relationship was observed between CAG repeat length in the three examined genes and age at onset (AO) of MSA.

Neurochemical abnormalities in premanifest and early spinocerebellar ataxias.

OBJECTIVE: To investigate whether early neurochemical abnormalities are detectable by high-field magnetic resonance spectroscopy (MRS) in individuals with spinocerebellar ataxias (SCAs) 1, 2, 3, and 6, including patients without manifestation of ataxia. METHODS: A cohort of 100 subjects (N = 18-21 in each SCA group, including premanifest mutation carriers; mean score on the Scale for the Assessment and Rating of Ataxia [SARA] <10 for all genotypes, and 22 matched controls) was scanned at 7 Tesla to obtain neurochemical profiles of the cerebellum and brainstem. A novel multivariate approach (distance-weighted discrimination) was used to combine regional profiles into an "MRS score." RESULTS: MRS scores robustly distinguished individuals with SCA from controls, with misclassification rates of 0% (SCA2), 2% (SCA3), 5% (SCA1), and 17% (SCA6). Premanifest mutation carriers with estimated disease onset within 10 years had MRS scores in the range of early-manifest SCA subjects. Levels of neuronal and glial markers significantly correlated with SARA and an Activities of Daily Living score in subjects with SCA. Regional neurochemical alterations were different between SCAs at comparable disease severity, with SCA2 displaying the most extensive neurochemical abnormalities, followed by SCA1, SCA3, and SCA6. INTERPRETATION: Neurochemical abnormalities are detectable in individuals before manifest disease, which may allow premanifest enrollment in future SCA trials. Correlations with ataxia and quality-of-life scores show that neurochemical levels can serve as clinically meaningful endpoints in trials. Ranking of SCA types by degree of neurochemical abnormalities indicates that the neurochemistry may reflect synaptic function or density. Ann Neurol 2018;83:816-829.

Destabilizing the AXH Tetramer by Mutations: Mechanisms and Potential Antiaggregation Strategies.

The AXH domain of protein Ataxin 1 is thought to play a key role in the misfolding and aggregation pathway responsible for Spinocerebellar ataxia 1. For this reason, a molecular level understanding of AXH oligomerization pathway is crucial to elucidate the aggregation mechanism, which is thought to trigger the disease. This study employs classical and enhanced molecular dynamics to identify the structural and energetic basis of AXH tetramer stability. Results of this work elucidate molecular mechanisms behind the destabilizing effect of protein mutations, which consequently affect the AXH tetramer assembly. Moreover, results of the study draw attention for the first time, to our knowledge, to the R638 protein residue, which is shown to play a key role in AXH tetramer stability. Therefore, R638 might be also implicated in the AXH oligomerization pathway and stands out as a target for future experimental studies focused on self-association mechanisms and fibril formation of full-length ATX1.

Docosahexaenoic acid is a beneficial replacement treatment for spinocerebellar ataxia 38.

OBJECTIVE: Spinocerebellar ataxia 38 (SCA38) is caused by mutations in the ELOVL5 gene, which encodes an elongase involved in the synthesis of polyunsaturated fatty acids, including docosahexaenoic acid (DHA). As a consequence, DHA is significantly reduced in the serum of SCA38 subjects. In the present study, we evaluated the safety of DHA supplementation, its efficacy for clinical symptoms, and changes of brain functional imaging in SCA38 patients. METHODS: We enrolled 10 SCA38 patients, and carried out a double-blind randomized placebo-controlled study for 16 weeks, followed by an open-label study with overall 40-week DHA treatment. At baseline and at follow-up visit, patients underwent standardized clinical assessment, brain 18-fluorodeoxyglucose positron emission tomography, electroneurography, and ELOVL5 expression analysis. RESULTS: After 16 weeks, we showed a significant pre-post clinical improvement in the DHA group versus placebo, using the Scale for the Assessment and Rating of Ataxia (SARA; mean difference [MD] = +2.70, 95% confidence interval [CI] = +0.13 to + 5.27, p = 0.042). At 40-week treatment, clinical improvement was found significant by both SARA (MD = +2.2, 95% CI = +0.93 to + 3.46, p = 0.008) and International Cooperative Ataxia Rating Scale (MD = +3.8, 95% CI = +1.39 to + 6.41, p = 0.02) scores; clinical data were corroborated by significant improvement of cerebellar hypometabolism (statistical parametric mapping analyses, false discovery rate corrected). We also showed a decreased expression of ELOVL5 in patients' blood at 40 weeks as compared to baseline. No side effect was recorded. INTERPRETATION: DHA supplementation is a safe and effective treatment for SCA38, showing an improvement of clinical symptoms and cerebellar hypometabolism. Ann Neurol 2017;82:615-621.

Large normal-range TBP and ATXN7 CAG repeat lengths are associated with increased lifetime risk of depression.

Depression is one of the most prevalent and debilitating psychiatric disorders worldwide. Recently, we showed that both relatively short and relatively long cytosine-adenine-guanine (CAG) repeats in the huntingtin gene (HTT) are associated with an increased risk of lifetime depression. However, to what extent the variations in CAG repeat length in the other eight polyglutamine disease-associated genes (PDAGs) are associated with depression is still unknown. We determined the CAG repeat sizes of ATXN1, ATXN2, ATXN3, CACNA1A, ATXN7, TBP, ATN1 and AR in two well-characterized Dutch cohorts-the Netherlands Study of Depression and Anxiety and the Netherlands Study of Depression in Older Persons-including 2165 depressed and 1058 non-depressed individuals-aged 18-93 years. The association between PDAG CAG repeat size and the risk for depression was assessed via binary logistic regression. We found that the odds ratio (OR) for lifetime depression was significantly higher for individuals with >10, compared with subjects with ≤10, CAG repeats in both ATXN7 alleles (OR=1.90, confidence interval (CI) 1.26-2.85). For TBP we found a similar association: A CAG repeat length exceeding the median in both alleles was associated with an increased risk for lifetime depression (OR=1.33, CI 1.00-1.76). In conclusion, we observed that carriers of either ATXN7 or TBP alleles with relatively large CAG repeat sizes in both alleles had a substantially increased risk of lifetime depression. Our findings provide critical evidence for the notion that repeat polymorphisms can act as complex genetic modifiers of depression.

SCA2 family presenting as typical Parkinson's disease: 34 year follow up.

OBJECTIVE: We describe a Korean family in SCA2 with long-duration levodopa-responsive parkinsonism without cerebellar ataxia. METHODS: Clinical evaluation, genetic testing, and extensive imaging studies were done. RESULTS: All family members showed a typical Parkinson's disease phenotype without cerebellar ataxia for a long disease duration (up to 34 years). Genetic testing showed 40 CAG repeats and 4 CAA interruptions which is the longest repeat number among the families or patients manifesting with a parkinsonian phenotype without ataxia. Structural imaging (7T MRI and brain CT) showed a normal cerebellum and functional images showed nigrostriatal dopaminergic degeneration and normal D2 receptor binding activity, in agreement with the clinical phenotype. CONCLUSION: SCA2 should be considered as a cause of typical Parkinson's disease phenotype even in the absence of cerebellar ataxia.

Gene co-expression network analysis for identifying modules and functionally enriched pathways in SCA2.

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disease caused by CAG repeat expansion in the ATXN2 gene. The repeat resides in an encoded region of the gene resulting in polyglutamine (polyQ) expansion which has been assumed to result in gain of function, predominantly, for the ATXN2 protein. We evaluated temporal cerebellar expression profiles by RNA sequencing of ATXN2Q127 mice versus wild-type (WT) littermates. ATXN2Q127 mice are characterized by a progressive motor phenotype onset, and have progressive cerebellar molecular and neurophysiological (Purkinje cell firing frequency) phenotypes. Our analysis revealed previously uncharacterized early and progressive abnormal patterning of cerebellar gene expression. Weighted Gene Coexpression Network Analysis revealed four gene modules that were significantly correlated with disease status, composed primarily of genes associated with GTPase signaling, calcium signaling and cell death. Of these genes, few overlapped with differentially expressed cerebellar genes that we identified in Atxn2-/- knockout mice versus WT littermates, suggesting that loss-of-function is not a significant component of disease pathology. We conclude that SCA2 is a disease characterized by gain of function for ATXN2.

SCA13 causes dominantly inherited non-progressive myoclonus ataxia.

INTRODUCTION: Spinocerebellar ataxia 13 (SCA13) is a rare autosomal dominant cerebellar ataxia. To our knowledge, its association to movement disorders has never been described. We aimed at reporting 8 new SCA13 cases with a focus on movement disorders especially myoclonus. METHODS: We performed a detailed neurological examination and neurophysiological recording in 8 patients consecutively diagnosed with SCA13 between December 2013 and October 2015 and followed up in two French tertiary centers. RESULTS: We identified mild subcortical myoclonus in all patients, with a homogenous clinical and electrophysiological pattern. Myoclonus ataxia was very slowly progressive, like the other symptoms of the disease, whatever the age of onset. Patients with R423H mutation had an earlier age of onset than patients with R420H mutation. CONCLUSIONS: Myoclonus appears to be frequent in SCA13. SCA13 should be considered facing non-progressive autosomal dominant myoclonus ataxia, and polymyographic recording should be included in the diagnosis work.

The Initial Symptom and Motor Progression in Spinocerebellar Ataxias.

The aim of this study is to determine whether the initial symptom associates with motor progression in spinocerebellar ataxias (SCAs). SCAs are clinically heterogeneous and the initial presentation may represent different subtypes of SCA with different motor progression. We studied 317 participants with SCAs1, 2, 3, and 6 from the Clinical Research Consortium for SCAs (CRC-SCA) and repeatedly measured the severity of ataxia for 2 years. SCA patients were divided into gait-onset and non-gait-onset (speech, vision, and hand dexterity) groups based on the initial presentation. In addition to demographic comparison, we employed regression models to study ataxia progression in these two groups after adjusting for age, sex, and pathological CAG repeats. The majority of SCA patients had gait abnormality as an initial presentation. The pathological CAG repeat expansions were similar between the gait-onset and non-gait-onset groups. In SCA1, gait-onset group progressed slower than non-gait-onset group, while gait-onset SCA6 group progressed faster than their counterpart. In addition, the disease presented 9 years later for SCA2 gait-onset group than non-gait-onset group. Initial symptoms of SCA3 did not influence age of onset or disease progression. The initial symptom in each SCA has a different influence on age of onset and motor progression. Therefore, gait and non-gait-onset groups of SCAs might represent different subtypes of the diseases.

Spinocerebellar ataxia: relationship between phenotype and genotype - a review.

Spinocerebellar ataxia (SCA) comprises a large group of heterogeneous neurodegenerative disorders inherited in an autosomal dominant fashion. It is characterized by progressive cerebellar ataxia with oculomotor dysfunction, dysarthria, pyramidal signs, extrapyramidal signs, pigmentary retinopathy, peripheral neuropathy, cognitive impairment and other symptoms. It is classified according to the clinical manifestations or genetic nosology. To date, 40 SCAs have been characterized, and include SCA1-40. The pathogenic genes of 28 SCAs were identified. In recent years, with the widespread clinical use of next-generation sequencing, the genes underlying SCAs, and the mutants as well as the affected phenotypes were identified. These advances elucidated the phenotype-genotype relationship in SCAs. We reviewed the recent clinical advances, genetic features and phenotype-genotype correlations involving each SCA and its differentiation. The heterogeneity of the disease and the genetic diagnosis might be attributed to the regional distribution and clinical characteristics. Therefore, recognition of the phenotype-genotype relationship facilitates genetic testing, prognosis and monitoring of symptoms.

Verification of Inter-laboratorial Genotyping Consistency in the Molecular Diagnosis of Polyglutamine Spinocerebellar Ataxias.

The polyglutamine spinocerebellar ataxias (SCAs) constitute a clinically and genetically heterogeneous group of rare late-onset neurodegenerative disorders, caused by CAG expansions in the coding region of the respective genes. Given their considerable clinical overlapping, differential diagnosis relies on molecular testing. Laboratory best practice guidelines for molecular genetic testing of the SCAs were released in 2010 by the European Molecular Genetics Quality Network, following the recognition of gross genotyping errors by some diagnostic laboratories. The main goal of this study was to verify the existence of inter-laboratorial consistency comparing genotypes for SCA1, SCA2, SCA3, SCA6 and SCA7 obtained by independent diagnostic laboratories. The individual impact of different methodological issues on the genotype for the several SCAs was also analysed. Four international collaborative diagnostic laboratories provided 79 samples and the respective SCA genotypes. Samples were genotyped in-house for all SCAs using an independent methodology; comparison of the allele size obtained with the one provided by the collaborative laboratories was performed. Globally, no significant differences were identified, a result which could be reflecting the fulfilment of recommendations for the molecular testing of SCAs and demonstrating an improvement in genotyping accuracy.