Spinocerebellar Ataxia Genes Research Articles

Identification and Copy Number Variant Analysis of Enhancer Regions of Genes Causing Spinocerebellar Ataxia.

Currently, routine diagnostics for spinocerebellar ataxia (SCA) look for polyQ repeat expansions and conventional variations affecting the proteins encoded by known SCA genes. However, ~40% of the patients still remain without a genetic diagnosis after routine tests. Increasing evidence suggests that variations in the enhancer regions of genes involved in neurodegenerative disorders can also cause disease. Since the enhancers of SCA genes are not yet known, it remains to be determined whether variations in these regions are a cause of SCA. In this pilot project, we aimed to identify the enhancers of the SCA genes ATXN1, ATXN3, TBP and ITPR1 in the human cerebellum using 4C-seq, publicly available datasets, reciprocal 4C-seq, and luciferase assays. We then screened these enhancers for copy number variants (CNVs) in a cohort of genetically undiagnosed SCA patients. We identified two active enhancers for each of the four SCA genes. CNV analysis did not reveal any CNVs in the enhancers of the four SCA genes in the genetically undiagnosed SCA patients. However, in one patient, we noted a CNV deletion with an unknown clinical significance near one of the ITPR1 enhancers. These results not only reveal elements involved in SCA gene regulation but can also lead to the discovery of novel SCA-causing genetic variants. As enhancer variations are being increasingly recognized as a cause of brain disorders, screening the enhancers of ATXN1, ATXN3, TBP and ITPR1 for variations other than CNVs and identifying and screening enhancers of other SCA genes might elucidate the genetic cause in undiagnosed patients.

Incidence of different pressure patterns of spinal cerebellar ataxia and analysis of imaging and genetic diagnosis.

Neurologists have a difficult time identifying sporadic cerebellar ataxia. Multiple system atrophy of the cerebellar type (MSA-C), spontaneous late cortical cerebellar atrophy, and prolonged alcohol use are a few possible causes. In a group of people with sporadic cerebellar ataxia that was not MSA-C, an autosomal-dominant spinocerebellar ataxia (SCA) mutation was recently discovered. Chinese single-hospital cohort will be used in this study to genetic screen for SCA-related genes. One hundred forty individuals with CA were monitored over 8 years. Thirty-one individuals had familial CA, 109 patients had sporadic CA, 73 had MSA-C, and 36 had non-MSA-C sporadic CA. In 28 of the 31 non-MSA-C sporadic patients who requested the test, we carried out gene analysis, including SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA12, SCA17, SCA31, and dentatorubro-pallidoluysian atrophy (DRPLA). The control group consisted of family members of the patients. In 57% of the instances with spontaneous CA that were not MSA-C, gene abnormalities were discovered. The most frequent exception among individuals with sporadic CA was SCA6 (36%), followed by monsters in SCA1, 2, 3, 8, and DRPLA. In contrast, 75% of the patients with familial CA had gene abnormalities, the most frequent of which was SCA6 abnormality. The age of 69 vs 59 was higher, and the CAG repeat length was a minor age of 23 vs 25 in the former instances compared to the last one among individuals with SCA6 anomalies that were sporadic as opposed to familial cases. In sporadic CA, autosomal-dominant mutations in SCA genes, notably in SCA6, are common. Although the cause of the increased incidence of SCA6 mutations is unknown, it may be related to a greater age of onset and varied penetrance of SCA6 mutations.

Gene therapy for selected neuromuscular and trinucleotide repeat disorders - An insight to subsume South Asia for multicenter clinical trials.

In this article, the authors discuss how they utilized the genetic mutation data in Sri Lankan Duchenne muscular dystrophy (DMD), Spinal muscular atrophy (SMA), Spinocerebellar ataxia (SCA) and Huntington's disease (HD) patients and compare the available literature from South Asian countries to identifying potential candidates for available gene therapy for DMD, SMA, SCA and HD patients. Rare disease patients (n=623) with the characteristic clinical findings suspected of HD, SCA, SMA and Muscular Dystrophy were genetically confirmed using Multiplex Ligation Dependent Probe Amplification (MLPA), and single plex PCR. A survey was conducted in the "Wiley database on Gene Therapy Trials Worldwide" to identify DMD, SMA, SCA, and HD gene therapy clinical trials performed worldwide up to April 2021. In order to identify candidates for gene therapy in other neighboring countries we compared our findings with available literature from India and Pakistan which has utilized the same molecular diagnostic protocol to our study. From the overall cohort of 623 rare disease patients with the characteristic clinical findings suspected of HD, SCA, SMA and Muscular Dystrophy, n=343 (55%) [Muscular Dystrophy- 65%; (DMD-139, Becker Muscular Dystrophy -BMD-11), SCA type 1-3-53% (SCA1-61,SCA2- 23, SCA3- 39), HD- 52% (45) and SMA- 34% (22)] patients were positive for molecular diagnostics by MLPA and single plex PCR. A total of 147 patients in Sri Lanka amenable to available gene therapy; [DMD-83, SMA-15 and HD-49] were identified. A comparison of Sri Lankan finding with available literature from India and Pakistan identified a total of 1257 patients [DMD-1076, SMA- 57, and HD-124] from these three South Asian Countries as amenable for existing gene therapy trials. DMD, SMA, and HD gene therapy clinical trials (113 studies) performed worldwide up to April 2021 were concentrated mostly (99%) in High Income Countries (HIC) and Upper Middle-Income Countries (UMIC). However, studies on the potential use of anti-sense oligonucleotides (ASO) for treatment of SCAs have yet to reach clinical trials. Most genetic therapies for neurodegenerative and neuromuscular disorders have been evaluated for efficacy primarily in Western populations. No multicenter gene therapy clinical trial sites for DMD, SMA and HD in the South Asian region, leading to lack of knowledge on the safety and efficacy of such personalized therapies in other populations, including South Asians. By fostering collaboration between researchers, clinicians, patient advocacy groups, government and industry in gene therapy initiatives for the inherited-diseases community in the developing world would link the Global North and Global South and breathe life into the motto "Together we can make a difference".

CAG Repeat Expansion in THAP11 Is Associated with a Novel Spinocerebellar Ataxia.

More than 50 loci are associated with spinocerebellar ataxia (SCA), and the most frequent subtypes share nucleotide repeats expansion, especially CAG expansion. The objective of this study was to confirm a novel SCA subtype caused by CAG expansion. We performed long-read whole-genome sequencing combined with linkage analysis in a five-generation Chinese family, and the finding was validated in another pedigree. The three-dimensional structure and function of THAP11 mutant protein were predicted. Polyglutamine (polyQ) toxicity of THAP11 gene with CAG expansion was assessed in skin fibroblasts of patients, human embryonic kidney 293 and Neuro-2a cells. We identified THAP11 as the novel causative SCA gene with CAG repeats ranging from 45 to 100 in patients with ataxia and from 20 to 38 in healthy control subjects. Among the patients, the number of CAA interruptions within CAG repeats was decreased to 3 (up to 5-6 in controls), whereas the number of 3' pure CAG repeats was up to 32 to 87 (4-16 in controls), suggesting that the toxicity of polyQ protein was length dependent on the pure CAG repeats. Intracellular aggregates were observed in cultured skin fibroblasts from patients. THAP11 polyQ protein was more intensely distributed in the cytoplasm of cultured skin fibroblasts from patients, which was replicated with in vitro cultured neuro-2a transfected with 54 or 100 CAG repeats. This study identified a novel SCA subtype caused by intragenic CAG repeat expansion in THAP11 with intracellular aggregation of THAP11 polyQ protein. Our findings extended the spectrum of polyQ diseases and offered a new perspective in understanding polyQ-mediated toxic aggregation. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Copy Number Variant Analysis of Spinocerebellar Ataxia Genes in a Cohort of Dutch Patients With Cerebellar Ataxia.

The spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of neurodegenerative disorders generally caused by single nucleotide variants (SNVs) or indels in coding regions or by repeat expansions in coding and noncoding regions of SCA genes. Copy number variants (CNVs) have now also been reported for 3 genes-ITPR1, FGF14, and SPTBN2-but not all SCA genes have been screened for CNVs as the underlying cause of the disease in patients. In this study, we aim to assess the prevalence of CNVs encompassing 36 known SCA genes. A cohort of patients with cerebellar ataxia who were referred to the University Medical Center Groningen for SCA genetic diagnostics was selected for this study. Genome-wide single nucleotide polymorphism (SNP) genotyping was performed using the Infinium Global Screening Array. Following data processing, genotyping data were uploaded into NxClinical software to perform CNV analysis per patient and to visualize identified CNVs in 36 genes with allocated SCA symbols. The clinical relevance of detected CNVs was determined using evidence from studies based on PubMed literature searches for similar CNVs and phenotypic features. Of the 338 patients with cerebellar ataxia, we identified putative clinically relevant CNV deletions in 3 patients: an identical deletion encompassing ITPR1 in 2 patients, who turned out to be related, and a deletion involving PPP2R2B in another patient. Although the CNV deletion in ITPR1 was clearly the underlying cause of SCA15 in the 2 related patients, the clinical significance of the deletion in PPP2R2B remained unknown. We showed that CNVs detectable with the limited resolution of SNP array are a very rare cause of SCA. Nevertheless, we suggest adding CNV analysis alongside SNV analysis to SCA gene diagnostics using next-generation sequencing approaches, at least for ITPR1, to improve the genetic diagnostics for patients.

Heterozygous PNPT1 Variants Cause Spinocerebellar Ataxia Type 25.

Dominant spinocerebellar ataxias (SCA) are characterized by genetic heterogeneity. Some mapped and named loci remain without a causal gene identified. Here we applied next generation sequencing (NGS) to uncover the genetic etiology of the SCA25 locus. Whole-exome and whole-genome sequencing were performed in families linked to SCA25, including the French family in which the SCA25 locus was originally mapped. Whole exome sequence data were interrogated in a cohort of 796 ataxia patients of unknown etiology. The SCA25 phenotype spans a slowly evolving sensory and cerebellar ataxia, in most cases attributed to ganglionopathy. A pathogenic variant causing exon skipping was identified in the gene encoding Polyribonucleotide Nucleotidyltransferase PNPase 1 (PNPT1) located in the SCA25 linkage interval. A second splice variant in PNPT1 was detected in a large Australian family with a dominant ataxia also mapping to SCA25. An additional nonsense variant was detected in an unrelated individual with ataxia. Both nonsense and splice heterozygous variants result in premature stop codons, all located in the S1-domain of PNPase. In addition, an elevated type I interferon response was observed in blood from all affected heterozygous carriers tested. PNPase notably prevents the abnormal accumulation of double-stranded mtRNAs in the mitochondria and leakage into the cytoplasm, associated with triggering a type I interferon response. This study identifies PNPT1 as a new SCA gene, responsible for SCA25, and highlights biological links between alterations of mtRNA trafficking, interferonopathies and ataxia. ANN NEUROL 2022;92:122-137.

Clinical Features and Genetic Spectrum of Patients With Clinically Suspected Hereditary Progressive Spastic Paraplegia.

Background and PurposeA variety of hereditary diseases overlap with neurological phenotypes or even share genes with hereditary spastic paraplegia (HSP). The aim of this study was to determine the clinical features and genetic spectrum of patients with clinically suspected HSPs.MethodsA total of 52 patients with clinically suspected HSPs were enrolled in this study. All the patients underwent next-generation sequencing (NGS) and triplet repeat primed PCR to screen for the dynamic mutations typical of spinocerebellar ataxia (SCA). Multiplex ligation-dependent probe amplification (MLPA) was further conducted in patients with no causative genetic mutations detected to examine for large deletions and duplications in genes of SPAST, ATL1, REEP1, PGN, and SPG11. Clinical characteristics and findings of brain MRI were analyzed in patients with definite diagnoses.ResultsThe mean age of the patients studied was 36.90 ± 14.57 years. 75% (39/52) of patients manifested a phenotype of complex form of HSPs. A genetic diagnosis was made in 51.9% (27/52) of patients, of whom 40.3% (21/52) of patients had mutations in HSPs genes (SPG4/SPG6/SPG8/SPG11/SPG15/SPG78/SPG5A) and 11.5% (6/52) of patients had mutations in SCAs genes (SCA3/SCA17/SCA28). SPG4 and SPG11 were the most common cause of pure form of HSPs (5/6, 83.3%) and complex form of HSPs (5/15, 33.3%), respectively. Gait disturbance was the most common initial symptom in both the patients with HSPs (15/21) and in patients with SCAs (5/6). Dysarthria and cerebellar ataxia were detected in 28.5% (6/21) and 23.8% (5/21) of patients with HSPs, respectively, and were the most common symptoms in addition to progressive weakness and spasticity of the lower limbs. Cerebellar atrophy was seen on the brain MRI of patients with SPG5A, SCA3, and SCA28.ConclusionCausative genetic mutations were identified in 51.9% of patients with clinically suspected HSPs by NGS and triplet repeat primed PCR. A final diagnosis of HSPs or SCAs was made in 40.3% and 11.5% of patients, respectively. The clinical manifestations and neuroimaging findings overlapped between patients with HSPs and patients with SCAs. Dynamic mutations should be screened in patients with clinically suspected HSPs, especially in those with phenotypes of complex form of HSPs.

Feasibility of Follow-Up Studies and Reclassification in Spinocerebellar Ataxia Gene Variants of Unknown Significance.

Spinocerebellar ataxia (SCA) is a heterogeneous group of neurodegenerative disorders with autosomal dominant inheritance. Genetic testing for SCA leads to diagnosis, prognosis and risk assessment for patients and their family members. While advances in sequencing and computing technologies have provided researchers with a rapid expansion in the genetic test content that can be used to unravel the genetic causes that underlie diseases, the large number of variants with unknown significance (VUSes) detected represent challenges. To minimize the proportion of VUSes, follow-up studies are needed to aid in their reclassification as either (likely) pathogenic or (likely) benign variants. In this study, we addressed the challenge of prioritizing VUSes for follow-up using (a combination of) variant segregation studies, 3D protein modeling, in vitro splicing assays and functional assays. Of the 39 VUSes prioritized for further analysis, 13 were eligible for follow up. We were able to reclassify 4 of these VUSes to LP, increasing the molecular diagnostic yield by 1.1%. Reclassification of VUSes remains difficult due to limited possibilities for performing variant segregation studies in the classification process and the limited availability of routine functional tests.

TGM6 might not be a specific causative gene for spinocerebellar ataxia resulting from genetic analysis and functional study.

To investigate whether TGM6 is a specific causative gene for spinocerebellar ataxia type 35 (SCA35). The next-generation sequencing (NGS) data consisted of 47 SCA, 762 non-SCA patients and 2827 normal controls were analyzed. The allele frequencies of low frequent and deleterious TGM6 variants were compared. Functional studies were performed in five widely distributed variants (V314M, R342Q, P347L, V391M, L517W). Two TGM6 detrimental variants were identified in one SCA patient, 14 in non-SCA patients and 43 in normal controls, the allele frequencies of TGM6 variants did not differ among the SCA and other controls. Seven reported pathogenic variants (c.7+1G>T, c.331C>T, c.1171G>A, c.1478C>T, c.1528G>C, c.1550T>G and c.1722_1724delAGA) were identified in patients with various neurologic diseases or normal controls. All the 5 widely distributed variants led to destabilization and significantly reduction of enzymatic activity of TG6 as the reported pathogenic mutations. TGM6 might not be a specific causative gene for SCA35, the relevant clinical consult or diagnostic should be pay more attention.

Frequency of spinocerebellar ataxia mutations in patients with multiple system atrophy.

Investigate single nucleotide variants and short tandem repeats in 39 genes related to spinocerebellar ataxia in clinical and pathologically defined cohorts of multiple system atrophy. Exome sequencing was conducted in 28 clinical multiple system atrophy patients to identify single nucleotide variants in spinocerebellar ataxia-related genes. Novel variants were validated in two independent disease cohorts: 86 clinically diagnosed multiple system atrophy patients and 166 pathological multiple system atrophy cases. Expanded repeat alleles in spinocerebellar ataxia genes were evaluated in 36 clinically diagnosed multiple system atrophy patients, and CAG/CAA repeats in TATA-Box Binding Protein (TBP, causative of SCA17) were screened in 216 clinical and pathological multiple system atrophy patients and 346 controls. No known pathogenic spinocerebellar ataxia single nucleotide variants or pathogenic range expanded repeat alleles of ATXN1, ATXN2, ATXN3, CACNA1A, AXTN7, ATXN8OS, ATXN10, PPP2R2B, and TBP were detected in any clinical multiple system atrophy patients. However, four novel variants were identified in four spinocerebellar ataxia-related genes across three multiple system atrophy patients. Additionally, four multiple system atrophy patients (1.6%) and one control (0.3%) carried an intermediate length 41 TBP CAG/CAA repeat allele (OR=4.11, P=0.21). There was a significant association between the occurrence of a repeat length of longer alleles (>38 repeats) and an increased risk of multiple system atrophy (OR=1.64, P=0.03). Occurrence of TBP CAG/CAA repeat length of longer alleles (>38 repeats) is significantly associated with increased multiple system atrophy risk. This discovery warrants further investigation and supports a possible genetic overlap of multiple system atrophy with SCA17.

Clinical, Imaging, and Laboratory Markers of Premanifest Spinocerebellar Ataxia 1, 2, 3, and 6: A Systematic Review

Background and PurposePremanifest mutation carriers with spinocerebellar ataxia (SCA) can exhibit subtle abnormalities before developing ataxia. We summarized the preataxic manifestations of SCA1, -2, -3, and -6, and their associations with ataxia onset.MethodsWe included studies of the premanifest carriers of SCA published between January 1998 and December 2019 identified in Scopus and PubMed by searching for terms including ‘spinocerebellar ataxia’ and several synonyms of ‘preataxic manifestation’. We systematically reviewed the results obtained in studies categorized based on clinical, imaging, and laboratory markers.ResultsWe finally performed a qualitative analysis of 48 papers. Common preataxic manifestations appearing in multiple SCA subtypes were muscle cramps, abnormal muscle reflexes, instability in gait and posture, lower Composite Cerebellar Functional Severity scores, abnormalities in video-oculography and transcranial magnetic stimulation, and gray-matter loss and volume reduction in the brainstem and cerebellar structures. Also, decreased sensory amplitudes in nerve conduction studies were observed in SCA2. Eotaxin and neurofilament light-chain levels were revealed as sensitive blood biomarkers in SCA3. Concerning potential predictive markers, hyporeflexia and abnormalities of somatosensory evoked potentials showed correlations with the time to ataxia onset in SCA2 carriers. However, no longitudinal data were found for the other SCA gene carriers.ConclusionsOur results suggest that preataxic manifestations vary among SCA1, -2, -3, and -6, with some subtypes sharing specific features. Combining various markers into a standardized index for premanifest carriers may be useful for early screening and assessing the risk of disease progression in SCA carriers.

Disrupted Calcium Signaling in Animal Models of Human Spinocerebellar Ataxia (SCA).

Spinocerebellar ataxias (SCAs) constitute a heterogeneous group of more than 40 autosomal-dominant genetic and neurodegenerative diseases characterized by loss of balance and motor coordination due to dysfunction of the cerebellum and its efferent connections. Despite a well-described clinical and pathological phenotype, the molecular and cellular events that underlie neurodegeneration are still poorly undaerstood. Emerging research suggests that mutations in SCA genes cause disruptions in multiple cellular pathways but the characteristic SCA pathogenesis does not begin until calcium signaling pathways are disrupted in cerebellar Purkinje cells. Ca2+ signaling in Purkinje cells is important for normal cellular function as these neurons express a variety of Ca2+ channels, Ca2+-dependent kinases and phosphatases, and Ca2+-binding proteins to tightly maintain Ca2+ homeostasis and regulate physiological Ca2+-dependent processes. Abnormal Ca2+ levels can activate toxic cascades leading to characteristic death of Purkinje cells, cerebellar atrophy, and ataxia that occur in many SCAs. The output of the cerebellar cortex is conveyed to the deep cerebellar nuclei (DCN) by Purkinje cells via inhibitory signals; thus, Purkinje cell dysfunction or degeneration would partially or completely impair the cerebellar output in SCAs. In the absence of the inhibitory signal emanating from Purkinje cells, DCN will become more excitable, thereby affecting the motor areas receiving DCN input and resulting in uncoordinated movements. An outstanding advantage in studying the pathogenesis of SCAs is represented by the availability of a large number of animal models which mimic the phenotype observed in humans. By mainly focusing on mouse models displaying mutations or deletions in genes which encode for Ca2+ signaling-related proteins, in this review we will discuss the several pathogenic mechanisms related to deranged Ca2+ homeostasis that leads to significant Purkinje cell degeneration and dysfunction.

Twenty-five years since the identification of the first SCA gene: history, clinical features and perspectives for SCA1.

Spinocerebellar ataxias (SCA) are a clinically and genetically heterogeneous group of monogenic diseases that share ataxia and autosomal dominant inheritance as the core features. An important proportion of SCAs are caused by CAG trinucleotide repeat expansions in the coding region of different genes. In addition to genetic heterogeneity, clinical features transcend motor symptoms, including cognitive, electrophysiological and imaging aspects. Despite all the progress in the past 25 years, the mechanisms that determine how neuronal death is mediated by these unstable expansions are still unclear. The aim of this article is to review, from an historical point of view, the first CAG-related ataxia to be genetically described: SCA 1.

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.

Exome sequencing and network analysis identifies shared mechanisms underlying spinocerebellar ataxia.

The autosomal dominant cerebellar ataxias, referred to as spinocerebellar ataxias in genetic nomenclature, are a rare group of progressive neurodegenerative disorders characterized by loss of balance and coordination. Despite the identification of numerous disease genes, a substantial number of cases still remain without a genetic diagnosis. Here, we report five novel spinocerebellar ataxia genes, FAT2, PLD3, KIF26B, EP300, and FAT1, identified through a combination of exome sequencing in genetically undiagnosed families and targeted resequencing of exome candidates in a cohort of singletons. We validated almost all genes genetically, assessed damaging effects of the gene variants in cell models and further consolidated a role for several of these genes in the aetiology of spinocerebellar ataxia through network analysis. Our work links spinocerebellar ataxia to alterations in synaptic transmission and transcription regulation, and identifies these as the main shared mechanisms underlying the genetically diverse spinocerebellar ataxia types.

Genetic Screening for Spinocerebellar Ataxia Genes in a Japanese Single-Hospital Cohort.

Objective Diagnosis of sporadic cerebellar ataxia is a challenge for neurologists. A wide range of potential causes exist, including chronic alcohol use, multiple system atrophy of cerebellar type (MSA-C), and sporadic late cortical cerebellar atrophy. Recently, an autosomal-dominant spinocerebellar ataxia (SCA) mutation was identified in a cohort of patients with non-MSA-C sporadic cerebellar ataxia. The aim of this study is to genetically screen genes involved in SCA in a Japanese single-hospital cohort. Methods Over an 8-year period, 140 patients with cerebellar ataxia were observed. There were 109 patients with sporadic cerebellar ataxia (no family history for at least four generations, 73 patients with MSA-C, and 36 patients with non-MSA-C sporadic cerebellar ataxia) and 31 patients with familial cerebellar ataxia. We performed gene analysis comprising SCA1, 2, 3, 6, 7, 8, 12, 17, 31, and dentatorubro-pallidoluysian atrophy (DRPLA) in 28 of 31 non-MSA-C sporadic patients who requested the test. Familial patients served as a control.Results Gene abnormalities were found in 57% of non-MSA-C sporadic cerebellar ataxia cases. Among patients with sporadic cerebellar ataxia, abnormalities in SCA6 were the most common (36%), followed by abnormalities in SCA1 (7.1%), SCA2 (3.6%), SCA3 (3.6%), SCA8 (3.6%), and DRPLA (3.6%). In contrast, gene abnormalities were found in 75% of familial cerebellar ataxia cases, with abnormalities in SCA6 being the most common (29%). For sporadic versus familial cases for those with SCA6 abnormalities, the age of onset was older (69 years vs. 59 years, respectively), and CAG repeat length was shorter (23 vs. 25, respectively) in the former than in the latter (not statistically significant).Conclusion Autosomal-dominant mutations in SCA genes, particularly in SCA6, are not rare in sporadic cerebellar ataxia. The reason for the frequency of mutations in SCA6 remains unclear; however, the reason may reflect a higher age at onset and variable penetrance of SCA6 mutations.

A panel study on patients with dominant cerebellar ataxia highlights the frequency of channelopathies.

Autosomal dominant cerebellar ataxias have a marked heterogeneous genetic background, with mutations in 34 genes identified so far. This large amount of implicated genes accounts for heterogeneous clinical presentations, making genotype-phenotype correlations a major challenge in the field. While polyglutamine ataxias, linked to CAG repeat expansions in genes such as ATXN1, ATXN2, ATXN3, ATXN7, CACNA1A and TBP, have been extensively characterized in large cohorts, there is a need for comprehensive assessment of frequency and phenotype of more 'conventional' ataxias. After exclusion of CAG/polyglutamine expansions in spinocerebellar ataxia genes in 412 index cases with dominantly inherited cerebellar ataxias, we aimed to establish the relative frequencies of mutations in other genes, with an approach combining panel sequencing and TaqMan® polymerase chain reaction assay. We found relevant genetic variants in 59 patients (14.3%). The most frequently mutated were channel genes [CACNA1A (n = 16), KCND3 (n = 4), KCNC3 (n = 2) and KCNA1 (n = 2)]. Deletions in ITPR1 (n = 11) were followed by biallelic variants in SPG7 (n = 9). Variants in AFG3L2 (n = 7) came next in frequency, and variants were rarely found in STBN2 (n = 2), ELOVL5, FGF14, STUB1 and TTBK2 (n = 1 each). Interestingly, possible risk factor variants were detected in SPG7 and POLG. Clinical comparisons showed that ataxias due to channelopathies had a significantly earlier age at onset with an average of 24.6 years, versus 40.9 years for polyglutamine expansion spinocerebellar ataxias and 37.8 years for SPG7-related forms (P = 0.001). In contrast, disease duration was significantly longer in the former (20.5 years versus 9.3 and 13.7, P=0.001), though for similar functional stages, indicating slower progression of the disease. Of interest, intellectual deficiency was more frequent in channel spinocerebellar ataxias, while cognitive impairment in adulthood was similar among the three groups. Similar differences were found among a single gene group, comparing 23 patients with CACNA1A expansions (spinocerebellar ataxia 6) to 22 patients with CACNA1A point mutations, which had lower average age at onset (25.2 versus 47.3 years) with longer disease duration (18.7 versus 10.9), but lower severity indexes (0.39 versus 0.44), indicating slower progression of the disease. In conclusion, we identified relevant genetic variations in up to 15% of cases after exclusion of polyglutamine expansion spinocerebellar ataxias, and confirmed CACNA1A and SPG7 as major ataxia genes. We could delineate firm genotype-phenotype correlations that are important for genetic counselling and of possible prognostic value.

Diagnosis of Spinocerebellar Ataxias Caused by Trinucleotide Repeat Expansions.

Spinocerebellar ataxias (SCAs) are a group of disorders that are both clinically and genetically heterogeneous. They usually demonstrate onset in adulthood, but some forms may have juvenile or infantile onset. There are many different types of SCA, demonstrating different modes of inheritance and types of mutation. The most common forms are due to dominantly inherited expansions in trinucleotide repeat sequences located within the coding region of the relevant genes, and these are readily identifiable by molecular genetic testing. In general, it is possible to test for these disorders using PCR-based assays, amplifying across the trinucleotide repeat regions and sizing the PCR products to determine the number of repeats. Larger expansions are generally associated with a more severe presentation of the disorder, and alternative methods may be necessary to detect these alleles. This protocol describes methods for detecting normal and expanded triplet repeat alleles in the most common SCA genes. © 2017 by John Wiley & Sons, Inc.

Clinical features of Chinese patients with Gerstmann–Sträussler–Scheinker identified by targeted next-generation sequencing

Gerstmann–Sträussler–Scheinker (GSS) is an autosomal dominant neurodegenerative disease due to mutations within prion protein (PRNP) gene. Clinically, it is not easy to distinguish GSS from spinocerebellar ataxia (SCA), especially in the early stage of disease. We aimed to identify genetic mutations in 8 Chinese pedigrees with dominant ataxia but excluded dynamic mutations of SCA genes. Targeted next-generation sequencing was performed in the 8 probands. A customized panel was designed to capture 24 known causative genes, including 15 autosomal dominant SCA genes and 9 dementia-related genes. A 2-year follow-up was performed in these patients who harbored mutation. Of the 8 probands, 5 were identified to harbor the p.P102L mutation within PRNP. All these 5 cases had progressive ataxia with age at onset ranging from 48 to 52 years (49.5 ± 4.51). Remarkable phenotypic heterogeneity was observed in them. Cognitive decline was found in 4/5 probands. The average duration from initial symptoms to cognitive decline is 32.5 months, ranging from 22 to 48 months. In this study, we presented the detailed clinical features of 5 GSS pedigrees with PRNP p.P102L mutation. The variable phenotypes among these GSS patients indicated other genetic or environmental factors might be involved in the phenotypic heterogeneity of GSS. Our findings also support the proposal that screening of PRNP mutations should be performed for the patients with dominant ataxia if dynamic mutations of SCA genes were excluded.

The preclinical stage of spinocerebellar ataxias.

The autosomal dominant spinocerebellar ataxias (SCAs) are a heterogeneous group of degenerative diseases of the cerebellum and connected regions. The discovery of various SCA genes and the subsequent possibility of predictive testing currently allow a genetic diagnosis to be established years or even decades before the actual appearance of ataxia symptoms. A growing body of evidence, however, indicates that this preclinical stage is subject to the earliest pathophysiologic changes. This review article comprehensively summarizes the studies conducted in preclinical carriers of a mutation in one of the SCA genes. From these data, it can indeed be concluded that the preclinical phase in SCA is already characterized by detectable central and peripheral nervous system changes, which are reflected by subtle abnormalities during a careful clinical examination, changes in structural and functional brain imaging, abnormal neurophysiologic measurements, and/or altered motor learning paradigms. As these may be compensated for a long time, ataxia symptoms probably only appear after a certain threshold of dysfunction or degeneration has been exceeded. Detailed knowledge of this disease stage is of particular relevance for a better understanding of the pathogenesis of SCAs, will allow us to determine the optimal point in time for interventions in future therapeutic trials, and points to objective, valid biomarkers to assess disease progression. Further studies will benefit from a consensus-based definition of the preclinical stage, from using one and the same validated ataxia rating scale with one fixed cutoff value, and from applying similar mathematical models to calculate time to predicted disease onset.