Inherited Cerebellar Ataxias: 5-Year Experience of the Irish National Ataxia Clinic

An Update on the Adult-Onset Hereditary Cerebellar Ataxias: Novel Genetic Causes and New Diagnostic Approaches

The hereditary cerebellar ataxias (HCAs) are rare, progressive neurologic disorders caused by variants in many different genes. Inheritance may follow autosomal dominant, autosomal recessive, X-linked or mitochondrial patterns. The list of genes associated with adult-onset cerebellar ataxia is continuously growing, with several new genes discovered in the last few years. This includes short-tandem repeat (STR) expansions in RFC1, causing cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS), FGF14-GAA causing spinocerebellar ataxia type 27B (SCA27B), and THAP11. In addition, the genetic basis for SCA4, has recently been identified as a STR expansion in ZFHX3. Given the large and growing number of genes, and different gene variant types, the approach to diagnostic testing for adult-onset HCA can be complex. Testing methods include targeted evaluation of STR expansions (e.g. SCAs, Friedreich ataxia, fragile X-associated tremor/ataxia syndrome, dentatorubral-pallidoluysian atrophy), next generation sequencing for conventional variants, which may include targeted gene panels, whole exome, or whole genome sequencing, followed by various potential additional tests. This review proposes a diagnostic approach for clinical testing, highlights the challenges with current testing technologies, and discusses future advances which may overcome these limitations. Implementing long-read sequencing has the potential to transform the diagnostic approach in HCA, with the overall aim to improve the diagnostic yield.

Genetic aspects of ataxias in a cohort of Turkish patients

INTRODUCTION

Ataxia is one of the clinical findings of the movement disorder disease group. Although there are many underlying etiological reasons, genetic etiology has an increasing significance thanks to the recently developing technology. The aim of this study is to present the variants detected in WES analysis excluding non-genetic causes, in patients with ataxia.

METHODS

Thirty-six patients who were referred to us with findings of ataxia and diagnosed through WES or other molecular genetic analysis methods were included in our study. At the same time, information such as the onset time of the complaints, consanguinity status between parents, and the presence of relatives with similar symptoms were evaluated. If available, the patient's biochemical and radiological test results were presented.

RESULTS

Thirty-six patients were diagnosed through WES or CES. The rate of detected autosomal recessive inheritance disease was 80.5%, while that of autosomal dominant inheritance disease was 19.5%. Abnormal cerebellum was detected on brain MRI images in 26 patients, while polyneuropathy was detected on EMG in eleven of them. While the majority of the patients were compatible with similar cases reported in the literature, five patients had different/additional features (variants in MCM3AP, AGTPBP1, GDAP2, and SH3TC2 genes).

CONCLUSIONS

The diagnosis of ataxia patients with unknown etiology is made possible thanks to these clues. Consideration of a genetic approach is recommended in patients with ataxia of unknown etiology.

The genetic basis of early-onset hereditary ataxia in Iran: results of a national registry of a heterogeneous population

BACKGROUND

To investigate the genetics of early-onset progressive cerebellar ataxia in Iran, we conducted a study at the Children's Medical Center (CMC), the primary referral center for pediatric disorders in the country, over a three-year period from 2019 to 2022. In this report, we provide the initial findings from the national registry.

METHODS

We selected all early-onset patients with an autosomal recessive mode of inheritance to assess their phenotype, paraclinical tests, and genotypes. The clinical data encompassed clinical features, the Scale for the Assessment and Rating of Ataxia (SARA) scores, Magnetic Resonance Imaging (MRI) results, Electrodiagnostic exams (EDX), and biomarker features. Our genetic investigations included single-gene testing, Whole Exome Sequencing (WES), and Whole Genome Sequencing (WGS).

RESULTS

Our study enrolled 162 patients from various geographic regions of our country. Among our subpopulations, we identified known and novel pathogenic variants in 42 genes in 97 families. The overall genetic diagnostic rate was 59.9%. Notably, we observed PLA2G6, ATM, SACS, and SCA variants in 19, 14, 12, and 10 families, respectively. Remarkably, more than 59% of the cases were attributed to pathogenic variants in these genes.

CONCLUSIONS

Iran, being at the crossroad of the Middle East, exhibits a highly diverse genetic etiology for autosomal recessive hereditary ataxia. In light of this heterogeneity, the development of preventive strategies and targeted molecular therapeutics becomes crucial. A national guideline for the diagnosis and management of patients with these conditions could significantly aid in advancing healthcare approaches and improving patient outcomes.

Health-Related Quality of Life in Patients with Inherited Ataxia in Ireland

Inherited cerebellar ataxias (CA) are heterogeneous progressive neurological conditions associated with significant functional limitations. This study aimed to assess the implications of inherited CA on patients' self-reported quality of life (QoL) and impairments in work and activities. 129 individuals with ataxia responded to a survey focused on QoL. Health-related QoL was measured using the RAND 36-Item Short Form Survey. An adaptation of the validated Work Productivity and Activity Impairment questionnaire was used to assess the effect of health on work productivity and ability to perform activities over the past week. Nine percent of respondents were currently employed. Individuals with inherited ataxia experienced significant activity impairment, and 75% required professional or informal care. Health-related quality of life (HRQoL) was significantly worse in all areas for the individuals with inherited ataxia compared with Irish population normative values. Participants with Friedreich's ataxia (n = 56) demonstrated worse physical functioning then those with undetermined ataxia (n = 55). Female gender, younger age at symptom onset, current employment, retirement due to age or ataxia, and living in a long-term care facility were associated with higher sub-scores in different domains of HRQoL, while disease duration correlated with worse physical functioning sub-scores. This study is the first cross-sectional study on HRQoL in patients with inherited ataxia in Ireland. It highlights high rates of unemployment, difficulty with daily activities and physical functioning limitations, which is worse than comparative international studies. Given the limited therapeutic options currently available, optimising HRQoL is an important aspect of managing ataxia.

Diagnostic Yield of NGS Tests for Hereditary Ataxia: a Systematic Review

Next-generation sequencing (NGS), comprising targeted panels (TP), exome sequencing (ES), and genome sequencing (GS) became robust clinical tools for diagnosing hereditary ataxia (HA). Determining their diagnostic yield (DY) is crucial for optimal clinical decision-making. We conducted a comprehensive systematic literature review on the DY of NGS tests for HA. We searched PubMed and Embase databases for relevant studies between 2016 and 2022 and manually examined reference lists of relevant reviews. Eligible studies described the DY of NGS tests in patients with ataxia as a significant feature. Data from 33 eligible studies showed a median DY of 43% (IQR = 9.5-100%). The median DY for TP and ES was 46% and 41.9%, respectively. Higher DY was associated with specific phenotype selection, such as episodic ataxia at 68.35% and early and late onset of ataxia at 46.4% and 54.4%. Parental consanguinity had a DY of 52.4% (p = 0.009), and the presumed autosomal recessive (AR) inheritance pattern showed 62.5%. There was a difference between the median DY of studies that performed targeted sequencing (tandem repeat expansion, TRE) screening and those that did not (p = 0.047). A weak inverse correlation was found between DY and the extent of previous genetic investigation (rho = - 0.323; p = 0.065). The most common genes were CACNA1A and SACS. DY was higher for presumed AR inheritance pattern, positive family history, and parental consanguinity. ES appears more advantageous due to the inclusion of rare genes that might be excluded in TP.

Inherited Ataxias in Children

The purpose of this review is to describe the current diagnostic approach to inherited ataxias during childhood. With the expanding use and availability of gene testing technologies including large sequencing panels, the ability to arrive at a precise genetic diagnosis in this group of disorders has been improving. We have reviewed all the gene sequencing studies of ataxias available by a comprehensive literature search and summarize their results. We provide a logical algorithm for a diagnostic approach in the context of this evolving information. We stress the fact that both autosomal recessive and autosomal dominant mutations can occur in children with ataxias and the need for keeping in mind nucleotide repeat expansions, which cannot be detected by sequencing technologies, as a possible cause of progressive ataxias in children. We discuss the traditional phenotype-based diagnostic approach in the context of gene testing technologies. Finally, we summarize those disorders in which a specific therapy may be indicated.

ANO10 Function in Health and Disease

Anoctamin 10 (ANO10), also known as TMEM16K, is a transmembrane protein and member of the anoctamin family characterized by functional duality. Anoctamins manifest ion channel and phospholipid scrambling activities and are involved in many physiological processes such as cell division, migration, apoptosis, cell signalling, and developmental processes. Several diseases, including neurological, muscle, blood disorders, and cancer, have been associated with the anoctamin family proteins. ANO10, which is the main focus of the present review, exhibits both scrambling and chloride channel activity; calcium availability is necessary for protein activation in either case. Additional processes implicating ANO10 include endosomal sorting, spindle assembly, and calcium signalling. Dysregulation of calcium signalling in Purkinje cells due to ANO10 defects is proposed as the main mechanism leading to spinocerebellar ataxia autosomal recessive type 10 (SCAR10), a rare, slowly progressive spinocerebellar ataxia. Regulation of the endolysosomal pathway is an additional ANO10 function linked to SCAR10 aetiology. Further functional investigation is essential to unravel the ANO10 mechanism of action and involvement in disease development.

NGS in Hereditary Ataxia: When Rare Becomes Frequent

The term hereditary ataxia (HA) refers to a heterogeneous group of neurological disorders with multiple genetic etiologies and a wide spectrum of ataxia-dominated phenotypes. Massive gene analysis in next-generation sequencing has entered the HA scenario, broadening our genetic and clinical knowledge of these conditions. In this study, we employed a targeted resequencing panel (TRP) in a large and highly heterogeneous cohort of 377 patients with a clinical diagnosis of HA, but no molecular diagnosis on routine genetic tests. We obtained a positive result (genetic diagnosis) in 33.2% of the patients, a rate significantly higher than those reported in similar studies employing TRP (average 19.4%), and in line with those performed using exome sequencing (ES, average 34.6%). Moreover, 15.6% of the patients had an uncertain molecular diagnosis. STUB1, PRKCG, and SPG7 were the most common causative genes. A comparison with published literature data showed that our panel would have identified 97% of the positive cases reported in previous TRP-based studies and 92% of those diagnosed by ES. Proper use of multigene panels, when combined with detailed phenotypic data, seems to be even more efficient than ES in clinical practice.

The Cost of Living with Inherited Ataxia in Ireland

Inherited ataxias are a heterogenous group of neurodegenerative disorders characterised by progressive impairment of balance and coordination, typically leading to permanent and progressive disability. Diagnosis and management of these disorders incurs a range of direct and indirect financial costs. The aim of this study was to collect individual ataxia-related healthcare resources in a large cohort of individuals with different subtypes of inherited ataxia and calculate the associated cost of illness in the Republic of Ireland. One hundred twenty-nine respondents completed a cross-sectional study on healthcare resource utilisation for progressive ataxia in Ireland. Costs were calculated using a prevalence-based approach and bottom-up methodology. The COI for inherited ataxia in 2016 was €59,993 per person per year. Results were similar between participants with Friedreich’s ataxia (FRDA, n = 56), non-FRDA (n = 18) and those with undetermined ataxia (n = 55). Indirect costs, based on productivity losses by participants or caregivers, accounted for 52% of the cost of illness. Inherited ataxia is associated with significant health and social care costs. Further funding for inherited ataxia to ease the financial burden on patients, caregivers and healthcare system and improve standards of care compliance is warranted.

Milestones in genetics of cerebellar ataxias

Cerebellar ataxias (CAs) comprise a group of rare, neurological disorders characterized by extensive phenotypic and genetic heterogeneity. The core clinical feature is the cerebellar syndrome, which is often accompanied by other neurological or non-neurological signs. In the last 30 years, our understanding of the CA etiology has increased significantly, and numerous ataxia-associated genes have been discovered. Conventional variants or tandem repeat expansions, localized in the coding or non-coding DNA sequences, lead to hereditary ataxia, which can display different patterns of inheritance. Advances in molecular techniques have enabled a rapid and cost-effective detection of causative variants in a significant number of CA patients. However, despite performing extensive investigations, a definite diagnosis is still unknown in the majority of affected individuals. In this review, we discuss the major advances in the genetics of CAs over the last 30 years, focusing on the impact of next-generation sequencing on the genetic landscape of childhood- and adult-onset CAs. Additionally, we outline possible directions for further genetic research in hereditary and sporadic CAs in the era of increasing application of whole-genome sequencing and genome-wide association studies in various neurological disorders.

Neurophysiological and ophthalmological findings of SPG7-related spastic ataxia: a phenotype study in an Irish cohort

BACKGROUND

Mutations in SPG7 are increasingly identified as a common cause of spastic ataxia. We describe a cohort of Irish patients with recessive SPG7-associated phenotype.

METHODS

Comprehensive phenotyping was performed with documentation of clinical, neurophysiological, optical coherence tomography (OCT) and genetic data from individuals with SPG7 attending two academic neurology units in Dublin, including the National Ataxia Clinic.

RESULTS

Thirty-two symptomatic individuals from 25 families were identified. Mean age at onset was 39.1 years (range 12-61), mean disease duration 17.8 years (range 5-45), mean disease severity as quantified with the scale for the assessment and rating of ataxia 9/40 (range 3-29). All individuals displayed variable ataxia and spasticity within a spastic-ataxic phenotype, and additional ocular abnormalities. Two had spasmodic dysphonia and three had colour vision deficiency. Brain imaging consistently revealed cerebellar atrophy (n = 29); neurophysiology demonstrated a length-dependent large-fibre axonal neuropathy in 2/27 studied. The commonest variant was c.1529C > T (p.Ala510Val), present in 21 families. Five novel variants were identified. No significant thinning of average retinal nerve fibre layer (RNFL) was demonstrated on OCT (p = 0.61), but temporal quadrant reduction was evident compared to controls (p < 0.05), with significant average and temporal RNFL decline over time. Disease duration, severity and visual acuity were not correlated with RNFL thickness.

CONCLUSIONS

Our results highlight that recessive SPG7 mutations may account for spastic ataxia with peripheral neuropathy in only a small proportion of patients. RNFL abnormalities with predominant temporal RNFL reduction are common and OCT should be considered part of the routine assessment in spastic ataxia.