Natural History, Phenotypic Spectrum, and Discriminative Features of Multisystemic RFC1 Disease

Comprehensive Analysis of a Japanese Pedigree with Biallelic ACAGG Expansions in RFC1 Manifesting Motor Neuronopathy with Painful Muscle Cramps

Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) is an autosomal recessive multisystem neurologic disorder caused by biallelic intronic repeats in RFC1. Although the phenotype of CANVAS has been expanding via diagnostic case accumulation, there are scant pedigree analyses to reveal disease penetrance, intergenerational fluctuations in repeat length, or clinical phenomena (including heterozygous carriers). We identified biallelic RFC1 ACAGG expansions of 1000 ~ repeats in three affected siblings having sensorimotor neuronopathy with spinocerebellar atrophy initially presenting with painful muscle cramps and paroxysmal dry cough. They exhibit almost homogeneous clinical and histopathological features, indicating motor neuronopathy. Over 10 years of follow-up, painful intractable muscle cramps ascended from legs to trunks and hands, followed by amyotrophy and subsequent leg pyramidal signs. The disease course combined with the electrophysical and imagery data suggest initial and prolonged hyperexcitability and the ensuing spinal motor neuron loss, which may progress from the lumbar to the rostral anterior horns and later expand to the corticospinal tract. Genetically, heterozygous ACAGG expansions of similar length were transmitted in unaffected family members of three successive generations, and some of them experienced muscle cramps. Leukocyte telomere length assays revealed comparatively shorter telomeres in affected individuals. This comprehensive pedigree analysis demonstrated a non-anticipating ACAGG transmission and high penetrance of manifestations with a biallelic state, especially motor neuronopathy in which muscle cramps serve as a prodromal and disease progress marker. CANVAS and RFC1 spectrum disorder should be considered when diagnosing lower dominant motor neuron disease, idiopathic muscle cramps, or neuromuscular hyperexcitability syndromes.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome: a neurogenic cough prototype

Chronic cough is a frequent disorder that is defined by cough of more than 8 weeks duration. Despite extensive investigation, some patients exhibit no aetiology and others do not respond to specific treatments directed against apparent causes of cough. Such patients are identified as having unexplained or refractory chronic cough. Recently, a high proportion of patients with chronic cough in the context of cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) was highlighted. CANVAS is a rare neurological disorder with a biallelic variation in the replication factor C subunit 1 (RFC1) gene corresponding mostly to an intronic AAGGG repeat expansion. Chronic cough in patients with CANVAS shares similar characteristics with cough hypersensitivity syndrome. The high prevalence of chronic cough in CANVAS gives the opportunity to better understand the neurogenic mechanism of chronic cough. In this review, we will describe the characteristics and mechanisms of CANVAS. We will also address the potential mechanisms responsible for chronic cough in CANVAS. Finally, we will address chronic cough management in the context of CANVAS.

Analysis and occurrence of biallelic pathogenic repeat expansions in RFC1 in a German cohort of patients with a main clinical phenotype of motor neuron disease

Biallelic pathogenic repeat expansions in RFC1 were recently identified as molecular origin of cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) as well as of one of the most common causes of adult-onset ataxia. In the meantime, the phenotypic spectrum has expanded massively and now includes mimics of multiple system atrophy or parkinsonism. After identifying a patient with a clinical diagnosis of amyotrophic lateral sclerosis (ALS) as a carrier of biallelic pathogenic repeat expansions in RFC1, we studied a cohort of 106 additional patients with a clinical main phenotype of motor neuron disease (MND) to analyze whether such repeat expansions are more common in MND patients. Indeed, two additional MND patients (one also with ALS and one with primary lateral sclerosis/PLS) have been identified as carrier of biallelic pathogenic repeat expansions in RFC1 in the absence of another genetic alteration explaining the phenotype, suggesting motor neuron disease as another extreme phenotype of RFC1 spectrum disorder. Therefore, MND might belong to the expanding phenotypic spectrum of pathogenic RFC1 repeat expansions, particularly in those MND patients with additional features such as sensory and/or autonomic neuropathy, vestibular deficits, or cerebellar signs. By systematically analyzing the RFC1 repeat array using Oxford nanopore technology long-read sequencing, our study highlights the high intra- and interallelic heterogeneity of this locus and allows the identification of the novel repeat motif 'ACAAG'.

An Updated Canvas of the RFC1-mediated CANVAS (Cerebellar Ataxia, Neuropathy and Vestibular Areflexia Syndrome)

Proliferation of specific nucleotide sequences within the coding and non-coding regions of numerous genes has been implicated in approximately 40 neurodegenerative disorders. Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS), a neurodegenerative disorder, is distinguished by a pathological triad of sensory neuropathy, bilateral vestibular areflexia and cerebellar impairments. It manifests in adults gradually and is autosomal recessive and multi-system ataxia. Predominantly, CANVAS is associated with biallelic AAGGG repeat expansions in intron 2 of the RFC1 gene. Although various motifs have been identified, only a subset induces pathological consequences, by forming stable secondary structures that disrupt gene functions both in vitro and in vivo. The pathogenesis of CANVAS remains a subject of intensive research, yet its precise mechanisms remain elusive. Herein, we aim to comprehensively review the epidemiology, clinical ramifications, molecular mechanisms, genetics, and potential therapeutics in light of the current findings, extending an overview of the most significant research on CANVAS.

Pathologic RFC1 repeat expansions do not contribute to the development of inflammatory neuropathies

Biallelic expansions of the AAGGG repeat in the replication factor C subunit 1 (RFC1) have recently been described to be responsible for cerebellar ataxia, peripheral neuropathy and vestibular areflexia syndrome. This genetic alteration has also allowed genetic classification in up to one-third of cases with idiopathic sensory neuropathy. Here, we screened a well-characterized cohort of inflammatory neuropathy patients for RFC1 repeat expansions to explore whether RFC1 was increased from background rates and possibly involved in the pathogenesis of inflammatory neuropathy. A total of 259 individuals with inflammatory neuropathy and 243 healthy controls were screened for the AAGGG repeat expansion using short-range flanking PCR and repeat-primed PCR. Cases without amplifiable PCR product on flanking PCR and positive repeat-primed PCR were also tested for the mostly non-pathogenic expansions of the AAAGG and AAAAG repeat units. None of the patients showed biallelic AAGGG expansion of RFC1, and their carrier frequency for AAGGG was comparable with controls [n = 27 (5.2%) and n = 23 (4.7%), respectively; P > 0.5]. Data suggest that the pathologic expansions of AAGGG repeats do not contribute to the development of inflammatory neuropathies nor lead to misdiagnosed cases. Accordingly, routine genetic screening for RFC1 repeat expansion is not indicated in this patient population.

RFC1 Repeat Distribution in the Cypriot Population: Study of a Large Cohort of Patients With Undiagnosed Ataxia and Non-Disease Controls

Background and Objectives

The intronic biallelic AAGGG expansion in the replication factor C subunit 1 (RFC1) gene was recently associated with a phenotype combining cerebellar ataxia, neuropathy, and vestibular areflexia syndrome, as well as with late-onset ataxia. Following this discovery, studies in multiple populations extended the phenotypic and genotypic spectrum of this locus. Multiple benign and additional pathogenic configurations are currently known. Our main objectives were to study the prevalence of the pathogenic AAGGG expansion in the Cypriot population, to further characterize the RFC1 repeat locus allele distribution, and to search for possible novel repeat configurations.

Methods

Cypriot undiagnosed patients, in the majority presenting at least with cerebellar ataxia and non-neurologic disease controls, were included in this study. A combination of conventional methods was used, including standard PCR flanking the repeat region, repeat-primed PCR, long-range PCR, and Sanger sequencing. Bioinformatics analysis of already available in-house short-read whole-genome sequencing data was also performed.

Results

A large group of undiagnosed patients (n = 194), mainly presenting with pure ataxia or with ataxia accompanied by neuropathy or additional symptoms, as well as a group of non-disease controls (n = 100), were investigated in the current study. Our findings include the diagnosis of 10 patients homozygous for the pathogenic AAGGG expansion and a high percentage of heterozygous AAGGG carriers in both groups. The benign AAAAGn, AAAGGn, and AAGAGn configurations were also identified in our cohorts. We also report and discuss the identification of 2 recently reported novel and possibly benign repeat configurations, AAAGGGn and AAGACn, thus confirming their existence in another distinct population, and we highlight an increased frequency of the AAAGGGn in the patient group, including a single case of homozygosity.

Discussion

Our findings indicate the existence of genetic heterogeneity regarding the RFC1 repeat configurations and that the AAGGG pathogenic expansion is a frequent cause of ataxia in the Cypriot population.

RFC1 and FGF14 Repeat Expansions in Serbian Patients with Cerebellar Ataxia

BACKGROUND

The newly discovered intronic repeat expansions in the genes encoding replication factor C subunit 1 (RFC1) and fibroblast growth factor 14 (FGF14) frequently cause late-onset cerebellar ataxia.

OBJECTIVES

To investigate the presence of RFC1 and FGF14 pathogenic repeat expansions in Serbian patients with adult-onset cerebellar ataxia.

METHODS

The study included 167 unrelated patients with sporadic or familial cerebellar ataxia. The RFC1 repeat expansion analysis was performed by duplex PCR and Sanger sequencing, while the FGF14 repeat expansion was tested for by long-range PCR, repeat-primed PCR, and Sanger sequencing.

RESULTS

We identified pathogenic repeat expansions in RFC1 in seven patients (7/167; 4.2%) with late-onset sporadic ataxia with neuropathy and chronic cough. Two patients also had bilateral vestibulopathy. Repeat expansions in FGF14 were found in nine unrelated patients (9/167; 5.4%) with ataxia, less than half of whom presented with neuropathy and two-thirds with global brain atrophy. Tremor and episodic features were the most frequent additional characteristics in carriers of uninterrupted FGF14 repeat expansions. Among the 122 sporadic cases, 12 (9.8%) carried an expansion in either RFC1 or FGF14, comparable to 4/45 (8.9%) among the patients with a positive family history.

CONCLUSIONS

Pathogenic repeat expansions in RFC1 and FGF14 are relatively frequent causes of adult-onset cerebellar ataxia, especially among sporadic patients, indicating that family history should not be considered when prioritizing ataxia patients for testing of RFC1 or FGF14 repeat expansions.

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.

Role of the repeat expansion size in predicting age of onset and severity in RFC1 disease

RFC1 disease, caused by biallelic repeat expansion in RFC1, is clinically heterogeneous in terms of age of onset, disease progression and phenotype. We investigated the role of the repeat size in influencing clinical variables in RFC1 disease. We also assessed the presence and role of meiotic and somatic instability of the repeat. In this study, we identified 553 patients carrying biallelic RFC1 expansions and measured the repeat expansion size in 392 cases. Pearson's coefficient was calculated to assess the correlation between the repeat size and age at disease onset. A Cox model with robust cluster standard errors was adopted to describe the effect of repeat size on age at disease onset, on age at onset of each individual symptoms, and on disease progression. A quasi-Poisson regression model was used to analyse the relationship between phenotype and repeat size. We performed multivariate linear regression to assess the association of the repeat size with the degree of cerebellar atrophy. Meiotic stability was assessed by Southern blotting on first-degree relatives of 27 probands. Finally, somatic instability was investigated by optical genome mapping on cerebellar and frontal cortex and unaffected peripheral tissue from four post-mortem cases. A larger repeat size of both smaller and larger allele was associated with an earlier age at neurological onset [smaller allele hazard ratio (HR) = 2.06, P < 0.001; larger allele HR = 1.53, P < 0.001] and with a higher hazard of developing disabling symptoms, such as dysarthria or dysphagia (smaller allele HR = 3.40, P < 0.001; larger allele HR = 1.71, P = 0.002) or loss of independent walking (smaller allele HR = 2.78, P < 0.001; larger allele HR = 1.60; P < 0.001) earlier in disease course. Patients with more complex phenotypes carried larger expansions [smaller allele: complex neuropathy rate ratio (RR) = 1.30, P = 0.003; cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) RR = 1.34, P < 0.001; larger allele: complex neuropathy RR = 1.33, P = 0.008; CANVAS RR = 1.31, P = 0.009]. Furthermore, larger repeat expansions in the smaller allele were associated with more pronounced cerebellar vermis atrophy (lobules I-V β = -1.06, P < 0.001; lobules VI-VII β = -0.34, P = 0.005). The repeat did not show significant instability during vertical transmission and across different tissues and brain regions. RFC1 repeat size, particularly of the smaller allele, is one of the determinants of variability in RFC1 disease and represents a key prognostic factor to predict disease onset, phenotype and severity. Assessing the repeat size is warranted as part of the diagnostic test for RFC1 expansion.

Spinocerebellar ataxia 27B (SCA27B), a frequent late-onset cerebellar ataxia

Genetic cerebellar ataxias are still a diagnostic challenge, and yet not all of them have been identified. Very recently, in early 2023, a new cause of late-onset cerebellar ataxia (LOCA) was identified, spinocerebellar ataxia 27B (SCA27B). This is an autosomal dominant ataxia due to a GAA expansion in intron 1 of the FGF14 gene. Thanks to the many studies carried out since its discovery, it is now possible to define the clinical phenotype, its particularities, and the progression of SCA27B. It has also been established that it is one of the most frequent causes of LOCA. The core phenotype of the disease consists of slowly progressive late-onset ataxia with cerebellar syndrome, oculomotor disorders including downbeat nystagmus, and episodic symptoms such as diplopia. Therapeutic approaches have been proposed, including acetazolamide, and 4-aminopyridine, the latter with a better benefit/tolerance profile.

RFC1 repeat expansions in downbeat nystagmus syndromes: frequency and phenotypic profile

OBJECTIVES

The cause of downbeat nystagmus (DBN) remains unknown in a substantial number of patients ("idiopathic"), although intronic GAA expansions in FGF14 have recently been shown to account for almost 50% of yet idiopathic cases. Here, we hypothesized that biallelic RFC1 expansions may also represent a recurrent cause of DBN syndrome.

METHODS

We genotyped the RFC1 repeat and performed in-depth phenotyping in 203 patients with DBN, including 65 patients with idiopathic DBN, 102 patients carrying an FGF14 GAA expansion, and 36 patients with presumed secondary DBN.

RESULTS

Biallelic RFC1 AAGGG expansions were identified in 15/65 patients with idiopathic DBN (23%). None of the 102 GAA-FGF14-positive patients, but 2/36 (6%) of patients with presumed secondary DBN carried biallelic RFC1 expansions. The DBN syndrome in RFC1-positive patients was characterized by additional cerebellar impairment in 100% (15/15), bilateral vestibulopathy (BVP) in 100% (15/15), and polyneuropathy in 80% (12/15) of cases. Compared to GAA-FGF14-positive and genetically unexplained patients, RFC1-positive patients had significantly more frequent neuropathic features on examination and BVP. Furthermore, vestibular function, as measured by the video head impulse test, was significantly more impaired in RFC1-positive patients.

DISCUSSION

Biallelic RFC1 expansions are a common monogenic cause of DBN syndrome.

Cognitive Impairment Is Part of the Phenotype of Cerebellar Ataxia, Neuropathy, Vestibular Areflexia Syndrome (CANVAS)

BACKGROUND

Little is known about the impact of the cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) on cognition.

OBJECTIVE

Our objective was to determine the frequency and severity of cognitive impairment in RFC1-positive patients and describe the pattern of deficits.

METHODS

Participants underwent a comprehensive neuropsychological assessment. Volume of the cerebellum and its lobules was measured in those who underwent a 3 Tesla-magnetic resonance scan.

RESULTS

Twenty-one patients underwent a complete assessment, including 71% scoring lower than the cutoff at the Montreal Cognitive assessment and 71% having a definite cerebellar cognitive affective/Schmahmann syndrome. Three patients had dementia and seven met the criteria of mild cognitive impairment. Severity of cognitive impairment did not correlate with severity of clinical manifestations. Performance at memory and visuospatial functions tests negatively correlated with the severity of cerebellar manifestations.

CONCLUSION

Cognitive manifestations are frequent in RFC1-related disorders. They should be included in the phenotype and screened systematically. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

RFC1: Motifs and phenotypes

Biallelic intronic expansions (AAGGG)exp in intron 2 of the RFC1 gene have been shown to be a common cause of late-onset ataxia. Since their first description, the phenotypes, neurological damage, and pathogenic variants associated with the RFC1 gene have been frequently updated. Here, we review the various motifs, genetic variants, and phenotypes associated with the RFC1 gene. We searched PubMed for scientific articles published between March 1st, 2019, and January 15th, 2024. The motifs and phenotypes associated with the RFC1 gene are highly heterogeneous, making molecular diagnosis and clinical screening and investigation challenging. In this review we will provide clues to give a better understanding of RFC1 disease. We briefly discuss new methods for molecular diagnosis, the origin of cough in RFC1 disease, and research perspectives.

Standards of NGS Data Sharing and Analysis in Ataxias: Recommendations by the NGS Working Group of the Ataxia Global Initiative

The Ataxia Global Initiative (AGI) is a worldwide multi-stakeholder research platform to systematically enhance trial-readiness in degenerative ataxias. The next-generation sequencing (NGS) working group of the AGI aims to improve methods, platforms, and international standards for ataxia NGS analysis and data sharing, ultimately allowing to increase the number of genetically ataxia patients amenable for natural history and treatment trials. Despite extensive implementation of NGS for ataxia patients in clinical and research settings, the diagnostic gap remains sizeable, as approximately 50% of patients with hereditary ataxia remain genetically undiagnosed. One current shortcoming is the fragmentation of patients and NGS datasets on different analysis platforms and databases around the world. The AGI NGS working group in collaboration with the AGI associated research platforms-CAGC, GENESIS, and RD-Connect GPAP-provides clinicians and scientists access to user-friendly and adaptable interfaces to analyze genome-scale patient data. These platforms also foster collaboration within the ataxia community. These efforts and tools have led to the diagnosis of > 500 ataxia patients and the discovery of > 30 novel ataxia genes. Here, the AGI NGS working group presents their consensus recommendations for NGS data sharing initiatives in the ataxia field, focusing on harmonized NGS variant analysis and standardized clinical and metadata collection, combined with collaborative data and analysis tool sharing across platforms.

Inflammatory sensory neuronopathies

Inflammatory sensory neuronopathies are rare disorders mediated by dysimmune mechanisms targeting sensory neurons in the dorsal root ganglia. They constitute a heterogeneous group of disorders with acute, subacute, or chronic courses, and occur with cancer, systemic autoimmune diseases, notably Sjögren syndrome, and viral infections but a noticeable proportion of them remains isolated. Identifying inflammatory sensory neuronopathies is crucial because they have the potential to be stabilized or even to improve with immunomodulatory or immunosuppressant treatments provided that the treatment is applied at an early stage of the disease, before a definitive degeneration of neurons. Biomarkers, and notably antibodies, are crucial for this early identification, which is the first step to develop therapeutic trials.

Autosomal recessive cerebellar ataxias: a diagnostic classification approach according to ocular features

Autosomal recessive cerebellar ataxias (ARCAs) are a heterogeneous group of neurodegenerative disorders affecting primarily the cerebellum and/or its afferent tracts, often accompanied by damage of other neurological or extra-neurological systems. Due to the overlap of clinical presentation among ARCAs and the variety of hereditary, acquired, and reversible etiologies that can determine cerebellar dysfunction, the differential diagnosis is challenging, but also urgent considering the ongoing development of promising target therapies. The examination of afferent and efferent visual system may provide neurophysiological and structural information related to cerebellar dysfunction and neurodegeneration thus allowing a possible diagnostic classification approach according to ocular features. While optic coherence tomography (OCT) is applied for the parametrization of the optic nerve and macular area, the eye movements analysis relies on a wide range of eye-tracker devices and the application of machine-learning techniques. We discuss the results of clinical and eye-tracking oculomotor examination, the OCT findings and some advancing of computer science in ARCAs thus providing evidence sustaining the identification of robust eye parameters as possible markers of ARCAs.

Unraveling the genetic landscape of undiagnosed cerebellar ataxia in Brazilian patients

INTRODUCTION

Hereditary ataxias (HAs) encompass a diverse and genetically intricate group of rare neurodegenerative disorders, presenting diagnostic challenges. Whole-exome sequencing (WES) has significantly improved diagnostic success. This study aimed to elucidate genetic causes of cerebellar ataxia within a diverse Brazilian cohort.

METHODS

Biological samples were collected from individuals with sporadic or familial cerebellar ataxia, spanning various ages and phenotypes, excluding common SCAs and Friedreich ataxia. RFC1 biallelic AAGGG repeat expansion was screened in all patients. For AAGGG-negative cases, WES targeting 441 ataxia-related genes was performed, followed by ExpansionHunter analysis for repeat expansions, including the recently described GGC-ZFHX3. Variant classification adhered to ClinGen guidelines, yielding definitive or probable diagnoses.

RESULTS

The study involved 76 diverse Brazilian families. 16 % received definitive diagnoses, and another 16 % received probable ones. RFC1-related ataxia was predominant, with two definitive cases, followed by KIF1A (one definitive and one probable) and SYNE-1 (two probable). Early-onset cases exhibited higher diagnostic rates. ExpansionHunter improved diagnosis by 4 %.We did not detected GGC-ZFHX3 repeat expansion in this cohort.

CONCLUSION

This study highlights diagnostic complexities in cerebellar ataxia, even with advanced genetic methods. RFC1, KIF1A, and SYNE1 emerged as prevalent mutations. ZFHX3 repeat expansion seem to be rare in Brazilian population. Early-onset cases showed higher diagnostic success. WES coupled with ExpansionHunter holds promise as a primary diagnostic tool, emphasizing the need for broader NGS accessibility in Brazil.

Intronic FGF14 GAA repeat expansions are a common cause of ataxia syndromes with neuropathy and bilateral vestibulopathy

BACKGROUND

Intronic GAA repeat expansions in the fibroblast growth factor 14 gene (FGF14) have recently been identified as a common cause of ataxia with potential phenotypic overlap with RFC1-related cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS). Our objective was to report on the frequency of intronic FGF14 GAA repeat expansions in patients with an unexplained CANVAS-like phenotype.

METHODS

We recruited 45 patients negative for biallelic RFC1 repeat expansions with a combination of cerebellar ataxia plus peripheral neuropathy and/or bilateral vestibulopathy (BVP), and genotyped the FGF14 repeat locus. Phenotypic features of GAA-FGF14-positive versus GAA-FGF14-negative patients were compared.

RESULTS

Frequency of FGF14 GAA repeat expansions was 38% (17/45) in the entire cohort, 38% (5/13) in the subgroup with cerebellar ataxia plus polyneuropathy, 43% (9/21) in the subgroup with cerebellar ataxia plus BVP and 27% (3/11) in patients with all three features. BVP was observed in 75% (12/16) of GAA-FGF14-positive patients. Polyneuropathy was at most mild and of mixed sensorimotor type in six of eight GAA-FGF14-positive patients. Family history of ataxia (59% vs 15%; p=0.007) was significantly more frequent and permanent cerebellar dysarthria (12% vs 54%; p=0.009) significantly less frequent in GAA-FGF14-positive than in GAA-FGF14-negative patients. Age at onset was inversely correlated to the size of the repeat expansion (Pearson's r, -0.67; R2=0.45; p=0.0031).

CONCLUSIONS

GAA-FGF14-related disease is a common cause of cerebellar ataxia with polyneuropathy and/or BVP, and should be included in the differential diagnosis of RFC1 CANVAS and disease spectrum.

Integration of multi-omics technologies for molecular diagnosis in ataxia patients

Background: Episodic ataxias are rare neurological disorders characterized by recurring episodes of imbalance and coordination difficulties. Obtaining definitive molecular diagnoses poses challenges, as clinical presentation is highly heterogeneous, and literature on the underlying genetics is limited. While the advent of high-throughput sequencing technologies has significantly contributed to Mendelian disorders genetics, interpretation of variants of uncertain significance and other limitations inherent to individual methods still leaves many patients undiagnosed. This study aimed to investigate the utility of multi-omics for the identification and validation of molecular candidates in a cohort of complex cases of ataxia with episodic presentation. Methods: Eight patients lacking molecular diagnosis despite extensive clinical examination were recruited following standard genetic testing. Whole genome and RNA sequencing were performed on samples isolated from peripheral blood mononuclear cells. Integration of expression and splicing data facilitated genomic variants prioritization. Subsequently, long-read sequencing played a crucial role in the validation of those candidate variants. Results: Whole genome sequencing uncovered pathogenic variants in four genes (SPG7, ATXN2, ELOVL4, PMPCB). A missense and a nonsense variant, both previously reported as likely pathogenic, configured in trans in individual #1 (SPG7: c.2228T>C/p.I743T, c.1861C>T/p.Q621*). An ATXN2 microsatellite expansion (CAG32) in another late-onset case. In two separate individuals, intronic variants near splice sites (ELOVL4: c.541 + 5G>A; PMPCB: c.1154 + 5G>C) were predicted to induce loss-of-function splicing, but had never been reported as disease-causing. Long-read sequencing confirmed the compound heterozygous variants configuration, repeat expansion length, as well as splicing landscape for those pathogenic variants. A potential genetic modifier of the ATXN2 expansion was discovered in ZFYVE26 (c.3022C>T/p.R1008*). Conclusion: Despite failure to identify pathogenic variants through clinical genetic testing, the multi-omics approach enabled the molecular diagnosis in 50% of patients, also giving valuable insights for variant prioritization in remaining cases. The findings demonstrate the value of long-read sequencing for the validation of candidate variants in various scenarios. Our study demonstrates the effectiveness of leveraging complementary omics technologies to unravel the underlying genetics in patients with unresolved rare diseases such as ataxia. Molecular diagnoses not only hold significant promise in improving patient care management, but also alleviates the burden of diagnostic odysseys, more broadly enhancing quality of life.

Investigation of RFC1 tandem nucleotide repeat locus in diverse neurodegenerative outcomes in an Indian cohort

An intronic bi-allelic pentanucleotide repeat expansion mutation, (AAGGG)400-2000, at AAAAG repeat locus in RFC1 gene, is known as underlying genetic cause in cases with cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) and late-onset sporadic ataxia. Biallelic positive cases carry a common recessive risk haplotype, "AAGA," spanning RFC1 gene. In this study, our aim is to find prevalence of bi-allelic (AAGGG)exp in Indian ataxia and other neurological disorders and investigate the complexity of RFC1 repeat locus and its potential association with neurodegenerative diseases in Indian population-based cohorts. We carried out repeat number and repeat type estimation using flanking PCR and repeat primed PCR (AAAAG/AAAGG/AAGGG) in four Indian disease cohorts and healthy controls. Haplotype assessment of suspected cases was done by genotyping and confirmed by Sanger sequencing. Blood samples and consent of all the cases and detailed clinical details of positive cases were collected in collaboration with A.I.I.M.S. Furthermore, comprehension of RFC1 repeat locus and risk haplotype analysis in Indian background was performed on the NGS data of Indian healthy controls by ExpansionHunter, ExpansionHunter Denovo, and PHASE analysis, respectively. Genetic screening of RFC1-TNR locus in 1998 uncharacterized cases (SCA12: 87; uncharacterized ataxia: 1818, CMT: 93) and 564 heterogenous controls showed that the frequency of subjects with bi-allelic (AAGGG)exp are 1.15%, < 0.05%, 2.15%, and 0% respectively. Two RFC1 positive sporadic late-onset ataxia cases, one bi-allelic (AAGGG)exp and another, (AAAGG)~700/(AAGGG)exp, had recessive risk haplotype and CANVAS symptoms. Long normal alleles, 15-27, are significantly rare in ataxia cohort. In IndiGen control population (IndiGen; N = 1029), long normal repeat range, 15-27, is significantly associated with A3G3 and some rare repeat motifs, AGAGG, AACGG, AAGAG, and AAGGC. Risk-associated "AAGA" haplotype of the original pathogenic expansion of A2G3 was found associated with the A3G3 representing alleles in background population. Apart from bi-allelic (AAGGG)exp, we report cases with a new pathogenic expansion of (AAAGG)exp/(AAGGG)exp in RFC1 and recessive risk haplotype. We found different repeat motifs at RFC1 TNR locus, like AAAAG, AAAGG, AAAGGG, AAAAGG, AAGAG, AACGG, AAGGC, AGAGG, and AAGGG, in Indian background population except ACAGG and (AAAGG)n/(AAGGG)n. Our findings will help in further understanding the role of long normal repeat size and different repeat motifs, specifically AAAGG, AAAGGG, and other rare repeat motifs, at the RFC1 locus.

Feasibility of a randomized, sham-controlled pilot study for accelerated rTMS-treatment of the cerebellum plus physiotherapy in CANVAS patients

BACKGROUND

Cerebellar ataxia, neuropathy and bilateral vestibular areflexia (CANVAS) is a rare neurodegenerative disease affecting the cerebellum, the peripheral nervous system and the vestibular system. Due to the lack of approved drugs, therapy comprises physiotherapy and speech therapy. Transcranial magnetic stimulation is a promising non-invasive therapeutic option to complement classical symptomatic therapies.

OBJECTIVE

To test feasibility of the combination of transcranial magnetic stimulation using an accelerated protocol and standard symptomatic therapy in patients with CANVAS.

METHODS

Eight patients with genetically confirmed CANVAS were assigned to either verum or sham cerebellar transcranial magnetic stimulation using an accelerated protocol. Treatment duration was limited to 5 days. Additionally, patients in both groups received symptomatic therapy (speech and physiotherapy) for the duration of the study.

RESULTS

All patients completed the stimulation protocol. Adverse events were rare. Ataxia severity improved in the verum group only.

CONCLUSION

The combination of transcranial magnetic stimulation and classic symptomatic therapy is feasible in a neuro-rehabilitation setting and potentially ameliorates ataxia severity.

Cranial Nerve Thinning Distinguishes RFC1-Related Disorder from Other Late-Onset Ataxias

BACKGROUND

RFC1-related disorder (RFC1/CANVAS) shares clinical features with other late-onset ataxias, such as spinocerebellar ataxias (SCA) and multiple system atrophy cerebellar type (MSA-C). Thinning of cranial nerves V (CNV) and VIII (CNVIII) has been reported in magnetic resonance imaging (MRI) scans of RFC1/CANVAS, but its specificity remains unclear.

OBJECTIVES

To assess the usefulness of CNV and CNVIII thinning to differentiate RFC1/CANVAS from SCA and MSA-C.

METHODS

Seventeen individuals with RFC1/CANVAS, 57 with SCA (types 2, 3 and 6), 11 with MSA-C and 15 healthy controls were enrolled. The Balanced Fast Field Echo sequence was used for assessment of cranial nerves. Images were reviewed by a neuroradiologist, who classified these nerves as atrophic or normal, and subsequently the CNV was segmented manually by an experienced neurologist. Both assessments were blinded to patient and clinical data. Non-parametric tests were used to assess between-group comparisons.

RESULTS

Atrophy of CNV and CNVIII, both alone and in combination, was significantly more frequent in the RFC1/CANVAS group than in healthy controls and all other ataxia groups. Atrophy of CNV had the highest sensitivity (82%) and combined CNV and CNVIII atrophy had the best specificity (92%) for diagnosing RFC1/CANVAS. In the quantitative analyses, CNV was significantly thinner in the RFC1/CANVAS group relative to all other groups. The cutoff CNV diameter that best identified RFC1/CANVAS was ≤2.2 mm (AUC = 0.91; sensitivity 88.2%, specificity 95.6%).

CONCLUSION

MRI evaluation of CNV and CNVIII using a dedicated sequence is an easy-to-use tool that helps to distinguish RFC1/CANVAS from SCA and MSA-C.

Spinocerebellar ataxia 27B: A novel, frequent and potentially treatable ataxia

Hereditary ataxias, especially when presenting sporadically in adulthood, present a particular diagnostic challenge owing to their great clinical and genetic heterogeneity. Currently, up to 75% of such patients remain without a genetic diagnosis. In an era of emerging disease-modifying gene-stratified therapies, the identification of causative alleles has become increasingly important. Over the past few years, the implementation of advanced bioinformatics tools and long-read sequencing has allowed the identification of a number of novel repeat expansion disorders, such as the recently described spinocerebellar ataxia 27B (SCA27B) caused by a (GAA)•(TTC) repeat expansion in intron 1 of the fibroblast growth factor 14 (FGF14) gene. SCA27B is rapidly gaining recognition as one of the most common forms of adult-onset hereditary ataxia, with several studies showing that it accounts for a substantial number (9-61%) of previously undiagnosed cases from different cohorts. First natural history studies and multiple reports have already outlined the progression and core phenotype of this novel disease, which consists of a late-onset slowly progressive pan-cerebellar syndrome that is frequently associated with cerebellar oculomotor signs, such as downbeat nystagmus, and episodic symptoms. Furthermore, preliminary studies in patients with SCA27B have shown promising symptomatic benefits of 4-aminopyridine, an already marketed drug. This review describes the current knowledge of the genetic and molecular basis, epidemiology, clinical features and prospective treatment strategies in SCA27B.

Pseudo-eye-of-the-tiger sign in cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS)

The well-known eye-of-the-tiger sign features bilateral and symmetrical changes in the globus pallidus, with a central area of high signal and peripheral low signal on T2-weighted MRI. Although formally considered pathognomonic of pantothenate kinase-associated neurodegeneration (PKAN), there are other neurodegenerative or genetic diseases showing similar findings. Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a late-onset ataxia, that was recently associated with biallelic AAGGG repeat expansion in the RFC1 gene. Although its predominant MRI finding is cerebellar atrophy, there may be other less common associated findings. Our aim is to present two cases of CANVAS with associated (pseudo-)eye-of-the-tiger sign, highlighting the possibility of yet another differential diagnosis for this imaging sign.

AAGGG repeat expansions trigger RFC1-independent synaptic dysregulation in human CANVAS Neurons

Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a late onset, recessively inherited neurodegenerative disorder caused by biallelic, non-reference pentameric AAGGG(CCCTT) repeat expansions within the second intron of replication factor complex subunit 1 (RFC1). To investigate how these repeats cause disease, we generated CANVAS patient induced pluripotent stem cell (iPSC) derived neurons (iNeurons) and utilized calcium imaging and transcriptomic analysis to define repeat-elicited gain-of-function and loss-of-function contributions to neuronal toxicity. AAGGG repeat expansions do not alter neuronal RFC1 splicing, expression, or DNA repair pathway functions. In reporter assays, AAGGG repeats are translated into pentapeptide repeat proteins that selectively accumulate in CANVAS patient brains. However, neither these proteins nor repeat RNA foci were detected in iNeurons, and overexpression of these repeats in isolation did not induce neuronal toxicity. CANVAS iNeurons exhibit defects in neuronal development and diminished synaptic connectivity that is rescued by CRISPR deletion of a single expanded allele. These phenotypic deficits were not replicated by knockdown of RFC1 in control neurons and were not rescued by ectopic expression of RFC1. These findings support a repeat-dependent but RFC1-independent cause of neuronal dysfunction in CANVAS, with important implications for therapeutic development in this currently untreatable condition.

The molecular mechanisms of spinocerebellar ataxias for DNA repeat expansion in disease

Spinocerebellar ataxias (SCAs) are a heterogenous group of neurodegenerative disorders which commonly inherited in an autosomal dominant manner. They cause muscle incoordination due to degeneration of the cerebellum and other parts of nervous system. Out of all the characterized (>50) SCAs, 14 SCAs are caused due to microsatellite repeat expansion mutations. Repeat expansions can result in toxic protein gain-of-function, protein loss-of-function, and/or RNA gain-of-function effects. The location and the nature of mutation modulate the underlying disease pathophysiology resulting in varying disease manifestations. Potential toxic effects of these mutations likely affect key major cellular processes such as transcriptional regulation, mitochondrial functioning, ion channel dysfunction and synaptic transmission. Involvement of several common pathways suggests interlinked function of genes implicated in the disease pathogenesis. A better understanding of the shared and distinct molecular pathogenic mechanisms in these diseases is required to develop targeted therapeutic tools and interventions for disease management. The prime focus of this review is to elaborate on how expanded 'CAG' repeats contribute to the common modes of neurotoxicity and their possible therapeutic targets in management of such devastating disorders.

Clinical and pathology characterization of small nerve fiber neuro(no)pathy in cerebellar ataxia with neuropathy and vestibular areflexia syndrome

BACKGROUND AND PURPOSE

Biallelic mutation/expansion of the gene RFC1 has been described in association with a spectrum of manifestations ranging from isolated sensory neuro(no)pathy to a complex presentation as cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS). Our aim was to define the frequency and characteristics of small fiber neuropathy (SFN) in RFC1 disease at different stages.

METHODS

RFC1 cases were screened for SFN using the Neuropathic Pain Symptom Inventory and Composite Autonomic Symptom Score 31 questionnaires. Clinical data were retrospectively collected. If available, lower limb skin biopsy samples were evaluated for somatic epidermal and autonomic subepidermal structure innervation and compared to healthy controls (HCs).

RESULTS

Forty patients, median age at onset 54 years (interquartile range [IQR] 49-61) and disease duration 10 years (IQR 6-16), were enrolled. Mild-to-moderate positive symptoms (median Neuropathic Pain Symptom Inventory score 12.1/50, IQR 5.5-22.3) and relevant autonomic disturbances (median Composite Autonomic Symptom Score 31 37.0/100, IQR 17.7-44.3) were frequently reported and showed scarce correlation with disease duration. A non-length-dependent impairment in nociception was evident in both clinical and paraclinical investigations. An extreme somatic denervation was observed in all patients at both proximal (fibers/mm, RFC1 cases 0.0 vs. HCs 20.5, p < 0.0001) and distal sites (fibers/mm, RFC1 cases 0.0 vs. HCs 13.1, p < 0.0001); instead only a slight decrease was observed in cholinergic and adrenergic innervation of autonomic structures.

CONCLUSIONS

RFC1 disease is characterized by a severe and widespread somatic SFN. Skin denervation may potentially represent the earliest feature and drive towards the suspicion of this disorder.

Chronic Cough as a Genetic Neurological Disorder? Insights from Cerebellar Ataxia with Neuropathy and Vestibular Areflexia Syndrome (CANVAS)

Chronic cough is common, and in many cases unexplained or refractory to otherwise effective treatment of associated medical conditions. Cough hypersensitivity has developed as a paradigm that helps to explain clinical and research observations that frequently point towards chronic cough as a neuropathic disorder. Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a recently described neurological condition whose clinical features include gait ataxia, unsteadiness, peripheral neuropathy, and autonomic dysfunction. Chronic cough is also a common feature of the syndrome, with features of hypersensitivity, often preceding core neurological symptoms by up to 30 years or more. The genetic basis in a majority of cases of CANVAS appears to be biallelic variable repeat intron expansion sequences within RFC1, a gene normally involved in the regulation of DNA replication and repair. The same polymorphism has now been identified at an increased frequency in patients with unexplained or refractory chronic cough in the absence of defining clinical features of CANVAS. This review expands on these points, aiming to increase the awareness of CANVAS amongst clinicians and researchers working with chronic cough. We discuss the implications of a link between RFC1 disease and cough. Improved understanding of CANVAS may lead to an enhanced grasp of the pathophysiology of chronic cough, and new approaches to antitussive treatments.

Normal and pathogenic variation of RFC1 repeat expansions: implications for clinical diagnosis

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is an autosomal recessive neurodegenerative disease, usually caused by biallelic AAGGG repeat expansions in RFC1. In this study, we leveraged whole genome sequencing data from nearly 10 000 individuals recruited within the Genomics England sequencing project to investigate the normal and pathogenic variation of the RFC1 repeat. We identified three novel repeat motifs, AGGGC (n = 6 from five families), AAGGC (n = 2 from one family) and AGAGG (n = 1), associated with CANVAS in the homozygous or compound heterozygous state with the common pathogenic AAGGG expansion. While AAAAG, AAAGGG and AAGAG expansions appear to be benign, we revealed a pathogenic role for large AAAGG repeat configuration expansions (n = 5). Long-read sequencing was used to characterize the entire repeat sequence, and six patients exhibited a pure AGGGC expansion, while the other patients presented complex motifs with AAGGG or AAAGG interruptions. All pathogenic motifs appeared to have arisen from a common haplotype and were predicted to form highly stable G quadruplexes, which have previously been demonstrated to affect gene transcription in other conditions. The assessment of these novel configurations is warranted in CANVAS patients with negative or inconclusive genetic testing. Particular attention should be paid to carriers of compound AAGGG/AAAGG expansions when the AAAGG motif is very large (>500 repeats) or the AAGGG motif is interrupted. Accurate sizing and full sequencing of the satellite repeat with long-read sequencing is recommended in clinically selected cases to enable accurate molecular diagnosis and counsel patients and their families.

Differential diagnosis of chorea (guidelines of the German Neurological Society)

INTRODUCTION

Choreiform movement disorders are characterized by involuntary, rapid, irregular, and unpredictable movements of the limbs, face, neck, and trunk. These movements often initially go unnoticed by the affected individuals and may blend together with seemingly intended, random motions. Choreiform movements can occur both at rest and during voluntary movements. They typically increase in intensity with stress and physical activity and essentially cease during deep sleep stages. In particularly in advanced stages of Huntington disease (HD), choreiform hyperkinesia occurs alongside with dystonic postures of the limbs or trunk before they typically decrease in intensity. The differential diagnosis of HD can be complex. Here, the authors aim to provide guidance for the diagnostic process. This guidance was prepared for the German Neurological Society (DGN) for German-speaking countries.

RECOMMENDATIONS

Hereditary (inherited) and non-hereditary (non-inherited) forms of chorea can be distinguished. Therefore, the family history is crucial. However, even in conditions with autosomal-dominant transmission such as HD, unremarkable family histories do not necessarily rule out a hereditary form (e.g., in cases of early deceased or unknown parents, uncertainties in familial relationships, as well as in offspring of parents with CAG repeats in the expandable range (27-35 CAG repeats) which may display expansions into the pathogenic range).

CONCLUSIONS

The differential diagnosis of chorea can be challenging. This guidance prepared for the German Neurological Society (DGN) reflects the state of the art as of 2023.

GAA-FGF14 ataxia (SCA27B): phenotypic profile, natural history progression and 4-aminopyridine treatment response

Ataxia due to an autosomal dominant intronic GAA repeat expansion in FGF14 [GAA-FGF14 ataxia, spinocerebellar ataxia 27B (SCA27B)] has recently been identified as one of the most common genetic late-onset ataxias. We here aimed to characterize its phenotypic profile, natural history progression, and 4-aminopyridine (4-AP) treatment response. We conducted a multi-modal cohort study of 50 GAA-FGF14 patients, comprising in-depth phenotyping, cross-sectional and longitudinal progression data (up to 7 years), MRI findings, serum neurofilament light (sNfL) levels, neuropathology, and 4-AP treatment response data, including a series of n-of-1 treatment studies. GAA-FGF14 ataxia consistently presented as late-onset [60.0 years (53.5-68.5), median (interquartile range)] pancerebellar syndrome, partly combined with afferent sensory deficits (55%) and dysautonomia (28%). Dysautonomia increased with duration while cognitive impairment remained infrequent, even in advanced stages. Cross-sectional and longitudinal assessments consistently indicated mild progression of ataxia [0.29 Scale for the Assessment and Rating of Ataxia (SARA) points/year], not exceeding a moderate disease severity even in advanced stages (maximum SARA score: 18 points). Functional impairment increased relatively slowly (unilateral mobility aids after 8 years in 50% of patients). Corresponding to slow progression and low extra-cerebellar involvement, sNfL was not increased relative to controls. Concurrent second diseases (including progressive supranuclear palsy neuropathology) represented major individual aggravators of disease severity, constituting important caveats for planning future GAA-FGF14 trials. A treatment response to 4-AP with relevance for everyday living was reported by 86% of treated patients. A series of three prospective n-of-1 treatment experiences with on/off design showed marked reduction in daily symptomatic time and symptom severity on 4-AP. Our study characterizes the phenotypic profile, natural history progression, and 4-AP treatment response of GAA-FGF14 ataxia. It paves the way towards large-scale natural history studies and 4-AP treatment trials in this newly discovered, possibly most frequent, and treatable late-onset ataxia.

Variability in the Results of Vestibular Assessment in Patients with Genetically Confirmed Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome

BACKGROUND

Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) presents an unpredictable and uneven clinical development of cerebellar ataxia, neuropathy, and vestibular areflexia. The aim of this study is to report the variability of vestibular test results in genetically confirmed patients with cerebellar ataxia, neuropathy, and vestibular areflexia syndrome.

METHODS

Caloric testing, video head impulse test (vHIT), and rotatory chair testing were performed in 7 patients who presented pathogenic repeat expansions in the replication factor complex unit 1 gene related to cerebellar ataxia, neuropathy, and vestibular areflexia syndrome.

RESULTS

Reduced vestibulo-ocular reflex (VOR) gain was observed in 100% of the patients in rotatory chair testing. Three of them had bilateral areflexia in caloric testing while 2 showed unilateral hypofunction and 2 had no alterations in the test. Only 1 patient had bilateral abnormal vHIT with gains under 0.6 in both ears.

CONCLUSION

Genetic testing allows an early diagnosis of cerebellar ataxia, neuropathy, and vestibular areflexia syndrome, whereby the vestibular system may be affected to different degrees. Rotatory chair testing has a higher sensitivity for the detection of vestibular hypofunction in these patients. Caloric testing can provide additional information. vHIT might underdiagnose patients with mild-to-moderate vestibulopathy.

Nigrostriatal dysfunction in RFC1-related disorder/CANVAS

INTRODUCTION

Parkinsonism is now recognized as an additional feature in RFC1/CANVAS syndrome; however, no systematic evaluation of nigrostriatal dopaminergic function has been published so far.

METHODS

This is an observational, single-center study, which analyzed 13 patients with molecular confirmation of RFC1/CANVAS. Disease severity was assessed with the SARA scale. Each subject was carefully evaluated for the presence of parkinsonian features. Dopamine transporter (DAT) imaging was acquired and reconstructed in the transverse, coronal and sagittal planes 4 h after venous injection of 99mTc-TRODAT-1. An experienced nuclear physician performed the visual analysis of all images.

RESULTS

Patients had a mean age of 62.3 ± 8.8 years, and there were 9 women. The mean SARA score was 15.5 ± 5.8. Nine patients had abnormal DAT imaging results. The putamen was more frequently affected than the caudate nucleus on both sides. Considering all regions, uptake of 99mTc-TRODAT-1 did not correlate with disease duration or SARA scores. Parkinsonism was noticed in 3/13 patients, all of which had abnormal DAT scans. Interestingly, six subjects had reduced DAT imaging uptake, but no clinical signs of parkinsonism.

CONCLUSION

Nigrostriatal dysfunction is frequent in RFC1/CANVAS even in the absence of clinical parkinsonism and may occur early in the disease course.

A patient with neuropathy and ataxia: what do I have to consider?

PURPOSE OF REVIEW

An increasing number of peripheral neuro(no)pathies are identified as involving other components of the neurological system, particularly those that further impair balance. Here we aim to outline an evidence-based approach to the diagnosis of patients who present with a somatosensory disorder which also involves at least one other area of neurological impairment such as the vestibular, auditory, or cerebellar systems.

RECENT FINDINGS

Detailed objective investigation of patients who present with sensory impairment, particularly where the degree of imbalance is greater than would be expected, aids the accurate diagnosis of genetic, autoimmune, metabolic, and toxic neurological disease.

SUMMARY

Diagnosis and management of complex somatosensory disorders benefit from investigation which extends beyond the presenting sensory impairment.

Multisystemic RFC1-Related Disorder: Expanding the Phenotype Beyond Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome

Background and Objectives

The RFC1 spectrum has become considerably expanded as multisystemic features beyond the triad of cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) have started to be unveiled, although many still require clinical replication. Here, we aimed to clinically characterize a cohort of RFC1-positive patients by addressing both classic and multisystemic features. In a second part of this study, we prospectively assessed small nerve fibers (SNF) and autonomic function in a subset of these RFC1-related patients.

Methods

We retrospectively enrolled 67 RFC1-positive patients from multiple neurologic centers in Portugal. All patients underwent full neurologic and vestibular evaluation, as well as neuroimaging and neurophysiologic studies. For SNF and autonomic testing (n = 15), we performed skin biopsies, quantitative sensory testing, sudoscan, sympathetic skin response, heart rate deep breathing, and tilt test.

Results

Multisystemic features beyond CANVAS were present in 82% of the patients, mainly chronic cough (66%) and dysautonomia (43%). Other features included motor neuron (MN) affection and motor neuropathy (18%), hyperkinetic movement disorders (16%), sleep apnea (6%), REM and non-REM sleep disorders (5%), and cranial neuropathy (5%). Ten patients reported an inverse association between cough and ataxia severity. A very severe epidermal denervation was found in skin biopsies of all patients. Autonomic dysfunction comprised cardiovascular (67%), cardiovagal (54%), and/or sudomotor (50%) systems.

Discussion

The presence of MN involvement, motor neuropathy, small fiber neuropathy, or extrapyramidal signs should not preclude RFC1 testing in cases of sensory neuronopathy. Indeed, the RFC1 spectrum can overlap not only with multiple system atrophy but also with hereditary motor and sensory neuropathy, hereditary sensory and autonomic neuropathy, and feeding dystonia phenotypes. Some clinical-paraclinical dissociations can pose diagnostic challenges, namely large and small fiber neuropathy and sudomotor dysfunction which are usually subclinical.

Natural History and Phenotypic Spectrum of GAA-FGF14 Sporadic Late-Onset Cerebellar Ataxia (SCA27B)

BACKGROUND

Heterozygous GAA expansions in the FGF14 gene have been related to autosomal dominant cerebellar ataxia (SCA27B-MIM:620174). Whether they represent a common cause of sporadic late-onset cerebellar ataxia (SLOCA) remains to be established.

OBJECTIVES

To estimate the prevalence, characterize the phenotypic spectrum, identify discriminative features, and model longitudinal progression of SCA27B in a prospective cohort of SLOCA patients.

METHODS

FGF14 expansions screening combined with longitudinal deep-phenotyping in a prospective cohort of 118 SLOCA patients (onset >40 years of age, no family history of cerebellar ataxia) without a definite diagnosis.

RESULTS

Prevalence of SCA27B was 12.7% (15/118). Higher age of onset, higher Spinocerebellar Degeneration Functional Score, presence of vertigo, diplopia, nystagmus, orthostatic hypotension absence, and sensorimotor neuropathy were significantly associated with SCA27B. Ataxia progression was ≈0.4 points per year on the Scale for Assessment and Rating of Ataxia.

CONCLUSIONS

FGF14 expansion is a major cause of SLOCA. Our natural history data will inform future FGF14 clinical trials. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Investigation of hearing loss in elderly vertigo and dizziness patients in the past 10 years

Background

Vertigo and hearing loss are both prevalent in the elderly. This study retrospectively analyzed hearing test results from elderly patients experiencing vertigo and dizziness at ENT outpatient over a 10-year period, in order to study the patterns of hearing loss in this patient population.

Methods

Nine thousand three hundred eighty four patients over 50 years old underwent retrospective collection and screening of outpatient diagnosis, pure tone audiometry, acoustic immittance measurement (tympanogram) and auditory brainstem response (ABR) test. The patient's audiograms are divided into 7 subtypes according to a set of fixed criteria. Meanwhile, K-Means clustering analysis method was used to classify the audiogram.

Results

The Jerger classification of tympanogram in elderly patients with vertigo and dizziness showed the majority falling under type A. The leading audiogram shapes were flat (27.81% in right ear and 26.89% in left ear), high-frequency gently sloping (25.97% in right ear and 27.34% in left ear), and high-frequency steeply sloping (21.60% in right ear and 22.53% in left ear). Meniere's disease (MD; 30.87%), benign recurrent vertigo (BRV; 19.07%), and benign paroxysmal positional vertigo (BPPV; 15.66%) were the most common etiologies in elderly vestibular diseases. We observed statistically significant differences in hearing thresholds among these vestibular diseases (P < 0.001). K-Means clustering analysis suggested that the optimal number of clusters was three, with sample sizes for the three clusters being 2,747, 2,413, and 4,139, respectively. The ANOVA statistical results of each characteristic value showed P < 0.001.

Conclusion

The elderly patients often have mild to moderate hearing loss as a concomitant symptom with vertigo. Female patients have better hearing thresholds than males. The dominant audiometric shapes in this patient population were flat, high-frequency gently sloping, and high-frequency steeply sloping according to a set of fixed criteria. This study highlights the need for tailored strategies in managing hearing loss in elderly patients with vertigo and dizziness.

ClinPrior: an algorithm for diagnosis and novel gene discovery by network-based prioritization

BACKGROUND

Whole-exome sequencing (WES) and whole-genome sequencing (WGS) have become indispensable tools to solve rare Mendelian genetic conditions. Nevertheless, there is still an urgent need for sensitive, fast algorithms to maximise WES/WGS diagnostic yield in rare disease patients. Most tools devoted to this aim take advantage of patient phenotype information for prioritization of genomic data, although are often limited by incomplete gene-phenotype knowledge stored in biomedical databases and a lack of proper benchmarking on real-world patient cohorts.

METHODS

We developed ClinPrior, a novel method for the analysis of WES/WGS data that ranks candidate causal variants based on the patient's standardized phenotypic features (in Human Phenotype Ontology (HPO) terms). The algorithm propagates the data through an interactome network-based prioritization approach. This algorithm was thoroughly benchmarked using a synthetic patient cohort and was subsequently tested on a heterogeneous prospective, real-world series of 135 families affected by hereditary spastic paraplegia (HSP) and/or cerebellar ataxia (CA).

RESULTS

ClinPrior successfully identified causative variants achieving a final positive diagnostic yield of 70% in our real-world cohort. This includes 10 novel candidate genes not previously associated with disease, 7 of which were functionally validated within this project. We used the knowledge generated by ClinPrior to create a specific interactome for HSP/CA disorders thus enabling future diagnoses as well as the discovery of novel disease genes.

CONCLUSIONS

ClinPrior is an algorithm that uses standardized phenotype information and interactome data to improve clinical genomic diagnosis. It helps in identifying atypical cases and efficiently predicts novel disease-causing genes. This leads to increasing diagnostic yield, shortening of the diagnostic Odysseys and advancing our understanding of human illnesses.

Frequency and Phenotype of RFC1 Repeat Expansions in Bilateral Vestibulopathy

BACKGROUND AND OBJECTIVES

Bilateral vestibulopathy (BVP) is a chronic debilitating neurologic disorder with no monogenic cause established so far despite familiar presentations. We hypothesized that replication factor complex subunit 1 (RFC1) repeat expansions might present a recurrent monogenic cause of BVP.

METHODS

The study involved RFC1 screening and in-depth neurologic, vestibulo-oculomotor, and disease evolution phenotyping of 168 consecutive patients with idiopathic at least "probable BVP" from a tertiary referral center for balance disorders, with127 of them meeting current diagnostic criteria of BVP (Bárány Society Classification).

RESULTS

Biallelic AAGGG repeat expansions in RFC1 were identified in 10/127 patients (8%) with BVP and 1/41 with probable BVP. Heterozygous expansions in 10/127 patients were enriched compared with those in reference populations. RFC1-related BVP manifested at a median age of 60 years (range 34-72 years) and co-occurred predominantly with mild polyneuropathy (10/11). Additional cerebellar involvement (7/11) was subtle and limited to oculomotor signs in early stages, below recognition of classic cerebellar ataxia, neuropathy, and vestibular areflexia syndrome. Clear dysarthria, appendicular ataxia, or cerebellar atrophy developed 6-8 years after onset. Dysarthria, absent patellar reflexes, and downbeat nystagmus best discriminated RFC1-positive BVP from RFC1-negative BVP, but neither sensory symptoms nor fine motor problems. Video head impulse gains of patients with RFC1-positive BVP were lower relative to those of patients with RFC1-negative BVP and decreased until 10 years disease duration, indicating a potential progression and outcome marker for RFC1-disease.

DISCUSSION

This study identifies RFC1 as the first-and frequent-monogenic cause of BVP. It characterizes RFC1-related BVP as part of the multisystemic evolution of RFC1 spectrum disease, with implications for designing natural history studies and future treatment trials.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that RFC1 repeat expansions cause BVP.

RFC1 in an Australasian neurological disease cohort: extending the genetic heterogeneity and implications for diagnostics

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome is a progressive, generally late-onset, neurological disorder associated with biallelic pentanucleotide expansions in Intron 2 of the RFC1 gene. The locus exhibits substantial genetic variability, with multiple pathogenic and benign pentanucleotide repeat alleles previously identified. To determine the contribution of pathogenic RFC1 expansions to neurological disease within an Australasian cohort and further investigate the heterogeneity exhibited at the locus, a combination of flanking and repeat-primed PCR was used to screen a cohort of 242 Australasian patients with neurological disease. Patients whose data indicated large gaps within expanded alleles following repeat-primed PCR, underwent targeted long-read sequencing to identify novel repeat motifs at the locus. To increase diagnostic yield, additional probes at the RFC1 repeat region were incorporated into the PathWest diagnostic laboratory targeted neurological disease gene panel to enable first-pass screening of the locus for all samples tested on the panel. Within the Australasian cohort, we detected known pathogenic biallelic expansions in 15.3% (n = 37) of patients. Thirty indicated biallelic AAGGG expansions, two had biallelic 'Māori alleles' [(AAAGG)exp(AAGGG)exp], two samples were compound heterozygous for the Māori allele and an AAGGG expansion, two samples had biallelic ACAGG expansions and one sample was compound heterozygous for the ACAGG and AAGGG expansions. Forty-five samples tested indicated the presence of biallelic expansions not known to be pathogenic. A large proportion (84%) showed complex interrupted patterns following repeat-primed PCR, suggesting that these expansions are likely to be comprised of more than one repeat motif, including previously unknown repeats. Using targeted long-read sequencing, we identified three novel repeat motifs in expanded alleles. Here, we also show that short-read sequencing can be used to reliably screen for the presence or absence of biallelic RFC1 expansions in all samples tested using the PathWest targeted neurological disease gene panel. Our results show that RFC1 pathogenic expansions make a substantial contribution to neurological disease in the Australasian population and further extend the heterogeneity of the locus. To accommodate the increased complexity, we outline a multi-step workflow utilizing both targeted short- and long-read sequencing to achieve a definitive genotype and provide accurate diagnoses for patients.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): diagnostic contribution of vestibular function tests

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is a recently recognised but underdiagnosed cause of late-onset hereditary ataxia. Symptoms may vary, and differential diagnoses can span several specialties. We report the case of a man in his 60 s who presented with a 10 year history of imbalance and progressive gait disturbance associated with a chronic spasmodic cough that preceded these symptoms by almost 30 years. He had previously undergone extensive testing for acquired and genetic causes of ataxia without a conclusive diagnosis. Brain MRI revealed cerebellar atrophy, and nerve conduction tests suggested a sensory ganglionopathy. Vestibular function testing was crucial for diagnosis, identifying a severe bilateral vestibulopathy. This led to the consideration of CANVAS, which was finally confirmed by genetic testing. This case raises awareness of this novel genetic disease, highlighting the importance of objective vestibular function tests in establishing an early diagnosis.

RFC1 repeat expansions and cerebellar ataxia, neuropathy and vestibular areflexia syndrome: Experience and perspectives from a neuromuscular disorders unit

INTRODUCTION

Pathogenic expansions in RFC1 have been described as a cause of a spectrum of disorders including late-onset ataxia, chronic cough, and cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS). Sensory neuronopathy/neuropathy appears to be a major symptom of RFC1-disorder, and RFC1 expansions are common in patients with sensory chronic idiopathic axonal neuropathy or sensory ganglionopathy. We aimed to investigate RFC1 expansions in patients with suspected RFC1-related disease followed-up in a Neuromuscular Diseases Unit, with a particular interest in the involvement of the peripheral nervous system.

METHODS

We recruited twenty consecutive patients based on the presence of at least two of the following features: progressive ataxia, sensory neuropathy/neuronopathy, vestibulopathy and chronic cough. Medical records were retrospectively reviewed for a detailed clinical description. More extensive phenotyping of the RFC1-positive patients and clinical comparison between RFC1 positive and negative patients were performed.

RESULTS

Biallelic AAGGG repeat expansions were identified in 13 patients (65%). The most frequent symptoms were chronic cough and sensory disturbances in the lower extremities (12/13). Only 4 patients (31%) had complete CANVAS. The phenotypes were sensory ataxia and sensory symptoms in extremities in 4/13; sensory ataxia, sensory symptoms, and vestibulopathy in 3/13; sensory symptoms plus chronic cough in 2/13. Chronic cough and isolated sensory neuronopathy were significantly more prevalent in RFC1-positive patients.

CONCLUSION

Pathogenic RFC1 expansions are a common cause of sensory neuropathy/neuronopathy and should be considered in the approach to these patients. Identification of key symptoms or detailed interpretation of nerve conduction studies may improve patient selection for genetic testing.

Ataxias: Hereditary, Acquired, and Reversible Etiologies

A variety of etiologies can cause cerebellar dysfunction, leading to ataxia symptoms. Therefore, the accurate diagnosis of the cause for cerebellar ataxia can be challenging. A step-wise investigation will reveal underlying causes, including nutritional, toxin, immune-mediated, genetic, and degenerative disorders. Recent advances in genetics have identified new genes for both autosomal dominant and autosomal recessive ataxias, and new therapies are on the horizon for targeting specific biological pathways. New diagnostic criteria for degenerative ataxias have been proposed, specifically for multiple system atrophy, which will have a broad impact on the future clinical research in ataxia. In this article, we aim to provide a review focus on symptoms, laboratory testing, neuroimaging, and genetic testing for the diagnosis of cerebellar ataxia causes, with a special emphasis on recent advances. Strategies for the management of cerebellar ataxia is also discussed.

Updates in neuro-otology

PURPOSE OF REVIEW

Recent updates with clinical implications in the field of neuro-otology are reviewed.

RECENT FINDINGS

Important updates relating to several neuro-otologic disorders have been reported in recent years. For benign positional paroxysmal vertigo (BPPV), we provide updates on the characteristics and features of the short arm variant of posterior canal BPPV. For the acute vestibular syndrome, we report important updates on the use of video-oculography in clinical diagnosis. For autoimmune causes of neuro-otologic symptoms, we describe the clinical and paraclinical features of kelch-like protein 11 encephalitis, a newly-identified antibody associated disorder. For cerebellar ataxia, neuropathy, vestibular areflexia syndrome, we report recent genetic insights into this condition.

SUMMARY

This review summarizes important recent updates relating to four hot topics in neuro-otology.

Truncating Variants in RFC1 in Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome

BACKGROUND AND OBJECTIVE

Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) is an autosomal recessive neurodegenerative disease characterized by adult-onset and slowly progressive sensory neuropathy, cerebellar dysfunction, and vestibular impairment. In most cases, the disease is caused by biallelic (AAGGG)n repeat expansions in the second intron of the replication factor complex subunit 1 (RFC1). However, a small number of cases with typical CANVAS do not carry the common biallelic repeat expansion. The objective of this study was to expand the genotypic spectrum of CANVAS by identifying sequence variants in RFC1-coding region associated with this condition.

METHODS

Fifteen individuals diagnosed with CANVAS and carrying only 1 heterozygous (AAGGG)n expansion in RFC1 underwent whole-genome sequencing or whole-exome sequencing to test for the presence of a second variant in RFC1 or other unrelated gene. To assess the effect of truncating variants on RFC1 expression, we tested the level of RFC1 transcript and protein on patients' derived cell lines.

RESULTS

We identified 7 patients from 5 unrelated families with clinically defined CANVAS carrying a heterozygous (AAGGG)n expansion together with a second truncating variant in trans in RFC1, which included the following: c.1267C>T (p.Arg423Ter), c.1739_1740del (p.Lys580SerfsTer9), c.2191del (p.Gly731GlufsTer6), and c.2876del (p.Pro959GlnfsTer24). Patient fibroblasts containing the c.1267C>T (p.Arg423Ter) or c.2876del (p.Pro959GlnfsTer24) variants demonstrated nonsense-mediated mRNA decay and reduced RFC1 transcript and protein.

DISCUSSION

Our report expands the genotype spectrum of RFC1 disease. Full RFC1 sequencing is recommended in cases affected by typical CANVAS and carrying monoallelic (AAGGG)n expansions. In addition, it sheds further light on the pathogenesis of RFC1 CANVAS because it supports the existence of a loss-of-function mechanism underlying this complex neurodegenerative condition.

Association of biallelic RFC1 expansion with early-onset Parkinson's disease

BACKGROUND AND PURPOSE

The biallelic repeat expansion (AAGGG)_exp in the replication factor C subunit 1 gene (RFC1) is a frequent cause of cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) as well as late-onset ataxia. The clinical spectrum of RFC1 disease has expanded since the first identification of biallelic (AAGGG)_exp and includes now various nonclassical phenotypes. Biallelic (AAGGG)_exp in RFC1 in patients with clinically confirmed Parkinson's disease (PD) has recently been found.

METHODS

A nationwide cohort of 273 Finnish patients with early-onset PD was examined for the biallelic intronic expansion in RFC1. The expansion (AAGGG)_exp was first screened using extra long polymerase chain reactions (Extra Large-PCRs) and flanking multiplex PCR. The presence of biallelic (AAGGG)_exp was then confirmed by repeat-primed PCR and, finally, the repeat length was determined by long-read sequencing.

RESULTS

Three patients were found with the biallelic (AAGGG)_exp in RFC1 giving a frequency of 1.10% (0.23%-3.18%; 95% confidence interval). The three patients fulfilled the diagnostic criteria of PD, none of them had ataxia or neuropathy, and only one patient had a mild vestibular dysfunction. The age at onset of PD symptoms was 40-48 years and their disease course had been unremarkable apart from the early onset.

CONCLUSIONS

Our results suggest that (AAGGG)_exp in RFC1 is a rare cause of early-onset PD. Other populations should be examined in order to determine whether our findings are specific to the Finnish population.

Evolution of Clinical Outcome Measures and Biomarkers in Sporadic Adult-Onset Degenerative Ataxia

BACKGROUND

Sporadic adult-onset ataxias without known genetic or acquired cause are subdivided into multiple system atrophy of cerebellar type (MSA-C) and sporadic adult-onset ataxia of unknown etiology (SAOA).

OBJECTIVES

To study the differential evolution of both conditions including plasma neurofilament light chain (NfL) levels and magnetic resonance imaging (MRI) markers.

METHODS

SPORTAX is a prospective registry of sporadic ataxia patients with an onset >40 years. Scale for the Assessment and Rating of Ataxia was the primary outcome measure. In subgroups, blood samples were taken and MRIs performed. Plasma NfL was measured via a single molecule assay. Regional brain volumes were automatically measured. To assess signal changes, we defined the pons and middle cerebellar peduncle abnormality score (PMAS). Using mixed-effects models, we analyzed changes on a time scale starting with ataxia onset.

RESULTS

Of 404 patients without genetic diagnosis, 130 met criteria of probable MSA-C at baseline and 26 during follow-up suggesting clinical conversion to MSA-C. The remaining 248 were classified as SAOA. At baseline, NfL, cerebellar white matter (CWM) and pons volume, and PMAS separated MSA-C from SAOA. NfL decreased in MSA-C and did not change in SAOA. CWM and pons volume decreased faster, whereas PMAS increased faster in MSA-C. In MSA-C, pons volume had highest sensitivity to change, and PMAS was a predictor of faster progression. Fulfillment of possible MSA criteria, NfL and PMAS were risk factors, CWM and pons volume protective factors for conversion to MSA-C.

CONCLUSIONS

This study provides detailed information on differential evolution and prognostic relevance of biomarkers in MSA-C and SAOA. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.