RFC1 expansions are a common cause of idiopathic sensory neuropathy

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.

Genetics of inherited peripheral neuropathies and the next frontier: looking backwards to progress forwards

Inherited peripheral neuropathies (IPNs) encompass a clinically and genetically heterogeneous group of disorders causing length-dependent degeneration of peripheral autonomic, motor and/or sensory nerves. Despite gold-standard diagnostic testing for pathogenic variants in over 100 known associated genes, many patients with IPN remain genetically unsolved. Providing patients with a diagnosis is critical for reducing their 'diagnostic odyssey', improving clinical care, and for informed genetic counselling. The last decade of massively parallel sequencing technologies has seen a rapid increase in the number of newly described IPN-associated gene variants contributing to IPN pathogenesis. However, the scarcity of additional families and functional data supporting variants in potential novel genes is prolonging patient diagnostic uncertainty and contributing to the missing heritability of IPNs. We review the last decade of IPN disease gene discovery to highlight novel genes, structural variation and short tandem repeat expansions contributing to IPN pathogenesis. From the lessons learnt, we provide our vision for IPN research as we anticipate the future, providing examples of emerging technologies, resources and tools that we propose that will expedite the genetic diagnosis of unsolved IPN families.

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.

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.

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.

Structural investigation of pathogenic RFC1 AAGGG pentanucleotide repeats reveals a role of G-quadruplex in dysregulated gene expression in CANVAS

An expansion of AAGGG pentanucleotide repeats in the replication factor C subunit 1 (RFC1) gene is the genetic cause of cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS), and it also links to several other neurodegenerative diseases including the Parkinson's disease. However, the pathogenic mechanism of RFC1 AAGGG repeat expansion remains enigmatic. Here, we report that the pathogenic RFC1 AAGGG repeats form DNA and RNA parallel G-quadruplex (G4) structures that play a role in impairing biological processes. We determine the first high-resolution nuclear magnetic resonance (NMR) structure of a bimolecular parallel G4 formed by d(AAGGG)2AA and reveal how AAGGG repeats fold into a higher-order structure composed of three G-tetrad layers, and further demonstrate the formation of intramolecular G4s in longer DNA and RNA repeats. The pathogenic AAGGG repeats, but not the nonpathogenic AAAAG repeats, form G4 structures to stall DNA replication and reduce gene expression via impairing the translation process in a repeat-length-dependent manner. Our results provide an unprecedented structural basis for understanding the pathogenic mechanism of AAGGG repeat expansion associated with CANVAS. In addition, the high-resolution structures resolved in this study will facilitate rational design of small-molecule ligands and helicases targeting G4s formed by AAGGG repeats for therapeutic interventions.

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.

Clinical and functional characteristics, possible causes, and impact of chronic cough in patients with cerebellar ataxia, neuropathy, and bilateral vestibular areflexia syndrome (CANVAS)

Cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS) is an hereditary autosomal recessive disease. Recent studies propose including chronic cough (CC) as a symptom of CANVAS. For 10 patients with CANVAS as genetically confirmed by biallelic expansion of the AAGG repeat motif (AAGGGexp) in intron 2 of replication factor C subunit 1 (RFC1), our aim was, as a multidisciplinary team, to describe clinical and functional characteristics and possible causes of CC following European Respiratory Society (ERS) recommendations, and to evaluate CC impact on quality of life (QoL) using self-administered questionnaires (Cough Severity Diary, Leicester Cough Questionnaire, Discrete Emotions Questionnaire, and EQ-5D-5L). In all 10 patients, the CC was a dry cough that developed several years prior to the neurological symptoms (mean 14.2 years); 7 patients had symptoms compatible with gastroesophageal reflux (GER), 5 with pathological GER diagnosed by 24-h esophageal pH testing, and 6 patients had impaired esophageal motility diagnosed by high-resolution esophageal manometry, most frequently ineffective peristalsis. Although further studies are required for confirmation, we conclude that CC may be a characteristic prodrome of CANVAS and may be related to GER and esophageal disorders. Furthermore, CC affects patients' QoL, especially in the psychosocial sphere.

Hereditary transthyretin amyloidosis in middle-aged and elderly patients with idiopathic polyneuropathy: a nationwide prospective study

BACKGROUND

Hereditary transthyretin amyloidosis (ATTRv) is an adult-onset autosomal dominant disease resulting from TTR gene pathogenic variants. ATTRv often presents as a progressive polyneuropathy, and effective ATTRv treatments are available.

METHODS

In this 5 year-long (2017-2021) nationwide prospective study, we systematically analysed the TTR gene in French patients with age >50 years with a progressive idiopathic polyneuropathy.

RESULTS

553 patients (70% males) with a mean age of 70 years were included. A TTR gene pathogenic variant was found in 15 patients (2.7%), including the Val30Met TTR variation in 10 cases. In comparison with patients with no TTR gene pathogenic variants (n = 538), patients with TTR pathogenic variants more often presented with orthostatic hypotension (53 vs. 21%, p = .007), significant weight loss (33 vs 11%, p = .024) and rapidly deteriorating nerve conduction studies (26 vs. 8%, p = .03). ATTRv diagnosis led to amyloid cardiomyopathy diagnosis in 11 cases, ATTRv specific treatment in all cases and identification of 15 additional ATTRv cases among relatives.

CONCLUSION

In this nationwide prospective study, we found ATTRv in 2.7% of patients with age >50 years with a progressive polyneuropathy. These results are highly important for the early identification of patients in need of disease-modifying treatments.

LUSTR: a new customizable tool for calling genome-wide germline and somatic short tandem repeat variants

BACKGROUND

Short tandem repeats (STRs) are widely distributed across the human genome and are associated with numerous neurological disorders. However, the extent that STRs contribute to disease is likely under-estimated because of the challenges calling these variants in short read next generation sequencing data. Several computational tools have been developed for STR variant calling, but none fully address all of the complexities associated with this variant class.

RESULTS

Here we introduce LUSTR which is designed to address some of the challenges associated with STR variant calling by enabling more flexibility in defining STR loci, allowing for customizable modules to tailor analyses, and expanding the capability to call somatic and multiallelic STR variants. LUSTR is a user-friendly and easily customizable tool for targeted or unbiased genome-wide STR variant screening that can use either predefined or novel genome builds. Using both simulated and real data sets, we demonstrated that LUSTR accurately infers germline and somatic STR expansions in individuals with and without diseases.

CONCLUSIONS

LUSTR offers a powerful and user-friendly approach that allows for the identification of STR variants and can facilitate more comprehensive studies evaluating the role of pathogenic STR variants across human diseases.

Can CANVAS due to RFC1 biallelic expansions present with pure ataxia?

BACKGROUND

Biallelic expansion of AAGGG in the replication factor complex subunit 1 (RFC1) was identified as a major cause of cerebellar ataxia, neuropathy (sensory ganglionopathy, or SG) and vestibular areflexia syndrome (CANVAS). We wanted to clarify if RFC1 expansions can present with pure ataxia and if such expansions could be responsible for some cases where an alternative diagnosis had been made.

METHODS

We identified patients with a combination of ataxia and SG and no other cause found, patients where an alternative diagnosis had been made, and patients with pure ataxia. Testing for RFC1 expansions was done using established methodology.

RESULTS

Among 54 patients with otherwise idiopathic sporadic ataxia without SG, none was found to have RFC1 expansions. Among 38 patients with cerebellar ataxia and SG in which all other causes were excluded, 71% had RFC1 expansions. Among 27 patients with cerebellar ataxia and SG diagnosed with coeliac disease or gluten sensitivity, 15% had RFC1 expansions.

CONCLUSIONS

Isolated cerebellar ataxia without SG makes the diagnosis of CANVAS due to RFC1 expansions highly improbable, but CANVAS is frequently the cause of the combination of idiopathic cerebellar ataxia with SG. It is important to screen patients diagnosed with other causes of acquired ataxia and SG as a small percentage were found to have RFC1 expansions.

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.

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.

Paraneoplastic neuropathies and peripheral nerve hyperexcitability disorders

Peripheral neuropathy is a common referral for patients to the neurologic clinics. Paraneoplastic neuropathies account for a small but high morbidity and mortality subgroup. Symptoms include weakness, sensory loss, sweating irregularity, blood pressure instability, severe constipation, and neuropathic pain. Neuropathy is the first presenting symptom of malignancy among many patients. The molecular and cellular oncogenic immune targets reside within cell bodies, axons, cytoplasms, or surface membranes of neural tissues. A more favorable immune treatment outcome occurs in those where the targets reside on the cell surface. Patients with antibodies binding cell surface antigens commonly have neural hyperexcitability with pain, cramps, fasciculations, and hyperhidrotic attacks (CASPR2, LGI1, and others). The antigenic targets are also commonly expressed in the central nervous system, with presenting symptoms being myelopathy, encephalopathy, and seizures with neuropathy, often masked. Pain and autonomic components typically relate to small nerve fiber involvement (nociceptive, adrenergic, enteric, and sudomotor), sometimes without nerve fiber loss but rather hyperexcitability. The specific antibodies discovered help direct cancer investigations. Among the primary axonal paraneoplastic neuropathies, pathognomonic clinical features do not exist, and testing for multiple antibodies simultaneously provides the best sensitivity in testing (AGNA1-SOX1; amphiphysin; ANNA-1-HU; ANNA-3-DACH1; CASPR2; CRMP5; LGI1; PCA2-MAP1B, and others). Performing confirmatory antibody testing using adjunct methods improves specificity. Antibody-mediated demyelinating paraneoplastic neuropathies are limited to MAG-IgM (IgM-MGUS, Waldenström's, and myeloma), with the others associated with cytokine elevations (VEGF, IL6) caused by osteosclerotic myeloma, plasmacytoma (POEMS), and rarely angiofollicular lymphoma (Castleman's). Paraneoplastic disorders have clinical overlap with other idiopathic antibody disorders, including IgG4 demyelinating nodopathies (NF155 and Contactin-1). This review summarizes the paraneoplastic neuropathies, including those with peripheral nerve hyperexcitability.

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.

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.

Detection and discovery of repeat expansions in ataxia enabled by next-generation sequencing: present and future

Hereditary cerebellar ataxias are a heterogenous group of progressive neurological disorders that are disproportionately caused by repeat expansions (REs) of short tandem repeats (STRs). Genetic diagnosis for RE disorders such as ataxias are difficult as the current gold standard for diagnosis is repeat-primed PCR assays or Southern blots, neither of which are scalable nor readily available for all STR loci. In the last five years, significant advances have been made in our ability to detect STRs and REs in short-read sequencing data, especially whole-genome sequencing. Given the increasing reliance of genomics in diagnosis of rare diseases, the use of established RE detection pipelines for RE disorders is now a highly feasible and practical first-step alternative to molecular testing methods. In addition, many new pathogenic REs have been discovered in recent years by utilising WGS data. Collectively, genomes are an important resource/platform for further advancements in both the discovery and diagnosis of REs that cause ataxia and will lead to much needed improvement in diagnostic rates for patients with hereditary ataxia.

Advances in diagnosis and management of distal sensory polyneuropathies

Distal sensory polyneuropathy (DSP) is characterised by length-dependent, sensory-predominant symptoms and signs, including potentially disabling symmetric chronic pain, tingling and poor balance. Some patients also have or develop dysautonomia or motor involvement depending on whether large myelinated or small fibres are predominantly affected. Although highly prevalent, diagnosis and management can be challenging. While classic diabetes and toxic causes are well-recognised, there are increasingly diverse associations, including with dysimmune, rheumatological and neurodegenerative conditions. Approximately half of cases are initially considered idiopathic despite thorough evaluation, but often, the causes emerge later as new symptoms develop or testing advances, for instance with genetic approaches. Improving and standardising DSP metrics, as already accomplished for motor neuropathies, would permit in-clinic longitudinal tracking of natural history and treatment responses. Standardising phenotyping could advance research and facilitate trials of potential therapies, which lag so far. This review updates on recent advances and summarises current evidence for specific treatments.

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.

The Diagnostic Landscape of Adult Neurogenetic Disorders

Neurogenetic diseases affect individuals across the lifespan, but accurate diagnosis remains elusive for many patients. Adults with neurogenetic disorders often undergo a long diagnostic odyssey, with multiple specialist evaluations and countless investigations without a satisfactory diagnostic outcome. Reasons for these diagnostic challenges include: (1) clinical features of neurogenetic syndromes are diverse and under-recognized, particularly those of adult-onset, (2) neurogenetic syndromes may manifest with symptoms that span multiple neurological and medical subspecialties, and (3) a positive family history may not be present or readily apparent. Furthermore, there is a large gap in the understanding of how to apply genetic diagnostic tools in adult patients, as most of the published literature focuses on the pediatric population. Despite these challenges, accurate genetic diagnosis is imperative to provide affected individuals and their families guidance on prognosis, recurrence risk, and, for an increasing number of disorders, offer targeted treatment. Here, we provide a framework for recognizing adult neurogenetic syndromes, describe the current diagnostic approach, and highlight studies using next-generation sequencing in different neurological disease cohorts. We also discuss diagnostic pitfalls, barriers to achieving a definitive diagnosis, and emerging technology that may increase the diagnostic yield of testing.

Neuromuscular ultrasound as a marker for inherited sensory neuronopathy

A review and detailed analysis of the literature over the past two decades has revealed a unique ultrasound feature of pathologically "small" nerves in inherited sensory neuronopathies. Although sample sizes were limited, due to the rarity of these diseases, this characteristic ultrasound finding has been consistently reported across a variety of inherited diseases that affect the dorsal root ganglia. Direct comparisons with both acquired and inherited diseases that primarily affect the axons in the peripheral nerves showed that the ultrasound finding of abnormally "small" cross-sectional areas (CSAs) in mixed nerves of the upper limbs has a high diagnostic accuracy for inherited sensory neuronopathy. Based on this review, ultrasound CSA of the mixed upper limb nerves can be proposed as a marker for inherited sensory neuronopathy.

CANVAS-related RFC1 mutations in patients with immune-mediated neuropathy

Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) has recently been attributed to biallelic repeat expansions in RFC1. More recently, the disease entity has expanded to atypical phenotypes, including chronic neuropathy without cerebellar ataxia or vestibular areflexia. Very recently, RFC1 expansions were found in patients with Sjögren syndrome who had neuropathy that did not respond to immunotherapy. In this study RFC1 was examined in 240 patients with acute or chronic neuropathies, including 105 with Guillain-Barré syndrome or Miller Fisher syndrome, 76 with chronic inflammatory demyelinating polyneuropathy, and 59 with other types of chronic neuropathy. Biallelic RFC1 mutations were found in three patients with immune-mediated neuropathies, including Guillain-Barré syndrome, idiopathic sensory ataxic neuropathy, or anti-myelin-associated glycoprotein (MAG) neuropathy, who responded to immunotherapies. In addition, a patient with chronic sensory autonomic neuropathy had biallelic mutations, and subclinical changes in Schwann cells on nerve biopsy. In summary, we found CANVAS-related RFC1 mutations in patients with treatable immune-mediated neuropathy or demyelinating neuropathy.

Hereditary Neuropathies

OBJECTIVE

This article provides an overview of hereditary neuropathies, describes the different hereditary neuropathy subtypes and the clinical approach to differentiating between them, and summarizes their clinical management.

LATEST DEVELOPMENTS

Increasingly available clinical genetic testing has broadened the clinical spectrum of hereditary neuropathy subtypes and demonstrated a significant overlap of phenotypes associated with a single gene. New subtypes such as SORD -related neuropathy and CANVAS (cerebellar ataxia, neuropathy, vestibular areflexia syndrome) have emerged. The optimization of clinical management has improved gait and motor function in the adult and pediatric populations. Novel therapeutic approaches are entering clinical trials.

ESSENTIAL POINTS

Hereditary neuropathies constitute a spectrum of peripheral nerve disorders with variable degrees of motor and sensory symptoms, patterns of involvement, and clinical courses.

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.

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.

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.

F-waves persistence in peripheral sensory syndromes

BACKGROUND

The distinction between sensory neuronopathies (SN), which is by definition purely sensory, and sensory polyneuropathies (SP) and sensory multineuropathies (SM) is important for etiologic investigation and prognosis estimation. However, this task is often challenging in clinical practice. We hypothesize that F-wave assessment might be helpful, since it is able to detect subtle signs of motor involvement, which are found in SP and SM, but not in SN.

OBJECTIVE

The aim of the present study was to determine whether F-waves are useful to distinguish SN from SP and SM.

METHODS

We selected 21 patients with SP (12 diabetes mellitus, 4 transthyretin familial amyloid polyneuropathy, 4 others), 22 with SM (22 leprosy), and 26 with SN (13 immune-mediated, 10 idiopathic, 3 others) according to clinical-electrophysiological-etiological criteria. For every subject, we collected data on height and performed 20 supramaximal distal stimuli in median, ulnar, peroneal, and tibial nerves, bilaterally, to record F-waves. Latencies (minimum and mean) and persistences were compared across groups using the Kruskal-Wallis and Bonferroni tests. P-values < 0.05 were considered significant.

RESULTS

All groups were age, gender, and height-matched. Overall, there were no significant between-group differences regarding F-wave latencies. In contrast, F-wave persistence was able to stratify the groups. Peroneal F-wave persistence was higher, bilaterally, in the SN group compared to SM and SP (p < 0.05). In addition, F-waves persistence of the ulnar and tibial nerves was also helpful to separate SN from SP (p < 0.05).

CONCLUSION

F-wave persistence of the peroneal nerves might be an additional and useful diagnostic tool to differentiate peripheral sensory syndromes.

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.

Late-onset sensory-motor axonal neuropathy, a novel SLC12A6-related phenotype

We describe five families from different regions in Norway with a late-onset autosomal-dominant hereditary polyneuropathy sharing a heterozygous variant in the SLC12A6 gene. Mutations in the same gene have previously been described in infants with autosomal-recessive hereditary motor and sensory neuropathy with corpus callosum agenesis and mental retardation (Andermann syndrome), and in a few case reports describing dominantly acting de novo mutations, most of them with onset in childhood. The phenotypes in our families demonstrated heterogeneity. Some of our patients only had subtle to moderate symptoms and some individuals even no complaints. None had CNS manifestations. Clinical and neurophysiological evaluations revealed a predominant sensory axonal polyneuropathy with slight to moderate motor components. In all 10 patients the identical SLC12A6 missense variant, NM_001365088.1 c.1655G>A p.(Gly552Asp), was identified. For functional characterization, the mutant potassium chloride cotransporter 3 was modelled in Xenopus oocytes. This revealed a significant reduction in potassium influx for the p.(Gly552Asp) substitution. Our findings further expand the spectrum of SLC12A6 disease, from biallelic hereditary motor and sensory neuropathy with corpus callosum agenesis and mental retardation and monoallelic early-onset hereditary motor and sensory neuropathy caused by de novo mutations, to late-onset autosomal-dominant axonal neuropathy with predominant sensory deficits.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

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.

Ultrasound of peripheral nerves distinguishes inherited sensory neuronopathy of cerebellar ataxia with neuropathy and vestibular areflexia syndrome from inherited axonopathy

Introduction/Aims Recent studies have shown that ultrasound of peripheral nerves can distinguish inherited sensory neuronopathy from acquired axonopathy with a high degree of accuracy. In this study we aimed to determine whether ultrasound can also distinguish inherited sensory neuronopathy from inherited axonopathy. Methods We compared the ultrasound cross-sectional areas (CSAs) of the median, ulnar, sural, and tibial nerves of retrospectively recruited patients with cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS), in whom sensory neuronopathy is a cardinal feature, with Charcot-Marie-Tooth type 2 (CMT2) disease patients, who have an inherited axonopathy, using the Kruskal-Wallis test and receiver-operating characteristic curves. Results There were 17 patients with CANVAS and 18 with CMT2. The upper limb nerve CSAs were significantly smaller in CANVAS than in CMT2 (P < .001), with the CSAs of the median nerve at mid-forearm and ulnar nerve at mid-arm being a third or less the size of those of the CMT2 patients. Nerve ultrasound reliably distinguished CANVAS from CMT2 with ROC areas under the curve between 0.97 and 0.99. The lower limb CSAs of the two patient groups were not significantly different. Discussion Ultrasound of the upper limb nerves distinguishes CANVAS sensory neuronopathy from inherited axonopathy with high accuracy and can therefore be proposed as a reliable additional tool in the investigation of these diseases.

Sjögren syndrome and RFC1-CANVAS sensory ganglionopathy: co-occurrence or misdiagnosis?

BACKGROUND AND AIMS

RFC1-CANVAS and primary Sjögren syndrome (pSS) are among the most frequent causes of sensory ganglionopathy (SG) and can present simultaneously in a given patient, sharing confounding signs and symptoms. We describe the clinical characteristics of patients with SG due to CANVAS who were suspected of having or had received a previous diagnosis of pSS.

METHODS

Patients with SG and a genetically confirmed RFC1-CANVAS followed in our centre were ascertained and their personal history of pSS was collected.

RESULTS

Among the 71 patients with CANVAS, six patients (all females) had been diagnosed with pSS by their clinicians. In these six patients, the mean age at onset of the SG was 61 years, four patients describing gait instability and positive sensory symptoms, one only gait instability and one only positive sensory symptoms. Five patients had a history of chronic cough; signs at examination included a cerebellar syndrome (n = 3), vestibulopathy (n = 2), pes cavus (n = 3) and dysautonomia (n = 2). On neurophysiological examination, SG was generally severe and symmetrical. All patients presented a sicca syndrome, but only four patients had a confirmed pSS as per 2016 ACR-EULAR classification criteria. Four patients were treated but did not respond to immunosuppressive therapy, one suffering from side effects.

INTERPRETATION

The suspicion or diagnosis of pSS in a patient with SG should not exclude RFC1 expansion analysis, as a CANVAS diagnosis may dissuade the clinician from initiating or continuing immunosuppressive therapy, especially in unconfirmed pSS patients. Importantly, a sicca syndrome may lead to pSS misdiagnosis.

RFC1-Related Disease

In 2019, a biallelic pentanucleotide repeat expansion in the gene encoding replication factor C subunit 1 (RFC1) was reported as a cause of cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS). In addition, biallelic expansions were shown to account for up to 22% of cases with late-onset ataxia. Since this discovery, the phenotypic spectrum reported to be associated with RFC1 expansions has extended beyond the initial conditions to include pure cerebellar ataxia, isolated somatosensory impairment, combinations of the 2, and parkinsonism, leading to a potentially broad differential diagnosis. Genetic studies suggest RFC1 expansions may be the most common genetic cause of ataxia and are likely underdiagnosed. This review summarizes the current molecular and clinical knowledge of RFC1-related disease, with a focus on the evaluation of recent phenotype associations and highlighting the current challenges in clinical pathways to diagnosis and molecular testing.

Sensory neuronopathies, diagnostic criteria and causes

PURPOSE OF REVIEW

To stress on the diagnostic strategy of sensory neuronopathies (SNN), including new genes and antibodies.

RECENT FINDING

SNN involve paraneoplastic, dysimmune, toxic, viral and genetic mechanisms. About one-third remains idiopathic. Recently, new antibodies and genes have reduced this proportion. Anti-FGFR3 and anti-AGO antibodies are not specific of SNN, although SNN is predominant and may occur with systemic autoimmune diseases. These antibodies are the only marker of an underlying dysimmune context in two-thirds (anti-FGFR3 antibodies) and one-third of the cases (anti-AGO antibodies), respectively. Patients with anti-AGO antibodies may improve with treatment, which is less clear with anti-FGFR3 antibodies. A biallelic expansion in the RFC1 gene is responsible for the cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) in which SNN is a predominant manifestation. Most of the patients have an adult onset and are sporadic. The RFC1 mutation may represent one-third of idiopathic sensory neuropathies. Finally, the criteria for the diagnosis of paraneoplastic SNN have recently been updated.

SUMMARY

The diagnostic of SNN relies on criteria distinguishing SNN from other neuropathies. The strategy in search of their cause now needs to include these recent findings.

Multi-type RFC1 repeat expansions as the most common cause of hereditary sensory and autonomic neuropathy

Non-coding repeat expansions within RFC1 and NOTCH2NLC genes have lately been linked to multisystem neurodegenerative diseases, which also shed light on yet undiagnosed patients with inherited peripheral neuropathies. The aim of this study was to identify the genetic basis of patients with hereditary sensory and autonomic neuropathy (HSAN). We collected 79 unrelated DNA samples clinically suspected with HSAN from multiple regions of Japan. Mutation screening was first performed using gene panel sequencing and whole-exome sequencing. Pathogenic/likely pathogenic variants were identified from genes of WNK1/HSN2 (6 cases), SCN9A (3 cases), NTRK1 (3 cases), and DNMT1 (2 cases). Subsequently, long-range flanking PCR and repeat-primed PCR were applied to analyze repeat expansions in RFC1 and NOTCH2NLC. Bi-allelic RFC1 repeat expansions were detected from 20 adult-onset HSAN patients, consisting of [(AAGGG)exp/(AAGGG)exp] (8 cases), [(ACAGG)exp/(ACAGG)exp] (8 cases), and [(AAGGG)exp/(ACAGG)exp] (4 cases). GGC repeat expansion in NOTCH2NLC was found in 1 case. Single-nucleotide variant-based haplotype analysis of patients harboring disease-associated repeat expansions in RFC1 revealed distinguishable haplotypes among subgroups with different repeat genotypes. These findings substantially redefine the genetic spectrum of HSAN, where multi-type RFC1 repeat expansions account for 25.3% of all patients, highlighting the necessity of genetic screening, particularly for adult-onset patients.

Prevalence of intronic repeat expansions in RFC1 in Dutch patients with CANVAS and adult-onset ataxia

Recently, an intronic biallelic (AAGGG)_n repeat expansion in RFC1 was shown to be a cause of CANVAS and adult-onset ataxia in multiple populations. As the prevalence of the RFC1 repeat expansion in Dutch cases was unknown, we retrospectively tested 9 putative CANVAS cases and two independent cohorts (A and B) of 395 and 222 adult-onset ataxia cases, respectively, using the previously published protocol and, for the first time optical genome mapping to determine the size of the expanded RFC1 repeat. We identified the biallelic (AAGGG)_n repeat expansion in 5/9 (55%) putative CANVAS patients and in 10/617 (1.6%; cohorts A + B) adult-onset ataxia patients. In addition to the AAGGG repeat motif, we observed a putative GAAGG repeat motif in the repeat expansion with unknown significance in two adult-onset ataxia patients. All the expanded (AAGGG)_n repeats identified were in the range of 800–1299 repeat units. The intronic biallelic RFC1 repeat expansion thus explains a number of the Dutch adult-onset ataxia cases that display the main clinical features of CANVAS, and particularly when ataxia is combined with neuropathy. The yield of screening for RFC1 expansions in unselected cohorts is relatively low. To increase the current diagnostic yield in ataxia patients, we suggest adding RFC1 screening to the genetic diagnostic workflow by using advanced techniques that attain long fragments.

Comprehensive Genetic Analyses of Inherited Peripheral Neuropathies in Japan: Making Early Diagnosis Possible

Various genomic variants were linked to inherited peripheral neuropathies (IPNs), including large duplication/deletion and repeat expansion, making genetic diagnosis challenging. This large case series aimed to identify the genetic characteristics of Japanese patients with IPNs. We collected data on 2695 IPN cases throughout Japan, in which PMP22 copy number variation (CNV) was pre-excluded. Genetic analyses were performed using DNA microarrays, next-generation sequencing-based gene panel sequencing, whole-exome sequencing, CNV analysis, and RFC1 repeat expansion analysis. The overall diagnostic rate and the genetic spectrum of patients were summarized. We identified 909 cases with suspected IPNs, pathogenic or likely pathogenic variants. The most common causative genes were MFN2, GJB1, MPZ, and MME. MFN2 was the most common cause for early-onset patients, whereas GJB1 and MPZ were the leading causes of middle-onset and late-onset patients, respectively. Meanwhile, GJB1 and MFN2 were leading causes for demyelinating and axonal subtypes, respectively. Additionally, we identified CNVs in MPZ and GJB1 genes and RFC1 repeat expansions. Comprehensive genetic analyses explicitly demonstrated the genetic basis of our IPN case series. A further understanding of the clinical characteristics of IPN and genetic spectrum would assist in developing efficient genetic testing strategies and facilitate early diagnosis.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

Movement disorders and neuropathies: overlaps and mimics in clinical practice

Movement disorders as well as peripheral neuropathies are extremely frequent in the general population; therefore, it is not uncommon to encounter patients with both these conditions. Often, the coexistence is coincidental, due to the high incidence of common causes of peripheral neuropathy, such as diabetes and other age-related disorders, as well as of Parkinson disease (PD), which has a typical late onset. Nonetheless, there is broad evidence that PD patients may commonly develop a sensory and/or autonomic polyneuropathy, triggered by intrinsic and/or extrinsic mechanisms. Similarly, some peripheral neuropathies may develop some movement disorders in the long run, such as tremor, and rarely dystonia and myoclonus, suggesting that central mechanisms may ensue in the pathogenesis of these diseases. Although rare, several acquired or hereditary causes may be responsible for the combination of movement and peripheral nerve disorders as a unique entity, some of which are potentially treatable, including paraneoplastic, autoimmune and nutritional aetiologies. Finally, genetic causes should be pursued in case of positive family history, young onset or multisystemic involvement, and examined for neuroacanthocytosis, spinocerebellar ataxias, mitochondrial disorders and less common causes of adult-onset cerebellar ataxias and spastic paraparesis. Deep phenotyping in terms of neurological and general examination, as well as laboratory tests, neuroimaging, neurophysiology, and next-generation genetic analysis, may guide the clinician toward the correct diagnosis and management.

Head impulse testing in bilateral vestibulopathy in patients with genetically defined CANVAS

BACKGROUND

To investigate the association between disease duration and the severity of bilateral vestibulopathy in individuals with complete or incomplete CANVAS (Cerebellar Ataxia with Neuropathy and Vestibular Areflexia Syndrome) and biallelic RFC1 repeat expansions.

METHODS

Retrospective analysis of clinical data and the vestibulo-ocular reflex quantified by the video head impulse test in 20 patients with confirmed biallelic RFC1 repeat expansions.

RESULTS

Vestibulo-ocular reflex gain at first admittance 6.9 ± 5.0 years after disease onset was 0.16 [0.15-0.31] (median [interquartile range]). Cross-sectional analysis revealed that gain reduction was associated with disease duration. Follow-up measurements were available for ten individuals: eight of them exhibited a progressive decrease of the vestibulo-ocular reflex gain over time. At the first visit, six of all patients (30%) did not show clinical signs of cerebellar ataxia.

CONCLUSIONS

Our data suggest a pathological horizontal head impulse test, which can easily be obtained in many outpatient clinics, as a sign of bilateral vestibulopathy in genetically confirmed CANVAS that can precede clinically accessible cerebellar ataxia at least in a subset of patients. The presumably continuous decline over time possibly reflects the neurodegenerative character of the disease. Thus, genetic testing for RFC1 mutations in (isolated) bilateral vestibulopathy might allow disease detection before the onset of cerebellar signs. Further studies including a wider spectrum of vestibular function tests are warranted in a prospective design.

Screening for RFC-1 pathological expansion in late-onset ataxias: a contribution to the differential diagnosis

We screened 62 late-onset ataxia patients for the AAGGG pathological expansion in the RFC-1 gene that, when biallelic, causes Cerebellar Ataxia, Neuropathy, Vestibular Areflexia Syndrome (CANVAS). Nine patients tested positive. Six had a previous diagnosis of sporadic adult-onset ataxia (SAOA) and three of multisystem atrophy type C (MSA-C). Further six patients were heterozygous for the pathological RFC-1 expansion, four with an initial diagnosis of MSA-C and two of SAOA. In comparison with CANVAS, MSA-C patients had faster progression and shorter disease duration to walking with aids. An abnormal DaTscan does not seem to contribute to differential diagnosis between CANVAS and MSA-C.