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

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.

Strategy for genetic analysis in hereditary neuropathy

Inherited neuropathies are a heterogeneous group of slowly progressive disorders affecting either motor, sensory, and/or autonomic nerves. Peripheral neuropathy may be the major component of a disease such as Charcot-Marie-Tooth disease or a feature of a more complex multisystemic disease involving the central nervous system and other organs. The goal of this review is to provide the clinical clues orientating the genetic diagnosis in a patient with inherited peripheral neuropathy. This review focuses on primary inherited neuropathies, amyloidosis, inherited metabolic diseases, while detailing clinical, neurophysiological and potential treatment of these diseases.

Whole-Genome and Long-Read Sequencing Identify a Novel Mechanism in RFC1 Resulting in CANVAS Syndrome

Objectives

Cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS) results from biallelic intronic pentanucleotide repeats in RFC1. We describe an adult male proband with progressive imbalance, cerebellar atrophy, somatosensory neuronopathy, and absence of peripheral vestibular function for whom clinical testing demonstrated a heterozygous RFC1 expansion consistent with an unaffected carrier.

Methods

We performed whole-genome sequencing (WGS) on peripheral blood DNA samples from the proband and his unaffected mother. We performed DNA long-read sequencing and synthesized complementary DNA from RNA using peripheral blood from the proband.

Results

WGS confirmed the maternally inherited RFC1 expansion and identified a rare, nonsense RFC1 variant: c.C1147T; p.R383X in the proband but not the maternal DNA sample. RFC1 variants were confirmed in trans with long-read sequencing. Functional studies demonstrated the absence of complementary DNA (cDNA) transcript from the c.C1147T; p.R383X variant supporting nonsense-mediated decay of this transcript.

Discussion

We report an adult with CANVAS due to compound heterozygous pathogenic RFC1 variants: the pathogenic intronic pentanucleotide expansion confirmed in trans with a nonsense variant. This report represents a novel molecular mechanism for CANVAS. Sequencing for RFC1 should be considered for adults meeting clinical criteria for the CANVAS phenotype if only a heterozygous pathogenic RFC1 expansion is identified.

RFC1 nonsense and frameshift variants cause CANVAS: clues for an unsolved pathophysiology

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is an inherited late-onset neurological disease caused by bi-allelic AAGGG pentanucleotide expansions within intron 2 of RFC1. Despite extensive studies, the pathophysiological mechanism of these intronic expansions remains elusive. We screened by clinical exome sequencing two unrelated patients presenting with late-onset ataxia. A repeat-primer polymerase chain reaction was used for RFC1 AAGGG intronic expansion identification. RFC1 mRNA expression was assessed by quantitative reverse transcription-polymerase chain reaction. We identified the first two CANVAS affected patients who are compound heterozygous for RFC1 truncating variants (p.Arg388* and c.575delA, respectively) and a pathological AAGGG expansion. RFC1 expression studies in whole blood showed a significant reduction of RFC1 mRNA for both patients compared to three patients with bi-allelic RFC1 expansions. In conclusion, this observation provides clues that suggest bi-allelic RFC1 conditional loss-of-function as the cause of the disease.

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.

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.

Cognitive impairment is not uncommon in patients with biallelic RFC1 AAGGG repeat expansion, but the expansion is rare in patients with cognitive disease

INTRODUCTION

The biallelic repeat expansion (AAGGG)_exp in RFC1 causes cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS). Recently, cognitive impairment has been reported in patients with CANVAS and a broader neurodegenerative process associated with RFC1 has been suggested. Furthermore, rare cases of multiple system atrophy, Parkinson's disease, amyotrophic lateral sclerosis or CANVAS with features of dementia with Lewy bodies have been found.

OBJECTIVE

We hypothesized that the biallelic (AAGGG)_exp is associated with neurodegeneration manifested as cognitive symptoms and that atypical RFC1 disease may be found among patients with cognitive disorder.

METHODS

Clinical data on nine patients with biallelic (AAGGG)_exp were reviewed and 564 patients with Alzheimer's disease or frontotemporal dementia (FTD) were investigated for biallelic RFC1 (AAGGG)_exp.

RESULTS

Five patients with biallelic (AAGGG)_exp were found with a cognitive impairment and in four of them the phenotype resembled FTD. However, biallelic (AAGGG)_exp was not detected among patients with Alzheimer's disease or FTD.

CONCLUSION

Cognitive impairment is a feature in patients with the biallelic (AAGGG)_exp, but the pathogenic expansion seems to be rare in patients with dementia. Studies on patients with diverse phenotypes would be useful to further explore the involvement of RFC1 in neuronal degeneration and to identify atypical phenotypes, which should be taken into account in clinical practice.

Multiple system atrophy

Multiple system atrophy (MSA) is a rare neurodegenerative disease that is characterized by neuronal loss and gliosis in multiple areas of the central nervous system including striatonigral, olivopontocerebellar and central autonomic structures. Oligodendroglial cytoplasmic inclusions containing misfolded and aggregated α-synuclein are the histopathological hallmark of MSA. A firm clinical diagnosis requires the presence of autonomic dysfunction in combination with parkinsonism that responds poorly to levodopa and/or cerebellar ataxia. Clinical diagnostic accuracy is suboptimal in early disease because of phenotypic overlaps with Parkinson disease or other types of degenerative parkinsonism as well as with other cerebellar disorders. The symptomatic management of MSA requires a complex multimodal approach to compensate for autonomic failure, alleviate parkinsonism and cerebellar ataxia and associated disabilities. None of the available treatments significantly slows the aggressive course of MSA. Despite several failed trials in the past, a robust pipeline of putative disease-modifying agents, along with progress towards early diagnosis and the development of sensitive diagnostic and progression biomarkers for MSA, offer new hope for patients.

Genetic and clinical features of cerebellar ataxia with RFC1 biallelic repeat expansions in Japan

The recessive intronic pentanucleotide repeat AAGGG expansion of replication factor complex subunit 1 (RFC1) is associated with cerebellar ataxia, sensory neuropathy, and vestibular areflexia syndrome. And the clinical spectrum has been continuously expanding. We conducted this study to demonstrate the clinical and genetic features of a large-scale case series of Japanese patients with cerebellar ataxia with RFC1 repeat expansions. We examined 1,289 Japanese patients with cerebellar ataxia and analyzed RFC1 repeat expansions in 840 patients, excluding those with genetic diagnoses or an autosomal dominant inheritance pattern. For individuals where no product was obtained by flanking polymerase chain reaction (PCR), repeat-primed PCR was performed using primers specific for the following four repeat motifs: AAAAG, AAAGG, AAGGG, and ACAGG. RFC1 analysis revealed multitype biallelic pathogenic repeat expansions in 15 patients, including (AAGGG)exp/(AAGGG)exp in seven patients, (ACAGG)exp/(ACAGG)exp in three patients, (AAGGG)exp/(ACAGG)exp in four patients, and (AAGGG)exp/(AAAGG)₁₅(AAGGG)exp in one patient. Clinical analysis showed various combinations of cerebellar ataxia, vestibular dysfunction, neuropathy, cognitive decline, autonomic dysfunction, chronic cough, pyramidal tract disorder, parkinsonism, involuntary movement, and muscle fasciculation. Pathological RFC1 repeat expansions account for 1.8% (15/840) of undiagnosed patients with cerebellar ataxia and sporadic/recessive/unclassified inheritance. Screening of RFC1 repeat expansions should be considered in patients with cerebellar ataxia, irrespective of their subtype and onset age.

Beyond canvas: behavioral onset of rfc1-expansion disease in an Italian family-causal or casual?

INTRODUCTION

Biallelic intronic AAGGG repeat expansion in the replication factor C subunit 1 (RFC1) gene was recently identified in two/third of patients with cerebellar ataxia, sensory neuropathy, and bilateral vestibular areflexia syndrome (CANVAS). The phenotypic spectrum has expanded since (i.e., parkinsonism, motor neuron involvement, cognitive decline); no behavioral symptoms have been reported yet.

CASE REPORT

We report an Italian family that met the diagnostic criteria for CANVAS, and RFC1-expansion was detected in five of seven. All the affected members presented behavioral-psychiatric symptoms (anxiety, panic attacks, alcohol abuse) before the multisystemic RFC1-expansion manifestation. The disease course was progressive, with ataxia and behavioral-cognitive aspects as the most disabling symptoms.

CONCLUSION

These behavioral-cognitive observations may broaden the RFC1-expansion phenotypic spectrum and highlight the importance of investigating the whole non-motor symptoms in ataxic patients.

Repeat expansions nested within tandem CNVs: a unique structural change in GLS exemplifies the diagnostic challenges of non-coding pathogenic variation

Glutaminase deficiency has recently been associated with ataxia and developmental delay due to repeat expansions in the 5'UTR of the glutaminase (GLS) gene. Patients with the described GLS repeat expansion may indeed remain undiagnosed due to the rarity of this variant, the challenge of its detection and the recency of its discovery. In this study, we combined advanced bioinformatics screening of ~3000 genomes and ~1500 exomes with optical genome mapping and long-read sequencing for confirmation studies. We identified two GLS families, previously intensely and unsuccessfully analyzed. One family carries an unusual and complex structural change involving a homozygous repeat expansion nested within a quadruplication event in the 5'UTR of GLS. Glutaminase deficiency and its metabolic consequences were validated by in-depth biochemical analysis. The identified GLS patients showed progressive early-onset ataxia, cognitive deficits, pyramidal tract damage and optic atrophy, thus demonstrating susceptibility of several specific neuron populations to glutaminase deficiency. This large-scale screening study demonstrates the ability of bioinformatics analysis-validated by latest state-of-the-art technologies (optical genome mapping and long-read sequencing)-to effectively flag complex repeat expansions using short-read datasets and thus facilitate diagnosis of ultra-rare disorders.

RFC1-Related Disorder: In Vivo Evaluation of Spinal Cord Damage

BACKGROUND

RFC1-related disorder is a novel heredodegenerative condition with a broad phenotypic spectrum. Its neuropathological bases are not yet fully understood, particularly regarding the pattern, extent, and clinical relevance of spinal cord (SC) damage.

OBJECTIVES

The objectives were to determine the SC structural signature in RFC1-related disorder in vivo and to identify potential clinical correlates for these imaging abnormalities.

METHODS

We enrolled 17 subjects with biallelic RFC1 (AAGGG)n expansions and 11 age- and sex-matched healthy controls that underwent multimodal magnetic resonance imaging SC acquisitions in a 3T Philips Achieva scanner. Both global morphometry and tract-specific analyses were then performed across all cervical levels. Between-group comparisons were assessed using nonparametric tests.

RESULTS

In the patient group, mean age and disease duration were 62.9 ± 9.3 and 9.3 ± 4.0, respectively. Compared to controls, patients had remarkable SC cross-sectional area reduction along all cervical levels but anteroposterior flattening only in the lower cervical levels. There was also prominent SC gray matter atrophy. Diffusivity abnormalities were identified in the dorsal columns but not in the lateral corticospinal tracts. Disease severity did not correlate with these imaging parameters.

CONCLUSION

SC damage is a hallmark of RFC1-related disorder and characterized by gray as well as white matter involvement. In particular, dorsal columns are severely and diffusely affected. The clinical correlates of these imaging abnormalities still deserve additional investigations. © 2022 International Parkinson and Movement Disorder Society.

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.

[Diagnosis and Treatment of Ataxias: An Up-To-Date Overview]

Ataxias are a heterogeneous group of diseases. They can occur at any age and have various causes. Most ataxias are rare diseases and many are genetic disorders. A large and steadily increasing number of underlying gene defects are known. The path to the correct diagnosis is often challenging. This overview summarizes the typical findings for the most important acquired, hereditary and non-hereditary degenerative ataxias. The focus is on ataxias with adult onset.

Cognitive Impairment in a Complex Family With AAGGG and ACAGG Repeat Expansions in RFC1 Detected by ExpansionHunter Denovo

Background and Objectives

We investigated the genetic basis and brain metabolism and blood flow of a Japanese family with spinocerebellar degeneration (SCD), with multiple affected members for 3 generations.

Methods

After excluding DNA repeat expansion (RE) of common SCD genes by fragment analysis, we performed whole-exome sequencing (WES) and whole-genome sequencing (WGS). Homozygosity mapping was performed using these data. REs were investigated with WGS data using ExpansionHunter Denovo and Expansion Hunter.

Results

WES and WGS were unable to identify likely pathogenic variants, and homozygosity mapping failed to narrow down the locus. However, ExpansionHunter Denovo detected REs in intron 2 of the RFC1 gene and led us to the diagnosis of RFC1-related disorders. Subsequent repeat-primed PCR and Southern blot hybridization analyses revealed that 3 of 6 patients and 1 suspected individual had expansions of AAGGG ((AAGGG)_exp) and (ACAGG)_exp repeats in a compound heterozygous state and 3 had a homozygous (ACAGG)_exp. The patients showed a variety of clinical features, including adult-onset ataxia, sensorimotor neuropathy, head tremor, parkinsonism, dystonia, and cognitive impairment. A comparison of previous reports with those of the family in study suggested that motor neuropathy could be a feature of compound heterozygous patients and biallelic (ACAGG)_exp patients. Cognitive function tests showed cognitive impairment with a predominance of frontal lobe dysfunction. Examination of MRI, SPECT, and ¹⁸F-fluorodeoxyglucose-PET showed clear cortical damage with frontal lobe predominance in 1 case, but no cerebral damage was evident in the other 2 cases.

Discussion

Our report shows the usefulness of WGS and RE detection tools for SCD of unknown cause. The studied family with RFC1-related disorders included patients with (ACAGG)_exp and (AAGGG)_exp in a compound heterozygous state and was characterized by motor neuropathy. Based on the results of cognitive function tests and imaging studies, 1 patient presented with cognitive impairment due to frontal lobe metabolic changes, but there were also patients who presented with cognitive impairment without apparent cerebral metabolic or blood flow, suggesting that other factors are also associated with cognitive impairment.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Cognitive Dysfunction in Repeat Expansion Diseases: A Review

With the development of the sequencing technique, more than 40 repeat expansion diseases (REDs) have been identified during the past two decades. Moreover, the clinical features of these diseases show some commonality, and the nervous system, especially the cognitive function was affected in part by these diseases. However, the specific cognitive domains impaired in different diseases were inconsistent. Here, we survey literature on the cognitive consequences of the following disorders presenting cognitive dysfunction and summarizing the pathogenic genes, epidemiology, and different domains affected by these diseases. We found that the cognitive domains affected in neuronal intranuclear inclusion disease (NIID) were widespread including the executive function, memory, information processing speed, attention, visuospatial function, and language. Patients with C9ORF72-frontotemporal dementia (FTD) showed impairment in executive function, memory, language, and visuospatial function. While in Huntington's disease (HD), the executive function, memory, and information processing speed were affected, in the fragile X-associated tremor/ataxia syndrome (FXTAS), executive function, memory, information processing speed, and attention were impaired. Moreover, the spinocerebellar ataxias showed broad damage in almost all the cognitive domains except for the relatively intact language ability. Some other diseases with relatively rare clinical data also indicated cognitive dysfunction, such as myotonic dystrophy type 1 (DM1), progressive myoclonus epilepsy (PME), Friedreich ataxia (FRDA), Huntington disease like-2 (HDL2), and cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS). We drew a cognitive function landscape of the related REDs that might provide an aspect for differential diagnosis through cognitive domains and effective non-specific interventions for these diseases.

Recessive cerebellar and afferent ataxias - clinical challenges and future directions

Cerebellar and afferent ataxias present with a characteristic gait disorder that reflects cerebellar motor dysfunction and sensory loss. These disorders are a diagnostic challenge for clinicians because of the large number of acquired and inherited diseases that cause cerebellar and sensory neuron damage. Among such conditions that are recessively inherited, Friedreich ataxia and RFC1-associated cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) include the characteristic clinical, neuropathological and imaging features of ganglionopathies, a distinctive non-length-dependent type of sensory involvement. In this Review, we discuss the typical and atypical phenotypes of Friedreich ataxia and CANVAS, along with the features of other recessive ataxias that present with a ganglionopathy or polyneuropathy, with an emphasis on recently described clinical features, natural history and genotype-phenotype correlations. We review the main developments in understanding the complex pathology that affects the sensory neurons and cerebellum, which seem to be most vulnerable to disorders that affect mitochondrial function and DNA repair mechanisms. Finally, we discuss disease-modifying therapeutic advances in Friedreich ataxia, highlighting the most promising candidate molecules and lessons learned from previous clinical trials.

Expanding the Clinical Spectrum of RFC1 Gene Mutations

Biallelic intronic repeat expansion in the replication factor complex unit 1 (RFC1) gene has recently been described as a cause of late onset ataxia with degeneration of the cerebellum, sensory pathways and the vestibular apparatus. This condition is termed cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS). Since the identification of this novel gene mutation, the phenotypic spectrum of RFC1 mutations continues to expand and includes not only CANVAS but also slowly progressive cerebellar ataxia, ataxia with chronic cough (ACC), isolated sensory neuropathy and multisystemic diseases. We present a patient with a genetically confirmed intronic repeat expansion in the RFC1 gene with a symptom complex not described previously.

MRI CNS Atrophy Pattern and the Etiologies of Progressive Ataxias

MRI shows the three archetypal patterns of CNS volume loss underlying progressive ataxias in vivo, namely spinal atrophy (SA), cortical cerebellar atrophy (CCA) and olivopontocerebellar atrophy (OPCA). The MRI-based CNS atrophy pattern was reviewed in 128 progressive ataxias. A CNS atrophy pattern was identified in 91 conditions: SA in Friedreich’s ataxia, CCA in 5 acquired and 72 (24 dominant, 47 recessive,1 X-linked) inherited ataxias, OPCA in Multi-System Atrophy and 12 (9 dominant, 2 recessive,1 X-linked) inherited ataxias. The MRI-based CNS atrophy pattern may be useful for genetic assessment, identification of shared cellular targets, repurposing therapies or the enlargement of drug indications in progressive ataxias.

Bilateral vestibulopathy: the causes, diagnosis, and treatments

PURPOSE OF REVIEW

The current review covers recent advances in bilateral vestibulopathy (BVP) in terms of its etiology, diagnosis, and treatments.

RECENT FINDINGS

The etiology of BVP depends on its clinical course and associated findings, and genetic abnormalities are increasingly recognized in isolated as well as complicated form of BVP. Recent developments in evaluation of the vestibular function have greatly enhanced the detection of BVP, and introduction of the consensus diagnostic criteria by Barany Society has facilitated research on BVP. Vestibular prosthesis may improve vestibular function, posture, gait and quality of life in patients with BVP and would expand the therapeutic options for BVP in near future.

SUMMARY

Genetics is expanding its role in identifying the causes of BVP of hitherto unknown etiology. The detection and investigation of BVP have been greatly enhanced by introduction of consensus diagnostic criteria and recent developments in methodology evaluating the vestibular function. Vestibular prothesis appears promising in managing BVP.

VIDEO ABSTRACT

http://links.lww.com/CONR/A59.

Biallelic expansion in RFC1 as a rare cause of Parkinson's disease

An intronic expansion (AAGGG)_exp in the RFC1 gene has recently been shown to cause recessively inherited cerebellar ataxia, neuropathy, and vestibular areflexia syndrome and, furthermore, a few patients with ataxia and parkinsonism have been reported. We investigated 569 Finnish patients with medicated parkinsonism for RFC1 and found biallelic (AAGGG)_exp in three non-consanguineous patients with clinically confirmed Parkinson’s disease without ataxia suggesting that RFC1-related disorders include Parkinson’s disease as well.

Motor neuron pathology in CANVAS due to RFC1 expansions

CANVAS caused by RFC1 biallelic expansions is a major cause of inherited sensory neuronopathy. Detection of RFC1 expansion is challenging and CANVAS can be associated with atypical features.

We clinically and genetically characterized 50 patients, selected based on the presence of sensory neuronopathy confirmed by EMG. We screened RFC1 expansion by PCR, repeat-primed PCR, and Southern blotting of long-range PCR products, a newly developed method. Neuropathological characterization was performed on the brain and spinal cord of one patient.

Most patients (88%) carried a biallelic (AAGGG)n expansion in RFC1. In addition to the core CANVAS phenotype (sensory neuronopathy, cerebellar syndrome, and vestibular impairment), we observed chronic cough (97%), oculomotor signs (85%), motor neuron involvement (55%), dysautonomia (50%), and parkinsonism (10%). Motor neuron involvement was found for 24 of 38 patients (63.1%). First motor neuron signs, such as brisk reflexes, extensor plantar responses, and/or spasticity, were present in 29% of patients, second motor neuron signs, such as fasciculations, wasting, weakness, or a neurogenic pattern on EMG in 18%, and both in 16%. Mixed motor and sensory neuronopathy was observed in 19% of patients. Among six non-RFC1 patients, one carried a heterozygous AAGGG expansion and a pathogenic variant in GRM1. Neuropathological examination of one RFC1 patient with an enriched phenotype, including parkinsonism, dysautonomia, and cognitive decline, showed posterior column and lumbar posterior root atrophy. Degeneration of the vestibulospinal and spinocerebellar tracts was mild. We observed marked astrocytic gliosis and axonal swelling of the synapse between first and second motor neurons in the anterior horn at the lumbar level. The cerebellum showed mild depletion of Purkinje cells, with empty baskets, torpedoes, and astrogliosis characterized by a disorganization of the Bergmann's radial glia. We found neuronal loss in the vagal nucleus. The pars compacta of the substantia nigra was depleted, with widespread Lewy bodies in the locus coeruleus, substantia nigra, hippocampus, entorhinal cortex, and amygdala.

We propose new guidelines for the screening of RFC1 expansion, considering different expansion motifs. Here, we developed a new method to more easily detect pathogenic RFC1 expansions. We report frequent motor neuron involvement and different neuronopathy subtypes. Parkinsonism was more prevalent in this cohort than in the general population, 10% versus the expected 1% (p < 0.001). We describe, for the first time, the spinal cord pathology in CANVAS, showing the alteration of posterior columns and roots, astrocytic gliosis and axonal swelling, suggesting motor neuron synaptic dysfunction.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): from clinical diagnosis towards genetic testing

The cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is a late-onset and recessively inherited ataxia. For many years, CANVAS has been diagnosed based on the clinical phenotype. Only recently, a large biallelic pentanucleotide repeat expansion in the replication factor C subunit 1 (RFC1) gene has been identified as the underlying genetic cause for the large majority of CANVAS cases. Subsequently, other phenotypes such as ataxia with chronic cough, incomplete CANVAS and MSA-C-like phenotypes have been associated with biallelic RFC1 repeat expansions. Because of this heterogeneity it has been suggested to change the name of the disease to "RFC1 disease". Chronic cough is characteristic and can precede neurological symptoms by years or decades. In the neurological examination signs of cerebellar, sensory, and vestibular ataxia are frequently observed. Nerve conduction studies usually show absent or markedly reduced sensory nerve action potentials. On brain MRI cerebellar degeneration and spinal cord alterations are common. In later disease stages more widespread neurodegeneration with additional involvement of the brainstem and basal ganglia is possible. As yet, the exact incidence of RFC1-associated neurological diseases remains uncertain although first studies suggest that RFC1-related ataxia is common. Moreover, the pathophysiological mechanisms caused by the large biallelic pentanucleotide repeat expansions in RFC1 remain elusive. Future molecular and genetic research as well as natural history studies are highly desirable to pave the way towards personalized treatment approaches.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): genetic and clinical aspects

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) typically presents in middle life with a combination of neuropathy, ataxia and vestibular disease, with patients reporting progressive imbalance, oscillopsia, sensory disturbance and a dry cough. Examination identifies a sensory neuropathy or neuronopathy and bilaterally impaired vestibulo-ocular reflex. The underlying genetic basis is of biallelic AAGGG expansions in the second intron of replication factor complex subunit 1 (RFC1). The frequency and phenotype spectrum of RFC1 disease is expanding, ranging from typical CANVAS to site-restricted variants affecting the sensory nerves, cerebellum and/or the vestibular system. Given the wide phenotype spectrum of RFC1, the differential diagnosis is broad. RFC1 disease due to biallelic AAGGG expansions is probably the most common cause of recessive ataxia. The key to suspecting the disease (and prompt genetic testing) is a thorough clinical examination assessing the three affected systems and noting the presence of chronic cough.

Brain Structural Signature of RFC1-Related Disorder

BACKGROUND

The cerebellar ataxia, neuropathy, and vestibular areflexia syndrome was initially described in the early 1990s as a late-onset slowly progressive condition. Its underlying genetic cause was recently mapped to the RFC1 gene, and additional reports have expanded on the phenotypic manifestations related to RFC1, although little is known about the pattern and extent of structural brain abnormalities in this condition.

OBJECTIVE

The aim is to characterize the structural signature of brain damage in RFC1-related disorder, correlating the findings with clinical symptoms and normal brain RFC1 expression.

METHODS

We recruited 22 individuals with molecular confirmation of RFC1 expansions and submitted them to high-resolution 3T magnetic resonance imaging scans. We performed multimodal analyses to assess separately cerebral and cerebellar abnormalities within gray and white matter (WM). The results were compared with a group of 22 age- and sex-matched controls.

RESULTS

The mean age and disease duration of patients were 62.8 and 10.9 years, respectively. Ataxia, sensory neuronopathy, and vestibular areflexia were the most frequent manifestations, but parkinsonism and pyramidal signs were also noticed. We found that RFC1-related disorder is characterized by widespread and relatively symmetric cerebellar and basal ganglia atrophy. There is brainstem volumetric reduction along all its segments. Cerebral WM is also involved-mostly the corpus callosum and deep tracts, but cerebral cortical damage is rather restricted.

CONCLUSION

This study adds new relevant insights into the pathophysiological mechanisms of RFC1-related disorder. It should no longer be considered a purely cerebellar and sensory pathway disorder. Basal ganglia and deep cerebral WM are additional targets of damage. © 2021 International Parkinson and Movement Disorder Society.

The ARCA Registry: A Collaborative Global Platform for Advancing Trial Readiness in Autosomal Recessive Cerebellar Ataxias

Autosomal recessive cerebellar ataxias (ARCAs) form an ultrarare yet expanding group of neurodegenerative multisystemic diseases affecting the cerebellum and other neurological or non-neurological systems. With the advent of targeted therapies for ARCAs, disease registries have become a precious source of real-world quantitative and qualitative data complementing knowledge from preclinical studies and clinical trials. Here, we review the ARCA Registry, a global collaborative multicenter platform (>15 countries, >30 sites) with the overarching goal to advance trial readiness in ARCAs. It presents a good clinical practice (GCP)- and general data protection regulation (GDPR)-compliant professional-reported registry for multicenter web-based capture of cross-center standardized longitudinal data. Modular electronic case report forms (eCRFs) with core, extended, and optional datasets allow data capture tailored to the participating site's variable interests and resources. The eCRFs cover all key data elements required by regulatory authorities [European Medicines Agency (EMA)] and the European Rare Disease (ERD) platform. They capture genotype, phenotype, and progression and include demographic data, biomarkers, comorbidity, medication, magnetic resonance imaging (MRI), and longitudinal clinician- or patient-reported ratings of ataxia severity, non-ataxia features, disease stage, activities of daily living, and (mental) health status. Moreover, they are aligned to major autosomal-dominant spinocerebellar ataxia (SCA) and sporadic ataxia (SPORTAX) registries in the field, thus allowing for joint and comparative analyses not only across ARCAs but also with SCAs and sporadic ataxias. The registry is at the core of a systematic multi-component ARCA database cluster with a linked biobank and an evolving study database for digital outcome measures. Currently, the registry contains more than 800 patients with almost 1,500 visits representing all ages and disease stages; 65% of patients with established genetic diagnoses capture all the main ARCA genes, and 35% with unsolved diagnoses are targets for advanced next-generation sequencing. The ARCA Registry serves as the backbone of many major European and transatlantic consortia, such as PREPARE, PROSPAX, and the Ataxia Global Initiative, with additional data input from SPORTAX. It has thus become the largest global trial-readiness registry in the ARCA field.

Cerebellar ataxia, neuropathy, vestibular areflexia syndrome: genetic and clinical insights

PURPOSE OF REVIEW

This review aims to summarise the present cerebellar ataxia, neuropathy, vestibular ataxia syndrome (CANVAS) literature, providing both clinical and genetic insights that might facilitate the timely clinical and genetic diagnosis of this disease.

RECENT FINDINGS

Recent advancements in the range of the clinical features of CANVAS have aided the development of a broader, more well-defined clinical diagnostic criteria. Additionally, the identification of a biallelic repeat expansion in RFC1 as the cause of CANVAS and a common cause of late-onset ataxia has opened the door to the potential discovery of a pathogenic mechanism, which in turn, may lead to therapeutic advancements and improved patient care.

SUMMARY

The developments in the clinical and genetic understanding of CANVAS will aid the correct and timely diagnosis of CANVAS, which continues to prove challenging within the clinic. The insights detailed within this review will raise the awareness of the phenotypic spectrum and currently known genetics. We also speculate on the future directions of research into CANVAS.

The Complex Genetic Landscape of Hereditary Ataxias in Turkey and Implications in Clinical Practice

BACKGROUND

The genetic and epidemiological features of hereditary ataxias have been reported in several populations; however, Turkey is still unexplored. Due to high consanguinity, recessive ataxias are more common in Turkey than in Western European populations.

OBJECTIVE

To identify the prevalence and genetic structure of hereditary ataxias in the Turkish population.

METHODS

Our cohort consisted of 1296 index cases and 324 affected family members. Polymerase chain reaction followed by Sanger sequencing or fragment analysis were performed to screen for the trinucleotide repeat expansions in families with a dominant inheritance pattern, as well as in sporadic cases. The expansion in the frataxin (FXN) gene was tested in all autosomal recessive cases and in sporadic cases with a compatible phenotype. Whole-exome sequencing was applied to 251 probands, selected based on the family history, age of onset, and phenotype.

RESULTS

Mutations in known ataxia genes were identified in 30% of 1296 probands. Friedreich's ataxia was found to be the most common recessive ataxia in Turkey, followed by autosomal recessive spastic ataxia of Charlevoix-Saguenay. Spinocerebellar ataxia types 2 and 1 were the most common dominant ataxias. Whole-exome sequencing was performed in 251 probands with an approximate diagnostic yield of 50%. Forty-eight novel variants were found in a plethora of genes, suggesting a high heterogeneity. Variants of unknown significance were discussed in light of clinical data.

CONCLUSION

With the large sample size recruited across the country, we consider that our results provide an accurate picture of the frequency of hereditary ataxias in Turkey. © 2021 International Parkinson and Movement Disorder Society.