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

A Preliminary Data Visualization Approach to Vestibulocochlear Diseases Based on Multiple Scalar Dimensions

For vestibulocochlear diseases, traditional clinical history-taking, and the terminology of widely taught nosologic taxonomy, have misleading implications that can lead to errors in diagnosis and therefore in treatment. In the interest of facilitating differential diagnosis while simultaneously recognizing that many of these diseases may not be as discrete as textbooks suggest, we propose a data visualization approach focusing on several continuous scalar dimensions in the domains of anatomy, physiology, and chronology. We illustrate the application of this approach to several categories of clinical presentation.

Spectrum disorder of RFC1 expansions/CANVAS: Clinical and electrophysiological characterization of a group of 31 patients

INTRODUCTION/OBJECTIVE

Biallelic expansion of the pentanucleotide AAGGG in the RFC1- gene is associated with cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS). This study aimed to comprehensively characterise this condition by conducting an in-depth neurophysiological examination of afflicted patients.

METHODS

A retrospective analysis was conducted in 31 RFC1-positive patients. Assessment included clinical examination and electrodiagnostic tests: nerve conduction studies, F waves, needle electromyography, H, blink and masseter reflexes; Autonomic Nervous System (ANS): Heart Rate Variability, Sympathetic Skin Response and Electrochemical Skin Conductance, and Quantitative Sensory Test (QST). Medullar tracts were assessed using Somatosensory Evoked Potentials (SEPs) and Transcranial Magnetic Stimulation.

RESULTS

All patients exhibited moderate to severe sensory axonal neuropathy/neuronopathy, whereas motor nerve conduction parameters and the pyramidal pathway showed no abnormalities. Tibial nerve SEPs were absent or severely attenuated in all patients, and were moderately affected by the median nerve. H-reflexes were preserved in 85.7% and blink reflex in 60% of tested patients. ANS tests yielded predominantly normal results, although 74% demonstrated impaired QST.

CONCLUSIONS

Our findings highlight the characteristic features of sensory neuropathy/neuronopathy and severe somatosensory deficits, with predominantly preserved H-reflexes and motor sparing. Thermoalgesic fibres are frequently involved, whereas the ANS is relatively preserved.

SIGNIFICANCE

Neurophysiological examinations can objectively characterize the spectrum of RFC1-related disease.

Pseudodominance in RFC1-Spectrum Disorder

Cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) and disease spectrum is an autosomal recessive disorder associated with biallelic repeat expansion (RE) in the RFC1 gene. A high carrier frequency in the healthy population determines the possibility of having affected members in two consecutive generations. We describe pseudodominance in two families affected with RFC1 disorder (10 affected, 5 oligo/asymptomatic individuals). In Family A, after the 75-year-old index case was diagnosed with CANVAS, the 73-year-old wife decided to undergo screening for carrier testing. Although she did not report any symptoms, she resulted positive for the biallelic AAGGG RE thus leading to a diagnosis in the asymptomatic offspring as well and revealing a pseudodominant pattern of inheritance. In Family B pseudodominance was suspected after the identification of the RFC1 RE in the proband affected by sensitive neuropathy because of a positive family history for undetermined polyneuropathy in the mother. The post-mortem identification of the RFC1 RE in a sample specimen from the deceased mother, who had been under our care, allowed the solution of a "cold case". Our report suggests that pseudodominance is a confounding phenomenon to consider in RFC1-spectrum disorder and genetic counselling is instrumental in families with affected individuals.

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

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

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

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

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

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