Novel Clinical Insights From a Swedish RFC1 Spectrum Disorder Cohort

BACKGROUND: Biallelic pentanucleotide expansions in RFC1 cause cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS), and a growing spectrum of presentations. We aimed to clinically characterize a cohort of patients from Sweden with biallelic expansions in RFC1. METHODS: We retrospectively enrolled patients with homozygous expansions in RFC1 from a tertiary center in Sweden, evaluating clinical and genetic data. Assessments included nerve conduction studies (NCS, n = 27), electromyography (n = 7), quantitative sensory testing (n = 18), brain MRI (n = 27), and vestibular/eye motor tests (n = 18-21). RESULTS: Of the 30 patients enrolled, 28 were ethnic Swedish; 17/30 from smaller regions, including eight (27%) from Norrbotten. Twenty-two patients met the CANVAS criteria. Mean age of onset was 52 ± 12 years (range 20-70), and disease duration was 14 ± 12 years. Symptoms matched the CANVAS acronym with multisystemic features in 83%, including dysautonomia (77%), dyskinesia (36%), and bradykinesia (17%). Phenotypes overlapped with MSA-C (n = 2) and mitochondrial ataxias (n = 1). Notably, one symptomatic patient lacked neuropathy on NCS. Annual disease progression was slow (0.3 by spinocerebellar degeneration functional score, 1.2 by SARA). At vestibular testing, 47% showed a preserved caloric response and pathologic angular VOR with a nonsignificant trend among younger patients and milder ataxia; otolith function was largely preserved. DISCUSSION: Our findings expand the RFC1 spectrum, suggesting a founder effect in Sweden and extensive subclinical involvement. RFC1-spectrum disorder should also be considered in patients with cerebellar and vestibular dysfunction but lacking neuropathy. A discordant VOR pattern may represent an incipient sign of RFC1-spectrum disorder; interestingly, otolith pathways seem to be generally spared.