RFC1 CANVAS: genotype phenotype correlations

<h3></h3> RFC1 disease, caused by biallelic AAGGG repeat expansion in RFC1, is clinically heterogeneous in terms of age of onset, disease progression and phenotype. In this study, we identified 553 patients carrying biallelic RFC1 expansions and measured the repeat expansion size in 392 cases. A larger repeat size of both smaller and larger allele was associated with an earlier age at neurological onset and with a higher hazard of developing disabling symptoms, such as dysarthria or dysphagia or loss of independent walking earlier in disease course. Patients with more complex phenotypes carried larger expansions. Furthermore, larger repeat expansions in the smaller allele were associated with more pronounced cerebellar vermis atrophy on MRI. The repeat did not show significant instability during vertical transmission and across different tissues and brain regions. Additionally, long-read sequencing was used to characterize the entire repeat sequence in selected cases, revealing a pathogenic role for large AAAGG repeat expansions, and identified novel rare pathogenic repeat motifs AGGGC, AAGGC, AGAGG. 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. Moreover full sequencing of the satellite is recommended in clinically selected cases to enable accurate molecular diagnosis and counsel patients and their families.