Role of the repeat expansion size in predicting age of onset and severity in RFC1 disease
Authors: Currò Riccardo , Dominik Natalia , Facchini Stefano , Vegezzi Elisa , Sullivan Roisin , Galassi Deforie Valentina , Fernández-Eulate Gorka , Traschütz Andreas , Rossi Salvatore , Garibaldi Matteo , Kwarciany Mariusz , Taroni Franco , Brusco Alfredo , Good Jean-Marc , Cavalcanti Francesca , Hammans Simon , Ravenscroft Gianina , Roxburgh Richard H ,   RFC1 repeat expansion study group, Albájar Inés , Ashton Catherine , Beauchamp Nick , Beecroft Sarah J , Bellone Emilia , Berciano Josè , Bogdanova-Mihaylova Petya , Borroni Barbara , Brais Bernard , Bugiardini Enrico , Campos Catarina , Carr Aisling , Carroll Liam , Castellani Francesca , Cavallaro Tiziana , Chinnery Patrick F , Colnaghi Silvia , Cosentino Giuseppe , Damasio Joana , Das Soma , Devigili Grazia , Di Bella Daniela , Dick David , Durr Alexandra , El-Saddig Amar , Faber Jennifer , Ferrarini Moreno , Filosto Massimiliano , Fuller Geraint , Gallone Salvatore , Gemelli Chiara , Grandis Marina , Hardy John , Hewamadduma Channa , Horvath Rita , Huin Vincent , Imperiale Daniele , Iruzubieta Pablo , Kaski Diego , King Andrew , Klockgether Thomas , Koç Müge , Kumar Kishore R , Kuntzer Thierry , Laing Nigel , Laurà Matilde , Lavin Timothy , Leigh Peter Nigel , Leonardis Lea , Lunn Michael P , Magri Stefania , Magrinelli Francesca , Malaquias Maria João , Mancuso Michelangelo , Manji Hadi , Massucco Sara , McConville John , Munhoz Renato P , Nagy Sara , Ndayisaba Alain , Nemeth Andrea Hilary , Novis Luiz Eduardo , Palmio Johanna , Pegoraro Elena , Pellerin David , Perrone Benedetta , Pisciotta Chiara , Polke James , Proudfoot Malcolm , Orsi Laura , Radunovic Aleksandar , Riva Nilo , Robert Aiko , Ronco Riccardo , Rossini Elena , Rossor Alex M , Şahbaz Irmak , Sa’di Qais , Salsano Ettore , Salvalaggio Alessandro , Santoro Lucio , Sarto Elisa , Schaefer Andrew , Schenone Angelo , Scriba Carolin , Shaw Joseph , Silvestri Gabriella , Stevens James , Strupp Michael , Sumner Charlotte J , Szymura Agnieszka , Tagliapietra Matteo , Tassorelli Cristina , Tessa Alessandra , Theaudin Marie , Tomaselli Pedro , Tozza Stefano , Tucci Arianna , Valente Enza Maria , Versino Maurizio , Walsh Richard A , Wood Nick W , Yau Way Yan , Zuchner Stephan , Parolin Schnekenberg Ricardo , Rugginini Bianca , Abati Elena , Manini Arianna , Quartesan Ilaria , Ghia Arianna , Lòpez de Munaìn Adolfo , Manganelli Fiore , Kennerson Marina , Santorelli Filippo Maria , Infante Jon , Marques Wilson , Jokela Manu , Murphy Sinéad M , Mandich Paola , Fabrizi Gian Maria , Briani Chiara , Gosal David , Pareyson Davide , Ferrari Alberto , Prados Ferran , Yousry Tarek , Khurana Vikram , Kuo Sheng-Han , Miller James , Troakes Claire , Jaunmuktane Zane , Giunti Paola , Hartmann Annette , Basak Nazli , Synofzik Matthis , Stojkovic Tanya , Hadjivassiliou Marios , Reilly Mary M , Houlden Henry , Cortese Andrea 
Published: January 2024

Abstract
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 &lt; 0.001; larger allele HR = 1.53, P &lt; 0.001] and with a higher hazard of developing disabling symptoms, such as dysarthria or dysphagia (smaller allele HR = 3.40, P &lt; 0.001; larger allele HR = 1.71, P = 0.002) or loss of independent walking (smaller allele HR = 2.78, P &lt; 0.001; larger allele HR = 1.60; P &lt; 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 &lt; 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 &lt; 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.