RFC1

Inheritance Pattern: AR

Disease ID:MONDO:0044720

Submitter: Labcorp Genetics (formerly Invitae)

Inheritance Pattern: AR

Disease ID:MONDO:0044720

Submitter: PanelApp Australia

Inheritance Pattern: AR

Disease ID:MONDO:0013809

Submitter: Ambry Genetics

Inheritance Pattern: AR

Disease ID:MONDO:0044720

Submitter: Orphanet

Inheritance Pattern: AR

Disease ID:MONDO:0044720

Submitter: ClinGen

Replication factor C subunit 1 (*RFC1*) biallelic repeat expansion variants were first implicated as causal for cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS - MONDO:0044720, MIM# 61979) in 2019 (PMID 30926972). Affected individuals often present initially with chronic cough (PMID 38755058) which progresses to neurological phenotypes and is nearly always a late-onset condition (median age 52yo) however patients have been observed to present with symptoms much earlier (PMIDs 32040566, 32851396). Deep intronic, biallelic, non-reference pentanucleotide repeat expansions in *RFC1* have been shown extensively in the literature to segregate with features of CANVAS. The pentanucleotide repeat motif and length both appear to be important in disease, with 400 to 2000 AAGGG repeats being most frequently reported as pathogenic (PMID 38627134). Rare cases of compound heterozygosity have been observed where a pathogenic repeat expansion is found in *trans* with a truncating allele (PMIDs 35883251, 36478048, 36289003). Patients can present with all three cardinal features of CANVAS, however, a majority of patients present with either partial features and/or in combination with additional phenotypes beyond the disease acronym including: chronic cough, autonomic dysfunction, and motor neuropathy (PMID 37674869). The pathogenic mechanism for the disease has been difficult to determine given that transcript levels, splicing, protein expression and DNA repair functionality appear to be unchanged in the context of biallelic repeat expansions (PMIDs 30926972, 32873692, 39231235). The genetic evidence includes both case-level and case-control data in which neurological phenotypes consistent with CANVAS and either biallelic *RFC1* intron 2 repeat expansion or compound heterozygous repeat expansion and null variant(s) are included in this curation. Case-control data from one study was also included in this curation. These probands and case-control data have been reported in six publications (PMIDs 30926972, 32851396, 35883251, 34600502, 31824583, 32873692). More evidence exists in the literature, however the maximum score for genetic evidence was reached. This curation is also supported by experimental evidence indicating pathogenesis may be due to the biallelic *RFC1* repeat expansions and not to loss of function of RFC1 coding product (PMID 39231235). This is supported by the embryonic lethality observed in *RFC1* -/- mouse line and lack of neurological phenotype recapitulation (PMID 11266438). Pathogenic mechanisms in other repeat expansion disorders can include the formation of RNA foci and the accumulation of RAN translation proteins in a cell (PMID 28851463). In CANVAS patient tissue, RNA foci and KGREG repeat protein aggregates were identified by immunohistochemistry studies. However, ectopic expression of AAGGG or CCCTT repeats in mouse brains did not affect cells significantly when compared to a GFP only control. Lastly, patient derived iPSC differentiated neuronal cells showed alterations to synaptic activity and network formation in culture which was rescued by deletion of single repeat expansion but not by overexpression of wildtype RFC1 (PMID 39231235). In summary, *RFC1* pathogenic repeat expansions are definitively associated with autosomal recessive cerebellar ataxia, neuropathy, and/or vestibular areflexia syndrome. This connection has been demonstrated repeatedly in the literature over time without emergence of contradictory evidence, leading to a Definitive classification. This classification was approved by the ClinGen Cerebellar Ataxia GCEP on the meeting date October 9, 2024 (SOP Version 10).