Truncating Variants in in Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome.

Background And Objective: Cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS) is an autosomal recessive neurodegenerative disease characterized by adult-onset and slowly progressive sensory neuropathy, cerebellar dysfunction, and vestibular impairment. In most cases, the disease is caused by biallelic (AAGGG) repeat expansions in the second intron of the replication factor complex subunit 1 (). However, a small number of cases with typical CANVAS do not carry the common biallelic repeat expansion. The objective of this study was to expand the genotypic spectrum of CANVAS by identifying sequence variants in -coding region associated with this condition.

Methods: Fifteen individuals diagnosed with CANVAS and carrying only 1 heterozygous (AAGGG) expansion in underwent whole-genome sequencing or whole-exome sequencing to test for the presence of a second variant in or other unrelated gene. To assess the effect of truncating variants on expression, we tested the level of RFC1 transcript and protein on patients' derived cell lines.

Results: We identified 7 patients from 5 unrelated families with clinically defined CANVAS carrying a heterozygous (AAGGG) expansion together with a second truncating variant in , which included the following: c.1267C>T (p.Arg423Ter), c.1739_1740del (p.Lys580SerfsTer9), c.2191del (p.Gly731GlufsTer6), and c.2876del (p.Pro959GlnfsTer24). Patient fibroblasts containing the c.1267C>T (p.Arg423Ter) or c.2876del (p.Pro959GlnfsTer24) variants demonstrated nonsense-mediated mRNA decay and reduced RFC1 transcript and protein.

Discussion: Our report expands the genotype spectrum of RFC1 disease. Full sequencing is recommended in cases affected by typical CANVAS and carrying monoallelic (AAGGG) expansions. In addition, it sheds further light on the pathogenesis of RFC1 CANVAS because it supports the existence of a loss-of-function mechanism underlying this complex neurodegenerative condition.