From the Department of Neuromuscular Diseases (R.R., R.C., N.D., S.F., Alice Gennari, R.S., S.S., S.N., A.T., A.S., L.E.N.D.F., M.M.R., N.W.W., H.H., A.C.), UCL Queen Square Institute of Neurology, London, United Kingdom; Department of Brain and Behavioral Sciences (R.R., R.C., I.Q., A.C.), University of Pavia, Pavia, Italy; Institute of Molecular Genetics IGM-CNR "Luigi Luca Cavalli-Sforza" (C.P., E.C.), Italy; Department of Neurology (S.N.), University Hospital Basel, University of Basel, Switzerland; IRCCS Mondino Foundation (E.V.), Pavia, Italy; Manchester Centre for Clinical Neurosciences (A.E.-S., T.L., D.G.), Salford Royal Hospital, Northern Care Alliance NHS Foundation Trust, Manchester, United Kingdom; Clinical Pharmacology (A.T.), William Harvey Research Institute, School of Medicine and Dentistry, Queen Mary University of London, United Kingdom; Departamento de Distúrbios do Movimento (L.E.N.D.F.), Hospital Das Clínicas Da Universidade Federal Do Paraná, Curitiba, Brazil; University of Chicago Medical Center (Alexander Gary, M.D., P.K., S.D.), The University of Chicago, IL; Department of Human Genetics (N.N., S.T.), The University of Chicago, IL; Neurogenetics (J.S., J.P.), University College London Hospitals NHS Foundation Trust, National Hospital for Neurology and Neurosurgery, London, United Kingdom; Department of Neurology (C.G.), The University of Chicago, IL; and Ataxia Center (J.Y.H.C., J.D.S.), Laboratory for Neuroanatomy and Cerebellar Neurobiology, Department of Neurology, Massachusetts General Hospital and Harvard Medical School, Boston.
Neurology. 2023 Jan 31;100(5):e543-e554. doi: 10.1212/WNL.0000000000201486. Epub 2022 Oct 26.
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.
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.
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.
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.
小脑性共济失调、感觉神经病、前庭眼反射消失综合征（CANVAS）是一种常染色体隐性神经退行性疾病，其特征为成年起病和进行性缓慢的感觉神经病、小脑功能障碍和前庭功能损害。在大多数情况下，该病是由复制因子复合物亚单位 1（）的第二个内含子中的双等位基因（AAGGG）重复扩展引起的。然而，一小部分具有典型 CANVAS 的病例不携带常见的双等位基因重复扩展。本研究的目的是通过鉴定与该疾病相关的编码区序列变异，扩展 CANVAS 的基因型谱。
15 名被诊断为 CANVAS 且仅携带 1 个杂合（AAGGG）扩展的个体接受了全基因组测序或全外显子组测序，以检测或其他无关基因中是否存在第二个变异。为了评估截断变异对 RFC1 表达的影响，我们测试了患者来源的细胞系中 RFC1 转录本和蛋白的水平。
我们从 5 个无关的家族中发现了 7 名具有临床定义的 CANVAS 患者，他们携带杂合（AAGGG）扩展以及第二个截断变异 ，包括以下内容：c.1267C>T（p.Arg423Ter）、c.1739_1740del（p.Lys580SerfsTer9）、c.2191del（p.Gly731GlufsTer6）和 c.2876del（p.Pro959GlnfsTer24）。含有 c.1267C>T（p.Arg423Ter）或 c.2876del（p.Pro959GlnfsTer24）变异的患者成纤维细胞表现出无意义介导的 mRNA 衰减，以及 RFC1 转录本和蛋白减少。
本报告扩展了 RFC1 疾病的基因型谱。建议对受典型 CANVAS 影响且携带单等位基因（AAGGG）扩展的病例进行全序列测序。此外，它进一步阐明了 RFC1 CANVAS 的发病机制，因为它支持这种复杂神经退行性疾病存在功能丧失机制。
