CANVAS is inherited in an autosomal recessive manner. This means that individuals of both sexes have the same probability of inheriting the mutated gene. A person will only develop symptoms of the disease if they inherit two mutant copies (alleles) of the RFC1 gene — one from the biological father and one from the mother. Those who inherit only one mutant copy will be asymptomatic carriers. In these cases, the 'normal' copy of the gene is sufficient to maintain function, preventing the appearance of symptoms. In individuals who inherit two pathogenic copies of the RFC1 gene, however, the disease manifests, provided that the expansion falls within the range considered pathogenic (as explained below).

 

Ranges of AAGGG expansion for CANVAS diagnostic confirmation

Different types of pentanucleotide repeats may occur in intron 2 of the RFC1 gene (AAAAG, AAAGG, AAGGG, ACAGG, etc.), but so far, the most clearly pathogenic form is the expanded AAGGG [2]:

• Normal range (not disease-causing):

  • AAAAG repeats: usually between 11 and ~200 repeats (stable, not disease-causing).

  • AAAGG repeats: may range from 40 up to >1000 repeats, also non-pathogenic.

• Pathogenic range:

  • Expanded AAGGG: considered pathogenic with 400 or more repeats.

It should be emphasized that the mutation must be biallelic to cause the disease — that is, the individual must inherit one RFC1 allele in the pathogenic range from the father and another from the mother in order to develop CANVAS symptoms.

 

Note: Other pentanucleotide repeat combinations have been reported in certain populations and may be pathogenic when present in the biallelic state, such as ACAGG (Japan) and AAAGG/AAGGG (New Zealand Māori).

 

Probability of children inheriting the disease

Statistically, if both parents are carriers:

• The probability that each child will develop the disease (= inherit one mutant allele from the mother and one from the father) is 25%.

• The probability that each child will not develop the disease but will also be a carrier (= inherit only one mutant allele from either parent) is 50%.

• The probability that each child will inherit no mutant allele (neither from the father nor the mother) — and thus will neither develop the disease nor be an asymptomatic carrier — is 25%.

 

The diagnosis of CANVAS requires specialized molecular genetic testing

A definitive diagnosis of CANVAS requires specialized molecular genetic testing. Since the causative mutation is a repeat expansion located in an intronic (non-coding) region of the RFC1 gene, it is not detected by conventional techniques such as NGS panels or clinical exome sequencing, which analyze only the exons (coding regions). To confirm the presence of the pathogenic expansion, specific methods are used, such as:

• Repeat-primed PCR (RP-PCR), in combination with flanking PCR, which enables identification of the repetitive pentanucleotide (AAGGG);

• Southern blot or long-read sequencing technologies (such as PacBio or Oxford Nanopore), which allow more accurate measurement of the expansion size.

Thus, exome sequencing alone is not sufficient to confirm the diagnosis of CANVAS, and these complementary approaches are required to detect and assess the size of the intronic expansion in RFC1.

 

Genetic counseling

Given the autosomal recessive inheritance of CANVAS, once a pathogenic variant in the RFC1 gene has been identified in a family member, genetic counseling is strongly recommended. It should be offered to adults who wish to have children as well as to first-degree relatives of affected individuals, in order to:

• Assess the likelihood of being asymptomatic carriers;

• Clarify the risks of transmission to offspring (25% chance of an affected child when both parents are carriers);

• Discuss reproductive planning strategies and available testing options.

 

Note: "Autosomal" means the gene is located on any chromosome except the X and Y sex chromosomes. Genes, like chromosomes, typically exist in pairs (we have a pair of each gene; one copy of the gene is inherited from the mother, the other from the father). Thus, men and women are equally likely to inherit a mutated gene that can cause hereditary ataxia. "Recessive" means that two mutated copies (alleles) of the gene must be inherited (one from the biological father, the other from the mother) for a person to develop the disease.

Figure 3 - Source: MedlinePlus, US National Library of Medicine .