No-Cost Genetic Testing Program for Friedreich Ataxia (FA)

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative condition caused by biallelic pathogenic variants in the FXN gene. The most common pathogenic variant is expansion of a GAA trinucleotide repeat within intron 1 of the FXN gene. An estimated 96% of individuals with Friedreich ataxia have biallelic expansions (i.e., a pathogenic expansion is found in both copies of FXN). The remaining 4% of individuals are typically compound heterozygous for a pathogenic expansion on one allele and a different type of pathogenic variant on the other (Delatycki and Bidichandani. 2019. PubMed ID: 31494282).

FRDA is the most common inherited ataxia, accounting for 50% of all hereditary ataxia cases (Williams et al. 2023. PubMed ID: 33085346). An estimated 1 in 40,000 individuals worldwide have FRDA (Bidichandani et al. 1998. PubMed ID: 20301458). It is estimated that 1 in 100 Europeans is a carrier of an expanded FXN allele (Pandolfo et al. 2008. PubMed ID: 18852343).

Major clinical features of FRDA include progressive ataxia (ataxic gait and limbs), dysarthria, dysphagia, decreased proprioception, distal muscle weakness, peripheral sensory neuropathy, absent lower limb tendon reflexes, spasticity, scoliosis, pes cavus, and hypertrophic cardiomyopathy. For the vast majority of FRDA patients, gait ataxia (caused by spinocerebellar degeneration) is the first presenting clinical feature, which is later followed by slurred speech and upperlimb ataxia as spinocerebellar degeneration progresses (Bidichandani et al. 1998. PubMed ID: 20301458). Less common features found in some individuals with FRDA include diabetes mellitis, lower urinary tract symptoms (urinary frequency and urgency), nystagmus, and optic nerve atrophy

Clinical features of FRDA typically present before the age of 25 (Bidichandani et al. 1998. PubMed ID: 20301458; Cook and Guinti 2017. PubMed ID: 29053830). The age of onset, presence of leg muscle weakness/wasting, duration until wheelchair use, and prevalence of cardiomyopathy, pes cavus, and scoliosis are all inversely correlated with the length of the expanded GAA repeat. In patients with biallelic pathogenic expansions, the size of the smaller of the two expanded alleles has been shown to correlate with age of onset and clinical manifestations more strongly than the larger of the two alleles (Bidichandani et al. 1998. PubMed ID: 20301458; Filla et al. 1996. PubMed ID: 8751856; Schols et al. 1997. PubMed ID: 9448568; La Pean et al. 2008. PubMed ID: 18759347).

Normal FXN alleles contain 5 to 33 GAA repeats. Alleles that have 34 to 43 repeats are known as normal mutable or premutation alleles. Repeats of this size are not associated with a clinical phenotype; however, they have a higher propensity to expand in subsequent generations. Borderline alleles have 44-65 repeats, which are also at risk for expansion in the next generation, and there has been inconclusive evidence about whether these individuals are clinically affected. Alleles with 66 or greater repeats are considered pathogenic; an individual with two pathogenic expansions or compound heterozygous for a pathogenic expansion and another type of pathogenic variant are clinically affected with FRDA.

Given the autosomal recessive nature of FRDA, carriers of pathogenic alleles (one expanded and one normal allele) are clinically not affected but have a 50% risk of passing the pathogenic allele on to their offspring. Individuals who are carriers of premutation or borderline alleles are also at risk for passing on a pathogenic allele to their offspring due to the spontaneous expansion of repeats during meiosis; however, the absolute risk of this occurring has not been well established (Schols et al. 1997. PubMed ID: 9448568; Montermini et al. 1997. PubMed ID: 9259271; Cossee et al. 1997. PubMed ID: 9207112). Of note, GAA repeat alleles including interruptions are much less likely to expand into the pathogenic range compared to GAA repeat alleles that are uninterrupted (Masnovo et al. 2022. PubMed ID: 35952488).

The repeat assay is specifically designed to detect pathogenic expansions of a GAA trinucleotide repeat in intron 1 of the FXN gene but does not assess for repeat interruptions or other potentially pathogenic variants within FXN. Our assay involves two amplicon-length assays from from both the 3’ and 5’ ends of the repeat region to determine the number of repeats, and a 5’ repeat-primed PCR assay with a locus specific primer. Pathogenic expansions (66 or greater repeats) are not able to be sized by amplicon-length analysis but are detected by the repeat primed assay. For this reason, sizing is not available for pathogenic expansions and are reported as “expanded” alleles.

Testing for sequence variants and copy number variants within the FXN gene will be performed via NGS reflexively in an automated fashion only for patients with a single positive GAA repeat expansion, to assess the other allele for a second causative variant.

Candidates for this test are patients in the United States and Puerto Rico with a suspected or clinical diagnosis of FA and are 16 years of age or older.

SPECIMEN REQUIREMENTS

Whole Blood

Collect 3 ml - 5 ml of whole blood in EDTA (purple top tube) or ACD (yellow top tube), minimum 1 ml for small infants.

Saliva

Oragene™ or GeneFiX™ Saliva Collection kit used according to manufacturer instructions. DNA from saliva specimens is invariably contaminated with microbial and food DNA, which can impact specimen quality and may result in delayed testing and/or the need for a second specimen.

OCD-100 Buccal Swab

OCD-100 Buccal Swab used according to manufacturer instructions.

Specimen collection kits: Buccal specimen collection kits, which contain the TRF and the shipping label, may be requested through the kit order form or via the online order form.

SHIPPING AND HANDLING INSTRUCTIONS

Label all specimen containers with the patient's name, date of birth, and/or ID number. At least two identifiers should be listed on specimen containers. Specimen deliveries are accepted Monday-Saturday for all specimen types. Holiday schedules will be posted on our website at least one week prior to major holidays.