Targeted Long-Read Sequencing as a Single Assay Improves the Diagnosis of Spastic-Ataxia Disorders.

Objective: The hereditary spastic-ataxia spectrum disorders are a group of disabling neurological diseases. The traditional genetic testing pathway is complex, multistep and leaves many cases unsolved. We aim to streamline and improve this process using long-read sequencing.

Methods: We developed a targeted long-read sequencing strategy with the capacity to characterise the genetic variation of all types and sizes within 469 disease-associated genes, in a single assay. We applied this to a cohort of 34 individuals with unsolved spastic-ataxia. An additional five individuals with a known genetic diagnosis were included as positive controls.

Results: We identified causative pathogenic variants that would be sufficient for genetic diagnosis in 14/34 (41%) unsolved participants. The success rate was 5/11 (45%) in those who were naïve to genetic testing and 9/23 (39%) in those who were undiagnosed after prior genetic testing, completed on a clinical basis. Short tandem repeat expansions in FGF14 were the most common (7/34, 21%). Two individuals (2/34, 6%) had biallelic pathogenic expansions in RFC1 and one individual had a monoallelic pathogenic expansion in ATXN8OS/ATXN8. Causative pathogenic sequence variants other than short tandem repeat expansions were found in four individuals, including in VCP, STUB1, ANO10 and SPG7. Furthermore, all five positive controls were identified.

Interpretation: Our results demonstrate the utility of targeted long-read sequencing in the genetic evaluation of patients with spastic-ataxia spectrum disorders, highlighting both the capacity to increase overall diagnostic yield and to streamline the testing pathway by capturing all known genetic causes in a single assay.