To assess the diagnostic utility of whole exome sequencing (WES) for cerebellar ataxia and related neurodegenerative disorders given current advances in technical and bioinformatic methodologies.

Genetic ataxias are associated with mutations in hundreds of genes with high phenotypic overlap complicating clinical diagnosis. Whole exome sequencing has increased the overall diagnostic rate considerably across all patients. However, the upper limit of this method remains ill-defined, hindering efforts to address the remaining diagnostic gap.

To further assess the role of rare coding variation in ataxic disorders, we reanalyzed our previously published exome cohort of 76 predominantly adult and sporadic onset patients, expanded the total number of cases to 260, and introduced analyses for copy number variation and repeat expansion in a representative subset.

For new cases (n=184), our resulting clinically relevant detection rate remained stable at 47% with 24% classified as pathogenic. Reanalysis of the previously sequenced 76 patients modestly improved the pathogenic rate by 7%. For the combined cohort (n=260), the total observed clinical detection rate was 52% with 25% classified as pathogenic. Published studies of similar neurological phenotypes report comparable rates.

The consistency in diagnostic rates across multiple cohorts suggests that, given our current knowledge of molecular pathogenesis, despite continued technical and analytical advancements, an ~50% diagnostic rate likely marks a relative ceiling for current WES-based methods and a more comprehensive genome-wide assessment will be needed to identify the missing causative genetic etiologies for cerebellar ataxia and related neurodegenerative diseases.