Clinical exome sequencing in early-onset generalized dystonia and large-scale resequencing follow-up

Background: Dystonia is clinically and genetically heterogeneous. Despite being a first-line testing tool for heterogeneous inherited disorders, whole-exome sequencing has not yet been evaluated in dystonia diagnostics. We set up a pilot study to address the yield of whole-exome sequencing for early-onset generalized dystonia, a disease subtype enriched for monogenic causation. Methods: Clinical whole-exome sequencing coupled with bioinformatics analysis and detailed phenotyping of mutation carriers was performed on 16 consecutive cases with genetically undefined early-onset generalized dystonia. Candidate pathogenic variants were validated and tested for cosegregation. The whole-exome approach was complemented by analyzing 2 mutated yet unestablished causative genes in another 590 dystonia cases. Results: Whole-exome sequencing detected clinically relevant mutations of known dystonia-related genes in 6 generalized dystonia cases (37.5%), among whom 3 had novel variants. Reflecting locus heterogeneity, identified unique variants were distributed over 5 genes (GCH1, THAP1, TOR1A, ANO3, ADCY5), of which only 1 (ANO3) was mutated recurrently. Three genes (GCH1, THAP1, TOR1A) were associated with isolated generalized dystonia, whereas 2 (ANO3, ADCY5) gave rise to combined dystonia-myoclonus phenotypes. Follow-up screening of ANO3 and ADCY5 revealed a set of distinct variants of interest, the pathogenicity of which was supported by bioinformatics testing and cosegregation work. Conclusions: Our study identified whole-exome sequencing as an effective strategy for molecular diagnosis of early-onset generalized dystonia and offers insights into the heterogeneous genetic architecture of this condition. Furthermore, it provides confirmatory evidence for a dystonia-relevant role of ANO3 and ADCY5, both of which likely associate with a broader spectrum of dystonic expressions than previously thought.