Tourette syndrome has long been conceptualized as part of a spectrum of developmental tic disorders, with transient tics at one end (1), and severe Tourette syndrome with multiple psychiatric comorbidities at the other. However, until recently, potential biological relationships between the various tic disorders were unknown, as were the underlying genetic contributions to tic severity. The results of this study further illuminate the genetic architecture of Tourette syndrome and its relationships to phenotypic expression. First, the PRS analyses probing the genetic architecture of tic disorders in the population-based Icelandic sample demonstrates that individuals with Tourette syndrome share the same underlying polygenic risk as those with other tic disorders. Furthermore, the observation that Tourette syndrome cases have a significantly higher mean PRS than those with non-Tourette tic disorders provides evidence for a liability spectrum of genetic risk within tic disorders. Lastly, within Tourette syndrome cases, the finding that higher Tourette PRS was associated with increased tic severity also builds on our previous analyses demonstrating a relationship between higher Tourette PRS and the presence of complex symmetry and socially inappropriate tics (29).
