In a sample of 811 TS subjects recruited for a genetics study, hair pulling (3.8%) and skin picking (13.0%) disorders by DSM-5 were surprisingly common \citep{29098466}.
Etiology
A national database study found that parents of children with chronic tics had significantly higher rates of psychiatric illness: more than twice as high in mothers and 39% higher in fathers \citep{28335873}. The maternal risk, which included a range of psychopathology, was significantly higher than the paternal risk, which comprised primarily OCD and anxiety disorders. Further work will be needed to clarify whether results reflect maternal-specific environmental risks, genetic risks, factors related to parental care-seeking, or (to a modest extent, given the typical ages of onset for various parental disorders identified) parental stress.
Genetics
A large mixed genetic sample yielded two heritable collections of symptoms that cross diagnostic boundaries, here named symmetry (including some other obsessions and compulsions) and disinhibition (including complex verbal tics) \citep{27809572}. Whole exome sequencing from over 500 trios identified a clear excess of likely gene-disrupting de novo mutations, and 4 risk genes that were altered by different mutations in multiple probands \citep{Willsey2017}. Another report described whole exome sequencing in a 3-generation family with TS, using induced pluripotent stem cells converted into neuronal cell types and assessing protein expression levels \citep{28894297}. The PNKD gene product was expressed at a lower rate in family members with TS or OCD.
Epigenetics
Environmental risk factors