Epigenetic abnormalities associated with a chromosome 18(q21-q22) inversion and a Gilles de la Tourette syndrome phenotype

Gilles de la Tourette syndrome (GTS) is a potentially debilitating neuropsychiatric disorder defined by the presence of both vocal and motor tics. Despite evidence that this and a related phenotypic spectrum, including chronic tics (CT) and Obsessive Compulsive Disorder (OCD), are genetically mediated, no gene involved in disease etiology has been identified. Chromosomal abnormalities have long been proposed to play a causative role in isolated cases of GTS spectrum phenomena, but confirmation of this hypothesis has yet to be forthcoming. We describe an i(18q2l.l-q22.2) inversion in a patient with CT and OCD. We have fine mapped the telomeric aspect of the rearrangement to within 1 Mb of a previously reported 18q22 breakpoint that cosegregated in a family with GTS and related phenotypes. A comprehensive characterization of this genomic interval led to the identification of two transcripts, neither of which was found to be structurally disrupted. Analysis of the epigenetic characteristics of the region demonstrated a significant increase in replication asynchrony in the patient compared to controls, with the inverted chromosome showing delayed replication timing across at least a 500-kb interval. These findings are consistent with long-range functional dysregulation of one or more genes in the region. Our data support a link between chromosomal aberrations and epigenetic mechanisms in GTS and suggest that the study of the functional consequences of balanced chromosomal rearrangements is warranted in patients with phenotypes of interest, irrespective of the findings regarding structurally disrupted transcripts.