The lack of significance for 5-HTTLPR and COMT polymorphisms in conferring liability to TS does not exclude a role of different functional polymorphisms in genes coding for serotonergic or dopaminergic structures in the etiology of TS.
As neuroimaging studies allude to dopaminergic and serotonergic dysfunction in GTS and serotonin as an important factor for dopamine release, genotyping of common polymorphisms in the serotonergic receptor (HTR1A: C-1019G; HTR2A: T102C, His452Tyr, A-1438G; HTR2C: C-759T, G-697C) and transporter genes (SLC6A4) was carried out in 87 patients with GTS, compared with 311 matched controls.
Likewise, the higher expressing LAC haplotype (5-HTTLPR/rs25531/rs25532) was more frequent in TD probands than in controls (P = 0.024; OR, 1.33) and also in the TD alone group versus the TD plus OCD group (P = 0.0013; OR, 2.14).