Objective: Abnormalities in neurotransmission within the cortico-striatal-thalamo-cortical circuitry are implicated in the pathogenesis of Tourette syndrome. Glutamate is a major excitatory neurotransmitter and an important member in the cortico-striatal-thalamo-cortical circuitry. To explore the role of glutamatergic neurotransmission in genetic susceptibility of Tourette syndrome, we carried out the genetic and functional characterization of sequence variants in SLC1A3 gene, which encodes the main glutamate transporter in astrocytes in individuals with well-characterized Tourette syndrome (n=256) and normal controls (n=224).
Methods: Exon-containing regions of SLC1A3 gene were screened using capillary electrophoresis-single strand conformation polymorphism followed by direct sequencing. Sequence variants were genotyped by restriction enzyme digestion and studied using glutamate uptake assay and membrane protein pull-down for transporter function.
Results: A missense variant involving a highly conserved residue, E219D, was identified in 11 heterozygous individuals with Tourette syndrome and four in the controls. The allele frequency for E219D was 2.4 folds higher in the Tourette syndrome (0.022) compared with the control cohort (0.009) although the difference did not reach statistical significance in the current cohorts (P=0.09). A 3H-glutamate-uptake assay showed that E219D conveys a significant increase (1.66 fold) in the SLC1A3-mediated glutamate uptake in HEK293 cells. A biotin-mediated membrane pull-down analysis showed a similar increase (1.5 fold) of mutant SLC1A3 protein in the membrane fraction of transfected HEK293 cells compared with that in the wild type controls.
Conclusion: These results indicate that E219D is a functional SLC1A3 variant that is presented in a small number of individuals with Tourette syndrome. Further studies on possible changes in glutamate transport in the pathogenesis of Tourette syndrome are warranted.