Role of silent polymorphisms within the dopamine D1 receptor associated with schizophrenia on D1–D2 receptor hetero-dimerization

Within the coding region of the dopamine D1 receptor (D1R), two synonymous polymorphisms, D1RG198A and D1RG1263A, have been identified and postulated to correlate with the schizophrenia phenotype. Binding studies revealed that the density of these genetic variants was much lower than the density of wild type D1R in the human embryonic kidney (HEK) 293 cell line, used as a model system. From the data obtained using MFOLD software it is apparent that the G198A mutation has a greater impact on the secondary structure of the mRNA, which may affect its translation. However, the G1263A mutation is localized within the serine 421 codon of D1R, which is predicted to be a potential site of phosphorylation according to the PHOSIDA database.In order to determine whether the studied synonymous polymorphisms influence the process of dopamine D1–D2 receptors hetero-dimerization, we employed fluorescence resonance energy transfer (FRET) technology. The dopamine D2 receptor (D2R) was tagged with cyan fluorescence protein and the D1R and its genetic variants were tagged with yellow fluorescence protein. The degree of D1–D2 receptor hetero-dimerization was significantly decreased when genetic variants of D1R were co-expressed with D2R. Since the D1R mutations affected the expression levels of the proteins in the cell membrane without affecting the cellular localization of the receptor proteins, we postulated that the D1R polymorphisms altered the translation rate and protein structure of the receptor. The altered hetero-dimerization that likely results from the lower expression of these genetic variants of D1R with D2R may be partially responsible for the association of both G198A and G1263A polymorphisms with the schizophrenia phenotype.