SLC6A3 and Parkinson disease: COMT: catechol-O-methyltransferase; DAT: dopamine transporter; DRD: dopamine receptor D; MAO: monoamine oxidase; mRNA: messenger ribonucleic acid; PD: Parkinson’s disease; SLC: solute carrier; SV2C: synaptic vesicle protein 2C; VNTR: variable number of tandem repeats