BDNF and Parkinson disease: COMT: catechol-O-methyltransferase; Met: Methionine; MTHFR: methylenetetrahydrofolate reductase; A allele: adenine allele; CA: cytosine-adenine; MLIC: motor levodopa-induced complications; PD: Parkinson's disease; AG: adenine-guanine; GG: guanine-guanine; AA: adenine-adenine; LID: levodopa-induced dyskinesia; BDNF: brain-derived neurotrophic factor; DAT: dopamine transporter; Val: Valine; ECT: electroconvulsive therapy; tDCS: transcranial direct current stimulation; MMT: methadone maintenance treatment; TC: thymine-cytosine; CC: cytosine-cytosine