DDC and Parkinson disease: Gene therapy for PD offers a promising strategy (Choong and Mochizuki 2017), including delivery of genes encoding proteins that can protect dopaminergic neurons from damage, such as neurotropic factors, anti-apoptotic and anti-free radical proteins (Lin et al. 2017; Valdés and Schneider 2016; Nam et al. 2021), and delivery of genes encoding enzymes involving in dopamine synthesis, for example, aromatic L-amino acid decarboxylase (AADC), tyrosine hydroxylase (TH), GTP cyclohydrolase (GCH) (Jamebozorgi et al. 2019; Lin et al. 2017; Christine et al. 2019; Ciesielska et al. 2017).