GCH1 and Dystonia: In addition to DYT5, the enzymatic defects in the dopamine biosynthetic pathway also cause dystonia [i.e., 6-pyruvoyltetrahydropterin synthase, sepiapterin reductase, dihydropteridine reductase, tyrosine hydroxylase (TH), and aromatic L-amino acid decarboxylase (AADC)] (10).