Related to New Topics Plasma Phenylalanine Level in Dopa-Responsive Dystonia

DYT5 is a condition characterized by levodopa (L-dopa) responsive dystonia (DRD) that shows childhood onset and marked diurnal fluctuation. Patients with DYT5 have heterozygous mutations in the GCH1 gene, which codes for GTP cyclohydrolase I (GTPCH), a rate-limiting enzyme in tetrahydrobiopterin (BH4) synthesis. 1 BH4 is a cofactor for aromatic amino acid hydroxylases including tyrosine hydroxylase (TH), phenylalanine hydroxylase (PAH), and tryptophan hydroxylase. In patients with DYT5, production of dopamine is suppressed due to the decrease of BH4 because TH is a rate-limiting enzyme in dopamine synthesis. The lack of BH4 may also affect the activity of PAH, and patients with complete GTPCH deficiency show hyperphenylalaninemia. However, hyperphenylalaninemia has not been reported in patients with DYT5. Therefore, we examined blood phenylalanine levels in patients with DYT5 compared with controls to determine whether the decrease of BH4 in DYT5 affects PAH activity. Blood samples for analysis of amino acids were obtained from seven patients with DRD and heterozygous mutations of GCH1, 24 patients with dopa nonresponsive dystonia (non-DRD), and 12 controls. The samples were collected in a tube containing EDTA, and plasma was obtained for analysis of phenylalanine and tyrosine levels using an auto-amino acid analyzer (HS-3000; Hitachi, Tokyo, Japan). All data are expressed as means 6 SD. The data were analyzed by analysis of variance (ANOVA) with multiple comparison using the Bonferroni method. A significant difference was defined as a P value< 0.05. The phenylalanine and tyrosine levels in the plasma of patients with DYT5, patients with non-DRD, and controls are shown in Figure 1. The phenylalanine levels in the DYT5 (81.1 6 26.6 lmol/L), non-DRD (60.5 6 14.5 lmol/L), and control (58.7 6 9.1 lmol/L) groups were all within the normal range. However, the phenylalanine level in patients with DYT5 was significantly higher than those in the other groups (P 5 0.027 by ANOVA). Multiple comparison also indicated that the level of plasma phenylalanine in patients with DYT5 was significantly higher than those in patients with non-DRD (P 5 0.043) and in controls (P 5 0.040). There was no significant difference in the plasma phenylalanine levels between patients with non-DRD and controls. There were no significant differences in plasma tyrosine levels among all the groups. BH4 deficiency causes hyperphenylalaninemia and a decrease of dopamine production, because it suppresses the activity of PAH and TH. Patients with a homozygous mutation in the GCH1 gene are reported to show hyperphenylalaninemia, in addition to DRD. Patients with DYT5 having only a heterozygous mutation in GCH1 also show symptoms of DRD, but do not have hyperphenylalaninemia. Our results indicated that blood phenylalanine levels in patients with DYT5 are within the normal range, but are higher than those in controls, which suggests that the activity of PAH is partially affected by the decrease in BH4 in DYT5. GTPCH is regulated by BH4 itself and phenylalanine via GTPCH feedback regulatory protein (GFRP). An increase of phenylalanine induces GFRP to upregulate GTPCH activity, whereas an increase of BH4 downregulates GTPCH activity via GFRP. Hyland et al. has reported prolonged hyperphenylalaninemia after oral phenylalanine loading in patients with DRD, which suggests that decreased BH4 production due to mutations in GTPCH restrict the catalysis of excessive phenylalanine by PAH. However, a defect in GFRP or in the interaction between GTPCH and GFRP would cause the same results. Our results indicate that the phenylalanine level in patients with DYT5 differs from those in controls without phenylalanine loading, which suggests that a partial defect of GTPCH affects the activity of PAH.