Background-(6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) is a cofactor for enzymes involved in catecholamine and nitric oxide generation whose synthesis is initiated by GTP cyclohydrolase I (GTPCH-1), encoded by GCH1. In the absence of a potent, specific GTPCH-1 inhibitor, natural BH4 deficiency caused by mutations in GCH1 in the rare movement disorder, DOPA-responsive dystonia (OMIM DYT5), offers the opportunity to study the role of endogenous BH4 in humans. Methods and Results-In 16 DOPA-responsive dystonia patients with mutations predicted to affect GTPCH-1 expression or function and in age-and sex-matched control subjects, we measured plasma biopterin and nitrogen oxides by highperformance liquid chromatography and the Griess reaction, respectively, endothelial function by brachial artery flowmediated dilation (FMD), sympathetic function by measurement of plasma norepinephrine, epinephrine, and heart rate and blood pressure in response. Cardiac function and structure were assessed by echocardiography. 9%, Pϭ0.91). In patients but not control subjects, FMD was insensitive to nitric oxide synthase inhibition (FMD at baseline, 6.7Ϯ2.1%; FMD during L-NMMA infusion, 6.2Ϯ2.5, Pϭ0.68). The heart rate at rest was higher in patients, but the heart rate and blood pressure response to sympathetic stimulation did not differ in patients and control subjects despite lower concentrations of norepinepherine (264Ϯ8 pg/mL versus 226Ϯ9 pg/mL, Pϭ0.006) and epinephrine (33.8Ϯ5.2 pg/mL versus 17.8Ϯ4.6 pg/mL, Pϭ0.03) in patients. There was also no difference in cardiac function and structure. Conclusions-Sympathetic, cardiac, and endothelial functions are preserved in patients with GCH1 mutations despite a neurological phenotype, reduced plasma biopterin, and norepinepherine and epinephrine concentrations. Lifelong endogenous BH4 deficiency may elicit developmental adaptation through mechanisms that are inaccessible during acquired BH4 deficiency in adulthood. (Circ Cardiovasc Genet. 2010;3:513-522.)Key Words: endothelium Ⅲ sympathetic function Ⅲ tetrahydrobiopterin Ⅲ DOPA-responsive dystonia T etrahydrobiopterin (BH4), synthesized from GTP by the rate-limiting enzyme GTP cyclohydrolase I (GTPCH-1) and encoded by the GCH1 gene, is an essential cofactor for the aromatic amino acid hydroxylases (phenylalanine, tyrosine, and tryptophan hydroxylase), which are responsible for the synthesis of the central and sympathetic neurotransmitters dopamine (DA), norepinephrine (NE), epinephrine, and 5-hydroxytryptamine (5HT), as well as the nitric oxide synthases (eNOS, iNOS, and nNOS). The Michaelis constant (K m ) for BH4 of the aromatic amino acid hydroxylases (100 mol/L) 1 is higher than that for the NOSs (100 nmol/ L), 2 suggesting that they may be more sensitive to the effect of inherited or acquired BH4 deficiency. Clinical Perspective on p 522In the absence of a specific, potent inhibitor of GTPCH-1 for use in humans, it has been difficult to evaluate the precise physiological role of BH4 in sympathetic, endothelial, and cardiac functions....