The purpose of this study is to identify the dose of Kuvan needed to normalize the levels of tetrahydrobiopterin (BH4) in patients with a Guanosine Triphosphate Cyclohydrolase (GTPCH) deficiency and to asses the potential impact of oral Kuvan on mood and function in individuals with GTPCH deficiency who may be experiencing symptoms of anxiety, depression, fatigue, trouble concentrating, or memory loss. This will be a phase one study. The investigators will monitor patients over a period of three to six months while on the medication. Medication levels will be monitored by measuring the BH4 levels in cerebral spinal fluid (CSF). Patients will undergo a baseline lumbar puncture and two follow-up lumbar punctures for this purpose. The investigators will also monitor emotional function and motor function.

Inclusion Criteria:

• established diagnosis of GTPCH deficiency, supported by appropriate family history, CSF neurotransmitter studies, skin fibroblast enzyme assay and/or mutation analysis
• minimum age 18 years
• identified by self or others to have symptoms of anxiety, depression, fatigue, or other neurocognitive dysfunction (trouble concentrating, memory loss, etc)
• willingness to undergo at least 2 CSF evaluations for BH4 and neurotransmitter levels over an 8 to 12 week period

Exclusion Criteria:

• age < 18 years old
• unwillingness to undergo repeated CSF analysis
• lack of supporting diagnostic criteria
• concomitant medical problems or medications which would increase risk of Kuvan®
• concomitant psychiatric state, such as severe depression with suicidal ideation that requires immediate referral and alternative treatment intervention
• prior history of back surgery, abnormality or chronic pain that in the opinion of the investigator would increase risks associated with lumbar puncture
• significant obesity that might increase difficulty or risk in performing lumbar puncture
• if female, unwillingness to use birth control during the period of study drug administration

Per protocol, adverse event data was collected from Baseline visit to 24 week Extension Visit

Any untoward or unfavorable medical occurrence in a human subject, including any abnormal sign (for example, abnormal physical exam or laboratory finding), symptom, or disease, temporally associated with the subject's participation in the research, whether or not considered related to the subject's participation in the research