Study of Kuvan Treatment in Adults With GTPCH Deficiency

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