Hyperphenylalaninemia due to impaired dihydrobiopterin biosynthesis

A fourteen month-old boy with atypical phenylketonuria was treated with 5-hydroxytryptophan, L-dopa and peripheral aromatic amino acid decarboxylase inhibitor (Ro 4-4602:benserazide). Despite the good control of plasma phenylalanine on a low phenylalanine diet, he had shown no improvement in his development but progressive neurological symptoms, such asiirritability, convulsions and decrease voluntary movement. After beginning neurotransmitter therapy, his irritability disappeared promptly and the other symptoms diminished. He gradually reached his developmental milestones. At two and a half years of age, he had recovered sufficiently to be able to walk freely on treatment with 13 mg/kg/day of 5-hydroxytryptophan, 11 mg/kg/day of L-dopa and 2.7 mg/kg/day of benserazide in combination with slight restriction of phenylalanine intake (100 mg/kg/day). Levels of serotonin and 5-hydroxyindoleacetic acid were low in the patient's CSF. His urinary biopterin (Crithidia factor) excretion was low. An increase in serum biopterin following L-phenylalanine loading was not found. Dihydropteridine reductase activity in his skin fibroblasts was normal. He excreated large amounts of erythro- and threo-neopterins (but only a trace of biopterin) in his urine. After loading with phenylalanine the urinary excretion of neopterins was even more enhanced, but biopterin remained at low levels. These findings indicated that the patient has a dihydrobiopterin synthetase deficiency.