Phenylketonuria due to a deficiency of dihydropteridine reductase.

The onset of neurologic symptoms in a child who had markedly elevated blood phenylalanine levels during the first two weeks of life and who was promptly treated with a low phenylalanine diet, with excellent control of serum phenylalanine levels, suggested that this child had an unusual form of phenylketonuria. In assays of the components of the phenylalanine hydroxylating system (open liver biopsy at 14 months), the activity of phenylalanine hydroxylase was 20 per cent of the average normal adult value. By contrast, no dihydropteridine reductase activity was detected in the patient's liver, brain or cultured skin fibroblasts. Since dihydropteridine reductase is also essential for the biosynthesis of dopamine, norepinephrine, and serotonin, disturbed neurotransmitter function may be responsible for the patient's neurologic deterioration. On the basis of these results, assay of reductase in cultured skin fibroblasts may be advisable in the initial diagnosis of phenylketonuria.