New variant of phenylketonuria with progressive neurological illness unresponsive to phenylalanine restriction.

Three children, two of them siblings, with an unusual type of phenylketonuria are described. The three patients, two of them observed from the neonatal period, had a progressive neurological illness which was unlike that of classical phenylketonuria, and which did not respond to a low phenylalanine diet. The biochemical features suggested that the block in the conversion of phenylalanine to tyrosine was less severe than in the classical disease, and phenylalanine rho-hydroxylase activity, measured in one patient was normal. It is suggested that the patients have a disorder of biopterin metabolism possibly due to a defect of the enzyme dihydropteridine reductase.