Neonatal hyperphenylalaninaemia presumably caused by a new variant of biopterin synthetase deficiency.

Systematic investigation of hyperphenylalaninaemic infants for tetrahydrobiopterin deficiency has recently led to the description of new variants of cofactor deficiency. In the present case, the initial observation was of hyperphenylalaninaemia with a significant increase in the neopterin to biopterin ratio in the urine. A tetrahydrobiopterin loading test resulted in a significant decrease of blood phenylalanine levels. Cerebrospinal fluid (CSF) biopterin and neurotransmitter metabolite levels were within the normal range. The in vivo clearance of phenylalanine remained altered despite a high dietary tolerance. At 9 months of age, the patient was clinically well, but minor neurological signs appeared when blood phenylalanine levels increased. These data were similar to those found in the so-called "peripheral form" of tetrahydrobiopterin deficiency. However, an unidentified pteridine-like compound had been found in the urine and CSF since the birth, suggesting the existence of an unknown block in the biosynthetic pathway of biopterin.