The spectrum of phenylalanine variations under tetrahydrobiopterin load in subjects affected by phenylalanine hydroxylase deficiency

A fall in blood phenylalanine (Phe) after tetrahydrobiopterin (BH4) administration is a common trait in phenylalanine hydroxylase (PAH, EC 1.14.16.1) deficiency (McKusick 261600). To explore the extent and biological correlates of this phenomenon we studied: (a) the spectrum of BH4 response in patients with PAH deficiency; (b) the variability of BH4 response according to the severity of the biochemical phenotype; and (c) the variability of the response to BH4 in subjects with the same genotype. Fifty PAH-deficient subjects (age 1 month-35 years) were enrolled for the study (5 with mild hyperphenylalaninaemia (MHPHE), 15 with mild phenylketonuria (MPKU) and 30 with classic phenylketonuria (CPKU) and underwent an identical schedule of blood samplings 24 h before and after oral BH4 challenge (6(R)-BH4, 20 mg/kg per day), leaving Phe intake unchanged. The effect of BH4 on blood Phe concentration was evaluated according to the percent decrease of Phe during the 24 h following the challenge (criterion a), and as variation exceeding the individual variability of blood Phe (criterion b). The number of BH4-responders according to criterion b was 31 (including all the 14 detected by criterion a): 17 out of 30 CPKU (57%), 9 out of 15 MPKU (60%), and all the MHPHE subjects (chi(2) = 3.45, df = 2, p = 0.178). The effect of BH4 showed a large interindividual variability unrelated to diagnostic classification, basal value of blood Phe, maximum percentage of Phe reduction, Phe intake, and genotype. Some inconsistencies were found in patients with identical genotype. The first responsive case homozygous for the severe R408W mutation was found. Two new mutations, Y387X and G352C, were identified (the former was BH4-responsive), and the responsiveness of three already reported mutations (R261Q, D338Y, T92I) was substantiated.