Genotype-predicted tetrahydrobiopterin (BH4)-responsiveness and molecular genetics in Croatian patients with phenylalanine hydroxylase (PAH) deficiency

a b s t r a c t Specific mutations in the gene encoding phenylalanine hydroxylase (PAH), located on chromosome 12q22-24.1, are linked to tetrahydrobiopterin (BH 4 ; sapropterin)-responsive phenylketonuria (PKU). Diagnosis is usually done through the newborn screening for PKU, followed by a BH 4 loading test. So far, more than 60 mutant alleles, presenting with a substantial residual PAH activity (average $47%), were identified in more than 500 patients worldwide. We investigated the predictive value of BH 4 -responsive PAH mutations in Croatian population. From a group of 127 PKU patients, 62 were selected (based on the genotype) as potentially BH 4 -responsive and 39 loaded with BH 4 (20 mg/kg). The overall frequency of BH 4 -responsiveness (>30% blood phenylalanine reduction within 24 h) was 36% (14 out of 39 patients with 23 different genotypes), significantly less than expected. The best responders were patients with mild hyperphenylalaninemia (4/4; 100%), followed by mild PKU (8/9; 89%), and classical PKU (2/26; 8%). The most common BH 4 -responsive genotypes were p.E390G/p.R408W and p.P281L/ p.E390G. These genotypes correspond for approximately >30% residual PAH activity. The p.E390G mutation was 100% associated with BH 4 -responsiveness, regardless of the second allele (p.R408W, p.P281L, p.F55Lfs, p.L249P). With regard to the predicted relative PAH activity of recombinantly expressed mutant alleles, there was a significant (p < 0.002) difference between BH 4 -responders and non-responders. In a general Croatian PKU population, disease-causing mutations were identified on 226 alleles (99%). There were 35 different mutations: 21 missense, 8 splice site, 3 nonsense, 2 single nucleotide deletions