PKU and NON-PKU Hyperphenylalaninemia: Differentiation, Indication for Therapy and Therapeutic Results

The differential diagnosis of phenylalanine hydroxylase deficiency (PAHD) by biochemical methods is difficult. Using standardized oral protein loading or the intravenous deuterated phenylalanine (phe) load in 46 patients with PAHD, three groups could be distinguished: 1) Phenylketonuria (PKU) with plasma phe levels over 20 mg% under the protein load and with residual activities of less than 1% of normal; 2) mild PKU with plasma phe of 10–20 mg% and residual enzyme activities of 1–3%; 3) Non-PKU hyperphenylalaninemia with plasma phe levels of less than 10 mg% and residual enzyme activities of more than 3%. Psychomotor development in 32 untreated patients with PAHD showed that there is a high risk of brain damage for all patients with in vivo residual activities of less than 3% of normal. Restriction fragment length polymorphism (RFLP) haplotypes at the phenylalanine hydroxylase (PAH) locus in 60 German patients with PAHD showed that 90% of the mutant alleles are confined to four distinct haplotypes. Using an oligonucleotide probe for the splicing mutation associated with mutant haplotype 3 a close association between the mutation and the haplotype 3 has been observed. There is also an association between haplotypes 2 and 3 and PKU patients with residual enzyme activities of less than 1%. However, in only 37% of our patients with PAHD could a direct diagnosis of the mutations in the PAH gene be made. More knowledge of other mutations in the PAH gene is necessary to differentiate patients with PAHD on the DNA level.