Phenylalanine hydroxylase deficiency: The inherited inability to process the amino acid phenylalanine normally, due to deficiency of the enzyme phenylalanine hydroxylase. This deficiency is caused by mutation in the PAH gene (on chromosome 12 in band 12q23.2). Hundreds of different disease-causing mutations have been found in the PAH gene. NTBC Phenylalanine hydroxylase deficiency causes a spectrum of disorders including:
In non-PKU HPA, there is elevation of phenylalanine (phe) in the blood but not enough to qualify as PKU or variant PKU. Non-PKU HPA is associated with the least risk of intellectual damage. Variant PKU is intermediate between PKU and non-PKU HPA.
The decision as to whether an individual needs to be on a low-phenylalanine diet is based on their blood level of phenylalanine (phe), rather on the precise condition causing the elevated phe, because phe is toxic to the brain.