Phenylketonuria (PKU) is a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene, which helps create the enzyme needed to break down phenylalanine. In a person with PKU, a change in the PAH gene causes a lack of or reduced amount of the enzyme thats needed to process phenylalanine, an amino acid. A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts or meat, or grains such as bread and pasta, or aspartame, an artificial sweetener.
PKU has different forms, including classic PKU, which is the most severe form of the disorder, and less severe forms such as variant PKU and non-PKU hyperphenylalaninemia. In classic PKU, the enzyme needed to break down phenylalanine is missing or severely reduced, resulting in high levels of phenylalanine that can cause severe brain damage. In mild or moderate forms, the enzyme still has some function, so phenylalanine levels are not as high, resulting in a smaller risk of significant brain damage. People with very mild cases may not require treatment.
PKU is diagnosed shortly after birth as part of routine newborn screening via a blood test. If a baby's phenylalanine levels in their blood sample are high, additional tests are administered to confirm the diagnosis and the type of PKU, normally with additional blood or urine tests. Without treatment, PKU can damage the brain and nervous system, which can lead to learning disabilities. Symptoms of untreated PKU include eczema, behavioral difficulties such as frequent temper tantrums and episodes of self-harm, and fairer skin, hair, and eyes than siblings who do not have the disorder. PKU is highly treatable with diet and supplements.
