Phenylketonuria affects one in 10,000 to 15,000 newborns in the U.S. according to the Genetics Home Reference, a service of the U.S. National Library of Medicine. The condition results from a mutated gene passed on from both of the child's parents, which prevents the break down of phenylalanine, an amino acid. As a result, untreated patients have a build up of phenylalanine in their bodies, which may lead to intellectual disabilities.
PAH Gene Mutations
Patients with phenylketonuria have a mutated PAH gene on chromosome 12, which is involved in the production of the enzyme phenylalanine hydroxylase. Normally, phenylalanine hydroxylase converts phenylalanine, but the mutation to the PAH gene causes toxic levels of phenylalanine to build up in the body, which can lead to damage to the brain cells. The Genetics Home Reference points out that scientists have identified more than 500 mutations of the PAH gene, but the most common mutation occurs at position 408 of the gene, in which tryptophan, an amino acid, replaces another amino acid, arginine.
Types
Different types of phenylketonuria exist, which vary by the mutation to the PAH gene. For example, the Genetic Home Reference notes that with classic phenylketonuria, patients have severely reduced or no phenylalanine hydroxylase activity. Other types of the disorder, such as variant phenylketonuria and non-phenylketonuria hyperphenylalaninemia, are less severe with some phenylalanine hydroxylase activity.
Identification
MedlinePlus explains that since phenylalanine affects pigmentation, phenylketonuria patients may have identifiable traits of the condition. For example, patients may have lighter eye, skin or hair color compared to siblings who do not have phenylketonuria. With untreated phenylketonuria, patients have a musty odor in their urine, breath or skin. Phenylketonuria can cause other physical signs, such as a smaller head size or jerking movements of the limbs. Patients may also position their hands strangely. Other identifiable symptoms of phenylketonuria include skin rashes, seizures, tremors, hyperactivity and delayed social and mental skills.
Testing
Doctors use a blood test called serum phenylalanine screening to test for phenylketonuria. MedlinePlus points out that the doctor does this test before the infant leaves the hospital or within 48 to 72 hours after birth for those born outside of a hospital. The test involves getting three drops of blood from the infant for three test circles. The laboratory technician mixes the blood sample with a bacteria that lives off phenylalanine and another substance which prevents other reactions.
