PHENYLKETONURIA

What is phenylketonuria (PKU)?

Phenylketonuria is a rare metabolic disorder that affects the way the body breaks down protein. If not treated shortly after birth, PKU can be destructive to the nervous system, causing mental retardation.

PKU is caused by a mutation in a gene on chromosome 12. The gene codes for a protein called PAH (phenylalanine hydroxylase), an enzyme in the liver. This enzyme breaks down the amino acid phenylalanine into other products the body needs. When this gene is mutated, the shape of the PAH enzyme changes and it is unable to properly break down phenylalanine. Phenylalanine builds up in the blood and poisons nerve cells (neurons) in the brain.

How do people get PKU?

PKU is an autosomal recessive disorder, meaning that you need to inherit mutations in both copies of the gene to develop the symptoms of the disorder. A carrier does not have symptoms of the disease, but can pass on the defective gene to his or her children. If both parents carry one copy of the faulty gene, each of their children have a 25 percent chance of being born with the disease.

What are the symptoms of PKU?

Babies born with PKU usually have no symptoms at first. But if the disease is left untreated, babies experience severe brain damage. This damage can cause epilepsy, behavioral problems, and stunt the growth of the baby. Other symptoms include: eczema (skin rash), a musty body odor (from too much phenylalanine), a small head (microcephaly), and fair skin (because phenylalanine is necessary for skin pigmentation).

How do doctors diagnose PKU?

Because PKU must be treated early, babies in every U.S. state are routinely tested for the disease. A small blood sample is taken from the baby's heel or arm and checked in a laboratory for high levels of phenylalanine.

How is PKU treated?

People who have PKU must eat a protein-free diet, because nearly all proteins contain phenylalanine. Infants are given a special formula without phenylalanine. Older children and adults have to avoid protein-rich foods such as meat, eggs, cheese, and nuts. They must also avoid artificial sweeteners with aspertame, which contains phenylalanine.

Interesting facts about PKU

Norwegian doctor Asbjørn Følling discovered PKU in 1934.

About 1 out of every 15,000 babies in the United States is born with PKU.