• What is phenylketonuria?
• How common is phenylketonuria?
• What genes are related to phenylketonuria?
• How do people inherit phenylketonuria?
• What other names do people use for phenylketonuria?

What is phenylketonuria?

Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the amount of the amino acid phenylalanine to harmful levels in the blood. (Amino acids are the building blocks of proteins.) If PKU is not treated, excess phenylalanine can cause mental retardation and other serious health problems.

Phenylketonuria is a subtype of hyperphenylalaninemia.

The signs and symptoms of this disorder vary from mild to very severe. The most severe form of the disorder is known as classic PKU. Infants with classic PKU appear normal until they are a few months old. Without treatment, these children develop permanent mental retardation and behavioral problems. Seizures, delayed development, and movement disorders are also common. Many children have a musty or "mouse-like" odor as a side effect of too much phenylalanine in the body. Children with classic PKU tend to have lighter skin and hair than unaffected family members, because phenylalanine is important for skin pigmentation. Children with the disorder are also likely to have skin disorders such as eczema.

Less severe forms of PKU (sometimes called moderate or mild PKU) have a smaller risk of brain damage. People with very mild cases may not require treatment with a special diet.

Babies born to mothers with high levels of phenylalanine have a high risk of mental retardation because they are exposed to very high levels of phenylalanine before birth. These infants may also grow more slowly than other children and may have heart defects or other heart problems, small head size (microcephaly), and behavioral problems. Affected women with uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.

How common is phenylketonuria?

PKU is found in about 1 in 10,000 Caucasian (white) newborns. The disorder occurs much less frequently in infants of Japanese, Ashkenazi Jewish, Finnish, and African backgrounds. PKU is most common in Turkey, where 1 in 2,600 newborns is affected.

What genes are related to phenylketonuria?

Mutations in the PAH gene cause phenylketonuria.

PAH produces an enzyme called phenylalanine hydroxylase, which converts the amino acid phenylalanine to other essential compounds in the body. Normal levels of phenylalanine are important for brain function. This amino acid is found in all proteins and in some artificial sweeteners. If gene mutations reduce the activity of phenylalanine hydroxylase, phenylalanine from the diet can build up to dangerous levels in the bloodstream and other tissues. Excess phenylalanine damages nerve cells, resulting in brain damage.

How do people inherit phenylketonuria?

PKU is inherited in an autosomal recessive pattern, which means two copies of the gene must be altered for a person to be affected by the disorder. Most often, the parents of a child with an autosomal recessive disorder are not affected but are carriers of one copy of the altered gene.

What other names do people use for phenylketonuria?

• Classical Phenylketonuria
• Deficiency Disease, Phenylalanine Hydroxylase
• Folling Disease
• Phenylalanine Hydroxylase Deficiency Disease
• Phenylketonuria I
• PKU

Source: Genetics Home Reference, National Library of Medicine, National Institutes of Health

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