Phenylketonuria

Alternative names:

PKU

Definition:

A rare hereditary condition in which the amino acid phenylalanine is not properly metabolized; this can cause severe mental retardation.

Causes, incidence, and risk factors:

PKU is inherited as an autosomal recessive trait. PKU occurs about once out of 16,000 births in the United States and is distributed equally between the sexes. Because PKU can be easily detected by a simple blood test and it is a treatable disease, most states require a screening test for all newborns. The genetically determined abnormality in phenylketonuria is a missing enzyme, phenylalanine hydroxylase.

Phenylalanine is one of the eight essential amino acids found in protein-containing foods. In PKU, phenylalanine cannot be used in a normal fashion because of the missing enzyme. Subsequently, high levels of phenylalanine and two closely related phenylalanine derivatives develop. These compounds are toxic to the central nervous system and cause brain damage.

Damage to the brain causes marked mental retardation by the end of the first year of life. Older children may develop movement disorders (athetosis), rocking, and hyperactivity. Because phenylalanine is involved indirectly in the production of melanin, the pigment responsible for skin and hair color, children with phenylketonuria often have lighter complexions than their unaffected siblings (PKU children often have blond hair and blue eyed). These children may have an odor described as mouse-like that results from the accumulation of phenylacetic acid. This odor may be detected on their breath, skin, and urine.