Related concepts:

PKU, Classic phenylketonuria, Benign hyperphenylalaninemia, Malignant hyperphenylalaninemia, Transient hyperphenylalaninemia.

Introduction ot PKU:

The sweetener aspartame, and many protein-rich foods, can act like poisons to children with PKU.

What is PKU?

Phenylalanine is an essential amino acid, one of the building blocks of protein that we need in our diets. Extra phenylalanine that we eat or drink is normally broken down by a specific enzyme and its cofactor.

If there is not enough of the enzyme or its cofactor, then the extra phenylalanine builds up in the blood (hyperphenylalaninemia) and spills into the urine (phenylketonuria). This excess can cause brain damage.

Classic PKU is deficiency of the key enzyme, phenylalanine hydroxylase.

Deficiency of the cofactor, tetrahydrobiopterin, is even worse, because the cofactor also has other important roles in the body. It is necessary for manufacturing neurotransmitters in the brain, so its absence can result in brain damage in at least two different ways. Deficiency of the cofactor is called malignant hyperphenylalaninemia, because it does not respond to ordinary PKU treatment.

Benign hyperphenylalaninemia is a mild deficiency of the key enzyme, allowing babies to develop normally if they do not eat excess protein.

Transient hyperphenylalaninemia is the name given to those babies who have a temporary inability to process phenylalanine. As they mature over the first few weeks of life, transient hyperphenylalaninemia disappears.

Who gets PKU?

PKU is an inherited condition, passed along as a recessive trait. There are over 100 different variations of the gene that can result in PKU. When both parents are carriers, about one in four of their children will have the disease.

Babies with classic PKU are often the blondest children in the family. They are typically blue-eyed, fair-skinned babies.

Transient hyperphenylalaninemia most often occurs in premature babies in the first month of life.

PKU is somewhat rare (seen in fewer than one in 10,000 children), but important to identify. Early treatment can make a huge difference for these children.

What are the symptoms of PKU?

Babies with classic PKU are normal at birth, except for missing the necessary enzyme to process phenylalanine. As they get phenylalanine in the diet, brain damage gradually occurs and accumulates and intelligence is lost at a rate of about 4 IQ points per month during the first year. If untreated for a year, babies lose almost 50 IQ points, resulting in severe mental retardation.

Sometimes these babies vomit – so much so that they might mistakenly be thought to have Seizures and rhythmic rocking are common. They have small heads, small bodies, and poorly developed teeth. Skin rashes may also result. These children often have an unpleasant musty or mousy odor from the excess by-products of unprocessed phenylalanine.