About PKU

PKUnderstood

Phenylketonuria, or PKU for short, is an inherited, genetic condition in which the body can’t process phenylalanine (Phe), an amino acid found in many foods.

PKU is caused by a defect in the enzyme in the body that works to process Phe. This enzyme is called phenylalanine hydroxylase (PAH). In order for the PAH enzyme to work, it needs BH4, a molecule that is naturally present in your body. When the PAH enzyme can’t do its job, too much Phe builds up in the body.

Phe is found in:

1. All protein-containing foods (eg, meat, eggs, dairy, nuts)
2. Many other foods that are not generally thought of as containing protein (eg, most wheat products, such as pasta and bread, and some fruit, such as oranges and cherries).

If people with PKU followed a normal diet, they would eat more Phe than their bodies can handle.

Too much Phe is toxic to the brain. High Phe levels over an extended period of time can lead to vomiting, irritability, eczema, seizures, psychological and behavioral issues, and severe mental retardation.

Currently, management of PKU consists of limiting Phe intake. This means following a Phe-restricted diet in order to limit the amount of Phe in the body. Several other approaches to management are under investigation.

The Genetics of PKU

PKU is caused by an inherited genetic alteration.²⁷ To understand how PKU is inherited, it is important to learn a little about genetics.

• Our genes carry the instructions for traits like eye and hair color as well as a multitude of instructions that our body needs to function properly, including instructions to make PAH and all other enzymes.
• We inherit genes in pairs from our mother and our father. People who have one working PAH gene and one nonworking gene are called carriers; they make enough PAH to remain healthy and do not have PKU.
• People who inherit one nonworking copy of the PAH gene from the mother and one nonworking copy of the PAH gene from father end up with PKU.

How PKU is inherited

If both parents are carriers of the nonworking gene (r), then there is a 25% chance that both of them will pass it on to their child (rr). There is a 50% chance that their child will also be a carrier but not have PKU (Rr), and a 25% chance that their child will not be a carrier and not have PKU (RR).

Parents might know if they are a carrier of PKU if they have a family history of PKU; but for some parents the diagnosis of their child is how they find out.

Parents of children with PKU should not feel as though they could have known or done anything to prevent PKU. We are all carriers of a few nonworking genes and have no way of knowing until, by chance, a disorder occurs.