FAQs About Phenylketonuria (PKU)

A diagnosis of phenylketonuria (PKU) for your child can lead to many questions about the disease and your child’s future. Here are several frequently asked questions (FAQs) and answers about PKU.

What is PKU?

PKU is a rare genetic disorder that affects the body’s ability to break down an amino acid called phenylalanine (Phe). Amino acids are the building blocks of proteins. The body normally converts Phe from the diet into another amino acid called tyrosine (Tyr). However, in people with PKU, this doesn’t happen effectively. The result is high levels of Phe and low levels of Tyr in the body.

What are the symptoms?

PKU can lead to a number of symptoms, especially if left untreated. These include problems in the skin and eyes, as well as neurological problems such as seizures, learning disabilities, behavioral problems and mood disorders.

What causes PKU?

Mutations in the PAH gene cause PKU. This gene contains information that tells cells in the liver how to produce an enzyme called phenylalanine hydroxylase (PAH) that converts Phe into Tyr. The mutations lead to a reduction or loss of the PAH enzyme, leading to high levels of Phe and low levels of Tyr in the body.

Tyr is necessary to make several neurotransmitters (chemicals that send messages between nerve cells or from nerve cells to other cells of the body) including dopamine, serotonin, and norepinephrine. A reduction in Tyr levels can affect the ability of the body to make these neurotransmitters and ultimately causes issues with the nervous system.

High Phe levels have been linked to increased oxidative stress in cells and the formation of amyloid plaques (clumps of protein), both of which can damage the nervous system.

Is PKU a heritable disease?

A baby usually inherits PKU from his or her parents. We all receive two copies of each chromosome, one from our mother and one from our father. PKU is an autosomal recessive disease, meaning that a person needs to receive an altered PAH gene from both parents in order to have the disorder. Some people have one copy of the defective gene and are carriers of the disease. They usually have no symptoms since the second, unaffected copy of the gene can still function properly.

People with PKU can have children who are not affected by the disorder if their partner does not have PKU. However, all their children will be carriers. If the partner is a PKU carrier, then their children will have a 50% chance of inheriting the disease and a 50% chance of being a carrier.

If both parents are carriers, then any children will have a 25% chance of inheriting the disease and a 50% chance of being carriers. They will have a 25% chance of neither having the disease nor being a carrier.

How common is PKU?

In the U.S., the prevalence (how common a disease is in a population) of PKU is estimated to be between one in 10,000 to 15,000 people. PKU may be more common in people of European, Native American, and Turkish descent, and less common in people of African and Asian descent.

How do doctors diagnose PKU?

Since it is important to start treatment for PKU as early as possible to limit developmental disorders and mental retardation, most countries perform screenings for newborns. For the screening, clinicians collect a few drops of blood from the baby’s heel at birth and send it to a laboratory for analysis. If the results come back positive, then follow-up testing of the baby’s urine and blood will be done. Genetic testing also may be performed on the child and parents to look for mutations in the PAH gene.

How do doctors treat PKU?

There is currently no cure for PKU. The best treatment for the management of PKU so far is to eat a diet low in Phe with supplemental protein shakes to ensure proper nutrition. This diet can successfully manage PKU symptoms for many patients. However, in some patients, their Phe blood levels may still rise above acceptable ranges. For these patients, treatment with an enzyme replacement therapy called Palynziq can provide enzymes to break down Phe in the body. Another medication that is approved for patients with PKU is Kuvan. Kuvan is a synthetic version of tetrahydrobiopterin (BH4). It binds to any residual PAH enzyme in the body and increases its activity.

Where can I find more about treatments and research?

We regularly publish news articles on our website about PKU and its treatment.

A number of other organizations offer information about PKU treatments and research including:

Last updated: Nov. 12, 2020

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Phenylketonuria News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.

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