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Phenylketonuria (PKU)

Phenylketonuria (PKU)

E70.0

PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene. This gene helps create the enzyme needed to break down phenylalanine, an amino acid.
Without the enzyme necessary to break down phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that contain protein or eats aspartame, an artificial sweetener. This can eventually lead to serious health problems.

The gene defect for PKU is an autosomal recessive genetic defect.
An affected person inherits two genetic defects for the disorder (one from each parent).
A person with one genetic defect for the disorder is called a 'carrier' for PKU. Carriers do not have symptoms of the disorder.
When two carriers conceive a child, there is a one in four (or 25%) chance for each pregnancy that the baby will have PKU.
The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is one in 2,500.

1 out of 8,000-15,000 Caucasian babies (data from US and UK).

Infancy

PKU does not usually cause any symptoms if treatment is started early.
Without treatment, PKU can damage the brain and nervous system, which can lead to learning disabilities.
Symptoms of untreated PKU include:
- Behavioural difficulties such as frequent temper tantrums and episodes of self-harm
- Fairer skin, hair, and eyes than siblings who do not have the condition (phenylalanine is involved in the body's production of melanin, the pigment responsible for skin and hair colour)
- Eczema
- Repeatedly being sick
- Jerking movements in arms and legs
- Tremors
- Epilepsy
- Musty smell to the breath, skin, and urine

Diet: The main treatment for PKU is a low-protein diet and controlled intake of many other foods such as potatoes and cereals.
High protein foods should be completely avoided. These include:
- Meat and fish
- Eggs and cheese
- Nuts and seeds
- Flour-based foods like bread, pasta, cakes, and biscuits
- Soya, quorn, and tofu
People with PKU must take an amino acid supplement to ensure they're getting all the nutrients required for normal growth and good health.
Aspartame*: People with PKU must also avoid food products that contain aspartame as it's converted into phenylalanine in the body.
*Aspartame is a sweetener found in:
- Sugar substitutes such as the artificial sweeteners often used in tea and coffee
- Diet versions of fizzy drinks
- Chewing gum
- Squash and cordial
- Some alcopops
In 2007, sapropterin dihydrochloride (Kuvan) was approved by the U.S. Food and Drug Administration (FDA) to treat PKU. Kuvan is an oral pharmaceutical formulation of BH4, the natural cofactor for the PAH enzyme, which stimulates activity of the residual PAH enzyme to metabolise phenylalanine into tyrosine.
In 2018, pegvaliase-pqpz (Palynziq) was approved by the FDA for adults with PKU. Palynziq is an injectable enzyme therapy for patients who have uncontrolled blood phenylalanine concentrations on current treatment.

Keep a record of the food eaten every day.
Measure food portions using standard measuring cups and spoons and a kitchen scale that reads in grams. The food amounts are compared with a food list or are used to calculate the amount of phenylalanine eaten every day.
A registered dietitian with experience in PKU can help to devise delicious low-phenylalanine dinners. A dietitian may also have great ideas for holiday meals and birthdays.