Phenylketonuria (PKU) is a rare genetic condition that is present from birth (congenital). In PKU, the body is unable to break down a chemical called phenylalanine which then builds up in the blood and in the brain.
Left untreated, high phenylalanine levels disrupt the normal development of a child’s brain and can cause severe learning difficulties. A person with the most severe learning difficulties has mental abilities similar to those you would expect to see in a one or two year old infant and would require 24 hour care for the whole of their life.
Phenylalanine
Phenylalanine is an amino acid. Amino acids are the ‘building blocks’ that the body uses to assemble proteins and other complex chemicals, such as some neurotransmitters (‘messenger chemicals’ that are used to transmit information around the brain).
The main source of phenylalanine for humans is food that is high in protein such us:
- meat
- fish
- eggs
- cheese
- milk
Genetic mutation
PKU is caused by a genetic mutation. A genetic mutation is where some of the instructions that are found in all living cells become scrambled, causing certain cells to no longer work in the right way.
In most people the phenylalanine that is found in food is broken down by an enzyme known as phenylalanine hydroxylase (PAH). Proper breakdown of phenylalanine is needed for the body to make certain neurotransmitters.
In people with PKU, the PAH enzyme does not work properly due to the genetic mutation. As a result, the phenylalanine levels in the blood and other tissues rise. Phenylalanine is thought to have a toxic (poisonous) effect on the brain, resulting in brain damage and learning difficulties.
The treatment for PKU is both simple and effective. It involves following a special, low-protein diet and taking regular dietary supplements which contain amino acids, vitamins and minerals. The aim is to keep the blood phenylalanine level within a specific target range. The target range changes with age; the diet is very strict in early childhood but can be gradually relaxed as people get older.
How common is PKU?
PKU is a very rare condition. It is estimated that only 1 in every 12,000 babies born in England will have PKU.
Both sexes are affected equally by PKU. The condition is slightly more common in white people, particularly those of Celtic descent (Scottish, Irish and Welsh). It is very rare in black people and those of Japanese descent.
Screening
All newborn babies who are born in the UK, and those who are born in other developed countries, are routinely screened for high phenylalanine levels using the heel prick test which is carried out during the first week of a baby’s life. The heel prick test involves taking a blood sample from the baby’s heel to test for a number of rare, but serious, conditions, such as PKU.
If a high phenylalanine level is found on screening and confirmed by further testing, the baby will immediately be started on a low protein diet and a special supplement. Regular testing ensures that the phenylalanine level remains low. Keeping phenylalanine levels within the target range will prevent any damage to the brain and ensure that the child grows up with normal levels of intelligence.
Outlook
With treatment, the outlook for PKU is excellent. As long as a person with the condition sticks to a low protein diet throughout childhood their natural intelligence should be unaffected. Many adults find that the dietary restrictions can be challenging and annoying. Fortunately, if phenylalanine levels rise in adulthood, intelligence is not affected although symptoms such as poor concentration and short temper can occur.
Without treatment early in life, the outlook for PKU is very poor. Most people will develop severe learning disabilities and will require constant care.
