Phenylketonuria (PKU) is a genetic condition that is passed to a baby by its parents.
Phenylketonuria (PKU) is a genetic disorder in which the body can't process part of a protein called phenylalanine (Phe). Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. If the Phe level gets too high, it can damage the brain and cause severe intellectual disability.
PKU doesn't usually cause any symptoms if treatment is started early – but if it isn't treated, damage to the brain and nervous system can lead to:
• learning disabilities
• behavioural difficulties
High levels of phenylalanine can also cause eczema.
What happens in PKU
PKU is caused by a mutation in a gene on chromosome 12. The gene codes for a protein called PAH (phenylalanine hydroxylase), an enzyme in the liver. This enzyme breaks down the amino acid phenylalanine into other products the body needs. When this gene is mutated, the shape of the PAH enzyme changes and it is unable to properly break down phenylalanine. Phenylalanine builds up in the blood and poisons nerve cells (neurons) in the brain.
PKU is caused by a genetic mutation (change to the DNA) in the human phenylalanine hydroxylase (PAH) gene.
There are many different types of mutations that can affect the PAH gene, all of which result in the body not producing a functioning PAH enzyme.
Enzymes are proteins that speed up and control chemical reactions. The lack of the PAH enzyme means that a chemical called phenylalanine, which is found in some foods, cannot be processed properly by the body.
Autosomal recessive mutation
All the genes in our body come in pairs. We receive one half of the pair from our mother and the other half from our father.
The mutation that causes PKU is an autosomal recessive mutation. This means we need to receive two mutated copies of the gene (one from our mother and one from our father) in order to develop the condition.
We will not develop PKU if we only receive one mutated copy of the gene from one of our parents, but we will carry the mutated gene.
If someone is a carrier of the mutated gene and conceives a baby with a partner who is also a carrier, there are possibilities of:
• 1 in 4 chance that the baby will receive a pair of mutated genes and develop PKU
• 1 in 4 chance that baby will receive a pair of normal genes
• 1 in 2 chance that baby will receive one normal gene and one mutated gene and become a carrier of PKU without developing symptoms.
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