Phenylketonuria (PKU) is an inherited disease caused by mutations in the PAH gene, which prevents the body from breaking down phenylalanine. The severity of the condition depends on the level of enzyme activity, with classic PKU being the most severe. The disorder is autosomal recessive and carriers may not show symptoms.
Phenylketonuria (PKU) is an inherited disease in which the body is unable to break down phenylalanine, an essential amino acid found in protein-containing foods such as cheese, meat, and milk. There aren’t many different causes of phenylketonuria, as a hereditary disorder means the condition runs in families. In this sense, it can be said that one of the causes of phenylketonuria is that both parents of the affected person carry and pass on the defective phenylalanine hydroxylase (PAH) gene to their child. Generally, however, mutations in the PAH gene are what constitute the causes of phenylketonuria, with different mutations determining the severity of the condition.
As an autosomal recessive disorder, phenylketonuria occurs when a child inherits a defective PAH gene from both parents. The affected person’s parents don’t necessarily have the disorder because they carry only one defective copy of the gene, not two, as their child does. A person who carries the gene but does not have the disorder is called a carrier. In most cases, carriers pass on the gene unknowingly. This is because carriers usually do not show any symptoms of the disorder and, therefore, do not realize they are a carrier of the defective gene.
Although mutations in the PAH gene cause phenylketonuria, different levels of these mutations are responsible for different forms of phenylketonuria. To figure this out, it’s best to first have some background information about the disorder itself. Essentially, the PAH gene carries instructions that enable the body to produce the enzyme phenylalanine hydroxylase, which is responsible for breaking down phenylalanine and converting it into other compounds. Mutations in the PAH gene can reduce or stop the production of this enzyme altogether, which then allows phenylalanine to build up in excessive amounts in the body. Accumulation of this amino acid is dangerous because it can damage the central nervous system and lead to brain damage.
Mild forms of the disorder, such as variant phenylketonuria, occur when the gene mutation still allows some enzyme activity to persist within the body, effectively contributing to the breakdown of phenylalanine. The most severe form, classic phenylketonuria, occurs when enzymatic activity is absent or reduced to the point of allowing accumulation of the amino acid. While various mutations in the PAH gene determine how severe an affected person’s condition is, it is thought that other genes could also play a role in the severity of phenylketonuria, although not much is known about this.
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