Phenylketurnia

Phenylketournia (PKU) is a disease that a person inherits from both of their parents, called a recessive disorder. It occurs in one in twenty thousand babies. It is a mutation on a single gene of chromosome 12, which has instructions for making a certain enzyme called phenylalanine hydroxylase (PAH). PAH is usually produced in the liver, where it helps convert the amino acid phenylalanine into another amino acid called tyrosine. PKU changes the shape of the PAH enzyme by mutating one of the thousand of DNA bases. Different mutations cause different effects. For example, a certain mutation will change the enzyme so that it doesn't recognize phenylalanine anymore, or they will make the enzyme work very slowly. Other mutations change the shape of the enzyme so that it will fall apart. No matter the mutation, the end result is the same: PAH cannot convert phenylalanine to tyrosine. Excess phenylalanine builds up in the brain and poisons the neurons.

PKU causes mental retardation in infancy. Scientists are not sure why, but one idea is that it slows down the development of neurons inside of the cerebral cortex. Another idea is that retardation is a an effect of a low level of dopamine which is found in people with untreated PKU. Dopamine is a chemical that transports messages from one neuron to another. The extra enzyme may also cause a musty odor, lighter hair and skin, a smaller than normal head, and epilepsy.

PKU is diagnosed at infancy. At birth, doctors take a few drops of blood from a baby to test levels of phenylalanine in the blood on Guthrie cards. A piece of this is cut out and put on a plate of bacteria that can't grow without the presence of phenylalanine. If there is a growth of bacteria then the doctors can conclude the baby has a higher level of phenylalanine in it's blood. The more bacteria around the blood, the more phenylalanine is present.

PKU is treated with a very strict diet that contains little phenylalanine. Doctors determine the amount of phenylalanine that is safe for a person with frequent tests of phenylalanine levels in the blood. People with PKU should avoid foods with high levels of protein. Babies are given a special formula so they can receive the nutrients they need.