What causes PKU?

The Cristine M. Trahms Program for Phenylketonuria. (2008) [4]

Phenylketonuria (PKU) is passed down through families. It is an autosomal recessive trait because both parents have to pass on the defective gene for the baby to get diagnosed with this disease.

Babies with PKU are missing the enzyme phenylalanine hydroxylase, which is used to break down amino acid phenylalanine. Phenylalanine is found in protein foods (example: meat, milk). 

Without the enzyme, levels of phenylalanine and 2 closely-related substances increase in the body. High levels of these substances are harmful to the central nervous system and results in severe and permanent damage to the developing brain.

Over 500 different genetic mutations have been found can result in the phenylalanine hydroxylase enzyme not working properly, resulting in significantly higher phenylalanine levels. Each different mutation or combination of mutations results in more or less enzyme activity in the affected person. Schuett, (2008) [5] Thus the treatment of each patient is tailored to his or her own condition, depending on the severity of their condition.