All of our genes code for specific proteins, which have specific roles in our bodies. In Canavan disease, a defect in the ASPA gene causes an absence of a vital enzyme called aspartoacylase (ASPA.) An enzyme is a type of protein. The job of the ASPA enzyme is to break down N-acetylaspartate acid (NAA). When NAA builds up in the brain it leads to the symptoms of Canavan Disease.
It is sometimes helpful to think of the substrate and the enzyme as a lock and a key. The substrate is the lock, and the enzyme is the key. If the key is made incorrectly, it is not going to fit into the lock. An enzyme is a type of protein. In order for any protein to function correctly, it must contain the correct parts (which are called amino acids) and also be folded correctly.
The enzyme is made incorrectly if the gene that codes for the enzyme contains a spelling error, which is called a mutation. Mutations in the gene can cause the enzyme to contain incorrect parts, or cause it to be folded incorrectly. Sometimes the mutations in the gene cause the enzyme to not be made at all. In infantile Canavan disease, no enzyme is made. In juvenile/milder forms of Canavan disease, some enzyme is made, so there is less build up of NAA in the brain.