Hunter syndrome develops when a defective chromosome is inherited from the child's mother. Because of that defective chromosome, an enzyme that's needed to break down complex sugars called glycosaminoglycans is missing or malfunctioning.
The missing or malfunctioning enzyme is called iduronate-2-sulfatase.
In unaffected people, these enzymes are found in parts of the body's cells known as lysosomes. The lysosomes use enzymes to break down glycosaminoglycans, as part of the body's normal recycling and renewal process. In a person with Hunter syndrome or another form of MPS, these enzymes either are missing or don't work correctly.
Normally, the nutrients that are broken down by lysosomes help your body build bone, cartilage, tendons, corneas, skin and connective tissue, and the fluid that lubricates your joints.
When this enzyme isn't working properly, undigested glycosaminoglycans collect in the cells, blood and connective tissues, causing permanent and progressive damage. Hunter syndrome and other forms of MPS are sometimes called lysosomal storage diseases.