MPS II or Hunter syndrome is a rare, X-linked lysosomal storage disorder. Lysosomes are important parts of our cells that work like a digestive system. When there are problems with lysosomes and their ability to break down waste and other materials, this can cause a build-up of waste that can be harmful. For MPS II, the cause behind the lysosomal storage disorder is caused by changes in the iduronate 2-sulfatase (IDS) gene.

The IDS gene is needed to make a special protein known as I2S, an enzyme responsible for breaking down large sugar molecules or cellular waste called glycosaminoglycans (GAGs). GAGs are naturally occurring substances that are key building blocks in the body. Normally, the I2S enzymes break down these GAGs, or cellular waste, so that just the right amount is present. In MPS II, changes in the IDS gene result in an I2S enzyme that is absent or does not function properly. Because the I2S enzyme is affected, this leads to a buildup of cellular waste throughout the body.

GAG buildup occurs in many organs, leading to limited range of motion, enlarged liver and spleen, skeletal changes, and breathing problems. Most patients with MPS II experience both peripheral (nonbrain and spine nervous system) and central nervous system (CNS/brain and spine nervous system) symptoms. The effect of MPS II on the brain can vary, with cognitive function such as thinking, memory, and decision-making ranging from normal to severely impaired. Severe Hunter syndrome results in toxic lysosomal accumulation of GAGs that causes progressive debilitation and decline in cognitive function. Regardless of CNS involvement, all individuals with MPS II experience severe peripheral symptoms. Hunter syndrome occurs in approximately 1 in 100,000 to 1 in 170,000 males. The severe form of MPS II can lead to a life expectancy of 10 to 20 years.