What is Mucopolysaccharidosis Type II?
Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, is a lysosomal storage disorder that almost always affects males. Lysosomal storage disorders are a group of diseases characterized by the build up of toxic materials in the body. MPS II is caused by deficiency in the enzyme, iduronate-2-sulphatase. Deficiency in iduronate-2-sulphatase leads to the build-up of glycosaminoglycans (GAGs), important complex carbohydrates responsible for many regulatory functions in the body. The build-up of GAGs occurs in numerous organs and tissues leading to symptoms of MPS II.
Onset of symptoms and disease severity can be variable, but characteristic features include coarse facial features (broad nose, large tongue, and thick lips), enlarged head, recurrent ear infections, enlarged liver and spleen, hernias, thickened, pebbled skin, and short stature. Generally, severe cases present with symptoms between 18 months and 4 years of age. Approximately two-thirds of affected males will have central nervous system (CNS) involvement, leading to severe neurological decline and intellectual disability. Other major features include skeletal abnormalities (short stature as well as joint deformities and limited joint mobility), hearing loss, heart abnormalities, and airway obstruction that leads to pauses in breathing and progressive respiratory disease.
For individuals with a milder form of MPS II, onset of non-CNS related symptoms can occur in infancy or early childhood, although onset can also occur later than those with a severe form of MPS II. The severity of disease and progression can vary significantly, but heart disease and hearing loss are still common. Individuals with a milder form of MPS II often have normal neurologic and motor development.
It is important to note that symptomatic carrier females have been documented. These rare cases in females are due to chromosomal structural abnormalities or inactivation of the paternal X chromosome. Symptoms may be milder due to residual enzyme activity.
How common is Mucopolysaccharidosis Type II?
The occurrence of MPS II varies by region but is approximately 1 in 100,000 to 1 in 170,000 males.
How is Mucopolysaccharidosis Type II treated?
There is no cure for MPS II. Treatment focuses on management of symptoms, for example, physical and occupational therapy for developmental delays or surgical valve replacement for heart abnormalities. Enzyme replacement therapy (idursulfase) is also available for treatment of non-CNS related complications, though the success of treatment is dependent on the severity of disease. Treatment via bone marrow transplant and stem cell transplant have been attempted, though more data is needed to determine their long-term effectiveness.
What is the prognosis for a person with Mucopolysaccharidosis Type II?
For those with severe disease, death typically occurs in the first or second decade of life. Individuals with a milder form of MPS II can have complications that lead to death in their twenties or thirties, though survival into the fifties and sixties has been reported.