Simply put, the human body produces mucopolysaccharides--specialized sugars, basically--for use in making connective tissues, mucous membranes, nerve coatings, and mucous itself. The body makes more of these carbohydrates than it normally needs, so there is a special enzyme series that breaks down the excess sugars so they can be excreted from the body.
When one or another enzyme in this series is missing or deficient, then the excess mucopolysaccharides can't be broken down and excreted, and the body must store them in places where they don't belong. There is a family of mucopolysaccharidoses, each associated with a particular missing enzyme, of which Hunter's Syndrome or MPS II is just one. Other syndromes in this family are Hurler's (MPS I), Sanfilippo (MPS III A-D) and several others.
As the mucopolysaccharides build up in the body over time, they cause deformations and systemic problems peculiar to each syndrome which ultimately become fatal. Though each case is unique and the course of the disease varies, Hunter's Syndrome is a degenerative disorder that typically reduces life span to 13-15 years. Though many bodily systems are severely affected, it is typically either congestive heart failure or obstructive airway disease that brings life to an end.
Here is a more technical description from Wikipedia...
The Wikipedia entry (overview):Hunter syndrome, or mucopolysaccharidosis II (MPS II), is a serious genetic disorder that affects males. It interferes with the body's ability to break down and recycle specific mucopolysaccharides, also known as glycosaminoglycans or GAG. Hunter syndrome is one of several related lysosomal storage diseases.
In Hunter syndrome, GAG build up in cells throughout the body due to a deficiency or absence of the enzyme iduronate-2-sulfatase (I2S).This buildup interferes with the way certain cells and organs in the body function and leads to a number of serious symptoms. As the buildup of GAG continues throughout the cells of the body, signs of Hunter syndrome become more visible.
Physical manifestations for some people with Hunter syndrome include distinct facial features, a large head, and an enlarged abdomen. People with Hunter syndrome may also experience hearing loss, thickening of the heart valves leading to a decline in cardiac function, obstructive airway disease, sleep apnea, and enlargement of the liver and spleen.
Range of motion and mobility may also be affected. In some cases of Hunter syndrome, central nervous system involvement leads to developmental delays and nervous system problems. Not all people with Hunter syndrome are affected by the disease in exactly the same way, and the rate of symptom progression varies widely.
However, Hunter syndrome is always severe, progressive, and life-limiting.
This 11 year old Canadian boy, Shimon, suffers from Hunter Syndrome:
About.com's entry on Hunter SyndromeMayo Clinic entry on Hunter Syndrome
