Hunter syndrome is a lysosomal storage disorder caused by a deficiency in the enzyme iduronate-2-sulfatase (I2S). The disease was first described in 1917 by physician Charles A. Hunter (1873-1955), a Scottish national who had a medical practice in Winnipeg, Manitoba. The disease is inherited in an X-linked recessive manner and usually affects males.
The I2S enzyme that is deficient in Hunter syndrome is required for the break down and recycling of specific mucopolysaccharides called glycosaminoglycans or GAGs. A deficiency in this enzyme leads to a build up of GAGs in cells throughout the body, which eventually interferes with the function of various tissues and organs. As the cellular GAG accumulation continues over time, the symptoms and complications associated with Hunter syndrome gradually worsen and become more noticeable.
The symptoms of this condition are not present when a child is born, but start to develop after the first year of life. If symptoms start to manifest early (between 2 and 4 years of age), they are generally more severe. One of the first symptoms to develop is often abdominal hernia, which occurs when an abdominal organ or adipose tissue start to protrude through a weakness in the abdominal wall.
Children with the syndrome often share characteristic facial features which include a prominent forehead, a nose that is shallow-bridged and an enlarged tongue. The head and abdomen may also seem larger than usual.
The continued accumulation GAG in cells can eventually affect the function of important organs. The walls of the heart and heart valves thicken which can lead to a decline in cardiac function, for example. The bronchial walls also thicken, which can lead to respiratory complications such as obstructive airway disease.
The accumulation of GAG in the brain can eventually lead to developmental delays and mental retardation. The extent to which function is lost varies between individuals.
Estimates suggest that Hunter’s syndrome affects around 2,000 people worldwide. About 500 of these sufferers live in the United States. There are 70 patients with the syndrome said to be living in Korea; 30 in Canada; 20 in the Philippines; six in Ireland, two in New Zealand and one in each of Iran, Saudi Arabia, Chile, Israel, India and Pakistan. Reports also suggest that 20 cases exist in Poland. In the UK, the incidence among males is though to be around 1 in 130,000 live births.