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Hunter syndrome is a hereditary disease in which the breakdown of a mucopolysaccharide (a chemical that is widely distributed in the body outside of cells) is defective. This chemical builds up and causes a characteristic facial appearance, abnormal function of multiple organs, and in severe cases, early death.
Hunter syndrome is inherited as an X-linked recessive disease. This means that boys will be the ones most often affected, because the defective gene is on the X chromosome. Boys have only 1 copy of the X chromosome. Girls have 2 copies.
Because girls have two X chromosomes , their normal copy on one X can provide a functioning gene, even if their other X has the abnormal gene. Women can carry the defective gene and pass it on, without being affected themselves, unless both their copies are abnormal. But because boys have an X and a Y, there is no normal X gene to fix the problem if the X is defective.
The metabolic abnormality that causes Hunter syndrome is a lack of the enzyme iduronate sulfatase. Without this enzyme, mucopolysaccharides collect in various body tissues, causing damage.
Affected children may develop an early-onset type (severe form) shortly after age 2 that causes a large skull, coarse facial features, profound mental retardation , spasticity , aggressive behavior, joint stiffness and death before age 20. A late-onset type (mild form) causes later and less severe symptoms.
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