Hunter syndrome, Iduronate sulfatase deficiency
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.
Causes, incidence, and risk factors
Hunter syndrome is inherited as an X-linked recessive disease. This means that women carry the disease and can pass it on to their sons, but are not themselves afffected.
Because girls have two X chromosomes, their normal X can provide a functioning gene even if their other X is defective. 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. In its absence, 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.
Juvenile form (early-onset, severe form):
- mental deterioration
- severe mental retardation
- aggressive behavior
Late (mild form):
- mild to no mental deficiency
- coarse facial features
- large head (macrocephaly)
- stiffening of joints
- increased hair (hypertrichosis)
- deafness (progressive)
- enlargement of internal organs such as liver and spleen
- abnormal retina (back of the eye)
- carpal tunnel syndrome
Signs and tests
Signs of the disorder that the doctor might look for include:
- hepatomegaly (enlargement of liver)
- splenomegaly (enlargement of spleen)
- inguinal hernia
- heart murmur and leaky heart valves
- joint contractures
- excretion of heparan sulfate and dermatan sulfate in urine
- decreased iduronate sulfatase enzyme activity in serum or cells
Tests that may indicate this disorder is present include:
- urine for heparan sulfate and dermatan sulfate
- enzyme study, decreased iduronosulfate sulfatase (may be studied in serum, WBCs, fibroblasts)
- genetic testing may show mutation in the iduronate sulfatase gene
There is no cure for Hunter syndrome. A specific treatment is being developed called enzyme replacement therapy. However, it is experimental and may not be able to prevent neurologic disease from getting worse. Individual problems should be addressed separately. Bone marrow transplant has been attempted for the early-onset form with variable results.
Life expectancy for the early-onset form (severe form) is 10-20 years. Life expectancy for the late-onset form (mild form) is 20-60 years.
- airway obstruction in late-onset form (mild form)
- progressive mental deterioration in early-onset, severe form
- progressive loss of activities of daily living in early-onset, severe form
- progressive hearing loss in both mild and severe forms
- progressive joint stiffness leading to contractures of joints in early-onset, severe form
- carpal tunnel syndrome
Calling your health care provider
Call your health care provider if you or your child manifest a group of these symptoms, or if you know you are a genetic carrier and are considering having children.
Genetic counseling is recommended for prospective parents with a family history of Hunter syndrome. Prenatal testing is available. Carrier testing for female relatives of affected males is available at a few specialized centers.
by Sharon M. Smith, M.D.