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Hurler syndrome, also known as mucopolysaccharidosis type I (MPS I) or "Hurler's disease", is a genetic disorder that results in the deficiency of alpha-L iduronidase, which is an enzyme that breaks down mucopolysaccharides. Without this enzyme, the buildup of heparan sulfate and dermatan sulfate occurs in the body (the heart, liver, brain etc.). Symptoms appear during childhood and early death can occur due to organ damage.
MPS I is divided into three subtypes based on severity of symptoms. All three types result from an absence of, or insufficient levels of, the enzyme alpha-L-iduronidase. MPS I H or Hurler syndrome is the most severe of the MPS I subtypes. The other two types are MPS I S or Scheie syndrome and MPS I H-S or Hurler-Scheie syndrome
Hurler's Syndrome is often classified as a lysosomal storage disease and is mechanistically related to Hunter's Syndrome (X-linked recessive).
Genetic Profile
Children born to an MPS I parent carry a defective IDUA gene, which has been mapped to the 4p16.3 site on chromosome 4. The gene is named IDUA because of its iduronidase enzyme protein product. As of 2001, 52 different mutations in the IDUA gene have been shown to cause Hurler syndrome.
Because Hurler syndrome is an autosomal recessive disorder, affected persons have two bad copies of the IDUA gene. If someone is born with one normal and one defective copy of the gene he is called a carrier and will produce less alpha-L-iduronidase than an individual with two normal copies of the gene. The reduced production of the enzyme, however, isn't strong enough to cause the person to show any symptoms of the disease.
Features
The condition is marked by progressive deterioration, hepatosplenomegaly, dwarfism and gargoyle-like faces. There is a progressive mental retardation, with death occurring by the age of 10 years.
Developmental delay is evident by the end of the first year, and patients usually stop developing between ages 2 and 4. This is followed by progressive mental decline and loss of physical skills. Language may be limited due to hearing loss and an enlarged tongue. In time, the clear layers of the cornea become clouded and retinas may begin to degenerate. Carpal tunnel syndrome (or similar compression of nerves elsewhere in the body) and restricted joint movement are common.
Affected children may be quite large at birth and appear normal but may have inguinal (in the groin) or umbilical (where the umbilical cord passes through the abdomen) hernias. Growth in height may be faster than normal but begins to slow before the end of the first year and often ends around age 3. Many children develop a short body trunk and a maximum stature of less than 4 feet. Distinct facial features (including flat face, depressed nasal bridge, and bulging forehead) become more evident in the second year. By age 2, the ribs have widened and are oar-shaped. The liver, spleen and heart are often enlarged. Children may experience noisy breathing and recurring upper respiratory tract and ear infections. Feeding may be difficult for some children, and many experience periodic bowel problems. Children with Hurler syndrome often die before age 10 from obstructive airway disease, respiratory infections, or cardiac complications.
There is some clinical similarity with Hunter syndrome.
Diagnosis
Diagnosis often can be made through clinical examination and urine tests (excess mucopolysaccharides are excreted in the urine). Enzyme assays (testing a variety of cells or body fluids in culture for enzyme deficiency) are also used to provide definitive diagnosis of one of the mucopolysaccharidoses. Prenatal diagnosis using amniocentesis and chorionic villus sampling can verify if a fetus either carries a copy of the defective gene or is affected with the disorder. Genetic counseling can help parents who have a family history of the mucopolysaccharidoses determine if they are carrying the mutated gene that causes the disorders.
Treatment
Enzyme replacement therapies are currently in use, BioMarin Pharmaceutical provides therapeutics for mucopolysaccaradosis type I (MPS I), by manufacturing Aldurazyme® (commercialized by Genzyme Corporation). Enzyme replacement therapy has proven useful in reducing non-neurological symptoms and pain. Bone marrow transplantation (BMT) and umbilical cord blood transplantation (UCBT) have had limited success in treating the mucopolysaccharidoses. Abnormal physical characteristics, except for those affecting the skeleton and eyes, may be improved, but neurologic outcomes have varied. BMT and UCBT are high-risk procedures and are usually performed only after family members receive extensive evaluation and counseling.
Epidemiology
Hurler syndrome has an overall frequency of ca. 1 per 100,000 population.
"Hurler syndrome"