Hurler syndrome

background

Hurler syndrome is a rare, inherited disorder in which a hereditary factor, or gene, is defective. The gene affected is one that controls production of an enzyme, called alpha-L-iduronidase. As a result, patients with Hurler syndrome typically experience developmental abnormalities, learning difficulties, intellectual disabilities, and various types of organ dysfunction.
Enzymes are proteins that bring about chemical reactions in the body. Alpha-L-iduronidase is the enzyme responsible for the breakdown of large, complex sugar molecules called glucosaminoglycans (formerly called mucopolysaccharides). Glucosaminoglycans are found in most of the body's tissues and organs. Like most substances in the body, they are continually produced and broken down in order to maintain constant levels in the body.
Hurler syndrome is one of eleven disorders that are classified as mucopolysaccaharidoses (MPS). Different classes of MPS syndromes are categorized based on the specific enzyme deficiency involved in the complex chemical breakdown pathway of the glucosaminoglycans. Hurler syndrome is considered MPS type I.
Because patients with Hurler syndrome have defective alpha-L-iduronidase, glucosaminoglycans build up in abnormally large amounts throughout the body.
Hurler syndrome is a lysosomal storage disease. Lysosomal storage diseases occur when the enzyme responsible for breaking down a particular substance inside the lysosomes, or sac-like structures within cells, is missing. As a result, the substance builds up inside the lysosomes.
Both parents must pass down a defective gene for a child to develop Hurler syndrome. However, each parent almost always has one normal copy of the gene, and therefore produce enough alpha-L-iduronidase so that glucosaminoglycans continue to be broken down normally. Thus, neither parent carrying a copy of the defective gene generally exhibits any disease symptoms.
Without treatment, survival beyond the early teen-aged years is rare. However, with treatments that are currently available, some children with Hurler syndrome have been able to live longer and more comfortable lives.

Related Terms

Alpha-L-iduronidase, bone marrow transplant, claw hand, cord blood, enzyme replacement therapy, gargoylism, gibbus deformity, glucosomaminoglycans, Hurler's disease, Hurler-Scheie syndrome, laronidase, lysosomal enzymes, lysosomal storage disease, lysosomes, Mongolian spots, MPS I H, MPS H-S, MPS I S, mucopolysaccharides, mucopolysaccharidosis, Scheie syndrome.

types of the disease

General: Hurler syndrome is one of 11 disorders that are classified as mucopolysaccaharidoses (MPS). Different classes of MPS syndromes (formerly called mucopolysaccharides) are categorized based on the specific enzyme deficiency involved in the complex chemical breakdown pathway of the glucosaminoglycans. Hurler syndrome is considered MPS type I.
There are three different subclasses of MPS I. All three forms of MPS I are characterized by a deficiency of the alpha-L-iduronidase enzyme. However, there is considerable disagreement among experts as to whether the three forms of MPS I represent fundamentally different types of specific genetic defects that affect alpha-L-iduronidase production, or alternatively, variations in severity due to other hereditary factors. Management and treatment is the same for all three MPH I subclasses.
Hurler syndrome (MPS I H): Hurler syndrome (also called MPS I H) is the most severe MPS I subclass. It is also the most common subclass, occurring in one out of every 100,000 births. Delay in growth and development in Hurler syndrome is often evident by the end of the first year, and children usually stop developing between ages two and four. This is followed by progressive decline in mental abilities and onset of potentially life-threatening complications due to obstructive airway disease, respiratory infections, or heart abnormalities.
Hurler-Scheie syndrome (MPS I H-S): In a less severe subclass of MPS I, known as Hurler-Scheie syndrome (also called MPS I H-S), onset of symptoms is usually between the ages of three and eight. Physical signs and symptoms are similar to Hurler syndrome. However, in children with Hurler-Scheie syndrome, progression of the disease is slower. Mental retardation and learning difficulties in Hurler-Scheie syndrome are also usually much less severe and progress less rapidly. While children with Hurler syndrome often die before age 10, life expectancy for children with Hurler-Scheie syndrome usually extends into the late teens or early twenties. The frequency of children born with Hurler-Scheie syndrome is about one out of 115,000 births.
Sheie syndrome (MPS I S): The mildest subclass of MPS I is called Sheie syndrome (also known as MPS I S). It is also the rarest form, and occurs in only about one out of 500,000 births. Physical symptoms of Scheie syndrome are similar to Hurler and Hurler-Sheie syndromes, but are less severe, and mental development is usually normal or nearly normal. However, some individuals with Scheie syndrome may have significant learning difficulties. Although signs and symptoms may appear earlier, diagnosis of most children with Scheie syndrome does not usually occur until about age 10, and they are frequently able to live to middle-age, when complications of the disease often become life-threatening.