Definition of Mucopolysaccharidosis

Mucopolysaccharidosis: One of a series of inherited metabolic disorders affecting a type of complex carbohydrate called a mucopolysaccharide that is deposited in body tissues because the person lacks the specific enzyme needed to metabolize it.

The deposition of mucopolysaccharide in tissues damages and distorts them, stunts the child's growth and development, limits their joint movement and in some (but not all) types of MPS causes mental retardation.

The condition usually becomes evident in early childhood. That something is wrong may be noticed by parents or doctors. The diagnosis may be suspected by the clinical features. Confirmation of the diagnosis, however, requires biochemical tests of blood, urine, or tissues. Prenatal diagnosis is feasible in all types of MPS.

The prognosis (long-term outlook) depends upon the particular type of MPS. There are a number of different types of MPS that are designated somewhat confusingly by number (and letter), by syndrome name, and by precising what enzyme is lacking. The classification (as of 2001) is as follows:

• MPS type I -- Hurler syndrome, Scheie syndrome, and Hurler-Scheie syndrome -- due to varying degrees of deficiency of the enzyme alpha-L-iduronidase
• MPS type II -- Hunter syndrome -- due to deficiency of the enzyme iduronate sulfatase
• MPS type IIIA -- Sanfilippo syndrome -- due to deficiency of the enzyme heparan N-sulfatase
• MPS type IIIB -- Sanfilippo syndrome -- due to deficiency of the enzyme alpha-N-acetylglucosaminidase
• MPS type IIIC -- Sanfilippo syndrome -- due to deficiency of the enzyme acetyl CoA:alpha-glucosaminide acetyltransferase
• MPS type IIID -- Sanfilippo syndrome -- due to deficiency of the enzyme N-acetylglucosamine 6-sulfatase
• MPS type IVA -- Morquio syndrome -- due to deficiency of the enzyme N-acetylgalactosamine-6-sulfate sulfatase
• MPS type IVB -- Morquio syndrome -- due to deficiency of the enzyme beta-galactosidase
• MPS type VI -- Maroteaux-Lamy syndrome -- due to deficiency of the enzyme N-acetylgalactosamine-4-sulfatase
• MPS type VII -- Sly syndrome -- due to deficiency of the enzyme beta-glucuronidase
• MPS type VIII -- DiFerrante syndrome -- due to deficiency of the enzyme glucosamine-6-sulfate

All types of MPS are inherited as recessive traits. With one exception, they are autosomal (not sex-linked). Boys and girls alike can have these diseases if they receive two copies of the relevant gene, one from each of their parents. The risk for each subsequent child is 1 in 4.

The sole exception to autosomal recessive inheritance is MPS type II (Hunter syndrome). It is X-linked recessive. The Hunter gene is on the X chromosome. It is carried by seemingly normal women who have a 50-50 chance of transmitting it and the disease to each of their sons.

All types of MPS are lysosomal storage diseases. They involve enzymes found within the cell in lysosomes, miniature structures that are packets of degradative enzymes. In this respect, MPS is like all the other disorders of lysosomal storage such as Gaucher disease, Fabry disease, and Pompe disease.

Bone marrow transplantation is an effective treatment for patients with MPS type I (which is called Hurler syndrome), especially if the transplant is performed before the patient's growth and development begin to decline.

Enzyme replacement, first done in Gaucher disease in 1991, was reported in MPS type I (Hurler syndrome) in 2001. Treatment for a year with the missing enzyme was found to "ameliorate some clinical manifestations of the disease." The salutary enzyme was human alpha-L-iduronidase made by recombinant DNA methods. Enzyme replacement has also been tested in MPS type VI (Maroteaux-Lamy syndrome).