Mucopolysaccharidosis type III (MPS III) is a rare disease in which the body is missing or does not have enough of certain enzymes needed to break down long chains of sugar molecules. These chains of molecules are called glycosaminoglycans (formerly called mucopolysaccharides). As a result, the molecules build up in different parts of the body and cause various health problems.

The condition belongs to a group of diseases called mucopolysaccharidoses (MPSs). MPS II is also known as Sanfilippo syndrome.

There are several other types of MPSs, including:

• MPS I (Hurler syndrome; Hurler-Scheie syndrome; Scheie syndrome)
• MPS II (Hunter syndrome)
• MPS IV (Morquio syndrome)

MPS III; Sanfilippo syndrome; MPS IIIA; MPS IIIB; MPS IIIC; MPS IIID; Lysosomal storage disease - mucopolysaccharidosis type III

MPS III is an inherited disorder. This means it is passed down through families. If both parents carry a nonworking copy of a gene related to this condition, each of their children has a 25% (1 in 4) chance of developing the disease. This is called an autosomal recessive trait.

MPS III occurs when the enzymes needed to break down the heparan sulfate sugar chain are missing or defective.

There are four main types of MPS III. The type a person has depends on which enzyme is affected.

• Type A is caused by a defect in the SGSH gene and is the most severe form. People with this type do not have a normal form of the enzyme called heparan N-sulfatase.
• Type B is caused by a defect in the NAGLU gene. People with this type are missing or do not produce enough alpha-N-acetylglucosaminidase.
• Type C is caused by a defect in the HGSNAT gene. People with this type are missing or do not produce enough acetyl-CoA:alpha-glucosaminide N-acetyltransferase.
• Type D is caused by a defect in the GNS gene. People with this type are missing or do not produce enough N-acetylglucosamine 6-sulfatase.

Symptoms often appear after the first year of life. A decline in learning ability typically occurs between ages 2 and 6. The child may have normal growth during the first few years, but final height is below average. Delayed development is followed by worsening mental status.

Other symptoms may include:

• Behavioral problems, including hyperactivity
• Coarse facial features with heavy eyebrows that meet in the middle of the face above the nose
• Chronic diarrhea
• Enlarged liver and spleen
• Sleep difficulties
• Stiff joints that may not extend fully
• Vision problems and hearing loss
• Walking problems

Treatment of MPS III is aimed at managing the symptoms. There is no specific treatment for this disease. New research is under development for gene therapy and enzyme replacement.

More information and support for people with MPS III and their families can be found at:

• National Organization for Rare Disorders -- rarediseases.org/rare-diseases/mucopolysaccharidosis-type-iii/
• NIH Genetic and Rare Diseases Information Center -- rarediseases.info.nih.gov/diseases/3807/mucopolysaccharidosis-type-iii
• Team Sanfilippo Foundation -- teamsanfilippo.org/

MPS III causes significant nervous system symptoms, including severe intellectual disability. Most people with MPS III live into their teenage years. Some live longer, while others with severe forms die at an earlier age. Symptoms are most severe in people with type A.

These complications can occur:

• Blindness
• Inability to care for self
• Intellectual disability
• Nerve damage that slowly gets worse and eventually requires wheelchair use
• Seizures

Call your child's provider if your child does not seem to be growing or developing normally.

See your provider if you plan to have children and you have a family history of MPS III.

Genetic counseling is recommended for couples who want to have children and who have a family history of MPS III. Prenatal testing is available.

Published Date: May 02, 2021
Published By: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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