MPS I disease is a rare inherited disease caused by a deficiency in an enzyme called alpha-L-iduronidase. Globally, MPS I occurs in about 1 in every 100,000 births.

About MPS I

Mucopolysaccharidosis I (MPS I) is a rare genetic disorder that affects many body systems and that leads to organ damage. It is caused by a defect in the gene that makes an enzyme called alpha-L-iduronidase. Because of this defect, cells either produce the enzyme in low amounts or cannot produce it at all. The enzyme is needed to break down substances called glycosaminoglycans (GAGs), which are long chains of complex sugar molecules used in the building of bones, cartilage, skin, tendons, and many other tissues. If GAGs are not broken down, they build up in the cell, eventually leading to cell, tissue, and organ damage.

MPS I is a lysosomal storage disorder, and it is one of the seven sub-types of MPS. Each MPS disorder is caused by accumulation of GAGs in the lysosomes, but the exact GAGs that accumulate are different in each case.

Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) are a group of rare genetic conditions caused by enzyme deficiencies. In a lysosomal storage disorder, certain structures in cells, called lysosomes, have trouble breaking down specific complex fatty molecules. As a result, the lysosomes fill up with these undigested molecules, impairing the cell’s ability to function properly.

Signs & Symptoms

MPS I has a wide range of symptoms, and people may experience different degrees of disease progression. Because the signs and symptoms are variable, the disease affects each individual differently.

Some of the symptoms that may occur in individuals with various severities of MPS I include:

  • Stiffened joints
  • Bone formation and growth problems
  • Carpal tunnel syndrome
  • Heart (valve) disease
  • Recurrent upper airway infections
  • Lung disease/sleep apnea
  • Corneal clouding
  • Spinal cord compression
  • Enlarged liver and spleen
  • Hernia (inguinal or umbilical)
  • Hearing loss
  • Delayed mental development
  • Coarse facial features
  • Communicating hydrocephalus (fluid on the brain)
  • Abnormally shaped teeth

Anisa, MPS I


MPS I is a progressive disease. Early diagnosis and intervention may prevent irreversible damage.

To diagnose MPS I, a doctor will typically first do a urine test to look for abnormally high levels of GAGs. A urine test is only one of the first steps in diagnosing MPS I; definitive diagnosis requires a test to measure enzyme activity levels in the blood or skin cells. In healthy individuals, the tests show white blood cells, serum, and skin cells that contain normal enzyme activity. In individuals with MPS I, the enzyme activity is much lower or absent.

Consult your doctor if you believe you or someone you know has exhibited symptoms of MPS I disease.

Last Updated: 12/3/2015

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