What Is Mucopolysaccharidosis Type I?
Mucopolysaccharidosis Type I (MPS I)is an inherited condition in which the body lacks an enzyme called alpha-L-iduronidase. Without this enzyme, the body cannot properly break down long chains of sugar molecules called glycosaminoglycans. As a result, these molecules accumulate in the body, causing numerous health problems. There are two forms of MPS I: a severe form and a milder (attenuated) form. MPS I is caused by mutations in the IDUA gene.
Severe Mucopolysaccharidosis Type I
Children with the condition appear normal at birth but may start to develop symptoms before the age of one. Infants may develop bulging of organs or tissue (hernia) at the belly button or inner groin, coarsening of facial features (broad mouth, square jaw), spine malformations, and frequent infections in the nose, sinuses, pharynx, or larynx. Delays in development are usually present in the first year of life and progress as the child ages. By the age of three, growth slows and hearing loss is common. Enlargement of the liver and spleen may be present and heart disease (including valve problems and narrowed arteries) and lung disease frequently develop. A dangerous accumulation of fluid around the brain (hydrocephalus) can also occur. Children with this condition tend to have a shortened lifespan but may live longer with treatment.
Attenuated Mucopolysaccharidosis Type I
This form of the condition is also known as Hurler-Scheie syndrome or Scheie syndrome. Children usually develop symptoms between the ages of 3 and 10 years. There is a wide range of disease severity in children with MPS I, ranging from individuals living normal lifespans to individuals having complications leading to death by the age of 20 to 30 years. Individuals with a more normal lifespan may still have significant issues from progressive joint disease (arthropathy) as well as heart and lung disease. Learning disabilities can be present, and hearing loss and cardiac valvular disease are common.
How Common Is Mucopolysaccharidosis Type I?
The prevalence of MPS I is 1 in 100,000 individuals for the severe form and 1 in 500,000 for the attenuated form. MPS I has been found in people of all ethnicities.
How Is Mucopolysaccharidosis Type I Treated?
The use of immature cells capable of developing into multiple blood cell types is the primary treatment for severe MPS I. Bone-marrow transplants can be effective in relieving physical aspects of MPS I, although it does not seem to help the bone or eye symptoms. Children who receive bone-marrow transplants early (before the age of two) tend to have better mental development, although they still have learning problems and progressive mental decline. Outcomes of the procedure do vary, but a bone-marrow transplant can prolong the lifespan of a person with MPS I, even though it will still be significantly shortened. This procedure carries a high risk of fatality.
Umbilical cord blood is a more recent treatment for MPS I, allowing for an unrelated donor and eliminating the need for total body radiation, as is the norm with a bone-marrow transplant. This treatment can prolong the lifespan of an affected child, but does not help the bone and eye issues. A cord blood transplant can help prevent a certain measure of developmental delay if it is performed before significant damage is done to the intellect, often before the age of 18 months. Like bone-marrow transplants, the procedure itself carries a high risk of fatality and can result in a variety of outcomes. For these reasons, these methods are used primarily for the severe form of MPS I.
Enzyme replacement therapy (ERT) using recombinant human alpha-L-iduronidase has also been shown to benefit individuals with MPS I, relieving many of the physical symptoms. ERT may be used alone in attenuated cases, or in tandem with the above surgical options in severe cases. The combination of these two approaches in severe MPS I may slow mental decline. Improvements in liver size, joint mobility, breathing, and liver size have been observed with ERT treatment.
It is important to note that the outcome of these treatments depends on the age of the individual and the severity of the disease. Early treatment leads to better outcomes, and less-severe disease shows the best improvement in quality of life and life expectancy. Other symptoms of the disease can be addressed as they arise. Examples of these treatments include special education for developmental delays, heart-valve replacement, shunting to remove excess fluid and relieve pressure from around the brain, sunglasses or hats to promote better vision, and physical therapy to aid in movement.
What Is the Prognosis for a Person with MPS I?
The prognosis for individuals with severe MPS I is generally poor. They need special education and assistance to perform ordinary daily functions and are often wheelchair-bound. Death usually occurs within the first ten years of life, although early treatment such as a bone-marrow transplant can extend the lifespan. Heart and breathing problems are often the cause of death among children with severe MPS I. Individuals with attenuated MPS I have a variable severity of disease and lifespan.