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Mucopolysaccharidosis, MPS-I

 

Condition
Mucopolysaccharidosis, MPS-I
Class
Metabolic disorders
Description
Mucopolysaccharidoses (MPS) are a group of rare, genetic metabolic diseases caused by the absence of enzymes capable of breaking down large sugar molecules called glycosaminoglycans. There are seven different types of mucopolysaccharidosis, named according to the type of enzyme that is missing.

Although each type of MPS differs clinically, most patients generally experience a period of normal development followed by a progressive decline in physical and mental abilities and which, depending on severity, can cause premature death.

Hurler, Hurler-Scheie and Scheie syndromes are different forms of mucopolysaccharidosis Type I (MPS-I). Individuals most severely affected are classified as having Hurler syndrome, those with Scheie syndrome are more mildly affected, while patients who seem not to fit clearly at either the severe or the less severe end of the disease are said to have Hurler-Scheie syndrome.

The number of cases of MPS-I is estimated to be between one in 144,000 births for Hurler syndrome, and one in 1.3 million births for Scheie syndrome.

The remainder of this article will focus on MPS-I.
Causes
Mucopolysaccharides are long chains of sugar molecules used in the building of connective tissues in the body. Connective tissue is an important part of many physical structures, including tendons, blood, cartilage and bone.

In the body there is a continuous process of replacing used materials and breaking them down for disposal. People with MPS-I are missing an enzyme called alpha-L-idurodinase which is essential in cutting up the mucopolysaccharides called dermatan and heparan sulphate. The incompletely broken down mucopolysaccharides remain stored in cells in the body causing progressive damage. Babies and children may show little sign of the disease, but as more and more cells become damaged, symptoms start to appear.
Symptoms
Symptoms may be divided into 3 types - physical, medical and intelligence.

Physical: These are variable depending upon severity. Growth in individuals with Hurler disease is severely restricted and many children will have typical physical characteristics such as large heads, broad flat noses, enlarged tongues and coarse hair. The appearance of children with the less severe form of the disease is very variable and some will grow to normal height.

Medical: Children with the severe form of the disease suffer from major ear, nose and throat problems and pose a significant risk if requiring anaesthesia. Deafness, corneal clouding, hernias, heart disease and sleep apnoea are common among children with the more severe form. Joint stiffness is common in all forms of the disease and joint movement may become limited.

Intelligence: There is great variation depending on the severity of the disease. In Hurler syndrome, intelligence is severely affected and any skills learnt will be gradually lost as the disease progresses. Children with Hurler-Scheie syndrome may be of normal intelligence but some will have moderate learning difficulties. People with Scheie syndrome are of normal intelligence.
Treatment
Bone Marrow Transplantation
A bone marrow transplant is a treatment option for children severely affected with Hurler syndrome. A bone marrow donor has to be found and the best results are usually achieved when there is a compatible match with the bone marrow from the child’s brother or sister. There is a significant risk of developing graft versus host disease or developing another life threatening complication following a bone marrow transplant.

Enzyme Replacement Therapy
Enzyme replacement therapy aims to replace alpha-L-iduronidase, the enzyme that is missing in MPS-I.

Laronidase (Aldurazyme) is a form of alpha-L-iduronidase that is produced by recombinant DNA technology, a techniques developed to allow the production of large quantities of enzyme.

Laronidase is given by once weekly intravenous infusions. By replacing the missing enzyme, laronidase helps the body break down the mucopolysaccharides that build up in cells and tissues. Laronidase can by used for the treatment of patients with Hurler and Hurler-Scheie forms of MPS-I and for patients with Scheie syndrome who have moderate to severe symptoms. Unfortunately, enzyme replacement therapy does not currently correct the neurological symptoms experienced by some MPS-I patients.
Living with MPS-I
The news that you, your baby or a member of your family has MPS-I can come as a shock and it is natural to feel saddened and overwhelmed by the fact. As parents, you may have feelings of guilt and helplessness, blaming yourselves for your baby’s condition and wondering what to do next. As an individual, you may be worried about how the disease will affect you.

These are perfectly natural reactions. The important thing to remember is that you are not alone. Members of the healthcare team are there to help and support you. Try to get as much information that you think you need to help you understand and cope with the condition.
Further information
Genzyme, the pharmaceutical company producing Aldurazyme, has produced two useful websites that provide information for parents and healthcare professionals. Although the websites are tailored for America, they do provide useful information on the diagnosis and treatment of MPS-I, and offer some practical guidance.

www.aldurazyme.com
www.mps1disease.com


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