Hunter syndrome, or MPS-II is a rare hereditary disorder affecting one in 100,000 people, where abnormal amounts of a group of substances called mucopolysaccharides (MPS for short)¹ build up in the tissues of the body. There are several such MPS storage disorders:

It is thought that one in 25,000 people are born with some form of MPS storage disorder.

How the Disease Works

The body is made up of billions of cells, held together into an ordered structure. Surrounding the cells is the extracellular matrix, a mixture of sugars and proteins which acts as the scaffolding and glue to keep the cells in their rightful place. One major constituent of the extracellular matrix is a large and complex molecule called a proteoglycan. Proteoglycans are continually broken down and replaced by the body, to help protect against wear and tear. They are broken down into mucopolysaccharides, which are in turn broken down into smaller molecules.

People with Hunter syndrome are unable to break down two specific mucopolysaccharides: Dermatan (found in skin, blood vessels and the heart) and heparan (found in the lungs, arteries and on cell membranes). This is due to a deficiency of an enzyme called iduronate-2-sulphatase (I2S for short). Because the dermatan and heparan are not being broken down and removed from the body, they can build up to toxic levels, causing the symptoms of the disease.

The Symptoms of Hunter Syndrome

Hunter syndrome can manifest itself in two forms: MPS-IIA and MPS-IIB. MPS-IIB is similar to MPS-IIA, though the symptoms are less severe and usually occur later in life.

MPS-IIA - the Severe Form

Children born with the type A variant usually begin to have symptoms after the first year of life. Initially the symptoms may be limited to colds, runny nose, ear infections and inguinal (groin) hernias. These are all relatively common in infants, and are unlikely to alert a doctor to the presence of Hunter syndrome. As time goes on, however, some or all of the following problems may be encountered:

MPS-IIB - the Mild Form

Even though the B variant is called the mild form, it can still cause significant problems for those who have it. This variant of Hunter syndrome usually manifests later in life (usually after childhood). People with MPS-IIB are often of normal intelligence, or may have mild mental retardation. Some or all of the following problems may be encountered:

How the Disease is Inherited:

Hunter syndrome is an inherited disorder - it is passed from one generation to the next. Every cell in the human body has 23 pairs of chromosomes, with one of each pair derived from each parent. One pair of chromosomes determines your sex: females have two X-chromosomes, one from each parent, while males have one X-chromosome inherited from their mother and one Y-chromosome inherited from their father. The X- (and to a lesser extent) the Y-chromosomes are not responsible for sex alone - they contain many genes that do other things.

The I2S gene is found on the X-chromosome. Females can therefore have two copies of the I2S gene, whereas males can only have one copy. There are five possible combinations of I2S gene:

• A male can have one X-chromosome with the normal I2S gene, and one Y-chromosome. This male will not have Hunter syndrome.

• A male can have one X-chromosome with a defective I2S gene, and one Y-chromosome. This male will have Hunter syndrome.

• A female can have two X-chromosomes, both with the normal I2S gene. This female will be unaffected by Hunter syndrome.

• A female can have one X-chromosome with the normal I2S gene, and one chromosome with an abnormal I2S gene. This female will be a carrier. Carriers do not experience any of the symptoms of Hunter syndrome.

• A female can have two X-chromosomes, both with an abnormal I2S gene. While technically this female would have Hunter syndrome, this is unheard of.

What this Means for your Family

If you are a male with Hunter syndrome your children will be:

• Boys who do not have Hunter syndrome - they do not carry the defective gene.

• Girls who are carriers of Hunter syndrome - they do not have any symptoms.

It is more complicated if you are a female carrier:

• If you have a son, he has a 50% chance of having Hunter syndrome.

• If you have a daughter, she has 50% chance of being a carrier of Hunter syndrome.

• This is all down to chance, so it is possible that all or none of your children are unaffected by Hunter syndrome.

On rare occasions, a child with or carrying Hunter syndrome is born to parents who do not have the defective gene. This is due to a new mutation of the I2S gene, and cannot be predicted.

Prognosis

People with the severe form of Hunter syndrome usually die by fifteen years of age, due to the overwhelming problems their symptoms can cause. Mental retardation is usually severe in these cases. Those with the mild form of Hunter syndrome live into their 50s or 60s, and there are people who have lived well into their 80s. Quality of life for mildly affected people is usually high, and there is usually no reason why such people cannot lead a relatively normal life. Mental retardation can be mild or non-existent.

Treatment

Sadly there is no cure for Hunter syndrome or many of its symptoms. Attempts have been made (with very limited success) to transfer I2S-producing cells from unaffected people into those with Hunter syndrome. These attempts have focused on the use of blood or bone marrow. Research is being carried out to examine the possibility of giving I2S infusions. Currently the only treatment on offer is supportive: physiotherapy; hydrotherapy; hearing aids; and symptomatic surgical procedures such as decompression of carpal tunnel syndrome.

Further Information

• Society of Mucopolysaccharide (MPS) Diseases provides support for those affected by Hunter syndrome world-wide. 46 Woodside Road,
  Amersham,
  Buckinghamshire HP6 6AJ
  UK
  WWW: www.mpssociety.co.uk
• The National MPS Society also provides help and support for those affected by Hunter syndrome. 102 Aspen Drive,
  Dowington, PA 19335
  USA
  WWW: www.mpssociety.org
• As does the Canadian Society for Mucopolysaccharide and Related Disorders. Postal Box 64714,
  Unionville,
  Ontario, L3R OM9
  WWW: www.mpssociety.ca
• Children Living with Inherited Metabolic Diseases (CLIMB) provides counselling and support for the families of children with an hereditary metabolic disease, as well as research funding and education for medical professionals. The Quadrangle,
  Crewe Hall,
  Weston Road,
  Crewe,
  Cheshire CW1 6UR
  UK
  WWW: www.climb.org.uk