MPS II (Hunter Syndrome)

Mucopolysarcharidosis type II (MPS II) is a rare, inherited disorder. MPS II is also known as Hunter syndrome. Children with this condition have an abnormal accumulation of complex sugars in their cells, which affects many systems in their bodies. Hunter syndrome primarily occurs in boys and is one of about 50 diseases classified as lysosomal storage disorders (LSD). In these disorders, genetic variations disrupt the normal activity of lysosomes in human cells.

Lysosomes contain specific proteins (enzymes) that are responsible for breaking down and recycling molecules such as fats and sugars. Individuals with a lysosomal storage disorder lack one of these necessary enzymes, or do not contain one of these enzymes in sufficient quantities to break down molecules for cells to function properly.

Newborns with Hunter syndrome typically do not have any signs or symptoms at birth. As children grow and develop, signs of disease may become more apparent.

Symptoms fall on a spectrum. Some children have only a few, mild symptoms. Other children experience significant complications of the disease.

Symptoms may include:

• Clouding of the front part of the eye (corneal clouding)
• Frequent upper respiratory infections
• Enlarged tonsils and/or adenoids
• Distinct facial features (coarse facial features)
• hernias

Over time, children might experience additional symptoms, including:

• Distinct white skin growths
• Deep, hoarse voice
• Enlargement of the heart chambers (ventricular hypertrophy)
• Enlargement of the liver and spleen (hepatosplenomegaly)
• Accumulation of fluid around the brain (hydrocephalus)
• Compression of the spinal cord
• Short stature
• Skeletal/joint abnormalities
• Intellectual or developmental decline

Hunter syndrome is caused by a variation in the IDS gene, which contains the instructions for the production of a specific enzyme known as I2S. This specialized protein is normally found in the lysosomes of cells, where it helps to break down complex sugars called glycosaminoglycans (GAGs). Genetic variations in the IDS gene result in a deficiency or a complete absence of I2S, which in turn results in an abnormal accumulation of GAGs in the body’s cells.

Babies with Hunter syndrome are born with an X-linked recessive pattern. The gene that causes the condition is located on the X chromosome, one of two sex chromosomes found in every cell. Because male babies have only one X chromosome, a single altered copy of the IDS gene in each cell is enough to cause disease in boys. Female babies have two X chromosomes so an altered copy of the IDS gene must be present in both of these X chromosomes to cause disease. Women who carry one normal X chromosome and one X chromosome with the genetic variation for disease are known as “carriers.”

Current approaches to Hunter syndrome are tailored to specific patients and may include enzyme replacement therapy (ERT) and targeted symptom management. Treatment plans require interdisciplinary collaboration and depend greatly on your child’s age and specific disease presentation.

The team of providers in the Boston Children's Lysosomal Storage Disorders (BoLD) Program is committed to the care of complex patients. As part of Boston Children’s Hospital, we are prepared to meet the challenge of providing multifaceted care by partnering with you and your child to deliver direct care in our BoLD clinic. We work with the broad array of world-class specialists at Boston Children’s to optimize the care we provide your child with Hunter syndrome.