Understanding Hunter syndrome

What is Hunter syndrome?1

Hunter syndrome, or mucopolysaccharidosis type II (MPS II), is a rare genetic disorder in which large sugar molecules called glycosaminoglycans (also known as GAGs, or mucopolysaccharides), build up in body tissues. Hunter syndrome is caused by a deficiency of the lysosomal enzyme iduronate-2-sulfatase (IDS).

What are the signs and symptoms of Hunter syndrome?2,3

Traditionally, Hunter syndrome is described as having two forms: the mild or attenuated form, affecting approximately one-third of patients, and the more severe or neuronopathic form, affecting the other two-thirds. However, it is understood that these two forms may actually represent a broad spectrum of disease severity. Patients with Hunter syndrome typically develop symptoms between 2-5 years of age and may present with coarse facial features, hepatosplenomegaly, joint and skeletal involvement, cardiopulmonary disease, and both central and peripheral nervous system dysfunction.

Some other features and symptoms of Hunter syndrome may include:

  • Carpal tunnel syndrome

  • Developmental delay

  • Disruptive behaviors

  • Hearing loss

  • Hydrocephalus (water on the brain)

  • Impaired cognition

  • Impaired sleep

  • Macrocephaly (large head)

  • Recurrent otitis media (ear infection)

  • Reduced pulmonary function

  • Reduced range of joint motion

  • Spinal stenosis

  • Valvular heart disease

  • Vision loss

What is the link between Hunter syndrome and genetics?2

Hunter syndrome is caused by a defective chromosome, and a child must have the defective chromosome to develop the disease. Hunter syndrome is what’s known as an X-linked recessive disease, which means that women who carry the defective disease-causing X chromosome can pass it on to their children, or the disease can occur if the X chromosome has a new mutation. The condition nearly always affects boys because males only have one X chromosome (inherited from their mother) while females have two X chromosomes (one inherited from each parent).

What is Iduronate-2-Sulfatase (IDS)?4

The IDS gene provides instructions for producing an enzyme called iduronate-2-sulfatase (IDS), which is essential for the breakdown of large sugar molecules called glycosaminoglycans (GAGs). Specifically, IDS removes a chemical group known as a sulfate from a molecule called sulfated alpha-L-iduronic acid, which is present in two GAGs called heparan sulfate and dermatan sulfate. IDS is located in lysosomes, compartments within cells that digest and recycle different types of molecules. When IDS is deficient, GAGs build up in the lysosomes and cause them to malfunction, leading to the symptoms of Hunter syndrome. The current standard treatment for Hunter syndrome, IV IDS replacement therapy, treats the peripheral symptoms of the disease but does not appear to impact the neurocognitive components of the disease.

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References: 1. https://www.mayoclinic.org/diseases-conditions/hunter-syndrome/symptoms-causes/syc-20350706 Accessed 07/06/2020. 2. Scarpa M. Mucopolysaccharidosis Type II. 2007 Nov 6 [Updated 2018 Oct 4]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020. 3. Ullman et al., Sci. Transl. Med. 12, eaay1163 (2020) 27 May 2020 4. https://ghr.nlm.nih.gov/gene/IDS Accessed 07/06/2020.