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Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

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Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

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Questions

USMLE® Step 1 style questions USMLE

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A 4-year-old boy is brought to the clinic for evaluation of increasing head size and difficulties at school. The patient has had trouble interacting with his peers and is unable to communicate with full sentences. His teacher has reported several episodes of aggressive behavior towards other students as well. His birth was unremarkable, and he started walking at the age of 15 months. His parents have noticed that he tends to walk with an unsteady gait over the last month. His past medical history is significant for recurrent upper respiratory infections since the age of 8 months. Vitals are within normal limits. Weight and height are tracking at the 30th percentile, but head circumference is above the 90th percentile. His facial features are notable for a prominent forehead, flat nose bridge and enlarged lips, gums and tongue. Echocardiogram reveals left ventricular hypertrophy. Abdominal examination shows hepatosplenomegaly and an umbilical hernia. Ophthalmic examination is normal.  Accumulation of which of the following metabolites is responsible for this patient’s symptoms?

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Hunter syndrome p. 58, 86

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Content Reviewers

Rishi Desai, MD, MPH

Hunter syndrome is a rare genetic metabolic disorder that occurs when lysosomal enzymes are absent.

Hunter syndrome is also known as mucopolysaccharidosis type 2.

Mucopolysaccharides or glycosaminoglycans are types of complex sugars, which are produced by the cells and exported to the extracellular space.

Examples include heparan sulfate and dermatan sulfate.

Both heparan sulfate and dermatan sulfate can be found on almost all cell surfaces as well as in the basement membrane, which separates epithelial cells from the connective tissue that lies beneath.

When mucopolysaccharides like heparin sulfate and dermatan sulfate need to get degraded, they are taken over to a lysosome, which contains enzymes needed to break down the mucopolysaccharides.

Each mucopolysaccharide requires multiple enzymes to fully degrade, and some mucopolysaccharides share certain enzymes in common.

For example, both heparan sulfate and dermatan sulfate need the iduronate sulfatase and alpha-L-iduronidase enzymes to get broken down.

Hunter syndrome is caused by a deficiency in iduronate sulfatase, and the result is that heparan sulfate and dermatan sulfate can’t be degraded, so they build up in various tissues.

Hunter Syndrome is an X-linked recessive disorder so it’s more common in males.

Summary

Mucopolysaccharide storage disease type 2, also known as Hunter syndrome, is an inherited disorder caused by a deficiency in the iduronate sulfatase enzyme. This enzyme is responsible for breaking down glycosaminoglycans (GAGs) which are large molecules that include heparan sulfate and dermatan sulfate.

Deficiency in iduronate sulfatase results in GAGs accumulating in organs and tissues throughout the body, leading to a wide range of symptoms, such as developmental delay, hearing loss, respiratory difficulties, enlarged organs, and skin abnormalities. Treatment focuses on managing symptoms, and may include physical, occupational, and speech therapies, enzyme replacement therapy, and bone marrow transplantation.