Sanfilippo Syndrome Type A Versus Type B

Sanfilippo syndrome (also called MPS III) is a rare genetic disease characterized by neurodegeneration, or the death of nerve cells. There are four subtypes of Sanfilippo syndrome, marked as A, B, C, and D. Patients have one of these types based on their genetic mutations.

Sanfilippo syndrome type A and B are the most prevalent worldwide.

What causes type A vs. type B?

Mutations in genes that play a role in the function and maintenance of  lysosomes cause all types of Sanfilippo syndrome. Lysosomes are the “recycling center” of cells. They digest old proteins, bits of membrane, and foreign particles inside cells.

Mutations in genes that encode for enzymes in the lysosomes, which play a role in breaking down large sugar-containing molecules called glycosaminoglycans (GAGs), cause both type A and type B Sanfilippo syndrome.

Mutations in the SGSH gene cause type A disease, while mutations in a gene called NAGLU cause type B disease. Without either gene, cells cannot break down GAGs, which build up inside cells and poison them.

Does my child have type A or type B disease?

The symptoms of Sanfilippo syndrome are very similar across disease types and appear at similar ages. However, there are indications that Sanfilippo syndrome type A may be more severe than other types. This can mean that symptoms appear sooner, and progress faster compared to the other types.

Elevated levels of GAGs in the blood and urine may indicate Sanfilippo syndrome in a child. A genetic test can give a definitive diagnosis and determine the disease’s type.

Do doctors treat these types in the same way?

There is currently no cure for Sanfilippo syndrome, and the treatments that exist aim to ease symptoms and improve patients’ quality of life. Their use does not depend on the type of Sanfilippo syndrome a child may have.

However, treatments in development, namely enzyme replacement therapy (ERT) and gene replacement therapy, will be tailored to the particular enzyme a patient is missing.

ERT is a treatment that supplies the enzyme that the patient is not able to make, via in an infusion or other means. For Sanfilippo syndrome, patients with type A disease would need to receive SGSH, while those with type B disease would need to receive NAGLU.

Similarly, in gene therapy, where the patient’s defective gene is replaced or supplemented with a healthy gene copy, patients would need to receive a different gene (SGSH for type A disease, and NAGLU for type B).

Both ERT and gene therapy are still experimental and cannot yet be used as treatments in a clinic.

Last updated: June 9, 2020

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