What causes MPS IIIA?

Mucopolysaccharidosis type IIIA (MPS IIIA), also known as Sanfilippo syndrome type A, is part of a family of lysosomal storage diseases called mucopolysaccharidoses.

These are genetic diseases that affect certain enzymes in the body that are found in lysosomes.

Enzymes icon

Enzymes

Proteins that facilitate a chemical reaction in the body, including breaking down other molecules.

Lysosome icon

Lysosome

Part of the cell that contains enzymes that break down complex molecules into smaller pieces that can be reused or recycled.

MPS IIIA is specifically caused by a change in a gene called SGSH

TYPICAL FORM OF SGSH
  • SGSH provides the instructions for a cell to create an enzyme called sulfamidase
  • Sulfamidase breaks down a complex sugar in the body called heparan sulfate
  • The broken-down parts of heparan sulfate can be recycled or reused
What causes MPS IIIA? What causes MPS IIIA?
FORM OF SGSH IN PEOPLE WITH MPS IIIA
  • The change in the SGSH gene may cause the cell to not make enough functional sulfamidase
  • Heparan sulfate isn’t broken down, so it builds up inside the cell
  • Too much heparan sulfate can damage cells, resulting in the signs and symptoms of MPS IIIA

What this means ...

It’s important that heparan sulfate stays at certain levels in the body. When it builds up, its toxic effects on the cells of the brain and body lead to the progressive effects of MPS IIIA.

How is MPS IIIA inherited?

MPS IIIA is inherited in what’s called an autosomal recessive manner.
Illustration depicting how MPS IIIA is inherited in what’s called an autosomal recessive manner from unaffected carrier parents
  1. Each parent carries 1 copy of the SGSH gene with a genetic change but does not have MPS IIIA because 2 copies are needed.
  2. Each child has a 1 in 4, or 25%, chance of not carrying a copy of the SGSH gene with a genetic change.
  3. Each child also has a 1 in 2, or 50%, chance of being a carrier, which means they carry 1 copy of the SGSH gene with a genetic change but they do not have MPS IIIA.
  4. Each child has a 1 in 4, or 25%, chance of having MPS IIIA, which means they have 2 copies of the SGSH gene with a genetic change, inheriting 1 copy from each parent.
Illustration depicting how MPS IIIA is inherited in what’s called an autosomal recessive manner from unaffected carrier parents
A parent and child embracing

How rare is MPS IIIA?

MPS IIIA occurs in fewer than

1 in 70,000 births

in the United States.*

*Data from 2017.

Discover the many signs and symptoms of MPS IIIA

Signs & Symptoms