MPS VII – Sly disease

For more information please download our Guide to MPS VII.

What is MPS VII?

Sly Disease is a mucopolysaccharide storage disorder also known as Mucopolysaccharidosis type VII (MPS VII). Sly disease takes its name from William Sly who originally first described the disorder in 1972.

Although there is no cure for MPS diseases, there are ways of managing and treating the problems they cause.

What causes this disease?

Mucopolysaccharides are long chains of sugar molecule used in the building of connective tissues in the body.

  • “saccharide” is a general term for a sugar molecule (think of saccharin)
  • “poly” means many
  • “muco” refers to the thick jelly-like consistency of the molecules

In the course of normal life there is a continuous process of building new mucopolysaccharides and breaking down old ones – a recycling process. The breakdown and recycling process requires a series of special biochemical tools called enzymes. Individuals with Sly disease are missing or deficient in an enzyme called betaglucuronidase which is essential in the breaking down mucopolysaccharides. The incompletely broken down mucopolysaccharides remain stored inside the cells of the body and begin to build up causing progressive damage.

Babies may show little sign of the disease but as more and more cells become damaged by accumulation of mucopolysaccharides, symptoms start to appear.

Does MPS VII affect individuals differently?

Sly Disease, like most MPS and related disorders, is very variable. In its most extreme form, children are born with a condition called hydrops fetalis. This is a very severe condition in which the child retains an enormous amount of fluid throughout its body. Babies with hydrops fetalis rarely survive beyond a few weeks to a few months of age. Many individuals with Sly Disease are less severely affected and have clinical symptoms similar to other MPS and Related Diseases.

It is important to remember that Sly Disease is extremely varied in its effects and there is a wide spectrum of severity.

How common is this disease?

In the United Kingdom, Sly Disease is the rarest Mucopolysaccharidosis with an average of only one baby every 10 years born with the condition. Worldwide it is estimated to have a frequency of less than 1:250,000 births.

How is the disease inherited?

MPS VII is an autosomal recessive disease whereby both parents must carry the same defective gene and each pass this same defective gene to their child. Where both parents are carriers of the MPS VII gene there is a 25% (1:4) chance of having an affected child with each pregnancy.

There is a 50% (1:2) chance of a child receiving only one copy of the defective gene and therefore being a carrier. A carrier will not be affected but can pass the defective gene to his/her offspring. The remaining 25% (1:4) will be neither affected nor a carrier. Using information from an affected individual’s DNA, it may be possible to determine whether brothers and sisters are carriers of, or are affected by MPS VII.

There is a more detailed explanation of this complex subject in the booklet on inheritance available from the MPS Society.