Mucolipidosis III – ML III

For more information please download our Guide to ML III.

What is ML III?

Mucolipidosis Type III (ML III) is one of the lysosomal storage disorders known collectively as Mucolipidoses and is closely related to Mucopolysaccharidoses.

ML III is sometimes referred to as Pseudo-Hurler Polydystrophy as it resembled a less severe form of Hurler Disease (MPS I). Polydystrophy means that many organs are abnormal.

ML III was first described in 1966 by Dr Maroteaux and Dr Lamy from France.

What causes this disease?

In the course of normal life there is a continuous recycling process of building new materials and breaking down old ones ready for disposal. This activity takes place in a special part of the body’s cells called the lysosome. This process requires a series of biochemical tools called enzymes which can only reach the lysosomes after a special signal has been attached to them. In individuals with ML III this signal is not attached so the enzymes are unable to get to the right place and are therefore lost outside the cell.

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

Does ML III affect individuals differently?

Mucolipidoses are a group of storage disorders displaying a spectrum of clinical symptoms. At the severe end, these are labelled ML II. Less severely affected individuals are considered to have ML III.

How common is ML III?

The MPS Society, which co-ordinates the Registry for Mucopolysaccharide and related diseases has shown that ML III is a rare condition. For example, between 1989 and 1999, 5 babies were born with ML III in the UK.

How is the disease inherited?

ML III 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 ML III 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.

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