Mucolipidosis II – ML II

For more information please download our Guide to ML II.

What is ML II?

Mucolipidosis Type II (ML II) is one of the lysosomal storage disorders known collectively as Mucolipidoses. These disorders are closely related to but different from the Mucopolysaccharidoses.

ML II is sometimes referred to as ‘I-Cell Disease’. This term is derived from the observation of changes within a cell known as the fibroblast. A healthy fibroblast cell has a specific shape but in individuals affected by ML II this shape changes as a result of the accumulation of storage materials.

Dr. Jules Leroy from Belgium was one of the first doctors to write about the condition in the 1960’s and his name is sometimes used to refer to ML II.

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. The process requires a series of biochemical tools called enzymes. Enzymes can only reach the lysosomes after a special signal has been attached to them. In children with ML II the 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 II affect individuals differently?

This disease encompasses a spectrum of clinical symptoms which, at the severe end of the spectrum are labelled ML II. Less severely affected individuals are considered to have ML III.

How common are these diseases?

The MPS Society, which co-ordinates the ‘Registry for MPS and Related Diseases’ has shown that ML II is a rare condition, for example, between 1989 and 1999 22 babies were born with ML II in the UK.

How is the disease inherited?

ML II is an autosomal recessive disease; both parents must carry the same defective gene and each pass this same gene on to their child. Where both parents are carriers of the ML II 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 affected by ML II.

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