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Mucolipidosis Type I


Mucolipidosis type I (ML I), also known as sialidosis, is a rare inherited lysosomal storage disease, belonging to the group of oligosaccharidosis that affects many organs and tissues, including the nervous system.

Read on for information about the condition or see the latest updates and resources.

Frequently asked questions

Oligosaccharides are chains of sugar molecules used in the building of bones, cartilage, skin, tendons and many other tissues in the body. “Oligo” means a few and “saccharide” is a general term for the sugar part of the molecule. In the course of normal life there is a continuous recycling process of building new oligosaccharides and breaking down old ones. In the course of normal life this recycling process requires a series of biochemical tools called enzymes.

People with ML I lack the specific enzyme or are low in the enzyme called alpha-neuraminidase which breaks down large oligosaccharides and removing a substance called sialic acid. When alpha-neuraminidase is not present or low in activity it is unable to break down oligosaccharides therefore sialic acid builds up the body causing damage to various tissues and organs in the body.

ML I is an autosomal recessive disease this means that both parents must carry the same affected gene and each pass this same affected gene to their child.

People probably carry from 5 to 10 genes with mutations in each of their cells. Problems happen when the particular gene is dominant or when a mutation is present in both copies of a recessive gene pair. Genes are the unique set of instructions inside our bodies that make each of us an individual. They are the blueprint for our growth and development, as well as controlling how our bodies function.

Genes are carried on structures called chromosomes and it is usual to have 23 pairs. A child will inherit half of the chromosomes from the mother and the other half from the father resulting in 23 pairs. 22 of these pairs look the same in both males and females. Pair 23 are the sex chromosomes, and this is the pair that differ between females and males. The X chromosome is inherited from the mother and the Y chromosome is inherited from the father. More information about inheritance is available in our publication.

For each pregnancy the chances of a baby inheriting ML I are completely independent of whether a previous child was affected with ML I. With each pregnancy there is a 1 in 4 chance that the baby will be affected by ML I.

All parents of children with ML I can benefit from genetic counselling, the counsellor can provide advice on the risk to close relatives and to suggest whether the wider family should be informed. To find out during a pregnancy, if the baby is affected by ML I, screening tests can be arranged early on during a pregnancy for those families who already have a child with ML I. Where only one parent is a carrier, they can opt for carrier screening but it is not 100% reliable or accurate and is not possible in all cases.

Amniocentesis and chorionic villus sampling are both available during the pregnancy to find out if the baby is affected by ML I.

It is estimated that nearly 6% of the UK population (around 3.5million people) will be affected by a rare disease at some point in their lives. A single rare disease may affect up to about 30,000 people however the vast majority of rare diseases affect far fewer than this.

ML I often goes unrecognised or misdiagnosed, determining the true frequency of ML I in the general population is difficult. It is estimated to occur between 1-4 people in every 200,000 of the general population.

People with ML I can experience some or many symptoms from a wide spectrum which range from severe to very mild. The severity of ML I varies widely across 2 types which are defined by the age of the patient when the first signs and symptoms start to appear.

  • Type 1, symptoms can develop anywhere from childhood to young adulthood.

  • Type 2, symptoms can be present at birth or develop shortly after birth.

ML I type 1

People with ML I type 1 may develop symptoms anywhere from childhood to young adulthood, with most people developing symptoms during their twenties and thirties. Development and growth is normal until problems with walking or when abnormalities require medical attention. ML I type 1 does not affect intelligence or life expectancy.

Muscle twitches (myoclonus), difficulty coordinating movements (ataxia), leg tremors, and seizures are symptoms of ML I type 1. Muscle twitching worsens over time, causing difficulty sitting, standing, or walking. People with ML I type 1 eventually require wheelchair assistance. Other symptoms of ML I type 1 include the development of distinct red spots in the eyes known as cherry-red macules. People with ML I may experience loss of clarity of vision and may develop impaired colour vision and night blindness. Rapid, involuntary eye movements (nystagmus) and clouding of the cornea may also occur.

ML I type 2

ML I type 2 the more severe, early onset form, is characterised by a progressive and severe mucopolysaccharidosis-like features such as coarse facial features, enlarged organs, and developmental delay. ML type 2 is further divided into congenital, infantile, and juvenile forms.

ML I type 2 congenital

The features of congenital ML type 2 can develop before birth. This form of ML I is associated with widespread swelling before birth caused by fluid accumulation. Affected infants may also have an enlarged liver and spleen, abnormal bone development, and distinctive coarse facial features. As a result of these serious health problems infants with congenital ML type 2 usually are stillborn or die soon after birth.

ML I type 2 infantile

Infantile form shares some features with the congenital form, although the signs and symptoms are slightly less severe and begin within the first year of life. As children with infantile ML I type 2 get older, they may develop involuntary muscle jerk movements and cherry-red macules in the eyes affecting vision. Other signs and symptoms include hearing loss, overgrowth of the gums, and widely spaced teeth. Affected children may survive into childhood or adolescence.

ML I type 2 juvenile

The juvenile form has the least severe signs and symptoms. Features of this type usually appear in late childhood and may include mildly coarse facial features, mild bone abnormalities, cherry-red macules, involuntary muscle jerk movements, intellectual disability, and dark red spots on the skin. The life expectancy of people with juvenile ML I type 2 varies depending on the severity of symptoms.

At present there is treatment for symptoms as they arise, but no cure for the underlying disease. More information on supportive care treatments for people with MPS and related diseases can be found in the treatments section.

For an up-to-date list of current UK based trials taking place visit Be Part of Research (resource provided by the National Institute for Health Research). For an international search visit Clinical Trials (resource provided by the U.S. National Library of Medicine).

This resource provides information on trial status including recruiting, completed or withdrawn and worldwide trial locations. To find out more about past or current trials speak to your doctor and learn about the risks and potential benefits.

The MPS Society is the only UK charity at the forefront of supporting people and families affected by MPS and related diseases. Our extensive support services offer you a wide range of support and resources.

The team can advise and sign post you to adequate needs-led support and services in your local area as well as social care, home adaptions, education and much more.

The support team can visit you in your home and provide you with vital support.

Get involved and support us in the community, volunteer or support fundraising; we are a small charity but with your support we can continue to offer a highly valued and essential service.

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