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HEREDITARY MONOGENIC DISEASES

Mucolipidosis type 2

The mucolipidoses (ML) are a group of inherited metabolic diseases that affect the body’s ability to carry out the normal turnover of various materials within cells. In ML, abnormal amounts of carbohydrates and fatty materials (lipids) accumulate in cells. Because our cells are not able to handle such large amounts of these substances, damage to the cells occurs, causing symptoms that range from mild learning disabilities to severe intellectual impairment and skeletal deformities.

Symptoms of ML can be congenital (present at birth) or begin in early childhood or adolescence. Early symptoms can include vision problems and developmental delays. Over time, many children with ML develop poor mental capacities, have difficulty reaching normal developmental milestones, and, in many cases, eventually die of the disease.

Patients with ML are born with a genetic defect in which their bodies either do not produce enough enzymes or, in some instances, produce ineffective forms of enzymes. Without functioning enzymes, which are proteins, lysosomes cannot break down carbohydrates and lipids and transport them to their normal destination. The molecules then accumulate in the cells of various tissues in the body, leading to damage of organs. In patients with ML, the molecules accumulate in nerve, liver, and muscle tissue as well as in bone marrow, and this abnormal storage causes the various symptoms associated with ML. For example, excess storage of these molecules in nerve tissues can cause mental retardation, accumulation in the tissues of the spleen and liver can cause poor functioning of these vital organs, and excess storage in the bone marrow can damage bones, leading to skeletal deformities.

The MLs are similar to another group of lysosomal storage diseases known as the mucopolysaccharidoses. While both conditions produce similar symptoms and are caused by the lack of enzymes necessary to break down and transport carbohydrates and lipids, the mucopolysaccharidoses result in an excess of sugars, known as mucopolysaccharides, in the urine. Mucopolysaccharides are not seen in urine of patients with ML, therefore screening of the urine can help doctors distinguish between the two groups of disorders.

There are four types of ML and each is classified according to the enzyme(s) or other protein that is deficient or mutated (altered). Symptoms can range from mild to severe.

The mucolipidosis type II is caused by a deficiency of the enzyme N-acetylglucosamine-1-phosphotransferase. It is a particularly severe form of mucolipidoses that looks like one of the mucopolysaccharidosis called Hurler syndrome. Some physical signs such as abnormal skeletal development, grotesque facial features, and restricted joint movement, can be present at birth. Children with ML II generally have enlarged organs such as the liver and spleen, and even heart valves. Often affected children do not grow or develop in the first months of life. Developmental delays in their motor skills are generally more pronounced than delays in their cognitive abilities. Children with ML II eventually develop corneal opacity and due to the lack of growth, develop short trunk dwarfism. They are frequent (pneumonia, otitis, bronchitis) respiratory infections and usually die before the age of seven years of life, often as a result of congestive heart failure or recurrent infections of the airways.

The mucolipidoses are similar to another group of lysosomal storage diseases known as mucopolysaccharidosis. Although both diseases and produce similar symptoms are caused by the lack of necessary for disintegrating and transporting carbohydrates and lipids enzymes, mucopolysaccharidosis produce excess sugars, known as mucopolysaccharides, in urine. Mucopolysaccharides are not seen in the urine of patients with mucolipidoses, so the evaluation of urine can help doctors distinguish between the two groups of disorders.

The diagnosis of mucolipidoses based on clinical symptoms, a complete medical history and laboratory tests. Diagnosing mucolipidosis I, II, and III can be confirmed by a blood test that measures the enzymatic activity of white blood cells of the patient. In addition, we have identified the genes responsible for the four types of mucolipidoses.

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