What is the definition of Mucopolysaccharidosis Type 4?
Mucopolysaccharidosis type IV (MPS IV), also known as Morquio syndrome, is a rare metabolic condition in which the body is unable to break down long chains of sugar molecules called glycosaminoglycans. As a result, toxic levels of these sugars accumulate in cell structures called lysosomes, leading to the various signs and symptoms associated with the condition. Affected people generally develop features of MPS IV between the ages of 1 and 3. These signs and symptoms may include abnormalities of the skeleton, eyes, heart and respiratory system. There are two forms of MPS IV:
- MPS IVA is caused by changes (mutations) in the GALNS gene.
- MPS IVB is caused by mutations in the GLB1 gene.
Both forms are inherited in an autosomal recessive manner. Treatment is based on the signs and symptoms present in each person.
What are the alternative names for Mucopolysaccharidosis Type 4?
- Mucopolysaccharidosis type 4
- Morquio disease
What are the causes for Mucopolysaccharidosis Type 4?
There are two forms of mucopolysaccharidosis type IV (MPS IV): type A and type B. Type A is caused by changes (mutations) in the GALNS gene and type B is caused by mutations in the GLB1 gene. Both of these genes encode enzymes that help the body breakdown large sugar molecules called glycosaminoglycans (GAGs). If mutations reduce or eliminate the amount of these enzymes functioning in the body, GAGs accumulate in specialized cell structures called lysosomes (compartments in the cell that break down and recycle different types of molecules). This buildup of GAGs is toxic to various tissues in the body and leads to the signs and symptoms associated with the condition.
What are the symptoms for Mucopolysaccharidosis Type 4?
People affected by mucopolysaccharidosis type IV (MPS IV) often develop signs and symptoms of the condition in early childhood. The condition is considered progressive; however, the rate at which symptoms worsen varies significantly among affected people. All people affected by MPS IV develop skeletal problems such as scoliosis, knock-knees, short stature, pectus carinatum and variety of other abnormalities of the ribs, chest, spine, hips, and wrists. Another common feature of MPS IV is an underdeveloped odontoid process (a peg-like bone in the neck that helps stabilize the cervical vertebrae). This can misalign, compress and damage the spinal cord, leading to paralysis or even death.
Other features of the condition include:
- Coarse facial features
- Hypermobile joints
- Corneal clouding and vision loss
- Heart valve abnormalities
- Respiratory complications, including airway obstruction, sleep apnea and restrictive lung disease
- Widely-spaced, discolored teeth with thin enamel
- Mild to moderate hearing loss
- Mild hepatomegaly
How is Mucopolysaccharidosis Type 4 diagnosed?
A diagnosis of mucopolysaccharidosis type IV is often suspected based on the presence of characteristic signs and symptoms. Additional testing can then be ordered to confirm the diagnosis, determine the severity of the condition and rule out other conditions that can be associated with similar features. These tests may include:
- Specialized urine and blood tests
- Eye exam
- Skin fibroblast culture
- Genetic testing to identify mutations in the GALNS or GLB1 genes
Is Mucopolysaccharidosis Type 4 an inherited disorder?
Mucopolysaccharidosis type IV is inherited in an autosomal recessive manner. This means that to be affected, a person must have a mutation in both copies of the responsible gene in each cell. Affected people inherit one mutated copy of the gene from each parent, who is referred to as a carrier. Carriers of an autosomal recessive condition typically do not have any signs or symptoms (they are unaffected). When 2 carriers of an autosomal recessive condition have children, each child has a:
- 25% (1 in 4) chance to be affected
- 50% (1 in 2) chance to be an unaffected carrier like each parent
- 25% chance to be unaffected and not be a carrier