Researchers believe that the buildup of GAGs may also cause the features of MPS IV by interfering with the functions of other proteins inside lysosomes and disrupting the movement of molecules inside the cell. The accumulation of GAGs causes the bone deformities in this disorder. In MPS IV, GAGs accumulate to toxic levels in many tissues and organs, particularly in the bones. Conditions such as MPS IV that cause molecules to build up inside the lysosomes are called lysosomal storage disorders. Lysosomes are compartments in the cell that break down and recycle different types of molecules. Without these enzymes, GAGs accumulate within cells, specifically inside the lysosomes. Mutations in the GALNS and GLB1 genes reduce or completely eliminate the activity of the enzymes produced from these genes. In general, the two types of MPS IV cannot be distinguished by their signs and symptoms. When MPS IV is caused by mutations in the GALNS gene it is called MPS IV type A (MPS IVA), and when it is caused by mutations in the GLB1 gene it is called MPS IV type B (MPS IVB). GAGs were originally called mucopolysaccharides, which is where this condition gets its name. These genes provide instructions for producing enzymes involved in the breakdown of large sugar molecules called glycosaminoglycans (GAGs). Mutations in the GALNS and GLB1 genes cause MPS IV. Spinal cord compression and airway obstruction are major causes of death in people with MPS IV. Those with milder forms of the disorder usually live into adulthood, although their life expectancy may be reduced. Severely affected individuals may survive only until late childhood or adolescence. The life expectancy of individuals with MPS IV depends on the severity of symptoms. Unlike some other types of mucopolysaccharidosis, MPS IV does not affect intelligence. Other common features of this condition include mildly "coarse" facial features, thin tooth enamel, multiple cavities, heart valve abnormalities, a mildly enlarged liver (hepatomegaly), and a soft out-pouching around the belly-button (umbilical hernia ) or lower abdomen (inguinal hernia). The airway may become narrow in some people with MPS IV, leading to frequent upper respiratory infections and short pauses in breathing during sleep (sleep apnea). Some affected individuals have recurrent ear infections and hearing loss. In people with MPS IV, the clear covering of the eye ( cornea ) typically becomes cloudy, which can cause vision loss. Odontoid hypoplasia can lead to misalignment of the cervical vertebrae, which may compress and damage the spinal cord, resulting in paralysis or death. The odontoid process helps stabilize the spinal bones in the neck ( cervical vertebrae ). A characteristic feature of this condition is underdevelopment (hypoplasia) of a peg-like bone in the neck called the odontoid process. People with MPS IV often have joints that are loose and very flexible (hypermobile), but they may also have restricted movement in certain joints. Affected individuals develop various skeletal abnormalities, including short stature, knock knees, and abnormalities of the ribs, chest, spine, hips, and wrists. The first signs and symptoms of MPS IV usually become apparent during early childhood. The rate at which symptoms worsen varies among affected individuals. Mucopolysaccharidosis type IV (MPS IV), also known as Morquio syndrome, is a progressive condition that mainly affects the skeleton.
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