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Mucopolysaccharidosis IVA, (MPS IVA, Morquio A syndrome) is an autosomal recessive mucopolysaccharidosis caused by reduced or absent N-acetylgalactosamine-6-sulfate sulfatase (GALNS) enzyme activity. The mucopolysaccharidoses are a group of disorders caused by the deficiency of any of the enzymes involved in the stepwise degradation of dermatan sulfate, heparan sulfate, keratan sulfate, or chondroitin sulfate (glycosaminoglycans, GAGs). Accumulation of GAGs (previously called mucopolysaccharides) in lysosomes interferes with normal functioning of cells, tissues, and organs.
Clinical features and severity of symptoms of MPS IVA are widely variable and affect multiple body systems. Clinical features may include skeletal dysplasia, short stature, dental anomalies, corneal clouding, respiratory insufficiency, and cardiac disease. Intelligence is usually normal. Treatment options are mostly limited to symptom management, however, more recently available enzyme replacement therapy has shown to be effective in improving some function and quality of life for individuals with Morquio A. Estimates of the incidence of Morquio A syndrome range from 1 in 200,000 to 1 in 300,000 live births.
A diagnostic workup in an individual with MPS IVA typically demonstrates elevated levels of urinary GAGs and increased keratan sulfate and chondroitin-6-sulfate detected via quantitative and qualitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of the specific sulfates. Morquio B is a genetically distinct disorder caused by a deficiency of beta-galactosidase and has a significant number of overlapping clinical features with Morquio A. Enzyme analysis is necessary to distinguish between the 2 types. Reduced or absent activity of N-acetylgalactosamine-6-sulfate sulfatase enzyme in leukocytes or fibroblasts can confirm a diagnosis of MPS IVA. Sequencing of the GALNS gene allows for detection of disease-causing mutations in affected patients and identification of familial mutations allows for testing of at-risk family members.
Assisting in the diagnosis of Morquio A disease
Very low enzyme levels are consistent with Morquio A disease.
The test cannot be used to establish carrier status for Morquio A disease.
> or =163 nmol/17 hour/mg protein
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