Iduronate-2-sulfatase, Whole Blood
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Mucopolysaccharidosis II, (MPS II, Hunter syndrome) is an X-linked lysosomal storage disorder caused by the deficiency of iduronate sulfatase (IDS). 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 the lysosomes interferes with normal functioning of cells, tissues, and organs. MPS II is caused by a reduced or absent activity of the IDS enzyme and gives rise to the physical manifestations of the disease.
Clinical features and severity of symptoms are widely variable ranging from severe disease to an attenuated form of the disease, which generally has a later onset with a milder clinical presentation. In general, symptoms may include coarse facies, short stature, enlarged liver and spleen, hoarse voice, stiff joints, cardiac disease, and profound neurologic involvement leading to developmental delays and regression. Because Hunter disease is an X-linked disorder, it occurs almost exclusively in males with an estimated incidence of 1 in 120,000 male births (though symptomatic carrier females have been reported). Treatment options include hematopoietic stem cell transplantation and/or enzyme replacement therapy.
A diagnostic workup in an individual with MPS II typically demonstrates elevated levels of urinary glycosaminoglycans and increased amounts of both dermatan and heparan sulfate. Reduced or absent activity of IDS in fibroblasts, leukocytes, and/or serum can confirm a diagnosis of MPS II; however, enzymatic testing is not reliable to detect carriers. Molecular genetic testing of the IDS gene allows for detection of the disease-causing mutation in affected patients and subsequent carrier detection in female relatives. Currently, no clear genotype-phenotype correlations have been established.
Diagnosis of mucopolysaccharidosis II (MPS II, Hunter syndrome) using dried blood spot specimens
Specimens with results <1.5 nmol/h/mL in properly submitted specimens are consistent with iduronate-2-sulfatase deficiency (mucopolysaccharidosis II: MPS II). If clinically indicated, consider further confirmation by molecular genetic analysis of the IDS gene. Please note that this enzyme's activity can also be reduced in multiple sulfatase deficiency (MSD; OMIM #272200). If clinically indicated, consider biochemical genetic testing of other sulfatases or molecular genetic testing of the SUMF1 gene to exclude MSD.
Normal results (> or =1.5 nmol/h/mL) are not consistent with iduronate-2-sulfatase deficiency.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test cannot reliably determine carrier status for mucopolysaccharidosis II.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
> or =1.5 nmol/h/mL
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Neufeld EF, Muenzer J: The mucopolysaccharidoses. In The Metabolic and Molecular Basis of Inherited Disease. Vol 3. Eighth edition. Edited by CR Scriver, AL Beaudet, WS Sly, et al. McGraw-Hill, Medical Publishing Division, 2001, p 3421
2. Chamoles NA, Blanco M, Gaggioli D, Casentini C: Hurler-like phenotype: enzymatic diagnosis in dried blood spots on filter paper. Clin Chem 2001;47:2098-2102