Institute of Genomic Medicine
Mucopolysaccharidoses: Screening by Analysis of Glycosaminoglycans
The mucopolysaccharidoses (MPS) are a family of heritable disorders caused by deficiency of lysosomal enzymes needed to degrade mucopolysaccharides (preferably called glycosaminoglycans, or GAGs). The undegraded or partially degraded GAGs are stored in lysosomes and/or excreted in the urine. Clinical laboratories that provide diagnosis of MPS need simple screening methods that narrow down the list of possible enzyme defects, since clinically they can resemble each other. All known MPS are characterized by abnormal excretion of specific GAGs. Analysis of urinary GAGs facilitates differential diagnosis of MPS by demonstrating the biochemical defect and pointing to the probable enzyme defect.
INDICATIONS FOR TESTING:
The MPS share many clinical features in varying degrees, including a chronic and progressive course, multisystem involvement, organomegaly, dysostosis multiplex, and abnormal facies. Hearing, vision, cardiovascular function, and joint mobility may be affected. Profound mental retardation may be found in the Hurler, Hunter and San Filippo syndromes (MPS Types I, II and III) but normal intellect may be retained in other MPS and some mildly affected Hunter patients.
PROCEDURE:
Isolation of urinary GAGs is based upon the ability of quaternary ammonium salts to precipitate polyanionic molecules. 10% cetylpyridinium chloride (CPC) is added to urine and the resulting CPC-GAG complex is separated by centrifugation. Quantitative methods to detect urinary GAGs utilize their ability to bind dyes. Alcian blue dye reagent is added to the isolated urinary GAGs and measured at 480nm on a spectrophotometer. Although quantitation of urinary GAGs is effective for detection of MPS, it does not provide the specificity required to establish a specific diagnosis, and must be coupled to qualitative analysis. The qualitative analysis is done by thin-layer chromatography (TLC). GAGs are separated by TLC based on solubility of their calcium salts in solvents of varying ethanol concentration.
SAMPLE REQUIREMENTS:
20-25cc of random urine is required for testing, preferably first morning urine.
INTERPRETATION:
Urinary GAG excretion is age-dependent. Infants excrete more GAGs in their urine than adults. The patient's results are compared to known reference ranges grouped by age to detect excessive excretion of GAGs. Differential diagnosis is based upon detection of abnormal distribution of sulfated GAGs in urine. Normal urine contains mostly chondroitin sulfate, with small amounts of heparan sulfate and dermatan sulfate. Most of the MPS present patterns of urinary GAGs that are clearly different from normal. Patient's results are compared against various standards, and against positive and negative controls.
