Diagnosis of Mucopolysaccharidoses

Abstract

The mucopolysaccharidoses (MPSs) include 11 different conditions caused by specific enzyme deficiencies in the degradation pathway of glycosaminoglycans (GAGs). Although most MPS types present increased levels of GAGs in tissues, including blood and urine, diagnosis is challenging as specific enzyme assays are needed for the correct diagnosis. Enzyme assays are usually performed in blood, with some samples (as leukocytes) providing a final diagnosis, while others (such as dried blood spots) still being considered as screening methods. The identification of variants in the specific genes that encode each MPS-related enzyme is helpful for diagnosis confirmation (when needed), carrier detection, genetic counseling, prenatal diagnosis (preferably in combination with enzyme assays) and phenotype prediction. Although the usual diagnostic flow in high-risk patients starts with the measurement of urinary GAGs, it continues with specific enzyme assays and is completed with mutation identification; there is a growing trend to have genotype-based investigations performed at the beginning of the investigation. In such cases, confirmation of pathogenicity of the variants identified should be confirmed by measurement of enzyme activity and/or identification and/or quantification of GAG species. As there is a growing number of countries performing newborn screening for MPS diseases, the investigation of a low enzyme activity by the measurement of GAG species concentration and identification of gene mutations in the same DBS sample is recommended before the suspicion of MPS is taken to the family. With specific therapies already available for most MPS patients, and with clinical trials in progress for many conditions, the specific diagnosis of MPS as early as possible is becoming increasingly necessary. In this review, we describe traditional and the most up to date diagnostic methods for mucopolysaccharidoses.

Keywords: enzyme replacement therapy; glycosaminoglycans; mucopolysaccharidoses; newborn screening.; tandem mass spectrometry.

Figure 1

Distribution of urinary glycosaminoglycans by age. MPS I: mucopolysaccharidosis type I; MPS II: mucopolysaccharidosis type II; MPS IIIA: mucopolysaccharidosis type IIIA; MPS VI: mucopolysaccharidosis type VI; MPS VII: mucopolysaccharidosis type VII; DMB: dimethyl methylene blue.

Figure 2

Electrophoresis of urinary glycosaminoglycans. CS: chondroitin sulfate, HS: heparan sulfate, KS: keratan sulfate. Top wells: 1, 2, 4, 5, 7, 8, 10, 11, 13, 15, 17 & 20 are not suggestive of MPS. 3, 16 & 18: Patients with DS suggestive of MPS VI (but confirmation with enzyme assay is needed). 6 & 12: Patients with DS and HS (perform enzyme assay for MPS I, II, and VII). 9, 14 & 19: patients with KS (perform enzyme assay for MPS IVA & IVB).

Figure 3

A. Chromatograms of glycosaminoglycans analyzed in dried blood spots from liquid chromatography tandem mass spectrometry in dried blood spots of a control and an MPS IIIB patient. B. Table shows the area counts of the chromatograms. HS-NS: heparan sulfate (0S and NS); MPS IIIB: mucopolysaccharidosis type IIIB.

Figure 4

Map with the regions where screening is regular or in pilot stages. Currently, several states of the United States of America (USA) are universally screening for MPS I, and the state of Illinois (IL) is also screening for MPS II. Some centers in Italy are conducting screening for MPS I and Taiwan is screening for MPS I with pilot studies for MPS II and VI. IL: Illinois, MPS I: mucopolysaccharidosis type I, MPS II: mucopolysaccharidosis type II.

Figure 5

Proposed flow-chart for the investigation of MPS in high-risk patients. GAG: glycosaminoglycans, DMB: dimethyl methylene blue, TLC: thin-layer chromatography, MPS: mucopolysaccharidosis, WBC: white blood cells, DBS: dried blood spots, DNA: deoxyribonucleic acid. *GAGs can also be analyzed by liquid chromatography tandem mass spectrometry.