An improved method for glycosaminoglycan analysis by LC–MS/MS of urine samples collected on filter paper

A simple method for early age phenotype confirmation using toe tissue from a mouse model of MPS IIIA

RATIONALE

Determination of genotype can be difficult, especially during the early stages of developing an animal model, e.g. when PCR primers are not yet available. An increase or decrease in specific metabolites can be used as a surrogate marker for genotype; for instance, in homozygous MPS IIIA mice heparan sulphate (HS) is increased.

METHODS

A simple method was developed for extracting and depolymerising HS from mouse toe tissue using methanolysis under acidic conditions. The sample was lyophilised and resuspended in methanolic HCl. The reaction products are desulphated disaccharides and readily analysable by liquid chromatography/tandem mass spectrometry (LC/MS/MS) in positive ion multiple reaction monitoring mode. Measurements were normalised to a spiked deuterated HS internal standard and to endogenous chondroitin sulphate (CS).

RESULTS

HS was measured in toe tissue taken from 30 mice in three groups of 10 (normal controls, MPS IIIA homozygotes and heterozygotes). A significant difference was observed between the MPS IIIA homozygotes and the other two groups, making it possible to identify mice with the MPS IIIA genotype based on the measurement of HS. Normalisation to CS was shown to correct for sample variability and reaction efficiency.

CONCLUSIONS

Analysis of toe tissue provides a simple and rapid way of determining a storage phenotype at 5 to 7 days of age. Significantly, this method does not require any additional samples to be taken from animals, as it utilises tissue that is a by-product of toe clipping, a method that is routinely used to permanently identify mice.

Assay for Glycosaminoglycans by Tandem Mass Spectrometry and its Applications

Glycosaminoglycans (GAGs) are distributed in the whole body and play a variety of important physiological roles associated with inflammation, growth, coagulation, fibrinolysis, lipolysis, and cell-matrix biology. Accumulation of undegraded GAGs in lysosomes gives rise to a distinct clinical syndrome, mucopolysaccharidoses. Measurement of each specific GAG in a variety of specimens is urgently required to understand GAG interaction with other molecules, physiological status of patients, and prognosis and pathogenesis of the disease. We established a highly sensitive and accurate tandem mass spectrometry (LC-MS/MS) method for measurements of disaccharides derived from four specific GAGs [dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS), and chondroitin sulfate (CS)]. Disaccharides were produced by specific enzyme digestion of each GAG, and quantified by negative ion mode of multiple reaction monitoring. Subclasses of HS and GAGs with identical molecular weights can be separated using a Hypercarbcolumn (2.0 mm×50 mm, 5 μm) with an aectonitrile gradient in ammonium acetate (pH 11.0). We also developed a GAG assay by RapidFire with tandem mass spectrometry (RF-MS/MS). The RF system consists of an integrated solid phase extraction robot that binds and de-salts samples from assay plates and directly injects them into a MS/MS detector, reducing sample processing time to ten seconds. RF-MS/MS consequently yields much faster throughput than conventional LC-MS/MS-based methods. However, the RF system does not have a chromatographic step, and therefore, cannot distinguish GAGs that have identical molecular weights. Both methods can be applied to analysis of dried blood spots, blood, and urine specimens. In this article, we compare the assay methods for GAGs and describe their potential applications.

Newborn screening and diagnosis of mucopolysaccharidoses

Mucopolysaccharidoses (MPS) are caused by deficiency of lysosomal enzyme activities needed to degrade glycosaminoglycans (GAGs), which are long unbranched polysaccharides consisting of repeating disaccharides. GAGs include: chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS), and hyaluronan. Their catabolism may be blocked singly or in combination depending on the specific enzyme deficiency. There are 11 known enzyme deficiencies, resulting in seven distinct forms of MPS with a collective incidence of higher than 1 in 25,000 live births. Accumulation of undegraded metabolites in lysosomes gives rise to distinct clinical syndromes. Generally, the clinical conditions progress if untreated, leading to developmental delay, systemic skeletal deformities, and early death. MPS disorders are potentially treatable with enzyme replacement therapy or hematopoietic stem cell transplantation. For maximum benefit of available therapies, early detection and intervention are critical. We recently developed a novel high-throughput multiplex method to assay DS, HS, and KS simultaneously in blood samples by using high performance liquid chromatography/tandem mass spectrometry for MPS. The overall performance metrics of HS and DS values on MPS I, II, and VII patients vs. healthy controls at newborns were as follows using a given set of cut-off values: sensitivity, 100%; specificity, 98.5-99.4%; positive predictive value, 54.5-75%; false positive rate, 0.62-1.54%; and false negative rate, 0%. These findings show that the combined measurements of these three GAGs are sensitive and specific for detecting all types of MPS with acceptable false negative/positive rates. In addition, this method will also be used for monitoring therapeutic efficacy. We review the history of GAG assay and application to diagnosis for MPS.

Mucopolysaccharidosis type III (Sanfilippo syndrome) and misdiagnosis of idiopathic developmental delay, attention deficit/hyperactivity disorder or autism spectrum disorder

Mucopolysaccharidosis III is a rare genetic disease characterized by progressive cognitive decline and severe hyperactivity that does not respond to stimulants. Somatic features are relatively mild. Patients are often initially misdiagnosed as having idiopathic developmental delay, attention deficit/hyperactivity disorder and/or autism spectrum disorders, putting them at risk for unnecessary testing and treatments.

Conclusion: Children with developmental or speech delay, especially those with a characteristic somatic feature or behavioural abnormalities, should be screened for MPS III.

Assay of heparan-N-sulfamidase in dried leukocytes impregnated in filter paper: a new tool for the identification of mucopolisaccharidosis IIIA and potentially other lysosomal disorders

Diagnosis of lysosomal storage disorders (LSDs) is mainly based on specific enzyme assays in leucocytes. Dried blood spots have also been used as sample for the enzyme assays. However, some lysosomal enzymes such as heparan-N-sulfamidase (HNS) and others cannot be assayed by this material. We developed an assay for HNS using dried leukocytes impregnated in filter paper (DLFP) as source of enzyme, and the results allowed the correct identification of Mucopolisaccharidosis IIIA. From this proof of concept we predict that the assay of lysosomal enzymes in DLFP samples, which still needs further development, could be a useful tool for the diagnosis of LSDs, especially in regions where transportation of liquid blood samples in appropriate conditions for long distances and/or across country borders is challenging.