Development of an assay for the detection of mucopolysaccharidosis type VI patients using dried blood-spots

Family study of a novel mutation of mucopolysaccharidosis type VI with a severe phenotype and good response to enzymatic replacement therapy: Case report

RATIONALE

Mucopolysaccharidosis type VI (MPS VI) or Maroteaux-Lamy syndrome is produced by the deficiency of the enzyme arylsulfatase B, responsible for the hydrolysis of N-acetyl-D-galactosamine, chondroitin sulfate, and dermatan sulfate.

PATIENT CONCERNS

A 3-year-old male with Moroccan origins is the index case. He had healthy consanguineous parents and 4 healthy brothers and sisters. The patient showed a wide spectrum of symptoms including skeletal dysplasia and short stature with elevated glycosaminoglycans (GAGs) in urine.

DIAGNOSES, INTERVENTIONS, AND OUTCOMES

GAGs were quantified by spectrometry method with 1,9-dimethylen blue in 24-hour urine samples. The qualitative analysis of urine GAGs was obtained by thin-layer chromatography to determine the predominant presence of dermatan sulfate. The activities of both arylsulfatase B and beta-galactosidase as well as genetic studies were performed in dried blood spots. The genetic study was performed with deoxyribonucleic acid by massive sequencing a of lisosomal storage diseases. Results showed a new mutation c.263A > C with the severe phenotype in homozygous in the patient. The familiar study of ARSB and GLB1 genes presented some asymptomatic SNPs but with a discrete decrease in the activity of arylsulfatase B and beta-galactosidase. After an early detection by pediatricians, and both enzymatic and genetic confirmation, the patient had a good response to substitutive enzymatic treatment with galsulfase.

LESSONS

Mucoplysaccharidosis type VI is an autosomal recessive rare disease characterized by a lysosomal storage disorder. Although a number of mutations have been already associated to the disease, we have found a new mutation located in the arylsulfatase B enzyme gene. We have described that this mutation is the ultimate cause of a severe presentation of the disease.

Alpha-l-iduronidase and arylsulfatase B in dried blood spots on filter paper: Biochemical parameters and time stability

BACKGROUND

The goal of this study was to assess the biochemical parameters of the enzymes α-l-iduronidase (IDUA) and arylsulfatase B (ASB), which are deficient in mucopolysaccharidosis (MPS) I and VI, respectively, in dried blood spot (DBS) samples impregnated on filter paper.

METHODS AND RESULTS

The optimal pH, Km, and Vmax of IDUA and ASB in DBS are hereby presented. After these analyses, the reference values for the activities of these enzymes in DBS with cutoff of 3.65nmol/h/mL for IDUA and 6.80nmol/h/mL for ASB were established. The research also showed that the stability (21days) of the IDUA activity is lower than ASB, which maintained its enzymatic activity stable up until 60days of analysis, after impregnating the filter paper with blood.

CONCLUSION

Currently, DBS ensures important advantages in handling storage and transportation of samples with respect to neonatal screening programs. This study contributes to characterizing and differentiating the biochemistry of deficient enzymes in MPSs I and VI of DBS samples.

Improving arylsulfatase activity determination in dried blood spots: Screening and diagnostic approaches for Maroteaux-Lamy syndrome (MPS VI)

BACKGROUND

Mucopolysaccharidosis type VI can be screened by measuring the lysosomal arylsulfatase B (ARSB) residual enzyme activity in dried blood spots (DBS) using synthetic substrates. However, we have found experimental obstacles when determining ARSB activity with the fluorescent method due to the significant quenching effect rendered by DBS components.

METHODS

We adapted the methods originally described by Chamoles et al. [1] and Civallero et al. [2] and put forward 2 distinct approaches for ARSB activity quantification from DBS samples by measuring the 4-methylumbelliferone (β-MU) fluorescence generated from the ARSB 4-methylumbelliferone sulfate (β-MUS) substrate.

RESULTS

We demonstrate the high throughput feasibility of a novel approach for measuring ARSB activities by incorporating tailor-made calibration curves according to each patient's DBS sample quenching properties. The second method is used to calculate ARSB activities by measuring the fluorescence and absorbance parameters in each reaction sample with a single DBS-free calibration curve.

CONCLUSIONS

The quantitative correlation between the DBS sample absorbance and its quenching effect can be used to calculate predictive ARSB activities and would serve as an affordable first tier screening test. The method described herein demonstrates the critical importance of adapting the β-MU calibration curves to each patient's unique DBS sample matrix and its positive impact on the accuracy and reliability of ARSB activity measurements.

Newborn screening for lysosomal storage disorders

Every newborn in the U.S. is screened for at least 29 disorders, where evidence suggests that early detection is possible and beneficial. With new or improved treatment options and development of high-throughput screening tests, additional conditions have been proposed for inclusion in newborn screening programs. Among those are several lysosomal storage disorders that have been evaluated in limited pilot studies or that are already included in a few national or international newborn screening programs. These conditions include Pompe disease, Niemann-Pick type A/B disease, Fabry disease, Krabbe disease, Mucopolysaccharidoses types I and II, and Gaucher disease. Here, we review the current state of newborn screening for these lysosomal storage disorders.

Newborn screening for lysosomal storage diseases

BACKGROUND

There is worldwide interest in newborn screening for lysosomal storage diseases because of the development of treatment options that give better results when carried out early in life. Screens with high differentiation between affected and nonaffected individuals are critical because of the large number of potential false positives.

CONTENT

This review summarizes 3 screening methods: (a) direct assay of enzymatic activities using tandem mass spectrometry or fluorometry, (b) immunocapture-based measurement of lysosomal enzyme abundance, and (c) measurement of biomarkers. Assay performance is compared on the basis of small-scale studies as well as on large-scale pilot studies of mass spectrometric and fluorometric screens.

SUMMARY

Tandem mass spectrometry and fluorometry techniques for direct assay of lysosomal enzymatic activity in dried blood spots have emerged as the most studied approaches. Comparative mass spectrometry vs fluorometry studies show that the former better differentiates between nonaffected vs affected individuals. This in turn leads to a manageable number of screen positives that can be further evaluated with second-tier methods.

Unresolving short stature in a possible case of mucopolysccharidosis

We present a metabolic disorder with main complaints of unresolving short stature following prolonged treatment for rickets. ES is a 4-year-old male who first presented to our hospital on self-referral but had been seen previously at another tertiary health facility. The complaints were a swelling on the back and poor growth since 1 year of age with associated skeletal deformities: Chest wall, wrists, knees and ankle joints, which were progressive. Examination revealed a severely stunted child with a large head and caput quadratum, craniofacial disproportion, coarse facial features, saddle-shaped nose, thick lips and bilateral corneal clouding/opacities. He had very poor language development for his age. His diagnoses based on clinical and radiological assessment was in keeping with Hurlers type of mucopolysaccharidoses. We highlight this case to emphasize the need for early consideration of other possible rare differential diagnoses in metabolic conditions in children.

Dried blood spots for the enzymatic diagnosis of lysosomal storage diseases in dogs and cats

BACKGROUND

In people, lysosomal storage diseases (LSD) can be diagnosed by assaying enzyme activities in dried blood spots (DBS).

OBJECTIVE

The aim of this study was to evaluate the feasibility of using DBS samples from dogs and cats to measure lysosomal enzymatic activities and diagnose LSD.

METHODS

Drops of fresh whole blood collected in EDTA from dogs and cats with known or suspected LSD and from clinically healthy dogs and cats were placed on neonatal screening cards, dried, and mailed to the Metabolic Laboratory, University Children's Hospital, Frankfurt, Germany. Activities of selected lysosomal enzymes were measured using fluorescent substrates in a 2-mm diameter disk (~2.6 μL blood) punched from the DBS. Results were expressed as nmol substrate hydrolyzed per mL of blood per minute or hour.

RESULTS

Reference values were established for several lysosomal enzyme activities in DBS from dogs and cats; for most enzymes, activities were higher than those published for human samples. Activities of β-glucuronidase, N-acetylglucosamine-4-sulfatase (arylsulfatase B), α-mannosidase, α-galactosidase, α-fucosidase, and hexosaminidase A were measureable in DBS from healthy cats and dogs; α-iduronidase activity was measureable only in cats. In samples from animals with LSD, markedly reduced activity of a specific enzyme was found. In contrast, in samples from cats affected with mucolipidosis II, activities of lysosomal enzymes were markedly increased.

CONCLUSIONS

Measurement of lysosomal enzyme activities in DBS provides an inexpensive, simple, and convenient method to screen animals for suspected LSD and requires only a small sample volume. For diseases in which the relevant enzyme activity can be measured in DBS, a specific diagnosis can be made.

Expert recommendations for the laboratory diagnosis of MPS VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease caused by a deficiency of N-acetylgalactosamine 4-sulfatase (arylsulfatase B, ASB). This enzyme is required for the degradation of dermatan sulfate. In its absence, dermatan sulfate accumulates in cells and is excreted in large quantities in urine. Specific therapeutic intervention is available; however, accurate and timely diagnosis is crucial for maximal benefit. To better understand the current practices for diagnosis and to establish diagnostic guidelines, an international MPS VI laboratory diagnostics scientific summit was held in February of 2011 in Miami, Florida. The various steps in the diagnosis of MPS VI were discussed including urinary glycosaminoglycan (uGAG) analysis, enzyme activity analysis, and molecular analysis. The following conclusions were reached. Dilute urine samples pose a significant problem for uGAG analysis and MPS VI patients can be missed by quantitative uGAG testing alone as dermatan sulfate may not always be excreted in large quantities. Enzyme activity analysis is universally acknowledged as a key component of diagnosis; however, several caveats must be considered and the appropriate use of reference enzymes is essential. Molecular analysis supports enzyme activity test results and is essential for carrier testing, subsequent genetic counseling, and prenatal testing. Overall the expert panel recommends caution in the use of uGAG screening alone to rule out or confirm the diagnosis of MPS VI and acknowledges enzyme activity analysis as a critical component of diagnosis. Measurement of another sulfatase enzyme to exclude multiple sulfatase deficiency was recommended prior to the initiation of therapy. When feasible, the use of molecular testing as part of the diagnosis is encouraged. A diagnostic algorithm for MPS VI is provided.

Mucopolysaccharidosis type VI in a Miniature Poodle-type dog caused by a deletion in the arylsulphatase B gene

AIM

To investigate and characterise an inborn error of metabolism in a dog with skeletal and ocular abnormalities.

METHODS

A 2.5-year-old small male Miniature Poodle-like dog was presented with gross joint laxity and bilateral corneal opacities. Clinical examination was augmented by routine haematology, serum chemistry, radiographs, pathology, enzymology and molecular genetic studies. Euthanasia was requested when the dog was 3 years of age because of progressively decreasing quality of life.

RESULTS

Radiology revealed generalised epiphyseal dysplasia, malformed vertebral bodies, luxation/subluxation of appendicular and lumbosacral joints with hypoplasia of the odontoid process and hyoid apparatus. These clinical and radiographic findings, together with a positive urinary Berry spot test for mucopolysaccharides, and metachromatic granules in leucocytes, were indicative of a mucopolysaccharidosis (MPS), a lysosomal storage disease. Histological lesions included vacuolation of stromal cells of the cornea, fibroblasts, chondrocytes, macrophages and renal cells. The brain was essentially normal except for moderate secondary Wallerian-type degeneration in motor and sensory tracts of the hind brain. Dermatan sulphate-uria was present and enzymology revealed negligible activity of N-acetylgalactosamine-4-sulphatase, also known as arylsulphatase B, in cultured fibroblasts and liver tissue. A novel homozygous 22 base pair (bp) deletion in exon 1 of this enzyme's gene was identified (c.103_124del), which caused aframe-shift and subsequent premature stop codon. The "Wisdom pure breed-mixed breed" test reported the dog as a cross between a Miniature and Toy Poodle.

CONCLUSIONS

The clinicopathological features are similar to those of MPS type VI as previously described in dogs, cats and other species, and this clinical diagnosis was confirmed by enzymology and molecular genetic studies. This is an autosomal recessively inherited lysosomal storage disease.

CLINICAL RELEVANCE

The prevalence of MPS VI in Miniature or Toy Poodles in New Zealand and elsewhere is currently unknown. Due to the congenital nature of the disorder, malformed pups may be subject to euthanasia without investigation and the potential genetic problem in the breed may not be fully recognised. The establishment of a molecular genetic test now permits screening for this mutation as a basis to an informed breeding policy.

Mucopolysaccharidosis VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease with progressive multisystem involvement, associated with a deficiency of arylsulfatase B leading to the accumulation of dermatan sulfate. Birth prevalence is between 1 in 43,261 and 1 in 1,505,160 live births. The disorder shows a wide spectrum of symptoms from slowly to rapidly progressing forms. The characteristic skeletal dysplasia includes short stature, dysostosis multiplex and degenerative joint disease. Rapidly progressing forms may have onset from birth, elevated urinary glycosaminoglycans (generally >100 microg/mg creatinine), severe dysostosis multiplex, short stature, and death before the 2nd or 3rd decades. A more slowly progressing form has been described as having later onset, mildly elevated glycosaminoglycans (generally <100 microg/mg creatinine), mild dysostosis multiplex, with death in the 4th or 5th decades. Other clinical findings may include cardiac valve disease, reduced pulmonary function, hepatosplenomegaly, sinusitis, otitis media, hearing loss, sleep apnea, corneal clouding, carpal tunnel disease, and inguinal or umbilical hernia. Although intellectual deficit is generally absent in MPS VI, central nervous system findings may include cervical cord compression caused by cervical spinal instability, meningeal thickening and/or bony stenosis, communicating hydrocephalus, optic nerve atrophy and blindness. The disorder is transmitted in an autosomal recessive manner and is caused by mutations in the ARSB gene, located in chromosome 5 (5q13-5q14). Over 130 ARSB mutations have been reported, causing absent or reduced arylsulfatase B (N-acetylgalactosamine 4-sulfatase) activity and interrupted dermatan sulfate and chondroitin sulfate degradation. Diagnosis generally requires evidence of clinical phenotype, arylsulfatase B enzyme activity <10% of the lower limit of normal in cultured fibroblasts or isolated leukocytes, and demonstration of a normal activity of a different sulfatase enzyme (to exclude multiple sulfatase deficiency). The finding of elevated urinary dermatan sulfate with the absence of heparan sulfate is supportive. In addition to multiple sulfatase deficiency, the differential diagnosis should also include other forms of MPS (MPS I, II IVA, VII), sialidosis and mucolipidosis. Before enzyme replacement therapy (ERT) with galsulfase (Naglazyme), clinical management was limited to supportive care and hematopoietic stem cell transplantation. Galsulfase is now widely available and is a specific therapy providing improved endurance with an acceptable safety profile. Prognosis is variable depending on the age of onset, rate of disease progression, age at initiation of ERT and on the quality of the medical care provided.

Dermatan sulfate and heparan sulfate as a biomarker for mucopolysaccharidosis I

Mucopolysaccharidosis I (MPS I) is an autosomal recessive disorder caused by deficiency of alpha-L-iduronidase leading to accumulation of its catabolic substrates, dermatan sulfate (DS) and heparan sulfate (HS), in lysosomes. This results in progressive multiorgan dysfunction and death in early childhood. The recent success of enzyme replacement therapy (ERT) for MPS I highlights the need for biomarkers that reflect response to such therapy. To determine which biochemical markers are better, we determined serum and urine DS and HS levels by liquid chromatography tandem mass spectrometry in ERT-treated MPS I patients. The group included one Hurler, 11 Hurler/Scheie, and two Scheie patients. Seven patients were treated from week 1, whereas the other seven were treated from week 26. Serum and urine DS (DeltaDi-4S/6S) and HS (DeltaDiHS-0S, DeltaDiHS-NS) were measured at baseline, week 26, and week 72. Serum DeltaDi-4S/6S, DeltaDiHS-0S, and DeltaDiHS-NS levels decreased by 72%, 56%, and 56%, respectively, from baseline at week 72. Urinary glycosaminoglycan level decreased by 61.2%, whereas urine DeltaDi-4S/6S, DeltaDiHS-0S, and DeltaDiHS-NS decreased by 66.8%, 71.8%, and 71%, respectively. Regardless of age and clinical severity, all patients showed marked decrease of DS and HS in blood and urine samples. We also evaluated serum DS and HS from dried blood-spot samples of three MPS I newborn patients, showing marked elevation of DS and HS levels compared with those in control newborns. In conclusion, blood and urine levels of DS and HS provide an intrinsic monitoring and screening tool for MPS I patients.

Expensive drugs for rare disorders: to treat or not to treat? The case of enzyme replacement therapy for mucopolysaccharidosis VI

BACKGROUND

Mucopolysaccharidosis VI (MPS VI) is a very rare, chronically debilitating lysosomal storage disorder that develops in people with an enzyme deficiency. Clinical characteristics and progression rates vary widely between patients. The recent introduction of enzyme replacement therapy (ERT) has improved considerably the lives of patients with MPS VI, at an annual cost of treatment between euro 150,000 and euro 450,000 per patient.

SCOPE

This Commentary article addresses the controversial topic of granting reimbursement for expensive treatment options for orphan diseases, such as MPS VI. The discussion reflects clinical, economic and ethical aspects and incorporates insights from the relevant literature (based on a Medline search to September 2008) on MPS VI, efficacy of ERT, orphan drugs, and the economics and ethics of health-care prioritisation.

FINDINGS

Although ERT for MPS VI received marketing authorisation in the European Union in January 2006, patients' access to this therapy varies geographically due to differences between national reimbursement schemes for orphan drugs. Some inclusion and exclusion criteria for treatment of MPS VI patients with ERT appear arbitrary and may contribute to the exclusion from treatment of patients who could benefit in the long term. Reimbursement schemes which rely on proof of short-term treatment effectiveness may discriminate against slowly progressive patients, as health gain can often not be confirmed over a short period of time in these patients. Conventional cost-effectiveness analysis remains silent on crucial issues related to budgetary impact, i.e. opportunity cost from a system perspective, and fair access to treatment.

CONCLUSIONS

To prevent patients from being deprived of effective treatment, it is suggested that inclusion and exclusion criteria for treatment should be primarily based on a careful individual assessment of expected long-term clinical benefits. Once treatment has been agreed to as the correct option on clinical grounds, it is further argued that the conventional cost-effectiveness criterion currently in widespread use does not offer a sufficient basis for rejecting reimbursement of expensive treatments for exceptionally rare disorders, providing that decisions on reimbursement are intended to reflect public preferences.

Mutational analysis of 105 mucopolysaccharidosis type VI patients

Mucopolysaccharidosis type VI (MPS VI; Maroteaux-Lamy syndrome) is a lysosomal storage disorder caused by mutations in the N-acetylgalactosamine-4-sulfatase (arylsulfatase B, ARSB) gene. ARSB is a lysosomal enzyme involved in the degradation of the glycosaminoglycans (GAG) dermatan and chondroitin sulfate. ARSB mutations reduce enzyme function and GAG degradation, causing lysosomal storage and urinary excretion of these partially degraded substrates. Disease onset and rate of progression is variable, producing a spectrum of clinical presentation. In this study, 105 MPS VI patients-representing about 10% of the world MPS VI population-were studied for molecular genetic and biochemical parameters. Direct sequencing of patient genomic DNA was used to identify ARSB mutations. In total, 83 different disease-causing mutations were found, 62 of which were previously unknown. The novel sequence changes included: 38 missense mutations, five nonsense mutations, 11 deletions, one insertion, seven splice-site mutations, and four polymorphisms. ARSB mutant protein and residual activity were determined on fibroblast extracts for each patient. The identification of many novel mutations unique to individuals/their families highlighted the genetic heterogeneity of the disorder and provided an appropriate cohort to study the MPS VI phenotypic spectrum. This mutation analysis has identified a clear correlation between genotype and urinary GAG that can be used to predict clinical outcome.

Management guidelines for mucopolysaccharidosis VI

Mucopolysaccharidosis VI (Maroteaux-Lamy syndrome) is a lysosomal storage disease that is characterized by systemic clinical manifestations and significant functional impairment. Diagnosis and management are often challenging because of the considerable variability in symptom presentation and rate of progression. The optimal standard of care should be based on evidence from randomized, controlled trials, meta-analyses, systematic reviews, and expert opinion. In support of this goal, comprehensive management guidelines have been drafted by an international group of experts in the management of patients with mucopolysaccharidosis VI. The guidelines provide a detailed outline of disease manifestations by body system, recommendations for regular assessments, and an overview of current treatment options.

Twelve different enzyme assays on dried-blood filter paper samples for detection of patients with selected inherited lysosomal storage diseases

BACKGROUND

Diagnoses of inherited lysosomal storage diseases are based on specific enzymatic assays performed on plasma, leukocytes, fibroblasts, and lately, dried-blood filter paper samples. We evaluated feasibility of detecting of patients with several inherited lysosomal storage diseases using dried-blood filter paper samples for appropriate enzyme assays.

METHODS

Fluorometric methods were used to evaluate the activities of arylsulfatase B, alpha-N-acetylglucosaminidase, chitotriosidase, alpha and beta-galactosidases, beta-glucosidase, beta-glucuronidase, total hexosaminidases, hexosaminidase A, alpha-iduronidase, and iduronate-2-sulfatase. A radiometric method was used for sphyngomyelinase determination. Single 3.0-mm diameter disks containing dried-blood samples were incubated at 37 degrees C with appropriate dilution buffers and artificial substrates, and the fluorescence or radioactivity was measured.

RESULTS

Our results showed a statistically significant difference of the enzyme activity between affected individuals and controls, in all the assays performed. In contrast, we have not obtained a complete differentiation between heterozygotes and controls with these assays.

CONCLUSIONS

Enzyme assay on dried-blood filter paper is a suitable method to screen for several lysosomal storage diseases. Despite the low individual incidence of these pathologies, the incorporation of individual enzyme assays in neonatal screening programs could be justified to screen for diseases with relatively high local frequency and therapeutic measures available.

N-acetylgalactosamine-6-sulfatase protein detection in MPS IVA patient and unaffected control samples

BACKGROUND

Mucopolysaccharidosis type IVA (MPS IVA; Morquio syndrome) is a lysosomal storage disorder caused by a deficiency in the activity of the lysosomal hydrolase N-acetylgalactosamine-6-sulfatase (GALNS). MPS IVA patients can present with severe myelopathy, hearing loss, heart valve involvement, short trunk/dwarfism and corneal clouding. Early diagnosis of MPS IVA will allow potential treatments to be implemented before the onset of irreversible pathology.

METHODS

We have developed a sensitive immune-quantification assay for the accurate detection of GALNS protein in skin fibroblasts, blood and plasma from unaffected control and MPS IVA patients.

RESULTS

MPS IVA patient fibroblast extracts (n=11) had non-detectable (ND)-10 ng/mg of 6-sulfatase protein compared to 3-82 ng/mg for normal controls (n=19). Dried blood-spots from MPS IVA patients (n=4) contained ND-1.3 ng/L of 6-sulfatase protein compared to 18-145 ng/L for normal controls (n=49). Plasma from MPS IVA patients (n=7) contained ND 6-sulfatase protein compared to 1-9 ng/L for normal controls (n=49).

CONCLUSIONS

The immune assay described here had the capacity to accurately measure the amount of GALNS protein in various biological samples, providing the basis of an assay that could be further developed to enable newborn and high-risk population screening for MPS IVA patients.

Newborn screening for lysosomal storage disorders

Lysosomal storage disorders (LSD) are chronic progressive diseases that have a devastating impact on the patient and family. Most patients are clinically normal at birth but develop symptoms early in childhood. Despite no curative treatment, a number of therapeutic options are available to improve quality of life. To achieve this, there is a pressing need for newborn screening to identify affected individuals early, before the onset of severe irreversible pathology. We have developed a multiplexed immune-quantification assay of 11 different lysosomal proteins for the identification of individuals with an LSD and evaluated this assay in a retrospective study using blood-spots from; newborns subsequently diagnosed with an LSD (n=19, six different LSD), individuals sampled after diagnosis of an LSD (n=92, 11 different LSD), newborn controls (n=433), and adult controls (n=200). All patients with mucopolysaccharidosis type I (MPS I), MPS II, MPS IIIA, MPS VI, metachromatic leukodystrophy, Niemann-Pick disease type A/B, and multiple sulfatase deficiency could be identified by reduced enzyme levels compared to controls. All mucolipidosis type II/III patients were identified by the elevation of several lysosomal enzymes, above the control range. Most Fabry, Pompe, and Gaucher disease patients were identified from either single protein differences or profiles of multiple protein markers. Newborn screening for multiple LSD is achievable using multiplexed immune-quantification of a panel of lysosomal proteins. With further validation, this method could be readily incorporated into existing screening laboratories and will have a substantial impact on patient management and counseling of families.

Immunochemistry of lysosomal storage disorders

BACKGROUND

Lysosomal storage disorders are a group of genetic diseases, each with a broad spectrum of clinical presentation that ranges from attenuated to severe. The immunochemical analysis of patient samples is aimed at several key aspects of patient management, including early detection of the disorder, prediction of clinical severity, determining the most appropriate therapeutic regimen, and monitoring of patients on therapy.

METHODS

In this study, we review the current and emerging technology available to achieve these assessments.

RESULTS

Immune assays have direct practical application for the early detection, diagnosis and prognosis of lysosomal storage disorder patients. Multiplexing of these assays may provide a platform to allow newborn screening for multiple lysosomal storage disorders.

CONCLUSIONS

We have reviewed the immunochemical techniques available for the analysis of lysosomal storage disorder patient samples and advise that these may be used in conjunction with other technologies for effective patient management.

Detection of mucopolysaccharidosis type II by measurement of iduronate-2-sulfatase in dried blood spots and plasma samples

BACKGROUND

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disorder related to a deficiency in the enzyme iduronate-2-sulfatase (IDS). Clinical trials of enzyme replacement therapy are in progress, but effective treatment will require screening assays to enable early detection and diagnosis of MPS II. Our study evaluated the diagnostic accuracy of IDS protein and enzyme activity measurements in dried blood spots and plasma.

METHODS

We collected dried-blood-spot and plasma samples from unaffected control individuals and from MPS II patients. We measured IDS protein concentration with a 2-step time-delayed dissociation-enhanced lanthanide fluorescence immunoassay. To measure enzyme activity, we immobilized anti-IDS antibody on microtiter plates to capture the enzyme and measured its activity with the fluorogenic substrate 4-methylumbelliferyl sulfate.

RESULTS

Dried-blood-spot samples from MPS II patients showed an almost total absence of IDS activity (0-0.075 micromol x h(-1) x L(-1)) compared with control blood spots (0.5-4.7 micromol x h(-1) x L(-1)) and control plasma (0.17-8.1 micromol x h(-1) x L(-1)). A dried-blood-spot sample from only 1 of 12 MPS II patients had detectable concentrations of IDS protein (24.8 microg/L), but no IDS protein was detected in plasma from MPS II patients. Ranges for IDS protein in control samples were 25.8-153 microg/L for blood spots and 22.8-349.4 microg/L for plasma.

CONCLUSION

Measurement of the IDS protein concentration and enzyme activity (as measured by a simple fluorogenic assay with an immune capture technique) enables identification of the majority of MPS II patient samples from both dried blood spots and plasma samples.