Liquid chromatographic assay for a glucose tetrasaccharide, a putative biomarker for the diagnosis of Pompe disease

The relation between dietary polysaccharide intake and urinary excretion of tetraglucoside

The urinary metabolite tetraglucoside (Glc4) is a potential biomarker for hepatic glycogen storage diseases (GSDs). Glc4 is believed to reflect body glycogen content and/or turnover. However, dietary polysaccharide intake may influence Glc4 excretion, potentially limiting the utility of Glc4 as a monitoring biomarker in hepatic GSDs. We aimed to investigate the association of dietary polysaccharide intake with Glc4 excretion. Urinary Glc4 excretion (mmol/mmol creatinine and mmol/24 h) was analyzed using a validated LC-MS/MS method. Data was analyzed from 65 kidney transplant recipients and 58 healthy kidney donors in the TransplantLines cohort study. Spearman's correlation and multivariable linear regression analyses were performed. In the multivariable analysis, dry lean body mass (DLBM), dietary polysaccharide intake, transplantation status, age, sex, and glycated hemoglobin (HbA1c) served as independent variables. Daily variation was examined in 21 healthy individuals of urinary Glc4 excretion in 2-h collections over a 24-h period. Mixed generalized additive models were built to study the association of prior polysaccharide intake with Glc4 excretion. No (univariate) associations were found between polysaccharide intake and Glc4 excretion. However, a significant interaction between DLBM and polysaccharide on 24 h Glc4 excretion was observed in the multivariate analysis. Glc4 excretion throughout the day exhibited no relationship to prior polysaccharide intake. Our findings suggest an indirect effect of polysaccharide intake on Glc4 excretion, potentially due to changes in muscle glycogen content and/or turnover. We have found no evidence that dietary polysaccharides under normal intakes increase urinary Glc4 directly.

First-in-Human Evaluation of Safety, Pharmacokinetics and Muscle Glycogen Lowering of a Novel Glycogen Synthase 1 Inhibitor for the Treatment of Pompe Disease

Pompe disease is a rare glycogen storage disease caused by mutations in the enzyme acid α-glucosidase (GAA) resulting in pathological accumulation of glycogen in muscle tissues leading to progressive weakness and respiratory dysfunction. Enzyme replacement therapy (ERT) with GAA is currently the sole treatment option for patients with Pompe disease. ERT burdens patients with frequent intravenous infusions while insufficiently halting disease progression due to incomplete ERT skeletal muscle distribution. Glycogen synthase 1 (GYS1) has been proposed as a substrate reduction therapy (SRT) target for Pompe disease. Here, we report results from the first-in-human study of the orally available GYS1 inhibitor MZE001 in healthy subjects. In 88 participants, MZE001 was well-tolerated up to a single dose of 480 mg BID and multiple doses of 720 mg BID for 10 days. Noncompartmental analysis determined that the half-life and Ctrough concentrations of MZE001 could provide efficacious exposures with once or twice daily oral dosing. Change from baseline of peripheral blood mononuclear cell (PBMC) glycogen, which correlated with muscle glycogen levels in preclinical models, was significantly reduced dose-dependently following 10 days of MZE001 treatment in healthy subjects. A muscle biopsy sub-study demonstrated that 10 days of MZE001 (480 mg BID) dosing safely and substantially lowered muscle glycogen stores in healthy adults. This correlated with the PBMC exposure response and supports the use of PBMC glycogen reduction as a surrogate for muscle response, and MZE001 potential for development as the first oral substrate reduction therapy for patients with Pompe disease.

Clinical insight meets scientific innovation to develop a next generation ERT for Pompe disease

Years of research into the structure, processing, and function of acid alpha-glucosidase led to the development and 2006 approval of alglucosidase alfa (recombinant human acid alpha-glucosidase, Myozyme®/Lumizyme®), an enzyme replacement therapy and the first approved treatment for Pompe disease. Alglucosidase alfa has been a lifesaving treatment for patients with infantile-onset Pompe disease and radically improved daily life for patients with late-onset Pompe disease; however, long-term experience with alglucosidase alfa unraveled key unmet needs in these populations. Despite treatment, Pompe disease continues to progress, especially from a skeletal muscle perspective, resulting in a multitude of functional limitations. Strong collaboration between the scientific and patient communities led to increased awareness of Pompe disease, a better understanding of disease pathophysiology, knowledge of the clinical course of the disease as patients surpassed the first decade of life, and the strengths and limitations of enzyme replacement therapy. Taken together, these advancements spurred the need for development of a next generation of enzyme replacement therapy and provided a framework for progress toward other novel treatments. This review provides an overview of the development of avalglucosidase alfa as a model to highlight the interaction between clinical experience with existing treatments, the role of the clinician scientist, translational research at both system and cellular levels, and the iterative and collaborative process that optimizes the development of therapeutics.

Development of a rapid simultaneous assay of two urinary tetrasaccharide metabolites using differential ion mobility and tandem mass spectrometry and its application to patients with glycogen storage disease (type Ib and II)

Glucose tetrasaccharide (Glc4) and maltotetraose (M4) are important biomarkers for Pompe disease and other glycogen storage diseases (GSDs). With the development of new treatments for GSDs, more specific and sensitive bioanalytical methods are needed to determine biomarkers. In recent years, differential mobility spectrometry (DMS) has become an effective analytical technique with high selectivity and specificity. This study aimed to develop an efficient analytical method for the two urinary tetrasaccharide metabolites using DMS and apply it to patients with GSDs (type Ib and II). Urine samples were directly diluted and injected into liquid chromatography-differential mobility spectrometry tandem mass spectrometry (LC-DMS-MS/MS). Chromatographic separation was performed on an Acquity™ UPLC BEH Amide column (2.1 × 50 mm, 1.7 μm) with a short gradient elution of 2.6 min. DMS-MS/MS was used to detect two urinary tetrasaccharide metabolites in a negative multiple reaction monitoring mode with isopropanol as a modifier. A total of 20 urine samples from 6 healthy volunteers and 10 patients with GSDs (type Ib and II) were collected for analysis. The method was linear over a concentration range of 0.5~100.0 µg/mL for each urinary tetrasaccharide (r≥0.99). The intra- and inter-day precision RSD% were less than 14.3%, and the accuracy RE% were in the range of -14.3~13.4%. The relative matrix effect was between 86.6 and 114.3%. No carryover or interference was observed. Patients with GSDs (type Ib and II) had significantly higher median urinary Glc4 (P=0.001) and M4 (P=0.012) excretion than healthy subjects. The developed method was simple, rapid, sensitive, and specific. It was successfully applied to healthy volunteers and patients with GSDs (type Ib and II). DMS technology greatly improved analysis efficiency and provided high sensitivity and specificity.

Rapid ultra-performance liquid chromatography-tandem mass spectrometry method for the simultaneous determination of three characteristic urinary saccharide metabolites in patients with glycogen storage diseases (type Ⅰb and Ⅱ)

Urinary 1,5-anhydroglucitol (1, 5-AG), 6-α-D-glucopyranosyl-maltotriose (Glc4) and maltotetraose (M4) are important biomarkers for glycogen storage disease (types Ib and Ⅱ). This study aimed to develop and validate an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) to detect these three urinary saccharide metabolites. Urine samples were diluted and then analyzed. Chromatographic separation was performed on an Acquity™ UPLC Amide column (2.1 × 100 mm, 1.7 μm) with gradient elution. The quantitation of analytes was achieved on a 5500 Qtrap mass spectrometer using negative multiple reaction monitoring (MRM) mode. The calibration curves for all analytes were linear over the range of 0.500 to 100 μg/mL with a correlation coefficient, R2 ≥ 0.999. The percent relative standard deviations (RSD%) were ≤12.8%, and the percent relative errors (RE%) were in the range of -11.7%-11.0%. The relative matrix effects of all analytes were between 87.2% and 104% with RSD% < 3.10% across three concentrations. The developed analytical method was simple, accurate, and reliable for rapid and simultaneous analysis of these three urinary saccharide metabolites. It was applied to healthy volunteers and patients. To our knowledge, it was the first validated assay for urinary maltotetraose quantification. This work provides support for exploring the potential of maltotetraose as a biomarker for Pompe disease.

Development of a kit for urine collection on filter paper as an alternative for Pompe disease screening and monitoring by LC-HRMS

Pompe disease (PD) is an inborn error of metabolism caused by α-glucosidase acid enzyme deficiency. It significantly impacts patients' health and life quality and may lead to death in the first few years of life. Among the well-established diagnostic methods, urinary glucose tetrasaccharide (Glc4) screening by high performance-liquid chromatography has been helpful in monitoring Glc4 levels in patients on enzyme replacement therapy, demonstrating therapy efficacy. However, the specimen shipping process from a sample collecting location to a specialized laboratory for monitoring the Glc4 is costly and presents preanalytical challenges. In this work, we developed a filter paper based-urine collection kit to facilitate specimen shipment, and liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS) analysis to determine Glc4 and creatinine in dried urine on filter paper. The LC-HRMS was based on a combination of targeted and untargeted screening on the same specimen injection and was successfully developed and validated. Bland-Altman statistics revealed a good relationship between dried and liquid urine samples and Glc4 and creatinine. Glc4 and other metabolites in dried urine showed stability for at least 7 days at 4 and 22 °C, and 3 days at 50 °C. The stability of the analytes and the efficiency of the kit were tested simulating real conditions by sending it by post. After two days in transit without refrigeration, the stability of compounds was maintained, showing the reliability of the urine collection kit and analysis method to determine the PD biomarker Glc4.

Combined targeted and untargeted high-resolution mass spectrometry analyses to investigate metabolic alterations in pompe disease

INTRODUCTION

Pompe disease is a rare, lysosomal disorder, characterized by intra-lysosomal glycogen accumulation due to an impaired function of α-glucosidase enzyme. The laboratory testing for Pompe is usually performed by enzyme activity, genetic test, or urine glucose tetrasaccharide (Glc4) screening by HPLC. Despite being a good preliminary marker, the Glc4 is not specific for Pompe.

OBJECTIVE

The purpose of the present study was to develop a simple methodology using liquid chromatography-high resolution mass spectrometry (LC-HRMS) for targeted quantitative analysis of Glc₄ combined with untargeted metabolic profiling in a single analytical run to search for complementary biomarkers in Pompe disease.

METHODS

We collected 21 urine specimens from 13 Pompe disease patients and compared their metabolic signatures with 21 control specimens.

RESULTS

Multivariate statistical analyses on the untargeted profiling data revealed Glc₄, creatine, sorbitol/mannitol, L-phenylalanine, N-acetyl-4-aminobutanal, N-acetyl-L-aspartic acid, and 2-aminobenzoic acid as significantly altered in Pompe disease. This panel of metabolites increased sample class prediction (Pompe disease versus control) compared with a single biomarker.

CONCLUSION

This study has demonstrated the potential of combined acquisition methods in LC-HRMS for Pompe disease investigation, allowing for routine determination of an established biomarker and discovery of complementary candidate biomarkers that may increase diagnostic accuracy, or improve the risk stratification of patients with disparate clinical phenotypes.

The Clinical Management of Pompe Disease: A Pediatric Perspective

Pompe disease (PD) is an inherited metabolic disorder caused by a deficiency of acid α-glucosidase (GAA), leading to lysosomal accumulation of glycogen, mainly in skeletal and cardiac muscles as well as the nervous system. Patients with PD develop cellular dysfunction and muscle damage. PD can be classified into two classic forms, namely infantile-onset PD (IOPD) and late-onset PD (LOPD). Delayed treatment, particularly in IOPD, would result in significant organ damage and early death. Nonetheless, early diagnosis and timely treatment are often hampered by the rarity of PD and its wide variety of, but overlapping, symptoms. This article reviews the common clinical presentations of PD and outlines the essentials of PD management. In particular, the implications of newborn screening (NBS) and clinical performance of enzyme replacement therapy (ERT) are highlighted.

Experience with the Urinary Tetrasaccharide Metabolite for Pompe Disease in the Diagnostic Laboratory

Following clinical indications, the laboratory diagnosis of the inherited metabolic myopathy, Pompe disease (PD), typically begins with demonstrating a reduction in acid alpha-glucosidase (GAA), the enzyme required for lysosomal glycogen degradation. Although simple in concept, a major challenge is defining reference intervals, as even carriers can have reduced GAA, and pseudodeficiencies complicate interpretation. Here, we developed a mass spectrometric assay for quantification of a urinary glycogen metabolite (tetrasaccharide) and reported on its utility as a confirmatory test for PD in a diagnostic laboratory. Using two age-related reference intervals, eight returned tetrasaccharide concentrations above the calculated reference interval but did not have PD, highlighting non-specificity. However, retrospective analysis revealed elevated tetrasaccharide in seven infantile-onset (IOPD) cases and sixteen late-onset (LOPD) cases, and normal concentrations in one heterozygote. Prospective tetrasaccharide analysis in nine individuals with reduced GAA confirmed IOPD in one, LOPD in six and identified two heterozygotes. Using this metabolite as a biomarker of therapeutic response was not overly informative; although most patients showed an initial drop following therapy initiation, tetrasaccharide concentrations fluctuated considerably and remained above reference intervals in all patients. While useful as a confirmation of PD, its utility as a biomarker for monitoring treatment warrants further investigation.

Biomarkers in Glycogen Storage Diseases: An Update

Glycogen storage diseases (GSDs) are a group of 19 hereditary diseases caused by a lack of one or more enzymes involved in the synthesis or degradation of glycogen and are characterized by deposits or abnormal types of glycogen in tissues. Their frequency is very low and they are considered rare diseases. Except for X-linked type IX, the different types are inherited in an autosomal recessive pattern. In this study we reviewed the literature from 1977 to 2020 concerning GSDs, biomarkers, and metabolic imbalances in the symptoms of some GSDs. Most of the reported studies were performed with very few patients. Classification of emerging biomarkers between different types of diseases (hepatics GSDs, McArdle and PDs and other possible biomarkers) was done for better understanding. Calprotectin for hepatics GSDs and urinary glucose tetrasaccharide for Pompe disease have been approved for clinical use, and most of the markers mentioned in this review only need clinical validation, as a final step for their routine use. Most of the possible biomarkers are implied in hepatocellular adenomas, cardiomyopathies, in malfunction of skeletal muscle, in growth retardation, neutropenia, osteopenia and bowel inflammation. However, a few markers have lost interest due to a great variability of results, which is the case of biotinidase, actin alpha 2, smooth muscle, aorta and fibroblast growth factor receptor 4. This is the first review published on emerging biomarkers with a potential application to GSDs.

Diurnal variability of glucose tetrasaccharide (Glc4) excretion in patients with glycogen storage disease type III

AIM

The urinary glucose tetrasaccharide, Glcα1-6Glcα1-4Glcα1-4Glc (Glc4), is a glycogen limit dextrin that is elevated in patients with glycogen storage disease (GSD) type III. We evaluated the potential of uncooked cornstarch therapy to interfere with Glc4 monitoring, by measuring the diurnal variability of Glc4 excretion in patients with GSD III.

METHODS

Voids were collected at home over 24 hours, stored at 4°C and frozen within 48 hours. Glc4 was analyzed using liquid chromatography-tandem mass spectrometry and normalized to creatinine.

RESULTS

Subjects with GSD III (median age: 13.5 years, range: 3.7-62; n = 18) completed one or more 24-hour urine collection, and 28/36 collections were accepted for analysis. Glc4 was elevated in 16/18 subjects (median: 13 mmol/mol creatinine, range: 2-75, reference range: <3). In collections with elevated Glc4 (23/28), two-thirds (15/23) had low diurnal variability in Glc4 excretion (coefficient of variation [CV%] <25). The diurnal variability was significantly correlated with the Glc4 concentration (Pearson R = .644, P < .05), but not with the dose of uncooked cornstarch. High intraday variability (>25%) was not consistently observed in repeat collections by the same subject.

CONCLUSIONS

The extent and variability of Glc4 excretion relative to creatinine was not correlated with cornstarch dose. A majority of collections showed low variability over 24 hours. These findings support the use of single time-point collections to evaluate Glc4 in patients with GSD III treated with cornstarch. However, repeat sampling over short time-periods will provide the most accurate assessment of Glc4 excretion, as intraday variability may be increased in patients with high Glc4 excretion.

The decision-making levels of urine tetrasaccharide for the diagnosis of Pompe disease in the Turkish population

Background Recently, urinary excretion of the tetrasaccharide 6-α-D-glucopyranosyl-maltotriose (Glc4) has been proposed as a marker for the diagnosis and monitoring of Pompe disease (PD). We aimed to determine the reference intervals and reliable decision-making levels of urine tetrasaccharide concentrations for the diagnosis of infantile- and late-onset Pompe patients in the Turkish population. Methods In this study, nine patients with PD (five of them with late-onset PD [LOPD]) and 226 healthy individuals (aged 0-64 years) were included. Urine Glc4 concentrations were determined using the ultra-high-performance liquid chromatography (UHPLC) tandem mass spectrometry (MS/MS) method. Results Our data showed that the urine tetrasaccharide levels decreased with age in healthy individuals (p < 0.001, r = -0.256). It was higher especially during the first year of life compared to that in the elder subjects. The tetrasaccharide level of Pompe patients was higher compared to that of healthy controls of the same age: 99 ± 68 mmol/mol creatinine for infantile onset vs. 4.0 ± 3.0 mmol/mol creatinine for healthy controls of the same age group and 12.1 ± 17.4 mmol/mol creatinine for late onset vs. 1.7±1.2 mmol/mol creatinine for healthy controls of the same age group. Conclusions The results of this study showed that the reference intervals of tetrasaccharide in urine changed over time; therefore, it is critically important to define age-based decision levels for the diagnosis of LOPD.

Diagnosis and management of glycogen storage diseases type VI and IX: a clinical practice resource of the American College of Medical Genetics and Genomics (ACMG)

PURPOSE

Glycogen storage disease (GSD) types VI and IX are rare diseases of variable clinical severity affecting primarily the liver. GSD VI is caused by deficient activity of hepatic glycogen phosphorylase, an enzyme encoded by the PYGL gene. GSD IX is caused by deficient activity of phosphorylase kinase (PhK), the enzyme subunits of which are encoded by various genes: ɑ (PHKA1, PHKA2), β (PHKB), ɣ (PHKG1, PHKG2), and δ (CALM1, CALM2, CALM3). Glycogen storage disease types VI and IX have a wide spectrum of clinical manifestations and often cannot be distinguished from each other, or from other liver GSDs, on clinical presentation alone. Individuals with GSDs VI and IX can present with hepatomegaly with elevated serum transaminases, ketotic hypoglycemia, hyperlipidemia, and poor growth. This guideline for the management of GSDs VI and IX was developed as an educational resource for health-care providers to facilitate prompt and accurate diagnosis and appropriate management of patients.

METHODS

A national group of experts in various aspects of GSDs VI and IX met to review the limited evidence base from the scientific literature and provided their expert opinions. Consensus was developed in each area of diagnosis, treatment, and management. Evidence bases for these rare disorders are largely based on expert opinion, particularly when targeted therapeutics that have to clear the US Food and Drug Administration (FDA) remain unavailable.

RESULTS

This management guideline specifically addresses evaluation and diagnosis across multiple organ systems involved in GSDs VI and IX. Conditions to consider in a differential diagnosis stemming from presenting features and diagnostic algorithms are discussed. Aspects of diagnostic evaluation and nutritional and medical management, including care coordination, genetic counseling, and prenatal diagnosis are addressed.

CONCLUSION

A guideline that will facilitate the accurate diagnosis and optimal management of patients with GSDs VI and IX was developed. This guideline will help health-care providers recognize patients with GSDs VI and IX, expedite diagnosis, and minimize adverse sequelae from delayed diagnosis and inappropriate management. It will also help identify gaps in scientific knowledge that exist today and suggest future studies.

4-epi-Isofagomine derivatives as pharmacological chaperones for the treatment of lysosomal diseases linked to β-galactosidase mutations: Improved synthesis and biological investigations

(5aR)-5a-C-pentyl-4-epi-isofagomine 1 is a powerful inhibitor of lysosomal β-galactosidase and a remarkable chaperone for mutations associated with GM1-gangliosidosis and Morquio disease type B. We report herein an improved synthesis of this compound and analogs (5a-C-methyl, pentyl, nonyl and phenylethyl derivatives), and a crystal structure of a synthetic intermediate that confirms its configuration resulting from the addition of a Grignard reagent. These compounds were evaluated as glycosidase inhibitors and their potential as chaperones for mutant lysosomal galactosidases determined. Based on these results and on docking studies, the 5-C-pentyl derivative 1 was selected as the optimal structure for further investigations: this compound induces the maturation of mutated β-galactosidase in fibroblasts of a GM1-gangliosidosis patient and promote the decrease of keratan sulfate and oligosaccharide load in patient cells. Compound 1 is clearly capable of restoring β-galactosidase activity and of promoting maturation of the protein, which should result in significant clinical benefit. These properties strongly support the development of compound 1 for the treatment of GM1-gangliosidosis and Morquio disease type B patients harboring β-galactosidase mutations sensitive to pharmacological chaperoning.

UPLC-MS/MS Analysis of Urinary Free Oligosaccharides for Lysosomal Storage Diseases: Diagnosis and Potential Treatment Monitoring

BACKGROUND

The glycoproteinoses are a subgroup of lysosomal storage diseases (LSDs) resulting from impaired degradation of N-linked oligosaccharide side chains of glycoproteins, which are commonly screened by detecting the accumulated free oligosaccharides (FOSs) in urine via thin layer chromatography (TLC). The traditional TLC method suffers from limited analytical sensitivity and specificity and lacks quantification capability. Therefore, we developed an analytically sensitive and relatively specific assay using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) for urinary FOS analysis and validated its use for urine screening of glycoproteinoses and other LSDs.

METHODS

Urine volumes equivalent to 30 μg of creatinine were derivatized with butyl-4-aminobenzoate and then purified through a solid-phase extraction cartridge. A 7-min UPLC-MS/MS analysis was performed on a triple quadrupole mass spectrometer using an amide column for separation of derivatized FOS. Urine samples from >100 unaffected controls and 37 patients with various LSDs were studied.

RESULTS

Relative quantification was conducted on 7 selected FOSs using a single internal standard, which allowed the identification of patients with 1 of 8 different LSDs: aspartylglucosaminuria, α-fucosidosis, α-mannosidosis, β-mannosidosis, β-galactosidase deficiency, Sandhoff disease, sialidosis, and galactosialidosis. Patients treated with hematopoietic stem cell transplant show decreased FOS responses compared with untreated patients.

CONCLUSIONS

This UPLC-MS/MS assay offers a valuable tool for screening of glycoproteinoses and other LSDs, with potential use for future treatment monitoring.

Contribution of tandem mass spectrometry to the diagnosis of lysosomal storage disorders

Tandem mass spectrometry (MS/MS) is a highly sensitive and specific technique. Thanks to the development of triple quadrupole analyzers, it is becoming more widely used in laboratories working in the field of inborn errors of metabolism. We review here the state of the art of this technique applied to the diagnosis of lysosomal storage disorders (LSDs) and how MS/MS has changed the diagnostic rationale in recent years. This fine technology brings more sensitive, specific, and reliable methods than the previous biochemical ones for the analysis of urinary glycosaminoglycans, oligosaccharides, and sialic acid. In sphingolipidoses, the quantification of urinary sphingolipids (globotriaosylceramide, sulfatides) is possible. The measurement of new plasmatic biomarkers such as oxysterols, bile acids, and lysosphingolipids allows the screening of many sphingolipidoses and related disorders (Niemann-Pick type C), replacing tedious biochemical techniques. Applied to amniotic fluid, a more reliable prenatal diagnosis or screening of LSDs is now available for fetuses presenting with antenatal manifestations. Applied to enzyme measurements, it allows high throughput assays for the screening of large populations, even newborn screening. The advent of this new method can modify the diagnostic rationale behind LSDs.

Newborn Screening for Lysosomal Storage Diseases: A Concise Review of the Literature on Screening Methods, Therapeutic Possibilities and Regional Programs

Newborn screening for lysosomal storage diseases (LSDs) is increasingly being considered as an option. The development of analytical screening methods, of second-tier methods, and of therapeutic possibilities, are paving the way for routine screening for LSDs in the coming years. Here, we give a brief description of the current status quo, what screening methods are currently available or are in the pipeline, what is the current status of therapeutic possibilities for LSDs, what LSDs are the most obvious candidates for introduction in screening programs, and what LSDs are already part of regional or national pilot or routine screening programs worldwide.

Liquid Chromatography-Tandem Mass Spectrometry Assay of Leukocyte Acid α-Glucosidase for Post-Newborn Screening Evaluation of Pompe Disease

BACKGROUND

Pompe disease (PD) is the first lysosomal storage disorder to be added to the Recommended Uniform Screening Panel for newborn screening. This condition has a broad phenotypic spectrum, ranging from an infantile form (IOPD), with severe morbidity and mortality in infancy, to a late-onset form (LOPD) with variable onset and progressive weakness and respiratory failure. Because the prognosis and treatment options are different for IOPD and LOPD, it is important to accurately determine an individual's phenotype. To date, no enzyme assay of acid α-glucosidase (GAA) has been described that can differentiate IOPD vs LOPD using blood samples.

METHODS

We incubated 10 μL leukocyte lysate and 25 μL GAA substrate and internal standard (IS) assay cocktail for 1 h. The reaction was purified by a liquid-liquid extraction. The extracts were evaporated and reconstituted in 200 μL methanol and analyzed by LC-MS/MS for GAA activity.

RESULTS

A 700-fold higher analytical range was observed with the LC-MS/MS assay compared to the fluorometric method. When GAA-null and GAA-containing fibroblast lysates were mixed, GAA activity could be measured accurately even in the range of 0%-1% of normal. The leukocyte GAA activity in IOPD (n = 4) and LOPD (n = 19) was 0.44-1.75 nmol · h^-1 · mg^-1 and 2.0-6.5 nmol · h^-1 · mg^-1, respectively, with no overlap. The GAA activity of pseudodeficiency patients ranged from 3.0-28.1 nmol · h^-1 · mg^-1, showing substantial but incomplete separation from the LOPD group.

CONCLUSIONS

This assay allows determination of low residual GAA activity in leukocytes. IOPD, LOPD, and pseudodeficiency patients can be partially differentiated by measuring GAA using blood samples.

The infantile-onset form of Pompe disease: an autopsy diagnosis

Pompe disease (PD) is a rare, inherited autosomal recessive metabolic disorder caused by the deficiency of the lysosomal acid alpha-glucosidase (GAA) enzyme described in 1932 by the Dutch pathologist Joannes Cassianus Pompe. The prevalence of PD ranges from 1:40,000 to 1:300,000 births and depends on geographic and ethnic factors. Clinical manifestations may vary from a rapidly progressive disabling disease with cardiomegaly, hepatomegaly, weakness, generalized hypotonia, and death within the first year of life, to a mild presentation characterized by slowly progressive myopathy predominantly involving the skeletal muscles. The laboratory diagnostic gold standard is represented by the determination of the alpha-glucosidase activity. However, the muscle histology may also yield the diagnosis by evaluating the tissular glycogen accumulation. Until recently, supportive measures constituted the unique available therapy. Currently, the administration of the recombinant GAA is being used with promising results. The authors present the case of a 5-month-old boy, previously diagnosed with hypertrophic cardiomyopathy since the age of 2 months, who presented acute heart failure accompanied by biventricular dilation followed by refractory shock and death. The autopsy findings confirmed the glycogen-accumulation disease.

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.

A chaperone enhances blood α-glucosidase activity in Pompe disease patients treated with enzyme replacement therapy

Enzyme replacement therapy is currently the only approved treatment for Pompe disease, due to acid α-glucosidase deficiency. Clinical efficacy of this approach is variable, and more effective therapies are needed. We showed in preclinical studies that chaperones stabilize the recombinant enzyme used for enzyme replacement therapy. Here, we evaluated the effects of a combination of enzyme therapy and a chaperone on α-glucosidase activity in Pompe disease patients. α-Glucosidase activity was analyzed by tandem-mass spectrometry in dried blood spots from patients treated with enzyme replacement therapy, either alone or in combination with the chaperone N-butyldeoxynojirimycin given at the time of the enzyme infusion. Thirteen patients with different presentations (3 infantile-onset, 10 late-onset) were enrolled. In 11 patients, the combination treatment resulted in α-glucosidase activities greater than 1.85-fold the activities with enzyme replacement therapy alone. In the whole patient population, α-glucosidase activity was significantly increased at 12 hours (2.19-fold, P = 0.002), 24 hours (6.07-fold, P = 0.001), and 36 hours (3.95-fold, P = 0.003). The areas under the curve were also significantly increased (6.78-fold, P = 0.002). These results suggest improved stability of recombinant α-glucosidase in blood in the presence of the chaperone.

Assessment of toxicity and biodistribution of recombinant AAV8 vector-mediated immunomodulatory gene therapy in mice with Pompe disease

A preclinical safety study was conducted to evaluate the short- and long-term toxicity of a recombinant adeno-associated virus serotype 8 (AAV2/8) vector that has been developed as an immune-modulatory adjunctive therapy to recombinant human acid α-glucosidase (rhGAA, Myozyme) enzyme replacement treatment (ERT) for patients with Pompe disease (AAV2/8-LSPhGAApA). The AAV2/8-LSPhGAApA vector at 1.6 × 10(13) vector particles/kg, after intravenous injection, did not cause significant short- or long-term toxicity. Recruitment of CD4(+) (but not CD8(+)) lymphocytes to the liver was elevated in the vector-dosed male animals at study day (SD) 15, and in group 8 animals at SD 113, in comparison to their respective control animals. Administration of the vector, either prior to or after the one ERT injection, uniformly prevented the hypersensitivity induced by subsequent ERT in males, but not always in female animals. The vector genome was sustained in all tissues through 16-week postdosing, except for in blood with a similar tissue tropism between males and females. Administration of the vector alone, or combined with the ERT, was effective in producing significantly increased GAA activity and consequently decreased glycogen accumulation in multiple tissues, and the urine biomarker, Glc4, was significantly reduced. The efficacy of the vector (or with ERT) was better in males than in females, as demonstrated both by the number of tissues showing significantly effective responses and the extent of response in a given tissue. Given the lack of toxicity for AAV2/8LSPhGAApA, further consideration of clinical translation is warranted in Pompe disease.

Quantitative computed tomography for enzyme replacement therapy in Pompe disease

OBJECTIVE

Pompe disease is an autosomal recessive disorder caused by deficiency of the lysosomal enzyme, acid alpha-glucosidase (GAA). To the best of our knowledge, no studies have reported the results of systematic and sequential CT analyses before and during ERT. In this study we have treated three patients with late onset Pompe disease by ERT, and investigated the efficacy of treatment by computed tomography number.

METHODS

We measured the serial changes in the computed tomography (CT) number of multiple organs in three patients with late onset of Pompe disease during 24 months of enzyme replacement therapy (ERT).

RESULTS

Before treatment, the liver and muscle CT numbers were higher in these patients than in the controls. The liver CT number decreased after performing ERT. Furthermore, the urinary glucose tetrasaccharide levels, a biomarker of glycogen accumulation, were elevated before ERT and reduced thereafter.

CONCLUSIONS

The findings in these cases suggest that the elevation of the liver CT number represents glycogen accumulation in the liver and that the analysis of the liver CT number is therefore a useful tool for assessing the efficacy of ERT.

Analysis of urinary oligosaccharides in lysosomal storage disorders by capillary high-performance anion-exchange chromatography-mass spectrometry

Many lysosomal storage diseases are characterized by an increased urinary excretion of glycoconjugates and oligosaccharides that are characteristic for the underlying enzymatic defect. Here, we have used capillary high-performance anion-exchange chromatography (HPAEC) hyphenated to mass spectrometry to analyze free oligosaccharides from urine samples of patients suffering from the lysosomal storage disorders fucosidosis, α-mannosidosis, G(M1)-gangliosidosis, G(M2)-gangliosidosis, and sialidosis. Glycan fingerprints were registered, and the patterns of accumulated oligosaccharides were found to reflect the specific blockages of the catabolic pathway. Our analytical approach allowed structural analysis of the excreted oligosaccharides and revealed several previously unpublished oligosaccharides. In conclusion, using online coupling of HPAEC with mass spectrometric detection, our study provides characteristic urinary oligosaccharide fingerprints with diagnostic potential for lysosomal storage disorders.

Rapid ultraperformance liquid chromatography-tandem mass spectrometry assay for a characteristic glycogen-derived tetrasaccharide in Pompe disease and other glycogen storage diseases

BACKGROUND

Urinary excretion of the tetrasaccharide 6-α-D-glucopyranosyl-maltotriose (Glc₄) is increased in various clinical conditions associated with increased turnover or storage of glycogen, making Glc₄ a potential biomarker for glycogen storage diseases (GSD). We developed an ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) assay to detect Glc₄ in urine without interference of the Glc₄ isomer maltotetraose (M₄).

METHODS

Urine samples, diluted in 0.1% ammonium hydroxide containing the internal standard acarbose, were filtered, and the filtrate was analyzed by UPLC-MS/MS.

RESULTS

We separated and quantified acarbose, M₄, and Glc₄ using the ion pairs m/z 644/161, 665/161, and 665/179, respectively. Response of Glc₄ was linear up to 1500 μmol/L and the limit of quantification was 2.8 μmol/L. Intra- and interassay CVs were 18.0% and 18.4% (10 μmol/L Glc₄), and 10.5% and 16.2% (200 μmol/L Glc₄). Glc₄ in control individuals (n = 116) decreased with increasing age from a mean value of 8.9 mmol/mol to 1.0 mmol/mol creatinine. M₄ was present in 5% of urine samples. Mean Glc₄ concentrations per age group in untreated patients with Pompe disease (GSD type II) (n = 66) were significantly higher, ranging from 39.4 to 10.3 mmol/mol creatinine (P < 0.001-0.005). The diagnostic sensitivity of Glc₄ for GSD-II was 98.5% and the diagnostic specificity 92%. Urine Glc₄ was also increased in GSD-III (8 of 9), GSD-IV (2 of 3) and GSD-IX (6 of 10) patients.

CONCLUSIONS

The UPLC-MS/MS assay of Glc₄ in urine was discriminative between Glc₄ and M₄ and confirmed the diagnosis in >98% of GSD-II cases.

Urine analysis of glucose tetrasaccharide by HPLC; a useful marker for the investigation of patients with Pompe and other glycogen storage diseases

A high performance liquid chromatography method, adapted from an established urinary sugars method, has been developed for the analysis of a tetraglucose oligomer (Glc(4)) in urine. Pompe disease results from defects in the activity of lysosomal acid α-glucosidase (GAA) with patients typically excreting increased amounts of Glc(4). Rapid determination of GAA in dried blood spots is now possible. However, enzymatic analysis is unable to discriminate between patients with Pompe disease and those individuals harbouring pseudo deficiency mutations. This method was able to quantify Glc(4) levels in all patients analysed with an established diagnosis of Pompe disease, and all controls analysed had Glc(4) levels below the limit of detection for this method. Importantly the method was able to discriminate between an individual known to harbour a pseudo Pompe mutation and patients with Pompe disease, providing a useful supporting test to enzymatic analysis. Sequential measurement of urinary Glc(4) has been proposed to monitor the effects of enzyme replacement therapy (ERT). We observed a clear decrease in Glc(4) levels following commencement of treatment in three patients studied. Additionally, raised levels of Glc(4) were observed in patients with glycogen storage disease (GSD) type Ia and type III suggesting that this method may have applications in other GSDs.

Assessing disease severity in Pompe disease: the roles of a urinary glucose tetrasaccharide biomarker and imaging techniques

Defining disease severity in patients with Pompe disease is important for prognosis and monitoring the response to therapies. Current approaches include qualitative and quantitative assessments of the disease burden, and clinical measures of the impact of the disease on affected systems. The aims of this manuscript were to review a noninvasive urinary glucose tetrasaccharide biomarker of glycogen storage, and to discuss advances in imaging techniques for determining the disease burden in Pompe disease. The glucose tetrasaccharide, Glcα1-6Glcα1-4Glcα1-4Glc (Glc(4) ), is a glycogen-derived limit dextrin that correlates with the extent of glycogen accumulation in skeletal muscle. As such, it is more useful than traditional biomarkers of tissue damage, such as CK and AST, for monitoring the response to enzyme replacement therapy in patients with Pompe disease. Glc(4) is also useful as an adjunctive diagnostic test for Pompe disease when performed in conjunction with acid alpha-glucosidase activity measurements. Review of clinical records of 208 patients evaluated for Pompe disease by this approach showed Glc(4) had 94% sensitivity and 84% specificity for Pompe disease. We propose Glc(4) is useful as an overall measure of disease burden, but does not provide information on the location and distribution of excess glycogen accumulation. In this manuscript we also review magnetic resonance spectroscopy and imaging techniques as alternative, noninvasive tools for quantifying glycogen and detailing changes, such as fibrofatty muscle degeneration, in specific muscle groups in Pompe disease. These techniques show promise as a means of monitoring disease progression and the response to treatment in Pompe disease. © 2012 Wiley Periodicals, Inc.

The impact of antibodies on clinical outcomes in diseases treated with therapeutic protein: lessons learned from infantile Pompe disease

PURPOSE

Enzyme replacement therapy with rhGAA (Myozyme®) has lead to improved survival, which is largely attributable to improvements in cardiomyopathy and skeletal muscle function. However, crossreactive immunologic material-negative patients have a poor clinical response to enzyme replacement therapy secondary to high sustained antibody titers. Furthermore, although the majority of crossreactive immunologic material-positive patients tolerize or experience a downtrend in anti-rhGAA antibody titers, antibody response is variable with some crossreactive immunologic material-positive infants also mounting high sustained antibody titers.

METHODS

We retrospectively analyzed 34 infants with Pompe disease: 11 crossreactive immunologic material-negative patients, nine high-titer crossreactive immunologic material-positive patients, and 14 low-titer crossreactive immunologic material-positive patients. Clinical outcome measures were overall survival, ventilator-free survival, left ventricular mass index, Alberta Infant Motor Scale score, and urine Glc(4) levels.

RESULTS

Clinical outcomes in the high-titer crossreactive immunologic material-positive group were poor across all areas evaluated relative to the low-titer crossreactive immunologic material-positive group. For the crossreactive immunologic material-negative and high-titer crossreactive immunologic material-positive groups, no statistically significant differences were observed for any outcome measures, and both patient groups did poorly.

CONCLUSIONS

Our data indicate that, irrespective of crossreactive immunologic material status, patients with infantile Pompe disease with high sustained antibody titer have an attenuated therapeutic response to enzyme replacement therapy. With the advent of immunomodulation therapies, identification of patients at risk for developing high sustained antibody titer is critical.

Urinary glycan markers for disease

Robust assays for the isolation and characterization of urinary FOS (free oligosaccharides) have been developed to screen patients for altered protein and/or lipid glycosylation. A FOS analysis can therefore identify potential biomarkers for hepatocellular carcinoma, since variations in glycosylation as a result of tumorigenecity should be detectable in the FOS of patients. HCC (hepatocellular carcinoma) accounts for 80-90% of all liver cancers. It occurs more often in men than women and occurs mostly in people 50-60 years old. The disease is more common in parts of Africa and Asia than in North or South America and Europe. Using a combination of solid-phase extraction techniques and affinity chromatography, followed by separation of urinary FOS by NP (normal phase)-HPLC and HIAX (hydrophilic interaction and anion-exchange)-HPLC, more than 200 different species have been identified in patient samples. The high incidence of small sialylated oligosaccharides in HCC patients suggests that pro-inflammatory markers may be detected as early indicators of disease progression. In addition, the methods developed here to isolate and analyse excreted glycoprotein- and glycosphingolipid-bound oligosaccharides have been used to characterize changes in metabolic processes that underlie a number of human genetic disorders. The ability to predict disease status in microlitre amounts of readily available non-invasive urine samples indicates that rapid methods for screening can be developed.

Towards a selected reaction monitoring mass spectrometry fingerprint approach for the screening of oligosaccharidoses

The oligosaccharidoses are a group of metabolic disorders resulting from a deficiency in enzymes responsible for the catabolism of protein bound oligosaccharides and are typified by the accumulation of corresponding sugars in the urine. Screening is typically accomplished using thin layer chromatography. However, analyte specificity can be a problem and thus complicate interpretation of results. For this reason we developed a mixed mode liquid chromatography tandem mass spectrometry assay for the screening of the oligosaccharidoses which potentially mitigates many of the problems associated with thin layer chromatography. Samples from patients previously diagnosed with I-Cell disease, mannosidosis, Pompe, galactosialidosis, and fucosidosis were derivatized with 3-methyl-1-phenyl-2-pyrazolin-5-one and subjected to analysis by liquid chromatography tandem mass spectrometry. Results were compared to normal control samples. Preliminary results suggest that each oligosaccharidoses produces a unique selected reaction monitoring fingerprint and that the developed method may be an effective screening and diagnostic tool for these disorders.

Glycan profiling of urine, amniotic fluid and ascitic fluid from galactosialidosis patients reveals novel oligosaccharides with reducing end hexose and aldohexonic acid residues

Urine, amniotic fluid and ascitic fluid samples of galactosialidosis patients were analyzed and structurally characterized for free oligosaccharides using capillary high-performance anion-exchange chromatography with pulsed amperometric detection and online mass spectrometry. In addition to the expected endo-beta-N-acetylglucosaminidase-cleaved products of complex-type sialylated N-glycans, O-sulfated oligosaccharide moieties were detected. Moreover, novel carbohydrate moieties with reducing-end hexose residues were detected. On the basis of structural features such as a hexose-N-acetylhexosamine-hexose-hexose consensus sequence and di-sialic acid units, these oligosaccharides are thought to represent, at least in part, glycan moieties of glycosphingolipids. In addition, C(1)-oxidized, aldohexonic acid-containing versions of most of these oligosaccharides were observed. These observations suggest an alternative catabolism of glycosphingolipids in galactosialidosis patients: oligosaccharide moieties from glycosphingolipids would be released by a hitherto unknown ceramide glycanase activity. The results show the potential and versatility of the analytical approach for structural characterization of oligosaccharides in various body fluids.

Cross-reactive immunologic material status affects treatment outcomes in Pompe disease infants

Deficiency of acid alpha glucosidase (GAA) causes Pompe disease, which is usually fatal if onset occurs in infancy. Patients synthesize a non-functional form of GAA or are unable to form native enzyme. Enzyme replacement therapy with recombinant human GAA (rhGAA) prolongs survival in infantile Pompe patients but may be less effective in cross-reactive immunologic material (CRIM)-negative patients. We retrospectively analyzed the influence of CRIM status on outcome in 21 CRIM-positive and 11 CRIM-negative infantile Pompe patients receiving rhGAA. Patients were from the clinical setting and from clinical trials of rhGAA, were 6 months of age, were not invasively ventilated, and were treated with IV rhGAA at a cumulative or total dose of 20 or 40 mg/kg/2 weeks. Outcome measures included survival, invasive ventilator-free survival, cardiac status, gross motor development, development of antibodies to rhGAA, and levels of urinary Glc(4). Following 52 weeks of treatment, 6/11 (54.5%) CRIM-negative and 1/21 (4.8%) CRIM-positive patients were deceased or invasively ventilated (p<0.0001). By age 27.1 months, all CRIM-negative patients and 4/21 (19.0%) CRIM-positive patients were deceased or invasively ventilated. Cardiac function and gross motor development improved significantly more in the CRIM-positive group. IgG antibodies to rhGAA developed earlier and serotiters were higher and more sustained in the CRIM-negative group. CRIM-negative status predicted reduced overall survival and invasive ventilator-free survival and poorer clinical outcomes in infants with Pompe disease treated with rhGAA. The effect of CRIM status on outcome appears to be mediated by antibody responses to the exogenous protein.

Diagnostic criteria for late-onset (childhood and adult) Pompe disease

The diagnosis of late-onset (childhood and adult) Pompe disease can often be challenging, as it is a rare disease and the heterogeneous clinical presentation can mimic the presentation of other neuromuscular disorders. The objective was to develop a consensus-based algorithm for the diagnosis of late-onset Pompe disease. A systematic literature search was conducted, and an expert panel composed of neuromuscular specialists and individuals with expertise in Pompe disease reviewed the literature and convened for consensus development. An algorithm for the diagnosis of late-onset Pompe disease was created. Patients presenting with either a limb-girdle syndrome or dyspnea secondary to diaphragm weakness should undergo further testing, including evaluations of muscle strength, motor function, and pulmonary function. A blood-based acid alpha-glucosidase (GAA) enzyme activity assay is the recommended tool to screen for GAA enzyme deficiency. The diagnosis should be confirmed by a second test: either a second GAA enzyme activity assay in another tissue or GAA gene sequencing.

A convenient LC-MS method for assessment of glucose kinetics in vivo with D-[13C6]glucose as a tracer

BACKGROUND

The isotope-labeled intravenous glucose tolerance test (IVGTT) combined with computer modeling is widely used to derive parameters related to glucose metabolism in vivo. Most of these methods involve use of either (2)H(2)-labeled or (13)C(1)-labeled D-glucose as a tracer with GC-MS to measure the isotope enrichment. These methods are challenging, both technologically and economically. We have developed a novel approach that is suitable for labeled-IVGTT studies involving a large cohort of individuals.

METHODS

The tracer, D-[(13)C(6)]glucose, is a low-cost alternative with the significant advantage that the sixth isotope of natural glucose has virtually zero natural abundance, which facilitates isotopomer analysis with <1% labeled glucose in the infusate. After deproteinization of plasma samples collected at various times, glucose is converted to a stable derivative, purified by solid-phase extraction (SPE), and analyzed by HPLC-electrospray ionization mass spectrometry to accumulate the isotope-abundance data for the A+2, A+3, and A+6 ions of the glucose derivative. A 2-pool modeling program was used to derive standard kinetic parameters.

RESULTS

With labeled-IVGTT data from 10 healthy male individuals, the values for insulin sensitivity, glucose effectiveness, and the plasma clearance rate estimated with the 2-pool minimal model compared well with values obtained via traditional methods.

CONCLUSIONS

The relative simplicity and robustness of the new method permit the preparation and analysis of up to 48 samples/day, a throughput equivalent to 2 complete IVGTT experiments, and this method is readily adaptable to existing 96 well-format purification and analytical systems.

Improvement with ongoing Enzyme Replacement Therapy in advanced late-onset Pompe disease: a case study

Benefits of enzyme replacement therapy with Myozyme (alglucosidase alfa), anecdotally reported in late-onset Pompe disease, range from motor and pulmonary improvement in less severely affected patients, to stabilization with minimal improvement in those with advanced disease. We report a case of a 63-year-old patient with significant morbidity who made notable motor and pulmonary function gains after two years on therapy. Thus, improvements in those with advanced disease may be possible after long-term treatment.

Comparison of fluorescent labels for oligosaccharides and introduction of a new postlabeling purification method

Labeling of oligosaccharides with fluorescent dyes is the prerequisite for their sensitive analysis by high-performance liquid chromatography (HPLC). In this work, we present a fast new postlabeling cleanup procedure that requires no device other than the reaction vial itself. The procedure can be applied to essentially all labeling reagents. We also compare the performance of 15 different labels for N-glycan analysis in various analytical procedures. We took special care to prevent obscuring influences from incomplete derivatization and signal quenching by impurities. Procainamide emerged as more sensitive than anthranilic acid for normal-phase HPLC, but its chromatographic performance was not convincing. 2-aminopyridine was the label with the lowest retention on reversed-phase and graphitic carbon columns and, thus, appears to be most suitable for glycan fractionation by multidimensional HPLC. Most glycan derivatives performed better than native sugars in matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and electrospray ionization-MS (ESI-MS), but the gain was small and hardly sufficient to compensate for sample loss during preparation.

Methods for a prompt and reliable laboratory diagnosis of Pompe disease: report from an international consensus meeting

Pompe disease is an autosomal recessive disorder of glycogen metabolism caused by a deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA). It presents at any age, with variable rates of progression ranging from a rapidly progressive course, often fatal by one-year of age, to a more slowly, but nevertheless relentlessly progressive course, resulting in significant morbidity and premature mortality. In infants, early initiation of enzyme replacement therapy is needed to gain the maximum therapeutic benefit, underscoring the need for early diagnosis. Several new methods for measuring GAA activity have been developed. The Pompe Disease Diagnostic Working Group met to review data generated using the new methods, and to establish a consensus regarding the application of the methods for the laboratory diagnosis of Pompe disease. Skin fibroblasts and muscle biopsy have traditionally been the samples of choice for measuring GAA activity. However, new methods using blood samples are rapidly becoming adopted because of their speed and convenience. Measuring GAA activity in blood samples should be performed under acidic conditions (pH 3.8-4.0), using up to 2 mM of the synthetic substrate 4-methylumbelliferyl-alpha-D-glucoside or glycogen (50 mg/mL), in the presence of acarbose (3-9 microM) to inhibit the isoenzyme maltase-glucoamylase. The activity of a reference enzyme should also be measured to confirm the quality of the sample. A second test should be done to support the diagnosis of Pompe disease until a program for external quality assurance and proficiency testing of the enzymatic diagnosis in blood is established.

Glucose tetrasaccharide as a biomarker for monitoring the therapeutic response to enzyme replacement therapy for Pompe disease

A tetraglucose oligomer, Glcalpha1-6Glcalpha1-4Glcalpha1-4Glc, designated Glc4, has been shown to be a putative biomarker for the diagnosis of Pompe disease. The purpose of this study was to assess whether Glc4 could be used to monitor the therapeutic response to recombinant human acid alpha glucosidase (rhGAA) enzyme replacement therapy (ERT) in patients with Pompe disease. Urinary Glc4 levels in 11 patients receiving rhGAA therapy was determined by both HPLC-UV and stable isotope dilution ESI-MS/MS. Combined Glc4 and maltotetraose, Glcalpha1-4Glcalpha1-4Glcalpha1-4Glc, (M4) concentrations, designated Hex4, in plasma from these patients were measured by HPLC-UV only. Baseline urinary Glc4 and plasma Hex4 in these patients (mean+/-SD: 34.2+/-11.3 mmol/mol creatinine and 1.7+/-0.8 microM, respectively) were higher than age-matched control values (mean+/-SD, 6.1+/-5.1 mmol/mol creatinine and 0.22+/-0.15 microM, respectively). Both urinary Glc4 and plasma Hex4 levels decreased after initiation of ERT for all patients. In the four patients with the best overall clinical response in both skeletal and cardiac muscle, levels decreased to within, or near, normal levels during the first year of treatment. In contrast, levels fluctuated and were persistently elevated above the control ranges in those patients with a less favorable clinical response (good cardiac response but limited motor improvement). These results suggest that urinary Glc4 and plasma Hex4 could serve as a valuable adjunct to clinical endpoints for monitoring the efficacy of therapeutic interventions such as rhGAA ERT in Pompe disease.

Evasion of immune responses to introduced human acid alpha-glucosidase by liver-restricted expression in glycogen storage disease type II

Glycogen storage disease type II (GSD-II; Pompe disease) is caused by a deficiency of acid alpha-glucosidase (GAA; acid maltase) and manifests as muscle weakness, hypertrophic cardiomyopathy, and respiratory failure. Adeno-associated virus vectors containing either a liver-specific promoter (LSP) (AAV-LSPhGAApA) or a hybrid CB promoter (AAV-CBhGAApA) to drive human GAA expression were pseudotyped as AAV8 and administered to immunocompetent GAA-knockout mice. Secreted hGAA was detectable in plasma between 1 day and 12 weeks postadministration with AAV-LSPhGAApA and only from 1 to 8 days postadministration for AAV-CBGAApA. No anti-GAA antibodies were detected in response to AAV-LSPhGAApA (<1:200), whereas AAV-CBhGAApA provoked an escalating antibody response starting 2 weeks postadministration. The LSP drove approximately 60-fold higher GAA expression than the CB promoter in the liver by 12 weeks following vector administration. Furthermore, the detected cellular immunity was provoked by AAV-CBhGAApA, as detected by ELISpot and CD4+/CD8+ lymphocyte immunodetection. GAA activity was increased to higher than normal and glycogen content was reduced to essentially normal levels in the heart and skeletal muscle following administration of AAV-LSPhGAApA. Therefore, liver-restricted GAA expression with an AAV vector evaded immunity and enhanced efficacy in GSD-II mice.

Efficacy of an adeno-associated virus 8-pseudotyped vector in glycogen storage disease type II

Glycogen storage disease type II (GSD-II; Pompe disease) causes death in infancy from cardiorespiratory failure. The underlying deficiency of acid alpha-glucosidase (GAA; acid maltase) can be corrected by liver-targeted gene therapy in GSD-II, if secretion of GAA is accompanied by receptor-mediated uptake in cardiac and skeletal muscle. An adeno-associated virus (AAV) vector encoding human (h) GAA was pseudotyped as AAV8 (AAV2/8) and injected intravenously into immunodeficient GSD-II mice. High levels of hGAA were maintained in plasma for 24 weeks following AAV2/8 vector administration. A marked increase in vector copy number in the liver was demonstrated for the AAV2/8 vector compared to the analogous AAV2/2 vector. GAA deficiency in the heart and skeletal muscle was corrected with the AAV2/8 vector in male GSD-II mice, consistent with receptor-mediated uptake of hGAA. Male GSD-II mice demonstrated complete correction of glycogen storage in heart and diaphragm with the AAV2/8 vector, while female GSD-II mice had correction only in the heart. A biomarker for GSD-II was reduced in both sexes following AAV2/8 vector administration. Therefore, GAA production with an AAV2/8 vector in a depot organ, the liver, generated evidence for efficacious gene therapy in a mouse model for GSD-II.

Analysis of a glucose tetrasaccharide elevated in Pompe disease by stable isotope dilution-electrospray ionization tandem mass spectrometry

Patients with glycogen storage disease type II (GSD II) typically excrete increased amounts of a glycogen-derived glucose tetrasaccharide, Glcalpha1-6Glcalpha1-4Glcalpha1-4Glc (Glc(4)), in the urine. With the advent of a new enzyme replacement therapy for GSD II, there is a need for early identification of patients with this disease and for monitoring the efficacy of treatment. Glc(4) is a good candidate biomarker for GSD II. A simple and robust method using stable isotope dilution-electrospray ionization-tandem mass spectrometry for the analysis of Glc(4) in biological samples was developed. A 13C(6)-labeled stable isotope internal standard was synthesized by transglycosylation using a recombinant alpha-amylase. Butyl 4-aminobenzoate derivatives of Glc(4) and the internal standard were analyzed using multiple reaction monitoring. This method was shown to be accurate and precise by the repeated analysis of calibrators and quality control samples in urine and plasma. There was good agreement with a high-performance liquid chromatography-UV method for urine samples, whereas there was less agreement with plasma samples. Accurate determination from dried urine spot samples was also demonstrated. This method is amenable to high-throughput analysis, a necessary prerequisite for mass screening for GSD II.

Determination of Oligosaccharides in Pompe Disease by Electrospray Ionization Tandem Mass Spectrometry

Background: The development of therapies for lysosomal storage disorders has created a need for biochemical markers to monitor the efficacy of therapy and methods to quantify these markers in biologic samples. In Pompe disease, the concentration of a tetrasaccharide, consisting of four glucose residues, is reputedly increased in urine and plasma, but faster and more sensitive methods are required for the analysis of this, and other oligosaccharides, from biologic fluids.

Methods: We optimized the derivatization of storage oligosaccharides with 1-phenyl-3-methyl-5-pyrazolone for the measurement, by electrospray ionization tandem mass spectrometry, of oligosaccharide concentrations in urine (n = 6), plasma (n = 11), and dried-blood spots (n = 17) from Pompe-affected individuals. Age-matched control samples of urine (n = 10), plasma (n = 28), and blood spots (n = 369) were also analyzed.

Results: The mean tetrasaccharide concentration was increased in urine from infantile-onset (0.69–12 mmol/mol of creatinine) and adult-onset (0.22–3.0 mmol/mol of creatinine) Pompe individuals compared with age-matched controls. In plasma samples, an increased tetrasaccharide concentration was observed in some infantile patients (up to 22 μmol/L) compared with age-matched controls (mean, 2.2 μmol/L). The method developed was sensitive enough to determine oligosaccharide concentrations in a single 3-mm blood spot, but no differences were observed between blood spots from control and Pompe-affected individuals.

Conclusions: Measurements of oligosaccharide concentrations in urine by this new method have potential application for the diagnosis and monitoring of patients with Pompe disease. Plasma analysis may have limited application for infantile patients, but analysis of blood spots does not discriminate between controls and affected individuals.

Are there useful biochemical markers of disease activity in lysosomal storage diseases?

The primary biochemical consequence of a defect in a gene encoding a functional component of the lysosomal system is disruption of the catabolism or processing of macromolecules in the lumen of the lysosome. Transport of the resulting digestion products through the lysosomal membrane may also be affected. This leads to the accumulation of specific metabolites within the lysosomes of affected cells. The nature of these storage products depends upon the functional protein affected and the cell type. The accumulation of storage products is progressive and leads to hypertrophy of the lysosomal system, the hallmark of lysosomal storage diseases (LSDs). Subsequent cell necrosis or, possibly, exocytosis results in the appearance in body fluids of the storage products and components of the lysosomes at much higher concentrations than seen in normal unaffected individuals. Measurement of these increased levels of metabolites and proteins provides disease-specific and generic biochemical markers for LSDs. Secondary changes in metabolism and cellular function may also produce characteristic changes in the levels of metabolites or proteins, which can also be used as markers of the disease process. Although the rate of appearance of these biochemical markers in an individual will depend upon the underlying mutation in the gene and on other genetic and environmental factors, it provides a good indicator of the progression of the disease. As the novel forms of treatment being developed may reverse the hypertrophy of the lysosomal system, biochemical markers could also be used to monitor the reversal of pathology and the efficacy of treatment.