Diagnosis of lysosomal storage disorders: current techniques and future directions

Sphingolipidoses in Morocco: Chemical profiling for an affordable and rapid diagnosis strategy

Sphingolipidoses are a group of metabolic diseases in which lysosomal hydrolases dysfunction disrupt normal sphingolipids' metabolism, leading to excess accumulation in cellular compartments and excretion in urine. These pathologies represent a significant burden among Moroccan population, for which an easy access to enzymatic assays and genetic tests is not guaranteed. Parallel analytical methods thus have to be developed for preliminary screening. In this study, 107 patients were addressed to the metabolic platform of the Marrakesh Faculty of Medicine for diagnosis confirmation. Thin-Layer Chromatography was used as a first step to perform chemical profiling of the patients' urinary lipids, allowing 36% of the patients to be efficiently oriented towards the adequate enzymatic assay. UPLC-MS/MS analyses of urinary sulfatides excreted in urines patient had been used to control the reliability of TLC analysis and to obtain more accurate information related to the sulfatides isoforms. This analytical process combining TLC with UPLC-MS/MS has enabled rapid and appropriate patient management in a reduced time and with reduced resources.

Intravenous delivery of adeno-associated viral gene therapy in feline GM1 gangliosidosis

GM1 gangliosidosis is a fatal neurodegenerative disease caused by a deficiency of lysosomal β-galactosidase. In its most severe form, GM1 gangliosidosis causes death by 4 years of age, and no effective treatments exist. Previous work has shown that injection of the brain parenchyma with an adeno-associated viral vector provides pronounced therapeutic benefit in a feline GM1 model. To develop a less invasive treatment for the brain and increase systemic biodistribution, intravenous injection of AAV9 was evaluated. AAV9 expressing feline β-galactosidase was intravenously administered at 1.5x1013 vector genomes/kilogram body weight to six GM1 cats at approximately 1 month of age. The animals were divided into two cohorts: 1) a long-term group, which was followed to humane endpoint, and 2) a short-term group, which was analyzed 16-weeks post treatment. Clinical assessments included neurological exams, cerebrospinal fluid and urine biomarkers, and 7-Telsa magnetic resonance imaging and spectroscopy. Postmortem analysis included β-galactosidase and virus distribution, histological analysis, and ganglioside content. Untreated GM1 animals survived 8.0 ± 0.6 months while intravenous treatment increased survival to an average of 3.5 years (n = 2) with substantial improvements in quality of life and neurologic function. Neurological abnormalities, which in untreated animals progress to the inability to stand and debilitating neurological disease by 8 months of age, were mild in all treated animals. Cerebrospinal fluid biomarkers were normalized, indicating decreased central nervous system cell damage in the treated animals. Urinary glycosaminoglycans decreased to normal levels in the long-term cohort. Magnetic resonance imaging and spectroscopy showed partial preservation of the brain in treated animals, which was supported by postmortem histological evaluation. β-galactosidase activity was increased throughout the central nervous system, reaching carrier levels in much of the cerebrum and normal levels in the cerebellum, spinal cord and cerebrospinal fluid. Ganglioside accumulation was significantly reduced by treatment. Peripheral tissues such as heart, skeletal muscle, and sciatic nerve also had normal β-galactosidase activity in treated GM1 cats. GM1 histopathology was largely corrected with treatment. There was no evidence of tumorigenesis or toxicity. Restoration of β-galactosidase activity in the central nervous system and peripheral organs by intravenous gene therapy led to profound increases in lifespan and quality of life in GM1 cats. This data supports the promise of intravenous gene therapy as a safe, effective treatment for GM1 gangliosidosis.

Registries for orphan drugs: generating evidence or marketing tools?

Independent disease registries for pre-and post-approval of novel treatments for rare diseases are increasingly important for healthcare professionals, patients, regulators and the pharmaceutical industry. Current registries for rare diseases to evaluate orphan drugs are mainly set up and owned by the pharmaceutical industry which leads to unacceptable conflicts of interest. To ensure independence from commercial interests, disease registries should be set up and maintained by healthcare professionals and patients. Public funding should be directed towards an early establishment of international registries for orphan diseases, ideally well before novel treatments are introduced. Regulatory bodies should insist on the use of data from independent disease registries rather than company driven, drug-oriented registries.

Hypomyelinating disorders in China: The clinical and genetic heterogeneity in 119 patients

OBJECTIVE

Hypomyelinating disorders are a group of clinically and genetically heterogeneous diseases characterized by neurological deterioration with hypomyelination visible on brain MRI scans. This study was aimed to clarify the clinical and genetic features of HMDs in Chinese population.

METHODS

119 patients with hypomyelinating disorders in Chinese population were enrolled and evaluated based on their history, clinical manifestation, laboratory examinations, series of brain MRI with follow-up, genetic etiological tests including chromosomal analysis, multiplex ligation probe amplification, Sanger sequencing, targeted enrichment-based next-generation sequencing and whole exome sequencing.

RESULTS

Clinical and genetic features of hypomyelinating disorders were revealed. Nine different hypomyelinating disorders were identified in 119 patients: Pelizaeus-Merzbacher disease (94, 79%), Pelizaeus-Merzbacher-like disease (10, 8%), hypomyelination with atrophy of the basal ganglia and cerebellum (3, 3%), GM1 gangliosidosis (5, 4%), GM2 gangliosidosis (3, 3%), trichothiodystrophy (1, 1%), Pol III-related leukodystrophy (1, 1%), hypomyelinating leukodystrophy type 9 (1, 1%), and chromosome 18q deletion syndrome (1, 1%). Of the sample, 94% (112/119) of the patients were genetically diagnosed, including 111 with mutations distributing across 9 genes including PLP1, GJC2, TUBB4A, GLB1, HEXA, HEXB, ERCC2, POLR3A, and RARS and 1 with mosaic chromosomal change of 46, XX,del(18)(q21.3)/46,XX,r(18)(p11.32q21.3)/45,XX,-18. Eighteen novel mutations were discovered. Mutations in POLR3A and RARS were first identified in Chinese patients with Pol III-related leukodystrophy and hypomyelinating leukodystrophy, respectively.

SIGNIFICANCE

This is the first report on clinical and genetic features of hypomyelinating disorders with a large sample of patients in Chinese population, identifying 18 novel mutations especially mutations in POLR3A and RARS in Chinese patients, expanding clinical and genetic spectrums of hypomyelinating disorders.

Emptying the stores: lysosomal diseases and therapeutic strategies

Lysosomal storage disorders (LSDs) - designated as 'orphan' diseases - are inborn errors of metabolism caused by defects in genes that encode proteins involved in various aspects of lysosomal homeostasis. For many years, LSDs were viewed as unattractive targets for the development of therapies owing to their low prevalence. However, the development and success of the first commercial biologic therapy for an LSD - enzyme replacement therapy for type 1 Gaucher disease - coupled with regulatory incentives rapidly catalysed commercial interest in therapeutically targeting LSDs. Despite ongoing challenges, various therapeutic strategies for LSDs now exist, with many agents approved, undergoing clinical trials or in preclinical development.

Epidemiology and diagnosis of lysosomal storage disorders; challenges of screening

The lysosomal storage disorders (LSDs) are a group of genetic disorders resulting from defective lysosomal metabolism and subsequent accumulation of substrates. Patients present with a large phenotypic spectrum of disease manifestations that are generally not specific for LSDs, leading to considerable diagnostic delay and missed cases. Introduction of new disease modifying therapies for LSDs has made early diagnosis a priority. Increased awareness, but particularly the introduction of screening programs allow for early diagnosis and timely initiation of treatment. This review will provide insight into the epidemiology and diagnostic process for LSDs. In addition, challenges for carrier screening, high-risk screening and newborn population screening for LSDs are discussed.

Urinary Glycosaminoglycan Estimation as a Routine Clinical Service

Mucopolysaccharidoses, a group of inherited disorders are associated with defects in glycosaminoglycan metabolism. Thus, assessment of urinary glycosaminoglycan is used as a screening test for mucopolysaccharidoses. The detection methods range from qualitative spot tests to quantification using metachromatic dyes. In our laboratory we optimized a spectrophotometric quantitative method using a metachromatic dye, dimethylmethylene blue. Heparan sulfate was used for quantification. The glycosaminoglycan-dye complex showed a marked shift in color with increase in concentration. The color complex was quantified at 520 nm. The method was linear from 10-89 mg/L. An age matched normal range was obtained in 177 healthy individuals, grouped in 8 different age groups from neonates to adults. Urinary glycosaminoglycan concentration varied distinctly amongst the study population wherein the lowest range in healthy neonates was more than 3 times the upper limit of healthy adults. Urine samples from 10 patients with mucopolysaccharidoses were also included in the study for clinical validation. The method qualified both analytical and clinical validation and was found to be simple, robust and ideal to be offered as a screening test for mucopplysaccharidoses in a routine clinical chemistry laboratory.

Impairment of homeostasis in lysosomal storage disorders

Lysosomal storage disorders (LSDs) are inherited metabolic diseases caused by deficiencies in lysosomal proteins, which result in accumulation of undegraded metabolites and disruption of lysosomal proteostasis. Despite significant progress in the molecular genetics and biochemistry underlying the cellular pathogenesis of LSDs, the mechanisms that link accumulation of storage material to development and progression of these diseases are still unclear. At the crossroad of degradative pathways, lysosomes play a fundamental role in the maintenance of cellular homeostasis. Through a series of examples, this review illustrates how defects in lysosomal biogenesis and function impact a number of cellular pathways that are involved in the pathogenic cascade.

Fast urinary screening of oligosaccharidoses by MALDI-TOF/TOF mass spectrometry

Background

Oligosaccharidoses, which belong to the lysosomal storage diseases, are inherited metabolic disorders due to the absence or the loss of function of one of the enzymes involved in the catabolic pathway of glycoproteins and indirectly of glycosphingolipids. This enzymatic deficiency typically results in the abnormal accumulation of uncompletely degraded oligosaccharides in the urine. Since the clinical features of many of these disorders are not specific for a single enzyme deficiency, unambiguous screening is critical to limit the number of costly enzyme assays which otherwise must be performed.

Methods

Here we provide evidence for the advantages of using a MALDI-TOF/TOF (matrix-assisted laser desorption ionization time-of-flight) mass spectrometric (MS) method for screening oligosaccharidoses. Urine samples from previously diagnosed patients or from unaffected subjects were randomly divided into a training set and a blind testing set. Samples were directly analyzed without prior treatment.

Results

The characteristic MS and MS/MS molecular profiles obtained allowed us to identify fucosidosis, aspartylglucosaminuria, GM1 gangliosidosis, Sandhoff disease, α-mannosidosis, sialidosis and mucolipidoses type II and III.

Conclusions

This method, which is easily run in less than 30 minutes, is performed in a single step, and is sensitive and specific. Invaluable for clinical chemistry purposes this MALDI-TOF/TOF mass spectrometry procedure is semi-automatizable and suitable for the urinary screening of oligosacharidoses.

The cell biology of disease: lysosomal storage disorders: the cellular impact of lysosomal dysfunction

Lysosomal storage diseases (LSDs) are a family of disorders that result from inherited gene mutations that perturb lysosomal homeostasis. LSDs mainly stem from deficiencies in lysosomal enzymes, but also in some non-enzymatic lysosomal proteins, which lead to abnormal storage of macromolecular substrates. Valuable insights into lysosome functions have emerged from research into these diseases. In addition to primary lysosomal dysfunction, cellular pathways associated with other membrane-bound organelles are perturbed in these disorders. Through selective examples, we illustrate why the term “cellular storage disorders” may be a more appropriate description of these diseases and discuss therapies that can alleviate storage and restore normal cellular function.

Conformationally-locked N-glycosides with selective β-glucosidase inhibitory activity: identification of a new non-iminosugar-type pharmacological chaperone for Gaucher disease

A series of conformationally locked N-glycosides having a cis-1,2-fused pyranose-1,3-oxazoline-2-thione structure and bearing different substituents at the exocyclic sulfur has been prepared. The polyhydroxylated bicyclic system was built in only three steps by treatment of the corresponding readily available 1,2-anhydrosugar with KSCN using TiO(TFA)(2) as catalyst, followed by S-alkylation and acetyl deprotection. In vitro screening against several glycosidase enzymes showed highly specific inhibition of mammalian β-glucosidase with a marked dependence of the potency upon the nature of the exocyclic substituent. The most potent representative, bearing an S-(ω-hydroxyhexadecyl) substituent, was further assayed as inhibitor of the human lysosomal β-glucocerebrosidase and as pharmacological chaperone in Gaucher disease fibroblasts. Activity enhancements in N370S/N370S mutants analogous to those achieved with the reference compound ambroxol were attained with a more favorable chaperone/inhibitor balance.

A Dysmorphometric Analysis to Investigate Facial Phenotypic Signatures as a Foundation for Non-invasive Monitoring of Lysosomal Storage Disorders

BACKGROUND

Some lysosomal storage disorders (LSDs), including Muccopolysaccharidosis type 1 (MPSI), are associated with characteristic facies. METHODS such as three-dimensional (3D) facial scanning and geometric morphometric techniques can potentially generate detailed objective descriptions of these facial phenotypes. This approach can facilitate discriminating the inherent overlap in facial phenotypes within these disease spectra, and the non-invasive monitoring of disease progression and treatment.

METHODS

3D facial images of three MPS I-affected individuals and 400 reference subjects (aged 5-25 years) were obtained using a 3dMD camera (Atlanta, Georgia). Images were fitted with an anthropometric mask, comprising a set of spatially dense quasi-landmarks. A statistical face-space was constructed from the reference image set and the MPS I-affected individuals were compared to this face-space utilising an emerging methodology known as dysmorphometrics. This facilitated simultaneous identification of harmonic and discordant facial regions. A relative significant discordance (RSD) score quantified proportional facial discordance for a given individual, whilst a root-mean-squared-error (RMSE) score measured the degree of facial discordance providing a severity measure.

RESULTS

A consistent facial pattern, with differential severities, primarily affecting the frontal, nasal, infraorbital and cheek regions, was detected in all three individuals. As expected, there was greater discordance (RMSE, RSD) with clinically severe MPS I when compared to attenuated disease.

CONCLUSIONS

Objective detection and localisation of MPS I facial characteristics was achieved, and severity scores were attributed. This spatially dense dysmorphometric facial phenotyping technique has the potential to be used for non-invasive treatment monitoring and as a discriminatory tool.

Noninvasive diagnosis of mucopolysaccharidosis via depth-resolved optical spectroscopy of the outer ear

Current diagnostics for lysosomal storage disorders such as mucopolysaccharidosis (MPS) rely on evaluation of ex vivo bodily fluids, which has several shortcomings. In this study, we evaluated whether Raman spectroscopy could noninvasively diagnose MPS in a murine model. Via confocal sampling of the murine outer ear, Raman spectra were obtained at multiple depths. Partial least-squares discriminant analysis of the processed Raman spectra showed a 93% sensitivity and 91% specificity for disease. The discriminant algorithm relied on several Raman bands related to glycosaminoglycans (GAGs) that typically accumulate in MPS. These findings indicate the possibility for a new, noninvasive diagnostic tool for MPS.

Investigation of the child with an acute metabolic disorder

Inherited biochemical defects may present with acute life-threatening illness with a high mortality and morbidity. Some are treatable and have a good outcome with early appropriate intervention. However, because of their rarity, diagnosis is often delayed; they are not considered or investigated appropriately. This is especially likely in those presenting in previously healthy adults. The collection of acute samples is crucial. There are numerous disorders, and front-line tests must cast a wide net. A small core of emergency tests generally indicates which metabolic pathway is defective and provides a working diagnosis and basis for treatment. Later confirmation and identification of the precise defect are essential for long-term management and for genetic counselling and prenatal diagnosis of future pregnancies. An escalating number of specialist tests and mutation analyses are undertaken by metabolic laboratories worldwide, but they are not widely available, are expensive, and must be requested selectively. Guidelines are presented here for the front-line investigation of acutely ill children with hypoglycaemia, metabolic acidosis, encephalopathy and intractable seizures, and for a dying child with a suspected, undiagnosed, inherited metabolic defect. With modification, these are also applicable to adults with a metabolic defect. In order to guide further investigation, selected disorders are described briefly along with their diagnostic work-up. Information about sample collection and processing is provided.

Hemoglobin precipitation greatly improves 4-methylumbelliferone-based diagnostic assays for lysosomal storage diseases in dried blood spots

Derivatives of 4-methylumbelliferone (4MU) are favorite substrates for the measurement of lysosomal enzyme activities in a wide variety of cell and tissue specimens. Hydrolysis of these artificial substrates at acidic pH leads to the formation of 4-methylumbelliferone, which is highly fluorescent at a pH above 10. When used for the assay of enzyme activities in dried blood spots the light emission signal can be very low due to the small sample size so that the patient and control ranges are not widely separated. We have investigated the hypothesis that quenching of the fluorescence by hemoglobin leads to appreciable loss of signal and we show that the precipitation of hemoglobin with trichloroacetic acid prior to the measurement of 4-methylumbelliferone increases the height of the output signal up to eight fold. The modified method provides a clear separation of patients' and controls' ranges for ten different lysosomal enzyme assays in dried blood spots, and approaches the conventional leukocyte assays in outcome quality.

Lysosomal storage disorders in the newborn

Lysosomal storage disorders are rare inborn errors of metabolism, with a combined incidence of 1 in 1500 to 7000 live births. These relatively rare disorders are seldom considered when evaluating a sick newborn. A significant number of the >50 different lysosomal storage disorders, however, do manifest in the neonatal period and should be part of the differential diagnosis of several perinatal phenotypes. We review the earliest clinical features, diagnostic tests, and treatment options for lysosomal storage disorders that can present in the newborn. Although many of the lysosomal storage disorders are characterized by a range in phenotypes, the focus of this review is on the specific symptoms and clinical findings that present in the perinatal period, including neurologic, respiratory, endocrine, and cardiovascular manifestations, dysmorphic features, hepatosplenomegaly, skin or ocular involvement, and hydrops fetalis/congenital ascites. A greater awareness of these features may help to reduce misdiagnosis and promote the early detection of lysosomal storage disorders. Implementing therapy at the earliest stage possible is crucial for several of the lysosomal storage disorders; hence, an early appreciation of these disorders by physicians who treat newborns is essential.

A mass spectrometric strategy for profiling glycoproteinoses, Pompe disease, and sialic acid storage diseases

Glycoproteinoses, Pompe disease, and sialic acid storage diseases are characterized by a massive accumulation of unprocessed oligosaccharides and/or glycoconjugates in urine. The identification of these glycocompounds is essential for a proper diagnosis. In this study, we investigated the potential of MALDI-TOF-MS to identify glycocompounds present in urine from patients with different inborn errors of glycan metabolism. Urinary glycocompounds were permethylated, and analyzed using GC-MS and MALDI-TOF-MS. In order to confirm tentative assignments, a second aliquot of urine was purified on a C18 Sep-Pak cartridge and glycocompounds were desalted on a column of nonporous graphitized carbon. The glycocompounds were then sequentially on-plate digested using an array of exoglycosidases. A range of disease-specific oligosaccharides as well as glycopeptides was identified for all oligosacchariduria models. In addition, free sialic acid accumulated in urine from a patient suffering from French-type sialuria, has been detected by a GC-MS approach, which could be applied to other sialic acid storage diseases. This procedure is simple, and can be performed in few simple steps in less than 24 h. This current method can be applied for newborn screening for other inherited metabolic diseases as well as for assessing treatments in clinical trials.

The biology of the Gaucher cell: the cradle of human chitinases

Gaucher disease (GD) is the most common lysosomal storage disorder and is caused by inherited deficiencies of glucocerebrosidase, the enzyme responsible for the lysosomal breakdown of the lipid glucosylceramide. GD is characterized by the accumulation of pathological, lipid laden macrophages, so-called Gaucher cells. Following the development of enzyme replacement therapy for GD, the search for suitable surrogate disease markers resulted in the identification of a thousand-fold increased chitinase activity in plasma from symptomatic Gaucher patients and that decreases upon successful therapeutic intervention. Biochemical investigations identified a single enzyme, named chitotriosidase, to be responsible for this activity. Chitotriosidase was found to be an excellent marker for lipid laden macrophages in Gaucher patients and is now widely used to assist clinical management of patients. In the wake of the identification of chitotriosidase, the presence of other members of the chitinase family in mammals was discovered. Amongst these is AMCase, an enzyme recently implicated in the pathogenesis of asthma. Chitinases are omnipresent throughout nature and are also produced by vertebrates in which they play important roles in defence against chitin-containing pathogens and in food processing.

[Neurological aspects of Fabry's disease]

Fabry disease is a rare X-linked disorder caused by deficient activity of the lysosomal enzyme alpha-galactosidase A. Progressive accumulation in lysosomes of the undegraded glycosphingolipids leads to a multi-system disease with dermatological, ocular, renal, cardiac, and neurological manifestations. Peripheral nerve involvement, neuropathic pain and chronic acroparesthesiae, are frequent and early-onset signs revealing the disease. They are due to the involvement of small nerve fiber, thus explaining the normality of electroneuromyography. Cochleo-vestibular and autonomic nervous system involvement is frequent. Besides rare aseptic meningitis, central nervous system involvement is essentially represented by cerebrovascular events (stroke, transient ischemic attack). Affecting essentially the posterior circulation, their etiologies have to be clarified: progressive stenosis of small vessels with globotriasocylceramide deposits, arterial remodeling, endothelial dysfunction, pro-thrombotic state, cerebral hypoperfusion consecutive to dysautonaumy, cardiac embolism. MRI shows numerous silent lesions, increasing with age, mainly in small perforant arteries (periventricular white matter, brainstem, cerebellum, basal ganglia). Pulvinar calcifications, due to an increase in cerebral hyperperfusion, could be specific of Fabry disease. Positon tomography analysis shows a reduced cerebral flow velocity and impaired cerebral autoregulation, secondary to the glycosphingolipid storage in vascular endothelial cells. Enzyme replacement therapy has to be carefully monitored.

National health budgets for expensive orphan drugs: Gaucher disease in Israel as a model

Drugs for orphan diseases are often disproportionately costly, although the patient population is by definition small. In Israel, with a high percentage of Ashkenazi Jews and therefore many patients with Gaucher disease, the expense of enzyme replacement therapy for all patients would be prohibitive. For this reason, with approval of enzyme replacement therapy in Israel, the low-dose regimen (30 U/kg/month), less than one quarter of the manufacturer's original recommended dosage, was instituted as the starting regimen. A Gaucher Committee of medical experts under the auspices of the Ministry of Health determines eligibility for enzyme replacement therapy based on criteria of disease severity. At the advent of 2006, 184 patients in Israel receive enzyme replacement therapy, about one third of all known Israeli patients with Gaucher disease. The national budget provides capitation for each patient to each Sick Fund via a health care basket for severe/expensive treatments. After nearly nine years, the benefits of these innovations include availability of budget for patients requiring enzyme replacement therapy, evidence-based data that low dose is safe and effective, and that untreated mildly affected patients generally continue a benign disease trajectory, but if necessary, have recourse to enzyme replacement therapy, that is, patient care is never compromised. Questionnaires of satisfaction with this system highlight good outcome scores. For countries with limited resources, the use of an impartial committee of experts is recommended, as is long-term surveillance of all patients; maintenance protocols or even drug vacations as enzyme-treated patients approach normalization of disease parameters should be considered.

Genetic correction of the fetal brain increases the lifespan of mice with the severe multisystemic disease mucopolysaccharidosis type VII

Neurogenetic diseases typically have globally distributed lesions, and pathology usually develops early in life, requiring early diagnosis and treatment. We investigated the effects of transferring a corrective gene into the fetal brain before the onset of pathology in the mucopolysaccharidosis (MPS) type VII mouse, a model of a lysosomal storage disease. A single adeno-associated virus serotype 1 vector injection into the ventricle at 15.5 days of gestation resulted in widespread distribution and lifelong expression of the normal gene in the brain and spinal cord. The normal enzyme was distributed to neighboring cells (as expected) and completely prevented the development of storage lesions throughout the central nervous system (CNS). No vector transfer was found outside the CNS, including the gonads, but a small amount of enzyme was present in visceral tissues, consistent with transfer from cerebrospinal fluid to venous circulation. The enzyme was present peripherally in such low amounts that it did not result in the severe skeletal dysmorphology that occurs readily when systemic treatment is used in neonates. However, the survival probability of the treated animals was significantly increased. The results suggest that the nervous system disease may contribute to the overall physiologic health of the animal in this type of disease.

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.

Newborn screening and the pediatric practitioner

Since the beginning of newborn screening for metabolic and other disorders in 1964, advances in the understanding of the disorders identified and development of new methods of testing newborn screening blood spots have contributed to improved health in children. Pediatricians and others involved in the health care of infants must be able to participate in the assessment and confirmatory testing of infants who have an abnormal test result and in the care of infants identified with a disorder. Expansion in the technology and number of disorders identified has complicated this process. As more and a greater variety of disorders are tested for and identified, a crucial collaborative role has emerged for the newborn screening programs and their public health professionals, the tertiary care specialists in the disorders and the primary care clinicians who comprise the medical home of the infants identified. This collaboration needs to provide prompt results of the newborn screening tests, expeditious and expert confirmatory testing and an effective care plan for the affected infant to realize the benefits of treatment for children with otherwise devastating disorders.

Enzyme-replacement therapy for metabolic storage disorders

BACKGROUND

After many years of intensive investigation, enzyme-replacement therapy (ERT) has become standard treatment for patients with type 1 (non-neuronopathic) Gaucher's disease. ERT has greatly changed the clinical course of this disorder by reducing hepatosplenomegaly, by improving anaemia and thrombocytopenia, and by ameliorating skeletal damage. This example has prompted the investigation of ERT for several other metabolic disorders. The results of several of these trials have recently been published.

RECENT DEVELOPMENTS

In addition to Gaucher's disease, the effects of ERT in four other major metabolic storage disorders have been reported. Among these rare orphan diseases are Fabry's disease, in which the heart, kidney, gastrointestinal tract, and peripheral nerves are damaged; Pompe's disease, in which the heart, skeletal muscles, and brain are involved; Hurler's disease and Maroteaux-Lamy syndrome in which the eyes, liver, joints, and skeleton are usually affected. Responses to ERT in these four disorders have generally been encouraging although the degree and extent of benefit vary considerably.

WHERE NEXT

There are several critical features of ERT that require attention and amelioration. Among these are the development of severity-score indices that can be used to explicitly quantify the benefit of ERT. The benefit of this treatment has been slight in Fabry's disease and is yet to be fully shown in the other three disorders. Secondly, novel technologies need to be developed to deliver therapeutic enzymes effectively to tissues such as the cardiac muscle and kidney in Fabry's disease, skeletal muscle in patients with Pompe's disease, and to joint tissues and structures in patients with Hurler's disease and Maroteaux-Lamy syndrome. Finally, an all-encompassing concern is to devise methods to ameliorate the damage to the central and peripheral nervous systems that occurs in specific phenotypes of these disorders. In this review we descibe emerging strategies that seem to be useful in these critical regards.