Substrate reduction therapy of glycosphingolipid storage disorders

Biomarkers in the diagnosis of lysosomal storage disorders: proteins, lipids, and inhibodies

A biomarker is an analyte indicating the presence of a biological process linked to the clinical manifestations and outcome of a particular disease. In the case of lysosomal storage disorders (LSDs), primary and secondary accumulating metabolites or proteins specifically secreted by storage cells are good candidates for biomarkers. Clinical applications of biomarkers are found in improved diagnosis, monitoring disease progression, and assessing therapeutic correction. These are illustrated by reviewing the discovery and use of biomarkers for Gaucher disease and Fabry disease. In addition, recently developed chemical tools allowing specific visualization of enzymatically active lysosomal glucocerebrosidase are described. Such probes, coined inhibodies, offer entirely new possibilities for more sophisticated molecular diagnosis, enzyme replacement therapy monitoring, and fundamental research.

Identification of potent and selective glucosylceramide synthase inhibitors from a library of N-alkylated iminosugars

Glucosylceramide synthase (GCS) is an important target for clinical drug development for the treatment of lysosomal storage disorders and a promising target for combating type 2 diabetes. Iminosugars are useful leads for the development of GCS inhibitors; however, the effective iminosugar type GCS inhibitors reported have some unwanted cross-reactivity toward other glyco-processing enzymes. In particular, iminosugar type GCS inhibitors often also inhibit to some extent human acid glucosylceramidase (GBA1) and the nonlysosomal glucosylceramidase (GBA2), the two enzymes known to process glucosylceramide. Of these, GBA1 itself is a potential drug target for the treatment of the lysosomal storage disorder, Gaucher disease, and selective GBA1 inhibitors are sought after as potential chemical chaperones. The physiological importance of GBA2 in glucosylceramide processing in relation to disease states is less clear, and here, selective inhibitors can be of use as chemical knockout entities. In this communication, we report our identification of a highly potent and selective N-alkylated l-ido-configured iminosugar. In particular, the selectivity of 27 for GCS over GBA1 is striking.

Glycosphingolipids and insulin resistance

Glycosphingolipids are structural membrane components, residing largely in the plasma membrane with their sugar-moieties exposed at the cell's surface. In recent times a crucial role for glycosphingolipids in insulin resistance has been proposed. A chronic state of insulin resistance is a rapidly increasing disease condition in Western and developing countries. It is considered to be the major underlying cause of the metabolic syndrome, a combination of metabolic abnormalities that increases the risk for an individual to develop Type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome and nonalcoholic fatty liver disease. As discussed in this chapter, the evidence for a direct regulatory interaction of glycosphingolipids with insulin signaling is still largely indirect. However, the recent finding in animal models that pharmacological reduction of glycosphingolipid biosynthesis ameliorates insulin resistance and prevents some manifestations of metabolic syndrome, supports the view that somehow glycosphingolipids act as critical regulators, Importantly, since reductions in glycosphingolipid biosynthesis have been found to be well tolerated, such approaches may have a therapeutic potential.

Substrate reduction therapy with miglustat in chronic GM2 gangliosidosis type Sandhoff: results of a 3-year follow-up

GM2 gangliosidosis type Sandhoff is caused by a defect of beta-hexosaminidase, an enzyme involved in the catabolism of gangliosides. It has been proposed that substrate reduction therapy using N-butyl-deoxynojirimycin (miglustat) may delay neurological progression, at least in late-onset forms of GM2 gangliosidosis. We report the results of a 3-year treatment with miglustat (100 mg t.i.d) in a patient with chronic Sandhoff disease manifesting with an atypical, spinal muscular atrophy phenotype. The follow-up included serial neurological examinations, blood tests, abdominal ultrasound, and neurophysiologic, cognitive, brain, and muscle MRI studies. We document some minor effects on neurological progression in chronic Sandhoff disease by miglustat treatment, confirming the necessity of phase II therapeutic trials including early-stage patients in order to assess its putative efficacy in chronic Sandhoff disease.

Dietary modifications in patients receiving miglustat

Weight loss and gastrointestinal disturbances are often seen during miglustat therapy for lysosomal storage diseases. A retrospective analysis of data from a mixed group of patients treated with miglustat at two UK centres was performed to evaluate the effect of two different dietary interventions on body weight and gastrointestinal tolerability during the initial 6 months of miglustat therapy. Neurological outcomes in these patients are not discussed herein. Data were analysed from a total of 29 patients with varied neurolipidoses (21 children/adolescents; 8 adults). Negative mean changes in body weight were seen in children/adolescents on an unmodified diet (-8.1%), and in adults (-4.1%) and children/adolescents (-5.2%) on a low-lactose diet. Patients on the low-disaccharide diet showed a positive mean change in body weight (+2.0%), although there was high variability in this group. Non-parametric sub-analysis of median body-weight change in children/adolescents also showed high variability both within and between diet groups, with no statistically significant difference between the effects of different diets on body weight (p = 0.062). The low-lactose diet reduced gastrointestinal disturbances; single small doses of loperamide were required in some patients. Patients on the low-disaccharide diet showed the lowest frequency of gastrointestinal effects. In conclusion, simple dietary modifications allowed the maintenance of body-weight gain in line with normal growth potential during miglustat therapy in young patients with lysosomal storage diseases, and reduced gastrointestinal disturbances.

Misdiagnosis of Niemann-Pick disease type C as Gaucher disease

Niemann-Pick disease type C (NPC) is a model for inborn errors of metabolism whose gene product mediates molecular trafficking rather than catabolizing macromolecules, as in classic lipidoses. We report the case of an infant who presented with hepatosplenomegaly without neurological abnormalities. Decreased activity of acid β-glucosidase and elevated serum chitotriosidase and tartrate-resistant acid phosphatase on repeated measurements led to initial diagnosis of Gaucher disease (GD). Failure to respond to enzyme replacement therapy after one year, however, put the diagnosis in question. Cholesterol esterification assays in cultured skin fibroblasts and NPC gene analysis led to the correct diagnosis of NPC. The patient had markedly reduced cholesterol esterification and was a compound heterozygote for a known and a novel mutation in the NPC gene (395delC and 2068insTCCC), which are both predicted to lead to protein truncation. Although the full phenotype of NPC involves hepatosplenomegaly and neurodegenerative disease, the initial presentation in a pediatric patient may be restricted to visceral disease. Of interest, this patient had decreased activity of leukocyte acid β-glucosidase activity and elevated serum chitotriosidase to levels often seen in GD. Although acid β-glucosidase activity in leukocytes was low, it was in the normal range in skin fibroblasts. Therefore, diagnostic delay may occur in NPC due to false positive testing for GD. Diagnosis of NPC requires a high index of suspicion and should be considered in a patient with hepatosplenomegaly even in the absence of neurodevelopmental signs. Prompt diagnosis will become increasingly important as effective therapies are developed for NPC.

The effect of N-octyl-β-valienamine on β-glucosidase activity in tissues of normal mice

Gaucher disease (GD), mainly caused by a defect of acid β-glucosidase (β-Glu), is the most common sphingolipidosis. We have previously shown that a carbohydrate mimic N-octyl-β-valienamine (NOV), an inhibitor of β-Glu, could increase the protein level and enzyme activity of various mutant β-Glu in cultured GD fibroblasts, suggesting that NOV acted as a pharmacological chaperone to accelerate transport and maturation of this mutant enzymes. In the present study, the NOV effect was evaluated for β-Glu activity, tissue distribution and adverse effects in normal mice. We measured the β-Glu activity in tissues of normal mice which received water containing increasing concentrations of NOV ad libitum for 1 week. Fluid intake and body weight were measured periodically throughout the study. Measurement of tissue NOV concentration, blood chemistry and urinalysis were performed at the end of the study. The results showed that NOV had no impact on the body weight but fluid intake in the 10mM NOV group mice decreased and there was a moderate increase in blood urea nitrogen (BUN). No other adverse effect was observed during this experiment. Tissue NOV concentration increased in all tissues examined with increasing NOV doses. No inhibitory effect of NOV on β-Glu was observed. Furthermore, NOV increased the β-Glu activity in the liver, spleen, muscle and cerebellum of the mice significantly. This study on NOV showed its oral availability and wide tissue distribution, including the brain and its lack of acute toxicity. These characteristics of NOV would make it a potential therapeutic chaperone in the treatment of GD with neurological manifestations and selected mutations.

Ultrasensitive in situ visualization of active glucocerebrosidase molecules

Deficiency of glucocerebrosidase (GBA) underlies Gaucher disease, a common lysosomal storage disorder. Carriership for Gaucher disease has recently been identified as major risk for parkinsonism. Presently, no method exists to visualize active GBA molecules in situ. We here report the design, synthesis and application of two fluorescent activity-based probes allowing highly specific labeling of active GBA molecules in vitro and in cultured cells and mice in vivo. Detection of in vitro labeled recombinant GBA on slab gels after electrophoresis is in the low attomolar range. Using cell or tissue lysates, we obtained exclusive labeling of GBA molecules. We present evidence from fluorescence-activated cell sorting analysis, fluorescence microscopy and pulse-chase experiments of highly efficient labeling of GBA molecules in intact cells as well as tissues of mice. In addition, we illustrate the use of the fluorescent probes to study inhibitors and tentative chaperones in living cells.

Pharmacological small molecules for the treatment of lysosomal storage disorders

IMPORTANCE OF THE FIELD

Inherited lysosomal storage diseases often cause severe disability and have a devastating effect on quality of life. Enzyme replacement therapy (ERT) forms a cornerstone in the treatment of lysosomal enzyme deficiencies. Although for some lysosomal disorders ERT is lifesaving, important intrinsic restrictions of the approach are limited access of infused enzyme to less accessible body compartments such as the CNS, the burden of frequent intravenous administration, the emergence of antibodies and the high associated costs. Pharmacological small molecules may overcome these limitations.

AREAS COVERED IN THIS REVIEW

Several novel therapeutic approaches using small molecules are emerging: substrate reduction therapy, pharmacological chaperone therapy, premature nonsense mutation suppressors and proteostasis regulators.

WHAT THE READER WILL GAIN

Based on an extensive literature search up until June 2010, we here review the various therapeutic approaches with small compounds, including those currently in clinical use and those that have entered clinical trials. Compounds that are still in the preclinical phase are also briefly discussed.

TAKE HOME MESSAGE

pharmacological small molecules are a new class of agents that show great promise for the treatment of lysosomal storage disorders.

Multi-system disorders of glycosphingolipid and ganglioside metabolism

Glycosphingolipids (GSLs) and gangliosides are a group of bioactive glycolipids that include cerebrosides, globosides, and gangliosides. These lipids play major roles in signal transduction, cell adhesion, modulating growth factor/hormone receptor, antigen recognition, and protein trafficking. Specific genetic defects in lysosomal hydrolases disrupt normal GSL and ganglioside metabolism leading to their excess accumulation in cellular compartments, particularly in the lysosome, i.e., lysosomal storage diseases (LSDs). The storage diseases of GSLs and gangliosides affect all organ systems, but the central nervous system (CNS) is primarily involved in many. Current treatments can attenuate the visceral disease, but the management of CNS involvement remains an unmet medical need. Early interventions that alter the CNS disease have shown promise in delaying neurologic involvement in several CNS LSDs. Consequently, effective treatment for such devastating inherited diseases requires an understanding of the early developmental and pathological mechanisms of GSL and ganglioside flux (synthesis and degradation) that underlie the CNS diseases. These are the focus of this review.

Dual-action lipophilic iminosugar improves glycemic control in obese rodents by reduction of visceral glycosphingolipids and buffering of carbohydrate assimilation

The lipophilic iminosugar N-[5-(adamantan-1-ylmethoxy)pentyl]-1-deoxynojirimycin (2, AMP-DNM) potently controls hyperglycemia in obese rodent models of insulin resistance. The reduction of visceral glycosphingolipids by 2 is thought to underlie its beneficial action. It cannot, however, be excluded that concomitant inhibition of intestinal glycosidases and associated buffering of carbohydrate assimilation add to this. To firmly establish the mode of action of 2, we developed a panel of lipophilic iminosugars varying in configuration at C-4/C-5 and N-substitution of the iminosugar. From these we identified the l-ido derivative of 2, l-ido-AMP-DNM (4), as a selective inhibitor of glycosphingolipid synthesis. Compound 4 lowered visceral glycosphingolipids in ob/ob mice and ZDF rats on a par with 2. In contrast to 2, 4 did not inhibit sucrase activity or sucrose assimilation. Treatment with 4 was significantly less effective in reducing blood glucose and HbA1c. We conclude that the combination of reduction of glycosphingolipids in tissue and buffering of carbohydrate assimilation by 2 produces a superior glucose homeostasis.

Glycosphingolipids--nature, function, and pharmacological modulation

The discovery of the glycosphingolipids is generally attributed to Johan L. W. Thudichum, who in 1884 published on the chemical composition of the brain. In his studies he isolated several compounds from ethanolic brain extracts which he coined cerebrosides. He subjected one of these, phrenosin (now known as galactosylceramide), to acid hydrolysis, and this produced three distinct components. One he identified as a fatty acid and another proved to be an isomer of D-glucose, which is now known as D-galactose. The third component, with an "alkaloidal nature", presented "many enigmas" to Thudichum, and therefore he named it sphingosine, after the mythological riddle of the Sphinx. Today, sphingolipids and their glycosidated derivatives are the subjects of intense study aimed at elucidating their role in the structural integrity of the cell membrane, their participation in recognition and signaling events, and in particular their involvement in pathological processes that are at the basis of human disease (for example, sphingolipidoses and diabetes type 2). This Review details some of the recent findings on the biosynthesis, function, and degradation of glycosphingolipids in man, with a focus on the glycosphingolipid glucosylceramide. Special attention is paid to the clinical relevance of compounds directed at interfering with the factors responsible for glycosphingolipid metabolism.

Common G102S polymorphism in chitotriosidase differentially affects activity towards 4-methylumbelliferyl substrates

Chitotriosidase (CHIT1) is a chitinase that is secreted by activated macrophages. Plasma chitotriosidase activity reflects the presence of lipid-laden macrophages in patients with Gaucher disease. CHIT1 activity can be conveniently measured using fluorogenic 4-methylumbelliferyl (4MU)-chitotrioside or 4MU-chitobioside as the substrate, however, nonsaturating concentrations have to be used because of apparent substrate inhibition. Saturating substrate concentrations can, however, be used with the newly designed substrate 4MU-deoxychitobioside. We studied the impact of a known polymorphism, G102S, on the catalytic properties of CHIT1. The G102S allele was found to be common in type I Gaucher disease patients in the Netherlands ( approximately 24% of alleles). The catalytic efficiency of recombinant Ser102 CHIT1 was approximately 70% that of wild-type Gly102 CHIT1 when measured with 4MU-chitotrioside at a nonsaturating concentration. However, the activity was normal with 4MU-deoxychitobioside as the substrate at saturating concentrations, consistent with predictions from molecular dynamics simulations. In conclusion, interpretation of CHIT1 activity measurements with 4MU-chitotrioside with respect to CHIT1 protein concentrations depends on the presence of Ser102 CHIT1 in an individual, complicating estimation of the body burden of storage macrophages. Use of the superior 4MU-deoxychitobioside substrate avoids such complications because activity towards this substrate under saturating conditions is not affected by the G102S substitution.

Real-world clinical experience with long-term miglustat maintenance therapy in type 1 Gaucher disease: the ZAGAL project

There are few published data from real-world clinical experience with miglustat (Zavesca), an oral inhibitor of glucosylceramide synthase, in type 1 Gaucher disease. We report data from a prospective, open-label investigational study that evaluated substrate reduction therapy with miglustat 100 mg t.i.d. as a maintenance therapy in patients with Type 1 Gaucher disease who had been switched from previous enzyme replacement therapy. Long-term data on changes in organ size, blood counts, disease severity bio-markers, bone marrow infiltration, overall clinical status and safety/tolerability were analyzed from 28 patients with Type 1 Gaucher disease who were attending routine clinic visits. Assessments were performed at six, 12, 24, 36 and 48 months of therapy. Disease severity biomarkers improved up to 48 months after initiation of miglustat, while other disease parameters remained stable. Miglustat showed an acceptable profile of safety and tolerability throughout treatment. In conclusion, miglustat is an effective therapy for the long-term maintenance of patients with Type 1 Gaucher disease previously stabilized with enzyme replacement therapy.

The potential action of galactose as a "chemical chaperone": increase of beta galactosidase activity in fibroblasts from an adult GM1-gangliosidosis patient

BACKGROUND

The glycosphingolipid storage disorder GM1-gangliosidosis is a severe neurodegenerative condition for which no therapy is currently available. Protein misfolding in lysosomal defects may have the potential to be corrected by chemical chaperones: in vitro and clinical approaches are being investigated.

AIMS

We investigated the in vitro effect of galactose on some lysosomal hydrolases, and its in vitro efficacy as a chemical chaperone in GM1-gangliosidosis.

METHODS

Galactose was added to the culture medium of fibroblasts from patients, controls and transfected COS-1 cells. Enzyme assays of lysosomal hydrolases, beta galactosidase in particular, were performed.

RESULTS

Our data show that galactose alters selectively alpha and beta galactosidases. A significant increase (2,5 fold) in beta galactosidase activity occurred when galactose was added to the cultured fibroblasts of an adult patient. Chemical chaperone therapy requires the presence of residual enzyme activity. The adult patient here reported is heterozygous for the p.T329A mutation that showed no beta galactosidase activity, and for the p.R442Q mutation with residual enzyme activity. The p.R442Q mutation was therefore selected as a potential target for the galactose chaperone; after the addition of galactose, COS-1 cells transfected with this mutation showed an increase in beta galactosidase activity from 6.9% to 12% of control values.

CONCLUSIONS

These results suggest that galactose or its derivatives with potential chaperone properties could be used in the development of non-invasive therapies for GM1-gangliosidosis.

Goal-oriented therapy with miglustat in Gaucher disease

BACKGROUND

Gaucher disease (GD) is a highly heterogeneous disorder with multisystem involvement. Specific therapeutic goals for each manifestation of type 1 GD (GD1) were established in 2004 by an international panel of experts, to facilitate better management of GD1 patients. The goals were defined based on experience with enzyme replacement therapy (ERT) using imiglucerase. Miglustat, a small iminosugar, is the only commercially available substrate reduction therapy (SRT) for patients with GD1. Several clinical studies have demonstrated the beneficial effects of miglustat on cardinal disease manifestations of GD1.

OBJECTIVE

To review the currently available data on miglustat, and provide guidance on the attainment of the GD therapeutic goals with miglustat therapy.

METHODS

A literature search identified publications on miglustat using MEDLINE, HighWire Press, and Google Scholar databases. Articles were identified using the terms 'miglustat' and 'Gaucher disease type 1'.

FINDINGS

Improvements in hematological manifestations and organomegaly can be expected with miglustat therapy, with disease stabilization achievable over the long term. Recent data suggest that miglustat can maintain stability in patients with mild to moderate GD1 who have been previously treated with ERT. Miglustat may be beneficial with regards to bone manifestations, with reduction in the incidence of patients reporting bone pain and improvements in bone mineral density seen within the first 24 months of therapy.

CONCLUSIONS

Several of the therapeutic goals for patients with GD1 can be achieved with miglustat therapy. In select cases, miglustat can be considered an alternative to ERT for the treatment of patients with GD1. Long-term experience with the use of miglustat will help define its overall safety and efficacy; this information will be useful in determining the role of SRT using miglustat in the management of the general adult GD1 patient population.

Current strategies in the management of lysosomal storage diseases

Lysosomal storage diseases (LSDs) comprise a diverse group of over 40 clinically distinct inherited disorders. LSDs are progressive and may present at any age affecting any number of tissues and organ systems. They result from a genetic defect in cellular transport or metabolism of molecules within the lysosome. Treatment is directed toward symptomatic care of secondary complications for most of these diseases. For some individuals, hematopoietic stem cell transplantation or enzyme-replacement therapy can be effective. However, limitations in these therapies still exist. To date, there is no cure for any of the LSDs. Early diagnosis and treatment is essential for optimal treatment; this lends support to implementing mass newborn screening for LSDs.

Potential artefacts in proteome analysis of plasma of Gaucher patients due to protease abnormalities

The plasma proteome of type I Gaucher disease patients was investigated by 2D gel electrophoresis (2DGE). Using the classical procedure with 8 M urea treated plasma, several high molecular weight proteins were absent from Gaucher plasma specimens, while additional low molecular weight proteins were visible. The latter were identified as proteolytic degradation products. Adding small amounts of patient plasma to control plasma gave extensive protein breakdown. The presence of 2.2 M thiourea/7.7 M urea in the rehydration solution totally prevented breakdown. In the 'urea only' solution, protease(s) uniquely present in Gaucher plasma, appear to be still active towards other denatured plasma proteins at low pH. Therapy of patients results in gradual disappearance of proteolytic capacity from plasma specimens, indicating it to be related to the presence of Gaucher storage cells. The proteolytic activity could be partly removed from Gaucher plasma samples by Concanavalin A, suggesting that glycoproteins are involved. Reduction of proteolysis by Pepstatin A and Leupeptin implies that cathepsins, proteases known to be overproduced by Gaucher storage cells, are involved. In conclusion, 2DGE Gaucher plasma proteomes should be interpreted cautiously given the abnormal high levels of proteases associated with this disorder.

Differential sensitivity of mouse strains to an N-alkylated imino sugar: glycosphingolipid metabolism and acrosome formation

This review deals with the pharmacological properties of an alkylated monosaccharide mimetic, N-butyldeoxynojirimycin (NB-DNJ). This compound is of pharmacogenetic interest because one of its biological effects in mice - impairment of spermatogenesis, leading to male infertility - depends greatly on the genetic background of the animal. In susceptible mice, administration of NB-DNJ perturbs the formation of an organelle, the acrosome, in early post-meiotic male germ cells. In all recipient mice, irrespective of reproductive phenotype, NB-DNJ has a similar biochemical effect: inhibition of the glucosylceramidase beta-glucosidase 2 and subsequent elevation of glucosylceramide, a glycosphingolipid. The questions that we now need to address are: how can glucosylceramide specifically affect early acrosome formation, and why is this contingent on genetic factors? Here we discuss relevant aspects of reproductive biology, the metabolism and cell biology of sphingolipids, and complex trait analysis; we also present a speculative model that takes our observations into account.

Biomarkers for lysosomal storage disorders: identification and application as exemplified by chitotriosidase in Gaucher disease

A biomarker is an analyte that indicates the presence of a biological process linked to the clinical manifestations and outcome of a particular disease. An ideal biomarker provides indirect but ongoing determinations of disease activity. In the case of lysosomal storage disorders (LSDs), metabolites or proteins specifically secreted by storage cells are good candidates for biomarkers. Potential clinical applications of biomarkers are found in improved diagnosis, monitoring of disease progression and assessment of therapeutic correction. These applications are illustrated by reviewing the use of plasma chitotriosidase in the clinical management of patients with Gaucher disease, the most common LSD. The ongoing debate on the value of biomarkers in patient management is addressed. Novel analytical methods have revolutionized the identification and measurement of biomarkers at the protein and metabolite level. Recent developments in biomarker discovery by proteomics are described and the future for biomarkers of LSDs is discussed.

CONCLUSION

Besides direct applications for biomarkers in patient management, biomarker searches are likely to render new insights into pathophysiological mechanisms and metabolic adaptations, and may provide new targets for therapeutic intervention.

Gaucher disease: unmet treatment needs

Gaucher disease is a multisystemic metabolic disorder arising from a deficiency of lysosomal glucocerebrosidase. The predominant clinical manifestations of the disease are hepatosplenomegaly, peripheral blood cytopenias and skeletal disease. Treatment with enzyme replacement therapy (ERT) and substrate reduction therapy (SRT) has been shown to be effective in improving organ volume, anaemia, thrombocytopenia, bone markers and biomarkers in patients with Gaucher disease. However, some patient needs remain unmet because of the limited availability of treatment, the inaccessibility of certain disease sites and emerging disease manifestations. An increase in haematological, lymphoreticular and immune system malignancies has been observed in patients with Gaucher disease, but mechanisms underlying the development of these are not fully understood. Mild neurological manifestations may also affect patients with type 1 Gaucher disease, but treatment with ERT or SRT does not improve neurological function. Potential new treatments for Gaucher disease include small molecules, which may penetrate tissues that are not accessible by ERT.

CONCLUSION

ERT currently remains the most effective treatment for Gaucher disease. New treatments are emerging, but deficiencies in understanding basic pathophysiological mechanisms hinder progress.

[Current development and usefulness of biomarkers for Gaucher disease follow up]

Gaucher's disease is due to glucocerebrosidase deficiency which is responsible for the accumulation of non degraded glucosylceramide within the lysosomes of macrophages: these "Gaucher cells", overloaded and alternatively activated, release in patient's plasma numerous compounds (cytokines, chemokines, hydrolases...) some of which contribute to the various tissue damages. Some of these compounds are surrogate biomarkers which contribute to the evaluation of disease severity, progression and stabilisation or regression during treatment. To date, the most interesting biomarkers are chitotriosidase and the chemokine CCL18/PARC, especially in chitotriosidase deficient patients. These biomarkers together with the clinical evaluation help to therapeutic choice (treatment by enzyme replacement therapy or substrate reduction therapy) and initiation decision, response follow-up and dose adjustments. Biomarkers should be assessed every 12 months together with clinical evaluation in patients not receiving specific treatments. An assessment every 3 months is recommended during the first year of treatment. Then when clinical goals have been achieved, the frequency can be reduced to every 12 months if the therapeutic scheme is not modified.

Substrate deprivation therapy: a new hope for patients suffering from neuronopathic forms of inherited lysosomal storage diseases

Lysosomal storage diseases are a group of disorders caused by defects in enzymes responsible for degradation of particular compounds in lysosomes. In most cases, these diseases are fatal, and until recently no treatment was available. Introduction of enzyme replacement therapy was a breakthrough in the treatment of some of the diseases. However, while this therapy is effective in reduction of many somatic symptoms, its efficacy in the treatment of the central nervous system is negligible, if any, mainly because of problems with crossing the blood-brain-barrier by intravenously administered enzyme molecules. On the other hand, there are many lysosomal storage diseases in which the central nervous system is affected. Results of very recent studies indicate that in at least some cases, another type of therapy, called substrate deprivation therapy (or substrate reduction therapy) may be effective in the treatment of neuronopathic forms of lysosomal storage diseases. This therapy, based on inhibition of synthesis of the compounds that cannot be degraded in cells of the patients, has been shown to be effective in several animal models of various diseases, and recent reports demonstrate its efficacy in the treatment of patients suffering from Niemann-Pick C disease and Sanfilippo disease.

Gaucher disease

Although Gaucher disease is a rare disorder, recent developments in novel means for therapeutic intervention have invigorated both academic research and pharmaceutical industry discovery programmes. The common mutations found in the lysosomal enzyme deficient in Gaucher disease, beta-glucocerebrosidase, earmark these proteins for destruction by the endoplasmic reticulum-localised protein folding machinery, resulting in enzyme insufficiency, lysosomal glycolipid storage and subsequent pathology. However, many of these mutants can be rescued from global misfolding to preserve glycolipid substrate binding and eventual catalysis in the lysosome, by the addition of subinhibitory concentrations of pharmacologically active small molecules. This novel, chaperon-mediated approach has benefited from insights into the molecular understanding of beta-glucocerebrosidase structure, drug design and development in cellular models for disease.

Increased plasma macrophage inflammatory protein (MIP)-1α and MIP-1β levels in type 1 Gaucher disease

Pancytopenia, hepatosplenomegaly and skeletal complications are hallmarks of Gaucher disease. Monitoring of the outcome of therapy on skeletal status of Gaucher patients is problematic since currently available imaging techniques are expensive and not widely accessible. The availability of a blood test that relates to skeletal manifestations would be very valuable. We here report that macrophage inflammatory protein (MIP)-1alpha and MIP-1beta, both implicated in skeletal complications in multiple myeloma (MM), are significantly elevated in plasma of Gaucher patients. Plasma MIP-1alpha of patients (median 78 pg/ml, range 21-550 pg/ml, n=48) is elevated (normal median 9 pg/ml, range 0-208 pg/ml, n=39). Plasma MIP-1beta of patients (median 201 pg/ml, range 59-647 pg/ml, n=49) is even more pronouncedly increased (normal median 17 pg/ml, range 1-41 pg/ml, n=39; one outlier: 122 pg/ml). The increase in plasma MIP-1beta levels of Gaucher patients is associated with skeletal disease. The plasma levels of both chemokines decrease upon effective therapy. Lack of reduction of plasma MIP-1beta below 85 pg/ml during 5 years of therapy was observed in patients with ongoing skeletal disease. In conclusion, MIP-1alpha and MIP-1beta are elevated in plasma of Gaucher patients and remaining high levels of MIP-1beta during therapy seem associated with ongoing skeletal disease.

A specific and potent inhibitor of glucosylceramide synthase for substrate inhibition therapy of Gaucher disease

An approach to treating Gaucher disease is substrate inhibition therapy which seeks to abate the aberrant lysosomal accumulation of glucosylceramide. We have identified a novel inhibitor of glucosylceramide synthase (Genz-112638) and assessed its activity in a murine model of Gaucher disease (D409V/null). Biochemical characterization of Genz-112638 showed good potency (IC(50) approximately 24nM) and specificity against the target enzyme. Mice that received drug prior to significant accumulation of substrate (10 weeks of age) showed reduced levels of glucosylceramide and number of Gaucher cells in the spleen, lung and liver when compared to age-matched control animals. Treatment of older mice that already displayed significant amounts of tissue glucosylceramide (7 months old) resulted in arrest of further accumulation of the substrate and appearance of additional Gaucher cells in affected organs. These data indicate that substrate inhibition therapy with Genz-112638 represents a viable alternate approach to enzyme therapy to treat the visceral pathology in Gaucher disease.

N-Azidoacetylmannosamine-mediated chemical tagging of gangliosides

Peracetylated N-alpha-azidoacetylmannosamine (Ac(4)ManNAz) is metabolized by cells to CMP-azidosialic acid. It has been demonstrated previously that in this way azidosialic acid-containing glycoproteins are formed that can be labeled on the cell surface by a modified Staudinger ligation. Here, we first demonstrate that the same procedure also results in the formation of azidosialic acid-containing gangliosides. Deoxymannojirimycin, an inhibitor of N-glycan processing in proteins, decreases the total cell surface labeling in Jurkat cells by approximately 25%. Inhibition of ganglioside biosynthesis with N-[5-(adamantan-1-yl-methoxy)-pentyl]1-deoxynojirimycin reduces cell surface labeling by approximately 75%. In conclusion, exposure of cells to Ac(4)ManNAz allows in vivo chemical tagging of gangliosides.

Enzyme enhancement activity of N-octyl-β-valienamine on β-glucosidase mutants associated with Gaucher disease

Gaucher disease (GD), caused by a defect of beta-glucosidase (beta-Glu), is the most common form of sphingolipidosis. We have previously shown that a carbohydrate mimic N-octyl-beta-valienamine (NOV), an inhibitor of beta-Glu, could increase the protein level and enzyme activity of F213I mutant beta-Glu in cultured GD fibroblasts, suggesting that NOV acted as a pharmacological chaperone to accelerate transport and maturation of this mutant enzyme. In the current study, NOV effects were evaluated in GD fibroblasts with various beta-Glu mutations and in COS cells transiently expressing recombinant mutant proteins. In addition to F213I, NOV was effective on N188S, G202R and N370S mutant forms of beta-Glu, whereas it was ineffective on G193W, D409H and L444P mutants. When expressed in COS cells, the mutant proteins as well as the wild-type protein were localized predominantly in the endoplasmic reticulum and were sensitive to Endo-H treatment. NOV did not alter this localization or Endo-H sensitivity, suggesting that it acted in the endoplasmic reticulum. Profiling of N-alkyl-beta-valienamines with various lengths of the acyl chain showed that N-dodecyl-beta-valienamine was as effective as NOV. These results suggest a potential therapeutic value of NOV and related compounds for GD with a broad range of beta-Glu mutations.

HPLC for Simultaneous Quantification of Total Ceramide, Glucosylceramide, and Ceramide Trihexoside Concentrations in Plasma

Background: Simple, reproducible assays are needed for the quantification of sphingolipids, ceramide (Cer), and sphingoid bases. We developed an HPLC method for simultaneous quantification of total plasma concentrations of Cer, glucosylceramide (GlcCer), and ceramide trihexoside (CTH).

Methods: After addition of sphinganine as internal calibrator, we extracted lipids from 50 μL plasma. We deacylated Cer and glycosphingolipids by use of microwave-assisted hydrolysis in methanolic NaOH, followed by derivatization of the liberated amino-group with o-phthaldialdehyde. We separated the derivatized sphingoid bases and lysoglycosphingolipids by HPLC on a C18 reversed-phase column with a methanol/water mobile phase (88:12, vol/vol) and quantified them by use of a fluorescence detector at λex 340 nm and λem 435 nm.

Results: Optimal conditions in the Solids/Moisture System SAM-155 microwave oven (CEM Corp.) for the complete deacylation of Cer and neutral glycosphingolipids without decomposition were 60 min at 85% power, fan setting 7. Intra- and interassay CVs were <4% and <14%, respectively, and recovery rates were 87%–113%. The limit of quantification was 2 pmol (0.1 pmol on column), and the method was linear over the interval of 2–200 μL plasma. In samples from 40 healthy individuals, mean (SD) concentrations were 9.0 (2.3) μmol/L for Cer, 6.3 (1.9) μmol/L for GlcCer, and 1.7 (0.5) μmol/L for CTH. Plasma concentrations of GlcCer were higher in Gaucher disease patient samples and of CTH in Fabry disease patient samples.

Conclusions: HPLC enables quantification of total Cer, GlcCer, and CTH in plasma and is useful for the follow-up of patients on therapy for Gaucher or Fabry disease.

Clinical features and molecular genetics of autosomal recessive cerebellar ataxias

Among the hereditary ataxias, autosomal recessive spinocerebellar ataxias comprise a diverse group of neurodegenerative disorders. Clinical phenotypes vary from predominantly cerebellar syndromes to sensorimotor neuropathy, ophthalmological disturbances, involuntary movements, seizures, cognitive dysfunction, skeletal anomalies, and cutaneous disorders, among others. Molecular pathogenesis also ranges from disorders of mitochondrial or cellular metabolism to impairments of DNA repair or RNA processing functions. Diagnosis can be improved by a systematic approach to the categorisation of these disorders, which is used to direct further, more specific, biochemical and genetic investigations. In this Review, we discuss the clinical characteristics and molecular genetics of the more common autosomal recessive ataxias and provide a framework for assessment and differential diagnosis of patients with these disorders.

Identification of the Non-lysosomal Glucosylceramidase as β-Glucosidase 2

The primary catabolic pathway for glucosylceramide is catalyzed by the lysosomal enzyme glucocerebrosidase that is defective in Gaucher disease patients. A distinct non-lysosomal glucosylceramidase has been described but its identity remained enigmatic for years. We here report that the non-lysosomal glucosylceramidase is identical to the earlier described bile acid beta-glucosidase, being beta-glucosidase 2 (GBA2). Expressed GBA2 is identical to the native non-lysosomal glucosylceramidase in various enzymatic features such as substrate specificity and inhibitor sensitivity. Expression of GBA2 coincides with increased non-lysosomal glucosylceramidase activity, and GBA2-targeted RNA interference reduces endogenous non-lysosomal glucosylceramidase activity in cells. GBA2 is found to be located at or close to the cell surface, and its activity is linked to sphingomyelin generation. Hydrophobic deoxynojirimycins are extremely potent inhibitors for GBA2. In mice pharmacological inhibition of GBA2 activity is associated with impaired spermatogenesis, a phenomenon also very recently reported for GBA2 knock-out mice (Yildiz, Y., Matern, H., Thompson, B., Allegood, J. C., Warren, R. L., Ramirez, D. M., Hammer, R. E., Hamra, F. K., Matern, S., and Russell, D. W. (2006) J. Clin. Invest. 116, 2985-2994). In conclusion, GBA2 plays a role in cellular glucosylceramide metabolism.

Selective action of the iminosugar isofagomine, a pharmacological chaperone for mutant forms of acid-beta-glucosidase

Gaucher disease is a lysosomal glycolipid storage disorder characterized by defects in acid-beta-glucosidase (GlcCerase), the enzyme responsible for the catabolism of glucosylceramide. We recently demonstrated that isofagomine (IFG), an iminosugar that binds to the active site of GlcCerase, enhances the folding, transport and activity of the N370S mutant form of GlcCerase. In this study we compared the effects of IFG on a number of other glucosidases and glucosyltransferases. We report that IFG has little or no inhibitory activity towards intestinal disaccharidase enzymes, ER alpha-glucosidase II or glucosylceramide synthase at concentrations previously shown to enhance N370S GlcCerase folding and trafficking in Gaucher fibroblasts. Furthermore, treatment of wild type fibroblasts with high doses of IFG did not alter the processing of newly synthesized N-linked oligosaccharides. These findings support further evaluation of IFG as a potential therapeutic agent in the treatment of some forms of Gaucher disease.

The iminosugar isofagomine increases the activity of N370S mutant acid beta-glucosidase in Gaucher fibroblasts by several mechanisms

Gaucher disease is a lysosomal storage disorder caused by deficiency in lysosomal acid beta-glucosidase (GlcCerase), the enzyme responsible for the catabolism of glucosylceramide. One of the most prevalent disease-causing mutations, N370S, results in an enzyme with lower catalytic activity and impaired exit from the endoplasmic reticulum. Here, we report that the iminosugar isofagomine (IFG), an active-site inhibitor, increases GlcCerase activity 3.0 +/- 0.6-fold in N370S fibroblasts by several mechanisms. A major effect of IFG is to facilitate the folding and transport of newly synthesized GlcCerase in the endoplasmic reticulum, thereby increasing the lysosomal pool of the enzyme. In addition, N370S GlcCerase synthesized in the presence of IFG exhibits a shift in pH optimum from 6.4 to 5.2 and altered sensitivity to SDS. Although IFG fully inhibits GlcCerase in the lysosome in an in situ assay, washout of the drug leads to partial recovery of GlcCerase activity within 4 h and full recovery by 24 h. These findings provide support for the possible use of active-site inhibitors in the treatment of some forms of Gaucher disease.