Synthesis and use of novel fluorescent glycosphingolipids for estimating beta-glucosidase activity in vitro in the absence of detergents and subtyping Gaucher disease variants following administration into intact cells

Identification and characterization of pharmacological chaperones to correct enzyme deficiencies in lysosomal storage disorders

Many human diseases result from mutations in specific genes. Once translated, the resulting aberrant proteins may be functionally competent and produced at near-normal levels. However, because of the mutations, the proteins are recognized by the quality control system of the endoplasmic reticulum and are not processed or trafficked correctly, ultimately leading to cellular dysfunction and disease. Pharmacological chaperones (PCs) are small molecules designed to mitigate this problem by selectively binding and stabilizing their target protein, thus reducing premature degradation, facilitating intracellular trafficking, and increasing cellular activity. Partial or complete restoration of normal function by PCs has been shown for numerous types of mutant proteins, including secreted proteins, transcription factors, ion channels, G protein-coupled receptors, and, importantly, lysosomal enzymes. Collectively, lysosomal storage disorders (LSDs) result from genetic mutations in the genes that encode specific lysosomal enzymes, leading to a deficiency in essential enzymatic activity and cellular accumulation of the respective substrate. To date, over 50 different LSDs have been identified, several of which are treated clinically with enzyme replacement therapy or substrate reduction therapy, although insufficiently in some cases. Importantly, a wide range of in vitro assays are now available to measure mutant lysosomal enzyme interaction with and stabilization by PCs, as well as subsequent increases in cellular enzyme levels and function. The application of these assays to the identification and characterization of candidate PCs for mutant lysosomal enzymes will be discussed in this review. In addition, considerations for the successful in vivo use and development of PCs to treat LSDs will be discussed.

Use of lissamine rhodamine ceramide trihexoside as a functional assay for alpha-galactosidase A in intact cells

Fabry disease is an X-linked disorder caused by mutations in the GLA gene encoding for alpha-galactosidase A (AGA, EC 3.2.1.22). Measurement of AGA enzyme activity using cell homogenates can easily identify men with Fabry disease, but in women, the degree of X-inactivation in the tested tissue may produce activities in homogenates that are indistinguishable from normal. Monti et al. developed a series of lissamine rhodamine-labeled glycosphingolipid substrates that can be used to measure clearance of these lipids in intact cells (1). We report here that one of these substrates, lissamine rhodamine ceramide trihexoside (LR-CTH), can be used as a probe for functional activity of AGA in intact fibroblasts, endothelial cells, and T-lymphocytes from patients with Fabry disease. By utilizing standard detection techniques, such as microscopic imaging, fluorescence microplate spectrophotometry, and flow cytometry, cells with impaired AGA activity can easily be distinguished from wild-type (WT) cells, and these two cell types can be isolated into separate populations using fluorescence-activated cell sorting (FACS). The assay we report here can be adapted to evaluate new therapies by high-throughput screening, can aid in the study of AGA activity in living cells, and can assist in the diagnosis of women with the Fabry trait.

Phenotypic heterogeneity of N370S homozygotes with type I Gaucher disease: an analysis of 798 patients from the ICGG Gaucher Registry

Gaucher disease is a lysosomal storage disorder caused by a deficiency of the enzyme acid beta-glucosidase. The most prevalent mutant genotype in type I Gaucher disease, N370S/N370S, is commonly thought to confer a mild phenotype presenting in adulthood. To characterize a subset of more severely affected N370S homozygotes, we assessed the phenotypes at or near the time of diagnosis of all N370S homozygotes with available data enrolled in the International Collaborative Gaucher Group Gaucher Registry. N370S compound heterozygotes were analyzed for comparison, as they are expected to present with a more severe phenotype. Of 798 N370S homozygotes and 1,278 N370S compound heterozygotes identified, 32% (251/788) and 65% (820/1269), respectively, were diagnosed before age 20 years. At diagnosis, N370S homozygotes as compared to N370S compound heterozygotes had the following clinical characteristics: irreversible skeletal lesions 17% (34/198) for N370S homozygotes versus 26% (76/290) for N370S compound heterozygotes; anaemia 18% (59/327) versus 29% (145/494); thrombocytopenia 52% (170/327) versus 62% (281/453); hepatomegaly 44% (83/190) versus 72% (141/195); splenomegaly 73% (142/194) versus 91% (178/195); and osteopenia or osteoporosis 48.6% (34/70) versus 51% (25/49). Some N370S homozygotes exhibited more severe clinical manifestations: 9% (29/327) had severe thrombocytopenia; 3% (5/190) had severe hepatomegaly; 11% (22/194) had severe splenomegaly; 7% (18/255) reported bone crises; 11% (8/70) had osteoporosis. In conclusion, N370S homozygosity does not consistently confer a mild, adult-onset phenotype. Gaucher disease patients with the N370S/N370S genotype exhibit a high degree of phenotypic heterogeneity and some may be at risk for early disease onset and severe clinical manifestations.

Sensitive assay for hormone-sensitive lipase using NBD-labeled monoacylglycerol to detect low activities in rat adipocytes

The recent finding that p-nitrobenzofurazan (NBD)-FA is incorporated into and released from the acylglycerols of isolated rat adipocytes in an insulin-sensitive manner [G. Muller, H. Jordan, C. Jung, H. Kleine, and S. Petry. 2003. Biochimie. 85: 1245-1246] suggests that NBD-FA-labeled acylglycerols are cleaved by rat adipocyte hormone-sensitive lipase (HSL) in vivo. In the present study, we developed a continuous, sensitive in vitro lipase assay using a monoacylglycerol (MAG) containing NBD (NBD-MAG). NBD-MAG was found to provide an efficient substrate for rat adipocyte and human recombinant HSL. Ultrasonic treatment applied in the presence of phospholipids leads to the incorporation of NBD-MAG into the phospholipid liposomes and to a concomitant change of its spectrophotometric properties. The enzymatic release of NBD-FA and its dissociation from the carrier liposomes is accompanied by the recovery of the original spectrophotometric characteristics. The rate of lipolysis was monitored by measuring the increase in optical density at 481 nm, which was found to be linear with time and linearly proportional to the amount of lipase added. To assess the specific activity of recombinant HSL, we determined the molar extinction coefficient of NBD-FA under the assay conditions. This convenient assay procedure based on NBD-MAG should facilitate the search for small molecule HSL inhibitors.

Gaucher disease: in vivo evidence for allele dose leading to neuronopathic and nonneuronopathic phenotypes

Gaucher disease, a common lysosomal storage disorder, is associated with mutations at the acid beta-glucosidase (GCase) locus. Two affected individuals are described to share a common mutant allele, but manifest different clinical categorical phenotypes. A 57-year-old female, with Gaucher disease type 1 and Cherokee ancestry, was homozygous for a rare mutant allele encoding Lys79Asn (K79N). A 2-year-old Caucasian male, with Gaucher disease type 3 and Cherokee ancestry, was a heteroallelic homozygote for this same allele (K79N) and a novel complex mutation (null allele). The shared alleles were identical as determined by complete gene sequencing, suggesting a founder effect. The discrepant phenotypes (types 1 and 3) in these two patients provide support for a threshold of residual activity necessary to "protect" the central nervous system (CNS) from the pathogenic effects of Gaucher disease, indicating an allele dose-effect. Designation of genotype associations with specific phenotypes must be assessed with this perspective.

Gaucher disease: gene frequencies and genotype/phenotype correlations

Gaucher disease is the most prevalent lysosomal storage disease and has its highest incidence in the Ashkenazi Jewish population. Over 100 mutant alleles have been identified in affected patients, but four alleles, termed N370S, L444P, 84GG, and IVS2, have significant frequencies in this population. In affected patients, genotype data show that the presence of a single N370S allele is diagnostic of the type 1 or nonneuronopathic variant, whereas the L444P/L444P genotype is highly associated with neuronopathic variants in the Caucasian population. Large screening studies also indicate a significant underestimation (approximately two-fold) of the prevalence of the N370S/N370S genotype in the affected Ashkenazi Jewish patient population. These results indicate that the N370S/N370S genotype provides a necessary but not sufficient condition for the development of the Gaucher disease phenotype. The genotype/phenotype correlations and gene frequencies have significant impact on genetic counseling of at-risk couples and the future need for therapy of affected patients.

Characterization of glucocerebrosidase in Greek Gaucher disease patients: mutation analysis and biochemical studies

Gaucher disease is the most frequent lysosomal storage disease in Greece, accounting for 24% of all lysosomal disorders diagnosed during the last 13 years at the Institute of Child Health in Athens. The nature of the defects in glucocerebrosidase in Greek Gaucher patients with non-neuronopathic (type 1) and neuronopathic (types 2 and 3) phenotypes was investigated at the level of the glucocerebrosidase gene and enzyme activity. Mutation analysis performed in 10/23 Gaucher patients with different types of the disorder led to the identification of four mutations, N370S, L444P, R463C and D409H, comprising 75% of the investigated alleles. N370S was only found in association with type 1 disease. The genotype D409H/R463C was identified for the first time and was associated with the severe type 2 disorder. There was no correlation between residual in vitro enzyme activity and either phenotype or genotype. However, in cultured fibroblast of the neuronopathic cases, glucocerebrosidase protein concentration was reduced and the capacity to degrade exogenous C6NBD-glucosylceramide was more severely impaired.

Analysis of glucocerebrosidase activity using N-(1-[14C]hexanoyl)-D-erythroglucosylsphingosine demonstrates a correlation between levels of residual enzyme activity and the type of Gaucher disease

Glucosylceramide, a degradation product of complex glycosphingolipids, is hydrolysed in lysosomes by glucocerebrosidase (GlcCerase). Mutations in the human GlcCerase gene cause a reduction in GlcCerase activity and accumulation of glucosylceramide, which results in the onset of Gaucher disease, the most common lysosomal storage disease. Significant clinical heterogeneity is observed in Gaucher disease, with three main types known, but no clear correlation has been reported between the different types and levels of residual GlcCerase activity. We now demonstrate that a correlation exists by using a radioactive, short-acyl chain substrate, N-(1-[14C]hexanoyl)-D-erythro-glucosylsphingosine ([14C]hexanoyl-GlcCer). This substrate rapidly transferred into biological membranes in the absence of detergent [Futerman and Pagano (1991) Biochem. J. 280, 295-302] and was hydrolyzed to N-(1-[14C]hexanoyl)-D-erythro-sphingosine ([14C]hexanoyl-Cer) both in vitro and in situ, with an acid pH optimum. A strict correlation was observed between levels of [14C]hexanoyl-GlcCer hydrolysis and Gaucher type in human skin fibroblasts. The mean residual activity measured in vitro for 3 h incubation in type 1 Gaucher fibroblasts (the mild form of the disease) was 46.3 +/- 4.6 nmol of [14C]hexanoyl-Cer formed per mg protein (n = 9), and in type 2 and 3 fibroblasts (the neuronopathic forms of the disease) was 19.6 +/- 6.5 (n = 9). A similar correlation was observed when activity was measured in situ, suggesting that the clinical severity of a lysosomal storage disease is related to levels of residual enzyme activity.