Treating lysosomal storage disorders: current practice and future prospects

Substrate reduction augments the efficacy of enzyme therapy in a mouse model of Fabry disease

Fabry disease is an X-linked glycosphingolipid storage disorder caused by a deficiency in the activity of the lysosomal hydrolase α-galactosidase A (α-gal). This deficiency results in accumulation of the glycosphingolipid globotriaosylceramide (GL-3) in lysosomes. Endothelial cell storage of GL-3 frequently leads to kidney dysfunction, cardiac and cerebrovascular disease. The current treatment for Fabry disease is through infusions of recombinant α-gal (enzyme-replacement therapy; ERT). Although ERT can markedly reduce the lysosomal burden of GL-3 in endothelial cells, variability is seen in the clearance from several other cell types. This suggests that alternative and adjuvant therapies may be desirable. Use of glucosylceramide synthase inhibitors to abate the biosynthesis of glycosphingolipids (substrate reduction therapy, SRT) has been shown to be effective at reducing substrate levels in the related glycosphingolipidosis, Gaucher disease. Here, we show that such an inhibitor (eliglustat tartrate, Genz-112638) was effective at lowering GL-3 accumulation in a mouse model of Fabry disease. Relative efficacy of SRT and ERT at reducing GL-3 levels in Fabry mouse tissues differed with SRT being more effective in the kidney, and ERT more efficacious in the heart and liver. Combination therapy with ERT and SRT provided the most complete clearance of GL-3 from all the tissues. Furthermore, treatment normalized urine volume and uromodulin levels and significantly delayed the loss of a nociceptive response. The differential efficacies of SRT and ERT in the different tissues indicate that the combination approach is both additive and complementary suggesting the possibility of an improved therapeutic paradigm in the management of Fabry disease.

Reference values for lysosomal enzymes activities using dried blood spots samples - a Brazilian experience

Background

Lysosomal storage diseases (LSD) are inherited disorders caused by deficiency of lysosomal enzymes in which early diagnosis is essential to provide timely treatment. This study reports interval values for the activity of lysosomal enzymes that are deficient in Mucopolysaccharidosis type I, Fabry, Gaucher and Pompe disease, using dried blood spots on filter paper (DBS) samples in a Brazilian population.

Results

Reference activity values were obtained from healthy volunteers samples for alpha-galactosidase A (4.57 ± 1.37 umol/L/h), beta-glucosidase (3.06 ± 0.99 umol/L/h), alpha-glucosidase (ratio: 13.19 ± 4.26; % inhibition: 70.66 ± 7.60), alpha-iduronidase (3.45 ± 1.21 umol/L/h) and beta-galactosidase (14.09 ± 4.36 umol/L/h).

Conclusion

Reference values of five lysosomal enzymes were determined for a Brazilian population sample. However, as our results differ from other laboratories, it highlights the importance of establishing specific reference values for each center.

Lessons learnt from animal models: pathophysiology of neuropathic lysosomal storage disorders

Approximately 50 inborn errors of metabolism known as lysosomal storage disorders have been discovered to date, most of which are due to a single mutation in a gene encoding a soluble lysosomal enzyme. Consequently, inadequate enzyme activity results in the accumulation of substrates for that enzyme, invariably accompanied by a wide variety of secondary pathological changes. Many of these conditions remain untreatable, and therefore, research into pathogenic processes and potential treatment strategies is intense. A key tool for researchers in this area is the availability of clinically relevant animal models in which to study disease manifestation and evaluate therapeutic outcomes. Large numbers of both naturally occurring and genetically modified animal models of neurodegenerative lysosomal storage disorders are in existence, with spontaneous models occurring in both large domestic (e.g., cat, dog, sheep) and small (e.g., mouse) animal species. Many have undergone rigorous phenotypic characterization and are now providing us with insights into neurological disease processes. The purpose of this review is to highlight some of the major lessons learnt from these studies.

Pathophysiology of neuropathic lysosomal storage disorders

Although neurodegenerative diseases are most prevalent in the elderly, in rare cases, they can also affect children. Lysosomal storage diseases (LSDs) are a group of inherited metabolic neurodegenerative disorders due to deficiency of a specific protein integral to lysosomal function, such as enzymes or lysosomal components, or to errors in enzyme trafficking/targeting and defective function of nonenzymatic lysosomal proteins, all preventing the complete degradation and recycling of macromolecules. This primary metabolic event determines a cascade of secondary events, inducing LSD's pathology. The accumulation of intermediate degradation affects the function of lysosomes and other cellular organelles. Accumulation begins in infancy and progressively worsens, often affecting several organs, including the central nervous system (CNS). Affected neurons may die through apoptosis or necrosis, although neuronal loss usually does not occur before advanced stages of the disease. CNS pathology causes mental retardation, progressive neurodegeneration, and premature death. Many of these features are also found in adult neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases. However, the nature of the secondary events and their exact contribution to mental retardation and dementia remains largely unknown. Recently, lysosomal involvement in the pathogenesis of these disorders has been described. Improved knowledge of secondary events may have impact on diagnosis, staging, and follow-up of affected children. Importantly, new insights may provide indications about possible disease reversal upon treatment. A discussion about the CNS pathophysiology involvement in LSDs is the aim of this review. The lysosomal involvement in adult neurodegenerative diseases will also be briefly described.

Mannose phosphorylation in health and disease

Lysosomal hydrolases catalyze the degradation of a variety of macromolecules including proteins, carbohydrates, nucleic acids and lipids. The biogenesis of lysosomes or lysosome-related organelles requires a continuous substitution of soluble acid hydrolases and lysosomal membrane proteins. The targeting of lysosomal hydrolases depends on mannose 6-phosphate residues (M6P) that are recognized by specific receptors mediating their transport to an endosomal/prelysosomal compartment. The key role in the formation of M6P residues plays the GlcNAc-1-phosphotransferase localized in the Golgi apparatus. Two genes have been identified recently encoding the type III alpha/beta-subunit precursor membrane protein and the soluble gamma-subunit of GlcNAc-1-phosphotransferase. Mutations in these genes result in two severe diseases, mucolipidosis type II (MLII) and III (MLIII), biochemically characterized by the missorting of multiple lysosomal hydrolases due to impaired formation of the M6P recognition marker, and general lysosomal dysfunction. This review gives an update on structural properties, localization and functions of the GlcNAc-1-phosphotransferase subunits and improvements of pre- and postnatal diagnosis of ML patients. Further, the generation of recombinant single-chain antibody fragments against M6P residues and of new mouse models of MLII and MLIII will have considerable impact to provide deeper insight into the cell biology of lysosomal dysfunctions and the pathomechanisms underlying these lysosomal disorders.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Substrate-reduction therapy with miglustat for glycosphingolipid storage disorders affecting the brain

A number of lysosomal storage disorders (LSDs) are characterized by glycosphingolipid (GSL) storage in the brain. Although enzyme-replacement therapy is an effective treatment for the visceral manifestations of some of these disorders, this approach has not, to date, been useful in CNS disease. Substrate reduction therapy (SRT) is an alternative approach to treatment in which the aim is to reduce the rate of synthesis of GSL to a level where the residual enzyme activity in the affected cell can prevent lysosomal storage. Miglustat, an iminosugar, is an inhibitor of the first step of GSL synthesis and has been used successfully for SRT in Gaucher disease. Miglustat is a small molecule and can enter the brain. It has been shown to delay symptom onset and prolong life in a number of animal models of GSL LSDs. This review describes the current progress in the clinical development of SRT with miglustat for these disorders.