Lysosomal Storage Diseases Gregory M. Pastores Abstract The lysosomal storage diseases (LSD) are a heterogeneous group of disorders, characterized by the progressive accumulation of various substrates in multiple cell types, as a consequence of defects in the degradation of by-products of cellular turnover. Several subtypes are associated with neurodegenerative features, which present as a major therapeutic challenge. Although the causal gene defects and corresponding enzyme, cofactor, or transport deficiency have been delineated, there r emains incomplete understanding of the downstream pathways leading to organ dysfunction and clinical symptomatology. Recent studies suggest that sev- eral processes, including inflammation, apoptosis, and defects of autophagy, may be involved. Therapy remains palliative for most LSDs, although enzyme replacement therapy is available for several disorders that are caused by a deficiency in a solu- ble hydrolase. Novel strategies, which involve the use of small molecular agents that inhibit substrate synthesis or act as pharmacological chaperones to rescue the mutant protein, are current subjects of investigation. In addition, gene therapy and stem cell therapy are being evaluated. The multifactorial basis of LSDs will likely necessitate a combination of approaches to optimize therapeutic outcome. Meanwhile, preim- plantation genetic diagnosis and prenatal detection are being offered as an option to families at risk. Newborn screening and carrier detection in populations at risk is also being undertaken, to enable early diagnosis, appropriate counseling, and timely intervention. Keywords Enzyme replacement therapy · Enzyme deficiency · Lysosomal storage disease · Substrate reduction therapy Contents 1 Introduction 786 2 Modes of Clinical Presentation 786 G.M. Pastores (B) Neurology and Pediatrics, New York University School of Medicine, New York, NY 10016, USA; Neurogenetics Laboratory, New York University School of Medicine, New York, NY 10016, USA e-mail: gregory.pastores@med.nyu.edu 785 J.P. Blass (ed.), Neurochemical Mechanisms in Disease, Advances in Neurobiology 1, DOI 10.1007/978-1-4419-7104-3_23, C Springer Science+Business Media, LLC 2011 786 G.M. Pastores 3 Diagnostic Confirmation 791 4 Pathophysiological Mechanisms 792 5 Therapeutic Approaches 793 6 Summary 795 References 796 1 Introduction The lysosomal storage diseases (LSD) are a heterogeneous group of disorders result- ing from an inherited defect in the metabolism of by-products of cellular turnover (Reuser and Drost, 2006). As a consequence, there is progressive accumulation of one or more substrates within the lysosome, eventually leading to multiple organ/system dysfunction. Although individual disorders may be rarely encoun- tered, collectively 1 in 5000 children will be found to have an LSD, caused primarily by a deficiency of a lysosomal hydrolase or its cofactor. Given the frequent delays in diagnosis, and the introduction of therapies for certain subtypes, several groups have advocated for screening programs of newborns or high-risk populations (i.e., based on ethnic group or defined clinical groups). There are at least 50 distinct LSDs, grouped according to the biochemical com- position of the storage material into the sphingolipidoses, mucopolysaccharidoses (MPS), oligosaccharidoses, and so on. Several LSDs have also been given an epony- mous designation (e.g., Gaucher disease, Fabry disease, Tay–Sachs) in recognition of the physician/scientist who played a role in their seminal description (Table 1). The majority of LSDs are inherited in an autosomal recessive fashion, except for three disorders: Fabry disease, Dannon disease, and Hunter syndrome (MPS type II). The diagnosis in suspected LSD cases can be confirmed by biochemical and/or molecular assays, which can be applied for prenatal and presymptomatic diagnosis (Meikle et al., 2004). Although most LSDs have onset in childhood, several subtypes have later-onset of disease, with symptoms that may not be evident until adulthood. The latter individuals often suffer from delayed diagnosis, unless there is a prior family history. However, there can be heterogeneity in clinical expression, and even siblings can have a distinct clinical course of disease, particularly among those with a chronic variant. Except for null alleles, which are often associated with the “classic phenotype,” studies of the correlation between genotype and phenotype suggest a role for factors that modifies disease expression. 2 Modes of Clinical Presentation A significant proportion of patients with an LSD have neurological involvement, which can be manifested as developmental delay and behavioral changes (Table 2). The presence of specific findings may suggest the diagnosis (e.g., leucodystrophy in patients with metachromatic leucodystrophy or globoid cell leucodystrophy; cherry Lysosomal Storage Diseases 787 Table 1 The lysosomal storage disorders classified according to relevant substrate involved Stored Substrate Disease Enzyme Deficiency Gene Locus A. Sphingolipids GM 2 -gangliosides, glycolipids, globoside oligosaccharides GM 1 -gangliosides, oligosaccharides, keratan sulfate, glycolipids Sulphatides GM 1 -gangliosides, sphingomyelin, glycolipids, sulphatide Galactosylceramides α-galactosyl-sphingolipids, oligosaccharides Glucosylceramide, globosides Glucosylceramide, globosides Ceramide Sphingomyelin B.Mucopolysaccharidoses (glycosaminoglycans) Dermatan sulfate (DS) and Heparan sulfate (HS) Tay–Sachs GM 2 gangliosidosis (three types) Sandhoff disease GM 2 gangliosidosis GM 2 gangliosidosis, AB variant GM 1 gangliosidosis (three types) Metachromatic leukodystrophy (MLD) MLD variant Krabbe disease Fabry disease Gaucher disease (GD) (three types) GD (variant) Farber disease (seven types) Niemann–Pick disease types AandB MPS I, Hurler, Scheie MPS II, Hunter α-subunit β-hexosaminidase β-subunit β-hexosaminidase G M2 activator β-galactosidase Arylsulphatase A (galactose-3-sulphatase) Saposin B activator Galactocerebrosidase α-galactosidase A β-glucosidase Saposin C Acid ceramidase Sphingomyelinase α-L-iduronidase Iduronate-2-sulphatase 15q23-4 5q13 5q32-33 3p21-3pter 22q13.31-qter 10q21 14q31 Xq22 1q2l 10q21 8p22-21.2 11p15.1-15.4 4p16.3 Xq27.3-28 788 G.M. Pastores Table 1 (continued) Stored Substrate Disease Enzyme Deficiency Gene Locus HS Keratan sulfate (KS) DS DS and HS Hyaluronan C. Glycogen Glycogen Glycogen D. Oligosaccharides/ Glycopeptides α-mannoside β-mannoside α-−fucosides, glycolipids α-N-acetylgalactosaminide sialyloligosaccharides aspartylglucosamine E. Multiple Enzyme deficiencies Glycolipids, oligosaccharides MPS IIIA, Sanfilippo A MPS IIIB, Sanfilippo B MPS IIIC, Sanfilippo C MPS IIID, Sanfilippo D MPS IVA, Morquio A MPS IVB, Morquio B MPS VI, Maroteaux–Lamy MPS VII, Sly MPS IX Natowicz Pompe, GSD IIA Danon disease α-mannosidosis β-mannosidosis α-fucosidosis Schindler/Kanzaki disease Sialidosis Aspartylglucosaminuria Mucolipidosis II (I-cell disease); mucolipidosis III (pseudo-Hurler polydystrophy) – three complementation groups Sulfamidase α-N-acetylglucosaminidase Acetyl CoA:α-glucosaminide-N- acetyltransferase N-acetylglucosamine-6-sulfatase Galactosamine-6-sulphatase β-D-galactosidase N-acetylgalactosamine-4-sulfatase β-D-glucuronidase Hyaluronidase α-D glucosidase Lysosomal associated membrane protein-2 (LAMP-2) α-mannosidase β-mannosidase α-fucosidase α-N-acetylgalactosaminidase α-neuraminidase Aspartylglucosaminidase N-acetylglucosamine-1- phosphotransferase 17q25.3 17q21.1 1412q14 16q24.3 3p21.33 5q13-14 7q21.1-22 3p21.3 17q25 Xq24 19p13.2-q12 4q22-25 1p34.1-36.1 22q13.1-13.2 6p21.3 4q34-35 4q21-q23; ML-III subtype C (gamma subunit mutations-16p) Lysosomal Storage Diseases 789 Table 1 (continued) Stored Substrate Disease Enzyme Deficiency Gene Locus Sulphatides, glycolipids, glycosaminoglycans F. Lipids Cholesterol esters Cholesterol, sphingomyelin, GM 2 -gangliosides G. Monosaccraides/amino acid/monomers Sialic acid, glucuronic acid cystine H. Peptides Bone proteins S-acylated proteins Palmitoylated proteins Pepstatin-insensitive lysosomal peptidase Cathepsin D Galactosialidosis (protective protein/cathepsin A) Multiple sulfatases (Austin disease) Wolman disease, CESD (cholesterol ester storage disease) Niemann–Pick disease type C Salla, ISSD Cystinosis Pycnodysostosis Infantile neuronal ceroid lipofuscinosis (NCL) Late-infantile NCL Congenital NCL protective protein/cathepsin A SUMF-1 Acid lipase NPC1; HE1 Sialin Cystinosis Cathepsin K Palmitoyl-protein thioesterase Pepstatin-insensitive lysosomal peptidase Lysosomal cysteine protease 20 3p26 10q23.2-q23.3 18q11-12; 14q24.3 6q14-15 17p13 1q21 1p32 11p15.5 11p15.5 790 G.M. Pastores Table 2 Neurological features encountered in patients with an LSD Cherry-red spot ∗ , Optic atrophy, Visual loss • Galactosialidosis • G M1 -gangliosidosis • Infantile free sialic acid storage disease (ISSD) • Mucolipidosis II (I-cell disease) • Mucopolysaccharidosis types IV (MPS IV) and VII (MPS VII) • Neuronal ceroid lipofuscinosis • Niemann-Pick disease type A • Sialidosis type 1 • Sandhoff disease • Tay–Sachs disease Retinitis pigmentosa • Neuronal ceroid lipofuscinosis Corneal opacities (clouding) • I-cell disease (ML II) • Mucolipidosis IV (MLIV) • MPS I, IV, VI • Oligosaccharidosis (late-onset α-mannosidosis) • Fabry disease Lenticular opacities (cataracts) • Oligosaccharidosis (sialidosis, α-mannosidosis) • Fabry disease Ophthalmoplegia (Abnormal eye movements), Nystagmus • Gaucher disease 3 • Niemann–Pick C Leukosdystrophy • Krabbe disease • MLD • Fabry disease ∗ Myoclonic seizures • Galactosialidosis • Gaucher disease III • G M2 -gangliosidosis • Neuronal ceroid lipofuscinosis • Niemann-Pick C • Oligosaccharidosis (α-N-acetylgalactosaminidase deficiency, fucosidosis, Sialidosis type 1) Deafness • Fabry disease • Galactosialidosis • Gaucher disease type 2 • I-cell disease • MPS I, II, IV • Oligosaccharidosis (α-and β-mannosidosis) • Metachromatic leukodystrophy • Infantile Pompe disease Macrocephaly • Tay–Sachs disease • Sandhoff disease • Krabbe disease Peripheral neuropathy • Krabbe disease • MLD (spastic paraplegia) • Multiple sulfatase deficiency Cortical atrophy • Late stage of G M1 -and G M2 -gangliosidosis (cerebellar atrophy) • MLD • I-cell disease • Neuronal ceroid lipofuscinosis Cerebrovascular or strokelike episodes and other vascular events (e.g., Raynaud’s phenomenon) • Fabry disease Ataxia • Galactosialidosis • Gaucher disease III • G M1 -gangliosidosis • Late-onset G M2 -gangliosidosis (cerebellar hypoplasia) • Krabbe disease • MLIV • MLD • Neuronal ceroid lipofucinosis • Niemann-Pick C • Salla disease • Sialidosis I Extrapyramidal signs • Gaucher disease 3 • G M1 -gangliosidosis (adult form) • Late-onset G M2 -gangliosidosis • Krabbe disease • Niemann–Pick C • Oligosaccharidosis Dementia, Psychosis • Fabry disease • Gaucher disease 3 • G M1 -gangliosidosis • Late-onset G M2 -gangliosidosis • Krabbe disease • MLD • MPS III (Sanfilippo disease) • Neuronal ceroid lipofuscinosis • Niemann–Pick C Lysosomal Storage Diseases 791 red spot in Tay–Sachs disease, G M1 -gangliosidosis, Niemann–Pick type A and Sialidosis; ophthalmoplegia in Gaucher disease type 3 and Niemann–Pick disease type C). Extraneurological features that should lead to consideration of an LSD diagnosis include hepatosplenomegaly, short stature, joint contractures, and cardiomyopathy. An early age of symptom onset often portends a r apidly progressive course, although each LSD subtype is associated with chronic subtypes, with a clinical course that can run into decades. In general, null alleles are associated with the classic early-onset phenotype, whereas missense mutations which lead to defective proteins that exhibit residual enzyme activity lead to attenuated phenotypes (Froissart et al., 2002). However, studies of genotype–phenotype correlation have revealed a lack of perfect concor- dance, which suggests other factors may be involved that influence disease outcome (Froissart et al., 2002). At present, the putative factors that modify LSD-phenotypes among patients with identical genotypes remain obscure. 3 Diagnostic Confirmation Diagnosis of an LSD is critical for several reasons: (1) it focuses attention on the needs of the patient, and the potential to intervene in subtypes for which treat- ment is available; (2) the inherited basis implies a risk of recurrence during future pregnancies, and as prenatal diagnosis is available for most, families are given the opportunity to plan accordingly; (3) although treatment is available for certain sub- types, early diagnosis is essential as current approaches are unlikely to restore organ function when there is considerable pre-existing pathology at the time of initiation. For disorders characterized by an underlying enzyme deficiency (e.g., Gaucher disease, Fabry disease, Tay–Sachs, Hurler syndrome), assays of enzyme activity in blood and/or tissues is generally available (Meikle et al., 2004). Mutation analysis is also available, particularly for populations in whom the common disease alleles are known (e.g., mutations among Ashkenazi Jews for Gaucher, Tay–Sachs, Niemann– Pick type A, and mucolipidosis type IV; Ostrer, 2001). In other cases, analysis of the gene defect responsible for rare subtypes is available through specialized laboratories. Examination of skin or other tissues (e.g., liver, bone marrow) may sug- gest the presence of lysosomal storage, however, this involves invasive proce- dures and requires expertise in interpretation of the findings (Alroy and Ucci, 2006). Analysis of urine for excess substrates (e.g., glycosaminoglycans in the Mucopolysaccharidoses, globotriaolsylceramide in Fabry disease) may also suggest the presence of an LSD. In any case, all patients suspected to have an LSD should have diagnostic confirmation by means of biochemical and/or molecular genetic analysis. 792 G.M. Pastores 4 Pathophysiological Mechanisms Intralysosomal substrate storage represents the initial insult to cells; by-products of intermediary metabolism (e.g., psychosine in globoid cell leukodytrorphy), a disruption of normal lysosome function, and/or the consequent deficiency in recy- cling of certain substrates are putative disease events (Ballabio and Gieselmann, 2009). In disorders characterized by primary (e.g., Tay–Sachs disease) or secondary (e.g., Niemann–Pick type C) ganglioside storage, neuronal cells develop ectopic dendritogenesis and meganeurite formation (Walkley, 2009). These changes may be associated with a disturbance in neuronal signal transmission and/or the trans- port of trophic factors along the length of the axon; partly accounting for the neurodegenerative features seen in these conditions. There is incomplete understanding of the disease mechanism beyond substrate storage, although several processes (such as inflammation, apoptosis, defects of autophagy and activation of the ER-stress response) may have a contributory role (Ballabio and Gieselmann, 2009). In globoid cell leucodystrophy the accumulation of galactosylsphingosine (psy- chosine) is believed to promote energy depletion, loss of oligodendrocytes, and the induction of gliosis and aberrant inflammation by astrocytes in the central nervous system (CNS) (Suzuki, 1998). Recently, psychosine has also been shown to down- regulate AMP-activated protein kinase (AMPK), the “cellular energy switch” in oligodendrocytes and astrocytes (Giri et al., 2008). In an oligodendrocyte cell line (MO3.13) and primary astrocytes, psychosine accumulation increased the biosyn- thesis of lipids, including cholesterol and free fatty acid. These findings delineate an explicit role for AMPK in psychosine-induced inflammation in astrocytes, without directly affecting the cell death of oligodendrocytes. In the brain obtained from the mucopolysaccharidosis type IIIB mouse model, the accumulating substrate—heparan sulfate oligosaccharides—activated microglial cells by signaling through the Toll-like receptor 4 and the adaptor protein MyD88 (Ausseil et al., 2008). Although intrinsic to the disease, the observed phenomenon was deemed not to be a major determinant of the neurodegenerative process, with a possibly greater role for inflammatory changes in the later stages of the disease (Ausseil et al., 2008). In multiple sulphatase deficiency and mucopolysaccharidosis type IIIA, stud- ies in the respective mouse models suggest defects in autophagy; a lysosomal- dependent catabolic pathway through which long-lived cytosolic proteins and organelles (such as mitochondria) are sequestered by double-membrane vesicles and ultimately degraded after fusion with lysosomes. In affected cells, reduced colocalization of the lysosomal membrane protein LAMP-1 with the autophago- some marker LC3 have been observed; indicative of an impairment of lyso- some/autophagosome fusion (Settembre et al., 2008). Accumulation of autophagic vacuoles in the heart and skeletal muscle are hallmarks of Danon disease (Yang and Vatta, 2007). LAMP2, which is defective in Danon disease, is believed to be normally involved in lysosome/autophagosome fusion, and may have a role Lysosomal Storage Diseases 793 in dynein-based centripetal motility. In Niemann–Pick type C, there is increased expression of Beclin-1 and LC3-II; the Purkinje neuron cell death encountered in this disorder is believed to be dependent on autophagy (Pacheco and Lieberman, 2008.). A disturbance of autophagy has also been found in the mouse model of Pompe disease; which interestingly has been linked to a deficiency in the traffick- ing/processing of recombinant enzyme along the endocytic pathway (Raben et al., 2008). Several endeavors are being directed towards identifying biomarkers that can serve as a surrogate indicator of disease severity, in terms of either overall disease burden or involvement of a particular organ/system in patients with an LSD. In mucopolysaccharidosis type I, the analysis of the levels of oligosaccharides derived from GAGs in cultured fibroblasts (as measured by electrospray ionization tandem mass spectrometry) combined with the residual α-L-iduronidase activity have been shown to distinguish patients with and without CNS involvement (Fuller et al., 2005). The practical application of these techniques in the final assignment of disease subtype remains to be determined, but may be relevant when combined with genotype information in the selection of appropriate therapy for diagnosed patients with mucopolysaccharidosis type I. Meanwhile, ongoing efforts, employ- ing proteomic-based screening tools (such as SELDI-TOF-MS), are anticipated to reveal markers that will help with prediction of disease severity and that may also be useful in monitoring of therapy (Hendriks et al., 2007). Protein profiling provides an opportunity to identify and analyze multiple markers, and enables a systems biology approach to ascertain the impact of the primary deficiency in lysosomal function. It is likely that one or more of these pathological events may promote cellular dysfunction or tissue damage in the LSDs (Table 3). At present, it is uncertain which of the processes that have been identified plays a dominant role. Certain mechanisms may also be cell-type-specific, but this remains to be clarified. Table 3 Putative mechanisms of disease • Altered trafficking of molecules through the endolysosomal network, including sequestration of membrane rafts, leading to a disruption in signaling • Aberrant inflammatory response, either through activation of resident microglia and/or recruitment and activation of peripheral monocytes • Oxidative stress and activation of ER-stress response • Disruption of autophagy • Initiation of apoptosis 5 Therapeutic Approaches The management of patients with an LSD is mainly palliative, particularly for sub- types associated with neurological involvement. Commonly encountered problems include seizures, altered sleep–wake cycles, and behavioral problems such as hyper- activity and aggression. Attempts at controlling or modifying these problems may help improve the quality of life of an affected individual and their relatives. 794 G.M. Pastores Observations of metabolic cross-correction provided the rationale for cellular replacement, achieved primarily through allogeneic hematopoietic stem cell or bone transplantation (HSCT) (Prasad and Kurtzberg, 2008). More recently, the use of neu- ral stem cells (NSC) implanted in the brain of patients with late-infantile neuronal ceroid lipofuscinosis has been contemplated (Pierret et al., 2008) but there are no reports as yet of its potential efficacy. Within the central nervous system there must be proper integration of donor cells, and differentiation into appropriate cell types. As specialized cell types within the nervous system elaborate neurotransmitters and are involved with conducting electrical impulses, functional differentiation may be a major hurdle for the neurodegenerative LSDs. Increasingly, donor material is isolated from umbilical cord blood (UCB); these cells are deemed to have greater potential for transdifferentiation into appropriate cell types, and thus may have greater facility for tissue-specific regeneration or repair (Gluckman and Rocha, 2009). In addition, the incidence of graft versus host disease appears to decrease following the use of UCB cells, potentially resulting in decreased morbidity. HSCT has been performed in several disorders associated with primary CNS involvement (e.g., globoid cell l eukodystrophy, Hurler syndrome, α-mannosidosis) (Prasad and Kurtzberg, 2008). The justification has been based on the presence of monocytes in the donor pool, which can traverse the blood–brain barrier (BBB) and differentiate into microglia; serving as the source of functional enzyme. The replacement of endogenous microglia by donor cells is estimated to take at least six to nine months, during which time pathogenic influences may remain; this may explain the potential limitations of HSCT, particularly in cases where the diagnosis is delayed. In globoid cell leukodystrophy, over 80% of infantile cases subjected to HSCT in the first few weeks of life develop gross motor problems after the age two years; often requiring assistance with ambulation (Prasad and Kurtzberg, 2008). Enzyme replacement therapy is available for several subtypes associated with a soluble hydrolase deficiency; this therapeutic approach has been shown to modify disease course, primarily features of the disorder resulting from extraneurological involvement (Grabowski, 2008). Unfortunately, the ultimate prognosis is not sig- nificantly altered in patients with neurodenegerative features, likely because the intravenously administered enzyme does not gain sufficient access across the blood– brain barrier (Pastores, 2003). In addition, therapeutic response is limited in patients with an advanced disease stage, wherein organ function may not be fully restored in cases with significant tissue damage from fibrosis or necrosis. Varying propor- tions of patients given recombinant enzymes have developed antibodies, which can lead to neutralization of enzyme activity and/or altered tissue distribution (Pastores, 2003). The significance of these observations on long-term outcome remains to be established. Substrate reduction therapy (SRT) involves the inhibition of substrate synthesis to a level where the load falls within the capacity of the mutant enzyme that exhibits residual function (Platt and Jeyakumar, 2008). Thus this approach, as in the case of pharmacological chaperones, may be dependent on the type of mutation responsible for disease in an individual. . lipofuscinosis (NCL) Late-infantile NCL Congenital NCL protective protein/cathepsin A SUMF-1 Acid lipase NPC1; HE1 Sialin Cystinosis Cathepsin K Palmitoyl-protein thioesterase Pepstatin-insensitive. (protective protein/cathepsin A) Multiple sulfatases (Austin disease) Wolman disease, CESD (cholesterol ester storage disease) Niemann–Pick disease type C Salla, ISSD Cystinosis Pycnodysostosis Infantile. of lipids, including cholesterol and free fatty acid. These findings delineate an explicit role for AMPK in psychosine-induced in ammation in astrocytes, without directly affecting the cell death