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:
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.