Metabolic and Toxic Disorders

Clinical Features

Most LSDs cause one of four basic clinical-pathologic phenotypes: neuronal lipidosis, leukodystrophy, storage histiocytosis, and mucopolysaccharidosis or Hurler phenotype. These phenotypes overlap in several LSDs. Some LSDs (Fabry disease, Niemann-Pick type C) exhibit protean manifestations involving many organ systems. Other LSDs (Farber lipogranulomatosis, Wolman disease, Pompe disease) cause distinct clinical and pathologic changes.

The neuronal lipidosis phenotype is caused by lysosomal storage in the neuronal perikaryon. This phenotype is characterized by slowing of psychomotor development and the eventual loss of acquired motor and perceptual skills, dementia, myoclonus, epilepsy, and visual loss, progressing to a vegetative state. The cherry red spot that is seen in some LSDs with this phenotype is caused by lysosomal storage in the perifoveal ganglion cell layer. While being the principal manifestation of GM ₁ and GM ₂ gangliosidosis and Niemann-Pick type A and B, neuronal lipidosis occurs also in Niemann-Pick type C and D, the NCLs, the MPS (in which neurons store gangliosides), and most glycoproteinoses.

The leukodystrophy phenotype, seen mainly in metachromatic leukodystrophy and Krabbe disease, is caused by loss of oligodendrocytes and myelin and is characterized clinically by psychomotor retardation, spasticity, ataxia, visual loss, and demyelinative peripheral neuropathy.

The storage histiocytosis phenotype is caused by lysosomal storage in monocyte-macrophage cells and is characterized by hepatosplenomegaly and, occasionally, also by hematopoietic and skeletal abnormalities. This phenotype is seen primarily in Gaucher disease and Niemann-Pick disease.

The MPS phenotype, seen in the MPS, glycoproteinoses, and GM ₁ gangliosidosis, is due to visceral, soft tissue, and skeletal changes resulting from lysosomal storage of water-soluble glycoconjugates and consists of coarse facial features, organomegaly, and dysostosis multiplex. The most exaggerated form of these changes is the Hurler phenotype (see Mucopolysaccharidoses further on). Many MPS have secondary neuronal lipidosis, which accounts for the central nervous system (CNS) manifestations.

Pathologic Features

Lysosomal storage distends the cytoplasm. This distension is most impressive when it affects large pyramidal cells ( Fig. 8.3 ). Water-soluble materials are washed out during processing, leaving a clear or vacuolated cytoplasm ( Fig. 8.4 ). Glycolipids have a finely granular or foamy appearance and are sudanophilic and periodic acid-Schiff (PAS)-positive. In the sphingolipidoses and other LSDs with neuronal storage, lysosomes accumulate also in axon hillocks and dendrites causing spherical or torpedo-like swellings ( Fig. 8.5 ). Increased cytoplasmic mass causes organomegaly. Thus, megalencephaly occurs early in the gangliosidoses. However, the end result of the storage is cell death followed by cerebral atrophy and gliosis ( Figs. 8.6 and 8.7 ). These changes are the pathologic substrate of the neuronal lipidosis phenotype. Neuronal lipidosis involves autonomic neurons and retinal ganglion cells. Rectal biopsy with detection of storage in enteric ganglia ( Fig. 8.8 ) was frequently used for the diagnosis of sphingolipidoses before biochemical methods were available. Ganglioside storage in the cellular perifoveal ganglion cell layer creates an opaque, yellowish halo that accentuates the red color of the fovea. The fovea appears as a cherry red spot on fundoscopic examination.

In metachromatic leukodystrophy and Krabbe disease, lipid accumulation involves oligodendroglial cells and Schwann cells. When oligodendroglial cells die, the lipid is picked up by macrophages that accumulate in the white matter. Dysfunction and loss of myelin-producing cells causes loss of myelin. These findings are the substrate of the leukodystrophy phenotype. Storage of water-soluble materials in mesenchymal and epithelial cells and in the extracellular matrix correlates with the MPS phenotype, and accumulation in monocyte-macrophage cells is the substrate of the storage histiocytosis phenotype.

The storage material accumulates in membrane-bound (i.e., intralysosomal) particles ( Fig. 8.9 ) that have acid phosphatase activity ( Fig. 8.10 ). Acid phosphatase has been used as a lysosomal marker since the time lysosomes were discovered by De Duve. The fine structure of the stored material gives a general idea of its chemical composition but is not specific. In general, sphingolipids assume membranous forms that are arranged concentrically or in parallel stacks (membranous cytoplasmic bodies [MCBs], zebra bodies) ( Figs. 8.11 and 8.12 ). Concentric lamellae are more common in the gangliosidoses and zebra bodies in the MPS (in which neurons store gangliosides), but none of these structures are specific for any given type of sphingolipidosis, and they are usually found in various combinations in all of them. Mucopolysaccharides (glycosaminoglycans [GAGs]) are highly soluble in water and are lost in processing, leaving behind clear cells by light microscopy and sparse reticulogranular structures by electron microscopy (see Figs. 8.1 and 8.9 ). Other LSDs with storage of water-soluble compounds have a similar appearance.

In some instances, such as Gaucher disease and globoid cell leukodystrophy, morphology is diagnostic. For the most part, however, light and electron microscopy are nonspecific. Thus, at the cellular level, all ganglioside-storing LSDs look the same and the MPS are indistinguishable from one another and from the glycoproteinoses.

Special techniques supplement light and electron microscopy. Loss of GAGs can be prevented to some extent by adding the cationic dye toluidine blue to the fixatives and buffers used for electron microscopy. Metachromatic stains are diagnostic in MLD and helpful in the MPS. Cystine crystals can be identified under polarized light ( Fig. 8.2 ), and ceroid-lipofuscin is autofluorescent ( Fig. 8.13 ). Because in most LSDs the stored material is a glycoconjugate, lectin histochemistry and immuno-electron microscopy can be used to characterize the storage products.

Pathogenesis: How Cells and Tissues are Damaged in the LSD

Cell injury in LSDs is primarily due to the mechanical effects of storage (i.e., displacement of organelles, disruption of cytoplasmic circulation, and, in the case of neurons, impaction of axons and dendrites). This process leads to cellular dysfunction and loss, which is most critical for neurons and oligodendrocytes. Unavailability of ganglioside precursors and other biomaterials due to their sequestration in lysosomes may interfere indirectly with synthetic processes. Impaired recycling and imbalance of membrane lipids in LSDs with ganglioside accumulation cause aberrant dendritic sprouting and abnormal recycling of neurotransmitter receptors (which are embedded in the neuronal membrane). These changes account in part for the seizures, which are common in the neuronal lipidoses, and also adversely affect neurotransmission and other signal transduction activity. Abnormal chemical composition of myelin lipids may play a role in myelin breakdown in the leukodystrophies, but loss of myelin-producing cells is more important.

Several LSDs cause accumulation of materials other than the primary substrate of the mutant enzyme. The prime example of this phenomenon is the MPS in which gangliosides (zebra bodies) accumulate in neurons, but similar changes are seen in several other LSDs. The mechanism of this secondary storage is unclear. One plausible explanation may be that, as lysosomes swell, their proton pumps cannot maintain their acid environment. Lysosomal pH rises, leading to generalized lysosomal exhaustion. The most common secondary storage products are gangliosides, phospholipids, and cholesterol. Secondary storage, especially of gangliosides, is clinically important, and accounts for the diverse fine structure of lysosomal products in some LSDs.

In most LSDs, the stored material is chemically inert. In some gangliosidoses, toxic by-products of the stored gangliosides are also produced and cause additional cell damage. One such example is globoid cell leukodystrophy in which psychosine, a metabolite of galactocerebroside, is toxic to oligodendrocytes. The primary pathology of LSDs is intracellular; however, in many of them, significant changes also develop in the extracellular matrix. Some of these changes represent scarring due to loss of parenchymal elements. In MPS and other LSDs with an MPS phenotype, extracellular accumulation of GAGs and other glycoconjugates causes severe soft tissue changes, which are central to their phenotype.

Biochemical-Phenotypic Correlation

The type and distribution of substrate in various tissues determine what cells and organ systems are affected in the LSDs. Lysosomal activity also varies between tissues and at different stages of development depending on substrate turnover. For instance, acid maltase deficiency affects most severely tissues with high glycogen turnover, such as liver, heart, and skeletal muscle; hexosaminidase A deficiency primarily affects neurons that have the highest GM ₂ content and turnover, especially during the phase of dendritic sprouting; and MPS affect connective tissues and the skeleton, which contain a large number of GAGs. Although LSDs affect all cells and organs, two classes of cells are most vulnerable: (1) neurons because of their large size, high ganglioside content of their membranes, and their inability to regenerate; and (2) phagocytic cells, which are easily overloaded because of their high burden of substrate turnover.

As lysosomes swell in LSDs, their proton pumps cannot maintain the acid environment. Lysosomal pH rises, leading to generalized loss of enzyme activity. This lysosomal exhaustion may also explain the accumulation of diverse products in LSDs (see the section on Mucopolysaccharidoses later in this chapter).

Genotype-Phenotype Correlation

The severity of each LSD depends primarily on residual enzyme activity, which correlates with the genotype. Several mutations of each lysosomal enzyme gene occur. Some of these mutations cause severe enzyme deficiency, while others allow some enzyme to be produced. Because of this allelic diversity and because many patients are compound heterozygotes, no two cases of any given LSD are exactly alike. Normally, cells produce about 10 times the amount of lysosomal enzymes that are needed to carry out cellular function. LSD heterozygotes, with about half normal enzyme activity, are phenotypically normal. In every tissue, there is a threshold of lysosomal enzyme activity, usually 10 % to 20 % of normal, that is required to maintain normal function. Storage develops when enzyme activity falls below this critical level. Some LSDs such as Gaucher disease, show significant clinical variability within the same genotype suggesting that additional genetic and environmental factors contribute to the phenotype.

The molecular defect of LSDs is present from the time of conception. The biochemical defect is expressed in amniocytes, trophoblasts, and fetal tissues, allowing for a prenatal diagnosis in the first trimester by enzyme testing. Typical pathologic changes also develop before the second trimester. Fetal and neonatal tissues have the highest regenerative ability, but by the same token, the period of growth and development challenges the capacity of the lysosomal system with the highest substrate turnover. This is especially true in the developing brain in which large numbers of primitive neurons are eliminated by programmed death. As a consequence, severe enzyme mutations are clinically apparent prenatally or in early childhood. Mutations allowing for higher, (but below threshold) residual activity cause symptoms later in life. For instance, in TSD, severe hexosaminidase deficiency causes diffuse neuronal storage in the first few months of life and is fatal within 1 or 2 years. Mutations resulting in somewhat higher hexosaminidase activity have a later onset, milder symptoms, and a more chronic course, and affect neuronal groups selectively.

Lysosomal enzymes are not substrate-specific. They are specific for certain residues and linkages and break these linkages in whatever molecules they occur. Thus, hexosaminidases cleave β-hexosamine from glycolipids and glycoproteins; enzymes involved in glycoprotein degradation also degrade glycosphingolipids, and GAG-cleaving enzymes participate in the degradation of several classes of GAGs. Deficiency of a single multicatalytic enzyme causes storage of diverse compounds, creating overlapping phenotypes. For example, β-galactosidase deficiency causes GM ₁ gangliosidosis and keratan sulfate storage (MPS IV B-Morquio syndrome type B). Conversely, several enzymes participate in the degradation of the same molecule. Deficiency of any of these enzymes may cause storage of similar compounds, resulting in a shared phenotype. Thus, Sanfilippo syndrome phenotype can be caused by four different gene mutations. The Morquio syndrome phenotype can result from either β-galactosidase deficiency or galactosamine-6-sulfatase deficiency.

Laboratory Diagnosis

The gold standard in the laboratory diagnosis of LSDs is measurement of enzyme activity by tandem mass spectrometry or fluorometry. Because the enzyme deficiency is expressed in all tissues, albeit not equally, a diagnosis can be made by testing samples obtained by relatively noninvasive methods, such as serum, plasma, white cells, amniocytes, CVS samples, and cultured skin fibroblasts. Enzyme assay can also be used to detect carriers. Some LSDs can be diagnosed by assaying a leukocyte pellet. In most instances where multiple enzymes need to be evaluated, cultured fibroblasts or amniocytes must be used.

Urine chromatography is a useful screening test for many LSDs. The pattern of glycoconjugates secreted in the urine can identify several MPS and glycoproteinoses, but is not specific because storage of similar compounds may result from different enzyme defects. Because of the great genotypic diversity that characterizes all LSDs, molecular analysis is impractical for primary diagnosis. However, when the mutation is known, molecular analysis can be used for carrier detection and prenatal diagnosis.

Molecular genetic analysis using next-generation sequencing (NGS) and other methods is used to confirm the results of enzyme assays, determine which mutations are involved, and for prenatal diagnosis.

Morphology, combining light microscopy, electron microscopy, and ancillary studies, is an important tool in the diagnosis and investigation of LSDs. Anatomical studies reliably reveal the presence of a storage process and can narrow it down to a presumptive diagnosis, which is important for guiding biochemical, molecular, and genetic studies. Because small amounts of substrates are asymptomatically stored in many cell types, characteristic pathologic changes can be detected by minimally invasive sampling of a variety of cells and tissues such as amniotic fluid, skin, conjunctiva, bone marrow, and circulating leukocytes ( Fig. 8.14 ). In several LSDs, although enzymatic diagnosis is available, morphology can also provide a specific diagnosis ( Table 8.2 ).

Table 8.2

Lysosomal storage disorders with diagnostic morphology

Disorder	Diagnostic finding
Gaucher disease	Gaucher cells, characteristic fine structure
Globoid cell leukodystrophy	Globoid cells, characteristic fine structure
Metachromatic leukodystrophy	Metachromasia, characteristic fine structure
Farber lipogranulomatosis	Lipid granulomas, characteristic fine structure
Fabry disease	Vascular, renal pathology
Pompe disease	Lysosomal glycogen, storage myopathy
Wolman disease	Adrenal calcification, lipid droplets, cholesterol crystals
Neuronal ceroid lipofuscinoses	Characteristic ultrastructure

In a few LSDs, morphology is the only diagnostic modality. Morphology is the only means of detecting drug-induced LSDs. It helps in understanding the pathogenesis of cell and tissue injury and is indispensable in evaluating the effects of treatment. The pathology of many LSDs is incompletely described and systematic anatomical study can still make significant contributions to our understanding.

Prognosis and Therapy

Hematopoietic stem cell transplantation (HSCT), still used for MPS I (Hurler syndrome), was the earliest attempt to treat LSDs. The most common approach to therapy of LSDs presently is enzyme replacement therapy (ERT) using intravenous or intrathecal infusions of recombinant enzymes. ERT is very expensive and does not reverse the neuropathologic changes because enzymes do not cross the blood-brain barrier. Its effectiveness is reduced because some patients develop neutralizing antibodies to the enzymes. Substrate reduction therapy (SRT) aims to reduce the amount of accumulating substrate by inhibiting its synthesis, thus balancing synthesis and catabolism and avoiding the harmful effects of storage. Chaperone therapy helps mutant enzymes regain their function by correcting their conformation. Gene therapy uses viral vectors to introduce genes into organs or infusions of the patient’s own ex vivo genetically corrected hematopoietic stem cells. ERT alone or in combination with SRT has been used to treat Gaucher disease, Fabry disease, MPS, Pompe disease, and other LSDs.

Based on the chemical structure of the stored material, LSDs can be divided into four major groups, the sphingolipidoses, MPS, glycoproteinoses, ceroid lipofuscinoses, and several other individual entities. An account of the core features of each group and brief descriptions of the most common LSDs follows.

Clinical Features

GM ₁ gangliosidosis presents in infancy with severe psychomotor retardation and a full-blown Hurler phenotype (see Mucopolysaccharidoses later in this chapter) including coarse facial features ( Fig. 8.15 ), dysostosis multiplex, corneal clouding, and organomegaly. A cherry red spot is seen in 50 % of patients. Patients with infantile onset usually die before 2 years of age. Later onset variants have a milder neurological picture (which includes dystonia and ataxia) and milder somatic and skeletal findings. An allelic variant called Morquio syndrome type B is phenotypically similar to Morquio A (MPS IVA), consisting primarily of skeletal changes (short trunk dwarfism) with normal intelligence and without organomegaly or corneal clouding. The neurological symptoms in Morquio type B develop not as a result of CNS storage but rather as a consequence of atlantoaxial dislocation.

Clinical Features

TSD is the prototype of LSDs and was the first LSD to be described. It is caused by an α subunit (and Hex A) deficiency. Patients with classic infantile TSD present in the first year of life and die by 5 years of age. They have profound psychomotor retardation, myoclonus, hypotonia, and cherry red spots. Variants caused by milder mutations begin later and have a subacute or chronic course characterized by seizures, ataxia, spasticity, choreoathetosis, and motor neuron disease with little or no cognitive dysfunction.

Clinical Features

Gaucher disease (GD) is an LSD characterized by storage of glucocerebroside (glucosylceramide) in monocyte-macrophage cells due to deficiency of glucocerebrosidase (glucosylceramidase). With an estimated birth frequency of 1:50,000, it is the most common LSD in Caucasians. GD is caused by mutations in the GBA1 gene, located on 1q21. Four mutations account for the vast majority of all forms of GD, although more than 300 mutations have been described. The phenotype is predictable in some mutations. In others, there is a marked phenotypic variability, even among monozygotic twins with GD. In addition to lysosomal deficiency, accumulation of misfolded mutant glucocerebrosidase in the endoplasmic reticulum is thought to have damaging effects that contribute to the pathology.

In addition to having GD, persons with pathogenic GBA1 mutations, including carriers, also have a 20- to 30-fold risk for developing PD, and 70 %–10 % of PD patients have GBA1 mutations. The clinical and pathologic phenotype of PD in GD is similar to idiopathic PD except that it has an earlier onset and causes more cognitive impairment. The mechanism by which GD causes PD is unknown, but it is thought to be a combination of lysosomal impairment, mitochondrial dysfunction, and oxidative and endoplasmic reticulum stress induced by GD. These changes impair α-synuclein degradation, leading to its accumulation.

Three clinical phenotypes are recognized. The most common by far is type 1, the prevalence of which in Ashkenazi Jews is 1:855 and carrier frequency 1:18. Type 1 GD presents from childhood to early adulthood and is characterized by hepatosplenomegaly with occasional splenic infarcts, bone disease (osteopenia, focal lytic or sclerotic lesions, osteonecrosis, pathologic fractures, chronic bone pain), anemia and thrombocytopenia due to hypersplenism, coagulation abnormalities, pulmonary interstitial infiltrates, and other manifestations. Type 1 GD patients may develop neurological manifestations from spinal cord or root compression secondary to bone disease but have no primary CNS involvement by GD.

In type 2 (acute neuronopathic) GD, neurological manifestations (stridor, strabismus and other oculomotor abnormalities, swallowing difficulty, opisthotonus, spasticity) appear before age 2 years and progress rapidly leading to death by 2 to 4 years of age. Some patients present at birth with nonimmune fetal hydrops and ichthyosiform or collodion skin changes. Type 2 GD patients also have hepatosplenomegaly similar to type 1. There is no special ethnic prevalence for type 2 GD. Type 3 (subacute neuronopathic) GD is frequent in Northern Sweden and has similar clinical manifestations to type 2 but is more slowly progressive such that patients may survive into their 20s and 30s. Type 2 and type 3 GD are not distinct entities but rather a spectrum.

Pathologic Features

The pathology of GD consists of lysosomal storage of glucocerebroside in cells of the monocyte-macrophage system. This storage leads to a characteristic cellular alteration of these cells–the Gaucher cells (GCs). GCs have a large cytoplasmic mass with a striated appearance that has been likened to wrinkled tissue paper or crumpled silk ( Fig. 8.17 ). This change is caused by storage of twisted bundles of tubules ( Fig. 8.18 ) in large, irregular, branching lysosomal compartments. These inclusions are unique and diagnostic for GD. Biochemically, they consist of glucocerebroside, the main sources of which are thought to be membranes of blood cells phagocytosed by monocytes-macrophages. A similar cellular alteration (pseudo-GC) is sometimes seen in the marrow of patients with chronic myelogenous leukemia.

GCs are present in the bone marrow, spleen, lymph nodes ( Fig. 8.18 ), hepatic sinusoids, and other organs and tissues in all forms of GD but account for a small fraction of the mass of the liver and spleen in GD. Cytokines released by GCs induce an inflammatory response that plays a significant role in the organomegaly and systemic manifestations of GD. An increased incidence of cancer including lymphoma, myeloma, and bone tumors has been reported in GD patients. Rare GCs without other pathology also occur in the leptomeninges and Virchow-Robin spaces in type 1 GD.

Types 2 and 3 GD involve the CNS. There are numerous GCs in perivascular spaces and rare GCs in brain parenchyma ( Fig. 8.19 ). No part of the CNS is spared, but the brainstem and deep nuclei are more severely affected than the cortex and account for most neurological deficits. Along with the presence of GCs, there is neuronophagia, neuronal loss, and gliosis. Except for the rare finding of tubular inclusions in neurons, no neuronal storage is seen. Neuronal degeneration and loss have been attributed to the neurotoxic action of glucosyl sphingosine, a by-product of glucocerebroside not normally present in the brain.

Prognosis and Therapy

GD is the first LSD that was successfully managed by ERT and its success in GD has opened the road for similar therapies in other LSDs. Current ERT uses imiglucerase, velaglucerase alfa, or taliglucerase alfa. Lifelong ERT is required. ERT reverses the hematologic manifestations and visceromegaly and reduces the bone pathology but has no effect on neuronopathic features because the drugs used for ERT do not cross the blood-brain barrier. SRT using miglustat or eliglustat is used for patients who are unable to receive ERT.

Clinical Features

Fabry disease (α-galactosidase [α-gal A] deficiency) is one of two X-linked LSDs, the other one being MPS II (Hunter syndrome). The gene for α-galactosidase, GLA , is located on Xq22.1. Currently, over 900 mutations have been reported. Full-blown Fabry disease becomes clinically apparent in childhood or adolescence with peripheral neuropathy, cutaneous lesions, renal disease, corneal opacities, and cataracts. The peripheral neuropathy (acroparesthesia) is a small fiber neuropathy characterized by intermittent episodes of disabling burning limb pain that are often precipitated by stress, exercise, and increased temperature. No significant weakness or muscle atrophy occurs. Cutaneous telangiectases (angiokeratoma corporis diffusum) cover the body in a bathing trunk distribution. Renal dysfunction progresses to renal failure with hypertension and cardiac and cerebrovascular complications. Heart involvement is manifested by mitral insufficiency, cardiac arrhythmias, cardiomyopathy, angina, and myocardial infarction. Four to 8 % of unselected patients with hypertrophic nonobstructive cardiomyopathy have Fabry disease. Generalized manifestations occur in patients with less than 1 % α-gal A activity. Patients with higher α-gal A activity have milder, later onset disease that affects one organ, usually the heart. Males with full-blown Fabry disease usually die in their 30s or 40s from renal failure, heart disease, or cerebrovascular disease. Because of random X chromosome inactivation, female carriers of Fabry disease may develop any of the manifestations seen in males but usually they have more variable, milder, and late-onset disease than affected males. The only other LSDs that cause severe heart disease are Pompe disease and the MPS (see further on).

ERT using recombinant α-gal A (agalsidase α and agalsidase β), starting as early as possible, slows the development of renal insufficiency and heart disease and decreases neuropathic pain but does not prevent stroke.

Pathologic Features

The pathology of Fabry disease consists of widespread lysosomal storage of birefringent lipids in epithelial, mesenchymal, and neural cells ( Fig. 8.20 ). On electron microscope examination, the membrane-bound deposits have a lamellar configuration ( Fig. 8.21 ). Endothelial cells and vascular smooth muscle are most severely affected. Vascular involvement causes endothelial injury, thrombosis, and ischemic changes and weakens vessel walls, leading to telangiectasia of cutaneous, conjunctival, and retinal vessels. The involvement of glomerular cells ( Fig. 8.20 ) and renal tubular epithelium explains the renal complications. Heart disease is due to lipid storage in cardiac myocytes (cardiomyopathy) and ischemic myocardial damage. The CNS complications of Fabry disease are primarily due to ischemia and hypertension, but storage in neurons, glial cells, and in the leptomeninges also occurs. Sural nerve biopsy in Fabry disease shows loss of small myelinated and unmyelinated fibers and lipid deposits in vascular cells, perineurial cells, and Schwann cells. Storage in sensory and autonomic ganglionic neurons also occurs.

The diagnosis of Fabry disease can be established by finding lower than 25 % of normal α-gal A activity in peripheral blood leukocytes or cultured skin fibroblasts. However, some female carriers have normal α-gal A activity because of random X chromosome inactivation. These carriers can be diagnosed by molecular genetic testing.

Clinical Features

Farber lipogranulomatosis, the rare autosomal recessive deficiency of acid ceramidase, encoded by the ASAH1 gene, has a wide phenotypic spectrum. In its classic infantile form, it presents in the first few months of life with a striking characteristic combination of painful, swollen, deformed joints, particularly in the hands and feet; subcutaneous nodules; and hoarseness due to development of granulomas in the larynx. As the disease progresses, granulomas cause skeletal deformities and affect the lungs, liver, heart, and other organs. Patients have psychomotor retardation and usually die in 2 to 3 years. Milder forms have a later onset and longer survival with mild neurological involvement. Cherry red spots associated with progressive neurological symptoms are seen in some patients. A severe rapidly fatal phenotype presents with neonatal hydrops and hepatosplenomegaly. Mutations in the ASAH1 gene also cause a combination of spinal muscular atrophy and progressive myoclonic epilepsy.

Clinical Features

The MPS share certain core clinical features. They are progressive multisystem disorders characterized by coarse facial features, skeletal and joint abnormalities, organomegaly, corneal clouding, cardiovascular disease, and CNS involvement. MPS I-H (Hurler syndrome), the most severe MPS phenotype, is described in detail as a prototype below. The variation among the other MPS in the expression of key clinical findings is discussed further on and presented in Table 8.5 .

Clinical Features

Patients with MPS I-H are normal at birth. Clinical manifestations begin to develop in the first year of life. Most patients are diagnosed by 18 months and die before they reach 10 years. The first abnormality to appear may be inguinal or umbilical hernia followed by coarsening of facial features. Corneal clouding occurs in all patients, and open angle glaucoma and retinal degeneration may cause additional visual impairment. The cardiovascular involvement consists of aortic and mitral insufficiency, hypertrophic cardiomyopathy with arrhythmias, and ischemic heart disease due to coronary artery stenosis. The skeletal manifestations of MPS I-H comprise the syndrome of dysostosis multiplex ( Fig. 8.22 ), which consists of enlarged skull, thick calvarium, J-shaped sella, broad clavicles, oar-shaped ribs, scoliosis, abnormal vertebrae, flared iliac wings, dysplastic acetabula, and shortened long bones with thickened cortices. Thickening of synovium and soft tissues causes progressive joint stiffening and distortion, claw-hand deformity, and carpal tunnel and other nerve entrapment syndromes. Thickening of the tongue, tonsils and adenoids and other soft tissues of the oropharynx causes upper airway obstruction, noisy breathing, and copious nasal discharge. Eustachian tube obstruction causes middle ear infections, which are the main cause of hearing loss seen in patients with severe MPS I-H. The abdomen protrudes due to hepatosplenomegaly. The Hurler phenotype, which has prompted the insensitive term “gargoylism,” consists of short stature, protruding abdomen, and coarse facial features ( Fig. 8.23 ). Delay in neurological development is usually evident by 1 year of age, followed by regression and severe psychomotor retardation. Imaging studies show hydrocephalus and arachnoid cysts. Additional neurological damage is caused by spinal cord compression resulting from spondylolisthesis or thickening of the dura.

Patients with mild MPS I (Scheie syndrome) and intermediate MPS I (Hurler-Scheie syndrome) have a later onset of the disease and may survive to adulthood. Corneal clouding and all other somatic manifestations develop at a lower pace but may become severe due to prolonged survival. Intellectual impairment is mild or absent.

Pathologic Features

Visceral and Skeletal Pathology

In extraneural tissues, the key cellular pathologyof MPS is cellular swelling with a clear or vacuolated appearance on light microscopic examination (see Fig. 8.4 ). GAGs form a loose branching fibrillary network but are lost in processing because they are highly soluble in water. Electron microscopy shows distended lysosomes that are either empty or contain sparse reticulogranular structures (see Figs. 8.9 and 8.24 ). Accumulation of GAGs in the extracellular matrix, presumably due to deficient recycling and probably also due to discharge from dying cells, is central to the phenotype of MPS I-H and other MPS. Extracellular GAGs are also lost during processing but can be observed in frozen sections with metachromatic stains and in paraffin sections with colloidal iron stains.

The tissue and organ pathology in the MPS is due to the combined intracellular and extracellular accumulation of GAGs. Cellular swelling is responsible for the organomegaly. GAG storage in connective tissue matrix and fibroblast dysfunction explain the thickening and deformity of connective tissues, the thick swollen skin, coarse facial features, macroglossia, arthropathy, cardiovascular pathology, and corneal opacity. The cardiac pathology of MPS I-H consists of the thickening of heart valves and the endocardium leading to mitral and aortic stiffening and regurgitation ( Fig. 8.25 ). This pathology is compounded by myocardial ischemia due to intimal thickening of the coronary arteries ( Fig. 8.26 ). The cornea is a fibrous structure consisting of fibroblasts in a collagenous stroma. GAG storage in corneal fibroblasts ( Fig. 8.27 ) causes corneal clouding and loss of vision. The skeletal pathology of dysostosis multiplex has not been described. Although there is GAG storage in chondrocytes and cartilage matrix, growth plates appear normal or slightly disorganized. Numerous clear cells are seen in the periosteum, and osteoblasts also store GAGs. Presumably, dysostosis is due to a disturbance of enchondral and periosteal osteogenesis caused by an excess of GAGs in the matrix and osteoblast dysfunction due to intracellular GAG storage.

Prognosis and Therapy

ERT uses Idursulfase, a recombinant enzyme that was recently approved by the FDA. In patients with MPS II (Hunter syndrome), weekly or every other week infusions of Idursulfase improve physical performance and respiratory function and reduce hepatosplenomegaly and GAG secretion.

Clinical Features

The multicatalytic action of the deficient lysosomal enzymes in the glycoproteinoses explains the diversity of storage products and their hybrid phenotypes, which include elements of MPS and sphingolipidoses. In particular, the role of glycoproteins as structural molecules and components of mucinous secretions explains the connective tissue matrix and skeletal pathology that resembles that of MPS.

Like other LSDs, the glycoproteinoses are progressive disorders. They resemble the MPS in that most of them have core clinical components of coarse facial features, skeletal abnormalities, and psychomotor retardation. However, even these core components may be absent or very mild in some glycoproteinoses. For instance, mannosidosis and fucosidosis have mild kyphosis and no true dysostosis multiplex, and Schindler disease has severe neurological deterioration without skeletal abnormalities. Other features such as corneal opacity, organomegaly, and heart disease are even more variable. Variations in severity within each group further widen the phenotypic spectrum. Diverse neurological manifestations including myoclonus are seen in sialidosis and Schindler disease. Moreover, several glycoproteinoses present unique and distinctive clinical features such as cherry red spot (sialidosis), angiokeratoma (β-mannosidosis and fucosidosis), and nephrotic syndrome (sialidosis).

The most significant phenotypic variation is seen in sialidosis. Three sialidosis phenotypes are distinguished. Sialidosis I is characterized by juvenile to adult onset, cherry red spot, and myoclonus without dysostosis or psychomotor retardation; sialidosis II (mucolipidosis I, childhood dysmorphic sialidosis) is characterized by severe neurological and somatic abnormalities; and severe neonatal sialidosis characterized by fetal and neonatal hydrops, nephrotic syndrome, and early death.

Clinical and Pathologic Features

Clinically and pathologically, the NCLs are indistinguishable from other LSDs that present with the neuronal lipidosis phenotype. Fourteen NCL entities, designated CLN1-CLN14 disease, have been identified (CLN9 and CLN4 are the same disease). The main NCL clinical phenotypes are infantile NCL (INCL), late infantile NCL (LINCL), JNCL (Batten disease), and adult NCL (ANCL [Kufs disease]) ( Table 8.7 ). INCL is most common in Finland. The other NCLs occur worldwide. The light microscopic changes are similar in all NCLs, but the fine structure of the storage material is different and is important for their pathologic recognition. Neuronal storage leads to neuronal loss and cerebral and cerebellar atrophy, which may be mild or extreme depending on the type of NCL and duration of illness. In addition to the retina and the CNS, subclinical storage with the same fine structure occurs in a variety of other tissues, including endothelial cells, Schwann cells, smooth and skeletal muscle, sweat glands, and lymphocytes. Examination of these tissues and cells may be used for diagnosis. Vacuolated lymphocytes are present in JNCL-CLN3.

INCL (CLN1 disease) is caused by mutations in the CLN1 gene, which encodes PPT1. Patients are normal at birth and develop normally for a few months but, within 1 year of age, they present with hypotonia, myoclonus, seizures, ataxia, progressive psychomotor retardation, and blindness and die between 8 and 13 years of age. Neurons and glial cells in INCL contain membrane-bound granular osmiophilic deposits ( Fig. 8.32 ). In patients surviving beyond 3 to 4 years of age, virtually all cortical neurons are lost and there is striking cerebellar atrophy. MRI shows cortical atrophy and thalamic hypointensity.

LINCL (CLN2 disease) is caused by mutations in the CLN2 gene, which encodes TTP1. Patients present between 2 and 4 years of age with seizures, myoclonus, and ataxia. Psychomotor regression and blindness ensue and death occurs between 10 and 30 years of age. The storage material in LINCL consists of curvilinear bodies ( Fig. 8.33 ). Patients with advanced disease have severe cerebral and cerebellar atrophy ( Fig. 8.34 ). MRI shows cerebral and cerebellar atrophy.

JNCL (CLN3 disease, Batten disease) is caused by mutations in the CLN3 gene, which encodes CLN3 protein (Battenin). It begins between 4 and 10 years of age with rapidly progressive visual loss, which leads to blindness in 2 to 4 years. Seizures and myoclonus appear later, and progressive psychomotor retardation and focal neurologic deficits develop, leading to death by 20 to 30 years of age. The storage material takes the form of fingerprint bodies, sometimes mixed with curvilinear and other inclusions ( Fig. 8.35 ). The brain shows neuronal loss and cortical atrophy but not of the extreme degree seen in INCL and LINCL. MRI shows cerebral and cerebellar atrophy.

ANCL (CLN4 disease, Kufs disease) is caused by mutations in the CLN4 / DNAJC5 gene, which encodes cysteine-string protein alpha (CSPa). It presents, on the average, by 30 years of age. Some patients present with myoclonus, ataxia, and dementia and others with behavioral abnormalities and dementia. Progressive neurodegeneration leads to death in 10 years. The storage material in ANCL is a mixture of granular osmophilic deposits and other inclusions ( Fig. 8.36 ). Neuronal loss and cortical atrophy are not as pronounced as in other NCLs.

It should be noted, however, that phenotypes similar to INCL, LINCL, JNCL, and ANCL are sometimes caused by a variety of mutations in other CLN genes. Moreover, the fine structure of the stored material is prevalent in the major forms of NCLs as described above but is not entirely disease-specific, and some NCLs contain diverse inclusions that may vary in different tissues.

Clinical Features

NPC is autosomal recessive. Ninety-five percent of cases, designated as NPC1, are caused by mutations in the NPC1 gene on 18q11-q12. The product of this gene is a membrane protein in the endoplasmic reticulum that is thought to be involved in intracellular sorting of cholesterol and glycosphingolipids. Approximately 380 mutations in NPC1 have been reported. Most affected individuals are compound heterozygotes. Five percent of NPC cases, designated as NPC2, are caused by mutations in the NPC2 gene on 14q24.3, which codes for a cholesterol-binding protein. At an estimated prevalence of 1:150,000 in Europe, NPC is much more frequent than Niemann-Pick A and B combined. Niemann-Pick type D refers to a genetic cluster of this disease in Nova Scotia.

Infantile-onset NPC may present with fetal ascites, neonatal hepatitis, hypotonia, and delay in psychomotor development. Neurological symptoms predominate when the onset is in childhood. In addition to psychomotor retardation, the neurological presentation includes impaired vertical gaze, dystonia, speech abnormality, and seizures. Similar neurological manifestations developing at a slower pace and psychiatric illness characterize adolescent and adult presentations.