GM Gangliosidosis Chapter 9

9.1 Clinical Features

and Laboratory Investigations GM

gangliosidosis is an autosomal recessive disor- der of GM

metabolism, resulting in variable neural and visceral accumulations. Three forms can be dis- tinguished: infantile or type 1 GM

gangliosidosis, ju- venile or type 2 GM

gangliosidosis, and adult, chron- ic or type 3 GM

gangliosidosis.

Type 1 infantile GM

gangliosidosis presents at or soon after birth, signs being poor sucking and feed- ing. The child is hypotonic and hypoactive and soon develops facial and peripheral edema. At times neonatal ascites and hydrocele are seen, and some- times generalized edema. There are characteristic coarse facial features similar to those found in mu- copolysaccharidoses. Facial abnormalities include frontal bossing, wide and depressed nasal bridge, long philtrum, and large low-set ears. The gums and tongue may appear hypertrophied. The cornea is clear. Hepatomegaly is present, and the spleen is often also enlarged. The dysmorphic features and he- patosplenomegaly gave this disease its other name:

pseudo-Hurler disease. The skin is usually thick and rough. Exceptional cases of angiokeratoma corporis diffusum have been described. Failure to thrive and severe psychomotor retardation are early signs of the disease. Bilateral, cherry-red spots are found on the maculae in about half the patients. Early blindness occurs, which is cortical or retinal in origin. The child remains hypoactive and is weak. Noise frequently provokes an exaggerated startle response. Movements are poorly coordinated. Reflexes are hyperactive.

Macrocephaly can develop, but is less marked than in Tay-Sachs disease. As the child gets older, broadness of the hands and shortness of the fingers become apparent. Joints become stiff. The wrist and ankle joints are often enlarged, but not tender. Flexion con- tractures frequently occur at elbows and knees.

Kyphoscoliosis is frequently found. After a year, neurological deterioration is rapid, with epileptic seizures, progressive spasticity, and finally decere- brate rigidity, deafness, blindness, and loss of social contact. In rare cases cardiomyopathy with cardiac failure occurs. Flexion contractures of the arms and legs may become extremely severe. Respiratory prob- lems are common, with frequent infections. Bron- chopneumonia is a frequent cause of death, which usually occurs between 12 and 24 months.

In type 2 GM

gangliosidosis the clinical course is slower and the onset insidious. The children initially appear normal. The first clinical signs begin between 6 months (late-infantile form) and 2 years (juvenile form) of age. They usually consist of developmental arrest and gait disturbances. The subsequent course is characterized by progressive dementia, lethargy, epilepsy, an abnormally pronounced startle response to noise, spastic tetraplegia, cerebellar ataxia, loss of speech, and extrapyramidal features, such as choreoathetosis. Blindness is also a feature, but starts late in the course. No retinal degeneration or cherry- red spots are seen. The patients are not characterized by organomegaly or coarsening of facial features sug- gestive of mucopolysaccharidosis. Skeletal deformi- ties are relatively mild. The average life span varies between 3 and 10 years. Death is usually caused by recurrent bronchopneumonia.

Type 3 GM

gangliosidosis is the adult or chronic variant. The clinical signs usually emerge in the sec- ond decade of life, but onset in the first decade has also been described. Gait disturbance and speech dis- turbance are early signs of the disease. Extrapyra- midal features are usually most prominent and can take the form of slowly progressive dystonia, choreoathetotic movements, facial grimacing, ble- pharospasm, dysarthria, rigidity, parkinsonism with immobile face, bradykinesia, and typical gait abnor- malities. In rare cases ataxia, pyramidal signs, mild intellectual impairment, and seizures occur. Bony ab- normalities are minimal, if present. Cherry-red spots, visceromegaly, and facial dysmorphism do not occur.

In type 1 GM

gangliosidosis vacuolated lympho- cytes are found in the peripheral blood smear, but these are not present in types 2 and 3. Large, foamy sea-blue histiocytes are present in the bone marrow in type 1; these cells are fewer in number in types 2 and 3. Rectal biopsy shows neuronal lipidosis in Meissner’s plexus. Ultrastructurally, membranous cytoplasmic bodies similar to those seen in GM

gan- gliosidosis are seen within neurons. The membra- nous cytoplasmic bodies consist of spirally wound lamellae enclosed within a membrane of lysosomal origin. Other inclusions are more pleiomorphic in nature. Evidence of neuronal lipidosis on rectal biopsy can be found in all types of GM

gangliosido- sis. Clear vacuoles can also be found in visceral histi- ocytes and parenchymal cells of visceral organs and

GM ₁ Gangliosidosis

Chapter 9

in epithelial cells in all types, but are less abundant in the later onset forms.

Radiological abnormalities in type 1 may be mini- mal at birth but become progressively more pro- nounced with time. By 6 months there is usually little difficulty in identifying them. The abnormalities in- clude kyphoscoliosis and hypoplasia and beaking of one or more vertebrae. The long bones are wide in the center, tapering to both ends. There is generalized rarefaction of the cortex of most bones. With increas- ing age the externally thickened cortical wall is re- moved by expansion of the medullary cavity. The metacarpals are wedge-shaped, being expanded dis- tally and constricted proximally. The sella turcica is shoe shaped, shallow, and elongated. In type 2, radio- logical changes are mild and involve mainly the verte- bral bodies. In type 3 the radiological changes are minimal or absent. Flattening of vertebral bodies may be seen.

In types 1 and 2 the EEG shows progressive deteri- oration, but epileptic discharges are rare. The ERG remains normal.

Biochemical investigations disclose abnormal uri- nary oligosaccharide excretion. The concentration of the urinary oligosaccharides appears to correlate with the severity of the disease, the excretion being highest in type 1 and lowest in type 3 GM

gangliosi- dosis. The CSF protein is normal. In types 1 and 2 an increased concentration of GM

ganglioside may be found in plasma and CSF.

Demonstration of a deficiency of β-galactosidase activity in leukocytes or cultured fibroblasts is the most effective means of establishing the diagnosis. It is important also to analyze neuraminidase activity in order to exclude galactosialidosis. A decreased hy- drolysis rate of GM

ganglioside can be demonstrated in cultured skin fibroblasts. DNA analysis for diag- nostic purposes is possible. Prenatal diagnosis is pos- sible by enzyme analysis in cultured amniotic fluid cells or chorionic villi or by DNA analysis.

9.2 Pathology

External examination of the brain in type 1 GM

gangliosidosis usually reveals no abnormalities, though sometimes some cortical atrophy is detect- ed. The consistency of the white matter may be in- creased.

Light microscopy reveals neuronal storage through- out the nervous system, mainly in cerebral and cere- bellar cortex, but also in basal ganglia, brain stem, spinal cord, and Meissner’s plexus. The neurons have ballooning, foamy cytoplasm, and the nucleus is dis- placed to the periphery. Accumulation of storage ma- terial in proximal nerve cell processes results in the formation of meganeurites and megadendrites. There

is degeneration and loss of neurons. Storage bodies are also present in glia.

Electron microscopy demonstrates that the neu- ronal inclusion bodies are identical to the membra- nous cytoplasmic bodies seen in GM

gangliosidosis.

They consist of spirally wound membranous lamellae enclosed within a limiting membrane of lysosomal origin.

The white matter in type 1 GM

gangliosidosis is gliotic, and there is a diffuse and profound paucity of myelin. Axons are relatively preserved. Where pre- sent, myelin is structurally normal, but the thickness of most myelin sheaths is reduced. The early myeli- nating structures have a myelin content that is normal for age, whereas the myelin content of later myelinat- ing structures is much lower than expected. Oligo- dendrocytes are decreased in number, and apoptosis of these cells can be observed. The white matter ab- normalities are probably the result of a combination of disturbed myelination and myelin loss.

In type 1 GM

gangliosidosis visceral storage is found. The liver is enlarged, and storage material is present in hepatocytes and in histiocytes in liver sinusoids. The renal glomerular epithelium shows marked vacuolization of the cytoplasm. The spleen, lymph nodes, thymus, and intestinal mucosa contain many foamy histiocytes. Large foamy histiocytes are present in bone marrow aspirates. Skin biopsies show foamy vacuolization of sweat gland epithelium, histi- ocytes, fibroblasts, and endothelium cells. The vac- uoles appear empty. The stored oligosaccharides in visceral organs are extremely soluble in water and are lost on fixation.

In types 2 and 3 GM

gangliosidosis identical neu- ronal storage is seen. In type 3 the neuronal storage is present predominantly in the basal ganglia. White matter is either not affected at all, or only to a mini- mal extent. The visceral storage varies considerably.

There may be no storage at all, or sparse histiocytes may be seen in spleen and liver. There may be foam cells in bone marrow and lymphatic tissue, and vac- uolization of hepatic and renal cells similar in form to but less severe than in type 1.

9.3 Chemical Pathology

Gangliosides are glycosphingolipids that contain

sialic acid in their oligosaccharide chain. In type 1

GM

gangliosidosis pronounced accumulation of the

normal monosialoganglioside GM

occurs, accompa-

nied by a minor accumulation of its asialo derivative

GA

and of other minor glycolipids and glycopep-

tides. The total ganglioside content of the brain is

increased, with an approximately 10-fold increase of

GM

in gray matter and a 2-fold increase in white

matter. In gray matter GM

ganglioside constitutes

70–90% of total ganglioside, instead of the normal about 25%. The level of total lipid in gray matter is slightly decreased, mainly due to a moderate decrease of phospholipids and glycolipids. In white matter a marked decrease of major myelin constituents is found, such as cholesterol, phospholipids, cerebro- sides, sulfatides, and proteolipid protein. Marked increases in free fatty acids and in cholesterol esters are found in most cases. The white matter chemical abnormalities are compatible with moderately severe myelin destruction. In the myelin membrane itself the concentration of GM

ganglioside is also several times the normal concentration. Other abnormalities in the composition of isolated myelin are a very high concentration of cholesterol, a low level of glyco- lipids, especially cerebroside, and a low concentration of phospholipids, especially ethanolamine phospho- lipids. These myelin abnormalities represent the transitional state of myelin undergoing nonspecific breakdown.

The neuronal inclusion bodies, the so-called mem- branous cytoplasmic bodies, have an extremely high ganglioside content. GM

accounts for approximately 95% of the total ganglioside content. Other compo- nents are proteolipid protein, phospholipids, and cerebroside. One of the glycolipids, cerebroside, con- sists mainly of glucocerebroside, which is an unusual cerebral constituent after the infantile period.

There is a 20- to 50-fold accumulation of GM

gan- glioside in liver and spleen. Furthermore, there is vis- ceral accumulation of galactose-containing oligosac- charides, which by far exceeds the accumulation of gangliosides. Some oligosaccharide fractions contain sialic acid. The oligosaccharide accumulation rather than the ganglioside storage is the chief cause of the visceral histiocytic vacuolation. Storage of galactose- containing, partially degraded, derivatives of keratan sulfate has been demonstrated in liver and brain.

In type 2 GM

gangliosidosis the concentration of GM

ganglioside and its asialo derivative are moder- ately elevated in the brain, to a considerably lesser de- gree than in type 1 GM

gangliosidosis. The myelin lipids, such as cholesterol, phospholipids, sulfatide, and cerebroside, are much closer to normal than in type 1 GM

gangliosidosis. Visceral accumulation of oligosaccharides is less marked.

In type 3 GM

gangliosidosis the accumulation of GM

ganglioside in the brain is more focal. Accumu- lation is most marked in the putamen and caudate nucleus, where GM

accounts for 50% or more of all gangliosides, whereas GM

accounts for about 30% of all gangliosides in the white matter and only a slight increase, if any, in the proportion of GM

ganglioside is noted in the cerebral cortex. Abnormal accumula- tion of asialo GM

is only noted in the basal ganglia.

There are no abnormalities in the concentrations of other lipids, such as cholesterol, phospholipids, and

glycolipids. Visceral accumulation of oligosaccha- rides is minor.

9.4 Pathogenetic Considerations

GM

gangliosidosis is caused by a deficiency of the lysosomal degradative enzyme acid β-galactosidase.

This enzyme is an acid hydrolase catalyzing cleavage of terminal β-linked galactose from a variety of substrates, including GM

ganglioside, its asialo de- rivative G

, lactosylceramide, galactose-containing oligosaccharides, and the mucopolysaccharide ker- atan sulfate. The gene coding for β-galactosidase, GLB1, is located on chromosome 3p21.33. Activity of β-galactosidase is also dependent on a functional protein, called protective protein/cathepsin A. This protein is encoded by PPGB, a gene located on chro- mosome 20q13.1. Protective protein associates with two enzymes, β-galactosidase and neuraminidase, in an early biosynthetic compartment, and by virtue of the association the two enzymes are correctly routed to the lysosome and protected against rapid break- down by intralysosomal proteases. Some hydrolytic enzymes need nonenzymic factors (activator pro- teins) for degradation of sphingolipids in the lyso- some. Substrates cleaved by β-galactosidase differ in their requirement of such activator proteins. Sphin- golipid activator protein 1 (called SAP-B or saposin B) is required for the cleavage of GM

ganglioside by β-galactosidase. SAP-B acts on GM

as a kind of solu- bilizer. SAP-B shares a common larger precursor pro- tein, called prosaposin, with some other SAPs. Pro- saposin is encoded by a gene on chromosome 10q22.1, PSAP. SAP-B not only activates β-galactosi- dase to cleave GM

ganglioside, but also activates the cleavage of sulfatide by arylsulfatase A and globo- triaosylceramide by α-galactosidase.

Two diseases have been recognized for deficiency of β-galactosidase: GM

gangliosidosis, a neuro- degenerative disorder with visceral involvement, and mucopolysaccharidosis type IV B, also called Morquio B disease, a generalized bone disease. Mole- cular analysis has confirmed allelic mutations of the same gene in these two diseases with diverse pheno- typic expressions. It has been suggested that this mul- tiplicity of phenotypes can be explained by different alterations in the catalytic activity of the mutant enzyme, which differentially alters its activity on a variety of substrates. There is evidence that patients with Morquio disease type B retain a higher catalytic activity for GM

ganglioside than for oligosaccha- rides and mucopolysaccharides. The phenotypic vari- ability in clinical symptomatology of GM

gangliosi- dosis may be explained in the same way. The early in- fantile form is characterized by neurological dysfunc- tion in combination with bony abnormalities and

Chapter 9 GM1Gangliosidosis 98

visceral storage. In later onset forms, progressive neu- rological symptoms are present but visceral and bony abnormalities are minimal or absent. Differences in residual enzyme activity, different types of mutations affecting different catalytic functions of the enzyme, different rates of turnover of the enzyme and sub- strates in brain, viscera, and bone may be responsible for these phenotypic variations. An attractive hypo- thesis is that in some cases the mutant enzyme pos- sesses about the same low residual activity for each natural substrate (infantile type), whereas the mutant enzyme possesses significantly different residual activities for different natural substrates in other cas- es (juvenile and adult types).

Apart from GM

gangliosidosis and Morquio dis- ease type B, β-galactosidase deficiency occurs in a number of disorders caused by different gene muta- tions: variant forms of metachromatic leukodystro- phy, galactosialidosis, and I cell disease. Deficiency of the protective protein caused by a protective protein gene mutation leads to galactosialidosis. Deficiency of the protective protein gives rise to a secondary deficiency of both β-galactosidase and neuramini- dase activity. SAP-B deficiency results in a form of metachromatic leukodystrophy, with a variable con- comitant accumulation of gangliosides and other glycosphingolipids in addition to sulfatide storage.

In I-cell disease, or mucolipidosis III, deficiency of β-galactosidase is caused by a defect in posttrans- lational processing of the enzyme molecule.

In GM

gangliosidosis, GM

ganglioside and its asialo derivative accumulate in lysosomes owing to the impairment of normal degradation. GM

is a nor- mal component of cellular membranes, and its con- tent is especially high in neuronal plasma mem- branes, particularly in the regions of nerve endings and dendrites. GM

gangliosides act as binding mole- cules for toxins and hormones and are involved in cell differentiation and cell–cell interaction. They stimu- late neurite outgrowth and enhance the action of nerve growth factor. In GM

gangliosidosis accumula- tion occurs predominantly in neurons, resulting in neuronal dysfunction and eventually neuronal cell death. There are several factors, which may contribute to neuronal dysfunction and death. The accumulated GM

ganglioside and its asialo derivative are relative- ly insoluble in water and aggregate within lamellated membranous bodies in lysosomes. Expanding lyso- somes with increasing amounts of stored products may disturb intracellular transport, in this way dis- turbing cellular metabolism. Leakage of toxic in- tralysosomal products of enzymes into the cytoplasm during the process of intralysosomal storage may cause damage. Accumulation of GM

ganglioside in the neuronal membrane results in alterations of membrane structure. Gangliosides contain long, satu- rated fatty acids, which increase the packing density

of the membrane and reduce its fluidity. GM

ganglio- side, specifically, has a pronounced effect on the re- duction in membrane fluidity as the carbohydrate moieties of GM

reduce the rotational freedom with- in the hydrophobic regions of the membrane. The increased cholesterol content also contributes to the reduction of membrane fluidity. The altered mem- brane fluidity may influence the synaptic transmis- sion and the activity of membrane-bound enzymes.

GM

gangliosidosis is also characterized by inappro- priate proliferation of secondary neurites and aber- rant formation of synapses. This abnormal sprouting may lead to changes in neuronal connectivity, result- ing in specific functional impairment. Evidence has been found for neurotransmitter dysfunction with disturbed neurotransmitter release and re-uptake and for specific dysfunction of cholinergic and GABAergic neurons.

The later onset forms of GM

gangliosidosis have more focal neuronal pathology, which is especially marked in basal ganglia and the spinal cord, whereas in the infantile form there is more generalized neu- ronal pathology, which is especially pronounced in cerebral and cerebellar cortex. We can only speculate on the explanation for this phenomenon. The regula- tion of substrate and enzyme synthesis and turnover may not be identical in different types of cells and may not be the same at all ages, which would change the distribution of cells in which saturation of the residual enzyme occurs earliest and is most promi- nent.

Significant white matter changes are only present in type 1 GM

gangliosidosis. There is a severe myelin deficiency, which is caused mainly by a disturbance of myelinogenesis with seriously delayed and arrested myelination. Myelin deposition occurs in close col- laboration between axons, oligodendrocytes, and astrocytes. Altered neuronal/axonal membrane prop- erties and disturbed oligodendroglial-axonal com- munication may be at the basis of the disturbed myelination process. Disturbances of oligoden- droglial maturation and myelin production and early oligodendroglial cell death through apoptosis are contributing factors. However, there is also a compo- nent of myelin loss. The myelin loss may be secondary to degeneration of neurons as well as primary owing to oligodendroglial cell death, altered myelin compo- sition, and myelin instability.

Galactose-containing oligosaccharides accumulate in the viscera, particularly in type 1 GM

gangliosido- sis. These oligosaccharides are derived from the in- complete degradation of glycoproteins in lysosomes.

The accumulation of these water-soluble compounds

is responsible for the cytoplasmic vacuolation of vis-

ceral cells, the foamy histiocytosis in bone marrow,

and the vacuoles in circulating lymphocytes. Storage

of galactose-containing, partially degraded deriva-

tives of keratan sulfate occurs in liver, spleen, brain, and bone, especially in type 1. Storage of these com- pounds in bones is responsible for the bony deformi- ties.

9.5 Therapy

At present there is no effective treatment for GM

gangliosidosis. Hematopoietic stem cell transplanta- tion can be attempted, especially in the later onset forms, but insufficient data are as yet available to assess its potential. In addition, there is evidence in animal experiments that the low molecular com- pound 1-deoxygalactonojirimycin may increase β- galactosidase activity. The compound passes through the blood–brain barrier. Human studies are in progress. Gene therapy is still at the level of experi- mental research.

9.6 Magnetic Resonance Imaging

In type 1 GM

gangliosidosis CT of the brain typical- ly shows a slightly increased density of the thalamus (Fig. 9.1). MRI shows diffuse white matter abnormal- ities, partly but not only explained by severely delayed and disturbed myelination. The signal abnormalities are more pronounced than would be expected if they were due merely to delayed myelination (Fig. 9.1).

White matter gliosis and some myelin destruction add to the signal changes. We observed a very subtle pattern of radiating lines with a more normal signal

intensity within a patient’s abnormal cerebral white matter, which we found to be related to a higher myelin content in perivascular regions. The brain stem, and sometimes the posterior part of the corpus callosum, are better myelinated, but the cerebellar white matter can be myelin deficient (Fig. 9.1). The basal ganglia and thalamus display subtle signal changes on MRI, with a slightly increased or decreased signal intensity of the thalamus and a slightly in- creased signal intensity of the basal ganglia on T

- weighted images and a variably increased signal in- tensity of the thalamus on T

-weighted images (Fig.

9.1, upper row). The pattern closely resembles the imaging findings in early-onset GM

gangliosidosis.

In type 2 GM

gangliosidosis, progressive atrophy of the cerebral hemispheres has been described with enlargement of the ventricular system and subarach- noid space and atrophy of the cerebellum and brain stem (Fig. 9.2). There are subtle white matter signal abnormalities, which are commonly seen in neuronal degenerative disorders and are secondary to loss of neurons and axons and their myelin sheaths (Fig. 9.2). The CT images that have been published suggest that the density of the thalamus and basal ganglia might possibly be mildly increased.

In type 3 GM

gangliosidosis, elevated T

signal and atrophy may be found bilaterally in the caudate nucleus and putamen. Hypointensity of the globus pallidus on T

-weighted images has also been report- ed. In addition, cerebral atrophy and slight white mat- ter signal changes secondary to neuronal degenera- tion can be found. The images are identical to those of adult GM

gangliosidosis.

Chapter 9 GM1Gangliosidosis 100

Fig. 9.1. CT scan and MR images in a 7-month-old boy with type 1 GM1gangliosidosis. Note the high density of the thala- mus on CT.The thalamus and basal ganglia are diffusely slight- ly abnormal on both T1- and T2-weighted MR images (first row).

The cerebral and cerebellar white matter has a diffusely dis- tributed high signal on T2-weighted images, which is higher than is compatible with hypomyelination only

Chapter 9 GM1Gangliosidosis 102

Fig. 9.2. Sagittal T1-weighted and axial T2-weighted images in a 9-year-old boy with type 2 GM1 gangliosidosis.The thick skull indicates a long-standing disease process. The cerebral white matter has a slightly higher signal than normal on these

T2-weighted images, and the cerebral hemispheres are mildly atrophic, suggestive of a primary neuronal degenerative process. The globus pallidus has a low signal