14.1 Clinical Features
and Laboratory Investigations The sialic acid storage disorders include severe infan- tile sialic acid storage disease (ISSD) and a milder variant, Salla disease (SD). Intermediate variants have also been reported.
ISSD is a rare disorder, presenting in the neonatal period with coarse facial features, hepatospleno- megaly, and often ascites or hydrops. Cardiomegaly and heart failure may also be present. The patients usually display generalized hypotonia. The subse- quent clinical course is invariably characterized by failure to thrive and grossly delayed development.
Most patients have hypopigmented skin and fair hair.
Albinoid fundi and optic atrophy may be present.
Mild hypertrophy of the tongue and gums may be seen. A nephrotic syndrome may complicate the course of the disease. Spastic tetraparesis develops with increased muscle tone and hyperactive tendon reflexes. Seizures may occur. There are recurrent res- piratory tract infections. Hypothyroidism has been reported infrequently. The age at death varies from soon after birth to 5 years of age.
SD occurs with a relatively high frequency in Fin- land. Pregnancy and the perinatal period are un- eventful. The first clinical signs usually appear at 6–9 months of age and include hypotonia, ataxia, and nys- tagmus. Gradually the patients develop spasticity.
Many develop signs of athetosis. Motor development is delayed, and about 30% of the patients never walk without support. Speech development is also delayed, and the speech is dysarthric. The nystagmus disap- pears. Most patients acquire a divergent squint. Many patients are growth-retarded with a height below the second percentile. Mental development is delayed from early on, and most adults are severely mentally handicapped. Facial features may become coarse late in the course of the disease. Epileptic seizures may oc- cur. Rarely, endocrine disturbances have been report- ed, including growth hormone deficiency and hypo- gonadotropic hypogonadism. The clinical course in SD patients is often static for many years, which de- lays evaluation for a metabolic disorder. The life span is relatively long. Most patients die in their thirties, but death in the seventies is also known to occur.
A few patients have been reported with a pheno- type intermediate between SD and ISSD, and in fact
there is a phenotypic continuum between ISSD and SD.
Light microscopic examination of blood smears and bone marrow specimens reveal vacuolated lym- phocytes in almost all ISSD patients and in most SD patients; young SD patients in particular may not have them. Electron microscopy of skin or conjuncti- val biopsy reveals vacuoles in many cell types includ- ing fibroblasts, smooth muscle cells, perineural cells, and Schwann cells. These vacuoles are bound by a single membrane and prove to be lysosomes. Most of them seem empty, but some contain small amounts of fibrillogranular material, membrane fragments, and occasional dark globules.
Dysostosis multiplex is found in about half of the patients with ISSD. Skeletal abnormalities are rare in SD and may include ovoid deformation of the verte- bral bodies and a thickened calvarium of the skull. In SD EEG initially shows a slowing of background activity; in some patients epileptic activity is seen.
With increasing age a gradual decrease in amplitude occurs; in adults a low-voltage EEG is a consistent finding. Motor and sensory nerve condition velocities are reduced in about half of the SD patients. So- matosensory evoked potentials are abnormal in the majority of SD patients, but visual and brain stem auditory evoked potentials are usually normal. The most prominent changes in nerve conduction and evoked responses are seen in the patients with the most severe disease.
Demonstration of increased urinary excretion of free sialic acid and sialic acid accumulation in cul- tured fibroblasts is the mainstay of the laboratory diagnosis in both ISSD and SD. The levels of sialic acid excretion vary widely among patients. The level reflects more or less the severity of the clinical dis- ease. Confirmation of the diagnosis by DNA analysis is possible.
Prenatal diagnosis is possible by assay of free sialic acid in a chorionic villus biopsy or by molecular studies. In all families in which the mutation is known, the latter option is the most reliable, in par- ticular in SD, in which the increase in sialic acid in chorionic villi of an affected fetus is less pronounced than in ISSD. The use of cultured amniotic fluid cells is less appropriate because the elevation of the free sialic acid content in these cells may be only moder- ate.
Free Sialic Acid Storage Disorder
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14.2 Pathology
In ISSD, the brain has a firmer consistency at autopsy.
There is some atrophy of the brain which a marked dilatation of the lateral ventricles. On sectioning, the white matter is firm and reduced in volume.
Histological examination shows clear vacuoles in neurons and astrocytes at all levels of the CNS. The stromal cells of the choroid plexus also contain these vacuoles. There may be neuronal loss in some areas.
Axonal spheroids are present in all gray matter struc- tures. The white matter is poorly myelinated and severely gliotic. The cerebellar white matter is also poorly myelinated and may show microcalcifications.
Axonal spheroids are present in the cerebral and cere- bellar white matter.
In SD, there is some external atrophy of the brain at autopsy. On sectioning, a marked reduction in white matter volume is found, whereas the cortex and basal ganglia are macroscopically normal. The corpus cal- losum is very thin. The cerebellum is atrophic with a markedly reduced white matter volume. Brain stem and spinal cord may appear thinner than normal.
Histological examination reveals storage of large amounts of lipofuscin in the perikarya of neurons in the cerebral cortex, thalamus, basal ganglia, brain stem nuclei, cerebellar cortex, and spinal cord. There may be extensive loss of Purkinje cells in the cerebel- lum. The cerebral and to a lesser extent the cerebellar white matter demonstrate severe myelin paucity with a marked loss of axons as well, accompanied by pro- nounced astrogliosis. The remaining axons frequent- ly show spheroids.
On electron microscopy, a single membrane binds the cellular inclusions, indicative of lysosomes. They are filled with sparse fibrillogranular material and occasionally small neutral lipid droplets.
The sural nerve is normal on light microscopic ex- amination, but electron microscopy shows vacuolar inclusions in the cytoplasm of Schwann cells.
In ISSD widespread storage of fibrillogranular ma- terial within large vacuoles is found in virtually all tissues of the body, including skin, conjunctiva, liver, kidney, myocardium, and bone marrow. Vacuolated lymphocytes can also been seen in blood. In SD the storage is less impressive.
14.3 Chemical Pathology
The material stored in ISSD and SD consists almost exclusively of N-acetylneuraminic acid. Studies of various tissues including the brain give identical results.
14.4 Pathogenetic Considerations
Free sialic acid storage disorders are autosomal reces- sive lysosomal storage disorders characterized by the accumulation of the acid monosaccharide sialic acid in lysosomes, caused by a defective efflux of sialic acid from the lysosomes. The basic defect in both ISSD and SD concerns the lysosomal free sialic acid trans- porter. The related gene SLC17A5 is located on chro- mosome 6q14–15. The gene has also been designated AST (for anion and sugar transporter). This gene en- codes the protein sialin, a predicted integral lysoso- mal membrane protein.
The anion and sugar transporter does not only transport sialic acid. The transporter carries acid monosaccharides, but also aliphatic nonsugar mono- and dicarboxylates. So, N-substituted acid monosac- charides (N-acetylneuraminic acid and N-glycolyl- neuraminic acid), glucuronic acids (the class of hex- oses with a carboxyl on C-6), and glucoaldonic acids as well as lactic acid and a-ketoglutarate are all trans- ported by the same carrier. Additionally, a large num- ber of organic anions bind to the carrier, inhibiting transport function.
Both ISSD and SD patients have mutations in the AST gene. Almost all Finnish SD patients have the same missense mutation in both alleles. The patients who are compound heterozygous for the Finnish mutation and another mutation usually have a more severe SD phenotype (intermediate phenotype). A variety of mutations, including deletions, insertions, missense, and nonsense mutations, are found in ISSD patients. These findings suggest that there is some genotypic–phenotypic correlation.
Sialic acids constitute a family of over 30 com- pounds derived from neuraminic acid. In humans, N-acetylneuraminic acid is the predominant sialic acid, referred to simply as “sialic acid.” Normally, a small portion of total sialic acid is free in tissues and body fluids. Most sialic acid is bound to glycoconju- gates, providing these macromolecules with a nega- tively charged terminal sugar that serves many func- tions. No biological role is attributed to free sialic acid. Free sialic acid is produced in the lysosome by the degradation of sialylated oligosaccharides by neuraminidase. Sialic acid is removed from lyso- somes for further metabolism by a specific mem- brane transporter system.
As a result of the defective transport of free sialic acid from lysosomes, patients store it excessively in many types of tissues and cultured fibroblasts and ex- crete large amounts in urine. Free sialic acid storage disorders must be discriminated from other forms of sialic acid storage, namely sialidosis, galactosialido-
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sis, and sialuria. In sialidosis and galactosialidosis, bound sialic acid accumulates as the terminal sugar of complex oligosaccharides, which remain undegraded as a result of deficient lysosomal sialidase (neu- raminidase) or as a result of a defect in protective protein. Sialuria results from the lack of feedback in- hibition of the enzyme epimerase, which is involved in the biosynthesis of sialic acid form N-acetylman- nosamine.
The pathophysiology of the myelin deficiency is unknown in ISSD and SD. Widespread storage of sial- ic acid in neurons and astrocytes may lead to dys- function of these cells from early on. Myelin is de- posited in a close collaboration and interdependence of neurons, oligodendrocytes, and astrocytes. This collaboration may be severely disturbed in the sialic acid storage disorders, leading to a profound defi- ciency of myelin.
14.5 Therapy
There is no causal treatment for sialic acid storage disorders. For both ISSD and SD, treatment is only supportive.
14.6 Magnetic Resonance Imaging
In ISSD MRI is characterized by a severe deficiency in myelin and a reduction in volume of the cerebral white matter, with enlargement of the lateral ventri- cles and in particular the subarachnoid spaces. The sylvian fissure may be wide open.
In SD MRI show a similarly serious myelin defi- ciency, but the reduction in cerebral white matter vol- ume is less severe (Fig. 14.1). The lateral ventricles are
normal or moderately increased in size and the sub- arachnoid spaces are prominent. The corpus callo- sum is thin (Fig. 14.1). The brain stem and cerebellar white matter are usually better myelinated, although hypomyelination of the cerebellar white matter has also been observed (Fig. 14.1). The internal capsule is myelinated to a variable extent. In older patients vari- able atrophy of the cerebellum and brain stem is found. There is a correlation between the imaging findings and the clinical phenotype: better myelina- tion is seen in patients with milder clinical symp- toms.
The images in ISSD and SD are similar to those in Pelizaeus–Merzbacher disease. Proton MRS, however, distinguishes ISSD and SD from Pelizaeus–Merz- bacher disease by showing elevations of sialic acid.
Sialic acid (= N-acetylneuraminic acid) co-resonates with N-acetylaspartate. The result is a very high
“NAA” peak at 2.02 ppm in a patient with a Peli- zaeus–Merzbacher-like MRI. NAA is not equally high in Pelizaeus–Merzbacher disease.
14.6 Magnetic Resonance Imaging 135
Fig. 14.1. The sagittal images in a 7-year-old girl with Salla disease demonstrate the thin corpus callosum and cerebellar atrophy. The axial T
2-weighted images show that the cerebral white matter has a high signal intensity throughout, consis- tent with myelin deficiency. The internal capsule is also hy- pomyelinated. The cerebellar white matter appears to contain more myelin, although still less than normal. The brain stem is myelinated best. Courtesy of Dr. J. Østergaard, Department of Pediatrics, and Dr. T. Christensen, MRI Research Center, Univer- sity Hospital of Aarhus, Denmark. (Fig. 14.1 see next page)
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