Increased urinary excretion of free N-acetylneuraminic acid in thirteen patients with Salla disease

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Contribution of tandem mass spectrometry to the diagnosis of lysosomal storage disorders

Tandem mass spectrometry (MS/MS) is a highly sensitive and specific technique. Thanks to the development of triple quadrupole analyzers, it is becoming more widely used in laboratories working in the field of inborn errors of metabolism. We review here the state of the art of this technique applied to the diagnosis of lysosomal storage disorders (LSDs) and how MS/MS has changed the diagnostic rationale in recent years. This fine technology brings more sensitive, specific, and reliable methods than the previous biochemical ones for the analysis of urinary glycosaminoglycans, oligosaccharides, and sialic acid. In sphingolipidoses, the quantification of urinary sphingolipids (globotriaosylceramide, sulfatides) is possible. The measurement of new plasmatic biomarkers such as oxysterols, bile acids, and lysosphingolipids allows the screening of many sphingolipidoses and related disorders (Niemann-Pick type C), replacing tedious biochemical techniques. Applied to amniotic fluid, a more reliable prenatal diagnosis or screening of LSDs is now available for fetuses presenting with antenatal manifestations. Applied to enzyme measurements, it allows high throughput assays for the screening of large populations, even newborn screening. The advent of this new method can modify the diagnostic rationale behind LSDs.

Evidence that herpes simplex virus DNA derived from quiescently infected cells in vitro, and latently infected cells in vivo, is physically damaged

Using polymerase chain reaction (PCR) and alkaline gel electrophoresis, the authors show that, compared with DNA derived from virions used to establish infection, herpes simplex virus DNA derived from quiescently infected rat pheochromocytoma (PC12) cells in culture accumulates alkaline-labile lesions. That is, compared with equivalent amounts of virion DNA, viral DNA from nerve growth factor-differentiated long-term infected cells in culture is consistently 3 to 10 times more refractory to amplification by PCR. Despite using equal mole amounts of DNA isolated from quiescently infected cells (determined by quantitative Southern blots), DNA from quiescently infected cells could not be detected by PCR under conditions in which the virion-derived DNA was easily detected. Refractoriness to PCR was confirmed by analysis with a ligation-mediated PCR technique. The refractoriness was not the result of genomic circularization. The refractoriness was, however, related to the time that the quiescently infected cells had been maintained in culture. The refractoriness to PCR was taken as an indication that the viral DNA was damaged. This hypothesis was confirmed by showing that viral DNA from quiescently infected PC12 cells accumulated alkaline-labile DNA lesions, as determined by alkaline gel electrophoresis. The phenomenon was not limited to tissue culture, because viral DNA derived from the ganglia of latently infected mice is also 3 to 10 times more refractory to amplification than are equivalent amounts of virion-derived genomes. Taken together, these results represent the first evidence that herpes simplex virus DNA is physically damaged as a function of long-term infection. Implications for viral reactivation and pathogenesis are discussed.

A novel mutation in the SLC17A5 gene causing both severe and mild phenotypes of free sialic acid storage disease in one inbred Bedouin kindred

Four members of an extended consanguineous Bedouin family presented with different phenotypic variants of an autosomal recessive lysosomal free sialic acid storage disease. One affected individual had congenital ascites followed by rapid clinical deterioration and death, a presentation concordant with the clinical course of infantile free sialic acid storage disorder. His three first cousins had a more slowly progressive neurodegenerative disease, in line with the clinical phenotype of the milder form (Salla type) of this lysosomal disorder. Diagnosis of free sialic acid storage disease was based on clinical findings, histology, and biochemical assays of sialic acid. Molecular studies showed that all four affected individuals were homozygous for the same novel 983G > A mutation in exon 8 of the SLC17A5 gene, replacing glycine with glutamic acid at position 328 of the sialin protein. This family demonstrates the significant phenotypic variability of the disease in affected members of a single inbred kindred with precisely the same mutation, suggesting a role for modifier genes or environmental factors. It also highlights the need to consider this rare disorder in the differential diagnosis of congenital ascites and of unexplained psychomotor retardation, ataxia, and hypomyelination in infancy.

Sialic acid storage disease and related disorders

This paper gives an overview of the two sialic acid storage disorders, Salla disease and infantile sialic acid storage disease, and the related disorders cystinosis, sialuria, sialidosis, and galactosialidosis. Sialic acid storage disease and cystinosis are models for a deficient lysosomal transport of monosaccharides and amino acids, respectively. Several gene mutations leading to the production of the faulty membrane proteins sialin and cystinosin have been identified in recent years. Knowledge of the underlying pathophysiology is a prerequisite for future research projects, which will focus on the expression of the disease genes in living systems and the physical characterization of these proteins by X-ray crystallography and nuclear magnetic resonance spectroscopy.

Determination of free N-acetylneuraminic acid in urine by high-performance liquid chromatography using 3-[(1-[[4-(5,6-dimethoxy-1-oxoisoindolin-2-yl)-2-methoxyphenyl]sulfonyl]pyrrolidin-2-yl)carbonylamino]phenylboronic acid as a fluorescent labeling reagent

A highly sensitive high-performance liquid chromatography (HPLC) method for the determination of urinary N-acetylneuraminic acid (NeuAc) using 3-[(1-[[4-(5,6-dimethoxy-1-oxoisoindolin-2-yl)-2-methoxyphenyl]sulfonyl]pyrrolidin-2-yl)carbonylamino]phenylboronic acid as a fluorescent labeling reagent was developed. The labeling reaction was carried out at 30 degrees C for 30 min in the presence of pyridine. The derivative was monitored at Ex 314 nm and Em 388 nm. The detection limit of NeuAc was about 48 fmol per injection. The relative standard deviations of within-day and between-day precisions were 2.6-3.3 and 1.7-3.3%, respectively. Urine diluted 10 times with distilled water was analyzed by employing the standard-addition method. The concentrations were 8-89 nmol/mg creatinine (30+/-28 nmol/mg creatinine, n=9).

Biochemical characterization of patients and prenatal diagnosis of sialic acid storage disease for three families

Modifications of the assay method of Aminoff (1961) for the determination of sialic acid levels in urine, amniotic fluid, cultured cell homogenates and tissue homogenates, which reduce the interference from proteins by precipitation and from interfering chromogens by second derivative spectroscopy are described. Biochemical profiles of patients from three families with different clinical forms of sialic acid storage disease were made using data obtained with the new method. A family with two patients with the clinically severe, early-onset form is described, in which a 9-fold elevation of sialic acid can be detected in the skin fibroblasts and a 12-fold elevation in the urine. A patient from the second family is described with very severe clinical features and with 160-fold and 16-fold elevations of sialic acid in the urine and skin fibroblasts respectively. A patient from a third family is described with mild clinical features but with a 160-fold and 6-fold elevation of sialic acid in urine and skin fibroblasts respectively. The data obtained in this study are compared with data obtained in other laboratories on other patients. The method was used to assess the levels of sialic acid present in amniotic cells and chorionic villus cells obtained prenatally from pregnancies in each of the three families. In one case, in which amniotic cells were elevated in sialic acid and were vacuolated, the pregnancy was terminated. Follow-up studies confirmed the diagnosis. Pregnancies from the other two families were assessed to be not affected.

Prenatal detection of Salla disease based upon increased free sialic acid in amniocytes

Salla disease is an autosomal recessive lysosomal storage disease relatively common in the Finnish population. The main manifestations of more than 70 patients detected to date are severe psychomotor retardation and ataxia of early onset. Intracellular free N-acetylneuraminic acid (sialic acid) is increased 10-20-fold and localized in the lysosomes. Four pregnancies at risk were monitored by quantitation of free and total sialic acid in amniocytes and supernatant amniotic fluid by high-performance liquid chromatography. In 3 children results were normal. Free sialic acid content of the amniocytes from one affected child was 2.6 nmol/mg protein, which was approximately 5 times higher than that of the 3 unaffected children (0.3 to 0.8) and 14 control samples (0.3 to 0.9). The ratio of free/total sialic acid of the amniocytes also clearly distinguished the affected pregnancy (13.8%) from the unaffected (2.3-4.8%) and control individuals (1.8-5.3%). This represents the first successful prenatal identification of a patient with Salla disease and indicates that both free sialic acid and free/total sialic acid ratio should be monitored in pregnancies at risk for the disease.

Free sialic acid storage disease. A new Italian case

Increased amounts of free sialic acid were found in cultured fibroblasts and urine of a 4-year-7-month-old Italian boy with mental retardation, hypotonia, failure to thrive, coarse facial features, convergent strabismus, pale skin and fair hair. Ultramicroscopic examination of conjunctival and skin tissues showed a number of membrane-bound vacuoles containing low-density granular material in the cytoplasm of the fibroblasts. The clinical, biochemical and ultrastructural findings are similar to those described in Salla disease. Neuraminidase activity is normal. The molecular basis of the sialic acid storage disease is not known. Evidence for defective transport of sialic acid across the lysosomal membrane has been demonstrated in the patient's fibroblasts. It is possible that this might represent the metabolic abnormality.

Detection of sialuria by cation-exchange high-performance liquid chromatography

A simple and selective method for the detection of sialuria by high-performance liquid chromatography is described. The urine sample (2 ml) is purified using a C18 cartridge or ion-exchange chromatography, and free N-acetylneuraminic acid is separated on an Aminex HPX-87 cation-exchange column using 3 mM sulphuric acid as the mobile phase. The retention time of N-acetylneuraminic acid is ca. 8 min and the detection limit ca. 1 mumol/l. The within-day coefficient of variation is less than 4.9% and the day-to-day coefficient of variation is less than 5.6%. The method was tested on twenty normal individuals and four sialuria patients.

Defective sialic acid egress from isolated fibroblast lysosomes of patients with Salla disease

Normal fibroblasts exposed to N-acetylmannosamine yielded lysosome-rich granular fractions loaded with free (unbound) sialic acid, whose velocity of egress increased with increasing initial loading. Fibroblast granular fractions of patients with Salla disease exhibited negligible egress of sialic acid, whether endogenous or derived from N-acetylmannosamine exposure. Salla disease represents the first disorder demonstrated to be caused by defective transport of a monosaccharide out of cellular lysosomes.

Studies on the defect underlying the lysosomal storage of sialic acid in Salla disease. Lysosomal accumulation of sialic acid formed from N-acetyl-mannosamine or derived from low density lipoprotein in cultured mutant fibroblasts

Salla disease is a lysosomal storage disorder characterized by mental retardation and disturbed sialic acid metabolism. To study endogenous synthesis and breakdown of sialic acid, fibroblasts were incubated for 5 d in the presence and then in the absence of N-[3H]acetylmannosamine. Labeling of free sialic acid was 5-10 times higher in mutant than in normal cells. Radioactivity decreased in 4 d by 75% in normal but only by 30% in mutant fibroblasts. The labeling pattern was not normalized upon coculture of mutant and normal cells. To study the metabolism of extracellular sialic acid, low-density lipoprotein (LDL) was labeled in the sialic acid moiety (periodate-NaB3H4) or in the protein moiety (125I). Binding, internalization, lysosomal degradation, and exit of products of protein catabolism were similar in normal and mutant fibroblasts. Upon incubation with LDL labeled in the sialic acid moiety, mutant cells accumulated 2-3 times more free sialic acid radioactivity than normal fibroblasts, mostly in the lysosomal fraction. After a 24-h chase incubation, radioactivity in free sialic acid decreased by 70-80% in normal but only by 10-30% in mutant cells. In mutant fibroblasts, 40% of the radioactivity remained in lysosomes, whereas no labeled free sialic acid was detected in lysosomes from normal fibroblasts. We conclude that in Salla disease, fibroblast endogenous synthesis of sialic acid and lysosomal cleavage of exogenous glycoconjugates is normal, but free sialic acid cannot leave the lysosome. These findings suggest that the basic defect in Salla disease is deficient transport of free sialic acid through the lysosomal membrane.

Hyperexcretion of free N-acetylneuraminic acid--a novel type of sialuria

A 26-yr-old female with increased urinary excretion of free N-acetylneuraminic acid is described. Her early history was normal but she had difficulties at school and developed epilepsy at 14. She is mildly retarded, has slight changes in the spine, but no hepato- or splenomegaly. Cerebrospinal fluid leucocytes and IgG are elevated. Liver and skin biopsies disclosed no morphological abnormality. The daily excretion of free N-acetylneuraminic acid is ca. 0.5 g, intermediate to that found in other known types of sialuria. The clinical and morphological findings are also different suggesting a novel type of sialuria.

Clinical and laboratory diagnosis of Salla disease in infancy and childhood

Salla disease is an autosomal recessive lysosomal storage disorder; increased amounts of free sialic acid (N-acetylneuraminic acid) are found in urine and tissues. The disease causes severe psychomotor retardation, with onset by 1 year of age, but the patients have an apparently normal life-span. This paper describes the clinical features of Salla disease in six infants and young children and provides the background for laboratory diagnosis by thin-layer chromatography or spectrophotometric determination of sialic acid in urine.

Free N-acetylneuraminic acid in tissues in Salla disease and the enzymes involved in its metabolism

Salla disease is a lysosomal storage disorder of unknown etiology, characterized biochemically by increased urinary excretion of N-acetylneuraminic acid. This compound has now been shown to occur in abnormally large amounts in liver and cultured skin fibroblasts from these patients. Quantification of N-acetylneuraminic acid was performed using a new gas-chromatography/mass spectrometric single-ion method which is sensitive and specific. No abnormalities in the activity of several enzymes involved in sialic acid metabolism (N-acetylneuraminate:pyruvate lyase, neuraminidase, CMP-N-acetylneuraminate N-acylneuraminohydrolase and CTP:N-acyl-neuraminate cytidylyltransferase) were demonstrable. A possible explanation for the defect is a malfunctioning active transport of N-acetylneuraminic acid across the lysosomal membrane.

Generalized N-acetylneuraminic acid storage disease: quantitation and identification of the monosaccharide accumulating in brain and other tissues

Brain and other tissues from a patient with extensive neonatal ascites and clinical symptoms suggestive of a severe neurovisceral storage disorder were examined following autopsy for the accumulation of oligosaccharides. This carbohydrate analysis revealed the presence of large amounts (3--21) mumol/g fresh weight) of sialic acid in brain, liver, and kidney tissue as the major abnormality. Exhaustive characterization of the accumulating material by gel filtration, gas-liquid chromatography, thin-layer chromatography, and GLC-mass spectrometry positively identified the saccharide as free N-acetylneuraminic acid. Based on the accumulation of only free N-acetylneuraminic acid in the tissue of this patient, and normal activities of lysosomal enzymes involved in the catabolism of cellular glycoproteins, this storage disorder appears to result from a previously unreported defect in glycoconjugate metabolism.

Urinary sialic acid levels in aspartylglycosaminuria

Urinary sialoglycoconjugates were studied in 22 patients with inherited deficiency of 1-aspartamido-beta-N-acetylglucosamine amidohydrolase (aspartylglycosaminuria), in eight obligate heterozygotes, and in age- and sex-matched control subjects. Total sialic acid excretion was significantly higher in the patients (38.3 +/- 17.7 mumol/mmol creatinine, mean +/- S.D.) than in the matched controls (17.7 +/- 7.3 mumol/mmol creatinine, p less than 0.001). The sialic acid output in the heterozygotes did not differ from that of the controls. Gel filtration studies revealed that the increase in urinary sialic acid in aspartylglycosaminuria is of bound type and confined to the low molecular mass region. A linear positive correlation was found between the output of sialic acid and glycoasparagine in the individual patients (r = 0.77, p less than 0.001). The amount of sialylated metabolites excreted in urine did not correlate with the severity of clinical manifestations in aspartyl-glycosaminuria.