Sialic acid storage diseases. A multiple lysosomal transport defect for acidic monosaccharides

Salla disease variant in a Dutch patient. Potential value of polymorphonuclear leucocytes for heterozygote detection

A Dutch child with psychomotor retardation, impaired speech, ataxia, sialic acid storage and vacuolized skin fibroblasts and lymphocytes was diagnosed as having free sialic acid storage disease. Slight corneal opacities, pale optic disks at the fundus oculi and vertebral abnormalities, not earlier reported in Salla disease, were peculiar to this case. Free sialic acid was about tenfold increased in urine and cultured fibroblasts, without changes in the glycoconjugate-bound sialic acid pool. A subsequent pregnancy of the patient's mother was monitored by assay of sialic acid in chorionic villi and amniotic fluid. An unaffected foetus was predicted. Sialic acid was also assayed in peripheral blood total leucocytes, and in mononuclear and polymorphonuclear (PMN) leucocyte subpopulations. Each of these leucocyte fractions from the patient showed 10- to 30-fold increase in sialic acid content. The PMN subpopulation provided the most restricted range of control values and showed slightly increased values for the patient's parents. These results suggest that the assay of sialic acid in PMN might be useful for the identification of heterozygotes in sialic acid storage disease. Studies on a larger number of obligate heterozygotes are needed to confirm this observation.

Functional reconstitution of the lysosomal sialic acid carrier into proteoliposomes

The lysosomal carrier for the acidic monosaccharides sialic acid and glucuronic acid was solubilized from rat liver lysosomal membranes and reconstituted into phospholipid vesicles. Membrane proteins were extracted from lysosomal membranes with Triton X-100. Upon removal of the detergent by absorption on Amberlite XAD-2 beads, the solubilized proteins were incorporated in egg yolk phospholipids. The reconstituted proteoliposomes show proton-driven carrier-mediated uptake of acidic monosaccharides. The reconstituted carrier was compared in several characteristics with the transporter as present in the native lysosomal membrane. Transporter substrate affinity (Kt for glucuronic acid = 0.4 mM) and specificity for acidic monosaccharides are completely retained. The proteoliposomes also demonstrate trans-stimulation properties with both substrates sialic acid and glucuronic acid. The transporter is inhibited, both in its native and in the reconstituted state, by the sulfhydryl-modifying agents p-chloromercuribenzoic acid, N-ethylmaleimide, and phenyl isothiocyanate. In native membrane vesicles, arginine and histidine modifiers phenylglyoxal and diethyl pyrocarbonate inactivated transport in a substrate-protectable manner. In reconstituted proteoliposomes, similar inhibition was observed. However, protection by substrates was achieved only after treatment with phenylglyoxal. These data suggest that arginine or histidine residues or both are present at or near the substrate binding site of the carrier. Possibly, other essential histidines become exposed in the reconstituted state. The successful functional reconstitution of the lysosomal sialic acid carrier represents an important step towards its purification and its detailed molecular characterization.

Exclusion map of Salla disease: attempts to localize the disease gene using a computer program

Salla disease is a lysosomal storage disorder due to impaired transport of free sialic acid across the lysosomal membrane. The clinical presentation of this autosomal recessive trait is severe psychomotor retardation from early infancy on. In order to determine the gene locus for the disease we have initiated a genetic linkage study using polymorphic gene markers in representative family material comprising about 60% of all families known to be affected with Salla disease. Here we present an exclusion map based on combined linkage data from 64 informative loci on 19 autosomes. Theoretically, at least 55% of the genome has been excluded as a locus for the disease gene, while some chromosome areas, particularly the long arm of chromosome 2, are highlighted as possible sites for the gene locus.

Confirmation of the chromosomal localization of human lamp genes and their exclusion as candidate genes for Salla disease

Salla disease is an inherited lysosomal storage disorder caused by accumulation of free sialic acid in the lysosomes. Lamp genes, lamp A and lamp B (lysosome associated membrane proteins), are the first known genes encoding for human lysosomal membrane proteins. Absence of linkage in a large group of families shows that lamp genes are not involved in Salla disease. The lamp genes were localized, using Southern hybridization in hamster--human hybrid cell panels, to chromosomes 13 (lamp A) and X (lamp B).