Additional biochemical findings in a patient and fetal sibling with a genetic defect in the sphingolipid activator protein (SAP) precursor, prosaposin. Evidence for a deficiency in SAP-1 and for a normal lysosomal neuraminidase.

It has been shown that sphingolipid activator proteins (SAPs) 1 and 2 are encoded on the same gene along with two other putative activator proteins [Fürst, Machleidt & Sandhoff (1988) Biol. Chem. Hoppe-Seyler 369, 317-328 and O'Brien, Kretz, Dewji, Wenger, Esch & Fluharty (1988) Science 241, 1098-1101]. We have undertaken further biochemical investigations on a patient and fetal sibling, who were previously shown to have a unique sphingolipid storage disorder associated with an SAP-2 deficiency [Harzer, Paton, Poulos, Kustermann-Kuhn, Roggendorf, Grisar & Popp (1989) Eur. J. Pediatr. 149, 31-39]. The severity of their disorder suggested that other products of the SAP precursor or prosaposin gene may also be deficient. The turnover of cerebroside sulphate and globotriaosylceramide were investigated and were both impaired in fibroblasts from the patient and fetus. However, the activities of cerebroside sulphate sulphatase and globotriaosylceramide alpha-galactosidase in vitro were normal in cells from the fetus and patient respectively. In addition, there was an increase in cerebroside sulphate concentration in the kidney of the affected fetus. These results indicate that, in addition to the SAP-2 deficiency, there was a defect in SAP-1 function in this disorder. Additional increases in the concentration of monohexosyl- and dihexosyl-ceramide in the fetal kidney probably reflect the deficiency of SAP-2 in the case of monohexosylceramides, and the combined activator deficiency in the case of dihexosylceramides. Lactosylceramide-loading studies confirmed that there was a defect in the turnover of this lipid in fibroblasts from the affected patient and fetus but not from a patient with an isolated SAP-1 deficiency, or from patients with Krabbe disease, GM1 gangliosidosis or galactosialidosis. It has been suggested [Potier, Lamontagne, Michaud & Tranchemontagne (1990) Biochem. Biophys. Res. Commun. 173, 449-456] that the prosaposin gene also codes for lysosomal neuroaminidase. However, we found normal neuraminidase activity in fibroblasts from our patient, using assay conditions which are diagnostic for sialidosis patients. The role of prosaposin gene products in sphingolipid metabolism is discussed in view of our biochemical findings in this genetic disorder.