Cholesterol esterification and Niemann–Pick disease: An approach to identifying the defect in fibroblasts

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Abstract
Fibroblasts from 13 patients with the clinical phenotype of type IIS, Niemann–Pick disease were evaluated for their ability to incorporate oleic acid into cholesterol esters via an LDL responsive mechanism. Eight patients displayed a severe deficiency (< 8% of normal) of cholesterol ester (CE) synthesis while a clinically less affected group displayed intermediate levels (36% of normal) of synthetic capacity with no detectable overlap between these groups and the control range. There was no deficiency in cholesterol ester production in fibroblasts from a patient with Zellweger's disease, a disorder characterized by altered peroxisomes and abnormal peroxisomal cholesterol metabolism, while in I-cell disease, characterized by a primary deficiency of a phosphotransferase which results in altered targeting of lysosomal hydrolases, it was reduced to 25% of the control level. To further implicate lysosomal proteins in the etiology of type IIS, Niemann–Pick disease we measured the effect of correction (conditioned) medium, and the lysosomotropic agent, NH4Cl on cholesterol ester synthesis in fibroblasts. NH4Cl completely inhibited incorporation into CE by normal cells, thus mimicking the CE defect in type IIS, Niemann–Pick cells. Conditioned medium had no effect on incorporation into CE synthesis but medium conditioned in the presence of 10 mM NH4Cl stimulated incorporation into CE in the control but not in Niemann–Pick cells. When Niemann–Pick cells cultured in the presence of NH4Cl were challenged to synthesize CE in the absence of NH4Cl, a significant enhancement of CE synthesis was noted in representative cell lines from both groups of patients. These results indicate that two levels of deficiency can be clearly identified and that this biochemical phenotype appears to correlate with the severity of clinical disease. We suggest that the defect in patients with intermediate levels of CE synthesis may reside in a phosphomannosyl-directed glycoprotein that is targeted to the lysosome via the phosphomannosyl receptor, while the most severely affected patients may be defective in another type of protein.