Reduction of ABCA1 Expression and Function in Cholesteryl Ester Storage Disease

Objective: Low levels of plasma HDL-C have been reported in genetic disorders of the intracellular cholesterol (CH) trafficking that reduce the availability of the metabolically active unesterified CH pool in the cells. Low HDL-C levels were observed in patients with Wolman Disease (WD) and Cholesteryl Ester Storage Disease (CESD), recessive disorders due to the deficiency of the Lysosomal Acidic Lipase (LAL), the enzyme that hydrolyzes CH-esters (CE) in lysosomes. The aim of our work was the molecular characterization of LAL defect and the study of cellular CH metabolism in four WD/CESD fibroblast cell lines. Results: The two CESD cell lines carried the common CESD mutation c.894 G>A (del p. S275_Q298) either in homozygous form or in combination with another mutation (p. H295Y) previously found in CESD patients. In these cell lines LAL activity and mRNA were greatly reduced. The WD cell lines carried two novel mutations (IVS3 c.229 +1 G>A or c.796 G>T p.G266X.) resulting in the complete absence of LAL activity and mRNA. Oil-Red-O staining confirmed the CE accumulation in these cells. Incubation of WD cells with LAL-containing medium caused a substantial reduction of CE accumulation. We observed an increased LDLR protein in the WD fibroblasts compared to CESD and control cells. ABCA1 protein was found to be reduced in WD/CESD cells cultured in 10% FBS compared to control cells. Restoration of normal ABCA1 expression was obtained in WD/CESD cells after loading with free CH (30µg/ml) or stimulation with LXR-RXR agonists. The ABCA1-mediated cell CH efflux was reduced in WD/CESD cells under LXR-RXR stimulation compared to control cells, irrespective of ABCA1 protein expression. Conclusion: Low HDL levels observed in WD/CESD patients could result from an impaired ABCA1 protein expression and function likely to result from a reduction of the intracellular pool of unesterified cholesterol.