This study was designed to investigate the mechanism(s) underlying the hypercatabolism of high density lipoprotein in Tangier disease (TD). Initially, the metabolism of normal HDL incubated in Tangier plasma in vitro was examined. Sufficient normal human HDL was added to TD plasma to raise the concentration of HDL-cholesterol to within normal levels. During incubation the concentration of HDL-cholesterol in the TD plasma fell by up to 50% in a time dependent manner. This was not seen in control samples treated in a similar manner. The loss of HDL-cholesterol in the TD could be completely accounted for by the loss of HDL-cholesteryl ester and was accompanied by a 2.3-fold increase in the concentration of HDL-triglyceride. These observations could not be accounted for by lecithin: cholesterol acytransferase activity, cholesteryl ester hydrolysis, or the triglyceride level in the TD plasma. However, preliminary evidence suggested that the activity of cholesteryl ester transfer protein in TD plasma is responsible for the changes in HDL-lipid composition. The resulting triglyceride-rich, cholesteryl-poor HDL was shown to have a normal affinity for the human skin fibroblast HDL receptor. However, this finding does not exclude other pathways of HDL catabolism that may contribute to the rapid turnover of modified HDL in TD plasma. The metabolism of normal HDL by TD fibroblasts and monocytes in vitro was also studied in an attempt to identify a cellular defect of HDL metabolism in TD. However, both TD fibroblasts and monocytes were normal with respect to their ability to bind/internalize and degrade normal HDL invitro. It is concluded that the hypercatabolism of normal HDL in TD involves alterations of HDL-lipid and protein composition prior to removal from the plasma component. Thus, these studies support the hypothesis that the defect in TD resides in the plasma and not in the cells of these patients. / Medicine, Faculty of / Pathology and Laboratory Medicine, Department of / Graduate

LCAT deficiency is a rare autosomal recessive disease characterized by low levels of plasma HDL and an inability of the enzyme lecithin:cholesterol acyltransferase (LCAT) to esterify cholesterol. An understanding of the structure and function of the LCAT protein will add significantly to the understanding of reverse cholesterol transport. This understanding can be gained, in part, by studying different mutations within the LCAT gene and their resultant phenotypes. Recombinant DNA technology has been used to determine the nature of a mutation in an LCAT gene of a previously described homozygote with this disorder. Southern blot analysis determined there were no major rearrangements in the genomic DNA at the LCAT locus. An attempt was made to follow segregation of the mutant alleles in three generations of a large pedigree by linkage analysis. There are known polymorphisms at the haptoglobin (Hp) locus, which is linked to LCAT on the long arm of chromosome 16, and in the adenosine phosphoribotransferase (APRT) and choesterol ester transfer protein (CETP) loci which are also on the long arm of chromosome 16, but have not been shown linked to LCAT. The information gained was uninformative in this pedigree. An extensive restriction fragment length polymorphism (RFLP) search in the immediate vicinity of the LCAT gene did not reveal any polymorphic sites. 2.4 kb of the ⋋ phage clone SF1020, obtained from one of the homozygotes, containing exons 1-5 plus 0.5 kb of DNA 5¹ to the LCAT gene, but not exon 6, was subcloned into M13 and sequenced. A cytosine to thymidine (C->T) transition was discovered in exon 4. This would result in a substitution of tryptophan for arginine at amino acid 135. The amino acid arginine is positively charged and resides in one of the most highly charged segments along the amino acid chain of the LCAT protein indicating that this region is likely involved in protein folding. Tryptophan, on the other hand is the most hydrophobic of the amino acids and would, therefore, severely disrupt the interaction of charged amino acids in that region, preventing normal folding of the LCAT protein. / Medicine, Faculty of / Medical Genetics, Department of / Graduate