A Cysteine-Containing Truncated Apo A-I Variant Associated With HDL Deficiency

Abstract

We identified a 50-year-old Japanese woman with a novel mutation in the apolipoprotein (apo) A-I gene causing high-density lipoprotein (HDL) deficiency. The patient had extremely low HDL cholesterol and apo A-I levels (0.14 mmol/L and 0.8 mg/dL, respectively) but no evidence of coronary heart disease. However, she had bilateral xanthomas of the Achilles tendon, elbow, and knee joint as well as corneal opacities. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of serum followed by immunoblotting revealed that the patient's apo A-I had a lower molecular weight (24,000) than normal apo A-I. A partial gene duplication encompassing 23 nucleotides was found by DNA sequence analysis, resulting in a tandem repeat of bases 333 to 355 from the 5' end of exon 4. This tandem repeat caused a frameshift mutation with premature termination after amino acid 207. The frameshift gives rise to a predicted protein sequence that contains two cysteines. We designated this mutant as apo A-ISasebo. Apo A-ISasebo formed heterodimers with apo A-II and apo E in the patient's plasma and was associated with both the low-density lipoprotein and HDL fractions. The patient's cholesterol esterification rate and lecithin-cholesterol acyltransferase activity were reduced to about 30% of normal, although specific enzyme activity was unaffected, suggesting that it remained functionally normal. In addition, cholesteryl ester transfer activity was reduced to about half of normal. Thus, apo A-ISasebo was associated with complex derangements of lipoprotein metabolism.

A Novel Mutant, ApoA-I Nichinan (Glu235→0), Is Associated With Low HDL Cholesterol Levels and Decreased Cholesterol Efflux From Cells

“…Plasma lipoproteins were isolated by sequential ultracentrifugation. 29 The following density fractions were obtained: VLDL (dϽ1.019 g/mL), LDL (dϭ1.019 to 1.063 g/mL), HDL 2 (dϭ1.063 to 1.125 g/mL), and HDL 3 (dϭ1.125 to 1.21 g/mL). Acetylated LDL (acetyl-LDL) was prepared by chemical modification of LDL with acetic anhydride 30 LCAT activity in plasma was determined by the dimyristoyl phosphatidylcholine method.…”

A Novel Homozygous Missense Mutation in the Apo A-I Gene With Apo A-I Deficiency

“…Apolipoprotein concentrations were measured by single radial immunodiffusion, and plasma lipoproteins were separated by sequential ultracentrifugation. 5 The following density fractions were obtained: VLDL (dϽ1.006 g/mL), LDL (dϭ1.019 to 1.063 g/mL), HDL 2 (dϭ1.063 to 1.125 g/mL), and HDL 3 (dϭ1.125 to 1.21 g/mL). Fractionation of plasma lipoproteins by Superose 6HR 10/30 gel chromatography was performed with a fast protein liquid chromatography system (Pharmacia Fine Chemicals).…”

“…[3][4][5] These data suggest that the functional domains of apoA-I contain central amphipathic ␣-helixes involving amino acid residues between 137 and 186, which are critical for both activation of LCAT and enhancing cholesterol efflux from peripheral cells.…”

“…[5][6][7][8][9][10][11][12][13][14][15] However, in subjects with HDL deficiency, CAD is not always present. In addition, Ϸ40 heterozygous structural apoA-I variants with a single amino acid substitution have been characterized.…”

“…1,2 In addition, naturally occurring mutations in the apoA-I gene have been structurally and functionally characterized to be the underlying molecular defects of low HDL levels. [3][4][5] These data suggest that the functional domains of apoA-I contain central amphipathic ␣-helixes involving amino acid residues between 137 and 186, which are critical for both activation of LCAT and enhancing cholesterol efflux from peripheral cells.…”