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

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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-I Sasebo . Apo A-I Sasebo 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-I Sasebo was associated with complex derangements of lipoprotein metabolism.

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