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Apolipoprotein A-I (Glu 198→Lys): A Mutant of the Major Apolipoprotein of High-Density Lipoproteins Occurring in a Family with Dyslipoproteinemia

ABSTRACT: To detect genetic mutants of apo A-I, the major structural protein of human HDL, we screened 530 unrelated Austrian prohands (168 children, 362 adults). An apo A-I mutant characterized hy an exchange of the acidic amino acid Glu in position 198 with the basic amino acid Lys was identified in the serum of the mother of a hyperlipoproteinemic girl. So far only two patients with this mutant, referred to as apo A-I (Glu 198→Lys) have been described. We detected six new patients (two children and four adults) with apo A-I (Glu 198→Lys) among 20 members in three generations of the affected family. An autosomal codominant inheritance of the apolipoprotein variant could be established. All affected individuals were heterozygous for the mutant. Among the six new subjects with apo A-I (Glu 198→Lys) two children and one adult presented with high-density lipoprotein (HDL) cholesterol concentrations below the fifth percentile for age and sex and with low serum apo A-I and A-II. Although there was no consistent relationship of the mutant with low serum HDL in this family, a moderate effect of apo A-I (Glu 198→Lys) on HDL levels cannot be ruled out. Hyperlipo-proteinemia of types IIa, IIb, and IV was observed in eight of the 20 family members studied, but did not cosegregate with the mutant apo A-I. There was no association of apo A-I (Glu 198→Lys) with premature clinical manifestations of atherosclerosis. The mutation occurred in a part of the apo A-I molecule, which is thought to be involved in lipid binding. The mutant apo A-I, however, was almost exclusively bound to HDL, similar to normal apo A-I. Although apo A-I (Glu 198→Lys) does not appear to be related to dyslipoproteinemia or premature atherosclerosis, this apolipoprotein variant may be of interest in studying the interaction of HDL with cell surface receptors and for genetic mapping studies.