FAMILIAL LIGAND-DEFECTIVE APOLIPOPROTEIN-B - IDENTIFICATION OF A NEW MUTATION THAT DECREASES LDL RECEPTOR-BINDING AFFINITY

Detection of new ligand-defective mutations of apolipoprotein B (apoB) will enable identification of sequences involved in binding to the LD L receptor. Genomic DNA from patients attending a lipid clinic was scr eened by single-strand conformation polymorphism analysis for novel mu tations in the putative LDL receptor-binding domain of apoB-100. A 46- yr-old woman of Celtic and Native American ancestry with primary hyper cholesterolemia (total cholesterol [TC] 343 mg/dl; LDL cholesterol [LD L-C] 231 mg/dl) and pronounced peripheral vascular disease was found t o be heterozygous for a novel Arg(3531)-->Cys mutation, caused by a C- ->T transition at nucleotide 10800. One unrelated 59-yr-old man of Ita lian ancestry was found with the same mutation after screening 1,560 i ndividuals. He had coronary heart disease, a TC of 310 mg/dl, and an L DL-C of 212 mg/dl. A total of eight individuals were found with the de fect in the families of the two patients. They had an age- and sex-adj usted TC of 240+/-14 mg/dl and LDL-C of 169+/-10 mg/dl. This compares with eight unaffected family members with age- and sex-adjusted TC of 185+/-12 mg/dl and LDL-C of 124+/-12 mg/dl. In a dual-label fibroblast binding assay, LDL from the eight subjects with the mutation had an a ffinity for the LDL receptor that was 63% that of control LDL. LDL fro m eight unaffected family members had an affinity of 91%. By way of co mparison, LDL from six patients heterozygous for the Arg(3500)-->Gln m utation had an affinity of 36%. The percentage mass ratio of the defec tive Cys(3531) LDL to normal LDL was 59:41, as determined using the mA b MB19 and dynamic laser light scattering. Thus, the defective LDL had accumulated in the plasma of these patients. Using this mass ratio, i t was calculated that the defective Cys(3531) LDL particles bound with 27% of normal affinity. Deduced haplotypes using 10 apoB gene markers showed the Arg(3531)-->Cys alleles to be different in the two kindred s and indicates that the mutations arose independently. The Arg(3531)- ->Cys mutation is the second reported cause of familial ligand-defecti ve apoB.