Familial ligand-defective apolipoprotein B. Identification of a new mutation that decreases LDL receptor binding affinity.

Detection of new ligand-defective mutations of apolipopro- tein B (apoB) will enable identification of sequences in- volved in binding to the LDL receptor. Genomic DNA from patients attending a lipid clinic was screened by singlestrand conformation polymorphism analysis for novel muta- tions in the putative LDL receptor-binding domain of apoB-100. A 46-yr-old woman of Celtic and Native Ameri- can ancestry with primary hypercholesterolemia (total cho- lesterol TC 343 mg/dl; LDL cholesterol LDL-C 241 mg/ dl) and pronounced peripheral vascular disease was found to be heterozygous for a novel Arg3531-Cys mutation, caused by a C-T transition at nucleotide 10800. One unrelated 59yr-old man of Italian ancestry was found with the same mutation after screening 1,560 individuals. He had coronary heart disease, a TC of 310 mg/dl, and an LDL-C of 212 mg/ dl. A total of eight individuals were found with the defect in the families of the two patients. They had an age-and sex-adjusted TC of 24014 mg/dl and LDL-C of 169+10 mg/dl. This compares with eight unaffected family members with age-and sex-adjusted TC of 18512 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 affin- ity for the LDL receptor that was 63% that of control LDL. LDL from eight unaffected family members had an affinity of 91%. By way of comparison, LDL from six patients het- erozygous for the Arg3500-Gln mutation had an affinity of 36%. The percentage mass ratio of the defective Cys3531 LDL to normal LDL was 59:41, as determined using the mAb MB19 and dynamic laser light scattering. Thus, the defec- tive LDL had accumulated in the plasma of these patients. Using this mass ratio, it was calculated that the defective Cys353, LDL particles bound with 27% of normal affinity.