IDENTIFICATION OF APOLIPOPROTEIN B100 POLYMORPHISMS THAT AFFECT LOW-DENSITY-LIPOPROTEIN METABOLISM - DESCRIPTION OF A NEW APPROACH INVOLVING MONOCLONAL-ANTIBODIES AND DYNAMIC LIGHT-SCATTERING

Rare mutations in apolipoprotein B (apoB) can cause defective binding of low-density lipoproteins (LDLs) to the LDL receptor, leading to ele vated plasma cholesterol levels and premature atherosclerosis. This co mmunication describes a novel approach to study the effects of apoB mu tations on LDL metabolism. Monoclonal antibody MB19 identifies a commo n polymorphism in apoB, an Ile/Thr substitution at residue 71, by bind ing with a 60-fold higher affinity to apoB (Ile(71))-containing LDL. B ecause each LDL contains a single apoB, a maximum of two LDLs may be b ound by the bivalent monoclonal antibody. Thus, at the appropriate con centration, an equivalent amount of MB19 will promote substantial dime r formation of LDL containing the strongly binding apoB(Ile(71)), but little dimer formation of LDL containing the weakly binding apoB(Thr(7 1)). For LDL isolated from heterozygous individuals, the amount of dim er formed, determined by dynamic light scattering, yields an estimate of the allelic ratio of the two forms of LDL. For such individuals, no t only the effect of the polymorphism recognized by MB19 but also the effects of other polymorphisms on the LDL allelic ratio can be determi ned. Examination of six normolipemic MB19 heterozygotes gave percent a llelic ratios between 48:52 and 51:49 tight:weak-binding LDL, not sign ificantly different from a 50:50 ratio. These individuals were also he terozygous for six common apoB polymorphisms, allowing calculation of the odds that each of these polymorphisms caused significant alteratio ns in lipid levels. In contrast, the rare mutation at residue 3500 cau sing defective binding to the LDL receptor and familial defective apoB 100 (FDB) resulted in substantial changes (26:74 and 13:87) in LDL all elic ratio in both of two FDB individuals examined.