Unexpected consequences of deletion of the first two repeats of the ligand-binding domain from the low density lipoprotein receptor. Evidence from a human mutation.

Heterozygosity for a 5-kilobase (kb) deletion of the first two ligand-binding repeats (exons 2 and 3) of the low density lipoprotein (LDL) receptor (R) gene (LDL-R delta 5kb) confers familial hypercholesterolemia (FH). The FH phenotype is unexpected based on previous site-directed mutagenesis showing that deletion of exons 2 and 3 resulted in little or no defect in LDL-R activity. In the present study, we took unique advantage of the ability to distinguish the LDL-R delta 5kb from the normal receptor on the basis of size, in order to resolve this apparent discrepancy. Fibroblasts from heterozygotes for the LDL-R delta 5kb displayed 50% of normal capacity to bind LDL and beta-VLDL, apparently due to lower receptor number. Cellular mRNA for the delta 5kb allele was at least as abundant as that for the normal allele. Immunoblotting and cell binding assays with anti-LDL-R antibody IgG-4A4 demonstrated normal synthesis and transport of the delta 5kb receptor. Ligand blotting demonstrated that the delta 5kb receptor displayed minimal or no ability to bind LDL or beta-VLDL. Thus, in contrast to transfected cell lines, in human fibroblasts, the first two cysteine rich repeats of the LDL-R appear functionally necessary. These characteristics of the LDL-R delta 5kb in human fibroblasts explain the in vivo phenotype of carriers.