Genetics of Low Density Lipoprotein Receptor Mutations in Fibroblasts of Patients with Homozygous Familial Hypercholesterolemia

Abstract

We have analyzed the low density lipoprotein (LDL) receptor activities of fibroblasts from 4 normal subjects, 5 patients with heterozygous familial hypercholesterolemia (FH) and 9 patients with homozygous FH.
In the cells from 4 normal subjects, the receptor binding, internalization and degradation of ¹²⁵I-LDL were 44±3ng/mg protein (mean±S. E. M.), 386±32ng/mg protein and 1335±214ng/mg protein/6hr, respectively. The cells from 5 heterozygotes showed about 46% of the normal activities of receptor. In the cells from 3 homozygotes, the receptor binding, internalization and degradation of ¹²⁵I-LDL were 0.5±0.3ng/mg protein, 11±5ng/mg protein and 5±3ng/mg protein/6hr, respectively. These homozygotes were thought as receptor-negative type. In the cells from other 6 homozygotes, the receptor binding, internalization and degradation of ¹²⁵I-LDL were 5±1ng/mg protein, 33±4ng/mg protein and 66±20ng/mg protein/6hr, respectively. These homozygotes were thought as receptor-defective type. In these 9 homozygotes, three pairs of siblings were included. One pairs of siblings were classified as receptor-negative and two pairs of siblings were classified as receptor-defective. The receptor-negative phenotypes and receptor-defective phenotypes bled true in individual families.
ML-236B, competitive inhibitor of HMG-CoA reductase, completely inhibited the incorporation of [¹⁴C]acetate into digitonin-precipitable sterols in fibroblasts from normal subjects and heterozygous and homozygous patients with FH with the concentration of 0.5μg/ml. However, at 0.05μg/ml of ML-236B sterol synthesis in fibroblasts from homozygotes was not completely suppressed in contrast to normal and heterozygous cells. Moreover, after preincubation with 0.05μg/ml of ML-236B for 24hr in medium containing lipoproteins sterol synthesis in the cells from receptor-negative homozygote showed 75% of the initial activity compared with that of 25% without preincubation. In the cells from a normal subject and a heterozygote, sterol synthesis was inhibited even after preincubation. These results suggest that 1) the inhibitory effect of ML-236B is overcome in homozygote cells by their high intracellular levels of HMG-CoA reductase and 2) that a higher dose of ML-236B may be required to lower serum cholesterol levels in FH homozygotes than in heterozygotes.