Abnormal regulation of the LDL-R and HMG CoA reductase genes in subjects with familial hypercholesterolemia with the “French Canadian Mutation”

Abstract

Familial hypercholesterolemia (FH) is seen with high frequency in the province of Québec, Canada. A large deletion (>10 kb) of the 5′-end of the low density lipoprotein receptor (LDL-R) gene is the major mutation of the LDL-R in FH subjects in Québec (~ 60% of FH subjects). No mRNA is produced from the allele bearing the mutation, and cellular cholesterol obtained by receptor-mediated endocytosis is under the control of the non-deletion allele. We have previously reported that some patients with the 10-kb deletion (~ 9%) fail to respond to the hydroxymethylglutaryl coenzyme A reductase (HMG CoA reductase) inhibitor class of medications. We studied mRNA levels of the LDL-R and HMG CoA reductase genes in response to the HMG CoA reductase inhibitor lovastatin in a time- and dose-dependent fashion in cultured human skin fibroblasts and we devised an in vitro model to study the response to drug therapy in subjects with FH. We determined mRNA levels by RNase protection assay in skin fibroblasts obtained from controls ( n = 3) and FH subjects with the >10-kb deletion (responders, n = 3; non responders, n = 3; to drug therapy). We measured 125I-LDL binding on skin fibroblasts grown in the presence of lipoprotein-deficient serum with or without 1 μM lovastatin, using 10 μg/mL of 125I-LDL protein. Control subjects exhibited coordinate regulation of the LDL-R and HMG CoA reductase genes in response to lovastatin, 0.1 – 25 μM, for 0–24 h. Correlation coefficients between mRNA levels of both genes were >0.9 in controls and FH subjects. However, by linear regression analysis, the corresponding slopes for the correlation between both genes were 0.98 (controls), 3.36 and 3.63 (FH responders and non-responders), indicating a pattern of dissociated but still coordinate regulation in FH subjects. The magnitude of increase of mRNA levels of the LDL-R gene was approximately five-fold over LPDS in controls, two-fold in FH responders and two-fold in non-responders. Binding studies using 125I-LDL reveal that a control subject and all responders had a 2 – 2.5-fold increase in binding to cell surface receptors but two out of three FH non-responders showed no increase in binding in response to 1 μM lovastatin. The LDL-R and HMG CoA reductase genes are expressed in coordinate regulation in fibroblasts from subjects with FH due to the >10-kb deletion, but with a proportionately greater up-regulation of the HMG CoA reductase gene. Some subjects, with FH caused by the >10-kb deletion of the LDL-R gene, who fail to respond to HMG CoA reductase inhibitors have abnormal LDL receptor binding activity at the cell surface in response to lovastatin in vitro.