Eran Leitersdorf

Common low-density lipoprotein receptor mutations in the French Canadian population.

Familial hypercholesterolemia (FH) has a frequency of 0.2% in most populations of the world. In selected populations such as the Afrikaners in South Africa, the Christian Lebanese, and the French Canadians, the disease is more frequent due to the founder effect. Previous studies demonstrated that a single mutation at the LDL receptor locus, the so-called French Canadian deletion, makes up 60% of the mutant genes responsible for FH in the French Canadian population. In this study, efforts were directed to determine if there were other common LDL receptor mutations in this population. Three missense mutations were identified and each mutation was reproduced and expressed in vitro. Two of the three mutations result in the production of an LDL receptor protein that is not processed to its mature form at a normal rate. Molecular assays were developed to detect the mutations directly, and the LDL receptor genes of 130 French Canadian FH heterozygotes were screened for the presence of the three missense mutations as well as two deletions. LDL receptor mutations were detected in 76% of individuals and 14% had one of the three missense mutations.

Disruption of the Sterol 27-Hydroxylase Gene in Mice Results in Hepatomegaly and Hypertriglyceridemia REVERSAL BY CHOLIC ACID FEEDING

Sterol 27-hydroxylase (CYP27) participates in the conversion of cholesterol to bile acids. We examined lipid metabolism in mice lacking the Cyp27 gene. On normal rodent chow,Cyp27 −/− mice have 40% larger livers, 45% larger adrenals, 2-fold higher hepatic and plasma triacylglycerol concentrations, a 70% higher rate of hepatic fatty acid synthesis, and a 70% increase in the ratio of oleic to stearic acid in the liverversus Cyp27 +/+ controls. InCyp27 −/− mice, cholesterol 7α-hydroxylase activity is increased 5-fold, but bile acid synthesis and pool size are 47 and 27%, respectively, of those in Cyp27 +/+mice. Intestinal cholesterol absorption decreases from 54 to 4% in knockout mice, while fecal neutral sterol excretion increases 2.5-fold. A compensatory 2.5-fold increase in whole body cholesterol synthesis occurs in Cyp27 −/− mice, principally in liver, adrenal, small intestine, lung, and spleen. The mRNA for the cholesterogenic transcription factor sterol regulatory element-binding protein-2 (SREBP-2) and mRNAs for SREBP-2-regulated cholesterol biosynthetic genes are elevated in livers of mutant mice. In addition, the mRNAs encoding the lipogenic transcription factor SREBP-1 and SREBP-1-regulated monounsaturated fatty acid biosynthetic enzymes are also increased. Hepatic synthesis of fatty acids and accumulation of triacylglycerols increases in Cyp27 −/− mice and is associated with hypertriglyceridemia. Cholic acid feeding reverses hepatomegaly and hypertriglyceridemia but not adrenomegaly inCyp27 −/− mice. These studies confirm the importance of CYP27 in bile acid synthesis and they reveal an unexpected function of the enzyme in triacylglycerol metabolism.

Identification of bile acid precursors as endogenous ligands for the nuclear xenobiotic pregnane X receptor

Sterol 27-hydroxylase (CYP27A1) is required for bile acid synthesis by both the classical and alternate pathways. Cyp27a1−/− mice exhibit a dramatic increase in the activity of cytochrome P450 3A (CYP3A), which catalyzes side-chain hydroxylations of bile acid intermediates, thereby facilitating their excretion in the bile and urine. We examine the role of the nuclear xenobiotic receptor PXR (pregnane X receptor) in this process. We demonstrate that expression of Cyp3a11 and other established PXR target genes is increased in the Cyp27a1−/− mice. WhenCyp27a1−/− mice are fed a diet containing either cholic acid or chenodeoxycholic acid, expression of CYP7A1, which catalyzes the rate-limiting step in bile acid biosynthesis, is strongly suppressed. In parallel, the induction of Cyp3a11 observed in these mice is reversed, suggesting that bile acid intermediates serve as PXR activators. In support of this hypothesis, three potentially toxic sterols (7α-hydroxy-4-cholesten-3-one, 5β-cholestan-3α,7α,12α-triol, and 4-cholesten-3-one), including two that are known to accumulate in Cyp27a1−/− mice, are efficacious activators of mouse PXR. All three compounds are more potent activators of mouse PXR than of human PXR, which may explain in part why humans who lack functional CYP27A1 do not display a corresponding increase in CYP3A activity and are stricken with the disease cerebrotendinous xanthomatosis. Taken together, these results reveal the existence of a feedforward regulatory loop by which potentially toxic bile acid intermediates activate PXR and induce their own metabolism. In addition, this study demonstrates that animal models with alterations in gene expression can be used to identify endogenous ligands for orphan nuclear receptors.

Two common low density lipoprotein receptor gene mutations cause familial hypercholesterolemia in Afrikaners.

Familial hypercholesterolemia (FH), an autosomal dominant disease caused by mutations in the LDL receptor gene, is five times more frequent in the Afrikaner population of South Africa than it is in the population of the United States and Europe. It has been proposed that the high frequency is due to a founder effect. In this paper, we characterized 24 mutant LDL receptor alleles from 12 Afrikaner individuals homozygous for FH. We identified two mutations that together makeup greater than 95% of the mutant LDL receptor genes represented in our sample. Both mutations were basepair substitutions that result in single-amino acid changes. Each mutation can be detected readily with the polymerase chain reaction and restriction analysis. The finding of two common LDL receptor mutations in the Afrikaner FH homozygotes predicts that these mutations will predominate in the Afrikaner population and that the high frequency of FH is due to a founder effect. The increased incidence of ischemic heart disease in the Afrikaner population may in part be due to the high frequency of these two mutations in the LDL receptor gene.

Evidence for a dominant gene that suppresses hypercholesterolemia in a family with defective low density lipoprotein receptors.

This paper describes an unusual kindred with familial hypercholesterolemia in which one-third of the relatives with a mutant LDL receptor gene have normal plasma cholesterol concentrations. The proband, a 9-yr-old boy with a plasma cholesterol value greater than 500 mg/dl, is homozygous for a point mutation that changes Ser156 to Leu in the LDL receptor. This substitution in the fourth repeat of the ligand binding domain slows the transport of the protein to the cell surface. The defective receptor cannot bind LDL, which contains apo B-100, but it does bind beta-migrating VLDL, which contains apo E in addition to apo B-100. Although the mother is heterozygous for this mutation, her LDL-cholesterol concentration is consistently in the 28th percentile for the population. Through direct examination of genomic DNA, we identified the mutant gene in heterozygous form in 17 of the mothers relatives, five of whom had normal LDL-cholesterol values. The pedigree was consistent with dominant transmission of a single gene that ameliorates or suppresses the hypercholesterolemic effect of the LDL receptor mutation. Through linkage analysis, we excluded the possibility that this suppressor gene was an allele at the LDL receptor locus. We also excluded the genes for the two ligands for the LDL receptor, apo B-100 and apo E. The existence of this putative suppressor gene may explain the occasional observation of normal LDL-cholesterol concentrations in heterozygotes for LDL receptor mutations.

Multiple crm- mutations in familial hypercholesterolemia. Evidence for 13 alleles, including four deletions.

The low density lipoprotein (LDL) receptors in fibroblasts from 132 subjects with the clinical syndrome of homozygous familial hypercholesterolemia were analyzed by immunoprecipitation with an anti-LDL receptor monoclonal antibody. 16 of the 132 cell strains (12%) synthesized no immunodetectable LDL receptor protein, indicating the presence of two mutant genes that failed to produce cross-reacting material (crm- mutations). DNA and mRNA from 15 of the 16 crm- patients, representing 30 crm- genes, were available for further study. Haplotype analysis based on 10 restriction fragment length polymorphisms (RFLPs) suggested that the 30 crm- genes represent 13 mutant alleles. Four of the alleles produced no mRNA. Three of these four mRNA- alleles had large deletions ranging from 6 to 20 kb that eliminated the promoter region of the gene. The fourth mRNA- allele did not contain any deletion or alteration in the promoter sequence; the reason for the mRNA- phenotype was not apparent. Nine alleles were positive for mRNAs, of which three encoded mRNAs of abnormal size. One of the abnormal mRNAs was produced by a gene harboring a deletion, and another was produced by a gene with a complex rearrangement. The third abnormal-sized mRNA (3.1 kb larger than normal) was produced by an allele that had no detectable alterations as judged by Southern blotting. The other six mRNA+ alleles appeared normal by Southern blotting and produced normal-sized mRNA but no receptor protein. The current studies demonstrate that mRNA analysis coupled with haplotype determination by Southern blot analysis can be used to classify crm- mutations at a genetic locus where multiple alleles exist.