Faculty Bibliography

Evidence linking mutations in ATP-binding-cassette transporter gene 1 (ABC1) to Tangier disease suggests it functions in the active transport of free cholesterol out of cells. Since its mRNA level is regulated in response to cellular cholesterol stores it is of interest to explore its promoter response elements, and to investigate polymorphisms for their contributions to the prevalence of low levels of HDL in the population that promotes premature coronary heart disease. Investigation of the 5' end of the gene by 5' RACE analysis revealed 455 nucleotides additional to published sequences, and predicts another 60 amino acid N-terminal residues, resulting in a 2261-residue protein. Protein sequence analysis predicts a membrane-spanning region and possible signal peptide. The 5' flanking region was located by a Human Research Project BLAST search. This region contains regulatory elements that potentially control ABC1 gene expression. In addition to numerous SP1 binding sites there are four putative sterol regulatory elements (SREs). Our studies uncovered three single nucleotide substitution polymorphisms, one in the promoter region and two in the 5' untranslated region (5'UTR), plus an insertion/deletion polymorphism.

Familial combined hyperlipidemia (FCH) is a common genetic lipid disorder with a frequency of 1-2% in the population. In addition to the hypercholesterolemia and/or hypertriglyceridemia that affected individuals exhibit, small, dense LDL particles and decreased HDL-cholesterol levels are traits frequently associated with FCH. Recently, we reported that families with FCH and families enriched for coronary artery disease (CAD) share genetic determinants for the atherogenic lipoprotein phenotype (ALP), a profile presenting with small, dense LDL particles, decreased HDL-cholesterol levels, and increased triglyceride levels. Other studies in normolipidemic populations have shown that the hepatic lipase (HL) gene is linked to HDL-cholesterol levels and that a polymorphism within the HL promoter (-514C-->T) is associated with increased HDL-cholesterol levels as well as larger, more buoyant LDL particles. In the present study, we tested whether the HL gene locus also contributes to ALP in a series of Dutch FCH families using nonparametric sibpair linkage analysis and association analysis. Evidence for linkage of LDL particle size (P < 0.019), HDL-cholesterol (P < 0.003), and triglyceride levels (P < 0.026) to the HL gene locus was observed. A genome scan in a subset of these families exhibited evidence for linkage of PPD (LOD = 2.2) and HDL-cholesterol levels (LOD = 1.2) to the HL gene locus as well. The -514C-->T promoter polymorphism was significantly associated (P < 0.0001) with higher HDL-cholesterol levels in the unrelated males of this population, but not in unrelated females. No association was observed between the polymorphism and LDL particle size or triglyceride levels. Our results provide support that ALP is a multigenic trait and suggest that the relationship between small, dense LDL particles, HDL-cholesterol, and triglyceride levels in FCH families is due, in part, to common genetic factors.

Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by elevated levels of plasma cholesterol and triglycerides that is present in 10% to 20% of patients with premature coronary artery disease. To study the pathophysiological basis and genetics of FCHL, we previously reported recruitment of 18 large families. We now report linkage studies of 14 candidate genes selected for their potential involvement in the aspects of lipid and lipoprotein metabolism that are altered in FCHL. We used highly polymorphic markers linked to the candidate genes, and these markers were analyzed using several complementary, nonparametric statistical allele-sharing linkage methodologies. This current sample has been extended over the one in which we identified an association with the apolipoprotein (apo) AI-CIII-AIV gene cluster. We observed evidence for linkage of this region and FCHL (P<0.001), providing additional support for its involvement in FCHL. We also identified a new locus showing significant evidence of linkage to the disorder: the lecithin:cholesterol acyltransferase (LCAT) locus (P<0.0006) on chromosome 16. In addition, analysis of the manganese superoxide dismutase locus on chromosome 6 revealed a suggestive linkage result in this sample (P<0.006). Quantitative traits related to FCHL also provided some evidence of linkage to these regions. No evidence of linkage to the lipoprotein lipase gene, the microsomal triglyceride transfer protein gene, or several other genes involved in lipid metabolism was observed. The data suggest that the lecithin:cholesterol acyltransferase and apolipoprotein AI-CIII-AIV loci may act as modifying genes contributing to the expression of FCHL.

Familial combined hyperlipidemia (FCHL) is a common familial lipid disorder characterized by a variable pattern of elevated levels of plasma cholesterol and/or triglycerides. It is present in 10%-20% of patients with premature coronary heart disease. The genetic etiology of the disease, including the number of genes involved and the magnitude of their effects, is unknown. Using a subset of 35 Dutch families ascertained for FCHL, we screened the genome, with a panel of 399 genetic markers, for chromosomal regions linked to genes contributing to FCHL. The results were analyzed by use of parametric-linkage methods in a two-stage study design. Four loci, on chromosomes 2p, 11p, 16q, and 19q, exhibited suggestive evidence for linkage with FCHL (LOD scores of 1.3-2.6). Markers within each of these regions were then examined in the original sample and in additional Dutch families with FCHL. The locus on chromosome 2 failed to show evidence for linkage, and the loci on chromosome 16q and 19q yielded only equivocal or suggestive evidence for linkage. However, one locus, near marker D11S1324 on the short arm of human chromosome 11, continued to show evidence for linkage with FCHL, in the second stage of this design. This region does not contain any strong candidate genes. These results provide evidence for a candidate chromosomal region for FCHL and support the concept that FCHL is complex and heterogeneous.

Familial combined hyperlipidemia (FCHL) is a complex genetic disorder of unknown etiology. Recently, 'modifier' genes of the FCHL phenotype, such as the apolipoprotein AI-CIII-AIV gene cluster and LPL, have been identified in several populations. A 'major' gene for FCHL has been identified in a Finnish isolate which maps to a region syntenic to murine chromosome 3 where a locus for combined hyperlipidemia has been identified. We review these and other recent studies which indicate that FCHL is genetically heterogeneous.

Small, dense LDL particles consistently have been associated with hypertriglyceridemia, premature coronary artery disease (CAD), and familial combined hyperlipidemia (FCH). Previously, we have observed linkage of LDL particle size with four separate candidate-gene loci in a study of families enriched for CAD. These loci contain the genes for manganese superoxide dismutase (MnSOD), on chromosome 6q; for apolipoprotein AI-CIII-AIV, on chromosome 11q; for cholesteryl ester transfer protein (CETP) and lecithin:cholesterol acyltransferase (LCAT), on chromosome 16q; and for the LDL receptor (LDLR), on chromosome 19p. We have now tested whether these loci also contribute to LDL particle size in families ascertained for FCH. The members of 18 families (481 individuals) were typed for genetic markers at the four loci, and linkage to LDL particle size was assessed by nonparametric sib-pair linkage analysis. The presence of small, dense LDL (pattern B) was much more frequent in the FCH probands (39%) than in the spouse controls (4%). Evidence for linkage was observed at the MnSOD (P=.02), CETP/LCAT (P=.03), and apolipoprotein AI-CIII-AIV loci (P=.005) but not at the LDLR locus. We conclude that there is a genetically based association between FCH and small, dense LDL and that the genetic determinants for LDL particle size are shared, at least in part, among FCH families and the more general population at risk for CAD.

The relationship between mitochondrial genotype and clinical phenotype is complicated in most instances by the heteroplasmic nature of pathogenic mitochondrial mutations. We have previously shown that maternally inherited hearing loss in a large Arab-Israeli kindred is due to the homoplasmic A1555G mutation in the mitochondrial 12S ribosomal RNA gene [Prezant et al., 1993: Nat Genet 4:289-294]. Family members with this mutation have phenotypes ranging from profound hearing loss to completely normal hearing, and we have shown that there is genetic and biochemical evidence for nuclear gene involvement in this family [Bu et al., 1993: Genet Epidemiol 9:27-44; Guan et al., 1996: Hum Mol Genet 5:963-971]. To identify such a nuclear locus, two candidate genes were excluded through linkage analysis and sequencing, and a genome-wide linkage search in family members who all have the identical homoplasmic mitochondrial mutation, but differ in their hearing status, was performed. In two stages a total of 560 polymorphic genetic markers was genotyped, and the data were analyzed under model-dependent and model-free assumptions. No chromosomal region was identified as a major contributor to the phenotypic expression of the mitochondrial mutation. Thus, in this simplified paradigm of a homoplasmic mitochondrial mutation in a single kindred who all live in the similar environment of a small village, the penetrance of the mitochondrial mutation appears to depend on the interaction of multiple nuclear genes.