The atherogenic lipoprotein phenotype is not caused by a mutation in the coding region of the low density lipoprotein receptor gene

Genetics of LDL particle heterogeneity

Substantial evidence exists suggesting that small, dense LDL particles are associated with an increased risk of coronary heart disease. This disease-related risk factor is recognized to be under both genetic and environmental influences. Several studies have been conducted to elucidate the genetic architecture underlying this trait, and a review of this literature seems timely. The methods and strategies used to determine its genetic component and to identify the genes have greatly changed throughout the years owing to the progress made in genetic epidemiology and the influence of the Human Genome Project. Heritability studies, complex segregation analyses, candidate gene linkage and association studies, genome-wide linkage scans, and animal models are all part of the arsenal to determine the susceptibility genes. The compilation of these studies clearly revealed the complex genetic nature of LDL particles. This work is an attempt to summarize the growing evidence of genetic control on LDL particle heterogeneity with the aim of providing a concise overview in one read.

Genetic evaluation for coronary artery disease

There is substantial evidence that genetic factors contribute to coronary artery disease (CAD). Currently, family history collection and interpretation is the best method for identifying individuals with genetic susceptibility to CAD. Family history reflects not only genetic susceptibility, but also interactions between genetic, environmental, cultural, and behavioral factors. Stratification of familial risk into different risk categories (e.g., average, moderate, or high) is possible by considering the number of relatives affected with CAD and their degree of relationship, the ages of CAD onset, the occurrence of associated conditions, and the gender of affected relatives. Familial risk stratification should improve standard CAD risk assessment methods and treatment guidelines (e.g., Framingham CAD risk prediction score and Adult Treatment Panel III guidelines). Individuals with an increased familial risk for CAD should be targeted for aggressive risk factor modification. Individuals with a high familial risk might also benefit from early detection strategies and biochemical and DNA-based testing, which can further refine risk for CAD. In addition, individuals with the highest familial risk might have mendelian disorders associated with a large magnitude of risk for premature CAD. In these cases, referral for genetic evaluation should be considered, including pedigree analysis, risk assessment, genetic counseling and education, discussion of available genetic tests, and recommendations for risk-appropriate screening and preventive interventions. Research is needed to assess the feasibility, clinical validity, clinical utility, and ethical, legal, and social issues of an approach that uses familial risk stratification and genetic evaluation to enhance CAD prevention efforts.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Characterization and in silico mapping of a novel murine zinc finger transcription factor

Transcription factors play important roles in development and homeostasis. We have completed an embryonic stem cell-based neural differentiation screen, which was carried out with a view to isolating early regulators of neurogenesis. Fifty eight of the expressed sequence tags isolated from this screen represent known transcription factors or sequences containing transcription factor motifs. We have determined the full-length sequence of a novel mouse zinc finger-containing gene (ZFEND; also known as Mus musculus zinc finger protein 358 (Zfp358)) that was identified from this screen. ZFEND has 87% nucleotide and 86% amino acid identity to a previously identified human cDNA, FLJ10390, which is moderately similar to zinc finger protein 135. Northern blotting and RPAs demonstrate highest expression of ZFEND during mid-late mouse embryogenesis. Expression is also observed in several adult tissues with highest expression in heart, brain, and liver. Whole-mount in situ hybridization studies reveal apparent ubiquitous expression of ZFEND during mid-gestation stages (embryonic days 11.5, 12.5), while sections of whole-mount embryos reveal much higher expression levels in the neural folds during neural tube closure and at the boundary between the forelimb buds and the body wall. Bioinformatic analysis maps ZFEND to mouse chromosome 8pter, while FLJ10390 resides on 19p13.3-p13.2, a gene-rich region to which a number of disorders have been mapped. More precise mapping indicates that the involvement of FLJ10390 in atherogenic lipoprotein phenotype, familial febrile convulsions 2, and psoriasis susceptibility cannot be ruled out.

cDNA and protein sequence, genomic organization, and analysis of cis regulatory elements of mouse and human PLVAP genes

PV-1 is a novel protein component of the endothelial fenestral and stomatal diaphragms. PV-1 cDNA and protein sequences are highly conserved across species. The conserved extracellular domain features found in rat, mouse, and human PV-1 protein are four N-glycosylation sites, two coiled-coil domains, a proline-rich region, and even cysteine spacing. No consensus site in the intracellular domain was found. Northern blotting of mouse and human tissues is in agreement with and expands those performed in rat and correlated well with the postulated presence of PV-1 in the endothelial diaphragms. The genomic organization of the human and mouse genes (HGMW-approved symbol PLVAP) has been determined, and the analysis of their 5' flanking regions has found a highly conserved classical TATA-driven promoter that shows several transcription factor consensus binding sites. Radiation hybrid panel mapping has localized the human and mouse PLVAP genes to chromosomes 19p13.2 and 8B3-C1, respectively.

Atherogenic lipoprotein phenotype and diet-gene interactions

Studies employing analysis of LDL subclasses have demonstrated heterogeneity of the LDL response to low fat, high carbohydrate diets in healthy nonobese subjects. In individuals with a genetically influenced atherogenic lipoprotein phenotype, characterized by a predominance of small dense LDL (LDL subclass pattern B), lowering of plasma LDL cholesterol levels by diets with < or =24% fat has been found to represent a reduction in numbers of circulating mid-sized and small LDL particles, and hence an expected lowering of cardiovascular disease risk. In contrast, in the majority of healthy individuals with larger LDL (pattern A, found in approximately 70% of men and a larger percentage of women), a significant proportion of the low fat diet-induced reduction in plasma LDL cholesterol is made by depletion of the cholesterol content of LDL particles. This change in LDL composition is accompanied by a shift from larger to smaller LDL particle diameters. Moreover, with progressive reduction of dietary fat and isocaloric substitution of carbohydrate, an increasing number of subjects with pattern A convert to the pattern B phenotype. Studies in families have indicated that susceptibility to induction of pattern B by low fat diets is under genetic influence. Thus, diet-gene interactions affecting LDL subclass patterns may contribute to substantial interindividual variability in the effects of low fat diets on coronary heart disease risk.

Inheritance of LDL peak particle diameter: results from a segregation analysis in Israeli families

Genetic and environmental determinants of LDL peak particle diameter (LDL-PPD) were investigated in a sample of 80 kindreds residing in kibbutz settlements in Israel. The sample included 182 males and 191 females ages 15-93 years. LDL-PPD levels were first adjusted for variability in sex and age. Commingling analysis demonstrated that a mixture of two normal distributions fit the adjusted LDL-PPD levels better than did a single normal distribution. Complex segregation analysis was first applied to these sex and age adjusted data but was not conclusive. However, when the regression model for sex and age allowed coefficients to be ousiotype (class) specific, the mixed environmental model was rejected while a major Mendelian model was not. These results suggest that the particular genotypes determined by the major gene, which are associated with different phenotypic variances, are likely to be more realistic, and that this analytic approach can contribute to improving our understanding of the genetics of LDL particle size.

Linkage of low-density lipoprotein size to the lipoprotein lipase gene in heterozygous lipoprotein lipase deficiency

Small low-density lipoprotein (LDL) particles are a genetically influenced coronary disease risk factor. Lipoprotein lipase (LpL) is a rate-limiting enzyme in the formation of LDL particles. The current study examined genetic linkage of LDL particle size to the LpL gene in five families with structural mutations in the LpL gene. LDL particle size was smaller among the heterozygous subjects, compared with controls. Among heterozygous subjects, 44% were classified as affected by LDL subclass phenotype B, compared with 8% of normal family members. Plasma triglyceride levels were significantly higher, and high-density lipoprotein cholesterol (HDL-C) levels were lower, in heterozygous subjects, compared with normal subjects, after age and sex adjustment. A highly significant LOD score of 6.24 at straight theta=0 was obtained for linkage of LDL particle size to the LpL gene, after adjustment of LDL particle size for within-genotype variance resulting from triglyceride and HDL-C. Failure to adjust for this variance led to only a modest positive LOD score of 1.54 at straight theta=0. Classifying small LDL particles as a qualitative trait (LDL subclass phenotype B) provided only suggestive evidence for linkage to the LpL gene (LOD=1. 65 at straight theta=0). Thus, use of the quantitative trait adjusted for within-genotype variance, resulting from physiologic covariates, was crucial for detection of significant evidence of linkage in this study. These results indicate that heterozygous LpL deficiency may be one cause of small LDL particles and may provide a potential mechanism for the increase in coronary disease seen in heterozygous LpL deficiency. This study also demonstrates a successful strategy of genotypic specific adjustment of complex traits in mapping a quantitative trait locus.

Linkage analysis of candidate genes and the small, dense low-density lipoprotein phenotype

There is accumulating evidence for the importance of small, dense low-density lipoprotein (LDL), the defining feature of the atherogenic lipoprotein phenotype, as a risk factor for coronary heart disease. Although both family studies and twin studies have demonstrated genetic influences on this phenotype, the specific gene(s) involved remain to be identified. The purpose of this study was to determine whether there was evidence for genetic linkage between small, dense LDL (LDL subclass phenotype B), as determined by gradient gel electrophoresis, and selected candidate genes known to be involved in lipid metabolism. The linkage analyses were based on a sample of 19 families, including 142 individual family members, using a lod score linkage analysis approach. Nine candidate genes were examined, including loci for manganese superoxide dismutase (Mn SOD2), apolipoproteins CIII, AII, and apo CII, lipoprotein lipase, hepatic lipase, microsomal triglyceride transport protein, the insulin receptor and the LDL receptor. The analyses did not provide significant evidence for genetic linkage between markers for any of these genes and LDL subclass phenotype B, nor did it confirm previous reports of linkage between the LDL receptor gene and LDL subclass phenotype B. Using three closely linked markers for the Mn SOD2 locus excluded close linkage between this candidate gene region and LDL subclass phenotype B. These findings demonstrate the complexity of genetically mapping risk factor phenotypes, and emphasize the necessity of identifying new genetic loci, other than known candidate genes, involved in susceptibility to atherosclerosis.

Families with familial combined hyperlipidemia and families enriched for coronary artery disease share genetic determinants for the atherogenic lipoprotein phenotype

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.