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Human Genetics of Lipoprotein Metabolism
Summary: Helen Hobbs is interested in defining the genetic factors responsible for differences in blood cholesterol levels and heart disease. She has discovered genetic defects causing both very high and low plasma levels of cholesterol. Her characterization of these defective genes has provided insights into cholesterol metabolism and identified new drug targets for the treatment of elevated levels of plasma cholesterol, the major risk factor for atherosclerosis and heart disease.
Throughout the course of human evolution, wide fluctuations in the food supply promoted the selection of DNA sequences that enhance the efficient extraction, storage, and utilization of dietary nutrients. With the advent of the industrial age, agricultural practices changed rapidly from subsistence farming to massive-scale food production. Our genome has had insufficient time to adapt to the dietary abundance and physical inactivity of the modern era. Consequently, diseases of dietary excess (e.g., diabetes and atherosclerosis) rather than insufficiency (malnutrition and infections) are the major cause of death and disability in the Western world. The increased intake of dietary cholesterol and saturated fats has had a profound effect on human health. The central theme of my research program is the role of cholesterol and triglyceride uptake and trafficking in human diseases, especially heart disease.
Multiple Factors Contribute to Heart Disease
Atherosclerosis is a complex and heterogeneous disorder resulting from the interplay of genetic susceptibilities and environmental challenges. To better define the genetic and nongenetic factors contributing to coronary heart disease (CHD), we established a population-based study in Dallas (the Dallas Heart Study). More than 3,500 individuals (50 percent African American) were characterized with respect to behavioral, environmental, metabolic, and genetic risk factors for CHD. Imaging studies were performed to provide quantification of atherosclerotic burden, heart size and function, and body fat distribution. Genomic DNA was obtained from each subject, and a comprehensive panel of blood and urinary analytes was measured. The extensive phenotypic database generated from this study has been used to identify new factors that will improve our ability to predict who will get heart disease. To interrogate the genetic and nongenetic factors associated with the progression of CHD, we have invited all study participants to return for a follow-up clinical assessment in 2007.
Cholesterol and Heart Attacks
The most important risk factor for CHD is the plasma level of cholesterol. Cholesterol is transported in the blood in lipid-protein complexes called lipoproteins. Low-density lipoprotein (LDL) is the major cholesterol-carrying lipoprotein, and the incidence of heart disease is related directly to the levels of LDL in the blood. Approximately 50 percent of the differences in plasma levels of LDL-cholesterol (LDL-C) between individuals is due to DNA sequence differences. A major focus of our laboratory is to define the sequence variants that contribute to the differences in plasma levels of LDL, with a goal of identifying new therapeutic approaches and treatments to prevent CHD.
A New Genetic Cause of Low Plasma Levels of LDL-C
We identified a new genetic cause of low plasma levels of LDL-C, which is due to inactivating mutations in proprotein convertase subtilisin/kexin type 9 (PCSK9). Selected amino acid substitutions in PCSK9 had been shown to cause severe hypercholesterolemia. We found that other mutations that disrupt PCSK9 function result in lower plasma levels of LDL-C. Surprisingly, 1 of every 50 African Americans and 1 of every 30 Caucasians has an inactivating mutation in PCSK9.
We used these sequence variations in PCSK9 to address the following question: What would be the effect of having a lower circulating level of LDL-C from the time of birth? In a large, longitudinal study, we compared the rates of CHD over a 15-year period in individuals with inactivating PCSK9 alleles to those with two normal alleles. In the African Americans, the mutant alleles that caused a 28 percent reduction in mean LDL-C level were associated with an 88 percent reduction in CHD. Caucasians with the PCSK9 variant had a 15 percent reduction in LDL-C and a 46 percent reduction in CHD. The magnitude of CHD reduction was much greater than that observed in clinical trials using cholesterol-lowering drugs such as statins, where the 5-year reduction in events is comparable to the reduction in LDL-C level. The dramatic reductions in CHD found in association with the PCSK9 variants were particularly striking since the population studied had a very high atherosclerotic risk factor burden (i.e., hypertension, diabetes, low HDL-C, smoking). These data confirm the importance of LDL levels in the development of heart disease and suggest that the initiation of treatment earlier in life might magnify the benefits of cholesterol-lowering therapy.
Currently, approximately half of the individuals taking cholesterol-lowering drugs do not achieve the target LDL-C level. Inactivation of PCSK9 provides a new therapeutic target for cholesterol-lowering therapy. A current focus of our laboratory is to define how PCSK9 normally inactivates LDL receptor–mediated uptake into the liver, which is the major pathway for clearance of circulating LDL.
A Common Genetic Variation Increases Heart Attack Risk
This year we performed a whole-genome association study, in which we compared the genomes of individuals with early symptomatic CHD to those of older asymptomatic subjects. Those sequence variations that were more frequent in the CHD group were tested in independent populations. We found a highly reproducible association between a 58-kilobase interval on chromosome 9 and CHD. Two other groups simultaneously identified the same result, thus confirming the association. We estimated that individuals with two copies of the "risk" allele have a 40 percent increase in CHD and found that the effect is not due to an association with other CHD risk factors. No known gene resides within the implicated genomic interval. One focus of our current studies is the mechanistic link between the risk allele and CHD.
Genetic Variations Protecting Individuals from Metabolic Risk
Recently we resequenced the coding region of the adipokine ANGPTL4 (angiopoietin-like protein 4) in the Dallas Heart Study (n = 3,500). To examine the relationship between sequence variations in ANGPTL4 and levels of lipids and lipoproteins, we used a strategy we previously employed to show that rare sequence variations cumulatively contribute to some complex traits. We compared the number of sequence variations in ANGPTL4 that are predicted to alter gene function in the extremes of the distributions of various traits. We found an excess of ANGPTL4 sequence variations in individuals with plasma triglyceride levels in the lowest quartile compared to the highest quartile. When we extended our findings to other populations, we showed that genetic variations in ANGPTL4 cause an increase in plasma levels of HDL and a reduction in triglyceride levels. We are now probing the mechanism underlying the improvement in metabolic profiles associated with inactivating mutations in ANGPTL4.
Dietary Cholesterol and Heart Attacks
Plasma LDL-C levels are influenced by the amount of saturated fat, calories, and sterols in the diet. The relative rates of cholesterol absorption and excretion are major determinants of plasma cholesterol levels. Americans consume ~650 mg of sterols each day, of which one-third (predominantly sitosterol and campesterol) is derived from plants and two-thirds (predominantly cholesterol) is from animals. Less than 5 percent of plant sterols and ~40 percent of cholesterol are absorbed from the diet and delivered to the liver. We identified two genes, ABCG5 and ABCG8, that limit the amount of sterols absorbed from the diet and promote the excretion of sterols from the liver into the bile. Mutations in either gene cause the autosomal-recessive disorder sitosterolemia. When patients with sitosterolemia eat a Western diet, cholesterol and plant sterols accumulate in their tissues, resulting in the development of CHD.
Using genetically modified mice, we showed that ABCG5 and ABCG8 protect against the accretion of sterols (both plant sterols and cholesterol). A major role of these transporters is to eliminate plant sterols from organisms. Why is the accumulation of plant sterols so actively avoided by mammals? Our characterization of mice expressing no ABCG5 or ABCG8 showed that certain plant sterols disrupt normal cholesterol homeostasis by activating a nuclear hormone receptor (LXR), thus promoting sterol efflux from cells, and by inhibiting cholesterol biosynthesis, even when tissue levels of cholesterol are low.
We have used both recombinant and native protein to purify and reconstitute sterol transport by ABCG5 and ABCG8, and we are now characterizing the mechanism by which this transporter promotes the movement of sterols across membranes.
Determining the DNA sequence variations that confer susceptibility to cardiovascular disease will enhance our understanding of the underlying processes that contribute to the disease in the population, providing the opportunity to identify new treatment targets, new diagnostic tests, and new therapeutic interventions for patients with heart disease.
Grants from the National Heart, Lung, and Blood Institute and the Donald W. Reynolds Foundation provide partial support for these studies.
Last updated: September 10, 2007
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