 |
Human Genetics of Lipoprotein Metabolism
Summary: Helen Hobbs's research is focused on defining genetic factors that influence susceptibility to heart disease. She has discovered genetic defects that elevate, and others that lower, plasma levels of cholesterol and triglycerides. Her work has provided insights into cholesterol and fat metabolism and identified new drug targets for the prevention of atherosclerotic heart disease. Recently, she has identified genetic variants that predispose to fatty liver disease, a burgeoning medical problem.
Throughout the course of human evolution, wide fluctuations in the food supply promoted the selection of DNA sequences that optimize the 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, the increased intake of dietary cholesterol and saturated fats has had a profound effect on human health. 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. My research program is focused on the identification of the genetic factors that contribute to (and protect from) diseases of dietary excess, especially heart and liver 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) in 2002. 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 quantify 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 factors contributing to progression of CHD, we invited all study participants to return for a follow-up clinical assessment, starting 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-cholesterol (LDL-C) in the blood. Approximately 50 percent of the variation in plasma levels of LDL-C is due to DNA sequence differences. A major focus of our laboratory has been to identify the genes that contribute to the differences in plasma levels of LDL-C, with a goal of identifying new therapeutic approaches to prevent CHD.
First we focused on rare diseases that cause very high plasma levels of LDL-C. We showed that mutations in genes required for the clearance of circulating LDL (ARH or LDLRAP) and for the excretion of cholesterol from the body (ABCG5 and ABCG8) both cause hypercholesterolemia and premature CHD. These studies confirmed that severe hypercholesterolemia, irrespective of the molecular etiology, is sufficient for the development of CHD.
A Genetic Cause of Low Plasma Levels of LDL-C
We identified a new genetic cause of low plasma levels of LDL-C that is due to inactivating mutations in a circulating protein, proprotein convertase subtilisin/kexin type 9 (PCSK9). 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. By screening the families of participants heterozygous for a loss-of-function mutation in PCSK9, we identified a 34-year-old woman with no functional PCSK9 and an LDL-C of only 14 mg/dL. These findings confirmed the central role that PCSK9 plays in LDL metabolism in humans and suggested that PCSK9 may be an excellent, effective (and safe) target for cholesterol-lowering therapy.
Identification of these sequence variations in PCSK9 allowed us to address this question: What would be the effect of having a lower circulating level of LDL-C starting at birth? When we compared the rates of CHD over a 15-year period in those individuals with and without an inactivating mutation in PCSK9, we found that the reduction in CHD in PCSK9 mutation carriers was significantly greater than had previously been observed in clinical trials using cholesterol-lowering drugs such as statins, presumably because the PCSK9 carriers have had lower blood cholesterol levels their entire life, whereas statin users only had lower levels of LDL-C starting in middle age, after the disease is already established. Our findings, taken together with other studies, suggest that initiating lipid-lowering treatment earlier in life, either by changing dietary composition or by taking low doses of cholesterol-reducing medications, would be the optimal strategy to prevent development of heart disease.
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 inactivates uptake of LDL by the liver, which is the major pathway for clearance of circulating LDL.
A Common Genetic Variation Increases Heart Attack Risk
We compared the genomes of individuals with early symptomatic CHD to those of older asymptomatic subjects and identified a highly reproducible association between a 58-kilobase interval on chromosome 9 and CHD. Individuals with two copies of the "risk" allele have a 40 percent increase in CHD that is not explained by any of the known risk factors. Other investigators have now shown that genetic variation in the same genomic interval also predisposes to aortic and cerebral aneurysms. The implicated sequences are in a region of the genome that does not code for any proteins, but does code for a long noncoding RNA. We are probing the mechanistic link between the risk allele and CHD.
Rare Genetic Defects Cumulatively Contribute to Complex Disorders, Such as Metabolic Syndrome
The obesity epidemic has resulted in a dramatic increase in the prevalence of metabolic risk factors for diabetes and heart disease, including insulin resistance, high plasma levels of triglycerides, low plasma levels of HDL-C, fatty liver disease, and hypertension. We have used genetics to identify new genes that confer sensitivity (and resistance) to the development of metabolic risk for heart disease. We have taken two different approaches to identify genetic variants that contribute to metabolic risk. First, we compared the number of mutations in selected genes in the extremes of the distribution of various metabolic traits, including plasma levels of LDL-C, HDL-C, and triglycerides. For many of the genes we analyzed, we found an excess of rare and low-frequency loss-of-function mutations that contribute to these traits, and we have shown that a significant proportion of the general population have mutations that result in a complete loss of function. Thus, mutations with large effects on protein function are not rare in the general population, and these mutations collectively contribute significantly to complex traits. Characterization of individuals with such mutations provides the opportunity to assess directly the role of a gene in humans.
Gene Defect in Fatty Liver Disease
Another approach we have taken to identify genes that contribute to metabolic risk is to perform an unbiased screen of the genome for variants that change the amino acid sequence of proteins. Previously, we showed that one-third of the subjects in the Dallas Heart Study had an excess of triglyceride stored in lipid droplets in the cytoplasm of hepatocytes (hepatic steatosis), with Hispanics having the highest and African Americans the lowest prevalence of fatty liver disease. We examined the association of more than 9,000 sequence variations that change the amino acid of a protein with hepatic fat content. A variant in a member of the patatin-like phospholipase domain–containing family of proteins (PNPLA), PNPLA3, was associated with hepatic fat content. This variant was most frequent in Hispanics and least frequent in African Americans. We found another sequence variation in PNPLA3 that was most common in African Americans and was associated with reduced hepatic fat content. These sequence variations were not associated with body weight, insulin sensitivity, or plasma lipid levels. Thus, the differences in hepatic fat content were not secondary to the major risk factors for hepatic steatosis.
These two sequence variations explained 70 percent of the variation in hepatic fat content between the Hispanics, African Americans, and European Americans.
PNPLA3 has a domain at its N terminus that resembles patanin, an enzyme with acylhydrolase activity that was originally discovered in the potato tuber. Structural modeling of PNPLA3 suggests that the mutation associated with hepatic steatosis masks the active site of the enzyme. Individuals with the at-risk allele in PNPLA3 also had evidence of liver injury. We are now attempting to determine if the sequence variations we have identified contribute to the development of hepatitis and cirrhosis in individuals with hepatic steatosis.
Determining the DNA sequence variations that confer susceptibility to metabolic and cardiovascular disease will enhance our understanding of the underlying processes that contribute to these diseases in the population, providing the opportunity to identify new treatment targets, new diagnostic tests, and new therapeutic interventions for patients with metabolic risk factors that contribute to 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 March 09, 2009
|
|
|
|
