Sequence Variation in the Apoa1 and Apoa4 Genes and Their Relationship with Plasma Hdl-Cholesterol Levels
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SEQUENCE VARIATION IN THE APOA1 AND APOA4 GENES AND THEIR RELATIONSHIP WITH PLASMA HDL-CHOLESTEROL LEVELS by Sarah Elizabeth Hill B.S., Arizona State University, 2006 Submitted to the Graduate Faculty of the Department of Human Genetics, Genetic Counseling Graduate School of Public Health in partial fulfillment of the requirements for the degree of Master of Science University of Pittsburgh 2009 UNIVERSITY OF PITTSBURGH GRADUATE SCHOOL OF PUBLIC HEALTH This thesis was presented by Sarah Elizabeth Hill It was defended on April 1, 2009 and approved by Thesis Advisor: M. Ilyas Kamboh, Ph.D. Professor and Department Chair Department of Human Genetics Graduate School of Public Health University of Pittsburgh Committee Member: F. Yesim Demirci, M.D. Research Assistant Professor Department of Human Genetics Graduate School of Public Health University of Pittsburgh Committee Member: David N. Finegold, M.D. Professor Department of Pediatrics and Department of Human Genetics School of Medicine and Graduate School of Public Health University of Pittsburgh ii Copyright © by Sarah Elizabeth Hill 2009 iii SEQUENCE VARIATION IN THE APOA1 AND APOA4 GENES AND THEIR RELATIONSHIP WITH PLASMA HDL-CHOLESTEROL LEVELS Sarah Elizabeth Hill, M.S. University of Pittsburgh, 2009 Heart disease continues to be the leading cause of death in the United States, making it one of the foremost public health concerns. Many factors influence the risk to develop heart disease, including abnormal blood lipid levels. High levels of plasma high-density lipoprotein (HDL)- cholesterol have been shown to have a protective effect. Recent genome-wide association studies (GWAS) and candidate gene studies have identified genes thought to contribute to HDL- cholesterol levels. Two genes, APOA1 and APOA4, have been associated with HDL-cholesterol levels in multiple studies with inconsistent results. The majority of these studies focused on the “common variant-common disease” hypothesis whereas only one study by Cohen et al. (2004) evaluated APOA1 using the “rare variant-common disease” hypothesis. The aim of this study was to further investigate the role of common and rare variation in these two genes by sequencing individuals having extremely low and high HDL-cholesterol levels in two populations, U.S. Non-Hispanic Whites (NHWs), and African Blacks, and then screening the identified variants in the entire sample. In the initial sequence analysis, 54 variants were identified in APOA1 (25 of which were new), and 43 in APOA4 (21 of which were new). According to preliminary analysis of the sequencing data for APOA1 and APOA4, no striking difference was noticed between the distribution of rare variants between high and low HDL groups in either population. To date, screening data was compiled for the entire NHWs and iv Black samples for a total of seven common variants: 2 for APOA1 (rs5070 and rs5072), and 5 in APOA4 (rs5092, rs5100, rs5104, rs5106, and rs5109). All 7 variants were present in the Black population; five were present in NHWs (rs5070, rs5072, rs5092, rs5100, and rs5104). Modest or marginal significant p-values were observed; however, none would maintain significance after multiple testing correction in either population. Additional variants identified in sequencing remain to be screened in the entire NHWs and Black samples. v TABLE OF CONTENTS PREFACE ..................................................................................................................................... X 1.0 BACKGROUND AND SIGNIFICANCE .......................................................................... 1 1.1 HIGH DENSITY LIPOPROTEIN (HDL) ................................................................ 1 1.1.1 The HDL Particle ............................................................................................. 1 1.1.2 HDL-Cholesterol Metabolism ......................................................................... 2 1.2 ATHEROSCLEROSIS AND HEART DISEASE .................................................... 3 1.2.1 The Public Health Impact of Heart Disease .................................................. 3 1.2.2 Risk Factors for Heart Disease ....................................................................... 4 1.2.3 Cholesterol Levels and Heart Disease ............................................................ 4 1.2.4 Atherosclerosis and Heart Disease ................................................................. 4 1.2.5 Epidemiological Evidence for the Antiatherogenic Properties of HDL- Cholesterol .................................................................................................................... 5 1.3 GENETIC STUDIES OF HDL-CHOLESTEROL LEVELS .................................. 6 1.3.1 Candidate Gene Studies ................................................................................... 6 1.3.2 Genome Wide Association Studies ................................................................. 6 1.3.3 Genetic Models for HLD Variation ................................................................ 7 1.3.3.1 Common Variant-Common Disease Hypothesis ................................ 7 1.3.3.2 Rare Variant-Common Disease Hypothesis ....................................... 8 1.4 APOLIPOPROTEIN A-I: THE APOA1 GENE ..................................................... 10 1.4.1 Protein Structure ............................................................................................ 10 1.4.2 Functional Considerations ............................................................................ 11 1.4.3 APOA1 Variants and Phenotypic Association ............................................. 12 1.5 APOLIPOPROTEIN A-IV AND THE APOA4 GENE .......................................... 19 1.5.1 Structure ......................................................................................................... 20 1.5.2 APOA4 Variants and Phenotypic Association ............................................. 21 1.6 SPECIFIC AIMS ....................................................................................................... 24 2.0 SUBJECTS AND METHODS .......................................................................................... 25 2.1 SUBJECTS ................................................................................................................. 25 2.1.1 Study Populations .......................................................................................... 25 2.1.2 Subset of the Study Population Used for Sequencing ................................. 26 2.2 DNA SEQUENCING ................................................................................................. 27 2.3 GENOTYPING WITH TAQMAN .......................................................................... 30 2.4 STATISTICAL METHODS ..................................................................................... 32 3.0 RESULTS ........................................................................................................................... 33 3.1 DNA SEQUENCING ................................................................................................. 33 3.1.1 APOA1 ............................................................................................................. 33 3.1.2 APOA4 ............................................................................................................. 38 vi 3.1.3 APOA1 and APOA4 Annotated Sequence .................................................... 42 3.2 DISTRIBUTION OF APOA1 AND APOA4 VARIANTS IN HIGH AND LOW HDL GROUPS .................................................................................................................... 46 3.2.1 APOA1 ............................................................................................................. 46 3.2.1.1 Non-Hispanic Whites .......................................................................... 46 3.2.1.2 Blacks ................................................................................................... 52 3.2.2 APOA4 ............................................................................................................. 59 3.2.2.1 Non-Hispanic Whites .......................................................................... 59 3.2.2.2 Blacks ................................................................................................... 64 3.3 LD AND TAGGER ANALYSES OF APOA1 AND APOA4 VARIANTS ............ 70 3.3.1 Non-Hispanic Whites ..................................................................................... 70 3.3.2 Blacks .............................................................................................................. 73 3.4 GENOTYPING OF ENTIRE NHW AND BLACK SAMPLES USING AVALIABLE TAQMAN SNP GENOTYPING ASSAYS .............................................. 76 3.4.1 LD Analysis of Variants Screened in Entire NWH and Black Samples ... 76 3.4.2 Association Analysis of the Variants Screened in the Entire NWH and Black Samples for their Effects on Plama HDL levels ........................................... 77 4.0 DISCUSSION ..................................................................................................................... 80 5.0 CONCLUSION .................................................................................................................. 87 BIBLIOGRAPHY ....................................................................................................................... 88 vii LIST OF TABLES Table 1. Allelic Variants in APOA1 ............................................................................................