Amy Luke

A Spectrum of PCSK9 Alleles Contributes to Plasma Levels of Low-Density Lipoprotein Cholesterol

Selected missense mutations in the proprotein convertase subtilisin/kexin type 9 serine protease gene (PCSK9) cause autosomal dominant hypercholesterolemia, whereas nonsense mutations in the same gene are associated with low plasma levels of low-density lipoprotein cholesterol (LDL-C). Here, DNA sequencing and chip-based oligonucleotide hybridization were used to determine whether other sequence variations in PCSK9 contribute to differences in LDL-C levels. The coding regions of PCSK9 were sequenced in the blacks and whites from the Dallas Heart Study (n=3,543) who had the lowest (<5th percentile) and highest (>95th percentile) plasma levels of LDL-C. Of the 17 missense variants identified, 3 (R46L, L253F, and A443T) were significantly and reproducibly associated with lower plasma levels of LDL-C (reductions ranging from 3.5% to 30%). None of the low-LDL-C variants were associated with increased hepatic triglyceride content, as measured by proton magnetic resonance spectroscopy. This finding is most consistent with the reduction in LDL-C being caused primarily by accelerating LDL clearance, rather than by reduced lipoprotein production. Association studies with 93 noncoding single-nucleotide polymorphisms (SNPs) at the PCSK9 locus identified 3 SNPs associated with modest differences in plasma LDL-C levels. Thus, a spectrum of sequence variations ranging in frequency (from 0.2% to 34%) and magnitude of effect (from a 3% increase to a 49% decrease) contribute to interindividual differences in LDL-C levels. These findings reveal that PCSK9 activity is a major determinant of plasma levels of LDL-C in humans and make it an attractive therapeutic target for LDL-C lowering.

Admixture mapping for hypertension loci with genome-scan markers

Identification of genetic variants that contribute to risk of hypertension is challenging. As a complement to linkage and candidate gene association studies, we carried out admixture mapping using genome-scan microsatellite markers among the African American participants in the US National Heart, Lung, and Blood Institutes Family Blood Pressure Program. This population was assumed to have experienced recent admixture from ancestral groups originating in Africa and Europe. We used a set of unrelated individuals from Nigeria to represent the African ancestral population and used the European Americans in the Family Blood Pressure Program to provide estimates of allele frequencies for the European ancestors. We genotyped a common set of 269 microsatellite markers in the three groups at the same laboratory. The distribution of marker location–specific African ancestry, based on multipoint analysis, was shifted upward in hypertensive cases versus normotensive controls, consistent with linkage to genes conferring susceptibility. This shift was largely due to a small number of loci, including five adjacent markers on chromosome 6q and two on chromosome 21q. These results suggest that chromosome 6q24 and 21q21 may contain genes influencing risk of hypertension in African Americans.

Linkage and Association Analysis of Angiotensin I–Converting Enzyme (ACE)–Gene Polymorphisms with ACE Concentration and Blood Pressure

Considerable effort has been expended to determine whether the gene for angiotensin I-converting enzyme (ACE) confers susceptibility to cardiovascular disease. In this study, we genotyped 13 polymorphisms in the ACE gene in 1,343 Nigerians from 332 families. To localize the genetic effect, we first performed linkage and association analysis of all the markers with ACE concentration. In multipoint variance-component analysis, this region was strongly linked to ACE concentration (maximum LOD score 7.5). Likewise, most of the polymorphisms in the ACE gene were significantly associated with ACE (P<.0013). The two most highly associated polymorphisms, ACE4 and ACE8, accounted for 6% and 19% of the variance in ACE, respectively. A two-locus additive model with an additive x additive interaction of these polymorphisms explained most of the ACE variation associated with this region. We next analyzed the relationship between these two polymorphisms (ACE4 and ACE8) and blood pressure (BP). Although no evidence of linkage was detected, significant association was found for both systolic and diastolic BP when a two-locus additive model developed for ACE concentration was used. Further analyses demonstrated that an epistasis model provided the best fit to the BP variation. In conclusion, we found that the two polymorphisms explaining the greatest variation in ACE concentration are significantly associated with BP, through interaction, in this African population sample. Our study also demonstrates that greater statistical power can be anticipated with association analysis versus linkage, when markers in strong linkage disequilibrium with a trait locus have been identified. Furthermore, allelic interaction may play an important role in the dissection of complex traits such as BP.

Estimating the changes in energy flux that characterize the rise in obesity prevalence

BACKGROUND The daily energy imbalance gap associated with the current population weight gain in the obesity epidemic is relatively small. However, the substantially higher body weights of populations that have accumulated over several years are associated with a substantially higher total energy expenditure (TEE) and total energy intake (TEI), or energy flux (EnFlux = TEE = TEI). OBJECTIVE The objective was to develop an equation relating EnFlux to body weight in adults for estimating the rise in EnFlux associated with the obesity epidemic. DESIGN Multicenter, cross-sectional data for TEE from doubly labeled water studies in 1399 adults aged 5.9 +/- 18.8 y (mean +/- SD) were analyzed in linear regression models with natural log (ln) weight as the dependent variable and ln EnFlux as the independent variable, adjusted for height, age, and sex. These equations were compared with those for children and applied to population trends in weight gain. RESULTS ln EnFlux was positively related to ln weight (beta = 0.71; 95% CI: 0.66, 0.76; R2 = 0.52), adjusted for height, age, and sex. This slope was significantly steeper than that previously described for children (beta = 0.45; 95% CI: 0.38, 0.51). CONCLUSIONS This relation suggests that substantial increases in TEI have driven the increases in body weight over the past 3 decades. Adults have a higher proportional weight gain than children for the same proportional increase in energy intake, mostly because of a higher fat content of the weight being gained. The obesity epidemic will not be reversed without large reductions in energy intake, increases in physical activity, or both.

Increasing body mass index and obesity in the incident ESRD population

An increase in obesity prevalence among patients who initiate dialysis may influence the growth of the total ESRD population as a result of improved survival and decreased likelihood for transplantation. Temporal trends in mean body mass index (BMI) and obesity prevalence were examined among incident patients with ESRD by year of dialysis initiation between 1995 and 2002, and these trends were compared with those in the US population during this same period. Among incident dialysis patients, BMI was calculated with the height and estimated dry weight collected from the Centers for Medicare and Medicaid Services End-Stage Renal Disease Medical Evidence Form. In the US population, self-reported height and weight were used. Prevalence of total obesity and obesity stage > or =2 were defined as a BMI > or =30 and > or =35 kg/m(2), respectively. Among incident patients with ESRD, mean BMI increased from 25.7 to 27.5 kg/m(2), and total obesity and obesity stage > or =2 increased by 33 and 63%, respectively, among incident patients with ESRD (P < 0.0001 for obesity trends). BMI slope was approximately two-fold higher in the incident ESRD population compared with the US population for all age groups. However, temporal increases in obesity prevalence were similar between the two populations. As a result of the survival advantage associated with obesity and decreased likelihood for transplantation, these trends most likely will influence the total number of patients who receive dialysis in the next decade.

Metabolic Syndrome and Self-Reported History of Kidney Stones: The National Health and Nutrition Examination Survey (NHANES III) 1988-1994

BACKGROUND Metabolic syndrome affects approximately 25% of the American population. Components of metabolic syndrome, such as obesity, hypertension, and diabetes, were associated with kidney stone disease, but no published large-scale study examined the association between metabolic syndrome and history of kidney stones. STUDY DESIGN Cross-sectional analysis. The American Heart Association and National Heart, Lung, and Blood Institute statement on metabolic syndrome was used to define metabolic syndrome. SETTING & PARTICIPANTS A national probability sample of the US population National Health and Nutrition Examination Survey aged 20 years and older. PREDICTOR Metabolic syndrome as defined by the American Heart Association and National Heart, Lung, and Blood Institute. OUTCOMES & MEASUREMENTS Self-reported history of kidney stones. RESULTS Of all adults older than 20 years, 4.7% reported a history of kidney stones. The prevalence of self-reported history of kidney stones increased with the number of metabolic syndrome traits from 3% with 0 traits to 7.5% with 3 traits to 9.8% with 5 traits. After adjustment for age and other covariates, the presence of 2 or more traits significantly increased the odds of self-reported kidney stone disease. The presence of 4 or more traits was associated with an approximate 2-fold increase in odds of self-reported kidney stone disease. LIMITATIONS Cross-sectional design, absence of dietary data. CONCLUSION Metabolic syndrome traits are associated with a self-reported history of kidney stones. This association should be verified in prospective studies.

An international comparative study of blood pressure in populations of European vs. African descent

BackgroundThe consistent finding of higher prevalence of hypertension in US blacks compared to whites has led to speculation that African-origin populations are particularly susceptible to this condition. Large surveys now provide new information on this issue.MethodsUsing a standardized analysis strategy we examined prevalence estimates for 8 white and 3 black populations (N = 85,000 participants).ResultsThe range in hypertension prevalence was from 27 to 55% for whites and 14 to 44% for blacks.ConclusionsThese data demonstrate that not only is there a wide variation in hypertension prevalence among both racial groups, the rates among blacks are not unusually high when viewed internationally. These data suggest that the impact of environmental factors among both populations may have been under-appreciated.

Set points, settling points and some alternative models: Theoretical options to understand how genes and environments combine to regulate body adiposity

The close correspondence between energy intake and expenditure over prolonged time periods, coupled with an apparent protection of the level of body adiposity in the face of perturbations of energy balance, has led to the idea that body fatness is regulated via mechanisms that control intake and energy expenditure. Two models have dominated the discussion of how this regulation might take place. The set point model is rooted in physiology, genetics and molecular biology, and suggests that there is an active feedback mechanism linking adipose tissue (stored energy) to intake and expenditure via a set point, presumably encoded in the brain. This model is consistent with many of the biological aspects of energy balance, but struggles to explain the many significant environmental and social influences on obesity, food intake and physical activity. More importantly, the set point model does not effectively explain the ‘obesity epidemic’ – the large increase in body weight and adiposity of a large proportion of individuals in many countries since the 1980s. An alternative model, called the settling point model, is based on the idea that there is passive feedback between the size of the body stores and aspects of expenditure. This model accommodates many of the social and environmental characteristics of energy balance, but struggles to explain some of the biological and genetic aspects. The shortcomings of these two models reflect their failure to address the gene-by-environment interactions that dominate the regulation of body weight. We discuss two additional models – the general intake model and the dual intervention point model – that address this issue and might offer better ways to understand how body fatness is controlled.

The Epidemiology of Cirrhosis in the United States: A Population-based Study.

Background and Aims: Liver cirrhosis is an important public health concern in the United States and a significant source of morbidity and mortality. However, the epidemiology of cirrhosis is incompletely understood. The aims of this study were to estimate the prevalence of cirrhosis in the general US population, determine characteristics of affected Americans with a focus on health disparities, and calculate excess mortality attributable to cirrhosis. Methods: National Health And Nutrition Examination Survey data conducted between 1999 and 2010 were used to estimate cirrhosis prevalence and factors associated with cirrhosis. The National Center for Health Statistics-linked death certificate data from the National Death Index were linked to the National Health And Nutrition Examination Survey database for the years 1999 to 2004, and attributable mortality was calculated using propensity score adjustment. Cirrhosis was ascertained by aspartate aminotransferase-to-platelet ratio of >2 and abnormal liver function tests. Results: The prevalence of cirrhosis in the United States was approximately 0.27%, corresponding to 633,323 adults. Sixty-nine percent reported that they were unaware of having liver disease. The prevalence was higher in non-Hispanic blacks and Mexican Americans, those living below the poverty level, and those with less than a 12th grade education. Diabetes, alcohol abuse, hepatitis C and B, male sex, and older age were all independently associated with cirrhosis, with a population attributable fraction of 53.5% from viral hepatitis (mostly hepatitis C), diabetes, and alcohol abuse. Mortality was 26.4% per 2-year interval in cirrhosis compared with 8.4% in propensity-matched controls. Conclusions: The prevalence of cirrhosis is higher than previously estimated. Many cases may be undiagnosed, and more than half are potentially preventable by controlling diabetes, alcohol abuse, and viral hepatitis. Public health efforts are needed to reduce this disease burden, particularly among racial/ethnic minorities and individuals at lower socioeconomic status.

A Combined Analysis of Genomewide Linkage Scans for Body Mass Index, from the National Heart, Lung, and Blood Institute Family Blood Pressure Program

A combined analysis of genome scans for obesity was undertaken using the interim results from the National Heart, Lung, and Blood Institute Family Blood Pressure Program. In this research project, four multicenter networks of investigators conducted eight individual studies. Data were available on 6,849 individuals from four ethnic groups (white, black, Mexican American, and Asian). The sample represents the largest single collection of genomewide scan data that has been analyzed for obesity and provides a test of the reproducibility of linkage analysis for a complex phenotype. Body mass index (BMI) was used as the measure of adiposity. Genomewide linkage analyses were first performed separately in each of the eight ethnic groups in the four networks, through use of the variance-component method. Only one region in the analyses of the individual studies showed significant linkage with BMI: 3q22.1 (LOD 3.45, for the GENOA network black sample). Six additional regions were found with an associated LOD >2, including 3p24.1, 7p15.2, 7q22.3, 14q24.3, 16q12.2, and 17p11.2. Among these findings, the linkage at 7p15.2, 7q22.3, and 17p11.2 has been reported elsewhere. A modified Fishers omnibus procedure was then used to combine the P values from each of the eight genome scans. A complimentary approach to the meta-analysis was undertaken, combining the average allele-sharing identity by descent (pi) for whites, blacks, and Mexican Americans. Using this approach, we found strong linkage evidence for a quantitative-trait locus at 3q27 (marker D3S2427; LOD 3.40, P=.03). The same location has been shown to be linked with obesity-related traits and diabetes in at least two other studies. These results (1) confirm the previously reported obesity-susceptibility locus on chromosomes 3, 7, and 17 and (2) demonstrate that combining samples from different studies can increase the power to detect common genes with a small-to-moderate effect, so long as the same gene has an effect in all samples considered.