Demonstrating Reduced Environmental and Genetic Diversity in Human Isolates by Analysis of Blood Lipid Levels
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE Brief Report Demonstrating Reduced Environmental and Genetic Diversity in Human Isolates by Analysis of Blood Lipid Levels Ozren Polašek1,2, Ivana Kolčić1, Ankica Smoljanović3, Dražen Stojanović4, Matijana Grgić5, Barbara Ebling6, Maja Klarić6, Josip Milas6, Dinko Puntarić6 1Department of Medical Statistics, Aim To test the hypothesis that phenotypic diversity in isolated hu- Epidemiology, and Medical Informatics, man populations is decreased in comparison with general outbred Andrija Štampar School of Public Health, population because of reduced genetic and environmental diversity. Zagreb University School of Medicine, Zagreb, Croatia; To demonstrate this in populations for which reduced genetic and 2Department of Public Health Sciences, environmental diversity had already been established, by studying the University of Edinburgh, Edinburgh, UK amount of variation in plasma lipid levels. 3 Institute of Public Health, Split, Croatia Methods Fasting plasma lipid levels (cholesterol, triglycerides, low 4Department of Epidemiology, Rijeka density lipoprotein [LDL], and high density lipoprotein [HDL]) were University School of Medicine, measured in randomly selected 300 inhabitants from 2 isolated hu- Rijeka, Croatia man populations, the island of Rab and the neighboring islands of Vis 5Institute of Public Health, and Lastovo, Croatia. The populations were chosen based on previous Zagreb, Croatia analyses of genetic diversity and lifestyle patterns, which were shown 6Department of Epidemiology, Osijek University School of Medicine, to be both less diverse and more uniform than the general Croatian Osijek, Croatia population. We studied whether the 25’-75’ and 5′-95’ interpercentile ranges in observed values were consistently smaller in 2 samples of 300 examinees from isolated populations in comparison with nearly 6000 examinees from an earlier study who were demographically targeted to > Correspondence to: represent the larger Croatian population. Ozren Polašek Results General population had much wider range of observed values Department of Medical Statistics, Epide- miology, and Medical Informatics of triglycerides and HDL than both isolated populations. However, Andrija Štampar School of Public Health both isolated populations exhibited greater extent of variation in the Zagreb University School of Medicine levels of LDL, while the ranges of cholesterol values were similar. Rockefellerova 4 10000 Zagreb, Croatia Conclusion Although reduced genetic and environmental diversity in [email protected] isolated human populations should necessarily reduce the variance in observed phenotypic values, it appears that specific population genetic processes in isolated populations could be acting to maintain the varia- > Received: March 9, 2006 tion. Departure from the Hardy-Weinberg equilibrium due to consan- > Accepted: June 8, 2006 guinity, sub-structuring and differentiation within the isolates, and in- creased rate of new mutations could theoretically explain this paradox. > Croat Med J. 2006;47:649-55 www.cmj.hr 649 Croat Med J 2006;47:649-655 Isolated human populations were proposed to identified (eg, ApoE, lipoprotein lipase, ApoC3, have major advantages in mapping genes for ADH3), and their interactions with environ- complex diseases and quantitative traits due to mental factors explored (5). Furthermore, the their reduced genetic and environmental diver- ApoB, TNFalpha, and low-density lipoprotein sity, which should reduce the underlying com- (LDL) receptor candidate genes seem to influ- plexity and facilitate the identification of the ence lipid profiles in youth, independently of en- genes controlling these traits (1). Subsequently, vironmental factors (7). Also, phenotypic corre- reduced genetic diversity in isolated human pop- lations among levels of various plasma lipids may ulations has been convincingly demonstrated be due to pleiotropic genes (eg, between LDL- (2). Reduced environment and lifestyle diversity C and ApoB and also between triglycerides and in isolated human populations was also demon- HDL-C) (8). Recent studies investigating the ef- strated in studies by many anthropologists over fects of common genetic variants on the determi- the past decades (3). The population genetic the- nation of levels of lipids in plasma have shown ory links phenotypic variation to its genetic and that they contribute minimally to the variance in environmental component through the con- values observed in the population, which high- cept of heritability (4). Based on this concept, lights the polygenic and multifactorial control we should intuitively expect that reduced genet- of these important quantitative traits through ic and environmental diversity in isolated human multiple rare variants (9,10). An understanding populations should necessarily reduce the vari- is emerging on the role of multiple genes with ance in observed phenotypic values. However, al- weak associations, which interact with environ- though this is an assumption in most studies in mental factors and determine levels of lipids in isolated human populations, we could find no plasma (11). studies where the presumed reduction in pheno- The aim of this study was to test the hypoth- typic variation was convincingly demonstrated esis that phenotypic diversity in isolated human in comparison to an appropriate general outbred populations is indeed decreased in comparison to population. general outbred population, because of reduced Levels of lipids in plasma represent a good genetic and environmental diversity, on an ex- candidate for traits that could be studied to dem- ample of serum lipid levels. onstrate such phenotypic reduction. They are among the major risk factors for cardiovascular Subjects and methods diseases, and it is known that many environmen- tal and genetic factors affect their concentrations This study aimed to address a simple but very in the general population (5). Twin studies re- specific question, which requires a careful study vealed substantial heritability for the plasma lipid design. In the first step we chose 4 plasma lipid levels, although comparisons of the twins reared levels: total cholesterol, triglycerides, high-density together with those reared apart suggested that lipoprotein (HDL), and low-density lipoprotein the effects of environment were also substantial (LDL) as phenotypes that need to be measured. (6). Besides, the effect of genetic factors on the All of them are determined by both genetic and lipid levels mostly decreases with age (5). Individ- environmental factors, which was convincingly ual variability in lipid levels and response to ther- shown in many previous studies (5-11). apeutic intervention is probably explained by the The second step was to define an expected interaction between genetic and environmen- distribution of measurements of the four chosen tal factors (5). Recently, a number of candidate phenotypes with appropriate ranges in the out- genes influencing plasma lipid levels have been bred general population. This was already per- 650 Polašek et al: Reduced Diversity in Human Isolates formed during the First Croatian Health Proj- to test the hypothesis that the phenotypic vari- ect (1995-1997) when all 4 plasma lipid levels of ance is decreased in both isolates in comparison interest were measured. The project gathered a with general population in all 4 lipid levels (con- sample of about 6000 examinees from 30 regions sistency), and that the reduction of phenotypic of Croatia. This sample was targeted to exactly variance is greater in the isolate with lower genet- correspond to the total population of the Repub- ic and environmental diversity (dose-response re- lic of Croatia, according to the last available pop- lationship). ulation census, by age, gender, and region of resi- Because of high level of genetic differentia- dence. Field methods and the description of the tion in the villages from which the examinees performed measurements were described in great were chosen, demonstrated by Vitart et al (14), detail elsewhere (12). it was important to choose two samples that will The third step was to choose isolated hu- fulfill the following requirements: to contain a man populations where measurements of the number of examinees large enough (at least 300 same phenotypes were made using comparable each) to allow firmer conclusions when com- field and laboratory methods. We chose the field pared with a sample of 6000 examinees from the study which collected data on 1001 examinees general population; to contain examinees from from the isolated populations of the islands of the villages that have shared genetic ancestry and Rab, Vis, and Lastovo between 2001 and 2003. similar genetic structure, and where gene diversi- The study was conducted by the same team, con- ty was proven to be reduced in comparison with sisting of the investigators from the Institute for the large outbred population; and to have a dem- Anthropological Research in Zagreb and medi- onstrated reduction in environment and lifestyle cal doctors affiliated to School of Public Health variability in comparison with the large outbred from Zagreb. Serum samples were collected in population. the same way as in the First Croatian Health Eventually, we compared a sample of 6000 Project. Laboratory analyses of serum lipid lev- examinees representative of the population of els were performed in a laboratory of a highly Croatia, a sample of 300 examinees from Rab is- trained