Genetic bottlenecks, perceived racism, and hypertension risk among African Americans and first-generation African immigrants

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1 (2001) 15, Nature Publishing Group All rights reserved /01 $ ORIGINAL ARTICLE Genetic bottlenecks, perceived racism, and hypertension risk among African Americans and first-generation African immigrants WS Carlos Poston 1, VN Pavlik 2, DJ Hyman 3, K Ogbonnaya 2, CL Hanis 4 CK Haddock 1, ML Hyder 1 and JP Foreyt 3 1 Mid America Heart Institute and University of Missouri-Kansas City, MO, USA; 2 Department of Family and Community Medicine, Baylor College of Medicine, TX, USA; 3 Department of Medicine, Baylor College of Medicine, TX, USA; 4 Human Genetic Center, University of Texas Houston Health Science Center, TX, USA The complexity of factors influencing the development of hypertension (HTN) in African Americans has given rise to theories suggesting that genetic changes occurred due to selection pressures/genetic bottleneck effects (ie, constriction of existing genetic variability) over the course of the slave trade. Ninety-nine US-born and 86 African-born health professionals were compared in a cross-sectional survey examining genetic and psychosocial predictors of HTN. We examined the distributions of three genetic loci (G-protein, AGT-235, and ACE I/D) that have been associated with increased HTN risk. There were no significant differences between USborn African Americans and African-born immigrants in the studied genetic loci or biological variables (eg, plasma renin and angiotensin converting enzyme activity), except that the AGT-235 homozygous T genotype was somewhat more frequent among African-born participants than US-born African Americans. Only age, body mass index, and birthplace consistently demonstrated associations with HTN status. Thus, there was no evidence of a genetic bottleneck in the loci studied, ie, that US-born African Americans have different genotype distributions that increase their risk for HTN. In fact, some of the genotypic distributions evidenced lower frequencies of HTN-related alleles among US-born African Americans, providing evidence of European admixture. The consistent finding that birthplace (ie, US vs Africa) was associated with HTN, even though it was not always significant, suggests potential and unmeasured cultural, lifestyle, and environmental differences between African immigrants and US-born African Americans that are protective against HTN. (2001) 15, Keywords: African Americans; genetics; G-proteins; first-generation African immigrants Introduction Hypertension (HTN) is more prevalent among African Americans than any other ethnic group in the US and is a primary contributor to excess cardiovascular-related mortality (ie, heart, kidney, and cerebrovascular disease) in this population. 1,2 For example, HTN accounts for 20% of all-cause mortality in African Americans but only 10% in Caucasians. 3 In addition, African Americans experience higher HTN prevalence rates than Afro-Caribbean or African populations. 3,4 Traditional risk factors (eg, Correspondence: Walker S Carlos Poston, MPH, PhD, 4825 Troost, Suite 124, University of Missouri-Kansas City, Kansas City, MO 64110, USA. postonwa umkc.edu Received 1 September 2000; revised 16 October 2000; accepted 13 November 2000 psychological, dietary, and environmental factors) have not consistently or adequately explained the increased HTN prevalence and associated cardiovascular morbidity and mortality in African Americans when they are compared to non-hispanic whites and Hispanics in the US. 5 8 The inability of these risk factors to fully account for differences in HTN risk between African Americans and other ethnic groups has given rise to a number of genetic theories to explain their increased susceptibility These theories suggest that the experience of slavery in the US may have increased the frequency of genotypes that predispose African Americans to HTN. In addition they suggest that genetic changes (ie, a genetic bottleneck effect resulting in the constriction of existing genetic variability) occurred due to artificial selection pressures over the course of slave transportation and as

2 342 a result of harsh living conditions in the US that favoured salt conservation and predisposed African Americans to HTN. 10 It also is possible that other genes common in West Africa may play a role in the aetiology of HTN in persons living a Western lifestyle. While the genetics of HTN remain poorly understood and the slavery hypothesis has been criticised, ie, it is unlikely that experience of slavery alone accounts for the increased HTN susceptibility in African Americans, 12 several candidate genes have been identified. The primary genes that have been examined include polymorphisms (ie, allelic variations) of the angiotensinogen gene (AGT) (ie, differences in the frequency of the M235T and AGT- 6 genotypes and alleles) and the insertion/deletion polymorphism of the angiotensin-converting enzyme (ACE) gene These genes are considered promising candidates because of their role in the renin-angiotensin system, which regulates sodium and water balance, the likely place where the sodium sensitivity hypothesis would have its effect. A recently examined HTN candidate gene is the G-protein (heterotrimeric GTP-binding proteins) subunit. 27 G-proteins may play a role in enhancing the activity of the ph regulating ion exchange system and appear to mediate the intracellular effects of many vasoactive substances in virtually all cardiovascular tissues by activating intracellular signalling A novel polymorphism was detected in exon 10, position 825 of the gene encoding the -3 subunit of the G-protein. Initial genotyping studies found that the presence of the T allele was associated with HTN in European populations For example, Siffert et al 32 found that individuals with HTN were 79% more likely to have the TT genotype than the CC genotype (OR = 1.79; 95% CI = ) when compared to normotensive individuals. Another study found that the TT genotype was associated with HTN (OR = 1.70; 95% CI = ) when compared to individuals with the CC genotype. 31 Benjafield and colleagues 33 also found a relationship between HTN and the G-protein T allele. In this study of 110 HTN patients and 189 normotensive individuals, all of whom were of European descent, the T allele frequency was 0.43 among HTN patients compared to 0.25 in the normotensive group (OR = 2.3; 95% CI = ). In addition, the T allele tracked with higher baseline blood pressure. Finally, a replication in a population-based sample of 608 European adults confirmed that the T allele was mildly associated with elevated diastolic blood pressure, higher aldosterone-to-renin ratio, and reduced renin levels. 34 When subjects were categorised as having HTN or being normotensive, the odds of being homozygous for the T allele was substantially increased (OR = 1.84; 95% CI = ). If subjects were taking two or more hypertensive medications, the odds of T allele homozygocity were even greater (OR = 3.60, 95% CI = ). 35 In contrast, Brand et al 35 did not find statistically significant increased HTN risk associated with the T allele in a casecontrol study with European adults. G-proteins also have been studied in some non- European populations. For example, Hegele and associates 36 examined the relationship between G- proteins and HTN in the Canadian Oji-Cree. They found that the frequency of the G-protein T allele was 0.50 in the 447 adult Oji-Cree examined, which is considerably higher than frequencies reported in populations of European descent. 31,34 The G-protein T allele also was significantly associated with variation in systolic, but not diastolic, blood pressure and was slightly higher in those taking antihypertensive medications. 36 Dong et al 37 found the distribution of the T genotypes (TT and CT) to be very different than those reported in European populations (ie, TT = 62.4%, CT = 33.6%, and CC = 4.0%) and that the presence of the T allele was associated with increased risk for HTN in West Africans and Afro-Caribbeans (OR = 3.71, 95% CI = ). Finally, Siffert et al 38 also found that the distribution of the T allele in African Americans and West Africans was higher than that reported in Europeans with some suggestion that the gene frequencies were not in the direction suggested by the slave ship hypothesis. The present study examined the role of the three genetic loci, perceptions of racism, and psychological status in predicting HTN using an immigration model. Studies of disease expression in first-generation immigrants offer the unique potential for understanding the interaction between genetic predisposition and environmental factors that influence HTN. The aims of study were to: (1) determine the prevalence and compare genotype distributions for G-proteins, AGT-235, and ACE I/D and determine if disease-related alleles are more common in African Americans than first generation African immigrants; and (2) examine the association between HTN and standard cardiovascular risk factors (eg, age, gender, body mass, and smoking), birthplace (US vs Africa), psychological status, experiences with racism, and the three genetic loci (ie, G-proteins, AGT-235, and ACE I/D). Subjects and methods Sample selection and recruitment This study was approved by the Baylor College of Medicine Institutional Review Board. Participants were registered nurses and pharmacists living in the Houston metropolitan area that self-identified themselves as Black/African-American in their respective profession s computerised membership roster. As described in Hyman et al, 39 African-born individuals were initially identified by their surname. African-born persons represented approximately 10% of the total membership roster, thus the non- African-born sample was formed by stratifying

3 non-african-born members by sex and 5-year age categories and randomly selecting one non-african born from the appropriate age-sex stratum to match each African-born person. All persons with African surnames on the pharmacist and nurse lists (n = 406) were invited to participate. All prospective participants were invited to attend a 1-day continuing education programme on the cardiovascular health of African-Americans. The programme also included free blood pressure and cholesterol screening prior to the start of the scientific programme and free continuing education credit. All participants signed an informed consent stating that data were being gathered for research purposes. Of the 912 persons invited to participate, 184 (20%) registered and 137 (74%) attended. An additional 124 registered after learning about the programme from friends or co-workers who had received a mailed invitation, and 84 (68%) of them attended. Because self-invited participants were similar in age, ethnicity, and professional membership to the study population, we decided to include them as long as they were US born African-Americans, African-born immigrants, and they completed all screening procedures. Thus, a total of 221 individuals participated in this study. Ninety-nine African American and 86 African-born participants had valid blood pressure measurements for assessment of HTN prevalence using the guidelines from the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure 40 and were included in the analysis. In order to determine whether HTN awareness was a significant factor in programme attendance, we conducted a telephone follow-up interview on a random subsample of 100 individuals who were on the original mailing list and on all persons who registered but did not attend the programme. No significant differences were found in prevalence of selfreported HTN among participants compared to nonparticipants. In addition, the accuracy of the African surname to identify African-born individuals was high, ie, 93% and 94% of African-born and African American participants, respectively, were correctly identified by surname. Laboratory and anthropometric measures All physical measurements were made between 7.00 am and 9.00 am with subjects fasting for 12 h before the measurement session. Prior to the physiological measurements, participants completed a questionnaire that elicited information on their social history (birthplace, time of residence in US, language preference), medical history, including awareness of and treatment for HTN, and psychosocial distress. Blood pressure measurement: Blood pressure was measured by certified research technicians according to the standardised method used by the HTN Detection and Follow-Up Program following the standard epidemiological protocol, ie, 5 min of rest in a seated position and then three separate blood pressure measurements with a mercury sphygmomanometer separated by 2 min between each reading. 40 The first blood pressure reading was omitted and the last two averaged to obtain each subject s blood pressure. Participants had not smoked for at least 1 h prior to the measurement session. G-protein, angiotensinogen, and-ace, genes: Approximately 20 ml of blood from each participant was processed, frozen and stored for genetic analyses. Genomic DNA was extracted using a NaCl extraction procedure 41 as implemented in commercially available kits (Gentra Systems, Inc. Puregene DNA Isolation Kits, Minneapolis, MN, USA). Three genetic loci polymorphisms were determined using standard polymerase chain reaction (PCR) protocols. The AGT-235 (M235T) polymorphism was determined as described in Russ et al 42 where, following amplification, PCR product was digested with Tth 1111 (Promega) and size differences visualised on ethidium bromide stained agarose gels. G-protein (GNB3) was similarly typed following amplification and BseDI digestion. 33 ACE I/D size differences were visualised directly on ethidium bromide stained agarose gels following amplification. 43 Renin and ACE activity: Renin was analysed by immunoradiometric (IRMA) non-competitive assay The amount of bound renin present is directly proportional to the amount of renin present in the sample (expected values: pg/ml). ACE activity was analysed using Spectrophotometric Stop Rate Determination. 47 Using this method, the linear range of ACE values is from less than 50 to 1300 U/L with expected values for this sample ranging from 8 52 U/L. Results of this method correlate with other established methods (r = 0.98) and it is useful due to its low cost and procedural simplicity. 47 Cholesterol and glucose: Twelve-hour fasting blood samples were drawn and analysed for total cholesterol, high-density lipoproteins (HDL), and blood glucose level. Cholesterol and HDL/LDL determinants were performed by a certified commercial laboratory. Body mass index (BMI): Height and weight were measured using balance beam scales with attached statiometers. Subjects were asked to remove their shoes for the height/weight measurement. BMI was calculated by dividing weight (kg) by height squared (m 2 ). 48 Heath behaviour and psychological measures Smoking status: Tobacco use was assessed using questions from the Behavioral Risk Factor Surveil- 343

4 344 lance Survey and the National Health and Nutrition Examination Survey III epidemiological survey Participants were asked if they currently smoke and, if yes, how many cigarettes they smoke per day and how many years they have smoked this amount. General well-being (GWB) schedule: The GWB schedule 53 is comprised of 18 items indicating subjective feelings of psychological well-being and distress. Scores range from 0 to 110, with low scores representing greater distress. 53 Cut-offs representing three levels of distress are 0 to 60 (severe distress), 61 to 72 (moderate distress), and 73 to 110 (positive well-being). Adequate test-retest reliability has been reported for the GWB total, with reliability coefficients ranging from 0.68 to 0.85 (53 54). High internal consistency has been demonstrated for the GWB, with all correlations reported to be over ,55 Previous studies also have demonstrated concurrent validity between the GWB and depression scales 54 and with use of psychiatric services. 56 The perceived racism scale (PRS): The experience of racism is a complex phenomenon with multiple dimensions and it has been recommended that selfreported experiences should be measured directly. 57,58 The PRS 58 was developed to assess the multidimensional experience of racism in African Americans. The PRS provides an indication of frequency of exposure to racism and includes subscales that assess exposure in public places, in jobrelated matters, and the overhearing of racist comments. It also measures emotional and behavioural coping responses to racism, although these scales were not used in the current study. Subjects are asked to indicate this exposure within the past year, and over their lifetime, but only the lifetime exposure subscales were used because it was thought that lifetime exposure to racism might be the most different for the two populations. Coefficient alphas, an indicator of the PRS internal consistency, are high and range from Twoweek test-retest coefficients, indicators of stability, also are high, ranging from for the frequency of exposure scales. The factor structures of both the racism exposure frequency and coping responses were supported in a factor analytic study. 58 Statistical analysis For the analysis, participants were classified dichotomously as hypertensive or normotensive using the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure guidelines. 40 HTN was defined as measured systolic blood pressure 140, or diastolic pressure 90 mm Hg, or currently taking antihypertensive medication. 40 Differences in the distribution of study variables between African-born and US-born participants were analysed using analysis of variance for continuous variables or the chi-square test for discrete variables. The association between hypertensive status, the genetic loci, and each study variable was assessed using univariate logistic regression analysis. Multivariate logistic models were used to evaluate the association between HTN, birthplace (US vs Africa) and the three genetic loci (ie, AGT-235, ACE I/D, and G-protein) after adjusting for age and BMI. Results A total of 99 African American and 86 African-born participants had valid blood pressure measurements for assessment of HTN prevalence using the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure criteria 40 and were included in the analysis. The majority of participants were women (88.0% of USborn and 67.3% of African-born participants). African American participants were slightly older (M = 44.3[8.2]; Median = 44.0) than African-born participants (M = 40.7[6.0]; Median = 41.0) and were more likely to be females. Among the African-born participants, 7% had lived in the US for 5 years, 13% for 5 10 years, and 80% for 10 years. Eightynine percent of the African-born participants were from Nigeria and the birth countries of the remaining African-born subjects were distributed approximately evenly among Ghana, Tanzania, South Africa, Ethiopia, Kenya, and Liberia. The distribution of study variables by birthplace is reported in Table 1. All data presented in Table 1 were age-adjusted using direct standardisation to the US general population (under 65) age distribution used to age-adjust NCHS survey data (NHES and NHANES). 59 The prevalence of HTN was higher among African Americans than first generation African immigrants, but the difference was not statistically significant. Systolic and diastolic blood pressure was not different in the two groups. It should be noted that the systolic and diastolic blood pressure values noted in Tables 1 and 2 are likely to be lower than their true values because of the number of subjects in both groups reporting use of antihypertensive medications (which lower blood pressure). However, African American subjects were three times as likely to report taking antihypertensive medications. In contrast, African-born subjects had significantly higher levels of plasma ACE and glucose. Finally, US born participants had significantly higher BMIs than African-born subjects. The comparison of health behaviour and psychosocial variables indicated that African-born participants were less likely to report that they currently smoked, but they did not differ with respect to psychological well-being (GWB) or experiences with racism (PRS). The calculation of allele frequencies using standard methods demonstrated that all three genetic loci were found to be in Hardy Weinberg equilib-

5 Table 1 Distribution of study variables (M [s.d.] or percent [%]) by birthplace a 345 Variables US born African born P-value African Americans US immigrants (n = 99) (n = 86) Demographic and physiological Age (years) 44.3 (8.2) 40.7 (6.0) Gender (% female) 88.0% 67.3% HTN prevalence (%) 42.8% 33.1% NS Currently taking 1 or more antihypertensive 47.6% 15.3% medications (%) Body mass index (kg/m 2 ) 30.2 (6.4) 27.3 (3.6) SBP (mm Hg) (14.4) (9.7) NS DBP (mm Hg) 82.5 (9.3) 80.1 (8.3) NS Cholesterol (mg/dl) (43.7) (66.8) NS HDL cholesterol (mg/dl) 60.1 (17.9) 58.0 (11.1) NS Glucose (mg/dl) 99.2 (28.3) 89.5 (12.3) 0.01 Renin (pg/ml) 11.0 (9.4) 10.0 (14.0) NS ACE activity (U/L) 26.0 (16.2) 33.4 (12.8) 0.01 Lifestyle and psychosocial Current smoker (% yes)* 8.1% 1.1% 0.05 Psychological well-being (GWB) 72.8 (15.6) 76.1 (9.9) NS Perceived racism On job 21.9 (7.6) 21.8 (10.9) NS In public places 29.0 (8.4) 27.6 (9.9) NS Overheard statements 17.6 (4.5) 17.0 (5.8) NS a All data presented in Table 1 are age-adjusted utilising direct standardisation to the US general population (under 65) age distribution used to age-adjust NCHS survey data (NHES and NHANES) (NCHS, 1978). HTN, hypertension (defined as systolic blood pressure 140 or diastolic blood pressure 90 mm Hg or taking antihypertensive medications); SBP, systolic blood pressure; DBP, diastolic blood pressure; HDL, high-density lipoprotein; ACE, angiotensin converting enzyme; GWB, general well-being schedule. rium. 60 Table 2 summarises the distribution of the genotypes by birthplace along with selected demographic variables, %HTN, systolic and diastolic blood pressure, and plasma renin and ACE levels. There were no significant differences between African Americans and African-born immigrants in the three genotype distributions, except that the AGT-235 homozygous T genotype was somewhat more frequent among African-born participants than African Americans while the heterzygous genotype was somewhat less frequent (Chi-square = 6.03; P = 0.049). Nevertheless, the AGT-235 T allele and TT genotype were very common among both African Americans and African immigrants (ie, the T allele frequency was 82.1% vs 91.1% among African Americans and African immigrants, respectively). The G-protein T allele frequency was 70.1% and 79.8% among African Americans and African immigrants, respectively. The ACE D polymorphism allele frequency was 60.3% among African Americans and 60.2% in the African immigrant group. None of these differences were significant. There was no significant relationship between HTN status and renin (Point Biserial Correlation (PBC) = 0.037; P = 0.723) or ACE activity (PBC = 0.079; P = 0.448) or the ACE D allele and ACE activity (PBC = 0.136; P = 0.208) among the African American subjects. Similarly, there was no relationship between HTN and renin levels (PBC = 0.039; P = 0.722) or ACE activity (PBC = 0.162; P = 0.141) or the ACE I/D D allele and ACE activity (PBC = 0.140; P = 0.208) among the African-born subjects. Finally, none of the genetic loci were significantly correlated with BMI in either subject group. Table 3 summarises the results of the univariate logistic regression models predicting HTN status. Age, BMI, and birthplace were significant predictors of HTN status in univariate logistic regression models. Hypertensive subjects were 49% and 75% more likely to be older and heavier. In addition, US born African Americans were twice as likely to be classified as hypertensive as African-born participants (OR = 2.16; 95% CI = ). None of the lifestyle or psychosocial variables were significant predictors. Gender was not a significant predictor of HTN, but hypertensives were somewhat more likely to be males. The small number of males in this study limits the exploration of gender differences in HTN risk. Because none of the psychosocial factors were significant predictors of HTN, multivariate logistic regression analyses were performed to evaluate the importance of the three genetic loci and their interaction with birthplace, after adjusting for age and BMI. Table 4 presents the results of the multivariate models for HTN risk. There were no consistent main effects for birthplace and any of the genetic loci, so interactions between birthplace and genotype were not evaluated. In addition, there were no significant interactions between birthplace and age, BMI, or gender. Birthplace demonstrated consistently posi-

6 346 Table 2 Genotype distributions, demographics, hypertension status and blood pressure, and plasma renin and ACE activity by birthplace Genetic loci US born African Americans African born US immigrants (n = 99) (n = 86) G-Protein 825 TT TC CC TT TC CC Genotype percent (%) Age (yrs) (8.7) (8.3) (6.5) (6.0) (6.2) (5.0) BMI (kg/m 2 ) (5.5) (7.7) (4.7) (4.1) (3.5) ( ) % Female % HTN SBP (mm Hg) (18.0) (14.9) (14.6) (18.4) (12.9) (4.2) DBP (mm Hg) (10.9) (7.4) (12.5) (12.0) (10.8) (5.7) Renin (pg/ml) (8.5) (12.9) (11.4) (15.2) (7.8) (3.0) ACE activity (U/L) (15.9) (14.6) (20.1) (16.1) (19.4) (12.5) AGT-235 TT MT MM TT MT MM Genotype percent (%) Age (yrs) N/A (8.0) (8.6) (8.0) (6.2) (5.0) BMI (kg/m 2 ) N/A (6.0) (7.6) (5.0) (4.2) (1.7) % Female N/A % HTN N/A SBP (mm Hg) N/A (16.1) (18.3) (7.0) (17.5) (11.7) DBP (mm Hg) N/A (10.1) (9.1) (8.5) (11.6) (10.6) Renin (pg/ml) N/A (9.7) (13.5) (2.0) (6.5) (28.4) ACE activity (U/L) N/A (15.8) (14.7) (18.2) (17.5) (15.9) ACE I/D DD ID II DD ID II Genotype percent (%) Age (yrs) (8.9) (7.8) (7.8) (5.1) (6.5) (6.2) BMI (kg/m 2 ) (6.8) (6.3) (7.0) (4.1) (3.9) (4.2) % Female % HTN SBP (mm Hg) (18.0) (13.5) (22.8) (17.3) (14.9) (18.9) DBP (mm Hg) (10.0) (8.6) (10.7) (12.5) (10.1) (13.1) Renin (pg/ml) (6.5) (13.7) (10.0) (14.7) (7.1) (19.9) ACE activity (U/L) (18.5) (14.7) (11.7) (20.6) (15.0) (9.8) HTN, hypertension (defined as systolic blood pressure 140 or diastolic blood pressure 90 mm Hg or taking antihypertensive medications); SBP, systolic blood pressure; DBP, diastolic blood pressure; BMI, body mass index; ACE, angiotensin converting enzyme. tive associations with HTN status, but was only significant in the G-protein multivariate model. Figure 1 illustrates the G-protein genotype distribution for the current sample and other published studies. Discussion As noted in Table 1, African-born immigrants were less likely to be female or take antihypertensive medications than African American subjects. They

7 Table 3 Univariate associations between study variables and hypertension a 347 Variables Odds ratio 95% confidence interval P-value Demographic Age (5-year increments) 1.49 ( ) BMI (5-unit increments) 1.76 ( ) Sex (0 = female, 1 = male) 1.12 ( ) Birthplace (0 = Africa, 1 = US) 2.16 ( ) Physiological Renin (1-unit increments) 1.00 ( ) ACE activity (1-unit increments) 1.00 ( ) Genetic Loci G-protein 825 T allele (0 = CC, 1 = TT or CT) 1.78 ( ) AGT-235 T allele (0 = CC, 1 = TT or CT) 0.64 ( ) ACE I/D D allele (0 = II, 1 = DD or ID) 1.29 ( ) Lifestyle and psychosocial Current smoker (0 = no, 1 = yes) 0.87 ( ) Psychological well-being (GWB) 0.98 ( ) Perceived racism On the job 0.98 ( ) In public places 0.99 ( ) Overheard statements 0.99 ( ) a Hypertension was classified dichotomously (1 = hypertensive, 0 = normotensive). Hypertension was defined as systolic blood pressure 140 or diastolic blood pressure 90 mm Hg or taking antihypertensive medications. BMI, body mass index; ACE, angiotensin converting enzyme; GWB, general well being schedule. Table 4 Multivariate models of the relationship between hypertension a and three general loci adjusted for age, BMI, and place of birth Variables Odds 95% P-value ratio confidence interval Model 1: G-Proteins Age (5-year increments) 1.46 ( ) BMI (5-unit increments) 1.61 ( ) Birthplace (0 = Africa, 1 = US) 2.11 ( ) G-protein T allele (0 = CC, 2.75 ( ) = TT or CT) Model 2: AGT-235 Age (5-year increments) 1.47 ( ) BMI (5-unit increments) 1.56 ( ) Birthplace (0 = Africa, 1 = US) 1.88 ( ) AGT-235 T allele (0 = CC, 0.37 ( ) = TT or CT) Model 3: ACE I/D Age (5-year increments) 1.44 ( ) BMI (5-unit increments) 1.52 ( ) Birthplace (0 = Africa, 1 = US) 1.81 ( ) ACE I/D D Allele (0 = II, 1 = DD 1.23 ( ) or ID) a Hypertension was classified dichotomously (0 = normotensive, 1 = hypertensive). Hypertension was defined as systolic blood pressure 140 or diastolic blood pressure 90 mm Hg or taking antihypertensive medications. BMI, body mass index. also weighed less, were slightly younger, and were less likely to smoke. The measures of psychological well-being (GWB) or experiences with racism did not differ statistically between the two groups. There were few differences between the two groups with respect to the biological and genetic variables (see Tables 1 and 2). There were no significant differences between African Americans or African immigrants in G-protein or ACE I/D distributions, renin, total cholesterol, or HDL levels. In addition, the distribution of the ACE. I/D genotypes was very similar to that reported in other African, African American, and European populations. For example, estimates for the DD genotype in African Americans and Africans range from 34% to 35% while they vary between 27% to 36% in various European populations. 61 The G-protein genotype distribution was substantially different from the European samples reported by Schunkert et al, 34 Siffert et al, 32 and others for Europeans and Native Americans. For example, the frequency of the TT genotype in the study samples was 48.9% in African Americans and 63.1% among African-born subjects while it was only 11% and 8% in large European samples. 32,34 However, these frequencies are similar to those reported by Dong et al 37 and Siffert et al 38 for individuals of African descent (see Figure 1). The magnitude of the association between HTN and the G-protein T allele in the present study is similar to that reported in other ethnic groups, even though this allele appears to be more prevalent among individuals of African descent.

8 348 Figure 1 Distribution of the G-protein genotypes in the study sample and previously published samples. A greater percentage of subjects who were African-born were homozygous for the AGT-235 T allele (83.3% vs 67.4%). This difference between African Americans and Africans in the AGT-235 T allele frequency and TT genotype has been reported before and is thought to be due to European admixture in African Americans. African immigrants also had higher ACE activity than African Americans and the levels were higher than those reported in other studies. 65 Regardless, there were no significant correlations between the ACE D allele and ACE activity or ACE activity and HTN status in either the African American or African-born subjects. Implications of the study The results of this study are not supportive of the slavery hypothesis of HTN. 11 As noted in this and other studies, there were no substantial differences in gene frequencies, renin, or ACE levels between African American and African-born subjects. When differences were found (eg, AGT-235 T allele frequency and TT genotype and ACE activity), they were not in the direction predicted by the slavery hypothesis. 3 For example, the AGT-235 T allele and TT genotype and the G-protein T allele and TT genotype were more common among first generation African immigrants than African Americans, suggesting European admixture, but the prevalence of HTN was significantly lower in this group. Finally, despite having lower AGT-235 T and G-protein T allele frequencies, being born in the US was associated with increased odds of being hypertensive in the univariate and multivariate models predicting HTN risk. While none of the lifestyle/psychological factors predicted HTN status, the reliable finding that BMI predicted HTN status and varied in a manner similar to that published in cross-national studies of persons of African descent, 66,67 in the absence of significant genetic variation among these groups, suggests that other lifestyle and environmental factors are likely to be the important contributors to HTN and probably will better explain racial differences in blood pressure. 68 It is unclear what other factors influenced the increased risk for HTN among the African Americans or why the prevalence of HTN is greater in this sample of African immigrants than HTN prevalence data on West Africans living in Africa. 66 Other lifestyle, social, economic, and environmental factors that were not measured in this study should be evaluated in future research. For example, some investigators are pursuing the interaction between ethnicity and geography (eg, latitude and UV exposure) and its impact on factors that may play a role in blood pressure, such as reduced vitamin D synthesis. 69 The study has several limitations. The sample size was small and the frequency of some of the alleles was very high (eg, AGT-235 T allele); thus, the ability to detect group differences or genetic associations was constrained by low statistical power. 62,70 Nevertheless, the sample size in this study was comparable

9 or larger than many of the reviewed genetic studies. In addition, the use of a cross-sectional design does not allow conclusions about causality because exposure and disease status were determined at the same time. While this is not a substantial problem when the genetic loci are considered (ie, because their current values are unalterable over time), it limits conclusions that can be drawn about any of the health behaviour or psychosocial measures. It also is possible that there was bias in disease ascertainment. The higher prevalence of HTN among African Americans in the study sample, ie 42.8% compared to estimates of 32.4% 4 and 32.6% 3,71 from studies with larger samples could reflect sampling bias or misclassification. Evaluation for potential selection bias did not reveal evidence that HTN awareness was a factor in study participation. Regression-dilution bias might have caused misclassification because only two out of three blood pressure readings were used to assess blood pressure, rather than ambulatory monitoring. 24 In addition, categorisation of HTN based on reported use of antihypertensive agents also could have resulted in misclassification of disease status, although it was unlikely to be differential. A final limitation is that the study was restricted to health professionals. This was done to maximise recruitment and increase the likelihood of educational and economic similarity between the two groups. On the other hand, professional status is not a complete marker of socioeconomic status and it restricts generalisability of results to the population of African immigrants and African Americans as a whole. Acknowledgements This work was partially supported by a Minority Scientist Development Award from the American Heart Association and with funds contributed by the AHA, Puerto Rico Affiliate. 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