(Hypertension. 1999;34:e7.)
© 1999 American Heart Association, Inc.
Letters to the Editor - Web |
Nativity and Race
Department of Epidemiology School of Public Health, University of Michigan, Ann Arbor, Mich
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To the Editor:
Lackland et al1 reported an association between Stroke Belt mortality and birthplace such that those born outside the Southeast had lower stroke rates than those born within this region and still lower rates when compared to those born in South Carolina, a finding more striking for blacks than for whites. Similar results have been reported for stroke mortality among blacks in New York City: the highest rates were for immigrants from the South.2 Lackland et al1 suggest that place of birth may be a useful area of investigation to illuminate the perplexing question of why stroke rates are so high in the southeastern United States. It is suggested here that examining black-white admixture rates in terms of geography may be one approach to understanding this question for the black population.
Reed3 showed that the presence of white-derived genes among black populations in the United States is not geographically uniform, with the lowest rates of white admixture occurring in the Southeast (Charleston, SC, 4%; Evans County, Georgia, 11%) and the highest rates occurring in northern cities, such as Detroit (26%). Blood pressure levels, a risk factor for stroke, appear to covary with these admixture estimates. Thus, albeit from different studies, blood pressures are correspondingly higher in Charleston, SC,4 and Evans County, Georgia,5 than in Detroit, Mich,6 for approximately similar age groups. More recent admixture estimates,7 using alleles that are unique to one population or have frequency differences greater than 45% between the parental populations, report the lowest admixture percentage for Charleston (12%); however, the northern cities do not show the marked differences from the southern cities that were reported by Reed.3
Admixture proportions within a single geographical population have been investigated using skin color as a marker for African genes and most, but not all, report positive findings of a relationship between darker skin pigmentation and higher blood pressure (for review, see Reference 8), suggesting the possibility that African-derived genes have an influence on blood pressure. However, skin color is highly confounded with socio-environmental events, so that the significance of this finding remains unclear. On the other hand, Maclean et al9 found, after estimating individual admixture percentages using 9 genetic loci, that there was a significant correlation between the percentage of African genes and higher diastolic blood pressure in a sample of self-identified American blacks residing in Rochester, NY. Estimation of individual admixture proportions and its application to understanding the causes of complex diseases is a tool that has been underutilized.
Thus, admixed populations represent "natural experiments"10 that can be useful in explaining genetic contributions to major chronic diseases. Different degrees of white admixture in black populations residing in various geographic regions may account, in part, for the findings of Lackland et al1 and Fang et al2 for blacks. Some suggestions for future investigations, using both individual and population approaches, would be to (1) design prospective studies that estimate admixture within individuals at baseline and determine if the estimates predict to various morbid cardiovascular outcomes; (2) correlate the levels of a quantitative genetic trait or an intermediate phenotype (known to differ markedly in frequency between blacks and whites, and known or suspected to be related to morbid cardiovascular events) with admixture percentages in individuals; (3) determine across a number of black populations, if, for example, blood pressure levels, plasma renin activity, or genes in the renin-angiotensin-aldosterone axis covary with such estimates. A generation has passed since Reed3 published his estimates; thus, the recently calculated rates of Parra et al10 using population-specific alleles, insofar as is possible, probably reflect more accurately thirty years of increasing and shifting proportions of white genes within US black local populations.
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1. Lackland DT, Egan BM, Jones PJ. Impact of nativity and race on "stroke belt" mortality. Hypertension. 1999;34:57–62.
2.
Fang J, Madhavan S, Alderman MH. The association
between birthplace and mortality from cardiovascular
causes among black and white residents of New York City. N
Engl J Med.. 1996;335:1545–1551.
3.
Reed TE. Caucasian genes in American negroes.
Science. 1969;165:762–768.
4.
Boyle E Jr. Biological patterns in hypertension by
race, sex, body weight, and skin color. JAMA. 1970;213:1637–1643.
5. McDonough JR, Garrison GE, Hames CG. Blood pressure and hypertensive disease among negroes and whites: A study in Evans County, Georgia. Annals Int Med. 1964;61:208–228.
6. Harburg E, Schork MA, Erfurt JC, Schull WJ, Chape C. Heredity, stress and blood pressure, a family set method, II: results of blood pressure measurement. J Chron Dis. 1977; 30:649–658.
7. Parra EJ, Marcini A, Akey J, Martinson J, Batzer MA, Cooper R, Forrester T, Allison DB, Deka R, Ferrell RE, Shriver MD. Estimating African American admixture proportions by use of population-specific alleles. Am J Hum Genet. 1998; 63:1839–1851.
8. Gleiberman L, Harburg E, Frone MR, Russell M, Cooper ML. Skin colour, measures of socioeconomic status, and blood pressure among blacks in Erie County, NY. Annals Hum Biol. 1995;22:69–73.[Medline] [Order article via Infotrieve]
9. MacLean CJ, Adams MS, Leyshon WC, Workman PL, Reed TE, Gershowitz H, Weitkamp LR. Genetic studies on hybrid populations, III: blood pressure in an American black community. Am J Hum Genet. 1974;26:614–626.[Medline] [Order article via Infotrieve]
10. Weiss, KM. Natural and unnatural crosses. Ethn Dis. 1991;1:231–235. Editorial.[Medline] [Order article via Infotrieve]
Response
Department of Biometry and Epidemiology
Department of Pharmacology Medical University of South Carolina, Charleston, South Carolina
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We agree with Dr Gleiberman’s recommendation for continued study of the reasons for the geographic variation in stroke mortality. As noted in our paper, genetics may indeed provide some explanation for the differences in risk. We did not consider racial admixture and appreciate the thoughtful recommendation. As Dr. Gleiberman has described, racial admixture population estimates have historically correlated with stroke rates. While the magnitude of the effects of nativity on stroke risks were greatest for blacks, the effect was also significant in whites, which suggests that factors other than racial admixture underlie our findings. It is conceivable that the larger impact of nativity on stroke mortality in blacks compared to whites would have been even greater if racial admixture were accounted for. Therefore, it could be instructive to assess genetic factors including racial admixture in future studies along with other variables noted in the paper, which include medical care practices and behaviors, compliance with treatment of hypertension, dietary habits and lifestyles, and fetal and early-life events as indicated by low birthweight.
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