- Email: [email protected]
Genome-wide linkage analyses for hypertension genes in two ethnically and geographically diverse populations
AJH
2003; 16:154 –157
Genome-Wide Linkage Analyses for Hypertension Genes in Two Ethnically and Geographically Diverse Populations Sharon L.R. Kardia, Laura S. Rozek, Julia Krushkal, Robert E. Ferrell, Stephen T. Turner, Richard Hutchinson, Andrew Brown, Charles F. Sing, and Eric Boerwinkle We report the results of a genome-wide linkage scan for hypertension genes in 450 African American hypertensive sibpairs from Jackson, MS, and 539 non-Hispanic white hypertensive sibpairs from Rochester, MN. In the Jackson samples we identified one LOD score peak ⬎1.0 on chromosome 1. In the Rochester sample, no genomic region had a LOD score ⬎1.0. These analyses provide no appreciable evidence of hypertension genes with strong effects
H
independent of other genetic and environmental contexts and suggest that stratified linkage analyses may be required to identify hypertension susceptibility genes in these populations. Am J Hypertens 2003;16:154 –157 © 2003 American Journal of Hypertension, Ltd. Key Words: Linkage, hypertension, genetics.
ypertension, defined by a systolic/diastolic blood pressure (BP) ⱖ140/90 mm Hg, affects approximately 50 million Americans and is one of the most important risk factors for coronary artery disease, heart failure, stroke, and renal failure.1,2 Although BP is straightforward to measure and antihypertensive therapies are readily available, it is estimated that only 27% of hypertensives have their BP adequately controlled.3 It is also recognized that 30% to 50% of interindividual variation in BP is attributable to genetic variation.4 Because BP is regulated by numerous metabolic and structural characteristics of the cardiovascular, renal, nervous, and endocrine systems that involve the products of many genes, identifying those genes whose variations influence interindividual variation in BP may facilitate improved strategies for prevention, detection, evaluation, and treatment of hypertension. To this end, the Genetic Epidemiology Network of Arteriopathy (GENOA) was established as a member of the National Heart, Lung and Blood Institute (NHLBI) Family Blood Pressure Program (FBPP) to identify susceptibility genes for human hypertension in ethnically
diverse populations. Genome-wide analyses including both linkage and association approaches are being used to localize and characterize gene regions influencing interindividual variation in BP and the occurrence of hypertension. Because features of hypertension (eg, prevalence, age of onset, severity, complications) differ among ethnic groups, it is important to determine whether there are particular genes whose effects are unique to a particular ethnic or geographic group. The objective of the current study is to assess whether there is significant evidence for linkage between genome-wide markers and genes that are associated with hypertension in two ethnically and geographically different populations—African Americans from Jackson, MS, and non-Hispanic whites from Rochester, MN.
Received January 17, 2002. First decision June 1, 2002. Accepted November 5, 2002. From the Departments of Epidemiology (SLRK, LSR) and Department of Human Genetics (CFS), University of Michigan, Ann Arbor, Michigan; Department of Biology and Biotechnology (JK), Worcester Polytechnic Institute, Worcester, Massachusetts; Department of Human Genetics (REF), University of Pittsburgh, Pittsburgh, Pennsylvania; Division of Hypertension (STT), Mayo Clinic, Rochester, Minnesota; Department of Medicine (RH, AB), University of Mississippi, Jackson,
Mississippi; and Institute of Molecular Medicine and Human Genetics Center (EB), The University of Texas–Houston Health Science Center, Houston, Texas.
0895-7061/03/$30.00 PII S0895-7061(02)03249-1
Methods Subjects The primary inclusion criteria for a sibship into the GENOA study was to have at least two full siblings with essential hypertension, clinically diagnosed before age 60
The GENOA Network of the NHLBI FBPP is supported by HL54464 and HL54457. LSR was supported by T32 HG00040 from NCHGR. Address correspondence and reprint requests to Dr. Sharon L.R. Kardia, Department of Epidemiology, 109 Observatory Street, Ann Arbor, MI 48109-2029; e-mail: [email protected] © 2003 by the American Journal of Hypertension, Ltd. Published by Elsevier Science Inc.
AJH–February 2003–VOL. 16, NO. 2
years. In Jackson, MS, probands with essential hypertension diagnosed ⬍60 years of age were identified from participants in the Atherosclerosis Risk in Communities Study (ARIC), a multicenter study of the risk factors and occurrence of atherosclerotic disease.5 In Rochester, MN, probands were identified through the Rochester Epidemiology Project and the Mayo Medical Center diagnostic index for residents of Olmsted County, MN. However, once a sibship was ascertained, all sibs within a sibship were invited to participate. After identifying participants, a standardized classification scheme was used to validate the BP diagnostic category of each participant. This classification scheme took into account current and past antihypertensive medication use, prior diagnosis of hypertension, and BP measurements made at examination. All BP measurements were made with a random zero sphygmomanometer with a cuff appropriate for arm size. Three readings were taken in the right arm with the patient in the sitting position. A rest period of 5 min was observed before the first measurement. The systolic and diastolic pressures were the first and fifth phase Korotkoff sounds, respectively. The last two BP readings were averaged to arrive at the value used for diagnostic categorization. A more detailed account of the sampling methods and hypertension diagnostic criteria are presented elsewhere (Daniels et al, pers. communication). Microsatellite Genotyping For the Jackson sample, 338 autosomal microsatellite markers (CHLC/Weber screening set 6.0) were genotyped using standard polymerase chain reaction (PCR)-based methods and independently scored by two separate individuals at the University of Texas–Houston Health Science Center. For the Rochester sample, 381 autosomal microsatellite markers (CHLC/Weber screening set 9.0) were genotyped by standard PCR-based methods by the Mammalian Genotyping Center of the Marshfield Medical Research Foundation. Marker order and genetic map distances were those provided by the Marshfield Medical Research Foundation (www.marshmed.org/genetics). Statistical Analysis Because of the expected ethnic differences in the genetic architecture of hypertension between African Americans and non-Hispanic whites, we analyzed the Jackson and Rochester samples separately. Potential half-sibs, identical twins, and nonsibs were identified using ASPEX and then removed. Biologic inconsistencies in marker genotypes within sibships were identified using PEDCHECK.6 After data cleaning, genotypes from all individuals in a particular sample were used to estimate allele frequencies and to calculate the multipoint identity-by-descent (IBD) probabilities every 1 cM using the hidden Markov method implemented in GENEHUNTER 2.0.7 The maximum likelihood estimates of IBD sharing, using the possible trian-
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
155
gle constraint,8,9 among the affected sibpairs was then compared to the expected IBD sharing under the null hypothesis of no linkage using the log likelihood ratio. The final sample included 682 Jackson subjects distributed among 229 sibships and 775 Rochester subjects distributed among 257 sibships. The 229 Jackson sibships contained 996 total sibpairs including the 450 hypertensive sibpairs used in the analysis presented later. The 257 Rochester sibships contained 997 total sibpairs including the 539 hypertensive sibpairs used in the linkage analysis. Although all individuals within a sibship were used in the calculation of the IBD probabilities, only hypertensive sibpairs were used in the calculation of the LOD scores. The final calculation of the likelihood score (LOD) was weighted by the number of siblings in a sibship.10
Results The baseline characteristics of the Jackson and Rochester hypertensive sibs recruited by the GENOA Network are presented in Table 1. The average age of the hypertensive participants was 58.8 years (range, 35.1 to 86.7 years) for Jackson subjects and 58.0 years (range, 31.4 to 82.8 years) for Rochester subjects. The average body mass index was 31.3 kg/m2 (range, 14.9 to 63.4 kg/m2) for Jackson subjects and 29.9 kg/m2 (range, 15.8 to 67.9 kg/m2) for Rochester subjects. The Jackson sample was 69.8% female and the Rochester sample was 53.3% female. Results from the genome-wide linkage analysis are presented in Table 1. In the Jackson sample we identified only one LOD score peak ⬎1.0 on chromosome 1 at ⬃140 cM (from the p terminus). The LOD score peaks between 0.5 and 1.0 were found in regions of 3q, 7p, 8p, and 12p in the Jackson sample. Notable was the lack of any appreciable LOD scores ⬎0.5 on chromosomes 10, 11, 13 to 17, and 20 to 22. In the Rochester sample, no genomic region had a LOD score ⬎1.0, although a peak at 49 cM on chromosome 7 had a LOD score of 1.0 exactly. There were only three additional chromosomal regions (6p, 7q, and 16q) that had LOD scores between 0.5 and 1.0 in the Rochester, MN, sample. Again, it was notable that the remainder of the chromosomes (4, 10, 12, 13, 18, and 19) had essentially no appreciable LOD score ⬎0.5. The only region with a measurable LOD score common to both the Jackson and Rochester samples was a broad region on the short arm of chromosome 7.
Discussion In this study we found no appreciable evidence of linkage to hypertension susceptibility genes, although our samples were larger than many other reported studies. In this linkage study and others, there are several biologic and nonbiologic reasons for such a negative linkage result including: 1) genotyping errors, 2) incorrect estimates of allele frequencies, 3) misspecification of the hypertension phenotype, 4) loss of information associated with dichot-
156
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
AJH–February 2003–VOL. 16, NO. 2
Table 1. Baseline characteristics of the hypertensive sibling pairs Jackson (N ⴝ 480) Age Height (cm) Weight (kg) BMI Systolic BP Diastolic BP % Female
Chromosome 1p 3q 6p 7p 7q 8p 12p 16q
Rochester (N ⴝ 571)
Mean (SD)
Range
Mean (SD)
Range
58.8 (9.1) 166.15 (8.77) 86.19 (17.34) 31.31 (6.71) 141.4 (22.2) 79.9 (12.8) 69.8
35.1–86.7 145.50–197.9 46.13–164.55 14.89–63.4 86.0–223.0 35.0–118.0
58.0 (9.2) 169.12 (9.34) 85.92 (19.85) 29.96 (6.23) 137.1 (16.6) 79.9 (9.3) 53.3
31.4–82.8 138.9–202.0 42.40–184.80 15.79–67.88 100.0–215 52.0–110.0
Peak LOD Score
Position (cm)
Peak LOD Score
Position (cm)
1.2 0.58 — — 0.60 0.63 0.83 —
140 183 — — 87 61 50 —
— — 0.72 1.0 0.54 — — 0.69
— — 18 42 133 — — 109
BMI ⫽ body mass index; BP ⫽ blood pressure.
omizing the BP distribution into hypertensive/normotensive, and 5) the inappropriateness of using a single locus linkage strategy for a complex chronic disease. Although there are always levels of inexactitude in any measurement and estimation, the high quality of our genotype data, stringent hypertension diagnostic criteria, and allele frequency estimates are not likely to have a large impact on our results. The potential impact of the loss of information incurred from dichotomizing continuously distributed traits and the inappropriateness of a single locus linkage strategy is more difficult to assess and will require future study. Finally, we note that the relatively large number of affected sibpairs in our sample provide more than 90% power to detect a LOD ⬎3 (under a variety of genetic models) given an estimated relative recurrence risk of 1.5 for hypertension. There have been only a modest number of studies that have reported evidence of linkage to genetic loci accounting for variation in BP levels or risk of hypertension. Using highly discordant non-Hispanic white sibships from Rochester, MN, Krushkal et al11 reported one of the first genome-wide linkage analyses for a gene influencing systolic BP. They identified four chromosomal regions (2p, 5q, 6q, and 15q) with significant evidence of linkage. We note that in our linkage analyses of a separate sample from Rochester, MN, we did have a modest LOD score peak on chromosome 2p. Xu et al12 also reported evidence of linkage on chromosomes 3, 11, 15, 16, and 17 from a genome-wide linkage analyses in a very large sample of Chinese hypertensive sibpairs. These studies, as well as others, suggest that there are likely to be different sets of genes contributing to the genetic architecture of hypertension in different ethnic groups.
Moreover, as BP homeostasis involves a multitude of traits with balancing pressor and depressor roles, the influence of variation in any one gene is expected to be distributed, and hence muted, across biochemical, physiologic, and anatomic levels of the cardiovascular, renal, nervous, and endocrine systems that regulate BP levels. Consequently, the effect of any one gene or gene region is expected to be small and inconsistent across different genetic and environmental backgrounds. Hence, it is not surprising that we did not detect strong evidence of linkage in these genome-wide linkage analyses. Our experiences with candidate gene association with complex disease risk factors13,14 indicate that context-dependent linkage analyses may be required to identify new hypertension susceptibility gene regions in these populations.
References 1.
2. 3.
4.
5.
Burt VL, Whelton P, Roccella EJ, Brown C, Cutler JA, Higgins M, Horan MJ, Labarthe D: Prevalence of hypertension in the US adult population: Results from the Third National Health and Nutrition Examination Survey, 1988 –91. Hypertension 1995;25:305–313. Whelton PK: Epidemiology of hypertension. Lancet 1994;334:101– 106. JNC: The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Arch Intern Med 1997;157:2413–2446. Ward R: Familial aggregation and genetic epidemiology of hypertension, in Laragh JH, Brenner BM (eds): Hypertension: Pathophysiology, Diagnosis, and Management, 2nd ed. New York, Raven Press, 1995; pp 67– 88. ARIC Investigators: The Atherosclerosis Risk in Communities (ARIC) Study: Design and objectives. Am J Epidemiol 1989;129: 687–702.
AJH–February 2003–VOL. 16, NO. 2
6.
O’Connell JR, Weeks DE: Pedcheck: A program for identification of genotypic incompatibilities in linkage analysis. Am J Hum Genet 1998;63:259 –266. 7. Kruglyak L, Daly MJ, Reeve-Daly MP, Lander ES: Parametric and nonparametric linkage analysis: a unified multipoint approach. Am J Hum Genet 1996;58:1347–1363. 8. Holmans P: Asymptotic properties of affected sib pair linkage analysis. Am J Hum Genet 1993;52:362–374. 9. Kruglyak L, Lander ES: Complete multipoint sib-pair analysis of qualitative and quantitative traits. Am J Human Genet 1995;57: 439 –454. 10. Lange K: A test statistic for the affected-sib-set method. Ann Hum Genet 1986;50:283–290. 11. Krushkal J, Ferrell R, Mockrin SC, Turner ST, Sing CF, Boerwinkle
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
157
E: Genome-wide linkage analyses of systolic blood pressure using highly discordant siblings. Circulation 1999;99:1407–1410. 12. Xu X, Rogus JJ, Terwedow HA, Yang J, Wang Z, Chen C, Niu T, Wang B, Xu H, Weiss S, Schork NJ, Fang Z: An extreme-sib-pair genome scan for genes regulating blood pressure. Am J Hum Genet 1999;64:1694 –1701. 13. Kardia SLR, Haviland MB, Ferrell RE, Sing CF: The relationship between risk factor levels and presence of coronary artery calcification is dependent on apolipoprotein E genotype. Arterioscler Thromb Vasc Biol 1999;19:427–435. 14. Turner ST, Boerwinkle E, Sing CF: Context dependent associations of the ACE I/D polymorphism with blood pressure. Hypertension 1999;34:773–778.
2003; 16:154 –157
Genome-Wide Linkage Analyses for Hypertension Genes in Two Ethnically and Geographically Diverse Populations Sharon L.R. Kardia, Laura S. Rozek, Julia Krushkal, Robert E. Ferrell, Stephen T. Turner, Richard Hutchinson, Andrew Brown, Charles F. Sing, and Eric Boerwinkle We report the results of a genome-wide linkage scan for hypertension genes in 450 African American hypertensive sibpairs from Jackson, MS, and 539 non-Hispanic white hypertensive sibpairs from Rochester, MN. In the Jackson samples we identified one LOD score peak ⬎1.0 on chromosome 1. In the Rochester sample, no genomic region had a LOD score ⬎1.0. These analyses provide no appreciable evidence of hypertension genes with strong effects
H
independent of other genetic and environmental contexts and suggest that stratified linkage analyses may be required to identify hypertension susceptibility genes in these populations. Am J Hypertens 2003;16:154 –157 © 2003 American Journal of Hypertension, Ltd. Key Words: Linkage, hypertension, genetics.
ypertension, defined by a systolic/diastolic blood pressure (BP) ⱖ140/90 mm Hg, affects approximately 50 million Americans and is one of the most important risk factors for coronary artery disease, heart failure, stroke, and renal failure.1,2 Although BP is straightforward to measure and antihypertensive therapies are readily available, it is estimated that only 27% of hypertensives have their BP adequately controlled.3 It is also recognized that 30% to 50% of interindividual variation in BP is attributable to genetic variation.4 Because BP is regulated by numerous metabolic and structural characteristics of the cardiovascular, renal, nervous, and endocrine systems that involve the products of many genes, identifying those genes whose variations influence interindividual variation in BP may facilitate improved strategies for prevention, detection, evaluation, and treatment of hypertension. To this end, the Genetic Epidemiology Network of Arteriopathy (GENOA) was established as a member of the National Heart, Lung and Blood Institute (NHLBI) Family Blood Pressure Program (FBPP) to identify susceptibility genes for human hypertension in ethnically
diverse populations. Genome-wide analyses including both linkage and association approaches are being used to localize and characterize gene regions influencing interindividual variation in BP and the occurrence of hypertension. Because features of hypertension (eg, prevalence, age of onset, severity, complications) differ among ethnic groups, it is important to determine whether there are particular genes whose effects are unique to a particular ethnic or geographic group. The objective of the current study is to assess whether there is significant evidence for linkage between genome-wide markers and genes that are associated with hypertension in two ethnically and geographically different populations—African Americans from Jackson, MS, and non-Hispanic whites from Rochester, MN.
Received January 17, 2002. First decision June 1, 2002. Accepted November 5, 2002. From the Departments of Epidemiology (SLRK, LSR) and Department of Human Genetics (CFS), University of Michigan, Ann Arbor, Michigan; Department of Biology and Biotechnology (JK), Worcester Polytechnic Institute, Worcester, Massachusetts; Department of Human Genetics (REF), University of Pittsburgh, Pittsburgh, Pennsylvania; Division of Hypertension (STT), Mayo Clinic, Rochester, Minnesota; Department of Medicine (RH, AB), University of Mississippi, Jackson,
Mississippi; and Institute of Molecular Medicine and Human Genetics Center (EB), The University of Texas–Houston Health Science Center, Houston, Texas.
0895-7061/03/$30.00 PII S0895-7061(02)03249-1
Methods Subjects The primary inclusion criteria for a sibship into the GENOA study was to have at least two full siblings with essential hypertension, clinically diagnosed before age 60
The GENOA Network of the NHLBI FBPP is supported by HL54464 and HL54457. LSR was supported by T32 HG00040 from NCHGR. Address correspondence and reprint requests to Dr. Sharon L.R. Kardia, Department of Epidemiology, 109 Observatory Street, Ann Arbor, MI 48109-2029; e-mail: [email protected] © 2003 by the American Journal of Hypertension, Ltd. Published by Elsevier Science Inc.
AJH–February 2003–VOL. 16, NO. 2
years. In Jackson, MS, probands with essential hypertension diagnosed ⬍60 years of age were identified from participants in the Atherosclerosis Risk in Communities Study (ARIC), a multicenter study of the risk factors and occurrence of atherosclerotic disease.5 In Rochester, MN, probands were identified through the Rochester Epidemiology Project and the Mayo Medical Center diagnostic index for residents of Olmsted County, MN. However, once a sibship was ascertained, all sibs within a sibship were invited to participate. After identifying participants, a standardized classification scheme was used to validate the BP diagnostic category of each participant. This classification scheme took into account current and past antihypertensive medication use, prior diagnosis of hypertension, and BP measurements made at examination. All BP measurements were made with a random zero sphygmomanometer with a cuff appropriate for arm size. Three readings were taken in the right arm with the patient in the sitting position. A rest period of 5 min was observed before the first measurement. The systolic and diastolic pressures were the first and fifth phase Korotkoff sounds, respectively. The last two BP readings were averaged to arrive at the value used for diagnostic categorization. A more detailed account of the sampling methods and hypertension diagnostic criteria are presented elsewhere (Daniels et al, pers. communication). Microsatellite Genotyping For the Jackson sample, 338 autosomal microsatellite markers (CHLC/Weber screening set 6.0) were genotyped using standard polymerase chain reaction (PCR)-based methods and independently scored by two separate individuals at the University of Texas–Houston Health Science Center. For the Rochester sample, 381 autosomal microsatellite markers (CHLC/Weber screening set 9.0) were genotyped by standard PCR-based methods by the Mammalian Genotyping Center of the Marshfield Medical Research Foundation. Marker order and genetic map distances were those provided by the Marshfield Medical Research Foundation (www.marshmed.org/genetics). Statistical Analysis Because of the expected ethnic differences in the genetic architecture of hypertension between African Americans and non-Hispanic whites, we analyzed the Jackson and Rochester samples separately. Potential half-sibs, identical twins, and nonsibs were identified using ASPEX and then removed. Biologic inconsistencies in marker genotypes within sibships were identified using PEDCHECK.6 After data cleaning, genotypes from all individuals in a particular sample were used to estimate allele frequencies and to calculate the multipoint identity-by-descent (IBD) probabilities every 1 cM using the hidden Markov method implemented in GENEHUNTER 2.0.7 The maximum likelihood estimates of IBD sharing, using the possible trian-
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
155
gle constraint,8,9 among the affected sibpairs was then compared to the expected IBD sharing under the null hypothesis of no linkage using the log likelihood ratio. The final sample included 682 Jackson subjects distributed among 229 sibships and 775 Rochester subjects distributed among 257 sibships. The 229 Jackson sibships contained 996 total sibpairs including the 450 hypertensive sibpairs used in the analysis presented later. The 257 Rochester sibships contained 997 total sibpairs including the 539 hypertensive sibpairs used in the linkage analysis. Although all individuals within a sibship were used in the calculation of the IBD probabilities, only hypertensive sibpairs were used in the calculation of the LOD scores. The final calculation of the likelihood score (LOD) was weighted by the number of siblings in a sibship.10
Results The baseline characteristics of the Jackson and Rochester hypertensive sibs recruited by the GENOA Network are presented in Table 1. The average age of the hypertensive participants was 58.8 years (range, 35.1 to 86.7 years) for Jackson subjects and 58.0 years (range, 31.4 to 82.8 years) for Rochester subjects. The average body mass index was 31.3 kg/m2 (range, 14.9 to 63.4 kg/m2) for Jackson subjects and 29.9 kg/m2 (range, 15.8 to 67.9 kg/m2) for Rochester subjects. The Jackson sample was 69.8% female and the Rochester sample was 53.3% female. Results from the genome-wide linkage analysis are presented in Table 1. In the Jackson sample we identified only one LOD score peak ⬎1.0 on chromosome 1 at ⬃140 cM (from the p terminus). The LOD score peaks between 0.5 and 1.0 were found in regions of 3q, 7p, 8p, and 12p in the Jackson sample. Notable was the lack of any appreciable LOD scores ⬎0.5 on chromosomes 10, 11, 13 to 17, and 20 to 22. In the Rochester sample, no genomic region had a LOD score ⬎1.0, although a peak at 49 cM on chromosome 7 had a LOD score of 1.0 exactly. There were only three additional chromosomal regions (6p, 7q, and 16q) that had LOD scores between 0.5 and 1.0 in the Rochester, MN, sample. Again, it was notable that the remainder of the chromosomes (4, 10, 12, 13, 18, and 19) had essentially no appreciable LOD score ⬎0.5. The only region with a measurable LOD score common to both the Jackson and Rochester samples was a broad region on the short arm of chromosome 7.
Discussion In this study we found no appreciable evidence of linkage to hypertension susceptibility genes, although our samples were larger than many other reported studies. In this linkage study and others, there are several biologic and nonbiologic reasons for such a negative linkage result including: 1) genotyping errors, 2) incorrect estimates of allele frequencies, 3) misspecification of the hypertension phenotype, 4) loss of information associated with dichot-
156
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
AJH–February 2003–VOL. 16, NO. 2
Table 1. Baseline characteristics of the hypertensive sibling pairs Jackson (N ⴝ 480) Age Height (cm) Weight (kg) BMI Systolic BP Diastolic BP % Female
Chromosome 1p 3q 6p 7p 7q 8p 12p 16q
Rochester (N ⴝ 571)
Mean (SD)
Range
Mean (SD)
Range
58.8 (9.1) 166.15 (8.77) 86.19 (17.34) 31.31 (6.71) 141.4 (22.2) 79.9 (12.8) 69.8
35.1–86.7 145.50–197.9 46.13–164.55 14.89–63.4 86.0–223.0 35.0–118.0
58.0 (9.2) 169.12 (9.34) 85.92 (19.85) 29.96 (6.23) 137.1 (16.6) 79.9 (9.3) 53.3
31.4–82.8 138.9–202.0 42.40–184.80 15.79–67.88 100.0–215 52.0–110.0
Peak LOD Score
Position (cm)
Peak LOD Score
Position (cm)
1.2 0.58 — — 0.60 0.63 0.83 —
140 183 — — 87 61 50 —
— — 0.72 1.0 0.54 — — 0.69
— — 18 42 133 — — 109
BMI ⫽ body mass index; BP ⫽ blood pressure.
omizing the BP distribution into hypertensive/normotensive, and 5) the inappropriateness of using a single locus linkage strategy for a complex chronic disease. Although there are always levels of inexactitude in any measurement and estimation, the high quality of our genotype data, stringent hypertension diagnostic criteria, and allele frequency estimates are not likely to have a large impact on our results. The potential impact of the loss of information incurred from dichotomizing continuously distributed traits and the inappropriateness of a single locus linkage strategy is more difficult to assess and will require future study. Finally, we note that the relatively large number of affected sibpairs in our sample provide more than 90% power to detect a LOD ⬎3 (under a variety of genetic models) given an estimated relative recurrence risk of 1.5 for hypertension. There have been only a modest number of studies that have reported evidence of linkage to genetic loci accounting for variation in BP levels or risk of hypertension. Using highly discordant non-Hispanic white sibships from Rochester, MN, Krushkal et al11 reported one of the first genome-wide linkage analyses for a gene influencing systolic BP. They identified four chromosomal regions (2p, 5q, 6q, and 15q) with significant evidence of linkage. We note that in our linkage analyses of a separate sample from Rochester, MN, we did have a modest LOD score peak on chromosome 2p. Xu et al12 also reported evidence of linkage on chromosomes 3, 11, 15, 16, and 17 from a genome-wide linkage analyses in a very large sample of Chinese hypertensive sibpairs. These studies, as well as others, suggest that there are likely to be different sets of genes contributing to the genetic architecture of hypertension in different ethnic groups.
Moreover, as BP homeostasis involves a multitude of traits with balancing pressor and depressor roles, the influence of variation in any one gene is expected to be distributed, and hence muted, across biochemical, physiologic, and anatomic levels of the cardiovascular, renal, nervous, and endocrine systems that regulate BP levels. Consequently, the effect of any one gene or gene region is expected to be small and inconsistent across different genetic and environmental backgrounds. Hence, it is not surprising that we did not detect strong evidence of linkage in these genome-wide linkage analyses. Our experiences with candidate gene association with complex disease risk factors13,14 indicate that context-dependent linkage analyses may be required to identify new hypertension susceptibility gene regions in these populations.
References 1.
2. 3.
4.
5.
Burt VL, Whelton P, Roccella EJ, Brown C, Cutler JA, Higgins M, Horan MJ, Labarthe D: Prevalence of hypertension in the US adult population: Results from the Third National Health and Nutrition Examination Survey, 1988 –91. Hypertension 1995;25:305–313. Whelton PK: Epidemiology of hypertension. Lancet 1994;334:101– 106. JNC: The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Arch Intern Med 1997;157:2413–2446. Ward R: Familial aggregation and genetic epidemiology of hypertension, in Laragh JH, Brenner BM (eds): Hypertension: Pathophysiology, Diagnosis, and Management, 2nd ed. New York, Raven Press, 1995; pp 67– 88. ARIC Investigators: The Atherosclerosis Risk in Communities (ARIC) Study: Design and objectives. Am J Epidemiol 1989;129: 687–702.
AJH–February 2003–VOL. 16, NO. 2
6.
O’Connell JR, Weeks DE: Pedcheck: A program for identification of genotypic incompatibilities in linkage analysis. Am J Hum Genet 1998;63:259 –266. 7. Kruglyak L, Daly MJ, Reeve-Daly MP, Lander ES: Parametric and nonparametric linkage analysis: a unified multipoint approach. Am J Hum Genet 1996;58:1347–1363. 8. Holmans P: Asymptotic properties of affected sib pair linkage analysis. Am J Hum Genet 1993;52:362–374. 9. Kruglyak L, Lander ES: Complete multipoint sib-pair analysis of qualitative and quantitative traits. Am J Human Genet 1995;57: 439 –454. 10. Lange K: A test statistic for the affected-sib-set method. Ann Hum Genet 1986;50:283–290. 11. Krushkal J, Ferrell R, Mockrin SC, Turner ST, Sing CF, Boerwinkle
GENOME-WIDE SCAN FOR HYPERTENSION: GENOA
157
E: Genome-wide linkage analyses of systolic blood pressure using highly discordant siblings. Circulation 1999;99:1407–1410. 12. Xu X, Rogus JJ, Terwedow HA, Yang J, Wang Z, Chen C, Niu T, Wang B, Xu H, Weiss S, Schork NJ, Fang Z: An extreme-sib-pair genome scan for genes regulating blood pressure. Am J Hum Genet 1999;64:1694 –1701. 13. Kardia SLR, Haviland MB, Ferrell RE, Sing CF: The relationship between risk factor levels and presence of coronary artery calcification is dependent on apolipoprotein E genotype. Arterioscler Thromb Vasc Biol 1999;19:427–435. 14. Turner ST, Boerwinkle E, Sing CF: Context dependent associations of the ACE I/D polymorphism with blood pressure. Hypertension 1999;34:773–778.