(Hypertension. 2001;37:425.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Division of Epidemiology and Institute of Human Genetics (L.D.A.), University of Minnesota, Minneapolis; the Department of Genetics (P.B.S., J.W.M.), Southwest Foundation for Biomedical Research, San Antonio, Tex; the Human Genetics Center (J.E.H.), University of Texas Health Science Center, Houston; and the Department of Medicine (M.P.S.), University of Texas Health Science Center, San Antonio.
Correspondence to Dr Larry Atwood, Division of Epidemiology, University of Minnesota, 1300 South Second St, Suite 300, Minneapolis, MN 55454-1015. E-mail atwood{at}epi.umn.edu
| Abstract |
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) of 0.02.
D7S1799 had a lod score of 2.04
(
=0.01), D8S1100 had a lod
score of 1.98 (
=0.08), and
D18S844 had a lod score of 1.95
(
=0.11). These results are highly correlated with results involving
systolic blood pressure, indicating that pulse pressure may not
be genetically distinct from systolic blood
pressure.
Key Words: population genetics blood pressure hypertension, genetic cardiovascular diseases
| Introduction |
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The strong effects of sex, age, and body mass index (BMI) on blood pressure in the general population are well known.4 The high correlation between systolic blood pressure (SBP) and PP suggests that these effects exist for PP as well. In most genetic analyses, a simple correction that is held constant across genotypes is used to account for these effects. A possible interaction between genes and PP has not been reported in any study. Indeed, only a few studies have looked for interactions between common covariates and SBP or diastolic blood pressure (DBP). A segregation analysis5 of a large number of randomly ascertained nuclear families showed that there were significant genotype-specific effects of both sex and age on SBP. Recently, Turner et al6 have shown that the ACE insertion/deletion polymorphism has genotype-specific effects on blood pressure. Furthermore, Kardia7 has called for a more explicit consideration of gene-gene and gene-environment interactions in the study of blood pressure traits. In the present study, we modeled PP with sex-specific and genotype-specific effects of age, age2, and BMI. Then, using the resulting model, we performed genome-wide 2-point lod score (z) analysis on PP. We used conservative criteria8 for claiming significant (z>3.3) and suggestive (z>1.9) linkage. We also report results that are weakly suggestive (z>0.59, which corresponds to a value of P<0.05). Given the known high correlation between PP and SBP, we also compare the genome scans for the 2 measures.
| Methods |
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16
years and living in the San Antonio area. Because these criteria show
no obvious selection bias with respect to blood pressure, this data set
can be considered a random sample from the San Antonio Mexican American
population. The proband and all first-, second-, and third-degree
relatives willing to participate were subjected to an extensive
data-gathering protocol in which demographic information on
morphometrics, cigarette smoking, alcohol consumption, dietary
behavior, and physical activity were obtained. For all individuals,
blood pressure was measured 3 times after a 5-minute seated rest period
with a random zero sphygmomanometer. Blood pressure was calculated by
dropping the first reading and averaging the latter 2 readings. PP was
calculated as the difference between SBP and DBP. The model-building
phase of the study included 41 extended families. The total number of
individuals was 1777 (1308 with blood pressure data). Several families
had marriages with no children with complete data. Removing these
marriage links occasionally had the effect of turning 1 large family
into multiple smaller families that are computationally more efficient.
This increased the actual number of families analyzed to
46. The SAFHS chose 10 families for initial genotyping, and the linkage reported in the present study is based on these 10 families. These 10 families (637 individuals, 495 with PP data) were chosen, on the basis of size, to maximize statistical power to detect linkage. Inasmuch as this selection shows no obvious bias with respect to PP, we regard the 10 families as a random sample with respect to PP. However, it must be recognized that this selection procedure may have introduced hidden bias with respect to PP.
All individuals who participated in SAFHS gave informed consent, and the Institutional Review Board of the University of Texas Health Science Center at San Antonio approved all protocols.
Genotyping
Genotyping of 399 polymorphic markers was
accomplished by using standard methods described by Atwood et
al.9 Polymorphic markers
used were from the MapPairs 6 and 8 Linkage Screening Sets (Research
Genetics, Inc). An AGT 3'
dinucleotide
polymorphism10 was typed
as described by Atwood et
al.11 There were 399
polymorphic markers, both candidate loci and anonymous marker loci,
that had heterozygosity of
0.60 and were included in the linkage
analysis. The average number of genotyped individuals
per marker used in the linkage analysis was 441 (SD 19) in 10
families. To save computation time in the linkage analysis, any
marker that had >7 alleles was reduced to 7 alleles by using
the DOWNCODE12 program, which
minimizes information loss. The average heterozygosity of all included
downcoded markers was 0.75 (SD 0.06), ranging from 0.60 to 0.85. All
genetic locations used in the present study are given in
centimorgans from the p-terminus and are taken from the Genetic
Location Database.13 The
Genetic Location Database is the only database that contained all
markers of interest and therefore provides consistent location
estimates.
Statistical Analysis
Using version 4 of the Pedigree Analysis
Package computer program,14
we defined a single-locus 2-allele mendelian model for PP. This
model incorporated sex-specific genotypic means and sex-specific and
genotype-specific effects of age,
age2, and BMI. Theoretical aspects of
gene-covariate interactions were first developed for this type of model
by Konigsberg et al.15 All 3
quantitative covariates were standardized to have a mean of 0 and an SD
of 1. Genotype frequencies were assumed to be in Hardy-Weinberg
equilibrium, and residual familial correlations were modeled with a
heritability parameter. Residual variance was held equal
across all genotypes. Under the assumption that these pedigrees
constitute a population-based sample of the San Antonio Mexican
American population, no ascertainment correction was necessary. Data
for individuals with extreme values of PP (SD
±4) were removed from
consideration. In a separate set of nuclear families, a Monte Carlo
test showed no effect of medication on PP. This is consistent
with the assumption that medication has the same effect on SBP and DBP.
Thus, no correction for medication was performed in this
analysis. Only 10% of the individuals in this data set were on
high blood pressure medication.
Maximum likelihood estimates for this model were computed with the use of NPSOL, one of the maximization options available in Pedigree Analysis Package. To ensure that the global maximum was found, the model was maximized 500 times with different random initial estimates.
Is the maximum likelihood model valid for linkage analysis? Blood pressure is a complex trait, and the single locus model is certainly incorrect. However, Williamson and Amos16 have shown that linkage analysis is valid when the genetic trait model is incorrect but the marker model is correct. Because this sample shows no selection bias with respect to blood pressure, allele frequencies based on the data are a random sample of the population; ie, the marker model is correct. Therefore, the linkage analysis is valid. Would a major gene from a prior segregation analysis improve power? An analysis of Genetic Analysis Workshop 9 data17 showed that requiring a major gene from a segregation analysis before linkage analysis can actually reduce overall power to detect linkage for a complex trait. Thus, a segregation analysis is not necessary to achieve a valid error rate and may reduce power; therefore, we did not perform one on this data set.
To determine whether the sex-specific and
genotype-specific parameters were significant,
several subset models were analyzed to test these
parameters with respect to age,
age2, and BMI. In these subsets, the
sex-specific or genotype-specific parameters were
removed, and the maximum likelihood of the resulting subset model was
computed. Examination of all possible subsets is computationally
impractical, so we tested subsets grouped by age (this group included
both age and age2) and BMI, in that order.
All subsets were compared with the full 27-parameter model
by a
2 test. If a sex-specific or
genotype-specific component was found to be nonsignificant, it
was removed from further consideration.
Using version 4 of the Pedigree Analysis
Package,14 we then performed
2-point linkage analysis on the 10 SAFHS families that had been
genotyped. In the linkage analysis, one locus that was
constructed on all 46 families was described by the mendelian model,
and the other locus was 1 of the 399 polymorphic markers. Marker
allele frequencies were estimated from the data.
Parameter estimates for both loci were fixed; thus, the
only free parameter in the linkage model was the
recombination fraction (
). The lod score
(z) at the maximum likelihood
estimate of
was computed as
logL(
max)-logL(
=0.5). For any
significant linkage, we also tested for sex-specific recombination and
heterogeneity between
families.
| Results |
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The
Table
shows the maximum lod score and recombination fraction for all markers
with maximum lod scores >0.59 (equivalent to
P=0.05 for a 1-tailed test) for
PP. There were no markers with a lod score >3.3, which is the
criterion for significant linkage in families proposed by Lander and
Kruglyak.8 However, there were
4 markers with maximum lod scores >1.9, which is their criterion for
suggestive linkage in families.
D21S1440 had a maximum lod
score of 2.78 at a
value of 0.02.
D7S1799 had a maximum lod score
of 2.04 (
=0.01), D8S1100 had
a maximum lod score of 1.98 (
=0.08), and
D18S844 had a maximum lod score
of 1.95 (
=0.11). The lod scores for flanking markers were generally
low, with the highest coming on the p-terminus side of
D8S1100 at marker
D8S373, which had a maximum lod
score of 0.59.
|
The raw variance of PP was 234.9. The polygenic model, ie, the model with no single locus component, yielded a heritability of 0.21 for PP and a residual variance of 128.4 after accounting for simple linear effects of sex, age, and BMI. Thus, the polygenic model accounted for 45% of the total variation in PP. The full model, ie, the polygenic model plus the single locus components, gave a residual variance of 62.4. Thus, the full model accounted for 73% of the total variation in PP. The correlation between PP and SBP was 0.84, and the correlation between PP and DBP was 0.
| Discussion |
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In the model used for PP in the present study, we found significant sex-specific and genotype-specific effects of age, age2, and BMI on PP. Given the strong correlation between SBP and PP, these results can be regarded as confirming the result of Perusse et al,5 who found sex-specific and genotype-specific effects of age on SBP.
Support for linkage of quantitative trait loci (QTL)
affecting PP to the 4 regions located by the present study can be
found in at least 1 independent study for each region. The suggestive
linkage of a QTL affecting PP to chromosome 21
(D21S1440,
z=2.78 and
=0.02) is
supported by evidence from 2 genome scans. Krushkal et
al,18 in a study of SBP,
found a maximum lod score of 0.86 with a locus 3 cM away from
D21S1440. Further support comes
from a genome scan of DBP performed by Xu et
al,19 who found a maximum lod
score of 2.20 with a locus 11 cM away from
D21S1440.
The suggestive linkage of a QTL affecting PP to chromosome 7
(D7S1799,
z=2.04 and
=0.01) is
supported by a genome scan of SBP reported by Rice et
al.20 They found a maximum
lod score of 2.26 with a locus 20 cM from
D7S1799.
The suggestive linkage of a QTL affecting PP to chromosome 8
(D8S1100,
z=1.98 and
=0.08) is
supported by several studies.
D8S1100 is 1.3 cM away from the
aldosterone synthase gene
(CYP11B2). A chimeric fusion of
the aldosterone synthase gene and the adjacent 11
ß-hydroxylase gene (CYP11B1)
is known to cause glucocorticoid-remediable aldosteronism, a rare
monogenic form of
hypertension.21 Further
evidence that variation at this candidate gene influences blood
pressure variation comes from Davies et
al,22 who performed a
case-control study of individuals from Glasgow, Scotland, in which
polymorphisms in CYP11B2
were associated with hypertension and aldosterone
excretion.
The suggestive linkage of a QTL affecting PP to chromosome
18 (D18S844,
z=1.95 and
=0.11) is
supported by 2 studies. Pankow et
al,23 in a genome scan of the
response of SBP to postural change, found a maximum lod score of 2.6
with a locus 27 cM away from
D18S844. DeStefano et
al,24 in a study of
orthostatic hypotension, found a significant maximum lod
score of 3.21 with a locus 8.0 cM away from
D18S844.
One of the 399 markers in the genome scan was a highly
polymorphic marker at the angiotensinogen
(AGT) locus.
AGT is the most widely studied
candidate gene for primary hypertension and has been linked to
hypertension in this data set using hypertensive
sibpairs.11
AGT is therefore a natural
candidate for linkage to PP; however, in this analysis,
AGT was excluded from linkage
to PP (z=-9.94 at
=0.00).
Thus, although AGT may have
some effect in hypertensive subjects, it may not have a significant
effect on the population.
In summary, we have found suggestive evidence for QTL that affects PP in 4 distinct chromosomal regions in a randomly ascertained Mexican American population. A comparison of genome scans indicates that PP and SBP are likely to have the same genetic etiology.
| Acknowledgments |
|---|
Received October 25, 2000; first decision December 11, 2000; accepted December 19, 2000.
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