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(Hypertension. 1995;25:320-326.)
© 1995 American Heart Association, Inc.
Articles |
-Adducin Locus With Essential Hypertension
From the Division of Nephrology, Dialysis and Hypertension, University of Milan, S. Raffaele Hospital; Prassis-Sigma-Tau Research Institute, Settimo Milanese, Milan; the Blood Transfusion and Immunology of Transplantation Centre, Ospedale Maggiore Policlinico, Milan; the Department of Neuroscience, University of Milan, S. Raffaele Hospital; and the Department of Genetics and Microbiology, University of Milan (Italy).
Correspondence to Prof Giuseppe Bianchi, University of Milan, Division of Nephrology, Dialysis and Hypertension, S. Raffaele Hospital, via Olgettina 60, 20132 Milan, Italy.
| Abstract |
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-adducin gene may regulate blood
pressure. Adducin is a cytoskeletal protein that may be involved in
cellular signal transduction and interacts with other membrane-skeleton
proteins that affect ion transport across the cell membrane. There is a
high homology between rat and human adducin and pathophysiological
similarities between the Milan hypertensive rat strain and a subgroup
of patients with essential hypertension. Thus, we designed a
case-control study to test the possible association between the
-adducin locus and hypertension. One hundred ninety primary
hypertensive patients were compared with 126 control subjects. All
subjects were white and unrelated. Four multiallelic markers
surrounding the
-adducin locus located in 4p16.3 were selected:
D4S125 and D4S95 mapping at 680 and 20 kb centromeric, and D4S43 and
D4S228/E24 mapping at 660 and 2500 kb telomeric. Alleles for each
marker were pooled into groups. Comparisons between control subjects
and hypertensive patients were carried out by testing the
allele-disease association relative to the marker genotype. The maximal
association occurred for D4S95 (
12 13.33),
which maps closest to
-adducin. These data suggest that a
polymorphism within the
-adducin gene may affect blood pressure in
humans.
Key Words: case-control studies hypertension, essential polymorphism (genetics) cytoskeleton genes
| Introduction |
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Adducin migrates from the cytoplasm to the cell-to-cell contact sites in a calcium- and phosphorylation-dependent manner in renal tubular cells in culture.11 Adducin favors the spectrin-actin interaction, thus affecting the structure of the cell membrane skeleton. The precise role of adducin is unknown; however, in view of the proposed role of actin, ankirin, and other skeleton membrane proteins in the regulation of cell membrane ion transport,12 13 we postulated that adducin polymorphism may also be involved in the regulation of BP.
Adducin is a dimer with
- and ß-subunits whose genes map in both
humans and rats on different chromosomes14 (G.C.,
unpublished observation). The determination of the MHS and MNS adducin
cDNA sequences showed one point mutation in each of the two genes
coding for the
- and ß-subunits of adducin.10 The
genetic analysis in rats showed that the mutation of
-adducin
alone could account for the most significant BP variation, whereas the
mutation on the ß-subunit was only modulating the effect of the
-subunit.10
On the assumption that hypertension in humans may also be
influenced by functional mutations within the
-adducin gene, which
in turn is in linkage disequilibrium with DNA markers mapping close to
the adducin locus, we designed a case-control study for testing a
possible association between 4p16.3 marker polymorphism and
hypertension. The results show a highly significant association of the
minisatellite D4S95 with hypertension. The association weakens and
disappears for markers mapping farther from the adducin locus.
Therefore, our results are consistent with the hypothesis that
-adducin polymorphism may account for a significant portion of BP
genetic variation in humans.
| Methods |
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To avoid recruitment of false-positive or secondary hypertensive patients and false-negative control subjects, we applied the following inclusion criteria. All hypertensive patients underwent standard screening for secondary forms of hypertension. Selected hypertensive patients either had a BP higher than 150/95 mm Hg, if recruited before the onset of antihypertensive treatment, or were under antihypertensive treatment at the time of blood sampling. Moreover, each patient was less than 60 years of age before the onset of hypertension. BP information and data on the initiation of pharmacological treatment were available for each subject. For those patients considered as urgently needing pharmacological treatment, only one pretreatment value was available. The less severe hypertensive patients (n=160) were routinely monitored for up to 3 months before the start of pharmacological treatment (so that three measurements of BP in three consecutive visits at the referral center were available). The mean duration of hypertension in our sample was 8.02±0.5 years.
To exclude false-negative control subjects with late onset of hypertension, subjects were older than 60 years, had not been diagnosed or treated as hypertensive earlier in life, had a negative (self-reported) family history of hypertension, and had a BP value less than 150/85 mm Hg. They were in all cases recruited before surgery, and their BP values were recorded several times during their stay in the hospital.
Informed consent to this study was obtained from each individual recruited.
Genotype Analysis and Clustering of Alleles
Genomic DNA was isolated from 3 mL of whole blood following a
standard procedure.15 The four markers surrounding the
-adducin geneD4S12516 and D4S9517
centromeric, and D4S4318 and D4S228/E2419
telomeric, mapping 680, 20, 660, and 2500 kb from the
-adducin
locus, respectivelywere selected for their high polymorphism
information content values and the possibility of rapid analysis by
polymerase chain reaction (PCR). To increase sensitivity and
interexperiment reproducibility, we used nondenaturing polyacrylamide
(5% or 6%) gel electrophoresis to separate D4S125 and D4S95 allelic
forms, thus increasing the number of resolvable alleles to 28 and 30,
respectively. Allelic bands were fixed and silver
stained.20 The 28 different alleles of D4S125 range from
1600 to 2400 bp as reported.16 The 30 allelic forms of
D4S95 span from 990 to 1600 bp as in Allitto et al.17
After PCR cycling, D4S43 and D4S228/E24 were resolved with a denaturing
6% or 5% polyacrylamide gel, respectively. Gels were blotted onto
positively charged nylon membranes and hybridized with
32P-labeled D4S43 or D4S228/E24 PCR product. Sixteen
alleles, from 184 to 478 bp, were detected with marker D4S43.
Microsatellite D4S228/E24 gave nine different alleles, differing from
each other by a single CA repeat. For convenience, all alleles of each
marker were numbered from the shortest (allele 1) to the longest
(allele n). To obtain an accurate and reproducible reading of the
genotypes from gel to gel, we loaded onto each gel a specific
minisatellite marker generated by pooling several single PCR products
from DNAs of known genotype and two commercial molecular weight DNA
markers (FX HaeIII digested and 1-kb ladder from BRL). All
genotype determination underwent double-blind scoring to minimize gel
reading errors.
To overcome the problem of dealing with the high number of different
allelic forms resolved for each marker, we clustered alleles, thus
reducing the number of comparisons and the risk of type 1 errors.
Clustering is implied by the fact that the different number of tandem
repeats resulting in the corresponding alleles is not evenly scattered
(between the shortest and longest); that is, they tend to concentrate
in groups separated by a "valley" of a few or, as for D4S43 and
D4S95, of missing allelic forms. Mutations generating alleles of new
lengths occur much more frequently at hypervariable loci than in coding
regions and are probably due to deletion or duplication of one or more
repeat units arising by sister chromatid exchange or DNA slippage
during replication.21 22 In fact, the gain or loss of one
or a few minisatellite repeats is much more frequent than the
appearance of new allelic forms deriving from the insertion or deletion
of a large number of minisatellite repeat units.23 This
supports the hypothesis that every cluster of alleles originated from a
common ancestor, which displayed a number of repeat units close (or
corresponding) to the average number of repeats of the considered
cluster. Thus, a progenitor of an allele cluster is assumed to be
linked to a cosegregating mutation in a gene close to the marker, in
our case
-adducin.
The clustering method is based on the pattern of allelic frequency
distribution shown by the examination of the normal probability plot
and the detrended normal probability plot and later confirmed by study
of the frequency distribution of the raw data. We used this descriptive
statistical method, which approximates to normal the commingling
distributions. In the normal probability plot, each observed value was
paired with its expected value from the normal distribution. The
expected value from the normal distribution is based on the number of
cases and the rank order of the case in the sample. If the sample is
from a normal distribution, we expect that the points will fall on a
straight line. Visually, it is more practical to look at the detrended
plot, which expresses the difference between observed and expected
values (Fig 1, bottom left). Each change of slope on the
normal probability plot or each inversion of direction for the
detrended plot corresponds to a minimum (a valley) for the
distribution, thereby representing the possible boundaries of
natural allele groups. Fig 1 summarizes the data for D4S95, the closest
marker to
-adducin. Alternatively, clusters of alleles were also
grouped "binomially" into short versus long ones, similar to what
was previously done for the apolipoprotein B 3' variable number tandem
repeat (VNTR).24 Although such binomial division is
evidently an arbitrary one, it has the practical consequence of
reducing to two the possible number of allelic forms for each marker,
making the analysis simpler and allowing the analysis for
genotype.
|
Statistical Methods
All clinical parameters are expressed as mean±SEM. Mean BP
values and anthropometric variables were compared with Student's
t test. Sex ratios and allele-disease association were
tested by
2. Statistical analysis was
performed with the use of SPSS (version 4) statistical software.
| Results |
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The frequency distribution obtained with D4S43 on 81 unrelated whites previously studied18 was compared with our normotensive and hypertensive groups: the first was quite similar, and the latter showed different frequencies, in particular for allele 1 (see Reference 1818 : 25%; normotensive, 26.2%; hypertensive, 36.3%; P<.01) and allele 11 (see Reference 18: 16%; normotensive, 15.1%; hypertensive, 9.5%; P<.05). No frequency distribution data were available for D4S228/E24. Previous articles involving linkage analysis with markers D4S125 and D4S95 by differential allelic migration on agarose gel electrophoresis detected 925 and 1526 different alleles, respectively. Therefore, no comparison can be made with our results because a larger number of alleles was detected in our experimental conditions.
Genotyping and Clustering of Alleles
From the data provided by the analysis of the distribution for
each marker, we extrapolated the following major groups of alleles (the
cutoff points are indicated by the arrows in Fig 1) for marker D4S95:
First group
13; second group >13 but
22; and third group >22. An
identical approach was followed for clustering the alleles of other
markers. D4S125 was divided into four major clusters of alleles (
7,
>7 but
14, >14 but
20, and >20), D4S43 into three major clusters
(
3, >3 but
8, and >8), and D4S228/E24 was again divided into
three major clusters (
4, >4 but
5, and >5).
Allele-Disease Association
Table 2 summarizes the data of the association
study for the four markers with hypertension. D4S95 and D4S43 allelic
distribution was significantly different between control subjects and
hypertensive patients. The binomial division cutoff points into short
versus long alleles were alleles 13 and 14 for D4S95, 14 and 15 for
D4S125, 3 and 4 for D4S43, and 5 and 6 for D4S228/E24. The results of
the allele-disease association using such binomial division cutoff
points are summarized in Fig 2. They are in agreement
with results obtained by considering all major clusters and show a
significant association between markers D4S125, D4S95, and D4S43 and
hypertension.
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Interference of Other Clinical Variables
Because of the study design, our control subjects were older than
the hypertensive patients. Also, differences in BMI and sex
distribution between the groups were present. Since any of these
differences may have contributed per se to determining the attainment
of the allele-disease association, we performed the following tests to
exclude this possibility.
Effect of Age
The effect of age was tested in three ways. First, 29 hypertensive
patients were older than 60 (mean age, 65.8±1.02 years) at the time of
blood sampling, since their hypertension was diagnosed many years
before. No difference of allelic distribution for each marker was found
when these subjects were compared with the hypertensive patients
younger than 60 (mean age, 48.1±0.62 years); however, the number of
old hypertensive patients is small. Second, the analysis was
repeated using as a cutoff the median of age (51 years for the
hypertensive patients, 69 for the normotensive subjects), so that two
groups of approximately the same size were found. No difference of
allelic distribution for each marker was found by comparing
hypertensive patients (or normotensive subjects) younger than the
median for their age versus hypertensive patients (or normotensive
subjects) older than the median for their age (data not shown). Third,
we compared the distribution of each marker only in hypertensive
patients older than 51 (the median value for the age of all
hypertensive patients) and normotensive subjects younger than 76 (which
corresponds to the oldest age of the hypertensive patients). Despite
having halved the difference in age between hypertensive patients and
control subjects (58.4±0.35 and 67.4±0.35, respectively), results
very similar to those for the total sample were obtained
(
2 and significance levels for D4S125, D4S95,
D4S43, and D4S228/E24, respectively:
12
2.31, P=.13;
12 7.0,
P=.008;
12 7.2,
P=.007; and
12 0.01,
P=.90).
Effect of BMI
The effect of BMI was tested by removing individuals with extreme
BMI values (<20 and >28 kg/m2). When the analysis was
limited to 102 control subjects and 137 hypertensive patients with mean
BMI values of 24.0±0.21 and 24.7±0.16 kg/m2,
respectively, the results were similar to those obtained for the total
sample (
2 and significance levels for D4S125,
D4S95, D4S43, and D4S228/E24, respectively:
12 1.95, P=.16;
12 11.75, P=.0006;
12 7.64, P=.006;
12 0.10, P=.75). As for age, the
analysis was also repeated using as a cutoff the median of BMI (for
hypertensive patients, 25.88 kg/m2; for normotensive
subjects, 23.82 kg/m2) so that two groups of the same size
were found. No difference of allelic distribution for each marker was
found by comparing hypertensive patients (or normotensive subjects)
whose BMI was smaller than the median of their BMI with hypertensive
patients (or normotensive subjects) whose BMI was greater than the
median of their BMI (data not shown).
Effect of Sex
No significant difference in allelic distribution between
men and women was seen when our sample was considered globally
(
2 and significance levels for D4S125, D4S95,
D4S43, and D4S228/E24, respectively:
12
0.056, P=.81;
12 2.86,
P=.91;
12 0.113,
P=.74;
12 0.773,
P=.38) or after hypertensive patients alone or normotensive
subjects alone were considered (data not shown).
| Discussion |
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-adducin gene. Moreover, the most
significant difference occurs with a DNA marker on the short arm of
chromosome 4 (4p16.3), very close (approximately 20 kb) to the
-adducin locus, whereas the marker mapping far from the
-adducin
locus (approximately 2500 kb) did not reach statistical significance.
These findings are consistent with the working hypothesis originating
from the studies on the rat model and point to a very narrow DNA region
close to (or corresponding to) the
-adducin locus. However, a
gene-disease association arising from a case-control study should be
considered with caution because of possible sampling
errors.27 We have previously shown that hypertension tends to be a recessive trait in rats.28 It is likely that such a mode of inheritance may also occur in humans if the same genetic mechanism is involved. For this reason, young normotensive offspring of normotensive parents could develop hypertension later in life because of homozygosity of recessive alleles. Therefore, we were compelled to use as control subjects only individuals who were still normotensive after the age of 60 years. By selecting the control subjects older than 60, we minimized the number of hypertensive alleles in this population. However, because of the age difference between hypertensive patients and control subjects, it is not possible to exclude a selective accumulation of alleles independent of hypertension as age progresses. Therefore, we analyzed the effect of age as well as the influence of BMI and gender in the present study. The distribution of the alleles of each marker studied was not affected by age. Despite 14.8 years of age difference between hypertensive patients younger and older than the median age of 51 years, no difference in allelic distribution for each marker was found. Similar results were obtained for the comparison of hypertensive patients younger and older than 60. On the contrary, when only the hypertensive patients older than the median are considered, the significant difference for allelic distribution between control subjects and hypertensive patients remains.
The removal from both groups of a subset of individuals who were at the extreme of BMI distribution did not affect the significance of the allele-disease association, ruling out the possible influence of this variable. Finally, no sex effect could be detected.
The different allelic distribution between hypertensive patients and
control subjects seems to focus on the narrow region containing the
-adducin locus. Most importantly, the allelic distribution of the
normotensive population reported here for the D4S43 marker is similar
to the distribution reported in a different white
population.18 On the contrary, the same comparison with
our hypertensive patients showed different frequencies for alleles 1
and 11.
To date, many candidate genes coding for proteins known to be involved in BP regulation have been considered for genetic studies in both animal models of hypertension and humans with primary hypertension. The results obtained in these studies have suggested a genetic role for angiotensinogen,29 aldosterone synthase,30 and, controversially, angiotensin-converting enzyme31 32 in humans as well as renin,33 34 11ß-hydroxylase,35 kallikrein,36 and angiotensin-converting enzyme37 38 in rats.
Our approach was different. We addressed our attention to adducin as a candidate for genetic studies in MHS after a long series of observations on renal transplantation39 and function40 41 and ion transport across the cell membrane of intact cells or membrane vesicles either with42 43 44 45 46 or without47 membrane skeleton. These observations were consistent with the hypothesis that hypertension in MHS was caused by a faster ion transport across cell membrane, which in turn was sustained by an abnormal membrane skeleton protein, subsequently identified as adducin.8
Because of these findings in rats, the high degree of amino acid
homology between rat and human adducin (94%, unpublished observation),
and pathophysiological similarities between MHS and a subgroup of
patients with primary hypertension,2 we tested the
hypothesis that the
-adducin locus could be involved in BP
regulation in humans. The results described here are consistent with
this hypothesis.
Our data do not exclude the possibility that other genes mapping in the
same region (both knownsuch as the one coding for fibroblast growth
factor receptor 3,48
-L-iduronidase,49 ß-polypeptide of
cyclic GMP phosphodiesterase [PDEB],50 G
proteincoupled receptor kinase,51 and the Huntington's
diseaserelated protein52 or unknown) could be in
linkage disequilibrium with the D4S95 marker and also contribute to the
genetic BP variation.
The adducin polymorphism might account for only a portion of genetic
variation of BP, in agreement with the common and well-supported
hypothesis that hypertension is a heterogeneous and polygenic disease.
In MHS the ß-adducin gene polymorphism modulates the influence of
-adducin mutation on BP, but certainly many other epistatic
interactions are possible in both rats and humans.
In conclusion, our findings relating to both rats and humans are consistent with the hypothesis that a constitutive cell membrane skeleton protein may contribute to BP control in two mammalian species that diverged approximately 50 to 60 million years ago.53
| Acknowledgments |
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Received March 29, 1994; first decision May 4, 1994; accepted October 25, 1994.
| References |
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