(Hypertension. 2000;35:568.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From the Centre de Recherche du CHUM, Université de Montréal, Montréal, Québec, Canada (P.D., Z.P., M.D., Y.S., J.T., P.H.); the Institute of Biology, First Medical Faculty, Charles University, Prague, Czech Republic (V.K., D.K., M.P.); the Institute of Physiology, Czech Academy of Sciences, Prague, Czech Republic (V.K., M.P.); and the Department of Biology, University of Akron, Akron, Ohio (D.E., M.T.).
Correspondence to Johanne Tremblay or Pavel Hamet, Laboratory of Cellular Biology of Hypertension and Laboratory of Molecular Medicine, Centre de Recherche-CHUM, 3850 Rue Saint-Urbain, Montréal, Québec, Canada H2W 1T8. E-mail hamet{at}ere.umontreal.ca
| Abstract |
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Key Words: stress linkage (genetic) quantitative trait sex chromosomes
| Introduction |
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| Methods |
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A total of 27 RIS were used for stress-response phenotyping. The high Na diet consisted of normal rat chow (0.6% Na) supplemented with NaCl to raise the Na content to 3.15% by weight (8% NaCl). Y Chr consomic strains, originally designated as SHR/y and SHR/a, were developed from Wistar-Kyoto (WKY) and SHR inbred colonies kept at the University of Akron, Ohio, as described previously.12 For uniformity of nomenclature, we designated them here as WKY.SHR-Y and SHR.WKY-Y, respectively, according to their somatic and sexual Chr origins.
The SHR.BN-Y consomic strain was produced by introgressing the Y Chr from BN males into the SHR genetic background (V. Kren, DSc, et al, unpublished data, 1998). The BN Y Chr is cytogenetically distinguishable from the Y Chr of SHR origin (M. Sladká, oral communication, 1998). SHR.BN-Rattus norvegicus Chr 10 (RNo10) congenics were obtained by introgressing a segment of BN Chr 10 onto the SHR genetic background. After 9 backcrosses, an intercross was performed, and Myh3 BB homozygotes were selected by polymerase chain reaction genotyping. Myh3 BBhomozygous congenic males from N9F3 with residual heterozygosity in Myh3 flanking markers were used. D10Wox11, D10Mit4, and D10Rat31 markers were BB homozygotes. Residual heterozygosity was observed for the following Myh3 flanking markers: D10Wox12, D10Wox14, D10Mit2, D10 Mgh6, D10Rat59, and D10Rat102. D10Mit1/Ace, D10Mit5, and D10Mit7 were HH homozygotes (D. Krenova, DSc, et al, unpublished results, 1998). A total of 2 to 12 rats per RIS were used for phenotyping. For congenic and consomic strains, 6 to 9 rats were used (see Figures).
Thermal Response to Stress
BT after immobilization served as a reproducible marker of the
stress response. To control the variation of the circadian rhythm of
BT, all measurements were conducted between 8:00 and 12:00
AM in a calm environment. Naive animals were placed in
transparent plastic restraint holders routinely used for tail-cuff
determination of BP in rats, and a rectal thermal probe was introduced
(3 to 5 cm) rapidly and secured around the tail. Rectal temperature was
monitored continuously during the 20-minute immobilization period
(T20). Two rats could be recorded simultaneously, with
randomization of temperature probes and strains. Temperature
thermistors with a linearity between 5 and 45°C and a precision of
0.1°C were used (YSI 44202, Yellow Springs Instrument Co).
Genetic Analysis
The heritability of T20 under both dietary conditions was
estimated using the following formula: heritability=Va/(Va+Ve), where
Va is the interstrain variance and Ve, the intrastrain variance. Strain
distribution patterns of the thermal response to stress observed on
high and normal Na diets were correlated with those of 475 biochemical,
morphological, immunogenetic, and molecular genetic markers available
from the ratmap Internet site.13 The nomenclature
and descriptions are reported in detail elsewhere.9 14 The
linkage map of RIS covers 1139 centimorgans (cM) of the rat genome,
with a mean of 20 markers per Chr, as described
previously.14
Total Genome Linkage Search
Linkage analysis was performed with MapManager software,
which was developed and adapted by Dr K.F. Manly for QTL
analysis (MapManager QT v3.0b21).15 16 Because of
multiple simultaneous comparisons, stringent statistical
criteria were used to avoid false-positive linkages.9 We
ran a nonparametric permutation test, as proposed by Doerge
and Churchill17 and implemented on MapManager, which
estimates thresholds of findings specified by linkage statistics on
total genome scan. The thresholds used were 0.5, 0.05, and 0.001, which
corresponded to suggestive, significant, and highly significant
linkages, respectively.18 The permutation test was run for
each phenotype every 5 cM with 500 permutations to simulate the
experimental conditions of marker density and multiple comparisons
required, respectively. Pearson product-moment correlation
analysis, which is equivalent to the Holms t-test,
was also used. It provided signs of association and an estimate of the
percentage of variance of the trait explained by the locus
(r2).
Statistics
Data are presented as the mean±SEM obtained for a
single measurement in several animals, as indicated in Results. We
evaluated the relative contribution of each significant locus with a
multiple linear regression model. The dependent variable was T20,
and the independent predictors were the individual significant loci
taken as dichotomous variables (-1 for BN.Lx genotypes; 1
for SHR genotypes). The diet effect was also studied (-1 for
normal diet; 1 for the high-salt diet). All analyses were
considered significant at the 0.05
level. Data from individual rats
were computed.
| Results |
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T), and initial rate of increase, for which
heritability was 59% and 49%, respectively.
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Total Genome Linkage Search
The T20 phenotype was measured in 25 strains on a normal
salt diet and in 24 strains on a high-salt diet. This analysis
showed 2 QTLs on Chrs 10 and 12 (Figure 3) that were significantly
associated with the T20 phenotype. The most significant
association was found on Chr 10 (Myh3; r=0.58;
P=0.0017; logarithm of odds [LOD] score, 2.2; Figure 3a). This effect had greater significance when the stress
response was measured in rats fed a high Na diet (r=0.72;
P=0.00002; LOD score, 4.0). The effect of this QTL explains
the 34% and 52% of the T20 variance on normal and high Na intakes,
respectively.
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Another QTL was found on Chr 12, with a peak at the Lsn2 marker (r=0.49; P=0.014; LOD, 1.5; Figure 3b). Its effect on T20 was increased by high Na intake (r=0.69; P=0.000094; LOD, 3.3). The statistical significance of these results satisfied the stringent statistical criteria proposed by Neumann20 that are required for genome scan when using RIS.
Impact of Chr 10 QTL on the Stress Response: RNo10 Congenic
Strain
Chr 10 (RNo10) congenic rats were used to further assess the
impact of the locus identified in RIS. Figure 4a shows the size of the segment of BN
Chr 10 transferred onto a SHR genetic background. On both normal and
high Na diets, SHR.BN-RNo10 congenic rats displayed a lower T20 than
SHR (Figure 4b). In accordance with the results obtained with
the genome scan in RIS, the observed difference between SHR.BN-RNo10
and SHR had a greater significance with high Na intake (Figure 4b). Furthermore, the congenic paradigm narrowed down the region
impacting on T20.
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Negatively Correlated QTLs
Negatively correlated QTLs (ie, where BN alleles are
associated with a higher T20 value) were also found when the stress
response was measured in rats under normal Na intake. The most
suggestive one was identified on Chr 11, with a peak at the
Mit2 marker (P=0.0008; LOD, 2.5;
r=-0.65). Another negative QTL for T20 on normal diet was
on Chr 8 by the Cebr92s2 marker (P=0.0046; LOD,
1.9; r=-0.55). Interestingly, a locus for hyperactivity in
the WKHA rat has been localized in the same
region.21
Consomic Strains for the Y Chr: Modulation of the Stress
Response
Because the set of RIS used in this work originates from
reciprocal crosses of BN.Lx and SHR,8 11 we could evaluate
the putative effect of the Y Chr of SHR origin on the stress response.
Strains fed a normal Na diet and bearing the Y Chr of the SHR
progenitor (BxH strains) had a higher T20 than strains bearing the Y
Chr from the BN.Lx progenitor (HxB strains) (Figure 5a; P=0.02). To further
analyze this effect, we studied Y Chr consomic strains on the
normal Na diet. The transfer of the BN Y Chr to a SHR genetic
background in SHR.BN-Y consomics significantly lowered T20 compared
with SHR (38.40±0.29°C versus 39.00±0.16°C, respectively;
P<0.05; Figure 5b). Similarly, in Y Chr consomic
strains originating from WKY and SHR reciprocal
crosses,22 23 T20 was significantly lower in
SHR.WKY-Y strains, which bear the Y Chr from WKY and the autosomes of
SHR, than in SHR (38.64±0.21°C versus 39.15±0.07°C;
P=0.014; Figure 5b). It is noteworthy that the
removal of SHR-Y and its replacement with either BN-Y or WKY-Y produces
a similar decrease in T20. However, transfer of the Y Chr from SHR
(SHR-Y) to a WKY genetic background (in WKY.SHR-Y) did not
increase T20 compared with WKY (38.34±0.19°C versus 38.49±0.24°C;
P=NS). Therefore, it seems that removing SHR-Y from the SHR
background has more impact than adding it to a WKY background, which
suggests that SHR-Y is needed, but not sufficient for, the expression
of the stress response observed in SHR. The Y Chr thus seems to have a
modulating effect on autosomal QTLs of the stress response.
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Relative Contribution of Significant QTLs on T20
We evaluated the relative contribution of significant QTLs on T20
by multiple linear regression analysis. Two significant markers
were chosen as predictors of T20 (Myh3 on Chr 10 and
Lsn2 on Chr 12), as was the Y Chr, because the impact of the
latter on T20 has been confirmed in consomic strains. Data from
individual rats were computed to increase the power of
analysis, and the effect of the diet was evaluated. Due to
multicolinearity between the primary effects and their interactions,
the model cannot evaluate the impact of diet as an independent
variable. We could, nevertheless, estimate the impact of the high
Na diet by comparing a subset of 209 rats (21 RIS) for which stress was
measured on normal and high Na diets. We observed a significant overall
diet effect (ANOVA, P=0.022), in accordance with the results
shown in Figure 3, which are suggestive of a salt-sensitive
component of T20 at these loci. By combining the individual data under
both dietary conditions (n=242), the following regression equation was
derived: T20=Int38.694+(0.19xMyh3)+ (0.26xLsn2)+(0.14xMyh3xLsn2)+ (0.13xMyh3xYChr)
in which the adjusted r2=0.46.
This model predicts 46% of T20 variance. The results from multiple linear regression clearly demonstrated that the effect of the significant QTLs was not only additive, but that the 3 loci also interacted: the locus on Chr 10 with the locus on Chr 12, and the locus on Chr 10 with the Y Chr. For an interaction to be present, the effect of 1 locus depends on the genotype of the other locus.24 This is clearly illustrated in the Table: the interaction between the loci of Chrs 10 and 12 in which the highest T20 is seen only occurs when the SHR alleles at both loci are combined (ANOVA, P<0.0001 versus the 3 other genotypic combinations).
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| Discussion |
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Human Chr 7q21-q22 is syntenic to rat Chr 12 near the Lsn2
locus. A malignant hyperthermia susceptibility locus (Mhs3)
has been linked to this segment of Chr 7q. Malignant hyperthermia is a
pharmacogenetic disease that susceptible individuals develop on
exposure to ether and related anesthetics.27 It is
noteworthy that SHR are also hypersensitive to heat and
ether.3 4 28 Genes of calcium channel subunits and Na
channel subunits mapped on human Chr 7q have been proposed as candidate
genes for malignant hyperthermia.29 30 31 The
Hsp27 stress gene is also a good positional candidate on Chr
12, because we have shown its modulation by SHR-Y.32
Discussion of positional candidates must, however, be tempered by the
size of the estimated confidence intervals (
30 cM) associated with
these QTLs.
Finally, of great interest is the demonstration of interactions between 2 autosomal loci (Chr 10 with Chr 12 loci) and between the Y and autosomal Chrs (Chrs 10 and Y). Interactions between primary QTLs should be expected with polygenic phenotypes, such as the stress response. Hence, the use of different crosses may further help discriminate between confounding and primary QTLs, because different genetic backgrounds may reveal or hinder significant linkages.
This work opens the way for finding the genetic determinants of susceptibility to environmental stressors. Two major QTLs were detected, with a modulating impact of the Y Chr, as ascertained in congenic and consomic strains. These results provide the first step toward a full understanding of the genetic basis of the hyperresponse to environmental stress and its relevance to cardiovascular diseases.
| Acknowledgments |
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Received August 30, 1999; first decision September 28, 1999; accepted October 4, 1999.
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